Effects of 5-hydroxytryptamine applied into nucleus raphe magnus on nociceptive thresholds and neuronal firing rate

The effect of iontophoretically applied 5-hydroxytryptamine on neurones in nucleus raphe magnus, and the effect of microinjection of 5-hydroxytryptamine into nucleus raphe magnus on nociceptive thresholds were examined in the rat. Iontophoretically applied 5-hydroxytryptamine excited 66% and inhibited 6% of the neurones encountered in the nucleus raphe magnus. The excitatory reponse to 5-hydroxytryptamine was reduced by the putative serotonergic antagonist cinanserin in 21 of 24 cases. In 12 of these neurones the responses to iontophoretically applied glutamate were also examined. In 11 of the 12 studies the responses to glutamate were reduced by cinanserin. Microinjection of 5 μg of 5-hydroxytryptamine into the nucleus raphe magnus produced analgesia as assessed by the tail-flick response to noxious heat stimulation, but no analgesia as assessed by the paw withdrawal response to pressure. Microinjection of 5 μg of 5-hydroxytryptamine into the adjacent nucleus reticularis paragigantocellularis had no analgesic effect in either test. These results indicate that 5-hydroxytryptamine mainly excites neurones in nucleus raphe magnus and that 5-hydroxytryptamine has an action on neurones in nucleus raphe magnus which modulate the nociceptive threshold.