Effects of intrathecally administered methysergide and yohimbine on microstimulation-produced antinociception in the rat.

This study examined whether intrathecal (i.t.) administration of the serotonergic antagonist methysergide, of the alpha 2 noradrenergic antagonist yohimbine, or of both drugs antagonized stimulation-produced antinociception (SPA) evoked from the nucleus raphe magnus (NRM) and the nucleus reticularis paragigantocellularis (NRPG) of lightly anesthetized rats. The increase in tail flick latency (TFL), but not the increase in paw pinch withdrawal threshold (PWT), evoked from NRM sites was antagonized by i.t. administration of methysergide. Intrathecal administration of yohimbine antagonized both the increase in TFL and the increase in PWT produced by stimulation of NRM sites. Stimulation of sites in the NRPG also increased TFL and PWT; these increases were not antagonized by i.t. administration of methysergide. Although i.t. administration of yohimbine antagonized the increase in TFL evoked from the NRPG, the increase in PWT was not antagonized. When coadministered intrathecally, methysergide and yohimbine antagonized the increases in TFL and PWT produced by stimulation of NRM and of NRPG sites. In contrast, i.v. administration of the same doses of methysergide and yohimbine did not antagonize either the increase in TFL or the increase in PWT evoked from either set of sites. These results support the concept that activation of serotonergic and noradrenergic bulbospinal neurons mediates SPA and additionally suggest that the noradrenergic component involves an alpha 2 noradrenergic receptor.