Differential effects of homotypic vs. heterotypic chronic stress regimens on microglial activation in the prefrontal cortex

How does chronic psychosocial distress induce pain? Focus on neuroinflammation and neuroplasticity changes

Chronic primary pain including fibromyalgia for the musculoskeletal system persists for more than 3 months. Its etiological factors and the pathophysiological mechanisms are not known, and therefore, there is no satisfactory therapy, it is an unmet medical need condition. The only etiological and aggravating factor is chronic psychosocial distress, which is known to cause neuroimmune and endocrine changes both in the periphery and the central nervous system. In this short review, we introduce our research perspective by summarizing the recent literature on the interactions between chronic pain, stress, and commonly co-morbid mood disorders. Immune activation, autoimmunity, neuro-immune-vascular crosstalks and neuroinflammation play roles in the pathophysiology of these conditions. Data on stress-induced neuroplasticity changes at cellular and molecular levels were also collected in relation to chronic primary pain both from clinical studies and animal experiments of translational relevance. Understanding these mechanisms could help to identify novel therapeutic targets for chronic primary pain including fibromyalgia.

Immuno-surveillance and protection of the human cochlea

Background

Despite its location near infection-prone areas, the human inner ear demonstrates remarkable resilience. This suggests that there are inherent instruments deterring the invasion and spread of pathogens into the inner ear. Here, we combined high-resolution light microscopy, super-resolution immunohistochemistry (SR-SIM) and synchrotron phase contrast imaging (SR-PCI) to identify the protection and barrier systems in the various parts of the human inner ear, focusing on the lateral wall, spiral ganglion, and endolymphatic sac.

Materials and methods

Light microscopy was conducted on mid-modiolar, semi-thin sections, after direct glutaraldehyde/osmium tetroxide fixation. The tonotopic locations were estimated using SR-PCI and 3D reconstruction in cadaveric specimens. The sections were analyzed for leucocyte and macrophage activity, and the results were correlated with immunohistochemistry using confocal microscopy and SR-SIM.

Results

Light microscopy revealed unprecedented preservation of cell anatomy and several macrophage-like cells that were localized in the cochlea. Immunohistochemistry demonstrated IBA1 cells frequently co-expressing MHC II in the spiral ganglion, nerve fibers, lateral wall, spiral limbus, and tympanic covering layer at all cochlear turns as well as in the endolymphatic sac. RNAscope assays revealed extensive expression of fractalkine gene transcripts in type I spiral ganglion cells. CD4 and CD8 cells occasionally surrounded blood vessels in the modiolus and lateral wall. TMEM119 and P2Y12 were not expressed, indicating that the cells labeled with IBA1 were not microglia. The round window niche, compact basilar membrane, and secondary spiral lamina may form protective shields in the cochlear base.

Discussion

The results suggest that the human cochlea is surveilled by dwelling and circulating immune cells. Resident and blood-borne macrophages may initiate protective immune responses via chemokine signaling in the lateral wall, spiral lamina, and spiral ganglion at different frequency locations. Synchrotron imaging revealed intriguing protective barriers in the base of the cochlea. The role of the endolymphatic sac in human inner ear innate and adaptive immunity is discussed.

Stress and Microglia: A Double-edged Relationship

Microglia are highly dynamic cells and acquire different activation states to modulate their multiple functions, which are tightly regulated by the central nervous system microenvironment in which they reside. In response to stress, that is to the appearance of non-physiological signals in their vicinity, microglia will adapt their function in order to promote a return to brain homeostasis. However, when these stress signals are chronically present, microglial response may not be adapted and lead to the establishment of a pathological state. The aim of this book chapter is to examine the substantial literature around the ability of acute and chronic stressors to affect microglial structure and function, with a special focus on psychosocial and nutritional stresses. We also discuss the molecular mechanisms known to date that explain the link between exposure to stressors and microglial activation.

Chemokine receptor 5 signaling in PFC mediates stress susceptibility in female mice

Chronic stress induces changes in the periphery and the central nervous system (CNS) that contribute to neuropathology and behavioral abnormalities associated with psychiatric disorders. In this study, we examined the impact of peripheral and central inflammation during chronic social defeat stress (CSDS) in female mice. Compared to male mice, we found that female mice exhibited heightened peripheral inflammatory response and identified C-C motif chemokine ligand 5 (CCL5), as a stress-susceptibility marker in females. Blocking CCL5 signaling in the periphery promoted resilience to CSDS. In the brain, stress-susceptible mice displayed increased expression of C-C chemokine receptor 5 (CCR5), a receptor for CCL5, in microglia in the prefrontal cortex (PFC). This upregulation was associated with microglia morphological changes, their increased migration to the blood vessels, and enhanced phagocytosis of synaptic components and vascular material. These changes coincided with neurophysiological alterations and impaired blood-brain barrier (BBB) integrity. By blocking CCR5 signaling specifically in the PFC were able to prevent stress-induced physiological changes and rescue social avoidance behavior. Our findings are the first to demonstrate that stress-mediated dysregulation of the CCL5-CCR5 axis triggers excessive phagocytosis of synaptic materials and neurovascular components by microglia, resulting in disruptions in neurotransmission, reduced BBB integrity, and increased stress susceptibility. Our study provides new insights into the role of cortical microglia in female stress susceptibility and suggests that the CCL5-CCR5 axis may serve as a novel sex-specific therapeutic target for treating psychiatric disorders in females.

Involvement of brain cell phenotypes in stress-vulnerability and resilience

Stress-related disorders' prevalence is epidemically increasing in modern society, leading to a severe impact on individuals' well-being and a great economic burden on public resources. Based on this, it is critical to understand the mechanisms by which stress induces these disorders. The study of stress made great progress in the past decades, from deeper into the hypothalamic-pituitary-adrenal axis to the understanding of the involvement of a single cell subtype on stress outcomes. In fact, many studies have used state-of-the-art tools such as chemogenetic, optogenetic, genetic manipulation, electrophysiology, pharmacology, and immunohistochemistry to investigate the role of specific cell subtypes in the stress response. In this review, we aim to gather studies addressing the involvement of specific brain cell subtypes in stress-related responses, exploring possible mechanisms associated with stress vulnerability versus resilience in preclinical models. We particularly focus on the involvement of the astrocytes, microglia, medium spiny neurons, parvalbumin neurons, pyramidal neurons, serotonergic neurons, and interneurons of different brain areas in stress-induced outcomes, resilience, and vulnerability to stress. We believe that this review can shed light on how diverse molecular mechanisms, involving specific receptors, neurotrophic factors, epigenetic enzymes, and miRNAs, among others, within these brain cell subtypes, are associated with the expression of a stress-susceptible or resilient phenotype, advancing the understanding/knowledge on the specific machinery implicate in those events.

Stressed Microglia: Neuroendocrine-Neuroimmune Interactions in the Stress Response

Stressful life experiences are associated with the development of neuropsychiatric disorders like depression. Emerging evidence indicates that microglia, the specialized resident macrophages of the brain, may be a key mediator of the relationship between psychosocial stressor exposure and adaptive or maladaptive responses at the level of synaptic, circuit, and neuroimmune alterations. Here, we review current literature regarding how psychosocial stressor exposure changes microglial structure and function, thereby altering behavioral and brain outcomes, with a particular focus on age- and sex-dependent effects. We argue that additional emphasis should be placed in future research on investigating sex differences and the impacts of stressor exposure during sensitive periods of development, as well as going beyond traditional morphological measurements to interrogate microglial function. The bidirectional relationship between microglia and the stress response, particularly the role of microglia in the neuroendocrine control of stress-related circuits, is also an important area for future investigation. Finally, we discuss emerging themes and future directions that point to the possibility of the development of novel therapeutics for stress-related neuropsychiatric disorders.

