Modulation of Inhibitory Activity Markers by Intermittent Theta-burst Stimulation in Rat Cortex is NMDA-receptor Dependent

Gamma oscillatory transcranial direct current stimulation of motor cortex enhances corticospinal excitability and brain connectivity in healthy individuals

Cortical excitability, the tendency of neurons to respond to various stimuli, is impaired in most neuropsychiatric conditions. Non-invasive brain stimulation can exert therapeutic effects by modulating the cortical excitability. Transcranial direct current stimulation (tDCS) and transcranial alternating current stimulation (tACS) have shown promise in various neuropsychiatric disorders, including improving cognitive abilities and motor function following stroke. Oscillatory transcranial direct current stimulation (otDCS), as a novel stimulation paradigm, combines tDCS and tACS to simultaneously regulate neuronal membrane potentials and oscillatory rhythms. This combination may produce more significant effects on neurons. To investigate this, participants received the following stimuli for 20 min on different days: (i) 2 mA 40 Hz otDCS, (ii) 2 mA 40 Hz tACS, (iii) 2 mA tDCS, and (iv) sham stimulation. Motor evoked potentials (MEPs) and transcranial magnetic stimulation combined with electroencephalography (TMS-EEG) were assessed both before and after stimulation. The increase in MEPs amplitudes was most pronounced under otDCS conditions compared with tACS and tDCS. Furthermore, analysis of TMS-EEG data revealed that changes in time-varying brain network patterns were most pronounced after otDCS, manifesting as enhanced brain-wide information connectivity. Our results indicate that gamma otDCS has significant potential for regulating cortical excitability and activating brain networks.

Acute Intermittent Theta-Burst Stimulation Produces Antidepressant-Like Effects by Modulating Neuronal Oscillations and Serotonin Levels of the Medial Prefrontal Cortex in Experimental Parkinson's Disease

Parkinson's disease (PD)-related depression is associated with aberrant neuronal oscillations and 5-hydroxytryptamine (5-HT) neurotransmission in the medial prefrontal cortex (mPFC). Intermittent theta-burst stimulation (iTBS), an updated pattern of high-frequency repetitive transcranial magnetic stimulation, has possible efficacy in PD-related depression. However, whether iTBS alleviates PD-related depression through modulating neuronal oscillations and 5-HT levels in the mPFC has not been determined. In this study, male Sprague-Dawley rats were used to establish a unilateral 6-hydroxydopamine-induced PD model. Then, acute iTBS was applied to the parkinsonian rats, and behavioral, neurochemical, and electrophysiological experiments were performed. We found that the parkinsonian rats exhibited increased immobility time and decreased sucrose preference accompanied by an increase of δ power and a decrease of θ power in the mPFC compared to sham-operated rats. One block of iTBS (1 block-iTBS, 300 stimuli) alleviated depressive-like behaviors in parkinsonian rats and elevated 5-HT levels in the mPFC compared to sham-iTBS. Additionally, it altered neuronal oscillations in the mPFC in the opposite fashion by suppressing the δ rhythm and enhancing the θ and β rhythms compared to sham-iTBS, suggesting that acute iTBS induces hyperactivity in the mPFC. With this iTBS paradigm, we also observed decreased parvalbumin expression in the mPFC, reflecting reduced cortical inhibition. Finally, correlation analyses showed strong correlation between immobility time and θ power after 1 block-iTBS. These findings suggest that the application of acute iTBS in parkinsonian rats produces antidepressant-like effects, which may be associated with elevated 5-HT levels and normalized neuronal oscillations in the mPFC.

The role of sex and age in the differential efficacy of 10 Hz and intermittent theta-burst (iTBS) repetitive transcranial magnetic stimulation (rTMS) treatment of major depressive disorder (MDD)

BACKGROUND

Sex- and age-dependent outcome differences have been observed in treatment of Major Depressive Disorder (MDD), including 10 Hz repetitive Transcranial Magnetic Stimulation (rTMS). We examined whether there are sex- and age-dependent differences in outcome with intermittent Theta Burst Stimulation (iTBS), another rTMS protocol.

METHODS

The relationship between biological sex, age, and treatment outcome was retrospectively examined among 414 patients with MDD treated with 10 Hz or iTBS rTMS. Linear mixed-effects modeling was used to examine the association between treatment and change in the 30-item Inventory of Depressive Symptomatology Self-Report (IDS-SR30) score from baseline to treatments 10 and 30, with biological sex (M/F), protocol (iTBS/10 Hz), age (≥/<50 years old), and time (treatment 1/10/30) included as fixed effects. The three-way sex-protocol-time and age-protocol-time interactions were used to determine any differential relationships between protocol and outcome dependent on sex and age. Post-hoc t-tests were conducted to examine differences in improvement.

RESULTS

There was a significant three-way sex-protocol-time interaction at treatments 10 (p = 0.016) and 30 (p = 0.031). Males showed significantly greater improvement with iTBS than females at treatments 10 (p = 0.041) and 30 (p = 0.035), while females showed numerically greater improvement with 10 Hz treatment. While there was not a significant three-way age-protocol-time interaction, there was a significant interaction between age (≥50 years old) and time at treatments 10 (p = 0.007) and 30 (p = 0.042), and among age, sex, and time at treatment 30 (p = 0.028).

LIMITATIONS

Retrospective naturalistic treatment protocol.

CONCLUSIONS

iTBS appeared less efficacious in females than in males, and rTMS overall was more efficacious in patients over fifty, particularly females.

Dose-Dependent Target Engagement of a Clinical Intermittent Theta Burst Stimulation Protocol: An Interleaved Transcranial Magnetic Stimulation-Functional Magnetic Resonance Imaging Study in Healthy People

BACKGROUND

Intermittent theta burst stimulation (iTBS) of the dorsolateral prefrontal cortex (DLPFC) is widely applied as a therapeutic intervention in mental health; however, the understanding of its mechanisms is still incomplete. Prior magnetic resonance imaging (MRI) studies have mainly used offline iTBS or short sequences in concurrent transcranial magnetic stimulation (TMS)-functional MRI (fMRI). This study investigated a full 600-stimuli iTBS protocol using interleaved TMS-fMRI in comparison with 2 control conditions in healthy subjects.

METHODS

In a crossover design, 18 participants underwent 3 sessions of interleaved iTBS-fMRI: 1) the left DLPFC at 40% resting motor threshold (rMT) intensity, 2) the left DLPFC at 80% rMT intensity, and 3) the left primary motor cortex (M1) at 80% rMT intensity. We compared immediate blood oxygen level-dependent (BOLD) responses during interleaved iTBS-fMRI across these conditions including correlations between individual fMRI BOLD activation and iTBS-induced electric field strength at the target sites.

