Early maternal separation and chronic variable stress in adulthood changes the neural activity and the expression of glucocorticoid receptor in limbic structures

Effects of early life adversity on immediate early gene expression: Systematic review and 3-level meta-analysis of rodent studies

Early-life adversity (ELA) causes long-lasting structural and functional changes to the brain, rendering affected individuals vulnerable to the development of psychopathologies later in life. Immediate-early genes (IEGs) provide a potential marker for the observed alterations, bridging the gap between activity-regulated transcription and long-lasting effects on brain structure and function. Several heterogeneous studies have used IEGs to identify differences in cellular activity after ELA; systematically investigating the literature is therefore crucial for comprehensive conclusions. Here, we performed a systematic review on 39 pre-clinical studies in rodents to study the effects of ELA (alteration of maternal care) on IEG expression. Females and IEGs other than cFos were investigated in only a handful of publications. We meta-analyzed publications investigating specifically cFos expression. ELA increased cFos expression after an acute stressor only if the animals (control and ELA) had experienced additional hits. At rest, ELA increased cFos expression irrespective of other life events, suggesting that ELA creates a phenotype similar to naïve, acutely stressed animals. We present a conceptual theoretical framework to interpret the unexpected results. Overall, ELA likely alters IEG expression across the brain, especially in interaction with other negative life events. The present review highlights current knowledge gaps and provides guidance to aid the design of future studies.

Interactions Between Experience, Genotype and Sex in the Development of Individual Coping Strategies

Coping strategies, the first line of defense against adversities, develop through experience. There is consistent evidence that both genotype and sex contribute to the development of dysfunctional coping, leading to maladaptive outcomes of adverse experiences or to adaptive coping that fosters rapid recovery even from severe stress. However, how these factors interact to influence the development of individual coping strategies is just starting to be investigated. In the following review, we will consider evidence that experience, sex, and genotype influence the brain circuits and neurobiological processes involved in coping with adversities and discuss recent results pointing to the specific effects of the interaction between early experiences, genotype, and stress in the development of functional and dysfunctional coping styles.

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

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

Unstable Maternal Environment Affects Stress Response in Adult Mice in a Genotype-Dependent Manner

Early postnatal events exert powerful effects on development, inducing persistent functional alterations in different brain network, such as the catecholamine prefrontal-accumbal system, and increasing the risk of developing psychiatric disorders later in life. However, a vast body of literature shows that the interaction between genetic factors and early environmental conditions is crucial for expression of psychopathologies in adulthood. We evaluated the long-lasting effects of a repeated cross-fostering (RCF) procedure in 2 inbred strains of mice (C57BL/6J, DBA/2), known to show a different susceptibility to the development and expression of stress-induced psychopathologies. Coping behavior (forced swimming test) and preference for a natural reinforcing stimulus (saccharine preference test) were assessed in adult female mice of both genotypes. Moreover, c-Fos stress-induced activity was assessed in different brain regions involved in stress response. In addition, we evaluated the enduring effects of RCF on catecholamine prefrontal-accumbal response to acute stress (restraint) using, for the first time, a new "dual probes" in vivo microdialysis procedure in mouse. RCF experience affects behavioral and neurochemical responses to acute stress in adulthood in opposite direction in the 2 genotypes, leading DBA mice toward an "anhedonic-like" phenotype and C57 mice toward an increased sensitivity for a natural reinforcing stimulus.

Neurobiology of early life stress and visceral pain: translational relevance from animal models to patient care

BACKGROUND

Epidemiological studies show that females are twice as likely to receive a diagnosis of irritable bowel syndrome (IBS) than their male counterparts. Despite evidence pointing to a role for sex hormones in the onset or exacerbation of IBS symptoms, the mechanism by which ovarian hormones may predispose women to develop IBS remains largely undefined. On the other hand, there is a growing body of research showing a correlation between reports of early life stress (ELS) and the diagnosis of IBS. Current treatments available for IBS patients target symptom relief including abdominal pain and alterations in bowel habits, but are not directed to the etiology of the disease.

PURPOSE

To better understand the mechanisms by which sex hormones and ELS contribute to IBS, animal models have been developed to mirror complex human experiences allowing for longitudinal studies that investigate the lifelong consequences of ELS. These preclinical models have been successful in recapitulating ELS-induced visceral pain. Moreover, in female rats the influence of cycling hormones on visceral hypersensitivity resembles that seen in women with IBS. Such studies suggest that rodent models of ELS may serve as pivotal tools in determining (i) the etiology of IBS, (ii) novel future treatments for IBS, and (iii) improving individualized patient care. The current review aims to shed light on the progress and the challenges observed by clinicians within the field of gastroenterology and the preclinical science aimed at addressing those challenges in an effort to understand and more efficiently treat functional gastrointestinal disorders (FGIDs) in both children and adults.

