Microglial dopamine receptor elimination defines sex-specific nucleus accumbens development and social behavior in adolescent rats

Emerging role of microglia in the developing dopaminergic system: Perturbation by early life stress

Early life stress correlates with a higher prevalence of neurological disorders, including autism, attention-deficit/hyperactivity disorder, schizophrenia, depression, and Parkinson's disease. These conditions, primarily involving abnormal development and damage of the dopaminergic system, pose significant public health challenges. Microglia, as the primary immune cells in the brain, are crucial in regulating neuronal circuit development and survival. From the embryonic stage to adulthood, microglia exhibit stage-specific gene expression profiles, transcriptome characteristics, and functional phenotypes, enhancing the susceptibility to early life stress. However, the role of microglia in mediating dopaminergic system disorders under early life stress conditions remains poorly understood. This review presents an up-to-date overview of preclinical studies elucidating the impact of early life stress on microglia, leading to dopaminergic system disorders, along with the underlying mechanisms and therapeutic potential for neurodegenerative and neurodevelopmental conditions. Impaired microglial activity damages dopaminergic neurons by diminishing neurotrophic support (e.g., insulin-like growth factor-1) and hinders dopaminergic axon growth through defective phagocytosis and synaptic pruning. Furthermore, blunted microglial immunoreactivity suppresses striatal dopaminergic circuit development and reduces neuronal transmission. Furthermore, inflammation and oxidative stress induced by activated microglia can directly damage dopaminergic neurons, inhibiting dopamine synthesis, reuptake, and receptor activity. Enhanced microglial phagocytosis inhibits dopamine axon extension. These long-lasting effects of microglial perturbations may be driven by early life stress-induced epigenetic reprogramming of microglia. Indirectly, early life stress may influence microglial function through various pathways, such as astrocytic activation, the hypothalamic-pituitary-adrenal axis, the gut-brain axis, and maternal immune signaling. Finally, various therapeutic strategies and molecular mechanisms for targeting microglia to restore the dopaminergic system were summarized and discussed. These strategies include classical antidepressants and antipsychotics, antibiotics and anti-inflammatory agents, and herbal-derived medicine. Further investigations combining pharmacological interventions and genetic strategies are essential to elucidate the causal role of microglial phenotypic and functional perturbations in the dopaminergic system disrupted by early life stress.

Potential roles for microglia in drug addiction: Adolescent neurodevelopment and beyond

Adolescence is a sensitive period for development of addiction-relevant brain circuits, and it is also when people typically start experimenting with drugs. Unfortunately, such substance use may cause lasting impacts on the brain, and might increase vulnerability to later-life addictions. Microglia are the brain's immune cells, but their roles in shaping neural connectivity and synaptic plasticity, especially in developmental sensitive periods like adolescence, may also contribute to addiction-related phenomena. Here, we overview how drugs of abuse impact microglia, and propose that they may play poorly-understood, but important roles in addiction vulnerability and progression.

Prefrontal cortical microglial transcriptome relates to mouse offspring executive function deficits after perinatal opioid exposure in a sex-dependent manner

Opioid use during pregnancy affects over 7% of pregnancies in the United States. While efforts have been directed at mitigating effects of prenatal opioid exposure acutely in the neonatal period, long-term neurodevelopmental studies in humans remain challenging. Using a preclinical model, we previously found that perinatal morphine (MO) exposure induces sex-dependent executive function deficits in adult offspring, and sexually divergent shifts in microglia phenotype. Therefore, this study used transcriptional profiling to test whether perinatal MO exposure would cause sex-specific transcriptional changes in microglia that would relate to offspring executive function outcomes in BXD F1 mice. Female C57BL/6 mice were given MO via the drinking water or saccharin only (SCH) one week prior to mating with DBA males, throughout gestation, and lactation until offspring were weaned. Offspring executive function was assessed in adulthood using the 5-choice serial reaction time task (5CSRTT), and microglia from the PFC were isolated and characterized via RNA-seq. In the 5CSRTT, male MO-exposed offspring had reduced accuracy and female MO-exposed offspring had increased inattentive behavior. There were a similar number of genes altered in female vs. male microglia, but only 3 differentially expressed genes were evident in both sexes. Further, hierarchical clustering analysis and WGCNA identified genes that related to behavioral deficits. Together, our data identify individual genes and pathways in microglia within each sex that may relate to executive function deficits observed after perinatal opioid exposure, even though the transcriptional profiles are highly divergent between the sexes.

The regulatory variant rs1950834 confers the risk of depressive disorder by reducing LRFN5 expression

BACKGROUND

Genome-wide association studies have identified 14q21.1 as a robust risk locus for major depressive disorder (MDD). However, the underlying mechanism remains elusive. Here, we aim to explore the regulatory function of rs1950834 on leucine-rich repeat and fibronectin type III domain containing 5 (LRFN5) expression in MDD.

METHODS

Clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated protein 9 (Cas9)-mediated genome knockout and single-base editing were used to determine the effects of rs1950834 on the binding of transcriptional factors and the expression of the target gene LRFN5. Meta-analysis of multiple transcriptomic datasets was performed to clarify the brain region responsible for LRFN5 downregulation in MDD patients. Adeno-associated virus (AAV)-mediated Lrfn5 overexpression or knockdown in the nucleus accumbens (NAc) was used to test their effects on depression-like behaviors and sensitivity to chronic unpredictable mild stress (CUMS) in male mice. Synaptic structure and functions were monitored by synaptic protein expression assay, Golgi staining, and electrophysiological analysis.

RESULTS

The risk allele (A) of rs1950834 reduced the binding affinity to RNA polymerase II subunit A (POLR2A) and the transcription factor RAD21 cohesin complex component (RAD21), leading to decreased expression of LRFN5. LRFN5 expression was downregulated specifically in the NAc of MDD patients as compared to healthy controls. Knockdown of Lrfn5 in NAc neurons induced depression-like behaviors and further exacerbated CUMS-induced phenotypes via synaptic damage, but overexpression of Lrfn5 in mouse NAc induced resilience to CUMS.

CONCLUSIONS

These findings reveal that the functional risk single nucleotide polymorphism rs1950834 at 14q21.1 regulates LRNN5 expression and function in NAc, providing a novel perspective for molecular diagnosis and targeted interventions of MDD.

Dopamine D1 receptor agonist alleviates post-weaning isolation-induced neuroinflammation and depression-like behaviors in female mice

BACKGROUND

Major depressive disorder is a significant global cause of disability, particularly among adolescents. The dopamine system and nearby neuroinflammation, crucial for regulating mood and processing rewards, are central to the frontostriatal circuit, which is linked to depression. This study aimed to investigate the effect of post-weaning isolation (PWI) on depression in adolescent mice, with a focus on exploring the involvement of microglia and dopamine D1 receptor (D1R) in the frontostriatal circuit due to their known links with mood disorders.

RESULTS

Adolescent mice underwent 8 weeks of PWI before evaluating their depression-like behaviors and the activation status of microglia in the frontostriatal regions. Selective D1-like dopamine receptor agonist SKF-81,297 was administered into the medial prefrontal cortex (mPFC) of PWI mice to assess its antidepressant and anti-microglial activation properties. The effects of SKF-81,297 on inflammatory signaling pathways were examined in BV2 microglial cells. After 8 weeks of PWI, female mice exhibited more severe depression-like behaviors than males, with greater microglial activation in the frontostriatal regions. Microglial activation in mPFC was the most prominent among the three frontostriatal regions examined, and it was positively correlated with the severity of depression-like behaviors. Female PWI mice exhibited increased expression of dopamine D2 receptors (D2R). SKF-81,297 treatment alleviated depression-like behaviors and local microglial activation induced by PWI; however, SKF-81,297 induced these alterations in naïve mice. In vitro, SKF-81,297 decreased pro-inflammatory cytokine release and phosphorylations of JNK and ERK induced by lipopolysaccharide, while in untreated BV2 cells, SKF-81,297 elicited inflammation.

CONCLUSIONS

This study highlights a sex-specific susceptibility to PWI-induced neuroinflammation and depression. While targeting the D1R shows potential in alleviating PWI-induced changes, further investigation is required to evaluate potential adverse effects under normal conditions.

