Genetic Liability for Internalizing Versus Externalizing Behavior Manifests in the Developing and Adult Hippocampus: Insight From a Meta-analysis of Transcriptional Profiling Studies in a Selectively Bred Rat Model

Adolescent environmental enrichment induces social resilience and alters neural gene expression in a selectively bred rodent model with anxious phenotype

Stress is a major influence on mental health status; the ways that individuals respond to or copes with stressors determine whether they are negatively affected in the future. Stress responses are established by an interplay between genetics, environment, and life experiences. Psychosocial stress is particularly impactful during adolescence, a critical period for the development of mood disorders. In this study we compared two established, selectively-bred Sprague Dawley rat lines, the "internalizing" bred Low Responder (bLR) line versus the "externalizing" bred High Responder (bHR) line, to investigate how genetic temperament and adolescent environment impact future responses to social interactions and psychosocial stress, and how these determinants of stress response interact. Male bLR and bHR rats were exposed to social and environmental enrichment in adolescence prior to experiencing social defeat and were then assessed for social interaction and anxiety-like behavior. Adolescent enrichment caused rats to display more social interaction, as well as nominally less social avoidance, less submission during defeat, and resilience to the effects of social stress on corticosterone, in a manner that seemed more notable in bLRs. For bHRs, enrichment also caused greater aggression during a neutral social encounter and nominally during defeat, and decreased anxiety-like behavior. To explore the neurobiology underlying the development of social resilience in the anxious phenotype bLRs, RNA-seq was conducted on the hippocampus and nucleus accumbens, two brain regions that mediate stress regulation and social behavior. Gene sets previously associated with stress, social behavior, aggression and exploratory activity were enriched with differential expression in both regions, with a particularly large effect on gene sets that regulate social behaviors. Our findings provide further evidence that adolescent enrichment can serve as an inoculating experience against future stressors. The ability to induce social resilience in a usually anxious line of animals by manipulating their environment has translational implications, as it underscores the feasibility of intervention strategies targeted at genetically vulnerable adolescent populations.

Resource: A Curated Database of Brain-Related Functional Gene Sets (Brain.GMT)

Transcriptional profiling has become a common tool for investigating the nervous system. During analysis, differential expression results are often compared to functional ontology databases, which contain curated gene sets representing well-studied pathways. This dependence can cause neuroscience studies to be interpreted in terms of functional pathways documented in better studied tissues (e.g., liver) and topics (e.g., cancer), and systematically emphasizes well-studied genes, leaving other findings in the obscurity of the brain "ignorome". To address this issue, we compiled a curated database of 918 gene sets related to nervous system function, tissue, and cell types ("Brain.GMT") that can be used within common analysis pipelines (GSEA, limma, edgeR) to interpret results from three species (rat, mouse, human). Brain.GMT includes brain-related gene sets curated from the Molecular Signatures Database (MSigDB) and extracted from public databases (GeneWeaver, Gemma, DropViz, BrainInABlender, HippoSeq) and published studies containing differential expression results. Although Brain.GMT is still undergoing development and currently only represents a fraction of available brain gene sets, "brain ignorome" genes are already better represented than in traditional Gene Ontology databases. Moreover, Brain.GMT substantially improves the quantity and quality of gene sets identified as enriched with differential expression in neuroscience studies, enhancing interpretation.

Corticosteroid Treatment During Sepsis Alters Hippocampal Function in Male and Female Survivors

Background

Millions of sepsis survivors annually face neuropsychiatric sequelae of their illness. Corticosteroids are frequently administered for sepsis, and their use improves neuropsychiatric outcomes, but the mechanisms are unknown. In light of prior work that has shown persistent inflammation in sepsis survivors, we hypothesized that short-term corticosteroid treatment during illness would reverse the long-term impact of sepsis on inflammatory gene expression in the hippocampus and rescue associated changes to affective behaviors.

Methods

Male and female mice underwent cecal ligation and puncture or a sham surgery to induce acute infection and were treated for 5 days with corticosterone or vehicle. Starting 2 weeks after the surgery, we performed functional phenotyping in the survivor mice followed by hippocampal RNA sequencing to identify underlying mechanisms.

Results

Long-term cecal ligation and puncture survivors exhibited anxiety-like behavior, increased central hypothalamic-pituitary-adrenal axis activity, and persistent systemic and neuroinflammation. Corticosterone treatment during illness did not reverse anxiety-like behavior or inflammation in survivors. Instead, corticosterone treatment impaired object memory and increased active coping behavior in females. History of corticosterone treatment influenced the expression of >10% of detectable transcripts in the dorsal and ventral hippocampus, including a coordinated downregulation of activity-dependent genes.

