Periodic maternal separation decreases hippocampal neurogenesis without affecting basal corticosterone during the stress hyporesponsive period, but alters HPA axis and coping behavior in adulthood

Coffee polyphenols ameliorate early-life stress-induced cognitive deficits in male mice

Stress exposure during the sensitive period of early development has been shown to program the brain and increases the risk to develop cognitive deficits later in life. We have shown earlier that early-life stress (ES) leads to cognitive decline at an adult age, associated with changes in adult hippocampal neurogenesis and neuroinflammation. In particular, ES has been shown to affect neurogenesis rate and the survival of newborn cells later in life as well as microglia, modulating their response to immune or metabolic challenges later in life. Both of these processes possibly contribute to the ES-induced cognitive deficits. Emerging evidence by us and others indicates that early nutritional interventions can protect against these ES-induced effects through nutritional programming. Based on human metabolomics studies, we identified various coffee-related metabolites to be part of a protective molecular signature against cognitive decline in humans. Caffeic and chlorogenic acids are coffee-polyphenols and have been described to have potent anti-oxidant and anti-inflammatory actions. Therefore, we here aimed to test whether supplementing caffeic and chlorogenic acids to the early diet could also protect against ES-induced cognitive deficits. We induced ES via the limited nesting and bedding paradigm in mice from postnatal(P) day 2-9. On P2, mice received a diet to which 0.02% chlorogenic acid (5-O-caffeoylquinic acid) + 0.02% caffeic acid (3',4'-dihydroxycinnamic acid) were added, or a control diet up until P42. At 4 months of age, all mice were subjected to a behavioral test battery and their brains were stained for markers for microglia and neurogenesis. We found that coffee polyphenols supplemented early in life protected against ES-induced cognitive deficits, potentially this is mediated by the survival of neurons or microglia, but possibly other mechanisms not studied here are mediating the effects. This study provides additional support for the potential of early nutritional interventions and highlights polyphenols as nutrients that can protect against cognitive decline, in particular for vulnerable populations exposed to ES.

Long-term effects of Preweaning environmental impoverishment on neurobehavioral and neurocognitive outcomes in Sprague Dawley rats: An early environmental stress model

Developmental stress, including low socioeconomic status (SES), can induce dysregulation of the hypothalamic-pituitary-adrenal axis and result in long-term changes in stress reactivity. Children in lower SES conditions often experience more stress than those in other SES groups. There are multiple model systems of early environmental stress (EES), one of which is reduced cage bedding. Here we tested the effects of both prenatal and lactational EES in rats on a range of long-term behavioral and cognitive outcomes. There were persistent reductions in body weight in the EES rats in both sexes. The behavioral results showed no effects on learning and memory using tests of spatial learning or cognitive flexibility in the Morris water maze, egocentric learning in the Cincinnati water maze, or working memory in the radial-arm maze. There were no effects on basic open-field activity, elevated zero-maze, or forced swim test, but EES rats had reduced time in the dark side of the light/dark test. When rats were drug challenged in the open-field with d-amphetamine or MK-801, there were no differential responses to d-amphetamine, but the EES group under responded compared with the drug-induced hyperactivity in the control group in both males and females. The objective was to establish a developmental stress model that induced cognitive deficits and to the extent that this method did not cause such effects it was not the model we sought. However, the data showed several long-term effects of EES, including the reduced response to the irreversible NMDA antagonist MK-801. This effect merits further investigation.

Early Life Stress Negatively Impacts Spatial Learning Acquisition and Increases Hippocampal CA1 Microglial Activation After a Mild Traumatic Brain Injury in Adult Male Rats

Early life stress (ELS) affects neurogenesis and spatial learning, and increases neuroinflammation after a pediatric mild traumatic brain injury (mTBI). Previous studies have shown that ELS has minimal effects in juveniles but shows age-dependent effects in adults. Hence, we aimed to evaluate the effects of ELS in adult male rats after an mTBI. Maternal separation for 180 min per day (MS180) during the first 21 post-natal (P) days was used as the ELS model. At P110, the rats were subjected to a mild controlled cortical impact injury (2.6 mm) or sham surgery. Spatial learning was evaluated in the Morris water maze (MWM) 14 days after surgery and both microglial activation and neurogenesis were quantified. The results indicate that MS180 + mTBI, but not control (CONT) + mTBI, rats show deficiencies in the acquisition of spatial learning. mTBI led to comparable increases in microglial activation in both the hilus and cortical regions for both groups. However, MS180 + mTBI rats exhibited a greater increase in microglial activation in the ipsilateral CA1 hippocampus subfield compared with CONT + mTBI. Interestingly, for the contralateral CA1 region, this effect was observed exclusively in MS180 + mTBI. ELS and mTBI independently caused a decrease in hippocampal neurogenesis and this effect was not increased further in MS180 + mTBI rats. The findings demonstrate that ELS and mTBI synergistically affect cognitive performance and neuroinflammation, thus supporting the hypothesis that increased inflammation resulting from the combination of ELS and mTBI could underlie the observed effects on learning.

Resilience to Early Life Adversity Effects on Stress Reactivity by Postnatal Knockdown of 5-HT 1A Autoreceptors

Early Life Adversity (ELA) predisposes to stress hypersensitivity in adulthood, but neurobiological mechanisms that can protect from long-lasting effects of ELA are poorly understood. Serotonin 1A (5HT 1A ) autoreceptors in the raphé nuclei regulate adult stress vulnerability, but if 5HT 1A could be targeted to prevent ELA effects on susceptibility to future stressors is unknown. Here, we exposed mice with postnatal knockdown of 5HT 1A autoreceptors to the limited bedding and nesting model of ELA from postnatal day (P)3-10. We then tested behavioral, neuroendocrine, neurogenic, and neuroinflammatory responses to an acute swim stress in male and female mice in adolescence (P35) and in adulthood (P56). In ELA-exposed females, adult swim stress exposure increased passive coping and despair-like behavior, corticosterone levels at baseline and after stress, and neuronal activity and corticotropin releasing hormone levels in the paraventricular nucleus of the hypothalamus. ELA also reduced neurogenesis and increased microglia activation in the ventral dentate gyrus (DG) of the hippocampus - an important mediator of individual differences in stress susceptibility. These effects of ELA were specific to females, but not males, and manifested predominantly in adulthood, but not earlier on in adolescence. Postnatal 5HT 1A autoreceptor knockdown prevented ELA effects on stress reactivity and on neurogenesis and neuroinflammation in the DG, indicating that reducing 5HT 1A autoreceptors confers resilience to ELA. Our findings demonstrate that ELA induces long-lasting and sex-specific impairments in stress reactivity and ventral DG function across development, and identify 5HT 1A autoreceptors as potential targets to prevent these persistent effects of ELA.

Neonatal kaempferol exposure attenuates impact of cerebral palsy model on neuromotor development, cell proliferation, microglia activation, and antioxidant enzyme expression in the hippocampus of rats

OBJECTIVES

This study aims to assess the effect of neonatal treatment with kaempferol on neuromotor development, proliferation of neural precursor cells, the microglia profile, and antioxidant enzyme gene expression in the hippocampus.

METHODS

A rat model of cerebral palsy was established using perinatal anoxia and sensorimotor restriction of hindlimbs during infancy. Kaempferol (1 mg/ kg) was intraperitoneally administered during the neonatal period.

RESULTS

Neonatal treatment with kaempferol reduces the impact of the cerebral palsy model on reflex ontogeny and on the maturation of physical features. Impairment of locomotor activity development and motor coordination was found to be attenuated by kaempferol treatment during the neonatal period in rats exposed to cerebral palsy. Neonatal treatment of kaempferol in cerebral palsy rats prevents a substantial reduction in the number of neural precursor cells in the dentate gyrus of the hippocampus, an activated microglia profile, and increased proliferation of microglia in the sub-granular zone and in the granular cell layer. Neonatal treatment with kaempferol increases gene expression of superoxide dismutase and catalase in the hippocampus of rats submitted to the cerebral palsy model.

DISCUSSION

Kaempferol attenuates the impact of cerebral palsy on neuromotor behavior development, preventing altered hippocampal microglia activation and mitigating impaired cell proliferation in a neurogenic niche in these rats. Neonatal treatment with kaempferol also increases antioxidant defense gene expression in the hippocampus of rats submitted to the cerebral palsy model.

