The physiology of GABAA receptors in retinal neurons

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.

Effect of GABA on melatonin content in golden hamster retina

The effect of GABA on melatonin content in vitro was studied in the golden hamster retina. GABA significantly increased melatonin levels in a dose-dependent manner, its effect being reversed by a GABA(A) receptor antagonist, bicuculline, but not by saclofen, a GABA(B) antagonist. Moreover, an equimolar concentration of muscimol, a GABA(A) receptor agonist, significantly increased retinal melatonin content, whereas baclofen, a GABA(B) receptor agonist, was ineffective. The darkness-induced increase in melatonin content in vitro was inhibited by bicuculline, whereas saclofen was ineffective. Retinal GABA turnover rate was significantly higher at midnight than at midday. GABA significantly decreased cyclic AMP and increased cyclic GMP accumulation in the golden hamster retina. The effect of GABA on both nucleotide levels was reversed by bicuculline, but baclofen had no effect. Cyclic GMP analogues (i.e., 8-bromoguanosine 3',5'-cyclic monophosphate and 2'-O-dibutyrylguanosine 3',5'-cyclic monophosphate) significantly increased retinal melatonin content in vitro. Taken together, these results support the hypothesis that GABA may be important for the "dark message" in the hamster retina.

Analysis of excitatory and inhibitory spontaneous synaptic activity in mouse retinal ganglion cells

Spontaneous inhibitory and excitatory postsynaptic currents (sIPSCs and sEPSCs) were identified and characterized with whole cell and perforated patch voltage-clamp recordings in adult mouse retinal ganglion cells. Pharmacological dissection revealed that all cells were driven by spontaneous synaptic inputs mediated by glutamate and gamma-aminobutyric acid-A (GABAA) receptors. One-half (7/14) of the cells also received glycinergic spontaneous synaptic inputs. Both GABAA and glycine receptor-mediated sIPSCs had rise times (10-90%) of < 1 ms. The decay times of the GABAA receptor-mediated sIPSCs were comparable with those of the glycine receptor-mediated sIPSCs. The average decay time constant for monoexponentially fitted sIPSCs was 63.2 +/- 74.1 ms (mean +/- SD, n = 3278). Glutamate receptor-mediated sEPSCs had an average rise time of 0.50 +/- 0.20 ms (n = 109) and an average monoexponential decay time constant of 5.9 +/- 8.6 ms (n = 2705). Slightly more than two-thirds of the spontaneous synaptic events were monoexponential (68% for sIPSCs and 76% for sEPSCs). The remainder of the events was biexponential. The amplitudes of the spontaneous synaptic events were not correlated with rise times, suggesting that the electrotonic filtering properties of the neurons and/or differences in the spatial location of synaptic inputs could not account for the difference between the decay time constants of the glutamate and GABAA/glycine receptor-mediated spontaneous synaptic events. The amplitudes of sEPSCs were similar to those recorded in tetrodotoxin (TTX), consistent with the events measured in control saline being the response to the release of a single quantum of transmitter. The range of the sIPSC amplitudes in control saline was wider than that recorded in TTX, consistent with some sIPSCs being evoked by presynaptic spikes having an average quantal size greater than one. The rates of sIPSCs and sEPSCs were determined under equivalent conditions by recording with perforated patch electrodes at potentials at which both types of event could be identified. Two groups of ganglion cell were observed; one group had an average sEPSCs/sIPSCs frequency ratio of 0.96 +/- 0.77 (n = 28) and another group had an average ratio of 6.63 +/- 0.82 (n = 7). These findings suggest that a subset of cells is driven much more strongly by excitatory synaptic inputs. We propose that this subset of cells could be OFF ganglion cells, consistent with the higher frequency of spontaneous action potentials found in OFF ganglion cells in other studies.

