Occlusion of bidirectional plasticity by preceding low-frequency stimulation in the human motor cortex

Exercise-induced cortical disinhibition mediates the relationship between fitness and memory in older adults

Regular exercise benefits learning and memory in older adults, but the neural mechanisms mediating these effects remain unclear. Evidence in young adults indicates that acute exercise creates a favourable environment for synaptic plasticity by enhancing cortical disinhibition. As such, we investigated whether plasticity-related disinhibition mediated the relationship between cardiorespiratory fitness and memory function in healthy older adults (n = 16, mean age = 66.06). Participants completed a graded maximal exercise test and assessments of visual and verbal memory, followed by two counterbalanced sessions involving 20 min of either high-intensity interval training exercise or rest. Disinhibition was measured following intermittent theta burst stimulation via paired-pulse transcranial magnetic stimulation. In line with our hypotheses, we observed a positive correlation between cardiorespiratory fitness and verbal memory, which was mediated by plasticity-related cortical disinhibition. Our novel finding implicates cortical disinhibition as a mechanism through which the effects of acute bouts of exercise may translate to improved memory in older adults. This finding extends current understanding of the physiological mechanisms underlying the positive influence of cardiorespiratory fitness for memory function in older adults, and further highlights the importance of promoting exercise engagement to maintain cognitive health in later life. KEY POINTS: There are well established benefits of regular exercise for memory function in older adults, but the mechanisms are unclear. Cortical disinhibition is important for laying down new memories, and is enhanced following acute exercise in young adults, suggesting it is a potential mechanism underlying these benefits in ageing. Older adults completed a fitness test and assessments of memory, followed by two sessions involving either 20 min of exercise or rest. Disinhibition was measured following intermittent theta burst stimulation via paired-pulse transcranial magnetic stimulation. Cardiorespiratory fitness was positively associated with memory performance. Higher fitness was associated with enhanced cortical disinhibition following acute exercise. Cortical disinhibition completely mediated the relationship between fitness and memory. This novel finding provides a mechanistic account for the positive influence of cardiorespiratory fitness on memory in later life, and emphasises the importance of regular exercise for cognitive health in older populations.

Can we manipulate brain connectivity? A systematic review of cortico-cortical paired associative stimulation effects

OBJECTIVE

Cortico-cortical paired associative stimulation (ccPAS) is a form of dual-site transcranial magnetic stimulation (TMS) entailing a series of single-TMS pulses paired at specific interstimulus intervals (ISI) delivered to distant cortical areas. The goal of this article is to systematically review its efficacy in inducing plasticity in humans focusing on stimulation parameters and hypotheses of underlying neurophysiology.

METHODS

A systematic review of the literature from 2009-2023 was undertaken to identify all articles utilizing ccPAS to study brain plasticity and connectivity. Six electronic databases were searched and included.

RESULTS

32 studies were identified. The studies targeted connections within the same hemisphere or between hemispheres. 28 ccPAS studies were in healthy participants, 1 study in schizophrenia, and 1 in Alzheimer's disease (AD) patients. 2 additional studies used cortico-cortical repetitive paired associative stimulation (cc-rPAS) in generalized anxiety disorder (GAD) patients. Outcome measures include electromyography (EMG), behavioral measures, electroencephalography (EEG), and functional magnetic resonance imaging (fMRI). ccPAS seems to be able to modulate brain connectivity depending on the ISI.

CONCLUSIONS

ccPAS can be used to modulate corticospinal excitability, brain activity, and behavior. Although the stimulation parameters used across studies reviewed in this paper are varied, ccPAS is a promising approach for basic research and potential clinical applications.

SIGNIFICANCE

Recent advances in neuroscience have caused a shift of interest from the study of single areas to a more complex approach focusing on networks of areas that orchestrate brain activity. Consequently, the TMS community is also witnessing a change, with a growing interest in targeting multiple brain areas rather than a single locus, as evidenced by an increasing number of papers using ccPAS. In light of this new enthusiasm for brain connectivity, this review summarizes existing literature and stimulation parameters that have proven effective in changing electrophysiological, behavioral, or neuroimaging-derived measures.

Improvement of synaptic plasticity and cognitive function in RASopathies-a monocentre, randomized, double-blind, parallel-group, placebo-controlled, cross-over clinical trial (SynCoRAS)

BACKGROUND

Cognitive impairment is a common medical issue in rat sarcoma (RAS) pathway disorders, so-called RASopathies, like Neurofibromatosis type 1 (NF1) or Noonan syndrome (NS). It is presumed to be caused by impaired synaptic plasticity. In animal studies, pathway-specific pharmacological interventions with lovastatin (LOV) and lamotrigine (LTG) have been shown to improve synaptic plasticity as well as cognitive function. The aim of this clinical trial is to translate the findings of animal studies to humans and to probe the effect of lovastatin (NS) and lamotrigine (NS and NF1) on synaptic plasticity and cognitive function/alertness in RASopathies.

METHODS

Within this phase IIa, monocentre, randomized, double-blind, parallel-group, placebo-controlled, cross-over clinical trial (syn. SynCoRAS), three approaches (approaches I-III) will be carried out. In patients with NS, the effect of LTG (approach I) and of LOV (approach II) is investigated on synaptic plasticity and alertness. LTG is tested in patients with NF1 (approach III). Trial participants receive a single dose of 300 mg LTG or placebo (I and III) and 200 mg LOV or placebo (II) daily for 4 days with a cross-over after at least 7 days. Synaptic plasticity is investigated using a repetitive high-frequency transcranial magnetic stimulation (TMS) protocol called quadri-pulse theta burst stimulation (qTBS). Attention is examined by using the test of attentional performance (TAP). Twenty-eight patients are randomized in groups NS and NF1 with n = 24 intended to reach the primary endpoint (change in synaptic plasticity). Secondary endpoints are attention (TAP) and differences in short interval cortical inhibition (SICI) between placebo and trial medication (LTG and LOV).

DISCUSSION

The study is targeting impairments in synaptic plasticity and cognitive impairment, one of the main health problems of patients with RASopathies. Recent first results with LOV in patients with NF1 have shown an improvement in synaptic plasticity and cognition. Within this clinical trial, it is investigated if these findings can be transferred to patients with NS. LTG is most likely a more effective and promising substance improving synaptic plasticity and, consecutively, cognitive function. It is expected that both substances are improving synaptic plasticity as well as alertness. Changes in alertness may be a precondition for improvement of cognition.

TRIAL REGISTRATION

The clinical trial is registered in ClinicalTrials.gov (NCT03504501; https://www.

CLINICALTRIALS

gov ; date of registration: 04/11/2018) and in EudraCT (number 2016-005022-10).

Effects of Paired Associative Stimulation on Cortical Plasticity in Agonist–Antagonist Muscle Representations

Paired associative stimulation (PAS) increases and decreases cortical excitability in primary motor cortex (M1) neurons, depending on the spike timing-dependent plasticity, i.e., long-term potentiation (LTP)- and long-term depression (LTD)-like plasticity, respectively. However, how PAS affects the cortical circuits for the agonist and antagonist muscles of M1 is unclear. Here, we investigated the changes in the LTP- and LTD-like plasticity for agonist and antagonist muscles during PAS: 200 pairs of 0.25-Hz peripheral electric stimulation of the right median nerve at the wrist, followed by a transcranial magnetic stimulation of the left M1 with an interstimulus interval of 25 ms (PAS-25 ms) and 10 ms (PAS-10 ms). The unconditioned motor evoked potential amplitudes of the agonist muscles were larger after PAS-25 ms than after PAS-10 ms, while those of the antagonist muscles were smaller after PAS-25 ms than after PAS-10 ms. The γ-aminobutyric acid A (GABAA)- and GABAB-mediated cortical inhibition for the agonist and antagonist muscles were higher after PAS-25 ms than after PAS-10 ms. The cortical excitability for the agonist and antagonist muscles reciprocally and topographically increased and decreased after PAS, respectively; however, GABAA and GABAB-mediated cortical inhibitory functions for the agonist and antagonist muscles were less topographically decreased after PAS-10 ms. Thus, PAS-25 ms and PAS-10 ms differentially affect the LTP- and LTD-like plasticity in agonist and antagonist muscles.

