ABSTRACT
One of the most likely explanations is that stress modifies, at the neurobiological level, the motivational and/or reinforcing properties of drugs of abuse. Indeed, one of the effects of stress is to increase the activity of those neurobiological systems involved in motivation and reward46·47• As these neurobiological systems also serve as a substrate for drug-induced reinforcement'·3·16•17, by changing their activity stressors could enhance the responsiveness to drugs of abuse. Why stressors increase the activity of biological substrate of reward still remains an open question. However, it has been recently proposed that this could constitute a compensatory attempt to counteract the aversive effects of stress47•
