Comparing GABA-dependent physiological measures of inhibition with proton magnetic resonance spectroscopy measurement of GABA using ultra-high-field MRI

Acupuncture for generalized anxiety disorder: a study protocol for a randomized controlled trial

During the COVID-19 outbreak, there was a sharp increase in generalized anxiety disorder (GAD). Acupuncture therapy has the advantages of accurate clinical efficacy, safety and reliability, few adverse reactions, and no dependence, and is gradually becoming one of the emerging therapies for treating GAD. We present a study protocol for a randomized clinical trial with the aim of exploring the mechanism of brain plasticity in patients with GAD and evaluate the effectiveness and reliability of acupuncture treatment. Transcranial magnetic stimulation (TMS) will be used to assess cortical excitability in GAD patients and healthy people. Sixty-six GAD patients meeting the inclusion criteria will be randomly divided into two groups: TA group, (treatment with acupuncture and basic western medicine treatment) and SA group (sham acupuncture and basic western medicine treatment). Twenty healthy people will be recruited as the control group (HC). The parameters that will be evaluated are amplitude of motor evoked potentials (MEPs), cortical resting period (CSP), resting motor threshold (RMT), and Hamilton Anxiety Scale (HAMA) score. Secondary results will include blood analysis of γ-aminobutyric acid (GABA), glutamate (Glu), glutamine (Gln), serotonin (5-HT), and brain-derived nerve growth factor (BDNF). Outcomes will be assessed at baseline and after the intervention (week 8). This study protocol is the first clinical trial designed to detect differences in cerebral cortical excitability between healthy subjects and patients with GAD, and the comparison of clinical efficacy and reliability before and after acupuncture intervention is also one of the main contents of the protocol. We hope to find a suitable non-pharmacological alternative treatment for patients with GAD.

Modulation of intracortical circuits in primary motor cortex during automatic action tendencies

Humans display automatic action tendencies toward emotional stimuli, showing faster automatic behavior (i.e., approaching a positive stimulus and avoiding a negative stimulus) than regulated behavior (i.e., avoiding a positive stimulus and approaching a negative stimulus). Previous studies have shown that the primary motor cortex is involved in the processing of automatic actions, with higher motor evoked potential amplitudes during automatic behavior elicited by single-pulse transcranial magnetic stimulation. However, it is unknown how intracortical circuits are involved with automatic action tendencies. Here, we measured short-interval intracortical inhibition and intracortical facilitation within the primary motor cortex by using paired-pulse transcranial magnetic stimulation protocols during a manikin task, which has been widely used to explore approaching and avoiding behavior. Results showed that intracortical facilitation was stronger during automatic behavior than during regulated behavior. Moreover, there was a significant negative correlation between reaction times and intracortical facilitation effect during automatic behavior: individuals with short reaction times had stronger faciliatory activity, as shown by higher intracortical facilitation. By contrast, no significant difference was found for short-interval intracortical inhibition between automatic behavior and regulated behavior. The results indicated that the intracortical facilitation circuit, mediated by excitatory glutamatergic neurons, in the primary motor cortex, plays an important role in mediating automatic action tendencies. This finding further supports the link between emotional perception and the action system.

Neurotransmitter levels in the basal ganglia are associated with intracortical circuit activity of the primary motor cortex in healthy humans

BACKGROUND

The basal ganglia are strongly connected to the primary motor cortex (M1) and play a crucial role in movement control. Interestingly, several disorders showing abnormal neurotransmitter levels in basal ganglia also present concomitant anomalies in intracortical function within M1.

OBJECTIVE/HYPOTHESIS

The main aim of this study was to clarify the relationship between neurotransmitter content in the basal ganglia and intracortical function at M1 in healthy individuals. We hypothesized that neurotransmitter content of the basal ganglia would be significant predictors of M1 intracortical function.

METHODS

We combined magnetic resonance spectroscopy (MRS) and transcranial magnetic stimulation (TMS) to test this hypothesis in 20 healthy adults. An extensive TMS battery probing common measures of intracortical, and corticospinal excitability was administered, and GABA and glutamate-glutamine levels were assessed from voxels placed over the basal ganglia and the occipital cortex (control region).

RESULTS

Regression models using metabolite concentration as predictor and TMS metrics as outcome measures showed that glutamate level in the basal ganglia significantly predicted short interval intracortical inhibition (SICI) and intracortical facilitation (ICF), while GABA content did not. No model using metabolite measures from the occipital control voxel was significant.

CONCLUSIONS

Taken together, these results converge with those obtained in clinical populations and suggest that intracortical circuits in human M1 are associated with the neurotransmitter content of connected but distal subcortical structures crucial for motor function.

Magnetic Resonance Spectroscopy (MRS) in Alzheimer's Disease

MRS is a noninvasive technique to measure different metabolites in the brain. Changes in the levels of certain metabolites can be used as surrogate markers for Alzheimer's disease. They can potentially be used for diagnosis, prediction of prognosis, or even assessing response to treatment.There are different techniques for MRS acquisitions including STimulated Echo Acquisition Mode (STEAM) and Point Resolved Spectroscopy (PRESS). In terms of localization, single or multi-voxel methods can be used. Based on current data: 1. NAA, marker of neuronal integrity and viability, reduces in AD with longitudinal changes over the time as the disease progresses. There are data claiming that reduction of NAA is associated with tau accumulation, early neurodegenerative processes, and cognitive decline. Therefore, it can be used as a stage biomarker for AD to assess the severity of the disease. With advancement of disease modifying therapies, there is a potential role for NAA in the future to be used as a marker of response to treatment. 2. mI, marker of glial cell proliferation and activation, is associated with AB pathology and has early changes in the course of the disease. The NAA/mI ratio can be predictive of AD development with high specificity and can be utilized in the clinical setting to stratify cases for further evaluation with PET for potential treatments. 3. The changes in the level of other metabolites such as Chol, Glu, Gln, and GABA are controversial because of the lack of standardization of MRS techniques, current technical limitations, and possible region specific changes. 4. Ultrahigh field MRS and more advanced techniques can overcome many of these limitations and enable us to measure more metabolites with higher accuracy. 5. Standardization of MRS techniques, validation of metabolites' changes against PET using PET-guided technique, and longitudinal follow-ups to investigate the temporal changes of the metabolites in relation to other biomarkers and cognition will be crucial to confirm the utility of MRS as a potential noninvasive biomarker for AD.

Increased cortical inhibition following brief motor memory reactivation supports reconsolidation and overnight offline learning gains

Practicing motor skills stabilizes and strengthens motor memories by repeatedly reactivating and reconsolidating them. The conventional view, by which a repetitive practice is required for substantially improving skill performance, has been recently challenged by behavioral experiments, in which even brief reactivations of the motor memory have led to significant improvements in skill performance. However, the mechanisms which facilitate brief reactivation-induced skill improvements remain elusive. While initial memory consolidation has been repeatedly associated with increased neural excitation and disinhibition, reconsolidation has been shown to involve a poorly understood mixture of both excitatory and inhibitory alterations. Here, we followed a 3-d reactivation-reconsolidation framework to examine whether the excitatory/inhibitory mechanisms which underlie brief reactivation and repetitive practice differ. Healthy volunteers practiced a motor sequence learning task using either brief reactivation or repetitive practice and were assessed using ultrahigh field (7T) magnetic resonance spectroscopy at the primary motor cortex (M1). We found that increased inhibition (GABA concentrations) and decreased excitation/inhibition (glutamate/GABA ratios) immediately following the brief reactivation were associated with overnight offline performance gains. These gains were on par with those exhibited following repetitive practice, where no correlations with inhibitory or excitatory changes were observed. Our findings suggest that brief reactivation and repetitive practice depend on fundamentally different neural mechanisms and that early inhibition-and not excitation-is particularly important in supporting the learning gains exhibited by brief reactivation.

Females with painful temporomandibular disorders present higher intracortical facilitation relative to pain-free controls

OBJECTIVES

This study aimed to investigate cortical excitability differences in the primary motor cortex (M1) hand representation between individuals with temporomandibular disorders (TMD) and healthy controls. We assessed resting motor thresholds, motor-evoked potentials (MEPs), intracortical inhibition, and intracortical facilitation and explored potential associations with clinical and psychosocial characteristics in the TMD group.

MATERIALS AND METHODS

We recruited 36 female participants with TMD and 17 pain-free controls. Transcranial magnetic stimulation (TMS) was used to assess M1 cortical excitability. Correlations between clinical and psychosocial factors and cortical excitability measures were also evaluated.

RESULTS

Patients with TMD showed significantly higher intracortical facilitation at 12 ms (z = 1.98, p = 0.048) and 15 ms (z = 2.65, p = 0.008) when compared to controls. Correlations revealed associations between intracortical facilitation and pain interference, sleep quality, depressive symptoms, and pain catastrophizing in the TMD group.