No robust online effects of transcranial direct current stimulation on corticospinal excitability

Transcranial direct current stimulation (tDCS) has been used for over twenty years to modulate cortical (particularly motor corticospinal) excitability both during (online) and outlasting (offline) the stimulation, with the former effects associated to the latter. However, tDCS effects are highly variable, partially because stimulation intensity is commonly not adjusted individually (in contrast to transcranial magnetic stimulation, TMS). In Experiment 1, we therefore explored an empirical approach of personalizing tDCS intensity for the primary motor cortex (M1) based on dose-response curves (DRCs), individually relating tDCS Intensity (in steps from 0.3 to 2.0 mA) and Polarity (anodal, cathodal) to the online modulation of concurrent TMS motor evoked potentials (MEP), assessing DRC reliability across two separate days. No robust DRCs could be observed, neither at the individual nor at the group level, with the only robust effect being a (paradoxical) MEP facilitation during cathodal tDCS at 2.0 mA, but no modulation at traditional intensities of or near 1 mA. In Experiment 2, we therefore attempted to replicate the classical bidirectional online MEP modulation during 1 mA tDCS that had been reported by several of the early seminal tDCS papers. We either closely recreated stimulation parameters and temporal protocol of these original studies (Experiment 2A) or slightly modernized them according to current standards (Experiment 2B). In neither experiment did we observed any significant online MEP modulation. We conclude that an empirical titration of individually effective tDCS intensities may not be feasible as online tDCS effects do not appear to be sufficiently robust.

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.

Protocols for inducing homeostatic plasticity reflected in the corticospinal excitability in healthy human participants: A systematic review and meta-analysis

Homeostatic plasticity complements synaptic plasticity by stabilising neural activity within a physiological range. In humans, homeostatic plasticity is investigated using two blocks of non-invasive brain stimulation (NIBS) with an interval without stimulation between blocks. The aim of this systematic review and meta-analysis was to investigate the effect of homeostatic plasticity induction protocols on motor evoked potentials (MEP) in healthy participants. Four databases were searched (Medline, Scopus, Embase and Cochrane library). Studies describing the application of two blocks of NIBS of the primary motor cortex with an interval of no stimulation between blocks reporting changes in corticospinal excitability by MEP amplitude were included. Thirty-seven reports with 55 experiments (700 participants) were included. Study quality was considered poor overall, with heterogeneity in study size, sample and designs. Two blocks of excitatory stimulation at the primary motor cortex produced a homeostatic response (decreased MEP) between 0 and 30 min post-protocols, when compared with a single stimulation block. Two blocks of inhibitory stimulation at the primary motor cortex using interval duration of 10 min or less produced a homeostatic response (increased MEP) between 0 and 30 min post-protocols, when compared with a single stimulation block. There were no differences in MEPs when compared with baseline MEPs. In conclusion, homeostatic plasticity induction using two blocks of NIBS with an interval of 10 min or less without stimulation between blocks produces a homeostatic response up to 30 min post-protocol. Improvements in participant selection, sample sizes and protocols of NIBS techniques are needed.

Double-Sine-Wave Quadri-Pulse Theta Burst Stimulation of Precentral Motor Hand Representation Induces Bidirectional Changes in Corticomotor Excitability

Neuronal plasticity is considered to be the neurophysiological correlate of learning and memory and changes in corticospinal excitability play a key role in the normal development of the central nervous system as well as in developmental disorders. In a previous study, it was shown that quadri-pulse theta burst stimulation (qTBS) can induce bidirectional changes in corticospinal excitability (1). There, a quadruple burst consisted of four single-sine-wave (SSW) pulses with a duration of 160 μs and inter-pulse intervals of 1.5 ms to match I-wave periodicity (666 Hz). In the present study, the pulse shape was modified applying double-sine-waves (DSW) rather than SSW pulses, while keeping the pulse duration at 160 μs. In two separate sessions, we reversed the current direction of the DSW pulse, so that its second component elicited either a mainly posterior-to-anterior (DSW PA-qTBS) or anterior-to-posterior (DSW AP-qTBS) directed current in the precentral gyrus. The after-effects of DSW qTBS on corticospinal excitability were examined in healthy individuals (n = 10) with single SSW TMS pulses. For single-pulse SSW TMS, the second component produced the same preferential current direction as DSW qTBS but had a suprathreshold intensity, thus eliciting motor evoked potentials (PA-MEP or AP-MEP). Single-pulse SSW TMS revealed bidirectional changes in corticospinal excitability after DSW qTBS, which depended on the preferentially induced current direction. DSW PA-qTBS at 666 Hz caused a stable increase in PA-MEP, whereas AP-qTBS at 666 Hz induced a transient decrease in AP-MEP. The sign of excitability following DSW qTBS at I-wave periodicity was opposite to the bidirectional changes after SSW qTBS. The results show that the pulse configuration and induced current direction determine the plasticity-effects of ultra-high frequency SSW and DSW qTBS at I-wave periodicity. These findings may offer new opportunities for short non-invasive brain stimulation protocols that are especially suited for stimulation in children and patients with neurological or neurodevelopmental disorders.

A framework to assess the impact of number of trials on the amplitude of motor evoked potentials

The amplitude of motor evoked potentials (MEPs) elicited by transcranial magnetic stimulation (TMS) is a common yet highly variable measure of corticospinal excitability. The tradeoff between maximizing the number of trials and minimizing experimental time remains a hurdle. It is therefore important to establish how many trials should be used. The aim of this study is not to provide rule-of-thumb answers that may be valid only in specific experimental conditions, but to offer a more general framework to inform the decision about how many trials to use under different experimental conditions. Specifically, we present a set of equations that show how the number of trials affects single-subject MEP amplitude, population MEP amplitude, hypothesis testing and test-retest reliability, depending on the variability within and between subjects. The equations are derived analytically, validated with Monte Carlo simulations, and representatively applied to experimental data. Our findings show that the minimum number of trials for estimating single-subject MEP amplitude largely depends on the experimental conditions and on the error considered acceptable by the experimenter. Conversely, estimating population MEP amplitude and hypothesis testing are markedly more dependent on the number of subjects than on the number of trials. These tools and results help to clarify the impact of the number of trials in the design and reproducibility of past and future experiments.

Indices of cortical plasticity after therapeutic sleep deprivation in patients with major depressive disorder

BACKGROUND

Therapeutic sleep deprivation (SD) presents a unique paradigm to study the neurobiology of major depressive disorder (MDD). However, the rapid antidepressant mechanism, which differs from today's slow first-line treatments, is not sufficiently understood. We recently integrated two prominent hypotheses of MDD and sleep, the synaptic plasticity hypothesis of MDD and the synaptic homeostasis hypothesis of sleep-wake regulation, into a synaptic plasticity model of therapeutic SD in MDD. Here, we further tested this model positing that homeostatically elevating net synaptic strength through therapeutic SD shifts the initially deficient inducibility of associative synaptic long-term potentiation (LTP)-like plasticity in patients with MDD into a more favorable window of associative plasticity.

