The up-regulation of spinal Toll-like receptor 4 in rats with inflammatory pain induced by complete Freund's adjuvant

• Inflammatory induced by CFA intra-plantar inject to rats.
• CFA induced pain hypersensitivity and paw edema in rats.
• Microglia and TLR4 display dramatic increase on 14 d post CFA.
• Minocycline i.t. reverses pain hypersensitivity, but has no effects on paw edema.
• Minocycline i.t. suppresses activation of microglia and TLR4 induced by CFA.

Peripheral inflammation induces central sensitization that displays the features by the development of pain hypersensitivity to the stimuli. It has been shown that activation of glia contributes to the development of behavioral hypersensitivity after peripheral inflammation. It has been suggested that Toll-like receptor 4 (TLR4) primarily expressed on microglia affects central pain response. The present study was designed to examine the expressions of TLR4 and microglia in the spinal cord in different time points of inflammatory pain induced by complete Freund's adjuvant (CFA). The results show that CFA induces significant pain hypersensitivity and paw edema as well as spinal dorsal horn (SDH) microglia activation with the increased expressions of OX-42 and TLR4 during the inflammatory pain, respectively. The quantification of TLR4 with Western Blot analysis also suggests the same patter with the morphological results during the progress of inflammatory pain. In addition, chronic minocycline hydrochloride intrathecal injection reverses pain hypersensitivity and suppresses activation of microglia and TLR4 induced by CFA, but has hardly any effects on paw edema. Taken together, our data demonstrate the importance of TLR4 and microglia in rats in CFA inflammatory pain states, and suggest that blockade of microglia should likely be considered as a therapeutic opportunity.