EXCITATION BY IRRITANT CHEMICAL-SUBSTANCES OF SENSORY AFFERENT UNITS IN THE CATS CORNEA

1. Single-unit electrical activity was recorded from thin myelinated sensory nerve fibres innervating the cornea of deeply anaesthetized cats. 2. Based on their responses to mechanical (calibrated von Frey hairs), chemical (10 mM-acetic acid and/or 616 mM-NaCl) and thermal (ice-cold or heat up to 51-degrees-C) stimuli, corneal A-delta fibres were classified as polymodal nociceptors (63%), high-threshold mechanoceptors (22%) and mechano-heat nociceptors (15%). Thin myelinated fibres responding only to cold were found in the limbus of the eye. 3. Application of 10 mM-acetic acid on the corneal surface for 30 s evoked in polymodal fibres a brisk discharge of impulses often followed by a low-frequency impulse activity. NaCl (616 mM) produced a more gradual and sustained firing response. 4. The responses of polymodal fibres to acid were proportional to extracellular pH values (pH range: 4.5-6.0). After sensitization to repeated heating, most mechano-heat units developed a sensitivity to acidic stimulation. 5. Topical 0.33 mM-capsaicin excited polymodal nociceptors of the cornea; 5 min after capsaicin about 15% of these fibres were inactivated to all subsequent stimuli. In the rest of the fibres, chemical and thermal sensitivity disappeared after 0.33-3.3 mM-capsaicin, but mechanosensitivity was preserved. 6. Corneal mechanoceptors and limbal cold receptors were not affected by capsaicin (up to 33 mM). 7. These experiments demonstrate that the cornea of the cat is innervated by polymodal as well as mechanoceptive A-delta nociceptors. In polymodal nociceptive fibres, mechanical and chemical sensitivities appear to be subserved by separate transduction mechanisms.