Voluntary running exercise modifies astrocytic population and features in the peri-infarct cortex

Rehabilitative exercise following a brain stroke has beneficial effects on the morphological plasticity of neurons. Particularly, voluntary running exercise after focal cerebral ischemia promotes functional recovery and ameliorates ischemia-induced dendritic spine loss in the peri-infarct motor cortex layer 5. Moreover, neuronal morphology is affected by changes in the perineuronal environment. Glial cells, whose phenotypes may be altered by exercise, are known to play a pivotal role in the formation of this perineuronal environment. Herein, we investigated the effects of voluntary running exercise on glial cells after middle cerebral artery occlusion. Voluntary running exercise increased the population of glial fibrillary acidic protein-positive astrocytes born between post-operative days (POD) 0 and 3 on POD15 in the peri-infarct cortex. After exercise, transcriptomic analysis of post-ischemic astrocytes revealed 10 upregulated and 70 downregulated genes. Furthermore, gene ontology analysis showed that the 70 downregulated genes were significantly associated with neuronal morphology. In addition, exercise reduced the number of astrocytes expressing lipocalin 2, a regulator of dendritic spine density, on POD15. Our results suggest that exercise modifies the composition of astrocytic population and their phenotype.

Impact of exercise on brain-bone marrow interactions in chronic stress: potential mechanisms preventing stress-induced hypertension

We examined the effect of chronic restraint stress and the counteractive effects of daily exercise on the molecular basis of the brain-bone marrow (BM) interactions, by especially focusing on the paraventricular nucleus (PVN) of the hypothalamus. Male Wistar rats were assigned into control, restraint stress, and stress + daily spontaneous exercise (SE) groups. BM and hypothalamic gene expression profiles were examined through the undertaking of RT-PCR and microarrays, respectively. The inflammatory blood cell population was investigated through flow cytometry. Through the use of immunohistochemistry, we examined the presence of BM-derived C-C chemokine receptor type 2 (CCR2)-expressing microglial cells in the rat PVN. The gene expression levels of BM inflammatory factors such as those of interleukin 1 beta and CCR2, and the inflammatory blood cell population were found to be significantly higher in both restrained groups compared with control group. Interestingly, chronic restraint stress alone activated the recruitment of BM-derived CCR2-expressing microglial cells into the PVN, whereas daily spontaneous exercise prevented it. A notable finding was that restraint stress upregulated relative gene expression of hypothalamic matrix metalloproteinase 3 (MMP3), which increases the permeability of the blood-brain barrier (BBB), and that exercise managed to normalize it. Moreover, relative expression of some hypothalamic genes directly involved in the facilitation of cell migration was downregulated by daily exercise. Our findings suggest that daily spontaneous exercise can reduce the numbers of BM-derived CCR2-expressing microglial cells into the PVN through the prevention of stress-induced changes in the hypothalamic gene expression.NEW & NOTEWORTHY Chronic restraint stress can upregulate MMP3 gene expression in the rat hypothalamus, whereas daily spontaneous exercise can prevent this stress-induced effect. Stress-induced BM-derived inflammatory cell recruitment into the rat PVN can be prevented by daily spontaneous exercise. Stress-induced increase of hypothalamic MMP3 gene expression may be responsible for BBB injury, thereby allowing for BM-derived inflammatory cells to be recruited and to accumulate in the rat PVN, and to be subsequently involved in the onset of stress-induced hypertension.

A consideration of the increased risk of schizophrenia due to prenatal maternal stress, and the possible role of microglia

Schizophrenia is caused by interaction of a combination of genetic and environmental factors. Of the latter, prenatal exposure to maternal stress is reportedly associated with elevated disease risk. The main orchestrators of inflammatory processes within the brain are microglia, and aberrant microglial activation/function has been proposed to contribute to the aetiology of schizophrenia. Here, we evaluate the epidemiological and preclinical evidence connecting prenatal stress to schizophrenia risk, and consider the possible mediating role of microglia in the prenatal stress-schizophrenia relationship. Epidemiological findings are rather consistent in supporting the association, albeit they are mitigated by effects of sex and gestational timing, while the evidence for microglial activation is more variable. Rodent models of prenatal stress generally report lasting effects on offspring neurobiology. However, many uncertainties remain as to the mechanisms underlying the influence of maternal stress on the developing foetal brain. Future studies should aim to characterise the exact processes mediating this aspect of schizophrenia risk, as well as focussing on how prenatal stress may interact with other risk factors.

Vulnerability and resilience to cardiovascular and neuroendocrine effects of stress in adult rats with historical of chronic stress during adolescence

AIMS

This study investigated the influence of exposure to stress during adolescence in autonomic, cardiovascular, neuroendocrine and somatic changes evoked by chronic stress in adult rats.

MAIN METHODS

Animals were subjected to a 10-days protocol of repeated restraint stress (RRS, habituating) or chronic variable stress (CVS, non-habituating) during adolescence, adulthood, or repeated exposure to either RRS or CVS in adolescence and adulthood (adolescence+adulthood group). The trials to measure autonomic, cardiovascular, neuroendocrine and somatic changes in all experimental groups were performed in adulthood.

KEY FINDINGS

CVS increased basal circulating corticosterone levels and caused adrenal hypertrophy in the adolescence+adulthood group, an effect not identified in animals subjected to this stressor only in adulthood or adolescence. CVS also caused a sympathetically-mediated resting tachycardia in the adulthood group. This effect of CVS was not identified in the adolescence+adulthood group once the increased cardiac sympathetic activity was buffered by a decrease in intrinsic heart rate in these animals. Moreover, the impairment in baroreflex function observed in the adulthood group subjected to CVS was shifted to an improvement in animals subjected to repeated exposure to this stressor during adolescence and adulthood. The RRS in the adolescence+adulthood group caused a sympathetically-mediated resting tachycardia, which was not observed in the adulthood group.

SIGNIFICANCE

Our findings suggest that enduring effects of adverse events during adolescence included a vulnerability to neuroendocrine changes and a resilience to autonomic and cardiovascular dysfunctions caused by the CVS. Furthermore, results of RRS indicated a vulnerability to cardiovascular and autonomic changes evoked by homotypic stressors.

Perinatal Morphine Exposure Leads to Sex-Dependent Executive Function Deficits and Microglial Changes in Mice

Children exposed prenatally to opioids are at an increased risk for behavioral problems and executive function deficits. The prefrontal cortex (PFC) and amygdala (AMG) regulate executive function and social behavior and are sensitive to opioids prenatally. Opioids can bind to toll-like receptor 4 (TLR4) to activate microglia, which may be developmentally important for synaptic pruning. Therefore, we tested the effects of perinatal morphine exposure on executive function and social behavior in male and female mouse offspring, along with microglial-related and synaptic-related outcomes. Dams were injected once daily subcutaneously with saline (n = 8) or morphine (MO; 10 mg/kg; n = 12) throughout pregestation, gestation, and lactation until offspring were weaned on postnatal day 21 (P21). Male MO offspring had impairments in attention and accuracy in the five-choice serial reaction time task, while female MO offspring were less affected. Targeted gene expression analysis at P21 in the PFC identified alterations in microglial-related and TLR4-related genes, while immunohistochemical analysis in adult brains indicated decreased microglial Iba1 and phagocytic CD68 proteins in the PFC and AMG in males, but females had an increase. Further, both male and female MO offspring had increased social preference. Overall, these data demonstrate male vulnerability to executive function deficits in response to perinatal opioid exposure and evidence for disruptions in neuron-microglial signaling.