Scopolamine affects fear learning and social recognition in adult zebrafish

Scopolamine is the secondary metabolite of the Datura stramonium and act as a muscarinic receptor antagonist. Previous studies showed that scopolamine caused attention and memory deficit. However, the effects of scopolamine on specific cognitive functions, such as fear learning and social recognition, remain poorly understood. Here, we demonstrate the effects of scopolamine on fear learning, social memory, and neural activity in zebrafish, providing a novel perspective on its impact on cognitive and social behaviors. Here, we used equal number of male and female zebrafish as an animal model and performed a series of behavioral tests after treatment with scopolamine (100 µM and 200 µM) for 1 h to evaluate social and cognitive functions. Treatment with scopolamine increased locomotion activity, reduced the level of anxiety in the novel tank diving test, and impaired memory retrieval in the active avoidance test. Scopolamine also increased the preference for newly introduced fish in the social recognition test. In situ hybridization of c-fos mRNA showed that scopolamine decreased the neural activity of the telencephalic regions that are crucial for social, cognitive, and memory functions. Our results demonstrate the effects of scopolamine on fear learning and social recognition in adult zebrafish.

Sex and Region-Specific Differences in Microglial Morphology and Function Across Development

Microglia are exceptionally dynamic resident innate immune cells within the central nervous system, existing on a continuum of morphologies and functions throughout their lifespan. They play vital roles in response to injuries and infections, clearing cellular debris, and maintaining neural homeostasis throughout development. Emerging research suggests that microglia are strongly influenced by biological factors, including sex, developmental stage, and their local environment. This review synthesizes findings on sex differences in microglial morphology and function in key brain regions, including the frontal cortex, hippocampus, amygdala, hypothalamus, basal ganglia, and cerebellum, across the lifespan. Where available, we examine how gonadal hormones influence these microglial characteristics. Additionally, we highlight the limitations of relying solely on morphology to infer function and underscore the need for comprehensive, multimodal approaches to guide future research. Ultimately, this review aims to advance the dialogue on these spatiotemporally heterogeneous cells and their implications for sex differences in brain function and vulnerability to neurological and psychiatric disorders.

Is the Relationship Between Adolescent Social Isolation and Anxiety-Like Behaviors Altered by Microglia Ablation in Female Long Evans Rats?

OBJECTIVE

Despite extensive, cross-disciplinary research revealing a relationship between early life stress (ELS) and an increased risk for neuropsychiatric disorders, the underlying processes mediating this relationship are not fully understood. Further, the majority of preclinical studies investigating this relationship have not taken sex differences into consideration. A growing body of work suggests that microglia, resident immune cells of the brain, are impacted by ELS and contribute to some of the maladaptive behavioral phenotypes in adulthood. Here, we utilized an adolescent social isolation (aSI) model of ELS in female rats to test the role of microglia in mediating the effects of ELS on anxiety-related behaviors.

METHODS

The present study sought to determine whether microglia ablation during aSI could prevent anxiety-like behaviors in female Long Evans rats. A colony-stimulating factor 1 receptor (CSF1-r) inhibitor, PLX3397, was provided in chow to ablate microglia at the start of the isolation period (postnatal day (P) 21-42). During the aSI period, animals performed a battery of behavioral assays including the open field test, elevated plus maze, and successive alleys test. Following completion of the behavioral assays, brain tissue was collected to confirm the efficacy of PLX3397 and identify changes in microglia population density.

RESULTS

Relative to group-housed (GH) controls, aSI rats showed increased locomotor activity in the open field test and higher closed-arm entries on the elevated plus maze. Although PLX3397 effectively ablated microglia across all animals, this treatment had minimal effects on observed aSI-associated phenotypes.

CONCLUSIONS

Together, these data suggest that microglia are not required for behavioral adaptations promoted by aSI. Future studies will be needed to assess the role of microglia in the relationship between ELS and maladaptive behavioral phenotypes.

Microglia and Astrocytes in Postnatal Neural Circuit Formation

Over the past two decades, microglia and astrocytes have emerged as critical mediators of neural circuit formation. Particularly during the postnatal period, both glial subtypes play essential roles in orchestrating nervous system development through communication with neurons. These functions include regulating synapse elimination, modulating neuronal density and activity, mediating synaptogenesis, facilitating axon guidance and organization, and actively promoting neuronal survival. Despite the vital roles of both microglia and astrocytes in ensuring homeostatic brain development, the extent to which the postnatal functions of these cells are regulated by sex and the manner in which these glial cells communicate with one another to coordinate nervous system development remain less well understood. Here, we review the critical functions of both microglia and astrocytes independently and synergistically in mediating neural circuit formation, focusing our exploration on the postnatal period from birth to early adulthood.

Microglia regulate nucleus accumbens synaptic development and circuit function underlying threat avoidance behaviors

While CNS microglia have well-established roles in synapse pruning during neurodevelopment, only a few studies have identified roles for microglia in synapse formation. These studies focused on the cortex and primary sensory circuits during restricted developmental time periods, leaving substantial gaps in our understanding of the early developmental functions of microglia. Here we investigated how the absence of microglia impacts synaptic development in the nucleus accumbens (NAc), a region critical for emotional regulation and motivated behaviors and where dysfunction is implicated in psychiatric disorders that arise early in life. Using a genetically modified mouse that lacks microglia (Csf1r ΔFIRE/ΔFIRE), we found blunted excitatory synapse formation in the NAc. This effect was most prominent during the second and third postnatal weeks, when we previously found microglia to be overproduced, and was accompanied by an increase in presynaptic release probability and alterations in postsynaptic kinetics. Tissue-level NAc proteomics confirmed that microglial absence impacted numerous proteins involved in synapse structure, trans-synaptic signaling, and pre-synaptic function. However, microglial absence did not perturb levels of astrocyte-derived cues and adhesive proteins that promote synaptogenesis, suggesting that reduced synapse number may be caused by absence of a microglial-derived synaptogenic cue. Although observed electrophysiological synaptic changes were largely normalized by adulthood, we identified lasting effects of microglial absence on threat avoidance behavior, and these behavioral effects were directly associated with alterations of NAc neuronal activity. Together, these results indicate a critical role for microglia in regulating the synaptic landscape of the developing NAc and in establishing functional circuits underlying adult behavioral repertoires.

Microglia regulate nucleus accumbens synaptic development and circuit function underlying threat avoidance behaviors

While CNS microglia have well-established roles in synapse pruning during neurodevelopment, only a few studies have identified roles for microglia in synapse formation. These studies focused on the cortex and primary sensory circuits during restricted developmental time periods, leaving substantial gaps in our understanding of the early developmental functions of microglia. Here we investigated how the absence of microglia impacts synaptic development in the nucleus accumbens (NAc), a region critical for emotional regulation and motivated behaviors and where dysfunction is implicated in psychiatric disorders that arise early in life. Using a genetically modified mouse that lacks microglia (Csf1r ΔFIRE/ΔFIRE), we found blunted excitatory synapse formation in the NAc. This effect was most prominent during the second and third postnatal weeks, when we previously found microglia to be overproduced, and was accompanied by an increase in presynaptic release probability and alterations in postsynaptic kinetics. Tissue-level NAc proteomics confirmed that microglial absence impacted numerous proteins involved in synapse structure, trans-synaptic signaling, and pre-synaptic function. However, microglial absence did not perturb levels of astrocyte-derived cues and adhesive proteins that promote synaptogenesis, suggesting that reduced synapse number may be caused by absence of a microglial-derived synaptogenic cue. Although observed electrophysiological synaptic changes were largely normalized by adulthood, we identified lasting effects of microglial absence on threat avoidance behavior, and these behavioral effects were directly associated with alterations of NAc neuronal activity. Together, these results indicate a critical role for microglia in regulating the synaptic landscape of the developing NAc and in establishing functional circuits underlying adult behavioral repertoires.

A, Guadarrama Gándara CG, Escobar C, Guerrero-Vargas NN. Maternal exposure to dim light at night induces behavioral alterations in the adolescent and adult offspring Wistar rat

Introduction

Access to electric light has exposed living organisms to varying intensities of light throughout the 24 h day. Dim light at night (DLAN) is an inappropriate signal for the biological clock, which is responsible for the circadian organization of physiology. During the gestational period, physiological adaptations occur to ensure a successful pregnancy and optimal fetal development. Environmental maternal conditions, such as disruptions of maternal circadian rhythms, could negatively affect offspring health. We have previously demonstrated that exposure of female Wistar rats to DLAN results in circadian, metabolic, and behavioral alterations. A relevant behavior during adolescence is social play, primarily regulated by the nucleus accumbens (NAc) which is crucial for the proper performance of important behaviors in adulthood. Throughout development, microglia are responsible for the remodeling of diverse brain regions via synaptic pruning. During adolescence, this process occurs within the NAc, where immune-mediated remodeling directly impacts social play behavior.

