Rapid postmortem changes in the cellular localisation of amino acid transmitters in the retina as assessed by immunocytochemistry

Vesicular Release of GABA by Mammalian Horizontal Cells Mediates Inhibitory Output to Photoreceptors

Feedback inhibition by horizontal cells regulates rod and cone photoreceptor calcium channels that control their release of the neurotransmitter glutamate. This inhibition contributes to synaptic gain control and the formation of the center-surround antagonistic receptive fields passed on to all downstream neurons, which is important for contrast sensitivity and color opponency in vision. In contrast to the plasmalemmal GABA transporter found in non-mammalian horizontal cells, there is evidence that the mechanism by which mammalian horizontal cells inhibit photoreceptors involves the vesicular release of the inhibitory neurotransmitter GABA. Historically, inconsistent findings of GABA and its biosynthetic enzyme, L-glutamate decarboxylase (GAD) in horizontal cells, and the apparent lack of surround response block by GABAergic agents diminished support for GABA's role in feedback inhibition. However, the immunolocalization of the vesicular GABA transporter (VGAT) in the dendritic and axonal endings of horizontal cells that innervate photoreceptor terminals suggested GABA was released via vesicular exocytosis. To test the idea that GABA is released from vesicles, we localized GABA and GAD, multiple SNARE complex proteins, synaptic vesicle proteins, and Cav channels that mediate exocytosis to horizontal cell dendritic tips and axonal terminals. To address the perceived relative paucity of synaptic vesicles in horizontal cell endings, we used conical electron tomography on mouse and guinea pig retinas that revealed small, clear-core vesicles, along with a few clathrin-coated vesicles and endosomes in horizontal cell processes within photoreceptor terminals. Some small-diameter vesicles were adjacent to the plasma membrane and plasma membrane specializations. To assess vesicular release, a functional assay involving incubation of retinal slices in luminal VGAT-C antibodies demonstrated vesicles fused with the membrane in a depolarization- and calcium-dependent manner, and these labeled vesicles can fuse multiple times. Finally, targeted elimination of VGAT in horizontal cells resulted in a loss of tonic, autaptic GABA currents, and of inhibitory feedback modulation of the cone photoreceptor Cai, consistent with the elimination of GABA release from horizontal cell endings. These results in mammalian retina identify the central role of vesicular release of GABA from horizontal cells in the feedback inhibition of photoreceptors.

Proper application of antibodies for immunohistochemical detection: antibody crimes and how to prevent them

For several decades antibodies raised against specific proteins, peptides, or peptide epitopes have proven to be versatile and very powerful tools to demonstrate molecular identity in cells and tissues. New techniques of immunohistochemistry and immunofluorescence have improved both the optical resolution of such protein identification as well as its sensitivity, particularly through the use of amplification methodology. However, this improved sensitivity has also increased the risks of false-positive and false-negative staining and thereby raised the necessity for proper and adequate controls. In this review, the authors draw on many years of experience to illuminate many of the more common errors and problematic issues in immunohistochemistry, and how these may be avoided. A key factor in all of this is that techniques need to be properly documented and especially antibodies and procedures must be adequately described. Antibodies are a valuable and shared resource within the scientific community; it is essential therefore that mistakes involving antibodies and their controls are not perpetuated through inadequate reporting in the literature.

The modulatory role of taurine in retinal ganglion cells

Taurine (2-aminoethylsuphonic acid) is present in nearly all animal tissues, and is the most abundant free amino acid in muscle, heart, CNS, and retina. Although it is known to be a major cytoprotectant and essential for normal retinal development, its role in retinal neurotransmission and modulation is not well understood. We investigated the response of taurine in retinal ganglion cells, and its effect on synaptic transmission between ganglion cells and their presynaptic neurons. We find that taurine-elicited currents in ganglion cells could be fully blocked by both strychnine and SR95531, glycine and GABA(A) receptor antagonists, respectively. This suggests that taurine-activated receptors might share the antagonists with GABA and glycine receptors. The effect of taurine at micromolar concentrations can effectively suppress spontaneous vesicle release from the presynaptic neurons, but had limited effects on light-evoked synaptic signals in ganglion cells. We also describe a metabotropic effect of taurine in the suppression of light-evoked response in ganglion cells. Clearly, taurine acts in multiple ways to modulate synaptic signals in retinal output neurons, ganglion cells.

