The Role And Mechanism Of Norepinephrine In The Medial Prefrontal Cortex In Chronic Pain After Surgery In Rats

Objective: To investigate the role and mechanism of norepinephrine in the medial prefrontal cortex(m PFC)in chronic pain after thoracotomy.Methods: To detect the changes of m PFC norepinephrine after thoracotomy.Twenty rats were randomly divided into four groups,Baseline group,T7 d group,T14 d group,T28 d group.No operation was performed in Baseline group,thoracotomy was performed in T7 d,T14d and T28 d groups.Mechanical pain threshold was measured with von frey filaments and cold pain threshold was assessed using aceton on 1d,7d,14 d,21d,28 d after operation.The expression of norepinephrine synthesis and metabolism related enzymes(DβH,MAOA,MAOB and COMT)in contralateral m PFC were detected by RT-PCR after the operation.To verify the role and mechanism of m PFC norepinephrine in chronic pain after thoracotomy.Sixty adult rats were randomly divided into sham group,sham+AAV.sh DβH group,T+AAV group and T+AAV.sh DβH group(n=15/group).In Sham+AAV.sh DβH group,adeno-associated virus(AAV)-mediated sh DβH was microinjected into the contralateral m PFC 3 days after pleurotomy alone.In T+AAV group and T+ AAV.sh DβH group,blank virus vector AAV or AAV.sh DβH was injected into contralateral m PFC 3 days after pleurotomy and rib dissection,respectively.Mechanical pain threshold and cold pain threshold were measured at 1d,7d,14 d,21d,28 d,35d postoperatively.RT-PCR was used to detect norepinephrine synthase DβH,pain-causing substance receptors(Bdkrb2,Tacr1),inflammatory factors(IL-6,TNF-α)in contralateral m PFC of rats after operation at 35 days.The expression of AQP4 and IL-33 in contralateral m PFC was detected by western blotting and immunofluorescence.The activation of contralateral m PFC microglia were detected by immunofluorescence.Results:Part I: After thoracotomy,chronic mechanical and cold hyperalgesia were observed.The expression of m PFC DβH on the contralateral side continued to increase on the 7th,14 th and 28 th day after surgery,but the expression of DβH on the ipsilateral m PFC and MAOA,MAOB and COMT of bilateral m PFC had no significant changes after surgery.Part Ⅱ: After thoracotomy,Bdkrb2 expression and neuroinflammation(microglial activation and inflammatory factor expression)increased,while AQP4 and IL-33 expression continued to decrease.3 days after thoracotomy,directional injection of sh DβH into contralateral m PFC inhibited the expression of DβH and reduced the level of norepinephrine in m PFC,alleviating chronic mechanical and cold hyperalgesia,reducing the expression of Bdkrb2,alleviating neuroinflammation,and partially reversing the decreased expression of AQP4 and IL-33.Conclusions: The continuous increase of norepinephrine in m PFC after thoracotomy may induce neuroinflammation and increase the expression of Bdkrb2 through down-regulating the expression of IL-33 and AQP4,ultimately lead to CPSP after thoracotomy.