The Role of Microglia in the (Mal)adaptive Response to Traumatic Experience in an Animal Model of PTSD

The present study investigates whether predator scent-stress (PSS) shifts the microglia from a quiescent to a chronically activated state and whether morphological alterations in microglial activation differ between individuals displaying resilient vs. vulnerable phenotypes. In addition, we examined the role that GC receptors play during PSS exposure in the impairment of microglial activation and thus in behavioral response. Adult male Sprague Dawley rats were exposed to PSS or sham-PSS for 15 min. Behaviors were assessed with the elevated plus-maze (EPM) and acoustic startle response (ASR) paradigms 7 days later. Localized brain expression of Iba-1 was assessed, visualized, and classified based on their morphology and stereological counted. Hydrocortisone and RU486 were administered systemically 10 min post PSS exposure and behavioral responses were measured on day 7 and hippocampal expression of Ionized calcium-binding adaptor molecule 1 (Iba-1) was subsequently evaluated. Animals whose behavior was extremely disrupted (PTSD-phenotype) selectively displayed excessive expression of Iba-1 with concomitant downregulation in the expression of CX3C chemokine receptor 1 (CX3CR1) in hippocampal structures as compared with rats whose behavior was minimally or partially disrupted. Changes in microglial morphology have also been related only to the PTSD-phenotype group. These data indicate that PSS-induced microglia activation in the hippocampus serves as a critical mechanistic link between the HPA-axis and PSS-induced impairment in behavioral responses.

Between Science and Big Business: Tapping Mary Jane's Uncharted Potential

At the intersection of science and medicine, government policy, and pop culture, cannabis has prompted society since the beginning of recorded history. And yet, there is comparatively little replicable data on the plant, its constituents, and their capacity to modify human physiology. Over the past decades, several findings have pointed toward the importance of the endogenous cannabinoid system in maintaining homeostasis, making it an important target for various diseases. Here, we summarize the current state of knowledge on endogenous- and plant-based cannabinoids, address the issues related to cannabinoid-based drug discovery, and incite efforts to utilize their polypharmacological profile toward tackling diseases with a complex underlying pathophysiology. By fusing modern science and technology with the empirical data that has been gathered over centuries, we propose an outlook that could help us overcome the dearth of innovation for new drugs and synchronously redefine the future of drug discovery. Simultaneously, we call attention to the startling disconnect between the scientific, regulatory, and corporate entities that is becoming increasingly evident in this booming industry.

Sex and region-specific effects of variable stress on microglia morphology

Major Depressive Disorder (MDD) is a common and debilitating mood disorder that is more prevalent in women than men. In humans, PET imaging of microglia activation is currently being explored as a potential biomarker of MDD and suicidal ideation. Stress is a trigger for many mood disorders, including MDD. Microglial changes in morphology and activation state in response to stress has been reported in various brain regions, but most studies only examined male subjects. Here we report changes in microglia morphology in the nucleus accumbens (NAc) and subregions of the hippocampus (HPC) in both male and female mice following variable stress of 6 or 28 days in duration. Our data demonstrate that after 6 days of stress, microglia in the female NAc and dentate gyrus have a reduction in homeostatic associated morphology and an increase in primed microglia. After 28 days some of these sex specific stress effects were still present in microglia within the NAc but not the dentate gyrus. There were no effects of stress in either sex at either timepoint in CA1. In female mice, anti-inflammatory activation of microglia using rosiglitazone promoted sociability behavior after 6 days of stress. Furthermore, both drug and stress have impact on microglia morphology and activation state in the NAc. These data suggest that microglia morphology and activation state are altered by 6 days of variable stress in a region-specific manner and may contribute to, or potentially compensate for, the onset of stress susceptibility rather than impacting long term exposure to stress.

The semantics of microglia activation: neuroinflammation, homeostasis, and stress

Microglia are emerging as critical regulators of neuronal function and behavior in nearly every area of neuroscience. Initial reports focused on classical immune functions of microglia in pathological contexts, however, immunological concepts from these studies have been applied to describe neuro-immune interactions in the absence of disease, injury, or infection. Indeed, terms such as 'microglia activation' or 'neuroinflammation' are used ubiquitously to describe changes in neuro-immune function in disparate contexts; particularly in stress research, where these terms prompt undue comparisons to pathological conditions. This creates a barrier for investigators new to neuro-immunology and ultimately hinders our understanding of stress effects on microglia. As more studies seek to understand the role of microglia in neurobiology and behavior, it is increasingly important to develop standard methods to study and define microglial phenotype and function. In this review, we summarize primary research on the role of microglia in pathological and physiological contexts. Further, we propose a framework to better describe changes in microglia1 phenotype and function in chronic stress. This approach will enable more precise characterization of microglia in different contexts, which should facilitate development of microglia-directed therapeutics in psychiatric and neurological disease.

Pubertal probiotics mitigate lipopolysaccharide-induced programming of the hypothalamic-pituitary-adrenal axis in male mice only

Puberty is a period of rapid cortical and neuronal development. Stress exposure during puberty programs the hypothalamic-pituitary-adrenal (HPA) axis responsiveness to future stressors. However, programming can result in an enduring maladaptation of the HPA axis activity and can be associated with long-term anxiety- and depression-like behaviours. Probiotic treatment mitigates the effect of stress on mental health, suggesting that the gut microbiome may mediate the programming of the HPA axis. However, the mechanism underlying this effect remains elusive. Thus, we investigated the effect of probiotic exposure on lipopolysaccharide (LPS)-induced programming of the HPA axis and glucocorticoid receptor (GR) expression in the paraventricular (PVN), basolateral amygdala (BLA), piriform cortex (PIR), and medial prefrontal cortex (mPFC). Male and female mice were exposed to either probiotics or control skim milk and were treated with either saline or LPS during puberty. Prior to euthanasia in adulthood, mice were restrained for 30 min. The results showed that pubertal LPS treatment permanently decreased GR expression in the PVN in milk fed control males. However, pubertal probiotic treatment blocked the LPS-induced decrease in GR expression in males. Given that this effect is limited to males, further research is required to better understand sex differences in the interactions between the gut microbiome and the programming of the HPA axis during puberty. Nevertheless, our findings suggest that the gut microbiome influences the neurophysiology of the HPA axis and mediates its programming in pubertal males. The prevention of GR reduction in the male PVN and PIR using probiotics illustrates the complexity of the gut-brain communication and compels continued investigation.

Type 1 interferon mediates chronic stress-induced neuroinflammation and behavioral deficits via complement component 3-dependent pathway

Chronic stress is a major risk factor in the pathophysiology of many neuropsychiatric disorders. Further, chronic stress conditions can promote neuroinflammation and inflammatory responses in both humans and animal models. Type I interferons (IFN-I) are critical mediators of the inflammatory response in the periphery and responsible for the altered mood and behavior. However, the underlying mechanisms are not well understood. In the present study, we investigated the role of IFN-I signaling in chronic stress-induced changes in neuroinflammation and behavior. Using the chronic restraint stress model, we found that chronic stress induces a significant increase in serum IFNβ levels in mice, and systemic blockade of IFN-I signaling attenuated chronic stress-induced infiltration of macrophages into prefrontal cortex and behavioral abnormalities. Furthermore, complement component 3 (C3) mediates systemic IFNβ-induced changes in neuroinflammation and behavior. Also, we found significant increases in the mRNA expression levels of IFN-I stimulated genes in the prefrontal cortex of depressed suicide subjects and significant correlation with C3 and inflammatory markers. Together, these findings from animal and human postmortem brain studies identify a crucial role of C3 in IFN-I-mediated changes in neuroinflammation and behavior under chronic stress conditions.

