The facilitatory effects of intermittent theta burst stimulation on corticospinal excitability are enhanced by nicotine

Managing substance use in patients receiving therapeutic repetitive transcranial magnetic stimulation: A scoping review

Repetitive Transcranial Magnetic Stimulation (rTMS) is an invaluable treatment option for neuropsychiatric disorders. Co-occurring recreational and nonmedical substance use can be common in those presenting for rTMS treatment, and it is unknown how it may affect the safety and efficacy of rTMS for the treatment of currently approved neuropsychiatric indications. This scoping review aimed to map the literature on humans receiving rTMS and had a history of any type of substance use. The search identified 274 articles providing information on inclusion/exclusion criteria, withdrawal criteria, safety protocols, type of rTMS and treatment parameters, adverse events and effect on primary outcomes that related to substance use. There are neurophysiological effects of substance use on cortical excitability, although the relevance to clinical rTMS practice is unknown. The current literature supports the safety and feasibility of delivering rTMS to those who have co-occurring neuropsychiatric disorder and substance use. However, specific details on how varying degrees of substance use alters the safety, efficacy, and mechanisms of rTMS remains poorly described.

The Effects of Nicotine on Cortical Excitability After Exercise: A Double-Blind Randomized, Placebo-controlled, Crossover Study

PURPOSE

The use of smokeless tobacco/nicotine products is common among athletes, but clear evidence for their positive or negative effect on sports performance is lacking. Nicotine is a psychoactive substance involved in numerous neuronal processes including cortical excitability. The aim of this study was to evaluate its effect on cortical excitability associated with aerobic exercise in nicotine-naive healthy volunteers.

METHODS

Ten nicotine-naive healthy volunteers were recruited for this double-blind, randomized, crossover study to compare the effect of snus (8 mg nicotine), an oral, smokeless tobacco product, to placebo on cortical excitability before and after aerobic exercise. Transcranial magnetic stimulation (TMS) was used to measure changes in corticomotor excitability (motor-evoked potentials, MEPs) and electromyography of leg muscles during maximal voluntary contractions (MVC) to assess changes in muscle contractions. Before and after aerobic exercise and with or without nicotine treatment, MEPs and MVCs were measured.

RESULTS

Analysis of TMS data showed lower motor cortex activation (lower MEP amplitude) after snus administration compared with placebo, whereas electromyography data showed no difference in muscle contraction between snus and placebo treatment.

CONCLUSIONS

These findings suggest a general reduction in cortical excitability, without no relevant effect on physical performance.

Effects of rTMS on the brain: is there value in variability?

The ability of repetitive transcranial magnetic stimulation (rTMS) to non-invasively induce neuroplasticity in the human cortex has opened exciting possibilities for its application in both basic and clinical research. Changes in the amplitude of motor evoked potentials (MEPs) elicited by single-pulse transcranial magnetic stimulation has so far provided a convenient model for exploring the neurophysiology of rTMS effects on the brain, influencing the ways in which these stimulation protocols have been applied therapeutically. However, a growing number of studies have reported large inter-individual variability in the mean MEP response to rTMS, raising legitimate questions about the usefulness of this model for guiding therapy. Although the increasing application of different neuroimaging approaches has made it possible to probe rTMS-induced neuroplasticity outside the motor cortex to measure changes in neural activity that impact other aspects of human behaviour, the high variability of rTMS effects on these measurements remains an important issue for the field to address. In this review, we seek to move away from the conventional facilitation/inhibition dichotomy that permeates much of the rTMS literature, presenting a non-standard approach for measuring rTMS-induced neuroplasticity. We consider the evidence that rTMS is able to modulate an individual's moment-to-moment variability of neural activity, and whether this could have implications for guiding the therapeutic application of rTMS.

The two-way relationship between nicotine and cortical activity: a systematic review of neurobiological and treatment aspects

Nicotine intake and cortical activity are closely related, as they can influence each other. Nicotine is implicated in the induction and modification of cortical plasticity and excitability, whereas a change on cortical plasticity and excitability can also lead to a modification of the smoking behaviour of an individual. The aim of this systematic review was, on the one hand, to evaluate the effects of nicotinergic modulation on cortical excitability and plasticity, and, on the other hand, to assess if modifying the brain's excitability and plasticity could influence one's smoking behaviour. Two systematic literature searches in the PubMed/MEDLINE and PsycINFO databases were conducted. Studies focusing either on the impact of nicotinergic modulation on cortical activity or the treatment effect of non-invasive brain stimulation techniques (NIBS) on smoking behaviour were included. A total of 22 studies for the first systematic search and 35 studies for the second one were included after full-text screening. Nicotine's effect on cortical activity appeared to depend on smoking status of the individual. While deprived smokers seem to generally profit from nicotine consumption in terms of cortical excitability and plasticity, the contrary was true for non-smokers. Regarding the questions of how changes in cortical excitability can influence smoking behaviour, a trend points towards NIBS being a potential intervention technique for smoking cessation.

