Interaction with stressed mothers affects corticosterone levels in pups after reunion and impairs the response to dexamethasone in adult mice

Long term effects of early life stress on HPA circuit in rodent models

Adaptation to environmental challenges represents a critical process for survival, requiring the complex integration of information derived from both external cues and internal signals regarding current conditions and previous experiences. The Hypothalamic-pituitary-adrenal axis plays a central role in this process inducing the activation of a neuroendocrine signaling cascade that affects the delicate balance of activity and cross-talk between areas that are involved in sensorial, emotional, and cognitive processing such as the hippocampus, amygdala, Prefrontal Cortex, Ventral Tegmental Area, and dorsal raphe. Early life stress, especially early critical experiences with caregivers, influences the functional and structural organization of these areas, affects these processes in a long-lasting manner and may result in long-term maladaptive and psychopathological outcomes, depending on the complex interaction between genetic and environmental factors. This review summarizes the results of studies that have modeled this early postnatal stress in rodents during the first 2 postnatal weeks, focusing on the long-term effects on molecular and structural alteration in brain areas involved in Hypothalamic-pituitary-adrenal axis function. Moreover, a brief investigation of epigenetic mechanisms and specific genetic targets mediating the long-term effects of these early environmental manipulations and at the basis of differential neurobiological and behavioral effects during adulthood is provided.

Rodent models of mental illness in polycystic ovary syndrome: the potential role of hypothalamic-pituitary-adrenal dysregulation and lessons for behavioral researchers

Polycystic ovary syndrome (PCOS) is the most commonly diagnosed endocrine disorder in women of reproductive age, with phenotypes including ovarian and metabolic dysfunctions. Women with PCOS also show increased rates of mental illness, dysregulation of the hypothalamic-pituitary-adrenal (HPA) axis, and altered responsiveness to stressors that may contribute to the higher rates of mental illness, specifically depression and anxiety. Animal models of PCOS have provided insight into the ovarian and metabolic mechanisms that underlie the syndrome, and several models have been used to study the behavioral consequences associated with PCOS in the laboratory. Several studies in rodent models of PCOS demonstrate changes in anxiety-like behavior, but researchers often neglect to report procedural details or behavioral data crucial to interpreting the differences observed in those studies. Additionally, the impact of potential HPA dysregulation in animal models of PCOS may influence behavioral findings, although only three studies to date have examined this. As such, researchers should consider and report stress-associated variables (e.g., time of day, light/dark cycle, light intensity, housing, and procedures to control experimenter and litter effects) that may influence depression- and anxiety-like behaviors in rodents. This review will summarize the behavioral and HPA-related studies in women with PCOS and rodent models of the disease, and provide considerations for future studies.

Risk Assessment of Neonatal Exposure to Low Frequency Noise Based on Balance in Mice

General electric devices and ventilation systems are known to generate low frequency noise (LFN) with frequencies of <100 Hz. Previous studies showed that exposure to LFN caused impairments of balance in humans and mice during adulthood. On the other hand, a previous study showed that noise levels in the neonatal intensive care unit (NICU) were greater than those in general home or office environments. Therefore, it is possible that neonates have a potential risk to be exposed to LFN in the NICU. However, the risk of neonatal exposure to LFN remains unclear in humans and mice. In this study, male ICR mice were exposed to LFN at 100 Hz for 4 weeks after birth and then subjected to rotarod and beam crossing tests in order to assess LFN-mediated risk of imbalance during the neonatal period. Exposure to LFN at 70 dB, but not exposure to LFN up to 60 dB, during the neonatal period significantly decreased performance scores for rotarod and beam crossing tests compared to the scores of the control group. The number of calbindin-positive hair cells in the saccule and utricle was decreased in mice exposed to LFN at 70 dB for 4 weeks in the neonatal phase. Cessation of exposure for 10 weeks did not result in recovery of the decreased performance in rotarod and beam crossing tests. Thus, our results suggest that 70 dB is a possible threshold for exposure to LFN for 4 weeks during the neonatal period causing unrecoverable imbalance in mice.

