Single-Cell Transcriptional Changes in Hypothalamic Corticotropin-Releasing Factor-Expressing Neurons After Early-Life Adversity Inform Enduring Alterations in Vulnerabilities to Stress

The paraventricular nucleus of the hypothalamus: a key node in the control of behavioural states

The paraventricular nucleus (PVN) of the hypothalamus contains diverse populations of neuropeptide-producing neurons. These include neurons that synthesise oxytocin, vasopressin, corticotropin-releasing hormone, thyrotropin-releasing hormone and somatostatin. While it is well established that these neurons control the secretion of neuroendocrine hormones, there is growing evidence that they also control the expression of important homeostatic behaviours. Here we review recent data showing a critical role of PVN neurons in controlling arousal, social behaviour, defensive behaviour and pain. Collectively, this suggests that the PVN is a key node in a wider neural network controlling behavioural states.

Infant Maltreatment Induces Early Alterations in Adrenal Glands and Stress Response in Juvenile Rat Offspring

Chronic stressors, such as infant maltreatment during early development, can have long-lasting effects on an individual's health and stress-coping abilities across the lifespan. In this study, we applied the scarcity-adversity model (SAM) to Wistar rats from postnatal days (PND) 8 to 12 to evaluate the impact of scarcity condition on adrenal glands morphology and stress response in juvenile offspring (PND 21-35) of both sexes. Our results demonstrate that SAM exposure leads to significant alterations in the adrenal glands of offspring. Notably, the cortex-to-medulla ratio was reduced in SAM pups starting at PND 28. Morphological analysis of the adrenal cortex revealed an increased thickness of the undifferentiated zone, suggesting enhanced cell proliferation, alongside a transient reduction in the thickness of the zona fasciculata, where glucocorticoid-synthesizing cells are located. Additionally, we observed a reduction in the cytoplasmic diameter of zona fasciculata cells, coupled with an increase in the thickness of blood capillaries in this region in SAM offspring of both sexes. An increase in BrdU + cells in the cortex at PND 28 was observed in male SAM pups, but not in females. Endocrine assessments revealed a marked increase in corticosterone levels following stress induced by movement restriction in juvenile SAM offspring of both sexes. In conclusion, our findings underscore the significant impact of early-life stress on adrenal gland morphology and stress responsiveness, with potential long-term consequences for health and stress-coping mechanisms in offspring.

Single-cell genomics meets systems neuroscience: Insights from mapping the brain circuitry of stress

Responses to external and internal dangers is essential for survival and homeostatic regulation. Hypothalamic corticotropin-releasing hormone neurons (CRHNs) play a pivotal role in regulating neuroendocrine responses to fear and stress. In recent years, the application of neurogenetic tools, such as fiber photometry, chemogenetics and optogenetics, have provided new insights into the dynamic neuronal responses of CRHNs during stressful events, offering new perspectives into their functional significance in mediating neurobehavioural responses to stress. Transsynaptic viral tracers have facilitated the comprehensive mapping of neuronal inputs to CRHNs. Furthermore, the development and application of innovative single-cell genomic tools combined with viral tracing have begun to pave the way for a deeper understanding of the transcriptional profiles of neural circuit components, enabling molecular-anatomical circuit mapping. Here, I will discuss how these systems neuroscience approaches and novel single-cell genomic methods are advancing the molecular and functional mapping of stress neurocircuits, their associated challenges and future directions.

Molecular and functional mapping of the neuroendocrine hypothalamus: a new era begins

BACKGROUND

Recent advances in neuroscience tools for single-cell molecular profiling of brain neurons have revealed an enormous spectrum of neuronal subpopulations within the neuroendocrine hypothalamus, highlighting the remarkable molecular and cellular heterogeneity of this brain area.

RATIONALE

Neuronal diversity in the hypothalamus reflects the high functional plasticity of this brain area, where multiple neuronal populations flexibly integrate a variety of physiological outputs, including energy balance, stress and fertility, through crosstalk mechanisms with peripheral hormones. Intrinsic functional heterogeneity is also observed within classically 'defined' subpopulations of neuroendocrine neurons, including subtypes with distinct neurochemical signatures, spatial organisation and responsiveness to hormonal cues.

