Animal models of early life stress: implications for understanding resilience

Seeding Stress Resilience through Inoculation

Stress is a generalized set of physiological and psychological responses observed when an organism is placed under challenging circumstances. The stress response allows organisms to reattain the equilibrium in face of perturbations. Unfortunately, chronic and/or traumatic exposure to stress frequently overwhelms coping ability of an individual. This is manifested as symptoms affecting emotions and cognition in stress-related mental disorders. Thus environmental interventions that promote resilience in face of stress have much clinical relevance. Focus of the bulk of relevant neurobiological research at present remains on negative aspects of health and psychological outcomes of stress exposure. Yet exposure to the stress itself can promote resilience to subsequent stressful episodes later in the life. This is especially true if the prior stress occurs early in life, is mild in its magnitude, and is controllable by the individual. This articulation has been referred to as "stress inoculation," reminiscent of resilience to the pathology generated through vaccination by attenuated pathogen itself. Using experimental evidence from animal models, this review explores relationship between nature of the "inoculum" stress and subsequent psychological resilience.

Early-life predictors of resilience and related outcomes up to 66 years later in the 6-day sample of the 1947 Scottish mental survey

PURPOSE

Psychological resilience, the ability to manage and quickly recover from stress and trauma, is associated with a range of health and wellbeing outcomes. Resilience is known to relate to personality, self-esteem and positive affect, and may also depend upon childhood experience and stress. In this study, we investigated the role of early-life contributors to resilience and related factors in later life.

METHODS

We used data from the 6-day sample of the Scottish mental survey 1947, an initially representative sample of Scottish children born in 1936. They were assessed on a range of factors between the ages of 11 and 27 years, and resilience and other outcomes at 77 years.

RESULTS

Higher adolescent dependability unexpectedly predicted lower resilience in older-age, as did childhood illnesses, while a count of specific stressors experienced throughout early life significantly predicted higher later-life resilience. We also observed significant cross-sectional correlations between resilience and measures of physical health, mental health, wellbeing and loneliness. Some of the associations between early-life predictors and later-life outcomes were significantly mediated by resilience.

CONCLUSIONS

Our results support the hypothesis that stress throughout early life may help to build resilience in later-life, and demonstrate the importance of resilience as a mediator of other influences on health and wellbeing in older age. We suggest that the mechanisms determining how early-life stress leads to higher resilience are worthy of further investigation, and that psychological resilience should be a focus of research and a target for therapeutic interventions aiming to improve older-age health and wellbeing.

Deconstructing the function of maternal stimulation in offspring development: Insights from the artificial rearing model in rats

This article is part of a Special Issue on "Parental Care". Maternal behavior has an important function in stimulating adequate growth and development of the young. Several approaches have been used in primates and rodents to deconstruct and examine the influence of specific components of maternal stimulation on offspring development. These approaches include observational studies of typical mother-infant interactions and studies of the effects of intermittent or complete deprivation of maternal contact. In this review, we focus on one unique approach using rats that enables the complete control of maternal variables by means of rearing rat pups artificially without contact with the mother or litter, while maintaining stable nutrition, temperature and exposure to stressful stimuli. This artificial rearing model permits the removal and controlled replacement of relevant maternal and litter stimuli and has contributed valuable insights regarding the influence of these stimuli on various developmental outcomes. It also enables the analysis of factors implicated in social isolation itself and their long-term influence. We provide an overview of the effects of artificial rearing on behavior, physiology, and neurobiology, including the influence of replacing maternal tactile stimulation and littermate contact on these outcomes. We then discuss the relevance of these effects in terms of the maternal role in regulating different aspects of offspring development and implications for human research. We emphasize that artificial rearing of rats does not lead to a global insult of nervous system development, making this paradigm useful in investigating specific developmental effects associated with maternal stimulation.

Sex differences and stress across the lifespan

Sex differences in stress responses can be found at all stages of life and are related to both the organizational and activational effects of gonadal hormones and to genes on the sex chromosomes. As stress dysregulation is the most common feature across neuropsychiatric diseases, sex differences in how these pathways develop and mature may predict sex-specific periods of vulnerability to disruption and increased disease risk or resilience across the lifespan. The aging brain is also at risk to the effects of stress, where the rapid decline of gonadal hormones in women combined with cellular aging processes promote sex biases in stress dysregulation. In this Review, we discuss potential underlying mechanisms driving sex differences in stress responses and their relevance to disease. Although stress is involved in a much broader range of diseases than neuropsychiatric ones, we highlight here this area and its examples across the lifespan.

