Receptive field of the retinal bipolar cell: a pharmacological study in the tiger salamander

Dopamine D1 and D4 receptors contribute to light adaptation in ON-sustained retinal ganglion cells

The adaptation of ganglion cells to increasing light levels is a crucial property of the retina. The retina must respond to light intensities that vary by 10-12 orders of magnitude, but the dynamic range of ganglion cell responses covers only ∼3 orders of magnitude. Dopamine is a crucial neuromodulator for light adaptation and activates receptors in the D1 and D2 families. Dopamine type D1 receptors (D1Rs) are expressed on horizontal cells and some bipolar, amacrine, and ganglion cells. In the D2 family, D2Rs are expressed on dopaminergic amacrine cells and D4Rs are primarily expressed on photoreceptors. However, the roles of activating these receptors to modulate the synaptic properties of the inputs to ganglion cells are not yet clear. Here, we used single-cell retinal patch-clamp recordings from the mouse retina to determine how activating D1Rs and D4Rs changed the light-evoked and spontaneous excitatory inputs to ON-sustained (ON-s) ganglion cells. We found that both D1R and D4R activation decrease the light-evoked excitatory inputs to ON-s ganglion cells, but that only the sum of the peak response decrease due to activating the two receptors was similar to the effect of light adaptation to a rod-saturating background. The largest effects on spontaneous excitatory activity of both D1R and D4R agonists was on the frequency of events, suggesting that both D1Rs and D4Rs are acting upstream of the ganglion cells.NEW & NOTEWORTHY Dopamine by bright light conditions allows retinal neurons to reduce sensitivity to adapt to bright light conditions. It is not clear how and why dopamine receptors modulate retinal ganglion cell signaling. We found that both D1 and D4 dopamine receptors in photoreceptors and inner retinal neurons contribute significantly to the reduction in sensitivity of ganglion cells with light adaptation. However, light adaptation also requires dopamine-independent mechanisms that could reflect inherent sensitivity changes in photoreceptors.

Antagonistic surround responses in different cones are mediated by feedback synapses from different horizontal cells

Cone photoreceptors are the first neurons along the visual pathway that exhibit center-surround antagonistic receptive fields, the basic building blocks for spatial information processing in the visual system. The surround responses in cones are mediated by the horizontal cells (HCs) via multiple feedback synaptic mechanisms. It has been controversial on which mechanisms are responsible for the surround-elicited depolarizing responses in cones (ΔVCone(s)), and whether the surround responses of various types of cones are mediated by the same HC feedback mechanisms. In this report, we studied ΔVCone(s)) of four types of cones in the salamander retina, and found that they are mediated by feedback synapses from A-type, B-type or A- and B-type HCs. ΔVCone(s) are observable in the presence of concomitant center light spots, and surround + center light stimuli of various intensity, size and wavelength differentially activate the feedback synapses from A- and B-type HCs to cones. We found that ΔVCone(s) of the L-cones are mediated by both A- and B-type HCs, those of the P- and S-cones by B-type HCs, and those of the A-cones by the A-type HCs. Moreover, our results suggest that B-type HCs mediate ΔVCone(s) through both GABAergic and GluT-ClC feedback synaptic mechanisms, and A-type HCs mediate ΔVCone(s) via the GluT-ClC feedback mechanism. Feedback synaptic mechanisms that increase calcium influx in cone synaptic terminals play important roles in mediating the antagonistic surround responses in the postsynaptic bipolar cells, but they may not generate enough current to depolarize the cones and significantly contribute to ΔVCone(s).

What the salamander eye has been telling the vision scientist's brain

Salamanders have been habitual residents of research laboratories for more than a century, and their history in science is tightly interwoven with vision research. Nevertheless, many vision scientists - even those working with salamanders - may be unaware of how much our knowledge about vision, and particularly the retina, has been shaped by studying salamanders. In this review, we take a tour through the salamander history in vision science, highlighting the main contributions of salamanders to our understanding of the vertebrate retina. We further point out specificities of the salamander visual system and discuss the perspectives of this animal system for future vision research.

Dopamine D1 receptor activation contributes to light-adapted changes in retinal inhibition to rod bipolar cells

Dopamine modulation of retinal signaling has been shown to be an important part of retinal adaptation to increased background light levels, but the role of dopamine modulation of retinal inhibition is not clear. We previously showed that light adaptation causes a large reduction in inhibition to rod bipolar cells, potentially to match the decrease in excitation after rod saturation. In this study, we determined how dopamine D1 receptors in the inner retina contribute to this modulation. We found that D1 receptor activation significantly decreased the magnitude of inhibitory light responses from rod bipolar cells, whereas D1 receptor blockade during light adaptation partially prevented this decline. To determine what mechanisms were involved in the modulation of inhibitory light responses, we measured the effect of D1 receptor activation on spontaneous currents and currents evoked from electrically stimulating amacrine cell inputs to rod bipolar cells. D1 receptor activation decreased the frequency of spontaneous inhibition with no change in event amplitudes, suggesting a presynaptic change in amacrine cell activity in agreement with previous reports that rod bipolar cells lack D1 receptors. Additionally, we found that D1 receptor activation reduced the amplitude of electrically evoked responses, showing that D1 receptors can modulate amacrine cells directly. Our results suggest that D1 receptor activation can replicate a large portion but not all of the effects of light adaptation, likely by modulating release from amacrine cells onto rod bipolar cells. NEW & NOTEWORTHY We demonstrated a new aspect of dopaminergic signaling that is involved in mediating light adaptation of retinal inhibition. This D1 receptor-dependent mechanism likely acts through receptors located directly on amacrine cells, in addition to its potential role in modulating the strength of serial inhibition between amacrine cells. Our results also suggest that another D2/D4 receptor-dependent or dopamine-independent mechanism must also be involved in light adaptation of inhibition to rod bipolar cells.

Dopamine D1 receptor modulation of calcium channel currents in horizontal cells of mouse retina

Horizontal cells form the first laterally interacting network of inhibitory interneurons in the retina. Dopamine released onto horizontal cells under photic and circadian control modulates horizontal cell function. Using isolated, identified horizontal cells from a connexin-57-iCre × ROSA26-tdTomato transgenic mouse line, we investigated dopaminergic modulation of calcium channel currents (ICa) with whole cell patch-clamp techniques. Dopamine (10 μM) blocked 27% of steady-state ICa, an action blunted to 9% in the presence of the L-type Ca channel blocker verapamil (50 μM). The dopamine type 1 receptor (D1R) agonist SKF38393 (20 μM) inhibited ICa by 24%. The D1R antagonist SCH23390 (20 μM) reduced dopamine and SKF38393 inhibition. Dopamine slowed ICa activation, blocking ICa by 38% early in a voltage step. Enhanced early inhibition of ICa was eliminated by applying voltage prepulses to +120 mV for 100 ms, increasing ICa by 31% and 11% for early and steady-state currents, respectively. Voltage-dependent facilitation of ICa and block of dopamine inhibition after preincubation with a Gβγ-blocking peptide suggested involvement of Gβγ proteins in the D1R-mediated modulation. When the G protein activator guanosine 5'-O-(3-thiotriphosphate) (GTPγS) was added intracellularly, ICa was smaller and showed the same slowed kinetics seen during D1R activation. With GTPγS in the pipette, additional block of ICa by dopamine was only 6%. Strong depolarizing voltage prepulses restored the GTPγS-reduced early ICa amplitude by 36% and steady-state ICa amplitude by 3%. These results suggest that dopaminergic inhibition of ICa via D1Rs is primarily mediated through the action of Gβγ proteins in horizontal cells.

