GABAergic synapses and benzodiazepine receptors are not identically distributed in the primate retina

Intrinsically photosensitive ganglion cells of the primate retina express distinct combinations of inhibitory neurotransmitter receptors

Intrinsically-photosensitive retinal ganglion cells (ipRGCs) express the photopigment melanopsin and function as irradiance detectors, responsible for crucial non-image forming visual functions. In addition to their intrinsic photosensitivity, ipRGCs are also activated by synaptic inputs originating at the classical photoreceptors, rods and cones. Little is known about inhibition through these retinal pathways, despite ipRGCs receiving massive synaptic inputs from inhibitory amacrine interneurons. We performed a wide anatomical screening for neurotransmitter receptors possibly involved in the inhibitory modulation of ipRGCs in the macaque retina. We investigated both subtypes of primate ipRGCs described so far and report that outer-stratifying (M1) cells possess mainly GlyR α2 and GABA(A)R α3 subunits, while inner-stratifying (M2) cells are overall subject to less inhibitory modulation. Our results suggest that M1 and M2 ipRGC subtypes are modulated via distinct inhibitory intraretinal circuits.

Functional polarity of dendrites and axons of primate A1 amacrine cells

The A1 cell is an axon-bearing amacrine cell of the primate retina with a diffusely stratified, moderately branched dendritic tree (approximately 400 microm diameter). Axons arise from proximal dendrites forming a second concentric, larger arborization (>4 mm diameter) of thin processes with bouton-like swellings along their length. A1 cells are ON-OFF transient cells that fire a brief high frequency burst of action potentials in response to light (Stafford & Dacey, 1997). It has been hypothesized that A1 cells receive local input to their dendrites, with action potentials propagating output via the axons across the retina, serving a global inhibitory function. To explore this hypothesis we recorded intracellularly from A1 cells in an in vitro macaque monkey retina preparation. A1 cells have an antagonistic center-surround receptive field structure for the ON and OFF components of the light response. Blocking the ON pathway with L-AP4 eliminated ON center responses but not OFF center responses or ON or OFF surround responses. Blocking GABAergic inhibition with picrotoxin increased response amplitudes without affecting receptive field structure. TTX abolished action potentials, with little effect on the sub-threshold light response or basic receptive field structure. We also used multi-photon laser scanning microscopy to record light-induced calcium transients in morphologically identified dendrites and axons of A1 cells. TTX completely abolished such calcium transients in the axons but not in the dendrites. Together these results support the current model of A1 function, whereby the dendritic tree receives synaptic input that determines the center-surround receptive field; and action potentials arise in the axons, which propagate away from the dendritic field across the retina.

Rabbit retinal ganglion cell survival after optic nerve section and its effect on the inner plexiform layer

Structural modifications of the inner retina were studied after optic nerve section (ONS) in the rabbit. Retinal ganglion cells (RGC) were labelled by injection of Fast Blue into the optic nerve, and counted under fluorescent light in control retina and retina 7, 14, 21 and 26 days post-axotomy. Studies on retinal cross-sections were also performed. For this purpose, retinal sections were stained with haematoxylin-eosin and immunohystochemistry for alpha1 and beta2/beta3 sub-units of the GABA(A) receptors. One week after axotomy, there was no significant loss in the number of ganglion cells with respect to control counts (1086+/-173cellsmm(-2) in the visual streak and 119+/-46cellsmm(-2) in the periphery, mean+/-SD, n=5). At 14 days post-axotomy, 271+/-46cellsmm(-2) remained in the visual streak and 33+/-6cellsmm(-2) in the periphery, corresponding to a mean survival of 27%. The number of ganglion cells decreased further on the following days, reaching 7.54% 1 month after ONS. A significant reduction in the thickness of the inner plexiform and ganglion cell layers was also observed in retinal cross-sections. Immunocytochemical studies show a remarkable disorganization of the layer stratification in the inner plexiform layer (IPL). We conclude that after ONS, RGC death occurs mainly between 7 and 14 days post-axotomy and a progressive death up to 26 days, causing a decrease in the thickness of the IPL and subsequent disorganization of its layers.

