The recovery trajectory of adolescent social defeat stress-induced behavioral, (1)H-MRS metabolites and myelin changes in Balb/c mice

Longitudinal trajectories of anterior cingulate glutamate and subclinical psychotic experiences in early adolescence: the impact of bullying victimization

Previous studies reported decreased glutamate levels in the anterior cingulate cortex (ACC) in non-treatment-resistant schizophrenia and first-episode psychosis. However, ACC glutamatergic changes in subjects at high-risk for psychosis, and the effects of commonly experienced environmental emotional/social stressors on glutamatergic function in adolescents remain unclear. In this study, adolescents recruited from the general population underwent proton magnetic resonance spectroscopy (MRS) of the pregenual ACC using a 3-Tesla scanner. We explored longitudinal data on the association of combined glutamate-glutamine (Glx) levels, measured by MRS, with subclinical psychotic experiences. Moreover, we investigated associations of bullying victimization, a risk factor for subclinical psychotic experiences, and help-seeking intentions, a coping strategy against stressors including bullying victimization, with Glx levels. Finally, path analyses were conducted to explore multivariate associations. For a contrast analysis, gamma-aminobutyric acid plus macromolecule (GABA+) levels were also analyzed. Negative associations were found between Glx levels and subclinical psychotic experiences at both Times 1 (n = 219, mean age 11.5 y) and 2 (n = 211, mean age 13.6 y), as well as for over-time changes (n = 157, mean interval 2.0 y). Moreover, effects of bullying victimization and bullying victimization × help-seeking intention interaction effects on Glx levels were found (n = 156). Specifically, bullying victimization decreased Glx levels, whereas help-seeking intention increased Glx levels only in bullied adolescents. Finally, associations among bullying victimization, help-seeking intention, Glx levels, and subclinical psychotic experiences were revealed. GABA+ analysis revealed no significant results. This is the first adolescent study to reveal longitudinal trajectories of the association between glutamatergic function and subclinical psychotic experiences and to elucidate the effect of commonly experienced environmental emotional/social stressors on glutamatergic function. Our findings may deepen the understanding of how environmental emotional/social stressors induce impaired glutamatergic neurotransmission that could be the underpinning of liability for psychotic experiences in early adolescence.

Brain metabolic derangements examined using 1H MRS and their (in)consistency among different rodent models of depression

Major depressive disorder (MDD) is underlined by neurochemical changes in the brain. Proton magnetic resonance spectroscopy (1H MRS) is a useful tool for their examination as it provides information about the levels of metabolites. This review summarises the current knowledge of 1H MRS findings from rodent models of MDD, assesses the results from both a biological and a technical perspective, and identifies the main sources of bias. From a technical point of view, bias-introducing factors are the diversity of the measured volumes and their positioning in the brain, the data processing, and the metabolite concentration expression. The biological variables are strain, sex, and species, as well as the model itself, and in vivo vs. ex vivo exploration. This review identified some consistency in the 1H MRS findings in the models of MDD: lower levels of glutamine, glutamate + glutamine, and higher levels of myo-inositol and taurine in most of the brain regions of MDD models. This may suggest changes in regional metabolism, neuronal dysregulation, inflammation, and a compensatory effect reaction in the MDD rodent models.

Daily stress, and mental health of professional degree graduate students in Chinese traditional medicine universities: the mediating role of learning career adaptation

BACKGROUND

With the expansion of professional degree graduate students' enrollment in China education, the mental health of these professional degree graduate students in medical-related majors who are under pressure of study, scientific research, clinical practice, and employment should not be ignored. What is the mental health level of these graduate students under the effect of learning career adaptation (internal resources) in the face of daily stress (external factors)? The purpose of this study is to discuss the relationship between these variables, and the mediating role of learning career adaptation of professional degree graduate students in traditional Chinese medicine colleges, and universities, to provide a theoretical basis for improving the learning career adaptation of students, and improving the level of mental health.

