Cell-type-specific role for nucleus accumbens neuroligin-2 in depression and stress susceptibility

Pre- and Post-Synaptic protein in the major depressive Disorder: From neurobiology to therapeutic targets

Major depressive disorder (MDD) has demonstrated its negative impact on various aspects of the lives of those affected. Although several therapies have been developed over the years, it remains a challenge for mental health professionals. Thus, understanding the pathophysiology of MDD is necessary to improve existing treatment options or seek new therapeutic alternatives. Clinical and preclinical studies in animal models of depression have shown the involvement of synaptic plasticity in both the development of MDD and the response to available drugs. However, synaptic plasticity involves a cascade of events, including the action of presynaptic proteins such as synaptophysin and synapsins and postsynaptic proteins such as postsynaptic density-95 (PSD-95). Additionally, several factors can negatively impact the process of spinogenesis/neurogenesis, which are related to many outcomes, including MDD. Thus, this narrative review aims to deepen the understanding of the involvement of synaptic formations and their components in the pathophysiology and treatment of MDD.

Mesocorticolimbic circuit mechanisms of social dominance behavior

Social animals, including rodents, primates, and humans, partake in competition for finite resources, thereby establishing social hierarchies wherein an individual's social standing influences diverse behaviors. Understanding the neurobiological underpinnings of social dominance is imperative, given its ramifications for health, survival, and reproduction. Social dominance behavior comprises several facets, including social recognition, social decision-making, and actions, indicating the concerted involvement of multiple brain regions in orchestrating this behavior. While extensive research has been dedicated to elucidating the neurobiology of social interaction, recent studies have increasingly delved into adverse social behaviors such as social competition and hierarchy. This review focuses on the latest advancements in comprehending the mechanisms of the mesocorticolimbic circuit governing social dominance, with a specific focus on rodent studies, elucidating the intricate dynamics of social hierarchies and their implications for individual well-being and adaptation.

The effects of different types of social interactions on the electrophysiology of neurons in the nucleus accumbens in rodents

BORLAND, J.M., The effects of different types of social interactions on the electrophysiology of neurons in the nucleus accumbens in rodents, NEUROSCI BIOBEH REV 21(1) XXX-XXX, 2024.-Sociality shapes an organisms' life. The nucleus accumbens is a critical brain region for mental health. In the following review, the effects of different types of social interactions on the physiology of neurons in the nucleus accumbens is synthesized. More specifically, the effects of sex behavior, aggression, social defeat, pair-bonding, play behavior, affiliative interactions, parental behaviors, the isolation from social interactions and maternal separation on measures of excitatory synaptic transmission, intracellular signaling and factors of transcription and translation in neurons in the nucleus accumbens in rodent models are reviewed. Similarities and differences in effects depending on the type of social interaction is then discussed. This review improves the understanding of the molecular and synaptic mechanisms of sociality.

Individual Differences in Volitional Social Self-Administration and Motivation in Male and Female Mice Following Social Stress

BACKGROUND

A key challenge in developing treatments for neuropsychiatric illness is the disconnect between preclinical models and the complexity of human social behavior. We integrate voluntary social self-administration into a rodent model of social stress as a platform for the identification of fundamental brain and behavior mechanisms underlying stress-induced individual differences in social motivation.

METHODS

Here, we introduced an operant social stress procedure in male and female mice composed of 3 phases: 1) social self-administration training, 2) social stress exposure concurrent with reinforced self-administration testing, and 3) poststress operant testing under nonreinforced and reinforced conditions. We used social-defeat and witness-defeat stress in male and female mice.

RESULTS

Social defeat attenuated social reward seeking in males but not females, whereas witness defeat had no effect in males but promoted seeking behavior in females. We resolved social stress-induced changes to social motivation by aggregating z-scored operant metrics into a cumulative social index score to describe the spectrum of individual differences exhibited during operant social stress. Clustering does not adequately describe the relative distributions of social motivation following stress and is better described as a nonbinary behavioral distribution defined by the social index score, capturing a dynamic range of stress-related alterations in social motivation inclusive of sex as a biological variable.

CONCLUSIONS

We demonstrated that operant social stress can detect stable individual differences in stress-induced changes to social motivation. The inclusion of volitional behavior in social procedures may enhance the understanding of behavioral adaptations that promote stress resiliency and their mechanisms under more naturalistic conditions.

Decoding depression: a comprehensive multi-cohort exploration of blood DNA methylation using machine learning and deep learning approaches

The causes of depression are complex, and the current diagnosis methods rely solely on psychiatric evaluations with no incorporation of laboratory biomarkers in clinical practices. We investigated the stability of blood DNA methylation depression signatures in six different populations using six public and two domestic cohorts (n = 1942) conducting mega-analysis and meta-analysis of the individual studies. We evaluated 12 machine learning and deep learning strategies for depression classification both in cross-validation (CV) and in hold-out tests using merged data from 8 separate batches, constructing models with both biased and unbiased feature selection. We found 1987 CpG sites related to depression in both mega- and meta-analysis at the nominal level, and the associated genes were nominally related to axon guidance and immune pathways based on enrichment analysis and eQTM data. Random forest classifiers achieved the highest performance (AUC 0.73 and 0.76) in CV and hold-out tests respectively on the batch-level processed data. In contrast, the methylation showed low predictive power (all AUCs < 0.57) for all classifiers in CV and no predictive power in hold-out tests when used with harmonized data. All models achieved significantly better performance (>14% gain in AUCs) with pre-selected features (selection bias), with some of the models (joint autoencoder-classifier) reaching AUCs of up to 0.91 in the final testing regardless of data preparation. Different algorithmic feature selection approaches may outperform limma, however, random forest models perform well regardless of the strategy. The results provide an overview over potential future biomarkers for depression and highlight many important methodological aspects for DNA methylation-based depression profiling including the use of machine learning strategies.

Neuroplasticity of children in autism spectrum disorder

Autism spectrum disorder (ASD) is a neurodevelopmental disorder that encompasses a range of symptoms including difficulties in verbal communication, social interaction, limited interests, and repetitive behaviors. Neuroplasticity refers to the structural and functional changes that occur in the nervous system to adapt and respond to changes in the external environment. In simpler terms, it is the brain's ability to learn and adapt to new environments. However, individuals with ASD exhibit abnormal neuroplasticity, which impacts information processing, sensory processing, and social cognition, leading to the manifestation of corresponding symptoms. This paper aims to review the current research progress on ASD neuroplasticity, focusing on genetics, environment, neural pathways, neuroinflammation, and immunity. The findings will provide a theoretical foundation and insights for intervention and treatment in pediatric fields related to ASD.

Balance Between Projecting Neuronal Populations of the Nucleus Accumbens Controls Social Behavior in Mice

BACKGROUND

Deficient social interactions are a hallmark of major neuropsychiatric disorders, and accumulating evidence points to altered social reward and motivation as key underlying mechanisms of these pathologies. In the present study, we further explored the role of the balance of activity between D1 and D2 receptor-expressing striatal projection neurons (D1R- and D2R-SPNs) in the control of social behavior, challenging the hypothesis that excessive D2R-SPN activity, rather than deficient D1R-SPN activity, compromises social behavior.

METHODS

We selectively ablated D1R- and D2R-SPNs using an inducible diphtheria toxin receptor-mediated cell targeting strategy and assessed social behavior as well as repetitive/perseverative behavior, motor function, and anxiety levels. We tested the effects of optogenetic stimulation of D2R-SPNs in the nucleus accumbens (NAc) and pharmacological compounds repressing D2R-SPN.

RESULTS

Targeted deletion of D1R-SPNs in the NAc blunted social behavior in mice, facilitated motor skill learning, and increased anxiety levels. These behaviors were normalized by pharmacological inhibition of D2R-SPN, which also repressed transcription in the efferent nucleus, the ventral pallidum. Ablation of D1R-SPNs in the dorsal striatum had no impact on social behavior but impaired motor skill learning and decreased anxiety levels. Deletion of D2R-SPNs in the NAc produced motor stereotypies but facilitated social behavior and impaired motor skill learning. We mimicked excessive D2R-SPN activity by optically stimulating D2R-SPNs in the NAc and observed a severe deficit in social interaction that was prevented by D2R-SPN pharmacological inhibition.

CONCLUSIONS

Repressing D2R-SPN activity may represent a promising therapeutic strategy to relieve social deficits in neuropsychiatric disorders.

Adaptations in Nucleus Accumbens Neuron Subtypes Mediate Negative Affective Behaviors in Fentanyl Abstinence

BACKGROUND

Opioid discontinuation generates a withdrawal syndrome marked by increased negative affect. Increased symptoms of anxiety and dysphoria during opioid discontinuation are significant barriers to achieving long-term abstinence in opioid-dependent individuals. While adaptations in the nucleus accumbens are implicated in opioid abstinence syndrome, the precise neural mechanisms are poorly understood. Additionally, our current knowledge is limited to changes following natural and semisynthetic opioids, despite recent increases in synthetic opioid use and overdose.