Changes in dam and pup behavior following repeated postnatal exposure to a predator odor (TMT): A preliminary investigation in Long-Evans rats

The present study investigated whether repeated early postnatal exposure to the predator odor 2,5-dihydro-2,4,5-trimethylthiazoline (TMT) alters behavioral responses to the stimulus later in life, at postnatal day (PN30). Long-Evans rat pups with their mothers were exposed for 20 min daily to TMT, water, or a noxious odor, butyric acid (BTA), during the first three weeks of life. Mothers exposed to TMT displayed more crouching and nursing behavior than those exposed to BTA, and TMT exposed pups emitted more ultrasonic vocalizations than BTA exposed pups. At PN30, rats were tested for freezing to TMT, water, or BTA. Rats exposed to TMT during the postnatal period displayed less freezing to TMT than rats exposed postnatally to water or BTA. Our data indicate that early-life experience with a predator cue has a significant impact on later fear responses to that same cue, highlighting the programming capacity of the postnatal environment on the development of behavior.

Social buffering of stress responses in nonhuman primates: Maternal regulation of the development of emotional regulatory brain circuits

Social buffering, the phenomenon by which the presence of a familiar individual reduces or even eliminates stress- and fear-induced responses, exists in different animal species and has been examined in the context of the mother-infant relationship, in addition to adults. Although it is a well-known effect, the biological mechanisms that underlie it as well as its developmental impact are not well understood. Here, we provide a review of evidence of social and maternal buffering of stress reactivity in nonhuman primates, and some data from our group suggesting that when the mother-infant relationship is disrupted, maternal buffering is impaired. This evidence underscores the critical role that maternal care plays for proper regulation and development of emotional and stress responses of primate infants. Disruptions of the parent-infant bond constitute early adverse experiences associated with increased risk for psychopathology. We will focus on infant maltreatment, a devastating experience not only for humans, but for nonhuman primates as well. Taking advantage of this naturalistic animal model of adverse maternal caregiving, we have shown that competent maternal care is critical for the development of healthy attachment, social behavior, and emotional and stress regulation, as well as of the neural circuits underlying these functions.

Extending injury- and disease-resistant CNS phenotypes by repetitive epigenetic conditioning

Significant reductions in the extent of acute injury in the CNS can be achieved by exposure to different preconditioning stimuli, but the duration of the induced protective phenotype is typically short-lasting, and thus is deemed as limiting its clinical applicability. Extending the period over which such adaptive epigenetic changes persist – in effect, expanding conditioning’s “therapeutic window” – would significantly broaden the potential applications of such a treatment approach in patients. The frequency of the conditioning stimulus may hold the key. While transient (1–3 days) protection against CNS ischemic injury is well established preclinically following a single preconditioning stimulus, repetitively presenting preconditioning stimuli extends the duration of ischemic tolerance by many weeks. Moreover, repetitive intermittent postconditioning enhances post-ischemic recovery metrics and improves long-term survival. Intermittent conditioning is also efficacious for preventing or delaying injury in preclinical models of chronic neurodegenerative disease, and for promoting long-lasting functional improvements in a number of other pathologies as well. Although the detailed mechanisms underlying these protracted kinds of neuroplasticity remain largely unstudied, accumulating empirical evidence supports the contention that all of these adaptive phenotypes are epigenetically mediated. Going forward, additional preclinical demonstrations of the ability to induce sustained beneficial phenotypes that reduce the burden of acute and chronic neurodegeneration, and experimental interrogations of the regulatory constructs responsible for these epigenetic responses, will accelerate the identification of not only efficacious but also practical, adaptive epigenetics-based treatments for individuals with neurological disease.

S100B blood levels and childhood trauma in adolescent inpatients

BACKGROUND

Serum levels of the astrocytic protein S100B have been reported to indicate disruption of the blood-brain barrier. In this study, we investigated the relationship between S100B levels and childhood trauma in a child psychiatric inpatient unit.

METHOD

Levels of S100B were measured in a group of youth with mood disorders or psychosis with and without history of childhood trauma as well as in healthy controls. Study participants were 93 inpatient adolescents admitted with a diagnosis of psychosis (N = 67), or mood disorder (N = 26) and 22 healthy adolescents with no history of trauma or psychiatric illness. Childhood trauma was documented using the Life Events Checklist (LEC) and Adverse Child Experiences (ACE).

