Plasticity-Induced Effects of Theta Burst Transcranial Ultrasound Stimulation in Parkinson's Disease.

BACKGROUND: Low-intensity transcranial ultrasound stimulation (TUS) is a noninvasive brain stimulation (NIBS) technique with high spatial specificity. Previous studies showed that TUS delivered in a theta burst pattern (tbTUS) increased motor cortex (MI) excitability up to 30 minutes due to long-term potentiation (LTP)-like plasticity. Studies using other forms of NIBS suggested that cortical plasticity may be impaired in patients with Parkinson's disease (PD). OBJECTIVE: The aim was to investigate the neurophysiological effects of tbTUS in PD patients off and on dopaminergic medications compared to healthy controls. METHODS: We studied 20 moderately affected PD patients in on and off dopaminergic medication states (7 with and 13 without dyskinesia) and 17 age-matched healthy controls in a case-controlled study. tbTUS was applied for 80 seconds to the MI. Motor-evoked potentials (MEP), short-interval intracortical inhibition (SICI), and short-interval intracortical facilitation (SICF) were recorded at baseline, and at 5 minutes (T5), T30, and T60 after tbTUS. Motor Unified Parkinson's Disease Rating Scale (mUPDRS) was measured at baseline and T60. RESULTS: tbTUS significantly increased MEP amplitude at T30 compared to baseline in controls and in PD patients on but not in PD patients off medications. SICI was reduced in PD off medications compared to controls. tbTUS did not change in SICI or SICF. The bradykinesia subscore of mUPDRS was reduced at T60 compared to baseline in PD on but not in the off medication state. The presence of dyskinesia did not affect tbTUS-induced plasticity. CONCLUSIONS: tbTUS-induced LTP plasticity is impaired in PD patients off medications and is restored by dopaminergic medications. © 2024 The Author(s). Movement Disorders published by Wiley Periodicals LLC on behalf of International Parkinson and Movement Disorder Society.

Effects of different sonication parameters of theta burst transcranial ultrasound stimulation on human motor cortex.

BACKGROUND: Theta burst TUS (tbTUS) can induce increased cortical excitability in human, but how different sonication parameters influence the effects are still unknown. OBJECTIVE: To examine how a range of sonication parameters, including acoustic intensity, pulse repetition frequency, duty cycle and sonication duration, influence the effects of tbTUS on human motor cortical excitability. METHODS: 14 right-handed healthy subjects underwent 8 sessions with different tbTUS parameters in a randomized, cross-over design on separate days. The original tbTUS protocol was studied in one session and one parameter was changed in each of the seven sessions. To examine changes in cortical excitability induced by tbTUS, we measured the motor-evoked potential (MEP) amplitude, resting motor threshold, short-interval intracortical inhibition and intracortical facilitation, as well as short-interval intracortical facilitation before and up to 90 min after tbTUS. RESULTS: All conditions increased MEP amplitudes except the condition with low acoustic intensity of 10 W/cm2. Pulse repetition frequency of 5 Hz produced higher MEP amplitudes compared to pulse repetition frequencies of 2 and 10 Hz. In addition, higher duty cycles (5%, 10%, and 15%) and longer sonication durations (40, 80, and 120 s) were associated with longer duration of increased MEP amplitudes. Resting motor threshold remained stable in all conditions. For paired-pulse TMS measures, tbTUS reduced short-interval intracortical inhibition and enhanced short-interval intracortical facilitation, but had no effect on intracortical facilitation. CONCLUSIONS: Ultrasound bursts repeated at theta (∼5 Hz) frequency is optimal to produce increased cortical excitability with the range of 2-10 Hz. Furthermore, there was a dose-response effect regarding duty cycle and sonication duration in tbTUS for plasticity induction. The aftereffects of tbTUS were associated with a shift of the inhibition/excitation balance toward less inhibition and more excitation in the motor cortex. These findings can be used to determine the optimal tbTUS parameters in neuroscience research and treatment of neurological and psychiatric disorders.

