Glutamatergic deficits and parvalbumin-containing inhibitory neurons in the prefrontal cortex in schizophrenia

Research Progress on NMDA Receptor Enhancement Drugs for the Treatment of Depressive Disorder

Major depressive disorder (MDD) is a severe mental illness with a complex etiology. Currently, many medications employed in clinical treatment exhibit limitations such as delayed onset of action and a high incidence of adverse reactions. Therefore, there is a pressing need to develop antidepressants that exhibit enhanced efficacy and safety. The N-methyl-D-aspartate receptor (NMDAR), a distinctive glutamate-gated ion channel receptor, has been implicated in the onset and progression of depressive disorder, as evidenced by both preclinical and clinical research. The NMDAR antagonist, ketamine, exhibits rapid and sustained antidepressant effects, holding promise as a novel therapeutic approach for depressive disorder. However, its psychotomimetic impact and potential for addiction have restricted its widespread clinical application. Notably, over the past decade, studies have suggested that enhancing NMDAR functionality can produce antidepressant effects with improved safety, especially with the emergence of NMDAR-positive allosteric modulators (PAMs). We view this as a potential novel strategy for treating depression, forming the basis for the narrative review that follows.

Transcriptional and cellular response of hiPSC-derived microglia-neural progenitor co-cultures exposed to IL-6

Elevated interleukin (IL-)6 levels during prenatal development have been linked to increased risk for neurodevelopmental disorders (NDD) in the offspring, but the mechanism remains unclear. Human-induced pluripotent stem cell (hiPSC) models offer a valuable tool to study the effects of IL-6 on features relevant for human neurodevelopment in vitro. We previously reported that hiPSC-derived microglia-like cells (MGLs) respond to IL-6, but neural progenitor cells (NPCs) in monoculture do not. Therefore, we investigated whether co-culturing hiPSC-derived MGLs with NPCs would trigger a cellular response to IL-6 stimulation via secreted factors from the MGLs. Using N=4 donor lines without psychiatric diagnosis, we first confirmed that NPCs can respond to IL-6 through trans-signalling when recombinant IL-6Ra is present, and that this response is dose-dependent. MGLs secreted soluble IL-6R, but at lower levels than found in vivo and below that needed to activate trans-signalling in NPCs. Whilst transcriptomic and secretome analysis confirmed that MGLs undergo substantial transcriptomic changes after IL-6 exposure and subsequently secrete a cytokine milieu, NPCs in co-culture with MGLs exhibited a minimal transcriptional response. Furthermore, there were no significant cell fate-acquisition changes when differentiated into post-mitotic cultures, nor alterations in synaptic densities in mature neurons. These findings highlight the need to investigate if trans-IL-6 signalling to NPCs is a relevant disease mechanism linking prenatal IL-6 exposure to increased risk for psychiatric disorders. Moreover, our findings underscore the importance of establishing more complex in vitro human models with diverse cell types, which may show cell-specific responses to microglia-released cytokines to fully understand how IL-6 exposure may influence human neurodevelopment.

Bone marrow-derived myeloid cells transiently colonize the brain during postnatal development and interact with glutamatergic synapses

Although the roles of embryonic yolk sac-derived, resident microglia in neurodevelopment were extensively studied, the possible involvement of bone marrow-derived cells remains elusive. In this work, we used a fate-mapping strategy to selectively label bone marrow-derived cells and their progeny in the brain (FLT3+IBA1+). FLT3+IBA1+ cells were confirmed to be transiently present in the healthy brain during early postnatal development. FLT3+IBA1+ cells have a distinct morphology index at postnatal day(P)0, P7, and P14 compared with neighboring microglia. FLT3+IBA1+ cells also express the microglial markers P2RY12 and TMEM119 and interact with VGLUT1 synapses at P14. Scanning electron microscopy indeed showed that FLT3+ cells contact and engulf pre-synaptic elements. Our findings suggest FLT3+IBA1+ cells might assist microglia in their physiological functions in the developing brain including synaptic pruning which is performed using their purinergic sensors. Our findings stimulate further investigation on the involvement of peripheral macrophages during homeostatic and pathological development.

Activation of prefrontal parvalbumin interneurons ameliorates working memory deficit even under clinically comparable antipsychotic treatment in a mouse model of schizophrenia

One of the critical unmet medical needs in schizophrenia is the treatment for cognitive deficits. However, the neural circuit mechanisms of them remain unresolved. Previous studies utilizing animal models of schizophrenia did not consider the fact that patients with schizophrenia generally cannot discontinue antipsychotic medication due to the high risk of relapse. Here, we used multi-dimensional approaches, including histological analysis of the prelimbic cortex (PL), LC-MS/MS-based in vivo dopamine D2 receptor occupancy analysis for antipsychotics, in vivo calcium imaging, and behavioral analyses of mice using chemogenetics to investigate neural mechanisms and potential therapeutic strategies for working memory deficit in a chronic phencyclidine (PCP) mouse model of schizophrenia. Chronic PCP administration led to alterations in excitatory and inhibitory synapses, specifically in dendritic spines of pyramidal neurons, vesicular glutamate transporter 1 (VGLUT1) positive terminals, and parvalbumin (PV) positive GABAergic interneurons located in layer 2-3 of the PL. Continuous administration of olanzapine, which achieved a sustained therapeutic window of dopamine D2 receptor occupancy (60-80%) in the striatum, did not ameliorate these synaptic abnormalities and working memory deficit in the chronic PCP-treated mice. We demonstrated that chemogenetic activation of PV neurons in the PL, as confirmed by in vivo calcium imaging, ameliorated working memory deficit in this model even under clinically comparable olanzapine treatment which by itself inhibited only PCP-induced psychomotor hyperactivity. Our study suggests that targeting prefrontal PV neurons could be a promising therapeutic intervention for cognitive deficits in schizophrenia in combination with antipsychotic medication.

Use of Quantitative Electroencephalography to Inform Age- and Sex-Related Differences in NMDA Receptor Function Following MK-801 Administration

Sex- and age-related differences in symptom prevalence and severity have been widely reported in patients with schizophrenia, yet the underlying mechanisms contributing to these differences are not well understood. N-methyl-D-aspartate (NMDA) receptor hypofunction contributes to schizophrenia pathology, and preclinical models often use NMDA receptor antagonists, including MK-801, to model all symptom clusters. Quantitative electroencephalography (qEEG) represents a translational approach to measure neuronal activity, identify targetable biomarkers in neuropsychiatric disorders and evaluate possible treatments. Abnormalities in gamma power have been reported in patients with schizophrenia and correspond to psychosis and cognitive impairment. Further, as gamma power reflects cortical glutamate and GABA signaling, it is highly sensitive to changes in NMDA receptor function, and NMDA receptor antagonists aberrantly increase gamma power in rodents and humans. To evaluate the role of sex and age on NMDA receptor function, MK-801 (0.03-0.3 mg/kg, SC) was administered to 3- and 9-month-old male and female Sprague-Dawley rats that were implanted with wireless EEG transmitters to measure cortical brain function. MK-801-induced elevations in gamma power were observed in 3-month-old male and female and 9-month-old male rats. In contrast, 9-month-old female rats demonstrated blunted maximal elevations across a wide dose range. Importantly, MK-801-induced hyperlocomotor effects, a common behavioral screen used to examine antipsychotic-like activity, were similar across all groups. Overall, sex-by-age-related differences in gamma power support using qEEG as a translational tool to evaluate pathological progression and predict treatment response across a heterogeneous population.

N-methyl-D-aspartate receptor hypofunction as a potential contributor to the progression and manifestation of many neurological disorders

N-methyl-D-aspartate receptors (NMDA) are glutamate-gated ion channels critical for synaptic transmission and plasticity. A slight variation of NMDAR expression and function can result in devastating consequences, and both hyperactivation and hypoactivation of NMDARs are detrimental to neural function. Compared to NMDAR hyperfunction, NMDAR hypofunction is widely implicated in many neurological disorders, such as intellectual disability, autism, schizophrenia, and age-related cognitive decline. Additionally, NMDAR hypofunction is associated with the progression and manifestation of these diseases. Here, we review the underlying mechanisms of NMDAR hypofunction in the progression of these neurological disorders and highlight that targeting NMDAR hypofunction is a promising therapeutic intervention in some neurological disorders.

Computational Modeling of Oddball Sequence Processing Exposes Common and Differential Auditory Network Changes in First-Episode Schizophrenia-Spectrum Disorders and Schizophrenia

BACKGROUND AND HYPOTHESIS

Differences in sound relevance filtering in schizophrenia are proposed to represent a key index of biological changes in brain function in the illness. This study featured a computational modeling approach to test the hypothesis that processing differences might already be evident in first-episode, becoming more pronounced in the established illness.