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.

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.

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.

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.

Regulation of endogenous dopamine release in amphibian retina by gamma-aminobutyric acid and glycine

Endogenous dopamine release in the retina of the African clawed frog (Xenopus laevis) increases in light and decreases in darkness. The roles of the inhibitory amino acid transmitters gamma-aminobutyric acid (GABA) and glycine in regulating this light/dark difference in dopamine release were explored in the present study. Exogenous GABA, the GABA-A receptor agonist muscimol, the GABA-B receptor agonist baclofen, and the GABA-C receptor agonist cis-aminocrotonic acid (CACA) suppressed light-evoked dopamine overflow from eyecups. The effects of GABA-A and -B receptor agonists were selectively reversed by their respective receptor-specific antagonists, whereas the effect of CACA was reversed by the competitive GABA-A receptor antagonist bicuculline. The benzodiazepine diazepam enhanced the effect of muscimol on light-evoked dopamine release. Both GABA-A and -B receptor antagonists stimulated dopamine release in light or darkness. Bicuculline was more potent in light than in darkness. These data suggest that retinal dopaminergic neurons are inhibited by GABA-A and -B receptor activation in both light and darkness but that GABA-mediated inhibitory tone may be greater in darkness than in light. Exogenous glycine inhibited light-stimulated dopamine release in a concentration-dependent and strychnine-sensitive manner. However, strychnine alone did not increase dopamine release in light or darkness, nor did it augment bicuculline-stimulated release in darkness. Additionally, both strychnine and 7-chlorokynurenate, an antagonist of the strychnine-insensitive glycine-binding site of the N-methyl-D-aspartate subtype of glutamate receptor, suppressed light-evoked dopamine release. Thus, the role of endogenous glycine in the regulation of dopamine release remains unclear.

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.

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.

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.

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.

Morphology and mosaics of VIP-like immunoreactive neurons in the retina of the rhesus monkey

Vasoactive intestinal peptide (VIP) is a 28-amino acid peptide that has been demonstrated to reside in cells ( = VIP+ cells) of the retinae of various vertebrate species. In an attempt to study the morphology and distribution of VIP+ cells in the retina of the rhesus monkey in more detail, we subjected VIP+ cells observed in cryostat sections or wholemounts rhesus monkey retinae to a quantitative analysis. VIP+ cells were found to reside in the innermost row of the inner nuclear layer (INL) and in the ganglion cell layer (GCL) in similar numbers (estimate: 50 cells/mm2 at 6-10 mm eccentricity each) and only on rare occasions (12% of all VIP+ cells) in varying positions within the inner plexiform layer (IPL). Somata of VIP+ cells were circular and had a mean diameter of 9.1 microns. They gave rise to 1-3 main dendrites, which were usually oriented toward the IPL. Main dendrites ramified widely into thin fibers (dendritic field diameter less than = 1 mm), carrying varicose swellings. The fibers that contributed to one and the same plexus of VIP+ fibers preferred the middle third of the IPL, independent of the positions of the parent somata. A quantitative analysis of nearest-neighbour distances in the retinal wholemount preparation suggested that VIP+ cells in the GCL and in the INL might be distributed according to 2 independent mosaics. A comparison with Golgi-stained material leads to the tentative equation of VIP+ cells with the "spiny" A12 amacrine cell of Mariani ('90). Whereas the low density and large dendritic field size of VIP+ cells might suggest a more widespread function, the varicose dendritic morphology seems to be more compatible with functionally independent dendritic subunits mediating localized effects.

