Nucleus accumbens medium spiny neuron subtypes mediate depression-related outcomes to social defeat stress

SIRT1 Coordinates Transcriptional Regulation of Neural Activity and Modulates Depression-Like Behaviors in the Nucleus Accumbens

BACKGROUND

Major depression and anxiety disorders are significant causes of disability and socioeconomic burden. Despite the prevalence and considerable impact of these affective disorders, their pathophysiology remains elusive. Thus, there is an urgent need to develop novel therapeutics for these conditions. We evaluated the role of SIRT1 in regulating dysfunctional processes of reward by using chronic social defeat stress to induce depression- and anxiety-like behaviors. Chronic social defeat stress induces physiological and behavioral changes that recapitulate depression-like symptomatology and alters gene expression programs in the nucleus accumbens, but cell type-specific changes in this critical structure remain largely unknown.

METHODS

We examined transcriptional profiles of D1-expressing medium spiny neurons (MSNs) lacking deacetylase activity of SIRT1 by RNA sequencing in a cell type-specific manner using the RiboTag line of mice. We analyzed differentially expressed genes using gene ontology tools including SynGO and EnrichR and further demonstrated functional changes in D1-MSN-specific SIRT1 knockout (KO) mice using electrophysiological and behavioral measurements.

RESULTS

RNA sequencing revealed altered transcriptional profiles of D1-MSNs lacking functional SIRT1 and showed specific changes in synaptic genes including glutamatergic and GABAergic (gamma-aminobutyric acidergic) receptors in D1-MSNs. These molecular changes may be associated with decreased excitatory and increased inhibitory neural activity in Sirt1 KO D1-MSNs, accompanied by morphological changes. Moreover, the D1-MSN-specific Sirt1 KO mice exhibited proresilient changes in anxiety- and depression-like behaviors.

CONCLUSIONS

SIRT1 coordinates excitatory and inhibitory synaptic genes to regulate the GABAergic output tone of D1-MSNs. These findings reveal a novel signaling pathway that has potential for the development of innovative treatments for affective disorders.

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.

Effects and neural mechanisms of different physical activity on major depressive disorder based on cerebral multimodality monitoring: a narrative review

Major depressive disorder (MDD) is currently the most common psychiatric disorder in the world. It characterized by a high incidence of disease with the symptoms like depressed mood, slowed thinking, and reduced cognitive function. Without timely intervention, there is a 20-30% risk of conversion to treatment-resistant depression (TRD) and a high burden for the patient, family and society. Numerous studies have shown that physical activity (PA) is a non-pharmacological treatment that can significantly improve the mental status of patients with MDD and has positive effects on cognitive function, sleep status, and brain plasticity. However, the physiological and psychological effects of different types of PA on individuals vary, and the dosage profile of PA in improving symptoms in patients with MDD has not been elucidated. In most current studies of MDD, PA can be categorized as continuous endurance training (ECT), explosive interval training (EIT), resistance strength training (RST), and mind-body training (MBT), and the effects on patients' depressive symptoms, cognitive function, and sleep varied. Therefore, the present study was based on a narrative review and included a large number of existing studies to investigate the characteristics and differences in the effects of different PA interventions on MDD. The study also investigated the characteristics and differences of different PA interventions in MDD, and explained the neural mechanisms through the results of multimodal brain function monitoring, including the intracranial environment and brain structure. It aims to provide exercise prescription and theoretical reference for future research in neuroscience and clinical intervention in MDD.

The nucleus accumbens in reward and aversion processing: insights and implications

The nucleus accumbens (NAc), a central component of the brain's reward circuitry, has been implicated in a wide range of behaviors and emotional states. Emerging evidence, primarily drawing from recent rodent studies, suggests that the function of the NAc in reward and aversion processing is multifaceted. Prolonged stress or drug use induces maladaptive neuronal function in the NAc circuitry, which results in pathological conditions. This review aims to provide comprehensive and up-to-date insights on the role of the NAc in motivated behavior regulation and highlights areas that demand further in-depth analysis. It synthesizes the latest findings on how distinct NAc neuronal populations and pathways contribute to the processing of opposite valences. The review examines how a range of neuromodulators, especially monoamines, influence the NAc's control over various motivational states. Furthermore, it delves into the complex underlying mechanisms of psychiatric disorders such as addiction and depression and evaluates prospective interventions to restore NAc functionality.

Hypocretin in the nucleus accumbens shell modulates social approach in female but not male California mice

The hypocretin (Hcrt) system modulates arousal and anxiety-related behaviors and has been considered as a novel treatment target for stress-related affective disorders. We examined the effects of Hcrt acting in the nucleus accumbens shell (NAcSh) and anterodorsal bed nucleus of the stria terminalis (adBNST) on social behavior in male and female California mice (Peromyscus californicus). In female but not male California mice, infusion of Hcrt1 into NAcSh decreased social approach. Weak effects of Hcrt1 on social vigilance were observed in both females and males. No behavioral effects of Hcrt1 infused into the adBNST were observed. Analyses of sequencing data from California mice and Mus musculus NAc showed that Hcrtr2 was more abundant than Hcrtr1, so we infused the selective Hcrt receptor 2 antagonist into the NAcSh, which increased social approach in females previously exposed to social defeat. A calcium imaging study in the NAcSh of females before and after stress exposure showed that neural activity increased immediately following the expression of social avoidance but not during freezing behavior. This observation is consistent with previous studies that identified populations of neurons in the NAc that drive avoidance. Intriguingly, calcium transients were not affected by stress. These data suggest that hypocretin acting in the NAcSh plays a key role in modulating stress-induced social avoidance.

Activation of Metabotropic Glutamate Receptor 3 Modulates Thalamo-accumbal Transmission and Rescues Schizophrenia-Like Physiological and Behavioral Deficits

BACKGROUND

Polymorphisms in the gene encoding for metabotropic glutamate receptor 3 (mGlu3) are associated with an increased likelihood of schizophrenia diagnosis and can predict improvements in negative symptoms following treatment with antipsychotics. However, the mechanisms by which mGlu3 can regulate brain circuits involved in schizophrenia pathophysiology are not clear.

METHODS

We employed selective pharmacological tools and a variety of approaches including whole-cell patch-clamp electrophysiology, slice optogenetics, and fiber photometry to investigate the effects of mGlu3 activation on phencyclidine (PCP)-induced impairments in thalamo-accumbal transmission and sociability deficits. A chemogenetic approach was used to evaluate the role of thalamo-accumbal transmission in PCP-induced sociability deficits.

RESULTS

We first established that PCP treatment augmented excitatory transmission onto dopamine D1 receptor-expressing medium spiny neurons (D1-MSNs) in the nucleus accumbens (NAc) and induced sociability deficits. Our studies revealed a selective increase in glutamatergic synaptic transmission from thalamic afferents to D1-MSNs in the NAc shell. Chemogenetic silencing of thalamo-accumbal inputs rescued PCP-induced sociability deficits. Pharmacological activation of mGlu3 normalized PCP-induced impairments in thalamo-accumbal transmission and sociability deficits. Mechanistic studies revealed that mGlu3 activation induced robust long-term depression at synapses from the thalamic projections onto D1-MSNs in the NAc shell.

CONCLUSIONS

These data demonstrate that activation of mGlu3 decreases thalamo-accumbal transmission and thereby rescues sociability deficits in mouse modeling schizophrenia-like symptoms. These findings provide novel insights into the NAc-specific mechanisms and suggest that agents modulating glutamatergic signaling in the NAc may provide a promising approach for treating negative symptoms in schizophrenia.