Gender specific effects of neonatal limited nesting on viscerosomatic sensitivity and anxiety-like behavior in adult rats

BACKGROUND

Evidence exists to suggest that early life stress (ELS), such as neglect or abuse has profound effects on the developing brain. The current study tests the hypothesis that ELS in the form of neonatal limited nesting (LN) may serve as a predisposing factor for the development of altered nociceptive processing and comorbid increases in anxiety-like behavior in adulthood.

METHODS

Both male and female neonatal Sprague-Dawley rats were subjected to LN from postnatal day (PND) 2-9, while a control group was exposed to standard cage bedding. In adulthood, visceral sensitivity was assessed by quantifying a visceromotor behavioral response to graded isobaric pressures of colorectal distension. Hindpaw withdrawal thresholds in response to von Frey filaments were used to measure somatic sensitivity. Anxiety-like behavior was assessed in adult life using both the elevated plus maze and open field assay.

KEY RESULTS

Early life stress in the form of neonatal LN induced visceral and somatic hypersensitivity in adult male rats and augmented anxiety-like behavior. However, in adult cycling females, neonatal LN did not alter nociceptive processing or lead to changes in the levels of anxiety-like behavior.

CONCLUSIONS & INFERENCES

Our findings suggest that in male rats the LN model is a novel tool to investigate the long-term consequences of adverse early life experience on adult health.

Severe life stress and oxidative stress in the brain: from animal models to human pathology

SIGNIFICANCE

Severe life stress (SLS), as opposed to trivial everyday stress, is defined as a serious psychosocial event with the potential of causing an impacting psychological traumatism.

RECENT ADVANCES

Numerous studies have attempted to understand how the central nervous system (CNS) responds to SLS. This response includes a variety of morphological and neurochemical modifications; among them, oxidative stress is almost invariably observed. Oxidative stress is defined as disequilibrium between oxidant generation and the antioxidant response.

CRITICAL ISSUES

In this review, we discuss how SLS leads to oxidative stress in the CNS, and how the latter impacts pathophysiological outcomes. We also critically discuss experimental methods that measure oxidative stress in the CNS. The review covers animal models and human observations. Animal models of SLS include sleep deprivation, maternal separation, and social isolation in rodents, and the establishment of hierarchy in non-human primates. In humans, SLS, which is caused by traumatic events such as child abuse, war, and divorce, is also accompanied by oxidative stress in the CNS.

FUTURE DIRECTIONS

The outcome of SLS in humans ranges from resilience, over post-traumatic stress disorder, to development of chronic mental disorders. Defining the sources of oxidative stress in SLS might in the long run provide new therapeutic avenues.

Epigenetics and the transition from acute to chronic pain

OBJECTIVE

The objective of this study was to review the epigenetic modifications involved in the transition from acute to chronic pain and to identify potential targets for the development of novel, individualized pain therapeutics.

BACKGROUND

Epigenetics is the study of heritable modifications in gene expression and phenotype that do not require a change in genetic sequence to manifest their effects. Environmental toxins, medications, diet, and psychological stresses can alter epigenetic processes such as DNA methylation, histone acetylation, and RNA interference. As epigenetic modifications potentially play an important role in inflammatory cytokine metabolism, steroid responsiveness, and opioid sensitivity, they are likely key factors in the development of chronic pain. Although our knowledge of the human genetic code and disease-associated polymorphisms has grown significantly in the past decade, we have not yet been able to elucidate the mechanisms that lead to the development of persistent pain after nerve injury or surgery.

DESIGN

This is a focused literature review of epigenetic science and its relationship to chronic pain.

RESULTS

Significant laboratory and clinical data support the notion that epigenetic modifications are affected by the environment and lead to differential gene expression. Similar to mechanisms involved in the development of cancer, neurodegenerative disease, and inflammatory disorders, the literature endorses an important potential role for epigenetics in chronic pain.

CONCLUSIONS

Epigenetic analysis may identify mechanisms critical to the development of chronic pain after injury, and may provide new pathways and target mechanisms for future drug development and individualized medicine.

Postnatal aversive experience impairs sensitivity to natural rewards and increases susceptibility to negative events in adult life

Evidence shows that maternal care and postnatal traumatic events can exert powerful effects on brain circuitry development but little is known about the impact of early postnatal experiences on processing of rewarding and aversive stimuli related to the medial prefrontal cortex (mpFC) function in adult life. In this study, the unstable maternal environment induced by repeated cross-fostering (RCF) impaired palatable food conditioned place preference and disrupted the natural preference for sweetened fluids in the saccharin preference test. By contrast, RCF increased sensitivity to conditioned place aversion (CPA) and enhanced immobility in the forced swimming test. Intracerebral microdialysis data showed that the RCF prevents mpFC dopamine (DA) outflow regardless of exposure to rewarding or aversive stimuli, whereas it induces a strong and sustained prefrontal norepinephrine (NE) release in response to different aversive experiences. Moreover, the selective mpFC NE depletion abolished CPA, thus indicating that prefrontal NE is required for motivational salience attribution to aversion-related stimuli. These findings demonstrate that an unstable maternal environment impairs the natural propensity to seek pleasurable sources of reward, enhances sensitivity to negative events in adult life, blunts prefrontal DA outflow, and modulates NE release in the reverse manner depending on the exposure to rewarding or aversive stimuli.