Neuromodulation techniques - From non-invasive brain stimulation to deep brain stimulation.

Over the past 30 years, the field of neuromodulation has witnessed remarkable advancements. These developments encompass a spectrum of techniques, both non-invasive and invasive, that possess the ability to both probe and influence the central nervous system. In many cases neuromodulation therapies have been adopted into standard care treatments. Transcranial magnetic stimulation (TMS), transcranial direct current stimulation (tDCS), and transcranial ultrasound stimulation (TUS) are the most common non-invasive methods in use today. Deep brain stimulation (DBS), spinal cord stimulation (SCS), and vagus nerve stimulation (VNS), are leading surgical methods for neuromodulation. Ongoing active clinical trials using are uncovering novel applications and paradigms for these interventions.

Trends and disparities in deep brain stimulation utilization in the United States: a Nationwide Inpatient Sample analysis from 1993 to 2017.

Background: Deep brain stimulation (DBS) is an approved treatment option for Parkinson's Disease (PD), essential tremor (ET), dystonia, obsessive-compulsive disorder and epilepsy in the United States. There are disparities in access to DBS, and clear understanding of the contextual factors driving them is important. Previous studies aimed at understanding these factors have been limited by single indications or small cohort sizes. The aim of this study is to provide an updated and comprehensive analysis of DBS utilization for multiple indications to better understand the factors driving disparities in access. Methods: The United States based National Inpatient Sample (NIS) database was utilized to analyze the surgical volume and trends of procedures based on indication, using relevant ICD codes. Predictors of DBS use were analyzed using a logistic regression model. DBS-implanted patients in each indication were compared based on the patient-, hospital-, and outcome-related variables. Findings: Our analysis of 104,356 DBS discharges from 1993 to 2017 revealed that the most frequent indications for DBS were PD (67%), ET (24%), and dystonia (4%). Although the number of DBS procedures has consistently increased over the years, radiofrequency ablation utilization has significantly decreased to only a few patients per year since 2003. Negative predictors for DBS utilization in PD and ET cohorts included age increase and female sex, while African American status was a negative predictor across all cohorts. Significant differences in patient-, hospital-, and outcome-related variables between DBS indications were also determined. Interpretation: Demographic and socioeconomic-based disparities in DBS use are evident. Although racial disparities are present across all indications, other disparities such as age, sex, wealth, and insurance status are only relevant in certain indications. Funding: This work was supported by Alan & Susan Hudson Cornerstone Chair in Neurosurgery at University Health Network.

Accelerated Transcranial Ultrasound Neuromodulation in Parkinson's Disease: A Pilot Study.

OBJECTIVE: Low-intensity transcranial focused ultrasound (TUS) is a novel method for neuromodulation. We aimed to study the feasibility of stimulating the bilateral primary motor cortices (M1) with accelerated theta-burst TUS (a-tbTUS) on neurophysiologic and clinical outcomes in Parkinson's disease (PD). METHODS: Patients were randomly assigned to receive active or sham a-tbTUS for the first visit and the alternate condition on the second visit, at least 10 days apart. a-tbTUS was administered in three consecutive sonications at 30-minute intervals. We used an accelerated protocol to produce an additive effect of stimulation. Patients were studied in the OFF-medication state. Transcranial magnetic stimulation (TMS)-elicited motor-evoked potentials (MEPs) were used to assess motor cortical excitability before and after TUS. Clinical outcomes after a-tbTUS administration were assessed using the Movement Disorder Society Unified Parkinson's Disease Rating Scale (MDS-UPDRS)-III. RESULTS: A total of 20 visits were conducted in 10 PD patients. Compared to the baseline, TMS-elicited MEP amplitudes significantly increased following active but not sham sonication (P = 0.0057). MEP amplitudes were also higher following a-tbTUS than sham sonication (P = 0.0064). There were no statistically significant changes in MDS-UPDRS-III scores with active or sham a-tbTUS. CONCLUSIONS: a-tbTUS increases motor cortex excitability and is a feasible non-invasive neuromodulation strategy in PD. Future studies should determine optimal dosing parameters and the durability of neurophysiologic and clinical outcomes in PD patients. © 2023 The Authors. Movement Disorders published by Wiley Periodicals LLC on behalf of International Parkinson and Movement Disorder Society.

