Impaired long-term depression in schizophrenia: a cathodal tDCS pilot study

Can transcranial direct current stimulation (tDCS) alleviate symptoms and improve cognition in psychiatric disorders?

OBJECTIVES

Since the discovery of psychopharmacological treatments in the early 1950s, followed by the development of second-generation antidepressants and antipsychotics, biological psychiatry has not achieved much progress. Recent technological advances in the field of non-invasive brain stimulation open new perspectives in the treatment of psychiatric disorders. Amongst them, transcranial direct current stimulation (tDCS) modulates cortical excitability and induces long-lasting effects. Here, we aimed at evaluating whether tDCS has potential to be developed as an innovative treatment in psychiatry.

METHODS

We conducted a systematic review of the current state of development and application of tDCS in psychiatric disorders, exploring clinical and cognitive effects, especially in major depressive disorder (MDD), schizophrenia and substance use disorder.

RESULTS

Systematic literature search yielded 40 publications: 22 in MDD, nine in schizophrenia, seven in substance use disorder, one in obsessive-compulsive disorder and one in mania. Our findings indicated beneficial clinical effects of tDCS for MDD and a promising literature in schizophrenia and substance use disorder.

CONCLUSIONS

Despite methodological differences, the data published to date are promising and supports the use of tDCS as a treatment for psychiatric disorders. However, its place regarding other treatments still has to be determined before becoming a routine clinical treatment.

Understanding tDCS effects in schizophrenia: a systematic review of clinical data and an integrated computation modeling analysis

Although recent clinical studies using transcranial direct current stimulation (tDCS) for schizophrenia showed encouraging results, several tDCS montages were employed and their current flow pattern has not been investigated. We performed a systematic review to identify clinical tDCS studies in schizophrenia. We then applied computer head modeling analysis for prediction of current flow. Out of 41 references, we identified 12 relevant studies. The most employed montage was anode and cathode over the left dorsolateral prefrontal and temporoparietal cortex, respectively. Computational model analysis predicted activation and under-activation under the anode and the cathode, respectively, occurring in areas respectively associated with negative and positive symptoms. We also identified tDCS-induced electrical currents in cortical areas between the electrodes (frontoparietal network) and, to a lesser extent, in deeper structures involved in schizophrenia pathophysiology. Mechanisms of tDCS effects in schizophrenia and the usefulness of computer modeling techniques for planning tDCS trials in schizophrenia are discussed.

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.

Transcranial direct current stimulation in schizophrenia

Transcranial direct current stimulation (tDCS) is an upcoming treatment modality for patients with schizophrenia. A series of recent observations have demonstrated improvement in clinical status of schizophrenia patients with tDCS. This review summarizes the research work that has examined the effects of tDCS in schizophrenia patients with respect to symptom amelioration, cognitive enhancement and neuroplasticity evaluation. tDCS is emerging as a safe, rapid and effective treatment for various aspects of schizophrenia symptoms ranging from auditory hallucinations-for which the effect is most marked, to negative symptoms and cognitive symptoms as well. An interesting line of investigation involves using tDCS for altering and examining neuroplasticity in patients and healthy subjects and is likely to lead to new insights into the neurological aberrations and pathophysiology of schizophrenia. The mechanistic aspects of the technique are discussed in brief. Future work should focus on establishing the clinical efficacy of this novel technique and on evaluating this modality as an adjunct to cognitive enhancement protocols. Understanding the mechanism of action of tDCS as well as the determinants and neurobiological correlates of clinical response to tDCS remains an important goal, which will help us expand the clinical applications of tDCS for the treatment of patients with schizophrenia.