Methods

This study investigated the effects of maternal exposure to DLAN or a light-dark cycle (LD) before (5 weeks) and during the gestational period (21-23 days) on the metabolism and behavior of offspring in adolescence and adulthood. Body mass was measured every 5 days from postnatal day 1 (PN1) to PN25 and every 10 days from PN40 to PN90; food consumption was monitored weekly from PN40 to PN90. Social play behavior was evaluated at PN40. The quantification and morphology of microglia in the NAc were measured on PN30. An open field test was conducted at PN60, and anhedonia test was assessed at PN90.

Results and discussion

Male and female offspring from mothers exposed to DLAN showed increased body mass gain at PN25. DLAN male offspring had lower food consumption, while DLAN females exhibited increased food consumption. In social play behavior, no differences were found between DLAN and LD female offspring. In contrast, DLAN male offspring exhibited a significant decrease in social play behavior compared to LD animals, which was associated with higher numbers of microglia in the NAc that had more ramified morphology. Importantly, at PN90, DLAN offspring presented increased anxiety-like behaviors. These results demonstrate that DLAN exposure induces intergenerational behavioral alterations that persist until adulthood.

Impact of Neuroimmune System Activation by Adolescent Binge Alcohol Exposure on Adult Neurobiology

Adolescence is a conserved neurodevelopmental period encompassing maturation of glia and the innate immune system that parallels refinement of brain structures, neurotransmitter systems, and neurocircuitry. Given the vast neurodevelopmental processes occurring during adolescence, spanning brain structural and neurocircuitry refinement to maturation of neurotransmitter systems, glia, and the innate immune system, insults incurred during this critical period of neurodevelopment, could have profound effects on brain function and behavior that persist into adulthood. Adolescent binge drinking is common and associated with many adverse outcomes that may underlie the lifelong increased risk of alcohol-related problems and development of an alcohol use disorder (AUD). In this chapter, we examined the impact of adolescent binge drinking models using the adolescent intermittent ethanol (AIE) model on adult neurobiology. These studies implicate proinflammatory neuroimmune signaling across glia and neurons in persistent AIE-induced neuropathology. Some of these changes are reversible, providing unique opportunities for the development of treatments to prevent many of the long-term consequences of adolescent alcohol misuse.

Emerging evidence of a link between inflammation and the neuropathology of prenatal opioid exposure

Opioid use continues to increase, particularly among women of reproductive age. As a result, increasing numbers of infants are born with prenatal exposure to opioids, suffering both acute and long-term negative consequences. Studies performed across the past 5 years have highlighted both peripheral and central inflammation as a consistent feature of prenatal opioid exposure. Dysregulated innate and adaptive immunity have been detected in human and rodent studies, highlighting a likely role of inflammation in the neuropathology associated with opioid exposure. Identifying immune changes occurring following prenatal opioid exposure will be critical for developing new therapeutic approaches in this field.

Complement and microglia activation mediate stress-induced synapse loss in layer 2/3 of the medial prefrontal cortex in male mice

Spatially heterogeneous synapse loss is a characteristic of many psychiatric and neurological disorders, but the underlying mechanisms are unclear. Here, we show that spatially-restricted complement activation mediates stress-induced heterogeneous microglia activation and synapse loss localized to the upper layers of the medial prefrontal cortex (mPFC) in male mice. Single cell RNA sequencing also reveals a stress-associated microglia state marked by high expression of the apolipoprotein E gene (Apoehigh) localized to the upper layers of the mPFC. Mice lacking complement component C3 are protected from stress-induced layer-specific synapse loss, and the Apoehigh microglia population is markedly reduced in the mPFC of these mice. Furthermore, C3 knockout mice are also resilient to stress-induced anhedonia and working memory behavioral deficits. Our findings suggest that region-specific complement and microglia activation can contribute to the disease-specific spatially restricted patterns of synapse loss and clinical symptoms found in many brain diseases.

Microglia signaling in health and disease - Implications in sex-specific brain development and plasticity

Microglia, the intrinsic neuroimmune cells residing in the central nervous system (CNS), exert a pivotal influence on brain development, homeostasis, and functionality, encompassing critical roles during both aging and pathological states. Recent advancements in comprehending brain plasticity and functions have spotlighted conspicuous variances between male and female brains, notably in neurogenesis, neuronal myelination, axon fasciculation, and synaptogenesis. Nevertheless, the precise impact of microglia on sex-specific brain cell plasticity, sculpting diverse neural network architectures and circuits, remains largely unexplored. This article seeks to unravel the present understanding of microglial involvement in brain development, plasticity, and function, with a specific emphasis on microglial signaling in brain sex polymorphism. Commencing with an overview of microglia in the CNS and their associated signaling cascades, we subsequently probe recent revelations regarding molecular signaling by microglia in sex-dependent brain developmental plasticity, functions, and diseases. Notably, C-X3-C motif chemokine receptor 1 (CX3CR1), triggering receptors expressed on myeloid cells 2 (TREM2), calcium (Ca2+), and apolipoprotein E (APOE) emerge as molecular candidates significantly contributing to sex-dependent brain development and plasticity. In conclusion, we address burgeoning inquiries surrounding microglia's pivotal role in the functional diversity of developing and aging brains, contemplating their potential implications for gender-tailored therapeutic strategies in neurodegenerative diseases.

The Influence of Strain and Sex on High Fat Diet-Associated Alterations of Dopamine Neurochemistry in Mice

Objective: The objective of this study was to determine the influence of sex and strain on striatal and nucleus accumbens dopamine neurochemistry and dopamine-related behavior due to a high-saturated-fat diet (HFD). Methods: Male and female C57B6/J (B6J) and Balb/cJ (Balb/c) mice were randomly assigned to a control-fat diet (CFD) containing 10% kcal fat/g or a mineral-matched HFD containing 60% kcal fat/g for 12 weeks. Results: Intraperitoneal glucose tolerance testing (IPGTT) and elevated plus maze experiments (EPM) confirmed that an HFD produced marked blunting of glucose clearance and increased anxiety-like behavior, respectively, in male and female B6J mice. Electrically evoked dopamine release in the striatum and reuptake in the nucleus accumbens (NAc), as measured by ex vivo fast scan cyclic voltammetry, was reduced for HFD-fed B6J females. Impairment in glucose metabolism explained HFD-induced changes in dopamine neurochemistry for B6J males and, to a lesser extent, Balb/c males. The relative expressions of protein markers associated with the activation of microglia, ionized calcium binding adaptor molecule (Iba1) and cluster of differentiation molecule 11b (CD11b) in the striatum were increased due to an HFD for B6J males but were unchanged or decreased amongst HFD-fed Balb/c mice. Conclusions: Our findings demonstrate that strain and sex influence the insulin- and microglia-dependent mechanisms of alterations to dopamine neurochemistry and associated behavior due to an HFD.

Abstinence from cocaine self-administration promotes microglia pruning of astrocytes which drives cocaine-seeking behavior

Rodent drug self-administration leads to compromised ability of astrocytes to maintain glutamate homeostasis within the brain's reward circuitry, as well as reductions in surface area, volume, and synaptic colocalization of astrocyte membranes. However, the mechanisms driving astrocyte responses to cocaine are unknown. Here, we report that long-access cocaine self-administration followed by prolonged home cage abstinence results in decreased branching complexity of nucleus accumbens astrocytes, characterized by the loss of peripheral processes. Using a combination of confocal fluorescence microcopy and immuno-gold electron microscopy, we show that alterations in astrocyte structural features are driven by microglia phagocytosis, as labeled astrocyte membranes are found within microglia phagolysosomes. Inhibition of complement C3-mediated phagocytosis using the neutrophil inhibitory peptide (NIF) rescued astrocyte structure and decreased cocaine seeking behavior following cocaine self-administration and abstinence. Collectively, these results provide evidence for microglia pruning of accumbens astrocytes across cocaine abstinence which mediates cocaine craving.

Associations of the immune system in aggression traits and the role of microglia as mediators

There is an important relationship between the immune system and aggressive behavior. Aggressive encounters acutely increase the levels of proinflammatory cytokines, and there are positive correlations between aggressive traits and peripheral proinflammatory cytokines. Endotoxin lipopolysaccharide (LPS) treatment, which results in peripheral immune activation, decreases aggressive behavior as one of the sickness behavioral symptoms. In contrast, certain brain infections and chronic interferon treatment are associated with increased aggression. Indeed, the effects of proinflammatory cytokines on the brain in aggressive behavior are bidirectional, depending on the type and dose of cytokine, target brain region, and type of aggression. Some studies have suggested that microglial activation and neuroinflammation influence intermale aggression in rodent models. In addition, pathological conditions as well as physiological levels of cytokines produced by microglia play an important role in social and aggressive behavior in adult animals. Furthermore, microglial function in early development is necessary for the establishment of the social brain and the expression of juvenile social behaviors, including play fighting. Overall, this review discusses the important link between the immune system and aggressive traits and the role of microglia as mediators of this link.

