Response of Corneal Cold Nerve Terminals to Thermal Changes

Abstract

Abstract: : Purpose: To study the characteristics of the corneal cold nerve terminals (CNT) response to thermal changes using extracellular recording. Methods: 164 CNT were recorded from guinea–pig eyes in vitro. The eyes were mounted in a recording chamber and superfused with physiological saline at 31–33°C. The optic and ciliary nerves were drawn into a suction stimulating electrode. A glass recording electrode (tip diameter ∼50µm) filled with physiological saline was applied to the corneal epithelium surface with light suction. Thermal stimulation was performed changing the temperature of the bathing solution between 20°C to 50°C. Spontaneous and stimulus–evoked nerve impulses occurring in single CNT were recorded using focal extracellular recording techniques. Electrical activity was recorded through an amplifier and analysed using Spike 2 version 4 program. Different parameters were measured for CNT recorded, as mean spontaneous activity at 32ºC, cooling threshold temperature, peak frequency in response to cold and the temperature at this time and temperature to get silent when heating. Menthol (0.1mM) was used to test its effect on those CNT. Results: CNT were identified by their rhythmic activity and response to cooling. All of them had spontaneous firing (8.1±0.3 imp/s) at 32ºC, that increased significantly with cooling and stopped during re–warming. With cooling pulses to 20ºC, mean threshold temperature for significant frequency increases was 28.7±0.2ºC and the peak CNT frequency was 19.4±0.6 imp/s at 26.1±0.2ºC. During re–heating to 50ºC CNT, firing stopped at 36.4±0.4ºC, although a 33% of the terminals exhibited a ‘paradoxical’ response when heating at 44.1±0.8ºC (n=61). When cooling from 50ºC to 32ºC was applied CNT discharge resumed at 41.1±0.4ºC. In all CNT tested (n=42) menthol (added to the perfusion solution increased significantly the firing frequency. Conclusions: CNT of the cornea behave as cold sensory fibers of the skin and oral and nasal mucosae and are particularly sensitive to dynamic temperature changes. Their firing pattern suggests that these cold–sensitive nerve endings are responsible of the cooling and freshness sensations evoked by temperature changes in the eye surface.

Keywords: cornea: basic science