Excitatory amino acid-induced toxicity in chick retina: amino acid release, histology, and effects of chloride channel blockers

[Amyloid β hypothesis in Alzheimer's disease and Cl--ATPase-Neuronal cell death via PI4KIIα inhibition and recovery agents]

In the brains of patients with Alzheimer's disease, a decrease in phosphatidylinositol phosphate (PIP) requiring Cl^--ATPase activity was found. In cultured rat hippocampal neurons, pathophysiological concentrations of amyloid β proteins (Aβs≤10 nM) lowered PIP levels and Cl^--ATPase activity with an increase in intracellular Cl^- concentrations, resulting in Cl^--dependent enhancements in glutamate neurotoxicity and, ultimately, neuronal cell death. Pathophysiological concentrations of Aβs(0.1-10 nM) directly lowered phosphatidylinositol-4-kinase. Non-toxic peptide fragments of Aβ, such as Ile-Gly-Leu, recovered Aβ-induced inhibition of recombinant human phosphatidylinositol-4-kinase IIα (PI4KIIα) and the intrahippocampally administered Aβ-induced degeneration of hippocampal neurons and impairment of spatial memory in mice. Agents with the potential to block these neurotoxic mechanisms of Aβ were summarized herein as (1) Aβ antagonists, (2) substrates of PI4K, (3) PI4K product, (4) PI4K activators, and (5) GABAc receptor stimulants.

Neuroprotection by α2-Adrenergic Receptor Stimulation after Excitotoxic Retinal Injury: A Study of the Total Population of Retinal Ganglion Cells and Their Distribution in the Chicken Retina

We have studied the effect of α2-adrenergic receptor stimulation on the total excitotoxically injured chicken retinal ganglion cell population. N-methyl-D-aspartate (NMDA) was intraocularly injected at embryonic day 18 and Brn3a positive retinal ganglion cells (Brn3a+ RGCs) were counted in flat-mounted retinas using automated routines. The number and distribution of the Brn3a+ RGCs were analyzed in series of normal retinas from embryonic day 8 to post-hatch day 11 retinas and in retinas 7 or 14 days post NMDA lesion. The total number of Brn3a+ RGCs in the post-hatch retina was approximately 1.9x10⁶ with a density of approximately 9.2x10³ cells/mm². The isodensity maps of normal retina showed that the density decreased with age as the retinal size increased. In contrast to previous studies, we did not find any specific region with increased RGC density, rather the Brn3a+ RGCs were homogeneously distributed over the central retina with decreasing density in the periphery and in the region of the pecten oculli. Injection of 5–10 μg NMDA caused 30–50% loss of Brn3a+ cells and the loss was more severe in the dorsal than in the ventral retina. Pretreatment with brimonidine reduced the loss of Brn3a+ cells both 7 and 14 days post lesion and the protective effect was higher in the dorsal than in the ventral retina. We conclude that α2-adrenergic receptor stimulation reduced the impact of the excitotoxic injury in chicken similarly to what has been shown in mammals. Furthermore, the data show that the RGCs are evenly distributed over in the retina, which challenges previous results that indicate the presence of specific high RGC-density regions of the chicken retina.

The effects of avermectin on amino acid neurotransmitters and their receptors in the pigeon brain

The objective of this study was to examine the effects of avermectin (AVM) on amino acid neurotransmitters and their receptors in the pigeon brain. Four groups two-month-old American king pigeons (n=20/group) were fed either a commercial diet or an AVM-supplemented diet (20mg/kg·diet, 40 mg/kg·diet, or 60 mg/kg·diet) for 30, 60, or 90 days. The contents of aspartic acid (ASP), glutamate (GLU), glycine (GLY), and γ-aminobutyric acid (GABA) in the brain tissues were determined using ultraviolet high-performance liquid chromatography (HPLC). The expression levels of the GLU and GABA receptor genes were analyzed using real-time quantitative polymerase chain reaction (qPCR). The results indicate that AVM exposure significantly enhances the contents of GABA, GLY, GLU, and ASP in the cerebrum, cerebellum, and optic lobe. In addition, AVM exposure increases the mRNA expression levels of γ-aminobutyric acid type A receptor (GABAAR), γ-aminobutyric acid type B receptor (GABABR), N-methyl-d-aspartate 1 receptor (NR1), N-methyl-d-aspartate 2A receptor (NR2A), and N-methyl-d-aspartate 2B receptor (NR2B) in a dose- and time-dependent manner. Moreover, we found that the most damaged organ was the cerebrum, followed by the cerebellum, and then the optic lobe. These results show that the AVM-induced neurotoxicity may be associated with its effects on amino acid neurotransmitters and their receptors. The information presented in this study will help supplement the available data for future AVM toxicity studies.

Vigabatrin-associated retinal damage: potential biochemical mechanisms

Vigabatrin (VGB), an irreversible inhibitor of gamma-aminobutyric acid (GABA) transaminase, is approved as adjunct treatment of refractory partial seizures as well as infantile spasms. Although VGB has been proven to be effective, its use is limited by the risk of retinopathy and associated peripheral visual field defects. This review describes and analyzes current literature related to potential pathophysiologic mechanisms underlying VGB-mediated cellular toxicity. Animal data suggest that GABA mediates neural excitotoxicity. The amino acid taurine is concentrated in retinal cells, and deficiency of this amino acid may be involved in VGB-mediated retinal degeneration and possible phototoxicity.

Restoration of the luteinizing hormone surge in middle-aged female rats by altering the balance of GABA and glutamate transmission in the medial preoptic area

Hypothalamic glutamate and gamma-aminobutyric acid (GABA) neurotransmission are involved in the ovarian hormone-induced GnRH-LH surge in rodents. We previously reported that middle-aged rats have significantly less glutamate release in the medial preoptic area than young rats on the day of the LH surge. The present study tested the hypothesis that the delayed and attenuated LH surge in ovariohysterectomized middle-aged rats primed with ovarian steroids results from reduced hypothalamic glutamate and increased GABA(A) neurotransmission. Microdialysis results show that middle-aged rats with attenuated LH surges had reduced extracellular glutamate and increased extracellular GABA levels in the medial preoptic area compared with young rats. Blocking GABA(A) receptors with bicuculline or inhibiting synaptic glutamate reuptake with L-trans-pyrrolidine-2,4-dicarboxylic acid increased extracellular Glu in the medial preoptic area and partially restored LH surge amplitude in middle-aged rats without altering LH surge onset. Complete recovery of LH surge amplitude was observed in middle-aged rats treated with the combination of bicuculline and L-trans-pyrrolidine-2,4-dicarboxylic acid. This treatment also restored the extracellular glutamate:GABA ratio in the medial preoptic area of middle-aged rats to the level of young rats. Immunoblot analysis revealed that estradiol and progesterone treatment reduced SLC32A1(formerly known as vesicular GABA transporter) levels and increased SLC17A6 (formerly known as vesicular glutamate transporter 2) levels in the anterior hypothalamus of ovariohysterectomized young but not middle-aged rats. These data suggest that both reduced availability of glutamate and increased activation of GABA(A) receptors under estrogen-positive feedback conditions contribute to the age-related delay in onset and attenuated amplitude of the LH surge.