STUDY DESIGN

Auditory event-related potentials to a typical oddball sequence (rare pitch deviations amongst regular sounds) were recorded from 90 persons with schizophrenia-spectrum disorders (40 first-episode schizophrenia-spectrum, 50 established illness) and age-matched healthy controls. The data were analyzed using dynamic causal modeling to identify the changes in effective connectivity that best explained group differences.

STUDY RESULTS

Group differences were linked to intrinsic (within brain region) connectivity changes. In activity-dependent measures these were restricted to the left auditory cortex in first-episode schizophrenia-spectrum but were more widespread in the established illness. Modeling suggested that both established illness and first-episode schizophrenia-spectrum groups expressed significantly lower inhibition of inhibitory interneuron activity and altered gain on superficial pyramidal cells with the data indicative of differences in both putative N-methyl-d-aspartate glutamate receptor activity-dependent plasticity and classic neuromodulation.

CONCLUSIONS

The study provides further support for the notion that examining the ability to alter responsiveness to structured sound sequences in schizophrenia and first-episode schizophrenia-spectrum could be informative to uncovering the nature and progression of changes in brain function during the illness. Furthermore, modeling suggested that limited differences present at first-episode schizophrenia-spectrum may become more expansive with illness progression.

Regulation of the E/I-balance by the neural matrisome

In the mammalian cortex a proper excitatory/inhibitory (E/I) balance is fundamental for cognitive functions. Especially γ-aminobutyric acid (GABA)-releasing interneurons regulate the activity of excitatory projection neurons which form the second main class of neurons in the cortex. During development, the maturation of fast-spiking parvalbumin-expressing interneurons goes along with the formation of net-like structures covering their soma and proximal dendrites. These so-called perineuronal nets (PNNs) represent a specialized form of the extracellular matrix (ECM, also designated as matrisome) that stabilize structural synapses but prevent the formation of new connections. Consequently, PNNs are highly involved in the regulation of the synaptic balance. Previous studies revealed that the formation of perineuronal nets is accompanied by an establishment of mature neuronal circuits and by a closure of critical windows of synaptic plasticity. Furthermore, it has been shown that PNNs differentially impinge the integrity of excitatory and inhibitory synapses. In various neurological and neuropsychiatric disorders alterations of PNNs were described and aroused more attention in the last years. The following review gives an update about the role of PNNs for the maturation of parvalbumin-expressing interneurons and summarizes recent findings about the impact of PNNs in different neurological and neuropsychiatric disorders like schizophrenia or epilepsy. A targeted manipulation of PNNs might provide an interesting new possibility to indirectly modulate the synaptic balance and the E/I ratio in pathological conditions.

Current advancements of modelling schizophrenia using patient-derived induced pluripotent stem cells

Schizophrenia (SZ) is a severe psychiatric disorder, with a prevalence of 1-2% world-wide and substantial health- and social care costs. The pathology is influenced by both genetic and environmental factors, however the underlying cause still remains elusive. SZ has symptoms including delusions, hallucinations, confused thoughts, diminished emotional responses, social withdrawal and anhedonia. The onset of psychosis is usually in late adolescence or early adulthood. Multiple genome-wide association and whole exome sequencing studies have provided extraordinary insights into the genetic variants underlying familial as well as polygenic forms of the disease. Nonetheless, a major limitation in schizophrenia research remains the lack of clinically relevant animal models, which in turn hampers the development of novel effective therapies for the patients. The emergence of human induced pluripotent stem cell (hiPSC) technology has allowed researchers to work with SZ patient-derived neuronal and glial cell types in vitro and to investigate the molecular basis of the disorder in a human neuronal context. In this review, we summarise findings from available studies using hiPSC-based neural models and discuss how these have provided new insights into molecular and cellular pathways of SZ. Further, we highlight different examples of how these models have shown alterations in neurogenesis, neuronal maturation, neuronal connectivity and synaptic impairment as well as mitochondrial dysfunction and dysregulation of miRNAs in SZ patient-derived cultures compared to controls. We discuss the pros and cons of these models and describe the potential of using such models for deciphering the contribution of specific human neural cell types to the development of the disease.

Targeting α6GABAA receptors as a novel therapy for schizophrenia: A proof-of-concept preclinical study using various animal models

GABA_A receptors containing α6 subunits (α6GABA_ARs) in the cerebellum have -been implicated in schizophrenia. It was reported that the GABA synthesizing enzymes were downregulated whereas α6GABA_ARs were upregulated in postmortem cerebellar tissues of patients with schizophrenia and in a rat model induced by chronic phencyclidine (PCP). We have previously demonstrated that pyrazoloquinolinone Compound 6, an α6GABA_AR-highly selective positive allosteric modulator (PAM), can rescue the disrupted prepulse inhibition (PPI) induced by methamphetamine (METH), an animal model mimicking the sensorimotor gating deficit based on the hyper-dopaminergic hypothesis of schizophrenia. Here, we demonstrate that not only Compound 6, but also its structural analogues, LAU463 and LAU159, with similarly high α6GABA_AR selectivity and their respective deuterated derivatives (DK-I-56-1, DK-I-58-1 and DK-I-59-1) can rescue METH-induced PPI disruption. Besides, Compound 6 and DK-I-56-I can also rescue the PPI disruption induced by acute administration of PCP, an animal model based on the hypo-glutamatergic hypothesis of schizophrenia. Importantly, Compound 6 and DK-I-56-I, at doses not affecting spontaneous locomotor activity, can also rescue impairments of social interaction and novel object recognition in mice induced by chronic PCP treatments. At similar doses, Compound 6 did not induce sedation but significantly suppressed METH-induced hyperlocomotion. Thus, α6GABA_AR-selective PAMs can rescue not only disrupted PPI but also hyperlocomotion, social withdrawal, and cognitive impairment, in both METH- and PCP-induced animal models mimicking schizophrenia, suggesting that they are a potential novel therapy for the three core symptoms, i.e. positive symptoms, negative symptoms, and cognitive impairment, of schizophrenia.

N-Methyl-d-Aspartate receptor and inflammation in dorsolateral prefrontal cortex in schizophrenia

Lower N-methyl-d-aspartate receptor (NMDAR) GluN1 subunit levels and heightened neuroinflammation are found in the cortex in schizophrenia. Since neuroinflammation can lead to changes in NMDAR function, it is possible that these observations are linked in schizophrenia. We aimed to extend our previous studies by measuring molecular indices of NMDARs that define key functional properties of this receptor - particularly the ratio of GluN2A and GluN2B subunits - in dorsolateral prefrontal cortex (DLPFC) from schizophrenia and control cases (37/37). We sought to test whether changes in these measures are specific to the subset of schizophrenia cases with high levels of inflammation-related mRNAs, defined as a high inflammatory subgroup. Quantitative autoradiography was used to detect 'functional' NMDARs ([³H]MK-801), GluN1-coupled-GluN2A subunits ([³H]CGP-39653), and GluN1-coupled-GluN2B subunits ([³H]Ifenprodil). Quantitative RT-PCR was used to measure NMDAR subunit transcripts (GRIN1, GRIN2A and GRIN2B). The ratios of GluN2A:GluN2B binding and GRIN2A:GRIN2B mRNAs were calculated as an index of putative NMDAR composition. We found: 1) GluN2A binding, and 2) the ratios of GluN2A:GluN2B binding and GRIN2A:GRIN2B mRNAs were lower in schizophrenia cases versus controls (p < 0.05), and 3) lower GluN2A:GluN2B binding and GRIN2A:GRIN2B mRNA ratios were exaggerated in the high inflammation/schizophrenia subgroup compared to the low inflammation/control subgroup (p < 0.05). No other NMDAR-related indices were significantly changed in the high inflammation/schizophrenia subgroup. This suggests that neuroinflammation may alter NMDAR stoichiometry rather than targeting total NMDAR levels overall, and future studies could aim to determine if anti-inflammatory treatment can alleviate this aspect of NMDAR-related pathology.