Ionotropic GABA Receptors and Distal Retinal ON and OFF Responses

In the vertebrate retina, visual signals are segregated into parallel ON and OFF pathways, which provide information for light increments and decrements. The segregation is first evident at the level of the ON and OFF bipolar cells in distal retina. The activity of large populations of ON and OFF bipolar cells is reflected in the b- and d-waves of the diffuse electroretinogram (ERG). The role of gamma-aminobutyric acid (GABA), acting through ionotropic GABA receptors in shaping the ON and OFF responses in distal retina, is a matter of debate. This review summarized current knowledge about the types of the GABAergic neurons and ionotropic GABA receptors in the retina as well as the effects of GABA and specific GABAA and GABAC receptor antagonists on the activity of the ON and OFF bipolar cells in both nonmammalian and mammalian retina. Special emphasis is put on the effects on b- and d-waves of the ERG as a useful tool for assessment of the overall function of distal retinal ON and OFF channels. The role of GABAergic system in establishing the ON-OFF asymmetry concerning the time course and absolute and relative sensitivity of the ERG responses under different conditions of light adaptation in amphibian retina is also discussed.

Effects of picrotoxin on light adapted frog electroretinogram are not due entirely to its action in proximal retina

In order to evaluate the site of action of picrotoxin (antagonist of ionotropic GABA receptors) on the electroretinographic (ERG) b- and d-waves, in this study we compared its effects on the intensity-response function of the ERG waves in intact light adapted frog eyecup preparations with its effects in eyecups, where the activity of proximal neurons was blocked by 1 mMN-methyl-d-aspartate (MNDA). Picrotoxin markedly enhanced the b- and d-wave amplitude and slowed the time course of the responses at all stimulus intensities in the intact eyecups. Perfusion with NMDA alone caused significant enhancement of the b-wave amplitude and diminution of the d-wave amplitude without altering their time course in the entire intensity range. When picrotoxin was applied in combination with NMDA, an enhancement of the b-wave amplitude and slowing of its time course were observed at all stimulus intensities. The increase of the b-wave amplitude was significantly higher than that seen in NMDA group. Combined application of picrotoxin and NMDA caused an enhancement of the d-wave amplitude at the lower stimulus intensities and its diminution at the higher ones, while the d-wave time course was delayed over the entire intensity range. The results obtained indicate that a part of picrotoxin effects on the amplitude and time course of the photopic ERG b- and d-waves are due to its action in the distal frog retina.

The projective field of retinal bipolar cells and its modulation by visual context

The receptive field of a sensory neuron spells out all the receptor inputs it receives. To understand a neuron's role in the circuit, one also needs to know its projective field, namely the outputs it sends to all downstream cells. Here we present the projective fields of the primary excitatory neurons in a sensory circuit. We stimulated single bipolar cells of the salamander retina and recorded simultaneously from a population of ganglion cells. Individual bipolar cell signals diverge through polysynaptic pathways into ganglion cells of many different types and over surprisingly large distance. However, the strength and polarity of the projection depend on the cell types involved. Furthermore, visual stimulation strongly modulates the bipolar cell projective field, in opposite direction for different cell types. In this way, the context from distant parts of the visual field can control the routing of signals in the inner retina.

Lateral interactions in the outer retina

Lateral interactions in the outer retina, particularly negative feedback from horizontal cells to cones and direct feed-forward input from horizontal cells to bipolar cells, play a number of important roles in early visual processing, such as generating center-surround receptive fields that enhance spatial discrimination. These circuits may also contribute to post-receptoral light adaptation and the generation of color opponency. In this review, we examine the contributions of horizontal cell feedback and feed-forward pathways to early visual processing. We begin by reviewing the properties of bipolar cell receptive fields, especially with respect to modulation of the bipolar receptive field surround by the ambient light level and to the contribution of horizontal cells to the surround. We then review evidence for and against three proposed mechanisms for negative feedback from horizontal cells to cones: 1) GABA release by horizontal cells, 2) ephaptic modulation of the cone pedicle membrane potential generated by currents flowing through hemigap junctions in horizontal cell dendrites, and 3) modulation of cone calcium currents (I(Ca)) by changes in synaptic cleft proton levels. We also consider evidence for the presence of direct horizontal cell feed-forward input to bipolar cells and discuss a possible role for GABA at this synapse. We summarize proposed functions of horizontal cell feedback and feed-forward pathways. Finally, we examine the mechanisms and functions of two other forms of lateral interaction in the outer retina: negative feedback from horizontal cells to rods and positive feedback from horizontal cells to cones.

The projective field of a retinal amacrine cell

In sensory systems, neurons are generally characterized by their receptive field, namely the sensitivity to activity patterns at the input of the circuit. To assess the role of the neuron in the system, one must also know its projective field, namely the spatiotemporal effects the neuron exerts on all of the outputs of the circuit. We studied both the receptive and projective fields of an amacrine interneuron in the salamander retina. This amacrine type has a sustained OFF response with a small receptive field, but its output projects over a much larger region. Unlike other amacrine cells, this type is remarkably promiscuous and affects nearly every ganglion cell within reach of its dendrites. Its activity modulates the sensitivity of visual responses in ganglion cells but leaves their kinetics unchanged. The projective field displays a center-surround structure: depolarizing a single amacrine suppresses the visual sensitivity of ganglion cells nearby and enhances it at greater distances. This change in sign is seen even within the receptive field of one ganglion cell; thus, the modulation occurs presynaptically on bipolar cell terminals, most likely via GABA(B) receptors. Such an antagonistic projective field could contribute to the mechanisms of the retina for predictive coding.

Center/surround organization of retinal bipolar cells: High correlation of fundamental responses of center and surround to sinusoidal contrasts

Receptive field organization of cone-driven bipolar cells was investigated by intracellular recording in the intact light-adapted retina of the tiger salamander (Ambystoma tigrinum). Centered spots and concentric annuli of optimum dimensions were used to selectively stimulate the receptive field center and surround with sinusoidal modulations of contrast at 3 Hz. At low contrasts, responses of both the center and surround of both ON and OFF bipolar cells were linear, showing high gain and thus contrast enhancement relative to cones. The contrast/response curves for the fundamental response, measured by a Fast Fourier Transform, reached half maximum amplitude quickly at 13% contrast followed by saturation at high contrasts. The variation of the normalized amplitude of the center and surround responses was remarkably similar, showing linear regression over the entire response range with very high correlations, r2 = 0.97 for both ON and OFF cells. The contrast/response curves of both center and surround for both ON and OFF cells were well fit (r2 = 0.98) by an equation for single-site binding. In about half the cells studied, the nonlinear waveforms of center and surround could be brought into coincidence by scaling and shifting the surround response in time. This implies that a nonlinearity, common to both center and surround, occurs after polarity inversion at the cone feedback synapse. Evidence from paired whole-cell recordings between single cones and OFF bipolar cells suggests that substantial nonlinearity is not due to transmission at the cone synapse but instead arises from intrinsic bipolar cell and network mechanisms. When sinusoidal contrast modulations were applied to the center and surround simultaneously, clear additivity was observed for small responses in both ON and OFF cells, whereas the interaction was strikingly nonadditive for large responses. The contribution of the surround was then greatly reduced, suggesting attenuation at the cone feedback synapse.

