Isolation rearing induces social and emotional function abnormalities and alters glutamate and neurodevelopment-related gene expression in rats

The role of social isolation stress in escalated aggression in rodent models

Anti-social behavior and violence are major public health concerns. Globally, violence contributes to more than 1.6 million deaths each year. Previous studies have reported that social rejection or neglect exacerbates aggression. In rodent models, social isolation stress is used to demonstrate the adverse effects of social deprivation on physiological, endocrinological, immunological, and behavioral parameters, including aggressive behavior. This review summarizes recent rodent studies on the effect of social isolation stress during different developmental periods on aggressive behavior and the underlying neural mechanisms. Social isolation during adulthood affects the levels of neurosteroids and neuropeptides and increases aggressive behavior. These changes are ethologically relevant for the adaptation to changes in local environmental conditions in the natural habitats. Chronic deprivation of social interaction after weaning, especially during the juvenile to adolescent periods, leads to the disruption of the development of appropriate social behavior and the maladaptive escalation of aggressive behavior. The understanding of neurobiological mechanisms underlying social isolation-induced escalated aggression will aid in the development of therapeutic interventions for escalated aggression.

Central amygdala is related to the reduction of aggressive behavior by monosodium glutamate ingestion during the period of development in an ADHD model rat

Introduction

Monosodium glutamate (MSG), an umami substance, stimulates the gut-brain axis communication via gut umami receptors and the subsequent vagus nerves. However, the brain mechanism underlying the effect of MSG ingestion during the developmental period on aggression has not yet been clarified. We first tried to establish new experimental conditions to be more appropriate for detailed analysis of the brain, and then investigated the effects of MSG ingestion on aggressive behavior during the developmental stage of an ADHD rat model.

Methods

Long-Evans, WKY/Izm, SHR/Izm, and SHR-SP/Ezo were individually housed from postnatal day 25 for 5 weeks. Post-weaning social isolation (PWSI) was given to escalate aggressive behavior. The resident-intruder test, that is conducted during the subjective night, was used for a detailed analysis of aggression, including the frequency, duration, and latency of anogenital sniffing, aggressive grooming, and attack behavior. Immunohistochemistry of c-Fos expression was conducted in all strains to predict potential aggression-related brain areas. Finally, the most aggressive strain, SHR/Izm, a known model of attention-deficit hyperactivity disorder (ADHD), was used to investigate the effect of MSG ingestion (60 mM solution) on aggression, followed by c-Fos immunostaining in aggression-related areas. Bilateral subdiaphragmatic vagotomy was performed to verify the importance of gut-brain interactions in the effect of MSG.

Results

The resident intruder test revealed that SHR/Izm rats were the most aggressive among the four strains for all aggression parameters tested. SHR/Izm rats also showed the highest number of c-Fos + cells in aggression-related brain areas, including the central amygdala (CeA). MSG ingestion significantly decreased the frequency and duration of aggressive grooming and attack behavior and increased the latency of attack behavior. Furthermore, MSG administration successfully increased c-Fos positive cell number in the intermediate nucleus of the solitary tract (iNTS), a terminal of the gastrointestinal sensory afferent fiber of the vagus nerve, and modulated c-Fos positive cells in the CeA. Interestingly, vagotomy diminished the MSG effects on aggression and c-Fos expression in the iNTS and CeA.

Conclusion

MSG ingestion decreased PWSI-induced aggression in SHR/Izm, which was mediated by the vagus nerve related to the stimulation of iNTS and modulation of CeA activity.

Better together: isolation impedes memory formation for configural learning in Lymnaea stagnalis

Social interactions play an important role in learning and memory. There is great variability in the literature regarding the effects of social isolation on cognition. Here, we investigated how memory formation was affected when Lymnaea stagnalis, our model system, were socially isolated at three different time periods: before, during or after the configural learning training procedure. Each group of snails underwent configural learning where we recorded and compared their feeding behaviour before and after the pairing of an appetitive food stimulus with predator kairomones (i.e. the training procedure). We found that isolating snails before the training procedure had no effect on their learning and memory. However, when snails were isolated either during the training procedure or immediately after the training procedure, they no longer formed memory. These data provide further insight into how isolation impacts cognitive functioning in the context of higher-order learning.

Social recognition memory differences between mouse strains: On the effects of social isolation, adult neurogenesis, and environmental enrichment

Remembering conspecifics is paramount for the establishment and maintenance of groups. Here we asked whether the variability in social behavior caused by different breeding strategies affects social recognition memory (SRM). We tested the hypothesis that the inbred Swiss and the outbred C57BL/6 mice behave differently on SRM. Social memory in C57BL/6 mice endured at least 14 days, while in Swiss mice lasted 24 h but not ten days. We showed previously that an enriched environment enhanced the persistence of SRM in Swiss mice. Here we reproduced this result and added that it also increases the survival of adult-born neurons in the hippocampus. Next, we tested whether prolonged SRM observed in C57BL/6 mice could be changed by diminishing the trial duration or using an interference stimulus after learning. Neither short acquisition time nor interference during consolidation affected it. However, social isolation impaired SRM in C57BL/6 mice, similar to what was previously observed in Swiss mice. Our results demonstrate that SRM expression can vary according to the mouse strain, which shows the importance of considering this variable when choosing the most suitable model to answer specific questions about this memory system. We also demonstrate the suitability of both C57BL/6 and Swiss strains for exploring the impact of environmental conditions and adult neurogenesis on social memory.

