Effects of clonidine and yohimbine on a C-fibre-evoked blood pressure reflex in the rat.
1. The involvement of alpha 2-adrenoceptors in a capsaicin-induced depressor reflex in the rat has been investigated. 2. The reflex fall in blood pressure following an intra-arterial injection of capsaicin was partially blocked by an intrathecal infusion of clonidine (1 microgram) at the spinal cord level L4-L5, an effect which was abolished by the prior intrathecal infusion of yohimbine (5 micrograms). 3. Clonidine (1 microgram) given intracisternally blocked the depressor reflex almost completely; however, it also produced a gradual, prolonged fall in mean carotid pressure. 4. Yohimbine (20 micrograms) given intracisternally did not change mean blood pressure but had a dual effect on the depressor reflex in response to the capsaicin injection: a short period of enhanced reflex response was followed by a long lasting inhibition of the response. 5. It was concluded that alpha 2-adrenoceptors in the spinal cord inhibit the capsaicin-evoked depressor reflex and that pre- and postsynaptic alpha 2-adrenoceptors in the brain stem modulate this reflex.
Donnerer
J
J
Department of Experimental and Clinical Pharmacology, University of Graz, Austria.
Yan
Z
Z
Lembeck
F
F
D000818:Animals; D001794:Blood Pressure / Q000187:drug effects*; D002211:Capsaicin / Q000037:antagonists & inhibitors / Q000494:pharmacology*; D002946:Cisterna Magna; D003000:Clonidine / Q000008:administration & dosage / Q000494:pharmacology*; D005260:Female; D007267:Injections; D007278:Injections, Spinal; D008297:Male; D009412:Nerve Fibers / Q000187:drug effects*; D051381:Rats; D011919:Rats, Inbred Strains; D012018:Reflex / Q000187:drug effects; D015016:Yohimbine / Q000008:administration & dosage / Q000494:pharmacology*
D015016:Yohimbine; D003000:Clonidine; D002211:Capsaicin