GABA and GAD immunoreactivity of photoreceptor terminals in primate retina

Neocortical post-traumatic epileptogenesis is associated with loss of GABAergic neurons

The subtle mechanisms of post-traumatic epileptogenesis remain unknown, although the incidence of chronic epilepsy after penetrating cortical wounds is high. Here, we investigated whether the increased frequency of seizures occurring within 6 weeks following partial deafferentation of the suprasylvian gyrus in cats is accompanied with a change in the ratio between the number of excitatory and inhibitory neurons. Immuno-histochemical labeling of all neurons with neuronal-specific nuclear protein (NeuN) antibody, and of the GABAergic inhibitory neurons with either gamma-aminobutyric acid (GABA) or glutamic acid decarboxylase (GAD 65&67) antibodies, was performed on sections obtained from control and epileptic animals with chronically deafferented suprasylvian gyrus. Quantification of the labeled neurons was performed in control animals and at 2, 4, and 6 weeks following cortical deafferentation, in the suprasylvian and marginal gyri, both ipsi- and contra-lateral to the cortical trauma. In all epileptic animals, the neuronal loss was circumscribed to the deafferented suprasylvian gyrus. Inhibitory GABAergic neurons were particularly more sensitive to cortical deafferentation than excitatory ones, leading to a progressively increasing ratio between excitation and inhibition towards excitation, potentially explaining the increased propensity to seizures in chronic undercut cortex.

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.

GABA-immunoreactive photoreceptors in the retina of an anuran, Pelobates fuscus

We have recently started to unravel the retinal neurochemistry of an anuran species, the spadefoot toad (Pelobates fuscus), because of its unique lifestyle. The immunolabelling experiments included tests to localize the major inhibitory transmitter, gamma-aminobutyric acid (GABA) to subsets of retinal neurons, using commercially available antibodies. Apart from the regular GABA-immunoreactive pattern observed formerly in other anurans, certain structures in the photoreceptor layer were also regularly labeled for GABA. The soma diameter of the labeled cells is 5-6 microm and the outer segment seems to be unlabeled. In resin-embedded preparations GABA-positive photoreceptor cells were identified as cones based on their sparse distribution and short outer segments. If these cells release GABA as a transmitter, it may act on the second order cells, from which certain horizontal and bipolar cells have functional GABA receptors. Alternatively, GABA may influence the cones themselves through autoreceptors.

Dopaminergic and GABAergic retinal cell populations in mammals

A number of modern techniques now allow histologists to characterize subpopulations of retinal neurons by their neurotransmitters. The morphologies and connections of these chemically defined neurons can be analyzed precisely at both light and electron microscope levels and lead to a better understanding of retinal circuitry. The dopaminergic neurons form a loose population of special wide-field amacrine cells bearing intraretinal axons within the inner plexiform layer. One subtype, the interplexiform cell, sends an axon to the outer plexiform and outer nuclear layers. The number of interplexiform cells is variable throughout mammalian species. The GABAergic neurons form a dense and heterogeneous population of amacrine cells branching at all levels of the inner plexiform layer. The presence of GABA in horizontal cells seems to be species-dependent. Close relationships occur between dopaminergic and GABAergic cells. GABA antagonizes a number of dopaminergic actions by inhibiting both the release and synthesis of dopamine. This inhibition can be supported by GABA synapses onto dopaminergic cells, but GABA can also diffuse to its targets. Finally, GABA is also contained and synthesized in dopaminergic cells. This colocalization might be the basis of an intracellular modulation of dopamine by GABA.

Catecholamine-, indoleamine-, and GABA-containing cells in the chameleon retina

Neurons containing catecholamine, indoleamine, and gamma-aminobutyric acid (GABA) were identified by immunohistochemistry in the chameleon retina. Tyrosine hydroxylase (TH) and serotonin (5HT) were observed mostly in two subtypes of orthotopic amacrine cells differing in their soma size and process distribution within the IPL. Some labelled cells were displaced either to the IPL (5HT) or the GCL (TH and 5HT). A multiplicity of retinal cell types contained GABA including cones, horizontal, amacrine, and ganglion cells. Our results confirmed those obtained in the retinas of other lizards except for the presence of interstitial and displaced amacrine cells containing TH or 5HT of which this is the first report.