Male-biased vulnerability of mouse brain tryptophan/kynurenine and glutamate systems to adolescent exposures to concentrated ambient ultrafine particle air pollution

Air pollution (AP) exposures have been associated with numerous neurodevelopmental and psychiatric disorders, including autism spectrum disorder, attention deficit hyperactivity disorder and schizophrenia, all male-biased disorders with onsets from early life to late adolescence/early adulthood. While prior experimental studies have focused on effects of AP exposures during early brain development, brain development actually extends well into early adulthood. The current study in mice sought to extend the understanding of developmental brain vulnerability during adolescence, a later but significant period of brain development and maturation to the ultrafine particulate (UFPs) component of AP, considered its most reactive component. Additionally, it examined adolescent response to UFPs when preceded by earlier developmental exposures, to ascertain the trajectory of effects and potential enhancement or mitigation of adverse consequences. Outcomes focused on shared features associated with multiple neurodevelopmental disorders. For this purpose, C57Bl/6 J mice of both sexes were exposed to ambient concentrated UFPs or filtered air from PND (postnatal day) 4-7 and PND10-13, and again at PND39-42 and 45-49, resulting in 3 exposure postnatal/adolescent treatment groups per sex: Air/Air, Air/UFP, and UFP/UFP. Features common to neurodevelopmental disorders were examined at PND50. Mass exposure concentration from postnatal exposure averaged 44.34 μg/m3 and the adolescent exposure averaged 49.18 μg/m3. Male brain showed particular vulnerability to UFP exposures in adolescence, with alterations in frontal cortical and striatal glutamatergic and tryptophan/serotonergic neurotransmitters and concurrent reductions in levels of astrocytes in corpus callosum and in serum cytokine levels, with combined exposures resulting in significant reductions in corpus callosum myelination and serum corticosterone. Reductions in serum corticosterone in males correlated with reductions in neurotransmitter levels, and reductions in striatal glutamatergic function specifically correlated with reductions in corpus callosum astrocytes. UFP-induced changes in neurotransmitter levels in males were mitigated by prior postnatal exposure, suggesting potential adaptation, whereas reductions in corticosterone and in corpus callosum neuropathological effects were further strengthened by combined postnatal and adolescent exposures. UFP-induced changes in females occurred primarily in striatal dopamine systems and as reductions in serum cytokines only in response to combined postnatal and adolescent exposures. Findings in males underscore the importance of more integrated physiological assessments of mechanisms of neurotoxicity. Further, these findings provide biological plausibility for an accumulating epidemiologic literature linking air pollution to neurodevelopmental and psychiatric disorders. As such, they support a need for consideration of the regulation of the UFP component of air pollution.

Behavioral neuroscience's inevitable SABV growing pains

The field of rodent behavioral neuroscience is undergoing two major sea changes: an ever-growing technological revolution, and worldwide calls to consider sex as a biological variable (SABV) in experimental design. Both have enormous potential to improve the precision and rigor with which the brain can be studied, but the convergence of these shifts in scientific practice has exposed critical limitations in classic and widely used behavioral paradigms. While our tools have advanced, our behavioral metrics - mostly developed in males and often allowing for only binary outcomes - have not. This opinion article explores how this disconnect has presented challenges for the accurate depiction and interpretation of sex differences in brain function, arguing for the expansion of current behavioral constructs to better account for behavioral diversity.

Post-weaning social isolation alters sociability in a sex-specific manner

Adolescence is a critical period for brain development in humans and stress exposure during this time can have lasting effects on behavior and brain development. Social isolation and loneliness are particularly salient stressors that lead to detrimental mental health outcomes particularly in females, although most of the preclinical work on social isolation has been done in male animals. Our lab has developed a model of post-weaning adolescent social isolation that leads to increased drug reward sensitivity and altered neuronal structure in limbic brain regions. The current study utilized this model to determine the impact of adolescent social isolation on a three-chamber social interaction task both during adolescence and adulthood. We found that while post-weaning isolation does not alter social interaction during adolescence (PND45), it has sex-specific effects on social interaction in young adulthood (PND60), potentiating social interaction in male mice and decreasing it in female mice. As early life stress can activate microglia leading to alterations in neuronal pruning, we next examined the impact of inhibiting microglial activation with daily minocycline administration during the first 3 weeks of social isolation on these changes in social interaction. During adolescence, minocycline dampened social interaction in male mice, while having no effect in females. In contrast, during young adulthood, minocycline did not alter the impact of adolescent social isolation in males, with socially isolated males exhibiting higher levels of social interaction compared to their group housed counterparts. In females, adolescent minocycline treatment reversed the effect of social isolation leading to increased social interaction in the social isolation group, mimicking what is seen in naïve males. Taken together, adolescent social isolation leads to sex-specific effects on social interaction in young adulthood and adolescent minocycline treatment alters the effects of social isolation in females, but not males.

Sex differences in patterns of white matter neuroplasticity after balance training in young adults

Introduction

In past work we demonstrated different patterns of white matter (WM) plasticity in females versus males associated with learning a lab-based unilateral motor skill. However, this work was completed in neurologically intact older adults. The current manuscript sought to replicate and expand upon these WM findings in two ways: (1) we investigated biological sex differences in neurologically intact young adults, and (2) participants learned a dynamic full-body balance task.

Methods

24 participants (14 female, 10 male) participated in the balance training intervention, and 28 were matched controls (16 female, 12 male). Correlational tractography was used to analyze changes in WM from pre- to post-training.

Results

Both females and males demonstrated skill acquisition, yet there were significant differences in measures of WM between females and males. These data support a growing body of evidence suggesting that females exhibit increased WM neuroplasticity changes relative to males despite comparable changes in motor behavior (e.g., balance).

Discussion

The biological sex differences reported here may represent an important factor to consider in both basic research (e.g., collapsing across females and males) as well as future clinical studies of neuroplasticity associated with motor function (e.g., tailored rehabilitation approaches).

Feminization of social play behavior depends on microglia

Many sex differences in brain and behavior are established developmentally by the opposing processes of feminization and masculinization, which manifest following differential steroid hormone exposure in early life. The cellular mechanisms underlying masculinization are well-documented, a result of the fact that it is steroid-mediated and can be easily induced in newborn female rodents via exogenous steroid treatment. However, the study of feminization of particular brain regions has largely been relegated to being "not masculinization" given the absence of an identified initiating trigger. As a result, the mechanisms of this key developmental process remain elusive. Here we describe a novel role for microglia, the brain's innate immune cell, in the feminization of the medial amygdala and a complex social behavior, juvenile play. In the developing amygdala, microglia promote proliferation of astrocytes equally in both sexes, with no apparent effect on rates of cell division, but support cell survival selectively in females through the trophic actions of Tumor Necrosis Factor α (TNFα). We demonstrate that disrupting TNFα signaling, either by depleting microglia or inhibiting the associated signaling pathways, prevents the feminization of astrocyte density and increases juvenile play levels to that seen in males. This data, combined with our previous finding that male-like patterns of astrocyte density are sculpted by developmental microglial phagocytosis, reveals that sexual differentiation of the medial amygdala involves opposing tensions between active masculinization and active feminization, both of which require microglia but are achieved via distinct processes.

Epigenetic heterogeneity shapes the transcriptional landscape of regional microglia

Microglia, the innate immune cells in the central nervous system, exhibit distinct transcriptional profiles across brain regions that are important for facilitating their specialized function. There has been recent interest in identifying the epigenetic modifications associated with these distinct transcriptional profiles, as these may improve our understanding of the underlying mechanisms governing the functional specialization of microglia. One obstacle to achieving this goal is the large number of microglia required to obtain a genome-wide profile for a single histone modification. Given the cellular and regional heterogeneity of the brain, this would require pooling many samples which would impede biological applications that are limited by numbers of available animals. To overcome this obstacle, we have adapted a method of chromatin profiling known as Cleavage Under Targets and Tagmentation (CUT&Tag-Direct) to profile histone modifications associated with regional differences in gene expression throughout the brain reward system. Consistent with previous studies, we find that transcriptional profiles of microglia vary by brain region. However, here we report that these regional differences also exhibit transcriptional network signatures specific to each region. Additionally, we find that these region-dependent network signatures are associated with differential deposition of H3K27ac and H3K7me3, and while the H3K27me3 landscape is remarkably stable across brain regions, the H3K27ac landscape is most consistent with the anatomical location of microglia which explain their distinct transcriptional profiles. Altogether, these findings underscore the established role of H3K27me3 in cell fate determination and support the active role of H3K27ac in the dynamic regulation of microglial gene expression. In this study, we report a molecular and computational framework that can be applied to improve our understanding of the role of epigenetic regulation in microglia in both health and disease, using as few as 2,500 cells per histone mark.

