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 was used as a marker of the intrinsic rhythmicity of the suprachiasmatic nucleus (SCN). A single melatonin administration during late day to rats maintained originally under a short photoperiod with 8 h of light 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 administration in vivo may instantaneously reset the intrinsic SCN rhythmicity.