Mechanisms of late-onset cognitive decline after early-life stress

Early-life stress produces muscle hyperalgesia and nociceptor sensitization in the adult rat

Chronic pain in adults has been associated with early-life stress. To examine the pronociceptive effect of early-life stress, we evaluated cutaneous and muscle nociception and activity in muscle nociceptors in an animal model of neonatal stress, limited bedding, in the rat. In this neonatal limited bedding (NLB) model, litters are exposed to limited bedding between postnatal days 2 and 9, and controls to standard bedding. In adult NLB-treated rats, mechanical nociceptive threshold in skeletal muscle was significantly lower (~22%) than in controls. Furthermore, administration of prostaglandin E(2) in skin as well as muscle produced markedly prolonged hyperalgesia, an effect prevented by spinal intrathecal injection of oligodeoxynucleotide antisense to protein kinase Cε (PKCε), a second messenger in nociceptors that has been implicated in the induction and maintenance of chronic pain. In electrophysiological studies, mechanical threshold of muscle nociceptors was reduced by ~31% and conduction velocity significantly increased (~28%). These findings indicate that neonatal stress induces a persistent hyperalgesia and nociceptor sensitization manifest in the adult and that the second messenger PKCε may be a target against which therapies might be directed to treat a chronic pain syndrome that is associated with early-life traumatic stress.

Ovarian hormones influence corticotropin releasing factor receptor colocalization with delta opioid receptors in CA1 pyramidal cell dendrites

Stress interacts with addictive processes to increase drug use, drug seeking, and relapse. The hippocampal formation (HF) is an important site at which stress circuits and endogenous opioid systems intersect and likely plays a critical role in the interaction between stress and drug addiction. Our prior studies demonstrate that the stress-related neuropeptide corticotropin-releasing factor (CRF) and the delta-opioid receptor (DOR) colocalize in interneuron populations in the hilus of the dentate gyrus and stratum oriens of CA1 and CA3. While independent ultrastructural studies of DORs and CRF receptors suggest that each receptor is found in CA1 pyramidal cell dendrites and dendritic spines, whether DORs and CRF receptors colocalize in CA1 neuronal profiles has not been investigated. Here, hippocampal sections of adult male and proestrus female Sprague-Dawley rats were processed for dual label pre-embedding immunoelectron microscopy using well-characterized antisera directed against the DOR for immunoperoxidase and against the CRF receptor for immunogold. DOR-immunoreactivity (-ir) was found presynaptically in axons and axon terminals as well as postsynaptically in somata, dendrites and dendritic spines in stratum radiatum of CA1. In contrast, CRF receptor-ir was predominantly found postsynaptically in CA1 somata, dendrites, and dendritic spines. CRF receptor-ir frequently was observed in DOR-labeled dendritic profiles and primarily was found in the cytoplasm rather than at or near the plasma membrane. Quantitative analysis of CRF receptor-ir colocalization with DOR-ir in pyramidal cell dendrites revealed that proestrus females and males show comparable levels of CRF receptor-ir per dendrite and similar cytoplasmic density of CRF receptor-ir. In contrast, proestrus females display an increased number of dual-labeled dendritic profiles and an increased membrane density of CRF receptor-ir in comparison to males. We further examined the functional consequences of CRF receptor-ir colocalization with DOR-ir in the same neuron using the hormone responsive neuronal cell line NG108-15, which endogenously expresses DORs, and assayed intracellular cAMP production in response to CRF receptor and DOR agonists. Results demonstrated that short-term application of DOR agonist SNC80 inhibited CRF-induced cAMP accumulation in NG108-15 cells transfected with the CRF receptor. These studies provide new insights on opioid-stress system interaction in the hippocampus of both males and females and establish potential mechanisms through which DOR activation may influence CRF receptor activity.

Stress effects on memory: an update and integration

It is well known that stressful experiences may affect learning and memory processes. Less clear is the exact nature of these stress effects on memory: both enhancing and impairing effects have been reported. These opposite effects may be explained if the different time courses of stress hormone, in particular catecholamine and glucocorticoid, actions are taken into account. Integrating two popular models, we argue here that rapid catecholamine and non-genomic glucocorticoid actions interact in the basolateral amygdala to shift the organism into a 'memory formation mode' that facilitates the consolidation of stressful experiences into long-term memory. The undisturbed consolidation of these experiences is then promoted by genomic glucocorticoid actions that induce a 'memory storage mode', which suppresses competing cognitive processes and thus reduces interference by unrelated material. Highlighting some current trends in the field, we further argue that stress affects learning and memory processes beyond the basolateral amygdala and hippocampus and that stress may pre-program subsequent memory performance when it is experienced during critical periods of brain development.

Early life stress impairs social recognition due to a blunted response of vasopressin release within the septum of adult male rats

Early life stress poses a risk for the development of psychopathologies characterized by disturbed emotional, social, and cognitive performance. We used maternal separation (MS, 3h daily, postnatal days 1-14) to test whether early life stress impairs social recognition performance in juvenile (5-week-old) and adult (16-week-old) male Wistar rats. Social recognition was tested in the social discrimination test and defined by increased investigation by the experimental rat towards a novel rat compared with a previously encountered rat. Juvenile control and MS rats demonstrated successful social recognition at inter-exposure intervals of 30 and 60 min. However, unlike adult control rats, adult MS rats failed to discriminate between a previously encountered and a novel rat after 60 min. The social recognition impairment of adult MS rats was accompanied by a lack of a rise in arginine vasopressin (AVP) release within the lateral septum seen during social memory acquisition in adult control rats. This blunted response of septal AVP release was social stimulus-specific because forced swimming induced a rise in septal AVP release in both control and MS rats. Retrodialysis of AVP (1 μg/ml, 3.3 μl/min, 30 min) into the lateral septum during social memory acquisition restored social recognition in adult MS rats at the 60-min interval. These studies demonstrate that MS impairs social recognition performance in adult rats, which is likely caused by blunted septal AVP activation. Impaired social recognition may be linked to MS-induced changes in other social behaviors like aggression as shown previously.

Emerging roles of epigenetic mechanisms in the enduring effects of early-life stress and experience on learning and memory

Epigenetic mechanisms are involved in programming gene expression throughout development. In addition, they are key contributors to the processes by which early-life experience fine-tunes the expression levels of key neuronal genes, governing learning and memory throughout life. Here we describe the long-lasting, bi-directional effects of early-life experience on learning and memory. We discuss how enriched postnatal experience enduringly augments spatial learning, and how chronic early-life stress results in persistent and progressive deficits in the structure and function of hippocampal neurons. The existing and emerging roles of epigenetic mechanisms in these fundamental neuroplasticity phenomena are illustrated.