The scheduling of adolescence with Netrin-1 and UNC5C

Dopamine axons are the only axons known to grow during adolescence. Here, using rodent models, we examined how two proteins, Netrin-1 and its receptor, UNC5C, guide dopamine axons toward the prefrontal cortex and shape behaviour. We demonstrate in mice (Mus musculus) that dopamine axons reach the cortex through a transient gradient of Netrin-1-expressing cells - disrupting this gradient reroutes axons away from their target. Using a seasonal model (Siberian hamsters; Phodopus sungorus) we find that mesocortical dopamine development can be regulated by a natural environmental cue (daylength) in a sexually dimorphic manner - delayed in males, but advanced in females. The timings of dopamine axon growth and UNC5C expression are always phase-locked. Adolescence is an ill-defined, transitional period; we pinpoint neurodevelopmental markers underlying this period.

Post-weaning social isolation alters sociability in a sex-specific manner

Adolescence is a critical period for brain development in humans and stress exposure during this time can have lasting effects on behavior and brain development. Social isolation and loneliness are particularly salient stressors that lead to detrimental mental health outcomes particularly in females, although most of the preclinical work on social isolation has been done in male animals. Our lab has developed a model of post-weaning adolescent social isolation that leads to increased drug reward sensitivity and altered neuronal structure in limbic brain regions. The current study utilized this model to determine the impact of adolescent social isolation on a three-chamber social interaction task both during adolescence and adulthood. We found that while post-weaning isolation does not alter social interaction during adolescence (PND45), it has sex-specific effects on social interaction in adulthood (PND60), potentiating social interaction in male mice and decreasing it in female mice. As early life stress can activate microglia leading to alterations in neuronal pruning, we next examined the impact of inhibiting microglial activation with daily minocycline administration during the first three weeks of social isolation on these changes in social interaction. During adolescence, minocycline dampened social interaction in male mice, while having no effect in females. In contrast, during adulthood, minocycline did not alter the impact of adolescent social isolation in males, with socially isolated males exhibiting higher levels of social interaction compared to their group housed counterparts. In females, adolescent minocycline treatment reversed the effect of social isolation leading to increased social interaction in the social isolation group, mimicking what is seen in naïve males. Taken together, adolescent social isolation leads to sex-specific effects on social interaction in adulthood and adolescent minocycline treatment alters the effects of social isolation in females, but not males.

Sex differences in membrane properties and cellular excitability of dopamine D1 receptor-expressing neurons within the shell of the nucleus accumbens of pre- and mid-adolescent mice

BACKGROUND

The transition from childhood to adulthood, or adolescence, a developmental stage, is characterized by psychosocial and biological changes. The nucleus accumbens (NAc), a striatal brain region composed of the core (NAcC) and shell (NAcSh), has been linked to risk-taking behavior and implicated in reward seeking and evaluation. Most neurons in the NAc are medium spiny neurons (MSNs) that express dopamine D1 receptors (D1R +) and/or dopamine D2 receptors (D2R +). Changes in dopaminergic and glutamatergic systems occur during adolescence and converge in the NAc. While there are previous investigations into sex differences in membrane excitability and synaptic glutamate transmission in both subdivisions of the NAc, to our knowledge, none have specified NAcSh D1R + MSNs from mice during pre- and mid-adolescence.

METHODS

Sagittal brain slices containing the NAc were prepared from B6.Cg-Tg(Drd1a-tdTomato)6Calak/J mice of both sexes from postnatal days 21-25 and 35-47, representing pre- and mid-adolescence, respectively. Whole-cell electrophysiology recordings were collected from NAcSh D1R + MSNs in the form of membrane-voltage responses to current injections, to assess membrane properties and action potential waveform characteristics, and spontaneous excitatory postsynaptic currents (sEPSCs) to assess glutamatergic synaptic activity.

RESULTS

Relative to pre-adolescent males, pre-adolescent female NAcSh D1R + MSNs exhibited a less hyperpolarized resting membrane potential, increased input resistance, and smaller action potential afterhyperpolarization amplitudes. During mid-adolescence, decreased input resistance and a shorter action potential duration in females were the only sex differences observed.

CONCLUSIONS

Taken together, our results indicate that NAcSh D1R + MSNs in mice exhibit sex differences in membrane properties and AP waveform during pre-adolescence that are overall indicative of increased cellular excitability in females and are suggestive of possible sex differences in glycine receptors, inwardly-rectifying potassium channels, and large conductance voltage-gated potassium channels. These differences do not appear to persist into mid-adolescence, when sex was observed to affect input resistance oppositely to that of pre-adolescence and AP waveform in a manner suggestive of differences in voltage-gated potassium channels.

Molecular mechanisms underlying microglial sensing and phagocytosis in synaptic pruning

Microglia are the main non-neuronal cells in the central nervous system that have important roles in brain development and functional connectivity of neural circuits. In brain physiology, highly dynamic microglial processes are facilitated to sense the surrounding environment and stimuli. Once the brain switches its functional states, microglia are recruited to specific sites to exert their immune functions, including the release of cytokines and phagocytosis of cellular debris. The crosstalk of microglia between neurons, neural stem cells, endothelial cells, oligodendrocytes, and astrocytes contributes to their functions in synapse pruning, neurogenesis, vascularization, myelination, and blood-brain barrier permeability. In this review, we highlight the neuron-derived "find-me," "eat-me," and "don't eat-me" molecular signals that drive microglia in response to changes in neuronal activity for synapse refinement during brain development. This review reveals the molecular mechanism of neuron-microglia interaction in synaptic pruning and presents novel ideas for the synaptic pruning of microglia in disease, thereby providing important clues for discovery of target drugs and development of nervous system disease treatment methods targeting synaptic dysfunction.

Early-life inflammation increases ethanol consumption in adolescent male mice

Recent studies have demonstrated that stress during the critical windows of development can evoke a cascade of neurological changes that can result in neuropsychiatric disorders later in life. In this study, we examined the effect of early-life inflammation on ethanol consumption in adolescent mice. C57BL/6J mice were assigned to either the control or Lipopolysaccharide (LPS) group on postnatal day 14 (P14). In the latter group, LPS at a dose of 50 μg/kg was injected intraperitoneally. The mice were weaned at P21, and behavior tests were performed at P45. Ethanol consumption was assessed using a two-bottle choice drinking paradigm. Anxiety-like behaviors were assessed by marble burying test (MBT), open field (OF), and elevated plus maze (EPM). Ethanol-induced loss of righting reflex (LORR), hypothermia and ethanol metabolism were assessed to evaluate ethanol intoxication. P14 LPS-injected adolescent male mice exhibited significantly increased ethanol preference and consumption, with a similar taste preference for saccharin and avoidance of quinine. The adolescent male mice showed increased anxiety-like behaviors in the OF and EPM tests, and an increased duration of LORR, without affecting the hypothermic effects of ethanol and ethanol metabolism. Interestingly, these behavioral changes were not obvious in female mice. In conclusion, our data indicate that early-life inflammation may be a risk factor for ethanol consumption in adolescents with greater changes observed in male mice. SIGNIFICANCE STATEMENT: Our study is the first preclinical model to report the enhancement effect of early-life inflammation on ethanol consumption in adolescent male mice and our findings provide a valuable mouse model to examine the neurobiological mechanisms mediating the long-lasting effects of early-life inflammation on alcohol use disorders vulnerability.

