The female urine sniffing test: a novel approach for assessing reward-seeking behavior in rodents

Estradiol, via estrogen receptor β signaling, mediates stress-susceptibility in the male brain

Dysregulation of normal reward processing via psychological stress contributes to the development of psychiatric disorders. Estrogen is involved in reward processing in females, but this effect has not been well studied in males despite the abundant conversion of androgens to estrogens in the male brain. Here, we used a combination of genetic deletions, behavioral assays, pharmacology, circuit dissection, electrophysiology, in vivo fiber photometry, and optogenetics/chemogenetics to determine the role of the most prevalent and potent estrogen, 17β-estradiol, in male stress-induced reward processing dysfunction. We found that absence of estrogen receptor (ER) β renders male but not female mice susceptible to stress-induced maladaptive reward-processing behaviors. We demonstrated that activation of ERβ-projecting neurons from the basolateral amygdala to nucleus accumbens induced rewarding effects in male, but not female mice. Moreover, we show that the activity of ERβ-expressing neurons projecting from the basolateral amygdala to nucleus accumbens is reduced in hypogonadal male mice subjected to stress, while activation of this circuit reverses stress-induced maladaptive reward processing behaviors and inhibition induces stress susceptibility. We identified that absence of estradiol, but not testosterone per se, underlies susceptibility to stress-mediated dysfunction of rewarding behaviors and that brain-selective delivery of estradiol and intra-basolateral amygdala administration of an ERβ-specific agonist prevent maladaptive reward-processing behaviors in hypogonadal male mice. These findings delineate an estrogen-based mechanism underlying stress susceptibility and provide a novel therapeutic strategy for the treatment of reward-related disorders associated with hypogonadal conditions.

The activation of the piriform cortex to lateral septum pathway during chronic social defeat stress is crucial for the induction of behavioral disturbance in mice

Chronic stress induces neural dysfunctions and risks mental illnesses. Clinical and preclinical studies have established the roles of brain regions underlying emotional and cognitive functions in stress and depression. However, neural pathways to perceive sensory stimuli as stress to cause behavioral disturbance remain unknown. Using whole-brain imaging of Arc-dVenus neuronal response reporter mice and machine learning analysis, here we unbiasedly demonstrated different patterns of contribution of widely distributed brain regions to neural responses to acute and chronic social defeat stress (SDS). Among these brain regions, multiple sensory cortices, especially the piriform (olfactory) cortex, primarily contributed to classifying neural responses to chronic SDS. Indeed, SDS-induced activation of the piriform cortex was augmented with repetition of SDS, accompanied by impaired odor discrimination. Axonal tracing and chemogenetic manipulation showed that excitatory neurons in the piriform cortex directly project to the lateral septum and activate it in response to chronic SDS, thereby inducing behavioral disturbance. These results pave the way for identifying a spatially defined sequence of neural consequences of stress and the roles of sensory pathways in perceiving chronic stress in mental illness pathology.

Early-life gut inflammation drives sex-dependent shifts in the microbiome-endocrine-brain axis

Despite recent advances in understanding the connection between the gut microbiota and the adult brain, significant knowledge gaps remain regarding how gut inflammation affects brain development. We hypothesized that gut inflammation during early life would negatively affect neurodevelopment by disrupting microbiota communication to the brain. We therefore developed a novel pediatric chemical model of inflammatory bowel disease (IBD), an incurable condition affecting millions of people worldwide. IBD is characterized by chronic intestinal inflammation, and is associated with comorbid symptoms such as anxiety, depression and cognitive impairment. Notably, 25% of patients with IBD are diagnosed during childhood, and the effects of chronic inflammation during this critical developmental period remain poorly understood. This study investigated the effects of early-life gut inflammation induced by DSS (dextran sulfate sodium) on a range of microbiota, endocrine, and behavioral outcomes, focusing on sex-specific impacts. DSS-treated mice exhibited increased intestinal inflammation and altered microbiota membership, which correlated with changes in microbiota-derived circulating metabolites. The majority of behavioral measures were unaffected, with the exception of impaired mate-seeking behaviors in DSS-treated males. DSS-treated males also showed significantly smaller seminal vesicles, lower circulating androgens, and decreased intestinal hormone-activating enzyme activity compared to vehicle controls. In the brain, DSS treatment led to chronic, sex-specific alterations in microglial morphology. These results suggest that early-life gut inflammation causes changes in gut microbiota composition, affecting short-chain fatty acid (SCFA) producers and glucuronidase (GUS) activity, correlating with altered SCFA and androgen levels. The findings highlight the developmental sensitivity to inflammation-induced changes in endocrine signalling and emphasize the long-lasting physiological and microbiome changes associated with juvenile IBD.

Plasma proteomic signature of chronic psychosocial stress in mice

Chronic stress significantly impacts both physical and mental wellbeing, increasing risk of cardiovascular disease, immune dysregulation, and psychiatric conditions such as depression and anxiety disorders. The plasma proteome is a valuable source of biomarkers of health and disease, but the limited number of studies exploring the potential of the plasma proteome as a biomarker for stress-related disorders underscores the importance of further investigation of the effects of chronic stress on the plasma proteome. The aim of this study was to examine the effect of a 5-week chronic psychosocial stress paradigm on the plasma proteome in mice and to determine if any affected proteins correlated with stress-induced changes in behaviour and physiology, and thus might represent biomarkers of negative impacts of chronic stress. Using LC-MS/MS proteomic analysis, 38 proteins in the mouse plasma proteome were identified to be affected by chronic psychosocial stress. Functional analysis revealed that these proteins clustered into biological functions including inflammatory response, regulation of the immune response, complement and coagulation cascades, lipid metabolic process, and high-density lipoprotein particles. Correlation analyses of the identified proteins with stress-induced behavioral or physiological changes stress revealed significant correlations between stress-induced anxiety-like behaviour and Phosphatidylinositol-glycan-specific phospholipase D, Complement C2, Epidermal growth factor receptor, Prosaposin, Actin-related protein 2/3 complex subunit 1B, Maltase-glucoamylase, Mannosyl-oligosaccharide 1,2-alpha-mannosidase IA and Fibrinogen-like protein 1. Chronic psychosocial stress blunted acute stress-induced corticosterone release, and this correlated with abundance of Pyrethroid hydrolase Ces2a; N-fatty-acyl-amino acid synthase/hydrolase Pm20d1, Mannosyl-oligosaccharide 1,2-alpha-mannosidase IA, Alpha-2-macroglobulin-P and L-selectin. Finally, stress-induced reductions in both brown and epididymal fat correlated with Phosphatidylinositol-glycan-specific phospholipase D, Complement C2, Epidermal growth factor receptor, Kininogen-1, Apolipoprotein M, Angiopoietin-related protein 3, Proprotein convertase subtilisin/kexin type 9, and Lipopolysaccharide-binding protein. These findings demonstrate that chronic psychosocial stress induces alterations in plasma proteins implicated in key biological processes and pathways related to stress response, immune function, and lipid metabolic regulation. Further investigation into these proteins may provide new avenues for identification of biomarkers or mediators of stress-induced pathology.

Deep phenotyping reveals CRH and FKBP51-dependent behavioral profiles following chronic social stress exposure in male mice

The co-chaperone FKBP51, encoded by FKBP5 gene, is recognized as a psychiatric risk factor for anxiety and depressive disorders due to its crucial role in the stress response. Another key modulator in stress response regulation is the corticotropin releasing hormone (CRH), which is co-expressed with FKBP51 in many stress-relevant brain-regions and cell-types. Together, they intricately influence the balance of the hypothalamic-pituitary-adrenal (HPA) axis, one of the primary stress response systems. Previous research underscores the potential moderating effects these genes have on the regulation of the stressful life events towards the vulnerability of major depressive disorder (MDD). However, the specific function of FKBP51 in CRH-expressing neurons remains largely unexplored. Here, through deep behavioral phenotyping, we reveal heightened stress effects in mice lacking FKBP51 in CRH co-expressing neurons (CRHFKBP5-/-), particularly evident in social contexts. Our findings highlight the importance of considering cell-type specificity and context in comprehending stress responses and advocate for the utilization of machine-learning-driven phenotyping of mouse models. By elucidating these intricacies, we lay down the groundwork for personalized interventions aimed at enhancing stress resilience and individual well-being.