High glucocorticoid levels during gestation activate the inflammasome in hippocampal oligodendrocytes of the offspring

Exposure to high levels of glucocorticoids (GCs) during early life induces long-lasting neuroinflammation. GCs induce rapid degranulation of mast cells, which release proinflammatory molecules promoting activation of microglia and astrocytes. The possible involvement of oligodendrocytes, however, remains poorly understood. It was studied whether high GC levels during gestation activates the inflammasome in hippocampal oligodendrocytes of mouse offspring. Oligodendrocytes of control pups showed expression of inflammasome components (NLRP3, ACS, and caspase-1) and their levels were increased by prenatal administration of dexamethasone (DEX), a synthetic GC. These cells also showed high levels of IL-1β and TNF-α, revealing activation of the inflammasome. Moreover, they showed increased levels of the P2X₇ receptor and pannexin1, which are associated to inflammasome activation. However, levels of connexins either were not affected (Cx29) or reduced (Cx32 and Cx47). Nonetheless, the functional states of pannexin1 and connexin hemichannels were elevated and directly associated to functional P2X₇ receptors. As observed in DEX-treated brain slices, hemichannel activity first increased in hippocampal mast cells and later in microglia and macroglia. DEX-induced oligodendrocyte hemichannel activity was mimicked by urocortin-II, which is a corticotropin-releasing hormone receptor (CRHR) agonist. Response to DEX and urocortin-II was inhibited by antalarmin (a CRHR blocker) or by mast cells or microglia inhibitors. The increase in hemichannel activity persisted for several weeks after birth and cross-fostering with a control mother did not reverse this condition. It is proposed that activation of the oligodendrocyte inflammasome might be relevant in demyelinating diseases associated with early life exposure to high GC levels. © 2016 Wiley Periodicals, Inc. Develop Neurobiol 77: 625-642, 2017.

Early handling and repeated cross-fostering have opposite effect on mouse emotionality

Early life events have a crucial role in programming the individual phenotype and exposure to traumatic experiences during infancy can increase later risk for a variety of neuropsychiatric conditions, including mood and anxiety disorders. Animal models of postnatal stress have been developed in rodents to explore molecular mechanisms responsible for the observed short and long lasting neurobiological effects of such manipulations. The main aim of this study was to compare the behavioral and hormonal phenotype of young and adult animals exposed to different postnatal treatments. Outbred mice were exposed to (i) the classical Handling protocol (H: 15 min-day of separation from the mother from day 1 to 14 of life) or to (ii) a Repeated Cross-Fostering protocol (RCF: adoption of litters from day 1 to 4 of life by different dams). Handled mice received more maternal care in infancy and showed the already described reduced emotionality at adulthood. Repeated cross fostered animals did not differ for maternal care received, but showed enhanced sensitivity to separation from the mother in infancy and altered respiratory response to 6% CO₂ in breathing air in comparison with controls. Abnormal respiratory responses to hypercapnia are commonly found among humans with panic disorders (PD), and point to RCF-induced instability of the early environment as a valid developmental model for PD. The comparisons between short- and long-term effects of postnatal handling vs. RCF indicate that different types of early adversities are associated with different behavioral profiles, and evoke psychopathologies that can be distinguished according to the neurobiological systems disrupted by early-life manipulations.

The effect of maternal stress activation on the offspring during lactation in light of vasopressin

Although it is obvious that preconceptional effects as well as stressors during pregnancy profoundly influence the progeny, the lactation period seems to be at least as important. Here we summarize how maternal stressors during the lactation period affect the offspring. As vasopressin is one of the crucial components both for stress adaptation and social behavior, special emphasis was given to this neuropeptide. We can conclude that stressing the mother does not have the same acute effect on the hypothalamo-pituitary-adrenocortical axis (as the main target of stress adaptation) of the pups as stressing the pups, but later endocrine and behavioral consequences can be similar. Vasopressin plays a role in acute and later consequences of perinatal stressor applied either to the mother or to the offspring, thereby contributing to transmitting the mothers' stress to the progeny. This mother-infant interaction does not necessarily mean a direct transmission of molecules, but rather is the result of programming the brain development through changes in maternal behavior. Thus, there is a time lag between maternal stress and stress-related changes in the offspring. The interactions are bidirectional as not only stress in the dam but also stress in the progeny has an effect on nursing.