A subgroup of bipolar cells in human retina is GABA-immunoreactive

The distribution of GABA in the perifoveal and the near and far peripheral region of human retina was studied with peroxidase anti-peroxidase immunocytochemistry applied on semithin epoxy resin sections. Among the labeled amacrine cells in these regions, four types can be identified: putative diffuse A2 amacrines, stratified semilunar amacrines, interstitial amacrines and small displaced amacrines. GABA-immunoreactive interplexiform cells and ganglion cells also occur. Contrary to previous post-embedding studies, our preparations show that some bipolar cells in the near and far peripheral region are GABA-immunopositive. This indicates that a number of bipolar cells in human retina does have an enhanced GABA content.

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.

Neurotransmitter-specific identification and characterization of neurons in the all-cone retina of Anolis carolinensis, I: Gamma-aminobutyric acid

The inhibitory amino-acid neurotransmitter, gamma-aminobutyric acid (GABA), was localized in the pure cone retina of the lizard Anolis carolinensis by autoradiographic and immunocytochemical techniques. Uptake of [3H]-GABA labeled horizontal cells, amacrine cells, numerous cells in the ganglion cell layer, both plexiform layers, and the nerve fiber layer. Label in the inner plexiform layer showed distinct lamination. The pattern of GABA immunoreactivity was similar to the pattern of [3H]-GABA uptake, although some differences, particularly in labeling of amacrine and ganglion cells, were observed. Immunocytochemistry revealed endogenous stores of GABA in a set of horizontal cells, amacrine cells, and cells in the ganglion cell layer. Both plexiform layers were labeled by the GABA antisera. Labeling in the inner plexiform layer (IPL) was highly stratified and GABA-immunoreactive strata were present in both sublaminae a and b. Six subtypes of conventionally placed GABA-immunoreactive amacrine cells and one displaced amacrine cell subtype were identified. Three of the six conventional amacrine cell subtypes were of pyriform morphology and three subtypes were of multipolar morphology. GABA-immunoreactive interstitial cells also were observed. Under certain conditions the GABA antiserum labeled the cones. Etching the resin eliminated cone labeling, suggesting that GABA in the cones is present in a labile pool, unlike GABA in horizontal or amacrine cells, or the observed labeling was not due to endogenous GABA. Cones did not demonstrate [3H]-GABA uptake.

Glutamate, GABA, and glycine in the human retina: an immunocytochemical investigation

The distribution of the neuroactive amino acids glutamate, GABA, and glycine in the human retina was examined in consecutive semithin sections treated with antisera specific for fixed glutamate, GABA, and glycine, respectively. Glutamate immunoreactivity was conspicuous in all photoreceptor cells (rods more strongly labelled than cones), and in a majority (85-89%) of the cells in the inner nuclear layer (INL). Rod spherules and cone pedicles showed a greater enrichment of glutamate immunoreactivity than the parent cell bodies and inner segments. Also, structures of the inner plexiform layer (IPL) were labelled. A large majority (83-91%) of cells in the ganglion cell layer (GCL) was strongly stained, as were most axons in the nerve fibre layer. Müller cell processes appeared unstained. GABA immunoreactivity was present in presumed amacrine but not in bipolar-like cells. The stained cells were restricted to the inner 1/3 of the INL and were more frequent in central than in peripheral retina (40% and 26% of all cells in the inner 1/2 of INL, respectively). GABA positive cell processes, probably originating from interplexiform cells, appeared to traverse the INL and end in the outer plexiform layer. Dense immunolabeling was found in the IPL. GABA immunoreactive cells (some also stained for glutamate) comprised 23% of all GCL cells in the peripheral retina, but only 5% in the central retina. Most of them were localized adjacent to the IPL. A few GABA positive (possibly ganglion) cells extended a single fibre toward the nerve fibre layer. Solitary GABA positive fibres were seen in this layer and in the optic nerve. Glycine immunoreactivity was observed in cells with the location typical of amacrine and bipolar (peripheral retina) cells, as well as in punctate structures of the IPL. In contrast to the GABA positive cells, the glycine positive cells were more frequent in the peripheral than in the central retina (42% and 23% of all cells in inner 1/3 of INL, respectively). A few cells in the GCL (0.5-1.5%) were glycine positive. Glutamate colocalized with GABA or glycine in a majority of the cells stained for either of these inhibitory transmitters (90-95% of the GABA positive cells, and 80-86% of the glycine positive cells, in the INL). Some bipolar cells were stained for both glutamate and glycine. Colocalization of GABA and glycine occurred in a subpopulation (3-4%) of presumed amacrine cells, about half of which was also glutamate positive.

