Early postnatal handling alters glucocorticoid receptor concentrations in selected brain regions

Developmental Neuroendocrinology of Early-Life Stress: Impact on Child Development and Behavior

Our internal balance, or homeostasis, is threatened or perceived as threatened by stressful stimuli, the stressors. The stress system is a highly conserved system that adjusts homeostasis to the resting state. Through the concurrent activation of the hypothalamic-pituitary-adrenal axis and the locus coeruleus/norepinephrine-autonomic nervous systems, the stress system provides the appropriate physical and behavioral responses, collectively termed as "stress response", to restore homeostasis. If the stress response is prolonged, excessive or even inadequate, several acute or chronic stress-related pathologic conditions may develop in childhood, adolescence and adult life. On the other hand, earlylife exposure to stressors has been recognized as a major contributing factor underlying the pathogenesis of non-communicable disorders, including neurodevelopmental disorders. Accumulating evidence suggests that early-life stress has been associated with an increased risk for attention deficit hyperactivity disorder and autism spectrum disorder in the offspring, although findings are still controversial. Nevertheless, at the molecular level, early-life stressors alter the chemical structure of cytosines located in the regulatory regions of genes, mostly through the addition of methyl groups. These epigenetic modifications result in the suppression of gene expression without changing the DNA sequence. In addition to DNA methylation, several lines of evidence support the role of non-coding RNAs in the evolving field of epigenetics. In this review article, we present the anatomical and functional components of the stress system, discuss the proper, in terms of quality and quantity, stress response, and provide an update on the impact of early-life stress on child development and behavior.

Sex differences in hippocampal structural plasticity and glycosaminoglycan disaccharide levels after neonatal handling

In this study we investigated the effects of a neonatal handling protocol that mimics the handling of sham control pups in protocols of neonatal exposure to brain insults on dendritic arborization and glycosaminoglycan (GAG) levels in the developing brain. GAGs are long, unbranched polysaccharides, consisting of repeating disaccharide units that can be modified by sulfation at specific sites and are involved in modulating neuronal plasticity during brain development. In this study, male and female Sprague-Dawley rats underwent neonatal handling daily between post-natal day (PD)4 and PD9, with brains analyzed on PD9. Neuronal morphology and morphometric analysis of the apical and basal dendritic trees of CA1 hippocampal pyramidal neurons were carried out by Golgi-Cox staining followed by neuron tracing and analysis with the software Neurolucida. Chondroitin sulfate (CS)-, Hyaluronic Acid (HA)-, and Heparan Sulfate (HS)-GAG disaccharide levels were quantified in the hippocampus by Liquid Chromatography/Mass Spectrometry analyses. We found sex by neonatal handling interactions on several parameters of CA1 pyramidal neuron morphology and in the levels of HS-GAGs, with females, but not males, showing an increase in both dendritic arborization and HS-GAG levels. We also observed increased expression of glucocorticoid receptor gene Nr3c1 in the hippocampus of both males and females following neonatal handling suggesting that both sexes experienced a similar stress during the handling procedure. This is the first study to show sex differences in two parameters of brain plasticity, CA1 neuron morphology and HS-GAG levels, following handling stress in neonatal rats.

A comprehensive approach to modeling maternal immune activation in rodents

Prenatal brain development is a highly orchestrated process, making it a very vulnerable window to perturbations. Maternal stress and subsequent inflammation during pregnancy leads to a state referred to as, maternal immune activation (MIA). If persistent, MIA can pose as a significant risk factor for the manifestation of neurodevelopmental disorders (NDDs) such as autism spectrum disorder and schizophrenia. To further elucidate this association between MIA and NDD risk, rodent models have been used extensively across laboratories for many years. However, there are few uniform approaches for rodent MIA models which make not only comparisons between studies difficult, but some established approaches come with limitations that can affect experimental outcomes. Here, we provide researchers with a comprehensive review of common experimental variables and potential limitations that should be considered when designing an MIA study based in a rodent model. Experimental variables discussed include: innate immune stimulation using poly I:C and LPS, environmental gestational stress paradigms, rodent diet composition and sterilization, rodent strain, neonatal handling, and the inclusion of sex-specific MIA offspring analyses. We discuss how some aspects of these variables have potential to make a profound impact on MIA data interpretation and reproducibility.

Early life adversity shapes neural circuit function during sensitive postnatal developmental periods

Early life adversity (ELA) is a major risk factor for mental illness, but the neurobiological mechanisms by which ELA increases the risk for future psychopathology are still poorly understood. Brain development is particularly malleable during prenatal and early postnatal life, when complex neural circuits are being formed and refined through an interplay of excitatory and inhibitory neural input, synaptogenesis, synaptic pruning, myelination, and neurogenesis. Adversity that influences these processes during sensitive periods of development can thus have long-lasting and pervasive effects on neural circuit maturation. In this review, we will discuss clinical and preclinical evidence for the impact of ELA on neural circuit formation with a focus on the early postnatal period, and how long-lasting impairments in these circuits can affect future behavior. We provide converging evidence from human and animal studies on how ELA alters the functional development of brain regions, neural circuits, and neurotransmitter systems that are crucial for cognition and affective behavior, including the hippocampus, the hypothalamus-pituitary-adrenal (HPA) axis, neural networks of fear responses and cognition, and the serotonin (5-HT) system. We also discuss how gene-by-environment (GxE) interactions can determine individual differences in susceptibility and resilience to ELA, as well as molecular pathways by which ELA regulates neural circuit development, for which we emphasize epigenetic mechanisms. Understanding the molecular and neurobiological mechanisms underlying ELA effects on brain function and psychopathology during early postnatal sensitive periods may have great potential to advance strategies to better treat or prevent psychiatric disorders that have their origin early in life.

Maternal deprivation effect on morphine-induced CPP is related to changes in Opioid receptors in selected rat brain regions (hippocampus, prefrontal cortex, and nucleus accumbens)

Early-life environmental conditions affect offspring's development. Maternal deprivation (MD) can induce persistent changes that give rise to neuropsychiatric diseases including substance abuse disorders. However, long-lasting mechanisms that determine vulnerability to drug addiction remain unknown. We hypothesized that MD could induce changes in Opioid system, HPA (hypothalamic-pituitary-adrenal) axis, and BDNF (brain-derived neurotrophic factor), so may be involved in the drug abuse in later life. Male offspring of Wistar rats (n=8 per group) were subjected to 3h of daily MD during postnatal days 1-14. In adulthood, morphine-induced CPP (conditioned place preference) was investigated using two doses of morphine (3 and 5mg/kg). Serum corticosterone level was measured by ELISA method. The expression level of genes in selected brain regions (hippocampus, prefrontal cortex, and nucleus accumbens) was determined by qPCR (quantitative PCR). A greater morphine-induced CPP was observed in MD rats with 3 and 5mg/kg morphine compared to controls. MD group had a higher corticosterone level. A significant decrease was observed in the expression of BDNF gene (in all of the selected brain regions) and GR (glucocorticoid receptor) gene (in the hippocampus and nucleus accumbens) in MD rats. Also, a significant increase in the expression of μ Opioid receptor (in all of the selected brain regions) and κ Opioid receptor (in the prefrontal cortex and nucleus accumbens) was observed in MD rats. Our results suggest that MD induces alterations in the HPA axis function, BDNF level, and Opioid receptors system that enhance vulnerability to morphine at adulthood.

