The sympathetic nervous system is involved in the inhibitory effect of morphine on the colon motility in rats

The role of the sympathetic nervous system in the inhibitory effect of morphine on colonic motility was investigated in male adult Wistar rats. The responses of colonic motility and blood pressure to the intravenous administration of morphine under urethane anesthesia were recorded. Sympathectomy (6-hydroxydopamine) or pretreatment with phentolamine, an α-adrenoreceptor antagonist (3.15 μM/kg, i.v.), or propranolol, a β-adrenoreceptor antagonist (3.38 μM/kg, i.v.), on the inhibitory effect of intravenously administered morphine on colonic motility were observed. The results of the investigation showed that a significant depression of colonic motility occurred in untreated rats following intravenous administration of morphine, while no significant effect was seen in the sympathectomized. Pretreatment with phentolamine or propranolol also significantly attenuated the depression of colonic motility induced by morphine. Morphine also produced a transient depression of the blood pressure. However, this inhibitory effect of morphine on blood pressure was intensified after sympathectomy or pretreatment with either phentolamine or propranolol. We conclude that sympathetic activity plays an important role in the inhibitory effects of morphine on colonic motility and that both α- and β-adrenoreceptors are involved.