Covariance modeling of MRI brain volumes in memory circuitry in schizophrenia: Sex differences are critical

Women have consistently demonstrated better verbal memory on tests that evaluate immediate and delayed free recall. In patients with schizophrenia, these verbal memory processes are relatively more preserved in women than men. However an understanding of the brain anatomy of the female advantage for verbal memory is still unclear. 29 females and 59 males with schizophrenia made comparable to 21 female and 27 male healthy volunteers were scanned using structural magnetic resonance imaging (sMRI) in order to assess volumes of regions across the entire brain. Sex differences within and between groups in the covariance structure of memory circuitry regions were evaluated using a novel approach to covariance analysis (the Box M Test). Brain areas of interest included the prefrontal cortex (PFC), inferior parietal lobule (iPAR), anterior cingulate gyrus (ACG), parahippocampus, and hippocampus (HIPP). Results showed significant differences in the covariance matrices of females and males with schizophrenia compared with their healthy counterparts, in particular the relationships between iPAR-PFC, iPAR-ACG, and HIPP-PFC. Sex differences in the iPAR-PFC relationship were significantly associated with sex differences in verbal memory performance. In control women, but not in men ACG volume correlated strongly with memory performance. In schizophrenia, ACG volume was reduced in females, but not in men, relative to controls. Findings suggest that the relationship between iPAR and PFC is particularly important for understanding the relative preservation of verbal memory processing in females with schizophrenia and may compensate for ACG volume reductions. These results illustrate the utility of a unique covariance structure modeling approach that yields important new knowledge for understanding the nature of schizophrenia.

Hippocampal volume and sensitivity to maternal aggressive behavior: a prospective study of adolescent depressive symptoms

It has been suggested that biological factors confer increased sensitivity to environmental influences on depressive symptoms during adolescence, a crucial time for the onset of depressive disorders. Given the critical role of the hippocampus in sensitivity to stress and processing of contextual aspects of the environment, investigation of its role in determining sensitivity to environmental context seems warranted. This study prospectively examined hippocampal volume as a measure of sensitivity to the influence of aggressive maternal behavior on change in depressive symptoms from early to midadolescence. The interaction between aggressive maternal behavior and hippocampal volume was found to predict change in depressive symptoms. Significant sex differences also emerged, whereby only for girls were larger bilateral hippocampal volumes more sensitive to the effects of maternal aggressive behavior, particularly with respect to experiencing the protective effects of low levels of maternal aggressiveness. These findings help elucidate the complex relationships between brain structure, environmental factors such as maternal parenting style, and sensitivity to (i.e., risk for, and protection from) the emergence of depression during this life stage. Given that family context risk factors are modifiable, our findings suggest the potential utility of targeted parenting interventions for the prevention and treatment of adolescent depressive disorder.

Mechanical-tactile stimulation (MTS) during neonatal stress prevents hyperinsulinemia despite stress-induced adiposity in weanling rat pups

Stress in early life negatively influences growth quality through perturbations in body composition including increased fat mass. At term (40 weeks) preterm infants have greater fat mass and abdominal visceral adipose tissue than term-born infants. Mechanical-tactile stimulation (MTS) attenuates the stress response in preterm infants and rodents. We tested the hypothesis that MTS, administered during an established model of neonatal stress, would decrease stress-driven adiposity and prevent associated metabolic imbalances in rat pups. Pups received one of three treatments from postnatal days 5 to P9: Neonatal Stress (Stress; n=20) = painful stimulus and hypoxic/hyperoxic challenge during 60 min of maternal separation; MTS (n=20) = neonatal stress+10 min of MTS; or Control (n=20). Body weight, DXA whole body fat mass (g), MRI subcutaneous and visceral adipose tissue, and fasting adiponectin, leptin, glucose, insulin, and corticosterone were measured at weaning (P21). Stress and MTS weight gain (g/d) were accelerated following neonatal stress with greater fat mass, abdominal subcutaneous adipose tissue, serum adiponectin, leptin, and fasting glucose at weaning (P21). Male Stress and MTS pups had greater visceral adipose tissue depot. Male and female Stress pups were hyperinsulinemic. In summary, neonatal stress compromised body composition by increasing fat mass and abdominal subcutaneous adipose tissue depot, and in males, visceral adipose tissue depot. Importantly, MTS prevented hyperinsulinemia despite of stress-induced adiposity. We conclude that MTS during neonatal stress has the potential to minimize metabolic consequences associated with stress-driven perturbations in fat mass and abdominal adipose depots.

Early-life stress and cognitive outcome

RATIONALE

Early-life stress is associated with later neuropsychiatric illness. While the association between early-life stress and brain development is well recognized, relatively few studies have examined the association between exposure to early-life stress and cognitive outcome.

OBJECTIVES

The objective of this paper is to examine the association between early-life stress and cognitive outcome in animal models and humans.

METHODS

In this article, we review alterations in cognitive function associated with early-life stress in animals and then discuss the association of early-life stress and cognitive function in humans.

RESULTS

Findings suggest that early-life stress is associated with abnormal cognitive function in animals and humans. Furthermore, cognitive deficits associated with exposure to early-life stress in humans may persist into at least early adulthood, although animal models of enriched environments and studies of children adopted from institutionalized care into foster families suggest that certain social factors may at least partially reverse cognitive deficits following exposure to early-life stress.

CONCLUSIONS

Exposure to stress in early life may be associated with later deficits in cognitive function.

Early maternal deprivation affects dentate gyrus structure and emotional learning in adult female rats

RATIONALE

Stress elicits functional and structural changes in the hippocampus. Early life stress is one of the major risk factors for stress-related pathologies like depression. Patients suffering from depression show a reduced hippocampal volume, and in women, this occurs more often when depression is preceded by childhood trauma. However, the underlying mechanisms that account for a reduced hippocampal volume are unknown.

OBJECTIVE

We examined the effects of maternal absence on structure and function of the hippocampus in female offspring.

METHODS

We studied whether 24 h of maternal deprivation (MD) on postnatal day 3 altered adult neurogenesis, individual neuronal morphology and dentate gyrus (DG) structure in young adult female rats. In addition, functional alterations were addressed by studying synaptic plasticity in vitro, and spatial as well as emotional learning was tested.

RESULTS

Adult females that were subjected to MD revealed significant reductions in DG granule cell number and density. In addition, DG neurons were altered in their dendritic arrangement. No effects on the rate of adult neurogenesis were found. Furthermore, MD did not alter synaptic plasticity in vitro, neither under normal nor high-stress conditions. In addition, spatial learning and contextual fear conditioning were comparable between control and MD animals. However, MD animals showed an improved amygdala-dependent fear memory.

CONCLUSION

Although early life stress exposure did not impair hippocampus-dependent functioning in female offspring, it irreversibly affected DG structure by reducing cell numbers. This may be relevant for the reduced hippocampal volume observed in depression and the increased vulnerability of women to develop depression.

Early life stress paradigms in rodents: potential animal models of depression?

RATIONALE

While human depressive illness is indeed uniquely human, many of its symptoms may be modeled in rodents. Based on human etiology, the assumption has been made that depression-like behavior in rats and mice can be modulated by some of the powerful early life programming effects that are known to occur after manipulations in the first weeks of life.