GABAA and GABAC receptors on mammalian rod bipolar cells

Rod bipolar (RB) cells of mammalian retinae receive synapses from different gamma-aminobutyric acid (GABAergic) amacrine cells in the inner plexiform layer (IPL). We addressed the question whether RB cells of the rabbit and of the rat retina express different types of GABA receptors at these synapses. RB cells were immunolabeled in vertical sections of rat retinae with an antibody against protein kinase C (PKC). The sections were double-labeled for the alpha 1, alpha 2, alpha 3, or gamma 2 subunits of the GABAA receptor. Punctate immunofluorescence, which represents synaptic localization, was found for all four subunits. Many of the alpha 1-, alpha 3-, or gamma 2-immunoreactive puncta coincided with the axon terminals of the PKC-immunolabeled RB cells. Sections and wholemounts of rabbit retinae were also double labeled for PKC and the rho subunits of the GABAC receptor. Rabbit RB cells were decorated by many rho-immunoreactive puncta, which were shown by electron microscopy to represent synaptic localization. Previous work from our laboratory has shown that the alpha 1, alpha 2, alpha 3, and rho subunits are not found within the same synapse but are expressed at different synaptic sites. Taken together, these results suggest that RB cells of mammalian retinae express at least three different types of GABA receptors at synaptic sites in the IPL: GABAC receptors, GABAA receptors containing the alpha 1 subunit, and GABAA receptors containing the alpha 3 subunit.

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.

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.

GABAergic and glycinergic IPSCs in ganglion cells of rat retinal slices

GABAergic and glycinergic IPSCs were studied in identified retinal ganglion cells (RGCs) of light-adapted rat retinal slices, using whole-cell recording techniques. GABAergic IPSCs were blocked specifically by SR95531 (3 microM) and bicuculline (3 microM) and glycinergic IPSCs by strychnine (0.3 microM). From 37 RGCs studied, 25 showed exclusively GABAergic IPSCs, 6 presented only glycinergic IPSCs, and 6 showed both. This distribution may result from differences in amacrine cells input rather than from receptor heterogeneity, because both GABA and glycine elicited Cl--selective currents in all RGCs tested. TTX markedly reduced GABAergic IPSCs frequency, whereas glycinergic IPSCs were unaffected. Ca2+-free media, with or without high Mg2+, blocked TTX-resistant GABAergic and glycinergic IPSCs. These results suggest that GABAergic IPSCs in RGCs can be elicited either by Na+-dependent action potentials or by local Ca2+ influx in medium or large dendritic field GABAergic amacrine cells, whereas glycinergic IPSCs are generated by action potential-independent Ca2+ influx in narrow field glycinergic amacrine cells. Both types of IPSCs had fast rise times and biexponential decays, but glycinergic IPSC decay was significantly slower than that of GABAergic IPSCs. An elementary conductance of 54 pS for the glycine-gated channels was estimated from single-channel events, clearly detected in the falling phase of glycinergic IPSCs, and from responses to exogenous glycine.

Time course of appearance of GABA and GABA receptors during retinal regeneration in the adult newt

Appearance and maturation of the GABA (gamma-aminobutyric acid) system during newt retinal regeneration were studied by electrophysiological, immunohistochemical, and biochemical techniques. (1) Responses to GABA appeared in neurons dissociated from regenerating retinae before the segregation of the plexiform layers; whereas (2) GABA immunoreactivity appeared at sites of the presumptive horizontal cell and amacrine cell layers at the beginning of the segregation of these layers. During subsequent regeneration, GABA-immunoreactive cells at the amacrine cell layer increased in number and extended lateral processes, forming a GABA-immunoreactive inner plexiform layer. Also GABA immunoreactivity increased in the region of the outer plexiform layer, but not their somata which showed decreased GABA immunoreactivity. (3) GABA synthesis in the retina increased significantly at the beginning of the segregation of the plexiform layers. These results suggest that the increase of GABA synthesis during retinal regeneration correlates well with the development of GABA-immunoreactive cells and that functional GABA receptors appear earlier than increased GABA synthesis.

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.

Cloning of a putative gamma-aminobutyric acid (GABA) receptor subunit rho 3 cDNA

Cloned cDNA encoding a putative member of GABA receptor rho-subunit class was isolated from rat-retina-mRNA-derived libraries. The cDNA encodes a signal peptide of 21 amino acids followed by the mature rho 3 subunit sequence of 443 amino acids. The proposed amino acid sequence exhibits 63 and 61% homology to the previously-reported human rho 1 and rat rho 2 sequences, respectively. Northern blot analysis demonstrated the expression of mRNA for rho 3 subunit in retina.

