Late cortical plasticity in motor and auditory cortex: role of met-allele in BDNF Val66Met polymorphism

Effect of presentation rate on auditory processing in Rett syndrome: event-related potential study

BACKGROUND

Rett syndrome (RS) is a rare neurodevelopmental disorder characterized by mutations in the MECP2 gene. Patients with RS have severe motor abnormalities and are often unable to walk, use hands and speak. The preservation of perceptual and cognitive functions is hard to assess, while clinicians and care-givers point out that these patients need more time to process information than typically developing peers. Neurophysiological correlates of auditory processing have been also found to be distorted in RS, but sound presentation rates were relatively quick in these studies (stimulus onset asynchrony, SOA < 1000 ms). As auditory event-related potential (ERP) is typically increased with prolongation of SOA we aim to study if SOA prolongation might compensate for observed abnormalities.

METHODS

We presented a repetitive stimulus (1000 Hz) at three different SOAs of 900 ms, 1800 ms, and 3600 ms in children with RS (N = 24, Mean age = 9.0 ± 3.1) and their typical development (TD) peers (N = 27, Mean age = 9.7 ± 3.4) while recording 28-channels electroencephalogram, EEG. Some RS participants (n = 10) did not show clear ERP and were excluded from the analysis.

RESULTS

Major ERP components (here assessed as N1P1 and P2N1 peak-to-peak values) were smaller at SOA 900 than at longer SOAs in both groups, pointing out that the basic mechanism of adaptation in the auditory system is preserved in at least in RS patients with evident ERPs. At the same time the latencies of these components were significantly delayed in the RS than in TD. Moreover, late components (P2N1 and N2P2) were drastically reduced in Rett syndrome irrespective of the SOA, suggesting a largely affected mechanism of integration of upcoming sensory input with memory. Moreover, developmental stagnation of auditory ERP characterized patients with RS: absence of typical P2N1 enlargement and P1 and N1 shortening with age at least for shortest SOA.

LIMITATIONS

We could not figure out the cause for the high percentage of no-evident ERP RS participants and our final sample of the RS group was rather small. Also, our study did not include a control clinical group.

CONCLUSIONS

Thus, auditory ERPs inform us about abnormalities within auditory processing that cannot be fully overcomed by slowing presentation rate.

Impact of transcranial direct current stimulation on white matter microstructure integrity in mild cognitive impairment patients according to effect modifiers as risk factors for Alzheimer's disease

Background

Little research exists on how individual risk factors for Alzheimer's disease (AD) affect the intermediate phenotype after transcranial direct current stimulation (tDCS), despite the importance of precision medicine-based therapeutic approaches.

Objective

To determine how an application of sequential tDCS (2 mA/day, left dorsolateral prefrontal cortex, 10 sessions) affects changes in white matter (WM) microstructure integrity in 63 mild cognitive impairment (MCI) patients with effect modifiers such as Aβ deposition, APOE ε4 carrier status, BDNF Val66Met polymorphism status, and sex.

Methods

We examined individual effect modifier-by-tDCS interactions and multiple effect modifiers-by-tDCS interactions for diffusion metrics. We also evaluated the association between baseline Aβ deposition and changes in WM microstructure integrity following tDCS.

Results

We found that APOE ε4 carrier status and sex had a significant interaction with tDCS, resulting in increased fractional anisotropy (FA) in the right uncinate fasciculus (UF) after stimulation. Additionally, we observed multiple effect modifiers-by-tDCS interactions on WM integrity of the right UF, leading to a more pronounced increase in FA values in APOE ε4 carriers and females with Val66 homozygotes. Finally, baseline Aβ deposition was positively associated with a difference in FA of the left cingulum in the hippocampal area, which showed a positive association with the changes in the score for delayed memory.

Conclusion

Our study shows the differential impact of individual AD risk factors on changes in the early intermediate phenotype after sequential tDCS in MCI patients. This research emphasizes the importance of precision medicine approaches in tDCS for the prodromal stages of AD.

Opposite effects of rapid auditory stimulation on tetanized and non-tetanized tone of adjacent frequency: Mismatch negativity study

Our study describes the effects of sensory tetanization on neurophysiological and behavioral measures in humans linking cellular studies of long-term potentiation with high-level brain processes. Rapid (every 75ms) presentation of pure tone (1020 Hz, 50ms) for 2 minutes was preceded and followed by oddball blocks that contained the same stimulus presented as deviant (probability of 5-10%) interspersed with standard (80-90%) and deviant tones (5-10%) of adjacent frequencies (1000 and 980Hz, respectively). Mismatch negativity (MMN) component in response to tetanized tone (1020Hz), while being similar to MMN for non-tetanized tone before tetanization, became larger than that after tetanization, pointing to the increase in cortical differentiation of these tones. However, this differentiation was partially due to the MMN decrease after tetanization for tones adjacent to tetanized frequency, suggesting the influence of lateral inhibition to this effect. Although MMN correlated with tone discriminability in a psychophysical task, the behavioral improvement after tetanization was not statistically detectable. To conclude, short-term auditory tetanization affects cortical representation of tones that are not limited to the tetanized stimuli.

