Potential physiological and pathophysiological roles of nitric oxide in the brain.

Nitric oxide (NO) is a free radical molecule which has been described to play a role as a messenger molecule in at least three systems: white blood cells, blood vessels and most recently in the nervous system. In the brain, NO is produced enzymatically in postsynaptic structures in response to activation of excitatory amino acid receptors. A major action of NO is to activate soluble guanylate cyclase and to raise cGMP level in target cells. The role of NO as a messenger in long-term potentiation and in long-term depression has been established and recent studies have directly implicated NO in neuronal damage associated with vascular strokes. Concerning the role of NO in the excitatory amino acid neurotoxicity, more studies will be necessary to elucidate the implication of NO mediating neuronal damage. Whatever the exact function of NO, it is sure that this substance play an important role in the brain and that pharmacological manipulations of NO pathway will constitute a novel approach for therapeutical applications in the future.