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.
Mosqueda-Garcia
R
R
Department of Pharmacology, McGill University, Montreal, Que., Canada.
Kunos
G
G
D000818:Animals; D001794:Blood Pressure / Q000187:drug effects; D002320:Cardiovascular Physiological Phenomena; D002319:Cardiovascular System / Q000187:drug effects; D003000:Clonidine / Q000008:administration & dosage*; D006339:Heart Rate / Q000187:drug effects; D007275:Injections, Intravenous; D008297:Male; D008526:Medulla Oblongata / Q000187:drug effects; D008845:Microinjections; D009270:Naloxone / Q000008:administration & dosage*; D010111:Oxymorphone / Q000494:pharmacology; D051381:Rats; D011919:Rats, Inbred Strains; D011957:Receptors, Opioid / Q000187:drug effects / Q000502:physiology*
D011957:Receptors, Opioid; D009270:Naloxone; D010111:Oxymorphone; D003000:Clonidine