Postnatal maternal separation enhances tonic GABA current of cortical layer 5 pyramidal neurons in juvenile rats and promotes genesis of GABAergic neurons in neocortical molecular layer and subventricular zone in adult rats

Spatiotemporal Mapping of the Oxytocin Receptor at Single-Cell Resolution in the Postnatally Developing Mouse Brain

The oxytocin receptor (OXTR) has garnered increasing attention for its role in regulating both mature behaviors and brain development. It has been established that OXTR mediates a range of effects that are region-specific or period-specific. However, the current studies of OXTR expression patterns in mice only provide limited help due to limitations in resolution. Therefore, our objective was to generate a comprehensive, high-resolution spatiotemporal expression map of Oxtr mRNA across the entire developing mouse brain. We applied RNAscope in situ hybridization to investigate the spatiotemporal expression pattern of Oxtr in the brains of male mice at six distinct postnatal developmental stages (P7, P14, P21, P28, P42, P56). We provide detailed descriptions of Oxtr expression patterns in key brain regions, including the cortex, basal forebrain, hippocampus, and amygdaloid complex, with a focus on the precise localization of Oxtr+ cells and the variance of expression between different neurons. Furthermore, we identified some neuronal populations with high Oxtr expression levels that have been little studied, including glutamatergic neurons in the ventral dentate gyrus, Vgat+Oxtr+ cells in the basal forebrain, and GABAergic neurons in layers 4/5 of the cortex. Our study provides a novel perspective for understanding the distribution of Oxtr and encourages further investigations into its functions.

Neonatal Maternal Separation Induces Sexual Dimorphism in Brain Development: The Influence on Amino Acid Levels and Cognitive Disorders

Repeated maternal separation (MS) is a useful experimental model in rodents for studying the long-term influence of early-life stress on brain neurophysiology. In our work, we assessed the effect of repeated MS (postnatal day (PND)1-21, 180 min/day) on the postnatal development of rat brain regions involved in memory using proton magnetic resonance spectroscopy (1HMRS) for tissue volume and the level of amino acids such as glutamate, aspartate, glutamine, glycine and gamma-aminobutyric acid (GABA) in the hippocampus. We assessed whether these effects are sex dependent. We also use novel object recognition (NOR) task to examine the effect of MS on memory and the effect of ethanol on it. Finally, we attempted to ameliorate postnatal stress-induced memory deficits by using VU-29, a positive allosteric modulator (PAM) of the metabotropic glutamate type 5 (mGlu5) receptor. In males, we noted deficits in the levels of glutamate, glycine and glutamine and increases in GABA in the hippocampus. In addition, the values of perirhinal cortex, prefrontal cortex and insular cortex and CA3 were decreased in these animals. MS females, in contrast, demonstrated significant increase in glutamate levels and decrease in GABA levels in the hippocampus. Here, the CA1 values alone were increased. VU-29 administration ameliorated these cognitive deficits. Thus, MS stress disturbs amino acids levels mainly in the hippocampus of adult male rats, and enhancement of glutamate neurotransmission reversed recognition memory deficits in these animals.

Low Fos expression in newly generated neurons of the main and accessory olfactory bulb following single maternal separation

The main and accessory olfactory bulbs (MOB and AOB) are unique in that they produce new neurons throughout adulthood. Despite the recent knowledge about the involvement of postnatally generated cells in several aspects of olfaction, the functional role of these neurons is still not sufficiently understood. The function of newly generated olfactory bulb neurons is primarily investigated in relation to activities related to smell. Stress-induced activation of new olfactory neurons has not yet been studied. Thus, our work was aimed to investigate whether a stressful event, such as maternal separation (MS) can induce Fos expression in postnatally-born neurons in the MOB and AOB. Rat pups were exposed to single maternal separation (SMS) for 2 h at the postnatal days: P7, P14, and P21. Quantification of immunohistochemically labeled Fos + cells revealed that exposure to SMS in different age stages during the first postnatal month stimulates activity in cells of individual MOB/AOB layers in an age-dependent manner. In order to find out whether newly generated cells in the MOB/AOB could express Fos protein as a response to SMS, newborn rats were administrated with the marker of proliferation, bromodeoxyuridine (BrdU) at P0, and three weeks later (at P21) colocalization of Fos and BrdU in the neurons of the MOB and AOB was assessed. Quantitative analysis of BrdU/Fos double-labeled cells showed that Fos is expressed only in a small number of postnatally generated cells within the MOB/AOB. Our results indicate that postnatally generated MOB/AOB neurons are less sensitive to stress caused by MS than preexisting ones. LAY SUMMARY Our results showed that single maternal separation (SMS) is a stressful event that in age-dependent manner stimulates cellular activity in the main and accessory olfactory bulb (AOB) - the structures dedicated to odor information processing. The low level of Fos expression in newborn neurons of the main and accessory bulb indicates that postnatally generated cells are less sensitive to neonatal stress than preexisting neurons.