Cellular distribution of L-glutamate decarboxylase (GAD) and gamma-aminobutyric acidA (GABAA) receptor mRNAs in the retina

1. Gamma-aminobutryic acid (GABA), a major inhibitory transmitter of the vertebrate retina, is synthesized from glutamate by L-glutamate decarboxylase (GAD) and mediates neuronal inhibition at GABAA receptors. GAD consists of two distinct molecular forms, GAD65 and GAD67, which have similar distribution patterns in the nervous system (Feldblum et al., 1990; Erlander and Tobin, 1991). GABAA receptors are composed of several distinct polypeptide subunits, of which the GABAA alpha 1 variant has a particularly extensive and widespread distribution in the nervous system. The aim of this study was to determine the cellular localization patterns of GAD and GABAA alpha 1 receptor mRNAs to define GABA- and GABAA receptor-synthesizing neurons in the rat retina. 2. GAD and GABAA alpha 1 mRNAs were localized in retinal neurons by in situ hybridization histochemistry with 35S-labeled antisense RNA probes complementary to GAD67 and GABAA alpha 1 mRNAs. 3. The majority of neurons expressing GAD67 mRNA is located in the proximal inner nuclear layer (INL) and ganglion cell layer (GCL). Occasional GAD67 mRNA-containing neurons are present in the inner plexiform layer. Labeled neurons are not found in the distal INL or in the outer nuclear layer (ONL). 4. GABAA alpha 1 mRNA is expressed by neurons distributed to all regions of the INL. Some discretely labeled cells are present in the GCL. Labeled cells are not observed in the ONL. 5. The distribution of GAD67 mRNA demonstrates that numerous amacrine cells (conventional, interstitial, and displaced) and perhaps interplexiform cells synthesize GABA. These cells are likely to employ GABA as a neurotransmitter. 6. The distribution of GABAA alpha 1 mRNA indicates that bipolar, amacrine, and perhaps ganglion cells express GABAA receptors having an alpha 1 polypeptide subunit, suggesting that GABA acts directly upon these cells.

GABAA receptors in the retina of the cat: an immunohistochemical study of wholemounts, sections, and dissociated cells

Gamma-aminobutyric acid (GABA) is an inhibitory neurotransmitter used by many neurons of the mammalian retina. To identify the synaptic targets of these cells, we undertook an immunohistochemical study with a monoclonal antibody that recognizes the GABAA receptors (62-3G1, generously donated by A. de Blas). This antibody labels the somata of at least one group of amacrine cells in the inner nuclear layer. It also labels two groups of somata in the ganglion cell layer; one small and the other much larger. The small cells are likely to be displaced amacrine cells based on their size, although some could be gamma ganglion cells. The much larger receptor-positive cells are clearly ganglion cells, based both on their size and the antibody labeling of the initial portion of their axon. In the peripheral retina, the size of these large somata suggests that many are beta ganglion cells. However, at any point across the retina the density of these cells never exceeded 50% of the density of beta cells as a whole. The antibody also labels a dense plexus of processes that extends throughout the inner plexiform layer (IPL), with a marked concentration in the inner third of the layer. This is the portion of the IPL in which the rod bipolar cells terminate. It is difficult to recognize processes of individual cells in the IPL, so retinae were dissociated. The rod bipolar cells were identified by protein kinase C immunoreactivity (Negishi et al., 1988; Karschin & Wäsle, 1990). They were not labeled by the GABAA receptor antibody. This is surprising in light of tight-seal, whole cell voltage-clamp recordings that have shown that the rod bipolars express functional GABAA receptors. One possible explanation is that the antibody recognizes only a subset of the GABAA receptors.

Mapping of benzodiazepine-like immunoreactivity in the rat brain as revealed by a monoclonal antibody to benzodiazepines

