Immunohistochemical localization of GABAA receptors in the retina of the new world primate Saimiri sciureus

Contribution of parasol-magnocellular pathway ganglion cells to foveal retina in macaque monkey

Parasol-magnocellular pathway ganglion cells form an important output stream of the primate retina and make a major contribution to visual motion detection. They are known to comprise ON and OFF type response polarities but the relative numbers of ON and OFF parasol cells, and the overall contribution of parasol cells to high-acuity foveal vision are not well understood. Here we use antibodies against carbonic anhydrase 8 (CA8) and intracellular injections of the liphilic dye DiI to show that CA8 selectively labels OFF parasol cells in macaque retina. By combined labeling with CA8 antibodies and a previously-described marker for parasol cells (GABA_A receptor antibodies), we show that ON and OFF parasol cells each comprise ∼ 6% of all ganglion cells in central retina (each peak density ∼ 3000 cells/mm² at 5 deg.), and each population comprises ∼ 10% of all ganglion cells in peripheral temporal retina. Thus, the spatial density of parasol cells in central retina is greater than reported by previous anatomical studies, and the central-peripheral gradient in parasol cell density is shallower than previously reported. The data nevertheless predict decline in spatial acuity with visual field eccentricity for both midget-parvocellular pathway and parasol-magnocellular pathway mediated visual functions. The spatial resolving power of the OFF parasol array (peak ∼ 7 cpd) falls short of macaque behavioral grating acuity by at least a factor of three throughout the retina.

Morphology, Molecular Characterization, and Connections of Ganglion Cells in Primate Retina

The eye sends information about the visual world to the brain on over 20 parallel signal pathways, each specialized to signal features such as spectral reflection (color), edges, and motion of objects in the environment. Each pathway is formed by the axons of a separate type of retinal output neuron (retinal ganglion cell). In this review, we summarize what is known about the excitatory retinal inputs, brain targets, and gene expression patterns of ganglion cells in humans and nonhuman primates. We describe how most ganglion cell types receive their input from only one or two of the 11 types of cone bipolar cell and project selectively to only one or two target regions in the brain. We also highlight how genetic methods are providing tools to characterize ganglion cells and establish cross-species homologies.

Extraretinal Spike Normalization in Retinal Ganglion Cell Axons

Spike conduction velocity characteristically differs between myelinated and unmyelinated axons. Here we test whether spikes of myelinated and unmyelinated paths differ in other respects by measuring rat retinal ganglion cell (RGC) spike duration in the intraretinal, unmyelinated nerve fiber layer and the extraretinal, myelinated optic nerve and optic chiasm. We find that rapid spike firing and illumination broaden spikes in intraretinal axons but not in extraretinal axons. RGC axons thus initiate spikes intraretinally and normalize spike duration extraretinally. Additionally, we analyze spikes that were recorded in a previous study of rhesus macaque retinogeniculate transmission and find that rapid spike firing does not broaden spikes in optic tract. The spike normalization we find reduces the number of spike properties that can change during RGC light responses. However, this is not because identical spikes fire in all axons. Instead, our recordings show that different subtypes of RGC generate axonal spikes of different durations and that the differences resemble spike duration increases that alter neurotransmitter release from other neurons. Moreover, previous studies have shown that RGC spikes of shorter duration can fire at higher maximum frequencies. These properties should facilitate signal transfer by different mechanisms at RGC synapses onto subcortical target neurons.

GABAergic neurotransmission and retinal ganglion cell function

Ganglion cells are the output retinal neurons that convey visual information to the brain. There are ~20 different types of ganglion cells, each encoding a specific aspect of the visual scene as spatial and temporal contrast, orientation, direction of movement, presence of looming stimuli; etc. Ganglion cell functioning depends on the intrinsic properties of ganglion cell's membrane as well as on the excitatory and inhibitory inputs that these cells receive from other retinal neurons. GABA is one of the most abundant inhibitory neurotransmitters in the retina. How it modulates the activity of different types of ganglion cells and what is its significance in extracting the basic features from visual scene are questions with fundamental importance in visual neuroscience. The present review summarizes current data concerning the types of membrane receptors that mediate GABA action in proximal retina; the effects of GABA and its antagonists on the ganglion cell light-evoked postsynaptic potentials and spike discharges; the action of GABAergic agents on centre-surround organization of the receptive fields and feature related ganglion cell activity. Special emphasis is put on the GABA action regarding the ON-OFF and sustained-transient ganglion cell dichotomy in both nonmammalian and mammalian 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.

