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Omar Gonzalez Gonzalez, Ignacio Alcalde, Almudena Íñigo-Portugués, Juana Gallar, Jesus Merayo-Lloves, Carlos Belmonte; FUNTIONAL ACTIVITY OF CORNEAL COLD SENSORY NERVE TERMINALS AND BASAL TEARING RATE IN YOUNG AND AGED MICE. Invest. Ophthalmol. Vis. Sci. 2015;56(7 ):292.
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© ARVO (1962-2015); The Authors (2016-present)
To determine in young and old mice the change in the electrophysiological characteristics of the different functional classes of cold corneal sensory nerve terminals, and the relation of these changes with basal tearing rate values.
C57BL6/J mice of 3 and 24 months were studied. Mice were sacrificed by CO2 exposure. The excised eye was drawn in a solution similar to tear and bubbled with carbogen gas. A peltier device was used to control the temperature. Extracellular electrical activity of single sensory nerve endings of the corneal surface was recorded with an Ag/AgCl borosilicate micropipette electrode filled with the same solution and placed with slight suction in to the cornea surface with a micromanipulator. All data obtained were filtered and analyzed in the computer with Spike2 program. Basal tearing was measured in anesthetized animals using phenol red threads.
Two different types of cold terminals were found: low- (LTC) and high- threshold (HTC). 3-months mice had 37% of low-threshold (>30.5 °C) cold sensitive fibers showing high background activity and vigorous firing responses to cooling. They were reduced to 26% in 24 months-old mice. Contrarily, cold sensitive fibers with a high cooling threshold (<30.5 °C), very low frequency background activity and weak responses to cooling was 17% in 3 months-old mice and 40% in 24 months-old mice. Other cold population was found only in 24 months old mice, sharing characteristics of both groups: Mixed-type cold endings. TRPM8 agonist menthol applied in LTC and HTC endings increased background activity in 100% and 87%, respectively. The proportion of polymodal and mechano-nociceptors endings did not change as a function of aging. Basal tearing rate increased significantly in old mice.
The reduced percentage of LTC in 24-months old mice suggests that this class of fibers changed their functional properties with age, evolving towards MTC, while the proportion of HTC, polymodal nociceptors and mechano-nociceptors remains constant. The changes in LTC fibers may be related with the changes in basal tearing rate developed by ageing mice.
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