Increased hippocampal tau phosphorylation and axonal mitochondrial transport in a mouse model of chronic stress

Corticotropin-releasing hormone (CRH) is considered the driving force of the hypothalamo-pituitary-adrenal (HPA) axis and plays an important role in mood regulation. The HPA axis is reported to be closely related to acute stress-induced tau phosphorylation in the rodent hippocampus. However, the relationship between the hyperactive HPA axis and tau phosphorylation in the hippocampus and hence the functional implications for chronic stress are not fully understood. In this study, we aimed to examine tau phosphorylation and the effect on axonal transport of mitochondria in the hippocampus of a chronic stress model. A mouse model was created by neonatal isolation before weaning, followed by chronic mild stress by social isolation after weaning. Behavioural tests showed that the model had a typical depression/anxiety-like behaviour accompanied by increased plasma corticosterone level and hypothalamic CRH mRNA expression. Phosphorylated tau increased significantly, accompanied by increased synaptosomal mitochondrial levels in hippocampus of the chronic stress model. CRH receptor 1 antagonist (CP154,526) treatment, not glucocorticoid receptor antagonist (RU486) treatment, decreased tau phosphorylation and synaptosomal mitochondrial levels in the hippocampus of the mouse model. Consistent with an in-vivo model, when hyperphosphorylated tau was inhibited by lithium in cultured primary hippocampal neurons, mitochondrial transport monitored by live imaging was also decreased. We show here for the first time that phosphorylated tau in the hippocampus of a chronic stress model, accompanied by increased mitochondrial transport, was mediated by CRH receptor 1, not by glucocorticoid receptors, which suggests that centrally derived CRH may be involved in the process of mitochondrial axon transport and hence play an important role in hippocampus of a chronic stress model.

Early postnatal migration and development of layer II pyramidal neurons in the rodent cingulate/retrosplenial cortex

The cingulate and retrosplenial regions are major components of the dorsomedial (dm) limbic cortex and have been implicated in a range of cognitive functions such as emotion, attention, and spatial memory. While the structure and connectivity of these cortices are well characterized, little is known about their development. Notably, the timing and mode of migration that govern the appropriate positioning of late-born neurons remain unknown. Here, we analyzed migratory events during the early postnatal period from ventricular/subventricular zone (VZ/SVZ) to the cerebral cortex by transducing neuronal precursors in the VZ/SVZ of newborn rats/mice with Tomato/green fluorescent protein-encoding lentivectors. We have identified a pool of postmitotic pyramidal precursors in the dm part of the neonatal VZ/SVZ that migrate into the medial limbic cortex during the first postnatal week. Time-lapse imaging demonstrates that these cells migrate on radial glial fibers by locomotion and display morphological and behavioral changes as they travel through the white matter and enter into the cortical gray matter. In the granular retrosplenial cortex, these cells give rise to a Satb2+ pyramidal subtype and develop dendritic bundles in layer I. Our observations provide the first insight into the patterns and dynamics of cell migration into the medial limbic cortex.

Maternal separation as a model of brain-gut axis dysfunction

RATIONALE

Early life stress has been implicated in many psychiatric disorders ranging from depression to anxiety. Maternal separation in rodents is a well-studied model of early life stress. However, stress during this critical period also induces alterations in many systems throughout the body. Thus, a variety of other disorders that are associated with adverse early life events are often comorbid with psychiatric illnesses, suggesting a common underlying aetiology. Irritable bowel syndrome (IBS) is a functional gastrointestinal disorder that is thought to involve a dysfunctional interaction between the brain and the gut. Essential aspects of the brain-gut axis include spinal pathways, the hypothalamic pituitary adrenal axis, the immune system, as well as the enteric microbiota. Accumulating evidence suggest that stress, especially in early life, is a predisposing factor to IBS.

OBJECTIVE

The objective of this review was to assess and compile the most relevant data on early life stress and alterations at all levels of the brain gut axis.

RESULTS

In this review, we describe the components of the brain-gut axis individually and how they are altered by maternal separation. The separated phenotype is characterised by alterations of the intestinal barrier function, altered balance in enteric microflora, exaggerated stress response and visceral hypersensitivity, which are all evident in IBS.

CONCLUSION

Thus, maternally separated animals are an excellent model of brain-gut axis dysfunction for the study of disorders such as IBS and for the development of novel therapeutic interventions.