Down-regulation of hypothalamic corticotropin-releasing hormone messenger ribonucleic acid (mRNA) precedes early-life experience-induced changes in hippocampal glucocorticoid receptor mRNA

The role of epigenetic mechanisms in the long-term effects of early-life adversity and mother-infant relationship on physiology and behavior of offspring in laboratory rats and mice

Maternal care during the early postnatal period of altricial mammals is a key factor in the survival and adaptation of offspring to environmental conditions. Natural variations in maternal care and experimental manipulations with maternal-child relationships modeling early-life adversity (ELA) in laboratory rats and mice have a strong long-term influence on the physiology and behavior of offspring in rats and mice. This literature review is devoted to the latest research on the role of epigenetic mechanisms in these effects of ELA and mother-infant relationship, with a focus on the regulation of hypothalamic-pituitary-adrenal axis and brain-derived neurotrophic factor. An important part of this review is dedicated to pharmacological interventions and epigenetic editing as tools for studying the causal role of epigenetic mechanisms in the development of physiological and behavioral profiles. A special section of the manuscript will discuss the translational potential of the discussed research.

Lasting impact of postnatal maternal separation on the developing BNST: Lifelong socioemotional consequences

Nearly one percent of children in the US experience childhood neglect or abuse, which can incite lifelong emotional and behavioral disorders. Many studies investigating the neural underpinnings of maleffects inflicted by early life stress have largely focused on dysregulation of the hypothalamic-pituitary-adrenal (HPA) axis. Newer veins of evidence suggest that exposure to early life stressors can interrupt neural development in extrahypothalamic areas as well, including the bed nucleus of the stria terminalis (BNST). One widely used approach in this area is rodent maternal separation (MS), which typically consists of separating pups from the dam for extended periods of time, over several days during the first weeks of postnatal life - a time when pups are highly dependent on maternal care for survival. MS has been shown to incite myriad lasting effects not limited to increased anxiety-like behavior, hyper-responsiveness to stressors, and social behavior deficits. The behavioral effects of MS are widespread and thus unlikely to be limited to hypothalamic mechanisms. Recent work has highlighted the BNST as a critical arbiter of some of the consequences of MS, especially socioemotional behavioral deficits. The BNST is a well-documented modulator of anxiety, reward, and social behavior by way of its connections with hypothalamic and extra-hypothalamic systems. Moreover, during the postnatal period when MS is typically administered, the BNST undergoes critical neural developmental events. This review highlights evidence that MS interferes with neural development to permanently alter BNST circuitry, which may account for a variety of behavioral deficits seen following early life stress. This article is part of the Special Issue on 'Fear, Anxiety and PTSD'.

A comprehensive approach to modeling maternal immune activation in rodents

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

Exposure to unpredictability and mental health: Validation of the brief version of the Questionnaire of Unpredictability in Childhood (QUIC-5) in English and Spanish

Unpredictability is increasingly recognized as a primary dimension of early life adversity affecting lifespan mental health trajectories; screening for these experiences is therefore vital. The Questionnaire of Unpredictability in Childhood (QUIC) is a 38-item tool that measures unpredictability in childhood in social, emotional and physical domains. The available evidence indicates that exposure to unpredictable experiences measured with the QUIC predicts internalizing symptoms including depression and anxiety. The purpose of the present study was to validate English and Spanish brief versions (QUIC-5) suitable for administration in time-limited settings (e.g., clinical care settings, large-scale epidemiological studies). Five representative items were identified from the QUIC and their psychometric properties examined. The predictive validity of the QUIC-5 was then compared to the QUIC by examining mental health in four cohorts: (1) English-speaking adult women assessed at 6-months postpartum (N = 116), (2) English-speaking male veterans (N = 95), (3) English-speaking male and female adolescents (N = 155), and (4) Spanish-speaking male and female adults (N = 285). The QUIC-5 demonstrated substantial variance in distributions in each of the cohorts and is correlated on average 0.84 (r's = 0.81-0.87) with the full 38-item version. Furthermore, the QUIC-5 predicted internalizing symptoms (anxiety and depression) in all cohorts with similar effect sizes (r's = 0.16-0.39; all p's < 0.05) to the full versions (r's = 0.19-0.42; all p's < 0.05). In sum, the QUIC-5 exhibits good psychometric properties and is a valid alternative to the full QUIC. These findings support the future use of the QUIC-5 in clinical and research settings as a concise way to measure unpredictability, identify risk of psychopathology, and intervene accordingly.

Clinical translational neuroimaging of the antioxidant effect of N-acetylcysteine on neural microstructure

PURPOSE

Oxidative stress and downstream effectors have emerged as important pathological processes that drive psychiatric illness, suggesting that antioxidants may have a therapeutic role in psychiatric disease. However, no imaging biomarkers are currently available to track therapeutic response. The purpose of this study was to examine whether advanced DWI techniques are able to sensitively detect the potential therapeutic effects of the antioxidant N-acetylcysteine (NAC) in a Disc1 svΔ2 preclinical rat model of psychiatric illness.

METHODS

Male and female Disc1 svΔ2 rats and age-matched, sex-matched Sprague-Dawley wild-type controls were treated with a saline vehicle or NAC before ex vivo MRI acquisition at P50. Imaging data were fit to DTI and neurite orientation dispersion and density imaging models and analyzed for region-specific changes in quantitative diffusion metrics. Brains were further processed for cellular quantification of microglial density and morphology. All experiments were repeated for Disc1 svΔ2 rats exposed to chronic early-life stress to test how gene-environment interactions might alter effectiveness of NAC therapy.

RESULTS

The DTI and neurite orientation dispersion and density imaging analyses demonstrated amelioration of early-life, sex-specific neural microstructural deficits with concomitant differences in microglial morphology across multiple brain regions relevant to neuropsychiatric illness with NAC treatment, but only in male Disc1 svΔ2 rats. Addition of chronic early-life stress reduced the ability of NAC to restore microstructural deficits.

CONCLUSION

These findings provide evidence for a treatment pathway targeting endogenous antioxidant capacity, and the clinical translational utility of neurite orientation dispersion and density imaging microstructural imaging to sensitively detect microstructural alterations resulting from antioxidant treatment.

Beyond maternal care: The effects of extra-maternal influences within the maternal environment on offspring neurodevelopment and later-life behavior

The early-life maternal environment has a profound and persistent effect on offspring neuroendocrine function, neurotransmitter systems, and behavior. Studies using rodent models suggest that early-life maternal care can influence the 'developmental programming' of offspring in part through altered epigenetic regulation of specific genes. The exploration of epigenetic regulation of these genes as a biological mechanism has been important to our understanding of how animals adapt to their environments and how these developmental trajectories may be altered. However, other non-maternal factors have been shown to act directly, or to interact with maternal care, to influence later-life phenotype. Based on accumulating evidence, including our research, we discuss other important influences on the developmental programming of offspring. We highlight early-life variations in temperature exposure and offspring genotype x environment interactions as prominent examples. We conclude with recommendations for future investigations on how early-life maternal care and extra-maternal influences lead to persistent changes in the brain and behavior of the offspring throughout development.

