Immunoreactivity to GABAA receptor in the outer plexiform layer of the cat retina

Vesicular Release of GABA by Mammalian Horizontal Cells Mediates Inhibitory Output to Photoreceptors

Feedback inhibition by horizontal cells regulates rod and cone photoreceptor calcium channels that control their release of the neurotransmitter glutamate. This inhibition contributes to synaptic gain control and the formation of the center-surround antagonistic receptive fields passed on to all downstream neurons, which is important for contrast sensitivity and color opponency in vision. In contrast to the plasmalemmal GABA transporter found in non-mammalian horizontal cells, there is evidence that the mechanism by which mammalian horizontal cells inhibit photoreceptors involves the vesicular release of the inhibitory neurotransmitter GABA. Historically, inconsistent findings of GABA and its biosynthetic enzyme, L-glutamate decarboxylase (GAD) in horizontal cells, and the apparent lack of surround response block by GABAergic agents diminished support for GABA's role in feedback inhibition. However, the immunolocalization of the vesicular GABA transporter (VGAT) in the dendritic and axonal endings of horizontal cells that innervate photoreceptor terminals suggested GABA was released via vesicular exocytosis. To test the idea that GABA is released from vesicles, we localized GABA and GAD, multiple SNARE complex proteins, synaptic vesicle proteins, and Cav channels that mediate exocytosis to horizontal cell dendritic tips and axonal terminals. To address the perceived relative paucity of synaptic vesicles in horizontal cell endings, we used conical electron tomography on mouse and guinea pig retinas that revealed small, clear-core vesicles, along with a few clathrin-coated vesicles and endosomes in horizontal cell processes within photoreceptor terminals. Some small-diameter vesicles were adjacent to the plasma membrane and plasma membrane specializations. To assess vesicular release, a functional assay involving incubation of retinal slices in luminal VGAT-C antibodies demonstrated vesicles fused with the membrane in a depolarization- and calcium-dependent manner, and these labeled vesicles can fuse multiple times. Finally, targeted elimination of VGAT in horizontal cells resulted in a loss of tonic, autaptic GABA currents, and of inhibitory feedback modulation of the cone photoreceptor Cai, consistent with the elimination of GABA release from horizontal cell endings. These results in mammalian retina identify the central role of vesicular release of GABA from horizontal cells in the feedback inhibition of photoreceptors.

Horizontal Cell Feedback to Cone Photoreceptors in Mammalian Retina: Novel Insights From the GABA-pH Hybrid Model

How neurons in the eye feed signals back to photoreceptors to optimize sensitivity to patterns of light appears to be mediated by one or more unconventional mechanisms. Via these mechanisms, horizontal cells control photoreceptor synaptic gain and enhance key aspects of temporal and spatial center-surround receptive field antagonism. After the transduction of light energy into an electrical signal in photoreceptors, the next key task in visual processing is the transmission of an optimized signal to the follower neurons in the retina. For this to happen, the release of the excitatory neurotransmitter glutamate from photoreceptors is carefully regulated via horizontal cell feedback, which acts as a thermostat to keep the synaptic transmission in an optimal range during changes to light patterns and intensities. Novel findings of a recently described model that casts a classical neurotransmitter system together with ion transport mechanisms to adjust the alkaline milieu outside the synapse are reviewed. This novel inter-neuronal messaging system carries feedback signals using two separate, but interwoven regulated systems. The complex interplay between these two signaling modalities, creating synaptic modulation-at-a-distance, has obscured it’s being defined. The foundations of our understanding of the feedback mechanism from horizontal cells to photoreceptors have been long established: Horizontal cells have broad receptive fields, suitable for providing surround inhibition, their membrane potential, a function of stimulus intensity and size, regulates inhibition of photoreceptor voltage-gated Ca²⁺ channels, and strong artificial pH buffering eliminates this action. This review compares and contrasts models of how these foundations are linked, focusing on a recent report in mammals that shows tonic horizontal cell release of GABA activating Cl⁻ and HCO₃⁻ permeable GABA autoreceptors. The membrane potential of horizontal cells provides the driving force for GABAR-mediated HCO₃⁻ efflux, alkalinizing the cleft when horizontal cells are hyperpolarized by light or adding to their depolarization in darkness and contributing to cleft acidification via NHE-mediated H⁺ efflux. This model challenges interpretations of earlier studies that were considered to rule out a role for GABA in feedback to cones.

Evidence for functional GABAA but not GABAC receptors in mouse cone photoreceptors

At the first retinal synapse, horizontal cells (HCs) contact both photoreceptor terminals and bipolar cell dendrites, modulating information transfer between these two cell types to enhance spatial contrast and mediate color opponency. The synaptic mechanisms through which these modulations occur are still debated. The initial hypothesis of a GABAergic feedback from HCs to cones has been challenged by pharmacological inconsistencies. Surround antagonism has been demonstrated to occur via a modulation of cone calcium channels through ephaptic signaling and pH changes in the synaptic cleft. GABAergic transmission between HCs and cones has been reported in some lower vertebrates, like the turtle and tiger salamander. In these reports, it was revealed that GABA is released from HCs through reverse transport and target GABA receptors are located at the cone terminals. In mammalian retinas, there is growing evidence that HCs can release GABA through conventional vesicular transmission, acting both on autaptic GABA receptors and on receptors expressed at the dendritic tips of the bipolar cells. The presence of GABA receptors on mammalian cone terminals remains equivocal. Here, we looked specifically for functional GABA receptors in mouse photoreceptors by recording in the whole-cell or amphotericin/gramicidin-perforated patch clamp configurations. Cones could be differentiated from rods through morphological criteria. Local GABA applications evoked a Cl- current in cones but not in rods. It was blocked by the GABAA receptor antagonist bicuculline methiodide and unaffected by the GABAC receptor antagonist TPMPA [(1,2,5,6-tetrahydropyridin-4-yl)methylphosphinic acid]. The voltage dependency of the current amplitude was as expected from a direct action of GABA on cone pedicles but not from an indirect modulation of cone currents following the activation of the GABA receptors of HCs. This supports a direct role of GABA released from HCs in the control of cone activity in the mouse retina.

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.

Inhibitory mechanisms that generate centre and surround properties in ON and OFF brisk-sustained ganglion cells in the rabbit retina

Lateral inhibition produces the centre-surround organization of retinal receptive fields, in which inhibition driven by the mean luminance enhances the sensitivity of ganglion cells to spatial and temporal contrast. Surround inhibition is generated in both synaptic layers; however, the synaptic mechanisms within the inner plexiform layer are not well characterized within specific classes of retinal ganglion cell. Here, we compared the synaptic circuits generating concentric centre-surround receptive fields in ON and OFF brisk-sustained ganglion cells (BSGCs) in the rabbit retina. We first characterized the synaptic inputs to the centre of ON BSGCs, for comparison with previous results from OFF BSGCs. Similar to wide-field ganglion cells, the spatial extent of the excitatory centre and inhibitory surround was larger for the ON than the OFF BSGCs. The results indicate that the surrounds of ON and OFF BSGCs are generated in both the outer and the inner plexiform layers. The inner plexiform layer surround inhibition comprised GABAergic suppression of excitatory inputs from bipolar cells. However, ON and OFF BSGCs displayed notable differences. Surround suppression of excitatory inputs was weaker in ON than OFF BSGCs, and was mediated largely by GABA(C) receptors in ON BSGCs, and by both GABA(A) and GABA(C) receptors in OFF BSGCs. Large ON pathway-mediated glycinergic inputs to ON and OFF BSGCs also showed surround suppression, while much smaller GABAergic inputs showed weak, if any, spatial tuning. Unlike OFF BSGCs, which receive strong glycinergic crossover inhibition from the ON pathway, the ON BSGCs do not receive crossover inhibition from the OFF pathway. We compare and discuss possible roles for glycinergic inhibition in the two cell types.

