Synaptology of physiologically identified ganglion cells in the cat retina: a comparison of retinal X- and Y-cells

General features of the retinal connectome determine the computation of motion anticipation

Motion anticipation allows the visual system to compensate for the slow speed of phototransduction so that a moving object can be accurately located. This correction is already present in the signal that ganglion cells send from the retina but the biophysical mechanisms underlying this computation are not known. Here we demonstrate that motion anticipation is computed autonomously within the dendritic tree of each ganglion cell and relies on feedforward inhibition. The passive and non-linear interaction of excitatory and inhibitory synapses enables the somatic voltage to encode the actual position of a moving object instead of its delayed representation. General rather than specific features of the retinal connectome govern this computation: an excess of inhibitory inputs over excitatory, with both being randomly distributed, allows tracking of all directions of motion, while the average distance between inputs determines the object velocities that can be compensated for.

DOI:http://dx.doi.org/10.7554/eLife.06250.001

The retina is a structure at the back of the eye that converts light into nerve impulses, which are then processed in the brain to produce the images that we see. It normally takes about one-tenth of a second for the retina to send a signal to the brain after an object first moves into view. This is about the same time it takes a tennis ball to travel several meters during a tennis match, yet we are still able to see where the moving tennis ball is in real time. This is because a process called ‘motion anticipation’ is able to compensate for the delay in processing the position of a moving object. However, it was not known precisely how motion anticipation occurs.

Inside the retina, cells called photoreceptors detect light and ultimately send signals (via some intermediate cell types) to nerve cells known as retinal ganglion cells. These signals can either excite a retinal ganglion cell to cause it to send an electrical signal to the brain, or inhibit it, which temporarily prevents electrical activity. Each cell receives signals from several photoreceptors, which each connect to a different site along branch-like structures called dendrites that project out of the retinal ganglion cells.

Johnston and Lagnado have now investigated how motion anticipation occurs in the retina by using electrical recordings of the activity in the retinas of goldfish combined with computer simulations of this activity. This revealed inhibitory signals, sent from photoreceptors to retinal ganglion cells via a type of intermediate cell (called amacrine cells), play a key role in motion anticipation. The ability to track motion effectively in all directions requires more inhibitory signals to be sent to the dendrites of a retinal ganglion cell than excitatory signals. These two types of input must also be randomly distributed across the cell. Furthermore, it is the density of these input sites on a dendrite that determines how well the retina can compensate for the motion of a fast-moving object. The building blocks required for motion anticipation in the retina are also found in visual areas higher in the brain. Therefore, further work may reveal that higher visual areas also use this mechanism to predict the future location of moving objects.

DOI:http://dx.doi.org/10.7554/eLife.06250.002

Receptor targets of amacrine cells

Amacrine cells are a morphologically and functionally diverse group of inhibitory interneurons. Morphologically, they have been divided into approximately 30 types. Although this diversity is probably important to the fine structure and function of the retinal circuit, the amacrine cells have been more generally divided into two subclasses. Glycinergic narrow-field amacrine cells have dendrites that ramify close to their somas, cross the sublaminae of the inner plexiform layer, and create cross talk between its parallel ON and OFF pathways. GABAergic wide-field amacrine cells have dendrites that stretch long distances from their soma but ramify narrowly within an inner plexiform layer sublamina. These wide-field cells are thought to mediate inhibition within a sublamina and thus within the ON or OFF pathway. The postsynaptic targets of all amacrine cell types include bipolar, ganglion, and other amacrine cells. Almost all amacrine cells use GABA or glycine as their primary neurotransmitter, and their postsynaptic receptor targets include the most common GABA(A), GABA(C), and glycine subunit receptor configurations. This review addresses the diversity of amacrine cells, the postsynaptic receptors on their target cells in the inner plexiform layer of the retina, and some of the inhibitory mechanisms that arise as a result. When possible, the effects of GABAergic and glycinergic inputs on the visually evoked responses of their postsynaptic targets are discussed.