RESULTS

In a multivariate regression model, suicidality scores and trauma were the only two variables which were independently related to serum S100B levels. Patients with greater levels of childhood trauma had significantly higher S100B levels even after controlling for intensity of suicidal ideation. Patients with psychotic diagnoses and mood disorders did not significantly differ in their levels of S100B. Patients exposed to childhood trauma were significantly more likely to have elevated levels of S100B (p < .001) than patients without trauma, and patients with trauma had significantly higher S100B levels (p < .001) when compared to the control group. LEC (p = 0.046), and BPRS-C suicidality scores (p = 0.001) significantly predicted S100B levels.

CONCLUSIONS

Childhood trauma can potentially affect the integrity of the blood-brain barrier as indicated by associated increased S100B levels.

Stress inoculation modeled in mice

Stress inoculation entails intermittent exposure to mildly stressful situations that present opportunities to learn, practice and improve coping in the context of exposure psychotherapies and resiliency training. Here we investigate behavioral and hormonal aspects of stress inoculation modeled in mice. Mice randomized to stress inoculation or a control treatment condition were assessed for corticosterone stress hormone responses and behavior during open-field, object-exploration and tail-suspension tests. Stress inoculation training sessions that acutely increased plasma levels of corticosterone diminished subsequent immobility as a measure of behavioral despair on tail-suspension tests. Stress inoculation also decreased subsequent freezing in the open field despite comparable levels of thigmotaxis in mice from both treatment conditions. Stress inoculation subsequently decreased novel-object exploration latencies and reduced corticosterone responses to repeated restraint. These results demonstrate that stress inoculation acutely stimulates glucocorticoid signaling and then enhances subsequent indications of active coping behavior in mice. Unlike mouse models that screen for the absence of vulnerability to stress or presence of traits that occur in resilient individuals, stress inoculation training reflects an experience-dependent learning-like process that resembles interventions designed to build resilience in humans. Mouse models of stress inoculation may provide novel insights for new preventive strategies or therapeutic treatments of human psychiatric disorders that are triggered and exacerbated by stressful life events.

Neurobiological mechanisms supporting experience-dependent resistance to social stress

Humans and other animals show a remarkable capacity for resilience following traumatic, stressful events. Resilience is thought to be an active process related to coping with stress, although the cellular and molecular mechanisms that support active coping and stress resistance remain poorly understood. In this review, we focus on the neurobiological mechanisms by which environmental and social experiences promote stress resistance. In male Syrian hamsters, exposure to a brief social defeat stressor leads to increased avoidance of novel opponents, which we call conditioned defeat. Also, hamsters that have achieved dominant social status show reduced conditioned defeat as well as cellular and molecular changes in the neural circuits controlling the conditioned defeat response. We propose that experience-dependent neural plasticity occurs in the prelimbic (PL) cortex, infralimbic (IL) cortex, and ventral medial amygdala (vMeA) during the maintenance of dominance relationships, and that adaptations in these neural circuits support stress resistance in dominant individuals. Overall, behavioral treatments that promote success in competitive interactions may represent valuable interventions for instilling resilience.

Stress, social behavior, and resilience: insights from rodents

The neurobiology of stress and the neurobiology of social behavior are deeply intertwined. The social environment interacts with stress on almost every front: social interactions can be potent stressors; they can buffer the response to an external stressor; and social behavior often changes in response to stressful life experience. This review explores mechanistic and behavioral links between stress, anxiety, resilience, and social behavior in rodents, with particular attention to different social contexts. We consider variation between several different rodent species and make connections to research on humans and non-human primates.

Perspectives on stress resilience and adolescent neurobehavioral function

Interest in adolescence as a crucial stage of neurobehavioral maturation is growing, as is the concern of how stress may perturb this critical period of development. Though it is well recognized that stress-related vulnerabilities increase during adolescence, not all adolescent individuals are uniformly affected by stress nor do stressful experiences inevitability lead to negative outcomes. Indeed, many adolescents show resilience to stress-induced dysfunctions. However, relatively little is known regarding the mechanisms that may mediate resilience to stress in adolescence. The goal of this brief review is to bring together a few separate, yet related lines of research that highlight specific variables that may influence stress resilience during adolescence, including early life programming of the hypothalamic-pituitary-adrenal (HPA) axis, stress inoculation, and genetic predisposition. Though we are far from a clear understanding of the factors that mediate resistance to stress-induced dysfunctions, it is imperative that we identify and delineate these aspects of resilience to help adolescents reach their full potential, even in the face of adversity.