CONCLUSIONS

Females with TMD exhibit heightened motor cortex intracortical facilitation in the hand representation, potentially indicating altered cortical excitability beyond the motor face area. This suggests a role for cortical excitability in TMD pathophysiology, influenced by psychosocial factors.

CLINICAL RELEVANCE

Understanding cortical excitability in TMD may inform targeted interventions. Psychosocial variables may play a role in cortical excitability, emphasizing the multidimensional nature of TMD-related pain. Further research is needed to confirm and expand upon these findings, with potential implications for the management of TMD and related pain conditions.

Excitatory-inhibitory modulation of transcranial focus ultrasound stimulation on human motor cortex

AIMS

Transcranial focus ultrasound stimulation (tFUS) is a promising non-invasive neuromodulation technology. This study aimed to evaluate the modulatory effects of tFUS on human motor cortex (M1) excitability and explore the mechanism of neurotransmitter-related intracortical circuitry and plasticity.

METHODS

Single pulse transcranial magnetic stimulation (TMS)-eliciting motor-evoked potentials (MEPs) were used to assessed M1 excitability in 10 subjects. Paired-pulse TMS was used to measure the effects of tFUS on GABA- and glutamate-related intracortical excitability and 1 H-MRS was used to assess the effects of repetitive tFUS on GABA and Glx (glutamine + glutamate) neurometabolic concentrations in the targeting region in nine subjects.

RESULTS

The etFUS significantly increased M1 excitability, decreased short interval intracortical inhibition (SICI) and long interval intracortical inhibition (LICI). The itFUS significantly suppressed M1 excitability, increased SICI, LICI, and decreased intracortical facilitation (ICF). Seven times of etFUS decreased the GABA concentration (6.32%), increased the Glx concentration (12.40%), and decreased the GABA/Glx ratio measured by MRS, while itFUS increased the GABA concentration (18.59%), decreased Glx concentration (0.35%), and significantly increased GABA/Glx ratio.

CONCLUSION

The findings support that tFUS with different parameters can exert excitatory and inhibitory neuromodulatory effects on the human motor cortex. We provide novel insights that tFUS change cortical excitability and plasticity by regulating excitatory-inhibition balance related to the GABAergic and glutamatergic receptor function and neurotransmitter metabolic level.

Dissecting the chain of information processing and its interplay with neurochemicals and fluid intelligence across development

Previous research has highlighted the role of glutamate and gamma-aminobutyric acid (GABA) in perceptual, cognitive, and motor tasks. However, the exact involvement of these neurochemical mechanisms in the chain of information processing, and across human development, is unclear. In a cross-sectional longitudinal design, we used a computational approach to dissociate cognitive, decision, and visuomotor processing in 293 individuals spanning early childhood to adulthood. We found that glutamate and GABA within the intraparietal sulcus (IPS) explained unique variance in visuomotor processing, with higher glutamate predicting poorer visuomotor processing in younger participants but better visuomotor processing in mature participants, while GABA showed the opposite pattern. These findings, which were neurochemically, neuroanatomically and functionally specific, were replicated ~21 mo later and were generalized in two further different behavioral tasks. Using resting functional MRI, we revealed that the relationship between IPS neurochemicals and visuomotor processing is mediated by functional connectivity in the visuomotor network. We then extended our findings to high-level cognitive behavior by predicting fluid intelligence performance. We present evidence that fluid intelligence performance is explained by IPS GABA and glutamate and is mediated by visuomotor processing. However, this evidence was obtained using an uncorrected alpha and needs to be replicated in future studies. These results provide an integrative biological and psychological mechanistic explanation that links cognitive processes and neurotransmitters across human development and establishes their potential involvement in intelligent behavior.

Transcranial focused ultrasound-mediated neurochemical and functional connectivity changes in deep cortical regions in humans

Low-intensity transcranial ultrasound stimulation (TUS) is an emerging non-invasive technique for focally modulating human brain function. The mechanisms and neurochemical substrates underlying TUS neuromodulation in humans and how these relate to excitation and inhibition are still poorly understood. In 24 healthy controls, we separately stimulated two deep cortical regions and investigated the effects of theta-burst TUS, a protocol shown to increase corticospinal excitability, on the inhibitory neurotransmitter gamma-aminobutyric acid (GABA) and functional connectivity. We show that theta-burst TUS in humans selectively reduces GABA levels in the posterior cingulate, but not the dorsal anterior cingulate cortex. Functional connectivity increased following TUS in both regions. Our findings suggest that TUS changes overall excitability by reducing GABAergic inhibition and that changes in TUS-mediated neuroplasticity last at least 50 mins after stimulation. The difference in TUS effects on the posterior and anterior cingulate could suggest state- or location-dependency of the TUS effect-both mechanisms increasingly recognized to influence the brain's response to neuromodulation.

Human neuronal excitation/inhibition balance explains and predicts neurostimulation induced learning benefits

Previous research has highlighted the role of the excitation/inhibition (E/I) ratio for typical and atypical development, mental health, cognition, and learning. Other research has highlighted the benefits of high-frequency transcranial random noise stimulation (tRNS)-an excitatory form of neurostimulation-on learning. We examined the E/I as a potential mechanism and studied whether tRNS effect on learning depends on E/I as measured by the aperiodic exponent as its putative marker. In addition to manipulating E/I using tRNS, we also manipulated the level of learning (learning/overlearning) that has been shown to influence E/I. Participants (n = 102) received either sham stimulation or 20-minute tRNS over the dorsolateral prefrontal cortex (DLPFC) during a mathematical learning task. We showed that tRNS increased E/I, as reflected by the aperiodic exponent, and that lower E/I predicted greater benefit from tRNS specifically for the learning task. In contrast to previous magnetic resonance spectroscopy (MRS)-based E/I studies, we found no effect of the level of learning on E/I. A further analysis using a different data set suggest that both measures of E/I (EEG versus MRS) may reflect, at least partly, different biological mechanisms. Our results highlight the role of E/I as a marker for neurostimulation efficacy and learning. This mechanistic understanding provides better opportunities for augmented learning and personalized interventions.

Neurochemistry-enriched dynamic causal models of magnetoencephalography, using magnetic resonance spectroscopy

We present a hierarchical empirical Bayesian framework for testing hypotheses about neurotransmitters' concertation as empirical prior for synaptic physiology using ultra-high field magnetic resonance spectroscopy (7T-MRS) and magnetoencephalography data (MEG). A first level dynamic causal modelling of cortical microcircuits is used to infer the connectivity parameters of a generative model of individuals' neurophysiological observations. At the second level, individuals' 7T-MRS estimates of regional neurotransmitter concentration supply empirical priors on synaptic connectivity. We compare the group-wise evidence for alternative empirical priors, defined by monotonic functions of spectroscopic estimates, on subsets of synaptic connections. For efficiency and reproducibility, we used Bayesian model reduction (BMR), parametric empirical Bayes and variational Bayesian inversion. In particular, we used Bayesian model reduction to compare alternative model evidence of how spectroscopic neurotransmitter measures inform estimates of synaptic connectivity. This identifies the subset of synaptic connections that are influenced by individual differences in neurotransmitter levels, as measured by 7T-MRS. We demonstrate the method using resting-state MEG (i.e., task-free recording) and 7T-MRS data from healthy adults. Our results confirm the hypotheses that GABA concentration influences local recurrent inhibitory intrinsic connectivity in deep and superficial cortical layers, while glutamate influences the excitatory connections between superficial and deep layers and connections from superficial to inhibitory interneurons. Using within-subject split-sampling of the MEG dataset (i.e., validation by means of a held-out dataset), we show that model comparison for hypothesis testing can be highly reliable. The method is suitable for applications with magnetoencephalography or electroencephalography, and is well-suited to reveal the mechanisms of neurological and psychiatric disorders, including responses to psychopharmacological interventions.

The phase of plasticity-induced neurochemical changes of high-frequency repetitive transcranial magnetic stimulation are different from visual perceptual learning

Numerous studies have found that repetitive transcranial magnetic stimulation (rTMS) modulates plasticity. rTMS has often been used to change neural networks underlying learning, often under the assumption that the mechanism of rTMS-induced plasticity should be highly similar to that associated with learning. The presence of visual perceptual learning (VPL) reveals the plasticity of early visual systems, which is formed through multiple phases. Hence, we tested how high-frequency (HF) rTMS and VPL modulate the effect of visual plasticity by investigating neurometabolic changes in early visual areas. We employed an excitatory-to-inhibitory (E/I) ratio, which refers to glutamate concentration divided by GABA+ concentration, as an index of the degree of plasticity. We compared neurotransmitter concentration changes after applying HF rTMS to the visual cortex with those after training in a visual task, in otherwise identical procedures. Both the time courses of the E/I ratios and neurotransmitter contributions to the E/I ratio significantly differed between HF rTMS and training conditions. The peak E/I ratio occurred 3.5 h after HF rTMS with decreased GABA+, whereas the peak E/I ratio occurred 0.5 h after visual training with increased glutamate. Furthermore, HF rTMS temporally decreased the thresholds for detecting phosphene and perceiving low-contrast stimuli, indicating increased visual plasticity. These results suggest that plasticity in early visual areas induced by HF rTMS is not as involved in the early phase of development of VPL that occurs during and immediately after training.

