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

Perispinal administration (into the lumbar intrathecal space) of phentolamine (40 micrograms), an alpha-adrenergic receptor blocking agent, reduced the analgesic effect of vaginal stimulation by 39.7% (measured by tail flick latency) and 57.1% (measured by vocalization threshold) as compared to controls. Perispinal administration of methysergide (10 micrograms), 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 into a superfusate of the spinal cord. During vaginal stimulation, levels of norepinephrine and serotonin increased about 2-fold above resting levels. These findings indicate that vaginal stimulation releases norepinephrine and serotonin into the spinal cord, thereby exerting an analgesic effect.