OBJECTIVE

Here we review the evidence that is available in literature for early life manipulation as risk factors for the development of depression-like symptoms such as anhedonia, passive coping strategies, and neuroendocrine changes. Early life paradigms that were evaluated include early handling, separation, and deprivation protocols, as well as enriched and impoverished environments. We have also included a small number of stress-related pharmacological models.

RESULTS

We find that for most early life paradigms per se, the actual validity for depression is limited. A number of models have not been tested with respect to classical depression-like behaviors, while in many cases, the outcome of such experiments is variable and depends on strain and additional factors.

CONCLUSION

Because programming effects confer vulnerability rather than disease, a number of paradigms hold promise for usefulness in depression research, in combination with the proper genetic background and adult life challenges.

Electrophysiological insights into the enduring effects of early life stress on the brain

Increasing evidence links exposure to stress early in life to long-term alterations in brain function, which in turn have been linked to a range of psychiatric and neurological disorders in humans. Electrophysiological approaches to studying these causal pathways have been relatively underexploited. Effects of early life stress on neuronal electrophysiological properties offer a set of potential mechanisms for these susceptibilities, notably in the case of epilepsy. Thus, we review experimental evidence for altered cellular and circuit electrophysiology resulting from exposure to early life stress. Much of this work focuses on limbic long-term potentiation, but other studies address alterations in electrophysiological properties of ion channels, neurotransmitter systems, and the autonomic nervous system. We discuss mechanisms which may mediate these effects, including influences of early life stress on key components of brain synaptic transmission, particularly glutamate, GABA and 5-HT receptors, and influences on neuroplasticity (primarily neurogenesis and synaptic density) and on neuronal network activity. The existing literature, although small, provides strong evidence that early life stress induces enduring, often robust effects on a range of electrophysiological properties, suggesting further study of enduring effects of early life stress employing electrophysiological methods and concepts will be productive in illuminating disease pathophysiology.

Epigenetic marking of the BDNF gene by early-life adverse experiences

Studies over the past half-century have made it clear that environmental influences in development, particularly stress and traumatic experiences, can remain pervasive across the lifespan. Though it has been hypothesized for some time that the long-term consequences of early-life adversity represent epigenetic influences, it has not been until recently that studies have begun to provide empirical support of experience-driven epigenetic modifications to the genome. Here we focus on this theme, and review current knowledge pertaining to the epigenetics of behavioral development. At the center of our discussion is the brain-derived neurotrophic factor (BDNF) gene, as abnormal BDNF gene activity is a leading etiological hypothesis by which early-life adverse experiences persistently modify brain and behavioral plasticity.

Hypericum perforatum treatment: effect on behaviour and neurogenesis in a chronic stress model in mice

Background

Extracts of Hypericum perforatum (St. John's wort) have been traditionally recommended for a wide range of medical conditions, in particular mild-to-moderate depression. The present study was designed to investigate the effect of Hypericum perforatum treatment in a mouse model of anxiety/depressive-like behavior, induced by chronic corticosterone administration.

Methods

CD1 mice were submitted to 7 weeks corticosterone administration and then behavioral tests as Open Field (OF), Novelty-Suppressed Feeding (NSF), Forced Swim Test (FST) were performed. Cell proliferation in hippocampal dentate gyrus (DG) was investigated by both 5-bromo-2'-deoxyuridine (BrdU) and doublecortin (DCX) immunohistochemistry techniques and stereological procedure was used to quantify labeled cells. Golgi-impregnation method was used to evaluate changes in dendritic spines in DG. Hypericum perforatum (30 mg/Kg) has been administered for 3 weeks and then neural development in the adult hippocampus and behavioral changes have been examined.

Results

The anxiety/depressive-like state due to chronic corticosterone treatment was reversed by exogenous administration of Hypericum perforatum; the proliferation of progenitor cells in mice hippocampus was significantly reduced under chronic corticosterone treatment, whereas a long term treatment with Hypericum perforatum prevented the corticosterone-induced decrease in hippocampal cell proliferation. Corticosterone-treated mice exhibited a reduced spine density that was ameliorated by Hypericum perforatum administration.

Conclusion

These results provide evidence of morphological adaptations occurring in mature hippocampal neurons that might underlie resilient responses to chronic stress and contribute to the therapeutic effects of chronic Hypericum perforatum treatment.

Presynaptic BDNF promotes postsynaptic long-term potentiation in the dorsal striatum

Brain-derived neurotrophic factor (BDNF) facilitates the formation of long-term potentiation (LTP) in hippocampus, but whether this involves release from presynaptic versus postsynaptic pools is unclear. We therefore tested whether BDNF is essential for LTP in dorsal striatum, a structure in which the neurotrophin is present only in afferent terminals. Whole-cell recordings were collected from medium spiny neurons in striatal slices prepared from adult mice. High-frequency stimulation (HFS) of neocortical afferents produced a rapid and stable NMDA receptor-dependent potentiation. The ratio of AMPA to NMDA receptor-mediated components of the EPSPs was substantially increased after inducing potentiation, suggesting that the response enhancement involved postsynaptic changes. In accord with this, paired-pulse response ratios, a measure of transmitter release kinetics, were reduced by elevated calcium but not by LTP. Infusion of the BDNF scavenger TrkB-Fc blocked the formation of potentiation, beginning with the second minute after HFS, without reducing responses to HFS. These results suggest that presynaptic pools of BDNF can act within 2 min of HFS to support the formation of a postsynaptic form of LTP in striatum.

Hippocampal dysfunction and cognitive impairments provoked by chronic early-life stress involve excessive activation of CRH receptors

Chronic stress impairs learning and memory in humans and rodents and disrupts long-term potentiation (LTP) in animal models. These effects are associated with structural changes in hippocampal neurons, including reduced dendritic arborization. Unlike the generally reversible effects of chronic stress on adult rat hippocampus, we have previously found that the effects of early-life stress endure and worsen during adulthood, yet the mechanisms for these clinically important sequelae are poorly understood. Stress promotes secretion of the neuropeptide corticotropin-releasing hormone (CRH) from hippocampal interneurons, activating receptors (CRF(1)) located on pyramidal cell dendrites. Additionally, chronic CRF(1) occupancy negatively affects dendritic arborization in mouse organotypic slice cultures, similar to the pattern observed in middle-aged, early-stressed (CES) rats. Here we found that CRH expression is augmented in hippocampus of middle-aged CES rats, and then tested whether the morphological defects and poor memory performance in these animals involve excessive activation of CRF(1) receptors. Central or peripheral administration of a CRF(1) blocker following the stress period improved memory performance of CES rats in novel-object recognition tests and in the Morris water maze. Consonant with these effects, the antagonist also prevented dendritic atrophy and LTP attenuation in CA1 Schaffer collateral synapses. Together, these data suggest that persistently elevated hippocampal CRH-CRF(1) interaction contributes importantly to the structural and cognitive impairments associated with early-life stress. Reducing CRF(1) occupancy post hoc normalized hippocampal function during middle age, thus offering potential mechanism-based therapeutic interventions for children affected by chronic stress.