A monoclonal antibody against benzodiazepines (21-7F9) was used to study the distribution of benzodiazepine-like immunoreactivity in the rat brain. Immunodensitometry in combination with image analysis were used for quantification. The results showed a ubiquitous distribution of benzodiazepine-like immunoreactivity throughout the brain. Very high levels of benzodiazepine-like immunoreactivity were found in the Purkinje cell layer of the cerebellum, in the primary olfactory cortex, in the stratum pyramidale of the hippocampus and in the mitral cell layer of the olfactory bulb. High densities of benzodiazepine-like immunoreactivity were found in the granule cell layer of the cerebellum, the pyramidal cell layer of the olfactory tubercle, the granule layer of the dentate gyrus, the arcuate nucleus of the hypothalamus, the mammillary bodies, the interstitial nucleus of Cajal and superficial grey layer of superior colliculus. The substantia nigra pars compacta, the islands of Calleja and layers II, III, V and VI of the cerebral cortex had moderate levels of benzodiazepine-like immunoreactivity. Lower densities were found in the internal granular layer and the external plexiform layer of the olfactory bulb, in the molecular layer of the dentate gyrus, in layers I and IV of the cerebral cortex, in the nucleus caudate-putamen and most of the thalamic nuclei. The lowest density of immunoreactivity was found in the globus pallidus, and the strata radiatum, oriens and lacunosum-moleculare of the hippocampus. The distribution of endogenous benzodiazepine-like immunoreactivity was compared with the distribution of the GABA/benzodiazepine receptor by using both immunocytochemistry and receptor autoradiography. Our studies have shown a clear mismatch between the localization of the benzodiazepine-like immunoreactivity and the GABA/benzodiazepine receptors.

Amacrine cells of the rhesus monkey retina

Amacrine cells of the rhesus monkey, Macaca mulatta, were studied in 38 retinas Golgi-impregnated as whole, flat preparations. By using criteria of dendritic morphology, span of arborization, and level of arborization in the inner plexiform layer, 26 types of amacrine cell ranging in size of dendritic span from 30 microns to 2 mm were identified and listed in increasing size of dendritic span. In some instances, different cell types could be grouped together due to similar morphological features. For example, 1 group, "knotty amacrine cells," has small cell bodies and a profusion of small, varicose, intertwined processes that span up to 30 microns and are essentially monostratified, but each of the 3 types ends in different strata. Another group is 2 types with about 20 fine radiating processes spanning 1 mm that possess some prominent varicosities. One of these has all of its processes terminating in the innermost stratum of the inner plexiform layer ("spidery"-type 2 amacrine cells). The other with predominantly similarly ending processes has some that also terminate in the outermost stratum ("spidery"-type 1 amacrines). These 2 cell types likely correspond to the type 1 and type 2 indolamine-accumulating amacrine cells in rabbit retina. Other types are individuals which cannot be grouped together but resemble familiar types in cat retina (AII and A13). Other types can be correlated with their putative neurotransmitter (type 1 CA-dopamine) or transmitter/drug receptor ("spiny"-benzodiazepine receptor) phenotype. Many types as yet have no known correlate from other Golgi studies or clues as to transmitter or receptor phenotype. This study provides evidence for an unprecedented number of amacrine cell types in the primate retina. The similar morphologies of different types of amacrine cell types within a group suggest other common features within these groups such as neurotransmitter phenotype.

Distribution of immunoreactivity to protein kinase C in the turtle retina

Immunocytochemical staining procedures using the HRP-complexed antibody to protein kinase C (PKC) have been carried out on the turtle retina. Wholemounts and frozen sections of retina have been studied by light microscopy to evaluate PKC immunoreactivity after stimulation of the retina with light and neurotransmitters known to be active in the vertebrate retina. The most dramatically stained sites are cone synaptic pedicles and bipolar cells under all conditions. Ganglion cells stain weakly under certain conditions. Applying the antibody to a 'control' retina under dark adapted conditions results in uniform background staining of both hyperpolarizing and depolarizing bipolar pathways, while stimulating the retina with K+ under dim light conditions results in discretely stained bipolar cells and a prominent band of staining in stratum 4 of the inner plexiform layer. Stronger stimulation of bipolar cells with their terminals contributing to strata 3 and 4 and the continuous dominant band in stratum 4 can be elicited with incubation of the retina in neurotransmitter agonists, GABA and dopamine. Incubation with dopamine, in particular, brings out the putative dopaminergic amacrine cell. The only condition in which a strong band in stratum 2 can be demonstrated is under stimulation with a flashing bar of spot of light. Thus K+ and neurotransmitter stimulation elicit PKC staining in neurons contributing to the ON or depolarizing sublamina of the IPL, while intermittent flashing light stimulus is required to elicit PKC staining in the OFF or hyperpolarizing sublamina of the IPL.

