Control of cortical GABA circuitry development by Nrg1 and ErbB4 signalling

Neuregulin 1 Type I Overexpression Is Associated with Reduced NMDA Receptor-Mediated Synaptic Signaling in Hippocampal Interneurons Expressing PV or CCK

Hypofunction of N-methyl-d-aspartate receptors (NMDARs) in inhibitory GABAergic interneurons is implicated in the pathophysiology of schizophrenia (SZ), a heritable disorder with many susceptibility genes. However, it is still unclear how SZ risk genes interfere with NMDAR-mediated synaptic transmission in diverse inhibitory interneuron populations. One putative risk gene is neuregulin 1 (NRG1), which signals via the receptor tyrosine kinase ErbB4, itself a schizophrenia risk gene. The type I isoform of NRG1 shows increased expression in the brain of SZ patients, and ErbB4 is enriched in GABAergic interneurons expressing parvalbumin (PV) or cholecystokinin (CCK). Here, we investigated ErbB4 expression and synaptic transmission in interneuronal populations of the hippocampus of transgenic mice overexpressing NRG1 type I (NRG1tg-type-I mice). Immunohistochemical analyses confirmed that ErbB4 was coexpressed with either PV or CCK in hippocampal interneurons, but we observed a reduced number of ErbB4-immunopositive interneurons in the NRG1tg-type-I mice. NMDAR-mediated currents in interneurons expressing PV (including PV+ basket cells) or CCK were reduced in NRG1tg-type-I mice compared to their littermate controls. We found no difference in AMPA receptor-mediated currents. Optogenetic activation (5 pulses at 20 Hz) of local glutamatergic fibers revealed a decreased NMDAR-mediated contribution to disynaptic GABAergic inhibition of pyramidal cells in the NRG1tg-type-I mice. GABAergic synaptic transmission from either PV+ or CCK+ interneurons, and glutamatergic transmission onto pyramidal cells, did not significantly differ between genotypes. The results indicate that synaptic NMDAR-mediated signaling in hippocampal interneurons is sensitive to chronically elevated NGR1 type I levels. This may contribute to the pathophysiological consequences of increased NRG1 expression in SZ.

Regulation of Synapse Development by Vgat Deletion from ErbB4-Positive Interneurons

GABA signaling has been implicated in neural development; however, in vivo genetic evidence is missing because mutant mice lacking GABA activity die prematurely. Here, we studied synapse development by ablating vesicular GABA transporter (Vgat) in ErbB4⁺ interneurons. We show that inhibitory axo-somatic synapses onto pyramidal neurons vary from one cortical layer to another; however, inhibitory synapses on axon initial segments (AISs) were similar across layers. Conversely, parvalbumin-positive (PV⁺)/ErbB4⁺ interneurons and PV-only interneurons receive a higher number of inhibitory synapses from PV⁺ErbB4⁺ interneurons compared with ErbB4-only interneurons. Vgat deletion from ErbB4⁺ interneurons reduced axo-somatic or axo-axonic synapses from PV⁺ErbB4⁺ interneurons onto excitatory neurons. This effect was associated with corresponding changes in neurotransmission. However, the Vgat mutation seemed to have little effect on inhibitory synapses onto PV⁺ and/or ErbB4⁺ interneurons. Interestingly, perineuronal nets, extracellular matrix structures implicated in maturation, survival, protection, and plasticity of PV⁺ interneurons, were increased in the cortex of ErbB4-Vgat^-/- mice. No apparent difference was observed between males and females. These results demonstrate that Vgat of ErbB4⁺ interneurons is essential for the development of inhibitory synapses onto excitatory neurons and suggest a role of GABA in circuit assembly.SIGNIFICANCE STATEMENT GABA has been implicated in neural development, but in vivo genetic evidence is missing because mutant mice lacking GABA die prematurely. Here, we ablated Vgat in ErbB4⁺ interneurons in an inducible manner. We provide evidence that the formation of inhibitory and excitatory synapses onto excitatory neurons requires Vgat in interneurons. In particular, inhibitory axo-somatic and axo-axonic synapses are more vulnerable. Our results suggest a role of GABA in circuit assembly.

Neural circuit dysfunction in mouse models of neurodevelopmental disorders

Neuropsychiatric disorders arise from the alteration of normal brain developmental trajectories disrupting the function of specific neuronal circuits. Recent advances in human genetics have greatly accelerated the identification of genes whose variation increases the susceptibility for neurodevelopmental disorders, most notably for autism spectrum disorder (ASD) and schizophrenia. In parallel, experimental studies in animal models-most typically in mice-are beginning to shed light on the role of these genes in the development and function of specific brain circuits. In spite of their limitations, understanding the impact of pathological gene variation in animal models at the level of specific neuronal populations and circuits will likely contribute to orienting human clinical studies in the search for precise disease mechanisms and novel treatments.

Inhibitory control of the excitatory/inhibitory balance in psychiatric disorders

Neuronal networks consist of different types of neurons that all play their own role in order to maintain proper network function. The two main types of neurons segregate in excitatory and inhibitory neurons, which together regulate the flow of information through the network. It has been proposed that changes in the relative strength in these two opposing forces underlie the symptoms observed in psychiatric disorders, including autism and schizophrenia. Here, we review the role of alterations to the function of the inhibitory system as a cause of psychiatric disorders. First, we explore both patient and post-mortem evidence of inhibitory deficiency. We then discuss the function of different interneuron subtypes in the network and focus on the central role of a specific class of inhibitory neurons, parvalbumin-positive interneurons. Finally, we discuss genes known to be affected in different disorders and the effects that mutations in these genes have on the inhibitory system in cortex and hippocampus. We conclude that alterations to the inhibitory system are consistently identified in animal models of psychiatric disorders and, more specifically, that mutations affecting the function of parvalbumin-positive interneurons seem to play a central role in the symptoms observed in these disorders.

An essential role for neuregulin-4 in the growth and elaboration of developing neocortical pyramidal dendrites

Neuregulins, with the exception of neuregulin-4 (NRG4), have been shown to be extensively involved in many aspects of neural development and function and are implicated in several neurological disorders, including schizophrenia, depression and bipolar disorder. Here we provide the first evidence that NRG4 has a crucial function in the developing brain. We show that both the apical and basal dendrites of neocortical pyramidal neurons are markedly stunted in Nrg4^-/- neonates in vivo compared with Nrg4^+/+ littermates. Neocortical pyramidal neurons cultured from Nrg4^-/- embryos had significantly shorter and less branched neurites than those cultured from Nrg4^+/+ littermates. Recombinant NRG4 rescued the stunted phenotype of embryonic neocortical pyramidal neurons cultured from Nrg4^-/- mice. The majority of cultured wild type embryonic cortical pyramidal neurons co-expressed NRG4 and its receptor ErbB4. The difference between neocortical pyramidal dendrites of Nrg4^-/- and Nrg4^+/+ mice was less pronounced, though still significant, in juvenile mice. However, by adult stages, the pyramidal dendrite arbors of Nrg4^-/- and Nrg4^+/+ mice were similar, suggesting that compensatory changes in Nrg4^-/- mice occur with age. Our findings show that NRG4 is a major novel regulator of dendritic arborisation in the developing cerebral cortex and suggest that it exerts its effects by an autocrine/paracrine mechanism.

