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

Investigational applications of transcranial magnetic stimulation (TMS) in Mood Disorders: Studies from a tertiary care center in India

The investigational potential of TMS in psychiatry is largely underutilized. In the current article, we present the results of five studies with similar TMS protocols that looked at the investigative applications of TMS via measuring cortical reactivity as potential biomarkers in mood disorders. The first two studies, evaluate potential of TMS parameters and Motor neuron system (MNS) as state or trait markers of BD. Third and fourth studies evaluate these as endophenotypic markers of BD. The fifth study which is an RCT evaluating add-on yoga in UD, evaluates if markers of CI can index the therapeutic response of yoga. In study one MT1 was significantly greater in the SM (symptomatic-mania) group compared to HC (healthy-control) (P=0.032). The cortical inhibition measures SICI was reduced in SM(P=0.021) and BD (remitted Bipolar) (P=0.023) groups compared to HC. LICI was increased in the SM(0.021) and BD(P=0.06) groups compared to HC. In study two, a significant group x time interaction effect was observed indicating higher putative MNS-activity mediation in patients compared to HC on SlCl(P=0.024), LlCl(P=0.033). There were no significant group differences noted in the endophenotype studies. The fifth study showed a significant time X group interaction for CSP, favoring improvement in YG (yoga-group) (p<0.01).No significant change was observed for LICI(p=0.2), SICI(p=0.5). Limitations of these studies notwithstanding, we conclude that cortical reactivity measured using TMS is a potential biomarker across the course of mood disorders, starting from state and trait markers to understanding the therapeutic mechanism of a particular treatment modality in these disorders.

Identifying misdiagnosed bipolar disorder using support vector machine: feature selection based on fMRI of follow-up confirmed affective disorders

Nearly a quarter of bipolar disorder (BD) patients were misdiagnosed as major depressive disorder (MDD) patients, which cannot be corrected until mania/hypomania develops. It is important to recognize these obstacles so that the appropriate treatment can be initiated. Thus, we sought to distinguish patients with BD from MDD, especially to identify misdiagnosed BD before mania/hypomania, and further explore potential trait features that allow accurate differential diagnosis independent of state matters. Functional magnetic resonance imaging scans were performed at baseline on 92 MDD patients and 48 BD patients. The MDD patients were then followed up for more than two years. After follow-up, 23 patients transformed into BD (tBD), and 69 patients whose diagnoses remained unchanged were eligible for unipolar depression (UD). A support vector machine classifier was trained on the amygdala-based functional connectivity (FC) of 48 BD and 50 UD patients using a novel region-based feature selection. Then, the classifier was tested on the dataset, encompassing tBD and the remaining UD. It performed well for known BD and UD and can also distinguish tBD from UD with an accuracy of 81%, sensitivity of 82.6%, specificity of 79%, and AUC of 74.6%, respectively. Feature selection results revealed that ten regions within the cortico-limbic neural circuit contributed most to classification. Furthermore, in the FC comparisons among diseases, BD and tBD shared almost overlapped FC patterns in the cortico-limbic neural circuit, and both of them presented pronounced differences in most regions within the circuit compared with UD. The FC values of the most discriminating brain regions had no prominent correlations with the severity of depression, anxiety, and mania/hypomania (FDR correction). It suggests that BD possesses some trait features in the cortico-limbic neural circuit, rendering it dichotomized by the classifier based on known-diagnosis data.

Hemispheric asymmetry of motor cortex excitability in mood disorders - Evidence from a systematic review and meta-analysis

OBJECTIVE

Mood disorders have been associated with lateralized brain dysfunction, on the left-side for depression and right-side for mania. Consistently, asymmetry of cortical excitability, as measured by transcranial magnetic stimulation (TMS) has been reported. Here, we reviewed and summarized work assessing such measures bilaterally in mood disorders.

METHODS

We performed a systematic review and extracted data to perform meta-analyses of interhemispheric asymmetry of motor cortex excitability, assessed with TMS, across different mood disorders and in healthy subjects. Additionally, potential predictors of interhemispheric asymmetry were explored.

RESULTS

Asymmetry of resting motor threshold (MT) among healthy volunteers was significant, favoring lower right relative to left-hemisphere excitability. MT was also significantly asymmetric in major depressive disorder (MDD), but with lower excitability of the left -hemisphere, when compared to the right, no longer observed in recovered patients. Findings on intracortical facilitation were similar. The few trials including bipolar depression revealed similar trends for imbalance, but with lower right hemisphere excitability, relative to the left.

CONCLUSIONS

There is interhemispheric asymmetry of motor cortical excitability in MDD, with lower excitability on left when compared to right-side. Interhemispheric asymmetry, with lower right relative to left-sided excitability, was found for bipolar depression and was also suggested for healthy volunteers, in a pattern that is clearly distinct from MDD.

SIGNIFICANCE

Mood disorders display asymmetric motor cortical excitability that is distinct from that found in healthy volunteers, supporting the presence of lateralized brain dysfunction in these disorders.