Modulating neural plasticity with non-invasive brain stimulation in schizophrenia

Schizophrenia is a severe mental disorder characterised by a complex phenotype including positive, negative, affective and cognitive symptoms. Various theories have been developed to integrate the clinical phenotype into a strong neurobiological framework. One theory describes schizophrenia as a disorder of impaired neural plasticity. Recently, non-invasive brain stimulation techniques have garnered much attention to their ability to modulate plasticity and treat schizophrenia. The aim of this review is to introduce the basic physiological principles of conventional non-invasive brain stimulation techniques and to review the available evidence for schizophrenia. Despite promising evidence for efficacy in a large number of clinical trials, we continue to have a rudimentary understanding of the underlying neurobiology. Additional investigation is required to improve the response rates to non-invasive brain stimulation, to reduce the interindividual variability and to improve the understanding of non-invasive brain stimulation in schizophrenia.

A similar but distinctive pattern of impaired cortical excitability in first-episode schizophrenia and ADHD

BACKGROUND

First-episode schizophrenia (FE-SZ) and attention deficit hyperactivity disorder (ADHD) are both neuropsychiatric disorders associated with an impaired dopaminergic transmission. Though displaying different clinical phenotypes, a common pathophysiological pathway is discussed controversially. Several studies using transcranial magnetic stimulation (TMS) revealed abnormalities in human motor cortex excitability in both schizophrenia and ADHD patients. Studies on cortical excitability comparing these two diseases directly are lacking.

METHOD

In this study, a total of 94 subjects were analyzed. Twenty-five FE-SZ patients were directly compared with 28 ADHD patients and 41 healthy controls (HC). We investigated cortical excitability (inhibitory and facilitatory networks) with single- and paired-pulse TMS to the left and right motor cortex.

RESULTS

Compared to HC, FE-SZ/ADHD patients displayed an impaired cortical inhibition over the left hemisphere. Apart from an enhanced intracortical facilitation, FE-SZ patients did not differ compared to ADHD patients in the main outcome measures. Both patient groups presented a dysfunctional hemispheric pattern of cortical inhibition and facilitation in comparison with HC.

CONCLUSION

The results of this study indicate a pattern of cortical disinhibition and abnormal hemispheric balance of intracortical excitability networks in two different psychiatric diseases. These effects might be associated with an imbalance in GABAergic and dopaminergic transmission and might provide evidence for a common pathophysiological pathway of both diseases.

Therapeutic effects of non-invasive brain stimulation with direct currents (tDCS) in neuropsychiatric diseases

Neuroplasticity, which is the dynamic structural and functional reorganization of central nervous system connectivity due to environmental and internal demands, is recognized as a major physiological basis for adaption of cognition, and behavior, and thus of utmost importance for normal brain function. Pathological alterations of plasticity are increasingly explored as pathophysiological foundation of diverse neurological and psychiatric diseases. Non-invasive brain stimulation techniques (NIBS), such as repetitive transcranial magnetic stimulation (rTMS), and transcranial direct current stimulation (tDCS), are able to induce and modulate neuroplasticity in humans. Therefore, they have potential to alter pathological plasticity on the one hand, and foster physiological plasticity on the other, in neuropsychiatric diseases to reduce symptoms, and enhance rehabilitation. tDCS is an emerging NIBS tool, which induces glutamatergic plasticity via application of relatively weak currents through the scalp in humans. In the last years its efficacy to treat neuropsychiatric diseases has been explored increasingly. In this review, we will give an overview of pathological alterations of plasticity in neuropsychiatric diseases, gather clinical studies involving tDCS to ameliorate symptoms, and discuss future directions of application, with an emphasis on optimizing stimulation effects.

The effect of non-competitive NMDA receptor antagonist MK-801 on neuronal activity in rodent prefrontal cortex: an animal model for cognitive symptoms of schizophrenia

Schizophrenia affects about 1% of the world population and is a major socio-economical problem in ours societies. Cognitive symptoms are particularly resistant to current treatments and are believed to be closely related to an altered function of prefrontal cortex (PFC). Particularly, abnormalities in the plasticity processes in the PFC are a candidate mechanism underlying cognitive symptoms, and the recent evidences in patients are in line with this hypothesis. Animal pharmacological models of cognitive symptoms, notably with non-competitive NMDA receptor antagonists such as MK-801, are commonly used to investigate the underlying cellular and molecular mechanisms of schizophrenia. However, it is still unknown whether in these animal models, impairments in plasticity of PFC neurons are present. In this article, we briefly summarize the current knowledge on the effect of non-competitive NMDA receptor antagonist MK-801 on medial PFC (mPFC) neuronal activity and then introduce a form of plasticity found after acute exposure to MK-801, which was accompanied by cognitive deficits. These observations suggest a potential correlation between cognitive deficits and the aberrant plasticity in the mPFC in the animal model of schizophrenia.

