Parvalbumin and GAD65 interneuron inhibition in the ventral hippocampus induces distinct behavioral deficits relevant to schizophrenia

Prenatal tetrahydrocannabinol and cannabidiol exposure produce sex-specific pathophysiological phenotypes in the adolescent prefrontal cortex and hippocampus

Clinical and preclinical evidence has demonstrated an increased risk for neuropsychiatric disorders following prenatal cannabinoid exposure. However, given the phytochemical complexity of cannabis, there is a need to understand how specific components of cannabis may contribute to these neurodevelopmental risks later in life. To investigate this, a rat model of prenatal cannabinoid exposure was utilized to examine the impacts of specific cannabis constituents (Δ9-tetrahydrocannabinol [THC]; cannabidiol [CBD]) alone and in combination on future neuropsychiatric liability in male and female offspring. Prenatal THC and CBD exposure were associated with low birth weight. At adolescence, offspring displayed sex-specific behavioural changes in anxiety, temporal order and social cognition, and sensorimotor gating. These phenotypes were associated with sex and treatment-specific neuronal and gene transcriptional alterations in the prefrontal cortex, and ventral hippocampus, regions where the endocannabinoid system is implicated in affective and cognitive development. Electrophysiology and RT-qPCR analysis in these regions implicated dysregulation of the endocannabinoid system and balance of excitatory and inhibitory signalling in the developmental consequences of prenatal cannabinoids. These findings reveal critical insights into how specific cannabinoids can differentially impact the developing fetal brains of males and females to enhance subsequent neuropsychiatric risk.

The Pathophysiological Underpinnings of Gamma-Band Alterations in Psychiatric Disorders

Investigating the biophysiological substrates of psychiatric illnesses is of great interest to our understanding of disorders' etiology, the identification of reliable biomarkers, and potential new therapeutic avenues. Schizophrenia represents a consolidated model of γ alterations arising from the aberrant activity of parvalbumin-positive GABAergic interneurons, whose dysfunction is associated with perineuronal net impairment and neuroinflammation. This model of pathogenesis is supported by molecular, cellular, and functional evidence. Proof for alterations of γ oscillations and their underlying mechanisms has also been reported in bipolar disorder and represents an emerging topic for major depressive disorder. Although evidence from animal models needs to be further elucidated in humans, the pathophysiology of γ-band alteration represents a common denominator for different neuropsychiatric disorders. The purpose of this narrative review is to outline a framework of converging results in psychiatric conditions characterized by γ abnormality, from neurochemical dysfunction to alterations in brain rhythms.

Ventral hippocampal cholecystokinin interneurons gate contextual reward memory

Associating contexts with rewards depends on hippocampal circuits, with local inhibitory interneurons positioned to play an important role in shaping activity. Here, we demonstrate that the encoding of context-reward memory requires a ventral hippocampus (vHPC) to nucleus accumbens (NAc) circuit that is gated by cholecystokinin (CCK) interneurons. In a sucrose conditioned place preference (CPP) task, optogenetically inhibiting vHPC-NAc terminals impaired the acquisition of place preference. Transsynaptic rabies tracing revealed vHPC-NAc neurons were monosynaptically innervated by CCK interneurons. Using intersectional genetic targeting of CCK interneurons, ex vivo optogenetic activation of CCK interneurons increased GABAergic transmission onto vHPC-NAc neurons, while in vivo optogenetic inhibition of CCK interneurons increased cFos in these projection neurons. Notably, CCK interneuron inhibition during sucrose CPP learning increased time spent in the sucrose-associated location, suggesting enhanced place-reward memory. Our findings reveal a previously unknown hippocampal microcircuit crucial for modulating the strength of contextual reward learning.

Auditory training remodels hippocampus-related memory in adult rats

Consequences of perceptual training, such as improvements in discriminative ability, are highly stimulus and task specific. Therefore, most studies on auditory training-induced plasticity in adult brain have focused on the sensory aspects, particularly on functional and structural effects in the auditory cortex. Auditory training often involves, other than auditory demands, significant cognitive components. Yet, how auditory training affects cognition-related brain regions, such as the hippocampus, remains unclear. Here, we found in female rats that auditory cue-based go/no-go training significantly improved the memory-guided behaviors associated with hippocampus. The long-term potentiations of the trained rats recorded in vivo in the hippocampus were also enhanced compared with the naïve rats. In parallel, the phosphorylation level of calcium/calmodulin-dependent protein kinase II and the expression of parvalbumin-positive interneurons in the hippocampus were both upregulated. These findings demonstrate that auditory training substantially remodels the processing and function of brain regions beyond the auditory system, which are associated with task demands.

Depletion of microglia with PLX3397 attenuates MK-801-induced hyperactivity associated with regulating inflammation-related genes in the brain

Acute administration of MK-801 (dizocilpine), an N-methyl-D-aspartate receptor (NMDAR) antagonist, can establish animal models of psychiatric disorders. However, the roles of microglia and inflammation-related genes in these animal models of psychiatric disorders remain unknown. Here, we found rapid elimination of microglia in the prefrontal cortex (PFC) and hippocampus (HPC) of mice following administration of the dual colony-stimulating factor 1 receptor (CSF1R)/c-Kit kinase inhibitor PLX3397 (pexidartinib) in drinking water. Single administration of MK-801 induced hyperactivity in the open-field test (OFT). Importantly, PLX3397-induced depletion of microglia prevented the hyperactivity and schizophrenia-like behaviors induced by MK-801. However, neither repopulation of microglia nor inhibition of microglial activation by minocycline affected MK-801-induced hyperactivity. Importantly, microglial density in the PFC and HPC was significantly correlated with behavioral changes. In addition, common and distinct glutamate-, GABA-, and inflammation-related gene (116 genes) expression patterns were observed in the brains of PLX3397- and/or MK-801-treated mice. Moreover, 10 common inflammation-related genes ( CD68, CD163, CD206, TMEM119, CSF3R, CX3CR1, TREM2, CD11b, CSF1R, and F4/80) with very strong correlations were identified in the brain using hierarchical clustering analysis. Further correlation analysis demonstrated that the behavioral changes in the OFT were most significantly associated with the expression of inflammation-related genes ( NLRP3, CD163, CD206, F4/80, TMEM119, and TMEM176a), but not glutamate- or GABA-related genes in PLX3397- and MK-801-treated mice. Thus, our results suggest that microglial depletion via a CSF1R/c-Kit kinase inhibitor can ameliorate the hyperactivity induced by an NMDAR antagonist, which is associated with modulation of immune-related genes in the brain.

Voluntary running-induced activation of ventral hippocampal GABAergic interneurons contributes to exercise-induced hypoalgesia in neuropathic pain model mice

The exact mechanism of exercise-induced hypoalgesia (EIH) in exercise therapy to improve chronic pain has not been fully clarified. Recent studies have suggested the importance of the ventral hippocampus (vHPC) in inducing chronic pain. We investigated the effects of voluntary running (VR) on FosB⁺ cells and GABAergic interneurons (parvalbumin-positive [PV⁺] and somatostatin-positive [SOM⁺]) in the vHPC-CA1 in neuropathic pain (NPP) model mice. VR significantly improved thermal hyperalgesia in the NPP model. The number of the FosB⁺ cells was significantly higher in partial sciatic nerve ligation-sedentary mice than in Sham and Naive mice, whereas VR significantly suppressed the FosB⁺ cells in the vHPC-CA1. Furthermore, VR significantly increased the proportion of activated PV⁺ and SOM⁺ interneurons in the vHPC-CA1, and tracer experiments indicated that approximately 24% of neurons projecting from the vHPC-CA1 to the basolateral nucleus of amygdala were activated in NPP mice. These results indicate that feedforward suppression of the activated neurons via VR-induced activation of GABAergic interneurons in the vHPC-CA1 may be a mechanism to produce EIH effects, and suggested that disappearance of negative emotions such as fear and anxiety by VR may play a critical role in improving chronic pain.

