Circadian rhythm of nitric oxide production in the dorsal region of the suprachiasmatic nucleus in rats

Extracellular concentration of nitrite (NO2-), an oxidized product of nitri c oxide (NO), was measured consecutively in the dorsal reg ion of the rat s uprachiasmatic nucleus (SCN) by means of in vivo microdialysis. The NO2- co ncentrations in the dialysates showed robust circadian rhythm under a 12:12 h light/dark cycle and were higher during the dark phase than during the l ight phase. When the rats were transferred to constant darkness, the 24 h r hythm of NO2- persisted without damping the amplitude. The NO; level was si gnificantly lowered by an injection of NO synthase inhibitor (N-G monomethy l-L-arginine, 10 mg/ka i.p.). These findings indicate that the daily fluctu ation of NO2- in the dorsal region of the SCN, which represents endogenous rhythm of NO, is regulated independently of photic inputs into the SCN and may be related to the circadian clock functions. (C) 2001 Published by Else vier Science Ireland Ltd.