RESULTS

Whole-brain analysis showed increased activation in several regions following iTBS. Specifically, the left DLPFC, as well as the bilateral M1, anterior cingulate cortex, and insula, showed increased activation during 80% rMT left DLPFC stimulation. Increased BOLD activity in the left DLPFC was observed with neither 40% rMT left DLPFC stimulation nor left M1 80% rMT iTBS, whereas activation in other regions was found to overlap between conditions. Of note, BOLD activation and electric field intensities were only correlated for M1 stimulation and not for the DLPFC conditions.

CONCLUSIONS

This interleaved TMS-fMRI study showed dosage- and target-specific BOLD activation during a 600-stimuli iTBS protocol in healthy individuals. Future studies may use our approach for investigating target engagement in clinical samples.

Neuromodulatory Responses Elicited by Intermittent versus Continuous Transcranial Focused Ultrasound Stimulation of the Motor Cortex in Rats

Transcranial focused ultrasound stimulation (tFUS) has emerged as a promising neuromodulation technique that delivers acoustic energy with high spatial resolution for inducing long-term potentiation (LTP)- or depression (LTD)-like plasticity. The variability in the primary effects of tFUS-induced plasticity could be due to different stimulation patterns, such as intermittent versus continuous, and is an aspect that requires further detailed exploration. In this study, we developed a platform to evaluate the neuromodulatory effects of intermittent and continuous tFUS on motor cortical plasticity before and after tFUS application. Three groups of rats were exposed to either intermittent, continuous, or sham tFUS. We analyzed the neuromodulatory effects on motor cortical excitability by examining changes in motor-evoked potentials (MEPs) elicited by transcranial magnetic stimulation (TMS). We also investigated the effects of different stimulation patterns on excitatory and inhibitory neural biomarkers, examining c-Fos and glutamic acid decarboxylase (GAD-65) expression using immunohistochemistry staining. Additionally, we evaluated the safety of tFUS by analyzing glial fibrillary acidic protein (GFAP) expression. The current results indicated that intermittent tFUS produced a facilitation effect on motor excitability, while continuous tFUS significantly inhibited motor excitability. Furthermore, neither tFUS approach caused injury to the stimulation sites in rats. Immunohistochemistry staining revealed increased c-Fos and decreased GAD-65 expression following intermittent tFUS. Conversely, continuous tFUS downregulated c-Fos and upregulated GAD-65 expression. In conclusion, our findings demonstrate that both intermittent and continuous tFUS effectively modulate cortical excitability. The neuromodulatory effects may result from the activation or deactivation of cortical neurons following tFUS intervention. These effects are considered safe and well-tolerated, highlighting the potential for using different patterns of tFUS in future clinical neuromodulatory applications.

Long-term intermittent theta burst stimulation enhanced hippocampus-dependent memory by regulating hippocampal theta oscillation and neurotransmitter levels in healthy rats

Intermittent theta burst stimulation (iTBS), an updated pattern of high-frequency repetitive transcranial magnetic stimulation, is a potential candidate for improving memory. The hippocampus has been shown to be involved in the memory-enhancing effect induced by iTBS. However, it remains largely unknown whether this effect is achieved by regulating hippocampal theta oscillation and neurotransmitters gamma-aminobutyric acid (GABA) and glutamate, which are strongly related to memory. Thus, we investigated the effect of 14 days of iTBS on hippocampus-dependent memory and further explored the roles of hippocampal theta oscillation and neurotransmitters GABA and glutamate in this effect. We found that compared to sham iTBS, real iTBS enhanced hippocampus-dependent memory measured by hole-board test and object place recognition test. Further, real iTBS increased the density of c-Fos positive neurons and normalized power of theta oscillation in the dorsal hippocampus (dHip) compared to sham iTBS. Interestingly, we observed a decrease in the level of extracellular GABA and an increase in the level of extracellular glutamate in the dHip after real iTBS. Our results suggest that long-term iTBS improved hippocampus-dependent memory, which may be attributed to the enhancement of theta oscillation and altered levels of extracellular GABA and glutamate in the dHip.

Magnetic Stimulation as a Therapeutic Approach for Brain Modulation and Repair: Underlying Molecular and Cellular Mechanisms

Neurological and psychiatric diseases generally have no cure, so innovative non-pharmacological treatments, including non-invasive brain stimulation, are interesting therapeutic tools as they aim to trigger intrinsic neural repair mechanisms. A common brain stimulation technique involves the application of pulsed magnetic fields to affected brain regions. However, investigations of magnetic brain stimulation are complicated by the use of many different stimulation parameters. Magnetic brain stimulation is usually divided into two poorly connected approaches: (1) clinically used high-intensity stimulation (0.5-2 Tesla, T) and (2) experimental or epidemiologically studied low-intensity stimulation (μT-mT). Human tests of both approaches are reported to have beneficial outcomes, but the underlying biology is unclear, and thus optimal stimulation parameters remain ill defined. Here, we aim to bring together what is known about the biology of magnetic brain stimulation from human, animal, and in vitro studies. We identify the common effects of different stimulation protocols; show how different types of pulsed magnetic fields interact with nervous tissue; and describe cellular mechanisms underlying their effects-from intracellular signalling cascades, through synaptic plasticity and the modulation of network activity, to long-term structural changes in neural circuits. Recent advances in magneto-biology show clear mechanisms that may explain low-intensity stimulation effects in the brain. With its large breadth of stimulation parameters, not available to high-intensity stimulation, low-intensity focal magnetic stimulation becomes a potentially powerful treatment tool for human application.

Pretreatment pupillary reactivity is associated with differential early response to 10 Hz and intermittent theta-burst repetitive transcranial magnetic stimulation (rTMS) treatment of major depressive disorder (MDD)

BACKGROUND

Repetitive Transcranial Magnetic Stimulation (rTMS) is an effective treatment for Major Depressive Disorder (MDD). Two common rTMS protocols, 10 Hz and intermittent theta burst stimulation (iTBS), have comparable rates of efficacy in groups of patients. Recent evidence suggests that some individuals may be more likely to benefit from one form of stimulation than the other. The pretreatment pupillary light reflex (PLR) is significantly associated with response to a full course of rTMS using heterogeneous stimulation protocols.

OBJECTIVE

To test whether the relationship between pretreatment PLR and early symptom improvement differed between subjects treated with iTBS or 10 Hz stimulation.

METHODS

PLR was measured in 52 subjects who received solely 10 Hz (n = 35) or iTBS (n = 17) to left dorsolateral prefrontal cortex (DLPFC) for the first ten sessions of their treatment course. Primary outcome measure was the percent change of Inventory of Depressive Symptomatology - Self Report (IDS-SR) from session 1 to session 10.