Coping and glucocorticoid receptor regulation by stress inoculation

Intermittent exposure to mildly stressful situations provides opportunities to practice coping in the context of exposure psychotherapies and stress inoculation training. Previously, we showed that stress inoculation modeled in juvenile monkeys enhances subsequent indications of resilience. Here we examine stress inoculation effects in adult female monkeys. We found that stress inoculation prevents social separation stress induced anhedonia measured using sucrose preference tests and reduces the hypothalamic pituitary adrenal (HPA) axis stress hormone response to a novel environment. Stress inoculation also increases glucocorticoid receptor (NR3C1) gene expression in anterior cingulate cortex but not hippocampus. Increased anterior cingulate cortex NR3C1 expression induced by stress inoculation is not associated with significant changes in GR1F promoter DNA methylation. On average, low levels of promoter DNA methylation and limited GR1F expression were evident in monkey anterior cingulate cortex as observed in corticolimbic brain regions of adult humans. Taken together these findings suggest that stress inoculation in adulthood enhances behavioral and hormonal aspects of coping without significantly influencing GR1F promoter DNA methylation as a mechanism for NR3C1 transcription regulation.

A mechanistic look at the effects of adversity early in life on cardiovascular disease risk during adulthood

Early origins of adult disease may be defined as adversity or challenges during early life that alter physiological responses and prime the organism to chronic disease in adult life. Adverse childhood experiences or early life stress (ELS) may be considered a silent independent risk factor capable of predicting future cardiovascular disease risk. Maternal separation (MatSep) provides a suitable model to elucidate the underlying molecular mechanisms by which ELS increases the risk to develop cardiovascular disease in adulthood. The aim of this review is to describe the links between behavioural stress early in life and chronic cardiovascular disease risk in adulthood. We will discuss the following: (i) adult cardiovascular outcomes in humans subjected to ELS, (ii) MatSep as an animal model of ELS as well as the limitations and advantages of this model in rodents and (iii) possible ELS-induced mechanisms that predispose individuals to greater cardiovascular risk. Overall, exposure to a behavioural stressor early in life sensitizes the response to a second stressor later in life, thus unmasking an exaggerated cardiovascular dysfunction that may influence quality of life and life expectancy in adulthood.

Neuroimaging resilience to stress: a review

There is a high degree of intra-individual variation in how individuals respond to stress. This becomes evident when exploring the development of posttraumatic symptoms or stress-related disorders after exposure to trauma. Whether or not an individual develops posttraumatic symptoms after experiencing a traumatic event is partly dependent on a person's resilience. Resilience can be broadly defined as the dynamic process encompassing positive adaptation within the context of significant adversity. Even though research into the neurobiological basis of resilience is still in its early stages, these insights can have important implications for the prevention and treatment of stress-related disorders. Neuroimaging studies contribute to our knowledge of intra-individual variability in resilience and the development of posttraumatic symptoms or other stress-related disorders. This review provides an overview of neuroimaging findings related to resilience. Structural, resting-state, and task-related neuroimaging results associated with resilience are discussed. There are a limited number of studies available and neuroimaging research of resilience is still in its infancy. The available studies point at brain circuitries involved in stress and emotion regulation, with more efficient processing and regulation associated with resilience.

Neurobiology of resilience

Humans exhibit a remarkable degree of resilience in the face of extreme stress, with most resisting the development of neuropsychiatric disorders. Over the past 5 years, there has been increasing interest in the active, adaptive coping mechanisms of resilience; however, in humans, most published work focuses on correlative neuroendocrine markers that are associated with a resilient phenotype. In this review, we highlight a growing literature in rodents that is starting to complement the human work by identifying the active behavioral, neural, molecular and hormonal basis of resilience. The therapeutic implications of these findings are important and can pave the way for an innovative approach to drug development for a range of stress-related syndromes.

