gamma-Aminobutyric acid system in developing and degenerating mouse retina

Retinitis Pigmentosa and Retinal Degenerations: Deciphering Pathways and Targets for Drug Discovery and Development

Inherited retinal diseases (IRDs) are a group of retinopathies generally caused by genetic mutations. Retinitis pigmentosa (RP) represents one of the most studied IRDs. RP leads to intense vision loss or blindness resulting from the degeneration of photoreceptor cells. To date, RP is mainly treated with palliative supplementation of vitamin A and retinoids, gene therapies, or surgical interventions. Therefore, a pharmacologically based therapy is an urgent need requiring a medicinal chemistry approach, to validate molecular targets able to deal with retinal degeneration. This Review aims at outlining the recent research efforts in identifying new drug targets for RP, especially focusing on the neuroprotective role of the Wnt/β-catenin/GSK3β pathway and apoptosis modulators (in particular PARP-1) but also on growth factors such as VEGF and BDNF. Furthermore, the role of spatiotemporally expressed G protein-coupled receptors (GPR124) in the retina and the emerging function of histone deacetylase inhibitors in promoting retinal neuroprotection will be discussed.

Differential alterations in the expression of neurotransmitter receptors in inner retina following loss of photoreceptors in rd1 mouse

Loss of photoreceptors leads to significant remodeling in inner retina of rd1 mouse, a widely used model of retinal degeneration. Several morphological and physiological alterations occur in the second- and third-order retinal neurons. Synaptic activity in the excitatory bipolar cells and the predominantly inhibitory amacrine cells is enhanced. Retinal ganglion cells (RGCs) exhibit hyperactivity and aberrant spiking pattern, which adversely affects the quality of signals they can carry to the brain. To further understand the pathophysiology of retinal degeneration, and how it may lead to aberrant spiking in RGCs, we asked how loss of photoreceptors affects some of the neurotransmitter receptors in rd1 mouse. Using Western blotting, we measured the levels of several neurotransmitter receptors in adult rd1 mouse retina. We found significantly higher levels of AMPA, glycine and GABAa receptors, but lower levels of GABAc receptors in rd1 mouse than in wild-type. Since GABAa receptor is expressed in several retinal layers, we employed quantitative immunohistochemistry to measure GABAa receptor levels in specific retinal layers. We found that the levels of GABAa receptors in inner plexiform layer of wild-type and rd1 mice were similar, whereas those in outer plexiform layer and inner nuclear layer combined were higher in rd1 mouse. Specifically, we found that the number of GABAa-immunoreactive somas in the inner nuclear layer of rd1 mouse retina was significantly higher than in wild-type. These findings provide further insights into neurochemical remodeling in the inner retina of rd1 mouse, and how it might lead to oscillatory activity in RGCs.

Loss of photoreceptors results in upregulation of synaptic proteins in bipolar cells and amacrine cells

Deafferentation is known to cause significant changes in the postsynaptic neurons in the central nervous system. Loss of photoreceptors, for instance, results in remarkable morphological and physiological changes in bipolar cells and horizontal cells. Retinal ganglion cells (RGCs), which send visual information to the brain, are relatively preserved, but show aberrant firing patterns, including spontaneous bursts of spikes in the absence of photoreceptors. To understand how loss of photoreceptors affects the circuitry presynaptic to the ganglion cells, we measured specific synaptic proteins in two mouse models of retinal degeneration. We found that despite the nearly total loss of photoreceptors, the synaptophysin protein and mRNA levels in retina were largely unaltered. Interestingly, the levels of synaptophysin in the inner plexiform layer (IPL) were higher, implying that photoreceptor loss results in increased synaptophysin in bipolar and/or amacrine cells. The levels of SV2B, a synaptic protein expressed by photoreceptors and bipolar cells, were reduced in whole retina, but increased in the IPL of rd1 mouse. Similarly, the levels of syntaxin-I and synapsin-I, synaptic proteins expressed selectively by amacrine cells, were higher after loss of photoreceptors. The upregulation of syntaxin-I was evident as early as one day after the onset of photoreceptor loss, suggesting that it did not require any massive or structural remodeling, and therefore is possibly reversible. Together, these data show that loss of photoreceptors results in increased synaptic protein levels in bipolar and amacrine cells. Combined with previous reports of increased excitatory and inhibitory synaptic currents in RGCs, these results provide clues to understand the mechanism underlying the aberrant spiking in RGCs.