Unexpected Transcriptional Programs Contribute to Hippocampal Memory Deficits and Neuronal Stunting after Early-Life Adversity

Early-life adversity (ELA) is associated with lifelong memory deficits, yet the responsible mechanisms remain unclear. We impose ELA by rearing rat pups in simulated poverty, assess hippocampal memory, and probe changes in gene expression, their transcriptional regulation, and the consequent changes in hippocampal neuronal structure. ELA rats have poor hippocampal memory and stunted hippocampal pyramidal neurons associated with ~140 differentially expressed genes. Upstream regulators of the altered genes include glucocorticoid receptor and, unexpectedly, the transcription factor neuron-restrictive silencer factor (NRSF/REST). NRSF contributes critically to the memory deficits because blocking its function transiently following ELA rescues spatial memory and restores the dendritic arborization of hippocampal pyramidal neurons in ELA rats. Blocking NRSF function in vitro augments dendritic complexity of developing hippocampal neurons, suggesting that NRSF represses genes involved in neuronal maturation. These findings establish important, surprising contributions of NRSF to ELA-induced transcriptional programming that disrupts hippocampal maturation and memory function.

Early Life Stress Preceding Mild Pediatric Traumatic Brain Injury Increases Neuroinflammation but Does Not Exacerbate Impairment of Cognitive Flexibility during Adolescence

Early life stress (ELS) followed by pediatric mild traumatic brain injury (mTBI) negatively impacts spatial learning and memory and increases microglial activation in adolescent rats, but whether the same paradigm negatively affects higher order executive function is not known. Hence, we utilized the attentional set-shifting test (AST) to evaluate executive function (cognitive flexibility) and to determine its relationship with neuroinflammation and hypothalamic-pituitary-adrenal (HPA) axis activity after pediatric mTBI in male rats. ELS was induced via maternal separation for 180 min per day (MS180) during the first 21 post-natal (P) days, while controls (CONT) were undisturbed. At P21, fully anesthetized rats received a mild controlled cortical impact (2.2 mm tissue deformation at 4 m/sec) or sham injury. AST was evaluated during adolescence on P35-P40 and cytokine expression and HPA activity were analyzed on P42. The data indicate that pediatric mTBI produced a significant reversal learning deficit on the AST versus sham (p < 0.05), but that the impairment was not exacerbated further by MS180. Additionally, ELS produced an overall elevation in set-loss errors on the AST, and increased hippocampal interleukin (IL)-1β expression after TBI. A significant correlation was observed in executive dysfunction and IL-1β expression in the ipsilateral pre-frontal cortex and hippocampus. Although the combination of ELS and pediatric mTBI did not worsen executive function beyond that of mTBI alone (p > 0.05), it did result in increased hippocampal neuroinflammation relative to mTBI (p < 0.05). These findings provide important insight into the susceptibility to incur alterations in cognitive and neuroimmune functioning after stress exposure and TBI during early life.

Early life stress increases vulnerability to the sequelae of pediatric mild traumatic brain injury

Early life stress (ELS) is a risk factor for many psychopathologies that happen later in life. Although stress can occur in cases of child abuse, studies on non-accidental brain injuries in pediatric populations do not consider the possible increase in vulnerability caused by ELS. Hence, we sought to determine whether ELS increases the effects of pediatric mild traumatic brain injury (mTBI) on cognition, hippocampal inflammation, and plasticity. Male rats were subjected to maternal separation for 180 min per day (MS180) or used as controls (CONT) during the first 21 post-natal (P) days. At P21 the rats were anesthetized with isoflurane and subjected to a mild controlled cortical impact or sham injury. At P32 the rats were injected with the cell proliferation marker bromodeoxyuridine (BrdU, 500 mg/kg), then evaluated for spatial learning and memory in a water maze (P35-40) and sacrificed for quantification of Ki67⁺, BrdU⁺ and Iba1⁺ (P42). Neither MS180 nor mTBI impacted cognitive outcome when provided alone but their combination (MS180 + mTBI) decreased spatial learning and memory relative to Sham controls (p < .01). mTBI increased microglial activation and affected BrdU⁺ cell survival in the ipsilateral hippocampus without affecting proliferation rates. However, only MS180 + mTBI increased microglial activation in the area adjacent to the injury and the contralateral CA1 hippocampal subfield, and decreased cell proliferation in the ipsilateral neurogenic niche. Overall, the data show that ELS increases the vulnerability to the sequelae of pediatric mTBI and may be mediated by increased neuroinflammation.

Maternal separation impairs long term-potentiation in CA3-CA1 synapses in adolescent female rats

Mother-infant interactions influence the development of physiology and behavior during the first weeks after birth. As an adverse early life experience, maternal separation (MS) produces behavioral and neuroendocrine functions disorders associated with the hippocampus. Considering the critical role of long-term potentiation (LTP) in learning and memory, we investigated whether MS affects LTP in adolescent female rats. In this study, female rat pups were exposed to daily 3-h (MS180) or 15-min (MS15) periods of maternal separation on postnatal days (PND) 1-14 and control offspring remained with the dams all the time before weaning. Extracellular evoked field excitatory postsynaptic potentials (fEPSPs) were recorded in the stratum radiatum of the CA1 area of the slice at 28-35 days of age. Our results indicate that a significant difference existed in the magnitude of LTP between the control group and MS180 group, but the MS15 group was not different from control. In conclusion, these findings suggest that MS may impair LTP induction in the CA1 area of the hippocampus in adolescent female rats.

Preclinical neuroimaging of gene-environment interactions in psychiatric disease

Psychiatric disease is one of the leading causes of disability worldwide. Despite the global burden and need for accurate diagnosis and treatment of mental illness, psychiatric diagnosis remains largely based on patient-reported symptoms, allowing for immense symptomatic heterogeneity within a single disease. In renewed efforts towards improved diagnostic specificity and subsequent evaluation of treatment response, a greater understanding of the underlying of the neuropathology and neurobiology of neuropsychiatric disease is needed. However, dissecting these mechanisms of neuropsychiatric illness in clinical populations are problematic with numerous experimental hurdles limiting hypothesis-driven studies including genetic confounds, variable life experiences, different environmental exposures, therapeutic histories, as well as the inability to investigate deeper molecular changes in vivo . Preclinical models, where many of these confounding factors can be controlled, can serve as a crucial experimental bridge for studying the neurobiological origins of mental illness. Furthermore, although behavioral studies and molecular studies are relatively common in these model systems, focused neuroimaging studies are very rare and represent an opportunity to link the molecular changes in psychiatric illness with advanced quantitative neuroimaging studies. In this review, we present an overview of well-validated genetic and environmental models of psychiatric illness, discuss gene-environment interactions, and examine the potential role of neuroimaging towards understanding genetic, environmental, and gene-environmental contributions to psychiatric illness.

