Chronic stress-induced disruption of the astrocyte network is driven by structural atrophy and not loss of astrocytes

N-3 polyunsaturated fatty acids promote astrocyte differentiation and neurotrophin production independent of cAMP in patient-derived neural stem cells

Evidence from epidemiological and laboratory studies, as well as randomized placebo-controlled trials, suggests supplementation with n-3 polyunsaturated fatty acids (PUFAs) may be efficacious for treatment of major depressive disorder (MDD). The mechanisms underlying n-3 PUFAs potential therapeutic properties remain unknown. There are suggestions in the literature that glial hypofunction is associated with depressive symptoms and that antidepressants may normalize glial function. In this study, induced pluripotent stem cells (iPSC)-derived neuronal stem cell lines were generated from individuals with MDD. Astrocytes differentiated from patient-derived neuronal stem cells (iNSCs) were verified by GFAP. Cells were treated with eicosapentaenoic acid (EPA), docosahexaenoic acid (DHA) or stearic acid (SA). During astrocyte differentiation, we found that n-3 PUFAs increased GFAP expression and GFAP positive cell formation. BDNF and GDNF production were increased in the astrocytes derived from patients subsequent to n-3 PUFA treatment. Stearic Acid (SA) treatment did not have this effect. CREB activity (phosphorylated CREB) was also increased by DHA and EPA but not by SA. Furthermore, when these astrocytes were treated with n-3 PUFAs, the cAMP antagonist, RP-cAMPs did not block n-3 PUFA CREB activation. However, the CREB specific inhibitor (666-15) diminished BDNF and GDNF production induced by n-3 PUFA, suggesting CREB dependence. Together, these results suggested that n-3 PUFAs facilitate astrocyte differentiation, and may mimic effects of some antidepressants by increasing production of neurotrophic factors. The CREB-dependence and cAMP independence of this process suggests a manner in which n-3 PUFA could augment antidepressant effects. These data also suggest a role for astrocytes in both MDD and antidepressant action.

Fractionated Low-Dose Radiation Induces Long-Lasting Inflammatory Responses in the Hippocampal Stem Cell Niche

PURPOSE

Despite major technical advances in hippocampus-sparing radiation therapy, radiation-induced injury to the neural stem cell compartment may affect neurocognitive functions. In the brain, glial cells modulate neuronal functions and are major mediators of neuroinflammation. In a preclinical mouse model with fractionated low-dose radiation (LDR), the complex response to radiation-induced injury was analyzed in the hippocampal stem cell compartment over a period of 6 months.

METHODS AND MATERIALS

Adult and juvenile C57BL/6NCrl mice were exposed to low doses of ionizing radiation (IR; 20 fractions of 0.1 Gy, for up to 4 weeks) daily. At 72 hours and 1, 3, and 6 months after fractionated LDR, magnetic resonance imaging (9.4 T) was conducted to detect structural and functional abnormalities in the hippocampal region. Using immunofluorescence and histologic studies, neuroglia cells (astrocytes, microglia, oligodendrocytes) were quantified and neuroinflammatory responses were characterized in the hippocampal dentate gyrus. Using in vivo bromodeoxyuridine labeling, the cell fate of newly generated progenitor cells was tracked in the subgranular zone of the dentate gyrus during fractionated LDR.

RESULTS

Low doses of IR induced long-lasting inflammatory responses with local increases of activated microglia and reactive astrocytes, which were most pronounced in the juvenile hippocampus within the first months after LDR. Glial activation with the consequent release of proinflammatory mediators increased local blood flow and vascular permeability in the hippocampal region. Cell fate mapping of progenitors located in the subgranular zone revealed a transient shift from neurogenesis to gliogenesis.

CONCLUSIONS

Glial cell activation and transient neuroinflammation may reflect radiation-induced neuronal damage in the hippocampal stem cell niche. The increased proliferative capacity of the developing brain may explain the enhanced hippocampal radiosensitivity, with stronger inflammatory reactions in the juvenile hippocampus. Thus, limiting the radiation dose to the hippocampal region is an important issue of clinical radiation therapy at all ages to preserve neurocognitive functions.

Neuroinflammatory Basis of Depression: Learning From Experimental Models

The neuroinflammatory basis of depression encompasses the detrimental role of otherwise supportive non-neuronal cells and neuroinflammation in hampering neuronal function, leading to depressive behavior. Animals subjected to different stress paradigms show glial cell activation and a surge in proinflammatory cytokines in various brain regions. The concept of sterile inflammation observed in animal models of depression has intrigued many researchers to determine the possible triggers of central immune cell activation. Notably, microglial activation and subsequent phenotypic polarization in depression have been strongly advocated by the wealth of recent preclinical studies; however, findings from human studies have shown contradictory results. Despite intensive investigation, many research gaps still exist to elucidate the molecular mechanisms of neuroinflammatory cascades underlying the pathophysiology of depression. In this mini-review, recent progress in understanding neuroinflammatory mechanisms in light of experimental models of depression will be thoroughly discussed. The challenges of mirroring depression in animal and in vitro models will also be highlighted. Furthermore, prospects of targeting neuroinflammation to treat depressive disorder will be covered.

Contributions of glucocorticoid receptors in cortical astrocytes to memory recall

Dysfunctions in memory recall lead to pathological fear; a hallmark of trauma-related disorders, like posttraumatic stress disorder (PTSD). Both, heightened recall of an association between a cue and trauma, as well as impoverished recall that a previously trauma-related cue is no longer a threat, result in a debilitating fear toward the cue. Glucocorticoid-mediated action via the glucocorticoid receptor (GR) influences memory recall. This literature has primarily focused on GRs expressed in neurons or ignored cell-type specific contributions. To ask how GR action in nonneuronal cells influences memory recall, we combined auditory fear conditioning in mice and the knockout of GRs in astrocytes in the prefrontal cortex (PFC), a brain region implicated in memory recall. We found that knocking out GRs in astrocytes of the PFC disrupted memory recall. Specifically, we found that knocking out GRs in astrocytes in the PFC (AstroGRKO) after fear conditioning resulted in higher levels of freezing to the CS+ tone when compared with controls (AstroGRintact). While we did not find any differences in extinction of fear toward the CS+ between these groups, AstroGRKO female but not male mice showed impaired recall of extinction training. These results suggest that GRs in cortical astrocytes contribute to memory recall. These data demonstrate the need to examine GR action in cortical astrocytes to elucidate the basic neurobiology underlying memory recall and potential mechanisms that underlie female-specific biases in the incidence of PTSD.

Neurobiological effects of perceived stress are different between adolescents and middle-aged adults

Stress is an inevitable element of everyday living. Developmental studies suggested that adolescents are more vulnerable and sensitive to the effect of stress due to their developing brains, especially in areas related to stress perception and processing. This voxel-based morphometry study examined the association between various neurobiological markers and the level of perceived stress experienced by adolescents (n = 26) and middle-aged adults (n = 26). Our findings indicated that differences existed in the relationships between perceived stress and the structural volume of the orbitofrontal cortex (OFC) extending to the insula and amygdala. Specifically, the levels of perceived stress and the grey matter volume of the orbitofrontal cortex, the insula, and the amygdala were positively related in adolescents but negatively related for adults. Furthermore, a significant negative correlation between perceived stress and cortisol levels was observed in adults, whereas the relationship between perceived stress and cortisol levels was not significant for adolescents. Perceived stress measurement may be better than cortisol levels in terms of reflecting the emotional states of adolescents. In sum, the relationships between perceived stress and neurobiological markers were different between adolescents and middle-aged adults and thus appeared to be age dependent.

