Nitric oxide synthase inhibitor decreases NMDA-induced elevations of extracellular glutamate and intracellular Ca2+ levels via a cGMP-independent mechanism in cerebellar granule neurons

These studies were designed to examine the differential effect of nitric ox ide (NO) and cGMP on glutamate neurotransmission. In primary cultures of ra t cerebellar granule cells, the glutamate receptor agonist N-methyl-D-aspar tate (NMDA) stimulates the elevation of intracellular calcium concentration ([Ca2+](i)), the release of glutamate, the synthesis of NO and an increase of cGMP. Although NO has been shown to stimulate guanylyl cyclase, it is u nclear yet whether NO alters the NMDA-induced glutamate release and [Ca2+]( i) elevation. We showed that the NO synthase inhibitor, NC-monomethyl-L-arg inine (NMMA), partially prevented the NMDA-induced release of glutamate and elevation of [Ca2+](i) and completely blocked the elevation of cGMP. These effects of NO on glutamate release and [Ca2+](i) elevation were unlikely t o be secondary to cGMP as the cGMP analogue, dibutyryl cGMP (dBcGMP), did n ot suppress the effects of NMDA. Rather, dBcGMP slightly augmented the NMDA -induced elevation of [Ca2+](i) with no change in the basal level of glutam ate or [Ca2+](i). The extracellular NO scavenger hydroxocobalamine prevente d the NMDA-induced release of glutamate providing indirect evidence that th e effect of NO may act on the NMDA receptor. These results suggest that low concentration of NO has a role in maintaining the NMDA receptor activation in a cGMP-independent manner.