RESULTS

There was a positive association between normalized maximum constriction velocity (nMCV) and early improvement in subjects receiving 10 Hz stimulation (R = 0.48, p = 0.004) and a negative association in subjects receiving iTBS (R = -0.52, p = 0.03). ANOVA revealed a significant interaction between nMCV and the type of initial stimulation (p = 0.001). Among subjects with low nMCV, those initially treated with iTBS showed 2.6 times greater improvement after 10 sessions (p = 0.01) than subjects initially receiving 10 Hz stimulation.

CONCLUSION

nMCV may detect physiologic differences between those likely to benefit from 10 Hz or iTBS treatment. Future studies should examine whether PLR could guide prospective treatment selection.

Intermittent Theta Burst Stimulation Improves Motor and Behavioral Dysfunction through Modulation of NMDA Receptor Subunit Composition in Experimental Model of Parkinson's Disease

Parkinson's disease (PD) is the second most common neurodegenerative disorder characterized by the progressive degeneration of the dopaminergic system, leading to a variety of motor and nonmotor symptoms. The currently available symptomatic therapy loses efficacy over time, indicating the need for new therapeutic approaches. Repetitive transcranial magnetic stimulation (rTMS) has emerged as one of the potential candidates for PD therapy. Intermittent theta burst stimulation (iTBS), an excitatory protocol of rTMS, has been shown to be beneficial in several animal models of neurodegeneration, including PD. The aim of this study was to investigate the effects of prolonged iTBS on motor performance and behavior and the possible association with changes in the NMDAR subunit composition in the 6-hydroxydopamine (6-OHDA)-induced experimental model of PD. Two-month-old male Wistar rats were divided into four groups: controls, 6-OHDA rats, 6-OHDA + iTBS protocol (two times/day/three weeks) and the sham group. The therapeutic effect of iTBS was evaluated by examining motor coordination, balance, spontaneous forelimb use, exploratory behavior, anxiety-like, depressive/anhedonic-like behavior and short-term memory, histopathological changes and changes at the molecular level. We demonstrated the positive effects of iTBS at both motor and behavioral levels. In addition, the beneficial effects were reflected in reduced degeneration of dopaminergic neurons and a subsequent increase in the level of DA in the caudoputamen. Finally, iTBS altered protein expression and NMDAR subunit composition, suggesting a sustained effect. Applied early in the disease course, the iTBS protocol may be a promising candidate for early-stage PD therapy, affecting motor and nonmotor deficits.

Repetitive Transcranial Magnetic Stimulation of the Brain After Ischemic Stroke: Mechanisms from Animal Models

Stroke is a common cerebrovascular disease with high morbidity, mortality, and disability worldwide. Post-stroke dysfunction is related to the death of neurons and impairment of synaptic structure, which results from cerebral ischemic damage. Currently, transcranial magnetic stimulation (TMS) techniques are available to provide clinically effective interventions and quantitative diagnostic and prognostic biomarkers. The development of TMS has been 40 years and a range of repetitive TMS (rTMS) protocols are now available to regulate neuronal plasticity in many neurological disorders, such as stroke, Parkinson disease, psychiatric disorders, Alzheimer disease, and so on. Basic studies in an animal model with ischemic stroke are significant for demonstrating potential mechanisms of neural restoration induced by rTMS. In this review, the mechanisms were summarized, involving synaptic plasticity, neural cell death, neurogenesis, immune response, and blood-brain barrier (BBB) disruption in vitro and vivo experiments with ischemic stroke models. Those findings can contribute to the understanding of how rTMS modulated function recovery and the exploration of novel therapeutic targets. The mechanisms of rTMS in treating ischemic stroke from animal models. rTMS can prompt synaptic plasticity by increasing NMDAR, AMPAR and BDNF expression; rTMS can inhibit pro-inflammatory cytokines TNF and facilitate the expression of anti-inflammatory cytokines IL-10 by shifting astrocytic phenotypes from A1 to A2, and shifting microglial phenotypes from M1 to M2; rTMS facilitated the release of angiogenesis-related factors TGFβ and VEGF in A2 astrocytes, which can contribute to vasculogenesis and angiogenesis; rTMS can suppress apoptosis by increasing Bcl-2 expression and inhibiting Bax, caspase-3 expression; rTMS can also suppress pyroptosis by decreasing caspase-1, IL-1β, ASC, GSDMD and NLRP1 expression. rTMS, repetitive transcranial magnetic stimulation; NMDAR, N-methyl-D-aspartic acid receptors; AMPAR: α-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors; BDNF, brain-derived neurotrophic factor; VEGF, vascular endothelial growth factor; GSDMD: cleaved Caspase-1 cleaves Gasdermin D; CBF: cerebral blood flow.

Dose and time-dependence of acute intermittent theta-burst stimulation on hippocampus-dependent memory in parkinsonian rats

Background

The treatment options for cognitive impairments in Parkinson's disease (PD) are limited. Repetitive transcranial magnetic stimulation has been applied in various neurological diseases. However, the effect of intermittent theta-burst stimulation (iTBS) as a more developed repetitive transcranial magnetic stimulation paradigm on cognitive dysfunction in PD remains largely unclear.

Objective

Our aim was to explore the effect of acute iTBS on hippocampus-dependent memory in PD and the mechanism underlying it.

Methods

Different blocks of iTBS protocols were applied to unilateral 6-hydroxidopamine-induced parkinsonian rats followed by the behavioral, electrophysiological and immunohistochemical analyses. The object-place recognition and hole-board test were used to assess hippocampus-dependent memory.

Results

Sham-iTBS and 1 block-iTBS (300 stimuli) didn't alter hippocampus-dependent memory, hippocampal theta rhythm and the density of c-Fos- and parvalbumin-positive neurons in the hippocampus and medial septum. 3 block-iTBS (900 stimuli) alleviated 6-hydroxidopamine-induced memory impairments, and increased the density of hippocampal c-Fos-positive neurons at 80 min post-stimulation but not 30 min compared to sham-iTBS. Interestingly, 3 block-iTBS first decreased and then increased normalized theta power during a period of 2 h following stimulation. Moreover, 3 block-iTBS decreased the density of parvalbumin-positive neurons in the medial septum at 30 min post-stimulation compared to sham-iTBS.

Conclusion

The results indicate that multiple blocks of iTBS elicit dose and time-dependent effects on hippocampus-dependent memory in PD, which may be attributed to changes in c-Fos expression and the power of theta rhythm in the hippocampus.