Anxiety-related activity of ventral hippocampal interneurons

Anxiety is an aversive mood reflecting the anticipation of potential threats. The ventral hippocampus (vH) is a key brain region involved in the genesis of anxiety responses. Recent studies have shown that anxiety is mediated by the activation of vH pyramidal neurons targeting various limbic structures. Throughout the cortex, the activity of pyramidal neurons is controlled by GABA-releasing inhibitory interneurons and the GABAergic system represents an important target of anxiolytic drugs. However, how the activity of vH inhibitory interneurons is related to different anxiety behaviours has not been investigated so far. Here, we integrated in vivo electrophysiology with behavioural phenotyping of distinct anxiety exploration behaviours in rats. We showed that pyramidal neurons and interneurons of the vH are selectively active when animals explore specific compartments of the elevated-plus-maze (EPM), an anxiety task for rodents. Moreover, rats with prior goal-related experience exhibited low-anxiety exploratory behaviour and showed a larger trajectory-related activity of vH interneurons during EPM exploration compared to high anxiety rats. Finally, in low anxiety rats, trajectory-related vH interneurons exhibited opposite activity to pyramidal neurons specifically in the open arms (i.e. more anxiogenic) of the EPM. Our results suggest that vH inhibitory micro-circuits could act as critical elements underlying different anxiety states.

Got milk? Maternal immune activation during the mid-lactational period affects nutritional milk quality and adolescent offspring sensory processing in male and female rats

Previous studies have underscored the importance of breastfeeding and parental care on offspring development and behavior. However, their contribution as dynamic variables in animal models of early life stress are often overlooked. In the present study, we investigated how lipopolysaccharide (LPS)-induced maternal immune activation (MIA) on postnatal day (P)10 affects maternal care, milk, and offspring development. MIA was associated with elevated milk corticosterone concentrations on P10, which recovered by P11. In contrast, both milk triglyceride and percent creamatocrit values demonstrated a prolonged decrease following inflammatory challenge. Adolescent MIA offspring were heavier, which is often suggestive of poor early life nutrition. While MIA did not decrease maternal care quality, there was a significant compensatory increase in maternal licking and grooming the day following inflammatory challenge. However, this did not protect against disrupted neonatal huddling or later-life alterations in sensorimotor gating, conditioned fear, mechanical allodynia, or reductions in hippocampal parvalbumin expression in MIA offspring. MIA-associated changes in brain and behavior were likely driven by differences in milk nutritional values and not by direct exposure to LPS or inflammatory molecules as neither LPS binding protein nor interleukin-6 milk levels differed between groups. These findings reflected comparable microbiome and transcriptomic patterns at the genome-wide level. Animal models of early life stress can impact both parents and their offspring. One mechanism that can mediate the effects of such stressors is changes to maternal lactation quality which our data show can confer multifaceted and compounding effects on offspring physiology and behavior.

Relevance of interactions between dopamine and glutamate neurotransmission in schizophrenia

Dopamine (DA) and glutamate neurotransmission are strongly implicated in schizophrenia pathophysiology. While most studies focus on contributions of neurons that release only DA or glutamate, neither DA nor glutamate models alone recapitulate the full spectrum of schizophrenia pathophysiology. Similarly, therapeutic strategies limited to either system cannot effectively treat all three major symptom domains of schizophrenia: positive, negative, and cognitive symptoms. Increasing evidence suggests extensive interactions between the DA and glutamate systems and more effective treatments may therefore require the targeting of both DA and glutamate signaling. This offers the possibility that disrupting DA-glutamate circuitry between these two systems, particularly in the striatum and forebrain, culminate in schizophrenia pathophysiology. Yet, the mechanisms behind these interactions and their contributions to schizophrenia remain unclear. In addition to circuit- or system-level interactions between neurons that solely release either DA or glutamate, here we posit that functional alterations involving a subpopulation of neurons that co-release both DA and glutamate provide a novel point of integration between DA and glutamate systems, offering a key missing link in our understanding of schizophrenia pathophysiology. Better understanding of mechanisms underlying DA/glutamate co-release from these neurons may therefore shed new light on schizophrenia pathophysiology and lead to more effective therapeutics.

Schizophrenia Hypothesis: Autonomic Nervous System Dysregulation of Fetal and Adult Immune Tolerance

The autonomic nervous system can control immune cell activation via both sympathetic adrenergic and parasympathetic cholinergic nerve release of norepinephrine and acetylcholine. The hypothesis put forward in this paper suggests that autonomic nervous system dysfunction leads to dysregulation of immune tolerance mechanisms in brain-resident and peripheral immune cells leading to excessive production of pro-inflammatory cytokines such as Tumor Necrosis Factor alpha (TNF-α). Inactivation of Glycogen Synthase Kinase-3β (GSK3β) is a process that takes place in macrophages and microglia when a toll-like receptor 4 (TLR4) ligand binds to the TLR4 receptor. When Damage-Associated Molecular Patterns (DAMPS) and Pathogen-Associated Molecular Patterns (PAMPS) bind to TLR4s, the phosphatidylinositol-3-kinase (PI3K)-protein kinase B (Akt) pathway should be activated, leading to inactivation of GSK3β. This switches the macrophage from producing pro-inflammatory cytokines to anti-inflammatory cytokines. Acetylcholine activation of the α7 subunit of the nicotinic acetylcholine receptor (α7 nAChR) on the cell surface of immune cells leads to PI3K/Akt pathway activation and can control immune cell polarization. Dysregulation of this pathway due to dysfunction of the prenatal autonomic nervous system could lead to impaired fetal immune tolerance mechanisms and a greater vulnerability to Maternal Immune Activation (MIA) resulting in neurodevelopmental abnormalities. It could also lead to the adult schizophrenia patient’s immune system being more vulnerable to chronic stress-induced DAMP release. If a schizophrenia patient experiences chronic stress, an increased production of pro-inflammatory cytokines such as TNF-α could cause significant damage. TNF-α could increase the permeability of the intestinal and blood brain barrier, resulting in lipopolysaccharide (LPS) and TNF-α translocation to the brain and consequent increases in glutamate release. MIA has been found to reduce Glutamic Acid Decarboxylase mRNA expression, resulting in reduced Gamma-aminobutyric acid (GABA) synthesis, which combined with an increase of glutamate release could result in an imbalance of glutamate and GABA neurotransmitters. Schizophrenia could be a “two-hit” illness comprised of a genetic “hit” of autonomic nervous system dysfunction and an environmental hit of MIA. This combination of factors could lead to neurotransmitter imbalance and the development of psychotic symptoms.

Increasing the Excitatory Drive Rescues Excitatory/Inhibitory Imbalance and Mismatch Negativity Deficit Caused by Parvalbumin Specific GluA1 Deletion

Disturbance in synaptic excitatory and inhibitory (E/I) transmission in the prefrontal cortex is considered a critical factor for cognitive dysfunction, a core symptom in schizophrenia. However, the cortical network pathophysiology induced by E/I imbalance is not well characterized, and an effective therapeutic strategy is lacking. In this study, we simulated imbalanced cortical network by using mice with parvalbumin neuron (PV) specific knockout of GluA1 (AMPA receptor subunit 1) (Gria1-PV KO) as an experimental model. Applying high-content confocal imaging and electrophysiological recordings in the medial prefrontal cortex (mPFC), we found structural and functional alterations in the local network of Gria1-PV KO mice. Additionally, we applied electroencephalography (EEG) to assess potential deficits in mismatch negativity (MMN), the standard readout in the clinic for measuring deviance detection and sensory information processing. Gria1-PV KO animals exhibited abnormal theta oscillation and MMN, which is consistent with clinical findings in cognitively impaired patients. Remarkably, we demonstrated that the glycine transporter 1 (GlyT1) inhibitor, Bitopertin, ameliorates E/I imbalance, hyperexcitability, and sensory processing malfunction in Gria1-PV KO mice. Our results suggest that PV-specific deletion of GluA1 might be an experimental approach for back translating the E/I imbalance observed in schizophrenic patients. Our work offers a systematic workflow to understand the effect of GlyT1 inhibition in restoring cortical network activity from single cells to local brain circuitry. This study highlights that selectively boosting NMDA receptor-mediated excitatory drive to enhance the network inhibitory transmission from interneurons to pyramidal neurons (PYs) is a potential therapeutic strategy for restoring E/I imbalance-associated cognitive-related abnormality.

