Redistribution of GABA immunoreactivity following central retinal artery occlusion

The Interplay between Neurotransmitters and Calcium Dynamics in Retinal Synapses during Development, Health, and Disease

The intricate functionality of the vertebrate retina relies on the interplay between neurotransmitter activity and calcium (Ca2+) dynamics, offering important insights into developmental processes, physiological functioning, and disease progression. Neurotransmitters orchestrate cellular processes to shape the behavior of the retina under diverse circumstances. Despite research to elucidate the roles of individual neurotransmitters in the visual system, there remains a gap in our understanding of the holistic integration of their interplay with Ca2+ dynamics in the broader context of neuronal development, health, and disease. To address this gap, the present review explores the mechanisms used by the neurotransmitters glutamate, gamma-aminobutyric acid (GABA), glycine, dopamine, and acetylcholine (ACh) and their interplay with Ca2+ dynamics. This conceptual outline is intended to inform and guide future research, underpinning novel therapeutic avenues for retinal-associated disorders.

Intraocular Adeno-Associated Virus-Mediated Transgene Endothelin-1 Delivery to the Rat Eye Induces Functional Changes Indicative of Retinal Ischemia-A Potential Chronic Glaucoma Model

Endothelin-1 (ET-1) overactivity has been implicated as a factor contributing to glaucomatous neuropathy, and it has been utilized in animal models of retinal ischemia. The functional effects of long-term ET-1 exposure and possible compensatory mechanisms have, however, not been investigated. This was therefore the purpose of our study. ET-1 was delivered into rat eyes via a single intravitreal injection of 500 µM or via transgene delivery using an adeno-associated viral (AAV) vector. Retinal function was assessed using electroretinography (ERG) and the retinal expression of potentially compensatory genes was evaluated by means of qRT-PCR. Acute ET-1 delivery led to vasoconstriction and a significant reduction in the ERG response. AAV-ET-1 resulted in substantial transgene expression and ERG results similar to the acute ET-1 injections and comparable to other models of retinal ischemia. Compensatory changes were observed, including an increase in calcitonin gene-related peptide (CGRP) gene expression, which may both counterbalance the vasoconstrictive effects of ET-1 and provide neuroprotection. This chronic ET-1 ischemia model might be especially relevant to glaucoma research, mimicking the mild and repeated ischemic events in patients with long-term vascular dysfunction. The compensatory mechanisms, and particularly the role of vasodilatory CGRP in mitigating the retinal damage, warrant further investigation with the aim of evaluating new therapeutic strategies.

Animal Models Used to Simulate Retinal Artery Occlusion: A Comprehensive Review

Purpose

To present an overview of animal models of retinal artery occlusion (RAO).

Methods

Through a systematic literature search in PubMed and Embase, papers describing methods of inducing RAO in animal models were included. The identified methodologic approaches were presented in a narrative synthesis and compared with RAO in humans.

Results

In total, 83 papers reporting on 88 experiments were included. Six different species were used with rodents and monkeys being the most common, and a minority were performed using cats, dogs, rabbits, or pigs. The anatomy of pigs and monkeys resemble that of humans most closely. The two most frequently used methods were laser-induced occlusion or ligation of the arteries. Other methods included raised intraocular pressure, arterial clamping, administration of vasoconstricting agents, the use of an occluder, embolization, and endovascular approaches to induce occlusion. In general, occlusions lasted for only 30 to 90 minutes, often followed by reperfusion.

Conclusions

Although a broad range of methods have previously been used, they all have limitations. Preferably, the methods should imitate the human disease as closely as possible and avoid damaging other structures. Therefore, monkeys followed by pigs are to be preferred and ligation or clamping may be a suitable model in larger animals as there is a potential to isolate and occlude the retinal artery only. Being less invasive, laser-induced occlusion is another suitable approach.

Translational Relevance

This review aims at assisting researchers in deciding on the most ideal experimental setting, and thereby increase the translational value to human disease.

