Abstract
Daily behavioral and physiological rhythms are linked to circadian oscillations of clock genes in the brain and periphery that are synchronized by the master clock in the suprachiasmatic nucleus. In addition, there are a number of inputs that can influence circadian oscillations in clock gene expression in a tissue-specific manner. Here we identify an influence on the circadian oscillation of the clock protein PER2, endogenous changes in ovarian steroids, within two nuclei of the limbic forebrain: the oval nucleus of the bed nucleus of the stria terminalis and central nucleus of the amygdala. We show that the daily rhythm of PER2 expression within these nuclei but not in the suprachiasmatic nucleus, dentate gyrus, or basolateral amygdala is blunted in the metestrus and diestrus phases of the estrus cycle. The blunting of the PER2 rhythm at these phases of the cycle is abolished by ovariectomy and restored by phasic estrogen replacement suggesting that fluctuations in estrogen levels or their sequelae are necessary to produce these effects. The finding that fluctuations in ovarian hormones have area-specific effects on clock gene expression in the brain introduces a new level of organizational complexity in the control of circadian rhythms of behavior and physiology.