Corneal nerve activity during ocular inflammatory processes

Purpose To study the changes in sensory nerve activity of the eye associated with ocular inflammation in two different experimental models developed in the guinea‐pig. Methods Allergic keratoconjunctivitis (AK): Ovalbumin sensitization was induced and blinking and tearing rate was measured before and after the allergic challenge. UV photokeratitis: One eye of the anesthetized guinea‐pig was exposed to 254 nm radiation during different times. Blinking rate and tear secretion were measured before and 48 h after UV. In both models, nerve activity was recorded in vitro in the whole eye or the excised cornea superfused with physiological saline at a controlled temperature. Spontaneous (SA) and stimulus‐evoked electrical activity were recorded. Mechanical, thermal and chemical sensitivity were tested in control and inflamed eyes. Results In AK model tearing and blinking rate increased significantly. In nociceptors, mechanical threshold decreased, the percentage of units with SA increased and the impulse response to chemical stimulation increased significantly in inflamed eyes compared with controls. On the contrary, SA and response to cold of cold thermoreceptors decreased during inflammation. Conclusions In ocular pathological processes primarily characterized by local inflammation, nociceptors became sensitized while cold thermoreceptors became desensitized, due to changes in the expression and/or activity of ion channels present in sensory nerve terminals. Disbalance in the sensory input from the different functional types of sensory nerve fibers innervating the ocular surface in inflammatory conditions may be in the bases of the unpleasant sensations evoked by inflammatory pathological processes affecting the ocular surface in humans.