METHODS

A cross-sectional survey was conducted among 1593 professional degree graduate students majoring in clinical medicine, traditional Chinese medicine, and nursing in five traditional Chinese medicine universities. Finally, 660 questionnaires were returned, with a recovery rate of 41.43%. The scores of daily stress, learning career adaptation, and mental health were measured by Daily Stressors Scale for graduate students, graduate-students learning career adaptation scale, and General Mental Health Questionnaire (GHQ-20). Descriptive statistics were used to analyze the status quo of daily stress, learning career adaptation, and mental health. Pearson correlation analysis were used to analyze the relationship between them. we undertake analyses using structural equation modeling to construct the latent variable path model of daily stress, learning career adaptation on mental health. The significance level of the mediating effect was tested by the non-parametric percentile bootstrap method.

RESULTS

The scores of mental health, daily stress, and learning career adaptation were 50.56 ± 10.80, 35.12 ± 19.55, and 67.13 ± 7.48 respectively. Daily stress was negatively correlated with the three dimensions of learning career adaptation: career confidence, focus on his career, and career control (P < 0.01). Daily stress was positively correlated with depression and anxiety (P < 0.01). Self-affirmation, depression, and anxiety were negatively correlated with career confidence, focus on his career, and career control (P < 0.05). Learning career adaptation plays a partial mediating role between daily stress, and mental health (p < 0.001), with an intermediate effect value of 0.127, representing 28.54% of the total effect.

CONCLUSIONS

Mental health, learning career adaption of medical-related professional degree graduate students in traditional Chinese medical universities were at a moderate degree, and an upper-middle level respectively, while daily stress is to a lesser extent. Learning career adaptation mediates the relationship between daily stress, and mental health partially. To some extent, it can buffer the impact of daily stress on mental health, especially anxiety. The educational administrator could take various measures to improve the mental health of professional degree graduate students. It can also enhance their learning career adaptation from the perspective of individuals, and organizations to improve their mental health.

Node of Ranvier remodeling in chronic psychosocial stress and anxiety

Differential expression of myelin-related genes and changes in myelin thickness have been demonstrated in mice after chronic psychosocial stress, a risk factor for anxiety disorders. To determine whether and how stress affects structural remodeling of nodes of Ranvier, another form of myelin plasticity, we developed a 3D reconstruction analysis of node morphology in C57BL/6NCrl and DBA/2NCrl mice. We identified strain-dependent effects of chronic social defeat stress on node morphology in the medial prefrontal cortex (mPFC) gray matter, including shortening of paranodes in C57BL/6NCrl stress-resilient and shortening of node gaps in DBA/2NCrl stress-susceptible mice compared to controls. Neuronal activity has been associated with changes in myelin thickness. To investigate whether neuronal activation is a mechanism influencing also node of Ranvier morphology, we used DREADDs to repeatedly activate the ventral hippocampus-to-mPFC pathway. We found reduced anxiety-like behavior and shortened paranodes specifically in stimulated, but not in the nearby non-stimulated axons. Altogether, our data demonstrate (1) nodal remodeling of the mPFC gray matter axons after chronic stress and (2) axon-specific regulation of paranodes in response to repeated neuronal activity in an anxiety-associated pathway. Nodal remodeling may thus contribute to aberrant circuit function associated with anxiety disorders.

The effects of social defeat stress on hippocampal glial cells and interleukin-6 in adolescence and adulthood

Adolescent social stress has been associated with the vulnerability to developing psychopathological disorders in adulthood that are accompanied by brain inflammatory processes. The purpose of this study is to investigate the dynamic changes of the hippocampal neuroinflammatory mediators, including microglia, astrocyte, and interleukin-6 (IL-6) levels in mice experiencing social defeat stress during adolescence. Adolescent mice were divided into the control group and stress group. Mice in the stress group were exposed to chronic intermittent social defeat for a total of 12 days, and control mice were reared in normal conditions. The hippocampal microglia, astrocyte, and IL-6 levels were measured 24 h and 3 weeks after the end of stress exposure. Microglia activation characterized by increased ionized calcium-binding adapter molecule 1 positive cell numbers or staining area in the CA1 and CA3 regions of the hippocampus were observed 24 h after the end of stress, which did not last into the adulthood. No short-term or long-term alterations of the number of hippocampal CA1 and CA3 glia fibrillary acidic protein astrocytes were found in mice experiencing adolescent social defeat, whereas IL-6 levels were only increased 3 weeks after the end of stress. These data suggested that exposure to chronic social defeat stress led to short-term and long-term neuroinflammatory changes in the hippocampus.