Effect of caffeine on hippocampal memory and levels of gene expression in social isolation stress

BACKGROUND

Caffeine (Cf) antagonizes the adenosine receptors and has neuroprotective properties. The effect of Cf has been seen on stress-induced deficits of cognitive. In this study, we have investigated the effect of Cf on learning and memory functions induced by social isolation (SI) stress.

MATERIALS AND METHODS

In the present study, 21-day-old Wistar albino male rats (n = 28) were divided into four groups: the control (C), the SI, the Cf, and the social isolation + caffeine (SICf). Cf (0.3 g/L) was added to the drinking water of the experimental animals for 4 weeks. The learning and memory functions were assessed using the Morris Water Maze Test (MWMT). Following, was performed histopathological evaluation and determined hippocampal gene expression levels by RT-qPCR.

RESULTS

According to MWMT findings, the time spent in the quadrant where the platform removed was decreased in the SI group compared with the C (p < 0.05). Histological evaluation showed morphological changes in SI by irregular appearance, cellular edema, and dark pycnotic appearance of nuclei in some neurons. However, it was observed that the histological structure of most of the neurons in the SICf group was similar to the C and Cf groups. Hippocampal SNAP25 expression was decreased in the Cf and SICf groups than in the C group (p < 0.05). The GFAP expression was increased in the SICf group than in the C group (p < 0.05). NR2A increased in the SI and SICf groups compared with C and Cf groups (p < 0.05). NR2B expression decreased in the Cf group compared with C and SI groups (p < 0.05).

CONCLUSIONS

SI impaired spatial memory and causes morphological changes in adolescent rats, but this effect of isolation was not seen in Cf-treated animals. The effects of SI on NR2A, Cf on NR2B, and SNAP25 are remarkable. Here, we propose that the impaired effect of SI on spatial memory may be mediated by NR2A, but further studies are needed to explain this effect.

Impact of prolonged chronic social isolation stress on behavior and multifractal complexity of metabolic rate in Octodon degus

Social interaction can improve animal performance through the prevention of stress-related events, the provision of security, and the enhancement of reproductive output and survival. We investigated the effects of prolonged chronic social isolation stress on behavioral, cognitive, and physiological performance in the social, long-lived rodent Octodon degus. Degu pups were separated into two social stress treatments: control (CTRL) and chronically isolated (CI) individuals from post-natal and post-weaning until adulthood. We quantified anxiety-like behavior and cognitive performance with a battery of behavioral tests. Additionally, we measured their basal metabolic rate (BMR) and analyzed the multifractal properties of the oxygen consumption time series using Multifractal Detrended Fluctuation Analysis, a well-known method for assessing the fractal characteristics of biological signals. Our results showed that CI induced a significant increase in anxiety-like behaviors and led to a reduction in social and working memory in male degus. In addition, CI-treated degus reduced the multifractal complexity of BMR compared to CTRL, which implies a decrease in the ability to respond to environmental stressors and, as a result, an unhealthy state. In contrast, we did not observe significant effects of social stress on BMR. Multivariate analyses showed a clear separation of behavior and physiological variables into two clusters, corresponding to CI and CTRL degus. This study provides novel insights into the effects of prolonged chronic social isolation stress on behavior, cognitive performance, and metabolic complexity in this rodent animal model. To the best of our knowledge, it is the first study to integrate cognitive-behavioral performance and multifractal dynamics of a physiological signal in response to prolonged social isolation. These findings highlight the importance of social interactions for the well-being and overall performance of social animals.

Role of Acorus calamus in preventing depression, anxiety, and oxidative stress in long-term socially isolated rats

Background and Aim

Social isolation stress (SIS) and individual housing have been shown to cause abnormal cognitive insufficiencies, altered anxiety levels, and signs of psychiatric diseases. Acorus calamus (AC), commonly known as Sweet Flag, has been widely used in India to treat neurological, metabolic, and respiratory disorders, indicating its potential therapeutic value. This study aimed to determine the antidepressant and antioxidative effects of AC on rats subjected to long-term, social isolation-induced stress.

Materials and Methods

This study involved 2-month-old male rats (24) weighing approximately 180-200 g bred in-house. The rats were divided into four groups (n = 6): Group 1 received saline, Group 2 received SIS, Group 3 received only 50 mg/kg AC, and Group 4 received 50 mg/kg AC and SIS for 6 weeks. After this, behavioral, biochemical, and neuronal assays were conducted.

Results

Behavioral experiments showed significantly higher activity levels (p < 0.001) in AC-treated rats than in the SIS group. In addition, rats subjected to SIS with AC treatment exhibited enhanced total antioxidants, superoxide dismutase, and neuronal assays compared to rats subjected to SIS alone.

Conclusion

Acorus calamus treatment improved the antidepressant and antioxidant potential against SIS in rat brain tissue. Moreover, we proved that AC can effectively reverse the neurotoxicity induced by SIS in animal models. As we battle against the coronavirus disease 2019 pandemic and social isolation, AC could be considered a supplementary treatment to alleviate depressive-like symptoms in our present-day lifestyle.