Elevated corticosterone levels during the first postpartum period influence subsequent pregnancy outcomes and behaviours of the dam

Postpartum depression affects 15% of new mothers and previous depressive episodes increase the risk for postpartum depression. Chronic administration of corticosterone (CORT) to dams during the postpartum period causes depressive-like behaviour and has been used as a model of postpartum depression. To better understand the subsequent progress of this model, we examined whether there were persistent effects of CORT treatment during the dam's first postpartum period on maternal care and mood following a subsequent pregnancy. Sprague-Dawley female rats received either sesame oil (control) or CORT (40 mg/kg) injections for 22 days during their first postpartum period. Subsequently, all females were re-mated for a second time and neither group received treatment during the second postpartum period. Maternal care was observed from days 2-8 of each postpartum period and dams were tested in the forced-swim test on days 21 and 22 of the first and days 4 and 21 of the second postpartum period. As expected, the amount of time spent immobile in the forced-swim test was increased in CORT dams at the end of the first postpartum period; however, the amount of time spent immobile was decreased at the end of the second postpartum period relative to oil dams. Furthermore, dams treated with CORT in first postpartum period gave birth to a smaller litter with a larger male/female sex ratio after their second pregnancy. This implies that elevated stress hormone levels during the first postpartum period have a substantial influence on subsequent postpartum behaviour and litter characteristics. Further investigations are necessary to fully understand the effect of parity, experience during first motherhood, and hypothalamic-pituitary-adrenal axis regulation on postpartum depression.

Reproductive experience affects parental retrieval behaviour associated with increased plasma oxytocin levels in wild-type and CD38-knockout mice

The transition to motherhood results in a number of hormonal, neurological and behavioural changes necessary to ensure offspring growth. Once motherhood is established, further neurological and behavioural changes may result in long-term memory in mothering. Recent research has shown that postpartum motherhood enhances both nurturing behaviour and oxytocin activities. The transmembrane glycoprotein, CD38, is expressed on many neuronal cells and has been shown to play a role in social behaviours through stimulating the release of oxytocin in the hypothalamus. The present study was performed to investigate the effects of reproductive experience (primi- and multiparity, dams and sires) on the degree of parental behaviour, such as retrieval. Comparisons were performed between wild-type (Cd38 (+/+) ) and Cd38 knockout (Cd38 (-/-) ) mice of the ICR strain. Multiparous Cd38 (-/-) dams retrieved pups much faster than primiparous mice, whereas there were no significant differences between primi- and multiparous Cd38 (+/+) dams. Plasma oxytocin levels were significantly increased in multiparous dams of both genotypes. In addition, oxytocin levels in the hypothalamus and pituitary were lower in Cd38 (-/-) than in wild-type mice. ADP-ribosyl cyclase activity in the hypothalamus, but not in the pituitary, was slightly increased in Cd38 (+/+) dams. In an identical test, 40% of first-time Cd38 (+/+) sires showed retrieval. The time required to retrieval was shorter in second-time Cd38 (+/+) sires. Both first- and second-time Cd38 (-/-) sires showed only 10% retrieval behaviour. These results indicate that parental behaviour is improved by reproductive experience, especially in Cd38 (-/-) dams, and suggest that these effects may be a result of increased oxytocin levels.

