Basic Biomedical Sciences & Center for Brain and Behavior Research, University of South Dakota Sanford School of Medicine, Vermillion, South Dakota, United States.
Department of Psychiatry, University of South Dakota Sanford School of Medicine, Sioux Falls, South Dakota, United States.
J Neurophysiol. 2024 Sep 1;132(3):791-807. doi: 10.1152/jn.00143.2023. Epub 2024 Jul 31.
Neurostimulation/neurorecording are tools to study, diagnose, and treat neurological/psychiatric conditions. Both techniques depend on volume conduction between scalp and excitable brain tissue. Here, we examine how neurostimulation with transcranial magnetic stimulation (TMS) is affected by hydration status, a physiological variable that can influence the volume of fluid spaces/cells, excitability, and cellular/global brain functioning. Normal healthy adult participants (32, 9 males) had common motor TMS measures taken in a repeated-measures design from dehydrated (12-h overnight fast/thirst) and rehydrated (identical dehydration protocol followed by rehydration with 1 L water in 1 h) testing days. The target region was left primary motor cortex hand area. Response at the target muscle was recorded with electromyography. Urinalysis confirmed hydration status. Motor hotspot shifted in half of participants. Motor threshold decreased in rehydration, indicating increased excitability. Even after redosing/relocalizing TMS to the new threshold/hotspot, rehydration still showed evidence of increased excitability: recruitment curve measures generally shifted upward and the glutamate-dependent paired-pulse protocol, short intracortical facilitation (SICF), was increased. Short intracortical inhibition (SICI), long intracortical inhibition (LICI), long intracortical facilitation (LICF), and cortical silent period (CSP) were relatively unaffected. The hydration perturbations were mild/subclinical based on the magnitude/speed and urinalysis. Motor TMS measures showed evidence of expected physiological changes of osmotic challenges. Rehydration showed signs of macroscopic and microscopic volume changes including decreased scalp-cortex distance (brain closer to stimulator) and astrocyte swelling-induced glutamate release. Hydration may be a source of variability affecting any techniques dependent on brain volumes/volume conduction. These concepts are important for researchers/clinicians using such techniques or dealing with the wide variety of disease processes involving water balance. Hydration status can affect brain volumes and excitability, which should affect techniques dependent on electrical volume conduction, including neurostimulation/recording. We test the previously unknown effects of hydration on neurostimulation with TMS and briefly review relevant physiology of hydration. Rehydration showed lower motor threshold, shifted motor hotspot, and generally larger responses even after compensating for threshold/hotspot changes. This is important for clinical and research applications of neurostimulation/neurorecording and the many clinical disorders related to water balance.
神经刺激/神经记录是用于研究、诊断和治疗神经/精神疾病的工具。这两种技术都依赖于头皮和可兴奋脑组织之间的容积传导。在这里，我们研究了经颅磁刺激 (TMS) 的神经刺激如何受到水合状态的影响，水合状态是一种可以影响液腔/细胞体积、兴奋性和细胞/整体大脑功能的生理变量。正常健康的成年参与者（32 名，男性 9 名）在重复测量设计中从脱水（12 小时禁食/口渴的夜间禁食）和再水合（相同的脱水方案后 1 小时内用 1 升水再水合）测试日接受常见的运动 TMS 测量。目标区域为左初级运动皮层手部区域。用肌电图记录目标肌肉的反应。尿分析证实了水合状态。一半的参与者的运动热点发生了转移。再水合时运动阈值降低，表明兴奋性增加。即使重新给药/重新定位 TMS 至新的阈值/热点后，再水合仍显示兴奋性增加的证据：募集曲线测量值通常向上移动，谷氨酸依赖性成对脉冲方案，短程皮质内易化（SICF）增加。短程皮质内抑制（SICI）、长程皮质内抑制（LICI）、长程皮质内易化（LICF）和皮质静息期（CSP）相对不受影响。根据幅度/速度和尿分析，水合扰动是轻微/亚临床的。运动 TMS 测量结果表明存在与渗透挑战相关的预期生理变化的证据。再水合显示出宏观和微观体积变化的迹象，包括头皮-皮层距离缩短（大脑更接近刺激器）和星形胶质细胞肿胀诱导的谷氨酸释放。水合可能是影响任何依赖于脑体积/容积传导的技术的可变性来源。这些概念对于使用此类技术的研究人员/临床医生或处理涉及水平衡的各种疾病过程非常重要。水合状态会影响脑体积和兴奋性，这会影响依赖于电容积传导的技术，包括神经刺激/记录。我们测试了以前未知的水合作用对 TMS 神经刺激的影响，并简要回顾了水合作用的相关生理学。再水合时运动阈值降低，运动热点转移，即使在补偿阈值/热点变化后，反应也普遍更大。这对于神经刺激/神经记录的临床和研究应用以及许多与水平衡相关的临床疾病非常重要。