Juvenile peripheral LPS exposure overrides female resilience to prenatal VPA effects on adult sociability in mice

Autism spectrum disorder (ASD) exhibits a gender bias, with boys more frequently affected than girls. Similarly, in mouse models induced by prenatal exposure to valproic acid (VPA), males typically display reduced sociability, while females are less affected. Although both males and females exhibit VPA effects on neuroinflammatory parameters, these effects are sex-specific. Notably, females exposed to VPA show increased microglia and astrocyte density during the juvenile period. We hypothesized that these distinct neuroinflammatory patterns contribute to the resilience of females to VPA. To investigate this hypothesis, we treated juvenile animals with intraperitoneal bacterial lipopolysaccharides (LPS), a treatment known to elicit brain neuroinflammation. We thus evaluated the impact of juvenile LPS-induced inflammation on adult sociability and neuroinflammation in female mice prenatally exposed to VPA. Our results demonstrate that VPA-LPS females exhibit social deficits in adulthood, overriding the resilience observed in VPA-saline littermates. Repetitive behavior and anxiety levels were not affected by either treatment. We also evaluated whether the effect on sociability was accompanied by heightened neuroinflammation in the cerebellum and hippocampus. Surprisingly, we observed reduced astrocyte and microglia density in the cerebellum of VPA-LPS animals. These findings shed light on the complex interactions between prenatal insults, juvenile inflammatory stimuli, and sex-specific vulnerability in ASD-related social deficits, providing insights into potential therapeutic interventions for ASD.

Social isolation postweaning alters reward-related dopamine dynamics in a region-specific manner in adolescent male rats

Early development is characterized by dynamic transitions in brain maturation, which may be impacted by environmental factors. Here, we sought to determine the effects of social isolation from postweaning and during adolescence on reward behavior and dopaminergic signaling in male rats. Subjects were socially isolated or group housed at postnatal day 21. Three weeks later, extracellular dopamine concentrations were examined in the medial prefrontal cortex (mPFC) and nucleus accumbens shell (NAc) during a feeding bout. Surprisingly, opposing effects were found in which increased mPFC dopamine concentrations were observed in group housed, but not isolated, rats. In stark contrast, increased dopamine levels were found in the NAc of isolated, but not group housed, rats. Moreover, the absence of an effect in the mPFC of the isolated rats could not be reversed by subsequent group housing, demonstrating the remarkable long-term effects on dopamine signaling dynamics. When provided a highly palatable food, the isolated subjects exhibited a dramatic increase in mPFC dopamine levels when the chocolate was novel, but no effects following chronic chocolate consumption. In contrast, the group housed subjects showed significantly increased dopamine levels only with chronic chocolate consumption. The dopamine changes were correlated with differences in behavioral measures. Importantly, the deficit in reward-related behavior during isolation could be reversed by microinjection of either dopamine or cocaine into the mPFC. Together, these data provide evidence that social isolation from postweaning and during adolescence alters reward-induced dopamine levels in a brain region-specific manner, which has important functional implications for reward-related behavior.

Microglia as integrators of brain-associated molecular patterns

Microglia are brain-resident macrophages that play key roles in brain development and experience dependent plasticity. In this review we discuss recent findings regarding the molecular mechanisms through which mammalian microglia sense the unique molecular patterns of the homeostatic brain. We propose that microglial function is acutely controlled in response to 'brain-associated molecular patterns' (BAMPs) that function as indicators of neuronal activity and neural circuit remodeling. A further layer of regulation comes from instructive cytokine cues that define unique microglial functional states. A systematic investigation of the receptors and signaling pathways that mediate these two regulatory axes may begin to define a functional code for microglia-neuron interactions.

The scheduling of adolescence with Netrin-1 and UNC5C

Dopamine axons are the only axons known to grow during adolescence. Here, using rodent models, we examined how two proteins, Netrin-1 and its receptor, UNC5C, guide dopamine axons towards the prefrontal cortex and shape behaviour. We demonstrate in mice ( Mus musculus ) that dopamine axons reach the cortex through a transient gradient of Netrin-1 expressing cells - disrupting this gradient reroutes axons away from their target. Using a seasonal model (Siberian hamsters; Phodopus sungorus ) we find that mesocortical dopamine development can be regulated by a natural environmental cue (daylength) in a sexually dimorphic manner - delayed in males, but advanced in females. The timings of dopamine axon growth and UNC5C expression are always phase-locked. Adolescence is an ill-defined, transitional period; we pinpoint neurodevelopmental markers underlying this period.

Dopaminergic signaling regulates microglial surveillance and adolescent plasticity in the frontal cortex

Adolescence is a sensitive period for frontal cortical development and cognitive maturation. The dopaminergic (DA) mesofrontal circuit is particularly malleable in response to changes in adolescent experience and DA activity. However, the cellular mechanisms engaged in this plasticity remain unexplored. Here, we report that microglia, the innate immune cells of the brain, are uniquely sensitive to adolescent mesofrontal DA signaling. Longitudinal in vivo two-photon imaging in mice shows that frontal cortical microglia respond dynamically to plasticity-inducing behavioral or optogenetic DA axon stimulation with increased parenchymal and DA bouton surveillance. Microglial-axon contact precedes new bouton formation, and both D1 and D2-type DA receptors regulate microglial-bouton interactions and axonal plasticity. Moreover, D2 antagonism in adults reinstates adolescent plasticity, including increased microglial surveillance and new DA bouton formation. Our results reveal that DA signaling regulates microglial surveillance and axonal plasticity uniquely in the adolescent frontal cortex, presenting potential interventions for restoring plasticity in the adult brain.

Investigating female versus male differences in white matter neuroplasticity associated with complex visuo-motor learning

Magnetic resonance imaging (MRI) has increasingly been used to characterize structure-function relationships during white matter neuroplasticity. Biological sex differences may be an important factor that affects patterns of neuroplasticity, and therefore impacts learning and rehabilitation. The current study examined a participant cohort before and after visuo-motor training to characterize sex differences in microstructural measures. The participants (N = 27) completed a 10-session (4 week) complex visuo-motor training task with their non-dominant hand. All participants significantly improved movement speed and their movement speed variability over the training period. White matter neuroplasticity in females and males was examined using fractional anisotropy (FA) and myelin water fraction (MWF) along the cortico-spinal tract (CST) and the corpus callosum (CC). FA values showed significant differences in the middle portion of the CST tract (nodes 38-51) across the training period. MWF showed a similar cluster in the inferior portion of the tract (nodes 18-29) but did not reach significance. Additionally, at baseline, males showed significantly higher levels of MWF measures in the middle body of the CC. Combining data from females and males would have resulted in reduced sensitivity, making it harder to detect differences in neuroplasticity. These findings offer initial insights into possible female versus male differences in white matter neuroplasticity during motor learning. This warrants investigations into specific patterns of white matter neuroplasticity for females versus males across the lifespan. Understanding biological sex-specific differences in white matter neuroplasticity may have significant implications for the interpretation of change associated with learning or rehabilitation.

Epigenetic regulation of microglia and neurons by proinflammatory signaling following adolescent intermittent ethanol (AIE) exposure and in human AUD

Adolescent alcohol drinking is linked to high rates of adult alcohol problems and alcohol use disorder (AUD). The Neurobiology of Alcohol Drinking in Adulthood (NADIA) consortium adolescent intermittent ethanol (AIE) models adolescent binge drinking, followed by abstinent maturation to adulthood to determine the persistent AIE changes in neurobiology and behavior. AIE increases adult alcohol drinking and preference, increases anxiety and reward seeking, and disrupts sleep and cognition, all risks for AUD. In addition, AIE induces changes in neuroimmune gene expression in neurons and glia that alter neurocircuitry and behavior. HMGB1 is a unique neuroimmune signal released from neurons and glia by ethanol that activates multiple proinflammatory receptors, including Toll-like receptors (TLRs), that spread proinflammatory gene induction. HMGB1 expression is increased by AIE in rat brain and in post-mortem human AUD brain, where it correlates with lifetime alcohol consumption. HMGB1 activation of TLR increase TLR expression. Human AUD brain and rat brain following AIE show increases in multiple TLRs. Brain regional differences in neurotransmitters and cell types impact ethanol responses and neuroimmune gene induction. Microglia are monocyte-like cells that provide trophic and synaptic functions, that ethanol proinflammatory signals sensitize or "prime" during repeated drinking cycles, impacting neurocircuitry. Neurocircuits are differently impacted dependent upon neuronal-glial signaling. Acetylcholine is an anti-inflammatory neurotransmitter. AIE increases HMGB1-TLR4 signaling in forebrain, reducing cholinergic neurons by silencing multiple cholinergic defining genes through upregulation of RE-1 silencing factor (REST), a transcription inhibitor known to regulate neuronal differentiation. HMGB1 REST induction reduces cholinergic neurons in basal forebrain and cholinergic innervation of hippocampus. Adult brain hippocampal neurogenesis is regulated by a neurogenic niche formed from multiple cells. In vivo AIE and in vitro studies find ethanol increases HMGB1-TLR4 signaling and other proinflammatory signaling as well as reducing trophic factors, NGF, and BDNF, coincident with loss of the cholinergic synapse marker vChAT. These changes in gene expression-transcriptomes result in reduced adult neurogenesis. Excitingly, HMGB1 antagonists, anti-inflammatories, and epigenetic modifiers like histone deacetylase inhibitors restore trophic the neurogenesis. These findings suggest anti-inflammatory and epigenetic drugs should be considered for AUD therapy and may provide long-lasting reversal of psychopathology.