Understanding Retinal Changes after Stroke

Stroke is the fifth leading cause of death and disability in the United States. According to World Heart Federation, every year, 15 million people suffer from stroke worldwide out of which nearly 6 million people die and another 5 million people are disabled. Out of many organs affected after stroke, one of them is eye. Majority of the stroke victims suffer vision loss due to stroke-induced retinal damage. However, stroke-induced retinal damage and microvascular changes have not been given paramount importance in understanding stroke pathophysiology and predicting its occurrence. Retinal imaging can be a very powerful tool to understand and predict stroke. This review will highlight the importance of retinal changes in predicting occurrence of stroke, major retinal changes, the relationship between retinal diseases and stroke and moreover, molecular mechanisms delineating the stroke induced-retinal changes and therapeutics associated with it.

GABA and Glutamate Uptake and Metabolism in Retinal Glial (Müller) Cells

Müller cells, the principal glial cells of the retina, support the synaptic activity by the uptake and metabolization of extracellular neurotransmitters. Müller cells express uptake and exchange systems for various neurotransmitters including glutamate and γ-aminobutyric acid (GABA). Müller cells remove the bulk of extracellular glutamate in the inner retina and contribute to the glutamate clearance around photoreceptor terminals. By the uptake of glutamate, Müller cells are involved in the shaping and termination of the synaptic activity, particularly in the inner retina. Reactive Müller cells are neuroprotective, e.g., by the clearance of excess extracellular glutamate, but may also contribute to neuronal degeneration by a malfunctioning or even reversal of glial glutamate transporters, or by a downregulation of the key enzyme, glutamine synthetase. This review summarizes the present knowledge about the role of Müller cells in the clearance and metabolization of extracellular glutamate and GABA. Some major pathways of GABA and glutamate metabolism in Müller cells are described; these pathways are involved in the glutamate-glutamine cycle of the retina, in the defense against oxidative stress via the production of glutathione, and in the production of substrates for the neuronal energy metabolism.

Pathophysiology of stroke and stroke-induced retinal ischemia: emerging role of stem cells

The current review focuses on pathophysiology, animal models and molecular analysis of stroke and retinal ischemia, and the role of stem cells in recovery of these disease conditions. Research findings associated with ischemic stroke and retinal ischemia have been discussed, and efforts towards prevention and limiting the recurrence of ischemic diseases, as well as emerging treatment possibilities with endothelial progenitor cells (EPCs) in ischemic diseases, are presented. Although most neurological diseases are still not completely understood and reliable treatment is lacking, animal models provide a major step in validating novel therapies. Stem cell approaches constitute an emerging form of cell-based therapy to treat ischemic diseases since it is an attractive source for regenerative therapy in the ischemic diseases. In this review, we highlight the advantages and limitations of this approach with a focus on key observations from preclinical animal studies and clinical trials. Further research, especially on treatment with EPCs is warranted.

Specific amacrine cell changes in an induced mouse model of glaucoma

BACKGROUND

To investigate retinal cell population changes under chronic elevated intraocular pressure in an inducible mouse model of glaucoma.

METHODS

Chronic unilateral ocular hypertension was induced in 40 C57BL6/J mice by ablation of the limbal episcleral veins. After 5, 20, 40 and 60 days of elevated intraocular pressure, specific retinal cell types were identified and/or quantified by immunohistochemistry for protein kinase C α, glial fibrillary acidic protein, parvalbumin and calretinin. Apoptotic cells were identified by TUNEL and cleaved caspase-3 immunohistochemistry.

