Ultrastructural identification of trigeminal nerve endings in the rat cornea and iris.

Trigeminal nerve terminals in the rat cornea and iris were ultrastructurally identified using anterograde tracing with Phaseolus vulgaris-leukoagglutinin (PHA-L). Electron microscopic immunohistochemistry was used to demonstrate the presence and localization of calcitonin gene-related peptide (CGRP) in cornea and iris. In the cornea and iris, nerve fibers were labelled with PHA-L throughout the stroma. Labelling was most obvious within varicosities, densely packed with mainly clear and a few granular vesicles and containing dark mitochondria. Numerous fibers in the stroma of cornea and iris were CGRP-positive. CGRP-positive staining was most intense within varicosities, containing mainly clear and incidentally granular vesicles and dark mitochondria, similar to the structures labelled with PHA-L. CGRP-positive varicosities packed with mainly clear and few granular vesicles also were demonstrated in fibers adjacent to the sphincter and dilator muscles of the iris. In the corneal epithelium, small terminals containing vesicles were CGRP-positive. Trigeminal nerve fibers innervating the rat cornea and iris contained numerous varicosities packed with vesicles. These areas are CGRP-positive, so it can be implied that CGRP is released from these varicosities as a response to triggering impulses. This agrees with the hypothesis that in addition to their afferent function, sensory fibers also exert an efferent modulating function.