Metabolic state and gustatory perception shapes dynamic interplay between cortical excitability and motor response

Abstract

The measurement of corticospinal excitability is a cornerstone for the field of transcranial magnetic stimulation (TMS) research. Though, how an individual’s metabolism and physiology modulate motor cortex (M1) excitability is amatter of some debate. In healthy individuals, consuming a high-sugar drink prior to TMS has been shown to increase M1 excitability [1] or have no effect [2]. The interaction between metabolism and corticospinal excitability can also depend on disease state. For example, patients with generalized epilepsy show greater inhibition of MEPs by paired-pulse TMS after consuming a meal [3]. Given the high-sugar content of Western diets and recent trends in ketogenic diets and intermittent fasting, we evaluated the effect of glucose consumption on corticospinal excitability in an experimenter-blinded crossover design study of glucose consumption on both the motor evoked potential (MEP) and the TMS evoked potential (TEP) measured by electroencephalography (EEG). We sought to clarify the role of glucose in modulating measurements of excitability with TMS in an effort to provide guidance for reducing variability and increasing the signal-to-noise in future TMS studies. We recruited 7 healthy participants with body mass indices less than 30 kg/m2 and a fasting blood glucose level (BGL) less than 95 mg/dL at initial screening. All participants provided informed consent in accordance with the Institutional Review Board at the University of North Carolina at Chapel Hill. Participants fasted for 12 hours and abstained from alcohol for 24 hours prior to two study visits where participants consumed either a 75 g glucose drink (298 mL; Azer Scientific Inc., Morgantown, PA, USA)dequivalent to two 12-ounce cans of a typical soft drinkdor a volumematched control drink (300 mL water). At each visit, we measured BGLs and combined TMS-EEG-EMG (electromyography) before and 0, 30, 60, and 120 minutes after the drink (Fig. 1A). Capillary BGLs were measured with a Hemocue Glucose 201 Analyzer (HemoCue America Inc., Brea, CA, USA) (Fig. 1B). EEG/EMG data were recorded using a 128-channel system at 1000 Hz sampling rate (Geodesic EEG system 410, Electrical Geodesics, Inc., OR, USA). Data from all