Treatments in context: transcranial direct current brain stimulation as a potential treatment in pediatric psychosis

Childhood-onset schizophrenia is a chronic, severe form of schizophrenia, and is typically treatment resistant. Even after optimized pharmacotherapy, a majority (over 70%) of these pediatric patients present lasting psychotic symptoms and impaired cognition, necessitating the need for novel treatment modalities. Recent work in transcranial magnetic stimulation suggests moderate efficacy in symptom reduction in adult patients with schizophrenia; however, the transcranial magnetic stimulation treatment is cumbersome for this severely ill population. Transcranial direct current stimulation may provide a safe and effective adjuvant treatment for continued residual symptoms of schizophrenia.

Impaired motor cortex responses in non-psychotic first-degree relatives of schizophrenia patients: a cathodal tDCS pilot study

BACKGROUND

Schizophrenia has recently been described as a disorder of impaired plasticity and dysconnectivity. Several lines of evidence suggest that alterations in glutamatergic neurotransmission underlie different symptom domains of schizophrenia. Little is known about the impact of genetic liability on cortical plasticity and connectivity in schizophrenia.

OBJECTIVE

To compare N-methyl-d-aspartate receptor (NMDAR)-dependent cortical plasticity and connectivity in schizophrenia patients and unaffected first-degree relatives to that in healthy subjects.

METHODS

Cortical plasticity can be induced in the motor cortex with cathodal transcranial direct current stimulation (tDCS). Animal and human research indicates that this long-term depression-like plasticity (LTD-like) is NMDAR dependent, and that these plasticity shifts can last for several hours. tDCS-induced plasticity was assessed by measuring motor-evoked potentials (MEPs) generated by applying transcranial magnetic stimulation (TMS) to both hemispheres in healthy controls, chronically ill schizophrenia patients and unaffected first-degree relatives.

RESULTS

Compared to healthy controls, both first-degree relatives and schizophrenia patients showed abolished motor-cortical LTD-like plasticity of the stimulated hemisphere. On the non-stimulated hemisphere, plasticity was again abolished in schizophrenia patients, whereas first-degree relatives had a reversed plasticity.

CONCLUSIONS

Non-psychotic and clinically unaffected first-degree relatives showed an alteration and a reversal of LTD-like cortical plasticity, indicating functional alterations of glutamatergic transmission as a result of a genetic liability for developing schizophrenia. These results provide new evidence for the association between plasticity dysregulation and functional cortical connectivity, and the importance of these networks in the pathophysiology of schizophrenia.

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

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

Cognitive and neuroplasticity mechanisms by which congenital or early blindness may confer a protective effect against schizophrenia

Several authors have noted that there are no reported cases of people with schizophrenia who were born blind or who developed blindness shortly after birth, suggesting that congenital or early (C/E) blindness may serve as a protective factor against schizophrenia. By what mechanisms might this effect operate? Here, we hypothesize that C/E blindness offers protection by strengthening cognitive functions whose impairment characterizes schizophrenia, and by constraining cognitive processes that exhibit excessive flexibility in schizophrenia. After briefly summarizing evidence that schizophrenia is fundamentally a cognitive disorder, we review areas of perceptual and cognitive function that are both impaired in the illness and augmented in C/E blindness, as compared to healthy sighted individuals. We next discuss: (1) the role of neuroplasticity in driving these cognitive changes in C/E blindness; (2) evidence that C/E blindness does not confer protective effects against other mental disorders; and (3) evidence that other forms of C/E sensory loss (e.g., deafness) do not reduce the risk of schizophrenia. We conclude by discussing implications of these data for designing cognitive training interventions to reduce schizophrenia-related cognitive impairment, and perhaps to reduce the likelihood of the development of the disorder itself.

