Understanding the Effects of Repetitive Transcranial Magnetic Stimulation on Neuronal Circuits

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.

A C-shaped miniaturized coil for transcranial magnetic stimulation in rodents

Objective.Transcranial magnetic stimulation (TMS) is a non-invasive technique widely used for neuromodulation. Animal models are essential for investigating the underlying mechanisms of TMS. However, the lack of miniaturized coils hinders the TMS studies in small animals, since most commercial coils are designed for humans and thus incapable of focal stimulation in small animals. Furthermore, it is difficult to perform electrophysiological recordings at the TMS focal point using conventional coils.Approach.We designed, fabricated, and tested a novel miniaturized TMS coil (4-by-7 mm) that consisted of a C-shaped iron powder core and insulated copper wires (30 turns). The resulting magnetic and electric fields were characterized with experimental measurements and finite element modeling. The efficacy of this coil in neuromodulation was validated with electrophysiological recordings of single-unit activities (SUAs), somatosensory evoked potentials (SSEPs), and motor evoked potentials (MEPs) in rats (n= 32) following repetitive TMS (rTMS; 3 min, 10 Hz).Main results.This coil could generate a maximum magnetic field of 460 mT and an electric field of 7.2 V m-1in the rat brain according to our simulations. With subthreshold rTMS focally delivered over the sensorimotor cortex, mean firing rates of primary somatosensory and motor cortical neurons significantly increased (154±5% and 160±9% from the baseline level, respectively); MEP and SSEP amplitude significantly increased (136±9%) and decreased (74±4%), respectively.Significance.This miniaturized C-shaped coil enabled focal TMS and concurrent electrophysiological recording/stimulation at the TMS focal point. It provided a useful tool to investigate the neural responses and underlying mechanisms of TMS in small animal models. Using this paradigm, we for the first time observed distinct modulatory effects on SUAs, SSEPs, and MEPs with the same rTMS protocol in anesthetized rats. These results suggested that multiple neurobiological mechanisms in the sensorimotor pathways were differentially modulated by rTMS.

Finding synaptic couplings from a biophysical model of motor evoked potentials after theta-burst transcranial magnetic stimulation

OBJECTIVE

We aimed to use measured input-output (IO) data to identify the best fitting model for motor evoked potentials.

METHODS

We analyzed existing IO data before and after intermittent and continuous theta-burst stimulation (iTBS & cTBS) from a small group of subjects (18 for each). We fitted individual synaptic couplings and sensitivity parameters using variations of a biophysical model. A best performing model was selected and analyzed.

RESULTS

cTBS gives a broad reduction in MEPs for amplitudes larger than resting motor threshold (RMT). Close to threshold, iTBS gives strong potentiation. The model captures individual IO curves. There is no change to the population average synaptic weights post TBS but the change in excitatory-to-excitatory synaptic coupling is strongly correlated with the experimental post-TBS response relative to baseline.

CONCLUSIONS

The model describes population-averaged and individual IO curves, and their post-TBS change. Variation among individuals is accounted for with variation in synaptic couplings, and variation in sensitivity of neural response to stimulation.

SIGNIFICANCE

The best fitting model could be applied more broadly and validation studies could elucidate underlying biophysical meaning of parameters.

