The role of substance P and calcitonin gene-related peptide containing nerve fibers in maintaining fungiform taste buds in the rat after a chronic chorda tympani nerve injury

Abstract

Taste buds in the anterior part of the tongue of adult rats were denervated by unilateral resection of the chorda tympani nerve in the middle ear. Three months later one group of animals was perfused and their tongues were processed for demonstration of substance P (SP) and calcitonin gene-related peptide (CGRP) immunoreactivity. Fungiform taste buds found on the denervated side showed increased numbers of intragemmal SP- and CGRP-immunoreactive (IR) fibers compared to the normal side. Compared to the normal side, the number of taste buds appeared to be fewer on the denervated side. Moreover, taste buds on this side seemed to be only partially restored. Another group of animals was given the neurotoxin capsaicin which causes a depletion of SP and CGRP from sensory axons. The animals were perfused 2 or 3 weeks after the capsaicin treatment, and their tongues prepared for SP and CGRP immunohistochemistry or for histological examination of taste buds. Very few SP- and CGRP-IR fibers were present in capsaicin-treated animals. In these animals almost all fungiform taste buds and papillae on the chorda tympani-injured side disappeared. In contrast, normal numbers of taste buds were still present on the contralateral side where the chorda tympani innervation remained intact. It is conceivable that taste buds on the chorda tympani-innervated part of the tongue, deprived of the normal chorda tympani-innervation, can regenerate and become reinnervated by SP- and CGRP-containing fibers, and that these are essential for partially restoring and maintaining the structure of the denervated taste buds and the fungiform papillae.