Effects of the mode of re-socialization after juvenile social isolation on medial prefrontal cortex myelination and function

Pathophysiology in cortico-amygdala circuits and excessive aversion processing: the role of oligodendrocytes and myelination

Stress-related psychiatric illnesses, such as major depressive disorder, anxiety and post-traumatic stress disorder, present with alterations in emotional processing, including excessive processing of negative/aversive stimuli and events. The bidirectional human/primate brain circuit comprising anterior cingulate cortex and amygdala is of fundamental importance in processing emotional stimuli, and in rodents the medial prefrontal cortex-amygdala circuit is to some extent analogous in structure and function. Here, we assess the comparative evidence for: (i) Anterior cingulate/medial prefrontal cortex<->amygdala bidirectional neural circuits as major contributors to aversive stimulus processing; (ii) Structural and functional changes in anterior cingulate cortex<->amygdala circuit associated with excessive aversion processing in stress-related neuropsychiatric disorders, and in medial prefrontal cortex<->amygdala circuit in rodent models of chronic stress-induced increased aversion reactivity; and (iii) Altered status of oligodendrocytes and their oligodendrocyte lineage cells and myelination in anterior cingulate/medial prefrontal cortex<->amygdala circuits in stress-related neuropsychiatric disorders and stress models. The comparative evidence from humans and rodents is that their respective anterior cingulate/medial prefrontal cortex<->amygdala circuits are integral to adaptive aversion processing. However, at the sub-regional level, the anterior cingulate/medial prefrontal cortex structure-function analogy is incomplete, and differences as well as similarities need to be taken into account. Structure-function imaging studies demonstrate that these neural circuits are altered in both human stress-related neuropsychiatric disorders and rodent models of stress-induced increased aversion processing. In both cases, the changes include altered white matter integrity, albeit the current evidence indicates that this is decreased in humans and increased in rodent models. At the cellular-molecular level, in both humans and rodents, the current evidence is that stress disorders do present with changes in oligodendrocyte lineage, oligodendrocytes and/or myelin in these neural circuits, but these changes are often discordant between and even within species. Nonetheless, by integrating the current comparative evidence, this review provides a timely insight into this field and should function to inform future studies-human, monkey and rodent-to ascertain whether or not the oligodendrocyte lineage and myelination are causally involved in the pathophysiology of stress-related neuropsychiatric disorders.

Functional myelin in cognition and neurodevelopmental disorders

In vertebrates, oligodendrocytes (OLs) are glial cells of the central nervous system (CNS) responsible for the formation of the myelin sheath that surrounds the axons of neurons. The myelin sheath plays a crucial role in the transmission of neuronal information by promoting the rapid saltatory conduction of action potentials and providing neurons with structural and metabolic support. Saltatory conduction, first described in the peripheral nervous system (PNS), is now generally recognized as a universal evolutionary innovation to respond quickly to the environment: myelin helps us think and act fast. Nevertheless, the role of myelin in the central nervous system, especially in the brain, may not be primarily focused on accelerating conduction speed but rather on ensuring precision. Its principal function could be to coordinate various neuronal networks, promoting their synchronization through oscillations (or rhythms) relevant for specific information processing tasks. Interestingly, myelin has been directly involved in different types of cognitive processes relying on brain oscillations, and myelin plasticity is currently considered to be part of the fundamental mechanisms for memory formation and maintenance. However, despite ample evidence showing the involvement of myelin in cognition and neurodevelopmental disorders characterized by cognitive impairments, the link between myelin, brain oscillations, cognition and disease is not yet fully understood. In this review, we aim to highlight what is known and what remains to be explored to understand the role of myelin in high order brain processes.

Social isolation leads to mild social recognition impairment and losses in brain cellularity

Chronic social stress is a significant risk factor for several neuropsychiatric disorders, mainly major depressive disorder (MDD). In this way, patients with clinical depression may display many symptoms, including disrupted social behavior and anxiety. However, like many other psychiatric diseases, MDD has a very complex etiology and pathophysiology. Because social isolation is one of the multiple depression-inducing factors in humans, this study aims to understand better the link between social stress and MDD using an animal model based on social isolation after weaning, which is known to produce social stress in mice. We focused on cellular composition and white matter integrity to establish possible links with the abnormal social behavior that rodents isolated after weaning displayed in the three-chamber social approach and recognition tests. We used the isotropic fractionator method to assess brain cellularity, which allows us to robustly estimate the number of oligodendrocytes and neurons in dissected brain regions. In addition, diffusion tensor imaging (DTI) was employed to analyze white matter microstructure. Results have shown that post-weaning social isolation impairs social recognition and reduces the number of neurons and oligodendrocytes in important brain regions involved in social behavior, such as the anterior neocortex and the olfactory bulb. Despite the limitations of animal models of psychological traits, evidence suggests that behavioral impairments observed in patients might have similar biological underpinnings.

