Effects of early adolescent environmental enrichment on cognitive dysfunction, prefrontal cortex development, and inflammatory cytokines after early life stress

Impacts of age and environment on postnatal microglial activity: Consequences for cognitive function following early life adversity

Early life adversity (ELA) increases the likelihood of later-life neuropsychiatric disorders and cognitive dysfunction. Importantly, ELA, neuropsychiatric disorders, and cognitive deficits all involve aberrant immune signaling. Microglia are the primary neuroimmune cells and regulate brain development. Microglia are particularly sensitive to early life insults, which can program their responses to future challenges. ELA in the form of maternal separation (MS) in rats alters later-life microglial morphology and the inflammatory profile of the prefrontal cortex, a region important for cognition. However, the role of microglial responses during MS in the development of later cognition is not known. Therefore, here we aimed to determine whether the presence of microglia during MS mediates long-term impacts on adult working memory. Clodronate liposomes were used to transiently deplete microglia from the brain, while empty liposomes were used as a control. We hypothesized that if microglia mediate the long-term impacts of ELA on working memory in adulthood, then depleting microglia during MS would prevent these deficits. Importantly, microglial function shifts throughout the neonatal period, so an exploratory investigation assessed whether depletion during the early versus late neonatal period had different effects on adult working memory. Surprisingly, empty liposome treatment during the early, but not late, postnatal period induced microglial activity changes that compounded with MS to impair working memory in females. In contrast, microglial depletion later in infancy impaired later life working memory in females, suggesting that microglial function during late infancy plays an important role in the development of cognitive function. Together, these findings suggest that microglia shift their sensitivity to early life insults across development. Our findings also highlight the potential for MS to impact some developmental processes only when compounded with additional neuroimmune challenges in a sex-dependent manner.

Prefrontal and Hippocampal Parvalbumin Interneurons in Animal Models for Schizophrenia: A Systematic Review and Meta-analysis

BACKGROUND

Consistent with postmortem findings in patients, most animal models for schizophrenia (SCZ) present abnormal levels of parvalbumin (PV), a marker of fast-spiking GABAergic interneurons, in the prefrontal cortex (PFC) and hippocampus (HIP). However, there are discrepancies in the literature. PV reductions lead to a functional loss of PV interneurons, which is proposed to underly SCZ symptoms. Given its complex etiology, different categories of animal models have been developed to study SCZ, which may distinctly impact PV levels in rodent brain areas.

STUDY DESIGN

We performed a quantitative meta-analysis on PV-positive cell number/density and expression levels in the PFC and HIP of animal models for SCZ based on pharmacological, neurodevelopmental, and genetic manipulations.

RESULTS

Our results confirmed that PV levels are significantly reduced in the PFC and HIP regardless of the animal model. By categorizing into subgroups, we found that all pharmacological models based on NMDA receptor antagonism decreased PV-positive cell number/density or PV expression levels in both brain areas examined. In neurodevelopmental models, abnormal PV levels were confirmed in both brain areas in maternal immune activation models and HIP of the methylazoxymethanol acetate model. In genetic models, negative effects were found in neuregulin 1 and ERBB4 mutant mice in both brain regions and the PFC of dysbindin mutant mice. Regarding sex differences, male rodents exhibited PV reductions in both brain regions only in pharmacological models, while few studies have been conducted in females.

CONCLUSION

Overall, our findings support deficits in prefrontal and hippocampal PV interneurons in animal models for SCZ.

Adolescent nonpharmacological interventions for early-life stress and their mechanisms

Those with a negative experience of psychosocial stress during the early stage of life not only have a high susceptibility of the psychiatric disorder in all phases of their life span, but they also demonstrate more severe symptoms and poorer response to treatment compared to those without a history of early-life stress. The interventions targeted to early-life stress may improve the effectiveness of treating and preventing psychiatric disorders. Brain regions associated with mood and cognition develop rapidly and own heightened plasticity during adolescence. So, manipulating nonpharmacological interventions in fewer side effects and higher acceptance during adolescence, which is a probable window of opportunity, may ameliorate or even reverse the constantly deteriorating impact of early-life stress. The present article reviews animal and people studies about adolescent nonpharmacological interventions for early-life stress. We aim to discuss whether those adolescent nonpharmacological interventions can promote individuals' psychological health who expose to early-life stress.

Adverse Childhood Experiences Predict Diurnal Cortisol Throughout Gestation

OBJECTIVE

Adverse childhood experiences (ACEs) are associated with negative prenatal and perinatal health outcomes and may, via these pathways, have intergenerational effects on child health and development. We examine the impact of ACEs on maternal salivary cortisol, a key measure of prenatal biology previously linked with pregnancy-related health outcomes.

METHODS

Leveraging assessments across three trimesters, we used linear mixed-effects models to analyze the influence of ACEs on maternal prenatal diurnal cortisol patterns in a diverse cohort of pregnant women (analytic sample, n = 207). Covariates included comorbid prenatal depression, psychiatric medications, and sociodemographic factors.

RESULTS

Maternal ACEs were significantly associated with flatter diurnal cortisol slopes (i.e., less steep decline), after adjusting for covariates, with effects consistent across gestation (estimate = 0.15, standard error = 0.06, p = .008).

CONCLUSIONS

ACEs experienced before pregnancy may have a robust and lasting influence on maternal prenatal hypothalamic-pituitary-adrenal activity throughout gestation, a key biological marker associated with perinatal and child health outcomes. The findings suggest one route of intergenerational transmission of early adverse experiences and underscore the potential value of assessing prepregnancy adverse experiences for promoting perinatal and maternal and child health.

Current Understanding of the Roles of Gut-Brain Axis in the Cognitive Deficits Caused by Perinatal Stress Exposure

The term 'perinatal environment' refers to the period surrounding birth, which plays a crucial role in brain development. It has been suggested that dynamic communication between the neuro-immune system and gut microbiota is essential in maintaining adequate brain function. This interaction depends on the mother's status during pregnancy and/or the newborn environment. Here, we show experimental and clinical evidence that indicates that the perinatal period is a critical window in which stress-induced immune activation and altered microbiota compositions produce lasting behavioral consequences, although a clear causative relationship has not yet been established. In addition, we discuss potential early treatments for preventing the deleterious effect of perinatal stress exposure. In this sense, early environmental enrichment exposure (including exercise) and melatonin use in the perinatal period could be valuable in improving the negative consequences of early adversities. The evidence presented in this review encourages the realization of studies investigating the beneficial role of melatonin administration and environmental enrichment exposure in mitigating cognitive alteration in offspring under perinatal stress exposure. On the other hand, direct evidence of microbiota restoration as the main mechanism behind the beneficial effects of this treatment has not been fully demonstrated and should be explored in future studies.

One day away from mum has lifelong consequences on brain and behaviour

This chapter presents a brief overview of attachment theory and discusses the importance of the neonatal period in shaping an individual's physiological and behavioural responses to stress later in life, with a focus on the role of the parent-infant relationship, particularly in rodents. In rodents, the role of maternal behaviours goes far beyond nutrition, thermoregulation and excretion, acting as hidden regulators of the pup's physiology and development. In this review, we will discuss the inhibitory role of specific maternal behaviours on the ACTH and corticosterone (CORT) stress response. The interest of our group to explore the long-term consequences of maternal deprivation for 24 h (DEP) at different ages (3 days and 11 days) in rats was sparked by its opposite effects on ACTH and CORT levels. In early adulthood, DEP3 animals (males and females alike) show greater negative impact on affective behaviours and stress related parameters than DEP11, indicating that the latter is more resilient in tests of anxiety-like behaviour. These findings create an opportunity to explore the neurobiological underpinnings of vulnerability and resilience to stress-related disorders. The chapter also provides a brief historical overview and highlights the relevance of attachment theory, and how DEP helps to understand the effects of childhood parental loss as a risk factor for depression, schizophrenia, and PTSD in both childhood and adulthood. Furthermore, we present the concept of environmental enrichment (EE), its effects on stress responses and related behavioural changes and its benefits for rats previously subjected to DEP, along with the clinical implications of DEP and EE.

Early life adversity as a risk factor for cognitive impairment and Alzheimer's disease

Neurological conditions, including cognitive impairment and Alzheimer's disease (AD), impose a huge burden on society, affecting millions of people globally. In addition to genetic factors, recent studies indicate that environmental and experiential factors may contribute to the pathogenesis of these diseases. Early life adversity (ELA) has a profound impact on brain function and health later in life. In rodent models, exposure to ELA results in specific cognitive deficits and aggravated AD pathology. Extensive concerns have been raised regarding the higher risk of developing cognitive impairments in people with a history of ELA. In this review, we scrutinize findings from human and animal studies focusing on the connection of ELA with cognitive impairment and AD. These discoveries suggest that ELA, especially at early postnatal stages, increases susceptibility to cognitive impairment and AD later in life. In terms of mechanisms, ELA could lead to dysregulation of the hypothalamus-pituitary-adrenal axis, altered gut microbiome, persistent inflammation, oligodendrocyte dysfunction, hypomyelination, and aberrant adult hippocampal neurogenesis. Crosstalks among these events may synergistically contribute to cognitive impairment later in life. Additionally, we discuss several interventions that may alleviate adverse consequences of ELA. Further investigation into this crucial area will help improve ELA management and reduce the burden of related neurological conditions.