Structural and Functional Deviations of the Hippocampus in Schizophrenia and Schizophrenia Animal Models

Schizophrenia is a grave neuropsychiatric disease which frequently onsets between the end of adolescence and the beginning of adulthood. It is characterized by a variety of neuropsychiatric abnormalities which are categorized into positive, negative and cognitive symptoms. Most therapeutical strategies address the positive symptoms by antagonizing D2-dopamine-receptors (DR). However, negative and cognitive symptoms persist and highly impair the life quality of patients due to their disabling effects. Interestingly, hippocampal deviations are a hallmark of schizophrenia and can be observed in early as well as advanced phases of the disease progression. These alterations are commonly accompanied by a rise in neuronal activity. Therefore, hippocampal formation plays an important role in the manifestation of schizophrenia. Furthermore, studies with animal models revealed a link between environmental risk factors and morphological as well as electrophysiological abnormalities in the hippocampus. Here, we review recent findings on structural and functional hippocampal abnormalities in schizophrenic patients and in schizophrenia animal models, and we give an overview on current experimental approaches that especially target the hippocampus. A better understanding of hippocampal aberrations in schizophrenia might clarify their impact on the manifestation and on the outcome of this severe disease.

MK212, a 5-hydroxytryptamine 2C receptor agonist, reverses prepulse inhibition deficits in the medial prefrontal cortex and ventral hippocampus

Prepulse inhibition (PPI) is disrupted in many neuropsychiatric diseases. Molecules such as 5-HT_2C receptor agonists alleviate PPI deficits in rodents; however, the precise mechanisms and critical regions of the brain responsible for the reversal effect of these agonists remain inconclusive. The present study aimed to investigate the areas of the brain critical for the reversal effect of 5-HT_2C receptor agonists on PPI deficits in mice. The results showed that systemic administration of the 5-HT_2C receptor agonist MK212 did not affect normal PPI behavior, but reversed the PPI deficits induced by the N-methyl d-aspartate receptor antagonist MK801 in mice. In addition, the 5-HT_2C receptor antagonist SB242084 had no effect on PPI behavior despite MK801 treatment. Moreover, local infusion of MK212 into the medial prefrontal cortex and ventral hippocampus, excluding the nucleus accumbens or ventral tegmental area, rescued the PPI deficits induced by MK801. These data suggest that the medial prefrontal cortex and ventral hippocampus are critical brain areas responsible for the reversal of 5-HT_2C agonists on PPI deficits. The results will contribute to our current knowledge on the molecular and neural mechanisms underlying the antipsychotic effects of 5-HT_2C receptor agonists, especially the neural circuits modulated by 5-HT_2C receptor activity.

Pimavanserin, a 5-hydroxytryptamine 2A receptor inverse agonist, reverses prepulse inhibition deficits in the nucleus accumbens and ventral hippocampus

Prepulse inhibition (PPI) is disrupted in many neuropsychiatric diseases. Although the inverse agonist of the 5-hydroxytryptamine 2A (5-HT2A) receptors, pimavanserin, alleviates PPI deficits in rodents, the precise mechanisms and critical brain areas in the reversal effect of 5-HT2A receptor inverse agonists remain unclear. The present study aimed to investigate the critical brain areas responsible for the reversal effect of the 5-HT2A receptor inverse agonist on PPI deficits in male mice. The results showed that intraperitoneal administration of pimavanserin was found to improve normal PPI behavior and reverse PPI deficits elicited by the dopamine D1/D2 receptor nonselective agonist, pergolide. Further, local infusion of pimavanserin into the nucleus accumbens and ventral hippocampus reversed PPI deficits, whereas the same manipulation in the medial prefrontal cortex or ventral tegmental area did not reverse PPI deficits. Overall, the nucleus accumbens and ventral hippocampus are the critical brain areas responsible for the reversal effect of 5-HT2A inverse agonists on PPI deficits. Such findings contribute to the extensive exploration of the accurate molecular and neural mechanisms underlying the antipsychotic effects of 5-HT2A receptor inverse agonists, especially the neural circuits modulated by 5-HT2A receptor activity.

A Selective Review of the Excitatory-Inhibitory Imbalance in Schizophrenia: Underlying Biology, Genetics, Microcircuits, and Symptoms

Schizophrenia is a chronic disorder characterized by specific positive and negative primary symptoms, social behavior disturbances and cognitive deficits (e.g., impairment in working memory and cognitive flexibility). Mounting evidence suggests that altered excitability and inhibition at the molecular, cellular, circuit and network level might be the basis for the pathophysiology of neurodevelopmental and neuropsychiatric disorders such as schizophrenia. In the past decades, human and animal studies have identified that glutamate and gamma-aminobutyric acid (GABA) neurotransmissions are critically involved in several cognitive progresses, including learning and memory. The purpose of this review is, by analyzing emerging findings relating to the balance of excitatory and inhibitory, ranging from animal models of schizophrenia to clinical studies in patients with early onset, first-episode or chronic schizophrenia, to discuss how the excitatory-inhibitory imbalance may relate to the pathophysiology of disease phenotypes such as cognitive deficits and negative symptoms, and highlight directions for appropriate therapeutic strategies.

Semantic priming and neurobiology in schizophrenia: A theoretical review

In this theoretical review we bridge the cognitive and neurobiological sciences to shed light on the neurocognitive foundations of the semantic priming effect in schizophrenia. We review and theoretically evaluate the neurotransmitter systems (dopaminergic, GABAergic and glutamatergic) and neurobiological underpinnings of behavioural and electrophysiological (N400) semantic priming in the pathology, and the main hypotheses on their geneses: a disinhibition of the semantic spread of activation, a disorganised semantic storage or noisy lexical-semantic associations, a psychomotor artefact, an artefact of relatedness proportions, or an inability to mobilise contextual information. We further assess the literature on the endophenotype of Formal Thought Disorder from multiple standpoints, ranging from neurophysiology to cognition: considerations are weaved on neuronal (PV basket cell, SST, VIP) and receptor deficits (DRD1, NMDA), neurotransmitter imbalances (dopamine), cortical and dopaminergic lateralisation, inter alia. In conclusion, we put forth novel postulates on the underlying causes of controlled hypopriming, automatic hyperpriming, N400 reversals (larger amplitudes for close associations), indirect versus direct hyperpriming, and the endophenotype of lexical-semantic disturbances in schizophrenia.

Viral Vector Delivery of DREADDs for CNS Therapy

Designer Receptors Exclusively Activated by Designer Drugs (DREADDs) are genetically modified G-protein-coupled receptors (GPCRs), that can be activated by a synthetic ligand which is otherwise inert at endogenous receptors. DREADDs can be expressed in cells in the central nervous system (CNS) and subsequently offer the opportunity for remote and reversible silencing or activation of the target cells when the synthetic ligand is systemically administered. In neuroscience, DREADDs have thus far shown to be useful tools for several areas of research and offer considerable potential for the development of gene therapy strategies for neurological disorders. However, in order to design a DREADD-based gene therapy, it is necessary to first evaluate the viral vector delivery methods utilised in the literature to deliver these chemogenetic tools. This review evaluates each of the prominent strategies currently utilised for DREADD delivery, discussing their respective advantages and limitations. We focus on adeno-associated virus (AAV)-based and lentivirus-based systems, and the manipulation of these through cell-type specific promoters and pseudotyping. Furthermore, we address how virally mediated DREADD delivery could be improved in order to make it a viable gene therapy strategy and thus expand its translational potential.

Behavioral Alterations and Decreased Number of Parvalbumin-Positive Interneurons in Wistar Rats after Maternal Immune Activation by Lipopolysaccharide: Sex Matters

Maternal immune activation (MIA) during pregnancy represents an important environmental factor in the etiology of schizophrenia and autism spectrum disorders (ASD). Our goal was to investigate the impacts of MIA on the brain and behavior of adolescent and adult offspring, as a rat model of these neurodevelopmental disorders. We injected bacterial lipopolysaccharide (LPS, 1 mg/kg) to pregnant Wistar dams from gestational day 7, every other day, up to delivery. Behavior of the offspring was examined in a comprehensive battery of tasks at postnatal days P45 and P90. Several brain parameters were analyzed at P28. The results showed that prenatal immune activation caused social and communication impairments in the adult offspring of both sexes; males were affected already in adolescence. MIA also caused prepulse inhibition deficit in females and increased the startle reaction in males. Anxiety and hypolocomotion were apparent in LPS-affected males and females. In the 28-day-old LPS offspring, we found enlargement of the brain and decreased numbers of parvalbumin-positive interneurons in the frontal cortex in both sexes. To conclude, our data indicate that sex of the offspring plays a crucial role in the development of the MIA-induced behavioral alterations, whereas changes in the brain apparent in young animals are sex-independent.