METHODS

We used paired associative stimulation (PAS), a transcranial magnetic stimulation protocol (TMS), to quantify cortical LTP-like plasticity after one night of therapeutic sleep deprivation in 28 patients with MDD.

RESULTS

We demonstrate a significantly different inducibility of associative plasticity in clinical responders to therapeutic SD (> 50% improvement on the 6-item Hamilton-Rating-Scale for Depression, n=13) compared to non-responders (n=15), which was driven by a long-term depression (LTD)-like response in SD-non-responders. Indices of global net synaptic strength (wake EEG theta activity, intracortical inhibition and BDNF serum levels) were increased after SD in both groups, with responders showing a generally lower intracortical inhibition than non-responders.

LIMITATIONS

Repetitive assessments prior to and after treatment would be needed to further determine potential mechanisms.

CONCLUSION

After a night of therapeutic SD, clinical responders show a significantly higher inducibility of associative LTP-like plasticity than non-responders.

The human pain system exhibits higher-order plasticity (metaplasticity)

The human pain system can be bidirectionally modulated by high-frequency (HFS; 100 Hz) and low-frequency (LFS; 1 Hz) electrical stimulation of nociceptors leading to long-term potentiation or depression of pain perception (pain-LTP or pain-LTD). Here we show that priming a test site by very low-frequency stimulation (VLFS; 0.05 Hz) prevented pain-LTP probably by elevating the threshold (set point) for pain-LTP induction. Conversely, prior HFS-induced pain-LTP was substantially reversed by subsequent VLFS, suggesting that preceding HFS had primed the human nociceptive system for pain-LTD induction by VLFS. In contrast, the pain elicited by the pain-LTP-precipitating conditioning HFS stimulation remained unaffected. In aggregate these experiments demonstrate that the human pain system expresses two forms of higher-order plasticity (metaplasticity) acting in either direction along the pain-LTD to pain-LTP continuum with similar shifts in thresholds for LTD and LTP as in synaptic plasticity, indicating intriguing new mechanisms for the prevention of pain memory and the erasure of hyperalgesia related to an already established pain memory trace. There were no apparent gender differences in either pain-LTP or metaplasticity of pain-LTP. However, individual subjects appeared to present with an individual balance of pain-LTD to pain-LTP (a pain plasticity "fingerprint").

How different priming stimulations affect the corticospinal excitability induced by noninvasive brain stimulation techniques: a systematic review and meta-analysis

Noninvasive brain stimulation (NIBS) techniques could induce changes in corticospinal excitability (CSE) and neuroplasticity. These changes could be affected by different factors, including having a session of stimulation called the ‘priming’ protocol before the main stimulation session called the ‘test’ protocol. Literature indicates that a priming protocol could affect the activity of postsynaptic neurons, form a neuronal history, and then modify the expected effects of the test protocol on CSE indicated by the amplitude of transcranial magnetic stimulation-induced motor-evoked potentials. This prior history affects a threshold to activate the necessary mechanism stabilizing the neuronal activity within a useful dynamic range. For studying the effects of this history and related metaplasticity mechanisms in the human primary motor cortex (M1), priming-test protocols are successfully employed. Thirty-two studies were included in this review to investigate how different priming protocols could affect the induced effects of a test protocol on CSE in healthy individuals. The results showed that if the history of synaptic activity were high or low enough to displace the threshold, the expected effects of the test protocol would be the reverse. This effect reversal is regulated by homeostatic mechanisms. On the contrary, the effects of the test protocol would not be the reverse, and at most we experience a prolongation of the lasting effects if the aforementioned history is not enough to displace the threshold. This effect prolongation is mediated by nonhomeostatic mechanisms. Therefore, based on the characteristics of priming-test protocols and the interval between them, the expected results of priming-test protocols would be different. Moreover, these findings could shed light on the different mechanisms of metaplasticity involved in NIBS. It helps us understand how we can improve the expected outcomes of these techniques in clinical approaches.

Metaplasticity: A Promising Tool to Disentangle Chronic Disorders of Consciousness Differential Diagnosis

The extent of cortical reorganization after brain injury in patients with Vegetative State/Unresponsive Wakefulness Syndrome (UWS) and Minimally Conscious State (MCS) depends on the residual capability of modulating synaptic plasticity. Neuroplasticity is largely abnormal in patients with UWS, although the fragments of cortical activity may exist, while patients MCS show a better cortical organization. The aim of this study was to evaluate cortical excitability in patients with disorders of consciousness (DoC) using a transcranial direct current stimulation (TDCS) metaplasticity protocol. To this end, we tested motor-evoked potential (MEP) amplitude, short intracortical inhibition (SICI), and intracortical facilitation (ICF). These measures were correlated with the level of consciousness (by the Coma Recovery Scale-Revised, CRS-R). MEP amplitude, SICI, and ICF strength were significantly modulated following different metaplasticity TDCS protocols only in the patients with MCS. SICI modulations showed a significant correlation with the CRS-R score. Our findings demonstrate, for the first time, a partial preservation of metaplasticity properties in some patients with DoC, which correlates with the level of awareness. Thus, metaplasticity assessment may help the clinician in differentiating the patients with DoC, besides the clinical evaluation. Moreover, the responsiveness to metaplasticity protocols may identify the subjects who could benefit from neuromodulation protocols.

No modulatory effects by transcranial static magnetic field stimulation of human motor and somatosensory cortex

BACKGROUND

Recently, it was reported that the application of a static magnetic field by placing a strong permanent magnet over the scalp for 10 min led to an inhibition of motor cortex excitability for at least 6 min after removing the magnet. When placing the magnet over the somatosensory cortex, a similar inhibitory after effect could be observed as well.

OBJECTIVE

Our aim was to replicate the inhibitory effects of transcranial static magnetic field stimulation in the motor and somatosensory system.

METHODS

The modulatory effect of static magnetic field stimulation was investigated in three experiments. In two experiments motor cortex excitability was measured before and after 10 or 15 min of magnet application, respectively. The second experiment included a sham condition and was designed in a double-blinded manner. In a third experiment, paired-pulse SSEPs were measured pre and four times post positioning the magnet over the somatosensory cortex for 10 min on both hemispheres, respectively. The SSEPs of the non stimulated hemisphere served as control condition.

RESULTS

We did not observe any systematic effect of the static magnetic field neither on motor cortex excitability nor on SSEPs. Moreover, no SSEP paired-pulse suppression was found.

CONCLUSION

We provide a detailed analysis of possible confounding factors and differences to previous studies on tSMS. After all, our results could not confirm the static magnetic field effect.

