Exogenous corticosterone induces the expression of the clock protein, PERIOD2, in the oval nucleus of the bed nucleus of the stria terminalis and the central nucleus of the amygdala of adrenalectomized and intact rats.

The cyclical expression of the clock protein PERIOD2 (PER2) in select regions of the limbic forebrain is contingent upon the rhythmic secretion of the adrenal glucocorticoid, corticosterone. Daily rhythmic PER2 expression in the oval nucleus of the bed nucleus of the stria terminalis (BNSTov) and the central nucleus of the amygdala (CEA) is abolished with the removal of the adrenal glands but restored with rhythmic hormone replacement via the drinking water at a time corresponding to the endogenous peak of circulating glucocorticoids. Here, we investigated the effects of serial or acute systemic injections of corticosterone on the expression of PER2 in the BNSTov and CEA of both adrenalectomized (ADX) and intact rats. We sought to determine whether there is a temporal window of corticosterone sensitivity by delivering the hormone at a time corresponding to trough levels of circulating glucocorticoids, at lights on. We found that daily morning injections of corticosterone induced PER2 expression in the BNSTov and CEA of ADX rats, with levels peaking 1 h after injection. In intact rats, daily morning injections significantly upregulated the expression of PER2 in the BNSTov and CEA 1 h after injection and dampened the evening peak, while a single injection abolished the rhythm of PER2 expression in the CEA but had no effect on PER2 in the BNSTov. Our findings suggest that despite the potential masking effect of signals from the light-entrained master clock, daytime chronic and acute corticosterone administration can alter the rhythmic expression of PER2 in the BNSTov and CEA, and that the response is region-specific and dependent on the duration of treatment.