Hippocampal atrophy is associated with psychotic symptom severity following traumatic brain injury

Psychosis is a rare, but particularly serious sequela of traumatic brain injury. However, little is known as to the neurobiological processes that may contribute to its onset. Early evidence suggests that psychotic symptom development after traumatic brain injury may co-occur with hippocampal degeneration, invoking the possibility of a relationship. Particularly regarding the hippocampal head, these degenerative changes may lead to dysregulation in dopaminergic circuits, as is reported in psychoses due to schizophrenia, resulting in the positive symptom profile typically seen in post-injury psychosis. The objective of this study was to examine change in hippocampal volume and psychotic symptoms across time in a sample of moderate-to-severe traumatic brain injury patients. We hypothesized that hippocampal volume loss would be associated with increased psychotic symptom severity. From a database of n = 137 adult patients with prospectively collected, longitudinal imaging and neuropsychiatric outcomes, n = 24 had complete data at time points of interest (5 and 12 months post-traumatic brain injury) and showed increasing psychotic symptom severity on the Personality Assessment Inventory psychotic experiences subscale of the schizophrenia clinical scale across time. Secondary analysis employing stepwise regression with hippocampal volume change (independent variable) and Personality Assessment Inventory psychotic symptom change (dependent variable) from 5 to 12 months post-injury was conducted including age, sex, marijuana use, family history of schizophrenia, years of education and injury severity as control variables. Total right hippocampal volume loss predicted an increase in the Personality Assessment Inventory psychotic experiences subscale (F_{(1, 22)} = 5.396, adjusted R² = 0.161, P = 0.030; β = −0.017, 95% confidence interval = −0.018, −0.016) as did volume of the right hippocampal head (F_{(1, 22)} = 5.764, adjusted R² = 0.172, P = 0.025; β = −0.019, 95% confidence interval = −0.021, −0.017). Final model goodness-of-fit was confirmed using k-fold (k = 5) cross-validation. Consistent with our hypotheses, the current findings suggest that hippocampal degeneration in the chronic stages of moderate-to-severe traumatic brain injury may play a role in the delayed onset of psychotic symptoms after traumatic brain injury. These findings localized to the right hippocampal head are supportive of a proposed aetiological mechanism whereby atrophy of the hippocampal head may lead to the dysregulation of dopaminergic networks following traumatic brain injury; possibly accounting for observed clinical features of psychotic disorder after traumatic brain injury (including prolonged latency period to symptom onset and predominance of positive symptoms). If further validated, these findings may bear important clinical implications for neurorehabilitative therapies following traumatic brain injury.

Functional Dysregulations in CA1 Hippocampal Networks of a 3-Hit Mouse Model of Schizophrenia

For a better translation from treatment designs of schizophrenia to clinical efficiency, there is a crucial need to refine preclinical animal models. In order to consider the multifactorial nature of the disorder, a new mouse model associating three factors (genetic susceptibility-partial deletion of the MAP6 gene, early-life stress-maternal separation, and pharmacological treatment-chronic Δ-9-tetrahydrocannabinol during adolescence) has recently been described. While this model depicts a schizophrenia-like phenotype, the neurobiological correlates remain unknown. Synaptic transmission and functional plasticity of the CA1 hippocampal region of male and female 3-hit mice were therefore investigated using electrophysiological recordings on the hippocampus slice. While basal excitatory transmission remained unaffected, NMDA receptor (NMDAr)-mediated long-term potentiation (LTP) triggered by theta-burst (TBS) but not by high-frequency (HFS) stimulation was impaired in 3-hit mice. Isolated NMDAr activation was not affected or even increased in female 3-hit mice, revealing a sexual dimorphism. Considering that the regulation of LTP is more prone to inhibitory tone if triggered by TBS than by HFS, the weaker potentiation in 3-hit mice suggests a deficiency of intrinsic GABA regulatory mechanisms. Indeed, NMDAr activation was increased by GABAA receptor blockade in wild-type but not in 3-hit mice. This electrophysiological study highlights dysregulations of functional properties and plasticity in hippocampal networks of 3-hit mice, one of the mechanisms suspected to contribute to the pathophysiology of schizophrenia. It also shows differences between males and females, supporting the sexual dimorphism observed in the disorder. Combined with the previously reported study, the present data reinforce the face validity of the 3-hit model that will help to consider new therapeutic strategies for psychosis.

Neurodevelopmental insights into circuit dysconnectivity in schizophrenia

Schizophrenia is increasingly being recognized as a disorder of brain circuits of developmental origin. Animal models, however, have been technically limited in exploring the effects of early developmental circuit abnormalities on the maturation of the brain and associated behavioural outputs. This review discusses evidence of the developmental emergence of circuit abnormalities in schizophrenia, followed by a critical assessment on how animal models need to be adapted through optimized tools in order to spatially and temporally manipulate early developmental events, thereby providing insight into the causal contribution of developmental perturbations to schizophrenia.

Behavioral Tasks Evaluating Schizophrenia-like Symptoms in Animal Models: A Recent Update

BACKGROUND

Schizophrenia is a serious mental illness that affects more than 21 million people worldwide. Both genetics and the environment play a role in its etiology and pathogenesis. Symptoms of schizophrenia are mainly categorized into positive, negative, and cognitive. One major approach to identify and understand these diverse symptoms in humans has been to study behavioral phenotypes in a range of animal models of schizophrenia.

OBJECTIVE

We aimed to provide a comprehensive review of the behavioral tasks commonly used for measuring schizophrenia-like behaviors in rodents together with an update of the recent study findings.

METHODS

Articles describing phenotypes of schizophrenia-like behaviors in various animal models were collected through a literature search in Google Scholar, PubMed, Web of Science, and Scopus, with a focus on advances over the last 10 years.

RESULTS

Numerous studies have used a range of animal models and behavioral paradigms of schizophrenia to develop antipsychotic drugs for improved therapeutics. In establishing animal models of schizophrenia, the candidate models were evaluated for schizophrenia-like behaviors using several behavioral tasks for positive, negative, and cognitive symptoms designed to verify human symptoms of schizophrenia. Such validated animal models were provided as rapid preclinical avenues for drug testing and mechanistic studies.

CONCLUSION

Based on the most recent advances in the field, it is apparent that a myriad of behavior tests are needed to confirm and evaluate the congruency of animal models with the numerous behaviors and clinical signs exhibited by patients with schizophrenia.

Hippocampal Hyperactivity as a Druggable Circuit-Level Origin of Aberrant Salience in Schizophrenia

The development of current neuroleptics was largely aiming to decrease excessive dopaminergic signaling in the striatum. However, the notion that abnormal dopamine creates psychotic symptoms by causing an aberrant assignment of salience that drives maladaptive learning chronically during disease development suggests a therapeutic value of early interventions that correct salience-related neural processing. The mesolimbic dopaminergic output is modulated by several interconnected brain-wide circuits centrally involving the hippocampus and key relays like the ventral and associative striatum, ventral pallidum, amygdala, bed nucleus of the stria terminalis, nucleus reuniens, lateral and medial septum, prefrontal and cingulate cortex, among others. Unraveling the causal relationships between these circuits using modern neuroscience techniques holds promise for identifying novel cellular—and ultimately molecular—treatment targets for reducing transition to psychosis and symptoms of schizophrenia. Imaging studies in humans have implicated a hyperactivity of the hippocampus as a robust and early endophenotype in schizophrenia. Experiments in rodents, in turn, suggested that the activity of its output region—the ventral subiculum—may modulate dopamine release from ventral tegmental area (VTA) neurons in the ventral striatum. Even though these observations suggested a novel circuit-level target for anti-psychotic action, no therapy has yet been developed along this rationale. Recently evaluated treatment strategies—at least in part—target excess glutamatergic activity, e.g. N-acetyl-cysteine (NAC), levetiracetam, and mGluR2/3 modulators. We here review the evidence for the central implication of the hippocampus-VTA axis in schizophrenia-related pathology, discuss its symptom-related implications with a particular focus on aberrant assignment of salience, and evaluate some of its short-comings and prospects for drug discovery.