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.

Chromatic bipolar cell pathways in the mouse retina

Like most mammals, mice feature dichromatic color vision based on short (S) and middle (M) wavelength-sensitive cone types. It is thought that mammals share a retinal circuit that in dichromats compares S- and M-cone output to generate blue/green opponent signals, with bipolar cells (BCs) providing separate chromatic channels. Although S-cone-selective ON-BCs (type 9 in mouse) have been anatomically identified, little is known about their counterparts, the M-cone-selective OFF-BCs. Here, we characterized cone connectivity and light responses of selected mouse BC types using immunohistochemistry and electrophysiology. Our anatomical data indicate that four (types 2, 3a/b, and 4) of the five mouse OFF-BCs indiscriminately contact both cone types, whereas type 1 BCs avoid S-cones. Light responses showed that the chromatic tuning of the BCs strongly depended on their position along the dorsoventral axis because of the coexpression gradient of M- and S-opsin found in mice. In dorsal retina, where coexpression is low, most type 2 cells were green biased, with a fraction of cells (≈ 14%) displaying strongly blue-biased responses, likely reflecting S-cone input. Type 1 cells were also green biased but did not comprise blue-biased "outliers," consistent with type 1 BCs avoiding S-cones. We therefore suggest that type 1 represents the green OFF pathway in mouse. In addition, we confirmed that type 9 BCs display blue-ON responses. In ventral retina, all BC types studied here displayed similar blue-biased responses, suggesting that color vision is hampered in ventral retina. In conclusion, our data support an antagonistically organized blue/green circuit as the common basis for mammalian dichromatic color vision.

Immunocytochemical evidence that monkey rod bipolar cells use GABA

Certain bipolar cells in most species immunostain for GABA or its synthesizing enzyme glutamic acid decarboxylase. However, it is unknown whether they actually release GABA and, if so, from which cellular compartment and by what release mechanism. We investigated these questions in monkey retina where rod bipolar cells immunostain for GABA. We found that rod bipolar cells immunostain for one isoform of GAD (GAD65) in their somas, dendrites and axon terminals. Near the fovea, the somatic stain of rod bipolar cells is weaker than that of horizontal cells but, at the periphery, it is stronger. Staining for the vesicular GABA transporter in monkey rod bipolar cells is negative. However, staining for the GABA transporter GAT3 is positive in the soma and primary dendrites (but not in the axon terminals). Staining for GAT3 is also positive in horizontal cells. Double staining of rod bipolar cells and the alpha subunit of the GABAA receptor reveals scarce GABAA puncta that appose rod bipolar dendrites. We conclude that monkey rod bipolar cells use GABA and discuss the possibility that they tonically release GABA from their dendrites using a reverse action of GAT3.

Assembly of the outer retina in the absence of GABA synthesis in horizontal cells

BACKGROUND

The inhibitory neurotransmitter gamma-amino-butyric acid (GABA) not only modulates excitability in the mature nervous system but also regulates neuronal differentiation and circuit development. Horizontal cells, a subset of interneurons in the outer retina, are transiently GABAergic during the period of cone photoreceptor synaptogenesis. In rodents, both horizontal cells and cone axonal terminals express GABAA receptors. To explore the possibility that transient GABA expression in mouse neonatal horizontal cells influences the structural development of synaptic connectivity in the outer retina, we examined a mutant in which expression of GAD67, the major synthesizing enzyme for GABA, is selectively knocked out in the retina.

RESULTS

Immunocytochemistry and electron microscopy revealed that the assembly of triad synapses involving cone axonal pedicles and the dendrites of horizontal and bipolar cells is unaffected in the mutant retina. Moreover, loss of GABA synthesis in the outer retina did not perturb the spatial distributions and cell densities of cones and horizontal cells. However, there were some structural alterations at the cellular level: the average size of horizontal cell dendritic clusters was larger in the mutant, and there was also a small but significant increase in cone photoreceptor pedicle area. Moreover, metabotropic glutamate receptor 6 (mGluR6) receptors on the dendrites of ON bipolar cells occupied a slightly larger proportion of the cone pedicle in the mutant.

CONCLUSIONS

Together, our analysis shows that transient GABA synthesis in horizontal cells is not critical for synapse assembly and axonal and dendritic lamination in the outer retina. However, pre- and postsynaptic structures are somewhat enlarged in the absence of GABA in the developing outer retina, providing for a modest increase in potential contact area between cone photoreceptors and their targets. These findings differ from previous results in which pharmacological blockade of GABAA receptors in the neonatal rabbit retina caused a reduction in cone numbers and led to a grossly disorganized outer retina.

Guinea pig horizontal cells express GABA, the GABA-synthesizing enzyme GAD 65, and the GABA vesicular transporter

Gamma-aminobutyric acid (GABA) is likely expressed in horizontal cells of all species, although conflicting physiological findings have led to considerable controversy regarding its role as a transmitter in the outer retina. This study has evaluated key components of the GABA system in the outer retina of guinea pig, an emerging retinal model system. The presence of GABA, its rate-limiting synthetic enzyme glutamic acid decarboxylase (GAD(65) and GAD(67) isoforms), the plasma membrane GABA transporters (GAT-1 and GAT-3), and the vesicular GABA transporter (VGAT) was evaluated by using immunohistochemistry with well-characterized antibodies. The presence of GAD(65) mRNA was also evaluated by using laser capture microdissection and reverse transcriptase-polymerase chain reaction. Specific GABA, GAD(65), and VGAT immunostaining was localized to horizontal cell bodies, as well as to their processes and tips in the outer plexiform layer. Furthermore, immunostaining of retinal whole mounts and acutely dissociated retinas showed GAD(65) and VGAT immunoreactivity in both A-type and B-type horizontal cells. However, these cells did not contain GAD(67), GAT-1, or GAT-3 immunoreactivity. GAD(65) mRNA was detected in horizontal cells, and sequencing of the amplified GAD(65) fragment showed approximately 85% identity with other mammalian GAD(65) mRNAs. These studies demonstrate the presence of GABA, GAD(65), and VGAT in horizontal cells of the guinea pig retina, and support the idea that GABA is synthesized from GAD(65), taken up into synaptic vesicles by VGAT, and likely released by a vesicular mechanism from horizontal cells.

