Brainstem lesions dissociate neural mechanisms of morphine analgesia in different kinds of pain.

The effects of brainstem lesions on morphine analgesia were examined using the formalin test which produces moderate pain that lasts about 2 h, and the tail-flick test which measures brief threshold-level pain. Lesions of the nucleus raphe magnus attenuated and small lesions of the central tegmental nucleus potentiated the effects of morphine in the tail-flick test. Lesions of the median raphe nucleus potentiated the effects of morphine in the formalin test. Large lesions of the pontine reticular formation had no effect in either pain test. These results indicate that the neural mechanisms underlying morphine analgesia are different in different kinds of pain.