Resilience priming: Translational models for understanding resiliency and adaptation to early life adversity

Despite the increasing attention to early life adversity and its long-term consequences on health, behavior, and the etiology of neurodevelopmental disorders, our understanding of the adaptations and interventions that promote resiliency and rescue against such insults are underexplored. Specifically, investigations of the perinatal period often focus on negative events/outcomes. In contrast, positive experiences (i.e. enrichment/parental care//healthy nutrition) favorably influence development of the nervous and endocrine systems. Moreover, some stressors result in adaptations and demonstrations of later-life resiliency. This review explores the underlying mechanisms of neuroplasticity that follow some of these early life experiences and translates them into ideas for interventions in pediatric settings. The emerging role of the gut microbiome in mediating stress susceptibility is also discussed. Since many negative outcomes of early experiences are known, it is time to identify mechanisms and mediators that promote resiliency against them. These range from enrichment, quality parental care, dietary interventions and those that target the gut microbiota.

Measuring novel antecedents of mental illness: the Questionnaire of Unpredictability in Childhood

Increasing evidence indicates that, in addition to poverty, maternal depression, and other well-established factors, unpredictability of maternal and environmental signals early in life influences trajectories of brain development, determining risk for subsequent mental illness. However, whereas most risk factors for later vulnerability to mental illness are readily measured using existing, clinically available tools, there are no similar measures for assessing early-life unpredictability. Here we validate the Questionnaire of Unpredictability in Childhood (QUIC) and examine its associations with mental health in the context of other indicators of childhood adversity (e.g., traumatic life events, socioeconomic status, and parenting quality). The QUIC was initially validated through administration to a cohort of adult females (N = 116) and then further refined in two additional independent cohorts (male Veterans, N = 95, and male and female adolescents, N = 175). The QUIC demonstrated excellent internal (α = 0.89) and test-retest reliability (r = 92). Scores on the QUIC were positively correlated with other prospective indicators of exposures to unpredictable maternal inputs in infancy and childhood (unpredictable maternal mood and sensory signals), and accuracy of recall also was confirmed with prospective data. Importantly, the QUIC predicted symptoms of anxiety, depression, and anhedonia in the three study cohorts, and these effects persisted after adjusting for other previously established risk factors. The QUIC, a reliable and valid self-report assessment of exposure to unpredictability in the social, emotional, and physical domains during early life, is a brief, comprehensive, and promising instrument for predicting risk for mental illness.

Effects of early adversity on the brain: Larger-volume anterior cingulate cortex in AIDS orphans

Multiple studies have revealed that adolescent AIDS orphans have more psychosocial problems than healthy adolescents. However, little is known about whether and how the brain structures of adolescent AIDS orphans differ from those of healthy adolescents. Here, we used magnetic resonance imaging to compare adolescent AIDS orphans reared in institutions (N = 20) with a sex- and age-matched group of healthy adolescents reared in families (N = 20) in China using a voxel-based morphometry analysis. First, we found that both total gray- and white-matter volumes did not differ between groups. Second, after correcting for age, sex, and total gray-matter volume, the AIDS orphan group demonstrated smaller hippocampal volumes, larger anterior cingulate cortex (ACC) volumes, and no differences in the amygdala. Third, a whole-brain analysis identified higher gray-matter volume of the ACC in the AIDS orphan group than in the control group. The preliminary findings of this study highlight the need for future research to confirm the sensitivity of the hippocampus and ACC to early adversity.

Vulnerability and resilience to Alzheimer's disease: early life conditions modulate neuropathology and determine cognitive reserve

BACKGROUND

Alzheimer's disease (AD) is a progressive neurodegenerative disorder with a high prevalence among the elderly and a huge personal and societal impact. Recent epidemiological studies have indicated that the incidence and age of onset of sporadic AD can be modified by lifestyle factors such as education, exercise, and (early) stress exposure. Early life adversity is known to promote cognitive decline at a later age and to accelerate aging, which are both primary risk factors for AD. In rodent models, exposure to 'negative' or 'positive' early life experiences was recently found to modulate various measures of AD neuropathology, such as amyloid-beta levels and cognition at later ages. Although there is emerging interest in understanding whether experiences during early postnatal life also modulate AD risk in humans, the mechanisms and possible substrates underlying these long-lasting effects remain elusive.

METHODS

We review literature and discuss the role of early life experiences in determining later age and AD-related processes from a brain and cognitive 'reserve' perspective. We focus on rodent studies and the identification of possible early determinants of later AD vulnerability or resilience in relation to early life adversity/enrichment.

RESULTS

Potential substrates and mediators of early life experiences that may influence the development of AD pathology and cognitive decline are: programming of the hypothalamic-pituitary-adrenal axis, priming of the neuroinflammatory response, dendritic and synaptic complexity and function, overall brain plasticity, and proteins such as early growth response protein 1 (EGR1), activity regulated cytoskeleton-associated protein (Arc), and repressor element-1 silencing transcription factor (REST).

CONCLUSIONS

We conclude from these rodent studies that the early postnatal period is an important and sensitive phase that influences the vulnerability to develop AD pathology. Yet translational studies are required to investigate whether early life experiences also modify AD development in human studies, and whether similar molecular mediators can be identified in the sensitivity to develop AD in humans.

Experience-dependent neuroplasticity of the developing hypothalamus: integrative epigenomic approaches

Augmented maternal care during the first postnatal week promotes life-long stress resilience and improved memory compared with the outcome of routine rearing conditions. Recent evidence suggests that this programming commences with altered synaptic connectivity of stress sensitive hypothalamic neurons. However, the epigenomic basis of the long-lived consequences is not well understood. Here, we employed whole-genome bisulfite sequencing (WGBS), RNA-sequencing (RNA-seq), and a multiplex microRNA (miRNA) assay to examine the effects of augmented maternal care on DNA cytosine methylation, gene expression, and miRNA expression. A total of 9,439 differentially methylated regions (DMRs) associated with augmented maternal care were identified in male offspring hypothalamus, as well as a modest but significant decrease in global DNA methylation. Differentially methylated and expressed genes were enriched for functions in neurotransmission, neurodevelopment, protein synthesis, and oxidative phosphorylation, as well as known stress response genes. Twenty prioritized genes were identified as highly relevant to the stress resiliency phenotype. This combined unbiased approach enabled the discovery of novel genes and gene pathways that advance our understanding of the epigenomic mechanisms underlying the effects of maternal care on the developing brain.