Exploring the links between gut microbiota and excitatory and inhibitory brain processes in alcohol use disorder: A TMS study

While the impact of the gut microbiota on brain and behavior is increasingly recognized, human studies examining this question are still scarce. The primary objective of the current study was to explore the potential relationships between the gut microbiota composition, motor cortical excitability at rest and during inhibitory control, as well as behavioral inhibition, in healthy volunteers and in patients suffering from alcohol use disorder. Motor cortical excitability was examined using a range of transcranial magnetic stimulation (TMS) measures probed at rest, including the recruitment curve, short and long intracortical inhibition, and intracortical facilitation within the primary motor cortex. Moreover, TMS was applied during a choice reaction time task to assess changes in motor excitability associated with inhibitory control. Finally, behavioral inhibition was investigated using a neuropsychological task (anti-saccade). Overall, our results highlight several interesting correlations between microbial composition and brain measures. Hence, higher bacterial diversity, as well as higher relative abundances of UGC-002 and Christensenellaceae R-7 group were correlated with stronger changes in motor excitability associated with inhibitory control. Also, higher abundance of Anaerostipes was associated with higher level of corticospinal excitability. Finally, relative abundances of Bifidobacterium and Faecalibacterium were positively related to performance in the neuropsychological task, suggesting that they might have a positive impact on behavioral inhibition. Although correlation is not causation, the present study suggests that excitatory and inhibitory brain processes might be related to gut microbiota composition. This article is part of the Special Issue on 'Microbiome & the Brain: Mechanisms & Maladies'.

Detection of physiological concentrations of GABA using dielectric spectroscopy - A pilot study

Gamma-aminobutyric acid (GABA) is a major inhibitory neurotransmitter that is present at a relatively low level throughout the normal adult human brain. Abnormal GABA levels are found in people with neurodegenerative disorders such as Parkinson's disease, epilepsy, schizophrenia, depression, and others. Being able to measure the GABA concentration would be beneficial for patient groups with fluctuating GABA levels for better diagnosis and treatment. In this study, we explore the feasibility of using dielectric relaxation spectroscopy for the detection of GABA concentrations within a physiological range, with the perspective of miniaturization and use during implantation. Utilizing machine learning techniques, we were able to differentiate GABA concentrations down to 5 μm. This work investigates a novel use of dielectric relaxation spectroscopy, to assess if physiological GABA concentrations can be detected through permittivity measurements.

Altered Medial Prefrontal Connectivity in Parkinson's Disease Patients with Somatic Symptoms

BACKGROUND

The high co-occurrence of somatic symptom disorder (SSD) in Parkinson's disease (PD) patients suggests overlapping pathophysiology. However, little is known about the neural correlates of SSD and their possible interactions with PD. Existing studies have shown that SSD is associated with reduced task-evoked activity in the medial prefrontal cortex (mPFC), a central node of the default-mode network (DMN). SSD is also associated with abnormal γ-aminobutyric acid (GABA) content, a marker of local inhibitory tone and regional hypoactivity, in the same area when SSD co-occurs with PD.

OBJECTIVES

To disentangle the individual and shared effects of SSD and PD on mPFC neurotransmission and connectivity patterns and help disclose the neural mechanisms of comorbidity in the PD population.

METHODS

The study cohort included 18 PD patients with SSD (PD + SSD), 18 PD patients, 13 SSD patients who did not exhibit neurologic disorders, and 17 healthy subjects (HC). Proton magnetic resonance (MR) spectroscopy evaluated GABA levels within a volume of interest centered on the mPFC. Resting-state functional MR imaging investigated the region's functional connectivity patterns.

RESULTS

Compared to HC or PD groups, the mPFC of SSD subjects exhibited higher GABA levels and connectivity. Higher mPFC connectivity involved DMN regions in SSD patients without PD and regions of the executive and attentional networks (EAN) in patients with PD comorbidity.

CONCLUSIONS

Aberrant reconfigurations of connectivity patterns between the mPFC and the EAN are distinct features of the PD + SSD comorbidity. © 2022 The Authors. Movement Disorders published by Wiley Periodicals LLC on behalf of International Parkinson and Movement Disorder Society.

The effect of parietal glutamate/GABA balance on test anxiety levels in early childhood in a cross-sectional and longitudinal study

The increased prevalence of test anxiety in our competitive society makes it a health issue of public concern. However, its neurobiological basis, especially during the years of formal education, is currently scant. Previous research has highlighted the association between neural excitation/inhibition balance and psychopathology and disease. We examined whether the glutamate/GABA profile tracks test anxiety levels in development, using a cross-sectional and longitudinal design in a cohort spanning from early childhood to early adulthood (N = 289), reassessed approximately 21 months later (N = 194). We used magnetic resonance spectroscopy to noninvasively quantify glutamate and gamma-Aminobutyric acid (GABA) levels in the intraparietal sulcus (IPS) and the middle frontal gyrus. We show that the glutamate/GABA balance within the IPS relates to current individual variation in test anxiety levels and predict future test anxiety approximately 21 months later. Critically, this relationship was observed during early childhood but not during the later developmental stages. Our results extend the use of the excitation/inhibition balance framework to characterize the psychopathology mechanisms of test anxiety, an underexplored yet widespread and debilitating condition that can impact early child development. Our findings provide a better understanding of the neurotransmitter basis underlying the emergence of anxiety disorders during development.

The interaction between endogenous GABA, functional connectivity, and behavioral flexibility is critically altered with advanced age

The flexible adjustment of ongoing behavior challenges the nervous system’s dynamic control mechanisms and has shown to be specifically susceptible to age-related decline. Previous work links endogenous gamma-aminobutyric acid (GABA) with behavioral efficiency across perceptual and cognitive domains, with potentially the strongest impact on those behaviors that require a high level of dynamic control. Our analysis integrated behavior and modulation of interhemispheric phase-based connectivity during dynamic motor-state transitions with endogenous GABA concentration in adult human volunteers. We provide converging evidence for age-related differences in the preferred state of endogenous GABA concentration for more flexible behavior. We suggest that the increased interhemispheric connectivity observed in the older participants represents a compensatory neural mechanism caused by phase-entrainment in homotopic motor cortices. This mechanism appears to be most relevant in the presence of a less optimal tuning of the inhibitory tone as observed during healthy aging to uphold the required flexibility of behavioral action. Future work needs to validate the relevance of this interplay between neural connectivity and GABAergic inhibition for other domains of flexible human behavior.

An integrated analysis of GABA concentration, interhemispheric phase-based connectivity behavior and in healthy human participants revealed a task-specific association between GABA concentration and motor-cortical connectivity. These results shed light on how GABA is involved in neural oscillations and behavioral performance.

Glutamate-Weighted Magnetic Resonance Imaging (GluCEST) Detects Effects of Transcranial Magnetic Stimulation to the Motor Cortex

Transcranial magnetic stimulation (TMS) is used in several FDA-approved treatments and, increasingly, to treat neurological disorders in off-label uses. However, the mechanism by which TMS causes physiological change is unclear, as are the origins of response variability in the general population. Ideally, objective in vivo biomarkers could shed light on these unknowns and eventually inform personalized interventions. Continuous theta-burst stimulation (cTBS) is a form of TMS observed to reduce motor evoked potentials (MEPs) for 60 min or longer post-stimulation, although the consistency of this effect and its mechanism continue to be under debate. Here, we use glutamate-weighted chemical exchange saturation transfer (gluCEST) magnetic resonance imaging (MRI) at ultra-high magnetic field (7T) to measure changes in glutamate concentration at the site of cTBS. We find that the gluCEST signal in the ipsilateral hemisphere of the brain generally decreases in response to cTBS, whereas consistent changes were not detected in the contralateral region of interest (ROI) or in subjects receiving sham stimulation.

Cortical Inhibition State-Dependent iTBS Induced Neural Plasticity

Background

Intermittent theta burst stimulation (iTBS) is an effective stimulus for long-term potentiation (LTP)-like plasticity. However, iTBS-induced effects varied greatly between individuals. Ample evidence suggested that an initial decrease in local γ-aminobutyric acid (GABA) or enhancement in N-methyl-D-aspartate (NMDA) facilitation neurotransmission is of vital importance for allowing LTP-like plasticity to occur. Therefore, we aimed to investigate whether the individual level of GABA or NMDA receptor-mediated activity before stimulation is correlated with the after-effect in cortical excitability induced by iTBS.

Methods

Fifteen healthy volunteers were recruited for the present study. We measured short-interval intracortical inhibitory (SICI), long-interval intracortical inhibitory (LICI), and intracortical facilitation (ICF), which index GABA_A receptor-, GABA_B receptor-, and glutamate receptor-mediated activity, respectively, in the cortex before conducting iTBS. After iTBS intervention, the changes of motor-evoked potential (MEP) amplitude were taken as a measure for cortical excitability in response to iTBS protocol.