Mechanisms of synaptic transmission dysregulation in the prefrontal cortex: pathophysiological implications

The prefrontal cortex (PFC) serves as the chief executive officer of the brain, controlling the highest level cognitive and emotional processes. Its local circuits among glutamatergic principal neurons and GABAergic interneurons, as well as its long-range connections with other brain regions, have been functionally linked to specific behaviors, ranging from working memory to reward seeking. The efficacy of synaptic signaling in the PFC network is profundedly influenced by monoaminergic inputs via the activation of dopamine, adrenergic, or serotonin receptors. Stress hormones and neuropeptides also exert complex effects on the synaptic structure and function of PFC neurons. Dysregulation of PFC synaptic transmission is strongly linked to social deficits, affective disturbance, and memory loss in brain disorders, including autism, schizophrenia, depression, and Alzheimer's disease. Critical neural circuits, biological pathways, and molecular players that go awry in these mental illnesses have been revealed by integrated electrophysiological, optogenetic, biochemical, and transcriptomic studies of PFC. Novel epigenetic mechanism-based strategies are proposed as potential avenues of therapeutic intervention for PFC-involved diseases. This review provides an overview of PFC network organization and synaptic modulation, as well as the mechanisms linking PFC dysfunction to the pathophysiology of neurodevelopmental, neuropsychiatric, and neurodegenerative diseases. Insights from the preclinical studies offer the potential for discovering new medical treatments for human patients with these brain disorders.

Current findings and perspectives on aberrant neural oscillations in schizophrenia

There is now consistent evidence that neural oscillation at low- and high-frequencies constitute an important aspect of the pathophysiology of schizophrenia. Specifically, impaired rhythmic activity may underlie the deficit to generate coherent cognition and behavior, leading to the characteristic symptoms of psychosis and cognitive deficits. Importantly, the generating mechanisms of neural oscillations are relatively well-understood and thus enable the targeted search for the underlying circuit impairments and novel treatment targets. In the following review, we will summarize and assess the evidence for aberrant rhythmic activity in schizophrenia through evaluating studies that have utilized Electro/Magnetoencephalography to examine neural oscillations during sensory and cognitive tasks as well as during resting-state measurements. These data will be linked to current evidence from post-mortem, neuroimaging, genetics, and animal models that have implicated deficits in GABAergic interneurons and glutamatergic neurotransmission in oscillatory deficits in schizophrenia. Finally, we will highlight methodological and analytical challenges as well as provide recommendations for future research.

Mechanisms governing activity-dependent synaptic pruning in the developing mammalian CNS

Almost 60 years have passed since the initial discovery by Hubel and Wiesel that changes in neuronal activity can elicit developmental rewiring of the central nervous system (CNS). Over this period, we have gained a more comprehensive picture of how both spontaneous neural activity and sensory experience-induced changes in neuronal activity guide CNS circuit development. Here we review activity-dependent synaptic pruning in the mammalian CNS, which we define as the removal of a subset of synapses, while others are maintained, in response to changes in neural activity in the developing nervous system. We discuss the mounting evidence that immune and cell-death molecules are important mechanistic links by which changes in neural activity guide the pruning of specific synapses, emphasizing the role of glial cells in this process. Finally, we discuss how these developmental pruning programmes may go awry in neurodevelopmental disorders of the human CNS, focusing on autism spectrum disorder and schizophrenia. Together, our aim is to give an overview of how the field of activity-dependent pruning research has evolved, led to exciting new questions and guided the identification of new, therapeutically relevant mechanisms that result in aberrant circuit development in neurodevelopmental disorders.

Early-life lead exposure and neurodevelopmental disorders

Lead (Pb²⁺) exposure is a global public health problem of major proportion with an alarming number of children with blood Pb²⁺ levels > 10 >g/dL, twice the current CDC reference level for Pb²⁺ exposure. Mounting evidence from population-based studies suggests an association between chronic early life Pb²⁺ exposure (CELLE) and psychiatric disorders, specifically schizophrenia (SZ). Preclinical studies suggest a common mechanism in the pathophysiology of CELLE and SZ, NMDA receptor hypofunction. Here we describe human and experimental animal studies providing the evidence for such an association. Further, recent preclinical studies indicate that Pb²⁺-induced changes in neurotransmitter receptors that mediate the action(s) of drugs of abuse are increased in brain regions associated with addiction circuits in adolescence, a period of increased susceptibility to drug use and abuse and expression of psychiatric disease in humans. In summary, the relationship between the global burden of childhood Pb²⁺ exposure and the latent onset of psychiatric disorders and predisposition to drug use requires further investigations in human populations.

Parvalbumin interneuron vulnerability and brain disorders

Parvalbumin-expressing interneurons (PV-INs) are highly vulnerable to stressors and have been implicated in many neuro-psychiatric diseases such as schizophrenia, Alzheimer's disease, autism spectrum disorder, and bipolar disorder. We examined the literature about the current knowledge of the physiological properties of PV-INs and gathered results from diverse research areas to provide insight into their vulnerability to stressors. Among the factors that confer heightened vulnerability are the substantial energy requirements, a strong excitatory drive, and a unique developmental trajectory. Understanding these stressors and elaborating on their impact on PV-IN health is a step toward developing therapies to protect these neurons in various disease states and to retain critical brain functions.

The effect of rs1076560 (DRD2) and rs4680 (COMT) on tardive dyskinesia and cognition in schizophrenia subjects

OBJECTIVE

The aim of the study is to test the association of a functional variant each in DRD2 and COMT genes with schizophrenia and its endophenotypes.

BASIC METHODS

Effect of two functional variants rs1076560 in DRD2 and rs4680 in COMT on (1) schizophrenia (502 cases, 448 controls) diagnosed by Diagnostic and Statistical Manual of Mental Disorders-IV criteria and in subsets with (2) tardive dyskinesia (80 positive, 103 negative), assessed by Abnormal Involuntary Movement Scale (AIMS), positive and negative symptoms assessed by Positive and Negative Syndrome Scale (PANSS) and (3) cognition (299 cases, 245 controls), estimated by Penn Computerized Neurocognitive Battery, were analysed either using analysis of variance (ANOVA) or regression analysis.

MAIN RESULTS

No association of two SNPs with schizophrenia, but association of rs4680 (P < 0.05) with tardive dyskinesia was observed. On ANOVA, main effect of smoking [F(2,148) = 16.3; P = 3.9 × 10]; rs4680 [F(2,148) = 3.3; P = 0.04] and interaction effect of tardive dyskinesia-status*Smoking [F(2,148) = 5.4, P = 0.006]; Smoking*rs1076560 [F(3,148) = 3.6; P = 0.01]; Smoking*rs4680 [F(4,148) = 5.3; P = 4.7 × 10] were significant with AIMS tardive dyskinesia score. The main effect of rs1076560 [F(2,148) = 4.5; P = 0.013] and rs4680 [F(2,148) = 4.0; P = 0.02] were significant with limb truncal tardive dyskinesia. Allelic/genotypic (P = 0.004/P = 0.01) association of rs1076560 with negative scale of PANSS in tardive dyskinesia-negative; diminished expression factor of PANSS in tardive dyskinesia-negative subcohort (allelic/genotypic P = 3.3 × 10/6.6 × 10) and tardive dyskinesia cohorts (P = 0.003/0.002); genotypic association (P = 0.05) with disorganised/concrete factor in tardive dyskinesia-positive subcohorts were observed by regression analysis using gPLINKv2.050. Further allelic/genotypic (P = 0.02) association of rs4680 with depressed factor of PANSS in tardive dyskinesia cohort was observed. Allelic/genotypic association of rs1076560 with abstraction and mental flexibilityaccuracy (P = 0.03/0.04), abstraction and mental flexibilityefficiency (P = 0.01/0.02); allelic association with spatial abilityprocessing speed (P = 0.03), emotionefficiency (P = 0.05); and with spatial abilityefficiency (genotypic, P = 0.05) in healthy controls and allelic association of rs4680 with emotionefficiency in cases with schizophrenia (P = 0.04) were notable.

PRINCIPAL CONCLUSION

Dopaminergic genes seem to contribute to tardive dyskinesia and cognition warranting replication.

Interactions of Antibodies to the Gram-Negative Gastric Bacterium Helicobacter pylori with the Synaptic Calcium Sensor Synaptotagmin 5, Correlate to Impaired Vesicle Recycling in SiMa Human Neuroblastoma Cells

Due to molecular mimicry, maternal antibacterial antibodies are suspected to promote neurodevelopmental changes in the offspring that finally can cause disorders like autism and schizophrenia. Using a human first trimester prenatal brain multiprotein array (MPA), we demonstrate here that antibodies to the digestive tract bacteria Helicobacter pylori (α-HPy) and Campylobacter jejuni (α-CJe) interact with different synaptic proteins, including the calcium sensor synaptotagmin 5 (Syt5). Interactions of both antisera with Syt5 were confirmed by Western blot with a HEK293-cells overexpression lysate of this protein. Immunofluorescence and Western blotting revealed SiMa cells to express Syt5, which also co-migrated with a band/spot labeled by either α-HPy or α-CJe. Functionally, a 12-h pretreatment of SiMa cells with 10 μg/ml of either α-HPy or α-CJe resulted in a significant reduction of acetylcholine(ACh)-dependent calcium signals as compared to controls. Also ACh-dependent vesicle recycling was significantly reduced in cells pretreated with either α-HPy or α-CJe. Similar effects were observed upon pretreatment of SiMa cells with Syt5-specific antibodies. In conclusion, the present study supports the view that prenatal maternal antibacterial immune responses towards HPy and by this to Syt5 are able to cause functional changes, which in the end might contribute also to neurodevelopmental disorders.