Stress, sex, and neural adaptation to a changing environment: mechanisms of neuronal remodeling

The adult brain is much more resilient and adaptable than previously believed, and adaptive structural plasticity involves growth and shrinkage of dendritic trees, turnover of synapses, and limited amounts of neurogenesis in the forebrain, especially the dentate gyrus of the hippocampal formation. Stress and sex hormones help to mediate adaptive structural plasticity, which has been extensively investigated in the hippocampus and to a lesser extent in the prefrontal cortex and amygdala, all brain regions that are involved in cognitive and emotional functions. Stress and sex hormones exert their effects on brain structural remodeling through both classical genomic as well as non-genomic mechanisms, and they do so in collaboration with neurotransmitters and other intra- and extracellular mediators. This review will illustrate the actions of estrogen on synapse formation in the hippocampus and the process of stress-induced remodeling of dendrites and synapses in the hippocampus, amygdala, and prefrontal cortex. The influence of early developmental epigenetic events, such as early life stress and brain sexual differentiation, is noted along with the interactions between sex hormones and the effects of stress on the brain. Because hormones influence brain structure and function and because hormone secretion is governed by the brain, applied molecular neuroscience techniques can begin to reveal the role of hormones in brain-related disorders and the treatment of these diseases. A better understanding of hormone-brain interactions should promote more flexible approaches to the treatment of psychiatric disorders, as well as their prevention through both behavioral and pharmaceutical interventions.

Early life programming and neurodevelopmental disorders

For more than a century, clinical investigators have focused on early life as a source of adult psychopathology. Early theories about psychic conflict and toxic parenting have been replaced by more recent formulations of complex interactions of genes and environment. Although the hypothesized mechanisms have evolved, a central notion remains: early life is a period of unique sensitivity during which experience confers enduring effects. The mechanisms for these effects remain almost as much a mystery today as they were a century ago. Recent studies suggest that maternal diet can program offspring growth and metabolic pathways, altering lifelong susceptibility to diabetes and obesity. If maternal psychosocial experience has similar programming effects on the developing offspring, one might expect a comparable contribution to neurodevelopmental disorders, including affective disorders, schizophrenia, autism, and eating disorders. Due to their early onset, prevalence, and chronicity, some of these disorders, such as depression and schizophrenia, are among the highest causes of disability worldwide according to the World Health Organization 2002 report. Consideration of the early life programming and transcriptional regulation in adult exposures supports a critical need to understand epigenetic mechanisms as a critical determinant in disease predisposition. Incorporating the latest insight gained from clinical and epidemiological studies with potential epigenetic mechanisms from basic research, the following review summarizes findings from a workshop on Early Life Programming and Neurodevelopmental Disorders held at the University of Pennsylvania in 2009.

Association of childhood trauma with cognitive function in healthy adults: a pilot study

BACKGROUND

Animal and human studies suggest that stress experienced early in life has detrimental consequences on brain development, including brain regions involved in cognitive function. Cognitive changes are cardinal features of depression and posttraumatic stress disorder. Early-life trauma is a major risk factor for these disorders. Only few studies have measured the long-term consequences of childhood trauma on cognitive function in healthy adults.

METHODS

In this pilot study, we investigated the relationship between childhood trauma exposure and cognitive function in 47 healthy adults, who were identified as part of a larger study from the general population in Wichita, KS. We used the Cambridge Neuropsychological Test Automated Battery (CANTAB) and the Wide-Range-Achievement-Test (WRAT-3) to examine cognitive function and individual achievement. Type and severity of childhood trauma was assessed by the Childhood Trauma Questionnaire (CTQ). Data were analyzed using multiple linear regression on CANTAB measures with primary predictors (CTQ scales) and potential confounders (age, sex, education, income).

RESULTS

Specific CTQ scales were significantly associated with measures of cognitive function. Emotional abuse was associated with impaired spatial working memory performance. Physical neglect correlated with impaired spatial working memory and pattern recognition memory. Sexual abuse and physical neglect were negatively associated with WRAT-3 scores. However, the association did not reach the significance level of p < 0.01.

CONCLUSIONS

Our results suggest that physical neglect and emotional abuse might be associated with memory deficits in adulthood, which in turn might pose a risk factor for the development of psychopathology.

Correlated memory defects and hippocampal dendritic spine loss after acute stress involve corticotropin-releasing hormone signaling

Stress affects the hippocampus, a brain region crucial for memory. In rodents, acute stress may reduce density of dendritic spines, the location of postsynaptic elements of excitatory synapses, and impair long-term potentiation and memory. Steroid stress hormones and neurotransmitters have been implicated in the underlying mechanisms, but the role of corticotropin-releasing hormone (CRH), a hypothalamic hormone also released during stress within hippocampus, has not been elucidated. In addition, the causal relationship of spine loss and memory defects after acute stress is unclear. We used transgenic mice that expressed YFP in hippocampal neurons and found that a 5-h stress resulted in profound loss of learning and memory. This deficit was associated with selective disruption of long-term potentiation and of dendritic spine integrity in commissural/associational pathways of hippocampal area CA3. The degree of memory deficit in individual mice correlated significantly with the reduced density of area CA3 apical dendritic spines in the same mice. Moreover, administration of the CRH receptor type 1 (CRFR(1)) blocker NBI 30775 directly into the brain prevented the stress-induced spine loss and restored the stress-impaired cognitive functions. We conclude that acute, hours-long stress impairs learning and memory via mechanisms that disrupt the integrity of hippocampal dendritic spines. In addition, establishing the contribution of hippocampal CRH-CRFR(1) signaling to these processes highlights the complexity of the orchestrated mechanisms by which stress impacts hippocampal structure and function.

Defining age limits of the sensitive period for attachment learning in rat pups

Enhanced odor preference learning and attenuated fear learning characterizes rat pups' attachment learning Sensitive Period for learning the maternal odor. This period terminates at 10 days old (PN10) with increasing endogenous levels of the stress hormone, corticosterone. Increasing Sensitive Period pups' corticosterone prematurely terminates the Sensitive Period, while decreasing corticosterone in older pups delays Sensitive Period termination. Here we extend these findings and define the age range corticosterone alters learning and question whether corticosterone permanently terminates the Sensitive Period. Pups were odor-0.5 mA shock conditioned with either corticosterone increased (PN5-6; 4 mg/kg vs. saline) or decreased (PN15-16; naturally by maternal presence or corticosterone synthesis blocker, Metyrapone). Finally, PN7-8 pups were conditioned with corticosterone and reconditioned without corticosterone to assess whether the Sensitive Period was permanently terminated. Results indicate developmental limits for corticosterone regulation of pup learning are PN6 through PN15. Furthermore, inducing precocious corticosterone induced fear learning was not permanent, since reconditioning without corticosterone enabled odor preference learning. Results suggest pups are protected from learning aversions to maternal odor until approaching weaning.