Retinal synaptic pathways underlying the response of the rabbit local edge detector

We studied the circuitry that underlies the behavior of the local edge detector (LED) retinal ganglion cell in rabbit by measuring the spatial and temporal properties of excitatory and inhibitory currents under whole cell voltage clamp. Previous work showed that LED excitation is suppressed by activity in the surround. However, the contributions of outer and inner retina to this characteristic and the neurotransmitters used are currently unknown. Blockage of retinal inhibitory pathways (GABA(A), GABA(C), and glycine) eliminated edge selectivity. Inverting gratings in the surround with 50-microm stripe sizes did not stimulate horizontal cells, but suppressed on and off excitation by roughly 60%, indicating inhibition of bipolar terminals (feedback inhibition). On pharmacologic blockage, we showed that feedback inhibition used both GABA(A) and GABA(C) receptors, but not glycine. Glycinergic inhibition suppressed GABAergic feedback inhibition in the center, enabling larger excitatory currents in response to luminance changes. Excitation, feedback inhibition, and direct (feedforward) inhibition responded to luminance-neutral flipping gratings of 20- to 50-microm widths, showing they are driven by independent subunits within their receptive fields, which confers sensitivity to borders between areas of texture and nontexture. Feedforward inhibition was glycinergic, its rise time was faster than decay time, and did not function to delay spiking at the onset of a stimulus. Both the on and off phases could be triggered by luminance shifts as short in duration as 33 ms and could be triggered during scenes that already produced a high baseline level of feedforward inhibition. Our results show how LED circuitry can use subreceptive field sensitivity to detect visual edges via the interaction between excitation and feedback inhibition and also respond to rapid luminance shifts within a rapidly changing scene by producing feedforward inhibition.

Modeling convergent ON and OFF pathways in the early visual system

For understanding the computation and function of single neurons in sensory systems, one needs to investigate how sensory stimuli are related to a neuron's response and which biological mechanisms underlie this relationship. Mathematical models of the stimulus-response relationship have proved very useful in approaching these issues in a systematic, quantitative way. A starting point for many such analyses has been provided by phenomenological "linear-nonlinear" (LN) models, which comprise a linear filter followed by a static nonlinear transformation. The linear filter is often associated with the neuron's receptive field. However, the structure of the receptive field is generally a result of inputs from many presynaptic neurons, which may form parallel signal processing pathways. In the retina, for example, certain ganglion cells receive excitatory inputs from ON-type as well as OFF-type bipolar cells. Recent experiments have shown that the convergence of these pathways leads to intriguing response characteristics that cannot be captured by a single linear filter. One approach to adjust the LN model to the biological circuit structure is to use multiple parallel filters that capture ON and OFF bipolar inputs. Here, we review these new developments in modeling neuronal responses in the early visual system and provide details about one particular technique for obtaining the required sets of parallel filters from experimental data.

Retinal adaptation to object motion

Due to fixational eye movements, the image on the retina is always in motion, even when one views a stationary scene. When an object moves within the scene, the corresponding patch of retina experiences a different motion trajectory than the surrounding region. Certain retinal ganglion cells respond selectively to this condition, when the motion in the cell's receptive field center is different from that in the surround. Here we show that this response is strongest at the very onset of differential motion, followed by gradual adaptation with a time course of several seconds. Different subregions of a ganglion cell's receptive field can adapt independently. The circuitry responsible for differential motion adaptation lies in the inner retina. Several candidate mechanisms were tested, and the adaptation most likely results from synaptic depression at the synapse from bipolar to ganglion cell. Similar circuit mechanisms may act more generally to emphasize novel features of a visual stimulus.

Neuromorphic excitable maps for visual processing

An excitable membrane is described which can perform different visual tasks such as contour detection, contour propagation, image segmentation, and motion detection. The membrane is designed to fit into a neuromorphic multichip system. It consists of a single two-dimensional (2-D) layer of locally connected integrate-and-fire neurons and propagates input in the subthreshold and the above-threshold range. It requires adjustment of only one parameter to switch between the visual tasks. The performance of two spiking membranes of different connectivity is compared, a hexagonally and an octagonally connected membrane. Their hardware and system suitability is discussed.

Cross-talk between ON and OFF channels in the salamander retina: Indirect bipolar cell inputs to ON–OFF ganglion cells

It has been widely accepted that ON and OFF channels in the visual system are segregated with little cross-communication, except for the mammalian rod bipolar cell-AII amacrine cell-ganglion cell pathway. Here, we show that in the tiger salamander retina the light responses of a subpopulation of ON-OFF ganglion cells are mediated by crossing the ON and OFF bipolar cell pathways. Although the majority of ON-OFF ganglion cells (type I cells) receive direct excitatory inputs from depolarizing and hyperpolarizing bipolar cells (DBCs and HBCs), about 5% (type II cells) receive indirect excitatory inputs from DBCs and 20% (type III cells) receive indirect excitatory inputs from HBCs. These indirect bipolar cell inputs are likely to be mediated by a subpopulation of amacrine cells that exhibit transient hyperpolarizing light responses (AC(H)s) and make GABAergic/glycinergic synapses on DBC or HBC axon terminals. GABA and glycine receptor antagonists enhanced the ON and OFF excitatory cation current (DeltaI(C)) in type I ganglion cells, but completely suppressed the ON DeltaI(C) mediated by DBCs in type II cells and the OFF DeltaI(C) mediated by HBCs in types III cells. Dendrites of type I cells ramify in both sublamina A and B, type II cells exclusively in sublamina A, and type III cells exclusively in sublamina B of the inner plexiform layer. These results demonstrate that indirect, amacrine cell-mediated bipolar cell-ganglion cell synaptic pathways exist in a non-mammalian retina, and that bidirectional cross-talk between ON and OFF channels is present in the vertebrate retina.

Immunocytochemical analysis of GABA-positive and calretinin-positive horizontal cells in the tiger salamander retina

By using immunocytochemical techniques, we demonstrate that there are two distinct, nonoverlapping populations of horizontal cells (HCs) in the tiger salamander retina: GABA-positive cells account for about 72% and GABA-negative (calretinin-positive) cells account for 28% of the total HC somas. The calretinin-positive HCs have relatively sparse and thick dendrites: soma diameter of 19.72 +/- 0.29 microm, and soma density of 140 +/- 13 cells/mm(2), morphological features very much like the A-type HCs described in the accompanying article. The GABA-positive HCs have thinner dendritic and coarse axon-terminal-like processes of higher density: soma diameter of 18 +/- 0.18 microm, and soma density of 364 +/- 18 cells/mm(2), features that very much resemble the B-type HCs and B-type HC axon terminals in the accompanying article. By using double and triple immunostaining techniques we found that only 18% of the non-GABAergic HC dendritic clusters contact rods, whereas the remaining 82% of the dendritic clusters contact cones. This is consistent with the physiological finding in the accompanying article that the A-type HCs are cone-dominated. On the other hand, 32% of GABAergic HC dendrites contact rod pedicles and 68% contact cone pedicles, consistent with the physiological finding that B-type HCs and B-type HC axon terminals receive mixed rod/cone inputs. Detailed confocal microscope analysis shows that 4% rods, 6% principal double cones/single cones, and 100% accessory double cones contact calretinin-positive HCs, and 79% rods, 100% principal double cones, 14% accessory double cones, and 82% single cones contact GABAergic HCs. These results suggest that GABAergic and non-GABAergic HC input/output synapses differ and they may mediate different functional pathways in the outer retina.