Translational neurophysiological biomarkers of N-methyl-d-aspartate receptor dysfunction in serine racemase knockout mice

Alterations in glutamatergic function are well established in schizophrenia (Sz), but new treatment development is hampered by the lack of translational pathophysiological and target engagement biomarkers as well as by the lack of animal models that recapitulate the pathophysiological features of Sz. Here, we evaluated the rodent auditory steady state response (ASSR) and long-latency auditory event-related potential (aERP) as potential translational markers. These biomarkers were assessed for their sensitivity to both the N-methyl-d-aspartate receptor (NMDAR) antagonist phencyclidine (PCP) and to knock-out (KO) of Serine Racemase (SR), which is known to lead to Sz-like alterations in function of parvalbumin (PV)-type cortical interneurons. PCP led to significant increases of ASSR that were further increased in SRKO−/−, consistent with PV interneuron effects. Similar effects were observed in mice with selective NMDAR KO on PV interneurons. By contrast, PCP but not SRKO reduced the amplitude of the rodent analog of the human N1 potential. Overall, these findings support use of rodent ASSR and long-latency aERP, along with previously described measures such as mismatch negativity (MMN), as translational biomarkers, and support SRKO mice as a potential rodent model for PV interneuron dysfunction in Sz.

Microglial Pruning of Synapses in the Prefrontal Cortex During Adolescence

Exaggerated synaptic elimination in the prefrontal cortex (PFC) during adolescence has been suggested to contribute to the neuropathological changes of schizophrenia. Recent data indicate that microglia (MG) sculpt synapses during early postnatal development. However, it is not known if MG contribute to the structural maturation of the PFC, which has a protracted postnatal development. We determined if MG are involved in developmentally specific synapse elimination in the PFC, focusing on adolescence. Layer 5 PFC pyramidal cells (PCs) were intracellularly filled with Lucifer Yellow for dendritic spine measurements in postnatal day (P) 24, P30, P35, P39, and P50 rats. In the contralateral PFC we evaluated if MG engulfed presynaptic (glutamatergic) and postsynaptic (dendritic spines) elements. Dendritic spine density increased from P24 to P35, when spine density peaked. There was a significant increase in MG engulfment of spines at P39 relative to earlier ages; this subsided by P50. MG also phagocytosed presynaptic glutamatergic terminals. These data indicate that MG transiently prune synapses of PFC PCs during adolescence, when the symptoms of schizophrenia typically first appear. An increase in MG-mediated synaptic remodeling of PFC PCs may contribute to the structural changes observed in schizophrenia.

A conceptualized model linking matrix metalloproteinase-9 to schizophrenia pathogenesis

Matrix metalloproteinase 9 (MMP-9) is an extracellularly operating zinc-dependent endopeptidase that is commonly expressed in the brain, other tissues. It is synthesized in a latent zymogen form known as pro-MMP-9 that is subsequently converted to the active MMP-9 enzyme following cleavage of the pro-domain. Within the central nervous system, MMP-9 is localized and released from neurons, astrocytes and microglia where its expression levels are modulated by cytokines and growth factors during both normal and pathological conditions as well as by reactive oxygen species generated during oxidative stress. MMP-9 is involved in a number of key neurodevelopmental processes that are thought to be affected in schizophrenia, including maturation of the inhibitory neurons that contain the calcium-binding protein parvalbumin, developmental formation of the specialized extracellular matrix structure perineuronal net, synaptic pruning, and myelination. In this context, the present article provides a narrative synthesis of the existing evidence linking MMP-9 dysregulation to schizophrenia pathogenesis. We start by providing an overview of MMP-9 involvement in brain development and physiology. We then discuss the potential mechanisms through which MMP-9 dysregulation may affect neural circuitry maturation as well as how these anomalies may contribute to the disease process of schizophrenia. We conclude by articulating a comprehensive, cogent, and experimentally testable hypothesis linking MMP-9 to the developmental pathophysiologic cascade that triggers the onset and sustains the chronicity of the illness.

Synaptic GluN2A-Containing NMDA Receptors: From Physiology to Pathological Synaptic Plasticity

N-Methyl-d-Aspartate Receptors (NMDARs) are ionotropic glutamate-gated receptors. NMDARs are tetramers composed by several homologous subunits of GluN1-, GluN2-, or GluN3-type, leading to the existence in the central nervous system of a high variety of receptor subtypes with different pharmacological and signaling properties. NMDAR subunit composition is strictly regulated during development and by activity-dependent synaptic plasticity. Given the differences between GluN2 regulatory subunits of NMDAR in several functions, here we will focus on the synaptic pool of NMDARs containing the GluN2A subunit, addressing its role in both physiology and pathological synaptic plasticity as well as the contribution in these events of different types of GluN2A-interacting proteins.

Sociability development in mice with cell-specific deletion of the NMDA receptor NR1 subunit gene

Social affiliative behavior is an important component of everyday life in many species and is likely to be disrupted in disabling ways in various neurodevelopmental and neuropsychiatric disorders. Therefore, determining the mechanisms involved in these processes is crucial. A link between N-methyl-d-aspartate (NMDA) receptor function and social behaviors has been clearly established. The cell types in which NMDA receptors are critical for social affiliative behavior, however, remain unclear. Here, we use mice carrying a conditional allele of the NMDA R1 subunit to address this question. Mice bearing a floxed NMDAR1 (NR1) allele were crossed with transgenic calcium/calmodulin-dependent kinase IIα (CaMKIIα)-Cre mice or parvalbumin (PV)-Cre mice targeting postnatal excitatory forebrain or PV-expressing interneurons, respectively, and assessed using the three-chambered Social Approach Test. We found that deletion of NR1 in PV-positive interneurons had no effect on social sniffing, but deletion of NR1 in glutamatergic pyramidal cells resulted in a significant increase in social approach behavior, regardless of age or sex. Therefore, forebrain excitatory neurons expressing NR1 play an important role in regulating social affiliative behavior.

Alterations in cortical interneurons and cognitive function in schizophrenia

Certain clinical features of schizophrenia, such as working memory disturbances, appear to emerge from altered gamma oscillatory activity in the prefrontal cortex (PFC). Given the essential role of GABA neurotransmission in both working memory and gamma oscillations, understanding the cellular substrate for their disturbances in schizophrenia requires evidence from in vivo neuroimaging studies, which provide a means to link markers of GABA neurotransmission to gamma oscillations and working memory, and from postmortem studies, which provide insight into GABA neurotransmission at molecular and cellular levels of resolution. Here, we review findings from both types of studies which converge on the notions that 1) inhibitory GABA signaling in the PFC, especially between parvalbumin positive GABAergic basket cells and excitatory pyramidal cells, is required for gamma oscillatory activity and working memory function; and 2) disturbances in this signaling contribute to altered gamma oscillations and working memory in schizophrenia. Because the PFC is only one node in a distributed cortical network that mediates working memory, we also review evidence of GABA abnormalities in other cortical regions in schizophrenia.

The impact of D-cycloserine and sarcosine on in vivo frontal neural activity in a schizophrenia-like model

BACKGROUND

N-methyl-D-aspartate receptor (NMDAR) hypofunction has been proposed to underlie the pathogenesis of schizophrenia. Specifically, reduced function of NMDARs leads to altered balance between excitation and inhibition which further drives neural network malfunctions. Clinical studies suggested that NMDAR modulators (glycine, D-serine, D-cycloserine and glycine transporter inhibitors) may be beneficial in treating schizophrenia patients. Preclinical evidence also suggested that these NMDAR modulators may enhance synaptic NMDAR function and synaptic plasticity in brain slices. However, an important issue that has not been addressed is whether these NMDAR modulators modulate neural activity/spiking in vivo.

METHODS

By using in vivo calcium imaging and single unit recording, we tested the effect of D-cycloserine, sarcosine (glycine transporter 1 inhibitor) and glycine, on schizophrenia-like model mice.