Brain-Derived Neurotrophic Factor Gene Polymorphism Predicts Response to Continuous Theta Burst Stimulation in Chronic Stroke Patients

OBJECTIVES

The efficacy of repetitive transcranial magnetic stimulation (rTMS) in clinically relevant neuroplasticity research depends on the degree to which stimulation induces robust, reliable effects. The high degree of interindividual and intraindividual variability observed in response to rTMS protocols, such as continuous theta burst stimulation (cTBS), therefore represents an obstacle to its utilization as treatment for neurological disorders. Brain-derived neurotrophic factor (BDNF) is a protein involved in human synaptic and neural plasticity, and a common polymorphism in the BDNF gene (Val66Met) may influence the capacity for neuroplastic changes that underlie the effects of cTBS and other rTMS protocols. While evidence from healthy individuals suggests that Val66Met polymorphism carriers may show diminished or facilitative effects of rTMS compared to their homozygous Val66Val counterparts, this has yet to be demonstrated in the patient populations where neuromodulatory therapies are most relevant.

MATERIALS AND METHODS

We examined the effects of BDNF Val66Met polymorphism on cTBS aftereffects in stroke patients. We compared approximately 30 log-transformed motor-evoked potentials (LnMEPs) obtained per time point: at baseline and at 0, 10, 20, and 30 min after cTBS-600, from 18 patients with chronic stroke using single TMS pulses. We used linear mixed-effects regression with trial-level data nested by subject for higher statistical power.

RESULTS

We found a significant interaction between BDNF genotype and pre-/post-cTBS LnMEPs. Val66Val carriers showed decrease in cortical excitability, whereas Val66Met carriers exhibited a modest increase in cortical excitability for 20 min poststimulation, followed by inhibition 30 min after cTBS-600.

CONCLUSIONS

Our findings strongly suggest that BDNF genotype differentially affects neuroplastic responses to TMS in individuals with chronic stroke. This provides novel insight into potential sources of variability in cTBS response in patients, which has important implications for optimizing the utility of this neuromodulation approach. Incorporating BDNF polymorphism genetic screening to stratify patients prior to use of cTBS as a neuromodulatory technique in therapy or research may optimize response rates.

Transcranial Direct Current Stimulation Targeting the Entire Motor Network Does Not Increase Corticospinal Excitability

Transcranial direct current stimulation (tDCS) over the contralateral primary motor cortex of the target muscle (conventional tDCS) has been described to enhance corticospinal excitability, as measured with transcranial magnetic stimulation. Recently, tDCS targeting the brain regions functionally connected to the contralateral primary motor cortex (motor network tDCS) was reported to enhance corticospinal excitability more than conventional tDCS. We compared the effects of motor network tDCS, 2 mA conventional tDCS, and sham tDCS on corticospinal excitability in 21 healthy participants in a randomized, single-blind within-subject study design. We applied tDCS for 12 min and measured corticospinal excitability with TMS before tDCS and at 0, 15, 30, 45, and 60 min after tDCS. Statistical analysis showed that neither motor network tDCS nor conventional tDCS significantly increased corticospinal excitability relative to sham stimulation. Furthermore, the results did not provide evidence for superiority of motor network tDCS over conventional tDCS. Motor network tDCS seems equally susceptible to the sources of intersubject and intrasubject variability previously observed in response to conventional tDCS.

Inter-Individual Variability in tDCS Effects: A Narrative Review on the Contribution of Stable, Variable, and Contextual Factors

Due to its safety, portability, and cheapness, transcranial direct current stimulation (tDCS) use largely increased in research and clinical settings. Despite tDCS's wide application, previous works pointed out inconsistent and low replicable results, sometimes leading to extreme conclusions about tDCS's ineffectiveness in modulating behavioral performance across cognitive domains. Traditionally, this variability has been linked to significant differences in the stimulation protocols across studies, including stimulation parameters, target regions, and electrodes montage. Here, we reviewed and discussed evidence of heterogeneity emerging at the intra-study level, namely inter-individual differences that may influence the response to tDCS within each study. This source of variability has been largely neglected by literature, being results mainly analyzed at the group level. Previous research, however, highlighted that only a half-or less-of studies' participants could be classified as responders, being affected by tDCS in the expected direction. Stable and variable inter-individual differences, such as morphological and genetic features vs. hormonal/exogenous substance consumption, partially account for this heterogeneity. Moreover, variability comes from experiments' contextual elements, such as participants' engagement/baseline capacity and individual task difficulty. We concluded that increasing knowledge on inter-dividual differences rather than undermining tDCS effectiveness could enhance protocols' efficiency and reproducibility.