The effects of preventative cannabidiol in a male neuregulin 1 mouse model of schizophrenia

Cannabidiol (CBD) is a non-intoxicating cannabinoid with antipsychotic-like properties, however it’s potential to prevent schizophrenia development has not been thoroughly investigated. Brain maturation during adolescence creates a window where CBD could potentially limit the development of schizophrenia. The neuregulin 1 transmembrane domain heterozygous (Nrg1 TM HET) mutant mouse shows face, predictive, and construct validity for schizophrenia. Here we sought to determine if CBD given in adolescence could prevent the development of the schizophrenia-relevant phenotype, as well as susceptibility to the psychoactive cannabinoid Δ⁹-tetrahydrocannabinol (THC) in Nrg1 TM HET mice. Adolescent male Nrg1 mutants and wild type-like (WT) animals were administered 30 mg/kg CBD i.p. daily for seven weeks, and were tested for locomotion, social behavior, sensorimotor gating and cognition, and sensitivity to acute THC-induced behaviors. GAD67, GluA1, and NMDAR1 protein levels were measured in the hippocampus, striatum, and prefrontal cortex. Chronic adolescent CBD increased locomotion in animals regardless of genotype, was anxiolytic, and increased social behavior when animals were tested for their acute THC response. CBD did not alleviate the schizophrenia-relevant hyperlocomotive phenotype of Nrg1 mutants, nor deficits in social behaviors. Nrg1 mutant mice treated with CBD and THC showed no habituation to a startle pulse, suggesting CBD increased vulnerability to the startle habituation-reducing effects of THC in mutant mice. CBD increased levels of GluA1, but reduced levels of GAD67 in the hippocampus of Nrg1 mutants. These results suggest adolescent CBD is not effective as a preventative of schizophrenia-relevant behavioral deficits in mutants and may actually contribute to pathological changes in the brain that increase sensitivity to THC in particular behavioral domains.

Behavioral effects of environmental enrichment on male and female wistar rats with early life stress experiences

Exposure to adverse childhood experiences or early life stress experiences (ELSs) increase the risk of non-adaptive behaviors and psychopathology in adulthood. Environmental enrichment (EE) has been proposed to minimize these effects. The vast number of methodological variations in animal studies underscores the lack of systematicity in the studies and the need for a detailed understanding of how enrichment interacts with other variables. Here we evaluate the effects of environmental enrichment in male and female Wistar rats exposed to adverse early life experiences (prenatal, postnatal, and combined) on emotional (elevated plus maze), social (social interaction chamber), memory (Morris water maze) and flexibility tasks. Our results—collected from PND 51 to 64—confirmed: 1) the positive effect of environmental enrichment (PND 28–49) on anxiety-like behaviors in animals submitted to ELSs. These effects depended on type of experience and type of enrichment: foraging enrichment reduced anxiety-like behaviors in animals with prenatal and postnatal stress but increased them in animals without ELSs. This effect was sex-dependent: females showed lower anxiety compared to males. Our data also indicated that females exposed to prenatal and postnatal stress had lower anxious responses than males in the same conditions; 2) no differences were found for social interactions; 3) concerning memory, there was a significant interaction between the three factors: A significant interaction for males with prenatal stress was observed for foraging enrichment, while physical enrichment was positive for males with postnatal stress; d) regarding cognitive flexibility, a positive effect of EE was found in animals exposed to adverse ELSs: animals with combined stress and exposed to physical enrichment showed a higher index of cognitive flexibility than those not exposed to enrichment. Yet, within animals with no EE, those exposed to combined stress showed lower flexibility than those exposed to both prenatal stress and no stress. On the other hand, animals with prenatal stress and exposed to foraging-type enrichment showed lower cognitive flexibility than those with no EE. The prenatal stress-inducing conditions used here 5) did not induced fetal or maternal problems and 6) did not induced changes in the volume of the dentate gyrus of the hippocampus.

Effects of early life adversities upon memory processes and cognition in rodent models

Exposure to stressors in early postnatal life induces long-lasting modifications in brainfunction.Thisplasticity,an essential characteristic of the brain that enables adaptation to the environment, may also induce impairments in some psychophysiological functions, including learning and memory. Early life stress (ELS) has long-term effects on thehypothalamic-pituitary-adrenal axisresponse to stressors, and has been reported to lead toneuroinflammation,altered levelsof neurotrophic factors, modifications inneurogenesis andsynaptic plasticity,with changes in neurotransmitter systems and network functioning. In this review, we focus on early postnatal stress in animal models and their effects on learning and memory.Many studies have reported ELS-induced impairments in different types of memories, including spatial memory, fear memory, recognition (both for objects and social) memory, working memory and reversal learning. Studies are not always in agreement, however, no effects, or sometimes facilitation, being reported, depending on the nature and intensity of the early intervention, as well as the age when the outcome was evaluated and the sex of the animals. When considering processes occurring after consolidation, related with memory maintenance or modification, there are a very reduced number of reports. Future studies addressing the mechanisms underlying memory changes for ELS should shed some light on the understanding of the different effects induced by stressors of different types and intensities on cognitive functions.

Impact of stress on inhibitory neuronal circuits, our tribute to Bruce McEwen

This manuscript is dedicated to the memory of Bruce S. McEwen, to commemorate the impact he had on how we understand stress and neuronal plasticity, and the profound influence he exerted on our scientific careers. The focus of this review is the impact of stressors on inhibitory circuits, particularly those of the limbic system, but we also consider other regions affected by these adverse experiences. We revise the effects of acute and chronic stress during different stages of development and lifespan, taking into account the influence of the sex of the animals. We review first the influence of stress on the physiology of inhibitory neurons and on the expression of molecules related directly to GABAergic neurotransmission, and then focus on specific interneuron subpopulations, particularly on parvalbumin and somatostatin expressing cells. Then we analyze the effects of stress on molecules and structures related to the plasticity of inhibitory neurons: the polysialylated form of the neural cell adhesion molecule and perineuronal nets. Finally, we review the potential of antidepressants or environmental manipulations to revert the effects of stress on inhibitory circuits.

The Relative Benefits of Environmental Enrichment on Learning and Memory are Greater When Stressed: A Meta-analysis of Interactions in Rodents

Environmental enrichment ("EE") is expected to alleviate the negative effects of stress on cognitive performance. However, there are complexities associated with interpreting interactions that obscure determining the benefit EE may play in mitigating the negative effects of stress. To clarify these complexities, we conducted a systematic review with meta-analysis on the main and interactive effects of EE and stress on learning and memory in rodents. We show that EE and stress interact 'synergistically' where EE provides a greater relative benefit to stressed individuals compared to those reared in conventional housing. Importantly, EE can fully-compensate for the negative effects of stress where stressed individuals with EE performed equally to enriched individuals without a stress manipulation. Additionally, we show the importance of other mediating factors, including the order of treatment exposure, duration and type of stress, type of EE, and type of cognitive assays used. This study not only quantifies the interactions between EE and stress, but also provides a clear example for how to conduct and interpret meta-analysis of interactions.

Glia-Driven Brain Circuit Refinement Is Altered by Early-Life Adversity: Behavioral Outcomes

Early-life adversity (ELA), often clinically referred to as "adverse childhood experiences (ACE)," is the exposure to stress-inducing events in childhood that can result in poor health outcomes. ELA negatively affects neurodevelopment in children and adolescents resulting in several behavioral deficits and increasing the risk of developing a myriad of neuropsychiatric disorders later in life. The neurobiological mechanisms by which ELA alters neurodevelopment in childhood have been the focus of numerous reviews. However, a comprehensive review of the mechanisms affecting adolescent neurodevelopment (i.e., synaptic pruning and myelination) is lacking. Synaptic pruning and myelination are glia-driven processes that are imperative for brain circuit refinement during the transition from adolescence to adulthood. Failure to optimize brain circuitry between key brain structures involved in learning and memory, such as the hippocampus and prefrontal cortex, leads to the emergence of maladaptive behaviors including increased anxiety or reduced executive function. As such, we review preclinical and clinical literature to explore the immediate and lasting effects of ELA on brain circuit development and refinement. Finally, we describe a number of therapeutic interventions best-suited to support adolescent neurodevelopment in children with a history of ELA.