Implication of cerebral astrocytes in major depression: A review of fine neuroanatomical evidence in humans

Postmortem investigations have implicated astrocytes in many neurological and psychiatric conditions. Multiple brain regions from individuals with major depressive disorder (MDD) have lower expression levels of astrocyte markers and lower densities of astrocytes labeled for these markers, suggesting a loss of astrocytes in this mental illness. This paper reviews the general properties of human astrocytes, the methods to study them, and the postmortem evidence for astrocyte pathology in MDD. When comparing astrocyte density and morphometry studies, astrocytes are more abundant and smaller in human subcortical than cortical brain regions, and immunohistochemical labeling for the astrocyte markers glial fibrillary acidic protein (GFAP) and vimentin (VIM) reveals fewer than 15% of all astrocytes that are present in cortical and subcortical regions, as revealed using other staining techniques. By combining astrocyte densities and morphometry, a model was made to illustrate that domain organization is mostly limited to GFAP-IR astrocytes. Using these markers and others, alterations of astrocyte densities appear more widespread than those for astrocyte morphologies throughout the brain of individuals having died with MDD. This review suggests how reduced astrocyte densities may relate to the association of depressive episodes in MDD with elevated S100 beta (S100B) cerebrospinal fluid serum levels. Finally, a potassium imbalance theory is proposed that integrates the reduced astrocyte densities generated from postmortem studies with a hypothesis for the antidepressant effects of ketamine generated from rodent studies.

Benefits of animal models to understand the pathophysiology of depressive disorders

Major depressive disorder (MDD) is a potentially life-threatening mental disorder imposing severe social and economic burden worldwide. Despite the existence of effective antidepressant treatment strategies the exact pathophysiology of the disease is still unknown. Large number of animal models of MDD have been developed over the years, but all of them suffer from significant shortcomings. Despite their limitations these models have been extensively used in academic research and drug development. The aim of this review is to highlight the benefits of animal models of MDD. We focus here on recent experimental data where animal models were used to examine current theories of this complex disease. We argue, that despite their evident imperfections, these models provide invaluable help to understand cellular and molecular mechanisms contributing to the development of MDD. Furthermore, animal models are utilized in research to find clinically useful biomarkers. We discuss recent neuroimaging and microRNA studies since these investigations yielded promising candidates for biomarkers. Finally, we briefly summarize recent progresses in drug development, i.e. the FDA approval of two novel antidepressant drugs: S-ketamine and brexanolone (allopregnanolone). Deeper understanding of the exact molecular and cellular mechanisms of action responsible for the antidepressant efficacy of these rapid acting drugs could aid us to design further compounds with similar effectiveness, but less side effects. Animal studies are likely to provide valuable help in this endeavor.

Opposing functions of α- and β-adrenoceptors in the formation of processes by cultured astrocytes

Astrocytes are glial cells with numerous fine processes which are important for the functions of the central nervous system. The activation of β-adrenoceptors induces process formation of astrocytes via cyclic AMP (cAMP) signaling. However, the role of α-adrenoceptors in the astrocyte morphology has not been elucidated. Here, we examined it by using cultured astrocytes from neonatal rat spinal cords and cortices. Exposure of these cells to noradrenaline and the β-adrenoceptor agonist isoproterenol increased intracellular cAMP levels and induced the formation of processes. Noradrenaline-induced process formation was enhanced with the α1-adrenoceptor antagonist prazosin and α2-adrenoceptor antagonist atipamezole. Atipamezole also enhanced noradrenaline-induced cAMP elevation. Isoproterenol-induced process formation was not inhibited by the α1-adrenoceptor agonist phenylephrine but was inhibited by the α2-adrenoceptor agonist dexmedetomidine. Dexmedetomidine also inhibited process formation induced by the adenylate cyclase activator forskolin and the membrane-permeable cAMP analog dibutyryl-cAMP. Moreover, dexmedetomidine inhibited cAMP-independent process formation induced by adenosine or the Rho-associated kinase inhibitor Y27632. In the presence of propranolol, noradrenaline inhibited Y27632-induced process formation, which was abolished by prazosin or atipamezole. These results demonstrate that α-adrenoceptors inhibit both cAMP-dependent and -independent astrocytic process formation.

Saikosaponin D Rescues Deficits in Sexual Behavior and Ameliorates Neurological Dysfunction in Mice Exposed to Chronic Mild Stress

Often associated with sexual dysfunction (SD), chronic stress is the main contributing risk factor for the pathogenesis of depression. Radix bupleuri had been widely used in traditional Chinese medicine formulation for the regulation of emotion and sexual activity. As the main active component of Radix bupleuri, saikosaponin D (SSD) has a demonstrated antidepressant effect in preclinical studies. Herein, we sought to investigate the effect of SSD to restore sexual functions in chronically stressed mice and elucidate the potential brain mechanisms that might underly these effects. SSD was gavage administered for three weeks during the induction of chronic mild stress (CMS), and its effects on emotional and sexual behaviors in CMS mice were observed. The medial posterodorsal amygdala (MePD) was speculated to be involved in the manifestation of sexual dysfunctions in CMS mice. Our results revealed that SSD not only alleviated CMS-induced depressive-like behaviors but also rescued CMS-induced low sexual motivation and poor sexual performance. CMS destroyed astrocytes and activated microglia in the MePD. SSD treatment reversed the changes in glial pathology and inhibited neuroinflammatory and oxidative stress in the MePD of CMS mice. The neuronal morphological and functional deficits in the MePD were also alleviated by SSD administration. Our results provide insights into the central mechanisms involving the brain associated with sexual dysfunction. These findings deepen our understanding of SSD in light of the psychopharmacology of stress and sexual disorders, providing a theoretical basis for its potential clinical application.

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.

Neurobiology of resilience in depression: immune and vascular insights from human and animal studies

Major depressive disorder (MDD) is a chronic and recurrent psychiatric condition characterized by depressed mood, social isolation and anhedonia. It will affect 20% of individuals with considerable economic impacts. Unfortunately, 30-50% of depressed individuals are resistant to current antidepressant treatments. MDD is twice as prevalent in women and associated symptoms are different. Depression's main environmental risk factor is chronic stress, and women report higher levels of stress in daily life. However, not every stressed individual becomes depressed, highlighting the need to identify biological determinants of stress vulnerability but also resilience. Based on a reverse translational approach, rodent models of depression were developed to study the mechanisms underlying susceptibility vs resilience. Indeed, a subpopulation of animals can display coping mechanisms and a set of biological alterations leading to stress resilience. The aetiology of MDD is multifactorial and involves several physiological systems. Exacerbation of endocrine and immune responses from both innate and adaptive systems are observed in depressed individuals and mice exhibiting depression-like behaviours. Increasing attention has been given to neurovascular health since higher prevalence of cardiovascular diseases is found in MDD patients and inflammatory conditions are associated with depression, treatment resistance and relapse. Here, we provide an overview of endocrine, immune and vascular changes associated with stress vulnerability vs. resilience in rodents and when available, in humans. Lack of treatment efficacy suggests that neuron-centric treatments do not address important causal biological factors and better understanding of stress-induced adaptations, including sex differences, could contribute to develop novel therapeutic strategies including personalized medicine approaches.

