Neonatal maternal separation affects endocrine and metabolic stress responses to ether exposure but not to restraint exposure in adult rats

Maternal separation induced resilience to depression and spatial memory deficit despite intensifying hippocampal inflammatory responses to chronic social defeat stress in young adult male rats

Early life adversity has been suggested to affect neuroendocrine responses to subsequent stressors and accordingly vulnerability for behavioral disorders. This is the first work to study the effects of maternal separation (MS) stress on the co-occurrence of depression and cognitive impairments along with hippocampal inflammatory response under chronic social defeat stress (CSDS) in young adult male rats. During the first two postnatal weeks, the male pups were either exposed to MS or left undisturbed with their mothers (Std). Subsequently, starting on postnatal day 50, the animals of each group were either left undisturbed in the standard group housing (Con) or underwent CSDS for three weeks. Totally, there were four groups (n = 10/group), namely Std-Con, Ms-Con, Std-CSDS, and MS-CSDS. Pup retrieval test was performed on daily basis from PND1 to PND14. During the last week of the CSDS exposure, in the light phase, the behavioral tests and the retro-orbital blood sampling were performed to assess basal plasma corticosterone levels. Afterwards, the hippocampus of the animals was removed to measure the interleukin 1β (IL-1β) content. Exposure to CSDS increased the plasma corticosterone levels and induced social avoidance along with memory deficit. Maternal separation intensified hippocampal IL-1β contents as well as the plasma corticosterone levels in response to CSDS. Meanwhile, it facilitated the spatial learning and potentiated resilience to social avoidance and memory deficit. In conclusion, although maternal separation increased the basal plasma corticosterone levels, it could facilitate the learning process and induce resilience to the onset of depression and memory deficit in response to CSDS, probably through the compensatory increase in maternal care and the induction of mild hippocampal inflammatory response.

Early maternal separation alters the activation of stress-responsive brain areas in adulthood

The expression of c-Fos protein has been extensively used as a marker of neuronal activation in response to stressful stimuli. Early maternal separation (MS) is a model of early life adversity that affects the responsiveness of the brain areas to stressors. Thus, this study examined the impact of early MS on activating stress-responsive areas in the brain of adult rats in response to physical (ether) or psychological (restraint) stressors. Male pups were divided for the MS or non-handled (NH) groups. The MS was carried out daily between the 2nd and 14th day of postnatal life and consisted in removing the dams from the cage for 180 min. The rats were then subjected to experimental protocols of restraint or ether exposure at 10-12 weeks old. The rats were anesthetized 90 min after exposure to the stressors, and their brains were prepared for immunohistochemical analysis of c-Fos immunoreactive (c-Fos-ir) neurons in the hypothalamic paraventricular nucleus (PVN), supraoptic nucleus (SON), medial preoptic area (MPA), medial amygdaloid nucleus (MeA), locus coeruleus (LC), and nucleus of the solitary tract (NST). The MS-group presented 86%, 125%, 73%, 56%, and 137% higher c-Fos-ir neurons in the LC, PVN, SON, MPA, and MeA, respectively, compared to NH-group in response to the restraint stressor. In addition, the MS-group presented 180%, 137%, 170%, and 138% higher c-Fos-ir neurons for the ether exposure in the LC, PVN, MPA, and MeA, respectively. Our results show a greater increase in neuronal activation in the MS group, indicating that early life adversity can induce reprogramming in the brain response to stress in adulthood.

Early postnatal stress impairs insulin secretion in response to psychological stress in adult rats

PURPOSE

Adversity in early life can induce metabolic defects in exposure to stress in adulthood. Therefore, the exploration of involving mechanisms can be helpful in the treatment of metabolic disorders. So, the present study was conducted in terms of exploring the effects of interaction between early postnatal stress and young adulthood psychological stress on insulin secretion and pancreatic GLUT-2 levels in male rats.

