Primary Motor Cortex Transcranial Direct Current Stimulation Modulates Temporal Summation of the Nociceptive Withdrawal Reflex in Healthy Subjects

Objective. Transcranial direct current stimulation (tDCS) over the primary motor cortex (M1) has shown efficacy in a number of chronic pain conditions. Despite attempts to dissect the analgesic mechanisms, it is unknown whether M1 tDCS modulates the central integration of spinal nociception. To test this, we investigated the top-down modulation of spinal excitability using temporal summation (TS) of the nociceptive withdrawal reflex (NWR). Methods. In this randomized, blinded, cross-over study, eight healthy subjects received electrically evoked TS of the NWR, which was delivered at 5Hz at a threshold and a suprathreshold (1.1x threshold) to elicit TS resulting in different levels of pain. Subjects were asked to rate their pain after each stimulation. Changes in the TS of the NWR and pain ratings were investigated following 20 minutes of 2-mA anodal tDCS or sham stimulation applied over M1. Results. Baseline recordings showed that TS of the NWR was induced with both threshold and suprathreshold stimulation. Suprathreshold stimulation was also associated with a higher pain intensity rating. After brain stimulation, there was no effect over the lower-intensity TS of the NWR or pain ratings in both the tDCS and sham conditions. However, tDCS reduced TS of the NWR and associated pain ratings following higher-intensity suprathreshold stimulation. Conclusions. These results indicate that M1 tDCS can indirectly modulate the central integration of suprathreshold nociceptive processing in the spinal cord. It is possible that the analgesic efficacy of tDCS is dependent on plasticity induced within pain pathways following repeated, high-intensity stimulation, which may explain the beneficial effects seen in chronic pain patients.