Diurnal changes in amplification of hormone rhythms in the adrenocortical system

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Abstract
To explore further the factors that contribute to the circadian rhythm in plasma corticosterone levels and to test the hypothesis that corticotropin-releasing factor (CRF) secretion during the day is rhythmic, resting hormone levels in untreated female rats, corticosterone responses to ACTH and metabolic clearance rates (MCR) of corticosterone were determined in the morning (near lights on) and evening (near lights off). A normal 2-fold increase in ACTH, 2-fold increase in adrenal responsiveness to ACTH and 7-fold increase in corticosterone but no change in the MCR of corticosterone in intact rats were found between morning and evening. There was no rhythm in plasma ACTH and corticosterone or adrenal corticosterone in rats with hypothalamic lesions that prevented corticosterone responses to known stimuli. In lesioned rats, ACTH and adrenal corticosterone levels were equal to the low values found in intact rats in the morning; plasma corticosterone levels were elevated at both times above the levels in intact rats in the morning. Elevated corticosterone levels in lesioned rats were explained by the fact that, at both times of the day, the MCR of corticosterone was decreased by at least 50% and the distribution volume of corticosterone was decreased by at least 40% compared to intact rats. There was a 2-fold increase in the ACTH and corticosterone responses to CRF between morning and evening in lesioned rats. Apparently, there is considerable autonomous CRF-independent ACTH secretion by the pituitary in the morning and there is amplification of the CRF signal at both pituitary and adrenal between morning and evening that results in a 4-fold amplification in plasma corticosterone.