Prenatal stress inhibits hippocampal neurogenesis but spares olfactory bulb neurogenesis

Repeated Ketamine Anesthesia during the Neonatal Period Impairs Hippocampal Neurogenesis and Long-Term Neurocognitive Function by Inhibiting Mfn2-Mediated Mitochondrial Fusion in Neural Stem Cells

The mechanism of ketamine-induced neurotoxicity development remains elusive. Mitochondrial fusion/fission dynamics play a critical role in regulating neurogenesis. Therefore, this study was aimed to evaluate whether mitochondrial dynamics were involved in ketamine-induced impairment of neurogenesis in neonatal rats and long-term synaptic plasticity dysfunction. In the in vivo study, postnatal day 7 (PND-7) rats received intraperitoneal (i.p.) injection of 40 mg/kg ketamine for four consecutive times at 1 h intervals. The present findings revealed that ketamine induced mitochondrial fusion dysfunction in hippocampal neural stem cells (NSCs) by downregulating Mitofusin 2 (Mfn2) expression. In the in vitro study, ketamine treatment at 100 μM for 6 h significantly decreased the Mfn2 expression, and increased ROS generation, decreased mitochondrial membrane potential and ATP levels in cultured hippocampal NSCs. For the interventional study, lentivirus (LV) overexpressing Mfn2 (LV-Mfn2) or control LV vehicle was microinjected into the hippocampal dentate gyrus (DG) 4 days before ketamine administration. Targeted Mfn2 overexpression in the DG region could restore mitochondrial fusion in NSCs and reverse the inhibitory effect of ketamine on NSC proliferation and its faciliatory effect on neuronal differentiation. In addition, synaptic plasticity was evaluated by transmission electron microscopy, Golgi-Cox staining and long-term potentiation (LTP) recordings at 24 h after the end of the behavioral test. Preconditioning with LV-Mfn2 improved long-term cognitive dysfunction after repeated neonatal ketamine exposure by reversing the inhibitory effect of ketamine on synaptic plasticity in the hippocampal DG. The present findings demonstrated that Mfn2-mediated mitochondrial fusion dysfunction plays a critical role in the impairment of long-term neurocognitive function and synaptic plasticity caused by repeated neonatal ketamine exposure by interfering with hippocampal neurogenesis. Thus, Mfn2 might be a novel therapeutic target for the prevention of the developmental neurotoxicity of ketamine.

Prenatal stress alters transcription of NMDA-type glutamate receptors in the hippocampus

Prenatal stress increases the risk of neurodevelopmental disorders. NMDA-type glutamate receptor (NMDAR) activity plays an important pathophysiological role in the cortico-hippocampal circuit in these disorders. We tested the hypothesis that transcription of NMDAR subunits is modified in the frontal cortex (FCx) and hippocampus after exposure to prenatal restraint stress (PRS) in mice. At 10 weeks of age, male PRS offspring (n = 20) and non-stressed controls (NS, n = 20) were treated with haloperidol (1 mg/kg), clozapine (5 mg/kg) or saline twice daily for 5 days, before measuring social approach (SOC). Saline-treated and haloperidol-treated PRS mice had reduced SOC relative to NS (P < 0.01), but clozapine-treated PRS mice had similar SOC to NS mice. These effects of PRS were associated with increased transcription of NMDAR subunits encoded by GRIN2A and GRIN2B genes in the hippocampus but not FCx. GRIN transcription in FCx correlated positively with SOC, but hippocampal GRIN transcription had negative correlation with SOC. The ratio of GRIN2A/GRIN2B transcription is known to increase during development but was lower in PRS mice. These results suggest that GRIN2A and GRIN2B transcript levels are modified in the hippocampus by PRS, leading to life-long deficits in social behavior. These data have some overlap with the molecular pathophysiology of schizophrenia. Similar to PRS in mice, schizophrenia, has been associated with social withdrawal, with increased GRIN2 expression in the hippocampus, and reduced GRIN2A/GRIN2B expression ratios in the hippocampus. These findings suggest that PRS in mice may have construct validity as a preclinical model for antipsychotic drug development.

Glucocorticoid effects on the brain: from adaptive developmental plasticity to allostatic overload

Exposure to stress during early life may alter the developmental trajectory of an animal by a mechanism known as adaptive plasticity. For example, to enhance reproductive success in an adverse environment, it is known that animals accelerate their growth during development. However, these short-term fitness benefits are often associated with reduced longevity, a phenomenon known as the growth rate-lifespan trade-off. In humans, early life stress exposure compromises health later in life and increases disease susceptibility. Glucocorticoids (GCs) are major stress hormones implicated in these processes. This Review discusses the evidence for GC-mediated adaptive plasticity in development, leading to allostatic overload in later life. We focus on GC-induced effects on brain structure and function, including neurogenesis; highlight the need for longitudinal studies; and discuss approaches to identify molecular mechanisms mediating GC-induced alteration of the brain developmental trajectory leading to adult dysfunctions. Further understanding of how stress and GC exposure can alter developmental trajectories at the molecular and cellular level is of critical importance to reduce the burden of mental and physical ill health across the life course.