Chronic stress disrupts the homeostasis and progeny progression of oligodendroglial lineage cells, associating immune oligodendrocytes with prefrontal cortex hypomyelination

Major depressive disorder (MDD) is a chronic debilitating illness affecting yearly 300 million people worldwide. Oligodendrocyte-lineage cells have emerged as important neuromodulators in synaptic plasticity and crucial components of MDD pathophysiology. Using the repeated social defeat (RSDS) mouse model, we demonstrate that chronic psychosocial stress induces long-lasting losses and transient proliferation of oligodendrocyte-precursor cells (OPCs), aberrant differentiation into oligodendrocytes, and severe hypomyelination in the prefrontal cortex. Exposure to chronic stress results in OPC morphological impairments, excessive oxidative stress, and oligodendroglial apoptosis, implicating integrative-stress responses in depression. Analysis of single-nucleus transcriptomic data from MDD patients revealed oligodendroglial-lineage dysregulation and the presence of immune-oligodendrocytes (Im-OL), a novel population of cells with immune properties and myelination deficits. Im-OL were also identified in mice after RSDS, where oligodendrocyte-lineage cells expressed immune-related markers. Our findings demonstrate cellular and molecular changes in the oligodendroglial lineage in response to chronic stress and associate hypomyelination with Im-OL emergence during depression.

Sex-Specific Brain Transcriptional Signatures in Human MDD and Their Correlates in Mouse Models of Depression

Major depressive disorder (MDD) is amongst the most devastating psychiatric conditions affecting several millions of people worldwide every year. Despite the importance of this disease and its impact on modern societies, still very little is known about the etiological mechanisms. Treatment strategies have stagnated over the last decades and very little progress has been made to improve the efficiency of current therapeutic approaches. In order to better understand the disease, it is necessary for researchers to use appropriate animal models that reproduce specific aspects of the complex clinical manifestations at the behavioral and molecular levels. Here, we review the current literature describing the use of mouse models to reproduce specific aspects of MDD and anxiety in males and females. We first describe some of the most commonly used mouse models and their capacity to display unique but also shared features relevant to MDD. We then transition toward an integral description, combined with genome-wide transcriptional strategies. The use of these models reveals crucial insights into the molecular programs underlying the expression of stress susceptibility and resilience in a sex-specific fashion. These studies performed on human and mouse tissues establish correlates into the mechanisms mediating the impact of stress and the extent to which different mouse models of chronic stress recapitulate the molecular changes observed in depressed humans. The focus of this review is specifically to highlight the sex differences revealed from different stress paradigms and transcriptional analyses both in human and animal models.

Neurochemical Alterations in Social Anxiety Disorder (SAD): A Systematic Review of Proton Magnetic Resonance Spectroscopic Studies

(1) Objective: Considering that current knowledge of mechanisms involved in the molecular pathogenesis of Social Anxiety Disorder (SAD) is limited, we conducted a systematic review to evaluate cumulative data obtained by Proton Magnetic Resonance Spectroscopic (¹H MRS) studies. (2) Methods: A computer-based literature search of Medline, EMBASE, PsycInfo, and ProQuest was performed. Only cross-sectional studies using ¹H MRS techniques in participants with SAD and healthy controls (HCs) were selected. (3) Results: The search generated eight studies. The results indicated regional abnormalities in the ‘fear neurocircuitry’ in patients with SAD. The implicated regions included the anterior cingulate cortex (ACC), dorsomedial prefrontal cortex (dmPFC), dorsolateral prefrontal cortex (dlPFC), insula, occipital cortex (OC), as well as the subcortical regions, including the thalamus, caudate, and the putamen. (4) Conclusions: The evidence derived from eight studies suggests that possible pathophysiological mechanisms of SAD include impairments in the integrity and function of neurons and glial cells, including disturbances in energy metabolism, maintenance of phospholipid membranes, dysregulations of second messenger systems, and excitatory/inhibitory neurocircuitry. Conducting more cross-sectional studies with larger sample sizes is warranted given the limited evidence in this area of research.