Sex Differences in Affective Dysfunction and Alterations in Parvalbumin in Rodent Models of Early Life Adversity

The early life environment markedly influences brain and behavioral development, with adverse experiences associated with increased risk of anxiety and depressive phenotypes, particularly in females. Indeed, early life adversity (ELA) in humans (i.e., caregiver deprivation, maltreatment) and rodents (i.e., maternal separation, resource scarcity) is associated with sex-specific emergence of anxious and depressive behaviors. Although these disorders show clear sex differences in humans, little attention has been paid toward evaluating sex as a biological variable in models of affective dysfunction; however, recent rodent work suggests sex-specific effects. Two widely used rodent models of ELA approximate caregiver deprivation (i.e., maternal separation) and resource scarcity (i.e., limited bedding). While these approaches model aspects of ELA experienced in humans, they span different portions of the pre-weaning developmental period and may therefore differentially contribute to underlying mechanistic risk. This is borne out in the literature, where evidence suggests differences in trajectories of behavior depending on the type of ELA and/or sex; however, the neural underpinning of these differences is not well understood. Because anxiety and depression are thought to involve dysregulation in the balance of excitatory and inhibitory signaling in ELA-vulnerable brain regions (e.g., prefrontal cortex, amygdala, hippocampus), outcomes are likely driven by alterations in local and/or circuit-specific inhibitory activity. The most abundant GABAergic subtypes in the brain, accounting for approximately 40% of inhibitory neurons, contain the calcium-binding protein Parvalbumin (PV). As PV-expressing neurons have perisomatic and proximal dendritic targets on pyramidal neurons, they are well-positioned to regulate excitatory/inhibitory balance. Recent evidence suggests that PV outcomes following ELA are sex, age, and region-specific and may be influenced by the type and timing of ELA. Here, we suggest the possibility of a combined role of PV and sex hormones driving differences in behavioral outcomes associated with affective dysfunction following ELA. This review evaluates the literature across models of ELA to characterize neural (PV) and behavioral (anxiety- and depressive-like) outcomes as a function of sex and age. Additionally, we detail a putative mechanistic role of PV on ELA-related outcomes and discuss evidence suggesting hormone influences on PV expression/function which may help to explain sex differences in ELA outcomes.

Maternal separation leads to regional hippocampal microglial activation and alters the behavior in the adolescence in a sex-specific manner

Early life adversities during childhood (such as maltreatment, abuse, neglect, or parental deprivation) may increase the vulnerability to cognitive disturbances and emotional disorders in both, adolescence and adulthood. Maternal separation (MS) is a widely used model to study stress-related changes in brain and behavior in rodents. In this study, we investigated the effect of MS (postnatal day 2–14, 3 ​h/day) in both, female and male adolescent mice. Specifically, we evaluated (i) the spatial working memory, anxiety and depressive-like behavior, (ii) the hippocampal synaptic gene expression, and (iii) the hippocampal neuroinflammatory response.

Our results show that MS significantly increased depressive-like behavior in adolescent female mice and altered the spatial memory in adolescent male mice. In addition, MS led to decreased expression of genes related to synaptic function (5ht6r, Synaptophysin, and Cox-2) and induced an exacerbated microglial activation in dentate gyrus (DG), CA1, and CA3. However, while the levels of hippocampal inflammatory cytokines were not modified by MS, they did follow a sex-specific expression in adolescent mice.

Taken together, our results suggest that MS induces long-term changes in hippocampal microglia and synaptic gene expression, alters the spatial memory, and induces depressive-like behavior in the adolescent mice, in a sex-specific manner.

In wildtype adolescent mice (6 weeks of age):•

Maternal separation alters spatial working memory in males and induces depressive-like behavior in females.

•

Maternal separation changes hippocampal synaptic gene expression.

•

Maternal separation activates microglia in dentate gyrus, CA1, and CA3 but does not affect hippocampal cytokine levels. However, males present higher levels of cytokines compared to females.

Parvalbumin interneuron alterations in stress-related mood disorders: A systematic review

Stress-related psychiatric disorders including depression involve complex cellular and molecular changes in the brain, and GABAergic signaling dysfunction is increasingly implicated in the etiology of mood disorders. Parvalbumin (PV)-expressing neurons are fast-spiking interneurons that, among other roles, coordinate synchronous neuronal firing. Mounting evidence suggests that the PV neuron phenotype is altered by stress and in mood disorders. In this systematic review, we assessed PV interneuron alterations in psychiatric disorders as reported in human postmortem brain studies and animal models of environmental stress. This review aims to 1) comprehensively catalog evidence of PV cell function in mood disorders (humans) and stress models of mood disorders (animals); 2) analyze the strength of evidence of PV interneuron alterations in various brain regions in humans and rodents; 3) determine whether the modulating effect of antidepressant treatment, physical exercise, and environmental enrichment on stress in animals associates with particular effects on PV function; and 4) use this information to guide future research avenues. Its principal findings, derived mainly from rodent studies, are that stress-related changes in PV cells are only reported in a minority of studies, that positive findings are region-, age-, sex-, and stress recency-dependent, and that antidepressants protect from stress-induced apparent PV cell loss. These observations do not currently translate well to humans, although the postmortem literature on the topic remains limited.

The impact of environmental and social factors on learning abilities: a meta-analysis

Since the 1950s, researchers have examined how differences in the social and asocial environment affect learning in rats, mice, and, more recently, a variety of other species. Despite this large body of research, little has been done to synthesize these findings and to examine if social and asocial environmental factors have consistent effects on cognitive abilities, and if so, what aspects of these factors have greater or lesser impact. Here, we conducted a systematic review and meta-analysis examining how different external environmental features, including the social environment, impact learning (both speed of acquisition and performance). Using 531 mean-differences from 176 published articles across 27 species (with studies on rats and mice being most prominent) we conducted phylogenetically corrected mixed-effects models that reveal: (i) an average absolute effect size |d| = 0.55 and directional effect size d = 0.34; (ii) interventions manipulating the asocial environment result in larger effects than social interventions alone; and (iii) the length of the intervention is a significant predictor of effect size, with longer interventions resulting in larger effects. Additionally, much of the variation in effect size remained unexplained, possibly suggesting that species differ widely in how they are affected by environmental interventions due to varying ecological and evolutionary histories. Overall our results suggest that social and asocial environmental factors do significantly affect learning, but these effects are highly variable and perhaps not always as predicted. Most notably, the type (social or asocial) and length of interventions are important in determining the strength of the effect.

Mini-review: Brain energy metabolism and its role in animal models of depression, bipolar disorder, schizophrenia and autism

Mitochondria are cellular organelles essential for energy metabolism and antioxidant defense. Mitochondrial impairment is implicated in many psychiatric disorders, including depression, bipolar disorder, schizophrenia, and autism. To characterize and eventually find effective treatments of bioenergetic impairment in psychiatric disease, researchers find animal models indispensable. The present review focuses on brain energetics in several environmental, genetic, drug-induced, and surgery-induced animal models of depression, bipolar disorder, schizophrenia, and autism. Most reported deficits included decreased activity in the electron transport chain, increased oxidative damage, decreased antioxidant defense, decreased ATP levels, and decreased mitochondrial potential. Models of depression, bipolar disorder, schizophrenia, and autism shared many bioenergetic deficits. This is in concordance with the absence of a disease-specific brain energy phenotype in human patients. Unfortunately, due to the absence of null results in examined literature, indicative of reporting bias, we refrain from making generalized conclusions. Present review can be a valuable tool for comparing current findings, generating more targeted hypotheses, and selecting fitting models for further preclinical research.

Maternal Deprivation in Rats Decreases the Expression of Interneuron Markers in the Neocortex and Hippocampus

Early life stress has profound effects on the development of the central nervous system. We exposed 9-day-old rat pups to a 24 h maternal deprivation (MD) and sacrificed them as young adults (60-day-old), with the aim to study the effects of early stress on forebrain circuitry. We estimated numbers of various immunohistochemically defined interneuron subpopulations in several neocortical regions and in the hippocampus. MD rats showed reduced numbers of parvalbumin-expressing interneurons in the CA1 region of the hippocampus and in the prefrontal cortex, compared with controls. Numbers of reelin-expressing and calretinin-expressing interneurons were also reduced in the CA1 and CA3 hippocampal areas, but unaltered in the neocortex of MD rats. The number of calbinin-expressing interneurons in the neocortex was similar in the MD rats compared with controls. We analyzed cell death in 15-day-old rats after MD and found no difference compared to control rats. Thus, our results more likely reflect the downregulation of markers than the actual loss of interneurons. To investigate synaptic activity in the hippocampus we immunostained for glutamatergic and inhibitory vesicular transporters. The number of inhibitory synapses was decreased in the CA1 and CA3 regions of the hippocampus in MD rats, with the normal number of excitatory synapses. Our results indicate complex, cell type-specific, and region-specific alterations in the inhibitory circuitry induced by maternal deprivation. Such alterations may underlie symptoms of MD at the behavioral level and possibly contribute to mechanisms by which early life stress causes neuropsychiatric disorders, such as schizophrenia.

