Intrathecal Injection of JWH-015 Attenuates Bone Cancer Pain Via Time-Dependent Modification of Pro-inflammatory Cytokines Expression and Astrocytes Activity in Spinal Cord

Cannabinoid receptor type 2 (CB2) agonists display potential analgesic effects in acute and neuropathic pain. However, its complex cellular and molecular mechanisms in bone cancer pain remain unclear. And less relevant reports concerned its time-dependent effects on the long-lasting modifications of behavior, spinal inflammatory cytokines levels, astrocytes activity induced by bone cancer pain. A rat model of bone cancer pain induced by intra-tibia inoculation of Walker 256 mammary gland carcinoma cells was utilized. Pain behaviors at different time points were assessed by ambulatory pain scores and paw withdrawal mechanical threshold (PWMT). Pro-inflammatory cytokines, such as interleukin (IL)-1 beta, IL-6, IL-18, and tumor necrosis factor alpha (TNF-alpha), were quantitated by Western blots. Glial activity was assessed by immunohistochemistry. Intra-tibia inoculation of Walker 256 mammary gland carcinoma cells induced progressive bone cancer pain; a long-term up-regulation of IL-1 beta, IL-6, IL-18, and TNF-alpha; and the activation of glia in spinal cord. Activation of microglia was first evident on day 4 after surgery and reached to a peak on day 7 while activation of astrocytes was on day 10. A single intrathecal injection of JWH-015 attenuated bone cancer induced spontaneous pain and mechanical allodynia, reduced the expression of pro-inflammatory cytokines, and inhibited the activity of astrocytes. All the modifications were transient and peaked at 24 h after JWH-015 administration. Furthermore, the protective effects of JWH-015 were reversed in the presence of CB2-selective antagonist AM630. Overall, our results provided evidences for the persistent participation of inflammation reaction in the progression of bone cancer pain, and demonstrated that JWH-015 reduced the expression of IL-1 beta, IL-6, IL-18, and TNF-alpha and inhibited astrocytes activation in a time-dependent manner, thereby displaying an analgesic effect.