Transcranial Direct Current Stimulation Targeting the Primary Motor Cortex Suppresses Activity in the Ventrolateral Periaqueductal Gray and Alleviates Chronic Pain in a Rat Model of Knee Osteoarthritis

Yifan Xu,1 Chenglei Fan,2 Junqiang Xue,1 Chengfei Gao,1 Yu Zhou,3 Tieshan Li1 1Department of Rehabilitation Medicine, The Affiliated Hospital of Qingdao University, Qingdao, Shandong Province, People’s Republic of China; 2School of Rehabilitation Sciences and Engineering, University of Health and Rehabilitation Sciences, Qingdao, Shandong Province, People’s Republic of China; 3Department of Life Sciences and Health, University of Health and Rehabilitation Sciences, Qingdao, Shandong Province, People’s Republic of ChinaCorrespondence: Tieshan Li; Yu Zhou, Email [email protected]; [email protected]: Transcranial direct current stimulation (tDCS) shows promise for chronic pain treatment; however, its efficacy and mechanisms in knee osteoarthritis (KOA)-associated pain remain unclear. This study aimed to evaluate the analgesic effects of primary motor cortex (M1)-targeted tDCS in a rat model of KOA and to investigate the underlying mechanisms.Methods: A chronic KOA model was established via unilateral intra-articular monoiodoacetate (MIA) injection. Mechanical allodynia and thermal hyperalgesia were assessed using von Frey filaments and a hot plate test. Neuronal activation and microglial responses in the ventrolateral periaqueductal gray (vlPAG) were evaluated by immunohistochemistry, and inflammatory cytokines were quantified using qPCR. The effects of M1-targeted tDCS on vlPAGactivity, microglial activation, and pain behaviors were examined. Minocycline was administered into the vlPAG to inhibit microglial. CTB-555 retrograde tracing was used to map M1-vlPAG connectivity.Results: MIA induced persistent pain hypersensitivity, accompanied by increased neuronal activity, microglial activation, and elevated pro-inflammatory cytokines in the contralateral vlPAG. Local administration of minocycline suppressed both microglial and neuronal activation and alleviated pain behaviors. M1-targeted tDCS significantly reduced pain hypersensitivity, suppressed vlPAG neuronal activity and microglial activation, and downregulated inflammatory cytokines expression.Conclusion: KOA-associated chronic pain is linked to increased neuronal activity and neuroinflammation in the vIPAG. M1-targeted tDCS alleviates pain, likely by suppressing vIPAG activity and microglial-mediated inflammation via the M1-vlPAG pathway. However, given differences between the MIA model and human osteoarthritis, caution is required in clinical translation.Keywords: transcranial direct current stimulation, knee osteoarthritis, chronic pain, microglial, neuroinflammation, M1-vlPAG pathway