Modulation of cortical excitability induced by repetitive transcranial magnetic stimulation: Influence of timing and geometrical parameters and underlying mechanisms

Neuroplasticity-based brain stimulation interventions in the study and treatment of schizophrenia: a review

We reviewed novel brain stimulation approaches that modify neuroplasticity and are used in the treatment and study of schizophrenia. We searched PubMed and Scholars Portal using search terms related to schizophrenia, brain stimulation, and neuroplasticity. Various brain stimulation approaches simulating a range of experimental protocols that induce synaptic long-term potentiation or depression have been developed. By far, repetitive transcranial magnetic stimulation (rTMS) has been the most widely used in the field of schizophrenia. Its application has been associated with mixed results in treating treatment-resistant symptoms and cognitive deficits associated with schizophrenia. Compared to the other approaches, rTMS is probably the least similar to plasticity-inducing cellular paradigms. Other approaches, such as paired associative stimulation, theta-burst stimulation, and transcranial direct current stimulation, are in their incipient stages in the study and treatment of schizophrenia, with promising early results. Numerous brain stimulation approaches have been developed to treat resistant dimensions of schizophrenia. Notwithstanding some promising reports, optimization of the methods and large randomized controlled trials are still needed.

Addiction

Drug and alcohol addiction is a debilitating disorder characterized by persistent drug-seeking behaviors despite negative physiological, medical, or social consequences. Neurobiological models of addiction propose that the reinforcing effects of addictive drugs are associated with altered neurotransmission within the reward 'mesocorticolimbic' circuitry in the brain. Immense efforts are therefore designed to target the mesocorticolimbic circuitry in attenuating drug dependence and addiction-related behaviors. Yet, to date, most addiction treatments have demonstrated only limited success in reducing addiction-related behaviors. Accumulating and compelling evidence suggests that novel nonsurgical brain stimulation techniques, such as transcranial magnetic stimulation and transcranial direct current stimulation, could serve as promising tools for indexing altered neurotransmission associated with repetitive drug use, and moreover, may hold therapeutic potential for the treatment of drug dependence and addiction-related behaviors. This chapter reviews and discusses the current and potential applications of such techniques in the study and treatment of addiction; we focus on a number of common drugs of abuse, including nicotine, alcohol, cocaine, cannabis, and ecstasy.

Mechanisms underlying long-interval cortical inhibition in the human motor cortex: a TMS-EEG study

Long-interval cortical inhibition (LICI) refers to suppression of neuronal activity following paired-pulse transcranial magnetic stimulation (TMS) with interstimulus intervals (ISIs) between 50 and 200 ms. LICI can be measured either from motor-evoked potentials (MEPs) in small hand muscles or directly from the cortex using concurrent electroencephalography (EEG). However, it remains unclear whether EEG inhibition reflects similar mechanisms to MEP inhibition. Eight healthy participants received single- and paired-pulse TMS (ISI = 100 ms) over the motor cortex. MEPs were measured from a small hand muscle (first dorsal interosseus), whereas early (P30, P60) and late (N100) TMS-evoked cortical potentials (TEPs) were measured over the motor cortex using EEG. Conditioning and test TMS intensities were altered, and modulation of LICI strength was measured using both methods. LICI of MEPs and both P30 and P60 TEPs increased in strength with increasing conditioning intensities and decreased with increasing test intensities. LICI of N100 TEPs remained unchanged across all conditions. In addition, MEP and P30 LICI strength correlated with the slope of the N100 evoked by the conditioning pulse. LICI of early and late TEP components was differentially modulated with altered TMS intensities, suggesting independent underlying mechanisms. LICI of P30 is consistent with inhibition of cortical excitation similar to MEPs, whereas LICI of N100 may reflect presynaptic autoinhibition of inhibitory interneurons. The N100 evoked by the conditioning pulse is consistent with the mechanism responsible for LICI, most likely GABAB-mediated inhibition of cortical activity.

Quantifying the effect of repetitive transcranial magnetic stimulation in the rat brain by μSPECT CBF scans

BACKGROUND

Repetitive transcranial magnetic stimulation (rTMS) is used to treat neurological and psychiatric disorders such as depression and addiction amongst others. Neuro-imaging by means of SPECT is a non-invasive manner of evaluating regional cerebral blood flow (rCBF) changes, which are assumed to reflect changes in neural activity.

