Role of TRPM8 channels in altered cold sensitivity of corneal primary sensory neurons induced by axonal damage

The cornea is extensively innervated by trigeminal ganglion cold thermoreceptor neurons expressing TRPM8. These neurons respond to cooling, hyperosmolarity and wetness of the corneal surface. Surgical injury of corneal nerve fibers alters tear production and often causes dry eye sensation. The contribution of TRPM8-expressing corneal cold-sensitive neurons (CCSNs) to these symptoms is unclear. Using extracellular recording of CCSNs nerve terminals combined with in vivo confocal tracking of re-innervation, Ca2+ imaging and patch-clamp recordings of fluorescent retrogradely labeled corneal neurons in culture, we analyzed the functional modifications of CCSNs induced by peripheral axonal damage in male mice. After injury, the percentage of CCSNs, the cold- and menthol-evoked intracellular [Ca2+] rises and TRPM8-current density in CCSNs were larger than in sham animals, with no differences in the brake K+ current IKD. Active and passive membrane properties of CCSNs from both groups were alike, and corresponded mainly to those of canonical low- and high-threshold cold thermoreceptor neurons. Ongoing firing activity and menthol-sensitivity were higher in CCSN terminals of injured mice, an observation accounted by mathematical modeling. These functional changes developed in parallel with a partial re-innervation of the cornea by TRPM8(+) fibers and with an increase in basal tearing in injured animals compared to sham mice. Our results unveil key TRPM8-dependent functional changes in CCSNs in response to injury, suggesting that increased tearing rate and ocular dryness sensation derived from deep surgical ablation of corneal nerves are due to enhanced functional expression of TRPM8 channels in these injured trigeminal primary sensory neurons. SIGNIFICANCE STATEMENT This paper unveils a key role of TRPM8 in the sensory and autonomic disturbances associated with surgical damage of eye surface nerves. We studied the damage-induced functional alterations of corneal cold-sensitive neurons using confocal tracking of re-innervation, extracellular corneal nerve terminal recordings, tearing measurements in vivo, Ca2+ imaging and patch-clamp recordings of cultured corneal neurons, and mathematical modeling. Corneal nerve ablation upregulates TRPM8 mainly in canonical cold thermoreceptors, enhancing their cold- and menthol-sensitivity, inducing a rise in the ongoing firing activity of TRPM8(+) nerve endings and an increase in basal tearing. Our results suggest that unpleasant dryness sensations together with augmented tearing rate following corneal nerve injury are largely due to upregulation of TRPM8 in cold thermoreceptor neurons.