Vanilloid receptor subtype-1 (VR1) is specifically localized to taste papillae

Pungent sensation of hot peppers is thought to be mediated by vanilloid receptor subtype-1 (VR1), which can be activated by capsaicin, but there is little information regarding its histological localization in the tongue. We examined the immunohistochemical distribution of VR1 in the rat tongue. Intensely labeled VR1-immunoreactive (VR1-IR) fibers were concentrated in the circumvallate, foliate, and fungiform papillae, while sparse VR1-IR fibers were scattered throughout the tongue. VR1-positive taste-bud cells were not observed. Many VR1-positive nerve fibers surrounded the furrows of the circumvallate and foliate papillae, forming plexuses. Some of these VR1-positive fibers penetrated the apical epithelium and the trench wall epithelium, while a few of them entered taste buds. These VR1 distribution patterns resembled those of substance P (SP) and the calcitonin gene-related peptide (CGRP). Double-labeling experiments revealed that most of the VR1-immunoreactivity coexisted with SP- or CGRP-immunoreactivity in the nerve terminals in the circumvallate and foliate papillae. On the other hand, in the fungiform papillae, most of the VR1-IR fibers were positive for SP, but fewer were also positive for CGRP. Although VR1-immunoreactivity was not observed in taste-bud cells, our findings that a large number of VR1-IR fibers concentrated in the taste papillae suggest that capsaicin easily reaches the VR1 nerve terminals because of its lipophilic nature.