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Latest update: 3/8/2007Listed alphabetically by first author
Vis Neurosci 20:189-209, 2003Cell density ratios in a foveal patch in macaque retina
Ahmad KM; Klug K; Herr S; Sterling P; Schein S
We examine the assumptions that the fovea contains equal numbers of inner (invaginating or ON) and outer (flat or OFF) midget bipolar cells and equal numbers of inner and outer diffuse bipolar cells. Based on reconstruction from electron photomicrographs of serial thin sections through the fovea of a macaque monkey, we reject both assumptions. First, every foveal L and M cone is presynaptic to one inner and one outer midget bipolar cell; however, S cones are presynaptic to one outer but no inner midget bipolar cell. Second, we measure the density of all foveal cells in the same patch of fovea, affording accurate cell density ratios. For each foveal cone pedicle, at a density of 26,500 mm^2, there is close to one (0.88) outer diffuse bipolar cell but only 0.40 inner diffuse bipolar cells. This asymmetry may be related to differences in resolution and sensitivity for light increments and decrements. We also find one (1.01) Müller cell, one (1.01) amacrine cell in the inner nuclear layer, and close to one (0.83) horizontal cell for each cone pedicle. In addition, for each S cone, there are two inner S-cone bipolar cells and two small bistratified ganglion cells. In total, there are 3.4 cone bipolar cells per cone but only 2.6 ganglion cells per cone. The latter ratio is enough to accommodate one midget ganglion cell for each midget bipolar cell.
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Vis Neurosci 21:913-924, 2004Postsynaptic calcium feedback between rods and rod bipolar cells in the mouse retina.
Berntson A; Smith RG; Taylor WR
Light-evoked currents were recorded from rod bipolar cells in a dark-adapted mouse retinal slice preparation. Low-intensity light steps evoked a sustained inward current. Saturating light steps evoked an inward current with an initial peak that inactivated, with a time constant of about 60-70 ms, to a steady plateau level that was maintained for the duration of the step. The inactivation was strongest at hyperpolarized potentials, and absent at positive potentials. Inactivation was mediated by an increase in the intracellular calcium concentration, as it was abolished in cells dialyzed with 10 mM BAPTA, but was present in cells dialyzed with 1 mM EGTA. Moreover, responses to brief flashes of light were broader in the presence of intracellular BAPTA indicating that the calcium feedback actively shapes the time course of the light responses. Recovery from inactivation observed for paired-pulse stimuli occurred with a time constant of about 375 ms. Calcium feedback could act to increase the dynamic range of the bipolar cells, and to reduce variability in the amplitude and duration of the single-photon signal. This may be important for nonlinear processing at downstream sites of convergence from rod bipolar cells to AII amacrine cells. A model in which intracellular calcium rapidly binds to the light-gated channel and reduces the conductance can account for the results.
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Vis Neurosci 21:693-702 (2004)Transmission of single photon signals through a binary synapse in the mammalian retina.
Berntson A; Smith RG; Taylor WR
At very low light levels the sensitivity of the visual system is determined by the efficiency with which single photons are captured, and the resulting signal transmitted from the rod photoreceptors through the retinal circuitry to the ganglion cells and on to the brain. Although the tiny electrical signals due to single photons have been observed in rod photoreceptors, little is known about how these signals are preserved during subsequent transmission to the optic nerve. We find that the synaptic currents elicited by single photons in mouse rod bipolar cells have a peak amplitude of 5-6 pA, and that about 20 rod photoreceptors converge upon each rod bipolar cell. The data indicates that the first synapse, between rod photoreceptors and rod bipolar cells, signals a binary event: the detection, or not, of a photon or photons in the connected rod photoreceptors. We present a simple model that demonstrates how a threshold nonlinearity during synaptic transfer allows transmission of the single photon signal, while rejecting the convergent neural noise from the 20 other rod photoreceptors feeding into this first synapse.
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J Comp Neurol453:100-111, 2002
How Müller glial cells in macaque fovea coat and isolate the synaptic terminals of cone photoreceptors
Burris C; Klug K; Ngo IT; SterlingP; Schein S
A cone synaptic terminal in macaque fovea releases quanta of glutamate from ~20 active zones at a high rate in the dark. The transmitter reaches ~500 receptor clusters on bipolar and horizontal cell processes by diffusion laterally along the terminal's 50 µm^2 secretory face and ~2 µm inward. To understand what shapes transmitter flow, we investigated from electron photomicrographs of serial sections the relationship between Müller glial processes and cone terminals. We find that each Müller cell has one substantial trunk that ascends in the outer plexiform layer below the space between the "footprints" of the terminals. We find exactly equal numbers of Müller cell trunks and foveal cone terminals, which may make the fovea particularly vulnerable to Müller cell dysfunction. The processes that emerge from the single trunk do not ensheathe a single terminal. Instead, each Müller cell partially coats two to three terminals; in turn, each terminal is completely coated by two to three Müller cells. Therefore, the Müller cells that coat one terminal also partially coat the surrounding (~ six) terminals, creating a common environment for the cones supplying the center/surround receptive field of foveal midget bipolar and ganglion cells. Upon reaching the terminals, the trunk divides into processes that coat the terminals' sides but not their secretory faces. This glial framework minimizes glutamate transporter (EAAT1) beneath a terminal's secretory face but maximizes EAAT1 between adjacent terminals, thus permitting glutamate to diffuse locally along the secretory face and inward toward inner receptor clusters but reducing its effective spillover to neighboring terminals.
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Nature 371(6492):70-72, 1994
M and L cones in macaque fovea connect to midget ganglion cells by different numbers of excitatory synapses.
Calkins DJ; Schein SJ; Tsukamoto Y; Sterling P
Visual acuity depends on the fine-grained neural image set by the foveal cone mosaic. To preserve this spatial detail, cones transmit through non-divergent pathways: cone-->midget bipolar cell-->midget ganglion cell. Adequate gain must be established along each pathway; crosstalk and sources of variation between pathways must be minimized. These requirements raise fundamental questions regarding the synaptic connections: (1) how many synapses from bipolar to ganglion cell transmit a cone signal a nd with what degree of crosstalk between adjacent pathways; (2) how accurately these connections are reproduced across the mosaic; and (3) whether the midget circuits for middle (M) and long (L) wavelength sensitive cones are the same. We report here that the midget ganglion cell collects without crosstalk either 28 ± 4 or 47 ± 3 midget bipolar synapses. Two cone types are defined by this difference; being about equal in number and distributing randomly in small clusters of like type, they are probably M and L.
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Nature 381(6583):613-615, 1996Absence of spectrally specific lateral inputs to midget ganglion cells in primate retina.
Calkins DJ; Sterling P
Visual information is conveyed to the brain by the retinal ganglion cells. Midget ganglion cells serve fine spatial vision by summing excitation from a receptive field 'centre', receiving input from a single cone in the central retina, with lateral inhibition from a receptive field 'surround', receiving input from many surrounding cones. Midget ganglion cells are also thought to serve colour opponent vision because the centre excitation is from a cone of one spectral type, while the surround inhibition is from cones of the other type. The two major cone types, middle (M)- and long-(L) wavelength sensitive, are equally numerous and randomly distributed in the primate central retina, so a spectrally homogeneous surround requires that the cells mediating lateral interactions (horizontal or amacrine cells) receive selective input from only one cone type. Horizontal cells cannot do this because they receive input indiscriminately from M and L cones. Here we report that the amacrine cells connected to midget ganglion cells are similarly indiscriminate. The absence of spectral specificity in the inhibitory wiring raises doubt about the involvement of midget ganglion cells in colour vision and suggest that colour opponency may instead be conveyed by a different type of ganglion cell.
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Vision Res 36(21):3373-3381, 1996Foveal cones form basal as well as invaginating junctions with diffuse ON bipolar cells.
Calkins DJ; Tsukamoto Y; Sterling P
The response of a mammalian bipolar cell is generally thought to be determined by the location and morphology of synapses from the cone terminal: ON bipolar cells are believed to be depolarized strictly at invaginating contacts and OFF bipolar cells hyperpolarized at basal contacts. This hypothesis was re-investigated in the macaque fovea (1 deg nasal) using electron micrographs of serial sections. We determined the number of invaginating sites available and then identified the contacts to bipolar cells with axons in the ON level of the inner plexiform layer. A cone terminal forms about 20 active zones marked by ribbons. A few active zones house two invaginating dendrites, so there are 22 invaginating sites per cone. A midget ON bipolar cell collects 18 invaginating contacts from one cone, thus only about four invaginating sites remain for diffuse ON bipolar cells. Two diffuse ON cells were reconstructed; each collects about 25 contacts from an estimated 10 cones. Only three or four of these contacts are invaginating; the rest are basal, adjacent to the triad. This suggests that basal contacts can be depolarizing. The distance from the vesicle release site at active zones to an invaginating contact is 140 ± 40 nm; to a basal contact adjacent to the t riad is 500 ± 160 nm, and to the next nearest basal contact is 950 ± 370 nm.
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J Neurosci 18(9):3373-3385, 1998Microcircuitry and mosaic of a blue-yellow ganglion cell in the primate retina.
Calkins DJ; Tsukamoto Y; Sterling P.
Perception of hue is opponent, involving the antagonistic comparison of signals from different cone types. For blue versus yellow opponency, the antagonism is first evident at a ganglion cell with firing that increases to stimulation of short wavelength-sensitive (S) cones and decreases to stimulation of middle wavelength-sensitive (M) and long wavelength-sensitive (L) cones. This ganglion cell, termed blue-yellow (B-Y), has a distinctive morphology with dendrites in both ON and OFF strata of the inner plexiform layer (Dacey and Lee, 1994). Here we report the synaptic circuitry of the cell and its spatial density. Reconstructing neurons in macaque fovea from electron micrographs of serial sections, we identified six ganglion cells that branch in both strata and have similar circuitry. In the ON stratum each cell collects approximately 33 synapses from bipolar cells traced back exclusively to invaginating contacts from S cones, and in the OFF stratum each cell collects approximately 14 synapses from bipolar cells (types DB2 and DB3) traced to basal synapses from approximately 20 M and L cones. This circuitry predicts that spatially coincident blue-yellow opponency arises at the level of the cone output via expression of different glutamate receptors. S cone stimuli suppress glutamate release onto metabotropic receptors of the S cone bipolar cell dendrite, thereby opening cation channels, whereas M and L cone stimuli suppress glutamate release onto ionotropic glutamate receptors of DB2 and DB3 cell dendrites, thereby closing cation channels. Although the B-Y cell is relatively rare (3% of foveal ganglion cells), its spatial density equals that of the S cone; thus it could support psychophysical discrimination of a blue-yellow grating down to the spatial cutoff of the S cone mosaic.
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Neuron 24(2):313-21, 1999Evidence that circuits for spatial and color vision segregate at the first retinal synapse
Calkins DJ; Sterling P
Different psychophysical "channels mediate our perception of fine spatial patterns and color. The spatial channel compares the photon catch between adjacent cones, regardless of type, and thus supports perceptions as fine as the cone mosaic (Williams, 1986). Color channels compare the photon catch between different cone types (Hurvich and Jameson, 1957; Krauskopf et al., 1982). The "blue/yellow" channel compares the catch in cones sensitive to short (S) wavelengths with the catch in neighboring cones sensitive to middle (M) and long (L) wavelengths (Pugh and Mollon, 1979). The "red/green" channel compares the photon catch in L and S cones with that in neighboring M cones (Thornton and Pugh, 1983; Calkins et al., 1992). Each comparison is equivalent to a subtraction: for blue/yellow, S-(M+L); for red/green, L-M+(S). The color channels are rather coarse, with an acuity of about one-third that of the spatial channel (Anderson et al., 1991; Sekiguchi et al., 1993). What neural circuits service the psychophysical channels for spatial acuity and color? The standard view is that messages for both channels are conveyed by one type of ganglion cell, termed "P", because it projects to a parvocellular neuron in the lateral geniculate nucleus. The parvocellular neuron would relay both messages to primary visual cortex where, finally, spatial and color information would begin to segregate (Ingling and Martinez-Uriegas, 1983; Lennie and D'Zmura, 1988; Hubel and Livingstone, 1990; De Valois and De Valois, 1993). This circuit design, by compressing two messages on one axon, would minimize the number of axons in the optic nerve and geniculo-striate radiation. But it would also compromise the filtering of one or both messages and would reduce their shares of bandwidth (Atick, 1992). Although this "double duty" hypothesis for the P cell still predominates (e.g., Boycott and Wässle, 1999), its anatomical and physiological basis has recently been eroded. Meanwhile new evidence has accumulated for an alternative view, that the densely distributed P cell serves only the fine spatial channel and that sparsely distributed ganglion cells serve the color channels (Rodieck, 1991). This circuit design would optimize filtering and bandwidth at the cost of additional axons. Here, we summarize the current evidence for both views, starting with the origin of the P cell hypothesis.
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J Neurosci 27(10):2646-2653, 2007Microcircuitry for two types of achromatic ganglion cell in primate fovea
Calkins D; Sterling P
Synaptic circuits in primate fovea have been quantified for midget/parvocellular ganglion cells. Here, based on partial reconstructions from serial electron micrographs, we quantify synaptic circuits for two other types of ganglion cell: the familiar parasol/magnocellular cell and a smaller type, termed “garland.” The excitatory circuits both derive from two types ofOFFdiffuse cone bipolar cell,DB3and DB2, which collected unselectively from at least 6±1 cones, including the S type. Cone contacts to DB3 dendrites were usually located between neighboring triads, whereas half of the cone contacts to DB2 were triad associated. Ribbon outputs were as follows: DB3, 69 ± 5; DB2, 48 ± 4. A complete parasol cell (30 µm dendritic field diameter) would collect from ~50 cones via ~120 bipolar and ~85 amacrine contacts; a complete garland cell (25 µm dendritic field) would collect from ~40 cones via ~75 bipolar and ~145 amacrine contacts. The bipolar types contributed differently: the parasol cell received most contacts (60%) from DB3, whereas the garland cell received most contacts (67%) from DB2. We hypothesize that DB3 is a transient bipolar cell and that DB2 is sustained. This would be consistent with their relative inputs to the brisk-transient (parasol) ganglion cell. The garland cell, with its high proportion of DB2 inputs plus its high proportion of amacrine synapses (70%) and dense mosaic, might correspond to the local-edge cell in nonprimate retinas, which serves finer acuity at low temporal frequencies. The convergence of S cones onto both types could contribute S-cone input for cortical areas primary visual cortex and the middle temporal area.
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Neuron 48:555-562, 2005Encoding light intensity by the cone photoreceptor synapse
Choi S-Y; Borghuis B; Rea R; Levitan ES; Sterling P; Kramer RH
How cone synapses encode light intensity determines the precision of information transmission at the first synapse on the visual pathway. Although it is known that cone photoreceptors hyperpolarize to light over 4–5 log units of intensity, the relationship between light intensity and transmitter release at the cone synapse has not been determined. Here, we use two-photon microscopy to visualize release of the synaptic vesicle dye FM1-43 from cone terminals in the intact lizard retina, in response to different stimulus light intensities. Wethen employ electron microscopy to translate these measurements into vesicle release rates. We find that from darkness to bright light, release decreases from 49 to w2 vesicles per 200 ms; therefore, cones compress their 10,000-fold operating range for phototransduction into a 25-fold range for synaptic vesicle release. Tonic release encodes ten distinguishable intensity levels, skewed to most finely represent bright light, assuming release obeys Poisson statistics.
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J Comp Neurol 250(1):1-7, 1986Accumulation of [3H]glycine by cone bipolar neurons in the cat retina.
Cohen E; Sterling P
Cone bipolar neurons in the cat retina were studied in serial sections prepared as electron microscope autoradiograms following intravitreal injection of [3H]glycine. The goal was to learn whether the cone bipolar types that accumulate glycine correspond to the types thought on other grounds to be inhibitory. About half of the cone bipolars in a given patch of retina showed specific accumulation of silver grains. The specificity of accumulation was similar to that shown by glycine-accumulating amacrines. All of the cone bipolars arborizing in sublamina b accumulated glycine but none of the cone bipolars arborizing in sublamina a did so. The types of cone bipolars accumulating glycine did not match the types thought to be inhibitory. Cone bipolar types CBb1 and CBb2 both form gap junctions with the glycine-accumulating AII amacrine, thus raising the possibility that glycine might accumulate in these cone bipolars by diffusion from the AII cell or vice versa. Thus it is logically impossible to tell which of these three cells contains a high-affinity uptake mechanism for glycine and consequently which of the three might actually use glycine as a neurotransmitter.
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Philos Trans R Soc Lond B Biol 330(1258):305-321, 1990Demonstration of cell types among cone bipolar neurons of cat retina.
Cohen E; Sterling P
We identified all the cone bipolar cells (80) in a small patch of one retina and then studied in detail the complete subset (42) that sends axons to sublamina b of the inner plexiform layer. The point was to learn whether the 'types' suggested previous ly, based on a few examples from a large population, could be substantiated or whether there would be intermediate forms. Tissue from the area centralis (1 degree eccentricity), was prepared as a series of 279 ultrathin sections and photographed in the el ectron microscope. Thirteen cells were reconstructed completely and parcelled into five categories (b1-b5) based on external morphology. For nine of these cells (two from categories b1-b4 and one from b5) most of the synaptic inputs and outputs were ident ified. When these nine cells were parcelled according to their synaptic patterns, they sorted into the same five categories. The remaining 29 cells in the population, though not reconstructed, were studied in detail by tracing their processes through the series. Ten of these cells, those near the margin of the series, were incomplete. The other 19 cells had essentially the same distribution of morphologies and synaptic patterns as the subset studied by total reconstruction: when plotted in multiparametric space, they formed distinct clusters corresponding to the five morphological categories. There was no hint of intermediate forms. That all the neurons in the population sort into some cluster (no intermediate forms), and that each neuron sorts into the same cluster by different criteria, argues that the clusters represent natural types. Each type forms a regular array in the region studied with an axonal 'coverage factor' that is close to one.
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Philos Trans R Soc Lond B Biol 330(1258):323-328, 1990Convergence and divergence of cones onto bipolar cells in the central area of cat retina.
Cohen E; Sterling P
In the central area of cat retina the cone bipolar cells that innervate sublamina b of the inner plexiform layer comprise five types, four with narrow dendritic fields and one with a wide dendritic field. This was shown in the preceding paper (Cohen &a mp; Sterling 1990 a) by reconstruction from electron micrographs of serial sections. Here we show by further analysis of the same material that the coverage factor (dendritic spread x cell density) is about one for each of the narrow-field types (b1, b2, and b4). The same is probably true for the other narrow-field type (b3), but this could not be proved because its dendrites were too fine to trace. The dendrites of types b1, b2, and b4 collect from all the cone pedicles within their reach and do not bypa ss local pedicles in favour of more distant ones. The dendrites of type b5, the wide-field cell, bypass many pedicles. On average 5.1 ± 1.0 pedicles coverage on a b1 bipolar cell; 6.0 ± 1.2 converge on a b2 cell and 5.7 ± 1.5 converge on a b4 cell. Divergence within a type is minimal: one pedicle contacts only 1.2 b1 cells, 1.0 b2 cells, and 1.0 b4 cells. Divergence across types is broad: each pedicle apparently contacts all four types of the narrow-field bipolar cells that innervate sublamina b. Ea ch pedicle probably also contacts an additional 4-5 types of narrow-field bipolar cell that innervate sublamina a. There are several possible advantages to encoding the cone signal into multiple, parallel, narrow-field pathways.
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J Neurophysiol 65(2):352-359, 1991Microcircuitry related to the receptive field center of the on-beta ganglion cell.
Cohen E; Sterling P
1. We have investigated the anatomic basis for the Gaussian-like receptive field center of the on-beta ("X") ganglion cell in the area centralis of cat retina. Three adjacent on-beta cells were reconstructed from electron micrographs of 279 s erial sections cut vertically through a patch of retina at approximately 1 degree eccentricity. 2. All the bipolar synapses on these cells were identified, and about one-half of these were traced to type b1 bipolar cells, which formed a regular array in t he plane of the retina. 3. On average, seven b1 cells contributed to a beta cell: bipolar axons near the middle of the beta dendritic field tended to give many contacts (12-33 contacts); axons near the edge of the field tended to give few contacts (3-4 co ntacts). 4. Each b1 cell collected from four to seven cones, and the mean number of cones converging through the b1 array to a beta cell was 30. 5. Assuming equal effectiveness for all b1->beta cell synapses, a spatial weighting function was derived fro m these results. The mean radius of this function at 1/e amplitude for three beta cells was 18.0 ± 1.1 (SD) microns. This is considerably narrower than the corresponding measurements of the beta cell receptive field center (28 ± 3 microns) at this ecc entricity. 6. It is concluded, in agreement with previous work, that all cones encompassed by the beta cell's dendritic field and those slightly beyond it connect directly to the beta cell via the b1 bipolar cell array. However, the center of the beta cel l receptive field is still broader by approximately 60%. This suggests that pooling of cone signals may occur at the level of the outer plexiform layer.
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Eur J Neurosci 4:506-520, 1992Parallel circuits from cones to the On-beta ganglion cell.
Cohen E; Sterling P
Neural integration depends critically upon circuit architecture; yet the architecture has never been established quantitatively (numbers of cells and synapses) for any vertibrate local circuit. Here we describe circuits in the cat retina that connect cones to the on-beta ganglion cell. This cell type is important because on- and off-beta cells contribute about 50% of the optic nerve fibers and the major retinal input to the striate cortex. Three adjacent on-beta cells in the area centralis and their bipolar connections to cones were reconstructed from electron micrographs of 279 serial section. The beta dendritic field is 34±2 micrometers in diameter and encompasses 35 cones. All of these cones connect to the beta cell via 14-17 bipolar cells. These bipolar cells were shown previously by cluster analysis to be of four types (b1-b4); three of these types (b1, b2 and b3) provided 97% of the bipolar contacts to the beta cell, in the ratio 4:2:1. On average, bipolar cells nearest the centre of the beta dendritic field contribute more synapses than those towards the edge, but the peaked distribution of bipolar synapses across the dendritic field is only slightly broader than the optical pointspread function of the cat's eye, and is narrower by half than the centre of the ganglion cell receptive field. This implies that the distribution of bipolar synapses across the beta cell dendritic field contributes little to the extent or shape of the receptive field. Since all three bipolar circuits connect to the same set of cones, they must carry the same spatial and chromatic information; they might convey different temporal frequencies. The numbers of bipolar synapses (mean±SD = 154±8) and amacrine synapses (59±5) converging on three adjacent beta cells are remarkably constant (SD approx. equals 5% of the mean). Thus, as the circuits repeat locally, the fundamental design is accurately reproduced.
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J Comp Neurol 188(4):599-627, 1979
Microcircuitry of cat visual cortex: classification of neurons in layer IV of area 17, and identification of the patterns of lateral geniculate input.
