Interrelationship between retinal ischaemic damage and turnover and metabolism of putative amino acid neurotransmitters, glutamate and GABA

Loss of Müller cell glutamine synthetase immunoreactivity is associated with neuronal changes in late-stage retinal degeneration

Introduction

A hallmark of photoreceptor degenerations is progressive, aberrant remodeling of the surviving retinal neurons and glia following photoreceptor loss. The exact relationship between neurons and glia remodeling in this late stage of retinal degeneration, however, is unclear. This study assessed this by examining Müller cell dysfunction via glutamine synthetase immunoreactivity and its spatial association with retinal neuron subpopulations through various cell markers.

Methods

Aged Rd1 mice retinae (P150 - P536, n = minimum 5 per age) and control heterozygous rd1 mice retinae (P536, n = 5) were isolated, fixed and cryosectioned. Fluorescent immunolabeling of glutamine synthetase was performed and retinal areas quantified as having low glutamine synthetase immunoreactivity if proportion of labeled pixels in an area was less than two standard deviations of the mean of the total retina. Other Müller cell markers such as Sox9 and Glial fibrillary acidic protein along with neuronal cell markers Calbindin, Calretinin, recoverin, Protein kinase C-α, Glutamic acid decarboxylase 67, and Islet-1 were then quantified within areas of low and normal synthetase immunoreactivity.

Results

Glutamine synthetase immunoreactivity was lost as a function of age in the rd1 mouse retina (P150 - P536). Immunoreactivity of other Müller cell markers, however, were unaffected suggesting Müller cells were still present in these low glutamine synthetase immunoreactive regions. Glutamine synthetase immunoreactivity loss affected specific neuronal populations: Type 2, Type 8 cone, and rod bipolar cells, as well as AII amacrine cells based on reduced recoverin, protein kinase Ca and parvalbumin immunoreactivity, respectively. The number of cell nuclei within regions of low glutamine synthetase immunoreactivity was also reduced suggesting possible neuronal loss rather than reduced cell marker immunoreactivity.

Conclusion

These findings further support a strong interplay between glia-neuronal alterations in late-stage degeneration and highlight a need for future studies and consideration in intervention development.

Ocular ischaemia: signs, symptoms, and clinical considerations for primary eye care practitioners

Ischaemic stroke is a major disease burden as well as a leading cause of death. Early signs of ischaemic stroke can manifest in the eye, placing primary eyecare practitioners in an important position to identify patients at risk of ischaemic stroke and initiate suitable referral pathways. The vascular supply to the brain is reviewed with reference to vision including the various retinal signs and ocular symptoms associated with transient ischaemic attacks and ischaemic stroke. Using a range of clinical cases, the diverse clinical presentations of retinal embolic events, as well as other forms of vascular occlusion, are highlighted and the underlying pathophysiology is discussed. A succinct scheme for the assessment and management of ischaemic events for primary eye care practitioners is provided.

Successful management of ivermectin-induced blindness in an African lion (Panthera leo) by intravenous administration of a lipid emulsion

Background

Ivermectin is widely used in veterinary practice for the treatment of ecto- and endo-parasites. In wildlife, an extra-label use this parasiticide is sometimes associated with toxicity. Different treatment regimens have been used in ivermectin toxicosis. The present report describes a successful reversal of ivermectin toxicity by intravenous administration of a commercially available lipid emulsion in a captive African lion (Panthera leo).

Case presentation

A 2-year old captive African lion (Panthera leo) weighing ~130 kg was presented with acute neurological impairment and bilateral blindness that had developed 24 h after ivermectin exposure. The animal was treated with a commercially available lipid emulsion along with supportive therapy and experienced complete recovery.

Conclusion

To our knowledge, this is the first case report of the use of lipid emulsion in the management of ivermectin induced blindness in an African lion and it appears that intravenous lipid emulsion may be an effective therapy in ivermectin toxicity in lions. Further testing in expanded clinical trials is clearly warranted.