Steroid hormones and hippocampal neurogenesis in the adult mammalian brain

Hippocampal neurogenesis persists across the lifespan in many species, including rodents and humans, and is associated with cognitive performance and the pathogenesis of neurodegenerative disease and psychiatric disorders. Neurogenesis is modulated by steroid hormones that change across development and differ between the sexes in rodents and humans. Here, we discuss the effects of stress and glucocorticoid exposure from gestation to adulthood as well as the effects of androgens and estrogens in adulthood on neurogenesis in the hippocampus. Throughout the review we highlight sex differences in the effects of steroid hormones on neurogenesis and how they may relate to hippocampal function and disease. These data highlight the importance of examining age and sex when evaluating the effects of steroid hormones on hippocampal neurogenesis.

Targeting the Stress System During Gestation: Is Early Handling a Protective Strategy for the Offspring?

The perinatal window is a critical developmental time when abnormal gestational stimuli may alter the development of the stress system that, in turn, influences behavioral and physiological responses in the newborns. Individual differences in stress reactivity are also determined by variations in maternal care, resulting from environmental manipulations. Despite glucocorticoids are the primary programming factor for the offspring's stress response, therapeutic corticosteroids are commonly used during late gestation to prevent preterm negative outcomes, exposing the offspring to potentially aberrant stress reactivity later in life. Thus, in this study, we investigated the consequences of one daily s.c. injection of corticosterone (25 mg/kg), from gestational day (GD) 14-16, and its interaction with offspring early handling, consisting in a brief 15-min maternal separation until weaning, on: (i) maternal behavior; and (ii) behavioral reactivity, emotional state and depressive-like behavior in the adolescent offspring. Corticosterone plasma levels, under non-shock- and shock-induced conditions, were also assessed. Our results show that gestational exposure to corticosterone was associated with diminished maternal care, impaired behavioral reactivity, increased emotional state and depressive-like behavior in the offspring, associated with an aberrant corticosterone response. The early handling procedure, which resulted in increased maternal care, was able to counteract the detrimental effects induced by gestational corticosterone exposure both in the behavioral- and neurochemical parameters examined. These findings highlight the potentially detrimental consequences of targeting the stress system during pregnancy as a vulnerability factor for the occurrence of emotional and affective distress in the adolescent offspring. Maternal extra-care proves to be a protective strategy that confers resiliency and restores homeostasis.

Neurobiological mechanisms underlying the sleep-pain relationship in adolescence: A review

Adolescence characterizes a period of significant change in brain structure and function, causing the neural circuitry to be particularly susceptible to the environment and various other experiences. Chronic pain and sleep deprivation represent major health issues that plague adolescence. A bidirectional relationship exists between sleep and pain; however, emerging evidence suggests that sleep disturbances have a stronger influence on subsequent pain than vice versa. The neurobiological underpinnings of this relationship, particularly during adolescence, are poorly understood. This review examines the current literature regarding sleep and pain in adolescence, with a particular focus on the neurobiological mechanisms underlying pain, sleep problems, and the neural circuitry that potentially links the two. Finally, a research agenda is outlined to stimulate future research on this topic. Given the high prevalence of these health issues during adolescence and the debilitating effects they inflict on nearly every domain of development, it is crucial that we determine the neurobiological mechanisms fundamental to this relationship and identify potential therapeutic strategies.

Effects of an early life experience on rat brain cannabinoid receptors in adolescence and adulthood

Neonatal handling is an experimental model of early life experience associated with resilience in later life challenges, altering the ability of animals to respond to stress. The endocannabinoid system of the brain modulates the neuroendocrine and behavioral effects of stress, while this system is also capable of being modulated by stress exposure itself. The present study has addressed the question of whether neonatal handling in rats could affect cannabinoid receptors, in an age- and sex-dependent manner, using in situ hybridization and receptor binding techniques. Different effects of neonatal handling were observed in adolescent and adult brain on CB1 receptor mRNA and [3H]CP55,940 binding levels, which in some cases were sexually dimorphic. Neonatal handling interfered in the developmental trajectories of CB1 receptor mRNA levels in striatum and amygdaloid nuclei, as well as of [3H]CP55,940 binding levels in almost all regions studied. Adult handled rats showed reduced [3H]CP55,940 binding levels in the prefrontal cortex, striatum, nucleus accumbens and basolateral amygdala, while binding levels in prefrontal cortex of adolescent handled rats were increased. Finally, handling resulted in decreases in female [3H]CP55,940 binding levels in the striatum, nucleus accumbens, CA3 and DG of dorsal hippocampus and basolateral amygdala. Our results suggest that a brief and repeated maternal separation during the neonatal period induces changes on cannabinoid receptors differently manifested between adolescence and adulthood, male and female brain, which could be correlated to their stress response.

Intergenerational transmission of trauma effects: putative role of epigenetic mechanisms

This paper reviews the research evidence concerning the intergenerational transmission of trauma effects and the possible role of epigenetic mechanisms in this transmission. Two broad categories of epigenetically mediated effects are highlighted. The first involves developmentally programmed effects. These can result from the influence of the offspring's early environmental exposures, including postnatal maternal care as well as in utero exposure reflecting maternal stress during pregnancy. The second includes epigenetic changes associated with a preconception trauma in parents that may affect the germline, and impact fetoplacental interactions. Several factors, such as sex-specific epigenetic effects following trauma exposure and parental developmental stage at the time of exposure, explain different effects of maternal and paternal trauma. The most compelling work to date has been done in animal models, where the opportunity for controlled designs enables clear interpretations of transmissible effects. Given the paucity of human studies and the methodological challenges in conducting such studies, it is not possible to attribute intergenerational effects in humans to a single set of biological or other determinants at this time. Elucidating the role of epigenetic mechanisms in intergenerational effects through prospective, multi-generational studies may ultimately yield a cogent understanding of how individual, cultural and societal experiences permeate our biology.

Cultural trauma and epigenetic inheritance

The question of whether and how the effects of cultural trauma can be transmitted intergenerationally from parents to offspring, or even to later generations, has evoked interest and controversy in academic and popular forums. Recent methodological advances have spurred investigations of potential epigenetic mechanisms for this inheritance, representing an exciting area of emergent research. Epigenetics has been described as the means through which environmental influences “get under the skin,” directing transcriptional activity and influencing the expression or suppression of genes. Over the past decade, this complex environment–biology interface has shown increasing promise as a potential pathway for the intergenerational transmission of the effects of trauma. This article reviews challenges facing research on cultural trauma, biological findings in trauma and posttraumatic stress disorder, and putative epigenetic mechanisms for transmission of trauma effects, including through social, intrauterine, and gametic pathways. Implications for transmission of cultural trauma effects are discussed, focused on the relevance of cultural narratives and the possibilities of resilience and adaptivity.

