TRPM8-dependent activity of corneal cold thermoreceptor neurons contributes to both reflex and spontaneous blink

Abstract

Purpose : To explore the contribution of TRPM8-mediated corneal sensory nerve activity to both reflex (RB) and spontaneous blink (SB) in the rat.

Methods : Orbicularis oculi muscle EMG signals and the activity of trigeminal cold thermoreceptor neurons (CTNs) were recorded in Wistar rats of both sexes. Frequency and amplitude of SB and RB, and CTN activity were measured before and after ocular instillation of saline drops reducing corneal temperature from -0.1°C to -14°C. The effects of TRPM8 antagonist AMTB (1mM) on EMG signals and CTN activity were analyzed. Corneal surface temperature was continuously measured during the experiments with an IR video camera and dedicated software.

Results : All stimuli that decreased ocular surface temperature evoked reflex blinking, regardless of the cooling intensity. EMG signal amplitude was proportional to corneal cooling between -0.1° and -14°C (r²=0.354, p<0.001). AMTB reduced the blink response to cold stimuli (p<0.01). Activity of high background-low threshold CTNs (HBLT; 75% of units) was proportional to low intensity cooling (-0.1 and -0.5°C; r²=0.367, p<0.001). Activity of low background-high threshold (LBHT; 25%) CTNs was proportional to cooling in a wider range (-0.1 to -14°C; r²=0.607, p<0.001). Corneal surface temperature increased by 0.8°C during eye closing and then decreased during the next 10s; CTN activity decreased with warming and started to increase (up to 40%) when the eyelids open after blink. Most CTNs did not respond to cold stimulation after AMTB; both background activity (p<0.01) and peak response to cooling drops (p<0.01) were significantly reduced in both HBLT and LBHT CTNs.

Conclusions : Increased sensory input from the ocular surface driven by TRPM8-dependent activity of trigeminal cold sensitive neurons evokes reflex blink responses. The amplitude and duration of reflex blink are proportional to the corneal temperature change. During the interblink intervals, corneal temperature slightly drops gradually. This mild cooling activates cold thermoreceptor neurons and, although it does not evoke conscious sensations, it seems sufficient to trigger the so-called spontaneous blinking.

This abstract was presented at the 2024 ARVO Annual Meeting, held in Seattle, WA, May 5-9, 2024.