Impairments of motor-cortex responses to unilateral and bilateral direct current stimulation in schizophrenia

Transcranial direct current stimulation (tDCS) is a non-invasive stimulation technique that can be applied to modulate cortical activity through induction of cortical plasticity. Since various neuropsychiatric disorders are characterized by fluctuations in cortical activity levels (e.g., schizophrenia), tDCS is increasingly investigated as a treatment tool. Several studies have shown that the induction of cortical plasticity following classical, unilateral tDCS is reduced or impaired in the stimulated and non-stimulated primary motor cortices (M1) of patients with schizophrenia. Moreover, an alternative, bilateral tDCS setup has recently been shown to modulate cortical plasticity in both hemispheres in healthy subjects, highlighting another potential treatment approach. Here we present the first study comparing the efficacy of unilateral tDCS (cathode left M1, anode right supraorbital) with simultaneous bilateral tDCS (cathode left M1, anode right M1) in patients with schizophrenia. tDCS-induced cortical plasticity was monitored by investigating motor-evoked potentials induced by single-pulse transcranial magnetic stimulation applied to both hemispheres. Healthy subjects showed a reduction of left M1 excitability following unilateral tDCS on the stimulated left hemisphere and an increase in right M1 excitability following bilateral tDCS. In schizophrenia, no plasticity was induced following both stimulation paradigms. The pattern of these results indicates a complex interplay between plasticity and connectivity that is impaired in patients with schizophrenia. Further studies are needed to clarify the biological underpinnings and clinical impact of these findings.

Deficient inhibitory cortical networks in antipsychotic-naive subjects at risk of developing first-episode psychosis and first-episode schizophrenia patients: a cross-sectional study

BACKGROUND

Impaired cortical inhibition is a well-established finding in schizophrenia patients and has been linked to dysfunctional gamma-aminobutyric acid (GABA)ergic transmission. However, there have been no previous studies investigating cortical excitability with particular regard to intracortical inhibitory networks in antipsychotic-naive subjects at risk of developing first-episode psychosis.

METHODS

A total of 18 subjects at risk, 18 first-episode schizophrenia patients, and 18 healthy control subjects were included in this study. Transcranial magnetic stimulation over the left primary motor cortex was used to determine short-latency intracortical inhibition, intracortical facilitation, and the contralateral silent period (CSP). Short-latency intracortical inhibition can be considered as a parameter of GABA type A (GABA(A))-mediated inhibition and it has been proposed that CSP can test GABA type B (GABA(B))-mediated inhibitory intracortical networks.

RESULTS

Subjects at risk and first-episode patients showed a reduced short-latency intracortical inhibition compared with healthy control subjects, suggesting reduced GABA(A)-mediated inhibition. First-episode patients had a prolonged CSP duration compared with the other two groups, implying a GABA(B) imbalance only in patients with full-blown psychosis. Analyses did not reveal group differences for intracortical facilitation.

CONCLUSIONS

These results indicate specific alterations in inhibitory cortical networks in subjects at risk and in first-episode patients. It appears that there is already a cortical inhibitory deficit in at-risk individuals. These results suggest a possible GABA(A) dysfunction early in the disease course, whereas alterations in GABA(B) functionality seem to occur later in the disease's progression. Future longitudinal studies will be needed to clarify this inhibitory deficit and its relation to the transition to psychosis.