Strain Differences and Effects of Environmental Manipulation on Astrocytes (Glial Fibrillary Acidic Protein), Glucocorticoid Receptor, and Microglia (Iba1) Immunoreactivity between Wistar-Kyoto and Wistar Females

BACKGROUND

Depression is often associated with an increase in hypothalamic-pituitary-adrenal (HPA) axis reactivity and immune response. To investigate this relationship, we examined the consequences of environmental manipulation on the neural correlates of the HPA axis and immune response in an animal model of depression, the Wistar-Kyoto (WKY) rat. Additionally, female animals are often overlooked in preclinical research because of the hormone fluctuations inherent in the estrous cycle.

METHODS

Female rats were randomly assigned to 1 of 3 environments for 30 days: (1) environmental enrichment (EE), (2) standard housing (SH), and (3) isolated housing (IH). Immunoreactivity of astrocytes (glial fibrillary acidic protein [GFAP]), glucocorticoid receptors (GRs), and microglia (Iba1) in the hippocampus and amygdala were measured using immunohistochemistry.

RESULTS

WKY animals had significantly more GR staining area and Iba1 staining intensity and area in the CA1 of the hippocampus. In enriched Wistar rats, GFAP staining intensity and area were greater in the CA1. A trend towards a greater percent of area stained with GR was found in WKY animals as compared to that of the Wistar animals. This was due to WKY females in EE having significantly higher GR staining intensity and area in the amygdala as compared to that of animals in SH.

DISCUSSION

These strain differences lend support to the use of WKY animals as an animal model of depression. Furthermore, due to the effects of EE on GFAP and GR staining in WKY females, we suggest that EE can be used as an intervention to potentially alleviate the negative effects of depression.

Early Life Stress, Depression And Parkinson's Disease: A New Approach

This review aims to shed light on the relationship that involves exposure to early life stress, depression and Parkinson's disease (PD). A systematic literature search was conducted in Pubmed, MEDLINE, EBSCOHost and Google Scholar and relevant data were submitted to a meta-analysis . Early life stress may contribute to the development of depression and patients with depression are at risk of developing PD later in life. Depression is a common non-motor symptom preceding motor symptoms in PD. Stimulation of regions contiguous to the substantia nigra as well as dopamine (DA) agonists have been shown to be able to attenuate depression. Therefore, since PD causes depletion of dopaminergic neurons in the substantia nigra, depression, rather than being just a simple mood disorder, may be part of the pathophysiological process that leads to PD. It is plausible that the mesocortical and mesolimbic dopaminergic pathways that mediate mood, emotion, and/or cognitive function may also play a key role in depression associated with PD. Here, we propose that a medication designed to address a deficiency in serotonin is more likely to influence motor symptoms of PD associated with depression. This review highlights the effects of an antidepressant, Fluvoxamine maleate, in an animal model that combines depressive-like symptoms and Parkinsonism.

Neonatal overfeeding by small-litter rearing sensitises hippocampal microglial responses to immune challenge: Reversal with neonatal repeated injections of saline or minocycline

The early-life period is extremely vulnerable to programming effects from the environment, many of which persist into adulthood. We have previously demonstrated that adult rats overfed as neonates have hypothalamic microglia that are hyper-responsive to an immune challenge, as well as hippocampal microglia that respond less efficiently to learning. We therefore hypothesised that neonatal overfeeding would alter the ability of hippocampal microglia to respond to an immune challenge with lipopolysaccharide (LPS) and that concomitant minocycline, a tetracycline antibiotic that suppresses microglial activity, could restore these responses. We induced neonatal overfeeding by manipulating the litter sizes in which Wistar rat pups were raised, so the pups were suckled in litters of four (neonatally overfed) or 12 (control-fed). We then examined the hippocampal microglial profiles 24 hour after an immune challenge with LPS and found that the neonatally overfed rats had dramatically increased microglial numbers in the hippocampus after immune challenge compared to control-fed rats. Attempts to reverse these effects with minocycline revealed repeated that neonatal injections, whether with minocycline or with saline, markedly suppressed microglial number and density throughout the hippocampus and abolished the difference between the groups in their responses to LPS. These data suggest that neonatal overfeeding not only can have lasting effects on hippocampal immune responses, but also that neonatal exposure to a protocol of repeated injections, irrespective of treatment, has a pronounced long-term impact, highlighting the importance of considering these effects when interpreting experimental data.

Impact of Stress and Glucocorticoids on Schema-Based Learning

Pre-existing knowledge, a 'schema', facilitates the encoding, consolidation, and retrieval of schema-relevant information. Such schema-based memory is key to every form of education and provides intriguing insights into the integration of new information and prior knowledge. Stress is known to have a critical impact on memory processes, mainly through the action of glucocorticoids and catecholamines. However, whether stress and these major stress mediators affect schema-based learning is completely unknown. To address this question, we performed two experiments, in which participants acquired a schema on day 1 and learned schema-related as well as schema-unrelated information on day 2. In the first experiment, participants underwent a stress or control manipulation either immediately or about 25 min before schema-based memory testing. The second experiment tested whether glucocorticoid and/or noradrenergic activation is sufficient to modulate schema-based memory. To this end, participants received orally a placebo, hydrocortisone, the α2-adrenoceptor-antagonist yohimbine, leading to increased noradrenergic stimulation, or both drugs, before completing the schema-based memory test. Our data indicate that stress, irrespective of the exact timing of the stress exposure, impaired schema-based learning, while leaving learning of schema-unrelated information intact. A very similar effect was obtained after hydrocortisone, but not yohimbine, administration. These data show that stress disrupts participants' ability to benefit from prior knowledge during learning and that glucocorticoid activation is sufficient to produce this effect. Our findings provide novel insights into the impact of stress and stress hormones on the dynamics of human memory and have important practical implications, specifically for educational contexts.