Low intensity repetitive transcranial magnetic stimulation modulates brain-wide functional connectivity to promote anti-correlated c-Fos expression

Repetitive transcranial magnetic stimulation (rTMS) induces action potentials to induce plastic changes in the brain with increasing evidence for the therapeutic importance of brain-wide functional network effects of rTMS; however, the influence of sub-action potential threshold (low-intensity; LI-) rTMS on neuronal activity is largely unknown. We investigated whether LI-rTMS modulates neuronal activity and functional connectivity and also specifically assessed modulation of parvalbumin interneuron activity. We conducted a brain-wide analysis of c-Fos, a marker for neuronal activity, in mice that received LI-rTMS to visual cortex. Mice received single or multiple sessions of excitatory 10 Hz LI-rTMS with custom rodent coils or were sham controls. We assessed changes to c-Fos positive cell densities and c-Fos/parvalbumin co-expression. Peak c-Fos expression corresponded with activity during rTMS. We also assessed functional connectivity changes using brain-wide c-Fos-based network analysis. LI-rTMS modulated c-Fos expression in cortical and subcortical regions. c-Fos density changes were most prevalent with acute stimulation, however chronic stimulation decreased parvalbumin interneuron activity, most prominently in the amygdala and striatum. LI-rTMS also increased anti-correlated functional connectivity, with the most prominent effects also in the amygdala and striatum following chronic stimulation. LI-rTMS induces changes in c-Fos expression that suggest modulation of neuronal activity and functional connectivity throughout the brain. Our results suggest that LI-rTMS promotes anticorrelated functional connectivity, possibly due to decreased parvalbumin interneuron activation induced by chronic stimulation. These changes may underpin therapeutic rTMS effects, therefore modulation of subcortical activity supports rTMS for treatment of disorders involving subcortical dysregulation.

Effect of whole-hand water flow stimulation on the neural balance between excitation and inhibition in the primary somatosensory cortex

Sustained peripheral somatosensory stimulations, such as high-frequency repetitive somatosensory stimulation (HF-RSS) and vibrated stimulation, are effective in altering the balance between excitation and inhibition in the somatosensory cortex (S1) and motor cortex (M1). A recent study reported that whole-hand water flow (WF) stimulation induced neural disinhibition in the M1. Based on previous results, we hypothesized that whole-hand WF stimulation would lead to neural disinhibition in the S1 because there is a strong neural connection between M1 and S1 and aimed to examine whether whole-hand WF stimulation would change the neural balance between excitation and inhibition in the S1. Nineteen healthy volunteers were studied by measuring excitation and inhibition in the S1 before and after each of the four 15-min interventions. The excitation and inhibition in the S1 were assessed using somatosensory evoked potentials (SEPs) and paired-pulse inhibition (PPI) induced by single- and paired-pulse stimulations, respectively. The four interventions were as follows: control, whole-hand water immersion, whole-hand WF, and HF-RSS. The results showed no significant changes in SEPs and PPI following any intervention. However, changes in PPI with an interstimulus interval (ISI) of 30 ms were significantly correlated with the baseline value before whole-hand WF. Thus, the present findings indicated that the whole-hand WF stimulation had a greater decreased neural inhibition in participants with higher neural inhibition in the S1 at baseline. Considering previous results on M1, the present results possibly show that S1 has lower plasticity than M1 and that the duration (15 min) of each intervention may not have been enough to alter the balance of excitation and inhibition in the S1.

La stimulation magnétique répétée pour le traitement des traumas spinaux

Les traumas spinaux induisent des déficits moteurs et sensoriels. La mise au point de thérapies visant à rétablir les fonctions altérées à la suite d’une lésion de la moelle épinière est donc nécessaire. La stimulation magnétique répétée (SMr) est une thérapie innovante et non invasive utilisée pour moduler l’activité de réseaux neuronaux dans diverses maladies neurologiques, telles que la maladie de Parkinson, ou psychiatriques, telles que le trouble bipolaire. Son utilisation chez les personnes atteintes de traumas spinaux pourrait avoir des effets fonctionnels bénéfiques. Des études réalisées in vitro, in vivo et ex vivo ont permis de comprendre en partie les mécanismes sous-jacents à la modulation de l’activité neuronale induite par les protocoles de SMr. Son utilisation dans des modèles précliniques de lésion médullaire a de plus montré des effets bénéfiques fonctionnels. Ainsi, la SMr pourrait potentialiser la récupération des fonctions perdues après un trauma spinal.

The Impact of Theta-Burst Stimulation on Cortical GABA and Glutamate in Treatment-Resistant Depression: A Surface-Based MRSI Analysis Approach

Background: Theta burst stimulation (TBS) belongs to one of the biological antidepressant treatment options. When applied bilaterally, excitatory intermittent TBS (iTBS) is commonly targeted to the left and inhibitory continuous TBS (cTBS) to the right dorsolateral prefrontal cortex. TBS was shown to influence neurotransmitter systems, while iTBS is thought to interfere with glutamatergic circuits and cTBS to mediate GABAergic neurotransmission. Objectives: We aimed to expand insights into the therapeutic effects of TBS on the GABAergic and glutamatergic system utilizing 3D-multivoxel magnetic resonance spectroscopy imaging (MRSI) in combination with a novel surface-based MRSI analysis approach to investigate changes of cortical neurotransmitter levels in patients with treatment-resistant depression (TRD). Methods: Twelve TRD patients (five females, mean age ± SD = 35 ± 11 years) completed paired MRSI measurements, using a GABA-edited 3D-multivoxel MEGA-LASER sequence, before and after 3 weeks of bilateral TBS treatment. Changes in cortical distributions of GABA+/tNAA (GABA+macromolecules relative to total N-acetylaspartate) and Glx/tNAA (Glx = mixed signal of glutamate and glutamine), were investigated in a surface-based region-of-interest (ROI) analysis approach. Results: ANCOVAs revealed a significant increase in Glx/tNAA ratios in the left caudal middle frontal area (p corr. = 0.046, F = 13.292), an area targeted by iTBS treatment. Whereas, contralateral treatment with cTBS evoked no alterations in glutamate or GABA concentrations. Conclusion: This study demonstrates surface-based adaptions in the stimulation area to the glutamate metabolism after excitatory iTBS but not after cTBS, using a novel surface-based analysis of 3D-MRSI data. The reported impact of facilitatory iTBS on glutamatergic neurotransmission provides further insight into the neurobiological effects of TBS in TRD.

Transcranial magnetic stimulation as a tool to induce and explore plasticity in humans

Activity-dependent synaptic plasticity is the main theoretical framework to explain mechanisms of learning and memory. Synaptic plasticity can be explored experimentally in animals through various standardized protocols for eliciting long-term potentiation and long-term depression in hippocampal and cortical slices. In humans, several non-invasive protocols of repetitive transcranial magnetic stimulation and transcranial direct current stimulation have been designed and applied to probe synaptic plasticity in the primary motor cortex, as reflected by long-term changes in motor evoked potential amplitudes. These protocols mimic those normally used in animal studies for assessing long-term potentiation and long-term depression. In this chapter, we first discuss the physiologic basis of theta-burst stimulation, paired associative stimulation, and transcranial direct current stimulation. We describe the current biophysical and theoretical models underlying the molecular mechanisms of synaptic plasticity and metaplasticity, defined as activity-dependent changes in neural functions that modulate subsequent synaptic plasticity such as long-term potentiation (LTP) and long-term depression (LTD), in the human motor cortex including calcium-dependent plasticity, spike-timing-dependent plasticity, the role of N-methyl-d-aspartate-related transmission and gamma-aminobutyric-acid interneuronal activity. We also review the putative microcircuits responsible for synaptic plasticity in the human motor cortex. We critically readdress the issue of variability in studies investigating synaptic plasticity and propose available solutions. Finally, we speculate about the utility of future studies with more advanced experimental approaches.

