EFFECTS OF DAILY INJECTIONS OF MELATONIN ON LOCOMOTOR-ACTIVITY RHYTHMS IN RATS MAINTAINED UNDER CONSTANT BRIGHT OR DIM LIGHT

It has been demonstrated that daily melatonin injections entrain free- running locomotor activity rhythms in rats kept in constant darkness, and synchronize disrupted circadian patterns of wheel-running activity under constant light. Contrary to these previous observations, our re sult did not show that daily injections of melatonin synchronize disru pted locomotor activity in rats maintained under constant bright light . On the other hand, daily treatment with melatonin entrained the inta ct free-running rhythm in rats kept in constant dim light. This entrai nment took place only when the time of injection corresponded to the a ctivity onset time, and similar results were obtained in blinded rats. Pinealectomy had no influences on either the free-running rhythm or m elatonin-induced entrainment. To examine whether a behavioral feedback mechanism is involved in melatonin-induced entrainment, rats were imm obilized for 3 h after each daily melatonin injections. This did not i nterfere with melatonin-induced entrainment. These results suggest tha t the mechanism underlying melatonin-induced entrainment of activity r hythms may be different from those in photic and behavioral entrainmen t.