GABA-like immunoreactivity in the developing chick retina: differentiation of GABAergic horizontal cell and its possible contacts with photoreceptors

The morphology of gamma-aminobutyric acid (GABA)-containing horizontal cells was examined in mature and developing chick retinas by GABA immunocytochemistry. In the outer plexiform layer of the mature retina, GABA-immunoreactive components were located in three different sublayers. In the inner (vitreal) layer most positively-stained fibres were laterally oriented processes from horizontal cells. Thick processes were found in the middle layer, and the relatively thin fibres in the outer (scleral) layer showed a concave curvature, suggesting their termination on photoreceptor terminals. By electron microscopy it was found that the principal cone pedicles were usually indented by immunoreactive lateral neurites of horizontal cells but that rod spherules faced only occasionally immunoreactive fibres. Accessory cones and single cones were also not usually indented by immunoreactive fibres. These observations may indicate that horizontal cells regulate the excitation of cone photoreceptors by several different inhibitory mechanisms. During retinal development, horizontal cells begin to extend lateral fibres by the ninth embryonic day, and some GABAergic horizontal cells also possess inwardly extending fibres until embryonic day 11. Between embryonic days 13 and 15, some immunoreactive cells were found among the bipolar cells, suggesting that they were still migrating to their final position. On embryonic day 17, the staining pattern was very similar to that of the mature retina. These results suggest that GABA immunohistochemistry may be an excellent tool for studying horizontal cell differentiation.

The mismatch problem for GABAergic amacrine cells in goldfish retina: resolution and other issues

GABAergic neurons in the vertebrate retina have received intensive study. Yet there are several notable examples of a "mismatch" among the cytochemical markers used to identify GABAergic neurons. The mismatch between [3H]GABA uptake autoradiography and all other indicators of GABAergic neurons as they pertain to amacrine cells in goldfish retina is examined in this overview. The discrepancies can be accounted for largely by barriers to diffusion presented by significant GABA uptake sinks at the inner and outer margins of the retina and by the differential subcellular distribution of the various markers for GABAergic neurons. Also, conditions producing a redistribution of [3H]-GABA and endogenous GABA stores within the retina are described and discussed.

Cholinergic amacrine cells in the rabbit retina accumulate muscimol

The cholinergic amacrine cells of the rabbit retina form two mosaics placed symmetrically on either side of the inner plexiform layer. Recently, these cells have been reported to contain immunocytochemical markers for GABA. In this paper, we labeled the cholinergic cells with DAPI, then incubated the retina in [3H]-muscimol, a neuronal marker for GABA. Subsequently, we converted the DAPI fluorescence of the displaced cholinergic matrix to an opaque product by photooxidation in the presence of DAB. Autoradiography showed that all of the displaced cholinergic amacrine cells were labeled with [3H]-muscimol, thus confirming the immunocytochemical results. The cholinergic cells account for approximately 80% of the cells in the ganglion cell layer which take up [3H]-muscimol.