Maternal separation modulates short-term behavioral and physiological indices of the stress response

Early-life stress produces an anxiogenic profile in adulthood, presumably by activating the otherwise quiescent hypothalamic-pituitary-adrenal (HPA) axis during the vulnerable 'stress hyporesponsive period'. While the long-term effects of such early-life manipulations have been extensively characterized, little is known of the short-term effects. Here, we compared the short-term effects of two durations of maternal separation stress and one unseparated group (US) on behavioral and physiological indices of the stress response in rat pups. Separations included 3h on each of 12days, from postnatal day (PND) 2 to 13 (MS2-13) and 3days of daily, 6-h separation from PND11-13 (MS11-13). On PND14 (Experiment 1), both MS2-13 and MS11-13 produced marked reductions in freezing toward an adult male conspecific along with reduced levels of glucocorticoid type 2 (GR) and CRF type-1 (CRF(1)) receptor mRNA in the hippocampus. Group MS2-13 but not MS11-13 produced deficits in stressor-induced corticosterone secretion, accompanied by reductions in body weight. Our results suggest that GR and/or CRF(1) levels, not solely the magnitude of corticosterone secretion, may be involved in the modulation of freezing. In a second experiment, we aimed to extend these findings by testing male and female separated and unseparated pups' unconditioned defensive behaviors to cat odor on PND26, and subsequent cue+context conditioning and extinction throughout postnatal days 27-32. Our results show that maternal separation produced reductions in unconditioned freezing on PND26, with MS2-13 showing stronger deficits than MS11-13. However, separation did not affect any other defensive behaviors. Furthermore, separated rats failed to show conditioned freezing, although they did avoid the no-odor block conditioned cue. There were no sex differences other than weight. We suggest that maternal separation may have produced these changes by disrupting normal development of hippocampal regions involved in olfactory-mediated freezing, not in mechanisms of learning and memory per se. These findings may have direct relevance for understanding the mechanisms by which early-life adverse experiences produce short-term and lasting psychopathologies.

Severe early life stress hampers spatial learning and neurogenesis, but improves hippocampal synaptic plasticity and emotional learning under high-stress conditions in adulthood

Early life stress increases the risk for developing stress-related pathologies later in life. Recent studies in rats suggest that mild early life stress, rather than being overall unfavorable, may program the hippocampus such that it is optimally adapted to a stressful context later in life. Here, we tested whether this principle of "adaptive programming" also holds under severely adverse early life conditions, i.e., 24 h of maternal deprivation (MD), a model for maternal neglect. In young adult male rats subjected to MD on postnatal day 3, we observed reduced levels of adult hippocampal neurogenesis as measured by cell proliferation, cell survival, and neuronal differentiation. Also, mature dentate granule cells showed a change in their dendritic morphology that was most noticeable in the proximal part of the dendritic tree. Lasting structural changes due to MD were paralleled by impaired water maze acquisition but did not affect long-term potentiation in the dentate gyrus. Importantly, in the presence of high levels of the stress hormone corticosterone, even long-term potentiation in the dentate gyrus of MD animals was facilitated. In addition to this, contextual learning in a high-stress environment was enhanced in MD rats. These morphological, electrophysiological, and behavioral observations show that even a severely adverse early life environment does not evolve into overall impaired hippocampal functionality later in life. Rather, adversity early in life can prepare the organism to perform optimally under conditions associated with high corticosteroid levels in adulthood.

Central role of the brain in stress and adaptation: links to socioeconomic status, health, and disease

The brain is the key organ of stress reactivity, coping, and recovery processes. Within the brain, a distributed neural circuitry determines what is threatening and thus stressful to the individual. Instrumental brain systems of this circuitry include the hippocampus, amygdala, and areas of the prefrontal cortex. Together, these systems regulate physiological and behavioral stress processes, which can be adaptive in the short-term and maladaptive in the long-term. Importantly, such stress processes arise from bidirectional patterns of communication between the brain and the autonomic, cardiovascular, and immune systems via neural and endocrine mechanisms underpinning cognition, experience, and behavior. In one respect, these bidirectional stress mechanisms are protective in that they promote short-term adaptation (allostasis). In another respect, however, these stress mechanisms can lead to a long-term dysregulation of allostasis in that they promote maladaptive wear-and-tear on the body and brain under chronically stressful conditions (allostatic load), compromising stress resiliency and health. This review focuses specifically on the links between stress-related processes embedded within the social environment and embodied within the brain, which is viewed as the central mediator and target of allostasis and allostatic load.

Early-life stress disrupts attachment learning: the role of amygdala corticosterone, locus ceruleus corticotropin releasing hormone, and olfactory bulb norepinephrine

Infant rats require maternal odor learning to guide pups' proximity-seeking of the mother and nursing. Maternal odor learning occurs using a simple learning circuit including robust olfactory bulb norepinephrine (NE), release from the locus ceruleus (LC), and amygdala suppression by low corticosterone (CORT). Early-life stress increases NE but also CORT, and we questioned whether early-life stress disrupted attachment learning and its neural correlates [2-deoxyglucose (2-DG) autoradiography]. Neonatal rats were normally reared or stressed-reared during the first 6 d of life by providing the mother with insufficient bedding for nest building and were odor-0.5 mA shock conditioned at 7 d old. Normally reared paired pups exhibited typical odor approach learning and associated olfactory bulb enhanced 2-DG uptake. However, stressed-reared pups showed odor avoidance learning and both olfactory bulb and amygdala 2-DG uptake enhancement. Furthermore, stressed-reared pups had elevated CORT levels, and systemic CORT antagonist injection reestablished the age-appropriate odor-preference learning, enhanced olfactory bulb, and attenuated amygdala 2-DG. We also assessed the neural mechanism for stressed-reared pups' abnormal behavior in a more controlled environment by injecting normally reared pups with CORT. This was sufficient to produce odor aversion, as well as dual amygdala and olfactory bulb enhanced 2-DG uptake. Moreover, we assessed a unique cascade of neural events for the aberrant effects of stress rearing: the amygdala-LC-olfactory bulb pathway. Intra-amygdala CORT or intra-LC corticotropin releasing hormone (CRH) infusion supported aversion learning with intra-LC CRH infusion associated with increased olfactory bulb NE (microdialysis). These results suggest that early-life stress disturbs attachment behavior via a unique cascade of events (amygdala-LC-olfactory bulb).

A review of adversity, the amygdala and the hippocampus: a consideration of developmental timing

A review of the human developmental neuroimaging literature that investigates outcomes following exposure to psychosocial adversity is presented with a focus on two subcortical structures – the hippocampus and the amygdala. Throughout this review, we discuss how a consideration of developmental timing of adverse experiences and age at measurement might provide insight into the seemingly discrepant findings across studies. We use findings from animal studies to suggest some mechanisms through which timing of experiences may result in differences across time and studies. The literature suggests that early life may be a time of heightened susceptibility to environmental stressors, but that expression of these effects will vary by age at measurement.

The timing of prenatal exposure to maternal cortisol and psychosocial stress is associated with human infant cognitive development

The consequences of prenatal maternal stress for development were examined in 125 full-term infants at 3, 6, and 12 months of age. Maternal cortisol and psychological state were evaluated 5 times during pregnancy. Exposure to elevated concentrations of cortisol early in gestation was associated with a slower rate of development over the 1st year and lower mental development scores at 12 months. Elevated levels of maternal cortisol late in gestation, however, were associated with accelerated cognitive development and higher scores at 12 months. Elevated levels of maternal pregnancy-specific anxiety early in pregnancy were independently associated with lower 12-month mental development scores. These data suggest that maternal cortisol and pregnancy-specific anxiety have programming influences on the developing fetus.

