Early experience as a determinant of adult behavioural responses to reward: the effects of repeated maternal separation in the rat

Childhood maltreatment influences adult brain structure through its effects on immune, metabolic, and psychosocial factors

Childhood maltreatment (CM) leads to a lifelong susceptibility to mental ill-health which might be reflected by its effects on adult brain structure, perhaps indirectly mediated by its effects on adult metabolic, immune, and psychosocial systems. Indexing these systemic factors via body mass index (BMI), C-reactive protein (CRP), and rates of adult trauma (AT), respectively, we tested three hypotheses: (H1) CM has direct or indirect effects on adult trauma, BMI, and CRP; (H2) adult trauma, BMI, and CRP are all independently related to adult brain structure; and (H3) childhood maltreatment has indirect effects on adult brain structure mediated in parallel by BMI, CRP, and AT. Using path analysis and data from N = 116,887 participants in UK Biobank, we find that CM is related to greater BMI and AT levels, and that these two variables mediate CM's effects on CRP [H1]. Regression analyses on the UKB MRI subsample (N = 21,738) revealed that greater CRP and BMI were both independently related to a spatially convergent pattern of cortical effects (Spearman's ρ = 0.87) characterized by fronto-occipital increases and temporo-parietal reductions in thickness. Subcortically, BMI was associated with greater volume, AT with lower volume and CPR with effects in both directions [H2]. Finally, path models indicated that CM has indirect effects in a subset of brain regions mediated through its direct effects on BMI and AT and indirect effects on CRP [H3]. Results provide evidence that childhood maltreatment can influence brain structure decades after exposure by increasing individual risk toward adult trauma, obesity, and inflammation.

Annual Research Review: Neuroimmune network model of depression: a developmental perspective

Depression is a serious public health problem, and adolescence is an 'age of risk' for the onset of Major Depressive Disorder. Recently, we and others have proposed neuroimmune network models that highlight bidirectional communication between the brain and the immune system in both mental and physical health, including depression. These models draw on research indicating that the cellular actors (particularly monocytes) and signaling molecules (particularly cytokines) that orchestrate inflammation in the periphery can directly modulate the structure and function of the brain. In the brain, inflammatory activity heightens sensitivity to threats in the cortico-amygdala circuit, lowers sensitivity to rewards in the cortico-striatal circuit, and alters executive control and emotion regulation in the prefrontal cortex. When dysregulated, and particularly under conditions of chronic stress, inflammation can generate feelings of dysphoria, distress, and anhedonia. This is proposed to initiate unhealthy, self-medicating behaviors (e.g. substance use, poor diet) to manage the dysphoria, which further heighten inflammation. Over time, dysregulation in these brain circuits and the inflammatory response may compound each other to form a positive feedback loop, whereby dysregulation in one organ system exacerbates the other. We and others suggest that this neuroimmune dysregulation is a dynamic joint vulnerability for depression, particularly during adolescence. We have three goals for the present paper. First, we extend neuroimmune network models of mental and physical health to generate a developmental framework of risk for the onset of depression during adolescence. Second, we examine how a neuroimmune network perspective can help explain the high rates of comorbidity between depression and other psychiatric disorders across development, and multimorbidity between depression and stress-related medical illnesses. Finally, we consider how identifying neuroimmune pathways to depression can facilitate a 'next generation' of behavioral and biological interventions that target neuroimmune signaling to treat, and ideally prevent, depression in youth and adolescents.

Maternal Substance Use and Early-Life Adversity: Inducing Drug Dependence in Offspring, Interactions, Mechanisms, and Treatments

The likelihood of substance dependency in offspring is increased in cases when there is a family history of drug or alcohol use. Mothering is limited by maternal addiction because of the separation. Maternal separation (MS) leads to the development of behavioural and neuropsychiatric issues in the future. Despite the importance of this issue, empirical investigations of the influences of maternal substance use and separation on substance use problems in offspring are limited, and studies that consider both effects are rare. This study aims to review a few studies on the mechanisms, treatments, genetics, epigenetics, molecular and psychological alterations, and neuroanatomical regions involved in the dependence of offspring who underwent maternal addiction and separation. The PubMed database was used. A total of 95 articles were found, including the most related ones in the review. The brain's lateral paragigantocellularis (LPGi), nucleus accumbens (NAc), caudate-putamen (CPu), prefrontal cortex (PFC), and hippocampus, can be affected by MS. Dopamine receptor subtype genes, alcohol biomarker minor allele, and preproenkephalin mRNA may be affected by alcohol or substance use disorders. After early-life adversity, histone acetylation in the hippocampus may be linked to brain-derived neurotrophic factor (BDNF) gene epigenetics and glucocorticoid receptors (GRs). The adverse early-life experiences differ in offspring›s genders and rewire the brain›s dopamine and endocannabinoid circuits, making offspring more susceptible to dependence. Related psychological factors rooted in early-life stress (ELS) and parental substance use disorder (SUD). Treatments include antidepressants, histone deacetylase inhibitors, lamotrigine, ketamine, choline, modafinil, methadone, dopamine, cannabinoid 1 receptor agonists/antagonists, vitamins, oxytocin, tetrahydrocannabinol, SR141716A, and dronabinol. Finally, the study emphasizes the need for multifaceted strategies to prevent these outcomes.

Maternal separation and TNBS-induced gut inflammation synergistically alter the sexually differentiated stress response in rats

BACKGROUND

Neonatal maternal separation (MS) has been used to model long-lasting changes in behavior caused by neuroplastic changes associated with exposure to early-life stress. Earlier studies showed that transient gut inflammation can influence the development of irritable bowel syndrome (IBS). A prevailing paradigm of the etiology of IBS is that transient noxious events lead to long-lasting sensitization of the neural pain circuit, despite complete resolution of the initiating event. This study characterizes the changes in behaviors and neuroendocrine parameters after MS and early-phase trinitrobenzene sulfonic acid (TNBS)-induced colitis. We tested the hypothesis that MS and gut inflammation synergistically induce (1) hyperactivity in male rats and anxiety-like behaviors in female rats and (2) activation of the HPA axis in female rats and deactivation of the HPA axis in male rats after colorectal distention (CRD).

METHODS

Male and female rat pups were separated from their dams for 180 min daily from postnatal day (PND) 2 to PND 14 (MS). Early-phase colitis was induced by colorectal administration with TNBS on PND 8. The elevated plus-maze test was performed at 7 weeks. Tonic CRD was performed at 60 mmHg for 15 min at 8 weeks. Plasma ACTH and serum corticosterone were measured at baseline or after the CRD. Analysis of variance was performed for comparison among controls, TNBS, MS, and MS + TNBS.

RESULTS

In male rats, the time spent in open arms significantly differed among the groups (p < 0.005). The time spent in open arms in male MS + TNBS rats was significantly higher than that of controls (p < 0.009) or TNBS rats (p < 0.031, post hoc test). Female rats showed no difference in the time spent in open arms among the groups. MS and gut inflammation induced an increase in plasma ACTH in female rats but not in male rats at baseline.

CONCLUSIONS

These findings suggest that MS and gut inflammation synergistically induce hyperactive behavior or exaggerated hypothalamic-pituitary-adrenal axis function depending on sex.

Sex-dependent effects of early life stress on reinforcement learning and limbic cortico-striatal functional connectivity

Major depressive disorder (MDD) is a stress-related condition hypothesized to involve aberrant reinforcement learning (RL) with positive and negative stimuli. The present study investigated whether repeated early maternal separation (REMS) stress, a procedure widely recognized to cause depression-like behaviour, affects how subjects learn from positive and negative feedback. The REMS procedure was implemented by separating male and female rats from their dam for 6 h each day from post-natal day 5-19. Control rat offspring were left undisturbed during this period. Rats were tested as adults for behavioral flexibility and feedback sensitivity on a probabilistic reversal learning task. A computational approach based on RL theory was used to derive latent behavioral variables related to reward learning and flexibility. To assess underlying brain substrates, a seed-based functional MRI connectivity analysis was applied both before and after an additional adulthood stressor in control and REMS rats. Female but not male rats exposed to REMS stress showed increased response 'stickiness' (repeated responses regardless of reward outcome). Following repeated adulthood stress, reduced functional connectivity from the basolateral amygdala (BLA) to the dorsolateral striatum (DLS), cingulate cortex (Cg), and anterior insula (AI) cortex was observed in females. By contrast, control male rats exposed to the second stressor showed impaired learning from negative feedback (i.e., non-reward) and reduced functional connectivity from the BLA to the DLS and AI compared to maternally separated males. RL in male rats exposed to REMS was unaffected. The fMRI data further revealed that connectivity between the mOFC and other prefrontal cortical and subcortical structures was positively correlated with response 'stickiness'. These findings reveal differences in how females and males respond to early life adversity and subsequent stress. These effects may be mediated by functional divergence in resting-state connectivity between the basolateral amygdala and fronto-striatal brain regions.