Maternal separation early in life induces excessive activity of the central amygdala related to abnormal aggression

Epidemiological studies have indicated that child maltreatment, such as neglect, is a risk factor of escalated aggression, potentially leading to delinquency and violent crime in the future. However, little is known about the mechanisms by which an early adverse environment may later cause violent behavior. In this study, we aimed to thoroughly examine the association between aggression against conspecific animals and the activity of amygdala subnuclei using the maternal separation (MS) model, which is a common model of early life stress. In the MS group, pups of Sprague-Dawley rats were separated from their dam during postnatal days 2-20 (twice a day, 3 h each). We only included 9-week-old male offspring for each analysis and compared the MS group with the mother-reared control group; both groups were raised by the same dam during postnatal days 2-20. The results revealed that the MS group exhibited higher aggression and excessive activity of only the central amygdala (CeA) among the amygdala subnuclei during the aggressive behavior test. Moreover, a significant positive correlation was observed between higher aggression and CeA activation. While CeA activity is known to be involved in hunting behavior for prey, some previous studies have also indicated a relationship between CeA and intraspecific aggression. It remains unclear, however, whether excessive CeA activity directly induces intraspecific aggression. Therefore, we stimulated the CeA using optogenetics with 8-week-old rats to clarify the relationship between intraspecific aggression and CeA activity. Notably, CeA activation resulted in higher aggression, even when the opponent was a conspecific animal. In particular, bilateral CeA activation resulted in more severe displays of aggressive behavior than necessary, such as biting a surrendered opponent. These findings suggest that an adverse environment during early development intensifies aggression through excessive CeA activation, which can increase the risk of escalating to violent behavior in the future.

Corticosterone effects on postnatal cerebellar development in mice

Glucocorticoids administered early in infancy can affect the architectonic organization of brain structures, particularly those with a postnatal development and resulting in long-term deficits of neuromotor function and cognition. The present study was undertaken to study the effects of daily corticosterone (CORT) injections at a pharmacological dose from postnatal days 8-15 on cerebellar and hippocampal development in mouse pups. Gene expression status for trophic factors involved in synaptic development and function as well as measures of layer thickness associated with cytochrome oxidase labelling were analyzed in the hippocampus, hypothalamus, and specific cerebellar lobules involved in motor control. Repeated CORT injections dysregulated the HPA axis with increased Crh and Nr3c1 mRNA levels in the hypothalamus and a resulting higher serum corticosterone level. The CORT treatment altered the morphology of the hippocampus and down-regulated gene transcription for corticotropin-releasing hormone (Crh) and its type-1 receptor (Crhr1), glucocorticoid receptor (Nr3c1), and brain-derived neurotrophic factor Bdnf and its receptor Ntrk2 (neurotrophic receptor tyrosine kinase 2). Similar mRNA expression decreases were found in the cerebellum for Crhr1, Crhr2, Nr3c1, and Grid2 (glutamatergic δ2 receptor). Morphological alterations and metabolic activity variations were observed in specific cerebellar lobules involved in motor control. The paramedian lobule, normally characterized by mitotic activity in the external germinative layer during the second postnatal week, was atrophic but metabolically hyperactive in its granule cell and molecular layers. On the contrary, lobules with an earlier cell proliferation displayed neurogenesis but a hypoactivated granule cell layer, suggesting a developmental delay in synaptogenesis. The results indicate that glucocorticoid, administered daily during the second postnatal week modulated the developmental programming of the hippocampus and cerebellum. These growth and metabolic alterations may lead possibly to morphological and functional changes later in life.

Repeated neonatal Needle-pricking stimulation alter neurodevelopment in adolescent rats

OBJECTIVE

To explore the repeated pain stimulation in neonatal rats affects their cognitive and memory abilities during puberty, and the proliferation expression of hippocampal neurons.

METHODS

Postnatal 1 day (P1) SD rats were randomly divided into two groups, and the skin of the needle group was pricked for seven days consecutively while the skin of the control group was stroked for the same period of time. The rats in both groups were weighed every week, and the Morris water maze experiment was performed from P44 to P49 to test the cognitive and memory abilities of the rats. On P50, the hippocampal tissue was extracted for observation of pathological features and the expressions of Ki-67 and caspase 3 were determined.

RESULTS

With the increase of the days, the body weight of the rats in the needle group increased slightly slower than that of the control group. The escape latency of the needle group was significantly higher than that of the control group in the water maze test at P45 and P48, and the number of times the rats crossing the platform in the needle group was lower than that of the control group. The HE staining of the hippocampal tissue showed that the cells in the needle group were disorganized, with irregular morphology. Under the electron microscope, the structure of neuron cells and organelles is changed in the hippocampal CA1 region of rats. It showed a decrease in the Ki-67 expression and an increase in caspase 3 in the needle group.

CONCLUSION

Repeated experience of needle-pricking stimulation in neonatal rats can cause cognitive impairment and memory loss in puberty, disrupt hippocampal organization, and diminish neuronal proliferation.

Impacts of maternal separation stress on ethanol-related responses, anxiety- and depressive-like behaviors in adolescent mice

The present work evaluated the consequences of chronic maternal separation (MS), an animal model of early-life stress, on ethanol intake and striatal Fos expression induced by ethanol consumption. Furthermore, we analyzed MS impacts on anxiety- and depressive-like behaviors and on locomotor and plasma corticosterone responses to intraperitoneal treatment with ethanol in adolescent mice. For that, male and female C57BL/6J mice were exposed or not to MS stress, for 3 h per day, from postnatal day (PND) 1 to 14, and submitted to behavioral tests from PND 28. In Experiment 1, MS and control groups of mice were submitted to an involuntary ethanol intake protocol, and striatal Fos expression following ethanol exposure was analyzed. In Experiment 2, mice behavior was assessed in elevated plus-maze, sucrose splash, saccharin preference, and open field tests. Locomotor and plasma corticosterone responses induced by a systemic dose of ethanol (1.75 g/kg) were also evaluated. Our results demonstrated that MS increased ethanol intake only in an acute manner and did not impact ethanol-induced Fos expression in the dorsal striatum and nucleus accumbens (NAc) core and shell subregions. MS did not change the parameters analyzed during elevated plus-maze, sucrose splash, preference for saccharin, and open field tests. MS did not affect locomotor activity following ethanol injection nor plasma corticosterone response to the drug. Thus, our data showed that MS transiently increased ethanol intake. However, early-life stress did not impact Fos, locomotor, or plasma corticosterone responses to the drug. In addition, MS did not affect anxiety- and depressive-like behaviors in adolescent mice.

Risky Decision-taking Task: a novel paradigm to assess the risk-taking behaviour in rats predisposed to early-life stress

One of the characteristic features of adolescence is risk-taking behavioural traits. Uncontrolled risk-taking without proper assessment may have harmful impact on mental health later in life. Therefore, it is essential to identify it early for the preventable health problems. In the present study, we have designed a novel paradigm, viz. Risky Decision-taking Task (RDTT), to evaluate the spontaneous risk-taking behavioural repertoire in adolescent rodents. The task was designed based on both risk and cognitive factors. To validate and compare the risk-taking tendency, we have used early maternal separation and isolation (MS) stress model, as it is known to increase anxiety and curiosity-like behaviour at adolescence. We have used Sprague-Dawley rats of both sexes. Rats were exposed to MS stress for 10 days daily for six hours during stress hyporesponsive period (SHRP) from postnatal day 4 to 13. These rats were subjected to RDTT during adolescence. This task is a reward-based task where the latency to collect reward in the presence or absence of a risk factor is assessed. It consists of habituation, training to find the location of small and large rewards, reward preference for small and large reward and testing period under risky situation. Rats were trained individually to retrieve the valuation-based rewards under the risky, but innate aversive environments. The results from RDTT showed that as compared to controls, MS rats from both sexes showed reduced latency to collect large reward in the presence of a risk element and a reduced risk-index which is indicative of a higher risk-taking tendency in these rats. In addition, MS rats showed a trend towards anxiety-like behaviour as compared to controls in the Light-Dark Test. These results together show decreased risk latency for the large reward and reduced risk assessment in MS rats which is suggestive of more risk-taking tendency in these rats. Thus, we propose that RDTT paradigm can be used to evaluate the spontaneous risk-taking behavioural repertoire based on innate, spontaneous aversion and cognitive factors in rats.

