Vicious cycle involving Na+ channels, glutamate release, and NMDA receptors mediates delayed neurodegeneration through nitric oxide formation

Cortical and striatal neuronal cultures of the same embryonic origin show intrinsic differences in glutamate receptor expression and vulnerability to excitotoxicity

Cortical and striatal cultures were prepared from the same embryonic rat brains and maintained in identical culture conditions. In this way, the intrinsic, genetically imprinted differences determine the responses of cortical and striatal neurons in comparative studies. Cortical and striatal neurons differed in their sensitivity to glutamate receptor-mediated neurotoxicity as measured by the MTT cell viability assay. On the 8th day in vitro, striatal cultures were less sensitive to N-methyl-d-aspartate (NMDA)-induced toxicity than cortical, although both cultures were equally vulnerable to alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA)- or kainate-induced toxicity. The AMPA receptor-mediated cell death in cortical cultures, however, was much more dependent on preventing AMPA receptor desensitization than in striatal cultures. Furthermore, glutamate-induced neurotoxicity was primarily mediated by NMDA receptors in cortical cultures, while blockade of either NMDA or AMPA receptors gave almost complete protection against glutamate in striatal cultures. To elucidate the molecular mechanisms responsible for the observed differences, we analyzed the expression of NMDA receptor subunits (NR1, NR2A-C) at the mRNA and the protein level in cortical and striatal cultures as well as in standard cerebellar granule cell cultures. The lowest expression level of NMDA receptor subunits was found in striatal cultures, thereby providing a possible explanation for their lower sensitivity to NMDA. Remarkable differences were found between the relative rates of mRNA and protein expression for NR1 and NR2B in the three cultures, indicative of intrinsic differences in the posttranscriptional regulation of NMDA receptor subunit expression in cultures from various brain regions.

Potentiation by histamine of synaptically mediated excitotoxicity in cultured hippocampal neurones: a possible role for mast cells

Excessive glutamatergic neurotransmission, particularly when mediated by the N:-methyl-D-aspartate (NMDA) subtype of glutamate receptor, is thought to underlie neuronal death in a number of neurological disorders. Histamine has been reported to potentiate NMDA receptor-mediated events under a variety of conditions. In the present study we have utilized primary hippocampal neurone cultures to investigate the effect of mast cell-derived, as well as exogenously applied, histamine on neurotoxicity evoked by excessive synaptic activity. Exposure of mature cultures for 15 min to an Mg(2+)-free/glycine-containing buffer to trigger synaptic transmission through NMDA receptors, caused a 30-35% neuronal loss over 24 h. When co-cultured with hippocampal neurones, activated mast cells increased excitotoxic injury to 60%, an effect that was abolished in the presence of histaminase. Similarly, addition of histamine during magnesium deprivation produced a concentration-dependent potentiation (+ 60%; EC(50) : 5 microM) of neuronal death which was inhibited by sodium channel blockers and NMDA receptor antagonists, although this effect did not involve known histamine receptors. The histamine effect was further potentiated by acidification of the culture medium. Cultures 'preconditioned' by sublethal (5 min) Mg(2+) deprivation exhibited less neuronal death than controls when exposed to a more severe insult. NMDA receptor activation and the extracellular regulated kinase cascade were required for preconditioning neuroprotection. The finding that histamine potentiates NMDA receptor-mediated excitotoxicity may have important implications for our understanding of conditions where enhanced glutamatergic neurotransmission is observed in conjunction with tissue acidification, such as cerebral ischaemia and epilepsy.

Nitric oxide modulation of interleukin-1[beta]-evoked intracellular Ca2+ release in human astrocytoma U-373 MG cells and brain striatal slices

Intracellular Ca(2+) mobilization and release into mammal CSF plays a fundamental role in the etiogenesis of fever induced by the proinflammatory cytokine interleukin-1beta (IL-1beta) and other pyrogens. The source and mechanism of IL-1beta-induced intracellular Ca(2+) mobilization was investigated using two experimental models. IL-1beta (10 ng/ml) treatment of rat striatal slices preloaded with (45)Ca(2+) elicited a delayed (30 min) and sustained increase (125-150%) in spontaneous (45)Ca(2+) release that was potentiated by l-arginine (300 microm) and counteracted by N-omega-nitro-l-arginine methyl ester (l-NAME) (1 and 3 mm). The nitric oxide (NO) donors diethylamine/NO complex (sodium salt) (0.3 and 1 mm) and spermine/NO (0.1 and 0.3 mm) mimicked the effect of IL-1beta on Ca(2+) release. IL-1beta stimulated tissue cGMP concentration, and dibutyryl cGMP enhanced Ca(2+) release. The guanyl cyclase inhibitors 1H-[1,2, 4]oxadiazole[4,3-a] quinoxalin-1-one (100 microm) and 6-[phenylamino]-5,8 quinolinedione (50 microm) counteracted Ca(2+) release induced by 2.5 but not 10 ng/ml IL-1beta. Ruthenium red (50 microm) and, to a lesser extent, heparin (3 mg/ml) antagonized IL-1beta-induced Ca(2+) release, and both compounds administered together completely abolished this response. Similar results were obtained in human astrocytoma cells in which IL-1beta elicited a delayed (30 min) increase in intracellular Ca(2+) concentration ([Ca(2+)](i)) (402 +/- 71.2% of baseline), which was abolished by 1 mm l-NAME. These data indicate that the NO/cGMP-signaling pathway is part of the intracellular mechanism transducing IL-1beta-evoked Ca(2+) mobilization in glial and striatal cells and that the ryanodine and the inositol-(1,4,5)-trisphosphate-sensitive Ca(2+) stores are involved.

Opportunities for neuroprotection in traumatic brain injury

Traumatic injury of the brain in man is normally followed by little or no recovery of function by the lesioned tissue. Neuroprotective strategies employed in the acute period after traumatic CNS injury attempt to use pharmacological tools to reduce the progressive secondary injury processes that follow after the initial lesion occurs to limit overall tissue damage. Results from experimental animal studies using a variety of drugs that modulate neurotransmitter function, scavenge free radicals, or interfere with cell death cascades point toward many new opportunities for pharmacological intervention in the acute and subacute period after traumatic brain injury.

Short- and long-term differential effects of neuroprotective drug NS-7 on voltage-dependent sodium channels in adrenal chromaffin cells

In cultured bovine adrenal chromaffin cells, NS-7 [4-(4-fluorophenyl)-2-methyl-6-(5-piperidinopentyloxy) pyrimidine hydrochloride], a newly-synthesized neuroprotective drug, inhibited veratridine-induced (22)Na(+) influx via voltage-dependent Na(+) channels (IC(50)=11.4 microM). The inhibition by NS-7 occurred in the presence of ouabain, an inhibitor of Na(+),K(+) ATPase, but disappeared at higher concentration of veratridine, and upon the washout of NS-7. NS-7 attenuated veratridine-induced (45)Ca(2+) influx via voltage-dependent Ca(2+) channels (IC(50)=20.0 microM) and catecholamine secretion (IC(50)=25.8 microM). Chronic (>/=12 h) treatment of cells with NS-7 increased cell surface [(3)H]-STX binding by 86% (EC(50)=10.5 microM; t(1/2)=27 h), but did not alter the K(D) value; it was prevented by cycloheximide, an inhibitor of protein synthesis, or brefeldin A, an inhibitor of vesicular transport from the trans-Golgi network, but was not associated with increased levels of Na(+) channel alpha- and beta(1)-subunit mRNAs. In cells subjected to chronic NS-7 treatment, (22)Na(+) influx caused by veratridine (site 2 toxin), alpha-scorpion venom (site 3 toxin) or beta-scorpion venom (site 4 toxin) was suppressed even after the extensive washout of NS-7, and veratridine-induced (22)Na(+) influx remained depressed even at higher concentration of veratridine; however, either alpha- or beta-scorpion venom, or Ptychodiscus brevis toxin-3 (site 5 toxin) enhanced veratridine-induced (22)Na(+) influx as in nontreated cells. These results suggest that in the acute treatment, NS-7 binds to the site 2 and reversibly inhibits Na(+) channels, thereby reducing Ca(2+) channel gating and catecholamine secretion. Chronic treatment with NS-7 up-regulates cell surface Na(+) channels via translational and externalization events, but persistently inhibits Na(+) channel gating without impairing the cooperative interaction between the functional domains of Na(+) channels.

Role of NMDA receptor functional domains in excitatory cell death

The mechanisms by which the NMDA receptor (NMDAR) induces excitotoxicity were investigated using a novel assay. We quantitated the capacity of wild type and mutant receptors for cell killing in CHO cells and cultured cortical neurons by measuring the activity of a co-transfected firefly luciferase expression plasmid. NR1 subunit pore mutations that block Ca(2+) influx, and deletion of the NR1 cytoplasmic C-terminal domain, which functions in Ca(2+) regulation of receptor currents, decreased NMDAR mediated cell killing. We also transfected the NR1 pore mutants and C-terminal truncations in the presence of co-expressed exogenous wild type subunits. The pore and C-terminal truncation mutants acted in a dominant negative fashion and increased the survival of NMDAR-expressing CHO cells. Although physiological studies of similar NMDA receptor mutants have been carried out in heterologous cell lines, their functions in neurons remain relatively unknown. We show that expression of pore mutants and specific C terminal truncation mutants in cultured cortical neurons also exerts dominant negative function and protects these primary cells from endogenous receptor induced excitotoxic death. These results implicate positive actions of the selectivity filter and of the NR1 C-terminal domain in a Ca(2+)-dependent mechanism for NMDAR excitotoxicity. They also indicate that the mutant receptors which show diminished excitotoxicity and dominant negative action in heterologous cells can co-assemble with endogenous subunits in primary neurons and block NMDAR-dependent excitotoxic death.