The effect of psychotropic drugs on cortical excitability and plasticity measured with transcranial magnetic stimulation: Implications for psychiatric treatment

OBJECTIVE

Repetitive transcranial magnetic stimulation (rTMS) is an emerging treatment for neuropsychiatric disorders. Patients in rTMS treatment typically receive concomitant psychotropic medications, which affect neuronal excitability and plasticity and may interact to affect rTMS treatment outcomes. A greater understanding of these drug effects may have considerable implications for optimizing multi-modal treatment of psychiatric patients, and elucidating the mechanism(s) of action (MOA) of rTMS.

METHOD

We summarized the empirical literature that tests how psychotropic drugs affect cortical excitability and plasticity, using varied experimental TMS paradigms.

RESULTS

Glutamate antagonists robustly attenuate plasticity, largely without changes in excitability per se; antiepileptic drugs show the opposite pattern of effects, while calcium channel blockers attenuate plasticity. Benzodiazepines have moderate and variable effects on plasticity, and negligible effects on excitability. Antidepressants with potent 5HT transporter inhibition reduce both excitability and alter plasticity, while antidepressants with other MOAs generally lack either effect. Catecholaminergic drugs, cholinergic agents and lithium have minimal effects on excitability but exhibit robust and complex, non-linear effects in TMS plasticity paradigms.

LIMITATIONS

These effects remain largely untested in sustained treatment protocols, nor in clinical populations. In addition, how these medications impact clinical response to rTMS remains largely unknown.

CONCLUSIONS

Psychotropic medications exert robust and varied effects on cortical excitability and plasticity. We encourage the field to more directly and fully investigate clinical pharmaco-TMS studies to improve outcomes.

Use of theta-burst stimulation in changing excitability of motor cortex: A systematic review and meta-analysis

Noninvasive brain stimulation has been demonstrated to modulate cortical activity in humans. In particular, theta burst stimulation (TBS) has gained notable attention due to its ability to induce lasting physiological changes after short stimulation durations. The present study aimed to provide a comprehensive meta-analytic review of the efficacy of two TBS paradigms; intermittent (iTBS) and continuous (cTBS), on corticospinal excitability in healthy individuals. Literature searches yielded a total of 87 studies adhering to the inclusion criteria. iTBS yielded moderately large MEP increases lasting up to 30 min with a pooled SMD of 0.71 (p<0.00001). cTBS produced a reduction in MEP amplitudes lasting up to 60 min, with the largest effect size seen at 5 min post stimulation (SMD=-0.9, P<0.00001). The collected studies were of heterogeneous nature, and a series of tests conducted indicated a degree of publication bias. No significant change in SICI and ICF was observed, with exception to decrease in SICI with cTBS at the early time point (SMD=0.42, P=0.00036). The results also highlight several factors contributing to TBS efficacy, including the number of pulses, frequency of stimulation and BDNF polymorphisms. Further research investigating optimal TBS stimulation parameters, particularly for iTBS, is needed in order for these paradigms to be successfully translated into clinical settings.

Short-latency afferent inhibition is a poor predictor of individual susceptibility to rTMS-induced plasticity in the motor cortex of young and older adults

Cortical plasticity, including long-term potentiation (LTP)-like plasticity, can be assessed non-invasively with repetitive transcranial magnetic stimulation (rTMS) protocols. In this study, we examined age differences in responses to intermittent theta burst stimulation (iTBS) in a group of 20 young and 18 healthy older adults. Because the cholinergic system plays a role in the neural processes underlying learning and memory, including LTP, we also investigated whether short latency afferent inhibition (SAI), a neurophysiological marker of central cholinergic activity, would be associated with age-related differences in LTP-like plasticity induced by iTBS.

Methods: SAI was first assessed by examining the modulation of motor evoked potentials (MEPs) in response to median nerve conditioning 20 ms prior to TMS. Participants then underwent iTBS (3 pulses at 50 Hz every 200 ms for 2 s with 8 s between trains, repeated 20 times). MEP responses (120% resting motor threshold (RMT)) were assessed immediately after iTBS and 5, 10, and 20 min post-application.

Results: Responses to iTBS were quite variable in both age groups, with only approximately 60% of the participants (n = 13 young and 10 older adults) showing the expected facilitation of MEP responses. There were no significant age group differences in MEP facilitation following iTBS. Although older adults exhibited reduced SAI, individual variations were not associated with susceptibility to express LTP-like induced plasticity after iTBS.