Functional and neurochemical development in the normal and degenerating mouse retina

The rd1 mouse is a well-established animal model for human retinitis pigmentosa (RP). We used electroretinography (ERG) to evaluate retinal function and postembedding immunocytochemistry to determine the changes in cellular amino acid expression in the normal (C57Bl6) and degenerating mouse retina (rd1), as a function of age during development and the onset of degeneration. In the normal mouse retina, photoreceptoral and post-photoreceptoral ERG responses improved simultaneously from eye-opening until adult levels were achieved at approximately postnatal day (P) 30. Maturation of amino acid neurochemistry preceded the development of retinal function in the normal retina. Amino acid levels increased immediately from birth and reached stable levels by eye-opening. In contrast, in the rd1 mouse, both rod and cone pathway function rapidly reduced from eye-opening and by P21 became undetectable. Interestingly, at P18 cone responses were still comparable between the normal and degenerating retina. Before eye opening, the pattern of amino acid immunoreactivity in the rd1 retina was similar to the normal retina. Alterations in neurochemistry were observed after the onset of rod photoreceptor cell death. The most obvious change was the reduction in neurotransmitter immunoreactivity within the synaptic layers and some cell classes of the rd1 retina. Reduction of glutamine and glutamate was observed in Müller cells before established gliosis markers. Overall, these results suggest the rapid maturation of neurochemistry by eye opening followed by functional maturation by P30 in the normal retina. The dystrophic retina displays similar neurochemistry to control retina before eye opening but a subsequent decline.

Functional integrity and modification of retinal dopaminergic neurons in the rd1 mutant mouse: roles of melanopsin and GABA

The progressive loss of rod and cone photoreceptors in human subjects with retinitis pigmentosa causes a gradual decline in vision and can result in blindness. Current treatment strategies for the disease rely on the integrity of inner retinal neurons, such as amacrine cells, that are postsynaptic to photoreceptors. Previous work has demonstrated that a specialized subclass of retinal amacrine cell that synthesizes and releases the key neurotransmitter dopamine remains morphologically intact during the disease; however, the pathophysiological function of these neurons remains poorly understood. Here we examined spontaneous and light-evoked spike activity of genetically labeled dopamine neurons from the retinas of retinal degeneration 1 (rd1) mice. Our results indicated that rd1 dopamine neurons remained functionally intact with preserved spontaneous spiking activity and light-evoked responses. The light responses were mediated exclusively by melanopsin phototransduction, not by surviving cones. Our data also suggested that dopamine neurons were altered during photoreceptor loss, as evidenced by less spontaneous bursting activity and increased light-evoked responses with age. Further evidence showed that these alterations were attributed to enhanced GABA/melanopsin signaling to dopamine neurons during disease progression. Taken together, our studies provide valuable information regarding the preservation and functional modification of the retinal dopamine neuronal system in rd1; this information should be considered when designing treatment strategies for retinitis pigmentosa.

Blocking GABA(C) receptors increases light responsiveness of retinal ganglion cells in a rat model of retinitis pigmentosa

Previous studies in a mouse model of retinitis pigmentosa indicate that the GABAergic system in the retina may be overactive. GABA is known to act on GABA(C) receptors present on the axon terminals of bipolar cells to inhibit the release of excitatory neurotransmitter from these cells. The present study examined the effects of a GABA(C) receptor antagonist on the light-evoked responses of retinal ganglion cells (RGCs) in a rat model of retinitis pigmentosa. Extracellular recordings were made from RGCs in retinas isolated from P23H transgenic rats and non-dystrophic Sprague-Dawley (SD) rats. Spike activity of RGCs was measured in response to brief flashes of light over a range of light intensities. Intensity-response curves were evaluated prior to and during bath application of the GABA(C) receptor antagonist TPMPA. I found that TPMPA consistently increased the sensitivity of P23H rat RGCs to light flashes. For ON-center RGCs (n = 21), the average increase in light sensitivity was 0.63 log unit. For OFF-center RGCs (n = 6), the average increase was 0.38 log unit. TPMPA increased the maximum peak response of ON-center RGCs by 22% and OFF-center RGCs by 11%. However, the increase in maximum peak response of OFF-center RGCs was not statistically significant. TPMPA had no significant effect on the dynamic operating range of either ON-center or OFF-center RGCs. Nine ON-center SD rat RGCs were also tested. In contrast to what was observed for P23H rat RGCs, TPMPA decreased the sensitivity of these RGCs to light flashes, on average by 0.20 log unit. In conclusion, GABA(C) receptors may be novel targets for therapeutic interventions aimed at increasing light responsiveness in patients with retinitis pigmentosa or other diseases involving degeneration of photoreceptors.