Epigenetic mechanisms of depression and antidepressant action

Epigenetic mechanisms, which control chromatin structure and function, mediate changes in gene expression that occur in response to diverse stimuli. Recent research has established that environmental events and behavioral experience induce epigenetic changes at particular gene loci and that these changes help shape neuronal plasticity and function and hence behavior. Some of these changes can be stable and can even persist for a lifetime. Increasing evidence supports the hypothesis that aberrations in chromatin remodeling and subsequent effects on gene expression within limbic brain regions contribute to the pathogenesis of depression and other stress-related disorders such as post-traumatic stress disorder and other anxiety syndromes. Likewise, the gradually developing but persistent therapeutic effects of antidepressant medications may be achieved in part via epigenetic mechanisms. This review discusses recent advances in our understanding of the epigenetic regulation of stress-related disorders and focuses on three distinct aspects of stress-induced epigenetic pathology: the effects of stress and antidepressant treatment during adulthood, the lifelong effects of early-life stress on subsequent stress vulnerability, and the possible transgenerational transmission of stress-induced abnormalities.

How can the study of physiological reactivity contribute to our understanding of adversity and resilience processes in development?

The focus of this article is to present current progress in understanding the interplay among adversity, physiological sensitivity to context, and adaptive functioning, with an emphasis on implications and future directions for resilience researchers. It includes a review of current literature that demonstrates (a) links between various levels of adversity exposure and variability in physiological reactivity, (b) how the interplay between children's physiological reactivity and different sources of risk and adversity relates to variability in adaptive functioning, and (c) various approaches for capturing a more dynamic nature of physiological reactivity and related processes. Throughout, important conceptual and empirical issues are highlighted.

Enhanced stimulus sequence-dependent repeated learning in male offspring after prenatal stress alone or in conjunction with lead exposure

Both lead (Pb) exposure and prenatal stress (PS) can produce cognitive deficits, and in a prior study we demonstrated enhanced cognitive deficits in repeated learning of female rats exposed to both of these developmental insults (Cory-Slechta et al., 2010). However, PS can also lead to improved cognitive outcomes that are both gender- and context-dependent. Thus, the current study examined whether Pb ± PS likewise produced repeated learning deficits in males, either after maternal or lifetime Pb exposure. Repeated learning was evaluated using a multiple schedule of repeated learning and performance that required learning 3-response sequences in male offspring that had been subjected to either maternal Pb (0 or 150 ppm) or lifetime Pb exposure (0 or 50 ppm) beginning two months prior to dam breeding, to prenatal immobilization restraint stress (gestational days 16-17), or to both Pb and PS. Blood Pb, corticosterone, hippocampal glucocorticoid receptor density and brain monoamines were also measured. In contrast to outcomes in females, sequence-specific enhancements of repeated learning accuracy were produced by PS, particularly when combined with Pb, results that appeared to be more robust in combination with lifetime than maternal Pb exposure. A common behavioral mechanism of these improvements appears to be an increased reinforcement density associated with increased response rates and shorter session times seen with PS ± Pb that could shorten time to reinforcement. Trends toward lower levels of nucleus accumbens dopamine activity seen after both maternal Pb and lifetime Pb combined with PS suggest a possible role for this region/neurotransmitter in enhanced accuracy, whereas PS ± Pb-induced corticosterone changes did not exhibit an obvious systematic relationship to accuracy enhancements. While PS ± Pb-based increases in accuracy appear to be an improved outcome, the benefits of increased response rate are by no means universal, but highly context-dependent and can lead to adverse behavioral effects in other conditions.

Antidepressants and the resilience to early-life stress in inbred mouse strains

RATIONALE

Selecting an effective treatment for patients with major depressive disorder is a perpetual problem for psychiatrists. It is of particular interest to explore the interaction between genetic predisposition and environmental factors.

OBJECTIVES

Mouse inbred strains vary in baseline performance in depression-related behaviour tests, which were originally validated as tests of antidepressant response. Therefore, we investigated interactions between environmental stress, genotype, and drug response in a multifactorial behaviour study.

METHOD

Our study design included four inbred mouse strains (129S1/SvlmJ, C57LB/6J, DBA/2J and FVB/NJ) of both sexes, two subjected to environmental manipulations (maternal separation and unpredictable chronic mild stress) and two representative of treatment with antidepressants (escitalopram and nortryptiline vs. vehicle). The mice treated with antidepressants were further divided into those administered acute (1 day) and subchronic (14 days) regimes, giving 144 experimental groups in all, each with at least seven animals. All animals were tested using the Porsolt forced-swim test (FST) and the hole-board test.

RESULTS

Despite a 24-h maternal separation (MS) or a 14-day unpredictable chronic mild stress protocol, most animals seemed to be resilient to the stress induced. One compelling finding is the long-lasting, strain-specific effect of MS resulting in an increased depression-like behaviour in the Porsolt FST and elevated anxiety-related behaviour in the hole-board test seen in 129S1/SvImJ mice. Nortriptyline was effective in reversing the effect of MS in the FST in 129S1/SvlmJ male mice.