Chronic Early-Life Social Isolation Enhances Spatial Memory in Male and Female Rats

Social adversity during childhood and adolescence can alter brain development in ways that may increase the likelihood of many prominent mental illnesses. To determine the underlying mechanisms, several animal models have been developed, such as Chronic Early-Life Social Isolation (CELSI), which sees rats isolated for several weeks after weaning. Although such a paradigm does cause many consistent changes in adult behaviour, one area where uncertainty exists concerns its effect upon hippocampal-dependent learning and memory. To help sort out how CELSI affects spatial learning and memory, male and female siblings from 15 Sprague-Dawley rat litters were stratified by sex and then randomly assigned to either group-housing (3 animals/cage), or social isolation (1 animal/cage) for 7 weeks. Spatial learning and memory were then tested over 5 days using the Morris water maze. Next, the animals were euthanised, and stress-sensitive biometrics, including serum corticosterone levels, were collected. Lastly, to determine whether CELSI affected neural cell density, the expression of key neuronal and glial proteins (such as PSD-95 and GFAP, respectively) was assessed in isolated hippocampal tissue using immunoblotting. Notably, both male and female rats that had experienced post-weaning social isolation displayed stronger spatial learning and memory abilities than their group-housed counterparts. As well, socially isolated male rats exhibited a clear increase in expression of PSD-95. However, housing condition did not seem to affect either stress-sensitive biometrics, or hippocampal GFAP expression. Our results support the possibility that CELSI may enhance some aspects of hippocampal-dependent behaviour in a fashion similar among male and female rats.

Promises and Pitfalls of NMDA Receptor Antagonists in Treating Violent Aggression

Treatment options for chronically aggressive individuals remain limited despite recent medical advances. Traditional pharmacological agents used to treat aggression, such as atypical antipsychotics, have limited efficacy and are often replete with dangerous side effects. The non-competitive NMDAR antagonists ketamine and memantine are promising alternatives, but their effects appear to be highly dependent on dosage, context, and personal experience. Importantly, these drugs can increase aggression when combined with substances of abuse or during periods of heightened stress. This is likely due to mechanistic differences operating at specific synapses under different contexts. Previous findings from our lab and others have shown that early life stress, substance abuse, and attack experience promote aggression through NMDAR-dependent synaptic plasticity within aggression-related brain circuits. Ketamine and memantine affect these types of aggression in opposite ways. This has led us to propose that ketamine and memantine oppositely affect aggression brought on by early life stress, substance abuse, or attack experience through opposite effects on NMDAR-dependent synaptic plasticity. This would account for the persistent effects of these drugs on aggression and suggest they could be leveraged as a more long-lasting treatment option. However, a more thorough examination of the effects of ketamine and memantine on cellular and synaptic function will be necessary for responsible administration. Additionally, because the effects of ketamine and memantine are highly dependent on prior drug use, traumatic stress, or a history of aggressive behavior, we propose a more thorough medical evaluation and psychiatric assessment will be necessary to avoid possible adverse interactions with these drugs.

Considering Organismal Physiology in Laboratory Studies of Rodent Behavior

Any experiment conducted in a rodent laboratory is done so against the backdrop of each animal's physiological state at the time of the experiment. This physiological state can be the product of multiple factors, both internal (e.g., animal sex, strain, hormone cycles, or circadian rhythms) and external (e.g., housing conditions, social status, and light/dark phases). Each of these factors has the potential to influence experimental outcomes, either independently or via interactions with others, and yet there is little consistency across laboratories in terms of the weight with which they are considered in experimental design. Such discrepancies-both in practice and in reporting-likely contribute to the perception of a reproducibility crisis in the field of behavioral neuroscience. In this review, we discuss how several of these sources of variability can impact outcomes within the realm of common learning and memory paradigms. Expected final online publication date for the Annual Review of Neuroscience, Volume 45 is July 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Anger management: Mechanisms of glutamate receptor-mediated synaptic plasticity underlying animal aggression

Excessive and recurring violent aggression is a serious concern for society and a symptom of many psychiatric diseases. Substance abuse, attack experience, and social and traumatic stress increase vulnerability to developing this type of aggression. Glutamate receptors are an intriguing target for long-term treatment. This review will assess the importance of glutamate receptors and glutamatergic pathways in aggression, focusing on the role of glutamate receptor-mediated synaptic plasticity in experience-dependent long-lasting aggression. By synthesizing what is known about glutamatergic systems in aggression, it is hoped more effective treatments can be developed.

Social Stress and Aggression in Murine Models

Throughout life, animals engage in a variety of social interactions ranging from the affiliative mother-offspring interaction and juvenile play to aggressive conflict. Deprivation of the appropriate social interaction during early development is stressful and disrupts the development of appropriate social behaviors and emotional responses later in life. Additionally, agonistic encounters can induce stress responses in both dominant and subordinate individuals. This review focuses on the social stress that escalates aggressive behavior of animals and discusses the known neurobiological and physiological mechanisms underlying the link between social stress and aggression. Social instigation, a brief exposure to a rival without physical contact, induces aggressive arousal in dominant animals and escalates aggressive behaviors in the following agonistic encounter. Furthermore, the experience of winning an aggressive encounter is known to be as rewarding as addictive drugs, and the experience of repeatedly winning induces addiction-like behavioral and neurobiological changes and leads to abnormal aggressive behaviors. Social isolation stress in early development from neonatal to juvenile and adolescent periods also affects aggressive behavior, but these effects largely depend on the strain, sex, and species as well as the stage of development in which isolation stress is experienced. In conclusion, understanding neurobiological mechanisms underlying the link between social stress and aggression will provide an important insight for the development of more effective and tolerable treatments for maladaptive aggression in humans.