The rostral ventromedial medulla modulates pain and depression-related behaviors caused by social stress

ABSTRACT

The rostral ventromedial medulla (RVM) is a crucial structure in the descending pain modulatory system, playing a key role as a relay for both the facilitation and inhibition of pain. The chronic social defeat stress (CSDS) model has been widely used to study stress-induced behavioral impairments associated with depression in rodents. Several studies suggest that CSDS also causes changes related to chronic pain. In this study, we aimed to investigate the involvement of the RVM in CSDS-induced behavioral impairments, including those associated with chronic pain. We used chemogenetics to activate or inhibit the RVM during stress. The results indicated that the RVM is a vital hub influencing stress outcomes. Rostral ventromedial medulla activation during CSDS ameliorates all the stress outcomes, including social avoidance, allodynia, hyperalgesia, anhedonia, and behavioral despair. In addition, RVM inhibition in animals exposed to a subthreshold social defeat stress protocol induces a susceptible phenotype, facilitating all stress outcomes. Finally, chronic RVM inhibition-without any social stress stimulus-induces chronic pain but not depressive-like behaviors. Our findings provide insights into the comorbidity between chronic pain and depression by indicating the involvement of the RVM in establishing social stress-induced behavioral responses associated with both chronic pain and depression.

Cell- and Pathway-Specific Disruptions in the Accumbens of Fragile X Mouse

Fragile X syndrome (FXS) is a genetic cause of intellectual disability and autism spectrum disorder. The mesocorticolimbic system, which includes the prefrontal cortex (PFC), basolateral amygdala (BLA), and nucleus accumbens core (NAcC), is essential for regulating socioemotional behaviors. We employed optogenetics to compare the functional properties of the BLA→NAcC, PFC→NAcC, and reciprocal PFC↔BLA pathways in Fmr1-/y::Drd1a-tdTomato male mice. In FXS mice, the PFC↔BLA reciprocal pathway was unaffected, while significant synaptic modifications occurred in the BLA/PFC→NAcC pathways. We observed distinct changes in D1 striatal projection neurons (SPNs) and separate modifications in D2 SPNs. In FXS mice, the BLA/PFC→NAcC-D2 SPN pathways demonstrated heightened synaptic strength. Focusing on the BLA→NAcC pathway, linked to autistic symptoms, we found increased AMPAR and NMDAR currents and elevated spine density in D2 SPNs. Conversely, the amplified firing probability of BLA→NAcC-D1 SPNs was not accompanied by increased synaptic strength, AMPAR and NMDAR currents, or spine density. These pathway-specific alterations resulted in an overall enhancement of excitatory-to-spike coupling, a physiologically relevant index of how efficiently excitatory inputs drive neuronal firing, in both BLA→NAcC-D1 and BLA→NAcC-D2 pathways. Finally, the absence of fragile X messenger ribonucleoprotein 1 (FMRP) led to impaired long-term depression specifically in BLA→D1 SPNs. These distinct alterations in synaptic transmission and plasticity within circuits targeting the NAcC highlight the potential role of postsynaptic mechanisms in selected SPNs in the observed circuit-level changes. This research underscores the heightened vulnerability of the NAcC in the context of FMRP deficiency, emphasizing its pivotal role in the pathophysiology of FXS.

Dopamine Dysregulation in Reward and Autism Spectrum Disorder

Autism spectrum disorder (ASD) is primarily characterized by core deficits in social skills, communication, and cognition and by repetitive stereotyped behaviors. These manifestations are variable between individuals, and ASD pathogenesis is complex, with over a thousand implicated genes, many epigenetic factors, and multiple environmental influences. The mesolimbic dopamine (DA) mediated brain reward system is held to play a key role, but the rapidly expanding literature reveals intricate, nuanced signaling involving a wide array of mesolimbic loci, neurotransmitters and receptor subtypes, and neuronal variants. How altered DA signaling may constitute a downstream convergence of the manifold causal origins of ASD is not well understood. A clear working framework of ASD pathogenesis may help delineate common stages and potential diagnostic and interventional opportunities. Hence, we summarize the known natural history of ASD in the context of emerging data and perspectives to update ASD reward signaling. Then, against this backdrop, we proffer a provisional framework that organizes ASD pathogenesis into successive levels, including (1) genetic and epigenetic changes, (2) disrupted mesolimbic reward signaling pathways, (3) dysregulated neurotransmitter/DA signaling, and finally, (4) altered neurocognitive and social behavior and possible antagonist/agonist based ASD interventions. This subdivision of ASD into a logical progression of potentially addressable parts may help facilitate the rational formulation of diagnostics and targeted treatments.

Distinct Thalamo-Subcortical Circuits Underlie Painful Behavior and Depression-Like Behavior Following Nerve Injury

Clinically, chronic pain and depression often coexist in multiple diseases and reciprocally reinforce each other, which greatly escalates the difficulty of treatment. The neural circuit mechanism underlying the chronic pain/depression comorbidity remains unclear. The present study reports that two distinct subregions in the paraventricular thalamus (PVT) play different roles in this pathological process. In the first subregion PVT posterior (PVP), glutamatergic neurons (PVPGlu) send signals to GABAergic neurons (VLPAGGABA) in the ventrolateral periaqueductal gray (VLPAG), which mediates painful behavior in comorbidity. Meanwhile, in another subregion PVT anterior (PVA), glutamatergic neurons (PVAGlu) send signals to the nucleus accumbens D1-positive neurons and D2-positive neurons (NAcD1→D2), which is involved in depression-like behavior in comorbidity. This study demonstrates that the distinct thalamo-subcortical circuits PVPGlu→VLPAGGABA and PVAGlu→NAcD1→D2 mediated painful behavior and depression-like behavior following spared nerve injury (SNI), respectively, which provides the circuit-based potential targets for preventing and treating comorbidity.

Neurobiology and systems biology of stress resilience

Stress resilience is the phenomenon that some people maintain their mental health despite exposure to adversity or show only temporary impairments followed by quick recovery. Resilience research attempts to unravel the factors and mechanisms that make resilience possible and to harness its insights for the development of preventative interventions in individuals at risk for acquiring stress-related dysfunctions. Biological resilience research has been lagging behind the psychological and social sciences but has seen a massive surge in recent years. At the same time, progress in this field has been hampered by methodological challenges related to finding suitable operationalizations and study designs, replicating findings, and modeling resilience in animals. We embed a review of behavioral, neuroimaging, neurobiological, and systems biological findings in adults in a critical methods discussion. We find preliminary evidence that hippocampus-based pattern separation and prefrontal-based cognitive control functions protect against the development of pathological fears in the aftermath of singular, event-type stressors [as found in fear-related disorders, including simpler forms of posttraumatic stress disorder (PTSD)] by facilitating the perception of safety. Reward system-based pursuit and savoring of positive reinforcers appear to protect against the development of more generalized dysfunctions of the anxious-depressive spectrum resulting from more severe or longer-lasting stressors (as in depression, generalized or comorbid anxiety, or severe PTSD). Links between preserved functioning of these neural systems under stress and neuroplasticity, immunoregulation, gut microbiome composition, and integrity of the gut barrier and the blood-brain barrier are beginning to emerge. On this basis, avenues for biological interventions are pointed out.

Mono-methylation of lysine 27 at histone 3 confers lifelong susceptibility to stress

Histone post-translational modifications are critical for mediating persistent alterations in gene expression. By combining unbiased proteomics profiling and genome-wide approaches, we uncovered a role for mono-methylation of lysine 27 at histone H3 (H3K27me1) in the enduring effects of stress. Specifically, mice susceptible to early life stress (ELS) or chronic social defeat stress (CSDS) displayed increased H3K27me1 enrichment in the nucleus accumbens (NAc), a key brain-reward region. Stress-induced H3K27me1 accumulation occurred at genes that control neuronal excitability and was mediated by the VEFS domain of SUZ12, a core subunit of the polycomb repressive complex-2, which controls H3K27 methylation patterns. Viral VEFS expression changed the transcriptional profile of the NAc, led to social, emotional, and cognitive abnormalities, and altered excitability and synaptic transmission of NAc D1-medium spiny neurons. Together, we describe a novel function of H3K27me1 in the brain and demonstrate its role as a "chromatin scar" that mediates lifelong stress susceptibility.