Davis TL; Sterling P
Neurons in the cerebral cortex have been classified primarily by their differences in axonal and dendritic branching patterns observed in material impregnated by the Golgi method. Although these morphological differences are widely believed to reflect differences in connectivity, very little is actually known about the patterns of synaptic input to different cell types. We have obtained such information for 32 adjacent neurons in layer IVab of cat cortical area 17 by reconstructing them from electron micrographs of 150 serial sections. Synaptic terminals from the lateral geniculate nucleus were labeled in this material by anterograde degeneration and their distribution, as well as that of normal terminals containing flat or round vesicles, was recorded . The neurons were divided into seven classes based on differences in size, shape, dendritic branching pattern and synaptic input. Class I cells were pyramidal with apical and basilar dendrites, dendritic spines, exclusively flat-vesicle terminals on the somas (11/100 micron2), and geniculate terminals on the basilar dendrites. Class II cells were large stellates (20 micron diameter) with dark cytoplasm and numerous flat-vesicle and round-vesicle terminals on the somas (48/100 micron2). Geniculate terminals contacted the cell bodies and primary, secondary, and tertiary dendrites. The Class III cell was stellate with varicose dendrites, a sparse distribution of flat-vesicle terminals (8/100 micron2) on the soma, and both geniculate and round-vesicle termi nals on the dendrites. Class IV cells had radially elongated somas with sharply tapered apical and basilar dendrites bearing spines. There was a medium distribution of flat-vesicles terminals (17/100 mu2), to the somas while geniculate terminals were restricted to the secondary dendrites. Class V cells were multipolar with flat-vesicle terminals on the somas (11/100 micron2) and a few geniculate terminals on the dendrites. Class VI cells were mostly small (as small as 7 micron diameter), with a sparse dis tribution on the somas of both flat-vesicle terminals (7/100 micron2). Two cells had geniculate terminals on their somas. Class VII cells had sharply tapered apical and basilar dendrites, both flat-vesicle and round-vesicle terminals on the somas (14/100 micron2), and no geniculate input. The results make clear that the neurons in layer IVab are quite heterogeneous, not merely in their intrinsic morphology, but also in their patterns of connectivity. The geniculate input is not funneled to a single type o f neuron but diverges widely, contacting at least six different cell types, and may form on each a pattern that is characteristic for the type. The reconstruction approach, in providing a detailed identification of the synaptic patterns on substantial num bers of adjacent cells, should make it possible to address directly certain unanswered questions about cortical circuitry.
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Proc Natl Acad Sci USA 94:13363-13366, 1997Brain activity in visual cortex predicts individual differences in reading performance
Demb JB; Boynton GM; Heeger DJ
The relationship between brain activity and reading performance was examined to test the magnocellular (M) pathway. Functional magnetic resonance imaging was used to measure brain activity in dyslexic and control subjects in conditions designed to preferentially stimulate the M pathway. Dyslexics showed reduced activity compared with controls both in the primary visual cortex and in a secondary cortical visual area (MT1) that is believed to receive a strong M pathway input. Most importantly, significant correlations were found between individual differences in reading rate and brain activity. These results support the hypothesis for an M pathway abnormality in dyslexia and imply a strong relationship between the integrity of the M pathway and reading ability.
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J Neurosci 19(22):9756-9767, 1999Functional circuitry of the retinal ganglion cell's nonlinear receptive field.
Demb JB; Haarsma L; Freed MA; Sterling P
A retinal ganglion cell commonly expresses two spatially overlapping receptive field mechanisms. One is the familiar "center/surround," which sums excitation and inhibition across a region somewhat broader than the ganglion cell's dendritic field. This mechanism responds to a drifting grating by modulating firing at the drift frequency (linear response). Less familiar is the "nonlinear" mechanism, which sums the rectified output of many small subunits that extend for millimeters beyond the dendritic field. This mechanism responds to a contrast-reversing grating by modulating firing at twice the reversal frequency (nonlinear response). We investigated this nonlinear mechanism by presenting visual stimuli to the intact guinea pig retina in vitro while recording intracellularly from large brisk and sluggish ganglion cells. A contrast-reversing grating modulated the membrane potential (in addition to the firing rate) at twice the reversal frequency. This response was initially hyperpolarizing for some cells (either ON or OFF center) and initially depolarizing for others. Experiments in which responses to bars were summed in-phase or out-of-phase suggested that the single class of bipolar cells (either ON or OFF) that drives the center/surround response also drives the nonlinear response. Consistent with this, nonlinear responses persisted in OFF ganglion cells when ON bipolar cell responses were blocked by L-AP-4. Nonlinear responses evoked from millimeters beyond the ganglion cell were eliminated by tetrodotoxin. Thus, to relay the response from distant regions of the receptive field requires a spiking interneuron. Nonlinear responses from different regions of the receptive field added linearly.
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J Neurosci 21:7447-7454, 2001Bipolar cells contribute to nonlinear spatial summation in the brisk-transient (Y) ganglion cell in mammalian retina.
Demb JB; Zaghloul K; Haarsma L; Sterling P
The receptive field of the Y-ganglion cell comprises two excitatory mechanisms: one integrates linearly over a narrow field, and the other integrates nonlinearly over a wide field. The linear mechanism has been attributed to input from bipolar cells, and the nonlinear mechanism has been attributed to input from a class of amacrine cells whose nonlinear "subunits" extend across the linear receptive field and beyond. However, the central component of the nonlinear mechanism could in theory be driven by bipolar input if that input were rectified. Recording intracellularly from the Y-cell in guinea pig retina, we blocked the peripheral component of the nonlinear mechanism with tetrodotoxin and found the remaining nonlinear receptive field to be precisely co-spatial with the central component of the linear receptive field. Both linear and nonlinear mechanisms were caused by an excitatory postsynaptic potential that reversed near 0 mV. The nonlinear mechanism depended neither on acetylcholine nor on feedback involving GABA or glycine. Thus the central components of the ganglion cell's linear and nonlinear mechanisms are apparently driven by synapses from the same rectifying bipolar cell.
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Neuron 32:711-721, 2001Cellular basis for the response to second-order motion cues in Y retinal ganglion cells.
Demb J; Zaghloul K; Sterling P
We perceive motion when presented with spatiotemporal changes in contrast (second-order cue). This requires linear signals to be rectified and then summed in temporal order to compute direction. Although both operations have been attributed to cortex, rectification might occur in retina, prior to the ganglion cell. Here we show that the Y ganglion cell does indeed respond to spatiotemporal contrast modulations of a second-order motion stimulus. Responses in an OFF ganglion cell are caused by an EPSP/IPSP sequence evoked from within the dendritic field; in ON cells inhibition is indirect. Inhibitory effects, which are blocked by tetrodotoxin, clamp the response near resting potential thus preventing saturation. Apparently the computation for second-order motion can be initiated by Y cells and completed by cortical cells that sum outputs of multiple Y cells in a directionally selective manner.
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J Neurophysiol 92:2510-2519, 2004Electrical coupling between mammalian cones
Demb JB; Sterling P; Freed MA
Synaptic vesicles are released stochastically, and therefore stimuli that increase a neuron’s synaptic input might increase noise at its spike output. Indeed this appears true for neurons in primary visual cortex, where spike output variability increases with stimulus contrast. But in retinal ganglion cells, although intracellular recordings (with spikes blocked) showed that stronger stimuli increase membrane fluctuations, extracellular recordings showed that noise at the spike output is constant. Here we show that these seemingly paradoxical findings occur in the same cell and explain why. We made intracellular recordings from ganglion cells, in vitro, and presented periodic stimuli of various contrasts. For each stimulus cycle, we measured the response at the stimulus frequency (F1) for both membrane potential and spikes as well as the spike rate. The membrane and spike F1 response increased with contrast, but noise (SD) in the F1 responses and the spike rate was constant. We also measured membrane fluctuations (with spikes blocked) during the response depolarization and found that they did increase with contrast. However, increases in fluctuation amplitude were small relative to the depolarization (10% at high contrast). A model based on estimated synaptic convergence, release rates, and membrane properties accounted for the relative magnitudes of fluctuations and depolarization. Furthermore, a cell’s peak spike response preceded the peak depolarization, and therefore fluctuation amplitude peaked as the spike response declined. We conclude that two extremely general properties of a neuron, synaptic convergence and spike generation, combine to minimize the effects of membrane fluctuations on spiking.
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Current Biology 12:1900-1907, 2002Electrical coupling between mammalian cones
DeVries S; Qi X; Smith R; Makous W; Sterling P
Background
Cone photoreceptors are noisy because of random fluctuations of photon absorption, signaling molecules, and ion channels. However, each cone’s noise is independent of the others, whereas their signals are partially shared. Therefore, electrically coupling the synaptic terminals prior to forward transmission and subsequent nonlinear processing can appreciably reduce noise relative to the signal. This signal-processing strategy has been demonstrated in lower vertebrates with rather coarse vision, but its occurrence in mammals with fine acuity has been doubted (even though gap junctions are present) because coupling would blur the neural image.
Results
In ground squirrel retina, whose triangular cone lattice resembles the human fovea, paired electrical recordings from adjacent cones demonstrated electrical coupling with an average conductance of approximately 320 pS. Blur caused by this degree of coupling had a space constant of approximately 0.5 cone diameters. Psychophysical measurements employing laser interferometry to bypass the eye’s optics suggest that human foveal cones experience a similar degree of neural blur and that it is invariant with light intensity. This neural blur is narrower than the eye's optical blur, and we calculate that it should improve the signal-to-noise ratio at the cone terminal by about 77%.
Conclusions
We conclude that the gap junctions observed between mammalian cones, including those in the human fovea, represent genuine electrical coupling. Because the space constant of the resulting neural blur is less than that of the optical blur, the signal-to-noise ratio can be markedly improved before the nonlinear stages with little compromise to visual acuity.
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J Neurosci. 20(24):9053-9058, 2000The light response of ON bipolar neurons requires G[alpha]o.
Dhingra A; Lyubarsky A; Jiang M; Pugh EN Jr; Birnbaumer L; Sterling P; Vardi N
ON bipolar neurons in retina detect the glutamate released by rods and cones via metabotropic glutamate receptor 6 (mGluR6), whose cascade is unknown. The trimeric G-protein G(o) might mediate this cascade because it colocalizes with mGluR6. To test this, we studied the retina in mice negative for the alpha subunit of G(o) (Galpha(o)-/-). Retinal layering, key cell types, synaptic structure, and mGluR6 expression were all normal, as was the a-wave of the electroretinogram, which represents the rod and cone photocurrents. However, the b-wave of the electroretinogram, both rod- and cone-driven components, was entirely missing. Because the b-wave represents the massed response of ON bipolar cells, its loss in the Galpha(o) null mouse establishes that the light response of the ON bipolar cell requires G(o). This represents the first function to be defined in vivo for the alpha subunit of the most abundant G-protein of the brain.
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J Neurosci. 22(12):4878-4884, 2002The light response of retinal ON bipolar cells requires a specific splice variant of Gao.
Dhingra A; Jiang M; Wang, T-L; Lyubarsky A; Savchenko A; Bar-Yehuda T; Sterling P; Birnbaumer L; Vardi N
Glutamate released onto retinal ON bipolar neurons binds to a metabotropic receptor to activate a heterotrimeric G-protein (Go) that ultimately closes a nonspecific cation channel. Signaling requires the alpha subunit (Gao), but its effector is unknown. Because Gao is transcribed into two splice variants (ao1 and ao2) that differ in the key GTPase domain, the next step in elucidating this pathway was to determine which splice variant carries the signal. Here we show by reverse transcription-PCR and Western blots that retina expresses both splice variants. Furthermore, in situ hybridization and immunostaining on mouse retina deficient in one splice variant or the other show that both ao1 and ao2 are expressed by ON bipolar cells but that ao1 is much more abundant. Finally, electroretinography performed on mice deficient for one splice variant or the other shows that the positive b-wave (response of ON bipolar cells to rod and cone input) requires ao1 but not ao2. Thus, the light response of the ON bipolar cell is probably carried by its strongly expressed splice variant, Gao1.
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J Neurosci 24(25):5684-5693, 2004A Retinal-Specific Regulator of G-Protein Signaling Interacts with G(alpha)o and Accelerates an Expressed Metabotropic Glutamate Receptor 6 Cascade.
Dhingra A; Faurobert E; Dascal N; Sterling P; Vardi N
Go is the most abundant G-protein in the brain, but its regulators are essentially unknown. In retina, Gao1 is obligatory in mediating themetabotropic glutamate receptor 6 (mGluR6)-initiated ON response. To identify the interactors of Go , we conducted a yeast two-hybrid screen with constituitively active Gao as a bait. The screen frequently identified a regulator of G-protein signaling (RGS), Ret-RGS1, the interaction of which we confirmed by coimmunoprecipitation with Gao in transfected cells and in retina. Ret-RGS1 localized to the dendritic tips of ON bipolar neurons, along with mGluR6 and Gao1.When Ret-RGS1 was coexpressed in Xenopus oocytes with mGluR6, Gao1, and a GIRK (G-protein-gated inwardly rectifyingK+) channel, it accelerated the deactivation of the channel response to glutamate in a concentration-dependent manner. Because light onset suppresses glutamate release from photoreceptors onto the ON bipolar dendrites, Ret-RGS1 should accelerate the rising phase of the light response of the ON bipolar cell. This would tend to match its kinetics to that of the OFF bipolar that arises directly from ligand-gated channels.
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Int J Dev Neurosci 19:533-540, 2001Synaptic development in semi-dissociated cultures of rat retina
Dhingra NK; Reddy R; Govindaiah; Hemavathy U; Raju TR; Ramamohan Y
Cultured neurons provide a simpler and more accessible environment to study the synaptic physiology. However, it is not clear if development of synapses in culture is similar to that in the in vivo condition. We studied the developmental sequence and morphological differentiation of chemical synapses in semi-dissociated rat retinal cultures that consisted of dissociated neurons as well as undissociated retinal aggregates. Synapses were quantified by synaptophysin immunoreactive puncta. During second week of in vitro development the average number of chemical synapses on the cell body decreased while that on the neurites increased significantly. Conventional synapses appeared both in aggregate and in dissociated neurons, with the developmental profile similar to that reported for in vivo retina. In contrast, the development of ribbon synapses was adversely affected by the in vitro microenvironment as suggested by following observations. The ribbon synapses were more frequently found in aggregate than in dissociated neurons, and were not associated with dyadic or triadic synaptic arrangement. The photoreceptor ribbons did not contact a postsynaptic process while bipolar ribbons made single (monadic) synapses. Further, photoreceptor ribbons in dissociated neurons were late to form and took more time to mature as compared to those in the aggregate cultures. Most of the rod bipolar cells, identified by their immunoreactivity to protein kinase C (PKC), had three or more neurites. Unlike in the in vivo retina, the dissociated rod bipolar cells did not show any PKC immunoreactive varicosities, suggesting that they failed to develop a well-differentiated synaptic terminal. Interestingly, we did not find any parvalbumin positive AII amacrine cells that are normally postsynaptic to rod bipolar cells. These results show that the conventional synapses of retina, which are similar to chemical synapses in other parts of the brain, develop normally both in aggregate and dissociated neurons. However, the highly specialized ribbon synapses have more stringent developmental requirements, and their normal development may require the presence of postsynaptic neurons in their close vicinity.
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J Neurophysiol 89(5):2360-2369, 2003Contrast threshold of a brisk-transient ganglion cell in vitro
Dhingra NK; Kao YH; Sterling P; Smith RG
We measured the contrast threshold for mammalian brisk-transient ganglion cells in vitro. Spikes were recorded extracellularly in the intact retina (guinea pig) in response to a spot with sharp onset, flashed for 100 ms over the receptive field center. Probability density functions were constructed from spike responses to stimulus contrasts that bracketed threshold. Then an "ideal observer" (IO) compared additional trials to these probability distributions and decided, using a single-interval, two-alternative forced-choice procedure, which contrasts had most likely been presented. From these decisions we constructed neurometric functions that yielded the threshold contrast by linear interpolation. Based on the number of spikes in a response, the IO detected contrasts as low as 1% [4.2 ± 0.4% (SE); n = 35]; based on the temporal pattern of spikes, the IO detected contrasts as low as 0.8% (2.8 ± 0.2%). Contrast increments above a very low "basal contrast" were discriminated with greater sensitivity than they were detected against the background. Performance was optimal near 37 degrees C and declined with a Q(10) of about 2, similar to that of retinal metabolism. By the method used by previous in vivo studies of brisk-transient cells, our most sensitive cells had similar thresholds. The in vitro measurements thus provide an important benchmark for comparing sensitivity of neurons upstream (cone and bipolar cell) and downstream to assess efficiency of retinal and central circuits.
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J Neurosci 24:2914-2922, 2004Spike generator limits efficiency of information transfer in a retinal ganglion cell
Dhingra N; Smith RGThe quality of the signal a retinal ganglion cell transmits to the brain is important for preception because it sets the minimum detectable stimulus. The ganglion cell converts graded potentials into a spike train with a selective filter but in the process adds noise. To explore how efficiently information is transferred to spikes, we measured contrast detection threshold and increment threshold from graded potential and spike responses of brisk-transient ganglion cells. Intracellular responses to a spot flashed over the receptive field center of the cell were recorded in an intact mammalian retina maintained in vitro at 37°C. Thresholds were measured in a single-interval forced-choice procedure with an ideal observer. The graded potential gave a detection threshold of 1.5% contrast, whereas spikes gave 3.8%. The graded potential also gave increment thresholds approximately twofold lower and carried ~60% more gray levels. Increment threshold "dipped" below the detection threshold at a low contrast (<5%) but increased rapidly at higher contrasts. The magnitude of the "dipper" for both graded potential and spikes could be predicted from a threshold nonlinearity in the responses. Depolarization of the cell by current injection reduced the detection threshold for spikes but also reduced the range of contrasts they can transmit. This suggests that contrast sensitivity and dynamic range are related in an essential trade-off.
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J Neurosci 25:8097-8103, 2005Voltage-gated sodium channels improve contrast sensitivity of a retinal ganglion cell.
Dhingra NK; Freed MA; Smith RG
Voltage-gated channels in a retinal ganglion cell are necessary for spike generation. However, they also add noise to the graded potential and spike train of the ganglion cell, which may degrade its contrast sensitivity, and they may also amplify the graded potential signal. We studied the effect of blocking Na+ channels in a ganglion cell on its signal and noise amplitudes and its contrast sensitivity. A spot was flashed at 1-4 Hz over the receptive field center of a brisk transient ganglion cell in an intact mammalian retina maintained in vitro. We measured signal and noise amplitudes from its intracellularly recorded graded potential light response and measured its contrast detection thresholds with an "ideal observer." When Na+ channels in the ganglion cell were blocked with intracellular lidocaine N-ethyl bromide (QX-314), the signal-to-noise ratio (SNR) decreased (r < 0.05) at all tested contrasts (2-100%). Likewise, bath application of tetrodotoxin (TTX) reduced the SNR and contrast sensitivity but only at lower contrasts (<50%), whereas at higher contrasts, it increased the SNR and sensitivity. The opposite effect of TTX at high contrasts suggested involvement of an inhibitory surround mechanism in the inner retina. To test this hypothesis, we blocked glycinergic and GABAergic inputs with strychnine and picrotoxin and found that TTX in this case had the same effect as QX-314: a reduction in the SNR at all contrasts. Noise analysis suggested that blocking Na+ channels with QX-314 or TTX attenuates the amplitude of quantal synaptic voltages. These results demonstrate that Na+ channels in a ganglion cell amplify the synaptic voltage, enhancing the SNR and contrast sensitivity.
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J Comp Neurol 260(1):63-75, 1987Ultrastructure of synapses from the A-laminae of the lateral geniculate nucleus in layer IV of the cat striate cortex.
Einstein G; Davis TL; Sterling P
The morphology of synapses in layer IV of the cat striate cortex was studied by electron microscope (EM) autoradiography of serial sections following injection of titrated amino acids into the lateral geniculate nucleus. Of the terminals in the neuropil, 22% had 2 or more silver grains in 10 successive sections and were labeled at 8-80 times the background level. These terminals were considered to be specifically labeled and to be derived from the lateral geniculate. Two forms of geniculate synapse were observed. One had medium-size, round vesicles and a modest postsynaptic asymmetry (RA); the other had smaller, pleomorphic vesicles and hardly any postsynaptic opacity; that is, it appeared symmetrical (PS). The geniculate RA terminals were presynaptic to dendritic spines, fine processes, and cell bodies; the geniculate PS terminals were presynaptic to dendrites and cell bodies but not to spines. The possible sources of geniculate PS terminals are discussed.
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J Comp Neurol 260(1):76-86, 1987Pattern of lateral geniculate synapses on neuron somata in layer IV of the cat striate cortex.
Einstein G; Davis TL; Sterling P
The distribution of geniculate synapses on neuron cell bodies in layers IVab and IVc of cat area 17 was studied. Electron microscope autoradiography was used to identify geniculate terminals that were labeled by anterograde transport of radioactivity injected into the A-laminae of the lateral geniculate nucleus. Thirty-eight cell bodies (19 in layer IVab and 19 in layer IVc) were examined in a series of 138 consecutive sections. Two pyramidal somas were studied and had no geniculate contacts. All of the other somas studied were nonpyramidal, and of these, 85% received geniculate contacts. The proportion of somas receiving somatic geniculate input differed in layers IVab and IVc. In layer IVab, 70% of the nonpyramidal somas received geniculate contacts; in IVc, 100%. Such high percentages indicate that geniculate afferents synapse with more types of layer IV neuron than the aspinous neurons that synthesize gamma-aminobutyric acid (GABA) (Freund et al., '85b). The pattern of input to somas was so diverse that it was impossible to form groups of neurons based on only this criterion. We wondered if it would be possible to form groups of neurons based on a range of characteristics among which would be pattern of synaptic input. To this end, pyramidal neurons and neurons that contained a cytoplasmic laminated body (CLB) (Winfield, '79; Einstein et al., '84) were treated as two separate classes. We found fair agreement among the features of these neurons within their own classes, with the CLB-cells in layer I Vab and IVc forming separate groups. Among the remaining neurons there was too little agreement within the range of features to enable us to treat them in this manner. Geniculate somatic contacts in both sublayers were of 2 forms, those with round vesicles and asymmetric thickenings (RA) and those with pleomorphic vesicles and symmetric thickenings (PS) (Einstein et al., '87). The distribution of these forms varied: some cells received contacts exclusively from one form or the other; other cells received contacts from both. On one cell that bore 33 somatic geniculate terminals, 61% were RA and 39% were PS. Such substantial numbers of geniculate contacts located near the site of impulse initiation are likely to contribute significantly to the receptive field properties of this neuron, and the possible effects are discussed.
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Review of Radical Polital Econimics 9: Spring 1977Stress-related mortality and social organization
Eyer J; Sterling P
Modern capitalist social organization, through intensified, conflicted work and the destruction of cooperative, supportive forms of social community, causes a large excess mortality among adults in developed countries. This excess morality is most strikingly evident in the comparison of vital rates for advanced capitalist societies with those of undisrupted hunter-gatherers. In the twentieth century United States, this excess mortality has varied markedly with the social fate of successive generations entering the labor market. The excess was high for the large cohort entering the labor market before the depression, low for the small cohort entering in the 1930s and with the boom, 1940-55; and now high once again for the baby boom children, entering the labor market since about 1960. Though death rates for heart disease and other stress-related diseases are now declining as the small cohort moves into older middle age, death rates are rising for the baby boom children at ages 25-30. If past experience with the first large cohort of the twentieth century is a valid guide, this new large cohort of baby boom children will suffer a large increase in death rates for cirrhosis of the liver, cancer, and heart disease, as it moves into maximal risk ages by the 1990s. Contemporary medicine transforms a largescale social problem into a problem in the motivation of individuals, for which marketable commodities, including therapy programs, surgery, and drugs are seen as the typical solutions. Therefore it mystifies and defuses potential autonomous awareness and organization looking to a different kind of society., The reintegration of cooperative community, with its consequences in the reduction of work intensity and the dealienation of labor, is however associated with a marked reduction of stress. The cooperative assertion of mass-scale cooperative community may therefore prove to be the most effective therapy for the diseases of modern capitalism.