The GABAergic system in the retina of neonate and adult Octodon degus, studied by immunohistochemistry and electroretinography

In the vertebrate retina, gamma-aminobutyric acid (GABA) mediates inhibitory processes that shape the visual response and is also thought to have neurotrophic functions during retinal development. To investigate the role of GABAergic signaling at the beginning of visual experience, we used immunohistochemistry to compare the distribution of GABA, the two isoforms of glutamic acid decarboxylase GAD65/67, and the GABA receptor types A, B, and C, in neonate versus adult Octodon degus, a native South American rodent with diurnal-crepuscular activity and a high cone-to-rod ratio. In parallel, we used electroretinography to evaluate retinal functionality and to test the contribution of fast GABAergic transmission to light responses at both developmental stages. Neonate O. degus opened their eyes on postnatal day (P)0 and displayed an adult-like retinal morphology at this time. GABA, its biosynthetic sources, and receptors had a similar cellular distribution in neonates and adults, but labeling of the outer plexiform layer and of certain amacrine and ganglion cells was more conspicuous at P0. In neonates, retinal sensitivity was 10 times lower than in adults, responses to ultraviolet light could not be detected, and oscillatory potentials were reduced or absent. Blockade of GABA(A/C) receptors by bicuculline and TPMPA had no noticeable effect in neonates, while it significantly altered the electroretinogram response in adults.

CONCLUSION

In spite of modest differences regarding retinal morphology and GABAergic expression, overall light response properties and GABAergic signaling are undeveloped in neonate O. degus compared to adults, suggesting that full retinal functionality requires a period of neural refinement under visual experience.

The classical receptive field surround of primate parasol ganglion cells is mediated primarily by a non-GABAergic pathway

Although the center-surround receptive field is a fundamental property of retinal ganglion cells, the circuitry that mediates surround inhibition remains controversial. We examined the contribution of horizontal cells and amacrine cells to the surround of parasol ganglion cells of macaque and baboon retina by measuring receptive field structure before and during the application of drugs that have been shown previously to affect surrounds in a range of mammalian and nonmammalian species. Carbenoxolone and cobalt, thought to attenuate feedback from horizontal cells to cones, severely reduced the surround. Tetrodotoxin, which blocks sodium spiking in amacrine cells, and picrotoxin, which blocks the inhibitory action of GABA, only slightly reduced the surround. These data are consistent with the hypothesis that the surrounds of light-adapted parasol ganglion cells are generated primarily by non-GABAergic horizontal cell feedback in the outer retina, with a small contribution from GABAergic amacrine cells of the inner retina.

Light- and electron-microscopic analysis of vasoactive intestinal polypeptide-immunoreactive amacrine cells in the guinea pig retina

Vasoactive intestinal polypeptide (VIP) is a neuroactive substance that is expressed in both nonmammalian and mammalian retinas. This study investigated the morphology and synaptic connections of VIP-containing neurons in the guinea pig retina by immunocytochemistry, by using antisera against VIP. Specific VIP immunoreactivity was localized to a population of wide-field and regularly spaced amacrine cells with processes ramifying mainly in strata 1 and 2 of the inner plexiform layer (IPL). Double-label immunohistochemistry demonstrated that all VIP-immunoreactive cells possessed gamma-aminobutyric acid immunoreactivity. The synaptic connectivity of VIP-immunoreactive amacrine cells was identified in the IPL by electron microscopy. The VIP-labeled amacrine cell processes received synaptic input from other amacrine cell processes and bipolar cell axon terminals in strata 1 to 3 of the IPL. The most frequent postsynaptic targets of VIP-immunoreactive amacrine cells were other amacrine cell processes in strata 1 to 3 of the IPL. Synaptic outputs to bipolar cells were also observed in strata 1 to 3 of the IPL. In addition, ganglion cell dendrites were also postsynaptic to VIP-immunoreactive neurons in the sublamina a of the IPL. These studies show that one type of VIP-immunoreactive amacrine cells make contact predominantly with other amacrine cell processes. This finding suggests that VIP-containing amacrine cells may influence inner retinal circuitry, thus mediating visual processing.