Generalization of sustained neurophysiological effects of short-term auditory 13-Hz stimulation to neighboring frequency representation in humans

A fuller understanding of the effects of auditory tetanization in humans would inform better language and sensory learning paradigms, however, there are still unanswered questions. Here, we probe sustained changes in the event-related potentials (ERPs) to 1020Hz and 980Hz tones following a rapid presentation of 1020Hz tone (every 75 ms, 13.3Hz, tetanization). Consistent with the previous studies (Rygvold, et al., 2021, Mears & Spencer 2012), we revealed the increase in the P2 ERP component after tetanization. Contrary to other studies (Clapp et al., 2005; Lei et al., 2017) we did not observe the expected N1 increase after tetanization even in the experimental sequence identical to Clapp. et al., 2005. We detected a significant N1 decrease after tetanization. Expanding previous research, we showed that P2 increase and N1 decrease is not specific to the stimulus type (tetanized 1020Hz and non-tetanized 980Hz), suggesting the generalizability of tetanization effect to the not-stimulated auditory tones, at least to those of the neighboring frequency. The ERPs tetanization effects were observed for at least 30 min - the most prolonged interval examined, consistent with the duration of long-term potentiation, LTP. In addition, the tetanization effects were detectable in the blocks where the participants watched muted videos, an experimental setting that can be easily used in children and other challenging groups. Thus, auditory 13-Hz stimulation affects brain processing of tones including those of neighboring frequencies.

Do Brain-Derived Neurotrophic Factor Genetic Polymorphisms Modulate the Efficacy of Motor Cortex Plasticity Induced by Non-invasive Brain Stimulation? A Systematic Review

Techniques of non-invasive brain stimulation (NIBS) of the human primary motor cortex (M1) are widely used in basic and clinical research to induce neural plasticity. The induction of neural plasticity in the M1 may improve motor performance ability in healthy individuals and patients with motor deficit caused by brain disorders. However, several recent studies revealed that various NIBS techniques yield high interindividual variability in the response, and that the brain-derived neurotrophic factor (BDNF) genotype (i.e., Val/Val and Met carrier types) may be a factor contributing to this variability. Here, we conducted a systematic review of all published studies that investigated the effects of the BDNF genotype on various forms of NIBS techniques applied to the human M1. The motor-evoked potential (MEP) amplitudes elicited by single-pulse transcranial magnetic stimulation (TMS), which can evaluate M1 excitability, were investigated as the main outcome. A total of 1,827 articles were identified, of which 17 (facilitatory NIBS protocol, 27 data) and 10 (inhibitory NIBS protocol, 14 data) were included in this review. More than two-thirds of the data (70.4–78.6%) on both NIBS protocols did not show a significant genotype effect of NIBS on MEP changes. Conversely, most of the remaining data revealed that the Val/Val type is likely to yield a greater MEP response after NIBS than the Met carrier type in both NIBS protocols (21.4–25.9%). Finally, to aid future investigation, we discuss the potential effect of the BDNF genotype based on mechanisms and methodological issues.

Does non-invasive brain stimulation modulate emotional stress reactivity?

Excessive emotional responses to stressful events can detrimentally affect psychological functioning and mental health. Recent studies have provided evidence that non-invasive brain stimulation (NBS) targeting the prefrontal cortex (PFC) can affect the regulation of stress-related emotional responses. However, the reliability and effect sizes have not been systematically analyzed. In the present study, we reviewed and meta-analyzed the effects of repetitive transcranial magnetic (rTMS) and transcranial direct current stimulation (tDCS) over the PFC on acute emotional stress reactivity in healthy individuals. Forty sham-controlled single-session rTMS and tDCS studies were included. Separate random effects models were performed to estimate the mean effect sizes of emotional reactivity. Twelve rTMS studies together showed no evidence that rTMS over the PFC influenced emotional reactivity. Twenty-six anodal tDCS studies yielded a weak beneficial effect on stress-related emotional reactivity (Hedges’ g = −0.16, CI_95% = [−0.33, 0.00]). These findings suggest that a single session of NBS is insufficient to induce reliable, clinically significant effects but also provide preliminary evidence that specific NBS methods can affect emotional reactivity. This may motivate further research into augmenting the efficacy of NBS protocols on stress-related processes.

Can Transcranial Direct Current Stimulation Enhance Poststroke Motor Recovery? Development of a Theoretical Patient-Tailored Model

New treatments that can facilitate neural repair and reduce persistent impairments have significant value in promoting recovery following stroke. One technique that has gained interest is transcranial direct current stimulation (tDCS) as early research suggested it could enhance plasticity and enable greater behavioral recovery. However, several studies have now identified substantial intersubject variability in response to tDCS and clinical trials revealed insufficient evidence of treatment effectiveness. A possible explanation for the varied and negative findings is that the physiologic model of stroke recovery that researchers have used to guide the application of tDCS-based treatments in stroke is overly simplistic and does not account for stroke heterogeneity or known determinants that affect the tDCS response. Here, we propose that tDCS could have a more clearly beneficial role in enhancing stroke recovery if greater consideration is given to individualizing treatment. By critically reviewing the proposed mechanisms of tDCS, stroke physiology across the recovery continuum, and known determinants of tDCS response, we propose a new, theoretical, patient-tailored approach to delivering tDCS after stroke. The proposed model includes a step-by-step principled selection strategy for identifying optimal neuromodulation targets and outlines key areas for further investigation. Tailoring tDCS treatment to individual neuroanatomy and physiology is likely our best chance at producing robust and meaningful clinical benefit for people with stroke and would therefore accelerate opportunities for clinical translation.

