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 Agonist Drastically Increases Delta Activity in the Thalamic Nucleus Reuniens: Potential Role in Communication between Prefrontal Cortex and Hippocampus

Network oscillations are essential for all cognitive functions. Oscillatory deficits are well established in psychiatric diseases and are recapitulated in animal models. They are significantly and specifically affected by pharmacological interventions using psychoactive compounds. Dopamine D4 receptor (D4R) activation was shown to enhance gamma rhythm in freely moving rats and to specifically affect slow delta and theta oscillations in the urethane-anesthetized rat model. The goal of this study was to test the effect of D4R activation on slow network oscillations at delta and theta frequencies during wake states, potentially supporting enhanced functional connectivity during dopamine-induced attention and cognitive processing. Network activity was recorded in the prefrontal cortex (PFC), hippocampus (HC) and nucleus reuniens (RE) in control conditions and after injecting the D4R agonist A-412997 (3 and 5 mg/kg; systemic administration). We found that A-412997 elicited a lasting (~40 min) wake state and drastically enhanced narrow-band delta oscillations in the PFC and RE in a dose-dependent manner. It also preferentially enhanced delta synchrony over theta coupling within the PFC-RE-HC circuit, strongly strengthening PFC-RE coupling. Thus, our findings indicate that the D4R may contribute to cognitive processes, at least in part, through acting on wake delta oscillations and that the RE, providing an essential link between the PFC and HC, plays a prominent role in this mechanism.

Individual/Peak Gamma Frequency: What Do We Know?

In recent years, the concept of individualized measures of electroencephalographic (EEG) activity has emerged. Gamma-band activity plays an important role in many sensory and cognitive processes. Thus, peak frequency in the gamma range has received considerable attention. However, peak or individual gamma frequency (IGF) is rarely used as a primary measure of interest; consequently, little is known about its nature and functional significance. With this review, we attempt to comprehensively overview available information on the functional properties of peak gamma frequency, addressing its relationship with certain processes and/or modulation by various factors. Here, we show that IGFs seem to be related to various endogenous and exogenous factors. Broad functional aspects that are related to IGF might point to the differences in underlying mechanisms. Therefore, research utilizing different types of stimulation for IGF estimation and covering several functional aspects in the same population is required. Moreover, IGFs span a wide range of frequencies (30-100 Hz). This could be partly due to the variability of methods used to extract the measures of IGF. In order to overcome this issue, further studies aiming at the optimization of IGF extraction would be greatly beneficial.

Behavioural effects of APH199, a selective dopamine D4 receptor agonist, in animal models

RATIONALE

The dopamine D4 receptors (DRD4) play a key role in numerous brain functions and are involved in the pathogenesis of various psychiatric disorders. DRD4 ligands have been shown to moderate anxiety, reward and depression-like behaviours, and cognitive impairments. Despite a series of promising but ambiguous findings, the therapeutic advantages of DRD4 stimulation remain elusive.

OBJECTIVES

The investigation focused on the behavioural effects of the recently developed DRD4 agonist, APH199, to evaluate its impact on anxiety, anhedonia, behavioural despair, establishment and retrieval of alcohol reinforcement, and amphetamine (AMPH)-induced symptoms.

METHODS

Male C57BL/6 J mice and Sprague-Dawley rats were examined in five independent experiments. We assessed APH199 (0.1-5 mg/kg, i.p.) effects on a broad range of behavioural parameters in the open field (OF) test, conditioned place preference test (CPP), elevated plus maze (EPM), light-dark box (LDB), novelty suppressed feeding (NSF), forced swim test (FST), sucrose preference test (SPT), AMPH-induced hyperlocomotion test (AIH), and prepulse inhibition (PPI) of the acoustic startle response in AMPH-sensitized rats.

RESULTS

APH199 caused mild and sporadic anxiolytic and antidepressant effects in EPM and FST, but no remarkable impact on behaviour in other tests in mice. However, we found a significant increase in AMPH-induced hyperactivity, suggesting an exaggeration of the psychotic-like responses in the AMPH-sensitized rats.