Chronic early-life social isolation affects NMDA and TrkB receptor expression in a sex-specific manner

Exposing mammals to adverse social environments early in life can affect brain development in ways that alter adult behaviour. For example, chronic, early-life social isolation (CELSI) has been found to cause novelty-induced hyperactivity, impaired pre-pulse inhibition, and enhanced anxiety-related behaviour. Although the molecular mechanism(s) underlying the embedding of CELSI have not been fully elucidated, evidence suggests changes in the level of excitatory neurotransmission and neurotrophic factor signalling may be quite important. Since much of the work in this area has focused upon mRNA-level analyses, and has shown variable responses across both brain region and animal sex, our study aimed to explore the impact of CELSI on the expression of two important plasticity-related proteins (Tropomyosin receptor kinase B and the GluN2B subunit of the NMDA receptor) in the pre-frontal cortex and hippocampus of both male and female rats. We observed that the expression of both proteins was clearly changed by CELSI, but that the effect occurred in a sex (but not region) specific manner. Our results support the growing view that early-life adversity can cause structural changes reasonably associated with adult behaviour, and emphasise that the study of such changes benefits from a sex-based analysis.

Prenatal MAM treatment altered fear conditioning following social isolation: Relevance to schizophrenia

Adolescent social isolation (SI) might change the trajectory of brain development. In the present study, we investigated the effect of short-term adolescent SI on fear memory, anxiety and protein levels in the adult medial prefrontal cortex of rats prenatally treated with methylazoxymethanol, MAM-E17 model of schizophrenia. The animals were maintained in standard housing (SH) or social isolation (P30-P40, SI) conditions. Behavioural tests (trace or delay fear conditioning, light/dark box) were performed in late adolescence and early adulthood. The results showed that MAM treatment did not alter fear memory, which was investigated with the use of either trace or delay fear conditioning, at any age, and SI decreased the fear response in adult control animals only under trace conditioning. Neither MAM nor SI influenced anxiety-related behaviour measured in the light/dark box. A proteomics study showed that both MAM and SI changed the protein levels related to synapse maturation and cytoskeletal organization, energy transfer and metabolic processes. Prenatal or adolescent environmental factors are able to change the expression of proteins that are correlated with behavioural impairments. Moreover, SI reversed some alterations in proteins induced by MAM. Thus, normally developing brains showed different responses to adolescent SI than those with altering courses of MAM administration.

Involvement of the dopamine system in the effect of chronic social isolation during adolescence on social behaviors in male C57 mice

In the early stage of life, experiencing social isolation can generate long-lasting deleterious effects on behaviors and brain development. However, the effects of chronic social isolation during adolescence on social behaviors and its underlying neurobiological mechanisms remain unclear. The present study found that four weeks of social isolation during adolescence impaired social recognition ability in the three-chamber test and five-trial social recognition test, and increased aggressive-like behaviors, but reduced environmental exploration, as showed in the social interaction test. Chronic social isolation decreased levels of dopamine D2 receptor in the shell of the nucleus accumbens (NAcc) and medial prefrontal cortex. It also reduced TH in the NAcc. Using in vivo fiber photometry, it was also found that isolated mice displayed a reduction in NAcc shell activity upon exploring unfamiliar social stimuli. An injection of a 100 ng dose of the D2R agonist quinpirole into the shell of the NAcc reversed behavioral abnormalities induced by chronic social isolation. These data suggest that the dopamine system is involved in alterations in social behaviors induced by chronic social isolation. This finding sheds light on the mechanism underlying abnormalities in social behavior induced by adolescent chronic social isolation and provides a promising target to treat mental diseases relevant to social isolation.

Social isolation causes downregulation of immune and stress response genes and behavioural changes in a social insect

Humans and other social mammals experience isolation from their group as stressful, triggering behavioural and physiological anomalies that reduce fitness. While social isolation has been intensely studied in social mammals, it is less clear how social insects, which evolved sociality independently, respond to isolation. Here we examined whether the typical mammalian responses to social isolation, e.g., an impaired ability to interact socially and immune suppression are also found in social insects. We studied the consequences of social isolation on behaviour and brain gene expression in the ant Temnothorax nylanderi. Following isolation, workers interacted moderately less with adult nestmates, increased the duration of brood contact, and reduced the time spent self-grooming, an important sanitary behaviour. Our brain transcriptome analysis revealed that only a few behaviour-related genes had altered their expression with isolation time. Rather, many genes linked to immune system functioning and stress response had been downregulated. This probably sensitizes isolated individuals to various stressors, in particular because isolated workers exhibit reduced sanitary behaviour. We provide evidence of the diverse consequences of social isolation in social insects, some of which resemble those found in social mammals, suggesting a general link between social well-being, stress tolerance, and immune competence in social animals.

Epigenetic-mediated N-methyl-D-aspartate receptor changes in the brain of isolated reared rats

Aim: We investigated: Grin1, Grin2a, Grin2b DNA methylation; NR1 and NR2 mRNA/protein in the prefrontal cortex (PFC); and hippocampus of male Wistar rats exposed to isolation rearing. Materials & methods: Animals were kept isolated or grouped (n = 10/group) from weaning for 10 weeks. Tissues were dissected for RNA/DNA extraction and N-methyl-D-aspartate receptor subunits were analyzed using quantitative reverse transcription (RT)-PCR, ELISA and pyrosequencing. Results: Isolated-reared animals had: decreased mRNA in PFC for all markers, increased NR1 protein in hippocampus and hypermethylation of Grin1 in PFC and Grin2b in hippocampus, compared with grouped rats. Associations between mRNA/protein and DNA methylation were found for both brain areas. Conclusion: This study indicates that epigenetic DNA methylation may underlie N-methyl-D-aspartate receptor mRNA/protein expression alterations caused by isolation rearing.

The Neural Processes Interlinking Social Isolation, Social Support, and Problem Alcohol Use

BACKGROUND

Subjective feeling of social isolation, as can be measured by perceived burdensomeness (PB), is a major risk factor for alcohol misuse. Heightened PB is associated with elevated stress response and diminished cognitive control, both of which contribute to problem drinking. Here, we sought to identify the neural substrates underlying the relationship between PB and alcohol misuse.

