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B. O. Ibitokun, K. E. Miller; Corneal Afferents Contain Synaptic Vesicle Proteins: Synaptophysin I and Synaptophysin II. Invest. Ophthalmol. Vis. Sci. 2010;51(13):1974.
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We propose that neurotransmitters are released from sensory afferents in the cornea by synaptic vesicle proteins. In the present study, we examined synaptophysin I and synaptophysin II (synaptoporin) with immunohistochemistry in the rat cornea.
Fluorescence immunohistochemistry was performed on paraformaldehyde/picric acid fixed rat corneas. Corneas (whole mounts with radial cuts) were incubated in rabbit anti-synaptophysin I and synaptophysin II (Chemicon). Cy3 labeled donkey anti-rabbit IgG (Jackson) was used to visualize immunoreactivity. Tissues were mounted on microscope slides and viewed and photographed with an Olympus BX51 epiflourescence microscope.
Corneal afferents were immunoreactive for both synaptophysin I and synaptophysin II. Abundant immunoreactive fibers were found in bundles in the corneal stroma. Immunoreactive varicose fibers were identified within the corneal epithelium.
Neurotransmitters such as Substance P (SP) and Calcitonin Gene Related Peptide (CGRP) are released from corneal afferent nerves in response to temperature changes, inflammation and nerve injury. They play a role in nociception, have a neuromodulatory effect on the physiology of corneal epithelium, and influence corneal epithelium wound healing. The mechanism of release, however, of these neurotransmitters is unknown. From the results of the current study, synaptic vesicle proteins are present in corneal afferents and most likely are involved in the release of neurotransmitters such as SP and CGRP. The presence of both synaptophysin I and synaptophysin II may indicate the existence of two major types of corneal afferent fibers. Understanding the release mechanisms from corneal afferents may help in developing therapeutic measures targeted at inhibiting neurotransmitter release in order to alleviate pain due to infective, inflammatory, and neurotropic keratitis.
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