Conditioned circadian rhythm of plasma corticosterone in the rat induced by food restriction.

Earlier observations that food restriction for a short period of time in the morning produced an altered circadian rhythm of plasma corticosterone having the peak just before feeding time, and that the elevated plasma corticosterone levels declined promptly immediately after food presentation were confirmed. After a 14-day restricted feeding schedule, where food was given from 11:00 to 13:00, if food was not given, elevated levels of plasma corticosterone were sustained for at least 1 hr and then declined gradually. On the other hand, if food was given 2 hr earlier than the scheduled time, the peak at 11:00 disappeared. The conditioned peak of plasma corticosterone was maintained for at least 3 days after the restricted feeding schedule if hypertonic saline, but not tap water, was given without food in male rats and ovariectomized female ones. When female rats were treated with lysine vasopressin for 5 days 10 min before the food presentation, highly elevated levels of plasma corticosterone were found at the time of food presentation. However, administration of cortisol 2 hr before the feeding time blocked the effect of vasopressin. The results suggest that vasopressin is involved in the acquisition and consolidation of the conditioned circadian rhythm of plasma corticosterone induced by restricted feeding.