Spinal Cord Stimulation for Parkinson's Disease: A Systematic Review and Meta-Analysis of Pain and Motor Outcomes.

BACKGROUND: Spinal cord stimulation (SCS) has been investigated as a potential therapeutic option for managing refractory symptoms in patients with Parkinson's disease (PD). OBJECTIVE: This systematic review and meta-analysis aimed to evaluate the safety and efficacy of SCS in PD. METHOD: A comprehensive literature search was conducted on PubMed and Web of Science to identify SCS studies reporting Unified Parkinson Disease Rating Scale-III (UPDRS-III) or Visual Analogue Scale (VAS) score changes in PD cohorts with at least 3 patients and a follow-up period of at least 1 month. Treatment effect was measured as the mean change in outcome scores and analyzed using an inverse variance random-effects model. The risk of bias was assessed using the Newcastle-Ottawa Scale and funnel plots. RESULTS: A total of 11 studies comprising 76 patients were included. Nine studies involving 72 patients reported an estimated decrease of 4.43 points (95% confidence interval [CI]: 2.11; 6.75, p < 0.01) in UPDRS-III score, equivalent to a 14% reduction. The axial subscores in 48 patients decreased by 2.35 points (95% CI: 1.26; 3.45, p < 0.01, 20% reduction). The pooled effect size of five studies on back and leg pain VAS scores was calculated as 4.38 (95% CI: 2.67; 6.09, p < 0.001), equivalent to a 59% reduction. CONCLUSIONS: Our analysis suggests that SCS may provide significant motor and pain benefits for patients with PD, although the results should be interpreted with caution due to several potential limitations including study heterogeneity, open-label designs, small sample sizes, and the possibility of publication bias. Further research using larger sample sizes and placebo-/sham-controlled designs is needed to confirm effectiveness.

Mechanisms of theta burst transcranial ultrasound induced plasticity in the human motor cortex.

BACKGROUND: Transcranial ultrasound stimulation (TUS) is a novel non-invasive brain stimulation technique with high depth penetrance and spatial resolution. Theta-burst TUS (tbTUS) is a plasticity-inducing protocol which increases motor cortical excitability for up to 30 min following 80s of sonication. While this protocol may have therapeutic potential for the treatment of psychiatric and neurological disorders, the mechanisms of action of TUS remain unclear. OBJECTIVE: We conducted the first pharmacological study to examine the mechanisms of TUS in human primary motor cortex. By administering brain-active drugs with known mechanisms of action, we aimed to elucidate the mechanisms of tbTUS. METHODS: Fourteen healthy subjects participated in a within-subjects randomized, double-blind, cross-over study with five visits. At each visit, one of four study drugs (carbamazepine - Na+ channel blocker, nimodipine - L-type Ca2+ channel blocker, lorazepam - positive allosteric modulator of gamma-aminobutyric acid (GABA) type A receptor, dextromethorphan - N-methyl-d-aspartate receptor antagonist) or placebo was administered in random order, followed by tbTUS. RESULTS: The plasticity effects of tbTUS on motor cortex excitability measured by motor-evoked potential amplitudes elicited by transcranial magnetic stimulation were reduced by all study drugs compared to placebo. CONCLUSION: tbTUS may induce NMDA-dependent synaptic plasticity since the effects are blocked by increased GABAA receptor activities and voltage-gated Na+ and Ca2+ channels blockers. These results are consistent with the hypotheses that tbTUS induced long-term potentiation-like mechanisms and that TUS involves activation of mechanosensitive Na+ and Ca2+ channels. Alternatively, non-specific pharmacologically induced changes in excitatory/inhibitory balance might have interfered with the effects of tbTUS.