GABA-like immunoreactivity in the macaque monkey retina: a light and electron microscopic study

The distribution of GABA-like immunoreactivity in the macaque monkey retina was studied by using postembedding techniques on semithin and ultrathin sections. At the light microscopic level, both inner and outer plexiform layers showed strong GABA-like immunoreactivity in the central retina. All the horizontal cells, some bipolar cells, 30-40% of amacrine cells, occasional interplexiform cells, and practically all displaced amacrine cells were labeled. In the peripheral retina (beyond 5 mm eccentricity), the outer plexiform layer and the horizontal cells were not labeled, but all other cell types showed the same labeling pattern as in the central retina. Synapses of the inner plexiform layer involving a pre- or postsynaptic GABA-labeled process were studied electron microscopically. Synapses involving a GABA-labeled presynaptic amacrine cell process made up 80% of the synapses observed. These GABA-labeled amacrine processes synapsed onto amacrine, bipolar, and ganglion cell processes as well as onto amacrine and ganglion cell bodies. Synapses involving a postsynaptic GABA-labeled process made up 20% of the synapses studied. The GABA-like immunoreactive processes were postsynaptic to bipolar cells at the dyads and to amacrine cells at conventional synapses.

Distribution of GABA-immunoreactive amacrine cell synapses in the inner plexiform layer of macaque monkey retina

The distribution patterns of GABA immunoreactive (+) and immunonegative (-) amacrine cell synapses and profiles in the inner plexiform layer (IPL) were analyzed in three macaque monkey retinas using postembedding electron-microscopic (EM) immunogold cytochemistry. Synapses and profiles were counted at 5% intervals throughout the IPL depth in three EM montages (total area = 6509 microns 2), with 0% depth at the inner nuclear layer/IPL border. Nearly 70% of all amacrine synapses were GABA+, and they contacted all major classes of neurons that arborize in the IPL: bipolars (45%), ganglion cells (25%), and GABA+ (20%) and GABA- (10%) amacrines. A major relationship was seen between GABA+ amacrine processes and bipolar terminals: 76% of all amacrine-to-bipolar synapses were GABA+, and 82% of bipolar output dyads contained at least one GABA+ amacrine. GABA+ amacrine profiles (N = 2455) were concentrated in three wide bands at IPL depths of 0-25%, 40-60%, and 75-100%, corresponding to the dense bands seen with light-microscopic immunocytochemistry. In contrast, GABA+ amacrine synapses (N = 1081) were distributed evenly throughout the IPL depth, rather than being confined to the three dense bands. GABA- amacrine synapses (N = 516) were concentrated at 40% and 60% depths. Each category of amacrine output synapses had a characteristic pattern of stratification in the IPL. GABA+ amacrine-to-bipolar synapses occurred throughout the IPL but were most frequent at 20% and 80% IPL depths, where the dendrites of midget cone bipolars arborize (Polyak, 1941). In contrast, GABA+ amacrine-to-ganglion cell synapses were concentrated at 30% and 70% IPL depths, near the dendritic arborizations of parasol ganglion cells (Watanabe & Rodieck, 1989). GABA+ synapses onto bipolars and amacrines were also concentrated at the level of rod bipolar terminals. GABA+ amacrines must play significant but different roles in ON and OFF midget and parasol pathways as well as the rod pathway.