Modulation of the Hypothalamic-Pituitary-Adrenal Axis by Early Life Stress Exposure

Exposure to stress during critical periods in development can have severe long-term consequences, increasing overall risk on psychopathology. One of the key stress response systems mediating these long-term effects of stress is the hypothalamic-pituitary-adrenal (HPA) axis; a cascade of central and peripheral events resulting in the release of corticosteroids from the adrenal glands. Activation of the HPA-axis affects brain functioning to ensure a proper behavioral response to the stressor, but stress-induced (mal)adaptation of the HPA-axis' functional maturation may provide a mechanistic basis for the altered stress susceptibility later in life. Development of the HPA-axis and the brain regions involved in its regulation starts prenatally and continues after birth, and is protected by several mechanisms preventing corticosteroid over-exposure to the maturing brain. Nevertheless, early life stress (ELS) exposure has been reported to have numerous consequences on HPA-axis function in adulthood, affecting both its basal and stress-induced activity. According to the match/mismatch theory, encountering ELS prepares an organism for similar ("matching") adversities during adulthood, while a mismatching environment results in an increased susceptibility to psychopathology, indicating that ELS can exert either beneficial or disadvantageous effects depending on the environmental context. Here, we review studies investigating the mechanistic underpinnings of the ELS-induced alterations in the structural and functional development of the HPA-axis and its key external regulators (amygdala, hippocampus, and prefrontal cortex). The effects of ELS appear highly dependent on the developmental time window affected, the sex of the offspring, and the developmental stage at which effects are assessed. Albeit by distinct mechanisms, ELS induced by prenatal stressors, maternal separation, or the limited nesting model inducing fragmented maternal care, typically results in HPA-axis hyper-reactivity in adulthood, as also found in major depression. This hyper-activity is related to increased corticotrophin-releasing hormone signaling and impaired glucocorticoid receptor-mediated negative feedback. In contrast, initial evidence for HPA-axis hypo-reactivity is observed for early social deprivation, potentially reflecting the abnormal HPA-axis function as observed in post-traumatic stress disorder, and future studies should investigate its neural/neuroendocrine foundation in further detail. Interestingly, experiencing additional (chronic) stress in adulthood seems to normalize these alterations in HPA-axis function, supporting the match/mismatch theory.

Paediatric stress: from neuroendocrinology to contemporary disorders

BACKGROUND

Stress is defined as a state of threatened or perceived as threatened homeostasis. A broad spectrum of extrinsic or intrinsic, real or perceived stressful stimuli, called 'stressors', activates a highly conserved system, the 'stress system', which adjusts homeostasis through central and peripheral neuroendocrine responses. Inadequate, excessive or prolonged adaptive responses to stress may underlie the pathogenesis of several disease states prevalent in modern societies. The development and severity of these conditions primarily depend on the genetic vulnerability of the individual, the exposure to adverse environmental factors and the timing of the stressful event(s), given that prenatal life, infancy, childhood and adolescence are critical periods characterized by increased vulnerability to stressors.

MATERIALS AND METHODS

We conducted a systematic review of original articles and reviews published in MEDLINE from 1975 through June 2016. The search terms were 'childhood stress', 'pediatric stress', 'stress and disorders' and 'stress management'.

RESULTS

In this review, we discuss the historical and neuroendocrine aspects of stress, and we present representative examples of paediatric stress system disorders, such as early-life adversity, obesity and bullying. We also discuss the adverse impact of a socio-economic crisis on childhood health. The tremendous progress of epigenetics has enabled us to have a deeper understanding of the molecular mechanisms underlying paediatric stress-related disorders.

CONCLUSIONS

The need for early successful stress management techniques to decrease the incidence of paediatric stress-related diseases, as well as to prevent the development of several pathologic conditions in adolescence and adulthood, is imperative.

Massage Therapy Facilitates Weight Gain in Preterm Infants

Studies from several labs have documented a 31 to 47% greater weight gain in preterm new-borns receiving massage therapy (three 15-min sessions for 5–10 days) compared with standard medical treatment. Although the underlying mechanism for this relationship between massage therapy and weight gain has not yet been established, possibilities that have been explored in studies with both humans and rats include (a) increased protein synthesis, (b) increased vagal activity that releases food-absorption hormones like insulin and enhances gastric motility, and (c) decreased cortisol levels leading to increased oxytocin. In addition, functional magnetic resonance imaging studies are being conducted to assess the effects of touch therapy on brain development. Further behavioral, physiological, and genetic research is needed to understand these effects of massage therapy on growth and development.

Social exclusion changes histone modifications H3K4me3 and H3K27ac in liver tissue of wild house mice

Wild house mice form social hierarchies with aggressive males defending territories, in which females, young mice and submissive adult males share nests. In contrast, socially excluded males are barred from breeding groups, have numerous bite wounds and patches of thinning fur. Since their feeding times are often disrupted, we investigated whether social exclusion leads to changes in epigenetic marks of metabolic genes in liver tissue. We used chromatin immunoprecipitation and quantitative PCR to measure enrichment of two activating histone marks at 15 candidate loci. The epigenetic profiles of healthy males sampled from nest boxes differed significantly from the profiles of ostracized males caught outside of nests and showing bite wounds indicative of social exclusion. Enrichment of histone-3 lysine-4 trimethylation (H3K4me3) changed significantly at genes Cyp4a14, Gapdh, Nr3c1, Pck1, Ppara, and Sqle. Changes at histone-3 lysine-27 acetylation (H3K27ac) marks were detected at genes Fasn, Nr3c1, and Plin5. A principal components analysis separated the socialized from the ostracized mice. This was independent of body weight for the H3K4me3 mark, and partially dependent for H3K27ac. There was no separation, however, between healthy males that had been sampled from two different nests. A hierarchical cluster analysis also separated the two phenotypes, which was independent of body weight for both markers. Our study shows that a period of social exclusion during adult life leads to quantitative changes in histone modification patterns in mouse liver tissue. Similar epigenetic changes might occur during the development of stress-induced metabolic disorders in humans.

Early-life experience, epigenetics, and the developing brain

Development is a dynamic process that involves interplay between genes and the environment. In mammals, the quality of the postnatal environment is shaped by parent-offspring interactions that promote growth and survival and can lead to divergent developmental trajectories with implications for later-life neurobiological and behavioral characteristics. Emerging evidence suggests that epigenetic factors (ie, DNA methylation, posttranslational histone modifications, and small non-coding RNAs) may have a critical role in these parental care effects. Although this evidence is drawn primarily from rodent studies, there is increasing support for these effects in humans. Through these molecular mechanisms, variation in risk of psychopathology may emerge, particularly as a consequence of early-life neglect and abuse. Here we will highlight evidence of dynamic epigenetic changes in the developing brain in response to variation in the quality of postnatal parent-offspring interactions. The recruitment of epigenetic pathways for the biological embedding of early-life experience may also have transgenerational consequences and we will describe and contrast two routes through which this transmission can occur: experience dependent vs germline inheritance. Finally, we will speculate regarding the future directions of epigenetic research and how it can help us gain a better understanding of the developmental origins of psychiatric dysfunction.