Modulating microglia activation prevents maternal immune activation induced schizophrenia-relevant behavior phenotypes via arginase 1 in the dentate gyrus

Prenatal infection during pregnancy increases the risk for developing neuropsychiatric disorders such as schizophrenia. This is linked to an inflammatory microglial phenotype in the offspring induced by maternal immune activation (MIA). Microglia are crucial for brain development and maintenance of neuronal niches, however, whether and how their activation is involved in the regulation of neurodevelopment remains unclear. Here, we used a MIA rodent model in which polyinosinic: polycytidylic acid (poly (I:C)) was injected into pregnant mice. We found fewer parvalbumin positive (PV+) cells and impaired GABAergic transmission in the dentate gyrus (DG), accompanied by schizophrenia-like behavior in the adult offspring. Minocycline, a potent inhibitor of microglia activation, successfully prevented the above-mentioned deficits in the offspring. Furthermore, by using microglia-specific arginase 1 (Arg1) ablation as well as overexpression in DG, we identified a critical role of Arg1 in microglia activation to protect against poly (I:C) imparted neuropathology and altered behavior in offspring. Taken together, our results highlight that Arg1-mediated alternative activation of microglia are potential therapeutic targets for psychiatric disorders induced by MIA.

Enhancing excitatory projections from the ventral subiculum to the nucleus accumbens shell contribute to the MK-801-induced impairment of prepulse inhibition

Prepulse inhibition (PPI), a measure of sensorimotor gating, has been shown to be disrupted in several animal models of neuropsychiatric disorders, such as schizophrenia. The neural circuits involving the hippocampus and nucleus accumbens (NAC) have been studied in rats to uncover the neurochemical and neuroanatomical substrates that regulate PPI. Majority of the studies of the hippocampus on PPI to date have been focused on CA1, CA2, and dentate gyrus (DG) area. Little is known about the role of the subiculum, which maintains the hippocampal formation intact, on the sensorimotor gating. In this study, the PPI disruption was induced by intraperitoneal injection of MK-801 in rats, and the neuronal activity in the dorsal and ventral subiculum by c-Fos immunostaining was examined. The projections from the subiculum to the nucleus accumbens (NAC) were detected by retrograde tracing of cholera toxin B subunit, in the PPI dysfunctional animals. The results showed an increase in neuronal activity in the ventral subiculum (vSub) while remaining constant in the dorsal subiculum during PPI disruption. The excitatory projections from the vSub to the NAC shell were significantly enhanced when PPI was disrupted. Muscimol Inhibition of vSub could significantly ameliorate the MK801-induced PPI deficit. This data suggests that the enhancement of neuronal activity in the vSub was associated with the PPI impairment, possibly due to the enhanced excitatory output from vSub the NAC shell.

Hyperexcitable Parvalbumin Interneurons Render Hippocampal Circuitry Vulnerable to Amyloid Beta

Parvalbumin (PV) interneuron dysfunction is associated with various brain disorders, including Alzheimer disease (AD). Here, we asked whether early PV neuron hyperexcitability primes the hippocampus for amyloid beta-induced functional impairment. We show that prolonged chemogenetic activation of PV neurons induces long-term hyperexcitability of these cells, disrupts synaptic transmission, and causes spatial memory deficits on the short-term. On the long-term, pyramidal cells also become hyperexcitable, and synaptic transmission and spatial memory are restored. However, under these conditions of increased excitability of both PV and pyramidal cells, a single low-dose injection of amyloid beta directly into the hippocampus significantly impairs PV neuron function, increases pyramidal neuron excitability, and reduces synaptic transmission, resulting in significant spatial memory deficits. Taken together, our data show that an initial hyperexcitable state of PV neurons renders hippocampal function vulnerable to amyloid beta and may contribute to an increased risk for developing AD.

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Hyperexcitable hippocampal PV neurons impair spatial memory

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When excitability of pyramidal neurons also increases, spatial memory is restored

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This overall network hyperstate is particularly sensitive to amyloid-beta toxicity

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PV neuron hyperexcitability increases risk for Alzheimer disease

Local Perturbations of Cortical Excitability Propagate Differentially Through Large-Scale Functional Networks

Electrophysiological recordings have established that GABAergic interneurons regulate excitability, plasticity, and computational function within local neural circuits. Importantly, GABAergic inhibition is focally disrupted around sites of brain injury. However, it remains unclear whether focal imbalances in inhibition/excitation lead to widespread changes in brain activity. Here, we test the hypothesis that focal perturbations in excitability disrupt large-scale brain network dynamics. We used viral chemogenetics in mice to reversibly manipulate parvalbumin interneuron (PV-IN) activity levels in whisker barrel somatosensory cortex. We then assessed how this imbalance affects cortical network activity in awake mice using wide-field optical neuroimaging of pyramidal neuron GCaMP dynamics as well as local field potential recordings. We report 1) that local changes in excitability can cause remote, network-wide effects, 2) that these effects propagate differentially through intra- and interhemispheric connections, and 3) that chemogenetic constructs can induce plasticity in cortical excitability and functional connectivity. These findings may help to explain how focal activity changes following injury lead to widespread network dysfunction.

Protein-protein interactions underlying the behavioral and psychological symptoms of dementia (BPSD) and Alzheimer's disease

Alzheimer’s Disease (AD) is a devastating neurodegenerative disorder currently affecting 45 million people worldwide, ranking as the 6^th highest cause of death. Throughout the development and progression of AD, over 90% of patients display behavioral and psychological symptoms of dementia (BPSD), with some of these symptoms occurring before memory deficits and therefore serving as potential early predictors of AD-related cognitive decline. However, the biochemical links between AD and BPSD are not known. In this study, we explored the molecular interactions between AD and BPSD using protein-protein interaction (PPI) networks built from OMIM (Online Mendelian Inheritance in Man) genes that were related to AD and two distinct BPSD domains, the Affective Domain and the Hyperactivity, Impulsivity, Disinhibition, and Aggression (HIDA) Domain. Our results yielded 8 unique proteins for the Affective Domain (RHOA, GRB2, PIK3R1, HSPA4, HSP90AA1, GSK3beta, PRKCZ, and FYN), 5 unique proteins for the HIDA Domain (LRP1, EGFR, YWHAB, SUMO1, and EGR1), and 6 shared proteins between both BPSD domains (APP, UBC, ELAV1, YWHAZ, YWHAE, and SRC) and AD. These proteins might suggest specific targets and pathways that are involved in the pathogenesis of these BPSD domains in AD.

A new inbred strain of Fawn-Hooded rats demonstrates mania-like behavioural and monoaminergic abnormalities

The Fawn-Hooded (FH) rat carries a gene mutation that results in a dysfunctional serotoninergic system. However, previous studies have reported differing features between the FH/Wjd and FH/Har strains. We aimed to compare the behavioural and neurobiological features of FH/HamSlc rats with those of Fischer 344 rats. We performed the open field, elevated minus-maze, Y-maze spontaneous alternation, and forced swim tests to investigate behavioural alterations. We also assessed neurobiological characteristics by quantifying monoamines and their related compounds in the prefrontal cortex, hippocampus, and striatum using high-performance liquid chromatography with an electrochemical detection system. FH/HamSlc rats showed hyperactivity and a high impulsivity tendency in the open field and the elevated minus maze test, but no cognitive dysfunction. In addition, the hyperactivity was suppressed immediately after the forced swim test. FH/HamSlc rats showed low dopamine levels, but high dopamine turnover in the striatum. Serotonin and noradrenaline levels were low in the prefrontal cortex and the hippocampus of FH/HamSlc rats, but high serotonin turnover was observed in the prefrontal cortex, hippocampus, and striatum. FH/HamSlc rats show (1) mania-like behavioural characteristics that are different from those of other strains of FH rats; (2) stimulus dependent suppression of hyperactivity similar to the clinical findings that exercise alleviates the symptoms of bipolar disorder; and (3) monoaminergic dysregulation such as monoamine imbalance and hyperturnover that may be associated with mania-related behavioural characteristics. Thus, the FH/HamSlc rat is a new animal model for mania including bipolar disorder.

