title: Inhibition of a peripheral sympathetic-cholinergic system by presynaptic alpha 2-adrenoceptors.
abstract: Intravenous administration of catecholamines produced dose-related inhibition of electrodermal responses (EDRs) evoked by electrical stimulation of the post-ganglionic sciatic nerve in anaesthetized rats, with relative potencies being (-)-adrenaline greater than (+/-)-adrenaline greater than (-)-noradrenaline = (+)-adrenaline. The suppression of EDRs by (+/-)-adrenaline and (-)-noradrenaline was blocked by pretreatment with yohimbine (0.75 mg/kg i.v.) but not by prazosin (0.3 mg/kg i.v.). The selective alpha 2-adrenoceptor agonist B-HT 920 also inhibited neurally evoked skin potential responses. This effect of B-HT 920 was antagonized by the selective alpha 2-adrenoceptor antagonist idazoxan (0.1 mg/kg i.v.) but was insensitive to prazosin. Idazoxan was more potent than yohimbine in blocking (+/-)-adrenaline-induced suppression of EDRs. Methacholine administered into the femoral artery evoked EDRs by an atropine-sensitive mechanism. Methacholine-induced EDRs were not suppressed by intravenous administration of (-)-adrenaline (1 microgram/kg or 3 micrograms/kg) whereas EDRs evoked by the sciatic nerve stimulation on the other hindpaw were inhibited. Increase in the endogenous catecholamines by asphyxia strongly inhibited EDRs by a mechanism which was also sensitive to yohimbine but not to prazosin. These results suggest that peripheral presynaptic alpha 2-adrenergic mechanisms are involved in inhibition of transmitter release in this sympathetic-cholinergic system.
mesh_terms: D000317:Adrenergic alpha-Antagonists / Q000494:pharmacology; D000818:Animals; D001237:Asphyxia / Q000503:physiopathology; D001381:Azepines / Q000494:pharmacology; D002395:Catecholamines / Q000494:pharmacology; D002799:Cholinergic Fibers / Q000187:drug effects / Q000502:physiology*; D004146:Dioxanes / Q000494:pharmacology; D004305:Dose-Response Relationship, Drug; D004558:Electric Stimulation; D005712:Galvanic Skin Response / Q000187:drug effects / Q000502:physiology*; D019329:Idazoxan; D008297:Male; D008688:Methacholine Compounds / Q000494:pharmacology; D051381:Rats; D011942:Receptors, Adrenergic, alpha / Q000187:drug effects / Q000502:physiology*; D012584:Sciatic Nerve / Q000502:physiology; D013564:Sympathetic Nervous System / Q000187:drug effects / Q000502:physiology*
chemical_list: D000317:Adrenergic alpha-Antagonists; D001381:Azepines; D002395:Catecholamines; D004146:Dioxanes; D008688:Methacholine Compounds; D011942:Receptors, Adrenergic, alpha; C024763:talipexole; D019329:Idazoxan