Spinal cord monoamines modulate the antinociceptive effects of vaginal stimulation in rats

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Abstract
Perispinal administration (into the lumbar intrathecal space) of phentolamine (40 .mu.g), an .alpha.-adrenergic receptor blocking agent, reduced the analgesic effect of vaginal stimulation of 39.7% (measured by tail flick latency) and 57.1% (measured by vocalization threshold) as compared to controls. Perispinal administration of methysergide (10 .mu.g), a serotoninergic receptor blocking agent, reduced the analgesic effect of vaginal stimulation by 48.5% (measured by vocalization threshold), although it did not significantly affect the tail flick measure. In a separate experiment, vaginal stimulation activated the release of norepinephrine and serotonin 5-hydroxytryptamine into a superfusate of the spinal cord. During vaginal stimulation, levels of norepinephrine and serotonin increased $ 2-fold above resting levels. Vaginal stimulation probably releases norepinephrine and serotonin into the spinal cord, thereby exerting an analgesic effect.