Dimensions of childhood maltreatment and adult risky behaviors: Differential affective and inhibitory control mechanisms

BACKGROUND

While childhood maltreatment has been identified as a robust risk factor for a myriad of poor outcomes, significant gaps in our understanding still remain regarding mechanisms of this risk transmission across the lifespan.

OBJECTIVE

Informed by recent dimensional models, the objective of this study was to examine how unique dimensions of childhood maltreatment relate to adult risky behaviors via interactions between cognitive control and affective processes across Positive and Negative Valence Systems.

PARTICIPANTS AND SETTING

Our sample consisted of 110 community adults (M/SD_age = 31.92/10.58 years old, 52.7 % male).

METHODS

Participants completed a neuropsychological test of inhibitory control and self-report measures of childhood maltreatment, recent adult risky behaviors, and indices of affective processing.

RESULTS

Moderated-mediation analyses revealed that childhood experiences of neglect and abuse exhibited distinct mechanisms of risk transmission related to adult risky behaviors. Specifically, disruptions in inhibitory control and reward-related processes were linked to adult risky behavior in the context of Childhood Deprivation (95%CI = -0.166/-0.002), whereas negative affective processes were associated with adult risky behavior in the context of Childhood Threat (95 % CI = 0.015/0.157).

CONCLUSIONS

Findings provide empirical support for novel dimensional theories of childhood maltreatment and suggest that distinct regulatory processes are impacted by these early life experiences, which may confer risk into adulthood.

The impact of early-life environment on absence epilepsy and neuropsychiatric comorbidities

This review discusses the long-term effects of early-life environment on epileptogenesis, epilepsy, and neuropsychiatric comorbidities with an emphasis on the absence epilepsy. The WAG/Rij rat strain is a well-validated genetic model of absence epilepsy with mild depression-like (dysthymia) comorbidity. Although pathologic phenotype in WAG/Rij rats is genetically determined, convincing evidence presented in this review suggests that the absence epilepsy and depression-like comorbidity in WAG/Rij rats may be governed by early-life events, such as prenatal drug exposure, early-life stress, neonatal maternal separation, neonatal handling, maternal care, environmental enrichment, neonatal sensory impairments, neonatal tactile stimulation, and maternal diet. The data, as presented here, indicate that some early environmental events can promote and accelerate the development of absence seizures and their neuropsychiatric comorbidities, while others may exert anti-epileptogenic and disease-modifying effects. The early environment can lead to phenotypic alterations in offspring due to epigenetic modifications of gene expression, which may have maladaptive consequences or represent a therapeutic value. Targeting DNA methylation with a maternal methyl-enriched diet during the perinatal period appears to be a new preventive epigenetic anti-absence therapy. A number of caveats related to the maternal methyl-enriched diet and prospects for future research are discussed.

Novel Dopamine Transporter Inhibitor, CE-123, Ameliorates Spatial Memory Deficits Induced by Maternal Separation in Adolescent Rats: Impact of Sex

Maternal separation (MS) is a key contributor to neurodevelopmental disorders, including learning disabilities. To test the hypothesis that dopamine signaling is a major factor in this, an atypical new dopamine transporter (DAT) inhibitor, CE-123, was assessed for its potential to counteract the MS-induced spatial learning and memory deficit in male and female rats. Hence, neonatal rats (postnatal day (PND)1 to 21) were exposed to MS (180 min/day). Next, the acquisition of spatial learning and memory (Barnes maze task) and the expression of dopamine D1 receptor, dopamine transporter (DAT), and the neuronal GTPase, RIT2, which binds DAT in the vehicle-treated rats were evaluated in the prefrontal cortex and hippocampus in the adolescent animals. The results show that MS impairs the acquisition of spatial learning and memory in rats, with a more severe effect in females. Moreover, the MS induced upregulation of DAT and dopamine D1 receptors expression in the prefrontal cortex and hippocampus in adolescent rats. Regarding RIT2, the expression was decreased in the hippocampus for both the males and females, however, in the prefrontal cortex, reduction was found only in the females, suggesting that there are region-specific differences in DAT endocytic trafficking. CE-123 ameliorated the behavioral deficits associated with MS. Furthermore, it decreased the MS-induced upregulation of D1 receptor expression level in the hippocampus. These effects were more noted in females. Overall, CE-123, an atypical DAT inhibitor, is able to restore cognitive impairment and dopamine signaling in adolescent rats exposed to MS—with more evident effect in females than males.

Encore: Behavioural animal models of stress, depression and mood disorders

Animal studies over the past two decades have led to extensive advances in our understanding of pathogenesis of depressive and mood disorders. Among these, rodent behavioural models proved to be of highest informative value. Here, we present a comprehensive overview of the most popular behavioural models with respect to physiological, circuit, and molecular biological correlates. Behavioural stress paradigms and behavioural tests are assessed in terms of outcomes, strengths, weaknesses, and translational value, especially in the domain of pharmacological studies.

Enduring disruption of reward and stress circuit activities by early-life adversity in male rats

In humans, early-life adversity (ELA) such as trauma, poverty, and chaotic environment is linked to increased risk of later-life emotional disorders including depression and substance abuse. These disorders involve underlying disruption of reward circuits and likely vary by sex. Accordingly, we previously found that ELA leads to anhedonia for natural rewards and cocaine in male rodents, whereas in females ELA instead increases vulnerability to addiction-like use of opioid drugs and palatable food. While these findings suggest that ELA-induced disruption of reward circuitry may differ between the sexes, the specific circuit nodes that are influenced by ELA in either sex remain poorly understood. Here, in adult male Sprague-Dawley rats, we ask how ELA impacts opioid addiction-relevant behaviors that we previously tested after ELA in females. We probe potential circuit mechanisms in males by assessing opioid-associated neuronal activation in stress and reward circuit nodes including nucleus accumbens (NAc), amygdala, medial prefrontal cortex (mPFC), and paraventricular thalamus. We find that ELA diminishes opioid-seeking behaviors in males, and alters heroin-induced activation of NAc, PFC, and amygdala, suggesting a potential circuit-based mechanism. These studies demonstrate that ELA leads to behavioral and neurobiological disruptions consistent with anhedonia in male rodents, unlike the increased opioid seeking we previously saw in females. Our findings, taken together with our prior work, suggest that men and women could face qualitatively different mental health consequences of ELA, which may be essential for individually tailoring future intervention strategies.

A Reciprocal Link Between Gut Microbiota, Inflammation and Depression: A Place for Probiotics?

Depression is a severe mental disorder that places a significant economic burden on public health. The reciprocal link between the trillions of bacteria in the gut, the microbiota, and depression is a controversial topic in neuroscience research and has drawn the attention of public interest and press coverage in recent years. Mounting pieces of evidence shed light on the role of the gut microbiota in depression, which is suggested to involve immune, endocrine, and neural pathways that are the main components of the microbiota-gut-brain axis. The gut microbiota play major roles in brain development and physiology and ultimately behavior. The bidirectional communication between the gut microbiota and brain function has been extensively explored in animal models of depression and clinical research in humans. Certain gut microbiota strains have been associated with the pathophysiology of depression. Therefore, oral intake of probiotics, the beneficial living bacteria and yeast, may represent a therapeutic approach for depression treatment. In this review, we summarize the findings describing the possible links between the gut microbiota and depression, focusing mainly on the inflammatory markers and sex hormones. By discussing preclinical and clinical studies on probiotics as a supplementary therapy for depression, we suggest that probiotics may be beneficial in alleviating depressive symptoms, possibly through immune modulation. Still, further comprehensive studies are required to draw a more solid conclusion regarding the efficacy of probiotics and their mechanisms of action.