RESULTS

In vivo neural activity is significantly higher in the schizophrenia-like model mice, compared to control mice. D-cycloserine and sarcosine showed no significant effect on neural activity in the schizophrenia-like model mice. Glycine induced a large reduction in movement in home cage and reduced in vivo brain activity in control mice which prevented further analysis of its effect in schizophrenia-like model mice.

CONCLUSIONS

We conclude that there is no significant impact of the tested NMDAR modulators on neural spiking in the schizophrenia-like model mice.

The origin of NMDA receptor hypofunction in schizophrenia

N-methyl-d-aspartate (NMDA) receptor (NMDAR) hypofunction plays a key role in pathophysiology of schizophrenia. Since NMDAR hypofunction has also been reported in autism, Alzheimer's disease and cognitive dementia, it is crucial to identify the location, timing, and mechanism of NMDAR hypofunction for schizophrenia for better understanding of disease etiology and for novel therapeutic intervention. In this review, we first discuss the shared underlying mechanisms of NMDAR hypofunction in NMDAR antagonist models and the anti-NMDAR autoantibody model of schizophrenia and suggest that NMDAR hypofunction could occur in GABAergic neurons in both models. Preclinical models using transgenic mice have shown that NMDAR hypofunction in cortical GABAergic neurons, in particular parvalbumin-positive fast-spiking interneurons, in the early postnatal period confers schizophrenia-related phenotypes. Recent studies suggest that NMDAR hypofunction can also occur in PV-positive GABAergic neurons with alterations of NMDAR-associated proteins, such as neuregulin/ErbB4, α7nAChR, and serine racemase. Furthermore, several environmental factors, such as oxidative stress, kynurenic acid and hypoxia, may also potentially elicit NMDAR hypofunction in GABAergic neurons in early postnatal period. Altogether, the studies discussed here support a central role for GABAergic abnormalities in the context of NMDAR hypofunction. We conclude by suggesting potential therapeutic strategies to improve the function of fast-spiking neurons.

Pre-frontal parvalbumin interneurons in schizophrenia: a meta-analysis of post-mortem studies

Parvalbumin interneurons are fast-spiking GABAergic neurons that provide inhibitory control of cortical and subcortical circuits and are thought to be a key locus of the pathophysiology underlying schizophrenia. In view of the contradictory results regarding the nature of parvalbumin post-mortem findings in schizophrenia, we conducted a quantitative meta-analysis of the data on parvalbumin cell density and parvalbumin mRNA levels in pre-frontal regions in the brains of patients with schizophrenia (n = 274) compared with healthy controls (n = 275). The results suggest that parvalbumin interneurons are reduced in density in the frontal cortex of patients with schizophrenia (Hedges’ g = − 0.27; p = 0.03) and there is a non-significant reduction in parvalbumin mRNA levels (g = − 0.44; p = 0.12). However, certain methodological issues need to be considered in interpreting such results and are discussed in more detail. A meta-regression was conducted for post-mortem interval and year of publication as covariates which were both non-significant, except in the mRNA meta-analysis where post-mortem interval was found to be significant. Overall our findings provide tentative support for the hypothesis that the GABAergic system is deficient in schizophrenia and that parvalbumin-containing interneurons offer a potential target for treatment. However, further well-controlled studies that examine multiple regions and layers are warranted to determine whether parvalbumin alterations are region or layer specific and to test the robustness of the findings further.

Transcranial Low-Intensity Pulsed Ultrasound Modulates Structural and Functional Synaptic Plasticity in Rat Hippocampus

Plasticity of synaptic structure and function play an essential role in neuronal development, cognitive functions, and degenerative diseases. Recently, low-intensity pulsed ultrasound (LIPUS) stimulation has been reported as a promising technology for neuromodulation. However, the effect of LIPUS stimulation on the structural and functional synaptic plasticity in rat hippocampus has not yet been addressed. The aim of this study was to investigate whether LIPUS stimulation could affect the dendritic structure, electrophysiological properties, and expression level of glutamate receptors GluN2A, GluN2B, and GluR1 subunits in rat hippocampus. Transcranial LIPUS was delivered to CA1 of the intact hippocampus of rats ( n = 40 ) for 10 days (10 min/day) with the following parameters: fundamental frequency of 0.5 MHz, pulse repetition frequency (PRF) of 500 Hz, peak negative pressure of 0.42 MPa, and I_spta of 360 mW/cm². The effect of LIPUS on dendritic structure, electrophysiological properties, and the expression of neurotransmitter receptors was measured using Golgi staining, electrophysiological recording, and western blotting, respectively. Golgi staining and electrophysiological recordings showed that LIPUS stimulation significantly increased the density of dendritic spines (0.72 ± 0.17 versus 0.94 ± 0.19 spines/ [Formula: see text], ) and the frequency of spontaneous excitatory postsynaptic current (0.37 ± 0.14 versus 1.77 ± 0.37 Hz, ) of CA1 hippocampal neurons. Furthermore, the western blotting analysis demonstrated a significant increase in the expression level of GluN2A ( ). The results illustrated the effect of LIPUS on the dendritic structure, function, and neurotransmitter receptors, which may provide a powerful tool for treating neurodegenerative diseases.

How could N-Methyl-D-Aspartate Receptor Antagonists Lead to Excitation Instead of Inhibition?

N-methyl-D-aspartate receptors (NMDARs) are a family of ionotropic glutamate receptors mainly known to mediate excitatory synaptic transmission and plasticity. Interestingly, low-dose NMDAR antagonists lead to increased, instead of decreased, functional connectivity; and they could cause schizophrenia- and/or antidepressant-like behavior in both humans and rodents. In addition, human genetic evidences indicate that NMDAR loss of function mutations underlie certain forms of epilepsy, a disease featured with abnormal brain hyperactivity. Together, they all suggest that under certain conditions, NMDAR activation actually lead to inhibition, but not excitation, of the global neuronal network. Apparently, these phenomena are rather counterintuitive to the receptor's basic role in mediating excitatory synaptic transmission. How could it happen? Recently, this has become a crucial question in order to fully understand the complexity of NMDAR function, particularly in disease. Over the past decades, different theories have been proposed to address this question. These include theories of “NMDARs on inhibitory neurons are more sensitive to antagonism”, or “basal NMDAR activity actually inhibits excitatory synapse”, etc. Our review summarizes these efforts, and also provides an introduction of NMDARs, inhibitory neurons, and their relationships with the related diseases. Advances in the development of novel NMDAR pharmacological tools, particularly positive allosteric modulators, are also included to provide insights into potential intervention strategies.

Can N-Methyl-D-Aspartate Receptor Hypofunction in Schizophrenia Be Localized to an Individual Cell Type?

Hypofunction of N-methyl-D-aspartate glutamate receptors (NMDARs), whether caused by endogenous factors like auto-antibodies or mutations, or by pharmacological or genetic manipulations, produces a wide variety of deficits which overlap with-but do not precisely match-the symptom spectrum of schizophrenia. In order to understand how NMDAR hypofunction leads to different components of the syndrome, it is necessary to take into account which neuronal subtypes are particularly affected by it in terms of detrimental functional alterations. We provide a comprehensive overview detailing findings in rodent models with cell type-specific knockout of NMDARs. Regarding inhibitory cortical cells, an emerging model suggests that NMDAR hypofunction in parvalbumin (PV) positive interneurons is a potential risk factor for this disease. PV interneurons display a selective vulnerability resulting from a combination of genetic, cellular, and environmental factors that produce pathological multi-level positive feedback loops. Central to this are two antioxidant mechanisms-NMDAR activity and perineuronal nets-which are themselves impaired by oxidative stress, amplifying disinhibition. However, NMDAR hypofunction in excitatory pyramidal cells also produces a range of schizophrenia-related deficits, in particular maladaptive learning and memory recall. Furthermore, NMDAR blockade in the thalamus disturbs thalamocortical communication, and NMDAR ablation in dopaminergic neurons may provoke over-generalization in associative learning, which could relate to the positive symptom domain. Therefore, NMDAR hypofunction can produce schizophrenia-related effects through an action on various different circuits and cell types.