Results

There was a significant negative correlation between the amount of SICI measured before iTBS and the after-effect of iTBS-induced LTP-like plasticity at the time points of 5, 10, and 15 min after inducing iTBS. A multiple linear regression model indicated that SICI was a good predictor of the after-effect in cortical excitability induced by iTBS at 5, 10, and 15 min following stimulation.

Conclusion

The present study found that GABA_A receptor-mediated activity measured before stimulation is negatively correlated with the after-effect of cortical excitability induced by iTBS. SICI, as the index of GABA_A receptor-mediated activity measured before stimulation, might be a good predictor of iTBS-induced LTP-like plasticity for a period lasting 15 min following stimulation.

Tools to explore neuroplasticity in humans: Combining interventional neurophysiology with functional and structural magnetic resonance imaging and spectroscopy

This chapter summarizes how brain imaging can be used in combination with non-invasive transcranial stimulation to probe and induce neuroplasticity in the human brain. We aim to give a conceptual account and highlight exemplary studies. We showcase the scientific and clinical potentials of studies focusing on the combination of transcranial magnetic stimulation (TMS) with Magnetic Resonance Imaging (MRI) or Magnetic Resonance Spectroscopy (MRS). MRI and MRS can be used before brain stimulation to identify target networks and loci but also to inform individual dosing. After a brain stimulation session, MRI and MRS can be used to pinpoint how the stimulation protocol alters brain function, structure, or metabolism and relate these after-effects to behavioral and clinical outcomes. Complementing these "offline" approaches, TMS can also be applied "online" during MRI or MRS to delineate how stimulation acutely engages the stimulated brain regions and networks. In this case, it is critical to account for confounds introduced by off-target stimulation of peripheral structures of the nervous system that may not only confound MR-based readouts but also induce neuroplastic phenomena.

Neurophysiological modulations in the (pre)motor-motor network underlying age-related increases in reaction time and the role of GABA levels - a bimodal TMS-MRS study

It has been argued that age-related changes in the neurochemical and neurophysiological properties of the GABAergic system may underlie increases in reaction time (RT) in older adults. However, the role of GABA levels within the sensorimotor cortices (SMC) in mediating interhemispheric interactions (IHi) during the processing stage of a fast motor response, as well as how both properties explain interindividual differences in RT, are not yet fully understood. In this study, edited magnetic resonance spectroscopy (MRS) was combined with dual-site transcranial magnetic stimulation (dsTMS) for probing GABA+ levels in bilateral SMC and task-related neurophysiological modulations in corticospinal excitability (CSE), and primary motor cortex (M1)-M1 and dorsal premotor cortex (PMd)-M1 IHi, respectively. Both CSE and IHi were assessed during the preparatory and premotor period of a delayed choice RT task. Data were collected from 25 young (aged 18-33 years) and 28 older (aged 60-74 years) healthy adults. Our results demonstrated that older as compared to younger adults exhibited a reduced bilateral CSE suppression, as well as a reduced magnitude of long latency M1-M1 and PMd-M1 disinhibition during the preparatory period, irrespective of the direction of the IHi. Importantly, in older adults, the GABA+ levels in bilateral SMC partially accounted for task-related neurophysiological modulations as well as individual differences in RT. In contrast, in young adults, neither task-related neurophysiological modulations, nor individual differences in RT were associated with SMC GABA+ levels. In conclusion, this study contributes to a comprehensive initial understanding of how age-related differences in neurochemical properties and neurophysiological processes are related to increases in RT.

Temperature dependence of the microwave dielectric properties of [Formula: see text]-aminobutyric acid

The GABA molecule is the major inhibitory neurotransmitter in the mammalian central nervous system. Through binding to post-synaptic neurons, GABA reduces the neuronal excitability by hyperpolarization. Correct binding between the GABA molecules and its receptors relies on molecular recognition. Earlier studies suggest that recognition is determined by the geometries of the molecule and its receptor. We employed dielectric relaxation spectroscopy (DRS) to study the conformation and dielectric properties of the GABA molecule under physiologically relevant laboratory conditions. The dielectric properties of GABA investigated have given us new insights about the GABA molecule, such as how they interact with each other and with water molecules at different temperatures (22°C and 37.5°C). Higher temperature leads to lower viscosity, thus lower relaxation time. The change in the GABA relaxation time due to concentration change is more associated with the solution viscosity than with the GABA dipole moment. A resonance behavior was observed with high GABA concentrations at physiological temperature, where there might be a phase transition at a certain temperature for a given GABA concentration that leads to a sudden change of the dielectric properties.

The relation between parietal GABA concentration and numerical skills

Several scientific, engineering, and medical advancements are based on breakthroughs made by people who excel in mathematics. Our current understanding of the underlying brain networks stems primarily from anatomical and functional investigations, but our knowledge of how neurotransmitters subserve numerical skills, the building block of mathematics, is scarce. Using 1H magnetic resonance spectroscopy (N = 54, 3T, semi-LASER sequence, TE = 32 ms, TR = 3.5 s), the study examined the relation between numerical skills and the brain's major inhibitory (GABA) and excitatory (glutamate) neurotransmitters. A negative association was found between the performance in a number sequences task and the resting concentration of GABA within the left intraparietal sulcus (IPS), a key region supporting numeracy. The relation between GABA in the IPS and number sequences was specific to (1) parietal but not frontal regions and to (2) GABA but not glutamate. It was additionally found that the resting functional connectivity of the left IPS and the left superior frontal gyrus was positively associated with number sequences performance. However, resting GABA concentration within the IPS explained number sequences performance above and beyond the resting frontoparietal connectivity measure. Our findings further motivate the study of inhibition mechanisms in the human brain and significantly contribute to our current understanding of numerical cognition's biological bases.

Hippocampal and striatal responses during motor learning are modulated by prefrontal cortex stimulation

While it is widely accepted that motor sequence learning (MSL) is supported by a prefrontal-mediated interaction between hippocampal and striatal networks, it remains unknown whether the functional responses of these networks can be modulated in humans with targeted experimental interventions. The present proof-of-concept study employed a multimodal neuroimaging approach, including functional magnetic resonance (MR) imaging and MR spectroscopy, to investigate whether individually-tailored theta-burst stimulation of the dorsolateral prefrontal cortex can modulate responses in the hippocampus and the basal ganglia during motor learning. Our results indicate that while stimulation did not modulate motor performance nor task-related brain activity, it influenced connectivity patterns within hippocampo-frontal and striatal networks. Stimulation also altered the relationship between the levels of gamma-aminobutyric acid (GABA) in the stimulated prefrontal cortex and learning-related changes in both activity and connectivity in fronto-striato-hippocampal networks. This study provides the first experimental evidence, to the best of our knowledge, that brain stimulation can alter motor learning-related functional responses in the striatum and hippocampus.

Disruption to normal excitatory and inhibitory function within the medial prefrontal cortex in people with chronic pain

BACKGROUND

Growing evidence indicates a link between changes in the medial prefrontal cortex and the pathophysiology of chronic pain. In particular, chronic pain is associated with altered medial prefrontal anatomy and biochemistry. Due to the comorbid affective disorders seen across all pain conditions, the medial prefrontal cortex is a region of significance as it is involved in emotional processing. We have recently reported that a decrease in medial prefrontal N-acetylaspartate and glutamate is associated with increased emotional dysregulation, indicating there are neurotransmitter imbalances in chronic pain. Therefore, we compared medial prefrontal neurochemistry in 24 people with chronic pain conditions to 24 age and sex-matched healthy controls with no history of chronic pain.

METHOD

GABA-edited MEGA-PRESS was used to measure GABA⁺ levels, and short TE PRESS was used to measure glutamate levels in the medial prefrontal cortex. Psychometric measures regarding pain intensity a week before scanning, during the scan and the total duration of chronic pain, were also recorded and compared to measured GABA⁺ and glutamate levels.

RESULTS

This study reveals that the presence of chronic pain is associated with significant decreases in medial prefrontal GABA⁺ and glutamate. These findings support the hypothesis that chronic pain is associated with altered medial prefrontal biochemistry.

CONCLUSION

The dysregulation of glutamatergic and GABAergic neurotransmitter systems supports a model of disinhibition of chronic pain, which may play a key role in both the experience of persistent pain and its associated affective disturbances.

SIGNIFICANCE

This study reveals a significant reduction in γ-aminobutyric acid (GABA⁺ ) and glutamate within the medial prefrontal cortex in chronic pain sufferers. While the current findings should be considered with reference to a small sample size, the disruption to normal excitatory and inhibitory medial prefrontal cortex function may be key in the development and maintenance of chronic pain and comorbid mental health disorders.