Conclusions

Corticosterone treatment during sepsis impaired memory formation in survivors and caused a lasting decrease in hippocampal neural activity, which could underlie its effect on memory. Future studies should focus on how this lasting effect of corticosteroid treatment on hippocampal activity and memory translates into improved neuropsychiatric outcomes in human sepsis survivors.

Genetic and epigenetic studies of opioid abuse disorder - the potential for future diagnostics

INTRODUCTION

Opioid use disorder (OUD) is a global problem that often begins with prescribed medications. The available treatment and maintenance plans offer solutions for the consumption rate by individuals leaving the outstanding problem of relapse, which is a major factor hindering the long-term efficacy of treatments.

AREAS COVERED

Understanding the neurobiology of addiction and relapse would help identifying the core causes of relapse and distinguish vulnerable from resilient individuals, which would lead to more targeted and effective treatment and provide diagnostics to screen individuals who have a propensity to OUD. In this review, we cover the neurobiology of the reward system highlighting the role of multiple brain regions and opioid receptors in the development of the disorder. We also review the current knowledge of the epigenetics of addiction and the available screening tools for aberrant use of opioids.

EXPERT OPINION

Relapse remains an anticipated limitation in the way of recovery even after long period of abstinence. This highlights the need for diagnostic tools that identify vulnerable patients and prevent the cycle of addiction. Finally, we discuss the limitations of the available screening tools and propose possible solutions for the discovery of addiction diagnostics.

Genome-wide association study in a rat model of temperament identifies multiple loci for exploratory locomotion and anxiety-like traits

Common genetic factors likely contribute to multiple psychiatric diseases including mood and substance use disorders. Certain stable, heritable traits reflecting temperament, termed externalizing or internalizing, play a large role in modulating vulnerability to these disorders. To model these heritable tendencies, we selectively bred rats for high and low exploration in a novel environment [bred High Responders (bHR) vs. Low Responders (bLR)]. To identify genes underlying the response to selection, we phenotyped and genotyped 538 rats from an F₂ cross between bHR and bLR. Several behavioral traits show high heritability, including the selection trait: exploratory locomotion (EL) in a novel environment. There were significant phenotypic and genetic correlations between tests that capture facets of EL and anxiety. There were also correlations with Pavlovian conditioned approach (PavCA) behavior despite the lower heritability of that trait. Ten significant and conditionally independent loci for six behavioral traits were identified. Five of the six traits reflect different facets of EL that were captured by three behavioral tests. Distance traveled measures from the open field and the elevated plus maze map onto different loci, thus may represent different aspects of novelty-induced locomotor activity. The sixth behavioral trait, number of fecal boli, is the only anxiety-related trait mapping to a significant locus on chromosome 18 within which the Pik3c3 gene is located. There were no significant loci for PavCA. We identified a missense variant in the Plekhf1 gene on the chromosome 1:95 Mb QTL and Fancf and Gas2 as potential candidate genes that may drive the chromosome 1:107 Mb QTL for EL traits. The identification of a locomotor activity-related QTL on chromosome 7 encompassing the Pkhd1l1 and Trhr genes is consistent with our previous finding of these genes being differentially expressed in the hippocampus of bHR vs. bLR rats. The strong heritability coupled with identification of several loci associated with exploratory locomotion and emotionality provide compelling support for this selectively bred rat model in discovering relatively large effect causal variants tied to elements of internalizing and externalizing behaviors inherent to psychiatric and substance use disorders.

Long-Term Calorie Restriction Alters Anxiety-like Behaviour and the Brain and Adrenal Gland Transcriptomes of the Ageing Male Rat

Further examination of the molecular regulators of long-term calorie restriction (CR), reported to have an anxiolytic effect, may highlight novel therapeutic targets for anxiety disorders. Here, adult male Hooded Wistar rats were exposed to a 25% CR whilst anxiety-like behaviour was assessed at 6-, 12-, and 18-months of age via the elevated plus maze, open field, and acoustic startle tests. Next-generation sequencing was then used to measure transcriptome-wide gene expression in the hypothalamus, amygdala, pituitary, and adrenal glands. Results showed an anxiolytic behavioural profile across early, middle, and late adulthood by CR, with the strongest effects noted at 6-months. Transcriptomic analysis by seven attribute weighting algorithms, including Info Gain Ratio, Rule, Chi Squared, Gini Index, Uncertainty, Relief, and Info Gain, led to the development of a signature of long-term CR, independent of region. Complement C1q A chain (C1qa), an extracellular protein, expression was significantly decreased by CR in most regions examined. Furthermore, text mining highlighted the positive involvement of C1qa in anxiety, depression, neurodegeneration, stress, and ageing, collectively identifying a suitable biomarker candidate for CR. Overall, the current study identified anxiety-related phenotypic changes and a novel transcriptome signature of long-term CR, indicating potential therapeutic targets for anxiety, depression, and neurodegeneration.