Early life stress as an influence on limbic epilepsy: an hypothesis whose time has come?

The pathogenesis of mesial temporal lobe epilepsy (MTLE), the most prevalent form of refractory focal epilepsy in adults, is thought to begin in early life, even though seizures may not commence until adolescence or adulthood. Amongst the range of early life factors implicated in MTLE causation (febrile seizures, traumatic brain injury, etc.), stress may be one important contributor. Early life stress is an a priori agent deserving study because of the large amount of neuroscientific data showing enduring effects on structure and function in hippocampus and amygdala, the key structures involved in MTLE. An emerging body of evidence directly tests hypotheses concerning early life stress and limbic epilepsy: early life stressors, such as maternal separation, have been shown to aggravate epileptogenesis in both status epilepticus and kindling models of limbic epilepsy. In addition to elucidating its influence on limbic epileptogenesis itself, the study of early life stress has the potential to shed light on the psychiatric disorder that accompanies MTLE. For many years, psychiatric comorbidity was viewed as an effect of epilepsy, mediated psychologically and/or neurobiologically. An alternative – or complementary – perspective is that of shared causation. Early life stress, implicated in the pathogenesis of several psychiatric disorders, may be one such causal factor. This paper aims to critically review the body of experimental evidence linking early life stress and epilepsy; to discuss the direct studies examining early life stress effects in current models of limbic seizures/epilepsy; and to suggest priorities for future research.

The pathways from mother's love to baby's future

Together with genetic factors, early-life experience governs the expression and function of stress-related genes throughout life. This, in turn, contributes to either resilience or vulnerability to depression and to aging-related cognitive decline. In humans and animal models, both the quality and quantity of early-life maternal care has been shown to be a predominant signal triggering bi-directional and enduring changes in expression profiles of genes including glucocorticoids and corticotropin releasing factor (CRH; hypothalamic and hippocampal), associated with the development of resilient or vulnerable phenotypes. However, many crucial questions remain unresolved. For examples, how is the maternal-derived signal transmitted to specific neuronal populations where enduring (likely epigenetic) regulation of gene expression takes place? What is the nature of this information? In other words, how do neurons know to ‘turn on’ epigenetic machinery? What are the direct functional consequences of altered gene expression? This review describes the voyage of recurrent bursts of sensory input from the mother (‘mother's love’) to CRH-expressing hypothalamic neurons that govern the magnitude of the response to stress. In addition, the acute and enduring effects of both nurturing and fragmented maternal care on the structure, cellular signaling and function of specific hippocampal and hypothalamic neurons are discussed. The evolving understanding of the processes initiated by the early life experience of ‘mother's love’ suggest novel molecular targets for prevention and therapy of stress-related affective and cognitive disorders.

Postnatal stress produces hyperglycemia in adult rats exposed to hypoxia-ischemia

Fetal or early postnatal stressors may predispose infants to develop diabetes, metabolic syndrome, or stroke. We hypothesized that postnatal stress will predispose animals to develop metabolic syndrome and impair the physiologic response to hypoxic-ischemic brain injury. We characterized the short- and long-term physiologic responses to postnatal stress by examining corticosterone (CS), glucose metabolism, and brain injury in neonatal and adult rats exposed to hypoxia-ischemia (H-I). Rat pups were divided into three levels of postnatal stress from postnatal day (P) 3 to P7. All rats underwent unilateral brain injury on either P7 or P134. We measured brain injury, growth, blood pressure, urine/plasma CS, plasma leptin, insulin, and glucose before and after H-I. Postnatal stress increased neonatal CS production, exacerbated neonatal white matter injury, and was associated with adult hyperglycemia after H-I despite increased insulin production. There were no group differences in adult weight, blood pressure, or leptin. Postnatal stress exacerbated brain injury and produced adult hyperglycemia, triggered after hypoxia exposure, consistent with the hypotheses that neonates exposed to early stress are more vulnerable to hypoxia and may be predisposed to develop metabolic syndrome in adulthood. Prolonged maternal separation produced more hyperglycemia than did brief daily handling.

Early-life experience alters response of developing brain to seizures

Prolonged seizures during childhood are associated with behavior problems, memory impairment and school failure. No effective treatment currently exists after seizures to mitigate neuronal injury and long-term neurological sequelae for children with epilepsy. We studied the therapeutic efficacy of early-life environment on seizure-induced behavioral deficits, neuronal injury and the inflammatory reaction using the kainic acid (KA) seizure model. Two rearing conditions, maternal separation for 3 h daily versus maternal care in an enriched environment, were followed by single housing for the former (Deprived) and group housing in an enriched environment for the latter (Enriched). To examine the influence of differential rearing on the behavioral effects of early-life seizures, KA was injected on P21. On P28, marked reduction in exploratory behavior was noted after seizures only in the Deprived group. To investigate seizure-induced hippocampal injury, a separate group of rats were injected with KA on P35 since consistent seizure-induced neuronal injury is observed only in mature rats. Brains of rats sacrificed on P37 displayed a significant reduction in DNA fragmentation and microglial activation in Enriched compared to Deprived animals. Our results suggest that a nurturing early environment can enhance the ability of the developing brain to recover from seizures and provide a buffer against their damaging effects. While the nurturing environment was neuroprotective, the combination of deprived rearing and the insult of early-life seizures resulted in significant behavioral deficits, an increase in neuronal injury and activation of microglia in young rats.

Prenatal psychosocial stress exposure is associated with subsequent working memory performance in young women

The aim of the present study was to examine the association between prenatal psychosocial stress exposure and subsequent prefrontal cortex-dependent working memory performance in human adults. Working memory performance was assessed using an item-recognition task under 10 mg hydrocortisone (cortisol) and placebo conditions in a sample of 32 healthy young women (mean age = 25 +/- 4.34 years) whose mothers experienced a major negative life event during their pregnancy (Prenatal Stress, PS group), and in a comparison group of 27 healthy young women (mean age = 24 +/- 3.4 years). The two groups did not differ in the placebo condition, however, subjects in the PS group showed longer reaction times after hydrocortisone administration compared with subjects in the comparison group (p = .02). These findings provide support for an association between prenatal stress exposure and the potential modulatory effect of cortisol on working memory performance in young adults, which may reflect compromised development of the prefrontal cortex in prenatal life.