Photoreceptor degeneration and loss of immunoreactive GABA in the Abyssinian cat retina

GABA (gamma-amino butyric acid) and its synthesizing enzyme, GAD (glutamate decarboxylase; EC 4.1.1.15) were localized in the retina of Abyssinian cats homozygous for a recessively inherited retinal degenerative disorder which in several respects is similar to the human disease, retinitis pigmentosa. Clinically normal mongrel cats and heterozygous Abyssinian cats were studied for comparison. The GABA and GAD immunoreactive neurons of the heterozygous or young homozygous (clinically unaffected animals) had the same distribution and morphology as normal mongrel European type cats. The neuronal GABA immunoreactivity in both the inner and outer parts of the retina gradually disappeared in the course of the disease, with little or no loss of GAD immunoreactive neurons. Early in the disease, the changes were most severe in patches in the mid periphery of the eye and then spread both centrally and peripherally. Loss of photoreceptors was a prerequisite for the loss of GABA immunoreactivity. The observations show that retinal changes are not limited to the photoreceptors. The GABA loss is not likely to be due to a loss of neurons, because of the persistence of GAD immunoreactive neurons.

A new cornea-positive component to the ERG of the aspartate-treated frog retina?

Using isolated bullfrog retinas treated with aspartate, we have found a new cornea-positive photo-response (positive response). The positive response could be detected only when the retina was stimulated by a dim flash. The peak amplitude and the rate of initial rise of the positive response were intensity dependent. The spectral sensitivity of the positive response peaked at 500 nm. The general characteristics of the response were different from those of the PIII response. The positive response was closely related to the extracellular Na+ and Ca2+ concentration and completely abolished by 50 microM La3+. On the basis of the present findings, it was suggested that some types of calcium channels or transporters are involved in the generation of the positive response.

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.

gamma-Aminobutyric acid system in developing and degenerating mouse retina

Freeze-dried sections (14 microns thick) of retinal layers were prepared from mice with retinal degeneration (C3H strain) and control mice (C57BL strain). The weighed sections (2-30 ng dry weight) were analyzed using our microassay methods. In the control retina, gamma-aminobutyric acid (GABA) concentration and glutamate decarboxylase (GAD) activity, on a dry weight basis, increased from birth to 9 weeks of age and decreased slightly at 20 weeks. In the degenerated retina, the levels of GABA and GAD activity were higher at birth than in the control retina, and continued to increase until 20 weeks of age, at which time the GAD activity reached a markedly high level. This increase was found when the total GABA and GAD levels per retina were determined. In the normal retinal layers, GABA and GAD were confined primarily to the inner plexiform layer. In the degenerated retina, GAD activity gradually increased in the inner layers during postnatal development, but by 20 weeks the increase was most prominent in the inner part of inner nuclear layer and in the outer part of inner plexiform layer. GABA transaminase activity and its distribution were not much different in both normal and degenerated retinas during development.

Localization of L-glutamic acid decarboxylase mRNA in cat retinal horizontal cells by in situ hybridization

Retinal horizontal cells receive synaptic input from photoreceptors and provide a pathway for lateral interactions in the vertebrate retina. In nonmammalian retinas, the H1 horizontal cells appear to use gamma-amino butyric acid (GABA) as their neurotransmitter. The transmitter used by mammalian horizontal cells, however, remains to be identified. In the present study, we have employed in situ hybridization to examine whether cat retinal horizontal cells contain L-glutamic acid decarboxylase (GAD) mRNA and hence might use GABA as their transmitter. In the cat retina, labeled cell bodies were found in the inner nuclear layer and the ganglion cell layer. No labeled cells were found in the photoreceptor layer. In the inner nuclear layer, labeled somata were present at two locations. The majority of them (approximately 72%) were located in the vitread side of the inner nuclear layer bordering the inner nuclear layer/inner plexiform layer boundary. A second class of labeled cells in the inner nuclear layer (approximately 20%) had larger somata and were present at the inner nuclear layer/outer plexiform layer boundary. Double labeling experiments with antisera to parvalbumin, a horizontal cell marker, showed that these perikarya belonged to horizontal cells. RNA blot analysis showed that cat retina contains a single species of GAD mRNA that is about 4 kb in size. These data show that in addition to GABAergic amacrine, displaced amacrine, and interplexiform cells described previously, horizontal cells contain GAD mRNA and may use GABA as their neurotransmitter. Hence, GABA may be a transmitter that is involved in lateral inhibition in both nonmammalian and mammalian retinas.

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.