Conclusion: Overall, these results are consistent with reports of high inter-individual variability in responses to iTBS. Although SAI was reduced in older adults, consistent with a deterioration of the cholinergic system with age, SAI levels were not associated with LTP-like plasticity as assessed with iTBS.

Cortical and brainstem plasticity in Tourette syndrome and obsessive-compulsive disorder

Gilles de la Tourette syndrome is characterized by motor/vocal tics commonly associated with psychiatric disorders, including obsessive-compulsive disorder. We investigated primary motor cortex and brainstem plasticity in Tourette patients, exposed and unexposed to chronic drug treatment, with and without psychiatric disturbances. We also investigated primary motor cortex and brainstem plasticity in obsessive-compulsive disorder. We studied 20 Tourette patients with and without psychiatric disturbances, 15 with obsessive-compulsive disorder, and 20 healthy subjects. All groups included drug-naïve patients. We conditioned the left primary motor cortex with intermittent/continuous theta-burst stimulation and recorded motor evoked potentials. We conditioned the supraorbital nerve with facilitatory/inhibitory high-frequency stimulation and recorded the blink reflex late response area. In healthy subjects, intermittent theta-burst increased and continuous theta-burst stimulation decreased motor evoked potentials. Differently, intermittent theta-burst failed to increase and continuous theta-burst stimulation failed to decrease motor evoked potentials in Tourette patients, with and without psychiatric disturbances. In obsessive-compulsive disorder, intermittent/continuous theta-burst stimulation elicited normal responses. In healthy subjects and in subjects with obsessive-compulsive disorder, the blink reflex late response area increased after facilitatory high-frequency and decreased after inhibitory high-frequency stimulation. Conversely, in Tourette patients, with and without psychiatric disturbances, facilitatory/inhibitory high-frequency stimulation left the blink reflex late response area unchanged. Theta-burst and high-frequency stimulation elicited similar responses in drug-naïve and chronically treated patients. Tourette patients have reduced plasticity regardless of psychiatric disturbances. These findings suggest that abnormal plasticity contributes to the pathophysiology of Gilles de la Tourette syndrome. However, obsessive-compulsive disorder patients have normal cortical and brainstem plasticity.

Rapid effect of nicotine intake on neuroplasticity in non-smoking humans

In various studies nicotine has shown to alter cognitive functions in non-smoking subjects. The physiological basis for these effects might be nicotine-generated modulation of cortical structure, excitability, and activity, as mainly described in animal experiments. In accordance, a recently conducted study demonstrated that application of nicotine for hours via nicotine patch in non-smoking humans alters the effects of neuroplasticity-inducing non-invasive brain stimulation techniques on cortical excitability. Specifically, nicotine abolished inhibitory plasticity independent from the focality of the stimulation protocol. While nicotine prevented also the establishment of non-focal facilitatory plasticity, focal synapse-specific facilitatory plasticity was enhanced. These results agree with a focusing effect of prolonged nicotine application on facilitatory plasticity. However, since nicotine induces rapid adaption processes of its receptors, this scenario might differ from the effect of nicotine in cigarette smoking. Thus in this study we aimed to gain further insight in the mechanism of nicotine on plasticity by exploring the effect of nicotine spray on non-focal and focal plasticity-inducing protocols in non-smoking subjects, a fast-acting agent better comparable to cigarette smoking. Focal, synapse-specific plasticity was induced by paired associative stimulation (PAS), while non-focal plasticity was elicited by transcranial direct current stimulation (tDCS). Forty eight non-smokers received nicotine spray respectively placebo combined with one of the following protocols (anodal tDCS, cathodal tDCS, PAS-25, and PAS-10). Corticospinal excitability was monitored via motor-evoked potentials elicited by transcranial magnetic stimulation (TMS). Nicotine spray abolished facilitatory plasticity irrespective of focality and PAS-10-induced excitability diminution, while tDCS-derived excitability reduction was delayed and weakened. Nicotine spray had thus a clear effect on neuroplasticity in non-smoking subjects. However, the effects of nicotine spray differ clearly from those of prolonged nicotine application, which might be due to missing adaptive nicotinic receptor alterations. These results enhance our knowledge about the dynamic impact of nicotine on plasticity, which might be related to its heterogenous effect on cognition.