Benzodiazepine-sensitive GABA(A) receptors limit the activity of the NMDA/NO/cyclic GMP pathway: a microdialysis study in the cerebellum of freely moving rats

In the cerebellum, infusion of NMDA (200 microM) for 20 min evoked a marked (200%) increase of extracellular cyclic GMP (cGMP) levels. The selective GABA(A) receptor agonist muscimol (0.01-100 microM) was able to counteract the NMDA effect with an EC(50) of 0.65 microM; the inhibitory effect of muscimol (10 microM) was prevented by bicuculline (50 microM). Diazepam (10 microM) significantly potentiated the muscimol (1 microM) inhibition; furthermore, when coinfused with 0.1 microM muscimol (a concentration not affecting, on its own, the cGMP response to NMDA), diazepam (10 microM) reduced the NMDA effect. Similar results were obtained with zolpidem (0.1-1 microM). Finally, local infusion of the benzodiazepine site antagonist flumazenil (10 microM), together with muscimol and diazepam, almost completely restored the effect of NMDA on extracellular cGMP levels. It is concluded that GABA(A) receptors potently control the NMDA/nitric oxide/cGMP pathway in the cerebellum in vivo. In terms of the alpha subunit composition, we can deduce that the cerebellar GABA(A) receptor does not contain alpha(6) or beta(4) subunits because it is diazepam-sensitive. Moreover, the observation that zolpidem is active at a rather low concentration, in combination with localization studies present in the literature, tend to exclude the presence of alpha(5) subunits in the receptor composition and suggest the involvement of an alpha(1) subunit.

On the regulation of ischaemia-induced glutamate efflux from rat cortex by GABA; in vitro studies with GABA, clomethiazole and pentobarbitone

Prisms of adult rat cortex were maintained in vitro in either aerobic conditions (control) or conditions simulating an acute ischaemic challenge (hypoxia with no added glucose). Endogenous glutamate efflux increased with time in ischaemic conditions, being 2.7 fold higher than control efflux at 45 min. Returning prisms to control solution after 20 min of simulated ischaemia resulted in glutamate efflux returning to near-control values. Endogenous GABA efflux in ischaemic conditions also increased, being 4.5 fold higher than control efflux at 45 min. Ischaemia-induced glutamate efflux was not accompanied by increased lactate dehydrogenase efflux and was unaltered by omitting calcium from the extra-cellular solution and adding EGTA (0.1 mM). Both GABA and the GABA-mimetic clomethiazole inhibited ischaemia-induced glutamate efflux, with IC(50) values of 26 and 24 microM respectively. The maximum inhibition by either drug was 60 - 70%. Bicuculline (10 microM) abolished the inhibitory effect of GABA (100 microM) but not clomethiazole (100 microM). Picrotoxin (100 microM) abolished the action of both GABA and clomethiazole. Pentobarbitone inhibited glutamate efflux at 100 - 300 microM (maximal inhibition: 39%). Bicuculline (10 microM) abolished this effect. These data suggest that ischaemia-induced glutamate efflux from rat cerebral cortex is calcium-independent and not due to cell damage up to 45 min. The inhibitory effect of GABA, clomethiazole and pentobarbitone on ischaemia-induced glutamate efflux appears to be mediated by GABA(A) receptors. The results suggest that clomethiazole, unlike pentobarbitone, is able to activate the GABAA receptor-linked chloride channel directly and not merely potentiate the effect of endogenous GABA.

3-Nitropropionic acid neurotoxicity in organotypic striatal and corticostriatal slice cultures is dependent on glucose and glutamate

Mitochondrial inhibition by 3-nitropropionic acid (3-NPA) causes striatal degeneration reminiscent of Huntington's disease. We studied 3-NPA neurotoxicity and possible indirect excitotoxicity in organotypic striatal and corticostriatal slice cultures. Neurotoxicity was quantified by assay of lactate dehydrogenase in the medium and glutamic acid decarboxylase in tissue homogenates. 3-NPA toxicity (25-100 microM in 5 mM glucose, 24-48 h) appeared to be highly dependent on culture medium glucose levels. 3-NPA treatment caused also a dose-dependent lactate increase, reaching a maximum of threefold increase above control at 100 microM. Both a high dose of glutamate (5 mM) and glutamate uptake blockade by dl-threo-beta-hydroxyaspartate potentiated 3-NPA neurotoxicity in corticostriatal slice cultures. Furthermore, striatum from corticostriatal cocultures was more sensitive to 3-NPA than striatum without cortex and tetrodotoxin, MK-801, and d-2-amino-5-phosphonopentanoic acid prevented or attenuated 3-NPA neurotoxicity, suggesting that membrane depolarization and/or neuronal activity of the glutamatergic corticostriatal pathway contributes to striatal pathology. The results indicate that in vivo characteristics of 3-NPA toxicity can be reproduced in organotypic corticostriatal slice cultures.

Exacerbated responses to oxidative stress by an Na(+) load in isolated nerve terminals: the role of ATP depletion and rise of [Ca(2+)](i)

We have explored the consequences of a [Na(+)](i) load and oxidative stress in isolated nerve terminals. The Na(+) load was achieved by veratridine (5-40 microM), which allows Na(+) entry via a voltage-operated Na(+) channel, and oxidative stress was induced by hydrogen peroxide (0.1-0.5 mM). Remarkably, neither the [Na(+)](i) load nor exposure to H(2)O(2) had any major effect on [Ca(2+)](i), mitochondrial membrane potential (Deltapsim), or ATP level. However, the combination of an Na(+) load and oxidative stress caused ATP depletion, a collapse of Deltapsim, and a progressive deregulation of [Ca(2+)](i) and [Na(+)](i) homeostasis. The decrease in the ATP level was unrelated to an increase in [Ca(2+)](i) and paralleled the rise in [Na(+)](i). The loss of Deltapsim was prevented in the absence of Ca(2+) but unaltered in the presence of cyclosporin A. We conclude that the increased ATP consumption by the Na,K-ATPase that results from a modest [Na(+)](i) load places an additional demand on mitochondria metabolically compromised by an oxidative stress, which are unable to produce a sufficient amount of ATP to fuel the ATP-driven ion pumps. This results in a deregulation of [Na(+)](i) and [Ca(2+)](i), and as a result of the latter, collapse of Deltapsim. The vicious cycle generated in the combined presence of Na(+) load and oxidative stress could be an important factor in the neuronal injury produced by ischemia or excitotoxicity, in which the oxidative insult is superimposed on a disturbed Na(+) homeostasis.

Electrophysiology of sipatrigine: a lamotrigine derivative exhibiting neuroprotective effects

Sipatrigine (BW619C89), a derivative of the antiepileptic agent lamotrigine, has potent neuroprotective properties in animal models of cerebral ischemia and head injury. In the present study we investigated the electrophysiological effects of sipatrigine utilizing intracellular current-clamp recordings obtained from striatal spiny neurons in rat corticostriatal slices and whole-cell patch-clamp recordings in isolated striatal neurons. The number of action potentials produced in response to a depolarizing current pulse in the recorded neurons was reduced by sipatrigine (EC(50) 4.5 microM). Although this drug preferentially blocked action potentials in the last part of the depolarizing current pulse, it also decreased the frequency of the first action potentials. Sipatrigine also inhibited tetrodotoxin-sensitive sodium (Na(+)) current recorded from isolated striatal neurons. The EC(50) for this inhibitory action was 7 microM at the holding potential (V(h)) of -65 mV, but 16 microM at V(h) = -105, suggesting a dependence of this pharmacological effect on the membrane potential. Moreover, although the inhibitory action of sipatrigine on Na(+) currents was maximal during high-frequency activation (20 Hz), it could also be detected at low frequencies. The amplitude of excitatory postsynaptic potentials (EPSPs), recorded following stimulation of the corticostriatal pathway, was depressed by sipatrigine (EC(50) 2 microM). This inhibitory action, however, was incomplete; in fact maximal concentrations of this drug reduced EPSP amplitude by only 45%. Sipatrigine produced no increase in paired-pulse facilitation, suggesting that the modulation of a postsynaptic site was the main pharmacological effect of this agent. The inhibition of voltage-dependent Na(+) channels exerted by sipatrigine might account for its depressant effects on both repetitive firing discharge and corticostriatal excitatory transmission. The modulation of Na(+) channels described here, as well as the previously observed inhibition of high-voltage-activated calcium currents, might contribute to the neuroprotective efficacy exerted by this compound in experimental models of in vitro and in vivo ischemia.