CONCLUSIONS

Our data challenged the hypothesis of the therapeutic benefits of DRD4 agonists, pointing out a possible aggravation of psychosis. We suggest a need for further preclinical studies to ensure the safety of antipsychotics with DRD4 stimulating properties.

Differential Effect of Dopamine D4 Receptor Activation on Low-Frequency Oscillations in the Prefrontal Cortex and Hippocampus May Bias the Bidirectional Prefrontal–Hippocampal Coupling

Dopamine D4 receptor (D4R) mechanisms are implicated in psychiatric diseases characterized by cognitive deficits, including schizophrenia, ADHD, and autism. The cellular mechanisms are poorly understood, but impaired neuronal synchronization in cortical networks was proposed to contribute to these deficits. In animal experiments, D4R activation was shown to generate aberrant increased gamma oscillations and to reduce performance on cognitive tasks requiring functional prefrontal cortex (PFC) and hippocampus (HPC) networks. While fast oscillations in the gamma range are important for local synchronization within neuronal ensembles, long-range synchronization between distant structures is achieved by slow rhythms in the delta, theta, alpha ranges. The characteristics of slow oscillations vary between structures during cognitive tasks. HPC activity is dominated by theta rhythm, whereas PFC generates unique oscillations in the 2–4 Hz range. In order to investigate the role of D4R on slow rhythms, cortical activity was recorded in rats under urethane anesthesia in which slow oscillations can be elicited in a controlled manner without behavioral confounds, by electrical stimulation of the brainstem reticular formation. The local field potential segments during stimulations were extracted and subjected to fast Fourier transform to obtain power density spectra. The selective D4R agonist A-412997 (5 and 10 mg/kg) and antagonists L-745870 (5 and 10 mg/kg) were injected systemically and the peak power in the two frequency ranges were compared before and after the injection. We found that D4R compounds significantly changed the activity of both HPC and PFC, but the direction of the effect was opposite in the two structures. D4R agonist enhanced PFC slow rhythm (delta, 2–4 Hz) and suppressed HPC theta, whereas the antagonist had an opposite effect. Analogous changes of the two slow rhythms were also found in the thalamic nucleus reuniens, which has connections to both forebrain structures. Slow oscillations play a key role in interregional cortical coupling; delta and theta oscillations were shown in particular, to entrain neuronal firing and to modulate gamma activity in interconnected forebrain structures with a relative HPC theta dominance over PFC. Thus, the results of this study indicate that D4R activation may introduce an abnormal bias in the bidirectional PFC–HPC coupling which can be reversed by D4R antagonists.

Frontal-posterior functional imbalance and aberrant function developmental patterns in schizophrenia

Schizophrenia (SZ) is a neurodevelopmental disorder. There remain significant gaps in understanding the neural trajectory across development in SZ. A major research focus is to clarify the developmental functional changes of SZ and to identify the specific timing, the specific brain regions, and the underlying mechanisms of brain alterations during SZ development. Regional homogeneity (ReHo) characterizing brain function was collected and analyzed on humans with SZ (hSZ) and healthy controls (HC) cross-sectionally, and methylazoxymethanol acetate (MAM) rats, a neurodevelopmental model of SZ, and vehicle rats longitudinally from adolescence to adulthood. Metabolomic and proteomic profiling in adult MAM rats and vehicle rats was examined and bioanalyzed. Compared to HC or adult vehicle rats, similar ReHo alterations were observed in hSZ and adult MAM rats, characterized by increased frontal (medial prefrontal and orbitofrontal cortices) and decreased posterior (visual and associated cortices) ReHo. Longitudinal analysis of MAM rats showed aberrant ReHo patterns as decreased posterior ReHo in adolescence and increased frontal and decreased posterior ReHo in adulthood. Accordingly, it was suggested that the visual cortex was a critical locus and adolescence was a sensitive window in SZ development. In addition, metabolic and proteomic alterations in adult MAM rats suggested that central carbon metabolism disturbance and mitochondrial dysfunction were the potential mechanisms underlying the ReHo alterations. This study proposed frontal-posterior functional imbalance and aberrant function developmental patterns in SZ, suggesting that the adolescent visual cortex was a critical locus and a sensitive window in SZ development. These findings from linking data between hSZ and MAM rats may have a significant translational contribution to the development of effective therapies in SZ.

