CIRCADIAN ACTIONS OF MELATONIN AT THE SUPRACHIASMATIC NUCLEUS

The biological clock in the suprachiasmatic nucleus (SCN) of the hypot halamus plays a well-defined role in regulating melatonin production b y the pineal. Emerging evidence indicates that melatonin itself can fe ed back upon the SCN and thereby influence circadian functions. Melato nin administration has been shown to entrain activity rhythms in roden ts and humans. Melatonin binds specifically within the SCN and alters SCN physiology by both acute and clock-resetting mechanisms. The circa dian clock in the SCN appears to temporally restrict its own sensitivi ty to melatonin, such that physiological sensitivity is greatest in th e subjective dusk period