Discovery of 1-(Hetero)aryl-β-carboline Derivatives as IDO1/TDO Dual Inhibitors with Antidepressant Activity

Depression is the leading cause of global burden of disease and disability. Abnormalities in the kynurenine pathway of tryptophan degradation have been closely linked to the pathogenesis of depression. An integrative bioinformatics analysis demonstrated that indoleamine 2,3-dioxygenase 1 (IDO1) and tryptophan 2,3-dioxygenase (TDO) are potential targets for the development of antidepressants. A series of 1-(hetero)aryl-β-carboline derivatives were designed, synthesized, and evaluated as novel IDO1/TDO dual inhibitors. Among them, compound 28 displayed potent inhibition of both IDO1 (IC₅₀ = 3.53 μM) and TDO (IC₅₀ = 1.15 μM) and had an acceptable safety profile and pharmacokinetic properties. Compound 28 also rescued lipopolysaccharide-induced depressive-like behavior in mice. Further studies revealed that 28 likely had unique antidepressant mechanisms involving suppressing microglial activation, lowering IDO1 expression, and reducing proinflammatory cytokine and kynurenine levels in the mouse brain. Overall, this work provides practical guidance for the development of IDO1/TDO dual inhibitors to treat inflammation-induced depression.

Sex-Specific Brain Transcriptional Signatures in Human MDD and Their Correlates in Mouse Models of Depression

Major depressive disorder (MDD) is amongst the most devastating psychiatric conditions affecting several millions of people worldwide every year. Despite the importance of this disease and its impact on modern societies, still very little is known about the etiological mechanisms. Treatment strategies have stagnated over the last decades and very little progress has been made to improve the efficiency of current therapeutic approaches. In order to better understand the disease, it is necessary for researchers to use appropriate animal models that reproduce specific aspects of the complex clinical manifestations at the behavioral and molecular levels. Here, we review the current literature describing the use of mouse models to reproduce specific aspects of MDD and anxiety in males and females. We first describe some of the most commonly used mouse models and their capacity to display unique but also shared features relevant to MDD. We then transition toward an integral description, combined with genome-wide transcriptional strategies. The use of these models reveals crucial insights into the molecular programs underlying the expression of stress susceptibility and resilience in a sex-specific fashion. These studies performed on human and mouse tissues establish correlates into the mechanisms mediating the impact of stress and the extent to which different mouse models of chronic stress recapitulate the molecular changes observed in depressed humans. The focus of this review is specifically to highlight the sex differences revealed from different stress paradigms and transcriptional analyses both in human and animal models.

Differences in microglia morphological profiles reflect divergent emotional temperaments: insights from a selective breeding model

Microglia play critical roles in healthy brain development and function, as well as the neuropathology underlying a range of brain diseases. Despite evidence for a role of microglia in affective regulation and mood disorders, little is known regarding how variation in microglia status relates to individual differences in emotionality. Using a selective breeding model, we have generated rat lines with unique temperamental phenotypes that reflect broad emotional traits: bred low responder rats (bLRs) are novelty-averse and model a passive coping style, whereas bred high responder rats (bHRs) are highly exploratory and model an active coping style. To identify a functional role of microglia in these phenotypes, we administered minocycline, an antibiotic with potent microglia inhibiting properties and observed shifts in forced swim, sucrose preference, and social interaction behaviors in bLRs. Using detailed anatomical analyses, we compared hippocampal microglia profiles of bHRs and bLRs and found that although the lines had similar numbers of microglia, selective breeding was associated with a shift in the morphological features of these cells. Specifically, microglia from bLRs were characterized by a hyper-ramified morphology, with longer processes and more complicated branching patterns than microglia from bHRs. This morphology is thought to reflect an early stage of microglia activation and suggests that bLR microglia are in a reactive state even when animals are not overtly challenged. Taken together, our results provide novel evidence linking variation in inborn temperament with differences in the baseline status of microglia and implicate a role for microglia in shaping enduring emotional characteristics.

