Learned helplessness: an experimental model of the DST in rats.

Elevated ratings of anxiety and agitation in Dexamethasone Suppression Test (DST) nonsuppressors suggest a role for psychological stress in the generation of the hypothalamic-pituitary-adrenal cortical (HPAC) abnormalities characteristic of depression. We employed the learned helplessness model of depression to test the effectiveness of psychological stress in inducing a resistance of plasma corticosterone levels to dexamethasone suppression. Inescapably shocked rats exhibited corticosterone levels that were significantly more resistant to dexamethasone suppression than were the levels of rats receiving an equivalent amount of escapable shock or no shock. These results confirm the hypothesis that HPAC resistance to dexamethasone suppression is enhanced by the distress associated with the inefficacy of behavioral coping responses. The present findings represent the first analog of the DST in the learned helplessness model of depression. This DST model allows investigations into neurobiological mechanisms underlying the HPAC alterations in depression.