Cortical excitability in smoking and not smoking individuals with and without nicotine

Acute aerobic exercise modulates cognition and cortical excitability in adults with attention-deficit hyperactivity disorder (ADHD) and healthy controls

Evidence suggests aerobic exercise has beneficial effects on cognitive performance in adults with attention-deficit hyperactivity disorder (ADHD). The underlying mechanisms might depend on mechanisms of exercise-mediated brain physiology. The study aims to investigate the effects of acute aerobic exercise on cortical excitability and cognitive performance, and the correlation between these phenomena in adults with ADHD. Twenty-six drug-naïve ADHD adults, and twenty-six age-, and gender-matched healthy controls were assessed with respect to cortical excitability and cognitive performance before and after acute aerobic exercise (a single session for 30 min) or a control intervention. The results show significantly enhanced intracortical facilitation (ICF) and decreased short intracortical inhibition (SICI) after aerobic exercise in healthy subjects. In contrast, SICI was significantly enhanced following acute aerobic exercise in ADHD. In ADHD, furthermore inhibitory control and motor learning were significantly improved after the acute aerobic exercise intervention. Alterations of SICI induced by aerobic exercise, and inhibitory control and motor learning improvement were significantly positively correlated in the ADHD group. Aerobic exercise had partially antagonistic effects in healthy controls, and ADHD patients. Furthermore, aerobic exercise-induced cognition-enhancing effects in ADHD depend on specific alterations of brain physiology, which differ from healthy humans.

TMS-derived short afferent inhibition discriminates cognitive status in older adults without dementia

Aging is a complex and diverse biological process characterized by progressive molecular, cellular, and tissue damage, resulting in a loss of physiological integrity and heightened vulnerability to pathology. This biological diversity corresponds with highly variable cognitive trajectories, which are further confounded by genetic and environmental factors that influence the resilience of the aging brain. Given this complexity, there is a need for neurophysiological indicators that not only discern physiologic and pathologic aging but also closely align with cognitive trajectories. Transcranial Magnetic Stimulation (TMS) may have utility in this regard as a non-invasive brain stimulation tool that can characterize features of cortical excitability. Particularly, as a proxy for central cholinergic function, short-afferent inhibition (SAI) dysfunction is robustly associated with cognitive deficits in the latter stages of Alzheimer's Disease and Related Dementia (ADRD). In this study, we evaluated SAI in healthy young adults and older adults who, though absent clinical diagnoses, were algorithmically classified as cognitively normal (CN) or cognitively impaired (CI) according to the Jak/Bondi actuarial criteria. We report that SAI is preserved in the Old-CN cohort relative to the young adults, and SAI is significantly diminished in the Old-CI cohort relative to both young and CN older adults. Additionally, diminished SAI was significantly associated with impaired sustained attention and working memory. As a proxy measure for central cholinergic deficits, we discuss the potential value of SAI for discerning physiological and pathological aging.

Decreased short-latency afferent inhibition in individuals with mild cognitive impairment: A TMS-EEG study

TMS combined with EEG (TMS-EEG) is a tool to characterize the neurophysiological dynamics of the cortex. Among the TMS paradigms, short-latency afferent inhibition (SAI) allows the investigation of inhibitory effects mediated by the cholinergic system. The aim of this study was to compare cholinergic function in the DLPFC between individuals with mild cognitive impairment (MCI) and healthy controls (HC) using TMS-EEG with the SAI paradigm. In this study, 30 MCI and 30 HC subjects were included. The SAI paradigm consisted of 80 single pulse TMS and 80 SAI stimulations applied to the left DLPFC. N100 components, global mean field power (GMFP) and total power were calculated. As a result, individuals with MCI showed reduced inhibitory effects on N100 components and GMFP at approximately 100 ms post-stimulation and on β-band activity at 200 ms post-stimulation compared to HC. Individuals with MCI showed reduced SAI, suggesting impaired cholinergic function in the DLPFC compared to the HC group. We conclude that these findings underscore the clinical applicability of the TMS-EEG method as a powerful tool for assessing cholinergic function in individuals with MCI.

An investigation of the acute effects of aerobic exercise on executive function and cortical excitability in adolescents with attention deficit hyperactivity disorder (ADHD)

Previous studies have shown that aerobic exercise has beneficial effects on executive function in adolescents with attention-deficit hyperactivity disorder (ADHD). The underlying mechanisms could be partially due to aerobic exercise-induced cortical excitability modulation. The aim of this study was to explore the effects of acute aerobic exercise on executive functions and cortical excitability and the association between these phenomena in adolescents with ADHD. The study was conducted using a complete crossover design. Executive functions (inhibitory control, working memory, and planning) and cortical excitability were assessed in twenty-four drug-naïve adolescents with ADHD before and after acute aerobic exercise or a control intervention. Inhibitory control, working memory, and planning improved after acute aerobic exercise in adolescents with ADHD. Moreover, cortical excitability monitored by transcranial magnetic stimulation (TMS) decreased after intervention in this population. Additionally, improvements in inhibitory control and working memory performance were associated with enhanced cortical inhibition. The findings provide indirect preliminary evidence for the assumption that changes in cortical excitability induced by aerobic exercise partially contribute to improvements in executive function in adolescents with ADHD.

The trial-by-trial fluctuations in primary motor cortex excitability during attentional bias among smokers: A transcranial magnetic stimulation study

Background

A relatively new computational approach called trial-level bias score (TL-BS) has shown that attentional bias to smoking-related stimuli in smokers fluctuates temporally, trial by trial, during attention tasks. Here, we investigated the reliability of using TL-BS values to assess attentional bias and the electrophysiology mechanisms undergirding fluctuations in attentional bias among smokers.

Method

In total, 26 male smokers and 26 male non-smokers performed a dot-probe task in Experiment 1. In Experiment 2, an additional 23 male smokers and 23 male non-smokers performed the same task while undergoing single-pulse transcranial magnetic stimulation, which was used to investigate corticospinal excitability.

Results

It showed that assessing TL-BS parameters for reaction time (RT) was more reliable than calculating the traditional mean attentional bias score; however, this superior reliability was no longer apparent after controlling for general RT variability. There was a significant difference between smokers and non-smokers in TL-BS parameters calculated for both RT and motor-evoked potential (MEP) amplitude. However, TL-BS parameters for RT and MEP amplitude were strongly correlated with general RT variability and general MEP variability, respectively.