Immunocytochemical evidence for SNARE protein-dependent transmitter release from guinea pig horizontal cells

Horizontal cells are lateral interneurons that participate in visual processing in the outer retina but the cellular mechanisms underlying transmitter release from these cells are not fully understood. In non-mammalian horizontal cells, GABA release has been shown to occur by a non-vesicular mechanism. However, recent evidence in mammalian horizontal cells favors a vesicular mechanism as they lack plasmalemmal GABA transporters and some soluble NSF attachment protein receptor (SNARE) core proteins have been identified in rodent horizontal cells. Moreover, immunoreactivity for GABA and the molecular machinery to synthesize GABA have been found in guinea pig horizontal cells, suggesting that if components of the SNARE complex are expressed they could contribute to the vesicular release of GABA. In this study we investigated whether these vesicular and synaptic proteins are expressed by guinea pig horizontal cells using immunohistochemistry with well-characterized antibodies to evaluate their cellular distribution. Components of synaptic vesicles including vesicular GABA transporter, synapsin I and synaptic vesicle protein 2A were localized to horizontal cell processes and endings, along with the SNARE core complex proteins, syntaxin-1a, syntaxin-4 and synaptosomal-associated protein 25 (SNAP-25). Complexin I/II, a cytosolic protein that stabilizes the activated SNARE fusion core, strongly immunostained horizontal cell soma and processes. In addition, the vesicular Ca(2+)-sensor, synaptotagmin-2, which is essential for Ca(2+)-mediated vesicular release, was also localized to horizontal cell processes and somata. These morphological findings from guinea pig horizontal cells suggest that mammalian horizontal cells have the capacity to utilize a regulated Ca(2+)-dependent vesicular pathway to release neurotransmitter, and that this mechanism may be shared among many mammalian species.

Robust syntaxin-4 immunoreactivity in mammalian horizontal cell processes

Horizontal cells mediate inhibitory feed-forward and feedback communication in the outer retina; however, mechanisms that underlie transmitter release from mammalian horizontal cells are poorly understood. Toward determining whether the molecular machinery for exocytosis is present in horizontal cells, we investigated the localization of syntaxin-4, a SNARE protein involved in targeting vesicles to the plasma membrane, in mouse, rat, and rabbit retinae using immunocytochemistry. We report robust expression of syntaxin-4 in the outer plexiform layer of all three species. Syntaxin-4 occurred in processes and tips of horizontal cells, with regularly spaced, thicker sandwich-like structures along the processes. Double labeling with syntaxin-4 and calbindin antibodies, a horizontal cell marker, demonstrated syntaxin-4 localization to horizontal cell processes; whereas, double labeling with PKC antibodies, a rod bipolar cell (RBC) marker, showed a lack of co-localization, with syntaxin-4 immunolabeling occurring just distal to RBC dendritic tips. Syntaxin-4 immunolabeling occurred within VGLUT-1-immunoreactive photoreceptor terminals and underneath synaptic ribbons, labeled by CtBP2/RIBEYE antibodies, consistent with localization in invaginating horizontal cell tips at photoreceptor triad synapses. Vertical sections of retina immunostained for syntaxin-4 and peanut agglutinin (PNA) established that the prominent patches of syntaxin-4 immunoreactivity were adjacent to the base of cone pedicles. Horizontal sections through the OPL indicate a one-to-one co-localization of syntaxin-4 densities at likely all cone pedicles, with syntaxin-4 immunoreactivity interdigitating with PNA labeling. Pre-embedding immuno-electron microscopy confirmed the subcellular localization of syntaxin-4 labeling to lateral elements at both rod and cone triad synapses. Finally, co-localization with SNAP-25, a possible binding partner of syntaxin-4, indicated co-expression of these SNARE proteins in the same subcellular compartment of the horizontal cell. Taken together, the strong expression of these two SNARE proteins in the processes and endings of horizontal cells at rod and cone terminals suggests that horizontal cell axons and dendrites are likely sites of exocytotic activity.

Distribution of plasma membrane-associated syntaxins 1 through 4 indicates distinct trafficking functions in the synaptic layers of the mouse retina

Background

Syntaxins 1 through 4 are SNAP receptor (SNARE) proteins that mediate vesicular trafficking to the plasma membrane. In retina, syntaxins 1 and 3 are expressed at conventional and ribbon synapses, respectively, suggesting that synaptic trafficking functions differ among syntaxin isoforms. To better understand syntaxins in synaptic signaling and trafficking, we further examined the cell- and synapse-specific expression of syntaxins 1 through 4 in the mouse retina by immunolabeling and confocal microscopy.

Results

Each isoform was expressed in the retina and showed a unique distribution in the synaptic layers of the retina, with little or no colocalization of isoforms. Syntaxin 1 was present in amacrine cell bodies and processes and conventional presynaptic terminals in the inner plexiform layer (IPL). Syntaxin 2 was present in amacrine cells and their processes in the IPL, but showed little colocalization with syntaxin 1 or other presynaptic markers. Syntaxin 3 was found in glutamatergic photoreceptor and bipolar cell ribbon synapses, but was absent from putative conventional glutamatergic amacrine cell synapses. Syntaxin 4 was localized to horizontal cell processes in the ribbon synaptic complexes of photoreceptor terminals and in puncta in the IPL that contacted dopaminergic and CD15-positive amacrine cells. Syntaxins 2 and 4 often were apposed to synaptic active zones labeled for bassoon.

Conclusion

These results indicate that each syntaxin isoform has unique, non-redundant functions in synaptic signaling and trafficking. Syntaxins 1 and 3 mediate presynaptic transmitter release from conventional and ribbon synapses, respectively. Syntaxins 2 and 4 are not presynaptic and likely mediate post-synaptic trafficking.

An ionotropic GABA receptor with novel pharmacology at bullfrog cone photoreceptor terminals

Characteristics of ionotropic gamma-aminobutyric acid (GABA) receptors at bullfrog cone terminals were studied by patch clamp techniques in isolated cell and retinal slice preparations. GABA-induced inward currents from isolated cones reversed in polarity at a potential, very close to the chloride equilibrium potential, and they were completely suppressed by picrotoxin. Unexpectedly, the GABA current was dose-dependently potentiated by the well-known GABA(A) receptor antagonist bicuculline (BIC), but was suppressed by gabazine, another GABA(A) antagonist, and imidazole-4-acetic acid (I4AA), a GABA(C) receptor antagonist. Similarly, currents induced by both GABA(A) agonist muscimol and GABA(C) agonist cis-4-aminocrotonic acid (CACA) were also potentiated by BIC. Furthermore, currents induced from cones by GABA and kainate-caused depolarization of horizontal cells in retinal slice preparations were both potentiated by BIC. All these results suggest that the ionotropic GABA receptor at the bullfrog cone terminal exhibits novel pharmacology, distinct from both traditional GABA(A) and GABA(C) receptors.

Depolarizing effect of GABA in rod bipolar cells of the mouse retina

Gamma-amino butyric acid (GABA) has been characterized as inhibitory neurotransmitter through chloride mediated channels in the adult nervous system. However, using gramicidin perforated patch-clamp recordings from rod bipolar cells dissociated from retinas of adult mice, we find that GABA is capable of inducing cell depolarization. Currents mediated by GABA(A) and GABA(C) receptors were further isolated by the use of GABA receptor specific blockers. In rod bipolar cells dissociated from the mouse retina, activation of GABA(A) receptors located at the cell dendrites induces ionic currents which show a reversal potential of -33 mV. However, local activation of GABA(C) receptors located at the axon terminal induces ionic currents with a reversal potential of -60 mV. According to Nernst equation, the dendrites of rod bipolar cells of the mouse retina would have a high intracellular chloride concentration ([Cl(-)](i)) and there must be an intracellular gradient in [Cl(-)](i), being the [Cl(-)](i) more elevated in the dendrites than in the axon terminal. The depolarizing effect of GABA at the dendrites of rod bipolar cells may contribute to the lateral interaction in the mammalian retina, thereby enhancing visual discrimination of stimuli input.