Interleukin-11Rα2 in the hypothalamic arcuate nucleus affects depression-related behaviors and the AKT-BDNF pathway

Depression is a widespread emotional disorder with complex pathogenesis. An essential function of the hypothalamus is to regulate emotional disorders. However, further investigation is required to identify the pathogenic genes and molecular mechanisms that contribute to the onset of depression within the hypothalamus. Through RNA-sequencing analysis, this study identified the upregulated expression of interleukin-11 receptor alpha 2 (IL-11Rα2) in the hypothalamus of mice with chronic unpredictable stress (CUS)-induced depression. This substantial increase in IL-11Rα2, not IL-11Rα1 expression levels in the hypothalamus under the influence of CUS was found to be associated with depression-related behaviors. We further showed that IL-11Rα2 is expressed in the arcuate nucleus (ARC) proopiomelanocortin (POMC) neurons of the hypothalamus. Male and female mice exhibited behaviors association with depression, when IL-11Rα2 or its ligand IL-11 was overexpressed in the ARC POMC neurons through the action of an adeno-associated virus. In addition, reductions in the expression levels of proteins involved in the protein kinase B signaling pathways and brain-derived neurotrophic factor were observed upon overexpression of IL-11Rα2 in the hypothalamic ARC. This study emphasizes the importance of IL-11Rα2 in the hypothalamus ARC in the development of depression, and presents it as a potential novel target for depression treatment.

Evaluation of a user-friendly CSDS cage apparatus for studying depressive-like behaviors in rodents

BACKGROUND

Previously, a chronic social defeat stress (CSDS) model has been widely-adopted for assessing depressive-like behaviors in animals. However, there is still room for improvement in the CSDS model to safeguard study accuracy and the welfare of lab rodents. Our study team developed a novel, standardized apparatus to induce CSDS in rodents and assessed the model's practical adaptability.

METHODS

An innovative CSDS cage apparatus and water bottle was designed. To evaluate the effectiveness of the newly developed tools, a variety of animal models, including the tail suspension test (TST), sucrose preference test, forced swimming test (FST), novelty-suppressed feeding test, female urine sniffing test, and open field test (OFT), were adopted to assess depressive-like behaviors in mice. Fluoxetine treatment was also administered to observe the reversal effect, as part of the validation.

RESULTS

The CSDS cage apparatus resulted in the manifestation of depressive-like behaviors in the model mice. Significant reductions in sucrose preference and urine sniffing time were observed, while the OFT revealed decreased central zone total distance, residence time, and frequency of entry. Moreover, increased immobility was found in the FST and TST. Fluoxetine treatment was found to successfully reverse the modeling effect.

CONCLUSION

The CSDS cage apparatus was validated for enhanced usability and addressed the previous challenges of water bottle leakage and lab rodent welfare issues. The consistent results from multiple behavioral tests also supported real-world application of the apparatus, offering researchers a promising alternative to conventional rodent cages.

IL-17A Mediates Depressive-Like Symptoms by Inducing Microglia Activation in Psoriasiform Dermatitis Mice

BACKGROUND

Psoriasis is recognized as a systemic disease for its accompanying comorbidities, among which psychological disorders present a high incidence rate and affect patients' life quality. Interleukin (IL)-17A is the central pathological factor in the pathogenesis and development of psoriasis.

OBJECTIVE

To clarify if psoriasis-induced systemic IL-17A increase can mediate the neuronal inflammation and result in depressive-like symptoms.

METHODS

Psoriasiform dermatitis model was established by imiquimod (IMQ) application on male BALB/c mice and IL-17A intervention was performed by lateral ventricular catheterization. Skin structural, histopathological characteristics, and behavioral tests were assessed. Serum IL-17A levels were detected by Enzyme-linked immunosorbent assay. mRNA expression of pro-inflammatory factors IL-1β, IL-6, and tumor necrosis factor-α (TNF-α) as well as anti-inflammatory factors IL-4 and IL-10 in the hippocampus and cortex were measured by RT-qPCR. The number of microglia and hippocampal neurons was quantified by immunofluorescent assay.

RESULTS

IMQ treatment resulted in significant skin structural and histopathological characters of psoriasiform dermatitis with elevated serum IL-17A levels, obvious depressive-like behaviors, microglia activation with increased IL-1β, IL-6, and TNF-α expression levels in the hippocampus and cortex, and notable inhibition of hippocampal neurogenesis. While, IL-17A neutralization by intracerebroventricular injection of anti-IL-17A antibody can remarkably inhibit microglia activation and decrease the abnormally increased expression levels of IL-1β, IL-6, and TNF-α in the hippocampus and cortex of psoriasiform dermatitis mice, promote hippocampal neurogenesis, thus alleviate the depressive-like behaviors.

CONCLUSION

In the pathological condition of psoriasis, systemic IL-17A elevation can trigger microglia activation, provoke pro-inflammation mediators to release, evoke neuroinflammation, subsequently inhibit hippocampal neurogenesis, and result in depression. IL-17A, as an important pathogenic factor in psoriasis, contributes to its critical role in mediating systemic inflammation and depression comorbidity.

Sex-specific GABAergic microcircuits that switch vulnerability into resilience to stress and reverse the effects of chronic stress exposure

Clinical and preclinical studies have identified somatostatin (SST)-positive interneurons as critical elements that regulate the vulnerability to stress-related psychiatric disorders. Conversely, disinhibition of SST neurons in mice results in resilience to the behavioral effects of chronic stress. Here, we established a low-dose chronic chemogenetic protocol to map these changes in positively and negatively motivated behaviors to specific brain regions. AAV-hM3Dq-mediated chronic activation of SST neurons in the prelimbic cortex (PLC) had antidepressant drug-like effects on anxiety- and anhedonia-like motivated behaviors in male but not female mice. Analogous manipulation of the ventral hippocampus (vHPC) had such effects in female but not male mice. Moreover, the activation of SST neurons in the PLC of male mice and the vHPC of female mice resulted in stress resilience. Activation of SST neurons in the PLC reversed prior chronic stress-induced defects in motivated behavior in males but was ineffective in females. Conversely, activation of SST neurons in the vHPC reversed chronic stress-induced behavioral alterations in females but not males. Quantitation of c-Fos+ and FosB+ neurons in chronic stress-exposed mice revealed that chronic activation of SST neurons leads to a paradoxical increase in pyramidal cell activity. Collectively, these data demonstrate that GABAergic microcircuits driven by dendrite targeting interneurons enable sex- and brain-region-specific neural plasticity that promotes stress resilience and reverses stress-induced anxiety- and anhedonia-like motivated behavior. The data provide a rationale for the lack of antidepressant efficacy of benzodiazepines and superior efficacy of dendrite-targeting, low-potency GABAA receptor agonists, independent of sex and despite striking sex differences in the relevant brain substrates.

Hippocampal contextualization of social rewards in mice

Acquiring and exploiting memories of rewarding experiences is critical for survival. The spatial environment in which a rewarding stimulus is encountered regulates memory retrieval. The ventral hippocampus (vH) has been implicated in contextual memories involving rewarding stimuli such as food, social cues or drugs. Yet, the neuronal representations and circuits underlying contextual memories of socially rewarding stimuli are poorly understood. Here, using in vivo electrophysiological recordings, in vivo one-photon calcium imaging, and optogenetics during a social reward contextual conditioning paradigm in male mice, we show that vH neurons discriminate between contexts with neutral or acquired social reward value. The formation of context-discriminating vH neurons following learning was contingent upon the presence of unconditioned stimuli. Moreover, vH neurons showed distinct contextual representations during the retrieval of social reward compared to fear contextual memories. Finally, optogenetic inhibition of locus coeruleus (LC) projections in the vH selectively disrupted social reward contextual memory by impairing vH contextual representations. Collectively, our findings reveal that the vH integrates contextual and social reward information, with memory encoding of these representations supported by input from the LC.

Altered reward processing following sucrose bingeing in male and female mice

Objectives: Binge eating disorder (BED) is the most prevalent eating disorder associated with multiple adverse health effects, especially mental health issues, including substance use disorders and mood and anxiety disorders. Given these high comorbidities, the objective of our study was to examine whether bingeing behavior would lead to altered perception of reinforcing properties of EtOH and changes in well-being. Methods: We used a sucrose bingeing model based on an intermittent access paradigm with a two-bottle choice, without fasting, in male and female mice. We examined the effect of 2-week sucrose paradigm on ethanol-reinforcing properties using a conditioned place preference test (CPP). Well-being, anxiety- and depressive-like behavioral tests were performed to assess emotional state following 2 and 8-week sucrose bingeing paradigm. Results: Mice with intermittent access to sucrose developed a binge-like behavior assessed by higher sucrose intake and escalation rate during the 1st hour of access, in comparison with mice with a continuous sucrose access. We show for the first time that sucrose bingeing in mice modifies positive reinforcing effect of EtOH in a CPP paradigm without marked alteration of emotional state. Interestingly, prolonging sucrose access for 8 weeks revealed an exacerbated bingeing behavior in female mice, and some signs of emotional state alterations in female with continuous access. Discussion: In sum, our findings broaden the understanding of behavioral alterations associated with bingeing, highlighting the need to investigate addictive-like properties of palatable food both in male and female mice.