AIM

The aim of this review is to critically evaluate past and current research on the functional diversity of hypothalamic neuroendocrine neurons and their plasticity. It focuses on how this neuronal plasticity in this brain area relates to metabolic control, feeding regulation and interactions with stress and fertility-related neural circuits.

CONCLUSION

Our analysis provides an original framework for improving our understanding of the hypothalamic regulation of hormone function and the development of neuroendocrine diseases.

Enduring memory consequences of early-life stress / adversity: Structural, synaptic, molecular and epigenetic mechanisms

Adverse early life experiences are strongly associated with reduced cognitive function throughout life. The link is strong in many human studies, but these do not enable assigning causality, and the limited access to the live human brain can impede establishing the mechanisms by which early-life adversity (ELA) may induce cognitive problems. In experimental models, artificially imposed chronic ELA/stress results in deficits in hippocampus dependent memory as well as increased vulnerability to the deleterious effects of adult stress on memory. This causal relation of ELA and life-long memory impairments provides a framework to probe the mechanisms by which ELA may lead to human cognitive problems. Here we focus on the consequences of a one-week exposure to adversity during early postnatal life in the rodent, the spectrum of the ensuing memory deficits, and the mechanisms responsible. We highlight molecular, cellular and circuit mechanisms using convergent trans-disciplinary approaches aiming to enable translation of the discoveries in experimental models to the clinic.

Hypothalamic sex-specific metabolic shift by canagliflozin during aging

The hypothalamus undergoes significant changes with aging and plays crucial roles in age-related metabolic alterations. Sodium-glucose co-transporter 2 inhibitors (SGLT2i) are anti-diabetic agents that promote glucose excretion, and metabolic homeostasis. Recent studies have shown that a SGLT2i, Canagliflozin (Cana), can extend the median survival of genetically heterogeneous UM-HET3 male mice and improve central metabolic control via increases in hypothalamic insulin responsiveness in aged males, as well as reduced age-associated hypothalamic inflammation. We studied the long- and short-term effects of Cana on hypothalamic metabolic control in UM-HET3 mice. Starting the treatment from 7 months of age, we show that 4 weeks of Cana treatment significantly reduced body weight and fat mass in male but not female mice that was associated with enhanced glucose tolerance and insulin sensitivity observed by 12 months. Indirect calorimetry showed that Cana treatment increased energy expenditure in male, but not female mice, at 12 months of age. Long-term Cana treatment increased metabolic rates in both sexes, and markedly increasing formation of both orexigenic and anorexigenic projections to the paraventricular nucleus of the hypothalamus (PVH) mostly in females by 25 months. Hypothalamic RNA-sequencing analysis revealed increased sex-specific genes and signaling pathways related to insulin signaling, glycogen catabolic pathway, neuropeptide signaling, and mitochondrial function upregulated by Cana, with males showing a more pronounced and sustained effect on metabolic pathways at both age groups. Overall, our data provide critical evidence for sex-specific mechanisms that are affected by Cana during aging suggesting key targets of hypothalamic Cana-induced neuroprotection for metabolic control.

Neonatal limited bedding and nesting experience may lead to a sex-dependent increase in panic-like defensive behaviours in adult mice

In humans, adverse physical and/or psychological traumas in childhood may predispose to developing psychiatric disorders in adulthood, including panic disorder. To model early life adversity in mice, we subjected male and female C57BL/6 J mice to a limited bedding and nesting (LBN) protocol between postnatal days 2-9 and investigated its effect on responsiveness to panicogenic challenges in adulthood. Panic-like escape behaviour was assessed during exposure to a high concentration of CO2 (20%) or in the beetle mania task (BMT), used to model respiratory and non-respiratory-related types of panic respectively. Neonatal exposure to LBN increased panic-like jumping during the CO2 challenge in male but not female mice. In an initial pharmacological validation of the BMT as a panic-inducing paradigm, undirected jumping and horizontal escape behaviours were reduced significantly by the panicolytic alprazolam (0.05 and 0.1mg.kg-1 i.p.) whilst tolerance to the close proximity of the aversive robo-beetle increased. The anxiolytic diazepam (1 mg.kg-1 i.p.) reduced only the number of horizontal escape attempts. In both sexes, previous experience of LBN significantly enhanced the number of horizontal escape episodes, indicating a pro-panic phenotype. Directed escape to access a safe ledge on the wall of the test arena, which was seen only in males, was also reduced significantly following LBN. These findings indicate that early life adversity produced by fragmented and unpredictable maternal care promotes a sex-specific increase in susceptibility to panic-like behaviour in adulthood. Whilst non-respiratory-related panic-like behaviour was enhanced in both sexes, females were resilient to respiratory-related challenges.