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J Comp Neurol 219(3):295-304, 1983Four types of amacrine in the cat retina that accumulate GABA.
Freed MA; Nakamura Y; Sterling P
Roughly one-quarter of neurons in the amacrine cell layer accumulate exogenous gamma-aminobutyric acid (GABA). Some of these (8%) are interplexiform cells; the remainder are true amacrine cells. We partially reconstructed, from serial electron microsco py autoradiograms, 25 GABA-accumulating amacrines and distinguished four types based on cytoplasmic appearance, soma size and shape, and the form of primary and secondary processes. Type 1 had a large (609 ± 60 microns3), dark soma, and multiple, medium -diameter (0.6 microns) processes splayed from the soma margins like the appendages from a crab. Type 2 had a medium (360 ± 40 microns3), helmet-shaped, pale soma, and medium-diameter (0.8 microns) processes that branched in sublamina alpha. Type 3 had a small (267 ± 44 microns3), dark, pyriform soma. The latter formed a single stout (3.0 microns) process that bifurcated in the middle of sublamina alpha. Type 4 had a very large, pale soma (860 microns3). This was pyriform, tapering into a stout (2.0 m icrons) process that descended into the middle of sublamina alpha where it emitted smaller tangential processes. It is to be expected that each of these amacrine cell types will have distinct functions in neurotransmitter retinal circuitry.
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J Comp Neurol 266(3):445-455, 1987Rod bipolar array in the cat retina: pattern of input from rods and GABA-accumulating amacrine cells.
Freed MA; Smith RG; Sterling P
The potential and actual connections between rod and rod bipolar arrays in the area centralis of the cat retina were studied by electron microscopy of serial ultrathin sections. In the region studied there were about 378,000 rods/mm2 and 36,000-47,000 rod bipolars/mm2. The tangential spread of rod bipolar dendrites was 11.2 microns in diameter, and the "coverage factor" for the rod bipolar cell was 3.5-4.6. We estimate that about 37 rods potentially converge on a rod bipolar cell and that one rod potentially diverges to about four rod bipolar cells. The actual connections, however, are less than this by about half: 16-20 rods actually converge on a bipolar cell and one rod actually diverges to slightly less than two rod bipolar cells. The deg ree of convergence appears to reflect a compromise between the need to signal graded stimulus intensities (requiring wide convergence) and the need to maintain a good signal/noise ratio (requiring narrow convergence). Amacrine varicosities that provide re ciprocal contact at the rod bipolar dyad were studied in serial electron microscopic autoradiograms following intraocular administration of 3H-GABA or 3H-glycine. More that 90% of the reciprocal amacrine processes accumulated GABA in a specific fashion. T his information, in conjunction with Nelson's recordings from the rod bipolar and amacrine cells postsynaptic at the dyad (Nelson et al: Invest. Ophthalmol. 15:946-953, '76; Kolb and Nelson: Vision Res. 23:301-312, '83), suggests that feedback at the rod bipolar output might be positive.
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J Neurosci 8(7):2303-2320, 1988The ON-alpha ganglion cell of the cat retina and its presynaptic cell types.
Freed MA; Sterling P
Anatomical circuits converging onto the ON-alpha (Y) ganglion cell were studied by computer-assisted reconstruction of substantial portions of 2 alpha cells from electron micrographs of serial sections. The alpha cells in the area centralis were labele d by a Golgi-like retrograde filling with horseradish peroxidase, and certain presynaptic amacrine processes were labeled by uptake of 3H-glycine. About 4400 synapses contacted the alpha cell. Eighty-six percent were from amacrine cells; the rest were fro m bipolar cells. About one-quarter of the amacrine synapses were specifically labeled by 3H-glycine and probably belong to the A4 amacrine. The bipolar inputs were provided by several types: cone bipolar CBb1 (85%), cone bipolar CBb5 (2%), the rod bipolar (5%), and some unidentified cone bipolars (11%). Contacts from each type occurred in specific strata, with the consequence that they tended to form spots or annulli over the alpha dendritic field. The CBb1 bipolars formed a moderately dense array (8000/m m2), with a nearest-neighbor distance of 8.6 ± 1.3 microns. Most members of the array (84%) contacted the alpha cell, providing 1-7 synapses (average, 2.7 ± 1.6). The placement of contacts from an individual CBb1 followed certain rules: they were rest ricted to a parent branch of the alpha arbor or to 2 daughter branches, but almost never crossed a branch of the alpha arbor. The synaptic territory of an individual CBb1 was not shared with other b1s (or cone bipolars of any sort), although it was shared with amacrine contacts. Rod bipolar cells also formed a very dense array (54,500/mm2) in the alpha dendritic field, but only a few of these (3%) contacted the alpha cell. The concentric receptive field of the CBb1, combined with the spatial organization of its array, is used to predict the CBb1 contribution to the alpha cell receptive field; this contribution resembles the spatial and temporal organization of the alpha receptive field itself.
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Proc Natl Acad Sci USA 89(1):236-240, 1992Computational model of the on-alpha ganglion cell receptive field based on bipolar cell circuitry.
Freed MA; Smith RG; Sterling P
The on-alpha ganglion cell in the area centralis of the cat retina receives approximately 450 synapses from type b1 cone bipolar cells. This bipolar type forms a closely spaced array (9 microns), which contributes from 1 to 7 synapses per b1 cell throu ghout the on-alpha dendritic field. Here we use a compartmental model of an on-alpha cell, based on a reconstruction from electron micrographs of serial sections, to compute the contribution of the b1 array to the on-alpha receptive field. The computation shows that, for a physiologic range of specific membrane resistance (9500-68,000 omega.cm2) and a linear synapse, inputs are equally effective at all points on the on-alpha dendritic tree. This implies that the electrotonic properties of the dendritic tree contribute very little to the domed shapes of the receptive field center and surround. Rather, these shapes arise from the domed distribution of synapses across the on-alpha dendritic field. Various sources of "jitter" in the anatomical circu it, such as variation in bipolar cell spacing and fluctuations in the number of synapses per bipolar cell, are smoothed by the overall circuit design. However, the computed center retains some minor asymmetries and lumps, due to anatomical jitter, as found in actual alpha-cell receptive fields.
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Prog Brain Res 90:107-131, 1992GABAergic circuits in the mammalian retina
Freed MA
This chapter's theme is that GABAergic circuits modify signals running through both rod and cone bipolar pathways. These modifiying circuits, in keeping with their name, are often circular chains of neurons, as distinct from the linear bipolar pathways, and therefore seem to mediate feedback. Another function of these modifying circuits can be understood in the context that, since each neuronal type is repeated across the retina, rod and cone bipolar pathways represent many parallel routes: the modifying circuits convey information laterally between these parallel routes. One modifying circuit cell is the horizontal cell, which interacts with rods, cones and bipolar cells in some manner not fully understood. In mammals, there are at least 2 types of horizontal cell (Ramon Y Cajal, 1972). There is equivocal evidence that both types of horizontal cell are GABAergic. Another modifying circuit cell is the amacrine cell, which interacts with bipolar cells, ganglion cells and other amacrine cells. There is good evidence that specific types of amacrine cells are GABAergic. A third kind of modifying neuron, the interplexiform cell, receives synapses from amacrine cells in the inner plexiform layer and synapses upon bipolar cell dendrites in the outer plexiform layer, constituting a longer feedback loop. In mammals, there is good evidence that 2 types of interplexiform cell are GABAergic.
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Vis Neurosci 11(2):261-269, 1994Conductances evoked by light in the ON-beta ganglion cell of cat retina.
Freed MA; Nelson R
When a bar of light (215 x 5000 microns) illuminates the receptive field of an ON-beta ganglion cell of cat retina, the cell depolarizes. Intracellular recording from the cat eyecup preparation shows that this depolarization is due to an increase in conductance (2.4 ± 0.6 nS). Different phases of this depolarization have different reversal potentials, but all of these reversal potentials are more positive than the cell's resting potential in the dark. When the light is turned on, there is an initial transient depolarization; the reversal potential measured for this transient is positive (23 ± 11 mV). As the light is left on, the cell partially repolarizes to a sustained depolarization; the reversal potential measured for this sustained depolarization is close to zero (-1 ± 5 mV). When the light is turned off, the cell repolarizes further; the reversal potential measured for this repolarization is negative (-18 ± 7 mV), but still above the resting potential in the dark (-50 mV). To explain this variety of reversal potentials, at least two different synaptic conductances are required: one to ions which have a positive reversal potential and another to ions which have a negative reversal potential. Comparing the responses to broad and narrow bars suggests that these two conductances are associated with the center and surround, respectively. Finally, since an ON-beta cell in the area centralis receives about 200 synapses, these results indicate that a single synapse produces an average conductance increase of about 15 pS during a near-maximal depolarization.
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J Comp Neurol 364(3):556-566, 1996ON-OFF amacrine cells in cat retina.
Freed MA; Pflug R; Kolb H;Nelson R
We studied the morphology, photic responses, and synaptic connections of ON-OFF amacrine cells in the cat retina by penetrating them with intracellular electrodes, staining them with horseradish peroxidase, and examining them with the electron microsco pe. In a sample of seven cells, we found two different morphological types: the A19, which ramifies narrowly in stratum 2 (sublamina a) of the inner plexiform layer, and the A22, which ramifies mostly in stratum 4 (sublamina b) but extends some dendrites to sublamina a. Both of these cell types have axon-like processes that extend > 800 microns from the conventional dendritic arbor. ON-OFF amacrine cells in our sample had receptive fields (1.7 ± 0.3 mm diameter) that were broader than their dendritic arbors (425 ± 35 microns diameter) and that extended over the region of axon-like processes. In addition, we found many features in common with ON-OFF amacrine cells in poikilotherm vertebrates: a broad receptive field without surround antagonism, two sizes of spike-like events, narrow dynamic range (1 log unit intensity), and excitatory postsynaptic potentials at light on and light off. Two A19 amacrine cells were examined in the electron microscope: most synaptic inputs (93 and 76%, respectively) to either cell were from amacrine cells, with minor inputs from cone bipolar cells. Synaptic outputs were to bipolar, amacrine, and ganglion cells, including the OFF-alpha cell.
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J Neurosci 20:3956-3963, 2000Parallel cone bipolar pathways to a ganglion cell use different rates and amplitudes of quantal excitation.
Freed MA
The cone signal reaches the cat's On-beta (X) ganglion cell via several parallel circuits (bipolar cell types b1, b2, and b3). These circuits might convey different regions of the cone's temporal bandwidth. To test this, I presented a step of light which elicited a transient depolarization followed by a sustained depolarization. The contribution of bipolar cells to these response components was isolated by blocking action potentials with tetrodotoxin and by blocking inhibitory synaptic potentials with bicuculline and strychnine. Stationary fluctuation analysis of the sustained depolarization gave the rate of quantal bombardment: about 5,100 quanta s-1 for small central cells and about 45,000 quanta s-1 for large peripheral cells. Normalizing these rates for the vastly different numbers of bipolar synapses (150-370 per small cell vs. 2000 per large cell), quantal rate was constant across the retina, about 22 quanta synapse-1 s-1. Non-stationary fluctuation analysis gave the mean quantal EPSP amplitude: about 240 microvolt for the transient depolarization and 30 microVolt for the sustained depolarization. The b1 bipolar cell is known from noise analysis of the On-alpha ganglion cell to have a near-maximal sustained release of only about 2 quanta synapse-1 s-1. This implies that the other bipolar types (b2 and b3) contribute many more quanta to the sustained depolarization (more than 46 synapse-1 s-1). Type b1 probably contributes large quanta to the transient depolarization. Thus, bipolar cell types b1 and b2/b3 apparently constitute parallel circuits that convey, respectively, high and low frequencies.
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J Neurophysiol 83:2956-2966, 2000Rate of quantal excitation to a retinal ganglion cell evoked by sensory input.
Freed MA
To determine the rate and statistics of light-evoked transmitter release from bipolar synapses, intracellular recordings were made from On-alpha ganglion cells in the periphery of the intact, superfused, cat retina. Sodium channels were blocked with tetrodotoxin to prevent action potentials. A light bar covering the receptive field center excited the bipolar cells that contact the alpha cell and evoked a transient then a sustained depolarization. The sustained depolarization was quantified as change in mean voltage (), and the increase in voltage noise that accompanied it was quantified as change in voltage variance (2). As light intensity increased, and 2 both increased, but their ratio held constant. This behavior is consistent with Poisson arrival of transmitter quanta at the ganglion cell. The response component attributable to glutamate quanta from bipolar synapses was isolated by application of CNQX (6-cyano-7-nitroquinoxaline). As CNQX concentration increased, the signal/noise ratio of this response component (CNQX/CNQX) held constant. This is also consistent with Poisson arrival and justified the application of fluctuation analysis. Two different methods of fluctuation analysis applied to CNQX and CNQX produced similar results, leading to an estimate that a just-maximal sustained response was caused by about 3700 quanta s-1. The transient response was caused by a rate which was no more than 10-fold greater. Since the On-alpha cell at this retinal locus has about 2200 bipolar synapses, one synapse released about 1.7 quanta s-1 for the sustained response and no more than 17 quanta s-1 for the transient. Consequently, within the ganglion cell's integration interval, here calculated to be about 16 ms, a bipolar synapse rarely releases more than one quantum. Thus for just-maximal sustained and transient depolarizations, the conductance modulated by a single bipolar cell synapse is limited to the quantal conductance (about 100 pS at its peak). This helps preserve linear summation of quanta. The 2/ ratio remained constant even as the ganglion cell's response saturated, which suggested that even at the peak of sensory input, summation remains linear, and that saturation occurs before the bipolar synapse.
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Neuron 38(1):89-101, 2003
Timing of quantal release from the retinal bipolar terminal is regulated by a feedback circuit.
Freed MA; Smith RG; Sterling P
In isolation, a presynaptic terminal generally releases quanta according to Poisson statistics, but in a circuit its release statistics might be shaped by synaptic interactions. We monitored quantal glutamate release from retinal bipolar cell terminals (which receive GABA-ergic feedback from amacrine cells) by recording spontaneous EPSCs (sEPSCs) in their postsynaptic amacrine and ganglion cells. In about one-third of these cells, sEPSCs were temporally correlated, arriving in brief bursts (10-55 ms) more often than expected from a Poisson process. Correlations were suppressed by antagonizing the GABA(C) receptor (expressed on bipolar terminals), and correlations were induced by raising extracellular calcium or osmolarity. Simulations of the feedback circuit produced "bursty" release when the bipolar cell escaped intermittently from inhibition. Correlations of similar duration were present in the light-evoked sEPSCs and spike trains of sluggish-type ganglion cells. These correlations were suppressed by antagonizing GABA(C) receptors, indicating that glutamate bursts from bipolar terminals induce spike bursts in ganglion cells.
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J Neurophysiol 94:1048-1056, 2005Quantal encoding of information in a retinal ganglion cell.
Freed MA
A retinal ganglion cell receives information about a white-noise stimulus as a flickering pattern of glutamate quanta. The ganglion cell reencodes this information as brief bursts of one to six spikes separated by quiescent periods. When the stimulus is repeated, the number of spikes in a burst is highly reproducible (variance < mean) and spike timing is precise to within 10 ms, leading to an estimate that each spike encodes about 2 bits. To understand how the ganglion cell reencodes information, we studied the quantal patterns by repeating a white-noise stimulus and recording excitatory currents from a voltage-clamped, brisk-sustained ganglion cell. Quanta occurred in synchronous bursts of 3 to 65; the resulting postsynaptic currents summed to form excitatory postsynaptic currents (EPSCs). The number of quanta in an EPSC was only moderately reproducible (variance = mean), quantal timing was precise to within 14 ms, and each quantum encoded 0.1-0.4 bit. In conclusion, compared to a spike, a quantum has similar temporal precision, but is less reproducible and encodes less information. Summing multiple quanta into discrete EPSCs improves the reproducibility of the overall quantal pattern and contributes to the reproducibility of the spike train.
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J Comp Neurol 495:658-667, 2006
Histamine receptors in mammalian retinas.
Gastinger M; Barber A; Vardi N; Marshak D
Mammalian retinas are innervated by histaminergic axons that originate from perikarya
in the posterior hypothalamus. To identify the targets of these retinopetal axons, we localized histamine receptors (HR) in monkey and rat retinas by light and electron microscopy. In monkeys, puncta containing HR3 were found at the tips of ON-bipolar cell dendrites in cone pedicles and rod spherules, closer to the photoreceptors than the other neurotransmitter receptors. This is the first ultrastructural localization of any histamine receptor and the first direct evidence that HR3 is present on postsynaptic membranes in the central nervous system. In rat retinas, most HR1 were localized to dopaminergic amacrine cells. The differences in histamine receptor localization may reflect the differences in the activity patterns of the two species. [FULL PDF]
J Physiol 88:98-106, 2002Roles of ATP in depletion and replenishment of the releasable pool of synaptic vesicles
Heidelberger R; Sterling P; Matthews G
Roles of ATP in Depletion and Replenishment of the Releasable Pool of Synaptic Vesicles. J. Neurophysiol. 88: 98-106, 2002.
Synaptic terminals of retinal bipolar neurons contain a pool of readily releasable synaptic vesicles that undergo rapid calcium-dependent release. ATP hydrolysis is required for the functional refilling of this vesicle pool. However, it was unclear which steps required ATP hydrolysis: delivery of vesicles to their anatomical release sites or preparation of synaptic vesicles and/or the secretory apparatus for fusion. To address this, we dialyzed single synaptic terminals with ATP or the poorly hydrolyzable analogue ATP-gS and examined the size of the releasable pool, refilling of the releasable pool, and the number of vesicles at anatomical active zones. After minutes of dialysis with ATP-gS, vesicles already in the releasable pool could still be discharged. This pool was not functionally refilled despite the fact that its anatomical correlate, the number of synaptic vesicles tethered to active zone synaptic ribbons, was completely normal. We conclude 1) because the existing releasable pool is stable during prolonged inhibition of ATP hydrolysis, whereas entry into the functional pool is blocked, a vesicle on entering the pool will tend to remain there until it fuses; 2) because the anatomical pool is unaffected by inhibition of ATP hydrolysis, failure to refill the functional pool is not caused by failure of vesicle movement; 3) local vesicle movements important for pool refilling and fusion are independent of conventional ATP-dependent motor proteins; and 4) ATP hydrolysis is required for the biochemical transition of vesicles and/or release sites to fusion-competent status.
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J Comp Neurol 457(2):185-201, 2003
Inner S-cone bipolar cells provide all of the central elements for S cones in macaque retina.
Herr S; Klug K; Sterling P; Schein S
Synaptic terminals of cones (pedicles) are presynaptic to numerous processes that arise from the dendrites of many types of bipolar cell. One kind of process, a central element, reaches deeply into invaginations of the cone pedicle just below an active zone associated with a synaptic ribbon. By reconstruction from serial electron micrographs, we show that L- and M-cone pedicles in macaque fovea are presynaptic to approximately 20 central elements that arise from two types of inner (invaginating) bipolar cell, midget and diffuse. In contrast, S-cone pedicles, with more synaptic ribbons, active zones/ribbon, and central elements/active zone, are presynaptic to approximately 33 central elements. Moreover, all of these arise from one type of bipolar cell, previously described by others, here termed an inner S-cone bipolar cell. Each provides approximately 16 central elements. Thirty-three is twice 16; correspondingly, these bipolar cells are twice as numerous as S cones. (Specifically, each S cone is presynaptic to four inner S-cone bipolar cells; in turn, each bipolar cell provides central elements to two S cones.) These bipolar cells are presynaptic to an equal number of small-field bistratified ganglion cells, giving cell numbers in 2G:2B:1S ratios. Each ganglion cell receives input from two or more inner S-cone bipolar cells and thereby collects signals from three or more S cones. This convergence, along with chromatic aberration of short-wavelength light, suggests that S-cone contributions to this ganglion cell's coextensive blue-ON/yellow-OFF receptive field are larger than opponent L/M-cone contributions via outer diffuse bipolar cells and that opponent L/M-cone signals are conveyed mainly by inner S-cone bipolar cells.
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In: Computation in Neurons and Neural Systems(1994)
Frank H. Eekman (Ed.) Kluwer Academic Publishers, Boston.Simulating the foveal cone receptive field
Hsu A and Smith RG
The foveal midget ganglion cell has a receptive field center fed by one cone. The surround might also be fed by the same center cone since a cone terminal laterally connects to neighboring cones through electrical coupling and horizontal cells. To explore the contributions of the cone lateral connections to the receptive field, we constructed a compartmental model of the primate foveal outer plexiform layer based on the known anatomy and physiology. The similarity between the computed cone receptive field and the measured midget cell receptive field suggest that much of the retina's spatial filtering occurs at the very first synapse.
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Vision Res 38: 2539-2549, 1998Functional architecture of primate cone and rod axons
Hsu A; Tsukamoto Y; Smith RG; Sterling P
The cone axon is nearly four times thicker than the rod axon (1.6 vs. 0.45 µm diameter). To assess how signal transfer and integration at the terminal depend on cable dimensions, a transducer (cone = ohmic conductance, rod = current source) coupled via passive cable to a sphere with a chloride conductance (representing GABAa receptor) was modelled. For a small signal in peripheral cone with a short axon, a steady photosignal transfers independently of axon diameter despite a significant chloride conductance at the cone terminal. A temporally varying photosignal also transfers independently of axon diameter up to 20 Hz and is attenuated only 20% at 50 Hz. Thus, to accomplish the basic electrical functions of a peripheral cone, a thin axon would suffice. For a foveal cone with a long axon, a steady photosignal transfers independently of axon diameter, but a temporally varying photosignal is attenuated 5-fold at 50 Hz for a thick axon and 10-fold for a thin axon. This might contribute to the lower sensitivity of central retina to high temporal frequencies. The cone axon contains 14-fold more microtubules than the rod axon, and its terminal contains at least 20-fold more ribbon synapses than the rod's. Since ribbon synapses sustain high rates of exocytosis, the additional microtubules (which require a thicker axon) may be needed to support a greater flux of synaptic vesicle components.
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J Optical Soc Am A 17(3):635-640 2000Cost of cone coupling to trichromacy in primate fovea.
Hsu A; Smith RG; Buchsbaum G; Sterling P
Cone synaptic terminals couple electrically to their neighbors. This reduces the amplitude of temporally uncorrelated voltage differences between neighbors. For an achromatic stimulus coarser than the cone mosaic, the uncorrelated voltage difference between neighbors represents mostly noise; so noise is reduced more than the signal. Here coupling improves signal-to-noise ratio and enhances contrast sensitivity. But for a chromatic stimulus the uncorrelated voltage difference between neighbors of different spectral type represents mostly signal; so signal would be reduced more than the noise. This cost of cone coupling to encoding chromatic signals was evaluated using a compartmental model of the foveal cone array. When cones sensitive to middle (M) and long (L) wavelengths alternated regularly, and the conductance between a cone and all of its immediate neighbors was 1000 pS (similar to 2 connexons/cone pair), coupling reduced the difference between the L and M action spectra by nearly fivefold, from about 38% to 8%. However, L and M cones distribute randomly in the mosaic, forming small patches of like type, and within a patch the responses to a chromatic stimulus are correlated. In such a mosaic, coupling still reduced the difference between the L and M action spectra, but only by 2.4-fold, to about 18%. This result is independent of the L/M ratio. Thus "patchiness" of the L/M mosaic allows cone coupling to improve achromatic contrast sensitivity while minimizing the cost to chromatic sensitivity.
