MELATONIN INSTANTANEOUSLY RESETS INTRINSIC CIRCADIAN RHYTHMICITY IN THE RAT SUPRACHIASMATIC NUCLEUS

In order to follow resetting of the rat circadian pacemaking system by melatonin in vivo, the rhythm in the light-induced c-fos expression w as used as a marker of the intrinsic rhythmicity of the suprachiasmati c nucleus (SCN). ii single melatonin administration during late day to rats maintained originally under a short photoperiod with 8 h of ligh t per day or under a long photoperiod with 16 h of light per day phase advanced immediately the evening rise in the light-induced SCN c-Fos immunochemistry by about 1.5 h. The data indicate that melatonin admin istration in vivo may instantaneously reset the intrinsic SCN rhythmic ity.