Stress induces microglia-associated synaptic circuit alterations in the dorsomedial prefrontal cortex

The mammalian dorsomedial prefrontal cortex (dmPFC) receives diverse inputs and plays important roles in adaptive behavior and cognitive flexibility. Stress, a major risk factor for many psychiatric disorders, compromises the structure and function of multiple brain regions and circuits. Here we show that 7-day restraint stress impairs reversal learning in the 4-choice odor discrimination test, a decision-making task requiring an intact dmPFC. In vivo two-photon imaging further reveals that stress increases dmPFC dendritic spine elimination, particularly those of the mushroom morphology, without affecting spine formation. In addition, stress alters dmPFC microglial branching complexity and elevates their terminal process dynamics. In stressed mice, dmPFC microglia contact dendrites more frequently, and dendritic spines with microglial contact are prone to elimination. In summary, our work suggests that stress-induced changes in glial-synapse interaction contributes to synaptic loss in dmPFC, resulting in neuronal circuit deficits and impaired cognitive flexibility.

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Restraint stress impairs cognitive flexibility in adolescent mice.

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Stress leads to synapse loss on pyramidal neurons in the dorsomedial prefrontal cortex.

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Stress decreases microglial complexity but increases their terminal dynamics and contacts with dendritic spines.

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Dendritic spines contacted by microglial processes are more prone to elimination.

Paternal exposure to exercise and/or caffeine and alcohol modify offspring behavioral and pathophysiological recovery from repetitive mild traumatic brain injury in adolescence

Only recently has the scope of parental research expanded to include the paternal sphere with epidemiological studies implicating stress, nutrition and alcohol consumption in the neurobiological and behavioral characteristics of offspring. This study was designed to determine if paternal exposure to caffeine, alcohol and exercise prior to conception would improve or exacerbate offspring recovery from adolescent repetitive mild traumatic brain injury (RmTBI). Sires received 7 weeks of standard drinking water, or caffeine and ethanol and were housed in regular cages or cages with running wheels, prior to being mated to control females. At postnatal day 40, offspring were administered RmTBI or sham injuries and were assessed for post concussive symptomology. Post-mortem quantitative real-time polymerase chain reaction (qRT-PCR) was used to assess gene expression in the prefrontal cortex (PFC), nucleus accumbens (NAc) and changes in telomere length. Additionally, enzyme-linked immunosorbent assay (ELISA's) were run on serum to detect levels of cytokines, chemokines and sex hormones. Paternal experience did not improve or exacerbate RmTBI behavioral outcomes. However, female and male offspring displayed unique responses to RmTBI and paternal experience, resulting in changes in physical, behavioral and molecular outcomes. Injury and paternal exercise modified changes in female offspring, whereas male offspring were affected by paternal exercise, caffeine and alcohol treatment. Additionally, paternal experience and RmTBI modified expression of many genes in the PFC, NAc, telomere length and levels of sex hormones. Although further exploration is required to understand the heterogeneity that exists in disease risk and resiliency, this study provides corroborating evidence that paternal experiences prior to conception influences offspring development.

Microglia Fighting for Neurological and Mental Health: On the Central Nervous System Frontline of COVID-19 Pandemic

Coronavirus disease 2019 (COVID-19) is marked by cardio-respiratory alterations, with increasing reports also indicating neurological and psychiatric symptoms in infected individuals. During COVID-19 pathology, the central nervous system (CNS) is possibly affected by direct severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) invasion, exaggerated systemic inflammatory responses, or hypoxia. Psychosocial stress imposed by the pandemic further affects the CNS of COVID-19 patients, but also the non-infected population, potentially contributing to the emergence or exacerbation of various neurological or mental health disorders. Microglia are central players of the CNS homeostasis maintenance and inflammatory response that exert their crucial functions in coordination with other CNS cells. During homeostatic challenges to the brain parenchyma, microglia modify their density, morphology, and molecular signature, resulting in the adjustment of their functions. In this review, we discuss how microglia may be involved in the neuroprotective and neurotoxic responses against CNS insults deriving from COVID-19. We examine how these responses may explain, at least partially, the neurological and psychiatric manifestations reported in COVID-19 patients and the general population. Furthermore, we consider how microglia might contribute to increased CNS vulnerability in certain groups, such as aged individuals and people with pre-existing conditions.

Neuroendocrine, neuroinflammatory and pathological outcomes of chronic stress: A story of microglial remodeling

Microglia, the resident macrophage cells of the central nervous system (CNS), are involved in a myriad of processes required to maintain CNS homeostasis. These cells are dynamic and can adapt their phenotype and functions to the physiological needs of the organism. Microglia rapidly respond to changes occurring in their microenvironment, such as the ones taking place during stress. While stress can be beneficial for the organism to adapt to a situation, it can become highly detrimental when it turns chronic. Microglial response to prolonged stress may lead to an alteration of their beneficial physiological functions, becoming either maladaptive or pro-inflammatory. In this review, we aim to summarize the effects of chronic stress exerted on microglia through the neuroendocrine system and inflammation at adulthood. We also discuss how these effects of chronic stress could contribute to microglial involvement in neuropsychiatric and sleep disorders, as well as neurodegenerative diseases.

How stress physically re-shapes the brain: Impact on brain cell shapes, numbers and connections in psychiatric disorders

Severe stress is among the most robust risk factors for the development of psychiatric disorders. Imaging studies indicate that life stress is integral to shaping the human brain, especially regions involved in processing the stress response. Although this is likely underpinned by changes to the cytoarchitecture of cellular networks in the brain, we are yet to clearly understand how these define a role for stress in human psychopathology. In this review, we consolidate evidence of macro-structural morphometric changes and the cellular mechanisms that likely underlie them. Focusing on stress-sensitive regions of the brain, we illustrate how stress throughout life may lead to persistent remodelling of the both neurons and glia in cellular networks and how these may lead to psychopathology. We support that greater translation of cellular alterations to human cohorts will support parsing the psychological sequalae of severe stress and improve our understanding of how stress shapes the human brain. This will remain a critical step for improving treatment interventions and prevention outcomes.

Hypothalamic Inflammation as a Potential Pathophysiologic Basis for the Heterogeneity of Clinical, Hormonal, and Metabolic Presentation in PCOS

Polycystic ovary syndrome (PCOS) is the most common endocrine disorder among women of reproductive age. It is a heterogeneous condition characterized by reproductive, endocrine, metabolic, and psychiatric abnormalities. More than one pathogenic mechanism is involved in its development. On the other hand, the hypothalamus plays a crucial role in many important functions of the body, including weight balance, food intake, and reproduction. A high-fat diet with a large amount of long-chain saturated fatty acids can induce inflammation in the hypothalamus. Hypothalamic neurons can sense extracellular glucose concentrations and participate, with a feedback mechanism, in the regulation of whole-body glucose homeostasis. When consumed nutrients are rich in fat and sugar, and these regulatory mechanisms can trigger inflammatory pathways resulting in hypothalamic inflammation. The latter has been correlated with metabolic diseases, obesity, and depression. In this review, we explore whether the pattern and the expansion of hypothalamic inflammation, as a result of a high-fat and -sugar diet, may contribute to the heterogeneity of the clinical, hormonal, and metabolic presentation in PCOS via pathophysiologic mechanisms affecting specific areas of the hypothalamus. These mechanisms could be potential targets for the development of effective therapies for the treatment of PCOS.

