CORTICOSTERONE IS INVOLVED IN FOOT SHOCK-INDUCED INACTIVITY IN RATS

Inescapable shock (IS) exposure induces behavioral inactivity, related to behavioral alterations in subsequent tests (i.e., escape failure, and inactivity during shuttle box task). Metyrapone (150 mg/kg, IF), a corticosterone (CS) synthesis inhibitor, administered 3 h prior to IS reduced inactivity during this aversive experience. Forty-eight hours later, when these rats were submitted to a shuttle box task, a reduct ion in both escape failure and inactivity was observed. These effects were reversed by CS (20 mg/kg, SC) and dose dependent of the synthetic glucocorticoid dexamethasone, both administered 1 h before IS. When m etyrapone was administered 3 h before the shuttle box task to IS-expos ed animals, escape failures and inactivity were markedly reduced. This effect was subsequently reversed by CS. The dynamics of changes in se rum CS concentrations after both IS and shuttle box task paralleled be havioral changes. Animals injected with metyrapone before IS, which di splayed active behavior, showed serum CS levels stable at their basal levels after shock, and their secretion pattern was quite attenuated a fter the shuttle box task, whereas vehicle-, CS alone-, and metyrapone + CS-injected animals showed higher serum CS concentrations post-IS, which slowly decreased to their corresponding basal levels. CS secreti on after the shuttle box task was similar for the three groups: it had the same magnitude as after IS, though the decrease was faster. In al l groups, animals displayed passive behavior. These results indicate t hat glucocorticoids are involved in the onset and expression of passiv e behaviors induced by uncontrolable stressors. Therefore, it is possi ble to suggest a functional relationship between CS released by exposu re to inescapable stressor and the behavioral strategies adopted by ra ts under this stressful condition.