Calcitonin gene‐related peptide and corneal innervation: A developmental study in the rat

Abstract

The development of calcitonin gene-related peptide-like immunoreactive (CGRP-LI) nerves was studied in neonatal and adult rat corneas stained immunohistochemically according to an avidin biotin peroxidase procedure. At birth, rat corneas already contained dense plexuses of CGRP-LI nerve fibers. Most of the nerves entered the cornea in 12-15 prominent stromal nerve bundles located at regular intervals around the circumference of the cornea. Fibers in these bundles entered the epithelium approximately midway between the limbus and the center of the cornea and supplied extensive central and pericentral areas of the tissue. In addition, smaller numbers of axons entered the cornea individually and in small fascicles located in between the larger bundles and supplied mainly peripheral territory. In the epithelium, the CGRP-LI nerves formed a complex, highly anastomotic meshwork that ramified uniformly throughout central and peripheral areas of the tissues. Fibers in the plexus gave origin to numerous short, stout terminal axons that extended into the adjacent epithelium in all directions with no preferred orientation. During the first week of neonatal life, several changes in CGRP-LI innervation occurred: 1) the innervation density of the central and pericentral cornea increased relative to the peripheral cornea; 2) intraepithelial axons became progressively longer, increased in branching complexity, and oriented preferentially towards the center of the cornea; and 3) a dense innervation of the corneoscleral limbus and, in particular, the branches of the marginal artery, developed. Midway through the second week of life, immature versions of corneal epithelial "leashes," the dominant feature of the adult corneal innervation, were first observed. Over the next 10 days, the leash formations in the central and pericentral cornea gradually became more complex and gave rise to greater numbers of terminal axons, compared to developing leashes in the peripheral cornea. The mature pattern of corneal CGRP-LI innervation was reached on day 21 and remained constant (except for compensatory growth-related elongation of axons) for at least the first 6 months of life. Transection of the ophthalmomaxillary nerve or neonatal administration of the sensory neurotoxin capsaicin resulted in the total loss of CGRP-LI staining from the cornea. In contrast, removal of the superior cervical ganglion had no effect on corneal CGRP-LI staining. The extraordinary density and complexity of the CGRP-LI innervation of the rat cornea demonstrated at all stages of development in this study suggests that these nerves may play important roles in corneal sensory, reflex, and trophic functions.

Electrophysiological properties and chemosensitivity of acutely dissociated trigeminal somata innervating the cornea

“…1A and B). The pattern of CNT labeling was similar to that observed using dextran-conjugated Oregon Green 488 BAPTA 1 (Gover et al, 2003), or to that revealed with immunostaining for substance P or CGRP (Miller et al, 1981;Jones and Marfurt, 1991). Four days after FM1-43 application, about one percent of the dissociated trigeminal ganglion neurons (TGNs) displayed FM1-43 fluorescence ( Fig.…”

Innervation of TRPV1-, PGP-, and CGRP-immunoreactive nerve fibers in the subepithelial layer of a whole mount preparation of the rat cornea

“…Common features of these three types of nerve fibers included concentration in proximity to the marginal area, a mesh-like structure, and sparsity in the central area of the cornea. The paucity of these nerve fibers in the central cornea does not necessarily indicate the absence of fibers because it has previously been shown to be richly innervated with peptidergic nerve fibers 17) . The innervation pattern of PGP-and CGRP-IR nerve fibers in the present study is in accord with that observed in previous reports 5,6,[15][16][17]19) .…”

“…The paucity of these nerve fibers in the central cornea does not necessarily indicate the absence of fibers because it has previously been shown to be richly innervated with peptidergic nerve fibers 17) . The innervation pattern of PGP-and CGRP-IR nerve fibers in the present study is in accord with that observed in previous reports 5,6,[15][16][17]19) . Dense innervation of PGP-IR nerve fibers was demonstrated in whole mount corneas from rat pups (age, <14 days) 6) and cryostat-sections of canine 19) corneas.…”

Capsaicin-resistant, Nonspecific Acetylcholinesterase (NsAchE) Reactive Nerve Fibers in the Rat Cornea: A Quantitative and Developmental Study

“…They argued that the remained nerve fibers may be sympathetic or capsaicin-insensitive corneal nerves. A similarity in the pattern and density between the rat corneal CGRPir nerves and those of NsAchE-reactive or stained with AuCl led to the suggestion that the CGRP-ir nerves are nearly equal to the NsAchE or AuCl reactive nerves (Jones and Marfurt, 1991 Shimizu et al (1984) initially demonstrated that various corneal lesions were induced in all rats and mice by 30 days after birth, irrespective of the dose of capsaicin (12.5 À 100 mg/kg, s.c). On the contrary, Kawakami et al (1996) reported that corneal abnormalities could not be observed under a slit-lamp microscope in 80 corneas from 40 rats neonatally treated with capsaicin (100 mg/kg, s.c).…”