Assessment of a combination of Serum Proteins as potential biomarkers to clinically predict Schizophrenia

Schizophrenia (SZ) is a devastating psychiatric disorder. Validation of potential serum biomarkers during first-episode psychosis (FEP) is especially helpful to understand the onset and prognosis of this disorder. To address this question, we examined multiple blood biomarkers and assessed the efficacy to diagnose SZ. The expression levels of Neuregulin1 (NRG1), ErbB4, brain-derived neurotrophic factor (BDNF), DNA methyltransferases 1 (DNMT1) and ten-eleven translocation 1 (TET1) proteins in peripheral blood of 53 FEP patients and 57 healthy controls were determined by enzyme-linked immunosorbent assay (ELISA). Multivariable logistic regression including biomarker concentration as covariates was used to predict SZ. Differentiating performance of these five serum protein levels was analyzed by Receiver Operating Characteristic (ROC) curve analysis. We found that patients with SZ present a higher concentration of DNMT1, and TET1 in peripheral blood, but a lower concentration of NRG1, ErbB4 and BDNF than controls. Multivariable logistic regression showed that ErbB4, BDNF and TET1 were independent predictors of SZ, and when combined, provided high diagnostic accuracy for SZ. Together, our findings highlight that altered expression of NRG1, ErbB4, BDNF, DNMT1 and TET1 are involved in schizophrenia development and they may serve as potential biomarkers for the diagnosis of the schizophrenia. Therefore, our study provides evidence that combination of ErbB4, BDNF and TET1 biomarkers could greatly improve the diagnostic performance.

ErbB4 signaling in dopaminergic axonal projections increases extracellular dopamine levels and regulates spatial/working memory behaviors

Genetic variants of Neuregulin 1 (NRG1) and its neuronal tyrosine kinase receptor ErbB4 are associated with risk for schizophrenia, a neurodevelopmental disorder characterized by excitatory/inhibitory imbalance and dopamine (DA) dysfunction. To date, most ErbB4 studies have focused on GABAergic interneurons in the hippocampus and neocortex, particularly fast-spiking parvalbumin-positive (PV+) basket cells. However, NRG has also been shown to modulate DA levels, suggesting a role for ErbB4 signaling in dopaminergic neuron function. Here we report that ErbB4 in midbrain DAergic axonal projections regulates extracellular DA levels and relevant behaviors. Mice lacking ErbB4 in tyrosine hydroxylase-positive (TH+) neurons, but not in PV+ GABAergic interneurons, exhibit different regional imbalances of basal DA levels and fail to increase DA in response to local NRG1 infusion into the dorsal hippocampus, medial prefrontal cortex and dorsal striatum measured by reverse microdialysis. Using Lund Human Mesencephalic (LUHMES) cells, we show that NRG/ErbB signaling increases extracellular DA levels, at least in part, by reducing DA transporter (DAT)-dependent uptake. Interestingly, TH-Cre;ErbB4^f/f mice manifest deficits in learning, spatial and working memory-related behaviors, but not in numerous other behaviors altered in PV-Cre;ErbB4^f/f mice. Importantly, microinjection of a Cre-inducible ErbB4 virus (AAV-ErbB4.DIO) into the mesencephalon of TH-Cre;ErbB4^f/f mice, which selectively restores ErbB4 expression in DAergic neurons, rescues DA dysfunction and ameliorates behavioral deficits. Our results indicate that direct NRG/ErbB4 signaling in DAergic axonal projections modulates DA homeostasis, and that NRG/ErbB4 signaling in both GABAergic interneurons and DA neurons contribute to the modulation of behaviors relevant to psychiatric disorders.

Mechanisms regulating dendritic arbor patterning

The nervous system is populated by diverse types of neurons, each of which has dendritic trees with strikingly different morphologies. These neuron-specific morphologies determine how dendritic trees integrate thousands of synaptic inputs to generate different firing properties. To ensure proper neuronal function and connectivity, it is necessary that dendrite patterns are precisely controlled and coordinated with synaptic activity. Here, we summarize the molecular and cellular mechanisms that regulate the formation of cell type-specific dendrite patterns during development. We focus on different aspects of vertebrate dendrite patterning that are particularly important in determining the neuronal function; such as the shape, branching, orientation and size of the arbors as well as the development of dendritic spine protrusions that receive excitatory inputs and compartmentalize postsynaptic responses. Additionally, we briefly comment on the implications of aberrant dendritic morphology for nervous system disease.

Neuregulin 3 Mediates Cortical Plate Invasion and Laminar Allocation of GABAergic Interneurons

Neural circuits in the cerebral cortex consist of excitatory pyramidal cells and inhibitory interneurons. These two main classes of cortical neurons follow largely different genetic programs, yet they assemble into highly specialized circuits during development following a very precise choreography. Previous studies have shown that signals produced by pyramidal cells influence the migration of cortical interneurons, but the molecular nature of these factors has remained elusive. Here, we identified Neuregulin 3 (Nrg3) as a chemoattractive factor expressed by developing pyramidal cells that guides the allocation of cortical interneurons in the developing cortical plate. Gain- and loss-of-function approaches reveal that Nrg3 modulates the migration of interneurons into the cortical plate in a process that is dependent on the tyrosine kinase receptor ErbB4. Perturbation of Nrg3 signaling in conditional mutants leads to abnormal lamination of cortical interneurons. Nrg3 is therefore a critical mediator in the assembly of cortical inhibitory circuits.

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Nrg3 acts a short-range chemoattractive molecule for cortical interneurons

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Nrg3 functions through ErbB4 to attract interneurons into the cortical plate

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Interneurons prefer Cxcl12 over Nrg3 during tangential migration

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Disruption of Nrg3 signaling causes abnormal interneuron lamination in the cortex

Hippocampal GABAergic Inhibitory Interneurons

In the hippocampus GABAergic local circuit inhibitory interneurons represent only ~10-15% of the total neuronal population; however, their remarkable anatomical and physiological diversity allows them to regulate virtually all aspects of cellular and circuit function. Here we provide an overview of the current state of the field of interneuron research, focusing largely on the hippocampus. We discuss recent advances related to the various cell types, including their development and maturation, expression of subtype-specific voltage- and ligand-gated channels, and their roles in network oscillations. We also discuss recent technological advances and approaches that have permitted high-resolution, subtype-specific examination of their roles in numerous neural circuit disorders and the emerging therapeutic strategies to ameliorate such pathophysiological conditions. The ultimate goal of this review is not only to provide a touchstone for the current state of the field, but to help pave the way for future research by highlighting where gaps in our knowledge exist and how a complete appreciation of their roles will aid in future therapeutic strategies.

A prenatal interruption of DISC1 function in the brain exhibits a lasting impact on adult behaviors, brain metabolism, and interneuron development

Mental illnesses like schizophrenia (SCZ) and major depression disorder (MDD) are devastating brain disorders. The SCZ risk gene, disrupted in schizophrenia 1 (DISC1), has been associated with neuropsychiatric conditions. However, little is known regarding the long-lasting impacts on brain metabolism and behavioral outcomes from genetic insults on fetal NPCs during early life. We have established a new mouse model that specifically interrupts DISC1 functions in NPCs in vivo by a dominant-negative DISC1 (DN-DISC1) with a precise temporal and spatial regulation. Interestingly, prenatal interruption of mouse Disc1 function in NPCs leads to abnormal depression-like deficit in adult mice. Here we took a novel unbiased metabonomics approach to identify brain-specific metabolites that are significantly changed in DN-DISC1 mice. Surprisingly, the inhibitory neurotransmitter, GABA, is augmented. Consistently, parvalbumin (PV) interneurons are increased in the cingulate cortex, retrosplenial granular cortex, and motor cortex. Interestingly, somatostatin (SST) positive and neuropeptide Y (NPY) interneurons are decreased in some brain regions, suggesting that DN-DISC1 expression affects the localization of interneuron subtypes. To further explore the cellular mechanisms that cause this change, DN-DISC1 suppresses proliferation and promotes the cell cycle exit of progenitors in the medial ganglionic eminence (MGE), whereas it stimulates ectopic proliferation of neighboring cells through cell non-autonomous effect. Mechanistically, it modulates GSK3 activity and interrupts Dlx2 activity in the Wnt activation. In sum, our results provide evidence that specific genetic insults on NSCs at a short period of time could lead to prolonged changes of brain metabolism and development, eventually behavioral defects.

Cornu Ammonis Regions-Antecedents of Cortical Layers?

