The effects of ACTH, adrenalectomy and dexamethasone on the acquisition of an avoidance response in rats

Exogenous elevation of circulating ACTH level facilitated the acquisition of a two-way avoidance response at a high but not at a moderate US intensity. Injections of ACTH had no general effect on spontaneous shuttling activity, escape behavior to light-onset, or intertrial responding during avoidance conditioning. Adrenalectomy, which resulted in minimal plasma levels of adrenocortical hormones and, presumably, elevated levels of ACTH, also facilitated avoidance responding. Injection of dexamethasone, a synthetic glucocorticoid and ACTH inhibitor, produced severe weight loss and hypophagia, but did not influence avoidance performance. However, the failure of dexamethasone to affect acquisition of avoidance responding was not related to its effect on body weight and food intake. The action of ACTH on avoidance conditioning is clearly extra-adrenal, but ACTH is not essential to normal performance, at least when high levels of glucocorticoids are present.