Cyclooxygenases 1 and 2 contribute to peroxynitrite-mediated inflammatory pain hypersensitivity.

Peroxynitrite (ONOO-), the reaction product of the interaction between superoxide (O2·-) and nitric oxide (·NO), is a potent proinflammatory and cytotoxic nitrooxidative species. Its role as a mediator of hyperalgesia (clinically defined as an augmented sensitivity to painful stimuli) is not known. In light of the known proinflammatory properties of ONOO-, our study addressed its potential involvement in the development of hyperalgesia associated with tissue damage and inflammation. Intraplantar injection in rats of the ONOO- precursor O2·- (1 μM) led to the development of thermal hyperalgesia associated with a profound localized inflammatory response. Both events were blocked by L-NAME (NG-nitro-L-arginine methyl ester, 3-30 mg/kg), a nitric oxide synthase inhibitor, or by FeTM-4-PyP 5+ [Fe(III)5,10,15,20-tetrakis(N-methylpyridinium-4-yl)porphyrin, 3-30 mg/kg], an ONOO- decomposition catalyst. These results suggested that locally synthesized ONOO- produced in situ by O 2·- and ·NO is key in the development of inflammatory hyperalgesia. The direct link between ONOO- and hyperalgesia was further supported by demonstrating that intraplantar injection of soluble ONOO- itself (1 μM) similarly led to inflammatory hyperalgesia. ONOO- generated by the interaction between exogenous administration of O2·- and endogenous ·NO, or provided by direct injection of ONOO-, activated the transcription factor NF-κB in paw tissues, enhancing expression of the inducible but not the constitutive cyclooxygenase enzyme (COX-2 and COX-1, respectively). ONOO--mediated hyperalgesia was blocked in a dose-dependent manner by intraperitoneal injections of indomethacin (10 mg/kg), a nonselective COX-1/ COX-2 inhibitor, or NS398 [N-(2-cyclohexyloxy-4-nitrophenyl) methanesulfonamide; 10 mg/kg] a selective COX-2 inhibitor, as well as by an anti-prostaglandin (PG) E2 antibody (200 μg). In another established model of inflammation-related hyperalgesia by intraplantar injection of carrageenan in rats, inhibition of ONOO- with FeTM-4-PyP5+ (3-30 mg/kg) inhibited the development of hyperalgesia and the release of PGE2 in paw tissue exudates. Furthermore, FeTM-4-PyP5+ synergized with indomethacin and NS397 (1-10 mg/kg) to block both hyperalgesia and edema. Taken together, these data show for the first time that ONOO- is a potent mediator of inflammation-derived hyperalgesia operating via the COX-to-PGE 2 pathway. These results provide a pharmacological rationale for the development of inhibitors of peroxynitrite biosynthesis as novel nonnarcotic analgesics. The broad implications of our study are that dual inhibition of both ONOO- formation and COX activity may provide an alternative therapeutic approach to the management of pain: effective analgesia with reduced side-effects typically associated with the use of COX inhibitors.