Inhibitory designer receptors aggravate memory loss in a mouse model of down syndrome

The pontine nucleus locus coeruleus (LC) is the primary source of noradrenergic (NE) projections to the brain and is important for working memory, attention, and cognitive flexibility. Individuals with Down syndrome (DS) develop Alzheimer's disease (AD) with high penetrance and often exhibit working memory deficits coupled with degeneration of LC-NE neurons early in the progression of AD pathology. Designer receptors exclusively activated by designer drugs (DREADDs) are chemogenetic tools that allow targeted manipulation of discrete neuronal populations in the brain without the confounds of off-target effects. We utilized male Ts65Dn mice (a mouse model for DS), and male normosomic (NS) controls to examine the effects of inhibitory DREADDs delivered via an AAV vector under translational control of the synthetic PRSx8, dopamine β hydroxylase (DβH) promoter. This chemogenetic tool allowed LC inhibition upon administration of the inert DREADD ligand, clozapine-N-oxide (CNO). DREADD-mediated LC inhibition impaired performance in a novel object recognition task and reversal learning in a spatial task. DREADD-mediated LC inhibition gave rise to an elevation of α-adrenoreceptors both in NS and in Ts65Dn mice. Further, microglial markers showed that the inhibitory DREADD stimulation led to increased microglial activation in the hippocampus in Ts65Dn but not in NS mice. These findings strongly suggest that LC signaling is important for intact memory and learning in Ts65Dn mice and disruption of these neurons leads to increased inflammation and dysregulation of adrenergic receptors.

Plasma neuropeptides as circulating biomarkers of multifactorial schizophrenia

BACKGROUND

Promising biomarkers would be used to improve the determination of diagnosis and severity, as well as the prediction of symptomatic and functional outcomes of schizophrenia.

BASIC PROCEDURES

In this study, we used three different mouse models induced by a genetic factor (PV-Cre; ErbB4^-/-, G group), an environmental stressor (adolescent social isolation, G group), and a combination of genetic factor and environmental stressor (PV-Cre; ErbB4^-/- mice with isolation, G × E group). Attenuated PPI (%) confirmed the successful establishment of three schizophrenia-like mouse models. To evaluate whether neuropeptide levels in plasma would be potential biomarkers of different schizophrenia models in our work, we used MILLIPLEX® MAP method to simultaneously measure 6 critical neuropeptides in plasma.

MAIN FINDINGS

Among the evaluated neuropeptides, increased neurotensin tends to be associated with genetic factors of schizophrenia, increased orexin A seems to be a biomarker of an interplay between genetic and social isolation, while higher plasma oxytocin might be more apt to be responsive to social isolation. The potential biomarkers are mostly independent of sex.

CONCLUSIONS

This research would provide novel clues to develop circulating biomarkers of plasma neuropeptides for multifactorial schizophrenia.

Schizophrenia-like reduced sensorimotor gating in intact inbred and outbred rats is associated with decreased medial prefrontal cortex activity and volume

Prepulse inhibition (PPI) of startle response is a measure of sensorimotor gating that is impaired in schizophrenia and in many other clinical conditions. Rat models using pharmacological or surgical strategies reveal that PPI is modulated by the cortico-striatal-pallido-thalamic (CSPT) circuit. Here, we explore whether spontaneous variation in PPI in intact inbred and outbred rats is associated with functional and structural differences in the CSPT circuit. Inbred Roman High-(RHA) and Low-avoidance (RLA) and outbred heterogeneous stock (HS) rats were assessed for PPI, brain activity, and brain volume. Brain activity was assessed by c-Fos expression and brain volume by magnetic resonance imaging. Relevant structures of the CSPT circuit were evaluated, such as the medial prefrontal cortex (mPFC), cingulate cortex, hippocampus (HPC), amygdala, nucleus accumbens (NAc), and dorsal striatum. RHA showed lower PPI than RLA rats, while HS rats were stratified by their PPI levels in three groups. Reduced PPI was accompanied by decreased mPFC activity in Roman and HS rats and increased NAc shell activity in HS rats. Low PPI was also associated with decreased mPFC and HPC volumes in Roman and HS rats. This study reports a consistent relationship between decreased function and volume of the mPFC and spontaneous low-PPI levels in inbred and outbred intact rats. Moreover, our findings suggest that, apart from a hypoactive and smaller mPFC, a hyperactive NAc and smaller HPC may underlie reduced PPI levels. Our results support the notion that sensorimotor gating is modulated by forebrain structures and highlight the importance of the mPFC in its regulation.

The impact of silencing feed-forward parvalbumin-expressing inhibitory interneurons in the cortico-thalamocortical network on seizure generation and behaviour

Feed-forward inhibition (FFI) is an essential mechanism within the brain, to regulate neuronal firing and prevent runaway excitation. In the cortico-thalamocortical (CTC) network, fast spiking parvalbumin-expressing (PV+) inhibitory interneurons regulate the firing of pyramidal cells in the cortex and relay neurons in the thalamus. PV+ interneuron dysfunction has been implicated in several neurological disorders, including epilepsy. Previously, we demonstrated that loss of excitatory AMPA-receptors, specifically at synapses on PV+ interneurons in CTC feedforward microcircuits, occurs in the stargazer mouse model of absence epilepsy. These mice present with absence seizures characterized by spike and wave discharges (SWDs) on electroencephalogram (EEG) and concomitant behavioural arrest, similar to childhood absence epilepsy. The aim of the current study was to investigate the impact of loss of FFI within the CTC on absence seizure generation and behaviour using new Designer Receptor Exclusively Activated by Designer Drug (DREADD) technology. We crossed PV-Cre mice with Cre-dependent hM4Di DREADD strains of mice, which allowed Cre-recombinase-mediated restricted expression of inhibitory G_i-DREADDs in PV+ interneurons. We then tested the impact of global and focal (within the CTC network) silencing of PV+ interneurons. CNO mediated silencing of all PV+ interneurons by intraperitoneal injection caused the impairment of motor control, decreased locomotion and increased anxiety in a dose-dependent manner. Such silencing generated pathological oscillations similar to absence-like seizures. Focal silencing of PV+ interneurons within cortical or thalamic feedforward microcircuits, induced SWD-like oscillations and associated behavioural arrest. Epileptiform activity on EEG appeared significantly sooner after focal injection compared to peripheral injection of CNO. However, the mean duration of each oscillatory burst and spike frequency was similar, irrespective of mode of CNO delivery. No significant changes were observed in vehicle-treated or non-DREADD wild-type control animals. These data suggest that dysfunctional feed-forward inhibition in CTC microcircuits may be an important target for future therapy strategies for some patients with absence seizures. Additionally, silencing of PV+ interneurons in other brain regions may contribute to anxiety related neurological and psychiatric disorders.

Selective Activation of Cholecystokinin-Expressing GABA (CCK-GABA) Neurons Enhances Memory and Cognition

Cholecystokinin-expressing GABAergic (CCK-GABA) neurons are perisomatic inhibitory cells that have been argued to regulate emotion and sculpt the network oscillations associated with cognition. However, no study has selectively manipulated CCK-GABA neuron activity during behavior in freely-moving animals. To explore the behavioral effects of activating CCK-GABA neurons on emotion and cognition, we utilized a novel intersectional genetic mouse model coupled with a chemogenetic approach. Specifically, we generated triple transgenic CCK-Cre;Dlx5/6-Flpe;RC::FL-hM3Dq (CCK-GABA/hM3Dq) mice that expressed the synthetic excitatory hM3Dq receptor in CCK-GABA neurons. Results showed that clozapine-N-oxide (CNO)-mediated activation of CCK-GABA neurons did not alter open field (OF) or tail suspension (TS) performance and only slightly increased anxiety in the elevated plus maze (EPM). Although CNO treatment had only modestly affected emotional behavior, it significantly enhanced multiple cognitive and memory behaviors including social recognition, contextual fear conditioning, contextual discrimination, object recognition, and problem-solving in the puzzle box. Collectively, these findings suggest that systemic activation of CCK-GABA neurons minimally affects emotion but significantly enhances cognition and memory. Our results imply that CCK-GABA neurons are more functionally diverse than originally expected and could serve as a potential therapeutic target for the treatment of cognitive/memory disorders.

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.

