Topical clonidine produces mydriasis by a central nervous system action

One drop of clonidine solution (0.125, 0.25 or 0.5%) was administered topically to one eye in cats anesthetized with pentobarbital in which the vagosympathetic nerves had been sectioned. Clonidine caused a simultaneous dose-related mydriasis in both eyes along with a decrease in heart rate. The peak effects were observed in about 20--30 min. Topical administration of clonidine (0.5%) produced no effect on the parasympathectomized, eserinized iris but did dilate the opposite pupil. Epinephrine (0.1--30 microgram, i.a.) produced equal pupillary dilation in both eyes. In addition, topical clonidine caused a dramatic decrease in postganglionic ciliary nerve activity. All of the effects of clonidine were antagonized by yohimbine (0.5 mg/kg, i.v.). These results demonstrate that topical administration of clonidine causes my driasis in the cat and that this effect is mediated totally by means of CNS inhibition of parasympathetic tone to the iris.