Conclusions

Our findings indicated that TL-BS parameters may not be ideal for measuring attentional bias at either the behavioral or electrophysiology level; however, larger general RT and MEP amplitude variabilities in non-smokers may indicate dysregulation of cognitive processing in smokers.

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.

Acute Aerobic Exercise at Different Intensities Modulates Motor Learning Performance and Cortical Excitability in Sedentary Individuals

Converging evidence indicates the beneficial effects of aerobic exercise on motor learning performance. Underlying mechanisms might be an impact of aerobic exercise on neuroplasticity and cortical excitability. Evidence suggests that motor learning and cortical excitability alterations correlate with the intensity of aerobic exercise and the activity level of participants. Thus, this study aims to investigate the effects of different aerobic exercise intensities on motor learning and cortical excitability in sedentary individuals. The study was conducted in a crossover and double-blind design. Twenty-six healthy sedentary individuals (13 women and 13 men) performed a motor learning task and received a cortical excitability assessment before and after a single session of low-, moderate-, and high-intensity aerobic exercise or a control intervention. The study revealed that motor learning performance and cortical excitability were significantly enhanced in the moderate-intensity aerobic exercise, compared with the other conditions. These findings suggest aerobic exercise intensity-dependent effects on motor learning in sedentary adults. The underlying mechanism might be an exercised-induced alteration of cortical excitability, specifically a reduction of GABA activity.

State-dependent effectiveness of cathodal transcranial direct current stimulation on cortical excitability

The extensive use of transcranial direct current stimulation (tDCS) in experimental and clinical settings does not correspond to an in-depth understanding of its underlying neurophysiological mechanisms. In previous studies, we employed an integrated system of Transcranial Magnetic Stimulation and Electroencephalography (TMS-EEG) to track the effect of tDCS on cortical excitability. At rest, anodal tDCS (a-tDCS) over the right Posterior Parietal Cortex (rPPC) elicits a widespread increase in cortical excitability. In contrast, cathodal tDCS (c-tDCS) fails to modulate cortical excitability, being indistinguishable from sham stimulation. Here we investigated whether an endogenous task-induced activation during stimulation might change this pattern, improving c-tDCS effectiveness in modulating cortical excitability. In Experiment 1, we tested whether performance in a Visuospatial Working Memory Task (VWMT) and a modified Posner Cueing Task (mPCT), involving rPPC, could be modulated by c-tDCS. Thirty-eight participants were involved in a two-session experiment receiving either c-tDCS or sham during tasks execution. In Experiment 2, we recruited sixteen novel participants who performed the same paradigm but underwent TMS-EEG recordings pre- and 10 min post- sham stimulation and c-tDCS. Behavioral results showed that c-tDCS significantly modulated mPCT performance compared to sham. At a neurophysiological level, c-tDCS significantly reduced cortical excitability in a frontoparietal network likely involved in task execution. Taken together, our results provide evidence of the state dependence of c-tDCS in modulating cortical excitability effectively. The conceptual and applicative implications are discussed.

Smoking status ameliorates cholinergic impairments in cortical inhibition in patients with schizophrenia

Rationale Modulation of cortical excitability, in particular inhibition, is impaired in patients with schizophrenia. Chronic nicotine consumption, which is prevalent in this group, has been shown to alter cortical excitability in healthy individuals and to increase inhibitory activity. Thus, beneficial effects of smoking on impaired cortical excitability in patients with schizophrenia have been proposed, though direct experimental evidence is still lacking.

OBJECTIVES

We aimed to explore the effect of chronic smoking on cortical excitability by comparing smoking and non-smoking patients with schizophrenia.

METHOD

Twenty-six smoking and 19 non-smoking patients diagnosed with schizophrenia were included. Transcranial magnetic stimulation (TMS) applied to the primary motor cortex served as experimental paradigm for measuring corticospinal and intracortical excitability as follows: Resting motor threshold (RMT) and the input/output curve (I/O curve) were obtained to assess corticospinal excitability. Intracortical excitability was explored using paired-pulse TMS techniques (intracortical facilitation (ICF), short-latency intracortical inhibition (SICI) and short-latency afferent inhibition (SAI)).

RESULTS

A significantly stronger inhibition in the cholinergically driven SAI protocol was observed in smokers compared to non-smokers. All other measures did not show significant differences between groups.

CONCLUSION

Our results suggest an increased inhibition within cholinergic circuits due to chronic nicotine consumption in schizophrenia. This increase may compensate impaired cholinergic neurotransmission and could explain the high rate of smokers in schizophrenia.

Older adults' episodic memory is related to a neurophysiological marker of brain cholinergic activity

Episodic memory is vulnerable to aging and may be influenced by age-related decline in the neurotransmitter acetylcholine. We probed this relation using a novel, minimally invasive transcranial magnetic stimulation marker of brain acetylcholine: short-latency afferent inhibition (SAI). We used neuropsychological testing to construct a composite score of episodic memory in N = 19 community-dwelling older adults, and stratified older adults into Higher- (N = 9) versus Lower-memory (N = 10) groups before SAI. The Higher-memory group showed significantly stronger SAI than the Lower-memory group, indicating an association between higher brain acetylcholine levels and better episodic memory. The two memory groups were equivalent in the potential confounds of age, education, mood, subjective sleep quality, and executive function. These data converge with others to suggest that episodic memory is related to acetylcholine in older adults. This relation should be further investigated, especially with pharmacology and neuroimaging.

Transcranial Direct Current Stimulation of the Dorsolateral Prefrontal Cortex Modulates Cognitive Function Related to Motor Execution During Sequential Task: A Randomized Control Study

In daily life, we perform a variety of sequential tasks while making cognitive decisions to achieve behavioral goals. If transcranial direct current electrical stimulation (tDCS) can be used to modulate cognitive functions involved in motor execution, it may provide a new rehabilitation method. In the present study, we constructed a new task in which cognitive decisions are reflected in motor actions and investigated whether the performance of the task can be improved by tDCS of the left dorsolateral prefrontal cortex (DLPFC). Forty healthy participants were randomly assigned to a real or sham tDCS group. The anode electrode was placed at F3 (left DLPFC), and the cathode electrode was positioned in the contralateral supraorbital area. Participants underwent one session of tDCS (1.5 mA, 20 min) and a sequential non-dominant hand task was performed for nine trials before and after tDCS. The task consisted of S1 (a manual dexterity task) and S2 (a manual dexterity task requiring a decision). The results showed the S2 trajectory length was significantly shorter after real tDCS than after sham tDCS (p = 0.017), though the S1 trajectory length was not significant. These results suggest that a single tDCS session of the left DLPFC can improve the performance of cognitive tasks complementary to motor execution, but not on dexterity tasks. By elucidating the modulating effect of tDCS on cognitive functions related to motor execution, these results may be used to improve the performance of rehabilitation patients in the future.