Quadri-Pulse Theta Burst Stimulation using Ultra-High Frequency Bursts - A New Protocol to Induce Changes in Cortico-Spinal Excitability in Human Motor Cortex

Patterned transcranial magnetic stimulation (TMS) such as theta burst stimulation (TBS) or quadri-pulse stimulation (QPS) can induce changes in cortico-spinal excitability, commonly referred to as long-term potentiation (LTP)-like and long-term depression (LTD)-like effects in human motor cortex (M1). Here, we aimed to test the plasticity-inducing capabilities of a novel protocol that merged TBS and QPS. 360 bursts of quadri-pulse TBS (qTBS) were continuously given to M1 at 90% of active motor threshold (1440 full-sine pulses). In a first experiment, stimulation frequency of each burst was set to 666 Hz to mimic the rhythmicity of the descending cortico-spinal volleys that are elicited by TMS (i.e., I-wave periodicity). In a second experiment, burst frequency was set to 200 Hz to maximize postsynaptic Ca²⁺ influx using a temporal pattern unrelated to I-wave periodicity. The second phase of sinusoidal TMS pulses elicited either a posterior-anterior (PA) or anterior-posterior (AP) directed current in M1. Motor evoked potentials (MEPs) were recorded before and after qTBS to probe changes in cortico-spinal excitability. PA-qTBS at 666 Hz caused a decrease in PA-MEP amplitudes, whereas AP-qTBS at 666 Hz induced an increase in mean AP-MEP amplitudes. At a burst frequency of 200 Hz, PA-qTBS and AP-qTBS produced an increase in cortico-spinal excitability outlasting for at least 60 minutes in PA- and AP-MEP amplitudes, respectively. Continuous qTBS at 666 Hz or 200 Hz can induce lasting changes in cortico-spinal excitability. Induced current direction in the brain appears to be relevant when qTBS targets I-wave periodicity, corroborating that high-fidelity spike timing mechanisms are critical for inducing bi-directional plasticity in human M1.

Effectiveness of repetitive trancranial or peripheral magnetic stimulation in neuropathic pain

PURPOSE

Maladaptive plasticity in the sensorimotor system, following neurological lesions or diseases, plays a central role in the generation and maintenance of neuropathic pain. Repetitive magnetic stimulation of the central and peripheral nervous system has gained relevance as noninvasive approach for neuromodulation and pain relief. Systematic reviews that evaluate the effectiveness and specificity of different protocols of repetitive magnetic stimulation to control neuropathic pain in clinical populations have the potential to improve the therapeutic applicability of this technique.

METHODS

Studies whose primary goal was to evaluate the effectiveness of repetitive magnetic stimulation for the treatment of various types of neuropathic pain published in PubMed until August 2015 have been included in this systematic review.

RESULTS

A total of 39 articles fulfilling the inclusion criteria were analyzed of which 37 studies investigated pain modulation using repetitive magnetic stimulation over the motor or non-motor cortices and two studies evaluated pain modulation using repetitive peripheral magnetic stimulation protocols.

CONCLUSIONS

Repetitive transcranial magnetic stimulation of the primary motor cortex using high frequency stimulation protocols can effectively reduce neuropathic pain, particularly in individuals with pain related to non-cerebral lesions. The application of multiple sessions can lead to long-lasting pain modulation and cumulative effects. Implications for Rehabilitation Maladaptive plasticity plays a central role in sensitization of nociceptive pathways, generation and maintainance of neuropathic pain; Most neuropathic pain conditions are refractory to pharmacological therapies; Repetitive magnetic stimulation of the central and peripheral nervous system has gained relevance as noninvasive approach for neuromodulation and pain relief.

Consensus Paper: Probing Homeostatic Plasticity of Human Cortex With Non-invasive Transcranial Brain Stimulation

Homeostatic plasticity is thought to stabilize neural activity around a set point within a physiologically reasonable dynamic range. Over the last ten years, a wide range of non-invasive transcranial brain stimulation (NTBS) techniques have been used to probe homeostatic control of cortical plasticity in the intact human brain. Here, we review different NTBS approaches to study homeostatic plasticity on a systems level and relate the findings to both, physiological evidence from in vitro studies and to a theoretical framework of homeostatic function. We highlight differences between homeostatic and other non-homeostatic forms of plasticity and we examine the contribution of sleep in restoring synaptic homeostasis. Finally, we discuss the growing number of studies showing that abnormal homeostatic plasticity may be associated to a range of neuropsychiatric diseases.

State-Dependent Partial Occlusion of Cortical LTP-Like Plasticity in Major Depression

The synaptic plasticity hypothesis of major depressive disorder (MDD) posits that alterations in synaptic plasticity represent a final common pathway underlying the clinical symptoms of the disorder. This study tested the hypotheses that patients with MDD show an attenuation of cortical synaptic long-term potentiation (LTP)-like plasticity in comparison with healthy controls, and that this attenuation recovers after remission. Cortical synaptic LTP-like plasticity was measured using a transcranial magnetic stimulation protocol, ie, paired associative stimulation (PAS), in 27 in-patients with MDD according to ICD-10 criteria and 27 sex- and age-matched healthy controls. The amplitude of motor-evoked potentials was measured before and after PAS. Patients were assessed during the acute episode and at follow-up to determine the state- or trait-character of LTP-like changes. LTP-like plasticity, the PAS-induced increase in motor-evoked potential amplitudes, was significantly attenuated in patients with an acute episode of MDD compared with healthy controls. Patients with remission showed a restoration of synaptic plasticity, whereas the deficits persisted in patients without remission, indicative for a state-character of impaired LTP-like plasticity. The results provide first evidence for a state-dependent partial occlusion of cortical LTP-like plasticity in MDD. This further identifies impaired LTP-like plasticity as a potential pathomechanism and treatment target of the disorder.

Recent History of Effector Use Modulates Practice-Dependent Changes in Corticospinal Excitability but Not Motor Learning

BACKGROUND

The theory of homeostatic metaplasticity has significant implications for human motor cortical plasticity and motor learning. Previous work has shown that the extent of recent effector use before exogenously-induced plasticity can affect the direction, magnitude and variability of aftereffects. However, the impact of recent effector use on motor learning and practice-dependent plasticity is not known.

HYPOTHESIS

We hypothesized that reducing effector use for 8 hours via hand/wrist immobilization would facilitate practice-dependent changes in corticospinal excitability and TMS-evoked thumb movement kinematics, while also promoting 24-hour retention of a ballistic motor skill.

METHODS

Subjects participated in a crossover study involving two conditions. During the immobilization condition, subjects wore a splint that restricted motion of the left hand and thumb for 8 hours. While wearing the splint, subjects were instructed to avoid using their left hand as much as possible. During the control condition, subjects did not wear a splint at any time nor were they instructed to avoid hand use. After either an 8 hour period of immobilization or normal hand use, we collected MEP and TMS-evoked thumb movement recruitment curves, and subjects practiced a ballistic motor skill involving rapid thumb extension. After motor practice, MEP and TMS-evoked thumb movement recruitment curves were re-tested. Retention of the motor skill was tested 30 minutes and 24 hours after motor practice.

RESULTS

Reduced effector use did not impact pre-practice corticospinal excitability but did facilitate practice-dependent changes in corticospinal excitability, and this enhancement was specific to the trained muscle. In contrast, reducing effector use did not affect practice-dependent changes in TMS-evoked thumb movements nor did it promote acquisition or retention of the skill. Finally, we detected some associations between pre-practice excitability levels, plasticity effects and learning effects, but these did not reach our adjusted criterion for significance.

CONCLUSION

Experimentally enhancing practice-dependent changes in corticospinal excitability is not sufficient to promote learning or memory of a ballistic motor skill.

A Comparison of Primed Low-frequency Repetitive Transcranial Magnetic Stimulation Treatments in Chronic Stroke

BACKGROUND

Preceding low-frequency repetitive transcranial magnetic stimulation (rTMS) with a bout of high-frequency rTMS called priming potentiates the after-effects of the former in healthy adults. The utility of primed rTMS in stroke remains under-explored despite its theoretical benefits in enhancing cortical excitability and motor function.

OBJECTIVE

To ascertain the efficacy of priming in chronic stroke by comparing changes in cortical excitability and paretic hand function following three types of primed low-frequency rTMS treatments.

