Naloxone does not prevent apomorphine-induced emesis or hypotension in dogs

Previous data have shown that apomorphine-induced respiratory depression can be reversed by the opiate antagonist, naloxone. The present study investigates the influence of naloxone on cardiovascular changes and vomiting elicited by apomorphine in dogs. In chloralose-anaesthetized animals, naloxone (0.02 mg/kg i.v.) failed to modify either the decrease in blood pressure and the biphasic changes (bradycardia followed by a long-lasting tachycardia in heart rate or the characteristics (occurrence, latency, duration) of the emesis elicited by apomorphine (200 micrograms/kg i.v.). In contrast, in conscious animals, naloxone (0.02 mg/kg i.v.) increased both the number and the duration (but not latency) of vomiting induced by a lower dose of apomorphine (30 micrograms/kg i.v.). These data show that apomorphine-induced vomiting and arterial hypotension do not involve opiate receptors.