METHODS

We used a combination of cell subtype-specific viral labeling and electrophysiology in male and female mice to investigate structural and functional plasticity in nucleus accumbens medium spiny neuron (MSN) subtypes after fentanyl abstinence. We characterized molecular adaptations after fentanyl abstinence with subtype-specific RNA sequencing and weighted gene co-expression network analysis. We used viral-mediated gene transfer to manipulate the molecular signature of fentanyl abstinence in D1-MSNs.

RESULTS

Here, we show that fentanyl abstinence increases anxiety-like behavior, decreases social interaction, and engenders MSN subtype-specific plasticity in both sexes. D1-MSNs, but not D2-MSNs, exhibit dendritic atrophy and an increase in excitatory drive. We identified a cluster of coexpressed dendritic morphology genes downregulated selectively in D1-MSNs that are transcriptionally coregulated by E2F1. E2f1 expression in D1-MSNs protects against loss of dendritic complexity, altered physiology, and negative affect-like behaviors caused by fentanyl abstinence.

CONCLUSIONS

Our findings indicate that fentanyl abstinence causes unique structural, functional, and molecular changes in nucleus accumbens D1-MSNs that can be targeted to alleviate negative affective symptoms during abstinence.

The role of the dorsal hippocampus in resistance to the development of posttraumatic stress disorder-like behaviours

This study aimed to determine the activity of the dorsal hippocampus (dHIP) in resistance to the development of posttraumatic stress disorder (PTSD)-like behaviours. Rats were divided into resistant, PTSD(-), and susceptible, PTSD(+) groups based on the time spent in the central area in an open field test and freezing duration during exposure to an aversive context one week after stress experience (electric foot shock). The PTSD(-) rats, compared to the PTSD(+) group, had an increased concentration of corticosterone in plasma and changes in the activity of the dHIP, specifically, increased c-Fos expression in the dentate gyrus (DG) and increased Neuroligin-2 (marker of GABAergic neurotransmission) expression in the DG and CA3 area of the dHIP. Moreover, in the hippocampus, the PTSD(-) group showed decreased mRNA expression for corticotropin-releasing factor receptors type 1 and 2, increased mRNA expression for orexin receptor type 1, and decreased miR-9 and miR-34c levels compared with the PTSD(+) group. This study may suggest that the increase in GABA signalling in the hippocampus attenuates the activity of the CRF system and enhances the function of the orexin system. Moreover, decreased expression of miR-34c and miR-9 could facilitate fear extinction and diminishes the anxiety response. These effects may lead to an anxiolytic-like effect and improve resistance to developing PTSD-like behaviours.

Plasma proteomic signature of major depressive episode in the elderly

Depression is a complex and multifactorial disease, affecting about 6.5% of the elderly population in what is referred to as late-life depression (LLD). Despite its public health relevance, there is still limited information about the molecular mechanisms of LLD. We analyzed the blood plasma of 50 older adults, 19 with LLD and 31 controls, through untargeted mass spectrometry, and used systems biology tools to identify biochemical pathways and biological processes dysregulated in the disease. We found 96 differentially expressed proteins between LLD patients and control individuals. Using elastic-net regression, we generated a panel of 75 proteins that comprises a potential model for determining the molecular signature of LLD. We also showed that biological pathways related to vesicle-mediated transport and voltage-dependent calcium channels may be dysregulated in LLD. These data can help to build an understanding of the molecular basis of LLD, offering an integrated view of the biomolecular alterations that occur in this disorder. SIGNIFICANCE: Major depressive disorder in the elderly, called late-life depression (LLD), is a common and disabling disorder, with recent prevalence estimates of 6.5% in the general population. Despite the public health relevance, there is still limited information about the molecular mechanisms of LLD. The findings in this paper shed light on LLD heterogeneous biological mechanisms. We uncovered a potential novel biomolecular signature for LLD and biological pathways related to this condition which can be targets for the development of novel interventions for prevention, early diagnosis, and treatment of LLD.

Historical and Modern Evidence for the Role of Reward Circuitry in Emergence

Increasing evidence supports a role for brain reward circuitry in modulating arousal along with emergence from anesthesia. Emergence remains an important frontier for investigation, since no drug exists in clinical practice to initiate rapid and smooth emergence. This review discusses clinical and preclinical evidence indicating a role for two brain regions classically considered integral components of the mesolimbic brain reward circuitry, the ventral tegmental area and the nucleus accumbens, in emergence from propofol and volatile anesthesia. Then there is a description of modern systems neuroscience approaches to neural circuit investigations that will help span the large gap between preclinical and clinical investigation with the shared aim of developing therapies to promote rapid emergence without agitation or delirium. This article proposes that neuroscientists include models of whole-brain network activity in future studies to inform the translational value of preclinical investigations and foster productive dialogues with clinician anesthesiologists.

Neuroligins in neurodevelopmental conditions: how mouse models of de novo mutations can help us link synaptic function to social behavior

Neurodevelopmental conditions (or neurodevelopmental disorders, NDDs) are highly heterogeneous with overlapping characteristics and shared genetic etiology. The large symptom variability and etiological heterogeneity have made it challenging to understand the biological mechanisms underpinning NDDs. To accommodate this individual variability, one approach is to move away from diagnostic criteria and focus on distinct dimensions with relevance to multiple NDDs. This domain approach is well suited to preclinical research, where genetically modified animal models can be used to link genetic variability to neurobiological mechanisms and behavioral traits. Genetic factors associated with NDDs can be grouped functionally into common biological pathways, with one prominent functional group being genes associated with the synapse. These include the neuroligins (Nlgns), a family of postsynaptic transmembrane proteins that are key modulators of synaptic function. Here, we review how research using Nlgn mouse models has provided insight into how synaptic proteins contribute to behavioral traits associated with NDDs. We focus on how mutations in different Nlgns affect social behaviors, as differences in social interaction and communication are a common feature of most NDDs. Importantly, mice carrying distinct mutations in Nlgns share some neurobiological and behavioral phenotypes with other synaptic gene mutations. Comparing the functional implications of mutations in multiple synaptic proteins is a first step towards identifying convergent neurobiological pathways in multiple brain regions and circuits.

The differentially expressed proteins related to clinical viral encephalitis revealed by proteomics

To screen out the prospective biomarkers of viral encephalitis (VE), analyze the biological process and signaling pathways involved by differentially expressed proteins (DEPs). A total of 11 cerebrospinal fluid (CSF) samples with VE and 5 with non-nervous system infection were used to perform label-free proteomic techniques. Then, the bioinformatic analysis of DEPs was applied by Interproscan software. Moreover, 73 CSF samples in the VE group and 53 in the control group were used to verify the changes of some DEPs by enzyme-linked immunosorbent assay (ELISA). Thirty-nine DEPs were identified, including 18 upregulated DEPs and 21 downregulated DEPs. DEPs were mainly enriched in cell adhesion molecules by Kyoto Encyclopedia of Genes and Genomes analysis pathway analysis. The DEPs related to axon tissue were obviously downregulated and the most significant downregulated proteins were neurexin 3, neurofascin, and neuroligin 2 (NLGN2). Moreover, the protein expression of NLGN2 in the VE group was significantly higher than that in the control group by ELISA. The correlation analysis of NLGN2 in the VE group revealed that there was a weak positive correlation with CSF protein and a weak negative correlation with CSF chloride. The clinical VE may be closely related to NLGN2 and the cell adhesion molecule pathway.

Cell-Type-Specific Adaptions in Striatal Medium-Sized Spiny Neurons and Their Roles in Behavioral Responses to Drugs of Abuse

Drug addiction is defined as a compulsive pattern of drug-seeking- and taking- behavior, with recurrent episodes of abstinence and relapse, and a loss of control despite negative consequences. Addictive drugs promote reinforcement by increasing dopamine in the mesocorticolimbic system, which alters excitatory glutamate transmission within the reward circuitry, thereby hijacking reward processing. Within the reward circuitry, the striatum is a key target structure of drugs of abuse since it is at the crossroad of converging glutamate inputs from limbic, thalamic and cortical regions, encoding components of drug-associated stimuli and environment, and dopamine that mediates reward prediction error and incentive values. These signals are integrated by medium-sized spiny neurons (MSN), which receive glutamate and dopamine axons converging onto their dendritic spines. MSN primarily form two mostly distinct populations based on the expression of either DA-D1 (D1R) or DA-D2 (D2R) receptors. While a classical view is that the two MSN populations act in parallel, playing antagonistic functional roles, the picture seems much more complex. Herein, we review recent studies, based on the use of cell-type-specific manipulations, demonstrating that dopamine differentially modulates dendritic spine density and synapse formation, as well as glutamate transmission, at specific inputs projecting onto D1R-MSN and D2R-MSN to shape persistent pathological behavioral in response to drugs of abuse. We also discuss the identification of distinct molecular events underlying the detrimental interplay between dopamine and glutamate signaling in D1R-MSN and D2R-MSN and highlight the relevance of such cell-type-specific molecular studies for the development of innovative strategies with potential therapeutic value for addiction. Because drug addiction is highly prevalent in patients with other psychiatric disorders when compared to the general population, we last discuss the hypothesis that shared cellular and molecular adaptations within common circuits could explain the co-occurrence of addiction and depression. We will therefore conclude this review by examining how the nucleus accumbens (NAc) could constitute a key interface between addiction and depression.