The impact of pubertal stress and adult hormone exposure on the transcriptome of the developing hypothalamus

Why individuals suffer negative consequences following stress is a complex phenomenon that is dictated by individual factors, the timing of stress within the lifespan, and when in the lifespan the consequences are measured. Women who undergo adverse childhood experiences are at risk for lasting biological consequences, including affective and stress dysregulation. We have shown that pubertal adversity is associated with a blunted hypothalamic-pituitary-adrenal axis glucocorticoid response in peripartum humans and mice. In mice, our prior examination of the paraventricular nucleus (PVN) of the hypothalamus showed that pubertal stress led to an upregulation of baseline mRNA expression of six immediate early genes (IEGs) in the PVN of adult, pregnant mice. Separately, we showed that the pregnancy-associated hormone allopregnanolone is necessary and sufficient to produce the blunted stress response phenotype in pubertally stressed mice. In the current study, we further examined a potential mechanistic role for the IEGs in the PVN. We found that in pubertally stressed adult female, but not male, mice, intra-PVN allopregnanolone was sufficient to recapitulate the baseline IEG mRNA expression profile previously observed in pubertally stressed, pregnant mice. We also examined baseline IEG mRNA expression during adolescence, where we found that IEGs have developmental trajectories that showed sex-specific disruption by pubertal stress. Altogether, these data establish that IEGs may act as a key molecular switch involved in increased vulnerability to negative outcomes in adult, pubertally stressed animals. How the factors that produce vulnerability combine throughout the lifespan is key to our understanding of the etiology of stress-related disorders.

Long-Term Impact of Early-Life Stress on Serotonin Connectivity

BACKGROUND

Chronic childhood stress is a prominent risk factor for developing affective disorders, yet mechanisms underlying this association remain unclear. Maintenance of optimal serotonin (5-HT) levels during early postnatal development is critical for the maturation of brain circuits. Understanding the long-lasting effects of early-life stress (ELS) on serotonin-modulated brain connectivity is crucial to develop treatments for affective disorders arising from childhood stress.

METHODS

Using a mouse model of chronic developmental stress, we determined the long-lasting consequences of ELS on 5-HT circuits and behavior in females and males. Using FosTRAP mice, we cross-correlated regional c-Fos density to determine brain-wide functional connectivity of the raphe nucleus. We next performed in vivo fiber photometry to establish ELS-induced deficits in 5-HT dynamics and optogenetics to stimulate 5-HT release to improve behavior.

RESULTS

Adult female and male mice exposed to ELS showed heightened anxiety-like behavior. ELS further enhanced susceptibility to acute stress by disrupting the brain-wide functional connectivity of the raphe nucleus and the activity of 5-HT neuron population, in conjunction with increased orbitofrontal cortex (OFC) activity and disrupted 5-HT release in medial OFC. Optogenetic stimulation of 5-HT terminals in the medial OFC elicited an anxiolytic effect in ELS mice in a sex-dependent manner.

CONCLUSIONS

These findings suggest a significant disruption in 5-HT-modulated brain connectivity in response to ELS, with implications for sex-dependent vulnerability. The anxiolytic effect of the raphe-medial OFC circuit stimulation has potential implications for developing targeted stimulation-based treatments for affective disorders that arise from early life adversities.