Using the rd1 mouse to understand functional and anatomical retinal remodelling and treatment implications in retinitis pigmentosa: A review

Retinitis Pigmentosa (RP) reflects a range of inherited retinal disorders which involve photoreceptor degeneration and retinal pigmented epithelium dysfunction. Despite the multitude of genetic mutations being associated with the RP phenotype, the clinical and functional manifestations of the disease remain the same: nyctalopia, visual field constriction (tunnel vision), photopsias and pigment proliferation. In this review, we describe the typical clinical phenotype of human RP and review the anatomical and functional remodelling which occurs in RP determined from studies in the rd/rd (rd1) mouse. We also review studies that report a slowing down or show an acceleration of retinal degeneration and finally we provide insights on the impact retinal remodelling may have in vision restoration strategies.

Mapping kainate activation of inner neurons in the rat retina

Kainate receptors mediate fast, excitatory synaptic transmission for a range of inner neurons in the mammalian retina. However, allocation of functional kainate receptors to known cell types and their sensitivity remains unresolved. Using the cation channel probe 1-amino-4-guanidobutane agmatine (AGB), we investigated kainate sensitivity of neurochemically identified cell populations within the structurally intact rat retina. Most inner retinal neuron populations responded to kainate in a concentration-dependent manner. OFF cone bipolar cells demonstrated the highest sensitivity of all inner neurons to kainate. Immunocytochemical localization of AGB and macromolecular markers confirmed that type 2 bipolar cells were part of this kainate-sensitive population. The majority of amacrine (ACs) and ganglion cells (GCs) showed kainate responses with different sensitivities between major neurochemical classes (γ-aminobutyric acid [GABA]/glycine ACs > glycine ACs > GABA ACs; glutamate [Glu]/weakly GABA GCs > Glu GCs). Conventional and displaced cholinergic ACs were highly responsive to kainate, whereas dopaminergic ACs do not appear to express functional kainate receptors. These findings further contribute to our understanding of neuronal networks in complex multicellular tissues.

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.

The significance of neuronal and glial cell changes in the rat retina during oxygen-induced retinopathy

Retinopathy of prematurity is a devastating vascular disease of premature infants. A number of studies indicate that retinal function is affected in this disease. Using the rat model of oxygen-induced retinopathy, it is possible to explore more fully the complex relationship between neuronal, glial and vascular pathology in this condition. This review examines the structural and functional changes that occur in the rat retina following oxygen-induced retinopathy. We highlight that vascular pathology in rats is characterized by aberrant growth of blood vessels into the vitreous at the expense of blood vessel growth into the body of the retina. Moreover, amino acid neurochemistry, a tool for examining neuronal changes in a spatially complete manner reveals widespread changes in amacrine and bipolar cells. In addition, neurochemical anomalies within inner retinal neurons are highly correlated with the absence of retinal vessels. The key cell types that link blood flow with neuronal function are macroglia. Macroglia cells, which in the retina include astrocytes and Müller cells, are affected by oxygen-induced retinopathy. Astrocyte loss occurs in the peripheral retina, while Müller cells show signs of reactive gliosis that is highly localized to regions that are devoid of intraretinal blood vessels. Finally, we propose that treatments, such as blockade of the renin-angiotensin system, that not only targets pathological angiogenesis, but that also promotes re-vascularization of the retina are likely to prove important in the treatment of those with retinopathy of prematurity.

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.

Changes of pyridoxal kinase expression and activity in the gerbil hippocampus following transient forebrain ischemia