Neurobiological basis of stress resilience

A majority of humans faced with severe stress maintain normal physiological and behavioral function, a process referred to as resilience. Such stress resilience has been modeled in laboratory animals and, over the past 15 years, has transformed our understanding of stress responses and how to approach the treatment of human stress disorders such as depression, post-traumatic stress disorder (PTSD), and anxiety disorders. Work in rodents has demonstrated that resilience to chronic stress is an active process that involves much more than simply avoiding the deleterious effects of the stress. Rather, resilience is mediated largely by the induction of adaptations that are associated uniquely with resilience. Such mechanisms of natural resilience in rodents are being characterized at the molecular, cellular, and circuit levels, with an increasing number being validated in human investigations. Such discoveries raise the novel possibility that treatments for human stress disorders, in addition to being geared toward reversing the damaging effects of stress, can also be based on inducing mechanisms of natural resilience in individuals who are inherently more susceptible. This review provides a progress report on this evolving field.

Pathological cell assembly dynamics in a striatal MSN network model

Under normal conditions the principal cells of the striatum, medium spiny neurons (MSNs), show structured cell assembly activity patterns which alternate sequentially over exceedingly long timescales of many minutes. It is important to understand this activity since it is characteristically disrupted in multiple pathologies, such as Parkinson's disease and dyskinesia, and thought to be caused by alterations in the MSN to MSN lateral inhibitory connections and in the strength and distribution of cortical excitation to MSNs. To understand how these long timescales arise we extended a previous network model of MSN cells to include synapses with short-term plasticity, with parameters taken from a recent detailed striatal connectome study. We first confirmed the presence of sequentially switching cell clusters using the non-linear dimensionality reduction technique, Uniform Manifold Approximation and Projection (UMAP). We found that the network could generate non-stationary activity patterns varying extremely slowly on the order of minutes under biologically realistic conditions. Next we used Simulation Based Inference (SBI) to train a deep net to map features of the MSN network generated cell assembly activity to MSN network parameters. We used the trained SBI model to estimate MSN network parameters from ex-vivo brain slice calcium imaging data. We found that best fit network parameters were very close to their physiologically observed values. On the other hand network parameters estimated from Parkinsonian, decorticated and dyskinetic ex-vivo slice preparations were different. Our work may provide a pipeline for diagnosis of basal ganglia pathology from spiking data as well as for the design pharmacological treatments.

Chronic defeat stress induces monoamine level dysregulation in the prefrontal cortex but not in the hippocampus of OF1 male mice

Chronic social stress can increase susceptibility to chronic diseases such as depression. One of the most used models to study the physiological mechanisms and behavioral outcomes of this type of stress is chronic defeat stress (CDS) in male mice. OF1 male mice were subjected to a stress period lasting 18 days. During that time, non-stressed animals were housed in groups. The cluster analysis of the behavioral profile displayed during the first social interaction divided subjects into two groups: active/aggressive (AA) and passive/reactive (PR). The day after the end of the stress period, the following behavioral analyses were performed: the sucrose preference test (SPT) on day 19, the open field test (OFT) on day 20, and the forced swim test (FST) on day 21. Immediately after completing the last test, animals were weighed, and blood samples were obtained. Then, they were sacrificed, and their prefrontal cortices and hippocampi were removed and stored to analyze monoamine levels. Stressed animals displayed anhedonia, and solely the PR mice continued to show higher levels of immobility in the OFT and FST. All stressed animals, regardless of the coping strategy, presented higher plasma corticosterone levels. In addition, stressed mice showed lower levels of tyrosine, dopamine, DOPAC, MHPG, kynurenine, kynurenic acid, and 5-HIAA levels but higher serotonin levels in the prefrontal cortex, not in the hippocampus. In conclusion, our results show that CSD induces differences in monoamine levels between brain areas, and these differences did not respond to the coping strategy adopted.

Adolescent nicotine exposure induces long-term, sex-specific disturbances in mood and anxiety-related behavioral, neuronal and molecular phenotypes in the mesocorticolimbic system

The majority of lifetime smokers begin using nicotine during adolescence, a critical period of brain development wherein neural circuits critical for mood, affect and cognition are vulnerable to drug-related insults. Specifically, brain regions such as the medial prefrontal cortex (mPFC), the ventral tegmental area (VTA), nucleus accumbens (NAc) and hippocampus, are implicated in both nicotine dependence and pathological phenotypes linked to mood and anxiety disorders. Clinical studies report that females experience higher rates of mood/anxiety disorders and are more resistant to smoking cessation therapies, suggesting potential sex-specific responses to nicotine exposure and later-life neuropsychiatric risk. However, the potential neural and molecular mechanisms underlying such sex differences are not clear. In the present study, we compared the impacts of adolescent nicotine exposure in male vs. female rat cohorts. We performed a combination of behavioral, electrophysiological and targeted protein expression analyses along with matrix assisted laser deionization imaging (MALDI) immediately post-adolescent exposure and later in early adulthood. We report that adolescent nicotine exposure induced long-lasting anxiety/depressive-like behaviors, disrupted neuronal activity patterns in the mPFC-VTA network and molecular alterations in various neural regions linked to affect, anxiety and cognition. Remarkably, these phenotypes were only observed in males and/or were expressed in the opposite direction in females. These findings identify a series of novel, sex-selective biomarkers for adolescent nicotine-induced neuropsychiatric risk, persisting into adulthood.

NAc-DBS corrects depression-like behaviors in CUMS mouse model via disinhibition of DA neurons in the VTA

Major depressive disorder (MDD) is characterized by diverse debilitating symptoms that include loss of motivation and anhedonia. If multiple medications, psychotherapy, and electroconvulsive therapy fail in some patients with MDD, their condition is then termed treatment-resistant depression (TRD). MDD can be associated with abnormalities in the reward-system-dopaminergic mesolimbic pathway, in which the nucleus accumbens (NAc) and ventral tegmental area (VTA) play major roles. Deep brain stimulation (DBS) applied to the NAc alleviates the depressive symptoms of MDD. However, the mechanism underlying the effects of this DBS has remained elusive. In this study, using the chronic unpredictable mild stress (CUMS) mouse model, we investigated the behavioral and neurobiological effects of NAc-DBS on the multidimensional depression-like phenotypes induced by CUMS by integrating behavioral, in vivo microdialysis coupled with high-performance liquid chromatography-electrochemical detector (HPLC-ECD), calcium imaging, pharmacological, and genetic manipulation methods in freely moving mice. We found that long-term and repeated, but not single, NAc-DBS induced robust antidepressant responses in CUMS mice. Moreover, even a single trial NAc-DBS led to the elevation of the γ-aminobutyric acid (GABA) neurotransmitter, accompanied by the increase in dopamine (DA) neuron activity in the VTA. Both the inhibition of the GABAA receptor activity and knockdown of the GABAA-α1 gene in VTA-GABA neurons blocked the antidepressant effect of NAc-DBS in CUMS mice. Our results showed that NAc-DBS could disinhibit VTA-DA neurons by regulating the level of GABA and the activity of VTA-GABA in the VTA and could finally correct the depression-like behaviors in the CUMS mouse model.

Chronic social defeat stress in prairie voles (Microtus ochrogaster): A preclinical model for the study of depression-related phenotypes

BACKGROUND

Stress-induced illnesses, like major depression, are among the leading causes of disability across the world. Consequently, there is a dire need for the validation of translationally-suited animal models incorporating social stress to uncover the etiology of depression. Prairie voles (Microtus ochrogaster) are more translationally relevant than many other rodent models as they display monogamous social and bi-parental behaviors. Therefore, we evaluated whether a novel social defeat stress (SDS) model in male prairie voles induces depression-relevant behavioral outcomes.

METHODS

Adult sexually-naïve male prairie voles experienced SDS bouts from a conspecific pair-bonded male aggressor, 10 min per day for 10 consecutive days. Non-stressed controls (same-sex siblings) were housed in similar conditions but never experienced physical stress. Twenty-four h later, voles were evaluated in social interaction, sucrose preference, and Morris water maze tests - behavioral endpoints validated to assess social withdrawal, anhedonia-related behavior, and spatial memory performance, respectively.

