AJP - Regulatory, Integrative and Comparative Physiology, Vol 255, Issue 2 237-R242, Copyright © 1988 by American Physiological Society
Melatonin administration entrains female rat activity rhythms in constant darkness but not in constant light
E. M. Thomas and S. M. Armstrong
Department of Physiology, Monash University, Clayton, Victoria, Australia.
In female rats the luteinizing hormone (LH) is timed by the circadian
system and is followed by a display of intense, estrogen-induced running
behavior. This proestrous running on the night of ovulation can be used as
a marker of the estrous cycle. Entrainment of the mammalian circadian
system by exogenous melatonin (MT) has been demonstrated only in the
activity rhythms of male rats. The present experiments were designed to
study the effect of daily MT injections on activity rhythms and proestrous
running of female rats in 1) continuous dim white light (LL) and 2)
continuous darkness (DD). In LL, MT injections (50 micrograms/kg or 1
mg/kg) had no discernible effect on activity rhythms. In DD, four of the
six MT-treated rats (100 micrograms/kg) entrained to the injection, and a
fifth animal showed phase advances in its activity rhythm when onset of
activity passed through injection time. The sixth animal was not injected
with MT at activity onset time. None of the six control animals showed
either effect. MT had no effect on the length of the estrous cycle. Thus MT
injections can entrain circadian rhythms of activity and proestrous running
in female rats in DD but not in LL.