Oligodendrocyte pathology in Huntington's disease: from mechanisms to therapeutics

Oligodendrocytes (OLGs), highly specialized glial cells that wrap axons with myelin sheaths, are critical for brain development and function. There is new recognition of the role of OLGs in the pathogenesis of neurodegenerative diseases (NDDs), including Huntington's disease (HD), a prototypic NDD caused by a polyglutamine tract expansion in huntingtin (HTT), which results in gain- and loss-of-function effects. Clinically, HD is characterized by a constellation of motor, cognitive, and psychiatric disturbances. White matter (WM) structures, representing myelin-rich regions of the brain, are profoundly affected in HD, and recent findings reveal oligodendroglia dysfunction as an early pathological event. Here, we focus on mechanisms that underlie oligodendroglial deficits and dysmyelination in the progression of the disease, highlighting the pathogenic contributions of mutant HTT (mHTT). We also discuss potential therapeutic implications involving these molecular pathways.

Environmental modifications to rehabilitate social behavior deficits after acquired brain injury: What is the evidence?

Social behavior deficits are a common, debilitating consequence of traumatic brain injury and stroke, particularly when sustained during childhood. Numerous factors influence the manifestation of social problems after acquired brain injuries, raising the question of whether environmental manipulations can minimize or prevent such deficits. Here, we examine both clinical and preclinical evidence addressing this question, with a particular focus on environmental enrichment paradigms and differing housing conditions. We aimed to understand whether environmental manipulations can ameliorate injury-induced social behavior deficits. In summary, promising data from experimental models supports a beneficial role of environmental enrichment on social behavior. However, limited studies have considered social outcomes in the chronic setting, and few studies have addressed the social context specifically as an important component of the post-injury environment. Clinically, limited high-caliber evidence supports the use of specific interventions for social deficits after acquired brain injuries. An improved understanding of how the post-injury environment interacts with the injured brain, particularly during development, is needed to validate the implementation of rehabilitative interventions that involve manipulating an individuals' environment.

Understanding the Role of Oxidative Stress, Neuroinflammation and Abnormal Myelination in Excessive Aggression Associated with Depression: Recent Input from Mechanistic Studies

Aggression and deficient cognitive control problems are widespread in psychiatric disorders, including major depressive disorder (MDD). These abnormalities are known to contribute significantly to the accompanying functional impairment and the global burden of disease. Progress in the development of targeted treatments of excessive aggression and accompanying symptoms has been limited, and there exists a major unmet need to develop more efficacious treatments for depressed patients. Due to the complex nature and the clinical heterogeneity of MDD and the lack of precise knowledge regarding its pathophysiology, effective management is challenging. Nonetheless, the aetiology and pathophysiology of MDD has been the subject of extensive research and there is a vast body of the latest literature that points to new mechanisms for this disorder. Here, we overview the key mechanisms, which include neuroinflammation, oxidative stress, insulin receptor signalling and abnormal myelination. We discuss the hypotheses that have been proposed to unify these processes, as many of these pathways are integrated for the neurobiology of MDD. We also describe the current translational approaches in modelling depression, including the recent advances in stress models of MDD, and emerging novel therapies, including novel approaches to management of excessive aggression, such as anti-diabetic drugs, antioxidant treatment and herbal compositions.

Juvenile social isolation affects the structure of the tanycyte-vascular interface in the hypophyseal portal system of the adult mice

Accumulating evidence indicates that social stress in the juvenile period affects hypothalamic-pituitary-adrenal (HPA) axis activity in adulthood. The biological mechanisms underlying this phenomenon remain unclear. We aimed to elucidate them by comparing adult mice that had experienced social isolation from postnatal day 21-35 (juvenile social isolation (JSI) group) with those reared normally (control group). JSI group mice showed an attenuated HPA response to acute swim stress, while the control group had a normal response to this stress. Activity levels of the paraventricular nucleus in both groups were comparable, as shown by c-Fos immunoreactivities and mRNA expression of c-Fos, Corticotropin-releasing factor (CRF), Glucocorticoid receptor, and Mineralocorticoid receptor. We found greater vascular coverage by tanycytic endfeet in the median eminence of the JSI group mice than in that of the control group mice under basal condition and after acute swim stress. Moreover, CRF content after acute swim stress was greater in the median eminence of the JSI group mice than in that of the control group mice. The attenuated HPA response to acute swim stress was specific to JSI group mice, but not to control group mice. Although a direct link awaits further experiments, tanycyte morphological changes in the median eminence could be related to the HPA response.