Functional Effects of Bilateral Dorsolateral Prefrontal Cortex Modulation During Sequential Decision-Making: A Functional Near-Infrared Spectroscopy Study With Offline Transcranial Direct Current Stimulation

The ability to learn sequential contingencies of actions for predicting future outcomes is indispensable for flexible behavior in many daily decision-making contexts. It remains open whether such ability may be enhanced by transcranial direct current stimulation (tDCS). The present study combined tDCS with functional near-infrared spectroscopy (fNIRS) to investigate potential tDCS-induced effects on sequential decision-making and the neural mechanisms underlying such modulations. Offline tDCS and sham stimulation were applied over the left and right dorsolateral prefrontal cortex (dlPFC) in young male adults (N = 29, mean age = 23.4 years, SD = 3.2) in a double-blind between-subject design using a three-state Markov decision task. The results showed (i) an enhanced dlPFC hemodynamic response during the acquisition of sequential state transitions that is consistent with the findings from a previous functional magnetic resonance imaging (fMRI) study; (ii) a tDCS-induced increase of the hemodynamic response in the dlPFC, but without accompanying performance-enhancing effects at the behavioral level; and (iii) a greater tDCS-induced upregulation of hemodynamic responses in the delayed reward condition that seems to be associated with faster decision speed. Taken together, these findings provide empirical evidence for fNIRS as a suitable method for investigating hemodynamic correlates of sequential decision-making as well as functional brain correlates underlying tDCS-induced modulation. Future research with larger sample sizes for carrying out subgroup analysis is necessary in order to decipher interindividual differences in tDCS-induced effects on sequential decision-making process at the behavioral and brain levels.

Genetic Polymorphisms Do Not Predict Interindividual Variability to Cathodal Transcranial Direct Current Stimulation of the Primary Motor Cortex

Introduction: High variability between individuals (i.e., interindividual variability) in response to transcranial direct current stimulation (tDCS) has become a commonly reported issue in the tDCS literature in recent years. Inherent genetic differences between individuals have been proposed as a contributing factor to observed response variability. This study investigated whether tDCS interindividual variability was genetically mediated. Methods: A large sample size of 61 healthy males received cathodal tDCS (c-tDCS) and sham-tDCS of the primary motor cortex at 1 mA and 10 min via 6 × 4 cm active and 7 × 5 cm return electrodes. Corticospinal excitability (CSE) was assessed via 25 single-pulse transcranial magnetic stimulation motor-evoked potentials (MEPs). Intracortical inhibition was assessed via twenty-five 3 msec interstimulus interval (ISI) paired-pulse MEPs, known as short-interval intracortical inhibition (SICI). Intracortical facilitation (ICF) was assessed via twenty-five 10 msec ISI paired-pulse MEPs. Gene variants encoding for excitatory and inhibitory neuroreceptors were determined via saliva samples. Predetermined thresholds and statistical cluster analyses were used to subgroup individuals. Results: Two distinct subgroups were identified, "responders" reducing CSE following c-tDCS and "nonresponders" showing no reduction or even increase in CSE. Differences in CSE between responders and nonresponders following c-tDCS were not explained by changes in SICI or ICF. Conclusions: No significant relationships were reported between gene variants and interindividual variability to c-tDCS, suggesting that the chosen gene variants did not influence the activity of the neuroreceptors involved in eliciting changes in CSE in responders following c-tDCS. In this largest c-tDCS study of its kind, novel insights were reported into the contribution genetic factors may play in observed interindividual variability to c-tDCS. Impact statement This study adds insight into the issue of interindividual variability to c-tDCS. It highlights not all individuals respond to c-tDCS similarly when exposed to the same stimulus parameters. This disparity in response to c-tDCS between individuals does not appear to be genetically mediated. For c-tDCS to progress to large-scale clinical application, reliability, predictability and reproducibility are essential. Systematically investigating factors contributing to interindividual variability take steps towards this progress the c-tDCS field towards the potential development of screening tools to determine clinical suitability to c-tDCS to ensure its application in those who may benefit the most.

Can genetic polymorphisms predict response variability to anodal transcranial direct current stimulation of the primary motor cortex?