METHODS

Eleven individuals with chronic stroke participated in this repeated-measures study receiving three treatments to the contralesional primary motor cortex in randomized order: 6 Hz primed 1 Hz rTMS, 1 Hz primed 1 Hz rTMS, and sham 6 Hz primed active 1 Hz rTMS. Within- and between-treatment differences from baseline in cortical excitability and paretic hand function from baseline were analyzed using mixed effects linear models.

RESULTS

6 Hz primed 1 Hz rTMS produced significant within-treatment differences from baseline in ipsilesional cortical silent period (CSP) duration and short-interval intracortical inhibition. Compared to 1 Hz priming and sham 6 Hz priming of 1 Hz rTMS, active 6 Hz priming generated significantly greater decreases in ipsilesional CSP duration. These heightened effects were not observed for intracortical facilitation or interhemispheric inhibition excitability measures.

CONCLUSION

Our findings demonstrate the efficacy of 6 Hz primed 1 Hz rTMS in probing homeostatic plasticity mechanisms in the stroke brain as best demonstrated by differences CSP duration and SICI from baseline. Though 6 Hz priming did not universally enhance cortical excitability across measures, our findings pose important implications in non-invasive brain stimulation application in stroke rehabilitation.

Effect of 30 Hz theta burst transcranial magnetic stimulation on the primary motor cortex in children and adolescents

Fourteen healthy children (13.8 ± 2.2 years, range 10–16; M:F = 5:9) received 30 Hz intermittent theta burst transcranial magnetic stimulation (iTBS) with a stimulation intensity of 70% of resting motor threshold (RMT) with a total of 300 (iTBS300) pulses. All volunteers were free of neurologic, psychiatric and serious medical illnesses, not taking any neuropsychiatric medications, and did not have any contraindications to transcranial magnetic stimulation. Changes in the mean amplitudes of motor-evoked potentials from baseline following iTBS were expressed as a ratio and assessed from 1 to 10 min (BLOCK1) and 1–30 min (BLOCK2) using repeated-measures analysis of variance. All 14 subjects completed iTBS300 over the dominant primary motor cortex (M1) without any clinically reported adverse events. ITBS300 produced significant M1 facilitation [F_{(5, 65)} = 3.165, p = 0.01] at BLOCK1 and trend level M1 facilitation at BLOCK2 [F_{(10, 129)} = 1.69, p = 0.089]. Although iTBS300 (stimulation duration of 92 s at 70% RMT) delivered over M1 in typically developed children was well-tolerated and produced on average significant facilitatory changes in cortical excitability, the post-iTBS300 neurophysiologic response was variable in our small sample. ITBS300-induced changes may represent a potential neuroplastic biomarker in healthy children and those with neuro-genetic or neuro-psychiatric disorders. However, a larger sample size is needed to address safety and concerns of response variability.

Improvements in hand function in adults with chronic tetraplegia following a multiday 10-Hz repetitive transcranial magnetic stimulation intervention combined with repetitive task practice

BACKGROUND AND PURPOSE

Evidence suggests that the use of stimulation to increase corticomotor excitability improves hand function in persons with cervical spinal cord injury. We assessed effects of a multiday application of 10-Hz repetitive transcranial magnetic stimulation (rTMS) applied to the corticomotor hand area combined with repetitive task practice (RTP) in participants with tetraplegia and neurologically healthy participants.

METHODS

Using a double-blind, randomized, crossover design, 11 participants with chronic tetraplegia and 10 neurologically healthy participants received 3 sessions of 10-Hz rTMS+RTP and 3 sessions of sham-rTMS+RTP to the corticomotor hand region controlling the weaker hand. Repetitive transcranial magnetic stimulation was interleaved with RTP of a skilled motor task between pulse trains. Hand function (Jebsen-Taylor Hand Function Test, pinch, and grasp strength) and corticomotor excitability (amplitude of motor-evoked potential) were assessed before and after the rTMS+RTP and sham-rTMS+RTP phases. We assessed significance, using paired t tests on pre-post differences, and effect sizes, using the standardized response mean.

RESULTS

RTMS+RTP was associated with larger effect sizes compared with sham-rTMS+RTP for improvement in Jebsen-Taylor Hand Function Test for both the trained hand (standardized response mean = 0.85 and 0.42, respectively) and non-trained hand (0.55 and 0.31, respectively), and for grasp strength of the trained hand in the group with cervical spinal cord injury (0.67 and 0.39, respectively) alone. Effect sizes for all other measures were small and there were no statistical between-condition differences in the outcomes assessed.

DISCUSSION AND CONCLUSIONS

Repetitive transcranial magnetic stimulation may be a valuable adjunct to RTP for improving hand function in persons with tetraplegia. Higher stimulation dose (frequency, intensity, and the number of sessions) may be associated with larger effects.

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Metaplasticity in human cortex

Metaplasticity refers to the modification of plasticity induction (direction, magnitude, duration) by previous activity of the same postsynaptic neuron or neuronal network. In recent years evidence from animal studies has been accumulated that metaplasticity significantly contributes to network function and behavior. Here, we review the evidence for metaplasticity at the system level of the human cortex as investigated by non-invasive brain stimulation. These studies support the notion that metaplasticity is also operative in the human brain and is mostly homeostatic in nature, that is, keeping network activity within a physiological range. However, non-homeostatic metaplasticity has also been described, which can increase non-invasive brain stimulation-induced aftereffects on cortical excitability, or learning. Current evidence further suggests that aberrant metaplasticity may underlie some neurological and psychiatric diseases. Finally, first proof-of-principle studies show that the concept of metaplasticity can be harnessed for treatment of patients suffering from brain diseases.

Induction of cortical plasticity for reciprocal muscles by paired associative stimulation

BACKGROUND

Paired associative stimulation (PAS) is widely used to induce plasticity in the human motor cortex. Although reciprocal inhibition of antagonist muscles plays a fundamental role in human movements, change in cortical circuits for reciprocal muscles by PAS is unknown.

METHODS

We investigated change in cortical plasticity for reciprocal muscles during PAS. PAS consisted of 200 pairs of peripheral electric stimulation of the right median nerve at the wrist at a frequency of 0.25 Hz followed by transcranial magnetic stimulation of the left M1 at the midpoint between the center of gravities of the flexor carpi radialis (FCR) and extensor carpi radialis (ECR) muscles. Measures of motor cortical excitability included resting motor threshold (RMT), GABAA-mediated short-interval intracortical inhibition (SICI), and GABAB-mediated long-interval intracortical inhibition (LICI).

RESULTS

Motor evoked potential amplitude-conditioned LICI for the FCR muscle was significantly decreased after PAS (P = 0.020), whereas that for the ECR muscle was significantly increased (P = 0.033). Changes in RMT and SICI for the FCR and ECR muscles were not significantly different before and after PAS. Corticospinal excitability for both reciprocal muscles was increased during PAS, but GABAB-mediated cortical inhibitory functions for the agonist and antagonist muscles were reciprocally altered after PAS.

CONCLUSION

These results implied that the cortical excitability for reciprocal muscles including GABAB-ergic inhibitory systems within human M1 could be differently altered by PAS.

