Metabotropic glutamate (mGlu) receptors have been implicated in a number of physiological and pathological responses to glutamate, but the exact role of group I mGlu receptors in causing postischaemic injury is not yet clear. In this study, we examined whether the recently-characterized and relatively selective mGlu1 receptor antagonists 1-aminoindan-1,5-dicarboxylic acid (AIDA) and (S)-(+)-2-(3'-carboxybicyclo[1.1.1]pentyl)-glycine (CBPG) could reduce neuronal death in vitro, following oxygen-glucose deprivation (OGD) in murine cortical cell and rat organotypic hippocampal cultures, and in vivo, after global ischaemia in gerbils. When present in the incubation medium during the OGD insult and the subsequent 24 h recovery period, AIDA and CBPG significantly reduced neuronal death in vitro. The extent of protection was similar to that observed with the nonselective mGlu receptor antagonist (+)-alpha-methyl-4-carboxyphenylglycine [(+)MCPG] and with typical ionotropic glutamate (iGlu) receptor antagonists. Neuroprotection was also observed when AIDA or CBPG were added only after the OGD insult was terminated. Neuronal injury was not attenuated by the inactive isomer (-)MCPG, but was significantly enhanced by the nonselective mGlu receptor agonist (1S,3R)-1-aminocyclopentane-1, 3-dicarboxylic acid [(1S,3R)-ACPD] and the group I mGlu receptor agonist 3,5-dihydroxyphenylglycine (3,5-DHPG). The antagonists (+)MCPG, AIDA and CBPG were also neuroprotective in vivo, because i. c.v. administration reduced CA1 pyramidal cell degeneration examined 7 days following transient carotid occlusion in gerbils. Our results point to a role of mGlu1 receptors in the pathological mechanisms responsible for postischaemic neuronal death and propose a new target for neuroprotection.

An excessive activation of poly(ADP-ribose) polymerase (PARP) has been proposed to play a key role in postischemic neuronal death. We examined the neuroprotective effects of the PARP inhibitors benzamide, 6(5H)-phenanthridinone, and 3,4-dihydro-5-[4-1(1-piperidinyl)buthoxy]-1(2H)-isoquinolinone in three rodent models of cerebral ischemia. Increasing concentrations of the three PARP inhibitors attenuated neuronal injury induced by 60 min oxygen-glucose deprivation (OGD) in mixed cortical cell cultures, but were unable to reduce CA1 pyramidal cell loss in organotypic hippocampal slices exposed to 30 min OGD or in gerbils following 5 min bilateral carotid occlusion. We then examined the necrotic and apoptotic features of OGD-induced neurodegeneration in cortical cells and hippocampal slices using biochemical and morphological approaches. Cortical cells exposed to OGD released lactate dehydrogenase into the medium and displayed ultrastructural features of necrotic cell death, whereas no caspase-3 activation nor morphological characteristics of apoptosis were observed at any time point after OGD. In contrast, a marked increase in caspase-3 activity was observed in organotypic hippocampal slices after OGD, together with fluorescence and electron microscope evidence of apoptotic neuronal death in the CA1 subregion. Moreover, the caspase inhibitor Z-VAD-FMK reduced OGD-induced CA1 pyramidal cell loss. These findings suggest that PARP overactivation may be an important mechanism leading to post-ischemic neurodegeneration of the necrotic but not of the apoptotic type. Cell Death and Differentiation (2001) 8, 921 ± 932.

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3OH-Kynurenine and quinolinic acid are tryptophan metabolites able to cause, at relatively elevated concentrations, neuronal death in vitro and in vivo. In primary cultures of mixed cortical cells, the minimal concentration of these compounds leading to a signi®cant degree of neurotoxicity decreased from 100 to 1 mM, when the exposure time was prolonged from 24 to 72 h. NMDA receptor antagonists and inhibitors of nitric oxide synthase or poly(ADP-ribose) polymerase reduced quinolinic acid, but not 3OH-kynurenine toxicity. In contrast, scavengers of free radicals, caspase inhibitors and cyclosporin preferentially reduced 3OH-kynurenine neurotoxicity. These observations suggest that quinolinic acid causes necrosis, whereas 3OH-kynurenine-exposed neurons primarily die in apoptosis. In line with this possibility, we found that ATP levels decreased more rapidly in quinolinate-than in 3OH-kynurenine-exposed cultures and that poly(ADP-ribose) polymer, the product of poly(ADPribose) polymerase activity, was more abundant in the nuclei of quinolinic acid than in those of 3OH-kynurenine-exposed neurons. Because minor changes in the physiological concentrations of 3OH-kynurenine and quinolinic acid may cause neuronal death, our data suggest that these metabolites play a key role in the pathogenesis of several neurological disorders.