Genetic mediation of cortical plasticity and the role genetic variants play in previously observed response variability to transcranial direct current stimulation (tDCS) have become important issues in the tDCS literature in recent years. This study investigated whether inter-individual variability to tDCS was in-part genetically mediated. In 61 healthy males, anodal-tDCS (a-tDCS) and sham-tDCS were administered to the primary motor cortex at 1 mA for 10-min via 6 × 4 cm active and 7 × 5 cm return electrodes. Twenty-five single-pulse transcranial magnetic stimulation (TMS) motor evoked potentials (MEP) were recorded to represent corticospinal excitability (CSE). Twenty-five paired-pulse MEPs were recorded with 3 ms inter-stimulus interval (ISI) to assess intracortical inhibition (ICI) via short-interval intracranial inhibition (SICI) and 10 ms ISI for intracortical facilitation (ICF). Saliva samples were tested for specific genetic polymorphisms in genes encoding for excitatory and inhibitory neuroreceptors. Individuals were sub-grouped based on a pre-determined threshold and via statistical cluster analysis. Two distinct subgroups were identified, increases in CSE following a-tDCS (i.e. Responders) and no increase or even reductions in CSE (i.e. Non-responders). No changes in ICI or ICF were reported. No relationships were reported between genetic polymorphisms in excitatory receptor genes and a-tDCS responders. An association was reported between a-tDCS responders and GABRA3 gene polymorphisms encoding for GABA-A receptors suggesting potential relationships between GABA-A receptor variations and capacity to undergo tDCS-induced cortical plasticity. In the largest tDCS study of its kind, this study presents an important step forward in determining the contribution genetic factors play in previously observed inter-individual variability to tDCS.

Do visual and auditory stimulus-specific response modulation reflect different mechanisms of neocortical plasticity?

Stimulus response modulation (SRM) of sensory evoked potentials represents a promising method as a non-invasive index of long-term potentiation (LTP)-like synaptic plasticity in the human sensory cortices. As of today, however, no consensus exists regarding which experimental parameters elicit the most robust SRM response. The aim of the current study was twofold; firstly, we aimed to replicate former studies demonstrating visual SRM in healthy adults. Second, we integrated visual and auditory stimuli within the same SRM recording session to assay potential cross-modal associations. Such an association between modalities would strengthen the assumption that the SRM effect reflects common mechanisms underlying synaptic plasticity rather than reflecting modality-specific phenomena. A replication of previous findings showing robust potentiation of the visual evoked potential was evident, supporting the majority of previous work using similar paradigms, lending further support to the notion that high-frequent visual stimulation is a viable probe into LTP-like synaptic plasticity in the human visual cortex. The auditory evoked potentials (AEPs) did not, however, fully replicate previous work, though a significant increase of temporally later AEP components was found. In contrast to our hypothesis, there were no significant within-subject cross-modality correlations between the visual and auditory SRM. This lack of significant association might suggest that auditory and visual SRM depend on different mechanisms, and that further SRM studies on non-invasive LTP-like synaptic plasticity should focus on optimizing paradigms within the visual modality.

Human Sensory LTP Predicts Memory Performance and Is Modulated by the BDNF Val66Met Polymorphism

Background: Long-term potentiation (LTP) is recognised as a core neuronal process underlying long-term memory. However, a direct relationship between LTP and human memory performance is yet to be demonstrated. The first aim of the current study was thus to assess the relationship between LTP and human long-term memory performance. With this also comes an opportunity to explore factors thought to mediate the relationship between LTP and long-term memory. The second aim of the current study was to explore the relationship between LTP and memory in groups differing with respect to brain-derived neurotrophic factor (BDNF) Val⁶⁶Met; a single-nucleotide polymorphism (SNP) implicated in memory function.

Methods: Participants were split into three genotype groups (Val/Val, Val/Met, Met/Met) and were presented with both an EEG paradigm for inducing LTP-like enhancements of the visually-evoked response, and a test of visual memory.

Results: The magnitude of LTP 40 min after induction was predictive of long-term memory performance. Additionally, the BDNF Met allele was associated with both reduced LTP and reduced memory performance.

Conclusions: The current study not only presents the first evidence for a relationship between sensory LTP and human memory performance, but also demonstrates how targeting this relationship can provide insight into factors implicated in variation in human memory performance. It is anticipated that this will be of utility to future clinical studies of disrupted memory function.

Biological and anatomical factors influencing interindividual variability to noninvasive brain stimulation of the primary motor cortex: a systematic review and meta-analysis

Noninvasive brain stimulation (NIBS) modifies corticospinal excitability (CSE) historically in a predictable manner dependent on stimulation parameters. Researchers, however, discuss high degrees of variability between individuals, either responding as expected or not responding as expected. The explanation for this interindividual variability remains unknown with suggested interplay between stimulation parameters and variations in biological, anatomical, and physiological factors. This systematic review and meta-analysis aimed to investigate the effect of variation in inherent factors within an individual (biological and anatomical factors) on CSE in response to NIBS of the primary motor cortex. Twenty-two studies were included investigating genetic variation (n=7), age variation (n=4), gender variation (n=7), and anatomical variation (n=5). The results indicate that variation in brain-derived neurotrophic factor genotypes may have an effect on CSE after NIBS. Variation between younger and older adults also affects CSE after NIBS. Variation between age-matched males and females does not affect CSE after NIBS, but variation across the menstrual cycle does. Variation between skull thickness and brain tissue morphology influences the electric field magnitude that ultimately reaches the primary motor cortex. These findings indicate that biological and anatomical variations may in part account for interindividual variability in CSE in response to NIBS of the primary motor cortex, categorizing individuals as responding as expected (responders) or not responding as expected (nonresponders).