A transdiagnostic evaluation of cortical inhibition in severe mental disorders using Transcranial Magnetic Stimulation

Multiple lines of investigations suggest the presence of cortical inhibition aberrations as central to the phenotypic manifestations of severe mental disorders. Transcranial Magnetic Stimulation (TMS) combined with electromyography can characterize these inhibitory processes in the motor cortex with satisfactory temporal precision. We examined TMS-evoked short- (SICI) and long-interval intracortical inhibition (LICI) and cortical silent period (CSP) as markers of GABA_A- (SICI) and GABA_B-mediated (LICI and CSP) cortical neurotransmission in symptomatic individuals with mania (n = 40), schizophrenia (n = 76), unipolar depression (n = 86), and OCD (n = 43), and compared them against similar recordings in healthy subjects (n = 125). We hypothesized transdiagnostic GABA_A deficits across all the clinical groups and diagnosis-specific GABA_B alterations in mania (increased) and OCD (decreased). After controlling for potential confounder variables (gender, education, benzodiazepine prescription, and motor threshold) using the ANCOVA, we observed no significant group difference in SICI (F = 1.04, P = 0.38), but a significant group effect in LICI (F = 16.29, P < 0.001) and CSP (F = 3.02, P = 0.018). Post-hoc analyses revealed that LICI was significantly reduced in OCD but increased in mania and schizophrenia with reference to the healthy group. Similarly, CSP was significantly reduced in OCD and depression groups as compared to the reference group. We observed that LICI and CSP, both followed similar descending gradients from mania through schizophrenia and depression to OCD; with significant elevation in mania, and reduction in depression and OCD, as compared to the healthy group. This pattern of GABA_B-mediated cortical inhibition aberrations needs independent validation as potential state-markers of distinct clinical categories.

Chronic lithium treatment alters the excitatory/ inhibitory balance of synaptic networks and reduces mGluR5-PKC signalling in mouse cortical neurons

Background

Bipolar disorder is characterized by cyclical alternation between mania and depression, often comorbid with psychosis and suicide. Compared with other medications, the mood stabilizer lithium is the most effective treatment for the prevention of manic and depressive episodes. However, the pathophysiology of bipolar disorder and lithium’s mode of action are yet to be fully understood. Evidence suggests a change in the balance of excitatory and inhibitory activity, favouring excitation in bipolar disorder. In the present study, we sought to establish a holistic understanding of the neuronal consequences of lithium exposure in mouse cortical neurons, and to identify underlying mechanisms of action.

Methods

We used a range of technical approaches to determine the effects of acute and chronic lithium treatment on mature mouse cortical neurons. We combined RNA screening and biochemical and electrophysiological approaches with confocal immunofluorescence and live-cell calcium imaging.

Results

We found that only chronic lithium treatment significantly reduced intracellular calcium flux, specifically by activating metabotropic glutamatergic receptor 5. This was associated with altered phosphorylation of protein kinase C and glycogen synthase kinase 3, reduced neuronal excitability and several alterations to synapse function. Consequently, lithium treatment shifts the excitatory–inhibitory balance toward inhibition.

Limitations

The mechanisms we identified should be validated in future by similar experiments in whole animals and human neurons.

Conclusion

Together, the results revealed how lithium dampens neuronal excitability and the activity of the glutamatergic network, both of which are predicted to be overactive in the manic phase of bipolar disorder. Our working model of lithium action enables the development of targeted strategies to restore the balance of overactive networks, mimicking the therapeutic benefits of lithium but with reduced toxicity.

A Systematic Review of Long-Interval Intracortical Inhibition as a Biomarker in Neuropsychiatric Disorders

Long-interval intracortical inhibition (LICI) is a paired-pulse transcranial magnetic stimulation (TMS) paradigm mediated in part by gamma-aminobutyric acid receptor B (GABA_B) inhibition. Prior work has examined LICI as a putative biomarker in an array of neuropsychiatric disorders. This review conducted in accordance with the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) sought to examine existing literature focused on LICI as a biomarker in neuropsychiatric disorders. There were 113 articles that met the inclusion criteria. Existing literature suggests that LICI may have utility as a biomarker of GABA_B functioning but more research with increased methodologic rigor is needed. The extant LICI literature has heterogenous methodology and inconsistencies in findings. Existing findings to date are also non-specific to disease. Future research should carefully consider existing methodological weaknesses and implement high-quality test-retest reliability studies.

Cortical inhibition in symptomatic and remitted mania compared to healthy subjects: A cross-sectional study

OBJECTIVES

Transcranial magnetic stimulation (TMS)-derived cortical reactivity studies provide a unique opportunity to non-invasively study gamma amino butyric acid (GABA)-mediated inhibitory neurotransmission in bipolar disorder (BD). Earlier studies were conducted in smaller samples and on patients who were on medications that can potentially confound the results. We aimed to study short-interval (SICI) and long-interval intracortical inhibition (LICI) in medication-naïve/free symptomatic (manic) BD patients (n=39), first episode mania (FEM) patients who had recently (≤6 months) remitted with treatment (remitted FEM; n = 28) and healthy subjects (HSs; n = 45).

METHODS

Resting motor threshold (RMT), stimulation intensity to elicit a 1-mV motor evoked potential (MEP) (SI_1 mV ), SICI and LICI were measured in three groups using single- and paired-pulse TMS.

RESULTS

Motor thresholds were higher in the manic BD and HS groups compared to the remitted FEM group (P < .001). SICI was lower (P = .026) but LICI was higher (P = .044) in the manic BD and remitted FEM groups compared to the HS group.

CONCLUSIONS

Lower motor thresholds in remitted FEM perhaps reflect the effect of treatment, and could be studied as potential prognostic neuromarkers. Inverse findings for SICI (reduced) and LICI (increased) in BD indicate a possible differential involvement of the GABA_A and GABA_B subreceptor systems. These could be trait markers as they are impaired in both mania and euthymia.