Lovastatin improves impaired synaptic plasticity and phasic alertness in patients with neurofibromatosis type 1

Background

Neurofibromatosis type 1 (NF1) is one of the most common genetic disorders causing learning disabilities by mutations in the neurofibromin gene, an important inhibitor of the RAS pathway. In a mouse model of NF1, a loss of function mutation of the neurofibromin gene resulted in increased gamma aminobutyric acid (GABA)-mediated inhibition which led to decreased synaptic plasticity and deficits in attentional performance. Most importantly, these defictis were normalized by lovastatin. This placebo-controlled, double blind, randomized study aimed to investigate synaptic plasticity and cognition in humans with NF1 and tried to answer the question whether potential deficits may be rescued by lovastatin.

Methods

In NF1 patients (n = 11; 19–44 years) and healthy controls (HC; n = 11; 19–31 years) paired pulse transcranial magnetic stimulation (TMS) was used to study intracortical inhibition (paired pulse) and synaptic plasticity (paired associative stimulation). On behavioural level the Test of Attentional Performance (TAP) was used. To study the effect of 200 mg lovastatin for 4 days on all these parameters, a placebo-controlled, double blind, randomized trial was performed.

Results

In patients with NF1, lovastatin revealed significant decrease of intracortical inhibition, significant increase of synaptic plasticity as well as significant increase of phasic alertness. Compared to HC, patients with NF1 exposed increased intracortical inhibition, impaired synaptic plasticity and deficits in phasic alertness.

Conclusions

This study demonstrates, for the first time, a link between a pathological RAS pathway activity, intracortical inhibition and impaired synaptic plasticity and its rescue by lovastatin in humans. Our findings revealed mechanisms of attention disorders in humans with NF1 and support the idea of a potential clinical benefit of lovastatin as a therapeutic option.

Microcircuit mechanisms involved in paired associative stimulation-induced depression of corticospinal excitability

Synaptic weight changes induced by temporal correlations between the spikes of pre- and postsynaptic neurons are referred to as spike-timing-dependent plasticity (STDP). Transcranial magnetic stimulation (TMS) induces long-lasting effects on corticospinal excitability, if it is repetitively paired with stimulation of afferents from a corresponding contralateral hand region at short intervals (paired associative stimulation, PAS). PAS-induced plasticity has been linked with synaptic STDP. We aimed to investigate which elements of the cortical microcircuitry sustain and govern PAS-induced depression of corticospinal excitability in the target muscle representation (and enhancement of excitability in its functional surround). We show that the time window during which the interaction between both stimulus-induced cortical events leads to immediate post-interventional depression is short (<4.5 ms). The depressant PAS effects at the target representation were completely blocked by applying a subthreshold magnetic pulse 3 ms before the principal TMS pulse, even when the strength of the latter was adjusted to generate a motor-evoked potential of similar amplitude to that with the unconditioned magnetic pulse. Epidural recordings from the cervical cord of a patient showed that under this condition late TMS-evoked I-waves remain suppressed. When the intensity of the TMS component during PAS was lowered - sufficient to allow activation of inhibitory neurons, but insufficient to activate corticospinal neurons - excitability of short-latency intracortical inhibition remained unchanged. PAS-induced facilitation in the functional surround followed the same pattern as the centre-depressant effects. These findings may suggest that excitability-depressant PAS-induced effects are due to weakening of excitatory synapses between upper cortical layer principal neurons, but not those located on the corticospinal neuron, or inhibitory synapses. Inhibitory interneurons involved in short-latency intracortical inhibition are gate-keepers to producing centre-depressant/surround-facilitatory PAS effects. Based on these and earlier findings we propose a model specifying the composition and laminar location of the involved microcircuit of PAS-induced plasticity that may enhance its utility as a model of STDP in humans.

Testing the nanoparticle-allostatic cross-adaptation-sensitization model for homeopathic remedy effects

Key concepts of the Nanoparticle-Allostatic Cross-Adaptation-Sensitization (NPCAS) Model for the action of homeopathic remedies in living systems include source nanoparticles as low level environmental stressors, heterotypic hormesis, cross-adaptation, allostasis (stress response network), time-dependent sensitization with endogenous amplification and bidirectional change, and self-organizing complex adaptive systems. The model accommodates the requirement for measurable physical agents in the remedy (source nanoparticles and/or source adsorbed to silica nanoparticles). Hormetic adaptive responses in the organism, triggered by nanoparticles; bipolar, metaplastic change, dependent on the history of the organism. Clinical matching of the patient's symptom picture, including modalities, to the symptom pattern that the source material can cause (cross-adaptation and cross-sensitization). Evidence for nanoparticle-related quantum macro-entanglement in homeopathic pathogenetic trials. This paper examines research implications of the model, discussing the following hypotheses: Variability in nanoparticle size, morphology, and aggregation affects remedy properties and reproducibility of findings. Homeopathic remedies modulate adaptive allostatic responses, with multiple dynamic short- and long-term effects. Simillimum remedy nanoparticles, as novel mild stressors corresponding to the organism's dysfunction initiate time-dependent cross-sensitization, reversing the direction of dysfunctional reactivity to environmental stressors. The NPCAS model suggests a way forward for systematic research on homeopathy. The central proposition is that homeopathic treatment is a form of nanomedicine acting by modulation of endogenous adaptation and metaplastic amplification processes in the organism to enhance long-term systemic resilience and health.

Long-term fluoxetine treatment induces input-specific LTP and LTD impairment and structural plasticity in the CA1 hippocampal subfield

Antidepressant drugs are usually administered for several weeks for the treatment of major depressive disorder. However, they are also prescribed in several additional psychiatric conditions as well as during long-term maintenance treatments. Antidepressants induce adaptive changes in several forebrain structures which include modifications at glutamatergic synapses. We recently found that repetitive administration of the selective serotonin reuptake inhibitor (SSRI) fluoxetine to naïve adult male rats induced an increase of mature, mushroom-type dendritic spines in several forebrain regions. This was associated with an increase of GluA2-containing α-amino-3-hydroxy-5-methylisoxazole-4-propionate receptors (AMPA-Rs) in telencephalic postsynaptic densities. To unravel the functional significance of such a synaptic re-arrangement, we focused on glutamate neurotransmission in the hippocampus. We evaluated the effect of four weeks of 0.7 mg/kg fluoxetine on long-term potentiation (LTP) and long-term depression (LTD) in the CA1 hippocampal subfield. Recordings in hippocampal slices revealed profound deficits in LTP and LTD at Schaffer collateral-CA1 synapses associated to increased spine density and enhanced presence of mushroom-type spines, as revealed by Golgi staining. However, the same treatment had neither an effect on spine morphology, nor on LTP and LTD at perforant path-CA1 synapses. Cobalt staining and immunohistochemical experiments revealed decreased AMPA-R Ca(2+) permeability in the stratum radiatum (s.r.) together with increased GluA2-containing Ca(2+) impermeable AMPA-Rs. Therefore, 4 weeks of fluoxetine treatment promoted structural and functional adaptations in CA1 neurons in a pathway-specific manner that were selectively associated with impairment of activity-dependent plasticity at Schaffer collateral-CA1 synapses.

Emerging roles of metaplasticity in behaviour and disease

Since its initial conceptualisation, metaplasticity has come to encompass a wide variety of phenomena and mechanisms, creating the important challenge of understanding how they contribute to network function and behaviour. Here, we present a framework for considering potential roles of metaplasticity across three domains of function. First, metaplasticity appears ideally placed to prepare for subsequent learning by either enhancing learning ability generally or by preparing neuronal networks to encode specific content. Second, metaplasticity can homeostatically regulate synaptic plasticity, and this likely has important behavioural consequences by stabilising synaptic weights while ensuring the ongoing availability of synaptic plasticity. Finally, we discuss emerging evidence that metaplasticity mechanisms may play a role in disease causally and may serve as a potential therapeutic target.