RESULTS

SDS-exposed voles displayed lower sociability and body weight, decreased preference for a sucrose solution, and impairment of spatial memory retrieval. Importantly, no differences in general locomotor activity were observed as a function of SDS exposure.

LIMITATIONS

This study does not include female voles in the experimental design.

CONCLUSIONS

We found that repeated SDS exposure, in male prairie voles, results in a depression-relevant phenotype resembling an anhedonia-like outcome (per reductions in sucrose preference) along with social withdrawal and spatial memory impairment - highlighting that the prairie vole is a valuable model with potential to study the neurobiology of social stress-induced depression-related outcomes.

Conditioning and pseudoconditioning differently change intrinsic excitability of inhibitory interneurons in the neocortex

Many studies indicate a broad role of various classes of GABAergic interneurons in the processes related to learning. However, little is known about how the learning process affects intrinsic excitability of specific classes of interneurons in the neocortex. To determine this, we employed a simple model of conditional learning in mice where vibrissae stimulation was used as a conditioned stimulus and a tail shock as an unconditioned one. In vitro whole-cell patch-clamp recordings showed an increase in intrinsic excitability of low-threshold spiking somatostatin-expressing interneurons (SST-INs) in layer 4 (L4) of the somatosensory (barrel) cortex after the conditioning paradigm. In contrast, pseudoconditioning reduced intrinsic excitability of SST-LTS, parvalbumin-expressing interneurons (PV-INs), and vasoactive intestinal polypeptide-expressing interneurons (VIP-INs) with accommodating pattern in L4 of the barrel cortex. In general, increased intrinsic excitability was accompanied by narrowing of action potentials (APs), whereas decreased intrinsic excitability coincided with AP broadening. Altogether, these results show that both conditioning and pseudoconditioning lead to plastic changes in intrinsic excitability of GABAergic interneurons in a cell-specific manner. In this way, changes in intrinsic excitability can be perceived as a common mechanism of learning-induced plasticity in the GABAergic system.

D1 and D2 medium spiny neurons in the nucleus accumbens core have distinct and valence-independent roles in learning

At the core of value-based learning is the nucleus accumbens (NAc). D1- and D2-receptor-containing medium spiny neurons (MSNs) in the NAc core are hypothesized to have opposing valence-based roles in behavior. Using optical imaging and manipulation approaches in mice, we show that neither D1 nor D2 MSNs signal valence. D1 MSN responses were evoked by stimuli regardless of valence or contingency. D2 MSNs were evoked by both cues and outcomes, were dynamically changed with learning, and tracked valence-free prediction error at the population and individual neuron level. Finally, D2 MSN responses to cues were necessary for associative learning. Thus, D1 and D2 MSNs work in tandem, rather than in opposition, by signaling specific properties of stimuli to control learning.

Genetic loss of the dopamine transporter significantly impacts behavioral and molecular responses to sub-chronic stress in mice

Dopaminergic neurotransmission has emerged as a critical determinant of stress susceptibility and resilience. Although the dopamine transporter (DAT) is known to play a key role in maintaining dopamine (DA) homeostasis, its importance for the regulation of stress susceptibility remains largely unknown. Indeed, while numerous studies have examined the neurochemical and behavioral consequences of genetic loss of DAT, very few have compared responses to stress in wild-type and DAT-knockout (KO) animals. The current study compared the responses of male and female WT and DAT-KO mice to a model of sub-chronic stress. Our results reveal that DAT-KO mice are resistant to stress-induced increases in the latency to enter the light chamber of the light-dark emergence test and demonstrate that DAT-KO mice exhibit baseline reductions in forced swim test immobility and grooming time in the splash test of grooming behavior. In addition to these behavioral changes, our results highlight the importance of sex and dopaminergic neurotransmission on stress-induced changes in the expression and phosphorylation of several signal transduction molecules in the nucleus accumbens that have previously been implicated in the regulation of stress susceptibility, including ERK, GSK3β, and ΔFosB. Overall, these results provide further evidence of the importance of dopaminergic neurotransmission in regulating stress susceptibility and suggest that genetic loss of DAT prevents stress-induced increases in anxiety-like behavior.

Circulating myeloid-derived MMP8 in stress susceptibility and depression

Psychosocial stress has profound effects on the body, including the immune system and the brain1,2. Although a large number of pre-clinical and clinical studies have linked peripheral immune system alterations to stress-related disorders such as major depressive disorder (MDD)3, the underlying mechanisms are not well understood. Here we show that expression of a circulating myeloid cell-specific proteinase, matrix metalloproteinase 8 (MMP8), is increased in the serum of humans with MDD as well as in stress-susceptible mice following chronic social defeat stress (CSDS). In mice, we show that this increase leads to alterations in extracellular space and neurophysiological changes in the nucleus accumbens (NAc), as well as altered social behaviour. Using a combination of mass cytometry and single-cell RNA sequencing, we performed high-dimensional phenotyping of immune cells in circulation and in the brain and demonstrate that peripheral monocytes are strongly affected by stress. In stress-susceptible mice, both circulating monocytes and monocytes that traffic to the brain showed increased Mmp8 expression following chronic social defeat stress. We further demonstrate that circulating MMP8 directly infiltrates the NAc parenchyma and controls the ultrastructure of the extracellular space. Depleting MMP8 prevented stress-induced social avoidance behaviour and alterations in NAc neurophysiology and extracellular space. Collectively, these data establish a mechanism by which peripheral immune factors can affect central nervous system function and behaviour in the context of stress. Targeting specific peripheral immune cell-derived matrix metalloproteinases could constitute novel therapeutic targets for stress-related neuropsychiatric disorders.

Latent-state and model-based learning in PTSD

Post-traumatic stress disorder (PTSD) is characterized by altered emotional and behavioral responding following a traumatic event. In this article, we review the concepts of latent-state and model-based learning (i.e., learning and inferring abstract task representations) and discuss their relevance for clinical and neuroscience models of PTSD. Recent data demonstrate evidence for brain and behavioral biases in these learning processes in PTSD. These new data potentially recast excessive fear towards trauma cues as a problem in learning and updating abstract task representations, as opposed to traditional conceptualizations focused on stimulus-specific learning. Biases in latent-state and model-based learning may also be a common mechanism targeted in common therapies for PTSD. We highlight key knowledge gaps that need to be addressed to further elaborate how latent-state learning and its associated neurocircuitry mechanisms function in PTSD and how to optimize treatments to target these processes.

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.

The cortico-striatal circuitry in autism-spectrum disorders: a balancing act

The basal ganglia are major targets of cortical inputs and, in turn, modulate cortical function via their projections to the motor and prefrontal cortices. The role of the basal ganglia in motor control and reward is well documented and there is also extensive evidence that they play a key role in social and repetitive behaviors. The basal ganglia influence the activity of the cerebral cortex via two major projections from the striatum to the output nuclei, the globus pallidus internus and the substantia nigra, pars reticulata. This modulation involves a direct projection known as the direct pathway and an indirect projection via the globus pallidus externus and the subthalamic nucleus, known as the indirect pathway. This review discusses the respective contribution of the direct and indirect pathways to social and repetitive behaviors in neurotypical conditions and in autism spectrum disorders.

Nucleus accumbens local circuit for cue-dependent aversive learning

Response to threatening environmental stimuli requires detection and encoding of important environmental features that dictate threat. Aversive events are highly salient, which promotes associative learning about stimuli that signal this threat. The nucleus accumbens is uniquely positioned to process this salient, aversive information and promote motivated output, through plasticity on the major projection neurons in the brain area. We describe a nucleus accumbens core local circuit whereby excitatory plasticity facilitates learning and recall of discrete aversive cues. We demonstrate that putative nucleus accumbens substance P release and long-term excitatory plasticity on dopamine 2 receptor-expressing projection neurons are required for cue-dependent fear learning. Additionally, we find that fear learning and recall is dependent on distinct projection neuron subtypes. Our work demonstrates a critical role for nucleus accumbens substance P in cue-dependent aversive learning.