Timely N-Acetyl-Cysteine and Environmental Enrichment Rescue Oxidative Stress-Induced Parvalbumin Interneuron Impairments via MMP9/RAGE Pathway: A Translational Approach for Early Intervention in Psychosis

Research in schizophrenia (SZ) emphasizes the need for new therapeutic approaches based on antioxidant/anti-inflammatory compounds and psycho-social therapy. A hallmark of SZ is a dysfunction of parvalbumin-expressing fast-spiking interneurons (PVI), which are essential for neuronal synchrony during sensory/cognitive processing. Oxidative stress and inflammation during early brain development, as observed in SZ, affect PVI maturation. We compared the efficacy of N-acetyl-cysteine (NAC) and/or environmental enrichment (EE) provided during juvenile and/or adolescent periods in rescuing PVI impairments induced by an additional oxidative insult during childhood in a transgenic mouse model with gluthation deficit (Gclm KO), relevant for SZ. We tested whether this rescue was promoted by the inhibition of MMP9/RAGE mechanism, both in the mouse model and in early psychosis (EP) patients, enrolled in a double-blind, randomized, placebo-controlled clinical trial of NAC supplementation for 6 months. We show that a sequential combination of NAC+EE applied after an early-life oxidative insult recovers integrity and function of PVI network in adult Gclm KO, via the inhibition of MMP9/RAGE. Six-month NAC treatment in EP patients reduces plasma sRAGE in association with increased prefrontal GABA, improvement of cognition and clinical symptoms, suggesting similar neuroprotective mechanisms. The sequential combination of NAC+EE reverses long-lasting effects of an early oxidative insult on PVI/perineuronal net (PNN) through the inhibition of MMP9/RAGE mechanism. In analogy, patients vulnerable to early-life insults could benefit from a combined pharmacological and psycho-social therapy.

Roles of inflammation in intrinsic pathophysiology and antipsychotic drug-induced metabolic disturbances of schizophrenia

Schizophrenia is a debilitating psychiatric illness that remains poorly understood. While the bulk of symptomatology has classically been associated with disrupted brain functioning, accumulating evidence demonstrates that schizophrenia is characterized by systemic inflammation and disturbances in metabolism. Indeed, metabolic disease is a major determinant of the high mortality rate associated with schizophrenia. Antipsychotic drugs (APDs) have revolutionized management of psychosis, making it possible to rapidly control psychotic symptoms. This has ultimately reduced relapse rates of psychotic episodes and improved overall quality of life for people with schizophrenia. However, long-term APD use has also been associated with significant metabolic disturbances including weight gain, dysglycemia, and worsening of the underlying cardiometabolic disease intrinsic to schizophrenia. While the mechanisms for these intrinsic and medication-induced metabolic effects remain unclear, inflammation appears to play a key role. Here, we review the evidence for roles of inflammatory mechanisms in the disease features of schizophrenia and how these mechanisms interact with APD treatment. We also discuss the effects of common inflammatory mediators on metabolic disease. Then, we review the evidence of intrinsic and APD-mediated effects on systemic inflammation in schizophrenia. Finally, we speculate about possible treatment strategies. Developing an improved understanding of inflammatory processes in schizophrenia may therefore introduce new, more effective options for treating not only schizophrenia but also primary metabolic disorders.

How stress physically re-shapes the brain: Impact on brain cell shapes, numbers and connections in psychiatric disorders

Severe stress is among the most robust risk factors for the development of psychiatric disorders. Imaging studies indicate that life stress is integral to shaping the human brain, especially regions involved in processing the stress response. Although this is likely underpinned by changes to the cytoarchitecture of cellular networks in the brain, we are yet to clearly understand how these define a role for stress in human psychopathology. In this review, we consolidate evidence of macro-structural morphometric changes and the cellular mechanisms that likely underlie them. Focusing on stress-sensitive regions of the brain, we illustrate how stress throughout life may lead to persistent remodelling of the both neurons and glia in cellular networks and how these may lead to psychopathology. We support that greater translation of cellular alterations to human cohorts will support parsing the psychological sequalae of severe stress and improve our understanding of how stress shapes the human brain. This will remain a critical step for improving treatment interventions and prevention outcomes.

Effects of Early-Life Stress, Postnatal Diet Modulation and Long-Term Western-Style Diet on Peripheral and Central Inflammatory Markers

Early-life stress (ES) exposure increases the risk of developing obesity. Breastfeeding can markedly decrease this risk, and it is thought that the physical properties of the lipid droplets in human milk contribute to this benefit. A concept infant milk formula (IMF) has been developed that mimics these physical properties of human milk (Nuturis®, N-IMF). Previously, we have shown that N-IMF reduces, while ES increases, western-style diet (WSD)-induced fat accumulation in mice. Peripheral and central inflammation are considered to be important for obesity development. We therefore set out to test the effects of ES, Nuturis® and WSD on adipose tissue inflammatory gene expression and microglia in the arcuate nucleus of the hypothalamus. ES was induced in mice by limiting the nesting and bedding material from postnatal day (P) 2 to P9. Mice were fed a standard IMF (S-IMF) or N-IMF from P16 to P42, followed by a standard diet (STD) or WSD until P230. ES modulated adipose tissue inflammatory gene expression early in life, while N-IMF had lasting effects into adulthood. Centrally, ES led to a higher microglia density and more amoeboid microglia at P9. In adulthood, WSD increased the number of amoeboid microglia, and while ES exposure increased microglia coverage, Nuturis® reduced the numbers of amoeboid microglia upon the WSD challenge. These results highlight the impact of the early environment on central and peripheral inflammatory profiles, which may be key in the vulnerability to develop metabolic derangements later in life.

Risky emotional family environment in childhood and depression-related cytokines in adulthood: The protective role of compassion

BACKGROUND

Previously, compassion has been found to protect against depressive symptoms, while emotional adversities in childhood are suggested to increase inflammatory responses. The current study investigated (a) whether emotional family environment in childhood predicts levels of such cytokines in adulthood that are previously found to be elevated in depression (interleukin [IL]-2, IL-6, IL-1b, monocyte chemoattractant protein-1, interferon-gamma [IFN-γ], and tumor necrosis factor alpha [TNF-α]) and (b) whether these associations are modified by compassion in adulthood.

METHODS

The participants (N = 1,198-1,523) came from the prospective population-based Young Finns data. Emotional family environment and parental socioeconomic factors were evaluated in 1980; participants' compassion in 2001; and participants' cytokine levels and adulthood covariates in 2007.

RESULTS

Risky emotional family environment in childhood predicted higher levels of IL-2, IL-6, IFN-γ, and TNF-α in adulthood. Additionally, there were significant interaction effects between compassion and emotional risk in childhood, when predicting IL-2, IL-6, and TNF-α. Specifically, individuals who grew up in a risky emotional family environment had on average higher levels of IL-2, IL-6, and TNF-α in adulthood when combined with low compassion.

CONCLUSIONS

In individuals coming from risky emotional family environments, high compassion for others may protect against elevated levels of cytokines previously linked with depression.

Early life stress due to repeated maternal separation alters the working memory acquisition brain functional network

Unfortunately, adverse environments in early life are frequently found in most human populations. Early life stress leads to diverse cognitive impairments, some of them related to learning and memory and executive functions such as working memory (WM). We employ an animal model of early stress using repeated maternal separation (MS) for 4 h a day on 21 consecutive days, pre-weaning. In adulthood, we tested their spatial WM using the Morris water maze. MS subjects showed a marked delay in the acquisition of the task. In addition, we explored brain energy oxidative metabolism and found an increase in cytochrome c oxidase (CCO) activity in the cingulate cortex, anterior thalamus, and supramammillary areas, indicating an intense effort to successfully solve the WM task. However, decreased CCO activity was found in the medial-medial mammillary nucleus in MS animals, which would partially explain the delayed acquisition of the WM task. Further studies are needed to explore the long-term alterations produced by early stress. LAY SUMMARY A stressful environment caused by the separation of baby rats from the mother for several hours a day in the first stages of postnatal life can be devastating to brain cells, making them look for alternative sources of energy, among other changes. These alterations in brain functional networks would lead to cognitive impairments such as the delayed acquisition of new learning and strategies.