Neurodevelopmental origins of substance use disorders: Evidence from animal models of early-life adversity and addiction

Addiction is a chronic relapsing disorder with devastating personal, societal, and economic consequences. In humans, early-life adversity (ELA) such as trauma, neglect, and resource scarcity are linked with increased risk of later-life addiction, but the brain mechanisms underlying this link are still poorly understood. Here, we focus on data from rodent models of ELA and addiction, in which causal effects of ELA on later-life responses to drugs and the neurodevelopmental mechanisms by which ELA increases vulnerability to addiction can be determined. We first summarize evidence for a link between ELA and addiction in humans, then describe how ELA is commonly modeled in rodents. Since addiction is a heterogeneous disease with many individually varying behavioral aspects that may be impacted by ELA, we next discuss common rodent assays of addiction-like behaviors. We then summarize the specific addiction-relevant behavioral phenotypes caused by ELA in male and female rodents and discuss some of the underlying changes in brain reward and stress circuits that are likely responsible. By better understanding the behavioral and neural mechanisms by which ELA promotes addiction vulnerability, we hope to facilitate development of new approaches for preventing or treating addiction in those with a history of ELA.

Discontinuation of hormonal contraception due to changes in mood and decreases in sexual desire: the role of adverse childhood experiences

PURPOSE

To evaluate if adverse childhood experiences are associated with hormonal contraception discontinuation due to mood and sexual side effects.

MATERIALS AND METHODS

Women, ages 18-40 (N = 826), with current and/or previous hormonal contraceptive use completed surveys on demographics, contraceptive history, and the Adverse Childhood Experiences Questionnaire. We characterised women into high (≥2 adverse experiences) and low (0 or 1) adverse childhood experience groups. We calculated risk ratios for associations between adverse childhood experiences and outcomes of interest using log binomial generalised linear models, and adjusted for relevant demographic variables.

RESULTS

Women in the high adverse childhood experiences group (n = 355) were more likely to report having discontinued hormonal contraception due to decreases in sexual desire (adjusted risk ratio 1.44, 1.03-2.00, p = .030). Covariates included age, current hormonal contraception use, and various demographic variables associated with discontinuation. Adverse childhood experiences were not associated with mood or sexual side effects among current (n = 541) hormonal contraceptive users.

CONCLUSIONS

Self-reported adverse childhood experiences were associated with greater likelihood of discontinuing hormonal contraception due to behavioural side effects, particularly decreases in sexual desire. Identification of risk factors for behavioural side effects can assist patients and clinicians in making informed choices on contraception that minimise risk of early discontinuation.

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.

Fluoxetine treatment supports predictive validity of the three hit model of depression in male PACAP heterozygous mice and underpins the impact of early life adversity on therapeutic efficacy

According to the three hit concept of depression, interaction of genetic predisposition altered epigenetic programming and environmental stress factors contribute to the disease. Earlier we demonstrated the construct and face validity of our three hit concept-based mouse model. In the present work, we aimed to examine the predictive validity of our model, the third willnerian criterion. Fluoxetine treatment was applied in chronic variable mild stress (CVMS)-exposed (environmental hit) CD1 mice carrying one mutated allele of pituitary adenylate cyclase-activating polypeptide gene (genetic hit) that were previously exposed to maternal deprivation (epigenetic hit) vs. controls. Fluoxetine reduced the anxiety level in CVMS-exposed mice in marble burying test, and decreased the depression level in tail suspension test if mice were not deprived maternally. History of maternal deprivation caused fundamental functional-morphological changes in response to CVMS and fluoxetine treatment in the corticotropin-releasing hormone-producing cells of the bed nucleus of the stria terminalis and central amygdala, in tyrosine-hydroxylase content of ventral tegmental area, in urocortin 1-expressing cells of the centrally projecting Edinger-Westphal nucleus, and serotonergic cells of the dorsal raphe nucleus. The epigenetic background of alterations was approved by altered acetylation of histone H3. Our findings further support the validity of both the three hit concept and that of our animal model. Reversal of behavioral and functional-morphological anomalies by fluoxetine treatment supports the predictive validity of the model. This study highlights that early life stress does not only interact with the genetic and environmental factors, but has strong influence also on therapeutic efficacy.

Potentiation of glutamatergic synaptic transmission onto lateral habenula neurons following early life stress and intravenous morphine self-administration in rats

Early life stress presents an important risk factor for drug addiction and comorbid depression and anxiety through persistent effects on the mesolimbic dopamine pathways. Using an early life stress model for child neglect (a single 24 h episode of maternal deprivation, MD) in rats, recent published works from our lab show that MD induces dysfunction in the ventral tegmental area and its negative controller, the lateral habenula (LHb). MD-induced potentiation of glutamatergic synaptic transmission onto LHb neurons shifts the coordination of excitation/inhibition (E/I) balance towards excitation, resulting in an increase in the overall spontaneous neuronal activity with elevation in bursting and tonic firing, and in the intrinsic excitability of LHb neurons in early adolescent male rats. Here, we explored how MD affects intravenous morphine self-administration (MSA) acquisition and sucrose preference as well as glutamatergic synaptic function in LHb neurons of adult male rats self-administering morphine. We found that MD-induced increases in LHb neuronal and glutamatergic synaptic activity and E/I ratio persisted into adulthood. Moreover, MD significantly reduced morphine intake, triggered anhedonia-like behaviour in the sucrose preference test and was associated with persistent glutamatergic potentiation 24 h after the last MSA session. MSA also altered the decay time kinetics of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor (AMPAR) currents in LHb neurons of control rats during this time period. Our data highlight that early life stress-induced glutamatergic plasticity in LHb may dampen the positive reinforcing and motivational properties of both natural rewards and opioids, and may contribute to the development of anhedonia and dysphoric states associated with opioids.

Different forms of childhood maltreatment have different impacts on the neural systems involved in the representation of reinforcement value

Background

The current study aimed to address two gaps in the literature on child maltreatment, reinforcement processing and psychopathology. First, the extent to which compromised reinforcement processing might be particularly associated with either neglect or abuse. Second, the extent to which maltreatment-related compromised reinforcement processing might be associated with particular symptom sets (depression, conduct problems, anxiety) or symptomatology more generally.

Methods

A sample of adolescents (N = 142) aged between 14 and 18 years with varying levels of prior maltreatment participated in this fMRI study. They were scanned while performing a passive avoidance learning task, where the participant learns to respond to stimuli that engender reward and avoid responding to stimuli that engender punishment. Maltreatment (abuse and neglect) levels were assessed with the Childhood Trauma Questionnaire (CTQ).

Results

We found that: (i) level of neglect, but not abuse, was negatively associated with differential BOLD responses to reward-punishment within the striatum and medial frontal cortex; and (ii) differential reward-punishment responses within these neglect-associated regions were particularly negatively associated with level of conduct problems.

Conclusion

Our findings demonstrate the adverse neurodevelopmental impact of childhood maltreatment, particularly neglect, on reinforcement processing. Moreover, they suggest a neurodevelopmental route by which neglect might increase the risk for conduct problems.