Three-layer model with absorption for conservative estimation of the maximum acoustic transmission coefficient through the human skull for transcranial ultrasound stimulation.

Transcranial ultrasound stimulation (TUS) has been shown to be a safe and effective technique for non-invasive superficial and deep brain stimulation. Safe and efficient translation to humans requires estimating the acoustic attenuation of the human skull. Nevertheless, there are no international guidelines for estimating the impact of the skull bone. A tissue independent, arbitrary derating was developed by the U.S. Food and Drug Administration to take into account tissue absorption (0.3 dB/cm-MHz) for diagnostic ultrasound. However, for the case of transcranial ultrasound imaging, the FDA model does not take into account the insertion loss induced by the skull bone, nor the absorption by brain tissue. Therefore, the estimated absorption is overly conservative which could potentially limit TUS applications if the same guidelines were to be adopted. Here we propose a three-layer model including bone absorption to calculate the maximum pressure transmission through the human skull for frequencies ranging between 100 kHz and 1.5 MHz. The calculated pressure transmission decreases with the frequency and the thickness of the bone, with peaks for each thickness corresponding to a multiple of half the wavelength. The 95th percentile maximum transmission was calculated over the accessible surface of 20 human skulls for 12 typical diameters of the ultrasound beam on the skull surface, and varies between 40% and 78%. To facilitate the safe adjustment of the acoustic pressure for short ultrasound pulses, such as transcranial imaging or transcranial ultrasound stimulation, a table summarizes the maximum pressure transmission for each ultrasound beam diameter and each frequency.

Long-Term Recording of Subthalamic Aperiodic Activities and Beta Bursts in Parkinson's Disease.

BACKGROUND: Local field potentials (LFPs) represent the summation of periodic (oscillations) and aperiodic (fractal) signals. Although previous studies showed changes in beta band oscillations and burst characteristics of the subthalamic nucleus (STN) in Parkinson's disease (PD), how aperiodic activity in the STN is related to PD pathophysiology is unknown. OBJECTIVES: The study aimed to characterize the long-term effects of STN-deep brain stimulation (DBS) and dopaminergic medications on aperiodic activities and beta bursts. METHODS: A total of 10 patients with PD participated in this longitudinal study. Simultaneous bilateral STN-LFP recordings were conducted in six separate visits during a period of 18 months using the Activa PC + S device in the off and on dopaminergic medication states. We used irregular-resampling auto-spectral analysis to separate oscillations and aperiodic components (exponent and offset) in the power spectrum of STN-LFP signals in beta band. RESULTS: Our results revealed a systematic increase in both the exponent and the offset of the aperiodic spectrum over 18 months following the DBS implantation, independent of the dopaminergic medication state of patients with PD. In contrast, beta burst durations and amplitudes were stable over time and were suppressed by dopaminergic medications. CONCLUSIONS: These findings indicate that oscillations and aperiodic activities reflect at least partially distinct yet complementary neural mechanisms, which should be considered in the design of robust biomarkers to optimize adaptive DBS. Given the link between increased gamma-aminobutyric acidergic (GABAergic) transmission and higher aperiodic activity, our findings suggest that long-term STN-DBS may relate to increased inhibition in the basal ganglia. © 2022 International Parkinson and Movement Disorder Society.

Multi-modal investigation of transcranial ultrasound-induced neuroplasticity of the human motor cortex.

