Morphological and functional changes in TRPM8‐expressing corneal cold thermoreceptor neurons during aging and their impact on tearing in mice

Abstract

Morphological and functional alterations of peripheral somatosensory neurons during the aging process lead to a decline of somatosensory perception. Here, we analyze the changes occurring with aging in trigeminal ganglion (TG), TRPM8-expressing cold thermoreceptor neurons innervating the mouse cornea, which participate in the regulation of basal tearing and blinking and have been implicated in the pathogenesis of dry eye disease (DED). TG cell bodies and axonal branches were examined in a mouse line (TRPM8 -EYFP) expressing a fluorescent reporter. In 3 months old animals, about 50% of TG cold thermoreceptor neurons were intensely fluorescent, likely providing strongly fluorescent axons and complex corneal nerve terminals with ongoing activity at 34°C and low-threshold, robust responses to cooling. The remaining TRPM8 corneal axons were weakly fluorescent with nonbeaded axons, sparsely ramified nerve terminals, and exhibited a low-firing rate at 34°C, responding moderately to cooling pulses as do weakly fluorescent TG neurons. In aged (24 months) mice, the number of weakly fluorescent TG neurons was strikingly high while the morphology of TRPM8 corneal axons changed drastically; 89% were weakly fluorescent, unbranched, and often ending in the basal epithelium. Functionally, 72.5% of aged cold terminals responded as those of young animals, but 27.5% exhibited very low-background activity and abnormal responsiveness to cooling pulses. These morpho-functional changes develop in parallel with an enhancement of tear's basal flow and osmolarity, suggesting that the aberrant sensory inflow to the brain from impaired peripheral cold thermoreceptors contributes to age-induced abnormal tearing and to the high incidence of DED in elderly people.

Functional and Morphologic Alterations in Mechanical, Polymodal, and Cold Sensory Nerve Fibers of the Cornea Following Photorefractive Keratectomy

“…The same abnormal firing has been reported in cold thermoreceptor axons of very old mice, where axonal terminal branching is drastically reduced. 54 Tearing disturbances and dryness sensation in humans appear often after PRK.…”

The Effects of Aging on Corneal and Ocular Surface Homeostasis in Mice

“…37 A number of other laboratories have reported agerelated changes to the ocular surface of C57BL/6J mice, including corneal sensitivity, nerve density, and surface irregularity 13 ; tear volume, surface irregularity, goblet cell changes, and CD4 þ T-cell accumulation in the conjunctiva 16 ; and tear volume, osmolarity, and alterations in TRPM8 þ coldsensing terminals. 15 Our study adds to these previous studies by examining correlations between quantitative clinical parameters (including corneal thickness, corneal sensitivity, tear osmolarity and tear volume) with ex vivo measures of corneal nerve density and corneal epithelial dendritic cell number and size in healthy aged female mice.…”

“…Tear osmolarity is a marker of tear film health that can be used to evaluate changes in the ocular surface environment, and has been shown to increase with aging in C57BL/6J mice. 15 Interestingly, McClellan et al 16 also reported an increase in tear volume and accompanying corneal surface irregularities with increasing age in mice.…”

“…14 Functionally, Stepp et al 13 demonstrated that mechanical corneal sensitivity in mice decreased with age, which paralleled a reduced density of sub-basal nerve axons, despite no change to SNTs. Interestingly, significant age-related changes to the function of cold-sensing TRPM8 þ nociceptors in mouse corneas were recently demonstrated, 15 with low-basal activity TRPM8 þ nociceptors exhibiting more unexpected high sensitivity to cold stimulation in aged mice compared with young mice. However, it is unclear whether corneal nerve architecture and distribution correlate with mechanical corneal sensitivity in mice.…”

Vers une meilleure compréhension des douleurs oculaires chroniques

“…Ainsi, l'ensemble de ces études électrophysiologiques a permis d'identifier trois populations de récepteurs sensitifs sur les terminaisons cornéennes : les récepteurs au froid (cold sensitive receptors) exprimés par les fibres Ad. Vers une meilleure compréhension des douleurs oculaires chroniques 3 aussi sensibles à la variation de l'osmolarité du film lacrymal (Hirata & Rosenblatt, 2014 ;Alcalde et al, 2018). En effet, un film lacrymal hyperosmolaire ayant des valeurs proches de celles trouvées chez les patients souffrant de sécheresse oculaire augmente la sensibilité des neurones sensibles au froid.…”