Chronic unpredictable mild stress alters odor hedonics and adult olfactory neurogenesis in mice

Experiencing chronic stress significantly increases the risk for depression. Depression is a complex disorder with varied symptoms across patients. However, feeling of sadness and decreased motivation, and diminished feeling of pleasure (anhedonia) appear to be core to most depressive pathology. Odorants are potent signals that serve a critical role in social interactions, avoiding danger, and consummatory behaviors. Diminished quality of olfactory function is associated with negative effects on quality of life leading to and aggravating the symptoms of depression. Odor hedonic value (I like or I dislike this smell) is a dominant feature of olfaction and guides approach or avoidance behavior of the odor source. The neural representation of the hedonic value of odorants is carried by the granule cells in the olfactory bulb, which functions to modulate the cortical relay of olfactory information. The granule cells of the olfactory bulb and those of the dentate gyrus are the two major populations of cells in the adult brain with continued neurogenesis into adulthood. In hippocampus, decreased neurogenesis has been linked to development or maintenance of depression symptoms. Here, we hypothesize that chronic mild stress can alter olfactory hedonics through effects on the olfactory bulb neurogenesis, contributing to the broader anhedonia phenotype in stress-associated depression. To test this, mice were subjected to chronic unpredictable mild stress and then tested on measures of depressive-like behaviors, odor hedonics, and measures of olfactory neurogenesis. Chronic unpredictable mild stress led to a selective effect on odor hedonics, diminishing attraction to pleasant but not unpleasant odorants, an effect that was accompanied by a specific decrease in adult neurogenesis and of the percentage of adult-born cells responding to pleasant odorants in the olfactory bulb.

Sex differences in the vulnerability of the hippocampus to prenatal stress

Distressing events during pregnancy that engage activity of the body's endocrine stress response have been linked with later life cognitive deficits in offspring and associated with developmental changes in cognitive-controlling neural regions. Interestingly, prenatal stress (PS)-induced alterations have shown some sex specificity. Here, we review the literature of animal studies examining sex-specific effect of physical PS on the function and structure of the hippocampus as hippocampal impairments likely underlie PS-associated deficits in learning and memory. Furthermore, the connectivity between the hypothalamic-pituitary-adrenal (HPA) axis and the hippocampus as well as the heavy presence of glucocorticoid receptors (GRs) in the hippocampus suggests this structure plays an important role in modulation of activity within stress circuitry in a sex-specific pattern. We hope that better understanding of sex-specific, PS-related hippocampal impairment will assist in uncovering the molecular mechanisms behind sex-based risk factors in PS populations across development, and perhaps contribute to greater precision in management of cognitive disturbances in this vulnerable population.

A Baldwin interpretation of adult hippocampal neurogenesis: from functional relevance to physiopathology

Hippocampal adult neurogenesis has been associated to many cognitive, emotional, and behavioral functions and dysfunctions, and its status as a selected effect or an "appendix of the brain" has been debated. In this review, we propose to understand hippocampal neurogenesis as the process underlying the "Baldwin effect", a particular situation in evolution where fitness does not rely on the natural selection of genetic traits, but on "ontogenetic adaptation" to a changing environment. This supports the view that a strong distinction between developmental and adult hippocampal neurogenesis is made. We propose that their functions are the constitution and the lifelong adaptation, respectively, of a basic repertoire of cognitive and emotional behaviors. This lifelong adaptation occurs through new forms of binding, i.e., association or dissociation of more basic elements. This distinction further suggests that a difference is made between developmental vulnerability (or resilience), stemming from dysfunctional (or highly functional) developmental hippocampal neurogenesis, and adult vulnerability (or resilience), stemming from dysfunctional (or highly functional) adult hippocampal neurogenesis. According to this hypothesis, developmental and adult vulnerability are distinct risk factors for various mental disorders in adults. This framework suggests new avenues for research on hippocampal neurogenesis and its implication in mental disorders.

Implication of microglia in ketamine-induced long-term cognitive impairment in murine pups

BACKGROUND

Ketamine, a non-competitive N-methyl-D-aspartate receptor (NMDAR) antagonist, is widely applicable to anesthesia, analgesia, and sedation. However, the function and mechanisms of ketamine in the long-term learning and memory function of neonatal mice are unclear.

OBJECTIVE

The present study aims to investigate whether long-term learning and memory function will be affected by multiple ketamine exposures in the early development period.

METHODS

The mRNA and protein levels were measured by RT-qPCR and western blot, respectively. The Morris Water Maze test was performed to assess spatial learning and memory.

RESULTS

We identified that neonatal exposure to ketamine downsized the positive neurons for microtubule-associated protein doublecortin (DCX) and Ki67 in hippocampal dentate gyrus at the juvenile and late adolescence stages. Double-labeling tests demonstrated that the counts of Iba1⁺ cells and Ki67⁺ cells were pronouncedly diminished with exposure to ketamine. Further, qPCR assays to screen the key factors predisposing the populations and maturation of microglia exhibited remarkable decline of CX3CR1 mRNA levels in ketamine group versus the control group. The close relation of microglia to synaptic plasticity was depicted by the significantly downregulated synaptic plasticity-related proteins NR2B and PSD-95 subsequent to multiple exposures to ketamine. Finally, we found that both the protein and mRNA levels of BDNF were markedly decreased in ketamine group versus the control group.

CONCLUSION

We found that multiple exposures to ketamine in neonatal mice lead to spatial learning and memory dysfunction. The alterations of microglial development and function are the possible mechanisms of long-term learning and memory impairment.