Studying neocortex and hippocampus in parallel, we are struck by the similarities. All three to four layered allocortices and the six layered mammalian neocortex arise in the pallium. All receive and integrate multiple cortical and subcortical inputs, provide multiple outputs and include an array of neuronal classes. During development, each cell positions itself to sample appropriate local and distant inputs and to innervate appropriate targets. Simpler cortices had already solved the need to transform multiple coincident inputs into serviceable outputs before neocortex appeared in mammals. Why then do phylogenetically more recent cortices need multiple pyramidal cell layers? A simple answer is that more neurones can compute more complex functions. The dentate gyrus and hippocampal CA regions-which might be seen as hippocampal antecedents of neocortical layers-lie side by side, albeit around a tight bend. Were the millions of cells of rat neocortex arranged in like fashion, the surface area of the CA pyramidal cell layers would be some 40 times larger. Even if evolution had managed to fold this immense sheet into the space available, the distances between neurones that needed to be synaptically connected would be huge and to maintain the speed of information transfer, massive, myelinated fiber tracts would be needed. How much more practical to stack the "cells that fire and wire together" into narrow columns, while retaining the mechanisms underlying the extraordinary precision with which circuits form. This demonstrably efficient arrangement presents us with challenges, however, not the least being to categorize the baffling array of neuronal subtypes in each of five "pyramidal layers." If we imagine the puzzle posed by this bewildering jumble of apical dendrites, basal dendrites and axons, from many different pyramidal and interneuronal classes, that is encountered by a late-arriving interneurone insinuating itself into a functional circuit, we can perhaps begin to understand why definitive classification, covering every aspect of each neurone's structure and function, is such a challenge. Here, we summarize and compare the development of these two cortices, the properties of their neurones, the circuits they form and the ordered, unidirectional flow of information from one hippocampal region, or one neocortical layer, to another.

Primary Cilia Signaling Shapes the Development of Interneuronal Connectivity

Appropriate growth and synaptic integration of GABAergic inhibitory interneurons are essential for functional neural circuits in the brain. Here, we demonstrate that disruption of primary cilia function following the selective loss of ciliary GTPase Arl13b in interneurons impairs interneuronal morphology and synaptic connectivity, leading to altered excitatory/inhibitory activity balance. The altered morphology and connectivity of cilia mutant interneurons and the functional deficits are rescued by either chemogenetic activation of ciliary G-protein-coupled receptor (GPCR) signaling or the selective induction of Sstr3, a ciliary GPCR, in Arl13b-deficient cilia. Our results thus define a specific requirement for primary cilia-mediated GPCR signaling in interneuronal connectivity and inhibitory circuit formation.

The Cytokine CXCL12 Promotes Basket Interneuron Inhibitory Synapses in the Medial Prefrontal Cortex

Prenatally, the cytokine CXCL12 regulates cortical interneuron migration, whereas its postnatal functions are poorly understood. Here, we report that CXCL12 is expressed postnatally in layer V pyramidal neurons and localizes on their cell bodies in the medial prefrontal cortex (mPFC), while its receptors CXCR4/CXCR7 localize to the axon terminals of parvalbumin (PV) interneurons. Conditionally eliminating CXCL12 in neonatal layer V pyramidal neurons led to decreased axon targeting and reduced inhibitory perisomatic synapses from PV+ basket interneurons onto layer V pyramidal neurons. Consequently, the mPFC of Cxcl12 conditional mutants displayed attenuated inhibitory postsynaptic currents onto layer V pyramidal neurons. Thus, postnatal CXCL12 signaling promotes a specific interneuron circuit that inhibits mPFC activity.

Developmental Dysfunction of VIP Interneurons Impairs Cortical Circuits

GABAergic interneurons play important roles in cortical circuit development. However, there are multiple populations of interneurons and their respective developmental contributions remain poorly explored. Neuregulin 1 (NRG1) and its interneuron-specific receptor ERBB4 are critical genes for interneuron maturation. Using a conditional ErbB4 deletion, we tested the role of vasoactive intestinal peptide (VIP)-expressing interneurons in the postnatal maturation of cortical circuits in vivo. ErbB4 removal from VIP interneurons during development leads to changes in their activity, along with severe dysregulation of cortical temporal organization and state dependence. These alterations emerge during adolescence, and mature animals in which VIP interneurons lack ErbB4 exhibit reduced cortical responses to sensory stimuli and impaired sensory learning. Our data support a key role for VIP interneurons in cortical circuit development and suggest a possible contribution to pathophysiology in neurodevelopmental disorders. These findings provide a new perspective on the role of GABAergic interneuron diversity in cortical development. VIDEO ABSTRACT.

Role of BACE1 in Alzheimer's synaptic function

Alzheimer's disease (AD) is the most common age-dependent disease of dementia, and there is currently no cure available. This hallmark pathologies of AD are the presence of amyloid plaques and neurofibrillary tangles. Although the exact etiology of AD remains a mystery, studies over the past 30 have shown that abnormal generation or accumulation of β-amyloid peptides (Aβ) is likely to be a predominant early event in AD pathological development. Aβ is generated from amyloid precursor protein (APP) via proteolytic cleavage by β-site APP cleaving enzyme 1 (BACE1). Chemical inhibition of BACE1 has been shown to reduce Aβ in animal studies and in human trials. While BACE1 inhibitors are currently being tested in clinical trials to treat AD patients, it is highly important to understand whether BACE1 inhibition will significantly impact cognitive functions in AD patients. This review summarizes the recent studies on BACE1 synaptic functions. This knowledge will help to guide the proper use of BACE1 inhibitors in AD therapy.

Neuregulin-Dependent Regulation of Fast-Spiking Interneuron Excitability Controls the Timing of the Critical Period

Maturation of excitatory drive onto fast-spiking interneurons (FS INs) in the visual cortex has been implicated in the control of the timing of the critical period for ocular dominance plasticity. However, the mechanisms that regulate the strength of these synapses over cortical development are not understood. Here we use a mouse model to show that neuregulin (NRG) and the receptor tyrosine kinase erbB4 regulate the timing of the critical period. NRG1 enhanced the strength of excitatory synapses onto FS INs, which inhibited ocular dominance plasticity during the critical period but rescued plasticity in transgenics with hypoexcitable FS INs. Blocking the effects of endogenous neuregulin via inhibition of erbBs rescued ocular dominance plasticity in postcritical period adults, allowing recovery from amblyopia induced by chronic monocular deprivation. Thus, the strength of excitation onto FS INs is a key determinant of critical period plasticity and is maintained at high levels by NRG-erbB4 signaling to constrain plasticity in adulthood.

SIGNIFICANCE STATEMENT

Despite decades of experimentation, the mechanisms by which critical periods of enhanced synaptic plasticity are initiated and terminated are not completely understood. Here we show that neuregulin (NRG) and the receptor tyrosine kinase erbB4 determine critical period timing by controlling the strength of excitatory synapses onto FS INs. NRG1 enhanced excitatory drive onto fast spiking interneurons, which inhibited ocular dominance plasticity in juveniles but rescued plasticity in transgenics with hypoexcitable FS INs. Blocking the effects of endogenous neuregulin via inhibition of erbBs rescued ocular dominance plasticity in adults, allowing recovery from amblyopia induced by chronic monocular deprivation. Thus, in contrast to prevailing views of the termination of the critical period, active maintenance of strong excitation onto FS INs constrains plasticity in adults.

