NITRIC-OXIDE PARTICIPATES IN EXCITOTOXIC MECHANISMS INDUCED BY CHEMICAL HYPOXIA

Changes in the activity of the NMDA receptor-gated ionic channels induced by potassium cyanide were studied in rat hippocampal slices utilizing a [H-3]MK-801 binding technique. A 30-min exposure of slices to potassium cyanide (KCN) increased MK-801 binding by 252%. Co-application of N omega-nitro-L-arginine (NNLA), a competitive antagonist of nitric oxide (NO) synthase, reduced this increase by 72%. This inhibition by NNLA was completely reversed by an excess of L-arginine, a substrate for NO synthase, suggesting that the KCN-induced increase in MK-801 binding is mediated by NO synthase activity. KCN had no effect on MK-801 binding in synaptic membranes. In Ca2+-containing medium, KCN increased the release of glutamate, aspartate and glycine by 4- to 5-fold, and this was blocked by application of NNLA. NNLA inhibition was reversed by an excess of L-arginine, indicating that KCN-stimulated release of these amino acids is mediated by NO synthase activity. In Ca2+-free medium, a KCN-induced increase in MK-801 binding and in excitatory amino acid release was also observed, however, this increase was not influenced by NO-related agents, suggesting that these changes were not mediated by NO synthase activation. NNLA given after the end of exposure to KCN did not reverse the increase in MK-801 binding. These findings suggest that NO is involved in the initial activation of NMDA receptor-gated ionic channels and in the enhanced amino acid transmitter release induced by KCN, but that KCN can also induce some of these effects by a Ca2+- and NO-independent mechanism.