An integrated single-cell RNA-seq atlas of the mouse hypothalamic paraventricular nucleus links transcriptomic and functional types

The hypothalamic paraventricular nucleus (PVN) is a highly complex brain region that is crucial for homeostatic regulation through neuroendocrine signaling, outflow of the autonomic nervous system, and projections to other brain areas. In the past years, single-cell datasets of the hypothalamus have contributed immensely to the current understanding of the diverse hypothalamic cellular composition. While the PVN has been adequately classified functionally, its molecular classification is currently still insufficient. To address this, we created a detailed atlas of PVN transcriptomic cell types by integrating various PVN single-cell datasets into a recently published hypothalamus single-cell transcriptome atlas. Furthermore, we functionally profiled transcriptomic cell types, based on relevant literature, existing retrograde tracing data, and existing single-cell data of a PVN-projection target region. Finally, we validated our findings with immunofluorescent stainings. In our PVN atlas dataset, we identify the well-known different neuropeptide types, each composed of multiple novel subtypes. We identify Avp-Tac1, Avp-Th, Oxt-Foxp1, Crh-Nr3c1, and Trh-Nfib as the most important neuroendocrine subtypes based on markers described in literature. To characterize the preautonomic functional population, we integrated a single-cell retrograde tracing study of spinally projecting preautonomic neurons into our PVN atlas. We identify these (presympathetic) neurons to cocluster with the Adarb2+ clusters in our dataset. Further, we identify the expression of receptors for Crh, Oxt, Penk, Sst, and Trh in the dorsal motor nucleus of the vagus, a key region that the pre-parasympathetic PVN neurons project to. Finally, we identify Trh-Ucn3 and Brs3-Adarb2 as some centrally projecting populations. In conclusion, our study presents a detailed overview of the transcriptomic cell types of the murine PVN and provides a first attempt to resolve functionality for the identified populations.

The impact of pubertal stress and adult hormone exposure on the transcriptome of the developing hypothalamus

Why individuals suffer negative consequences following stress is a complex phenomenon that is dictated by individual factors, the timing of stress within the lifespan, and when in the lifespan the consequences are measured. Women who undergo adverse childhood experiences are at risk for lasting biological consequences, including affective and stress dysregulation. We have shown that pubertal adversity is associated with a blunted hypothalamic-pituitary-adrenal axis glucocorticoid response in peripartum humans and mice. In mice, our prior examination of the paraventricular nucleus (PVN) of the hypothalamus showed that pubertal stress led to an upregulation of baseline mRNA expression of six immediate early genes (IEGs) in the PVN of adult, pregnant mice. Separately, we showed that the pregnancy-associated hormone allopregnanolone is necessary and sufficient to produce the blunted stress response phenotype in pubertally stressed mice. In the current study, we further examined a potential mechanistic role for the IEGs in the PVN. We found that in pubertally stressed adult female, but not male, mice, intra-PVN allopregnanolone was sufficient to recapitulate the baseline IEG mRNA expression profile previously observed in pubertally stressed, pregnant mice. We also examined baseline IEG mRNA expression during adolescence, where we found that IEGs have developmental trajectories that showed sex-specific disruption by pubertal stress. Altogether, these data establish that IEGs may act as a key molecular switch involved in increased vulnerability to negative outcomes in adult, pubertally stressed animals. How the factors that produce vulnerability combine throughout the lifespan is key to our understanding of the etiology of stress-related disorders.

Sex shapes cell-type-specific transcriptional signatures of stress exposure in the mouse hypothalamus

Stress-related psychiatric disorders and the stress system show prominent differences between males and females, as well as strongly divergent transcriptional changes. Despite several proposed mechanisms, we still lack the understanding of the molecular processes at play. Here, we explore the contribution of cell types to transcriptional sex dimorphism using single-cell RNA sequencing. We identify cell-type-specific signatures of acute restraint stress in the paraventricular nucleus of the hypothalamus, a central hub of the stress response, in male and female mice. Further, we show that a history of chronic mild stress alters these signatures in a sex-specific way, and we identify oligodendrocytes as a major target for these sex-specific effects. This dataset, which we provide as an online interactive app, offers the transcriptomes of thousands of individual cells as a molecular resource for an in-depth dissection of the interplay between cell types and sex on the mechanisms of the stress response.