Neonatal Anesthesia by Ketamine in Neonatal Rats Inhibits the Proliferation and Differentiation of Hippocampal Neural Stem Cells and Decreases Neurocognitive Function in Adulthood via Inhibition of the Notch1 Signaling Pathway

The Notch signaling pathway plays an important role in the regulation of neurogenesis. The objective of this study was to investigate whether the Notch signaling pathway was involved in the neurogenesis impairment and long-term neurocognitive dysfunction caused by neonatal exposure to ketamine. On postnatal day 7 (PND-7), male Sprague-Dawley (SD) rats were intraperitoneally injected with 40 mg/kg ketamine four consecutive times (40 mg/kg × 4) at 1-h intervals. Notch ligand Jagged1 (0.5 mg/kg) and lentivirus overexpressing the Notch1 intracellular domain (LV-NICD1) were microinjected into the hippocampal dentate gyrus (DG) 1 h or 4 days before ketamine administration, respectively. The expression of Notch1 signaling pathway-related proteins was detected by Western blotting 24 h after ketamine administration. The proliferation and differentiation of the neural stem cells (NSCs) in the hippocampal DG were evaluated by double immunofluorescence staining 24 h after treatment. Moreover, changes in hippocampus-dependent spatial memory of 2-month-old rats were investigated with the Morris water maze test. Ketamine anesthesia in neonatal rats decreased the expression levels of Jagged1, Notch1, NICD1, and hairy enhancer of split 1 (Hes1); inhibited the proliferation and astrocytic differentiation of NSCs; and promoted the differentiation of neurons. Neonatal exposure to ketamine caused deficits in hippocampus-dependent spatial reference memory tasks in 2-month-old rats. Microinjection of Jagged1 or LV-NICD1 reversed the inhibitory effect of ketamine on the expression of Notch1-related proteins in the hippocampal DG, attenuated the ketamine-mediated decrease in NSC proliferation and differentiation, and improved the cognitive function of 2-month-old rats after neonatal exposure to ketamine. These results suggest that neonatal exposure to ketamine in rats inhibits the proliferation and differentiation of hippocampal NSCs and impairs neurocognitive function in adulthood. The Notch1 signaling pathway may be involved in the impairment of hippocampus-dependent learning and memory during adulthood caused by neonatal exposure to ketamine. These findings contribute to further understanding the neurotoxicity induced by neonatal exposure to ketamine and the underlying mechanisms.

Unilateral vocal nerve resection alters neurogenesis in the avian song system in a region-specific manner

New neurons born in the adult brain undergo a critical period soon after migration to their site of incorporation. During this time, the behavior of the animal may influence the survival or culling of these cells. In the songbird song system, earlier work suggested that adult-born neurons may be retained in the song motor pathway nucleus HVC with respect to motor progression toward a target song during juvenile song learning, seasonal song restructuring, and experimentally manipulated song variability. However, it is not known whether the quality of song per se, without progressive improvement, may also influence new neuron survival. To test this idea, we experimentally altered song acoustic structure by unilateral denervation of the syrinx, causing a poor quality song. We found no effect of aberrant song on numbers of new neurons in HVC, suggesting that song quality does not influence new neuron culling in this region. However, aberrant song resulted in the loss of left-side dominance in new neurons in the auditory region caudomedial nidopallium (NCM), and a bilateral decrease in new neurons in the basal ganglia nucleus Area X. Thus new neuron culling may be influenced by behavioral feedback in accordance with the function of new neurons within that region. We propose that studying the effects of singing behaviors on new neurons across multiple brain regions that differentially subserve singing may give rise to general rules underlying the regulation of new neuron survival across taxa and brain regions more broadly.

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.

In vivo biomarkers of structural and functional brain development and aging in humans

Brain aging is a major determinant of aging. Along with the aging population, prevalence of neurodegenerative diseases is increasing, therewith placing economic and social burden on individuals and society. Individual rates of brain aging are shaped by genetics, epigenetics, and prenatal environmental. Biomarkers of biological brain aging are needed to predict individual trajectories of aging and the risk for age-associated neurological impairments for developing early preventive and interventional measures. We review current advances of in vivo biomarkers predicting individual brain age. Telomere length and epigenetic clock, two important biomarkers that are closely related to the mechanistic aging process, have only poor deterministic and predictive accuracy regarding individual brain aging due to their high intra- and interindividual variability. Phenotype-related biomarkers of global cognitive function and brain structure provide a much closer correlation to age at the individual level. During fetal and perinatal life, autonomic activity is a unique functional marker of brain development. The cognitive and structural biomarkers also boast high diagnostic specificity for determining individual risks for neurodegenerative diseases.

Smell and Stress Response in the Brain: Review of the Connection between Chemistry and Neuropharmacology

The stress response in the brain is not fully understood, although stress is one of the risk factors for developing mental disorders. On the other hand, the stimulation of the olfactory system can influence stress levels, and a certain smell has been empirically known to have a stress-suppressing effect, indeed. In this review, we first outline what stress is and previous studies on stress-responsive biomarkers (stress markers) in the brain. Subsequently, we confirm the olfactory system and review previous studies on the relationship between smell and stress response by species, such as humans, rats, and mice. Numerous studies demonstrated the stress-suppressing effects of aroma. There are also investigations showing the effects of odor that induce stress in experimental animals. In addition, we introduce recent studies on the effects of aroma of coffee beans and essential oils, such as lavender, cypress, α-pinene, and thyme linalool on the behavior and the expression of stress marker candidates in the brain. The transfer of volatile components into the brain is also discussed while using the results of thyme linalool as an example. These studies may provide a good opportunity to connect chemical research at the molecular level with neuropharmacological approaches in the future.

Update on the neurodevelopmental theory of depression: is there any 'unconscious code'?

