The role of stimulus intensity and stress in opioid-mediated analgesia.

Rats exposed to a Pavlovian conditioning paradigm developed naloxone-reversible analgesia only when the intensity of a noxious unconditioned stimulus was suprathreshold and the level of stress was augmented. The time course of the onset of this conditioned analgesia was reproduced by systemic administration of morphine. These findings suggest that both a minimal level of stimulus intensity and stress are necessary for the activation of endogenous opioid-mediated analgesia.