BACKGROUND Transcranial direct current stimulation (tDCS) is used to induce neuroplasticity in the human brain. Within certain limits of stimulation duration, anodal tDCS (a-tDCS) over the primary motor cortex induces long term potentiation- (LTP) like plasticity. A reversal of the direction of plasticity has however been described with prolonged a-tDCS protocols. OBJECTIVE We aimed to systematically investigate the intervention duration threshold for reversal of a-tDCS-induced effects on corticospinal excitability (CSE) and to determine the probable mechanisms involved in these changes. METHODS Fifteen healthy participants received a-tDCS of 1 mA for five different durations in pseudo-random session order. Transcranial magnetic stimulation (TMS) was delivered over the left M1, and motor evoked potentials (MEPs) of a contralateral hand muscle were recorded before, immediately and 30 min following intervention to measure CSE changes. Short-interval intracortical inhibition (SICI), intracortical facilitation (ICF), and long interval facilitation (LIF) were assessed via paired-pulse TMS protocols. RESULTS A-tDCS significantly increased CSE as expected at stimulation durations of 22 and 24 min. However, this effect of a-tDCS on CSE decreased and even reversed when stimulation duration increased to 26, 28, and 30 min. Respective alterations of ICF, LIF, and SICI indicate the involvement of glutamatergic, and GABAergic systems in these effects. CONCLUSIONS These results confirm a duration threshold for reversal of the excitability-enhancing effect of a-tDCS with stimulation durations ≥ 26 min. Counter-regulatory mechanisms are discussed as a mechanistic foundation for these effects, which might prevent excessive brain activation.

中文翻译：

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皮质脊髓兴奋性逆转的阳极 tDCS 持续时间阈值的测定：反调节机制诱导的研究

背景技术经颅直流电刺激(tDCS)用于诱导人脑中的神经可塑性。在刺激持续时间的某些限制内，初级运动皮层上的阳极 tDCS (a-tDCS) 会诱导长时程增强 (LTP) 样可塑性。然而，使用延长的 a-tDCS 协议描述了可塑性方向的逆转。目的 我们旨在系统地研究逆转 a-tDCS 诱导的皮质脊髓兴奋性 (CSE) 影响的干预持续时间阈值，并确定这些变化所涉及的可能机制。方法 15 名健康参与者以伪随机会话顺序在五个不同的持续时间内接受 1 mA 的 a-tDCS。经颅磁刺激（TMS）通过左侧 M1 传递，在干预前、即刻和干预后 30 分钟记录对侧手部肌肉的运动诱发电位 (MEP) 和运动诱发电位 (MEP)，以测量 CSE 变化。短间隔皮质内抑制 (SICI)、皮质内促进 (ICF) 和长间隔促进 (LIF) 通过成对脉冲 TMS 协议进行评估。结果 A-tDCS 在刺激持续时间为 22 和 24 分钟时如预期的那样显着增加了 CSE。然而，当刺激持续时间增加到 26、28 和 30 分钟时，a-tDCS 对 CSE 的这种影响会降低甚至逆转。ICF、LIF 和 SICI 的相应改变表明谷氨酸能和 GABA 能系统参与了这些影响。结论这些结果证实了在刺激持续时间≥ 26 分钟时逆转 a-tDCS 的兴奋性增强作用的持续时间阈值。