Hydrogen peroxide is a novel mediator of inflammatory hyperalgesia,acting via transient receptor potential vanilloid 1-dependent and independent mechanisms

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 (H₂O₂) in hyperalgesia. In the present study, intraplantar injection of H₂O₂-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. H₂O₂ also induced significant nociceptive behavior such as increased paw licking and decreased body liftings. H₂O₂ 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 H₂O₂ (i.pl.) was longer lasting in TRPV1 wild type mice compared to TRPV1 knockouts. It is unlikely that downstream lipid peroxidation was increased by H₂O₂. In conclusion, we demonstrate a notable effect of H₂O₂ in mediating inflammatory hyperalgesia, thus highlighting H₂O₂ removal as a novel therapeutic target for anti-hyperalgesic drugs in the clinic.