Pre-frontal stimulation does not reliably increase reward responsiveness

Depression is the leading cause of disability worldwide and its effects can be fatal, with over 800,000 people dying by suicide each year. Neuromodulatory treatments such as transcranial magnetic stimulation (TMS) are being used to treat depression. Despite its endorsement by two regulatory bodies: NICE (2016) and the FDA (2008), there are major questions about the treatment efficacy and biological mechanisms of TMS. Ahn et al.'s (2013) justified the use of TMS in a clinical context in an important study indicating that excitatory TMS increases reward responsiveness. A pseudo-replication of this study by Duprat et al., (2016) also found a similar effect of active TMS, but only with the addition of an exploratory covariate to the analyses-trait reward responsiveness. Here we replicate Ahn et al.'s (2013) key study, and to test the reliability of the effects, and their dependency on trait reward responsiveness as described by Duprat et al., (2016). Using excitatory and sham TMS, we tested volunteers using the probabilistic learning task to measure their reward responsiveness both before and after stimulation. We also examined affect (positive, negative) following stimulation. Irrespective of TMS, the task was shown to be sensitive to reward responsiveness. However, we did not show TMS to be effective in increasing reward responsiveness and we did not replicate Ahn et al., (2013) or Duprat et al., (2016)'s key findings for TMS efficacy, where we provide evidence favouring the null. Moreover, exploratory analyses suggested following active stimulation, positive affect was reduced. Given our findings, we question the basic effects, which support the use of TMS for depression, particularly considering potential deleterious effects of reduced positive affect in patients with depression.

Effects of non-invasive brain stimulation on motor function after spinal cord injury: a systematic review and meta-analysis

BACKGROUND

In recent years, non-invasive brain stimulation (NIBS) has been used for motor function recovery. However, the effects of NIBS in populations with spinal cord injury (SCI) remain unclear. This study aims to conduct a meta-analysis of the existing evidence on the effects and safety of NIBS against sham groups for motor dysfunction after SCI to provide a reference for clinical decision-making.

METHODS

Two investigators systematically screened English articles from PubMed, MEDLINE, Embase, and Cochrane Library for prospective randomized controlled trials regarding the effects of NIBS in motor function recovery after SCI. Studies with at least three sessions of NIBS were included. We assessed the methodological quality of the selected studies using the evidence-based Cochrane Collaboration's tool. A meta-analysis was performed by pooling the standardized mean difference (SMD) with 95% confidence intervals (CI).

RESULTS

A total of 14 randomized control trials involving 225 participants were included. Nine studies used repetitive transcranial magnetic stimulation (rTMS) and five studies used transcranial direct current stimulation (tDCS). The meta-analysis showed that NIBS could improve the lower extremity strength (SMD = 0.58, 95% CI = 0.02-1.14, P = 0.004), balance (SMD = 0.64, 95% CI = 0.05-1.24, P = 0.03), and decrease the spasticity (SMD = - 0.64, 95% CI = - 1.20 to - 0.03, P = 0.04). However, the motor ability of the upper extremity in the NIBS groups was not statistically significant compared with those in the control groups (upper-extremity strength: P = 0.97; function: P = 0.56; and spasticity: P = 0.12). The functional mobility in the NIBS groups did not reach statistical significance when compared with the sham NIBS groups (sham groups). Only one patient reported seizures that occurred during stimulation, and no other types of serious adverse events were reported.

CONCLUSION

NIBS appears to positively affect the motor function of the lower extremities in SCI patients, despite the marginal P-value and the high heterogeneity. Further high-quality clinical trials are needed to support or refute the use and optimize the stimulation parameters of NIBS in clinical practice.

Influence of High-Frequency Repetitive Transcranial Magnetic Stimulation on Neurobehavioral and Electrophysiology in Patients with Disorders of Consciousness

Objective

High-frequency repetitive transcranial magnetic stimulation (HF-rTMS) has been proposed as a promising therapeutic intervention for patients with disorders of consciousness (DOC). However, its therapeutic effects in the literature are inconsistently documented. The primary aim of this study was to explore the alterations in neural connectivity and neurobehavioral reactivity during rTMS modulation in patients with DOC. In addition, safety was investigated as a secondary aim.