Role of the frontal eye field in human microsaccade responses: A TMS study

Microsaccade is a type of fixational eye movements that is modulated by various sensory and cognitive processes, and impact our visual perception. Although studies in monkeys have demonstrated a functional role for the superior colliculus and frontal eye field (FEF) in controlling microsaccades, our understanding of the neural mechanisms underlying the generation of microsaccades is still limited. By applying continuous theta-burst stimulation (cTBS) over the right FEF and the vertex, we investigated the role of the FEF in generating human microsaccade responses evoked by salient stimuli or by changes in background luminance. We observed higher microsaccade rates prior to target appearance, and larger rebound in microsaccade occurrence following salient stimuli, when disruptive cTBS was applied over FEF compared to vertex stimulation. Moreover, the microsaccade direction modulation after changes in background luminance was disrupted with FEF stimulation. Together, our results constitute the first evidence of FEF modulation in human microsaccade responses.

Excitatory Repetitive Transcranial Magnetic Stimulation Over Prefrontal Cortex in a Guinea Pig Model Ameliorates Tinnitus

Tinnitus, a phantom auditory perception that can seriously affect quality of life, is generally triggered by cochlear trauma and associated with aberrant activity throughout the auditory pathways, often referred to as hyperactivity. Studies suggest that non-auditory structures, such as prefrontal cortex (PFC), may be involved in tinnitus generation, by affecting sensory gating in auditory thalamus, allowing hyperactivity to reach the cortex and lead to perception. Indeed, human studies have shown that repetitive transcranial magnetic stimulation (rTMS) of PFC can alleviate tinnitus. The current study investigated whether this therapeutic effect is achieved through inhibition of thalamic hyperactivity, comparing effects of two common clinical rTMS protocols with sham treatment, in a guinea pig tinnitus model. Animals underwent acoustic trauma and once tinnitus developed were treated with either intermittent theta burst stimulation (iTBS), 20 Hz rTMS, or sham rTMS (10 days, 10 min/day; weekdays only). Tinnitus was reassessed and extracellular recordings of spontaneous tonic and burst firing rates in auditory thalamus made. To verify effects in PFC, densities of neurons positive for calcium-binding proteins, calbindin and parvalbumin, were investigated using immunohistochemistry. Both rTMS protocols significantly reduced tinnitus compared to sham. However, spontaneous tonic firing decreased following 20 Hz stimulation and increased following iTBS in auditory thalamus. Burst rate was significantly different between 20 Hz and iTBS stimulation, and burst duration was increased only after 20 Hz treatment. Density of calbindin, but not parvalbumin positive neurons, was significantly increased in the most dorsal region of PFC indicating that rTMS directly affected PFC. Our results support the involvement of PFC in tinnitus modulation, and the therapeutic benefit of rTMS on PFC in treating tinnitus, but indicate this is not achieved solely by suppression of thalamic hyperactivity.

Transcranial Magnetic Stimulation-Induced Plasticity Mechanisms: TMS-Related Gene Expression and Morphology Changes in a Human Neuron-Like Cell Model

Transcranial Magnetic Stimulation (TMS) is a form of non-invasive brain stimulation, used to alter cortical excitability both in research and clinical applications. The intermittent and continuous Theta Burst Stimulation (iTBS and cTBS) protocols have been shown to induce opposite after-effects on human cortex excitability. Animal studies have implicated synaptic plasticity mechanisms long-term potentiation (LTP, for iTBS) and depression (LTD, for cTBS). However, the neural basis of TMS effects has not yet been studied in human neuronal cells, in particular at the level of gene expression and synaptogenesis. To investigate responses to TBS in living human neurons, we differentiated human SH-SY5Y cells toward a mature neural phenotype, and stimulated them with iTBS, cTBS, or sham (placebo) TBS. Changes in (a) mRNA expression of a set of target genes (previously associated with synaptic plasticity), and (b) morphological parameters of neurite outgrowth following TBS were quantified. We found no general effects of stimulation condition or time on gene expression, though we did observe a significantly enhanced expression of plasticity genes NTRK2 and MAPK9 24 h after iTBS as compared to sham TBS. This specific effect provides unique support for the widely assumed plasticity mechanisms underlying iTBS effects on human cortex excitability. In addition to this protocol-specific increase in plasticity gene expression 24 h after iTBS stimulation, we establish the feasibility of stimulating living human neuron with TBS, and the importance of moving to more complex human in vitro models to understand the underlying plasticity mechanisms of TBS stimulation.

Probing rapid network reorganization of motor and language functions via neuromodulation and neuroimaging

Motor and cognitive functions are organized in large-scale networks in the human brain that interact to enable flexible adaptation of information exchange to ever-changing environmental conditions. In this review, we discuss the unique potential of the consecutive combination of repetitive transcranial magnetic stimulation (rTMS) and functional neuroimaging to probe network organization and reorganization in the healthy and lesioned brain. First, we summarize findings highlighting the flexible (re-)distribution and short-term reorganization in motor and cognitive networks in the healthy brain. Plastic after-effects of rTMS result in large-scale changes on the network level affecting both local and remote activity within the stimulated network as well as interactions between the stimulated and distinct functional networks. While the number of combined rTMS-fMRI studies in patients with brain lesions remains scarce, preliminary evidence suggests that the lesioned brain flexibly (re-)distributes its computational capacities to functionally reorganize impaired brain functions, using a similar set of mechanisms to achieve adaptive network plasticity compared to short-term reorganization observed in the healthy brain after rTMS. In general, both short-term reorganization in the healthy brain and stroke-induced reorganization seem to rely on three general mechanisms of adaptive network plasticity that allow to maintain and recover function: i) interhemispheric changes, including increased contribution of homologous regions in the contralateral hemisphere and increased interhemispheric connectivity, ii) increased interactions between differentially specialized networks and iii) increased contributions of domain-general networks after disruption of more specific functions. These mechanisms may allow for computational flexibility of large-scale neural networks underlying motor and cognitive functions. Future studies should use complementary approaches to address the functional relevance of adaptive network plasticity and further delineate how these general mechanisms interact to enable network flexibility. Besides furthering our neurophysiological insights into brain network interactions, identifying approaches to support and enhance adaptive network plasticity may result in clinically relevant diagnostic and treatment approaches.