The pharmacology of neuroplasticity induced by non-invasive brain stimulation: building models for the clinical use of CNS active drugs

The term neuroplasticity encompasses structural and functional modifications of neuronal connectivity. Abnormal neuroplasticity is involved in various neuropsychiatric diseases, such as dystonia, epilepsy, migraine, Alzheimer's disease, fronto-temporal degeneration, schizophrenia, and post cerebral stroke. Drugs affecting neuroplasticity are increasingly used as therapeutics in these conditions. Neuroplasticity was first discovered and explored in animal experimentation. However, non-invasive brain stimulation (NIBS) has enabled researchers recently to induce and study similar processes in the intact human brain. Plasticity induced by NIBS can be modulated by pharmacological interventions, targeting ion channels, or neurotransmitters. Importantly, abnormalities of plasticity as studied by NIBS are directly related to clinical symptoms in neuropsychiatric diseases. Therefore, a core theme of this review is the hypothesis that NIBS-induced plasticity can explore and potentially predict the therapeutic efficacy of CNS-acting drugs in neuropsychiatric diseases. We will (a) review the basics of neuroplasticity, as explored in animal experimentation, and relate these to our knowledge about neuroplasticity induced in humans by NIBS techniques. We will then (b) discuss pharmacological modulation of plasticity in animals and humans. Finally, we will (c) review abnormalities of plasticity in neuropsychiatric diseases, and discuss how the combination of NIBS with pharmacological intervention may improve our understanding of the pathophysiology of abnormal plasticity in these diseases and their purposeful pharmacological treatment.

Synaptic plasticity in neurodegenerative diseases evaluated and modulated by in vivo neurophysiological techniques

Several studies demonstrated in experimental models and in humans synaptic plasticity impairment in some neurodegenerative and neuropsychiatric diseases such as Parkinson's disease, Alzheimer's disease, Huntington's disease, and schizophrenia. Recently new neurophysiological tools, such as repetitive transcranial magnetic stimulation and transcranial direct current stimulation, have been introduced in experimental and clinical settings for studying physiology of the brain and modulating cortical activity. These techniques use noninvasive transcranial electrical or magnetic stimulation to modulate neurons activity in the human brain. Cortical stimulation might enhance or inhibit the activity of cortico-subcortical networks, depending on stimulus frequency and intensity, current polarity, and other stimulation parameters such as the configuration of the induced electric field and stimulation protocols. On this basis, in the last two decades, these techniques have rapidly become valuable tools to investigate physiology of the human brain and have been applied to treat drug-resistant neurological and psychiatric diseases. Here we describe these techniques and discuss the mechanisms that may explain these effects.

Abnormal bihemispheric responses in schizophrenia patients following cathodal transcranial direct stimulation

Post-mortem and in vivo studies provide evidence for a link between reduced plasticity and dysconnectivity in schizophrenia patients. It has been suggested that the association between plasticity and connectivity contributes to the pathophysiology and symptomatology of schizophrenia. However, little is known about the impact of glutamate-dependent long-term depression (LTD)-like cortical plasticity on inter-hemispheric connectivity in schizophrenia patients. The aim of the present study was to investigate LTD-like cortical plasticity following excitability-diminishing cathodal transcranial direct current stimulation (tDCS) of the left primary motor cortex (M1) and its effects on the non-stimulated right M1. Eighteen schizophrenia patients and 18 matched (age, gender, handedness, and smoking status) control subjects were investigated in this study. Corticospinal excitability changes following tDCS and intra-cortical inhibitory circuits were monitored with transcranial magnetic stimulation. On the stimulated hemisphere, cathodal tDCS increased resting motor thresholds (RMT) in both groups and decreased motor-evoked potential (MEP) sizes in healthy controls to a greater extent compared to schizophrenia patients. On the non-stimulated hemisphere, RMTs were increased and MEPs were decreased only in the healthy control group. Our results confirm previous findings of reduced LTD-like plasticity in schizophrenia patients and offer hypothetical and indirect in vivo evidence for an association between LTD-like cortical plasticity and inter-hemispheric connectivity in schizophrenia patients. Moreover, our findings highlight the impact of plasticity on connectivity. Dysfunctional N-methyl D-aspartate receptors or modulation of dopaminergic transmission can explain these findings. Nevertheless, the effects of antipsychotic medication still need to be considered.