Feasibility of Implementing a Modified SENSE Program to Increase Positive Sensory Experiences for Preterm Infants in the Neonatal Intensive Care Unit (NICU): A Pilot Study

AIMS

To evaluate the feasibility of implementing the Supporting and Enhancing NICU Sensory Experiences (SENSE) program with adaptations to increase positive sensory exposure for infants born preterm, support neonatal neurodevelopment, and decrease parent stress.

METHODS

Eight infants born between 28 and 33 weeks were recruited within one week of birth. Parents, trained in the program, delivered the SENSE protocol. The first author provided up to 1 h of sensory input on weekdays when a parent could not be present.

RESULTS

Recruitment and retention rates were 87.5% and 100%, respectively. Recruitment and initial parent education and training averaged 37.5 min. On average, parents were present in the NICU 85.1% of days; they participated in SENSE for an average of 515.5 min. SENSE dose recommendations were not consistently met. Weekly infant assessments and regular parent check ins averaged 22.5 and 13.8 min, respectively. Post-assessments revealed normal scores on a neurodevelopmental assessment, low parent stress, and high parent satisfaction.

CONCLUSIONS

The recruitment and retention rates suggest high demand to participate. Outcomes for parent stress and neonatal neurodevelopment support continuation of SENSE. Time commitment for implementation, coupled with supporting families in meeting dose recommendations, suggest a need for a neonatal therapist to promote sustainability.

Isoflurane stress induces region-specific glucocorticoid levels in neonatal mouse brain

The profound programming effects of early life stress (ELS) on brain and behavior are thought to be primarily mediated by adrenal glucocorticoids (GCs). However, in mice, stressors are often administered between postnatal days 2 and 12 (PND2-12), during the stress hyporesponsive period (SHRP), when adrenal GC production is greatly reduced at baseline and in response to stressors. During the SHRP, specific brain regions produce GCs at baseline, but it is unknown if brain GC production increases in response to stressors. We treated mice at PND1 (pre-SHRP), PND5 (SHRP), PND9 (SHRP), and PND13 (post-SHRP) with an acute stressor (isoflurane anesthesia), vehicle control (oxygen), or neither (baseline). We measured a panel of progesterone and six GCs in the blood, hippocampus, cerebral cortex, and hypothalamus via liquid chromatography tandem mass spectrometry. At PND1, baseline corticosterone levels were high and did not increase in response to stress. At PND5, baseline corticosterone levels were very low, increases in brain corticosterone levels were greater than the increase in blood corticosterone levels, and stress had region-specific effects. At PND9, baseline corticosterone levels were low and increased similarly and moderately in response to stress. At PND13, blood corticosterone levels were higher than those at PND9, and corticosterone levels were higher in blood than in brain regions. These data illustrate the rapid and profound changes in stress physiology during neonatal development and suggest that neurosteroid production is a possible mechanism by which ELS has enduring effects on brain and behavior.

Antibiotic-induced microbiota depletion in normally-reared adult rats mimics the neuroendocrine effects of early life stress

Early life stress induced by maternal separation (MS) causes neuroendocrine, behavioral, and metabolic alterations that are related to gut dysbiosis. MS also increases microglial activation and decreases neurogenesis. Whether these long-term alterations are maintained or worsened in the absence of gut microbiota remains unknown. Hence, this study evaluated the effect of MS symptomatology after antibiotic-induced microbiota depletion (AIMD) in adult rats. Control and maternally separated (3 h per day from postnatal day one to 14, MS180) rats were subjected to AIMD for one month, then assessed for behavioral, metabolic, and neuroendocrine responses. Effects of MS180 and AIMD on gut microbiota were confirmed by qPCR. The data indicate that MS180 caused a passive coping strategy in the forced swimming test and decreased hippocampal neurogenesis. In addition, fasting glucose, cholesterol, and corticosterone levels increased, which correlated with a decrease in Lactobacillus spp counts in the caecum. AIMD also increased immobility in the forced swimming test, decreased hippocampal neurogenesis, and augmented corticosterone levels. However, it had no effects on glucose homeostasis or plasma lipid levels. Furthermore, the MS180-induced long-term effects on behavior and neurogenesis were not affected by microbiota depletion. Meanwhile, the metabolic imbalance was partially reversed in MS180 + AIMD rats. These results show that AIMD mimics the behavioral consequences of MS180 but may prevent metabolic imbalance, suggesting that gut dysbiosis could be part of the mechanisms involved in the maintenance of the long-term consequences of early life stress.

Olive oil-rich diet during pregnancy/lactation attenuated the early life stress effects on depressive-like behavior and altered energy metabolism in the dorsal hippocampus in a sex-specific manner

METHODS

and results: Pregnant Wistar rats received diets enriched in soybean oil (SO) or OO during gestation/lactation. At birth, litters were subdivided into MS or intact groups. After weaning, the pups received standard chow until adulthood, when they were subjected to behavioral tasks. At PND90 biochemical analyses were performed. Maternal OO-enriched diet prevented MS-induced higher weight gain, and decreased MS-induced anhedonic behavior. Increased latency to immobility and shorter immobility time were observed in the maternal OO-enrich diet groups. Maternal OO-enrich diet groups also presented reduced reactive oxygen species and increased activity of antioxidant enzymes. In addition, this diet showed sex-specific effects, by decreasing mitochondrial mass and potential, reducing AMPK activation, and increasing synaptophysin and PSD-95 immunocontent in the DH of male rats. Early stress, on the other hand, decreased production of free radicals and decreased levels of SIRT1 in the DH of male rats. In females, OO prevented the anhedonic behavior induced by MS.

CONCLUSIONS

Maternal OO-enrich diet attenuated MS-induced depressive behavior in both sexes. In addition, it affected energy metabolism in the DH of male rats, favored synaptic plasticity, and contributed to reducing pathophysiological conditions.

Early life stress induces a transient increase in hippocampal corticotropin-releasing hormone in rat neonates that precedes the effects on hypothalamic neuropeptides

Early life stress (ELS) programs hypothalamus-pituitary-adrenal (HPA) axis activity and affects synaptic plasticity and cognitive performance in adults; however, the effects of ELS during the temporal window of vulnerability are poorly understood. This study aimed to thoroughly characterize the effects of ELS in the form of periodic maternal separation (MS180) during the time of exposure to stress. Hippocampal corticotropin-releasing hormone (CRH) gene expression and baseline HPA axis activity were analyzed at postnatal (P) days 6, 12, 15, and 21, and in adulthood (P75); these factors were correlated with plasticity markers and adult behavior. Our results indicate that MS180 induces an increase in hippocampal CRH expression at P9, P12, and P15, whereas an increase in hypothalamic CRH expression was observed from P12 to P21. Increased arginine-vasopressin expression and corticosterone levels were observed only at P21. Moreover, MS180 caused transient alterations in hypothalamic synaptophysin expression during early life. As adults, MS180 rats showed a passive coping strategy in the forced swimming test, cognitive impairments in the object location test, increased hypothalamic CRH expression, and decreased oxytocin (OXT) expression. Spearman's analysis indicated that cognitive impairments correlated with CRH and OXT expression. In conclusion, our data indicate that MS180 induces a transient increase in hippocampal CRH expression in neonates that precedes the effects on hypothalamic neuropeptides, confirming the role of increased CRH during the temporal window of vulnerability as a mediator of some of the detrimental effects of ELS on brain development and adult behavior.

Maternal deprivation effect on morphine-induced CPP is related to changes in Opioid receptors in selected rat brain regions (hippocampus, prefrontal cortex, and nucleus accumbens)

Early-life environmental conditions affect offspring's development. Maternal deprivation (MD) can induce persistent changes that give rise to neuropsychiatric diseases including substance abuse disorders. However, long-lasting mechanisms that determine vulnerability to drug addiction remain unknown. We hypothesized that MD could induce changes in Opioid system, HPA (hypothalamic-pituitary-adrenal) axis, and BDNF (brain-derived neurotrophic factor), so may be involved in the drug abuse in later life. Male offspring of Wistar rats (n=8 per group) were subjected to 3h of daily MD during postnatal days 1-14. In adulthood, morphine-induced CPP (conditioned place preference) was investigated using two doses of morphine (3 and 5mg/kg). Serum corticosterone level was measured by ELISA method. The expression level of genes in selected brain regions (hippocampus, prefrontal cortex, and nucleus accumbens) was determined by qPCR (quantitative PCR). A greater morphine-induced CPP was observed in MD rats with 3 and 5mg/kg morphine compared to controls. MD group had a higher corticosterone level. A significant decrease was observed in the expression of BDNF gene (in all of the selected brain regions) and GR (glucocorticoid receptor) gene (in the hippocampus and nucleus accumbens) in MD rats. Also, a significant increase in the expression of μ Opioid receptor (in all of the selected brain regions) and κ Opioid receptor (in the prefrontal cortex and nucleus accumbens) was observed in MD rats. Our results suggest that MD induces alterations in the HPA axis function, BDNF level, and Opioid receptors system that enhance vulnerability to morphine at adulthood.