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Proc Natl Acad Sci USA 93(25):14598-14601, 1996Phospholipase C beta 4 is involved in modulating the visual response in mice.
Jiang H; Lyubarsky A; Dodd R; Vardi N; Pugh E; Baylor D; Simon MI; Wu D
Expression of G protein-regulated phospholipase C (PLC) beta 4 in the retina, lateral geniculate nucleus, and superior colliculus implies that PLC beta 4 may play a role in the mammalian visual process. A mouse line that lacks PLC beta 4 was generated and the physiological significance of PLC beta 4 in murine visual function was investigated. Behavioral tests using a shuttle box demonstrated that the mice lacking PLC beta 4 were impaired in their visual processing abilities, whereas they showed no defi cit in their auditory abilities. In addition, the PLC beta 4-null mice showed 4-fold reduction in the maximal amplitude of the rod a- and b-wave components of their electroretinograms relative to their littermate controls. However, recording from single r od photoreceptors did not reveal any significant differences between the PLC beta 4-null and wild-type littermates, nor were there any apparent differences in retinas examined with light microscopy. While the behavioral and electroretinographic results in dicate that PLC beta 4 plays a significant role in mammalian visual signal processing, isolated rod recording shows little or no apparent deficit, suggesting that the effect of PLC beta 4 deficiency on the rod signaling pathway occurs at some stage after the initial phototransduction cascade and may require cell-cell interactions between rods and other retinal cells.
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Visual Neurosci 15(4):743-753, 1998Regional differences in GABA and GAD immunoreactivity in rabbit horizontal cells.
Johnson MA; Vardi N
Mammalian horizontal cells are believed to be GABAergic because, in most species, they contain both GABA and glutamic acid decarboxylase (GAD), and their terminals are presynaptic to GABA receptors. In adult rabbit, however, GABA and GAD immunoreactivity have not been consistently demonstrated in horizontal cells, casting doubts on the assumption that they too are GABAergic. Here we demonstrate that all rabbit horizontal cell terminals--dendritic terminals of type A, and both dendritic and axonal terminals of type B--immunostain for one isoform of GAD, GAD67, In addition, we show that type A horizontal cell somas and primary dendrites in the visual streak (identified by their immunoreactivity to calbindin) are immunoreactive for the other GAD isoform, GAD65. Double-labeling experiments for GAD65 and GABA reveal that every cell that stains for GAD65 also stains for GABA. The presence of GAD67 in horizontal cell terminals suggests that rabbit horizontal cells are GABAergic. The segregation of the two GAD isoforms to different cell compartments suggests that GABA is released at different sites, possibly by two different mechanisms.
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Vis Neurosci 26:931-939, 2006Displaced GAD65 amacrine cells of the guinea pig retina are morphologically diverse
Kao Y-H; Sterling P
The ganglion cell layer of mammalian retina contains numerous amacrine cells. Many belong to one type, the cholinergic starburst cell, but the other types have not been systematically identified. Using a new method to target sparsely represented cell types, we filled about 200 amacrine neurons in the ganglion cell layer of the guinea pig visual streak and identified 11 types. Ten of these resemble types identified in other species with somas in the inner nuclear layer, but one type has not been previously reported. Most of the types and nearly all the injected cells (95%) arborized low in the synaptic layer where they would co-stratify with various classes of ON ganglion cell. The displaced somas (7% of all amacrine cells) thus represent a heterogeneous pool, which are relatively accessible for study of their interactions with ON ganglion cells.
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J Neurocytol 89:2360-2369, 2003Matching neural morphology to molecular expression: single cell injection following immunostaining.
Kao Y-H; Sterling P
To match a neuron's morphology with its expression of a particular protein, it is useful to first identify the cell by immunostaining and then inject it with fluorescent dye. Such targeted injection cannot be performed with a hydrophilic dye (such as Lucifer yellow) because the neuron, once rendered porous to antibodies, does not retain it. But a lipophilic dye (such as DiI) injected iontophoretically into the soma forms a crystal and is thereby trapped. From this intracellular depot dye diffuses into the cell membrane to reveal the detailed morphology. We have used this strategy to identify the morphology of a GABAergic retinal bipolar cell and several types of GABAergic amacrine cell. In addition, we demonstrate probable connections from a narrow-field, GABAergic amacrine cell to the OFF brisk-transient ganglion cell. Finally, we show that the strategy works in the cortical slice, showing a layer IV cell immunostained for parvalbumin to be a “nest basket cell”.
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J Comp Neurol 478:207-218, 2004Evidence That Certain Retinal Bipolar Cells Use Both Glutamate and GABA.
Kao Y-H; Lassová L; Bar-Yehuda T; Edwards RH; Sterling P; Vardi N
Retinal bipolar neurons release the excitatory transmitter, glutamate. However, certain ipolar cells contain GABA, raising the question whether a neuron might release both transmitters and, if so, what function might the inhibitory transmitter play in a particular circuit? Here we identify a subset of cone bipolar cells in cat retina that contain glutamate, plus its vesicular transporter (VGLUT1), and GABA, plus its synthetic enzyme (GAD65) and its vesicular transporter (VGAT). These cells are negative for a marker of ON bipolar cells and restrict their axons to the OFF strata of the inner synaptic layer. They do not colocalize with the neurokinin 3 receptor that stains a type (or two) of OFF bipolar cells. By “targeted injection,” we identified two types of OFF bipolar cell with the machinery to make and package both transmitters. One of these types costratifies with a dopamine plexus.
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Physica 22D 268-82, 1986Adaptive automata based on Darwinian selection
Kauffman SA; Smith RG
The principle of natural selection is general to assess its implications for achieving automata with desired dynamical or structural properties. The following issues arise: 1)Appropriate definition of the analogue of "genotype" and "phenotype". 2) Definition of the ensemble of "possible" automata in which mutational search for desired properties is occurring. 3) Kinematic properties of the "fitness landscape" in the ensemble governing the statistical features of connected walks through fitter variants. 4) Optimal mutation selection strategies given a particular fitness landscape.
We assess these questions in two ensembles of automata under selection for one attractor which matches a predetermined "target pattern". The results are: 1) The continuity of desired dynamical properties differs in the two ensembles. 2) When the best automaton seeds each generation, selection follows a characteristic curve, and asymptotes at automata which approach but fail to achieve the desired attractors. 3) Designation of a subset of the variables as hidden from fitness estimation as part of the target pattern does not improve approach to the target pattern. We discuss limitations in the capacity of mutation selection procedures, and approaches to overcoming them.
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J Neurosci 19(8):2954-2959, 1999AMPA receptor activates a G-protein that suppresses a cGMP-gated current.
Kawai F; Sterling P
The AMPA receptor, ubiquitous in brain, is termed "ionotropic" because it gates an ion channel directly. We found that an AMPA receptor can also modulate a G-protein to gate an ion channel indirectly. Glutamate applied to a retinal ganglion cell briefly suppresses the inward current through a cGMP-gated channel. AMPA and kainate also suppress the current, an effect that is blocked both by their general antagonist CNQX and also by the relatively specific AMPA receptor antagonist GYKI-52466. Neither NMDA nor agonists of metabotropic glutamate receptors are effective. The AMPA-induced suppression of the cGMP-gated current is blocked when the patch pipette includes GDP-beta-S, whereas the suppression is irreversible when the pipette contains GTP-gamma-S. This suggests a G-protein mediator, and, consistent with this, pertussis toxin blocks the current suppression. Nitric oxide (NO) donors induce the current suppressed by AMPA, and phosphodiesterase inhibitors prevent the suppression. Apparently, the AMPA receptor can exhibit a "metabotropic" activity that allows it to antagonize excitation evoked by NO.
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Vis Neurosci 19:373-380, 2002
cGMP modulates spike responses of retinal ganglion cells via a cGMP-gated current
Kawai F; Sterling P
Certain ganglion cells in the mammalian retina are known to express a cGMP-gated cation channel. WE found that a cGMP-gated current modulates spike responses of the gangion cells in mammalian retinal slice preparation. In such cells under current clamp, bath application of the membrane-permeant cGMP analog (8-bormo-cGMP, 8-p-chlorophenylthio-cGMP) or a nitric oxide donor (sodium mnitroprusside, S-nitroso-N-acetyl-penicillamine) depolarized the membrane potential by 5-15 mV, and reduced the amount of current needed to evoke action potentials. Similar effects were observed when the membrane potential was simply depolarized by steady current. The responses to cGMP are unaffected by inhibitors of cGMP-dependent protein kinase and Ca2+/calmodulin-dependent protein kinase. The response to cGMP persisted in Ca2+-free bath solution with Ca2+ buffers in the pipette. Under voltage clamp, cGMP analogs did not affect the response kinetics of voltage-gated currents. We conclude that cGMP modulates ganglion cell spiking simply by depolarizing the membrane potential via the inward current through the cGMP-gated channel. Modulation of this channel via the long-range NP-synthase amacrine cell may contribute to control of contrast gain by peripheral mechanisms.
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J Neurosci 15(11):7673-7683, 1995How retinal microcircuits scale for ganglion cells of different size.
Kier CK; Buchsbaum G; Sterling P
Ganglion cell receptive field centers are small in central retina and larger toward periphery. Accompanying this expansion, the distribution of sensitivity across the centers remain Gaussian, but peak sensitivities decline. To identify circuitry that might explain this physiology, we measured the density of bipolar cell synapses on the dendritic membrane of beta (X) and alpha (Y) ganglion cells and the distribution of dendritic membrane across their dendritic fields. Both central and peripheral beta cells receive bipolar cell synapses at a density of approximately 28/100 microns2 of dendritic membrane; central and peripheral alpha cells receive approximately 13/100 micron2. The distribution of dendritic membrane across the dendritic field is dome-like; therefore, the distribution of bipolar cell synapses is also dome-like. As the dendritic field enlarges, total postsynaptic membrane increases with field radius, but only linearly. Consequently, density of postsynaptic membrane in the dendritic field declines, and so does density of synapses within the field. The results suggest a simple model in which the receptive field center's Gaussian profile and peak sensitivity are both set by the density of bipolar cell synapses across the dendritic field.
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J Neurosci 1983 23(30):9881-9887Macaque retina contains an S-cone OFF midget pathway.
Klug K; Herr S; Ngo IT; Sterling P; Schein S
Psychophysical results suggest that the primate visual system is equally sensitive to both the onset and offset of short-wavelength light and that these responses are carried by separate pathways. However, physiological studies of cells in the retina and lateral geniculate nucleus find far fewer OFF-center than ON-center cells whose receptive-field centers are driven by short-wavelength-sensitive (S) cones. To determine whether S cones contact ON and OFF midget bipolar cells as well as (ON) “blue-cone bipolar” cells (Mariani, 1984), we examined 118 contiguous cone terminals and their bipolar cells in electron micrographs of serial sections from macaque foveal retina. Five widely spaced cone terminals do not contact ON midget bipolar cells. These five cone terminals contact the dendrites of “blue-cone bipolar” cells instead, showing that they are the terminals of S cones. These S-cone terminals are smaller and contain more synaptic ribbons than other terminals. Like neighboring cones, each S cone contacts its own OFF midget bipolar cell via triad-associated (flat) synaptic contacts. Moreover, each S-cone OFF midget bipolar cell has a synaptic terminal in the outer half of the inner plexiform layer, where it contacts an OFF midget ganglion cell.
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Curr Biol 2006 16:1428-1434How much the eye tells the brain
Koch K, McLean J, Segev R, Freed MA, Berry MJ II, Balasubramanian V, Sterling P
In the classic ‘‘What the frog’s eye tells the frog’s brain,’’ Lettvin and colleagues [1] showed that different types of retinal ganglion cell send specific kinds of information. For example, one type responds best to a dark, convex form moving centripetally (a fly). Here we consider a complementary question: how much information does the retina send and how is it apportioned among different cell types? Recording from guinea pig retina on a multi-electrode array and presenting various types of motion in natural scenes, we measured information rates for seven types of ganglion cell. Mean rates varied across cell types (6–13 bits -2) more than across stimuli. Sluggish cells transmitted information at lower rates than brisk cells, but because of trade-offs between noise and temporal correlation, all types had the same coding efficiency. Calculating the proportions of each cell type from receptive field size and coverage factor, we conclude (assuming independence) that the approximately 105 ganglion cells transmit on the order of 875,000 bits -2. Because sluggish cells are equally efficient but more numerous, they account for most of the information. With approximately 106 ganglion cells, the human retina would transmit data at roughly the rate of an Ethernet connection.
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Curr Biol 2006 14:1523-1530Efficiency of information transmission by retinal ganglion cells
Koch K, McLean J, Berry MJ II, Sterling P, Balasubramanian V, Freed MA
Background: Different types of retinal ganglion cells convey different messages to the brain. Messages are in the form of spike patterns, and the number of possible patterns per second sets the coding capacity. We asked if different ganglion cell types make equally efficient use of their coding capacity or whether efficiency depends do real cells approach this limit?
Results: We recorded spike trains from retinal ganglion cells in an in vitro preparation of the guinea pig retina. By calculating, for the observed spike rate, the number of possible spike patterns per second, we calculated coding capacity, and by counting the actual number of patterns, we estimated information rate. Cells with “brisk” responses, i.e., high firing rates, and a general message transmitted information at high rates (21 ± 9 bits s1). Cells with “sluggish” responses, i.e., lower firing rates, and specific messages (direction of motion, local-edge) transmitted information at lower rates (13 ± 7 bits s1). Yet, for every type of ganglion cell examined, the information rate was about one-third of coding capacity. For every ganglion cell, information rate was very close (within 4%) to that predicted from Poisson noise and the cell’s actual time-modulated rate.
Conclusions: Different messages are transmitted with similar efficiency. Efficiency is limited by temporal correlations, but correlations may be essential to improve decoding in the presence of irreducible noise.
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J Comp Neurol 1983 Apr 20;215(4):465-471Granule cells in the rat olfactory tubercle accumulate 3H-gamma-aminobutyric acid.
Krieger NR; Megill JR; Sterling P
The rat olfactory tubercle contains high concentrations of gamma-aminobutyric acid (GABA) and its synthetic enzyme, glutamic acid decarboxylase (GAD). We previously demonstrated that GABA and GAD are most concentrated in the polymorphic layer of the tubercle and relatively absent from the plexiform and pyramidal layers. Here we report that the granule cells (the islands of Calleja) in the polymorphic layer accumulate 3H-GABA. 3H-GABA (34.5 Ci/mmole; 1.5 microliter) was injected into the tubercle and an hour later the rat was perfused with a mixture of paraformaldehyde and glutaraldehyde. The tissue was osmicated, dehydrated, and embedded in epon. Silver grains were sparse over the pyramidal and polymorphic cell bodies but numerous over the granule cell bodies in the islands of Calleja and dendrites in the surrounding neuropil. Grain densities for the granule cells were 41/100 micrometer3 compared to 4.2 for the pyramidal and polymorphic cells. Within the island, all the granule cells appeared to be labeled. These results, combined with previous demonstrations of the presence in this region of endogenous GABA and GAD, suggest that the granule neurons of the rat olfactory tubercle are GABA-ergic. These neurons also appear to receive dopamine input and therefore form part of a circuit that includes targets for both major and minor tranquilizers.
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J Neurosci 4(12):2920-2938, 1984Microcircuitry of bipolar cells in cat retina.
McGuire BA; Stevens JK; Sterling P
We have studied 15 bipolar neurons from a small patch (14 X 120 micron) of adult cat retina located within the area centralis. From electron micrographs of 189 serial ultrathin sections, the axon of each bipolar cell was substantially reconstructed with its synaptic inputs and outputs by means of a computer-controlled reconstruction system. Based on differences in stratification, cytology, and synaptic connections, we identified eight different cell types among the group of 15 neurons: one type of rod bipolar and seven types of cone bipolar neurons. These types correspond to those identified by the Golgi method and by intracellular recording. Those bipolar cell types for which we reconstructed three or four examples were extremely regular in form, size, and cytology, and also in the quantitative details of their synaptic connections. They appeared quite as specific in these respects as invertebrate "identified" neurons. The synaptic patterns observed for each type of bipolar neuron were complex but may be summarized as follows: the rod bipolar axon ended in sublamina b of the inner plexiform layer and provided major input to the AII amacrine cell. The axons of three types of cone bipolar cells also terminated in sublamina b and provided contacts to dendrites of on-beta and other ganglion cells. All three types, but especially the Cb1, received gap junction contacts from the AII amacrine cell. Axons of four types of cone bipolar cells terminated in sublamina a of the inner plexiform layer and contacted dendrites of off-beta and other ganglion cells. One of these cone bipolar cell types, CBa1, made reciprocal chemical contacts with the lobular appendage of the AII amacrine cell. These results show that the pattern of cone bipolar cell input to beta (X) and probably alpha (Y) ganglion cells is substantially more complex than had been suspected. At least two types of cone bipolar contribute to each type of ganglion cell where only a single type had been anticipated. In addition, many of the cone bipolar cell pathways in the inner plexiform layer are available to the rod system, since at least four types of cone bipolar receive electrical or chemical inputs from the AII amacrine cell. This may help to explain why, in a retina where rods far outnumber the cones, there should be so many types of cone bipolar cells.
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J Neurosci 6(4):907-918, 1986Microcircuitry of beta ganglion cells in cat retina.
McGuire BA; Stevens JK; Sterling P
We reconstructed from electron micrographs of 189 serial ultrathin sections a major portion of the dendritic tree of an on-beta ganglion cell through its sixth order of branching. One hundred three contacts from three cone bipolar cells were identified. Forty-seven contacts were from a single CBb1 cone bipolar. These were distributed widely over the dendritic tree but were frequently found on the slender "basal tuft" dendrites. Twenty-two additional contacts from a second CBb1 cell were found but not studied in detail. Thirty-four contacts were from a single CBb2 cone bipolar; these also were distributed widely but were primarily on the branches of the main dendritic arborization. A major portion of the dendritic tree of an off-beta cell was also reconstructed through its seventh order of branching. Thirty-five contacts from two cone bipolar cells were identified. Twenty-three contacts were from a single CBa1 cone bipolar and 12 widely distributed over the off-beta cell dendritic tree. We propose that the photopic receptive field center of a beta cell corresponds to the envelope of the receptive fields of the bipolar cells that connect it to the cones. The center response of a beta cell may be generated by a "push-pull" mechanism. For the on-beta cell there would be excitation at light on from CBb1 and disinhibition from CBb2 and the reverse at light off. For the off-beta cell there would be inhibition at light on from CBa2 and withdrawal of excitation from CBa1. Should the bipolars have antagonistic surrounds (so far reported only for CBb1), the beta cell surrounds as well as their centers might be generated by this push-pull mechanism.
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J Comp Neurol 455(1):100-12, 2002Two ribbon synaptic units in rod photoreceptors of macaque, human, and cat.
Migdale K; Herr S; Klug K; Ahmad K; Linberg K; Sterling P; Schein S
The rod photoreceptor's synaptic terminal (or spherule) uses an elaborate synaptic structure to signal absorption of one or more photons to its postsynaptic targets. This structure includes one or two synaptic ribbons inside the terminal and a pouch-like "invagination" outside the terminal, into which enter a widely variable number of incoming fibers and postsynaptic targets-central elements supplied by rod bipolar cells and lateral elements supplied by horizontal cells. Nonetheless, our three-dimensional reconstructions of this synaptic structure in foveal retina of macaque monkey and peripheral retina of human and cat reveal several features that are highly conserved across species and with eccentricity: 1). every spherule has one invagination; 2). with rare exceptions, every spherule has two ribbon synaptic units with these features: a). on the presynaptic side, each ribbon synaptic unit has a ribbon or part of a ribbon and one trough-shaped arciform density that demarcates its active zone; b). on the postsynaptic side, each ribbon synaptic unit has two apposed lateral elements and one or more central elements; 3). the volume of the extracellular space in the single invagination is small, approximately 0.1 microm(3); and 4). the largest distance from active zone to receptor regions on bipolar cells is small, less than approximately 1.5 microm. With such small dimensions, release of one quantum of transmitter can pulse glutamate to a concentration comparable to the EC(50) of the metabotropic glutamate receptors on the central elements associated with both synaptic units. We speculate that two ribbon synaptic units are required to sustain the high quantal release rate needed to signal a single photon.
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J Comp Neurol 202(3):385-396, 1981Neurons and glia in cat superior colliculus accumulate [3H]gamma-aminobutyric acid (GABA).
Mize RR; Spencer RF; Sterling P
We have examined by autoradiography the labeling pattern in the cat superior colliculus following injection of tritiated gamma-aminobutyric acid (GABA). Silver grains were heavily distributed within the zonal layer and the upper 200 micrometer of the superficial gray. Fewer grains were observed deeper within the superficial gray, and still fewer were found within the optic and intermediate gray layers. The accumulation of label was restricted to certain classes of neuron and glia. Densely labeled neurons were small (8-12 micrometer in diameter) and located primarily within the upper 200 micrometer. Dark oligodendrocytes and astrocytes showed a moderate accumulation of label while pale oligodendrocytes and microglia were unlabeled. Label was also selectively accumulated over several other types of profile within the neuropil, including presynaptic dendrites, axons, and axon terminals.
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J Comp Neurol 206(2):180-192, 1982
Two types of GABA-accumulating neurons in the superficial gray layer of the cat superior colliculus.
Mize RR; Spencer RF; Sterling P
Two types of neuron in the upper superficial gray layer of the cat superior colliculus accumulated exogenous 3H-gamma-aminobutyric acid intensely. The first type was a horizontal cell with a fusiform cell body, horizontal dendrites, a low synaptic density, but a high percentage of cortical synaptic contacts. This cell had presynaptic dendrites. The second type was a granule cell (type A) with a small round cell body, thin and obliquely oriented dendrites, a moderate synaptic density, and few cortical synaptic contacts. These two types differed in size, shape, dendritic morphology, and patterns of synaptic input. They likely participate in different inhibitory mechanisms. Four types of unlabeled neurons were also identified. Type B granule cells were found only within the upper subdivision of the superficial gray layer. They had moderate-sized cell bodies, a high synaptic density, and numerous somatic spines. A third type of granule cell (type C) was found only in the deep subdivision of the superficial gray. This type had a low synaptic density and spines that contained synaptic vesicles. Vertical fusiform and stellate forms were also found. We conclude that at least six types of neurons populate the upper superficial gray layer of the cat superior colliculus.
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Nature Neurosci 29:85-92, 2006Axons and dendrites originate from neuroepithelial-like processes of retinal bipolar cells
Morgan JL; Dhingra A; Vardi N; Wong ROL
The cellular mechanisms underling axogenesis and dendritogenesis are not completely understood. The axons and dendrites of retinal bipolar cells, which contact their synaptic partners within specific laminae in the inner and outer retina, provide a good system for exploring these issues. Using trangenic mice expressing enhanced green fluorescent protein (GFP) in a subset of bipolar cells, we determined that axonal and dendritic arbors of these interneurons develop directly from apical and basal processes attached to the outer and inner limiting membranes, respectively. Selective stabilization of processes contributed to stratification of axonal and dendritic arobors within the appropriate synaptic layer. This unusual mode of axogenesis and dendritogenesis from neuroepithelial-like processes may act to preserve neighbor-neighbor relationships in syanaptic wiring between the outer and inner retina.