Chronic stress followed by social isolation promotes depressive-like behaviour, alters microglial and astrocyte biology and reduces hippocampal neurogenesis in male mice

Unpredictable chronic mild stress (UCMS) is one of the most commonly used, robust and translatable models for studying the neurobiological basis of major depression. Although the model currently has multiple advantages, it does not entirely follow the trajectory of the disorder, whereby depressive symptomology can often present months after exposure to stress. Furthermore, patients with depression are more likely to withdraw in response to their stressful experience, or as a symptom of their depression, and, in turn, this withdrawal/isolation can further exacerbate the stressful experience and the depressive symptomology. Therefore, we investigated the effect(s) of 6 weeks of UCMS followed by another 6 weeks of social isolation (referred to as UCMSI), on behaviour, corticosterone stress responsivity, immune system functioning, and hippocampal neurogenesis, in young adult male mice. We found that UCMSI induced several behavioural changes resembling depression but did not induce peripheral inflammation. However, UCMSI animals showed increased microglial activation in the ventral dentate gyrus (DG) of the hippocampus and astrocyte activation in both the dorsal and ventral DG, with increased GFAP-positive cell immunoreactivity, GFAP-positive cell hypertrophy and process extension, and increased s100β-positive cell density. Moreover, UCMSI animals had significantly reduced neurogenesis in the DG and reduced levels of peripheral vascular endothelial growth factor (VEGF) - a trophic factor produced by astrocytes and that stimulates neurogenesis. Finally, UCMSI mice also had normal baseline corticosterone levels but a smaller increase in corticosterone following acute stress, that is, the Porsolt Swim Test. Our work gives clinically relevant insights into the role that microglial and astrocyte functioning, and hippocampal neurogenesis may play in the context of stress, social isolation and depression, offering a potentially new avenue for therapeutic target.

Levodopa partially rescues microglial numerical, morphological, and phagolysosomal alterations in a monkey model of Parkinson's disease

Parkinson's disease (PD) is the most common neurodegenerative motor disorder. The mechanisms underlying the onset and progression of Levodopa (L-Dopa)-induced dyskinesia (LID) during PD treatment remain elusive. Emerging evidence implicates functional modification of microglia in the development of LID. Thus, understanding the link between microglia and the development of LID may provide the knowledge required to preserve or promote beneficial microglial functions, even during a prolonged L-Dopa treatment. To provide novel insights into microglial functional alterations in PD pathophysiology, we characterized their density, morphology, ultrastructure, and degradation activity in the sensorimotor functional territory of the putamen, using 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) cynomolgus monkeys. A subset of MPTP monkeys was treated orally with L-Dopa and developed LID similar to PD patients. Using a combination of light, confocal and transmission electron microscopy, our quantitative analyses revealed alterations of microglial density, morphology and phagolysosomal activity following MPTP intoxication that were partially normalized with L-Dopa treatment. In particular, microglial density, cell body and arborization areas were increased in the MPTP monkeys, whereas L-Dopa-treated MPTP animals presented a microglial phenotype similar to the control animals. At the ultrastructural level, microglia did not differ between groups in their markers of cellular stress or aging. Nevertheless, microglia from the MPTP monkeys displayed reduced numbers of endosomes, compared with control animals, that remained lower after L-Dopa treatment. Microglia from MPTP monkeys treated with L-Dopa also had increased numbers of primary lysosomes compared with non-treated MPTP animals, while secondary and tertiary lysosomes remained unchanged. Moreover, a decrease microglial immunoreactivity for CD68, considered a marker of phagocytosis and lysosomal activity, was measured in the MPTP monkeys treated with L-Dopa, compared with non-treated MPTP animals. Taken together, these findings revealed significant changes in microglia during PD pathophysiology that were partially rescued by L-Dopa treatment. Albeit, this L-Dopa treatment conferred phagolysosomal insufficiency on microglia in the dyskinetic Parkinsonian monkeys.

Restoring tripartite glutamatergic synapses: A potential therapy for mood and cognitive deficits in Gulf War illness

Gulf War illness is associated with a combination of exposure to war-related chemical agents and traumatic stress. Currently, there are no effective treatments, and the pathophysiology remains elusive. Neurological problems are among the most commonly reported symptoms. In this study, we investigated the glutamatergic system in the hippocampi of mice exposed to war-related chemical agents and stress. Mice developed Gulf War illness-like symptoms, including mood deficits, cognitive impairments, and fatigue. They exhibited the following pathological changes in hippocampi: elevated extracellular glutamate levels, impaired glutamatergic synapses, astrocyte atrophy, loss of interneurons, and decreased neurogenesis. LDN/OSU-215111 is a small-molecule that can strengthen the structure and function of both the astrocytic processes and the glutamatergic synapses that together form the tripartite synapses. We found that LDN/OSU-215111 effectively prevented the development of mood and cognitive deficits in mice when treatment was implemented immediately following the exposure. Moreover, when symptoms were already present, LDN/OSU-215111 still significantly ameliorated these deficits; impressively, benefits were sustained one month after treatment cessation, indicating disease modification. LDN/OSU-215111 effectively normalized hippocampal pathological changes. Overall, this study provides strong evidence that restoration of tripartite glutamatergic synapses by LDN/OSU-215111 is a potential therapy for Gulf War illness.

The type of stress matters: repeated injection and permanent social isolation stress in male mice have a differential effect on anxiety- and depressive-like behaviours, and associated biological alterations

Chronic stress can alter the immune system, adult hippocampal neurogenesis and induce anxiety- and depressive-like behaviour in rodents. However, previous studies have not discriminated between the effect(s) of different types of stress on these behavioural and biological outcomes. We investigated the effect(s) of repeated injection vs. permanent social isolation on behaviour, stress responsivity, immune system functioning and hippocampal neurogenesis, in young adult male mice, and found that the type of stress exposure does indeed matter. Exposure to 6 weeks of repeated injection resulted in an anxiety-like phenotype, decreased systemic inflammation (i.e., reduced plasma levels of TNFα and IL4), increased corticosterone reactivity, increased microglial activation and decreased neuronal differentiation in the dentate gyrus (DG). In contrast, exposure to 6 weeks of permanent social isolation resulted in a depressive-like phenotype, increased plasma levels of TNFα, decreased plasma levels of IL10 and VEGF, decreased corticosterone reactivity, decreased microglial cell density and increased cell density for radial glia, s100β-positive cells and mature neuroblasts-all in the DG. Interestingly, combining the two distinct stress paradigms did not have an additive effect on behavioural and biological outcomes, but resulted in yet a different phenotype, characterized by increased anxiety-like behaviour, decreased plasma levels of IL1β, IL4 and VEGF, and decreased hippocampal neuronal differentiation, without altered neuroinflammation or corticosterone reactivity. These findings demonstrate that different forms of chronic stress can differentially alter both behavioural and biological outcomes in young adult male mice, and that combining multiple stressors may not necessarily cause more severe pathological outcomes.

Adult Neurogenesis, Glia, and the Extracellular Matrix

In the adult mammalian hippocampus, new neurons arise from stem and progenitor cell division, in a process known as adult neurogenesis. Adult-generated neurons are sensitive to experience and may participate in hippocampal functions, including learning and memory, anxiety and stress regulation, and social behavior. Increasing evidence emphasizes the importance of new neuron connectivity within hippocampal circuitry for understanding the impact of adult neurogenesis on brain function. In this Review, we discuss how the functional consequences of new neurons arise from the collective interactions of presynaptic and postsynaptic neurons, glial cells, and the extracellular matrix, which together form the "tetrapartite synapse."

Endothelial Cdk5 deficit leads to the development of spontaneous epilepsy through CXCL1/CXCR2-mediated reactive astrogliosis

Liu et al. reveal a key mechanism that mediating the transition from cerebrovascular damage to epilepsy. They identify the endothelial cyclin-dependent kinase 5 (CDK5) regulates astrocytic glutamate reuptake and increased glutamate synaptic function through CXCL1/CXCR2-mediated astrogliosis.