How postnatal insults may program development: studies in animal models

During the postnatal period, the nervous system is modified and shaped by experience, in order to adjust it to the particular environment in which the animal will live. This plasticity, one of the most remarkable characteristics of the nervous system, promotes adaptive changes, but it also makes brain more vulnerable to insults. This chapter will focus on the effects of interventions during the postnatal development in animal models of neonatal handling (usually up to 15 min of handling) and maternal separation (usually at least for 3 h). Sex-specific changes and effects of prepubertal stress such as social isolation later on in life were also considered. These interventions during development induce long-lasting traces in the pups' nervous system, which will be reflected in changes in neuroendocrine functions, including the hypothalamus-pituitary-adrenal and hypothalamus-pituitary-gonadal axes; anxiety and cognitive performance; and feeding, sexual, and social behavior. These enduring changes may be adaptive or maladaptive, depending on the environment in which the animal will live. The challenge researchers facing now is to determine how to reverse the deleterious effects that may result from early-life stress exposure.

Early-life adversity programs emotional functions and the neuroendocrine stress system: the contribution of nutrition, metabolic hormones and epigenetic mechanisms

Clinical and pre-clinical studies have shown that early-life adversities, such as abuse or neglect, can increase the vulnerability to develop psychopathologies and cognitive decline later in life. Remarkably, the lasting consequences of stress during this sensitive period on the hypothalamic-pituitary-adrenal axis and emotional function closely resemble the long-term effects of early malnutrition and suggest a possible common pathway mediating these effects. During early-life, brain development is affected by both exogenous factors, like nutrition and maternal care as well as by endogenous modulators including stress hormones. These elements, while mostly considered for their independent actions, clearly do not act alone but rather in a synergistic manner. In order to better understand how the programming by early-life stress takes place, it is important to gain further insight into the exact interplay of these key elements, the possible common pathways as well as the underlying molecular mechanisms that mediate their effects. We here review evidence that exposure to both early-life stress and early-life under-/malnutrition similarly lead to life-long alterations on the neuroendocrine stress system and modify emotional functions. We further discuss how the different key elements of the early-life environment interact and affect one another and next suggest a possible role for the early-life adversity induced alterations in metabolic hormones and nutrient availability in shaping later stress responses and emotional function throughout life, possibly via epigenetic mechanisms. Such knowledge will help to develop intervention strategies, which gives the advantage of viewing the synergistic action of a more complete set of changes induced by early-life adversity.

Stress, the stress system and the role of glucocorticoids

All living organisms have developed a highly conserved and regulatory system, the stress system, to cope with a broad spectrum of stressful stimuli that threaten, or are perceived as threatening, their dynamic equilibrium or homeostasis. This neuroendocrine system consists of the hypothalamic-pituitary-adrenal (HPA) axis and the locus caeruleus/norepinephrine-autonomic nervous system. In parallel with the evolution of the homeostasis and stress concepts from ancient Greek to modern medicine, significant advances in the field of neuroendocrinology have identified the physiologic biochemical effector molecules of the stress response. Glucocorticoids, the end-products of the HPA axis, play a fundamental role in the maintenance of both resting and stress-related homeostasis and, undoubtedly, influence the physiologic adaptive reaction of the organism against stressors. If the stress response is dysregulated in terms of magnitude and/or duration, homeostasis is turned into cacostasis with adverse effects on many vital physiologic functions, such as growth, development, metabolism, circulation, reproduction, immune response, cognition and behavior. A strong and/or long-lasting stressor may precipitate and/or cause many acute and chronic diseases. Moreover, stressors during pre-natal, post-natal or pubertal life may have a critical impact on our expressed genome. This review describes the central and peripheral components of the stress system, provides a comprehensive overview of the stress response, and discusses the role of glucocorticoids in a broad spectrum of stress-related diseases. © 2014 S. Karger AG, Basel.

Early maternal separation: a rodent model of depression and a prevailing human condition

The early life of most mammals is spent in close contact with the mother, and for the neonate, early maternal separation is a traumatic event that, depending on various conditions, may shape its behavioral and neurochemical phenotype in adulthood. Studies on rodents demonstrated that a very brief separation followed by increased maternal care may positively affect the development of the offspring but that prolonged separation causes significant amounts of stress. The consequences of this stress (particularly the hyperreactivity of the HPA (hypothalamic-pituitary-adrenal) axis are expressed in adulthood and persist for life. Maternal separation in rodents, particularly rats, was used as a model for various psychotic conditions, especially depression. The most popular separation procedure of a 3-h daily separation from the second to the 12th postpartum day yields a depression model of high construct and predictive validity. The results of studies on maternal separation in rats and monkeys prompt a discussion of the consequences of traditional procedures in the maternity wards of developed countries where attention is focused on the hygiene of the neonates and not on their psychological needs. This alternate focus results in a drastic limitation of mother-infant contact and prolonged periods of separation. It is tempting to speculate that differences in the course and severity of various mental disorders, which are usually less prevalent in underdeveloped countries than in developed countries (as noted by Kraepelin), may be related to different modes of infant care. Only recently has so-called kangaroo mother care (establishing mother-infant skin-to-skin contact immediately after birth) become popular in developed countries. In addition to its instant benefits for the neonates, this procedure may also be beneficial for the mental health of the offspring in adulthood.

Neonatal handling: an overview of the positive and negative effects

As one of the first rodent models designed to investigate the effects of early-life experiences, the neonatal handling paradigm has helped us better understand how subtle changes in the infant environment can powerfully drive neurodevelopment of the immature brain in typical or atypical trajectories. Here, we review data from more than 50 years demonstrating the compelling effects of neonatal handling on behavior, physiology, and neural function across the lifespan. Moreover, we present data that challenge the classical view of neonatal handling as an animal model that results only in positive/beneficial outcomes. Indeed, the overall goal of this review is to offer the suggestion that the effects of early-life experiences-including neonatal handling-are nuanced rather than unidirectional. Both beneficial and negative outcomes may occur, depending on the parameters of testing, sex of the subject, and neurobehavioral system analyzed.

Maternal separation attenuates the effect of adolescent social isolation on HPA axis responsiveness in adult rats

Adverse early life experiences that occur during childhood and adolescence can have negative impacts on behavior later in life. The main goal of our work was to assess how the association between stressful experiences during neonatal and adolescent periods may influence stress responsiveness and brain plasticity in adult rats. Stressful experiences included maternal separation and social isolation at weaning. Three hours of separation from the pups (3-14 PND) significantly increased frequencies of maternal arched-back nursing and licking-grooming across the first two weeks postpartum. Separation also induced a long-lasting increase in dams blood levels of corticosterone. Maternal separation did not modify brain and plasma allopregnanolone and corticosterone levels in adult offspring, but they demonstrate partial recovery from the reduction induced by social isolation during adolescence. Moreover, the enhancement of corticosterone and allopregnanolone levels induced by foot shock stress in socially isolated animals that were subjected to maternal separation was markedly reduced with respect to that observed in animals that were just socially isolated. All experimental groups showed a significant reduction of BDNF and Arc protein expression in the hippocampus. However, the reduction of BDNF observed in animals that were maternally separated and subjected to social isolation was less significantly pronounced than in animals that were just socially isolated. The results sustained the mismatch hypothesis stating that aversive experiences early in life trigger adaptive processes, thereby rendering an individual to be better adapted to aversive challenges later in life.