Postnatal glucocorticoid excess due to pituitary glucocorticoid receptor deficiency: differential short- and long-term consequences

A tight regulation of hypothalamic-pituitary-adrenal (HPA) axis activity is essential for successful adaptation to stressful stimuli. Disruption of normal HPA axis development is a main risk factor for diseases such as posttraumatic stress disorder or depression, but the molecular mechanisms that lead to these long-term consequences are poorly understood. Here, we test the hypothesis that the pituitary glucocorticoid receptor (GR) is involved in regulating HPA axis function in neonatal and adult animals. Furthermore, we investigate whether postnatal hypercortisolism induced by pituitary GR deficiency is a main factor contributing to the persistent effects of early-life stress. Conditional knockout mice with a deletion of the GR at the pituitary (GR(POMCCre)) show excessive basal corticosterone levels during postnatal development, but not in adulthood. The hypercortisolemic state of neonatal GR(POMCCre) mice is accompanied by central gene expression changes of CRH and vasopressin in the paraventricular nucleus, but these alterations normalize at later ages. In adult mice, pituitary GR deficiency results in impaired glucocorticoid negative feedback. Furthermore, adult GR(POMCCre) mice display a more active coping strategy in the forced swim test, with no alterations in anxiety like behavior or cognitive functions. Postnatal GR antagonist treatment is able to prevent the long-term behavioral effects in GR(POMCCre) mice. In conclusion, we show that pituitary GRs are centrally involved in regulating HPA axis activity in neonates and mediate negative feedback regulation in adult animals. Postnatal glucocorticoid excess results in an altered stress-coping behavior in adult animals, with no effects on anxiety like behavior or cognition.

The neuro-symphony of stress

The impact of stress on brain function is increasingly recognized. Various substances are released in response to stress and can influence distinct neuronal circuits, but the functional advantages of having such a diversity of stress mediators remain unclear. Individual neurotransmitter, neuropeptide and steroid stress mediators have specific spatial and temporal niches, but these niches also overlap. In addition, the effects of individual mediators on neuronal function and plasticity are integrated, and emerging evidence suggests that there is crosstalk between them. Together, this results in the stress instruments producing an orchestrated 'symphony' that enables fine-tuned responses to diverse challenges.

Age matters

The age of an experimental animal can be a critical variable, yet age matters are often overlooked within neuroscience. Many studies make use of young animals, without considering possible differences between immature and mature subjects. This is especially problematic when attempting to model traits or diseases that do not emerge until adulthood. In this commentary we discuss the reasons for this apparent bias in age of experimental animals, and illustrate the problem with a systematic review of published articles on long-term potentiation. Additionally, we review the developmental stages of a rat and discuss the difficulty of using the weight of an animal as a predictor of its age. Finally, we provide original data from our laboratory and review published data to emphasize that development is an ongoing process that does not end with puberty. Developmental changes can be quantitative in nature, involving gradual changes, rapid switches, or inverted U-shaped curves. Changes can also be qualitative. Thus, phenomena that appear to be unitary may be governed by different mechanisms at different ages. We conclude that selection of the age of the animals may be critically important in the design and interpretation of neurobiological studies.

Opposite effects of early maternal deprivation on neurogenesis in male versus female rats

Background

Major depression is more prevalent in women than in men. The underlying neurobiological mechanisms are not well understood, but recent data shows that hippocampal volume reductions in depressed women occur only when depression is preceded by an early life stressor. This underlines the potential importance of early life stress, at least in women, for the vulnerability to develop depression. Perinatal stress exposure in rodents affects critical periods of brain development that persistently alter structural, emotional and neuroendocrine parameters in adult offspring. Moreover, stress inhibits adult hippocampal neurogenesis, a form of structural plasticity that has been implicated a.o. in antidepressant action and is highly abundant early postnatally. We here tested the hypothesis that early life stress differentially affects hippocampal structural plasticity in female versus male offspring.

Principal Findings

We show that 24 h of maternal deprivation (MD) at PND3 affects hippocampal structural plasticity at PND21 in a sex-dependent manner. Neurogenesis was significantly increased in male but decreased in female offspring after MD. Since no other structural changes were found in granule cell layer volume, newborn cell survival or proliferation rate, astrocyte number or gliogenesis, this indicates that MD elicits specific changes in subsets of differentiating cells and differentially affects immature neurons. The MD induced sex-specific effects on neurogenesis cannot be explained by differences in maternal care.

Conclusions

Our data shows that early environment has a critical influence on establishing sex differences in neural plasticity and supports the concept that the setpoint for neurogenesis may be determined during perinatal life. It is tempting to speculate that a reduced level of neurogenesis, secondary to early stress exposure, may contribute to maladaptation of the HPA axis and possibly to the increased vulnerability of women to stress-related disorders.

Enriched environment experience overcomes learning deficits and depressive-like behavior induced by juvenile stress

Mood disorders affect the lives and functioning of millions each year. Epidemiological studies indicate that childhood trauma is predominantly associated with higher rates of both mood and anxiety disorders. Exposure of rats to stress during juvenility (JS) (27–29 days of age) has comparable effects and was suggested as a model of induced predisposition for these disorders. The importance of the environment in the regulation of brain, behavior and physiology has long been recognized in biological, social and medical sciences. Here, we studied the effects of JS on emotional and cognitive aspects of depressive-like behavior in adulthood, on Hypothalamic-Pituitary-Adrenal (HPA) axis reactivity and on the expression of cell adhesion molecule L1 (L1-CAM). Furthermore, we combined it with the examination of potential reversibility by enriched environment (EE) of JS – induced disturbances of emotional and cognitive aspects of behavior in adulthood. Three groups were tested: Juvenile Stress –subjected to Juvenile stress; Enriched Environment – subjected to Juvenile stress and then, from day 30 on to EE; and Naïves. In adulthood, coping and stress responses were examined using the elevated plus-maze, open field, novel setting exploration and two way shuttle avoidance learning. We found that, JS rats showed anxiety- and depressive-like behaviors in adulthood, altered HPA axis activity and altered L1-CAM expression. Increased expression of L1-CAM was evident among JS rats in the basolateral amygdala (BLA) and Thalamus (TL). Furthermore, we found that EE could reverse most of the effects of Juvenile stress, both at the behavioral, endocrine and at the biochemical levels. The interaction between JS and EE resulted in an increased expression of L1-CAM in dorsal cornu ammonis (CA) area 1 (dCA1).

Divergent maternal behavioral patterns in two genetic animal models of depression

Maternal behavior was examined in Flinders Sensitive-Line (FSL) and Wistar-Kyoto (WKY) rats, two different genetic animal models of depression. Behavioral patterns were assessed by undisturbed observations in the nest [Post-Partum Days (PPD) 4 and 9] and post-disturbance observations ("retrieval tests") on PPD 10. Litters were randomly allocated to a mild chronic-stress condition (limiting available bedding between PPD 2 and 9) or a standard rearing condition. The findings indicated that FSL dams did not differ from control dams in the undisturbed observations. However, in the post-disturbance observations FSL dams exhibited less pup-directed behaviors, a shorter latency to first pup carrying/retrieval and more self-directed behaviors than controls (the latter effect only in dams' interaction with whole litter). In contrast, WKY dams performed more pup-directed activities (e.g., nursing and licking) and less self-directed activities in both the undisturbed and post-disturbance observations (in both dams' interaction with single-pup and with the whole-litter) compared to controls. Accordingly, WKY dams exhibited a shorter latency for first pup-licking bout (in both post-disturbance observations). The early life mild chronic-stress used in the study ('limited-bedding') had a minor effect on the dams' behavior. Overall, the study investigated for the first time the maternal behavior of WKY dams and suggests that these dams show an almost opposite behavioral pattern to that of FSL dams. The results are discussed with regard to earlier findings in the FSL strain and behavioral patterns documented in depressed human mothers.

