Hypoalgesia induced by the local injection of phentolamine in the nucleus raphe magnus: blockade by depletion of spinal cord monoamines.

Blockade of the noradrenergic input to the nucleus raphe magnus (NRM) by the injection of alpha-adrenergic antagonists produces hypoalgesia. Previous studies show that this hypoalgesia is blocked by the intrathecal injection of either phentolamine or methysergide. The present study demonstrates that depletion of spinal cord serotonin, norepinephrine, or both also blocks this hypoalgesia. Together these studies suggest that the hypoalgesia produced by microinjection of noradrenergic antagonists in the NRM is mediated by the activation of both raphe-spinal serotonergic neurons and bulbospinal noradrenergic neurons.