OFF response of bullfrog cones is shaped by terminal ionotropic GABA receptors

We recently reported an ionotropic GABA receptor expressed at the bullfrog retinal cone terminal that is potentiated by the GABA(A) receptor antagonist bicuculline (BIC) and suppressed by the GABA(C) receptor antagonist imidazole-4-acetic acid (I4AA) . In this study, by using the patch clamp technique in current clamp mode, we show that activation of this GABA receptor causes voltage changes of cones, which are closely dependent on the membrane potential level in relation to the chloride equilibrium potential of the cells. Furthermore, the OFF overshoot of cone light responses is enhanced or diminished when this receptor is potentiated by BIC or suppressed by I4AA, suggesting the involvement of this GABA receptor in shaping OFF light responses of bullfrog cones.

Presynaptic Inhibition Modulates Spillover, Creating Distinct Dynamic Response Ranges of Sensory Output

Sensory information is thought to be modulated by presynaptic inhibition. Although this form of inhibition is a well-studied phenomenon, it is still unclear what role it plays in shaping sensory signals in intact circuits. By visually stimulating the retinas of transgenic mice lacking GABAc receptor-mediated presynaptic inhibition, we found that this inhibition regulated the dynamic range of ganglion cell (GC) output to the brain. Presynaptic inhibition acted differentially upon two major retinal pathways; its elimination affected GC responses to increments, but not decrements, in light intensity across the visual scene. The GC dynamic response ranges were different because presynaptic inhibition limited glutamate release from ON, but not OFF, bipolar cells, which modulate the extent of glutamate spillover and activation of perisynaptic NMDA receptors at ON GCs. Our results establish a role for presynaptic inhibitory control of spillover in determining sensory output in the CNS.

Two-photon imaging reveals somatodendritic chloride gradient in retinal ON-type bipolar cells expressing the biosensor Clomeleon

A somatodendritic gradient of Cl(-) concentration ([Cl(-)](i)) has been postulated to generate GABA-evoked responses of different polarity in retinal bipolar cells, hyperpolarizing in OFF cells with low dendritic [Cl(-)](i), and depolarizing in ON cells with high dendritic [Cl(-)](i). As glutamate released by the photoreceptors depolarizes OFF cells and hyperpolarizes ON cells, the bipolars' antagonistic receptive field (RF) could be computed by simply integrating glutamatergic inputs from the RF center and GABAergic inputs from horizontal cells in the RF surround. Using ratiometric two-photon imaging of Clomeleon, a Cl(-) indicator transgenically expressed in ON bipolar cells, we found that dendritic [Cl(-)](i) exceeds somatic [Cl(-)](i) by up to 20 mM and that GABA application can lead to Cl(-) efflux (depolarization) in these dendrites. Blockers of Cl(-) transporters reduced the somatodendritic [Cl(-)](i) gradient. Hence, our results support the idea that ON bipolar cells employ a somatodendritic [Cl(-)](i) gradient to invert GABAergic horizontal cell input.

Functional GABA(B) receptors are expressed at the cone photoreceptor terminals in bullfrog retina

GABA(B) receptors at the cone terminals in bullfrog retina were characterized by immunocytochemical and whole-cell patch clamp techniques in retinal slice preparations. Somata, axons and synaptic terminals (pedicles) of cones were both GABA(B) receptor (GABA(B)R) 1 and GABA(B)R2 immunoreactive. Physiologically, barium/calcium currents of cones to voltage steps were significantly reduced in size when GABA was puffed to cone terminals in the presence of picrotoxin that is supposed to block both GABA(A) and GABA(C) receptors. Similar reduction in barium currents was obtained with puff application of baclofen to cone terminals. These results suggest the presence of functional GABA(B) receptors at the bullfrog cone terminals. Suppression of barium currents of cones by baclofen was dose-dependent. Moreover, barium currents of cones were potentiated by background illumination, as compared with those recorded in the dark. 6,7-Dinitroquinoxaline-2,3-dione, an antagonist of non-NMDA receptors that hyperpolarizes horizontal cells and reduces GABA release from these cells, and saclofen, a GABA(B) receptor antagonist, both potentiated barium currents of cones in the dark, thereby mimicking the effects of background illumination. It is suggested that changes in calcium influx into the cone synaptic terminals due to activation of GABA(B) receptors may provide a negative feedback mechanism for regulating signal transmission between cones and second-order neurons in the retina by modifying the amount of glutamate released from the cones.

Retinal bipolar cells: contrast encoding for sinusoidal modulation and steps of luminance contrast

Contrast encoding for sinusoidal modulations of luminance contrast was investigated by intracellular recording in the intact salamander retina. In what appears to be the first study of this kind for vertebrate bipolar cells, responses of the central receptive-field mechanism of cone-driven cells to modulation of 3 Hz were analyzed quantitatively via both signal averaging and a Fast Fourier Transform (FFT) while the retina was light adapted to 20 cd/m2. Depolarizing and hyperpolarizing bipolar cells showed very similar encoding. Both responded with sinusoidal waveforms whose amplitude varied linearly with modulation depths ranging up to 7-8%. The slope of the modulation/response curve was very steep in this range. Thus, the contrast gain was high, reaching values of 6-7, and the half-maximal response was achieved at modulations of 9% or less. At modulations above approximately 15%, the responses typically showed strong compressive nonlinearity and the waveform was increasingly distorted. At maximum modulation, the higher harmonics of the FFT constituted about 30% of the amplitude of the fundamental. Measurements were also made for cones and horizontal cells. Both cell types showed predominantly linear responses and low contrast gain, in marked contrast to bipolar cells. These results suggest that the high contrast gain and strong nonlinearity of bipolar cells largely arise postsynaptic to cone transmitter release. Further experiments were performed to compare responses to contrast steps versus those to sinusoidal modulation. In the linear range, we show that the contrast gains of cones and horizontal cells are low and virtually identical for both steps and sinusoidal modulations. In bipolar cells, on the other hand, the contrast gain is about two times greater for steps than that for the 3-Hz sine waves. These results suggest that mechanisms intrinsic to bipolar cells act like a high-pass filter with a short time constant to selectively emphasize contrast transients over slower changes in contrast.