Depolarizing effect of GABA in horizontal cells of the rabbit retina

Gamma-amino butyric acid (GABA) has been characterized as an inhibitory neurotransmitter acting through chloride mediated channels in the adult nervous system. Using gramicidin-perforated patch clamp recordings from horizontal cells dissociated from the retinas of adult rabbits, we found that GABA is able to induce cell depolarization. Ionic currents induced by GABA in dissociated horizontal cells showed a reversal potential close to -30 mV. This value is more positive than the resting potential of these cells (ca. -70 mV). Therefore, according to the Nernst equation, the intracellular chloride concentration in horizontal cells was estimated to be of 44 mM. The depolarizing effect of GABA at the dendrites of horizontal cells may serve to shape the center-surround organization of the receptive fields in retinal cells, thereby securing the shape discrimination of visual 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.

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.

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.

Cellular localization of the vesicular inhibitory amino acid transporter in the mouse and human retina

Horizontal cells are classically thought to mediate lateral inhibition by gamma-aminobutyric acid (GABA)-transporter mediated release. In the mammalian retina, however, GABA uptake and cloned GABA transporter were not detected in horizontal cells. Furthermore, the vesicular inhibitory amino acid transporter (VIAAT or VGAT) that loads GABA and glycine into synaptic vesicles was reported recently to be expressed in horizontal cells. To further assess synaptic transmission in mammalian horizontal cells, we examined the subcellular distribution of VIAAT in mouse and human retina by confocal microscopy with specific cell markers. VIAAT was observed in the mouse outer plexiform layer as punctate structures that localized in calbindin-positive horizontal cells. These structures were in close apposition with synaptophysin-, PSD-95-, dystrophin-, and bassoon-immunopositive photoreceptor terminals, suggesting that VIAAT is localized in horizontal cell tips at photoreceptor terminals. VIAAT-positive puncta were also in apposition to lectin-labeled cone terminals or dendrites of PKCalpha-immunopositive rod bipolar cells, indicating that VIAAT is expressed in horizontal cell tips at both rod and cone terminals. By contrast, only a very few puncta were observed in the human outer plexiform layer, whereas the inner plexiform layer remained labeled as in the mouse retina. When using adult human retinal cells in culture, horizontal cells identified by parvalbumin immunostaining were found to contain VIAAT, either at their terminals or throughout the entire cell similarly as in syntaxin-immunopositive cells. These differences between human retinal tissue and cultured cells were attributed to VIAAT degradation in postmortem retinal tissue. VIAAT localization in mouse and human horizontal cells further support the role of inhibitory transmitters in lateral inhibition at the photoreceptor terminals.

GABA transporters regulate inhibition in the retina by limiting GABA(C) receptor activation

Inhibition is mediated by two classes of ionotropic receptors in the retina, GABA(A) and GABA(C) receptors. We used the GABA transport blocker NO-711 to examine the role of GABA transporters in shaping synaptic responses mediated by these two receptors in the salamander retinal slice preparation. Focal applications (puffs) of GABA onto GABA(C) receptors on bipolar cells terminals or GABA(A) receptors on ganglion cells elicited currents that were enhanced by NO-711, demonstrating the presence of transporters in the inner plexiform layer (IPL). IPSCs were evoked in bipolar and ganglion cells by puffing kainate into the IPL. NO-711 enhanced the IPSCs only in bipolar cells, suggesting that, when GABA uptake was blocked, the GABA(C) receptors were more strongly activated by spillover transmission than the GABA(A) receptors on ganglion cells. NO-711 enhanced the light-evoked IPSCs mediated by GABA(C) receptors on bipolar cell axon terminals, which resulted in reduced transmission between bipolar and ganglion cells. NO-711 also shifted the intensity-response relationship of the ganglion cell, reducing its sensitivity to light. Surround illumination has been shown by others to produce similar shifts in ganglion cell light sensitivity. Our results show that GABA transporters limit the extent of inhibitory transmission at the inner retina during light-evoked signal processing.

Vesicular gamma-aminobutyric acid transporter expression in amacrine and horizontal cells

The vesicular gamma-aminobutyric acid (GABA) transporter (VGAT), which transports the inhibitory amino acid transmitters GABA and glycine, is localized to synaptic vesicles in axon terminals. The localization of VGAT immunoreactivity to mouse and rat retina was evaluated with light and electron microscopy by using well-characterized VGAT antibodies. Specific VGAT immunoreactivity was localized to numerous varicose processes in all laminae of the inner plexiform layer (IPL) and to the outer plexiform layer (OPL). Amacrine cell somata characterized by weak VGAT immunoreactivity in the cytoplasm were located in the ganglion cell layer and proximal inner nuclear layer (INL) adjacent to the IPL. In rat retina, VGAT-immunoreactive cell bodies also contained GABA, glycine, or parvalbumin (PV) immunoreactivity, suggesting vesicular uptake of GABA or glycine by these cells. A few varicose VGAT-immunoreactive processes entered the OPL from the IPL. VGAT immunoreactivity in the OPL was predominantly localized to horizontal cell processes. VGAT and calcium binding protein-28K immunoreactivities (CaBP; a marker for horizontal cells) were colocalized in processes and terminals distributed to the OPL. Furthermore, VGAT immunoreactivity overlapped or was immediately adjacent to postsynaptic density-95 (PSD-95) immunoreactivity, which is prominent in photoreceptor terminals. Preembedding immunoelectron microscopy of mouse and rat retinae showed that VGAT immunoreactivity was localized to horizontal cell processes and their terminals. Immunoreactivity was distributed throughout the cytoplasm of the horizontal cell processes. Taken together, these findings demonstrate VGAT immunoreactivity in both amacrine and horizontal cell processes, suggesting these cells contain vesicles that accumulate GABA and glycine, possibly for vesicular release.

Evidence that different cation chloride cotransporters in retinal neurons allow opposite responses to GABA

GABA gating an anion channel primarily permeable to chloride can hyperpolarize or depolarize, depending on whether the chloride equilibrium potential (E(Cl)) is negative or positive, respectively, to the resting membrane potential (E(rest)). If the transmembrane Cl(-) gradient is set by active transport, those neurons or neuronal regions that exhibit opposite responses to GABA should express different chloride transporters. To test this, we immunostained retina for the K-Cl cotransporter (KCC2) that normally extrudes chloride and for the Na-K-Cl cotransporter (NKCC) that normally accumulates chloride. KCC2 was expressed wherever E(Cl) is either known or predicted to be negative to E(rest) (ganglion cells, bipolar axons, and OFF bipolar dendrites), whereas NKCC was expressed wherever E(Cl) is either known or predicted to be positive to E(rest) (horizontal cells and ON bipolar dendrites). Thus, in the retina, the opposite effects of GABA on different cell types and on different cellular regions are probably primarily determined by the differential targeting of these two chloride transporters.

Opposite effects of GABA(A) and GABA(C) receptor antagonists on the b-wave of ERG recorded from the isolated rat retina

The largest component in the fully dark-adapted ERG is a corneal-positive response, known as the b-wave, and believed to originate from depolarizing (ON-type) bipolar cells. The two types of GABA receptors, GABA(A) and GABA(C) have been reported to exist on bipolar cells in rat retina. The goal of these experiments was to find whether these GABA receptors participate in the generation of the b-wave of electroretinogram (ERG). ERGs were recorded from the isolated rat retinas. The P(2)(t) component, obtained by subtracting the ERGs measured before the application of 50 micrograms APB from those measured after the application of 50 micrograms APB, was used as an indicator of depolarizing bipolar cell activity. Photovoltages, the fast P(3)(t) component of ERG, were registered between the two microelectrodes across the rod outer segments. Bicuculline and 3-aminopropylphosphonic acid (3-APA) were used as selective antagonists of GABA(A) and GABA(C) receptors, respectively. It was found that the GABA(A) and GABA(C) receptors antagonists have opposite effects on the b-wave: bicuculline increased the b-wave amplitude, while 3-APA reduced the amplitude of the b-wave. Neither bicuculline nor 3-APA affect photoreceptors.