Role of prostanoid production and receptors in the regulation of retinal endogenous amino acid neurotransmitters by 8-isoprostaglandin E2, ex vivo

The role of enzymes and receptors of the prostanoid pathway in the inhibitory effect of 8-isoprostaglandin E2 (8-isoPGE2) on endogenous amino acid neurotransmitter levels was examined, ex vivo. Freshly isolated bovine eyeballs were injected intravitreally with IsoPs, incubated in Krebs buffer for 30 min and retina prepared for HPLC-ECD detection of amino acids. 8-isoPGE2 attenuated retinal glutamate and its metabolite, glutamine and glycine in a concentration-dependent manner. The nonselective cyclooxygenase (COX)-inhibitor, flurbiprofen, COX-2 selective inhibitor, NS-398 and thromboxane (Tx) synthase inhibitor, furegrelate had no effect on both basal amino acid levels and the inhibitory effects of 8-isoPGE2 (1-100 μM) on the retinal amino acids. Whereas the TP-receptor antagonist SQ-29548(10 μM) exhibited no effect, SC-19220(EP1; 30 μM), AH-6809(EP(1-3); 30 μM) and AH-23848(EP4; 30 μM) reversed the inhibitory effects of 8-isoPGE2 (0.01-100 μM) on glutamate, glutamine and glycine levels. We conclude that prostanoid EP-receptors regulate the inhibitory effect of 8-isoPGE2 on basal levels of endogenous amino acids in bovine retina, ex vivo.

Expression of multiple glutamate transporter splice variants in the rodent testis

Glutamate is a regulated molecule in the mammalian testis. Extracellular regulation of glutamate in the body is determined largely by the expression of plasmalemmal glutamate transporters. We have examined by PCR, western blotting and immunocytochemistry the expression of a panel of sodium-dependent plasmalemmal glutamate transporters in the rat testis. Proteins examined included: glutamate aspartate transporter (GLAST), glutamate transporter 1 (GLT1), excitatory amino acid carrier 1 (EAAC1), excitatory amino acid transporter 4 (EAAT4) and EAAT5. We demonstrate that many of the glutamate transporters in the testis are alternately spliced. GLAST is present as exon-3- and exon-9-skipping forms. GLT1 was similarly present as the alternately spliced forms GLT1b and GLT1c, whereas the abundant brain form (GLT1a) was detectable only at the mRNA level. EAAT5 was also strongly expressed, whereas EAAC1 and EAAT4 were absent. These patterns of expression were compared with the patterns of endogenous glutamate localization and with patterns of d-aspartate accumulation, as assessed by immunocytochemistry. The presence of multiple glutamate transporters in the testis, including unusually spliced forms, suggests that glutamate homeostasis may be critical in this organ. The apparent presence of many of these transporters in the testis and sperm may indicate a need for glutamate transport by such cells.

Mammalian retinal horizontal cells are unconventional GABAergic neurons

Lateral interactions at the first retinal synapse have been initially proposed to involve GABA by transporter-mediated release from horizontal cells, onto GABA(A) receptors expressed on cone photoreceptor terminals and/or bipolar cell dendrites. However, in the mammalian retina, horizontal cells do not seem to contain GABA systematically or to express membrane GABA transporters. We here report that mouse retinal horizontal cells express GAD65 and/or GAD67 mRNA, and were weakly but consistently immunostained for GAD65/67. While GABA was readily detected after intracardiac perfusion, it was lost during classical preparation for histology or electrophysiology. It could not be restored by incubation in a GABA-containing medium, confirming the absence of membrane GABA transporters in these cells. However, GABA was synthesized de novo from glutamate or glutamine, upon addition of pyridoxal 5'-phosphate, a cofactor of GAD65/67. Mouse horizontal cells are thus atypical GABAergic neurons, with no functional GABA uptake, but a glutamate and/or glutamine transport system allowing GABA synthesis, probably depending physiologically from glutamate released by photoreceptors. Our results suggest that the role of GABA in lateral inhibition may have been underestimated, at least in mammals, and that tissue pre-incubation with glutamine and pyridoxal 5'-phosphate should yield a more precise estimate of outer retinal processing.