Decreased cortical Nrf2 gene expression in autism and its relationship to thiol and cobalamin status

Nuclear factor erythroid 2-related factor 2 (Nrf2) promotes expression of a large number of antioxidant genes and multiple studies have described oxidative stress and impaired methylation in autism spectrum disorder (ASD), including decreased brain levels of methylcobalamin(III) (MeCbl). Here we report decreased expression of the Nrf2 gene (NFE2L2) in frontal cortex of ASD subjects, as well as differences in other genes involved in redox homeostasis. In pooled control and ASD correlation analyses, hydroxocobalamin(III) (OHCbl) was inversely correlated with NFE2L2 expression, while MeCbl and total cobalamin abundance were positively correlated with NFE2L2 expression. Levels of methionine, S-adenosylmethionine (SAM), S-adenosylhomocysteine (SAH) and cystathionine were positively correlated with NFE2L2 expression, while homocysteine (HCY) was negatively correlated. The relationship between Nrf2 activity and cobalamin was further supported by a bioinformatics-based comparison of cobalamin levels in different tissues with expression of a panel of 40 Nrf2-regulated genes, which yielded a strong correlation. Lastly, Nrf2-regulated gene expression was also correlated with expression of intracellular cobalamin trafficking and processing genes, such as MMADHC and MTRR. These findings highlight a previously unrecognized relationship between the antioxidant-promoting role of Nrf2 and cobalamin status, which is dysfunctional in ASD.

Chronic Restraint Stress Affects Network Oscillations in the Anterior Cingulate Cortex in Mice

The anterior cingulate cortex (ACC) is vulnerable to stress. Its dysfunction is observed in psychiatric disorders manifested as alterations in network oscillations. Mechanisms linking stress load to disturbed emotional-cognitive behaviors are of essential importance to further elucidate therapeutic strategies for psychiatric diseases. Here, we analyzed the effects of chronic restraint stress (CRS) load in juvenile mice on kainic acid (KA)-induced network oscillations in ACC slice preparations and on the forced swim test (FST). The immobility time (IT) was shortened at the beginning of the FST in CRS mice. Power spectral density (PSD) obtained from KA-induced oscillations in field potentials in the superficial layers of the ACC were altered in slices from the CRS mice. The PSD was decreased in CRS mice at the alpha (8-12 Hz), beta (13-30 Hz), low gamma (30-50 Hz), and high gamma (50-80 Hz) components. Noradrenaline increased the PSD of the theta (3-8 Hz) components in both the control and CRS groups, and also in alpha components only in the CRS group. Dopamine did not modulate the PSD of any frequency components in the control mice, whereas it enhanced the PSD of theta and alpha components in CRS mice. It was suggested that chronic stress load affects the dynamics of the network oscillations in the ACC with enhanced cathecolaminergic modulation.

Dopamine Modulation of Prefrontal Cortex Activity Is Manifold and Operates at Multiple Temporal and Spatial Scales

Although the function of dopamine in subcortical structures is largely limited to reward and movement, dopamine neurotransmission in the prefrontal cortex (PFC) is critical to a multitude of temporally and functionally diverse processes, such as attention, working memory, behavioral flexibility, action planning, and sustained motivational and affective states. How does dopamine influence computation of these temporally complex functions? We find causative links between sustained and burst patterns of phasic dopamine neuron activation and modulation of medial PFC neuronal activity at multiple spatiotemporal scales. These include a multidirectional and weak impact on individual neuron rate activity but a robust influence on coordinated ensemble activity, gamma oscillations, and gamma-theta coupling that persisted for minutes. In addition, PFC network responses to burst pattern of dopamine firing were selectively strengthened in behaviorally active states. This multiplex mode of modulation by dopamine input may enable PFC to compute and generate spatiotemporally diverse and specialized outputs.

