Expression of substance P and calcitonin gene-related peptide in bone cancer pain-morphine tolerance rat model

Background and purpose:More than half of the advanced cancer patients suffer from severe pain.Opioid is the overwhelming drug in cancer pain management.But repeated exposure to opioids can lead to tolerance and hyperalgesia.Substance P(SP) and calcitonin gene-related peptide(CGRP) are the key neurotransmitters in pain signaling pathway.The expression of neurotransmitter has been widely investigated in many pain models but poorly explored in cancer pain-opioid tolerance model.This research is to investigate SP and CGRP expression characteristics at dorsal root ganglion(DRG) level in the development of bone cancer pain-morphine tolerance of rat model.Methods:Sixty Wistar rats with successful intrathecal lidocaine were randomly divided into 3 groups(n=20 each): sham group,morphine tolerance group and bone cancer pain-morphine tolerance group.Bone cancer pain were induced by intra-tibia inoculation of Walker256 mammary gland carcinoma cells in bone cancer pain-morphine tolerance group,while in morphine tolerance group heat-inactivated Walkore256 mammary gland carcinoma cells were given instead.Morphine 20 μg/kg was administered intrathecally twice a day for 9 consecutive days 10 days later.The mechanical paw withdrawal threshold(PWT) was measured before inoculation,at day 1,3,6 and 9 after inoculation,and at day l,3,5,7 and 9 of morphine administration.The rats were sacrificed after stopping the last stimulus.DRG of L4-L5 segment was isolated to determine the SP-immunoreactivity and CGRP-immunoreactivity expression.Results:PWT was significantly decreased in bone cancer pain-morphine tolerance group 5 days after morphine intrathecal compared with sham group and morphine tolerance group(P0.001).The IOD values of SP and CGRP(9 917.9±2 246.1 and 15 021.5±2 989.7) by immunohistochemical methods in bone cancer pain-morphine tolerance group were much higher than that in morphine tolerance group(5 191.7±1 052.6 and 9 737.1±2 239.8) and sham group(4 821.6±843.1 and 8 180.3±1 242.2,P0.001,respectively).Conclusion:It suggests that SP and CGRP may play an important role in morphine antinociceptive tolerance by up-regulation in DRG.This research would provide more scientific evidence of new strategy for inhibiting morphine tolerance in cancer pain.