METHODS

We employed resting-state functional magnetic resonance imaging data collected from 61 problem drinkers to characterize the functional connectivity of the hypothalamus and ventral striatum (VS) in relation to PB. We specifically examined whether the connectivities of the hypothalamus and VS were differentially influenced by PB to produce contrasting effects on alcohol use. Finally, we evaluated how individual differences in social support modulate the inter-relationships of social isolation, neural connectivity, and the severity of problem drinking.

RESULTS

Whole-brain multiple regressions show a positive relationship between PB and hypothalamic connectivity with the hippocampus and an inverse pattern for VS connectivity with the middle frontal gyrus. Difference in strength between the 2 connectivities predicted the severity of problem drinking, suggesting an imbalance involving elevated hypothalamic and diminished prefrontal cortical modulation in socially isolated problem drinkers. A path analysis further revealed that the lack of social support was associated with a bias toward low prefrontal connectivity, which in turn increased PB and facilitated problem drinking.

CONCLUSIONS

Altered hypothalamus and VS connectivity may underlie problem drinking induced by social isolation. The current findings also highlight the important role of social support as a potential protective factor against alcohol misuse.

Effects of long-lasting social isolation and re-socialization on cognitive performance and brain activity: a longitudinal study in Octodon degus

Social isolation is considered a stressful situation that results in increased physiological reactivity to novel stimuli, altered behaviour, and impaired brain function. Here, we investigated the effects of long-term social isolation on working memory, spatial learning/memory, hippocampal synaptic transmission, and synaptic proteins in the brain of adult female and male Octodon degus. The strong similarity between degus and humans in social, metabolic, biochemical, and cognitive aspects, makes it a unique animal model that can be highly applicable for further social, emotional, cognitive, and aging studies. These animals were socially isolated from post-natal and post-weaning until adulthood. We also evaluated if re-socialization would be able to compensate for reactive stress responses in chronically stressed animals. We showed that long-term social isolation impaired the HPA axis negative feedback loop, which can be related to cognitive deficits observed in chronically stressed animals. Notably, re-socialization restored it. In addition, we measured physiological aspects of synaptic transmission, where chronically stressed males showed more efficient transmission but deficient plasticity, as the reverse was true on females. Finally, we analysed synaptic and canonical Wnt signalling proteins in the hypothalamus, hippocampus, and prefrontal cortex, finding both sex- and brain structure-dependent modulation, including transient and permanent changes dependent on stress treatment.

A novel social fear conditioning procedure alters social behavior and mTOR signaling in differentially housed adolescent rats

Vulnerabilities to fear-related disorders can be enhanced following early life adversity. This study sought to determine whether post-weaning social isolation (PSI), an animal model of early life adversity, alters the development of social fear in an innovative model of conditioned social fear. Male and female Sprague-Dawley rats underwent either social rearing (SR) or PSI for 4 weeks following weaning. Rats were then assigned to groups consisting of either Footshock only, Social conditioned stimulus (CS) only, or Paired footshock with a social CS. Social behavior was assessed the next day. We observed a novel behavioral response in PSI rats, running in circles, that was rarely observed in SR rats; moreover, this behavior was augmented after Paired treatment in PSI rats. Other social behaviors were altered by both PSI and Paired footshock and social CS. The mammalian target of rapamycin (mTOR) pathway was assessed using immunohistochemistry for phosphorylated ribosomal protein S6 (pS6) in subregions of the prefrontal cortex (PFC) and amygdala. Paired treatment produced opposite effects in the PFC and amygdala in males, but no differences were observed in females. Conditioned social fear produced alterations in social behavior and the mTOR pathway that are dependent upon rearing condition and sex.

A Novel Neighbor Housing Environment Enhances Social Interaction and Rescues Cognitive Deficits from Social Isolation in Adolescence

Adolescence is characterized by high levels of playful social interaction, cognitive development, and increased risk-taking behavior. Juvenile exposure to social isolation or social stress can reduce myelin content in the frontal cortex, alter neuronal excitability, and disrupt hypothalamic pituitary adrenal (HPA) axis function. As compared to group housed animals, social isolation increases anxiety-like phenotypes and reduces social and cognitive performance in adulthood. We designed a neighbor housing environment to alleviate issues related to social isolation that still allowed individual homecages. Neighbor housing consists of four standard mouse cages fused together with semi-permeable ports that allow visual, olfactory, and limited social contact between mice. Adolescent C57BL/6J males and females were group housed (4/cage), single housed (1/cage), or neighbor housed (4/complex). As adults, mice were tested for social, anxiety-like, and cognitive behaviors. Living in this neighbor environment reduced anxiety-like behavior in the social interaction task and in the light-dark task. It also rescued cognitive deficits from single housing in the novel object recognition task. These data suggest that neighbor housing may partially ameliorate the social anxiety and cognitive deficits induced by social isolation. These neighbor cage environments may serve as a conduit by which researchers can house mice in individual cages while still enabling limited social interactions to better model typical adolescent development.

