Short-term high-frequency transcutaneous electrical nerve stimulation decreases human motor cortex excitability Tatsuya Mima a, * , Tatsuhide Oga a , John Rothwell a,b , Takeshi Satow a,c , Jun-ichi Yamamoto a,c , Keiichiro Toma d , Hidenao Fukuyama a , Hiroshi Shibasaki a,e , Takashi Nagamine a a Human Brain Research Center, Kyoto University Graduate School of Medicine, Human and Brain Research Center, Kyoto 606-8507, Japan b Sobell Department of Neurophysiology, Queen Square, London WC1N 3BG, UK c Department of Neurosurgery, Kyoto University Graduate School of Medicine, Human and Brain Research Center, Kyoto 606-8507, Japan d Institute of Biomedical Research and Innovation, Kobe 650-0047, Japan e NINDS, National Institutes of Health, Bethesda, MD 20892, USA Received 12 September 2003; received in revised form 14 October 2003; accepted 20 October 2003 Abstract Several pervious studies have shown that periods of changed sensory input can have after effects on the excitability of the corticospinal system. Here we test whether the parameters of peripheral stimulation conventionally used to treat pain with transcutaneous electrical nerve stimulation (TENS: 90 Hz) also have modulatory effects on the motor system. We measured the amplitude of motor evoked potentials (MEPs) elicited by the focal transcranial magnetic stimulation in the right abductor pollicis brevis and first dorsal interosseous muscles before and after 30 min TENS over the right thenar eminence. In addition, we evaluated tactile and 2-point discrimination thresholds at the same site. TENS transiently reduced MEPs and increased sensory thresholds. This suggests that short-term TENS might have an inhibitory effect on both the sensory and motor systems. q 2003 Elsevier Ireland Ltd. All rights reserved. Keywords: Transcutaenous electrical nerve stimulation; Transcranial magnetic stimulation; Sensorimotor cortex; Plasticity; Sensory threshold; Motor excitability The functional organization of human sensorimotor cortex has the capability to change even in adults [2,5]. One of the primary factors in producing such effects is a sustained change in sensory input, which can affect not only the sensory but also the motor representations in the cortex. For example, in human work, prolonged electrical stimulation of peripheral nerve has been shown to increase the excitability of the corticospinal pathway, at least as measured by the responses to transcranial magnetic stimulation (TMS) [10,13,17]. Indeed, several authors have attempted to use this approach to increase excitability in paretic muscles after stroke [4,7,18]. Other studies have focused on long-term changes in sensory function: TENS has been used for many years as a method for reducing pain perception in patients with chronic pain [11]. The exact mechanism of action is unknown and may involve changes at many levels in the sensory pathways, but effects on sensory cortex may be involved since somatosensory evoked potentials are reduced after TENS [6,8,12]. TENS for pain control usually involves high frequency stimulation of peripheral nerve, whilst the previous reports of excitatory effects of sensory input on the motor system have generally used lower frequencies (, 10 Hz). Given the suggestion of Fraser et al that the motor effects are likely to depend critically on the frequency of sensory stimulation, we tested whether TENS at conventional high frequency had effects on the motor system that were different to those reported previously with lower frequency stimulation [10,13,17]. Eight right-handed normal male volunteers (mean age 31 years old, from 26 to 36) without any neurological or psychiatric disorders participated in the study. The protocol was approved by the Ethics Committee of the Kyoto University Graduate School of Medicine. Subjects gave their written informed consent before the experiment. For continuous somatosensory stimulation, TENS was 0304-3940/03/$ - see front matter q 2003 Elsevier Ireland Ltd. All rights reserved. doi:10.1016/j.neulet.2003.10.045 Neuroscience Letters 355 (2004) 85–88 www.elsevier.com/locate/neulet * Corresponding author. Tel.: þ 81-75-751-3695; fax: þ81-75-751-3202. E-mail address: mima@kuhp.kyoto-u.ac.jp (T. Mima).