From recent advances in underlying neurocircuitry of fear and anxiety to promising pharmacotherapies for PTSD: The saga of heart, sex and the developing brain

Available pharmacotherapies for anxiety disorders and post-traumatic stress-disorder (PTSD) have limited efficacy, but no new anxiolytic drug has been approved for treatment since the 1980s. In this issue of Neuropharmacology on "Fear, anxiety and PTSD: from cellular mechanisms to translational approaches", we review the currently recommended pharmacotherapy for PTSD and discuss promising pharmacotherapies being revisited or newly developed. Novel strategies for pharmaceuticals in PTSD treatment include the use of serotonergic psychedelics as low-dose adjunct therapies combined with psychotherapy. We also discuss the use of glucocorticoids targeting the temporal window shortly following trauma exposure to interfere with fear memory consolidation. Although many factors have impeded progress in pharmacotherapy development for anxiety disorders and PTSD, we highlight three: (1) the sparsity of preclinical studies investigating the neurobiology of fear processing in female animal models despite the higher prevalence of anxiety disorders in women, (2) the poor implementation of the knowledge of how stress affects fear circuitry development across the lifetime into clinical practice, and (3) our paucity of knowledge of canonical fear circuitry in adaptive vs. maladaptive fear processing. Finally, we emphasize the functional link between interoceptive signals and emotion regulation and discuss how these interoceptive signals may be an inroad into PTSD treatment, which is often accompanied by cardiovascular dysregulation. A better understanding of the neurobiological underpinnings of adaptive and maladaptive fear processing is critical for identifying risk factors that will spur the development of sex- and developmental trauma-specific interventions, ushering in a new era of precision medicine for anxiety disorders and PTSD.

Chronic intermittent ethanol exposure disrupts stress-related tripartite communication to impact affect-related behavioral selection in male rats

Alcohol use disorder (AUD) is characterized by loss of intake control, increased anxiety, and susceptibility to relapse inducing stressors. Both astrocytes and neurons contribute to behavioral and hormonal consequences of chronic intermittent ethanol (CIE) exposure in animal models. Details on how CIE disrupts hypothalamic neuro-glial communication, which mediates stress responses are lacking. We conducted a behavioral battery (grooming, open field, reactivity to a single, uncued foot-shock, intermittent-access two-bottle choice ethanol drinking) followed by Ca²⁺ imaging in ex-vivo slices of paraventricular nucleus of the hypothalamus (PVN) from male rats exposed to CIE vapor or air-exposed controls. Ca²⁺ signals were evaluated in response to norepinephrine (NE) with or without selective α-adrenergic receptor (αAR) or GluN2B-containing N-methyl-D-aspartate receptor (NMDAR) antagonists, followed by dexamethasone (DEX) to mock a pharmacological stress response. Expectedly, CIE rats had altered anxiety-like, rearing, grooming, and drinking behaviors. Importantly, NE-mediated reductions in Ca²⁺ event frequency were blunted in both CIE neurons and astrocytes. Administration of the selective α1AR antagonist, prazosin, reversed this CIE-induced dysfunction in both cell types. Additionally, the pharmacological stress protocol reversed the altered basal Ca²⁺ signaling profile of CIE astrocytes. Signaling changes in astrocytes in response to NE were correlated with anxiety-like behaviors, such as the grooming:rearing ratio, suggesting tripartite synaptic function plays a role in switching between exploratory and stress-coping behavior. These data show how CIE exposure causes persistent changes to PVN neuro-glial function and provides the groundwork for how these physiological changes manifest in behavioral selection.

Transcriptional glucocorticoid effects in the brain: Finding the relevant target genes

Glucocorticoids are powerful modulators of brain function. They act via mineralocorticoid and glucocorticoid receptors (MR and GR). These are best understood as transcription factors. Although many glucocorticoid effects depend on the modulation of gene transcription, it is a major challenge to link gene expression to function given the large-scale, apparently pleiotropic genomic responses. The extensive sets of MR and GR target genes are highly specific per cell type, and the brain contains many different (neuronal and non-neuronal) cell types. Next to the set "trait" of cellular context, the "state" of other active signaling pathways will affect MR and GR transcriptional activity. Here, we discuss receptor specificity and contextual factors that determine the transcriptional outcome of MR/GR signaling, experimental possibilities offered by single-cell transcriptomics approaches, and reflect on how to make sense of lists of target genes in relation to understanding the functional effects of steroid receptor activation.