The Role of the N-Methyl-D-Aspartate Receptors in Social Behavior in Rodents

The appropriate display of social behaviors is essential for the well-being, reproductive success and survival of an individual. Deficits in social behavior are associated with impaired N-methyl-D-aspartate (NMDA) receptor-mediated neurotransmission. In this review, we describe recent studies using genetically modified mice and pharmacological approaches which link the impaired functioning of the NMDA receptors, especially of the receptor subunits GluN1, GluN2A and GluN2B, to abnormal social behavior. This abnormal social behavior is expressed as impaired social interaction and communication, deficits in social memory, deficits in sexual and maternal behavior, as well as abnormal or heightened aggression. We also describe the positive effects of pharmacological stimulation of the NMDA receptors on these social deficits. Indeed, pharmacological stimulation of the glycine-binding site either by direct stimulation or by elevating the synaptic glycine levels represents a promising strategy for the normalization of genetically-induced, pharmacologically-induced or innate deficits in social behavior. We emphasize on the importance of future studies investigating the role of subunit-selective NMDA receptor ligands on different types of social behavior to provide a better understanding of the underlying mechanisms, which might support the development of selective tools for the optimized treatment of disorders associated with social deficits.

Impact of social rearing-environment on performance in a complex maze in females of a cichlid fish

Spatial orientation is an important skill as it improves, for example, foraging, localisation of recourses, predator avoidance or navigation. Habitat complexity positively affects spatial abilities in various fish species with a more complex environment promoting learning ability. However, to what extent a complex social environment affects cognitive abilities in fishes has received less attention. Here, we investigated differences in maze performance of adult females of the West African cichlid fish Pelvicachromis taeniatus, which had been reared and maintained either in a group or in isolation from an early age on. Fish had to master the route through a maze in order to gain a food reward. Our results indicate marked differences in performance contingent upon social rearing-environment: isolation fish ran successful trials (i.e. locating the food reward) significantly more often than group fish and were faster during trials, also in a reversed maze. However, the number of mistakes did not differ between isolation and group fish and the time needed to relocate the food reward did not diminish with elapsed training days. In a second experiment, the activity of group and isolation fish was analysed in an open field test. Here, isolation fish were less active than group fish. We discuss different possibilities for performance differences of group and isolation fish including enhanced cognitive abilities of isolation fish, motivational/emotional differences and hyperactivity.

Long-Term Behavioral Effects of Post-weaning Social Isolation in Males and Females

Adolescence is a developmental period associated with vast neural and behavioral changes which are accompanied by altered sensitivity to stimuli, both stressful and rewarding. Perturbations, especially stressful stimuli, during this period have been shown to alter behavior in adulthood. Social isolation rearing is one such perturbation. This review highlights the long-term behavioral consequences of adolescent social isolation rearing in rodents with a specific focus on anxiety- and addiction-related behaviors. Sex-specific effects are discussed where data are available. We then consider changes in monoaminergic neurotransmission as one possible mechanism for the behavioral effects described. This research on both normative and perturbed adolescent development is crucial to understanding and treating the increased vulnerability to psychiatric disorders seen in humans during this life stage.

Long-term social isolation inhibits autophagy activation, induces postsynaptic dysfunctions and impairs spatial memory

Social isolation in adolescence leads to lasting deficits in hippocampal-dependent tasks. The reported effects of isolation on learning and memory in the Morris water maze and synaptic-related proteins have been inconsistent. Moreover, the autophagy level and its effect on cognition in the isolation model are also not clear. In the present study, we did an extended isolation period up to six months to establish a stable and appropriate isolation model to investigate the cognitive changes associated with it. The mTOR inhibitor rapamycin was systemically administered to mice to determine the roles of autophagy activation on cognitive changes. We discovered that long-term post-weaning social isolation (L-PWSI) produced marked deficits in spatial learning and memory and inhibited CA1 long-term potentiation (LTP), but paired-pulse facilitation (PPF) and input/output (I/O) curve were unaffected. The results further showed that the L-PWSI significantly decreased the protein expression levels of PSD-95, GluA1, NR1 and NR2B in the hippocampus, and no significant changes in the extracellular release of glutamate and the protein expression levels of synaptophysin, synapsin I, GAP-43, NR2A and GABAA. Moreover, we found that L-PWSI increased the protein expression of p-AKT/AKT, p-mTOR/mTOR and p62, whereas the protein levels of LC3B and Beclin1 were decreased indicating an inhibition in autophagy activity. Intraperitoneal injection of rapamycin significantly potentiated fEPSP slope and cognition-related proteins expression in the L-PWSI mice. These results therefore suggest that L-PWSI induces postsynaptic dysfunction by disrupting the interaction between AMPAR, NMDAR and PSD-95, and inhibit the autophagy activity which led to impaired spatial memory and cognitive function.

Low plasma concentrations of N-methyl-d-aspartate receptor subunits as a possible biomarker for psychosis

BACKGROUND

N-methyl-d-aspartate receptor (NMDAR) has been largely implicated in the neurobiology of schizophrenia and other psychosis. Aiming to evaluate their potential as peripheral biomarkers for psychosis, we quantified the plasma concentrations of NR1 and NR2 NMDAR subunits of first-episode psychosis patients in their first contact with mental health services due to psychotic symptoms, compared with siblings and matched community-based controls.

METHODS

The quantifications of NR1 and NR2 plasma concentrations were performed by ELISA. Data were analysed by nonparametric tests and Receiver Operating Curve (ROC) analysis.

RESULTS

We included 166 first-episode psychosis patients (mean age = 30.3 ± 12.2 years; 64% men), with the diagnosis of schizophrenia spectrum (n = 84), bipolar disorder (n = 51) and psychotic depression (n = 31), 76 siblings (mean age = 31.5 ± 11.0 years; 30.3% men) and 166 healthy community-based controls (mean age = 31.4 ± 12.0 years; 63.9% men). NMDAR subunits were significantly lower in patients compared with siblings and controls (p < 0.001), except by NR1 plasma concentrations of bipolar patients compared with siblings and controls. NR1 plasma concentrations lower than 17.65 pg/ml (AUC = 0.621) showed sensitivity of 42.8%, specificity of 84.3%, positive predictive value (PPV) of 73.2% and negative predictive value (NPV) of 59.6%. Individuals with NR2 plasma concentrations lower than 2.92 ng/ml (AUC = 0.801) presented a 10.61-fold increased risk of psychosis, with a sensibility of 71.9%, specificity of 80.6%, PPV of 79.0% and NPV of 73.9%.