Metabolic and functional profiling of the ischemic/reperfused rat retina

We quantitatively tracked the recovery in amino acid labeling and cation channel functionality within distinct retinal elements for up to 2 weeks after an ischemic insult. Pattern recognition analysis of multiple amino acid and agmatine (a cation channel probe; 1-amino-4-guanidobutane; AGB) immunocytochemical patterns was used to classify all neural elements within the retina. This classification was spatially complete and with single-cell resolution. By 48 hours of reperfusion the amino acid labeling pattern of virtually all cell populations had returned to near preischemic levels, with the exception of glutamine and alanine levels, which remained significantly higher in many cell populations. Classification resulted in a total of 18 statistically separable theme classes (including neurons, glia, and extraretinal classes), a reduction of 10 theme classes from the normal retina (Sun et al. [ 2007a, b] J Comp Neurol, this issue). In addition to the known selective losses of amacrine cell types within the inner nuclear layer, we now demonstrate a selective loss of theme classes representing cone bipolar cells within the bipolar cell population. While there was a recovery in the amino acid labeling pattern, there were persistent cation channel gating anomalies (as reflected by AGB labeling) within several theme classes, including the theme class representing all the remaining rod bipolar cells, suggesting aberrant neuronal function secondary to metabolic insult.

Changes in Localization of Amino Acids in the Detached Cat Retina

PURPOSE

To investigate the distribution of amino acids (glutamate, aspartate, glutamine, GABA, glycine) in detached retinas with minimum postmortem artifact and to clarify the relation between amino acid distribution and histopathological change in the outer portion of detached retinas.

METHODS

Unilateral retinal detachment was produced in cats by injecting 0.25% sodium hyaluronate into the subretinal space using a glass micropipet. The eyes were fixed by perfusion for 10 min, 1, 3, 6 and 24 h, 2, 3 and 7 days after detachment and then examined under conventional light- and electron-microscopic immunocytochemistry.

RESULTS

For glutamate, aspartate and glutamine, the inner segments and perikarya of the photoreceptor cells, which were not immunopositive in the normal retinas, showed various degrees of immunoreactivity immediately after retinal detachment. Photoreceptor cells with the strong immunoreactivity developed necrosis. The staining pattern of GABA and glycine scarcely changed during the course of retinal detachment.

CONCLUSIONS

Excess intracellular glutamate, aspartate and glutamine in photoreceptor cells may cause a part of neuronal death after retinal detachment.

Immunoelectron microscopic study of polyamines in the gastrointestinal tract of rat

Polyamines (PAs) are ubiquitous polycationic metabolites in the eukaryotic and prokaryotic cells and are believed to be intimately involved in the regulation of DNA, RNA, and protein biosynthesis. However, the subcellular localization of PAs has not yet been fully elucidated in a variety of cell types. In the present study, a pre-embedding indirect immunoperoxidase approach was used to define the fine structural localization of PAs in the gastrointestinal tract of rat, which was fixed with glutaraldehyde and the monoclonal antibody ASPM-29 specific for spermine (Spm) and spermidine (Spd). Examination by a transmission electron microscopy showed that the peroxidase end products were commonly and predominantly localized in the free and attached ribosomes of the rough endoplasmic reticulum (rER) in the active protein- or peptide-secreting cells, and in rapidly proliferating cells including the gastric chief cells, mucous neck cells, and intestinal crypt cells. The nuclei, mitochondria, and secretory vesicles were devoid of PAs. Of note is the new finding that PAs are also located even on the small number of ribosomes in the cytoplasm of the parietal cells and of the villus-tip cells, because these were the cell types that were found to be almost PA-negative at the light microscopic level. These results seem to be completely consistent with those recently obtained for rat neurons. Thus, the present study generalized the subcellular localization of PAs on the ribosomes, and demonstrated that PAs are one of the components of biologically active ribosomes, possibly in any type of cell, that are closely involved in the translation processes of protein biosynthesis.

Monocarboxylate transport inhibition alters retinal function and cellular amino acid levels