Synaptic inputs to two types of koniocellular pathway ganglion cells in marmoset retina

The retinal connectivity of the diverse group of cells contributing to koniocellular visual pathways (widefield ganglion cells) is largely unexplored. Here we examined the synaptic inputs onto two koniocellular-projecting ganglion cell types named large sparse and broad thorny cells. Ganglion cells were labeled by retrograde tracer injections targeted to koniocellular layer K3 in the lateral geniculate nucleus in marmosets (Callithrix jacchus) and subsequently photofilled. Retinal preparations were processed with antibodies against the C-terminal binding protein 2, the AMPA receptor subunit GluR4, and against CD15 to identify bipolar (excitatory) and/or antibodies against gephyrin to identify amacrine (inhibitory) input. Large sparse cells are narrowly stratified close to the ganglion cell layer. Broad thorny ganglion cells are broadly stratified in the center of the inner plexiform layer. Bipolar input to large sparse cells derives from DB6 and maybe other ON bipolar types, whereas that to broad thorny cells derives from ON and OFF bipolar cell types. The total number of putative synapses on broad thorny cells is higher than the number on large sparse cells but the density of inputs (between 2 and 5 synapses per 100 μm(2) dendritic area) is similar for the two cell types, indicating that the larger number of synapses on broad thorny cells is attributable to the larger membrane surface area of this cell type. Synaptic input density is comparable to previous values for midget-parvocellular and parasol-magnocellular pathway cells. This suggests functional differences between koniocellular, parvocellular, and magnocellular pathways do not arise from variation in synaptic input densities.

The spatial distribution of glutamatergic inputs to dendrites of retinal ganglion cells

The spatial pattern of excitatory glutamatergic input was visualized in a large series of ganglion cells of the rabbit retina, by using particle-mediated gene transfer of an expression plasmid for postsynaptic density 95-green fluorescent protein (PSD95-GFP). PSD95-GFP was confirmed as a marker of excitatory input by co-localization with synaptic ribbons (RIBEYE and kinesin II) and glutamate receptor subunits. Despite wide variation in the size, morphology, and functional complexity of the cells, the distribution of excitatory synaptic inputs followed a single set of rules: 1) the linear density of synaptic inputs (PSD95 sites/linear mum) varied surprisingly little and showed little specialization within the arbor; 2) the total density of excitatory inputs across individual arbors peaked in a ring-shaped region surrounding the soma, which is in accord with high-resolution maps of receptive field sensitivity in the rabbit; and 3) the areal density scaled inversely with the total area of the dendritic arbor, so that narrow dendritic arbors receive more synapses per unit area than large ones. To achieve sensitivity comparable to that of large cells, those that report upon a small region of visual space may need to receive a denser synaptic input from within that space.

Developmental patterning of glutamatergic synapses onto retinal ganglion cells

Background

Neurons receive excitatory synaptic inputs that are distributed across their dendritic arbors at densities and with spatial patterns that influence their output. How specific synaptic distributions are attained during development is not well understood. The distribution of glutamatergic inputs across the dendritic arbors of mammalian retinal ganglion cells (RGCs) has long been correlated to the spatial receptive field profiles of these neurons. Thus, determining how glutamatergic inputs are patterned onto RGC dendritic arbors during development could provide insight into the cellular mechanisms that shape their functional receptive fields.

Results

We transfected developing and mature mouse RGCs with plasmids encoding fluorescent proteins that label their dendrites and glutamatergic postsynaptic sites. We found that as dendritic density (dendritic length per unit area of dendritic field) decreases with maturation, the density of synapses along the dendrites increases. These changes appear coordinated such that RGCs attain the mature average density of postsynaptic sites per unit area (areal density) by the time synaptic function emerges. Furthermore, stereotypic centro-peripheral gradients in the areal density of synapses across the arbor of RGCs are established at an early developmental stage.

Conclusion

The spatial pattern of glutamatergic inputs onto RGCs arises early in synaptogenesis despite ensuing reorganization of dendritic structure. We raise the possibility that these early patterns of synaptic distributions may arise from constraints placed on the number of contacts presynaptic neurons are able to make with the RGCs.

