Role of chemokine receptor CXCR4 in spinal dorsal horn in development of morphine tolerance in rats with bone cancer pain

Abstract

Objective To evaluate the role of chemokine receptor CXCR4 in spinal dorsal horn in the development of morphine tolerance in rats with bone cancer pain (BCP).Methods Forty female Sprague-Dawley rats,weighing 180-220 g,were equally randomized into 5 groups using a random number table:sham operation group (group S),BCP group (group B),BCP + AMD3100 (specific CXCR4 antagonist) group (group BA),BCP + morphine group (group BM),BCP + morphine + AMD3100 group (group BMA).BCP was induced by injecting Walker 256 mammary gland cancer cell suspension (4 × 105 cells/ml) 5 μl into the bone marrow of the right tibia of rats anesthetized with chloral hydrate.On 6 days after injection of mammary gland cancer cells,AMD3100 2 mg/kg was intraperitoneally injected twice a day for 9 days in BA group,and morphine 10 mg/kg was subcutaneously injected twice a day for 9 days in BM group.AMD3100 was intraperitoneally injected and morphine was subcutaneously injected as previously described at the corresponding time point in BMA group.Before injection of mammary gland cancer cells (T0) and on 4,6,8,10,12 and 14 days after injection of mammary gland cancer cells (T1-6),paw withdrawal threshold to von Frey hair mechanical stimulation (PWMT) was measured.The rats were then sacrificed and L3-5 segments of the spinal cord were removed for determination of c-fos expression in spinal dorsal horn using immunofluorescence.Results Compared with S group,PMWT was significantly decreased at T2-6 in B and BA groups and at T4-6 in BM group,and c-fos expression was up-regulated at T6 in BM group (P ＜0.01).PMWT was significantly higher at T3-5 in BM group and at T3-6 in BMA group than in group B (P ＜ 0.01).Compared with BM group,PMWT was significantly increased at T5,6 and c-fos expression was down-regulated at T6 in BMA group (P ＜ 0.01).Conclusion Chemokine receptor CXCR4 in spinal dorsal horn is involved in the development of morphine tolerance in rats with BCP and the mechanism may be related to activation of c-fos. Key words: Receptors,CXCR4; Spinal cord; Bone neoplasms; Morphine; Drug tolerance