Interaction of stress and morphine in the rat using a classical conditioning design.

Previous s tudies have suggested that aversive and stressful stimulation may affect endogenous opioid sys tems and produce physiological changes (e.g., analgesia, Straub tail response) typically associated with opiate stimulation. The present experiment addressed the effect of s tressful but not immediately painful stimulation upon an opiate-mediated syndrome using a novel procedure. Noise s tress and morphine administration were factorially varied in the classical condit ioning of environmental preference in a two-choice apparatus . In compar i son with control subjects which showed no change, morphine produced a preference shift toward the conditioned environment which was further potentiated by a noise stimulation. Thus , s t ress may potentiate the reinforcing effects of opiate alkaloids.