Sepsis induced cognitive impairments by disrupting hippocampal parvalbumin interneuron-mediated inhibitory network via a D4-receptor mechanism

Patients who suffer sepsis often develop cognitive impairments, yet the underlying mechanisms largely remain to be elucidated. Increasing evidence has suggested that parvalbumin (PV) interneurons are required for the synchronization of neural activities and higher brain processes, whereas its dysfunction is implicated in many psychiatric disorders. In the present study, we examined the role of hippocampal PV interneuron-mediated inhibitory network in a rat model of polymicrobial sepsis induced by cecal ligation and puncture (CLP) and also explored the underlying mechanism. Here we showed that CLP-induced cognitive impairments, which were accompanied by significantly decreased expressions of PV and dopamine 4 (D4) receptor, decreased slow γ oscillation band, and reduced frequency of miniature inhibitory postsynaptic currents (mIPSCs). Notably, D4 receptor agonist RO-10-5824 treatment was able to reverse most of these abnormities. In summary, our study suggests that sepsis might disrupt PV interneuron-mediated network function that is dependent on the D4 receptor, leading to abnormal γ oscillation and consequent cognitive impairments.

Methylation-related metabolic effects of D4 dopamine receptor expression and activation

D4 dopamine receptor (D4R) activation uniquely promotes methylation of plasma membrane phospholipids, utilizing folate-derived methyl groups provided by methionine synthase (MS). We evaluated the impact of D4R expression on folate-dependent phospholipid methylation (PLM) and MS activity, as well as cellular redox and methylation status, in transfected CHO cells expressing human D4R variants containing 2, 4, or 7 exon III repeats (D4.2R, D4.4R, D4.7R). Dopamine had no effect in non-transfected CHO cells, but increased PLM to a similar extent for both D4.2R- and D4.4R-expressing cells, while the maximal increase was for D4.7R was significantly lower. D4R expression in CHO cells decreased basal MS activity for all receptor subtypes and conferred dopamine-sensitive MS activity, which was greater with a higher number of repeats. Consistent with decreased MS activity, D4R expression decreased basal levels of methylation cycle intermediates methionine, S-adenosylmethionine (SAM), and S-adenosylhomocysteine (SAH), as well as cysteine and glutathione (GSH). Conversely, dopamine stimulation increased GSH, SAM, and the SAM/SAH ratio, which was associated with a more than 2-fold increase in global DNA methylation. Our findings illustrate a profound influence of D4R expression and activation on MS activity, coupled with the ability of dopamine to modulate cellular redox and methylation status. These previously unrecognized signaling activities of the D4R provide a unique link between neurotransmission and metabolism.

Inferring the direction of rhythmic neural transmission via inter-regional phase-amplitude coupling (ir-PAC)

Phase-amplitude coupling (PAC) estimates the statistical dependence between the phase of a low-frequency component and the amplitude of a high-frequency component of local field potentials (LFP). To date PAC has been mainly applied to one signal. In this work, we introduce a new application of PAC to two LFPs and suggest that it can be used to infer the direction and strength of rhythmic neural transmission between distinct brain networks. This hypothesis is based on the accumulating evidence that transmembrane currents related to action potentials contribute a broad-band component to LFP in the high-gamma band, and PAC calculated between the amplitude of high-gamma (>60 Hz) in one LFP and the phase of a low-frequency oscillation (e.g., theta) in another would therefore relate the output (spiking) of one area to the input (somatic/dendritic postsynaptic potentials) of the other. We tested the hypothesis on theta-band long range communications between hippocampus and prefrontal cortex (PFC) and theta-band short range communications between dentate gyrus (DG) and the Ammon’s horn (CA1) within the hippocampus. The ground truth was provided by the known anatomical connections predicting hippocampus → PFC and DG → CA1, i.e., theta transmission is unidirectional in both cases: from hippocampus to PFC and from DG to CA1 along the tri-synaptic pathway within hippocampus. We found that (1) hippocampal high-gamma amplitude was significantly coupled to PFC theta phase, but not vice versa; (2) similarly, DG high-gamma amplitude was significantly coupled to CA1 theta phase, but not vice versa, and (3) the DG high-gamma-CA1 theta PAC was significantly correlated with DG → CA1 Granger causality, a well-established analytical measure of directional neural transmission. These results support the hypothesis that inter-regional PAC (ir-PAC) can be used to relate the output of a rhythmic “driver” network (i.e., high gamma) to the input of a rhythmic “receiver” network (i.e., theta) and thereby establish the direction and strength of rhythmic neural transmission.

