Xylazine-induced sedation in chicks is inhibited by opiate receptor antagonists.

The sedative effect of xylazine and its interaction with antagonists of alpha-adrenoceptors or opiate receptors was examined in chicks. The duration of the sleep-like behavioral state induced by xylazine was determined by measuring the time interval during which the chicks failed to exhibit the righting reflex. In these chicks, intramuscular administration of xylazine (0.3-4.8 mg/kg) induced a loss of the righting reflex, the duration of which was dose-dependent. Both alpha-adrenoceptor antagonists of the alpha2 type, i.e. yohimbine (0.1-1.0 mg/kg) and compound 170150 (0.1-1.0 mg/kg) (a benzodioxane derivative), and opiate receptor antagonists, i.e. naloxone (0.4-1.6 mg/kg) and nalorphine (3.0-30.0 mg/kg), effectively inhibited the sedative effect of xylazine while prazosin (0.1-10.0 mg/kg), an alpha 1-adrenoceptor antagonist, failed to antagonize xylazine-induced sedation. These findings suggest that in addition to the stimulation of central alpha 2-adrenoceptors, activation of an endogenous opiate mechanism may be involved in the sedative effect of xylazine.