Maternal hormonal milieu influence on fetal brain development

An adverse maternal hormonal environment during pregnancy can be associated with abnormal brain growth. Subtle changes in fetal brain development have been observed even for maternal hormone levels within the currently accepted physiologic ranges. In this review, we provide an update of the research data on maternal hormonal impact on fetal neurodevelopment, giving particular emphasis to thyroid hormones and glucocorticoids. Thyroid hormones are required for normal brain development. Despite serum TSH appearing to be the most accurate indicator of thyroid function in pregnancy, maternal serum free T4 levels in the first trimester of pregnancy are the major determinant of postnatal psychomotor development. Even a transient period of maternal hypothyroxinemia at the beginning of neurogenesis can confer a higher risk of expressive language and nonverbal cognitive delays in offspring. Nevertheless, most recent clinical guidelines advocate for targeted high-risk case finding during first trimester of pregnancy despite universal thyroid function screening. Corticosteroids are determinant in suppressing cell proliferation and stimulating terminal differentiation, a fundamental switch for the maturation of fetal organs. Not surprisingly, intrauterine exposure to stress or high levels of glucocorticoids, endogenous or synthetic, has a molecular and structural impact on brain development and appears to impair cognition and increase anxiety and reactivity to stress. Limbic regions, such as hippocampus and amygdala, are particularly sensitive. Repeated doses of prenatal corticosteroids seem to have short-term benefits of less respiratory distress and fewer serious health problems in offspring. Nevertheless, neurodevelopmental growth in later childhood and adulthood needs further clarification. Future studies should address the relevance of monitoring the level of thyroid hormones and corticosteroids during pregnancy in the risk stratification for impaired postnatal neurodevelopment.

Anhedonia Following Early-Life Adversity Involves Aberrant Interaction of Reward and Anxiety Circuits and Is Reversed by Partial Silencing of Amygdala Corticotropin-Releasing Hormone Gene

BACKGROUND

Anhedonia, the diminished ability to experience pleasure, is an important dimensional entity linked to depression, schizophrenia, and other emotional disorders, but its origins and mechanisms are poorly understood. We have previously identified anhedonia, manifest as decreased sucrose preference and social play, in adolescent male rats that experienced chronic early-life adversity/stress (CES). Here we probed the molecular, cellular, and circuit processes underlying CES-induced anhedonia and tested them mechanistically.

METHODS

We examined functional brain circuits and neuronal populations activated by social play in adolescent CES and control rats. Structural connectivity between stress- and reward-related networks was probed using high-resolution diffusion tensor imaging, and cellular/regional activation was probed using c-Fos. We employed viral-genetic approaches to reduce corticotropin-releasing hormone (Crh) expression in the central nucleus of the amygdala in anhedonic rats, and tested for anhedonia reversal in the same animals.

RESULTS

Sucrose preference was reduced in adolescent CES rats. Social play, generally considered an independent measure of pleasure, activated brain regions involved in reward circuitry in both control and CES groups. In CES rats, social play activated Crh-expressing neurons in the central nucleus of the amygdala, typically involved in anxiety/fear, indicating aberrant functional connectivity of pleasure/reward and fear circuits. Diffusion tensor imaging tractography revealed increased structural connectivity of the amygdala to the medial prefrontal cortex in CES rats. Crh-short hairpin RNA, but not control short hairpin RNA, given into the central nucleus of the amygdala reversed CES-induced anhedonia without influencing other emotional measures.

CONCLUSIONS

These findings robustly demonstrate aberrant interactions of stress and reward networks after early-life adversity and suggest mechanistic roles for Crh-expressing amygdala neurons in emotional deficits portending major neuropsychiatric disorders.

NRSF-dependent epigenetic mechanisms contribute to programming of stress-sensitive neurons by neonatal experience, promoting resilience

Resilience to stress-related emotional disorders is governed in part by early-life experiences. Here we demonstrate experience-dependent re-programming of stress-sensitive hypothalamic neurons, which takes place through modification of neuronal gene expression via epigenetic mechanisms. Specifically, we found that augmented maternal care reduced glutamatergic synapses onto stress-sensitive hypothalamic neurons and repressed expression of the stress-responsive gene, Crh. In hypothalamus in vitro, reduced glutamatergic neurotransmission recapitulated the repressive effects of augmented maternal care on Crh, and this required recruitment of the transcriptional repressor repressor element-1 silencing transcription factor/neuron restrictive silencing factor (NRSF). Increased NRSF binding to chromatin was accompanied by sequential repressive epigenetic changes which outlasted NRSF binding. chromatin immunoprecipitation-seq analyses of NRSF targets identified gene networks that, in addition to Crh, likely contributed to the augmented care-induced phenotype, including diminished depression-like and anxiety-like behaviors. Together, we believe these findings provide the first causal link between enriched neonatal experience, synaptic refinement and induction of epigenetic processes within specific neurons. They uncover a novel mechanistic pathway from neonatal environment to emotional resilience.

The impact of early-life stress on the expression of HPA-associated genes in the adult murine brain

Early life is an important period for the development of the nervous system and for the programming of behavioural phenotypes in adulthood. In our study, two types of early-life stress were used: prolonged separation of pups from their mothers (for 3 h/day, maternal separation (MS)) and brief separation (for 15 min/day, handling (HD)). We analysed the effects of early-life stress on behaviour and the expression of HPA-associated genes in the hypothalamus, hippocampus, and frontal cortex of male mice. Adult mice in the MS group demonstrated reduced locomotor activity and deficiencies in spatial long-term memory, while the HD showed no significant changes. Additionally, early-life MS resulted in reduced hippocampal Crhr1 mRNA, increased MR/GR mRNA in the hippocampus and hypothalamus. Both groups, HD and MS, showed increased Avp mRNA in the hypothalamus. Thus, prolonged maternal separation but not brief leads to adverse behavioural changes and influences the expression of HPA-associated genes in a brain region-specific manner.

Effects of Crocin on Learning and Memory in Rats Under Chronic Restraint Stress with Special Focus on the Hippocampal and Frontal Cortex Corticosterone Levels

Background:

Chronic stress adversely influences brain functions while crocin, as an effective component of saffron, exhibits positive effects on memory processes. This study investigated the effects of different doses of crocin on the improvement of learning and memory as well as corticosterone (CORT) levels in the hippocampus and frontal cortex of rats subjected to chronic stress.

Materials and Methods:

Forty male rats were randomly allocated to five different groups (n = 8): Control, sham; stress (6 h/day for 21 days) groups, and two groups receiving daily intraperitoneal injections of one of two doses (30 and 60 mg/kg) of crocin accompanied by 21 days of restraint stress. Latency was evaluated as a brain function using the passive avoidance test before and one-day after a foot shock. CORT levels were measured in the homogenized hippocampus and frontal cortex.

Results:

Results revealed that chronic stress had a significantly (P < 0.01) negative effect on memory. Crocin (30 and 60 mg/kg), however, gave increase to significantly (P < 0.01 and P < 0.05; respectively) improved memory functions in the stressed rats. Furthermore, the CORT levels in the hippocampus and frontal cortex declined significantly (P < 0.05) in the stress group compared to the control. Only a crocin dose of 30 mg/kg was observed modulate significantly (P < 0.05) the CORT levels in the hippocampus and frontal cortex in the stressed group.