Global brain delivery of neuroligin 2 gene ameliorates seizures in a mouse model of epilepsy

BACKGROUND

Despite the increasing availability of effective drugs, around one-third of patients with epilepsy are still resistant to pharmacotherapy. Gene therapy has been suggested as a plausible approach to achieve seizure control, in particular for patients with focal epilepsy. Because seizures develop across wide spans of the brain in many forms of epilepsy, global delivery of the vectors is necessary to tackle such generalized seizures. Neuroligin 2 (NL2) is a postsynaptic cell adhesion molecule that induces or strengthens inhibitory synaptic function by specifically combining with neurexin 1.

METHODS

In the present study, we applied an adeno-associated virus (AAV) type 9 vector expressing NL2 to modulate neuronal excitability in broad areas of the brain in epileptic (EL) mice, a model of polygene epilepsy. We administered the AAV vector expressing Flag-tagged NL2 under the synapsin I promoter (AAV-NL2) via cardiac injection 6 weeks after birth.

RESULTS

Significant reductions in the duration, strength and frequency of seizure were observed during a 14-week observation period in NL2-treated EL mice compared to untreated or AAV-green fluorescent protein-treated EL mice. No behavioral abnormality was observed in NL2-treated EL mice in an open-field test. Immunohistochemical examination at 14 weeks after AAV-NL2 injection revealed the expression of exogenous NL2 in broad areas of the brain, including the hippocampus and, in these areas, NL2 co-localized with postsynaptic inhibitory molecule gephyrin.

CONCLUSIONS

Global brain delivery of NL2 by systemic administration of AAV vector may provide a non-invasive therapeutic approach for generalized epilepsy.

Clinical Relevance and Prognostic Value of the Neuronal Protein Neuroligin 2 in Breast Cancer

Neuroligin 2 (NLGN2) is a well-recognized transmembrane scaffolding protein that functions in synapse development and neuronal signal transduction. It has recently been implicated in multiple diseases of peripheral ectodermal origin. However, the potential roles of NLGN2 in tumors remain ill-defined. The aim of this study was to determine the clinical relevance and prognostic value of NLGN2 in breast cancer. To this end, breast cancer datasets were extracted from TCGA and other public databases, and subjected to Kaplan-Meier potter for survival analysis, GEPIA2 for assessing the immunological relevance of NLGN2 and THPA for identifying its subcellular localization. The in-silico results were further validated by immunohistochemistry analysis of in-house tumor tissue specimens. NLGN2 was identified as a prognostic factor in breast cancer subtypes, and its high expression correlated to a favorable survival outcome. Moreover, NLGN2 overexpression in breast cancer was significantly associated with large tumor size, lymph node metastasis, late TNM stage, and high histological grade. Interestingly, there was a significant correlation between the expression level of NLGN2 and the immunomodulatory molecules, along with increased interstitial infiltration of lymphocytes. Furthermore, NLGN2 was predominantly localized in the mitochondria of breast cancer cells. In conclusion, NLGN2 has a prognostic role and immunoregulatory potential in breast cancer, and its functions likely have a mitochondrial basis. It is a promising therapeutic target in breast cancer and should be explored further.

Individual history of winning and hierarchy landscape influence stress susceptibility in mice

Social hierarchy formation is strongly evolutionarily conserved. Across species, rank within social hierarchy has large effects on health and behavior. To investigate the relationship between social rank and stress susceptibility, we exposed ranked male and female mice to social and non-social stressors and manipulated social hierarchy position. We found that rank predicts same sex social stress outcomes: dominance in males and females confers resilience while subordination confers susceptibility. Pre-existing rank does not predict non-social stress outcomes in females and weakly does so in males, but rank emerging under stress conditions reveals social interaction deficits in male and female subordinates. Both history of winning and rank of cage mates affect stress susceptibility in males: rising to the top rank through high mobility confers resilience and mice that lose dominance lose stress resilience, although gaining dominance over a subordinate animal does not confer resilience. Overall, we have demonstrated a relationship between social status and stress susceptibility, particularly when taking into account individual history of winning and the overall hierarchy landscape in male and female mice.

μ-Opioid Receptor (Oprm1) Copy Number Influences Nucleus Accumbens Microcircuitry and Reciprocal Social Behaviors

The μ-opioid receptor regulates reward derived from both drug use and natural experiences, including social interaction, through actions in the nucleus accumbens. Here, we studied nucleus accumbens microcircuitry and social behavior in male and female mice with heterozygous genetic knockout of the μ-opioid receptor (Oprm1+/-). This genetic condition models the partial reduction of μ-opioid receptor signaling reported in several neuropsychiatric disorders. We first analyzed inhibitory synapses in the nucleus accumbens, using methods that differentiate between medium spiny neurons (MSNs) expressing the D1 or D2 dopamine receptor. Inhibitory synaptic transmission was increased in D2-MSNs of male mutants, but not female mutants, while the expression of gephyrin mRNA and the density of inhibitory synaptic puncta at the cell body of D2-MSNs was increased in mutants of both sexes. Some of these changes were more robust in Oprm1+/- mutants than Oprm1-/- mutants, demonstrating that partial reductions of μ-opioid signaling can have large effects. At the behavioral level, social conditioned place preference and reciprocal social interaction were diminished in Oprm1+/- and Oprm1-/- mutants of both sexes. Interaction with Oprm1 mutants also altered the social behavior of wild-type test partners. We corroborated this latter result using a social preference task, in which wild-type mice preferred interactions with another typical mouse over Oprm1 mutants. Surprisingly, Oprm1-/- mice preferred interactions with other Oprm1-/- mutants, although these interactions did not produce a conditioned place preference. Our results support a role for partial dysregulation of μ-opioid signaling in social deficits associated with neuropsychiatric conditions.SIGNIFICANCE STATEMENT Activation of the μ-opioid receptor plays a key role in the expression of normal social behaviors. In this study, we examined brain function and social behavior of female and male mice, with either partial or complete genetic deletion of μ-opioid receptor expression. We observed abnormal social behavior following both genetic manipulations, as well as changes in the structure and function of synaptic input to a specific population of neurons in the nucleus accumbens, which is an important brain region for social behavior. Synaptic changes were most robust when μ-opioid receptor expression was only partially lost, indicating that small reductions in μ-opioid receptor signaling can have a large impact on brain function and behavior.

Altered theta and beta oscillatory synchrony in a genetic mouse model of pathological anxiety

While the neural circuits mediating normal, adaptive defensive behaviors have been extensively studied, substantially less is currently known about the network mechanisms by which aberrant, pathological anxiety is encoded in the brain. Here we investigate in mice how deletion of Neuroligin-2 (Nlgn2), an inhibitory synapse-specific adhesion protein that has been associated with pathological anxiety and other psychiatric disorders, alters the communication between key brain regions involved in mediating defensive behaviors. To this end, we performed multi-site simultaneous local field potential (LFP) recordings from the basolateral amygdala (BLA), centromedial amygdala (CeM), bed nucleus of the stria terminalis (BNST), prefrontal cortex (mPFC) and ventral hippocampus (vHPC) in an open field paradigm. We found that LFP power in the vHPC was profoundly increased and was accompanied by an abnormal modulation of the synchrony of theta frequency oscillations particularly in the vHPC-mPFC-BLA circuit. Moreover, deletion of Nlgn2 increased beta and gamma frequency synchrony across the network, and this increase was associated with increased center avoidance. Local deletion of Nlgn2 in the vHPC and BLA revealed that they encode distinct aspects of this avoidance phenotype, with vHPC linked to immobility and BLA linked to a reduction in exploratory activity. Together, our data demonstrate that alterations in long-range functional connectivity link synaptic inhibition to abnormal defensive behaviors, and that both exaggerated activation of normal defensive circuits and recruitment of fundamentally distinct mechanisms contribute to this phenotype. Nlgn2 knockout mice therefore represent a highly relevant model to study the role of inhibitory synaptic transmission in the circuits underlying anxiety disorders.