Depression is currently one of the most common psychiatric disorders and the number of patients receiving antidepressant treatment is increasing every year. Therefore, it is essential to understand the underlying mechanisms that are associated with higher prevalence of depression. The main component leading to the change in functioning, in the form of apathy, anhedonia, lack of motivation and sleep disturbances, is stress. This is the factor that in recent decades—due to the civilization speed, dynamic technological development as well as competitiveness and competition in relationships—significantly affects the psychophysical condition, which results in an increase in the prevalence of civilization diseases, including depression. To understand the mechanism of susceptibility to this disease, one should consider the significant role of the interaction between immune and nervous systems. Their joint development from the moment of conception is a matrix of later predispositions, both associated with the mobilization of the proinflammatory pathways (TNFα, IL-1β, IL-6) and associated with psychological coping with stress. Such an early development period is associated with epigenetic processes that are strongly marked in prenatal development up to 1 year of age and determinate the characteristic phenotype for various forms of pathology, including depression. Regarding the inflammatory hypothesis of depression, interleukin 17 (IL-17), among other proinflammatory cytokines, might play an important role in the development of depressive disorders. It is secreted by Th17 cells, crossed the placental barrier and acts on the brain structures of the fetus by increasing IL-17 receptor levels and affecting the intensity of its signaling in the brain.

Effects of Maternal Chewing on Prenatal Stress-Induced Cognitive Impairments in the Offspring via Multiple Molecular Pathways

We aimed to investigate the effects of maternal chewing on prenatal stress-induced cognitive impairments in the offspring and to explore the molecular pathways of maternal chewing in a mice model. Maternal chewing ameliorated spatial learning impairments in the offspring in a Morris water maze test. Immunohistochemistry and Western blot findings revealed that maternal chewing alleviated hippocampal neurogenesis impairment and increased the expression of hippocampal brain-derived neurotrophic factor in the offspring. In addition, maternal chewing increased the expression of glucocorticoid receptor (GR) and 11β-hydroxysteroid dehydrogenase isozyme 2 (11β-HSD2) and decreased the expression of 11β-HSD1 in the placenta, thereby attenuating the increase of glucocorticoid in the offspring. Furthermore, maternal chewing increased the expression of 11β-HSD2, FK506-binding protein 51 (FKBP51) and FKBP52 and decreased the expression of 11β-HSD1, thereby increasing hippocampal nuclear GR level. In addition, maternal chewing attenuated the increase in expression of DNMT1 and DNMT3a and the decrease in expression of histone H3 methylation at lysine 4, 9, 27 and histone H3 acetylation at lysine 9 induced by prenatal stress in the offspring. Our findings suggest that maternal chewing could ameliorate prenatal stress-induced cognitive impairments in the offspring at least in part by protecting placenta barrier function, alleviating hippocampal nuclear GR transport impairment and increasing the hippocampal brain-derived neurotrophic factor (BDNF) level.

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.

The novel antidepressant ketamine enhances dentate gyrus proliferation with no effects on synaptic plasticity or hippocampal function in depressive-like rats

AIM

Major depressive disorder is a common and debilitating condition with substantial economic impact. Treatment options, although effective, are aimed at relieving the symptoms with limited disease modification. Ketamine, a commonly used anaesthetic, has received substantial attention as it shows rapid antidepressant effects clinically. We studied the effects of ketamine on hippocampal function and dentate gyrus proliferation in rats showing a depressive-like phenotype.

METHODS

Adolescent and adult animals were pre-natally exposed to the glucocorticoid analog dexamethasone, and we verified a depressive-like phenotype using behavioural tests, such as the sucrose preference. We subsequently studied the effects of ketamine on hippocampal synaptic transmission, plasticity and dentate gyrus proliferation. In addition, we measured hippocampal glutamate receptor expression. We also tested the ketamine metabolite hydroxynorketamine for NMDA-receptor independent effects.

RESULTS

Surprisingly, our extensive experimental survey revealed limited effects of ketamine or its metabolite on hippocampal function in control as well as depressive-like animals. We found no effects on synaptic efficacy or induction of long-term potentiation in adolescent and adult animals. Also there was no difference when comparing the dorsal and ventral hippocampus. Importantly, however, ketamine 24 hours prior to experimentation significantly increased the dentate gyrus proliferation, as revealed by Ki-67 immunostaining, in the depressive-like phenotype.

CONCLUSION

We find limited effects of ketamine on hippocampal glutamatergic transmission. Instead, alterations in dentate gyrus proliferation could explain the antidepressant effects of ketamine.

Effects of physical exercise and stress on hippocampal CA1 and dentate gyrus synaptic transmission and long-term potentiation in adolescent and adult Wistar rats

It is commonly recognized that physical exercise positively affects several CNS regions and improves cognitive abilities. For example, exercise is associated with an increase in neurogenesis and facilitation of long-term potentiation in the hippocampus. Conversely, animal models for depression are associated with a decrease in neurogenesis and a reduction of long-term potentiation in the hippocampus. Although exercise could be a viable option in the treatment of some forms of depression, the mechanisms responsible for such improvements have not been elucidated. In this study, we examine hippocampal function using electrophysiological field recordings in CA1 and dentate gyrus to study baseline synaptic transmission and long-term potentiation in adolescent and adult rats prenatally exposed to the glucocorticoid dexamethasone. One group of animals was allowed to run voluntarily for 10 or 21 days using an exercise wheel before the experiments, and the control group was prevented from running (i.e. the exercise wheel was locked). In adult saline-exposed animals, exercise was associated with increased long-term potentiation in the dentate gyrus. Unexpectedly, in dexamethasone-exposed animals, dentate gyrus long-term potentiation was facilitated, whereas long-term potentiation in CA1 was unaffected by prenatal dexamethasone or by 10 or 21 days of voluntary running. Irrespective of age, prenatal dexamethasone and running had limited effects on synaptic transmission and presynaptic release in CA1 and dentate gyrus. In summary, running facilitates dentate gyrus long-term potentiation in adult animals that resembles the effects of prenatal dexamethasone.

