Evidence for a free-running circadian rhythm in pituitary-adrenal function in blinded adult female rats.

To study the eyes' role in maintaining the circadian rhythm in pituitary-adrenal function, 24-h patterns of corticosterone levels were compared in intact and blinded adult female rats. Rats were blinded by optic enucleation at approximately 80 days of age. Nonstress plasma corticosterone levels were determined fluorometrically in serial blood samples obtained from a tail vein at 4-h intervals for 44-h periods, 3 and 10 weeks after surgery. At 3 weeks after surgery, blinded and intact rats demonstrated comparable rhythms in corticosterone levels. At 10 weeks, steroid fluctuations in individual blinded rats still had an approximate 24-h periodicity. However, these fluctuations were no longer synchronized with the light-dark cycle or with those of other rats. These findings suggest that rats blinded as adults have a free-running pituitary-adrenal circadian rhythm.