Inner and outer retinal mechanisms engaged by epiretinal stimulation in normal and rd mice

Retinal prosthetic devices are being developed to bypass degenerated retinal photoreceptors by directly activating retinal neurons with electrical stimulation. However, the retinal circuitry that is activated by epiretinal stimulation is not well characterized. Whole-cell patch clamp recordings were obtained from ganglion cells in normal and rd mice using flat-mount and retinal slice preparations. A stimulating electrode was positioned along the ganglion cell side of the preparation at different distances from the stimulated tissue. Pulses of cathodic current evoked action potentials in ganglion cells and less frequently evoked sustained inward currents that appeared synaptic in origin. Sustained currents reversed around E(Cl) and were inhibited by blockade of α-amino-3-hydroxyl-5-methyl-4-isoxazole-proprionate (AMPA)-type glutamate receptors with 2,3-dihydroxy-6-nitro-sulfamoyl-benzo(f)-quinoxaline-2,3-dione (NBQX), γ aminobutyric acid a/c (GABA(a/c)) receptors with picrotoxinin, or glycine receptors with strychnine. This suggests that epiretinal stimulation activates glutamate release from bipolar cell terminals, which in turn evokes release of GABA and glycine from amacrine cells. Synaptic current thresholds were lower in ON ganglion cells than OFF cells, but the modest difference did not attain statistical significance. Synaptic currents were rarely observed in rd mice lacking photoreceptors compared to normal retina. In addition, confocal calcium imaging experiments in normal mice retina slices revealed that epiretinal stimulation evoked calcium increases in the outer plexiform layer. These results imply a contribution from photoreceptor inputs to the synaptic currents observed in ganglion cells. The paucity of synaptic responses in rd mice retina slices suggests that it is better to target retinal ganglion cells directly rather than to attempt to engage the inner retinal circuitry.

Effects of GABA receptor antagonists on thresholds of P23H rat retinal ganglion cells to electrical stimulation of the retina

An electronic retinal prosthesis may provide useful vision for patients suffering from retinitis pigmentosa (RP). In animal models of RP, the amount of current needed to activate retinal ganglion cells (RGCs) is higher than in normal, healthy retinas. In this study, we sought to reduce the stimulation thresholds of RGCs in a degenerate rat model (P23H-line 1) by blocking GABA receptor mediated inhibition in the retina. We examined the effects of TPMPA, a GABA(C) receptor antagonist, and SR95531, a GABA(A) receptor antagonist, on the electrically evoked responses of RGCs to biphasic current pulses delivered to the subretinal surface through a 400 µm diameter electrode. Both TPMPA and SR95531 reduced the stimulation thresholds of ON-center RGCs on average by 15% and 20% respectively. Co-application of the two GABA receptor antagonists had the greatest effect, on average reducing stimulation thresholds by 32%. In addition, co-application of the two GABA receptor antagonists increased the magnitude of the electrically evoked responses on average three-fold. Neither TPMPA nor SR95531, applied alone or in combination, had consistent effects on the stimulation thresholds of OFF-center RGCs. We suggest that the effects of the GABA receptor antagonists on ON-center RGCs may be attributable to blockage of GABA receptors on the axon terminals of ON bipolar cells.

IL-4 increases GABAergic phenotype in rat retinal cell cultures: involvement of muscarinic receptors and protein kinase C

Interleukin-4 (IL-4) is an anti-inflammatory cytokine. During injuries, infections and neurodegenerative diseases, high levels of this molecule are expressed in the brain. In the present work, we investigated the effect of IL-4 on GABAergic differentiation of retinal cells kept in vitro. We analyzed either the uptake of [3H]-gamma-aminobutyric acid (GABA) or the expression of glutamic acid decarboxylase (GAD-67) following IL-4 treatment. We have also investigated the pharmacological modulation of the [3H]-GABA uptake by cholinergic activation. Our results demonstrate that IL-4 increases the uptake of [3H]-GABA after 48 h in culture in a dose-dependent manner (0.5-100 U/ml). The maximal effect was obtained with 5 U/ml (75% increase). This effect was blocked by 1 mM of nipecotic acid, demonstrating the involvement of the GAT-1 subtype of GABA transporter. The IL-4 effect depends on M1 muscarinic activity, an increase in intracellular calcium levels, tyrosine kinase activity and protein kinase C (PKC) activity. Treatment with IL-4 for 48 h induced an increase of 90% in the number of GAD- and GABA-immunoreactive cells when compared with control cultures. Our results indicate that IL-4 modulates the GABAergic phenotype of retinal cells in culture. This result can suggest an important role for this cytokine either during the normal development of retinal circuitry or during neuroprotection after injuries.

