Microglial Response After Chronic Implantation of Epidural Spinal Cord Electrode

Spinal cord stimulation (SCS) is currently used to treat chronic pain and has been shown to be effective in the treatment of other neurological disorders, such as Parkinson’s disease. The electrodes used for SCS are placed adjacent to the nervous tissue and thus can induce inflammation that may interfere with its efficacy. One of our previous studies showed that acute implantation of spinal cord electrodes resulted in substantial microglial activation throughout the transversal sections of the spinal cord at the region of implant, but data on microglial activation after longer periods is still lacking. To investigate microglial activation in the spinal cord after a chronic period of implantation, 3 wistar rats were implanted with SCS electrodes in the epidural space under thoracic vertebra 4 (T4), while 4 animals underwent a sham surgery. Thirty seven days after the surgery, the animals were perfused and spinal cord sections of the implanted region (T4) and regions close (thoracic levels 3 and 5—T3 and T5) and distant to the implanted tissue (cervical level 4—C4 and lumbar level 1—L1) were analyzed by calcium-binding adaptor protein-1 (IBA-1) immunohistochemistry to determine microglial amount and morphology. There was a greater microglial activation at the implant site (T4) and in nearby regions (T3 and T5) in animals of the electrode group. In addition, at T4, the amount of activated microglia was higher at regions in close contact with the electrode than in regions farther from the electrode, with the highest percentage of activated microglia at the dorsal funiculus (18%). To our knowledge, this is the first study describing the microglial activation following chronic implantation of epidural electrodes, and it provides important new information regarding the spinal cord tissue response to chronic implants.