Synaptic activation of GABA(B) receptors regulates neuronal network activity and entrainment

Suppression of gamma-oscillations in the dorsolateral prefrontal cortex following long interval cortical inhibition: a TMS-EEG study

Gamma (gamma)-oscillations (30-50 Hz) represent important electrophysiological measures, which are generated through the execution of higher order cognitive tasks (eg, working memory) in the dorsolateral prefrontal cortex (DLPFC). By contrast, cortical inhibition (CI) refers to a neurophysiological process in which GABAergic inhibitory interneurons selectively suppress the activation of other neurons in the cortex. Recently, abnormalities in both CI and gamma-oscillations have been associated with various neuropsychiatric disorders including schizophrenia. Animal research suggests that suppression of gamma-oscillations is, in part, mediated through GABAergic inhibitory neurotransmission. However, no such evidence has been demonstrated in human, largely because of technological limitations. Recently, we reported on novel methods permitting the recording of CI from the DLPFC through transcranial magnetic stimulation (TMS) combined with electroencephalography (EEG). The aim of this study was to examine the effects of GABAergic inhibitory neurotransmission on gamma-oscillations by combining TMS with EEG. Long interval cortical inhibition (LICI), a paired TMS paradigm, was used to index GABA(B) receptor mediated inhibitory neurotransmission in the motor cortex and DLPFC of healthy individuals. Gamma-oscillations were significantly inhibited by LICI (38.1+/-26.5%; p< or =0.013) in the DLPFC but not in the motor cortex. These results provide neurophysiological evidence to demonstrate gamma-oscillations are inhibited by LICI in the DLPFC but not in the motor cortex. Such specificity suggests that the modulation of gamma-oscillations may represent an important neurophysiological process that may, in part, be responsible for optimal DLPFC functioning in healthy human subjects.

N-methyl-d-aspartic acid receptor antagonist-induced frequency oscillations in mice recreate pattern of electrophysiological deficits in schizophrenia

INTRODUCTION

Electrophysiological responses to auditory stimuli have provided a useful means of elucidating mechanisms and evaluating treatments in psychiatric disorders. Deficits in gating during paired-click tasks and lack of mismatch negativity following deviant stimuli have been well characterized in patients with schizophrenia. Recently, analyses of basal, induced, and evoked frequency oscillations have gained support as additional measures of cognitive processing in patients and animal models. The purpose of this study is to examine frequency oscillations in mice across the theta (4-7.5 Hz) and gamma (31-61 Hz) bands in the context of N-methyl-d-aspartic acid receptor (NMDAR) hypofunction and dopaminergic hyperactivity, both of which are thought to serve as pharmacological models of schizophrenia.

EXPERIMENTAL PROCEDURES

Electroencephalograms (EEG) were recorded from mice in five treatment groups that consisted of haloperidol, risperidone, amphetamine, ketamine, or ketamine plus haloperidol during an auditory task. Basal, induced and evoked powers in both frequencies were calculated.

RESULTS

Ketamine increased basal power in the gamma band and decreased the evoked power in the theta band. The increase in basal gamma was not blocked by treatment with a conventional antipsychotic. No other treatment group was able to fully reproduce this pattern in the mice.

CONCLUSIONS

Ketamine-induced alterations in EEG power spectra are consistent with abnormalities in the theta and gamma frequency ranges reported in patients with schizophrenia. Our findings support the hypothesis that NMDAR hypofunction contributes to the deficits in schizophrenia and that the dopaminergic pathways alone may not account for these changes.

Evaluating the relationship between long interval cortical inhibition, working memory and gamma band activity in the dorsolateral prefrontal cortex

Recent reports have demonstrated that long interval cortical inhibition (LICI) can be indexed in the dorsolateral prefrontal cortex (DLPFC) in healthy controls. LICI is a neurophysiologic process indexed using transcranial magnetic stimulation and is closely associated with cortical GABA(B) receptor mediated inhibitory neurotransmission. Several previous studies have also reported that gamma band activity represents a neurophysiological process that is mediated, in part, through GABAergic inhibitory neurotransmission and may subserve several cognitive operations including working memory (WM) in the DLPFC. The intension of the current study, therefore, was to directly evaluate the relationship between these neurophysiological processes in healthy subjects. Eleven right-handed healthy subjects participated in this experiment in which gamma band activity was measured through simultaneous recording of electroencephalography (EEG) during the N-back task, a cognitive task designed to index WM. LICI was recorded through EEG from the left DLFPC, left motor cortex and through EMG of peripheral hand muscles in a separate session according to previously published methods. There was no evidence for a relationship in the DLPFC between LICI and gamma band activity elicited during the N-back task, though there was a significant relationship between LICI and performance on the 3-back condition, the N-back condition of greatest difficulty. In conclusion these data provide evidence to suggest that in the DLPFC, there is no direct relationship between GABA(B) receptor mediated inhibitory neurotransmission and gamma band activity. However, our data does suggest that LICI was related to 3-back performance providing evidence implicating DLPFC GABAergic inhibitory neurotransmission in WM performance.