Understanding the aging hypothalamus, one cell at a time

The hypothalamus is a brain region that integrates signals from the periphery and the environment to maintain organismal homeostasis. To do so, specialized hypothalamic neuropeptidergic neurons control a range of processes, such as sleep, feeding, the stress response, and hormone release. These processes are altered with age, which can affect longevity and contribute to disease status. Technological advances, such as single-cell RNA sequencing, are upending assumptions about the transcriptional identity of cell types in the hypothalamus and revealing how distinct cell types change with age. In this review, we summarize current knowledge about the contribution of hypothalamic functions to aging. We highlight recent single-cell studies interrogating distinct cell types of the mouse hypothalamus and suggest ways in which single-cell 'omics technologies can be used to further understand the aging hypothalamus and its role in longevity.

Sex-specific behavioral outcomes of early-life adversity and emerging microglia-dependent mechanisms

Early-life adversity (ELA) is known to alter brain circuit maturation as well as increase vulnerability to cognitive and emotional disorders. However, the importance of examining sex as a biological variable when researching the effects of ELA has not been considered until recently. This perspective discusses the sex-specific behavioral outcomes of ELA in both humans and animal models, then proposes microglia-mediated mechanisms as a potential underlying cause. Recent work in rodent models suggests that ELA provokes cognitive deficits, anhedonia, and alcohol abuse primarily in males, whereas females exhibit greater risk-taking and opioid addiction-related behaviors. In addition, emerging evidence identifies microglia as a key target of ELA. For example, we have recently shown that ELA inhibits microglial synapse engulfment and process dynamics in male mice, leading to an increase in excitatory synapse number onto corticotrophin-releasing hormone (CRH)-expressing neurons in the paraventricular nucleus of the hypothalamus (PVN) and aberrant stress responses later in life. However, ELA-induced synaptic rewiring of neural circuits differs in females during development, resulting in divergent behavioral outcomes. Thus, examining the role of microglia in the sex-specific mechanisms underlying ELA-induced neuropsychiatric disorders is an important topic for future research.

Resource scarcity but not maternal separation provokes unpredictable maternal care sequences in mice and both upregulate Crh-associated gene expression in the amygdala

Early life adversity (ELA) is a major risk factor for the development of pathology, including anxiety disorders. Neurodevelopmental and behavioral outcomes following ELA are multifaceted and are influenced heavily by the type of adversity experienced and sex of the individual experiencing ELA. It remains unclear what properties of ELA portend differential neurobiological risk and the basis of sex-differences for negative outcomes. Predictability of the postnatal environment has emerged as being a core feature supporting development, with the most salient signals deriving from parental care. Predictability of parental care may be a distinguishing feature of different forms of ELA, and the degree of predictability afforded by these manipulations may contribute to the diversity of outcomes observed across models. Further, questions remain as to whether differing levels of predictability may contribute to differential effects on neurodevelopment and expression of genes associated with risk for pathology. Here, we tested the hypothesis that changes in maternal behavior in mice would be contingent on the type of ELA experienced, directly comparing predictability of care in the limited bedding and nesting (LBN) and maternal separation (MS) paradigms. We then tested whether the predictability of the ELA environment altered the expression of corticotropin-releasing hormone (Crh), a sexually-dimorphic neuropeptide that regulates threat-related learning, in the amygdala of male and female mice. The LBN manipulation reliably increased the entropy of maternal care, a measure that indicates lower predictability between sequences of dam behavior. LBN and MS rearing similarly increased the frequency of nest sorties and licking of pups but had mixed effects on other aspects of dam-, pup-, and nest-related behaviors. Increased expression of Crh-related genes was observed in pups that experienced ELA, with gene expression measures showing a significant interaction with sex and type of ELA manipulation. Specifically, MS was associated with increased expression of Crh-related genes in males, but not females, and LBN primarily increased expression of these genes in females, but not males. The present study provides evidence for predictability as a distinguishing feature of models of ELA and demonstrates robust consequences of these differing experience on sex-differences in gene expression critically associated with stress responding and sex differences in risk for pathology.

