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M. C. Acosta, C. Luna, C. Belmonte, J. Gallar; Effects of Microkeratome Lesion on Corneal Nerves Recorded From the Ciliary Nerves of the Guinea-Pig Eye ‘in vitro’. Invest. Ophthalmol. Vis. Sci. 2007;48(13):3471.
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To study impulse activity in corneal nerve fibers at different time periods after a microkeratome lesion of the cornea in an in vitro preparation of the guinea pig eye and to test the effect of drugs that reduce nerve activity in experimental cutaneous neuromas.
In anesthetized guinea-pigs, a corneal flap of 4mm diameter was cut at mid-stromal depth in the left eye using a custom-made microkeratome. After 24 hours or 3-10 days, animals were killed with an overdose of anesthesia and both eyes (control and lesioned) immediately excised. The eye was mounted in a recording chamber divided in two compartments, with the cornea in the anterior compartment continuously perfused with physiological solution. The posterior part, including the optic and ciliary nerves occupied the rear compartment filled with mineral oil. Ciliary nerve filaments were placed on Ag-AgCl electrodes and single or few units’ impulse activity was recorded using conventional electrophysiological equipment. Receptive fields of corneal afferents and mechanical thresholds were determined with calibrated von Frey hairs. Thermal stimulation was performed increasing (45ºC) or decreasing (20ºC) the temperature of the solution perfusing the cornea. Gas pulses containing 98% CO2 and applied on the corneal receptive field during 30s were used for acidic stimulation. Conduction velocity was measured stimulating the corneal receptive field with 0.5ms suprathreshold electrical pulses. Drugs were tested via application of a piece of tissue paper soaked in the drug solution to the receptive field for 10min.
AΔ mechano- and polymodal fibers innervating the injured corneas did not show a higher incidence of spontaneous activity or clear changes in mechanical threshold. However, 24 h after lesion the incidence of heat responses, and the magnitude of the response to CO2 were increased. 3-10 days after injury responsiveness of AΔ fibers to mechanical, thermal and chemical stimuli was normal. In both control and lesioned corneas, Lidocaine 0.01%, Carbamazepine 0.002% and Phenytoin 0.002% reduced significantly the response of polymodal nociceptors to CO2 while mechanical threshold of polymodal and mechano-nociceptor fibers remained normal.
Mechanical injury of polymodal and mechano-nociceptor AΔ corneal fibers with a microkeratome did not alter their mechanosensitivity and produced transient changes in the responsiveness to chemical stimuli of polymodal nociceptors. Drugs that reduce neuronal excitability decrease chemosensitivity of these fibers.
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