In the field mouse Mus booduga melatonin phase response curves (PRCs) havea different time course and wave form relative to light PRC

The phase shifting effects of the pineal hormone melatonin on the circadian locomotor activity rhythm of the field mouse Mus booduga was examined at v arious phases of the circadian cycle using single melatonin injections of t wo concentrations (10 mg/kg, high dose; and 1 mg/kg, low dose) and two phas e response curves (PRCs) were constructed. A single dose of melatonin admin istered during the early subjective day evoked maximum phase delays, and du ring the late subjective night evoked phase advances in the locomotor activ ity rhythm. Other phases of the circadian cycle also responded to melatonin . The interval between circadian time 19 (CT19) and CT2 of the high dose me latonin PRC is marked by significant phase advances, whereas the interval b etween CT2 and 19 is marked by significant phase delays. A single dose of m elatonin of strength 10 mg/kg was found to evoke phase shifts that were of comparable magnitude to those of the phase shifts evoked by natural dayligh t pulses. Control animals, treated with 50% dimethyl sulfoxide (DMSO), did not respond with phase shifts significantly greater than zero. Significant differences between the shapes of the two melatonin PRCs exist. Further mel atonin PRCs appear to have a different time course and wave form relative t o light-induced PRC.