Low-intensity transcranial magnetic stimulation promotes the survival and maturation of newborn oligodendrocytes in the adult mouse brain

Neuronal activity is a potent extrinsic regulator of oligodendrocyte generation and central nervous system myelination. Clinically, repetitive transcranial magnetic stimulation (rTMS) is delivered to noninvasively modulate neuronal activity; however, the ability of rTMS to facilitate adaptive myelination has not been explored. By performing cre‐lox lineage tracing, to follow the fate of oligodendrocyte progenitor cells in the adult mouse brain, we determined that low intensity rTMS (LI‐rTMS), administered as an intermittent theta burst stimulation, but not as a continuous theta burst or 10 Hz stimulation, increased the number of newborn oligodendrocytes in the adult mouse cortex. LI‐rTMS did not alter oligodendrogenesis per se, but instead increased cell survival and enhanced myelination. These data suggest that LI‐rTMS can be used to noninvasively promote myelin addition to the brain, which has potential implications for the treatment of demyelinating diseases such as multiple sclerosis.

Critical role of glutamatergic and GABAergic neurotransmission in the central mechanisms of theta-burst stimulation

Theta-burst stimulation (TBS) is a varied form of repetitive transcranial magnetic stimulation (rTMS) and has more rapid and powerful effects than rTMS. Experiments on the human motor cortex have demonstrated that intermittent TBS has facilitatory effects, whereas continuous TBS has inhibitory effects. Huang's simplified model provides a solid basis for elucidating such after-effects. However, evidence increasingly indicates that not all after-effects of TBS are as expected, and high variability among individuals has been observed. Studies have suggested that the GABAergic and glutamatergic neurotransmission play a vital role in the aforementioned after-effects, which might explain the interindividual differences in these after-effects. Herein, we reviewed the latest findings on TBS from animal and human experiments on glutamatergic and GABAergic neurotransmissions in response to TBS. Furthermore, an updated theoretical model integrating glutamatergic and GABAergic neurotransmissions is proposed.

Nil effects of μ-rhythm phase-dependent burst-rTMS on cortical excitability in humans: A resting-state EEG and TMS-EEG study

Repetitive transcranial magnetic stimulation (rTMS) can induce excitability changes of a stimulated brain area through synaptic plasticity mechanisms. High-frequency (100 Hz) triplets of rTMS synchronized to the negative but not the positive peak of the ongoing sensorimotor μ-rhythm isolated with the concurrently acquired electroencephalography (EEG) resulted in a reproducible long-term potentiation like increase of motor evoked potential (MEP) amplitude, an index of corticospinal excitability (Zrenner et al. 2018, Brain Stimul 11:374–389). Here, we analyzed the EEG and TMS-EEG data from (Zrenner et al., 2018) to investigate the effects of μ-rhythm-phase-dependent burst-rTMS on EEG-based measures of cortical excitability. We used resting-state EEG to assess μ- and β-power in the motor cortex ipsi- and contralateral to the stimulation, and single-pulse TMS-evoked and induced EEG responses in the stimulated motor cortex. We found that μ-rhythm-phase-dependent burst-rTMS did not significantly change any of these EEG measures, despite the presence of a significant differential and reproducible effect on MEP amplitude. We conclude that EEG measures of cortical excitability do not reflect corticospinal excitability as measured by MEP amplitude. Most likely this is explained by the fact that rTMS induces complex changes at the molecular and synaptic level towards both excitation and inhibition that cannot be differentiated at the macroscopic level by EEG.

Low-intensity repetitive transcranial magnetic stimulation requires concurrent visual system activity to modulate visual evoked potentials in adult mice

Repetitive transcranial stimulation (rTMS) is an increasingly popular method to non-invasively modulate cortical excitability in research and clinical settings. During rTMS, low-intensity magnetic fields reach areas perifocal to the target brain region, however, effects of these low-intensity (LI-) fields and how they interact with ongoing neural activity remains poorly defined. We evaluated whether coordinated neural activity during electromagnetic stimulation alters LI-rTMS effects on cortical excitability by comparing visually evoked potentials (VEP) and densities of parvalbumin-expressing (PV+) GABAergic interneurons in adult mouse visual cortex after LI-rTMS under different conditions: LI-rTMS applied during visually evoked (strong, coordinated) activity or in darkness (weak, spontaneous activity).We also compared response to LI-rTMS in wildtype and ephrin-A2A5^−/− mice, which have visuotopic anomalies thought to disrupt coherence of visually-evoked cortical activity. Demonstrating that LI-rTMS effects in V1 require concurrent sensory-evoked activity, LI-rTMS delivered during visually-evoked activity increased PV+ immunoreactivity in both genotypes; however, VEP peak amplitudes changed only in wildtypes, consistent with intracortical disinhibition. We show, for the first time, that neural activity and the degree of coordination in cortical population activity interact with LI-rTMS to alter excitability in a context-dependent manner.

Low intensity repetitive transcranial magnetic stimulation modulates skilled motor learning in adult mice

Repetitive transcranial magnetic stimulation (rTMS) is commonly used to modulate cortical plasticity in clinical and non-clinical populations. Clinically, rTMS is delivered to targeted regions of the cortex at high intensities (>1 T). We have previously shown that even at low intensities, rTMS induces structural and molecular plasticity in the rodent cortex. To determine whether low intensity rTMS (LI-rTMS) alters behavioural performance, daily intermittent theta burst LI-rTMS (120 mT) or sham was delivered as a priming or consolidating stimulus to mice completing 10 consecutive days of skilled reaching training. Relative to sham, priming LI-rTMS (before each training session), increased skill accuracy (~9%) but did not alter the rate of learning over time. In contrast, consolidating LI-rTMS (after each training session), resulted in a small increase in the rate of learning (an additional ~1.6% each day) but did not alter the daily skill accuracy. Changes in behaviour with LI-rTMS were not accompanied with long lasting changes in brain-derived neurotrophic factor (BDNF) expression or in the expression of plasticity markers at excitatory and inhibitory synapses for either priming or consolidation groups. These results suggest that LI-rTMS can alter specific aspects of skilled motor learning in a manner dependent on the timing of intervention.

Humans with Type-2 Diabetes Show Abnormal Long-Term Potentiation-Like Cortical Plasticity Associated with Verbal Learning Deficits

BACKGROUND

Type-2 diabetes mellitus (T2DM) accelerates cognitive aging and increases risk of Alzheimer's disease. Rodent models of T2DM show altered synaptic plasticity associated with reduced learning and memory. Humans with T2DM also show cognitive deficits, including reduced learning and memory, but the relationship of these impairments to the efficacy of neuroplastic mechanisms has never been assessed.

OBJECTIVE

Our primary objective was to compare mechanisms of cortical plasticity in humans with and without T2DM. Our secondary objective was to relate plasticity measures to standard measures of cognition.