Schizophrenia-related cognitive dysfunction in the Cyclin-D2 knockout mouse model of ventral hippocampal hyperactivity

Elevated activity at the output stage of the anterior hippocampus has been described as a physiological endophenotype of schizophrenia, and its development maps onto the transition from the prodromal to the psychotic state. Interventions that halt the spreading glutamatergic over-activity in this region and thereby the development of overt schizophrenia could be promising therapies. However, animal models with high construct validity to support such pre-clinical development are scarce. The Cyclin-D2 knockout (CD2-KO) mouse model shows a hippocampal parvalbumin-interneuron dysfunction, and its pattern of hippocampal over-activity shares similarities with that seen in prodromal patients. Conducting a comprehensive phenotyping of CD2-KO mice, we found that they displayed novelty-induced hyperlocomotion (a rodent correlate of positive symptoms of schizophrenia), that was largely resistant against D1- and D2-dopamine-receptor antagonism, but responsive to the mGluR2/3-agonist LY379268. In the negative symptom domain, CD2-KO mice showed transiently reduced sucrose-preference (anhedonia), but enhanced interaction with novel mice and objects, as well as normal nest building and incentive motivation. Also, unconditioned anxiety, perseveration, and motor-impulsivity were unaltered. However, in the cognitive domain, CD2-knockouts showed reduced executive function in assays of rule-shift and rule-reversal learning, and also an impairment in working memory, that was resistant against LY379268-treatment. In contrast, sustained attention and forms of spatial and object-related memory that are mediated by short-term habituation of stimulus-specific attention were intact. Our results suggest that CD2-KO mice are a valuable model in translational research targeted at the pharmacoresistant cognitive symptom domain in causal relation to hippocampal over-activity in the prodrome-to-psychosis transition.

Role of Endogenous Metabolite Alterations in Neuropsychiatric Disease

The potential role in neuropsychiatric disease risk arising from alterations and derangements of endogenous small-molecule metabolites remains understudied. Alterations of endogenous metabolite concentrations can arise in multiple ways. Marked derangements of single endogenous small-molecule metabolites are found in a large group of rare genetic human diseases termed "inborn errors of metabolism", many of which are associated with prominent neuropsychiatric symptomology. Whether such metabolites act neuroactively to directly lead to distinct neural dysfunction has been frequently hypothesized but rarely demonstrated unequivocally. Here we discuss this disease concept in the context of our recent findings demonstrating that neural dysfunction arising from accumulation of the schizophrenia-associated metabolite l-proline is due to its structural mimicry of the neurotransmitter GABA that leads to alterations in GABA-ergic short-term synaptic plasticity. For cases in which a similar direct action upon neurotransmitter binding sites is suspected, we lay out a systematic approach that can be extended to assessing the potential disruptive action of such candidate disease metabolites. To address the potentially important and broader role in neuropsychiatric disease, we also consider whether the more subtle yet more ubiquitous variations in endogenous metabolites arising from natural allelic variation may likewise contribute to disease risk but in a more complex and nuanced manner.

Restoring wild-type-like CA1 network dynamics and behavior during adulthood in a mouse model of schizophrenia

Schizophrenia is a severely debilitating neurodevelopmental disorder. Establishing a causal link between circuit dysfunction and particular behavioral traits that are relevant to schizophrenia is crucial to shed new light on the mechanisms underlying the pathology. We studied an animal model of the human 22q11 deletion syndrome, the mutation that represents the highest genetic risk of developing schizophrenia. We observed a desynchronization of hippocampal neuronal assemblies that resulted from parvalbumin interneuron hypoexcitability. Rescuing parvalbumin interneuron excitability with pharmacological or chemogenetic approaches was sufficient to restore wild-type-like CA1 network dynamics and hippocampal-dependent behavior during adulthood. In conclusion, our data provide insights into the network dysfunction underlying schizophrenia and highlight the use of reverse engineering to restore physiological and behavioral phenotypes in an animal model of neurodevelopmental disorder.

Optogenetic induction of the schizophrenia-related endophenotype of ventral hippocampal hyperactivity causes rodent correlates of positive and cognitive symptoms

Pathological over-activity of the CA1 subfield of the human anterior hippocampus has been identified as a potential predictive marker for transition from a prodromal state to overt schizophrenia. Psychosis, in turn, is associated with elevated activity in the anterior subiculum, the hippocampal output stage directly activated by CA1. Over-activity in these subfields may represent a useful endophenotype to guide translationally predictive preclinical models. To recreate this endophenotype and study its causal relation to deficits in the positive and cognitive symptom domains, we optogenetically activated excitatory neurons of the ventral hippocampus (vHPC; analogous to the human anterior hippocampus), targeting the ventral subiculum. Consistent with previous studies, we found that vHPC over-activity evokes hyperlocomotion, a rodent correlate of positive symptoms. vHPC activation also impaired performance on the spatial novelty preference (SNP) test of short-term memory, regardless of whether stimulation was applied during the encoding or retrieval stage of the task. Increasing dopamine transmission with amphetamine produced hyperlocomotion, but was not associated with SNP impairments. This suggests that short-term memory impairments resulting from hippocampal over-activity likely arise independently of a hyperdopaminergic state, a finding that is consistent with the pharmaco-resistance of cognitive symptoms in patients.

Medial septum modulates hippocampal gamma activity and prepulse inhibition in an N-methyl-d-aspartate receptor antagonist model of schizophrenia

We reviewed the participation of the septohippocampal system in an animal model of schizophrenia that was acutely induced by systemic injection of an N-methyl-d-aspartate (NMDA) receptor antagonist such as phencyclidine, MK-801 and ketamine. The NMDA receptor antagonist-induced model of schizophrenia is characterized by behavioral and electrophysiological disruptions, including a decrease in prepulse inhibition of the acoustic startle response (PPI), hyperlocomotion, decrease in gating of hippocampal auditory evoked potentials and robust increase in hippocampal gamma (30-100Hz) oscillations. Similar disruptions were also induced by a single electrographic seizure in the hippocampus. The behavioral and electrophysiological disruptions induced by an NMDA receptor antagonist can be reduced by inactivation or lesion of GABAergic neurons in the medial septum, deep brain stimulation of the medial septum or nucleus accumbens, or positive modulation of GABA_B receptors. Our results suggest a close association between high-amplitude hippocampal gamma oscillations and psychosis-relevant behaviors including PPI loss, behavioral hyperactivity and loss in auditory gating. Abnormal electrophysiology suggests a disruption of somatic and apical dendritic inhibition in the hippocampus, resulting in distorted sensory integration, and impaired cognitive and memory processing. The hippocampus is suggested to be a hub in a brain network that participates in psychosis-relevant behaviors, through its direct projection to the nucleus accumbens, or through indirect connections via the entorhinal, cingulate and prefrontal cortices.

GABAergic deficits and schizophrenia-like behaviors in a mouse model carrying patient-derived neuroligin-2 R215H mutation

Schizophrenia (SCZ) is a severe mental disorder characterized by delusion, hallucination, and cognitive deficits. We have previously identified from schizophrenia patients a loss-of-function mutation Arg215→His215 (R215H) of neuroligin 2 (NLGN2) gene, which encodes a cell adhesion molecule critical for GABAergic synapse formation and function. Here, we generated a novel transgenic mouse line with neuroligin-2 (NL2) R215H mutation. The single point mutation caused a significant loss of NL2 protein in vivo, reduced GABAergic transmission, and impaired hippocampal activation. Importantly, R215H KI mice displayed anxiety-like behavior, impaired pre-pulse inhibition (PPI), cognition deficits and abnormal stress responses, recapitulating several key aspects of schizophrenia-like behaviors. Our results demonstrate a significant impact of a single point mutation NL2 R215H on brain functions, providing a novel animal model for the study of schizophrenia and neuropsychiatric disorders.

Dorsal CA1 interneurons contribute to acute stress-induced spatial memory deficits

Exposure to severely stressful experiences disrupts the activity of neuronal circuits and impairs declarative memory. GABAergic interneurons coordinate neuronal network activity, but their involvement in stress-evoked memory loss remains to be elucidated. Here, we provide evidence that interneurons in area CA1 of the dorsal hippocampus partially modulate acute stress-induced memory deficits. In adult male mice, both acute forced swim stress and restraint stress impaired hippocampus-dependent spatial memory and increased the density of c-fos-positive interneurons in the dorsal CA1. Selective activation of dorsal CA1 interneurons by chemogenetics disrupted memory performance in the spatial object recognition task. In comparison, anxiety-related behavior, spatial working memory and novel object recognition memory remained intact when dorsal CA1 interneurons were overactivated. Moreover, chemogenetic activation of dorsal CA1 interneurons suppressed the activity of adjacent pyramidal neurons, whereas a single exposure to forced swim stress but not restraint stress increased the activity of CA1 pyramidal neurons. However, chemogenetic inhibition of dorsal CA1 interneurons led to spatial memory impairments and failed to attenuate acute stress-induced memory loss. These findings suggest that acute stress may overactivate interneurons in the dorsal CA1, which reduces the activity of pyramidal neurons and in turn disrupts long-term memory.