Non-invasive brain stimulation and neuroenhancement

Attempts to enhance human memory and learning ability have a long tradition in science. This topic has recently gained substantial attention because of the increasing percentage of older individuals worldwide and the predicted rise of age-associated cognitive decline in brain functions. Transcranial brain stimulation methods, such as transcranial magnetic (TMS) and transcranial electric (tES) stimulation, have been extensively used in an effort to improve cognitive functions in humans. Here we summarize the available data on low-intensity tES for this purpose, in comparison to repetitive TMS and some pharmacological agents, such as caffeine and nicotine. There is no single area in the brain stimulation field in which only positive outcomes have been reported. For self-directed tES devices, how to restrict variability with regard to efficacy is an essential aspect of device design and function. As with any technique, reproducible outcomes depend on the equipment and how well this is matched to the experience and skill of the operator. For self-administered non-invasive brain stimulation, this requires device designs that rigorously incorporate human operator factors. The wide parameter space of non-invasive brain stimulation, including dose (e.g., duration, intensity (current density), number of repetitions), inclusion/exclusion (e.g., subject's age), and homeostatic effects, administration of tasks before and during stimulation, and, most importantly, placebo or nocebo effects, have to be taken into account. The outcomes of stimulation are expected to depend on these parameters and should be strictly controlled. The consensus among experts is that low-intensity tES is safe as long as tested and accepted protocols (including, for example, dose, inclusion/exclusion) are followed and devices are used which follow established engineering risk-management procedures. Devices and protocols that allow stimulation outside these parameters cannot claim to be "safe" where they are applying stimulation beyond that examined in published studies that also investigated potential side effects. Brain stimulation devices marketed for consumer use are distinct from medical devices because they do not make medical claims and are therefore not necessarily subject to the same level of regulation as medical devices (i.e., by government agencies tasked with regulating medical devices). Manufacturers must follow ethical and best practices in marketing tES stimulators, including not misleading users by referencing effects from human trials using devices and protocols not similar to theirs.

Dissociating the role of dACC and dlPFC for emotion appraisal and mood regulation using cathodal tDCS

Several neuroimaging studies have shown that a distributed network of brain regions is involved in our ability to appraise the emotions we experience in daily life. In particular, scholars suggested that the dorsal anterior cingulate cortex (dACC) may play a role in the appraisal of emotional stimuli together with subcortical regions, especially when stimuli are negatively valenced, and the dorsolateral prefrontal cortex (dlPFC) may play a role in regulating emotions. However, proofs of the causal role of these regions are lacking. In the present study, we aim at testing this model by stimulating both the dACC and the left dlPFC via cathodal tDCS. Twenty-four participants were asked to attend and rate the arousal and valence of negative and neutral emotional stimuli (pictures and words) in three different experimental sessions: cathodal stimulation of dACC, left dlPFC, or sham. In addition to the experimental task, the baseline affective state was measured before and after the stimulation to further assess the effect of stimulation over the baseline affective state after the experimental session. Results showed that cathodal stimulation of dACC, but not the left dlPFC, was associated with reduced arousal ratings of emotional stimuli, both compared with the sham condition. Moreover, cathodal stimulation of left dlPFC decreased participant's positive affective state after the session. These findings suggest for the first time, a dissociation between the dACC and dlPFC, with the former more involved in emotion appraisal, and the latter more involved in mood modulation.

NMDA receptor-related mechanisms of dopaminergic modulation of tDCS-induced neuroplasticity

Dopamine is a key neuromodulator of neuroplasticity and an important neuronal substrate of learning, and memory formation, which critically involves glutamatergic N-methyl-D-aspartate (NMDA) receptors. Dopamine modulates NMDA receptor activity via dopamine D1 and D2 receptor subtypes. It is hypothesized that dopamine focuses on long-term potentiation (LTP)-like plasticity, i.e. reduces diffuse widespread but enhances locally restricted plasticity via a D2 receptor-dependent NMDA receptor activity reduction. Here, we explored NMDA receptor-dependent mechanisms underlying dopaminergic modulation of LTP-like plasticity induced by transcranial direct current stimulation (tDCS). Eleven healthy, right-handed volunteers received anodal tDCS (1 mA, 13 min) over the left motor cortex combined with dopaminergic agents (the D2 receptor agonist bromocriptine, levodopa for general dopamine enhancement, or placebo) and the partial NMDA receptor agonist D-cycloserine (dosages of 50, 100, and 200 mg, or placebo). Cortical excitability was monitored by transcranial magnetic stimulation-induced motor-evoked potentials. We found that LTP-like plasticity was abolished or converted into LTD-like plasticity via dopaminergic activation, but reestablished under medium-dose D-cycloserine. These results suggest that diffuse LTP-like plasticity is counteracted upon via D2 receptor-dependent reduction of NMDA receptor activity.

Alzheimer disease and neuroplasticity

Alzheimer's disease (AD) is considered the most harmful form of dementia in the elderly population. At present, there are no effective treatments and this is likely due to the incomplete understanding of the pathophysiology. Recent data indicate that synaptic dysfunction could be a central element of AD pathophysiology. It was found that a synaptic breakdown is an early event that heralds neuronal degeneration. Transcranial magnetic stimulation (TMS) has been recently introduced as a novel approach to identify the early signatures of synaptic dysfunction characterizing AD pathophysiology. In this chapter, we review the new neurophysiologic signatures of AD that have been emphasized by TMS studies. We show how TMS measurement of neuroplasticity identified long-term potentiation (LTP)-like cortical plasticity as a key element of AD synaptic dysfunction. These measurements are useful to increase the accuracy of differential diagnosis, predict disease progression, and anticipate response to therapy. Moreover, enhancing neuroplasticity holds as a promising therapeutic approach to improve cognition in AD. In recent years, studies showed treatments with multiple sessions of rTMS can influence cognition in people with neurodegenerative diseases. In the second part of this chapter, we also consider novel therapeutic approaches based on the clinical use of rTMS.