mGluR5-Mediated eCB Signaling in the Nucleus Accumbens Controls Vulnerability to Depressive-Like Behaviors and Pain After Chronic Social Defeat Stress

Stress contributes to major depressive disorder (MDD) and chronic pain, which affect a significant portion of the global population, but researchers have not clearly determined how these conditions are initiated or amplified by stress. The chronic social defeat stress (CSDS) model is a mouse model of psychosocial stress that exhibits depressive-like behavior and chronic pain. We hypothesized that metabotropic glutamate receptor 5 (mGluR5) expressed in the nucleus accumbens (NAc) normalizes the depressive-like behaviors and pain following CSDS. Here, we show that CSDS induced both pain and social avoidance and that the level of mGluR5 decreased in susceptible mice. Overexpression of mGluR5 in the NAc shell and core prevented the development of depressive-like behaviors and pain in susceptible mice, respectively. Conversely, depression-like behaviors and pain were exacerbated in mice with mGluR5 knockdown in the NAc shell and core, respectively, compared to control mice subjected to 3 days of social defeat stress. Furthermore, (RS)-2-chloro-5-hydroxyphenylglycine (CHPG), an mGluR5 agonist, reversed the reduction in the level of the endocannabinoid (eCB) 2-arachidonoylglycerol (2-AG) in the NAc of susceptible mice, an effect that was blocked by 3-((2-methyl-1, 3-thiazol-4-yl) ethynyl) pyridine hydrochloride (MTEP), an mGluR5 antagonist. In addition, the injection of CHPG into the NAc shell and core normalized depressive-like behaviors and pain, respectively, and these effects were inhibited by AM251, a cannabinoid type 1 receptor (CB1R) antagonist. Based on these results, mGluR5-mediated eCB production in the NAc relieves stress-induced depressive-like behaviors and pain.

Proteomics Study on the Cerebrospinal Fluid of Patients with Encephalitis

OBJECTIVE

Label-free quantitative proteomics was applied to analyze differentially expressed proteins (DEPs) in the cerebrospinal fluid (CSF) of patients with encephalitis. The database was used to screen for possible biomarkers in encephalitis, followed by validation and preliminary investigation of the role of some DEPs in the pathogenesis of encephalitis using enzyme-linked immunosorbent assay (ELISA).

METHODS

We performed label-free quantitative proteomics on 16 cerebrospinal fluid samples (EM group, encephalitis with mental and behavioral disorders patients, n = 5; NED group, encephalitis without mental and behavioral disorders patients, n = 6; N group, healthy individuals, n = 5). The extracted CSF proteins were examined by mass spectrometry and enzymatic digestion and detected using protein profiling and data analysis. Interproscan was used to perform Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment analysis of the DEPs. ELISA was used to verify the changes in the levels of some DEPs in the CSF.

RESULTS

A total of 941 proteins were found to be significantly differentially expressed, including 250 upregulated DEPs and 691 downregulated DEPs. GO analysis suggested that there were six enriched functions that intersect among the EM, NED, and N groups, including synapse organization, membrane, integral component of membrane, membrane part, G-protein-coupled receptor signaling pathway, and transmembrane signaling receptor activity. KEGG analysis revealed that there were three signaling pathways that intersect among the EM, NED, and N groups, including fructose and mannose metabolism, inositol phosphate metabolism, and Jak-STAT signaling pathway. Furthermore, four downregulated encephalitis-related neurological synapse proteins were identified after screening for differentially expressed proteins, including NRXN3, NFASC, LRRC4B, and NLGN2. The result of ELISA further verified that the expression of NLGN2 and LRRC4B was obviously higher in the NED group than in the N group.

CONCLUSIONS

These findings demonstrated that NLGN2 and LRRC4B proteins were upregulated in the NED group and could be potential biomarkers for the diagnosis of encephalitis, but still needs a lot of multiomics studies to be used in clinical.

Nucleus Accumbens Medium Spiny Neuron Subtypes Differentially Regulate Stress-Associated Alterations in Sleep Architecture

BACKGROUND

Stress is implicated in the pathophysiology of major depression and posttraumatic stress disorder. These conditions share core features, including motivational deficits, heighted anxiety, and sleep dysregulation. Chronic stress produces these same features in rodents, with some individuals being susceptible or resilient, as seen in humans. While stress-induced neuroadaptations within the nucleus accumbens are implicated in susceptibility-related dysregulation of motivational and emotional behaviors, their effects on sleep are unclear.

METHODS

We used chemogenetics (DREADDs [designer receptors exclusively activated by designer drugs]) to examine the effects of selective alterations in activity of nucleus accumbens medium spiny neurons expressing dopamine D₁ receptors (D1-MSNs) or dopamine D₂ receptors (D2-MSNs) on sleep-related end points. Mice were implanted with wireless transmitters enabling continuous collection of data to quantify vigilance states over a 20-day test period. Parallel cohorts were examined in behavioral tests assessing stress susceptibility.

RESULTS

D1- and D2-MSNs play dissociable roles in sleep regulation. Stimulation of inhibitory or excitatory DREADDs expressed in D1-MSNs exclusively affects rapid eye movement sleep, whereas targeting D2-MSNs affects slow wave sleep. The combined effects of D1-MSN inhibition and D2-MSN activation on sleep resemble those of chronic social defeat stress. Alterations in D1-MSN function also affect stress susceptibility in social behavior tests. Elevation of CREB (cAMP response element-binding protein) within D1-MSNs is sufficient to produce stress-like effects on rapid eye movement sleep.

CONCLUSIONS

In addition to regulation of motivational and emotional behaviors, the nucleus accumbens also influences sleep, an end point with high translational relevance. These findings provide a neural basis for comorbidity in key features of stress-related illness.

Using social rank as the lens to focus on the neural circuitry driving stress coping styles

Social hierarchy position in humans is negatively correlated with stress-related psychiatric disease risk. Animal models have largely corroborated human studies, showing that social rank can impact stress susceptibility and is considered to be a major risk factor in the development of psychiatric illness. Differences in stress coping style is one of several factors that mediate this relationship between social rank and stress susceptibility. Coping styles encompass correlated groupings of behaviors associated with differential physiological stress responses. Here, we discuss recent insights from animal models that highlight several neural circuits that can contribute to social rank-associated differences in coping style.

Cognitive behavioral therapy for patients with mild to moderate depression: Treatment effects and neural mechanisms

In this study, we combined clinical assessment and magnetic resonance imaging (MRI) techniques to investigate the brain mechanisms in mild to moderate depression (MMD) patients following cognitive behavioral therapy (CBT). Data were collected from 30 MMD patients and 18 healthy controls, and we divided patients into two treatment periods (4 weeks, 8 weeks). Clinical assessment indicated that depression characteristics, as quantified by Hamilton Depression Rating Scale (HAMD), were significantly higher in MMD patients than in healthy controls. At the baseline, MRI data revealed abnormalities in the hippocampus and nucleus accumbens (NAc) of patients with MMD, e.g., smaller gray matter volumes of the hippocampus and nucleus accumbens (NAc), as well as weaker functional connectivity between NAc and the posterior cingulate cortex/precuneus. Moreover, the hippocampus and NAc volumes were negatively correlated with the HAMD scores in MMD patients. After CBT intervention, the HAMD scores decreased, and the structural and functional characteristics of NAc in MMD patients obtained at 8-week were improved; e.g., no significant differences in NAc volume or NAc-based functional connectivity between the two groups. Taken together, our results provided evidence suggesting that CBT is an effective treatment for MMD patients. Alterations of gray matter volume and resting-state functional connectivity after 8 weeks of CBT indicated a potential modulation mechanism in brain structural modifications and functional connectivity plasticity within the NAc in MMD patients.

Prognostic plasma exosomal microRNA biomarkers in patients with substance use disorders presenting comorbid with anxiety and depression

Psychiatric disorders such as anxiety and depression precipitated by substance use occurred during both use and withdrawal. Exosomes play significant roles in biological functions and regulate numerous physiological and pathological processes in various diseases, in particular substance use disorders (SUDs) and other psychiatric disorders. To better understand the role of exosomal miRNAs in the pathology of symptoms of anxiety and depression in patients with SUDs, we first isolated circulating exosomes from heroin-dependent patients (HDPs) and methamphetamine-dependent patients (MDPs) and identified exosomal miRNAs that were differentially expressed between patients and healthy controls (HCs). Furthermore, the correlations between exosomal DE-miRNAs and symptoms of anxiety and depression which were measured using Hamilton-Anxiety (HAM-A)/Hamilton-Depression (HAM-D) Rating Scales in the participants. Notably, the expression level of exosomal hsa-miR-16-5p, hsa-miR-129-5p, hsa-miR-363-3p, and hsa-miR-92a-3p showed significantly negative correlations with HAM-A scores in both HDPs and MDPs. But all of the 4 DE-miRNAs lost significant correlations with HAM-D scores in HDPs. Functional annotation analyses showed that the target genes of the DE-miRNAs were mainly enriched for “synapse”, “cell adhesion”, “focal adhesion” and “MHC class II protein complex”. Our study suggests that a set of circulating exosomal miRNAs were associated with anxiety and depression in SUD patients and may have clinical utility as diagnostic and prognostic biomarkers.