Pharmacological activation of mGlu2/3 metabotropic glutamate receptors protects retinal neurons against anoxic damage in the goldfish Carassius auratus

We examined the expression of mGlu2/3 metabotropic glutamate receptors in the retina of the goldfish Carassius auratus. mGlu2/3 receptors were expressed in all retinal layers internal to the photoreceptor layer, particularly in the outer and inner nuclear layers. Although the goldfish brain is able to tolerate prolonged periods of anoxia, we examined whether anoxia could induce retinal damage. Three hours of anoxia induced in the retina the development of apoptotic cell death, as assessed 48 h later by TUNEL staining. TUNEL-positive cells were particularly found in the inner and outer nuclear layers, and were also present in the ganglion cell layer. Pharmacological activation of mGlu2/3 receptors by systemic injection of LY379268 (0.5 mg/kg, i.p., 15 min before the onset of anoxia) substantially protected retinas against anoxia-induced cell death. In contrast, systemic injection of the mGlu2/3 receptor antagonist, LY341495 (1 mg/kg, i.p., 15 min before the onset of anoxia), significantly amplified cell death. Finally, as mGlu2/3 receptors are implicated in the control of extracellular glutamate concentrations, we examined the stimulation of glutamate release in isolated goldfish retinas. Depolarizing medium containing 30 mM KCl led to a significant increase in glutamate release, which was substantially reduced by LY379268. We conclude that activation of mGlu2/3 receptors may provide a major defensive mechanism against ischemic/anoxic retinal damage.

Chloride-dependency of amyloid β protein-induced enhancement of glutamate neurotoxicity in cultured rat hippocampal neurons

In our previous studies, pathophysiological concentrations of amyloid-beta (Abeta) proteins increased intracellular Cl(-) concentration ([Cl(-)]i) and enhanced glutamate neurotoxicity in primary cultured neurons, suggesting Cl(-)-dependent changes in glutamate signaling. To test this possibility, we examined the effects of isethionate-replaced low Cl(-) medium on the Abeta-induced enhancement of glutamate neurotoxicity in the primary cultured rat hippocampal neurons. In a normal Cl(-) (135 mM) medium, treatment with 10 nM Abeta25-35 for 2 days increased neuronal [Cl(-)]i to a level three times higher than that of control as assayed using a Cl(-)-sensitive fluorescent dye, while in a low Cl(-) (16 mM) medium such an Abeta25-35-induced increase in [Cl(-)]i was not observed. The Abeta treatment aggravated glutamate neurotoxicity in a normal Cl(-) medium as measured by mitochondrial reducing activity and lactate dehydrogenase (LDH) release, while in a low Cl(-) medium the Abeta treatment did not enhance glutamate toxicity. Upon such Abeta plus glutamate treatment under a normal Cl(-) condition, activated anti-apoptotic molecule Akt (Akt-pS473) level monitored by Western blot significantly decreased to 74% of control. Under a low Cl(-) condition, a resting Akt-pS473 level was higher than that under a normal Cl(-) condition and did not significantly change upon Abeta plus glutamate treatment. Tyrosine phosphorylation levels of 110 and 60 kDa proteins (pp110 and pp60) increased upon Abeta plus glutamate treatment under a normal Cl(-), but not low Cl(-), condition. These findings indicated that Abeta-induced enhancement of glutamate neurotoxicity is Cl(-)-dependent. Chloride-sensitive Akt pathway and tyrosine phosphorylation of proteins (pp110 and pp60) may be involved in this process.

Microglia/macrophages responses to kainate-induced injury in the rat retina

The present study was aimed to elucidate how retinal microglia/macrophages would respond to neuronal death after intravitreal kainate injection. An increased expression of the complement receptor type 3 (CR3) and an induction of the major histocompatibility complex (MHC) class II and ED-1 antigens were mainly observed in the inner retina after kainate injection. Prominent cell death revealed by Fluoro Jade B (FJB) staining and ultrastructural examination appeared at the inner border of the inner nuclear layer (INL) at 1 day post-injection. Interestingly, some immunoreactive cells appeared at the outer segment of photoreceptor layer (OSPRL) at different time intervals. Our quantitative analysis further showed that CR3 immunoreactivity was drastically increased peaking at 7 days but subsided thereafter. MHC class II and ED-1 immunoreactivities showed a moderate but steady increase peaking at 3 days and declined thereafter. Double labeling study further revealed that retinal microglia/macrophages expressed concurrently CR3 and ED-1 antigens (OX-42+/ED-1+) or MHC class II molecules (OX-42+/OX-6+) and remained branched in shape at early stage of kainate challenge. By electron microscopy, microglia/macrophages with CR3 immunoreactivity displayed abundant cytoplasm containing a few vesicles and phagosomes. Other cells ultrastructurally similar to Müller cells or astrocytes could also engulf exogenous substances. In conclusion, retinal microglia/macrophages responded vigorously to kainate-induced neuronal cell death that may also trigger the recruitment of macrophages from neighboring tissues and induce the phagocytotic activity of cells other than retinal microglia/macrophages.

Neuronal versus glial cell swelling in the ischaemic retina

Under normal conditions, the pigment epithelium dehydrates the outer retina while Müller glial cells mediate the rapid water transport within the inner retina. Gliotic alterations of Müller cells may be implicated in the development of oedema in the post-ischaemic retina. Here, we suggest a mechanism of Müller cell-supported neuronal cell swelling and apoptosis in the ischaemic retina. During ischaemia, over-excitation of ionotropic glutamate receptors leads to neuronal cell depolarization that causes excess Ca(2+) influx into the cells, and to activation of the apoptosis machinery. The ion fluxes into the retinal neurons are associated with water movements that are mediated by aquaporin-4 water channels expressed by Müller cells and result in neuronal cell swelling. After reperfusion, the glial cells may swell due to the down-regulation of their K(+) conductance, which results in intracellular K(+) overload and water movements from the blood and vitreous into the cells. An inhibition of the glial cell-mediated water movements during ischaemic episodes should reduce the ion shifts at the neuronal synapses, resulting in decreased neuronal cell swelling and apoptosis. An inhibition of the water movements in the post-ischaemic phase may prevent cytotoxic Müller cell swelling but may impair the fluid clearance from retinal tissue in the presence of vasogenic oedema. Thus, pharmacological modification of the ion and fluid clearance functions of Müller cells may become a novel way to resolve both cytotoxic and vasogenic oedema in the retina.

The emerging role of proteases in retinal ganglion cell death

Retinal ganglion cell (RGC) death is an important issue in Primary Open Angle-Glaucoma (POAG) in terms of both vision loss and health care costs. Yet, the pathophysiology underlying RGC death in glaucoma is unclear. A growing body of evidence indicates that proteases that modulate the extracellular matrix (ECM) milieu in the retina, either directly or indirectly, play an important role in dictating the fate of RGCs. Recent evidence indicates that proteases, in addition to ECM-remodeling, have broader functional roles in glutamate receptor processing and predisposing RGCs to secondary damage. This review is focused on discussing the role of two groups of proteases, the matrix metalloproteinases (MMPs) and the plasminogen activators (PAs), in RGC death. In a long-run, a better understanding of the mechanisms involved in the regulation of proteases may lead to the development of adjunctive treatment options to attenuate RGC death and improve vision loss in glaucoma.