Impaired induction of long-term potentiation-like plasticity in patients with high-functioning autism and Asperger syndrome

AIM

We aimed to investigate the induction of long-term potentiation (LTP)-like plasticity by paired associative stimulation (PAS) in patients with high-functioning autism and Asperger syndrome (HFA/AS).

METHOD

PAS with an interstimulus interval between electrical and transcranial magnetic stimulation of 25 ms (PAS(25)) was performed in patients with HFA/AS (n=9; eight males, one female; mean age 17 y 11 mo, SD 4 y 5 mo) and in typically developing age-matched volunteers (n=9; five males, four females; mean age 22 y 4 mo, SD 5 y 2 mo). The amplitude of motor-evoked potentials was measured before PAS(25), immediately after stimulation, and 30 minutes and 60 minutes later. A PAS protocol adapted to individual N20 latency (PAS(N20+2)) was performed in six additional patients with HFA/AS. Short-interval intracortical inhibition was measured using paired-pulse stimulation.

RESULTS

In contrast to the typically developing participants, the patients with HFA/AS did not show a significant increase in motor-evoked potentials after PAS(25). This finding could also be demonstrated after adaptation for N20 latency. Short-interval intracortical inhibition of patients with HFA/AS was normal compared with the comparison group and did not correlate with PAS effect.

INTERPRETATION

Our results show a significant impairment of LTP-like plasticity induced by PAS in individuals with HFA/AS compared with typically developing participants. This finding is in accordance with results from animal studies as well as human studies. Impaired LTP-like plasticity in patients with HFA/AS points towards reduced excitatory synaptic connectivity and deficits in sensory-motor integration in these patients.

Transcranial electric and magnetic stimulation: technique and paradigms

Transcranial electrical and magnetic stimulation techniques encompass a broad physical variety of stimuli, ranging from static magnetic fields or direct current stimulation to pulsed magnetic or alternating current stimulation with an almost infinite number of possible stimulus parameters. These techniques are continuously refined by new device developments, including coil or electrode design and flexible control of the stimulus waveforms. They allow us to influence brain function acutely and/or by inducing transient plastic after-effects in a range from minutes to days. Manipulation of stimulus parameters such as pulse shape, intensity, duration, and frequency, and location, size, and orientation of the electrodes or coils enables control of the immediate effects and after-effects. Physiological aspects such as stimulation at rest or during attention or activation may alter effects dramatically, as does neuropharmacological drug co-application. Non-linear relationships between stimulus parameters and physiological effects have to be taken into account.

Plasticity

A variety of noninvasive brain stimulation techniques have been used to study neuronal plasticity. Mostly, noninvasive techniques have been employed, and the bulk of studies have focused on the motor system, because its physiology is more readily accessible and physiological properties can be studied with greater detail than in other systems. In many of the protocols, general conclusions have been drawn from the motor potentials evoked by transcranial magnetic stimulation of the primary motor cortex. Several of the phenomena induced by noninvasive brain stimulation have been mapped on to cellular physiological mechanisms such as synaptic long-term potentiation or long-term depression. Although some parallelisms are intriguing, this approach has also its limitations, and more direct verification of physiological phenomena by animal studies is needed.

Homeostatic metaplasticity of corticospinal excitatory and intracortical inhibitory neural circuits in human motor cortex

Homeostatic metaplasticity, a fundamental principle for maintaining overall synaptic weight in the physiological range in neuronal networks, was demonstrated at the cellular and systems level predominantly for excitatory synaptic neurotransmission. Although inhibitory networks are crucial for regulating excitability, it is largely unknown to what extent homeostatic metaplasticity of inhibition also exists. Here, we employed intermittent and continuous transcranial magnetic theta burst stimulation (iTBS, cTBS) of the primary motor cortex in healthy subjects for induction of long-term potentiation (LTP)-like and long-term depression (LTD)-like plasticity. We studied metaplasticity by testing the interactions of priming TBS with LTP/LTD-like plasticity induced by subsequent test TBS. Changes in excitatory neurotransmission were measured by the input-output curve of motor-evoked potentials (IO-MEP), and changes in GABA(A)ergic inhibitory neurotransmission by the IO of short-interval intracortical inhibition (IO-SICI, four conditioning stimulus intensities of 70-100% active motor threshold, interstimulus interval 2.0 ms). Non-primed iTBS increased IO-MEP, while non-primed cTBS decreased IO-MEP. Pairing of identical protocols (iTBSiTBS, cTBScTBS) resulted in suppression of the non-primed TBS effects on IO-MEP, and pairing of different protocols (cTBSiTBS, iTBScTBS) enhanced the test TBS effects on IO-MEP. While non-primed TBS did not result in significant changes of IO-SICI, iTBSiTBS resulted in IO-SICI decrease, and cTBScTBS in IO-SICI increase compared with the non-primed conditions. The changes in SICI induced by priming TBS correlated with the changes in MEP induced by subsequent test TBS. Findings demonstrate that plasticity in both excitatory and inhibitory circuits in the human motor cortex are regulated by homeostatic metaplasticity, and that priming effects on inhibition contribute to the homeostatic regulation of metaplasticity in excitatory circuits.

Relation of brain stimulation induced changes in MEP amplitude and BOLD signal

BACKGROUND

Non-invasive human brain stimulation can induce long-term plasticity reflected by changes in putative markers of synaptic activation, such as the motor evoked potential (MEP) amplitude elicited by transcranial magnetic stimulation or the task-dependent blood oxygenation level-dependent (BOLD) signal measured by functional magnetic resonance imaging.

OBJECTIVE

To study the relationship between brain stimulation induced changes in MEP amplitude and BOLD signal.

METHODS

Paired associative stimulation of the hand area of the left primary somatosensory cortex (S1-PAS) was applied in 15 healthy subjects to induce excitability change in the adjacent primary motor cortex (M1) [Kriváneková et al. 2011, Eur J Neurosci 34:1292-1300]. Before and after S1-PAS, MEP amplitude in a right hand muscle, and the BOLD signal during a right hand motor or somatosensory activation task were measured.

RESULTS

S1-PAS resulted in substantial individual MEP and BOLD signal changes, but these changes did not correlate in M1 or S1.

CONCLUSIONS

Findings indicate that MEP amplitude and BOLD signal within the tested M1 reflect physiologically distinct aspects of synaptic excitability change. Therefore, it is suggested that MEP amplitude and BOLD signal are complementary rather than interchangeable markers of synaptic excitability.

Effects of lamotrigine on human motor cortex plasticity

OBJECTIVE

Besides its use in epilepsy, lamotrigine (LTG) is also effective as mood stabilizer. The pathophysiology of mood disorders may incorporate a dysfunction of neuronal plasticity and animal experiments suggest that mood stabilizers influence induction of long-term potentiation (LTP) and -depression (LTD), two major forms of synaptic plasticity. However, the exact modes of action of LTG and its impact on neuronal plasticity in humans remain unclear.

METHODS

Here, we tested the effects of a single oral dose of LTG (300 mg) on motor cortical plasticity induced by paired associative stimulation (PAS(25)), a protocol that typically induces LTP-like plasticity, in 26 young healthy adults in a placebo-controlled, randomized, double-blind crossover design. We stratified analysis of the LTG effects according to the individual PAS(25) response in the placebo session (14 LTP-responders vs. 12 LTD-responders). Plasticity was indexed by motor evoked potential (MEP) amplitudes recorded before and for 60 min after PAS(25).

