NMDA receptor activation induces glutamate release through nitric oxide synthesis in guinea pig dentate gyrus

We tested the hypothesis that the release of glutamate following activation of N-methyl-d-aspartate (NMDA) receptors is mediated by nitric oxide (NO) production, using slices of the guinea pig hippocampus. The NMDA-induced glutamate release from slices of dentate gyrus or CA1, which was both concentration-dependent and Ca²⁺-dependent, was also Mg²⁺-sensitive and abolished by MK-801, a selective non-competitive NMDA receptor antagonist. In dentate gyrus, the NMDA-induced glutamate release was inhibited non-significantly by tetrodotoxin, whereas the NO synthase (NOS) inhibitor N^G-nitro-l-arginine (l-NNA) blocked the NMDA-induced release of glutamate in a concentration-dependent manner, but not a high K⁺-evoked release of glutamate. In addition, the l-NNA blockade of NMDA-induced release of glutamate was recovered by pretreatment with l-arginine, the normal substrate for NOS. These results suggest that activation of NMDA receptors in dentate gyrus, as well as subsequent Ca²⁺ fluxes, is required for the neuronal glutamate release mediated by NO production. On the other hand, the NMDA-evoked glutamate release from CA1 region was tetrodotoxin-sensitive and was not inhibited by l-NNA, thereby suggesting that activation of NMDA receptors in CA1 results in increased glutamate release in an NO-independent manner. Taken together, the NMDA receptor-mediated neuronal release of glutamate from the guinea pig dentate gyrus likely involves the recruitment of NOS activity.