METHODS

Footshock as a model of early life stress (at 2 weeks of age) and psychological stress induced by communication box as a model of young adulthood stress (at 8-10 weeks of age) were induced in male Wistar rats for five consecutive days (2 times/day). Blood samples were drawn to measure glucose, insulin, homeostatic model assessment of insulin resistance (HOMA-IR) and homeostasis model assessment of β-cell dysfunction (HOMA-B), before and after stress protocol in young adult rats. Corticosterone was measured on days 1 and 5 of stress induction. The day after the stress period, factors including glucose tolerance, TNF-alpha, isolated islets' insulin output and levels of pancreatic GLUT-2 protein via western blotting were determined.

RESULTS

The combination of early footshock exposure and psychological stress during adulthood did not affect plasma corticosterone, but increased plasma insulin, HOMA-IR, HOMA-B and TNF-alpha levels. Plasma TNF was not only increased by the combination of both stressors, but also after only E STR exposure. HOMA-IR was increased in both Psy STR and E + Psy-STR groups. Plasma glucose just increased in Psy STR group. The combination of these two life stressors further increased the in vitro insulin secretion from isolated islets in response to 16.7-mM glucose. The level of Glut2 was increased in Psy STR and decreased in both E STR and E + Psy STR groups. Finally, glucose tolerance was impaired and glucose-stimulated insulin secretion was increased in E + Psy STR group.

CONCLUSIONS

In conclusion, inducing stress in early life makes the organism more susceptible to metabolic defects in exposure to psychological stress later in life.

Early-life stress altered pancreatic Krebs cycle-related enzyme activities in response to young adulthood physical and psychological stress in male rat offspring

OBJECTIVES

Early-life stress (ELS) increases the risk of metabolic disorders in later life. The present study investigated the ELS effect on pancreatic pyruvate dehydrogenase (PDH) protein level, α-ketoglutarate dehydrogenase (α-KGDH), and aconitase activities as metabolic enzymes in response to young adulthood stress in male rat offspring.

METHODS

Male Wistar rats were divided into six groups: Control, early life stress (Early STR), young adult foot-shock stress (Y. adult F-SH STR), early + young adult foot-shock stress (Early + Y. adult F-SH STR), young adult psychological stress (Y. adult Psy STR) and early + young adult psychological stress (Early + Y. adult Psy STR). Stress was induced by a communication box at 2 weeks of age and young adulthood for five consecutive days. The blood samples were collected in young adult rats, then pancreases were removed to measure its PDH protein level and aconitase and α-KGDH activities.

RESULTS

In ELS animals, applying foot-shock stress in young adulthood increased PDH protein level, decreased α-KGDH and aconitase activities, and increased plasma glucose, insulin, and corticosterone concentrations. However, exposure to young adulthood psychological stress only decreased α-KGDH and aconitase activities.

CONCLUSIONS

It seems that ELS altered metabolic response to young adulthood stress through changes of Krebs cycle-related enzymes activities, though the type of adulthood stress was determinant.

Corticosterone, Adrenal, and the Pituitary-Gonadal Axis in Neonatal Rats: Effect of Maternal Separation and Hypoxia

Hypoxia, a common stressor in prematurity, leads to sexually dimorphic, short- and long-term effects on the adult hypothalamic-pituitary-adrenal (HPA) and hypothalamic-pituitary-gonadal (HPG) axes. We hypothesized that these effects are due to stress-induced increases in testosterone during early postnatal life. We evaluated this phenomenon by systematically assessing the short-term effects of normoxic or hypoxic separation on male and female pups at birth, postnatal hours (H) 2, 4, and 8, and postnatal days (PD) 2 to 7. Our findings were (a) hypoxic separation led to a large increase in plasma corticosterone from 4H-PD4, (b) neither normoxic nor hypoxic separation affected critical adrenal steroidogenic pathway genes; however, a significant decrease in baseline Cyp11a1, Mc2r, Mrap, and Star adrenal expression during the first week of neonatal life confirmed the start of the adrenal stress hyporesponsive period, (c) a luteinizing hormone/follicle-stimulating hormone-independent increase in plasma testosterone occurred in normoxic and hypoxic separated male pups at birth, (d) testicular Cyp11a1, Lhcgr, and Star expression was high at birth and decreased thereafter suggesting a hyporesponsive period in the testes, and (e) elevated estrogen in the early neonatal period occurred independently of gonadotropin stimulation. We conclude that a large corticosterone response to hypoxia during the first 5 days of life occurs as an adaptation to neonatal stress, that the testosterone surge during the first hours after birth occurs independently of gonadotropins but is associated with upregulation of the steroidogenic pathway genes in the testes, and that high postnatal estrogen production also occurs independently of gonadotropins.

