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Abdulhakeem S Alamri, James Alexander Brock, Jason John Ivanusic; The neurochemistry and morphology of functionally identified corneal polymodal and cold-sensitive receptors. Invest. Ophthalmol. Vis. Sci. 2016;57(12):4377. doi: https://doi.org/.
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© ARVO (1962-2015); The Authors (2016-present)
Confocal microscopy studies have revealed that sensory nerve terminations within the corneal epithelium can be distinguished on the basis of their morphology and neurochemistry, but it has not been confirmed that these different types of nerve terminal represent distinct functional classes of sensory receptor. Here we extended these studies by identifying the morphology and neurochemistry of electrophysiologically defined corneal polymodal and cold-sensitive receptors.
Eyes were isolated from guinea pigs that had been euthanized by stunning and exsanguination. The eyes were mounted in a recording chamber and superfused with physiological saline (PS). Extracellular recordings from single polymodal or cold receptors were made using glass pipette electrodes applied to the corneal surface. The recording sites were marked by perfusing the recording electrode with fluoro-gold (0.25% in PS). The corneas were fixed and processed to reveal immunoreactivity for the transient receptor potential channels TRPV1 or TPRM8.
Polymodal receptors had either no ongoing impulse activity or a low level of impulse activity (< 1 Hz), and were strongly activated capsaicin (0.5 μM). At these recording sites (n=6), the axons located immediately beneath the electrode were TRPV1-immunoreactive (IR) and derived from an axon that ascended from the sub-basal plexus to the superficial layer of the epithelium where it branched into a number of fibres that each ran parallel to the surface for a short distance before terminating in a small bulbar ending (ramifying endings). Cold receptors had a continuous ongoing rhythmic discharge of impulses at ~8 Hz that was increased by cooling and inhibited by warming. At these recording sites (n=6), the axons immediately beneath the electrode were TRPM8-IR and originated from an axon that branched as it ascended from the sub-basal plexus through the wing cell and squamous cell layers, and terminated with large bulbar endings close to the surface of the cornea (complex endings). Polymodal receptive endings (n=3) were not TRPM8-IR and cold-sensitive endings (n=6) were not TRPV1-IR.
The findings confirm that in the most superficial layer of the corneal epithelium that TRPV1 expressing nerve terminals are polymodal receptors and TRPM8 expressing nerve terminals are cold receptors.
This is an abstract that was submitted for the 2016 ARVO Annual Meeting, held in Seattle, Wash., May 1-5, 2016.
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