Sensory and Sympathetic Nerve Localization in Mouse Temporomandibular Joint and Knee Joint Neuro-Musculoskeletal Tissues

Abstract

Objective: Sympathetic-sensory coupling is increasingly implicated in joint homeostasis and disease. Towards the long-term goal of deciphering its role in temporomandibular disorders (TMDs), we characterised the spatial relationships of sympathetic and sensory nerves and their receptors in the mouse temporomandibular joint (TMJ) and trigeminal ganglion (TG), using the knee joint for comparison.

Materials and methods: RNAscope was used for localisation of sympathetic nerve markers tyrosine hydroxylase (Th) and adrenergic receptor alpha 2a (Adrα2a); and brain-derived neurotrophic factor (Bdnf) and its receptor neurotrophic receptor tyrosine kinase 2 (Ntrk2) that stimulate sympathetic nerve growth. Sensory marker calcitonin gene-related peptide (CGRP) was detected through immunofluorescence.

Results: Th, Adrα2a, Bdnf and Ntrk2 were expressed in the TMJ and knee joint tissues, including condylar and disc cartilages, muscles and bone marrow. CGRP+ sensory neurons were abundant in the TG and, to a lesser extent, in TMJ and knee tissues, where they were often located near Th- or Adrα2a-expressing sympathetic cells. Some CGRP+ neurons in the TG also co-expressed Adrα2a.

Conclusion: Our findings reveal the presence of sympathetic and sensory nerves within the TMJ, knee joint and TG, with these nerves often located near each other. Along with the presence of Adrα2a on CGRP+ sensory fibres, this suggests sympathetic-sensory coupling and potential crosstalk in physiologic and pathologic responses. Expression of Bdnf and Ntrk2 in the TMJ and knee implicates this neurotrophin signalling in modulating sympathetic activity via neural sprouting. Further studies will clarify the functional contributions of this neurological architecture to tissue homeostasis, disease and pain.

Keywords: TMJ; knee joint; sensory nerve; sympathetic nerve; trigeminal ganglion.

FIGURE 1

Sympathetic and neurotrophic signalling genes in the TMJ and Knee Joint. RNAscope illustrates the localisation of sympathetic nerve genes (Adrα2a, Th) and neurotrophic signalling genes (Ntrk2, Bdnf) in various tissues of the TMJ (A, B) and knee joint (C, D). Scale bars: 10 μm in A1–4, B1–4, C1–4, D1–4; 260 μm in A–D.

FIGURE 2

Localisation of sympathetic and neurotrophic signalling genes in the cartilage of TMJ and Knee joint. RNAscope of (A, B) Bdnf and its receptor Ntrk2; (C, D) Adrα2a and Th, a sympathetic nerve marker in TMJ and knee cartilage. Enlargements of the areas are outlined in yellow in A2, B1, C2, D1. Anterior (Ant), Middle (Mid), Posterior (Post). Scale bars: 10 μm in A1–2, C1–3; 130 μm in B1, D1; 260 μm in A–D; 1 μm in yellow boxes in A2, C2; 13 μm in yellow boxes in B1, D1.

FIGURE 3

Sensory nerves found in close proximity to sympathetic nerves in TG, TMJ and knee joint. RNAscope with immunostaining reveals the close association of sensory and sympathetic neurons and fibres in the TG (A, A1, A2, B, B1, B2), TMJ (C, D), and knee joint (E, F). Scale bars: 10 μm in A to F; 5 μm in A1, A2, B1; 2 μm in B2.

FIGURE 4

Sensory, sympathetic and neurotrophic signalling genes in the TMJ and knee joint. This schematic illustrates the spatial localisation of sensory nerve protein (CGRP), sympathetic genes (Adrα2a, Th) and neurotrophic signalling genes (Ntrk2, Bdnf) in the TMJ (top panels) and the knee joint (bottom panels). Gene and protein expression are mapped across various anatomical structures, including the articular disc, bone marrow, cartilage, epiphyseal growth plate, meniscus, metaphyseal bone, muscle and subchondral bone.