In the previous study, we observed chronological alterations of glutamic acid decarboxylase (GAD), which is the enzyme converting glutamate into GABA. GAD isoforms (GAD65 and GAD67) differ substantially in their interactions with cofactor pyridoxal 5'-phosphate, which is catalyzed by pyridoxal kinase (PLK). In the present study, we examined the chronological changes of PLK expression and activity in the hippocampus after 5 min transient forebrain ischemia in gerbils. PLK immunoreactivity in the sham-operated group was detected weakly in the hippocampus. Ischemia-related change of PLK immunoreactivity in the hippocampus was significant in the hippocampal cornu ammonis (CA1)region, not in the hippocampal CA2/3 region and dentate gyrus. PLK immunoreactivity was observed in non-pyramidal GABAergic neurons at 30 min to 3 h after ischemic insult. At 12 h after ischemic insult, PLK immunoreactivity was shown in many CA1 pyramidal cells as well as some non-pyramidal cells. At this time point, PLK immunoreactivity and protein content was highest after ischemia. Thereafter, PLK immunoreactivity and protein content is decreased time-dependently by 4 days after ischemic insult. Four days after ischemia, some astrocytes expressed PLK in the CA1 region. The specific PLK activity was not altered following ischemic insult up to 2 days after ischemic insult. Thereafter, the specific PLK activity decreased time-dependently. However, total activity of PLK was significantly increased 12-24 h after ischemic insult, and thereafter total activity of PLK decreased. Therefore, we suggest that the over-expression of PLK in the CA1 pyramidal cells at 12 h after ischemia may induce increase of GAD in the CA1 pyramidal cells, which plays an important role in delayed neuronal death via the increase of GABA or enhancement of GABA shunt pathway.

Short- and long-term enzymatic regulation secondary to metabolic insult in the rat retina

Changes in oxygen and/or glucose availability may result in altered levels of ATP production and amino acid levels, and alteration in lactic acid production. However, under certain metabolic insults, the retina demonstrates considerable resilience and maintains ATP production, and/or retinal function. We wanted to investigate whether this resilience would be reflected in alterations in the activity of key enzymes of retinal metabolism, or enzymes associated with amino acid production that may supply their carbon skeleton for energy production. Enzymatic assays were conducted to determine the activity of key retinal metabolic enzymes total ATPase and Na(+)/K(+)-ATPase, aspartate aminotransferase and lactate dehydrogenase. In vitro anoxia led to an increase in retinal lactate dehydrogenase activity and to a decrease in retinal aspartate aminotransferase activity, without significant changes in Na(+)/K(+)-ATPase activity. In vivo inhibition of glutamine synthetase resulted in a short-term significant decrease in retinal aspartate aminotransferase activity. An increase in retinal aspartate aminotransferase and lactate dehydrogenase activities was accompanied by altered levels of amino acids in neurons and glia after partial inhibition of glial metabolism, implying that short- and long-term up- and down-regulation of key metabolic enzymes occurs to supply carbon skeletons for retinal metabolism. ATPase activity does not appear to fluctuate under the metabolic stresses employed in our experimental procedures.

Inner retinal neurons display differential responses to N-methyl-D-aspartate receptor activation

The N-methyl-D-aspartate (NMDA) responses of neurons from within the inner rabbit retina were mapped using a channel permeable cation, 1-amino-4-guanidobutane (agmatine, AGB). Serial sections were subsequently probed with immunoglobulins targeting AGB, glutamate, gamma-aminobutyric acid (GABA), and glycine to visualize the NMDA responses of neurochemical subpopulations of neurons. Most inner retinal subpopulations of neurons demonstrated an NMDA concentration-dependent increase in activation. This NMDA-induced activation displayed a distinct pattern, with the most sensitive class to least sensitive class ranking being GC > GABA cAC > GABA/Gly cAC > Gly cAC > GABA dAC (GC, ganglion cells; AC, amacrine cells; c, conventional; d, displaced; Gly, glycine). The variable NMDA response may reflect differences in NMDA receptor subunit disposition or differences in receptor density. In addition to the variable NMDA activation pattern, we found that virtually all ganglion cells (87%) showed NMDA-gated AGB entry, compared with only 58% of amacrine cells. We conclude that a large cohort of amacrine cells do not possess functional NMDA receptors. In addition to most ganglion cells being activated by NMDA, a large subpopulation displayed the highest sensitivity to NMDA application. The functional significance of this finding is that the ganglion cell population will be the first neuronal class to be susceptible to glutamate-induced neurotoxicity mediated through the NMDA receptor. The addition of betaxolol significantly reduced NMDA-mediated AGB entry into most neuronal groups (ganglion cells, GABA, and glycine amacrine cells), with the greatest effect being on ganglion cells. Betaxolol had no significant effect on NMDA-gated entry of AGB on the GABA/Gly amacrine cell population.

