VICIOUS CYCLE INVOLVING NA-OXIDE FORMATION( CHANNELS, GLUTAMATE RELEASE, AND NMDA RECEPTORS MEDIATES DELAYED NEURODEGENERATION THROUGH NITRIC)

The mechanisms by which neurons die after cerebral ischemia and relate d conditions in vivo are unclear, but they are thought to involve volt age-dependent Na+ channels, glutamate receptors, and nitric oxide (NO) formation because selective inhibition of each provides neuroprotecti on. It is not known precisely what their roles are, nor whether they i nteract within a single cascade or in parallel pathways. These questio ns were investigated using an in vitro primary cell culture model in w hich striatal neurons undergo a gradual and delayed neurodegeneration after a brief (5 min) challenge with the glutamate receptor agonist NM DA. Unexpectedly, NO was generated continuously by the cultures for up to 16 hr after the NMDA exposure. Neuronal death followed the same ge neral time course except that its start was delayed by similar to 4 hr . Application of the NO synthase inhibitor nitroarginine after, but no t during, the NMDA exposure inhibited NO formation and protected again st delayed neuronal death. Blockade of NMDA receptors or of voltage-se nsitive Na+ channels [with tetrodotoxin (TTX)] during the postexposure period also inhibited both NO formation and cell death. The NMDA expo sure resulted in a selective accumulation of glutamate in the culture medium during the period preceding cell death. This glutamate release could be inhibited by NMDA antagonism or by mt, but not by nitroargini ne. These data suggest that Na+ channels, glutamate receptors, and NO operate interdependently and sequentially to cause neurodegeneration, At the core of the mechanism is a vicious cycle in which NMDA receptor stimulation causes activation of mt-sensitive Na+ channels, leading t o glutamate release and further NMDA receptor stimulation. The output of the cycle is an enduring production of NO from neuronal sources, an d this is responsible for delayed neuronal death. The same neurons, ho wever, could be induced to undergo more rapid NMDA receptor-dependent death that required neither mt-sensitive Na+ channels nor NO.