RESULTS

LTG resulted in a significant reduction of the LTP-like MEP increase in the LTP-responders and a reduction of the LTD-like MEP decrease in the LTD-responders, with the majority of LTD-responders even showing an MEP increase.

CONCLUSIONS

In summary, LTG differentially modulated cortical plasticity induced by non-invasive brain stimulation in human subjects depending on their individual intrinsic propensity for expressing LTP-like or LTD-like plasticity.

SIGNIFICANCE

Findings contribute to our understanding of the anticonvulsant and antidepressant clinical effects of LTG, which have been suggested to occur, at least in part, through downregulation of LTP (epilepsy) and LTD (depressive disorders).

Occlusion of LTP-like plasticity in human primary motor cortex by action observation

Passive observation of motor actions induces cortical activity in the primary motor cortex (M1) of the onlooker, which could potentially contribute to motor learning. While recent studies report modulation of motor performance following action observation, the neurophysiological mechanism supporting these behavioral changes remains to be specifically defined. Here, we assessed whether the observation of a repetitive thumb movement--similarly to active motor practice--would inhibit subsequent long-term potentiation-like (LTP) plasticity induced by paired-associative stimulation (PAS). Before undergoing PAS, participants were asked to either 1) perform abductions of the right thumb as fast as possible; 2) passively observe someone else perform thumb abductions; or 3) passively observe a moving dot mimicking thumb movements. Motor evoked potentials (MEP) were used to assess cortical excitability before and after motor practice (or observation) and at two time points following PAS. Results show that, similarly to participants in the motor practice group, individuals observing repeated motor actions showed marked inhibition of PAS-induced LTP, while the "moving dot" group displayed the expected increase in MEP amplitude, despite differences in baseline excitability. Interestingly, LTP occlusion in the action-observation group was present even if no increase in cortical excitability or movement speed was observed following observation. These results suggest that mere observation of repeated hand actions is sufficient to induce LTP, despite the absence of motor learning.

Plasticity-inducing TMS protocols to investigate somatosensory control of hand function

Hand function depends on sensory feedback to direct an appropriate motor response. There is clear evidence that somatosensory cortices modulate motor behaviour and physiology within primary motor cortex. However, this information is mainly from research in animals and the bridge to human hand control is needed. Emerging evidence in humans supports the notion that somatosensory cortices modulate motor behaviour, physiology and sensory perception. Transcranial magnetic stimulation (TMS) allows for the investigation of primary and higher-order somatosensory cortices and their role in control of hand movement in humans. This review provides a summary of several TMS protocols in the investigation of hand control via the somatosensory cortices. TMS plasticity inducing protocols reviewed include paired associative stimulation, repetitive TMS, theta-burst stimulation as well as other techniques that aim to modulate cortical excitability in sensorimotor cortices. Although the discussed techniques may modulate cortical excitability, careful consideration of experimental design is needed to isolate factors that may interfere with desired results of the plasticity-inducing protocol, specifically events that may lead to metaplasticity within the targeted cortex.

Plasticity of motor threshold and motor-evoked potential amplitude--a model of intrinsic and synaptic plasticity in human motor cortex?

BACKGROUND

Neuronal plasticity is the physiological correlate of learning and memory. In animal experiments, synaptic (i.e. long-term potentiation (LTP) and depression (LTD)) and intrinsic plasticity are distinguished. In human motor cortex, cortical plasticity can be demonstrated using transcranial magnetic stimulation (TMS). Changes in motor-evoked potential (MEP) amplitudes most likely represent synaptic plasticity and are thus termed LTP-like and LTD-like plasticity.

OBJECTIVE/HYPOTHESIS

We investigated the role of changes of motor threshold and their relation to changes of MEP amplitudes.

METHODS

We induced plasticity by paired associative stimulation (PAS) with 25 ms or 10 ms inter-stimulus interval or by motor practice (MP) in 64 healthy subjects aged 18-31 years (median 24.0).

RESULTS

We observed changes of MEP amplitudes and motor threshold after PAS[25], PAS[10] and MP. In all three protocols, long-term individual changes in MEP amplitude were inversely correlated to changes in motor threshold (PAS[25]: P = .003, n = 36; PAS[10]: P = .038, n = 19; MP: P = .041, n = 19).

CONCLUSION

We conclude that changes of MEP amplitudes and MT represent two indices of motor cortex plasticity. Whereas increases and decreases in MEP amplitude are assumed to represent LTP-like or LTD-like synaptic plasticity of motor cortex output neurons, changes of MT may be considered as a correlate of intrinsic plasticity.

Altered bidirectional plasticity and reduced implicit motor learning in concussed athletes

Persistent motor/cognitive alterations and increased prevalence of Alzheimer's disease are known consequences of recurrent sports concussions, the most prevalent cause of mild traumatic brain injury (TBI) among youth. Animal models of TBI demonstrated that impaired learning was related to persistent synaptic plasticity suppression in the form of long-term potentiation (LTP) and depression (LTD). In humans, single and repeated concussive injuries lead to lifelong and cumulative enhancements of gamma-aminobutyric acid (GABA)-mediated inhibition, which is known to suppress LTP/LTD plasticity. To test the hypothesis that increased GABAergic inhibition after repeated concussions suppresses LTP/LTD and contributes to learning impairments, we used a paired associative stimulation (PAS) protocol to induce LTP/LTD-like effects in primary motor cortex (M1) jointly with an implicit motor learning task (serial reaction time task, SRTT). Our results indicate that repeated concussions induced persistent elevations of GABA(B)-mediated intracortical inhibition in M1, which was associated with suppressed PAS-induced LTP/LTD-like synaptic plasticity. This synaptic plasticity suppression was related to reduced implicit motor learning on the SRTT task relative to normal LTP/LTD-like synaptic plasticity in unconcussed teammates. These findings identify GABA neurotransmission alterations after repeated concussions and suggest that impaired learning after multiple concussions could at least partly be related to compromised GABA-dependent LTP/LTD synaptic plasticity.

Correlation between cortical plasticity, motor learning and BDNF genotype in healthy subjects

There is good evidence that synaptic plasticity in human motor cortex is involved in behavioural motor learning; in addition, it is now possible to probe mechanisms of synaptic plasticity using a variety of transcranial brain-stimulation protocols. Interactions between these protocols suggest that they both utilise common mechanisms. The aim of the present experiments was to test how well responsiveness to brain-stimulation protocols and behavioural motor learning correlate with each other in a sample of 21 healthy volunteers. We also examined whether any of these measures were influenced by the presence of a Val66Met polymorphism in the BDNF gene since this is another factor that has been suggested to be able to predict response to tests of synaptic plasticity. In 3 different experimental sessions, volunteers underwent 5-Hz rTMS, intermittent theta-burst stimulation (iTBS) and a motor learning task. Blood samples were collected from each subject for BDNF genotyping. As expected, both 5-Hz rTMS and iTBS significantly facilitated MEPs. Similarly, as expected, kinematic variables of finger movement significantly improved during the motor learning task. Although there was a significant correlation between the effect of iTBS and 5-Hz rTMS, there was no relationship in each subject between the amount of TMS-induced plasticity and the increase in kinematic variables during motor learning. Val66Val and Val66Met carriers did not differ in their response to any of the protocols. The present results emphasise that although some TMS measures of cortical plasticity may correlate with each other, they may not always relate directly to measures of behavioural learning. Similarly, presence of the Val66Met BDNF polymorphism also does not reliably predict responsiveness in small groups of individuals. Individual success in behavioural learning is unlikely to be closely related to any single measure of synaptic plasticity.