Cancer pain is one of the most severe type of chronic pain.The most common cancer pain is caused by tumors that metastasize from distant sites such as breast, prostate, and lung to the bone.Morphine is a potent analgesic.However, its clinical application is restricted due to morphine tolerance.This study investigated upstream and downstream mechanisms of microglia-expressed p-p38 underlying the induction of bone cancer pain and tolerance to morphine analgesia in rats.Following bone cancer pain and morphine tolerance development, p38/MAPK was robustly activated in spinal microglia.Upstream, P2X7R expressed on spinal microglia was upregulated after inoculation of tumor cells into the tibia medullary cavity and chronic exposure to morphine.Intrathecal administration of Brilliant Blue G (BBG), a selective P2X7R inhibitor, or knock-down P2X7R significantly attenuated bone cancer pain and the loss of morphine analgesic potency, as well as upregulation of p-p38 in the spinal cord.Downstream, the expression of IL-18 by microglia and IL-18 receptor (IL-18R) by astrocytes was increased with bone cancer morphine tolerance development, which was blocked by p38 inhibitor or P2X7R antagonist.Moreover, the expression levels of NMDA receptor NR2A and Glu transporter GLT-1 in the spinal cord were significantly enhanced in bone cancer rats.