Mechanisms of chloride influx during KCl-induced swelling in the chicken retina

An increase in extracellular KCl ([KCl]o) occurs under various pathological conditions in the retina, leading to retinal swelling and possible neuronal damage. The mechanisms of this KCl o-induced retinal swelling were investigated in the present study, with emphasis on the Cl- transport mechanisms. Increasing [KCl]o (from 5 to 70 mM) led to concentration-dependent swelling in chicken retinas. The curve relating the degree of swelling to [KCl]o was biphasic, with one component from 5 to 35 mM [KCl]o and another from 35 to 100 mM. As Cl- omission prevented swelling in all conditions, the effect of cotransporter or Cl- channel blockers was examined to investigate the mechanisms of Cl- influx. The cotransporter blockers bumetanide and DIOA reduced swelling by 68% and 76%, respectively at [KCl]o 25 mM (K25), and by 14-17% at [KCl]o 54 mM (K54). The Cl- channel blockers NPPB and niflumic acid did not affect swelling at K25 but reduced it by 90-95% at K54 (NPPB IC50 60.7 microM). Furosemide showed an atypical effect, decreasing swelling by 14% at K25 and by 95% at K54 (IC50 173.9 microM). Na+ omission decreased swelling at K25 but not at K54. These results suggest the contribution of cotransporters to Cl- influx at K25 and of Cl- channels at K54. At K25, swelling was found in the ganglion cell layer and in the lower half of the inner nuclear layer. With K54, swelling occurred in all inner retinal layers. The ganglion cell layer swelling was due to both Muller cell end-foot and ganglion cell soma swelling. K54 also induced ganglion cell damage as shown by disorganized, pyknotic and refringent nuclei.

Control of programmed cell death by neurotransmitters and neuropeptides in the developing mammalian retina

It has long been known that a barrage of signals from neighboring and connecting cells, as well as components of the extracellular matrix, control cell survival. Given the extensive repertoire of retinal neurotransmitters, neuromodulators and neurotrophic factors, and the exhuberant interconnectivity of retinal interneurons, it is likely that various classes of released neuroactive substances may be involved in the control of sensitivity to retinal cell death. The aim of this article is to review evidence that neurotransmitters and neuropeptides control the sensitivity to programmed cell death in the developing retina. Whereas the best understood mechanism of execution of cell death is that of caspase-mediated apoptosis, current evidence shows that not only there are many parallel pathways to apoptotic cell death, but non-apoptotic programs of execution of cell death are also available, and may be triggered either in isolation or combined with apoptosis. The experimental data show that many upstream signaling pathways can modulate cell death, including those dependent on the second messengers cAMP-PKA, calcium and nitric oxide. Evidence for anterograde neurotrophic control is provided by a variety of models of the central nervous system, and the data reviewed here indicate that an early function of certain neurotransmitters, such as glutamate and dopamine, as well as neuropeptides such as pituitary adenylyl cyclase-activating polypeptide and vasoactive intestinal peptide is the trophic support of cell populations in the developing retina. This may have implications both regarding the mechanisms of retinal organogenesis, as well as pathological conditions leading to retinal dystrophies and to dysfunctional cellular behavior.

Altered membrane physiology in Müller glial cells after transient ischemia of the rat retina

Inwardly rectifying K+ (Kir) channels have been implicated in the mediation of retinal K+ homeostasis by Muller glial cells. To assess possible involvement of altered glial K+ channel expression in ischemia-reperfusion injury, transient retinal ischemia was induced in rat eyes. Acutely isolated Muller cells from postischemic retinae displayed a fast downregulation of their Kir currents, which began within 1 day and reached a maximum at 3 days of reperfusion, with a peak decrease to 20% as compared with control. This strong decrease of Kir currents was accompanied by an increase of the incidence of cells which displayed depolarization-evoked fast transient (A-type) K+ currents. While no cell from untreated control rats expressed A-type K+ currents, all cells investigated from 3- and 7-day postischemic retinae displayed such currents. An increased incidence of cells displaying fast transient Na+ currents was observed at 7 days after ischemia. These results suggest a role of altered glial Kir channel expression in postischemic neuronal degeneration via disturbance of retinal K+ siphoning.

Glutamate-induced elevations in intracellular chloride concentration in hippocampal cell cultures derived from EYFP-expressing mice

The homeostasis of intracellular Cl(-) concentration ([Cl(-)](i)) is critical for neuronal function, including gamma-aminobutyric acid (GABA)ergic synaptic transmission. Here, we investigated activity-dependent changes in [Cl(-)](i) using a transgenetically expressed Cl(-)-sensitive enhanced yellow-fluorescent protein (EYFP) in cultures of mouse hippocampal neurons. Application of glutamate (100 microm for 3 min) in a bath perfusion to cell cultures of various days in vitro (DIV) revealed a decrease in EYFP fluorescence. The EYFP signal increased in amplitude with increasing DIV, reaching a maximal response after 7 DIV. Glutamate application resulted in a slight neuronal acidification. Although EYFP fluorescence is sensitive to pH, EYFP signals were virtually abolished in Cl(-)-free solution, demonstrating that the EYFP signal represented an increase in [Cl(-)](i). Similar to glutamate, a rise in [Cl(-)](i) was also induced by specific ionotropic glutamate receptor agonists and by increasing extracellular [K(+)], indicating that an increase in driving force for Cl(-) suffices to increase [Cl(-)](i). To elucidate the membrane mechanisms mediating the Cl(-) influx, a series of blockers of ion channels and transporters were tested. The glutamate-induced increase in [Cl(-)](i) was resistant to furosemide, bumetanide and 4,4'-diisothiocyanato-stilbene-2,2'-disulphonic acid (DIDS), was reduced by bicuculline to about 80% of control responses, and was antagonized by niflumic acid (NFA) and 5-nitro-2-(3-phenylpropylamino)benzoic acid (NPPB). We conclude that membrane depolarization increases [Cl(-)](i) via several pathways involving NFA- and NPPB-sensitive anion channels and GABA(A) receptors, but not through furosemide-, bumetanide- or DIDS-sensitive Cl(-) transporters. The present study highlights the vulnerability of [Cl(-)](i) homeostasis after membrane depolarization in neurons.

Retinal ischemia: mechanisms of damage and potential therapeutic strategies

Retinal ischemia is a common cause of visual impairment and blindness. At the cellular level, ischemic retinal injury consists of a self-reinforcing destructive cascade involving neuronal depolarisation, calcium influx and oxidative stress initiated by energy failure and increased glutamatergic stimulation. There is a cell-specific sensitivity to ischemic injury which may reflect variability in the balance of excitatory and inhibitory neurotransmitter receptors on a given cell. A number of animal models and analytical techniques have been used to study retinal ischemia, and an increasing number of treatments have been shown to interrupt the "ischemic cascade" and attenuate the detrimental effects of retinal ischemia. Thus far, however, success in the laboratory has not been translated to the clinic. Difficulties with the route of administration, dosage, and adverse effects may render certain experimental treatments clinically unusable. Furthermore, neuroprotection-based treatment strategies for stroke have so far been disappointing. However, compared to the brain, the retina exhibits a remarkable natural resistance to ischemic injury, which may reflect its peculiar metabolism and unique environment. Given the increasing understanding of the events involved in ischemic neuronal injury it is hoped that clinically effective treatments for retinal ischemia will soon be available.

Na-K-Cl cotransporter contributes to glutamate-mediated excitotoxicity

We hypothesized that cation-dependent Cl- transport protein Na-K-Cl cotransporter isoform 1 (NKCC1) plays a role in the disruption of ion homeostasis in cerebral ischemia. In the current study, a role for NKCC1 in neuronal death was elucidated in neurotoxicity induced by glutamate and oxygen and glucose deprivation (OGD). Incubation of cortical neurons cultured for 14-15 d in vitro (DIV) with 100 microm glutamate for 24 hr resulted in 50% cell death. Three hours of OGD followed by 21 hr of reoxygenation led to 70% cell death. Inhibition of NMDA receptors with dizocilpine hydrogen maleate (1 microm) prevented both OGD- and glutamate-mediated cell death. Moreover, blocking of NKCC1 activity with bumetanide (5-10 microm) abolished glutamate- or OGD-induced neurotoxicity. Bumetanide was ineffective if added after 10-120 min of glutamate incubation or 3-6 hr of OGD treatment. Accumulation of intracellular Na+ and 36Cl content after NMDA receptor activation was inhibited by bumetanide. Blockage of NKCC1 significantly attenuated cell swelling after OGD or NMDA receptor activation. This neuroprotection was age dependent. Inhibition of NKCC1 did not protect DIV 7-8 neurons against OGD-mediated cell death. In contrast, cell death in DIV 7-8 neurons was prevented by the protein-synthesis inhibitor, cycloheximide. Taken together, the results suggest that NKCC1 activity is involved in the acute excitotoxicity as a result of excessive Na+ and Cl- entry and disruption of ion homeostasis.