Gene polymorphisms and response to transcranial direct current stimulation for auditory verbal hallucinations in schizophrenia

OBJECTIVE

Recent observations demonstrate a significant ameliorative effect of add-on transcranial direct current stimulation (tDCS) on auditory verbal hallucinations (AVHs) in schizophrenia. Of the many SNPs, NRG1 rs35753505 and catechol-o-methyl transferase (COMT) rs4680 polymorphisms have shown to have a strong association with neuroplasticity effect in schizophrenia.

METHODS

Schizophrenia patients (n=32) with treatment resistant auditory hallucinations were administered with an add-on tDCS. The COMT (rs4680) and NRG1 (rs35753505) genotypes were determined. The COMT genotypes were categorised into Val group (GG; n=15) and Met group (GG/AG; n=17) and NRG1 genotypes were categorised into AA group (n=12) and AG/GG group (n=20).

RESULTS

The reduction in auditory hallucination sub-scale score was significantly affected by COMT-GG genotype [Time×COMT interaction: F(1,28)=10.55, p=0.003, ɳ2=0.27]. Further, COMT-GG effect was epistatically influenced by the co-occurrence of NRG1-AA genotype [Time×COMT×NRG1 interaction: F(1,28)=8.09, p=0.008, ɳ2=0.22]. Irrespective of genotype, females showed better tDCS response than males [Time×Sex interaction: F(1,21)=4.67, p=0.04, ɳ2=0.18].

CONCLUSION

COMT-GG and NRG1-AA genotypes aid the tDCS-induced improvement in AVHs in schizophrenia patients. Our preliminary observations need replication and further systematic research to understand the neuroplastic gene determinants that modulate the effect of tDCS.

Comparison of Three Non-Invasive Transcranial Electrical Stimulation Methods for Increasing Cortical Excitability

Transcranial direct current stimulation (tDCS) is a representative non-invasive brain stimulation method (NIBS). tDCS increases cortical excitability not only in healthy individuals, but also in stroke patients where it contributes to motor function improvement. Recently, two additional types of transcranial electrical stimulation (tES) methods have been introduced that may also prove beneficial for stimulating cortical excitability; these are transcranial random noise stimulation (tRNS) and transcranial alternating current stimulation (tACS). However, comparison of tDCS with tRNS and tACS, in terms of efficacy in cortical excitability alteration, has not been reported thus far. We compared the efficacy of the three different tES methods for increasing cortical excitability using the same subject population and same current intensity. Fifteen healthy subjects participated in this study. Similar stimulation patterns (1.0 mA and 10 min) were used for the three conditions of stimulation (tDCS, tRNS, and tACS). Cortical excitability was explored via single-pulse TMS elicited motor evoked potentials (MEPs). Compared with pre-measurements, MEPs significantly increased with tDCS, tACS, and tRNS (p < 0.05). Compared with sham measurements, significant increases in MEPs were also observed with tRNS and tACS (p < 0.05), but not with tDCS. In addition, a significant correlation of the mean stimulation effect was observed between tRNS and tACS (p = 0.019, r = 0.598). tRNS induced a significant increase in MEP compared with the Pre or Sham at all time points. tRNS resulted in the largest significant increase in MEPs. These findings suggest that tRNS is the most effective tES method and should be considered as part of a treatment plan for improving motor function in stroke patients.

Effects of a common transcranial direct current stimulation (tDCS) protocol on motor evoked potentials found to be highly variable within individuals over 9 testing sessions

Transcranial direct current stimulation (tDCS) uses a weak electric current to modulate neuronal activity. A neurophysiologic outcome measure to demonstrate reliable tDCS modulation at the group level is transcranial magnetic stimulation engendered motor evoked potentials (MEPs). Here, we conduct a study testing the reliability of individual MEP response patterns following a common tDCS protocol. Fourteen participants (7m/7f) each underwent nine randomized sessions of 1 mA, 10 min tDCS (3 anode; 3 cathode; 3 sham) delivered using an M1/orbito-frontal electrode montage (sessions separated by an average of ~5.5 days). Fifteen MEPs were obtained prior to, immediately following and in 5 min intervals for 30 min following tDCS. TMS was delivered at 130 % resting motor threshold using neuronavigation to ensure consistent coil localization. A number of non-experimental variables were collected during each session. At the individual level, considerable variability was seen among different testing sessions. No participant demonstrated an excitatory response ≥20 % to all three anodal sessions, and no participant demonstrated an inhibitory response ≥20 % to all three cathodal sessions. Intra-class correlation revealed poor anodal and cathodal test-retest reliability [anode: ICC(2,1) = 0.062; cathode: ICC(2,1) = 0.055] and moderate sham test-retest reliability [ICC(2,1) = 0.433]. Results also revealed no significant effect of tDCS at the group level. Using this common protocol, we found the effects of tDCS on MEP amplitudes to be highly variable at the individual level. In addition, no significant effects of tDCS on MEP amplitude were found at the group level. Future studies should consider utilizing a more strict experimental protocol to potentially account for intra-individual response variations.