Center surround receptive field structure of cone bipolar cells in primate retina

In non-mammalian vertebrates, retinal bipolar cells show center-surround receptive field organization. In mammals, recordings from bipolar cells are rare and have not revealed a clear surround. Here we report center-surround receptive fields of identified cone bipolar cells in the macaque monkey retina. In the peripheral retina, cone bipolar cell nuclei were labeled in vitro with diamidino-phenylindole (DAPI), targeted for recording under microscopic control, and anatomically identified by intracellular staining. Identified cells included 'diffuse' bipolar cells, which contact multiple cones, and 'midget' bipolar cells, which contact a single cone. Responses to flickering spots and annuli revealed a clear surround: both hyperpolarizing (OFF) and depolarizing (ON) cells responded with reversed polarity to annular stimuli. Center and surround dimensions were calculated for 12 bipolar cells from the spatial frequency response to drifting, sinusoidal luminance modulated gratings. The frequency response was bandpass and well fit by a difference of Gaussians receptive field model. Center diameters were all two to three times larger than known dendritic tree diameters for both diffuse and midget bipolar cells in the retinal periphery. In one instance intracellular staining revealed tracer spread between a recorded cell and its nearest neighbors, suggesting that homotypic electrical coupling may contribute to receptive field center size. Surrounds were around ten times larger in diameter than centers and in most cases the ratio of center to surround strength was approximately 1. We suggest that the center-surround receptive fields of the major primate ganglion cell types are established at the bipolar cell, probably by the circuitry of the outer retina.

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.

The cone pedicle, a complex synapse in the retina

Cone pedicles, the synaptic terminals of cone photoreceptors, are connected in the macaque monkey retina to several hundred postsynaptic dendrites. Using light and electron microscopy, we found underneath each cone pedicle a laminated distribution of dendritic processes of bipolar and horizontal cells. Superimposed were three strata of glutamate receptor (GluR) aggregates, including a novel layer of glutamate receptors clustered at desmosome-like junctions. They are, most likely, postsynaptic densities on horizontal cell dendrites. GABA(A) and GABA(C) receptors are aggregated on bipolar cell dendrites in a narrow band underneath the cone pedicle. Glutamate released from cone pedicles and GABA released from horizontal cell dendrites act not only through direct synaptic contacts but also (more so) through diffusion to the appropriate receptors.

Localization and developmental expression patterns of the neuronal K-Cl cotransporter (KCC2) in the rat retina

The processing of signals by integrative neurons in the retina and CNS relies strongly on inhibitory synaptic inputs, principally from GABAergic and glycinergic neurons that serve primarily to hyperpolarize postsynaptic neurons. Recent evidence indicates that the neuron-specific K-Cl cotransporter 2 (KCC2) is the major chloride extrusion system permitting hyperpolarizing inhibitory responses. It has been hypothesized that depolarizing GABA responses observed in immature neurons are converted to hyperpolarizing responses in large part by the expression of KCC2 during the second week of postnatal development. The cell-specific localization and developmental expression of KCC2 protein have been examined in relatively few neural tissues and have never been studied in retina, of which much is known physiologically and morphologically about inhibitory synaptic circuits. We examined the localization of KCC2 in adult rat retina with immunohistochemical techniques and determined the time course of its postnatal expression. KCC2 expression was localized in horizontal cells, bipolar cells, amacrine cells, and, most likely, ganglion cells, all of which are known to express GABA receptor subtypes. Developmentally, KCC2 expression in the retina increased gradually from postnatal day 1 (P1) until P14 in the inner retina, whereas expression was delayed in the outer plexiform layer until P7 but reached its adult level by P14. These data support the hypothesis that the function of KCC2 is intimately involved in GABAergic synaptic processing. Furthermore, the delayed temporal expression of KCC2 in the outer plexiform layer indicates that GABAergic function may be differentially regulated in retina during postnatal development and that GABA may produce depolarizing responses in the outer plexiform layer at times when it generates hyperpolarizing responses in the inner plexiform layer.

Functional architecture of primate cone and rod axons

The cone axon is nearly four times thicker than the rod axon (1.6 vs 0.45 microns diameter). To assess how signal transfer and integration at the terminal depend on cable dimensions, a transducer (cone = ohmic conductance, rod = current source) coupled via passive cable to a sphere with a chloride conductance (representing GABAA receptor) was modelled. For a small signal in peripheral cone with a short axon, steady photosignal transfers independently of axon diameter despite a significant chloride conductance at the cone terminal. Temporally varying photosignal also transfers independently of axon diameter up to 20 Hz and is attenuated only 20% at 50 Hz. Thus, to accomplish the basic electrical functions of a peripheral cone, a thin axon would suffice. For a foveal cone with a long axon steady photosignal transfers independently of axon diameter, but temporally varying photosignal is attenuated 5-fold at 50 Hz for a thick axon and 10-fold for a thin axon. This might contribute to the lower sensitivity of central retina to high temporal frequencies. The cone axon contains 14-fold more microtubules than the rod axon, and its terminal contains at least 20-fold more ribbon synapses than the rod's. Since ribbon synapses sustain high rates of exocytosis, the additional microtubules (which require a thicker axon) may be needed to support a greater flux of synaptic vesicle components.

Neurochemistry of the mammalian cone 'synaptic complex'

The cone 'synaptic complex' is a unique structure in which a single presynaptic axon secretes glutamate onto processes of bipolar cells (both ON and OFF) and horizontal cells. In turn, the horizontal cell processes antagonize cone and bipolar responses to glutamate (probably by GABA). What still remains largely unknown is the molecular identity of the postsynaptic receptors and their exact locations. We identified several subunits of the glutamate receptor and the GABAA receptor expressed at the cone synaptic complex and localized them ultrastructurally. Glutamate receptors: (i) Invaginating (probably ON) bipolar dendrites in the monkey and rat express the metabotropic glutamate receptor, mGluR6. The stain is intense on the dendritic membrane where it first enters the invagination, and weak at the tip nearest to the ribbon. The cone membrane is electron-dense where it apposes the intense stain for mGluR6. Surprisingly, invaginating bipolar dendrites in the cat also express the AMPA receptor subunits, GluR2/3 and GluR4. (ii) Dendrites forming basal contacts in the cat (probably OFF) express the AMPA subunits GluR2/3, GluR4, and also the kainate subunit, GluR6/7. The stain is especially intense at the dendritic tips in apposition to electron-dense regions of cone membrane. (iii) Horizontal cells in the cat express the AMPA subunits GluR2/3, GluR4 and the kainate subunit, GluR6/7. The stain is strongest in the cytosol of somas and primary dendrites, but is also present in the invaginating terminals where it localizes to the membrane subjacent to the ribbon. GABAA receptors: (i) ON and OFF bipolar dendrites in the monkey express the alpha 1 and beta 2/3 subunits. The stain is localized to the bipolar cell membrane in apposition to horizontal cell processes. (ii) Cones did not express the GABAA subunits tested by immunocytochemistry, but beta 3 mRNA was amplified by RT-PCR from rat photoreceptors.