Conclusions:

It was found that the lower crocin dose (30 mg/kg) had more beneficial effects than its higher (60 mg/kg) dose on learning and memory under chronic stress conditions. Moreover, it was speculated that different doses of crocin act on different neurotransmitters and biochemical factors in the brain.

Emotional learning, stress, and development: An ever-changing landscape shaped by early-life experience

The capacity to learn to associate cues with negative outcomes is a highly adaptive process that appears to be conserved across species. However, when the cue is no longer a valid predictor of danger, but the emotional response persists, this can result in maladaptive behaviors, and in humans contribute to debilitating emotional disorders. Over the past several decades, work in neuroscience, psychiatry, psychology, and biology have uncovered key processes underlying, and structures governing, emotional responding and learning, as well as identified disruptions in the structural and functional integrity of these brain regions in models of pathology. In this review, we highlight some of this elegant body of work as well as incorporate emerging findings from the field of developmental neurobiology to emphasize how development contributes to changes in the ability to learn and express emotional responses, and how early experiences, such as stress, shape the development and functioning of these circuits.

Early postnatal handling reduces hippocampal amyloid plaque formation and enhances cognitive performance in APPswe/PS1dE9 mice at middle age

In rodents, fragmented and low levels of maternal care have been implicated in age-related cognitive decline and the incidence and progression of Alzheimer's pathology. In contrast, enhancing early postnatal maternal care has been associated with improved cognitive function later in life. Here we examined whether early postnatal handling of mouse pups from postnatal days 2-9 enhanced maternal care and whether this affected cognition and Alzheimer pathology at 5 and 11months of age in the APPswe/PS1dE9 mouse model for Alzheimer's disease. Brief, 15min daily episodes of separating offspring from their dams from postnatal days 2-9 (early handling, EH) increased maternal care of the dam towards her pups upon reunion. At 11 (but not 5) months of age, EH APPswe/PS1dE1 mice displayed significantly reduced amyloid plaque pathology in the hippocampus. At this age, EH also prevented short-term working memory deficits while restoring impairments in contextual fear memory formation in APPswe/PS1dE9 mice. EH did not modulate amyloid pathology in the amygdala, nor did it affect auditory fear conditioning deficits in APPswe/PS1dE9 mice. We conclude that increased levels of maternal care during the early life period delays amyloid accumulation and cognitive decline in an Alzheimer's mouse model, involving the hippocampus, but not to the amygdala. These studies highlight the importance of the early postnatal period in modulating resilience to develop Alzheimer's pathology later in life.

Early life stress leads to developmental and sex selective effects on performance in a novel object placement task

Disruptions in early life care, including neglect, extreme poverty, and trauma, influence neural development and increase the risk for and severity of pathology. Significant sex disparities have been identified for affective pathology, with females having an increased risk of developing anxiety and depressive disorder. However, the effects of early life stress (ELS) on cognitive development have not been as well characterized, especially in reference to sex specific impacts of ELS on cognitive abilities over development. In mice, fragmented maternal care resulting from maternal bedding restriction, was used to induce ELS. The development of spatial abilities were tracked using a novel object placement (NOP) task at several different ages across early development (P21, P28, P38, P50, and P75). Male mice exposed to ELS showed significant impairments in the NOP task compared with control reared mice at all ages tested. In female mice, ELS led to impaired NOP performance immediately following weaning (P21) and during peri-adolescence (P38), but these effects did not persist into early adulthood. Prior work has implicated impaired hippocampus neurogenesis as a possible mediator of negative outcomes in ELS males. In the hippocampus of behaviorally naïve animals there was a significant decrease in expression of Ki-67 (proliferative marker) and doublecortin (DCX-immature cell marker) as mice aged, and a more rapid developmental decline in these markers in ELS reared mice. However, the effect of ELS dissipated by P28 and no main effect of sex were observed. Together these results indicate that ELS impacts the development of spatial abilities in both male and female mice and that these effects are more profound and lasting in males.

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ELS leads to sex differences in spatial memory abilities in mice.

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Female mice show impaired performance that resolve prior to adolescence.

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Male mice show persistent impairments across early life.

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Effects are restricted to spatial abilities and not other task dimensions.

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Effects are not related to markers of proliferation and differentiation in hippocampus.

Influence of parental care on offspring hippocampal volume in young adults varies as a function of overprotection

Parental care results in increased hippocampal volumes through adaptive stress responses in developing animals. However, human studies have not yet provided consistent findings analogous to the animal literature, possibly because parental care in humans is likely intermingled with parental overprotection, as suggested by the optimal parenting theory. Here, we tested the hypothesis that the effect of parental care on offspring hippocampal volume varies as a function of parental overprotection with a large cohort of young adult participants (N = 257). Consistent with some previous human studies, we found that parental care in childhood alone had little association with the hippocampal volume in adulthood. However, when parental overprotection was low, parental care was positively correlated with offspring hippocampal volume, whereas there was no association between parental care and offspring hippocampal volume when parental overprotection was high. Thus, an interaction exists between parental care and overprotection in human’s hippocampal development, which contributes to the elucidation of the complex relationship between brain structure and environmental factors.

Late-Onset Cognitive Impairments after Early-Life Stress Are Shaped by Inherited Differences in Stress Reactivity

Early-life stress (ELS) has been associated with lasting cognitive impairments and with an increased risk for affective disorders. A dysregulation of the hypothalamus-pituitary-adrenal (HPA) axis, the body’s main stress response system, is critically involved in mediating these long-term consequences of adverse early-life experience. It remains unclear to what extent an inherited predisposition for HPA axis sensitivity or resilience influences the relationship between ELS and cognitive impairments, and which neuroendocrine and molecular mechanisms may be involved. To investigate this, we exposed animals of the stress reactivity mouse model, consisting of three independent lines selectively bred for high (HR), intermediate (IR), or low (LR) HPA axis reactivity to a stressor, to ELS and assessed their cognitive performance, neuroendocrine function and hippocampal gene expression in early and in late adulthood. Our results show that HR animals that were exposed to ELS exhibited an HPA axis hyper-reactivity in early and late adulthood, associated with cognitive impairments in hippocampus-dependent tasks, as well as molecular changes in transcript levels involved in the regulation of HPA axis activity (Crh) and in neurotrophic action (Bdnf). In contrast, LR animals showed intact cognitive function across adulthood, with no change in stress reactivity. Intriguingly, LR animals that were exposed to ELS even showed significant signs of enhanced cognitive performance in late adulthood, which may be related to late-onset changes observed in the expression of Crh and Crhr1 in the dorsal hippocampus of these animals. Collectively, our findings demonstrate that the lasting consequences of ELS at the level of cognition differ as a function of inherited predispositions and suggest that an innate tendency for low stress reactivity may be protective against late-onset cognitive impairments after ELS.