Neuronal Nitric Oxide Synthase in Nucleus Accumbens Specifically Mediates Susceptibility to Social Defeat Stress through Cyclin-Dependent Kinase 5

Stress-induced depression is common worldwide. NAc, a "reward" center, is recently reported to be critical to confer the susceptibility to chronic social defeat stress (CSDS) and the depression-related outcome. However, the underlying molecular mechanisms have not been well characterized. In this study, we induced depression-like behaviors with CSDS and chronic mild stress in male mice to mimic social and environmental factors, respectively, and observed animal behaviors with social interaction test, tail suspension test, and sucrose preference test. To determine the role of neuronal nitric oxide synthase (nNOS) and its product nitric oxide (NO), we used brain region-specifically nNOS overexpression and stereotaxic injection of NO inhibitor or donor. Moreover, the downstream molecular cyclin-dependent kinase 5 (CDK5) was explored by conditional KO and gene mutation. We demonstrate that nNOS-implicated mechanisms in NAc shell (NAcSh), including increased cell number, increased protein expression levels, and increased specific enzyme activity, contribute the susceptibility to social defeat and the following depression-like behaviors. NAcSh nNOS does not directly respond to chronic mild stress but facilitates the depression-like behaviors. The increased NAcSh nNOS expression after CSDS leads to the social avoidance and depression-like behaviors in defeated mice, which is dependent on the nNOS enzyme activity and NO production. Moreover, we identify the downstream signal in NAcSh. S-nitrosylation of CDK5 by NO contributes to enhanced CDK5 activity, leading to depression-related behaviors in susceptible mice. Therefore, NAcSh nNOS mediates susceptibility to social defeat stress and the depression-like behaviors through CDK5.SIGNIFICANCE STATEMENT Stress-induced depression is common worldwide, and chronic exposure to social and psychological stressors is important cause of human depression. Our study conducted with chronic social defeat stress mice models demonstrates that nNOS in NAcSh is crucial to regulate the susceptibility to social defeat stress and the following depression-like behaviors, indicating NAcSh nNOS as the responding molecule to social factors of depression. Moreover, we discover the downstream mechanism of NAcSh nNOS in mediating the susceptibility is NO and S-nitrosylation of CDK5. Thus, NAcSh nNOS mediates susceptibility to social defeat stress through CDK5 is a potential mechanism for depression, which may interpret how the brain transduces social stress exposure into depression.

Collapsin response mediator protein 5 (CRMP5) modulates susceptibility to chronic social defeat stress in mice

Collapsin response mediator protein 5 (CRMP5), a member of the CRMP family, is expressed in the brain, particularly in the hippocampus, an area of the brain that can modulate stress responses. Social stress has a well-known detrimental effect on health and can lead to depression, but not all individuals are equally sensitive to stress. To date, researchers have not conclusively determined how social stress increases the susceptibility of the brain to depression. Here, we used the chronic social defeat stress (CSDS) model and observed higher hippocampal CRMP5 expression in stress-susceptible (SS) mice than in control and stress-resilient (RES) mice. A negative correlation was observed between the expression levels of CRMP5 and the social interaction (SI) ratio. Reduced hippocampal CRMP5 expression increased the SI ratio in SS mice, whereas CRMP5 overexpression was sufficient to induce social avoidance behaviors in control mice following exposure to subthreshold social stress induced by lentivirus-based overexpression and inducible tetracycline-on strategies to upregulate CRMP5. Interestingly, increased CRMP5 expression in SS and lenti-CRMP5-treated mice also caused serum corticosterone concentrations to increase. These findings improve our understanding of the potential mechanism by which CRMP5 triggers susceptibility to social stress, and they support the further development of therapeutic agents for the treatment of stress disorders in humans.

HDAC3 Activity within the Nucleus Accumbens Regulates Cocaine-Induced Plasticity and Behavior in a Cell-Type-Specific Manner

Epigenetic mechanisms regulate processes of neuroplasticity critical to cocaine-induced behaviors. This includes the Class I histone deacetylase (HDAC) HDAC3, known to act as a negative regulator of cocaine-associated memory formation within the nucleus accumbens (NAc). Despite this, it remains unknown how cocaine alters HDAC3-dependent mechanisms. Here, we profiled HDAC3 expression and activity in total NAc mouse tissue following cocaine exposure. Although chronic cocaine did not affect expression of Hdac3 within the NAc, chronic cocaine did affect promoter-specific changes in HDAC3 and H4K8Ac occupancy. These changes in promoter occupancy correlated with cocaine-induced changes in expression of plasticity-related genes. To causally determine whether cocaine-induced plasticity is mediated by HDAC3's deacetylase activity, we overexpressed a deacetylase-dead HDAC3 point mutant (HDAC3-Y298H-v5) within the NAc of adult male mice. We found that disrupting HDAC3's enzymatic activity altered selective changes in gene expression and synaptic plasticity following cocaine exposure, despite having no effects on cocaine-induced behaviors. In further assessing HDAC3's role within the NAc, we observed that chronic cocaine increases Hdac3 expression in Drd1 but not Drd2-cells of the NAc. Moreover, we discovered that HDAC3 acts selectively within D1R cell-types to regulate cocaine-associated memory formation and cocaine-seeking. Overall, these results suggest that cocaine induces cell-type-specific changes in epigenetic mechanisms to promote plasticity important for driving cocaine-related behaviors.

SIGNIFICANCE STATEMENT Drugs of abuse alter molecular mechanisms throughout the reward circuitry that can lead to persistent drug-associated behaviors. Epigenetic regulators are critical drivers of drug-induced changes in gene expression. Here, we demonstrate that the activity of an epigenetic enzyme promotes neuroplasticity within the nucleus accumbens (NAc) critical to cocaine action. In addition, we demonstrate that these changes in epigenetic activity drive cocaine-seeking behaviors in a cell-type-specific manner. These findings are key in understanding and targeting cocaine's impact of neural circuitry and behavior.

Dysregulation of brain dopamine systems in major depressive disorder

Major depressive disorder (MDD or depression) is a debilitating neuropsychiatric syndrome with genetic, epigenetic, and environmental contributions. Depression is one of the largest contributors to chronic disease burden; it affects more than one in six individuals in the United States. A wide array of cellular and molecular modifications distributed across a variety of neuronal processes and circuits underlie the pathophysiology of depression-no established mechanism can explain all aspects of the disease. MDD suffers from a vast treatment gap worldwide, and large numbers of individuals who require treatment do not receive adequate care. This mini-review focuses on dysregulation of brain dopamine (DA) systems in the pathophysiology of MDD and describing new cellular targets for potential medication development focused on DA-modulated micro-circuits. We also explore how neurodevelopmental factors may modify risk for later emergence of MDD, possibly through dopaminergic substrates in the brain.

Proper synaptic adhesion signaling in the control of neural circuit architecture and brain function

Trans-synaptic cell-adhesion molecules are critical for governing various stages of synapse development and specifying neural circuit properties via the formation of multifarious signaling pathways. Recent studies have pinpointed the putative roles of trans-synaptic cell-adhesion molecules in mediating various cognitive functions. Here, we review the literature on the roles of a diverse group of central synaptic organizers, including neurexins (Nrxns), leukocyte common antigen-related receptor protein tyrosine phosphatases (LAR-RPTPs), and their associated binding proteins, in regulating properties of specific type of synapses and neural circuits. In addition, we highlight the findings that aberrant synaptic adhesion signaling leads to alterations in the structures, transmission, and plasticity of specific synapses across diverse brain areas. These results seem to suggest that proper trans-synaptic signaling pathways by Nrxns, LAR-RPTPs, and their interacting network is likely to constitute central molecular complexes that form the basis for cognitive functions, and that these complexes are heterogeneously and complexly disrupted in many neuropsychiatric and neurodevelopmental disorders.