CONCLUSIONS

This is the first study reporting the measurement and the reduction of NR1 and NR2 NMDAR subunits plasma concentrations in psychiatric disorders. In particular, the NR2 subunit may be a possible plasma biomarker for psychosis.

Neurobiology and consequences of social isolation stress in animal model-A comprehensive review

The brain is a vital organ, susceptible to alterations under genetic influences and environmental experiences. Social isolation (SI) acts as a stressor which results in alterations in reactivity to stress, social behavior, function of neurochemical and neuroendocrine system, physiological, anatomical and behavioral changes in both animal and humans. During early stages of life, acute or chronic SIS has been proposed to show signs and symptoms of psychiatric and neurological disorders such as anxiety, depression, schizophrenia, epilepsy and memory loss. Exposure to social isolation stress induces a variety of endocrinological changes including the activation of hypothalamic-pituitary-adrenal (HPA) axis, culminating in the release of glucocorticoids (GCs), release of catecholamines, activation of the sympatho-adrenomedullary system, release of Oxytocin and vasopressin. In several regions of the central nervous system (CNS), SIS alters the level of neurotransmitter such as dopamine, serotonin, gamma aminobutyric acid (GABA), glutamate, nitrergic system and adrenaline as well as leads to alteration in receptor sensitivity of N-methyl-D-aspartate (NMDA) and opioid system. A change in the function of oxidative and nitrosative stress (O&NS) mediated mitochondrial dysfunction, inflammatory factors, neurotrophins and neurotrophicfactors (NTFs), early growth response transcription factor genes (Egr) and C-Fos expression are also involved as a pathophysiological consequences of SIS which induce neurological and psychiatric disorders.

Antidepressant effect of recombinant NT4-NAP/AAV on social isolated mice through intranasal route

The purpose of the present study was to observe the depression-like behavior induced by social isolation; detect the antidepressant effect of a recombinant adeno-associated virus (AAV) expressing NAP on social isolation mice by intranasal delivery. After construction of NT4-NAP/AAV, expression of NAP was confirmed in vitro. 3-week-old C57/BL mice were bred individually in cages as social isolation-rearing. Six weeks later, the first subset of mice underwent behavioral tests and western blot; the second was for enzyme-linked immunosorbent assay. NT4-NAP/AAV was delivered quaque die by nasal administration for consecutive 10 days before behavioral test. Several depression-like behaviors were observed in social isolation mice, including decreased relative sucrose preference, longer immobility time in forced swimming test, lower plasma corticosterone and decreased brain-derived neurotrophic factor in hippocampus. Thus, social isolation procedure appears to be an animal model of depression with good face and construct validity. What's more, the antidepressant effect in social isolation-rearing mice was observed after intranasal administration of NT4-NAP/AAV, suggesting that this might be a promising therapeutic strategy for depressive disorder.

Monoacylglycerol lipase inhibition alters social behavior in male and female rats after post-weaning social isolation

Post-weaning social isolation (PSI) has been shown to increase aggressive behavior and alter medial prefrontal cortex (mPFC) function in rats. The present study sought to determine whether this phenotype would be normalized by increasing levels of the endocannabinoid 2-arachidonoylglycerol (2-AG) using pharmacological inhibition of monoacylglycerol lipase (MAGL). Male and female Sprague-Dawley rats were exposed to either 4 weeks of PSI or social rearing (SR) starting on postnatal day 21, then underwent a 15 min trial of social interaction with a novel, same-sex juvenile rat. Rats were administered an acute injection of the MAGL inhibitor MJN110 or vehicle prior to the social interaction. Rats received either 0 mg/kg (vehicle), 1 mg/kg, or 5 mg/kg of MJN110. Both doses of MJN110 decreased aggressive grooming, a measure of agonistic behavior, in both males and females, largely driven by decreased aggressive grooming in PSI rats. There were no effects of MJN110 on overall social behavior or play behavior, while modest effects were observed on locomotor activity in SR rats only. While social interaction increased c-Fos expression in the mPFC of both males and females, MJN110 reduced c-Fos preferentially in females. These results suggest that 2-AG can modulate specific social behaviors during adolescence, and may affect mPFC function differentially in males and females.

Impact of juvenile chronic stress on adult cortico-accumbal function: Implications for cognition and addiction