Fostering in mice induces cardiovascular and metabolic dysfunction in adulthood

Cross-fostering is widely used in developmental programming studies to determine the relative contribution of the in utero and suckling periods in establishing the adult offspring phenotype in response to an environmental challenge. We have investigated whether the process of fostering per se influences cardiovascular and metabolic function in adult offspring of C57BL/6J mice in comparison with animals suckled by their biological dams. Cross-fostered (CF) mice demonstrated juvenile onset hyperphagia and significantly higher body weight (from weaning to 12 weeks: male control (CON) vs. CF: P < 0.01, female CON vs. CF: P < 0.001; RM ANOVA) accompanied by increased abdominal adiposity in males only (white adipose tissue mass (mg): CON 280.5 ± 13.4 [mean ± SEM] (n = 7) vs. CF, 549.8 ± 99.3 (n = 8), P < 0.01). Both male and female CF mice demonstrated significantly enhanced glucose tolerance. A marked increase in systolic blood pressure (SBP) was observed in male CF mice (SBP (mmHg), day: CON 100.5 ± 1.4 (n = 6) vs. CF 114.3 ± 0.7 (n = 6), P < 0.001; night: CON 108.0 ± 2.0 (n = 6) vs. CF 123.2 ± 1.1 (n = 6), P < 0.001). Endothelium-dependent relaxation was enhanced in male CF mice, and renal noradrenaline was increased in female CF mice. Concentration of serum triglycerides, cholesterol, insulin and leptin were increased in CF vs. CON. The process of cross-fostering profoundly affects cardiovascular and metabolic phenotype in mice. The findings have implications for the inclusion of appropriate controls in the design of future studies and in the interpretation of previous cross-fostering studies in mice.

The WAG/Rij strain: a genetic animal model of absence epilepsy with comorbidity of depression [corrected]

A great number of clinical observations show a relationship between epilepsy and depression. Idiopathic generalized epilepsy, including absence epilepsy, has a genetic basis. The review provides evidence that WAG/Rij rats can be regarded as a valid genetic animal model of absence epilepsy with comorbidity of depression. WAG/Rij rats, originally developed as an animal model of human absence epilepsy, share many EEG and behavioral characteristics resembling absence epilepsy in humans, including the similarity of action of various antiepileptic drugs. Behavioral studies indicate that WAG/Rij rats exhibit depression-like symptoms: decreased investigative activity in the open field test, increased immobility in the forced swimming test, and decreased sucrose consumption and preference (anhedonia). In addition, WAG/Rij rats adopt passive strategies in stressful situations, express some cognitive disturbances (reduced long-term memory), helplessness, and submissiveness, inability to make choice and overcome obstacles, which are typical for depressed patients. Elevated anxiety is not a characteristic (specific) feature of WAG/Rij rats; it is a characteristic for only a sub-strain of WAG/Rij rats susceptible to audiogenic seizures. Interestingly, WAG/Rij rats display a hyper-response to amphetamine similar to anhedonic depressed patients. WAG/Rij rats are sensitive only to chronic, but not acute, antidepressant treatments, suggesting that WAG/Rij rats fulfill a criterion of predictive validity for a putative animal model of depression. However, more and different antidepressant drugs still await evaluation. Depression-like behavioral symptoms in WAG/Rij rats are evident at baseline conditions, not exclusively after stress. Experiments with foot-shock stress do not point towards higher stress sensitivity at both behavioral and hormonal levels. However, freezing behavior (coping deficits) and blunted response of 5HT in the frontal cortex to uncontrollable sound stress, increased c-fos expression in the terminal regions of the meso-cortico-limbic brain systems and greater DA response of the mesolimbic system to forced swim stress suggest that WAG/Rij rats are vulnerable to some, but not to all types of stressors. We propose that genetic absence epileptic WAG/Rij rats have behavioral depression-like symptoms, are vulnerable to stress and might represent a model of chronic low-grade depression (dysthymia). Both 5HT and DAergic abnormalities detected in the brain of WAG/Rij rats are involved in modulation of vulnerability to stress and provocation of behavioral depression-like symptoms. The same neurotransmitter systems modulate SWDs as well. Recent studies suggest that the occurrence and repetition of absence seizures are a precipitant of depression-like behavior. Whether the neurochemical changes are primary to depression-like behavioral alterations remains to be determined. In conclusion, the WAG/Rij rats can be considered as a genetic animal model for absence epilepsy with comorbidity of dysthymia. This model can be used to investigate etiology, pathogenic mechanisms and treatment of a psychiatric comorbidity, such as depression in absence epilepsy, to reveal putative genes contributing to comorbid depressive disorder, and to screen novel psychotropic drugs with a selective and/or complex (dual) action on both pathologies.

