Yohimbine stimulates colonic motility through a central action in conscious dogs

The effects of the α2‐adrenoceptor antagonist yohimbine on colonic motility were examined before and after vagotomy in conscious dogs chronically fitted with strain gauge transducers on the proximal, transverse and distal colon, and a cannula placed in a lateral ventricle of the brain. They were compared to those of another α2‐adrenoceptor antagonist, SKωF 86466, which poorly crosses the blood‐brain barrier. Intravenous administration of yohimbine (0.2 mg/kg) stimulated the motility of the proximal and transverse colon, the motility index being increased for the first hour by 59 and 55%, respectively, in comparison with controls. No significant increase in motility index was observed on the distal colon. After vagotomy the effects of intravenous yohimbine were limited to 1 to 3 high amplitude contractions. Intracerebroventricular administration of yohimbine (20 μg/kg) reproduced the effects of intravenous administration and was ineffective after vagotomy. Intracerebroventricular but not intravenous administration of SKωF 86466 stimulated the motility of the proximal and transverse colon; this effect was also abolished after vagotomy. It is concluded that yohimbine stimulates colonic motility through a blockade of central α2‐adrenoceptor and that these effects are mediated by the vagus.