Topiramate reduces excitotoxic and ischemic injury in the rat retina

The effects of topiramate, a drug used clinically as an anti-epileptic, were investigated in excitotoxin-induced neurotoxicity models involving two different retinal primary cultures and in a rat model of retinal ischemic injury. For the in vitro studies, we used retinal-neuron cultures from rat embryos and purified retinal ganglion cells (RGCs) from newborn rats. In the retinal-neuron cultures, neurotoxicity was induced by a 10-min exposure to 1 mM glutamate or (+/-)-a-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA). In RGCs, neurotoxicity was induced by incubation for 3 days in a culture medium containing 25 microM glutamate. For the in vivo study, retinal ischemia was induced by elevating intraocular pressure to 130 mmHg for 45 min, and topiramate was administered intraperitoneally before and after the ischemia. Retinal damage was evaluated by measuring the number of cells in the ganglion cell layer (GCL) and the thickness of the inner plexiform layer (IPL), and by examining the a- and b-waves of the electroretinogram (ERG). Topiramate (> or =1 microM) markedly reduced the neuronal cell death induced by each of the excitotoxins in rat retinal-neuron cultures and in RGCs. Ischemia caused a decrease in GCL cells and in IPL thickness, and a diminution of the ERG waves. Histopathologic and functional analyses indicated that systemic treatment with topiramate prevented ischemia-induced damage in a dose-dependent manner. In conclusion, topiramate was protective against excitotoxic and ischemic retinal-neuron damage in vitro and in vivo, respectively. Therefore, it may be useful for treatment of the retina-related diseases such as central retinal artery occlusion, diabetic retinopathy, and glaucoma.

Protection of cortical neurons against oxygen-glucose deprivation and N-methyl-D-aspartate by DIDS and SITS

The Cl(-) channel blockers, 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS) or 4-acetamido-4'-isothiocyanatostilbene-2,2'-disulfonic acid (SITS) dose-dependently protected against oxygen-glucose deprivation in cultured rat cortical neurons. DIDS or SITS attenuated oxygen-glucose deprivation-induced increases in extracellular glutamate concentrations and intracellular Ca(2+). DIDS or SITS provided moderate protection against N-methyl-D-aspartate (NMDA) toxicity and decreased NMDA receptor-mediated increases in intracellular Ca(2+). Whole-cell NMDA receptor currents were attenuated 39+/-2% and 21+/-3% by 1 mM DIDS and SITS, respectively. Other Cl(-) channel blockers as equipotent as DIDS and SITS did not decrease oxygen-glucose deprivation- or NMDA-mediated neuronal Ca(2+) influx or toxicity. Neurotoxicity by exogenous glutamate was not prevented by SITS and was exacerbated by DIDS. Reductions in oxygen-glucose deprivation-induced increases in intracellular Ca(2+) levels underlie neuroprotection by DIDS and SITS. This was a reflection of lower extracellular [glutamate], direct inhibition of Ca(2+) influx through postsynaptic NMDA receptors, and possibly through other protective properties associated with DIDS and SITS.

Ion Dependence and Receptor Mediation of Glutamate Toxicity in the Immature Rat Hippocampal Slice

Glutamate (glu) is a major excitatory transmitter and a toxin in the brain. In the present study, the immature rat hippocampal slice was used to determine the morphology, topography, ionic mediation and receptor specificity of glu toxicity. Slices were exposed to glu for 30 min, and the damage was evaluated after 3 h of recovery in regular medium. The effects on glu toxicity of changes of [Ca2+], [Cl-] and [Na+] were determined. The receptor preference of glu was assessed by using the N-methyl-D-aspartate (NMDA) antagonist MK-801 and the kainate (KA)/quisqualate (QA) antagonist DNQX, alone or in combination. Further, to see whether glu produces cytotoxicity via osmolysis, the effects of hyperosmolal sucrose on glu toxicity were studied. Glu toxicity was similar to the previously described NMDA toxicity with regard to cytopathology, but differed in some aspects from that caused by KA and QA. The severity of the lesion was determined by the proximity of neurons to the incubation fluid, probably as a consequence of cellular accumulation of the amino acid. Omission of Ca2+ abolished glu toxicity in all neurons except the granule cells of the outer blade. This population was completely protected when Ca2+ was omitted and [Cl-] was reduced. Elevation of [Ca2+] markedly aggravated the lesion caused by glu. Substitution of isethionate for Cl- worsened the glu-induced damage, whilst the amino acid produced qualitatively different neuropathology when choline substituted for Na+. Apparently glu did not damage hippocampal nerve cells through an osmolytic mechanism as medium supplemented with 100 mM sucrose increased the toxicity of glu. Since the lesion produced by glu was more widespread in the presence of high [Ca2+], the effects of receptor antagonists were studied under this condition. MK-801 inhibited glu toxicity whereas DNQX had no effect. Combination of MK-801 and DNQX did not offer better protection than did MK-801 alone. The results suggest that Ca2+ is the main (but not single) determinant of glu toxicity in the immature hippocampal slice. The ionic requirements of glu neurotoxicity are identical to those of NMDA, but differ from those of KA and QA. The notion that glu is a selective NMDA agonist in the present model was confirmed by the protection of MK-801, and by the lack of an effect of DNQX. This is the first report demonstrating that the toxicity of glu is mediated by NMDA receptors in brain tissue which has developed normally. The findings indicate that specific blockade of NMDA receptors may be the most rational strategy in the prevention of glu-related neuronal death occurring in certain neurological anomalies.

Protection of malonate-induced GABA but not dopamine loss by GABA transporter blockade in rat striatum

Previous work has shown that overstimulation of GABA(A) receptors can potentiate neuronal cell damage during excitotoxic or metabolic stress in vitro and that GABA(A) antagonists or GABA transport blockers are neuroprotective under these situations. Malonate, a reversible succinate dehydrogenase/mitochondrial complex II inhibitor, is frequently used in animals to model cell loss in neurodegenerative diseases such as Parkinson's and Huntington's diseases. To determine if GABA transporter blockade during mitochondrial impairment can protect neurons in vivo as compared with in vitro studies, rats received a stereotaxic infusion of malonate (2 micromol) into the left striatum to induce a metabolic stress. The nonsubstrate GABA transport blocker, NO711 (20 nmol) was infused in some rats 30 min before and 3 h following malonate infusion. After 1 week, dopamine and GABA levels in the striata were measured. Malonate caused a significant loss of striatal dopamine and GABA. Blockade of the GABA transporter significantly attenuated GABA, but not dopamine loss. In contrast with several in vitro reports, GABA(A) receptors were not a downstream mediator of protection by NO711. Intrastriatal infusion of malonate (2 micromol) plus or minus the GABA(A) receptor agonist muscimol (1 micromol), the GABA(A) Cl- binding site antagonist picrotoxin (50 nmol) or the GABA(B) receptor antagonist saclofen (33 nmol) did not modify loss of striatal dopamine or GABA when examined 1 week following infusion. These data show that GABA transporter blockade during mitochondrial impairment in the striatum provides protection to GABAergic neurons. GABA transporter blockade, which is currently a pharmacological strategy for the treatment of epilepsy, may thus also be beneficial in the treatment of acute and chronic conditions involving energy inhibition such as stroke/ischemia or Huntington's disease. These findings also point to fundamental differences between immature and adult neurons in the downstream involvement of GABA receptors during metabolic insult.