Postnatal exposure to fluoxetine led to cognitive-emotional alterations and decreased parvalbumin positive neurons in the hippocampus of juvenile Wistar rats

The exposure to selective serotonin reuptake inhibitors (SSRIs) during development results in behavioural impairment in adulthood in humans and animal models. Indeed, serotonergic overexpression in early life leads to structural and functional changes in brain circuits that control cognition and emotion. However, the effects of developmental exposure to these substances on the behaviour of adolescent rats are conflicting and remain poorly characterised. We performed a behavioural screening to investigate the effects of postnatal exposure to fluoxetine on memory and behaviours related to anxiety, anhedonia, and depression, as well we evaluate the parvalbumin expression in hippocampus of juvenile (~PND45) female and male rats. Fluoxetine (daily 20 mg/kg s.c. injections from PND7-PND21)- or vehicle-treated adolescent rats went through several behavioural tasks (from PND 38 to PND52) and were subject to transcardial perfusion and brain removal for immunohistochemical analysis (PND53). We found that postnatal exposure to fluoxetine increased anxiety- and depression-like behaviours in the open field and sucrose preference and forced swimming tests, respectively. In addition, this treatment induced working memory and short-term (but not long-term) recognition memory impairments, and reduced parvalbumin-positive interneurons in the hippocampus. In addition, the results revealed developmental sex-dependent effects of fluoxetine postnatal treatment on adolescent rats' behaviour. These outcomes indicate that affective disorders and mnemonic alterations caused by SSRIs perinatal exposure can be present at adolescence.

A Painful Beginning: Early Life Surgery Produces Long-Term Behavioral Disruption in the Rat

Early life surgery produces peripheral nociceptive activation, inflammation, and stress. Early life nociceptive input and inflammation have been shown to produce long-term processing changes that are not restricted to the dermatome of injury. Additionally stress has shown long-term effects on anxiety, depression, learning, and maladaptive behaviors including substance abuse disorder and we hypothesized that early life surgery would have long-term effects on theses complex behaviors in later life. In this study surgery in the rat hindpaw was performed to determine if there are long-term effects on anxiety, depression, audiovisual attention, and opioid reward behaviors. Male animals received paw incision surgery and anesthesia or anesthesia alone (sham) at postnatal day 6. At 10 weeks after surgery, open field center zone entries were decreased, a measure of anxiety (n = 20) (P = 0.03) (effect size, Cohen's d = 0.80). No difference was found in the tail suspension test as a measure of depression. At 16-20 weeks, attentional performance in an operant task was similar between groups at baseline and decreased with audiovisual distraction in both groups (P < 0.001) (effect size, η² = 0.25), but distraction revealed a persistent impairment in performance in the surgery group (n = 8) (P = 0.04) (effect size, η² = 0.13). Opioid reward was measured using heroin self-administration at 16-24 weeks. Heroin intake increased over time in both groups during 24-h free access (P < 0.001), but was greater in the surgery group (P = 0.045), with a significant interaction between time and treatment (P < 0.001) (effect size, Cohen f ² = 0.36). These results demonstrate long-term disruptions in complex behaviors from surgical incision under anesthesia. Future studies to explore sex differences in early life surgery and the attendant peripheral neuronal input, stress, and inflammation will be valuable to understand emerging learning deficits, anxiety, attentional dysfunction, and opioid reward and their mechanisms. This will be valuable to develop optimal approaches to mitigate the long-term effects of surgery in early life.

Early Life Stress and Risks for Opioid Misuse: Review of Data Supporting Neurobiological Underpinnings

A robust body of research has shown that traumatic experiences occurring during critical developmental periods of childhood when neuronal plasticity is high increase risks for a spectrum of physical and mental health problems in adulthood, including substance use disorders. However, until recently, relatively few studies had specifically examined the relationships between early life stress (ELS) and opioid use disorder (OUD). Associations with opioid use initiation, injection drug use, overdose, and poor treatment outcome have now been demonstrated. In rodents, ELS has also been shown to increase the euphoric and decrease antinociceptive effects of opioids, but little is known about these processes in humans or about the neurobiological mechanisms that may underlie these relationships. This review aims to establish a theoretical model that highlights the mechanisms by which ELS may alter opioid sensitivity, thereby contributing to future risks for OUD. Alterations induced by ELS in mesocorticolimbic brain circuits, and endogenous opioid and dopamine neurotransmitter systems are described. The limited but provocative evidence linking these alterations with opioid sensitivity and risks for OUD is presented. Overall, the findings suggest that better understanding of these mechanisms holds promise for reducing vulnerability, improving prevention strategies, and prescribing guidelines for high-risk individuals.

The Microbiome-Gut-Brain Axis and Resilience to Developing Anxiety or Depression under Stress

Episodes of depression and anxiety commonly follow the experience of stress, however not everyone who experiences stress develops a mood disorder. Individuals who are able to experience stress without a negative emotional effect are considered stress resilient. Stress-resilience (and its counterpart stress-susceptibility) are influenced by several psychological and biological factors, including the microbiome-gut-brain axis. Emerging research shows that the gut microbiota can influence mood, and that stress is an important variable in this relationship. Stress alters the gut microbiota and plausibly this could contribute to stress-related changes in mood. Most of the reported research has been conducted using animal models and demonstrates a relationship between gut microbiome and mood. The translational evidence from human clinical studies however is rather limited. In this review we examine the microbiome-gut-brain axis research in relation to stress resilience.

Maternal Separation Followed by Chronic Mild Stress in Adulthood Is Associated with Concerted Epigenetic Regulation of AP-1 Complex Genes

Depression is one of the most prevalent mental diseases worldwide. Patients with psychiatric diseases often have a history of childhood neglect, indicating that early-life experiences predispose to psychiatric diseases in adulthood. Two strong models were used in the present study: the maternal separation/early deprivation model (MS) and the chronic mild stress model (CMS). In both models, we found changes in the expression of a number of genes such as Creb and Npy. Strikingly, there was a clear regulation of expression of four genes involved in the AP-1 complex: c-Fos, c-Jun, FosB, and Jun-B. Interestingly, different expression levels were observed depending on the model, whereas the combination of the models resulted in a normal level of gene expression. The effects of MS and CMS on gene expression were associated with distinct histone methylation/acetylation patterns of all four genes. The epigenetic changes, like gene expression, were also dependent on the specific stressor or their combination. The obtained results suggest that single life events leave a mark on gene expression and the epigenetic signature of gene promoters, but a combination of different stressors at different life stages can further change gene expression through epigenetic factors, possibly causing the long-lasting adverse effects of stress.

The Developmental Origins of Opioid Use Disorder and Its Comorbidities

Opioid use disorder (OUD) rarely presents as a unitary psychiatric condition, and the comorbid symptoms likely depend upon the diverse risk factors and mechanisms by which OUD can arise. These factors are heterogeneous and include genetic predisposition, exposure to prescription opioids, and environmental risks. Crucially, one key environmental risk factor for OUD is early life adversity (ELA). OUD and other substance use disorders are widely considered to derive in part from abnormal reward circuit function, which is likely also implicated in comorbid mental illnesses such as depression, bipolar disorder, and schizophrenia. ELA may disrupt reward circuit development and function in a manner predisposing to these disorders. Here, we describe new findings addressing the effects of ELA on reward circuitry that lead to OUD and comorbid disorders, potentially via shared neural mechanisms. We discuss some of these OUD-related problems in both humans and animals. We also highlight the increasingly apparent, crucial contribution of biological sex in mediating the range of ELA-induced disruptions of reward circuitry which may confer risk for the development of OUD and comorbid neuropsychiatric disorders.

Impact of preweaning stress on long-term neurobehavioral outcomes in Sprague-Dawley rats: Differential effects of barren cage rearing, pup isolation, and the combination

Two developmental stressors were compared in preweaning rats exposed to either one stressor or both. Stressors were barren cage rearing or maternal separation (pup isolation). 40 gravid Sprague-Dawley CD/IGS rats were randomly assigned to two cage conditions: standard (Std) cage or barren cage (Bar), 20 litters/condition throughout gestation and lactation. After delivery, litters were randomly culled to 4 males and 4 females. The second stressor was maternal separation: Two male/female pairs per litter were isolated from their dam 4 h/day (Iso) and two pairs were not (Norm). Hence, there were 4 conditions: Std-Norm, Std-Iso, Bar-Norm, and Bar-Iso. One pair/litter/stress condition received the following: elevated zero-maze (EZM), open-field, swim channel, Cincinnati water maze, conditioned fear, and open-field with methamphetamine challenge. The second pair/litter/condition received the light-dark test, swim channel, Morris water maze, forced swim, and EZM with diazepam challenge. Barren rearing reduced EZM time-in-open, whereas isolation rearing reduced open-field activity in males and increased it in females. Effects on straight channel swimming were minor. In the Cincinnati water maze test of egocentric learning, isolation rearing increased errors whereas barren cage housing reduced errors in combination with normal rearing. Barren cage with maternal separation (pup isolation) increased Cincinnati water maze escape latency but not errors. Barren cage housing reduced hyperactivity in response to methamphetamine. Isolation rearing increased time in open in the EZM after diazepam challenge. Trends were seen in the Morris water maze. These suggested that barren cage and isolation rearing in combination reduced latency on acquisition on days 1 and 2 in males, whereas females had increased latency on days 2 and 3. Combined exposure to two developmental stressors did not induce additive or synergistic effects, however the data show that these stressors had long-term effects with some evidence that the combination of both caused effects when either stressor alone did not, but synergism was not observed.