The effect of foot reflexotherapy on the dynamics of cortical oscillatory waves in healthy humans: An EEG study

BACKGROUND

Foot reflexotherapy is a noninvasive complementary therapy that has gained considerable application in several fields of human endeavor. The therapy is used to relieve the symptoms of several ailments. For instance, foot reflexotherapy when applied to the cortical areas of the left or right foot relieves pain and stress. However, the electrophysiological mechanisms of the effect of foot reflexotherapy on cortical activity are not completely understood. While it has been shown that foot reflexotherapy exert positive effects on brain functions, little is known about the effects of this therapy on cortical activities as recorded with electroencephalogram (EEG) in healthy humans. Cortical activity is widely investigated with EEG, a noninvasive recording that is used to study brain activity in different functional states and conditions.

AIM

The aim of this study was to investigate the effect of foot reflexotherapy on EEG rhythms in healthy humans.

MATERIAL AND METHODS

EEG recording before and after reflexological therapy was carried out in seven healthy right-handed males who volunteered for the study.

RESULTS

Analysis of EEG data revealed activation offrontal cortex that resulted to significant increase in beta and gamma spectral powers after foot reflexotherapy (p ˂ 0.05).

CONCLUSION

Foot reflexotherapy is associated with increase in spectral powers in beta and gamma frequency bands. Therefore cortical beta and gamma waves of the EEG could be used as measures of functional activation of the brain, related to foot reflexotherapy.

Dopamine Receptors Influence Internally Generated Oscillations during Rule Processing in Primate Prefrontal Cortex

Neural oscillations in distinct frequency bands in the prefrontal cortex (pFC) are associated with specialized roles during cognitive control. How dopamine modulates oscillations to structure pFC functions remains unknown. We trained macaques to switch between two numerical rules and recorded local field potentials from pFC while applying dopamine receptor targeting drugs using microiontophoresis. We show that the D1 and D2 family receptors (D1Rs and D2Rs, respectively) specifically altered internally generated prefrontal oscillations, whereas sensory-evoked potentials remained unchanged. Blocking D1Rs or stimulating D2Rs increased low-frequency theta and alpha oscillations known to be involved in learning and memory. In contrast, only D1R inhibition enhanced high-frequency beta oscillations, whereas only D2R stimulation increased gamma oscillations linked to top–down and bottom–up attentional processing. These findings suggest that dopamine alters neural oscillations relevant for executive functioning through dissociable actions at the receptor level.

Isopeptide and ester bond ubiquitination both regulate degradation of the human dopamine receptor 4

How an optimal level of human dopamine D4 receptor (hD4R) is maintained in synaptic membranes is not known. We show here that hD4R is ubiquitinated in primary neurons. We go on to show that ubiquitin is attached to hD4R through isopeptide and ester bonds. When lysine (Lys) residues of the hD4R are substituted with arginine (Arg) residues, cellular hD4R protein levels increase. A synergistic effect on hD4R levels is noted when cytoplasmic serine (Ser) and threonine (Thr) residues are mutated. Chloroquine, an inhibitor of lysosomal degradation, did not have an effect on hD4R protein levels. However, treatment with bortezomib, an inhibitor of the 20S proteasome, caused a dose-dependent increase in hD4R protein levels. The effect of bortezomib was attenuated in the receptor variants that lacked Lys or Ser/Thr residues, and the hD4R mutant that lacked 17 cytoplasmic Lys, Ser, and Thr residues was nearly insensitive to bortezomib treatment. We conclude that both isopeptide and ester bond ubiquitination regulate proteasomal degradation of hD4R.