The α7 nicotinic acetylcholine receptors regulate hippocampal adult-neurogenesis in a sexually dimorphic fashion

Disruption in cholinergic signaling has been linked to many environmental and/or pathological conditions known to modify adult neurogenesis. The α7 nAChRs are in the family of cys-loop receptor channels which have been shown to be neuroprotective in adult neurons and are thought to be critical for survival and integration of immature neurons. However, in developing neurons, poor calcium buffering may cause α7 nAChR activation to be neurotoxic. To investigate whether the α7 nAChR regulates neurogenesis in the hippocampus, we used a combination of mouse genetics and imaging to quantify neural stem cell (NSC) densities located in the dentate gyrus of adult mice. In addition, we considered whether the loss of α7 nAChRs had functional consequences on a spatial discrimination task that is thought to rely on pattern separation mechanisms. We found that the loss of α7 nAChRs resulted in increased neurogenesis in male mice only, while female mice showed increased cell divisions and intermediate progenitors but no change in neurogenesis. Knocking out the α7 nAChR from nestin⁺ NSCs and their progeny showed signaling in these cells contributes to regulating neurogenesis. In addition, male, but not female, mice lacking α7 nAChRs performed significantly worse in the spatial discrimination task. This task was sexually dimorphic in wild-type mice, but not in the absence of α7 nAChRs. We conclude that α7 nAChRs regulate adult neurogenesis and impact spatial discrimination function in male, but not female mice, via a mechanism involving nestin⁺ NSCs and their progeny.

Placental glucocorticoid receptor and 11β-hydroxysteroid dehydrogenase-2 recruitment indicates impact of prenatal adversity upon postnatal development in mice

Prenatal stress may increase concentrations of maternal glucocorticoids, which restrict fetal growth, with variable impact upon postnatal development. Among key regulators of stress hormone effects are the glucocorticoid receptor (GR) and 11β-hydroxysteroid dehydrogenase-2 (11βHSD2), the enzyme that inactivates glucocorticoid. This study utilized mice selectively bred for social dominance (Dom) or submissiveness (Sub), respectively exhibiting resilience or sensitivity to stress, to test whether stress-induced alterations in placental GR and 11βHSD2 protein expression may mediate divergent effects of prenatal adversity upon postnatal development. Pregnant Dom and Sub dams underwent prenatal restraint stress (PRS) for 45 min on gestational days (GD) 15-17. PRS induced a similar spike in serum corticosterone concentrations of dams from each strain on GD15 (p < .001, n = 8), and impaired fetal growth (p < .01, n = 5 litters), although Dom placentae were larger than Sub placentae (p < .01). Among placentae from Dom dams, PRS elevated protein contents of both GR (p < .05, n = 5 litters) and 11βHSD2 (p < .01) on GD19. In contrast, GR contents were reduced among placentae from PRS-exposed Sub mice (p < .01), without changes in 11βHSD2 content. Correspondingly, Dom PRS pup growth recovered by PND14, yet Sub PRS pups remained underweight into adolescence (p < .0001, n = 40 pups). Thus, prenatal stress more strongly increased placental GR and 11βHSD2 levels among Dom mice than in Subs. Increased GR may improve placental function and up-regulate 11βHSD2 expression, protecting fetuses from effects of prenatal stress upon postnatal development. Placental recruitment of GR and 11βHSD2 are potential markers of stress-induced developmental disorders, in accordance with maternal resilience or sensitivity to stress.

Corticosterone response to gestational stress and postpartum memory function in mice

Maternal stress is a common adversity during pregnancy. Gestational corticosterone alternations are thought to contribute to the etiology of postpartum behavioral disturbances. However, the impact of stress during pregnancy, in particular noise exposure, on gestational corticosterone fluctuations and spatial cognition in postpartum mice has not been fully understood yet. We hypothesized that noise exposure during pregnancy negatively affects gestational corticosterone levels and postpartum memory function in the dams similar to the physical stressors. Pregnant C57BL/6 mice were randomly assigned to either one of two stress conditions or a control condition. The noise stress (NS) was induced by presenting a loud intermittent 3000 Hz frequency on gestational days (GDs) 12, 14, and 16 for 24 hours, whereas the physical stress (PS) consisted of restraint and exposure to an elevated platform on GDs 12–16. Plasma corticosterone level was collected on GDs 11 and 17, and Morris water task (MWT) was carried out 30 days after parturition. Compared to the control group, the level of corticosterone in the stressed groups was significantly increased on GD17 relative to GD11. Significantly longer swim time and lower swim speed were observed in both stressed groups relative to the control group. Probe time was significantly shorter in the NS group than the other groups. The delta corticosterone level was significantly correlated with the swim time as well as the probe time in the three groups. Given the results, the adverse effects of gestational noise exposure on the hypothalamic pituitary-adrenal (HPA) axis activation and postpartum spatial learning and memory function were as large as/ or a bit stronger than the physical stresses. The findings suggest the significance of conservation against loud noise exposure in daily living, as well as need to further notice to the different aspects of gestational stress in mothers’ behavior like offspring.

