Analysis of clonidine-induced mydriasis

In addition to its centrally mediated hypotensive action, clonidine causes a decrease in intraocular pressure associated with a long-lasting mydriasis. The present study was conducted to determine to what extent this drug-induced pupillary dilation is of central or peripheral origin. Pupil size was observed in cats anesthetized with pentobarbital. Clonidine (1 to 100 mug per kilogram, intravenously) resulted in a dose-dependent increase in pupillary diameter in intact as well as sympathectomized preparations. These same doses of clonidine produce no effect on the eserinized, parasympathectomized iris. Epinephrine administration (0.1 to 30 mug, intra-arterially) produced an equivalent pupillary dilation in all preparations. In addition, clonidine caused a dramatic decrease in postganglionic ciliary nerve activity and both the decreased nerve activity and pupillary dilation were reversed by intravenous administration of yohimbine hydrochloride. These results suggest that the inhibition of parasympathetic tone by clonidine may involve a central adrenergic inhibitory mechanism.