A Note of the Corneal Anesthesia Produced by Pilocarpine Administration

Abstract

The mydriasis which the administration of pilocarpine causes in the rat (Waddell, 1 Koppanyi 2 ) is one of the numerous paradoxical responses elicited by this drug. During the course of an investigation on the nature of iris sphincter tone in this animal, it was noticed that the topical or subcutaneous administration of pilocarpine resulted in corneal anesthesia. Inasmuch as it has been shown that substances capable of acting as surface anesthetics would cause the pupil of the rat to dilate (Barnard 3 ) it was thought, at first, that we held the key to the explanation of the pilocarpine mydriasis. Certain facts, however, indicate that pilocarpine does not owe its pupillo-dilator properties to the corneal anesthesia following its administration. The mydriasis is of a greater extent than that following the application of some local anesthetic. Furthermore, pilocarpine constricts the pupil of the mouse, whereas local anesthetics such as hexylresorcinol and butyn dilate the pupil in this animal as they do in the rat (Heidgen and Barnard 4 ).

Using graduated hairs for stimulation of the cornea, and accepting an absence of the winking reflex as a criterion for anesthesia, the effect of subcutaneous administration of pilocarpine was determined.

The number of experimental animals was, 34 albino rats, 9 albino mice, 5 guinea pigs, and 2 rabbits. Before the administration of pilocarpine the application of a pledget of cotton to the cornea would result in a closing of the eye, it being sufficient merely to touch the cornea very lightly. All animals were thus tested both before the administration of pilocarpine and after the removal of the pilocarpine anesthesia by atropinization. This constituted the control.