Dysregulation of prefrontal parvalbumin interneurons leads to adult aggression induced by social isolation stress during adolescence

Social isolation during the juvenile stage results in structural and functional impairment of the brain and deviant adult aggression. However, the specific subregions and cell types that underpin this deviant behavior are still largely unknown. Here, we found that adolescent social isolation led to a shortened latency to attack onset and extended the average attack time, accompanied by anxiety-like behavior and deficits in social preference in adult mice. However, when exposed to social isolation during adulthood, the mice did not show these phenotypes. We also found that the structural plasticity of prefrontal pyramidal neurons, including the dendritic complexity and spine ratio, was impaired in mice exposed to adolescent social isolation. The parvalbumin (PV) interneurons in the prefrontal infralimbic cortex (IL) are highly vulnerable to juvenile social isolation and exhibit decreased cell numbers and reduced activation in adulthood. Moreover, chemogenetic inactivation of IL-PV interneurons can mimic juvenile social isolation-induced deviant aggression and social preference. Conversely, artificial activation of IL-PV interneurons significantly attenuated deviant aggression and rescued social preference during adulthood in mice exposed to adolescent social isolation. These findings implicate juvenile social isolation-induced damage to IL-PV interneurons in long-term aggressive behavior in adulthood.

Atypical structural connectivity of language networks in autism spectrum disorder: A meta-analysis of diffusion tensor imaging studies

Patients with autism spectrum disorder (ASD) often show pervasive and complex language impairments that are closely associated with aberrant structural connectivity of language networks. However, the characteristics of white matter connectivity in ASD have remained inconclusive in previous diffusion tensor imaging (DTI) studies. The current meta‐analysis aimed to comprehensively elucidate the abnormality in language‐related white matter connectivity in individuals with ASD. We searched PubMed, Web of Science, Scopus, and Medline databases to identify relevant studies. The standardized mean difference was calculated to measure the pooled difference in DTI metrics in each tract between the ASD and typically developing (TD) groups. The moderating effects of age, sex, language ability, and symptom severity were investigated using subgroup and meta‐regression analysis. Thirty‐three DTI studies involving 831 individuals with ASD and 836 TD controls were included in the meta‐analysis. ASD subjects showed significantly lower fractional anisotropy or higher mean diffusivity across language‐associated tracts than TD controls. These abnormalities tended to be more prominent in the left language networks than in the right. In addition, children with ASD exhibit more pronounced and pervasive disturbances in white matter connectivity than adults. These results support the under‐connectivity hypothesis and demonstrate the widespread abnormal microstructure of language‐related tracts in patients with ASD. Otherwise, white matter abnormalities in the autistic brain could vary depending on the developmental stage and hemisphere.

Activation of liver X receptors protects oligodendrocytes in CA3 of stress-induced mice

Depression is a complex disorder that is associated with various structural abnormalities. Oligodendrocyte (OL) dysfunction is associated with the pathogenesis of depression and the promotion of hippocampal oligodendrocyte maturation and myelination could be a novel therapeutic strategy for ameliorating depressive behaviors. Recent studies have shown that activation of liver X receptors (LXRs) by GW3965 improves depressive phenotypes, but the effects of GW3965 on OL function and myelination in the hippocampus of depression remain relatively unclear. To address this issue, we investigated the effects of GW3965 on mature OL in the hippocampus and on the myelin sheaths of mice subjected to chronic unpredictable stress (CUS). Behavioral tests were performed to assess depressive behaviors. Then, the number of mature OLs (CC1⁺) in each hippocampal subregion was precisely quantified with immunohistochemical and stereological methods, and the density of newborn mature OLs (BrdU⁺/Olig2⁺/CC1⁺ cells) in each hippocampal subregion was quantified with immunofluorescence. In addition, myelin basic protein (MBP) staining intensity in the cornu ammonis 3 (CA3) region was assessed by using immunofluorescence. We found that both the number of CC1⁺ OLs and the density of BrdU⁺/Olig2⁺/CC1⁺ cells were obviously decreased in each hippocampal subregion of mice subjected to CUS, and 4 weeks of GW3965 treatment reversed these effects only in the CA3 region. Furthermore, the decreased MBP expression in the CA3 region of mice subjected to CUS was ameliorated by GW3965 treatment. Collectively, these results suggested that improvement of OL maturation and enhancement of myelination may be structural mechanisms underlying the antidepressant effects of LXR agonists.

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.

