Identification of trigeminal sensory neuronal types innervating masseter muscle

Abstract

Understanding masseter muscle (MM) innervation is critical for the study of cell-specific mechanisms of pain induced by temporomandibular disorder (TMDs) or after facial surgery. Here, we identified trigeminal (TG) sensory neuronal subtypes (MM TG neurons) innervating MM fibers, masseteric fascia, tendons, and adjusted tissues. A combination of patch clamp electrophysiology and immunohistochemistry (IHC) on TG neurons back-traced from reporter mouse MM found nine distinct subtypes of MM TG neurons. Of these neurons, 24% belonged to non-peptidergic IB-41/TRPA1– or IB-41/TRPA11 groups, while two TRPV11 small-sized neuronal groups were classified as peptidergic/CGRP1. One small-sized CGRP1 neuronal group had a unique electrophysiological profile and were recorded from Nav1.8– or trkC1 neurons. The remaining CGRP1 neurons were medium-sized, could be divided into Nav1.8–/trkC– and Nav1.8low/trkC1 clusters, and showed large 5HT-induced current. The final two MM TG neuronal groups were trkC1 and had no Nav1.8 and CGRP. Among MM TG neurons, TRPV11/CGRP– (somatostatin1), tyrosine hydroxylase (TH)1 (C-LTMR), TRPM81, MrgprA31, or trkB1 (Ad-LTMR) subtypes have not been detected. Masseteric muscle fibers, tendons and masseteric fascia in mice and the common marmoset, a new world monkey, were exclusively innervated by either CGRP1/ NFH1 or CGRP–/NFH1 medium-to-large neurons, which we found using a Nav1.8-YFP reporter, and labeling with CGRP, TRPV1, neurofilament heavy chain (NFH) and pgp9.5 antibodies. These nerves were mainly distributed in tendon and at junctions of deep-middle-superficial parts of MM. Overall, the data presented here demonstrates that MM is innervated by a distinct subset of TG neurons, which have unique characteristics and innervation patterns.