Modulation of the Hypothalamic-Pituitary-Adrenal Axis by Early Life Stress Exposure

Exposure to stress during critical periods in development can have severe long-term consequences, increasing overall risk on psychopathology. One of the key stress response systems mediating these long-term effects of stress is the hypothalamic-pituitary-adrenal (HPA) axis; a cascade of central and peripheral events resulting in the release of corticosteroids from the adrenal glands. Activation of the HPA-axis affects brain functioning to ensure a proper behavioral response to the stressor, but stress-induced (mal)adaptation of the HPA-axis' functional maturation may provide a mechanistic basis for the altered stress susceptibility later in life. Development of the HPA-axis and the brain regions involved in its regulation starts prenatally and continues after birth, and is protected by several mechanisms preventing corticosteroid over-exposure to the maturing brain. Nevertheless, early life stress (ELS) exposure has been reported to have numerous consequences on HPA-axis function in adulthood, affecting both its basal and stress-induced activity. According to the match/mismatch theory, encountering ELS prepares an organism for similar ("matching") adversities during adulthood, while a mismatching environment results in an increased susceptibility to psychopathology, indicating that ELS can exert either beneficial or disadvantageous effects depending on the environmental context. Here, we review studies investigating the mechanistic underpinnings of the ELS-induced alterations in the structural and functional development of the HPA-axis and its key external regulators (amygdala, hippocampus, and prefrontal cortex). The effects of ELS appear highly dependent on the developmental time window affected, the sex of the offspring, and the developmental stage at which effects are assessed. Albeit by distinct mechanisms, ELS induced by prenatal stressors, maternal separation, or the limited nesting model inducing fragmented maternal care, typically results in HPA-axis hyper-reactivity in adulthood, as also found in major depression. This hyper-activity is related to increased corticotrophin-releasing hormone signaling and impaired glucocorticoid receptor-mediated negative feedback. In contrast, initial evidence for HPA-axis hypo-reactivity is observed for early social deprivation, potentially reflecting the abnormal HPA-axis function as observed in post-traumatic stress disorder, and future studies should investigate its neural/neuroendocrine foundation in further detail. Interestingly, experiencing additional (chronic) stress in adulthood seems to normalize these alterations in HPA-axis function, supporting the match/mismatch theory.

Maternal administration of magnesium sulfate promotes cell proliferation in hippocampus dentate gyrus in offspring mice after exposing to prenatal stress

Prenatal stress (PS) inhibits cell proliferation in the hippocampal dentate gyrus (DG), which is related to hippocampal anatomy and function abnormality. The aim of the study was to investigate the effects of magnesium sulfate (MgSO₄) on PS-induced cell proliferation suppression in offspring during embryonic stage and postnatal spatial learning. MgSO₄ administration was performed after PS treatment on pregnant mice. Mice were randomly divided into four groups: non-PS or PS maternal mice injected with MgSO₄ or saline (P+NS, P+MG, C+MG and C+NS group). Corticosterone was collected from amniotic fluid of mother mice on day 17 of embryonic stage (E17). The ability for spatial learning and memory of pups postnatal 3 week was evaluated using water maze assay. Additionally, cell proliferation was detected by assessing the expression of Ki67 using immunohistochemistry in mice fetuses or pups. PS significantly increased corticosterone level in amniotic fluid (P<0.05) and impaired the spatial learning and memory (P+NS vs C+NS of latency time and track path length: P<0.05) of offspring on postnatal day 21. However, MgSO₄ administration could reverse PS-induced spatial learning and memory disability (P+MG vs P+NS, P<0.05). Additionally, PS reduced the number of Ki67-positive cell in hippocampal DG on E17, E19 and postnatal day 21 (P+NS vs C+NS, P<0.05), which were also abrogated by maternal administration of MgSO₄ (P+MG vs P+NS, P<0.05). Collectively, prenatal administration of MgSO₄ can reverse PS-induced reduction of cell proliferation in hippocampal DG during embryonic stage and postnatal spatial learning.

Plasticity in the olfactory bulb of the maternal mouse is prevented by gestational stress

Maternal stress is associated with an altered mother-infant relationship that endangers offspring development, leading to emotional/behavioral problems. However, little research has investigated the stress-induced alterations of the maternal brain that could underlie such a disruption of mother-infant bonding. Olfactory cues play an extensive role in the coordination of mother-infant interactions, suggesting that motherhood may be associated to enhanced olfactory performances, and that this effect may be abolished by maternal stress. To test this hypothesis, we analyzed the impact of motherhood under normal conditions or after gestational stress on olfactory functions in C57BL/6 J mice. We report that gestational stress alters maternal behavior and prevents both mothers' ability to discriminate pup odors and motherhood-induced enhancement in odor memory. We investigated adult bulbar neurogenesis as a potential mechanism of the enhanced olfactory function in mothers and found that motherhood was associated with an increased complexity of the dendritic tree of newborn neurons. This motherhood-evoked remodeling was totally prevented by gestational stress. Altogether, our results may thus provide insight into the neural changes that could contribute to altered maternal behavior in stressed mothers.