CONCLUSION

A single 24-h maternal separation of pups from their mother on postnatal day 9 is a sufficient insult to result in a depression-like phenotype in adult 129S1/SvImJ mice but not in C57LB/6 J, DBA/2 J, and FVB/NJ mice.

Chronic corticosterone exposure during adolescence reduces impulsive action but increases impulsive choice and sensitivity to yohimbine in male Sprague-Dawley rats

Chronic stress during adolescence is associated with an increased risk for alcoholism and addictive disorders. Addiction is also associated with increased impulsivity, and stress during adolescence could alter cortical circuits responsible for response inhibition. Therefore, the present study determined the effect of chronic exposure to the stress hormone corticosterone (CORT) during adolescence on tests of impulsivity in adulthood and examined possible biochemical mechanisms. Male Sprague-Dawley rats were exposed to CORT by their drinking water during adolescence (post-natal day 30-50). The rats were then tested in adulthood to assess behavior on the 5-choice serial reaction time task (5CSRTT), stop-signal reaction time task (SSRTT), and the delay-discounting task, which differentially assess attention, impulsive action, and impulsive choice. Yohimbine-induced impulsivity on the 5CSRTT and biochemical analysis of the lateral orbital frontal cortex (lOFC) was also assessed owing to the ability of yohimbine to activate the hypothalamic-pituitary-adrenal axis and influence impulsivity. Adolescent CORT-treated rats were found to behave largely like controls on the 5CSRTT, but did show reduced premature responses when the intertrial interval was increased. Nevertheless, the CORT-treated rats tended to have more yohimbine-induced impulsive responses at low doses on this task, which was not found to be due to increased pCREB in the lOFC, but could be related to a higher expression/activity of the AMPA receptor subunit GluR1. Adolescent CORT-treated rats performed more accurately on the SSRTT, but showed greater impulsivity on the delay-discounting task, as indicated by steeper discounting functions. Therefore, adolescent CORT exposure reduced impulsive action but increased impulsive choice, indicating that chronic stress hormone exposure in adolescence can have long-term consequences on behavior.

Racial and ethnic differences in diurnal cortisol rhythms in preadolescents: the role of parental psychosocial risk and monitoring

Racial/ethnic minorities experience persistent health disparities due in part to their exposure to chronic SES and psychosocial risk. The hypothalamic-pituitary-adrenal axis and its hormonal end product, cortisol, are believed to mediate the associations between chronic stress and poor health. In this study, racial/ethnic differences in diurnal salivary cortisol rhythms in 179 preadolescent youths and the contributing roles of SES risk, psychosocial risk, perceived discrimination, harsh parenting, and parental monitoring were examined. The analyses revealed racial/ethnic differences in diurnal cortisol rhythms, with African Americans having significantly flatter morning-to-evening cortisol slopes than Caucasians and with Latinos having significantly lower evening cortisol levels than Caucasians. Greater psychosocial risk and less parental monitoring were associated with flatter cortisol slopes. Racial/ethnic differences on the cortisol measures persisted when controlling for SES, psychosocial risk, and parenting quality. The need to assess chronic risk across the lifespan and disentangle possible genetic from environmental contributors is discussed.

Post-weaning to pre-pubertal ('juvenile') stress: a model of induced predisposition to stress-related disorders

Human studies suggest that childhood trauma predisposes individuals to develop stress-related disorders such as depression and post-traumatic stress disorder (PTSD). Recent years have witnessed growing interest in effectively modeling in animals the long-term effects of childhood emotional trauma on stress responses in adulthood. Most studies concerned with the impact of early-life stress on subsequent stress responses in adulthood in rodents have focused on the post-natal pre-weaning period. However, psychiatric studies often refer to human childhood rather than infancy when investigating the patients' traumatic history of stress-related psychopathologies. In accordance with that, we have examined the consequences of stress exposure at a later early-life period, the post-weaning, pre-puberty (juvenile) period, which holds greater resemblance to human childhood. This review summarizes a series of studies examining the impact of exposure of rats to stressors during 'juvenility' ('juvenile stress') on the ability of these animals to cope with stress later in life. Exposure to relatively brief but significant stress experience during juvenility was found to impair the ability of animals to cope with stressful challenges in adulthood. These behavioral manifestations were associated with lasting alterations in limbic system brain regions of neuromodulatory pathways, such as alterations in the expression of cell adhesion molecules, GABAergic system functioning and alterations in levels of circulating corticosterone. Importantly, these studies have also demonstrated considerable individual and sex differences, which call for the development of adequate analysis approaches. The juvenile stress model combined with characterization of individual profiles is presented as a useful model to study in rodents different facets of stress-related disorders and neural mechanisms of vulnerability and resilience to stress.