Methods

The presence of bilateral N20 components in DOC patients was determined by somatosensory-evoked potential (SEP) before enrollment in the study. A total of 64 patients were enrolled and randomly placed into the active and sham groups. Ultimately, 50 patients completed the study. Twenty-five patients in the active group underwent real HF-rTMS, and 25 patients in the sham group underwent sham HF-rTMS, which was delivered over the left dorsolateral prefrontal cortex (DLPFC). The outcome measures of performed pre- and postintervention included the latencies of the N20 and N20-P25 amplitudes of SEP, brainstem auditory-evoked potential (BAEP) grade, JFK Coma Recovery Scale-Revised (CRS-R) score, and Glasgow Coma Scale (GCS) score; any adverse events were recorded at any time during the intervention.

Result

Following six weeks of treatment, a significant increase was observed in the total CRS-R and GCS scores, and the N20-P25 amplitudes of patients in the two groups were compared with that obtained from preintervention (all p values < 0.05). The waves of BAEP in the two groups also showed a trend toward normalized activity compared with preintervention grades (p values < 0.05). A significant decrease in the latencies of N20 (p values < 0.001) was observed in the active group compared with measurements obtained from preintervention, whereas no significant decrease was observed in the sham group (p values = 0.013). The improvement in total CRS-R scores (p values = 0.002), total GCS scores (p values = 0.023), and N20-P25 amplitudes (p values = 0.011) as well as the decrease in latencies of N20 (p values = 0.018) and change in BAEP grades (p values = 0.013) were significantly different between the two groups. The parameters in neural connectivity (N20-P25 amplitudes, N20 latencies, and BAEP grades) were significantly correlated with the total CRS-R and GCS scores at postintervention, and the changes of CRS-R before and after interventions have a positive relationship with N20-P25 amplitudes. No adverse events related to the rTMS protocol were recorded.

Conclusion

Neural connectivity levels are affected by HF-rTMS and are significantly related to clinical responses in DOC patients with the presence of bilateral N20. The elevation of neural connectivity levels may lay a foundation for successful HF-rTMS treatment for DOC patients.

TMS Does Not Increase BOLD Activity at the Site of Stimulation: A Review of All Concurrent TMS-fMRI Studies

Transcranial magnetic stimulation (TMS) is widely used for understanding brain function in neurologically intact subjects and for the treatment of various disorders. However, the precise neurophysiological effects of TMS at the site of stimulation remain poorly understood. The local effects of TMS can be studied using concurrent TMS-functional magnetic resonance imaging (fMRI), a technique where TMS is delivered during fMRI scanning. However, although concurrent TMS-fMRI was developed over 20 years ago and dozens of studies have used this technique, there is still no consensus on whether TMS increases blood oxygen level-dependent (BOLD) activity at the site of stimulation. To address this question, here we review all previous concurrent TMS-fMRI studies that reported analyses of BOLD activity at the target location. We find evidence that TMS increases local BOLD activity when stimulating the primary motor (M1) and visual (V1) cortices but that these effects are likely driven by the downstream consequences of TMS (finger twitches and phosphenes). However, TMS does not appear to increase BOLD activity at the site of stimulation for areas outside of the M1 and V1 when conducted at rest. We examine the possible reasons for such lack of BOLD signal increase based on recent work in nonhuman animals. We argue that the current evidence points to TMS inducing periods of increased and decreased neuronal firing that mostly cancel each other out and therefore lead to no change in the overall BOLD signal.

Spontaneous brain activity underlying auditory hallucinations in the hearing-impaired

Auditory hallucinations, the perception of a sound without a corresponding source, are common in people with hearing impairment. Two forms can be distinguished: simple (i.e., tinnitus) and complex hallucinations (speech and music). Little is known about the precise mechanisms underlying these types of hallucinations. Here we tested the assumption that spontaneous activity in the auditory pathways, following deafferentation, underlies these hallucinations and is related to their phenomenology. By extracting (fractional) Amplitude of Low Frequency Fluctuation [(f)ALFF] scores from resting state fMRI of 18 hearing impaired patients with complex hallucinations (voices or music), 18 hearing impaired patients with simple hallucinations (tinnitus or murmuring), and 20 controls with normal hearing, we investigated differences in spontaneous brain activity between these groups. Spontaneous activity in the anterior and posterior cingulate cortex of hearing-impaired groups was significantly higher than in the controls. The group with complex hallucinations showed elevated activity in the bilateral temporal cortex including Wernicke's area, while spontaneous activity of the group with simple hallucinations was mainly located in the cerebellum. These results suggest a decrease in error monitoring in both hearing-impaired groups. Spontaneous activity of language-related areas only in complex hallucinations suggests that the manifestation of the spontaneous activity represents the phenomenology of the hallucination. The link between cerebellar activity and simple hallucinations, such as tinnitus, is new and may have consequences for treatment.