Unmoving and uninflamed: Characterizing neuroinflammatory dysfunction in the Wistar-Kyoto rat model of depression

Reductionistic research on depressive disorders has been hampered by the limitations of animal models. Recently, it has been hypothesized that neuroinflammation is a key player in depressive disorders. The Wistar-Kyoto (WKY) rat is an often-used animal model of depression, but no information so far exists on its neuroinflammatory profile. As such, we compared male young adult WKY rats to Wistar (WS) controls, with regard to both behavioral performance and brain levels of key neuroinflammatory markers. We first assessed anxiety- and depression-like behaviors in a battery consisting of the Elevated Plus Maze (EPM), the Novelty Suppressed Feeding (NSFT), Open Field (OFT), Social Interaction (SIT), Forced Swim (FST), Sucrose Preference (SPT), and Splash tests (ST). We found that WKY rats displayed increased NSFT feeding latency, decreased OFT center zone permanence, decreased EPM open arm permanence, decreased SIT interaction time, and increased immobility in the FST. However, WKY rats also evidenced marked hypolocomotion, which is likely to confound performance in such tests. Interestingly, WKY rats performed similarly, or even above, to WS levels in the SPT and ST, in which altered locomotion is not a significant confound. In a separate cohort, we assessed prefrontal cortex (PFC), hippocampus and amygdala levels of markers of astrocytic (GFAP, S100A10) and microglial (Iba1, CD86, Ym1) activation status, as well as of three key proinflammatory cytokines (IL-1β, IL-6, TNF-α). There were no significant differences between strains in any of these markers, in any of the regions assessed. Overall, results highlight that behavioral data obtained with WKY rats as a model of depression must be carefully interpreted, considering the marked locomotor activity deficits displayed. Furthermore, our data suggest that, despite WKY rats replicating many depression-associated neurobiological alterations, as shown by others, this is not the case for neuroinflammation-related alterations, thus representing a novel limitation of this model.

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.

Antidepressant-like effects of tomatidine and tomatine, steroidal alkaloids from unripe tomatoes, via activation of mTORC1 in the medial prefrontal cortex in lipopolysaccharide-induced depression model mice

ABSTRACTKetamine, an N-methyl-D-aspartate receptor antagonist, produces rapid antidepressant effects in patients with treatment-resistant depression. However, owing to the undesirable adverse effects of ketamine, there is an urgent need for developing safer and more effective prophylactic and therapeutic interventions for depression. Preclinical studies have demonstrated that activation of the mechanistic target of rapamycin complex 1 (mTORC1) in the medial prefrontal cortex (mPFC) mediates the rapid antidepressant effects of ketamine. The steroidal alkaloid tomatidine and its glycoside α-tomatine (tomatine) can activate mTORC1 signaling in peripheral tissues/cells. We examined whether tomatidine and tomatine exerted prophylactic and therapeutic antidepressant-like actions via mPFC mTORC1 activation using a mouse model of lipopolysaccharide (LPS)-induced depression. Male mice were intraperitoneally (i.p.) administered tomatidine/tomatine before and after the LPS challenge to test their prophylactic and therapeutic effects, respectively. LPS-induced depression-like behaviors in the tail suspension test (TST) and forced swim test (FST) were significantly reversed by prophylactic and therapeutic tomatidine/tomatine administration. LPS-induced anhedonia in the female urine sniffing test was reversed by prophylactic, but not therapeutic, injection of tomatidine, and by prophylactic and therapeutic administration of tomatine. Intra-mPFC infusion of rapamycin, an mTORC1 inhibitor, blocked the prophylactic and therapeutic antidepressant-like effects of tomatidine/tomatine in TST and FST. Moreover, both tomatidine and tomatine produced antidepressant-like effects in ovariectomized female mice, a model of menopause-associated depression. These results indicate that tomatidine and tomatine exert prophylactic and therapeutic antidepressant-like effects via mTORC1 activation in the mPFC and suggest these compounds as promising candidates for novel prophylactic and therapeutic agents for depression.

Sex-specific GABAergic microcircuits that switch vulnerability into resilience to stress and reverse the effects of chronic stress exposure

Clinical and preclinical studies have identified somatostatin (SST)-positive interneurons as key elements that regulate the vulnerability to stress-related psychiatric disorders. Conversely, disinhibition of SST neurons in mice results in resilience to the behavioral effects of chronic stress. Here we established a low-dose chronic chemogenetic protocol to map these changes in positively and negatively motivated behaviors to specific brain regions. AAV-hM3Dq mediated chronic activation of SST neurons in the prelimbic cortex (PLC) had antidepressant drug-like effects on anxiety- and anhedonia-related motivated behaviors in male but not female mice. Analogous manipulation of the ventral hippocampus (vHPC) had such effects in female but not male mice. Moreover, activation of SST neurons in the PLC of male and the vHPC of female mice resulted in stress resilience. Activation of SST neurons in the PLC reversed prior chronic stress-induced defects in motivated behavior in males but was ineffective in females. Conversely, activation of SST neurons in the vHPC reversed chronic stress-induced behavioral alterations in females but not males. Quantitation of c-Fos+ and FosB+ neurons in chronic stress-exposed mice revealed that chronic activation of SST neurons leads to a paradoxical increase in pyramidal cell activity. Collectively, these data demonstrate that GABAergic microcircuits driven by dendrite targeting interneurons enable sex- and brain-region-specific neural plasticity that promotes stress resilience and reverses stress-induced anxiety- and anhedonia-like motivated behavior. Our studies provide a mechanistic rationale for antidepressant efficacy of dendrite-targeting, low-potency GABAA receptor agonists, independent of sex and despite striking sex differences in the relevant brain substrates.

Lysophosphatidic Acid (LPA) and Its Receptors in Mood Regulation: A Systematic Review of the Molecular Mechanisms and Therapeutic Potential

Mood disorders affect over 300 million individuals worldwide, often characterized by their chronic and refractory nature, posing significant threats to patient life. There has been a notable increase in mood disorders among American adolescents and young adults, with a rising number of suicide attempts and fatalities, highlighting a growing association between mood disorders and suicidal outcomes. Dysregulation within the neuroimmune-endocrine system is now recognized as one of the fundamental biological mechanisms underlying mood and mood disorders. Lysophosphatidic acid (LPA), a novel mediator of mood behavior, induces anxiety-like and depression-like phenotypes through its receptors LPA1 and LPA5, regulating synaptic neurotransmission and plasticity. Consequently, LPA has garnered substantial interest in the study of mood regulation. This study aimed to elucidate the molecular mechanisms of lysophosphatidic acid and its receptors, along with LPA receptor ligands, in mood regulation and to explore their potential therapeutic efficacy in treating mood disorders. A comprehensive literature search was conducted using the PubMed and Web of Science databases, identifying 208 articles through keyword searches up to June 2024. After excluding duplicates, irrelevant publications, and those restricted by open access limitations, 21 scientific papers were included in this review. The findings indicate that LPA/LPA receptor modulation could be beneficial in treating mood disorders, suggesting that pharmacological agents or gintonin, an extract from ginseng, may serve as effective therapeutic strategies. This study opens new avenues for future research into how lysophosphatidic acid and its receptors, as well as lysophosphatidic acid receptor ligands, influence emotional behavior in animals and humans.

Repetitive transcranial magnetic stimulation (rTMS) for depressive-like symptoms in rodent animal models

Repetitive transcranial magnetic stimulation (rTMS) emerged as a non-invasive brain stimulation technique in the treatment of psychiatric disorders. Both preclinical and clinical studies as well as systematic reviews provide a heterogeneous picture, particularly concerning the stimulation protocols used in rTMS. Here, we present a review of rTMS effects in rodent models of depressive-like symptoms with the aim to identify the most relevant factors that lead to an increased therapeutic success. The influence of different factors, such as the stimulation parameters (stimulus frequency and intensity, duration of stimulation, shape and positioning of the coil), symptom severity and individual characteristics (age, species and genetic background of the rodents), on the therapeutic success are discussed. Accumulating evidence indicates that rTMS ameliorates a multitude of depressive-like symptoms in rodent models, most effectively at high stimulation frequencies (≥5 Hz) especially in adult rodents with a pronounced pathological phenotype. The therapeutic success of rTMS might be increased in the future by considering these factors and using more standardized stimulation protocols.

Paraventricular Thalamus Neuronal Ensembles Encode Early-life Adversity and Mediate the Consequent Sex-dependent Disruptions of Adult Reward Behaviors

Early-life adversity increases risk for mental illnesses including depression and substance use disorders, disorders characterized by dysregulated reward behaviors. However, the mechanisms by which transient ELA enduringly impacts reward circuitries are not well understood. In mice, ELA leads to anhedonia-like behaviors in males and augmented motivation for palatable food and sex-reward cues in females. Here, the use of genetic tagging demonstrated robust, preferential, and sex-specific activation of the paraventricular nucleus of the thalamus (PVT) during ELA and a potentiated reactivation of these PVT neurons during a reward task in adult ELA mice. Chemogenetic manipulation of specific ensembles of PVT neurons engaged during ELA identified a role for the posterior PVT in ELA-induced aberrantly augmented reward behaviors in females. In contrast, anterior PVT neurons activated during ELA were required for the anhedonia-like behaviors in males. Thus, the PVT encodes adverse experiences early-in life, prior to the emergence of the hippocampal memory system, and contributes critically to the lasting, sex-modulated impacts of ELA on reward behaviors.

