Glucocorticoids facilitate the retention of acquired immobility during forced swimming.

The adrenalectomy-induced decrease in the level of immobility during a 5 min retest period in the Porsolt swimming test could be reversed by glucocorticoids administered s.c. 15 min after the initial forced swimming exposure. The synthetic glucocorticoids dexamethasone and RU 28362 were active in the microgram dose range while corticosterone was only active at a 500 times higher dose. Aldosterone and progesterone were both ineffective. Treatment of adrenalectomized rats with the synthetic antiglucocorticoid RU 38486 prior to dexamethasone administration dose dependently blocked the effect of the glucocorticoid. Intact rats treated with the antiglucocorticoid RU 38486 prior to the initial forced swimming exposure behaved like adrenalectomized animals during the 5 min retest period. Removal of the adrenal medulla only temporarily impaired swimming behavior. It is concluded that intact adrenocortical secretion of glucocorticoids is sufficient for retention of acquired immobility during forced swimming.