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Treating bone cancer pain continues to be a clinical challenge and underlying mechanisms of bone cancer pain remain elusive. Here, we report that EphB1 receptor forward signaling in the spinal cord is critical to the development of bone cancer pain and morphine tolerance in treating bone cancer pain. Tibia bone cavity tumor cell implantation (TCI) produces bone cancer--related thermal hyperalgesia, mechanical allodynia, spontaneous and movement-evoked pain behaviors, and bone destruction. Production and persistence of these pain behaviors are well correlated with TCI-induced upregulation of EphB1 receptor and its ligand ephrinB2 in the dorsal horn and primary sensory neurons. Spinal administration of an EphB1 receptor blocking reagent EphB2-Fc prevents and reverses bone cancer pain behaviors and the associated induction of c-Fos and activation of astrocytes and microglial cells, NR1 and NR2B receptors, Src within the N-methyl-D-aspartate receptor complex, and the subsequent Ca2+-dependent signals. The exogenous ligand ephrinB2-Fc upregulates level of phosphorylation of NR1 and NR2B receptors depending on the activation of EphB1 receptor. Spinal administration of EphB2-Fc and ephrinB2-Fc induces downregulation of EphB1 and ephrinB2, respectively, accompaniedwith increased activity of matrix metalloproteinase (MMP)-2/9. Blocking MMP-2 or MMP-9 reverses EphB1-Fc treatment--induced down-regulation of EphB1 receptor. In addition, spinal blocking or targeted mutation of EphB1 receptor reverses morphine tolerance in treating bone cancer pain in rats and defensive pain inmice. These findings show a critical mechanism underlying the pathogenesis of bone cancer pain and suggest a potential target for treating bone cancer pain and improving analgesic effect of morphine clinically. Cancer Res; 71(13); 4392-402. ©2011 AACR. [ABSTRACT FROM AUTHOR]