Selective disruption of mTORC1 and mTORC2 in VTA astrocytes induces depression and anxiety-like behaviors in mice

Dysfunction of the mechanistic target of rapamycin (mTOR) signaling pathway is implicated in neuropsychiatric disorders including depression and anxiety. Most studies have been focusing on neurons, and the function of mTOR signaling pathway in astrocytes is less investigated. mTOR forms two distinct complexes, mTORC1 and mTORC2, with key scaffolding protein Raptor and Rictor, respectively. The ventral tegmental area (VTA), a vital component of the brain reward system, is enrolled in regulating both depression and anxiety. In the present study, we aimed to examine the regulation effect of VTA astrocytic mTOR signaling pathway on depression and anxiety. We specifically deleted Raptor or Rictor in VTA astrocytes in mice and performed a series of behavioral tests for depression and anxiety. Deletion of Raptor and Rictor both decreased the immobility time in the tail suspension test and the latency to eat in the novelty suppressed feeding test, and increased the horizontal activity and the movement time in locomotor activity. Deletion of Rictor decreased the number of total arm entries in the elevated plus-maze test and the vertical activity in locomotor activity. These data suggest that VTA astrocytic mTORC1 plays a role in regulating depression-related behaviors and mTORC2 is involved in both depression and anxiety-related behaviors. Our results indicate that VTA astrocytic mTOR signaling pathway might be new targets for the treatment of psychiatric disorders.

Rodent tests of depression and anxiety: Construct validity and translational relevance

Behavioral testing constitutes the primary method to measure the emotional states of nonhuman animals in preclinical research. Emerging as the characteristic tool of the behaviorist school of psychology, behavioral testing of animals, particularly rodents, is employed to understand the complex cognitive and affective symptoms of neuropsychiatric disorders. Following the symptom-based diagnosis model of the DSM, rodent models and tests of depression and anxiety focus on behavioral patterns that resemble the superficial symptoms of these disorders. While these practices provided researchers with a platform to screen novel antidepressant and anxiolytic drug candidates, their construct validity-involving relevant underlying mechanisms-has been questioned. In this review, we present the laboratory procedures used to assess depressive- and anxiety-like behaviors in rats and mice. These include constructs that rely on stress-triggered responses, such as behavioral despair, and those that emerge with nonaversive training, such as cognitive bias. We describe the specific behavioral tests that are used to assess these constructs and discuss the criticisms on their theoretical background. We review specific concerns about the construct validity and translational relevance of individual behavioral tests, outline the limitations of the traditional, symptom-based interpretation, and introduce novel, ethologically relevant frameworks that emphasize simple behavioral patterns. Finally, we explore behavioral monitoring and morphological analysis methods that can be integrated into behavioral testing and discuss how they can enhance the construct validity of these tests.

Role and mechanism of PVN-sympathetic-adipose circuit in depression and insulin resistance induced by chronic stress

Chronic stress induces depression and insulin resistance, between which there is a bidirectional relationship. However, the mechanisms underlying this comorbidity remain unclear. White adipose tissue (WAT), innervated by sympathetic nerves, serves as a central node in the interorgan crosstalk through adipokines. Abnormal secretion of adipokines is involved in mood disorders and metabolic morbidities. We describe here a brain-sympathetic nerve-adipose circuit originating in the hypothalamic paraventricular nucleus (PVN) with a role in depression and insulin resistance induced by chronic stress. PVN neurons are labelled after inoculation of pseudorabies virus (PRV) into WAT and are activated under restraint stress. Chemogenetic manipulations suggest a role for the PVN in depression and insulin resistance. Chronic stress increases the sympathetic innervation of WAT and downregulates several antidepressant and insulin-sensitizing adipokines, including leptin, adiponectin, Angptl4 and Sfrp5. Chronic activation of the PVN has similar effects. β-adrenergic receptors translate sympathetic tone into an adipose response, inducing downregulation of those adipokines and depressive-like behaviours and insulin resistance. We finally show that AP-1 has a role in the regulation of adipokine expression under chronic stress.

Medial prefrontal cortical PPM1F alters depression-related behaviors by modifying p300 activity via the AMPK signaling pathway

AIMS

Protein phosphatase Mg2+/Mn2+-dependent 1F (PPM1F) is a serine/threonine phosphatase, and its dysfunction in depression in the hippocampal dentate gyrus has been previously identified. Nevertheless, its role in depression of another critical emotion-controlling brain region, the medial prefrontal cortex (mPFC), remains unclear. We explored the functional relevance of PPM1F in the pathogenesis of depression.

METHODS

The gene expression levels and colocalization of PPM1F in the mPFC of depressed mice were measured by real-time PCR, western blot and immunohistochemistry. An adeno-associated virus strategy was applied to determine the impact of knockdown or overexpression of PPM1F in the excitatory neurons on depression-related behaviors under basal and stress conditions in both male and female mice. The neuronal excitability, expression of p300 and AMPK phosphorylation levels in the mPFC after knockdown of PPM1F were measured by electrophysiological recordings, real-time PCR and western blot. The depression-related behavior induced by PPM1F knockdown after AMPKα2 knockout or the antidepressant activity of PPM1F overexpression after inhibiting acetylation activity of p300 was evaluated.

RESULTS

Our results indicate that the expression levels of PPM1F were largely decreased in the mPFC of mice exposed to chronic unpredictable stress (CUS). Behavioral alterations relevant to depression emerged with short hairpin RNA (shRNA)-mediated genetic knockdown of PPM1F in the mPFC, while overexpression of PPM1F produced antidepressant activity and ameliorated behavioral responses to stress in CUS-exposed mice. Molecularly, PPM1F knockdown decreased the excitability of pyramidal neurons in the mPFC, and restoring this low excitability decreased the depression-related behaviors induced by PPM1F knockdown. PPM1F knockdown reduced the expression of CREB-binding protein (CBP)/E1A-associated protein (p300), a histone acetyltransferase (HAT), and induced hyperphosphorylation of AMPK, resulting in microglial activation and upregulation of proinflammatory cytokines. Conditional knockout of AMPK revealed an antidepressant phenotype, which can also block depression-related behaviors induced by PPM1F knockdown. Furthermore, inhibiting the acetylase activity of p300 abolished the beneficial effects of PPM1F elevation on CUS-induced depressive behaviors.

CONCLUSION

Our findings demonstrate that PPM1F in the mPFC modulates depression-related behavioral responses by regulating the function of p300 via the AMPK signaling pathway.

Impaired Ventrolateral Periaqueductal Gray-Ventral Tegmental area Pathway Contributes to Chronic Pain-Induced Depression-Like Behavior in Mice

Chronic pain conditions within clinical populations are correlated with a high incidence of depression, and researchers have reported their high rate of comorbidity. Clinically, chronic pain worsens the prevalence of depression, and depression increases the risk of chronic pain. Individuals suffering from chronic pain and depression respond poorly to available medications, and the mechanisms underlying the comorbidity of chronic pain and depression remain unknown. We used spinal nerve ligation (SNL) in a mouse model to induce comorbid pain and depression. We combined behavioral tests, electrophysiological recordings, pharmacological manipulation, and chemogenetic approaches to investigate the neurocircuitry mechanisms of comorbid pain and depression. SNL elicited tactile hypersensitivity and depression-like behavior, accompanied by increased and decreased glutamatergic transmission in dorsal horn neurons and midbrain ventrolateral periaqueductal gray (vlPAG) neurons, respectively. Intrathecal injection of lidocaine, a sodium channel blocker, and gabapentin ameliorated SNL-induced tactile hypersensitivity and neuroplastic changes in the dorsal horn but not depression-like behavior and neuroplastic alterations in the vlPAG. Pharmacological lesion of vlPAG glutamatergic neurons induced tactile hypersensitivity and depression-like behavior. Chemogenetic activation of the vlPAG-rostral ventromedial medulla (RVM) pathway ameliorated SNL-induced tactile hypersensitivity but not SNL-elicited depression-like behavior. However, chemogenetic activation of the vlPAG-ventral tegmental area (VTA) pathway alleviated SNL-produced depression-like behavior but not SNL-induced tactile hypersensitivity. Our study demonstrated that the underlying mechanisms of comorbidity in which the vlPAG acts as a gating hub for transferring pain to depression. Tactile hypersensitivity could be attributed to dysfunction of the vlPAG-RVM pathway, while impairment of the vlPAG-VTA pathway contributed to depression-like behavior.