Enduring disruption of reward and stress circuit activities by early-life adversity in male rats

In humans, early-life adversity (ELA) such as trauma, poverty, and chaotic environment is linked to increased risk of later-life emotional disorders including depression and substance abuse. These disorders involve underlying disruption of reward circuits and likely vary by sex. Accordingly, we previously found that ELA leads to anhedonia for natural rewards and cocaine in male rodents, whereas in females ELA instead increases vulnerability to addiction-like use of opioid drugs and palatable food. While these findings suggest that ELA-induced disruption of reward circuitry may differ between the sexes, the specific circuit nodes that are influenced by ELA in either sex remain poorly understood. Here, in adult male Sprague-Dawley rats, we ask how ELA impacts opioid addiction-relevant behaviors that we previously tested after ELA in females. We probe potential circuit mechanisms in males by assessing opioid-associated neuronal activation in stress and reward circuit nodes including nucleus accumbens (NAc), amygdala, medial prefrontal cortex (mPFC), and paraventricular thalamus. We find that ELA diminishes opioid-seeking behaviors in males, and alters heroin-induced activation of NAc, PFC, and amygdala, suggesting a potential circuit-based mechanism. These studies demonstrate that ELA leads to behavioral and neurobiological disruptions consistent with anhedonia in male rodents, unlike the increased opioid seeking we previously saw in females. Our findings, taken together with our prior work, suggest that men and women could face qualitatively different mental health consequences of ELA, which may be essential for individually tailoring future intervention strategies.

Effects of Early Life Adversity on Tooth Enamel Formation

In a systemic effort to survive environmental stress, organ systems fluctuate and adapt to overcome external pressures. The evolutionary drive back toward homeostasis makes it difficult to determine if an organism experienced a toxic exposure to stress, especially in early prenatal and neonatal periods of development. Previous studies indicate that primary human teeth may provide historical records of experiences related to stressors during that early time window. To assess the molecular effects of early life adversity on enamel formation, we used a limited bedding and nesting (LBN) mouse model of early life adversity (ELA) to assess changes in the enamel organ gene expression and enamel matrix mineralization. On average, postnatal day 12 (P12) ELA mice weighed significantly less than the controls. When adjusted for animal weight, ELA molar enamel volume was reduced as compared with the controls, and the relative mineral density of molar enamel was significantly increased. There were no obvious changes in enamel matrix crystal morphology or structure in ELA as compared with the control mouse enamel. RNAseq showed extracellular matrix organization to be the most significantly affected GO and reactome pathways, whereas butanote metabolism was the most significantly altered KEGG pathway. Transcripts expressing the enamel matrix proteins amelogenin (Amelx) and enamelin (Enam) were among the top 4 most differentially expressed genes. When evaluating molecular mechanisms for the changes in gene expression in ELA enamel organs, we found significantly increased expression of Dlx3, while transcripts for clock genes Per1 and Nrd1 were downregulated. These findings support the possibility that the developing enamel organ is sensitive to the pressures of early life adversity and produces molecular and structural biomarkers reflecting these challenges.

Microglia Don't Treat All Neurons the Same: The Importance of Neuronal Subtype in Microglia-Neuron Interactions in the Developing Hypothalamus

Microglia are now well-known as integral regulators of brain development, phagocytosing whole neurons, and pruning weak or excess synapses in order to sculpt and refine immature circuits. However, the importance of neuronal subtype in guiding microglial activity has not received much attention until recently. This perspective will delineate what is known about this topic so far, starting with the developing brain as a whole and then focusing on the developing hypothalamus in particular. There is emerging evidence that subpopulations of microglia treat excitatory and inhibitory neurons differently, and our recent work has shown that even the type of neuropeptide produced by the nearby neurons is important. For example, microglia abutting corticotropin-releasing hormone (CRH)-expressing neurons in the paraventricular nucleus of the hypothalamus (PVN) engulf fewer excitatory synapses than do microglia on the borders of the PVN that are not contacting CRH+ neurons. Potential future directions and technical considerations will be discussed in an effort to catalyze this emerging and exciting area of research. Applications of this research may hold promise in creating more specific therapies that target unique subtypes of microglia-neuron interactions in the atypically developing brain.