Zinc in the retina

Experimental evidence exists to suggest that zinc can have positive and negative effects on the physiology of cells depending on the "local" concentration, localisation (extracellular vs. intracellular) and/or state (bound vs. free). The retina contains particularly high amounts of zinc suggesting a pivotal role in the tissue. There is also suggestive evidence that zinc deficiency in humans may result in abnormal dark adaptation and/or age-related macular degeneration. The purpose of this article is to provide an overview of various proposed functions for zinc, particularly in the retina. Endogenous chelatable zinc in the retina is localised mainly to the photoreceptors and retinal pigment epithelial cells. Moreover, the zinc localisation in the photoreceptors varies in dark and light, suggesting a role for zinc in a light-regulated process. Some zinc is also located to other areas of the retina but clearly defined zinc-enriched neurones could not be identified as has been shown to occur in certain areas of the brain. Neurones post-synaptic to zinc-enriched neurones in the brain have been suggested to be particularly vulnerable in ischaemia. The role of zinc in retinal ischaemia has been investigated to determine how it is involved in the process. It would appear that when zinc is administered in low concentrations it generally has a positive effect on an insulted retina as in ischaemia. However, higher concentrations of zinc exacerbates the influence of the insult and also acts as a toxin. Use of zinc supplements in diet must, therefore, be taken with caution.

In glaucoma, should enthusiasm about neuroprotection be tempered by the experience obtained in other neurodegenerative disorders?

Some in vitro and in vivo evidence, as well as rare observations in human eyes with glaucoma, suggests that retinal ganglion cells could be lost by apoptosis during the course of glaucomatous optic neuropathy. There exist also observations indicating that in the vitreous of patients with glaucoma it is possible to measure an increased concentration of glutamate (an excitotoxic amino acid known to induce neuronal apoptosis in animal models). These observations, among others, suggest the possibility of an excitotoxicity mechanism in the pathogenesis of glaucoma and as a consequence the potential for a neuroprotective approach to treating this disorder. Amazingly, not only in glaucoma but also in other neurodegenerative disorders (Parkinson's disease, amyotrophic lateral sclerosis, stroke, etc.) it has been postulated that neurons could be lost through an excitotoxic mechanism. In these non-glaucomatous disorders, quite a large number of clinical trials have already been conducted to determine the potential benefit of different neuroprotective therapies. Unfortunately, with a few rare exceptions, the results of these clinical studies have been very disappointing (in contrast to encouraging results obtained in preclinical trials). The experience acquired in other neurodegenerative disorders should probably be kept in mind when addressing the question of neuroprotection in glaucoma. In particular, the hope raised by preclinical studies showing that drugs could have a beneficial effect on the survival of retinal ganglion cells should certainly be tempered until such an effect is confirmed by clinical trials conducted in patients with glaucoma.