Epidermal growth factor signals attenuate phenotypic and functional development of neocortical GABA neurons

Phenotypic development of neocortical GABA neurons is highly plastic and promoted by various neurotrophic factors such as neuregulin-1. A subpopulation of GABA neurons expresses not only neuregulin receptor (ErbB4) but also epidermal growth factor (EGF) receptor (ErbB1) during development, but the neurobiological action of EGF on this cell population is less understood than that of neuregulin-1. Here, we examined the effects of exogenous EGF on immature GABA neurons both in culture and in vivo and also explored physiological consequences in adults. We prepared low density cultures from the neocortex of rat embryos and treated neocortical neurons with EGF. EGF decreased protein levels of glutamic acid decarboxylases (GAD65 and GAD67), and EGF influences on neuronal survival and glial proliferation were negligible or limited. The EGF treatment also diminished the frequency of miniature inhibitory postsynaptic currents (mIPSCs). In vivo administration of EGF to mouse pups reproduced the above GABAergic phenomena in neocortical culture. In EGF-injected postnatal mice, GAD- and parvalbumin-immunoreactivities were reduced in the frontal cortex. In addition, postnatal EGF treatment decreased mIPSC frequency in, and the density of, GABAergic terminals on pyramidal cells. Although these phenotypic influences on GABA neurons became less marked during development, it later resulted in the reduced β- and γ-powers of sound-evoked electroencephalogram in adults, which is regulated by parvalbumin-positive GABA neurons and implicated in the schizophrenia pathophysiology. These findings suggest that, in contrast to the ErbB4 ligand of neuregulin-1, the ErbB1 ligand of EGF exerts unique maturation-attenuating influences on developing cortical GABAergic neurons.

ErbB4 signaling in the prelimbic cortex regulates fear expression

Many psychiatric diseases such as post-traumatic stress disorder (PTSD) are characterized by abnormal processing of emotional stimuli particularly fear. The medial prefrontal cortex (mPFC) is critically involved in fear expression. However, the molecular mechanisms underlying this process are largely unknown. Neuregulin-1 (NRG1) reportedly regulates pyramidal neuronal activity via ErbB4 receptors, which are abundant in parvalbumin (PV)-expressing interneurons in the PFC. In this study, we aimed to determine how NRG1/ErbB4 signaling in the mPFC modulates fear expression and found that tone-cued fear conditioning increased NRG1 expression in the mPFC. Tone-cued fear conditioning was inhibited following neutralization of endogenous NRG1 and specific inhibition or genetic ablation of ErbB4 in the prelimbic (PL) cortex but not in the infralimbic cortex. Furthermore, ErbB4 deletion specifically in PV neurons impaired tone-cued fear conditioning. Notably, overexpression of ErbB4 in the PL cortex is sufficient to reverse impaired fear conditioning in PV-Cre;ErbB4^-/- mice. Together, these findings identify a previously unknown signaling pathway in the PL cortex that regulates fear expression. As both NRG1 and ErbB4 are risk genes for schizophrenia, our study may shed new light on the pathophysiology of this disorder and help to improve treatments for psychiatric disorders such as PTSD.

Oxidative stress-driven parvalbumin interneuron impairment as a common mechanism in models of schizophrenia

Parvalbumin inhibitory interneurons (PVIs) are crucial for maintaining proper excitatory/inhibitory balance and high-frequency neuronal synchronization. Their activity supports critical developmental trajectories, sensory and cognitive processing, and social behavior. Despite heterogeneity in the etiology across schizophrenia and autism spectrum disorder, PVI circuits are altered in these psychiatric disorders. Identifying mechanism(s) underlying PVI deficits is essential to establish treatments targeting in particular cognition. On the basis of published and new data, we propose oxidative stress as a common pathological mechanism leading to PVI impairment in schizophrenia and some forms of autism. A series of animal models carrying genetic and/or environmental risks relevant to diverse etiological aspects of these disorders show PVI deficits to be all accompanied by oxidative stress in the anterior cingulate cortex. Specifically, oxidative stress is negatively correlated with the integrity of PVIs and the extracellular perineuronal net enwrapping these interneurons. Oxidative stress may result from dysregulation of systems typically affected in schizophrenia, including glutamatergic, dopaminergic, immune and antioxidant signaling. As convergent end point, redox dysregulation has successfully been targeted to protect PVIs with antioxidants/redox regulators across several animal models. This opens up new perspectives for the use of antioxidant treatments to be applied to at-risk individuals, in close temporal proximity to environmental impacts known to induce oxidative stress.

Erbb4 Deletion from Medium Spiny Neurons of the Nucleus Accumbens Core Induces Schizophrenia-Like Behaviors via Elevated GABAA Receptor α1 Subunit Expression

Medium spiny neurons (MSNs), the major GABAergic projection neurons in the striatum, are implicated in many neuropsychiatric diseases such as schizophrenia, but the underlying mechanisms remain unclear. We found that a deficiency in Erbb4, a schizophrenia risk gene, in MSNs of the nucleus accumbens (NAc) core, but not the dorsomedial striatum, markedly induced schizophrenia-like behaviors such as hyperactivity, abnormal marble-burying behavior, damaged social novelty recognition, and impaired sensorimotor gating function in male mice. Using immunohistochemistry, Western blot, RNA interference, electrophysiology, and behavior test studies, we found that these phenomena were mediated by increased GABA_A receptor α1 subunit (GABA_AR α1) expression, which enhanced inhibitory synaptic transmission on MSNs. These results suggest that Erbb4 in MSNs of the NAc core may contribute to the pathogenesis of schizophrenia by regulating GABAergic transmission and raise the possibility that GABA_AR α1 may therefore serve as a new therapeutic target for schizophrenia.SIGNIFICANCE STATEMENT Although ErbB4 is highly expressed in striatal medium spiny neurons (MSNs), its role in this type of neuron has not been reported previously. The present study demonstrates that Erbb4 deletion in nucleus accumbens (NAc) core MSNs can induce schizophrenia-like behaviors via elevated GABA_A receptor α1 subunit (GABA_AR α1) expression. To our knowledge, this is the first evidence that ErbB4 signaling in the MSNs is involved in the pathology of schizophrenia. Furthermore, restoration of GABA_AR α1 in the NAc core, but not the dorsal medium striatum, alleviated the abnormal behaviors. Here, we highlight the role of the NAc core in the pathogenesis of schizophrenia and suggest that GABA_AR α1 may be a potential pharmacological target for its treatment.

Behavioral, Neurophysiological, and Synaptic Impairment in a Transgenic Neuregulin1 (NRG1-IV) Murine Schizophrenia Model

Schizophrenia is a chronic, disabling neuropsychiatric disorder with complex genetic origins. The development of strategies for genome manipulation in rodents provides a platform for understanding the pathogenic role of genes and for testing novel therapeutic agents. Neuregulin 1 (NRG1), a critical developmental neurotrophin, is associated with schizophrenia. The NRG1 gene undergoes extensive alternative splicing and, to date, little is known about the neurobiology of a novel NRG1 isoform, NRG1-IV, which is increased in the brains of individuals with schizophrenia and associated with genetic risk variation. Here, we developed a transgenic mouse model (NRG1-IV/NSE-tTA) in which human NRG1-IV is selectively overexpressed in a neuronal specific manner. Using a combination of molecular, biochemical, electrophysiological, and behavioral analyses, we demonstrate that NRG1-IV/NSE-tTA mice exhibit abnormal behaviors relevant to schizophrenia, including impaired sensorimotor gating, discrimination memory, and social behaviors. These neurobehavioral phenotypes are accompanied by increases in cortical expression of the NRG1 receptor, ErbB4 and the downstream signaling target, PIK3-p110δ, along with disrupted dendritic development, synaptic pathology, and altered prefrontal cortical excitatory-inhibitory balance. Pharmacological inhibition of p110δ reversed sensorimotor gating and cognitive deficits. These data demonstrate a novel role for NRG1-IV in learning, memory, and neural circuit formation and a potential neurobiological mechanism for schizophrenia risk; show that deficits are pharmacologically reversible in adulthood; and further highlight p110δ as a target for antipsychotic drug development.