Activation of N-methyl-D-aspartate receptors in rat brain in vivo following acute ammonia intoxication: characterization by in vivo brain microdialysis

Ammonia is considered the main agent responsible for the neurological alterations in hepatic encephalopathy. It was suggested that ammonia toxicity is mediated by activation of N-methyl-D-aspartate (NMDA) receptors. The aim of this work was to assess, by in vivo brain microdialysis in freely moving rats, whether acute ammonia intoxication leads to activation of NMDA receptors in the cerebellum of the rat in vivo. We measured the effects of ammonia intoxication on the neuronal glutamate-nitric oxide-cyclic guanosine monophosphate (cGMP) pathway, by measuring the ammonia-induced increase of extracellular cGMP. Ammonia intoxication increases extracellular cGMP, and this increase is prevented by (5R,10S)-5-methyl-10,11-dihydro-5H-dibenzo[a, d]cyclohepten-5,10-imine hydrogen maleate (MK-801). There is a good correlation between the increase in cGMP and the seriousness of the neurological symptoms elicited by different doses of ammonia. Ammonia doses inducing coma did not affect extracellular glutamate, while doses leading to death increased it by 349%. The time courses of ammonia-induced increases in extracellular ammonia, cGMP, and glutamate indicate that NMDA receptor activation occurs before the increase in extracellular glutamate. Ammonia-induced increase in glutamate is prevented by MK-801. These results indicate that ammonia intoxication leads to activation of NMDA receptors in the animal in vivo, and that this activation is not caused by increased extracellular glutamate. The possible underlying mechanism is discussed.

Free radicals are involved in the damage to protein synthesis after anoxia/aglycemia and NMDA exposure

Neuronal protein synthesis is inhibited in CA1 pyramidal neurons for many hours after ischemia, hypoxia or hypoglycemia. This inhibition precedes cell death, is a hallmark characteristic of necrotic damage and may play a key role in the death of vulnerable neurons after these insults. The sequence of events leading to this inhibition remains to be fully elucidated. The protein synthesis failure after 7.5 min anoxia/aglycemia in the rat hippocampal slice can be prevented by blocking N-methyl-D-aspartate receptors in a reduced calcium environment during the insult. In this study, we demonstrate that N-methyl-D-aspartate exposure directly causes a dose-dependent, receptor-mediated and prolonged protein synthesis inhibition in CA1 pyramidal neurons. The free radical scavenger Vitamin E significantly attenuates this damage due to low concentrations of N-methyl-D-aspartate (10 microM). Free radical generation by xanthine/xanthine oxidase (XOD) can directly damage protein synthesis in neurons of the slice. Vitamin E, ascorbic acid and N-acetylcysteine can each prevent the damage due to anoxia/aglycemia and to higher concentrations of N-methyl-D-aspartate (50 microM), provided calcium levels are reduced concomitantly. These findings indicate that both free radicals and calcium play a role in the sequence of events leading to protein synthesis failure after energetic stress like anoxia/aglycemia. They further suggest that the mechanism by which N-methyl-D-aspartate receptor activation damages protein synthesis involves free radical generation.

Novel treatments after experimental brain injury

Perinatal hypoxic-ischaemic encephalopathy(HIE) is being studied in laboratory models that allow the delayed cascade of events triggered by the energetic insult to be examined in detail. The concept of the 'excitotoxic cascade' provides a conceptual framework for thinking about the pathogenesis of HIE. Major events in the cascade triggered by hypoxia-ischaemia include overstimulation of N-methyl-D-aspartate type glutamate receptors, calcium entry into cells, activation of calcium-sensitive enzymes such as nitric oxide synthase, production of oxygen free radicals, injury to mitochondria, leading in turn to necrosis or apoptosis. New experimental approaches to salvaging brain tissue from the effects of HIE include inhibition of neuronal nitric oxide synthase, administration of neuronal growth factors, and inhibition of the caspase enzymes that execute apoptosis. Recent experimental work suggests that these approaches may be effective during a longer 'therapeutic window' after the insult, because they are acting on events that are relatively delayed. Application of modest hypothermia may allow these agents to be neuroprotective at even longer intervals after hypoxia-ischaemia.

Routes of zinc entry in mouse cortical neurons: role in zinc-induced neurotoxicity

Exposure of central neurons to Zn2+ triggers neuronal death. The routes of Zn2+ entry were investigated in living cortical neurons from the mouse using the specific Zn2+ fluorescent dye N-(6-methoxy-8-quinolyl)-p-toluene sulphonamide (TSQ), which preferentially detects membrane-bound Zn2+. Exposure of cortical neurons to increasing concentrations of Zn2+ (1-100 microM) induced a progressive increase in the fluorescence of TSQ. This fluorescence signal was not attenuated by the permeation of plasma membrane with digitonin. Accordingly, the major part of TSQ fluorescence (two-thirds) was associated to the particulate fraction of cortical neurons exposed to Zn2+. These results suggest that Zn2+ detected with TSQ in neurons is mainly bound to membranes. TSQ fluorescence measured in neurons exposed to 3 microM Zn2+ was enhanced by Na+-pyrithione, a Zn2+ ionophore, alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA), N-methyl-D-aspartate (NMDA) or KCl-induced depolarization. However, in the absence of any treatment, TSQ labelling of neurons exposed to 3 microM Zn2+ was only decreased by NMDA receptor antagonists, whereas it remained unaltered in the presence of antagonists of AMPA receptors or L-type voltage-gated Ca2+ channels. Zn2+ entry through NMDA receptors did not contribute to Zn2+-induced neuronal death, as it was prevented by antagonists of NMDA receptors only when they were added after the Zn2+ exposure. Finally, Zn2+ induced a delayed accumulation of extracellular glutamate which might be responsible for the delayed NMDA receptor activation that leads to neuronal death.

Protective effect of aminoguanidine on hypoxic-ischemic brain damage and temporal profile of brain nitric oxide in neonatal rat

Nitric oxide (NO) produced by inducible NO synthase contributes to ischemic brain damage. However, the role of inducible NO synthase-derived NO on neonatal hypoxic-ischemic encephalopathy has not been clarified. We demonstrate here that aminoguanidine, a relatively selective inhibitor of inducible NO synthase, ameliorated neonatal hypoxic-ischemic brain damage and that temporal profiles of NO correlated with the neuroprotective effect of aminoguanidine. Seven-day-old Wister rat pups were subjected to left carotid artery occlusion followed by 2.5 h of hypoxic exposure (8% oxygen). Infarct volumes (cortical and striatal) were assessed 72 h after the onset of hypoxia-ischemia by planimetric analysis of coronal brain slices stained with hematoxylin-eosin. Aminoguanidine (300 mg/kg i.p.), administered once before the onset of hypoxia-ischemia and then three times daily, significantly ameliorated infarct volume (89% reduction in the cerebral cortex and 90% in the striatum; p<0.001). NO metabolites were measured by means of chemiluminescence using an NO analyzer. In controls, there was a significant biphasic increase in NO metabolites in the ligated side at 1 h (during hypoxia) and at 72 h after the onset of hypoxia (p<0.05). Aminoguanidine did not suppress the first peak but significantly reduced the second one (p<0.05), and markedly reduced infarct size in a neonatal ischemic rat model. Suppression of NO production after reperfusion is a likely mechanism of this neuroprotection.

Excitotoxicity of quinolinic acid: modulation by endogenous antagonists

Quinolinic acid (QUIN), a product of tryptophan metabolism by the kynurenine pathway, produces excitotoxicity by activation of NMDA receptors. Focal injections of QUIN can deplete the biochemical markers for dopaminergic, cholinergic, gabaergic, enkephalinergic and NADPH diaphorase neurons, which differ in their sensitivity to its neurotoxic action. This effect of QUIN differs from that of other NMDA receptor agonists in terms of its dependency on the afferent glutamatergic input and its sensitivity to the receptor antagonists. The enzymatic pathway yielding QUIN produces metabolites that inhibit QUIN-induced neurotoxicity. The most active of these metabolites, kynurenic acid (KYNA), blocks NMDA and non-NMDA receptor activity. Treatment with kynurenine hydroxylase and kynureinase inhibitors increases levels of endogenous KYNA in the brain and protects against QUIN-induced neurotoxicity. Other neuroprotective strategies involve reduction in QUIN synthesis from its immediate precursor, or endogenous synthesis of 7-chloro-kynurenic acid, a NMDA antagonist, from its halogenated precursor. Several other tryptophan metabolites--quinaldic acid, hydroxyquinaldic acid and picolinic acid--also inhibit excitotoxic damage but their presence in the brain is uncertain. Picolinic acid is of interest since it inhibits excitotoxic but not neuroexcitatory responses. The mechanism of its anti-excitotoxic action is unclear but might involve zinc chelation. Neurotoxic actions of QUIN are modulated by nitric oxide (NO). Treatment with inhibitors of NO synthase can augment QUIN toxicity in some models of excitotoxicity suggesting a neuroprotective potential of endogenous NO. In recent studies, certain nitroso compounds which could be NO donors, have been reported to reduce the NMDA receptor-mediated neurotoxicity. The existence of endogenous compounds which inhibit excitotoxicity provides a basis for future development of novel and effective neuroprotectants.