Low frequency transcranial magnetic stimulation in subacute ischemic stroke: Number of sessions that altered cortical excitability

BACKGROUND

Cortical reorganization between both cerebral hemispheres plays an important role in regaining the affected upper extremity motor function post-stroke.

OBJECTIVES

The purpose of the current study was to investigate the recommended number of contra-lesion low frequency repetitive transcranial magnetic stimulation (LF-rTMS) sessions that could enhance cortical reorganization post-stroke.

METHODS

Forty patients with right hemiparetic subacute ischemic stroke with an age range between 50-65 yrs were randomly assigned into two equal groups: control (GA) and study (GB) groups. Both groups were treated with a selected physical therapy program for the upper limb. Sham and real contra-lesion LF-rTMS was conducted for both groups daily for two consecutive weeks. Sequential changes of cortical excitability were calculated by the end of each session.

RESULTS

The significant enhancement in the cortical excitability was observed at the fourth session in favor of the study group (GB). Sequential rate of change in cortical excitability was significant for the first eight sessions. From the ninth session onwards, no difference could be detected between groups.

CONCLUSION

The pattern of recovery after stroke is extensive and not all factors could be controlled. Application of LF-rTMS in conjugation with a selected physical therapy program for the upper limb from four to eight sessions seems to be efficient.

High-Frequency Repetitive Transcranial Magnetic Stimulation Mediates Autophagy Flux in Human Bone Mesenchymal Stromal Cells via NMDA Receptor-Ca2+-Extracellular Signal-Regulated Kinase-Mammalian Target of Rapamycin Signaling

Aim

Repetitive transcranial magnetic stimulation (rTMS) is a non-invasive and safe technique for treatment of central and peripheral nerve injury. In recent years, this technique has been widely used in clinic, and an increasing number of studies have reported its mechanisms. In this study, we investigated the mechanisms of rTMS-mediated autophagy flux in human bone mesenchymal stromal cells (BMSCs).

Methods

A frequency of 50 Hz was employed. Cells were divided into five groups: (1) normal, (2) sham, (3) 0.5 T, (4) 1.0 T, and (5) 1.5 T. Cells were stimulated for 20 min/day. The levels of p62, LC3-II/I, phosphorylated extracellular signal-regulated kinase (p-ERK), ERK, phosphorylated-AKT (p-AKT), AKT, phosphorylated mammalian target of rapamycin (p-mTOR), mTOR, phosphorylated protein kinase A (p-PKA), PKA, phosphorylated epidermal growth factor receptor (p-EGFR), EGFR, Nanog, Oct4, Sox2, and NMDA receptor (NMDAR1) were investigated by western blotting. Intracellular calcium (Ca²⁺) levels were quantified by flow cytometry. p62 and LC3 expression was also assessed by immunofluorescence analysis.

Results

In the 0.5 T group, rTMS increased the expression of LC3-II/I, p-ERK/ERK, and NMDAR1 and decreased the levels of p62 and p-mTOR/mTOR than in the normal group. The ratio of p-AKT/AKT, p-PKA/PKA, and p-EGFR/EGFR and the expression of Nanog, Oct4, and Sox2 remained unchanged. Immunofluorescence analysis revealed colocalization of p62 with LC3 puncta, and flow cytometry analysis displayed that Ca²⁺ levels were elevated. However, in the 1.0 and 1.5 T groups, no changes in the expression of these autophagy markers were observed.