Diazepam limits microglia-mediated neuronal remodeling in the prefrontal cortex and associated behavioral consequences following chronic unpredictable stress

Chronic stress induces neuronal atrophy and synaptic loss in the medial prefrontal cortex (PFC), and this leads to behavioral and cognitive impairments. Our recent findings indicate that microglia contribute to structural remodeling of neurons via increased colony-stimulating factor (CSF)-1 in the medial PFC. Other work shows that chronic stress induces aberrant neuronal activity in the medial PFC, and that neuronal hyperactivity increases CSF1 signaling and alters microglia function. Thus, the present studies were designed to examine the role of neuronal activity in stress-induced CSF1 signaling and microglia-mediated neuronal remodeling in the medial PFC. Additional analyses probed stress effects on the dorsal hippocampus (HPC), basolateral amygdala (BLA), and somatosensory cortex (SSCTX). Mice were exposed to chronic unpredictable stress (CUS) or handled intermittently as controls, and received daily injection of vehicle or diazepam (1 mg/kg). As anticipated, diazepam attenuated CUS-induced behavioral despair and cognitive impairments. Further studies showed that diazepam normalized Csf1 and C3 mRNA in the PFC, and prevented increases in Csf1r and Cd11b in frontal cortex microglia following CUS. Stress had no effect on neuroimmune gene expression in the HPC. Confocal imaging in Thy1-GFP(M) mice demonstrated that diazepam limited microglial engulfment of neuronal elements and blocked CUS-induced dendritic spine loss in the medial PFC. Altogether, these findings indicate that modulation of chronic stress-induced neuronal activity limits microglia-mediated neuronal remodeling in the medial PFC, and subsequent behavioral and cognitive consequences.

Neuroinflammatory alterations in trait anxiety: modulatory effects of minocycline

High trait anxiety is a substantial risk factor for developing anxiety disorders and depression. While neuroinflammation has been identified to contribute to stress-induced anxiety, little is known about potential dysregulation in the neuroinflammatory system of genetically determined pathological anxiety or high trait anxiety individuals. We report microglial alterations in various brain regions in a mouse model of high trait anxiety (HAB). In particular, the dentate gyrus (DG) of the hippocampus of HABs exhibited enhanced density and average cell area of Iba1+, and density of phagocytic (CD68+/Iba1+) microglia compared to normal anxiety (NAB) controls. Minocycline was used to assess the capacity of a putative microglia 'inhibitor' in modulating hyperanxiety behavior of HABs. Chronic oral minocycline indeed reduced HAB hyperanxiety, which was associated with significant decreases in Iba1+ and CD68+Iba1+ cell densities in the DG. Addressing causality, it was demonstrated that longer (10 days), but not shorter (5 days), periods of minocycline microinfusions locally into the DG of HAB reduced Iba-1+ cell density and attenuated hyperanxiety-related behavior, indicating that neuroinflammation in the DG is at least partially involved in the maintenance of pathological anxiety. The present data reveal evidence of disturbances in the microglial system of individuals with high trait anxiety. Minocycline attenuated HAB hyperanxiety, likely by modulation of microglial activity within the DG. Thus, the present data suggest that drugs with microglia-targeted anti-inflammatory properties could be promising as novel alternative or complimentary anxiolytic therapeutic approaches in specific subgroups of individuals genetically predisposed to hyperanxiety.

Ovariectomy Induces Microglial Cell Activation and Inflammatory Response in Rat Prefrontal Cortices to Accelerate the Chronic Unpredictable Stress-Mediated Anxiety and Depression

Perimenopausal women are associated with increased risks of depression and anxiety, which may be potentially related to the lack of ovarian hormone with antidepression activity in the body. However, the precise mechanism remains unclear so far. This study first adopted the Sprague-Dawley (SD) female rats to construct the ovariectomy (OVX) combined with a chronic unpredictable stress (CUS) model. Then, a series of behavioral experimental results revealed that the ovariectomized rats receiving CUS had remarkably elevated anxiety and depression behaviors relative to those in sham group rats, and the sucrose preference rate in the sucrose preference test (SPT) was evidently reduced. In elevated plus maze test (EPM) experiment, the open arm entry time and open arm duration were decreased. In the open field test (OFT), the number of line crossings, rearing number, center square entries, and center square duration were reduced; the grooming time was extended; and the number of fecal particles in rats was increased. In the forced swimming test (FST), the rat immobility rate was increased, while the numbers of swimming and crawling were decreased. Afterwards, we discovered that OVX downregulated the serum levels of estradiol and corticosterone in rats. Thereafter, IF results suggested that OVX dramatically induced the increasing of the number of activated microglial cells in prefrontal cortices and the level of M1-type marker iNOS. Finally, PCR results demonstrated that, compared with the sham group, the proinflammatory and prooxidative genes, such as IL-1β, IL-6, TNF-α, iNOS, and CX3CR1, were upregulated in the prefrontal cortices of OVX rats after CUS stimulation, whereas the anti-inflammatory factor Arg1 and microglial cell negative regulatory factor CD200 were downregulated. To sum up, OVX enhances the CUS-mediated anxiety and depression phenomena in rats, and its mechanism may be related to inducing the activation and polarization of microglial cells in the prefrontal cortex of animal and to accelerating the inflammatory response.

Neuroinflammation and depressive disorder: The role of the hypothalamus

Data accumulated over the last two decades has demonstrated that hypothalamic inflammation plays an important role in the etiopathogenesis of the most prevalent diseases, such as cardiovascular diseases, metabolic syndrome, and even cancer. Recent findings indicate that hypothalamic inflammation is also associated with stress exposure and certain psychiatric diseases, such as depressive disorder. Mechanistic studies have shown that intense and/or chronic stress exposure is accompanied by the synthesis of inflammatory molecules in the hypothalamus, altered hypothalamic-pituitary-adrenal axis activity, and development of glucocorticoid resistance. Consequently, these factors might play a role in the etiopathogenesis of psychiatric disorders. We propose that hypothalamic inflammation represents an interconnection between somatic diseases and depressive disorder. These assumptions are discussed in this mini-review in the light of available data from studies focusing on hypothalamic inflammation.

Microglial Dysregulation and Suicidality: A Stress-Diathesis Perspective

According to the stress-diathesis model of suicidal behavior, completed suicide depends on the interaction between psychosocial stressors and a trait-like susceptibility. While there are likely multiple biological processes at play in suicidal behavior, recent findings point to over-activation of microglia, the resident macrophages of the central nervous system, as implicated in stress-induced suicidal behavior. However, it remains unclear how microglial dysregulation can be integrated into a clinical model of suicidal behavior. Therefore, this narrative review aims to (1) examine the findings from human post-mortem and neuroimaging studies that report a relationship between microglial activation and suicidal behavior, and (2) update the clinical model of suicidal behavior to integrate the role of microglia. A systematic search of SCOPUS, PubMed, PsycINFO, and Embase databases revealed evidence of morphological alterations in microglia and increased translocator protein density in the brains of individuals with suicidality, pointing to a positive relationship between microglial dysregulation and suicidal behavior. The studies also suggested several pathological mechanisms leading to suicidal behavior that may involve microglial dysregulation, namely (1) enhanced metabolism of tryptophan to quinolinic acid through the kynurenine pathway and associated serotonin depletion; (2) increased quinolinic acid leading to excessive N-methyl-D-aspartate-signaling, resulting in potential disruption of the blood brain barrier; (3) increased quinolinic acid resulting in higher neurotoxicity, and; (4) elevated interleukin 6 contributing to loss of inhibition of glutamatergic neurons, causing heightened glutamate release and excitotoxicity. Based on these pathways, we reconceptualized the stress-diathesis theory of suicidal behavior to incorporate the role of microglial activity.