Sex differences in prefrontal cortex microglia morphology: Impact of a two-hit model of adversity throughout development

Neuroimmune mechanisms play critical roles in brain development and can be impacted by early life adversity. Microglia are the resident immune cells in the brain, with both sex-specific and region-specific developmental profiles. Since early life adversity is associated with several neuropsychiatric disorders with developmental pathogeneses, here we investigated the degree to which maternal separation (MS) impacted microglia over development. Microglia are dynamic cells that alter their morphology in accordance with their functions and in response to stressors. While males and females reportedly display different microglial morphology in several brain regions over development and following immune and psychological challenges, little is known about such differences in the prefrontal cortex (PFC), which regulates several early life adversity-attributable disorders. Additionally, little is known about the potential for early life adversity to prime microglia for later immune challenges. In the current study, male and female rats were exposed to MS followed by lipopolysaccharide administration in juvenility or adolescence. The prelimbic and infralimbic PFC were then separately analyzed for microglial density and morphology. Typically developing males expressed smaller soma and less arborization than females in juvenility, but larger soma than females in adolescence. MS led to fewer microglia in the infralimbic PFC of adolescent males. Both MS and lipopolysaccharide administration affected morphological characteristics in juvenile males and females, with MS exposure leading to a greater increase in soma size following lipopolysaccharide. Interestingly, effects of MS and lipopolysaccharide were not observed in adolescence, while notable sex differences in PFC microglial morphology were apparent. Taken together, these findings provide insight into how PFC microglia may differentially respond to challenges over development in males and females.

Enduring neuroimmunological consequences of developmental experiences: From vulnerability to resilience

The immune system is crucial for normal neuronal development and function (neuroimmune system). Both immune and neuronal systems undergo significant postnatal development and are sensitive to developmental programming by environmental experiences. Negative experiences from infection to psychological stress at a range of different time points (in utero to adolescence) can permanently alter the function of the neuroimmune system: given its prominent role in normal brain development and function this dysregulation may increase vulnerability to psychiatric illness. In contrast, positive experiences such as exercise and environmental enrichment are protective and can promote resilience, even restoring the detrimental effects of negative experiences on the neuroimmune system. This suggests the neuroimmune system is a viable therapeutic target for treatment and prevention of psychiatric illnesses, especially those related to stress. In this review we will summarise the main cells, molecules and functions of the immune system in general and with specific reference to central nervous system development and function. We will then discuss the effects of negative and positive environmental experiences, especially during development, in programming the long-term functioning of the neuroimmune system. Finally, we will review the sparse but growing literature on sex differences in neuroimmune development and response to environmental experiences.

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The immune system is essential for development and function of the central nervous system (neuroimmune system)

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Environmental experiences can permanently alter neuroimmune function and associated brain development

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Altered neuroimmune function following negative developmental experiences may play a role in psychiatric illnesses

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Positive experiences can promote resilience and rescue the effects of negative experiences on the neuroimmune system

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The neuroimmune system is therefore a viable therapeutic target for preventing and treating psychiatric illnesses

Neuroimmunological effects of early life experiences

Exposure to adverse experiences during development increases the risk of psychiatric illness later in life. Growing evidence suggests a role for the neuroimmune system in this relationship. There is now substantial evidence that the immune system is critical for normal brain development and behaviour, and responds to environmental perturbations experienced early in life. Severe or chronic stress results in dysregulated neuroimmune function, concomitant with abnormal brain morphology and function. Positive experiences including environmental enrichment and exercise exert the opposite effect, promoting normal brain and immune function even in the face of early life stress. The neuroimmune system may therefore provide a viable target for prevention and treatment of psychiatric illness. This review will briefly summarise the neuroimmune system in brain development and function, and review the effects of stress and positive environmental experiences during development on neuroimmune function. There are also significant sex differences in how the neuroimmune system responds to environmental experiences early in life, which we will briefly review.

The Inflamed Brain in Schizophrenia: The Convergence of Genetic and Environmental Risk Factors That Lead to Uncontrolled Neuroinflammation

Schizophrenia is a disorder with a heterogeneous etiology involving complex interplay between genetic and environmental risk factors. The immune system is now known to play vital roles in nervous system function and pathology through regulating neuronal and glial development, synaptic plasticity, and behavior. In this regard, the immune system is positioned as a common link between the seemingly diverse genetic and environmental risk factors for schizophrenia. Synthesizing information about how the immune-brain axis is affected by multiple factors and how these factors might interact in schizophrenia is necessary to better understand the pathogenesis of this disease. Such knowledge will aid in the development of more translatable animal models that may lead to effective therapeutic interventions. Here, we provide an overview of the genetic risk factors for schizophrenia that modulate immune function. We also explore environmental factors for schizophrenia including exposure to pollution, gut dysbiosis, maternal immune activation and early-life stress, and how the consequences of these risk factors are linked to microglial function and dysfunction. We also propose that morphological and signaling deficits of the blood-brain barrier, as observed in some individuals with schizophrenia, can act as a gateway between peripheral and central nervous system inflammation, thus affecting microglia in their essential functions. Finally, we describe the diverse roles that microglia play in response to neuroinflammation and their impact on brain development and homeostasis, as well as schizophrenia pathophysiology.

Alleviation of Depression by Glucagon-Like Peptide 1 Through the Regulation of Neuroinflammation, Neurotransmitters, Neurogenesis, and Synaptic Function

Depression has emerged as a major cause of mortality globally. Many studies have reported risk factors and mechanisms associated with depression, but it is as yet unclear how these findings can be applied to the treatment and prevention of this disorder. The onset and recurrence of depression have been linked to diverse metabolic factors, including hyperglycemia, dyslipidemia, and insulin resistance. Recent studies have suggested that depression is accompanied by memory loss as well as depressive mood. Thus, many researchers have highlighted the relationship between depressive behavior and metabolic alterations from various perspectives. Glucagon-like peptide-1 (GLP-1), which is secreted from gut cells and hindbrain areas, has been studied in metabolic diseases such as obesity and diabetes, and was shown to control glucose metabolism and insulin resistance. Recently, GLP-1 was highlighted as a regulator of diverse pathways, but its potential as the therapeutic target of depressive disorder was not described comprehensively. Therefore, in this review, we focused on the potential of GLP-1 modulation in depression.

Effects of Early and Current Environmental Enrichment on Behavior and Growth in Pigs

Enriched environments are known to beneficially affect the behavior of pigs, as compared with barren pens. The influence of enrichment may, however, depend on pigs' early life housing experiences. The aim of this study was to investigate the long-term effects of early and later life environmental enrichment on behavior and growth in pigs with different coping styles. Pigs were housed in either barren pens or in larger pens enriched with rooting substrates from birth, and half of them experienced a housing switch, i.e., a loss or gain of enrichment, at 7 weeks of age, creating four treatment groups. Home pen behavior and body weight were recorded until 19 weeks of age. Pigs were classified as reactive or proactive based on a backtest at 2 weeks of age. Enrichment increased time spent exploring, chewing, and play and decreased oral manipulation of penmates and pen-directed exploring and chewing. Behavior of pigs that switched from barren to enriched pens or vice versa reflected not only their actual environment, but also their early life housing. As early and later life enrichment affected most behaviors in opposite directions, effects of enrichment, or lack thereof, after the switch were more pronounced in pigs that had experienced a different early life condition. For instance, pigs experiencing an upgrade from barren to enriched pens seemed to "catch-up" by showing more exploration and play. Conversely, pigs exposed to a downgrade displayed more oral manipulation of penmates than ones kept barren throughout, which particularly held for pigs with a reactive coping style. Effects of early life and current housing on several other behaviors depended on coping style too. Pigs housed in enriched conditions appeared better able to cope with weaning than barren housed pigs, as they gained more weight and had higher feed intake post-weaning. Barren housed pigs had a lower body weight than enriched pigs just before the switch, after which growth was mainly determined by actual housing, with enriched kept pigs having a higher feed intake and body weight. Thus, not only current housing conditions, but also a (mis)match with the early life environment may affect behavior and growth of pigs.