In the neuronal cell line SH-SY5Y, oxidative stress-induced free radical overproduction causes cell death without any participation of intracellular Ca(2+) increase

Adding the membrane-permeant oxidant tert-butylhydroperoxide (t-BOOH) to the incubation medium, in SH-SY5Y human neuroblastoma cells, induced a marked and progressive concentration-dependent (300, 500 and 1000 microM) increase of free radical production, as evaluated by the fluorescent probe 2',7'-dichlorodihydrofluorescein diacetate (DCFH-DA) and of the intracellular Ca(2+) ion concentrations [Ca(2+)](i). The removal of extracellular Ca(2+) ions did not prevent t-BOOH-induced [Ca(2+)](i) elevation, whereas the intracellular Ca(2+) ion chelator 1,2-bis(o-aminophenoxy) ethane-N,N, N',N'-tetraacetic acid (BAPTA) (10 microM) was shown to be effective. Both t-BOOH-induced free radical formation and the [Ca(2+)](i) increase were completely prevented by the peroxyl scavenger alpha-tocopherol (50 microM). t-BOOH induced a time-dependent SH-SY5Y cell injury, monitored by a 3-(4,5-dimethylthiazol-2-yl)-2, 5-diphenyltetrazolium bromide (MTT) assay (approximately 25% at 1 h, 50% at 3 h, 80% at 5 h) and by fluorescein diacetate (FDA)-propidium iodide (PI) fluorescent staining. The entity of t-BOOH-induced cell damage was the same both in the absence and in the presence of the intracellular Ca(2+) ion chelator BAPTA. By contrast, the peroxyl scavenger alpha-tocopherol (50 microM) completely prevented cell injury due to oxidative stress. Finally, superoxide dismutase (SOD) (500 ng/ml) caused a 30% reduction of t-BOOH-induced 2', 7'-dichlorofluorescein (DCF) fluorescence, whereas it did not modify the extent of cell injury produced by the oxidant. Collectively, the results of the present study demonstrated that in SH-SY5Y human neuroblastoma cells, the rise of [Ca(2+)](i) which occurs during oxidative stress is not involved in cell injury. Therefore, oxidative stress-induced cell death may be exclusively attributed to free radical overproduction.

Excitotoxic mitochondrial depolarisation requires both calcium and nitric oxide in rat hippocampal neurons

1. Glutamate neurotoxicity has been attributed to cellular Ca2+ overload. As mitochondrial depolarisation may represent a pivotal step in the progression to cell death, we have used digital imaging techniques to examine the relationship between cytosolic Ca2+ concentration ([Ca2+]c) and mitochondrial potential (DeltaPsim) during glutamate toxicity, and to define the mechanisms underlying mitochondrial dysfunction. 2. In cells of > 11 days in vitro (DIV), exposure to 50 mM potassium or 100 microM glutamate had different consequences for DeltaPsim. KCl caused a small transient loss of DeltaPsim but in response to glutamate there was a profound loss of DeltaPsim. In cells of 7-10 DIV, glutamate caused only a modest and reversible drop in DeltaPsim. 3. Using fura-2 to measure [Ca2+]c, responses to KCl and glutamate did not appear significantly different. However, use of the low affinity indicator fura-2FF revealed a difference in the [Ca2+]c responses to KCl and glutamate, which clearly correlated with the loss of DeltaPsim. Neurons exhibiting a profound mitochondrial depolarisation also showed a large secondary increase in the fura-2FF ratio. 4. The glutamate-induced loss of DeltaPsim was dependent on Ca2+ influx. However, inhibition of nitric oxide synthase (NOS) by L-NAME significantly attenuated the loss of DeltaPsim. Furthermore, photolysis of caged NO at levels that had no effect alone promoted a profound mitochondrial depolarisation when combined with high [Ca2+]c, either in response to KCl or to glutamate in cultures at 7-10 DIV. 5. In cells that showed only modest mitochondrial responses to glutamate, induction of a mitochondrial depolarisation by the addition of NO was followed by a secondary rise in [Ca2+]c. These data suggest that [Ca2+]c and nitric oxide act synergistically to cause mitochondrial dysfunction and impaired [Ca2+]c homeostasis during glutamate toxicity.

Glutathione efflux induced by NMDA and kainate: implications in neurotoxicity?

Neurotoxicity in acute as well as chronic neurological diseases may be partly mediated by oxidative stress caused by overactivation of glutamate receptors. A key component of the cellular defense against oxidative stress is reduced glutathione. In our earlier work, we have shown that ischemia in brain induces increased efflux, elevated metabolism, and decreased tissue concentrations of glutathione. In this study, we have evaluated the effect of glutamate receptor activation on the efflux of glutathione from hippocampus in vitro. NMDA and kainate induced a delayed increase in glutathione, taurine, and phosphoethanolamine efflux. Extracellular glutathione was recovered mainly in the reduced form (85-95%); the efflux was dependent on extracellular calcium but unrelated to dantrolene-sensitive intracellular calcium release and independent of glutathione or NO synthesis. The NMDA-induced efflux of glutathione was enhanced by blockage of gamma-glutamyl transpeptidase, indicating an increased transpeptidation of glutathione after NMDA receptor activation. Our results suggest that increased efflux of glutathione could be a factor in initiating nerve cell death via a change in intracellular redox potential and/or a decrease in the intracellular capacity for inactivation of reactive oxygen species.

Ischemic cell death in brain neurons

This review is directed at understanding how neuronal death occurs in two distinct insults, global ischemia and focal ischemia. These are the two principal rodent models for human disease. Cell death occurs by a necrotic pathway characterized by either ischemic/homogenizing cell change or edematous cell change. Death also occurs via an apoptotic-like pathway that is characterized, minimally, by DNA laddering and a dependence on caspase activity and, optimally, by those properties, additional characteristic protein and phospholipid changes, and morphological attributes of apoptosis. Death may also occur by autophagocytosis. The cell death process has four major stages. The first, the induction stage, includes several changes initiated by ischemia and reperfusion that are very likely to play major roles in cell death. These include inhibition (and subsequent reactivation) of electron transport, decreased ATP, decreased pH, increased cell Ca(2+), release of glutamate, increased arachidonic acid, and also gene activation leading to cytokine synthesis, synthesis of enzymes involved in free radical production, and accumulation of leukocytes. These changes lead to the activation of five damaging events, termed perpetrators. These are the damaging actions of free radicals and their product peroxynitrite, the actions of the Ca(2+)-dependent protease calpain, the activity of phospholipases, the activity of poly-ADPribose polymerase (PARP), and the activation of the apoptotic pathway. The second stage of cell death involves the long-term changes in macromolecules or key metabolites that are caused by the perpetrators. The third stage of cell death involves long-term damaging effects of these macromolecular and metabolite changes, and of some of the induction processes, on critical cell functions and structures that lead to the defined end stages of cell damage. These targeted functions and structures include the plasmalemma, the mitochondria, the cytoskeleton, protein synthesis, and kinase activities. The fourth stage is the progression to the morphological and biochemical end stages of cell death. Of these four stages, the last two are the least well understood. Quite little is known of how the perpetrators affect the structures and functions and whether and how each of these changes contribute to cell death. According to this description, the key step in ischemic cell death is adequate activation of the perpetrators, and thus a major unifying thread of the review is a consideration of how the changes occurring during and after ischemia, including gene activation and synthesis of new proteins, conspire to produce damaging levels of free radicals and peroxynitrite, to activate calpain and other Ca(2+)-driven processes that are damaging, and to initiate the apoptotic process. Although it is not fully established for all cases, the major driving force for the necrotic cell death process, and very possibly the other processes, appears to be the generation of free radicals and peroxynitrite. Effects of a large number of damaging changes can be explained on the basis of their ability to generate free radicals in early or late stages of damage. Several important issues are defined for future study. These include determining the triggers for apoptosis and autophagocytosis and establishing greater confidence in most of the cellular changes that are hypothesized to be involved in cell death. A very important outstanding issue is identifying the critical functional and structural changes caused by the perpetrators of cell death. These changes are responsible for cell death, and their identity and mechanisms of action are almost completely unknown.

Peroxynitrite mediated damage and lowered superoxide tolerance in primary cortical glial cultures after induction of the inducible isoform of NOS

The effect of the induction of i-NOS in primary glial cultures was studied with respect to the protein levels of reactive oxygen species (ROS) scavenging enzymes and the cytotoxicity of nitric oxide (.NO) formation at different levels of artificially generated superoxide. Stimulation of the cultures by bacterial lipopolysaccharides and gamma-interferon resulted in an induction of i-NOS exclusively in microglial cells. Among the ROS scavenging enzymes superoxide dismutase (Cu/Zn- and Mn-isoform), glutathione peroxidase and catalase only mitochondrial Mn-SOD was found to be upregulated in the course of i-NOS induction (Western blots). Although .NO formation did not affect cell viability at physiological levels of superoxide over a time period of 4 days, it caused an oxidative load particularly in microglial cells as observed by monitoring the oxidation of dichloro-dihydrofluorescein, an indicator for the formation of peroxynitrite and ROS. Elevated levels of superoxide, generated either intracellularly by paraquat or extracellularly via xanthine oxidase and hypoxanthine, resulted dose-dependently in a larger decline of cell viability in the .NO forming cultures compared to controls (release of lactate dehydrogenase, citrate synthase, stainability by propidium iodide, and tetramethylrhodamine). NOS-inhibitors reduced the degree of cell damage to that seen for control cultures, indicating an ONOO--/.NO mediated mechanism of cell damage. Our data support the concept that i-NOS catalyzed .NO-formation leads to an ONOO--mediated increased oxidative load. At physiological levels of superoxide and within a wide range of higher superoxide levels this nitrosative stress is well balanced in cultured glial cells by protective mechanisms.

