N-methyl d-aspartate receptor antagonists ketamine and MK-801 induce wake-related aberrant gamma oscillations in the rat neocortex

Traumatic brain injury detection using electrophysiological methods

Measuring neuronal activity with electrophysiological methods may be useful in detecting neurological dysfunctions, such as mild traumatic brain injury (mTBI). This approach may be particularly valuable for rapid detection in at-risk populations including military service members and athletes. Electrophysiological methods, such as quantitative electroencephalography (qEEG) and recording event-related potentials (ERPs) may be promising; however, the field is nascent and significant controversy exists on the efficacy and accuracy of the approaches as diagnostic tools. For example, the specific measures derived from an electroencephalogram (EEG) that are most suitable as markers of dysfunction have not been clearly established. A study was conducted to summarize and evaluate the statistical rigor of evidence on the overall utility of qEEG as an mTBI detection tool. The analysis evaluated qEEG measures/parameters that may be most suitable as fieldable diagnostic tools, identified other types of EEG measures and analysis methods of promise, recommended specific measures and analysis methods for further development as mTBI detection tools, identified research gaps in the field, and recommended future research and development thrust areas. The qEEG study group formed the following conclusions: (1) Individual qEEG measures provide limited diagnostic utility for mTBI. However, many measures can be important features of qEEG discriminant functions, which do show significant promise as mTBI detection tools. (2) ERPs offer utility in mTBI detection. In fact, evidence indicates that ERPs can identify abnormalities in cases where EEGs alone are non-disclosing. (3) The standard mathematical procedures used in the characterization of mTBI EEGs should be expanded to incorporate newer methods of analysis including non-linear dynamical analysis, complexity measures, analysis of causal interactions, graph theory, and information dynamics. (4) Reports of high specificity in qEEG evaluations of TBI must be interpreted with care. High specificities have been reported in carefully constructed clinical studies in which healthy controls were compared against a carefully selected TBI population. The published literature indicates, however, that similar abnormalities in qEEG measures are observed in other neuropsychiatric disorders. While it may be possible to distinguish a clinical patient from a healthy control participant with this technology, these measures are unlikely to discriminate between, for example, major depressive disorder, bipolar disorder, or TBI. The specificities observed in these clinical studies may well be lost in real world clinical practice. (5) The absence of specificity does not preclude clinical utility. The possibility of use as a longitudinal measure of treatment response remains. However, efficacy as a longitudinal clinical measure does require acceptable test-retest reliability. To date, very few test-retest reliability studies have been published with qEEG data obtained from TBI patients or from healthy controls. This is a particular concern because high variability is a known characteristic of the injured central nervous system.

Reverse translation of clinical electrophysiological biomarkers in behaving rodents under acute and chronic NMDA receptor antagonism

Electroencephalogram (EEG) stands out as a highly translational tool for psychiatric research, yet rodent and human EEG are not typically obtained in the same way. In this study we developed a tool to record skull EEG in awake-behaving rats in a similar manner to how human EEG are obtained and then used this technique to test whether acute NMDA receptor antagonism alters rodent EEG signals in a similar manner as in humans. Acute MK-801 treatment elevated gamma power and reduced beta band power, which closely mirrored EEG data from healthy volunteers receiving acute ketamine. To explore the mechanisms behind these oscillatory changes, we examined the effects of GABA-A receptor blockade, finding that picrotoxin (PTX) recapitulated the decrease in sound-evoked beta oscillations observed with acute MK-801, but did not produce changes in gamma band power. Chronic treatment with either PTX or MK-801 did not affect frequency-specific oscillatory activity when tested 24 h after the last drug injection, but decreased total broadband oscillatory power. Overall, this study validated a novel platform for recording rodent EEG and demonstrated similar oscillatory changes after acute NMDA receptor antagonism in both humans and rodents, suggesting that skull EEG may be a powerful tool for further translational studies.

Neural oscillations as a translational tool in schizophrenia research: rationale, paradigms and challenges

Neural oscillations have received recently a great deal of interest in schizophrenia research because of the possibility to integrate findings from non-invasive electro/magnetoencephalographical recordings with pre-clinical research, which could potentially lead to the identification of pathophysiological mechanisms and novel treatment targets. In the current paper, we review the potential as well as the challenges of this approach by summarizing findings on alterations in rhythmic activity from both animal models and human data which have implicated dysfunctional neural oscillations in the explanation of cognitive deficits and certain clinical symptoms of schizophrenia. Specifically, we will focus on findings that have examined neural oscillations during 1) perceptual processing, 2) working memory and executive processes and 3) spontaneous activity. The importance of the development of paradigms suitable for human and animal models is discussed as well as the search for mechanistic explanation for oscillatory dysfunctions.

Reduced susceptibility to induced seizures in the Neuroligin-3(R451C) mouse model of autism

Epilepsy is a common comorbidity in patients with autism spectrum disorder (ASD) and several gene mutations are associated with both of these disorders. In order to determine whether a point mutation in the gene for the synaptic protein, Neuroligin-3 (Nlgn3, R451C), identified in patients with ASD alters seizure susceptibility, we administered the proconvulsant pentylenetetrazole (PTZ) to adult male Neuroligin-3(R451C) (NL3(R451C)) and wild type (WT) mice. It has previously been reported that NL3(R451C) mice show altered inhibitory GABAergic activity in brain regions relevant to epilepsy, including the hippocampus and somatosensory cortex. PTZ administration induces absence-seizures at low dose, and generalised convulsive seizures at higher dose. Susceptibility to absence seizures was examined by analysing the frequency and duration of spike-and-wave discharge (SWD) events and accompanying motor seizure activity induced by subcutaneous administration of low dosage (20 or 30mg/kg) PTZ. Susceptibility to generalised convulsive seizures was tested by measuring the response to high dosage (60mg/kg) PTZ using a modified Racine scale. There was no change in the number of SWD events exhibited by NL3(R451C) compared to WT mice following administration of both 20mg/kg PTZ (1.17±0.31 compared to 16.0±11.16 events/30min, NL3(R451C) versus WT, respectively) and 30mg/kg PTZ (7.5±6.54 compared with 27.8±19.9 events/30min, NL3(R451C) versus WT, respectively). NL3(R451C) mice were seizure resistant to generalised convulsive seizures induced by high dose PTZ compared to WT littermates (median latency to first >3s duration clonic seizure; 14.5min versus 7.25min, 95% CI: 1.625-2.375, p=0.0009, NL3(R451C) versus WT, respectively). These results indicate that the R451C mutation in the Nlgn3 gene, associated with ASD in humans, confers resistance to induced seizures, suggesting dysfunction of PTZ-sensitive GABAergic signalling in this mouse model of ASD.

Effects of NMDA receptor antagonists and antipsychotics on high frequency oscillations recorded in the nucleus accumbens of freely moving mice

RATIONALE

Abnormal oscillatory activity associated with N-methyl-D-aspartate (NMDA) receptor hypofunction is widely considered to contribute to the symptoms of schizophrenia.

OBJECTIVE

This study aims to characterise the changes produced by NMDA receptor antagonists and antipsychotics on accumbal high-frequency oscillations (HFO; 130-180 Hz) in mice.

METHODS

Local field potentials were recorded from the nucleus accumbens of freely moving mice.

RESULTS

Systemic injection of ketamine and MK801 both dose-dependently increased the power of HFO and produced small increases in HFO frequency. The atypical antipsychotic drug, clozapine, produced a robust dose-dependent reduction in the frequency of MK801-enhanced HFO, whilst haloperidol, a typical antipsychotic drug, had little effect. Stimulation of NMDA receptors (directly or through the glycine site) as well as activation of 5-HT1A receptors, reduced the frequency of MK801-enhanced HFO, but other receptors known to be targets for clozapine, namely 5-HT2A, 5-HT7 and histamine H3 receptors had no effect.

CONCLUSIONS

NMDA receptor antagonists and antipsychotics produce broadly similar fundamental effects on HFO, as reported previously for rats, but we did observe several notable differences. In mice, HFO at baseline were weak or not detectable unlike rats. Post-injection of NMDA receptor antagonists HFO was also weaker but significantly faster. Additionally, we found that atypical antipsychotic drugs may reduce the frequency of HFO by interacting with NMDA and/or 5-HT1A receptors.