Cellular basis for contrast gain control over the receptive field center of mammalian retinal ganglion cells

Retinal ganglion cells fire spikes to an appropriate contrast presented over their receptive field center. These center responses undergo dynamic changes in sensitivity depending on the ongoing level of contrast, a process known as "contrast gain control." Extracellular recordings suggested that gain control is driven by a single wide-field mechanism, extending across the center and beyond, that depends on inhibitory interneurons: amacrine cells. However, recordings in salamander suggested that the excitatory bipolar cells, which drive the center, may themselves show gain control independently of amacrine cell mechanisms. Here, we tested in mammalian ganglion cells whether amacrine cells are critical for gain control over the receptive field center. We made extracellular and whole-cell recordings of guinea pig Y-type cells in vitro and quantified the gain change between contrasts using a linear-nonlinear analysis. For spikes, tripling contrast reduced gain by approximately 40%. With spikes blocked, ganglion cells showed similar levels of gain control in membrane currents and voltages and under conditions of low and high calcium buffering: tripling contrast reduced gain by approximately 20-25%. Gain control persisted under voltage-clamp conditions that minimize inhibitory conductances and pharmacological conditions that block inhibitory neurotransmitter receptors. Gain control depended on adequate stimulation, not of ganglion cells but of presynaptic bipolar cells. Furthermore, horizontal cell measurements showed a lack of gain control in photoreceptor synaptic release. Thus, the mechanism for gain control over the ganglion cell receptive field center, as measured in the subthreshold response, originates in the presynaptic bipolar cells and does not require amacrine cell signaling.

Structure-function relations of parasol cells in the normal and glaucomatous primate retina

PURPOSE

The purpose of this study was to examine the effect that chronic elevation of intraocular pressure has on the intrinsic and visual response properties of parasol cells in the primate retina.

METHODS

A primate model of experimental glaucoma was combined with intracellular recording and staining techniques using an isolated retina preparation. Intrinsic electrical properties were examined by injection of depolarizing and hyperpolarizing currents. Visual responses were studied using drifting and counterphased gratings. Morphologic comparisons were made by injecting recorded cells with Neurobiotin and analyzing them quantitatively with a computer-based neuron reconstruction system.

RESULTS

Structurally, parasol cells from glaucomatous eyes had smaller somata and smaller, less complex dendritic arbors, resulting in a significant reduction in total dendrite length and surface area. Functionally, these neurons did not differ from normal in their mean resting membrane potentials, input resistances, or thresholds to electrical activation, but did differ in membrane time constants and spike duration. Parasol cells from both normal and glaucomatous eyes preferred low-spatial-frequency stimuli, but significantly fewer glaucoma-related cells were driven visually-in particular, by patterned stimuli. Glaucomatous cells also did not respond as well to visual stimuli presented at increased temporal frequencies.

CONCLUSIONS

Ganglion cells in the glaucomatous eye retain most of their normal intrinsic electrical properties, but are less responsive, both spatially and temporally, to visual stimuli. The reduction in visual responsiveness most likely results from significant changes in dendritic architecture, which affects their level of innervation by more distal retinal neurons.

Synaptic organization of complex ganglion cells in rabbit retina: type and arrangement of inputs to directionally selective and local-edge-detector cells

The type and topographic distribution of synaptic inputs to a directionally selective (DS) rabbit retinal ganglion cell (GC) were examined and were compared with those received by two other complex GC types. The percentage of cone bipolar cell (BC) input, presumably an index of sustained responses and simple receptive field properties, is much higher than expected for complex GCs in reference to previous reports in other species: approximately 20% for the type 1 bistratified ON-OFF DS GC and for a multistratified GC, and approximately 40% for the small-tufted local-edge-detector GC. Consistent with a previous study (Famiglietti [1991] J. Comp. Neurol. 309:40-70), no ultrastructural evidence is found for inhibitory synapses from starburst amacrine cells to the ON-OFF DS GC. The density of inputs to the ON-OFF DS GC is high and rather evenly distributed over the dendritic tree. Clustering of inputs brings excitatory and inhibitory inputs into proximity, but the strict on-path condition of more proximal inhibitory inputs, favoring shunting inhibition, is not satisfied. Prominent BC input and its regional variation suggest that BCs play key roles in DS neural circuitry, both pre- and postsynaptic to the ON-OFF DS GC, according to a bilayer model (Famiglietti [1993] Invest. Ophthalmol. Vis. Sci. 34:S985). Asymmetry of inhibitory amacrine cell input may signify a region on the preferred side of the receptive field, the inhibition-free zone (Barlow and Levick [1965] J. Physiol. (Lond.) 178:477-504), supporting a role for postsynaptic integration in the DS mechanism. Prominent BC input to the local-edge-detector, often without accompanying amacrine cell input, indicates presynaptic integration in forming its trigger feature.