Ketamine Affects the Neurogenesis of the Hippocampal Dentate Gyrus in 7-Day-Old Rats

Ketamine has been reported to cause neonatal neurotoxicity via a neuronal apoptosis mechanism; however, no in vivo research has reported whether ketamine could affect postnatal neurogenesis in the hippocampal dentate gyrus (DG). A growing number of experiments suggest that postnatal hippocampal neurogenesis is the foundation of maintaining normal hippocampus function into adulthood. Therefore, this study investigated the effect of ketamine on hippocampal neurogenesis. Male Sprague-Dawley rats were divided into two groups: the control group (equal volume of normal saline), and the ketamine-anesthesia group (40 mg/kg ketamine in four injections at 1 h intervals). The S-phase marker 5-bromodeoxyuridine (BrdU) was administered after ketamine exposure to postnatal day 7 (PND-7) rats, and the neurogenesis in the hippocampal DG was assessed using single- or double-immunofluorescence staining. The expression of GFAP in the hippocampal DG was measured by western blot analysis. Spatial reference memory was tested by Morris water maze at 2 months after PND-7 rats exposed to ketamine treatment. The present results showed that neonatal ketamine exposure significantly inhibited neural stem cell (NSC) proliferation, decreased astrocytic differentiation, and markedly enhanced neuronal differentiation. The disruptive effect of ketamine on the proliferation and differentiation of NSCs lasted at least 1 week and disappeared by 2 weeks after ketamine exposure. Moreover, the migration of newborn neurons in the granule cell layer and the growth of astrocytes in the hippocampal DG were inhibited by ketamine on PND-37 and PND-44. Finally, ketamine caused a deficit in hippocampal-dependent spatial reference memory tasks at 2 months old. Our results suggested that ketamine may interfere with hippocampal neurogenesis and long-term neurocognitive function in PND-7 rats. These findings may provide a new perspective to explain the adult neurocognitive dysfunction induced by neonatal ketamine exposure.

Effects of postnatal alcohol exposure on hippocampal gene expression and learning in adult mice

Fetal alcohol syndrome (FAS) is a condition resulting from excessive drinking by pregnant women. Symptoms of FAS include abnormal facial features, stunted growth, intellectual deficits and attentional dysfunction. Many studies have investigated FAS, but its underlying mechanisms remain unknown. This study evaluated the relationship between alcohol exposure during the synaptogenesis period in postnatal mice and subsequent cognitive function in adult mice. We delivered two injections, separated by 2 h, of ethanol (3 g/kg, ethanol/saline, 20% v/v) to ICR mice on postnatal day 7. After 10 weeks, we conducted a behavioral test, sacrificed the animals, harvested brain tissue and analyzed hippocampal gene expression using a microarray. In ethanol-treated mice, there was a reduction in brain size and decreased neuronal cell number in the cortex, and also cognitive impairment. cDNA microarray results indicated that 1,548 genes showed a > 2-fold decrease in expression relative to control, whereas 974 genes showed a > 2-fold increase in expression relative to control. Many of these genes were related to signal transduction, synaptogenesis and cell membrane formation, which are highlighted in our findings.

Long-term effects of early life stress exposure: Role of epigenetic mechanisms

Stress is an adaptive response to demands of the environment and thus essential for survival. Exposure to stress during the first years of life has been shown to have profound effects on the growth and development of an adult individual. There are evidences demonstrating that stressful experiences during gestation or in early life can lead to enhanced susceptibility to mental disorders. Early-life stress triggers hypothalamic-pituitary-adrenocortical (HPA) axis activation and the associated neurochemical reactions following glucocorticoid release are accompanied by a rapid physiological response. An excessive response may affect the developing brain resulting in neurobehavioral and neurochemical changes later in life. This article reviews the data from experimental studies aimed to investigate hormonal, functional, molecular and epigenetic mechanisms involved in the stress response during early-life programming. We think these studies might prove useful for the identification of novel pharmacological targets for more effective treatments of mental disorders.

Developmental pathways from prenatal tobacco and stress exposure to behavioral disinhibition

Prenatal tobacco exposure (PTE) and prenatal stress exposure (PSE) both have been linked to externalizing behavior, although their effects generally have been considered in isolation. Here, we aimed to characterize the joint or interactive roles of PTE and PSE in early developmental pathways to behavioral disinhibition, a profile of cognitive and behavioral under-control that presages severe externalizing behavior. As part of a prospective, longitudinal study, 296 children were assessed at a mean age of 5 years. Exposures were assessed via repeated interviews across the prenatal period and bioassays of cotinine were obtained. Behavioral disinhibition was assessed using temperament measures in infancy, performance-based executive control tasks and measures of disruptive and inattentive behavior. PSE was associated with a higher probability of difficult temperament in infancy. Each exposure independently predicted poorer executive control at age 5 years. Difficult temperament and executive control difficulties in turn predicted elevated levels of disruptive behavior, although links from PTE and PSE to parent-reported attention problems were less robust. Children who experienced these prenatal exposures in conjunction with higher postnatal stress exposure showed the lowest executive control and highest levels of disruptive behavior. Findings highlight the compounding adverse impact of PTE and PSE on children's behavioral trajectories. Given their high concordance, prenatal health campaigns should target these exposures in tandem.

Regulation of Adult Neurogenesis and Plasticity by (Early) Stress, Glucocorticoids, and Inflammation

Exposure to stress is one of the best-known negative regulators of adult neurogenesis (AN). We discuss changes in neurogenesis in relation to exposure to stress, glucocorticoid hormones, and inflammation, with a particular focus on early development and on lasting effects of stress. Although the effects of acute and mild stress on AN are generally brief and can be quickly overcome, chronic exposure or more severe forms of stress can induce longer lasting reductions in neurogenesis that can, however, in part, be overcome by subsequent exposure to exercise, drugs targeting the stress system, and some antidepressants. Exposure to stress, particularly during the sensitive period of early life, may (re)program brain plasticity, in particular, in the hippocampus. This may increase the risk to develop cognitive or anxiety symptoms, common to brain diseases like dementia and depression in which plasticity changes occur, and a normalization of neurogenesis may be required for a successful treatment response and recovery.