Stress relief as a natural resilience mechanism against depression-like behaviors

Relief, the appetitive state after the termination of aversive stimuli, is evolutionarily conserved. Understanding the behavioral role of this well-conserved phenomenon and its underlying neurobiological mechanisms are open and important questions. Here, we discover that the magnitude of relief from physical stress strongly correlates with individual resilience to depression-like behaviors in chronic stressed mice. Notably, blocking stress relief causes vulnerability to depression-like behaviors, whereas natural rewards supplied shortly after stress promotes resilience. Stress relief is mediated by reward-related mesolimbic dopamine neurons, which show minute-long, persistent activation after stress termination. Circuitry-wise, activation or inhibition of circuits downstream of the ventral tegmental area during the transient relief period bi-directionally regulates depression resilience. These results reveal an evolutionary function of stress relief in depression resilience and identify the neural substrate mediating this effect. Importantly, our data suggest a behavioral strategy of augmenting positive valence of stress relief with natural rewards to prevent depression.

The different cell-specific mechanisms of voluntary exercise and forced exercise in the nucleus accumbens

Physical inactivity is a global epidemic. People who take the initiative to exercise will feel pleasure during the exercise process and stick with it for a long time, while people who passively ask for exercise will feel pain and cannot stick with it. However, the neural mechanisms underlying voluntary and forced exercise remain unclear. Here, we report that voluntary running increased the frequency and amplitude of miniature excitatory postsynaptic currents (mEPSC) but decreased membrane excitability in D1R-MSNs, whereas D2R-MSNs did not change in mEPSC and membrane excitability. Forced running increased the frequency of mEPSC and membrane excitability in D2R-MSNs, but D1R-MSNs did not change, which may be the mechanism by which forced exercise has a non-rewarding effect. These findings provide new insights into how voluntary and forced exercise mediate reward and non-reward effects.

Future Directions in the Mental Health of Transgender Youth: Towards a Social-Affective Developmental Model of Health Disparity

Mental health disparities in transgender and gender diverse (TGD) youth are well-documented. These disparities are often studied in the context of minority stress theory, and most of this research focuses on experiences of trauma and discrimination TGD youth experience after coming out. However, TGD youth may be targets of violence and victimization due to perceived gender nonconformity before coming out. In this Future Directions, we integrate research on attachment, developmental trauma, and effects of racism and homophobia on mental health to propose a social-affective developmental framework for TGD youth. We provide a clinical vignette to highlight limitations in current approaches to mental health assessment in TGD youth and to illustrate how using a social-affective developmental framework can improve clinical assessment and treatment approaches and deepen our understanding of mental health disparities in TGD people.

Ventral striatal islands of Calleja neurons bidirectionally mediate depression-like behaviors in mice

The ventral striatum is a reward center implicated in the pathophysiology of depression. It contains islands of Calleja, clusters of dopamine D3 receptor-expressing granule cells, predominantly in the olfactory tubercle (OT). These OT D3 neurons regulate self-grooming, a repetitive behavior manifested in affective disorders. Here we show that chronic restraint stress (CRS) induces robust depression-like behaviors in mice and decreases excitability of OT D3 neurons. Ablation or inhibition of these neurons leads to depression-like behaviors, whereas their activation ameliorates CRS-induced depression-like behaviors. Moreover, activation of OT D3 neurons has a rewarding effect, which diminishes when grooming is blocked. Finally, we propose a model that explains how OT D3 neurons may influence dopamine release via synaptic connections with OT spiny projection neurons (SPNs) that project to midbrain dopamine neurons. Our study reveals a crucial role of OT D3 neurons in bidirectionally mediating depression-like behaviors, suggesting a potential therapeutic target.

Nucleus Accumbens Local Circuit for Cue-Dependent Aversive Learning

Response to threatening environmental stimuli requires detection and encoding of important environmental features that dictate threat. Aversive events are highly salient which promotes associative learning about stimuli that signal this threat. The nucleus accumbens is uniquely positioned to process this salient, aversive information and promote motivated output, through plasticity on the major projection neurons in the brain area. We uncovered a nucleus accumbens core local circuit whereby excitatory plasticity facilitates learning and recall of discrete aversive cues. We demonstrate that putative nucleus accumbens substance P release and long-term excitatory plasticity on dopamine 2 receptor expressing projection neurons is required for cue-dependent fear learning. Additionally, we found fear learning and recall were dependent on distinct projection-neuron subtypes. Our work demonstrates a critical role for Nucleus Accumbens substance P in cue-dependent aversive learning.

Sex and cell-specific gene expression in corticolimbic brain regions associated with psychiatric disorders revealed by bulk and single-nuclei RNA sequencing

BACKGROUND

There are sex-specific differences in the prevalence, symptomology and course of psychiatric disorders. However, preclinical models have primarily used males, such that the molecular mechanisms underlying sex-specific differences in psychiatric disorders are not well established.

METHODS

In this study, we compared transcriptome-wide gene expression profiles in male and female rats within the corticolimbic system, including the cingulate cortex, nucleus accumbens medial shell (NAcS), ventral dentate gyrus and the basolateral amygdala (n = 22-24 per group/region).

FINDINGS

We found over 3000 differentially expressed genes (DEGs) in the NAcS between males and females. Of these DEGs in the NAcS, 303 showed sex-dependent conservation DEGs in humans and were significantly enriched for gene ontology terms related to blood vessel morphogenesis and regulation of cell migration. Single nuclei RNA sequencing in the NAcS of male and female rats identified widespread sex-dependent expression, with genes upregulated in females showing a notable enrichment for synaptic function. Female upregulated genes in astrocytes, Drd3+MSNs and oligodendrocyte were also enriched in several psychiatric genome-wide association studies (GWAS).

INTERPRETATION

Our data provide comprehensive evidence of sex- and cell-specific molecular profiles in the NAcS. Importantly these differences associate with anxiety, bipolar disorder, schizophrenia, and cross-disorder, suggesting an intrinsic molecular basis for sex-based differences in psychiatric disorders that strongly implicates the NAcS.

FUNDING

This work was supported by funding from the Hope for Depression Research Foundation (MJM).

GABAA receptors as targets for treating affective and cognitive symptoms of depression

In the past 20 years, our understanding of the pathophysiology of depression has evolved from a focus on an imbalance of monoaminergic neurotransmitters to a multifactorial picture including an improved understanding of the role of glutamatergic excitatory and GABAergic inhibitory neurotransmission. FDA-approved treatments targeting the glutamatergic [esketamine for major depressive disorder (MDD)] and GABAergic (brexanolone for peripartum depression) systems have become available. This review focuses on the GABAA receptor (GABAAR) system as a target for novel antidepressants and discusses the mechanisms by which modulation of δ-containing GABAARs with neuroactive steroids (NASs) or of α5-containing GABAARs results in antidepressant or antidepressant-like actions and discusses clinical data on NASs. Moreover, a potential mechanism by which α5-GABAAR-positive allosteric modulators (PAMs) may improve cognitive deficits in depression is presented.

Reactivation of Early-Life Stress-Sensitive Neuronal Ensembles Contributes to Lifelong Stress Hypersensitivity

Early-life stress (ELS) is one of the strongest lifetime risk factors for depression, anxiety, suicide, and other psychiatric disorders, particularly after facing additional stressful events later in life. Human and animal studies demonstrate that ELS sensitizes individuals to subsequent stress. However, the neurobiological basis of such stress sensitization remains largely unexplored. We hypothesized that ELS-induced stress sensitization would be detectable at the level of neuronal ensembles, such that cells activated by ELS would be more reactive to adult stress. To test this, we leveraged transgenic mice to genetically tag, track, and manipulate experience-activated neurons. We found that in both male and female mice, ELS-activated neurons within the nucleus accumbens (NAc), and to a lesser extent the medial prefrontal cortex, were preferentially reactivated by adult stress. To test whether reactivation of ELS-activated ensembles in the NAc contributes to stress hypersensitivity, we expressed hM4Dis receptor in control or ELS-activated neurons of pups and chemogenetically inhibited their activity during experience of adult stress. Inhibition of ELS-activated NAc neurons, but not control-tagged neurons, ameliorated social avoidance behavior following chronic social defeat stress in males. These data provide evidence that ELS-induced stress hypersensitivity is encoded at the level of corticolimbic neuronal ensembles.SIGNIFICANCE STATEMENT Early-life stress enhances sensitivity to stress later in life, yet the mechanisms of such stress sensitization are largely unknown. Here, we show that neuronal ensembles in corticolimbic brain regions remain hypersensitive to stress across the life span, and quieting these ensembles during experience of adult stress rescues stress hypersensitivity.

