Tumor Necrosis Factor-alpha Stimulates Isolated Peripheral Human Nociceptors: Mechanism for Pain

Objective: Tumor Necrosis Factor-a (TNFa) is a proinflamatory cytokine that modulates the development of pain and hyperalgesia. Increased levels of endogenous TNFa in peripheral tissues have been demonstrated in animal models of inflammatory pain as well as in clinical studies. These and other studies support the hypothesis that TNFa mediates pain in humans although the mechanism remains unknown. In this study, we evaluated the hypothesis that TNFa directly activates human peripheral nociceptors as measured by release of immunoreactive substance P (iSP). We conducted a clinical trial using a newly developed method of collecting isolated terminals of human nociceptors innervating the periodontal ligament of extracted teeth. Initial studies have demonstrated that this method permits evaluation of capsaicin-sensitive nociceptors using release of iSP as a dependent measure. Method: In this study, the terminal 6mm apical sections of freshly extracted vital human teeth were sectioned and pulpal tissue was removed (to exclude potential pulpal nociceptor contribution). The isolated roottips were then placed in sequential concentrations of TNFa (0.001 to 10ng/mL) or vehicle (20 min) and then both groups were stimulated with capsaicin (50mM for 20 min). Aliquots were acidified and heated (to inactivate peptidases), lyophilized and then assayed by radioimmunoassay for iSP. The data were normalized by calculating SP release as percentage of total iSP in the tissue and analyzed by repeated measures ANOVA. Results: The results indicate that administration of exogenous TNFa evokes release of iSP from human nociceptors in a concentration-dependent manner with peak effects (1-10ng/mL) similar to a 10mM concentration of capsaicin. Conclusion: We conclude that increased levels of TNFa during conditions of peripheral inflammation such as acute periapical periodontitis stimulate peripheral nociceptors and thereby contribute to the development of mechanical allodynia and hyperalgesia in pain patients. Supported by NIDCR T-32 DE14318 and DEIZ888 R37