Sensory encoding in Neuregulin 1 mutants

Schizophrenic patients show altered sensory perception as well as changes in electrical and magnetic brain responses to sustained, frequency-modulated sensory stimulation. Both the amplitude and temporal precision of the neural responses differ in patients as compared to control subjects, and these changes are most pronounced for stimulation at gamma frequencies (20-40 Hz). In addition, patients display enhanced spontaneous gamma oscillations, which has been interpreted as 'neural noise' that may interfere with normal stimulus processing. To investigate electrophysiological markers of aberrant sensory processing in a model of schizophrenia, we recorded neuronal activity in primary somatosensory cortex of mice heterozygous for the schizophrenia susceptibility gene Neuregulin 1. Sensory responses to sustained 20-70 Hz whisker stimulation were analyzed with respect to firing rates, spike precision (phase locking) and gamma oscillations, and compared to baseline conditions. The mutants displayed elevated spontaneous firing rates, a reduced gain in sensory-evoked spiking and gamma activity, and reduced spike precision of 20-40 Hz responses. These findings present the first in vivo evidence of the linkage between a genetic marker and altered stimulus encoding, thus suggesting a novel electrophysiological endophenotype of schizophrenia.

Gamma oscillations in the midbrain spatial attention network: linking circuits to function

Gamma-band (25-140Hz) oscillations are ubiquitous in mammalian forebrain structures involved in sensory processing, attention, learning and memory. The optic tectum (OT) is the central structure in a midbrain network that participates critically in controlling spatial attention. In this review, we summarize recent advances in characterizing a neural circuit in this midbrain network that generates large amplitude, space-specific, gamma oscillations in the avian OT, both in vivo and in vitro. We describe key physiological and pharmacological mechanisms that produce and regulate the structure of these oscillations. The extensive similarities between midbrain gamma oscillations in birds and those in the neocortex and hippocampus of mammals, offer important insights into the functional significance of a midbrain gamma oscillatory code.

Chronic administration of antipsychotics attenuates ongoing and ketamine-induced increases in cortical γ oscillations

Noncompetitive N-methyl-d-aspartate receptor (NMDAr) antagonists can elicit many of the symptoms observed in schizophrenia in healthy humans, and induce a behavioural phenotype in animals relevant to psychosis. These compounds also elevate the power and synchrony of gamma (γ) frequency (30-80 Hz) neural oscillations. Acute doses of antipsychotic medications have been shown to reduce ongoing γ power and to inhibit NMDAr antagonist-mediated psychosis-like behaviour in rodents. This study aimed to investigate how a chronic antipsychotic dosing regimen affects ongoing cortical γ oscillations, and the electrophysiological and behavioural responses induced by the NMDAr antagonist ketamine. Male Wistar rats were chronically treated with haloperidol (0.25 mg/kg/d), clozapine (5 mg/kg/d), LY379268 (0.3 mg/kg/d) or vehicle for 28 d, delivered by subcutaneous (s.c.) osmotic pumps. Weekly electrocorticogram (ECoG) recordings were acquired. On day 26, ketamine (5 mg/kg, s.c.) was administered, and ECoG and locomotor activity were simultaneously measured. These results were compared with data generated previously following acute treatment with these antipsychotics. Sustained and significant decreases in ongoing γ power were observed during chronic administration of haloperidol (64%) or clozapine (43%), but not of LY379268 (2% increase), compared with vehicle. Acute ketamine injection concurrently increased γ power and locomotor activity in vehicle-treated rats, and these effects were attenuated in rats chronically treated with all three antipsychotics. The ability of haloperidol or clozapine to inhibit ketamine-induced elevation in γ power was not observed following acute administration of these drugs. These results indicate that modulation of γ power may be a useful biomarker of chronic antipsychotic efficacy.

Computational study of NMDA conductance and cortical oscillations in schizophrenia

N-methyl-D-aspartate (NMDA) receptor hypofunction has been implicated in the pathophysiology of schizophrenia. The illness is also characterized by gamma oscillatory disturbances, which can be evaluated with precise frequency specificity employing auditory cortical entrainment paradigms. This computational study investigates how synaptic NMDA hypofunction may give rise to network level oscillatory deficits as indexed by entrainment paradigms. We developed a computational model of a local cortical circuit with pyramidal cells and fast-spiking interneurons (FSI), incorporating NMDA, α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic (AMPA), and γ-aminobutyric acid (GABA) synaptic kinetics. We evaluated the effects of varying NMDA conductance on FSIs and pyramidal cells, as well as AMPA to NMDA ratio. We also examined the differential effects across a broad range of entrainment frequencies as a function of NMDA conductance. Varying NMDA conductance onto FSIs revealed an inverted-U relation with network gamma whereas NMDA conductance onto the pyramidal cells had a more monotonic relationship. Varying NMDA vs. AMPA conductance onto FSIs demonstrated the necessity of AMPA in the generation of gamma while NMDA receptors had a modulatory role. Finally, reducing NMDA conductance onto FSI and varying the stimulus input frequency reproduced the specific reductions in gamma range (~40 Hz) as observed in schizophrenia studies. Our computational study showed that reductions in NMDA conductance onto FSIs can reproduce similar disturbances in entrainment to periodic stimuli within the gamma range as reported in schizophrenia studies. These findings provide a mechanistic account of how specific cellular level disturbances can give rise to circuitry level pathophysiologic disturbance in schizophrenia.

Neurophysiologic and antipsychotic profiles of TASP0433864, a novel positive allosteric modulator of metabotropic glutamate 2 receptor

Excess glutamatergic neurotransmission has been implicated in the pathophysiology of schizophrenia, and the activation of metabotropic glutamate 2 (mGlu2) receptor may exert antipsychotic effects by normalizing glutamate transmission. In the present study, we investigated the neurophysiologic and antipsychotic profiles of TASP0433864 [(2S)-2-[(4-tert-butylphenoxy)methyl]-5-methyl-2,3-dihydroimidazo[2,1-b][1,3]oxazole-6-carboxamide], a newly synthesized positive allosteric modulator (PAM) of mGlu2 receptor. TASP0433864 exhibited PAM activity at human and rat mGlu2 receptors with EC50 values of 199 and 206 nM, respectively, without exerting agonist activity at rat mGlu2 receptor. TASP0433864 produced a leftward and upward shift in the concentration-response curve of glutamate-increased guanosine 5'-O-(3-[(35)S]thio)triphosphate binding to mGlu2 receptor. In contrast, TASP0433864 had negligible activities for other mGlu receptors, including mGlu3 receptor, and did not have any affinity for other receptors or transporters. In hippocampal slices, TASP0433864 potentiated an inhibitory effect of DCG-IV [(2S,2'R,3'R)-2-(2',3'-dicarboxylcyclopropyl)glycine], a mGlu2/3 receptor agonist, on the field excitatory postsynaptic potentials in the dentate gyrus, indicating that TASP0433864 potentiates the mGlu2 receptor-mediated presynaptic inhibition of glutamate release. Moreover, TASP0433864 inhibited both MK-801 [(5S,10R)-(+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine hydrogen maleate]- and ketamine-increased cortical γ band oscillation in the rat cortical electroencephalogram, which have been considered to reflect the excess activation of cortical pyramidal neurons. The inhibitory effect of TASP0433864 on cortical activation was also observed in the mouse 2-deoxy-glucose uptake study. In a behavioral study, TASP0433864 significantly inhibited both ketamine- and methamphetamine-increased locomotor activities in mice and rats, respectively. Collectively, these findings indicate that TASP0433864 is a selective mGlu2 receptor PAM with antipsychotic activity, and the attenuation of excess glutamatergic neurotransmission may be involved in the action of TASP0433864.

