Effects of intracerebral injection of 6-hydroxydopamine in nucleus raphe magnus on electro-puncture analgesia in rats

The effects of injection of 6-OH-DA into the nucleus raphe magnus were studied with both behavioral and fluorescence histocliernical methods in rats. Eight rats were injected intracerebrally 6-OH-DA and 9 rats received a vehicle. Pain threshold was measured by tail-flick and vocalization following electric stimulation of the tail. The electropuncture analgesic effect was enhanced 3 d later. The fluorescence of NA terminals in the nucleus raphe magnus disappeard almost completely, while the 5-HT-containing neurons in the nucleus remained undamaged. The control rats showed no changes in electropuncture analgesia nor in histo-fluorescence findings. The results provide an evidence for the antagonism between NA and 5-HT in the nucleus raphe magnus in modulation of pain.