Modulation of the neonatal pituitary and adrenocortical responses to stress by thyroid hormones in the rat: effects of hypothyroidism and hyperthyroidism.

Neonatal rats exhibit a period of diminished pituitary and adrenocortical responses to stress during the first 2 weeks of life. Since thyroid hormones are known to affect brain development, modulation of these responses to stress by alterations in thyroid hormone status have been investigated in hypothyroid (Hypo) and hyperthyroid (Hyper) rat pups. Changes in ACTH and corticosterone (B) levels were measured under basal and stress conditions (3 min exposure to ether vapors) in neonates of various ages (day 5-21). Basal T4 and corticosterone-binding globulin (CBG) levels were also measured. Hypo pups were obtained from methimazole-treated mothers and hyperthyroidism was induced by daily subcutaneous injections of L-T4 (100 micrograms/kg BW) from birth on. In Hyper rats, premature onset of ACTH and B responses to stress was observed in 5-day-old rats while significant ACTH and B secretion only appeared by day 10 in vehicle-injected rats. By contrast, ACTH and B responses to stress were delayed in Hypo pups and only occurred by day 21. The lack of ACTH and B responses to stress of 14-day-old Hypo rats could be reversed by one single L-T4 injection (100 micrograms/kg BW) given 24 h, but not 4 h prior to exposure to stress. On day 21, smaller (p less than 0.05) stress-induced ACTH release was observed both in Hypo and Hyper rats compared to intact rats, concomitant with a diminished ACTH secretion following exogenous ovine CRF (10 micrograms/kg BW, i.p.) administration.(ABSTRACT TRUNCATED AT 250 WORDS)