Hybrid vigour and maternal environment in mice. II. Water escape learning, open-field activity and spatial memory

A combination of ovarian grafting and fostering was used to study the effects of pre- and postnatal maternal factors on adult behaviour. Inbred mice (BALB/c and C57BL/6) and F1 hybrids were compared to assess differential sensitivity to early maternal effects. In addition to the peculiar behavioural profile of BALB mice, results revealed the existence of a cognitive process in mice and confirmed the greater ability of F1 mice to adopt efficient escape strategies. Only postnatal maternal factors were shown to exert long lasting effects on behaviour, appearing to affect the general ability to react to a new situation rather than specific psychological functions. F1 hybrids proved less sensitive to such effects than inbred mice.

Neonatal handling alters the structure of maternal behavior and affects mother-pup bonding

During early life, a mother and her pups establish a very close relationship, and the olfactory learning of the nest odor is very important for the bond formation. The olfactory bulb (OB) is a structure that plays a fundamental role in the olfactory learning (OL) mechanism that also involves maternal behavior (licking and contact). We hypothesized that handling the pups would alter the structure of the maternal behavior, affect OL, and alter mother-pup relationships. Moreover, changes in the cyclic AMP-response element binding protein phosphorylation (CREB) and neurotrophic factors could be a part of the mechanism of these changes. This study aimed to analyze the effects of neonatal handling, 1 min per day from postpartum day 1 to 10 (PPD 1 to PPD 10), on the maternal behavior and pups' preference for the nest odor in a Y maze (PPD 11). We also tested CREB's phosphorylation and BDNF signaling in the OB of the pups (PPD 7) by Western blot analysis. The results showed that handling alters mother-pups interaction by decreasing mother-pups contact and changing the temporal pattern of all components of the maternal behavior especially the daily licking and nest-building. We found sex-dependent changes in the nest odor preference, CREB and BDNF levels in pups OB. Male pups were more affected by alterations in the licking pattern, and female pups were more affected by changes in the mother-pup contact (the time spent outside the nest and nursing).

Seasonal variations of basal cortisol and high stress response to captivity in Octodon degus, a mammalian model species

Across vertebrates, the hypothalamic-pituitary-adrenal axis is a conserved neuroendocrine network that responds to changing environments and involves the release of glucocorticoids into the blood. Few studies have been carried out concerning mammalian adrenal regulation in wild species either in the laboratory or field, and even fewer have been able to determine true glucocorticoid baselines. We studied the South-American caviomorph rodent Octodon degus, a diurnal and social mammal that has become an important species in the biological research. First, we determined the plasma cortisol baseline and the acute stress concentrations during the non-reproductive and mating seasons in free-living individuals. Second, using the same protocol we assessed the impact of long-term captivity on the adrenal function in wild-caught degus and degus born in laboratory. Third, we examined laboratory groups formed with degus taken from two distant natural populations; one of them originally occurs at the Andes Mountains in high altitude conditions. The data revealed seasonal modulation of basal cortisol in the wild associated with mating. In laboratory, degus presented higher cortisol stress responses, with greater magnitudes shown in degus born and reared in captivity. No differences between populations were found. The results suggest differential regulatory mechanisms between basal and stress-induced cortisol levels, and context dependence of cortisol modulation in a mammalian species.

Epigenetics and the regulation of stress vulnerability and resilience

The human brain has a remarkable capacity to adapt to and learn from a wide range of variations in the environment. However, environmental challenges can also precipitate psychiatric disorders in susceptible individuals. Why any given experience should induce one brain to adapt while another is edged toward psychopathology remains poorly understood. Like all aspects of psychological function, both nature (genetics) and nurture (life experience) sculpt the brain's response to stressful stimuli. Here we review how these two influences intersect at the epigenetic regulation of neuronal gene transcription, and we discuss how the regulation of genomic DNA methylation near key stress-response genes may influence psychological susceptibility or resilience to environmental stressors. Our goal is to offer a perspective on the epigenetics of stress responses that works to bridge the gap between the study of this molecular process in animal models and its potential usefulness for understanding stress vulnerabilities in humans.

Early handling attenuates enhancement of glucocorticoid receptors in the prefrontal cortex in an animal model of post-traumatic stress disorder

BACKGROUND

Changes in glucocorticoid receptors (GRs) have been implicated in the pathogenesis of stress related psychiatric disorders such as depression and post-traumatic stress disorder (PTSD). Abnormal adaptation of the stress-response system following traumatic stress can lead to an altered hypothalamic-pituitary-adrenal axis that may contribute to PTSD development. Indeed, elevated GR expression in the hippocampus and prefrontal cortex linked to PTSD-like characteristics have been reported in the validated animal model of PTSD, single-prolonged stress. These findings implicate increased levels of GRs in the development of post-traumatic psychopathology and suggest that exploration of GR-targeted interventions may have potential for PTSD prevention. Early handling during the neonatal phase alters GR expression and is proposed to confer resilience to stress. We therefore examined the effects of combined early handling and single prolonged stress treatments on GR expression.

METHODS

Timed pregnant dams gave birth to pups that were subjected to early handling (n = 11) or control (n = 13) procedures during the neonatal phase. At postnatal day 45 animals underwent single prolonged stress or a control procedure. Rats were euthanized one day later and GR levels were assayed using western blot electrophoresis.

RESULTS

Single prolonged stress exposure enhanced GR expression in the hippocampus and prefrontal cortex. Early handling treatment protected against single prolonged stress-induced enhancement of GR expression in the prefrontal cortex, but not in the hippocampus.

CONCLUSIONS

These data are a first step in highlighting the importance of targeting GR systems in prevention/resilience and may suggest that preventive strategies targeting GR upregulation might be particularly effective when prefrontal rather than hippocampal GRs are the target.

Increased susceptibility to mild neonatal stress in MTHFR deficient mice

Early life stress is shown to have a life-span outcome on human and animal behavior, increasing the risk for psychopathology. The gene methylenetetrahydrofolate reductase (MTHFR), which encodes for a key enzyme in one carbon metabolism, shows a high prevalence of polymorphism in patients with developmental disorders. Here we examined the hypothesis that MTHFR deficiency results in an increased susceptibility of the developing brain to mild neonatal stress (NS). Mild NS failed to alter corticosterone levels in young and adult Wt mice. However, an elevated level of corticosterone was found in the MTHFR deficient-NS female, exemplifying enhanced sensitivity to NS. Behavioral phenotyping of Wt and MTHFR deficient mice provides evidence that the effect of mild NS may be amplified by the MTHFR deficient genotype. Distinct behavioral characteristics were altered in male and female mice. In general, three patterns of influence on mice behavior were observed: (1) an additive suppressive effect of NS and MTHFR deficiency on exploration and activity was evident in females; (2) stress related parameters were significantly sensitive to genotype in females, presenting an interaction between genotype and sex; (3) various aspects of behavior in a social setting were modified preferably in males by genotype, NS and the interaction between the two, while females exhibited a smaller effect that was restricted to NS with no genotype effect. Overall, our results support an interaction between mild NS, the MTHFR genotype and sex. We suggest using this animal model to study the molecular mechanism linking these two risk factors and their involvement in neurodevelopmental disorders such as schizophrenia and autism.