Inner and outer retinal pathways both contribute to surround inhibition of salamander ganglion cells

Illumination of the receptive-field surround reduces the sensitivity of a retinal ganglion cell to centre illumination. The steady, antagonistic receptive-field surround of retinal ganglion cells is classically attributed to the signalling of horizontal cells in the outer plexiform layer (OPL). However, amacrine cell signalling in the inner plexiform layer (IPL) also contributes to the steady receptive-field surround of the ganglion cell. We examined the contributions of these two forms of presynaptic lateral inhibition to ganglion cell light sensitivity by measuring the effects of surround illumination on EPSCs evoked by centre illumination. GABA(C) receptor antagonists reduced inhibition attributed to dim surround illumination, suggesting that this inhibition was mediated by signalling to bipolar cell axon terminals. Brighter surround illumination further reduced the light sensitivity of the ganglion cell. The bright surround effects on the EPSCs were insensitive to GABA receptor blockers. Perturbing outer retinal signalling with either carbenoxolone or cobalt blocked the effects of the bright surround illumination, but not the effects of dim surround illumination. We found that the light sensitivities of presynaptic, inhibitory pathways in the IPL and OPL were different. GABA(C) receptor blockers reduced dim surround inhibition, suggesting it was mediated in the IPL. By contrast, carbenoxolone and cobalt reduced bright surround, suggesting it was mediated by horizontal cells in the OPL. Direct amacrine cell input to ganglion cells, mediated by GABA(A) receptors, comprised another surround pathway that was most effectively activated by bright illumination. Our results suggest that surround activation of lateral pathways in the IPL and OPL differently modulate the sensitivity of the ganglion cell to centre illumination.

Synaptic mechanisms that shape visual signaling at the inner retina

The retina is a layered structure that processes information in two stages. The outer plexiform layer (OPL) comprises the first stage and is where photoreceptors, bipolar cells, and horizontal cells interact synaptically. This is the synaptic layer where ON and OFF responses to light are formed, as well as the site where receptive field center and surround organization is first thought to occur. The inner plexiform layer (IPL) is where the second stage of synaptic interactions occurs. This synaptic layer is where subsequent visual processing occurs that may contribute to the formation of transient responses, which may underlie motion and direction sensitivity. In addition, synaptic interactions in the IPL may also contribute to the classical ganglion cell receptive field properties. This chapter will focus on the synapse and network properties at the IPL that sculpt light-evoked ganglion cell responses. These include synaptic mechanisms that may shape ganglion cell responses like desensitizing glutamate receptors and transporters, which remove glutamate from the synapse. Recent work suggests that inhibitory signaling at the IPL contributes to the surround receptive field organization of ganglion cells. A component of this amacrine cell inhibitory signaling is mediated by GABAC receptors, which are found on bipolar cell axon terminals in the IPL. Pharmacological experiments show that a component of the ganglion cell surround signal is mediated by these receptors, indicating that the ganglion cell center and surround receptive field organization is not formed entirely in the outer plexiform layer, as earlier thought.

Characterization of receptors for glutamate and GABA in retinal neurons

Glutamate and gamma-aminobutyric acid (GABA) are major excitatory and inhibitory neurotransmitters in the vertebrate retina, "a genuine neural center" (Ramón y Cajal, 1964, Recollections of My Life, C.E. Horne (Translater) MIT Press, Cambridge, MA). Photoreceptors, generating visual signals, and bipolar cells, mediating signal transfer from photoreceptors to ganglion cells, both release glutamate, which induces and/or changes the activity of the post-synaptic neurons (horizontal and bipolar cells for photoreceptors; amacrine and ganglion cells for bipolar cells). Horizontal and amacrine cells, which mediate lateral interaction in the outer and inner retina respectively, use GABA as a principal neurotransmitter. In recent years, glutamate receptors and GABA receptors in the retina have been extensively studied, using multi-disciplinary approaches. In this article some important advances in this field are reviewed, with special reference to retinal information processing. Photoreceptors possess metabotropic glutamate receptors and several subtypes of GABA receptors. Most horizontal cells express AMPA receptors, which may be predominantly assembled from flop slice variants. In addition, these cells also express GABAA and GABAC receptors. Signal transfer from photoreceptors to bipolar cells is rather complicated. Whereas AMPA/KA receptors mediate transmission for OFF type bipolar cells, several subtypes of glutamate receptors, both ionotropic and metabotropic, are involved in the generation of light responses of ON type bipolar cells. GABAA and GABAC receptors with distinct kinetics are differentially expressed on dendrites and axon terminals of both ON and OFF bipolar cells, mediating inhibition from horizontal cells and amacrine cells. Amacrine cells possess ionotropic glutamate receptors, whereas ganglion cells express both ionotropic and metabotropic glutamate receptors. GABAA receptors exist in amacrine and ganglion cells. Physiological data further suggest that GABAC receptors may be involved in the activity of these neurons. Moreover, responses of these retinal third order neurons are modulated by GABAB receptors, and in ganglion cells there exist several subtypes of GABAB receptors. A variety of glutamate receptor and GABA receptor subtypes found in the retina perform distinct functions, thus providing a wide range of neural integration and versatility of synaptic transmission. Perspectives in this research field are presented.

pH changes in the invaginating synaptic cleft mediate feedback from horizontal cells to cone photoreceptors by modulating Ca2+ channels

Feedback from horizontal cells (HCs) to cone photoreceptors plays a key role in the center-surround–receptive field organization of retinal neurons. Recordings from cone photoreceptors in newt retinal slices were obtained by the whole-cell patch-clamp technique, using a superfusate containing a GABA antagonist (100 μM picrotoxin). Surround illumination of the receptive field increased the voltage-dependent calcium current (I_Ca) in the cones, and shifted the activation voltage of I_Ca to negative voltages. External alkalinization also increased cone I_Ca and shifted its activation voltage toward negative voltages. Enrichment of the pH buffering capacity of the extracellular solution increased cone I_Ca, and blocked any additional increase in cone I_Ca by surround illumination. Hyperpolarization of the HCs by a glutamate receptor antagonist-augmented cone I_Ca, whereas depolarization of the HCs by kainate suppressed cone I_Ca. From these results, we propose the hypothesis that pH changes in the synaptic clefts, which are intimately related to the membrane voltage of the HCs, mediate the feedback from the HCs to cone photoreceptors. The feedback mediated by pH changes in the synaptic cleft may serve as an additional mechanism for the center-surround organization of the receptive field in the outer retina.

Control of intracellular chloride concentration and GABA response polarity in rat retinal ON bipolar cells

GABAergic modulation of retinal bipolar cells plays a crucial role in early visual processing. It helps to form centre-surround receptive fields which filter the visual signal spatially at the bipolar cell dendrites in the outer retina, and it produces temporal filtering at the bipolar cell synaptic terminals in the inner retina. The observed chloride transporter distribution in ON bipolar cells has been predicted to produce an intracellular chloride concentration, [Cl(-)](i), that is significantly higher in the dendrites than in the synaptic terminals. This would allow dendritic GABA-gated Cl(-) channels to generate the depolarization needed for forming the lateral inhibitory surround of the cell's receptive field, while synaptic terminal GABA-gated Cl(-) channels generate the hyperpolarization needed for temporal shaping of the light response. In contrast to this idea, we show here that in ON bipolar cells [Cl(-)](i) is only slightly higher in the dendrites than in the synaptic terminals, and that GABA-gated channels in the dendrites may generate a hyperpolarization rather than a depolarization. We also show that [Cl(-)](i) is controlled by movement of Cl(-) through ion channels in addition to transporters, that changes of [K(+)](o) alter [Cl(-)](i) and that voltage-dependent equilibration of [Cl(-)](i) in bipolar cells will produce a time-dependent adaptation of GABAergic modulation with a time constant of 8 s after illumination-evoked changes of membrane potential. Time-dependent adaptation of [Cl(-)](i) to voltage changes in retinal bipolar cells may add a previously unsuspected layer of temporal processing to signals as they pass through the retina.