Neuroligin-2 as a central organizer of inhibitory synapses in health and disease

Postsynaptic organizational protein complexes play central roles both in orchestrating synapse formation and in defining the functional properties of synaptic transmission that together shape the flow of information through neuronal networks. A key component of these organizational protein complexes is the family of synaptic adhesion proteins called neuroligins. Neuroligins form transsynaptic bridges with presynaptic neurexins to regulate various aspects of excitatory and inhibitory synaptic transmission. Neuroligin-2 (NLGN2) is the only member that acts exclusively at GABAergic inhibitory synapses. Altered expression and mutations in NLGN2 and several of its interacting partners are linked to cognitive and psychiatric disorders, including schizophrenia, autism, and anxiety. Research on NLGN2 has fundamentally shaped our understanding of the molecular architecture of inhibitory synapses. Here, we discuss the current knowledge on the molecular and cellular functions of mammalian NLGN2 and its role in the neuronal circuitry that regulates behavior in rodents and humans.

Effect of Aggressive Experience in Female Syrian Hamsters on Glutamate Receptor Expression in the Nucleus Accumbens

Our social relationships determine our health and well-being. In rodent models, there is now strong support for the rewarding properties of aggressive or assertive behaviors to be critical for the expression and development of adaptive social relationships, buffering from stress and protecting from the development of psychiatric disorders such as depression. However, due to the false belief that aggression is not a part of the normal repertoire of social behaviors displayed by females, almost nothing is known about the neural mechanisms mediating the rewarding properties of aggression in half the population. In the following study, using Syrian hamsters as a well-validated and translational model of female aggression, we investigated the effects of aggressive experience on the expression of markers of postsynaptic structure (PSD-95, Caskin I) and excitatory synaptic transmission (GluA1, GluA2, GluA4, NR2A, NR2B, mGluR1a, and mGluR5) in the nucleus accumbens (NAc), caudate putamen and prefrontal cortex. Aggressive experience resulted in an increase in PSD-95, GluA1 and the dimer form of mGluR5 specifically in the NAc 24 h following aggressive experience. There was also an increase in the dimer form of mGluR1a 1 week following aggressive experience. Aggressive experience also resulted in an increase in the strength of the association between these postsynaptic proteins and glutamate receptors, supporting a common mechanism of action. In addition, 1 week following aggressive experience there was a positive correlation between the monomer of mGluR5 and multiple AMPAR and NMDAR subunits. In conclusion, we provide evidence that aggressive experience in females results in an increase in the expression of postsynaptic density, AMPARs and group I metabotropic glutamate receptors, and an increase in the strength of the association between postsynaptic proteins and glutamate receptors. This suggests that aggressive experience may result in an increase in excitatory synaptic transmission in the NAc, potentially encoding the rewarding and behavioral effects of aggressive interactions.

Dietary phytochemicals modulate experience-dependent changes in Neurexin gene expression and alternative splicing in mice after chronic variable stress exposure

Neurexins (NRXNs) are cell-adhesion molecules important in the formation and remodeling of neural circuits. It has been shown that aversive environmental stimuli can affect the expression pattern of Neurexin genes (Nrxns) impacting the regulation of synaptic strength. Accumulated evidence suggests that, after chronic exposure to psychological stress, the triggered changes in gene expression and splicing patterns of Nrxns may be involved in aversive conditioning. Previously, we have demonstrated that a novel treatment using dietary phytochemicals can modulate the response to chronic variable stress (CVS) in mice. Here, we aimed to further investigate the long-term plasticity changes after CVS by focusing on the regulation of NRXNs at synapses. We found that CVS differentially triggers the region-specific gene expression of Nrxns in mice Nucleus Accumbens (NAc) and Hippocampus (HIPP). The prophylactic treatment with the combination of two phytochemicals dihydrocaffeic acid (DHCA) and Malvidin-3-O-glucoside (Mal-gluc) differentially modulated the stress-induced effects on Nrxn1 and 3 mRNA expression in these brain areas and promoted the alternative splicing of Nrxn3 in HIPP. Overall, our data supports the prophylactic effect of dietary phytochemicals in the restoration of stress-induced plasticity changes in mouse brain. By intervening in activity-dependent plasticity at synapses, these compounds may attenuate the effects of chronic aversive conditioning. We propose that an early therapeutic intervention may help with disorders of negative affect, such as depression or post-traumatic stress disorder. Our future studies will address how DHCA/Mal-gluc might serve as a potential complement for current therapies in depression and other mood disorders.

Dendritic spine density is increased on nucleus accumbens D2 neurons after chronic social defeat

Stress alters the structure and function of brain reward circuitry and is an important risk factor for developing depression. In the nucleus accumbens (NAc), structural and physiological plasticity of medium spiny neurons (MSNs) have been linked to increased stress-related and depression-like behaviors. NAc MSNs have opposing roles in driving stress-related behaviors that is dependent on their dopamine receptor expression. After chronic social defeat stress, NAc MSNs exhibit increased dendritic spine density. However, it remains unclear if the dendritic spine plasticity is MSN subtype specific. Here we use viral labeling to characterize dendritic spine morphology specifically in dopamine D2 receptor expressing MSNs (D2-MSNs). After chronic social defeat, D2-MSNs exhibit increased spine density that is correlated with enhanced social avoidance behavior. Together, our data indicate dendritic spine plasticity is MSN subtype specific, improving our understanding of structural plasticity after chronic stress.

Depression and Social Defeat Stress Are Associated with Inhibitory Synaptic Changes in the Nucleus Accumbens

Chronic stress in both humans and rodents induces a robust downregulation of neuroligin-2, a key component of the inhibitory synapse, in the NAc that modifies behavioral coping mechanisms and stress resiliency in mice. Here we extend this observation by examining the role of two other inhibitory synapse constituents, vesicular GABA transporter (vGAT) and gephyrin, in the NAc of male mice that underwent chronic social defeat stress (CSDS) and in patients with major depressive disorder (MDD). We first performed transcriptional profiling of vGAT and gephyrin in postmortem NAc samples from a cohort of healthy controls, medicated, and nonmedicated MDD patients. In parallel, we conducted whole-cell electrophysiology recordings in the NAc of stress-susceptible and stress-resilient male mice following 10 d of CSDS. Finally, we used immunohistochemistry to analyze protein levels of vGAT and gephyrin in the NAc of mice after CSDS. We found that decreased vGAT and gephyrin mRNA in the NAc of nonmedicated MDD patients is paralleled by decreased inhibitory synapse markers and decreased frequency of mini inhibitory postsynaptic currents (mIPSC) in the NAc of susceptible mice, indicating a reduction in the number of NAc inhibitory synapses that is correlated with depression-like behavior. Overall, these findings suggest a common state of reduced inhibitory tone in the NAc in depression and stress susceptibility.SIGNIFICANCE STATEMENT Existing studies focus on excitatory synaptic changes after social stress, although little is known about stress-induced inhibitory synaptic plasticity and its relevance for neuropsychiatric disease. These results extend our previous findings on the critical role of impaired inhibitory tone in the NAc following stress and provide new neuropathological evidence for reduced levels of inhibitory synaptic markers in human NAc from nonmedicated major depressive disorder patients. This finding is corroborated in stress-susceptible male mice that have undergone chronic social defeat stress, a mouse model of depression, at both the level of synaptic function and protein expression. These data support the hypothesis that reduced inhibitory synaptic transmission within the NAc plays a critical role in the stress response.

Dendritic remodeling of D1 neurons by RhoA/Rho-kinase mediates depression-like behavior

Depression alters the structure and function of brain reward circuitry. Preclinical evidence suggests that medium spiny neurons (MSNs) in the nucleus accumbens (NAc) undergo structural plasticity; however, the molecular mechanism and behavioral significance is poorly understood. Here we report that atrophy of D1, but not D2 receptor containing MSNs is strongly associated with social avoidance in mice subject to social defeat stress. D1-MSN atrophy is caused by cell-type specific upregulation of the GTPase RhoA and its effector Rho-kinase. Pharmacologic and genetic reduction of activated RhoA prevents depressive outcomes to stress by preventing loss of D1-MSN dendritic arbor. Pharmacologic and genetic promotion of activated RhoA enhances depressive outcomes by reducing D1-MSN dendritic arbor and is sufficient to promote depressive-like behaviors in the absence of stress. Chronic treatment with Rho-kinase inhibitor Y-27632 after chronic social defeat stress reverses depression-like behaviors by restoring D1-MSN dendritic complexity. Taken together, our data indicate functional roles for RhoA and Rho-kinase in mediating depression-like behaviors via dendritic remodeling of NAc D1-MSNs and may prove a useful target for new depression therapeutics.