Pyruvate and lactate protect striatal neurons against N-methyl-D-aspartate-induced neurotoxicity

A sustained release of glutamate contributes to neuronal loss during cerebral ischaemia. Using cultured mouse striatal neurons, we observed that glucose deprivation, which occurs in this pathological process, enhanced the N-Methyl-D-aspartate (NMDA)- or alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA)-induced neurotoxicity. The end products of glycolysis, lactate and pyruvate, strongly protected neurons from these neurotoxic effects. The neuroprotective effect of pyruvate (which is more prominent in the absence of glucose) was not related to its ability to react with H2O2 by a decarboxylation process. Pyruvate and L-lactate strongly counteracted the deep decrease in the neuronal ATP content induced by NMDA, indicating that they might protect striatal neurons by rescuing cellular energy charge. Addition of MK-801 after the NMDA withdrawal completely protected neurons, suggesting that NMDA neurotoxicity resulted from a delayed NMDA receptor activation probably linked to a delayed release of an endogenous agonist in the extracellular medium. The strong accumulation of extracellular glutamate which was found in both sham and NMDA-treated cultures was markedly decreased by pyruvate. Thus, pyruvate might also exert its protecting activity by decreasing the delayed accumulation of glutamate which seemed to be neurotoxic only after a preexposure of neurons to NMDA.

Increase in external glutamate and NMDA receptor activation contribute to H2O2-induced neuronal apoptosis

The present study aims to investigate the role of extracellular glutamate and NMDA receptor stimulation in the neuronal death induced by a transient exposure to H2O2 of cultured neurons originating from mouse cerebral cortex. Most of the neuronal loss following a transient exposure to H2O2 of cortical neurons results from an apoptotic process involving a secondary stimulation of NMDA receptors, which occurs after H2O2 washout. Indeed, (a) the neurotoxic effect of H2O2 was strongly reduced by antagonists of NMDA receptors, (b) the neurotoxic effect of H2O2 was enhanced in the absence of Mg2+, (c) the protective effect of MK-801 progressively decayed when it was applied with increasing delay time after H2O2 exposure, and (d), finally, the extracellular concentration of glutamate was increased after H2O2 exposure. The major part of H2O2-induced neurotoxicity is mediated by the formation of hydroxyl radicals, which might be involved in (a) the delayed accumulation of extracellular glutamate and NMDA receptor activation and (b) the poly(ADP-ribose) polymerase activation and the related NAD content decrease. The combination of these two mechanisms could lead to both an increase in ATP consumption and a decrease of ATP synthesis. The resulting large decrease in ATP content might be finally responsible for the neuronal death.

Role of Na+/H+ exchangers, excitatory amino acid receptors and voltage-operated Ca2+ channels in human immunodeficiency virus type 1 gp120-mediated increases in intracellular Ca2+ in human neurons and astrocytes

Human immunodeficiency virus type 1 (HIV-1) dementia is the commonest form of dementia in North American people less than 60 years of age. HIV-1 envelope glycoprotein gp120 has been implicated in the neurotoxicity observed in, and the pathogenesis of, HIV-1 dementia. Recombinant gp120 (gp120) was pressure-applied on to cultured human fetal neurons and astrocytes and, by using single-cell calcium imaging, we determined the mechanisms responsible for gp120-induced increases in the levels of intracellular calcium ([Ca2+]i). Significant dose-related increases in [Ca2+]i were observed in neurons and astrocytes. In neurons, 5 pM gp120 increased [Ca2+]i by 290+/-13 nM and increases of 2210+/-211 nM were found at 209 nM, the highest concentration of gp120 tested. The apparent EC50 value for gp120 of 223+/-40 pM in neurons was not significantly different from that in astrocytes. Immunoelution of gp120 with polyclonal anti-gp120 and Ca2+-free conditions blocked increases in [Ca2+]i by gp120. Increases in [Ca2+]i were significantly (P < 0.005) attenuated by the Na+/H+ exchange blocker 5-(N-methyl-N-isobutyl)-amiloride in neurons and astrocytes. The L-type calcium channel blockers nimodipine, diltiazem and CdCl2 + NiCl2 significantly (P < 0.005) reduced increases in [Ca2+]i in neurons, but not astrocytes. Increases in [Ca2+]i by gp120 were not significantly affected by blockers of N-, P- and Q-type calcium channels. The N-methyl-D-aspartate receptor antagonists (+/-)-2-amino-5-phosphonopentanoic acid (AP5), memantine and dizocilpine significantly (P < 0.01) lowered gp120-induced increases in [Ca2+]i in neurons. AP5 and memantine, but not dizocilpine, significantly (P < 0.01) reduced increases in [Ca2+]i by gp120 in astrocytes. Gp120 appears to activate astrocyte Na+/H+ exchangers to release glutamate and potassium and, subsequent to this, increases in [Ca2+]i in neurons and astrocytes result from activation of excitatory amino acid receptors on astrocytes and neurons, and voltage-operated calcium channels on neurons. Drugs that block gp120-induced changes in [Ca2+]i in neurons and astrocytes may help in the treatment of HIV-1 dementia.

Effect of arginine on basal and high potassium-induced efflux of [3H]D-aspartate from rat striatal slices

There are conflicting reports in the literature regarding the effects of nitric oxide as well as the involvement of the cyclic GMP pathway on the transmitter release. To study the influence of the availability of the nitric oxide precursor arginine on the glutamate transmission process, rat striatal slices preloaded with the tritiated glutamate analogue D-aspartate were used. L-Arginine stimulated in a concentration-dependent way (0.01-10.0 mM) the high potassium-induced efflux of [3H]D-aspartate. The basal release was increased only by 10 mM L-arginine. Neither the basal nor the depolarization-induced efflux of [3H]D-aspartate was affected by D-arginine. The L-arginine effect was abolished by the nitric oxide synthase inhibitor L-arginine methyl ester and was not modified by cyclic GMP. Only at high concentrations of L-arginine (10 mM) could an elevation of cyclic GMP level be demonstrated. The results are discussed in terms of direct presynaptic action of nitric oxide on [3H]D-aspartate efflux and a possible modulation of glutamate release by the availability of arginine.

Nitric oxide and mitochondrial respiration

Nitric oxide (NO) and its derivative peroxynitrite (ONOO-) inhibit mitochondrial respiration by distinct mechanisms. Low (nanomolar) concentrations of NO specifically inhibit cytochrome oxidase in competition with oxygen, and this inhibition is fully reversible when NO is removed. Higher concentrations of NO can inhibit the other respiratory chain complexes, probably by nitrosylating or oxidising protein thiols and removing iron from the iron-sulphur centres. Peroxynitrite causes irreversible inhibition of mitochondrial respiration and damage to a variety of mitochondrial components via oxidising reactions. Thus peroxynitrite inhibits or damages mitochondrial complexes I, II, IV and V, aconitase, creatine kinase, the mitochondrial membrane, mitochondrial DNA, superoxide dismutase, and induces mitochondrial swelling, depolarisation, calcium release and permeability transition. The NO inhibition of cytochrome oxidase may be involved in the physiological regulation of respiration rate, as indicated by the finding that isolated cells producing NO can regulate cellular respiration by this means, and the finding that inhibition of NO synthase in vivo causes a stimulation of tissue and whole body oxygen consumption. The recent finding that mitochondria may contain a NO synthase and can produce significant amounts of NO to regulate their own respiration also suggests this regulation may be important for physiological regulation of energy metabolism. However, definitive evidence that NO regulation of mitochondrial respiration occurs in vivo is still missing, and interpretation is complicated by the fact that NO appears to affect tissue respiration by cGMP-dependent mechanisms. The NO inhibition of cytochrome oxidase may also be involved in the cytotoxicity of NO, and may cause increased oxygen radical production by mitochondria, which may in turn lead to the generation of peroxynitrite. Mitochondrial damage by peroxynitrite may mediate the cytotoxicity of NO, and may be involved in a variety of pathologies.