Effects of aberrant gamma frequency oscillations on prepulse inhibition

Emerging literature implicates abnormalities in gamma frequency oscillations in the pathophysiology of schizophrenia, with hypofunction of N-methyl-D-aspartate (NMDA) receptors implicated as a key factor. Prepulse inhibition (PPI) is a behavioural measure of sensorimotor gating, which is disrupted in schizophrenia. We studied relationships between ongoing and sensory-evoked gamma oscillations and PPI using pharmacological interventions designed to increase gamma oscillations (ketamine, MK-801); reduce gamma oscillations (LY379268); or disrupt PPI (amphetamine). We predicted that elevating ongoing gamma power would lead to increased 'neural noise' in cortical circuits, dampened sensory-evoked gamma responses and disrupted behaviour. Wistar rats were implanted with EEG recording electrodes. They received ketamine (5 mg/kg), MK-801 (0.16 mg/kg), amphetamine (0.5 mg/kg), LY379268 (3 mg/kg) or vehicle and underwent PPI sessions with concurrent EEG recording. Ketamine and MK-801 increased the power of ongoing gamma oscillations and caused time-matched disruptions of PPI, while amphetamine marginally affected ongoing gamma power. In contrast, LY379268 reduced ongoing gamma power, but had no effect on PPI. The sensory gamma response evoked by the prepulse was reduced following treatment with all psychotomimetics, associating with disruptions in PPI. This was most noticeable following treatment with NMDA receptor antagonists. We found that ketamine and MK-801 increase ongoing gamma power and reduce evoked gamma power, both of which are related to disruptions in sensorimotor gating. This appears to be due to antagonism of NMDA receptors, since amphetamine and LY379268 differentially impacted these outcomes and possess different neuropharmacological substrates. Aberrant gamma frequency oscillations caused by NMDA receptor hypofunction may mediate the sensory processing deficits observed in schizophrenia.

Involvement of glutamatergic and GABAergic transmission in MK-801-increased gamma band oscillation power in rat cortical electroencephalograms

Hypofunction of the N-methyl-D-aspartic acid receptor (NMDAr) has been considered to play a crucial role in the pathophysiology of schizophrenia. In rodent electroencephalogram (EEG) studies, non-competitive NMDAr antagonists have been reported to produce aberrant basal gamma band oscillation (GBO), as observed in schizophrenia. Aberrations in GBO power have attracted attention as a translational biomarker for the development of novel antipsychotic drugs. However, the neuronal mechanisms as well as the pharmacological significance of NMDAr antagonist-induced aberrant GBO power have not been fully investigated. In the present study, to address the above questions, we examined the pharmacological properties of MK-801 (0.1 mg/kg)-increased basal GBO power in rat cortical EEG. Riluzole (3-10 mg/kg), a glutamate release inhibitor, reduced the MK-801-increased basal GBO power. In contrast, L-838,417 (1-3 mg/kg), an α2/3/5 subunit-selective GABAA receptor-positive allosteric modulator, enhanced the GBO increase. Antipsychotics such as haloperidol (0.05-0.3 mg/kg) and clozapine (1-10 mg/kg) dose-dependently attenuated the MK-801-increased GBO power. Likewise, LY379268 (0.3-3 mg/kg), an metabotropic glutamate 2/3 receptor (mGlu2/3 receptor) agonist, reduced the GBO increase in a dose-dependent manner, which was antagonized by an mGlu2/3 receptor antagonist LY341495. These results suggest that an increase in cortical GBO power induced by NMDAr hypofunction can be attributed to the aberrant activities of both excitatory pyramidal neurons and inhibitory interneurons in local circuits. The aberrant cortical GBO power reflecting cortical network dysfunction observed in schizophrenia might be a useful biomarker for the discovery of novel antipsychotic drugs.

Brain areas that influence general anesthesia

This document reviews the literature on local brain manipulation of general anesthesia in animals, focusing on behavioral and electrographic effects related to hypnosis or loss of consciousness. Local inactivation or lesion of wake-active areas, such as locus coeruleus, dorsal raphe, pedunculopontine tegmental nucleus, perifornical area, tuberomammillary nucleus, ventral tegmental area and basal forebrain, enhanced general anesthesia. Anesthesia enhancement was shown as a delayed emergence (recovery of righting reflex) from anesthesia or a decrease in the minimal alveolar concentration that induced loss of righting. Local activation of various wake-active areas, including pontis oralis and centromedial thalamus, promoted behavioral or electrographic arousal during maintained anesthesia and facilitated emergence. Lesion of the sleep-active ventrolateral preoptic area resulted in increased wakefulness and decreased isoflurane sensitivity, but only for 6 days after lesion. Inactivation of any structure within limbic circuits involving the medial septum, hippocampus, nucleus accumbens, ventral pallidum, and ventral tegmental area, amygdala, entorhinal and piriform cortex delayed emergence from anesthesia, and often reduced anesthetic-induced behavioral excitation. In summary, the concept that anesthesia works on the sleep-wake system has received strong support from studies that inactivated/lesioned or activated wake-active areas, and weak support from studies that lesioned sleep-active areas. In addition to the conventional wake-sleep areas, limbic structures such as the medial septum, hippocampus and prefrontal cortex are also involved in the behavioral response to general anesthesia. We suggest that hypnosis during general anesthesia may result from disrupting the wake-active neuronal activities in multiple areas and suppressing an atropine-resistant cortical activation associated with movements.

Brain state-dependent abnormal LFP activity in the auditory cortex of a schizophrenia mouse model

In schizophrenia, evoked 40-Hz auditory steady-state responses (ASSRs) are impaired, which reflects the sensory deficits in this disorder, and baseline spontaneous oscillatory activity also appears to be abnormal. It has been debated whether the evoked ASSR impairments are due to the possible increase in baseline power. GABAergic interneuron-specific NMDA receptor (NMDAR) hypofunction mutant mice mimic some behavioral and pathophysiological aspects of schizophrenia. To determine the presence and extent of sensory deficits in these mutant mice, we recorded spontaneous local field potential (LFP) activity and its click-train evoked ASSRs from primary auditory cortex of awake, head-restrained mice. Baseline spontaneous LFP power in the pre-stimulus period before application of the first click trains was augmented at a wide range of frequencies. However, when repetitive ASSR stimuli were presented every 20 s, averaged spontaneous LFP power amplitudes during the inter-ASSR stimulus intervals in the mutant mice became indistinguishable from the levels of control mice. Nonetheless, the evoked 40-Hz ASSR power and their phase locking to click trains were robustly impaired in the mutants, although the evoked 20-Hz ASSRs were also somewhat diminished. These results suggested that NMDAR hypofunction in cortical GABAergic neurons confers two brain state-dependent LFP abnormalities in the auditory cortex; (1) a broadband increase in spontaneous LFP power in the absence of external inputs, and (2) a robust deficit in the evoked ASSR power and its phase-locking despite of normal baseline LFP power magnitude during the repetitive auditory stimuli. The “paradoxically” high spontaneous LFP activity of the primary auditory cortex in the absence of external stimuli may possibly contribute to the emergence of schizophrenia-related aberrant auditory perception.

High-frequency oscillations and the neurobiology of schizophrenia

Neural oscillations at low- and high-frequency ranges are a fundamental feature of large-scale networks. Recent evidence has indicated that schizophrenia is associated with abnormal amplitude and synchrony of oscillatory activity, in particular, at high (beta/gamma) frequencies. These abnormalities are observed during task-related and spontaneous neuronal activity which may be important for understanding the pathophysiology of the syndrome. In this paper, we shall review the current evidence for impaired beta/gamma-band oscillations and their involvement in cognitive functions and certain symptoms of the disorder. In the first part, we will provide an update on neural oscillations during normal brain functions and discuss underlying mechanisms. This will be followed by a review of studies that have examined high-frequency oscillatory activity in schizophrenia and discuss evidence that relates abnormalities of oscillatory activity to disturbed excitatory/inhibitory (E/I) balance. Finally, we shall identify critical issues for future research in this area.

Differential effects of NMDA receptor antagonists at lower and higher doses on basal gamma band oscillation power in rat cortical electroencephalograms

Schizophrenic patients have been shown to exhibit abnormal cortical gamma band oscillation (GBO), which is thought to be related to the symptoms of schizophrenia, including cognitive impairment. Recently, non-competitive NMDA receptor (NMDAr) antagonists such as MK-801 and ketamine have been reported to increase the basal GBO power in rat cortical electroencephalograms. However, the mechanisms underlying the increase in basal GBO power induced by non-competitive NMDAr antagonists remain unclear. In the present study, we characterized the non-competitive NMDAr antagonists-increased GBO (30-80 Hz) power. MK-801 (0.05-0.2 mg/kg) increased the GBO power, exhibiting an inverted U-shape dose-response curve; at higher doses (0.3-1 mg/kg), the increase in GBO was reversed. The GBO power was closely correlated with the high-frequency oscillation (130-180 Hz) power following MK-801 administration, while the GBO power was inversely correlated with the increase in delta oscillation (0.5-4 Hz) power at higher doses. PCP (1.25-10 mg/kg) and ketamine (2.5-30 mg/kg) also exhibited the inverted U-shape dose-responses for the basal GBO power similar to MK-801. Interestingly, memantine (10-30 mg/kg) dose-dependently and potently increased the GBO power without remarkably affecting the other frequency band. In contrast, other psychotomimetics, such as methamphetamine (1-10 mg/kg) and DOI (0.5-2 mg/kg), did not induce noticeable changes in the basal GBO power even at doses that induce abnormal behaviors, indicating that the increase in GBO power induced by NMDAr antagonists is not necessarily attributed to psychotomimetic effects. In conclusion, the basal GBO power increase in response to non-competitive NMDAr antagonists may reflect the cortical hyperglutamatergic state through GABAergic disinhibition.