The GAbAergic and cholinergic systems in the retina are differentially affected by postnatal malnutrition during the suckling period

Malnutrition by severe protein deprivation induces deleterious consequences in the nervous system particularly in the initial period of development. These deficits can alter several important events during development, such as the expression of neurotransmitters. The induction of nutritional deficiency by using low protein diet, similar to that consumed by low income populations in Brazil, was applied in rats to investigate the effect of malnutrition on cells containing gamma-aminobutyric acid (GABA) and acetylcholine in the retina. GABA immunoreactivity was present in cells in the inner nuclear and ganglion cell layers and in processes in the inner and outer plexiform layers in retinas of control and malnourished animals. At postnatal day 8, there is a decrease (ca. 40%) of the GABAergic neurons in malnourished animals. At P13 and P21 the percentage of these neurons increased and was equivalent to control animals in the adult. Glutamic acid decarboxylase activity did not show significant changes between the two groups along development. Choline acetyltransferase immunoreactivity was localized in amacrine cells in the inner nuclear and ganglion cell layers and their processes in the inner plexiform layer. The percentage of cholinergic cells was always higher in malnourished animals than that observed in the control until postnatal day 30, when the same proportion of cholinergic neurons was found in the retinas of both groups. Choline acetyltransferase activity did not show significant changes between the two groups along development. In conclusion, our results show that despite the extreme somatic and behavioral changes observed the neurotransmitter systems studied were at a certain extent shielded from the insult.

Alterations in neurochemistry during retinal degeneration

Retinitis pigmentosa refers to a family of hereditary retinal degenerations that lead to photoreceptor death and vision loss. The underlying cause(s) are not known. In recent years there has been accumulating evidence of neurochemical changes during degeneration. In particular, the amino acids glutamate, GABA, and glycine show alterations in labelling intensity in subsets of neurons. Furthermore, there are differences in the labelling of the precursors, glutamine and aspartate, prior to, during, and following loss of photoreceptors, suggesting that the metabolic pathways involved in neurotransmitter formation and degradation may be abnormal. In addition, there is an elevation in glutamine and arginine content within Müller cells prior to the onset of photoreceptor death. Investigations evaluating Müller cell function indicate that formation and degradation of glutamate, in particular, is abnormal in the degenerating retina from an early age. These studies suggest that even though the primary genetic defect of the RCS rat is within the retinal pigment epithelium, Müller cells develop abnormally, and may contribute to the observed photoreceptor loss.

GABAergic system in the developing mammalian retina: dual sources of GABA at early stages of postnatal development

In the present work, we have characterized the maturation of the GABAergic system in mammalian retina. Immunoreactivity for GABA, GAD (glutamic acid decarboxylase, EC 4.1.1.15) -65 and -67 in the adult rat retina was localized in cells in the inner nuclear and ganglion cell layers. This pattern was established around postnatal day 8 and included transient GABA and GAD-67 expression in horizontal cells. GAD activity was very low at P1 and P4, increasing after P8, reaching maximal activity by P21 and decreasing to attain adult values by P30. GABA content was approximately constant from P1 to P13, increasing thereafter to reach adult levels. GAD protein content increased progressively with postnatal development and the two isoforms could be distinguished at P8. The disparity between retinal GABA content vs. presence and activity of the synthesizing enzyme, led us to investigate the alternative pathway for GABA synthesis that utilizes putrescine as a substrate. Highest levels of ornithine decarboxylase activity (the limiting step for putrescine synthesis) were found between P1 and P4, decreasing to very low levels after P13. The same pattern was observed for putrescine content in the retina. Highest amounts were found at P1, that decreased and remained constant after P13. Additionally, approximately 40% of tritiated putrescine incorporated by P1, P4 and adult retinas was converted into GABA. Our results suggest the existence of two different sources of GABA in mammalian retina, one that uses glutamate as a precursor and predominates in the mature nervous system and another that utilizes putrescine and is present transiently at early developmental stages.

