Pupillary constriction in response to light in rodents, which does not depend on central neural pathways

Abstract

We show here that the widely held belief that reflex constriction of the mammalian pupil in response to light depends exclusively upon neural pathways between eye and brain is in need of revision. We investigated the response of the pupil to light in dark-adapted rodents (golden hamsters; hooded rats; albino rats) subjected to a variety of surgical and pharmacological interventions designed to destroy or block all of the neural pathways and structures through which the reflex could be mediated. The interventions included bilateral intraorbital optic nerve section, or unilateral intracranial optic nerve section with enucleation of the contralateral eye, combined in some cases with bilateral removal of the superior cervical ganglia and/or pinealectomy; topical application of atropine; intraocular injection of tetrodotoxin (TTX). Golden hamsters and hooded rats, but not albino rats, retained an effective constriction of the pupil in response to light after all of these interventions, although the constriction was less and slower than in normal animals. These findings show that hamsters and hooded rats have both a neurally mediated fast light reflex that can be eliminated by severing connections between eye and brain, by blockade of cholinergic transmission to iris smooth muscle, and by blockade of action potentials by TTX; and a local, slower constriction in response to light, which remains after all these procedures. We have also confirmed previous observations of Bito and Turansky (1975) that pupillary constriction in response to light occurs in isolated in vitro anterior chamber preparations of hamster and hooded rat eyes. However, we were not able to demonstrate constriction in identical preparations of albino rat eyes. The data suggest on the basis of both in vivo and in vitro studies, that the hamster and hooded rat, but not the albino rat, possess an autonomous local mechanism for controlling pupillary diameter that is completely independent of the retina or the brain.