Noninvasive brain stimulation for motor recovery after stroke: mechanisms and future views

Repetitive transcranial magnetic stimulation and transcranial direct current stimulation are noninvasive brain stimulation (NIBS) techniques that can alter excitability of the human cortex. Considering the interhemispheric competition occurring after stroke, improvement in motor deficits can be achieved by increasing the excitability of the affected hemisphere or decreasing the excitability of the unaffected hemisphere. Many reports have shown that NIBS application improves motor function in stroke patients by using their physiological peculiarity. For continuous motor improvement, it is important to impart additional motor training while NIBS modulates the neural network between both hemispheres and remodels the disturbed network in the affected hemisphere. NIBS can be an adjuvant therapy for developed neurorehabilitation strategies for stroke patients. Moreover, recent studies have reported that bilateral NIBS can more effectively facilitate neural plasticity and induce motor recovery after stroke. However, the best NIBS pattern has not been established, and clinicians should select the type of NIBS by considering the NIBS mechanism. Here, we review the underlying mechanisms and future views of NIBS therapy and propose rehabilitation approaches for appropriate cortical reorganization.

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.

Safety of theta burst transcranial magnetic stimulation: a systematic review of the literature

Theta burst stimulation (TBS) protocols have recently emerged as a method to transiently alter cortical excitability in the human brain through repetitive transcranial magnetic stimulation. TBS involves applying short trains of stimuli at high frequency repeated at intervals of 200 milliseconds. Because repetitive transcranial magnetic stimulation is known to carry a risk of seizures, safety guidelines have been established. TBS has the theoretical potential of conferring an even higher risk of seizure than other repetitive transcranial magnetic stimulation protocols because it delivers high-frequency bursts. In light of the recent report of a seizure induced by TBS, the safety of this new protocol deserves consideration. We performed an English language literature search and reviewed all studies published from May 2004 to December 2009 in which TBS was applied. The adverse events were documented, and crude risk was calculated. The majority of adverse events attributed to TBS were mild and occurred in 5% of subjects. Based on this review, TBS seems to be a safe and efficacious technique. However, given its novelty, it should be applied with caution. Additionally, this review highlights the need for rigorous documentation of adverse events associated with TBS and intensity dosing studies to assess the seizure risk associated with various stimulation parameters (e.g., frequency, intensity, and location).

Nicotinergic impact on focal and non-focal neuroplasticity induced by non-invasive brain stimulation in non-smoking humans

Nicotine improves cognitive performance and modulates neuroplasticity in brain networks. The neurophysiological mechanisms underlying nicotine-induced behavioral changes have been sparsely studied, especially in humans. Global cholinergic activation focuses on plasticity in humans. However, the specific contribution of nicotinic receptors to these effects is unclear. Henceforth, we explored the impact of nicotine on non-focal neuroplasticity induced by transcranial direct current stimulation (tDCS) and focal, synapse-specific plasticity induced by paired associative stimulation (PAS) in healthy non-smoking individuals. Forty-eight subjects participated in the study. Each subject received placebo and nicotine patches combined with one of the stimulation protocols to the primary motor cortex in different sessions. Transcranial magnetic stimulation (TMS)-elicited motor-evoked potential (MEP) amplitudes were recorded as a measure of corticospinal excitability until the evening of the second day following the stimulation. Nicotine abolished or reduced both PAS- and tDCS-induced inhibitory neuroplasticity. Non-focal facilitatory plasticity was also abolished, whereas focal facilitatory plasticity was slightly prolonged by nicotine. Thus, nicotinergic influence on facilitatory, but not inhibitory plasticity mimics that of global cholinergic enhancement. Therefore, activating nicotinic receptors has clearly discernable effects from global cholinergic activation. These nicotine-generated plasticity alterations might be important for the effects of the drug on cognitive function.

Determinants of the induction of cortical plasticity by non-invasive brain stimulation in healthy subjects

The ability to induce cortical plasticity with non-invasive brain stimulation (NBS) techniques has provided novel and exciting opportunities for examining the role of the human cortex during a variety of behaviours. Additionally, and importantly, the induction of lasting changes in cortical excitability can, under some conditions, reversibly modify behaviour and interact with normal learning. Such findings have driven a large number of recent studies examining whether by using such approaches it might be possible to induce functionally significant changes in patients with a large variety of neurological and psychiatric conditions including stroke, Parkinson's disease and depression. However, even in neurologically normal subjects the variability in the neurophysiological and behavioural response to such brain stimulation techniques is high. This variability at present limits the therapeutic usefulness of these techniques. The cause of this variability is multifactorial and to some degree still unknown. However, a number of factors that can influence the induction of plasticity have been identified. This review will summarise what is known about the causes of variability in healthy subjects and propose additional factors that are likely to be important determinants. A greater understanding of these determinants is critical for optimising the therapeutic applications of non-invasive brain stimulation techniques.