α-MSH-catabolic enzyme prolylcarboxypeptidase in nucleus accumbens shell ameliorates stress susceptibility in mice through regulating synaptic plasticity

Emerging evidence demonstrates the vital role of synaptic transmission and structural remodeling in major depressive disorder. Activation of melanocortin receptors facilitates stress-induced emotional behavior. Prolylcarboxypeptidase (PRCP) is a serine protease, which splits the C-terminal amino acid of α-MSH and inactivates it. In this study, we asked whether PRCP, the endogenous enzyme of melanocortin system, might play a role in stress susceptibility via regulating synaptic adaptations. Mice were subjected to chronic social defeat stress (CSDS) or subthreshold social defeat stress (SSDS). Depressive-like behavior was assessed in SIT, SPT, TST and FST. Based on to behavioral assessments, mice were divided into the susceptible (SUS) and resilient (RES) groups. After social defeat stress, drug infusion or viral expression and behavioral tests, morphological and electrophysiological analysis were conducted in PFX-fixed and fresh brain slices containing the nucleus accumbens shell (NAcsh). We showed that PRCP was downregulated in NAcsh of susceptible mice. Administration of fluoxetine (20 mg·kg-1·d-1, i.p., for 2 weeks) ameliorated the depressive-like behavior, and restored the expression levels of PRCP in NAcsh of susceptible mice. Pharmacological or genetic inhibition of PRCP in NAcsh by microinjection of N-benzyloxycarbonyl-L-prolyl-L-prolinal (ZPP) or LV-shPRCP enhanced the excitatory synaptic transmission in NAcsh, facilitating stress susceptibility via central melanocortin receptors. On the contrary, overexpression of PRCP in NAcsh by microinjection of AAV-PRCP alleviated the depressive-like behavior and reversed the enhanced excitatory synaptic transmission, abnormal dendritogenesis and spinogenesis in NAcsh induced by chronic stress. Furthermore, chronic stress increased the level of CaMKIIα, a kinase closely related to synaptic plasticity, in NAcsh. The elevated level of CaMKIIα was reversed by overexpression of PRCP in NAcsh. Pharmacological inhibition of CaMKIIα in NAcsh alleviated stress susceptibility induced by PRCP knockdown. This study has revealed the essential role of PRCP in relieving stress susceptibility through melanocortin signaling-mediated synaptic plasticity in NAcsh.

Social threat, fronto-cingulate-limbic morphometry, and symptom course in depressed adolescents: a longitudinal investigation

BACKGROUND

Psychosocial stressors characterized by social threat, such as interpersonal loss and social rejection, are associated with depression in adolescents. Few studies, however, have examined whether social threat affects fronto-cingulate-limbic systems implicated in adolescent depression.

METHODS

We assessed lifetime stressor severity across several domains using the Stress and Adversity Inventory (STRAIN) in 57 depressed adolescents (16.15 ± 1.32 years, 34 females), and examined whether the severity of social threat and non-social threat stressors was associated with gray matter volumes (GMVs) in the anterior cingulate cortex (ACC), amygdala, hippocampus, and nucleus accumbens (NAcc). We also examined how lifetime social threat severity and GMVs in these regions related to depressive symptoms at baseline and over 9 months.

RESULTS

General stressor severity was related to greater depression severity at baseline and over 9 months. Moreover, greater severity of social threat (but not non-social threat) stressors was associated with smaller bilateral amygdala and NAcc GMVs, and smaller bilateral surface areas of caudal and rostral ACC (all pFDR ⩽ 0.048). However, neither social threat nor non-social threat stressor severity was related to hippocampal GMVs (all pFDR ⩾ 0.318). All fronto-cingulate-limbic structures that were associated with the severity of social threat were negatively associated with greater depression severity over 9 months (all pFDR ⩽ 0.014). Post-hoc analyses suggested that gray matter morphometry of bilateral amygdala, NAcc, and rostral and caudal ACC mediated the association between social threat and depression severity in adolescents over 9 months (all pFDR < 0.048).

CONCLUSIONS

Social threat specifically affects fronto-cingulate-limbic pathways that contribute to the maintenance of depression in adolescents.

Mitofusin-2 in nucleus accumbens D2-MSNs regulates social dominance and neuronal function

The nucleus accumbens (NAc) is a brain hub regulating motivated behaviors, including social competitiveness. Mitochondrial function in the NAc links anxiety with social competitiveness, and the mitochondrial fusion protein mitofusin 2 (Mfn2) in NAc neurons regulates anxiety-related behaviors. However, it remains unexplored whether accumbal Mfn2 levels also affect social behavior and whether Mfn2 actions in the emotional and social domain are driven by distinct cell types. Here, we found that subordinate-prone highly anxious rats show decreased accumbal Mfn2 levels and that Mfn2 overexpression promotes dominant behavior. In mice, selective Mfn2 downregulation in NAc dopamine D2 receptor-expressing medium spiny neurons (D2-MSNs) induced social subordination, accompanied by decreased accumbal mitochondrial functions and decreased excitability in D2-MSNs. Instead, D1-MSN-targeted Mfn2 downregulation affected competitive ability only transiently and likely because of an increase in anxiety-like behaviors. Our results assign dissociable cell-type specific roles to Mfn2 in the NAc in modulating social dominance and anxiety.

Involvement of brain cell phenotypes in stress-vulnerability and resilience

Stress-related disorders' prevalence is epidemically increasing in modern society, leading to a severe impact on individuals' well-being and a great economic burden on public resources. Based on this, it is critical to understand the mechanisms by which stress induces these disorders. The study of stress made great progress in the past decades, from deeper into the hypothalamic-pituitary-adrenal axis to the understanding of the involvement of a single cell subtype on stress outcomes. In fact, many studies have used state-of-the-art tools such as chemogenetic, optogenetic, genetic manipulation, electrophysiology, pharmacology, and immunohistochemistry to investigate the role of specific cell subtypes in the stress response. In this review, we aim to gather studies addressing the involvement of specific brain cell subtypes in stress-related responses, exploring possible mechanisms associated with stress vulnerability versus resilience in preclinical models. We particularly focus on the involvement of the astrocytes, microglia, medium spiny neurons, parvalbumin neurons, pyramidal neurons, serotonergic neurons, and interneurons of different brain areas in stress-induced outcomes, resilience, and vulnerability to stress. We believe that this review can shed light on how diverse molecular mechanisms, involving specific receptors, neurotrophic factors, epigenetic enzymes, and miRNAs, among others, within these brain cell subtypes, are associated with the expression of a stress-susceptible or resilient phenotype, advancing the understanding/knowledge on the specific machinery implicate in those events.

CD200 in dentate gyrus improves depressive-like behaviors of mice through enhancing hippocampal neurogenesis via alleviation of microglia hyperactivation

BACKGROUND

Neuroinflammation and microglia play critical roles in the development of depression. Cluster of differentiation 200 (CD200) is an anti-inflammatory glycoprotein that is mainly expressed in neurons, and its receptor CD200R1 is primarily in microglia. Although the CD200-CD200R1 pathway is necessary for microglial activation, its role in the pathophysiology of depression remains unknown.