SIGNIFICANCE STATEMENT

Schizophrenia is a disabling psychiatric disorder with neurodevelopmental origins. Genes that increase risk for schizophrenia have been identified. Understanding how these genes affect brain development and function is necessary. This work is the first report of a newly generated humanized transgenic mouse model engineered to express human NRG1-IV, an isoform of the NRG1 (Neuregulin 1) gene that is increased in the brains of patients with schizophrenia in association with genetic risk. Using behavioral neuroscience, molecular biology, electrophysiology, and pharmacology, we identify a role for NRG1-IV in learning, memory, and cognition and determine that this relates to brain excitatory-inhibitory balance and changes in ErbB4/PI3K/AKT signaling. Moreover, the study further highlights the potential of targeting the PI3K pathway for the treatment of schizophrenia.

Abnormal wiring of CCK+ basket cells disrupts spatial information coding

The function of cortical GABAergic interneurons is largely determined by their integration into specific neural circuits, but the mechanisms controlling the wiring of these cells remain largely unknown. This is particularly true for a major population of basket cells that express the neuropeptide cholecystokinin (CCK). Here we found that the tyrosine kinase receptor ErbB4 was required for the normal integration into cortical circuits of basket cells expressing CCK and vesicular glutamate transporter 3 (VGlut3). The number of inhibitory synapses made by CCK+VGlut3+ basket cells and the inhibitory drive they exerted on pyramidal cells were reduced in conditional mice lacking ErbB4. Developmental disruption of the connectivity of these cells diminished the power of theta oscillations during exploratory behavior, disrupted spatial coding by place cells, and caused selective alterations in spatial learning and memory in adult mice. These results suggest that normal integration of CCK+ basket cells in cortical networks is key to support spatial coding in the hippocampus.

Layer 3 Excitatory and Inhibitory Circuitry in the Prefrontal Cortex: Developmental Trajectories and Alterations in Schizophrenia

Convergent evidence suggests that schizophrenia is a disorder of neurodevelopment with alterations in both early and late developmental processes hypothesized to contribute to the disease process. Abnormalities in certain clinical features of schizophrenia, such as working memory impairments, depend on distributed neural circuitry including the dorsolateral prefrontal cortex (DLPFC) and appear to arise during the protracted maturation of this circuitry across childhood and adolescence. In particular, the neural circuitry substrate for working memory in primates involves the coordinated activity of excitatory pyramidal neurons and a specific population of inhibitory gamma-aminobutyric acid neurons (i.e., parvalbumin-containing basket cells) in layer 3 of the DLPFC. Understanding the relationships between the normal development of-and the schizophrenia-associated alterations in-the DLPFC circuitry that subserves working memory could provide new insights into the nature of schizophrenia as a neurodevelopmental disorder. Consequently, we review the following in this article: 1) recent findings regarding alterations of DLPFC layer 3 circuitry in schizophrenia, 2) the developmental refinements in this circuitry that occur during the period when the working memory alterations in schizophrenia appear to arise and progress, and 3) how various adverse environmental exposures could contribute to developmental disturbances of this circuitry in individuals with schizophrenia.

GABA metabolism and its role in gamma-band oscillatory activity during auditory processing: An MRS and EEG study

Gamma-aminobutyric acid (GABA) and glutamate are believed to have inhibitory and exhibitory neuromodulatory effects that regulate the brain's response to sensory perception. Furthermore, frequency-specific synchronization of neuronal excitability within the gamma band (30-80 Hz) is attributable to a homeostatic balance between excitation and inhibition. However, our understanding of the physiological mechanism underlying gamma rhythms is based on animal models. Investigations of the relationship between GABA concentrations, glutamate concentrations, and gamma band activity in humans were mostly restricted to the visual cortex and are conflicting. Here, we performed a multimodal imaging study combining magnetic resonance spectroscopy (MRS) with electroencephalography (EEG) in the auditory cortex. In 14 healthy subjects, we investigated the impact of individual differences in GABA and glutamate concentration on gamma band response (GBR) following auditory stimulus presentation. We explored the effects of bulk GABA on the GBR across frequency (30-200 Hz) and time (-200 to 600 ms) and found no significant relationship. Furthermore, no correlations were found between gamma peak frequency or power measures and metabolite concentrations (GABA, glutamate, and GABA/glutamate ratio). These findings suggest that, according to MRS measurements, and given the auditory stimuli used in this study, GABA and glutamate concentrations are unlikely to play a significant role in the inhibitory and excitatory drive in the generation of gamma band activity in the auditory cortex. Hum Brain Mapp 38:3975-3987, 2017. © 2017 Wiley Periodicals, Inc.

A role for GABAergic interneuron diversity in circuit development and plasticity of the neonatal cerebral cortex

GABAergic interneurons are a highly heterogeneous group of cells that are critical for the mature function and development of the neocortex. In terms of the latter, much attention has focused on the well-established role of parvalbumin (PV+)-expressing, fast spiking, basket cells in determining the critical period plasticity. However recent endeavours have started to shed the light on the contribution of other interneuron subtypes to early circuit formation and plasticity. Data suggests that there are significant interactions between PV+ cells and other interneuron subtypes that regulate circuit development in rodents in the first postnatal week. Moreover, a number of these early interactions are transient which points to an important, distinct role for interneuron diversity in setting up emergent neocortical processing.

Cannabinoids as hippocampal network administrators

Extensive pioneering studies performed in the hippocampus have greatly contributed to our knowledge of an endogenous cannabinoid system comprised of the molecular machinery necessary to process endocannabinoid lipid messengers and their associated cannabinoid receptors. Moreover, a foundation of knowledge regarding the function of hippocampal circuits, and its role in supporting synaptic plasticity has facilitated our understanding of the roles cannabinoids play in the diverse behaviors in which the hippocampus participates, in both normal and pathological states. In this review, we present an historical overview of research pertaining to the hippocampal cannabinoid system to provide context in which to understand the participation of the hippocampus in cognition, behavior, and epilepsy. We also examine potential roles for the hippocampal formation in mediating dysfunctional behavior, and assert that these phenomena reflect disordered physiological activity within the hippocampus and its interactions with other brain regions after exposure to synthetic cannabinoids, and the phytocannabinoids found in marijuana, such as Δ⁹-THC and cannabidiol. In this regard, we examine contemporary hypotheses concerning the hippocampal endocannabinoid system's participation in psychotic disorders, schizophrenia, and epilepsy, and examine cannabinoid-sensitive cellular mechanisms contributing to coherent network oscillations as potential contributors to these disorders. This article is part of the Special Issue entitled "A New Dawn in Cannabinoid Neurobiology".

Axonal Type III Nrg1 Controls Glutamate Synapse Formation and GluA2 Trafficking in Hippocampal-Accumbens Connections

Altered neuregulin 1 (Nrg1)/ErbB signaling and glutamatergic hypofunction have been implicated in the pathophysiology of schizophrenia. Here, we employed gene chimeric ventral hippocampus (vHipp)-nucleus accumbens (nAcc) coculture from mouse, electrophysiology, immunocytochemistry, FM1-43 vesicle fusion, and electron microscopy techniques to examine the pre- and postsynaptic mechanisms of genetic deficits in Nrg1/ErbB signaling-induced glutamatergic dysfunctions. Reduced presynaptic type III Nrg1 expression along vHipp axons decreases the number of glutamate synapses and impairs GluA2 trafficking in the postsynaptic nAcc neurons, resulting in decreased frequency and amplitude of miniature EPSCs (mEPSCs). Reduced expression of axonal type III Nrg1 along vHipp projections also decreases functional synaptic vesicle (SV) clustering and vesicular trafficking to presynaptic vHipp axonal terminals. These findings suggest that Nrg1/ErbB signaling modulate glutamatergic transmission via both pre- and postsynaptic mechanisms.

Association between ErbB4 single nucleotide polymorphisms and susceptibility to schizophrenia: A meta-analysis of case-control studies

BACKGROUND

Accumulating studies have reported inconsistent association between ErbB4 single nucleotide polymorphisms (SNPs) and predisposition to schizophrenia. To better interpret this issue, here we conducted a meta-analysis using published case-control studies.

