Analgesic effects of serotonin microinjection into nucleus raphe magnus and nucleus raphe dorsalis evaluated by the monosodium urate (MSU) tonic pain model in the rat.

The effects of 5-hydroxytryptamine (5-HT) microinjection into the nucleus raphe magnus (NRM) and the nucleus raphe dorsalis (NRD) on tonic pain were studied using the monosodium urate (MSU) tonic pain model in the rat. For the NRM, 5-HT microinjection produced significant analgesic effects, which were antagonized by systemic naloxone administration and also by subsequent microinjection of naloxone into the NRM. For the NRD, systemic naloxone administration did not antagonize these analgesic effects, although 5-HT microinjection produced significant analgesic effects. Therefore, as far as tonic pain is concerned, it was suggested that neural transmission mediated by 5-HT in the NRM and NRD plays an antinociceptive action, but via different neural mechanisms.