Reviewing the ketamine model for schizophrenia

The observation that antagonists of the N-methyl-D-aspartate receptor (NMDAR), such as phencyclidine (PCP) and ketamine, transiently induce symptoms of acute schizophrenia had led to a paradigm shift from dopaminergic to glutamatergic dysfunction in pharmacological models of schizophrenia. The glutamate hypothesis can explain negative and cognitive symptoms of schizophrenia better than the dopamine hypothesis, and has the potential to explain dopamine dysfunction itself. The pharmacological and psychomimetic effects of ketamine, which is safer for human subjects than phencyclidine, are herein reviewed. Ketamine binds to a variety of receptors, but principally acts at the NMDAR, and convergent genetic and molecular evidence point to NMDAR hypofunction in schizophrenia. Furthermore, NMDAR hypofunction can explain connectional and oscillatory abnormalities in schizophrenia in terms of both weakened excitation of inhibitory γ-aminobutyric acidergic (GABAergic) interneurons that synchronize cortical networks and disinhibition of principal cells. Individuals with prenatal NMDAR aberrations might experience the onset of schizophrenia towards the completion of synaptic pruning in adolescence, when network connectivity drops below a critical value. We conclude that ketamine challenge is useful for studying the positive, negative, and cognitive symptoms, dopaminergic and GABAergic dysfunction, age of onset, functional dysconnectivity, and abnormal cortical oscillations observed in acute schizophrenia.

Animal Models of Psychosis: Current State and Future Directions

Psychosis is an abnormal mental state characterized by disorganization, delusions and hallucinations. Animal models have become an increasingly important research tool in the effort to understand both the underlying pathophysiology and treatment of psychosis. There are multiple animal models for psychosis, with each formed by the coupling of a manipulation and a measurement. In this manuscript we do not address the diseases of which psychosis is a prominent comorbidity. Instead, we summarize the current state of affairs and future directions for animal models of psychosis. To accomplish this, our manuscript will first discuss relevant behavioral and electrophysiological measurements. We then provide an overview of the different manipulations that are combined with these measurements to produce animal models. The strengths and limitations of each model will be addressed in order to evaluate its cross-species comparability.

Deep brain stimulation of the medial septum or nucleus accumbens alleviates psychosis-relevant behavior in ketamine-treated rats

Deep brain stimulation (DBS) has been shown to be effective for relief of Parkinson's disease, depression and obsessive-compulsive disorder in humans, but the effect of DBS on psychosis is largely unknown. In previous studies, we showed that inactivation of the medial septum or nucleus accumbens normalized the hyperactive and psychosis-related behaviors induced by psychoactive drugs. We hypothesized that DBS of the medial septum or nucleus accumbens normalizes the ketamine-induced abnormal behaviors and brain activity in freely moving rats. Male Long-Evans rats were subcutaneously injected with ketamine (3 mg/kg) alone, or given ketamine and DBS, or injected with saline alone. Subcutaneous injection of ketamine resulted in loss of gating of hippocampal auditory evoked potentials (AEPs), deficit in prepulse inhibition (PPI) and hyperlocomotion, accompanied by increased hippocampal gamma oscillations of 70-100 Hz. Continuous 130-Hz stimulation of the nucleus accumbens, or 100-Hz burst stimulation of the medial septum (1s on and 5s off) significantly attenuated ketamine-induced PPI deficit and hyperlocomotion. Medial septal stimulation also prevented the loss of gating of hippocampal AEPs and the increase in hippocampal gamma waves induced by ketamine. Neither septal or accumbens DBS alone without ketamine injection affected spontaneous locomotion or PPI. The results suggest that DBS of the medial septum or nucleus accumbens may be an effective method to alleviate psychiatric symptoms of schizophrenia. The effect of medial septal DBS in suppressing both hippocampal gamma oscillations and abnormal behaviors induced by ketamine suggests that hippocampal gamma oscillations are a correlate of disrupted behaviors.

Sub-anesthetic ketamine modulates intrinsic BOLD connectivity within the hippocampal-prefrontal circuit in the rat

Dysfunctional connectivity within the hippocampal-prefrontal circuit (HC-PFC) is associated with schizophrenia, major depression, and neurodegenerative disorders, and both the hippocampus and prefrontal cortex have dense populations of N-methyl-D-aspartate (NMDA) receptors. Ketamine, a potent NMDA receptor antagonist, is of substantial current interest as a mechanistic model of glutamatergic dysfunction in animal and human studies, a psychotomimetic agent and a rapidly acting antidepressant. In this study, we sought to understand the modulatory effect of acute ketamine administration on functional connectivity in the HC-PFC system of the rat brain using resting-state fMRI. Sprague-Dawley rats in four parallel groups (N=9 per group) received either saline or one of three behaviorally relevant, sub-anesthetic doses of S-ketamine (5, 10, and 25 mg/kg, s.c.), and connectivity changes 15- and 30-min post-injection were studied. The strongest effects were dose- and exposure-dependent increases in functional connectivity within the prefrontal cortex and in anterior-posterior connections between the posterior hippocampus and retrosplenial cortex, and prefrontal regions. The increased prefrontal connectivity is consistent with ketamine-induced increases in HC-PFC electroencephalographic gamma band power, possibly reflecting a psychotomimetic aspect of ketamine's effect, and is contrary to the data from chronic schizophrenic patients suggesting that ketamine effect does not necessarily parallel the disease pattern but might rather reflect a hyperglutamatergic state. These findings may help to clarify the brain systems underlying different dose-dependent behavioral profiles of ketamine in the rat.

Ketamine alters oscillatory coupling in the hippocampus

Recent studies show that higher order oscillatory interactions such as cross-frequency coupling are important for brain functions that are impaired in schizophrenia, including perception, attention and memory. Here we investigated the dynamics of oscillatory coupling in the hippocampus of awake rats upon NMDA receptor blockade by ketamine, a pharmacological model of schizophrenia. Ketamine (25, 50 and 75 mg/kg i.p.) increased gamma and high-frequency oscillations (HFO) in all depths of the CA1-dentate axis, while theta power changes depended on anatomical location and were independent of a transient increase of delta oscillations. Phase coherence of gamma and HFO increased across hippocampal layers. Phase-amplitude coupling between theta and fast oscillations was markedly altered in a dose-dependent manner: ketamine increased hippocampal theta-HFO coupling at all doses, while theta-gamma coupling increased at the lowest dose and was disrupted at the highest dose. Our results demonstrate that ketamine alters network interactions that underlie cognitively relevant theta-gamma coupling.

Frontal gamma noise power and cognitive domains in schizophrenia

The cognitive deficit profile is different among individuals with schizophrenia. We quantified the amount of electroencephalographic activity unlocked to stimuli onset (noise power) over frontal regions regarding deficit in cognitive domains. Forty-six patients with schizophrenia and 27 healthy controls underwent clinical, cognitive and electrophysiological assessments. Noise power studies may be considered complementary but not equivalent to induced power studies. We compared gamma and theta noise power magnitude during a P300 paradigm between subsets of patients divided according to cognitive deficit in key domains and controls. Patients displayed higher gamma noise power activity at Fz site and significantly lower performance in all cognitive domains when compared to controls. The subset of patients with cognitive deficit for working memory and problem solving/executive functions domains displayed significantly higher frontal-lateral noise power values in comparison to the subset of patients without cognitive deficit and controls. Patients with significant cognitive deficits in domains with greater frontal contribution are also characterized by an abnormally higher gamma band noise power over the frontal region. Our data may endorse various biological subsets within schizophrenia, characterized by the presence or absence of a significant cognitive deficit in frontal domains.