Prior maternal separation stress alters the dendritic complexity of new hippocampal neurons and neuroinflammation in response to an inflammatory stressor in juvenile female rats

Stress during critical periods of neurodevelopment is associated with an increased risk of developing stress-related psychiatric disorders, which are more common in women than men. Hippocampal neurogenesis (the birth of new neurons) is vulnerable to maternal separation (MS) and inflammatory stressors, and emerging evidence suggests that hippocampal neurogenesis is more sensitive to stress in the ventral hippocampus (vHi) than in the dorsal hippocampus (dHi). Although research into the effects of MS stress on hippocampal neurogenesis is well documented in male rodents, the effect in females remains underexplored. Similarly, reports on the impact of inflammatory stressors on hippocampal neurogenesis in females are limited, especially when female bias in the prevalence of stress-related psychiatric disorders begins to emerge. Thus, in this study we investigated the effects of MS followed by an inflammatory stressor (lipopolysaccharide, LPS) in early adolescence on peripheral and hippocampal inflammatory responses and hippocampal neurogenesis in juvenile female rats. We show that MS enhanced an LPS-induced increase in the pro-inflammatory cytokine IL-1β in the vHi but not in the dHi. However, microglial activation was similar following LPS alone or MS alone in both hippocampal regions, while MS prior to LPS reduced microglial activation in both dHi and vHi. The production of new neurons was unaffected by MS and LPS. MS and LPS independently reduced the dendritic complexity of new neurons, and MS exacerbated LPS-induced reductions in the complexity of distal dendrites of new neurons in the vHi but not dHi. These data highlight that MS differentially primes the physiological response to LPS in the juvenile female rat hippocampus.

Glia-Driven Brain Circuit Refinement Is Altered by Early-Life Adversity: Behavioral Outcomes

Early-life adversity (ELA), often clinically referred to as "adverse childhood experiences (ACE)," is the exposure to stress-inducing events in childhood that can result in poor health outcomes. ELA negatively affects neurodevelopment in children and adolescents resulting in several behavioral deficits and increasing the risk of developing a myriad of neuropsychiatric disorders later in life. The neurobiological mechanisms by which ELA alters neurodevelopment in childhood have been the focus of numerous reviews. However, a comprehensive review of the mechanisms affecting adolescent neurodevelopment (i.e., synaptic pruning and myelination) is lacking. Synaptic pruning and myelination are glia-driven processes that are imperative for brain circuit refinement during the transition from adolescence to adulthood. Failure to optimize brain circuitry between key brain structures involved in learning and memory, such as the hippocampus and prefrontal cortex, leads to the emergence of maladaptive behaviors including increased anxiety or reduced executive function. As such, we review preclinical and clinical literature to explore the immediate and lasting effects of ELA on brain circuit development and refinement. Finally, we describe a number of therapeutic interventions best-suited to support adolescent neurodevelopment in children with a history of ELA.

Heterothermy as a mechanism to offset energetic costs of environmental and homeostatic perturbations

Environmental and biotic pressures impose homeostatic costs on all organisms. The energetic costs of maintaining high body temperatures (T_b) render endotherms sensitive to pressures that increase foraging costs. In response, some mammals become more heterothermic to conserve energy. We measured T_b in banner-tailed kangaroo rats (Dipodomys spectabilis) to test and disentangle the effects of air temperature and moonlight (a proxy for predation risk) on thermoregulatory homeostasis. We further perturbed homeostasis in some animals with chronic corticosterone (CORT) via silastic implants. Heterothermy increased across summer, consistent with the predicted effect of lunar illumination (and predation), and in the direction opposite to the predicted effect of environmental temperatures. The effect of lunar illumination was also evident within nights as animals maintained low T_b when the moon was above the horizon. The pattern was accentuated in CORT-treated animals, suggesting they adopted an even further heightened risk-avoidance strategy that might impose reduced foraging and energy intake. Still, CORT-treatment did not affect body condition over the entire study, indicating kangaroo rats offset decreases in energy intake through energy savings associated with heterothermy. Environmental conditions receive the most attention in studies of thermoregulatory homeostasis, but we demonstrated here that biotic factors can be more important and should be considered in future studies.

The Effects of Early Life Stress on the Brain and Behaviour: Insights From Zebrafish Models

The early life period represents a window of increased vulnerability to stress, during which exposure can lead to long-lasting effects on brain structure and function. This stress-induced developmental programming may contribute to the behavioural changes observed in mental illness. In recent decades, rodent studies have significantly advanced our understanding of how early life stress (ELS) affects brain development and behaviour. These studies reveal that ELS has long-term consequences on the brain such as impairment of adult hippocampal neurogenesis, altering learning and memory. Despite such advances, several key questions remain inadequately answered, including a comprehensive overview of brain regions and molecular pathways that are altered by ELS and how ELS-induced molecular changes ultimately lead to behavioural changes in adulthood. The zebrafish represents a novel ELS model, with the potential to contribute to answering some of these questions. The zebrafish offers some important advantages such as the ability to non-invasively modulate stress hormone levels in a whole animal and to visualise whole brain activity in freely behaving animals. This review discusses the current status of the zebrafish ELS field and its potential as a new ELS model.

Stress-Related Dysfunction of Adult Hippocampal Neurogenesis-An Attempt for Understanding Resilience?

Newborn neurons in the adult hippocampus are regulated by many intrinsic and extrinsic cues. It is well accepted that elevated glucocorticoid levels lead to downregulation of adult neurogenesis, which this review discusses as one reason why psychiatric diseases, such as major depression, develop after long-term stress exposure. In reverse, adult neurogenesis has been suggested to protect against stress-induced major depression, and hence, could serve as a resilience mechanism. In this review, we will summarize current knowledge about the functional relation of adult neurogenesis and stress in health and disease. A special focus will lie on the mechanisms underlying the cascades of events from prolonged high glucocorticoid concentrations to reduced numbers of newborn neurons. In addition to neurotransmitter and neurotrophic factor dysregulation, these mechanisms include immunomodulatory pathways, as well as microbiota changes influencing the gut-brain axis. Finally, we discuss recent findings delineating the role of adult neurogenesis in stress resilience.

Prolonged maternal separation alters neurogenesis and synaptogenesis in postnatal dentate gyrus of mice

OBJECTIVES

As a common model for adverse early experience and depression, maternal separation (MS) is always used to investigate the psychological disease. Despite extensive and strong evidence verified the depression-like state induced by MS, little is known about the specific mechanism of MS. Therefore, the present study aimed to investigate the neurobiology mechanism of the MS-induced depression-like state.

METHODS

To verify the depression-like behaviors of offspring induced by MS, a series of behavioral tests were performed. Then, in vivo electroporation and three-dimensional reconstruction, combining with immunohistochemistry and BrdU labeling, were mainly used to explore the neurogenesis and synaptogenesis in postnatal dentate gyrus.

RESULTS

Prolonged MS indeed induced the depression-like behaviors of offspring in adulthood. Surprisingly, learning and memory were enhanced by prolonged MS. Further investigation indicated that prolonged MS inhibited the proliferation of neural stem cells, impaired the survival, and altered the fate decision of newborn cells, whereas the total length and terminal tips of dendrite, and the spine density, especially thin spine, were significantly increased in prolonged MS mice.

CONCLUSIONS

Our results elucidated that prolonged MS induced the depression-like state by impairing postnatal neurogenesis of dentate gyrus. Importantly, our results emphasized that prolonged MS increased the spine density, especially thin spine, by increasing the total length and number of terminal tips of dendrite, thereby enhancing learning and memory.