Predation Stress Causes Excessive Aggression in Female Mice with Partial Genetic Inactivation of Tryptophan Hydroxylase-2: Evidence for Altered Myelination-Related Processes

The interaction between brain serotonin (5-HT) deficiency and environmental adversity may predispose females to excessive aggression. Specifically, complete inactivation of the gene encoding tryptophan hydroxylase-2 (Tph2) results in the absence of neuronal 5-HT synthesis and excessive aggressiveness in both male and female null mutant (Tph2−/−) mice. In heterozygous male mice (Tph2+/−), there is a moderate reduction in brain 5-HT levels, and when they are exposed to stress, they exhibit increased aggression. Here, we exposed female Tph2+/− mice to a five-day rat predation stress paradigm and assessed their emotionality and social interaction/aggression-like behaviors. Tph2+/− females exhibited excessive aggression and increased dominant behavior. Stressed mutants displayed altered gene expression of the 5-HT receptors Htr1a and Htr2a, glycogen synthase kinase-3 β (GSK-3β), and c-fos as well as myelination-related transcripts in the prefrontal cortex: myelin basic protein (Mbp), proteolipid protein 1 (Plp1), myelin-associated glycoprotein (Mag), and myelin oligodendrocyte glycoprotein (Mog). The expression of the plasticity markers synaptophysin (Syp) and cAMP response element binding protein (Creb), but not AMPA receptor subunit A2 (GluA2), were affected by genotype. Moreover, in a separate experiment, naïve female Tph2+/− mice showed signs of enhanced stress resilience in the modified swim test with repeated swimming sessions. Taken together, the combination of a moderate reduction in brain 5-HT with environmental challenges results in behavioral changes in female mice that resemble the aggression-related behavior and resilience seen in stressed male mutants; additionally, the combination is comparable to the phenotype of null mutants lacking neuronal 5-HT. Changes in myelination-associated processes are suspected to underpin the molecular mechanisms leading to aggressive behavior.

Beyond the neuron: Role of non-neuronal cells in stress disorders

Stress disorders are leading causes of disease burden in the U.S. and worldwide, yet available therapies are fully effective in less than half of all individuals with these disorders. Although to date, much of the focus has been on neuron-intrinsic mechanisms, emerging evidence suggests that chronic stress can affect a wide range of cell types in the brain and periphery, which are linked to maladaptive behavioral outcomes. Here, we synthesize emerging literature and discuss mechanisms of how non-neuronal cells in limbic regions of brain interface at synapses, the neurovascular unit, and other sites of intercellular communication to mediate the deleterious, or adaptive (i.e., pro-resilient), effects of chronic stress in rodent models and in human stress-related disorders. We believe that such an approach may one day allow us to adopt a holistic "whole body" approach to stress disorder research, which could lead to more precise diagnostic tests and personalized treatment strategies. Stress is a major risk factor for many psychiatric disorders. Cathomas et al. review new insight into how non-neuronal cells mediate the deleterious effects, as well as the adaptive, protective effects, of stress in rodent models and human stress-related disorders.

Persistent behavioral and neurobiological consequences of social isolation during adolescence

Meaningful social interactions are a fundamental human need, the lack of which can pose serious risks to an individual's physical and mental health. Across species, peer-oriented social behaviors are dramatically reshaped during adolescence, a developmental period characterized by dynamic changes in brain structure and function as individuals transition into adulthood. Thus, the experience of social isolation during this critical developmental stage may be especially pernicious, as it could permanently derail typical neurobiological processes that are necessary for establishing adaptive adult behaviors. The purpose of this review is to summarize investigations in which rodents were isolated during adolescence, then re-housed in typical social groups prior to testing, thus allowing the investigators to resolve the long-term consequences of social adversity experienced during adolescent sensitive periods, despite subsequent normalization of the social environment. Here, we discuss alterations in social, anxiety-like, cognitive, and decision-making behaviors in previously isolated adult rodents. We then explore corresponding neurobiological findings, focusing on the prefrontal cortex, including changes in synaptic densities and protein levels, white matter and oligodendrocyte function, and neuronal physiology. Made more urgent by the recent wave of social deprivation resulting from the COVID-19 pandemic, especially amongst school-aged adolescents, understanding the mechanisms by which even transient social adversity can negatively impact brain function across the lifespan is of paramount importance.

ASD-like behaviors, a dysregulated inflammatory response and decreased expression of PLP1 characterize mice deficient for sialyltransferase ST3GAL5

Gangliosides are glycosphingolipids, which are abundant in brain, are known to modulate ion channels and cell-to-cell communication. Deficiencies can result in aberrant myelination and altered immune responses, which can give rise to neurodevelopmental psychiatric disorders. However, to date, little mechanistic data is available on how ganglioside deficiencies contribute to the behavioural disorders. In humans, the loss of lactosylceramide-alpha-2,3-sialyltransferase (ST3Gal5) leads to a severe neuropathology, but in ST3Gal5 knock-out (St3gal5−/−) mice the absence of GM3 and associated a-, b- and c-series gangliosides is partially compensated by 0-series gangliosides and there is no overt behavioural phenotype. Here, we sought to examine the behavioural and molecular consequences of GM3 loss more closely. Mutants of both sexes exhibited impaired conditioned taste aversion in an inhibitory learning task and anxiety-like behaviours in the open field, moderate motor deficits, abnormal social interactions, excessive grooming and rearing behaviours. Taken together, the aberrant behaviours are suggestive of an autism spectrum disorder (ASD)-like syndrome. Molecular analysis showed decreased gene and protein expression of proteolipid protein-1 (Plp1) and over expression of proinflammatory cytokines, which has been associated with ASD-like syndromes. The inflammatory and behavioural responses to lipopolysaccharide (LPS) were also altered in the St3gal5−/− mice compared to wild-type, which is indicative of the importance of GM3 gangliosides in regulating immune responses. Together, the St3gal5−/− mice display ASD-like behavioural features, altered response to systemic inflammation, signs of hypomyelination and neuroinflammation, which suggests that deficiency in a- and b-series gangliosides could contribute to the development of an ASD-like pathology in humans.