Tactile stimulation during different developmental periods modifies hippocampal BDNF and GR, affecting memory and behavior in adult rats

Recent studies have shown that tactile stimulation (TS) in pups is able to prevent and/or minimize fear, anxiety behaviors, and addiction to psychostimulant drugs in adult rats. In these studies, animals have been exposed to handling from postnatal day (PND) 1-21. This study was designed to precisely establish which period of preweaning development has a greater influence of TS on neuronal development. After birth, male pups were exposed to TS from PND1-7, PND8-14, and PND15-21. In adulthood, the different periods of postnatal TS were assessed through behavioral, biochemical, and molecular assessments. Animals that received TS from PND8-14 showed lower anxiety-like symptoms, as observed by decreased anxiety index in elevated plus maze. This same TS period was able to improve rats' working memory by increasing the percentage of alternation rate in Y-maze, and induce better ability to cope with stressful situations, as showed in the defensive burying test by a reduced time of burying behavior. On the other hand, animals receiving TS in the first week of life showed longest cumulative burying time, which is directly related to increased anxiety-like behavior. Moreover, TS from PND8-14 showed lower corticosterone levels and better oxidative status, as observed by decreased lipid peroxidation and increased catalase activity in the hippocampus. Brain-derived neurotrophic factor (BDNF) immunocontent was increased in the hippocampus of animals receiving TS from PND8-14, while glucocorticoid receptors immunocontent was decreased in both TS_1-7 and TS_15-21 , but not TS_8-14 . To the best of our knowledge, this study is the first to show TS can be more efficient if applied over a focused period of neonatal development (PND8-14) and this beneficial influence can be reflected on reduced emotionality and increased ability to address stressful situations in adulthood. © 2016 Wiley Periodicals, Inc.

Neonatal repetitive pain in rats leads to impaired spatial learning and dysregulated hypothalamic-pituitary-adrenal axis function in later life

Preterm birth is a major health issue. As part of their life-saving care, most preterm infants require hospitalization and are inevitably exposed to repetitive skin-breaking procedures. The long-term effects of neonatal repetitive pain on cognitive and emotional behaviors involving hypothalamic-pituitary-adrenal (HPA) axis function in young and adult rats are unknown. From P8 to P85, mechanical hypersensitivity of the bilateral hindpaws was observed in the Needle group (P < 0.001). Compared with the Tactile group, the Needle group took longer to find the platform on P30 than on P29 (P = 0.03), with a decreased number of original platform site crossings during the probe trial of the Morris water maze test (P = 0.026). Moreover, the Needle group spent more time and took longer distances in the central area than the Tactile group in the Open-field test, both in prepubertal and adult rats (P < 0.05). The HPA axis function in the Needle group differed from the Tactile group (P < 0.05), with decreased stress responsiveness in prepuberty and puberty (P < 0.05) and increased stress responsiveness in adulthood (P < 0.05). This study indicates that repetitive pain that occurs during a critical period may cause severe consequences, with behavioral and neuroendocrine disturbances developing through prepuberty to adult life.

Pathway to neural resilience: Self-esteem buffers against deleterious effects of poverty on the hippocampus

Human neuroimaging studies have shown that people living in poverty tend to suffer hippocampal atrophy, which leads to impaired memory and learning throughout life. However, behavioral studies demonstrate that poor people with high self-esteem are often exempt from the deleterious effect of poverty and instead possess a happy and successful life. Here we investigated whether high self-esteem can buffer against the deleterious effects of poverty, as indicated by low subjective socioeconomic status (SSS), on the hippocampal gray matter volume (GMV) in a large cohort of young participants (N = 280). As expected, findings revealed that although low (vs. high) SSS was linked with a smaller hippocampal GMV, the deleterious effect of low SSS on hippocampal GMV was alleviated when the participants have high self-esteem. Commonality analyses further confirmed this observation. The current study suggests that positive psychological resources such as self-esteem may provide protection for the hippocampal atrophy in adversity. Hum Brain Mapp 37:3757-3766, 2016. © 2016 Wiley Periodicals, Inc.

The Effect of Preventive, Therapeutic and Protective Exercises on Hippocampal Memory Mediators in Stressed Rats

BACKGROUND

Exercise plays a significant role in learning and memory. The present study focuses on the hippocampal corticosterone (CORT), interleukin-1 beta (IL-1β), glucose, and brain-derived neurotrophic factor (BDNF) levels in preventive, therapeutic, and protective exercises in stressful conditions.

METHODS

Forty male rats were randomly divided into four groups: the control group and the preventive, therapeutic, and protective exercise groups. The treadmill running was applied at a speed of 20-21m/min and a chronic stress of 6 hours/day for 21 days. Subsequently, the variables were measured in the hippocampus.

RESULTS

The findings revealed that the hippocampal CORT levels in the preventive exercise group had a significant enhancement compared to the control group. In the protective and particularly the therapeutic exercise groups, the hippocampal CORT levels declined. Furthermore, the hippocampal BDNF levels in the preventive and the therapeutic exercise groups indicated significantly decreased and increased, respectively, in comparison with the control group. In the preventive exercise group, however, the hippocampal glucose level turned out to be substantially higher than that in the control group.

CONCLUSION

It appears that the therapeutic exercise group had the best exercise protocols for improving the hippocampal memory mediators in the stress conditions. By contrast, the preventive exercise group could not improve these mediators that had been altered by stress. It is suggested that exercise time, compared to stress, can be considered as a crucial factor in the responsiveness of memory mediators.

Positive and negative early life experiences differentially modulate long term survival and amyloid protein levels in a mouse model of Alzheimer's disease

Stress has been implicated as a risk factor for the severity and progression of sporadic Alzheimer's disease (AD). Early life experiences determine stress responsivity in later life, and modulate age-dependent cognitive decline. Therefore, we examined whether early life experiences influence AD outcome in a bigenic mouse model which progressively develops combined tau and amyloid pathology (biAT mice).Mice were subjected to either early life stress (ELS) or to 'positive' early handling (EH) postnatally (from day 2 to 9). In biAT mice, ELS significantly compromised long term survival, in contrast to EH which increased life expectancy. In 4 month old mice, ELS-reared biAT mice displayed increased hippocampal Aβ levels, while these levels were reduced in EH-reared biAT mice. No effects of ELS or EH were observed on the brain levels of APP, protein tau, or PSD-95. Dendritic morphology was moderately affected after ELS and EH in the amygdala and medial prefrontal cortex, while object recognition memory and open field performance were not affected. We conclude that despite the strong transgenic background, early life experiences significantly modulate the life expectancy of biAT mice. Parallel changes in hippocampal Aβ levels were evident, without affecting cognition of young adult biAT mice.