Neonatal intestinal injury induced by maternal separation: pathogenesis and pharmacological targets 1

Maternal separation (MS) is a well-studied phenomenon thought to play a role in the pathogenesis of many diseases ranging from neuropsychiatric to early intestinal disorders such as necrotizing enterocolitis. The existing evidence suggests that MS initiates a variety of processes that in turn lead to early intestinal injury. Although there are many theories as to how MS alters normal physiological processes, the exact mechanism of action remains to be elucidated. This review aims to describe some of the pathological processes affecting the intestine that are caused by MS, including (i) brain-gut axis, (ii) intestinal epithelial barrier function, (iii) microbiome, (iv) oxidative stress and endoplasmic reticulum stress, and (v) gut inflammation.

Maternal separation blunted spatial memory formation independent of peripheral and hippocampal insulin content in young adult male rats

This study explores the effects of maternal separation as a chronic early life stress (ELS) on pancreatic islets insulin content and secretion, and their potential relationship with the hippocampus insulin content and spatial memory in young adulthood. Male rat offspring were divided into two groups: stress (STR) and non-stress (non-STR) groups. The animals of the STR group were separated from their mothers during postnatal days (PND) 1 to 21. During the weaning time, that is, PND-0 to PND-21, the body weight and length of the pups were measured. Blood samples were collected on PND-1, 21, 29 and 34 and during young adulthood (53±2 days) to determine plasma corticosterone and insulin levels. The young adult animals were also tested for spatial memory. One day after the memory test, the animals were decapitated and their pancreases were removed to measure the islets insulin content and secretion. Finally, the animals' hippocampi were isolated to determine their insulin content and insulin receptor protein amounts. During the period of weaning, the body weight and length of pups belonging to the STR group were significantly lower as compared to those in the non-STR group. Maternal separation did not change the plasma levels of insulin but increased plasma corticosterone levels from PND-21 to young adulthood and also reduced the islets insulin content but did not affect insulin secretion and the hippocampus insulin content and insulin receptor protein amount. Although, at the end of the memory tests, rats of the STR group reached the escape box at almost the same time and distance and with the same errors as rats of the non-STR group, the distance traveled to reach the escape box showed a steep reduction in the non-STR group as compared to the STR group after the first trial. Moreover, as compared to the STR group, the non-STR group showed an increasing trend for direct strategy to find the escape box. The islets insulin content and secretion, and the plasma insulin concentration were not significantly correlated with the hippocampus insulin content. From the results of the present study, it appears that the main behavioral effect of the maternal separation stress in the spatial memory task was to impair the strategy used by the animals to reach the escape box. This may indicate that maternal separation stress affects brain regions other than the hippocampus. Moreover, due to the reduction of the body weight and length of offspring belonging to the STR group, it should be further considered that both maternal separation and early life malnutrition are directly (and mechanistically) linked to cognitive alterations later in life in ways that are not dependent on peripheral and hippocampal insulin content.

Programming of the Adult HPA Axis After Neonatal Separation and Environmental Stress in Male and Female Rats