Acute NMDA receptor antagonism disrupts synchronization of action potential firing in rat prefrontal cortex

Antagonists of N-methyl-D-aspartate receptors (NMDAR) have psychotomimetic effects in humans and are used to model schizophrenia in animals. We used high-density electrophysiological recordings to assess the effects of acute systemic injection of an NMDAR antagonist (MK-801) on ensemble neural processing in the medial prefrontal cortex of freely moving rats. Although MK-801 increased neuron firing rates and the amplitude of gamma-frequency oscillations in field potentials, the synchronization of action potential firing decreased and spike trains became more Poisson-like. This disorganization of action potential firing following MK-801 administration is consistent with changes in simulated cortical networks as the functional connections among pyramidal neurons become less clustered. Such loss of functional heterogeneity of the cortical microcircuit may disrupt information processing dependent on spike timing or the activation of discrete cortical neural ensembles, and thereby contribute to hallucinations and other features of psychosis induced by NMDAR antagonists.

The role of α5 GABAA receptor agonists in the treatment of cognitive deficits in schizophrenia

Currently available pharmacotherapies for the treatment of schizophrenia are ineffective in restoring the disrupted cognitive function associated with this disorder. As such, there is a continued search for more viable novel drug targets. Engaging in cognitive behaviors is associated with distinct coordinated oscillatory activity across brain regions, in particular the hippocampus and prefrontal cortex. In schizophrenia patients, pathological alterations in the functionality of GABAergic interneurons in the PFC and HPC responsible for generating network oscillations are thought to contribute to impaired cognition. Destabilized GABAergic interneuron activity in the HPC is further associated with aberrant increases in HPC output and enhanced dopamine neuron activity. Consequently, drugs directed at restoring HPC function could impact both oscillatory activity along with dopamine tone. There is compelling evidence from animal models of schizophrenia that allosteric modulation of the α5 subunit of the GABAA receptor is a viable means of resolving aberrant dopamine system activity through indirect alteration of HPC output. Consequently, these compounds are promising for their potential in also ameliorating cognitive deficits attributed to dysfunction in HPC network activity.

Anesthetic regimes modulate the temporal dynamics of local field potential in the mouse olfactory bulb

Anesthetized preparations have been widely used to study odor-induced temporal dynamics in the olfactory bulb. Although numerous recent data of single-cell recording or imaging in the olfactory bulb have employed ketamine cocktails, their effects on networks activities are still poorly understood, and odor-induced oscillations of the local field potential have not been characterized under these anesthetics. Our study aimed at describing the impact of two ketamine cocktails on oscillations and comparing them to awake condition. Anesthesia was induced by injection of a cocktail of ketamine, an antagonist of the N-methyl-d-aspartate receptors, combined with one agonist of α2-adrenergic receptors, xylazine (low affinity) or medetomidine (high affinity). Spontaneous and odor-induced activities were examined in anesthetized and awake conditions, in the same mice chronically implanted with an electrode in the main olfactory bulb. The overall dynamic pattern of oscillations under the two ketamine cocktails resembles that of the awake state. Ongoing activity is characterized by gamma bursts (>60 Hz) locked on respiration and beta (15-40 Hz) power increases during odor stimulation. However, anesthesia decreases local field potential power and leads to a strong frequency shift of gamma oscillations from 60-90 Hz to 100-130 Hz. We conclude that similarities between oscillations in anesthetized and awake states make cocktails of ketamine with one α2-agonist suitable for the recordings of local field potential to study processing in the early stages of the olfactory system.

Optogenetically evoked gamma oscillations are disturbed by cocaine administration

Drugs of abuse have enormous societal impact by degrading the cognitive abilities, emotional state and social behavior of addicted individuals. Among other events involved in the addiction cycle, the study of a single exposure to cocaine, and the contribution of the effects of that event to the continuous and further use of drugs of abuse are fundamental. Gamma oscillations are thought to be important neural correlates of cognitive processing in the prefrontal cortex (PFC) which include decision making, set shifting and working memory. It follows that cocaine exposure might modulate gamma oscillations, which could result in reduced cognitive ability. Parvalbumin-positive fast-spiking interneurons play an orchestrating role in gamma oscillation induction and it has been shown recently that gamma oscillations can be induced in an anesthetized animal using optogenetic techniques. We use a knock-in mouse model together with optogenetics and in vivo electrophysiology to study the effects of acute cocaine on PFC gamma oscillation as a step toward understanding the cortical changes that may underlie continuous use of stimulants. Our results show that acute cocaine administration increases entrainment of the gamma oscillation to the optogentically induced driving frequency. Our results also suggest that this modulation of gamma oscillations is driven trough activation of D1 receptors. The acute cocaine-mediated changes in mPFC may underlie the enhancement of attention and awareness commonly reported by cocaine users and may contribute to the further use and abuse of psychostimulants.

Disruption of thalamocortical activity in schizophrenia models: relevance to antipsychotic drug action

Non-competitive NMDA receptor antagonists are widely used as pharmacological models of schizophrenia due to their ability to evoke the symptoms of the illness. Likewise, serotonergic hallucinogens, acting on 5-HT(2A) receptors, induce perceptual and behavioural alterations possibly related to psychotic symptoms. The neurobiological basis of these alterations is not fully elucidated. Data obtained in recent years revealed that the NMDA receptor antagonist phencyclidine (PCP) and the serotonergic hallucinogen 1-(2,5-dimethoxy-4-iodophenyl-2-aminopropane; DOI) produce a series of common actions in rodent prefrontal cortex (PFC) that may underlie psychotomimetic effects. Hence, both agents markedly disrupt PFC function by altering pyramidal neuron discharge (with an overall increase) and reducing the power of low frequency cortical oscillations (LFCO; < 4 Hz). In parallel, PCP increased c-fos expression in excitatory neurons of various cortical areas, the thalamus and other subcortical structures, such as the amygdala. Electrophysiological studies revealed that PCP altered similarly the function of the centromedial and mediodorsal nuclei of the thalamus, reciprocally connected with PFC, suggesting that its psychotomimetic properties are mediated by an alteration of thalamocortical activity (the effect of DOI was not examined in the thalamus). Interestingly, the observed effects were prevented or reversed by the antipsychotic drugs clozapine and haloperidol, supporting that the disruption of PFC activity is intimately related to the psychotomimetic activity of these agents. Overall, the present experimental model can be successfully used to elucidate the neurobiological basis of schizophrenia symptoms and to examine the potential antipsychotic activity of new drugs in development.

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

Distinct frequency bands can be differentiated from neuronal ensemble recordings, such as local field potentials or electrocorticogram recordings. Recent years have witnessed a rapid acceleration of research examining how N-methyl-D-aspartate receptor (NMDAR) antagonists influence fundamental frequency bands in cortical and subcortical brain regions. Herein, we systematically review findings from in vivo studies with a focus on delta, theta, gamma and more recently identified high-frequency oscillations. We also discuss some of the current hypotheses that are considered to account for the actions of NMDAR antagonists on these frequency bands. The data emphasize a close relationship between altered oscillatory activity and NMDAR blockade, with both local and large-scale networks accounting for their effects. These findings may have fundamental implications for the psychotomimetic effects produced by NMDAR antagonists.

Relationship of resting brain hyperconnectivity and schizophrenia-like symptoms produced by the NMDA receptor antagonist ketamine in humans

N-methyl-D-aspartate glutamate receptor (NMDA-R) antagonists produce schizophrenia-like positive and negative symptoms in healthy human subjects. Preclinical research suggests that NMDA-R antagonists interfere with the function of gamma-aminobutyric acid (GABA) neurons and alter the brain oscillations. These changes have been hypothesized to contribute to psychosis. In this investigation, we evaluated the hypothesis that the NMDA-R antagonist ketamine produces alterations in cortical functional connectivity during rest that are related to symptoms. We administered ketamine to a primary sample of 22 subjects and to an additional, partially overlapping, sample of 12 subjects. Symptoms before and after the experimental session were rated with the Positive and Negative Syndrome Scale (PANSS). In the primary sample, functional connectivity was measured via functional magnetic resonance imaging almost immediately after infusion began. In the additional sample, this assessment was repeated after 45 min of continuous ketamine infusion. Global, enhanced functional connectivity was observed at both timepoints, and this hyperconnectivity was related to symptoms in a region-specific manner. This study supports the hypothesis that pathological increases in resting brain functional connectivity contribute to the emergence of positive and negative symptoms associated with schizophrenia.