We assessed the effect of the in vivo application of monocarboxylate transport inhibitors on retinal function and amino acid immunocytochemistry. We wanted to determine the impact that altered aerobic metabolite availability has on retinal function and the characteristics of amino acid shunting into metabolic pools. Electroretinograms were collected from anaesthetized rats at various times after intravitreal injection of the monocarboxylate transport inhibitors alpha-cyano-4-hydroxycinnamate (4-CIN; 2 micro L, 0.1-10 mm) or p-(dipropylsulphamoyl)benzoic acid (probenecid; 1-10 mm). Changes in retinal function were compared with quantitative amino acid immunocytochemical changes in retinas harvested 20 and 40 min after either 4-CIN or vehicle treatment. The injection of 4-CIN resulted in a dose-dependent reduction of the ON-bipolar cell P2 wave amplitude (20-80%) and delay in its implicit time. The phototransduction sensitivity was mildly reduced whereas the ON-bipolar cell P2 sensitivity was unaffected. Probenecid induced functional changes similar to those observed with 4-CIN. We also mapped the amino acid alterations within specific cell classes induced by 4-CIN application. All neurones displayed a reduced glutamate content averaging 48%; reduced GABA (31%) and glycine (28%) were found within amacrine cells and glutamine was reduced in all cell classes except photoreceptor and Müller cells. All cell classes in the retina demonstrated increases in aspartate (57%), whereas leucine (24%) and ornithine (21%) were only significantly increased in photoreceptor and bipolar cells. The reduction in glutamate immunolabelling in specific retinal cell classes was mirrored by an increase in aspartate levels at these locations. In addition, attenuated glutamine immunolabelling also closely matched the spatial pattern observed for glutamate. Our immunocytochemical analysis provides evidence that monocarboxylate transport inhibition induces a shift in the equilibrium of glutamate transamination reactions involving aspartate throughout the retina whereas photoreceptor and bipolar cells also use glutamate transamination reactions involving ornithine and leucine. The distribution pattern of glutamine secondary to monocarboxylate inhibition suggests that this amino acid is a major precursor for glutamate throughout the retina.

Quantification of amino acid neurochemistry secondary to NMDA or betaxolol application

BACKGROUND

Alterations in retinal amino acid neurochemistry are an indicator of metabolic function. Glutamate is the primary excitatory amino acid neurotransmitter within the retina, and excessive levels of glutamate can potentially cause excitotoxicity, in particular, through the N-methyl-D-aspartate (NMDA) subtype of glutamate receptor. Anomalies in NMDA receptor function have been implicated as causing many neurodegenerative disorders, and overactivation leads to neuronal death secondary to metabolic insult. Several pharmaceutical agents have been proposed as potential neuroprotective agents against excitotoxicity (e.g. betaxolol), yet any effects such drugs have on retinal neurochemistry have not been determined. Therefore, the aim of this study was to quantify the changes in retinal amino acid neurochemistry secondary to the application of NMDA with and without betaxolol.

METHODS

Functional NMDA channel activation was confirmed in both amacrine and ganglion cells by quantifying the entry into these neurones of a channel permeable probe (agmatine: 1-amino-4-guanidobutane [AGB]). By probing serial thin sections with immunoglobulins targeting AGB, glutamate, gamma-aminobutyric acid (GABA) and glycine, it was possible to simultaneously study the neurochemical characteristic as well as the NMDA-evoked AGB responses of different neurochemical populations of inner retinal neurones.

RESULTS

The authors have previously shown no accumulation of glutamate or GABA within Muller cells following NMDA application. Herein they report altered GABA and glycine immunoreactivity, but not glutamate immunoreactivity within neurones of the amacrine and ganglion cell layers following NMDA application. Finally, the addition of betaxolol did not significantly alter the normal neurochemistry of the retina.

CONCLUSION

The retina possesses intrinsic mechanisms that allow it to maintain metabolic integrity during short periods of high NMDA application.

Characterization of gabaergic neurons within the human medial mamillary nucleus

The morphology, distribution and relative frequency of GABAergic neurons in the medial mamillary nucleus (MMN) of normal human individuals was studied using a glutamic acid decarboxylase (GAD) antiserum. GAD-immunoreactive (GAD-IR) neurons were found sparsely distributed throughout the MMN and most displayed a simple bipolar morphology. A small population of large diameter GAD-IR neurons was found in the white matter capsule adjacent to the ventral border of the MMN. Results of double-labeling experiments revealed no evidence of calretinin, parvalbumin or calbindin immunoreactivities co-localizing with GAD-IR neurons. GAD-IR neurons of the MMN had an average somal area of 138+/-41 microm2, compared with the average somal area of 384+/-137 microm2 for the population of MMN neurons as a whole. GAD-IR neurons had a tendency to cluster in groups of two (and occasionally three) and showed a distribution gradient across the MMN with higher densities being found near the insertion of the fornix, the origin of the mamillo-thalamic tract and toward the medial MMN border. Quantitative estimates of GAD-IR neuron frequency revealed the GAD-IR phenotype to constitute an average of 1.7% percent of the total neuron population within the human MMN. These findings suggest that inhibitory activity within the human MMN is regulated in part by a small population of intrinsic GABAergic interneurons.

