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, IP), 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 reduction 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 metyrapone was administered 3 h before the shuttle box task to IS-exposed animals, escape failures and inactivity were markedly reduced. This effect was subsequently reversed by CS. The dynamics of changes in serum CS concentrations after both IS and shuttle box task paralleled behavioral changes. Animals injected with metyrapone before IS, which displayed active behavior, showed serum CS levels stable at their basal levels after shock, and their secretion pattern was quite attenuated after 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 secretion 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 all groups, animals displayed passive behavior. These results indicate that glucocorticoids are involved in the onset and expression of passive behaviors induced by uncontrollable stressors. Therefore, it is possible to suggest a functional relationship between CS released by exposure to inescapable stressor and the behavioral strategies adopted by rats under this stressful condition.