Blood–brain barrier (BBB) dysfunction has been suggested to play an important role in epilepsy. However, the mechanism mediating the transition from cerebrovascular damage to epilepsy remains unknown. Here, we report that endothelial cyclin-dependent kinase 5 (CDK5) is a central regulator of neuronal excitability. Endothelial-specific Cdk5 knockout led to spontaneous seizures in mice. Knockout mice showed increased endothelial chemokine (C-X-C motif) ligand 1 (Cxcl1) expression, decreased astrocytic glutamate reuptake through the glutamate transporter 1 (GLT1), and increased glutamate synaptic function. Ceftriaxone restored astrocytic GLT1 function and inhibited seizures in endothelial Cdk5-deficient mice, and these effects were also reversed after silencing Cxcl1 in endothelial cells and its receptor chemokine (C-X-C motif) receptor 2 (Cxcr2) in astrocytes, respectively, in the CA1 by AAV transfection. These results reveal a previously unknown link between cerebrovascular factors and epileptogenesis and provide a rationale for targeting endothelial signaling as a potential treatment for epilepsy.

Enhanced wall shear stress prevents obstruction by astrocytes in ventricular catheters

The treatment of hydrocephalus often involves the placement of a shunt catheter into the cerebrospinal ventricular space, though such ventricular catheters often fail by tissue obstruction. While diverse cell types contribute to the obstruction, astrocytes are believed to contribute to late catheter failure that can occur months after shunt insertion. Using in vitro microfluidic cultures of astrocytes, we show that applied fluid shear stress leads to a decrease of cell confluency and the loss of their typical stellate cell morphology. Furthermore, we show that astrocytes exposed to moderate shear stress for an extended period of time are detached more easily upon suddenly imposed high fluid shear stress. In light of these findings and examining the range of values of wall shear stress in a typical ventricular catheter through computational fluid dynamics (CFD) simulation, we find that the typical geometry of ventricular catheters has low wall shear stress zones that can favour the growth and adhesion of astrocytes, thus promoting obstruction. Using high-precision direct flow visualization and CFD simulations, we discover that the catheter flow can be formulated as a network of Poiseuille flows. Based on this observation, we leverage a Poiseuille network model to optimize ventricular catheter design such that the distribution of wall shear stress is above a critical threshold to minimize astrocyte adhesion and growth. Using this approach, we also suggest a novel design principle that not only optimizes the wall shear stress distribution but also eliminates a stagnation zone with low wall shear stress, which is common to current ventricular catheters.

Stress-Induced Morphological, Cellular and Molecular Changes in the Brain-Lessons Learned from the Chronic Mild Stress Model of Depression

Major depressive disorder (MDD) is a severe illness imposing an increasing social and economic burden worldwide. Numerous rodent models have been developed to investigate the pathophysiology of MDD. One of the best characterized and most widely used models is the chronic mild stress (CMS) model which was developed more than 30 years ago by Paul Willner. More than 2000 published studies used this model, mainly to assess novel compounds with potential antidepressant efficacy. Most of these studies examined the behavioral consequences of stress and concomitant drug intervention. Much fewer studies focused on the CMS-induced neurobiological changes. However, the stress-induced cellular and molecular changes are important as they may serve as potential translational biomarkers and increase our understanding of the pathophysiology of MDD. Here, we summarize current knowledge on the structural and molecular alterations in the brain that have been described using the CMS model. We discuss the latest neuroimaging and postmortem histopathological data as well as molecular changes including recent findings on microRNA levels. Different chronic stress paradigms occasionally deliver dissimilar findings, but the available experimental data provide convincing evidence that the CMS model has a high translational value. Future studies examining the neurobiological changes in the CMS model in combination with clinically effective antidepressant drug intervention will likely deliver further valuable information on the pathophysiology of MDD.

Emotional Stress Induces Structural Plasticity in Bergmann Glial Cells via an AC5-CPEB3-GluA1 Pathway

Stress alters brain function by modifying the structure and function of neurons and astrocytes. The fine processes of astrocytes are critical for the clearance of neurotransmitters during synaptic transmission. Thus, experience-dependent remodeling of glial processes is anticipated to alter the output of neural circuits. However, the molecular mechanisms that underlie glial structural plasticity are not known. Here we show that a single exposure of male and female mice to an acute stress produced a long-lasting retraction of the lateral processes of cerebellar Bergmann glial cells. These cells express the GluA1 subunit of AMPA-type glutamate receptors, and GluA1 knockdown is known to shorten the length of glial processes. We found that stress reduced the level of GluA1 protein and AMPA receptor-mediated currents in Bergmann glial cells, and these effects were absent in mice devoid of CPEB3, a protein that binds to GluA1 mRNA and regulates GluA1 protein synthesis. Administration of a β-adrenergic receptor blocker attenuated the reduction in GluA1, and deletion of adenylate cyclase 5 prevented GluA1 suppression. Therefore, stress suppresses GluA1 protein synthesis via an adrenergic/adenylyl cyclase/CPEB3 pathway, and reduces the length of astrocyte lateral processes. Our results identify a novel mechanism for GluA1 subunit plasticity in non-neuronal cells and suggest a previously unappreciated role for AMPA receptors in stress-induced astrocytic remodeling.SIGNIFICANCE STATEMENT Astrocytes play important roles in synaptic transmission by extending fine processes around synapses. In this study, we showed that a single exposure to an acute stress triggered a retraction of lateral/fine processes in mouse cerebellar astrocytes. These astrocytes express GluA1, a glutamate receptor subunit known to lengthen astrocyte processes. We showed that astrocytic structural changes are associated with a reduction of GluA1 protein levels. This requires activation of β-adrenergic receptors and is triggered by noradrenaline released during stress. We identified adenylyl cyclase 5, an enzyme that elevates cAMP levels, as a downstream effector and found that lowering GluA1 levels depends on CPEB3 proteins that bind to GluA1 mRNA. Therefore, stress regulates GluA1 protein synthesis via an adrenergic/adenylyl cyclase/CPEB3 pathway in astrocytes and remodels their fine processes.

Characterization of astrocytes throughout life in wildtype and APP/PS1 mice after early-life stress exposure

Background

Early-life stress (ES) is an emerging risk factor for later life development of Alzheimer’s disease (AD). We have previously shown that ES modulates amyloid-beta pathology and the microglial response to it in the APPswe/PS1dE9 mouse model. Because astrocytes are key players in the pathogenesis of AD, we studied here if and how ES affects astrocytes in wildtype (WT) and APP/PS1 mice and how these relate to the previously reported amyloid pathology and microglial profile.

Methods

We induced ES by limiting nesting and bedding material from postnatal days (P) 2–9. We studied in WT mice (at P9, P30, and 6 months) and in APP/PS1 mice (at 4 and 10 months) (i) GFAP coverage, cell density, and complexity in hippocampus (HPC) and entorhinal cortex (EC); (ii) hippocampal gene expression of astrocyte markers; and (iii) the relationship between astrocyte, microglia, and amyloid markers.

Results

In WT mice, ES increased GFAP coverage in HPC subregions at P9 and decreased it at 10 months. APP/PS1 mice at 10 months exhibited both individual cell as well as clustered GFAP signals. APP/PS1 mice when compared to WT exhibited reduced total GFAP coverage in HPC, which is increased in the EC, while coverage of the clustered GFAP signal in the HPC was increased and accompanied by increased expression of several astrocytic genes. While measured astrocytic parameters in APP/PS1 mice appear not be further modulated by ES, analyzing these in the context of ES-induced alterations to amyloid pathology and microglial shows alterations at both 4 and 10 months of age.

