Vasopressin, immobilization, and the dexamethasone suppression test in rats

To investigate the role of the hormone vasopressin (VP) in mediating the response of the body to stress, corticosterone levels of VP-containing (LE) rats and VP-deficient (DI) rats were compared following administration of the dexamethasone suppression test (DST) under stressed and nonstressed conditions. The stressor utilized was immobilization, an acute physical stressor. Dexamethasone (DEX), a synthetic glucocorticoid, was injected subcutaneously at a dose of 0.025 mg/kg. This dose of DEX was found to significantly suppress plasma corticosterone in the nonstressed animals (both DI and LE) via feedback inhibition of the hypothalamic-pituitary-adrenocortical (HPA) axis. In the stressed situation, however, LE animals exhibited "escape" from DEX suppression, whereas DI animals did not. Escape indicates a resistance of the HPA axis to the suppressive action of DEX. Thus, in the absence of corticotropin-releasing factor, which is inhibited by DEX, VP alone appears to be sufficient to elicit significant corticosterone release. These results support the hypothesis that VP plays an important role in the regulation of glucocorticoid release in acute stress via the HPA axis.