A systematic review of the effects of NMDA receptor antagonists on oscillatory activity recorded in vivo

Effects of Ketamine on Resting-State EEG Activity and Their Relationship to Perceptual/Dissociative Symptoms in Healthy Humans

N-methyl-D-aspartate (NMDA) receptor antagonists administered to healthy humans results in schizophrenia-like symptoms, which preclinical research suggests are due to glutamatergically altered brain oscillations. Here, we examined resting-state electroencephalographic activity in 21 healthy volunteers assessed in a placebo-controlled, double-blind, randomized study involving administration of either a saline infusion or a sub-anesthetic dose of ketamine, an NMDA receptor antagonist. Frequency-specific current source density (CSD) was assessed at sensor-level and source-level using eLORETA within regions of interest of a triple network model of schizophrenia (this model posits a dysfunctional switching between large-scale Default Mode and Central Executive networks by the monitor-controlling Salience Network). These CSDs were measured in each session along with subjective symptoms as indexed with the Clinician Administered Dissociative States Scale. Ketamine-induced CSD reductions in slow (delta/theta and alpha) and increases in fast (gamma) frequencies at scalp electrode sites were paralleled by frequency-specific CSD changes in the Default Mode, Central Executive, and Salience networks. Subjective symptoms scores were increased with ketamine and ratings of depersonalization in particular were associated with alpha CSD reductions in general and in specific regions of interest in each of the three networks. These results tentatively support the hypothesis that pathological brain oscillations associated with hypofunctional NMDA receptor activity may contribute to the emergence of the perceptual/dissociate symptoms of schizophrenia.

GluN2D N-Methyl-d-Aspartate Receptor Subunit Contribution to the Stimulation of Brain Activity and Gamma Oscillations by Ketamine: Implications for Schizophrenia

The dissociative anesthetic ketamine elicits symptoms of schizophrenia at subanesthetic doses by blocking N-methyl-d-aspartate receptors (NMDARs). This property led to a variety of studies resulting in the now well-supported theory that hypofunction of NMDARs is responsible for many of the symptoms of schizophrenia. However, the roles played by specific NMDAR subunits in different symptom components are unknown. To evaluate the potential contribution of GluN2D NMDAR subunits to antagonist-induced cortical activation and schizophrenia symptoms, we determined the ability of ketamine to alter regional brain activity and gamma frequency band neuronal oscillations in wild-type (WT) and GluN2D-knockout (GluN2D-KO) mice. In WT mice, ketamine (30 mg/kg, i.p.) significantly increased [(14)C]-2-deoxyglucose ([(14)C]-2DG) uptake in the medial prefrontal cortex (mPFC), entorhinal cortex and other brain regions, and decreased activity in the somatosensory cortex and inferior colliculus. In GluN2D-KO mice, however, ketamine did not significantly increase [(14)C]-2DG uptake in any brain region examined, yet still decreased [(14)C]-2DG uptake in the somatosensory cortex and inferior colliculus. Ketamine also increased locomotor activity in WT mice but not in GluN2D-KO mice. In electrocorticographic analysis, ketamine induced a 111% ± 16% increase in cortical gamma-band oscillatory power in WT mice, but only a 15% ± 12% increase in GluN2D-KO mice. Consistent with GluN2D involvement in schizophrenia-related neurologic changes, GluN2D-KO mice displayed impaired spatial memory acquisition and reduced parvalbumin (PV)-immunopositive staining compared with control mice. These results suggest a critical role of GluN2D-containing NMDARs in neuronal oscillations and ketamine's psychotomimetic, dissociative effects and hence suggests a critical role for GluN2D subunits in cognition and perception.

MK-801 and amphetamine result in dissociable profiles of cognitive impairment in a rodent paired associates learning task with relevance for schizophrenia

RATIONALE

Paired associates learning (PAL) has been suggested to be predictive of functional outcomes in first episode psychosis and of conversion from mild cognitive impairment to Alzheimer's disease. An automated touch screen-based rodent PAL (rPAL) task has been developed and is sensitive to manipulations of the dopaminergic and glutamatergic system. Accordingly, rPAL when used with pharmacological models of schizophrenia, like NMDA receptor blockade with MK-801 or dopaminergic stimulation with amphetamine, may have utility as a translational model of cognitive impairment in schizophrenia.