Ambra1 Shapes Hippocampal Inhibition/Excitation Balance: Role in Neurodevelopmental Disorders

Imbalances between excitatory and inhibitory synaptic transmission cause brain network dysfunction and are central to the pathogenesis of neurodevelopmental disorders. Parvalbumin interneurons are highly implicated in this imbalance. Here, we probed the social behavior and hippocampal function of mice carrying a haploinsufficiency for Ambra1, a pro-autophagic gene crucial for brain development. We show that heterozygous Ambra1 mice (Ambra^+/−) are characterized by loss of hippocampal parvalbumin interneurons, decreases in the inhibition/excitation ratio, and altered social behaviors that are solely restricted to the female gender. Loss of parvalbumin interneurons in Ambra1^+/− females is further linked to reductions of the inhibitory drive onto principal neurons and alterations in network oscillatory activity, CA1 synaptic plasticity, and pyramidal neuron spine density. Parvalbumin interneuron loss is underlined by increased apoptosis during the embryonic development of progenitor neurons in the medial ganglionic eminence. Together, these findings identify an Ambra1-dependent mechanism that drives inhibition/excitation imbalance in the hippocampus, contributing to abnormal brain activity reminiscent of neurodevelopmental disorders.

Adolescent Synthetic Cannabinoid Exposure Produces Enduring Changes in Dopamine Neuron Activity in a Rodent Model of Schizophrenia Susceptibility

BACKGROUND

Epidemiological studies recognize cannabis intake as a risk factor for schizophrenia, yet the majority of adolescents who use marijuana do not develop psychosis. Similarly, the abuse of synthetic cannabinoids poses a risk for psychosis. For these reasons, it is imperative to understand the effects of adolescent cannabinoid exposure in susceptible individuals.

METHODS

We recently developed a novel rodent model of schizophrenia susceptibility, the F2 methylazoxymethanol acetate rat, where only a proportion (~40%) of rats display a schizophrenia-like phenotype. Using this model, we examined the effects of adolescent synthetic cannabinoid exposure (0.2 mg/kg WIN55, 212-2, i.p.) or adolescent endocannabinoid upregulation (0.3 mg/kg URB597, i.p.) on dopamine neuron activity and amphetamine sensitivity in adulthood.

RESULTS

Adolescent synthetic cannabinoid exposure significantly increased the proportion of susceptible rats displaying a schizophrenia-like hyperdopaminergic phenotype after puberty without producing any observable alterations in control rats. Furthermore, this acquired phenotype appears to correspond with alterations in parvalbumin interneuron function within the hippocampus. Endocannabinoid upregulation during adolescence also increased the proportion of susceptible rats developing an increase in dopamine neuron activity; however, it did not alter the behavioral response to amphetamine, further emphasizing differences between exogenous and endogenous cannabinoids.

CONCLUSIONS

Taken together, these studies provide experimental evidence that adolescent synthetic cannabinoid exposure may contribute to psychosis in susceptible individuals.

Cytosolic Accumulation of L-Proline Disrupts GABA-Ergic Transmission through GAD Blockade

Proline dehydrogenase (PRODH), which degrades L-proline, resides within the schizophrenia-linked 22q11.2 deletion suggesting a role in disease. Supporting this, elevated L-proline levels have been shown to increase risk for psychotic disorders. Despite the strength of data linking PRODH and L-proline to neuropsychiatric diseases, targets of disease-relevant concentrations of L-proline have not been convincingly described. Here, we show that Prodh-deficient mice with elevated CNS L-proline display specific deficits in high-frequency GABA-ergic transmission and gamma-band oscillations. We find that L-proline is a GABA-mimetic and can act at multiple GABA-ergic targets. However, at disease-relevant concentrations, GABA-mimesis is limited to competitive blockade of glutamate decarboxylase leading to reduced GABA production. Significantly, deficits in GABA-ergic transmission are reversed by enhancing net GABA production with the clinically relevant compound vigabatrin. These findings indicate that accumulation of a neuroactive metabolite can lead to molecular and synaptic dysfunction and help to understand mechanisms underlying neuropsychiatric disease.

Parvalbumin-positive interneurons mediate neocortical-hippocampal interactions that are necessary for memory consolidation

Following learning, increased coupling between spindle oscillations in the medial prefrontal cortex (mPFC) and ripple oscillations in the hippocampus is thought to underlie memory consolidation. However, whether learning-induced increases in ripple-spindle coupling are necessary for successful memory consolidation has not been tested directly. In order to decouple ripple-spindle oscillations, here we chemogenetically inhibited parvalbumin-positive (PV⁺) interneurons, since their activity is important for regulating the timing of spiking activity during oscillations. We found that contextual fear conditioning increased ripple-spindle coupling in mice. However, inhibition of PV⁺ cells in either CA1 or mPFC eliminated this learning-induced increase in ripple-spindle coupling without affecting ripple or spindle incidence. Consistent with the hypothesized importance of ripple-spindle coupling in memory consolidation, post-training inhibition of PV⁺ cells disrupted contextual fear memory consolidation. These results indicate that successful memory consolidation requires coherent hippocampal-neocortical communication mediated by PV⁺ cells.

Ketamine: differential neurophysiological dynamics in functional networks in the rat brain

Recently, the N-methyl-d-aspartate-receptor (NMDAR) antagonist ketamine has emerged as a fast-onset mechanism to achieve antidepressant activity, whereas its psychomimetic, dissociative and amnestic effects have been well documented to pharmacologically model schizophrenia features in rodents. Sleep-wake architecture, neuronal oscillations and network connectivity are key mechanisms supporting brain plasticity and cognition, which are disrupted in mood disorders such as depression and schizophrenia. In rats, we investigated the dynamic effects of acute and chronic subcutaneous administration of ketamine (2.5, 5 and 10 mg kg-1) on sleep-wake cycle, multichannels network interactions assessed by coherence and phase-amplitude cross-frequency coupling, locomotor activity (LMA), cognitive information processing as reflected by the mismatch negativity-like (MMN) component of event-related brain potentials (ERPs). Acute ketamine elicited a short, lasting inhibition of rapid eye movement (REM) sleep, increased coherence in higher gamma frequency oscillations independent of LMA, altered theta-gamma phase-amplitude coupling, increased MMN peak-amplitude response and evoked higher gamma oscillations. In contrast, chronic ketamine reduced large-scale communication among cortical regions by decreasing oscillations and coherent activity in the gamma frequency range, shifted networks activity towards slow alpha rhythm, decreased MMN peak response and enhanced aberrant higher gamma neuronal network oscillations. Altogether, our data show that acute and chronic ketamine elicited differential changes in network connectivity, ERPs and event-related oscillations (EROs), supporting possible underlying alterations in NMDAR-GABAergic signaling. The findings underscore the relevance of intermittent dosing of ketamine to accurately maintain the functional integrity of neuronal networks for long-term plastic changes and therapeutic effect.

IgSF21 promotes differentiation of inhibitory synapses via binding to neurexin2α

Coordinated development of excitatory and inhibitory synapses is essential for higher brain function, and impairment in this development is associated with neuropsychiatric disorders. In contrast to the large body of accumulated evidence regarding excitatory synapse development, little is known about synaptic adhesion and organization mechanisms underlying inhibitory synapse development. Through unbiased expression screens and proteomics, we identified immunoglobulin superfamily member 21 (IgSF21) as a neurexin2α-interacting membrane protein that selectively induces inhibitory presynaptic differentiation. IgSF21 localizes postsynaptically and recruits axonal neurexin2α in a trans-interaction manner. Deleting IgSF21 in mice impairs inhibitory presynaptic organization, especially in the hippocampal CA1 stratum radiatum, and also diminishes GABA-mediated synaptic transmission in hippocampal CA1 neurons without affecting their excitatory synapses. Finally, mice lacking IgSF21 show a sensorimotor gating deficit. These findings suggest that IgSF21 selectively regulates inhibitory presynaptic differentiation through interacting with presynaptic neurexin2α and plays a crucial role in synaptic inhibition in the brain.

Molecular mechanisms regulating the development of inhibitory synapses are poorly understood. Here the authors show that IgSF21 interacts with neurexin2α to induce presynaptic differentiation of inhibitory synapses, and that mice lacking IgSF21 exhibit deficits in inhibitory synaptic transmission.