Correlating retinal function and amino acid immunocytochemistry following post-mortem ischemia

We wanted to determine the characteristics associated with electrophysiological and neurochemical changes secondary to ischemic insult as well as correlate these electrophysiological and neurochemical changes. A Ganzfeld source was used to elicit electroretinograms in anesthetized adult Sprague-Dawley rats. Following baseline recordings, one eye was removed for control quantitative amino acid immunocytochemistry, and ischemic insult was induced by cervical dislocation. Following the induction of ischemia, a single electroretinogram signal was collected at 1, 2, 4, 6, 8, 16, 32 or 64 min, after which the eye was removed for immunocytochemistry. The post-receptoral b-wave was undetectable after 1 min post-ischemia, whereas phototransduction declined more gradually and persisted for up to 16 min post-mortem. Both phototransduction saturated amplitude and sensitivity decayed with a similar time course (tc=3.06 (2.73, 3.48) versus 3.29 (2.61, 4.62)min). Significant elevation of amino acid neurotransmitter levels was not observed until 6 min post-mortem. Between 8 and 16 min post-ischemia, glutamate and GABA were significantly accumulated in neurons and Müller cells (p<0.05). Beyond 16 min, the neurotransmitter elevation in neurons and Müller cells was relatively attenuated. Aspartate immunoreactivity was significantly elevated at 4 and 6 min post-ischemia in neurons, prior to a change in any other amino acid. Moreover, of the amino acids assessed the post-ischemic change in aspartate immunoreactivity showed the best correlation with phototransduction decay (r2=0.68). Our findings show that complete impairment of phototransduction coincides with the accumulation of amino acid neurotransmitter. The correlation of aspartate immunoreactivity and phototransduction provides evidence of heightened glutamate oxidation during ischemic insult.

Taurine: evidence of physiological function in the retina

Taurine is a free amino acid found in high millimolar concentrations in mammalian tissue and is particularly abundant in the retina. Mammals synthesize taurine endogenously with varying abilities, with some species more dependent on dietary sources of taurine than others. Human children appear to be more dependent on dietary taurine than adults. Specifically, it has been established that visual dysfunction in both human and animal subjects results from taurine deficiency. Moreover, the deficiency is reversed with simple nutritional supplementation with taurine. The data suggest that taurine is an important neurochemical factor in the visual system. However, the exact function or functions of taurine in the retina are still unresolved despite continuing scientific study. Nevertheless, the importance of taurine in the retina is implied in the following experimental findings: (1) Taurine exhibits significant effects on biochemical systems in vitro. (2) The distribution of taurine is tightly regulated in the different retinal cell types through the development of the retina. (3) Taurine depletion results in significant retinal lesions. (4) Taurine release and uptake has been found to employ distinct regulatory mechanisms in the retina.

Retinal neurochemical changes following application of glutamate as a metablolic substrate

BACKGROUND

Retinal neural and glial cells share an intricate relationship that includes uptake and recycling of the amino acid neurotransmitters, glutamate and gamma-amino butyric acid (GABA), as well as metabolic links. The aim of this work was to determine the neurochemical and morphological changes induced by the removal of glucose but with the provision of exogenous glutamate in the isolated retinal preparation incubated under aerobic conditions. The carbon skeleton of glutamate can enter the tricarboxylic acid cycle as alpha-ketogluterate, providing an alternative metabolic substrate in cases of glucose deprivation.

METHODS

Isolated rat retinas were incubated in physiological media with and without glucose, using a range of glutamate concentrations to provide an alternative source of metabolic substrate. We conducted post-embedding immunocytochemistry and quantified the change in glutamate and GABA immunoreactivity within Müller cells under these different incubation conditions.

RESULTS

The provision of glutamate with normal (6 mM) glucose levels resulted in a gradual accumulation of glutamate and GABA in Müller cells, with Müller loading when exogenous glutamate concentrations were above 0.1 mM. However, when these varying levels of glutamate were applied in the absence of glucose, glutamate accumulation in Müller cells was decreased compared to the 6 mM glucose condition and GABA accumulation in Müller cells was at a minimum at moderate (0.5 and 1 mM) glutamate levels. Under hypoglycaemic conditions, exogenous glutamate (0.5 to 1 mM) is rapidly metabolised by Müller cells to the extent that no glial loading is evident, despite the high concentrations.