RESULTS

Elevations in intraocular pressure in the range 22-30 mmHg were developed and sustained in mice for up to 60 days. Protein kinase C α immunoreactivity localized to bipolar cells was unchanged. We observed a rapid increase in glial fibrillary acidic protein expression in Müller cells and a progressive loss of parvalbumin-labelled ganglion cells. After 60 days of elevated intraocular pressure, calretinin-immunoreactive cell counts declined by 55.4% and 46.4% in the inner nuclear and ganglion cell layers, respectively. However, at all time points examined, the markers of cell death were only observed in the ganglion cell layer, not in the inner nuclear layer.

CONCLUSIONS

In addition to ganglion cell death and reactive Müller cell changes, chronic experimental elevation of intraocular pressure alters calcium-binding protein immunohistochemistry in amacrine cells. However, these changes are not indicative of amacrine cell loss but may represent early indicators of cellular distress that precede physiological dysfunction or cell death.

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.

Metabolic and functional profiling of the normal rat retina

We established a metabolic and functional profile map of the normal rat retina, given the premise that: 1) amino acid neurochemistry reflects metabolic integrity and cellular identity, and 2) the permeation of a cation channel probe, agmatine (1-amino-4-guanidobutane, AGB), reflects cation channel functionality. The purpose was to provide a unique method of simultaneously assessing the metabolic and functional characteristics of the normal retina, upon which a comparison can be made to disease models. Quantitative pattern recognition analysis of overlapping amino acid and AGB expression profiles was used to provide a statistically robust classification of all neural elements according to their metabolic and functional characteristics. This classification was spatially complete and with single-cell resolution. The resulting classification demonstrated 28 statistically separable theme classes dominated by characteristic glutamate, GABA, glycine, and/or taurine profiles, with each of the neuronal theme classes containing further subtypes. The inclusion of a functional parameter (AGB mapping) in the classification process nearly doubled the number of neural elements that could be ascribed a neurochemical/cation profile, compared to when amino acid labeling was used alone. Strong endogenous glutamate gated AGB labeling was observed in horizontal cells, rod bipolar cells, cholinergic amacrine cells, and AII amacrine cells. The resulting amino acid and AGB profile matrix constitutes a nomogram for assessing cellular responses to experimental challenges in models of ocular disease.

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.

Neuroprotection in ophthalmology: a review

Evidence has accumulated that damaged neural cells may not inevitably degenerate, and that in vivo cells which are not directly injured by an insult may be adversely affected by adjacent dying cells. Neuroprotection is a strategy which aims to maximize recovery of injured neural cells and minimize secondary damage to neighboring cells. In this work, we review the current knowledge from neuroprotection research using in vitro and animal models of eye diseases, and clinical data.

Optic nerve ligation leads to astrocyte-associated matrix metalloproteinase-9 induction in the mouse retina

Ischemic damage results in irreversible ganglion cell loss in the retina. While the mechanisms underlying ischemia-induced ganglion cell loss are not clearly understood, we have recently reported that ischemia, induced by optic nerve ligation, results in increased nerve fiber layer-associated matrix metalloproteinase-9 (MMP-9) induction and loss of ganglion cells from the retina. This study was conducted to determine the cellular source of MMP-9 using antibodies against MMP-9 and various cell types in the inner retina. The results presented in this study show that optic nerve ligation leads to induction of MMP-9 and activation of astrocytes. Double labeling studies using antibodies against MMP-9 and GFAP showed a greater overlap of MMP-9 with GFAP, compared to antibodies against glutamine synthetase and MMP-9 which showed no overlapping, suggesting that activated astrocytes contribute to MMP-9 expression in the retina. Further, double labeling studies using antibodies against von Willebrand factor and MMP-9 or Mac-1 and MMP-9 showed no overlapping of MMP-9 with any antibodies indicating that endothelial cells and microglial cells are not the principal source of MMP-9 in the retina following optic nerve ligation.

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.