Neuroprotection in relation to retinal ischemia and relevance to glaucoma

Management of glaucoma is directed at the control of intraocular pressure (IOP), yet it is recognized now that increased IOP isjust an important risk factor in glaucoma. Therapy that prevents the death of ganglion cells is the main goal of treatment, but an understanding of the causes of ganglion cell death and precisely how it occurs remains speculative. Present information supports the working hypothesis that ganglion cell death may result from a particular form of ischemia. Support for this view comes from the fact that not all types of retinal ischemia lead to the pathologic findings seen in glaucomatous retinas or to cupping in the optic disk area. Moreover, in animal experiments in which ischemia is caused by elevated IOP, a retinal abnormality similar to that seen in true glaucoma is produced, whereas after occlusion of the carotid arteries a different pattern of damage is found. In ischemia, glutamate is released, and this initiates the death of neurons that contain ionotropic glutamate (NMDA) receptors. Elevated glutamate levels exist in the vitreous humor of patients with glaucoma, and NMDA receptors exist on ganglion cells and a subset of amacrine cells. Experimental studies have shown that a variety of agents can be used to prevent the death of retinal neurons (particularly ganglion cells) induced by ischemia. These agents are generally those that block NMDA receptors to prevent the action of the released glutamate or substances that interfere with the subsequent cycle of events that lead to cell death. The major causes of cell death after activation of NMDA receptors are the influx of calcium into cells and the generation of free radicals. Substances that prevent this cascade of events are, therefore, often found to act as neuroprotective agents. For a substance to have a role as a neuroprotective agent in glaucoma, it would ideally be delivered topically to the eye and used repeatedly. It is, therefore, of interest that betaxolol, a beta-blocker presently used to reduce IOP in humans, also has calcium channel-blocking functions. Moreover, experimental studies show that betaxolol is an efficient neuro protective agent against retinal ischemia in animals, when injected directly into the eye or intraperitoneally.

Pregnenolone sulfate modulates NMDA receptors, inducing and potentiating acute excitotoxicity in isolated retina

Pregnenolone sulfate (PS) acts as a positive allosteric modulator of N-methyl-D-aspartate (NMDA) receptor-mediated responses. In the retina, we previously observed that the synthesis of pregnenolone and PS increases after stimulation of NMDA receptors and blockade of the synthesis reduces retinal cell death. This study was carried out to explore in the isolated and intact retina the possible role of PS in NMDA-induced excitotoxicity. Lactate dehydrogenase (LDH) measurements and morphological analysis revealed that a 90-min exogenous application of PS at 0.1-500 microM concentrations potentiated NMDA-induced cell death and at 50-500 microM concentrations caused cytotoxicity. After 45 min, either NMDA or PS caused no significant LDH release; but their co-application resulted in a high degree of toxicity. In addition, we found that a mild NMDA insult developed into serious damage when even low PS concentrations (0.1-10 microM) were used. Toxicity-inducing and -potentiating effects were specific to PS modulatory action on NMDA receptors, in that they were blocked by 4-(3-phosphonopropyl)2-piperazinecarboxylic acid (CPP) and MK-801 but not by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), and neither dehydroepiandrosterone sulfate nor pregnenolone caused LDH release. Prevention of degenerative signs was seen in retinae pretreated with 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS), a Cl- channel blocker, thus indicating a Na+/Cl--dependent acute mode of excitotoxic cell death responsible for PS toxicity. The positive interaction between the neurosteroid and NMDA receptors was further proved by a PS dose-dependent increase in NMDA-induced stimulation of [3H] MK-801 binding to retinal membranes. The results suggest a crucial role of PS in retinal vulnerability and propose the toxicity-potentiating effects as an important key in linking NMDA-induced endogenous synthesis to acute excitotoxicity.

Induction of neurosteroid synthesis by NMDA receptors in isolated rat retina: a potential early event in excitotoxicity