OBJECTIVE

rCBF changes induced by rTMS are evaluated by comparing stimulation on/off in different stimulation paradigms using microSPECT of the rat brain.

METHODS

Rats (n = 6) were injected with 10 mCi of (99m)Tc-HMPAO during application of two rTMS paradigms (1 Hz and 10 Hz, 1430 A at each wing of a 20 mm figure-of-eight coil) and sham. SPM- and VOI-based analysis was performed.

RESULTS

rTMS caused widespread significant hypoperfusion throughout the entire rat brain. Differences in spatial extent and intensity of hypoperfusion were observed between both stimulation paradigms: 1 Hz caused significant hypoperfusion (P < 0.05) in 11.9% of rat brain volume while 10 Hz caused this in 23.5%; the minimal t-value induced by 1 Hz was -24.77 while this was -17.98 due to 10 Hz. Maximal percentage of hypoperfused volume due to 1 Hz and 10 Hz was reached at tissue experiencing 0.03-0.15 V/m.

CONCLUSION

High-frequency (10 Hz) stimulation causes more widespread hypoperfusion, while 1 Hz induces more pronounced hypoperfusion. The effect of rTMS is highly dependent on the electric field strength in the brain tissue induced by the TMS coil. This innovative imaging approach can be used as a fast screening tool in quantifying and evaluating the effect of various stimulation paradigms and coil designs for TMS and offers a means for research and development.

Complex magnetic field exposure system for in vitro experiments at intermediate frequencies

In occupational environments, an increasing number of electromagnetic sources emitting complex magnetic field waveforms in the range of intermediate frequencies is present, requiring an accurate exposure risk assessment with both in vitro and in vivo experiments. In this article, an in vitro exposure system able to generate complex magnetic flux density B-fields, reproducing signals from actual intermediate frequency sources such as magnetic resonance imaging (MRI) scanners, for instance, is developed and validated. The system consists of a magnetic field generation system and an exposure apparatus realized with a couple of square coils. A wide homogeneity (99.9%) volume of 210 × 210 × 110 mm(3) was obtained within the coils, with the possibility of simultaneous exposure of a large number of standard Petri dishes. The system is able to process any numerical input sequence through a filtering technique aimed at compensating the coils' impedance effect. The B-field, measured in proximity to a 1.5 T MRI bore during a typical examination, was excellently reproduced (cross-correlation index of 0.99). Thus, it confirms the ability of the proposed setup to accurately simulate complex waveforms in the intermediate frequency band. Suitable field levels were also attained. Moreover, a dosimetry index based on the weighted-peak method was evaluated considering the induced E-field on a Petri dish exposed to the reproduced complex B-field. The weighted-peak index was equal to 0.028 for the induced E-field, indicating an exposure level compliant with the basic restrictions of the International Commission on Non-Ionizing Radiation Protection. Bioelectromagnetics 34:211-219, 2013. © 2012 Wiley Periodicals, Inc.

Fundamentals of transcranial electric and magnetic stimulation dose: definition, selection, and reporting practices

BACKGROUND

The growing use of transcranial electric and magnetic (EM) brain stimulation in basic research and in clinical applications necessitates a clear understanding of what constitutes the dose of EM stimulation and how it should be reported.

METHODS

This paper provides fundamental definitions and principles for reporting of dose that encompass any transcranial EM brain stimulation protocol.

RESULTS

The biologic effects of EM stimulation are mediated through an electromagnetic field injected (via electric stimulation) or induced (via magnetic stimulation) in the body. Therefore, transcranial EM stimulation dose ought to be defined by all parameters of the stimulation device that affect the electromagnetic field generated in the body, including the stimulation electrode or coil configuration parameters: shape, size, position, and electrical properties, as well as the electrode or coil current (or voltage) waveform parameters: pulse shape, amplitude, width, polarity, and repetition frequency; duration of and interval between bursts or trains of pulses; total number of pulses; and interval between stimulation sessions and total number of sessions. Knowledge of the electromagnetic field generated in the body may not be sufficient but is necessary to understand the biologic effects of EM stimulation.

CONCLUSIONS

We believe that reporting of EM stimulation dose should be guided by the principle of reproducibility: sufficient information about the stimulation parameters should be provided so that the dose can be replicated.