The synergistic mechanism of action of Dajianzhong decoction in conjunction with ketamine in the treatment of depression

Depression is a multifactorial syndrome with a variety of underlying pathological mechanisms. While ketamine, an N-methyl-D-aspartate receptor (NMDAR) antagonist, exhibits a rapid antidepressant action in the central never system (CNS), the potential addiction and psychotomimetic adverse effects of ketamine limit its chronic use in clinical practice. Therefore, it is necessary to discover an additional agent that shows a synergistic antidepressant activity with ketamine to sustain its therapeutic action so as to reduce its use frequency in depression treatment. The present study indicated that Dajianzhong decoction (DJZT), an empirical herbal formula used for the clinical treatment of several inflammation-related intestinal disorders, sustains behavioral and synaptic action of ketamine in depressive mouse models. Additionally, ketamine was also demonstrated to exert a synergistic action with DJZT to alleviate the chronic unpredictable mild stress (CUMS)-induced abnormalities in gut barrier proteins and colonic histology, and subsequently to normalize the diversity and composition of gut microbiota. Furthermore, DJZT was shown to possess an anti-inflammatory activity to prevent activation of NF-κB from releasing proinflammatory cytokines, specifically through inhibiting Th17 cells/IL-17A pathway. Our results uncovered the mechanism of action of DJZT in conjunction with ketamine in depression treatment by which these agents target different pathological factors across biological systems and exert a synergistic activity through a bidirectional communication in the gut-brain axis, and also provided new insights into the systematic treatment of depression.

Microglial NLRP3 inflammasome activation mediates diabetes-induced depression-like behavior via triggering neuroinflammation

BACKGROUND

Abundant evidence suggests that the prevalence and risk of depression in people with diabetes is high. However, the pathogenesis of diabetes-related depression remains unclear. Since neuroinflammation is associated with the pathophysiology of diabetic complications and depression, this study aims to elucidate the neuroimmune mechanism of diabetes-related depression.

METHODS

Male C57BL/6 mice were injected with streptozotocin to establish a diabetes model. After screening, diabetic mice were treated with the NLRP3 inhibitor MCC950. Then, metabolic indicators and depression-like behaviors were evaluated in these mice, as well as their central and peripheral inflammation. To explore the mechanism of high glucose-induced microglial NLRP3 inflammasome activation, we performed in vitro studies focusing on its canonical upstream signal I (TLR4/MyD88/NF-κB) and signal II (ROS/PKR/P2X7R/TXNIP).

RESULTS

Diabetic mice exhibited depression-like behaviors and activation of NLRP3 inflammasome in hippocampus. In vitro high-glucose (50 mM) environment primed microglial NLRP3 inflammasome by promoting NF-κB phosphorylation in a TLR4/MyD88-independent manner. Subsequently, high glucose activated the NLRP3 inflammasome via enhancing intracellular ROS accumulation, upregulating P2X7R, as well as promoting PKR phosphorylation and TXNIP expression, thereby facilitating the production and secretion of IL-1β. Inhibition of NLRP3 with MCC950 significantly restored hyperglycemia-induced depression-like behavior and reversed the increase in IL-1β levels in the hippocampus and serum.

CONCLUSION

The activation of NLRP3 inflammasome, probably mainly in hippocampal microglia, mediates the development of depression-like behaviors in STZ-induced diabetic mice. Targeting the microglial inflammasome is a feasible strategy for the treatment of diabetes-related depression.

Changes in social, sexual, and hedonic behaviors in rats in response to stress and restoration by a negative allosteric modulator of α5-subunit containing GABA receptor

Major depressive disorder (MDD) is a debilitating and costly human condition. Treatment for MDD relies heavily on the use of antidepressants that are slow to produce mood-related changes and are not effective in all patients, such as selective serotonin reuptake inhibitors (SSRIs). Several novel compounds, including negative allosteric modulators of GABA-A receptors containing the α5-subunit (GABA-NAMs), are under investigation for potential fast acting therapeutic use in MDD. Preclinical evidence that these compounds produce a rapid antidepressant-like response comes primarily from simple tests of escape behavior and preference for rewarding stimuli after chronic stress. To increase the ethological relevance of these compounds, we tested the hypothesis that the GABA-NAM, L-655,708, would produce an antidepressant-like response in more complex stress-sensitive social and sex behaviors, which are of relevance to the symptoms of human depression. In male rats subjected to chronic restraint stress, injection of L-655,708 increased reward in a sexual conditioned place preference task, increased male sexual activity with a receptive female, and re-established male social dominance hierarchies within 24 h. We also report increased sucrose preference in the social defeat stress (SDS) model of depression following GABA-NAM administration, demonstrating that its antidepressant-like actions are independent of the type of chronic stress administered. This work extends the impact of GABA-NAMs beyond traditional tests of anhedonia and further supports the development of alpha5 subunit-selective GABA-NAMs as a potential fast-acting therapeutic approach for treating human MDD.

Maternal neglect alters reward-anticipatory behavior, social status stability, and reward circuit activation in adult male rats

Introduction

Adverse early life experiences affect neuronal growth and maturation of reward circuits that modify behavior under reward predicting conditions. Previous studies demonstrate that rats undergoing denial of expected reward in the form of maternal contact (DER-animal model of maternal neglect) during early post-natal life developed anhedonia, aggressive play-fight behaviors and aberrant prefrontal cortex structure and neurochemistry. Although many studies revealed social deficiency following early-life stress most reports focus on individual animal tasks. Thus, attention needs to be given on the social effects during group tasks in animals afflicted by early life adversity.

Methods

To investigate the potential impact of the DER experience on the manifestation of behavioral responses induced by natural rewards, we evaluated: 1) naïve adult male sexual preference and performance, and 2) anticipatory behavior during a group 2-phase food anticipation learning task composed of a context-dependent and a cue-dependent learning period.

Results

DER rats efficiently spent time in the vicinity of and initiated sexual intercourse with receptive females suggesting an intact sexual reward motivation and consummation. Interestingly, during the context-dependent phase of food anticipation training DER rats displayed a modified exploratory activity and lower overall reward-context association. Moreover, during the cue-dependent phase DER rats displayed a mild deficit in context-reward association while increased cue-dependent locomotion. Additionally, DER rats displayed unstable food access priority following food presentation. These abnormal behaviours were accompanied by overactivation of the ventral prefrontal cortex and nucleus accumbens, as assessed by pCREB levels.

Conclusions/discussion

Collectively, these data show that the neonatal DER experience resulted in adulthood in altered activation of the reward circuitry, interfered with the normal formation of context-reward associations, and disrupted normal reward access hierarchy formation. These findings provide additional evidence to the deleterious effects of early life adversity on reward system, social hierarchy formation, and brain function.

Role of Hippocampal miR-132-3p in Modifying the Function of Protein Phosphatase Mg2+/Mn2+-dependent 1 F in Depression

Depression is a common, severe, and debilitating psychiatric disorder of unclear etiology. Our previous study has shown that protein phosphatase Mg2+/Mn2+-dependent 1F (PPM1F) in the hippocampal dentate gyrus (DG) displays significant regulatory effects in depression-related behaviors. miR-132-3p plays a potential role in the etiology of depression. This study explored the effect of miR-132-3p on the onset of depression and the possible underlying mechanism for modulating PPM1F expression during the pathology of depression. We found that miR-132-3p levels in the hippocampus of depressed mice subjected to chronic unpredictable stress (CUS) were dramatically reduced, which were correlated with depression-related behaviors. Knockdown of miR-132-3p in hippocampal DG resulted in depression-related phenotypes and increased susceptibility to stress. miR-132-3p overexpression in hippocampal DG alleviated CUS-induced depression-related performance. We then screened out the potential target genes of miR-132-3p, and we found that the expression profiles of sterol regulatory element-binding transcription factor 1 (Srebf1) and forkhead box protein O3a (FOXO3a) were positively correlated with PPM1F under the condition of miR-132-3p knockdown. Finally, as anticipated, we revealed that the activities of Ca2+/calmodulin-dependent protein kinase II (CAMKII) and adenosine 5'-monophosphate (AMP)-activated protein kinase (AMPK) were reduced, which underlies the target signaling pathway of PPM1F. In conclusion, our study suggests that miR-132-3p was designed to regulate depression-related behaviors by indirectly regulating PPM1F and targeting Srebf1 and FOXO3a, which have been linked to the pathogenesis and treatment of depression.

