Establishment of a rat model of bone cancer pain-chronic morphine tolerance

Abstract

Objective To establish a rat model of bone cancer pain-chronic morphine tolerance. Methods Thirty-six adult female Sprague-Dawley rats weighing 180-200 g in which intrathecal catheters were successfully placed without complications were randomly divided into 3 groups ( n = 12 each) :group sham operation (group S),group chronic morphine tolerance (group M) and group bone cancer pain + chronic morphine tolerance (group BM). Bone cancer pain was induced by intra-tibia inoculation of Walker256 mammary gland carcinoma cells (4 ×102 cells/μl) in group BM, while in group M heat-inactivated Walker256 mammary gland carcinoma cells were given instead, and then 10 days later, intrathecal morphine 20 μg,/kg was administered twice a day for 9 consecutive days. The mechanical paw withdrawal threshold (MWT) and mechanical paw withdrawal duration (MWD) were measured before inoculation, at day 1, 3, 6 and 9 after inoculation, and at day 1, 3, 5, 7 and 9 of morphine administration. The degree of bone destruction was assessed by radiological analysis at day 9 after inoculation. After the last measurement of pain threshold, the rats were given innoxious touch-stimulus. The rats were sacrificed 3 h after stopping the stimulus, and L4-6 segment of the spinal cord was isolated to determine the expression of Fos protein in the spinal dorsal horn. Results Compared with group S, MWT was significantly decreased, MWD was significantly prolonged and the expression of Fos protein was up-regulated in group M ( P ＜ 0.05 or 0.01 ). MWT was significantly decreased, MWD was significantly prolonged, bone destruction scores were significantly increased,and the expression of Fos protein was up-regulated in group BM compared with group M ( P ＜ 0.05 or 0.01 ). Conclusion A rat model of bone cancer pain-chronic morphine tolerance is successfully established. Key words: Disease models,animal; Morphine; Drug tolerance; Bone neoplasms; Pain