Cortical inhibitory dysfunction in bipolar disorder: a study using transcranial magnetic stimulation

Evidence for gamma inhibition deficits in the dorsolateral prefrontal cortex of patients with schizophrenia

Previous studies have shown that patients with schizophrenia and bipolar disorder have deficits in cortical inhibition. Through the combination of interleaved transcranial magnetic stimulation and electroencephalography, we have recently reported on methods in which cortical inhibition can be measured from the dorsolateral prefrontal cortex, a cortical region that is more closely associated with the pathophysiology of schizophrenia. Furthermore, it is possible to index cortical inhibition of specific oscillatory frequencies including the gamma band (30-50 Hz) whose modulation has been related to higher order cortical processing. In this study, we show that patients with schizophrenia have significant deficits of cortical inhibition of gamma oscillations in the dorsolateral prefrontal cortex compared to healthy subjects and patients with bipolar disorder, while no deficits are demonstrated in the motor cortex. These results suggest that the lack of inhibition of gamma oscillations in the dorsolateral prefrontal cortex may represent an important frontal neurophysiological deficit, which may be responsible for the spectrum of deficits commonly found in schizophrenia.

Evidence of cortical inhibitory deficits in major depressive disorder

BACKGROUND

Several lines of evidence suggest that major depressive disorder is associated with deficits in gamma-aminobutyric acid (GABA) inhibitory neurotransmission. Transcranial magnetic stimulation represents a noninvasive technique to measure cortical inhibition. In this study, we endeavored to measure cortical inhibition in medicated patients with treatment resistant major depressive disorder (TRD), unmedicated patients with major depressive disorder, and medicated euthymic patients with a history of major depressive disorder and compare them with healthy subjects.

METHODS

Twenty-five patients with TRD, 16 unmedicated patients with major depressive disorder, 19 medicated euthymic patients with previous major depressive disorder (i.e., 17-item Hamilton Rating Scale for Depression < 8), and 25 healthy subjects were enrolled. Cortical inhibition was measured with transcranial magnetic stimulation paradigms known as short-interval cortical inhibition and the cortical silent period, which index GABA(A) and GABA(B) receptor-mediated inhibitory neurotransmission, respectively.

RESULTS

All major depressive disorder patient groups demonstrated significant cortical silent period deficits compared with healthy subjects. By contrast, only TRD patients demonstrated significant deficits in short-interval cortical inhibition compared with healthy subjects, medicated euthymic major depressive disorder patients, and unmedicated major depressive disorder patients. The TRD patients also demonstrated a significantly greater resting motor threshold compared with all other clinical subgroups and healthy subjects, suggesting that TRD was also associated with hypoexcitability of the frontal cortex.

CONCLUSIONS

Our findings suggest that GABA(B) neurophysiological deficits are closely related to pathophysiology of major depressive disorder. Our findings also suggest that more severe illness is selectively associated with GABA(A) receptor-mediated inhibitory deficits.

Molecular neurobiology of bipolar disorder: a disease of 'mood-stabilizing neurons'?

Although the role of a genetic factor is established in bipolar disorder, causative genes or robust genetic risk factors have not been identified. Increased incidence of subcortical hyperintensity, altered calcium levels in cells derived from patients and neuroprotective effects of mood stabilizers suggest vulnerability or impaired resilience of neurons in bipolar disorder. Mitochondrial dysfunction or impaired endoplasmic reticulum stress response is suggested to play a role in the neurons' vulnerability. Progressive loss or dysfunction of 'mood-stabilizing neurons' might account for the characteristic course of the illness. The important next step in the neurobiological study of bipolar disorder is identification of the neural systems that are responsible for this disorder.

The role of the cerebellum in the pathophysiology and treatment of neuropsychiatric disorders: a review

The cerebellum has traditionally been looked upon as a brain area primarily involved in motor behaviour. The last decade has however heralded the cerebellum as a brain region of renewed interest for neuropsychiatric disorders. This renewed interest is fuelled by new insights obtained from neuroanatomical research, modern functional neuroimaging and transcranial magnetic stimulation studies. In this review, evidence in support of cerebellar involvement in neuropsychiatric disorders will be presented. In addition, transcranial magnetic stimulation will be introduced as a novel way to study cerebellar contributions to the pathophysiology of psychiatric disorders. In conclusion, a new functional concept of the cerebellum as more than simply a brain area regulating motor control appears mandatory and the involvement of the cerebellum should be considered when studying the neurological basis of neuropsychiatric disorders.

Brain oscillatory responses in patients with bipolar disorder manic episode before and after valproate treatment

BACKGROUND

GABA/Glutamatergic dysfunction and neural circuits which regulate cognitive processing are involved in the underlying pathology of bipolar disorder. Event related oscillatory neuroelectrical activity reflects integrative brain functioning, different frequency bands representing different cognitive functions.

METHODS

Event Related Potentials to visual odd-ball paradigm in ten manic/hypomanic medication free, DSM-IV bipolar patients were measured before and after six weeks of valproate monotherapy in comparison to ten sex and age matched healthy controls. Different frequency band responses were obtained by digital filtration of ERPs. Young mania rating scale (YMRS) was used to assess clinical response. Repeated measures ANOVA, Wilcoxon and Mann Whitney U tests were used for statistical analysis.

RESULTS

Patients showed significantly higher baseline occipital beta (18-30 Hz) (p: 0.014) response than healthy controls. They were devoid of the occipito-frontal alpha (8-13 Hz) dominance presented by the control group. Occipital beta response reduced significantly (p: 0.009) and became similar to controls after treatment. Post-treatment alpha responses were significantly lower than baseline in anterior temporal (p: 0.038) and occipital (p: 0.027) locations. Healthy controls displayed a significantly increased frontal alpha response at the second assessment but the patients did not. Mean YMRS score reduced significantly compared to baseline at the end of six weeks (p: 0.004).

CONCLUSIONS

Alpha response is the universal operator in the brain. Increased occipital beta response in mania may be compensatory to the dysfunctional alpha operation. Its reduction after valproate may be through modulation of glutamatergic and GABAergic mechanisms and indicate medication's corrective effect on the underlying pathogenesis.

Quantification of the corticospinal silent period evoked via transcranial magnetic stimulation

A magnetic pulse to the cortex during a muscle contraction produces a motor evoked potential (MEP) followed by electrical quiescence before activity resumes that is indicative of corticospinal inhibition and commonly referred to as the corticospinal slient period (SP). The purpose of the present study was to determine the effect of stimulus intensity and quantification method on the between-visit variability of the SP in healthy individuals. On two occasions we recorded the SP from 9 humans at 3 stimulus intensities (10, 20 and 30% above active motor threshold [AMT]) and quantified the SP based on 8 common criteria. We evaluated the effect of stimulus intensity on reliability by using the limits of agreement, and this analysis revealed that the lower stimulus intensities (10 and 20% AMT) exhibited heteroscedasticity, which indicates the amount of random error increases as the silent period increases. The 30% AMT intensity was homoscedastic. We used both visual and mathematical approaches to quantify the SP, and observed that the between-visit coefficient of variation (CV) was less for the visual methods, and that the CV was reduced when the SP onset was earliest in the temporal occurrence of events (i.e. MEP onset to EMG return CV=12%). Inter-rater reliability for the visual analyses were high (r=0.91-0.99). These results suggest that SPs evoked with a stimulus intensity >or=30% AMT and quantified visually by defining the start of the SP at stimulus delivery or the start of the MEP be utilized to decrease the between visit variability.