[Role of nitric oxide in learning and memory processes].

Nitric oxide (NO) is a free radical gas that is synthesized from L-arginine by NO synthase (NOS). Activation of NMDA, non-NMDA or metabotropic glutamate receptors causes NO formation through NOS activation. From data obtained in experiments performed by microdialysis together with nitrate assay, we have proposed that NO production in the cerebellum following non-NMDA and metabotropic glutamate receptor activation may be independent of NOS activity, while NMDA receptor-mediated NO production depends on its activity. Glial cells appear to play a role in modulating NO production by regulating L-arginine availability. Activation of NMDA receptors and the increase in intracellular calcium concentration is a trigger for the long-term potentiation (LTP). NO acts as a retrograde messenger in the hippocampal LTP to enhance glutamate release from presynaptic nerve terminal, in which cyclic GMP may be involved. Behavioral studies demonstrate that NO is involved in some forms of learning and memory. Our studies suggest that NMDA/NO/cyclic GMP signaling plays a role in spatial working memory. Further, it is suggested that NO production in the brain is altered by aging. These results support the hypothesis that NO plays a role in mechanism of synaptic plasticity.