Cortisol levels in relation to maternal interaction and child internalizing behavior in preterm and full-term children at 18 months corrected age

Cortisol levels were compared in children born preterm at extremely low gestational age (ELGA; 24-28 weeks), very low gestational age (VGLA; 29-32 weeks), and full-term in response to cognitive assessment at 18 months corrected age (CA). Further, we investigated the relationship between maternal interactive behaviors and child internalizing behaviors (rated by the mother) in relation to child cortisol levels. EGLA children had higher "pretest" cortisol levels and a different pattern of cortisol response to cognitive assessment compared to VGLA and full-terms. Higher cortisol levels in ELGA, but not full-term, children were associated with less optimal mother interactive behavior. Moreover, the pattern of cortisol change was related to internalizing behaviors among ELGA, and to a lesser degree VLGA children. In conclusion, our findings suggest altered programming of the hypothalamic-pituitary-adrenal (HPA) axis in preterm children, as well as their greater sensitivity to environmental context such as maternal interactive behavior.

Depression during pregnancy and postpartum: contribution of stress and ovarian hormones

Depression is the most common psychiatric disease among women, exhibiting a prevalence which is 2-3x higher than in men. The postpartum period is considered the time of greatest risk for women to develop major depression and postpartum depression affects approximately 15% of women. A dysregulation of the hypothalamus-pituitary-adrenal (HPA) axis is the most prominent endocrine change seen in depression and normalization of the HPA axis is a major target of recent therapies. Females exhibit different stress sensitivities than males which might contribute to their increased vulnerability for depression. Maternal stress or depression during pregnancy and/or postpartum is particularly concerning as early developmental influences can affect the maturation of the offspring as well as the mental health of the mother. Despite the urgent need for more information on depression in females, especially during pregnancy and postpartum, most animal models of depression have utilized only males. Given the sex differences in incidence of depression and treatment, it is vitally important to create or validate animal models of depression in females. This review will focus on the association between stress, glucocorticoids and depression in humans, with a special focus on depression in women during pregnancy and postpartum and on animal models of postpartum depression and the consequences for the offspring.

Maternal deprivation induces a rapid decline in circulating leptin levels and sexually dimorphic modifications in hypothalamic trophic factors and cell turnover

Pathological outcomes, including metabolic and endocrine disturbances, of maternal deprivation (MD) in Wistar rats depend on gender and the timing of deprivation during development. We analyzed the effect of MD between postnatal days 9 and 10, a critical period in hypothalamic development, on circulating hormones and local production of trophic factors involved in this process, as well as on markers of cell turnover and maturation. Males and females were studied 12 and 24 h after MD and 12 h (MD36) after returning the dam to her pups. Circulating corticosterone levels were increased and glucose and leptin levels decreased throughout the study in both sexes. Hypothalamic mRNA levels of leptin receptor increased significantly at MD24 in both sexes, normalizing in females at MD36, but not in males. In male rats insulin-like growth factor mRNA levels were significantly decreased at MD24 and brain derived neurotrophic factor mRNA levels decreased at MD12 and MD24, with both trophic factors unaffected in females. In males cell proliferation was significantly decreased at MD36, as were the glial structural proteins, glial fibrillary acidic protein and vimentin. In females, nestin levels decreased significantly at MD24. These results indicate that MD differently affects trophic factors and cell-turnover in the hypothalamus of males and females, which may underlie the sex differences seen in the endocrine and metabolic outcome.

