Transcranial direct current stimulation attenuates the chronic pain of osteoarthritis in rats via the top-down modulation

Abstract Purpose Osteoarthritis (OA) has been the common cause to lead to chronic pain. Numerous studies have demonstrated that transcranial direct current stimulation (tDCS) can revert maladaptive changes and relieve chronic pain. TDCS is effective in the treatment of a number of chronic pain conditions, but the top-down analgesic mechanism involved is still unclear. This study observed the analgesic effects of tDCS and the changes of NMDA receptors levels in the spinal cord after tDCS treatment in rats to explore the top-down analgesic mechanism of tDCS. Methods Monosodium iodoacetate (MIA) was injected into the ankle joint of rats to establish OA chronic pain model. After 21 days, the rats received tDCS for 14 consecutive days (20 min/day). As indicators of mechanical allodynia and thermal hyperalgesia, we used Von Frey test and hot plate test to assess the pain-related behaviors at different time points. Western blot and Immunohistochemistry were performed to observe the expression level of NMDAR2B in the spinal cord after tDCS treatment. Results After MIA injection, rats developed apparent mechanical hyperalgesia and thermal hyperalgesia. However, the pain-related behaviors of rats were significantly improved after tDCS treatment. In addition, the expression of NMDAR2B and the proportion of positive stained cells of NMDAR2B were reversed by tDCS treatment. Conclusion The results demonstrated that tDCS can attenuate OA-induced chronic pain in rats via reducing NMDAR2B expressions in the spinal cord. We believe that this may be the result of tDCS participating in the top-down modulation of pain pathway in the endogenous analgesic system.