Participation of a bulbospinal serotonergic pathway in the rat brain in clonidine-induced hypotension and bradycardia.
The effects of microinjection of clonidine (1-10 micrograms in 1 microliter) into a region adjacent to the ventrolateral surface of the medulla oblongata on cardiovascular function were assessed in urethane-anesthetized rats. Intramedullary administration of clonidine, but not saline vehicle, caused a dose-dependent decrease in both the mean arterial pressure and the heart rate. The clonidine-induced hypotension was antagonized by prior spinal transection, but not bilateral vagotomy. On the other hand, the clonidine-induced bradycardia was antagonized by prior bilateral vagotomy, but not spinal transection. Furthermore, selective destruction of the spinal 5-HT nerves, produced by bilateral spinal injection of 5,7-dihydroxytryptamine, reduced the magnitude of the vasodepressor or the bradycardiac responses to clonidine microinjected into the area near the ventrolateral surface of the medulla oblongata in rats. The data indicate that a bulbospinal serotonergic pathway is involved in development of clonidine-induced hypotension and bradycardia. The induced hypotension is brought about by a decrease in sympathetic efferent activity, whereas the induced bradycardia was due to an increase in vagal efferent activity.
Department of Physiology, College of Medicine, National Cheng Kung University, Taman, Taiwan, ROC.
M L
ML
Tsai
M T
MT
Lin
D015116:5,7-Dihydroxytryptamine / Q000494:pharmacology; D000818:Animals; D001794:Blood Pressure / Q000187:drug effects*; D003000:Clonidine / Q000494:pharmacology*; D004558:Electric Stimulation; D006339:Heart Rate / Q000187:drug effects*; D008297:Male; D008526:Medulla Oblongata / Q000187:drug effects*; D009434:Neural Pathways / Q000187:drug effects*; D051381:Rats; D011919:Rats, Inbred Strains; D012701:Serotonin / Q000097:blood / Q000502:physiology*; D013116:Spinal Cord / Q000187:drug effects* / Q000502:physiology; D013268:Stimulation, Chemical; D014628:Vagotomy
D015116:5,7-Dihydroxytryptamine; D012701:Serotonin; D003000:Clonidine