OBJECTIVE

The purpose of this study was to determine if amphetamine- and MK-801-induced impairment represent distinct models of cognitive impairment by testing their sensitivity to common antipsychotics and determine the relative contributions of D1 versus D2 receptors on performance of PAL.

METHOD

Rats were trained in rPAL and were then treated with MK-801, amphetamine, risperidone, haloperidol, quinpirole, SK-82958, or SCH-23390 alone and in combination.

RESULTS

While both amphetamine and MK-801 caused clear impairments in accuracy, MK-801 induced a profound "perseverative" type behavior that was more pronounced when compared to amphetamine. Moreover, amphetamine-induced impairments, but not the effects of MK-801, could be reversed by antipsychotics as well as the D1 receptor antagonist SCH-23390, suggesting a role for both the D1 and D2 receptor in the amphetamine impairment model.

CONCLUSIONS

These data suggest that amphetamine and MK-801 represent dissociable models of impairment in PAL, dependent on different underlying neurobiology. The ability to distinguish dopaminergic versus glutamatergic effects on performance in rPAL makes it a unique and useful tool in the modeling of cognitive impairments in schizophrenia.

Ketamine Dysregulates the Amplitude and Connectivity of High-Frequency Oscillations in Cortical-Subcortical Networks in Humans: Evidence From Resting-State Magnetoencephalography-Recordings

Hypofunctioning of the N-methyl-D-aspartate receptor (NMDA-R) has been prominently implicated in the pathophysiology of schizophrenia (ScZ). The current study tested the effects of ketamine, a dissociative anesthetic and NMDA-R antagonist, on resting-state activity recorded with magnetoencephalography (MEG) in healthy volunteers. In a single-blind cross-over design, each participant (n = 12) received, on 2 different sessions, a subanesthetic dose of S-ketamine (0.006 mg/Kg) and saline injection. MEG-data were analyzed at sensor- and source-level in the beta (13-30 Hz) and gamma (30-90 Hz) frequency ranges. In addition, connectivity analysis at source-level was performed using transfer entropy (TE). Ketamine increased gamma-power while beta-band activity was decreased. Specifically, elevated 30-90 Hz activity was pronounced in subcortical (thalamus and hippocampus) and cortical (frontal and temporal cortex) regions, whilst reductions in beta-band power were localized to the precuneus, cerebellum, anterior cingulate, temporal and visual cortex. TE analysis demonstrated increased information transfer in a thalamo-cortical network after ketamine administration. The findings are consistent with the pronounced dysregulation of high-frequency oscillations following the inhibition of NMDA-R in animal models of ScZ as well as with evidence from electroencephalogram-data in ScZ-patients and increased functional connectivity during early illness stages. Moreover, our data highlight the potential contribution of thalamo-cortical connectivity patterns towards ketamine-induced neuronal dysregulation, which may be relevant for the understanding of ScZ as a disorder of disinhibition of neural circuits.

Coupling in the cortico-basal ganglia circuit is aberrant in the ketamine model of schizophrenia

