Circadian variation in the sensitivity of the pituitary-adrenal system to dexamethasone suppression.

Intact female rats were injected with 100 microgram dexamethasone/kg body weight or saline at 12.00, 24.00 or 04.00 h. Four hours later, i.e. 16.00, 04.00 or 08.00 h respectively, a double bleeding procedure was used to obtain blood samples from stressed and unstressed rats for subsequent fluorometric determination of corticosterone levels in the plasma. The difference between the level of corticosterone is stressed and unstressed rats (the stress increment) was determined and used as an index of the response of corticosterone in the plasma to stress. Increments in the levels of corticosterone in the plasma evoked by stress in rats injected at 12.00 h with dexamethasone and bled 4 h later were significantly (P less than 0.05) less than those in rats given dexamethasone at 24.00 or 04.00 and bled at 04.00 or 08.00 h respectively. In contrast, stress-induced increments in the level of corticosterone in the plasma of rats treated with saline did not vary with the time of day.