Effects of L-Dopa, SKF-38393, and quinpirole on exploratory, anxiety- and depressive-like behaviors in pubertal female and male mice

Adolescence is a phase of substantial changes in the brain, characterized by maturational remodeling of many systems. This remodeling allows functional plasticity to adapt to a changing environment. The dopaminergic system is under morphological and physiological changes during this phase. In the present study, we investigated if changes in the dopaminergic tone alter mice behavior in a receptor and sex-specific manner, specifically at the beginning of the puberty period. We administered L-Dopa, SKF-38393 (D1 dopamine receptor agonist), and Quinpirole (D2 dopamine receptor agonist) and tested male and female mice's motor, anxiety- and depressive-like behavior. While females displayed an impaired exploratory drive, males presented an intense depressive-like response. Our results provide insights into the function of dopaminergic development in adolescent behavior and highlight the importance of studies in this time window with male and female subjects.

Perinatal Western-style diet exposure associated with decreased microglial counts throughout the arcuate nucleus of the hypothalamus in Japanese macaques

Perinatal exposure to a high-fat, high-sugar Western-style diet (WSD) is associated with altered neural circuitry in the melanocortin system. This association may have an underlying inflammatory component, as consumption of a WSD during pregnancy can lead to an elevated inflammatory environment. Our group previously demonstrated that prenatal WSD exposure was associated with increased markers of inflammation in the placenta and fetal hypothalamus in Japanese macaques. In this follow-up study, we sought to determine whether this heightened inflammatory state persisted into the postnatal period, as prenatal exposure to inflammation has been shown to reprogram offspring immune function and long-term neuroinflammation would present a potential means for prolonged disruptions to microglia-mediated neuronal circuit formation. Neuroinflammation was approximated in 1-yr-old offspring by counting resident microglia and peripherally derived macrophages in the region of the hypothalamus examined in the fetal study, the arcuate nucleus (ARC). Microglia and macrophages were immunofluorescently stained with their shared marker, ionized calcium-binding adapter molecule 1 (Iba1), and quantified in 11 regions along the rostral-caudal axis of the ARC. A mixed-effects model revealed main effects of perinatal diet (P = 0.011) and spatial location (P = 0.003) on Iba1-stained cell count. Perinatal WSD exposure was associated with a slight decrease in the number of Iba1-stained cells, and cells were more densely located in the center of the ARC. These findings suggest that the heightened inflammatory state experienced in utero does not persist postnatally. This inflammatory response trajectory could have important implications for understanding how neurodevelopmental disorders progress.NEW & NOTEWORTHY Prenatal Western-style diet exposure is associated with increased microglial activity in utero. However, we found a potentially neuroprotective reduction in microglia count during early postnatal development. This trajectory could inform the timing of disruptions to microglia-mediated neuronal circuit formation. Additionally, this is the first study in juvenile macaques to characterize the distribution of microglia along the rostral-caudal axis of the arcuate nucleus of the hypothalamus. Nearby neuronal populations may be greater targets during inflammatory insults.

Adolescent alcohol drinking interaction with the gut microbiome: implications for adult alcohol use disorder

Reciprocal communication between the gut microbiota and the brain, commonly referred to as the "gut-brain-axis" is crucial in maintaining overall physiological homeostasis. Gut microbiota development and brain maturation (neuronal connectivity and plasticity) appear to be synchronized and to follow the same timeline during childhood (immature), adolescence (expansion) and adulthood (completion). It is important to note that the mesolimbic reward circuitry develops early on, whereas the maturation of the inhibitory frontal cortical neurons is delayed. This imbalance can lead to increased acquirement of reward-seeking and risk-taking behaviors during adolescence, and consequently eventuate in heightened risk for substance abuse. Thus, there is high initiation of alcohol drinking in early adolescence that significantly increases the risk of alcohol use disorder (AUD) in adulthood. The underlying causes for heightened AUD risk are not well understood. It is suggested that alcohol-associated gut microbiota impairment during adolescence plays a key role in AUD neurodevelopment in adulthood. Furthermore, alcohol-induced dysregulation of microglia, either directly or indirectly through interaction with gut microbiota, may be a critical neuroinflammatory pathway leading to neurodevelopmental impairments and AUD. In this review article, we highlight the influence of adolescent alcohol drinking on gut microbiota, gut-brain axis and microglia, and eventual manifestation of AUD. Furthermore, novel therapeutic interventions via gut microbiota manipulations are discussed briefly.

Social Interaction in Adolescent Rats with Neonatal Ethanol Exposure: Impact of Sex and CE-123, a Selective Dopamine Reuptake Inhibitor

Children with fetal alcohol spectrum disorders (FASDs) demonstrate deficits in social functioning that contribute to early withdrawal from school and delinquency, as well as the development of anxiety and depression. Dopamine is involved in reward, motivation, and social behavior. Thus, we evaluated whether neonatal ethanol exposure (in an animal model of FASDs) has an impact on social recognition memory using the three-chamber social novelty discrimination test during early and middle adolescence in male and female rats, and whether the modafinil analog, the novel atypical dopamine reuptake inhibitor CE-123, can modify this effect. Our study shows that male and female rats neonatally exposed to ethanol exhibited sex- and age-dependent deficits in social novelty discrimination in early (male) and middle (female) adolescence. These deficits were specific to the social domain and not simply due to more general deficits in learning and memory because these animals did not exhibit changes in short-term recognition memory in the novel object recognition task. Furthermore, early-adolescent male rats that were neonatally exposed to ethanol did not show changes in the anxiety index but demonstrated an increase in locomotor activity. Chronic treatment with CE-123, however, prevented the appearance of these social deficits. In the hippocampus of adolescent rats, CE-123 increased BDNF and decreased its signal transduction TrkB receptor expression level in ethanol-exposed animals during development, suggesting an increase in neuroplasticity. Thus, selective dopamine reuptake inhibitors, such as CE-123, represent interesting drug candidates for the treatment of deficits in social behavior in adolescent individuals with FASDs.

Gains and Losses: Resilience to Social Defeat Stress in Adolescent Female Mice

BACKGROUND

Adolescence is a unique period of psychosocial growth during which social adversity can negatively influence mental health trajectories. Understanding how adolescent social stress impacts males and females and why some individuals are particularly affected is becoming increasingly urgent. Social defeat stress models for adolescent male mice have been effective in reproducing some physical/psychological aspects of bullying. Designing a model suitable for females has proven challenging.

METHODS

We report a version of the adolescent male accelerated social defeat stress (AcSD) paradigm adapted for females. Early adolescent C57BL/6J female mice (N = 107) were exposed to our modified AcSD procedure twice a day for 4 days and categorized as resilient or susceptible based on a social interaction test 24 hours later. Mice were then assessed for changes in Netrin-1/DCC guidance cue expression in dopamine systems, for inhibitory control in adulthood using the Go/No-Go task, or for alterations in dopamine connectivity organization in the matured prefrontal cortex.

RESULTS

Most adolescent females showed protection against stress-induced social avoidance, but in adulthood, these resilient females developed inhibitory control deficits and showed diminution of prefrontal cortex presynaptic dopamine sites. Female mice classified as susceptible were protected against cognitive and dopaminergic alterations. AcSD did not alter Netrin-1/DCC in early adolescent females, contrary to previous findings with males.

CONCLUSIONS

Preserving prosocial behavior in adolescent females may be important for survival advantage but seems to come at the price of developing persistent cognitive and dopamine deficiencies. The female AcSD paradigm produced findings comparable to those found in males, allowing mechanistic investigation in both sexes.