Here we investigated the possible regulation of neurosteroidogenesis by N-methyl-D-aspartic acid (NMDA) receptor activation and addressed the hypothesis that neurosteroid synthesis may be involved in acute excitotoxicity. In the isolated retina, exposure to NMDA modified pregnenolone and pregnenolone sulphate formation. This effect was dose and time dependent, the synthesis being increased by relatively moderate NMDA doses (1-100 microM) within 30 min exposure and reduced to its control value by 60 min or by raising drug concentrations. NMDA-stimulated neurosteroid synthesis was blocked by (+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclo-hepten-5,10-imine hydrogen maleate (MK-801) and 3(2-carboxypiperazine-4-yl)propyl-1-phosphonic acid (CPP), depended on extracellular calcium and reproduced by glutamate. Lactate dehydrogenase (LDH) release and morphological analysis revealed that retinal cell viability was not significantly affected after 30 min exposure to 50 microM NMDA, but severe cell damage occurred by 60 min. When the GABAA (gamma-aminobutyric acid) receptor agonist muscimol (1-1000 microM), known to activate retinal neurosteroidogenesis, was added together with NMDA, no additional increase in neurosteroid synthesis was observed, and NMDA-induced LDH release remained unchanged. However, exposure to a high concentration of muscimol alone (500 microM) provoked a similar degree of toxicity to NMDA. By contrast, bicuculline abolished the increase in neurosteroidogenesis and LDH release. Similarly, pretreatment with R (+)-p-aminoglutethimide (AMG), an inhibitor of cholesterol side-chain cleavage cytochrome P450, attenuated acute retinal cell damage. The inhibitory nature of AMG on NMDA-stimulated neurosteroidogenesis was confirmed in the observation that drug treatment reduced pregnenolone content and did not affect the bindings of [3H] MK-801 and [3H] muscimol. The results demonstrate that NMDA receptors regulate neurosteroidogenesis through a transneuronal mechanism, which implies GABAA receptor activation. The early NMDA-mediated stimulation of neurosteroid synthesis seems to play a critical role in acute excitotoxicity; consequently, its inhibition is likely to delay neuronal cell death.

The localization of endogenous zinc and the in vitro effect of exogenous zinc on the GABA immunoreactivity and formation of reactive oxygen species in the retina

1. Endogenous zinc is localized mainly in the retinal photoreceptors and retinal pigment epithelial cells in the mammalian retina. No other types of retinal neurons contain large amounts of zinc. 2. Low concentrations of exogenous zinc, like the N-methyl-D-aspartate (NMDA) antagonist MK-801, counteract the NMDA-induced changes in the gamma-aminobutyric acid (GABA) immunoreactivity in the rabbit retina. However, greater concentrations of zinc exacerbate the effects of NMDA and ischemic-like insults (lack of glucose and oxygen) on GABA immunoreactivity. The data suggest that low concentrations of zinc are neuroprotective, but higher concentrations of zinc have a negative effect. 3. When low concentrations of zinc are present during ischemic-like insults to the retina, the GABA immunoreactivity is localized to the Müller cells, suggesting that the metabolism of GABA in the Müller glial cells is prevented. 4. Ascorbate/iron-induced generation of reactive oxygen species (ROS) in the retina is prevented by deferoxamine but not by zinc. High concentrations of zinc potentiate the ascorbate/iron induced formation of ROS.

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.

Amino acid signatures in the primate retina

Pattern recognition of amino acid signals partitions virtually all of the macaque retina into 16 separable biochemical theme classes, some further divisible by additional criteria. The photoreceptor-->bipolar cell-->ganglion cell pathway is composed of six separable theme classes, each possessing a characteristic glutamate signature. Neuronal aspartate and glutamine levels are always positively correlated with glutamate signals, implying that they largely represent glutamate precursor pools. Amacrine cells may be parsed into four glycine-dominated (including one glycine/GABA immunoreactive population) and four GABA-dominated populations. Horizontal cells in central retina possess a distinctive GABA signature, although their GABA content is constitutively lower than that of amacrine cells and shows both regional and sample variability. Finally, a taurine-glutamine signature defines Müller's cells. We thus have established the fundamental biochemical signatures of the primate retina along with multiple metabolic subtypes for each neurochemical class and demonstrated that virtually all neuronal space can be accounted for by cells bearing characteristic glutamate, GABA, or glycine signatures.

Disturbances in the distribution of neurotransmitters in the rat retina after ischemia

PURPOSE

Disturbances in neurotransmitter distribution have been observed in cerebral ischemia in the pathophysiologic process of excitotoxicity. The goal of this study was to examine the effect of pressure-induced retinal ischemia on the distribution of the retinal neurotransmitters glutamate and gamma-aminobutyric acid (GABA) within the rat retina.

METHODS

Animals were subjected to increased intraocular pressure of 110 mm Hg for 45 min using an intracameral hydrostatic pressure device. The distribution of glutamate and GABA immunoreactivity (IR) was determined at 0, 2, 4, 8 and 24 hrs after reperfusion by immunogold with silver intensification.