Predicting learning and achievement using GABA and glutamate concentrations in human development

Previous research has highlighted the role of glutamate and gamma-aminobutyric acid (GABA) in learning and plasticity. What is currently unknown is how this knowledge translates to real-life complex cognitive abilities that emerge slowly and how the link between these neurotransmitters and human learning and plasticity is shaped by development. While some have suggested a generic role of glutamate and GABA in learning and plasticity, others have hypothesized that their involvement shapes sensitive periods during development. Here we used a cross-sectional longitudinal design with 255 individuals (spanning primary school to university) to show that glutamate and GABA in the intraparietal sulcus explain unique variance both in current and future mathematical achievement (approximately 1.5 years). Furthermore, our findings reveal a dynamic and dissociable role of GABA and glutamate in predicting learning, which is reversed during development, and therefore provide novel implications for models of learning and plasticity during childhood and adulthood.

Recent advances in the role of excitation-inhibition balance in motor recovery post-stroke

Stroke affects millions of people worldwide each year, and stroke survivors are often left with motor deficits. Current therapies to improve these functional deficits are limited, making it a priority to better understand the pathophysiology of stroke recovery and find novel adjuvant options. The excitation-inhibition balance undergoes significant changes post-stroke, and the inhibitory neurotransmitter γ-aminobutyric acid (GABA) appears to play an important role in stroke recovery. In this review, we summarise the most recent studies investigating GABAergic inhibition at different stages of stroke. We discuss the proposed role of GABA in counteracting glutamate-mediated excitotoxicity in hyperacute stroke as well as the evidence linking decreased GABAergic inhibition to increased neuronal plasticity in early stroke. Then, we discuss two types of interventions that aim to modulate the excitation-inhibition balance to improve functional outcomes in stroke survivors: non-invasive brain stimulation (NIBS) and pharmacological interventions. Finding the optimal NIBS administration or adjuvant pharmacological therapies would represent an important contribution to the currently scarce therapy options.

Input-output slope curve estimation in neural stimulation based on optimal sampling principles. J Neural Eng 2021;18:10.1088/1741-2552/abffe5. [PMID: 33975287 PMCID: PMC8384062 DOI: 10.1088/1741-2552/abffe5]

This paper discusses some of the practical limitations and issues, which exist for the input-output (IO) slope curve estimation (SCE) in neural, brain and spinal, stimulation techniques. The drawbacks of the SCE techniques by using existing uniform sampling and Fisher-information-based optimal IO curve estimation (FO-IOCE) methods are elaborated. A novel IO SCE technique is proposed with a modified sampling strategy and stopping rule which improve the SCE performance compared to these methods. The effectiveness of the proposed IO SCE is tested on 1000 simulation runs in transcranial magnetic stimulation (TMS), with a realistic model of motor evoked potentials. The results show that the proposed IO SCE method successfully satisfies the stopping rule, before reaching the maximum number of TMS pulses in 79.5% of runs, while the estimation based on the uniform sampling technique never converges and satisfies the stopping rule. At the time of successful termination, the proposed IO SCE method decreases the 95th percentile (mean value in the parentheses) of the absolute relative estimation errors (AREs) of the slope curve parameters up to 7.45% (2.2%), with only 18 additional pulses on average compared to that of the FO-IOCE technique. It also decreases the 95th percentile (mean value in the parentheses) of the AREs of the IO slope curve parameters up to 59.33% (16.71%), compared to that of the uniform sampling method. The proposed IO SCE also identifies the peak slope with higher accuracy, with the 95th percentile (mean value in the parentheses) of AREs reduced by up to 9.96% (2.01%) compared to that of the FO-IOCE method, and by up to 46.29% (13.13%) compared to that of the uniform sampling method.

Influence of Brain-Derived Neurotrophic Factor Genotype on Short-Latency Afferent Inhibition and Motor Cortex Metabolites

The Met allele of the brain-derived neurotrophic factor (BDNF) gene confers reduced cortical BDNF expression and associated neurobehavioral changes. BDNF signaling influences the survival, development, and synaptic function of cortical networks. Here, we compared gamma-aminobutyric acid (GABA)ergic network activity in the human primary motor cortex (M1) between the Met (Val/Met and Met/Met) and non-Met (Val/Val) genotype groups. Short- and long-interval intracortical inhibition, short-latency afferent inhibition (SAI), and long-latency afferent inhibition were measured using transcranial magnetic stimulation (TMS) as indices of GABAergic activity. Furthermore, the considerable inter-individual variability in inhibitory network activity typically measured by TMS may be affected not only by GABA but also by other pathways, including glutamatergic and cholinergic activities; therefore, we used 3-T magnetic resonance spectroscopy (MRS) to measure the dynamics of glutamate plus glutamine (Glx) and choline concentrations in the left M1, left somatosensory cortex, and right cerebellum. All inhibitory TMS conditions produced significantly smaller motor-evoked potentials than single-pulses. SAI was significantly stronger in the Met group than in the Val/Val group. Only the M1 Glx concentration was significantly lower in the Met group, while the BDNF genotype did not affect choline concentration in any region. Further, a positive correlation was observed between SAI and Glx concentrations only in M1. Our findings provide evidence that the BDNF genotype regulates both the inhibitory and excitatory circuits in human M1. In addition, lower Glx concentration in the M1 of Met carriers may alter specific inhibitory network on M1, thereby influencing the cortical signal processing required for neurobehavioral functions.

GABAergic cortical network physiology in frontotemporal lobar degeneration

The clinical syndromes caused by frontotemporal lobar degeneration are heterogeneous, including the behavioural variant frontotemporal dementia (bvFTD) and progressive supranuclear palsy. Although pathologically distinct, they share many behavioural, cognitive and physiological features, which may in part arise from common deficits of major neurotransmitters such as γ-aminobutyric acid (GABA). Here, we quantify the GABAergic impairment and its restoration with dynamic causal modelling of a double-blind placebo-controlled crossover pharmaco-magnetoencephalography study. We analysed 17 patients with bvFTD, 15 patients with progressive supranuclear palsy, and 20 healthy age- and gender-matched controls. In addition to neuropsychological assessment and structural MRI, participants undertook two magnetoencephalography sessions using a roving auditory oddball paradigm: once on placebo and once on 10 mg of the oral GABA reuptake inhibitor tiagabine. A subgroup underwent ultrahigh-field magnetic resonance spectroscopy measurement of GABA concentration, which was reduced among patients. We identified deficits in frontotemporal processing using conductance-based biophysical models of local and global neuronal networks. The clinical relevance of this physiological deficit is indicated by the correlation between top-down connectivity from frontal to temporal cortex and clinical measures of cognitive and behavioural change. A critical validation of the biophysical modelling approach was evidence from parametric empirical Bayes analysis that GABA levels in patients, measured by spectroscopy, were related to posterior estimates of patients’ GABAergic synaptic connectivity. Further evidence for the role of GABA in frontotemporal lobar degeneration came from confirmation that the effects of tiagabine on local circuits depended not only on participant group, but also on individual baseline GABA levels. Specifically, the phasic inhibition of deep cortico-cortical pyramidal neurons following tiagabine, but not placebo, was a function of GABA concentration. The study provides proof-of-concept for the potential of dynamic causal modelling to elucidate mechanisms of human neurodegenerative disease, and explains the variation in response to candidate therapies among patients. The laminar- and neurotransmitter-specific features of the modelling framework, can be used to study other treatment approaches and disorders. In the context of frontotemporal lobar degeneration, we suggest that neurophysiological restoration in selected patients, by targeting neurotransmitter deficits, could be used to bridge between clinical and preclinical models of disease, and inform the personalized selection of drugs and stratification of patients for future clinical trials.

Relationship between GABA levels and task-dependent cortical excitability in children with attention-deficit/hyperactivity disorder

OBJECTIVE

Compared to typically developing (TD) peers, children with attention deficit hyperactivity disorder (ADHD) manifest reduced short interval cortical inhibition (SICI) in the dominant motor cortex measured with transcranial magnetic stimulation (TMS). This multimodal study investigates the inhibitory neurophysiology and neurochemistry by evaluating the relationship between SICI and γ-amino butyric acid (GABA+) levels, measured with magnetic resonance spectroscopy (MRS).

METHODS

Across two sites, 37 children with ADHD and 45 TD children, ages 8-12 years, participated. Single and paired pulse TMS to left motor cortex quantified SICI during REST and at times of action selection (GO) and inhibition (STOP) during a modified Slater-Hammel stop signal reaction task. MRS quantified GABA+ levels in the left sensorimotor cortex. Relationships between SICI and GABA+, as well as stopping efficiency and clinical symptoms, were analyzed with correlations and repeated-measure, mixed-models.

RESULTS

In both groups, higher GABA+ levels correlated with less SICI. In TD children only, higher GABA+ levels correlated with larger TMS motor evoked potentials (MEPs) at REST. In GO and STOP trials, higher GABA+ was associated with smaller MEP amplitudes, for both groups. Overall, GABA+ levels did not differ between groups or correlate with ADHD clinical symptoms.