CONCLUSIONS

(i) mGluR6 receptors concentrate on dendrites at the base of the invagination rather than at the apex. This implies that receptors at both 'invaginating' and 'basal' contacts lie roughly equidistant from the release sites and should therefore receive similar spatiotemporal concentrations of glutamate. (ii) The 'cone' membrane is electron-dense opposite to the receptor sites on both ON and OFF bipolar cells. This suggests a special role for this region in synaptic transmission. Possibly, these densities signify a transporter that would regulate glutamate concentration at sites remote (> 200 nm) from the locus of vesicle release.

Glycine and GABA receptors in the mammalian retina

Molecular cloning has introduced an unexpected diversity of neurotransmitter receptors. In this study we review the types, the localization and possible synaptic function of the inhibitory neurotransmitter receptors in the mammalian retina. Glycine receptors (GlyRs) and their localization in the mammalian retina were analyzed immunocytochemically. Specific antibodies against the alpha 1 subunit of the GlyR (mAb2b) and against all subunits of the GlyR (mAb4a) were used. Both antibodies produced a punctate immunofluorescence, which was shown by electron microscopy to represent clustering of GlyRs at synaptic sites. Synapses expressing the alpha 1 subunit of the GlyR were found on ganglion cell dendrites and on bipolar cell axons. GlyRs were also investigated in the oscillator mutant mouse. The complete loss of the alpha 1 subunit was compensated for by an apparent upregulation of the other subunits of the GlyR. GABAA receptors (GABAARs) and their retinal distribution were studied with specific antibodies that recognize the alpha 1, alpha 2, alpha 3, beta 1, beta 2, beta 3, gamma 2 and delta subunits. Most antibodies produced a punctate immunofluorescence in the inner plexiform layer (IPL) which was shown by electron microscopy to represent synaptic clustering of GABAARs. The density of puncta varied across the IPL and different subunits were found in characteristic strata. This stratification pattern was analyzed with respect to the ramification of cholinergic amacrine cells. Using intracellular injection with Lucifer yellow followed by immunofluorescence, we found that GABAARs composed of different subunits were expressed by the same ganglion cell, however, they were clustered at different synaptic sites. The distribution of GABAC receptors was studied in the mouse and in the rabbit retina using an antiserum that recognizes the rho 1, rho 2 and rho 3 subunits. GABAC receptors were found to be clustered at postsynaptic sites. Most, if not all of the synapses were found on rod and cone bipolar axon terminals. In conclusion we find a great diversity of glycine and GABA receptors in the mammalian retina, which might match the plethora of morphological types of amacrine cells. This may also point to subtle differences in synaptic function still to be elucidated.

Synaptic clustering of GABA(C) receptor rho-subunits in the rat retina

Polyclonal antibodies which recognize the rho-subunits of the GABA(C) receptor were applied to sections of the rat retina. Strong punctate immunoreactivity was found in the inner plexiform layer (IPL), which was shown by electron microscopy to represent a clustering of the GABA(C) receptors at synaptic sites. During postnatal development diffuse rho-immunoreactivity was first observed at postnatal day P3. Distinct labelling of bipolar cells appeared at P7 and punctate, synaptic labelling was observed at P10. In order to show that the rho-immunoreactive puncta coincide with the axons of bipolar cells, double immunostainings of retinal sections with an antiserum against syntaxin 3 and with the rho-antiserum were performed. The experiments showed that rho-immunoreactive puncta are preferentially located on the axon terminals of rod and cone bipolar cells. In order to determine whether GABA(C) receptor rho-subunits coassemble with GABA(A) receptor subunits, double-labelling experiments were performed with subunit specific antisera. Punctate, putative synaptic clustering was observed with all antisera applied, however, GABA(C) receptor expressing puncta did not coincide with GABA(A) receptor containing puncta. This suggests that there are no synaptic GABA receptors in the retina in which GABA(A) and GABA(C) receptor subunits are coassembled. Similar double-labelling experiments were also performed to find out whether GABA(C) receptors and glycine receptors are colocalized. They were clustered at different synapses. This suggests that synaptic GABA(C) receptors consist of rho-subunits and are not coassembled with GABA(A)- or glycine-receptor subunits.

GABA-gated Cl- channels in the rat retina

gamma-Aminobutyric acid (GABA) is a major inhibitory neurotransmitter in the mammalian retina, and its physiological action is well established. GABA receptors have been localized immunocytochemically in the retina of different mammalian species, and all major retinal cell types have been found to express GABAA receptor subunits. Recently, a new type of GABA receptor with pharmacological and electrophysiological properties different from the known GABAA and GABAB receptors, has been described. These GABAC receptors are found predominantly in the vertebrate retina. This review concentrates on the electrophysiological characterization of GABA receptors expressed by amacrine and bipolar cells of the rat retina. We recorded GABA-induced currents from cultured neonatal amacrine and bipolar cells as well as from isolated bipolar cells of adult animals. While amacrine cells contain a homogeneous population of GABAA receptors, bipolar cells exhibit both GABAA and GABAC responses. Although both receptors gate chloride-selective ion channels, their biophysical and pharmacological properties differ markedly. These functional differences and the cellular distribution of GABAA and GABAC receptors suggest that they have different inhibitory functions in the rat retina.

Differences in the retinal GABA system among control, spastic mutant and retinal degeneration mutant mice

Immunocytochemical methods were used to compare the GABA system in control mice and two mutant strains: spastic which has reduced glycine receptors and retinal degeneration mutant in which the photoreceptors degenerate and reportedly have increased GABA and GAD levels. We found that the spastic mutant retina had reduced GABA-immunoreactivity (IR) in the proximal retina, reduced staining for GAD-1440 in the OPL, and reduced GABAA receptor staining in the OPL, compared to control. The retinal degeneration mutant retinas had enhanced GABA-IR throughout the retina, particularly in Müller cells, bipolar cells and IPL, and enhancement of GABAA receptor staining in the OPL, compared to control. The distributions of GABA-IR, GAD-1440-IR and GABAA receptor-IR in retinas of spastic mutant mice that also expressed the retinal degeneration phenotype resembled those found in retinas of mice that expressed only the retinal degeneration phenotype rather than those that expressed only the spastic mutation. No differences were observed among the conditions for GAD-65, GAD-67 or GABA-T. Our results with the spastic and retinal degeneration mutant mice demonstrate that attenuation in the glycinergic system and photoreceptor degeneration, respectively, is accompanied by alterations in different aspects of the GABA system, giving impetus for caution in the interpretation of experiments involving genetic manipulation of complex phenotypes.

Immunocytochemical localization of GABA, GABAA receptors, and synapse-associated proteins in the developing and adult ferret retina

Gamma-aminobutyric acid (GABA) modulates the pattern of correlated spontaneous bursting activity between amacrine cells and ganglion cells of the ferret retina during the first postnatal month. Here, we demonstrate the presence of an anatomical network which may underlie these interactions throughout the period when correlated bursting activity is observed, by immunolabelling the neonatal ferret retina for GABA, GABAA receptors, and synapse-associated proteins. GABA immunoreactivity was detected in cell somata in the ganglion cell layer (GCL), in amacrine cells, and in the inner plexiform layer (IPL) by embryonic day 38. This pattern remained largely unchanged throughout neonatal development and in the adult. By contrast to other mammals, the outer plexiform layer (OPL) was only very weakly labelled for GABA, at all ages studied. Strong, punctate, immunolabelling for the beta 2/3 subunit of the GABAA receptor was apparent in the IPL by birth, and appeared in the OPL by the second postnatal week. The possibility that synaptic interactions in the IPL occur during bursting activity was examined by immunolabelling for synapse-associated proteins. Strong immunoreactivity for synaptic vesicle proteins, Synapsin I and II, and synaptic vesicle-2 (SV2), a synaptic vesicle transporter protein, was observed in the IPL by birth. Immunoreactivity for SNAP-25, a protein associated with vesicle fusion, was also intense at the level of the IPL and in the nerve fiber layer of the retina at birth. Taken together, these patterns of immunoreactivity suggest the presence of a GABAergic network in the IPL of the ferret retina by birth, coinciding with the appearance of correlated bursting activity in the inner retina.