Enduring influence of pubertal stressors on behavioral response to hormones in female mice

This article is part of a Special Issue "Puberty and Adolescence". The pubertal period is a time of change in an animal's response to stress, and it is a second period of sexual differentiation of the brain. Recently, it was discovered that particular stressors during the prolonged pubertal period of female mice result in enduring changes in behavioral responsiveness of the brain to estradiol and progesterone. Depending on the behavior, pubertal immune challenge or shipping from suppliers may decrease, eliminate, or even reverse the effects of estradiol. Pubertal immune challenge results in changes in the number of estrogen receptor-immunoreactive cells in key brain areas suggesting a cellular mechanism for this remodeling of the brain's response to hormones. A hypothesis is put forward that predicts that particular adverse experiences in girls may cause long-term alterations in the brain's response to estradiol and/or progesterone via activation of the immune system. This could lead to mood disorders or altered response to any behavior influenced by estradiol in humans.

Changes in heat shock protein 70, blood parameters, and fear-related behavior in broiler chickens as affected by pleasant and unpleasant human contact

An experiment was conducted to determine the effects of combining both pleasant and unpleasant contacts with human beings on physiology and behavior of broiler chickens. Birds were subjected to the following treatments: (i) received no physical or visual contact with humans (control); (ii) from d 1 to 28, chicks were individually stroked gently for 30 s once daily (PL); (iii) from d 1 to 28, chicks were picked up individually, suspended by both legs, exposed to recorded noise, and swung gently for 15 s once daily (UNPL); (iv) from d 1 to 14 and from d 15 to 28, chicks were subjected to PL and UNPL, respectively (PL-UNPL); and (v) from d 1 to 14 and from d 15 to 28, chicks were subjected to UNPL and PL, respectively (UNPL-PL). On d 42, birds from each treatment group were road-transported for 3 h. Heat shock protein (hsp) 70 expression, plasma levels of corticosterone, serum creatine kinase concentration, heterophil/lymphocyte ratios (HLR), and tonic immobility duration were determined pre- and posttransit. There were significant (P < 0.05) duration of transportation × human contact treatment interactions for HLR and hsp 70 density. Following transit, the PL chicks had significantly (P < 0.05) lower HLR and greater hsp 70 density than the other groups. The corticosterone of PL and UNPL chicks were lower than their control, PL-UNPL, and UNPL-PL counterparts. The PL and PL-UNPL treatments were effective in shortening tonic immobility duration significantly (P < 0.05). Except for UNPL-PL, the serum creatine kinase activity of PL was significantly lower than the other groups. In conclusion, subjecting birds to pleasant human contact reduced stress and fear reactions to transportation by enhancing the ability to express hsp 70 in the brain. Unpleasant human contact had adverse effect on the birds' response to transportation. Early age pleasant experience with humans failed to negate the adverse effects of subsequent unpleasant contact.

Sex and litter effects on anxiety and DNA methylation levels of stress and neurotrophin genes in adolescent rats

Maternal care variations associate with DNA methylation of the glucocorticoid receptor gene, Nr3c1, in hippocampus at a nerve-growth factor-inducible protein 1 binding site. Epigenetic regulation of brain-derived neurotrophin factor is affected by early stress. These systems contribute to anxiety and fear. Early stress has sex-dependent effects perhaps reflecting sex differences in maternal care. Altering litter gender composition affects maternal behavior and DNA methylation levels of another gene in hippocampus and nucleus accumbens (NAc). We now test if DNA methylation levels of Nr3c1, Egr1, and Bdnf differ by litter composition or sex. Rats from mixed- or single-sex litters were tested for anxiety and fear on postnatal day 35. Brain tissues were collected and analyzed using direct sequencing methods. Females showed hypermethylation of Nr3c1 of hippocampal DNA and litter composition modified sex effects on methylation of Egr1 in NAc. Few differences were seen for Bdnf. LGC modified some sex differences in behavior.

Have studies of the developmental regulation of behavioral phenotypes revealed the mechanisms of gene-environment interactions?

This review addresses the recent convergence of our long-standing knowledge of the regulation of behavioral phenotypes by developmental experience with recent advances in our understanding of mechanisms regulating gene expression. This review supports a particular perspective on the developmental regulation of behavioral phenotypes: That the role of common developmental experiences (e.g. maternal interactions, peer interactions, exposure to a complex environment, etc.) is to fit individuals to the circumstances of their lives within bounds determined by long-standing (evolutionary) mechanisms that have shaped responses to critical and fundamental types of experience via those aspects of gene structure that regulate gene expression. The phenotype of a given species is not absolute for a given genotype but rather variable within bounds that is determined by mechanisms regulated by experience (e.g. epigenetic mechanisms). This phenotypic variation is not necessarily random, or evenly distributed along a continuum of description or measurement, but often highly disjointed, producing distinct, even opposing, phenotypes. The potentiality for these varying phenotypes is itself the product of evolution, the potential for alternative phenotypes itself conveying evolutionary advantage. Examples of such phenotypic variation, resulting from environmental or experiential influences, have a long history of study in neurobiology, and a number of these will be discussed in this review: neurodevelopmental experiences that produce phenotypic variation in visual perception, cognitive function, and emotional behavior. Although other examples will be discussed, particular emphasis will be made on the role of social behavior on neurodevelopment and phenotypic determination. It will be argued that an important purpose of some aspects of social behavior is regulation of neurobehavioral phenotypes by experience via genetic regulatory mechanisms.

Early life manipulations alter learning and memory in rats

Much research shows that early life manipulations have enduring behavioral, neural, and hormonal effects. However, findings of learning and memory performance vary widely across studies. We reviewed studies in which pre-weaning rat pups were exposed to stressors and tested on learning and memory tasks in adulthood. Tasks were classified as aversive conditioning, inhibitory learning, or spatial/relational memory. Variables of duration, type, and timing of neonatal manipulation and sex and strain of animals were examined to determine if any predict enhanced or impaired performance. Brief separations enhanced and prolonged separations impaired performance on spatial/relational tasks. Performance was impaired in aversive conditioning and enhanced in inhibitory learning tasks regardless of manipulation duration. Opposing effects on performance for spatial/relational memory also depended upon timing of manipulation. Enhanced performance was likely if the manipulation occurred during postnatal week 3 but performance was impaired if it was confined to the first two postnatal weeks. Thus, the relationship between early life experiences and adulthood learning and memory performance is multifaceted and decidedly task-dependent.

