Daily variation of plasma testosterone, androstenedione, and corticosterone in rats under food restriction.

Food availability is an important synchronizer of the pituitary-adrenal axis and daytime restriction of food access phase-shifts the diurnal periodicity of plasma corticosterone (B) concentration in rats. However, little is known about the synchronizers of circulating androgens in male rats. We studied intact and castrated male rats with free access to food (control groups, C) and with access to food only from 0900 to 1100 hr (food-restricted groups, FR) for 14 days. Blood samples were collected on the 15th day by decapitation at 4-hr intervals. Plasma B concentration in C groups presented diurnal variation with higher values at 2000 than at 0800 hr. In the FR groups there was a 12-hr shift of peak B values. Apparently, castration had no effect on plasma B diurnal variation. In intact rats, plasma testosterone presented similar diurnal variation in both the C and the FR groups. In castrated rats, plasma testosterone was undetectable. Plasma androstenedione similarly varied over time in both C and FR intact rats. However, in castrated animals, the diurnal variation of plasma androstenedione was abolished. Our results indicate that the daily variation of plasma testosterone and androstenedione is dependent on testicular secretion and is not influenced by food availability in the male rat.