Immunocytochemistry of neurotransmitter receptors

Over the last several years our knowledge of neurotransmitter receptors has increased dramatically as receptor types and subtypes have been identified through the development of selective antagonists, neuropharmacological studies, and radioactive ligand binding studies. At the same time major advances were made in the immunocytochemical localization of neurotransmitters and their related enzymes. However, only recently has immunocytochemistry been used to localize neurotransmitter receptors, and these studies have been limited. Four receptors have been localized in the CNS with immunocytochemistry: the nicotinic acetylcholine receptor, the beta-adrenergic receptor, the GABA/benzodiazepine receptor, and the glycine receptor. Of these the glycine receptor has been the most thoroughly characterized. Glycine receptor immunoreactivity is highly concentrated at postsynaptic sites, and the distribution of immunoreactivity appears to correlate closely with glycinergic neurons. However, immunocytochemical studies done on other receptors suggest such a distribution may not always be the case. Some receptors may not be concentrated at postsynaptic sites, and receptor distribution may not always closely fit the distribution of the respective neurotransmitter. Work is rapidly progressing on the purification of other receptors and on the production of selective antibodies which will allow immunocytochemical studies which address these and other questions.

Properties of GABA-activated whole-cell currents in bipolar cells of the rat retina

This paper describes experiments on GABA-activated whole-cell membrane currents in bipolar cells freshly isolated from the adult rat retina. The main goal was to determine whether bipolar cell responses to GABA could be resolved in terms of mediation by the GABAA receptor, the GABAB receptor, or both. Bipolar cells were isolated by gentle enzymatic dissociation and identified by their distinct morphology. GABA agonists and antagonists were applied focally by pressure and the resultant currents were recorded under whole-cell voltage clamp. In all bipolar cells tested, GABA (0.1-100 microM) induced a monophasic response associated with a conductance increase (IGABA). The shift in reversal potential for IGABA as a function of pipet [Cl-] paralleled that predicted based on the Nernst equation for Cl-. IGABA was mimicked by muscimol (5-20 microM) and antagonized by bicuculline (20-100 microM). Baclofen (0.1-1.0 mM) produced no apparent conductance change. "Hot spots" of sensitivity to GABA which might be associated with regions of synaptic contact were not found; both the soma and processes of all bipolar cells were responsive to focally applied GABA. Furthermore, all bipolar cells tested responded to glycine. In conclusion, we have established the presence of GABAA receptors on rat retinal bipolar cells. Our data suggest further that these cells lack GABAB receptors. Finally, our observation that bipolar cells in the rat retina are relatively homogeneous in terms of their sensitivity to GABA and glycine lead us to postulate that the functional significance of the presence of receptors and their distribution on a neuron may be dictated more by the topography of the presynaptic inputs than by its inherent chemosensitivity.

Immunocytochemical localization of the GABAA/benzodiazepine receptor in the guinea pig cochlear nucleus: evidence for receptor localization heterogeneity

Immunocytochemistry with a monoclonal antibody against the GABAA/benzodiazepine receptor showed labeled axo-dendritic synapses in the anteroventral cochlear nucleus. In the dorsal cochlear nucleus, label was seen apposing both axo-somatic and axo-dendritic terminals. The results suggest a heterogeneous distribution of GABA receptors, together with a possible segregation of receptor subtypes between somata and dendrites in certain neurons. The presence of cytoplasmic labeling in some neurons might reflect a higher receptor turnover rate in these neurons.

Localization of L-glutamic acid decarboxylase mRNA in monkey and human retina by in situ hybridization

Immunocytochemical studies with gamma-aminobutyric acid (GABA) antibodies and glutamic acid decarboxylase antibodies have shown that the primate retina contains GABAergic amacrine, interplexiform, and displaced amacrine cells. In addition, subpopulations of photoreceptors and horizontal cells have also been suggested to be GABAergic in this retina. In the present study, we have used in situ hybridization to localize GABAergic neurons in human and monkey retinas. In situ hybridizations were carried out with 35S-labeled DNA and RNA probes derived from human and cat glutamic acid decarboxylase cDNA clones. In the monkey retina, labeled cells were present in the inner nuclear and ganglion cells layers. The outer nuclear layer or the inner segment layer had only background levels of labeling. In the inner nuclear layer, all labeled somata were located in the vitread-half bordering the inner nuclear layer/inner plexiform layer boundary. These cells constituted approximately 83% of all labeled cells. Labeled cells were also seen in the ganglion cell layer. In the human retina, labeled somata were observed only in the inner nuclear and the ganglion cell layers. In the inner nuclear layer, the majority of labeled cells were located close to the inner nuclear layer/inner plexiform layer boundary although a minor population of labeled somata (approximately 20%) were found deeper in the inner nuclear layer. The distribution of glutamic acid decarboxylase mRNA-containing cells we observed is in good agreement with the known location of GABAergic neurons. We, however, did not find glutamic acid decarboxylase mRNA in either horizontal cells or photoreceptors in monkey and human retina.