Involvement of P2 purinoceptors and the nitric oxide pathway in [3H]purine outflow evoked by short-term hypoxia and hypoglycemia in rat hippocampal slices

The objective of this study was to study how the outflow of [3H]purines is altered during a brief period of ischemic-like conditions in superfused hippocampal slices and to show whether it is regulated by P2 purinoceptors and the nitric oxide (NO) pathway. The outflow of [3H]purines increased in response to 5 min of combined hypoxia/hypoglycemia. High performance liquid chromatography analysis verified the efflux of [3H]adenosine-triphosphate, [3H]adenosine-diphosphate, [3H]adenosine-monophosphate, [3H]adenosine, [3H]inosine, and [3H]hypoxanthine in response to ischemic-like conditions. The P2 receptor antagonists suramin and pyridoxal-phosphate-6-azophenyl-2'-4'-disulphonic-acid-tetrasodium (PPADS) reduced significantly the [3H]purine efflux evoked by ischemic-like conditions, showing that P2 purinoceptors are involved in the initiation of purine outflow. The NO synthase inhibitor N-nitro-l-arginine-methyl-ester (l-NAME) attenuated significantly the [3H]purine outflow, evoked by ischemic-like conditions, while 7-nitroindazole (7-NI) caused only a mild decrease in the outflow. The NO donor sodium nitroprusside increased significantly the basal efflux of [3H]purines. In summary, a brief period of combined hypoxia/hypoglycemia induced the efflux of ATP in addition to the outflow of other purines. Since P2 receptor antagonists decreased the [3H]purine outflow evoked by ischemic-like conditions we propose that ATP, acting on P2 purinoceptors, is responsible for further efflux of purines after ischemic-like period. It seems likely that NO is also involved in the regulation of purine outflow, since inhibition of NO production attenuated the [3H]purine outflow, evoked by ischemic-like conditions, while exogenous NO facilitated the basal outflow.

Role of endogenous nitric oxide and peroxynitrite formation in the survival and death of motor neurons in culture

Motor neuron survival is highly dependent on trophic factor supply. Deprivation of trophic factors results in induction of neuronal NOS, which is also found in pathological conditions. Growing evidence suggests that motor neuron degeneration involves peroxynitrite formation. Trophic factors modulate peroxynitrite toxicity (Estévez et al., 1995; Shin et al., 1996; Spear et al., 1997). Whether a trophic factor prevents or potentiates peroxynitrite toxicity depends upon when the cells are exposed to the trophic factor (Table 1). These results strongly suggest that a trophic factor that can protect neurons under optimal conditions, but under stressful conditions can increase cell death. In this context, it is possible that trophic factors or cytokines produced as a response to damage may potentiate rather than prevent motor neuron death. A similar argument may apply to the therapeutic administration of trophic factors to treat neurodegenerative diseases. Similarly, the contrasting actions of NO on motor neurons may have important consequences for the potential use of nitric oxide synthase inhibitors in the treatment of ALS and other related neurodegenerative diseases.

Excitotoxic death of a subset of embryonic rat motor neurons in vitro

We have used cultures of purified embryonic rat spinal cord motor neurons to study the neurotoxic effects of prolonged ionotropic glutamate receptor activation. NMDA and non-NMDA glutamate receptor agonists kill a maximum of 40% of the motor neurons in a concentration- and time-dependent manner, which can be blocked by receptor subtype-specific antagonists. Subunit-specific antibodies stain all of the motor neurons with approximately the same intensity and for the same repertoire of subunits, suggesting that the survival of the nonvulnerable population is unlikely to be due to the lack of glutamate receptor expression. Extracellular Ca2+ is required for excitotoxicity, and the route of entry initiated by activation of non-NMDA, but not NMDA, receptors is L-type Ca2+ channels. Ca2+ imaging of motor neurons after application of specific glutamate receptor agonists reveals a sustained rise in intracellular Ca2+ that is present to a similar degree in most motor neurons, and can be blocked by appropriate receptor/channel antagonists. Although the lethal effects of glutamate receptor agonists are seen in only a subset of cultured motor neurons, the basis of this selectivity is unlikely to be simply the glutamate receptor phenotype or the level/pattern of rise in agonist-evoked intracellular Ca2+.

mGluR7-like metabotropic glutamate receptors inhibit NMDA-mediated excitotoxicity in cultured mouse cerebellar granule neurons

Glutamate-induced glutamate release may be involved in the delayed neuronal death induced by N-methyl-D-aspartate (NMDA). In order to examine a possible modulatory effect of the presynaptic group III mGluRs on glutamate excitotoxicity, the effect of L-2-amino-4-phosphonobutyrate (L-AP4) was examined on NMDA-induced delayed death of mouse cerebellar granule neurons in culture. We found that L-AP4, at high concentration (in the millimolar range), inhibited in a non-competitive manner the NMDA-induced toxicity. This effect was mimicked by high concentration of L-serine-o-phosphate (L-SOP), and was inhibited by pertussis toxin (PTX) indicating the involvement of a Gi/o protein. This suggests the involvement of mGluR7 in the L-AP4 effect, and this was consistent with the detection of both mGluR7 protein and mRNA in these cultured neurons. To examine the mechanism of the L-AP4-induced protection from excitotoxic damage, the effect of L-AP4 on glutamate release was examined. L-AP4 (> or = 1 mM) noncompetitively inhibited by more than 60% the glutamate release induced by NMDA during the insult. We also observed that the 10-min NMDA receptor stimulation resulted in a dramatic increase in the extracellular glutamate concentration reaching 6000% of the control value 24 h after the insult. This large increase was also inhibited when NMDA was applied in the presence of > or = 1 mM L-AP4. Part of the L-AP4-induced protection from excitotoxic damage of granule neurons may therefore result from the inhibition of the vicious cycle: dying cells release glutamate, glutamate induced cell death. The present results add to the hypothesis that presynaptic mGluRs, probably mGluR7, may be the targets of drugs decreasing glutamate release and then neuronal death observed in some pathological situations.

Characterization of mitochondrial glutaminase and amino acids at prolonged times after experimental focal cerebral ischemia

The mitochondrial enzyme glutaminase is a significant contributor to extracellular glutamate after neuronal injury in vitro [R. Newcomb, X. Sun, L. Taylor, N. Curthoys, R.G. Giffard, Increased production of extracellular glutamate by the mitochondrial glutaminase following neuronal death, J. Biol. Chem. 272 (1997) 11276-11282.]. As a step towards characterizing the role of the enzyme in neuronal injury in vivo, glutaminase activity was measured in central and peripheral regions of the ischemic distribution in rat brain at 6, 24, and 48 h after permanent focal ischemia. Although glutaminase activity decreases in the central ischemic area, significant activity remains in peripheral areas of evolving damage, even after 24 and 48 h ischemia. Western blots show no detectable change in glutaminase molecular weight or total immunoreactivity, regardless of the degree of inactivation. Significant amounts of glutamine remain in ischemic tissue at prolonged times after focal ischemia, while reductions in tissue amounts of glutamate are highly correlated with decreases in glutaminase activity. In vivo microdialysis probes were inserted into the ischemic periphery after 24 h focal ischemia. Glutamate is significantly elevated in these dialysates. Perfusion of the glutaminase substrate glutamine and the enzyme activator phosphate results in further and specific elevations in dialysate glutamate. In sum, significant mitochondrial glutaminase activity remains in the periphery of the ischemic lesion at 24 and 48 h, where it can contribute directly to elevated extracellular glutamate. Inactivation of the glutaminase in central areas of the ischemic lesion does not involve significant proteolytic degradation, and likely involves a specific molecular event.

Absence of nitric oxide synthase in rat oligodendrocytes: a light and electron microscopic study

The existence of nitric oxide synthase (NOS) in oligodendroglial cells is still a matter of debate. Therefore, by using immunocytochemistry, NADPH-diaphorase (NADPH-d) histochemistry and in-situ hybridization, NOS was analysed in oligodendroglial cells of rat brain tissue and in tissue cultures. Independent of the method utilized, NOS was found to be absent in oligodendrocytes. An exception was the presence of NADPH-d activity in some oligodendroglial cells when the electron microscopic BSPT-technique was used. However, this activity is probably not related to NOS. The absence of NO-producing capacity in these cells may explain their high vulnerability to radical-mediated toxicity.

Mechanisms of neuronal damage in brain hypoxia/ischemia: focus on the role of mitochondrial calcium accumulation

Following a hypoxic-ischemic insult, the collapse of ion gradients results in the inappropriate release of excitatory neurotransmitters. Although excitatory amino acids such as glutamate are the likely extracellular mediators of the ensuing neuronal cell death, the intracellular events occurring downstream of glutamate receptor activation are much less clear. The present review attempts to summarize how Ca2+ overload of neurons following a hypoxic-ischemic insult is neurotoxic. In particular, the interlocked relation between mitochondrial Ca2+ accumulation and subsequent neuronal cell death is examined.