Sluggish and brisk ganglion cells detect contrast with similar sensitivity

Roughly half of all ganglion cells in mammalian retina belong to the broad class, termed "sluggish." Many of these cells have small receptive fields and project via lateral geniculate nuclei to visual cortex. However, their possible contributions to perception have been largely ignored because sluggish cells seem to respond weakly compared with the more easily studied "brisk" cells. By selecting small somas under infrared DIC optics and recording with a loose seal, we could routinely isolate sluggish cells. When a spot was matched spatially and temporally to the receptive field center, most sluggish cells could detect the same low contrasts as brisk cells. Detection thresholds for the two groups determined by an "ideal observer" were similar: threshold contrast for sluggish cells was 4.7 +/- 0.5% (mean +/- SE), and for brisk cells was 3.4 +/- 0.3% (Mann-Whitney test: P > 0.05). Signal-to-noise ratios for the two classes were also similar at low contrast. However, sluggish cells saturated at somewhat lower contrasts (contrast for half-maximum response was 14 +/- 1 vs. 19 +/- 2% for brisk cells) and were less sensitive to higher temporal frequencies (when the stimulus frequency was increased from 2 to 4 Hz, the response rate fell by 1.6-fold). Thus the sluggish cells covered a narrower dynamic range and a narrower temporal bandwidth, consistent with their reported lower information rates. Because information per spike is greater at lower firing rates, sluggish cells may represent "cheaper" channels that convey less urgent visual information at a lower energy cost.

How Retinal Ganglion Cells Prevent Synaptic Noise From Reaching the Spike Output

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.

Different circuits for ON and OFF retinal ganglion cells cause different contrast sensitivities

The theory of "parallel pathways" predicts that, except for a sign reversal, ON and OFF ganglion cells are driven by a similar presynaptic circuit. To test this hypothesis, we measured synaptic inputs to ON and OFF cells as reflected in the subthreshold membrane potential. We made intracellular recordings from brisk-transient (Y) cells in the in vitro guinea pig retina and show that ON and OFF cells in fact express significant asymmetries in their synaptic inputs. An ON cell receives relatively linear input that modulates a single excitatory conductance; whereas an OFF cell receives rectified input that modulates both inhibitory and excitatory conductances. The ON pathway, blocked by L-AP-4, tonically inhibits an OFF cell at mean luminance and phasically inhibits an OFF cell during a light increment. Our results suggest that basal glutamate release is high at ON but not OFF bipolar terminals, and inhibition between pathways is unidirectional: ON --> OFF. These circuit asymmetries explain asymmetric contrast sensitivity observed in spiking behavior.

The receptive fields of cat retinal ganglion cells in physiological and pathological states: where we are after half a century of research

Studies on the receptive field properties of cat retinal ganglion cells over the past half-century are reviewed within the context of the role played by the receptive field in visual information processing. Emphasis is placed on the work conducted within the past 20 years, but a summary of key contributions from the 1950s to 1970s is provided. We have sought to review aspects of the ganglion cell receptive field that have not been featured prominently in previous review articles. Our review of the receptive field properties of X- and Y-cells focuses on quantitative studies and includes consideration of the function of the receptive field in visual signal processing. We discuss the non-classical as well as the classical receptive field. Attention is also given to the receptive field properties of the less well-studied cat ganglion cells-the W-cells-and the effect of pathology on cat ganglion cell properties. Although work from our laboratories is highlighted, we hope that we have given a reasonably balanced view of the current state of the field.

Bipolar cells contribute to nonlinear spatial summation in the brisk-transient (Y) ganglion cell in mammalian retina

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.