Brain-Wide Mapping of Afferent Inputs to Accumbens Nucleus Core Subdomains and Accumbens Nucleus Subnuclei

The nucleus accumbens (NAc) is the ventral part of the striatum and the interface between cognition, emotion, and action. It is composed of three major subnuclei: i.e., NAc core (NAcC), lateral shell (NAcLS), and medial shell (NAcMS), which exhibit functional heterogeneity. Thus, determining the synaptic inputs of the subregions of the NAc is important for understanding the circuit mechanisms involved in regulating different functions. Here, we simultaneously labeled subregions of the NAc with cholera toxin subunit B conjugated with multicolor Alexa Fluor, then imaged serial sections of the whole brain with a fully automated slide scanning system. Using the interactive WholeBrain framework, we characterized brain-wide inputs to the NAcC subdomains, including the rostral, caudal, dorsal, and ventral subdomains (i.e., rNAcC, cNAcC, dNAcC, and vNAcC, respectively) and the NAc subnuclei. We found diverse brain regions, distributed from the cerebrum to brain stem, projecting to the NAc. Of the 57 brain regions projecting to the NAcC, the anterior olfactory nucleus (AON) exhibited the greatest inputs. The input neurons of rNAcC and cNAcC are two distinct populations but share similar distribution over the same upstream brain regions, whereas the input neurons of dNAcC and vNAcC exhibit slightly different distributions over the same upstream regions. Of the 55 brain regions projecting to the NAcLS, the piriform area contributed most of the inputs. Of the 72 brain regions projecting to the NAcMS, the lateral septal nucleus contributed most of the inputs. The input neurons of NAcC and NAcLS share similar distributions, whereas the NAcMS exhibited brain-wide distinct distribution. Thus, the NAcC subdomains appeared to share the same upstream brain regions, although with distinct input neuron populations and slight differences in the input proportions, whereas the NAcMS subnuclei received distinct inputs from multiple upstream brain regions. These results lay an anatomical foundation for understanding the different functions of NAcC subdomains and NAc subnuclei.

The molecular and cellular mechanisms of depression: a focus on reward circuitry

Depression is a complex disorder that takes an enormous toll on individual health. As affected individuals display a wide variation in their clinical symptoms, the precise neural mechanisms underlying the development of depression remain elusive. Although it is impossible to phenocopy every symptom of human depression in rodents, the preclinical field has had great success in modeling some of the core affective and neurovegetative depressive symptoms, including social withdrawal, anhedonia, and weight loss. Adaptations in select cell populations may underlie these individual depressive symptoms and new tools have expanded our ability to monitor and manipulate specific cell types. This review outlines some of the most recent preclinical discoveries on the molecular and neurophysiological mechanisms in reward circuitry that underlie the expression of behavioral constructs relevant to depressive symptoms.

Autism Spectrum Disorder and miRNA: An Overview of Experimental Models

Autism spectrum disorder (ASD) is a complex neuropsychiatric disorder characterized by deficits in social interactions, communication, language, and in a limited repertoire of activities and interests. The etiology of ASD is very complex. Genetic, epigenetic, and environmental factors contribute to the onset of ASD. Researchers have shown that microRNAs (miRNAs) could be one of the possible causes associated with ASD. miRNAs are small noncoding mRNAs that regulate gene expression, and they are often linked to biological processes and implicated in neurodevelopment. This review aims to provide an overview of the animal models and the role of the different miRNAs involved in ASD. Therefore, the use of animal models that reproduce the ASD and the identification of miRNAs could be a useful predictive tool to study this disorder.

The molecular mechanism underlying GABAergic dysfunction in nucleus accumbens of depression-like behaviours in mice

Depression is the most frequent psychiatric disorder in the world. Recent evidence has shown that stress‐induced GABAergic dysfunction in the nucleus accumbens (NAc) contributed to the pathophysiology of depression. However, the molecular mechanisms underlying these pathological changes remain unclear. In this study, mice were constantly treated with the chronic unpredictable mild stress (CUMS) till showing depression‐like behaviours expression. GABA synthesis, release and uptake in the NAc tissue were assessed by analysing the expression level of genes and proteins of Gad‐1, VGAT and GAT‐3 by qRT‐PCR and Western blotting. The miRNA/mRNA network regulating GABA was constructed based on the bioinformatics prediction software and further validated by dual‐luciferase reporter assay in vitro and qRT‐PCR in vivo, respectively. Our results showed that the expression level of GAT‐3, Gad‐1 and VGAT mRNA and protein significantly decreased in the NAc tissue from CUMS‐induced depression‐like mice than that of control mice. However, miRNA‐144‐3p, miRNA‐879‐5p, miR‐15b‐5p and miRNA‐582‐5p that directly down‐regulated the expression of Gad‐1, VGAT and GAT‐3 were increased. In the mRNA/miRNA regulatory GABA network, Gad‐1 and VGAT were directly regulated by binding seed sequence of miR‐144‐3p, and miR‐15b‐5p, miR‐879‐5p could be served negative post‐regulators by binding to the different sites of VGAT 3′‐UTR. Chronic stress causes the impaired GABA synthesis, release and uptake by up‐regulating miRNAs and down‐regulating mRNAs and proteins, which may reveal the molecular mechanisms for the decreased GABA concentrations in the NAc tissue of CUMS‐induced depression.

Region-specific myelin differences define behavioral consequences of chronic social defeat stress in mice

Exposure to stress increases the risk of developing mood disorders. While a subset of individuals displays vulnerability to stress, others remain resilient, but the molecular basis for these behavioral differences is not well understood. Using a model of chronic social defeat stress, we identified region-specific differences in myelination between mice that displayed social avoidance behavior (‘susceptible’) and those who escaped the deleterious effect to stress (‘resilient’). Myelin protein content in the nucleus accumbens was reduced in all mice exposed to stress, whereas decreased myelin thickness and internodal length were detected only in the medial prefrontal cortex (mPFC) of susceptible mice, with fewer mature oligodendrocytes and decreased heterochromatic histone marks. Focal demyelination in the mPFC was sufficient to decrease social preference, which was restored following new myelin formation. Together these data highlight the functional role of mPFC myelination as critical determinant of the avoidance response to traumatic social experiences.

Editorial note: This article has been through an editorial process in which the authors decide how to respond to the issues raised during peer review. The Reviewing Editor's assessment is that all the issues have been addressed (see decision letter).

High levels of stress do not have the same effect on everybody: some individuals can show resilience and recover quickly, while other struggle to cope. Scientists have started to investigate how these differences may find their origin in biological processes, mainly by focusing on the role of neurons. However, neurons represent only one type of brain cells, and there is increasing evidence that interactions between neuronal and non-neuronal cells play an important role in the response to stress.

Oligodendrocytes are a common type of non-neuronal cells which shield and feed nerve cells. In particular, their membrane constitutes the myelin sheath, a protective coating that insulates neurons and allows them to better communicate with each other using electric signals.

Bonnefil et al. explored whether differences in oligodendrocytes could affect how mice responded to social stress. The rodents were exposed to repeated attacks from an aggressive mouse five minutes a day for ten days. After this period, ‘susceptible’ mice then avoided future contact with any other mice, while resilient animals remained interested in socializing.

Comparing the brain areas of resilient and susceptible mice revealed differences in the oligodendrocytes of the medial prefrontal cortex, the part of the brain that controls emotions and thinking. Susceptible animals had fewer mature oligodendrocytes and their neurons were covered in thinner and shorter segments of myelin sheaths. There was also evidence that, in these animals, the genes that regulate the maturation of oligodendrocytes were more likely to be switched off. Taken together, these results may suggest that, in certain animals, social stress disrupts the genetic program that controls how oligodendrocytes develop, potentially leading to neurons communicating less well.

To explore whether reduced amounts of myelin could be linked to decreased social behavior, Bonnefil et al. then damaged the myelin in the medial prefrontal cortex in another group of rodents. The mice were then less willing to interact with other animals until new sheaths had formed.

The results by Bonnefil et al. undercover how changes in non-neuronal cells can at least in part explain differences in the way individuals respond to stress. Ultimately, this knowledge may be useful to design new strategies to foster resilience.

Heat-killed Lactobacillus helveticus strain MCC1848 confers resilience to anxiety or depression-like symptoms caused by subchronic social defeat stress in mice

The gut microbiota is involved in the pathogenesis of stress-related disorders. Probiotics can benefit the central nervous system via the microbiota–gut–brain axis, which raises the possibility that probiotics are effective in managing depression. In the present study, we examined the effects of heat-killed Lactobacillus helveticus strain MCC1848 in subchronic and mild social defeat stress (sCSDS) model mice (a widely used animal model of depression). MCC1848 supplementation significantly enhanced the interaction time in the social interaction test and sucrose preference ratio in the sucrose preference test, suggesting that MCC1848 improved anxiety- or depressive-like behaviors in sCSDS mice. The gene expression profile analysis of the nucleus accumbens, which plays an important role in stress resilience, indicated that MCC1848 ameliorated sCSDS-induced gene expression alterations in signal transduction or nervous system development. These findings suggest that MCC1848 supplementation is useful as a preventive strategy for chronic-stress-induced depression.