The formative role of microglia in stress-induced synaptic deficits and associated behavioral consequences

Psychological stress can precipitate depression, and emerging preclinical data suggest a link between stress-induced alterations in microglia function and development of depressive-like behaviors. Microglia are highly dynamic, and play an integral role in maintaining neuronal homeostasis and synaptic plasticity. In this capacity, microglial dysfunction represents a compelling avenue through which stress might disrupt neuronal integrity and induce psychopathology. This review examines preclinical and clinical postmortem findings that indicate microglia-neuron interactions contribute to stress-induced synaptic deficits and associated behavioral and cognitive consequences. We focus on pathways that are implicated in microglia-mediated neuronal remodeling, including CSF1-CSF1R, CX3CL1-CX3CR1, and CD11b (CR3)-C3, as well as purinergic signaling via P2RX7 and P2RY12. We also highlight sex differences in stress effects on microglia, and the potential for microglia in the development of sex-specific treatments for depressive disorders.

The AT1 Receptor Antagonist Losartan Does Not Affect Depressive-Like State and Memory Impairment Evoked by Chronic Stressors in Rats

The present study investigated the effect of the treatment with the angiotensin II type 1 receptor (AT₁) antagonist losartan in the depressive-like state and memory impairment evoked by exposure to either homotypic (i.e., repeated exposure to the same type of stressor) or heterotypic (i.e., exposure to different aversive stimuli) chronic stressors in rats. For this, male Wistar rats were subjected to a 10 days regimen of repeated restraint stress (RRS, homotypic stressor) or chronic variable stress (CVS, heterotypic stressor) while being concurrently treated daily with losartan (30 mg/kg/day, p.o.). Depressive-like state was evaluated by analysis of the alterations considered as markers of depression (decreased sucrose preference and body weight and coat state deterioration), whereas cognitive non-emotional performance was tested using the novel object recognition (NOR) test. Locomotor activity was also evaluated in the open field test. Both RRS and CVS impaired sucrose preference and caused coat state deterioration, whereas only CVS impaired body weight gain. Besides, RRS impaired short-term memory (but not long-term memory) in the NOR test. Neither depressive-like state nor memory impairment evoked by the chronic stressors was affected by the treatment with losartan. Nevertheless, CVS increased the locomotion, which was inhibited by losartan. Taken together, these results provide evidence that the chronic treatment with losartan does not affect the depressive-like state and memory impairment evoked by either homotypic or heterotypic chronic stress regimens in rats.

Microglia: Neuroimmune-sensors of stress

Exposure to stressors disrupts homeostasis and results in the release of stress hormones including glucocorticoids, epinepherine and norepinepherine. Interestingly, stress also has profound affects on microglia, which are tissue-resident macrophages in the brain parenchyma. Microglia express a diverse array of receptors, which also allows them to respond to stress hormones derived from peripheral as well as central sources. Here, we review studies of how exposure to acute and chronic stressors alters the immunophenotype and function of microglia. Further, we examine a causal for stress hormones in these effects of stress on microglia. We propose that microglia serve as immunosensors of the stress response, which puts them in the unique position to sense and respond rapidly to alterations in homeostasis and integrate the neural response to threats.

Chronic stress-induced gut dysfunction exacerbates Parkinson's disease phenotype and pathology in a rotenone-induced mouse model of Parkinson's disease

Recent evidence provides support for involvement of the microbiota-gut-brain axis in Parkinson's disease (PD) pathogenesis. We propose that a pro-inflammatory intestinal milieu, due to intestinal hyper-permeability and/or microbial dysbiosis, initiates or exacerbates PD pathogenesis. One factor that can cause intestinal hyper-permeability and dysbiosis is chronic stress which has been shown to accelerate neuronal degeneration and motor deficits in Parkinsonism rodent models. We hypothesized that stress-induced intestinal barrier dysfunction and microbial dysbiosis lead to a pro-inflammatory milieu that exacerbates the PD phenotype in the low-dose oral rotenone PD mice model. To test this hypothesis, mice received unpredictable restraint stress (RS) for 12 weeks, and during the last six weeks mice also received a daily administration of low-dose rotenone (10 mg/kg/day) orally. The initial six weeks of RS caused significantly higher urinary cortisol, intestinal hyperpermeability, and decreased abundance of putative "anti-inflammatory" bacteria (Lactobacillus) compared to non-stressed mice. Rotenone alone (i.e., without RS) disrupted the colonic expression of the tight junction protein ZO-1, increased oxidative stress (N-tyrosine), increased myenteric plexus enteric glial cell GFAP expression and increased α-synuclein (α-syn) protein levels in the colon compared to controls. Restraint stress exacerbated these rotenone-induced changes. Specifically, RS potentiated rotenone-induced effects in the colon including: 1) intestinal hyper-permeability, 2) disruption of tight junction proteins (ZO-1, Occludin, Claudin1), 3) oxidative stress (N-tyrosine), 4) inflammation in glial cells (GFAP + enteric glia cells), 5) α-syn, 6) increased relative abundance of fecal Akkermansia (mucin-degrading Gram-negative bacteria), and 7) endotoxemia. In addition, RS promoted a number of rotenone-induced effects in the brain including: 1) reduced number of resting microglia and a higher number of dystrophic/phagocytic microglia as well as (FJ-C+) dying cells in the substantia nigra (SN), 2) increased lipopolysaccharide (LPS) reactivity in the SN, and 3) reduced dopamine (DA) and DA metabolites (DOPAC, HVA) in the striatum compared to control mice. Our findings support a model in which chronic stress-induced, gut-derived, pro-inflammatory milieu exacerbates the PD phenotype via a dysfunctional microbiota-gut-brain axis.

Putative Inflammatory Sensitive Mechanisms Underlying Risk or Resilience to Social Stress

It has been well recognized that exposure to stress can lead to the onset of psychosocial disorders such as depression. While there are a number of antidepressant therapies currently available and despite producing immediate neurochemical alterations, they require weeks of continuous use in order to exhibit antidepressant efficacy. Moreover, up to 30% of patients do not respond to typical antidepressants, suggesting that our understanding of the pathophysiology underlying stress-induced depression is still limited. In recent years inflammation has become a major focus in the study of depression as several clinical and preclinical studies have demonstrated that peripheral and central inflammatory mediators, including interleukin (IL)-1β, are elevated in depressed patients. Moreover, it has been suggested that inflammation and particularly neuroinflammation may be a direct and immediate link in the emergence of stress-induced depression due to the broad neural and glial effects that are elicited by proinflammatory cytokines. Importantly, individual differences in inflammatory reactivity may further explain why certain individuals exhibit differing susceptibility to the consequences of stress. In this review article, we discuss sources of individual differences such as age, sex and coping mechanisms that are likely sources of distinct changes in stress-induced neuroimmune factors and highlight putative sources of exaggerated neuroinflammation in susceptible individuals. Furthermore, we review the current literature of specific neural and glial mechanisms that are regulated by stress and inflammation including mitochondrial function, oxidative stress and mechanisms of glutamate excitotoxicity. Taken together, the impetus for this review is to move towards a better understanding of mechanisms regulated by inflammatory cytokines and chemokines that are capable of contributing to the emergence of depressive-like behaviors in susceptible individuals.