Different effects of the NMDA receptor antagonists ketamine, MK-801, and memantine on postsynaptic density transcripts and their topography: role of Homer signaling, and implications for novel antipsychotic and pro-cognitive targets in psychosis

Administration of NMDA receptor antagonists, such as ketamine and MK-801, may induce psychotic-like behaviors in preclinical models of schizophrenia. Ketamine has also been observed to exacerbate psychotic symptoms in schizophrenia patients. However, memantine, a non-competitive NMDA receptor antagonist approved for Alzheimer's disease and proposed for antipsychotic augmentation, may challenge this view. To date, the molecular mechanisms by which these NMDA receptor antagonists cause different neurochemical, behavioral, and clinical effects are still a matter of debate. Here, we investigated by molecular imaging whether these agents could differently modulate gene expression and topographical distribution of glutamatergic postsynaptic density (PSD) proteins. We focused on Homer1a/Homer1b/PSD-95 signaling network, which may be implicated in glutamate-dependent synaptic plasticity, as well as in psychosis pathophysiology and treatment. Ketamine (25 and 50mg/kg) and MK-801 (0.8mg/kg) significantly induced the transcripts of immediate-early genes (Arc, c-fos, and Homer1a) in cortical regions compared to vehicle, whereas they reduced Homer1b and PSD-95 expression in cortical and striatal regions. Differently, memantine (5mg/kg) did not increase Homer1a signal compared to vehicle, whereas it induced c-fos in the somatosensory and in the medial agranular cortices. Moreover, memantine did not affect Homer1b and PSD-95 expression. When compared to ketamine and MK-801, memantine significantly increased the expression of c-fos, Homer1b and PSD-95. Overall, ketamine and MK-801 prominently increased Homer1a/Homer1b expression ratio, whereas memantine elicited the opposite effect. These data may support the view that ketamine, MK-801 and memantine exert divergent effects on PSD transcripts, which may contribute to their partially different behavioral and clinical effects.

Progressive sleep and electroencephalogram changes in mice carrying the Huntington's disease mutation

Sleep disturbances in Huntington's disease may be deleterious to the cognitive performance, affective behaviour, and general well-being of patients, but a comprehensive description of the progression of changes in sleep and electroencephalogram in Huntington's disease has never been conducted. Here we studied sleep and electroencephalogram disturbances in a transgenic mouse model of Huntington's disease (R6/2 mice). We implanted 10 R6/2 mice and five wild-type littermates with electromyography electrodes, frontofrontal and frontoparietal electroencephalogram electrodes and then recorded sleep/wake behaviour at presymptomatic, symptomatic and late stages of the disease. In addition to sleep-wake scoring, we performed a spectral analysis of the sleep electroencephalogram. We found that sleep and electroencephalogram were already significantly disrupted in R6/2 mice at 9 weeks of age (presymptomatic stage). By the time they were symptomatic, R6/2 mice were unable to maintain long periods of wakefulness and had an increased propensity for rapid eye movement sleep. In addition, the peak frequency of theta rhythm was shifted progressively from 7 Hz to 6 Hz during rapid eye movement sleep, whereas slow wave activity decreased gradually during non-rapid eye movement sleep. Finally, as the disease progressed, an abnormal electroencephalogram gamma activity (30-40 Hz) emerged in R6/2 mice irrespective of sleep states. This is reminiscent of the increased gamma power described in schizophrenic patients during sleep and events of psychosis. Gaining a better understanding of sleep and electroencephalogram changes in patients with Huntington's disease should be a priority, since it will enable clinicians to initiate appropriate investigations and to instigate treatments that could dramatically improve patients' quality of life.

Enhancement of gamma activity after selective activation of dopamine D4 receptors in freely moving rats and in a neurodevelopmental model of schizophrenia

Dopamine D4 receptor (D4R) mechanisms have been implicated in several psychiatric diseases, including schizophrenia, attention-deficit hyperactivity disorder (ADHD), and autism, which are characterized by cognitive deficits. The cellular mechanisms are poorly understood but impaired neuronal synchronization within cortical networks in the gamma frequency band has been proposed to contribute to these deficits. A D4R polymorphism was recently linked to variations in gamma power in both normal and ADHD subjects, and D4R activation was shown to enhance kainate-induced gamma oscillations in brain slices in vitro. The goal of this study was to investigate the effect of D4R activation on gamma oscillations in freely moving rats during natural behavior. Field potentials were recorded in the frontal, prefrontal, parietal, and occipital cortex and hippocampus. Gamma power was assessed before and after subcutaneous injection of a D4R agonist, A-412997, in several doses between 0.3 and 10.0 mg/kg. The experiments were also repeated in a neurodevelopmental model of schizophrenia, in which rats are prenatally treated with methylazoxymethanol (MAM). We found that the D4R agonist increased gamma power in all regions at short latency and lasted for ~2 h, both in normal and MAM-treated rats. The effect was dose dependent indicated by the significant difference between the effects after 3 and 10 mg/kg in pair-wise comparison, whereas 0.3 and 1.0 mg/kg injections were ineffective. This study demonstrates the involvement of D4R in cortical gamma oscillations in vivo and identifies this receptor as potential target for pharmacological treatment of cognitive deficits.

Dopamine, cognitive function, and gamma oscillations: role of D4 receptors

Cognitive deficits in individuals with schizophrenia (SCZ) are considered core symptoms of this disorder, and can manifest at the prodromal stage. Antipsychotics ameliorate positive symptoms but only modestly improve cognitive symptoms. The lack of treatments that improve cognitive abilities currently represents a major obstacle in developing more effective therapeutic strategies for this debilitating disorder. While D4 receptor (D4R)-specific antagonists are ineffective in the treatment of positive symptoms, animal studies suggest that D4R drugs can improve cognitive deficits. Moreover, recent work from our group suggests that D4Rs synergize with the neuregulin/ErbB4 signaling pathway, genetically identified as risk factors for SCZ, in parvalbumin (PV)-expressing interneurons to modulate gamma oscillations. These high-frequency network oscillations correlate with attention and increase during cognitive tasks in healthy subjects, and this correlation is attenuated in affected individuals. This finding, along with other observations indicating impaired GABAergic function, has led to the idea that abnormal neural activity in the prefrontal cortex (PFC) in individuals with SCZ reflects a perturbation in the balance of excitation and inhibition. Here we review the current state of knowledge of D4R functions in the PFC and hippocampus, two major brain areas implicated in SCZ. Special emphasis is given to studies focusing on the potential role of D4Rs in modulating GABAergic transmission and to an emerging concept of a close synergistic relationship between dopamine/D4R and neuregulin/ErbB4 signaling pathways that tunes the activity of PV interneurons to regulate gamma frequency network oscillations and potentially cognitive processes.

Imaging patients with psychosis and a mouse model establishes a spreading pattern of hippocampal dysfunction and implicates glutamate as a driver

VIDEO ABSTRACT

The hippocampus in schizophrenia is characterized by both hypermetabolism and reduced size. It remains unknown whether these abnormalities are mechanistically linked. Here we addressed this question by using MRI tools that can map hippocampal metabolism and structure in patients and mouse models. In at-risk patients, hypermetabolism was found to begin in CA1 and spread to the subiculum after psychosis onset. CA1 hypermetabolism at baseline predicted hippocampal atrophy, which occurred during progression to psychosis, most prominently in similar regions. Next, we used ketamine to model conditions of acute psychosis in mice. Acute ketamine reproduced a similar regional pattern of hypermetabolism, while repeated exposure shifted the hippocampus to a hypermetabolic basal state with concurrent atrophy and pathology in parvalbumin-expressing interneurons. Parallel in vivo experiments using the glutamate-reducing drug LY379268 and direct measurements of extracellular glutamate showed that glutamate drives both neuroimaging abnormalities. These findings show that hippocampal hypermetabolism leads to atrophy in psychotic disorder and suggest glutamate as a pathogenic driver.

