The effect of different lesions of the median raphe on morphine analgesia.

The effect of different manipulations of the nucleus medianus raphe (MR) on morphine analgesia was investigated in rats using the tail-immersion test. Electrolytic lesions of this structure antagonized morphine analgesia, while injections of 5,7-dihydroxytryptamine (to destroy serotonergic neurons) or ibotenic acid (to destroy cell bodies) in the medianus raphe did not alter the effect of morphine. Injection of naloxone (0.5 and 0.1 micrograms) in the MR antagonized morphine analgesia. These results suggest the importance of this structure for morphine analgesia in this test, although the substrates within the nucleus that mediate this action are still unknown.