Confronting complexity: strategies for understanding the microcircuitry of the retina

The mammalian retina contains upward of 50 distinct functional elements, each carrying out a specific task. Such diversity is not rare in the central nervous system, but the retina is privileged because its physical location, the distinctive morphology of its neurons, the regularity of its architecture, and the accessibility of its inputs and outputs permit a unique variety of experiments. Recent strategies for confronting the retina's complexity attempt to marry genetic approaches to new kinds of anatomical and electrophysiological techniques.

Distribution of GABA and glycine receptors on bipolar and ganglion cells in the mammalian retina

The amino acids GABA and glycine mediate synaptic transmission via specific neurotransmitter receptors. Molecular cloning studies have shown that there is a great diversity of GABA and glycine receptors. In the present article, the distribution of GABA and glycine receptors on identified bipolar and ganglion cell types in the mammalian retina is reviewed. Immunofluorescence obtained with antibodies against GABA and glycine receptors is punctate. Electron microscopy shows that the puncta represent a cluster of receptors at synaptic sites. Bipolar cell types were identified with immunohistochemical markers. Double immunofluorescence with subunit-specific antibodies was used to analyze the distribution of receptor clusters on bipolar axon terminals. The OFF cone bipolar cells seem to be dominated by glycinergic input, whereas the ON cone bipolar and rod bipolar cells are dominated by GABAergic input. Ganglion cells were intracellularly injected with Neurobiotin, visualized with Streptavidin coupled to FITC, and subsequently stained with subunit specific antibodies. The distribution and density of receptor clusters containing the alpha1 subunit of the GABA(A) receptor and the alpha1 subunit of the glycine receptor, respectively, were analyzed on midget and parasol cells in the marmoset (a New World monkey). Both GABA(A) and glycine receptors are distributed uniformly along the dendrites of ON and OFF types of parasol and midget ganglion cells, indicating that functional differences between these subtypes of ganglion cells are not determined by GABA or glycinergic input.

Synaptic input to small bistratified (blue-ON) ganglion cells in the retina of a new world monkey, the marmoset Callithrix jacchus

Small bistratified (blue-ON) ganglion cells in the primate retina are involved in processing short wavelength sensitive cone signals. These ganglion cells stratify in both the ON- and OFF-sublamina of the inner plexiform layer. We investigated the origin of synaptic input to the small bistratified ganglion cell in the retina of a New World primate, the marmoset Callithrix jacchus. Two small bistratified cells from peripheral retina were intracellularly filled with Lucifer Yellow, subsequently photoconverted and processed for electron microscopy. Serial ultrathin sections were cut through portions of each cell, and these were analysed in the electron microscope. The majority of synaptic input (about 84%) to both the inner and outer tier of dendrites was from amacrine cells. Both dendritic tiers also received bipolar cell input. These findings are consistent with predictions from physiological studies that synaptic input to the inner and outer tier of small bistratified cells should be excitatory. However, the tiny fraction of total input supplied from bipolar cells to the outer tier is not consistent with the strong excitatory OFF response in cells of this pathway.

Transmitter-specific input to OFF-alpha ganglion cells in the cat retina

The synaptic input to OFF-center alpha ganglion cells in the cat retina was analyzed by electron microscopic reconstruction to quantify the bipolar and amacrine cell input and to determine the neurotransmitter content of the presynaptic cells. Cone bipolar cells were found to comprise 11% of the total input with their dyad synapses distributed across the dendritic tree. The remaining contacts were conventional synapses indicative of amacrine cells; postembedding immunogold labeling was used to characterize these cells as either GABA- or glycine-immunoreactive. Results showed the amacrine input to be equally divided between GABA and glycinergic contacts at each order of dendritic branching of the alpha cells. Among the GABA-positive neurons were A19 amacrine cells, the processes of which are characterized by a dense array of neurotubules. A major source of glycinergic input was from lobular appendages of AII amacrine cells with lesser contributions from other glycine-positive amacrine cells. The physiological role(s) of these amino acids must be interpreted in view of the multiple subpopulations of amacrine cells, which provide input to OFF-alpha cells, and the diversity in receptors at their synapses.