On the effect of social cue valence in contextual memory persistence

Social cues are valuable sensorial stimuli to the acquisition and retrieval of contextual memories. Here, we asked whether the valence of social cues would impact the formation of contextual memories. Adult male C57/BL6 mice were exposed to either conditioned place preference (CPP) or avoidance (CPA). As positive stimuli we used social interaction with a female (IF), while interaction with a male CD1 mice (IM) was used as negative stimulus. Contextual memory was tested 24 h and 7 days after conditioning. Aggressive behavior of CD1, as well as interaction with the female were quantified along the conditioning sessions. IM, but not IF, was salient enough to induce contextual memory estimated by the difference between the time in the conditioned context during test and habituation. Next, we chose two odors with innate behavioral responses and opposite valence to narrow down the sociability to one of its sensorial sources of information - the olfaction. We used urine from females in proestrus (U) and 2,4,5-trimethyl thiazoline (TMT), a predator odor. TMT decreased and U increased the time in the conditioned context during the test performed 24 h and 7 days after conditioning. Taken together, our results suggest that contextual memories conditioned to social encounters are difficult to stablish in mice, specially the one with positive valence. On the other hand, using odors with ecological relevance is a promising strategy to study long-term contextual memories with opposite valences. Ultimately, the behavioral protocol proposed here offers the advantage of studying contextual memories with opposite valences using unconditioned stimulus from the same sensorial category such as olfaction.

Increased intrinsic and synaptic excitability of hypothalamic POMC neurons underlies chronic stress-induced behavioral deficits

Chronic stress exposure induces maladaptive behavioral responses and increases susceptibility to neuropsychiatric conditions. However, specific neuronal populations and circuits that are highly sensitive to stress and trigger maladaptive behavioral responses remain to be identified. Here we investigate the patterns of spontaneous activity of proopiomelanocortin (POMC) neurons in the arcuate nucleus (ARC) of the hypothalamus following exposure to chronic unpredictable stress (CUS) for 10 days, a stress paradigm used to induce behavioral deficits such as anhedonia and behavioral despair [1, 2]. CUS exposure increased spontaneous firing of POMC neurons in both male and female mice, attributable to reduced GABA-mediated synaptic inhibition and increased intrinsic neuronal excitability. While acute activation of POMC neurons failed to induce behavioral changes in non-stressed mice of both sexes, subacute (3 days) and chronic (10 days) repeated activation of POMC neurons was sufficient to induce anhedonia and behavioral despair in males but not females under non-stress conditions. Acute activation of POMC neurons promoted susceptibility to subthreshold unpredictable stress in both male and female mice. Conversely, acute inhibition of POMC neurons was sufficient to reverse CUS-induced anhedonia and behavioral despair in both sexes. Collectively, these results indicate that chronic stress induces both synaptic and intrinsic plasticity of POMC neurons, leading to neuronal hyperactivity. Our findings suggest that POMC neuron dysfunction drives chronic stress-related behavioral deficits.

Stress-induced plasticity of a CRH/GABA projection disrupts reward behaviors in mice

Disrupted operations of the reward circuit underlie major emotional disorders, including depression, which commonly arise following early life stress / adversity (ELA). However, how ELA enduringly impacts reward circuit functions remains unclear. We characterize a stress-sensitive projection connecting basolateral amygdala (BLA) and nucleus accumbens (NAc) that co-expresses GABA and the stress-reactive neuropeptide corticotropin-releasing hormone (CRH). We identify a crucial role for this projection in executing disrupted reward behaviors provoked by ELA: chemogenetic and optogenetic stimulation of the projection in control male mice suppresses several reward behaviors, recapitulating deficits resulting from ELA and demonstrating the pathway's contributions to normal reward behaviors. In adult ELA mice, inhibiting-but not stimulating-the projection, restores typical reward behaviors yet has little effect in controls, indicating ELA-induced maladaptive plasticity of this reward-circuit component. Thus, we discover a stress-sensitive, reward inhibiting BLA → NAc projection with unique molecular features, which may provide intervention targets for disabling mental illnesses.

Introducing a depression-like syndrome for translational neuropsychiatry: a plea for taxonomical validity and improved comparability between humans and mice

Depressive disorders are the most burdensome psychiatric disorders worldwide. Although huge efforts have been made to advance treatment, outcomes remain unsatisfactory. Many factors contribute to this gridlock including suboptimal animal models. Especially limited study comparability and replicability due to imprecise terminology concerning depressive-like states are major problems. To overcome these issues, new approaches are needed. Here, we introduce a taxonomical concept for modelling depression in laboratory mice, which we call depression-like syndrome (DLS). It hinges on growing evidence suggesting that mice possess advanced socioemotional abilities and can display non-random symptom patterns indicative of an evolutionary conserved disorder-like phenotype. The DLS approach uses a combined heuristic method based on clinical depression criteria and the Research Domain Criteria to provide a biobehavioural reference syndrome for preclinical rodent models of depression. The DLS criteria are based on available, species-specific evidence and are as follows: (I) minimum duration of phenotype, (II) significant sociofunctional impairment, (III) core biological features, (IV) necessary depressive-like symptoms. To assess DLS presence and severity, we have designed an algorithm to ensure statistical and biological relevance of findings. The algorithm uses a minimum combined threshold for statistical significance and effect size (p value ≤ 0.05 plus moderate effect size) for each DLS criterion. Taken together, the DLS is a novel, biologically founded, and species-specific minimum threshold approach. Its long-term objective is to gradually develop into an inter-model validation standard and microframework to improve phenotyping methodology in translational research.

The live biotherapeutic Blautia stercoris MRx0006 attenuates social deficits, repetitive behaviour, and anxiety-like behaviour in a mouse model relevant to autism

Autism spectrum disorder (ASD) is a neurodevelopmental disorder characterised by deficits in social behaviour, increased repetitive behaviour, anxiety and gastrointestinal symptoms. The aetiology of ASD is complex and involves an interplay of genetic and environmental factors. Emerging pre-clinical and clinical studies have documented a potential role for the gut microbiome in ASD, and consequently, the microbiota represents a potential target in the development of novel therapeutics for this neurodevelopmental disorder. In this study, we investigate the efficacy of the live biotherapeutic strain, Blautia stercoris MRx0006, in attenuating some of the behavioural deficits in the autism-relevant, genetic mouse model, BTBR T+ Itpr3tf/J (BTBR). We demonstrate that daily oral administration with MRx0006 attenuates social deficits while also decreasing repetitive and anxiety-like behaviour. MRx0006 administration increases the gene expression of oxytocin and its receptor in hypothalamic cells in vitro and increases the expression of hypothalamic arginine vasopressin and oxytocin mRNA in BTBR mice. Additionally at the microbiome level, we observed that MRx0006 administration decreases the abundance of Alistipes putredinis, and modulates the faecal microbial metabolite profile. This alteration in the metabolite profile possibly underlies the observed increase in expression of oxytocin, arginine vasopressin and its receptors, and the consequent improvements in behavioural outcomes. Taken together, these findings suggest that the live biotherapeutic MRx0006 may represent a viable and efficacious treatment option for the management of physiological and behavioural deficits associated with ASD.

IGF-1 release in the medial prefrontal cortex mediates the rapid and sustained antidepressant-like actions of ketamine

Ketamine, an N-methyl-D-aspartate receptor antagonist, exerts rapid and sustained antidepressant actions. Preclinical studies demonstrated that the release of brain-derived neurotrophic factor (BDNF) and vascular endothelial growth factor in the medial prefrontal cortex (mPFC) is essential for the antidepressant-like effects of ketamine. However, the role of other neurotrophic factors in the antidepressant-like effects of ketamine has not been fully investigated. Since the intra-mPFC infusion of insulin-like growth factor 1 (IGF-1) reportedly produced antidepressant-like effects, the present study examined the role of endogenous intra-mPFC IGF-1 signaling in the antidepressant-like actions of ketamine. In vivo microdialysis showed that ketamine (10 and 30 mg/kg) significantly increased extracellular IGF-1 levels in the mPFC of male C57BL/6J mice for at least 5 h. Infusion of an IGF-1 neutralizing antibody (nAb; 160 ng/side) into the mPFC 15 min before or 2 h after ketamine injection blocked the antidepressant-like effects of ketamine in three different behavioral paradigms (forced swim, female urine sniffing, and novelty-suppressed feeding tests were conducted 1, 3 and 4 days post-ketamine, respectively). The ketamine-like antidepressant-like actions of the intra-mPFC infusion of BDNF (100 ng/side) and IGF-1 (50 ng/side) respectively were not blocked by co-infused IGF-1 nAb and BDNF nAb (200 ng/side). Moreover, intra-mPFC infusion of IGF-1 nAb 2 h post-ketamine blocked the antidepressant-like effects of ketamine in a murine lipopolysaccharide (LPS)-induced depression model. Intra-mPFC IGF-1 infusion also produced antidepressant-like effects in the LPS-challenged mice via mechanistic target of rapamycin complex 1 activation. These results suggest that persistent release of IGF-1, independently of BDNF, in the mPFC is essential for the antidepressant-like actions of ketamine.