Deficiency in matrix metalloproteinase gelatinase B (MMP-9) protects against retinal ganglion cell death after optic nerve ligation

Loss of retinal ganglion cells is the final end point in blinding diseases of the optic nerve such as glaucoma. To enable the use of mouse genetics to investigate mechanisms underlying ganglion cell loss, we adapted an experimental model of optic nerve ligation to the mouse and further characterized post-surgical outcome. We made the novel finding that apoptosis of retinal ganglion cells correlates with specific degradation of laminin from the underlying inner limiting membrane and an increase in gelatinolytic metalloproteinase activity. These changes co-localize with a specific increase in levels of the matrix metalloproteinase, gelatinase B (GelB; MMP-9). Using a transgenic mouse line harboring a reporter gene driven by the GelB promoter, we further show that increased GelB is controlled by activation of the GelB promoter. These findings led us to hypothesize that GelB activity plays a role in ganglion cell death and degradation of laminin. Applying the genetic approach, we demonstrate that GelB-deficient mice are protected against these pathological changes. This is the first report demonstrating a causal connection between GelB activity and pathological changes to the inner retina after optic nerve ligation.

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.

Differential perturbation of neuronal and glial glutamate transport systems in retinal ischaemia

Glutamate is the major excitatory neurotransmitter in the retina and is removed from the extracellular space by an energy-dependent process involving neuronal and glial cell transporters. The radial glial Müller cells express the glutamate transporter, GLAST, and preferentially accumulate glutamate. However, during an ischaemic episode, extracellular glutamate concentrations may rise to excitotoxic levels. Is this catastrophic rise in extracellular glutamate due to a failure of GLAST? Using immunocytochemistry, we monitored the transport of the glutamate transporter substrate, D-aspartate, in the retina under normal and ischaemic conditions. Two models of compromised retinal perfusion were compared: (1) Anaesthetised rats had their carotid arteries occluded for 7 days to produce a chronic reduction in retinal blood flow. Retinal function was assessed by electroretinography. D-aspartate was injected into the eye for 45 min. Following euthanasia, the retina was processed for D-aspartate, GLAST and glutamate immunocytochemistry. Although reduced retinal perfusion suppresses the electroretinogram b-wave, neither retinal histology, GLAST expression, nor the ability of Müller cells to uptake D-aspartate is affected. As this insult does not appear to cause excitotoxic neuronal damage, these data suggest that GLAST function and glutamate clearance are maintained during periods of reduced retinal perfusion. (2) Occlusion of the central retinal artery for 60 min abolishes retinal perfusion, inducing histological damage and electroretinogram suppression. Although GLAST expression appears to be normal, its ability to transport D-aspartate into Müller cells is greatly reduced. Interestingly, D-aspartate is transported into neuronal cells, i.e. photoreceptors, bipolar and ganglion cells. This suggests that while GLAST is vitally important for the clearance of excess extracellular glutamate, its capability to sustain inward transport is particularly susceptible to an acute ischaemic attack. Manipulation of GLAST function could alleviate the degeneration and blindness that result from ischaemic retinal disease.

Retinas from albino rats are more susceptible to ischaemic damage than age-matched pigmented animals

Age- and sex-matched pigmented (Lister Hooded) and albino (Wistar) rats were used in this study. The retinas of the animals were subjected to pressure-induced ischaemia (35 min, 120 mmHg) and reperfusion (3 days) in precisely the same way. The b-wave of the electroretinogram (ERG) in the pigmented animals recovered to normal levels while those of the albino rats were reduced by more than 80%. Moreover, the choline acetyltransferase (ChAT) immunoreactivity associated with a sub-set of amacrine cells was almost completely obliterated in the retinas from the albino rats but unaffected in the retinas of the pigmented rats. Also, in certain areas of the retina from albino rats there was a suggestion that the calretinin-immunoreactivity was affected. This was never seen in the retinas of the pigmented animals. The GABA-immunoreactivity in the retina of both albino and pigmented rats appeared to be unaffected by ischaemia/reperfusion. The data presented show that retinas from albino rats are more susceptible to ischaemia/reperfusion than retinas from pigmented animals. The results also show that reduction of the b-wave of the ERG and changes in the nature of the ChAT immunoreactivity represent sensitive markers to detect the effect of ischaemia/reperfusion to the retina.