Juvenile exposure to acute traumatic stress leads to long-lasting alterations in grey matter myelination in adult female but not male rats

Stress early in life can have a major impact on brain development, and there is increasing evidence that childhood stress confers vulnerability for later developing psychiatric disorders. In particular, during peri-adolescence, brain regions crucial for emotional regulation, such as the prefrontal cortex (PFC), amygdala (AMY) and hippocampus (HPC), are still developing and are highly sensitive to stress. Changes in myelin levels have been implicated in mental illnesses and stress effects on myelin and oligodendrocytes (OLs) are beginning to be explored as a novel and underappreciated mechanism underlying psychopathologies. Yet there is little research on the effects of acute stress on myelin during peri-adolescence, and even less work exploring sex-differences. Here, we used a rodent model to test the hypothesis that exposure to acute traumatic stress as a juvenile would induce changes in OLs and myelin content across limbic brain regions. Male and female juvenile rats underwent 3 h of restraint stress with exposure to a predator odor on postnatal day (p) 28. Acute stress induced a physiological response, increasing corticosterone release and reducing weight gain in stress-exposed animals. Brain sections containing the PFC, AMY and HPC were taken either in adolescence (p40), or in adulthood (p95) and stained for markers of OLs and myelin. We found that acute stress induced sex-specific changes in grey matter (GM) myelination and OLs in both the short- and long-term. Exposure to a single stressor as a juvenile increased GM myelin content in the AMY and HPC in p40 males, compared to the respective control group. At p40, corticosterone release during stress exposure was also positively correlated with GM myelin content in the AMY of male rats. Single exposure to juvenile stress also led to long-term effects exclusively in female rats. Compared to controls, stress-exposed females showed reduced GM myelin content in all three brain regions. Acute stress exposure decreased PFC and HPC OL density in p40 females, perhaps contributing towards this observed long-term decrease in myelin content. Overall, our findings suggest that the juvenile brain is vulnerable to exposure to a brief severe stressor. Exposure to a single short traumatic event during peri-adolescence produces long-lasting changes in GM myelin content in the adult brain of female, but not male, rats. These findings highlight myelin plasticity as a potential contributor to sex-specific sensitivity to perturbation during a critical window of development.

Intrauterine inflammation induced white matter injury protection by fibrinogen-like protein 2 deficiency in perinatal mice

BACKGROUND

White matter injury (WMI) induced by intrauterine inflammation can cause adverse neurological outcomes. Fibrinogen-like protein 2 (FGL2)/fibroleukin is an important trigger of inflammatory responses and is involved in some cerebral diseases. However, the role of FGL2 in intrauterine inflammation-induced WMI remains unclear.

METHODS

Lipopolysaccharide (LPS) was intraperitoneally injected into wild-type and FGL2 knockout mice to induce intrauterine inflammation. Body weight and brain weight of offspring were monitored. Major basic protein (MBP) expression was evaluated to demonstrate the myelination of offspring. To investigate the regulatory mechanism of FGL2, cytokine expression, microglial polarization, and the activation of mitogen-activated protein kinase (MAPK) signaling pathway in the offspring were analyzed.

RESULTS

Upon LPS exposure, FGL2 knockout offspring showed a significant increase in body weight loss. MBP reduction induced by LPS was prevented in FGL2 knockout offspring. Expression levels of proinflammatory cytokines interleukin-1β (IL-1β) and tumor necrosis factor-α, and M1 marker CD86 were suppressed, while the expression levels of anti-inflammatory cytokines IL-10 and M2 marker CD206 were increased. FGL2 deficiency significantly inhibited the phosphorylation of p38MAPK and c-Jun N-terminal kinase (JNK) protein.

CONCLUSIONS

FGL2 deficiency can ameliorate WMI induced by intrauterine inflammation, reducing inflammatory cascade and improving hypomyelination, through the regulation of microglial polarization and MAPK signaling pathways.