Regional selective neuronal degeneration after protein phosphatase inhibition in hippocampal slice cultures: evidence for a MAP kinase-dependent mechanism

The regional selectivity and mechanisms underlying the toxicity of the serine/threonine protein phosphatase inhibitor okadaic acid (OA) were investigated in hippocampal slice cultures. Image analysis of propidium iodide-labeled cultures revealed that okadaic acid caused a dose- and time-dependent injury to hippocampal neurons. Pyramidal cells in the CA3 region and granule cells in the dentate gyrus were much more sensitive to okadaic acid than the pyramidal cells in the CA1 region. Electron microscopy revealed ultrastructural changes in the pyramidal cells that were not consistent with an apoptotic process. Treatment with okadaic acid led to a rapid and sustained tyrosine phosphorylation of the mitogen-activated protein kinases ERK1 and ERK2 (p44/42(mapk)). The phosphorylation was markedly reduced after treatment of the cultures with the microbial alkaloid K-252a (a nonselective protein kinase inhibitor) or the MAP kinase kinase (MEK1/2) inhibitor PD98059. K-252a and PD98059 also ameliorated the okadaic acid-induced cell death. Inhibitors of protein kinase C, Ca2+/calmodulin-dependent protein kinase II, or tyrosine kinase were ineffective. These results indicate that sustained activation of the MAP kinase pathway, as seen after e.g., ischemia, may selectively harm specific subsets of neurons. The susceptibility to MAP kinase activation of the CA3 pyramidal cells and dentate granule cells may provide insight into the observed relationship between cerebral ischemia and dementia in Alzheimer's disease.

Design of isoform-selective inhibitors of nitric oxide synthase

Nitric oxide synthase, the mammalian enzyme catalyzing the oxidation of L-arginine to L-citrulline and nitric oxide, is present in three isoforms that have distinct physiological roles. Overstimulation or overexpression of individual nitric oxide synthase isoforms plays a role in a wide range of disorders including septic shock, arthritis, diabetes, ischemia-reperfusion injury, pain and various neurodegenerative diseases. Animal studies and early clinical trials suggest that nitric oxide synthase inhibitors could be therapeutic in many of these disorders, but preservation of physiologically important nitric oxide synthase functions might require use of isoform-selective inhibitors. Within the past few years both amino acid and nonamino acid nitric oxide synthase inhibitors with pharmacologically useful isoform selectivity have been reported. Selectivity has been achieved on the basis of initial binding affinity and, for mechanism-based inactivators, on the basis of isoform-dependent catalytic activation; particularly interesting are N5-(1-imino-3-butenyl)-L-ornithine, ARL 17477, 1400W and S-(2-aminoethyl)isothiourea.

Differential expression of superoxide dismutase isoforms in neuronal and glial compartments in the course of excitotoxically mediated neurodegeneration: relation to oxidative and nitrergic stress

To examine the cellular distribution of radical scavenging enzymes in glia, in comparison to that in neurons and their behaviour during excitotoxically induced neurodegenerative processes, protein levels and the cellular localization of cytosolic and mitochondrial superoxide dismutase (Cu/Zn- and Mn-SOD) were investigated in the rat brain undergoing quinolinic acid (Quin)-induced neurodegeneration. Evidence for the specificity of the applied antibodies to detect immunocytochemically these SOD isoforms was obtained from electron microscopy and Western blotting. In control striatum Mn-SOD was clearly confined to neurons, whereas Cu/Zn-SOD was found, rather delicately, only in astrocytes. Microglia failed to stain with antibodies to both SOD isoforms. Quin application resulted in an initial formation of oxygen and nitrogen radicals as determined by the decline in the ratio of ascorbic to dehydroascorbic acid and by increased levels of nitrated proteins, an indicator for elevated peroxynitrite formation. Morphologically, massive neuronal damage was seen in parallel. Astroglia remained intact but showed initially decreased glutamine synthetase activities. The levels of Mn-SOD protein increased 2-fold 24 h after Quin injection (Western blotting) and declined only slowly over the time period considered (10 days). Cu/Zn-SOD levels increased only 1.3-fold. Immunocytochemical studies revealed that the increase in Mn-SOD is confined to neurons, whereas that of Cu/Zn-SOD was observed only in astroglial cells. Quiescent microglial cells were, as a rule, free of immunocytochemically detectable SOD, whereas in activated microglia a few Mn-SOD immunolabeled mitochondria occurred. Our results suggest a differential protective response in the Quin lesioned striatum in that Mn-SOD is upregulated in neurons and Cu/Zn-SOD in astroglia. Both SOD-isoforms are assumed to be induced to prevent oxidative and nitric oxide/peroxynitrite-mediated damage. In the border zone of the lesion core this strategy may contribute to resist the noxious stimulus.

Melatonin prevents the delayed death of hippocampal neurons induced by enhanced excitatory neurotransmission and the nitridergic pathway

The mechanisms by which neurons die after stroke and status epilepticus and related neuropathological conditions are unclear, but may involve voltage-dependent Na+ channels, glutamate receptors, and nitric oxide (NO.). These questions were investigated using an in vitro primary cell culture model in which hippocampal pyramidal neurons undergo a gradual and delayed neurodegeneration induced by enhanced excitatory neurotransmission. When cells were treated with Mg2+-free, glycine-supplemented medium for a brief period (15 min) and examined 24 h later, approximately 30-40% of the neurons had died. Cell death could be inhibited by blockers of voltage-sensitive Na+ channels and by N-methyl-D-aspartate receptor antagonists. Application of either the endogenous antioxidant melatonin (EC50: 19.2+/-2.8 microM) or the NO. synthase inhibitor Nomega-nitro-L-arginine after, but not during, Mg2+-free exposure protected against delayed neuronal death; significant neuroprotection was observed when the addition was delayed for up to 4 h. This operational time window suggests that an enduring production of NO. and reactive oxygen species from neuronal sources is responsible for delayed cell death. A role for reactive oxygen species in this injury process was strengthened by the finding that, whereas neurons cocultured with astroglia were more resistant to killing, agents capable of lowering intracellular glutathione negated this protection. Because secretion levels of melatonin are decreased with aging, reductions in this pineal hormone may place neurons at a heightened risk for damage by excitatory synaptic transmission.

Nitric oxide-dependent production of cGMP supports the survival of rat embryonic motor neurons cultured with brain-derived neurotrophic factor

Trophic factor deprivation induces neuronal nitric oxide synthase (NOS) and apoptosis of rat embryonic motor neurons in culture. We report here that motor neurons constitutively express endothelial NOS that helps support the survival of motor neurons cultured with brain-derived neurotrophic factor (BDNF) by activating the nitric oxide-dependent soluble guanylate cyclase. Exposure of BDNF-treated motor neurons to nitro-L-arginine methyl ester (L-NAME) decreased cell survival 40-50% 24 hr after plating. Both low steady-state concentrations of exogenous nitric oxide (<0.1 microM) and cGMP analogs protected BDNF-treated motor neurons from death induced by L-NAME. Equivalent concentrations of cAMP analogs did not affect cell survival. Inhibition of nitric oxide-sensitive guanylate cyclase with 2 microM 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ) reduced the survival of BDNF-treated motor neurons by 35%. cGMP analogs also protected from ODQ-induced motor neuron death, whereas exogenous nitric oxide did not. In all cases, cell death was prevented with caspase inhibitors. Our results suggest that nitric oxide-stimulated cGMP synthesis helps to prevent apoptosis in BDNF-treated motor neurons.

Hyperpolarizing after-potentials regulate generation of long-duration plateau depolarizations in rat paraventricular nucleus neurons

Activation of N-methyl-D-aspartate (NMDA) receptors in a population of neurons of the paraventricular nucleus (PVN) results in long-duration plateau depolarizations during which the membrane rapidly depolarizes, reaching a stable plateau near -20 mV. These responses were observed in 29% of the Type II PVN neurons tested with 1 microM NMDA agonist (n = 84). The stable plateau phase is characterized by an increase in ionic conductance, from 1.19+/-0.11 nS to 5.24+/-2.17 nS (n = 5). Bath application of tetrodotoxin (n = 4) or alternatively inclusion of QX-314 in the pipette solution (n = 3) prevented the generation of these events. The remaining cells tested (n = 56) also depolarized in response to NMDA agonist, but long duration plateau depolarizations were not observed. Previous evidence from hypothalamic cultures has demonstrated synaptically driven plateau potentials following the blockade of repolarizing conductances. Pharmacological blockade of the post-spike hyperpolarizing afterpotential with 4-aminopyridine (200 microM), in cells that did not generate plateaux, resulted in the observance of long duration plateau depolarizations in response to a subsequent application of NMDA agonist (n = 4). Our results demonstrate that this 4-aminopyridine-sensitive ionic conductance plays a critical role in determining whether a cell will depolarize for a prolonged duration in response to NMDA receptor activation. As a prolonged depolarization of the postsynaptic membrane and accompanying membrane permeability changes are essential for neurotoxicity, these findings provide evidence for a potential protective mechanism that depends solely on the ability of the cell, through its ionic conductances, to control imposed changes in membrane potential.

