PERIODICALLY VARYING SENSITIVITY TO MELATONIN IN A MAMMALIAN CIRCADIAN SYSTEM

In this communication we examine the effect of the pineal hormone melatonin on the circadian locomotor activity rhythm of the field mouse Mus booduga. Phase shifts in the circadian rhythm of locomotor activity were measured and a phase response curve was constructed following a single dose of 2 mu l melatonin (1 mg/kg) at various circadian times. Melatonin administered from circadian time 4 (CT4) to CT16 induced delay phase shifts whereas at phases CT18 to CT2 advance phase shifts were evoked, The magnitude and direction of the phase shifts evoked were a function of the time of the melatonin injections. The results suggest that the circadian tinting system controlling the locomotor activity rhythm in the M. booduga is responsive to melatonin and also that the daily endogenous rhythm of melatonin may be involved in phasing or entraining the circadian system of mice.