Locomotor activity rhythm in the field mouse Mus booduga phase-shifts to melatonin injections in a dose-dependent manner

Melatonin is known to shift the phase of the locomotor activity rhythm in t he field mouse Mus booduga in accordance with a type-I phase response curve (PRC), with phase delays during the subjective day and phase advances duri ng late subjective night and the early subjective day. At CT4 (circadian ti me 4; i.e. 16 hr. after activity onset) and CT22 of the circadian cycle, a single dose of melatonin (1 mg/kg) is known to evoke maximum delay and maxi mum advance phase-shifts, respectively. We investigated the dose-dependent responses of the circadian pacemaker of these mice to a single dose of mela tonin at the times for maximum delay and maximum advance. The circadian pac emaker responsible for the locomotor activity rhythm in these mice responde d to various doses of melatonin in a dose-dependent manner with the magnitu de of phase shifts increasing with dose.