Behavioural effects of task-relevant neuromodulation by rTMS on giving-up

Recent studies suggest that online repetitive transcranial magnetic stimulation (rTMS) can induce local entrainment of ongoing endogenous oscillatory activity during a task. This effect may impact cognitive performance, depending on the function of the oscillation. In this study, we aimed to investigate the effects of stimulation frequency and target location that are relevant to the cognitive processes of giving-up. We first investigated the correlations between the EEG oscillations and cognitive giving-up processes during problem-solving tasks (Experiment 1). We then conducted online rTMS to examine the frequency-dependent stimulation effects of rTMS on the performance of problem-solving tasks and ongoing oscillations (Experiment 2). The results of Experiment 1 suggested that the frontal theta rhythm is associated with the giving-up processes and that the frontal alpha rhythm is associated with problem-solving behaviour. Accordingly, we hypothesised that rTMS at the theta frequency would induce ongoing theta activity and accelerate the giving-up behaviour, while rTMS at the alpha frequency would induce ongoing alpha activity and slow down the giving-up behaviour in Experiment 2. The results showed that theta-frequency rTMS application induced an increase in theta amplitudes and shortened the giving-up response. Alpha-frequency rTMS application induced an increase in alpha amplitudes, but did not change giving-up responses. Considering the close resemblance between giving-up behaviour and rumination in depression, neuromodulation of cognitive giving-up processes may lead to a new intervention to treat depression by rTMS. Furthermore, this study strengthens the hypothesis that modulating task-relevant oscillations by rTMS could induce behavioural changes related to cognitive performance.

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.

The effect of acute and chronic nicotine consumption on intra-cortical inhibition and facilitation: A transcranial magnetic stimulation study

OBJECTIVE

The aim of the present study was to explore the impact of acute and chronic nicotine consumption on measures of intracortical inhibition and facilitation.

METHODS

This study involved 50 chronic heavy cigarette smokers and 40 healthy subjects matched for age, sex and educational level, with no history of chronic nicotine intake. Intracortical inhibition and facilitation were assessed using transcranial magnetic stimulation (TMS) measures of motor threshold (MT), short- and long-interval intra-cortical inhibition (SICI, LICI), cortical silent period (CSP) and intra-cortical facilitation (ICF). Basal serum levels of cotinine were measured in the healthy group and at ½ and 2 h after smoking a single cigarette in the chronic smokers.

RESULTS

There was enhanced SICI and reduced ICF in smokers (independent of time after smoking) compared with non-smokers. The former suggests a chronic effect of increased nicotine levels on GABA-A neurotransmission whereas the latter suggests an additional effect on glutamatergic transmission. There were no significant differences between smokers and non-smokers in other TMS parameters. There was a significant negative correlation between cotinine levels at ½ h after smoking and SICI at 3 ms ISI (P < 0.001). There were no significant differences in any of the neurophysiological measures between smokers at ½ h versus 2 h after smoking a single cigarette.

CONCLUSION

Chronic nicotine consumption enhances SICI, and reduces ICF, supporting the hypothesis that nicotine acts as a neuromodulator of GABA-A and glutamate neurotransmission.

Inhibitory Effect of Apomorphine on Focal and Nonfocal Plasticity in the Human Motor Cortex

Dopamine is crucial for neuroplasticity, which is considered to be the neurophysiological foundation of learning and memory. The specific effect of dopamine on plasticity such as long-term potentiation (LTP) and long-term depression (LTD) is determined by receptor subtype specificity, concentration level, and the kind of plasticity induction technique. In healthy human subjects, the dopamine precursor levodopa (L-DOPA) exerts a dosage-dependent non-linear effect on motor cortex plasticity. Low and high dosage L-DOPA impaired or abolished plasticity, while medium-dose preserved and reversed plasticity in previous studies. Similar dosage-dependent effects were also observed for selective D1-like and D2-like receptor activation that favor excitatory and inhibitory plasticity, respectively. However, such a dosage-dependent effect has not been explored for a nonselective dopamine agonist such as apomorphine in humans. To this aim, nonfocal and focal motor cortex plasticity induction using paired associative stimulation (PAS) and transcranial direct current stimulation (tDCS) were performed respectively in healthy participants under 0.1, 0.2, 0.3 mg apomorphine or placebo drug. Transcranial magnetic stimulation-elicited motor-evoked potentials were used to monitor motor cortical excitability alterations. We hypothesized that, similar to L-DOPA, apomorphine will affect motor cortex plasticity. The results showed that apomorphine with the applied dosages has an inhibitory effect for focal and nonfocal LTP-like and LTD-like plasticity, which was either abolished, diminished or reversed. The detrimental effect on plasticity induction under all dosages of apomorphine suggests a predominantly presynaptic mechanism of action of these dosages.

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 Influence of Recreational Substance Use in TMS Research

(1) Background: Transcranial magnetic stimulation (TMS) approaches are widely used to study cortical and corticospinal function. However, responses to TMS are subject to significant intra-and inter-individual variability. Acute and chronic exposure to recreational substances alters the excitability of the sensorimotor system and may contribute to the variability in TMS outcome measures. The increasing prevalence of recreational substance use poses a significant challenge for executing TMS studies, but there is a lack of clarity regarding the influence of these substances on sensorimotor function. (2) Methods: The literature investigating the influence of alcohol, nicotine, caffeine and cannabis on TMS outcome measures of corticospinal, intracortical and interhemispheric excitability was reviewed. (3) Results: Both acute and chronic use of recreational substances modulates TMS measures of excitability. Despite the abundance of research in this field, we identify knowledge gaps that should be addressed in future studies to better understand the influence of these substances on TMS outcomes. (4) Conclusions: This review highlights the need for TMS studies to take into consideration the history of participant substance use and to control for acute substance use prior to testing.

