Activity of corneal nociceptive nerve fibers during allergic challenge of the ocular surface

Abstract

Abstract Purpose The aim of this work was to study in vitro the spontaneous and stimulus‐evoked electrical activity of corneal nociceptive nerve fibers during acute allergic inflammation of the ocular surface induced in the guinea‐pig. Methods Animals received i.p. 10% ovalbumin (OVA). 14 days later, a 10µl drop of OVA was applied topically to each eye. Blinking and scratching movements directed to the eye were measured during 10 min, and ocular symptoms (edema and hyperemia) and tear rate were measured. Animals were killed afterwards and both eyes were immediately excised and mounted in a superfused (32°C) recording chamber. Electrical activity of corneal sensory receptors was recorded from nerve filaments dissected from the ciliary nerves. Mechanical (calibrated von Frey hairs), thermal (bath solution temperature down to 20°C or up to 52°C), and chemical stimulation (30s‐pulses of 98%CO2) were performed. Spontaneous (SA) and stimulus‐evoked activity were analyzed. Results After the allergic challenge, eye‐scratching behavior was present in 4 out of 15 animals and blinking movements increased from 1±0.05 to 26±5. Tearing also increased compared to control (33±3 vs. 5±1 mm). Compared to naive eyes, proportion of nociceptors with SA (17% vs. 5%) and spontaneous discharge rate (0.13±0.07 vs. 0.01±0.01 imp/s) were increased. Mechanical threshold of mechano‐nociceptive units decreased significantly (0.37±0.05 vs. 0.89±0.13 mN). Chemosensitivity of polymodal nociceptors was slightly increased (1.87±0.42 vs. 1.34±0.23 imp/s). Conclusion Changes in corneal sensory nerve activity observed acutely after allergic challenge of the eye surface may constitute the basis of itching and discomfort sensations, and hypersensitivity observed in allergic patients.