CONCLUSIONS

In children with higher motor cortex GABA+, motor cortex is less responsive to inhibitory TMS (SICI). Comparing the relationships between MRS-GABA+ levels and responses to TMS at REST vs. GO/STOP trials suggests differences in inhibitory neurophysiology and neurotransmitters in children with ADHD. These differences are more prominent at rest than during response inhibition task engagement.

SIGNIFICANCE

Evaluating relationships between GABA+ and SICI may provide a biomarker useful for understanding behavioral diagnoses.

GABAergic polygenic risk for cocaine use disorder is negatively correlated with precuneus activity during cognitive control in African American individuals

Impaired cognitive control has been implicated in cocaine use disorder (CUD). GABAergic treatments have been proposed for CUD. Here we examined relationships between GABAergic genes and neural correlates of cognitive control in CUD. We analyzed two independent African American cohorts: one of >3000 genomewide-genotyped subjects with substance dependence and another of 40 CUD and 22 healthy control (HC) subjects who were exome-array genotyped and completed an fMRI Stroop task. We used five association thresholds to select variants of GABAergic genes in the reference cohort, yielding five polygenic risk scores (i.e., CUD-GABA-PRSs) for the fMRI cohort. At p < 0.005, the CUD-GABA-PRSs, which aggregated relative risks of CUD from 89 variants harboring in 16 genes, differed between CUD and HC individuals in the fMRI sample (p = 0.013). This CUD-GABA-PRS correlated inversely with Stroop-related activity in the left precuneus in CUD (r = -80.58, p_FWE < 0.05) but not HC participants. Post-hoc seed-based connectivity analysis of the left precuneus identified reduced functional connectivity to the posterior cingulate cortex (PCC) in CUD compared to HC subjects (p = 0.0062) and the degree of connectivity correlated with CUD-GABA-PRSs in CUD individuals (r = 0.287, p = 0.036). Our findings suggest that the GABAergic genetic risk of CUD in African Americans relates to precuneus/PCC functional connectivity during cognitive control. Identification of these GABAergic processes may be relevant targets in CUD treatment. The novel identification of 16 GABAergic genes may be investigated further to inform treatment development efforts for this condition that currently has no medication with a formal indication for its treatment.

Realistic head-shaped phantom with brain-mimicking metabolites for 7 T spectroscopy and spectroscopic imaging

PURPOSE

Moving to ultra-high fields (≥7 T), the inhomogeneity of both RF (B1 ) and static (B0 ) magnetic fields increases, which further motivates us to design a realistic head-shaped phantom, especially for spectroscopic imaging. Such phantoms provide images similar to the human brain and serve as a reliable tool for developing and examining methods in MRI. This study aims to develop and characterize a realistic head-shaped phantom filled with brain-mimicking metabolites for MRS and magnetic resonance spectroscopic imaging in a 7 T MRI scanner.

METHODS

A 3D head-shaped container with three sections-mimicking brain, muscle and precranial lipid-was constructed. The phantom was designed to provide robustness to heating, mechanical damage and leakage, with easy refilling. The head's shape and the agarose mixture were optimized to provide B0 and B1 distributions and T1 /T2 relaxation values similar to those of human brain. Eight brain-tissue-mimicking metabolites were included for spectroscopy. The phantom was evaluated for localized spectroscopy, fast spectroscopic imaging and fat suppression.

RESULTS

The B0 and B1 maps showed distribution similar to that of human brain, with increased B0 inhomogeneity near the nasal and ear areas and reduced B1 in the temporal lobe and brain stem regions, as expected in vivo. The metabolites' concentrations were verified by single-voxel spectroscopy, showing an average deviation of 11%. Fast spectroscopic imaging and imaging with fat suppression were demonstrated.

CONCLUSION

A 3D head-shaped phantom for human brain imaging and spectroscopic imaging in 7 T MRI was demonstrated, making it a realistic phantom for methodology development at 7 T.

GABA and glutamate in pediatric migraine

Migraine is one of the top 5 most prevalent childhood diseases; however, effective treatment strategies for pediatric migraine are limited. For example, standard adult pharmaceutical therapies are less effective in children and can carry undesirable side effects. To develop more effective treatments, improved knowledge of the biology underlying pediatric migraine is necessary. One theory is that migraine results from an imbalance in cortical excitability. Magnetic resonance spectroscopy (MRS) studies show changes in GABA and glutamate levels (the primary inhibitory and excitatory neurotransmitters in the brain, respectively) in multiple brain regions in adults with migraine; however, they have yet to be assessed in children with migraine. Using MRS and GABA-edited MRS, we show that children (7-13 years) with migraine and aura had significantly lower glutamate levels in the visual cortex compared to controls, the opposite to results seen in adults. In addition, we found significant correlations between metabolite levels and migraine characteristics; higher GABA levels were associated with higher migraine burden. We also found that higher glutamate in the thalamus and higher GABA/Glx ratios in the sensorimotor cortex were associated with duration since diagnosis, i.e., having migraines longer. Lower GABA levels in the sensorimotor cortex were associated with being closer to their next migraine attack. Together, this indicates that GABA and glutamate disturbances occur early in migraine pathophysiology and emphasizes that evidence from adults with migraine cannot be immediately translated to pediatric sufferers. This highlights the need for further mechanistic studies of migraine in children, to aid in development of more effective treatments.

Cerebellar GABA Levels and Cognitive Interference in Parkinson's disease and Healthy Comparators

The neuroanatomical and molecular substrates for cognitive impairment in Parkinson Disease (PD) are far from clear. Evidence suggests a non-dopaminergic basis, and a crucial role for cerebellum in cognitive control in PD. We investigated whether a PD cognitive marker (response inhibition) was differently controlled by g-amino butyric acid (GABA) and/or by glutamate-glutamine (Glx) levels in the cerebellum of idiopathic PD patients, and healthy comparators (HC). Magnetic resonance spectroscopy of GABA/Glx (MEGA-PRESS acquisition sequence) was performed at 3 Tesla, and response inhibition assessed by the Stroop Word-Color Test (SWCT) and the Wisconsin Card Sorting Test (WCST). Linear correlations between cerebellar GABA/Glx levels, SWCT time/error interference effects and WCST perseverative errors were performed to test differences between correlation coefficients in PD and HC. Results showed that higher levels of mean cerebellar GABA were associated to SWCT increased time and error interference effects in PD, and the contrary in HC. Such effect dissociated by hemisphere, while correlation coefficients differences were significant in both right and left cerebellum. We conclude that MRS measured levels of cerebellar GABA are related in PD patients with decreased efficiency in filtering task-irrelevant information. This is crucial for developing pharmacological treatments for PD to potentially preserve cognitive functioning.

Transcranial Magnetic Stimulation and H1-Magnetic Resonance Spectroscopy Measures of Excitation and Inhibition Following Lorazepam Administration

This study aimed at better understanding the neurochemistry underlying transcranial magnetic stimulation (TMS) and magnetic resonance spectroscopy (MRS) measurements as it pertains to GABAergic activity following administration of allosteric GABA_A receptor agonist lorazepam. Seventeen healthy adults (8 females, 26.0 ± 5.4 years old) participated in a double-blind, crossover, placebo-controlled study, where participants underwent TMS and MRS two hours after drug intake (placebo or lorazepam; 2.5 mg). Neuronavigated TMS measures reflecting cortical inhibition and excitation were obtained in the left primary motor cortex. Sensorimotor cortex and occipital cortex MRS data were acquired using a 3T scanner with a MEGA-PRESS sequence, allowing water-referenced [GABA] and [Glx] (glutamate + glutamine) quantification. Lorazepam administration decreased occipital [GABA], decreased motor cortex excitability and increased GABA_A-receptor mediated motor cortex inhibition (short intracortical inhibition (SICI)). Lorazepam intake did not modulate sensorimotor [GABA] and TMS measures of intra-cortical facilitation, long-interval cortical inhibition, cortical silent period, and resting motor threshold. Furthermore, higher sensorimotor [GABA] was associated with higher cortical inhibition (SICI) following lorazepam administration, suggesting that baseline sensorimotor [GABA] may be valuable in predicting pharmacological or neuromodulatory treatment response. Finally, the differential effects of lorazepam on MRS and TMS measures, with respect to GABA, support the idea that TMS measures of cortical inhibition reflect synaptic GABAergic phasic inhibitory activity while MRS reflects extrasynaptic GABA.

GABA Modulates Frequency-Dependent Plasticity in Humans

Frequency-dependent reorganization of the primary somatosensory cortex, together with perceptual changes, arises following repetitive sensory stimulation. Here, we investigate the role of GABA in this process. We co-stimulated two finger tips and measured GABA and Glx using magnetic resonance (MR) spectroscopy at the beginning and end of the stimulation. Participants performed a perceptual learning task before and after stimulation. There were 2 sessions with stimulation frequency either at or above the resonance frequency of the primary somatosensory cortex (23 and 39 Hz, respectively). Perceptual learning occurred following above resonance stimulation only, while GABA reduced during this condition. Lower levels of early GABA were associated with greater perceptual learning. One possible mechanism underlying this finding is that cortical disinhibition “unmasks” lateral connections within the cortex to permit adaptation to the sensory environment. These results provide evidence in humans for a frequency-dependent inhibitory mechanism underlying learning and suggest a mechanism-based approach for optimizing neurostimulation frequency.