Region-specific effects of maternal separation on oxidative stress accumulation in parvalbumin neurons of male and female rats

Early life adversity in humans is linked to cognitive deficits and increased risk of mental illnesses, including depression, bipolar disorder, and schizophrenia, with evidence for different vulnerabilities in men versus women. Modeling early life adversity in rodents shows similar neuropsychological deficits that may partially be driven by sex-dependent dysfunction in parvalbumin (PV) interneurons in the prefrontal cortex (PFC), hippocampus (HPC), and basolateral amygdala (BLA). Research demonstrates that PV interneurons are particularly susceptible to oxidative stress; therefore, accumulation of oxidative damage may drive PV dysfunction following early life adversity. The goal of this study was to quantify oxidative stress accumulation in PV neurons in rats exposed to maternal separation (MS). Pups were separated from their dam and littermates for 4 h per day from postnatal day (P)2 to 20. Serial sections from the PFC, HPC, and BLA of juvenile (P20) rats of both sexes were immunohistochemically stained with antibodies against PV and 8-oxo-dG, a marker for oxidative DNA damage. PV cell counts, colocalization with 8-oxo-dG, and intensity of each signal were measured in each region to determine the effects of MS and establish whether MS-induced oxidative damage varies between sexes. A significant increase in colocalization of PV and 8-oxo-dG was found in the PFC and HPC, indicating increased oxidative stress in that cell population following MS. Region-specific sex differences were also revealed in the PFC, BLA, and HPC. These data identify oxidative stress during juvenility as a potential mechanism mediating PV dysfunction in individuals with a history of early life adversity.

The Effects of Early Life Stress, Postnatal Diet Modulation, and Long-Term Western-Style Diet on Later-Life Metabolic and Cognitive Outcomes

Early life stress (ES) increases the risk to develop metabolic and brain disorders in adulthood. Breastfeeding (exclusivity and duration) is associated with improved metabolic and neurocognitive health outcomes, and the physical properties of the dietary lipids may contribute to this. Here, we tested whether early life exposure to dietary lipids mimicking some physical characteristics of breastmilk (i.e., large, phospholipid-coated lipid droplets; Concept Nuturis® infant milk formula (N-IMF)), could protect against ES-induced metabolic and brain abnormalities under standard circumstances, and in response to prolonged Western-style diet (WSD) in adulthood. ES was induced by exposing mice to limited nesting material from postnatal day (P) 2 to P9. From P16 to P42, male offspring were fed a standard IMF (S-IMF) or N-IMF, followed by either standard rodent diet (SD) or WSD until P230. We then assessed body composition development, fat mass, metabolic hormones, hippocampus-dependent cognitive function, and neurogenesis (proliferation and survival). Prolonged WSD resulted in an obesogenic phenotype at P230, which was not modulated by previous ES or N-IMF exposure. Nevertheless, ES and N-IMF modulated the effect of WSD on neurogenesis at P230, without affecting cognitive function, highlighting programming effects of the early life environment on the hippocampal response to later life challenges at a structural level.

Microglial Function in the Effects of Early-Life Stress on Brain and Behavioral Development

The putative effects of early-life stress (ELS) on later behavior and neurobiology have been widely investigated. Recently, microglia have been implicated in mediating some of the effects of ELS on behavior. In this review, findings from preclinical and clinical literature with a specific focus on microglial alterations induced by the exposure to ELS (i.e., exposure to behavioral stressors or environmental agents and infection) are summarized. These studies were utilized to interpret changes in developmental trajectories based on the time at which the stress occurred, as well as the paradigm used. ELS and microglial alterations were found to be associated with a wide array of deficits including cognitive performance, memory, reward processing, and processing of social stimuli. Four general conclusions emerged: (1) ELS interferes with microglial developmental programs, including their proliferation and death and their phagocytic activity; (2) this can affect neuronal and non-neuronal developmental processes, which are dynamic during development and for which microglial activity is instrumental; (3) the effects are extremely dependent on the time point at which the investigation is carried out; and (4) both pre- and postnatal ELS can prime microglial reactivity, indicating a long-lasting alteration, which has been implicated in behavioral abnormalities later in life.

Early-life stress and inflammation: A systematic review of a key experimental approach in rodents

Repeated maternal separation is the most widely used pre-clinical approach to investigate the relationship between early-life chronic stress and its neuropsychiatric and physical consequences. In this systematic review, we identified 46 studies that conducted repeated maternal separation or single-episode maternal separation and reported measurements of interleukin-1b, interleukin-6, interleukin-10, tumour necrosis factor-alpha, or microglia activation and density. We report that in the short-term and in the context of later-life stress, repeated maternal separation has pro-inflammatory immune consequences in diverse tissues. Repeated maternal separation animals exhibit greater microglial activation and elevated pro-inflammatory cytokine signalling in key brain regions implicated in human psychiatric disorders. Notably, repeated maternal separation generally has no long-term effect on cytokine expression in any tissue in the absence of later-life stress. These observations suggest that the elevated inflammatory signalling that has been reported in humans with a history of early-life stress may be the joint consequence of ongoing stressor exposure together with potentiated neural and/or immune responsiveness to stressors. Finally, our findings provide detailed guidance for future studies interrogating the causal roles of early-life stress and inflammation in disorders such as major depression.

Region-specific Effects of Maternal Separation on Perineuronal Net and Parvalbumin-expressing Interneuron Formation in Male and Female Rats

Early life experiences play a vital role in contributing to healthy brain development. Adverse experiences have a lasting impact on the prefrontal cortex (PFC) and basolateral amygdala (BLA), brain regions associated with emotion regulation. Early life adversity via maternal separation (MS) has sex-specific effects on expression of parvalbumin (PV), which is expressed in fast-spiking GABAergic interneurons that are preferentially enwrapped by perineuronal nets (PNNs). Importantly, PNN formation coincides with the closure of developmental critical periods and regulates PV-expressing interneuron activity. Since aberrant PNN organization has been reported following adverse experiences in adolescent and adult rats, we investigated the impact of adversity early in life in the form of MS on the developing brain. Rat pups were separated from their dams for 4 h per day from postnatal day (P) 2-20. Tissue sections from juvenile (P20), adolescent (P40), and early adult (P70) animals containing the PFC and BLA were fluorescently stained to visualize Wisteria floribunda agglutinin+ PNNs and PV-expressing interneurons, and density and intensity was quantified. Our results confirm past reports that PFC PNNs form gradually throughout development; however, PNN density plateaus in adolescence, while intensity continues to increase into adulthood. Importantly, MS delays PNN formation in the prelimbic PFC and results in sex-specific aberrations in PNN structural integrity that do not appear until adulthood. The present findings reveal sex-, age-, and region-specific effects of early life adversity on PNN and PV maturation, implicating neuroplastic alterations following early life adversity that may be associated with sex differences in psychopathology and resilience.

Acute neuroinflammation elicited by TLR-3 systemic activation combined with early life stress induces working memory impairments in male adolescent mice

Toll-like Receptors (TLRs) are implicated with the pathogenesis of cognitive impairment induced by inflammation. Early life stress is associated with altered trajectories of neuroimmune signaling with implications for cognitive development. However, effects of TLR-3 activation on early life stress-related cognitive outcomes are understudied. We investigated the effects of maternal separation (MS) during postnatal development and a viral immune challenge during adolescence on working memory performance. BALB/c mice exposed to MS were separated from their dams daily for 180-min from postnatal day (PND) 2 to 15. At PND 45, animals were challenged with a single i.p. injection of either Poly (I:C) or sterile saline, and then subjected to a spatial working memory test in a Y-maze apparatus. Gene expression was determined by qPCR. Protein levels of oxidative stress markers were also assessed. A single peripheral administration of a TLR-3 agonist was able to induce working memory impairments in adolescent mice exposed to MS. At a molecular level, exposure to MS was associated with lower mRNA levels of Tlr3 in the medial prefrontal cortex (mPFC). However, when MS animals were exposed to Poly (I:C), a more robust activation of Tlr3, Il6 and Nfkb1 gene transcription was observed in these mice compared with control animals. These modifications did not result in oxidative stress. Finally, higher mRNA levels of Nfkb1 in the mPFC were correlated with lower working memory performance, suggesting that altered NF-κB signaling might be related with poor cognitive functioning. These results have implications for how ELS affects neuroimmune signaling in the mPFC.

Maternal separation in rodents: a journey from gut to brain and nutritional perspectives

The developmental period constitutes a critical window of sensitivity to stress. Indeed, early-life adversity increases the risk to develop psychiatric diseases, but also gastrointestinal disorders such as the irritable bowel syndrome at adulthood. In the past decade, there has been huge interest in the gut-brain axis, especially as regards stress-related emotional behaviours. Animal models of early-life adversity, in particular, maternal separation (MS) in rodents, demonstrate lasting deleterious effects on both the gut and the brain. Here, we review the effects of MS on both systems with a focus on stress-related behaviours. In addition, we discuss more recent findings showing the impact of gut-directed interventions, including nutrition with pre- and probiotics, illustrating the role played by gut microbiota in mediating the long-term effects of MS. Overall, preclinical studies suggest that nutritional approaches with pro- and prebiotics may constitute safe and efficient strategies to attenuate the effects of early-life stress on the gut-brain axis. Further research is required to understand the complex mechanisms underlying gut-brain interaction dysfunctions after early-life stress as well as to determine the beneficial impact of gut-directed strategies in a context of early-life adversity in human subjects.