METHODS

We conducted a systematic search of MEDLINE (Pubmed), Embase (Ovid), Web of Science (Thomson-Reuters) to identify relevant references. The association between ErbB4 SNPs and schizophrenia was assessed by odds ratios (ORs) and 95% confidence intervals (CIs). Between-study heterogeneity was evaluated by I squared (I) statistics and Cochran's Q test. To appraise the stability of results, we employed sensitivity analysis by omitting 1 single study each time. To assess the potential publication bias, we conducted trim and fill analysis.

RESULTS

Seven studies published in English comprising 3162 cases and 4264 controls were included in this meta-analysis. Meta-analyses showed that rs707284 is statistically significantly associated with schizophrenia susceptibility among Asian and Caucasian populations under the allelic model (OR = 0.91, 95% CI: 0.83-0.99, P = 0.035). Additionally, a marginal association (P < 0.1) was observed between rs707284 and schizophrenia risk among Asian and Caucasian populations under the recessive (OR = 0.85, 95% CI: 0.72-1.01, P = 0.065) and homozygous (OR = 0.84, 95% CI: 0.68-1.03, P = 0.094) models. In the Asian subgroup, rs707284 was also noted to be marginally associated with schizophrenia under the recessive model (OR = 0.84, 95% CI: 0.70-1.00, P = 0.053). However, no statistically significant association was found between rs839523, rs7598440, rs3748962, and rs2371276 and schizophrenia risk.

CONCLUSION

This meta-analysis suggested that rs707284 may be a potential ErbB4 SNP associated with susceptibility to schizophrenia. Nevertheless, due to the limited sample size in this meta-analysis, more large-scale association studies are still needed to confirm the results.

Variation at NRG1 genotype related to modulation of small-world properties of the functional cortical network

Functional brain networks possess significant small-world (SW) properties. Genetic variation relevant to both inhibitory and excitatory transmission may contribute to modulate these properties. In healthy controls, genotypic variation in Neuregulin 1 (NRG1) related to the risk of psychosis (risk alleles) would contribute to functional SW modulation of the cortical network. Electroencephalographic activity during an odd-ball task was recorded in 144 healthy controls. Then, small-worldness (SWn) was calculated in five frequency bands (i.e., theta, alpha, beta1, beta2 and gamma) for baseline (from -300 to the stimulus onset) and response (150-450 ms post-target stimulus) windows. The SWn modulation was defined as the difference in SWn between both windows. Association between SWn modulation and carrying the risk allele for three single nucleotide polymorphisms (SNP) of NRG1 (i.e., rs6468119, rs6994992 and rs7005606) was assessed. A significant association between three SNPs of NRG1 and the SWn modulation was found, specifically: NRG1 rs6468119 in alpha and beta1 bands; NRG1 rs6994992 in theta band; and NRG1 rs7005606 in theta and beta1 bands. Genetic variation at NRG1 may influence functional brain connectivity through the modulation of SWn properties of the cortical network.

Peripheral Transcription of NRG-ErbB Pathway Genes Are Upregulated in Treatment-Resistant Schizophrenia

Investigation of peripheral gene expression patterns of transcripts within the NRG-ErbB signaling pathway, other than neuregulin-1 (NRG1), among patients with schizophrenia and more specifically treatment-resistant schizophrenia (TRS) is limited. The present study built on our previous work demonstrating elevated levels of NRG1 EGFα, EGFβ, and type I_(Ig2) containing transcripts in TRS by investigating 11 NRG-ErbB signaling pathway mRNA transcripts (NRG2, ErbB1, ErbB2, ErbB3, ErbB4, PIK3CD, PIK3R3, AKT1, mTOR, P70S6K, eIF4EBP1) in whole blood of TRS patients (N = 71) and healthy controls (N = 57). We also examined the effect of clozapine exposure on transcript levels using cultured peripheral blood mononuclear cells (PBMCs) from 15 healthy individuals. Five transcripts (ErbB3, PIK3CD, AKT1, P70S6K, eIF4EBP1) were significantly elevated in TRS patients compared to healthy controls but only expression of P70S6K (P_corrected = 0.018), a protein kinase linked to protein synthesis, cell growth, and cell proliferation, survived correction for multiple testing using the Benjamini-Hochberg method. Investigation of clinical factors revealed that ErbB2, PIK3CD, PIK3R3, AKT1, mTOR, and P70S6K expression were negatively correlated with duration of illness. However, no transcript was associated with chlorpromazine equivalent dose or clozapine plasma levels, the latter supported by our in vitro PBMC clozapine exposure experiment. Taken together with previously published NRG1 results, our findings suggest an overall upregulation of transcripts within the NRG-ErbB signaling pathway among individuals with schizophrenia some of which attenuate over duration of illness. Follow-up studies are needed to determine if the observed peripheral upregulation of transcripts within the NRG-ErbB signaling pathway are specific to TRS or are a general blood-based marker of schizophrenia.

Gene-environment interactions in cortical interneuron development and dysfunction: A review of preclinical studies

Cortical interneurons (cINs) are a diverse group of locally projecting neurons essential to the organization and regulation of neural networks. Though they comprise only ∼20% of neurons in the neocortex, their dynamic modulation of cortical activity is requisite for normal cognition and underlies multiple aspects of learning and memory. While displaying significant morphological, molecular, and electrophysiological variability, cINs collectively function to maintain the excitatory-inhibitory balance in the cortex by dampening hyperexcitability and synchronizing activity of projection neurons, primarily through use of the inhibitory neurotransmitter gamma-aminobutyric acid (GABA). Disruption of the excitatory-inhibitory balance is a common pathophysiological feature of multiple seizure and neuropsychiatric disorders, including epilepsy, schizophrenia, and autism. While most studies have focused on genetic disruption of cIN development in these conditions, emerging evidence indicates that cIN development is exquisitely sensitive to teratogenic disruption. Here, we review key aspects of cIN development, including specification, migration, and integration into neural circuits. Additionally, we examine the mechanisms by which prenatal exposure to common chemical and environmental agents disrupt these events in preclinical models. Understanding how genetic and environmental factors interact to disrupt cIN development and function has tremendous potential to advance prevention and treatment of prevalent seizure and neuropsychiatric illnesses.

Chronic Stress Increases Prefrontal Inhibition: A Mechanism for Stress-Induced Prefrontal Dysfunction

BACKGROUND

Multiple neuropsychiatric disorders, e.g., depression, are linked to imbalances in excitatory and inhibitory neurotransmission and prefrontal cortical dysfunction, and are concomitant with chronic stress.

METHODS

We used electrophysiologic (n = 5-6 animals, 21-25 cells/group), neuroanatomic (n = 6-8/group), and behavioral (n = 12/group) techniques to test the hypothesis that chronic stress increases inhibition of medial prefrontal cortex (mPFC) glutamatergic output neurons.

RESULTS

Using patch clamp recordings from infralimbic mPFC pyramidal neurons, we found that chronic stress selectively increases the frequency of miniature inhibitory postsynaptic currents with no effect on amplitude, which suggests that chronic stress increases presynaptic gamma-aminobutyric acid release. Elevated gamma-aminobutyric acid release under chronic stress is accompanied by increased inhibitory appositions and terminals onto glutamatergic cells, as assessed by both immunohistochemistry and electron microscopy. Furthermore, chronic stress decreases glucocorticoid receptor immunoreactivity specifically in a subset of inhibitory neurons, which suggests that increased inhibitory tone in the mPFC after chronic stress may be caused by loss of a glucocorticoid receptor-mediated brake on interneuron activity. These neuroanatomic and functional changes are associated with impairment of a prefrontal-mediated behavior. During chronic stress, rats initially make significantly more errors in the delayed spatial win-shift task, an mPFC-mediated behavior, which suggests a diminished impact of the mPFC on decision making.