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

A large population of amacrine cells in the retina are thought to use GABA as an inhibitory neurotransmitter in their synaptic interactions within the inner plexiform layer. However, little is known about their synaptic targets; the neurons that express the receptors for GABA have not been clearly identified. Recently, the GABAA receptor has been isolated and antibodies have been raised against it. These antibodies have proven useful for the immunocytochemical localization of the receptor, and two brief reports describing the distribution of GABAA receptor immunoreactivity in the retina have appeared (Richards et al., 1987; Mariani et al., 1987). We used a monoclonal antibody (62-3G1) against the GABAA receptor to study the retina of the New World primate Saimiri sciureus. Labeled somata were found in the inner nuclear layer (INL) and ganglion cell layer (GCL). The staining was confined to what appeared to be the cell's plasmalemma and small cytoplasmic granules. Most of the labeled neurons in the INL had small somata (5-7 microns in diameter) located at the vitreal edge of the layer. They arborized in two laminae (approximately 2 and 4) of inner plexiform layer (IPL). Ventral to the optic disc (2.5 mm) they comprised 29% of the cells present. A few of the labeled neurons appeared to be interplexiform cells or flat bipolar cells, with labeled processes that extended into both the IPL and the inner half of the outer plexiform layer. In the GCL, the labeled somata were among the largest present (13-20 microns in diameter), and 2.5 mm ventral to the optic disc they made up 15% of the cells present. Experiments in which immunoreactive somata were retrogradely labeled following the injection of fluorescent tracers into the optic tract provided a conclusive demonstration that some of the immunoreactive somata were ganglion cells. The antibody often labeled their axons in the optic fiber layer. This suggests that the GABAA receptors are transported anterogradely to the retinal terminal fields. The dendrites of the immunoreactive ganglion cells extended into the 2 laminae of labeled processes in the IPL, and their primary dendritic arbors were, at any given eccentricity, quite similar in appearance. This homogeneity suggests that they comprise a particular subset of the ganglion cells. Sections simultaneously labeled with the monoclonal antibody against the GABAA receptor and antisera against either L-glutamic acid decarboxylase (GAD) or GABA revealed that the GAD/GABA was distributed much more widely in the IPL than the GABAA receptor.(ABSTRACT TRUNCATED AT 400 WORDS)

Benzodiazepines and the mammalian retina. I. Autoradiographic localisation of receptor sites and the lack of effect on the electroretinogram

The majority of specific benzodiazepine binding sites were found to be restricted to the innerplexiform layer of the rat retina, although there were minor amounts of binding in the inner nuclear and ganglion cell layers. Relatively high levels of non-specific benzodiazepine binding sites were, on the other hand, found in the pigment epithelium and ciliary body, as well as in the corneal epithelium. The specific binding was enhanced by the GABA-A agonist, muscimol. In both rats and cats, neither acute nor chronic administration of benzodiazepines or their antagonists altered the retinal functions, as determined by the electroretinogram. These results suggest that retinal benzodiazepine receptors do not influence visually induced preganglionic retinal activity.