Conclusions

Our data suggest that ES leads to alterations to the astrocytic response to amyloid-β pathology.

Caffeine prevents neurodegeneration and behavioral alterations in a mice model of agitated depression

Longitudinal and some experimental studies have showed the potential of caffeine to counteract some depressive behaviors and synaptic dysfunctions. In this study, we investigated the potential of caffeine in preventing behavioral outcomes, neurodegeneration and synaptic proteins alterations in a mice model of agitated depression by bilateral olfactory bulbectomy (OB). For this purpose, bulbectomized mice received caffeine (0.3 g/L and 1.0 g/L, drinking water), during the active cycle, for seven weeks (two before the surgery and throughout five weeks after OB). Caffeine prevented OB-induced hyperactivity and recognition memory impairment and rescue self care and motivational behavior. In the frontal cortex, bulbectomized mice presented increase in the adenosine A₁ receptors (A₁R) and GFAP, while adenosine A_2A receptors (A_2AR) increased in the hippocampus and striatum and SNAP-25 was decreased in frontal cortex and striatum. Caffeine increased A₁R in the striatum of bulbectomized mice and in SHAM-water group caffeine increased A_2AR in the striatum and decreased SNAP-25 in the frontal cortex. Astrogliosis observed in the polymorphic layer of the dentate gyrus of OB mice was prevented by caffeine as well as the neurodegeneration in the striatum and piriform cortex. Based on these behavioral and neurochemical evidences, caffeine confirms its efficacy in preventing neurodegeneration associated with memory impairment and may be considered as a promising therapeutic tool in the prophylaxis and/or treatment of depression.

Retinal-glia ischemia and inflammation induced by chronic stress: The SABPA study

Background

Psychobiological processes linking stress and vascular diseases remain poorly understood. The retina and the brain share a common embryonic-diencephalon origin and blood-barrier physiology e.g. ongoing ischemia facilitates S100B release with astrocytic activity and glial-fibrillary-acidic-protein expression (GFAP). However, GFAP decreases revealed astrocyte pathology in the prefrontal cortex of depression/suicide cases; and might be a key mechanism in stress - disease pathways.

Methods

A chronic emotional stress phenotype independent of age, ethnicity or sex was used to stratify the current prospective cohort (N ​= ​359; aged 46 ​± ​9 years) into Stress (N ​= ​236) and no-Stress groups (N ​= ​123). Prospective data for glia ischemia risk markers were obtained, including 24 ​h BP, fasting S100B, GFAP, HbA1C and tumor-necrosis-factor-α (TNF-α). At 3-yr follow-up: diastolic-ocular-perfusion-pressure (indicating hypo-perfusion risk) was measured and retinal vessel calibers were quantified from digital images in the mydriatic eye.

Results

Higher hypertension (75% vs. 16%), diabetes (13% vs. 0%) and retinopathy (57% vs. 45%) prevalence was observed in Stress compared to no-Stress individuals. Stressed individuals had consistently raised S100B, TNF-α, HbA1C and higher diastolic-ocular-perfusion-pressure, but decreases in GFAP and GFAP:S100B. Furthermore stroke risk markers, arterial narrowing and venous widening were associated with consistently raised S100B, GFAP:S100B (p ​= ​0.060), TNF-α and higher diastolic-ocular-perfusion-pressure [Adj. R2 0.39-0.41, p ​≤ ​0.05]. No retinal-glia associations were evident in the no-Stress group.

Conclusions

Retinal-glia ischemia and inflammation was induced by chronic stress. Persistent higher inflammation and S100B with GFAP decreases further reflected stress-induced astrocyte pathology in the human retina. It is recommended to increase awareness on chronic stress and susceptibility for brain ischemia.

Daily changes in GFAP expression in radial glia of the olfactory bulb in rabbit pups entrained to circadian feeding

When food is restricted daily to a fixed time, animals show uncoupled molecular, physiological and behavioral circadian rhythms from those entrained by light and controlled by the suprachiasmatic nucleus. The loci of the food-entrainable oscillator and the mechanisms by which rhythms emerge are unclear. Using animals entrained to the light-dark cycle, recent studies indicate that astrocytes in the suprachiasmatic nucleus play a key role in the regulation of circadian rhythms. However, it is unknown whether astrocytic cells can be synchronized by circadian restricted feeding. Studying the olfactory bulb (OB) of rabbit pups entrained to daily feeding, we hypothesized that the expression of glial fibrillary acidic protein (GFAP) and the morphology of GFAP-immunopositive cells change in synchrony with timing of feeding. By using pups fed at 1000 h or 2200 h, we found that GFAP protein expression in the OB changes with a nadir at feeding time and a peak 16 h after feeding. We also found that length of radial glia processes, the most abundant GFAP⁺ cell in the rabbit pup OB, shows a daily change also coupled to feeding time. These temporal changes of GFAP were expressed in anti-phase to the rhythms of locomotor activity and c-Fos immunoreactivity. The results indicate that GFAP expression and elongation-retraction of radial glia processes are coupled by feeding time and suggest that glia cells may play an important functional role in food entraining of the OB circadian oscillator.

Evidence for Similar Prefrontal Structural and Functional Alterations in Male and Female Rats Following Chronic Stress or Glucocorticoid Exposure

Previous work of ours and others has documented regressive changes in neuronal architecture and function in the medial prefrontal cortex (mPFC) of male rats following chronic stress. As recent focus has shifted toward understanding whether chronic stress effects on mPFC are sexually dimorphic, here we undertake a comprehensive analysis to address this issue. First, we show that chronic variable stress (14-day daily exposure to different challenges) resulted in a comparable degree of adrenocortical hyperactivity, working memory impairment, and dendritic spine loss in mPFC pyramidal neurons in both sexes. Next, exposure of female rats to 21-day regimen of corticosterone resulted in a similar pattern of mPFC dendritic spine attrition and increase in spine volume. Finally, we examined the effects of another widely used regimen, chronic restraint stress (CRS, 21-day of daily 6-h restraint), on dendritic spine changes in mPFC in both sexes. CRS resulted in response decrements in adrenocortical output (habituation), and induced a pattern of consistent, but less widespread, dendritic spine loss similar to the foregoing challenges. Our data suggest that chronic stress or glucocorticoid exposure induces a relatively undifferentiated pattern of structural and functional alterations in mPFC in both males and females.

Spatiotemporal analysis of impaired microglia process movement at sites of secondary neurodegeneration post-stroke

It has recently been identified that after motor cortex stroke, the ability of microglia processes to respond to local damage cues is lost from the thalamus, a major site of secondary neurodegeneration (SND). In this study, we combine a photothrombotic stroke model in mice, acute slice and fluorescent imaging to analyse the loss of microglia process responsiveness. The peri-infarct territories and thalamic areas of SND were investigated at time-points 3, 7, 14, 28 and 56 days after stroke. We confirmed the highly specific nature of non-responsive microglia processes to sites of SND. Non-responsiveness was at no time observed at the peri-infarct but started in the thalamus seven days post-stroke and persisted for 56 days. Loss of directed process extension is not a reflection of general functional paralysis as phagocytic function continued to increase over time. Additionally, we identified that somal P₂Y₁₂ was present on non-responsive microglia in the first two weeks after stroke but not at later time points. Finally, both classical microglia activation and loss of process extension are highly correlated with neuronal damage. Our findings highlight the importance of microglia, specifically microglia dynamic functions, to the progression of SND post-stroke, and their potential relevance as modulators or therapeutic targets during stroke recovery.