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J Comp Neurol 405(2):173-184, 1999Differential expression of ionotropic glutamate receptor subunits in the outer retina.
Morigiwa K; Vardi N
Ionotropic glutamate receptors (iGluRs) are extremely diverse in their subunit compositions. To understand the functional consequences of this diversity, it is necessary to know the subunits that are expressed by known cell types. By using immunocytochemistry with light and electron microscopy, we localized several subunits (GluR2/3, GluR4, and GluR6/7) in cat retinal neurons, postsynaptic to photoreceptors. Type A horizontal cells express all three subunits strongly, whereas type B horizontal cells express GluR2/3 strongly, GluR6/7 weakly, and do not express GluR4. When they are present, the subunits are expressed strongly throughout the cytoplasm of the somata and primary dendrites; however, in the terminals, they are concentrated at the postsynaptic region, just opposite the presumed site of photoreceptor glutamate release. Surprisingly, all bipolar cell classes (OFF cone bipolar cells, ON cone bipolar cells, and rod bipolar cells) express at least one iGluR subunit at their dendritic tips. Cone bipolar cells forming basal contacts with the cones (presumably OFF cells) express all three subunits in association with the electron-dense postsynaptic membrane. Invaginating dendrites of cone bipolar cells (presumably ON cells) express GluR2/3 and GluR4. Rod bipolar cells (ON cells) express GluR2/3 in their invaginating dendrites. The function of iGluRs in horizontal cells and OFF bipolar cells clearly is to mediate their light responses. GluR6/7 subunit in the receptor of these cells may be responsible for the dopamine-mediated enhancement of glutamate responses that have been observed previously in these cells. The function of iGluRs in ON bipolar cells remains an enigma.
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Proc Natl Acad Sci USA 77(1):658-661, 1980Interplexiform cell in cat retina: identification by uptake of gamma-[3H]aminobutyric acid and serial reconstruction.
Nakamura Y; McGuire BA; Sterling P
After intravitreal injection of gamma-[3H] aminobutyric acid (GAB), 2% of the neurons at the outer margin of the inner plexiform layer were intensely labeled. Reconstructions of these neurons from serial electron microscope autoradiograms showed that they are interplexiform cells, which synapse on bipolar processes in the outer plexiform layer and on amacrine and bipolar processes in the inner plexiform layer.
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J Comp Neurol 329(1):68-84, 1993OFF-alpha and OFF-beta ganglion cells in cat retina. I: Intracellular electrophysiology and HRP stains.
Nelson R; Kolb H; Freed MA
Six OFF-alpha ganglion cells and a single OFF-beta ganglion cell were penetrated with intracellular microelectrodes and marked with horseradish peroxidase (HRP) in a perfused cat eyecup. Gaussian center radii (Rc) ranging from 40 to 217 microns were measured for receptive fields mapped with slits, values in agreement with previous extracellular reports. ON and OFF response components revealed nearly identical Rc's and center locations. Although Gaussian diameters (2Rc) were about 80% of dendritic field diameters overall, in this sample dendritic and receptive fields were not well correlated. Spatial tuning of ganglion cells was evidenced in peaked amplitude-vs.-width functions, fit by difference-of-Gaussians models. Such plots yielded Rc values about 40% less than position-vs amplitude plots. Rs values for surrounds ranged from 200 to 1,700 microns. Rod and cone signals were investigated with flicker. Rod flicker signals in OFF-alpha cells were larger and of shorter latency than in either horizontal or AII amacrine cells. Cone flicker signals were also short in latency, with an ON response time constant of 9 msec, and an OFF response time constant of 3 msec. The OFF-alpha rod-cone transition involved a latency increase of 20-30 msec. The spontaneous and light-evoked impulse rates of OFF-alpha responses varied linearly with extrinsic current, but the amplitude of ON hyperpolarization was little affected. After injection of staining current, the OFF-beta cell transiently depolarized at ON, suggestive of ON inhibition with reversed chloride gradient, a result not seen in OFF-alpha responses. Events (peaked, depolarizing voltage fluctuations) of high, low, and intermediate amplitudes were studied in OFF-alpha responses. High amplitude events (impulses), were OFF-correlated with the stimulus, and exhibited mean rise times (transit time from 25 to 75% of peak amplitude) from 255 to 392 microseconds. Intermediate level events (presumed synaptic origin) were also OFF correlated and had longer rise times (325 microseconds to 1.56 microseconds). Low level events (234-685 microseconds) revealed either ON, ON/OFF, or not stimulus correlation.
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Intl J Biol Chem 280:1248-1256, 2005Evaluation of the 17 kDa prenyl binding protein as a regulatory protein for phototransduction in retinal receptors
Norton AW; Hosier S; Terew JM; Li N; Dhingra A; Vardi N; Baehr W; Cote RH
The mammalian rod photoreceptor phosphodiesterase (PDE6) holoenzyme is isolated in both a membrane associated and a soluble form. Membrane binding is a consequence of prenylation of PDE6 catalytic subunits, whereas soluble PDE6 is purified with a 17-kDa prenylbinding protein (PDE) tightly bound. This protein, here termed PrBP/d, has been hypothesized to reduce activation of PDE6 by transducin, thereby desensitizing the photoresponse. To test the potential role of PrBP/d in regulating phototransduction, we examined the abundance, localization, and potential binding partners of PrBP/d in retina and in purified rod outer segment (ROS) suspensions whose physiological and biochemical properties are well characterized. The amphibian homologue of PrBP/d was cloned and sequenced and found to have 82% amino acid sequence identity with mammalian PrBP/d. In contrast to bovine ROS, all of the PDE6 in purified frog ROS is membrane-associated. However, addition of recombinant frog PrBP/d can solubilize PDE6 and prevent its activation by transducin. PrBP/d also binds other prenylated photoreceptor proteins invitro, including opsin kinase (GRK1/GRK7) and rab8.Quantitative immunoblot analysis of the PrBP/d contentof purified ROS reveals insufficient amounts of PrBP/d (<0.1 PrBP/d per PDE6) to serve as a subunit of PDE6 in either mammalian or amphibian photoreceptors.The immunolocalization of PrBP/d in frog and bovine retina shows greatest PrBP/ immunolabeling outside the photoreceptor cell layer. Within photoreceptors,only the inner segments of frog double cones are strongly labeled, whereas bovine photoreceptors reveal more PrBP/d labeling near the junction of the inner and outer segments (connectingcilium) of photoreceptors. Together, these results ruleout PrBP/d as a PDE6 subunit and implicate PrBP/d inthe transport and membrane targeting of prenylatedproteins (including PDE6) from their site of synthesis in the inner segment to their final destination in the outer segment of rods and cones.
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J Comp Neurol 361(3):479-490, 1995Retinal neurons and vessels are not fractal but space-filling.
Panico J; Sterling P
Many branched patterns in nature are hypothesized to be fractal, i.e., statistically self-similar across a range of scales. We tested this hypothesis on the two-dimensional arbors of retinal neurons and blood vessels. First, we measured fractalness on synthetic fractal and nonfractal patterns. The synthetic fractal patterns exhibited self-similarity over a decade of scale, but the nonfractal "controls" showed hardly any self-similarity. Neuronal and vascular patterns showed no greater self-similarity than the controls. Second, we manipulated a synthetic fractal pattern to remove its self-similarity and found this to be reflected in a loss of measured fractalness. The same manipulation of the nonfractal control and also of the neural and vascular patterns did not alter their measured fractalness. Third, we "grew" patterns of branched line segments according to a variety of nonfractal algorithms. These patterns were, if anything slightly more fractal than the neural and vascular patterns. We conclude that the biological patterns studied here are not fractal. Finally, we measured extended versions of these patterns: a contiguous array of homotypic neuron arbors and a vascular pattern with a high degree of total detail. These patterns showed a "fractal dimension" of 2, which implies that down to some cut-off scale they fill space completely. Thus, neural and vascular patterns might best be described as quasi-regular lattices.
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Neuron 37(3):379-82, 2003
Synaptic ribbon. conveyor belt or safety belt?
Parsons TD; Sterling P
The synaptic ribbon in neurons that release transmitter via graded potentials has been considered as a conveyor belt that actively moves vesicles toward their release sites. But evidence has accumulated to the contrary, and it now seems plausible that the ribbon serves instead as a safety belt to tether vesicles stably in mutual contact and thus facilitate multivesicular release by compound exocytosis.
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J Neurosci 23:4092-4099, 2003Endocytosis and vesicle recycling at a ribbon synapse.
Paillart C; Li J; Matthews G; Sterling P
At ribbon synapses, where exocytosis is regulated by graded depolarization, vesicles can fuse very rapidly with the plasma membrane (complete discharge of the releasable pool in approximately 200 msec). Vesicles are also retrieved very rapidly (time constant of approximately 1 sec), leading us to wonder whether their retrieval uses an unusual mechanism. To study this, we exposed isolated bipolar neurons from goldfish retina to cationized ferritin. This electron-dense marker uniformly decorated the cell membrane and was carried into the cell during membrane retrieval. Endocytosis was activity-dependent and restricted to the synaptic terminal. The labeling pattern was consistent with direct retrieval from the plasma membrane of large, uncoated endosomes 60-200 nm in diameter. Even after extensive synaptic activity lasting several minutes, most of the ferritin remained in large endosomes and was present in only approximately 10% of the small vesicles that constitute the reserve pool. By contrast, after brief stimulation at a conventional terminal, ferritin did not reside in endosomes but was present in approximately 63% of the small vesicles. We suggest that the bipolar ribbon synapse sustains its rapid exocytosis by retrieving membrane in larger "bites" than the clathrin-dependent mechanism thought to dominate at conventional synapses. The resulting large endosomes bud off small vesicles, which reenter the reserve pool and finally the releasable pool.
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Neuron 14(3):561-569, 1995Mammalian rod terminal: architecture of a binary synapse.
Rao-Mirotznik R; Harkins AB; Buchsbaum G; Sterling P
The mammalian rod synapse transmits a binary signal (one photon or none) using tonic, rapid exocytosis. We constructed a quantitative, physical model of the synapse. Presynaptically, a single, linear active zone provides docking sites for approximately 130 vesicles, and a "ribbon" anchored to the active zone provides a depot for approximately 640 vesicles. Postsynaptically, 4 processes invaginate the terminal: 2 (known to have low affinity glutamate receptors) lie near the active zone (16 nm), and 2 (known to have high affinity glutamate receptors) lie at a distance (130-640 nm). The presynaptic structure seems designed to minimize fluctuations in tonic rate owing to empty docking sites, whereas the postsynaptic geometry may permit 1 vesicle to evoke an all-or-none response at all 4 postsynaptic processes.
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J Neurophysiol 80(6):3163-3172, 1998Transmitter concentration at a three-dimensional synapse.
Rao-Mirotznik R; Buchsbaum G; Sterling P.
Intensities from starlight to 1000-fold brighter, the mammalian rod synapse transmits a binary signal, the capture of 0 or 1 photon. Zero is signified by tonic exocytosis, and 1 is signified by a brief pause. The synapse is three dimensional: vesicles discharge at the apex of a deep cleft created by the invagination of four postsynaptic processes. Two horizontal cell spines bearing alpha-amino-3-hydroxy-5- methyl- 4-isoxazolepropionic acid (AMPA) receptors reach near to the release sites (16 nm), and two bipolar dendrites bearing mGluR6 receptors end far from the release sites (up to 640 nm). We considered two hypotheses for signal transfer: transmitter quanta might be integrated in the cleft and sensed as a steady concentration (high for 0 and low for 1); or quanta might be sensed at the postsynaptic membrane as discrete postsynaptic potentials (PSPs) and integrated within the dendrite. We calculate from a passive diffusion model that the invagination empties rapidly (tau approximately 1.7 ms). Further calculations suggest that a glutamate concentration high enough to hold a bipolar cell in darkness at one end of its response range would require approximately 4,000 vesicles/s. On the other hand, the glutamate pulse from a single vesicle would reach both nearby AMPA receptors (low affinity) and distant mGluR6 receptors (high affinity) at spatiotemporal concentrations matched to their apparent binding affinities. Thus one vesicle could evoke a discrete PSP in all four postsynaptic processes. We calculate from a stochastic model that PSPs could transfer the binary signal at approximately 100 vesicles/s. Thus dendritic integration of unitary PSPs is both plausible and 40-fold more efficient than the alternative mechanism. The rod's deep invagination, rather than serving to pool transmitter, may serve to prevent "spillover" of transmitter to neighboring rods. Spillover, by pooling the noise from neighboring rods, would impair transmission of their binary signals.
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Biophys J 67(1):57-63, 1994Rate of quantal transmitter release at the mammalian rod synapse.
Rao R; Buchsbaum G; Sterling P
Under scotopic conditions, the mammalian rod encodes either one photon or none within its integration time. Consequently the signal presented to its synaptic terminal is binary. The synapse has a single active zone that releases neurotransmitter quanta tonically in darkness and pauses briefly in response to a rhodopsin isomerization by a photon. We asked: what minimum tonic rate would allow the postsynaptic bipolar cell to distinguish this pause from an extra-long interval between quanta due to the stochastic timing of release? The answer required a model of the circuit that included the rod convergence onto the bipolar cell and the bipolar cell's signal-to-noise ratio. Calculations from the model suggest that tonic release must be at least 40 quanta/s. This tonic rate is much higher than at conventional synapses where reliability is achieved by employing multiple active zones. The rod's synaptic mechanism makes efficient use of space, which in the retina is at a premium.
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Neuron 41:755-766, 2004Streamlined synaptic vesicle cycle in cone photoreceptor terminals
Rea R; Li J; Dharia A; Levitan ES; Sterling P; Kramer RH
Cone photoreceptors tonically release neurotransmitter in the dark through a continuous cycle of exocytosis and endocytosis. Here, using the synaptic vesicle marker FM1-43, we elucidate specialized features of the vesicle cycle. Unlike retinal bipolar cell terminals, where stimulation triggers bulk membrane retrieval, cone terminals appear to exclusively endocytose small vesicles. These retain their integrity until exocytosis, without pooling their membranes in endosomes. Endocytosed vesicles rapidly disperse through the terminal and are reused with no apparent delay. Unlike other synapses where most vesicles are immobilized and held in reserve, only a small fraction (<15%) becomes immobilized in cones. Photobleaching experiments suggest that vesicles move by diffusion and not by molecular motors on the cytoskeleton and that vesicle movement is not rate limiting for release. The huge reservoir of vesicles that move rapidly throughout cone terminals and the lack of a reserve pool are unique features, providing cones with a steady supply for continuous release.
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J Neurosci 24(38):8366-8378, 2004Evidence That Each S Cone in Macaque Fovea Drives One
Narrow-Field and Several Wide-Field Blue-Yellow
Ganglion CellsSchein S; Sterling P; Ngo IT; Huang TM; Herr S
A rule of retinal wiring is that many receptors converge onto fewer bipolar cells and still fewer ganglion cells. However, for each S cone in macaque fovea, there are two S-cone ON bipolar cells and two blue-yellow (BY) ganglion cells. To understand this apparent rule reversal, we reconstructed synaptic patterns of divergence and convergence and determined the basic three-tiered unit of connectivity that repeats across the retina. Each foveal S cone diverges to four S-coneONbipolar cells but contacts them unequally, providing 1–16 ribbon synapses per cell. Next, each bipolar cell diverges to two BY ganglion cells and also contacts them unequally, providing 14 and 28 ribbon synapses per cell. Overall, each S cone diverges to approximately six BY ganglion cells, dominating one and contributing more modestly to the others. Conversely, of each pair of BY ganglion cells, one is dominated by a single S cone and one is diffusely driven by several. This repeating circuit extracts blue/yellow information on two different spatiotemporal scales and thus parallels the circuits for achromatic, spatial vision, in which each cone dominates one narrow-field ganglion cell (midget) and contributes some input to several wider-field ganglion cells (parasol). Finally, because BY ganglion cells have coextensiveS and (LM) receptive fields, and each S cone contributes different weights to differentBYganglion cells, the coextensive receptive fields must be already present in the synaptic terminal of the S cone. The S-cone terminal thus constitutes the first critical locus for BY color vision.
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Intl J Biochem & Cell Biol 37:1681-1695, 2005Novel ryanodine-binding properties in mammalian retina
Shoshan-Barmatz V; Orr I; Martin C; Vardi N
The ryanodine receptor (RyR)/Ca2+ release channel mobilizes Ca2+ from internal calcium stores to support a variety of neuronal functions. To investigate the presence of such a protein in mammalian retina, we applied ryanodine binding, PCR and antibodies against known RyRs. Surprisingly, ryanodine-binding properties of retinal endoplasmic reticulum-enriched membrane fraction were vastly different from those of skeletal and cardiac muscles ryanodine-binding proteins. In common with the skeletal and cardiac muscle, ryanodine bound with high-affinity to two or more types of binding site (Kd1 = 20.6 and Kd2 = 114 nM); binding was strongly stimulated by high concentrations of NaCl; it was inhibited by tetracaine and the protein appeared to possess an ATP-binding site. Unlike cardiac and skeletal muscle, RyRs in retina binding was Ca2+-independent; inhibited by caffeine and dantrolene; less sensitive to ruthenium red; and unaffected by La3+. Also, in retina, ryanodine rapidly associated to and dissociated from its binding sites. Furthermore, although the protein bound the ATP analog BzATP, retinal ryanodine binding was not stimulated by nucleotides. Immunostaining of bovine retinal sections with anti-RyR2 showed a strong staining of amacrine, horizontal and ganglion cells. Finally, using RT-PCR, the three known RyR isoforms were identified in retina. However, consistent with the novel binding properties, the peptide maps yielded by trypsin treatment and Western blotting demonstrate different patterns. Together, the results suggest that retina expresses a novel ryanodine-binding protein, likely to be a ryanodine receptor. Its presence in retina suggests that this protein might play a role in controlling intracellular Ca2+ concentration.
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J Neurosci 6(12):3505-3517, 1986Microcircuitry of the dark-adapted cat retina: functional architecture of the rod-cone network.
Smith RG; Freed MA; Sterling P
The structure of the rod-cone network in the area centralis of cat retina was studied by reconstruction from serial electron micrographs. About 48 rods converge on each cone via gap junctions between the rod spherules and the basal processes of the cone pedicle. One rod diverges to 2.4 cones through these gap junctions, and each cone connects to 8 other cones, also through gap junctions. A static cable model of this network showed that at mesopic intensities, when all rods converging on a cone pedicle are continuously active, the collective rod signal would be efficiently conveyed to the pedicle. At scotopic intensities sufficiently low for only one of the converging rods to receive a single photon within its integration time, the quantal rod signal would be poorly transmitted to the cone pedicle. This is because the tiny signal would be dissipated by the large network into which the individual rod diverges. Under this condition, the rod signal would also be poorly conveyed to the rod spherule. If, however, the rods are electrically disconnected from the network, the quantal signal would be efficiently conveyed to the rod spherule. This analysis suggests that the rod signal is conveyed at mesopic intensities by the cone bipolar pathway and, at scotopic intensities, by the rod bipolar pathway, in accordance with the results of Nelson (1977, 1982; Nelson and Kolb, 1985).
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J Neurosci Methods 21(1):55-69, 1987Montage: a system for three-dimensional reconstruction by personal computer.
Smith RG
This paper describes a simplified system for serial section three-dimensional (3-D) reconstruction. A set of 9 software programs runs on a standard personal computer and produces camera-ready illustrations suitable for publication. The user enters trace points on a digitizing tablet from sections that have been already aligned. A 3-D view of the reconstructed object is generated which can be displayed with hidden lines removed. Analysis of volume, surface area and autoradiographic grain density are performed automatically. A relational database query language allows display and analysis of a selected subset of the data. The system runs under the UNIX operating system which allows the programs to be easily transported to new hardware or modified for other purposes.
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Vis Neurosci 5(5):453-461, 1990Cone receptive field in cat retina computed from microcircuitry.
Smith RG; Sterling P
The receptive-field profile of the cone in cat-retina was computed. The computation was based on (1) the known anatomical circuit connecting cones via narrow-field bipolar cells to the on-beta ganglion cell; (2) the known physiological receptive-field profile of the on-beta (X) cell at the corresponding eccentricity; and (3) a model in which the beta receptive field arises by linear superposition of cone receptive fields. The computed cone receptive field has a center/surround organization with a center almost as broad as that of the beta cell center. The cone surround is comparably broad to that of the beta cell but somewhat lower in peak amplitude. The problems to which the center/surround receptive field are the solution, namely, signal compression and noise reduction, apparently must be solved before the first synapse of the visual pathway.
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J Neurosci Methods 43(2-3):83-108, 1992NeuronC: a computational language for investigating functional architecture of neural circuits.
Smith RG
A computational language was developed to simulate neural circuits. A model of a neural circuit with up to 50,000 compartments is constructed from predefined parts of neurons, called "neural elements". A 2-dimensional (2-D) light stimulus and a photoreceptor model allow simulating a visual physiology experiment. Circuit function is computed by integrating difference equations according to standard methods. Large-scale structure in the neural circuit, such as whole neurons, their synaptic connections, and arrays of neurons, are constructed with procedural rules. The language was evaluated with a simulation of the receptive field of a single cone in cat retina, which required a model of cone-horizontal cell network on the order of 1000 neurons. The model was calibrated by adjusting biophysical parameters to match known physiological data. Eliminating specific synaptic connections from the circuit suggested the influence of individual neuron types on the receptive field of a single cone. An advantage of using neural elements in such a model is to simplify the description of a neuron's structure. An advantage of using procedural rules to define connections between neurons is to simplify the network definition.
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In: Computation in Neurons and Neural Systems
Frank H. Eeckman (Ed.), Kluwer Academic Publishers (1994)Measurement of simulation speed: its relation to simulation accuracy
Smith RG
This article presents an unbiased method for measuring simulation speed for compartmental simulators. The method measures how long it takes to simulate a neural circuit component at a given overall accuracy. Because both spatial and temporal discretizations influence overall accuracy, it is possible to optimize both spatial and temporal accuracy to maximize overall speed.
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Vis Neurosci 1995 Sep;12(5):851-860Simulation of the AII amacrine cell of mammalian retina: functional consequences of electrical coupling and regenerative membrane properties.
Smith RG; Vardi N
The AII amacrine cell of mammalian retina collects signals from several hundred rods and is hypothesized to transmit quantal "single-photon" signals at scotopic (starlight) intensities. One problem for this theory is that the quantal signal from one rod when summed with noise from neighboring rods would be lost if some mechanism did not exist for removing the noise. Several features of the AII might together accomplish such a noise removal operation: The AII is interconnected into a syncytial network by gap junctions, suggesting a noise-averaging function, and a quantal signal from one rod appears in five AII cells due to anatomical divergence. Furthermore, the AII contains voltage-gated Na+ and K+ channels and fires slow action potentials in vitro, suggesting that it could selectively amplify quantal photon signals embedded in uncorrelated noise. To test this hypothesis, we simulated a square array of AII somas (Rm = 25,000 Ohm-cm2) interconnected by gap junctions using a compartmental model. Simulated noisy inputs to the AII produced noise (3.5 mV) uncorrelated between adjacent cells, and a gap junction conductance of 200 pS reduced the noise by a factor of 2.5, consistent with theory. Voltage-gated Na+ and K+ channels (Na+: 4 nS, K+: 0.4 nS) produced slow action potentials similar to those found in vitro in the presence of noise. For a narrow range of Na+ and coupling conductance, quantal photon events (approximately 5-10 mV) were amplified nonlinearly by subthreshold regenerative events in the presence of noise. A lower coupling conductance produced spurious action potentials, and a greater conductance reduced amplification. Since the presence of noise in the weakly coupled circuit readily initiates action potentials that tend to spread throughout the AII network, we speculate that this tendency might be controlled in a negative feedback loop by up-modulating coupling or other synaptic conductances in response to spiking activity.
