Corneal sensory nerve activity in an experimental model of UV keratitis

Authors
M. C. Acosta, C. Luna, S. Quirce, C. Belmonte, J. Gallar

Lab
Universidad Miguel Hernandez-CSIC, San Juan de Alicante, Spain

Journal
Investigate Ophtalmology and Visual Science

Abstract
Purpose: To produce in guinea-pigs an ultraviolet (UV)-induced keratitis, to analyze the effects of this pathology on corneal nerve activity. Methods: In anesthetized animals, one eye was exposed to 254 nm UV-C radiation (500-1000 mJ/cm2), excised 24-48 h later and superfused 'in vitro'. Nerve impulse activity was recorded in ciliary nerve filaments or in corneal sensory terminals of intact and UV-irradiated eyes. Impulse activity in response to mechanical (von Frey hairs), chemical (98.5% CO2 gas jets) and thermal stimulation (cooling from 34ºC to 20°C; heating to 50°C) was analyzed. Duration of eyelid closure and blinking and tearing rates were evaluated in control and in UV-irradiated eyes, before and after application of TRPV1, TRPA1 and TRPM8 agonists (100µM capsaicin; 10mM AITC and 200µM menthol, respectively). Results: After irradiation, mechanical threshold of mechano-nociceptor corneo-scleral fibers was reduced (0.59±0.4 vs 0.27±0.07 mN; p<0.05) while polymodal nociceptors increased their response to chemical stimulation (1.7±0.2 vs 3.4±0.5 imps/s; p<0.05). In contrast, cold thermoreceptors showed a significantly lower ongoing activity at 34ºC (8.6±0.5 vs 6.1±0.9 imp/s; p<0.05) and a reduced responsiveness to cooling pulses (peak frequency=29.8±1.3 vs 18.9±1.8 imp/s; p<0.001). Blinking but not tearing rate was significantly higher; behavioral responses to topical capsaicin and AITC, but not to menthol were enhanced in UV-irradiated animals. Conclusions: Sensitization of nociceptor and depression of cold thermoreceptor activity following UV radiation appear to result from an action of inflammatory mediators on TRP channels selectively expressed by sensory nerve terminals. Changes in nerve activity possibly underlie discomfort sensations associated to corneo-conjunctival inflammation induced by UV exposure.

BIOSEB Instruments Used:
Von Frey Filaments (Bio-VF-M)