Mild prenatal stress-modulated behavior and neuronal spine density without affecting amphetamine sensitization

The present study investigated the effect of prenatal stress (PS) on juvenile behavior and adult amphetamine (AMPH) sensitization, as well as the effect of the interaction between experience (i.e. PS) and drug (i.e. AMPH) on cortical thickness and neuronal morphology in corticolimbic regions in rats. Juvenile male and female rats, exposed to gestational stress, were tested in behavioral tasks that included open field locomotion, elevated plus maze, novel object recognition, and play fighting behavior. The development and persistence of drug-induced behavioral sensitization in adults were tested by chronic AMPH administration and challenge, respectively. Spine density in corticolimbic regions was examined for structural plasticity. The findings showed that PS produced anxiety-like behavior in males. Furthermore, PS in males resulted in female-like play and enhanced partial rotation defense, whereas in females PS increased the probability of evasion in response to an attack. AMPH administration resulted in gradual increase in behavioral sensitization that persisted at least for 2 weeks; however, PS did not influence AMPH-induced behavioral sensitization in either male or female rats. Moreover, PS increased the spine density in the nucleus accumbens (NAc) and decreased it in the medial prefrontal cortex (mPFC) without any alteration in the orbital frontal cortex (OFC). Similarly, AMPH administration increased spine density in the NAc and mPFC, whereas a decrease was observed in the OFC. However, PS prevented the drug-induced alterations in the spine density observed in controls. In sum, PS modulated juvenile behavior and altered brain morphology without influencing AMPH-induced behavioral sensitization substantially.

From Freud to a modern understanding of behavioral, physiological, and brain development

Seymour Levine's first "early experience" experiments were inspired by Freud. Yet, Levine's lifetime of work, and the work of his colleagues and scientists who followed, unveiled a myriad of early experience effects that even Freud himself could not have imagined. Related to and extending beyond his work on early experience, Levine also made important, often seminal, contributions to overlapping and related areas, such as early maternal separation and deprivation, maternal behavior and physiology, sexual differentiation, perinatal malnutrition, attachment in non-human primates, hypothalamic-pituitary-adrenal (HPA) stress reactivity and its adaptive significance, and the development of the HPA system. Moreover, his work spawned new lines of research by investigators active today. The papers contained in this special issue provide a sampling of research demonstrating some of the important directions in which those earliest experiments have led, many with clinical applications.

Evolutionary functions of early social modulation of hypothalamic-pituitary-adrenal axis development in humans

The hypothalamic-pituitary-adrenal axis (HPAA) is highly responsive to social challenges. Because stress hormones can have negative developmental and health consequences, this presents an evolutionary paradox: Why would natural selection have favored mechanisms that elevate stress hormone levels in response to psychosocial stimuli? Here we review the hypothesis that large brains, an extended childhood and intensive family care in humans are adaptations resulting from selective forces exerted by the increasingly complex and dynamic social and cultural environment that co-evolved with these traits. Variations in the modulation of stress responses mediated by specific HPAA characteristics (e.g., baseline cortisol levels, and changes in cortisol levels in response to challenges) are viewed as phenotypically plastic, ontogenetic responses to specific environmental signals. From this perspective, we discuss relations between physiological stress responses and life history trajectories, particularly the development of social competencies. We present brief summaries of data on hormones, indicators of morbidity and social environments from our long-term, naturalistic studies in both Guatemala and Dominica. Results indicate that difficult family environments and traumatic social events are associated with temporal elevations of cortisol, suppressed reproductive functioning and elevated morbidity. The long-term effects of traumatic early experiences on cortisol profiles are complex and indicate domain-specific effects, with normal recovery from physical stressors, but some heightened response to negative-affect social challenges. We consider these results to be consistent with the hypothesis that developmental programming of the HPAA and other neuroendocrine systems associated with stress responses may facilitate cognitive targeting of salient social challenges in specific environments.