Pharmacological evaluation of in vivo tests for alpha 2-adrenoceptor blockade in the central nervous system and the effects of the enantiomers of mianserin and its aza-analog ORG 3770.

A series of compounds with actions on the central nervous system was tested for antagonism of clonidine-induced sleep in chicks and clonidine-induced mydriasis in rats for the purpose of evaluating these methods as tests for demonstrating an in vivo alpha 2-adrenoceptor blocking effect of a novel compound. Clonidine-induced mydriasis was found to be the most selective method. The order of potency for compounds fully antagonizing clonidine-induced mydriasis was MSD 26 greater than physostigmine = idazoxan greater than aptazapine greater than piperoxan greater than yohimbine greater than mianserin greater than tolazoline. Partial antagonism was found for quipazine and sulpiride. Misleading results can arise from the involvement of cholinergic mechanisms in the control of the pupil diameter. The order of potency for compounds antagonizing clonidine-induced sleep in chicks was apomorphine greater than yohimbine greater than idazoxan greater than aptazapine = MSD 26 greater than quipazine greater than methysergide greater than piperoxan = mianserin = bepridil = metergoline = cyproheptadine = desipramine greater than tolazoline greater than dexchlorpheniramine, although antagonism was not complete for all of these compounds. Misleading results can arise from effects on arousal of the chicks but cholinergic mechanisms do not play a disturbing role so that the method with chicks can be a useful supplement to the mydriasis method. The enantiomers of mianserin and of a compound related to mianserin, Org 3770, were tested in the 2 methods and the alpha 2-blocking effect of these compounds was found to be residing in the S(+)-enantiomers.