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

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

Localisation of amino acid neurotransmitters during postnatal development of the rat retina

We used postembedding immunocytochemistry to determine the localisation of the amino acid neurotransmitters glutamate, gamma-aminobutyrate (GABA), and glycine, potential neurotransmitter precursors (aspartate and glutamine), and taurine in the rat retina during postnatal development. All amino acids investigated were present at birth; however, only the inhibitory neurotransmitters GABA and glycine displayed neuronal localisation. GABA was localised in a sparse population of amacrine cells, and glycine immunoreactivity was found in cells within the ventricular zone that appeared to migrate through the neuroblastic layer. Glutamate labelling was diffuse across the retina until postnatal day (PND) 8. Localisation of glutamine was evident within Müller's cells by PND 6, in agreement with the known age of onset of glutamine synthetase activity. Based on the findings of uptake of radiolabelled glutamate and GABA by PND 8 and changes in immunoreactivity, we propose that Müller's cells evolve at PND 6-8 from their precursor cells, the radial glial cells. Evidence for differences in glutamate turnover in the infant retina was seen on examination of aspartate and glutamine immunoreactivity. Aspartate labelling was weak until PND 11, when ganglion cells and some amacrine cells were labelled. Unlike the mature retina, a large number of amacrine cells were glutamine immunoreactive in the PND 6 retina. One reason for the observed differences in precursor pooling may be a lack of neuronal neurotransmitter release and overall low metabolic activity. We also investigated the response of the developing retina to ischaemic insult to test the physiological hypoxia model of vascular development. Our findings are consistent with the hypothesis that the developing retina has increased tolerance to ischaemic insult. Our findings suggest that, although the retina is morphologically adult like by PND 8, there are differences in neurotransmitter turnover in the immature rat retina.

Neurochemical architecture of the normal and degenerating rat retina

We used post-embedding immunocytochemistry to determine the cellular localization of glutamate, gamma-amino butyric acid (GABA), glycine, aspartate, glutamine, arginine, and taurine in the normal and degenerating rat retina. Müller's cell function was also evaluated by determining the uptake and degradation characteristics for glutamate. Immunocytochemical localization of amino acids in adult Royal College of Surgeons (RCS) and control rat retinas were similar with respect to cell classes. Differences in the intensity of labelling for glutamate, aspartate, glutamine, and glycine were observed in several classes of neurons, but the most prominent differences were shown by bipolar cells of the adult RCS rat retina. In addition, glutamine labelling within Müller's cells was higher in the RCS rat than the control. These changes may have occurred because of alterations in the glutamate production or degradation pathways. We tested this hypothesis by determining Müller's cells glutamate uptake and degradation characteristics in adult and postnatal day 16 RCS retinas. High affinity uptake of 3[H]-glutamate revealed an accumulation of grains over Müller's cell bodies in the adult RCS retina implying glutamate degradation anomalies. We confirmed anomalies in glutamate metabolism in RCS Müller's cells by showing that exogenously applied glutamate was degraded over a longer time course in postnatal day 16 RCS retinas, compared to control retinas. Differences in arginine immunoreactivity in adult and immature RCS retinas conform to the presumed dysfunction of Müller's cells in these degenerating retinas. The anomalies of amino acid localization, uptake and degradation lead us to conclude that Müller's cells in the RCS retina show abnormal function by postnatal day 16; an earlier time to previously reported anatomical and functional changes in this animal model of retinal degeneration.

Nerve growth factor delays retinal degeneration in C3H mice

BACKGROUND

The aim of the present study was to investigate the biological role of nerve growth factor (NGF) on retinal degeneration in the C3H mouse strain. This strain is characterized by a single gene mutation (rd) which leads to photoreceptor degeneration resembling human retinitis pigmentosa.

METHODS

Neural retinas from 1- to 25 day-old C3H mice were dissected from outer ocular tissues, dissociated in cell suspension, stained with a vital dye and counted in a hemocytometer. For in vivo study, NGF was injected into the intraocular or retro-ocular area, and at the end of the treatment the mice were killed. The eyes were enucleated, fixed and cut by cryostat into 14-microns serial sections. The serial sections were stained with hematoxylin-eosin and the outer nuclear layer (ONL) was measured using a computerized image analysis system.