Effects of alpha-7 nicotinic allosteric modulator PNU 120596 on depressive-like behavior after lipopolysaccharide administration in mice

Evidence suggests that α7 nicotinic acetylcholine receptor (α7 nAChR) in the central nervous system has a critical role in the regulation of microglial function and neuroinflammation associated with the pathophysiology of major depressive disorder. The objectives of the present study were to determine the effects of PNU 120596, an α7 nAChR positive allosteric modulator (PAM), on depressive-like behavior and expression of ionized calcium binding adaptor molecule 1 (Iba-1), a microglial marker, in male C57BL/6J mice following lipopolysaccharide (LPS) administration, an animal model for depressive-like behavior. Forced swim test (FST), tail suspension test (TST), and sucrose preference test were used to determine the effects of PNU 120596 on depressive-like behavior, measured by increased immobility time or decreased sucrose preference. We also examined the effects of PNU 120596 on Iba-1 expression by using Western blot analysis and immunofluorescence staining in the hippocampus and prefrontal cortex, the brain regions implicated in major depressive disorder. Administration of LPS (1 mg/kg, i.p.) significantly increased immobility time during FST and TST and decreased sucrose preference. The PNU 120596 (1 or 4 mg/kg, i.p.) dose-dependently prevented LPS-induced depressive-like behavior during FST, TST, and sucrose preference test. The PNU 120596 (1 or 4 mg/kg) alone did not show any significant alteration on immobility time and sucrose preference. Pretreatment of methyllycaconitine (3 mg/kg, i.p.), an α7 nAChR antagonist, significantly prevented the antidepressant-like effects of PNU (4 mg/kg). Similarly, the PNU 120596 (4 mg/kg, i.p.) significantly reduced LPS-induced increased expression of Iba-1 in the hippocampus or prefrontal cortex. Overall, these results suggest that PNU 120596 reduces LPS-induced depressive-like behavior and microglial activation in the hippocampus and prefrontal cortex in mice. Therefore, α7 nAChR PAMs could be developed as potential therapeutic utility for the treatment of major depressive disorder in humans.

Stress in Regulation of GABA Amygdala System and Relevance to Neuropsychiatric Diseases

The amygdala is an almond-shaped nucleus located deep and medially within the temporal lobe and is thought to play a crucial role in the regulation of emotional processes. GABAergic neurotransmission inhibits the amygdala and prevents us from generating inappropriate emotional and behavioral responses. Stress may cause the reduction of the GABAergic interneuronal network and the development of neuropsychological diseases. In this review, we summarize the recent evidence investigating the possible mechanisms underlying GABAergic control of the amygdala and its interaction with acute and chronic stress. Taken together, this study may contribute to future progress in finding new approaches to reverse the attenuation of GABAergic neurotransmission induced by stress in the amygdala.

Chronic stress from adolescence to aging in the prefrontal cortex: A neuroimmune perspective

The development of the organism is a critical variable which influences the magnitude, duration, and reversibility of the effects of chronic stress. Such factors are relevant to the prefrontal cortex (PFC), as this brain region is the last to mature, the first to decline, and is highly stress-sensitive. Therefore, this review will examine the intersection between the nervous system and immune system at glutamatergic synapses in the PFC across three developmental periods: adolescence, adulthood, and aging. Glutamatergic synapses are tightly juxtaposed with microglia and astrocytes, and each of these cell types exhibits their own developmental trajectory. Not only does chronic stress differentially impact each of these cell types across development, but chronic stress also alters intercellular communication within this quad-partite synapse. These observations suggest that developmental shifts in both neural and immune function across neurons, microglia, and astrocytes mediate shifting effects of chronic stress on glutamatergic transmission.

Enhanced fear and altered neuronal activation in forebrain limbic regions of CX3CR1-deficient mice

Mounting evidence supports immune dysfunction in psychiatric conditions such as post-traumatic stress disorder (PTSD). The association of immunomodulatory mechanisms with PTSD-relevant behavior and physiology is not well understood. Communication between neurons and microglia, resident immune cells of the central nervous system, is crucial for optimal regulation of behavior and physiology. In this regard, the fractalkine CX3CL1, secreted from neurons and its target, the microglial CX3CR1 receptor represent a primary neuron-microglia inter-regulatory system important for synaptic plasticity and function. The current study investigated the impact of CX3CR1 deficiency on behaviors relevant to PTSD, such as fear acquisition and memory, acoustic startle response and anxiety-like behavior. Morphological analysis of microglia and neuronal activation within PTSD-relevant forebrain nuclei regulating stress and fear behaviors was also conducted. CX3CR1-deficient (CX3CR1-/-) mice elicited increased fear acquisition as well as reinstatement of fear as compared to wild type (CX3CR1+/+) mice. Conditioned fear and extinction were not significantly different between genotypes. No significant differences were observed in unconditioned acoustic startle response between genotypes. CX3CR1-/- mice showed reduced anxiety-like behaviors as compared with CX3CR1+/+ mice. Morphological assessment of microglia showed region-selective effects of CX3CR1 deficiency, primarily within hypothalamic and cortical areas. Lastly, CX3CR1-/- mice elicited elevated neuronal activity in the PVN and the ventral tegmental-interpeduncular area following reinstatement of fear. Collectively, our data suggest that impaired CX3CR1 function may evoke region-selective alterations in forebrain circuits regulating stress, anxiety and fear, impacting behaviors relevant to disorders such as PTSD.

Behavioral stress alters corticolimbic microglia in a sex- and brain region-specific manner

Women are more susceptible to numerous stress-linked psychological disorders (e.g., depression) characterized by dysfunction of corticolimbic brain regions critical for emotion regulation and cognitive function. Although sparsely investigated, a number of studies indicate sex differences in stress effects on neuronal structure, function, and behaviors associated with these regions. We recently demonstrated a basal sex difference in- and differential effects of stress on- microglial activation in medial prefrontal cortex (mPFC). The resident immune cells of the brain, microglia are implicated in synaptic and dendritic plasticity, and cognitive-behavioral function. Here, we examined the effects of acute (3h/day, 1 day) and chronic (3h/day, 10 days) restraint stress on microglial density and morphology, as well as immune factor expression in orbitofrontal cortex (OFC), basolateral amygdala (BLA), and dorsal hippocampus (DHC) in male and female rats. Microglia were visualized, classified based on their morphology, and stereologically counted. Microglia-associated transcripts (CD40, iNOS, Arg1, CX3CL1, CX3CR1, CD200, and CD200R) were assessed in brain punches from each region. Expression of genes linked with cellular stress, neuroimmune state, and neuron-microglia communication varied between unstressed male and female rats in a region-specific manner. In OFC, chronic stress upregulated a wider variety of immune factors in females than in males. Acute stress increased microglia-associated transcripts in BLA in males, whereas chronic stress altered immune factor expression in BLA more broadly in females. In DHC, chronic stress increased immune factor expression in males but not females. Moreover, acute and chronic stress differentially affected microglial morphological activation state in male and female rats across all brain regions investigated. In males, chronic stress altered microglial activation in a pattern consistent with microglial involvement in stress-induced dendritic remodeling across OFC, BLA, and DHC. Together, these data suggest the potential for microglia-mediated sex differences in stress effects on neural structure, function, and behavior.