Neuronal nitric oxide synthase immunoreactive neurons in the mammalian retina

The development of immunocytochemistry has led to a better understanding of synaptic transmission carried out by neuroactive substances in the mammalian brain, including the retina. In the mammalian retina, nitric oxide (NO) is widely accepted as a neuromodulator. Histochemistry based on NADPH-d and immunocytochemistry based on nitric oxide synthase (NOS) have been used to identify the presence of nitric oxide in the mammalian retina. Certain types of amacrine cells and a class of displaced amacrine cells have been labeled consistently in all mammalian retinae studied to date. Other cell types showing NADPH-d reactivity or NOS immunoreactivity varied between species. NADPH-d reactive or NOS immunoreactive amacrine cells may serve as a source of NO for amacrine, bipolar, and ganglion cells in the inner retina, whereas interplexiform cells, bipolar cells, and horizontal cells may serve as a source of NO for the outer retina of mammals.

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.

Distribution of GABAA and glycine receptors in the mammalian retina

1. GABA and glycine mediate synaptic inhibition via specific neurotransmitter receptors. Molecular cloning studies have shown that there is a great diversity of receptors for these two neurotransmitters. In the present paper, the distribution of GABAA and glycine receptors in the mammalian retina is reviewed. 2. In situ hybridization, immunocytochemistry with subunit-specific antibodies and single cell injection were used to analyse the localization of receptor subunits. Specific subunits are expressed in characteristic strata of the inner plexi-form layer, suggesting that different functional circuits involve specific subtypes of neurotransmitter receptors. 3. Different cell types express different combinations of receptor subunits and an individual neuron can express several receptor isoforms at distinct post-synaptic sites.

GABAA and GABAC receptors in adult porcine cones: evidence from a photoreceptor-glia co-culture model

1. Edge contrast enhancement is an integrated visual function based on the complex centre-surround organization of the cone photoreceptor light response. While centre responses result from direct light activation, surround responses are thought to result from lateral inhibition mediated by horizontal cells. This feedback signal has been attributed to GABAA receptors which have been found in lower vertebrate cones. 2. In order to study the GABA response of adult mammalian photoreceptors, we designed a culture system consisting of isolated photoreceptors seeded on a layer of retinal glial cells. Mature rods and cones required the presence of Muller glial cells to survive and develop neurites; they degenerated in the absence of glial cells. 3. Cone photoreceptors generated large GABA responses whereas rod photoreceptors did not respond to GABA applications. 4. Cone GABA responses consisted of two distinct components, one suppressed by the GABAA receptor blockers bicuculline and SR95531, and the second by the GABAC receptor antagonists TPMPA and imidazole-4-acetic acid (I4AA). Pentobarbital greatly increased the GABAA receptor component whereas it did not affect, or even reduced, the GABAC receptor component. During long GABA applications, GABAA receptor currents desensitized by 78%, contrasting with the sustained GABAC response. 5. Expression of GABAC receptors in cone photoreceptors was confirmed by anti-rho-subunit immunolabelling of porcine retinal sections. 6. These results indicate that both GABAA and GABAC receptors may participate in the feedback synapse from horizontal cells to cone photoreceptors in the mammalian 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.

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.

Presynaptic and postsynaptic localization of GABA(B) receptors in neurons of the rat retina

The recently cloned GABA(B) receptors were localized in rat retina using specific antisera. Immunolabelling was detected in the inner and outer plexiform layers (IPL, OPL), and in a number of cells in the inner nuclear layer and the ganglion cell layer. Double-labelling experiments for GABA (gamma-aminobutyric acid) and GABA(B) receptors, respectively, demonstrated a co-localization in horizontal cells and amacrine cells. Electron microscopy showed that GABA(B) receptors of the OPL were localized presynaptically in horizontal cell processes invaginating into photoreceptor terminals. In the IPL, GABA(B) receptors were present presynaptically in amacrine cells, as well as postsynaptically in amacrine and ganglion cells. The postnatal development of GABA(B) receptors was also studied, and immunoreactivity was observed well before morphological and synaptic differentiation of retinal neurons. The present results suggest a presynaptic (autoreceptor) as well as postsynaptic role for GABA(B) receptors. In addition, the extrasynaptic localization of GABA(B) receptors could indicate a paracrine function of GABA in the retina.

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.

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.

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.

