Hypoalgesia induced by blockade of noradrenergic projections to the raphe magnus: reversal by blockade of noradrenergic projections to the spinal cord.

Previous studies have demonstrated that microinjection of noradrenergic (NA) antagonists such as phentolamine in the nucleus raphe magnus (NRM) produces hypoalgesia. This hypoalgesia appears to result from disinhibition of raphe-spinal serotonergic neurons since it is blocked by intrathecally injected methysergide. The present studies demonstrate that the hypoalgesia produced by microinjection of phentolamine in the NRM is also abolished by intrathecal administration of phentolamine. The results suggest that the hypoalgesia produced by microinjection of NA antagonists in the NRM is also mediated, at least in part, by the activation of spinally projecting NA neurons. Such hypoalgesia does not appear to be mediated by activation of enkephalinergic neurons since intrathecal injection of the opiate antagonist naloxone did not attenuate the hypoalgesia.