Transport-mediated synapses in the retina

Most synapses rely on regulated exocytosis for determining the concentration of transmitter in the synaptic cleft. However, this mechanism may not be universal. Several synapses in the retina appear to use a synaptic machinery in which transmitter transporters play an essential role. Two types of transport-mediated synapses have been proposed. These synapses have been best observed in horizontal cells and cones of nonmammalian retinas. Horizontal cells use a transporter to mediate a bidirectional shuttle, whose balance point is set by ion concentrations and voltage. Nonmammalian cones combine exocytosis and the activity of a transporter. Because exocytosis is voltage independent over most of a cone's physiological voltage range, a voltage-dependent transporter determines the concentration of transmitter in the synaptic cleft. These two synapses may be models for transport-mediated synapses that operate in other parts of the brain.

The Hermann-Hering grid illusion demonstrates disruption of lateral inhibition processing in diabetes mellitus

BACKGROUND/AIM

The Hermann-Hering grid illusion consists of dark illusory spots perceived at the intersections of horizontal and vertical white bars viewed against a dark background. The dark spots originate from lateral inhibition processing. This illusion was used to investigate the hypothesis that lateral inhibition may be disrupted in diabetes mellitus.

METHOD

A computer monitor based psychophysical test was developed to measure the threshold of perception of the illusion for different bar widths. The contrast threshold for illusion perception at seven bar widths (range 0.09 degrees to 0.60 degrees) was measured using a randomly interleaved double staircase. Convolution of Hermann-Hering grids with difference of Gaussian receptive fields was used to generate model sensitivity functions. The method of least squares was used to fit these to the experimental data. 14 diabetic patients and 12 control subjects of similar ages performed the test.

RESULTS

The sensitivity to the illusion was significantly reduced in the diabetic group for bar widths 0.22 degrees, 0.28 degrees, and 0.35 degrees (p = 0.01). The mean centre:surround ratio for the controls was 1:9.1 (SD 1.6) with a mean correlation coefficient of R(2) = 0.80 (SD 0.16). In the diabetic group, two subjects were unable to perceive the illusion. The mean centre:surround ratio for the 12 remaining diabetic patients was 1:8.6 (SD 2.1). However, the correlation coefficients were poor with a mean of R(2) = 0.54 (SD 0.27), p = 0.04 in comparison with the control group.

CONCLUSIONS

A difference of Gaussian receptive field model fits the experimental data well for the controls but does not fit the data obtained for the diabetics. This indicates dysfunction of the lateral inhibition processes in the post-receptoral pathway.

Localization of neurotransmitters and calcium binding proteins to neurons of salamander and mudpuppy retinas

We wished to identify the different types of retinal neurons on the basis of their content of neuroactive substances in both larval tiger salamander and mudpuppy retinas, favored species for electrophysiological investigation. Sections and wholemounts of retinas were labeled by immunocytochemical methods to demonstrate three calcium binding protein species and the common neurotransmitters, glycine, GABA and acetylcholine. Double immunostained sections and single labeled wholemount retinas were examined by confocal microscopy. Immunostaining patterns appeared to be the same in salamander and mudpuppy. Double and single cones, horizontal cells, some amacrine cells and ganglion cells were strongly calbindin-immunoreactive (IR). Calbindin-IR horizontal cells colocalized GABA. Many bipolar cells, horizontal cells, some amacrine cells and ganglion cells were strongly calretinin-IR. One type of horizontal cell and an infrequently occurring amacrine cell were parvalbumin-IR. Acetylcholine as visualized by ChAT-immunoreactivity was seen in a mirror-symmetric pair of amacrine cells that colocalized GABA and glycine. Glycine and GABA colocalized with calretinin, calbindin and occasionally with parvalbumin in amacrine cells.

Multireceptor GABAergic regulation of synaptic communication in amphibian retina

The synaptic output of retinal bipolar cells was monitored by recording light-evoked EPSCs in ganglion cells. Application of (RS)-2-amino-3-(3-hydroxy-5-tert-butyl-4-isoxazolyl (ATPA), a selective agonist at kainate receptors, depolarized amacrine cells and reduced the light-evoked excitatory current (L-EPSC) in ganglion cells. ATPA had only a slight effect on the light responses of bipolar cells. Therefore, ATPA suppresses bipolar cell synaptic output to ganglion cells. ATPA reduced the transient L-EPSC, but had comparatively little effect on sustained L-EPSC, of ganglion cells. The transient ON L-EPSC was more suppressed than the transient OFF L-EPSC. Thus, ATPA preferentially suppressed transient output from bipolar cells.GABA receptor antagonists blocked the effect of ATPA. This indicates that ATPA stimulated an endogenous inhibitory feedback pathway that suppressed bipolar cell output.CGP55845 and CGP35348 reduced the ATPA-induced suppression of L-EPSCs in ganglion cells, signifying that part of the feedback pathway is mediated by metabotropic GABA receptors.(1,2,5,6-Tetrahydropyridine-4-yl)-methylphosphinic acid (TPMPA) and picrotoxin, GABAC receptor antagonists, reduced the ATPA effect. Picrotoxin was more effective than ATPA. However, picrotoxin blocked only a part of this GABAC effect, while imidazole-4-acetic acid (I4AA) blocked another segment of the effect. This indicates that two pharmacologically distinct GABAC receptors mediate feedback to bipolar cells. SR95531 produced a very small suppression of the ATPA effect. Thus, GABAA receptors provide a negligible component to this feedback pathway. The experiments indicate that endogenous GABAergic feedback to bipolar cells suppresses their output, and that this feedback is mediated by at least three types of GABA receptor, both metabotropic and ionotropic.In conjunction with previous studies, the results indicate that feedback inhibition is the predominant factor regulating transient signalling in ganglion cells, while feedforward inhibition is the primary regulator of tonic ganglion cell signals.