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Vis Neurosci 12(3):545-561, 1995Simulation of an anatomically defined local circuit: the cone-horizontal cell network in cat retina.
Smith RG
The outer plexiform layer of the retina contains a neural circuit in which cone synaptic terminals are electrically coupled and release glutamate onto wide-field and narrow-field horizontal cells. These are also electrically coupled and feed back through a GABAergic synapse to cones. In cat this circuit's structure is known in some detail, and much of the chemical architecture and neural responses are also known, yet there has been no attempt to synthesize this knowledge. We constructed a large-scale compartmental model (up to 50,000 compartments) to incorporate the known anatomical and biophysical facts. The goal was to discover how the various circuit components interact to form the cone receptive field, and thereby what possible function is implied. The simulation reproduced many features known from intracellular recordings: (1) linear response of cone and horizontal cell to intensity, (2) some aspects of temporal responses of cone and horizontal cell, (3) broad receptive field of the wide-field horizontal cell, and (4) center-surround cone receptive field (derived from a "deconvolution model"). With the network calibrated in this manner, we determined which of its features are necessary to give the cone receptive field a Gaussian center-surround shape. A Gaussian-like center that matches the center derived from the ganglion cell requires both optical blur and cone coupling: blur alone is too narrow, coupling alone gives an exponential shape without a central dome-shaped peak. A Gaussian-like surround requires both types of horizontal cell: the narrow-field type for the deep, proximal region and the wide-field type for the shallow, distal region. These results suggest that the function of the cone-horizontal cell circuit is to reduce the influence of noise by spatio-temporally filtering the cone signal before it passes through the first chemical synapse on the pathway to the brain.
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Proc Natl Acad Sci USA 81(12):3898-3900, 1984Numbers of specific types of neuron in layer IVab of cat striate cortex.
Solnick B; Davis TL; Sterling P
Layer IVab of the visual cortex (area 17) of the cat contains about 51,400 neurons per mm3, including about 400-1200 per mm3 of each of three categories of neuron believed from previous work to represent discrete types. Each type forms about 0.5-1.5% of all the IVab neurons, which suggests that the total number of types in this layer might be much greater than previously supposed, perhaps as many as 50 or more. From their densities and estimates of their dendritic fields, we calculate that each type completely "covers" layer IVab in the tangential plane but only by a small factor (1.3-4.2).
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Sterling & Kuypers 1966
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Sterling & Wickelgren 1969
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Sterling & Wickelgren 1970
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Sterling 1971
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Sterling & Gestrin 1975
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Sterling 1978
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Sterling 1979
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J Comp Neurol 1980 Aug 15;192(4):737-749Neurons in cat lateral geniculate nucleus that concentrate exogenous [3H]-gamma-aminobutyric acid (GABA).
Sterling P; Davis TL
About one-quarter of the neurons in the A-laminae of the cat lateral geniculate selectively accumulate exogenous [3H]-gamma-aminobutyric acid (GABA), its analog, [3H]-2,4-diaminobutyric acid (DABA), and the GABA agonist, [3H] muscimol. These neurons are small (12-18 micrometers diameter) and lack a laminar body, which suggests that they correspond to the class III cell identified in Golgi material. GABA and DABA are also accumulated by F-terminals which are post-synaptic to retinal terminals and presynaptic to relay cell dendrites. It is suggested that GABA may be the transmitter for these small neurons which appear to mediate by means of local circuits a feed-forward inhibition onto the relay cells.
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Sterling & Eyer 1981
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Sterling 1982
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Ann. Rev. Neurosci. 6:149-185, 1983Microcircuitry of the cat retina.
Sterling P
As a device for extracting information from a visual image, the vertebrate retina is unparalleled in its range, reliability, and compactness. Signaling in the retina is slower by six orders of magnitude than in an integrated digital circuit. The advantage of the biological structure must therefore derive from the variety of its fundamental elements and from the subtlety of their connections. Each of the five major classes of retinal neuron, whose synaptic contacts were first described systematically by Dowling & Boycott (1966) is now known to have multiple types, totaling in the cat about 60. Specific local circuits involving about one-third of these neurons have been recognized in the electron microscope. Physiological responses have also been documented for about one-third of the types, and evidence regarding the neural transmitter, or at least the sign of the synapse, has accumulated also for about one-third. These discoveries have abundantly supported certain concepts of retina function developed in the 1960s by Lettvin & Maturana. The function of the retina, they proposed, "is not to transmit information about the point-to-point distribution of light and dark in the image, but to analyze this image at every point in terms of ... arbitrary contexts ..." (Maturana et al., 1960). Each of these "contexts," they suggested, corresonds to some operation on the local image performed by a ganglion cell of particular size and shape (Lettvin et al., 1961). This idea, based on studies of the frog, seemed for a time inapplicable to the cat, which was thought to have a "simple" retina with only center-surround type ganglion cells. Subsequent studies to be reviewed here have firmly established for the cat the validity of this idea. Lettvin & Maturana also paid special attention to the stratification of processes in the frog's inner plexiform layer, believing tthat the operation performed by a ganglion cell is determined by specific bipolar inputs delivered to the strata of its dendritic arbor. This idea, too, was thought to be inapplicable to the cat, whose inner plexiform layer is less obvioiusly stratified than the frog's. Sutdies to be reviewed here now strongly support this concept for the cat. Nothing of the actual circuits between particular neuron types was known to Lettvin & Maturana, but fragments of such knowledge accumulated for the cat during the 1970s. Some of the first observations were extremely puzzling. It turned out, for example, that rods and ccones have separate bipolars and thus apparently separate pathways to ganglion cells. But rod signals are also transmitted directly to cones, so why the separate bipolars? Further, the rod bipolar does not contact most ganglion cells, as one might have anticipated, but contacts an amacrine, which in turn contacts, not ganglion cells, but cone bipolar axons! What could be the meaning of this second convergence of rod and cone pathways, and why send the rod signal through such a tortuous route? The functions of such apparently bizarre paths have been difficult to comprehend for the same reasons that fragments of an integrated circuit cannot be grasped except in the context of its larger diagram. Now, however, with links established between about one-third of the neuron types, broad pathways can be identified and specific hypotheses can be suggested regarding their function. The outline of a detailed mechanistic account of retinal function emerges, and many of the necessary strategies and techniques for achieving it seem at hand. In reviewing this subject now, when many puzzling findings of the past 15 years begin to fit, one is deeply impressed with the intelligence and care of the many individual studies from laboratories that literally girdle the globe. In this article the first section reviews our knowledge of particular cell types comprising each major class of retinal neuron. Information is presented, where available, for each type regarding morphology, circuitry, distribution, transmitter, and physiology. Such details constitute the primary evidence that the retina is composed of many discrete cell types arranged in a regular mosaic. This section also serves in effect as a "parts list" for the second section, which describes a complex circuit involving thirteen types of neuron and suggests how the circuit might function.
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J Neurosci 6(5):1314-1324, 1986Molecular specificity of defined types of amacrine synapse in cat retina.
Sterling P; Lampson LA
The inner plexiform layer of cat retina contains synaptic structures belonging to 50 or more types of "identified" neurons. To learn whether there are antigens confined to subsets of these synaptic structures, we raised monoclonal antibodies to homogenates of neural retina. Binding patterns of these antibodies were visualized by the peroxidase-antiperoxidase method and studied in serial, ultrathin sections by electron microscopy. Four antibodies stained the synaptic varicosities of certain amacrine cells. Many of the stained varicosities formed reciprocal synapses with a rod bipolar axon terminal, but only about half of the reciprocal synapses associated with a rod bipolar were stained. Other stained varicosities formed synapses with cone bipolar axons, ganglion cell dendrites, and unstained amacrine processes. The patterns were essentially the same for each antibody and were not altered by staining with the antibodies two at a time; therefore, it is likely that all four antibodies stain the same subset of synaptic structures. These patterns would be accounted for if there were staining of all the synaptic varicosities of three of the four types of identified amacrine reciprocally connected to the rod bipolar (A6, A8, A13). This localization suggests that the antigen responsible for the binding pattern is not associated with synaptic transmission. Staining is present in the inner plexiform layer during the period of synaptogenesis and consequently the antibodies are serving as markers for following the development of identified synapses in an identified neural circuit.
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Trends Neurosci 9:186-192, 1986Microcircuitry and functional architecture of the cat retina.
Sterling P; Freed M; Smith RG
Neurons in cat retina belong to specific types. Each type is characterized by a specific correspondence between morphology and physiology and forms a regular array that connects lawfully to the arrays of certain other types. Two circuits have been traced quantitatively through these arrays from photoreceptors to alpha- and beta- ganglion cells. The 'cone-bipolar circuit' appears to convey the center-surround receptive field to ganglion cells, using cones in daylight and rods (via gap junctions to cones) in twilight. A 'rod-bipolar circuit' appears to convey the quantal signal and the pure center receptive field to the ganglion cells in starlight.
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Neurosci Res Suppl 6:S269-S285, 1987Microcircuitry of the on-beta ganglion cell in daylight, twilight, and starlight.
Sterling P, Cohen E, Freed MA, Smith RG
Between noon and the end of nightfall, the intensity of light in the environment declines by about ten billion-fold. MOst of the drama in the human experience of this change occurs during the hours that we call "twilight". Colors gradually shift in hue and then desaturate, but spatial resolution is preserved for a while longer. Thus, in a garden the red roses turn purple and then black, but the structure of the bush remains distinct. Only later, as the stars appear, do the details of the foliage dissolve into shadow. Our experience of these transitions is paralleled to some extent by the behavior of individual ganglion cells in cat retina. So remarkable is their capacity to adapt that they remain responsive to visual stimuli over the full ten log unit range of envioronmental light intensity [1]. In this essay, we review some salient features of this adaptation process. We then summarize recent anatomical studies of the circuits connecting photoreceptors to the ganglion cells and speculate upon the relation of the neural architecture to the function. Only one type of ganglion cell is considered: the ON-center cell known to physiologists as "X" or "brisk-sustained" and to morphologists as "beta" [2-5]. All of the measurements considered here, both physiological and anatomical, refer to neurons in the area centralis.
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In: Handbook of Life Stress, Cogintion and Health
S Fisher & J Reason (eds) J. Wiley & Sons 1988
Allostasis: a new paradigm to explain arousal pathology.
Sterling P; Eyer J
This chapter summarizes our joint effort as epidemiologist (J.E.) and neurobiologist (P.S.) to understand the physiological basis for certain broad patterns of human morbidity and mortality. Age-specific death rates rise when intimate social relations are disrupted. This is observed in contemporary statistics, for example the mortality associated with bereavement, divorce, migration and overwork. It is also observed historically in the increased mortality of urban versus rural populations and in the rise of age-specific death rates that accompanies modern economic development. The increases in all these examples are large (two- to ten-fold) and are observed for essentially all causes, so they cannot be explained by any single environmental factor such as air pollution or nutrition (Berkson, 1962; Eyer and Sterling, 1977).[FULL PDF]
J Neurosci 8(2):623-642, 1988Architecture of rod and cone circuits to the On-beta ganglion cell.
Sterling P; Freed MA; Smith RG
Photoreceptors connect to the on-beta ganglion cell through parallel circuits involving rod bipolar (RB) and cone bipolar (CB) neurons. We estimated for a small patch in the area centralis of one retina the 3-dimensional architecture of both circuits. This was accomplished by reconstructing neurons and synapses from electron micrographs of 189 serial sections. There were (per mm2) 27,000 cones, 450,000 rods, 6500 CBb1, 30,300 RB, 4100 All amacrines, and 2000 on-beta ganglion cells. The tangential spread of processes was determined for each cell type, and, with the densities, this allowed us to calculate the potential convergence and divergence of each array upon the next. The actual numbers of cells converging and diverging were estimated from serial sections, as were the approximate numbers of chemical synapses involved. The cone bipolar circuit showed narrow convergence and divergence: 16 cones->4 CBb1->1 on-beta 1 cone->1 CBb1->1.2 on-beta This circuit is thought to contribute significantly to the on-beta cell's photopic receptive field because the CBb1 has a center-surround receptive field whose center diameter is greater than the spacing between adjacent CBb1s. Consequently, the receptive fields of the CBb1s converging on a beta cell are probably largely concentric and thus mutually reinforcing in their contributions to the on-beta. The rod bipolar circuit showed a wider convergence and divergence: 1500 rods->100 RB->5 AII->4 CBb1->1 on-beta 1 rod->2 RB->5 AII->8 CBb1->2 on-beta The 1500 rods converging via this circuit account for the spatial extent of the beta cell's dark-adapted receptive field. This convergence also accounts for the ganglion cell's maintained discharge, which is thought to arise from about 6 quantal "dark events" per second. This many dark events would appear in the ganglion cell if each rod in the circuit contributed 0.004 dark events per second, and this is close to what has been measured in monkey rods (Baylor et al., 1984). Divergence in this circuit serves to expand the number of copies of the quantal signal (1 rod->8 CBb1) and so to engage large numbers of chemical synapses that provide amplification. Reconvergence at the last stage (8 CBb1->2 on-beta) may reduce (by signal averaging) the synaptic noise that would otherwise accumulate along the pathway.
[FULL PDF]
In: Analysis and Modeling of Neural Systems (1992)
Ed. by Frank H. Eeckman. Kluwer Academic PublishersRetinal circuits for daylight: why ballplayers don't wear shades.
Sterling, P., Cohen, E., Smith, R.G., and Tsukamoto, Y.
A natural scene contains fine spatial detail at low contrast (Srinivasan et al., 1982), and to represent it as an optical image on the retina requires quite a lot of light. This is because the number of photons arriving at a given locus fluctuates according to Poisson statistics. For an image to emerge above this fluctuating background of "photon noise" an object must be brighter than the mean luminance by at least one standard deviation (corresponding to the square root of the mean luminance) (Rose, 1973). Consider an example from baseball, a high fly ball just barely visible against the bright sky. At 100 meters from the outfielder's eye the ball subtends only 12 cones and is brighter than the sky by about 0.3%. To represent this low contrast spot requires that the ball reflect onto the retina at least 12,000 photons per cone integration time (sqrt(12000) / 12000 = 0.3%). The number of extra photons per cone above the mean is only 30. Were an outfielder to wear sunglasses (which they do not, except to follow the ball directly across the sun), the mean luminance would be reduced by, say, ten-fold. The least detectable contrast at 100 meters would then be 0.9% (sqrt(1200)/1200=0.9%). Since the contrast of the ball against the sky is independent of mean luminance (and thus still only 0.3%) the ball would be invisible. To provide the minimum number of photons requisite for detection (12,000) the ball's image on the retina must be larger. Now it must subtend 120 cones but this occurs only when the ball closes the distance to 32 meters. Thus, for the optical image on the outfielder's retina, every photon is precious, even in broad daylight (see Pelli, 1990; Banks et al., 1987). Noise in the optical image from photon fluctuations is only the first problem, however, because creating the neural image involves additional noise. Each step in visual transduction and synaptic transmission involves Poisson statistics whose noise levels also follow the "square root law" (Attwell, 1986). Thus, how soon the outfielder sees the ball will depend on the efficiency of transduction and also on efficient encoding by the neural circuits leading from the cones. One anticipates that the urgent need to preserve the signal/noise (S/N) ratio of the neural image would be expressed in the design of these circuits. Here we review the functional archkitecture of the circuit in cat retina that connects cones to one type of ganglion cell. This is the "on-beta" ("X") cell (Boycott and Wassle, 1974), which responds linearly (Enroth-Cugell and Robson, 1966), and whose receptive field is fitted by a difference-of-Gaussians function (Linsenmeier et al., 1982). The on-beta cell and its complement, the "off-beta", have the highest sampling densities and the narrowest sampling apertures in the cat retina (Wassle et al., 1981; Cleland et al., 1979). The beta cell arrays apparently relay to the higher visual areas the finest grained neural image. We describe the structure of the cone circuit to the on-beta and suggest how the architecture might contribute to efficient coding of a fine, low contrast, neural image.
[FULL PDF]
In: Neurobiology and Clinical Aspects of the Outer Retina (1995)
(Archer S, Djamgoz MBA, Vallerga S eds), pp 325-348. London UK: Chapman & Hall, Ltd.Functional architecture of mammalian outer retina and bipolar cells.
Sterling P; Smith RG; Rao R; Vardi N
Function in the outer retina has mainly gbeen studied by recording in situ from single neurons. In lower vertebrates this approach to bipolar cells has been extremely fruitful (e.g. Chapter 12), but in mammals bipolar cell recordings can be counted on the fingers of (at most) two hands (Nelson and Kolb, 1983; Dacheux and Raviola, 1986). And, considering that the recordings include both rod bipolar and multiple types of cone bipolar cell (Chapter 11), the electrophysiological data regarding mammalian bipolar neurons are thinly spread. On the other hand, in lower vertebrates information essential to understanding the contribution of the outer retina to image processing (such as optics, sampling frequencies, and synaptic circuitry) hardly exists. So, in lower vertebrates how single neuron responses in the outer contribute to vision remains unclear. Yet, single cell recording is not the only possible approach to understanding retinal function. An alternative strategy is to determine complete circuit structure ('wiring diagram') plus the chemical architecture and to incorporate this informatlion, together with the optics and ganglion cell electrophysiology, into various computational models. Then, one might calculate backwards to the properties of the bipolar cells and photoreceptors. Such an effort leads to specific predictions regarding the photoreceptor and bipolar cell function, and with this approach a little electrophysiology goes a surprisingly long way. At least, that is our argument in this review. We emphasize cat retina, which is known in most detail, but also note recent data from reabbit and primate that indicate conservation of certain basic circuits and functions. In the exact center of the area centralis, cone density reaches 30,000-40,000/mm2 (Wässle and Riemann, 1978; Williams et al., 1993), but the circuitry has been studied slightly off center (1 deg eccentricity) where the cone density is about 24,000/mm2 and rod density is about 350,000/mm2 (Sterling et al., 1988). Here, due to the natural blur of the cat's optics (Wässle, 1971; Robson and Enroth-Cugell, 1978), the minimum number of photoreceptors stimulated from a point source is about 10 cones and 140 rods (Figure 13.1a, 2b). This is many more receptors than converging on a single bipolar cell, so even the finest spatial stimulus falling on either type of receptor will affect many bipolar cells. We review first the circuits for daylight that lead from cones because various portions of this circuit are parasitized by the circuits for twilight and starlight that lead from rods (Figure 13.2).
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Nature 377:676-677, 1995Tuning retinal circuits.
Sterling P
While studying the retina more than 100 years ago, Santiago Ramon y Cajal noted that deposits of silver dichromate completely filled the arborization of a single neuron but stopped at the cell boundary. He concluded that contiguous neurons are discrete and that signals between them must cross an extracellular gap, now known as a chemical synapse. He vigorously defended this idea against 'reticularism', the view that neurons form continuous networks, with his penetrating observations and towering polemics, and by 1935 reticularism was apparently crushed [1]. But we now know that the reticularists were also right: retinal neurons can couple with one another by means of clusters of fine intercellular channels called gap junctions [2] and it seems that this coupling is crucial for retinal function. New results described on page 734 of this issue by Mills and Massey [3] offer a deeper insight into reticularism with the demonstration that these gap junctions differ in pore size at different points in a circuit and that they are regulated by different second messengers. The authors injected tracers of different molecular weights into an AII amacrine cell of the rabbit retina (maintained in vitro) and followed their spread into adjacent neurons to which the AII is known to be coupled. As expected [4], the smaller tracer spread into the neighbouring AII cells and cone bipolar cells. A larger tracer with the same charge also spread readily into AII cells, but penetrated poorly into cone bipolars (see Figure 2 and Figure 3 of Mills and Massey's paper [3]). High concentrations of cyclic AMP were already known to curtail tracer spread to AII cells [4], and the authors found that cyclic GMP had a similar effect on tracer spread to cone bipolar cells (see their Figure 4). This discovery of a difference in pore size and in second messengers raises a host of questions that may eventually link the architecture of microcircuits to that of their computationally important molecules. To grasp the key issues requires some knowledge of the overall circuitry in which the AII cell is a critical link (see Figure 1). Figure 1. To convey the full range of environmental intensities requires three different but partially overlapping neural circuits. The mammalian retina accomplishes this at no extra cost in space or noise by using four sets of gap junctions (red), at least three of which are regulated. In daylight, cone terminals excite (depolarize) cone bipolar cells that chemically excite the ganglion cell. This last excitatory stage occupies most of the ganglion cell's dendritic surface (some space is reserved for inhibitory synapses). Gap junctions couple the cone terminals strongly enough to improve signal-to-noise ratio, but not so much as to degrade acuity. In twilight, cones are less active, but the 50 rods surrounding each cone couple to it via their terminals. In effect, rods 'parasitize' the cone terminals and thus the rest of the cone bipolar circuit. In starlight, only one rod per thousand transduces a photon over one second, so the rod-cone junction conveys mostly noise. Consequently, this junction uncouples, and rod terminals excite rod bipolar cells to excite All amacrine cells chemically. The All cell couples to the cone bipolar axon, in effect parasitizing the cone bipolar-to-ganglion cell synapses. Gap junctions couple All cells strongly enough to spread current widely and thus enlarge the ganglion cell's summation area well beyond its dendritic tree. This would improve signal-to-noise ratio in the dimmest light, but degrade acuity in brighter light, so this junction is also regulated.
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Neuron 21(4):643-644, 1998"Knocking out" a neural circuit.
Sterling P
Preview of Soucy et al., 1998
Although the visual system occupies nearly half of the mammalian brain, we still do not completely understand its first synaptic stage. One reason is that the dendrites postsynaptic to photoreceptors comprise such a maze of fine processes that doubt remains whether all the second order circuits have been identified -- even after four decades of electron microscopy. Now advanced functional methods applied to a mammalian rod pathway suggest a circuit previously unsuspected from anatomy.
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Nature Neurosci 2(10):851-853, 1999Deciphering the retina's wiring diagram.
Sterling P
By simultaneously recording from retinal ganglion cells while stimulating a single cone, Chichilnisky and Baylor demonstrate that the strength of physiological connections within a retinal microcircuit is linearly proportional to the number of anatomically defined synapses. Over the last few decades, considerable effort has been devoted to constructing a detailed 'wiring diagram' for the retina, that is, a quantitative map of all its excitatory and inhibitory synaptic connections1, 2. Propelling this effort has been a faith that the diagram would ultimately make functional sense—that we would be able to 'read' the diagram of a neural circuit just as we do for an electronic circuit. Were this faith to prove unjustified—if, for example, the physiological strength of an input proved to be unrelated to the number of anatomically defined synapses—then the retina's synaptic diagram would be far less informative than we might otherwise hope. It is certainly reasonable to question the usefulness of a purely structural diagram, given that neurons contain nonlinear elements such as voltage-gated channels and second-messenger cascades. However, a report by Chichilnisky and Baylor in this issue of Nature Neuroscience3 provides reassurance. By analyzing how individual cones contribute to the receptive field of a retinal gangion cell, the authors provide support for a key hypothesis derived from the wiring diagram, that the strength of an excitatory input is proportional to the number of chemical synapses that underlie it.