RESULTS

Three phases of neurotransmitter immunoreactivity patterns were discernible following retinal ischemia. Immediately following reperfusion (Phase I), a shift of GABA-IR from inner retinal neurons to the Mueller cells and their processes was noted. In contrast, despite marked decreases in neuronal glutamate-IR, a less pronounced shift of glutamate-IR to the Muller cells was simultaneously noted. This shift of neurotransmitter IR to the Mueller cells was transient with the gradual reappearance of IR within the inner retinal neurons noted 2-8 hrs after reperfusion (Phase II). Phase III began at 8 hrs after reperfusion with progressive loss of GABA-IR noted in the inner retina; by 24 hrs, secondary loss of inner retinal glutamate-IR was evident with corresponding dropout and pyknosis of inner retinal neurons apparent.

CONCLUSIONS

The distribution of glutamate-IR and GABA-IR was significantly altered following retinal ischemia. The alteration noted in Phase I suggested that the regulation of glutamate by Mueller cells was disrupted by this ischemic insult leading to glutamate excitotoxicity, and delayed neuronal cell degeneration as evidenced by the subsequent loss of inner retinal immunoreactivity in Phase III.

Distribution of phosphate-activated glutaminase-like immunoreactivity in the retina of rodents

The distribution of phosphate-activated glutaminase-like immunoreactivity was examined in the retinas of rodents. Intense glutaminase immunoreactivity was observed in many neuronal perikarya in the ganglion cell layer and inner nuclear layer including those of ganglion, bipolar and amacrine cells and possibly horizontal cells. Almost all bipolar cells containing protein kinase C were immunoreactive for glutaminase, suggesting that the majority of glutaminase immunoreactive bipolar cells were of the ON type. Intense glutaminase immunoreactivity was also found in the neuropil of the inner and outer plexiform layers and around the outer limiting membrane. Weak to moderate immunoreactivity was seen in the outer nuclear layer and photoreceptor inner and outer segments. Under electron microscopy, glutaminase immunoreactivity was seen in bipolar cell axons and amacrine cell processes in the inner plexiform layer. In the outer plexiform layer, immunoreactivity was found in the Müller cell processes, but not in the photoreceptor cell terminals. These results indicate that ganglion cells and ON type bipolar cells use glutaminase to produce transmitter glutamate and suggest glutaminase has additional roles in Müller cells. A population of amacrine cells and horizontal cells showed immunoreactivity to glutaminase.

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

We have assessed by means of immunocytochemistry, the cellular distributions of the amino acid transmitters GABA, glycine and glutamate, and the free-radical scavenger taurine, in the retinae of adult rabbits at various times after death. Within 10 min of death, horizontal cells began to display immunoreactivity for GABA, whilst displaced amacrine cells began to display immunoreactivity for glycine. By 40 min postmortem, GABA was present in glial cells. Glutamate, which is not normally detectable in retinal glia, was detected in such glia by 20 min postmortem. By contrast immunocytochemically detectable glycine did not accumulate in glia. There was a gradual diminution of immunoreactivity for taurine in glial cells and photoreceptors. By 2 h postmortem, most immunoreactivity had disappeared from the retina. We conclude that amino acid transmitters show rapid changes in their distributions immediately after death, which may be related to changes in the patterns of transmitter release and uptake, and changes in degradation mechanisms. The rapid changes in cellular localisation of amino acid immunoreactivity illustrated in this study, indicate that the fixation of nervous tissues must be performed rapidly. Moreover, the massive loss of immunoreactivity by 2 h postmortem suggests that any assays for content of these transmitters at this, and subsequent time-points, will bear little resemblance to the values obtained at the time of death.