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St3gal5−/− mice exhibit aberrant social, motor and cognitive behavior that is reminiscent of ASD-like syndrome.

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Interleukin1β is upregulated in the brain and spleen of St3gal5−/− of both sexes.

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Mutants display reduced gene and protein expression of the myelin protein Plp1.

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LPS induces sex-dependent abnormalities in the inflammatory response and social behavior in the St3gal5−/− mice.

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St3gal5−/− mice can be used to study the behavioural consequence of a- and b-series ganglioside deficiency

Preference uncertainty accounts for developmental effects on susceptibility to peer influence in adolescence

Adolescents are prone to social influence from peers, with implications for development, both adaptive and maladaptive. Here, using a computer-based paradigm, we replicate a cross-sectional effect of more susceptibility to peer influence in a large dataset of adolescents 14 to 24 years old. Crucially, we extend this finding by adopting a longitudinal perspective, showing that a within-person susceptibility to social influence decreases over a 1.5 year follow-up time period. Exploiting this longitudinal design, we show that susceptibility to social influences at baseline predicts an improvement in peer relations over the follow-up period. Using a Bayesian computational model, we demonstrate that in younger adolescents a greater tendency to adopt others’ preferences arises out of a higher uncertainty about their own preferences in the paradigmatic case of delay discounting (a phenomenon called ‘preference uncertainty’). This preference uncertainty decreases over time and, in turn, leads to a reduced susceptibility of one’s own behaviour to an influence from others. Neuro-developmentally, we show that a measure of myelination within medial prefrontal cortex, estimated at baseline, predicts a developmental decrease in preference uncertainty at follow-up. Thus, using computational and neural evidence, we reveal adaptive mechanisms underpinning susceptibility to social influence during adolescence.

People often change their preferences to conform with others. Using a longitudinal design, the authors show that such conformity decreases over the course of adolescence and that this reduction in conformity is accompanied by a decreasing degree of uncertainty about what to like.

"Live together, die alone": The effect of re-socialization on behavioural performance and social-affective brain-related proteins after a long-term chronic social isolation stress

Loneliness affects group-living mammals triggering a cascade of stress-dependent physiological disorders. Indeed, social isolation stress is a major risk factor for several neuropsychiatric disorders including anxiety and depression. Furthermore, social isolation has a negative impact on health and fitness. However, the neurobiological consequences of long-term chronic social isolation stress (LTCSIS) manifested during the adulthood of affected individuals are not fully understood. Our study assessed the impact of LTCSIS and social buffering (re-socialization) on the behavioural performance and social-affective brain-related proteins in diurnal, social, and long-lived Octodon degus (degus). Thereby, anxiety-like and social behaviour, and social recognition memory were assessed in male and female animals subjected to a variety of stress-inducing treatments applied from post-natal and post-weaning until their adulthood. Additionally, we evaluated the relationship among LTCSIS, Oxytocin levels (OXT), and OXT-Ca2+-signalling proteins in the hypothalamus, the hippocampus, and the prefrontal cortex. Our findings suggest that LTCSIS induces anxiety like-behaviour and impairs social novelty preference whereas sociability is unaffected. On the other hand, re-socialization can revert both isolation-induced anxiety and social memory impairment. However, OXT and its signalling remained reduced in the abovementioned brain areas, suggesting that the observed changes in OXT-Ca2+ pathway proteins were permanent in male and female degus. Based on these findings, we conclude degus experience social stress differently, suggesting the existence of sex-related mechanisms to cope with specific adaptive challenges.

Social isolation impairs the prefrontal-nucleus accumbens circuit subserving social recognition in mice

Although medial prefrontal cortex (mPFC) is known to play important roles in social behaviors, how early social experiences affect the mPFC and its subcortical circuit remains unclear. We report that mice singly housed (SH) for 8 weeks after weaning show a social recognition deficit, even after 4 weeks of resocialization. In SH mice, prefrontal infralimbic (IL) neurons projecting to the shell region of nucleus accumbens (NAcSh) show decreased excitability compared with group-housed (GH) mice. NAcSh-projecting IL neurons are activated when GH mice encounter a familiar conspecific, which is not observed in SH mice. Chemogenetic inhibition of NAcSh-projecting IL neurons in normal mice impairs social recognition without affecting social preference, whereas activation of these neurons reverses social recognition deficit in SH mice. Our findings demonstrate that early social experience critically affects mPFC IL-NAcSh projection, the activation of which is required for social recognition by encoding information for social familiarity.