Maternal separation, hypoxia, and hypothermia are common stressors in the premature neonate. Using our rat model of human prematurity, we evaluated sexual dimorphisms in the long-term effects of these neonatal stressors on behavior of the hypothalamic-pituitary-adrenal (HPA) axis in adult rats. Neonatal rats were exposed daily on postnatal days 2 to 6 to maternal separation with normoxia, with hypoxia allowing spontaneous hypothermia, with hypothermia per se, and with hypoxia while maintaining isothermia with external heat. The major findings were that (a) prior maternal-neonatal separation during the first week of postnatal life attenuated the plasma ACTH and corticosterone response to restraint stress in adult male but not female rats, (b) prior neonatal hypothermia augmented the plasma ACTH and corticosterone response to restraint stress in adult male rats, but not female rats, and (c) changes in hypothalamic, pituitary, and adrenal mRNA expression did not account for most of these HPA axis effects. Most of the programming effects on adult HPA axis was attributed to prior maternal-neonatal separation alone (with normoxia) because the addition of hypoxia with spontaneous hypothermia, hypothermia per se, and hypoxia while preventing hypothermia during maternal-neonatal separation had minimal effects on the HPA axis. These results may inform strategies to prevent sexually dimorphic sequelae of neonatal stress including those due to medical interventions.

Sex differences in adult rat insulin and glucose responses to arginine: programming effects of neonatal separation, hypoxia, and hypothermia

Acute neonatal hypoxia, a common stressor, causes a spontaneous decrease in body temperature which may be protective. There is consensus that hypothermia should be prevented during acute hypoxia in the human neonate; however, this may be an additional stress with negative consequences. We hypothesize that maintaining body temperature during hypoxia in the first week of postnatal life alters the subsequent insulin, glucose, and glucagon secretion in adult rats. Rat pups were separated from their dam daily from postnatal days (PD) 2-6 for the following 90 min experimental treatments: (1) normoxic separation (control), (2) hypoxia (8% O2) allowing spontaneous hypothermia, (3) normoxic hypothermia with external cold, and (4) exposure to 8% O2 while maintaining body temperature using external heat. An additional normoxic non-separated control group was performed to determine if separation per se changed the adult phenotype. Plasma insulin, glucose, and glucagon responses to arginine stimulation were evaluated from PD105 to PD133. Maternal separation (compared to non-separated neonates) had more pronounced effects on the adult response to arginine compared to the hypoxic, hypothermic, and hypoxic-isothermic neonatal treatments. Adult males exposed to neonatal maternal separation had augmented insulin and glucose responses to arginine compared to unseparated controls. Additionally, neonatal treatment had a significant effect on body weight gain; adults exposed to neonatal maternal separation were significantly heavier. Female adults had significantly smaller insulin and glucose responses to arginine regardless of neonatal treatment. Neonatal maternal separation during the first week of life significantly altered adult beta-cell function in a sexually dimorphic manner.

Effect of early life stress on pancreatic isolated islets' insulin secretion in young adult male rats subjected to chronic stress

Early stressful experiences may predispose organisms to certain disorders, including those of metabolic defects. This study aimed to explore the effects of early life stress on pancreatic insulin secretion and glucose transporter 2 (GLUT2) protein levels in stressed young adult male rats. Foot shock stress was induced in early life (at 2 weeks of age) and/or in young adulthood (at 8-10 weeks of age) for five consecutive days. Blood samples were taken before and after stress exposure in young adult rats. At the end of the experiment, glucose tolerance, isolated islets' insulin secretion, and pancreatic amounts of GLUT2 protein were measured. Our results show that early life stress has no effect on basal plasma corticosterone levels and adrenal weight, either alone or combined with young adulthood stress, but that early life + young adulthood stress could prevent weight gain, and cause an increase in basal plasma glucose and insulin. The homeostasis model assessment of insulin resistance index did not increase, when the rats were subjected to early life stress alone, but increased when combined with young adulthood stress. Moreover, glucose tolerance was impaired by the combination of early life + young adult stress. There was a decrease in islet's insulin secretion in rats subjected to early life stress in response to 5.6 mM glucose concentration, but an increase with a concentration of 16.7 mM glucose. However, in rats subjected to early life + young adulthood stress, islet's insulin secretion increased in response to both the levels of glucose concentrations. GLUT2 protein levels decreased in response to early life stress and early life + young adulthood stress, but there was a greater decrease in the early life stress group. In conclusion, perhaps early life stress sensitizes the body to stressors later in life, making it more susceptible to metabolic syndrome only when the two are in combination.