Microglia under psychosocial stressors along the aging trajectory: Consequences on neuronal circuits, behavior, and brain diseases

Mounting evidence indicates the importance of microglia for proper brain development and function, as well as in complex stress-related neuropsychiatric disorders and cognitive decline along the aging trajectory. Considering that microglia are resident immune cells of the brain, a homeostatic maintenance of their effector functions that impact neuronal circuitry, such as phagocytosis and secretion of inflammatory factors, is critical to prevent the onset and progression of these pathological conditions. However, the molecular mechanisms by which microglial functions can be properly regulated under healthy and pathological conditions are still largely unknown. We aim to summarize recent progress regarding the effects of psychosocial stress and oxidative stress on microglial phenotypes, leading to neuroinflammation and impaired microglia-synapse interactions, notably through our own studies of inbred mouse strains, and most importantly, to discuss about promising therapeutic strategies that take advantage of microglial functions to tackle such brain disorders in the context of adult psychosocial stress or aging-induced oxidative stress.

Microglia: An Interface between the Loss of Neuroplasticity and Depression

Depression has been widely accepted as a major psychiatric disease affecting nearly 350 million people worldwide. Research focus is now shifting from studying the extrinsic and social factors of depression to the underlying molecular causes. Microglial activity is shown to be associated with pathological conditions, such as psychological stress, pathological aging, and chronic infections. These are primary immune effector cells in the CNS and regulate the extensive dialogue between the nervous and the immune systems in response to different immunological, physiological, and psychological stressors. Studies have suggested that during stress and pathologies, microglia play a significant role in the disruption of neuroplasticity and have detrimental effects on neuroprotection causing neuroinflammation and exacerbation of depression. After a systematic search of literature databases, relevant articles on the microglial regulation of bidirectional neuroimmune pathways affecting neuroplasticity and leading to depression were reviewed. Although, several hypotheses have been proposed for the microglial role in the onset of depression, it is clear that all molecular pathways to depression are linked through microglia-associated neuroinflammation and hippocampal degeneration. Molecular factors such as an excess of glucocorticoids and changes in gene expression of neurotrophic factors, as well as neuro active substances secreted by gut microbiota have also been shown to affect microglial morphology and phenotype resulting in depression. This review aims to critically analyze the various molecular pathways associated with the microglial role in depression.

A multispecies approach for understanding neuroimmune mechanisms of stress

The relationship between stress challenges and adverse health outcomes, particularly for the development of affective disorders, is now well established. The highly conserved neuroimmune mechanisms through which responses to stressors are transcribed into effects on males and females have recently garnered much attention from researchers and clinicians alike. The use of animal models, from mice to guinea pigs to primates, has greatly increased our understanding of these mechanisms on the molecular, cellular, and behavioral levels, and research in humans has identified particular brain regions and connections of interest, as well as associations between stress-induced inflammation and psychiatric disorders. This review brings together findings from multiple species in order to better understand how the mechanisms of the neuroimmune response to stress contribute to stress-related psychopathologies, such as major depressive disorder, schizophrenia, and bipolar disorder.

Repeated exposure to two stressors in sequence demonstrates that corticosterone and paraventricular nucleus of the hypothalamus interleukin-1β responses habituate independently

A wide range of stress-related pathologies such as post-traumatic stress disorder are considered to arise from aberrant or maladaptive forms of stress adaptation. The hypothalamic-pituitary-adrenal (HPA) axis readily adapts to repeated stressor exposure, yet little is known about adaptation in neuroimmune responses to repeated or sequential stress challenges. In Experiment 1, rats were exposed to 10 days of restraint alone (60 minutes daily), forced swim alone (30 minutes daily) or daily sequential exposure to restraint (60 minutes) followed immediately by forced swim (30 minutes), termed sequential stress exposure. Habituation of the corticosterone (CORT) response occurred to restraint by 5 days and swim at 10 days, whereas rats exposed to sequential stress exposure failed to display habituation to the combined challenge. Experiment 2 compared 1 or 5 days of forced swim with sequential stress exposure and examined how each affected expression of several neuroimmune and cellular activation genes in the paraventricular nucleus of the hypothalamus (PVN), prefrontal cortex (PFC) and hippocampus (HPC). Sequential exposure to restraint and swim increased interleukin (IL)-1β in the PVN, an effect that was attenuated after 5 days. Sequential stress exposure also elicited IL-6 and tumour necrosis factor-α responses in the HPC and PFC, respectively, which did not habituate after 5 days. Experiment 3 tested whether prior habituation to restraint (5 days) would alter the IL-1β response evoked by swim exposure imposed immediately after the sixth day of restraint. Surprisingly, a history of repeated exposure to restraint attenuated the PVN IL-1β response after swim in comparison to acutely-exposed subjects despite an equivalent CORT response. Overall, these findings suggest that habituation of neuroimmune responses to stress proceeds: (i) independent of HPA axis habituation; (ii) likely requires more daily sessions of stress to develop; and (iii) IL-1β displays a greater tendency to habituate after repeated stress challenges compared to other stress-reactive cytokines.

Behavioral and physiological consequences of enrichment loss in rats

Significant loss produces the highest degree of stress and compromised well-being in humans. Current rodent models of stress involve the application of physically or psychologically aversive stimuli, but do not address the concept of loss. We developed a rodent model for significant loss, involving removal of long-term access to a rewarding enriched environment. Our results indicate that removal from environmental enrichment produces a profound behavioral and physiological phenotype with depression-like qualities, including helplessness behavior, hypothalamo-pituitary-adrenocortical axis dysregulation and overeating. Importantly, this enrichment removal phenotype was prevented by antidepressant treatment. Furthermore, the effects of enrichment removal do not occur following relief from chronic stress and are not duplicated by loss of exercise or social contact.

A multispecies approach for understanding neuroimmune mechanisms of stress

The relationship between stress challenges and adverse health outcomes, particularly for the development of affective disorders, is now well established. The highly conserved neuroimmune mechanisms through which responses to stressors are transcribed into effects on males and females have recently garnered much attention from researchers and clinicians alike. The use of animal models, from mice to guinea pigs to primates, has greatly increased our understanding of these mechanisms on the molecular, cellular, and behavioral levels, and research in humans has identified particular brain regions and connections of interest, as well as associations between stress-induced inflammation and psychiatric disorders. This review brings together findings from multiple species in order to better understand how the mechanisms of the neuroimmune response to stress contribute to stress-related psychopathologies, such as major depressive disorder, schizophrenia, and bipolar disorder.

Reconsidering the role of glial cells in chronic stress-induced dopaminergic neurons loss within the substantia nigra? Friend or foe?

Exposure to psychological stress is known to seriously disrupt the operation of the substantia nigra (SN) and may in fact initiate the loss of dopaminergic neurons within the SN. In this study, we aimed to investigate how chronic stress modified the SN in adult male mice. Using a paradigm of repeated restraint stress (an average of 20h per week for 6weeks), we examined changes within the SN using western blotting and immunohistochemistry. We demonstrated that chronic stress was associated with a clear loss of dopaminergic neurons within the SN. The loss of dopaminergic neurons was accompanied by higher levels of oxidative stress damage, indexed by levels of protein carbonylation and strong suppression of both microglial and astrocytic responses. In addition, we demonstrated for the first time, that chronic stress alone enhanced the aggregation of α-synuclein into the insoluble protein fraction. These results indicate that chronic stress triggered loss of dopaminergic neurons by increasing oxidative stress, suppressing glial neuroprotective functions and enhancing the aggregation of the neurotoxic protein, α-synuclein. Collectively, these results reinforce the negative effects of chronic stress on the viability of dopaminergic cells within the SN.