Challenges in researching the immune pathways between early life adversity and psychopathology

Exposure to childhood adversity is a critical risk factor for the development of psychopathology. A growing field of research examines how exposure to childhood adversity is translated into biological risk for psychopathology through alterations in immune system functioning, most notably heightened levels of inflammation biomarkers. Though our knowledge about how childhood adversity can instantiate biological risk for psychopathology is growing, there remain many challenges and gaps in the field to understand how inflammation from childhood adversity contributes to psychopathology. This paper reviews research on the inflammatory outcomes arising from childhood adversity and presents four major challenges that future research must address: (a) the measurement of childhood adversity, (b) the measurement of inflammation, (c) the identification of mediators between childhood adversity and inflammation, and (d) the identification of moderators of inflammatory outcomes following childhood adversity. We discuss synergies and inconsistencies in the literature to summarize the current understanding of the association between childhood adversity, a proinflammatory phenotype, and the biological risk for psychopathology. We discuss the clinical implications of the inflammatory links between childhood adversity and psychopathology, including possibilities for intervention. Finally, this review conclude by delineates future directions for research, including issues of how best to detect, prevent, and understand these “hidden wounds” of childhood adversity.

Understanding stress: Insights from rodent models

Through incorporating both physical and psychological forms of stressors, a variety of rodent models have provided important insights into the understanding of stress physiology. Rodent models also have provided significant information with regards to the mechanistic basis of the pathophysiology of stress-related disorders such as anxiety disorders, depressive illnesses, cognitive impairment and post-traumatic stress disorder. Additionally, rodent models of stress have served as valuable tools in the area of drug screening and drug development for treatment of stress-induced conditions. Although rodent models do not accurately reproduce the biochemical or physiological parameters of stress response and cannot fully mimic the natural progression of human disorders, yet, animal research has provided answers to many important scientific questions. In this review article, important studies utilizing a variety of stress models are described in terms of their design and apparatus, with specific focus on their capabilities to generate reliable behavioral and biochemical read-out. The review focusses on the utility of rodent models by discussing examples in the literature that offer important mechanistic insights into physiologically relevant questions. The review highlights the utility of rodent models of stress as important tools for advancing the mission of scientific research and inquiry.

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Stressful life events may lead to the onset of severe psychopathologies in humans.

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Rodents may model many features of stress exposure in human populations.

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Induction of stress via pharmacological and psychological manipulations alter rodent behavior.

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Mechanistic rodent studies reveal key molecular targets critical for new therapeutic targets.

Hexanal inhalation affects cognition and anxiety-like behavior in mice

Hexanal is a 6-carbon aldehyde that smells like green leaves and urine to mammals. However, its physiological effects remain unclear. In particular, the effects of hexanal inhalation on the central nervous system have not been clarified. We investigated hexanal inhalation in mice and conducted a series of behavioral experiments to examine the neuropsychological effects of hexanal. After inhaling hexanal emissions for 30 min, mice were subjected to an open field test, a hot plate test, a grip strength test, an elevated plus maze test, a Y-maze test, a tail suspension test, and a forced swim test to examine the effects of hexanal odor on mouse behavior. Compared to controls, mice that inhaled hexanal exhibited reduced anxiety-like behavior in the elevated plus maze test. In addition, mice that inhaled hexanal displayed significantly improved spatial cognitive ability in the Y-maze test. However, in some behavioral experiments there was no significant difference between control mice and mice that inhaled hexanal. The results of this study suggest that hexanal inhalation causes anxiolytic effects and improves cognitive function in mice. These findings may have implications for safety management procedures and determining the effective use of household goods containing hexanal, though further work is required.

Maternal deprivation induces persistent adaptations in putative dopamine neurons in rat ventral tegmental area: in vivo electrophysiological study

Early life aversive experiences can trigger persistent deficits in neuronal signaling within the mesolimbic pathway, most notably in the dopamine (DA) neurons of the ventral tegmental area (VTA). The identity of such cellular mechanisms currently appears as an important issue. To address this concern, we investigated whether early life maternal deprivation (MD) would affect the electrical activity of VTA DA neurons, via in vivo extracellular single-unit recording. Male Wistar rats were deprived of their dams for 3 h per day from postnatal days (PND) 1-14. Thereafter, the adult animals (PND 70-80) were tested for the discharge activity of putative VTA DA neurons. The VTA DA neurons displayed a decrease in firing rate and an increase in the variability of baseline discharge activity in deprived animals. MD also caused a decrease in burst firing of VTA DA neurons compared to control subjects. In summary, early life MD induces a hypoactive VTA DA system, which may contribute to lifespan psychopathologies.

Parsing inter- and intra-individual variability in key nervous system mechanisms of stress responsivity and across functional domains

Exposure to stressful events is omnipresent in modern human life, yet people show considerable heterogeneity in the impact of stress exposure(s) on their functionality and overall health. Encounter with stressor(s) is counteracted by an intricate repertoire of nervous-system responses. This narrative review starts with a brief summary of the vast evidence that supports heart rate variability, cortisol secretion, and large-scale cortical network interactions as kay physiological, endocrinological, and neural mechanisms of stress responsivity, respectively. The second section highlights potential sources for inter-individual variability in these mechanisms, by focusing on biological, environmental, social, habitual, and psychological factors that may influence stress responsivity patterns and thus contribute to heterogeneity in the impact of stress exposure on functionality and health. The third section introduces intra-individually variability in stress responsivity across functional domains as a novel putative source for heterogeneity in the impact of stress exposure. Challenges and future directions are further discussed. Parsing inter- and intra-individual variability in nervous-system mechanisms of stress responsivity and across functional domains is critical towards potential clinical translation.

Chemogenetic manipulation of the bed nucleus of the stria terminalis counteracts social behavioral deficits induced by early life stress in C57BL/6J mice

Trauma during critical periods of development can induce long-lasting adverse effects. To study neural aberrations resulting from early life stress (ELS), many studies utilize rodent maternal separation, whereby pups are intermittently deprived of maternal care necessary for proper development. This can produce adulthood behavioral deficits related to anxiety, reward, and social behavior. The bed nucleus of the stria terminalis (BNST) encodes aspects of anxiety-like and social behaviors, and also undergoes developmental maturation during the early postnatal period, rendering it vulnerable to effects of ELS. Mice underwent maternal separation (separation 4 hr/day during postnatal day (PD)2-5 and 8 hr/day on PD6-16) with early weaning on PD17, which induced behavioral deficits in adulthood performance on two-part social interaction task designed to test social motivation (choice between a same-sex novel conspecific or an empty cup) and social novelty preference (choice between the original-novel conspecific vs. a new-novel conspecific). We used chemogenetics to non-selectively silence or activate neurons in the BNST to examine its role in social motivation and social novelty preference, in mice with or without the history of ELS. Manipulation of BNST produced differing social behavior effects in non-stressed versus ELS mice; social motivation was decreased in non-stressed mice following BNST activation, but unchanged following BNST silencing, while ELS mice showed no change in social behavior after BNST activation, but exhibited enhancement of social motivation-for which they were deficient prior-following BNST silencing. Findings emphasize the BNST as a potential therapeutic target for social anxiety disorders instigated by childhood trauma.

