RESUMO
INTRODUCTION: Neurostimulation/neurorecording are tools to study, diagnose, and treat neurologic/psychiatric conditions. Both techniques depend on volume conduction between scalp and excitable brain tissue. We examine how neurostimulation with TMS is affected by hydration status, a physiologic variable which can influence the volume of fluid spaces/cells, excitability and cellular/global brain functioning. Compared to dehydration, we expected rehydration to show signs of macroscopic and microscopic volume changes including decreased scalp-cortex distance (brain closer to stimulator) and astrocyte swelling-induced glutamate release. METHODS: Normal healthy adult participants (32, 9 male) had common motor TMS measures taken in a repeated measures design from dehydrated (12-hour overnight fast/thirst) and rehydrated (identical dehydration protocol followed by rehydration with 1 L water in 1 hour) 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. RESULTS: Motor hotspot shifted in half of participants. Motor threshold decreased in rehydration, indicating increased excitability. Even after re-dosing/re-localizing TMS to the new threshold/hotspot, rehydration still showed evidence of increased excitability: recruitment curve measures generally shifted upwards and SICF was increased. SICI, LICI, LICF, and CSP were relatively unaffected. The hydration perturbations were mild/subclinical, based on the magnitude/speed and urinalysis. DISCUSSION: Motor TMS measures showed evidence of expected physiologic changes of osmotic challenges. Hydration may be a source of variability affecting techniques dependant 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.