Effects of enriched environment on depression and anxiety-like behavior induced by early life stress: A comparison between different periods

BACKGROUND

Brain development is a prolonged process and it is sensitive to the environment during critical periods. Stress in early life is believed to increase vulnerability to depression, while enriched environment (EE) has beneficial effects on neural plasticity and depression. In this study, we compared the therapeutic effect of EE during different periods on early life stress-induced depression, and investigated the role of brain-derived neurotrophic factor (BDNF) and protein kinase B (AKT) on the effect of EE. Plasma corticosterone level was also detected to evaluate the reactivity of hypothalamic-pituitary-adrenal axis.

METHODS

C57BL/6 mice were subjected to a 4-h maternal separation (MS) procedure during postnatal days 2-21. After this separation, the mice were assigned to standard environment groups (SE), EE in the early period groups (3-8 weeks, EEE) and EE in the adult groups (8-13 weeks, EEA). Depression and anxiety behavior were evaluated at 14-weeks of age. The plasma corticosterone was quantified utilizing enzyme-linked immunosorbent assay. Hippocampus BDNF and AKT/p-AKT were detected using Western blotting.

RESULTS

The results showed that MS increased depression and anxiety level, while EE in both intervention periods alleviated the symptoms of depression and anxiety. The EEE group showed better effects in terms of anhedonia and anxiety than the EEA group. The difference in despair behavior between the EEE and EEA groups was not significant. MS increased plasma corticosterone level, while EE decreased corticosterone level in both intervention periods. EE increased BDNF and p-AKT expression in the hippocampus, with stronger effects in the EEE group.

CONCLUSION

EE during the early development period was more effective in alleviating depression and anxiety induced by early life stress. BDNF and AKT may play a significant role in the effect of EE, and further research is needed to explore the detailed neurobiological mechanisms.

Perinatal Stress and Methamphetamine Exposure Decreases Anxiety-Like Behavior in Adult Male Rats

Methamphetamine (MA) is an illicit synthetic psychostimulant drug, and its abuse is growing worldwide. MA has been reported as the primary drug of choice, by drug-abusing women, during pregnancy. Since MA easily crosses the placental barrier, the fetus is exposed to MA in a similar fashion to the mother. This study aimed to evaluate the effect of long-term perinatal stressors and drug exposure on anxiety-like behavior in adult male rats using the open field test (OF) and elevated plus maze (EPM). Dams were divided into three groups according to drug treatment during pregnancy: controls (C), saline-SA [subcutaneous (s.c.), 1 ml/kg], and MA (s.c., 5 mg/kg). Litters were divided into four groups according to postnatal stressors: non-stressed controls (N), maternal separation (S), maternal cold water stress (W), and maternal separation plus maternal cold water stress (SW). Forty-five minutes before testing (in both OF and EPM), one-half of adult male rats received an (s.c.) injection of MA and the other half received an SA injection. Prenatal MA/stress exposure did not affect anxiety-like behavior in adult male rats in both tests. In the OF, an acute MA dose in adulthood increased the time spent in the central disk area, decreased time spent in the corners, and decreased time spent immobile and grooming. Also, postnatal stress increased time spent in the central disk area, decreased time spent in corners, and increased mobility compared to controls. All groups of rats exposed to postnatal stressors spent significantly less time in the closed arms of the EPM compared to controls. Overall, our results indicate that early postnatal stress and a single acute MA administration in adulthood decreases the parameters of anxiety-like behavior in adult male rats regardless of prenatal MA exposure. Moreover, postnatal stress via maternal separation impacts the effect of acute MA administration in adulthood. Long-term postnatal stress may thus result in improved adaptation to subsequent stressful experiences later in life.

Long-term activation of hippocampal glial cells and altered emotional behavior in male and female adult rats after different neonatal stressors

Early life stress increases the risk of developing psychiatric diseases in adulthood. Severe neonatal infections can also contribute to the development of affective illnesses. Stress and infections both trigger the immediate activation of the neuroimmune system. We compared the long-term effects of neonatal single or combined stress-immune challenges on emotional behavior and glial cell responses in the hippocampus. Male and female Sprague Dawley rats were randomly allocated across four conditions: (1) control + vehicle; (2) maternal separation (MS, 3 h/day on postnatal days [PN] 1-14) + vehicle; (3) control + lipopolysaccharide (LPS, 0.5. mg/kg, PN14); (4) MS + LPS. The rats' behaviors were analyzed from PN120 in males and from PN150 in diestrous females. LPS, but not MS, increased anxiety-like behavior in male rats; however, in females, it increased with both challenges. Depressive-like behavior increased after MS-but not LPS-in males and females. Combined stressors increased depressive-like behavior in both sexes. All stressors promoted microglial activation in CA3 and hilus in males and females. MS and LPS increased the astrocytic density within the male hilus, but LPS only increased it in CA3. MS prevented the rise in astrocytic density with LPS. In females, MS reduced the astrocytic population of the hilus and CA3 areas. Taken together, the behavioral and glial cell responses to early life challenges are sex-dependent and cell-type specific. This suggests a sexual dimorphism in the nature of the adverse event faced. These results have implications for understanding the emergence of psychiatric illnesses.

Prenatal programming of depression: cumulative risk or mismatch in the Ontario Child Health Study?

Consistent with cumulative risk hypotheses of psychopathology, studies examining prenatal adversity and later mental health largely suggest that pre and postnatal stress exposures have summative effects. Fewer data support that a mismatch in stress levels between pre- and postnatal life increases risk (the mismatch hypothesis). In this retrospective cohort study using data from the 1983 Ontario Child Health Study (OCHS), we examined interactions between birth weight status and childhood/adolescent stress to predict major depression in adulthood. Ninety-five participants born at low birth weight (LBW; <2500 g) and 972 normal birth weight (NBW) control participants completed the Composite International Diagnostic Interview Short-Form Major Depression module at 21-34 years of age. A youth risk scale consisting of five stressful exposures (family dysfunction, socioeconomic disadvantage, parental criminality, maternal mental illness, exposure to other life stresses) indexed child/adolescent adversity. Birth weight groups did not differ by childhood risk score nor depression levels. A significant interaction was observed between birth weight and the youth risk scale whereby exposure to increasing levels of exposure to childhood/adolescent adversity predicted increased levels of depression in the NBW group, but lower rates in those born at LBW. Consistent with the mismatch hypothesis, data from a large, longitudinally followed cohort suggest that the mental health of adults born LBW may be more resilient to the adverse effects of childhood/adolescent stress. Taken in the context of previous studies of low birth weight infants, these findings suggest that the nature of associations between gestational stress and later mental health may depend on the magnitude of prenatal stress exposure, as well as the degree of resilience and/or plasticity conferred by their early-life environment.

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.

Impact of preweaning stress on long-term neurobehavioral outcomes in Sprague-Dawley rats: Differential effects of barren cage rearing, pup isolation, and the combination

Two developmental stressors were compared in preweaning rats exposed to either one stressor or both. Stressors were barren cage rearing or maternal separation (pup isolation). 40 gravid Sprague-Dawley CD/IGS rats were randomly assigned to two cage conditions: standard (Std) cage or barren cage (Bar), 20 litters/condition throughout gestation and lactation. After delivery, litters were randomly culled to 4 males and 4 females. The second stressor was maternal separation: Two male/female pairs per litter were isolated from their dam 4 h/day (Iso) and two pairs were not (Norm). Hence, there were 4 conditions: Std-Norm, Std-Iso, Bar-Norm, and Bar-Iso. One pair/litter/stress condition received the following: elevated zero-maze (EZM), open-field, swim channel, Cincinnati water maze, conditioned fear, and open-field with methamphetamine challenge. The second pair/litter/condition received the light-dark test, swim channel, Morris water maze, forced swim, and EZM with diazepam challenge. Barren rearing reduced EZM time-in-open, whereas isolation rearing reduced open-field activity in males and increased it in females. Effects on straight channel swimming were minor. In the Cincinnati water maze test of egocentric learning, isolation rearing increased errors whereas barren cage housing reduced errors in combination with normal rearing. Barren cage with maternal separation (pup isolation) increased Cincinnati water maze escape latency but not errors. Barren cage housing reduced hyperactivity in response to methamphetamine. Isolation rearing increased time in open in the EZM after diazepam challenge. Trends were seen in the Morris water maze. These suggested that barren cage and isolation rearing in combination reduced latency on acquisition on days 1 and 2 in males, whereas females had increased latency on days 2 and 3. Combined exposure to two developmental stressors did not induce additive or synergistic effects, however the data show that these stressors had long-term effects with some evidence that the combination of both caused effects when either stressor alone did not, but synergism was not observed.