The Role of Chemokines in the Pathophysiology of Major Depressive Disorder

Major depressive disorder (MDD) is a debilitating condition, whose high prevalence and multisymptomatic nature set its standing as a leading contributor to global disability. To better understand this psychiatric disease, various pathophysiological mechanisms have been proposed, including changes in monoaminergic neurotransmission, imbalance of excitatory and inhibitory signaling in the brain, hyperactivity of the hypothalamic-pituitary-adrenal (HPA) axis, and abnormalities in normal neurogenesis. While previous findings led to a deeper understanding of the disease, the pathogenesis of MDD has not yet been elucidated. Accumulating evidence has confirmed the association between chronic inflammation and MDD, which is manifested by increased levels of the C-reactive protein, as well as pro-inflammatory cytokines, such as Interleukin 1 beta, Interleukin 6, and the Tumor necrosis factor alpha. Furthermore, recent findings have implicated a related family of cytokines with chemotactic properties, known collectively as chemokines, in many neuroimmune processes relevant to psychiatric disorders. Chemokines are small (8–12 kDa) chemotactic cytokines, which are known to play roles in direct chemotaxis induction, leukocyte and macrophage migration, and inflammatory response propagation. The inflammatory chemokines possess the ability to induce migration of immune cells to the infection site, whereas their homeostatic chemokine counterparts are responsible for recruiting cells for their repair and maintenance. To further support the role of chemokines as central elements to healthy bodily function, recent studies suggest that these proteins demonstrate novel, brain-specific mechanisms including the modulation of neuroendocrine functions, chemotaxis, cell adhesion, and neuroinflammation. Elevated levels of chemokines in patient-derived serum have been detected in individuals diagnosed with major depressive disorder, bipolar disorder, and schizophrenia. Furthermore, despite the considerable heterogeneity of experimental samples and methodologies, existing biomarker studies have clearly demonstrated the important role of chemokines in the pathophysiology of psychiatric disorders. The purpose of this review is to summarize the data from contemporary experimental and clinical studies, and to evaluate available evidence for the role of chemokines in the central nervous system (CNS) under physiological and pathophysiological conditions. In light of recent results, chemokines could be considered as possible peripheral markers of psychiatric disorders, and/or targets for treating depressive disorders.

Effects of early life stress on cocaine conditioning and AMPA receptor composition are sex-specific and driven by TNF

Exposure to early life adversity can predispose adolescents to the formation of substance abuse disorders. In rodents, early stressors such as repeated maternal separation (MS) impact AMPAR activity in the prefrontal cortex (PFC) and nucleus accumbens (NAc), regions involved in drug-cue association after cocaine-induced conditioned place preference (CPP). Notably, previous reports suggest that the pro-inflammatory cytokine tumor necrosis factor (TNF) regulates AMPAR subunit composition; increased TNF levels are reported to reduce GluA2-positive AMPARs. Since MS can elevate adolescent TNF levels, the stressor may therefore alter AMPAR subunit composition via neuroimmune signaling, thereby affecting cocaine-induced CPP. We tested the specific role of soluble TNF in MS-induced GluA2 loss and cocaine-induced CPP with biologic disruption of TNF signaling. TNF gene and protein expression were elevated in both PFC and NAc of MS males, but not females. GluA2 expression was reduced in both regions in only male MS rats, and systemic treatment with either ibudilast - a phosphodiesterase inhibitor, or XPro1595 - a blood-brain barrier-permeable blocker of soluble TNF - reversed such loss. MS males also formed greater preference for a cocaine-paired environment, the expression of which returned to control levels after XPro1595 administration. These data suggest a sex-specific mechanistic link between TNF signaling and changes in GluA2 expression and drug-cue conditioning, thereby providing further evidence for a role of MS and neuro-immune activity in cortical and striatal AMPAR changes. Moreover, manipulation of the TNF signaling pathway represents a novel approach for influencing response to reinforcing effects of drug use.

Pharmacological restoration of gut barrier function in stressed neonates partially reverses long-term alterations associated with maternal separation

RATIONALE

Intestinal permeability plays an important role in gut-brain axis communication. Recent studies indicate that intestinal permeability increases in neonate pups during maternal separation (MS).

OBJECTIVES

The present study aims to determine whether pharmacological inhibition of myosin light chain kinase (MLCK), which regulates tight junction contraction and controls intestinal permeability, in stressed neonates, protects against the long-term effects of MS.

METHODS

Male Wistar rats were exposed to MS (3 h per day from post-natal day (PND)2 to PND14) or left undisturbed and received daily intraperitoneal injection of a MLCK inhibitor (ML-7, 5 mg/kg) or vehicle during the same period. At adulthood, emotional behaviors, corticosterone response to stress, and gut microbiota composition were analyzed.

RESULTS

ML-7 restored gut barrier function in MS rats specifically during the neonatal period. Remarkably, ML-7 prevented MS-induced sexual reward-seeking impairment and reversed the alteration of corticosterone response to stress at adulthood. The effects of ML-7 were accompanied by the normalization of the abundance of members of Lachnospiraceae, Clostridiales, Desulfovibrio, Bacteroidales, Enterorhabdus, and Bifidobacterium in the feces of MS rats at adulthood.

CONCLUSIONS

Altogether, our work suggests that improvement of intestinal barrier defects during development may alleviate some of the long-term effects of early-life stress and provides new insight on brain-gut axis communication in a context of stress.

The Mitochondrion as Potential Interface in Early-Life Stress Brain Programming

Mitochondria play a central role in cellular energy-generating processes and are master regulators of cell life. They provide the energy necessary to reinstate and sustain homeostasis in response to stress, and to launch energy intensive adaptation programs to ensure an organism’s survival and future well-being. By this means, mitochondria are particularly apt to mediate brain programming by early-life stress (ELS) and to serve at the same time as subcellular substrate in the programming process. With a focus on mitochondria’s integrated role in metabolism, steroidogenesis and oxidative stress, we review current findings on altered mitochondrial function in the brain, the placenta and peripheral blood cells following ELS-dependent programming in rodents and recent insights from humans exposed to early life adversity (ELA). Concluding, we propose a role of the mitochondrion as subcellular intersection point connecting ELS, brain programming and mental well-being, and a role as a potential site for therapeutic interventions in individuals exposed to severe ELS.

Sex Differences in the Neuroimmune System

While sex differences in the peripheral immune response have been studied extensively, sex differences in the neuroimmune response, including glial activation and associated cytokine production in the brain, is a recently emerging field. Advances in our understanding of sex differences in the neuroimmune response have important implications for understanding how neural circuits are shaped during early brain development, how activation of the immune system may impact cognitive function and behavior, and how inflammation may be associated with the risk of mental health disorders that have strong sex-biases. The goal of this mini review is to highlight recent work in the field of sex differences in neuroimmune function, with a particular focus on how microglia function is influenced by age and sex hormone exposure.

Stress, alcohol and infection during early development: A brief review of common outcomes and mechanisms

Although stress is an adaptive physiological response to deal with adverse conditions, its occurrence during the early stages of life, such as infancy or adolescence, can induce adaptations in multiple physiological systems, including the reproductive axis, the hypothalamic-pituitary-adrenal (HPA) axis, the limbic cortex and the immune system. These early changes have consequences in adult life, as seen in the physiological and behavioural responses to stress. This review highlights the impact of several stress challenges incurred at various stages of development (perinatal, juvenile, adolescent periods) and how the developmental timing of early-life stress confers unique physiological adaptations that may persist across the lifespan. In doing so, we emphasise how intrinsic sex differences in the stress response might contribute to sex-specific vulnerabilities, the molecular processes underlying stress in the adult, and potential therapeutic interventions to mitigate the effects of early stage stress, including the novel molecular mechanism of SUMOylation as a possible key target of HPA regulation during early-life stress.

Adolescent environmental enrichment prevents behavioral and physiological sequelae of adolescent chronic stress in female (but not male) rats

The late adolescent period is characterized by marked neurodevelopmental and endocrine fluctuations in the transition to early adulthood. Adolescents are highly responsive to the external environment, which enhances their ability to adapt and recover from challenges when given nurturing influences, but also makes them vulnerable to aberrant development when exposed to prolonged adverse situations. Female rats are particularly sensitive to the effects of chronic stress in adolescence, which manifests as passive coping strategies and blunted hypothalamo-pituitary adrenocortical (HPA) stress responses in adulthood. We sought to intervene by exposing adolescent rats to environmental enrichment (EE) immediately prior to and during chronic stress, hypothesizing that EE would minimize or prevent the long-term effects of stress that emerge in adult females. To test this, we exposed male and female rats to EE on postnatal days (PND) 33-60 and implemented chronic variable stress (CVS) on PND 40-60. CVS consisted of twice-daily unpredictable stressors. Experimental groups included: CVS/unenriched, unstressed/EE, CVS/EE and unstressed/unenriched (n = 10 of each sex/group). In adulthood, we measured behavior in the open field test and forced swim test (FST) and collected blood samples following the FST. We found that environmental enrichment given during the adolescent period prevented the chronic stress-induced transition to passive coping in the FST and reversed decreases in peak adrenocortical responsiveness observed in adult females. Adolescent enrichment had little to no effect on males or unstressed females tested in adulthood, indicating that beneficial effects are specific to females that were exposed to chronic stress.