GABA and GABA-A receptors are maximally expressed in association with cone synaptogenesis in neonatal rabbit retina

Previous studies have shown the cone photoreceptors form reciprocal synapses with horizontal cells during the first week after birth in rabbits. These synapses constitute pioneering elements of the developing outer plexiform layer. We now report that antibodies against the alpha-1 and against the beta-2/3 subunits of the GABA-A receptor label a highly restricted sublamina in the developing outer plexiform layer known to contain nascent cone photoreceptor terminals. Staining is relatively weak at birth, increases to maximal levels between postnatal days 5 and 7, and is significantly reduced in the adult. These results support recent calcium imaging studies which have shown that the activation of GABA-A receptors causes an increase in intracellular free calcium in cones, an effect which is observed only at 3-9 days after birth. The transient expression of GABA-A receptors in this region coincides with the period of peak expression of GABA immunoreactivity in horizontal cells. A direct functional link between GABAergic transmission and cone synaptogenesis is suggested by previous reports that GABA-A receptor antagonists cause disruption of cone synaptogenesis. Together these findings support the notion that GABA functions as a developmental neurotransmitter which is produced by horizontal cells and interacts with developing cone axons in order to facilitate synaptic linkage between these two cells types.

Immunocytochemical localization of the alpha 6 subunit of the gamma-aminobutyric acidA receptor in the rat nervous system

The localization in the rat central nervous system and retina of the alpha 6 subunit peptide of the gamma-aminobutyric acid (GABAA) receptor has been studied by light microscopy immunocytochemistry with a specific anti-alpha 6 antibody. The alpha 6 subunit was present in the granule cells of the cerebellum, the granule cells of the dorsal cochlear nucleus, axons of the olfactory nerve including the glomerular endings, layer II of the dorsal horn of the spinal cord, and in the retinal synaptic layers, particularly the inner plexiform layer. Thus, contrary to the general belief, the alpha 6 subunit is not exclusively localized in the granule cells of the cerebellum. It is also expressed in some sensory neurons and other neurons involved in the initial processing of sensory information.

Brain GABAA receptors studied with subunit-specific antibodies

Brain GABAA/benzodiazepine receptors are highly heterogeneous. This heterogeneity is largely derived from the existence of many pentameric combinations of at least 16 different subunits that are differentially expressed in various brain regions and cell types. This molecular heterogeneity leads to binding differences for various ligands, such as GABA agonists and antagonists, benzodiazepine agonists, antagonists, and inverse agonists, steroids, barbiturates, ethanol, and Cl- channel blockers. Different subunit composition also leads to heterogeneity in the properties of the Cl- channel (such as conductance and open time); the allosteric interactions among subunits; and signal transduction efficacy between ligand binding and Cl- channel opening. The study of recombinant receptors expressed in heterologous systems has been very useful for understanding the functional roles of the different GABAA receptor subunits and the relationships between subunit composition, ligand binding, and Cl- channel properties. Nevertheless, little is known about the complete subunit composition of the native GABAA receptors expressed in various brain regions and cell types. Several laboratories, including ours, are using subunit-specific antibodies for dissecting the heterogeneity and subunit composition of native (no reconstituted) brain GABAA receptors and for revealing the cellular and subcellular distribution of these subunits in the nervous system. These studies are also aimed at understanding the ligand-binding, transduction mechanisms, and channel properties of the various brain GABAA receptors in relation to synaptic mechanisms and brain function. These studies could be relevant for the discovery and design of new drugs that are selective for some GABAA receptors and that have fewer side effects.

Immunocytochemical evidence for an axonal localization of GABA in the optic nerves of rabbits, rats, and cats

We have examined, by light-microscopic immunocytochemistry, the distribution of GABA in the optic nerves of adult rabbits, rats, and cats. Within the optic nerves, immunoreactivity for GABA was restricted to a small subset of axons; some axons were strongly labelled, others weakly labelled, whilst most axons were unlabelled. Glia and other non-neuronal elements were always unlabelled. Our ability to detect GABA in optic nerve axons of adult mammals contrasts with previous reports that indicate a lack of GABA immunoreactivity in such axons. We suggest that this discrepancy may be due to the sensitivity of our immunocytochemical techniques which enable us to detect low concentrations of GABA.

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.