Hepatic retinopathia. Changes in retinal function

In patients suffering from hepatic failure, the brain is subject to defined morphological and functional changes known as hepatic encephalopathia (HE). The morphological changes are dominated by glial cells (Alzheimer-type II astrocytes). It has recently been possible to demonstrate, that the retinal glia (Müller) cells undergo similar morphological changes. The present study was carried out in order to reveal if these Müller cell changes cause any characteristic functional deficits. We examined 11 patients with different stages of HE due to liver cirrhosis. Six patients were at stage 0 or 1 (group I) and five at stage 2 or 3 (group II). They underwent ophthalmological routine examination, colour vision testing and standard ERG recording. None of the patients reported impaired vision, in daylight or at night. There were no fundus abnormalities except very mild changes of the pigment epithelium and abnormal reflexes of the inner limiting membrane, especially in the higher HE stages. The number of confusions in the colour arrangement test increased with the higher stages of HE, preferably in the tritan axis. The scotopic a- and b-waves of the electroretinogram (ERG) were almost unchanged in group I and significantly decreased and delayed in group II. The photopic ERG b-wave amplitudes were changed in a similar fashion. Oscillatory potentials proved to be most sensitive to hepatotoxic changes. Their latencies were significantly delayed even in group I. Amplitudes were decreased significantly only in group II. Patients suffering from hepatic failure and accompanying HE display functional abnormalities of the retina. These are best demonstrated by the ERG, and correlate well with the degree of HE. A hypothesis is presented that relates the observed functional changes to altered neurotransmitter levels and impaired retinal glial-neuronal interaction, due to Müller cell damage caused by elevated ammonia levels.

Immunocytochemical localization of the GABA(C) receptor rho subunits in the cat, goldfish, and chicken retina

Polyclonal antibodies against the N-terminus of the rat rho1 subunit were used to study the distribution of gamma-aminobutyric acid C (GABA(C)) receptors in the cat, goldfish, and chicken retina. Strong punctate immunoreactivity was present in the inner plexiform layer (IPL) of all three species. The punctate labelling suggests a clustering of the GABA(C) receptors at synaptic sites. Weak label was also found in the outer plexiform layer (OPL) and over the cell bodies of bipolar cells. Double immunostaining of vertical sections with an antibody against protein kinase C (PKC) showed the punctate immunofluorescence to colocalize with bipolar cell axon terminals. In the goldfish retina, the axon terminals of Mb1 bipolar cells were enclosed by rho-immunoreactive puncta. In the chicken retina, several distinct strata within the IPL showed a high density of rho-immunoreactive puncta. The results suggest a high degree of sequence homology between the rho subunits of different vertebrate species, and they show that the retinal localization of GABA(C) receptors is similar across different species.

The effects of GABA and glycine on horizontal cells of the rabbit retina

Intracellular and patch-clamp recordings have been used to characterize GABA-activated channels in axonless horizontal cells (ALHC) of the rabbit retina. In our intracellular recordings on an everted eyecup preparation, GABA depolarized the horizontal cells (HC), diminished their light response amplitude and slowed the response rise time. Glycine showed similar effects on the HC light responses. In our whole cell patch-clamp recordings on dissociated ALHC, all HCs responded to 3 microM GABA but none to glycine, even at 100 microM. Dose-response relationship for GABA gave EC50 values around 10 microM and Hill slopes of 1.3. Whole-cell current-voltage (I-V) relationships of GABA-activated currents reversed close to the predicted Cl- equilibrium potential. Partial replacement of intracellular Cl- with isothetionate shifted the GABA reversal potential to a more negative value. Muscimol (30 microM), a GABAA agonist mimicked the effect of GABA, but baclofen (30 microM), a GABAB agonist and cis-aminocaprionic acid (30 microM), a GABAC agonist did not elicit any effect on ALHC. Responses to GABA were blocked by the GABAA receptor antagonist bicuculline (10 microM) and picrotoxin (100 microM). According to our results, we conclude that ALHC express GABA receptors coupled to ion channels, and they correspond to GABAA receptor subtypes.

Immunocytochemical localization of the GABAc receptor rho subunits in the mammalian retina

Polyclonal antibodies against the N terminus of the rat rho 1 subunit were generated to study the distribution of GABAc receptors in the mammalian retina. The specificity of the antibodies was tested in Western blots and transfected HEK-293 cells. No cross-reactivity with the GABAA receptor subunits alpha 1-3, beta 1-3, gamma 2, delta or with the glycine receptor subunits alpha 1 and beta could be detected. In contrast, the rho 1, rho 2, and rho 3 subunits were all recognized by the antibodies. In vertical sections of rat, rabbit, cat, and macaque monkey retinae, strong punctate immunoreactivity was present in the inner plexiform layer. Weaker immunoreactivity was also present in the outer-plexiform layer, and cell bodies of bipolar cells were faintly labeled. Double immunostaining of vertical sections and immunostaining of dissociated rat retinae showed the punctate immunofluorescence to colocalize with bipolar cell axon terminals. The puncta possibly represent clustering of the rho subunits at postsynaptic sites.

From ion currents to genomic analysis: recent advances in GABAA receptor research

The gamma-aminobutyric acid type A (GABAA) receptor represents an elementary switching mechanism integral to the functioning of the central nervous system and a locus for the action of many mood- and emotion-altering agents such as benzodiazepines, barbiturates, steroids, and alcohol. Anxiety, sleep disorders, and convulsive disorders have been effectively treated with therapeutic agents that enhance the action of GABA at the GABAA receptor or increase the concentration of GABA in nervous tissue. The GABAA receptor is a multimeric membrane-spanning ligand-gated ion channel that admits chloride upon binding of the neurotransmitter GABA and is modulated by many endogenous and therapeutically important agents. Since GABA is the major inhibitory neurotransmitter in the CNS, modulation of its response has profound implications for brain functioning. The GABAA receptor is virtually the only site of action for the centrally acting benzodiazepines, the most widely prescribed of the anti-anxiety medications. Increasing evidence points to an important role for GABA in epilepsy and various neuropsychiatric disorders. Recent advances in molecular biology and complementary information derived from pharmacology, biochemistry, electrophysiology, anatomy and cell biology, and behavior have led to a phenomenal growth in our understanding of the structure, function, regulation, and evolution of the GABAA receptor. Benzodiazepines, barbiturates, steroids, polyvalent cations, and ethanol act as positive or negative modulators of receptor function. The description of a receptor gene superfamily comprising the subunits of the GABAA, nicotinic acetylcholine, and glycine receptors has led to a new way of thinking about gene expression and receptor assembly in the nervous system. Seventeen genetically distinct subunit subtypes (alpha 1-alpha 6, beta 1-beta 4, gamma 1-gamma 4, delta, p1-p2) and alternatively spliced variants contribute to the molecular architecture of the GABAA receptor. Mysteriously, certain preferred combinations of subunits, most notably the alpha 1 beta 2 gamma 2 arrangement, are widely codistributed, while the expression of other subunits, such as beta 1 or alpha 6, is severely restricted to specific neurons in the hippocampal formation or cerebellar cortex. Nervous tissue has the capacity to exert control over receptor number, allosteric uncoupling, subunit mRNA levels, and posttranslational modifications through cellular signal transduction mechanisms under active investigation. The genomic organization of the GABAA receptor genes suggests that the present abundance of subtypes arose during evolution through the duplication and translocations of a primordial alpha-beta-gamma gene cluster. This review describes these varied aspects of GABAA receptor research with special emphasis on contemporary cellular and molecular discoveries.