Epigenetics of Aggression

Aggression is a complex behavioral trait modulated by both genetic and environmental influences on gene expression. By controlling gene expression in a reversible yet potentially lasting manner in response to environmental stimulation, epigenetic mechanisms represent prime candidates in explaining both individual differences in aggression and the development of elevated aggressive behaviors following life adversity. In this manuscript, we review the evidence for an epigenetic basis in the development and expression of aggression in both humans and related preclinical animal models. In particular, we discuss reports linking DNA methylation, histone post-translational modifications, as well as non-coding RNA, to the regulation of a variety of genes implicated in the neurobiology of aggression including neuropeptides, the serotoninergic and dopaminergic systems, and stress response related systems. While clinical reports do reveal interesting patterns of DNA methylation underlying individual differences and experience-induced aggressive behaviors, they do, in general, face the challenge of linking peripheral observations to central nervous system regulations. Preclinical studies, on the other hand, provide detailed mechanistic insights into the epigenetic reprogramming of gene expression following life adversities. Although the functional link to aggression remains unclear in most, these studies together do highlight the involvement of epigenetic events driven by DNA methylation, histone modifications, and non-coding RNA in the neuroadaptations underlying the development and expression of aggression.

Effects of Voluntary Running Wheel Exercise-Induced Extracellular Vesicles on Anxiety

Anxiety disorders are the most frequently diagnosed psychological condition, associated with serious comorbidities including excessive fear and interference with daily life. Drugs for anxiety disorders are typically prescribed but the side effects include weight gain, nausea, and sleepiness. Exercise is an effective treatment for anxiety. Exercise induces the release of extracellular vesicles (EVs) into the circulation, which transmit signals between organs. However, the effects of exercise-induced EVs on anxiety remain poorly understood. Here, we isolated EVs from the sera of mice that were sedentary or that voluntarily exercised. We characterized the changes in the miRNA profile of serum EVs after 4 weeks of voluntary exercise. miRNA sequencing showed that 82 miRNAs (46 of which were positive and 36 negative regulators) changed after exercise. We selected genes affected by at least two miRNAs. Of these, 27.27% were associated with neurotrophin signaling (9.09% with each of central nervous system neuronal development, cerebral cortical cell migration, and peripheral neuronal development). We also analyzed behavioral changes in mice with 3 weeks of restraint stress-induced anxiety after injection of 20 μg amounts of EVs from exercised or sedentary mice into the left cerebral ventricle. We found that exercise-derived EVs reduced anxiety (compared to a control group) in a nest-building test but found no between-group differences in the rotarod or open field tests. Exercise-derived EVs enhanced the expression of neuroactive ligand-receptor interaction genes. Thus, exercise-derived EVs may exhibit anti-anxiety effects and may be of therapeutic utility.

Examining the Role of Microbiota in Emotional Behavior: Antibiotic Treatment Exacerbates Anxiety in High Anxiety-Prone Male Rats

Intestinal microbiota are essential for healthy gastrointestinal function and also broadly influence brain function and behavior, in part, through changes in immune function. Gastrointestinal disorders are highly comorbid with psychiatric disorders, although biological mechanisms linking these disorders are poorly understood. The present study utilized rats bred for distinct emotional behavior phenotypes to examine relationships between emotionality, the microbiome, and immune markers. Prior work showed that Low Novelty Responder (LR) rats exhibit high levels of anxiety- and depression-related behaviors as well as myriad neurobiological differences compared to High Novelty Responders (HRs). Here, we hypothesized that the divergent HR/LR phenotypes are accompanied by changes in fecal microbiome composition. We used next-generation sequencing to assess the HR/LR microbiomes and then treated adult HR/LR males with an antibiotic cocktail to test whether it altered behavior. Given known connections between the microbiome and immune system, we also analyzed circulating cytokines and metabolic factors to determine relationships between peripheral immune markers, gut microbiome components, and behavioral measures. There were no baseline HR/LR microbiome differences, and antibiotic treatment disrupted the microbiome in both HR and LR rats. Antibiotic treatment exacerbated aspects of HR/LR behavior, increasing LRs' already high levels of anxiety-like behavior while reducing passive stress coping in both strains. Our results highlight the importance of an individual's phenotype to their response to antibiotics, contributing to the understanding of the complex interplay between gut microbes, immune function, and an individual's emotional phenotype.