Transgenerational effects of social stress on social behavior, corticosterone, oxytocin, and prolactin in rats

Social stressors such as depressed maternal care and family conflict are robust challenges which can have long-term physiological and behavioral effects on offspring and future generations. The current study investigates the transgenerational effects of an ethologically relevant chronic social stress on the behavior and endocrinology of juvenile and adult rats. Exposure to chronic social stress during lactation impairs maternal care in F0 lactating dams and the maternal care of the F1 offspring of those stressed F0 dams. The overall hypothesis was that the male and female F2 offspring of stressed F1 dams would display decreased social behavior as both juveniles and adults and that these behavioral effects would be accompanied by changes in plasma corticosterone, prolactin, and oxytocin. Both the female and male F2 offspring of dams exposed to chronic social stress displayed decreased social behavior as juveniles and adults, and these behavioral effects were accompanied by decreases in basal concentrations of corticosterone in both sexes, as well as elevated juvenile oxytocin and decreased adult prolactin in the female offspring. The data support the conclusion that social stress has transgenerational effects on the social behavior of the female and male offspring which are mediated by changes in the hypothalamic-pituitary-adrenal axis and hypothalamic-pituitary-gonadal axis. Social stress models are valuable resources in the study of the transgenerational effects of stress on the behavioral endocrinology of disorders such as depression, anxiety, autism, and other disorders involving disrupted social behavior.

Early-Life Stress, HPA Axis Adaptation, and Mechanisms Contributing to Later Health Outcomes

Stress activates the hypothalamic-pituitary-adrenal (HPA) axis, which then modulates the degree of adaptation and response to a later stressor. It is known that early-life stress can impact on later health but less is known about how early-life stress impairs HPA axis activity, contributing to maladaptation of the stress-response system. Early-life stress exposure (either prenatally or in the early postnatal period) can impact developmental pathways resulting in lasting structural and regulatory changes that predispose to adulthood disease. Epidemiological, clinical, and experimental studies have demonstrated that early-life stress produces long term hyper-responsiveness to stress with exaggerated circulating glucocorticoids, and enhanced anxiety and depression-like behaviors. Recently, evidence has emerged on early-life stress-induced metabolic derangements, for example hyperinsulinemia and altered insulin sensitivity on exposure to a high energy diet later in life. This draws our attention to the contribution of later environment to disease vulnerability. Early-life stress can alter the expression of genes in peripheral tissues, such as the glucocorticoid receptor and 11-beta hydroxysteroid dehydrogenase (11β-HSD1). We propose that interactions between altered HPA axis activity and liver 11β-HSD1 modulates both tissue and circulating glucocorticoid availability, with adverse metabolic consequences. This review discusses the potential mechanisms underlying early-life stress-induced maladaptation of the HPA axis, and its subsequent effects on energy utilization and expenditure. The effects of positive later environments as a means of ameliorating early-life stress-induced health deficits, and proposed mechanisms underpinning the interaction between early-life stress and subsequent detrimental environmental exposures on metabolic risk will be outlined. Limitations in current methodology linking early-life stress and later health outcomes will also be addressed.

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.

Maternal stress affects postnatal growth and the pituitary expression of prolactin in mouse offspring

Maternal stress exerts long-lasting psychiatric and somatic on offspring, which persist into adulthood. However, the effect of maternal stress on the postnatal growth of pups has not been widely reported. In this study, we found that maternal immobilization stress (IS) during lactation resulted in low body weight of male mouse offspring, which persisted after weaning. Despite free access to chow, IS induced maternal malnutrition and decreased the serum insulin-like growth factor-1 (IGF-1) levels in the mothers and in the pups. mRNA expression analysis of anterior pituitary hormones in the pups revealed that growth hormone (GH) and prolactin (PRL), but no other hormones, were decreased by IS. Expression of the pituitary transcription factor PIT1 and isoforms of PITX2, which are essential for the development and function of GH-producing somatotropes and PRL-producing lactotropes, was decreased, whereas that of PROP1, which is critical for the earlier stages of pituitary development, was unchanged. Immunohistochemistry also showed a decrease in pituitary PRL protein expression. These results suggest that stress in a postpartum mother has persistent effects on the body weight of the offspring. Reduced PRL expression in the offspring's pituitary gland may play a role in these effects.