Visual attentional load influences plasticity in the human motor cortex

Neural plasticity plays a critical role in learning, memory, and recovery from injury to the nervous system. Although much is known about the physical and physiological determinants of plasticity, little is known about the influence of cognitive factors. In this study, we investigated whether selective attention plays a role in modifying changes in neural excitability reflecting long-term potentiation (LTP)-like plasticity. We induced LTP-like effects in the hand area of the human motor cortex using transcranial magnetic stimulation (TMS). During the induction of plasticity, participants engaged in a visual detection task with either low or high attentional demands. Changes in neural excitability were assessed by measuring motor-evoked potentials in a small hand muscle before and after the TMS procedures. In separate experiments plasticity was induced either by paired associative stimulation (PAS) or intermittent theta-burst stimulation (iTBS). Because these procedures induce different forms of LTP-like effects, they allowed us to investigate the generality of any attentional influence on plasticity. In both experiments reliable changes in motor cortex excitability were evident under low-load conditions, but this effect was eliminated under high-attentional load. In a third experiment we investigated whether the attentional task was associated with ongoing changes in the excitability of motor cortex, but found no difference in evoked potentials across the levels of attentional load. Our findings indicate that in addition to their role in modifying sensory processing, mechanisms of attention can also be a potent modulator of cortical plasticity.

Exendin-4 protected against cognitive dysfunction in hyperglycemic mice receiving an intrahippocampal lipopolysaccharide injection

Background

Chronic hyperglycemia-associated inflammation plays critical roles in disease initiation and the progression of diabetic complications, including Alzheimer’s disease (AD). However, the association of chronic hyperglycemia with acute inflammation of the central nervous system in the progression of AD still needs to be elucidated. In addition, recent evidence suggests that Glucagon-like peptide-1 receptor (GLP-1R) protects against neuronal damage in the brain. Therefore, the neuroprotective effects of the GLP-1R agonist exendin-4 (EX-4) against hyperglycemia/lipopolysaccharides (LPS) damage were also evaluated in this study.

Methodology/Principal Findings

Ten days after streptozotocin (STZ) or vehicle (sodium citrate) treatment in mice, EX-4 treatment (10 µg/kg/day) was applied to the mice before intrahippocampal CA1 injection of LPS or vehicle (saline) and continued for 28 days. This study examined the molecular alterations in these mice after LPS and EX4 application, respectively. The mouse cognitive function was evaluated during the last 6 days of EX-4 treatment. The results showed that the activation of NF-κB-related inflammatory responses induced cognitive dysfunction in both the hyperglycemic mice and the mice that received acute intrahippocampal LPS injection. Furthermore, acute intrahippocampal LPS injection exacerbated the impairment of spatial learning and memory through a strong decrease in monoaminergic neurons and increases in astrocytes activation and apoptosis in the hyperglycemic mice. However, EX-4 treatment protected against the cognitive dysfunction resulting from hyperglycemia or/and intrahippocampal LPS injection.

Conclusions/Significance

These findings reveal that both hyperglycemia and intrahippocampal LPS injection induced cognitive dysfunction via activation of NF-κB-related inflammatory responses. However, acute intrahippocampal LPS injection exacerbated the progression of cognitive dysfunction in the hyperglycemic mice via a large increase in astrocytes activation-related responses. Furthermore, EX-4 might be considered as a potential adjuvant entity to protect against neurodegenerative diseases.

Translational neuromodulation: approximating human transcranial magnetic stimulation protocols in rats

OBJECTIVE

Transcranial magnetic stimulation (TMS) is a well-established clinical protocol with numerous potential therapeutic and diagnostic applications. Yet, much work remains in the elucidation of TMS mechanisms, optimization of protocols, and in development of novel therapeutic applications. As with many technologies, the key to these issues lies in the proper experimentation and translation of TMS methods to animal models, among which rat models have proven popular. A significant increase in the number of rat TMS publications has necessitated analysis of their relevance to human work. We therefore review the essential principles for the approximation of human TMS protocols in rats as well as specific methods that addressed these issues in published studies.

MATERIALS AND METHODS

We performed an English language literature search combined with our own experience and data. We address issues that we see as important in the translation of human TMS methods to rat models and provide a summary of key accomplishments in these areas.

RESULTS

  An extensive literature review illustrated the growth of rodent TMS studies in recent years. Current advances in the translation of single, paired-pulse, and repetitive stimulation paradigms to rodent models are presented. The importance of TMS in the generation of data for preclinical trials is also highlighted.