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.

The b-wave of the electroretinogram as an index of retinal ischemia

1. The b-wave of the electroretinogram (ERG) is a particularly sensitive index of retinal ischemia. The present paper summarizes the changes in the b-wave observed in five in vivo models of retinal ischemia. 2. Although the amount of reduction in b-wave amplitude during ischemia corresponds to the severity of the insult, the degree of recovery of the b-wave during reperfusion depends on the duration of ischemia. 3. A massive release of glutamate, intracellular overload with calcium and enhanced production of free radicals are suggested to be three major pathophysiological processes that contribute to retinal ischemic damage. The b-wave of the ERG represents a functional measure for potential therapeutic efficacy of drugs interacting with these pathophysiological processes. 4. Several glutamate antagonists, such as MK-801, memantine, flupirtine or GYKI 52466, along with the free radical scavengers vitamin E, lipoate, superoxide dismutase and catalase, all reduce the depression of the b-wave during ischemia or accelerate the recovery of the b-wave during reperfusion or both. The calcium channel antagonists nimodipine and levemopamil exert only a slight beneficial effect on the recovery of the amplitude of the b-wave during reperfusion, provided that the blood pressure is not potently reduced.

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.

Retinal ischemia induced by the intraluminal suture method in rats

In a model of transient focal cerebral ischemia in male Sprague-Dawley rats, which is induced by the intraluminal suture method, the acute effects on the electrical function of the retina were monitored by recording the electroretinogram. The electroretinogram was recorded from halothane-anesthetized rats before, during and after vascular occlusion through the intraluminal suture method for 180 min. During vascular occlusion the amplitude of the a- and b-wave were markedly suppressed. Upon reperfusion the a-wave recovered immediately. During reperfusion up to 48 h the amplitude of the b-wave increased to approximately 50% of the pre-occlusion value did not fully recover. Immunohistochemistry of the retinas revealed that the vascular occlusion induced the expression of glial fibrillary acidic protein in retinal Müller cells. The present data suggest that the intraluminal suture method leads to retinal ischemia.

Ultrastructural and biochemical studies on the neuroprotective effects of excitatory amino acid antagonists in the ischemic rat retina

The effects of glutamate receptor agonists were evaluated, by utilizing the electron microscope, in a photothrombotic occlusion model of rat retinal vessels in order to study the ischemic damage and its antagonism in each morphologically identified population of retinal neurons. Rats were systemically injected with rose bengal fluorescein dye and one of their eyes was then exposed to bright light. This treatment caused neuronal damage and reduced the activities of the neuronal marker enzymes, choline acetyltransferase and glutamate decarboxylase, by approximately 75%. A single intravitreal injection of 2,3-dihydroxy-6-nitro-7-sulfamoylbenzoquinoxaline (NBQX, 10-50 nmol), an antagonist of alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptors, or of thiokynurenate (100-400 nmol), which also antagonizes N-methyl-D-aspartate (NMDA) receptors, performed immediately after the lesion, significantly reduced this loss. The electron microscope examination showed major damage in each type of retinal neuron, the pigment epithelium, and the microvessels. NBQX or thiokynurenic acid reduced, in a comparable manner, the effects of ischemia on the pigment epithelium, the photoreceptors, and the bipolar and the horizontal cells. NBQX was particularly efficient in reducing the damage to the amacrine cells located in the inner nuclear layer. The displaced amacrine and ganglion cells were not protected by NBQX but were almost completely spared in animals treated with thiokynurenate. These results show that antagonism of AMPA receptors is sufficient to reduce ischemic damage in a large number of retinal neurons, but that neuroprotection in the ganglion cell layer may be obtained only with agents which also antagonize NMDA receptors.