Release of endogenous excitatory amino acids from turtle photoreceptors

Responses to light are transmitted from photoreceptors to second-order retinal neurons by chemical synapses that may use an excitatory amino acid (EAA) as the neurotransmitter. This hypothesis is based primarily on the pharmacological actions of EAA agonists and antagonists on the membrane potentials and light responses of second-order neurons. But the release of endogenous EAAs, which is a critical criterion for the identification of EAAs as transmitters, has not been demonstrated. Here we report the use of outside-out membrane patches excised from rat hippocampal neurons to detect the release of EAAs from synaptic terminals of isolated turtle photoreceptors. Electrical stimulation of or application of lanthanum chloride to photoreceptors induced an increase in the frequency of opening of 50-pS channels in the patches. These channels were identified as the class of glutamate-activated channels that are also gated by aspartate and NMDA (N-methyl-D-aspartate). In several photoreceptor-patch pairs, spontaneous channel activity was observed near the synaptic terminals. These results provide strong evidence to support the hypothesis that both rods and cones of the turtle use an EAA as their neurotransmitter.

Immunocytochemical evidence for the presence of histamine and GABA in photoreceptors of the barnacle (Balanus nubilus)

Biochemical evidence indicates that GABA and histamine may both be synthesized by barnacle photoreceptors (Koike & Tsuda, 1980; Timpe & Stuart, 1984; Callaway & Stuart, 1989b). We used antisera against GABA- and histamine-protein conjugates to determine whether the photoreceptors contain either or both of these antigens. Both antisera labeled all of the photoreceptors in each of the three ocelli. Histamine-like immunoreactivity was found throughout each photoreceptor cell but was most intense at their presynaptic terminals. Histamine-like immunoreactivity was blocked by preincubation of the antibody either with histamine or with a histamine-protein conjugate. GABA-like immunoreactivity was found in all parts of the photoreceptors including the cell body, axon, rhabdomeric dendrites, and presynaptic terminals. GABA-protein conjugates blocked the GABA-like labeling of the photoreceptors, while protein conjugates with histamine, L-glutamate, L-glutamine, beta-alanine, and taurine did not. Histamine-like immunoreactivity in the supraesophageal ganglion was confined to the photoreceptor terminals and a second, loose plexus of endings in the main neuropil. GABA-like immunoreactivity, in contrast, was found in approximately twenty-five pairs of neurons of this ganglion. In the cirral nerves, which are expected to contain inhibitory motoneurons, unidentified axons also labeled with the GABA antiserum.

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.

GABA-like immunoreactivity in the cat retina: electron microscopy

The synaptic organization of the cat retina was studied with antibodies against the GABA-GA (glutaraldehyde)-BSA (bovine serum albumin) complex. The postembedding technique combined with immunogold labelling ensured ultrastructural preservation and made identification of synapses possible. The most common putative GABA-ergic synapses in the inner plexiform layer were amacrine-to-bipolar-cell synapses followed by amacrine-to-ganglion-cell and amacrine-to-amacrine-cell synapses. GABA-immunoreactive amacrine cells received most of their synaptic input from bipolar cells followed by other amacrine cells. Synapses between two labelled amacrine cells were common. Rod bipolar cells were the predominant input source and also the preferred output target of GABA-labelled amacrine cells. OFF- and ON-ganglion cells received putative GABA-ergic synapses at their dendrites in laminas a and b, respectively, and also at their somata. In the outer plexiform layer, synapses of interplexiform cells onto bipolar cell dendrites expressed GABA-like immunoreactivity. In both the cone pedicles and the rod spherules, GABA-like immunoreactivity was observed in horizontal cell processes.

Quantitative distribution of six amino acids in rat retinal layers

Concentrations of glutamate, aspartate, glutamine, glycine, GABA, and taurine were determined in samples microdissected from rat retinal layers and assayed by HPLC. Glutamate and glutamine were relatively high in the inner nuclear (INL) and ganglion cell (GCL) layers; aspartate was relatively high in the outer nuclear layer (ONL), outer plexiform layer, and INL. Distributions of glutamate and aspartate did not correlate well with those of enzymes involved in their metabolism. Glycine and GABA were highest in the inner plexiform layer, with increasing concentrations through the INL, and were relatively high in the GCL. Taurine was highest in the ONL.