A Checklist to Reduce Response Variability in Studies Using Transcranial Magnetic Stimulation for Assessment of Corticospinal Excitability: A Systematic Review of the Literature

Response variability between individuals (interindividual variability) and within individuals (intraindividual variability) is an important issue in the transcranial magnetic stimulation (TMS) literature. This has raised questions of the validity of TMS to assess changes in corticospinal excitability (CSE) in a predictable and reliable manner. Several participant-specific factors contribute to this observed response variability with a current lack of consensus on the degree each factor contributes. This highlights a need for consistency and structure in reporting study designs and methodologies. Currently, there is no summarized review of the participant-specific factors that can be controlled and may contribute to response variability. This systematic review aimed to develop a checklist of methodological measures taken by previously published research to increase the homogeneity of participant selection criteria, preparation of participants before experimental testing, participant scheduling, and the instructions given to participants throughout experimental testing to minimize their effect on response variability. Seven databases were searched in full. Studies were included if CSE was measured via TMS and included methodological measures to increase the homogeneity of the participants. Eighty-four studies were included. Twenty-three included measures to increase participant selection homogeneity, 21 included measures to increase participant preparation homogeneity, while 61 included measures to increase participant scheduling and instructions during experimental testing homogeneity. These methodological measures were summarized into a user-friendly checklist with considerations, suggestions, and rationale/justification for their inclusion. This may provide the framework for further insights into ways to reduce response variability in TMS research.

The change of motor cortical excitability between eyes open and closed conditions

The effect of the eyes closed and eyes open states on transcranial magnetic stimulation (TMS)-induced motor cortex response remains unclear. This study evaluated the difference in TMS-induced motor cortical responses between the eyes open and eyes closed states. Ten healthy right-handed participants participated in three experiments. The stimulation-response curve of motor-evoked potential, short-interval intracortical inhibition, intracortical facilitation, and cortical silent period were determined in both the eyes open and eyes closed states, in random order. The order of performance of the eyes open and eyes closed states was also random. The stimulation-response curve obtained in the eyes open state was steeper than that obtained in the eyes closed state. The resting and active motor thresholds, cortical silent period, short-interval intracortical inhibition, and intracortical facilitation were similar in the eyes open and eyes closed states. These data demonstrate that the eyes closed state may affect the recruitment of cortical circuits and thus diminish the TMS-evoked motor output.

Nicotine modulates human brain plasticity via calcium-dependent mechanisms

KEY POINTS

Nicotine (NIC) modulates cognition and memory function by targeting the nicotinic ACh receptor and releasing different transmitter systems postsynaptically. With both NIC-generated mechanisms, calcium influx and calcium permeability can be regulated, which is a key requirement for the induction of long-term potentiation, comprising the physiological basis of learning and memory function. We attempt to unmask the underlying mechanism of nicotinic effects on anodal transcranial direct current stimulation (tDCS)-induced long-term potentiation-like plasticity based on the hypothesis of calcium-dependency. Abolished tDCS-induced neuroplasticity as a result of NIC administration is reversed by calcium channel blockade with flunarizine in a dose-dependent manner. The results of the present study suggest that there is a dose determination of NIC/NIC agonists in therapeutical settings when treating cognitive dysfunction, which partially explains the heterogeneous results on cognition observed in subjects in different experimental settings.

ABSTRACT

Nicotine (NIC) modulates neuroplasticity and improves cognitive performance in animals and humans mainly by increased calcium permeability and modulation of diverse transmitter systems. NIC administration impairs calcium-dependent plasticity induced by non-invasive brain stimulation with transcranial direct current stimulation (tDCS) in non-smoking participants probably as a result of intracellular calcium overflow. To test this hypothesis, we analysed the effect of calcium channel blockade with flunarizine (FLU) on anodal tDCS-induced cortical excitability changes in healthy non-smokers under NIC. We applied anodal tDCS combined with NIC patch and FLU at three different doses (2.5, 5 and 10 mg) or with placebo medication. NIC abolished anodal tDCS-induced neuroplasticity. Under medium dosage (but not under low and high dosage) of FLU combined with NIC, plasticity was re-established. For FLU alone, the lowest dosage weakened long-term potentiation (LTP)-like plasticity, whereas the highest dosage again abolished tDCS-induced plasticity. The medium dosage turned LTP-like plasticity in long-term depression-like plasticity. The results of the present study suggest a key role of calcium influx and calcium levels in nicotinic effects on LTP-like plasticity in humans. This knowledge might be relevant for the development of new therapeutic strategies in cognitive dysfunction.

Modulating Neural Circuits with Transcranial Magnetic Stimulation: Implications for Addiction Treatment Development

Although the last 50 years of clinical and preclinical research have demonstrated that addiction is a brain disease, we still have no neural circuit-based treatments for substance dependence or cue reactivity at large. Now, for the first time, it appears that a noninvasive brain stimulation technique known as transcranial magnetic stimulation (TMS), which is Food and Drug Administration approved to treat depression, may be the first tool available to fill this critical void in addiction treatment development. The goals of this review are to 1) introduce TMS as a tool to induce causal change in behavior, cortical excitability, and frontal-striatal activity; 2) describe repetitive TMS (rTMS) as an interventional tool; 3) provide an overview of the studies that have evaluated rTMS as a therapeutic tool for alcohol and drug use disorders; and 4) outline a conceptual framework for target selection when designing future rTMS clinical trials in substance use disorders. The manuscript concludes with some suggestions for methodological innovation, specifically with regard to combining rTMS with pharmacotherapy as well as cognitive behavioral training paradigms. We have attempted to create a comprehensive manuscript that provides the reader with a basic set of knowledge and an introduction to the primary experimental questions that will likely drive the field of TMS treatment development forward for the next several years.

Nicotinic Restoration of Excitatory Neuroplasticity Is Linked to Improved Implicit Motor Learning Skills in Deprived Smokers

Nicotine has been shown to modulate neuroplasticity, cognition, and learning processes in smokers and non-smokers. A possible mechanism for its effect on learning and memory formation is its impact on long-term depression and long-term potentiation (LTP). Nicotine abstinence in smokers is often correlated with impaired cognitive performance. As neuroplasticity is closely connected to learning and memory formation, we aimed to explore the effect of nicotine spray administration in deprived smokers on paired-associative stimulation (PAS25)-induced neuroplasticity and on performance of the serial reaction time task (SRTT), a sequential motor learning paradigm. Deprived smokers (n = 12) under placebo medication displayed reduced excitatory neuroplasticity induced by PAS25. Plasticity was restored by nicotine spray administration. Likewise, SRTT-performance improved after nicotine spray administration compared to placebo administration (n = 19). The results indicate a restitutional effect of nicotine spray in deprived smokers on both: LTP-like neuroplasticity and motor learning. These results present a possible explanation for persistence of nicotine addiction and probability of relapse.