Stress biomarker responses to different protocols of forced exercise in chronically stressed rats

Stress is one of the most significant causes of major health problems on a global scale. The beneficial effects of exercise on combating stress, however, are well-established. The present study investigated the stress biomarker responses, such as serum corticosterone, interlukin-1β, and glucose levels, to different (preventive, therapeutic, protective, and continuous) protocols of forced exercise under stress. Male rats were randomly allocated to the following five groups: stressed, preventive, therapeutic, protective, and continuous (and/or pre-stress, post-stress, stress-accompanied, and both pre-stress and stress-accompanied exercise respectively) exercise groups. Stress was applied 6 h/day for 21 days and the treadmill running was employed at a speed of 20-21 m/min for 21 and 42 days. The findings showed that the therapeutic, protective, and continuous exercises led to reduced corticosterone and glucose levels. Whereas, the preventive exercise did not reverse the stress responses, and that the therapeutic exercise led to a significant decline in serum interlukin-1β. It is concluded that protective, therapeutic, and, particularly, continuous exercises lead to significant reductions in serum corticosterone and the associated stress-induced hyperglycemia. Moreover, it appears that the timing and duration of exercise are the two factors contributing to changes in stress biomarker responses.

Early life programming of pain: focus on neuroimmune to endocrine communication

Chronic pain constitutes a challenge for the scientific community and a significant economic and social cost for modern societies. Given the failure of current drugs to effectively treat chronic pain, which are based on suppressing aberrant neuronal excitability, we propose in this review an integrated approach that views pain not solely originating from neuronal activation but also the result of a complex interaction between the nervous, immune, and endocrine systems. Pain assessment must also extend beyond measures of behavioural responses to noxious stimuli to a more developmentally informed assessment given the significant plasticity of the nociceptive system during the neonatal period. Finally integrating the concept of perinatal programming into the pain management field is a necessary step to develop and target interventions to reduce the suffering associated with chronic pain. We present clinical and animal findings from our laboratory (and others) demonstrating the importance of the microbial and relational environment in programming pain responsiveness later in life via action on hypothalamo-pituitary adrenal (HPA) axis activity, peripheral and central immune system, spinal and supraspinal mechanisms, and the autonomic nervous system.

Tactile stimulation effects on hippocampal neurogenesis and spatial learning and memory in prenatally stressed rats

Neurogenesis in the dentate gyrus (DG) of the hippocampus is increased by spatial learning and postnatal stimulation. Conversely, prenatal stress (PS) produces a decrease in the proliferation of hippocampal granular cells. This work evaluated the effect of postnatal tactile stimulation (PTS), when applied from birth to adulthood, on cognitive performance and hippocampal neurogenesis (survival and differentiation) in PS female and male rats. The response of the adrenal axis to training in the Morris water maze (MWM) was also analyzed. PS was provided during gestational days 15 through 21. Hippocampal neurogenesis and cognitive performance in the MWM were assessed at an age of three months. Results showed that escape latencies of both female and male PS rats were longer compared to those of their controls (CON). DG cell survival increased in the PS female rats. Corticosterone concentrations were significantly higher in the male and female PS rats after MWM training. PTS improved escape latencies and increased the number of new neurons in the DG of PS animals, and their corticosterone concentrations were similar to those in CON. In CON, PTS diminished DG cell survival but increased differentiation and reduces latency in the MWM. These results show that long-term PTS in PS animals might prevent learning deficits in adults through increase in the number of DG new cells and decrease of the reactivity of the adrenal axis to MWM training.

Stress-related synaptic plasticity in the hypothalamus

Stress necessitates an immediate engagement of multiple neural and endocrine systems. However, exposure to a single stressor causes adaptive changes that modify responses to subsequent stressors. Recent studies examining synapses onto neuroendocrine cells in the paraventricular nucleus of the hypothalamus demonstrate that stressful experiences leave indelible marks that alter the ability of these synapses to undergo plasticity. These adaptations include a unique form of metaplasticity at glutamatergic synapses, bidirectional changes in endocannabinoid signalling and bidirectional changes in strength at GABAergic synapses that rely on distinct temporal windows following stress. This rich repertoire of plasticity is likely to represent an important building block for dynamic, experience-dependent modulation of neuroendocrine stress adaptation.

GABAA receptor-acting neurosteroids: a role in the development and regulation of the stress response

Regulation of hypothalamic-pituitary-adrenocortical (HPA) axis activity by stress is a fundamental survival mechanism and HPA-dysfunction is implicated in psychiatric disorders. Adverse early life experiences, e.g. poor maternal care, negatively influence brain development and programs an abnormal stress response by encoding long-lasting molecular changes, which may extend to the next generation. How HPA-dysfunction leads to the development of affective disorders is complex, but may involve GABAA receptors (GABAARs), as they curtail stress-induced HPA axis activation. Of particular interest are endogenous neurosteroids that potently modulate the function of GABAARs and exhibit stress-protective properties. Importantly, neurosteroid levels rise rapidly during acute stress, are perturbed in chronic stress and are implicated in the behavioural changes associated with early-life adversity. We will appraise how GABAAR-active neurosteroids may impact on HPA axis development and the orchestration of the stress-evoked response. The significance of these actions will be discussed in the context of stress-associated mood disorders.

How postnatal insults may program development: studies in animal models

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

Synaptic rewiring of stress-sensitive neurons by early-life experience: a mechanism for resilience?

Genes and environment interact to influence cognitive and emotional functions throughout life. Early-life experiences in particular contribute to vulnerability or resilience to a number of emotional and cognitive illnesses in humans. In rodents, early-life experiences directly lead to resilience or vulnerability to stress later in life, and influence the development of cognitive and emotional deficits. The mechanisms for the enduring effects of early-life experiences on cognitive and emotional outcomes are not completely understood. Here, we present emerging information supporting experience-dependent modulation of the number and efficacy of synaptic inputs onto stress-sensitive neurons. This synaptic 'rewiring', in turn, may influence the expression of crucial neuronal genes. The persistent changes in gene expression in resilient versus vulnerable rodent models are likely maintained via epigenetic mechanisms. Thus, early-life experience may generate resilience by altering synaptic input to neurons, which informs them to modulate their epigenetic machinery.

Role of sensory, social, and hormonal signals from the mother on the development of offspring

For mammals, sensory, social, and hormonal experience early in life is essential for the continuity of the infant's development. These experiences come from the mother through maternal care, and have enduring effects on the physiology and behavior of the adult organism. Disturbing the mother-offspring interaction by maternal deprivation (neglect) or exposure to adverse events as chronic stress, maltreatment, or sexual abuse has negative effects on the mental, psychological, physiological, and behavioral health. Indeed, these kinds of negative experiences can be the source of some neuropsychiatric diseases as depression, anxiety, impulsive aggression, and antisocial behavior. The purpose of this chapter is to review the most relevant evidence that supports the participation of cues from the mother and/or littermates during the postnatal preweaning period for the development of nervous system of the offspring. These findings come from the most frequently utilized experimental paradigms used in animal models, such as natural variations in maternal behavior, handling, partial maternal deprivation, and total maternal deprivation and artificial rearing. Through the use of these experimental procedures, it is possible to positively (handling paradigm), or negatively (maternal deprivation paradigms), affect the offspring's development. Finally, this chapter reviews the importance of the hormones that pups ingest through the maternal milk during early lactation on the development of several physiological systems, including the immune, endocrine systems, as well as on the adult behavior of the offspring.

