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C. L. Luna, S. Quirce, C. Belmonte, J. Gallar, M. C. Acosta; Corneal Nerve Activity in UV-Inflamed Corneas. Invest. Ophthalmol. Vis. Sci. 2010;51(13):1962.
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To study the changes in spontaneous and stimulus-evoked impulse activity of corneal sensory nerve fibers after exposure of the cornea to ultraviolet radiation (UVR).
Albino and pigmented guinea-pigs were anesthetized and one eye was exposed during 7.5-49 min to 254 nm UV-C radiation (100, 500 or 1000 mJ/cm2). Tear secretion was measured before and 24-48 h after UVR using phenol-red threads. 24-48 h after UVR, animals were killed with an overdose of anesthesia and both eyes immediately excised. Whole eyes or isolated corneas were mounted in a recording chamber and superfused with physiological saline at 32°C. Electrical activity was recorded from nerve fibers in the back of the eye or from individual endings in the cornea, using conventional electrophysiological equipment. For thermal stimulation, the basal temperature of the bath solution was changed down to 20°C or up to 52°C. Mechanical stimulation was performed with calibrated von Frey hairs. Chemical stimulation was tested applying 30s gas jets of 98% CO2 in air. The characteristics of the spontaneous and evoked activity were analyzed in the control and the irradiated eye.
UVR did not change tear secretion but produced intensity and time-dependent changes in corneal sensory receptor activity. 24 h after 1000 mJ/cm2 UVR, mechanical threshold of corneal nociceptors was significantly decreased (0.29±0.04 mN versus 0.65±0.08 mN, n=22-92 in irradiated and control eyes, respectively) and the impulse response to chemical stimulus increased significantly (3.04±1 imp/s in treated versus 1.34±0.23 imp/s, n=8-18). Also, the percentage of corneal nociceptors with spontaneous activity increased from 3.1% to 14.8% after UVR. Cold receptors of irradiated eyes reached their maximum frequency of discharge with smaller decreases of temperature. No differences were detected between pigmented and albino animals.
UV radiation produced an increase in the proportion of corneal nociceptors firing spontaneously in absence of intended stimulation and augmented their spontaneous and stimulus-evoked nerve impulse activity. UV radiation also changed the responsiveness of corneal cold receptors. These findings are consistent with a sensitization of sensory nerve endings associated to corneal inflammation induced by UV exposure.
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