Astrocytic plasticity at the dorsal dentate gyrus on an animal model of recurrent depression

Astrocytes are now known to play crucial roles in the central nervous system, supporting and closely interacting with neurons and therefore able to modulate brain function. Both human postmortem studies in brain samples from patients diagnosed with Major Depressive Disorder and from animal models of depression reported numerical and morphological astrocytic changes specifically in the hippocampus. In particular, these studies revealed significant reductions in glial cell density denoted by a decreased number of S100B-positive cells and a decrease in GFAP expression in several brain regions including the hippocampus. To reveal plastic astrocytic changes in the context of recurrent depression, we longitudinally assessed dynamic astrocytic alterations (gene expression, cell densities and morphologic variations) in the hippocampal dentate gyrus under repeated exposure to unpredictable chronic mild stress (uCMS) and upon treatment with two antidepressants, fluoxetine and imipramine. Both antidepressants decreased astrocytic complexity immediately after stress exposure. Moreover, we show that astrocytic alterations, particularly an increased number of S100B-positive cells, are observed after recurrent stress exposure. Interestingly, these alterations were prevented at the long-term by either fluoxetine or imipramine treatment.

Apelin attenuates depressive-like behavior and neuroinflammation in rats co-treated with chronic stress and lipopolysaccharide

Several experimental studies have proved that activation of neuroinflammation pathways may contribute to the development of depression, a neuropsychiatric disorder disease. Our previous studies have shown the antidepressant properties of apelin, but the mechanism was unkown. This study was performed to verify whether the antidepressant effect of apelin was related to its anti-inflammation effect in the central nervous system. To achieve our aim, we selected the co-treatment of chronic stress and LPS to induced an inflammatory process in rats. The effect of this co-treatment was evaluated through the expression of inflammatory markers and glial cell activation. LPS injection co-treated with unpredictable chronic mild stress resulted in the activation of microglial cell and astrocyte, expression of inflammatory markers and depressive behaviors. Treatment with apelin significantly attenuates the deleterious effects in these rats. Our results showed that apelin improved depressive phenotype and decreased the activation of glial cells in stress co-treatment group. The down-regulations of p-NF-κB and p-IKKβ suggested that the effects are possibly mediated by inhibition of the NF-κB-mediated inflammatory response. These findings speculated that intracerebroventricular injection of apelin could be a therapeutic approach for the treatment of depression, and the antidepressant function of apelin may closely associated with its alleviation in neuroinflammation.

The association of psychosocial risk factors for mental health with a brain marker altered by inflammation: A translocator protein (TSPO) PET imaging study

Psychiatric disorders associated with psychosocial risk factors, including depression and psychosis, have been shown to demonstrate increased microglia activity. Whilst preclinical studies indicate that psychosocial stress leads to increased levels of microglia in the frontal cortex, no study has yet been performed in humans. This study aimed at investigating whether psychosocial risk factors for depression and/or psychosis would be associated with alterations in a brain marker expressed by microglia, the translocator specific protein (TSPO) in humans. We used [¹¹C]-PBR28 Positron Emission Tomography on healthy subjects exposed to childhood and adulthood psychosocial risk factors (high-risk group, N = 12) and age- and sex-matched healthy controls not exposed to childhood and adulthood psychosocial risk factors (low-risk group, N = 12). The [¹¹C]-PBR28 volume of distribution (V_T) and Distribution Volume Ratio (DVR) were measured in the total gray matter, and frontal, parietal, temporal, occipital lobes. Levels of childhood trauma, anxiety and depression were measured using respectively the Childhood Trauma Questionnaire, State-anxiety questionnaire and Beck Depression Inventory. Compared to the low-risk group, the high-risk group did not exhibit significant differences in the mean [¹¹C]-PBR28 V_T (F(1,20) = 1.619, p = 0.218) or DVR (F(1,22) = 0.952, p = 0.340) on any region. There were no significant correlations between the [¹¹C]-PBR28 V_T and DVRs in total gray matter and frontal lobe and measures of childhood trauma, anxiety and depression. Psychosocial risk factors for depression and/or psychosis are unlikely to be associated with alterations in [¹¹C]-PBR28 binding, indicating that alterations in TSPO expression reported in these disorders is unlikely to be caused by psychosocial risk factors alone.

Gonadal hormones differentially regulate sex-specific stress effects on glia in the medial prefrontal cortex

Women are more susceptible to various stress-linked psychopathologies, including depression. Dysfunction of the medial prefrontal cortex (mPFC) has been implicated in depression, and studies indicate sex differences in stress effects on mPFC structure and function. For example, chronic stress induces dendritic atrophy in the mPFC in male rats, yet dendritic growth in females. Recent findings suggest glial pathways toward depression. Glia are highly responsive to neuronal activity and function as critical regulators of synaptic plasticity. Preclinical models demonstrate stress-induced microglial activation in mPFC in males, yet deactivation in females. By contrast, stress reduces astrocyte complexity in mPFC in male rats, whereas the effects in females are unknown. Glia possess receptors for most gonadal hormones and gonadal hormones are known to modulate neuronal activity. Thus, gonadal hormones represent a potential mechanism underlying sex differences in glia, as well as divergent stress effects. Therefore, we examined the role of gonadal hormones in sex-specific stress effects on neuronal activity (ie FosB/ ΔFosB induction) and glia in the mPFC. The findings obtained indicate greater microglial activation in mPFC in females and a greater astrocyte area in males. Basal astrocyte morphology is modulated by androgens, whereas androgens or oestrogens dampen the microglial state in males. Astrocyte morphology is associated with neuronal activity in both sexes, regardless of hormonal condition. Chronic stress induced astrocytic atrophy in males, yet hypertrophy in females, with gonadal hormones partly regulating this difference. Stress effects on microglia are oestradiol-dependent in females. Taken together, these data suggest sex-specific, gonadal hormone-dependent stress effects on astrocytes and microglia in the mPFC.

Sleep Disturbance in Bipolar Disorder: Neuroglia and Circadian Rhythms

The worldwide prevalence of sleep disorders is approximately 50%, with an even higher occurrence in a psychiatric population. Bipolar disorder (BD) is a severe mental illness characterized by shifts in mood and activity. The BD syndrome also involves heterogeneous symptomatology, including cognitive dysfunctions and impairments of the autonomic nervous system. Sleep abnormalities are frequently associated with BD and are often a good predictor of a mood swing. Preservation of stable sleep-wake cycles is therefore a key to the maintenance of stability in BD, indicating the crucial role of circadian rhythms in this syndrome. The symptom most widespread in BD is insomnia, followed by excessive daytime sleepiness, nightmares, difficulty falling asleep or maintaining sleep, poor sleep quality, sleep talking, sleep walking, and obstructive sleep apnea. Alterations in the structure or duration of sleep are reported in all phases of BD. Understanding the role of neuroglia in BD and in various aspects of sleep is in nascent state. Contributions of the different types of glial cells to BD and sleep abnormalities are discussed in this paper.