METHODS

A prospective cross-sectional cohort study was conducted on 21 adults with T2DM and 15 demographically-similar non-diabetic controls. Long-term potentiation-like plasticity was assessed in primary motor cortex by comparing the amplitude of motor evoked potentials (MEPs) from single-pulse transcranial magnetic stimulation before and after intermittent theta-burst stimulation (iTBS). Plasticity measures were compared between groups and related to neuropsychological scores.

RESULTS

In T2DM, iTBS-induced modulation of MEPs was significantly less than controls, even after controlling for potential confounds. Furthermore, in T2DM, modulation of MEPs 10-min post-iTBS was significantly correlated with Rey Auditory Verbal Learning Task (RAVLT) performance.

CONCLUSION

Humans with T2DM show abnormal cortico-motor plasticity that is correlated with reduced verbal learning. Since iTBS after-effects and the RAVLT are both NMDA receptor-dependent measures, their relationship in T2DM may reflect brain-wide alterations in the efficacy of NMDA receptors. These findings offer novel mechanistic insights into the brain consequences of T2DM and provide a reliable means to monitor brain health and evaluate the efficacy of clinical interventions.

Can neurophysiologic measures serve as biomarkers for the efficacy of repetitive transcranial magnetic stimulation treatment of major depressive disorder?

Repetitive transcranial magnetic stimulation (rTMS) is an effective treatment for Major Depressive Disorder (MDD). There are clinical data that support the efficacy of many different approaches to rTMS treatment, and it remains unclear what combination of stimulation parameters is optimal to relieve depressive symptoms. Because of the costs and complexity of studies that would be necessary to explore and compare the large number of combinations of rTMS treatment parameters, it would be useful to establish reliable surrogate biomarkers of treatment efficacy that could be used to compare different approaches to treatment. This study reviews the evidence that neurophysiologic measures of cortical excitability could be used as biomarkers for screening different rTMS treatment paradigms. It examines evidence that: (1) changes in excitability are related to the mechanism of action of rTMS; (2) rTMS has consistent effects on measures of excitability that could constitute reliable biomarkers; and (3) changes in excitability are related to the outcomes of rTMS treatment of MDD. An increasing body of evidence indicates that these neurophysiologic measures have the potential to serve as reliable biomarkers for screening different approaches to rTMS treatment of MDD.

Effect of antipsychotic pharmacotherapy on clinical outcomes of intermittent theta-burst stimulation for refractory depression

Theta-burst stimulation is an emerging protocol for repetitive transcranial magnetic stimulation that takes 1-3 min to administer, yet offers equal/superior potency to conventional protocols lasting 30-60 min. However, preclinical evidence suggests that D2 receptor blockade may abolish the acute effects of theta-burst stimulation on synaptic facilitation or inhibition. As many patients presenting for repetitive transcranial magnetic stimulation are taking antipsychotic medications as augmentation for treatment-resistant depression, this finding is potentially concerning for the implementation of theta-burst stimulation in clinical settings. Here, we examined whether treatment-resistant depression patients taking antipsychotics have worse outcomes after a course of intermittent theta-burst stimulation. A chart review identified 105 treatment-resistant depression patients who underwent dorsomedial prefrontal-intermittent theta-burst stimulation; clinical outcomes on Hamilton Depression Rating Scale and Beck Depression Inventory were compared for those taking and not taking antipsychotics. The 29 of 105 patients who were taking antipsychotics showed non-significantly better response and remission rates, and non-significantly larger percentage improvements on both scales, with a positive but non-significant correlation between higher antipsychotic dose and larger percentage improvement. Contrary to expectations, outcomes were not significantly worse, and in some analyses trended towards being better, in patients taking antipsychotics. Future randomized controlled studies of repetitive transcranial magnetic stimulation combined with standardized dopaminergic manipulations may be justified and warranted.

Transcranial Magnetic and Direct Current Stimulation in Children

Promising results in adult neurologic and psychiatric disorders are driving active research into transcranial brain stimulation techniques, particularly transcranial magnetic stimulation (TMS) and transcranial direct current stimulation (tDCS), in childhood and adolescent syndromes. TMS has realistic utility as an experimental tool tested in a range of pediatric neuropathologies such as perinatal stroke, depression, Tourette syndrome, and autism spectrum disorder (ASD). tDCS has also been tested as a treatment for a number of pediatric neurologic conditions, including ASD, attention-deficit/hyperactivity disorder, epilepsy, and cerebral palsy. Here, we complement recent reviews with an update of published TMS and tDCS results in children, and discuss developmental neuroscience considerations that should inform pediatric transcranial stimulation.

Releasing the Cortical Brake by Non-Invasive Electromagnetic Stimulation? rTMS Induces LTD of GABAergic Neurotransmission

Repetitive Transcranial Magnetic Stimulation (rTMS) is a non-invasive brain stimulation technique which modulates cortical excitability beyond the stimulation period. However, despite its clinical use rTMS-based therapies which prevent or reduce disabilities in a functionally significant and sustained manner are scarce. It remains unclear how rTMS-mediated changes in cortical excitability, which are not task- or input-specific, exert beneficial effects in some healthy subjects and patients. While experimental evidence exists that repetitive magnetic stimulation (rMS) is linked to the induction of long-term potentiation (LTP) of excitatory neurotransmission, less attention has been dedicated to rTMS-induced structural, functional and molecular adaptations at inhibitory synapses. In this review article we provide a concise overview on basic neuroscience research, which reveals an important role of local disinhibitory networks in promoting associative learning and memory. These studies suggest that a reduction in inhibitory neurotransmission facilitates the expression of associative plasticity in cortical networks under physiological conditions. Hence, it is interesting to speculate that rTMS may act by decreasing GABAergic neurotransmission onto cortical principal neurons. Indeed, evidence has been provided that rTMS is capable of modulating inhibitory networks. Consistent with this suggestion recent basic science work discloses that a 10 Hz rTMS protocol reduces GABAergic synaptic strength on principal neurons. These findings support a model in which rTMS-induced long-term depression (LTD) of GABAergic synaptic strength mediates changes in excitation/inhibition-balance of cortical networks, which may in turn facilitate (or restore) the ability of stimulated networks to express input- and task-specific associative synaptic plasticity.