CONCLUSIONS

Rat TMS has several limitations when considering parallels between animal and human stimulation. However, it has proven to be a useful tool in the field of translational brain stimulation and will likely continue to aid in the design and implementation of stimulation protocols for therapeutic and diagnostic applications.

Spike suppression in a local cortical circuit induced by transcranial magnetic stimulation

Transcranial magnetic stimulation (TMS) noninvasively interferes with human cortical function, and is widely used as an effective technique for probing causal links between neural activity and cognitive function. However, the physiological mechanisms underlying TMS-induced effects on neural activity remain unclear. We examined the mechanism by which TMS disrupts neural activity in a local circuit in early visual cortex using a computational model consisting of conductance-based spiking neurons with excitatory and inhibitory synaptic connections. We found that single-pulse TMS suppressed spiking activity in a local circuit model, disrupting the population response. Spike suppression was observed when TMS was applied to the local circuit within a limited time window after the local circuit received sensory afferent input, as observed in experiments investigating suppression of visual perception with TMS targeting early visual cortex. Quantitative analyses revealed that the magnitude of suppression was significantly larger for synaptically-connected neurons than for isolated individual neurons, suggesting that intracortical inhibitory synaptic coupling also plays an important role in TMS-induced suppression. A conventional local circuit model of early visual cortex explained only the early period of visual suppression observed in experiments. However, models either involving strong recurrent excitatory synaptic connections or sustained excitatory input were able to reproduce the late period of visual suppression. These results suggest that TMS targeting early visual cortex disrupts functionally distinct neural signals, possibly corresponding to feedforward and recurrent information processing, by imposing inhibitory effects through intracortical inhibitory synaptic connections.

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.

Insights into cortical mechanisms of behavior from microstimulation experiments

Even the simplest behaviors depend on a large number of neurons that are distributed across many brain regions. Because electrical microstimulation can change the activity of localized subsets of neurons, it has provided valuable evidence that specific neurons contribute to particular behaviors. Here we review what has been learned about cortical function from behavioral studies using microstimulation in animals and humans. Experiments that examine how microstimulation affects the perception of stimuli have shown that the effects of microstimulation are usually highly specific and can be related to the stimuli preferred by neurons at the stimulated site. Experiments that ask subjects to detect cortical microstimulation in the absence of other stimuli have provided further insights. Although subjects typically can detect microstimulation of primary sensory or motor cortex, they are generally unable to detect stimulation of most of cortex without extensive practice. With practice, however, stimulation of any part of cortex can become detected. These training effects suggest that some patterns of cortical activity cannot be readily accessed to guide behavior, but that the adult brain retains enough plasticity to learn to process novel patterns of neuronal activity arising anywhere in cortex.

Transcranial pulsed magnetic field stimulation facilitates reorganization of abnormal neural circuits and corrects behavioral deficits without disrupting normal connectivity

Although the organization of neuronal circuitry is shaped by activity patterns, the capacity to modify and/or optimize the structure and function of whole projection pathways using external stimuli is poorly defined. We investigate whether neuronal activity induced by pulsed magnetic fields (PMFs) alters brain structure and function. We delivered low-intensity PMFs to the posterior cranium of awake, unrestrained mice (wild-type and ephrin-A2A5(-/-)) that have disorganized retinocollicular circuitry and associated visuomotor deficits. Control groups of each genotype received sham stimulation. Following daily stimulation for 14 d, we measured biochemical, structural (anterograde tracing), and functional (electrophysiology and behavior) changes in the retinocollicular projection. PMFs induced BDNF, GABA, and nNOS expression in the superior colliculus and retina of wild-type and ephrin-A2A5(-/-) mice. Furthermore, in ephrin-A2A5(-/-) mice, PMFs corrected abnormal neuronal responses and selectively removed inaccurate ectopic axon terminals to improve structural and functional organization of their retinocollicular projection and restore normal visual tracking behavior. In contrast, PMFs did not alter the structure or function of the normal projection in wild-type mice. Sham PMF stimulation had no effect on any mice. Thus, PMF-induced biochemical changes are congruent with its capacity to facilitate beneficial reorganization of abnormal neural circuits without disrupting normal connectivity and function.