INTRODUCTION: There is currently a gap in accessibility to neuromodulation tools that can approximate the efficacy and spatial resolution of invasive methods. Low intensity transcranial focused ultrasound stimulation (TUS) is an emerging technology for non-invasive brain stimulation (NIBS) that can penetrate cortical and deep brain structures with more focal stimulation compared to existing NIBS modalities. Theta burst TUS (tbTUS, TUS delivered in a theta burst pattern) is a novel repetitive TUS protocol that can induce durable changes in motor cortex excitability, thereby holding promise as a novel neuromodulation tool with durable effects. OBJECTIVE: The aim of the present study was to elucidate the neurophysiologic effects of tbTUS motor cortical excitability, as well on local and global neural oscillations and network connectivity. METHODS: An 80-s train of active or sham tbTUS was delivered to the left motor cortex in 15 healthy subjects. Motor cortical excitability was investigated through transcranial magnetic stimulation (TMS)-elicited motor-evoked potentials (MEPs), short-interval intracortical inhibition (SICI), and intracortical facilitation (ICF) using paired-pulse TMS. Magnetoencephalography (MEG) recordings during resting state and an index finger abduction-adduction task were used to assess oscillatory brain responses and network connectivity. The correlations between the changes in neural oscillations and motor cortical excitability were also evaluated. RESULTS: tbTUS to the motor cortex results in a sustained increase in MEP amplitude and decreased SICI, but no change in ICF. MEG spectral power analysis revealed TUS-mediated desynchronization in alpha and beta spectral power. Significant changes in alpha power were detected within the supplementary motor cortex (Right > Left) and changes in beta power within bilateral supplementary motor cortices, right basal ganglia and parietal regions. Coherence analysis revealed increased local connectivity in motor areas. MEP and SICI changes correlated with both local and inter-regional coherence. CONCLUSION: The findings from this study provide novel insights into the neurophysiologic basis of TUS-mediated neuroplasticity and point to the involvement of regions within the motor network in mediating this sustained response. Future studies may further characterize the durability of TUS-mediated neuroplasticity and its clinical applications as a neuromodulation strategy for neurological and psychiatric disorders.

Human Studies of Transcranial Ultrasound neuromodulation: A systematic review of effectiveness and safety.

BACKGROUND: Transcranial ultrasound stimulation (TUS) is gaining traction as a safe and non-invasive technique in human studies. There has been a rapid increase in TUS human studies in recent years, with more than half of studies to date published after 2020. This rapid growth in the relevant body of literature necessitates comprehensive reviews to update clinicians and researchers. OBJECTIVE: The aim of this work is to review human studies with an emphasis on TUS devices, sonication parameters, outcome measures, results, and adverse effects, as well as highlight future directions of investigation. METHODS: A systematic review was conducted by searching the Web of Science and PubMed databases on January 12, 2022. Human studies of TUS were included. RESULTS: A total of 35 studies were identified using focused/unfocused ultrasound devices. A total of 677 subjects belonging to diverse cohorts (i.e., healthy, chronic pain, dementia, epilepsy, traumatic brain injury, depression) were enrolled. The stimulation effects vary in a sonication parameter-dependant fashion. Clinical, neurophysiological, radiological and histological outcome measures were assessed. No severe adverse effects were reported in any of the studies surveyed. Mild symptoms were observed in 3.4% (14/425) of the subjects, including headache, mood deterioration, scalp heating, cognitive problems, neck pain, muscle twitches, anxiety, sleepiness and pruritis. CONCLUSIONS: Although increasingly being used, TUS is still in its early phases. TUS can change short-term brain excitability and connectivity, induce long-term plasticity, and modulate behavior. New techniques should be used to further elucidate its underlying mechanisms and identify its application in novel populations.

Induction of Human Motor Cortex Plasticity by Theta Burst Transcranial Ultrasound Stimulation.

