PEPTIDERGIC INNERVATION OF THE RAT CORNEA

Corneal nerves regulate corneal epithelial integrity, proliferation, a nd wound healing. The mechanisms by which the nerves mediate their eff ects remain poorly understood; however, the release of biologically ac tive neuropeptides has been hypothesized. The purpose of the current i nvestigation was to determine the relative densities, distribution pat terns, and origins of rat corneal nerves containing each of eight diff erent neuropeptides, calcitonin gene-related peptide (CGRP), substance P (SP), galanin (GAL), neuropeptide Y (NPY), methionine-enkephalin (M -ENK), vasoactive intestinal polypeptide (VIP), somatostatin (SOM), an d cholecystokinin (CCK). In the first set of experiments, immunohistoc hemical demonstrations of the above neuropeptides were performed on fr ee-floating corneal sections cut tangential to the corneal surface. Th e results showed that six of the peptides, CGRP, SP, GAL, NPY, M-ENK, and VIP were present in rat corneal nerves. The innervation patterns o f corneal nerves containing each of these six peptides were then docum ented by mapping all fibers in serial sections from select corneal qua drants onto a series of Line drawings by using a drawing tube. In the second set of experiments, the origins of the corneal peptidergic nerv e fibers were determined by selective ocular denervations. Unilateral combined sensory and sympathetic ocular denervations or unilateral sym pathetic ocular denervations were performed in adult rats by transecti ng the ophthalmomaxillary nerve and/or removing the superior cervical ganglion. After 5-7 days, each of the ipsilateral corneas was sectione d and processed immunohistochemically for the presence of one of the s ix peptides found in experiment one, and the fibers that survived the ocular denervations were plotted onto line drawings. Ocular denervatio ns revealed that corneal peptidergic nerves have sensory (CGRP, SP, an d GAL), sympathetic (NPY), and parasympathetic (GAL, NPY, M-ENK, and V IP) origins, The results of this investigation have shown that the pep tidergic innervation of the rat cornea is more extensive and complex t han previously reported. This is the first investigation to show the p resence of GAL in the rat cornea, and the first to demonstrate the pre sence of NPY-, VIP-, and M-ENK-IR nerve fibers in the cornea of any sp ecies. (C) 1998 Academic Press Limited.