Peripheral opiate receptors are not involved in the naloxone-sensitive cardiovascular effects of clonidine in rats.

The cardiovascular effects of clonidine and their inhibition by naloxone or naloxone methylbromide were tested in urethane-anesthetized, normotensive Sprague-Dawley rats. Clonidine administered intravenously (5 micrograms/kg) or directly into the nucleus tractus solitarii (NTS, 5 nmol) caused hypotension and bradycardia. The effects of intra-NTS clonidine were dose-dependently inhibited by intra-NTS administration of either antagonist, naloxone being 10 times more potent than naloxone methylbromide. The effects of i.v. clonidine were significantly inhibited by 2 mg/kg of i.v. naloxone, but were unaffected by 20 mg/kg of i.v. naloxone methylbromide. Naloxone alone had no effect on blood pressure or heart rate when given either centrally or systemically, whereas naloxone methylbromide given i.v., but not intra-NTS, caused transient hypotension and tachycardia. It is concluded that central but not peripheral opiate receptors are involved in the cardiovascular effects of clonidine.