•

In the context of repetitive sensory stimulation, GABA release is frequency dependent

•

Stimulating above the resonance frequency of the somatosensory cortex reduces GABA

•

Perceptual learning is associated with a reduction in GABA

•

Early GABA reduction opens a window for plasticity and learning

Predicting Working Memory Capacity Based on Glutamatergic Concentration and its Modulation of Functional Connectivity

Working memory (WM) capacity, the amount of information one can hold online in mind, has a central role in cognition. Previous electrophysiological and imaging studies revealed the pivotal role of persistent activity within parietal and frontal regions as the neural foundations underpinning WM capacity. The best candidate molecules determining persistent activity are the brain's major excitatory and inhibitory neurotransmitters, glutamate and gamma-aminobutyric acid (GABA), respectively. However, our knowledge of these neurophysiological determinants in forming WM capacity is still poor. Using magnetic resonance spectroscopy (MRS), we examined the contribution of glutamate and GABA within the left intraparietal sulcus (IPS) and the left inferior/middle frontal gyrus (FG) in tracking WM capacity. A positive association was found between glutamate within the left IPS and WM capacity. By utilising resting-state functional MRI, we identified a negative association between parieto-cingulate connectivity and WM capacity. Individual variation in parieto-cingulate connectivity was explained by glutamatergic concentration in the IPS. Moreover, we found that parieto-cingulate connectivity mediated the relationship between interparietal sulcus glutamate and WM capacity. This set of findings reveals a novel mechanistic insight by which glutamatergic concentration within the IPS shapes WM capacity via parieto-cingulate connectivity.

Age-related GABAergic differences in the primary sensorimotor cortex: A multimodal approach combining PET, MRS and TMS

Healthy aging is associated with mechanistic changes in gamma-aminobutyric acid (GABA), the most abundant inhibitory neurotransmitter in the human brain. While previous work mainly focused on magnetic resonance spectroscopy (MRS)-based GABA+ levels and transcranial magnetic stimulation (TMS)-based GABA_A receptor (GABA_AR) activity in the primary sensorimotor (SM1) cortex, the aim of the current study was to identify age-related differences in positron emission tomography (PET)-based GABA_AR availability and its relationship with GABA+ levels (i.e. GABA with the contribution of macromolecules) and GABA_AR activity. For this purpose, fifteen young (aged 20–28 years) and fifteen older (aged 65–80 years) participants were recruited. PET and MRS images were acquired using simultaneous time-of-flight PET/MR to evaluate age-related differences in GABA_AR availability (distribution volume ratio with pons as reference region) and GABA+ levels. TMS was applied to identify age-related differences in GABA_AR activity by measuring short-interval intracortical inhibition (SICI). Whereas GABA_AR availability was significantly higher in the SM cortex of older as compared to young adults (18.5%), there were neither age-related differences in GABA+ levels nor SICI. A correlation analysis revealed no significant associations between GABA_AR availability, GABA_AR activity and GABA+ levels. Although the exact mechanisms need to be further elucidated, it is possible that a higher GABA_AR availability in older adults is a compensatory mechanism to ensure optimal inhibitory functionality during the aging process.

Brain GABA+ changes in primary hypothyroidism patients before and after levothyroxine treatment: A longitudinal magnetic resonance spectroscopy study

OBJECTIVE

Increasing evidence indicates the involvement of the GABAergic system in the pathophysiology of hypothyroidism. We aimed to investigate longitudinal changes of brain GABA in primary hypothyroidism before and after levothyroxine (L-T4) treatment.

MATERIAL AND METHODS

In 18 patients with hypothyroidism, we used the MEGA-PRESS (Mescher-Garwood point-resolved spectroscopy) editing sequence to measure brain GABA levels from medial prefrontal cortex (mPFC) and posterior cingulate cortex (PCC) at baseline and after 6-months of L-T4 treatment. Sex- and age-matched healthy controls (n = 18) were scanned at baseline. Thyroid function and neuropsychological tests were also performed.

RESULTS

GABA signals were successfully quantified from all participants with fitting errors lower than 15%. GABA signal was labeled as GABA+ due to contamination from co-edited macromoleculars and homocarnosine. In hypothyroid patients, mean GABA+ was significantly lower in the mPFC region compared with controls (p = 0.031), and the mPFC GABA+ measurements were significantly correlated with depressive symptoms and memory function (r = -0.558, p = 0.016; r = 0.522, p = 0.026, respectively). After adequate L-T4 treatment, the mPFC GABA+ in hypothyroid patients increased to normal level, along with relieved neuropsychological impairments.

CONCLUSION

The study suggested the decrease of GABA+ may be an important neurobiological factor in the pathophysiology of hypothyroidism. Treatment of L-T4 may reverse the abnormal GABA+ and hypothyroidism-induced neuropsychiatric impairments, indicating the action mode of L-T4 in adjunctive treatment of affective disorders.

The neurometabolic profiles of GABA and Glutamate as revealed by proton magnetic resonance spectroscopy in type 1 and type 2 diabetes

Glucose metabolism is pivotal for energy and neurotransmitter synthesis and homeostasis, particularly in Glutamate and GABA systems. In turn, the stringent control of inhibitory/excitatory tonus is known to be relevant in neuropsychiatric conditions. Glutamatergic neurotransmission dominates excitatory synaptic functions and is involved in plasticity and excitotoxicity. GABAergic neurochemistry underlies inhibition and predicts impaired psychophysical function in diabetes. It has also been associated with cognitive decline in people with diabetes. Still, the relation between metabolic homeostasis and neurotransmission remains elusive.

Two 3T proton MR spectroscopy studies were independently conducted in the occipital cortex to provide insight into inhibitory/excitatory homeostasis (GABA/Glutamate) and to evaluate the impact of chronic metabolic control on the levels and regulation (as assessed by regression slopes) of the two main neurotransmitters of the CNS in type 2 diabetes (T2DM) and type 1 diabetes (T1DM).

Compared to controls, participants with T2DM showed significantly lower Glutamate, and also GABA. Nevertheless, higher levels of GABA/Glx (Glutamate+Glutamine), and lower levels of Glutamate were associated with poor metabolic control in participants with T2DM. Importantly, the relationship between GABA/Glx and HbA_1c found in T2DM supports a relationship between inhibitory/excitatory balance and metabolic control. Interestingly, this neurometabolic profile was undetected in T1DM. In this condition we found strong evidence for alterations in MRS surrogate measures of neuroinflammation (myo-Inositol), positively related to chronic metabolic control.

Our results suggest a role for Glutamate as a global marker of T2DM and a sensitive marker of glycemic status. GABA/Glx may provide a signature of cortical metabolic state in poorly controlled patients as assessed by HbA_1c levels, which indicate long-term blood Glucose control. These findings are consistent with an interplay between abnormal neurotransmission and metabolic control in particular in type 2 diabetes thereby revealing dissimilar contributions to the pathophysiology of neural dysfunction in both types of diabetes.

Intracortical Circuits in the Contralesional Primary Motor Cortex in Patients With Chronic Stroke After Botulinum Toxin Type A Injection: Case Studies

Spasticity and motor recovery are both related to neural plasticity after stroke. A balance of activity in the primary motor cortex (M1) in both hemispheres is essential for functional recovery. In this study, we assessed the intracortical inhibitory and facilitatory circuits in the contralesional M1 area in four patients with severe upper limb spasticity after chronic stroke and treated with botulinum toxin-A (BoNT-A) injection and 12 weeks of upper limb rehabilitation. There was little to no change in the level of spasticity post-injection, and only one participant experienced a small improvement in arm function. All reported improvements in quality of life. However, the levels of intracortical inhibition and facilitation in the contralesional hemisphere were different at baseline for all four participants, and there was no clear pattern in the response to the intervention. Further investigation is needed to understand how BoNT-A injections affect inhibitory and facilitatory circuits in the contralesional hemisphere, the severity of spasticity, and functional improvement.

