Role Of Mu-Opioid Receptor In Nociceptive Modulation In Anterior Cingulate Cortex Of Rats

Objective: To investigate the role of mu opioid receptor(MOR)in nociceptive modulation in ACC in normal rats,as well as in rats with neuropathic pain and rats with inflammatory pain.Methods: 1.To determine the effects of morphine-induced antinociception in ACC,different dose of morphine were injected intra-ACC in normal rats,rats with neuropathic pain and rats with inflammatory pain.2.To observe whether opioid receptor was involved in the morphine-induced antinociception in ACC,normal rats,rats with neuropathic pain and rats with inflammatory pain received intra-ACC injection of morphine and naloxone.3.To further explore MOR was involved in the morphine-induced antinociception in ACC,normal rats,rats with neuropathic pain and rats with inflammatory pain received intra-ACC injection of morphine and MOR antagonist ?-FNA.4.To confirm whether MOR was involved in the morphine-induced antinociception in ACC,normal rats received intra-ACC injection of siRNA targeting MOR or MOR meaningless sequence,the brain ACC tissue were tested by RT-PCR and western blotting.5.To further confirm whether MOR was involved in the morphine-induced antinociception in ACC,normal rats were received siRNA targeting MOR or MOR meaningless sequence,and the antinociception was observed by injecting morphine on days 3,6,and 9,respectively.6.Normal rats,rats with neuropathic pain and the rats with inflammatory pain were prepared.The rats were sacrificed at specific time to take the ACC region.The plastic changes of MOR in different pain models were detected by RT-PCR and western blotting.7.After intra-ACC injection of morphine in normal rats,CaMKII inhibitor,PKC inhibitor and ERK inhibitor were injected respectively to observe whether CaMKII,PKC and ERK signaling pathways were involved.Results: 1.After intra-ACC injection of morphine,there were significant increases in HWLs to thermal and mechanical stimulation in normal rats,the results demonstrated that intra-ACC injection of morphine induced significant antinociceptive effects in normal rats.2.The increased HWLs deceased significantly after intra-ACC administration of naloxone,the results indicate that opioid receptor was involved in the morphine-induced antinociception in the ACC of normal rats.3.The antinociceptive effects induced by morphine were attenuated by intra-ACC injection of the MOR antagonist ?-FNA,indicating that MOR medicates the morphine-induced antinociception in ACC in normal rats.4.The normal rats received intra-ACC administration of siRNA targeting MOR and found that there was a down-regulation in MOR mRNA levels,as well as a down-regulation in MOR expression in ACC tested by RT-PCR and western blotting.5.The morphine-induced antinociception decreased significant in rats with siRNA targeting MOR tested by hot-plate and Randall Selitto Test,and the effect lasted for 6 days,indicating that knockdown MOR in ACC also attenuated the morphine-induced antinociception.6.The morphine induced antinociception could be much significantly attenuated by followed intra-ACC injection of selective CaMKII and PKC inhibitors respectively,indicating that CaMKII and PKC signaling pathway may be involved in the morphine-induced antinociception in ACC in normal rats,but the ERK signaling pathway is not involved.7.The intra-ACC administration of morphine induced antinociceptive effects in a dose-dependent manner in rats with neuropathic pain and inflammatory pain,indicating that the morphine induced antinociception in ACC in rats with neuropathic pain and rats with inflammatory pain.8.The antinociceptive effects induced by morphine were attenuated by intra-ACC injection of the naloxone,indicating an involvement of opioid receptor in nociceptive modulation in ACC in rats with neuropathic pain and rats with inflammatory pain.9.The antinociceptive effects induced by morphine were attenuated by intra-ACC injection of the MOR antagonist ?-FNA,indicating an involvement of MOR in nociceptive modulation in ACC in rats with neuropathic pain and rats with inflammatory pain.10.There was a decreased trend in the content of MOR and mRNA lever in ACC in rats with neuropathic pain than that in normal rats tested by western blotting and RT-PCR,and there is a decreased significantly in the content of MOR in ACC in rats with inflammatory than that in normal rats tested by western blotting RT-PCR.At the same time,we further found that the morphine-induced antinociception was lower in rats withinflammatory pain or neuropathic pain than that in normal rats.Conclusion: These results suggest that MOR plays an important role in nociceptive modulation in ACC in normal rats,rats with neuropathic pain and rats with inflammatory pain.Both CaMK? and PKC signaling molecules are involved in morphine-induced antinociception in ACC in normal rats.Innovation: This paper firstly found the role of MOR in nociceptive modulation in ACC in rats with neuropathic pain and rats with inflammatory pain,and signaling pathways involved in morphine-induced antinociception in ACC in normal rats.