Effects of retinal lesions upon the distribution of nicotinic acetylcholine receptor subunits in the chick visual system

Immunohistochemistry was used in this study to evaluate the effects of retinal lesions upon the distribution of neuronal nicotinic acetylcholine receptor subunits in the chick visual system. Following unilateral retinal lesions, the neuropil staining with an antibody against the beta 2 receptor subunit, a major component of alpha-bungarotoxin-insensitive nicotinic receptors, was dramatically reduced or completely eliminated in all of the contralateral retinorecipient structures. On the other hand, neuropil staining with antibodies against two alpha-bungarotoxin-sensitive receptor subunits, alpha 7 and alpha 8, was only slightly affected after retinal lesions. Decreased neuropil staining for alpha 7-like immunoreactivity was only observed in the nucleus of the basal optic root and layers 2-4 and 7 of the optic tectum. For alpha 8-like immunoreactivity, slight reduction of neuropil staining could be observed in the ventral geniculate complex, griseum tecti, nucleus lateralis anterior, nucleus lentiformis mesencephali, layers 4 and 7 of the tectum, and nucleus suprachiasmaticus. Taken together with previous data on the localization of nicotinic receptors in the retina, the present results indicate that the beta 2 subunit is transported from retinal ganglion cells to their central targets, whereas the alpha 7 and alpha 8 subunit immunoreactivity appears to have a central origin. The source of these immunoreactivities could be, at least in part, the stained perikarya that were observed to contain alpha 7 and alpha 8 subunits in all retinorecipient areas. In agreement with this hypothesis, the beta 2 subunit of the nicotinic acetylcholine receptors was not frequently found in perikarya of the same areas.

Expression and localization of gamma-aminobutyric acid A (GABAA) receptor alpha 1 subunit and L-glutamate decarboxylase (GAD) mRNAs in rat retina: an analysis by in situ hybridization

The localization of the mRNAs encoding gamma-aminobutyric acidA receptor alpha 1 subunit (GABAA alpha 1) and L-glutamate decarboxylase (GAD) was elucidated in the rat retina by in situ hybridization. Soma diameter analysis of signal positive cells in the ganglion cell layer demonstrated that a subpopulation including alpha-cells of retinal ganglion cells expressed GABAA alpha 1 mRNA and a subpopulation of ganglion cells smaller than alpha-cells expressed GAD mRNA.

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.

Immunocytochemical localization of the GABAA receptor in the cerebral cortex of the lizard Psammodromus algirus

This study examined the distribution and localization of the gamma-aminobutyric acid (GABA)A receptor in the brain cortex of a reptile by light and electron microscopy, to test whether cortical GABA inhibition is mainly mediated through the GABAA receptor complex. We used preembedding immunocytochemistry and a monoclonal antibody, raised against the receptor complex, that recognizes the beta 2 and beta 3 subunits of the receptor. GABAA receptors were distributed throughout the entire cerebral cortex except the dorsomedial cortex. The immunostaining consisted of fine granules restricted to the plexiform layers of the cortex as seen by light microscopy. This granular aspect of the immunoreactivity most likely corresponds to the immunopositive dendritic and axonal profiles observed under the electron microscope. Some neurons in the medial and lateral cortices displayed patches of immunoreactivity along the cell body and processes, and as a result their morphology was outlined. We discuss the possibility that these neurons were GABAergic as well. The immunocytochemical data demonstrate that the distribution and localization of GABAA receptors in discrete regions of the reptilian cerebral cortex resemble that of parts of the hippocampal formation of humans and rats, suggesting that the basic configuration of the GABA system in these regions is conserved.

Nicotinic acetylcholine receptors in the ground squirrel retina: localization of the beta 4 subunit by immunohistochemistry and in situ hybridization

Immunohistochemical and in situ hybridization techniques were used to localize the beta 4 subunit of the neuronal nicotinic acetylcholine receptors (nAChRs) in the ground squirrel retina. The beta 4 nAChR subunit was detected in both transverse and horizontal sections of the retina using a subunit-specific antiserum and the avidin-biotin complex technique. Two bands of labeled processes were seen in the inner plexiform layer, corresponding approximately to the laminae where the cholinergic cells arborize. Labeled cells were found in the ganglion cell layer and the inner third of the inner nuclear layer. The cells in the ganglion cell layer were medium- to large-sized and were frequently observed to give rise to axon-like processes. Most of the labeled neurons in the inner nuclear layer were small presumptive amacrine cells, but a few medium-to-large cells were also labeled. These could constitute a different class of amacrine cells or displaced ganglion cells. The latter possibility is supported by the existence of nAChR-containing displaced ganglion cells in the avian retina. In situ hybridization with a 35S-labeled cRNA probe revealed the expression of mRNA coding for the nAChR beta 4 subunit in the ganglion cell layer and the inner third of the inner nuclear layer. This finding confirmed the immunohistochemical data of the cellular localization of beta 4 nAChR subunit. These results indicate that the beta 4 nAChR subunit is expressed by specific subtypes of neurons on the ground squirrel retina.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Horizontal cells in cat and monkey retina express different isoforms of glutamic acid decarboxylase

