Effect of adrenalectomy on a diurnal (circadian) rhythm in hypothalamic multiple unit activity in the female rat

In freely moving female rats multiple unit activity (MUA) was recorded from medial hypothalamic and septal structures for 4-7 days throughout the estrous cycle. A consistent elevation of MUA during paradoxial sleep (PS) was superimposed on a clearcut circadian rhythm with maximal activity during the night and minimal activity during the day. The circadian rhythm of MUA did not show variations related to the phases of the 4 day estrous cycle and was not altered either by ovariectomy or by administration of exogenous estrogen. In contrast, the diurnal cycle of MUA was completely abolished within 3 wk following adrenalectomy. It is suggested that the circadian rhythm of hypothalamic MUA in the female rat may be related to the rhythm of secretion of adrenal corticotropic hormone and/or hypothalamic corticotropin releasing factor.