Pituitary and adrenal glands in neonatal rats studied by metopirone (Su4885).

One half of each litter of the 2- and 7-day-old rats was injected twice daily for 3 consecutive days with 15 mg/100 g body weight of metopirone (MET) in 0.05 ml of saline, and the remaining littermates with saline only. At the ages of 5 and 10 days, one half of the pups was sacrificed immediately, and the second half 20 min after s.c. injection of histamine as a stressor. MET depressed body weight and induced adrenal hypertrophy in the 5- and 10- day-old rats, and pituitary hypertrophy in the 5-day-old rats only. In resting condition the adrenal glands of the 5-day-old rats treated with MET contained less corticosterone (determined fluorometrically) than the 5-day-old control and the 10-day-old pups treated also with MET. Furthermore, neonatal MET treatment induced (histamine) stress response by the increase in adrenal corticosterone content in the 10-day-old rats, and by the increase in corticosterone content in plasma in both ages. The results indicate high sensitivity of the young neonatal rats to hormonal manipulations. They also yield additional support to the concept of functional integrity of the hypothalamo-pituitary-adrenocortical system in neonatal rats, and to the flexibility of the postnatal developmental pattern of stress response.