The neurotransmitter used by horizontal cells in mammals has not been identified. GABA has been the leading candidate, but doubt has remained because of failure to clearly demonstrate the GABA synthetic enzyme, glutamic acid decarboxylase (GAD) in these cells. Because GAD was recently shown to exist as two isoforms, 65 kDa and 67 kDa, we considered whether there might be a mismatch between the forms of GAD expressed in horizontal cells and the probes used to detect it. Accordingly, we stained sections of mammalian retina with antibodies specific for each isoform. Cat horizontal cells of both types (A and B) were immunoreactive for GAD67 but negative for GAD65; monkey horizontal cells of both types (H(I) and HII) were positive for GAD65 and negative for GAD67. The findings reconcile previous, apparently conflicting, observations and strengthen considerably the hypothesis that mammalian horizontal cells are GABAergic.

Bipolar cells of the chick retina containing alpha-bungarotoxin-sensitive nicotinic acetylcholine receptors

Two cDNA clones for nicotinic acetylcholine receptor (nAChR) subunits sensitive to alpha-bungarotoxin (alpha-Bgt) have been isolated, the so-called alpha-Bgt binding proteins alpha 1 (or alpha 7 nAChR subunit) and alpha 2 (or alpha 8 nAChR subunit). Immunohistochemical experiments have shown that both alpha 7 and alpha 8 subunits, as well as subunits insensitive to alpha-Bgt (beta 2 and alpha 3), are present in amacrine and ganglion cells of the chick retina. However, only the alpha 8 subunit was observed in presumptive bipolar cells. The present study investigated in detail the pattern of distribution of the bipolar cells containing the alpha 8 nAChR subunit and its relation to the pattern of distribution of bipolar cells immunoreactive to protein kinase C (PKC). Presumptive alpha 8- and PKC-like immunoreactive (alpha 8-LI and PKC-LI) bipolar cells were observed sending their dendrites to the outer plexiform layers and their axons to the inner plexiform layer. Whereas alpha 8-LI bipolar cells corresponded to 40-53% of the whole population of bipolar cells, PKC-LI bipolar cells represented only 6-8% of the same population. The soma sizes of the alpha 8-LI bipolar cells were slightly smaller (mean +/- S.D.; 4.9 +/- 0.8 microns) than the soma sizes of the PKC-LI bipolar cells (5.4 +/- 0.9 microns). Double-labeling experiments indicated that probably all PKC-LI bipolar cells also contain alpha 8-LI. This indicates that two distinct groups of cholinoceptive bipolar cells exist in the chick retina, one that contains PKC-LI, and another one that does not.

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.

Visual dysfunction in the spontaneously hypertensive rat

We report two forms of visual defect in the spontaneously hypertensive rat (SHR) compared to its normotensive control strain (WKY). Ten-month-old male SHR and WKY were tested for intensity discrimination ability using a Y-maze and food reward. The SHR took more trials in the acquisition phase of the task using white light. In the test phase, in which the white light intensity difference was reduced, there was no significant difference in performance between SHR and WKY. Also, there was no significant difference between the strains when red light was used. The defect in SHR occurred in the blue range of the spectrum. Using blue light, the performance of SHR declined significantly at an intensity difference of 1.15 lux, whereas the performance of WKY did not decline significantly until the difference reached 0.01 lux. The defect in SHR for intensity discrimination was not related to cataract formation, but we detected a strain difference in the nuclei of the photoreceptor cells. Thus, this visual dysfunction may involve the rod photoreceptors. Further testing revealed a deficit in visual acuity in SHR aged 40-66 days, prior to the development of hypertension. Thus, the visual dysfunction of SHR is not secondary to their development of hypertension, but rather a discrete characteristic of the strain.

Glutamate and GABA in retinal circuitry

Glutamate and GABA have been identified as the major neurotransmitters in the radial and lateral synaptic pathways, respectively, of the vertebrate retina. Over the past year or so new information has appeared that has significantly increased the knowledge of how these compounds can elicit a range of responses. Key features of this new information are the identification and localization of many receptor subtypes within the retina, the recognition that glutamate can modulate membrane potential through cGMP-gated ion channels, and the finding that GABA can be released through non-vesicular pathways.