White-Matter Repair as a Novel Therapeutic Target for Early Adversity

Early adversity (EA) impairs myelin development in a manner that persists later in life across diverse mammalian species including humans, non-human primates, and rodents. These observations, coupled with the highly conserved nature of myelin development suggest that animal models can provide important insights into the molecular mechanisms by which EA impairs myelin development later in life and the impact of these changes on network connectivity, cognition, and behavior. However, this area of translational research has received relatively little attention and no comprehensive review is currently available to address these issues. This is particularly important given some recent mechanistic studies in rodents and the availability of new agents to increase myelination. The goals of this review are to highlight the need for additional pre-clinical work in this area and to provide specific examples that demonstrate the potential of this work to generate novel therapeutic interventions that are highly needed.

Decomposing complex links between the childhood environment and brain structure in school-aged youth

Childhood experiences play a profound role in conferring risk and resilience for brain and behavioral development. However, how different facets of the environment shape neurodevelopment remains largely unknown. Here we sought to decompose heterogeneous relationships between environmental factors and brain structure in 989 school-aged children from the Adolescent Brain Cognitive Development Study. We applied a cross-modal integration and clustering approach called 'Similarity Network Fusion', which combined two brain morphometrics (i.e., cortical thickness and myelin-surrogate markers), and key environmental factors (i.e., trauma exposure, neighborhood safety, school environment, and family environment) to identify homogeneous subtypes. Depending on the subtyping resolution, results identified two or five subgroups, each characterized by distinct brain structure-environment profiles. Notably, more supportive caregiving and school environments were associated with greater myelination, whereas less supportive caregiving, higher family conflict and psychopathology, and higher perceived neighborhood safety were observed with greater cortical thickness. These subtypes were highly reproducible and predicted externalizing symptoms and overall mental health problems. Our findings support the theory that distinct environmental exposures are differentially associated with alterations in structural neurodevelopment. Delineating more precise associations between risk factors, protective factors, and brain development may inform approaches to enhance risk identification and optimize interventions targeting specific experiences.

Maturation of amygdala inputs regulate shifts in social and fear behaviors: A substrate for developmental effects of stress

Stress can negatively impact brain function and behaviors across the lifespan. However, stressors during adolescence have particularly harmful effects on brain maturation, and on fear and social behaviors that extend beyond adolescence. Throughout development, social behaviors are refined and the ability to suppress fear increases, both of which are dependent on amygdala activity. We review rodent literature focusing on developmental changes in social and fear behaviors, cortico-amygdala circuits underlying these changes, and how this circuitry is altered by stress. We first describe changes in fear and social behaviors from adolescence to adulthood and parallel developmental changes in cortico-amygdala circuitry. We propose a framework in which maturation of cortical inputs to the amygdala promote changes in social drive and fear regulation, and the particularly damaging effects of stress during adolescence may occur through lasting changes in this circuit. This framework may explain why anxiety and social pathologies commonly co-occur, adolescents are especially vulnerable to stressors impacting social and fear behaviors, and predisposed towards psychiatric disorders related to abnormal cortico-amygdala circuits.

Post-weaning social isolation increases ΔFosB/FosB protein expression in sex-specific patterns in the prelimbic/infralimbic cortex and hippocampus in mice

Social isolation is a growing public health concern across the lifespan. Specifically, isolation early in life, during critical periods of brain development, increases the risk of psychiatric disorders later in life. Previous studies of isolation models in mice have shown distinct neurological abnormalities in various regions of the brain, but the mechanism linking the experience of isolation to these phenotypes is unclear. In this study, we show that ΔFosB, a long-lived transcription factor associated with neuronal activity, chronic stress, and drug-induced neuroplasticity, is upregulated in the prelimbic/infralimbic (PL/IL) region of the cortex and hippocampus of adult C57BL/6J mice transiently isolated for two weeks post-weaning. Additionally, a related transcription factor, FosB, is also increased in the PL/IL in socially isolated females.In contrast, both ΔFosB and FosB are increased in male mice isolated for six weeks from weaning until tissue collection. These results show that short-term isolation during the critical post-weaning period has long-lasting and sex-dependent effects on gene expression in brain and that FosB/ΔFosB expression provides a potential mechanistic link between post-weaning social isolation and associated neurological abnormalities.