CONCLUSIONS

Taken together, the data suggest that chronic stress increases synaptic inhibition onto prefrontal glutamatergic output neurons, limiting the influence of the prefrontal cortex in control of stress reactivity and behavior. Thus, these data provide a mechanistic link among chronic stress, prefrontal cortical hypofunction, and behavioral dysfunction.

Network abnormalities and interneuron dysfunction in Alzheimer disease

The function of neural circuits and networks can be controlled, in part, by modulating the synchrony of their components' activities. Network hypersynchrony and altered oscillatory rhythmic activity may contribute to cognitive abnormalities in Alzheimer disease (AD). In this condition, network activities that support cognition are altered decades before clinical disease onset, and these alterations predict future pathology and brain atrophy. Although the precise causes and pathophysiological consequences of these network alterations remain to be defined, interneuron dysfunction and network abnormalities have emerged as potential mechanisms of cognitive dysfunction in AD and related disorders. Here, we explore the concept that modulating these mechanisms may help to improve brain function in these conditions.

Neuregulin-1 Regulates Cortical Inhibitory Neuron Dendrite and Synapse Growth through DISC1

Cortical inhibitory neurons play crucial roles in regulating excitatory synaptic networks and cognitive function and aberrant development of these cells have been linked to neurodevelopmental disorders. The secreted neurotrophic factor Neuregulin-1 (NRG1) and its receptor ErbB4 are established regulators of inhibitory neuron connectivity, but the developmental signalling mechanisms regulating this process remain poorly understood. Here, we provide evidence that NRG1-ErbB4 signalling functions through the multifunctional scaffold protein, Disrupted in Schizophrenia 1 (DISC1), to regulate the development of cortical inhibitory interneuron dendrite and synaptic growth. We found that NRG1 increases inhibitory neuron dendrite complexity and glutamatergic synapse formation onto inhibitory neurons and that this effect is blocked by expression of a dominant negative DISC1 mutant, or DISC1 knockdown. We also discovered that NRG1 treatment increases DISC1 expression and its localization to glutamatergic synapses being made onto cortical inhibitory neurons. Mechanistically, we determined that DISC1 binds ErbB4 within cortical inhibitory neurons. Collectively, these data suggest that a NRG1-ErbB4-DISC1 signalling pathway regulates the development of cortical inhibitory neuron dendrite and synaptic growth. Given that NRG1, ErbB4, and DISC1 are schizophrenia-linked genes, these findings shed light on how independent risk factors may signal in a common developmental pathway that contributes to neural connectivity defects and disease pathogenesis.

Nicotine-induced neuroplasticity counteracts the effect of schizophrenia-linked neuregulin 1 signaling on NMDAR function in the rat hippocampus

A high rate of heavy tobacco smoking among people with schizophrenia has been suggested to reflect self-medication and amelioration of cognitive dysfunction, a core feature of schizophrenia. NMDAR hypofunction is hypothesized to be a mechanism of cognitive dysfunction, and excessive schizophrenia-linked neuregulin 1 (NRG1) signaling through its receptor ErbB4 can suppress NMDAR function by preventing Src-mediated enhancement of NMDAR responses. Here we investigated whether chronic nicotine exposure in rats by subcutaneous injection of nicotine (0.5-1 mg/kg, twice daily for 10-15 days) counteracts the suppressive effect of NRG1β on NMDAR-mediated responses recorded from CA1 pyramidal cells in acute hippocampal slices. We found that NRG1β, which prevents the enhancement of NMDAR responses by the Src-family-kinase-activating peptide pYEEI in naive rats, failed to block the effect of pYEEI in nicotine-exposed rats. In naive rats, NRG1β acts only on GluN2B-NMDARs by blocking their Src-mediated upregulation. Chronic nicotine exposure causes enhanced GluN2B-NMDAR responses via Src upregulation and recruits Fyn for the enhancement of GluN2A-NMDAR responses. NRG1β has no effect on both enhanced basal GluN2B-NMDAR responses and Fyn-mediated enhancement of GluN2A-NMDAR responses. Src-mediated enhancement of GluN2B-NMDAR responses and Fyn-mediated enhancement of GluN2A-NMDAR responses initiate long-term potentiation (LTP) of AMPAR synaptic responses in naive and nicotine-exposed CA1 pyramidal cells, respectively. These results suggest that NRG1β suppresses LTP by blocking Src-mediated enhancement of GluN2B-NMDAR responses, but has no effect on LTP in nicotine-exposed rats. These effects of chronic nicotine exposure may counteract the negative effect of increased NRG1-ErbB4 signaling on the cellular mechanisms of learning and memory in individuals with schizophrenia, and therefore may motivate heavy smoking.

Nox-2-Mediated Phenotype Loss of Hippocampal Parvalbumin Interneurons Might Contribute to Postoperative Cognitive Decline in Aging Mice

Postoperative cognitive decline (POCD) is a common complication following anesthesia and surgery, especially in elderly patients; however, the precise mechanisms of POCD remain unclear. Here, we investigated whether nicotinamide adenine dinucleotide phosphate (NADPH) oxidase mediated-abnormalities in parvalbumin (PV) interneurons play an important role in the pathophysiology of POCD. The animal model was established using isoflurane anesthesia and exploratory laparotomy in 16-month-old male C57BL/6 mice. For interventional experiments, mice were chronically treated with the NADPH oxidase inhibitor apocynin (APO). Open field and fear conditioning behavioral tests were performed on day 6 and 7 post-surgery, respectively. In a separate experiment, brain tissue was harvested and subjected to biochemical analysis. Primary hippocampal neurons challenged with lipopolysaccharide (LPS) in vitro were used to investigate the mechanisms underlying the oxidative stress-induced abnormalities in PV interneurons. Our results showed that anesthesia and surgery induced significant hippocampus-dependent memory impairment, which was accompanied by PV interneuron phenotype loss and increased expression of interleukin-1β (IL-1β), markers of oxidative stress and NADPH oxidase 2 (Nox2) in the hippocampus. In addition, LPS exposure increased Nox2 level and decreased the expression of PV and the number of excitatory synapses onto PV interneurons in the primary hippocampal neurons. Notably, treatment with APO reversed these abnormalities. Our study suggests that Nox2-derived reactive oxygen species (ROS) production triggers, at least in part, anesthesia- and surgery-induced hippocampal PV interneuron phenotype loss and consequent cognitive impairment in aging mice.

Increased GABAB receptor signaling in a rat model for schizophrenia

Schizophrenia is a complex disorder that affects cognitive function and has been linked, both in patients and animal models, to dysfunction of the GABAergic system. However, the pathophysiological consequences of this dysfunction are not well understood. Here, we examined the GABAergic system in an animal model displaying schizophrenia-relevant features, the apomorphine-susceptible (APO-SUS) rat and its phenotypic counterpart, the apomorphine-unsusceptible (APO-UNSUS) rat at postnatal day 20-22. We found changes in the expression of the GABA-synthesizing enzyme GAD67 specifically in the prelimbic- but not the infralimbic region of the medial prefrontal cortex (mPFC), indicative of reduced inhibitory function in this region in APO-SUS rats. While we did not observe changes in basal synaptic transmission onto LII/III pyramidal cells in the mPFC of APO-SUS compared to APO-UNSUS rats, we report reduced paired-pulse ratios at longer inter-stimulus intervals. The GABAB receptor antagonist CGP 55845 abolished this reduction, indicating that the decreased paired-pulse ratio was caused by increased GABAB signaling. Consistently, we find an increased expression of the GABAB1 receptor subunit in APO-SUS rats. Our data provide physiological evidence for increased presynaptic GABAB signaling in the mPFC of APO-SUS rats, further supporting an important role for the GABAergic system in the pathophysiology of schizophrenia.

