Sympathetic innervation of the temporomandibular joint modulates local hyperalgesia and IL-6 levels in zymosan-induced inflammatory pain

The temporomandibular joint (TMJ) is critical for essential orofacial functions including chewing and speaking. Chronic pain and dysfunction of the TMJ and its associated structures are classified as temporomandibular disorders (TMDs) and represent a significant public health burden. However, the precise neurobiological mechanisms driving these conditions are still unclear. The autonomic nervous system, in particular the sympathetic nervous system, plays a modulatory role in joint homeostasis, inflammation, and pain. This study investigated the sympathetic innervation of the rat TMJ and its functional role in inflammatory hyperalgesia. We first mapped the origin of sympathetic innervation to the TMJ using retrograde tracing combined with immunostaining. After confirming that the TMJ receives sympathetic input predominantly from the ipsilateral superior cervical ganglion (SCG), we performed unilateral superior cervical ganglionectomy (SCG-X) to assess the impact of sympathetic denervation on zymosan-induced TMJ inflammatory hyperalgesia and local cytokine production. SCG-X procedure significantly exacerbated zymosan-induced mechanical hyperalgesia in the TMJ and markedly increased intra-articular interleukin-6 (IL-6) levels, while tumor necrosis factor-alpha (TNF-alpha) remained unaffected. These results demonstrate that the sympathetic nervous system exerts anti-hyperalgesic and anti-inflammatory roles in the TMJ by modulating local cytokine responses during acute inflammation. This study reveals a previously unidentified neuroimmune mechanism within the TMJ, providing a foundation for developing novel, targeted therapeutic strategies for TMDs.