Neonatal dexamethasone administration. I. Temporary delay of development of the circadian serum corticosterone rhythm in rats.

A series of experiments was undertaken to examine the effects of neonatal dexamethasone administration on the development of the circadian serum corticosterone rhythm in rats. Initial experiments showed that the rhythm began normally on postnatal day 18 in untreated animals. Then 1-day-old rats were injected sc with dexamethasone acetate (1, 10, or 100 microgram), dexamethasone phosphate (1, 3, or 5 microgram) beginning on day 1. Appropriate vehicle controls were included. At most doses and dosage regimens, rat pups were sacrificed at 0700 and 1900 h on postnatal days 16, 18, 20, and 22, but at the highest doses, they were sacrificed only on days 25 and 30. The single dexamethasone acetate injection (100 microgram) delayed the appearance of the corticosterone rhythm for 2 days compared to that in the vehicle-injected controls. Similarly, the 3 consecutive injections of dexamethasone phosphate (5 microgram) delayed the appearance of the rhythm for 4 days. All other dexamethasone dosage regimens were ineffective in altering the onset of the circadian corticosterone rhythm.