Mechanisms of Nicotinic Modulation of Glutamatergic Neuroplasticity in Humans

The impact of nicotine (NIC) on plasticity is thought to be primarily determined via calcium channel properties of nicotinic receptor subtypes, and glutamatergic plasticity is likewise calcium-dependent. Therefore glutamatergic plasticity is likely modulated by the impact of nicotinic receptor-dependent neuronal calcium influx. We tested this hypothesis for transcranial direct current stimulation (tDCS)-induced long-term potentiation-like plasticity, which is abolished by NIC in nonsmokers. To reduce calcium influx under NIC, we blocked N-methyl-d-aspartate (NMDA) receptors. We applied anodal tDCS combined with 15 mg NIC patches and the NMDA-receptor antagonist dextromethorphan (DMO) in 3 different doses (50, 100, and 150 mg) or placebo medication. Corticospinal excitability was monitored by single-pulse transcranial magnetic stimulation-induced motor-evoked potential amplitudes after plasticity induction. NIC abolished anodal tDCS-induced motor cortex excitability enhancement, which was restituted under medium dosage of DMO. Low-dosage DMO did not affect the impact of NIC on tDCS-induced plasticity and high-dosage DMO abolished plasticity. For DMO alone, the low dosage had no effect, but medium and high dosages abolished tDCS-induced plasticity. These results enhance our knowledge about the proposed calcium-dependent impact of NIC on plasticity in humans and might be relevant for the development of novel nicotinic treatments for cognitive dysfunction.

Diverging effects of nicotine on motor learning performance: Improvement in deprived smokers and attenuation in non-smokers

Nicotine modulates cognition and neuroplasticity in smokers and non-smokers. A possible mechanism for its effect on learning and memory performance is its impact on long-term potentiation (LTP) and long-term depression (LTD). As neuroplasticity is closely connected to learning processes, we aimed to explore the effect of nicotine in healthy, young smokers and non-smokers on performance of the serial reaction time task (SRTT), a sequential motor learning paradigm. 20 nicotine-deprived smokers and 20 non-smokers participated in the study and were exposed to nicotine or placebo medication. Deprived smokers under placebo medication displayed reduced performance in terms of reaction time and error rates compared to the non-smoking group. After application of nicotine, performance in smokers improved while it deteriorated in non-smokers. These results indicate a restituting effect of nicotine in smokers in terms of cognitive parameters. This sheds further light on the proposed mechanism of nicotine on learning processes, which might be linked to the addictive component of nicotine, the probability of relapse and thus needs also be addressed in cessation treatment.

Olfactory dysfunction in early Parkinson's disease is associated with short latency afferent inhibition reflecting central cholinergic dysfunction

OBJECTIVE

Our study aimed to determine whether the short latency afferent inhibition (SAI) response could be associated with the severity of olfactory dysfunction in PD patients.

METHODS

A total of 71 PD patients and 20 controls were enrolled. All PD patients were classified into 3 groups by the severity of the olfactory deficit. Single-pulse transmagnetic stimulation (TMS) parameters and SAI were assessed.

RESULTS

The integrated SAI in the PD with anosmia and PD with hyposomia groups was significantly less inhibited than that in the PD with normosmia and control groups [64.79 {Interquartile range (IQR): 59.96, 71.33}, 84.79 {IQR: 75.03, 90.63} versus 36.72 {IQR: 32.28, 48.33}, 42.15 {IQR: 34.60, 44.96}, respectively]. In PD subjects, the severity of olfactory dysfunction also showed a significant negative correlation with the SAI response (r=-0.829, p<0.001).

CONCLUSIONS

Considering that the SAI response partly reflects central cholinergic dysfunction and that our study shows a relationship between the SAI response and the severity of olfactory dysfunction in PD, our findings indicate that cholinergic dysfunction could possibly contribute to the pathogenesis of olfactory dysfunction in early PD.

SIGNIFICANCE

SAI could be a useful marker to detect severe olfactory dysfunction in PD.

Differential effects of cannabis dependence on cortical inhibition in patients with schizophrenia and non-psychiatric controls

BACKGROUND

Cannabis is the most commonly used illicit substance among patients with schizophrenia. Cannabis exacerbates psychotic symptoms and leads to poor functional outcomes. Dysfunctional cortical inhibition has been implicated in the pathophysiology of schizophrenia; however, the effects of cannabis on this mechanism have been relatively unexamined. The goal of this study was to index cortical inhibition from the motor cortex among 4 groups: schizophrenia patients and non-psychiatric controls dependent on cannabis as well as cannabis-free schizophrenia patients and non-psychiatric controls.

METHODS

In this cross-sectional study, GABA-mediated cortical inhibition was index with single- and paired-pulse transcranial magnetic stimulation (TMS) paradigms to the left motor cortex in 12 cannabis dependent and 11 cannabis-free schizophrenia patients, and in 10 cannabis dependent and 13 cannabis-free controls.

RESULTS

Cannabis-dependent patients with schizophrenia displayed greater short-interval cortical inhibition (SICI) compared to cannabis-free schizophrenia patients (p = 0.029), while cannabis-dependent controls displayed reduced SICI compared to cannabis-free controls (p = 0.004). SICI did not differ between cannabis dependent patients and cannabis-free controls, or between dependent schizophrenia patients compared to dependent controls. No significant differences were found for long-interval cortical inhibition (LICI) or intra-cortical facilitation (ICF) receptor function, suggesting a selective effect on SICI.

CONCLUSION

These findings suggest that cannabis dependence may have selective and differing effects on SICI in schizophrenia patients compared to controls, which may provide insight into the pathophysiology of co-morbid cannabis dependence in schizophrenia.

Trait- and state-dependent cortical inhibitory deficits in bipolar disorder

OBJECTIVES

Euthymic patients with bipolar disorder (BD) have deficits in cortical inhibition. However, whether cortical inhibitory deficits are trait- or state-dependent impairments is not yet known and their relationship with psychiatric symptoms is not yet understood. In the present study, we examined trait- and state-dependent cortical inhibitory deficits and evaluated the potential clinical significance of these deficits.