OBJECTIVE: Transcranial ultrasound stimulation (TUS) is a promising noninvasive brain stimulation technique with advantages of high spatial precision and ability to target deep brain regions. This study aimed to develop a TUS protocol to effectively induce brain plasticity in human subjects. METHODS: An 80-second train of theta burst patterned TUS (tbTUS), regularly patterned TUS (rTUS) with the same sonication duration, and sham tbTUS was delivered to the motor cortex in healthy subjects. Transcranial magnetic stimulation (TMS) was used to examine changes in corticospinal excitability, intracortical inhibition and facilitation, and the site of plasticity induction. The effects of motor cortical tbTUS on a visuomotor task and the effects of occipital cortex tbTUS on motor cortical excitability were also tested. RESULTS: The tbTUS produced consistent increase in corticospinal excitability for at least 30 minutes, whereas rTUS and sham tbTUS produced no significant change. tbTUS decreased short-interval intracortical inhibition and increased intracortical facilitation. The effects of TMS in different current directions suggested that the site of the plastic changes was within the motor cortex. tbTUS to the occipital cortex did not change motor cortical excitability. Motor cortical tbTUS shortened movement time in a visuomotor task. INTERPRETATION: tbTUS is a novel and efficient paradigm to induce cortical plasticity in humans. It has the potential to be developed for neuromodulation treatment for neurological and psychiatric disorders, and to advance neuroscience research. ANN NEUROL 2022;91:238-252.

The effects of NMDA receptor blockade on TMS-evoked EEG potentials from prefrontal and parietal cortex.

Measuring the brain's response to transcranial magnetic stimulation (TMS) with electroencephalography (EEG) offers unique insights into the cortical circuits activated following stimulation, particularly in non-motor regions where less is known about TMS physiology. However, the mechanisms underlying TMS-evoked EEG potentials (TEPs) remain largely unknown. We assessed TEP sensitivity to changes in excitatory neurotransmission mediated by n-methyl-d-aspartate (NMDA) receptors following stimulation of non-motor regions. In fourteen male volunteers, resting EEG and TEPs from prefrontal (PFC) and parietal (PAR) cortex were measured before and after administration of either dextromethorphan (NMDA receptor antagonist) or placebo across two sessions in a double-blinded pseudo-randomised crossover design. At baseline, there were amplitude differences between PFC and PAR TEPs across a wide time range (15-250 ms), however the signals were correlated after ~80 ms, suggesting early peaks reflect site-specific activity, whereas late peaks reflect activity patterns less dependent on the stimulated sites. Early TEP peaks were not reliably altered following dextromethorphan compared to placebo, although findings were less clear for later peaks, and low frequency resting oscillations were reduced in power. Our findings suggest that early TEP peaks (<80 ms) from PFC and PAR reflect stimulation site specific activity that is largely insensitive to changes in NMDA receptor-mediated neurotransmission.

Effects of antiepileptic drugs on cortical excitability in humans: A TMS-EMG and TMS-EEG study.

Brain responses to transcranial magnetic stimulation (TMS) recorded by electroencephalography (EEG) are emergent noninvasive markers of neuronal excitability and effective connectivity in humans. However, the underlying physiology of these TMS-evoked EEG potentials (TEPs) is still heavily underexplored, impeding a broad application of TEPs to study pathology in neuropsychiatric disorders. Here we tested the effects of a single oral dose of three antiepileptic drugs with specific modes of action (carbamazepine, a voltage-gated sodium channel (VGSC) blocker; brivaracetam, a ligand to the presynaptic vesicle protein VSA2; tiagabine, a gamma-aminobutyric acid (GABA) reuptake inhibitor) on TEP amplitudes in 15 healthy adults in a double-blinded randomized placebo-controlled crossover design. We found that carbamazepine decreased the P25 and P180 TEP components, and brivaracetam the N100 amplitude in the nonstimulated hemisphere, while tiagabine had no effect. Findings corroborate the view that the P25 represents axonal excitability of the corticospinal system, the N100 in the nonstimulated hemisphere propagated activity suppressed by inhibition of presynaptic neurotransmitter release, and the P180 late activity particularly sensitive to VGSC blockade. Pharmaco-physiological characterization of TEPs will facilitate utilization of TMS-EEG in neuropsychiatric disorders with altered excitability and/or network connectivity.

The effects of a single dose of fluoxetine on practice-dependent plasticity.

