Optochemical modulation of corneal cold nerve terminal impulse activity with a photochromic ion channel blocker.

Background and purpose: The functional organization of corneal cold nerve endings, critical structures in maintaining the ocular surface, remains poorly understood. Here, the photoisomerizable small-molecule diethylamine-azobenzene-quaternary ammonium (DENAQ) was used to photomodulate activity of cold-sensing nerve terminals in control and chronic tear-deficient corneas. Furthermore, DENAQ was used for in vivo photochemical regulation of the thermally induced blink reflex.

Experimental approach: Extracellular nerve terminal impulse activity was recorded on cold terminals in excised corneas of naïve and tear-deficient guinea pigs pre-incubated with DENAQ. Pulses of light at a wavelength of 460 nm were delivered to the perfused corneas. The thermally induced blink reflex was assessed using orbicularis oculi electromyography in anaesthetised rats after topical administration of DENAQ to the eye under blue light and darkness conditions.

Key results: Exposure to blue light robustly reduced spontaneous activity of both naïve and tear-deficient cold nerve terminals pre-incubated with DENAQ, while cold-evoked responses remained unaffected. Pre-incubation of excised corneas with DENAQ, along with pharmacological P2X receptor antagonists, prevented the DENAQ-mediated photoreduction of the cold nerve terminal spontaneous activity. In addition, blue light increased cold-evoked reflex blink in eyes pre-treated with DENAQ.

Conclusion and implications: DENAQ enters guinea pig cold sensory nerve endings primarily through P2X channels in excised corneas. Subsequently, DENAQ decreases the spontaneous nerve terminal impulse activity upon light irradiation by modulating voltage-gated potassium (K_V) channel activity. Furthermore, the cold-evoked blink reflex is modulated by light in DENAQ-treated eyes. Chemical photoswitches like DENAQ might be potential new treatments for ocular discomfort and pain in dry eye disease.