The effects of corticosterone implantation on schedule-induced wheelrunning and HPA function in intact rats

Previous research has shown that Schedule-induced wheelrunning (SIW) is dependent on the function of the pituitary-adrenal axis and that the nature of pituitary-adrenal involvement is mainly through circulating corticosterone levels. In the present study it was shown that a 10-20 day period of subcutaneous implantation of exogenous corticosterone in intact rats alters HPA function but does not influence SIW. Plasma corticosterone levels in corticosterone-treated rats significantly decreased when compared with either the vehicle treated or untreated controls, and atrophy of adrenals occurred in corticosterone-treated animals, suggesting that exogenous corticosterone implantation impairs HPA function. However, the behavioural results showed that corticosterone implantation to intact rats did not affect the acquisition and development of SIW. These results raise the question of whether corticosterone exerts a permissive action or not.