Hypoalgesia induced by microinjection of a norepinephrine antagonist in the raphe magnus: reversal by intrathecal administration of a serotonin antagonist.

Microinjection of phentolamine in the nucleus raphe magnus produces hypoalgesia as assessed by both the tail flick and hot plate tests. The elevation of tail flick latency appears to result from activation of serotonergic raphe-spinal projections, while the elevation of hot plate latency appears to be mediated by activation of non-serotonergic raphe-spinal neurons and/or by activation of raphe neurons which project to supraspinal nuclei.