IMPACT

Intrauterine inflammation induces WMI leading to severe neurological sequelae. FGL2 plays an important role in the progression of WMI induced by intrauterine inflammation. FGL2 deficiency can protect against WMI by inhibiting p38 MAPK and JNK phosphorylation, regulating microglia polarization, and reducing inflammation response. FGL2 could be a novel molecular target for protecting against WMI induced by intrauterine inflammation.

Maternal separation with early weaning impairs neuron-glia integrity: non-invasive evaluation and substructure demonstration

Astrocytes and oligodendrocytes play essential roles in regulating neural signal transduction along neural circuits in CNS. The perfect coordination of neuron/astrocyte and neuron/oligodendrocyte entities was termed as neuron-glia integrity recently. Here we monitored the status of neuron-glia integrity via non-invasive neuroimaging methods and demonstrated the substructures of it using other approaches in an animal model of maternal separation with early weaning (MSEW), which mimics early life neglect and abuse in humans. Compared to controls, MSEW rats showed higher glutamate level, but lower GABA in prefrontal cortex (PFC) detected by chemical exchange saturation transfer and ¹H-MRS methods, lower levels of glial glutamate transporter-1 and ATP-α, but increased levels of glutamate decarboxylase-65 and glutamine synthetase in PFC; reduced fractional anisotropy in various brain regions revealed by diffusion tensor imaging, along with increased levels of N-acetyl-aspartate measured by ¹H-MRS; and hypomyelination in PFC as evidenced by relevant cellular and molecular changes.

Shared Transcriptional Signatures in Major Depressive Disorder and Mouse Chronic Stress Models

BACKGROUND

Most of our knowledge of the biological basis of major depressive disorder (MDD) is derived from studies of chronic stress models in rodents. While these models capture certain aspects of the behavioral and neuroendocrine features of MDD, the extent to which they reproduce the molecular pathology of the human syndrome remains unknown.

METHODS

We systematically compared transcriptional signatures in two brain regions implicated in depression-medial prefrontal cortex and nucleus accumbens-of humans with MDD and of 3 chronic stress models in mice: chronic variable stress, adult social isolation, and chronic social defeat stress. We used differential expression analysis combined with weighted gene coexpression network analysis to create interspecies gene networks and assess the capacity of each stress paradigm to recapitulate the transcriptional organization of gene networks in human MDD.

RESULTS

Our results show significant overlap between transcriptional alterations in medial prefrontal cortex and nucleus accumbens in human MDD and the 3 mouse chronic stress models, with each of the chronic stress paradigms capturing distinct aspects of MDD abnormalities. Chronic variable stress and adult social isolation better reproduce differentially expressed genes, while chronic social defeat stress and adult social isolation better reproduce gene networks characteristic of human MDD. We also identified several gene networks and their constituent genes that are most significantly associated with human MDD and mouse stress models.

CONCLUSIONS

This study demonstrates the ability of 3 chronic stress models in mice to recapitulate distinct aspects of the broad molecular pathology of human MDD, with no one mouse model apparently better than another.

Social Defeat Stress in Adolescent Mice Induces Depressive-like Behaviors with Reduced Oligodendrogenesis

Strong stress related to adverse experiences during adolescence can cause mental disorders, as well as affecting brain structure and function. However, the underlying neurobiological mechanisms remain largely unknown. To investigate whether stress induced by adverse experience during adolescence affects oligodendrocyte (OL) remodeling, social defeat stress was applied to 6-week-old adolescent mice for 10 days, followed by behavioral tests and assessments of oligodendrogenesis. Socially defeated mice showed depressive-like behaviors in behavioral experiments. Stress led to a decrease in the number of newly born OLs in the anterior cortical region and the number of proteolipid protein-positive mature OLs in the corpus callosum and posterior cerebral cortex. Fewer bromodeoxyuridine-incorporated CC1-positive mature OLs were observed in these regions in socially defeated mice. To assess whether decreased oligodendrogenesis caused by social defeat stress is related to depressive-like symptoms under stress, clemastine, a drug that induces OL generation, was administered to socially defeated adolescent mice, resulting in the rescue of the behavioral abnormalities accompanied by increased oligodendrogenesis. These findings suggest that oligodendrogenesis in adverse environments during adolescence plays a role in psychiatric disorders, and clemastine may provide a potential therapeutic drug for adolescent mental disorders, targeting OLs.

