Resistance to kynurenic acid of the NMDA receptor-dependent toxicity of 3-nitropropionic acid and cyanide in cerebellar granule neurons.

During cerebral hypoxia or ischaemia, mitochondrial dysfunction is induced which can lead to free radical production and cell death. This phenomenon is mimicked by the acute administration of mitochondrial poisons such as 3-nitropropionic acid (3-NPA) and potassium cyanide (KCN), with the production of reactive molecular species secondary to the activation of glutamate receptors. Also during ischaemia, the kynurenine pathway of tryptophan metabolism is activated, leading to the production of quinolinic acid and kynurenic acid which can modulate N-methyl-D-aspartate (NMDA) receptors as agonist and antagonist respectively. Since kynurenic acid is known to be neuroprotective, we have now examined its ability to prevent the neurotoxic effects of mitochondrial dysfunction in primary cultures of postnatal rat cerebellar granule neurons. Viability was quantified using the Alamar Blue (AB) assay and by direct morphological examination. Both 3-NPA and KCN (10 microM-1 mM) reduced neuronal viability in a concentration-dependent manner. The NMDA receptor antagonists 2-amino-5-phosphonopentanoic acid (D-AP5) at a concentration of 50 microM, and a 10 microM dose of (+)-5-Methyl-10,11-dihydro-5H-dibenzo(a,d)cyclohepten-5,10-imine hydrogen maleate (MK-801) prevented cell death, although the non-NMDA receptor blocker 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) at a concentration of 10 microM did not. The antioxidant enzymes catalase and superoxide dismutase, and the nitric oxide synthase inhibitor Nomega-Nitro-L-arginine methyl ester hydrochloride (L-NAME) afforded partial protection. Kynurenic acid, a glutamate antagonist with preference for the glycine site of the NMDA receptors, had no protective effect at all against 3-NPA or KCN toxicity at concentrations up to 1 mM. Although these data confirm a major role for NMDA receptors and oxidative stress in the neurotoxic effects of mitochondrial inhibitors, they reveal a resistance to kynurenic acid which suggests a non-classical activation of NMDA receptors by mitochondrial inhibitors that is independent of the glycine site or which occurs distal to the site of action of kynurenic acid.