Adrenal corticoids in hamsters: role in circadian timing.

The 24-h patterns of circulating cortisol and corticosterone were determined in male hamsters housed under a 14:10 light-dark cycle. Corticoid levels varied significantly over the 24-h sampling period with peak levels of both hormones occurring near the onset of the daily dark phase. The ratio of cortisol to corticosterone changed dramatically during the day. Corticosterone levels were significantly higher than cortisol during the early part of the light phase; however, cortisol levels became significantly higher than corticosterone when both hormones began their daily rise. To examine whether the circadian rhythm of cortisol secretion could be involved in the physiological control of hamster circadian organization, cortisol was infused at approximately physiological levels into adrenalectomized hamsters either continuously or in a 24-h rhythm. No significant differences were observed in the timing of circadian wheel-running rhythms in hamsters housed in LD 16:8, LD 14:10, or LL when cortisol was infused continuously, in a 24-h rhythm that mimicked the cortisol rhythm of intact hamsters, or in a 24-h rhythm several hours out of phase with the rhythm of intact hamsters. Provision of cortisol in a 24-h rhythm appeared to promote the survival of adrenalectomized hamsters since hamsters receiving a 24-h pattern of cortisol survived the experimental protocol significantly longer than those receiving the same dose of cortisol continuously.