A prefrontal-paraventricular thalamus circuit requires juvenile social experience to regulate adult sociability in mice

Juvenile social isolation reduces sociability in adulthood, but the neural circuit mechanisms are poorly understood. We found that, in male mice, 2 weeks of social isolation immediately following weaning leads to a failure to activate medial prefrontal cortex (mPFC) neurons projecting to the posterior paraventricular thalamus (pPVT) during social exposure in adulthood. Chemogenetic or optogenetic suppression of mPFC->pPVT activity in adulthood was sufficient to induce sociability deficits without affecting anxiety-related behaviors or preference toward rewarding food. Juvenile isolation led to both reduced excitability of mPFC->pPVT neurons and increased inhibitory input drive from low-threshold spiking somatostatin interneurons in adulthood, suggesting a circuit mechanism underlying sociability deficits. Chemogenetic or optogenetic stimulation of mPFC->pPVT neurons in adulthood could rescue the sociability deficits caused by juvenile isolation. Our study identifies a pair of specific mPFC excitatory and inhibitory neuron populations required for sociability that are profoundly affected by juvenile social experience.

White matter alterations in Williams syndrome related to behavioral and motor impairments

Myelin is the electrical insulator surrounding the neuronal axon that makes up the white matter (WM) of the brain. It helps increase axonal conduction velocity (CV) by inducing saltatory conduction. Damage to the myelin sheath and WM is associated with many neurological and psychiatric disorders. Decreasing myelin deficits, and thus improving axonal conduction, has the potential to serve as a therapeutic mechanism for reducing the severity of some of these disorders. Myelin deficits have been previously linked to abnormalities in social behavior, suggesting an interplay between brain connectivity and sociability. This review focuses on Williams syndrome (WS), a genetic disorder characterized by neurocognitive characteristics and motor abnormalities, mainly known for its hypersociability characteristic. We discuss fundamental aspects of WM in WS and how its alterations can affect motor abilities and social behavior. Overall, findings regarding changes in myelin genes and alterations in WM structure in WS suggest new targets for drug therapy aimed at improving conduction properties and altering brain-activity synchronization in this disorder.

Social training reconfigures prediction errors to shape Self-Other boundaries

Selectively attributing beliefs to specific agents is core to reasoning about other people and imagining oneself in different states. Evidence suggests humans might achieve this by simulating each other’s computations in agent-specific neural circuits, but it is not known how circuits become agent-specific. Here we investigate whether agent-specificity adapts to social context. We train subjects on social learning tasks, manipulating the frequency with which self and other see the same information. Training alters the agent-specificity of prediction error (PE) circuits for at least 24 h, modulating the extent to which another agent’s PE is experienced as one’s own and influencing perspective-taking in an independent task. Ventromedial prefrontal myelin density, indexed by magnetisation transfer, correlates with the strength of this adaptation. We describe a frontotemporal learning network, which exploits relationships between different agents’ computations. Our findings suggest that Self-Other boundaries are learnable variables, shaped by the statistical structure of social experience.

The human brain can simulate other people’s mental processes with Self-specific and Other-specific neural circuits, but it is not known how these circuits emerge. Here, the authors show that these circuits adapt to social experience, to determine whether a computation is attributed to Self or Other.

Environmental Enrichment and Physical Exercise Attenuate the Depressive-Like Effects Induced by Social Isolation Stress in Rats

We assessed the antidepressant-like effects of environmental enrichment (EE) and physical exercise (PE) compared with the selective serotonin reuptake inhibitor fluoxetine against the depression-related neurobehavioral alterations induced by postweaning social isolation (SI) in rats. After 1 month of SI, rats were submitted to PE (treadmill), EE, or fluoxetine (10 mg/kg), which were compared with naïve SI and group-housed rats. After 1 month, behavior was analyzed in the open field (OFT), the sucrose preference (SPT), and the forced swimming (FST) tests. Afterward, the hippocampal serotonin contents, its metabolite, and turnover were measured. SI induced a depression-related phenotype characterized by a marginal bodyweight gain, anxiety, anhedonia, behavioral despair, and alterations of serotonin metabolism. EE produced the widest and largest antidepressive-like effect, followed by PE and fluoxetine, which were almost equivalent. The treatments, however, affected differentially the neurobehavioral domains investigated. EE exerted its largest effect on anhedonia and was the only treatment inducing anxiolytic-like effects. Fluoxetine, in contrast, produced its largest effect on serotonin metabolism, followed by its anti-behavioral despair action. PE was a middle-ground treatment with broader behavioral outcomes than fluoxetine, but ineffective to reverse the serotonergic alterations induced by SI. The most responsive test to the treatments was the FST, followed closely by the SPT. Although OFT locomotion and body weight varied considerably between groups, they were barely responsive to PE and fluoxetine. From a translational standpoint, our data suggest that exercise and recreational activities may have broader health benefits than antidepressants to overcome confinement and the consequences of chronic stress.

