Stress, habits, and drug addiction: a psychoneuroendocrinological perspective.

It is well known that stress is a significant risk factor for the development of drug addiction and addiction relapse. Remarkably, the cognitive processes involved in the effects of stress on addictive behavior remain poorly understood. Here it is proposed that stress-induced changes in the neural circuits controlling instrumental action provide a potential mechanism by which stress affects the development of addiction and relapse vulnerability. Instrumental action can be controlled by two anatomically distinct systems: a goal-directed system that involves learning of action-outcome associations, and a habit system that learns stimulus-response associations. The transition from initial voluntary drug use to subsequent involuntary, compulsive drug use represents a switch from goal-directed to habitual control of action. Recent evidence indicates that this switch from goal-directed to habit action can be prompted by stress and stress hormones. We argue (i) that acute stressors reinstate habitual responding to drug-related cues and thus trigger relapse to addictive behavior, and (ii) that prolonged or repeated stress may accelerate the transition from voluntary to involuntary drug use and thus promote the development of addiction. The suggested mechanism encompasses cognitive processes that may contribute to the effects of stress on addictive behavior and could have important implications for the treatment of addiction and the prevention of relapse.