Ontogenetic diurnal variation of adrenal responsiveness to ACTH and stress in rats

We performed studies in 8-, 16-, 24-, 30 and 35-day-old Wistar rats at 8.00 h (AM) and 20.00 h (PM) to investigate the relationship between the diurnal variations of basal plasma corticosterone (compound B, CB) and its responses to ACTH and ether stress during the postnatal period. Basal plasma CB levels increased at PM from 8 to 35 days of age and an AM-PM difference was observed at 16 days. Although an AM-PM difference in CB responsiveness to ACTH was detected only at 24 and 35 days, ACTH induced an increasingly higher CB response at PM than at AM from 8 to 35 days. A stress-induced CB response was observed starting at 8 days of age and presented an age-dependent increase; however, no AM-PM difference was observed at any age. The stress-induced CB levels were higher than ACTH-induced CB values at all ages tested except at PM in 8-day old rats. These data demonstrate that the basal CB levels and adrenal sensitivity to ACTH rise during the evening as a function of neonatal development.