Maternal immune activation and repeated maternal separation alter offspring conditioned avoidance response learning and antipsychotic response in male rats

Previous work shows that repeated administration of several commonly used antipsychotic drugs, such as olanzapine (OLZ) over several days, induces an enhanced disruption of conditioned avoidance response (CAR) (termed antipsychotic sensitization) in normal adolescent and adult rats. However, it is unclear whether the same phenomenon can also be demonstrated in rat models of schizophrenia. The present study investigated OLZ sensitization in a combined maternal immune activation (MIA) and repeated maternal separation (RMS) model of schizophrenia. Sprague-Dawley male rats were first subjected to an early prenatal exposure to polyinosinic:polycytidylic acid (PolyI:C) on gestation days 13 (4 mg/kg, iv) and 15 (6 mg/kg, iv). They were then repeatedly separated from their mothers for 3 h daily during the first two weeks of postpartum. After they became adolescent (on postnatal day, PND 43), acute and OLZ sensitization effects in the CAR model was assessed. Adolescent MIA rats showed an impaired acquisition of conditioned avoidance response, but displayed a normal acute OLZ-induced avoidance suppression and OLZ sensitization effect. In adulthood (PND 81), MIA rats again showed an impairment in the acquisition of CAR. However, they showed a reduced response to OLZ (1.0 mg/kg; sc) treatment during the repeated drug test days, indicating a disruption of the induction of OLZ sensitization. In the OLZ sensitization challenge test, both MIA and control rats exhibited a robust and similar sensitization effect. In both adolescence and adulthood, RMS alone had no effect on any of the behavioral outcomes, and combined MIA-RMS even abolished the MIA alone-induced disruption of avoidance acquisition and the induction of OLZ sensitization. These results indicate that MIA disrupts associative learning and may reduce antipsychotic efficacy in the early stage of OLZ treatment. RMS does not appear to affect associative learning and behavioral responses to OLZ, and may possibly attenuate MIA-induced deficits. Our findings demonstrate that OLZ sensitization is a robust phenomenon but its magnitude can be altered by early MIA.

Glucocorticoid-mediated mechanisms of hippocampal damage: Contribution of subgranular neurogenesis

A comprehensive overview of the interplay between glucocorticoids (GCs) and adult hippocampal neurogenesis (AHN) is presented, particularly, in the context of a diseased brain. The effectors of GCs in the dentate gyrus neurogenic niche of the hippocampal are reviewed, and the consequences of the GC signaling on the generation and integration of new neurons are discussed. Recent findings demonstrating how GC signaling mediates impairments of the AHN in various brain pathologies are overviewed. GC-mediated effects on the generation and integration of adult-born neurons in the hippocampal dentate gyrus depend on the nature, severity, and duration of the acting stress factor. GCs realize their effects on the AHN primarily via specific glucocorticoid and mineralocorticoid receptors. Disruption of the reciprocal regulation between the hypothalamic-pituitary-adrenal (HPA) axis and the generation of the adult-born granular neurons is currently considered to be a key mechanism implicating the AHN into the pathogenesis of numerous brain diseases, including those without a direct hippocampal damage. These alterations vary from reduced proliferation of stem and progenitor cells to increased cell death and abnormalities in morphology, connectivity, and localization of young neurons. Although the involvement of the mutual regulation between the HPA axis and the AHN in the pathogenesis of cognitive deficits and mood impairments is evident, several unresolved critical issues are stated. Understanding the details of GC-mediated mechanisms involved in the alterations in AHN could enable the identification of molecular targets for ameliorating pathology-induced imbalance in the HPA axis/AHN mutual regulation to conquer cognitive and psychiatric disturbances.

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.

Maternal separation induces changes in TREK-1 and 5HT1A expression in brain areas involved in the stress response in a sex-dependent way

Depression is a prevalent disease in modern society, and has been linked to stressful events at early ages. Women are more susceptible to depression, and the neural basis for this are still under investigation. Serotonin is known to be involved in depression, and a decrease in 5HT_1A expression is observed on temporal and cortical areas in both men and women with depression. As knockout animals for TREK-1 are resilient to depression, this channel has emerged as a new potential pharmacological target for depression treatment. In this study, maternal separation (MS) was used to emulate early-life stress, and evaluate behaviour, as well as TREK-1 and 5HT_1A expression in the brain using immunohistochemistry. In juvenile females, 5HT_1A reduction coupled to increased TREK-1 in the dentate gyrus (DG) was associated with behavioural despair, as well as increased TREK-1 expression in basolateral amygdala (BLA) and prelimbic cortex (PL). In juvenile males, MS induced an increase in 5HT_1A in the BLA, and in TREK-1 in the PL, while no behavioural despair was observed. Anhedonia and anxiety-like behaviour were not induced by MS. We conclude stress-induced increase in TREK-1 in PL and GD is associated to depression, while 5HT_1A changes coupled to TREK-1 changes may be necessary to induce depression, with females being more vulnerable to MS effects than males. Thus, TREK-1 and 5HT_1A may be potential pharmacological targets for antidepressants development.

Early Life Stress Exacerbates Outcome after Traumatic Brain Injury

The neurocognitive impairments associated with mild traumatic brain injury (TBI) often resolve within 1-2 weeks; however, a subset of people exhibit persistent cognitive dysfunction for weeks to months after injury. The factors that contribute to these persistent deficits are unknown. One potential risk factor for worsened outcome after TBI is a history of stress experienced by a person early in life. Early life stress (ELS) includes maltreatment such as neglect, and interferes with the normal construction of cortical and hippocampal circuits. We hypothesized that a history of ELS contributes to persistent learning and memory dysfunction following a TBI. To explore this interaction, we modeled ELS by separating Sprague Dawley pups from their nursing mothers from post-natal days 2-14 for 3 h daily. At 2 months of age, male rats received sham surgery or mild to moderate parasagittal fluid-percussion brain injury. We found that the combination of ELS with TBI in adulthood impaired hippocampal-dependent learning, as assessed with contextual fear conditioning, the water maze task, and spatial working memory. Cortical atrophy was significantly exacerbated in TBI animals exposed to ELS compared with normal-reared TBI animals. Changes in corticosterone in response to restraint stress were prolonged in TBI animals that received ELS compared with TBI animals that were normally reared or sham animals that received ELS. Our findings indicate that ELS is a risk factor for worsened outcome after TBI, and results in persistent learning and memory deficits, worsened cortical pathology, and an exacerbation of the hormonal stress response.

Maternal Separation in ABA Rats Promotes Cell Proliferation in the Dentate Gyrus of the Hippocampus

Anorexia nervosa (AN) is a serious eating disorder characterized by self-starvation and excessive weight loss. Several studies support the idea that life stressors during the postnatal period could play a pivotal role in the pathogenesis of AN, underlying the multifactorial etiology of this disease. The activity-based anorexia (ABA) animal model mimics core features of the mental disorder, including severe food restriction, weight loss, and hyperactivity. Previous results obtained in our lab showed that maternal separation (MS) induces behavioral changes in anorexic-like ABA rats in a sexually dimorphic way: in females, the MS promoted hyperactivity and a less anxious-like phenotype in ABA animals; in males, instead, the MS attenuated the anxiolytic effect of the ABA protocol. These results led us to investigate the effect of the MS on brain areas involved in the control of the anxiety-like behavior. We focused our attention on the adult hippocampal neurogenesis, a process involved in the response to environmental stimuli and stressful condition. We analyzed the volume of the whole hippocampus and the proliferation rate in the dentate gyrus (DG) by quantifying Ki67-cells density and characterizing neuronal phenotype (DCX) and glial cells (GFAP) with double-fluorescence technique. The results obtained showed that only in maternally separated anorexic rats there is an increase of proliferation in DG, underlying the presence of a synergic effect of MS and ABA that boost the proliferation of new neurons and glia progenitors in a more evident way in females in comparison to males.