The Long-Term Effects of Adolescent Social Defeat Stress on Oligodendrocyte Lineage Cells and Neuroinflammatory Mediators in Mice

OBJECTIVE

Adverse childhood and adolescent experiences are associated with the emergences of psychopathology later in life and have negative consequences on white matter integrity. However, this adversity-induced white matter impairment remains not fully investigated.

METHODS

Adolescent Balb/c mice were subjected to intermittent social defeat stress once a day during postnatal days 25 to 40. Then, the subjects were allowed to recover for three weeks before sacrifice. At the end, oligodendrocyte (OL) lineage cells, cell proliferation, and microglia activation, as well as myelin basic protein (MBP) levels in frontal cortex and hippocampus were evaluated. The levels of interleukin (IL)-1β and IL-6 in the brain regions were assessed.

RESULTS

MBP protein level in frontal cortex, but not in the hippocampus of defeated mice, decreased significantly compared to controls. The numeral densities of mature OLs, oligodendrocyte progenitor cells, and proliferating cells in medial prefrontal cortex were comparable between the defeated mice and controls. The defeated mice, however, showed significantly higher IL-1β level, although IL-6 level and numeral density of microglia in frontal cortex did not change relative to controls.

CONCLUSION

These results indicate that effects of intermittent social defeat stress on the white matter integrity and OL lineage cells in mouse brain are region- and developmental stage-specific. Upregulated IL-1β may contribute to this negative consequence though the underlying mechanism remains to be investigated.

Elevated Neuroglobin Lessens Neuroinflammation and Alleviates Neurobehavioral Deficits Induced by Acute Inhalation of Combustion Smoke in the Mouse

Acute inhalation of combustion smoke produces long-term neurologic deficits in survivors. To study the mechanisms that contribute to the development of neurologic deficits and identify targets for prevention, we developed a mouse model of acute inhalation of combustion smoke, which supports longitudinal investigation of mechanisms that underlie the smoke induced inimical sequelae in the brain. Using a transgenic mouse engineered to overexpress neuroglobin, a neuroprotective oxygen-binding globin protein, we previously demonstrated that elevated neuroglobin preserves mitochondrial respiration and attenuates formation of oxidative DNA damage in the mouse brain after smoke exposure. In the current study, we show that elevated neuronal neuroglobin attenuates the persistent inflammatory changes induced by smoke exposure in the mouse brain and mitigates concordant smoke-induced long-term neurobehavioral deficits. Specifically, we found that increases in hippocampal density of GFAP and Iba-1 positive cells that are detected post-smoke in wild-type mice are absent in the neuroglobin overexpressing transgenic (Ngb-tg) mice. Similarly, the smoke induced hippocampal myelin depletion is not observed in the Ngb-tg mice. Importantly, elevated neuroglobin alleviates behavioral and memory deficits that develop after acute smoke inhalation in the wild-type mice. Taken together, our findings suggest that the protective effects exerted by neuroglobin in the brains of smoke exposed mice afford protection from long-term neurologic sequelae of acute inhalation of combustion smoke. Our transgenic mouse provides a tool for assessing the potential of elevated neuroglobin as possible strategy for management of smoke inhalation injury.

Region-specific myelin differences define behavioral consequences of chronic social defeat stress in mice

Exposure to stress increases the risk of developing mood disorders. While a subset of individuals displays vulnerability to stress, others remain resilient, but the molecular basis for these behavioral differences is not well understood. Using a model of chronic social defeat stress, we identified region-specific differences in myelination between mice that displayed social avoidance behavior (‘susceptible’) and those who escaped the deleterious effect to stress (‘resilient’). Myelin protein content in the nucleus accumbens was reduced in all mice exposed to stress, whereas decreased myelin thickness and internodal length were detected only in the medial prefrontal cortex (mPFC) of susceptible mice, with fewer mature oligodendrocytes and decreased heterochromatic histone marks. Focal demyelination in the mPFC was sufficient to decrease social preference, which was restored following new myelin formation. Together these data highlight the functional role of mPFC myelination as critical determinant of the avoidance response to traumatic social experiences.