Conclusion

In the 0.5 T group, high-frequency rTMS can induce autophagy through NMDAR–Ca²⁺–ERK–mTOR signaling in BMSCs. In the 1.0 and 1.5 T groups, autophagy is not activated.

Low-intensity contralesional electrical theta burst stimulation modulates ipsilesional excitability and enhances stroke recovery

Targeting interhemispheric inhibition using brain stimulation has shown potential for enhancing stroke recovery. Following stroke, increased inhibition originating from the contralesional hemisphere impairs motor activation in ipsilesional areas. We have previously reported that low-intensity electrical theta burst stimulation (TBS) applied to an implanted electrode in the contralesional rat motor cortex reduces interhemispheric inhibition, and improves functional recovery when commenced three days after cortical injury. Here we apply this approach at more clinically relevant later time points and measure recovery from photothrombotic stroke, following three weeks of low-intensity intermittent TBS (iTBS), continuous TBS (cTBS) or sham stimulation applied to the contralesional motor cortex. Interhemispheric inhibition and cellular excitability were measured in the same rats from single pyramidal neurons in the peri-infarct area, using in vivo intracellular recording. A minimal dose of iTBS did not enhance motor function when applied beginning one month after stroke. However both a high and a low dose of iTBS improved recovery to a similar degree when applied 10 days after stroke, with the degree of recovery positively correlated with ipsilesional excitability. The final level of interhemispheric inhibition was negatively correlated with excitability, but did not independently correlate with functional recovery. In contrast, contralesional cTBS left recovery unaltered, but decreased ipsilesional excitability. These data support focal contralesional iTBS and not cTBS as an intervention for enhancing stroke recovery and suggest that there is a complex relationship between functional recovery and interhemispheric inhibition, with both independently associated with ipsilesional excitability.

Intermittent theta burst stimulation enhances upper limb motor function in patients with chronic stroke: a pilot randomized controlled trial

BACKGROUND

Intermittent theta burst stimulation (iTBS) is a form of repetitive transcranial stimulation that has been used to enhance upper limb (UL) motor recovery. However, only limited studies have examined its efficacy in patients with chronic stroke and therefore it remains controversial.

METHODS

This was a randomized controlled trial that enrolled patients from a rehabilitation department. Twenty-two patients with first-ever chronic and unilateral cerebral stroke, aged 30-70 years, were randomly assigned to the iTBS or control group. All patients received 1 session per day for 10 days of either iTBS or sham stimulation over the ipsilesional primary motor cortex in addition to conventional neurorehabilitation. Outcome measures were assessed before and immediately after the intervention period: Modified Ashworth Scale (MAS), Fugl-Meyer Assessment Upper Extremity (FMA-UE), Action Research Arm Test (ARAT), Box and Block test (BBT), and Motor Activity Log (MAL). Analysis of covariance was adopted to compare the treatment effects between groups.

RESULTS

The iTBS group had greater improvement in the MAS and FMA than the control group (η² = 0.151-0.233; p < 0.05), as well as in the ARAT and BBT (η² = 0.161-0.460; p < 0.05) with large effect size. Both groups showed an improvement in the BBT, and there were no significant between-group differences in MAL changes.

CONCLUSIONS

The iTBS induced greater gains in spasticity decrease and UL function improvement, especially in fine motor function, than sham TBS. This is a promising finding because patients with chronic stroke have a relatively low potential for fine motor function recovery. Overall, iTBS may be a beneficial adjunct therapy to neurorehabilitation for enhancing UL function. Further larger-scale study is warranted to confirm the findings and its long-term effect.

TRIAL REGISTRATION

This trial was registered under ClinicalTrials.gov ID No. NCT01947413 on September 20, 2013.