GABA as a potential transmitter in lizard photoreceptors: immunocytochemical and biochemical evidence

The retina of the desert night lizard, Xantusia vigilis, was examined for immunoreactivity to antibodies against gamma-aminobutyric acid and L-glutamate decarboxylase. At the electron microscopic level it was found that a distinct population of the photoreceptor cells was immunoreactive to both antibodies. Computer-assisted reconstruction of serial sections positively identified the immunoreactive receptors as cones. These cones constituted 15% of the photoreceptors in the retinal sections, and they were morphologically distinct. The mean diameter of the labeled cone synaptic pedicles was 5.8 micron whereas that of the unlabeled pedicles was 7.9 micron, a statistically significant difference. L-glutamate decarboxylase was extracted from the lizard brain, positively identified radiometrically, and shown by immunodiffusion to crossreact with the antibody used for localization. The authors suggest that the immunoreactive cones synthesize and accumulate gamma-aminobutyric acid. Whether or not it is used by those cones as a neurotransmitter should be tested directly.

The effects of gabaculine in vivo on the distribution of GABA-like immunoreactivity in the rat retina

gamma-Aminobutyric acid (GABA) is an inhibitory transmitter found in the retinae of mammals largely within certain amacrine cells. In previous studies from this laboratory, subcutaneous administration to rats of gabaculine, an enzyme-activated irreversible inhibitor of gamma-aminobutyric acid (GABA)-transaminase, produced large, rapid and long-lasting increases in levels of retinal GABA. We employed immunocytochemistry to determine whether such changes in the levels of retinal GABA are accompanied by changes in the cellular distribution of GABA. Using a recently developed antiserum to a GABA-protein conjugate, and the peroxidase-antiperoxidase method, we examined retinae from control rats and from rats 2 or 8 h after administration of 10 mg/kg gabaculine. From previous work, retinal levels of GABA were respectively elevated 3- or 6-fold at those postgabaculine times. In the present study, marked changes in the distribution of GABA-like immunoreactivity (GABA-LIR) were apparent by 2 h after injection of gabaculine, and were more striking at 8 h postgabaculine. The pattern of staining for GABA-LIR strongly suggested that much of the GABA in gabaculine-treated retinae was within Müller glial cells. That observation provides evidence for the importance of those cells in the uptake and degradation of GABA after its release from retinal neurons.

Immunohistochemical analysis of the localization of guanine nucleotide-binding protein in the mouse brain

A guanine nucleotide-binding protein, G0, is a heterotrimer with the alpha- and beta gamma-subunits (referred to here as alpha 0 and beta gamma, respectively). We examined the distribution pattern of the anti-alpha 0 and anti-beta gamma immunoreactive products in the hippocampus, and cerebral and cerebellar cortices of the mouse brain. In the hippocampus, alpha 0- and beta gamma-immunoreactivities were localized in the neuropil of the stratum oriens, stratum radiatum and stratum lacunosum-moleculare, but were absent from the cell bodies of the pyramidal cells and their apical dendritic shafts. In the cerebral cortex, alpha 0- and beta gamma-immunoreactivities were seen in the neuropil of all 6 layers of the cerebral cortex, especially in the uppermost molecular layer (layer I), and were absent from cell bodies of neurons and their apical dendritic shafts. In the cerebellar cortex, the molecular layer was heavily stained with anti-alpha 0 and beta gamma-antibodies. The present study revealed that the distribution pattern of beta gamma-immunoreactivities in these structures of the mouse brain was strikingly similar to that of the alpha 0-immunoreactivities.

