The Research On The Mechanism Of Neuropathic Pain Participated By The Glucocorticoid Receptor And Spinal Cord Microglia

BackgroundNeuropathic pain is usually caused by the damage and dysfunction of nervous system. It is a kind of chronic pain, accompanied by the characteristics of scope widen and hyperpathia. However, the present treatment of neuropathic pain is still lack of effective method. So the research on mechanism of neuropathic pain is of particularly important.When the body is stimulated by trauma and infection, peripheral nociceptors would transfer the signal to the spinal cord, then diver it to the cerebral cortex and lead to pain. This procedure contains various electrochemical signals, such as inflammatory mediators and neurotransmitter, leading to central sensitization and the decline of threshold. In the past, the research on neuropathic pain was focused on neurons sensitization and the remodeling of synapse and the study on glia cells activation was limited.Recent studies showed that there were activation of glia cells in the peripheral nerve injury model, peripheral inflammatory pain model and the cancer pain model. This phenomenon indicates that the activation of glia cells in the spinal cord may play a significant role in neuropathic pain. Glucocorticoid is an important substance in normal human body. It plays an extensive role including anti-inflammatory, anti-shock, anti-toxic and so on. Glucocorticoid receptor(GR) is a hormone dependent transcription factor, which is expressed widely in the peripheral nervous system and the central nervous system. GR plays a considerable role in the nociceptive information transmission and the central sensitization, but the effect of GR in central nervous system is controversial. Therefore, how about the role of GR and microglia playing in neuropathic pain? What is the relationship between GR and microglia? These issues are needed to verify.In this study, we established the spared nervous injury model in rats in order to explore the activation state of microglia and the expression of GR in the spinal cord. Then we injected Dex(GR agonist), RU38486(GR antagonist) and Fluorocitrate(microglia antagonist) in spinal subarachnoid space. We observed the change of pain, detected the expression of microglia OX42 and GR, and explored the possible mechanism of neuropathic pain caused by peripheral nerve injury in order to provide the theoretical basis for the treatment. ObjectiveTo evaluate the relationship of OX42、GR and neuropathic pain, we established the spared nerve injury(SNI) model in rats and observed the change of 50% mechanical withdrawal threshold(MWT) 、the activation of OX42 and GR、the expression of GR protein in the spinal cord through injecting GR agonist Dex, GR antagonist RU38486 and microglia antagonist Fluorocitrate. Methods1. Animal grouping:90 SD male rats were randomly assigned to nine groups: model group(SNI group, n=24) and sham operation(sham group,n=24); SNI+Dex group(n=6), SNI+NS group(n=6), SNI+RU38486 group(n=6),RU38486 group(n=6) and NS group(n=6);SNI+FC group(n=6) and SNI+NS group(n=6).2.The establishment of SNI model: rats were weighed and administered intraperitoneally 10% chloral hydrate.Cut the skin and muscle,explored the sciatic nerve and the branch of sciatic nerve. Ligated the tibial nerve and peroneal nerve, sheared and removed a part of nerve stem. The Sham group exposed to sciatic nerve without surgery.3.Subarachnoid catheterization: PE10 was put into the endorhachis at L5-L6. The outflow of clear CSF from the catheter or the tail flick reflex showed the success of subarachnoid catheterization.4.50%MWT: 50%MWT of SNI group and Sham group were monitored on preoperative day, postoperative day of 3,7,14 and 21. 50%MWT of SNI+Dex group, SNI+NS group and SNI+RU38486 group were monitored on preoperative day, postoperative day of 3,7,9 to 13. 50%MWT of RU38486 group and NS group were monitored before catheterization day and after catheterization day of 5 to 10. 50%MWT of SNI+FC group and SNI+NS group were monitored on preoperative day, postoperative day of 3,7,9 to 13.5.Immunofluorescence: detected the change of OX42 and GR in SNI and Sham group on 14 th day postoperatively and the situation of GR in SNI+FC and SNI+NS group on postoperative day of 13.6.Western-blot: detected the expression of GR protein in SNI group on postoperative day of 14 and in SNI+FC、SNI+NS group on postoperative day of 13. Result1.Compared with the Sham group, 50%MWT of SNI group was significantly decreased after operation(P<0.05); Western-blot showed that the level of GR protein in SNI group was significantly decreased(P<0.05).2.Compared with the SNI+NS group, 50%MWT of SNI+Dex group was significantly increased after operation(P<0.05), but there was no significant difference between SNI+RU38486 group and SNI+NS group(P>0.05). Compared with the NS group, 50%MWT of RU38486 group was significantly decreased after catheterization(P<0.05). Western-blot showed that the level of GR protein in SNI+Dex group was significantly increased(P<0.05)and the expression of GR in SNI+RU38486 group was decreased(P<0.05).3.Compared with the Sham group, immunofluorescence in SNI group showed the expression of GR was significantly decreased and the expression of OX42 was increased(P<0.05). In addition, GR was expressed in microglia in the spinal cord.4.Compared with the SNI+NS group, 50%MWT of SNI+FC group was significantly increased after operation(P<0.05). Compared with the SNI+NS group, immunofluorescence in SNI+FC group showed the expression of OX42 was decreased(P<0.05)and the expression of GR was significantly increased(P<0.05). Conclusion1.There is a substantial reduction of mechanical pain sensitivity by intrathecal injection of Dexamethasone(Glucocorticoid Receptor agonist). Glucocorticoid Receptor may mediate the neuropathic pain.2. Glucocorticoid Receptor may regulate the neuropathic pain mediated by microglia in the spinal cord.