Transcranial direct current stimulation and neuroplasticity genes: implications for psychiatric disorders

BACKGROUND AND AIM

Transcranial direct current stimulation (tDCS) is a non-invasive and well-tolerated brain stimulation technique with promising efficacy as an add-on treatment for schizophrenia and for several other psychiatric disorders. tDCS modulates neuroplasticity; psychiatric disorders are established to be associated with neuroplasticity abnormalities. This review presents the summary of research on potential genetic basis of neuroplasticity-modulation mechanism underlying tDCS and its implications for treating various psychiatric disorders.

METHOD

A systematic review highlighting the genes involved in neuroplasticity and their role in psychiatric disorders was carried out. The focus was on the established genetic findings of tDCS response relationship with BDNF and COMT gene polymorphisms.

RESULT

Synthesis of these preliminary observations suggests the potential influence of neuroplastic genes on tDCS treatment response. These include several animal models, pharmacological studies, mentally ill and healthy human subject trials.

CONCLUSION

Taking into account the rapidly unfolding understanding of tDCS and the role of synaptic plasticity disturbances in neuropsychiatric disorders, in-depth evaluation of the mechanism of action pertinent to neuroplasticity modulation with tDCS needs further systematic research. Genes such as NRG1, DISC1, as well as those linked with the glutamatergic receptor in the context of their direct role in the modulation of neuronal signalling related to neuroplasticity aberrations, are leading candidates for future research in this area. Such research studies might potentially unravel observations that might have potential translational implications in psychiatry.

Does transcranial direct current stimulation enhance cognitive and motor functions in the ageing brain? A systematic review and meta- analysis

The use of transcranial direct current stimulation (tDCS) to enhance cognitive and motor functions has enjoyed a massive increase in popularity. Modifying neuroplasticity via non-invasive cortical stimulation has enormous potential to slow or even reverse declines in functions associated with ageing. The current meta-analysis evaluated the effects of tDCS on cognitive and motor performance in healthy older adults. Of the 81 studies identified, 25 qualified for inclusion. A random effects model meta-analysis revealed a significant overall standardized mean difference equal to 0.53 (SE=0.09; medium heterogeneity: I(2)=57.08%; and high fail-safe: N=448). Five analyses on moderator variables indicated significant tDCS beneficial effects: (a) on both cognitive and motor task performances, (b) across a wide-range of cognitive tasks, (c) on specific brain areas, (d) stimulation offline (before) or online (during) the cognitive and motor tasks. Although the meta-analysis revealed robust support for enhancing both cognitive and motor performance, we outline a number of caveats on the use of tDCS.

LTP-like plasticity in the visual system and in the motor system appear related in young and healthy subjects

LTP-like plasticity measured by visual evoked potentials (VEP) can be induced in the intact human brain by presenting checkerboard reversals. Also associated with LTP-like plasticity, around two third of participants respond to transcranial magnetic stimulation (TMS) with a paired-associate stimulation (PAS) protocol with a potentiation of their motor evoked potentials. LTP-like processes are also required for verbal and motor learning tasks. We compared effect sizes, responder rates and intercorrelations as well as the potential influence of attention between these four assessments in a group of 37 young and healthy volunteers. We observed a potentiation effect of the N75 and P100 VEP component which positively correlated with plasticity induced by PAS. Subjects with a better subjective alertness were more likely to show PAS and VEP potentiation. No correlation was found between the other assessments. Effect sizes and responder rates of VEP potentiation were higher compared to PAS. Our results indicate a high variability of LTP-like effects and no evidence for a system-specific nature. As a consequence, studies wishing to assess individual levels of LTP-like plasticity should employ a combination of multiple assessments.

Duration-dependent effects of the BDNF Val66Met polymorphism on anodal tDCS induced motor cortex plasticity in older adults: a group and individual perspective

The brain derived neurotrophic factor (BDNF) Val66Met polymorphism and stimulation duration are thought to play an important role in modulating motor cortex plasticity induced by non-invasive brain stimulation (NBS). In the present study we sought to determine whether these factors interact or exert independent effects in older adults. Fifty-four healthy older adults (mean age = 66.85 years) underwent two counterbalanced sessions of 1.5 mA anodal transcranial direct current stimulation (atDCS), applied over left M1 for either 10 or 20 min. Single pulse transcranial magnetic stimulation (TMS) was used to assess corticospinal excitability (CSE) before and every 5 min for 30 min following atDCS. On a group level, there was an interaction between stimulation duration and BDNF genotype, with Met carriers (n = 13) showing greater post-intervention potentiation of CSE compared to Val66Val homozygotes homozygotes (n = 37) following 20 min (p = 0.002) but not 10 min (p = 0.219) of stimulation. Moreover, Met carriers, but not Val/Val homozygotes, exhibited larger responses to TMS (p = 0.046) after 20 min atDCS, than following 10 min atDCS. On an individual level, two-step cluster analysis revealed a considerable degree of inter-individual variability, with under half of the total sample (42%) showing the expected potentiation of CSE in response to atDCS across both sessions. Intra-individual variability in response to different durations of atDCS was also apparent, with one-third of the total sample (34%) exhibiting LTP-like effects in one session but LTD-like effects in the other session. Both the inter-individual (p = 0.027) and intra-individual (p = 0.04) variability was associated with BDNF genotype. In older adults, the BDNF Val66Met polymorphism along with stimulation duration appears to play a role in modulating tDCS-induced motor cortex plasticity. The results may have implications for the design of NBS protocols for healthy and diseased aged populations.