Time-of-day effects in arousal: disrupted diurnal cortisol profiles in children with ADHD

BACKGROUND

  Fluctuations in attention-deficit hyperactivity disorder (ADHD) symptoms related to regulatory deficits in arousal states are themselves characterized by circadian rhythms. Although cortisol is an important circadian arousal-related marker, studies focusing on across-the-day cortisol variations in ADHD are scarce. There is no study with multiple measurements to take into account interday and intraday variability.

METHODS

  Salivary cortisol was sampled five times a day (awakening, 30 min after awakening, noon, 4 p.m., 8 p.m.) across five consecutive days in 33 children with ADHD (22 with and 11 without oppositional defiant disorder; ODD) and 33 class- and sex-matched controls (aged 6-12). The cortisol awakening response (increase from awakening to 30 min after awakening) and the diurnal cortisol profile (across-the-day variations) were compared for ADHD with ODD (ADHD + ODD) and without ODD (ADHD) subgroups and the control group.

RESULTS

  The cortisol awakening response was not significantly different between groups. However, longitudinal analyses to evaluate cortisol profiles across the day revealed a significant Group × Time effect (p < .001). More specifically, compared to each other, the ADHD subgroup showed a flatter slope with relative morning hypo-arousal and evening hyperarousal, whereas the ADHD + ODD subgroup showed a steeper slope with relative morning hyperarousal and evening hypo-arousal (p < .001).

CONCLUSIONS

  Findings support time-related arousal disruptions in children with ADHD associated with the presence or absence of ODD comorbidity. We recommend research on cortisol in larger samples for a better understanding of arousal mechanisms involved in ADHD not only with and without ODD but also with other comorbidities which may have implications for timing of arousal-based treatments.

Risk assessment behaviors associated with corticosterone trigger the defense reaction to social isolation in rats: role of the anterior cingulate cortex

The extent to which the hypothalamic-pituitary-adrenal axis is activated by short-term and long-term consequences of stress is still open to investigation. This study aimed to determine (i) the correlation between plasma corticosterone and exploratory behavior exhibited by rats subjected to the elevated plus maze (EPM) following different periods of social isolation, (ii) the effects of the corticosterone synthesis blocker, metyrapone, on the behavioral consequences of isolation, and (iii) whether corticosterone produces its effects through an action on the anterior cingulate cortex, area 1 (Cg1). Rats were subjected to 30-min, 2-h, 24-h, or 7-day isolation periods before EPM exposure and plasma corticosterone assessments. Isolation for longer periods of time produced greater anxiogenic-like effects on the EPM. However, stretched attend posture (SAP) and plasma corticosterone concentrations were increased significantly after 30 min of isolation. Among all of the behavioral categories measured in the EPM, only SAP positively correlated with plasma corticosterone. Metyrapone injected prior to the 24 h isolation period reversed the anxiogenic effects of isolation. Moreover, corticosterone injected into the Cg1 produced a selective increase in SAP. These findings indicate that risk assessment behavior induced by the action of corticosterone on Cg1 neurons initiates a cascade of defensive responses during exposure to stressors.

Chewing ameliorates stress-induced suppression of spatial memory by increasing glucocorticoid receptor expression in the hippocampus

Chewing alters hypothalamic-pituitary-adrenal axis function and improves the ability to cope with stress in rodents. Given that stress negatively influences hippocampus-dependent learning and memory, we aimed to elucidate whether masticatory movements, namely chewing, improve the stress-induced impairment of spatial memory in conjunction with increased hippocampal glucocorticoid receptor expression. Male Sprague-Dawley rats were subjected to restraint stress by immobilization for 2h: the stress with chewing (SC) group were allowed to chew on a wooden stick during the latter half of the immobilization period, whereas the stress without chewing (ST) group were not allowed to do so. Performance in the Morris water maze test was significantly impaired in the ST group compared with the SC group. Further, the numbers of glucocorticoid receptor immunopositive neurons in the hippocampal cornu ammonis 1 region were significantly lower in the ST group than in the control and SC groups. The control and SC rats showed no significant differences in both the water maze performance and the numbers of glucocorticoid receptor-immunopositive neurons. The immunohistochemical finding correlated with the performance in the water maze test. These results suggest that chewing is a behavioral mechanism to cope with stress by increasing hippocampal glucocorticoid receptor expression.

Single prolonged stress disrupts retention of extinguished fear in rats

Clinical research has linked post-traumatic stress disorder (PTSD) with deficits in fear extinction. However, it is not clear whether these deficits result from stress-related changes in the acquisition or retention of extinction or in the regulation of extinction memories by context, for example. In this study, we used the single prolonged stress (SPS) animal model of PTSD and fear conditioning procedures to examine the effects of prior traumatic stress on the acquisition, retention, and context-specificity of extinction. SPS administered one week prior to fear conditioning had no effect on the acquisition of fear conditioning or extinction but disrupted the retention of extinction memories for both contextual and cued fear. This SPS effect required a post-stress incubation period to manifest. The results demonstrate that SPS disrupts extinction retention, leading to enhanced fear renewal; further research is needed to identify the neurobiological processes through which SPS induces these effects.

Environmental-induced differences in corticosterone and glucocorticoid receptor blockade of amphetamine self-administration in rats

RATIONALE

Rats raised in isolation self-administer more amphetamine than rats raised in enrichment.

OBJECTIVE

This study examined whether differential rearing alters basal and amphetamine-stimulated corticosterone and whether blocking glucocorticoid receptors alters amphetamine self-administration in differentially reared rats.

METHODS

The rats were raised from 21 to 51 days of age in either an enriched condition (EC), social condition (SC), or isolated condition (IC). Following the repeated collection of basal blood samples, the rats were administered amphetamine (0.5 or 2.0 mg/kg, i.p.) or saline, and blood samples were collected again. In another experiment, EC and IC rats were trained to i.v. self-administer amphetamine (0.003 or 0.03 mg/kg/infusion) and then were pretreated with the glucocorticoid receptor antagonist RU-486 (5, 10, or 20 mg/kg; i.p.) or vehicle prior to the session.

RESULTS

Basal-free corticosterone levels were ~4 times higher in IC rats than in either EC or SC rats with the first blood collection, but not with repeated collections. IC rats showed a more rapid amphetamine-induced increase in corticosterone levels than EC and SC rats. RU-486 pretreatment decreased amphetamine self-administration dose-dependently in both EC and IC rats; however, using an amphetamine unit dose of 0.03 mg/kg/infusion, the effect of RU-486 was blunted in IC rats (maximal decrease of ~40% in IC and ~90% in EC), suggesting an environment-induced difference in the role of glucocorticoid receptors in stimulant reinforcement.

CONCLUSION

The increase in stimulant self-administration produced by social isolation may involve enhanced reactivity of the hypothalamo-pituitary-adrenal stress axis.