LTD-like plasticity of the human primary motor cortex can be reversed by γ-tACS

• Published in: Brain stimulation 2019-11, Vol.12 (6), p.1490-1499
• Main Authors: Guerra, Andrea, Suppa, Antonio, Asci, Francesco, De Marco, Giovanna, D'Onofrio, Valentina, Bologna, Matteo, Di Lazzaro, Vincenzo, Berardelli, Alfredo
• Format: Article
• Language: English
• Subjects: Adult; Beta; cTBS; Evoked Potentials, Motor - physiology; Female; Gamma; Humans; Inhibition, Psychological; Interneurons - physiology; Long-Term Synaptic Depression - physiology; Male; Motor Cortex - physiology; Neuronal Plasticity - physiology; Plasticity; tACS; TMS; Transcranial Direct Current Stimulation - methods; Transcranial Magnetic Stimulation - methods
• ISSN: 1935-861X; 1876-4754
• DOI: 10.1016/j.brs.2019.06.029

Cortical oscillatory activities play a role in regulating several brain functions in humans. However, whether motor resonant oscillations (i.e. β and γ) modulate long-term depression (LTD)-like plasticity of the primary motor cortex (M1) is still unclear. To address this issue, we combined transcranial alternating current stimulation (tACS), a technique able to entrain cortical oscillations, with continuous theta burst stimulation (cTBS), a transcranial magnetic stimulation (TMS) protocol commonly used to induce LTD-like plasticity in M1. Motor evoked potentials (MEPs) elicited by single-pulse TMS, short-interval intracortical inhibition (SICI) and intracortical facilitation (ICF) were evaluated before and 5, 15 and 30 min after cTBS alone or cTBS delivered during β-tACS (cTBS-β) or γ-tACS (cTBS-γ). Moreover, we tested the effects of β-tACS (alone) on short-latency afferent inhibition (SAI) and γ-tACS on SICI in order to verify whether tACS-related interneuronal modulation contributes to the effects of tACS-cTBS co-stimulation. cTBS-γ turned the expected after-effects of cTBS from inhibition to facilitation. By contrast, responses to cTBS-β were similar to those induced by cTBS alone. β- and γ-tACS did not change MEPs evoked by single-pulse TMS. β-tACS reduced SAI and γ-tACS reduced SICI. However, the degree of γ-tACS-induced modulation of SICI did not correlate with the effects of cTBS-γ. γ-tACS reverses cTBS-induced plasticity of the human M1. γ-oscillations may therefore regulate LTD-like plasticity mechanisms. •γ-tACS turned the expected after-effects of cTBS from inhibition to facilitation.•The after-effects of cTBS were not modified by a concurrent stimulation with β-tACS.•γ-tACS, given alone, reduced SICI and β-tACS reduced SAI.•The effects of γ-tACS on cTBS did not correlate with γ-tACS-induced changes of SICI.