Effects of Diurnal Variation in Plasma Corticosterone Levels on Adrenocortical Response to Stress.

Abstract

Adult female rats were subjected to 3-minute ether or immobilization stress during the trough (08:30) or peak (16:30) of the circadian rhythm in adrenocortical function. Two samples of peripheral plasma were obtained from each rat for fluorometric determination of corticosterone concentrations; the first was collected in less than 3 minutes and the second at 15 minutes following initiation of stress. Adrenals were also collected 15 minutes after stress for determination of corticosterone levels. Corticosterone concentrations in the first or “non-stress” plasma samples collected in the afternoon were 2-3 times higher than those obtained in the morning. Nevertheless, stress-induced increments in steroid levels did not differ significantly at the two times of day. Although adrenal concentrations of corticosterone 15 minutes following onset of stress in the morning were similar to those following stress in the afternoon, the changes observed in plasma suggest that acute pituitary-adrenal responses to the types of stress used are not altered by marked diurnal variation in plasma corticosterone levels.