RESULTS

An intraocular injection of NGF, or repeated retro-ocular injections, induced a significant increase in ONL thickness compared to controls.

CONCLUSION

Our data show that NGF inhibits retinal degeneration in C3H mice. The mechanism(s) underlying the protective action of NGF against retinal cell death remains to be established.

Postnatal developmental expression of glutamine and related amino acids in the rat retinas

PURPOSE

To evaluate postnatal developmental changes in the amounts of retinal glutamate, glutamine and GABA, and in the distribution of retinal glutamine in the rat.

METHODS

Free amino acids were extracted from rat retinas of different postnatal stages, and the concentrations of glutamate, glutamine and GABA were determined by HPLC. Also, anti-glutamine antibody was raised and an immunocytochemistry was performed with paraffin-embedded retinal sections in parallel with free amino acid analyses.

RESULTS

Glutamate occurred in high concentrations at the birth and showed a stable pool, while glutamine and GABA remained low until postnatal day 3 or 5, and gradually increased in the developing rat retinas. Glutamine immunolabeling was observed in the retinal pigment epithelium and in a subpopulation of presumed amacrine cells in the early postnatal days. It was also found in Muller cells and in some ganglion cells or displaced amacrine cells in the ganglion cells layer. Glutamine immunolabeling was transiently observed also in horizontal cells. Finally, the immunolabeling was dominant in the inner and outer plexiform layers in the adult retinas.

CONCLUSIONS

Postnatal developmental increase in the levels of glutamine and GABA might be dependent on the maturation of neurons or glial cells that possess the activity of the key enzymes of each amino acid. It was suggested that an expression of glutamine immunolabeling can be a marker of neurons that utilize glutamine as a precursor for glutamate or GABA, and of Müller cell maturations in postnatal early stage of the retina, while it changes to demonstrate the locations of glutamine cycle in the retina with adult characteristics.

Changes in GABA metabolism in streptozotocin-induced diabetic rat retinas

We examined the pathogenesis of reduced amplitude in electroretinogram (ERG) oscillatory potentials (OPs) in diabetes, in relation to possible changes in the metabolisms involving retinal amino acid neurotransmitters. With use of streptozotocin diabetic rats, flash ERGs were recorded and quantitative analyses of retinal free amino acids were performed. Immunocytochemical localizations of retinal glycine and GABA were examined. In addition, activities of glutamic acid decarboxylase (GAD) and GABA transaminase (GABA-T) were measured. Our results revealed that the amplitudes of OP 1 and OP 2 decreased, and retinal glycine and GABA content significantly increased in the diabetic rats. An increased immunoreactivity of GABA was observed in Müller cells in the diabetic rat retinas, while no apparent changes were found in glycine immunoreactivity. Finally, increased activations of GAD with reduced activities of GABA-T were observed in the diabetic rat retinas. Thus, reduced amplitudes of OPs were associated with changes in content, localization, and enzyme activities related to GABA in the retinas, implying that changes in GABA metabolism can be a candidate for the pathogenesis of the abnormal OPs in diabetes.

Immunocytochemical localization of three subtypes of GABA transporter in rat retina

Cellular distributions of three subtypes of GABA transporter (GAT1, GAT2, GAT3) in the eye were examined using polyclonal antisera for each subtype. GAT1 was present in the inner plexiform layer and proximal part of the inner nuclear layer, while GAT3 was distributed throughout the entire sensory retina, being predominant in the distal part of the inner plexiform layer and in the outer plexiform layer. GAT2 immunoreactivity was seen in the retina, including the retinal pigment epithelium layer and nerve fiber layer, also in the ciliary body epithelium. Confocal scanning laser fluorescence microscopy disclosed that the GAT1 immunoreactivity consisted of a number of small deposits in the inner plexiform layer and that GAT1-immunoreactive dots encircle immunonegative neurons in the inner nuclear layer. GAT2 immunoreactivity was present in the fiber bundle of the optic nerve and in the retinal pigment epithelium within the retina. GAT3 immunoreactive cells had long processes running vertically throughout the sensory retina. GAT1 is suggested to be present mainly in the processes of amacrine cells and GAT3 to be distributed in Müller cells. We conclude that GAT1, GAT2 and GAT3 are expressed in different cells, that they are involved in distinct GABAergic transmission in the retina, and that GAT2 may be involved in non-neuronal functions in the eye.