Neuronal correlates of ketamine and walking induced gamma oscillations in the medial prefrontal cortex and mediodorsal thalamus

Alterations in the function of the medial prefrontal cortex (mPFC) and its major thalamic source of innervation, the mediodorsal (MD) thalamus, have been hypothesized to contribute to the symptoms of schizophrenia. The NMDAR antagonist ketamine, used to model schizophrenia, elicits a brain state resembling early stage schizophrenia characterized by cognitive deficits and increases in cortical low gamma (40-70 Hz) power. Here we sought to determine how ketamine differentially affects spiking and gamma local field potential (LFP) activity in the rat mPFC and MD thalamus. Additionally, we investigated the ability of drugs targeting the dopamine D4 receptor (D4R) to modify the effects of ketamine on gamma activity as a measure of potential cognitive therapeutic efficacy. Rats were trained to walk on a treadmill to reduce confounds related to hyperactivity after ketamine administration (10 mg/kg s.c.) while recordings were obtained from electrodes chronically implanted in the mPFC and MD thalamus. Ketamine increased gamma LFP power in mPFC and MD thalamus in a similar frequency range, yet did not increase thalamocortical synchronization. Ketamine also increased firing rates and spike synchronization to gamma oscillations in the mPFC but decreased both measures in MD thalamus. Conversely, walking alone increased both firing rates and spike-gamma LFP correlations in both mPFC and MD thalamus. The D4R antagonist alone (L-745,870) had no effect on gamma LFP power during treadmill walking, although it reversed increases induced by the D4R agonist (A-412997) in both mPFC and MD thalamus. Neither drug altered ketamine-induced changes in gamma power or firing rates in the mPFC. However, in MD thalamus, the D4R agonist increased ketamine-induced gamma power and prevented ketamine's inhibitory effect on firing rates. Results provide new evidence that ketamine differentially modulates spiking and gamma power in MD thalamus and mPFC, supporting a potential role for both areas in contributing to ketamine-induced schizophrenia-like symptoms.

Decoding emotion of the other differs among schizophrenia patients and schizoaffective patients: A pilot study

The deficit in ability to attribute mental states such as thoughts, beliefs, and intentions of another person is a key component in the functional impairment of social cognition in schizophrenia. In the current study, we compared the ability of persons with first episode schizophrenia (FE-SZ) and individuals with schizophrenia displaying symptomatic remission (SZ-CR) to decode the mental state of others with healthy individuals and schizoaffective patients. In addition, we analyzed the effect of dopamine-related genes polymorphism on the ability to decode the mental state of another, and searched for different genetic signatures. Our results show that overall, individuals with schizophrenia performed worse in the "Reading the Mind in the Eyes" (eyes) test, a simple well-defined task to infer the mental state of others than healthy individuals. Within the schizophrenia group, schizoaffective scored significantly higher than FE-SZ, SZ-CR, and healthy individuals. No difference was observed in performance between FE-SZ and SZ-CR subjects. Interestingly, FE-SZ and SZ-CR, but not schizoaffective individuals, performed worse in decoding negative and neutral emotional valance than the healthy control group. At the genetic level, we observed a significant effect of the DAT genotype, but not D4R genotype, on the eyes test performance. Our data suggest that understanding the mental state of another person is a trait marker of the illness, and might serve as an intermediate phenotype in the diagnostic process of schizophrenia disorders, and raise the possibility that DA-related DAT gene might have a role in decoding the mental state of another person.

Cellular and circuit models of increased resting-state network gamma activity in schizophrenia

Schizophrenia (SCZ) is a disorder characterized by positive symptoms (hallucinations, delusions), negative symptoms (blunted affect, alogia, reduced sociability, and anhedonia), as well as persistent cognitive deficits (memory, concentration, and learning). While the biology underlying subjective experiences is difficult to study, abnormalities in electroencephalographic (EEG) measures offer a means to dissect potential circuit and cellular changes in brain function. EEG is indispensable for studying cerebral information processing due to the introduction of techniques for the decomposition of event-related activity into its frequency components. Specifically, brain activity in the gamma frequency range (30-80Hz) is thought to underlie cognitive function and may be used as an endophenotype to aid in diagnosis and treatment of SCZ. In this review we address evidence indicating that there is increased resting-state gamma power in SCZ. We address how modeling this aspect of the illness in animals may help treatment development as well as providing insights into the etiology of SCZ.

Aberrant high frequency oscillations recorded in the rat nucleus accumbens in the methylazoxymethanol acetate neurodevelopmental model of schizophrenia

BACKGROUND

Altered activity of the nucleus accumbens (NAc) is thought to be a core feature of schizophrenia and animal models of the disease. Abnormal high frequency oscillations (HFO) in the rat NAc have been associated with pharmacological models of schizophrenia, in particular the N-methyl-d-aspartate receptor (NMDAR) hypofunction model. Here, we tested the hypothesis that abnormal HFO are also associated with a neurodevelopmental rat model.