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.

Parasol (P alpha) ganglion-cells of the primate fovea: immunocytochemical staining with antibodies against GABAA-receptors

Retinae of macaque monkeys were immuno-stained with antibodies against GABAA-receptors. In peripheral retina most ganglion cells were immunoreactive. In central retina, around the fovea, staining in the ganglion cell layer was selective and only 5-8% of all ganglion cells were labelled: these had the largest cell bodies and their dendrites occupied a broad stratum in the middle of the inner plexiform layer. From comparison with Golgi-stained ganglion cells it is concluded that the entire population of parasol (P alpha)-cells at the fovea was labelled. The mosaic and sampling properties of parasol cells were determined by combining dendritic field measurements of Golgi-stained cells with their density when immuno-stained. There is convergence of 30-50 cones onto each foveal parasol ganglion cell. The dendritic fields of both ON- and OFF-parasol cells provide complete retinal coverage. The Nyquist limits of their mosaics are 4 min of arc.

Neurons of the chick brain and retina expressing both alpha-bungarotoxin-sensitive and alpha-bungarotoxin-insensitive nicotinic acetylcholine receptors: an immunohistochemical analysis

Immunohistochemical methods were used to study the possible co-localization of two alpha-bungarotoxin-sensitive (alpha 7 and alpha 8) and two alpha-bungarotoxin-insensitive (beta 2 and alpha 3) subunits of the nicotinic acetylcholine receptors in neurons of the chick brain and retina. Several structures contained neurons that were doubly-labeled with antibodies against the alpha 7 subunit and the beta 2 subunit. These structures included, for example, the interpeduncular nucleus, nucleus spiriformis lateralis, optic tectum, pretectal visual nuclei, and the lateral hypothalamus. Double-labeling with antibodies against the alpha 7 and alpha 8 subunits was also seen in several regions, which included the interpeduncular nucleus, visual pretectum, lateral hypothalamus, dorsal thalamus, and the habenular complex. In the retina, many cells in the inner nuclear layer were observed to contain alpha 8 and alpha 3 subunits, whereas neurons in the ganglion cell layer were seen to contain alpha 7 and alpha 8 or, less frequently, alpha 7 and alpha 3 subunits. These results indicate that alpha-bungarotoxin-sensitive and alpha-bungarotoxin-insensitive subunits of the nicotinic receptors are co-expressed by neurons of the chick brain and retina.

Actions of GABAergic ligands on brisk ganglion cells in the cat retina

Gamma-aminobutyric acid (GABA) is the major inhibitory neurotransmitter in the mammalian retina. We tested the actions of iontophoretically applied GABAergic ligands on the spontaneous and stimulus-evoked activity of retinal ganglion cells recorded extracellularly in the in vivo cat eye. GABA as well as GABAA receptor agonists inhibited all brisk ganglion cell types. This action was antagonized by bicuculline. Bicuculline on its own increased the activity of ON-ganglion cells but suppressed OFF-ganglion cells. This suppression effect was abolished during the blockade of glycinergic transmission by strychnine. The GABAB receptor agonist baclofen inhibited OFF-ganglion cells whereas the activity of ON-ganglion cells was either increased or decreased depending on the stimulus contrast. The antagonists, phaclofen and 2-hydroxy saclofen, produced opposite effects to baclofen and antagonized its action. The present study demonstrates that both GABAA and GABAB receptors modulate the activity of ON- and OFF-ganglion cells in the cat retina.

Immunohistochemical localization of nicotinic acetylcholine receptor subunits in the mesencephalon and diencephalon of the chick (Gallus gallus)