Antipsychotic compounds differentially modulate high-frequency oscillations in the rat nucleus accumbens: a comparison of first- and second-generation drugs

Improved understanding of the actions of antipsychotic compounds is critical for a better treatment of schizophrenia. Abnormal oscillatory activity has been found in schizophrenia and in rat models of the disease. N-Methyl-D-aspartic acid receptor (NMDAR) antagonists, used to model certain features of schizophrenia, increase the frequency and power of high-frequency oscillations (HFO, 130-180 Hz) in the rat nucleus accumbens, a brain region implicated in schizophrenia pathology. Antipsychotics can be classified as first- and second-generation drugs, the latter often reported to have wider benefit in humans and experimental models. This prompted the authors to examine the pre- and post-treatment effects of clozapine, risperidone (second-generation drugs) and sulpiride and haloperidol (first-generation drugs) on ketamine and MK801-enhanced accumbal HFO. Both NMDAR antagonists increased HFO frequency. In contrast, clozapine and risperidone markedly and dose-dependently reduced the frequency of spontaneous and NMDAR-antagonist-enhanced HFO, whilst a moderate effect was found for sulpiride and a much weaker effect for haloperidol. Unexpectedly, we found reductions in HFO frequency were associated with an increase in its power. These findings indicate that modulation of accumbal HFO frequency may be a fundamental effect produced by antipsychotic compounds. Of the drugs investigated, first- and second-generation compounds could be dissociated by their potency on this measure. This effect may partially explain the differences in the clinical profile of these drugs.

Impact of ketamine on neuronal network dynamics: translational modeling of schizophrenia-relevant deficits

Subanesthetic doses of the psychomimetic, ketamine, have been used for many years to elicit behavioral effects reminiscent of schizophrenia in both healthy humans and in animal models of the disease. More recently, there has been a move toward the use of simple neurophysiological measures (event-related potentials, brain oscillations) to assay the functional integrity of neuronal circuits in schizophrenia as these measures can be assessed in patients, healthy controls, intact animals, and even in brain slices. Furthermore, alterations of these measures are correlated with basic information processing deficits that are now considered central to the disease. Thus, here we review recent studies that determine the effect of ketamine on these measures and discuss to what extent they recapitulate findings in patients with schizophrenia. In particular, we examine methodological differences between human and animal studies and compare in vivo and in vitro effects of ketamine. Ketamine acts on multiple cortical and subcortical sites, as well as on receptors other than the N-methyl-d-aspartate receptor. Acute ketamine models' changes correlated with psychotic states (e.g. increased baseline gamma-band oscillations), whereas chronic ketamine causes cortical circuit changes and neurophysiological deficits (e.g. impaired event-related gamma-band oscillations) correlated with cognitive impairments in schizophrenia.

Increased spontaneous gamma power and synchrony in schizophrenia patients having higher minor physical anomalies

The higher frequency of minor physical anomalies (MPAs) in schizophrenia provides morphological evidence for the neurodevelopmental theory. Abnormal gamma oscillations (>30 Hz) seen in the electroencephalogram (EEG) in schizophrenia have been hypothesized to result from developmental insults. This study investigated spontaneous gamma oscillations in schizophrenia patients having higher and lower number of MPAs. Forty drug naïve/free schizophrenia patients and 20 matched healthy controls were assessed for MPAs on the Extended Waldrop Scale (EWS). All participants underwent an awake, resting 192-channel EEG recording. Spontaneous gamma spectral power and coherence were estimated in the low- (30-50 Hz) and high-gamma (51-70 and 71-100 Hz) bands. Significantly higher power was observed in high-MPA than healthy control group in low-gamma band over right frontal, parietal and temporal regions. Spectral power in the high-gamma band (71-100 Hz) was also significantly higher in the high-MPA schizophrenia subgroup than in the healthy control group over left frontal, parietal and temporal regions. Additionally, regional intra-hemispheric and inter-hemispheric coherence in the low-gamma band was significantly higher in the high-MPA schizophrenia subgroup than on the healthy control group. This study is the first to provide evidence of increased spontaneous gamma power and synchrony in schizophrenia patients having higher MPAs, supporting the idea that it may represent a distinct subgroup of schizophrenia with a neurodevelopmental basis.

MK-801 disrupts and nicotine augments 40 Hz auditory steady state responses in the auditory cortex of the urethane-anesthetized rat

Patients with schizophrenia show marked deficits in processing sensory inputs including a reduction in the generation and synchronization of 40 Hz gamma oscillations in response to steady-state auditory stimulation. Such deficits are not readily demonstrable at other input frequencies. Acute administration of NMDA antagonists to healthy human subjects or laboratory animals is known to reproduce many sensory and cognitive deficits seen in schizophrenia patients. In the following study, we tested the hypothesis that the NMDA antagonist MK-801 would selectively disrupt steady-state gamma entrainment in the auditory cortex of urethane-anesthetized rat. Moreover, we further hypothesized that nicotinic receptor activation would alleviate this disruption. Auditory steady state responses were recorded in response to auditory stimuli delivered over a range of frequencies (10-80 Hz) and averaged over 50 trials. Evoked power was computed under baseline condition and after vehicle or MK-801 (0.03 mg/kg, iv). MK-801 produced a significant attenuation in response to 40 Hz auditory stimuli while entrainment to other frequencies was not affected. Time-frequency analysis revealed deficits in both power and phase-locking to 40 Hz. Nicotine (0.1 mg/kg, iv) administered after MK-801 reversed the attenuation of the 40 Hz response. Administered alone, nicotine augmented 40 Hz steady state power and phase-locking. Nicotine's effects were blocked by simultaneous administration of the α4β2 antagonist DHßE. Thus we report for the first time, a rodent model that mimics a core neurophysiological deficit seen in patients with schizophrenia and a pharmacological approach to alleviate it.

How Nox2-containing NADPH oxidase affects cortical circuits in the NMDA receptor antagonist model of schizophrenia

SIGNIFICANCE

Schizophrenia is a complex neuropsychiatric disorder affecting around 1% of the population worldwide. Its mode of inheritance suggests a multigenic neurodevelopmental disorder with symptoms appearing during late adolescence/early adulthood, with its onset strongly influenced by environmental stimuli. Many neurotransmitter systems, including dopamine, glutamate, and gamma-aminobutyric acid, show alterations in affected individuals, and the behavioral and physiological characteristics of the disease can be mimicked by drugs that produce blockade of N-methyl-d-aspartate glutamate receptors (NMDARs).

RECENT ADVANCES

Mounting evidence suggests that drugs that block NMDARs specifically impair the inhibitory capacity of parvalbumin-expressing (PV+) fast-spiking neurons in adult and developing rodents, and alterations in these inhibitory neurons is one of the most consistent findings in the schizophrenic postmortem brain. Disruption of the inhibitory capacity of PV+ inhibitory neurons will alter the functional balance between excitation and inhibition in prefrontal cortical circuits producing impairment of working memory processes such as those observed in schizophrenia.

CRITICAL ISSUES

Mechanistically, the effect of NMDAR antagonists can be attributed to the activation of the Nox2-dependent reduced form of nicotinamide adenine dinucleotide phosphate oxidase pathway in cortical neurons, which is consistent with the emerging role of oxidative stress in the pathogenesis of mental disorders, specifically schizophrenia. Here we review the mechanisms by which NMDAR antagonists produce lasting impairment of the cortical PV+ neuronal system and the roles played by Nox2-dependent oxidative stress mechanisms.

FUTURE DIRECTIONS

The discovery of the pathways by which oxidative stress leads to unbalanced excitation and inhibition in cortical neural circuits opens a new perspective toward understanding the biological underpinnings of schizophrenia.

First and second generation antipsychotics influence hippocampal gamma oscillations by interactions with 5-HT3 and D3 receptors

BACKGROUND AND PURPOSE

Disturbed cortical gamma band oscillations (30-80 Hz) have been observed in schizophrenia: positive symptoms of the disease correlate with an increase in gamma oscillation power, whereas negative symptoms are associated with a decrease.

EXPERIMENTAL APPROACH

Here we investigated the effects of first and second generation antipsychotics (FGAs and SGAs, respectively) on gamma oscillations. The FGAs haloperidol, flupenthixol, chlorpromazine, chlorprothixene and the SGAs clozapine, risperidone, ziprasidone, amisulpride were applied on gamma oscillations induced by acetylcholine and physostigmine in the CA3 region of rat hippocampal slices.

