PHYSIOLOGICAL CONCENTRATIONS OF MELATONIN INHIBIT NITRIC-OXIDE SYNTHASE IN RAT CEREBELLUM

In the present paper we show the inhibitory effect of melatonin on rat cerebellar nitric oxide synthase (NOS) activity. NO production was mo nitored by the stoichiometric conversion of L-arginine to L-citrulline . The inhibitory effect of melatonin was dose-dependent, with an IC50 value of about 0.1 mM. However, a significant inhibition of enzyme act ivity (>22%) was observed at 1 nM melatonin which is in the range of t he physiological serum concentration of the hormone at night. The inhi bitory effect of melatonin was observed exclusively in the presence of Ca++. Results suggest a new and important role of the pineal hormone melatonin on central nervous system processes, i.e., by modulating NO production.