Objective. To investigate the involvement of transient receptor potential ankyrin 1 (TRPA1) in inflammatory hyperalgesia mediated by tumor necrosis factor ␣ (TNF␣) 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␣ 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␣ 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␣, 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␣ inhibited bilateral hyperalgesia. Intrathecal treatment with AP-18 also reduced TNF␣-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␣-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.Sensory nerves consisting of C and A␦ nerve fibers innervate joints and skin and are often located in close association with blood vessels (1-3). Nerve stimulation in inflamed joints is considered to play a primary role in arthritis-related pain, and sensory afferent nerves have been located in joints, where they terminate in subsynovial connective tissue (4). Certain members of the transient receptor potential (TRP) receptor family, which are expressed on sensory nerves, are involved as key molecular integrators in the initiation and maintenance of joint pain, although the precise mechanisms involved are unclear (5,6).

The cold-induced vascular response, consisting of vasoconstriction followed by vasodilatation, is critical for protecting the cutaneous tissues against cold injury. Whilst this physiological reflex response is historic knowledge, the mechanisms involved are unclear. Here by using a murine model of local environmental cold exposure, we show that TRPA1 acts as a primary vascular cold sensor, as determined through TRPA1 pharmacological antagonism or gene deletion. The initial cold-induced vasoconstriction is mediated via TRPA1-dependent superoxide production that stimulates α2C-adrenoceptors and Rho-kinase-mediated MLC phosphorylation, downstream of TRPA1 activation. The subsequent restorative blood flow component is also dependent on TRPA1 activation being mediated by sensory nerve-derived dilator neuropeptides CGRP and substance P, and also nNOS-derived NO. The results allow a new understanding of the importance of TRPA1 in cold exposure and provide impetus for further research into developing therapeutic agents aimed at the local protection of the skin in disease and adverse climates.

TRPA1 is involved in mediating vasodilation. TRPA1 can also influence changes in blood pressure of possible relevance to autonomic system reflexes and potentially to vasovagal/neurocardiogenic syncope disorders.

Inflammatory diseases associated with pain are often difficult to treat in the clinic due to insufficient understanding of the nociceptive pathways involved. Recently, there has been considerable interest in the role of reactive oxygen species (ROS) in inflammatory disease, but little is known of the role of hydrogen peroxide (H(2)O(2)) in hyperalgesia. In the present study, intraplantar injection of H(2)O(2)-induced a significant dose- and time-dependent mechanical and thermal hyperalgesia in the mouse hind paw, with increased c-fos activity observed in the dorsal horn of the spinal cord. H(2)O(2) also induced significant nociceptive behavior such as increased paw licking and decreased body liftings. H(2)O(2) levels were significantly raised in the carrageenan-induced hind paw inflammation model, showing that this ROS is produced endogenously in a model of inflammation. Moreover, superoxide dismutase and catalase significantly reduced carrageenan-induced mechanical and thermal hyperalgesia, providing evidence of a functionally significant endogenous role. Thermal, but not mechanical, hyperalgesia in response to H(2)O(2) (i.pl.) was longer lasting in TRPV1 wild type mice compared to TRPV1 knockouts. It is unlikely that downstream lipid peroxidation was increased by H(2)O(2). In conclusion, we demonstrate a notable effect of H(2)O(2) in mediating inflammatory hyperalgesia, thus highlighting H(2)O(2) removal as a novel therapeutic target for anti-hyperalgesic drugs in the clinic.