Differences in Motor Evoked Potentials Induced in Rats by Transcranial Magnetic Stimulation under Two Separate Anesthetics: Implications for Plasticity Studies

Repetitive transcranial magnetic stimulation (rTMS) is primarily used in humans to change the state of corticospinal excitability. To assess the efficacy of different rTMS stimulation protocols, motor evoked potentials (MEPs) are used as a readout due to their non-invasive nature. Stimulation of the motor cortex produces a response in a targeted muscle, and the amplitude of this twitch provides an indirect measure of the current state of the cortex. When applied to the motor cortex, rTMS can alter MEP amplitude, however, results are variable between participants and across studies. In addition, the mechanisms underlying any change and its locus are poorly understood. In order to better understand these effects, MEPs have been investigated in vivo in animal models, primarily in rats. One major difference in protocols between rats and humans is the use of general anesthesia in animal experiments. Anesthetics are known to affect plasticity-like mechanisms and so may contaminate the effects of an rTMS protocol. In the present study, we explored the effect of anesthetic on MEP amplitude, recorded before and after intermittent theta burst stimulation (iTBS), a patterned rTMS protocol with reported facilitatory effects. MEPs were assessed in the brachioradialis muscle of the upper forelimb under two anesthetics: a xylazine/zoletil combination and urethane. We found MEPs could be induced under both anesthetics, with no differences in the resting motor threshold or the average baseline amplitudes. However, MEPs were highly variable between animals under both anesthetics, with the xylazine/zoletil combination showing higher variability and most prominently a rise in amplitude across the baseline recording period. Interestingly, application of iTBS did not facilitate MEP amplitude under either anesthetic condition. Although it is important to underpin human application of TMS with mechanistic examination of effects in animals, caution must be taken when selecting an anesthetic and in interpreting results during prolonged TMS recording.

Low-intensity repetitive magnetic stimulation lowers action potential threshold and increases spike firing in layer 5 pyramidal neurons in vitro

Repetitive transcranial magnetic stimulation (rTMS) has become a popular method of modulating neural plasticity in humans. Clinically, rTMS is delivered at high intensities to modulate neuronal excitability. While the high-intensity magnetic field can be targeted to stimulate specific cortical regions, areas adjacent to the targeted area receive stimulation at a lower intensity and may contribute to the overall plasticity induced by rTMS. We have previously shown that low-intensity rTMS induces molecular and structural plasticity in vivo, but the effects on membrane properties and neural excitability have not been investigated. Here we investigated the acute effect of low-intensity repetitive magnetic stimulation (LI-rMS) on neuronal excitability and potential changes on the passive and active electrophysiological properties of layer 5 pyramidal neurons in vitro. Whole-cell current clamp recordings were made at baseline prior to subthreshold LI-rMS (600 pulses of iTBS, n=9 cells from 7 animals) or sham (n=10 cells from 9 animals), immediately after stimulation, as well as 10 and 20min post-stimulation. Our results show that LI-rMS does not alter passive membrane properties (resting membrane potential and input resistance) but hyperpolarises action potential threshold and increases evoked spike-firing frequency. Increases in spike firing frequency were present throughout the 20min post-stimulation whereas action potential (AP) threshold hyperpolarization was present immediately after stimulation and at 20min post-stimulation. These results provide evidence that LI-rMS alters neuronal excitability of excitatory neurons. We suggest that regions outside the targeted region of high-intensity rTMS are susceptible to neuromodulation and may contribute to rTMS-induced plasticity.

Repetitive Transcranial Magnetic Stimulation of the Brain: Mechanisms from Animal and Experimental Models

Since the development of transcranial magnetic stimulation (TMS) in the early 1980s, a range of repetitive TMS (rTMS) protocols are now available to modulate neuronal plasticity in clinical and non-clinical populations. However, despite the wide application of rTMS in humans, the mechanisms underlying rTMS-induced plasticity remain uncertain. Animal and in vitro models provide an adjunct method of investigating potential synaptic and non-synaptic mechanisms of rTMS-induced plasticity. This review summarizes in vitro experimental studies, in vivo studies with intact rodents, and preclinical models of selected neurological disorders-Parkinson's disease, depression, and stroke. We suggest that these basic research findings can contribute to the understanding of how rTMS-induced plasticity can be modulated, including novel mechanisms such as neuroprotection and neurogenesis that have significant therapeutic potential.

Construction and Evaluation of Rodent-Specific rTMS Coils

Rodent models of transcranial magnetic stimulation (TMS) play a crucial role in aiding the understanding of the cellular and molecular mechanisms underlying TMS induced plasticity. Rodent-specific TMS have previously been used to deliver focal stimulation at the cost of stimulus intensity (12 mT). Here we describe two novel TMS coils designed to deliver repetitive TMS (rTMS) at greater stimulation intensities whilst maintaining spatial resolution. Two circular coils (8 mm outer diameter) were constructed with either an air or pure iron-core. Peak magnetic field strength for the air and iron-cores were 90 and 120 mT, respectively, with the iron-core coil exhibiting less focality. Coil temperature and magnetic field stability for the two coils undergoing rTMS, were similar at 1 Hz but varied at 10 Hz. Finite element modeling of 10 Hz rTMS with the iron-core in a simplified rat brain model suggests a peak electric field of 85 and 12.7 V/m, within the skull and the brain, respectively. Delivering 10 Hz rTMS to the motor cortex of anaesthetized rats with the iron-core coil significantly increased motor evoked potential amplitudes immediately after stimulation (n = 4). Our results suggest these novel coils generate modest magnetic and electric fields, capable of altering cortical excitability and provide an alternative method to investigate the mechanisms underlying rTMS-induced plasticity in an experimental setting.

Metaplasticity in human primary somatosensory cortex: effects on physiology and tactile perception

Theta-burst stimulation (TBS) over human primary motor cortex evokes plasticity and metaplasticity, the latter contributing to the homeostatic balance of excitation and inhibition. Our knowledge of TBS-induced effects on primary somatosensory cortex (SI) is limited, and it is unknown whether TBS induces metaplasticity within human SI. Sixteen right-handed participants (6 females, mean age 23 yr) received two TBS protocols [continuous TBS (cTBS) and intermittent TBS (iTBS)] delivered in six different combinations over SI in separate sessions. TBS protocols were delivered at 30 Hz and were as follows: a single cTBS protocol, a single iTBS protocol, cTBS followed by cTBS, iTBS followed by iTBS, cTBS followed by iTBS, and iTBS followed by cTBS. Measures included the amplitudes of the first and second somatosensory evoked potentials (SEPs) via median nerve stimulation, their paired-pulse ratio (PPR), and temporal order judgment (TOJ). Dependent measures were obtained before TBS and at 5, 25, 50, and 90 min following stimulation. Results indicate similar effects following cTBS and iTBS; increased amplitudes of the second SEP and PPR without amplitude changes to SEP 1, and impairments in TOJ. Metaplasticity was observed such that TOJ impairments following a single cTBS protocol were abolished following consecutive cTBS protocols. Additionally, consecutive iTBS protocols altered the time course of effects when compared with a single iTBS protocol. In conclusion, 30-Hz cTBS and iTBS protocols delivered in isolation induce effects consistent with a TBS-induced reduction in intracortical inhibition within SI. Furthermore, cTBS- and iTBS-induced metaplasticity appear to follow homeostatic and nonhomeostatic rules, respectively.