Neurochemical signatures revealed by glutamine labeling in the chicken retina

Postembedding immunocytochemistry was used to determine the retinal distribution of the amino acid glutamine, and characterize amino acid signatures in the avian retinal ganglion cell layer. Glutamine is a potential precursor of glutamate and some glutamatergic neurons may use this amino acid to sustain production of glutamate for neurotransmission. Ganglion cells, cells in the inner nuclear layer, and some photoreceptors exhibited glutamine immunoreactivity of varying intensity. Ganglion cells demonstrated the highest level of immunoreactivity which indicates either slow glutamine turnover or active maintenance of a large standing glutamine pool relative to other glutamatergic neurons. Müller's cells in the avian retina are involved in glutamate uptake and carbon recycling by the rapid conversion of glutamate to glutamine, thus explaining the low glutamate and high glutamine immunoreactivity found throughout Müller's cells. Most chicken retinal ganglion cells are glutamate (E) and glutamine (Q) immunoreactive but display diverse signatures with presumed functional subsets of cells displaying admixtures of E and Q with GABA (gamma) and/or glycine (G). The four major ganglion cell signatures are (1) EQ; (2) EQ gamma; (3) EQG; and (4) EQ gamma G.

Immunocytochemical localization of the amino acid neurotransmitters in the chicken retina

Postembedding immunocytochemistry was used to determine the cellular localization of the amino acid neurotransmitters glutamate, aspartate, gamma-aminobutyric acid (GABA), and glycine in the avian retina. The through retinal pathway was glutamatergic, with all photoreceptors, bipolar cells, and ganglion cells being immunoreactive for glutamate. Bipolar cells displayed the highest level of glutamate immunoreactivity, with the cell bodies terminating just below the middle of the inner nuclear layer. All lateral elements, horizontal cells, amacrine cells, and interplexiform cells were immunoreactive for glycine or GABA. The GABAergic neurons consisted of two classes of horizontal cells and amacrine cells located in the lower part of the inner nuclear layer. GABA was also localized in displaced amacrine cells in the ganglion cell layer, and a population of ganglion cells that co-localize glutamate and GABA. Both the horizontal cells and GABAergic amacrine cells had high levels of glutamate immunoreactivity, which probably reflects a metabolic pool. At least two types of horizontal cells in the avian retina could be discriminated on the basis of the presence of aspartate immunoreactivity in the H2 horizontal cells. Glycine was contained in a subclass of amacrine cells, with their cell bodies located between the bipolar cells and GABAergic amacrine cells, two subclasses of bipolar cells, displaced amacrine cells in the ganglion cell layer, and ganglion cells that colocalize glutamate and glycine. Glycinergic amacrine cells had low levels of glutamate. We have also identified a new class of glycinergic interplexiform cell, with its stellate cell body located in the middle of the inner nuclear layer among the cell bodies of bipolar cells. Neurochemical signatures obtained by analyzing data from serial sections allowed the classification of subclasses of horizontal cells, bipolar cells, amacrine cells, and ganglion cells.

Colour vision anomalies following experimental glaucoma in monkeys

Spectral sensitivity defects, associated with chronic elevated intraocular pressure (IOP) produced by Argon laser trabeculoplasty, were studied in monkeys. Increment-threshold spectral sensitivity (ITSS) and threshold versus intensity (TVI) functions were measured using a behavioural model. Elevated IOP resulted in short wavelength (SW) sensitivity losses characteristic of many ocular diseases. The amount of SW sensitivity loss for ITSS functions depended upon the intensity level and chromatic composition of the background field. The optimum condition identifying the greatest SW sensitivity reduction was a yellow background of moderate intensity (100-1000 Td). In the early stages of experimental glaucoma, the cone mechanisms and the rod mechanism typically showed decreased test and field sensitivities. The SW cone pathway has slightly greater threshold elevation (approximately 0.3 log unit) compared to the rod and cone pathways. On the other hand, in the advanced stages of experimental glaucoma, the largest sensitivity losses were in the longer-wavelength, red-green opponent mechanisms, with the rod and SW cone pathways showing smaller losses. The similarities of the colour vision anomalies in this animal model with those of patients with glaucoma, provides support for its use as an experimental model for human glaucoma.