Neural stem cells respond to stress hormones: distinguishing beneficial from detrimental stress

Neural stem cells (NSCs), the progenitors of the nervous system, control distinct, position-specific functions and are critically involved in the maintenance of homeostasis in the brain. The responses of these cells to various stressful stimuli are shaped by genetic, epigenetic, and environmental factors via mechanisms that are age and developmental stage-dependent and still remain, to a great extent, elusive. Increasing evidence advocates for the beneficial impact of the stress response in various settings, complementing the extensive number of studies on the detrimental effects of stress, particularly in the developing brain. In this review, we discuss suggested mechanisms mediating both the beneficial and detrimental effects of stressors on NSC activity across the lifespan. We focus on the specific effects of secreted factors and we propose NSCs as a “sensor,” capable of distinguishing among the different stressors and adapting its functions accordingly. All the above suggest the intriguing hypothesis that NSCs are an important part of the adaptive response to stressors via direct and indirect, specific mechanisms.

Influence of sex and stress exposure across the lifespan on endophenotypes of depression: focus on behavior, glucocorticoids, and hippocampus

Sex differences exist in vulnerability, symptoms, and treatment of many neuropsychiatric disorders. In this review, we discuss both preclinical and clinical research that investigates how sex influences depression endophenotypes at the behavioral, neuroendocrine, and neural levels across the lifespan. Chronic exposure to stress is a risk factor for depression and we discuss how stress during the prenatal, postnatal, and adolescent periods differentially affects males and females depending on the method of stress and metric examined. Given that the integrity of the hippocampus is compromised in depression, we specifically focus on sex differences in how hippocampal plasticity is affected by stress and depression across the lifespan. In addition, we examine how female physiology predisposes depression in adulthood, specifically in postpartum and perimenopausal periods. Finally, we discuss the underrepresentation of women in both preclinical and clinical research and how this limits our understanding of sex differences in vulnerability, presentation, and treatment of depression.

Influences of prenatal and postnatal stress on adult hippocampal neurogenesis: the double neurogenic niche hypothesis

Adult hippocampal neurogenesis (AHN) is involved in learning, memory, and stress, and plays a significant role in neurodegenerative and psychiatric disorders. As an age-dependent process, AHN is largely influenced by changes that occur during the pre- and postnatal stages of brain development, and constitutes an important field of research. This review examines the current knowledge regarding the regulators of AHN and the influence of prenatal and postnatal stress on later AHN. In addition, a hypothesis is presented suggesting that each kind of stress influences a specific neurogenic pool, developmental or postnatal, that later becomes a precursor with important repercussions for AHN. This hypothesis is referred to as "the double neurogenic niche hypothesis." Discovering what receptors, transcription factors, or genes are specifically activated by different stressors is proposed as an essential line of future research in the field. Such knowledge shall constitute an important starting point toward the goal of modifying AHN in neurodegenerative or psychiatric diseases.

Distinct neurobehavioral dysfunction based on the timing of developmental binge-like alcohol exposure

Gestational exposure to alcohol can result in long-lasting behavioral deficiencies generally described as fetal alcohol spectrum disorder (FASD). FASD-modeled rodent studies of acute ethanol exposure typically select one developmental window to simulate a specific context equivalent of human embryogenesis, and study consequences of ethanol exposure within that particular developmental epoch. Exposure timing is likely a large determinant in the neurobehavioral consequence of early ethanol exposure, as each brain region is variably susceptible to ethanol cytotoxicity and has unique sensitive periods in their development. We made a parallel comparison of the long-term effects of single-day binge ethanol at either embryonic day 8 (E8) or postnatal day 7 (P7) in male and female mice, and here demonstrate the differential long-term impacts on neuroanatomy, behavior and in vivo electrophysiology of two systems with very different developmental trajectories. The significant long-term differences in odor-evoked activity, local circuit inhibition, and spontaneous coherence between brain regions in the olfacto-hippocampal pathway that were found as a result of developmental ethanol exposure, varied based on insult timing. Long-term effects on cell proliferation and interneuron cell density were also found to vary by insult timing as well as by region. Finally, spatial memory performance and object exploration were affected in P7-exposed mice, but not E8-exposed mice. Our physiology and behavioral results are conceptually coherent with the neuroanatomical data attained from these same mice. Our results recognize both variable and shared effects of ethanol exposure timing on long-term circuit function and their supported behavior.

Age- and sex-dependent effects of early life stress on hippocampal neurogenesis

Early life stress is a well-documented risk factor for the development of psychopathology in genetically predisposed individuals. As it is hard to study how early life stress impacts human brain structure and function, various animal models have been developed to address this issue. The models discussed here reveal that perinatal stress in rodents exerts lasting effects on the stress system as well as on the structure and function of the brain. One of the structural parameters strongly affected by perinatal stress is adult hippocampal neurogenesis. Based on compiled literature data, we report that postnatal stress slightly enhances neurogenesis until the onset of puberty in male rats; when animals reach adulthood, neurogenesis is reduced as a consequence of perinatal stress. By contrast, female rats show a prominent reduction in neurogenesis prior to the onset of puberty, but this effect subsides when animals reach young adulthood. We further present preliminary data that transient treatment with a glucocorticoid receptor antagonist can normalize cell proliferation in maternally deprived female rats, while the compound had no effect in non-deprived rats. Taken together, the data show that neurogenesis is affected by early life stress in an age- and sex-dependent manner and that normalization may be possible during critical stages of brain development.