Sex steroid hormones, the estrous cycle, and rapid modulation of glutamatergic synapse properties in the striatal brain regions with a focus on 17β-estradiol and the nucleus accumbens

The striatal brain regions encompassing the nucleus accumbens core (NAcc), shell (NAcs) and caudate-putamen (CPu) regulate cognitive functions including motivated behaviors, habit, learning, and sensorimotor action, among others. Sex steroid hormone sensitivity and sex differences have been documented in all of these functions in both normative and pathological contexts, including anxiety, depression and addiction. The neurotransmitter glutamate has been implicated in regulating these behaviors as well as striatal physiology, and there are likewise documented sex differences in glutamate action upon the striatal output neurons, the medium spiny neurons (MSNs). Here we review the available data regarding the role of steroid sex hormones such as 17β-estradiol (estradiol), progesterone, and testosterone in rapidly modulating MSN glutamatergic synapse properties, presented in the context of the estrous cycle as appropriate. Estradiol action upon glutamatergic synapse properties in female NAcc MSNs is most comprehensively discussed. In the female NAcc, MSNs exhibit development period-specific sex differences and estrous cycle variations in glutamatergic synapse properties as shown by multiple analyses, including that of miniature excitatory postsynaptic currents (mEPSCs). Estrous cycle-differences in NAcc MSN mEPSCs can be mimicked by acute exposure to estradiol or an ERα agonist. The available evidence, or lack thereof, is also discussed concerning estrogen action upon MSN glutamatergic synapse in the other striatal regions as well as the underexplored roles of progesterone and testosterone. We conclude that there is strong evidence regarding estradiol action upon glutamatergic synapse function in female NAcs MSNs and call for more research regarding other hormones and striatal regions.

Receptors on Microglia

Microglial cells are the most receptive cells in the central nervous system (CNS), expressing several classes of receptors reflecting their immune heritage and newly acquired neural specialisation. Microglia possess, depending on the particular context, receptors to neurotransmitters and neuromodulators as well as immunocompetent receptors. This rich complement allows microglial cells to monitor the functional status of the nervous system, contribute actively to the regulation of neural activity and plasticity and homeostasis, and guard against pathogens as well as other challenges to the CNS's integrity and function.

Substance Use and Addiction

Efforts to reveal the molecular, cellular, and circuit mechanisms of addiction have largely focused on neurons. Yet accumulating data regarding the ability of glial cells to impact synaptic function, circuit activity, and behavior demands that we explore how these nonneuronal cells contribute to substance use disorders and addiction. Important work has shown that glial cells, including microglia, exhibit changes in phenotype following exposure to drugs of abuse and that modification of glial responses can impact behaviors related to drug seeking and drug taking. While these are critical first steps to understanding how microglia can impact addiction, there are still substantial gaps in knowledge that need to be addressed. This chapter reviews some of the key studies that have shown how microglia are affected by and can contribute to addiction. It also discusses areas where more knowledge is urgently needed to reveal new therapeutic and preventative approaches.

Gonadal hormones impart male-biased behavioral vulnerabilities to immune activation via microglial mitochondrial function

There is a strong male bias in the prevalence of many neurodevelopmental disorders such as autism spectrum disorder. However, the mechanisms underlying this sex bias remain elusive. Infection during the perinatal period is associated with an increased risk of neurodevelopmental disorder development. Here, we used a mouse model of early-life immune activation that reliably induces deficits in social behaviors only in males. We demonstrate that male-biased alterations in social behavior are dependent upon microglial immune signaling and are coupled to alterations in mitochondrial morphology, gene expression, and function specifically within microglia, the innate immune cells of the brain. Additionally, we show that this behavioral and microglial mitochondrial vulnerability to early-life immune activation is programmed by the male-typical perinatal gonadal hormone surge. These findings demonstrate that social behavior in males over the lifespan are regulated by microglia-specific mechanisms that are shaped by events that occur in early development.

Sexually dimorphic effects of a modified adolescent social isolation paradigm on behavioral risk factors of alcohol use disorder in Long Evans Rats

Early life stress (ELS) is a major risk factor for alcohol use disorder (AUD) and comorbid neuropsychiatric conditions. We previously demonstrated that an adolescent social isolation (aSI) model of ELS significantly increased behavioral risk factors for these disorders (e.g. anxiety-like behaviors, alcohol drinking) in male, but not female rats. Since many neurodevelopmental milestones are accelerated in females, we investigated whether an earlier/shorter isolation window (PND 21-38) would yield comparable phenotypes in both sexes. In two experiments, Long Evans rats were socially isolated (SI) or group-housed (GH) on postnatal day (PND) 21 and locomotion was assessed in the open field test (OFT; PND 30). Experiment 1 also assessed behavior on the elevated plus-maze (EPM) (PND 32). In Experiment 2, all rats were single housed on PND 38 to assess home cage alcohol drinking. Experiment 1 revealed that SI females had increased locomotor activity in the OFT but did not differ from GH subjects on the EPM. The OFT results were replicated in both sexes in Experiment 2 and both male and female SI rats had significantly greater ethanol consumption during an eight day continuous access paradigm. In contrast, during subsequent intermittent two-bottle choice drinking, only SI females displayed greater ethanol intake and preference and increased consumption of a quinine-adulterated alcohol solution. These findings demonstrate that early life social isolation can promote AUD vulnerability-related phenotypes in female rats but that there are profound sex differences in the vulnerability window to this early life stressor. Uncovering the neural mechanisms responsible for these sexually dimorphic differences in sensitivity to ELS may shed light on the biological substrates associated with vulnerability to AUD and comorbid disorders of negative emotion in men and women.

Microglia at the Tripartite Synapse during Postnatal Development: Implications for Autism Spectrum Disorders and Schizophrenia

Synapses are the fundamental structures of neural circuits that control brain functions and behavioral and cognitive processes. Synapses undergo formation, maturation, and elimination mainly during postnatal development via a complex interplay with neighboring astrocytes and microglia that, by shaping neural connectivity, may have a crucial role in the strengthening and weakening of synaptic functions, that is, the functional plasticity of synapses. Indeed, an increasing number of studies have unveiled the roles of microglia and astrocytes in synapse formation, maturation, and elimination as well as in regulating synaptic function. Over the past 15 years, the mechanisms underlying the microglia- and astrocytes-dependent regulation of synaptic plasticity have been thoroughly studied, and researchers have reported that the disruption of these glial cells in early postnatal development may underlie the cause of synaptic dysfunction that leads to neurodevelopmental disorders such as autism spectrum disorder (ASD) and schizophrenia.

Glial regulation of critical period plasticity

Animal behavior, from simple to complex, is dependent on the faithful wiring of neurons into functional neural circuits. Neural circuits undergo dramatic experience-dependent remodeling during brief developmental windows called critical periods. Environmental experience during critical periods of plasticity produces sustained changes to circuit function and behavior. Precocious critical period closure is linked to autism spectrum disorders, whereas extended synaptic remodeling is thought to underlie circuit dysfunction in schizophrenia. Thus, resolving the mechanisms that instruct critical period timing is important to our understanding of neurodevelopmental disorders. Control of critical period timing is modulated by neuron-intrinsic cues, yet recent data suggest that some determinants are derived from neighboring glial cells (astrocytes, microglia, and oligodendrocytes). As glia make up 50% of the human brain, understanding how these diverse cells communicate with neurons and with each other to sculpt neural plasticity, especially during specialized critical periods, is essential to our fundamental understanding of circuit development and maintenance.

The effect of morphine on rat microglial phagocytic activity: An in vitro study of brain region-, plating density-, sex-, morphine concentration-, and receptor-dependency

Opioids have long been used for clinical pain management, but also have addictive properties that have contributed to the ongoing opioid epidemic. While opioid activation of opioid receptors is well known to contribute to reward and reinforcement, data now also suggest that opioid activation of immune signaling via toll-like receptor 4 (TLR4) may also play a role in addiction-like processes. TLR4 expression is enriched in immune cells, and in the nervous system is primarily expressed in microglia. Microglial phagocytosis is important for developmental, homeostatic, and pathological processes. To examine how morphine impacts microglial phagocytosis, we isolated microglia from adult male and female rat cortex and striatum and plated them in vitro at 10,000 (10K) or 50,000 cells/well densities. Microglia were incubated with neutral fluorescent microbeads to stimulate phagocytosis in the presence of one of four morphine concentrations. We found that the brain region from which microglia are isolated and plating density, but not morphine concentration, impacts cell survival in vitro. We found that 10-12 M morphine, but not higher concentrations, increases phagocytosis in striatal microglia in vitro independent of sex and plating density, while 10-12 M morphine increased phagocytosis in cortical microglia in vitro independent of sex, but contingent on a plating density. Finally, we demonstrate that the effect of 10-12 M morphine in striatal microglia plated at 10 K density is mediated via TLR4, and not μORs. Overall, our data suggest that in rats, a morphine-TLR4 signaling pathway increases phagocytic activity in microglia independent of sex. This may is useful information for better understanding the possible neural outcomes associated with morphine exposures.