Circadian adrenocortical periodicity changes in adult rats stimulated during early development--I. Effect of prednisolone.

The aim of this study was to test the feasibility of manipulating the adrenocortical circadian rhythm in adult rats by early postnatal prednisolone treatment. Prednisolone, when injected at 7-9 or 17-19 days after birth, produced a permanent suppression of the circadian rhythm of the basal levels of plasma 11-OH-corticosteroids and the rhythm of its responsiveness to stress. The administration of prednisolone at age of 2-4 or 12-14 days did not affect the circadian adrenocortical patterns in adults. Evidence was obtained for the existence of two critical periods during early development. Stimulation with prednisolone during these periods caused a profound modification of circadian periodicity in the performance of the pituitary-adrenocortical system. This modification was not related to changes in adrenal cortex ACTH responsiveness and also to altered stress reactivity of the pituitary-adrenocortical system. It was the presumable consequence of a blockage of a regulatory central mechanism initiating circadian variations in the pituitary-adrenocortical function. The existence of two distinct critical periods suggests that some prednisolone-sensitive links of this central pacemaker mechanism mature asynchronously during early postnatal life.