Alcohol, psychomotor-stimulants and behaviour: methodological considerations in preclinical models of early-life stress

BACKGROUND

In order to assess the risk associated with early-life stress, there has been an increase in the amount of preclinical studies investigating early-life stress. There are many challenges associated with investigating early-life stress in animal models and ensuring that such models are appropriate and clinically relevant.

OBJECTIVES

The purpose of this review is to highlight the methodological considerations in the design of preclinical studies investigating the effects of early-life stress on alcohol and psychomotor-stimulant intake and behaviour.

METHODS

The protocols employed for exploring early-life stress were investigated and summarised. Experimental variables include animals, stress models, and endpoints employed.

RESULTS

The findings in this paper suggest that there is little consistency among these studies and so the interpretation of these results may not be as clinically relevant as previously thought.

CONCLUSION

The standardisation of these simple stress procedures means that results will be more comparable between studies and that results generated will give us a more robust understanding of what can and may be happening in the human and veterinary clinic.

Beneficial effects of environmental enrichment on behavior, stress reactivity and synaptophysin/BDNF expression in hippocampus following early life stress

Exposure to environmental enrichment can beneficially influence the behavior and enhance synaptic plasticity. The aim of the present study was to investigate the mediated effects of environmental enrichment on postnatal stress-associated impact with regard to behavior, stress reactivity as well as synaptic plasticity changes in the dorsal hippocampus. Wistar rat pups were submitted to a 3 h maternal separation (MS) protocol during postnatal days 1-21, while another group was left undisturbed. On postnatal day 23, a subgroup from each rearing condition (maternal separation, no-maternal separation) was housed in enriched environmental conditions until postnatal day 65 (6 weeks duration). At approximately three months of age, adult rats underwent behavioral testing to evaluate anxiety (Elevated Plus Maze), locomotion (Open Field Test), spatial learning and memory (Morris Water Maze) as well as non-spatial recognition memory (Novel Object Recognition Test). After completion of behavioral testing, blood samples were taken for evaluation of stress-induced plasma corticosterone using an enzyme-linked immunosorbent assay (ELISA), while immunofluorescence was applied to evaluate hippocampal BDNF and synaptophysin expression in dorsal hippocampus. We found that environmental enrichment protected against the effects of maternal separation as indicated by the lower anxiety levels and the reversal of spatial memory deficits compared to animals housed in standard conditions. These changes were associated with increased BDNF and synaptophysin expression in the hippocampus. Regarding the neuroendocrine response to stress, while exposure to an acute stressor potentiated corticosterone increases in maternally-separated rats, environmental enrichment of these rats prevented this effect. The current study aimed at investigating the compensatory role of enriched environment against the negative outcomes of adverse experiences early in life concurrently on emotional and cognitive behaviors, HPA function and neuroplasticity markers.

Influence of early stress on memory reconsolidation: Implications for post-traumatic stress disorder treatment

Post-traumatic stress disorder (PTSD) is a common consequence of exposure to a life-threatening event. Currently, pharmacological treatments are limited by high rates of relapse, and novel treatment approaches are needed. We have recently demonstrated that propranolol, a β-adrenergic antagonist, inhibited aversive memory reconsolidation in animals. Following this, in an open-label study 70% of patients with PTSD treated with propranolol during reactivation of traumatic memory exhibited full remission. However, the reason why 30% of these patients did not respond positively to propranolol treatment is still unclear. One of the major candidates as factor of treatment resistance is the patient's early-life traumatic history. To test the role of this factor, mice with pre- or postnatal stress are being tested in fear conditioning and in a new behavioral task, the "city-like", specifically designed as a mouse model of PTSD. After reactivation of the traumatic event, mice received propranolol injection to block the noradrenergic system during memory reconsolidation. Results show that, in the “city-like” test, control mice strongly avoided the shock compartment but also the compartments containing cues associated with the electric shocks. Injection of propranolol after reactivation greatly reduced the memory of the traumatic event, but this effect was not present when mice had received pre- or postnatal stress. Moreover, propranolol produced only a very weak effect in the fear conditioning test, and never changed the corticosterone level whatever the behavioral experiment. Taken together our results suggest that our new behavioural paradigm is well adapted to PTSD study in mice, and that early stress exposure may have an impact on propranolol PTSD treatment outcome. These data are critical to understanding the effect of propranolol treatment, in order to improve the therapeutic protocol currently used in humans.

Implications of the gut microbiota in vulnerability to the social avoidance effects of chronic social defeat in male mice

Appreciable evidence suggests that perturbations within the gut microbiome and the immune system may play a key role in the pathogenesis of depression stemming from earlier stressful experiences. In the present investigation we examined whether microbial changes in cecum contents were associated with social avoidance behaviors, a feature of depression, and pro-inflammatory variations among socially stressed mice. Male C57BL/6 mice experienced social defeat or a control condition once a day for 10 consecutive days. Social avoidance behaviors were examined three weeks after the last defeat or control episode and blood, brain, and cecum contents were collected 24h afterward for the determination of corticosterone, pro-inflammatory cytokines, and microbial populations. Mice that were most susceptible to the behavioral effects of chronic social defeat (reflected by severe social avoidance behaviors) displayed the greatest changes within particular sets of bacteria at the phylum and genus taxonomic ranks. Although plasma and brain cytokines were not significantly altered in socially defeated mice, changes in the mRNA expression of interleukin (IL)-1β and IL-6 within the prefrontal cortex were associated with elevated abundance of Flavobacterium spp. and reduced abundance of Turicibacter spp., which were also strongly correlated to social avoidance severity. Although at this time a causal connection cannot be inferred, these results point to the possibility that specific clusters of bacterial communities in cecum contents may be linked to vulnerability to social deficits stemming from prolonged social stressor experiences.

Early-life stress lastingly alters the neuroinflammatory response to amyloid pathology in an Alzheimer's disease mouse model

Exposure to stress during the sensitive period of early-life increases the risk to develop cognitive impairments and psychopathology later in life. In addition, early-life stress (ES) exposure, next to genetic causes, has been proposed to modulate the development and progression of Alzheimer's disease (AD), however evidence for this hypothesis is currently lacking. We here tested whether ES modulates progression of AD-related neuropathology and assessed the possible contribution of neuroinflammatory factors in this. We subjected wild-type (WT) and transgenic APP/PS1 mice, as a model for amyloid neuropathology, to chronic ES from postnatal day (P)2 to P9. We next studied how ES exposure affected; 1) amyloid β (Aβ) pathology at an early (4month old) and at a more advanced pathological (10month old) stage, 2) neuroinflammatory mediators immediately after ES exposure as well as in adult WT mice, and 3) the neuroinflammatory response in relation to Aβ neuropathology. ES exposure resulted in a reduction of cell-associated amyloid in 4month old APP/PS1 mice, but in an exacerbation of Aβ plaque load at 10months of age, demonstrating that ES affects Aβ load in the hippocampus in an age-dependent manner. Interestingly, ES modulated various neuroinflammatory mediators in the hippocampus of WT mice as well as in response to Aβ neuropathology. In WT mice, immediately following ES exposure (P9), Iba1-immunopositive microglia exhibited reduced complexity and hippocampal interleukin (IL)-1β expression was increased. In contrast, microglial Iba1 and CD68 were increased and hippocampal IL-6 expression was decreased at 4months, while these changes resolved by 10months of age. Finally, Aβ neuropathology triggered a neuroinflammatory response in APP/PS1 mice that was altered after ES exposure. APP/PS1 mice exhibited increased CD68 expression at 4months, which was further enhanced by ES, whereas the microglial response to Aβ neuropathology, as measured by Iba1 and CD11b, was less prominent after ES at 10months of age. Finally, the hippocampus appears to be more vulnerable for these ES-induced effects, since ES did not affect Aβ neuropathology and neuroinflammation in the entorhinal cortex of adult ES exposed mice. Overall, our results demonstrate that ES exposure has both immediate and lasting effects on the neuroinflammatory response. In the context of AD, such alterations in neuroinflammation might contribute to aggravated neuropathology in ES exposed mice, hence altering disease progression. This indicates that, at least in a genetic context, ES could aggravate AD pathology.