Cytoskeleton disruption causes apoptotic degeneration of dentate granule cells in hippocampal slice cultures

Colchicine, a potent microtubule-depolymerizing agent, is well known to selectively kill dentate granule cells in the hippocampal formation in vivo. Using organotypic cultures of rat entorhino-hippocampal slices, we confirmed that in vitro exposure to 1 microM and 10 microM of colchicine reproduced a specific degeneration of the granule cells after 24 h. Similar results were obtained with other types of microtubule-disrupting agents, i.e., nocodazole, vinblastine, and Taxol. Interestingly, the actin-depolymerizing agents cytochalasin D and latrunculin A also elicited selective neurotoxicity in the dentate gyrus without affecting survival of hippocampal pyramidal cells. The selective pattern of degeneration was observable 24 h after a brief treatment with the toxins as short as 5 min, but this delayed neuronal death was unlikely to be a result of excitotoxicity because it was virtually unaffected by glutamate receptor antagonists, tetrodotoxin, or extracellular Ca(2+)-free conditions. The damaged tissues contained a large number of TUNEL-positive neurons and exhibited an increased level in caspase-3-like activity, suggesting that cytoskeleton disruption triggers an apoptosis-like process in dentate granule cells. Thus, this study may provide a basis for understanding the distinctive mechanism that supports granule cell survival.

Interleukin-1beta mediates ischemic injury in the rat retina

Two types of experiment were performed to examine the role of interleukin-1beta in ischemia-induced damage in the rat retina. In the in vivo study, enzyme-linked immunosorbent assay was used to investigate the expression of immunoreactive interleukin-1beta in the rat retina following a hypertension-induced ischemia/reperfusion, while the effect of a recombinant human interleukin-1 receptor antagonist or an anti-interleukin-1beta neutralizing antibody on the ischemia-induced damage was examined histologically. A transient increase in the expression of immunoreactive interleukin-1beta was observed in the retina 3-12 hr after reperfusion, and morphometric evaluation at 7 days after the ischemia showed a decrease in cell numbers in the ganglion cell layer and a decreased thickness of the inner plexiform layer with no change in the other retinal layers. Intravitreal injection of interleukin-1 receptor antagonist (1 or 10 ng per eye) or anti-interleukin-1beta antibody (50 or 500 ng per eye) 5 min before the onset of the ischemia reduced the damage. In the in vitro study, interleukin-1 receptor antagonist (500 ng ml(-1)) significantly reduced glutamate-induced neurotoxicity in rat cultured retinal neurons. These results suggest that interleukin-1 plays an important role in mediating ischemic and excitotoxic damage in the retina, and that interleukin-1 inhibitors may be therapeutically useful against neuronal injury caused by optic nerve or retinal diseases such as glaucoma and central retinal artery or vein occlusion.

Neuroprotective effects of lamotrigine and remacemide on excitotoxicity induced by glutamate agonists in isolated chick retina

The possible neuroprotective effects of two recently developed antiepileptic compounds, lamotrigine (LTG) and remacemide (REMA), against glutamate agonist-induced excitotoxicity have been investigated in the isolated chick embryo retina model. Retina segments from 15- or 16-day-old embryos were incubated in 1 ml of balanced salt solution, at 25 degrees C for 30 min, in the presence or absence of N-methyl-d-aspartate (NMDA), kainic acid (KA), or alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) (10 to 200 microM). LTG, REMA, and the active desglycinyl metabolite of REMA (d-REMA) (10-200 microM) were added separately 5 min before glutamate agonists. Retina damage was assessed after 24 h (i) by measuring LDH activity present in the medium, expressed as percentage of total retina LDH activity, and (ii) by histological analysis of retina specimens through scoring for the presence or absence of edema, necrosis, nuclear pyknosis, and cell layer damage. LTG, REMA, and d-REMA reduced LDH release produced by NMDA 58-70% in a dose-dependent manner, with d-REMA being the most potent (EC(50): d-REMA, 25.75 +/- 3.27 microM; REMA, 64.75 +/- 7.75 microM; LTG, 60.50 +/- 6.80 microM; P < 0.001). The drugs had less effect on the LDH release produced by AMPA and KA. Histological analysis confirmed these biochemical results, with all three compounds reducing edema and the number of necrotic and pyknotic nuclei in the ganglion layer. d-REMA provided almost complete protection of the ganglion cell layer against damage produced by NMDA. Combinations of d-REMA and LTG showed additive rather than potentiative effects against NMDA-induced cell injury. The present data provide pharmacological evidence that LTG, REMA, and d-REMA decrease glutamate agonist-induced excitotoxicity in isolated chick retina, findings that might have therapeutic implications for various neurological disorders.

Modulation of glycine responses by dihydropyridines and verapamil in rat spinal neurons

Although glycine receptors (GlyRs) are responsible for the main spinal inhibitory responses in adult vertebrates, in the embryo they have been reported to mediate depolarizing responses, which can sometimes activate dihydropyridine-sensitive L-type calcium channels. However, these channels are not the only targets of dihydropyridines (DHPs), and we questioned whether GlyRs might be directly modulated by DHPs. By whole-cell recording of cultured spinal neurons, we investigated modulation of glycine responses by the calcium channel antagonists, nifedipine, nitrendipine, nicardipine and (R)-Bay K 8644, and by the calcium channel, agonist (S)-Bay K 8644. At concentrations between 1 and 10 microM, all these DHPs could block glycine responses, even in the absence of extracellular Ca2+. The block was stronger at higher glycine concentrations, and increased with time during each glycine application. Nicardipine blocked GABAA responses from the same neurons in a similar manner. In addition to their blocking effects, nitrendipine and nicardipine potentiated the peak responses to low glycine concentrations. Both effects of extracellular nitrendipine on glycine responses persisted when the drug was present in the intracellular solution. Thus, these modulations are related neither to calcium channel modulation nor to possible intracellular effects of DHPs. Another type of calcium antagonist, verapamil (10-50 microM), also blocked glycine responses. Our results suggest that some of the effects of calcium antagonists, including the neuroprotective and anticonvulsant effects of DHPs, might result partly from their interactions with ligand-gated chloride channels.

Attenuation of malonate toxicity in primary mesencephalic cultures using the GABA transport blocker, NO-711

Cultured rat mesencephalic neurons were used to assess the effects of gamma-aminobutyric acid (GABA) transport blockers on toxicity caused by malonate, a reversible, competitive inhibitor of succinate dehydrogenase. Previous studies utilizing an ex vivo chick retinal preparation have shown that GABA release and cell swelling are early consequences of acute energy impairment and that GABA transport blockers attenuate this toxicity. The present results demonstrate that the nonsubstrate GABA transport blocker, NO-711 (1 nM-1 microM), dose-dependently protected cultured mesencephalic dopamine (DA) and GABA neurons from malonate-induced toxicity. Similar protection was demonstrated with nipecotic acid (1 mM) and SKF89976A (100 nM), substrate and nonsubstrate GABA transport blockers, respectively. These compounds by themselves produced no signs of toxicity, although nipecotic acid caused a long-term decrease in GABA uptake not associated with toxicity. Compounds which decrease intracellular reactive oxygen species (ROS) are protective in this model, but NO-711 did not prevent the rise in intracellular ROS induced by malonate, indicating its protective effects were downstream of ROS production. Supplementation of malonate treated cultures with the GABA(A) agonist, muscimol (10 microM), increased the toxicity toward the DA and GABA neuron populations. Antagonists at the GABA(A) and glycine receptors provided partial protection to both the GABA and DA neurons. These findings suggest that the GABA transporter, GABA(A), and/or glycine channels contribute to cell damage associated with energy impairment in this model.