Synaptic inputs to retrogradely labeled ganglion cells in the retina of the cane toad, Bufo marinus

The entire population of ganglion cells in the retina of the toad Bufo marinus was labeled by retrograde transport of a lysine-fixable biotinylated dextran amine of 3000 molecular weight. Synaptic connections between bipolar, amacrine, and ganglion cells in the inner plexiform layer were quantitatively analyzed, with emphasis on synaptic inputs to labeled ganglion cell dendrites. Synapses onto ganglion cell dendrites comprised 47% of a total of 1234 identified synapses in the inner plexiform layer. Approximately half of the bipolar or amacrine cell synapses were directed onto ganglion cell dendrites, while the rest were made mainly onto amacrine cell dendrites. Most of the synaptic inputs to ganglion cell dendrites derived from amacrine cell dendrites (84%), with the rest from bipolar cell terminals. Synaptic inputs to ganglion cell dendrites were distributed relatively uniformly throughout all sublaminae of the inner plexiform layer. The present study provides unambiguous identification of ganglion cell dendrites including very fine processes, enabling a detailed analysis of the types and distribution of synaptic inputs from the bipolar and amacrine cell to the ganglion cells. The retrograde tracing technique used in the present study will prove to be a useful tool for identifying synaptic inputs to ganglion cell dendrites from neurochemically identified bipolar and amacrine cell types in the retina.

Synaptic inputs to ON parasol ganglion cells in the primate retina

In primates, the retinal ganglion cells that project to the magnocellular layers of the lateral geniculate nucleus have distinctive responses to light, and one of these has been identified morphologically as the parasol ganglion cell. To investigate their synaptic connections, we injected parasol cells with Neurobiotin in lightly fixed baboon retinas. The five ON-center cells we analyzed by electron microscopy received approximately 20% of their input from bipolar cells. The major synaptic input to parasol cells was from amacrine cells via conventional synapses and, in this respect, they resembled alpha ganglion cells of the cat retina. We also found the gap junctions between amacrine cells and parasol ganglion cells that had been predicted from tracer-coupling experiments. To identify the presynaptic amacrine cells, ON-center parasol cells were injected with Neurobiotin and Lucifer yellow in living macaque retinas, which were then fixed and labeled by immunofluorescence. Two kinds of amacrine cells were filled with Neurobiotin via gap junctions: a large, polyaxonal cell containing cholecystokinin and a smaller one without cholecystokinin. There were also appositions between cholecystokinin-containing amacrine cell processes and parasol cell dendrites. Cholinergic amacrine cell processes often followed parasol cell dendrites and made extensive contacts. In other mammals, the light responses of polyaxonal amacrine cells like these and cholinergic amacrine cells have been recorded, and the effects of acetylcholine and cholecystokinin on ganglion cells are known. Using this information, we developed a model of parasol cells that accounts for some properties of their light responses.

Gain control and hyperpolarization level in cat horizontal cells as a function of light and dark adaptation

First a model is presented that accurately summarizes the dynamic properties of cat horizontal (H-) cells under photopic conditions as measured in our previous work. The model predicts that asymmetries in response to dark as compared to light flashes are flash-duration dependent. This somewhat surprising prediction is tested and confirmed in intracellular recordings from the optically intact in vivo eye of the cat (Experiment 1). The model implies that the gain of H-cells should be related rather directly to the sustained (baseline) membrane potential. We performed three additional experiments to test this idea. Experiment 2 concerns response vs intensity (R-I-) curves for various flash-diameters and background-sizes with background luminance varying over a 4 log unit range. Results support the assumption of a rather strict coupling between flash sensitivity (gain) and the sustained level of hyperpolarization. In Experiment 3 we investigate this relation for both dark and light flashes given on each of four background light levels. The results suggest that there are fixed minimum and maximum hyperpolarization levels, and that the baseline hyperpolarization for a given illumination thus also sets the available range for dark and light flash-responses. The question then arises whether, or how this changes during dark adaptation, when the rod contribution to H-cell responses gradually increases. The fourth experiment therefore studies the relationship between gain and hyperpolarization level during prolonged dark-adaptation. The results show that the rod contribution increases the polarization range of H-cells, but that the gain and polarization level nevertheless remain directly coupled. H-cell models relying on a close coupling between polarization level and gain thus remain attractive options.