Parvalbumin expression and gamma oscillation occurrence increase over time in a neurodevelopmental model of NMDA receptor dysfunction

Dysfunction of the N-methyl-d-aspartate receptor (NMDAR) is thought to play a role in the pathophysiology of neurodevelopmental diseases like schizophrenia. To study the effects of NMDAR dysfunction on synaptic transmission and network oscillations, we used hippocampal tissue of NMDAR subunit GluN2A knockout (KO) mice. Field excitatory postsynaptic potentials were recorded in acute hippocampal slices of adult animals. Synaptic transmission was impaired in GluN2A KO slices compared to wild-type (WT) slices. Further, to investigate whether NMDAR dysfunction would alter neurodevelopment in vitro, we used organotypic hippocampal slice cultures of WT and GluN2A KO mice. Immunostaining performed with cultures kept two, seven, 14, 25 days in vitro (DIV) revealed an increasing expression of parvalbumin (PV) over time. As a functional readout, oscillatory activity induced by the cholinergic agonist carbachol was recorded in cultures kept seven, 13, and 26 DIV using microelectrode arrays. Initial analysis focused on the occurrence of delta, theta, beta and gamma oscillations over genotype, DIV and hippocampal area (CA1, CA3, dentate gyrus (DG)). In a follow-up analysis, we studied the peak frequency and the peak power of each of the four oscillation bands per condition. The occurrence of gamma oscillations displayed an increase by DIV similar to the PV immunostaining. Unlike gamma occurrence, delta, theta, and beta occurrence did not change over time in culture. The peak frequency and peak power in the different bands of the oscillations were not different in slices of WT and GluN2A KO mice. However, the level of PV expression was lower in GluN2A KO compared to WT mice. Given the role of PV-containing fast-spiking basket cells in generation of oscillations and the decreased PV expression in subjects with schizophrenia, the study of gamma oscillations in organotypic hippocampal slices represents a potentially valuable tool for the characterization of novel therapeutic drugs.

Presynaptic Effects of N-Methyl-D-Aspartate Receptors Enhance Parvalbumin Cell-Mediated Inhibition of Pyramidal Cells in Mouse Prefrontal Cortex

BACKGROUND

Testing hypotheses regarding the role of N-methyl-D-aspartate receptor (NMDAR) hypofunction in schizophrenia requires understanding the mechanisms of NMDAR regulation of prefrontal cortex (PFC) circuit function. NMDAR antagonists are thought to produce pyramidal cell (PC) disinhibition. However, inhibitory parvalbumin-positive basket cells (PVBCs) have modest NMDAR-mediated excitatory drive and thus are unlikely to participate in NMDAR antagonist-mediated disinhibition. Interestingly, recent studies demonstrated that presynaptic NMDARs enhance transmitter release at central synapses. Thus, if presynaptic NMDARs enhance gamma-aminobutyric acid release at PVBC-to-PC synapses, they could participate in NMDAR-dependent PC disinhibition. Here, we examined whether presynaptic NMDAR effects could modulate gamma-aminobutyric acid release at PVBC-to-PC synapses in mouse PFC.

METHODS

Using whole-cell recordings from synaptically connected pairs in mouse PFC, we determined whether NMDA or NMDAR antagonist application affects PVBC-to-PC inhibition in a manner consistent with a presynaptic mechanism.

RESULTS

NMDAR activation enhanced by ∼40% the synaptic current at PVBC-to-PC pairs. This effect was consistent with a presynaptic mechanism given that it was 1) observed with postsynaptic NMDARs blocked by intracellular MK801, 2) associated with a lower rate of transmission failures and a higher transmitter release probability, and 3) blocked by intracellular MK801 in the PVBC. NMDAR antagonist application did not affect the synaptic currents in PVBC-to-PC pairs, but it reduced the inhibitory currents elicited in PCs with simultaneous glutamate release by extracellular stimulation.

CONCLUSIONS

We demonstrate that NMDAR activation enhances PVBC-to-PC inhibition in a manner consistent with presynaptic mechanisms, and we suggest that the functional impact of this presynaptic effect depends on the activity state of the PFC network.

(Micro)Glia as Effectors of Cortical Volume Loss in Schizophrenia

Contrary to the notion that neurology but not psychiatry is the domain of disorders evincing structural brain alterations, it is now clear that there are subtle but consistent neuropathological changes in schizophrenia. These range from increases in ventricular size to dystrophic changes in dendritic spines. A decrease in dendritic spine density in the prefrontal cortex (PFC) is among the most replicated of postmortem structural findings in schizophrenia. Examination of the mechanisms that account for the loss of dendritic spines has in large part focused on genes and molecules that regulate neuronal structure. But the simple question of what is the effector of spine loss, ie, where do the lost spines go, is unanswered. Recent data on glial cells suggest that microglia (MG), and perhaps astrocytes, play an important physiological role in synaptic remodeling of neurons during development. Synapses are added to the dendrites of pyramidal cells during the maturation of these neurons; excess synapses are subsequently phagocytosed by MG. In the PFC, this occurs during adolescence, when certain symptoms of schizophrenia emerge. This brief review discusses recent advances in our understanding of MG function and how these non-neuronal cells lead to structural changes in neurons in schizophrenia.

Associations between adolescent cannabis use and brain structure in psychosis

Associations between cannabis use and psychotic disorders suggest that cannabis may be a contributory risk factor in the neurobiology of psychosis. In this study, we examined brain structure characteristics, total and regional gray matter density (GMD), using Voxel Based Morphometry, in psychotic individuals, stratified by history of cannabis use (total n = 109). We also contrasted GMD estimates in individual diagnostic groups (schizophrenia/bipolar I disorder) with and without history of adolescent cannabis use (ACU). Individuals with psychosis as a whole, both with and without history of ACU, had lower total and regional GMD, compared to healthy controls. ACU was associated with attenuated GMD reductions, compared to non-users, especially in the schizophrenia cases, who showed robust GMD reductions in fronto-temporal and parietal cortex, as well as subcortical regions. Notably, total and regional GMD estimates in individuals with psychosis and ACU were not different from controls with no ACU. These data indicate that the history of ACU in psychotic individuals is associated with attenuated GMD abnormalities. Future investigations targeting potential unique etiological and risk factors associated with psychosis in individuals with ACU may help in understanding of the neurobiology of psychotic disorders and novel treatment options for these individuals.

Novel rare variations in genes that regulate developmental change in N-methyl-d-aspartate receptor in patients with schizophrenia

The mechanism underlying the vulnerability to developing schizophrenia (SCZ) during adolescence remains elusive. Hypofunction of N-methyl-d-aspartate receptors (NMDARs) has been implicated in the pathophysiology of SCZ. During development, the composition of synaptic NMDARs dramatically changes from NR2B-containing NMDARs to NR2A-containing NMDARs through the phosphorylation of NR2B S1480 or Y1472 by CDK5, CSNK2A1, and EphB2, which plays a pivotal role in the maturation of neural circuits. We hypothesized that the dysregulation of developmental change in NMDARs could be involved in the onset of SCZ. Using next-generation sequencing, we re-sequenced all the coding regions and splice sites of CDK5, CSNK2A1, and EphB2 in 474 patients with SCZ and 475 healthy controls. Variants on the database for human control subjects of Japanese origin were removed and all the nonsynonymous and nonsense variants were validated using Sanger sequencing. Four novel variants in CDK5 were observed in patients with SCZ but were not observed in controls. The total number of variants, however, was not significantly different between the SCZ and control groups (P=0.062). In silico analyses predicted P271T to be damaging. Further genetic research using a larger sample is required to examine whether CDK5 is involved in the pathophysiology of SCZ.

Oscillatory Synchronous Inhibition in the Basolateral Amygdala and its Primary Dependence on NR2A-containing NMDA Receptors

Synchronous, rhythmic firing of GABAergic interneurons is a fundamental mechanism underlying the generation of brain oscillations, and evidence suggests that NMDA receptors (NMDARs) play a key role in oscillatory activity by regulating the activity of interneurons. Consistent with this, derangement of brain rhythms in certain neuropsychiatric disorders, notably schizophrenia and autism, is associated with NMDAR hypofunction and loss of inhibitory interneurons. In the basolateral amygdala (BLA)-dysfunction of which is involved in a host of neuropsychiatric diseases-, principal neurons display spontaneous, rhythmic "bursts" of inhibitory activity, which could potentially be involved in the orchestration of oscillations in the BLA network; here, we investigated the role of NMDARs in these inhibitory oscillations. Rhythmic bursts of spontaneous IPSCs (0.5 Hz average burst frequency) recorded from rat BLA principal cells were blocked or significantly suppressed by D-AP5, and could be driven by NMDAR activation alone. BLA interneurons generated spontaneous bursts of suprathreshold EPSCs at a similar frequency, which were also blocked or reduced by D-AP5. PEAQX (GluN2A-NMDAR antagonist; 0.4 μM) or Ro-25-6981 (GluN2B-NMDAR antagonist; 5 μM) suppressed the IPSC and EPSC bursts; suppression by PEAQX was significantly greater than that by Ro-25-6981. Immunohistochemical labeling revealed the presence of both GluN2A- and GluN2B-NMDARs on GABAergic BLA interneurons, while, functionally, GluN2A-NMDARs have the dominant role, as suggested by a greater reduction of NMDA-evoked currents by PEAQX versus Ro-25-6981. Entrainment of BLA principal neurons in an oscillatory generation of inhibitory activity depends primarily on activation of GluN2A-NMDARs, and interneuronal GluN2A-NMDARs may play a significant role.

