Neuropeptides in intact and denervated tympanic membranes. An immunohistofluorescence study in the rat

The distribution and origin of peptide-containing and sympathetic nerve fibers were studied in the tympanic membrane of rats with intact innervation, and in rats following sympathectomy, vagotomy, or capsaicin treatment. Nerve fibers showing substance-P-like immunoreactivity (SP-LI), calcitonin gene-related peptide (CGRP)-LI, vasoactive intestinal polypeptide (VIP)-LI, enkephalin-LI, neuropeptide Y (NPY)-LI and tyrosine hydroxylase (TH)-LI were detected along blood vessels in the pars flaccida, the external auditory canal and in the fibrocartilaginous ring of the pars tensa. In the pars flaccida there were numerous fibers demonstrating SP-LI and CGRP-LI, while there were few such fibers in the pars tensa. In both portions of the tympanic membrane these fibers were present within and beneath the keratinized stratified squamous epithelium. In the pars flaccida, nerve fibers showing SP-LI and CGRP-LI were also seen near mast cells. Sympathectomy led to a loss of nerve fibers showing TH-LI and NPY-LI, whereas the other peptide-containing nerve fibers remained unaffected. Vagotomy did not reduce the immunoreactivity for any of the neuropeptides studied. Capsaicin treatment caused a reduction in nerve fibers displaying SP-LI and CGRP-LI. The abundance of nerve fibers showing SP-LI and CGRP-LI in the keratinized squamous epithelium indicates that the tympanic membrane is richly supplied with sensory nerves. The localization of nerve fibers exhibiting these latter substances in the vicinity of mast cells in the pars flaccida suggests that this part of the tympanic membrane is a site where neurogenic inflammation occurs.