METHODS

Nineteen patients with bipolar I disorder were evaluated using the paired-pulse transcranial stimulation protocol, which assessed cortical inhibition during an acute manic episode. Cortical inhibition measures were compared with those obtained in 28 demographically matched healthy controls. A follow-up assessment was performed in 15 of these patients three months later, when there was remission from their mood and psychotic symptoms. The association between cortical inhibitory measures and severity of psychiatric symptoms was also studied.

RESULTS

During mania, patients showed decreased short-interval intracortical and transcallosal inhibition, as well as a normal cortical silent period and long-interval cortical inhibition. These findings were the same during euthymia. Symptoms associated with motor hyperactivity were correlated negatively with the degree of cortical inhibition. These correlations were not significant when a Bonferroni correction was applied.

CONCLUSIONS

The present longitudinal study showed cortical inhibitory deficits in patients with BD, and supports the hypothesis that cortical inhibitory deficits in BD are trait dependent. Further research is necessary to confirm the clinical significance of these deficits.

Double dissociation of working memory and attentional processes in smokers and non-smokers with and without nicotine

Nicotine has been shown to affect cortical excitability measured using transcranial magnetic stimulation in smoking and non-smoking subjects in different ways. In tobacco-deprived smokers, administration of nicotine restores compromised cortical facilitation while in non-smokers, it enhances cortical inhibition. As cortical excitability and activity are closely linked to cognitive processes, we aimed to explore whether nicotine-induced physiological alterations in non-smokers and smokers are associated with cognitive changes. Specifically, we assessed the impact of nicotine on working memory performance (n-back letter task) and on attentional processes (Stroop interference test) in healthy smokers and non-smokers. Both tasks have been shown to rely on prefrontal areas, and nicotinic receptors are relevantly involved in prefrontal function. Sixteen smoking and 16 non-smoking subjects participated in the 3-back letter task and 21 smoking and 21 non-smoking subjects in the Stroop test after the respective application of placebo or nicotine patches. The results show that working memory and attentional processes are compromised in nicotine-deprived smokers compared to non-smoking individuals. After administration of nicotine, working memory performance in smokers improved, while non-smoking subjects displayed decreased accuracy with increased number of errors. The effects have been shown to be more apparent for working memory performance than attentional processes. In summary, cognitive functions can be restored by nicotine in deprived smokers, whereas non-smokers do not gain additional benefit. The respective changes are in accordance with related effects of nicotine on cortical excitability in both groups.

Smoking restores impaired LTD-like plasticity in schizophrenia: a transcranial direct current stimulation study

Impaired neuroplastic responses following noninvasive brain stimulation have been reported repeatedly in schizophrenia patients. These findings have been associated with deficits in GABAergic, glutamatergic, and cholinergic neurotransmission. Although various neurophysiological studies have indicated a relationship between nicotine and neuroplasticity in healthy individuals, the present study is the first investigation into the impact of nicotine on LTD-like plasticity in patients with schizophrenia. Cortical excitability and cortical plasticity were explored in 30 schizophrenia patients (17 smoker, 13 nonsmoker) and 45 healthy controls (13 smoker, 32 nonsmoker) by using single-pulse transcranial magnetic stimulation (TMS) before and following cathodal transcranial direct current stimulation (tDCS) applied to the left primary motor cortex. Our analysis revealed abolished LTD-like plasticity in nonsmoking schizophrenia patients. However, these plasticity deficits were not present in smoking schizophrenia patients. In healthy controls, significant MEP reductions following cathodal tDCS were observed in nonsmoking individuals, but only trend-level reductions in smokers. In smoking schizophrenia patients, the severity of negative symptoms correlated positively with reduced neuroplasticity, whereas nonsmoking patients displayed the opposite effect. Taken together, the data of our study support the notion of an association between chronic smoking and the restitution of impaired LTD-like plasticity in schizophrenia patients. Although replication and further research are needed to better understand this relationship, our findings indicate that nicotine intake might stabilize the impaired inhibition-facilitation balance in the schizophrenic brain through a complex interaction between cortical plasticity, and GABAergic and cholinergic neurotransmission, and might explain the reduced prevalence of negative symptoms in this population.

TMS and drugs revisited 2014

The combination of pharmacology and transcranial magnetic stimulation to study the effects of drugs on TMS-evoked EMG responses (pharmaco-TMS-EMG) has considerably improved our understanding of the effects of TMS on the human brain. Ten years have elapsed since an influential review on this topic has been published in this journal (Ziemann, 2004). Since then, several major developments have taken place: TMS has been combined with EEG to measure TMS evoked responses directly from brain activity rather than by motor evoked potentials in a muscle, and pharmacological characterization of the TMS-evoked EEG potentials, although still in its infancy, has started (pharmaco-TMS-EEG). Furthermore, the knowledge from pharmaco-TMS-EMG that has been primarily obtained in healthy subjects is now applied to clinical settings, for instance, to monitor or even predict clinical drug responses in neurological or psychiatric patients. Finally, pharmaco-TMS-EMG has been applied to understand the effects of CNS active drugs on non-invasive brain stimulation induced long-term potentiation-like and long-term depression-like plasticity. This is a new field that may help to develop rationales of pharmacological treatment for enhancement of recovery and re-learning after CNS lesions. This up-dated review will highlight important knowledge and recent advances in the contribution of pharmaco-TMS-EMG and pharmaco-TMS-EEG to our understanding of normal and dysfunctional excitability, connectivity and plasticity of the human brain.