CONCLUSIONS

Normal neurochemical function appears to be maintained secondary to exogenous glutamate provision of 0.5 to 1 mM when glucose is not in the incubation medium, implying that glutamate can be used as an alternative metabolic substrate. We also show that Müller cells possess more rapid glutamate metabolic capabilities compared to the metabolism of GABA.

Quantitative analysis of glutamic acid decarboxylase-immunoreactive neurons in the anterior thalamus of the human brain

Local circuit neurons in the human anterior thalamus (AT) were identified on the basis of glutamic acid decarboxylase immunoreactivity (GAD-IR). GAD-IR neurons of the AT displayed small diameter somas with thin, sparsely-branching dendrites, consistent with the morphological characteristics of local circuit neurons found in the thalami of other mammals. Sampling techniques revealed an average of 42% of all neurons within the AT were GAD-IR, one of the highest reported percentages of local circuit neurons in the mammalian thalamus. The presence of high proportion of local circuit neurons in the AT may indicate the extent to which the Papez circuit has evolved within the human brain in comparison to other mammals.

Glycinergic amacrine cells of the rat retina

Physiological studies of neurons of the inner retina, e.g., of amacrine cells, are now possible in a mammalian retinal slice preparation. The present anatomical study characterizes glycinergic amacrine cells of the rat retina and thus lays the ground for such future physiological and pharmacological experiments. Rat retinae were immunolabeled with antibodies against glycine and the glycine transporter-1 (GLYT-1), respectively. Glycine immunoreactivity was found in approximately 50% of the amacrine and 25% of the bipolar cells. GLYT-1 immunoreactivity was restricted to glycinergic amacrine cells. They were morphologically characterized by the intracellular injection of Lucifer Yellow followed by GLYT-1 immunolabeling. Eight different types of glycinergic amacrine cells could be distinguished. They were all small-field amacrine cells with bushy dendritic trees terminating at different levels within the inner plexiform layer. The well-known AII amacrine cell was encountered most frequently. From our measurements of the dendritic field sizes and the density of glycinergic cells, we estimate that there are enough glycinergic amacrine cells available to make sure that all eight types and possibly more tile the retina regularly with their dendritic fields.

Effects of aging and dementia on the levels of thiobarbituric-acid-reactive products stimulated by L-glutamic acid in human autopsy and biopsy brain tissue

Basal and stimulated (by L-glutamic acid, GA) levels of thiobarbituric-acid-reactive products were estimated in the brain tissue (hippocampus, cortex and cerebellum) from autopsy samples of people with Alzheimer disease (AD), multi-infarct dementia (MID) and from nondemented control patients. The experiment was also performed on biopsy brain tissue (cortex) of nondemented controls. The biopsy brain tissue influenced by normal aging in vivo showed a limited susceptibility to undergo lipid peroxidation stimulated by GA in vitro in comparison with the younger tissue. A significant decrease in the ratio of stimulated to basal levels was found in the cerebellum of MID patients in comparison with nondemented controls and AD patients.

Distribution of N-acetylaspartylglutamate immunoreactivity in human brain and its alteration in neurodegenerative disease

The dipeptide N-acetylaspartylglutamate (NAAG) may be involved in the process of glutamatergic signaling by both acting at glutamate receptors and as a glutamate protransmitter. In the present study we determined the cellular localization and distribution of NAAG-like immunoreactivity (NAAG-LI) in normal human brain and in neurodegenerative disorders to ascertain the degree of NAAG's colocalization to putative glutamatergic pathways. Immunohistochemistry with an antibody against NAAG was performed on control, Huntington's disease (HD) and Alzheimer's disease (AD) human autopsy and biopsy brain sections from the cerebral cortex, hippocampus, amygdala, neostriatum, brainstem and spinal cord. In normal human brain, NAAG-LI was widespread localized to putative glutamatergic pyramidal neurons of the cerebral cortex and hippocampus. Punctate NAAG-LI was present in areas known to receive neuronal glutamatergic input, such as layer IV of the cerebral cortex, striatal neuropil, and the outer portion of the molecular layer of the hippocampal dentate gyrus. In the two pathologic brain regions examined, the HD neostriatum and the AD temporal cortex, we observed a widespread loss of NAAG-LI neurons. In addition NAAG-LI reactive microglia surrounding plaques were seen in AD temporal cortex but not in the HD striatum. Our results suggest that NAAG is substantially localized to putative glutamatergic pathways in human brain and that NAAG-LI neurons are vulnerable to the neurodegenerative process in HD and AD.