Glial pathology in neuropsychiatric disorders: a brief review

Neurons have been considered the major functional entities of the nervous system that are responsible for most of the functions even though glial cells largely outnumber them. However, recent reports have proved that glial cells do not function just like glue in the nervous system but also substantially affect neuronal function and activities, and are significantly involved in the underlying pathobiology of various psychiatric disorders. Dysfunctional astrocytes and degeneration of glial cells are postulated to be critical factors contributing to the aggravation of depressive-like symptoms in humans, which was proved using animal models. Alteration in glial cell function predominantly targets three main brain regions - the prefrontal cortex, limbic areas including the hippocampus, and the amygdala, which have been extensively studied by various researchers across the globe. These studies have postulated that failure in adopting to the changing neurophysiology due to stress will lead to regressive plasticity in the hippocampus and prefrontal cortex, but to progressive plasticity in the amygdala. In this present review, an effort has been made to understand the different alterations in chronic stress models in correlation with clinical conditions, providing evidence on the defective maintenance of glial function and its potential role in the precipitation of neuropsychiatric disorders.

The Impact of Ethologically Relevant Stressors on Adult Mammalian Neurogenesis

Adult neurogenesis—the formation and functional integration of adult-generated neurons—remains a hot neuroscience topic. Decades of research have identified numerous endogenous (such as neurotransmitters and hormones) and exogenous (such as environmental enrichment and exercise) factors that regulate the various neurogenic stages. Stress, an exogenous factor, has received a lot of attention. Despite the large number of reviews discussing the impact of stress on adult neurogenesis, no systematic review on ethologically relevant stressors exists to date. The current review details the effects of conspecifically-induced psychosocial stress (specifically looking at the lack or disruption of social interactions and confrontation) as well as non-conspecifically-induced stress on mammalian adult neurogenesis. The underlying mechanisms, as well as the possible functional role of the altered neurogenesis level, are also discussed. The reviewed data suggest that ethologically relevant stressors reduce adult neurogenesis.

The involvement of astrocytes in early-life adversity induced programming of the brain

Early‐life adversity (ELA) in the form of stress, inflammation, or malnutrition, can increase the risk of developing psychopathology or cognitive problems in adulthood. The neurobiological substrates underlying this process remain unclear. While neuronal dysfunction and microglial contribution have been studied in this context, only recently the role of astrocytes in early‐life programming of the brain has been appreciated. Astrocytes serve many basic roles for brain functioning (e.g., synaptogenesis, glutamate recycling), and are unique in their capacity of sensing and integrating environmental signals, as they are the first cells to encounter signals from the blood, including hormonal changes (e.g., glucocorticoids), immune signals, and nutritional information. Integration of these signals is especially important during early development, and therefore we propose that astrocytes contribute to ELA induced changes in the brain by sensing and integrating environmental signals and by modulating neuronal development and function. Studies in rodents have already shown that ELA can impact astrocytes on the short and long term, however, a critical review of these results is currently lacking. Here, we will discuss the developmental trajectory of astrocytes, their ability to integrate stress, immune, and nutritional signals from the early environment, and we will review how different types of early adversity impact astrocytes.

Peripheral Biomarkers of Inflammation in Depression: Evidence from Animal Models and Clinical Studies

Depression is a highly prevalent psychiatric condition, with over 300 million sufferers, and is an important comorbidity for other conditions, like cardiovascular disorders or diabetes. Therapy is largely based on psychotherapy and/or pharmacological intervention, particularly aimed at altering neurotransmitter levels in the central nervous system, but inadequate response to treatment remains a significant clinical problem. Herein, evidence supporting a molecular link between inflammation and depression will be discussed, particularly the increased prevalence of depression in chronic inflammatory diseases and the evidence on the use of anti-inflammatory drugs to treat depression. Moreover, the potential for the levels of peripheral inflammatory molecules to act as depression biomarkers, in the diagnosis and monitoring of depression will be examined, considering clinical- and animal model-based evidence.

Astroglial correlates of neuropsychiatric disease: From astrocytopathy to astrogliosis

Complex roles for astrocytes in health and disease continue to emerge, highlighting this class of cells as integral to function and dysfunction of the nervous system. In particular, escalating evidence strongly implicates a range of changes in astrocyte structure and function associated with neuropsychiatric diseases including major depressive disorder, schizophrenia, and addiction. These changes can range from astrocytopathy, degeneration, and loss of function, to astrogliosis and hypertrophy, and can be either adaptive or maladaptive. Evidence from the literature indicates a myriad of changes observed in astrocytes from both human postmortem studies as well as preclinical animal models, including changes in expression of glial fibrillary protein, as well as changes in astrocyte morphology and astrocyte-mediated regulation of synaptic function. In this review, we seek to provide a comprehensive assessment of these findings and consequently evidence for common themes regarding adaptations in astrocytes associated with neuropsychiatric disease. While results are mixed across conditions and models, general findings indicate decreased astrocyte cellular features and gene expression in depression, chronic stress and anxiety, but increased inflammation in schizophrenia. Changes also vary widely in response to different drugs of abuse, with evidence reflective of features of astrocytopathy to astrogliosis, varying across drug classes, route of administration and length of withdrawal.

Self-injurious behaviours in rhesus macaques: Potential glial mechanisms

BACKGROUND

Self-injurious behaviour (SIB) can be classified as intentional, direct injuring of body tissue usually without suicidal intent. In its non-suicidal form it is commonly seen as a clinical sign of borderline personality disorder, autism, PTSD, depression, and anxiety affecting a wide range of ages and conditions. In rhesus macaques SIB is most commonly manifested through hair plucking, self-biting, self-hitting, and head banging. SIB in the form of self-biting is observed in approximately 5-15% of individually housed monkeys. Recently, glial cells are becoming recognised as key players in regulating behaviours.

METHOD

The goal of this study was to determine the role of glial activation, including astrocytes, in macaques that had displayed SIB. To this end, we performed immunohistochemistry and next generation sequence of brain tissues from rhesus macaques with SIB.

RESULTS

Our studies showed increased vimentin, but not nestin, expression on astrocytes of macaques displaying SIB. Initial RNA Seq analyses indicate activation of pathways involved in tissue remodelling, neuroinflammation and cAMP signalling.

CONCLUSIONS

Glia are most probably activated in primates with self-injury, and are therefore potential novel targets for therapeutics.

Sex Differences in Risk and Resilience: Stress Effects on the Neural Substrates of Emotion and Motivation

Risk for stress-sensitive psychopathologies differs in men and women, yet little is known about sex-dependent effects of stress on cellular structure and function in corticolimbic regions implicated in these disorders. Determining how stress influences these regions in males and females will deepen our understanding of the mechanisms underlying sex-biased psychopathology. Here, we discuss sex differences in CRF regulation of arousal and cognition, glucocorticoid modulation of amygdalar physiology and alcohol consumption, the age-dependent impact of social stress on prefrontal pyramidal cell excitability, stress effects on the prefrontal parvalbumin system in relation to emotional behaviors, contributions of stress and gonadal hormones to stress effects on prefrontal glia, and alterations in corticolimbic structure and function after cessation of chronic stress. These studies demonstrate that, while sex differences in stress effects may be nuanced, nonuniform, and nonlinear, investigations of these differences are nonetheless critical for developing effective, sex-specific treatments for psychological disorders.