Cl(-)-dependent excitotoxicity is associated with 3H2O influx in chick embryonic retina

The aim of this study was to show that Cl(-)-dependent excitotoxicity, with its characteristic cell swelling, involves actual water influx into the intracellular compartment. Taking advantage of the Ca2+ omission paradigm of Cl(-)-dependent excitotoxicity, in the chick embryonic neural retina ex vivo, which is associated with toxicity levels (lactate dehydrogenase (LDH) release) considerably higher than those seen after simple exposure of the retinas to glutamate agonists, we have demonstrated that an intracellular water intake of 4.2 microl into retinal cells is associated with 13.3% total retinal LDH release. The fact that mannitol blocks both water inflow and LDH release appears to link both events from a pathogenic point of view.

Ca2+-dependent kainate excitotoxicity in the chick embryonic neural retina ex vivo

The chick embryonic neural retina ex vivo has been singled out as a unique example of Cl(-)-dependent/Ca2+-independent excitotoxicity. However, after continuous incubation with 100 microM kainate, we have demonstrated the susceptibility of the chick retina to Ca2+-mediated damage, which becomes apparent after 12 h of exposure to the agonist in the absence of Cl-. Of the 20.8% lactate dehydrogenase released after 24 h incubation with kainate, some 11% is Cl(-)-dependent and the rest (9.8%) is presumably Ca2+-dependent. Upon omission of both Cl- and Ca2+, a 5% residual toxicity can still be detected after 24 h. This can be overcome by inclusion of EGTA in the incubation medium to neutralize Ca2+ released during incubation. A Ca2+-dependent toxicity mechanism is then operative in the embryonic chick retina ex vivo.

Excitotoxic cell death dependent on inhibitory receptor activation

Although excitotoxic cell death is usually considered a Ca(2+)-dependent process, in certain neuronal systems there is strong evidence that excitotoxic cell death is independent of Ca2+ and is instead remarkably dependent on extracellular Cl-. We have shown (in isolated chick embryo retina) that at least some of the lethal Cl- entry is through GABA and glycine receptors. Here we show that when all the GABA and glycine receptors are blocked by using an appropriate cocktail of inhibitors, agonist-induced excitotoxic cell death can be completely prevented. To determine if ligand-gated Cl- channels contribute to excitotoxic cell death in other neurons, we examined KA-induced cell death in cultured rat cerebellar granule cells. GABA receptor blockade with either a competitive or noncompetitive antagonist provides complete neuroprotection. KA stimulates Cl- uptake by the granule cells, and this is blocked by the GABA antagonists. Granule cell cultures take up [3H]GABA and release it in response to KA treatment. A subpopulation of neurons in the cultures is shown to have GAD and high concentrations of GABA, and this presumably is the source of the GABA that leads to receptor activation and lethal Cl- entry. Finally, we show that retinal cell death due to 1 h of simulated ischemia (combined oxygen and glucose deprivation) is completely prevented by blocking the inhibitory receptors. These results indicate that, paradoxically, excitotoxic cell death is completely dependent on activation of inhibitory receptors, in at least some neuronal systems, and this pathological process may contribute to disease.

Amino acids as osmolytes in the retina

Amino acids play a role as osmolytes during the regulatory volume decrease subsequent to hyposmotic swelling, but less is known about its role when swelling occurs in isosmotic conditions. In this work we examined the efflux of labelled GABA, taurine and glutamate (traced as D-aspartate) from the chick retina, after isosmotic swelling evoked by KCl-containing solutions, and compared its features to those in hyposmotic swelling. In both conditions, GABA and taurine efflux were more sensitive to swelling than glutamate, as assessed by the activation threshold and the amount released. The amino acid efflux in hyposmotic media was decreased by DIDS, tamoxifen and NPPB, agents acting as Cl channels blockers, which also inhibit the osmosensitive Cl efflux. The component associated with swelling in the KCl-stimulated efflux was assessed by the reduction observed when Cl is replaced by an impermeant anion, or by the influence of hyperosmotic media. GABA and taurine efflux exhibited a large swelling-dependent component, which was lower for D-aspartate. This component was markedly decreased by NPPB, but this was due to an effect of the blocker preventing swelling. These results suggest that the influx of Cl, acting as K counterion, which is responsible for cell swelling, occurs through a pathway sensitive to NPPB, similarly to that activated by hyposmolarity. This finding may be of interest in studies aiming at preventing the cell edema which occurs in a number of pathologies.

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.

Influence of gamma-aminobutyric acid on retinal cells excitotoxicity upon glucose deprivation

The role of extracellular endogenous gamma-aminobutyric acid (GABA) in rescuing retinal cells in culture from the decrease in viability induced by Glu under metabolic inhibition is analyzed. Glutamate (10 microM-10 mM) dose-dependently decreased the intracellular GABA content, but increased the extracellular accumulation of GABA. In the absence of glucose, Glu (10-100 microM) decreased the intracellular GABA (2-fold), whereas the extracellular accumulation of GABA was increased by about 4-fold. Glu-mediated decrement in cell survival was not affected by inhibiting the GABA(A) receptors with bicuculline (1 or 10 microM) or by blocking the Na+ -dependent release of GABA with 1-(4,4-diphenyl-3-butenyl)-3-piperidinecarboxylic acid (SKF89976-A). Data suggest a non-protective role of endogenous GABA release after metabolic deprivation of retinal cells submitted to Glu.

The intact isolated (ex vivo) retina as a model system for the study of excitotoxicity

Excitotoxicity is defined as a mode of neural cell death triggered by overactivation of receptors for the amino acid transmitter glutamate. There is considerable evidence that excitotoxicity is responsible for cell death in several neuropathological states, including some retinal diseases. The isolated retina, particularly from chick embryos, has been used extensively as an experimental system to characterize this process. This paper summarizes the use of isolated retina as a model system for studies of excitotoxicity from a theoretical and methodological point of view, and reviews results obtained from studies utilizing this system.

Guanine nucleotides protect against kainate toxicity in an ex vivo chick retinal preparation

Ex vivo preparations of chick neural retina have been successfully used in the assessment of excitotoxicity and in the evaluation of the protective effects of glutamate antagonists. Using a variation of this approach, and measuring the acute and delayed toxic effects of kainate (KA) in terms of lactate dehydrogenase release, we have shown that guanine nucleotides behave as effective neuroprotecting agents. The anti-excitotoxic potency of guanine nucleotides (in the case of GMP and GDPbetaS it is about 100 times lower than that of DNQX, a powerful kainate antagonist) correlates well with their ability to displace KA from retinal KA receptors.

Excitotoxic damage of retinal glial cells depends upon normal neuron-glial interactions

Glutamate, the principal retinal neurotransmitter, can also act as a toxin when present in excessive concentrations as may occur in pathologies such as retinal ischemia or more generally in cerebral neuronal degenerative disease. As glial cells play pivotal roles in transfer of blood-borne molecules and in glutamate clearance, we investigated the effects of the excitatory amino acids glutamic and kainic acid on different in vitro preparations of retinal Müller glial cells. Glial viability or morphology were not influenced by excitatory amino acid exposure in either pure glial cultures or in monolayer cultures of mixed neonatal neurons and glia, whereas kainic acid specifically lysed amacrine cells in mixed or pure neuronal cultures. When retinal fragments were pre-incubated in excitatory amino acids prior to dissociation and seeding into culture, under these conditions Müller glial cells exhibited a dramatic loss of their normal epithelioid form to a retracted morphology. However, glial cell viability was not compromised, and rapid restoration of epithelioid in vitro glial morphology could be achieved by addition of exogenous epidermal and basic fibroblast growth factor to the culture medium. This study demonstrates that glial cells are structurally perturbed by excitotoxic conditions and that such effects are dependent on normal glial-neuronal interactions.