Finding intestinal fortitude: Integrating the microbiome into a holistic view of depression mechanisms, treatment, and resilience

Depression affects at least 322 million people globally, or approximately 4.4% of the world's population. While the earnestness of researchers and clinicians to understand and treat depression is not waning, the number of individuals suffering from depression continues to increase over and above the rate of global population growth. There is a sincere need for a paradigm shift. Research in the past decade is beginning to take a more holistic approach to understanding depression etiology and treatment, integrating multiple body systems into whole-body conceptualizations of this mental health affliction. Evidence supports the hypothesis that the gut microbiome, or the collective trillions of microbes inhabiting the gastrointestinal tract, is an important factor determining both the risk of development of depression and persistence of depressive symptoms. This review discusses recent advances in both rodent and human research that explore bidirectional communication between the gut microbiome and the immune, endocrine, and central nervous systems implicated in the etiology and pathophysiology of depression. Through interactions with circulating inflammatory markers and hormones, afferent and efferent neural systems, and other, more niche, pathways, the gut microbiome can affect behavior to facilitate the development of depression, exacerbate current symptoms, or contribute to treatment and resilience. While the challenge of depression may be the direst mental health crisis of our age, new discoveries in the gut microbiome, when integrated into a holistic perspective, hold great promise for the future of positive mental health.

miR-138 Increases Depressive-Like Behaviors by Targeting SIRT1 in Hippocampus

BACKGROUND

Major depressive disorder (MDD) is a serious and common mood disorder with unknown etiology. Emerging evidence has demonstrated the critical roles of SIRT1 and microRNAs (miRNAs) in the progression of MDD. However, the underlying molecular mechanisms remain to be fully understood.

METHODS

In the present study, the expression level of miR-138 and SIRT1 were analyzed by RT-PCR or Western blotting in a chronic unpredictable mild stress (CUMS) model. The depressive-like behaviors were analyzed by forced swimming test (FST) and sucrose preference test (SPT) in mice injected with miR-138 and SIRT1 overexpression lentivirus. The luciferase reporter assay was used to assess the direct regulation of miR-138 on SIRT1 expression.

RESULTS

The upregulation of miR-138 was found in the hippocampus of the CUMS mice and correlated with decreased SIRT1 expression. C57BL/6J mice treated with SIRT1- and miR-138-expressing (miR-138) lentivirus showed increased depressive-like behaviors. In contrast, SIRT1 or si-miR-138 lentivirus treated C57BL/6J mice showed decreased depressive-like behaviors. Moreover, the Sirt1/PGC-1α/FNDC5/BDNF pathway was downregulated following miR-138 overexpression and increased upon miR-138 knockdown in hippocampus in CUMS mice and cultured primary neuronal cells. Mechanistically, luciferase reporter assay demonstrated that SIRT1 gene was a downstream transcriptional target of miR-138.

CONCLUSION

Our data demonstrated the regulation role of miR-138 on SIRT1 gene expression, miR-138 increased depressive-like behaviors by regulating SIRT1 expression in hippocampus.

Evidence that neuropsychological deficits following early life adversity may underlie vulnerability to depression

Early life adversity (ELA) is a risk factor for major depressive disorder (MDD), however the underlying mechanisms are not well understood. Clinical studies suggest that negative affective biases (the process, whereby cognitive processes such as learning and memory and decision-making are modified by emotional state) represent a vulnerability factor for MDD. In this study we investigate the impact of ELA on affective biases and reward-associated behaviours in rats. Sprague Dawley rat pups underwent 14 days of postnatal maternal separation (180 min/day from postnatal day 1: MS180) whilst control pups remained unhandled. In adulthood, affective biases associated with reward learning and decision-making were assessed using the affective bias test (ABT), or judgement bias task (JBT) respectively. Changes in motivation and reward sensitivity were tested in a progressive ratio (PR) schedule of operant responding and the sucrose preference test (SPT) respectively. We observed that MS180 animals expressed enhanced negative biases in response to acute corticosterone treatment but without effects on antidepressant-induced positive biases. ELA animals were impaired in their ability to develop appropriate biases in response to changes in reward value in a modified ABT but in the absence of any changes in reward sensitivity or motivation. No effects on decision-making were observed in the JBT but MS180 animals failed to develop the same more optimistic behavioural profile as controls in response to an increase in reward value. These findings suggest that ELA in rats increases vulnerability to negative affective biases and impairs animals' ability to appropriately learn reward value, independent of a reward sensitivity or changes in motivation. These data provide important evidence linking ELA with relevant neuropsychological impairments that may explain increased risk of developing MDD.

Sex-dependent effects of early unstable post-natal environment on response to positive and negative stimuli in adult mice

Alterations in early environmental conditions that interfere with the creation of a stable mother-pup bond have been suggested to be a risk factor for the development of stress-related psychopathologies later in life. The long-lasting effects of early experiences are mediated by changes in various cerebral circuits, such as the corticolimbic system, which processes aversive and rewarding stimuli. However, it is evident that the early environment is not sufficient per se to induce psychiatric disorders; interindividual (eg, sex-based) differences in the response to environmental challenges exist. To examine the sex-related effects that are induced by an early experience on later events in adulthood, we determine the enduring effects of repeated cross-fostering (RCF) in female and male C57BL/6J mice. To this end, we assessed the behavioral phenotype of RCF and control (male and female) mice in the saccharine preference test and cocaine-induced conditioned place preference to evaluate the response to natural and pharmacological stimuli and in the elevated plus maze test and forced swimming test to measure their anxiety- and depression-like behavior. We also evaluated FST-induced c-Fos immunoreactivity in various brain regions that are engaged in the response to acute stress exposure (FST). Notably, RCF has opposing effects on the adult response to these tests between sexes, directing male mice toward an "anhedonia-like" phenotype and increasing the sensitivity for rewarding stimuli in female mice.

Functional and Structural Brain Correlates of Socioeconomic Status

Socioeconomic status (SES) is a multidimensional construct that includes not only measures of material wealth, but also education, social prestige, and neighborhood quality. Socioeconomic correlates between wealth and cognitive functions have been well established in behavioral studies. However, functional and structural brain correlates of SES remain unclear. Here, we sought to uncover the most likely neural regions to be affected by low SES, specifically associated with age. Using effect size–seed-based d Mapping, we compiled studies that examined individuals with low SES and performed functional magnetic resonance imaging and voxel-based morphometry meta-analyses. The results revealed that as from early to late age, individuals exposed to low SES are less likely to have sustained executive network activity yet a greater likelihood to enhanced activity within reward-related regions. A similar activity was shown for gray matter volume across early to older age. These findings provide the first quantitative integration of neuroimaging results pertaining to the neural basis of SES. Hypoactivation of the executive network and hyperactivation of the reward network in low SES individuals may support the scarcity hypothesis and animal models of the effects of early adversity.

Sustained corticosterone rise in the prefrontal cortex is a key factor for chronic stress-induced working memory deficits in mice

Exposure to prolonged, unpredictable stress leads to glucocorticoids-mediated long-lasting neuroendocrine abnormalities associated with emotional and cognitive impairments. Excessive levels of serum glucocorticoids (cortisol in humans, corticosterone in rodents) contribute notably to deficits in working memory (WM), a task which heavily relies on functional interactions between the medial prefrontal cortex (PFC) and the dorsal hippocampus (dHPC). However, it is unknown whether stress-induced increases in plasma corticosterone mirror corticosterone levels in specific brain regions critical for WM. After a 6 week-UCMS exposure, C57BL/6 J male mice exhibited increased anxiety- and depressive-like behaviors when measured one week later and displayed WM impairments timely associated with increased plasma corticosterone response. In chronically stressed mice, basal phosphorylated/activated CREB (pCREB) was markedly increased in the PFC and the CA1 area of the dHPC and WM testing did not elicit any further increase in pCREB in the two regions. Using microdialysis samples from freely-moving mice, we found that WM testing co-occurred with a rapid and sustained increase in corticosterone response in the PFC while there was a late, non-significant rise of corticosterone in the dHPC. The results also show that non-stressed mice injected with corticosterone (2 mg/kg i.p.) before WM testing displayed behavioral and molecular alterations similar to those observed in stressed animals while a pre-WM testing metyrapone injection (35 mg/kg i.p.), a corticosterone synthesis inhibitor, prevented the effects of UCMS exposure. Overall, the abnormal regional increase of corticosterone concentrations mainly in the PFC emerges as a key factor of enduring WM dysfunctions in UCMS-treated animals.