Assessing positive and negative valence systems to refine animal models of bipolar disorders: the example of GBR 12909-induced manic phenotype

Bipolar disorders are defined by recurrences of depressive and manic episodes. The pathophysiology is still unknown, and translating clinical symptoms into behaviors explorable in animal models is challenging. Animal models of bipolar disorder do not exist because cyclicity of the disease is impossible to mimic, and it is therefore necessary to study mania and depression models separately. Beyond mood, emotional biases differentiate bipolar states in humans. Mania is associated with positive biases, e.g. emotional stimuli become more rewarding and less aversive, and the opposite for depression. We propose to assess behavioral hedonic responses to innately appetitive and aversive olfactory and gustatory cues in mice as proxies for the assigned emotional valence. A mania model is therefore supposed to exhibit positive hedonic bias. Using the GBR 12909 mania model, we observed the classical hyperactivity phenotype, along with low depressive-like but high anxiety-like behaviors. Unexpectedly, GBR 12909-treated mice exhibited strong negative hedonic biases. Consequently, the GBR 12909 model of mania might not be appropriate for studying emotional disturbances associated with mania states. We propose olfactory and gustatory preference tests as crucial assessment for positive and negative valence biases, necessary for precisely characterizing animal models of bipolar disorders.

Peptide LCGA-17 Attenuates Behavioral and Neurochemical Deficits in Rodent Models of PTSD and Depression

We have previously described the LCGA-17 peptide as a novel anxiolytic and antidepressant candidate that acts through the α2δ VGCC (voltage-gated calcium channel) subunit with putative synergism with GABA-A receptors. The current study tested the potential efficacy of acute and chronic intranasal (i.n.) LCGA-17 (0.05 mg/kg and 0.5 mg/kg) in rats on predator odor-induced conditioned place aversion (POCPA), a model of post-traumatic stress disorder (PTSD), and chronic unpredictable stress (CUS) that produce a range of behavioral and physiological changes that parallel symptoms of depression in humans. CUS and LCGA-17 treatment effects were tested in the sucrose preference (SPT) social interaction (SI), female urine sniffing (FUST), novelty-suppressed feeding (NSFT), and forced swim (FST) tests. Analysis of the catecholamines content in brain structures after CUS was carried out using HPLC. The efficacy of i.n. LCGA-17 was also assessed using the Elevated plus-maze (EPM) and FST. Acute LCGA-17 administration showed anxiolytic and antidepressant effects in EPM and FST, similar to diazepam and ketamine, respectively. In the POCPA study, LCGA-17 significantly reduced place aversion, with efficacy greater than doxazosin. After CUS, chronic LCGA-17 administration reversed stress-induced alterations in numerous behavioral tests (SI, FUST, SPT, and FST), producing significant anxiolytic and antidepressant effects. Finally, LCGA-17 restored the norepinephrine levels in the hippocampus following stress. Together, these results support the further development of the LCGA-17 peptide as a rapid-acting anxiolytic and antidepressant.

Midbrain projection to the basolateral amygdala encodes anxiety-like but not depression-like behaviors

Anxiety disorders are complex diseases, and often co-occur with depression. It is as yet unclear if a common neural circuit controls anxiety-related behaviors in both anxiety-alone and comorbid conditions. Here, utilizing the chronic social defeat stress (CSDS) paradigm that induces singular or combined anxiety- and depressive-like phenotypes in mice, we show that a ventral tegmental area (VTA) dopamine circuit projecting to the basolateral amygdala (BLA) selectively controls anxiety- but not depression-like behaviors. Using circuit-dissecting ex vivo electrophysiology and in vivo fiber photometry approaches, we establish that expression of anxiety-like, but not depressive-like, phenotypes are negatively correlated with VTA → BLA dopamine neuron activity. Further, our optogenetic studies demonstrate a causal link between such neuronal activity and anxiety-like behaviors. Overall, these data establish a functional role for VTA → BLA dopamine neurons in bi-directionally controlling anxiety-related behaviors not only in anxiety-alone, but also in anxiety-depressive comorbid conditions in mice.

Leptin enhances social motivation and reverses chronic unpredictable stress-induced social anhedonia during adolescence

Social anhedonia, a loss of interest and pleasure in social interactions, is a common symptom of major depression as well as other psychiatric disorders. Depression can occur at any age, but typically emerges in adolescence or early adulthood, which represents a sensitive period for social interaction that is vulnerable to stress. In this study, we evaluated social interaction reward using a conditioned place preference (CPP) paradigm in adolescent male and female mice. Adolescent mice of both sexes exhibited a preference for the social interaction-associated context. Chronic unpredictable stress (CUS) impaired the development of CPP for social interaction, mimicking social anhedonia in depressed adolescents. Conversely, administration of leptin, an adipocyte-derived hormone, enhanced social interaction-induced CPP in non-stressed control mice and reversed social anhedonia in CUS mice. By dissecting the motivational processes of social CPP into social approach and isolation avoidance components, we demonstrated that leptin treatment increased isolation aversion without overt social reward effect. Further mechanistic exploration revealed that leptin stimulated oxytocin gene transcription in the paraventricular nucleus of the hypothalamus, while oxytocin receptor blockade abolished the leptin-induced enhancement of socially-induced CPP. These results establish that chronic unpredictable stress can be used to study social anhedonia in adolescent mice and provide evidence that leptin modulates social motivation possibly via increasing oxytocin synthesis and oxytocin receptor activation.

Developmental Trajectories of Anhedonia in Preclinical Models

This chapter discusses how the complex concept of anhedonia can be operationalized and studied in preclinical models. It provides information about the development of anhedonia in the context of early-life adversity, and the power of preclinical models to tease out the diverse molecular, epigenetic, and network mechanisms that are responsible for anhedonia-like behaviors.Specifically, we first discuss the term anhedonia, reviewing the conceptual components underlying reward-related behaviors and distinguish anhedonia pertaining to deficits in motivational versus consummatory behaviors. We then describe the repertoire of experimental approaches employed to study anhedonia-like behaviors in preclinical models, and the progressive refinement over the past decade of both experimental instruments (e.g., chemogenetics, optogenetics) and conceptual constructs (salience, valence, conflict). We follow with an overview of the state of current knowledge of brain circuits, nodes, and projections that execute distinct aspects of hedonic-like behaviors, as well as neurotransmitters, modulators, and receptors involved in the generation of anhedonia-like behaviors. Finally, we discuss the special case of anhedonia that arises following early-life adversity as an eloquent example enabling the study of causality, mechanisms, and sex dependence of anhedonia.Together, this chapter highlights the power, potential, and limitations of using preclinical models to advance our understanding of the origin and mechanisms of anhedonia and to discover potential targets for its prevention and mitigation.

Towards Modeling Anhedonia and Its Treatment in Zebrafish

Mood disorders, especially depression, are a major cause of human disability. The loss of pleasure (anhedonia) is a common, severely debilitating symptom of clinical depression. Experimental animal models are widely used to better understand depression pathogenesis and to develop novel antidepressant therapies. In rodents, various experimental models of anhedonia have already been developed and extensively validated. Complementing rodent studies, the zebrafish (Danio rerio) is emerging as a powerful model organism to assess pathobiological mechanisms of affective disorders, including depression. Here, we critically discuss the potential of zebrafish for modeling anhedonia and studying its molecular mechanisms and translational implications.

Specific sub-regions of the longitudinal axis of the hippocampus mediate behavioural responses to chronic psychosocial stress

Accumulating evidence suggests that the hippocampus is functionally segregated along its longitudinal axis into a dorsal (dHi) sub-region, shown to play roles in learning & memory and a ventral sub-region (vHi), involved in anxiety and antidepressant action. Recent studies also suggest that the intermediate hippocampus (iHi) might be functionally independent, but it has received relatively little attention. We recently found that the iHi is involved in the behavioural effects of chronic treatment with the antidepressant fluoxetine in the forced swim test. However, the roles of specific sub-regions of the longitudinal axis of the hippocampus in the response to chronic stress, a risk factor for depression and anxiety disorders, has not yet been investigated. Therefore, we used excitotoxic lesions of the dHi, iHi or vHi in male C57BL/6 mice to investigate the roles of these sub-regions in the behavioural (anxiety, anhedonia, depression) responses to chronic psychosocial stress. We found that stress-induced increases in anxiety in the novelty-induced hypophagia and marble burying tests were prevented by each of the sub-region lesions, but only vHi lesions attenuated stress-induced anxiety in the open field test. Stress-induced anhedonia was reduced in dHi- and vHi- but not iHi-lesioned mice. In stressed mice, only vHi lesions induced an antidepressant-like effect in the forced swim test and prolonged latency to adopt a defeat posture during social defeat, suggesting an increase in stress resilience. Interestingly, iHi lesions increased stress-induced social avoidance in the social interaction test. In summary, we found that all hippocampal sub-regions are involved in the anxiogenic effects of chronic stress but that the iHi plays a predominant role in stress-induced social avoidance and the vHi has a predominant role in active coping behaviours and antidepressant-like behaviour following chronic stress.

