Aim of Investigation Cancer pain, especially bone cancer-induced pain, is one of the most severe types of chronic pain.Treatment for this kind of pain is far from satisfied.It has been demonstrated that the activation of purinergic P2X7 receptor (P2X7R) in the spinal cord contributes to the development of hyperalgesia and allodynia in neuropathic and inflammatory state.However, the role of P2X7R in the bone cancer-induced pain has not been elucidated.The present study investigated the possible role of spinal P2X7R in the pain state induced by bone cancer.Methods Rat bone cancer model was established by inoculating of syngeneic Walker 256 mammary gland carcinoma cell into tibia of female rats.Immunohistochemistry and Western blot were used to analyze the cell type and the expression of protein.Small RNA interference in vivo is used to knock down the expression of P2X7R in the spinal cord.A set of calibrated Von Frey filaments was used to determine the mechanical threshold during behavioral test.Results lntra-tibia inoculation of mamnary gland carcinoma cell robustly increased the expression of P2X7R, phosph-p38 (p-p38), IL-18, IL-18R and p-NR2B in the lumbar spinal dorsal horn.The increased P2X7R immunoreactivity was located mainly in Iba-l-positive microglia.Additionally, the increased p-p38 and IL-18 immunoreactivity were also observed in Iba-1-positive microglia, suggesting that P2X7R and IL18 might be the up-and down-stream of microglia p38 activation.Intrathecal injection of P2X7 receptor siRNA significantly downregulated the expression of spinal p-p38, IL-18, IL-18R, pNR2B and NR1, and attenuated painrelated behavior.Intrathecal injection of selective P2X7R inhibitor BBG also attenuated pain-related behavior.Moreover, intrathecal injection of selective p38 inhibitor SB239063 not only downregulated the expression of IL18 and IL-18 receptor, but also attenuated pain-related behavior.Conclusion In conclusion, our results suggest that P2X7R is involved in the development of bone cancer-induced pain, which might be via the activation of p38/MAPK signaling and lead to the release of IL-18, and then, activates NMDA receptor in neuron.Inhibition of the P2X7R-p38/MAPK-IL-18-NMDAR signaling pathway may provide new therapy targets for bone cancer-induced pain.