Glutamate in life and death of retinal amacrine cells

1. Glutamate is the neurotransmitter released by bipolar cells at their synapses with amacrine cells. The amacrine cells express ionotropic (NMDA, AMPA and kainate) and metabotropic (mGluR1, mGluR2, mGluR4 and mGluR7) glutamate receptors and may take up glutamate from the synaptic cleft. 2. Activation of the ionotropic glutamate receptors increases the intracellular free calcium concentration ([Ca2+]i), owing to Ca2+ entry through the receptor-associated channels as well as through voltage-gated Ca2+ channels. The [Ca2+]i response to glutamate may be amplified by Ca2+-induced Ca2+ release from intracellular sources. 3. Activation of NMDA and non-NMDA glutamate receptors stimulates the release of GABA and acetylcholine from amacrine cells. GABA is released by a Ca2+-dependent mechanism and by reversal of the neurotransmitter transporter. 4. Excessive activation of glutamate receptors during ischemia leads to amacrine cell death. An increase in [Ca2+]i due to Ca2+ influx through NMDA and AMPA/kainate receptor channels is related to cell death in studies in vitro. In other studies, it was shown that nitric oxide may also take part in the process of cell damage during ischemia.

Ca2+-independent excitotoxic neurodegeneration in isolated retina, an intact neural net: a role for Cl- and inhibitory transmitters

Rapidly triggered excitotoxic cell death is widely thought to be due to excessive influx of extracellular Ca2+, primarily through the N-methyl-D-aspartate subtype of glutamate receptor. By devising conditions that permit the maintenance of isolated retina in the absence of Ca2+, it has become technically feasible to test the dependence of excitotoxic neurodegeneration in this intact neural system on extracellular Ca2+. Using biochemical, Ca2+ imaging, and electrophysiological techniques, we found that (1) rapidly triggered excitotoxic cell death in this system occurs independently of both extracellular Ca2+ and increases in intracellular Ca2+; (2) this cell death is highly dependent on extracellular Cl-; and (3) lethal Cl- entry occurs by multiple paths, but a significant fraction occurs through pathologically activated gamma-aminobutyric acid and glycine receptors. These results emphasize the importance of Ca2+-independent mechanisms and the role that local transmitter circuitry plays in excitotoxic cell death.

Activity at the GABA transporter contributes to acute cellular swelling produced by metabolic impairment in retina

The role of the GABA transporter in acute toxicity in chick retina due to metabolic inhibition was investigated by the use of several substrate (nipecotic acid, THPO) and nonsubstrate (SKF 89976A, NO711) GABA transport inhibitors. Metabolic stress-induced acute toxicity in the retina is characterized by swelling of distinct populations of retinal neurons and selective release of GABA into the medium. Inhibitor concentrations were based on that needed to attenuate 14C-GABA uptake at its approximate KM concentration by > or = 70%. Under basal conditions, substrate, but not nonsubstrate, inhibitors increased extracellular GABA, but did not cause histological swelling per se. Under conditions of glycolytic inhibition, nonsubstrate, but not substrate, inhibitors significantly attenuated acute toxicity. Metabolic stress-induced acute toxicity was not altered by the GABA agonist muscimol, nor did muscimol reverse the protective effects of nonsubstrate transport inhibitors, suggesting that an increase in extracellular GABA during metabolic stress was not a component of the acute phase of toxicity. The results indicate that during metabolic inhibition, activity at the GABA transporter contributes to acute cellular swelling.

Contribution of glial metabolism to neuronal damage caused by partial inhibition of energy metabolism in retina

Glial cells are relatively resistant to energy impairment, although little is known of the extent to which glial metabolism is affected during partial energy impairment and how this influences neurons. Fluorocitrate has been shown to be a glial specific metabolic inhibitor. Its selective effect on chick retinal Müller cells was verified by measuring incorporation of radiolabel from 3H-acetate and U-14C-glucose into glutamate and glutamine following exposure of isolated embryonic day 15-18 chick retina to 20 microm fluorocitrate. Fluorocitrate significantly reduced the incorporation of radiolabel from acetate and glucose into glutamine, with less effect on incorporation of label from acetate into glutamate and no reduction of label from glucose into glutamate. The relative specific activity (RSA; ratio of glutamine to glutamate) increased between embryonic day 15 and 18 consistent with the increase in glutamine synthetase activity that occurs in Müller cells at this time in chick retinal development. As with previous findings, mild energy stress produced by inhibiting glycolysis with the general inhibitor iodoacetate (IOA) for up to 45 min, caused acute neuronal damage that was predominately NMDA receptor mediated and occurred in the absence of a net efflux of excitatory amino acids. Acute NMDA-mediated toxicity in this preparation is characterized by the selective damage to amacrine and ganglion cells and quantitatively, by GABA release into the medium. When IOA was combined with fluorocitrate, acute toxicity was potentiated and temporally accelerated. Acute damage was first noted at 15 min, occurred throughout all retinal layers and was accompanied by an overflow of excitatory amino acids at 30 and 45 min. Blocking NMDA receptors with MK-801 during IOA plus fluorocitrate exposure attenuated the rise in excitatory amino acids and prevented the swelling in neuronal, but not Müller cells. Following incorporation of radiolabel from acetate and glucose into glutamate and glutamine after different times of exposure to IOA showed that while the effects of incorporation of label from glucose were immediate, glutamine synthesis from acetate was preserved for a longer period of time. These findings suggest that during a partial energy impairment, neuronal metabolism is affected to a greater extent than is glial metabolism. Glial cells may play a protective role in this situation, and can delay the onset of acute neuronal damage.

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.

Glutamine synthetase protects against neuronal degeneration in injured retinal tissue

The neurotransmitter glutamate is neurotoxic when it is accumulated in a massive amount in the extracellular fluid. Excessive release of glutamate has been shown to be a major cause of neuronal degeneration after central nervous system injury. Under normal conditions, accumulation of synaptically released glutamate is prevented, at least in part, by a glial uptake system in which the glia-specific enzyme glutamine synthetase (GS) plays a key role. We postulated that glial cells cannot cope with glutamate neurotoxicity because the level of GS is not high enough to catalyze the excessive amounts of glutamate released by damaged neurons. We examined whether elevation of GS expression in glial cells protects against neuronal degeneration in injured retinal tissue. Analysis of lactate dehydrogenase efflux, DNA fragmentation, and histological sections revealed that hormonal induction of the endogenous GS gene in retinal glial cells correlates with a decline in neuronal degeneration, whereas inhibition of GS activity by methionine sulfoximine leads to increased cell death. A supply of purified GS enzyme to the culture medium of retinal explants or directly to the embryo in ovo causes a dose-dependent decline in the extent of cell death. These results show that GS is a potent neuroprotectant and that elevation of GS expression in glial cells activates an endogenous mechanism whereby neurons are protected from the deleterious effects of excess glutamate in extracellular fluid after trauma or ischemia. Our results suggest new approaches to the clinical handling of neuronal degeneration.

