Evidence for the involvement of alpha 2-adrenoceptors in the emetic action of xylazine in cats.

Intramuscular injection of xylazine induced dose-dependent vomiting in cats (ED50 = 0.277 mg/kg); administration of standard dose of xylazine (2 mg/kg, 2 times the 100% emetic dose) induced vomiting in 100% of the cats studied. The xylazine-induced vomiting was antagonized by adrenoceptor antagonists possessing alpha 2-blocking activity, which were yohimbine, tolazoline, and phentolamine. Of these antagonists, yohimbine was the most effective; the maximal antagonistic effect was seen at 1 mg of yohimbine/kg, a dose at which the other drugs had little or no effect. At the doses studied, prazosin and phenoxybenzamine, adrenoceptor antagonists with alpha 1-blocking activity, did not prevent vomiting induced by xylazine. Beta-Adrenoceptor (propranolol), dopamine receptor (domperidone and chlorpromazine), a cholinoceptor (atropine), an opiate receptor (naloxone), and a histamine-receptor (diphenhydramine) antagonists, at the doses studied, did not prevent xylazine-induced vomiting. Pretreatment with 6-hydroxydopamine failed to prevent xylazine-induced vomiting. These results indicated that xylazine-induced vomiting in cats is mediated by alpha 2-adrenoceptors and suggested that the alpha 2-adrenoceptors mediating the vomiting attributable to xylazine may not be presynaptic alpha 2-receptors located on noradrenergic nerve terminals.