Analysis of the distribution of glycine and GABA in amacrine cells of the developing rabbit retina: a comparison with the ontogeny of a functional GABA transport system in retinal neurons

The objectives of this study were to (1) determine whether the glycinergic and GABAergic amacrine cells in the developing rabbit retina were neurochemically distinct at birth, (2) determine if the ratio of GABAergic to glycinergic amacrine cells was constant during development, (3) determine whether the capacity to take up a GABA analogue was restricted to GABAergic neurons, and (4) whether initiation of GABA transport into GABAergic neurons preceded the presence of a content of GABA in these neurons. We have used a novel strategy to immunolocalize a non-endogenous GABA analogue, gamma-vinyl GABA, which is taken up into neurons by a GABA transporter. Examination of serial semithin resin-embedded sections of neonatal rabbit retinae that had been immunolabelled for glycine, GABA or gamma-vinyl GABA revealed that at 1 day postnatum, 60% of amacrine cells contain glycine but not GABA and did not accumulate gamma-vinyl GABA, which is similar to the percentage of glycinergic amacrine cells in the adult retina. The vast majority of the remaining amacrine cells contained GABA and many also transported gamma-vinyl GABA; however, a significant number of GABA-containing cells failed to accumulate gamma-vinyl GABA suggesting that possession of a content of GABA did not have to be preceded by, or be concomitant with, the presence of a GABA transport system. By 10 days postnatum, over 99% of GABA-containing amacrine cells also transported gamma-vinyl GABA indicating their functional maturity. Analysis of the horizontal cells revealed no evidence for uptake of gamma-vinyl GABA, but another GABA analogue, diaminobutyric acid, which is a substrate both for the neuron-associated GABA transporter and the glial GABA transporter, was accumulated into some horizontal cells at 21 days postnatum, a time point when these cells also contain endogenous GABA. We conclude that amacrine cells are committed to being GABAergic or glycinergic at, or prior to birth, and that in some amacrine cells, expression of a content of GABA may occur prior to the capacity to transport GABA. Conversely, in some ganglion cells transport of gamma-vinyl GABA may precede a content of GABA.

The DAPI-3 amacrine cells of the rabbit retina

In the rabbit retina, the nuclear dye, 4,6,diamidino-2-phenylindole (DAPI), selectively labels a third type of amacrine cell, in addition to the previously characterized type a and type b cholinergic amacrine cells. In this study, these "DAPI-3" amacrine cells have been characterized with respect to their somatic distribution, dendritic morphology, and neurotransmitter content by combining intracellular injection of biotinylated tracers with wholemount immunocytochemistry. There are about 100,000 DAPI-3 amacrine cells in total, accounting for 2% of all amacrine cells in the rabbit retina, and their cell density ranges from about 130 cells/mm2 in far-peripheral retina to 770 cells/mm2 in the visual streak. The thin varicose dendrites of the DAPI-3 amacrine cells form a convoluted dendritic tree that is symmetrically bistratified in S1/S2 and S4 of the inner plexiform layer. Tracer coupling shows that the DAPI-3 amacrine cells have a fivefold dendritic-field overlap in each sublamina, with the gaps in the arborization of each cell being occupied by dendrites from neighboring cells. The DAPI-3 amacrine cells consistently show the strongest glycine immunoreactivity in the rabbit retina and they also accumulate exogenous [3H]-glycine to a high level. By contrast, the AII amacrine cells, which are the best characterized glycinergic cells in the retina, are amongst the most weakly labelled of the glycine-immunopositive amacrine cells. The DAPI-3 amacrine cells costratify narrowly with the cholinergic amacrine cells and the On-Off direction-selective ganglion cells, suggesting that they may play an important role in movement detection.

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.