Presynaptic initiation by action potentials of retrograde signals in developing neurons

Until recently, the only means by which electrical activity was believed to initiate retrograde signals was via postsynaptic events: modulated synthesis or release of trophic factors. We have evidence in chick embryos for a presynaptic initiation of retrograde signals from the retina to the isthmo-optic nucleus, which is known to undergo 55% neuron death between embryonic days 12 and 17 and to become laminated during this period. Intraocular injections of saxitoxin just before embryonic day 14 reduce neuron death and prevent lamination in the isthmo-optic nucleus within as few as 6 hr. We show that these rapid effects are attributable to the direct action of saxitoxin on the isthmo-optic terminals. Alternative possibilities, such as an indirect effect via the target cells, are ruled out by control experiments. Normally, action potentials may lead to a chain of second messenger events in the axon terminal that is signaled retrogradely via the transport of a long-lived second messenger.

Ameliorative effect of MK-801 on retinal ischemia

The efficacy of MK-801, an N-methyl-D-aspartate receptor antagonist, was evaluated in a rat model of retinal ischemia induced by elevated intraocular pressure. Intraperitoneal injection of MK-801 at 0, 1, 3 and 10 mg/kg was given immediately after reperfusion. At 7 days after reperfusion, the inner retinal thickness, as measured from histologic sections of the retinas, of the 10 mg/kg treated group showed significant beneficial effect, while the other doses had no significant effect. Retinal ganglion cell counts on flat preparations of the retinas showed a beneficial dose dependent effect of MK-801 with the lowest dose showing no effect, 3 mg/kg showing marginal effects and 10 mg/kg showing significant effects. Intravitreal infusion of MK-801 during the ischemic period suppressed ischemia/reperfusion-induced internucleosomal DNA fragmentation measured at 18 hours after the insult as well as retinal tissue responses measured at 7 days. These findings suggested that the NMDA receptors may have an important role in ischemia-reperfusion insult as well as in mediating ischemia-induced apoptosis of retinal neurons. In addition, we demonstrated that pharmacological modulation of apoptotic cell death may affect the final tissue responses in vivo.

Attenuation of excitotoxic cell swelling and GABA release by the GABA transport inhibitor SKF 89976A

Acute excitotoxicity in the chick retina is characterized by cellular swelling and the subsequent selective release of GABA. In order to understand the source of GABA release, embryonic day 15 retina were incubated with 1 mM glutamate for 30 min in the presence or absence of the GABA transport inhibitor SKF 89976A (1-100 microM). SKF 89976A dose-dependently attentuated glutamate-induced GABA release (IC50, 39 microM). Histological examination of retina showed that SKF 89976A greatly reduced cellular swelling caused by glutamate exposure. Interaction of SKF 89976A with glutamate receptors was ruled out as a possible reason for protection vs acute glutamate excitotoxicity, since SKF 89976A had no effect on glutamate receptor-induced 22Na+ influx. In contrast, the NMDA antagonist, MK-801, significantly blocked glutamate-evoked 22NA+ uptake. These studies indicate that reversal of the GABA transporter contributes to the bulk of GABA release during acute excitotoxicity in retina. Further, a net effect of the presence of SKF 89976A during glutamate exposure is reduction in cellular swelling. It is not clear at present if attenuation of swelling is mediated specifically by an interaction with the GABA transporter or by a nonspecific or indirect effect of SKF 89976A.

Nitric oxide: a review of its role in retinal function and disease

Nitric oxide synthase (NOS), the enzyme that catalyzes the formation of nitric oxide from L-arginine, exists in three major isoforms, neuronal, endothelial, and immunologic. Neuronal and endothelial isoforms are constitutively expressed, and require calcium for activation. Both of these isoforms can be induced (i.e., new protein synthesis occurs) under appropriate conditions. The immunologic isoform is not constitutively expressed, and requires induction usually by immunologic activation; calcium is not necessary for its activation. Neuronal and immunologic NOS have been detected in the retina. Neuronal NOS may be responsible for producing nitric oxide in photoreceptors and bipolar cells. Nitric oxide stimulates guanylate cyclase of photoreceptor rod cells and increases calcium channel currents. In the retina of cats, NOS inhibition impairs phototransduction as assessed by the electroretinogram. Inducible nitric oxide synthase, found in Müller cells and in retinal pigment epithelium, may be involved in normal phagocytosis of the retinal outer segment, in infectious and ischemic processes, and in the pathogenesis of diabetic retinopathy. Nitric oxide contributes to basal tone in the retinal circulation. To date, findings are conflicting with respect to its role in retinal autoregulation. During glucose and oxygen deprivation, nitric oxide may increase blood flow and prevent platelet aggregation, but it may also mediate the toxic effects of excitatory amino acid release. This reactive, short-lived gas is involved in diverse processes within the retina, and its significance continues to be actively studied.

Glutamate inhibition of the adrenergic-stimulated production of melatonin in rat pineal gland in vitro

The effect of L-glutamate on the adrenergic-stimulated release of melatonin in the rat pineal gland was examined using an in vitro perfusion system. L-Glutamate by itself had no effect on melatonin secretion whereas L-glutamate administered prior to (-)-isoproterenol (beta-adrenergic agonist) and L-phenylephrine (alpha-adrenergic agonist) inhibited melatonin production by 42%. L-Glutamate did not inhibit melatonin secretion when glands were stimulated with (-)-isoproterenol alone. D-Glutamate, as well as the L-glutamate agonists kainate, N-methyl-D-aspartate, quisqualate, and trans-1-aminocyclopentane-1,3-dicarboxylic acid, had no effect on the (-)-isoproterenol- and L-phenylephrine-stimulated secretion of melatonin, which suggests that the inhibitory effects of glutamate are not mediated via any of the known glutamate receptor subtypes. The possibility that L-glutamate may be converted to another neuroactive compound (GABA) prior to the addition of (-)-isoproterenol and L-phenylephrine is suggested by the observation that simultaneous administration of L-glutamate with (-)-isoproterenol and L-phenylephrine did not inhibit melatonin production.

Hypothermia, metabolic stress, and NMDA-mediated excitotoxicity

Isolated embryonic retinas were metabolically stressed by inhibition of glycolysis either with iodoacetate (IOA) or by glucose withdrawal plus 10 mM 2-deoxy-D-glucose, and the effects of hypothermia were examined. Incubation at 30 versus 37 degrees C during 30 min of hypoglycemia with IOA completely reduced the rapid swelling-related GABA release [6 +/- 2 vs. 68 +/- 10 nmol/100 mg of protein (mean +/- SEM) for 30 and 37 degrees C, respectively]. Histology of the retina immediately following 30 min of metabolic stress at 30 degrees C appeared normal, whereas that at 37 degrees C showed a pattern of acute edema, characteristic of NMDA-mediated acute excitotoxicity. Coincubation with a competitive or noncompetitive NMDA antagonist, respectively, CGS-19755 (10 microM) or MK-801 (1 microM), during 30 min of hypoglycemia at 37 degrees C completely prevented tissue swelling, whereas extracellular GABA content remained at basal levels, indicating that the cytotoxic effects of IOA treatment for 30 min at 37 degrees C were NMDA receptor mediated. Longer periods of hypoglycemia at 37 degrees C produced acute toxicity that was only partially NMDA receptor mediated. Hypothermia delayed the onset of NMDA-mediated toxicity by 30-60 min. At 30 degrees C, the rate of loss of ATP was slowed during the first several minutes of hypoglycemia (82 and 58% of maximal tissue levels at 30 and 37 degrees C, respectively, at 5 min, but by 10 min, ATP levels were comparably reduced. After a transient exposure of retina to 50 microM NMDA in Mg(2+)-free medium, hypothermia significantly attenuated acute GABA release by 30%.(ABSTRACT TRUNCATED AT 250 WORDS)