Nonfunctional mutant Wrn protein leads to neurological deficits, neuronal stress, microglial alteration, and immune imbalance in a mouse model of Werner syndrome

Werner syndrome (WS) is a premature aging disorder caused by mutations in a RecQ-family DNA helicase, WRN. Mice lacking part of the helicase domain of the WRN orthologue exhibit many phenotypic features of WS, including metabolic abnormalities and a shorter lifespan. Yet, little is known about the impact of WRN mutations on the central nervous system in both humans and mouse models of WS. In the current study, we have performed a longitudinal behavioral assessment on mice bearing a Wrn helicase deletion. Behavioral tests demonstrated a loss of motor activity and coordination, reduction in perception, increase in repetitive behavior, and deficits in both spatial and social novelty memories in Wrn mutant mice compared to age-matched wild type mice. These neurological deficits were associated with biochemical and histological changes in the brain of aged Wrn mutant mice. Microglia, resident immune cells that regulate neuronal plasticity and function in the brain, were hyper-ramified in multiple regions involved with the behavioral deficits of Wrn mutant mice. Furthermore, western analyses indicated that Wrn mutant mice exhibited an increase of oxidative stress markers in the prefrontal cortex. Supporting these findings, electron microscopy studies revealed increased cellular aging and oxidative stress features, among microglia and neurons respectively, in the prefrontal cortex of aged Wrn mutant mice. In addition, multiplex immunoassay of serum identified significant changes in the expression levels of several pro- and anti-inflammatory cytokines. Taken together, these findings indicate that microglial dysfunction and neuronal oxidative stress, associated with peripheral immune system alterations, might be important driving forces leading to abnormal neurological symptoms in WS thus suggesting potential therapeutic targets for interventions.

Glucocorticoid receptor activation induces decrease of hippocampal astrocyte number in rats

RATIONALE

The decrease of astrocyte number and hypothalamic-pituitary-adrenal (HPA) axis overactivity are observed in individuals with major depressive disorder. Elevated levels of glucocorticoids induced by hyperactivation of the HPA axis may result in glucocorticoid receptor (GR) activation. However, it is unclear whether there is a direct link between GR activation and the decrease of astrocyte number.

METHODS

Animals were exposed to chronic unpredictable stress (CUS) for 28 days and treated with continuous subcutaneous injections of vehicle or corticosterone (CORT; 40 mg/kg/day) for 21 days. We then administered mifepristone on day 21 after CUS and on day 18 after the CORT treatment. We observed behavioral deficits in the sucrose preference test, open field test, and forced swim test. Protein expression was analyzed using immunofluorescence (IF) and western blot (WB).

RESULTS

Animals exposed to CUS exhibited behavioral deficits in tests measuring anhedonia, anxiety, and despair state. They also had decreases in glial fibrillary acidic protein (GFAP) expression and numbers of GFAP-positive cells in the hippocampus. The behavioral and cellular alterations induced by CUS were reversed by subchronic treatment with the GR antagonist mifepristone. We also found that the subcutaneous injection of glucocorticoids may induce depression-like behavior and reduce GFAP protein expression in rats, which was similarly reversed by mifepristone.

CONCLUSIONS

These findings provide experimental evidence that GR activation due to elevated CORT levels induces the decrease of hippocampal astrocyte number in rats.

Chemogenetic Manipulation of Dorsal Hippocampal Astrocytes Protects Against the Development of Stress-enhanced Fear Learning

Maladaptive behavioral outcomes following stress have been associated with immune dysregulation. For example, we have previously reported that stress-induced dorsal hippocampal interleukin-1β signaling is critical to the development of stress-enhanced fear learning (SEFL). In parallel, astroglial signaling has been linked to the development of post-traumatic stress disorder (PTSD)-like phenotypes and our most recent studies have revealed astrocytes as the predominant cellular source of stress-induced IL-1β. Here, we used chemogenetic technology and morphological analyses to further explore dorsal hippocampal astrocyte function in the context of SEFL. Using a glial-expressing DREADD construct (AAV8-GFAP-hM4Di(Gi)-mCherry), we show that dorsal hippocampal astroglial G_i activation is sufficient to attenuate SEFL. Furthermore, our data provide the first initial evidence to support the function of the glial-DREADD construct employed. Specifically, we find that CNO (clozapine-n-oxide) significantly attenuated colocalization of the G_i-coupled DREADD receptor and cyclic adenosine monophosphate (cAMP), indicating functional inhibition of cAMP production. Subsequent experiments examined dorsal hippocampal astrocyte volume, surface area, and synaptic contacts (colocalization with postsynaptic density 95 (PSD95)) following exposure to severe stress (capable of inducing SEFL). While severe stress did not alter dorsal hippocampal astrocyte volume or surface area, the severe stressor exposure reduced dorsal hippocampal PSD95 immunoreactivity and the colocalization analysis showed reduced PSD95 colocalized with astrocytes. Collectively, these data provide evidence to support the functional efficacy of the glial-expressing DREADD employed, and suggest that an astrocyte-specific manipulation, activation of astroglial G_i signaling, is sufficient to protect against the development of SEFL, a PTSD-like behavior.

Hippocampal astrocyte atrophy in a mouse depression model induced by corticosterone is reversed by fluoxetine instead of benzodiazepine diazepam

Astrocytes have become promising new agents against major depressive disorders (MDD) primarily due to the crucial role they play in the pathogenesis of such disorders. However, a simple and reliable animal model that can be used to screen for astrocyte-targeting antidepressants has not yet been developed. In this study, we utilized a repeated corticosterone (CORT) injection paradigm to develop a mouse depression model wherein we examined the occurrence of alterations in hippocampal astrocyte population by using two astrocytic markers, namely, glial fibrillary acidic protein (GFAP) and S100β. Moreover, we determined the effects of fluoxetine and diazepam on CORT-induced astrocytic alterations to assess the predictive validity. Results showed that repeated CORT injections showed no effects on the number of GFAP⁺ and S100β⁺ astrocytes, but they decreased the protrusion length of GFAP⁺ astrocytes and GFAP protein expression in the hippocampus. Furthermore, repeated CORT injections produced a sustained increase of S100β protein levels in the entire hippocampus of male mice. CORT-induced hippocampal astrocyte disruption was antagonized by chronic fluoxetine treatment. By contrast, the anxiolytic drug diazepam was ineffective in the same experimental setting. All these findings suggest that the repeated CORT injection paradigm produces the astrocytic alterations similar to those in MDD and can serve as a useful mouse model to screen antidepressants meant to target astrocytes. These observations can also help in further discussing the underlying mechanisms of CORT-induced astrocytic alterations.

Growth Hormone Improves Cognitive Function After Experimental Stroke

BACKGROUND AND PURPOSE

Cognitive impairment is a common outcome for stroke survivors. Growth hormone (GH) could represent a potential therapeutic option as this peptide hormone has been shown to improve cognition in various clinical conditions. In this study, we evaluated the effects of peripheral administration of GH at 48 hours poststroke for 28 days on cognitive function and the underlying mechanisms.

METHODS

Experimental stroke was induced by photothrombotic occlusion in young adult mice. We assessed the associative memory cognitive domain using mouse touchscreen platform for paired-associate learning task. We also evaluated neural tissue loss, neurotrophic factors, and markers of neuroplasticity and cerebrovascular remodeling using biochemical and histology analyses.

RESULTS

Our results show that GH-treated stroked mice made a significant improvement on the paired-associate learning task relative to non-GH-treated mice at the end of the study. Furthermore, we observed reduction of neural tissue loss in GH-treated stroked mice. We identified that GH treatment resulted in significantly higher levels of neurotrophic factors (IGF-1 [insulin-like growth factor-1] and VEGF [vascular endothelial growth factor]) in both the circulatory and peri-infarct regions. GH treatment in stroked mice not only promoted protein levels and density of presynaptic marker (SYN-1 [synapsin-1]) and marker of myelination (MBP [myelin basic protein]) but also increased the density and area coverage of 2 major vasculature markers (CD31 and collagen-IV), within the peri-infarct region.

CONCLUSIONS

These findings provide compelling preclinical evidence for the usage of GH as a potential therapeutic tool in the recovery phase of patients after stroke.

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.