Recent studies have suggested the implication of the basal ganglia in the pathogenesis of schizophrenia. To investigate this hypothesis, here we have used the ketamine model of schizophrenia to determine the oscillatory abnormalities induced in the rat motor circuit of the basal ganglia. The activity of free moving rats was recorded in different structures of the cortico-basal ganglia circuit before and after an injection of a subanesthesic dose of ketamine (10mg/kg). Spectral estimates of the oscillatory activity, phase-amplitude cross-frequency coupling interactions (CFC) and imaginary event-related coherence together with animals׳ behavior were analyzed. Oscillatory patterns in the cortico-basal ganglia circuit were highly altered by the effect of ketamine. CFC between the phases of low-frequency activities (delta, 1-4; theta 4-8Hz) and the amplitude of high-gamma (~80Hz) and high-frequency oscillations (HFO) (~150Hz) increased dramatically and correlated with the movement increment shown by the animals. Between-structure analyses revealed that ketamine had also a massive effect in the low-frequency mediated synchronization of the HFO's across the whole circuit. Our findings suggest that ketamine administration results in an aberrant hypersynchronization of the whole cortico-basal circuit where the tandem theta/HFO seems to act as the main actor in the hyperlocomotion shown by the animals. Here we stress the importance of the basal ganglia circuitry in the ketamine model of schizophrenia and leave the door open to further investigations devoted to elucidate to what extent these abnormalities also reflect the prominent neurophysiological deficits observed in schizophrenic patients.

Brain rhythms connect impaired inhibition to altered cognition in schizophrenia

In recent years, schizophrenia research has focused on inhibitory interneuron dysfunction at the level of neurobiology and on cognitive impairments at the psychological level. Reviewing both experimental and computational findings, we show how the temporal structure of the activity of neuronal populations, exemplified by brain rhythms, can begin to bridge these levels of complexity. Oscillations in neuronal activity tie the pathophysiology of schizophrenia to alterations in local processing and large-scale coordination, and these alterations in turn can lead to the cognitive and perceptual disturbances observed in schizophrenia.

An animal model of female adolescent cannabinoid exposure elicits a long-lasting deficit in presynaptic long-term plasticity

Cannabis continues to be the most accessible and popular illicit recreational drug. Whereas current data link adolescence cannabinoid exposure to increased risk for dependence on other drugs, depression, anxiety disorders and psychosis, the mechanism(s) underlying these adverse effects remains controversial. Here we show in a mouse model of female adolescent cannabinoid exposure deficient endocannabinoid (eCB)-mediated signaling and presynaptic forms of long-term depression at adult central glutamatergic synapses in the prefrontal cortex. Increasing endocannabinoid levels by blockade of monoacylglycerol lipase, the primary enzyme responsible for degrading the endocannabinoid 2-arachidonoylglycerol (2-AG), with the specific inhibitor JZL 184 ameliorates eCB-LTD deficits. The observed deficit in cortical presynaptic signaling may represent a neural maladaptation underlying network instability and abnormal cognitive functioning. Our study suggests that adolescent cannabinoid exposure may permanently impair brain functions, including the brain's intrinsic ability to appropriately adapt to external influences.

High-frequency oscillations are prominent in the extended amygdala

Previously, it was reported that various cortical and subcortical structures display high-frequency local field potential (LFP) oscillations in the 110- to 160-Hz range (HFOs), distinct from sharp-wave ripples. In the present study, we characterize HFOs in the extended amygdala. Rats were implanted with tetrode bundles in the bed nucleus of the stria terminalis (BNST), central amygdala (CeA), as well as adjacent regions (pallidum, caudate-putamen, and lateral septum). At all recorded sites, HFO power showed a systematic dependence on behavioral state: highest during quiet wakefulness, intermediate during paradoxical sleep, and lowest during active waking or slow-wave sleep. CO2 asphyxiation as well as anesthesia with isoflurane or urethane abolished HFOs. HFOs stood out relative to all other fast-frequency LFP components because they were highly coherent between distant sites of the extended amygdala, ipsi- and contralaterally. HFOs affected neuronal firing in two ways: firing rate could vary as a function of HFO power (rate modulation) or HFOs could entrain firing on a cycle-to-cycle basis (phase modulation). The incidence of phase-modulated neurons was about twice higher in BNST and CeA (20-40%) than in adjacent regions (≤8%). Among BNST and CeA neurons, many more were phase-modulated than rate-modulated, although about half of the latter were also phase-modulated. Overall, these results indicate that HFOs entrain the activity of a high proportion of neurons in the extended amygdala. A major challenge for future studies will be to identify the mechanisms supporting the high coherence of HFOs within and across hemispheres.