Early life stress increases anxiety-like behavior in Balb c mice despite a compensatory increase in levels of postnatal maternal care

A better understanding of the molecular and cellular mechanisms by which early life stress (ELS) modifies brain development and adult behavior is necessary for diagnosing and treating psychopathology associated with exposure to ELS. For historical reasons, most of the work in rodents has been done in rats and attempts to establish robust and reproducible paradigms in the mouse have proven to be challenging. Here we show that under normal rearing conditions, increased levels of postnatal maternal care are associated with a decrease in anxiety-like behavior in BALB/cByj offspring. Brief daily pup-dam separation (BDS) during the postnatal period was associated with increased postnatal maternal care but was surprisingly associated with increased anxiety-like behavior in adult offspring, providing the first example in which offspring receiving higher levels of postnatal maternal care are more anxious in adulthood. Plasma corticosterone levels were elevated in BDS pups even 3 h after the pups were reunited with the dam, suggesting that this paradigm represents a form of early life stress. We also show that levels of total RNA and DNA in the hippocampus reach a peak at postnatal day 14 and that exposure to BDS seems to inhibit this developmental growth spurt. We propose that exposure to stress during the postnatal period overrides the ability of high levels of postnatal maternal care to program anxiety-like behavior by inhibiting the normal growth spurt that characterizes this period.

Living in a dangerous world: the shaping of behavioral profile by early environment and 5-HTT genotype

Anxiety and anxiety disorders are influenced by both, environmental and genetic factors. One genetic factor under scrutiny for anxiety disorders is the genetically encoded variation of the serotonin transporter (5-HTT). The aim of this study was to elucidate the effects of a threatening environment during early phases of life on anxiety-like (ANX) and exploratory behavior (EXP) in adult mice, varying in serotonin transporter (5-HTT) genotype. For this purpose, pregnant and lactating 5-HTT +/− dams were repeatedly exposed to olfactory cues of unfamiliar adult males by introducing small amounts of soiled bedding to their home cage. These stimuli signal the danger of infanticide and simulate a threatening environment. Control females were treated with neutral bedding. The offspring (5-HTT +/+, +/−, −/−) were examined for their ANX and EXP. The main results were: (1) a main effect of genotype existed, with 5-HTT −/− showing higher levels of ANX and lower levels of EXP than 5-HTT +/− and wildtypes. (2) When mothers had lived in a threatening environment, their offspring showed increased ANX and reduced EXP compared to controls. (3) These effects were most pronounced in 5-HTT −/− mice. By applying a new ecologically relevant paradigm we conclude: If 5-HTT +/− mothers live in a threatening environment during pregnancy and lactation, their offspring behavioral profile will, in principle, be shaped in an adaptive way preparing the young for an adverse environment. This process is, however, modulated by 5-HTT genotype, bearing the risk that individuals with impaired serotonergic neurotransmission (5-HTT −/−) will develop an exaggerated, potentially pathological level of anxiety from gene × environment interactions.

Postnatal handling reverses social anxiety in serotonin receptor 1A knockout mice

Mice lacking the serotonin receptor 1A (Htr1a knockout, Htr1a(KO)) show increased innate and conditioned anxiety. This phenotype depends on functional receptor activity during the third through fifth weeks of life and thus appears to be the result of long-term changes in brain function as a consequence of an early deficit in serotonin signaling. To evaluate whether this phenotype can be influenced by early environmental factors, we subjected Htr1a knockout mice to postnatal handling, a procedure known to reduce anxiety-like behavior and stress responses in adulthood. Offspring of heterozygous Htr1a knockout mice were separated from their mother and exposed 15 min each day from postnatal day 1 (PD1) to PD14 to clean bedding. Control animals were left undisturbed. Maternal behavior was observed during the first 13 days of life. Adult male offspring were tested in the open field, social approach and resident-intruder tests and assessed for corticosterone response to restraint stress. Knockout mice showed increased anxiety in the open field and in the social approach test as well as an enhanced corticosterone response to stress. However, while no effect of postnatal handling was seen in wild-type mice, handling reduced anxiety-like behavior in the social interaction test and the corticosterone response to stress in knockout mice. These findings extend the anxiety phenotype of Htr1a(KO) mice to include social anxiety and demonstrate that this phenotype can be moderated by early environmental factors.