Transcranial magnetic stimulation and magnetic resonance spectroscopy: Opportunities for a bimodal approach in human neuroscience

Over the last decade, there has been an increasing number of studies combining transcranial magnetic stimulation (TMS) and magnetic resonance spectroscopy (MRS). MRS provides a manner to non-invasively investigate molecular concentrations in the living brain and thus identify metabolites involved in physiological and pathological processes. Particularly the MRS-detectable metabolites glutamate, the major excitatory neurotransmitter, and gamma-aminobutyric acid (GABA), the major inhibitory neurotransmitter, are of interest when combining TMS and MRS. TMS is a non-invasive brain stimulation technique that can be applied either as a neuromodulation or neurostimulation tool, specifically targeting glutamatergic and GABAergic mechanisms. The combination of TMS and MRS can be used to evaluate alterations in brain metabolite levels following an interventional TMS protocol such as repetitive TMS (rTMS) or paired associative stimulation (PAS). MRS can also be combined with a variety of non-interventional TMS protocols to identify the interplay between brain metabolite levels and measures of excitability or receptor-mediated inhibition and facilitation. In this review, we provide an overview of studies performed in healthy and patient populations combining MRS and TMS, both as a measurement tool and as an intervention. TMS and MRS may reveal complementary and comprehensive information on glutamatergic and GABAergic neurotransmission. Potentially, connectivity changes and dedicated network interactions can be probed using the combined TMS-MRS approach. Considering the ongoing technical developments in both fields, combined studies hold future promise for investigations of brain network interactions and neurotransmission.

The exploration of optimized protocol for repetitive transcranial magnetic stimulation in the treatment of methamphetamine use disorder: A randomized sham-controlled study

Background

The prefrontal-striatal circuit is a core circuit related to substance dependence. Previous studies have found that repetitive transcranial magnetic stimulation (rTMS) targeting the dorsolateral prefrontal cortex (DLPFC) (key region of executive network) had limited responses, while inhibiting hyperactivation of ventromedial prefrontal cortex (vmPFC) (key region of limbic network) may be another strategy. However, there is currently no comparison between these two treatment locations.

Methods

Seventy-four methamphetamine-dependent patients were randomly assigned to one of treatment groups with two-week treatment: (1) Group A: intermittent theta-burst stimulation (iTBS) targeting the left DLPFC; (2) Group B: continuous theta-burst stimulation (cTBS) targeting the left vmPFC; (3) Group C: a combination of treatment protocol of Group A and Group B; (4) Group D: sham theta-burst stimulation. The primary endpoint was the change of cue-induced craving. The trial was registered at ClinicalTrials.gov (NCT03736317).

Findings

The three real TBS groups had more craving decrease effect than the sham group (p<0.01). The changes of craving were positively correlated with the improvement of anxiety and withdrawal symptom. With the highest respondence rate, group C also had shorter respondence time than Group A (p = 0.03). Group C was effective in improve depression symptoms (p = 0.04) and withdrawal symptom (p = 0.02) compared with Group D. Besides, Group C was significant in improve sleep quality (p = 0.04) compared with Group A. Baseline depression scores and spatial working memory were positively predicting the intervention response.

Interpretation

The rTMS paradigms involving vmPFC with cTBS are optimized protocols and well-tolerated for methamphetamine-dependent individuals, and they may have better efficacies compared with DLPFC iTBS. Emotion and cognitive function are rTMS treatment response predictors for methamphetamine-dependent patients.

Funding

This work was supported by the National Key R&D Program of China (2017YFC1310400), National Natural Science Foundation of China (81,771,436, 81,801,319, 81,601,164), Shanghai Municipal Health and Family Planning Commission (2017ZZ02021), Municipal Human Resources Development Program for Outstanding Young Talents in Medical and Health Sciences in Shanghai (2017YQ013), Qihang Project of Shanghai Mental Health Center (2019-QH-05), Shanghai Sailing Program (19YF1442100), Shanghai Key Laboratory of Psychotic Disorders (13DZ2260500), Program of Shanghai Academic Research Leader (17XD1403300), Shanghai Municipal Science and Technology Major Project (2018SHZDZX05), and Shanghai Clinical Research Center for Mental Health (19MC1911100).

Chronic postnatal chemogenetic activation of forebrain excitatory neurons evokes persistent changes in mood behavior

Early adversity is a risk factor for the development of adult psychopathology. Common across multiple rodent models of early adversity is increased signaling via forebrain Gq-coupled neurotransmitter receptors. We addressed whether enhanced Gq-mediated signaling in forebrain excitatory neurons during postnatal life can evoke persistent mood-related behavioral changes. Excitatory hM3Dq DREADD-mediated chemogenetic activation of forebrain excitatory neurons during postnatal life (P2–14), but not in juvenile or adult windows, increased anxiety-, despair-, and schizophrenia-like behavior in adulthood. This was accompanied by an enhanced metabolic rate of cortical and hippocampal glutamatergic and GABAergic neurons. Furthermore, we observed reduced activity and plasticity-associated marker expression, and perturbed excitatory/inhibitory currents in the hippocampus. These results indicate that Gq-signaling-mediated activation of forebrain excitatory neurons during the critical postnatal window is sufficient to program altered mood-related behavior, as well as functional changes in forebrain glutamate and GABA systems, recapitulating aspects of the consequences of early adversity.

Stress and adversity in early childhood can have long-lasting effects, predisposing people to mental illness and mood disorders in adult life. The weeks immediately before and after birth are critical for establishing key networks of neurons in the brain. Therefore, any disruption to these neural circuits during this time can be detrimental to emotional development. However, it is still unclear which cellular mechanisms cause these lasting changes in behavior.

Studies in animals suggest that these long-term effects could result from abnormalities in a few signaling pathways in the brain. For example, it has been proposed that overstimulating the cells that activate circuits in the forebrain – also known as excitatory neurons – may contribute to the behavioral changes that persist into adulthood.

To test this theory, Pati et al. used genetic engineering to modulate a signaling pathway in male mice, which is known to stimulate excitatory neurons in the forebrain. The experiments showed that prolonged activation of excitatory neurons in the first two weeks after birth resulted in anxious and despair-like behaviors as the animals aged. The mice also displayed discrepancies in how they responded to certain external sensory information, which is a hallmark of schizophrenia-like behavior. However, engineering the same changes in adolescent and adult mice had no effect on their mood-related behaviors.

This animal study reinforces just how critical the first few weeks of life are for optimal brain development. It provides an insight into a possible mechanism of how disruption during this time could alter emotional behavior. The findings are also relevant to psychiatrists interested in the underlying causes of mental illness after early childhood adversity.

Baseline sensorimotor GABA levels shape neuroplastic processes induced by motor learning in older adults

Previous research in young adults has demonstrated that both motor learning and transcranial direct current stimulation (tDCS) trigger decreases in the levels of gamma-aminobutyric acid (GABA) in the sensorimotor cortex, and these decreases are linked to greater learning. Less is known about the role of GABA in motor learning in healthy older adults, a knowledge gap that is surprising given the established aging-related reductions in sensorimotor GABA. Here, we examined the effects of motor learning and subsequent tDCS on sensorimotor GABA levels and resting-state functional connectivity in the brains of healthy older participants. Thirty-six older men and women completed a motor sequence learning task before receiving anodal or sham tDCS to the sensorimotor cortex. GABA-edited magnetic resonance spectroscopy of the sensorimotor cortex and resting-state (RS) functional magnetic resonance imaging data were acquired before and after learning/stimulation. At the group level, neither learning nor anodal tDCS significantly modulated GABA levels or RS connectivity among task-relevant regions. However, changes in GABA levels from the baseline to post-learning session were significantly related to motor learning magnitude, age, and baseline GABA. Moreover, the change in functional connectivity between task-relevant regions, including bilateral motor cortices, was correlated with baseline GABA levels. These data collectively indicate that motor learning-related decreases in sensorimotor GABA levels and increases in functional connectivity are limited to those older adults with higher baseline GABA levels and who learn the most. Post-learning tDCS exerted no influence on GABA levels, functional connectivity or the relationships among these variables in older adults.

Fitness Level Influences White Matter Microstructure in Postmenopausal Women

Aerobic exercise has both neuroprotective and neurorehabilitative benefits. However, the underlying mechanisms are not fully understood and need to be investigated, especially in postmenopausal women, who are at increased risk of age-related disorders such as Alzheimer’s disease and stroke. To advance our understanding of the potential neurological benefits of aerobic exercise in aging women, we examined anatomical and functional responses that may differentiate women of varying cardiorespiratory fitness using neuroimaging and neurophysiology. A total of 35 healthy postmenopausal women were recruited (59 ± 3 years) and cardiorespiratory fitness estimated (22–70 mL/kg/min). Transcranial magnetic stimulation was used to assess -aminobutyric acid (GABA) and glutamate (Glu) receptor function in the primary motor cortex (M1), and magnetic resonance spectroscopy (MRS) was used to quantify GABA and Glu concentrations in M1. Magnetic resonance imaging was used to assess mean cortical thickness (MCT) of sensorimotor and frontal regions, while the microstructure of sensorimotor and other white matter tracts was evaluated through diffusion tensor imaging. Regression analysis revealed that higher fitness levels were associated with improved microstructure in pre-motor and sensory tracts, and the hippocampal cingulum. Fitness level was not associated with MCT, MRS, or neurophysiology measures. These data indicate that, in postmenopausal women, higher cardiorespiratory fitness is linked with preserved selective white matter microstructure, particularly in areas that influence sensorimotor control and memory.