Effect of combined perinatal thyroxine and corticosterone treatment on the development of the diurnal pituitary-adrenal rhythm.

Daily corticosterone injections (20 microgram/g b.w., i.p.), on either days 2, 3 and 4 or days 16, 17 and 18, abolished the normal development of the a.m.-p.m. difference in plasma corticosterone levels, as no a.m.-p.m. difference was present in either group on day 21. Thyroxine treatment (10 microgram/g b.w., s.c.) on days 8, 9 and 10 was able to counteract the effect of corticosterone at both ages, i.e. animals treated with thyroxine and corticosterone exhibited a significant a.m.-p.m. difference on day 18, the same day as controls. On the other hand, thyroxine-treated rats with prior corticosterone injections continued to show significantly earlier eye-opening than controls. These results suggest that thyroxine advances the maturation of central structures which are involved in the control of pituitary-adrenal activity and which are sensitive to high perinatal plasma levels of corticosterone.