Beta-carbolines as tools in memory research: human data with the beta-carboline ZK 93426

The discovery of substances which bind with high affinity to benzodiazepine receptors but have no pharmacological effects (antagonists) or effects opposite to those of benzodiazepines (inverse agonists) have introduced a new approach to elucidating mechanisms underlying memory and other cognitive processes. Since benzodiazepines induce anterograde amnesia and sedation, these substances should show an opposite effect and so enhance memory and/or increase vigilance. In the present report we present data obtained in humans with a benzodiazepine receptor antagonist with weak inverse agonist properties, ZK 93426. The drug was given intravenously to human volunteers in double-blind, placebo-controlled designs and performance on several psychometric tests was evaluated. In a general estimation of behavioural changes volunteers experienced a stimulatory, activating effect of the drug. An improvement was observed in two cognitive tasks, the logical reasoning task and the pictures difference task, which estimate concentration and attentional processes respectively. No effects were found in a letter cancellation test or in time estimation. In another study utilizing EEG recording, we demonstrated that ZK 93426 increased wakefulness (vigilance) in healthy subjects during the daytime. The effect of ZK 93426 upon memory processes was also investigated utilizing a visual memory test and word lists. A slight improvement in some memory processes, especially long-term retrieval, was found. The present data suggest that benzodiazepine receptor antagonists with weak inverse intrinsic activity possess some effects opposite to those of benzodiazepines.

Coexistence of GABA- and choline acetyltransferase (ChAT)-like immunoreactivity in the hypoglossal nucleus of the rat

Single and sequential double immunocytochemical techniques were applied to localize gamma-aminobutyric acid (GABA)- and choline acetyltransferase (ChAT)- like immunoreactivity (-LI) in the hypoglossal nucleus of the rat. After subsequential double staining a relatively high number of hypoglossal motor neurons showed the coexistence of both ChAT- and GABA-LI. Coexistence of both substances was also revealed in the axons of the hypoglossal nerve situated within the medulla oblongata. Cells showing only ChAT- or GABA-LI were also observed. Differences in immunostaining between the different cell groups of the hypoglossal nucleus were established. Following axotomy of the right hypoglossal nerve, a decrease or loss of the immunoreactivity for both ChAT and GABA in the motor neurons was established until the 3rd week after the operation. The results obtained do not give evidence on the origin of the GABA-like immunoreactive material and its functional significance in the cholinergic neurons. It can be only speculated that the GABA-like material is either taken up from the intercellular space or is synthesized by the ChAT-LI nerve cells. Functionally, the importance of GABA for the synthesis of gamma-hydroxybutyrate (a novel neurotransmitter candidate) and its postsynaptic transmitter action or presynaptic regulatory action (through autoreceptors in the membrane of the nerve endings) on the release of acetylcholine (ACh) should be taken into consideration.

The emergence of GABA-accumulating neurons during retinal histogenesis in the embryonic chick

The emergence of GABA-accumulating neurons was studied from stages 29 to 40 during retinal histogenesis in the chick, covering embryonic (E) days E6-E14, using autoradiographic analysis following incubation of isolated retinas with [3H]GABA (2 microM). Analysis was restricted to central retina which is more advanced in its differentiation than the periphery. On E6 numerous mitotic figures were present along the scleral border of the unstratified neuroepithelium. Specific localization of [3H]GABA was associated initially with somata situated in middle regions of the retinal expanse. Occasionally contiguous pairs of labeled cells were seen. The inner plexiform layer makes its appearance during E7; at that time silver grains were present over cell bodies located in the ganglion cell layer and the proximal portion of the inner nuclear layer, those of probable amacrine cells. As retinal stratification continued, more cells were observed to have elaborated membrane systems for GABA uptake with varying degrees of affinity. By E8, although dividing, non-labeled cells were in close proximity, GABA-labeled cells were observed in positions of horizontal cells. By E14, the pattern of label distribution appeared essentially similar to that reported for adult retina, i.e. [3H]GABA labeling was observed over horizontal cells and their processes, subpopulations of amacrine cells which appear to ramify extensively across the inner plexiform layer, selected perikarya of the ganglion cell layer, and the nerve fiber layer. In addition, a subpopulation of labeled photoreceptors, some identified as cones by virtue of oil droplets, was observed. Thus, preferential accumulation of GABA appears during E6, prior to formation of either inner or outer plexiform layers. The localization of [3H]GABA demonstrates that ganglion and amacrine cell bodies are labeled initially, followed by horizontal cells. Specific accumulation of [3H]muscimol, a potent agonist of GABA receptors, appears about E12 over cells located in proximal regions of the inner nuclear layer; these somata later ramify in sublaminae 2 and 4 of the inner plexiform layer.

