Role of interleukin-1β and tumor necrosis factor-α-dependent expression of cyclooxygenase-2 mRNA in thermal hyperalgesia induced by chronic inflammation in mice

The present study investigated whether the endogenous pro-inflammatory cytokines [interleukin (IL)-1β and tumor necrosis factor-α (TNF-α)]-dependent expression of cyclooxygenase-2 (COX-2) mRNA within the spinal cord could be involved in the development of chronic inflammatory pain-like behaviors in mice. We demonstrated that the expression of COX-2 mRNA on the ipsilateral side of the spinal cord was significantly increased 6 h and 3 days after intraplantar injection of complete Freund's adjuvant (CFA), compared with the expression in saline-treated mice. In addition, the chronic pain-like behaviors following CFA injection were markedly suppressed by repeated intrathecal (i.t.) pre-treatment with the COX-2 inhibitor etodolac, but not with the COX-1 inhibitor mofezolac. The cytosolic level of the activated form of nuclear factor-kappa B (NF-κB), which is a major contributor to the induction of COX-2, on the ipsilateral side of the mouse spinal cord was also increased compared with that in the saline-treated mice. The key finding in the present study was that a single i.t. injection with either IL-1β or TNF-α induced a marked increase in spinal COX-2 mRNA and persistent thermal hyperalgesia in mice. Furthermore, CFA-induced hypersensitivity to inflammatory pain was significantly reduced by repeated i.t. pre-injection of the recombinant Fc chimera of IL-1 receptor I or soluble TNF receptor I, which sequesters endogenous IL-1β or TNF-α, respectively. In contrast, the expression of spinal COX-2 mRNA in CFA-treated mice was similar to that in saline-treated mice at 7 days after CFA injection. The present findings strongly indicate the early intrathecal use of the COX-2 inhibitor for the relief of chronic inflammatory pain. Furthermore, together with the result in a previous study that pro-inflammatory cytokines lead to stimulation of a NF-κB-dependent transcriptional pathway, these findings suggest that a spinal cytokine/NF-κB/COX-2 pathway may play an important role in the development, but not maintenance, of chronic pain following peripheral tissue inflammation.