Monoclonal antibodies against two alpha-bungarotoxin-binding subunits (alpha 7 and alpha 8) of the nicotinic acetylcholine receptors (nAChRs) were used as immunohistochemical probes to map their distribution in the chick diencephalon and mesencephalon. The distribution of the alpha 7 and alpha 8 nAChR subunits was compared to the distribution of immunoreactivity produced by a monoclonal antibody against the beta 2 structural subunit of the nAChRs. Structures that contained high numbers of alpha 7-like immunoreactive (LI) somata included the intergeniculate leaflet, nucleus intercalatus thalami, nucleus ovoidalis, organum paraventricularis, nucleus rotundus, isthmic nuclei, nucleus trochlearis, oculomotor complex, nucleus interstitio-pretecto-subpretectalis, stratum griseum centrale of the optic tectum, and nucleus semilunaris. Neuropil staining for alpha 7-LI was intense in the nucleus dorsomedialis hypothalami, nucleus geniculatus lateralis ventralis, griseum tecti, isthmic nuclei, nucleus lentiformis mesencephali, nucleus of the basal optic root, and stratum griseum et fibrosum superficiale of the tectum. High numbers of alpha 8-LI somata were found in the stratum griseum et fibrosum superficiale of the tectum and the nucleus interstitio-pretecto-subpretectalis, and intense neuropil staining for alpha 8-LI was found in the dorsal thalamus, nucleus geniculatus lateralis ventralis, lateral hypothalamus, griseum et fibrosum superficiale of the tectum. High numbers of beta 2-LI somata were found only in the nucleus spiriformis lateralis, whereas neuropil staining for beta 2-LI was intense in the nucleus geniculatus lateralis ventralis, nucleus suprachiasmaticus, nucleus lateralis anterior, nucleus habenularis lateralis, area pretectalis, griseum tecti, nucleus lentiformis mesencephalis, nucleus externus, and nucleus interpeduncularis, and in the stratum griseum centrale, stratum griseum et fibrosum superficiale, and stratum opticum of the tectum. These results indicate that there are major disparities in the localization of the alpha-bungarotoxin-binding alpha 7 and alpha 8 nAChR subunits and the beta 2 structural nAChR subunit in the chick diencephalon and mesencephalon. These nAChR subunits appear, however, to coexist in several regions of the chick brain.

Localization of GABAA receptor subtypes in the tiger salamander retina

Dry autoradiography was used to determine the distribution of GABAA binding sites in tiger salamander retina. High-affinity binding of [3H]-flunitrazepam [( 3H]-FNZ) was used to localize benzodiazepine receptors (BZR) and [3H]-muscimol was used to localize the GABAA recognition site. Specific [3H]-FNZ binding was present only in the inner retina, primarily in the inner plexiform layer (IPL). Co-incubation with GABA enhanced [3H]-FNZ binding by 20-50%. [3H]-muscimol binding was found throughout the IPL and in the outer plexiform layer (OPL). Mouse monoclonal antibodies 62-3G1 and BD-17, that recognize the GABAA beta 2, beta 3 polypeptides, and BD-24, that recognizes the GABAA alpha 1 polypeptide, did not label either the OPL or IPL, despite numerous variations in the fixation and immunoprocessing methods. GABAA receptor location, as revealed by [3H]-muscimol binding, matches the distribution of presumed GABAergic terminals in the OPL and IPL. We suggest that there are at least two subtypes of GABAA receptor in the tiger salamander retina: one type is present only in the inner retina, primarily in the IPL and is functionally coupled to BZRs; the other type is located in the OPL and is not coupled to the BZRs. Furthermore, GABAA receptors in the tiger salamander retina appear to be of a different epitope than GABAA receptors in numerous other preparations that are recognized by mAbs 62-3G1, BD-17, and BD-24.

GABA-like immunoreactive cells containing nicotinic acetylcholine receptors in the chick retina

The possibility that GABA-like immunoreactive cells of the chick retina also contain neuronal nicotinic acetylcholine receptors was investigated by means of immunohistochemical techniques. Double-labeled cell bodies containing GABA-like immunoreactivity and nicotinic receptor-like immunoreactivity were seen in the inner third of the inner nuclear layer and were presumably amacrine cells. Approximately 29-36% of the GABA-positive cells in the inner nuclear layer contained nicotinic receptor immunoreactivity. Their soma sizes ranged from 5-12 microns. Some double-labeled cells ranging from 7-21 microns were observed in the ganglion cell layer as well. Between 9-37% of the GABA-positive cells in this layer contained nicotinic receptor-like immunoreactivity. Following injection of a retrograde tracer into the optic tectum, some of the retrogradely labeled cells were also double labeled with antibodies against GABA and nicotinic receptors. This indicates that at least some of the GABA-positive cells containing nicotinic acetylcholine receptors in the ganglion cell layer are indeed ganglion cells. The present data appear to represent the first demonstration of the presence of acetylcholine receptors in GABA-containing cells in the retina, thus providing a basis for a possible influence of acetylcholine upon those presumptive GABAergic cells.