METHODS

The chronic social defeat stress (CSDS) with behavioral tests were performed to investigate the effect of CD200 on the depressive-like behaviors. Viral vectors were used to overexpress or knockdown of CD200. The levels of CD200 and inflammatory cytokines were tested with molecular biological techniques. The status of microglia, the expression of BDNF and neurogenesis were detected with immunofluorescence imaging.

RESULTS

We found that the expression of CD200 was decreased in the dentate gyrus (DG) region of mice experienced CSDS. Overexpression of CD200 alleviated the depressive-like behaviors of stressed mice and inhibition of CD200 facilitated the susceptibility to stress. When CD200R1 receptors on microglia were knocked down, CD200 was unable to exert its role in alleviating depressive-like behavior. Microglia in the DG brain region were morphologically activated after exposure to CSDS. In contrast, exogenous administration of CD200 inhibited microglia hyperactivation, alleviated neuroinflammatory response in hippocampus, and increased the expression of BDNF, which in turn ameliorated adult hippocampal neurogenesis impairment in the DG induced by CSDS.

CONCLUSIONS

Taken together, these results suggest that CD200-mediated alleviation of microglia hyperactivation contributes to the antidepressant effect of neurogenesis in dentate gyrus in mice.

Emotionally clocked out: cell-type specific regulation of mood and anxiety by the circadian clock system in the brain

Circadian rhythms are self-sustained oscillations of biological systems that allow an organism to anticipate periodic changes in the environment and optimally align feeding, sleep, wakefulness, and the physiological and biochemical processes that support them within the 24 h cycle. These rhythms are generated at a cellular level by a set of genes, known as clock genes, which code for proteins that inhibit their own transcription in a negative feedback loop and can be perturbed by stress, a risk factor for the development of mood and anxiety disorders. A role for circadian clocks in mood and anxiety has been suggested for decades on the basis of clinical observations, and the dysregulation of circadian rhythms is a prominent clinical feature of stress-related disorders. Despite our understanding of central clock structure and function, the effect of circadian dysregulation in different neuronal subtypes in the suprachiasmatic nucleus (SCN), the master pacemaker region, as well as other brain systems regulating mood, including mesolimbic and limbic circuits, is just beginning to be elucidated. In the brain, circadian clocks regulate neuronal physiological functions, including neuronal activity, synaptic plasticity, protein expression, and neurotransmitter release which in turn affect mood-related behaviors via cell-type specific mechanisms. Both animal and human studies have revealed an association between circadian misalignment and mood disorders and suggest that internal temporal desynchrony might be part of the etiology of psychiatric disorders. To date, little work has been conducted associating mood-related phenotypes to cell-specific effects of the circadian clock disruptions. In this review, we discuss existing literature on how clock-driven changes in specific neuronal cell types might disrupt phase relationships among cellular communication, leading to neuronal circuit dysfunction and changes in mood-related behavior. In addition, we examine cell-type specific circuitry underlying mood dysfunction and discuss how this circuitry could affect circadian clock. We provide a focus for future research in this area and a perspective on chronotherapies for mood and anxiety disorders.

The effects of lipopolysaccharide exposure on social interaction, cytokine expression, and alcohol consumption in male and female mice

Much recent research has demonstrated a role of inflammatory pathways in depressive-like behavior and excess alcohol consumption. Lipopolysaccharide (LPS) is a cell wall component of gram-negative bacteria that can be used to trigger a strong inflammatory response in rodents in a preclinical research setting to study the mechanisms behind this relationship. In our study, we exposed male and female mice to LPS and assessed depressive-like behavior using the social interaction (SI) test, alcohol consumption in the two-bottle choice procedure, and expression of inflammatory mediators using quantitative PCR. We found that LPS administration decreased SI in female mice but had no significant impact on male mice when assessed 24 h after injection. LPS resulted in increased proinflammatory cytokine expression in both male and female mice; however, some aspects of the cytokine upregulation observed was greater in female mice as compared to males. A separate cohort of male and female mice underwent drinking for 12 days before receiving a saline or LPS injection, which we found to increase alcohol intake in both males and females. We have previously observed a role of the neurokinin-1 receptor (NK1R) in escalated alcohol intake, and in the inflammatory and behavioral response to LPS. The NK1R is the endogenous target of the neuropeptide SP, and this system has wide ranging roles in depression, anxiety, drug/alcohol seeking, pain, and inflammation. Thus, we administered a NK1R antagonist prior to alcohol access. This treatment reduced escalated alcohol consumption in female mice treated with LPS but did not affect drinking in males. Taken together, these results indicate that females are more sensitive to some physiological and behavioral effects of LPS administration, but that LPS escalates alcohol consumption in both sexes. Furthermore, NK1R antagonism can reduce alcohol consumption that is escalated by LPS treatment, in line with our previous findings.

Investigating cell-specific effects of FMRP deficiency on spiny projection neurons in a mouse model of Fragile X syndrome

Introduction

Fragile X syndrome (FXS), resulting from a mutation in the Fmr1 gene, is the most common monogenic cause of autism and inherited intellectual disability. Fmr1 encodes the Fragile X Messenger Ribonucleoprotein (FMRP), and its absence leads to cognitive, emotional, and social deficits compatible with the nucleus accumbens (NAc) dysfunction. This structure is pivotal in social behavior control, consisting mainly of spiny projection neurons (SPNs), distinguished by dopamine D1 or D2 receptor expression, connectivity, and associated behavioral functions. This study aims to examine how FMRP absence differentially affects SPN cellular properties, which is crucial for categorizing FXS cellular endophenotypes.

Methods

We utilized a novel Fmr1-/y::Drd1a-tdTomato mouse model, which allows in-situ identification of SPN subtypes in FXS mice. Using RNA-sequencing, RNAScope and ex-vivo patch-clamp in adult male mice NAc, we comprehensively compared the intrinsic passive and active properties of SPN subtypes.

Results

Fmr1 transcripts and their gene product, FMRP, were found in both SPNs subtypes, indicating potential cell-specific functions for Fmr1. The study found that the distinguishing membrane properties and action potential kinetics typically separating D1- from D2-SPNs in wild-type mice were either reversed or abolished in Fmr1-/y::Drd1a-tdTomato mice. Interestingly, multivariate analysis highlighted the compound effects of Fmr1 ablation by disclosing how the phenotypic traits distinguishing each cell type in wild-type mice were altered in FXS.

Discussion

Our results suggest that the absence of FMRP disrupts the standard dichotomy characterizing NAc D1- and D2-SPNs, resulting in a homogenous phenotype. This shift in cellular properties could potentially underpin select aspects of the pathology observed in FXS. Therefore, understanding the nuanced effects of FMRP absence on SPN subtypes can offer valuable insights into the pathophysiology of FXS, opening avenues for potential therapeutic strategies.

Circuit- and laminar-specific regulation of medial prefrontal neurons by chronic stress

BACKGROUND

Chronic stress exposure increases the risk of mental health problems such as anxiety and depression. The medial prefrontal cortex (mPFC) is a hub for controlling stress responses through communicating with multiple limbic structures, including the basolateral amygdala (BLA) and nucleus accumbens (NAc). However, considering the complex topographical organization of the mPFC neurons in different subregions (dmPFC vs. vmPFC) and across multiple layers (Layer II/III vs. Layer V), the exact effects of chronic stress on these distinct mPFC output neurons remain largely unknown.

RESULTS

We first characterized the topographical organization of mPFC neurons projecting to BLA and NAc. Then, by using a typical mouse model of chronic restraint stress (CRS), we investigated the effects of chronic stress on the synaptic activity and intrinsic properties of the two mPFC neuronal populations. Our results showed that there was limited collateralization of the BLA- and NAc-projecting pyramidal neurons, regardless of the subregion or layer they were situated in. CRS significantly reduced the inhibitory synaptic transmission onto the BLA-projecting neurons in dmPFC layer V without any effect on the excitatory synaptic transmission, thus leading to a shift of the excitation-inhibition (E-I) balance toward excitation. However, CRS did not affect the E-I balance in NAc-projecting neurons in any subregions or layers of mPFC. Moreover, CRS also preferentially increased the intrinsic excitability of the BLA-projecting neurons in dmPFC layer V. By contrast, it even caused a decreasing tendency in the excitability of NAc-projecting neurons in vmPFC layer II/III.

