A Topographical and Ultrastructural Study of Sensory Trigeminal Nerve Endings in the Rat Temporomandibular Joint as Demonstrated by Anterograde Transport of Wheat Germ Agglutinin-Horseradish Peroxidase (WGA-HRP)

Abstract

To extend our previous light microscopic observations concerning the distribution of trigeminal sensory nerves in the synovium of the rat temporomandibular joint, we investigated the detailed distribution and fine structure of sensory nerve endings at the light and electron microscopic level by the anterograde transport method using wheat germ agglutinin-horseradish peroxidase (WGA-HRP) injected into the trigeminal ganglion. At the light microscopic level, HRP-labeled nerve fibers were observed in the joint capsule and peripheral portion of the disc. The anterior portion of the disc was more densely innervated than the posterior portion, while no nerves were found in the central portion. At the electron microscopic level, HRP reaction products were observed intra-axonally in the thinly myelinated (A delta) and unmyelinated (C) axons in the anterior portion of the joint capsule, and were also localized in the extracellular space surrounding the unmyelinated fibers and terminals. In the subsynovial layer of the synovial membrane, the majority of labeled axons located near blood vessels or among the collagenous fibrils were covered by Schwann cell sheaths, although some naked axon terminals without sheaths were also found. These unsheathed terminals contained mitochondria, small clear vesicles, and large granular vesicles, and were close to the synovial A and/or B cells near the joint cavity. The minimum distance between the terminals and synovial cells was 75 nm. This is the first demonstration of trigeminal sensory nerve terminals close to synovial lining cells or joint cavity and suggests that neuropeptides such as substance P may be released close to the synovial lining cells or joint cavity.

Capsaicin receptor expression in the rat temporomandibular joint

“…The distribution pattern of TRPV1 nerves is similar to that of trigeminal sensory nerves and CGRP-or substance P-immunoreactive nerves, as previously reported (Kido et al 1993(Kido et al , 1995. TRPV1-immunoreactive nerves with NF200 are considered as A-delta fibers.…”

Differential Effects of Estradiol on Encoding Properties of TMJ Units in Laminae I and V at the Spinomedullary Junction in Female Rats

“…Studies in rat and cat indicate the TMJ region is innervated mainly by small-diameter myelinated and unmyelinated fibers (Ioi et al 2006;Kido et al 1995;Takeuchi and Toda 2003;Takeuchi et al 2001) that project to second-order neurons at the trigeminal subnucleus caudalis/upper cervical cord (Vc/C1-2) junction (Shigenaga et al 1986(Shigenaga et al , 1988. Lesion of the Vc/C1-2 junction, but not more rostral trigeminal brain stem areas, prevents increases in masseter muscle activity after TMJ inflammation (Hu et al 1997).…”

Pathogenesis of degenerative temporomandibular joint arthritides

“…[33][34][35][36][37][38][39] Previous immunohistochemical studies have shown that retrodiscal tissues and the anterior region of the capsular ligament are densely populated by terminals of these neurons. 33,35,38,39 It has been proposed that with mechanical stimulation of these regions, neuropeptides, including substance P and calcitonin gene-related peptides, are released from excited nerve terminals into adjacent articular tissues and synovial fluid. [17][18][19]21,27 These neuropeptides may then stimulate adjacent resident cell populations, resulting in the secretion of other soluble molecules by these cells and promoting a local inflammatory response.…”