Multidimensional Connectomics and Treatment-Resistant Schizophrenia: Linking Phenotypic Circuits to Targeted Therapeutics

Schizophrenia is a very complex syndrome that involves widespread brain multi-dysconnectivity. Neural circuits within specific brain regions and their links to corresponding regions are abnormal in the illness. Theoretical models of dysconnectivity and the investigation of connectomics and brain network organization have been examined in schizophrenia since the early nineteenth century. In more recent years, advancements have been achieved with the development of neuroimaging tools that have provided further clues to the structural and functional organization of the brain and global neural networks in the illness. Neural circuitry that extends across prefrontal, temporal and parietal areas of the cortex as well as limbic and other subcortical brain regions is disrupted in schizophrenia. As a result, many patients have a poor response to antipsychotic treatment and treatment failure is common. Treatment resistance that is specific to positive, negative, and cognitive domains of the illness may be related to distinct circuit phenotypes unique to treatment-refractory disease. Currently, there are no customized neural circuit-specific and targeted therapies that address this neural dysconnectivity. Investigation of targeted therapeutics that addresses particular areas of substantial regional dysconnectivity is an intriguing approach to precision medicine in schizophrenia. This review examines current findings of system and circuit-level brain dysconnectivity in treatment-resistant schizophrenia based on neuroimaging studies. Within a connectome context, on-off circuit connectivity synonymous with excitatory and inhibitory neuronal pathways is discussed. Mechanistic cellular, neurochemical and molecular studies are included with specific emphasis given to cell pathology and synaptic communication in glutamatergic and GABAergic systems. In this review we attempt to deconstruct how augmenting treatments may be applied within a circuit context to improve circuit integration and treatment response. Clinical studies that have used a variety of glutamate receptor and GABA interneuron modulators, nitric oxide-based therapies and a variety of other strategies as augmenting treatments with antipsychotic drugs are included. This review supports the idea that the methodical mapping of system-level networks to both on (excitatory) and off (inhibitory) cellular circuits specific to treatment-resistant disease may be a logical and productive approach in directing future research toward the advancement of targeted pharmacotherapeutics in schizophrenia.

Cortical high gamma network oscillations and connectivity: a translational index for antipsychotics to normalize aberrant neurophysiological activity

Oscillatory activity in the gamma frequency range is a critical mechanism, which integrates neural networks within and across brain structures during cognitive processes. In schizophrenia, abnormalities in high gamma oscillations are ubiquitous and most likely reflect dysfunction in neuronal networks. In conscious rats, disturbed network oscillations associated with positive symptoms and cognitive deficits were modeled in different cortical areas by the dopaminergic agonist (amphetamine) and the N-methyl-D-aspartate (NMDA) receptor antagonists (PCP and MK801). Subsequently, the efficacies of marketed atypical antipsychotics (olanzapine, risperidone, and clozapine) to normalize dysfunctional oscillations and network connectivity were examined. Acute NMDA antagonists elicited aberrant synchrony in the gamma frequency oscillations. In addition, coherent slow alpha network activity was observed with MK801 and amphetamine, both of whose oscillatory rhythms were correlated with pronounced locomotor activity. All antipsychotics commonly decreased slow alpha and high gamma network oscillations in different cortical regions as well as motion behavior. In the combined treatments, antipsychotics attenuated NMDA antagonist-induced abnormalities in functional network oscillations and connectivity, whose effects on motor behavior is mechanistically related. These results suggest that pharmacologically induced disruption of cortical gamma oscillations and network connectivity in rats is a candidate model to study dysfunctional oscillatory patterns described in positive and negative symptoms of schizophrenia. The efficacy of antipsychotics to rescue cortical network oscillatory patterns is in line with the idea that glutamatergic and dopaminergic systems play a role in maintaining the integrity of cortical circuits. Thus, gamma oscillations could provide a powerful translational index to assess the integrity of neural networks and to evaluate the efficacy of drugs with potential antipsychotic properties.

A lack of GluN2A-containing NMDA receptors confers a vulnerability to redox dysregulation: Consequences on parvalbumin interneurons, and their perineuronal nets

The GluN2A subunit of NMDA receptors (NMDARs) plays a critical role during postnatal brain development as its expression increases while Glun2B expression decreases. Mutations and polymorphisms in GRIN2A gene, coding for GluN2A, are linked to developmental brain disorders such as mental retardation, epilepsy, schizophrenia. Published data suggest that GluN2A is involved in maturation and phenotypic maintenance of parvalbumin interneurons (PVIs), and these interneurons suffer from a deficient glutamatergic neurotransmission via GluN2A-containing NMDARs in schizophrenia. In the present study, we find that although PVIs and their associated perineuronal nets (PNNs) appear normal in anterior cingulate cortex of late adolescent/young adult GRIN2A KO mice, a lack of GluN2A delays PNN maturation. GRIN2A KO mice display a susceptibility to redox dysregulation as sub-threshold oxidative stress and subtle alterations in antioxidant systems are observed in their prefrontal cortex. Consequently, an oxidative insult applied during early postnatal development increases oxidative stress, decreases the number of parvalbumin-immunoreactive cells, and weakens the PNNs in KO but not WT mice. These effects are long-lasting, but preventable by the antioxidant, N-acetylcysteine. The persisting oxidative stress, deficit in PVIs and PNNs, and reduced local high-frequency neuronal synchrony in anterior cingulate of late adolescent/young adult KO mice, which have been challenged by an early-life oxidative insult, is accompanied with microglia activation. Altogether, these indicate that a lack of GluN2A-containing NMDARs alters the fine control of redox status, leading to a delayed maturation of PNNs, and conferring vulnerability for long-term oxidative stress, microglial activation, and PVI network dysfunction.

The antipsychotic drugs olanzapine and haloperidol modify network connectivity and spontaneous activity of neural networks in vitro

Impaired neural synchronization is a hallmark of psychotic conditions such as schizophrenia. It has been proposed that schizophrenia-related cognitive deficits are caused by an unbalance of reciprocal inhibitory and stimulatory signaling. This supposedly leads to decreased power of induced gamma oscillations during the performance of cognitive tasks. In light of this hypothesis an efficient antipsychotic treatment should modify the connectivity and synchronization of local neural circuits. To address this issue, we investigated a model of hippocampal neuronal networks in vitro. Inhibitory and excitatory innervation of GABAergic and glutamatergic neurons was quantified using immunocytochemical markers and an automated routine to estimate network connectivity. The first generation (FGA) and second generation (SGA) antipsychotic drugs haloperidol and olanzapine, respectively, differentially modified the density of synaptic inputs. Based on the observed synapse density modifications, we developed a computational model that reliably predicted distinct changes in network activity patterns. The results of computational modeling were confirmed by spontaneous network activity measurements using the multiple electrode array (MEA) technique. When the cultures were treated with olanzapine, overall activity and synchronization were increased, whereas haloperidol had the opposite effect. We conclude that FGAs and SGAs differentially affect the balance between inhibition and excitation in hippocampal networks.

Systematic variation of the benzenesulfonamide part of the GluN2A selective NMDA receptor antagonist TCN-201

GluN2A subunit containing N-methyl-d-aspartate receptors (NMDARs) are highly involved in various physiological processes in the central nervous system, but also in some diseases, such as anxiety, depression and schizophrenia. However, the role of GluN2A subunit containing NMDARs in pathological processes is not exactly elucidated. In order to obtain potent and selective inhibitors of GluN2A subunit containing NMDARs, the selective negative allosteric modulator 2 was systematically modified at the benzenesulfonamide part. The activity of the test compounds was recorded in two electrode voltage clamp experiments using Xenopus laevis oocytes expressing exclusively NMDARs with GluN1a and GluN2A subunits. It was found that halogen atoms in 3-position of the benzenesulfonamide part result in high GluN2A antagonistic activity. With an IC50 value of 204 nM the 3-bromo derivative 5i (N-{4-[(2-benzoylhydrazino)carbonyl]benzyl}-3-bromobenzenesulfonamide) has 2.5-fold higher antagonistic activity than the lead compound 2 and represents our new lead compound.

