EFFECTS OF AGING ON NEUROCHEMICAL PROPERTIES OF MOUSE CORNEAL COLD SENSORY NEURONS, CHANGES IN MORPHOLOGY IN THEIR AFFERENT PROJECTIONS AND IMPLICATIONS IN BASAL TEAR SECRETION

Abstract

Purpose: To determine in aging mice the proportion, morphology and neurochemical characteristics of trigeminal ganglion cold primary sensory neurons and of their cold peripheral axons innervating the cornea, and the relationship of morphological changes with basal tearing rate.

Methods: Corneas and trigeminal ganglion (TG) obtained from transgenic TRPM8-EYFP and TRPM8 knock out mice of ages ranging from 90 to 720 postnatal days were studied. Basal tearing was measured in anesthetized animals using phenol red threads. Corneal fibers expressing EYFP were identified in whole mount corneas using ABC-Peroxidase immunohistochemical technique. TG neurons projecting to the cornea were traced with Fast Blue applied onto the cornea of the mice and were identified in TG sections using immunofluorescence techniques against peripherin, neurofilaments 200, TrkA neurotrophic receptor and calcitonin gene related peptide (CGRP).

Results: The density of TRPM8 fibers in the subbasal plexus and of epithelial nerve terminals in the cornea decreased with age. Subbasal nerve fibers of young TRPM8-EYFP mice showed characteristic beaded morphology and their nerve terminals ended in the uppermost ephitelium layers as a cluster of free endings. In old mice, TRPM8 fibers did not show beads, ending as single or double bulbous terminal branches. In the TG two population of TRPM8 corneal neurons with different neurochemical signature were identified. The proportion and molecular pattern of these two neuronal populations changed in old mice. In parallel, basal tearing rate was altered in aged mice.

Conclusions: Changes in the in morphological and neurochemical properties of corneal cold sensory neurons and their peripheral axons may be associated with the disturbances in basal tearing rate observed in mice with aging.