Alteration of nitric oxide synthase activity upon oxidative stress in cultured retinal cells

In the present study we examined the effect of oxidative stress-mediated hydroperoxide formation on the activity of nitric oxide synthase (NOS) in retinal cells in culture. Oxidative stress was induced in the presence of Fe2+ and ascorbate or Fe2+ alone and compared to H2O2-induced maximal cellular oxidation, and was measured by following the formation of intracellular hydroperoxides with the probe DCFH2 (2',7'-dichlorodihydrofluorescein). After a 15-min exposure to the oxidants, formation of hydroperoxides was significantly increased in the presence of 100 microM Fe2+ (about twofold), as compared to the control. Coadministration of Fe2+ and ascorbate (Fe-Asc) did not affect DCF fluorescence, but highly reduced the intracellular pH (pHi = 6.32 +/- 0.08), in comparison with control conditions (pHi = 7.05 +/- 0.11), as determined with the probe BCECF (2',7'-bis-(carboxyethyl)-5(and-6) carboxyfluorescein). Nevertheless, preincubation of Fe-Asc at acidic pH also increased the formation of hydroperoxides. Oxidative stress induced in the presence of Fe-Asc (at pH 6.5) significantly decreased the activity of NOS by 20% of control activity, as determined by the formation of [14C]citrulline. Fe-Asc (pH 6.5) also reduced the production of cyclic GMP (cGMP) in retinal cells by 1.5-fold, although a decrement in pH from 7.4 to 6.5 was not sufficient to decrease cGMP production. These data suggest that NO. production may be compromised in the presence of Fe-Asc. Moreover, neither 4 mM dithiotreitol (DTT) nor 4 mM glutathione (GSH) altered the production of cGMP in retinal cells submitted to oxidative stress. A reduction in NO. generation upon oxidative stress may reduce major damaging effects induced by ONOO- in cultured retinal cells.

Ion channels and exchangers that mediate ischemic neuronal injury

Preventing the loss of ion homeostasis or promoting its recovery are two primary targets of neuroprotective strategies. The contribution of excitatory neurotransmitter receptor linked ion channels is now well established. Sodium and calcium may also enter neurons and glia through voltage sensitive channels and exchangers, contributing directly and indirectly to injury.

Cyclic guanosine 3',5'-monophosphate-mediated neuroprotection by nitric oxide in dissociated cultures of rat dorsal root ganglion neurones

In dissociated cultures of dorsal root ganglia (DRG) derived from 15-day-old rats, many neurones expressed nitric oxide synthase (NOS) and this expression was found to be reduced by nerve growth factor. The application of blockers of NOS caused selective death of those neurones expressing NOS. The soluble guanylate cyclase (sGC) blocker ODQ also caused neuronal death. The appearance of the neurones undergoing cell death was typical of apoptosis. This suggests that NO has a neuroprotective action in DRG neurones which is probably mediated by its activation of cyclic guanosine 3',5'-monophosphate. These observations are discussed in relation to the developmental and neuropathic changes in NOS expression by DRG neurones.

Differential inhibition by riluzole, lamotrigine, and phenytoin of sodium and calcium currents in cortical neurons: implications for neuroprotective strategies

Among the several classes of drugs currently studied as neuroprotective agents, glutamate release blockers have been indicated as being rather effective. In particular, lamotrigine and riluzole have shown promise in the treatment of either acutely developing cellular damages (stroke, posttraumatic lesions) or slowly progressing neurodegenerative diseases as amyotrophic lateral sclerosis. These drugs are supposed to interfere with the release of endogenous glutamate in situ, yet the mechanisms underlying this effect are not fully defined. One possibility is that lamotrigine and riluzole act by inhibiting voltage-dependent inward conductances active in the soma and/or in the axon terminal region. Therefore, we have investigated the effects of lamotrigine and riluzole on the voltage-gated sodium and calcium currents of acutely isolated neurons from the adult rat neocortex. In addition, since phenytoin is a well-known blocker of the sodium channel, we have compared lamotrigine and riluzole responses with the peak current inhibition produced by phenytoin in the same cells. Lamotrigine produced a large reduction of the high-voltage-activated calcium currents and a smaller; use-dependent inhibition of the sodium conductance. Riluzole inhibited significantly the sodium current at surprisingly low concentrations (nanomolar range) and by up to 80% at saturating doses (1-10 microM). Furthermore, riluzole inhibited both high- and low-voltage-activated calcium currents in neocortical neurons isolated from adult and young animals. By contrast, phenytoin caused only a slight reduction of high-voltage-activated calcium currents even at supratherapeutic doses (by < 12% at 10 microM). Taken together, the different pharmacological profiles of the tested agents might indicate that glutamate release blockers do not represent a homogenous class of drugs. Conversely, our findings could support their selective utilization in different disease status.

Sodium channel modulators prevent oxygen and glucose deprivation injury and glutamate release in rat neocortical cultures

Neocortical cultures were deprived of oxygen and glucose to model ischemic neuronal injury. We used a graded series of periods of oxygen and glucose deprivation, providing graded insults. Cell death was measured by release of lactate dehydrogenase (LDH). One hundred and twenty to 240 min of deprivation caused graded increases in glutamate overflow, LDH release and 45Ca influx. Curves of LDH release with respect to deprivation time were shifted to longer intervals by treatment with tetrodotoxin (TTX; 3, 30 or 300 nM), phenytoin (10, 30 or 100 microM), lidocaine (10, 30 or 100 microM) or the N-methyl-D-aspartate antagonist CPP [3(2-carboxypiperazine-4-yl)propyl-1-phosphonic acid, 3, 10, 30 or 100 microM]. Combined treatment with TTX and CPP caused pronounced rightward shifts of LDH deprivation curves. Our results indicate that Na+ channel blockade is neuroprotective in neocortex cultures. Our results also suggest that neuroprotection with Na+ channel blockers may be due to inhibition of glutamate release.

Neuroprotective effects of the antioxidant LY231617 and NO synthase inhibitors in global cerebral ischaemia

Recent studies have shown that the novel antioxidant LY231617 protects against ischaemia-induced neuronal damage in rat models of global cerebral ischaemia. In the present studies we have examined the effects of LY231617 in the gerbil model of global cerebral ischaemia. We also examined the effects of four nitric oxide synthase inhibitors (3-bromo-7-nitroindazole, N(G)-nitro-L-arginine methyl ester, aminoguanidine and S-methylisothiourea sulphate) in this model. LY231617 (50 mg/kg p.o. or 30 mg/kg i.p.) was administered either 30 min prior to occlusion or immediately post-occlusion followed by three further doses at 4, 24 and 48 h after the initial dose. 3-Bromo-7-nitroindazole was administered at 40 mg/kg i.p. immediately after occlusion followed by 20 mg/kg i.p. at 3, 6, 24 and 48 h, N(G)-nitro-L-arginine methyl ester was administered at 10 mg/kg i.p. immediately after occlusion followed by 5 mg/kg i.p. at 3, 6, 24 and 48 h. Aminoguanidine was administered at 80 mg/kg i.p. immediately after occlusion followed by 40 mg/kg i.p. at 3, 6, 24 and 48 h and S-methylisothiourea sulphate was administered at 10 mg/kg i.p. immediately, 3, 6, 24 and 48 h after occlusion. We also examined the effects of aminoguanidine administered at 80 mg/kg i.p. immediately after occlusion followed by 40 mg/kg i.p. at 3, 6, 24, 48, 72 and 96 h and S-methylisothiourea sulphate administered at 10 mg/kg i.p. immediately, 3, 6, 24, 48, 72 and 96 h after occlusion. Control animals were either sham operated or subjected to 5 min bilateral carotid occlusion. Extensive neuronal death was observed in the CA1 layer of the hippocampus in 5-min bilateral carotid artery occluded animals 5 days after surgery. LY231617 provided significant neuroprotection against the ischaemia-induced brain damage when administration was initiated before or after occlusion (P < 0.05). The neuronal NO synthase inhibitors, 3-bromo-7-nitroindazole and a general NO synthase inhibitor, N(G)-nitro-L-arginine methyl ester also provided significant neuroprotection (P < 0.05). In contrast aminoguanidine and S-methylisothiourea sulphate (two inducible NO synthase inhibitors) failed to protect against the ischaemia-induced brain damage. These results indicate that free radicals and nitric oxide are involved in ischaemia-induced brain damage following global cerebral ischaemia. Antioxidants such as LY231617 or neuronal NO synthase inhibitors can prevent the ischaemia-induced neurodegeneration and may be useful as anti-ischaemic agents.

Glial cell line-derived neurotrophic factor protects against ischemia-induced injury in the cerebral cortex

Glial cell line-derived neurotrophic factor (GDNF), a recently described and cloned member of the transforming growth factor (TGF)-beta superfamily, has been shown to have marked trophic activity on several populations of central neurons. Survival-promoting and injury protectant activity in vitro and in vivo, using several paradigms, has been demonstrated for ventral mesencephalic dopaminergic neurons and spinal cord motoneurons. In view of a proposed commonality of mechanisms, involving intracellular free radical generation, depolarization-induced Ca2+ influx, and mitochondrial respiratory enzyme injury, between such GDNF-responsive paradigms and those of ischemia-induced injury, we tested the effects of GDNF on the extent of neural degeneration induced by transient middle cerebral artery (MCA) occlusion. We now report that intracerebroventricular and intraparenchymal administration of GDNF potently protects the cerebral hemispheres from damage induced by MCA occlusion. In addition, the increase in nitric oxide that accompanies MCA occlusion and subsequent reperfusion is blocked almost completely by GDNF. Thus, this protein may play an important role in the treatment of cerebrovascular occlusive disease.