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IN: The Retinal Basis of Vision
J Toyoda et al. editors; Elsevier, Tokyo 1999
The ganglion cell receptive field.
Sterling P
A ganglion cell's functional connection to the receptor mosaic determines its "receptive field". Receptive fields have been mapped in all verebrate classes from fish to mammal. In accord with an animal's lifestyle, certain receptive fields reflect specialized computations to extract particular types of information, such as speed and direction of motion, color, etc. [1,2]. However, this chapter treats only two types of receptive field that are common to all species, narrow- and wide-field. They have been most thoroughly studied in cat, where they are termed, respectively, beta (X) and alpha (Y) [3,.4].
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Sterling 2000 ECT
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Neuron 34:670-672, 2002
Needle from a haystack: optimal signaling by a nonlinear synapse
Sterling P
Commonly, a neuron must separate a small, rare event carried by one of its inputs from the noise carried by many others. In this issue of Neuron, Field and Rieke (2002) demonstrate that to solve this problem, the rod bipolar neuron in mouse retina selectively amplifies a rod's single-photon signal only when it is larger than average. This nonlinearity rejects nearly three-fourths of the single-photon signals. Yet, by also rejecting noise, it provides nearly optimal filtering near absolute visual threshold.
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Nature 42:375-376, 2002
Neuroscience: how neurons compute direction.
Sterling P
Review of Euler et al., 2002; Taylor & Vaney, 2002; Fried et al., 2002
Certain retinal neurons fire specifically in response to stimuli moving in one direction. This apparently occurs when branches of an upstream nerve cell respond asymmetrically, and link asymmetrically to the firing retinal neuron.
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IN: Allostasis, Homeostasis, and the Costs of Adaptation. J Schulkin, Cambridge University Press, 2004
Principles of allostasis: optimal design, predictive regulation, pathophysiology and rational therapeutics.
Sterling P
This chapter compares two alternative models of physiological regulation. The first model, homeostasis ("stability through constancy"), has dominated physiology and medicine since Claude Bernard declared, “All the vital mechanisms...have only one object "to preserve constant the conditions of ... the internal environment"”. His dictum has been interpreted literally to mean that the purpose of physiological regulation is to clamp each internal parameter at a "setpoint" by sensing errors and correcting them with negative feedback ( Figure 1; Cannon, 1935). Based on this model physicians reason that when a parameter deviates from its setpoint value, some internal mechanism must be broken. Consequently they design therapies to restore the "inappropriate" value to "normal".
The homeostasis model has contributed immeasurably to the theory and practice of scientific medicine, so to criticize it might almost seem absurd. Yet, all scientific models eventually encounter new facts that do not fit, and this is now the case for homeostasis. In physiology, evidence accumulates that parameters are not constant. And their variations, rather than signifying error, are apparently designed to reduce error. In medicine, major diseases now rise in prevalence, such as essential hypertension and type 2 diabetes, whose causes the homeostasis model cannot explain. For in contrast to the hypertension caused by a constricted renal artery and the diabetes caused by immune destruction of insulin-secreting cells, these newer disorders present no obviously defective mechanism. And treating these diseases with drugs to fix low-level mechanisms that are not broken turns out not to work particularly well. The chapter will expand upon each of these points.
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IN: The Synaptic Organization of the Brain
Gordon Shepherd, (Ed.) Oxford University Press.5th Edition (2004)
Retina
Sterling P; Demb JB
The retina is a thin sheet of neural tissue lining the posterior hemisphere of the eyeball. It is actually part of the brain itself (~0.5%), evaginating from the lateral wall of the neural tube during embryonic development. The optic stalk grows out from the brain toward the ectoderm, inducing it to form an optical system (cornea, pupil, lens), which projects a physical image of the world onto the retina. The retina's task is to convert this optical image into a "neural image" for transmission down the optic nerve to a multitude of centers for further analysis. The task is complex - which is reflected in the synaptic organization.
The transformation from optical to neural image involves three stages: (1) transduction of the image by photoreceptors; (2) transmission of these signals by excitatory chemical synapses to bipolar cells; and (3) further transmission by excitatory chemical synapses to ganglion cells. Axons from the latter collect in the optic nerve and project forward to the brain. At each synaptic stage there are specialized laterally connecting neurons called, respectively, horizontal and amacrine cells. These modify (largely by inhibitory chemical synapses) forward transmission across the synaptic layers. These elements are shown schematically in Fig. 6.1. A closer look at this apparently simple design (three interconnected layers and five broad classes of neuron) reveals additional complexity (Figs. 6.2, 6.3). Each neuron class is represented by several or many specific types. Each cell type is distinguished from others in its class by its characteristic morphology, connections, neurochemistry, and function (Rodieck and Brening, 1983; Sterling, 1983). This diversity, amounting to some 80 cellular types (Kolb et al., 1981; Sterling, 1983; Vaney, 1990), was puzzling at first, but a broad explanation has gradually emerged: it is impossible to encode all the information in an optical image using a single neural image. Therefore, the retina uses different cell types to create parallel circuits for different light levels - daylight, twilight, and starlight - but these share certain circuit components and use the same final pathways to the brain (Smith et al., 1986). This chapter describes key cell types and their interconnection in parallel circuits. It also discusses how the functional architecture of a circuit depends on the functional architecture of its synapses. Finally, it suggests how the flow of visual information shifts between circuits that are specialized for different light levels and how the circuits are switched. The chapter focuses on mammalian retina because that is where the combined knowledge of circuitry and cell physiology is best known. Early efforts centered on cat, so specific measurements, counts, etc., cited here refer to cat central retina. But recent efforts have broadened to include rabbit, rat, monkey, and human. These demonstrate strongly conserved patterns in the circuitry, as well as special adaptations, and some of both will be mentioned.
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In: The Visual Neurosciences
L. Chalupa & J.S. Werner (eds.) MIT Press, Cambridge MA (2004)Chapter 17
How retinal circuits optimize the transfer of visual information.
Sterling P
What It Means to "Understand" the Retina
The retina is a thin sheet of brain tissue (100 to 250 µm thick) that grows out into the eye to provide neural processing for photoreceptor signals (Fig. 17.1). In cats and macaque monkeys, it weighs about 0.1 g and covers 800 mm2, about twice the area of a U.S. quarter (Packer et al., 1989). The retina includes both photoreceptors and the first two to four stages of neural processing. Its output projects centrally over many axons (1.6 x 10^5 in cats [Williams et al., 1993]; 1.3 x10^6 in humans, and 1.8 x 106 in macaques [Potts et al., 1972]), and analysis of these information channels occupies about half of the cerebral cortex (van Essen et al., 1992). Because the retina constitutes a significant fraction of the brain (roughly 0.3%), to "solve" it completely would be a significant achievement for neuroscience. This overview considers what a "solution" would entail and summarizes progress toward that goal.
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Neuron 91:313-315, 2004Design for a binary synapse
Sterling P; Smith RG
The mammalian rod transfers a binary signal, the capture of 0 or 1 photon. In this issue of Neuron, Sampath and Rieke show in mouse that the rod’s tonic exocytosis in darkness completely saturates a G protein cascadeto close nearly all postsynaptic channels. A full-sized photon event supresses exocytosis sufficiently to allow ~30 postsynaptic channels to open simultaneously. Thus, the synapse behaves like a digital gate, whose hallmark is reliability and resistance to noise.
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TINS 28:20-29, 2005Structure and function of ribbon synapses
Sterling P; Matthews G
Sensory neurons with short conduction distances can use nonregenerative, graded potentials to modulate transmitter release continuously. This mechanism can transmit information at much higher rates than spiking. Graded signaling requires a synapse to sustain high rates of exocytosis for relatively long periods, and this capacity is the special virtue of ribbon synapses. Vesicles tethered to the ribbon provide a pool for sustained release that is typically fivefold greater than the docked pool available for fast release. The current article, which is part of the TINS Synaptic Connectivity series, reviews recent evidence for this fundamental computational strategy and its underlying cell biology.
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Science 207(4428):317-319, 1980Toward a functional architecture of the retina: serial reconstruction of adjacent ganglion cells.
Stevens JK; McGuire BA; Sterling P
Twenty adjacent ganglion cells in cat retina were partially reconstructed from electron micrographs of serial thin sections. Cells were classified by size and by dendritic branching patterns as alpha, beta, or gamma cells. The alpha and beta cells were further subdivided by differences in the laminar distribution of their dendrites in the inner plexiform layer. The distribution of synaptic contacts on the cells was distinctive for each of the five major classes. Contacts on the alpha and beta cells were mainly on the dendrites in the sublamina in which a cell's major dendritic arborization was contained.
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Brain Res 2(3):265-293, 1980A systematic approach to reconstructing microcircuitry by electron microscopy of serial sections.
Stevens JK; Davis TL; Friedman N; Sterling P
To observe certain quantitative features of neuronal geometry and microcircuitry, it is necessary to reconstruct neurons from electron micrographs of serial, ultra-thin sections. We describe here an approach to preparing, photographing, and analyzing moderately long series (100-500 sections). A series is prepared using an assembly line approach: one operator cuts while a second mounts ribbons of sections using various mechanical aids. Photographs are taken in the electron microscope at low magnification and high accelerating voltage. Sequential negatives are aligned using an image combiner and copied, using quasi-coherent illumination, onto 35 mm film. The resulting "movie' is mounted on a precision film transport mounted on an X-Y stage controlled by stepping motors. The movie is viewed through a high resolution video system while a video storage device and switching system permit rapid alternation between frames for comparisons. The profiles of a process in successive frames are "microaligned' by small adjustments of the transport's X-Y position. The absolute X-Y biological coordinates for each frame and the correction necessary to bring it into alignment are stored in a Z80 microprocessor as a process vector. When the movie is re-examined with the stepping motors under control of the computer, the microaligned process shows almost no frame-to-frame jitter. The process vector may be used to generate a "branch schematic' of the neuron. The microaligned profiles can also be digitized and displayed as a reconstruction using a PDP 11/34 computer. Uses of the approach are presented with examples from the cat retina and visual cortex.
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Vis Res443269-3276 (2004)Transmission of scotopic signals from the rod to rod-bipolar cell in the mammalian retina.
Taylor WR; Smith RG
Mammals can see at low scotopic light levels where only 1 rod in several thousand transduces a photon. The single photon signal is transmitted to the brain by the ganglion cell, which collects signals from more than 1000 rods to provide enough amplification. If the system were linear, such convergence would increase the neural noise enough to overwhelm the tiny rod signal. Recent studies provide evidence for a threshold nonlinearity in the rod to rod bipolar synapse, which removes much of the background neural noise. We argue that the height of the threshold should be 0.85 times the amplitude of the single photon signal, consistent with the saturation observed for the single photon signal. At this level, the rate of false positive events due to neural noise would be masked by the higher rate of dark thermal events. The evidence presented suggests that this synapse is optimized to transmit the single photon signal at low scotopic light levels.
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Proc Natl Acad Sci USA 87(5):1860-1864, 1990"Collective coding" of correlated cone signals in the retinal ganglion cell.
Tsukamoto Y; Smith RG; Sterling P
The signals in neighboring cones are partially correlated due to local correlations of luminance in the visual scene. By summing these partially correlated signals, the retinal ganglion cell improves its signal/noise ratio (compared to the signal/noise ratio in a cone) and expands the variance of its response to fill its dynamic range. Our computations prove that the optimal weighting function for this summation is dome-shaped. The computations also show that (assuming a particular space constant for the correlation function) ganglion cell collecting area and cone density are matched at all eccentricities such that the signal/noise ratio improves by a constant factor. The signal/noise improvement factor for beta ganglion cells in cat retina is about 4.
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Neurosci Res Suppl S15:S185-S198, 1991Spatial summation by ganglion cells: Some consequences for the efficient encoding of natural scenes.
Tsukamoto Y; Sterling P
A basic feature of retinal structure is the anatomical convergence of cones onto ganglion cells. This was appreciated by 19th Century anatomists, Cajal formost among them, and it was demonstrated functionally as "spatial summation" by physiologists starting about 50 years ago with Hartline (14) and later Barlow (3) and Kuffler (17). The processes of anatomical convergence and phnhysiological summation are accomplished through architecturally precise circuits. That is, for a particular type of ganglion cell at a given retinal locus, the dendritic spread, number of cones, and receptive field dimensions are invariant (7-11, 13, 23, 25). Each locus has several types of ganglion cell that form parallel systems, summming signals from the same cone array through equally precise, but quantitatively different ar4chitectures. Across the retina, on the other hand, the details of each circuit vary markedly and systematically (7, 20, 25, 37). We hypothesize that the local precision of the ganglion cell circuit and its variation with eccentricity reflect tunint (through evolution) fo efficiency in the enciding of natural scenes. The pressure to achieve a certain level of efficiency arises from the need to achieve a certain standard of performance on the one hand and physical constraints on the encoding processes on the other. In this essay we review both the standards of performance and the constraints and summarize the circkuit architecture used by the on-beta ganglion cell in cat area centralis for spatially summing cone signals. We compare this architecture to that of beta cells across the retina and to that of two other types of ganglion cell: the on-alpha cell (cat) and the parasol cell (monkey). The goal is to discover some consequences of these architectures for the efficient coding of natural scenes.
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Vision Res 32(10):1809-1815, 1992Gap junctions between the pedicles of macaque foveal cones.
Tsukamoto Y; Masarachia P; Schein SJ; Sterling P
Cone terminals ("pedicles") in the fovea of macaque retina were studied in electron micrographs of serial sections. Pedicles were sheathed in glia except for small (0.2 microns 2) fenestrations, 4.8 ± 1.7 per pedicle. At each fenestration the membranes of adjacent pedicles were contiguous and marked by an adherent junction, which in turn was invariably associated with gap junctions. There were 3.2 ± 1.4 gap junctions per adherent junction and thus, about fifteen gap junctions per pedicle. The gap junctions were small, 1.6 x 10(-3) ± 1.8 x 10(-3) microns 2 (mean ± SD) and were formed indiscriminately with all neighboring pedicles. An upper bound was estimated of 170 connexons per pedicle and thus a coupling conductance of 1.7 x 10(4) pS. Available psychophysical data suggest that the junctions are uncoupled at high luminance. They may couple at lower luminance where spatial averaging would improve contrast sensitivity without cost to spatial acuity.
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J Neurosci 21(21):8616-8623, 2001Microcircuits for night vision in mouse retina.
Tsukamoto Y; Morigiwa K; Ueda M; Sterling P
Because the mouse retina has become an important model system, we have begun to identify its specific neuron types and their synaptic connections. Here, based on electron micrographs of serial sections, we report that the wild-type mouse retina expresses the standard rod pathways known in other mammals: (1) rod -> cone (via gap junctions) to inject rod signals into the cone bipolar circuit; and (2) rod -> rod bipolar -> AII amacrine -> cone bipolar -> ganglion cell. The mouse also expresses another rod circuit: a bipolar cell with cone input also receives rod input at symmetrical contacts that express ionotropic glutamate receptors (Hack et al., 1999, 2001). We show that this rod-cone bipolar cell sends an axon to the outer (OFF) strata of the inner plexiform layer to form ribbon synapses with ganglion and amacrine cells. This rod-cone bipolar cell receives direct contacts from only 20% of all rod terminals. However, we also found that rod terminals form gap junctions with each other and thus establish partial syncytia that could pool rod signals for direct chemical transmission to the OFF bipolar cell. This third rod pathway probably explains the rod responses that persist in OFF ganglion cells after the well known rod pathways are blocked (Soucy et al., 1998).
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Vis Neurosci 21:611-625 (2004)Direction selectivity in a model of the starburst amacrine cell.
Tukker JJ; Taylor WR; Smith RG
The starburst amacrine cell (SBAC), found in all mammalian retinas, is thought to provide the directional inhibitory input recorded in On-Off direction-selective ganglion cells (DSGCs). While voltage recordings from the somas of SBACs have not shown robust direction selectivity (DS), the dendritic tips of these cells display direction-selective calcium signals, even when [gamma]-aminobutyric acid (GABAa,c) channels are blocked, implying that inhibition is not necessary to generate DS. This suggested that the distinctive morphology of the SBAC could generate a DS signal at the dendritic tips, where most of its synaptic output is located. To explore this possibility, we constructed a compartmental model incorporating realistic morphological structure, passive membrane properties, and excitatory inputs. We found robust DS at the dendritic tips but not at the soma. Two-spot apparent motion and annulus radial motion produced weak DS, but thin bars produced robust DS. For these stimuli, DS was caused by the interaction of a local synaptic input signal with a temporally delayed "global" signal, that is, an excitatory postsynaptic potential (EPSP) that spread from the activated inputs into the soma and throughout the dendritic tree. In the preferred direction the signals in the dendritic tips coincided, allowing summation, whereas in the null direction the local signal preceded the global signal, preventing summation. Sine-wave grating stimuli produced the greatest amount of DS, especially at high velocities and low spatial frequencies. The sine-wave DS responses could be accounted for by a simple mathematical model, which summed phase-shifted signals from soma and dendritic tip. By testing different artificial morphologies, we discovered DS was relatively independent of the morphological details, but depended on having a sufficient number of inputs at the distal tips and a limited electrotonic isolation. Adding voltage-gated calcium channels to the model showed that their threshold effect can amplify DS in the intracellular calcium signal.
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Visual Neurosci 15(5):809-821,1998Noise removal at the rod synapse of mammalian retina.
van Rossum MC; Smith RG.
Mammalian rods respond to single photons with a hyperpolarization of about 1 mV which is accompanied by continuous noise. Since the mammalian rod bipolar cell collects signals from 20-100 rods, the noise from the converging rods would overwhelm the single-photon signal from one rod at scotopic intensities (starlight) if the bipolar cell summed signals linearly (Baylor et al., 1984). However, it is known that at scotopic intensities the retina preserves single-photon responses (Barlow et al., 1971; Mastronarde, 1983). To explore noise summation in the rod bipolar pathway, we simulated an array of rods synaptically connected to a rod bipolar cell using a compartmental model. The performance of the circuit was evaluated with a discriminator measuring errors in photon detection as false positives and false negatives, which were compared to physiologically and psychophysically measured error rates. When only one rod was connected to the rod bipolar, a Poisson rate of 80 vesicles/s was necessary for reliable transmission of the single-photon signal. When 25 rods converged through a linear synapse the noise caused an unacceptably high false positive rate, even when either dark continuous noise or synaptic noise where completely removed. We propose that a threshold nonlinearity is provided by the mGluR6 receptor in the rod bipolar dendrite (Shiells & Falk, 1994) to yield a synapse with a noise removing mechanism. With the threshold nonlinearity the synapse removed most of the noise. These results suggest that a threshold provided by the mGluR6 receptor in the rod bipolar cell is necessary for proper functioning of the retina at scotopic intensities and that the metabotropic domains in the rod bipolar are distinct. Such a nonlinear threshold could also reduce synaptic noise for cortical circuits in which sparse signals converge.
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J Comp Neurol 288(4):601-611, 1989Structure of the starburst amacrine network in the cat retina and its association with alpha ganglion cells.
Vardi N; Masarachia PJ; Sterling P
To investigate indirect pathways to ganglion cells we studied the starburst amacrine cell network and its relationship to the alpha ganglion cell. Starburst cells were identified by an antiserum to choline acetyltransferase and alpha cells by injection of Lucifer yellow. The density of on and off starburst cells peaks at the area centralis and decreases with eccentricity by a factor of seven. The fine amacrine processes, interrupted by distinct varicosities, arborize in a planar fashion in the inner plexiform layer. The on network, at the junction of strata 3 and 4, and the off network, in stratum 2, have a similar appearance. Neighboring starburst processes run in intimate association to form a network of bundles. As bundles cross each other, loops of irregular size and shape are formed. The loops are smallest in the area centralis and increase by a factor of three towards the periphery; correspondingly, bundle length per unit area decreases with eccentricity. However, the number of varicosities/bundle length stays constant with eccentricity as does the number of processes per bundle. Segments of the starburst network associate over fairly long distances with dendrites of alpha ganglion cells. About 26% of the alpha ganglion dendritic tree shows such association, and this is significantly greater than would be expected if the alpha and starburst processes were independent. We conclude that the functional unit of the starburst cell is a linear bundle of processes and that the starburst network may connect synaptically to the alpha cell.
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J Comp Neurol 320(3):394-397, 1992Immunoreactivity to GABAA receptor in the outer plexiform layer of the cat retina.
Vardi N; Masarachia P; Sterling P
The distribution of GABAA receptor in the outer plexiform layer of cat retina was studied by immunocytochemistry with monoclonal antibodies. Staining was observed at the base of the cone pedicle, extracellularly, in association with the "triad" synaptic complex. Some bipolar dendrites and the basal processes that interconnect the cone pedicles were also stained. Rod spherules and horizontal cells were negative. The findings support the idea that the cone horizontal cells are GABAergic.
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Vis Neurosci 10(3):473-478, 1993Identification of a G-protein in depolarizing rod bipolar cells.
Vardi N; Matesic DF; Manning DR; Liebman PA; Sterling P
Synaptic transmission from photoreceptors to depolarizing bipolar cells is mediated by the APB glutamate receptor. This receptor apparently is coupled to a G-protein which activates cGMP-phosphodiesterase to modulate cGMP levels and thus a cGMP-gated cation channel. We attempted to localize this system immunocytochemically using antibodies to various components of the rod phototransduction cascade, including Gt (transducin), phosphodiesterase, the cGMP-gated channel, and arrestin. All of these antibodies reacted strongly with rods, but none reacted with bipolar cells. Antibodies to a different G-protein, G(o), reacted strongly with rod bipolar cells of three mammalian species (which are depolarizing and APB-sensitive). Also stained were subpopulations of cone bipolar cells but not the major depolarizing type in cat (b1). G(o) antibody also stained certain salamander bipolar cells. Thus, across a wide range of species, G(o) is present in retinal bipolar cells, and at least some of these are depolarizing and APB-sensitive.
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Vis Neurosci 11(1):135-142, 1994Horizontal cells in cat and monkey retina express different isoforms of glutamic acid decarboxylase.
Vardi N; Kaufman DL; Sterling P
The neurotransmitter used by horizontal cells in mammals has not been identified. GABA has been the leading candidate, but doubt has remained because of failure to clearly demonstrate the GABA synthetic enzyme, glutamic acid decarboxylase (GAD) in these cells. Because GAD was recently shown to exist as two isoforms, 65 kDa and 67 kDa, we considered whether there might be a mismatch between the forms of GAD expressed in horizontal cells and the probes used to detect it. Accordingly, we stained sections of mammalian retina with antibodies specific for each isoform. Cat horizontal cells of both types (A and B) were immunoreactive for GAD67 but negative for GAD65; monkey horizontal cells of both types (H(I) and HII) were positive for GAD65 and negative for GAD67. The findings reconcile previous, apparently conflicting, observations and strengthen considerably the hypothesis that mammalian horizontal cells are GABAergic.
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Vision Res 34(10):1235-1246, 1994Subcellular localization of GABAA receptor on bipolar cells in macaque and human retina.
