TNFα in the Trigeminal Nociceptive System Is Critical for Temporomandibular Joint Pain

Previous studies have shown that tumor necrosis factor alpha (TNFα) is significantly increased in complete Freund’s adjuvant (CFA)-treated temporomandibular joint (TMJ) tissues. However, it is unclear whether TNFα in the trigeminal nociceptive system contributes to the development of TMJ pain. In the present study, we investigated the role of TNFα in trigeminal ganglia (TG) and spinal trigeminal nucleus caudalis (Sp5C) in CFA-induced inflammatory TMJ pain. Intra-TMJ injection of CFA (10 μl, 5 mg/ml) induced inflammatory pain in the trigeminal nerve V2- and V3-innervated skin areas of WT mice, which was present on day 1 after CFA and persisted for at least 10 days. TNFα in both TG and Sp5C of WT mice was upregulated after CFA injection. The CFA-induced TMJ pain was significantly inhibited in TNFα KO mice. The immunofluorescence staining showed that intra-TMJ CFA injection not only enhanced co-localization of TNFα with Iba1 (a marker for microglia) in both TG and Sp5C but also markedly increased the expression of TNFα in the Sp5C neurons. By the methylated DNA immunoprecipitation assay, we also found that DNA methylation at the TNF gene promoter region in the TG was dramatically diminished after CFA injection, indicating that epigenetic regulation may be involved in the CFA-enhanced TNFα expression in our model. Our results suggest that TNFα in the trigeminal nociceptive system plays a critical role in CFA-induced inflammatory TMJ pain.