Regulation by chronic-mild stress of glucocorticoids, monocyte chemoattractant protein-1 and adiposity in rats fed on a high-fat diet

Stress has been reported as a widespread problem and several studies have linked obesity and inflammation-related diseases. Moreover, the combination of suffering from chronic stress and high energy intake might be related to the onset of some metabolic diseases. To study the possible relationships between stress, inflammatory status and obesity, a chronic-mild stress (CMS) paradigm with a high-fat dietary intake model (Cafeteria diet) was implemented on male Wistar rats for 11 weeks. Stress and dietary intake effects on animal adiposity, serum biochemical as well as glucocorticoids and inflammation markers were all analyzed. As expected, consuming a high-fat diet increased body weight, adiposity and insulin resistance in non-stressed animals. A decrease of total white adipose tissue (WAT) and an increase of fecal glucocorticoids, as well as angiotensinogen, and monocyte chemoattractant protein-1 (MCP-1) expression level in retroperitoneal WAT were found only on control-stressed rats. Regarding the serum MCP-1, a decrease was observed on animals under CMS while being fed Cafeteria diet. Furthermore, 11β-hydroxysteroid dehydrogenase activity, a glucocorticoid and obesity biomarker in the liver, was influenced by high-fat diet intake but not by stress. Finally, statistical analysis showed a strong relation between MCP-1 expression levels in retroperitoneal WAT, fecal corticosterone and total WAT. This trial proved that CMS induced a glucocorticoid-mediated response, which was reduced by the intake of a Cafeteria diet. These findings suggest that a high-fat diet could protect against a stress condition and revealed a different behavior to a stressful environment depending on the nutritional status.

Impact of early parental child-rearing behavior on young adults' cardiometabolic risk profile: a prospective study

OBJECTIVE

To examine prospectively whether early parental child-rearing behavior is a predictor of cardiometabolic outcome in young adulthood when other potential risk factors are controlled. Metabolic factors associated with increased risk for cardiovascular disease have been found to vary, depending on lifestyle as well as genetic predisposition. Moreover, there is evidence suggesting that environmental conditions, such as stress in pre- and postnatal life, may have a sustained impact on an individual's metabolic risk profile.

METHODS

Participants were drawn from a prospective, epidemiological, cohort study followed up from birth into young adulthood. Parent interviews and behavioral observations at the age of 3 months were conducted to assess child-rearing practices and mother-infant interaction in the home setting and in the laboratory. In 279 participants, anthropometric characteristics, low-density lipoprotein and high-density lipoprotein cholesterol, apolipoproteins, and triglycerides were recorded at age 19 years. In addition, structured interviews were administered to the young adults to assess indicators of current lifestyle and education.

RESULTS

Adverse early-life interaction experiences were significantly associated with lower levels of high-density lipoprotein cholesterol and apolipoprotein A1 in young adulthood. Current lifestyle variables and level of education did not account for this effect, although habitual smoking and alcohol consumption also contributed significantly to cardiometabolic outcomes.

CONCLUSIONS

These findings suggest that early parental child-rearing behavior may predict health outcome in later life through its impact on metabolic parameters in adulthood.

Acute restraint stress produces behavioral despair in weanling rats in the forced swim test

Stressful experiences in the rat during early life increase the vulnerability to later signs of behavioral despair in adulthood, reflected in increased immobility in the forced swim test (FST). However, the possible immediate effects of stress in weanling rats have only been partially described. The present study tested whether a single session of mild restraint stress modifies immobility in the FST in 21-day-old Wistar rats. After evaluating any possible changes in locomotion using the open field test (OFT), the latency and total duration of immobility were assessed in a single FST session. Regardless of gender, mild restraint stress significantly reduced crossings in the OFT, shortened the latency to the first period of immobility, and increased immobility in the FST compared with a control group devoid of stress. We conclude that a single mild physical stress session, as early as postnatal day 21, produces signs of behavioral despair.