OBJECTIVE: To determine whether a single dose of fluoxetine increases corticomotoneuronal excitability, motor performance and practice-dependent plasticity. METHODS: Twelve healthy adults completed this placebo-controlled, pseudo-randomized, double-blind crossover study. Transcranial magnetic stimulation (TMS) was used to assess corticomotoneuronal excitability, and two uni-axial accelerometers measured kinetics of fastest possible ballistic voluntary thumb movements and TMS-evoked thumb movements. Six hours after administration of either 20 mg of the serotonin reuptake inhibitor fluoxetine or placebo, participants practiced ballistic thumb movements in the direction opposite to the TMS-evoked thumb movements. The primary outcome of this study was the proportion of thumb movements that fell within the target-training zone (TTZ) during and for 30 min after the practice. RESULTS: All participants demonstrated practice-dependent plasticity evidenced by an increase of TMS-evoked thumb movements falling into the TTZ (P = 0.045), with no difference between drugs. There was a significant increase in peak acceleration of the practiced voluntary thumb movements (P = 0.002), but no DRUG by TIME interaction. Motor-evoked potential amplitudes were not changed by drug intake or practice. CONCLUSIONS: A single dose of 20 mg of fluoxetine did not enhance corticomotoneuronal excitability, performance of a ballistic thumb movement task, or practice-dependent plasticity in healthy adults. SIGNIFICANCE: Longer administration fluoxetine may be necessary to enhance motor performance and plasticity.

Effects of the Selective α5-GABAAR Antagonist S44819 on Excitability in the Human Brain: A TMS-EMG and TMS-EEG Phase I Study.

Alpha-5 gamma-aminobutyric acid type A receptors (α5-GABAARs) are located extrasynaptically, regulate neuronal excitability through tonic inhibition, and are fundamentally important for processes such as plasticity and learning. For example, pharmacological blockade of α5-GABAAR in mice with ischemic stroke improved recovery of function by normalizing exaggerated perilesional α5-GABAAR-dependent tonic inhibition. S44819 is a novel competitive selective antagonist of the α5-GABAAR at the GABA-binding site. Pharmacological modulation of α5-GABAAR-mediated tonic inhibition has never been investigated in the human brain. Here, we used transcranial magnetic stimulation (TMS) to test the effects of a single oral dose of 50 and 100 mg of S44819 on electromyographic (EMG) and electroencephalographic (EEG) measures of cortical excitability in 18 healthy young adults in a randomized, double-blinded, placebo-controlled, crossover phase I study. A dose of 100 mg, but not 50 mg, of S44819 decreased active motor threshold, the intensity needed to produce a motor evoked potential of 0.5 mV, and the amplitude of the N45, a GABAAergic component of the TMS-evoked EEG response. The peak serum concentration of 100 mg S44819 correlated directly with the decrease in N45 amplitude. Short-interval intracortical inhibition, a TMS-EMG measure of synaptic GABAAergic inhibition, and other components of the TMS-evoked EEG response remained unaffected. These findings provide first time evidence that the specific α5-GABAAR antagonist S44819 reached human cortex to impose an increase in cortical excitability. These data warrant further development of S44819 in a human clinical trial to test its efficacy in enhancing recovery of function after ischemic stroke. SIGNIFICANCE STATEMENT: The extrasynaptic α-5 gamma-aminobutyric acid type A receptor (α5-GABAAR) regulates neuronal excitability through tonic inhibition in the mammalian brain. Tonic inhibition is important for many fundamental processes such as plasticity and learning. Pharmacological modulation of α5-GABAAR-mediated tonic inhibition has never been investigated in the human brain. This study demonstrates that S44819, a selective α5-GABAAR antagonist, increases cortical excitability in healthy human subjects, as indicated by specific markers of transcranial magnetic stimulation-induced muscle and brain responses measured by electromyography and electroencephalography. Our findings imply that tonic inhibition in human cortex can be modified effectively and that this modification can be quantified with noninvasive brain stimulation methods. The actions of S44819 may be suitable to improve plasticity and learning.