The developmental support hypothesis: adaptive plasticity in neural development in response to cues of social support

Across mammals, cues of developmental support, such as touching, licking or attentiveness, stimulate neural development, behavioural exploration and even overall body growth. Why should such fitness-related traits be so sensitive to developmental conditions? Here, we review what we term the 'developmental support hypothesis', a potential adaptive explanation of this plasticity. Neural development can be a costly process, in terms of time, energy and exposure. However, environmental variability may sometimes compromise parental care during this costly developmental period. We propose this environmental variation has led to the evolution of adaptive plasticity of neural and behavioural development in response to cues of developmental support, where neural development is stimulated in conditions that support associated costs. When parental care is compromised, offspring grow less and adopt a more resilient and stress-responsive strategy, improving their chances of survival in difficult conditions, similar to existing ideas on the adaptive value of early-life programming of stress. The developmental support hypothesis suggests new research directions, such as testing the adaptive value of reduced neural growth and metabolism in stressful conditions, and expanding the range of potential cues animals may attend to as indicators of developmental support. Considering evolutionary and ecologically appropriate cues of social support also has implications for promoting healthy neural development in humans. This article is part of the theme issue 'Life history and learning: how childhood, caregiving and old age shape cognition and culture in humans and other animals'.

Expression of synaptophysin and BDNF in the medial prefrontal cortex following early life stress and neonatal hypoxia-ischemia

This study aims at investigating whether early stress interacts with brain injury due to neonatal hypoxia-ischemia (HI). To this end, we examined possible changes in synaptophysin (SYN) and brain-derived neurotrophic factor (BDNF) expression in the medial prefrontal cortex (mPFC) of maternally separated rats that were subsequently exposed to a HI episode. Rat pups (n = 11) were maternally separated during postnatal days 1 to 6 (3hr/day), while another group was left undisturbed (n = 11). On postnatal day 7, a subgroup (n = 12) from each postnatal manipulation was exposed to HI. Synaptophysin and BDNF expression was estimated in mPFC prelimbic and anterior cingulate subregions of the ipsilateral and contralateral to the occluded common carotid artery hemispheres. Maternally separated rats expressed significantly less BDNF and SYN in both hemispheres. Neonatal HI significantly reduced BDNF and SYN expression in the ipsilateral mPFC only and this reduction was not further altered by early stress. Our findings indicate the enduring negative effect of a short period of maternal separation on the expression of mPFC SYN and BDNF. They, also, reveal that the HI-associated decreases in these markers are limited to the ipsilateral mPFC and are not exacerbated by early stress. These decreases may have important functional implications given the role of prefrontal area in high-order cognition.

Ultrasound stress compromises the correlates of emotional-like states and brain AMPAR expression in mice: effects of antioxidant and anti-inflammatory herbal treatment

The modern lifestyle is associated with exposure to "psychological" or "emotional" stress. A growing portion of the population is exposed to emotional stress that results in a high incidence of anxiety disorders, a serious social problem. With this rise, there is a need for understanding the neurobiological causes of stress-induced anxiety and to offer safe remedies for this condition. Side effects of existing pharmaceuticals necessitate the search for alternatives. Having fewer adverse effects than classic remedies, natural extract-based therapies can be a promising solution. Here, we applied a model of emotional stress in BALB/c mice using ultrasound exposure to evoke the signs of anxiety-like behavior. We examined the behavioral and molecular impact of ultrasound and administration of herbal antioxidant/anti-inflammatory treatment (HAT) on AMPA receptor expression, markers of plasticity, inflammation and oxidative stress. A 3-week ultrasound exposure increased scores of anxiety-like behaviors in the standard tests and altered hippocampal expression as well as internalization of AMPA receptor subunits GluA1-A3. Concomitant treatment with HAT has prevented increases of anxiety-like behaviors and other behavioral changes, normalized hippocampal malondialdehyde content, GSK3β and pro-inflammatory cytokines Il-1β and Il-6, and the number of Ki67-positive cells. Levels of malondialdehyde, a common measure of oxidative stress, significantly correlated with the investigated end-points in stressed, but not in non-stressed animals. Our results emphasize the role of oxidative stress in neurobiological abnormalities associated with experimentally induced condition mimicking emotional stress in rodents and highlight the potential therapeutic use of anti-oxidants like herbal compositions for management of stress-related emotional disturbances within the community.

Systematic review and meta-analysis: effects of maternal separation on anxiety-like behavior in rodents

The mechanisms by which childhood maltreatment increases anxiety is unclear, but a propensity for increased defensive behavior in rodent models of early life stress (ELS) suggests that work in rodents may clarify important mechanistic details about this association. A key challenge in studying the effects of ELS on defensive behavior in rodents is the plethora of inconsistent results. This is particularly prominent with the maternal separation (MS) literature, one of the most commonly used ELS models in rodents. To address this issue we conducted a systematic review and meta-analysis, examining the effects of MS on exploratory-defensive behavior in mice and rats using the open field test (OFT) and the elevated plus maze (EPM). This search yielded a total of 49 studies, 24 assessing the effect of MS on behavior in the EPM, 11 tested behavior in the OFT, and 14 studies provided data on both tasks. MS was associated with increased defensive behavior in rats (EPM: Hedge's g = -0.48, p = 0.02; OFT: Hedge's g = -0.33, p = 0.05), effect sizes that are consistent with the anxiogenic effect of early adversity reported in humans. In contrast, MS did not alter exploratory behavior in mice (EPM: Hedge's g = -0.04, p = 0.75; OFT: Hedge's g = -0.03, p = 0.8). There was a considerable amount of heterogeneity between studies likely related to the lack of standardization of the MS protocol. Together, these findings suggest important differences in the ability of MS to alter circuits that regulate defensive behaviors in mice and rats.

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.

Animal Models for Functional Gastrointestinal Disorders

Functional gastrointestinal disorders (FGID), such as functional dyspepsia (FD) and irritable bowel syndrome (IBS) are characterized by chronic abdominal symptoms in the absence of an organic, metabolic or systemic cause that readily explains these complaints. Their pathophysiology is still not fully elucidated and animal models have been of great value to improve the understanding of the complex biological mechanisms. Over the last decades, many animal models have been developed to further unravel FGID pathophysiology and test drug efficacy. In the first part of this review, we focus on stress-related models, starting with the different perinatal stress models, including the stress of the dam, followed by a discussion on neonatal stress such as the maternal separation model. We also describe the most commonly used stress models in adult animals which brought valuable insights on the brain-gut axis in stress-related disorders. In the second part, we focus more on models studying peripheral, i.e., gastrointestinal, mechanisms, either induced by an infection or another inflammatory trigger. In this section, we also introduce more recent models developed around food-related metabolic disorders or food hypersensitivity and allergy. Finally, we introduce models mimicking FGID as a secondary effect of medical interventions and spontaneous models sharing characteristics of GI and anxiety-related disorders. The latter are powerful models for brain-gut axis dysfunction and bring new insights about FGID and their comorbidities such as anxiety and depression.

Treadmill running attenuates neonatal hypoxia induced adult depressive symptoms and promoted hippocampal neural stem cell differentiation via modulating AMPK-mediated mitochondrial functions

Neonatal hypoxia can induce the persisting brain dysfunctions and subsequently result in the behavioral abnormalities in adulthood. Improving mitochondrial functions were suggested as the effective strategy for brain functional recovery. In this study, we tested the effects of physical exercise, a well-established way benefits mitochondrion, for its functions to prevent hypoxia induced adult behavioral dysfunctions and the underlying molecular mechanism. Mice was induced with hypoxia and treadmill running were then administrated until the adulthood. The treadmill running resulted in the improved behavioral performance in depressive and anxiety tests together with the enhancement of hippocampal neurogenesis. We then detected treadmill running restored the mitochondrial morphology in adult neural stem cells (NSCs) as well as the ATP production in hippocampal tissue. In addition, activity of AMPK, which playing key roles in regulating mitochondrial functions, was also elevated by treadmill running. Blockage of AMPK with selective inhibitor compound C prohibited effects of treadmill running in attenuating neonatal hypoxia induced neurogenic impairment and antidepressant behavioral deficits in adulthood. In conclusion, treadmill running could prevent neonatal hypoxia induced adult antidepressant dysfunctions and neurogenic dampening via AMPK-mediated mitochondrial regulation.