Activity-dependent transport of GABA analogues into specific cell types demonstrated at high resolution using a novel immunocytochemical strategy

We have raised antisera against the GABA analogues gamma-vinyl GABA, diaminobutyric acid and gabaculine. These analogues are thought to be substrates for high-affinity GABA transporters. Retinae were exposed to micromolar concentrations of these analogues in the presence or absence of uptake inhibitors and then fixed and processed for immunocytochemistry at the light and electron microscopic levels. Immunolabelling for gamma-vinyl GABA revealed specific labelling of GABAergic amacrine cells and displaced amacrine cells in retinae of rabbits, cats, chickens, fish and a monkey. GABA-containing horizontal cells of cat and monkey retinae failed to exhibit labelling for gamma-vinyl GABA, suggesting that they lacked an uptake system for this molecule. In light-adapted fish, gamma-vinyl GABA was readily detected in H1 horizontal cells; similar labelling was also observed in light-adapted chicken retinae. The pattern of labelling in the fish and chicken retinae was modified by dark adaptation, when labelling was greatly reduced in the horizontal cells, indicating the activity dependence of GABA (analogue) transport. Intraperitoneal injection of gamma-vinyl GABA into rats resulted in its transport across the blood-brain barrier and subsequent uptake into populations of GABAergic neurons. The other analogues investigated in this study exhibited different patterns of transport; gabaculine was taken up into glial cells, whilst diaminobutyric acid was taken up into neurons, glial cells and retinal pigment epithelia. Thus, these analogues are probably substrates for different GABA transporters. We conclude that immunocytochemical detection of the high-affinity uptake of gamma-vinyl GABA permits the identification of GABAergic neurons which are actively transporting GABA, and suggest that this novel methodology will be a useful tool in rapidly assessing the recent activity of GABAergic neurons at the cellular level.

Synaptic organization of an organotypic slice culture of the mammalian retina

Vertical slices of postnatal day 6 (P6) rat retina were cut and cultured using the roller-tube technique. The organotypic differentiation during a culture period of up to 30 days has been described in a previous study (Feigenspan et al., 1993a). Here we concentrated on the synaptic organization in the retinal slice culture. Electron microscopy revealed the presence of ribbon synapses in the outer plexiform layer and conventional and ribbon synapses in the inner plexiform layer. Immunofluorescence with antibodies that recognize specific subunits of GABAA or glycine receptors revealed a punctate distribution of the receptors. They were aggregated in "hot spots" that correspond to a concentration of receptors at postsynaptic sites. Different isoforms of GABAA and glycine receptors occurred in the slice cultures. The experiments show that there is a differentiation of synapses and a diversity of transmitter receptors in the slice cultures that is comparable to the in vivo retina.

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

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

Redistribution of GABA immunoreactivity following central retinal artery occlusion

gamma-Aminobutyric acid (GABA) is normally primarily in amacrine cells in the rat retina. Immediately after an ischaemic insult, attained by occlusion of the central retinal artery for 60 min, GABA is then found to be associated with Müller cells. During subsequent reperfusion, the distribution of GABA immunoreactivity gradually reverts from the glial cells back into neuronal elements of the retina. Twenty-four hours after ischaemia, GABA staining is indistinguishable from that seen in control animals. It is suggested that during central retinal artery occlusion, Müller cell energy levels are sufficient to allow the active uptake of released GABA, but insufficient to metabolise it to glutamine. The normal cycle of GABA metabolites from Müller cells to neurones is thus inhibited. Restoration of blood flow and the consequent increase in retinal energy levels, as indicated by a slight recovery of the electroretinogram b-wave, facilitates glutamine shunting between glial cells and amacrine cells, resulting in the synthesis of neuronal GABA.

Taurine, amino acid transmitters, and related molecules in the retina of the Australian lungfish Neoceratodus forsteri: a light-microscopic immunocytochemical and electron-microscopic study

The morphology of the retina of the Australian lungfish Neoceratodus forsteri was investigated by means of light- and electron microscopy, whilst immunocytochemical studies were performed to determine the cellular distributions of the major amino acid neurotransmitters and other amino acids. The distributions of glycine and GABA were similar to those previously described for teleost, amphibian and mammalian retinae. Labelling was abundant in amacrine cells, whilst GABA was also present in one layer of horizontal cells and some bipolar cells. Taurine was present in both rods and cones, but, unlike the mammalian or avian retina, was absent from other cellular structures, including glial elements. Unexpectedly, the photoreceptor terminals lacked an apparent content of the excitatory amino acid transmitter glutamate. The glutamate that was present in the rods and cones occupied a crescentic arc corresponding to the location of glycogen-rich paraboloids. Asparagine was also present in rods, albeit in the modified mitochondria that formed the elipsoids of the rod inner segments. Arginine, the precursor for formation of nitric oxide, was present in glial cells, and in the paraboloids of both rods and cones.