Free nerve endings are key structures in sensory transduction of noxious stimuli. In spite of this, little is known about their functional organization. Transient receptor potential (TRP) channels have emerged as key molecular identities in the sensory transduction of pain-producing stimuli, yet the vast majority of our knowledge about sensory TRP channel function is limited to data obtained from in vitro models which do not necessarily reflect physiological conditions. In recent years, the development of novel optical methods such as genetically encoded calcium indicators and photo-modulation of ion channel activity by pharmacological tools has provided an invaluable opportunity to directly assess nociceptive TRP channel function at the nerve terminal.

Purpose Photo‐isomerizable small molecules (photoswitches) allow modulation of neural activity acting on native ion channels without requiring exogenous gene expression. DENAQ, a synthetic photoswitch, confers light‐sensitivity to retinal ganglion cells in a mouse model of retinitis pigmentosa. However, there are no direct evidences on whether photoswitches can modulate electrical activity of neural structures located far away from the neuron cell body, such as corneal sensory nerve terminals. Our objective was to photo‐modulate the activity of corneal cold sensory nerve terminals (CSNTs) in excised guinea‐pig corneas pre‐incubated with DENAQ. Methods Corneas were incubated with 2 mM DENAQ (40 min at 34°C). Then, electrical activity of CSNTs was recorded extracellularly from corneas superfused at 34°C. A LED was used to deliver pulses of blue light (460 nm, 125 mW/cm2) to the perfused corneas. Light stimulation protocol consisted of 5 cycles of alternating 15s light/dark intervals. Results Blue light had no effect on the ongoing activity (OA) of CSNTs recorded from naïve corneas, not pre‐incubated with DENAQ (7.4±0.4 vs 7.4±0.4 imp·s‐1, dark vs blue light, n=8, p=0.79, paired t‐test). In corneas pre‐incubated with DENAQ, OA of CSNTs was significantly higher in the dark than under blue light (4.5±0.8 vs 2.3± 0.4 imp·s‐1, p=0.001, n=8). OA of CSNTs under blue light was significantly lower in DENAQ pre‐incubated corneas than in naïve corneas (p=0.013, t‐test). Conclusions DENAQ produces a robust decrease of the spontaneous electrical activity of guinea pig corneal cold sensory nerve endings in the presence of blue light. Small molecules acting as chemical photoswitches may be potentially useful as drugs for patients with ectopic corneal nerve activity, such as in dry eye and ocular neuropathic pain. (SAF2014‐54518‐C3‐1‐R and ‐2‐R, MINECO, Spain, and ERDF, EC)