The effects of in vivo inactivation of GABA-transaminase and glutamic acid decarboxylase on levels of GABA in the rat retina

Gabaculine and gamma-vinyl GABA (GVG) are specific enzyme-activated irreversible inhibitors of GABA-transaminase (GABA-T). gamma-Acetylenic GABA (GAG) irreversibly inhibits both GABA-T and glutamic acid decarboxylase (GAD). Subcutaneous injection of any of those compounds rapidly elevated levels of GABA in the retinae of rats. After injection of 10 mg/kg gabaculine, levels of retinal GABA climbed 5-fold in 4 h, and peaked 16 h after injection at levels approximately 7 times those from water-injected control rats. They remained significantly elevated compared to control levels for at least 6 days after injection. The postgabaculine increase in levels of retinal GABA was linear with time between 0.5 and 4 h after injection. In contrast, retinal GABA levels peaked at less than 3 times control levels within 8 h of injection of 50 mg/kg GAG and returned to baseline levels within 4 days. GAG, upon coadministration with gabaculine, significantly attenuated the postgabaculine rise in levels of GABA in retinae. Neither the rate of rise, nor the maximum levels, of retinal GABA was so great after injection of GAG plus gabaculine, compared to those after injection of gabaculine alone. The degree to which postgabaculine GABA accumulation was inhibited in the retina by 50 mg/kg GAG closely corresponded with the extent to which that dose of GAG inactivated retinal GAD activity. The results of this study extend previous reports from this laboratory that systemically administered gabaculine, GVG and GAG all inactivate target enzymes more potently in retina than in other brain regions.(ABSTRACT TRUNCATED AT 250 WORDS)

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

The distribution of benzodiazepine receptors (BZR) was compared to the distribution of gamma-aminobutyric acid (GABA)-ergic synapses in the rhesus monkey retina using monoclonal antibodies against the BZR and polyclonal antisera to glutamate decarboxylase (GAD), the GABA-synthesizing enzyme which labels the presynaptic terminals of the GABAergic synapses. Indirect immunofluorescence including dual fluorochroming for both BZR and GAD indicates that although both were localized to the inner plexiform layer and adjacent cell body layers, their distributions were largely non-overlapping. Thus, in the primate retina, BZRs are not exclusively associated with GABAergic synapses.

Immunohistochemical localization of guanine nucleotide-binding protein in rat retina

The localization of a guanine nucleotide-binding protein, Go, in rat retina has been immunohistochemically examined by use of affinity-purified antibody against the alpha-subunit of Go. Dense Go-immunoreactive products are localized in the inner and outer plexiform layers which are considered as the region of synaptic interplay between neuronal components of the retina. Weak Go-immunoreactivity is also found in the neuropil of the ganglion cell layer and the inner nuclear layer. The selective enrichment of Go in synaptic zones of the retina suggests its major role in neuronal transmission within the retina.

The presence of glutamic acid decarboxylase and gamma-aminobutyric acid immunoreactivity in photoreceptors of hatching quail retina

The presence of glutamic acid decarboxylase (GAD) and gamma-aminobutyric acid (GABA) was investigated in neuroretina sections from hatching quail embryos by immunocytochemistry. The photoreceptors were found to be intensely immunoreactive to anti-GAD antiserum and to two distinct anti-GABA antisera.

GABA and GAD-like immunoreactivity in the primate retina

GABA immunoreactivity was studied and compared with GAD immunoreactivity in the retinae of baboon, cynomolgus monkey and man. The central and peripheral parts of the retinae were investigated separately in cynomolgus monkey and in man. The same kinds of structures were stained with both antisera. Cells with a position corresponding to amacrine cells were stained, as well as processes in the inner plexiform layer and some cells in the ganglion cell layer. The outer plexiform layer and some cells with the position and configuration of horizontal cells also appeared immunoreactive. Staining was also observed in bipolar-like cells, in man most clearly when using the GABA antiserum in sections from the central parts of the retina. It is possible that horizontal cells, as well as bipolar-like cells, may play a previously unsuspected role in GABAergic transmission in the primate retina.