I4AA-Sensitive chloride current contributes to the center light responses of bipolar cells in the tiger salamander retina

Light-evoked currents in depolarizing and hyperpolarizing bipolar cells (DBCs and HBCs) were recorded under voltage-clamp conditions in living retinal slices of the larval tiger salamander. Responses to illumination at the center of the DBCs' and HBCs' receptive fields were mediated by two postsynaptic currents: DeltaI(C), a glutamate-gated cation current with a reversal potential near 0 mV, and DeltaI(Cl), a chloride current with a reversal potential near -60 mV. In DBCs DeltaI(C) was suppressed by L-2-amino-4-phosphonobutyric acid (L-AP4), and in HBCs it was suppressed by 6,7-dinitroquinoxaline-2,3-dione (DNQX). In both DBCs and HBCs DeltaI(Cl) was suppressed by imidazole-4-acetic acid (I4AA), a GABA receptor agonist and GABA(C) receptor antagonist. In all DBCs and HBCs examined, 10 microM I4AA eliminated DeltaI(Cl) and the light-evoked current became predominately mediated by DeltaI(C). The addition of 20 microM L-AP4 to the DBCs or 50 microM DNQX to HBCs completely abolished DeltaI(C). Focal application of glutamate at the inner plexiform layer elicited chloride currents in bipolar cells by depolarizing amacrine cells that release GABA at synapses on bipolar cell axon terminals, and such glutamate-induced chloride currents in DBCs and HBCs could be reversibly blocked by 10 microM I4AA. These experiments suggest that the light-evoked, I4AA-sensitive chloride currents (DeltaI(Cl)) in DBCs and HBCs are mediated by narrow field GABAergic amacrine cells that activate GABA(C) receptors on bipolar cell axon terminals. Picrotoxin (200 microM) or (1,2,5,6-tetrahydropyridine-4yl) methyphosphinic acid (TPMPA) (2 other GABA(C) receptor antagonists) did not block (but enhanced and broadened) the light-evoked DeltaI(Cl), although they decreased the chloride current induced by puff application of GABA or glutamate. The light response of narrow field amacrine cells were not affected by I4AA, but were substantially enhanced and broadened by picrotoxin. These results suggest that there are at least two types of GABA(C) receptors in bipolar cells: one exhibits stronger I4AA sensitivity than the other, but both can be partially blocked by picrotoxin. The GABA receptors in narrow field amacrine cells are I4AA insensitive and picrotoxin sensitive. The light-evoked DeltaI(Cl) in bipolar cells are mediated by the more strongly I4AA-sensitive GABA(C) receptors. Picrotoxin, although acting as a partial GABA(C) receptor antagonist in bipolar cells, does not suppress DeltaI(Cl) because its presynaptic effects on amacrine cell light responses override its antagonistic postsynaptic actions.

Distinct ionotropic GABA receptors mediate presynaptic and postsynaptic inhibition in retinal bipolar cells

Ionotropic GABA receptors can mediate presynaptic and postsynaptic inhibition. We assessed the contributions of GABA(A) and GABA(C) receptors to inhibition at the dendrites and axon terminals of ferret retinal bipolar cells by recording currents evoked by focal application of GABA in the retinal slice. Currents elicited at the dendrites were mediated predominantly by GABA(A) receptors, whereas responses evoked at the terminals had GABA(A) and GABA(C) components. The ratio of GABA(C) to GABA(A) (GABA(C):GABA(A)) was highest in rod bipolar cell terminals and variable among cone bipolars, but generally was lower in OFF than in ON classes. Our results also suggest that the GABA(C):GABA(A) could influence the time course of responses. Currents evoked at the terminals decayed slowly in cell types for which the GABA(C):GABA(A) was high, but decayed relatively rapidly in cells for which this ratio was low. Immunohistochemical studies corroborated our physiological results. GABA(A) beta2/3 subunit immunoreactivity was intense in the outer and inner plexiform layers (OPL and IPL, respectively). GABA(C) rho subunit labeling was weak in the OPL but strong in the IPL in which puncta colocalized with terminals of rod bipolars immunoreactive for protein kinase C and of cone bipolars immunoreactive for calbindin or recoverin. These data demonstrate that GABA(A) receptors mediate GABAergic inhibition on bipolar cell dendrites in the OPL, that GABA(A) and GABA(C) receptors mediate inhibition on axon terminals in the IPL, and that the GABA(C):GABA(A) on the terminals may tune the response characteristics of the bipolar cell.

Three levels of lateral inhibition: A space-time study of the retina of the tiger salamander

The space-time patterns of activity generated across arrays of retinal neurons can provide a sensitive measurement of the effects of neural interactions underlying retinal activity. We measured the excitatory and inhibitory components associated with these patterns at each cellular level in the retina and further dissected inhibitory components pharmacologically. Using perforated and loose patch recording, we measured the voltages, currents, or spiking at 91 lateral positions covering approximately 2 mm in response to a flashed 300-microm-wide bar. First, we showed how the effect of well known lateral inhibition at the outer retina, mediated by horizontal cells, evolved in time to compress the spatial representation of the stimulus bar at ON and OFF bipolar cell bodies as well as horizontal cells. Second, we showed, for the first time, how GABA(C) receptor mediated amacrine cell feedback to bipolar terminals compresses the spatial representation of the stimulus bar at ON bipolar terminals over time. Third, we showed that a third spatiotemporal compression exists at the ganglion cell layer that is mediated by feedforward amacrine cells via GABA(A) receptors. These three inhibitory mechanisms, via three different receptor types, appear to compensate for the effects of lateral diffusion of activity attributable to dendritic spread and electrical coupling between retinal neurons. As a consequence, the width of the final representation at the ganglion cell level approximates the dimensions of the original stimulus bar.

The nature of surround-induced depolarizing responses in goldfish cones

Cones in the vertebrate retina project to horizontal and bipolar cells and the horizontal cells feedback negatively to cones. This organization forms the basis for the center/surround organization of the bipolar cells, a fundamental step in the visual signal processing. Although the surround responses of bipolar cells have been recorded on many occasions, surprisingly, the underlying surround-induced responses in cones are not easily detected. In this paper, the nature of the surround-induced responses in cones is studied. Horizontal cells feed back to cones by shifting the activation function of the calcium current in cones to more negative potentials. This shift increases the calcium influx, which increases the neurotransmitter release of the cone. In this paper, we will show that under certain conditions, in addition to this increase of neurotransmitter release, a calcium-dependent chloride current will be activated, which polarizes the cone membrane potential. The question is, whether the modulation of the calcium current or the polarization of the cone membrane potential is the major determinant for feedback-mediated responses in second-order neurons. Depolarizing light responses of biphasic horizontal cells are generated by feedback from monophasic horizontal cells to cones. It was found that niflumic acid blocks the feedback-induced depolarizing responses in cones, while the shift of the calcium current activation function and the depolarizing biphasic horizontal cell responses remain intact. This shows that horizontal cells can feed back to cones, without inducing major changes in the cone membrane potential. This makes the feedback synapse from horizontal cells to cones a unique synapse. Polarization of the presynaptic (horizontal) cell leads to calcium influx in the postsynaptic cell (cone), but due to the combined activity of the calcium current and the calcium-dependent chloride current, the membrane potential of the postsynaptic cell will be hardly modulated, whereas the output of the postsynaptic cell will be strongly modulated. Since no polarization of the postsynaptic cell is needed for these feedback-mediated responses, this mechanism of synaptic transmission can modulate the neurotransmitter release in single synaptic terminals without affecting the membrane potential of the entire cell.

Lateral inhibition in the inner retina is important for spatial tuning of ganglion cells

The center-surround receptive-field organization in retinal ganglion cells is widely believed to result mainly from lateral inhibition at the first synaptic level (in the outer retina). Inhibition at the second synaptic level (in the inner retina) is thought to mediate more complex response properties. Here we show that much of the sustained surround antagonism in certain on-center ganglion cells results from lateral inhibition in the inner retina, via GABAergic amacrine cells, and that the lateral conduction of this signal requires voltage-gated sodium currents. Blocking lateral inhibition in the inner retina eliminates the preference of small-center ganglion cells for small stimuli but has little effect on ganglion cells with large receptive-field centers. These results illustrate how lateral inhibition at successive synaptic stages can selectively control the size of neural receptive-field centers.