Sleep in the United States Military

The military lifestyle often includes continuous operations whether in training or deployed environments. These stressful environments present unique challenges for service members attempting to achieve consolidated, restorative sleep. The significant mental and physical derangements caused by degraded metabolic, cardiovascular, skeletomuscular, and cognitive health often result from insufficient sleep and/or circadian misalignment. Insufficient sleep and resulting fatigue compromises personal safety, mission success, and even national security. In the long-term, chronic insufficient sleep and circadian rhythm disorders have been associated with other sleep disorders (e.g., insomnia, obstructive sleep apnea, and parasomnias). Other physiologic and psychologic diagnoses such as post-traumatic stress disorder, cardiovascular disease, and dementia have also been associated with chronic, insufficient sleep. Increased co-morbidity and mortality are compounded by traumatic brain injury resulting from blunt trauma, blast exposure, and highly physically demanding tasks under load. We present the current state of science in human and animal models specific to service members during- and post-military career. We focus on mission requirements of night shift work, sustained operations, and rapid re-entrainment to time zones. We then propose targeted pharmacological and non-pharmacological countermeasures to optimize performance that are mission- and symptom-specific. We recognize a critical gap in research involving service members, but provide tailored interventions for military health care providers based on the large body of research in health care and public service workers.

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.

Characterization of behavioral phenotypes in the BTBR T+ Itpr3tf/J mouse model of autism spectrum disorder under social housing conditions using the multiple animal positioning system

The BTBR T⁺ Itpr3^tf/J (BTBR) mouse strain is a widely used model of autism spectrum disorder (ASD). The BTBR mice display behavior consistent with the three diagnostic categories of ASD. However, the behavioral phenotypes of the BTBR mice in a long-term group housing setting are largely unknown because conventional behavioral tests for ASD model mice are designed for use under simplified artificial conditions over a short observation period. In this study, we applied a newly developed assay system, the Multiple Animal Positioning System (MAPS), to quantify behaviors under group housing conditions over four days of continuous observation. Using MAPS, we showed that in a group housing condition, the BTBR mice exhibited lower activity levels in the dark phase and alteration of social behavior in comparison with the C57BL/6J mice. The phenotypes of the BTBR mice were affected by co-housing with the C57BL/6J mice for four days, but the influence was weak and limited. Our results by MAPS differ from those obtained using conventional behavioral tests. The present study demonstrated that MAPS would be useful for evaluating the usual/natural behaviors of various animal models in detail and under more ethological conditions.

Female mice are protected from space radiation-induced maladaptive responses

Interplanetary exploration will be humankind's most ambitious expedition and the journey required to do so, is as intimidating as it is intrepid. One major obstacle for successful deep space travel is the possible negative effects of galactic cosmic radiation (GCR) exposure. Here, we investigate for the first time how combined GCR impacts long-term behavioral and cellular responses in male and female mice. We find that a single exposure to simulated GCR induces long-term cognitive and behavioral deficits only in the male cohorts. GCR exposed male animals have diminished social interaction, increased anxiety-like phenotype and impaired recognition memory. Remarkably, we find that the female cohorts did not display any cognitive or behavioral deficits after GCR exposure. Mechanistically, the maladaptive behavioral responses observed only in the male cohorts correspond with microglia activation and synaptic loss in the hippocampus, a brain region involved in the cognitive domains reported here. Furthermore, we measured reductions in AMPA expressing synaptic terminals in the hippocampus. No changes in any of the molecular markers measured here are observed in the females. Taken together these findings suggest that GCR exposure can regulate microglia activity and alter synaptic architecture, which in turn leads to a range of cognitive alterations in a sex dependent manner. These results identify sex-dependent differences in behavioral and cognitive domains revealing promising cellular and molecular intervention targets to reduce GCR-induced chronic cognitive deficits thereby boosting chances of success for humans in deep space missions such as the upcoming Mars voyage.

Effects of cross-rearing with social peers on myelination in the medial prefrontal cortex of a mouse model with autism spectrum disorder

Autism spectrum disorder (ASD) is a neurodevelopmental disorder characterized by impaired social interaction, poor communication skills, and repetitive/restrictive behaviors. Recent studies have indicated that early rehabilitative intervention can alleviate the symptoms of individuals with ASD. However, it remains unknown whether rehabilitative intervention can restore brain structures such as myelin, which generally shows abnormalities in individuals with ASD. Therefore, in the present study, we used a mouse model of ASD (BTBR mice) that demonstrated asocial behaviors and hypomyelination in the medial prefrontal cortex (mPFC) to investigate whether interaction with social peers (C57BL/6J mice) has an effect on myelination. We found that housing with C57BL/6J mice after weaning through adulthood increased the myelin thickness in mPFC, but not in the motor cortex, of BTBR mice. There was no effect of cross-rearing with C57BL/6J mice on axon diameter in mPFC of BTBR mice. This finding suggests that early rehabilitative intervention may alleviate myelin abnormalities in mPFC as well as clinical symptoms in individuals with ASD.