Complete Freunds adjuvant-induced peripheral inflammation evokes glial activation and proinflammatory cytokine expression in the CNS

Peripheral inflammation induces central sensitization characterized by the development of allodynia and hyperalgesia to mechanical and thermal stimuli. Recent evidence suggests that activation of glial cells and a subsequent increase in proinflarnmatory cytokines contribute to the development of behavioral hypersensitivity after nerve injury or peripheral inflammation. In the present study, we examined mRNA and protein expression of glial markers and proinflammatory cytokines at the lumbar spinal cord, brainstem and forebrain following intraplantar administration of complete Freunds adjuvant (CFA) in rats. Gene expression studied by real-time reverse transcriptase-polymerase chain reaction (RT-PCR) for microglial markers (Mac-1, TLR4 and CD14) showed a significant increase in their expression during all phases (acute, subacute and chronic) of inflammation. Conversely, up-regulation of astroglial markers [glial fibrillary acidic protein (GFAP) and S100B] was observed only at the subacute and chronic phases of inflammation. Increased immunoreactivity for OX-42 (CR3/CD11b) and GFAP at various brain regions was also observed after the acute and subacute phases of the inflammation, respectively. Quantification of proinflarnmatory cytokines (IL-1beta, IL-6 and TNF-alpha) at the mRNA (by real-time RT-PCR) and protein level (by ELISA) revealed enhanced expression during the acute, subacute and chronic phases of CFA-induced peripheral inflammation. This study demonstrates that CFA-induced peripheral inflammation induces robust glial activation and proinflammatory cytokines both spinally and supraspinally. In addition, similar to nerve injury-induced behavioral hypersensitivity microglial activation preceded astrocytic activation following CFA-induced peripheral inflammation, supporting a role of microglia in the initiation phase and astrocytes in maintaining hypersensitivity. These findings further support a unifying theory that glial activation and enhanced cytokine expression at the CNS have a role in eliciting behavioral hypersensitivity.