Neuregulin-1/ErbB4 Signaling Regulates Visual Cortical Plasticity

Experience alters cortical networks through neural plasticity mechanisms. During a developmental critical period, the most dramatic consequence of occluding vision through one eye (monocular deprivation) is a rapid loss of excitatory synaptic inputs to parvalbumin-expressing (PV) inhibitory neurons in visual cortex. Subsequent cortical disinhibition by reduced PV cell activity allows for excitatory ocular dominance plasticity. However, the molecular mechanisms underlying critical period synaptic plasticity are unclear. Here we show that brief monocular deprivation during the critical period downregulates neuregulin-1(NRG1)/ErbB4 signaling in PV neurons, causing retraction of excitatory inputs to PV neurons. Exogenous NRG1 rapidly restores excitatory inputs onto deprived PV cells through downstream PKC-dependent activation and AMPA receptor exocytosis, thus enhancing PV neuronal inhibition to excitatory neurons. NRG1 treatment prevents the loss of deprived eye visual cortical responsiveness in vivo. Our findings reveal molecular, cellular, and circuit mechanisms of NRG1/ErbB4 in regulating the initiation of critical period visual cortical plasticity.

Redox dysregulation, neuroinflammation, and NMDA receptor hypofunction: A "central hub" in schizophrenia pathophysiology?

Accumulating evidence points to altered GABAergic parvalbumin-expressing interneurons and impaired myelin/axonal integrity in schizophrenia. Both findings could be due to abnormal neurodevelopmental trajectories, affecting local neuronal networks and long-range synchrony and leading to cognitive deficits. In this review, we present data from animal models demonstrating that redox dysregulation, neuroinflammation and/or NMDAR hypofunction (as observed in patients) impairs the normal development of both parvalbumin interneurons and oligodendrocytes. These observations suggest that a dysregulation of the redox, neuroimmune, and glutamatergic systems due to genetic and early-life environmental risk factors could contribute to the anomalies of parvalbumin interneurons and white matter in schizophrenia, ultimately impacting cognition, social competence, and affective behavior via abnormal function of micro- and macrocircuits. Moreover, we propose that the redox, neuroimmune, and glutamatergic systems form a "central hub" where an imbalance within any of these "hub" systems leads to similar anomalies of parvalbumin interneurons and oligodendrocytes due to the tight and reciprocal interactions that exist among these systems. A combination of vulnerabilities for a dysregulation within more than one of these systems may be particularly deleterious. For these reasons, molecules, such as N-acetylcysteine, that possess antioxidant and anti-inflammatory properties and can also regulate glutamatergic transmission are promising tools for prevention in ultra-high risk patients or for early intervention therapy during the first stages of the disease.

Language deficits in schizophrenia and autism as related oscillatory connectomopathies: An evolutionary account

Schizophrenia (SZ) and autism spectrum disorders (ASD) are characterised by marked language deficits, but it is not clear how these arise from gene mutations associated with the disorders. Our goal is to narrow the gap between SZ and ASD and, ultimately, give support to the view that they represent abnormal (but related) ontogenetic itineraries for the human faculty of language. We will focus on the distinctive oscillatory profiles of the SZ and ASD brains, in turn using these insights to refine our understanding of how the brain implements linguistic computations by exploring a novel model of linguistic feature-set composition. We will argue that brain rhythms constitute the best route to interpreting language deficits in both conditions and mapping them to neural dysfunction and risk alleles of the genes. Importantly, candidate genes for SZ and ASD are overrepresented among the gene sets believed to be important for language evolution. This translational effort may help develop an understanding of the aetiology of SZ and ASD and their high prevalence among modern populations.

Functions of the Alzheimer's Disease Protease BACE1 at the Synapse in the Central Nervous System

Inhibition of the protease β-site amyloid precursor protein-cleaving enzyme 1 (BACE1) is a promising treatment strategy for Alzheimer's disease, and a number of BACE inhibitors are currently progressing through clinical trials. The strategy aims to decrease production of amyloid-β (Aβ) peptide from the amyloid precursor protein (APP), thus reducing or preventing Aβ toxicity. Over the last decade, it has become clear that BACE1 proteolytically cleaves a number of substrates in addition to APP. These substrates are not known to be involved in the pathogenesis of Alzheimer's disease but have other roles in the developing and/or mature central nervous system. Consequently, BACE inhibition and knockout in mice results in synaptic and other neuronal dysfunctions and the key substrates responsible for these deficits are still being elucidated. Of the BACE1 substrates that have been validated to date, a number may contribute to the synaptic deficits seen with BACE blockade, including neuregulin 1, close homologue of L1 and seizure-related gene 6. It is important to understand the impact that BACE blockade may have on these substrates and other proteins detected in substrate screens and, if necessary, develop substrate-selective BACE inhibitors.

The reemergence of long-term potentiation in aged Alzheimer's disease mouse model

Mouse models of Alzheimer’s disease (AD) have been developed to study the pathophysiology of amyloid β protein (Aβ) toxicity, which is thought to cause severe clinical symptoms such as memory impairment in AD patients. However, inconsistencies exist between studies using these animal models, specifically in terms of the effects on synaptic plasticity, a major cellular model of learning and memory. Whereas some studies find impairments in plasticity in these models, others do not. We show that long-term potentiation (LTP), in the CA1 region of hippocampal slices from this mouse, is impared at Tg2576 adult 6–7 months old. However, LTP is inducible again in slices taken from Tg2576 aged 14–19 months old. In the aged Tg2576, we found that the percentage of parvalbumin (PV)-expressing interneurons in hippocampal CA1-3 region is significantly decreased, and LTP inhibition or reversal mediated by NRG1/ErbB signaling, which requires ErbB4 receptors in PV interneurons, is impaired. Inhibition of ErbB receptor kinase in adult Tg2576 restores LTP but impairs depotentiation as shown in aged Tg2576. Our study suggests that hippocampal LTP reemerges in aged Tg2576. However, this reemerged LTP is an insuppressible form due to impaired NRG1/ErbB signaling, possibly through the loss of PV interneurons.

A Transient Translaminar GABAergic Interneuron Circuit Connects Thalamocortical Recipient Layers in Neonatal Somatosensory Cortex

GABAergic activity is thought to influence developing neocortical sensory circuits. Yet the late postnatal maturation of local layer (L)4 circuits suggests alternate sources of GABAergic control in nascent thalamocortical networks. We show that a population of L5b, somatostatin (SST)-positive interneuron receives early thalamic synaptic input and, using laser-scanning photostimulation, identify an early transient circuit between these cells and L4 spiny stellates (SSNs) that disappears by the end of the L4 critical period. Sensory perturbation disrupts the transition to a local GABAergic circuit, suggesting a link between translaminar and local control of SSNs. Conditional silencing of SST+ interneurons or conversely biasing the circuit toward local inhibition by overexpression of neuregulin-1 type 1 results in an absence of early L5b GABAergic input in mutants and delayed thalamic innervation of SSNs. These data identify a role for L5b SST+ interneurons in the control of SSNs in the early postnatal neocortex.

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Early postnatal thalamic synaptic input onto L5b somatostatin interneurons

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Transient reciprocal connectivity between L5b INs and L4 spiny stellate cells

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Sensory activity is required for the transition to a local L4 GABAergic circuit

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Molecular bias toward early local IN synapses delays thalamic innervation of SSNs

Involvement of cortical fast-spiking parvalbumin-positive basket cells in epilepsy

GABAergic interneurons of the parvalbumin-positive fast-spiking basket cells subtype (PV INs) are important regulators of cortical network excitability and of gamma oscillations, involved in signal processing and cognition. Impaired development or function of PV INs has been associated with epilepsy in various animal models of epilepsy, as well as in some genetic forms of epilepsy in humans. In this review, we provide an overview of some of the experimental data linking PV INs dysfunction with epilepsy, focusing on disorders of the specification, migration, maturation, synaptic function, or connectivity of PV INs. Furthermore, we reflect on the potential therapeutic use of cell-type specific stimulation of PV INs within active networks and on the transplantation of PV INs precursors in the treatment of epilepsy and its comorbidities.