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.

Developmental expression of anxiety and depressive behaviours after prenatal predator exposure and early life homecage enhancement

Stressful events during gestation can have sex-specific effects on brain and behaviour, and may contribute to some of the differences observed in adult stress responding and psychopathology. We investigated the impact of a novel repeated prenatal psychological stress (prenatal predator exposure - PPS) during the last week of gestation in rats on offspring behaviours related to social interaction (play behaviour), open field test (OFT), forced swim test (FST) and sucrose preference test (SP) during the juvenile period and in adulthood. We further examined the role of postnatal environmental, using an enhanced housing condition (EHC), to prevent/rescue any changes. Some effects on anxiety, anhedonia, and stress-related coping behaviours (e.g., OFT, SP and OFT) did not emerge until adulthood. PPS increased OFT anxiety behaviours in adult males, and some OFT and SP behaviours in adult females. Contrary to this, EHC had few independent effects; most were apparent only when combined with PPS. In keeping with age-group differences, juvenile behaviours did not necessarily predict the same adult behaviours although juvenile OFT rearing and freezing, and juvenile FST immobility did predict adult FST immobility and sucrose preference, suggesting that some aspects of depressive behaviours may emerge early and predict adult vulnerability or coping behaviours. Together, these results suggest an important, though complex, role for early life psychological stressors and early life behaviours in creating an adult vulnerability to anxiety or depressive disorders and that environmental factors further modulate the effects of the prenatal stressors.

Exposure to early adversity: Points of cross-species translation that can lead to improved understanding of depression

The relationship between developmental exposure to adversity and affective disorders is reviewed. Adversity discussed herein includes physical and sexual abuse, neglect, or loss of a caregiver in humans. While these stressors can occur at any point during development, the unique temporal relationship to specific depressive symptoms was the focus of discussion. Further influences of stress exposure during sensitive periods can vary by gender and duration of abuse as well. Data from animal studies are presented to provide greater translational and causal understanding of how sensitive periods, different types of psychosocial stressors, and sex interact to produce depressive-like behaviors. Findings from maternal separation, isolation rearing, chronic variable stress, and peer-peer rearing paradigms clarify interpretation about how various depressive behaviors are influenced by age of exposure. Depressive behaviors are broken down into the following categories: mood and affect, anhedonia, energy, working memory, sleep-wake, appetite changes, suicide, and general malaise. Cross-species evidence from humans, nonhuman primates, rats, and mice within each of these categories is discussed. In conclusion, sensitive periods for affective-related behaviors (anxiety, mood, and controllability) occur earlier in life, while other aspects of depression are associated with adversity later during adolescence.

Cortical and subcortical gamma amino acid butyric acid deficits in anxiety and stress disorders: Clinical implications

Anxiety and stress disorders are a major public health issue. However, their pathophysiology is still unclear. The gamma amino acid butyric acid (GABA) neurochemical system has been strongly implicated in their pathogenesis and treatment by numerous preclinical and clinical studies, the most recent of which have been highlighted and critical review in this paper. Changes in cortical GABA appear related to normal personality styles and responses to stress. While there is accumulating animal and human neuroimaging evidence of cortical and subcortical GABA deficits across a number of anxiety conditions, a clear pattern of findings in specific brain regions for a given disorder is yet to emerge. Neuropsychiatric conditions with anxiety as a clinical feature may have GABA deficits as an underlying feature. Different classes of anxiolytic therapies support GABA function, and this may be an area in which newer GABA neuroimaging techniques could soon offer more personalized therapy. Novel GABAergic pharmacotherapies in development offer potential improvements over current therapies in reducing sedative and physiologic dependency effects, while offering rapid anxiolysis.

Early life stress and hippocampal neurogenesis in the neonate: sexual dimorphism, long term consequences and possible mediators

Adverse early life experience decreases adult hippocampal neurogenesis and results in increased vulnerability to neuropsychiatric disorders. Despite that the effects of postnatal stress on neurogenesis have been widely studied in adult individuals, few efforts have been done to evaluate its immediate effects on the developing hippocampus. Moreover, it is not clear whether postnatal stress causes a differential impact in hippocampus development in male and female neonates that could be related to emotional deficits in adulthood. It has been proposed that the long term effects of early stress exposure rise from a persistent HPA axis activation during sensitive time windows; nevertheless the exact mechanisms and mediators remain unknown. Here, we summarize the immediate and late effects of early life stress on hippocampal neurogenesis in male and female rat pups, compare its later consequences in emotionality, and highlight some relevant mediator peptides that could be potentially involved in programming.