The pinna reflex and its inhibition by clonidine: relationship to sedation and quantitation of central alpha 2-antagonist potency.
The relationship between sedation and pinna reflex inhibition has been measured for a range of centrally acting drugs. Ability to abolish the pinna reflex was not related to sedative activity as assessed by a behavioural method. Thus, at equisedative doses, diazepam, haloperidol, mianserin, prazosin and indoramin failed to abolish the pinna reflex while phenobarbitone and chlorpromazine caused partial- and clonidine complete-inhibition. At the ED50 for pinna reflex inhibition, guanabenz and guanfacine were significantly less sedative than clonidine. Mepyramine, yohimbine, RS-21361, idazoxan and phenylephrine produced little or no sedation and did not inhibit the reflex. When these agents (except for guanabenz and guanfacine) were tested for their ability to prevent clonidine-induced pinna reflex inhibition, all except the drugs with alpha 2-adrenoceptor antagonist activity were inactive. The potency order of the active agents was idazoxan greater than yohimbine greater than RS-21361 = mianserin. Antagonism of clonidine-induced pinna reflex inhibition may therefore prove to be a useful quantitative model for assessing the central potency of alpha 2-adrenoceptor antagonists.
D000317:Adrenergic alpha-Antagonists / Q000494:pharmacology*; D000818:Animals; D001921:Brain / Q000187:drug effects*; D002746:Chlorpromazine / Q000494:pharmacology; D003000:Clonidine / Q000494:pharmacology*; D004431:Ear, External; D006993:Hypnotics and Sedatives / Q000494:pharmacology*; D008297:Male; D051379:Mice; D010634:Phenobarbital / Q000494:pharmacology; D012018:Reflex / Q000187:drug effects*
D000317:Adrenergic alpha-Antagonists; D006993:Hypnotics and Sedatives; D003000:Clonidine; D002746:Chlorpromazine; D010634:Phenobarbital