Antagonism of the emetic action of xylazine by alpha-adrenoceptor blocking agents.

The intramuscular injection of xylazine (2 mg/kg) evoked vomiting in 81% of the dogs studied. Adrenoceptor antagonists showing alpha 2-blocking activity, yohimbine, tolazoline and phentolamine, antagonized the xylazine-induced vomiting in a dose-dependent manner. Of these antagonists, yohimbine was the most effective, since the maximal antagonistic effect was seen at 0.5 mg/kg yohimbine, a dose at which the other drugs had less or no effect. The adrenoceptor antagonists showing alpha 1-blocking activity, prazosin and phenoxybenzamine, at the doses studied did not prevent the emesis induced by xylazine. A beta-adrenoceptor antagonists, propranolol, was ineffective in reducing xylazine-induced vomiting. The dopamine receptor antagonists, metoclopramide and domperidone, did not prevent xylazine-induced vomiting nor did yohimbine antagonize apomorphine-induced vomiting. The xylazine-induced vomiting was not prevented by atropine, naloxone or hexamethonium. These results indicate that the xylazine-induced vomiting is mediated by alpha 2-adrenoceptors and does not appear to involve beta-adrenoceptors, cholinoceptors, dopamine or opiate receptors in the emetic pathway.