Alternation in dopamine D2-like and metabotropic glutamate type 5 receptor density caused by differing housing conditions during abstinence from cocaine self-administration in rats

BACKGROUND

Environmental conditions have an important function in substance use disorder, increasing or decreasing the risks of relapse. Several studies strongly support the role of the dopamine D₂-like and metabotropic glutamate type 5 receptors in maladaptive neurobiological responses to cocaine reward and relapse.

AIMS

The present study employed cocaine self-administration with yoked-triad procedure in rats to explore whether drug abstinence in different housing conditions affects the drug-seeking behaviour and the dopamine D₂-like and metabotropic glutamate type 5 receptor density and affinity in several regions of the animal brain.

METHODS

Rats were trained to self-administer cocaine and later they were forced to abstain either in: (a) enriched environment or (b) isolation cage conditions to evaluate the effect of housing conditions on the drug-seeking behaviour and to assess changes concerning receptors in animals brain.

RESULTS

Our results show that exposure to enriched environment conditions strongly reduced active lever presses during cue-induced drug-seeking. At the neurochemical level, we demonstrated a significant increase in the dopamine D₂-like receptor density in the prefrontal cortex in animals following drug abstinence in isolation cage or enriched environment conditions, and the reduction in their density in the dorsal striatum provoked by isolation cage conditions. The metabotropic glutamate type 5 receptor density decreased only in the prefrontal cortex after isolation cage and enriched environment abstinence.

CONCLUSIONS

This study shows the different impacts caused by the type of housing conditions during abstinence from cocaine self-administration on drug-seeking behaviour in rats. The observed changes in the dopamine D₂-like and metabotropic glutamate type 5 receptor B_max and/or K_d values were brain-region specific and related to either pharmacological and/or motivational features of cocaine.

Using a Developmental Ecology Framework to Align Fear Neurobiology Across Species

Children's development is largely dependent on caregiving; when caregiving is disrupted, children are at increased risk for numerous poor outcomes, in particular psychopathology. Therefore, determining how caregivers regulate children's affective neurobiology is essential for understanding psychopathology etiology and prevention. Much of the research on affective functioning uses fear learning to map maturation trajectories, with both rodent and human studies contributing knowledge. Nonetheless, as no standard framework exists through which to interpret developmental effects across species, research often remains siloed, thus contributing to the current therapeutic impasse. Here, we propose a developmental ecology framework that attempts to understand fear in the ecological context of the child: their relationship with their parent. By referring to developmental goals that are shared across species (to attach to, then, ultimately, separate from the parent), this framework provides a common grounding from which fear systems and their dysfunction can be understood, thus advancing research on psychopathologies and their treatment.

Effects of neonatal inhalation exposure to ultrafine carbon particles on pathology and behavioral outcomes in C57BL/6J mice

Background

Recent epidemiological studies indicate early-life exposure to air pollution is associated with adverse neurodevelopmental outcomes. Previous studies investigating neonatal exposure to ambient fine and ultrafine particles have shown sex specific inflammation-linked pathological changes and protracted learning deficits. A potential contributor to the adverse phenotypes from developmental exposure to particulate matter observed in previous studies may be elemental carbon, a well-known contributor to pollution particulate. The present study is an evaluation of pathological and protracted behavioral alterations in adulthood following subacute neonatal exposure to ultrafine elemental carbon. C57BL/6J mice were exposed to ultrafine elemental carbon at 50 μg/m³ from postnatal days 4–7 and 10–13 for 4 h/day. Behavioral outcomes measured were locomotor activity, novel object recognition (short-term memory), elevated plus maze (anxiety-like behavior), fixed interval (FI) schedule of food reward (learning, timing) and differential reinforcement of low rate (DRL) schedule of food reward (impulsivity, inability to inhibit responding). Neuropathology was assessed by measures of inflammation (glial fibrillary-acidic protein), myelin basic protein expression in the corpus callosum, and lateral ventricle area.

Results

Twenty-four hours following the final exposure day, no significant differences in anogenital distance, body weight or central nervous system pathological markers were observed in offspring of either sex. Nor were significant changes observed in novel object recognition, elevated plus maze performance, FI, or DRL schedule-controlled behavior in either females or males.

Conclusion

The limited effect of neonatal exposure to ultrafine elemental carbon suggests this component of air pollution is not a substantial contributor to the behavioral alterations and neuropathology previously observed in response to ambient pollution particulate exposures. Rather, other more reactive constituent species, organic and/or inorganic, gas-phase components, or combinations of constituents may be involved. Defining these neurotoxic components is critical to the formulation of better animal models, more focused mechanistic assessments, and potential regulatory policies for air pollution.

Electronic supplementary material

The online version of this article (10.1186/s12989-019-0293-5) contains supplementary material, which is available to authorized users.

Limited developmental neurotoxicity from neonatal inhalation exposure to diesel exhaust particles in C57BL/6 mice

Background

Recent epidemiological studies indicate early-life exposure to pollution particulate is associated with adverse neurodevelopmental outcomes. The need is arising to evaluate the risks conferred by individual components and sources of air pollution to provide a framework for the regulation of the most relevant components for public health protection. Previous studies in rodent models have shown diesel particulate matter has neurotoxic potential and could be a health concern for neurodevelopment. The present study shows an evaluation of pathological and protracted behavioral alterations following neonatal exposure to aerosolized diesel exhaust particles (NIST SRM 1650b). The particular behavioral focus was on temporal control learning, a broad and fundamental cognitive domain in which reward delivery is contingent on a fixed interval schedule. For this purpose, C57BL/6 J mice were exposed to aerosolized NIST SRM 1650b, a well-characterized diesel particulate material, from postnatal days 4–7 and 10–13, for four hours per day. Pathological features, including glial fibrillary-acidic protein, myelin basic protein expression in the corpus callosum, and ventriculomegaly, as well as learning alterations were measured to determine the extent to which NIST SRM 1650b would induce developmental neurotoxicity.

Results

Twenty-four hours following exposure significant increases in glial-fibrillary acidic protein (GFAP) in the corpus callosum and cortex of exposed male mice were present. Additionally, the body weights of juvenile and early adult diesel particle exposed males were lower than controls, although the difference was not statistically significant. No treatment-related differences in males or females on overall locomotor activity or temporal learning during adulthood were observed in response to diesel particulate exposure.

Conclusion

While some sex and regional-specific pathological alterations in GFAP immunoreactivity suggestive of an inflammatory reaction to SRM 1650b were observed, the lack of protracted behavioral and pathological deficits suggests further clarity is needed on the developmental effects of diesel emissions prior to enacting regulatory guidelines.

Electronic supplementary material

The online version of this article (10.1186/s12989-018-0287-8) contains supplementary material, which is available to authorized users.

Relevance of Rodent Models of Depression in Clinical Practice: Can We Overcome the Obstacles in Translational Neuropsychiatry?

The diagnosis of a mental disorder generally depends on clinical observations and phenomenological symptoms reported by the patient. The definition of a given diagnosis is criteria based and relies on the ability to accurately interpret subjective symptoms and complex behavior. This type of diagnosis comprises a challenge to translate to reliable animal models, and these translational uncertainties hamper the development of new treatments. In this review, we will discuss how depressive-like behavior can be induced in rodents, and the relationship between these models and depression in humans. Specifically, we suggest similarities between triggers of depressive-like behavior in animal models and human conditions known to increase the risk of depression, for example exhaustion and bullying. Although we acknowledge the potential problems in comparing animal findings to human conditions, such comparisons are useful for understanding the complexity of depression, and we highlight the need to develop clinical diagnoses and animal models in parallel to overcome translational uncertainties.