KEY RESULTS

Antipsychotics inhibited the power of gamma oscillations and increased the bandwidth of the gamma band. Haloperidol and clozapine had the highest inhibitory effects. To determine which receptor is responsible for the alterations in gamma oscillations, the effects of the antipsychotics were plotted against their pK(i) values for 19 receptors and analysed for correlation. Our results indicated that 5-HT(3) receptors have an enhancing effect on gamma oscillations whereas dopamine D(3) receptors inhibit them. To test this prediction, m-chlorophenylbiguanide, PD 128907 and CP 809101, selective agonists at 5-HT(3) , D(3) and 5-HT(2C) receptors were applied and revealed that 5-HT(3) receptors indeed enhanced the gamma power whereas D(3) receptors reduced it. As predicted, 5-HT(2C) receptors had no effects on gamma oscillations.

CONCLUSION AND IMPLICATIONS

Our data suggest that antipsychotics alter hippocampal gamma oscillations by interacting with 5-HT(3) and dopamine D(3) receptors. Moreover, a correlation of receptor affinities with the biological effects can be used to predict targets for the pharmacological effects of multi-target drugs.

Deficits in high- (>60 Hz) gamma-band oscillations during visual processing in schizophrenia

Current theories of the pathophysiology of schizophrenia have focused on abnormal temporal coordination of neural activity. Oscillations in the gamma-band range (>25 Hz) are of particular interest as they establish synchronization with great precision in local cortical networks. However, the contribution of high gamma (>60 Hz) oscillations toward the pathophysiology is less established. To address this issue, we recorded magnetoencephalographic (MEG) data from 16 medicated patients with chronic schizophrenia and 16 controls during the perception of Mooney faces. MEG data were analysed in the 25–150 Hz frequency range. Patients showed elevated reaction times and reduced detection rates during the perception of upright Mooney faces while responses to inverted stimuli were intact. Impaired processing of Mooney faces in schizophrenia patients was accompanied by a pronounced reduction in spectral power between 60–120 Hz (effect size: d = 1.26) which was correlated with disorganized symptoms (r = −0.72). Our findings demonstrate that deficits in high gamma-band oscillations as measured by MEG are a sensitive marker for aberrant cortical functioning in schizophrenia, suggesting an important aspect of the pathophysiology of the disorder.

Impaired GABAergic neurotransmission in schizophrenia underlies impairments in cortical gamma band oscillations

Impairment of cortical circuit function is increasingly believed to be central to the pathophysiology of schizophrenia (Sz). Such impairments are suggested to result in abnormal gamma band oscillatory activity observed in Sz patients, and likely underlie the psychosis and cognitive deficits linked to this disease. Development of improved therapeutic strategies to enhance functional outcome of Sz patients is contingent upon a detailed understanding of the mechanisms behind cortical circuit development and maintenance. Convergent evidence from both Sz clinical and preclinical studies suggests impaired activity of a particular subclass of interneuron which expresses the calcium binding protein parvalbumin is central to the cortical circuit impairment observed. Here we review our current understanding of the Sz related cortical circuit dysfunction with a particular focus on the role of fast spiking parvalbumin interneurons in both normal cortical circuit activity and in NMDA receptor hypofunction models of the Sz disease state.

The auditory steady-state response (ASSR): a translational biomarker for schizophrenia

Electrophysiological methods have demonstrated disturbances of neural synchrony and oscillations in schizophrenia which affect a broad range of sensory and cognitive processes. These disturbances may account for a loss of neural integration and effective connectivity in the disorder. The mechanisms responsible for alterations in synchrony are not well delineated, but may reflect disturbed interactions within GABAergic and glutamatergic circuits, particularly in the gamma range. Auditory steady-state responses (ASSRs) provide a non-invasive technique used to assess neural synchrony in schizophrenia and in animal models at specific response frequencies. ASSRs are electrophysiological responses entrained to the frequency and phase of a periodic auditory stimulus generated by auditory pathway and auditory cortex activity. Patients with schizophrenia show reduced ASSR power and phase locking to gamma range stimulation. We review alterations of ASSRs in schizophrenia, schizotypal personality disorder, and first-degree relatives of patients with schizophrenia. In vitro and in vivo approaches have been used to test cellular mechanisms for this pattern of findings. This translational, cross-species approach provides support for the role of N-methyl-D-aspartate and GABAergic dysregulation in the genesis of perturbed ASSRs in schizophrenia and persons at risk.

Serotonergic hallucinogens differentially modify gamma and high frequency oscillations in the rat nucleus accumbens

RATIONALE

The nucleus accumbens (NAc) is a site critical for the actions of many drugs of abuse. Psychoactive compounds, such as N-methyl-D-aspartate receptor (NMDAR) antagonists, modify gamma (40-90) and high frequency oscillations (HFO, 130-180 Hz) in local field potentials (LFPs) recorded in the NAc. Lysergic acid diethylamide (LSD) and 2,5-dimethoxy-4-iodoamphetamine (DOI) are serotonergic hallucinogens and activation of 5HT2A receptors likely underlies their hallucinogenic effects. Whether these compounds can also modulate LFP oscillations in the NAc is unclear.

OBJECTIVE

This study aims to examine the effect of serotonergic hallucinogens on gamma and HFO recorded in the NAc and to test whether 5HT2A receptors mediate the effects observed.

METHODS

LFPs were recorded from the NAc of freely moving rats. Drugs were administered intraperitoneally.

RESULTS

LSD (0.03-0.3 mg/kg) and DOI (0.5-2.0 mg/kg) increased the power and reduced the frequency of HFO. In contrast, the hallucinogens produced a robust reduction in the power of low (40-60 Hz), but not high gamma oscillations (70-90 Hz). MDL 11939 (1.0 mg/kg), a 5HT2A receptor antagonist, fully reversed the changes induced by DOI on HFO but only partially for the low gamma band. Equivalent increases in HFO power were observed after TCB-2 (5HT2A receptor agonist, 0.1-1.5 mg/kg), but not CP 809101 (5H2C receptor agonist, 0.1-3 mg/kg). Notably, hallucinogen-induced increases in HFO power were smaller than those produced by ketamine (25 mg/kg).

CONCLUSIONS

Serotonergic hallucinogen-induced changes in HFO and gamma are mediated, at least in part, by stimulation of 5HT2A receptors. Comparison of the oscillatory changes produced by serotonergic hallucinogens and NMDAR antagonists are also discussed.

Chronic Ketamine Reduces the Peak Frequency of Gamma Oscillations in Mouse Prefrontal Cortex Ex vivo

Abnormalities in EEG gamma band oscillations (GBO, 30-80 Hz) serve as a prominent biomarker of schizophrenia (Sz), associated with positive, negative, and cognitive symptoms. Chronic, subanesthetic administration of antagonists of N-methyl-D-aspartate receptors (NMDAR), such as ketamine, elicits behavioral effects, and alterations in cortical interneurons similar to those observed in Sz. However, the chronic effects of ketamine on neocortical GBO are unknown. Thus, here we examine the effects of chronic (five daily i.p. injections) application of ketamine (5 and 30 mg/kg) and the more specific NMDAR antagonist, MK-801 (0.02, 0.5, and 2 mg/kg), on neocortical GBO ex vivo. Oscillations were generated by focal application of the glutamate receptor agonist, kainate (KA), in coronal brain slices containing the prelimbic cortex. This region constitutes the rodent analog of the human dorsolateral prefrontal cortex, a brain region strongly implicated in Sz-pathophysiology. Here we report the novel finding that chronic ketamine elicits a reduction in the peak oscillatory frequency of KA-elicited oscillations (from 47 to 40 Hz at 30 mg/kg). Moreover, the power of GBO in the 40-50 Hz band was reduced. These findings are reminiscent of both the reduced resonance frequency and power of cortical oscillations observed in Sz clinical studies. Surprisingly, MK-801 had no significant effect, suggesting care is needed when equating Sz-like behavioral effects elicited by different NMDAR antagonists to alterations in GBO activity. We conclude that chronic ketamine in the mouse mimics GBO abnormalities observed in Sz patients. Use of this ex vivo slice model may be useful in testing therapeutic compounds which rescue these GBO abnormalities.