CONCLUSION

Our findings indicate that chronic stress exposure preferentially modulates the activity of the mPFC-BLA circuit in a subregion (dmPFC) and laminar (layer V) -dependent manner.

Monomethylation of Lysine 27 at Histone 3 Confers Lifelong Susceptibility to Stress

Histone post-translational modifications are critical for mediating persistent alterations in gene expression. By combining unbiased proteomics profiling, and genome-wide approaches, we uncovered a role for mono-methylation of lysine 27 at histone H3 (H3K27me1) in the enduring effects of stress. Specifically, mice exposed to early life stress (ELS) or to chronic social defeat stress (CSDS) in adulthood displayed increased enrichment of H3K27me1, and transient decreases in H3K27me2, in the nucleus accumbens (NAc), a key brain-reward region. Stress induction of H3K27me1 was mediated by the VEFS domain of SUZ12, a core subunit of the polycomb repressive complex-2, which is induced by chronic stress and controls H3K27 methylation patterns. Overexpression of the VEFS domain led to social, emotional, and cognitive abnormalities, and altered excitability of NAc D1 mediums spiny neurons. Together, we describe a novel function of H3K27me1 in brain and demonstrate its role as a "chromatin scar" that mediates lifelong stress susceptibility.

Neural and Molecular Investigation into the Paraventricular Thalamic-Nucleus Accumbens Circuit for Pain Sensation and Non-opioid Analgesia

The paucity of medications with novel mechanisms for pain treatment combined with the severe adverse effects of opioid analgesics has led to an imperative pursuit of non-opioid analgesia and a better understanding of pain mechanisms. Here, we identify the putative glutamatergic inputs from the paraventricular thalamic nucleus to the nucleus accumbens (PVT^Glut→NAc) as a novel neural circuit for pain sensation and non-opioid analgesia. Our in vivo fiber photometry and in vitro electrophysiology experiments found that PVT^Glut→NAc neuronal activity increased in response to acute thermal/mechanical stimuli and persistent inflammatory pain. Direct optogenetic activation of these neurons in the PVT or their terminals in the NAc induced pain-like behaviors. Conversely, inhibition of PVT^Glut→NAc neurons or their NAc terminals exhibited a potent analgesic effect in both naïve and pathological pain mice, which could not be prevented by pretreatment of naloxone, an opioid receptor antagonist. Anterograde trans-synaptic optogenetic experiments consistently demonstrated that the PVT^Glut→NAc circuit bi-directionally modulates pain behaviors. Furthermore, circuit-specific molecular profiling and pharmacological studies revealed dopamine receptor 3 as a candidate target for pain modulation and non-opioid analgesic development. Taken together, these findings provide a previously unknown neural circuit for pain sensation and non-opioid analgesia and a valuable molecular target for developing future safer medication.

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.

Nucleus accumbens in the pathogenesis of major depressive disorder: A brief review

Major depressive disorder (MDD) is the most prevalent mental disorder characterized by anhedonia, loss of motivation, avolition, behavioral despair and cognitive abnormalities. Despite substantial advancements in the pathophysiology of MDD in recent years, the pathogenesis of this disorder is not fully understood. Meanwhile,the treatment of MDD with currently available antidepressants is inadequate, highlighting the urgent need for clarifying the pathophysiology of MDD and developing novel therapeutics. Extensive studies have demonstrated the involvement of nuclei such as the prefrontal cortex (PFC), hippocampus (HIP), nucleus accumbens (NAc), hypothalamus, etc., in MDD. NAc,a region critical for reward and motivation,dysregulation of its activity seems to be a hallmark of this mood disorder. In this paper, we present a review of NAc related circuits, cellular and molecular mechanisms underlying MDD and share an analysis of the gaps in current research and possible future research directions.

GABAergic neurons in the nucleus accumbens core mediate the antidepressant effects of sevoflurane

General anaesthetics have been widely applied to induce reversible loss and recovery of consciousness in clinical practice and have been shown to have reliably safe profiles. Since brief exposure to general anaesthetics can result in long-lasting and global changes in neuronal structures and function, these drugs also exhibit strong therapeutic potential for mood disorders. Preliminary and clinical studies have suggested that the inhalational anaesthetic drug sevoflurane might relieve symptoms of depression. However, the antidepressant effects of sevoflurane and the underlying mechanisms remain elusive. In the present study, we confirmed that the antidepressant and anxiolytic effects of inhaling 2.5% sevoflurane for 30 min were comparable to those of ketamine and could be sustained for 48 h. Activation of GABAergic (γ-aminobutyric acidergic) neurons in the nucleus accumbens core by chemogenetics was shown to mimic the antidepressant effects of inhaled sevoflurane, whereas inhibition of these neurons significantly prevented these effects. Considered together, these results suggested that sevoflurane might exert rapid and long-lasting antidepressant effects via modulation of neuronal activities in the nucleus accumbens core nucleus.

Excitatory Projections from the Prefrontal Cortex to Nucleus Accumbens Core D1-MSNs and κ Opioid Receptor Modulate Itch-Related Scratching Behaviors

Itch is an uncomfortable and complex sensation that elicits the desire to scratch. The nucleus accumbens (NAc) activity is important in driving sensation, motivation, and emotion. Excitatory afferents from the medial prefrontal cortex (mPFC), amygdala, and hippocampus are crucial in tuning the activity of dopamine receptor D1-expressing and D2-expressing medium spiny neurons (Drd1-MSN and Drd2-MSN) in the NAc. However, a cell-type and neural circuity-based mechanism of the NAc underlying acute itch remains unclear. We found that acute itch induced by compound 48/80 (C48/80) decreased the intrinsic membrane excitability in Drd1-MSNs, but not in Drd2-MSNs, in the NAc core of male mice. Chemogenetic activation of Drd1-MSNs alleviated C48/80-induced scratching behaviors but not itch-related anxiety-like behaviors. In addition, C48/80 enhanced the frequency of spontaneous EPSCs (sEPSCs) and reduced the paired-pulse ratio (PPR) of electrical stimulation-evoked EPSCs in Drd1-MSNs. Furthermore, C48/80 increased excitatory synaptic afferents to Drd1-MSNs from the mPFC, not from the basolateral amygdala (BLA) or ventral hippocampus (vHipp). Consistently, the intrinsic excitability of mPFC-NAc projecting pyramidal neurons was increased after C48/80 treatment. Chemogenetic inhibition of mPFC-NAc excitatory synaptic afferents relieved the scratching behaviors. Moreover, pharmacological activation of κ opioid receptor (KOR) in the NAc core suppressed C48/80-induced scratching behaviors, and the modulation of KOR activity in the NAc resulted in the changes of presynaptic excitatory inputs to Drd1-MSNs in C48/80-treated mice. Together, these results reveal the neural plasticity in synapses of NAc Drd1-MSNs from the mPFC underlying acute itch and indicate the modulatory role of the KOR in itch-related scratching behaviors.SIGNIFICANCE STATEMENT Itch stimuli cause strongly scratching desire and anxiety in patients. However, the related neural mechanisms remain largely unclear. In the present study, we demonstrated that the pruritogen compound 48/80 (C48/80) shapes the excitability of dopamine receptor D1-expressing medium spiny neurons (Drd1-MSNs) in the nucleus accumbens (NAc) core and the glutamatergic synaptic afferents from medial prefrontal cortex (mPFC) to these neurons. Chemogenetic activation of Drd1-MSNs or inhibition of mPFC-NAc excitatory synaptic afferents relieves the scratching behaviors. In addition, pharmacological activation of κ opioid receptor (KOR) in the NAc core alleviates C48/80-induced itch. Thus, targeting mPFC-NAc Drd1-MSNs or KOR may provide effective treatments for itch.