The contribution of interindividual factors to variability of response in transcranial direct current stimulation studies

There has been an explosion of research using transcranial direct current stimulation (tDCS) for investigating and modulating human cognitive and motor function in healthy populations. It has also been used in many studies seeking to improve deficits in disease populations. With the slew of studies reporting “promising results” for everything from motor recovery after stroke to boosting memory function, one could be easily seduced by the idea of tDCS being the next panacea for all neurological ills. However, huge variability exists in the reported effects of tDCS, with great variability in the effect sizes and even contradictory results reported. In this review, we consider the interindividual factors that may contribute to this variability. In particular, we discuss the importance of baseline neuronal state and features, anatomy, age and the inherent variability in the injured brain. We additionally consider how interindividual variability affects the results of motor-evoked potential (MEP) testing with transcranial magnetic stimulation (TMS), which, in turn, can lead to apparent variability in response to tDCS in motor studies.

Sleep quality, BDNF genotype and gene expression in individuals with chronic abdominal pain

BACKGROUND

Sleep quality and genetics may contribute to the etiology of gastrointestinal (GI) symptoms. Individuals with impaired sleep often have a number of associated symptoms including chronic abdominal pain (CAP). The current study examined the interactions of brain-derived neurotrophic factor (BDNF) genotype with sleep quality in persons with CAP and healthy controls. In addition, associations among sleep quality, BDNF genotype, and gene expression were explored in the participants.

METHODS

Data were collected on 59 participants (46% male, 61% White, 26.9 ± 6.6 years; CAP (n=19) and healthy controls (n=40)). Participants with CAP reported poorer sleep quality compared to healthy controls. BDNF genotype, categorized as Val/Val homozygotes versus the Met carriers.

RESULTS

Microarray analysis found twenty-four differentially expressed genes by a two-fold magnitude in participants with poor sleep quality compared to good sleep quality, and seven differentially expressed genes comparing CAP to healthy control. Three specific genes in the pain group overlap with sleep quality, including insulin-like growth factor 1 (IGF1), spermatogenesis associated serine-rich 2-like (SPATS2L), and immunoglobulin heavy constant gamma 1 or mu (IGHG1/// IGHM). BDNF was shown to have an interaction effect with GI and sleep symptoms.

CONCLUSIONS

Participants with CAP reported poor sleep quality compared to healthy controls. The role of the BDNF Met allele on differential gene expression was not distinct as main factor, but impacted interactions with sleep quality and CAP. Down-regulation of IGF1, SPATS2L, and IGHG1 expression may be related to the etiology of poor sleep quality and CAP.

TRIAL REGISTRATION

Clinicaltrial.gov # NCT00824941.

Identification of BDNF sensitive electrophysiological markers of synaptic activity and their structural correlates in healthy subjects using a genetic approach utilizing the functional BDNF Val66Met polymorphism

Increasing evidence suggests that synaptic dysfunction is a core pathophysiological hallmark of neurodegenerative disorders. Brain-derived neurotropic factor (BDNF) is key synaptogenic molecule and targeting synaptic repair through modulation of BDNF signalling has been suggested as a potential drug discovery strategy. The development of such "synaptogenic" therapies depend on the availability of BDNF sensitive markers of synaptic function that could be utilized as biomarkers for examining target engagement or drug efficacy in humans. Here we have utilized the BDNF Val66Met genetic polymorphism to examine the effect of the polymorphism and genetic load (i.e. Met allele load) on electrophysiological (EEG) markers of synaptic activity and their structural (MRI) correlates. Sixty healthy adults were prospectively recruited into the three genetic groups (Val/Val, Val/Met, Met/Met). Subjects also underwent fMRI, tDCS/TMS, and cognitive assessments as part of a larger study. Overall, some of the EEG markers of synaptic activity and brain structure measured with MRI were the most sensitive markers of the polymorphism. Met carriers showed decreased oscillatory activity and synchrony in the neural network subserving error-processing, as measured during a flanker task (ERN); and showed increased slow-wave activity during resting. There was no evidence for a Met load effect on the EEG measures and the polymorphism had no effects on MMN and P300. Met carriers also showed reduced grey matter volume in the anterior cingulate and in the (left) prefrontal cortex. Furthermore, anterior cingulate grey matter volume, and oscillatory EEG power during the flanker task predicted subsequent behavioural adaptation, indicating a BDNF dependent link between brain structure, function and behaviour associated with error processing and monitoring. These findings suggest that EEG markers such as ERN and resting EEG could be used as BDNF sensitive functional markers in early clinical development to examine target engagement or drug related efficacy of synaptic repair therapies in humans.