A novel model of early experiences involving neonatal learning of a T-maze using maternal contact as a reward or its denial as an event of mild emotional adversity

We developed a novel animal model of early life experiences in which rat pups are trained during postnatal days (PND) 10-13 in a T-maze with maternal contact as a reward (RER group) or its denial (DER group) as a mildly aversive event. Both groups of animals learn the T-maze, albeit the RER do so more efficiently. Training results in activation of the basal ganglia in the RER and of the hippocampus and prefrontal cortex in the DER. Moreover, on PND10 DER training leads to increased corticosterone levels and activation of the amygdala. In adulthood, male DER animals show better mnemonic abilities in the Morris water maze while the RER exhibit enhanced fear memory. Furthermore, DER animals have a hypofunctioning serotonergic system and express depressive-like behavior and increased aggression. However, they have increased hippocampal glucocorticoid receptors, indicative of efficient hypothalamic-pituitary-adrenal axis function, and an adaptive pattern of stress-induced corticosterone response. The DER experience with its relatively negative emotional valence results in a complex behavioral phenotype, which cannot be considered simply as adaptive or maladaptive.

Neonatal handling: an overview of the positive and negative effects

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

Corticotropin releasing factor in neuroplasticity

Stress is among the strongest signals promoting neuroplasticity: Stress signals, indicating real or perceived danger, lead to alterations of neuronal function and often structure, designed to adapt to the changed conditions and promote survival. Corticotropin releasing factor (CRF) is expressed and released in several types of neuronal populations that are involved in cognition, emotion and the regulation of autonomic and endocrine function. CRF expressing neurons undergo functional and structural plasticity during stress and, in addition, the peptide acts via specific receptors to promote plasticity of target neurons.

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

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

Altered nociceptive, endocrine, and dorsal horn neuron responses in rats following a neonatal immune challenge

The neonatal period is characterized by significant plasticity where the immune, endocrine, and nociceptive systems undergo fine-tuning and maturation. Painful experiences during this period can result in long-term alterations in the neurocircuitry underlying nociception, including increased sensitivity to mechanical or thermal stimuli. Less is known about the impact of neonatal exposure to mild inflammatory stimuli, such as lipopolysaccharide (LPS), on subsequent inflammatory pain responses. Here we examine the impact of neonatal LPS exposure on inflammatory pain sensitivity and HPA axis activity during the first three postnatal weeks. Wistar rats were injected with LPS (0.05mg/kg IP, Salmonella enteritidis) or saline on postnatal days (PNDs) 3 and 5 and later subjected to the formalin test at PNDs 7, 13, and 22. One hour after formalin injection, blood was collected to assess corticosterone responses. Transverse spinal cord slices were also prepared for whole-cell patch clamp recording from lumbar superficial dorsal horn neurons (SDH). Brains were obtained at PND 22 and the hypothalamus was isolated to measure glucocorticoid (GR) and mineralocorticoid receptor (MR) transcript expression using qRT-PCR. Behavioural analyses indicate that at PND 7, no significant differences were observed between saline- or LPS-challenged rats. At PND 13, LPS-challenged rats exhibited enhanced licking (p<.01), and at PND 22, increased flinching in response to formalin injection (p<.05). LPS-challenged rats also displayed increased plasma corticosterone at PND 7 and PND 22 (p<.001) but not at PND 13 following formalin administration. Furthermore, at PND 22 neonatal LPS exposure induced decreased levels of GR mRNA and increased levels of MR mRNA in the hypothalamus. The intrinsic properties of SDH neurons were similar at PND 7 and PND 13. However, at PND 22, ipsilateral SDH neurons in LPS-challenged rats had a lower input resistance compared to their saline-challenged counterparts (p<.05). These data suggest neonatal LPS exposure produces developmentally regulated changes in formalin-induced behavioural responses, corticosterone levels, and dorsal horn neuron properties following noxious stimulation later in life. These findings highlight the importance of immune activation during the neonatal period in shaping pain sensitivity later in life. This programming involves both spinal cord neurons and the HPA axis.

Early paternal deprivation alters levels of hippocampal brain-derived neurotrophic factor and glucocorticoid receptor and serum corticosterone and adrenocorticotropin in a sex-specific way in socially monogamous mandarin voles

In monogamous mammals, fathers play an important role in the development of the brain and typical behavior in offspring, but the exact nature of this process is not well understood. In particular, little research has addressed whether the presence or absence of paternal care alters levels of hippocampal glucocorticoid receptor (GR) and brain-derived neurotrophic factor (BDNF), and basal levels of serum corticosterone (CORT) and adrenocorticotropin (ACTH). Here, we explored this concept using socially monogamous mandarin voles (Microtus mandarinus), a species in which fathers display high levels of paternal care toward their pups. Our immunohistochemical study shows that paternal deprivation (PD) significantly decreased levels of GR and BDNF protein in the CA1 and CA2/3 of the hippocampus. In the dental gyrus, decreases in GR and BDNF induced by PD were evident in females but not in males. Additionally, enzyme-linked immunosorbent assay results show that PD significantly upregulated levels of serum CORT and ACTH in females, but not males. These findings demonstrate that PD alters HPA axis activity in a sex-specific way. The changes in stress hormones documented here may be associated with alteration in hippocampal BDNF and GR levels.

Early life trauma and attachment: immediate and enduring effects on neurobehavioral and stress axis development

Over half a century of converging clinical and animal research indicates that early life experiences induce enduring neuroplasticity of the HPA-axis and the developing brain. This experience-induced neuroplasticity is due to alterations in the frequency and intensity of stimulation of pups' sensory systems (i.e., olfactory, somatosensory, gustatory) embedded in mother-infant interactions. This stimulation provides "hidden regulators" of pups' behavioral, physiological, and neural responses that have both immediate and enduring consequences, including those involving the stress response. While variation in stimulation can produce individual differences and adaptive behaviors, pathological early life experiences can induce maladaptive behaviors, initiate a pathway to pathology, and increase risk for later-life psychopathologies, such as mood and affective disorders, suggesting that infant-attachment relationships program later-life neurobehavioral function. Recent evidence suggests that the effects of maternal presence or absence during this sensory stimulation provide a major modulatory role in neural and endocrine system responses, which have minimal impact on pups' immediate neurobehavior but a robust impact on neurobehavioral development. This concept is reviewed here using two complementary rodent models of infant trauma within attachment: infant paired-odor-shock conditioning (mimicking maternal odor attachment learning) and rearing with an abusive mother that converge in producing a similar behavioral phenotype in later-life including depressive-like behavior as well as disrupted HPA-axis and amygdala function. The importance of maternal social presence on pups' immediate and enduring brain and behavior suggests unique processing of sensory stimuli in early life that could provide insight into the development of novel strategies for prevention and therapeutic interventions for trauma experienced with the abusive caregiver.