Editorial note: This article has been through an editorial process in which the authors decide how to respond to the issues raised during peer review. The Reviewing Editor's assessment is that all the issues have been addressed (see decision letter).

High levels of stress do not have the same effect on everybody: some individuals can show resilience and recover quickly, while other struggle to cope. Scientists have started to investigate how these differences may find their origin in biological processes, mainly by focusing on the role of neurons. However, neurons represent only one type of brain cells, and there is increasing evidence that interactions between neuronal and non-neuronal cells play an important role in the response to stress.

Oligodendrocytes are a common type of non-neuronal cells which shield and feed nerve cells. In particular, their membrane constitutes the myelin sheath, a protective coating that insulates neurons and allows them to better communicate with each other using electric signals.

Bonnefil et al. explored whether differences in oligodendrocytes could affect how mice responded to social stress. The rodents were exposed to repeated attacks from an aggressive mouse five minutes a day for ten days. After this period, ‘susceptible’ mice then avoided future contact with any other mice, while resilient animals remained interested in socializing.

Comparing the brain areas of resilient and susceptible mice revealed differences in the oligodendrocytes of the medial prefrontal cortex, the part of the brain that controls emotions and thinking. Susceptible animals had fewer mature oligodendrocytes and their neurons were covered in thinner and shorter segments of myelin sheaths. There was also evidence that, in these animals, the genes that regulate the maturation of oligodendrocytes were more likely to be switched off. Taken together, these results may suggest that, in certain animals, social stress disrupts the genetic program that controls how oligodendrocytes develop, potentially leading to neurons communicating less well.

To explore whether reduced amounts of myelin could be linked to decreased social behavior, Bonnefil et al. then damaged the myelin in the medial prefrontal cortex in another group of rodents. The mice were then less willing to interact with other animals until new sheaths had formed.

The results by Bonnefil et al. undercover how changes in non-neuronal cells can at least in part explain differences in the way individuals respond to stress. Ultimately, this knowledge may be useful to design new strategies to foster resilience.

Genetic Control of Myelin Plasticity after Chronic Psychosocial Stress

Anxiety disorders often manifest in genetically susceptible individuals after psychosocial stress, but the mechanisms underlying these gene-environment interactions are largely unknown. We used the chronic social defeat stress (CSDS) mouse model to study resilience and susceptibility to chronic psychosocial stress. We identified a strong genetic background effect in CSDS-induced social avoidance (SA) using four inbred mouse strains: 69% of C57BL/6NCrl (B6), 23% of BALB/cAnNCrl, 19% of 129S2/SvPasCrl, and 5% of DBA/2NCrl (D2) mice were stress resilient. Furthermore, different inbred mouse strains responded differently to stress, suggesting they use distinct coping strategies. To identify biological pathways affected by CSDS, we used RNA-sequencing (RNA-seq) of three brain regions of two strains, B6 and D2: medial prefrontal cortex (mPFC), ventral hippocampus (vHPC), and bed nucleus of the stria terminalis (BNST). We discovered overrepresentation of oligodendrocyte (OLG)-related genes in the differentially expressed gene population. Because OLGs myelinate axons, we measured myelin thickness and found significant region and strain-specific differences. For example, in resilient D2 mice, mPFC axons had thinner myelin than controls, whereas susceptible B6 mice had thinner myelin than controls in the vHPC. Neither myelin-related gene expression in several other regions nor corpus callosum thickness differed between stressed and control animals. Our unbiased gene expression experiment suggests that myelin plasticity is a substantial response to chronic psychosocial stress, varies across brain regions, and is genetically controlled. Identification of genetic regulators of the myelin response will provide mechanistic insight into the molecular basis of stress-related diseases, such as anxiety disorders, a critical step in developing targeted therapy.