Changes in Neuronal Oscillations Accompany the Loss of Hippocampal LTP that Occurs in an Animal Model of Psychosis

The first-episode of psychosis is followed by a transient time-window of ca. 60 days during which therapeutic interventions have a higher likelihood of being effective than interventions that are started with a greater latency. This suggests that, in the immediate time-period after first-episode psychosis, functional changes occur in the brain that render it increasingly resistant to intervention. The precise mechanistic nature of these changes is unclear, but at the cognitive level, sensory and hippocampus-based dysfunctions become increasingly manifest. In an animal model of first-episode psychosis that comprises acute treatment of rats with the irreversible N-methyl-D-aspartate receptor (NMDAR)-antagonist, MK801, acute but also chronic deficits in long-term potentiation (LTP) and spatial memory occur. Neuronal oscillations, especially in the form of information transfer through θ and γ frequency oscillations are an intrinsic component of normal information processing in the hippocampus. Changes in θ-γ coupling and power are known to accompany deficits in hippocampal plasticity. Here, we examined whether changes in δ, θ, α, β and γ oscillations, or θ-γ coupling accompany the chronic loss of LTP that is observed in the MK801-animal model of psychosis. One and 4 weeks after acute systemic treatment of adult rats with MK801, a potent loss of hippocampal in vivo LTP was evident compared to vehicle-treated controls. Overall, the typical pattern of θ-γ oscillations that are characteristic for the successful induction of LTP was altered. In particular, θ-power was lower and an uncoupling of θ-γ oscillations was evident in MK801-treated rats. The alterations in network oscillations that accompany LTP deficits in this animal model may comprise a mechanism through which disturbances in sensory information processing and hippocampal function occur in psychosis. These data suggest that the hippocampus is likely to comprise a very early locus of functional change after instigation of a first-episode psychosis-like state in rodents.

GRIN3B missense mutation as an inherited risk factor for schizophrenia: whole-exome sequencing in a family with a familiar history of psychotic disorders

Glutamate is the most important excitatory neurotransmitter in the brain. The N-methyl-D-aspartate (NMDA) receptor is a glutamate-gated ionotropic cation channel that is composed of several subunits and modulated by a glycine binding site. Many forms of synaptic plasticity depend on the influx of calcium ions through NMDA receptors, and NMDA receptor dysfunction has been linked to a number of neuropsychiatric disorders, including schizophrenia. Whole-exome sequencing was performed in a family with a strong history of psychotic disorders over three generations. We used an iterative strategy to obtain condense and meaningful variants. In this highly affected family, we found a frameshift mutation (rs10666583) in the GRIN3B gene, which codes for the GluN3B subunit of the NMDA receptor in all family members with a psychotic disorder, but not in the healthy relatives. Matsuno et al., also reported this null variant as a risk factor for schizophrenia in 2015. In a broader sample of 22 patients with psychosis, the allele frequency of the rs10666583 mutation variant was increased compared to those of healthy population samples and unaffected relatives. Compared to the 1000 Genomes Project population, we found a significant increase of this variant with a large effect size among patients. The amino acid shift degrades the S1/S2 glycine binding domain of the dominant modulatory GluN3B subunit of the NMDA receptor, which subsequently affects the permeability of the channel pore to calcium ions. A decreased glycine affinity for the GluN3B subunit might cause impaired functional capability of the NMDA receptor and could be an important risk factor for the pathogenesis of psychotic disorders.

Weaving a Net of Neurobiological Mechanisms in Schizophrenia and Unraveling the Underlying Pathophysiology

Perineuronal nets (PNNs) are enigmatic structures composed of extracellular matrix molecules that encapsulate the soma, dendrites, and axon segments of neurons in a lattice-like fashion. Although most PNNs condense around parvalbumin-expressing gamma-aminobutyric acidergic interneurons, some glutamatergic pyramidal cells in the brain are also surrounded by PNNs. Experimental findings suggest pivotal roles of PNNs in the regulation of synaptic formation and function. Also, an increasing body of evidence links PNN abnormalities to schizophrenia. The number of PNNs progressively increases during postnatal development until plateauing around the period of late adolescence and early adulthood, which temporally coincides with the age of onset of schizophrenia. Given the established role of PNNs in modulating developmental plasticity, the PNN represents a possible candidate for altering the onset and progression of schizophrenia. Similarly, the reported function of PNNs in regulating the trafficking of glutamate receptors places them in a critical position to modulate synaptic pathology, considered a cardinal feature of schizophrenia. We discuss the physiologic role of PNNs in neural function, synaptic assembly, and plasticity as well as how they interface with circuit/system mechanisms of cognition. An integrated understanding of these neurobiological processes should provide a better basis to elucidate how PNN abnormalities influence brain function and contribute to the pathogenesis of neurodevelopmental disorders such as schizophrenia.

Diverse modes of NMDA receptor positive allosteric modulation: Mechanisms and consequences

NMDA Receptors (NMDARs) play key roles in synaptic physiology and NMDAR hypofunction has been implicated in various neurological conditions. In recent years an increasing number of positive allosteric modulators (PAMs) of NMDARs have been discovered and characterized. These diverse PAM classes vary not only in their binding sites and GluN2 subunit selectivity profiles, but also in the nature of their impacts on channel function. Major differences exist in the degree of slowing of channel deactivation and shifting of apparent agonist affinity between different classes of PAMs. Here we review the diverse modes of potentiation by the currently known classes of NMDAR PAMs and discuss the potential consequences of different types of potentiation in terms of desirable and undesirable effects on brain function. This article is part of the Special Issue entitled 'Ionotropic glutamate receptors'.

Gamma band oscillations: a key to understanding schizophrenia symptoms and neural circuit abnormalities

PURPOSE OF REVIEW

We review our current understanding of abnormal γ band oscillations in schizophrenia, their association with symptoms and the underlying cortical circuit abnormality, with a particular focus on the role of fast-spiking parvalbumin gamma-aminobutyric acid (GABA) neurons in the disease state.

RECENT FINDINGS

Clinical electrophysiological studies of schizophrenia patients and pharmacological models of the disorder show an increase in spontaneous γ band activity (not stimulus-evoked) measures. These findings provide a crucial link between preclinical and clinical work examining the role of γ band activity in schizophrenia. MRI-based experiments measuring cortical GABA provides evidence supporting impaired GABAergic neurotransmission in schizophrenia patients, which is correlated with γ band activity level. Several studies suggest that stimulation of the cortical circuitry, directly or via subcortical structures, has the potential to modulate cortical γ activity, and improve cognitive function.

SUMMARY

Abnormal γ band activity is observed in patients with schizophrenia and disease models in animals, and is suggested to underlie the psychosis and cognitive/perceptual deficits. Convergent evidence from both clinical and preclinical studies suggest the central factor in γ band abnormalities is impaired GABAergic neurotransmission, particularly in a subclass of neurons which express parvalbumin. Rescue of γ band abnormalities presents an intriguing option for therapeutic intervention.

Knockout of NMDA-receptors from parvalbumin interneurons sensitizes to schizophrenia-related deficits induced by MK-801

It has been suggested that a functional deficit in NMDA-receptors (NMDARs) on parvalbumin (PV)-positive interneurons (PV-NMDARs) is central to the pathophysiology of schizophrenia. Supportive evidence come from examination of genetically modified mice where the obligatory NMDAR-subunit GluN1 (also known as NR1) has been deleted from PV interneurons by Cre-mediated knockout of the corresponding gene Grin1 (Grin1(ΔPV) mice). Notably, such PV-specific GluN1 ablation has been reported to blunt the induction of hyperlocomotion (a surrogate for psychosis) by pharmacological NMDAR blockade with the non-competitive antagonist MK-801. This suggests PV-NMDARs as the site of the psychosis-inducing action of MK-801. In contrast to this hypothesis, we show here that Grin1(ΔPV) mice are not protected against the effects of MK-801, but are in fact sensitized to many of them. Compared with control animals, Grin1(ΔPV)mice injected with MK-801 show increased stereotypy and pronounced catalepsy, which confound the locomotor readout. Furthermore, in Grin1(ΔPV)mice, MK-801 induced medial-prefrontal delta (4 Hz) oscillations, and impaired performance on tests of motor coordination, working memory and sucrose preference, even at lower doses than in wild-type controls. We also found that untreated Grin1(ΔPV)mice are largely normal across a wide range of cognitive functions, including attention, cognitive flexibility and various forms of short-term memory. Taken together these results argue against PV-specific NMDAR hypofunction as a key starting point of schizophrenia pathophysiology, but support a model where NMDAR hypofunction in multiple cell types contribute to the disease.