| Background: Knee osteoarthritis(KOA)is one of the common causes of chronic pain.The mechanism of pain is very complex,which may be related to the occurrence of central sensitization.Studies have shown that the dysregulation of the descending pain regulatory pathways can lead to central sensitization and contribute to the development of chronic pain.Brain-derived neurotrophic factor(BDNF)is involved in maintaining central sensitization and plays an important role in the occurrence of chronic pain by binding to its specific receptor tropomyosin-related kinase B(TrkB),but its role in KOA chronic pain is unclear.Transcranial direct current stimulation(tDCS)has been shown to relieve pain,but its analgesic mechanisms are still being explored.Objective: In this study,monosodium iodoacetate(MIA)rat model was used to investigate the role of BDNF/ TrkB signaling pathway in chronic pain of KOA.In addition,the analgesic effect and mechanism of tDCS on KOA chronic pain were also explored by applying tDCS to MIA model rats.Method:Experiment 1.In this study,40 SD rats were randomly selected according to the random number table method,and the chronic pain model of KOA was established by injecting60μL MIA into the left knee cavity.In addition,10 rats were randomly selected to inject the same dose of normal saline into the left knee joint cavity as Sham group.The modeling rats were randomly divided into MIA group,MIA+ANA-12(an inhibitor of TrkB receptor)group and MIA+1%DMSO(ANA-12 solvent)group,with 10 rats in each group.The MIA+ANA-12 group was injected with ANA-12 in rostral ventromedial medulla(RVM),while the MIA+1%DMSO group was injected with 1%DMSO solvent at the same site as the control.Pain behavior of all rats was measured at 1 day before modeling and at 1,3,7,14 and 21 days after modeling.After the last pain behavior measurement,protein expression levels of BDNF and TrkB in RVM and L4-5 spinal dorsal horn(SDH)of the rats in each group were detected by western blot and immunohistochemistry.Experiment 2.In this study,60 SD rats were randomly selected according to the random number table method for modeling.In addition,10 rats were randomly selected as Sham group.The rats with successful modeling were randomly divided into MIA group,MIA+tDCS group,MIA+StDCS(Sham tDCS)group,MIA+tDCS+BDNF group and MIA+tDCS+PBS(exogenous BDNF solvent)group,with 10 rats in each group.After 21 days of modeling,the MIA+ tDCS group received tDCS for 8 consecutive days,20 minutes a day,while the MIA+StDCS group received sham stimulation for 8 consecutive days under the same conditions.The MIA+tDCS+BDNF group was injected with exogenous BDNF in periaqueductal gray(PAG),while the MIA+tDCS+PBS group was injected with PBS solvent at the same site as the control.The pain behavior of all rats was measured at 1 day before modeling,at 1,3,7,14,21 days after modeling and at 0,1 days after treatment,respectively.After the last pain behavior measurement,protein expression levels of BDNF and TrkB in PAG,RVM and L4-5 SDH of the rats in each group were detected by western blot and immunohistochemistry.Results:Experiment 1.(1)Compared with the Sham group,the paw withdrawal threshold(PWT)and paw withdrawal latency(PWL)of the MIA group were significantly decreased(P < 0.0001).However,after ANA-12 injection,PWT and PWL were significantly increased in MIA+ANA-12 group compared with MIA+1%DMSO group(P < 0.001).(2)Western blot results showed that the protein expressions of BDNF and TrkB in RVM and SDH of rats in MIA group were significantly higher than those in Sham group21 days after MIA injection(P < 0.05).However,ANA-12 inhibited the overexpression of BDNF/TrkB,and the protein expressions of BDNF and TrkB in MIA+ANA-12 group were significantly decreased(P < 0.05).No significant difference in the expression of BDNF and TrkB was observed between the MIA+1% DMSO and MIA groups.(3)Immunohistochemistry results showed that MIA injection significantly increased the proportion of BDNF and TrkB positive staining cells in RVM and SDH(P < 0.01).However,compared with MIA+1%DMSO group,the proportion of BDNF and TrkB positive staining cells in RVM and SDH of rats in MIA+ANA-12 group was significantly decreased(P < 0.05).There was no significant difference in the expression of BDNF and TrkB in MIA+1%DMSO group compared with MIA group.Experiment 2.(1)After treatment with tDCS,PWT and PWL were significantly increased in MIA+tDCS group compared with MIA+StDCS group(P < 0.001).Interestingly,exogenous BDNF injection reversed the therapeutic effect of tDCS.Compared with MIA+tDCS+PBS group,PWT and PWL in MIA+tDCS+BDNF group were significantly decreased(P <0.0001).(2)Western blot results showed that after tDCS treatment,the expression level of BDNF protein in PAG in MIA+tDCS group was significantly lower than that in MIA+StDCS group(P < 0.05).Similarly,tDCS treatment also significantly decreased the protein expressions of BDNF and TrkB in RVM and SDH(P < 0.05).However,no significant difference was found between MIA group and MIA+StDCS group.In addition,after injection of exogenous BDNF,it was found that the protein expressions of BDNF in PAG,as well as the protein expressions of BDNF and TrkB in RVM and SDH in MIA+tDCS+BDNF group were significantly increased(P < 0.05).No significant difference was found between MIA+tDCS and MIA+tDCS+PBS.(3)Immunohistochemistry results showed that compared with MIA+StDCS group,tDCS treatment could reduce the proportion of BDNF positive staining cells in PAG and the proportion of BDNF and TrkB positive staining cells in RVM and SDH(P < 0.05),but there was no significant difference in the expression levels of BDNF and TrkB between MIA group and MIA+StDCS group.After injection of exogenous BDNF,compared with MIA+tDCS+PBS group,MIA+tDCS+BDNF group significantly increased the proportion of BDNF positive staining cells in PAG and the proportion of BDNF and TrkB positive staining cells in RVM and SDH(P < 0.05).However,there was no significant difference in the proportion of BDNF and TrkB positive stained cells between MIA+tDCS and MIA+tDCS+PBS groups.Conclusion:1.In the MIA-induced KOA chronic pain model,overexpression of BDNF/TrkB signaling in the descending pain modulation pathway may promote descending facilitation and participate in the occurrence of central sensitization,thus leading to chronic pain of KOA.2.tDCS can alleviate MIA-induced KOA chronic pain,and its analgesic mechanism may be to inhibit overexpression of BDNF/TrkB signal in descending pain pathway,reduce descending pain facilitation,improve central sensitization,and thus relieve chronic pain in KOA. |
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