Possible involvement of central adrenergic and histaminergic systems in clonidine-induced inhibition of pinna reflex

Clonidine produced dose-dependent inhibition of pinna reflex in rats, the ED 50 being 1.20 ± 0.28 mg/kg. The effect of agents affecting adrenergic, tryptaminergic and histaminergic systems were evaluated on clonidine-induced inhibition of pinna reflex. All these agents had no effect on pinna reflex. Phentolamine, phenoxybenzamine, prazosin, proprarolol, haloperidol, cyproheptadine and mepyramine did not affect significantly the action of clonidine. However, reserpine, alpha—methyl-p-tyrosine, alpha—methyldopa, diethyldithiocarbamate, 6-hydroxydopamine, yohimbine and cimetidine significantly blocked clonidine-induced inhibition of pinna reflex, indicating the involvement of central adrenergic-histaminergic systems.