Multidimensional nature of dominant behavior: Insights from behavioral neuroscience

Social interactions for many species of animals are critical for survival, wellbeing, and reproduction. Optimal navigation of a social system increases chances for survival and reproduction, therefore there is strong incentive to fit into social structures. Social animals rely heavily on dominant-submissive behaviors in establishment of stable social hierarchies. There is a link between extreme manifestation of dominance/submissiveness and behavioral deviations. To understand neural substrates affiliated with a specific hierarchical rank, there is a real need for reliable animal behavioral models. Different paradigms have been consolidated over time to study the neurobiology of social rank behavior in a standardized manner using rodent models to unravel the neural pathways and substrates involved in normal and abnormal intraspecific social interactions. This review summarizes and discusses the commonly used behavioral tests and new directions for the assessment of dominance in rodents. We discuss the hierarchy inheritable nature and other critical issues regarding hierarchical rank manifestation which may help in designing social-rank-related studies that serve as promising pre-clinical tools in behavioral psychiatry.

Subchronic and mild social defeat stress downregulates peripheral expression of sweet and umami taste receptors in male mice

Depression is associated with taste disorders; however, the mechanisms by which mental stress affects taste perception are not well understood. This study aimed to elucidate the effects of psychosocial stress on peripheral taste-sensing systems using a mouse depression model. Male mice were subjected to subchronic and mild social defeat stress (sCSDS). Results showed that sCSDS significantly increased body weight, food and water intake, and social avoidance behavior and that sCSDS did not change reward-seeking behavior on sucrose preference but tended to decrease pheromonal preference for female urine. Furthermore, sCSDS downregulated the mRNA levels of sweet and umami taste receptor subunits, i.e., sweet taste receptor type 1 members 2 and 3 (T1R2 and T1R3), but not the umami taste receptor subunit, i.e., taste receptor type 1 member 1 (T1R1), in the circumvallate papillae of mice. It is known that sucrose preference is mediated by the gut-brain axis without taste perception; thus, it was considered that sCSDS affected the peripheral taste-sensing systems, rather than the central reward systems, which mediate sucrose preference. This is the first study to report that psychosocial stress affects peripheral sweet and umami taste-sensing systems.

Ingestion of Bis(2-ethylhexyl) phthalate (DEHP) during adolescence causes depressive-like behaviors through hypoactive glutamatergic signaling in the medial prefrontal cortex

Over the past decades, the production and use of hazardous chemicals has increased worldwide, and the incidence of neurological diseases is increasing proportionately. Among these chemicals, Bis(2-ethylhexyl) phthalate (DEHP) is the most common member of the phthalate family used as a plasticizer. The present study assessed the consequences of daily DEHP ingestion and its effects on brain functions related to depressive-like behaviors. Adolescent C57BL/6 male mice ingested different concentrations of DEHP in their diet (2, 20, and 200 mg/kg of diet), and behavioral changes in anxiety, despair, anhedonia, and sociality were investigated. DEHP exposure evoked depressive-like behaviors in a dose-dependent manner for each symptom. The levels of corticosterone and reactive oxygen species/reactive nitrogen species increased in DEHP-exposed groups, suggesting chronic stress-like responses. In the medial prefrontal cortex (mPFC), glutamate and glutamine were decreased, and glutamine synthetase showed lower activity compared to the control group, suggesting imbalanced glutamatergic signaling. Measuring the spontaneous excitatory postsynaptic current of glutamatergic neurons, we found that DEHP ingestion resulted in hypoactive glutamatergic signaling in the mPFC.

Can animals develop depression? An overview and assessment of ‘depression-like’ states

Describing certain animal behaviours as ‘depression-like’ or ‘depressive’ has become common across several fields of research. These typically involve unusually low activity or unresponsiveness and/or reduced interest in pleasure (anhedonia). While the term ‘depression-like’ carefully avoids directly claiming that animals are depressed, this narrative review asks whether stronger conclusions can be legitimate, with animals developing the clinical disorder as seen in humans (cf., DSM-V/ICD-10). Here, we examine evidence from animal models of depression (especially chronically stressed rats) and animals experiencing poor welfare in conventional captive conditions (e.g., laboratory mice and production pigs in barren environments). We find troubling evidence that animals are indeed capable of experiencing clinical depression, but demonstrate that a true diagnosis has yet to be confirmed in any case. We thus highlight the importance of investigating the co-occurrence of depressive criteria and discuss the potential welfare and ethical implications of animal depression.

Reactivating a positive feedback loop VTA-BLA-NAc circuit associated with positive experience ameliorates the attenuated reward sensitivity induced by chronic stress

Both genetic predisposition and life events, particularly life stress, are thought to increase the risk for depression. Reward sensitivity appears to be attenuated in major depressive disorder (MDD), suggesting deficits in reward processing in these patients. We identified the VTA-BLA-NAc circuit as being activated by sex reward, and the VTA neurons that respond to sex reward are mostly dopaminergic. Acute or chronic reactivation of this circuit ameliorates the reward insensitivity induced by chronic restraint stress. Our histological and electrophysiological results show that the VTA neuron subpopulation responding to restraint stress, predominantly GABAergic neurons, inhibits the responsiveness of VTA dopaminergic neurons to reward stimuli, which is probably the mechanism by which stress modulates the reward processing neural circuits and subsequently disrupts reward-related behaviours. Furthermore, we found that the VTA-BLA-NAc circuit is a positive feedback loop. Blocking the projections from the BLA to the NAc associated with sex reward increases the excitability of VTA GABAergic neurons and decreases the excitability of VTA dopaminergic neurons, while activating this pathway decreases the excitability of VTA GABAergic neurons and increases the excitability of VTA dopaminergic neurons, which may be the cellular mechanism by which the VTA-BLA-NAc circuit associated with sex reward ameliorates the attenuated reward sensitivity induced by chronic stress.

Ablation of Nampt in AgRP neurons leads to neurodegeneration and impairs fasting- and ghrelin-mediated food intake

Agouti-related protein (AgRP) neurons in the arcuate nucleus of the hypothalamus regulates food intake and whole-body metabolism. NAD⁺ regulates multiple cellular processes controlling energy metabolism. Yet, its role in hypothalamic AgRP neurons to control food intake is poorly understood. Here, we aimed to assess whether genetic deletion of nicotinamide phosphoribosyltransferase (Nampt), a rate-limiting enzyme in NAD⁺ production, affects AgRP neuronal function to impact whole-body metabolism and food intake. Metabolic parameters during fed and fasted states, and upon systemic ghrelin and leptin administration were studied in AgRP-specific Nampt knockout (ARNKO) mice. We monitored neuropeptide expression levels and density of AgRP neurons in ARNKO mice from embryonic to adult age. NPY cells were used to determine effects of NAMPT inhibition on neuronal viability, energy status, and oxidative stress in vitro. In these cells, NAD⁺ depletion reduced ATP levels, increased oxidative stress, and promoted cell death. Agrp expression in the hypothalamus of ARNKO mice gradually decreased after weaning due to progressive AgRP neuron degeneration. Adult ARNKO mice had normal glucose and insulin tolerance, but exhibited an elevated respiratory exchange ratio (RER) when fasted. Remarkably, fasting-induced food intake was unaffected in ARNKO mice when evaluated in metabolic cages, but fasting- and ghrelin-induced feeding and body weight gain decreased in ARNKO mice when evaluated outside metabolic cages. Collectively, deletion of Nampt in AgRP neurons causes progressive neurodegeneration and impairs fasting and ghrelin responses in a context-dependent manner. Our data highlight an essential role of Nampt in AgRP neuron function and viability.

The paraventricular thalamus input to central amygdala controls depression-related behaviors

The dysregulation of neuronal networks may contribute to the etiology of major depressive disorder (MDD). However, the neural connections underlying the symptoms of MDD have yet to be elucidated. Here, we observed that glutamatergic neurons in the paraventricular thalamus (PVT) were activated by chronic unpredictable stress (CUS) with higher expression numbers of ΔFosB-labeled neurons and protein expression levels, activation of PVT neurons caused depressive-like phenotypes, whereas suppression of PVT neuronal activity induced an antidepressant effect in male, but not female mice, which were achieved by using a chemogenetic approach. Moreover, we found that PVT glutamatergic neurons showed strong neuronal projections to the central amygdala (CeA), activation of the CeA-projecting neurons in PVT or the neuronal terminals of PVT-CeA projection neurons induced depression-related behaviors or showed enhanced stress-induced susceptibility. These results suggest that PVT is a key depression-controlling nucleus, and PVT-CeA projection regulates depression-related behaviors in a sex-dependent manner, which could be served as an essential pathway for morbidity and treatment of depression.