Repeated stress at the same time of day does not mimic timed feeding in its effects on the plasma corticosterone rhythm in rats.

The present study examined whether mild restraint stress occurring at the same time each day would entrain an anticipatory peak in the circadian plasma corticosterone rhythm associated with the time of stress. Rats were stressed by tube restraint for 2 h in the morning on 23 consecutive days, and plasma corticosterone concentrations were measured at 4h intervals over the next 2 days. Plasma corticosterone patterns were similar in control and restrained rats, and no anticipatory corticosterone peak occurred in stressed rats before the time when stress would have occurred. However, periodic regression analysis of the data indicated that timed stress did advance the acrophase of the circadian corticosterone rhythm by 1.7 h. This effect was minimal and could not explain the anticipatory rise in corticosterone concentrations seen in restricted feeding paradigms. Thus, it is unlikely that any stress associated with restricted feeding entrains corticosterone rhythms to anticipate the time of feeding, and some aspect of feeding per se is likely involved in producing the corticosterone peak that anticipates the time of restricted feeding.