A novel mouse model for acute and long-lasting consequences of early life stress

Chronic early-life stress (ES) exerts profound acute and long-lasting effects on the hypothalamic-pituitary-adrenal system, with relevance to cognitive function and affective disorders. Our ability to determine the molecular mechanisms underlying these effects should benefit greatly from appropriate mouse models because these would enable use of powerful transgenic methods. Therefore, we have characterized a mouse model of chronic ES, which was provoked in mouse pups by abnormal, fragmented interactions with the dam. Dam-pup interaction was disrupted by limiting the nesting and bedding material in the cages, a manipulation that affected this parameter in a dose-dependent manner. At the end of their week-long rearing in the limited-nesting cages, mouse pups were stressed, as apparent from elevated basal plasma corticosterone levels. In addition, steady-state mRNA levels of CRH in the hypothalamic paraventricular nucleus of ES-experiencing pups were reduced, without significant change in mRNA levels of arginine vasopressin. Rearing mouse pups in this stress-provoking cage environment resulted in enduring effects: basal plasma corticosterone levels were still increased, and CRH mRNA levels in paraventricular nucleus remained reduced in adult ES mice, compared with those of controls. In addition, hippocampus-dependent learning and memory functions were impaired in 4- to 8-month-old ES mice. In summary, this novel, robust model of chronic early life stress in the mouse results in acute and enduring neuroendocrine and cognitive abnormalities. This model should facilitate the examination of the specific genes and molecules involved in the generation of this stress as well as in its consequences.

Regional variability in age-related loss of neurons from the primary visual cortex and medial prefrontal cortex of male and female rats

During aging, changes in the structure of the cerebral cortex of the rat have been seen, but potential changes in neuron number remain largely unexplored. In the present study, stereological methods were used to examine neuron number in the medial prefrontal cortex and primary visual cortex of young adult (85-90 days of age) and aged (19-22 months old) male and female rats in order to investigate any age-related losses. Possible sex differences in aging were also examined since sexually dimorphic patterns of aging have been seen in other measures. An age-related loss of neurons (18-20%), which was mirrored in volume losses, was found to occur in the primary visual cortex in both sexes in all layers except IV. Males, but not females, also lost neurons (15%) from layer V/VI of the ventral medial prefrontal cortex and showed an overall decrease in volume of this region. In contrast, dorsal medial prefrontal cortex showed no age-related changes. The effects of aging clearly differ among regions of the rat brain and to some degree, between the sexes.

Dysfunctional nurturing behavior in rat dams with limited access to nesting material: a clinically relevant model for early-life stress

BACKGROUND

Early-life emotional stress may be associated with affective and cognitive disorders later in life, yet satisfactory animal models for studying the underlying mechanisms are limited. Because maternal presence and behavior critically influence molecular and behavioral stress responses in offspring, we sought to create a model of dysfunctional, fragmented maternal nurturing behavior that would, in turn, provoke chronic early-life stress in the offspring.

METHODS

Sprague-Dawley rat dams' nursing and nurturing behaviors were altered by limiting their ability to create satisfactory nests during postpartum days 2-9. Maternal behavior was observed throughout the diurnal cycle, and the frequency and duration of nurturing behaviors were scored. In addition, potential stress and anxiety of the dams were assessed using behavioral, molecular and hormonal measures.

RESULTS

Both the quantity and the quality of dams' care of their pups were profoundly influenced by restriction of nesting materials in their cages: licking/grooming activities decreased and the frequency of leaving the pups increased, resulting in fragmented interactions between the dams and pups. The abnormal activity patterns of the dams were accompanied by increased anxiety-like behavior in the open field, but not in the elevated plus maze tests. Additionally, dams' plasma corticosterone levels and adrenal weights were augmented, suggesting chronic stress of these dams. By the end of the limited-nesting, stress-inducing period, hypothalamic corticotropin releasing hormone (CRH) mRNA expression was reduced in the limited-nesting dams, while arginine-vasopressin (AVP) mRNA levels were not significantly affected.

CONCLUSION

Limiting dams' ability to construct a nest for their pups leads to an abnormal repertoire of nurturing behaviors, possibly as a result of chronic stress and mild anxiety of the dams. Because the fragmented and aberrant maternal behavior provoked chronic stress in the pups, the limited-nesting paradigm provides a useful tool for studying the mechanisms and consequences of such early-life stress experience in the offspring.

Central effects of stress hormones in health and disease: Understanding the protective and damaging effects of stress and stress mediators

Stress begins in the brain and affects the brain, as well as the rest of the body. Acute stress responses promote adaptation and survival via responses of neural, cardiovascular, autonomic, immune and metabolic systems. Chronic stress can promote and exacerbate pathophysiology through the same systems that are dysregulated. The burden of chronic stress and accompanying changes in personal behaviors (smoking, eating too much, drinking, poor quality sleep; otherwise referred to as "lifestyle") is called allostatic overload. Brain regions such as hippocampus, prefrontal cortex and amygdala respond to acute and chronic stress and show changes in morphology and chemistry that are largely reversible if the chronic stress lasts for weeks. However, it is not clear whether prolonged stress for many months or years may have irreversible effects on the brain. The adaptive plasticity of chronic stress involves many mediators, including glucocorticoids, excitatory amino acids, endogenous factors such as brain neurotrophic factor (BDNF), polysialated neural cell adhesion molecule (PSA-NCAM) and tissue plasminogen activator (tPA). The role of this stress-induced remodeling of neural circuitry is discussed in relation to psychiatric illnesses, as well as chronic stress and the concept of top-down regulation of cognitive, autonomic and neuroendocrine function. This concept leads to a different way of regarding more holistic manipulations, such as physical activity and social support as an important complement to pharmaceutical therapy in treatment of the common phenomenon of being "stressed out". Policies of government and the private sector play an important role in this top-down view of minimizing the burden of chronic stress and related lifestyle (i.e. allostatic overload).

The central corticotropin releasing factor system during development and adulthood

Corticotropin releasing factor (CRH) has been shown to contribute critically to molecular and neuroendocrine responses to stress during both adulthood and development. This peptide and its receptors are expressed in the hypothalamus, as well as in limbic brain areas including amygdala and hippocampus. This is consistent with roles for CRH in mediating the influence of stress on emotional behavior and cognitive function. The expression of CRH and of its receptors in hypothalamus, amygdala and hippocampus is age-dependent, and is modulated by stress throughout life (including the first postnatal weeks). Uniquely during development, the cardinal influence of maternal care on the central stress response governs the levels of central CRH expression, and may alter the 'set-point' of CRH-gene sensitivity to stress in a lasting manner.