A Distinct Role for Transient Receptor Potential Ankyrin 1, in Addition to Transient Receptor Potential Vanilloid 1, in Tumor Necrosis Factor α-Induced Inflammatory Hyperalgesia and Freund's Complete Adjuvant-Induced Monarthritis

Objective. To investigate the involvement of transient receptor potential ankyrin 1 (TRPA1) in inflammatory hyperalgesia mediated by tumor necrosis factor alpha (TNF alpha) and joint inflammation. Methods. Mechanical hyperalgesia was assessed in CD1 mice, mice lacking functional TRP vanilloid 1 (TRPV1(-/-)) or TRPA1 (TRPA1(-/-)), or respective wildtype (WT) mice. An automated von Frey system was used, following unilateral intraplantar injection of TNF alpha or intraarticular injection of Freund's complete adjuvant (CFA). Knee swelling and histologic changes were determined in mice treated with intraarticular injections of CFA. Results. TNF alpha induced cyclooxygenase-independent bilateral mechanical hyperalgesia in CD1 mice. The selective TRPV1 receptor antagonist SB-366791 had no effect on mechanical hyperalgesia when it was coinjected with TNF alpha, but intrathecally administered SB-366791 attenuated bilateral hyperalgesia, indicating the central but not peripheral involvement of TRPV1 receptors. A decrease in pain sensitivity was also observed in TRPV1(-/-) mice. Intraplantar coadministration of the TRPA1 receptor antagonist AP-18 with TNF alpha inhibited bilateral hyperalgesia. Intrathecal treatment with AP-18 also reduced TNF alpha-induced hyperalgesia. CFA-induced mechanical hyperalgesia in CD1 mice was attenuated by AP-18 (administered by intraarticular injection 22 hours after the administration of CFA). Furthermore, intraarticular CFA-induced ipsilateral mechanical hyperalgesia was maintained for 3 weeks in TRPA1 WT mice. In contrast, TRPA1(-/-) mice exhibited mechanical hyperalgesia for only 24 hours after receiving CFA. Conclusion. Evidence suggests that endogenous activation of peripheral TRPA1 receptors plays a critical role in the development of TNF alpha-induced mechanical hyperalgesia and in sustaining the mechanical hyperalgesia observed after intraaarticular injection of CFA. These results suggest that blockade of TRPA1 receptors may be beneficial in reducing the chronic pain associated with arthritis.