METHODS

Using prenatal administration of the mitotoxin methylazoxymethanol acetate (MAM) we obtained the offspring MAM rats. Adult MAM and Sham rats were implanted with electrodes, for local field potential recordings, in the NAc.

RESULTS

Spontaneous HFO (spHFO) in MAM rats were characterized by increased power and frequency relative to Sham rats. MK801 dose-dependently increased the power of HFO in both groups. However, the dose-dependent increase in HFO frequency found in Sham rats was occluded in MAM rats. The antipsychotic compound, clozapine reduced the frequency of HFO which was similar in both MAM and Sham rats. Further, HFO were modulated in a similar manner by delta oscillations in both MAM and Sham rats.

CONCLUSION

Together these findings suggest that increased HFO frequency represents an important feature in certain animal models of schizophrenia. These findings support the hypothesis that altered functioning of the NAc is a core feature in animal models of schizophrenia.

Spontaneous Gamma Activity in Schizophrenia

IMPORTANCE

A major goal of translational neuroscience is to identify neural circuit abnormalities in neuropsychiatric disorders that can be studied in animal models to facilitate the development of new treatments. Oscillations in the gamma band (30-100 Hz) of the electroencephalogram have received considerable interest as the basic mechanisms underlying these oscillations are understood, and gamma abnormalities have been found in schizophrenia (SZ). Animal models of SZ based on hypofunction of the N-methyl-d-aspartate receptor (NMDAR) demonstrate increased spontaneous broadband gamma power, but this phenomenon has not been identified clearly in patients with SZ.

OBJECTIVE

To examine spontaneous gamma power and its relationship to evoked gamma oscillations in the auditory cortex of patients with SZ.

DESIGN, SETTING, AND PARTICIPANTS

We performed a cross-sectional study including 24 patients with chronic SZ and 24 matched healthy control participants at the Veterans Affairs Boston Healthcare System from January 1, 2009, through December 31, 2012. Electroencephalograms were obtained during auditory steady-state stimulation at multiple frequencies (20, 30, and 40 Hz) and during a resting state in 18 participants in each group.

MAIN OUTCOMES AND MEASURES

Electroencephalographic activity in the auditory cortex was estimated using dipole source localization. Auditory steady-state response (ASSR) measures included the phase-locking factor and evoked power. Spontaneous gamma power was measured as induced (non-phase-locked) gamma power in the ASSR data and as total gamma power in the resting-state data.

RESULTS

The ASSR phase-locking factor was reduced significantly in patients with SZ compared with controls for the 40-Hz stimulation (mean [SD], 0.075 [0.028] vs 0.113 [0.065]; F1,46 = 6.79 [P = .012]) but not the 20- or the 30-Hz stimulation (0.042 [0.038] vs 0.043 [0.034]; F1,46 = 0.006 [P = .938] and 0.084 [0.040] vs 0.098 [0.050]; F1,46 = 1.605 [P = .212], respectively), repeating previous findings. The mean [SD] broadband-induced (30-100 Hz) gamma power was increased in patients with SZ compared with controls during steady-state stimulation (6.579 [3.783] vs 3.984 [1.843]; F1,46 = 9.128 [P = .004]; d = 0.87) but not during rest (0.006 [0.003] vs 0.005 [0.002]; F1,34 = 1.067 [P = .309]; d = 0.35). Induced gamma power in the left hemisphere of the patients with SZ during the 40-Hz stimulation was positively correlated with auditory hallucination symptoms (tangential, ρ = 0.587 [P = .031]; radial, ρ = 0.593 [P = .024]) and negatively correlated with the ASSR phase-locking factor (baseline: ρ = -0.572 [P = .024]; ASSR: ρ = -0.568 [P = .032]).