Vardi N; Sterling P
The subcellular distribution of GABAA receptor in the macaque and human retina was studied by immunocytochemistry with monoclonal antibodies for the alpha and beta subunits with a particular focus on bipolar cells. Immunoreactivity to GABAA receptor was present on dendritic tips of all bipolar cells. The stain was strongest on bipolar membranes in apposition to horizontal cell processes. Stain was concentrated on the tips of flat and invaginating cone bipolar cells at the base of the cone pedicle and on the invaginating tips of rod bipolar cells. Stain on the cone pedicle membrane was restricted to sites of apposition to stained bipolar dendrites; pedicle membrane in apposition to horizontal cell processes was unstained. Stain was also present on bipolar axon terminals in both on and off strata of the inner plexiform layer. All bipolar cell somas stained faintly; horizontal and Muller cell somas were unstained. The alpha and beta subunits distributed similarly in monkey and human retina. Presence of GABAA receptor on the bipolar dendritic tips suggests that horizontal cells directly affect bipolar cells. Thus, GABAA receptor might mediate the receptive field surround of both off and on bipolar cells. Presence of GABAA receptor on bipolar axon terminals suggests that much of the inhibition feeding back from GABAergic amacrine to bipolar cells is GABAA-mediated.
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J Comp Neurol 351(3):374-384, 1995Specific cell types in cat retina express different forms of glutamic acid decarboxylase.
Vardi N; Auerbach P
We studied the expression of glutamate decarboxylase (GAD), GAD65 and GAD67, in cat retina by immunocytochemistry. About 10% of GABAergic amacrine cells express only GAD65 and 30% express only GAD67. Roughly 60% contain both forms of the enzyme, but GAD67 is present only at low levels in the majority of these double-labeled amacrine cells. The staining pattern in the inner plexiform layer (IPL) for the two GAD forms was also different. GAD65 was restricted to strata 1-4, and GAD67 was apparent throughout the IPL but was strongest in strata 1 and 5. This indicates that somas, as well as their processes, are differentially stained for the two forms of GAD. Cell types expressing only GAD65 include interplexiform cells, one type of cone bipolar cell, and at least one type of serotonin-accumulating amacrine cell. Cell types expressing only GAD67 include amacrine cells synthesizing dopamine, amacrine cells synthesizing nitric oxide (NO), and amacrine cells accumulating serotonin. Cholinergic amacrine cells express a low level of both GAD forms. Our findings in the retina are consistent with previous observations in the brain that GAD65 expression is greater in terminals than in somas. In addition, in retina most neurons expressing GAD67 also contain a second neurotransmitter as well as GABA, and they tend to be larger than neurons expressing GAD65. We propose that large cells have a greater demand for GABA than small cells, and thus require the constant, relatively unmodulated level of GABA that is provided by GAD67.
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Vision Res 36(23):3743-3757, 1996The AII amacrine network: coupling can increase correlated activity.
Vardi N; Smith RG
Retinal ganglion cells in the cat respond to single rhodopsin isomerizations with one to three spikes. This quantal signal is transmitted in the retina by the rod bipolar pathway: rod ->rod bipolar->AII->cone bipolar->ganglion cell. The two-dimensional circuit underlying this pathway includes extensive convergence from rods to an AII amacrine cell, divergence from a rod to several AII and ganglion cells, and coupling between the AII amacrine cells. In this study we explored the function of coupling by reconstructing several AII amacrine cells and the gap junctions between them from electron micrographs; and simulating the AII network with and without coupling. The simulation showed that coupling in the AII network can: (1) improve the signal/noise ratio in the AII network; (2) improve the signal/noise ratio for a single rhodopsin isomerization striking in the periphery of the ganglion cell receptive field center, and therefore in most ganglion cells responding to a single isomerization; (3) expand the AII and ganglion cells' receptive field center; and (4) expand the "correlation field". All of these effects have one major outcome: an increase in correlation between ganglion cell activity. Well correlated activity between the ganglion cells could improve the brain's ability to discriminate few absorbed external photons from the high background of spontaneous thermal isomerizations. Based on the possible benefits of coupling in the AII network, we suggest that coupling occurs at low scotopic luminances.
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Visual Neurosci 14(4):789-794, 1997ON cone bipolar cells in rat express the metabotropic receptor mGluR6.
Vardi N; Morigiwa K
The rod bipolar cell and about five types of ON cone bipolar cells depolarize to light by employing a sign-reversing metabotropic glutamate receptor. Glutamate responses are similar in both rod bipolar and cone bipolar cells, but the receptor mediating this response (mGluR6) was so far demonstrated only in rod bipolar cells. To test if ON cone bipolar cells also express mGluR6, we immunoreacted rat retina with an antibody specific for mGluR6, and studied the staining from serial ultrathin sections. We demonstrate that mGluR6 is indeed expressed in the dendritic tips of cone bipolar cells, the majority of which receive a ribbon synapse, and thus probably are ON cone bipolar cells. We further show that half of the dendritic tips contacting the cones stain for mGluR6, thus implying that all ON cone bipolar cell types express mGluR6.
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J Comp Neurol 395(1):43-52, 1998Alpha subunit of Go localizes in the dendritic tips of ON bipolar cells.
Vardi N
The metabotropic glutamate receptor (mGluR6), expressed by rod bipolar cells and ON cone bipolar cells, activates a trimeric guanine nucleotide-binding protein (G-protein) that ultimately closes a cation channel. The G-protein remains unidentified, but the alpha subunit of Go (Go(alpha)) has been suggested as a candidate because it is present in rod bipolar cells. However, the precise subcellular distribution of Go within the rod bipolar cell, and its distribution among cone bipolar cells was not determined. This information is important in assessing the hypothesis that Go couple mGluR6 to its effector. Here I report the distribution of Go (alpha subunit) by immunostaining in several mammalian retinas. The overall distribution is conserved across mammalian species: strongest in the dendrites of ON bipolar cells, moderate in their somas, weak in their axons, and absent from their terminals. Go(alpha) is also present in some amacrine somas and processes. In monkey fovea, where rods and rod bipolar cells are absent, Go(alpha) is present in about half of the bipolar somas which occupy the upper tiers of the bipolar layer, and are therefore identified as ON cone bipolar cells. Ultrastructurally, in monkey and cat, Go(alpha) is present in the dendritic tips of rod bipolar cells and ON cone bipolar cells, which are identified by their invaginating contacts. It is absent from OFF cone bipolar dendrites, which are identified by their flat contacts. It is also absent from axons entering the inner plexiform layer, and their terminals. In the primary dendrites, stain for Go(alpha) mainly associates with the plasma membrane, but in the dendritic tips it is also present in the cytosol. Apparently, Go(alpha) is expressed by the same bipolar cells that also express mGluR6, and is concentrated at the same subcellular location. Thus, Go(alpha) could serve to couple mGluR6 to later stages of its signaling cascade.
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Vision Res 38(10):1359-1369, 1998Neurochemistry of the mammalian cone 'synaptic complex'.
Vardi N; Morigiwa K; Wang TL; Shi YJ; Sterling P
The cone 'synaptic complex' is a unique structure in which a single presynaptic axon secretes glutamate onto processes of bipolar cells (both ON and OFF) and horizontal cells. In turn, the horizontal cell processes antagonize cone and bipolar responses to glutamate (probably by GABA). What still remains largely unknown is the molecular identity of the postsynaptic receptors and their exact locations. We identified several subunits of the glutamate receptor and the GABAA receptor expressed at the cone synaptic complex and localized them ultrastructurally. Glutamate receptors: (i) Invaginating (probably ON) bipolar dendrites in the monkey and rat express the metabotropic glutamate receptor, mGluR6. The stain is intense on the dendritic membrane where it first enters the invagination, and weak at the tip nearest to the ribbon. The cone membrane is electron-dense where it apposes the intense stain for mGluR6. Surprisingly, invaginating bipolar dendrites in the cat also express the AMPA receptor subunits, GluR2/3 and GluR4. (ii) Dendrites forming basal contacts in the cat (probably OFF) express the AMPA subunits GluR2/3, GluR4, and also the kainate subunit, GluR6/7. The stain is especially intense at the dendritic tips in apposition to electron-dense regions of cone membrane. (iii) Horizontal cells in the cat express the AMPA subunits GluR2/3, GluR4 and the kainate subunit, GluR6/7. The stain is strongest in the cytosol of somas and primary dendrites, but is also present in the invaginating terminals where it localizes to the membrane subjacent to the ribbon. GABAA receptors: (i) ON and OFF bipolar dendrites in the monkey express the alpha 1 and beta 2/3 subunits. The stain is localized to the bipolar cell membrane in apposition to horizontal cell processes. (ii) Cones did not express the GABAA subunits tested by immunocytochemistry, but beta 3 mRNA was amplified by RT-PCR from rat photoreceptors. Conclusions: (i) mGluR6 receptors concentrate on dendrites at the base of the invagination rather than at the apex. This implies that receptors at both 'invaginating' and 'basal' contacts lie roughly equidistant from the release sites and should therefore receive similar spatiotemporal concentrations of glutamate. (ii) The 'cone' membrane is electron-dense opposite to the receptor sites on both ON and OFF bipolar cells. This suggests a special role for this region in synaptic transmission. Possibly, these densities signify a transporter that would regulate glutamate concentration at sites remote (> 200 nm) from the locus of vesicle release.
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J Comp Neurol 423(3):402-12, 2000Localization of mGluR6 to dendrites of ON bipolar cells in primate retina.
Vardi N; Duvoisin R; Wu G; Sterling P
We prepared antibodies selective for the C-terminus of the human mGluR6 receptor and used confocal and electron microscopy to study the patterns of immunostaining in retina of monkey, cat, and rabbit. In all three species punctate stain was restricted to the outer plexiform layer. In monkey, stain was always observed in the central element of the postsynaptic "triad" of rod and cone terminals. In monkey peripheral retina, stain was seen only in central elements, but in the fovea, stain was also observed in some dendrites contacting the base of the cone terminal. S-cone terminals, identified by staining for S opsin, showed staining of postsynaptic dendrites. These were identified as dendrites of the ON S-cone bipolar cell by immunostaining for the marker cholecystokinin precursor. The staining pattern suggests that all types of ON bipolar cells, despite their marked differences in function, express a single isoform of mGluR6. Ultrastructurally, mGluR6 was located not on the tip of the central element, near the site of vesicle release, but on its base at the mouth of the invagination, 400-800 nm from the release site. Thus, the mGluR6 receptors of ON bipolar cells lie at about the same distance from sites of vesicle release as the iGluR receptors of OFF bipolar cells at the basal contacts.
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J Neurosci 20(20):7657-63, 2000Evidence that different cation chloride cotransporters in retinal neurons allow opposite responses to GABA.
Vardi N; Zhang LL; Payne JA; Sterling P
GABA gating an anion channel primarily permeable to chloride can hyperpolarize or depolarize, depending on whether the chloride equilibrium potential (E(Cl)) is negative or positive, respectively, to the resting membrane potential (E(rest)). If the transmembrane Cl(-) gradient is set by active transport, those neurons or neuronal regions that exhibit opposite responses to GABA should express different chloride transporters. To test this, we immunostained retina for the K-Cl cotransporter (KCC2) that normally extrudes chloride and for the Na-K-Cl cotransporter (NKCC) that normally accumulates chloride. KCC2 was expressed wherever E(Cl) is either known or predicted to be negative to E(rest) (ganglion cells, bipolar axons, and OFF bipolar dendrites), whereas NKCC was expressed wherever E(Cl) is either known or predicted to be positive to E(rest) (horizontal cells and ON bipolar dendrites). Thus, in the retina, the opposite effects of GABA on different cell types and on different cellular regions are probably primarily determined by the differential targeting of these two chloride transporters.
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Keio J Medicine 51(3):154-164, 2002Neurochemical organization of the first visual synapse
Vardi N; Dhingra A; Zhang L; Lyubarsky A; Wang TL; Morigiwa K
The retina employs two main synaptic relay in which information converges to higher order cells, and at the same time is modified by lateral inhibitory interneurons. At the first synpatic layer, rod and cone terminals contact second order neurons (horizontal and bipolar cells), and in turn, horizontal cells contact cones and bipolar cells. In this talk/review we describe the structures and the neurochemicals involved in transmitting the visual signal at this synaptic complex.
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Biochemistry 42(3):5640-5648, 2002Calcium-modulated guanylate cyclase transduction machinery in the photoreceptor-bipolar synaptic region.
Venkataraman V; Duda T; Vardi N; Koch K-W; Sharma RK
ABSTRACT: Rod outer segment membrane guanylate cyclase (ROS-GC) transduction system is a central component of the Ca2+-sensitive phototransduction machinery. The system is composed of two parts: Ca2+ sensor guanylate cyclase activating protein (GCAP) and ROS-GC. GCAP senses Ca2+ impulses and inhibits the cyclase. This operational feature of the cyclase is considered to be unique and exclusive in the phototransduction machinery. A combination of reconstitution, peptide competition, cross-linking, and immunocytochemical studies has been used in this study to show that the GCAP1/ROS-GC1 transduction system also exists in the photoreceptor synaptic (presynaptic) termini. Thus, the presence of this system and its linkage is not unique to the phototransduction machinery. A recent study has demonstrated that the photoreceptor-bipolar synaptic region also contains a Ca2+-stimulated ROS-GC1 transduction system [Duda, T., et al. (2002) EMBO J. 21, 2547-2556]. In this case, S100ß senses Ca2+ and stimulates the cyclase. The inhibitory and stimulatory Ca2+-modulated ROS-GC1 sites are distinct. These findings allow the formation of a new topographic model of ROS-GC1 transduction. In this model, the catalytic module of ROS-GC1 at its opposite ends is flanked by GCAP1 and S100ß modules. GCAP1 senses the Ca2+ impulse and inhibits the catalytic module; S100ß senses the impulse and stimulates the catalytic module. Thus, ROS-GC1 acts as a bimodal Ca2+ signal transduction switch in the photoreceptor bipolar synapse.
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Neuron 16(6):1221-1227, 1996Evidence that vesicles on the synaptic ribbon of retinal bipolar neurons can be rapidly released.
von Gersdorff H; Vardi E; Matthews G; Sterling P
We relate the ultrastructure of the giant bipolar synapse in goldfish retina to the jump in capacitance that accompanies depolarization-evoked exocytosis. Mean vesicle diameter is 29 ± 4 nm, giving 26.4 aF/vesicle, so the maximum evoked capacitance (150 fF within 200 ms) represents fusion of about 5700 vesicles. Two terminals contained, respectively, 45 and 65 ribbon-type synaptic outputs, and a fully loaded ribbon tethers about 110 vesicles. Thus, the tethered pool, about 6000 vesicles, corresponds to the rapidly released pool. Further, the difference between small and large terminals in number of tethered vesicles matches their difference in capacitance jump. This suggests, within a "fire and reload" model of exocytosis, that the ribbon translocates synaptic vesicles very rapidly to membrane docking sites, supporting a maximum release rate of 500 vesicles/active zone/s, until the population of tethered vesicles is exhausted.
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J Neurosci 19(11):4221-4228, 1999Localization of type I inositol 1,4,5-triphosphate receptor in the outer segments of mammalian cones.
Wang TL; Sterling P; Vardi N
Calcium enters the outer segment of a vertebrate photoreceptor through a cGMP-gated channel and is extruded via a Na/Ca, K exchanger. We have identified another element in mammalian cones that might help to control cytoplasmic calcium. Reverse transcription-PCR performed on isolated photoreceptors identified mRNA for the SII- splice variant of the type I receptor for inositol 1,4,5-triphosphate (IP3), and Western blots showed that the protein also is expressed in outer segments. Immunocytochemistry showed type I IP3 receptor to be abundant in red-sensitive and green-sensitive cones of the trichromatic monkey retina, but it was negative or weakly expressed in blue-sensitive cones and rods. Similarly, the green-sensitive cones expressed the receptor in dichromatic retina (cat, rabbit, and rat), but the blue-sensitive cones did not. Immunostain was localized to disk and plasma membranes on the cytoplasmic face. To restore sensitivity after a light flash, cytoplasmic cGMP must rise to its basal level, and this requires cytoplasmic calcium to fall. Cessation of calcium release via the IP3 receptor might accelerate this fall and thus explain why the cone recovers much faster than the rod. Furthermore, because its own activity of the IP3 receptor depends partly on cytoplasmic calcium, the receptor might control the set point of cytoplasmic calcium and thus affect cone sensitivity.
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Nature 224:1032-1033, 1969Effect on the superior colliculus of cortical removal in visually deprived cats
Wickelgren BG; Sterling P
One of the principal goals of analyses of the processing of sensory information has been an understanding of the receptive field properties of central neurones in terms of the convergence of afferents from more peripheral levels. Although it is well known that most cell groups receiving higher centres, there have been relatively few studies of descending effects on the organization of the receptive field. We are concerned here with the superficial layers of the superior colliculus which receive optic nerve fibres, chiefly from the contralateral retina, and descending input from the ipsilateral visual cortex. Most collicular cells in the normal cat are binocularly driven by slits, bars or edges and are directionally selective, that is they respond to the same types of visual stimuli, but most cells are driven only by the contralateral eye. The cells also lose their directional selectivity and respond equally well to all directions of stimulus movement. It therefore seems clear that some features of collicular cells depend on the cortex. The work described here with visually deprived animals demonstrates in another way the influence of the cortex on collicular cells.
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Wickelgren & Sterling 1969
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PNAS 99(11):7711-7716, 2003Identification of a family of calcium sensors as protein ligands of inositol trisphosphate receptor Ca2+ release channels.
Yang J; McBride S.; Mak ,DO; Vardi N; Palczewski K; Haeseleer F
The inositol trisphosphate (InsP3) receptor (InsP3R) is a ubiquitously expressed intracellular Ca2+ channel that mediates complex cytoplasmic Ca2+ signals, regulating diverse cellular processes, including synaptic plasticity. Activation of the InsP3R channel is normally thought to require binding of InsP3 derived from receptor-mediated activation of phosphatidylinositol lipid hydrolysis. Here we identify a family of neuronal Ca2+-binding proteins as high-affinity protein agonists of the InsP3R, which bind to the channel and activate gating in the absence of InsP3. CaBP/caldendrin, a subfamily of the EF-hand-containing neuronal calcium sensor family of calmodulin-related proteins, bind specifically to the InsP3-binding region of all three InsP3R channel isoforms with high affinity (Ka ~ 25 nM) in a Ca2+-dependent manner (Ka ~ 1 µM). Binding activates single-channel gating as efficaciously as InsP3, dependent on functional EF-hands in CaBP. In contrast, calmodulin neither bound with high affinity nor activated channel gating. CaBP1 and the type 1 InsP3R associate in rat whole brain and cerebellum lysates, and colocalize extensively in subcellular regions in cerebellar Purkinje neurons. Thus, InsP3R-mediated Ca2+ signaling in cells is possible even in the absence of InsP3 generation, a process that may be particularly important in responding to and shaping changes in intracellular Ca2+ concentration by InsP3-independent pathways and for localizing InsP3-mediated Ca2+ signals to individual synapses.
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J Neurosci 26(47)12351–12361, 2006Chromatic properties of horizontal and ganglion cell responses follow a dual gradient in cone opsin expression.
Yin L; Smith RG; Sterling P; Brainard DH
In guinea pig retina, immunostaining reveals a dual gradient of opsins: cones expressing opsin sensitive to medium wavelengths (M) predominate in the upper retina, whereas cones expressing opsin sensitive to shorter wavelengths (S) predominate in the lower retina. Whether these gradients correspond to functional gradients in postreceptoral neurons is essentially unknown. Using monochromatic flashes, we measured the relative weights with which M, S, and rod signals contribute to horizontal cell responses. For a background that produced 4.76 log10 photoisomerizations per rod per second (Rh*/rod/s), mean weights in superior retina were 52% (M), 2% (S), and 46% (rod). Mean weights in inferior retina were 9% (M), 50% (S), and 41% (rod). In superior retina, cone opsin weights agreed quantitatively with relative pigment density estimates from immunostaining. In inferior retina, cone opsin weights agreed qualitatively with relative pigment density estimates, but quantitative comparison was impossible because individual cones coexpress both opsins to varying and unquantifiable degrees. We further characterized the functional gradients in horizontal and brisk-transient ganglion cells using flickering stimuli produced by various mixtures of blue and green primary lights. Cone weights for both cell types resembled those obtained for horizontal cells using monochromatic flashes. Because the brisk-transient ganglion cell is thought to mediate behavioral detection of luminance contrast, our results are consistent with the hypothesis that the dual gradient of cone opsins assists achromatic contrast detection against different spectral backgrounds. In our preparation, rod responses did not completely saturate, even at background light levels typical of outdoor sunlight (5.14 log10 Rh*/rod/s).
J Neurosci 24:9752-9759, 2004Visualizing synaptic ribbons in the living cell.
Zenisek D; Horst N; Merrifield C; Sterling P; Matthews G
Visual and auditory information is encoded by sensory neurons that tonically release neurotransmitter at high rates. The synaptic ribbon is an essential organelle in nerve terminals of these neurons. Its precise function is unknown, but if the ribbon could be visualized in a living terminal, both its own dynamics and its relation to calcium and vesicle dynamics could be studied.Wedesigned a short fluorescent peptide with affinity for a known binding domain of RIBEYE, a protein unique to the ribbon. When introduced via a whole-cell patch pipette, the peptide labeled structures at the presynaptic plasma membrane of ribbon-type terminals. The fluorescent spots match in size, location, number, and distribution the known features of synaptic ribbons. Furthermore, fluorescent spots mapped by confocal microscopy directly match the ribbons identified by electron microscopy in the same cell. Clearly the peptide binds to the synaptic ribbon, but even at saturating concentrations it affects neither the morphology of the ribbon nor its tethering of synaptic vesicles. It also does not inhibit exocytosis. Using the peptide label, we observed that the ribbon is immobile over minutes and that calcium influx is concentrated at the ribbon. Finally, we find that each ribbon in a retinal bipolar cell contains4000 molecules of RIBEYE, indicating that it is the major component of the synaptic ribbon.
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J Neurophys 95:2404-2416, 2006Shift of intracellular chloride concentration in ganglion and amacrine cells of developing mouse retina
Zhang LL; Pathak HR; Coulter DA; Freed MA; Vardi N
Shift of intracellular chloride concentration in ganglion and amacrine cells of developing mouse retina. J Neurophysiol 95: 2404–2416, 2006. First published December 21, 2005; doi:10.1152/jn.00578.2005. GABA and glycine provide excitatory action during early development: they depolarize neurons and increase intracellular calcium concentration. As neurons mature, GABA and glycine become inhibitory. This switch from excitation to inhibition is thought to result from a shift of intracellular chloride concentration ([Cl-]i) from high to low, but in retina, measurements of [Cl-]i) or chloride equilibrium potential (ECl) during development have not been made. Using the developing mouse retina, we systematically measured [Cl-]i) in parallel with GABA’s actions on calcium and chloride. In ganglion and amacrine cells, fura-2 imaging showed that before postnatal day (P) 6, exogenous GABA, acting via ionotropic GABA receptors, evoked calcium rise, which persisted in HCO3 - free buffer but was blocked with 0 extracellular calcium. After P6, GABA switched to inhibiting spontaneous calcium transients. Concomitant with this switch we observed the following: 6-methoxy-N-ethylquinolinium iodide (MEQ) chloride imaging showed that GABA caused an efflux of chloride before P6 and an influx afterward; gramicidin-perforated-patch recordings showed that the reversal potential for GABA decreased from -45 mV, near threshold for voltage-activated calcium channel, to -60 mV, near resting potential; MEQ imaging showed that [Cl-]i) shifted steeply around P6 from 29 to 14 mM, corresponding to a decline of ECl from -39 to -58 mV. We also show that GABAergic amacrine cells became stratified by P4, potentially allowing GABA’s excitatory action to shape circuit connectivity. Our results support the hypothesis that a shift from high [Cl-]i) to low causes GABA to switch from excitatory to inhibitory.
