CORTICOSTERONE INFLUENCES FORCED SWIM-INDUCED IMMOBILITY

The effect of corticosterone (CS) synthesis inhibition with metyrapone -a blocker of the 11 beta-hydroxylase (150 mg/kg IP)-on immobility tim e during the forced swim test was recorded. Immobility time was measur ed during a 15-min forced swim (test). Twenty-four hours later rats we re subjected to an additional 5 min forced swim (retest). In one exper iment, metyrapone or vehicle was administered 3 h before the initial t est, while CS (0, 5, 10, or 20 mg/kg SC) was administered 1 h prior to the initial test. Metyrapone significantly reduced immobility time du ring both test and retest. This effect was reverted in a dose-dependen t fashion by CS. In a second experiment, animals exposed to the initia l test 24 h before were injected with metyrapone or vehicle 3 h before the retest, while CS (0, 10, or 20 mg/kg SC) was administered 1 h pri or the retest. Metyrapone, administered before the retest, reduced imm obility time and CS partially reverted metyrapone effect. In another g roup of animals, serum CS concentrations were evaluated before and aft er test and retest. In vehicle groups, the high immobility time during test and retest was associated with high CS serum concentrations post stress. In animals receiving metyrapone prior to the initial test, the reduced immobility time was related to low levels of CS after the tes t and an attenuated secretion following the retest. Moreover, CS (20 m g/kg) and metyrapone + CS groups had high CS levels before the test, w hich remained high 2 h after the test, although after the retest, both groups showed a pattern of CS secretion similar to that observed in v ehicle animals. These findings suggest that CS plays a critical role o n the behavioral strategies adopted by rats when they are forced to fa ce an aversive and inescapable stressful situation. Thus, behavioral i mmobility would require higher CS levels although active behavior woul d be related to low hormone concentrations.