Dosage-dependent effect of dopamine D2 receptor activation on motor cortex plasticity in humans

The neuromodulator dopamine plays an important role in synaptic plasticity. The effects depend on receptor subtypes, affinity, concentration level, and the kind of neuroplasticity induced. In animal experiments, dopamine D2-like receptor stimulation revealed partially antagonistic effects on plasticity, which might be explained by dosage dependency. In humans, D2 receptor block abolishes plasticity, and the D2/D3, but predominantly D3, receptor agonist ropinirol has a dosage-dependent nonlinear affect on plasticity. Here we aimed to determine the specific affect of D2 receptor activation on neuroplasticity in humans, because physiological effects of D2 and D3 receptors might differ. Therefore, we combined application of the selective D2 receptor agonist bromocriptine (2.5, 10, and 20 mg or placebo medication) with anodal and cathodal transcranial direct current stimulation (tDCS), which induces nonfocal plasticity, and with paired associative stimulation (PAS) generating a more focal kind of plasticity in the motor cortex of healthy humans. Plasticity was monitored by transcranial magnetic stimulation-induced motor-evoked potential amplitudes. For facilitatory tDCS, bromocriptine prevented plasticity induction independent from drug dosage. However, its application resulted in an inverted U-shaped dose-response curve on inhibitory tDCS, excitability-diminishing PAS, and to a minor degree on excitability-enhancing PAS. These data support the assumption that modulation of D2-like receptor activity exerts a nonlinear dose-dependent effect on neuroplasticity in the human motor cortex that differs from predominantly D3 receptor activation and that the kind of plasticity-induction procedure is relevant for its specific impact.

How do high- and low-frequency repetitive transcranial magnetic stimulations modulate the temporal cortex

Few studies have examined the impact of repetitive transcranial magnetic stimulation (rTMS) on the cortical excitability of nonmotor cortices; current treatments often target the temporal or prefrontal cortex. We used auditory evoked potentials recorded in 24 healthy subjects to evaluate the neuromodulatory effects of low- and high-frequency rTMS in the temporal lobe. Both auditory evoked potential P50 amplitude, a marker of cortical excitability, and P50 ratio, a marker of sensory gating known to be impaired in patients with auditory verbal hallucinations, were compared before and after rTMS. We observed a similar effect after both stimulation frequencies, with a decrease in P50 amplitude and no significant effect on P50 ratio. Low- and high-frequency rTMS applied to the temporal lobe seemed to exert the same cortical neuromodulation effect, while auditory sensory gating may not be modulated by temporal rTMS.

Enhanced corticobulbar excitability in chronic smokers during visual exposure to cigarette smoking cues

BACKGROUND

Neuroimaging studies of chronic smokers report altered activity of several neural regions involved in the processing of rewarding outcomes. Neuroanatomical evidence suggests that these regions are directly connected to the tongue muscle through the corticobulbar pathways. Accordingly, we examined whether corticobulbar excitability might be considered a somatic marker for nicotine craving.

METHODS

We compared motor-evoked potential (MEP) amplitudes recorded from the tongue and the extensor carpi radialis (control muscle) of chronic smokers under drug withdrawal and intake conditions as well as a nonsmoker group. All participants were tested during passive exposure to pictures showing a smoking cue or a meaningless stimulus. In the intake condition, chronic smokers were asked to smoke a real cigarette (CSn: group 1) or a placebo (CSp: group 2).

RESULTS

Results show that MEP amplitudes recorded from the tongues of participants in the CSn and CSp groups under the withdrawal condition were selectively enhanced during exposure to a smoking cue. However, this effect on tongue MEP amplitudes disappeared in the intake condition for both the CSn and CSp groups.

LIMITATIONS

Limitations include the fact that the study was conducted in 2 different laboratories, the small sample size, the absence of data on chronic smoker craving strength and the different tastes of the real and placebo cigarettes.

CONCLUSION

These results suggest that, in chronic smokers, tongue muscle MEP amplitudes are sensitive to neural processes active under the physiological status of nicotine craving. This finding implicates a possible functional link between neural excitability of the corticobulbar pathway and the reward system in chronic smokers.

Nonlinear dose-dependent impact of D1 receptor activation on motor cortex plasticity in humans

The neuromodulator dopamine plays an important role in synaptic plasticity. The effects are determined by receptor subtype specificity, concentration level, and the kind of neuroplasticity induced. D1-like receptors have been proposed to be involved in cognitive processes via their impact on plasticity. Cognitive studies in humans and animals revealed a dosage-dependent effect of D1-like receptor activation on task performance. In humans, D1-like receptor activation re-establishes plasticity under D2 receptor block. However, a dosage-dependent effect has not been explored so far. To determine the impact of the amount of D1-like receptor activation on neuroplasticity in humans, we combined sulpiride, a selective D2 receptor antagonist, with the dopamine precursor l-DOPA (25, 100, and 200 mg) or applied placebo medication. The impact on plasticity induced by anodal and cathodal transcranial direct current stimulation (tDCS) was compared with the impact on plasticity induced by excitatory and inhibitory paired associative stimulation (PAS) at the primary motor cortex of healthy humans. Stimulation-generated cortical excitability alterations were monitored by transcranial magnetic stimulation-induced motor-evoked potential amplitudes. D1-like receptor activation produced an inverted U-shaped dose-response curve on plasticity induced by both facilitatory tDCS and PAS. For excitability-diminishing tDCS and PAS, aftereffects were abolished or converted trendwise into facilitation. These data extend findings of dose-dependent inverted U-shaped effects of D1 receptor activation on neuroplasticity of the motor cortex.

Acute nicotine improves social decision-making in non-smoking but not in smoking schizophrenia patients

Schizophrenia patients are characterized by severe social impairments. Recently, social cognition has been put forward as an important mediator in schizophrenia between the often-reported neurocognitive deficits and functional outcome and is thus an important target for treatments. Nicotine has been reported to improve neurocognitive processes in schizophrenia patients but no studies have investigated possible nicotine-induced facilitation of social cognition. The current placebo-controlled crossover study aimed at bridging this gap by investigating whether the administration of active (1 mg or 2 mg) or placebo oromucosal nicotine spray resulted in improved social decision-making in non-smoking (N = 15) and smoking (N = 16) schizophrenia patients. All patients played the role of responder in a variant of the ultimatum game that allowed detailed measurements of fairness and intentionality considerations. The results showed impaired social decision-making in the non-smoking patients under placebo, but not in the smoking patients. Interestingly, this impairment normalized after administration of 1 mg of nicotine, but not after 2 mg of nicotine. Nicotine had no effect on performance in the smoking patients. The present study indicates that nicotine improves social decision-making in non-smoking patients. The present results suggest that acute nicotine effects may result in a facilitation of proactive control through improved attentional processes. However, the efficacy seems limited and although nicotine may thus be an interesting target for (social) cognitive enhancement in the subset of patients that do not smoke, more research is needed on the long-lasting effects of nicotine-based treatments.