Abstract
Purpose: :
To study the changes in spontaneous (SA) and stimulus-evoked electrical activity of corneal sensory nerves in a guinea-pig model of allergic conjunctivitis.
Methods: :
Guinea-pigs of both sexes were used. On day 1, control tearing was measured with a modified Schirmer test. Thereafter, ovalbumin (OVA) sensitization was induced by two i.p. injections (500µl) of 100µg OVA+20mg Al(OH)3 in 1ml PBS. On day 14th, a 10µl drop of 10% OVA in PBS was applied to each eye. Blinking and scratching movements directed to the challenged eye were measured during the following 10 min, and tearing measurements were performed. Animals were killed afterwards with an overdose of anesthesia and both eyes were immediately excised. The whole eye or the isolated cornea were mounted in a recording chamber superfused with physiological saline at 32°C. Electrical activity of corneal sensory receptors was recorded from nerve filaments dissected from the ciliary nerves (NF) or from corneal nerve terminals (CNT), using conventional electrophysiological equipment. Thermal stimulation was performed changing the temperature of the bath solution from 32-34ºC down to 20°C or up to 52°C. Mechanical stimulation was applied using calibrated von Frey hairs (NF recordings) or applying pressure with the pipette electrode (CNT recordings). Chemical stimulation was tested, applying for 30s a 98% CO2 gas pulse. The characteristics of the SA and stimulus-evoked activity were analyzed in sensory receptors recorded from control (n=465 NF, 68 CNT) and allergic eyes (n=77 NF, 47 CNT).
Results: :
In animals with allergic conjunctivitis, at day 14th tearing was significantly increased (11.0±1.3 vs. 1.1±0.4 mm; n=14, p<0.001). The percentage of nociceptive units with SA was higher in allergic animals compared to control (11.8 vs. 5.2%, p<0.002) and their mechanical threshold was significantly lower (0.34±0.03 vs. 0.63±0.04 mN) although the response to chemical stimulation was not modified immediately after allergic challenge. The response of CNT to cooling stimuli was also altered during allergic reaction: peak frequency values were significantly lower (20.7±1.4 vs. 28.5±1.3 imp/s; p<0.001) and attained with smaller temperature decreases (-4.7±0.4 vs. -6.7±0.4 ºC; p<0.005).
Conclusions: :
Allergic challenge of the front of the eye produces changes in corneal sensory nerve activity which may contribute to hypersensitivity and discomfort sensations evoked from the eye surface in allergic conditions.
Keywords: innervation: sensation • cornea: epithelium • conjunctivitis