The effects of early life stress on reward processing

Early life stress (ELS), in the form of childhood maltreatment, abuse, or neglect, increases the risk for psychiatric sequelae later in life. The neurobiology of response to early stress and of reward processing overlap substantially, leading to the prediction that reward processing may be a primary mediator of the effects of early life stress. We describe a growing body of literature investigating the effects of early life stressors on reward processing in animals and humans. Despite variation in the reviewed studies, an emerging pattern of results indicates that ELS results in deficits of ventral striatum-related functions of reward responsiveness and approach motivation, especially when the stressor is experienced in early in development. For stressors experienced later in the juvenile period and adolescence, the animal literature suggests an opposite effect, in which ELS results in increased hedonic drive. Future research in this area will help elucidate the transdiagnostic impact of early life stress, and therefore potentially identify and intervene with at-risk youth, prior to the emergence of clinical psychopathology.

Anhedonic behavior and γ-amino butyric acid during a sensitive period in female rats exposed to early adversity

Early life adversity increases depressive behavior that emerges during adolescence. Sensitive periods have been associated with fewer GABAergic interneurons, especially parvalbumin (PV), brain derived growth factor, and its receptor, TrkB. Here, maternal separation (MS) and social isolation (ISO) were used to establish a sensitive period for anhedonic depression using the learned helplessness (LH) paradigm. Female Sprague-Dawley rat pups underwent MS for 4-h/day or received typical care (CON) between postnatal days 2-20; for the ISO condition, separate cohorts were individually housed between days 20-40 or served as controls (CON2). Anhedonia was defined by dichotomizing subjects into two groups based on one standard deviation of the mean number of escapes for the CON group (<14). This approach categorized 22% of CON subjects and 44% of MS subjects as anhedonic (p < 0.05), similar to the prevalence in maltreated human populations. Only 12.5% of ISO rats met criterion versus 28.5% in CON2 rats. Levels of PV and TrkB were reduced in the amygdala and prelimbic prefrontal cortex (PFC) in MS rats with <14 escapes, but elevated in behaviorally resilient MS rats (>13 escapes). The number of escapes in MS subjects significantly correlated with PV and TrkB levels (PFC: r = 0.93 and 0.91 and amygdala: r = 0.63 and 0.81, respectively; n = 9), but not in CON/ISO/CON2 subjects. Calretinin, but not calbindin, was elevated in the amygdala of MS subjects. These data suggest that low levels of PV and TrkB double the risk for anhedonia in females with an MS history compared to normal adolescent females.

Maternal separation increases alcohol-drinking behaviour and reduces endocannabinoid levels in the mouse striatum and prefrontal cortex

Childhood adversity is associated with an increased risk of mood, anxiety and substance use disorders. Maternal separation is a reliable rodent model of early life adversity that leads to depression-like symptoms, which may increase the vulnerability to alcohol consumption during adolescence. However, the specific alterations in the pattern of alcohol consumption induced by maternal separation and the underlying molecular mechanisms are still unclear. The purpose of this study is to evaluate the long-term effects of maternal separation with early weaning (MSEW) on emotional and social behaviour, alcohol rewarding properties, and alcohol consumption, abstinence and relapse in adolescent male C57BL/6 mice. In addition, endocannabinoid and monoamine levels were analysed in discrete brain areas. Results showed that MSEW mice presented emotional alterations related to depressive-like behaviour and modified endocannabinoid levels in the striatum and the prefrontal cortex. MSEW mice also showed impairments in alcohol-induced conditioned place preference and higher alcohol intake in a model of binge drinking. Moreover, MSEW animals displayed a higher propensity to relapse in the two-bottle choice paradigm following a period of alcohol abstinence associated with reduced monoamine levels in the striatum. Such results indicate that exposure to early life stress increased the vulnerability to alcohol binge-drinking during adolescence, which may be partially explained by decreased sensitivity to alcohol rewarding properties and the ability to potentiate alcohol intake following a period of abstinence.

Early childhood stress exposure, reward pathways, and adult decision making

Individuals who have experienced high levels of stress in early childhood are at risk for a wide range of behavioral problems, yet the neurobiological processes underlying these associations are poorly understood. In this experiment, we uncover a potential mechanism leading to maladaptive decision making: altered brain activation during anticipation of rewards and losses. Individual differences in neural responses to cues of potential reward or loss were associated with early life stress and both laboratory and real-world measures of risk-taking behaviors. These effects were predicted only by childhood stress exposure, not by current levels of stress in the participants’ adult lives. Early life stress was assessed prospectively, when the participants were children, thereby avoiding the potential biases associated with adult retrospective recall.

Individuals who have experienced chronic and high levels of stress during their childhoods are at increased risk for a wide range of behavioral problems, yet the neurobiological mechanisms underlying this association are poorly understood. We measured the life circumstances of a community sample of school-aged children and then followed these children for a decade. Those from the highest and lowest quintiles of childhood stress exposure were invited to return to our laboratory as young adults, at which time we reassessed their life circumstances, acquired fMRI data during a reward-processing task, and tested their judgment and decision making. Individuals who experienced high levels of early life stress showed lower levels of brain activation when processing cues signaling potential loss and increased responsivity when actually experiencing losses. Specifically, those with high childhood stress had reduced activation in the posterior cingulate/precuneus, middle temporal gyrus, and superior occipital cortex during the anticipation of potential rewards; reduced activation in putamen and insula during the anticipation of potential losses; and increased left inferior frontal gyrus activation when experiencing an actual loss. These patterns of brain activity were associated with both laboratory and real-world measures of individuals’ risk taking in adulthood. Importantly, these effects were predicated only by childhood stress exposure and not by current levels of life stress.

Altered ventral striatal-medial prefrontal cortex resting-state connectivity mediates adolescent social problems after early institutional care

Early caregiving adversity is associated with increased risk for social difficulties. The ventral striatum and associated corticostriatal circuitry, which have demonstrated vulnerability to early exposures to adversity, are implicated in many aspects of social behavior, including social play, aggression, and valuation of social stimuli across development. Here, we used resting-state functional magnetic resonance imaging to assess the degree to which early caregiving adversity was associated with altered coritocostriatal resting connectivity in previously institutionalized youth (n = 41) relative to youth who were raised with their biological families from birth (n = 47), and the degree to which this connectivity was associated with parent-reported social problems. Using a seed-based approach, we observed increased positive coupling between the ventral striatum and anterior regions of medial prefrontal cortex (mPFC) in previously institutionalized youth. Stronger ventral striatum-mPFC coupling was associated with parent reports of social problems. A moderated-mediation analysis showed that ventral striatal-mPFC connectivity mediated group differences in social problems, and more so with increasing age. These findings show that early institutional care is associated with differences in resting-state connectivity between the ventral striatum and the mPFC, and this connectivity seems to play an increasingly important role in social behaviors as youth enter adolescence.

A neurocomputational investigation of reinforcement-based decision making as a candidate latent vulnerability mechanism in maltreated children

Alterations in reinforcement-based decision making may be associated with increased psychiatric vulnerability in children who have experienced maltreatment. A probabilistic passive avoidance task and a model-based functional magnetic resonance imaging analytic approach were implemented to assess the neurocomputational components underlying decision making: (a) reinforcement expectancies (the representation of the outcomes associated with a stimulus) and (b) prediction error signaling (the ability to detect the differences between expected and actual outcomes). There were three main findings. First, the maltreated group (n = 18; mean age = 13), relative to nonmaltreated peers (n = 19; mean age = 13), showed decreased activity during expected value processing in a widespread network commonly associated with reinforcement expectancies representation, including the striatum (especially the caudate), the orbitofrontal cortex, and medial temporal structures including the hippocampus and insula. Second, consistent with previously reported hyperresponsiveness to negative cues in the context of childhood abuse, the maltreated group showed increased prediction error signaling in the middle cingulate gyrus, somatosensory cortex, superior temporal gyrus, and thalamus. Third, the maltreated group showed increased activity in frontodorsal regions and in the putamen during expected value representation. These findings suggest that early adverse environments disrupt the development of decision-making processes, which in turn may compromise psychosocial functioning in ways that increase latent vulnerability to psychiatric disorder.