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

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Abstract
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. Depletion of spinal cord serotonin, 5-hydroxytryptamine norepinephrine or both also blocks this hypoalgesia. Evidently 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.