Interference of TRPA1 function affects background activity of corneal cold thermoreceptors in ageing mice

Abstract

PurposeTRPA1 and TRPM8 channels provide cold sensitivity to cold thermoreceptor neurons, being activated by high‐ and low‐intensity cooling stimuli respectively. Corneal cold sensory nerve terminals (CSNTs) encode both corneal surface temperature reductions and tear osmolarity increases associated with tear evaporation, acting as `dryness detectors′. TRPM8‐dependent activity of CSNTs contribute to basal tearing and is disturbed with ageing, while the role of TRPA1‐dependent activity is still unknown. In the present work, electrical activity of CSNTs from TRPA1‐KO mice was studied to define the contribution of TRPA1 channels to cold thermoreceptor sensory input to CNS.MethodsNerve terminal impulse (NTI) activity of CSNTs was recorded in vitro with a glass micropipette applied to the corneal surface of eyes from C57BL/6J (WT) and TRPA1‐KO mice (18‐22 months old) continuously superfused with physiological saline solution at 34°C. Background activity at basal temperature (34°C) and the impulse response to cooling ramps (from 34 to 15°C) and to TRP channel agonists (menthol, AITC, capsaicin; added to the perfusion solution) were analyzed. Tearing rate was measured with phenol red threads.ResultsBackground NTI activity of CSNTs at 34°C was slightly reduced in old TRPA1‐KO mice compared with aged‐matched WT (4.5±2.2 imp/s, n=10, vs 7.7±1.2 imp/s, n=11, p=0.22, t‐test), due to a reduced proportion of high background‐low threshold cold CSNTs. No significant differences were found in response to cooling ramps nor in tearing rate between WT and TRPA1‐KO mice.ConclusionsKnocking TRPA1 alters background activity of CSNTs but not their response to temperature decreases, suggesting that TRPA1‐dependent input to CNS is no essential for regulation of basal tearing.(Supported by SAF2014‐54518‐C3‐1‐R, MINECO, Spain, and ERDF, European Commission)