Colocalization of gephyrin and GABAA-receptor subunits in the rat retina

Gephyrin is a protein that copurifies with the glycine receptor (GlyR) and is required for the clustering of GlyRs at postsynaptic sites. Previously, it was thought that antibody mAb 7a, directed against gephyrin, was a specific marker for GlyR. However, there is evidence that gephyrin can also be found at nonglycinergic synapses. Here, immunocytochemistry was applied to show this directly for the rat retina. Both gephyrin and different subunits of the gamma-aminobutyric acid (GABA)A receptor were localized to discrete puncta in the inner plexiform layer, and these puncta were shown by electron microscopy to represent synaptic sites. Double immunocytochemistry revealed that GABAA receptors and GlyRs are not colocalized. However, gephyrin and different subunits of GABAA receptors were found to occur at the same synapses. The amount of colocalization varied with the GABAA receptor subunit composition and was most extensive for the alpha 2 subunit, less for the alpha 3 subunit, and minimal for the alpha 1 subunit. The gephyrin present at GABAergic synapses of the retina might also be involved with clustering of receptors at the postsynaptic sites. Hence, localization of gephyrin can no longer be considered as a unique marker of glycinergic synapses.

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.

Co-stratification of GABAA receptors with the directionally selective circuitry of the rat retina

Direction-selective (DS) ganglion cells of the mammalian retina have their dendrites in the inner plexiform layer (IPL) confined to two narrow strata. The same strata are also occupied by the dendrites of cholinergic amacrine cells which are probably presynaptic to the DS ganglion cells. GABA is known to play a crucial role in creating DS responses. We examined the types of GABAA receptors expressed by the cholinergic amacrine cells and also those expressed by their presynaptic and postsynaptic neurons, by applying immunocytochemical markers to vertical sections of rat retinas. Double-labelling experiments with antibodies against choline acetyltransferase (ChAT) and specific antibodies against different GABAA receptor subunits were performed. Cholinergic amacrine cells seem to express an unusual combination of GABAA receptor subunits consisting of alpha 2-, beta 1-, beta 2/3-, gamma 2-, and delta-subunits. Bipolar cells, which could provide synaptic input to the DS circuitry, were stained with antibodies against the glutamate transporter GLT-1. The axon terminals of these bipolar cells are narrowly stratified in close proximity to the dendritic plexus of displaced cholinergic amacrine cells. The retinal distribution of synaptoporin, a synaptic vesicle associated protein, was studied. Strong reduction of immunolabelling was observed in the two cholinergic strata. The anatomical findings are discussed in the context of models of the DS circuitry of the mammalian retina.

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.

Immunohistochemical localization of GABAA receptors in the scotopic pathway of the cat retina

The distribution of GABAA receptors in the inner plexiform layer of cat retina was studied using monoclonal antibodies against the beta 2/beta 3 subunits. A dense band of receptor labeling was found in the inner region of the inner plexiform layer where the rod bipolar axons terminate. Three forms of evidence indicate that the GABAA receptor labeling is on the indoleamine-accumulating, GABAergic amacrine cell that is synaptically interconnected with the rod bipolar cell terminal. (1) Electron microscopy showed that the anti-GABAA receptor antibody (62-3G1) labeled profiles that were postsynaptic to rod bipolar axons and made reciprocal synapses. (2) Indoleamine uptake (and the subsequent autofluorescence) combined with GABAA receptor immunohistochemistry showed co-localization of the two markers in half of the receptor-positive amacrine cells. (3) Double labeling demonstrated that half of the receptor-positive somata also contained GABA. These results indicate that a GABAergic amacrine cell interconnected with the rod bipolar cell, most likely the so-called A17 amacrine cell, itself bears GABAA receptors.

Localization of GABAA receptors in the rat retina

Gamma-aminobutyric acid (GABA) is the major inhibitory neurotransmitter in the mammalian retina. The present paper describes the localization of GABAA receptors in the rat retina as revealed by in situ hybridization and immunocytochemistry. In situ hybridization with probes against various alpha subunits revealed a marked differential expression pattern. The alpha 1 subunit gene is expressed mainly in the bipolar and horizontal cell layer, the alpha 2 gene in the amacrine and ganglion cell layer, and the alpha 4 gene in a subpopulation of amacrine cells. beta subunit mRNA is present diffusely throughout the entire inner nuclear layer and in the ganglion cell layer. The monoclonal antibody bd 17 (against beta 2/beta 3 subunits) stained subpopulations of GABAergic and glycinergic amacrine cells as well as some ganglion cells and bipolar cells. Immunoreactivity was not restricted to synaptic input sites. In the outer plexiform layer bipolar cell dendrites were immunoreactive; in the inner plexiform layer mainly amacrine and ganglion cell processes were labeled, and bipolar cell axons appeared unstained. The results demonstrate a strong heterogeneity of GABAA receptors in the retina.