Objective: Gout is a common cause of inflammatory arthritis and is provoked by the accumulation of monosodium urate (MSU) crystals. However, the underlying mechanisms of the pain associated with acute attacks of gout are poorly understood. The aim of this study was to evaluate the role of transient receptor potential ankyrin 1 (TRPA‐1) and TRPA‐1 stimulants, such as H₂O₂, in a rodent model of MSU‐induced inflammation.Methods: MSU or H₂O₂ was injected into the hind paws of rodents or applied in cultured sensory neurons, and the intracellular calcium response was measured in vitro. Inflammatory or nociceptive responses in vivo were evaluated using pharmacologic, genetic, or biochemical tools and methods.Results: TRPA‐1 antagonism, TRPA‐1 gene deletion, or pretreatment of peptidergic TRP‐expressing primary sensory neurons with capsaicin markedly decreased MSU‐induced nociception and edema. In addition to these neurogenic effects, MSU increased H₂O₂ levels in the injected tissue, an effect that was abolished by the H₂O₂‐detoxifying enzyme catalase. H₂O₂, but not MSU, directly stimulated sensory neurons through the activation of TRPA‐1. The nociceptive responses evoked by MSU or H₂O₂ injection were attenuated by the reducing agent dithiothreitol. In addition, MSU injection increased the expression of TRPA‐1 and TRP vanilloid channel 1 (TRPV‐1) and also enhanced cellular infiltration and interleukin‐1β levels, and these effects were blocked by TRPA‐1 antagonism.Conclusion: Our results suggest that MSU injection increases tissue H₂O₂, thereby stimulating TRPA‐1 on sensory nerve endings to produce inflammation and nociception. TRPV‐1, by a previously unknown mechanism, also contributes to these responses.

The transient receptor potential vanilloid 1 (TRPV1) receptor is relevant to the perception of noxious information and has been studied as a therapeutic target for the development of new analgesics. The goal of this study was to perform in vivo and in vitro screens to identify novel, efficacious, and safe TRPV1 antagonists isolated from leaves of the medicinal plant Vernonia tweedieana Baker. All of the fractions and the hydroalcoholic extract produced antinociception in mice during the capsaicin test, but the dichloromethane fraction also had antioedematogenic effect. Among the compounds isolated from the dichloromethane fraction, only α-spinasterol reduced the nociception and edema induced by capsaicin injection. Moreover, α-spinasterol demonstrated good oral absorption and high penetration into the brain and spinal cord of mice. α-Spinasterol was able to displace [³H]resiniferatoxin binding and diminish calcium influx mediated by capsaicin. Oral administration of the dichloromethane fraction and α-spinasterol also produced antinociceptive effect in the noxious heat-induced nociception test; however, they did not change the mechanical threshold of naive mice. The treatment with α-spinasterol did not produce antinociceptive effect in mice systemically pretreated with resiniferatoxin. In addition, α-spinasterol and the dichloromethane fraction reduced the edema, mechanical, and heat hyperalgesia elicited by complete Freund9s adjuvant paw injection. The dichloromethane fraction and α-spinasterol did not affect body temperature or locomotor activity. In conclusion, α-spinasterol is a novel efficacious and safe antagonist of the TRPV1 receptor with antinociceptive effect.