Acute inhalation of combustion smoke triggers neuroinflammation and persistent anxiety-like behavior in the mouse

CONTEXT

Acute inhalation of combustion smoke triggers neurologic sequelae in survivors. Due to the challenges posed by heterogeneity of smoke exposures in humans, mechanistic links between acute smoke inhalation and neuropathologic sequelae have not been systematically investigated.

METHODS

Here, using mouse model of acute inhalation of combustion smoke, we studied longitudinal neurobehavioral manifestations of smoke exposures and molecular/cellular changes in the mouse brain.

RESULTS

Immunohistochemical analyses at eight months post-smoke, revealed hippocampal astrogliosis and microgliosis accompanied by reduced myelination. Elevated expression of proinflammatory cytokines was also detected. Longitudinal testing in different neurobehavioral paradigms in the course of post-smoke recovery, revealed lasting anxiety-like behavior. The examined paradigms included the open field exploration/anxiety testing at two, four and six months post-smoke, which detected decreases in total distance traveled and time spent in the central arena in the smoke-exposed compared to sham-control mice, suggestive of dampened exploratory activity and increased anxiety-like behavior. In agreement with reduced open field activity, cued fear conditioning test revealed increased freezing in response to conditioned auditory stimulus in mice after acute smoke inhalation. Similarly, elevated plus maze testing demonstrated lesser presence in open arms of the maze, consistent with anxiety-like behavior, for the post-smoke exposure mice.

CONCLUSIONS

Taken together, our data demonstrate for the first time persistent neurobehavioral manifestations of acute inhalation of combustion smoke and provide new insights into long-term progression of events initiated by disrupted brain oxygenation that might contribute to lasting adverse sequelae in survivors of smoke inhalation injuries.

Chronic social defeat reduces myelination in the mouse medial prefrontal cortex

The medial prefrontal cortex (mPFC) plays a key role in top-down control of the brain’s stress axis, and its structure and function are particularly vulnerable to stress effects, which can lead to depression in humans and depressive-like states in animals. We tested whether chronic social defeat produces structural alterations in the mPFC in mice. We first performed a microarray analysis of mPFC gene expression changes induced by defeat, and biological pathway analysis revealed a dominant pattern of down-regulation of myelin-associated genes. Indeed, 69% of the most significantly down-regulated genes were myelin-related. The down regulation was confirmed by in situ hybridization histochemistry for two strongly down-regulated genes, myelin oligodendrocyte glycoprotein (Mog) and ermin (Ermn), and by immunohistochemistry for myelin basic protein. To test for stress-induced changes in myelin integrity, aurophosphate (Black Gold) myelin staining was performed on mPFC sections. Quantitative stereologic analysis showed reduced myelinated fiber length and density. Behavioral analysis confirmed that the 14-day social defeat sessions resulted in induction of depressive-like states measured in social interaction and light/dark tests. The combined data suggest that chronic social defeat induces molecular changes that reduce myelination of the prefrontal cortex, which may be an underlying basis for stress-induced depressive states.

Evaluation of neuron-glia integrity by in vivo proton magnetic resonance spectroscopy: Implications for psychiatric disorders

Proton magnetic resonance spectroscopy (¹H-MRS) has been widely applied in human studies. There is now a large literature describing findings of brain MRS studies with mental disorder patients including schizophrenia, bipolar disorder, major depressive disorder, and anxiety disorders. However, the findings are mixed and cannot be reconciled by any of the existing interpretations. Here we proposed the new theory of neuron-glia integrity to explain the findings of brain ¹H-MRS stuies. It proposed the neurochemical correlates of neuron-astrocyte integrity and axon-myelin integrity on the basis of update of neurobiological knowledge about neuron-glia communication and of experimental MRS evidence for impairments in neuron-glia integrity from the authors and the other investigators. Following the neuron-glia integrity theories, this review collected evidence showing that glutamate/glutamine change is a good marker for impaired neuron-astrocyte integrity and that changes in N-acetylaspartate and lipid precursors reflect impaired myelination. Moreover, this new theory enables us to explain the differences between MRS findings in neuropsychiatric and neurodegenerative disorders.