Decreased basal tear in aged mice is linked to morphological and functional changes in corneal sensory nerve fibers

Purpose To determine whether the effects of aging on density, morphology and functional activity of corneal cold sensory nerve fibers are associated with changes in basal tearing rate. Methods TRPM8-EYFP and TRPM8-KO mice of different ages ranging from 90 to 720 postnatal days were studied. Basal tearing was measured in anesthetized animals, using phenol red threads. Corneal nerves expressing EYFP protein were identified in whole mount corneas using immunohistochemical techniques. Also, trigeminal ganglia (TG) were processed for immunofluorescence techniques against TrkA and CGRP. Extracellular electrical activity of single sensory nerve endings of the corneal surface was recorded in excised and superfused eyes. Results The number of TRPM8+ fibers and nerve terminals decreased with age. Also, fibers of p90 mice were morphologically different from those of p720 TRPM8-EYFP mice resembling those found in TRPM8-KO mice. Basal tearing rate decreased in parallel with the reduction in number of TRPM8+ fibers. Two different populations of sensory nerve endings responding to cold were distinguished electrophysiologically: Low and High-threshold cold terminals. With aging, the incidence of low-threshold cold nerve endings decreased while more high-threshold nerve endings were found. Also, a population of non-peptidergic and another of peptidergic TRPM8+ neurons were distinguished in the TG. Conclusion Corneal cold sensory fibers and terminals of TRPM8-EYFP mice developed parallel changes in number and morphology, functional activity and basal tearing rate during aging, thus supporting the hypothesis that cold thermoreceptor activity and basal tearing are functionally associated.