Activity-induced circadian clock resetting in the Syrian hamster: effects of melatonin

Circadian rhythms in the Syrian hamster can be phase advanced by arousal during the mid-rest period. Similar phase shifts are induced by 5-HT(7) receptor activation in vivo and in vitro. Shifts in vitro are dependent on mobilization of intracellular cyclic AMP (cAMP), and can be blocked by melatonin, which opposes cAMP accumulation. If phase shifts to arousal in vivo are also dependent on cAMP, then these shifts may also be attenuated by melatonin. Hamsters were confined to a novel running wheel for 1.5 or 3 h in the mid-rest period. Melatonin (1 mg/kg i.p.) as a single bolus did not induce phase shifts, and single or multiple doses did not affect shifts to arousal. These data suggest that stimulation of cAMP by 5-HT(7) receptor activation is not necessary for clock resetting by behavioral arousal.