Modulating the brain at work using noninvasive transcranial stimulation

This paper proposes a shift in the way researchers currently view and use transcranial brain stimulation technologies. From a neuroscience perspective, the standard application of both transcranial magnetic stimulation (TMS) and transcranial direct current stimulation (tDCS) has been mainly to explore the function of various brain regions. These tools allow for noninvasive and painless modulation of cortical tissue. In the course of studying the function of an area, many studies often report enhanced performance of a task during or following the stimulation. However, little follow-up research is typically done to further explore these effects. Approaching this growing pool of cognitive neuroscience literature with a neuroergonomics mindset (i.e., studying the brain at work), the possibilities of using these stimulation techniques for more than simply investigating the function of cortical areas become evident. In this paper, we discuss how cognitive neuroscience brain stimulation studies may complement neuroergonomics research on human performance optimization. And, through this discussion, we hope to shift the mindset of viewing transcranial stimulation techniques as solely investigatory basic science tools or possible clinical therapeutic devices to viewing transcranial stimulation techniques as interventional tools to be incorporated in applied science research and systems for the augmentation and enhancement of human operator performance.

Deep transcranial magnetic stimulation add-on for treatment of negative symptoms and cognitive deficits of schizophrenia: a feasibility study

Treatment for negative symptoms and cognitive deficits, core elements of schizophrenia, remains inadequate. Stimulation of the prefrontal cortex via transcranial magnetic stimulation (TMS) yields only moderate results, possibly due to limited stimulation depth. Deep-TMS enables deeper and wider stimulation than before. This preliminary study is the first to examine deep-TMS as a possible add-on treatment for negative symptoms and cognitive deficits of schizophrenia. The effect of 20 daily deep-TMS sessions (20 Hz, 120% motor threshold) over the prefrontal cortex of 15 patients indicated improvement in cognition and negative symptoms that was maintained at 2-wk post-treatment follow-up.

Long-term effects of repetitive transcranial magnetic stimulation on markers for neuroplasticity: differential outcomes in anesthetized and awake animals

Long-term effects of repetitive transcranial magnetic stimulation (rTMS) have been associated with neuroplasticity, but most physiological studies have evaluated only the immediate effects of the stimulation on neurochemical markers. Furthermore, although it is known that baseline excitability state plays a major role in rTMS outcomes, the role of spontaneous neural activity in metaplasticity has not been investigated. The first aim of this study was to evaluate and compare the long-term effects of high- and low-frequency rTMS on the markers of neuroplasticity such as BDNF and GluR1 subunit of AMPA receptor. The second aim was to assess whether these effects depend on spontaneous neural activity, by comparing the neurochemical alterations induced by rTMS in anesthetized and awake rats. Ten daily sessions of high- or low-frequency rTMS were applied over the rat brain, and 3 d later, levels of BDNF, GluR1, and phosphorylated GluR1 were assessed in the hippocampus, prelimbic cortex, and striatum. We found that high-frequency stimulation induced a profound effect on neuroplasticity markers; increasing them in awake animals while decreasing them in anesthetized animals. In contrast, low-frequency stimulation did not induce significant long-term effects on these markers in either state. This study highlights the importance of spontaneous neural activity during rTMS and demonstrates that high-frequency rTMS can induce long-lasting effects on BDNF and GluR1 which may underlie the clinical benefits of this treatment in neuroplasticity-related disorders.

Modulation of cortical inhibition by rTMS - findings obtained from animal models

Transcranial magnetic stimulation (TMS) has become a popular method to non-invasively stimulate the human brain. The opportunity to modify cortical excitability with repetitive stimulation (rTMS) has especially gained interest for its therapeutic potential. However, details of the cellular mechanisms of the effects of rTMS are scarce. Currently favoured are long-term changes in the efficiency of excitatory synaptic transmission, with low-frequency rTMS depressing it, but high-frequency rTMS augmenting. Only recently has modulation of cortical inhibition been considered as an alternative way to explain lasting changes in cortical excitability induced by rTMS. Adequate animal models help to highlight stimulation-induced changes in cellular processes which are not assessable in human rTMS studies. In this review article, we summarize findings obtained with our rat models which indicate that distinct inhibitory cell classes, like the fast-spiking cells characterized by parvalbumin expression, are most sensitive to certain stimulation protocols, e.g. intermittent theta burst stimulation. We discuss how our findings can support the recently suggested models of gating and homeostatic plasticity as possible mechanisms of rTMS-induced changes in cortical excitability.