Anatomical specificity as the critical determinant of the interrelationship between raphe lesions and morphine analgesia.

Electrolytic lesions were produced in three separate parts of the raphe mesencephalic area: the nucleus raphe medianus, the nucleus raphe dorsalis and an area between these two nuclei. Seven days after the surgery, the animals were tested for morphine analgesia using the tail compression method and then sacrificed for the estimation of brain serotonin. It was found that the analgesic effect of morphine was significantly reduced in the rats lesioned in the nucleus raphe medianus but not in the animals lesioned either in the nucleus raphe dorsalis or in the 'intermediate raphe area'. Since a decrease of forebrain serotonin was observed in each experimental group, the reduction of morphine analgesia does not involve a simple direct correlation with a decrease of serotonin in the forebrain. The results are discussed in view of the possibility that the reduction of morphine analgesia after lesions of the nucleus raphe medianus is due to a disruption of a specific brain serotonergic system.