Spatiotemporal patterns at the retinal output

Edge enhancement in the retina is thought to be mediated by classical center-surround antagonism, first encountered as the interactions between horizontal cells and cones. But in the salamander retina these interactions do little to enhance edges. Instead, a robust dynamic interaction between amacrine and bipolar cells appears to be responsible for a sharp edge enhancement. To demonstrate this we recorded extracellularly from a single ganglion cell and moved a flashed square, 300 micro(m) on a side, over a 1.5 x 1.0 mm2 grid at 25-micro(m) increments. Playing back all of these recordings simultaneously simulated the pattern of responses that would have been measured from an array of ganglion cells. The emerging pattern of ganglion cell activity first faithfully represented the flashed square, but after approximately 60 ms the center of the representation collapsed, leaving a representation of only the edges. We inferred that the feedback synapse from amacrine to bipolar cells at gamma-aminobutyric acid-C (GABAC) receptors mediated this effect: bicuculline and strychnine were ineffective in altering the response pattern, but in picrotoxin the center of the representation did not collapse. The GABAergic amacrine cells thought to mediate this effect have quite narrow spread of processes, so the existence of this edge-enhancing effect suggests a mechanism quite different from classical lateral inhibition, namely the delayed inhibition of a spatially expanding input pattern.

GABA modulates color-opponent bipolar cells in carp retina

In the present work, the effects of gamma-aminobutyric acid (GABA) were examined while recording intracellularly from color-opponent bipolar cells in the isolated, superfused carp retina. Bath-applied GABA differentially suppressed light responses of R/G cells, which depolarized and hyperpolarized, respectively, in response to red and green flashes, and G/R cells, which showed the opposite responses. In either R/G or G/R cells, the response driven by input from red cones was invariably suppressed to a greater extent. Such effects of GABA were completely blocked by picrotoxin, but only partially by bicuculline, suggesting that the GABAC receptor, in addition to the GABAA receptor, may also be involved. It is postulated that GABA modulates the color-opponent bipolar cells either through activating GABA receptors on these cells directly or those on cone terminals indirectly.

Amino acid neurotransmitters in the retina: a functional overview

Physiological and pharmacological mechanisms of glutamatergic, GABAergic and glycinergic synapses in the tiger salamander retina were studied. We used immunocytochemical and autoradiographic methods to study localizations of these neurotransmitters and their uptake transporters; and electrophysiological methods (intracellular, extracellular and whole cell patch electrode recordings) to study the light responses, miniature postsynaptic currents and neurotransmitter-induced postsynaptic currents in various retinal neurons. Our results are consistent with the following scheme: Glutamate is used by the photoreceptor and bipolar cell output synapses and the release of glutamate is largely mediated by calcium-dependent vesicular processes. The postsynaptic glutamate receptors in DBCs are L-AP4 receptors, in HBCs, HCs and ganglion cells are the kainate/AMPA and NMDA receptors. Subpopulations of HCs make GABAergic synapses on cones and gate chloride condunctance through GABAA receptors. GABAergic HCs do not make feedforward synapses on bipolar cell dendrites and the neurotransmitter identity of the HCs making feedforward synapses is unknown. Subpopulations of amacrine cells make GABAergic synapses on bipolar cell synaptic terminals, other amacrine cells and ganglion cells and GABA gates chloride conductances in theses cells. Glycinergic amacrine cells make synapses on bipolar cell synaptic terminals, other amacrine cells and ganglion cells and glycine opens postsynaptic chloride channels. Glycinergic interplexiform cells make synapses on bipolar cells in the outer retina and glycine released from these cells open chloride channels in bipolar cell dendrites.

Synchronous period-doubling in flicker vision of salamander and man

Periodic flashes of light have long served to probe the temporal properties of the visual system. Here we show that during rapid flicker of high contrast and intensity the eye reports to the brain only every other flash of light. In this regime, retinal ganglion cells of the salamander fire spikes on alternating flashes. Neurons across the entire retina are locked to the same flashes. The effect depends sharply on contrast and flash frequency. It results from a period-doubling bifurcation in retinal processing, and a simple model of nonlinear feedback reproduces the phenomenon. Pharmacological studies indicate that the critical feedback interactions require only cone photoreceptors and bipolar cells. Analogous period-doubling is observed in the human visual system. Under bright full-field flicker, the electroretinogram (ERG) shows a regime of period-doubling between 30 and 70 Hz. In visual evoked potentials from the occiput, the subharmonic component is even stronger. By analyzing the accompanying perceptual effects, we find that retinal period-doubling begins in the periphery of the visual field, and that it is the cause of a long mysterious illusory flicker pattern.

Modulation of sustained and transient lateral inhibitory mechanisms in the mudpuppy retina during light adaptation

Two functionally and anatomically distinct types of lateral inhibition contribute to the receptive field organization of ganglion cells in the vertebrate retina: sustained lateral inhibition (SLI), which is present during steady illumination and transient lateral inhibition (TLI), evoked by changes in illumination. We studied adaptive changes in these two lateral inhibitory mechanisms in the mudpuppy retina by measuring the responses of ON-OFF ganglion cells to spots of light in the receptive field center, in the absence and presence of a concentric broken annulus (windmill) pattern, which was either stationary or rotating. SLI was measured as the percent suppression of the centered spot response by the stationary windmill and TLI was measured as the additional suppression produced when the windmill was rotating. In dark-adapted retinas SLI was elicited by windmills of 600 or 1,200 micron ID, but TLI could not be elicited by windmills of any size, over a wide range of windmill intensities and rotation rates. Exposure of dark-adapted retinas to diffuse adapting light caused an immediate decrease in the response to the spot alone, followed by slowly developing changes in both SLI and TLI: SLI produced by 1,200 micron ID windmills became weaker, whereas SLI produced by 600 micron ID windmills became stronger. After several minutes strong TLI could be elicited by both 600 and 1,200 micron ID windmills. The changes in SLI and TLI were usually complete within 5 and 15 min, respectively, and recovered to dark-adapted levels slightly more slowly after the adapting light was turned off. However the changes in sensitivity of the spot response were complete within one minute after onset and termination of the adapting light. The adaptive changes in SLI and TLI did not depend on the presence of the adapting light; after a brief (1 min) exposure to the adapting light, the changes in SLI and TLI slowly developed and then decayed back to the dark-adapted level. The effects of the adapting light on SLI were mimicked by dopamine and blocked by D1 dopamine receptor antagonists. However dopamine did not enable TLI in dark-adapted retinas and dopamine antagonists did not prevent enablement of TLI when dark-adapted retinas were exposed to light or disable TLI when applied to light-adapted retinas. The results suggest that light-adaptive changes in SLI are mediated by dopamine and are consistent with a reduction in electrical coupling between neurons that conduct the SLI signal laterally in the retina. In contrast, TLI appears to be switched off or suppressed in the dark-adapted retina and enabled in light-adapted retinas, by a relatively slow modulatory mechanism that does not involve dopamine.