Prenatal immobilization stress and postnatal maternal separation cause differential neuroendocrine responses to fasting stress in adult male rats

Prenatal immobilization stress (PNS) and postnatal maternal separation (MS180) are two widely used rodent models of early-life stress (ELS) that affect the hypothalamus-pituitary-adrenal (HPA) axis, cause behavioral alterations, and affect glucose tolerance in adults. We compared anxiety-like behavior, coping strategies, and HPA axis activity in PNS and MS180 adult (4-month-old) male rats and assessed their glucose tolerance and HPA axis response after mild fasting stress. Both PNS and MS180 induced a passive coping strategy in the forced swimming test, without affecting anxiety-like behavior in the elevated plus-maze. Moreover, both PNS and MS180 increased the hypothalamic corticotropin-releasing hormone expression; however, only MS180 increased the circulating corticosterone levels. Both early life stressors increased fasting glucose levels and this effect was significantly higher in PNS rats. MS180 rats showed impaired glucose tolerance 120 min after intravenous glucose administration, whereas PNS rats displayed an efficient homeostatic response. Moreover, MS180 rats showed higher circulating corticosteroid levels in response to fasting stress (overnight fasting, 12 hr), which were restored after glucose administration. In conclusion, early exposure to postnatal MS180, unlike PNS, increases the HPA axis response to moderate fasting stress, indicating a differential perception of fasting as a stressor in these two ELS models.

Presence of Mother Reduces Early-Life Social Stress: Linking the Alteration in Hypothalamic-Pituitary-Adrenal Axis and Serotonergic System

In this study, we examined whether the presence of mother suppresses early-life stressful social experience (SSE)-induced anxiety-like behavior and impairment of short-term memory later in life. On postnatal day (PND)-5, mothers with pups were grouped as follows: (i) control; (ii) maternal separation (MS); (iii) pups with mother experience the presence of a stranger (M+P-ST); and (iv) maternal separated pups experience the presence of a stranger (MSP-ST). Individuals were subjected to light-dark box and spontaneous alternation from PND-29 to 32. We observed that the MSP-ST group exhibits anxiety-like behavior and impairment in short-term memory. Further, SSE significantly elevated the adrenocorticotropic hormone, corticosterone and expression of glucocorticoid receptor (GR) in MSP-ST pups. Similarly, serotonin (5-hydroxytryptamine; 5-HT), dopamine, noradrenaline and expression of serotonin transporter levels were significantly elevated in MSP-ST pups. These observations suggest that during early postnatal days, the pups may recognize strangers by the sense of smell, and the presence of mother reduces the SSE-induced stress.

Oxidative stress, inflammatory reactions and apoptosis mediated the negative effect of chronic stress induced by maternal separation on the reproductive system in male mice

Exposure to severe and long-lasting stressors during early postnatal life negatively affects development of the brain and associated biological networks. Maternal separation (MS) is a valid stressful experience in early life that adversely affects neurobiological circuits. In the present study, we aimed to evaluate the effects of MS on sperm quality and histology of the testis in adult male mice. In this study, male mice were subjected to MS during post-natal days (PND) 2-14. Sperm parameters, histological alterations in the testicular tissue, ROS production (using DCFH-DA assay), gene expression of TLR4, NLRP3, TNFα, BAX, ASC, caspase-1 and BCL-2 (using RT-PCR), protein levels of caspase-3 and caspase-8 (using western blotting), and protein levels of IL-1β, IL-18, GPx and ATP (using ELISA) as well as protein expression of caspase-1 and NLRP3 (using immunocytochemistry) were evaluated. Findings showed that MS decreased count, morphology and viability of spermatozoa. MS decreased the diameter of seminiferous tubules and decreased the thickness of seminiferous epithelium. Furthermore, MS increased the level of ROS production and decreased the concentrations of GPx and ATP. MS led to increased expression of TLR4, NlRP3, TNFα, caspase-1, ASC, IL-1β and IL-18. In addition, MS induced apoptosis as evidenced by increased BAX, caspase-3 and caspase-8 as well as decreased BCL-2 expression. We concluded that early life stress induced by MS has detrimental effects on sperm parameters and testicular tissue. Our results suggest that these effects are mediated by activation of ROS production, and alterations in mitochondrial function, inflammatory processes and apoptosis pathways.

Neurobiological mechanisms underlying sex-related differences in stress-related disorders: Effects of neuroactive steroids on the hippocampus

Men and women differ in their vulnerability to a variety of stress-related illnesses, but the underlying neurobiological mechanisms are not well understood. This is likely due to a comparative dearth of neurobiological studies that assess male and female rodents at the same time, while human neuroimaging studies often don't model sex as a variable of interest. These sex differences are often attributed to the actions of sex hormones, i.e. estrogens, progestogens and androgens. In this review, we summarize the results on sex hormone actions in the hippocampus and seek to bridge the gap between animal models and findings in humans. However, while effects of sex hormones on the hippocampus are largely consistent in animals and humans, methodological differences challenge the comparability of animal and human studies on stress effects. We summarise our current understanding of the neurobiological mechanisms that underlie sex-related differences in behavior and discuss implications for stress-related illnesses.

Opposed cannabinoid 1 receptor (CB1R) expression in the prefrontal cortex vs. nucleus accumbens is associated with alcohol consumption in male rats

Abusive alcohol consumption is a health problem, worldwide. There is extensive literature indicating that cannabinoid 1 receptor (CB1R) plays a crucial role in mediating alcohol's reward effects. Maternal care deprivation (MCD) is a reliable rodent model of early life stress that leads to high levels of anxiety and alterations in motivation, which may increase vulnerability to alcohol consumption. The present study researched whether anxiety-like behaviors and the level of motivation for a natural reward, and CB1R expression in the prefrontal cortex (PFC) and nucleus accumbens (NAcc) can predict alcohol consumption in non-MCD and MCD male rats. Results indicate that MCD increases anxiety-like behaviors, i.e., reduces time in open arms in the elevated plus maze and increases alcohol intake. In turn, the motivation for a palatable reward, i.e., a chocolate flavored pellet, was not affected by MCD. MCD reduces CB1R expression in the PFC and increases it in the NAcc. Hence, both higher anxiety-like behaviors and higher CB1R expression in the NAcc and lower CB1R expression in the PFC are associated with higher alcohol intake. These results suggest that early life adverse experiences induce a reprogramming of the brain's endocannabinoid system that very likely contributes to making the brain vulnerable to develop alcohol abuse and dependence.

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.

Resilience and Vulnerability to Trauma: Early Life Interventions Modulate Aversive Memory Reconsolidation in the Dorsal Hippocampus

Early life experiences program lifelong responses to stress. In agreement, resilience and vulnerability to psychopathologies, such as posttraumatic stress disorder (PTSD), have been suggested to depend on the early background. New therapies have targeted memory reconsolidation as a strategy to modify the emotional valence of traumatic memories. Here, we used animal models to study the molecular mechanism through which early experiences may later affect aversive memory reconsolidation. Handling (H)—separation of pups from dams for 10 min—or maternal separation (MS) — 3-h separation—were performed from PDN1–10, using non-handled (NH) litters as controls. Adult males were trained in a contextual fear conditioning (CFC) task; 24 h later, a short reactivation session was conducted in the conditioned or in a novel context, followed by administration of midazolam 3 mg/kg i.p. (mdz), known to disturb reconsolidation, or vehicle; a test session was performed 24 h after. The immunocontent of relevant proteins was studied 15 and 60 min after memory reactivation in the dorsal hippocampus (dHc) and basolateral amygdala complex (BLA). Mdz-treated controls (NH) showed decreased freezing to the conditioned context, consistent with reconsolidation impairment, but H and MS were resistant to labilization. Additionally, MS males showed increased freezing to the novel context, suggesting fear generalization; H rats showed lower freezing than the other groups, in accordance with previous suggestions of reduced emotionality facing adversities. Increased levels of Zif268, GluN2B, β-actin and polyubiquitination found in the BLA of all groups suggest that memory reconsolidation was triggered. In the dHc, only NH showed increased Zif268 levels after memory retrieval; also, a delay in ERK1/2 activation was found in H and MS animals. We showed here that reconsolidation of a contextual fear memory is insensitive to interference by a GABAergic drug in adult male rats exposed to different neonatal experiences; surprisingly, we found no differences in the reconsolidation process in the BLA, but the dHc appears to suffer temporal desynchronization in the engagement of reconsolidation. Our results support a hippocampal-dependent mechanism for reconsolidation resistance in models of early experiences, which aligns with current hypotheses for the etiology of PTSD.