Regulation of spinal neuroimmune responses by prolonged morphine treatment in a rat model of cancer induced bone pain

Cancer induced bone pain (CIBP) is a major clinical problem. Although opioids remain the principal axis in drug therapies for CIBP, their sustained application is known to induce cellular and molecular adaptations including enhanced neuroimmune reactivity. This is generally characterized by glial activation and proinflammatory cytokine production which frequently results in pharmacological tolerance. This research was performed to investigate spinal neuroimmune responses after prolonged systemic morphine treatment in a rat model of CIBP. The model was established using a unilateral intra-tibia injection of Walker 256 mammary gland carcinoma cells. Subcutaneous morphine was repeatedly administered from postoperative days 14 to 19. Mechanical allodynia to von Frey filaments and ambulatory pain scores were recorded to investigate changes of nociceptive behaviors. Spinal glial activation was detected by immunohistochemistry and real-time PCR; the production of proinflammatory cytokines (IL-1β and TNF-α) was examined through real-time PCR and ELISA. Results showed that chronic morphine use failed to elicit analgesic tolerance in the rat CIBP model. Moreover, the treatment had no significant influence on the activated spinal glia morphology, cell density and expression of special cytomembrane markers, whereas it significantly down-regulated the local proinflammatory cytokine production at the mRNA and protein level. Collectively, these data suggest that chronic morphine treatment in CIBP is not concomitant with pharmacological tolerance, at least partially because the treatment fails to amplify spinal neuroimmune responses.