Functional role of GABA in cat retina: I. Effects of GABAA agonists

Putative GABAergic mechanisms were studied in perfused cat retina by means of intracellular recording and application of GABA and the GABAA agonists delta-amino valeric acid (dAVA), muscimol, and THIP. In contrast to results reported previously for cold-blooded vertebrates, introduction of 20 mM GABA into the superfusate had no influence upon the response properties of cat retinal horizontal cells (HCs). In common with results reported in cold-blooded vertebrates, introduction of the GABAA agonists dAVA (2-12 mM) and THIP or muscimol (0.2-1 mM) had four consistent reversible influences upon the response properties of cat retinal HCs: (1) they reduced photic-response amplitude, (2) slowed response onset, (3) slowed response offset, and (4) depolarized the dark membrane potential. Both rod and cone signal components were affected. GABAA agonists had similar influences upon both the time course and amplitude of responses recorded from amacrine and ganglion cells. In all cell types examined, the influence upon response kinetics was made particularly apparent with rapidly flickering stimuli. Flicker responses were reduced in amplitude much more than sustained responses. These results suggest that, in addition to other influences, GABAergic action serves to modify the time course of photic responses in both the inner and outer plexiform layer of mammalian retina making responses slower and less phasic.

GABAA receptor subunits have differential distributions in the rat retina: in situ hybridization and immunohistochemistry

The distributions of nine different subunits of the gamma-aminobutyric acidA (GABAA) receptor (alpha 1, alpha 2, alpha 3, alpha 5; beta 1, beta 2, beta 3; gamma 2; delta) were investigated in the rat retina using immunocytochemistry and in situ hybridization. With the exception of the alpha 5 subunit, all subunits could be localized. Each subunit was expressed in characteristic strata within the inner plexiform layer (IPL). Some subunits (e.g., gamma 2) showed a ubiquitous distribution, while others (e.g., delta) were restricted to narrow sublayers. Double labeling experiments using different combinations of the subunit-specific antibodies revealed colocalizations of subunits within individual neurons. Additionally, GABAA receptor subunits were mapped to distinct populations of retinal neurons by coapplication of defined immunocytochemical markers and subunit-specific antibodies. Cholinergic amacrine cells were found to express the alpha 2, beta 1, beta 2/3 and delta subunits, while dopaminergic amacrine cells express the alpha 2, alpha 3 and gamma 2 subunits. Dissociated rod bipolar cells express the alpha 1 and gamma 2 subunits. In summary, this study provides evidence for the existence of multiple GABAA receptor subtypes in the retina. The distinct stratification pattern of the subunits in the IPL suggests that different functional circuits involve specific subtypes of GABAA receptors.

Heterogeneity of GABAA receptor in goldfish retina

The possibility of GABAA receptor heterogeneity in goldfish retina was studied with immunocytochemical and biochemical approaches: 1) immunoblotted membrane particulates of goldfish retina with mAb 62-3G1; 2) immunoprecipitation of the detergent-solubilized membrane proteins with mAb 62-3G1 followed by the receptor binding assay; 3) photoaffinity labeling of the membrane particulates with 3H-flunitrazepam (FNZ) and visualization of the labeled receptors by SDS-PAGE and fluorography; 4) dry autoradiography of 3H-muscimol and 3H-FNZ binding sites on frozen sections. Immunoblots showed that 62-3G1 reacted with 55-57.5 kDa M(r) polypeptides, similar to the muscimol-binding subunit of the receptor complex in bovine brain; while 3H-FNZ photoaffinity labeled the 52.5 kDa and 41-43 kDa M(r) polypeptides. Immunoprecipitated receptors bound only 3H-muscimol, not 3H-FNZ. An attempt to precipitate the 3H-FNZ photolabeled polypeptides failed. Dry autoradiography showed 3H-FNZ binding only in the inner plexiform layer (IPL); the binding was enhanced with gamma-aminobutyric acid (GABA) and blocked by clonazepam. In contrast, 3H-muscimol was bound in both the outer plexiform layer (OPL) and IPL, similar to that observed with 62-3G1 immunocytochemistry. We suggest that there are two subtypes of GABAA receptor in the goldfish retina: 1) GABAA receptors that are not linked to a benzodiazepine (BZD) receptor are located in the OPL and at amacrine-to-amacrine and amacrine-to-ganglion cell synapses in the IPL and are recognized by 62-3G1; 2) GABAA receptors that are linked to a BZD receptor are located only in the IPL, largely at amacrine-to-bipolar cell synapses and are not recognized by mAb 62-3G1.

Subcellular localization of GABAA receptor on bipolar cells in macaque and human retina

The subcellular distribution of GABAA receptor in the macaque and human retina was studied by immunocytochemistry with monoclonal antibodies for the alpha and beta subunits with a particular focus on bipolar cells. Immunoreactivity to GABAA receptor was present on dendritic tips of all bipolar cells. The stain was strongest on bipolar membranes in apposition to horizontal cell processes. Stain was concentrated on the tips of flat and invaginating cone bipolar cells at the base of the cone pedicle and on the invaginating tips of rod bipolar cells. Stain on the cone pedicle membrane was restricted to sites of apposition to stained bipolar dendrites; pedicle membrane in apposition to horizontal cell processes was unstained. Stain was also present on bipolar axon terminals in both on and off strata of the inner plexiform layer. All bipolar cell somas stained faintly; horizontal and Müller cell somas were unstained. The alpha and beta subunits distributed similarly in monkey and human retina. Presence of GABAA receptor on the bipolar dendritic tips suggests that horizontal cells directly affect bipolar cells. Thus, GABAA receptor might mediate the receptive field surround of both off and on bipolar cells. Presence of GABAA receptor on bipolar axon terminals suggests that much of the inhibition feeding back from GABAergic amacrine to bipolar cells is GABAA-mediated.

Localization of GABAA receptors in the rabbit retina

The distribution of gamma-aminobutyric acidA (GABAA) receptors in the rabbit retina is investigated and compared with the distribution of GABAergic neurons using immunocytochemical methods. Antibodies against the alpha 1, beta 2/3, and gamma 2 subunits of the GABAA receptor label subpopulations of bipolar, amacrine and ganglion cells. Double labeling experiments show that the gamma 2 subunit is colocalized with the alpha 1 and the beta 2/3 subunits in bipolar, amacrine and ganglion cells. Electron microscopy reveals that in the outer plexiform layer, GABAA receptor immunoreactivity is present on dendrites of cone bipolar cells adjacent to the cone pedicles. Bipolar cell dendrites are also receptor-positive at synapses from interplexiform cells. Some receptor immunoreactivity is found intracellularly in processes of horizontal cells. In the inner plexiform layer, GABAA receptor immunoreactivity is present on both rod bipolar and cone bipolar axon terminals at putative GABAergic input sites. Amacrine and ganglion cell processes in sublamina a and b are also labeled.