CONCLUSIONS AND RELEVANCE

Spontaneous gamma activity is increased during auditory steady-state stimulation in SZ, reflecting a disruption in the normal balance of excitation and inhibition. This phenomenon interacts with evoked oscillations, possibly contributing to the gamma ASSR deficit found in SZ. The similarity of increased spontaneous gamma power in SZ to the findings of increased spontaneous gamma power in animal models of NMDAR hypofunction suggests that spontaneous gamma power could serve as a biomarker for the integrity of NMDARs on parvalbumin-expressing inhibitory interneurons in humans and in animal models of neuropsychiatric disorders.

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.

Altered resting-state EEG source functional connectivity in schizophrenia: the effect of illness duration

Despite the increasing body of evidence supporting the hypothesis of schizophrenia as a disconnection syndrome, studies of resting-state EEG Source Functional Connectivity (EEG-SFC) in people affected by schizophrenia are sparse. The aim of the present study was to investigate resting-state EEG-SFC in 77 stable, medicated patients with schizophrenia (SCZ) compared to 78 healthy volunteers (HV). In order to study the effect of illness duration, SCZ were divided in those with a short duration of disease (SDD; n = 25) and those with a long duration of disease (LDD; n = 52). Resting-state EEG recordings in eyes closed condition were analyzed and lagged phase synchronization (LPS) indices were calculated for each ROI pair in the source-space EEG data. In delta and theta bands, SCZ had greater EEG-SFC than HV; a higher theta band connectivity in frontal regions was observed in LDD compared with SDD. In the alpha band, SCZ showed lower frontal EEG-SFC compared with HV whereas no differences were found between LDD and SDD. In the beta1 band, SCZ had greater EEG-SFC compared with HVs and in the beta2 band, LDD presented lower frontal and parieto-temporal EEG-SFC compared with HV. In the gamma band, SDD had greater connectivity values compared with LDD and HV. This study suggests that resting state brain network connectivity is abnormally organized in schizophrenia, with different patterns for the different EEG frequency components and that EEG can be a powerful tool to further elucidate the complexity of such disordered connectivity.

Fast oscillatory activity in the anterior cingulate cortex: dopaminergic modulation and effect of perineuronal net loss

Dopamine release in the prefrontal cortex plays a critical role in cognitive function such as working memory, attention and planning. Dopamine exerts complex modulation on excitability of pyramidal neurons and interneurons, and regulates excitatory and inhibitory synaptic transmission. Because of the complexity of this modulation, it is difficult to fully comprehend the effect of dopamine on neuronal network activity. In this study, we investigated the effect of dopamine on local high-frequency oscillatory neuronal activity (in β band) in slices of the mouse anterior cingulate cortex (ACC). We found that dopamine enhanced the power of these oscillations induced by kainate and carbachol, but did not affect their peak frequency. Activation of D2R and in a lesser degree D1R increased the oscillation power, while activation of D4R had no effect. These high-frequency oscillations in the ACC relied on both phasic inhibitory and excitatory transmission and functional gap junctions. Thus, dopamine released in the ACC promotes high-frequency synchronized local cortical activity which is known to favor information transfer, fast selection and binding of distributed neuronal responses. Finally, the power of these oscillations was significantly enhanced after degradation of the perineuronal nets (PNNs) enwrapping most parvalbumin interneurons. This study provides new insights for a better understanding of the abnormal prefrontal gamma activity in schizophrenia (SZ) patients who display prefrontal anomalies of both the dopaminergic system and the PNNs.

Autism: a redox/methylation disorder

While autism is still a mysterious developmental disorder, expansion of research efforts over the past 10 to 15 years has yielded a number of important clues implicating both genetic and environmental factors. We can now assert with a measure of confidence that contemporary autism reflects the combined impact of multiple environmental factors on the processes that regulate development in genetically vulnerable individuals. Since epigenetic regulation of gene expression is acknowledged as the most critical factor in development and DNA methylation (the addition of a carbon atom at discrete locations) is the fundamental event for epigenetic regulation, dysfunctional methylation can be considered as a likely cause of autism. Since methylation activity is highly sensitive to oxidative stress (an abnormal redox state) and many environmental factors promote oxidative stress, we have proposed a redox/methylation hypothesis for autism causation. The narrative herein describes the evolution of this hypothesis, which is essentially a series of linked discoveries about how the brain uniquely relies on oxidation and methylation to guide its development and to carry out its cognitive functions.