Neuroligin-2 Determines Inhibitory Synaptic Transmission in the Lateral Septum to Optimize Stress-Induced Neuronal Activation and Avoidance Behavior

BACKGROUND

Investigations in the neocortex have revealed that the balance of excitatory and inhibitory synaptic transmission (E/I ratio) is important for proper information processing. The disturbance of this balance underlies many neuropsychiatric illnesses, including autism spectrum disorder and schizophrenia. However, little is known about the contribution of E/I balance to the functioning of subcortical brain regions, such as the lateral septum (LS), a structure that plays important roles in regulating anxiety-related behavior.

METHODS

We manipulated E/I balance in the mouse LS by localized conditional deletion of neuroligin-2, a postsynaptic cell adhesion protein located at gamma-aminobutyric acidergic synapses and important for inhibitory synaptic transmission. We then performed analyses of synaptic transmission in the LS, stress-induced expression of immediate early gene c-fos, and anxiety-related and depression-related behavior.

RESULTS

The absence of neuroligin-2 in the LS in the mature mouse brain resulted in postsynaptic impairment of inhibitory synaptic transmission. Importantly, the reduced inhibition and resulting E/I imbalance decreased the responsiveness of LS neurons to stress. Furthermore, this E/I imbalance in the LS was associated with impaired stress-induced activation of downstream hypothalamic nuclei and reduced avoidance behavior of the animals in the elevated plus maze.

CONCLUSIONS

Our results described the synaptic function of neuroligin-2 in the LS, uncovered a positive association between c-Fos-expressing neurons in the LS and downstream hypothalamic areas and avoidance behavior, and demonstrated that intact inhibitory synaptic transmission and proper E/I balance are required for the optimal functioning of this subcortical circuit.

Role of basal ganglia neurocircuitry in the pathology of psychiatric disorders

Over the last few decades, advances in human and animal-based techniques have greatly enhanced our understanding of the neural mechanisms underlying psychiatric disorders. Many of these studies have indicated connectivity between and alterations within basal ganglia structures to be particularly pertinent to the development of symptoms associated with several of these disorders. Here we summarize the connectivity, molecular composition, and function of sites within basal ganglia neurocircuits. Then we review the current literature from both human and animal studies concerning altered basal ganglia function in five common psychiatric disorders: obsessive-compulsive disorder, substance-related and addiction disorders, major depressive disorder, generalized anxiety disorder, and schizophrenia. Finally, we present a model based upon the findings of these studies that highlights the striatum as a particularly attractive target for restoring normal function to basal ganglia neurocircuits altered within psychiatric disorder patients.

A Motivational and Neuropeptidergic Hub: Anatomical and Functional Diversity within the Nucleus Accumbens Shell

The mesocorticolimbic pathway is canonically known as the "reward pathway." Embedded within the center of this circuit is the striatum, a massive and complex network hub that synthesizes motivation, affect, learning, cognition, stress, and sensorimotor information. Although striatal subregions collectively share many anatomical and functional similarities, it has become increasingly clear that it is an extraordinarily heterogeneous region. In particular, the nucleus accumbens (NAc) medial shell has repeatedly demonstrated that the rules dictated by more dorsal aspects of the striatum do not apply or are even reversed in functional logic. These discrepancies are perhaps most easily captured when isolating the functions of various neuromodulatory peptide systems within the striatum. Endogenous peptides are thought to play a critical role in modulating striatal signals to either amplify or dampen evoked behaviors. Here we describe the anatomical-functional backdrop upon which several neuropeptides act within the NAc to modulate behavior, with a specific emphasis on nucleus accumbens medial shell and stress responsivity. Additionally, we propose that, as the field continues to dissect fast neurotransmitter systems within the NAc, we must also provide considerable contextual weight to the roles local peptides play in modulating these circuits to more comprehensively understand how this important subregion gates motivated behaviors.

Gadd45b mediates depressive-like role through DNA demethylation

Animal studies using chronic social defeat stress (CSDS) in mice showed that brain-derived neurotrophic factor (BDNF) signaling in the mesolimbic dopamine (DA) circuit is important for the development of social aversion. However, the downstream molecular targets after BDNF release from ventral tegmental area (VTA) DA terminals are unknown. Here, we show that depressive-like behaviors induced by CSDS are mediated in part by Gadd45b downstream of BDNF signaling in the nucleus accumbens (NAc). We show that Gadd45b mRNA levels are increased in susceptible but not resilient mice. Intra-NAc infusion of BDNF or optical stimulation of VTA DA terminals in NAc enhanced Gadd45b expression levels in the NAc. Importantly, Gadd45b downregulation reversed social avoidance in susceptible mice. Together, these data suggest that Gadd45b in NAc contributes to susceptibility to social stress. In addition, we investigated the function of Gadd45b in demethylating CpG islands of representative gene targets, which have been associated with a depressive phenotype in humans and animal models. We found that Gadd45b downregulation changes DNA methylation levels in a phenotype-, gene-, and locus-specific fashion. Together, these results highlight the contribution of Gadd45b and changes in DNA methylation in mediating the effects of social stress in the mesolimbic DA circuit.

JHU-083 selectively blocks glutaminase activity in brain CD11b+ cells and prevents depression-associated behaviors induced by chronic social defeat stress

There are a number of clinically effective treatments for stress-associated psychiatric diseases, including major depressive disorder (MDD). Nonetheless, many patients exhibit resistance to first-line interventions calling for novel interventions based on pathological mechanisms. Accumulating evidence implicates altered glutamate signaling in MDD pathophysiology, suggesting that modulation of glutamate signaling cascades may offer novel therapeutic potential. Here we report that JHU-083, our recently developed prodrug of the glutaminase inhibitor 6-diazo-5-oxo-L-norleucine (DON) ameliorates social avoidance and anhedonia-like behaviors in mice subjected to chronic social defeat stress (CSDS). JHU-083 normalized CSDS-induced increases in glutaminase activity specifically in microglia-enriched CD11b⁺ cells isolated from the prefrontal cortex and hippocampus. JHU-083 treatment also reverses the CSDS-induced inflammatory activation of CD11b⁺ cells. These results support the importance of altered glutamate signaling in the behavioral abnormalities observed in the CSDS model, and identify glutaminase in microglia-enriched CD11b⁺ cells as a pharmacotherapeutic target implicated in the pathophysiology of stress-associated psychiatric conditions such as MDD.

Neuropharmacology of the mesolimbic system and associated circuits on social hierarchies

Most socially living species are organized hierarchically, primarily based on individual differences in social dominance. Dominant individuals typically gain privileged access to important resources, such as food, mating partners and territories, whereas submissive conspecifics are often devoid of such benefits. The benefits associated with a high social status provide a strong incentive to become dominant. Importantly, motivational- and reward-related processes are regulated, to a large extent, by the mesolimbic system. Consequently, several studies point to a key role for the mesolimbic system in social hierarchy formation. This review summarizes the growing body of literature that implicates the mesolimbic system, and associated neural circuits, on social hierarchies. In particular, we discuss the neurochemical and pharmacological studies that have highlighted the contributions of the mesolimbic system and associated circuits including dopamine signaling through the D1 or D2 receptors, GABAergic neurotransmission, the androgen receptor system, and mitochondria and bioenergetics. Given that low social status has been linked to the emergence of anxiety- and depressive-like disorders, a greater understanding of the neurochemistry underlying social dominance could be of tremendous benefit for the development of pharmacological treatments to dysfunctions in social behaviors. This article is part of the Special Issue entitled 'The neuropharmacology of social behavior: from bench to bedside'.

Targeted Ubiquitin-Proteasomal Proteolysis Pathway in Chronic Social Defeat Stress

Stressful events promote psychopathogenic changes that might contribute to the development of mental illnesses. Some individuals tend to recover from the stress response, while some do not. However, the molecular mechanisms of stress resilience during stress are not well-characterized. Here, we identify proteomic changes in the hippocampus using proteomic technique to examine mice following chronic social defeat stress. We showed that small ubiquitin-like modifier (SUMO)-1 expression was significantly decreased in susceptible mice following chronic social defeat stress. We also examined a protein inhibitor of activated signal transducer of transcription (PIAS)1 levels, an E3 SUMO-protein ligase protein inhibitor of activated STAT1, which is known to interact with SUMO-1. PIAS1 was shown to be profoundly decreased and monoamine oxidase (MAO)-A increased in the hippocampus of susceptible mice following chronic social defeat stress. Furthermore, the manipulated PIAS1 expression in the hippocampus also has an influence on glucocorticoid receptor (GR) translocation. We also found that knockdown of PIAS1 expression in the hippocampus then subject to submaximal stress increased GR to glucocorticoid response element (GRE)-binding site on the MAO-A promoter. The present study raises the possibility of different levels of PIAS1 between individuals in response to chronic social defeat stress and that such differences may contribute to the susceptibility to stress.