The development of the serum corticosterone rhythm in rats.

The purpose of this study was to determine whether the sequence of changes that occur in the adrenal rhythm in maturing female rats (development of a peak, shift in acrophase and amplitude) requires experience with a photoperiodic stimulus or a change in ovarian status. The emergence of the serum corticosterone (CS) rhythm occured more quickly in adult rats placed in a 14 h light, 10 h dark (14:10) cycle at 50 days of age after rearing in constant light (LL) than in weanling rats placed in 14:10. Ovariectomy at weaning age did not alter the pattern of CS development in 14:10 although the amplitude of the peak was reduced even in 25-day-old rats. Adult rats reared in 14:10 held a population rhythm of CS longer after they were placed in LL than did weanling rats placed in LL. This difference was not dependent upon the presence of the ovaries since acutely and chronically ovariectomized (OVX) adult rats responded in a similar manner to adult controls. It can be concluded that the adrenal rhythm emerges as a function of age rather than as a result of a change in ovarian status. The capacity to synchronize serum CS to light-dark cycles develops in the absence of photoperiodic cues.