Immunocytochemical localization of GABAA receptors in goldfish and chicken retinas

A monoclonal antibody (mAb 62-3G1) to the GABAA receptor/benzodiazepine receptor/Cl- channel complex from bovine brain was used with light and electron microscopy in goldfish retina and light microscopy in chicken retina to localize GABAA receptor immunoreactivity (GABAr-IR). GABAr-IR was found in the outer plexiform layer (OPL) in both species, in three broad bands in the inner plexiform layer (IPL) of goldfish, and in seven major bands of the chicken IPL. A small percentage of amacrine cell bodies (composing at least three types) were stained in chicken. In goldfish OPL, GABAr-IR was localized intracellularly and along the plasma membrane of cone pedicles, whereas rod spherules were lightly stained, but always only intracellularly. In chicken, all three sublayers of the OPL were GABAr-IR. The presence of GABAr-IR on photoreceptor terminals is consistent with data indicating feedback from GABAergic horizontal cells to cones. In the goldfish IPL, GABAr-IR was localized to postsynaptic sites of amacrine cell synapses; intracellular staining of processes in the IPL also was observed in presumed "GABAergic" targets. A comparison of GABAr-IR with the distributions of 3H-muscimol uptake/binding, glutamate decarboxylase-IR, GABA-IR, and 3H-GABA uptake in the IPL showed either a reasonable correspondence or mismatch, depending on the marker, species, and lamina within the IPL. The distribution of GABAr-IR in the retina corresponded better with the 3H-muscimol than with 3H-benzodiazepine binding patterns yet overall was in excellent agreement with many other physiological and anatomical indicators of GABAergic function. We suggest that intracellular GABAr-IR represents the biosynthetic and/or degradative pathway of the receptor and we conclude that mAb 62-3G1 is a valid marker of GABAA receptors in these retinas and will serve as a useful probe with which to address the issue of mismatches between the localization of GABAA receptors and indicators of presynaptic GABAergic terminals.

Electrophysiology of benzodiazepine receptor ligands: multiple mechanisms and sites of action

Electrophysiology of BZR ligands has been reviewed from different points of view. A great effort was made to critically discuss the arguments for and against the temporarily leading hypothesis of the mechanism of action of BZR ligands, the GABA hypothesis. As has been discussed at length in the present article, an impressive body of electrophysiological and biochemical evidence suggests an enhancement of GABAergic inhibition in CNS as a mechanism of action of BZR agonists. Biochemical data even indicate a physical coupling between GABA recognition sites and BZR which, together with the effector site build-up by Cl- channels, form a supramolecular GABAA/BZR complex. By binding to a specific site on this complex, BZR agonists allosterically increase and BZR inverse agonists decrease the gating of GABA-linked Cl- channels, whereas BZR antagonists bind to the same site without an appreciable intrinsic activity and block the binding and action of both agonists as well as inverse agonists. While this model is supported by many electrophysiological experiments performed with BZR ligands in higher nanomolar and lower micromolar concentrations, it does not explain much controversial data from animal behavior and, more importantly, is not in line with electrophysiological effects obtained with low nanomolar BZ concentrations. The latter actions of BZR ligands in brain slices occur within a concentration range compatible with concentrations of BZ observed in CSF fluid, which would be expected to be found in the biophase (receptor level) during anxiolytic therapy in man. Enhanced K+ conductance seems to be a suitable candidate for this effect of BZR ligands. This direct action on neuronal membrane properties may underlie the many electrophysiological observations with extremely low systemic doses of BZR ligands in vivo which demonstrated a depressant effect on spontaneous neuronal firing in various CNS regions. Skeletomuscular spasticity and epilepsy are two neurological disorders, where both the enhanced GABAergic inhibition and increased K+ conductance may contribute to the therapeutic effect of BZR agonists, since electrophysiological and behavioral studies strongly support GABA-dependent as well as GABA-independent action of BZR ligands elicited by low to intermediate doses of BZ necessary to evoke anticonvulsant and muscle relaxant effects. Somewhat higher doses of BZR ligands, inducing sedation and sleep, lead perhaps to the only pharmacologically relevant CNS concentrations (ca. 1 microM) which might be due entirely to increased GABAergic inhibition.(ABSTRACT TRUNCATED AT 400 WORDS)