Repeated exposure to stress during childhood is associated with increased risk for neuropsychiatric illness, substance use disorders and other behavioral problems in adulthood. However, it is not clear how chronic childhood stress can lead to emergence of such a wide range of symptoms and disorders in later life. One possible explanation lies in stress-induced disruption to the development of specific brain regions associated with executive function and reward processing, deficits in which are common to the disorders promoted by childhood stress. Evidence of aberrations in prefrontal cortex and nucleus accumbens function following repeated exposure of juvenile (pre- and adolescent) organisms to a variety of different stressors would account not only for the similarity in symptoms across the wide range of childhood stress-associated mental illnesses, but also their persistence into adulthood in the absence of further stress. Therefore, the goal of this review is to evaluate the current knowledge regarding disruption to executive function and reward processing in adult animals or humans exposed to chronic stress over the juvenile period, and the underlying neurobiology, with particular emphasis on the prefrontal cortex and nucleus accumbens. First, the role of these brain regions in mediating executive function and reward processing is highlighted. Second, the neurobehavioral development of these systems is discussed to illustrate how juvenile stress may exert long-lasting effects on prefrontal cortex-accumbal activity and related behavioral functions. Finally, a critical review of current animal and human findings is presented, which strongly supports the supposition that exposure to chronic stress (particularly social aggression and isolation in animal studies) in the juvenile period produces impairments in executive function in adulthood, especially in working memory and inhibitory control. Chronic juvenile stress also results in aberrations to reward processing and seeking, with increased sensitivity to drugs of abuse particularly noted in animal models, which is in line with greater incidence of substance use disorders seen in clinical studies. These consequences are potentially mediated by monoamine and glutamatergic dysfunction in the prefrontal cortex and nucleus accumbens, providing translatable therapeutic targets. However, the predominant use of male subjects and social-based stressors in preclinical studies points to a clear need for determining how both sex differences and stressor heterogeneity may differentially contribute to stress-induced changes to substrates mediating executive function and reward processing, before the impact of chronic juvenile stress in promoting adult psychopathology can be fully understood.

The Regulation of GluN2A by Endogenous and Exogenous Regulators in the Central Nervous System

The NMDA receptor is the most widely studied ionotropic glutamate receptor, and it is central to many physiological and pathophysiological processes in the central nervous system. GluN2A is one of the two main types of GluN2 NMDA receptor subunits in the forebrain. The proper activity of GluN2A is important to brain function, as the abnormal regulation of GluN2A may induce some neuropsychiatric disorders. This review will examine the regulation of GluN2A by endogenous and exogenous regulators in the central nervous system.

Early-Life Social Isolation-Induced Depressive-Like Behavior in Rats Results in Microglial Activation and Neuronal Histone Methylation that Are Mitigated by Minocycline

Early-life stress is a potent risk factor for development of psychiatric conditions such as depression. The underlying mechanisms remain poorly understood. Here, we used the early-life social isolation (ESI) model of early-life stress in rats to characterize development of depressive-like behavior, the role of microglia, levels of histone methylation, as well as expression of glutamate receptor subunits in the hippocampus. We found that depressive-like behavior was induced after ESI as determined by sucrose preference and forced swimming tests. Increased expression of microglial activation marker, Iba1, was observed in the hippocampus of the ESI group, while expression of the microglial CD200 receptor, which promotes microglial quiescence, significantly decreased. In addition, increased levels of proinflammatory cytokines, interleukin 1β (IL-1β), interleukin 6 (IL-6), and tumor necrosis factor-alpha (TNF-α) were observed in the hippocampus of the ESI group. Moreover, ESI increased levels of neuronal H3K9me2 (a repressive marker of transcription) and its associated "writer" enzymes, G9a and G9a-like protein, in the hippocampus. ESI also decreased expression of hippocampal NMDA receptor subunits, NR1, and AMPA receptor subunits, GluR1 and GluR2, which are involved in synaptic plasticity, but it did not affect expression of PSD95 and NR2B. Interestingly, treatment with minocycline to block microglial activation induced by ESI inhibited increases in hippocampal microglia and prevented ESI-induced depressive-like behavior as well as increases in IL-1β, IL-6, and TNF-α. Notably, minocycline also triggered downregulation of H3K9me2 expression and restored expression of NR1, GluR1, and GluR2. These results suggest that ESI induces depressive-like behavior, which may be mediated by microglial signaling.

A novel escapable social interaction test reveals that social behavior and mPFC activation during an escapable social encounter are altered by post-weaning social isolation and are dependent on the aggressiveness of the stimulus rat

Post-weaning social isolation (PSI) has been shown to increase aggressive behavior and alter medial prefrontal cortex (mPFC) function in social species such as rats. Here we developed a novel escapable social interaction test (ESIT) allowing for the quantification of escape and social behaviors in addition to mPFC activation in response to an aggressive or nonaggressive stimulus rat. Male rats were exposed to 3 weeks of PSI (ISO) or group (GRP) housing, and exposed to 3 trials, with either no trial, all trials, or the last trial only with a stimulus rat. Analysis of social behaviors indicated that ISO rats spent less time in the escape chamber and more time engaged in social interaction, aggressive grooming, and boxing than did GRP rats. Interestingly, during the third trial all rats engaged in more of the quantified social behaviors and spent less time escaping in response to aggressive but not nonaggressive stimulus rats. Rats exposed to nonaggressive stimulus rats on the third trial had greater c-fos and ARC immunoreactivity in the mPFC than those exposed to an aggressive stimulus rat. Conversely, a social encounter produced an increase in large PSD-95 punctae in the mPFC independently of trial number, but only in ISO rats exposed to an aggressive stimulus rat. The results presented here demonstrate that PSI increases interaction time and aggressive behaviors during escapable social interaction, and that the aggressiveness of the stimulus rat in a social encounter is an important component of behavioral and neural outcomes for both isolation and group-reared rats.

The neurobiological basis of human aggression: A review on genetic and epigenetic mechanisms

Aggression is an evolutionary conserved behavior present in most species including humans. Inadequate aggression can lead to long-term detrimental personal and societal effects. Here, we differentiate between proactive and reactive forms of aggression and review the genetic determinants of it. Heritability estimates of aggression in general vary between studies due to differing assessment instruments for aggressive behavior (AB) as well as age and gender of study participants. In addition, especially non-shared environmental factors shape AB. Current hypotheses suggest that environmental effects such as early life stress or chronic psychosocial risk factors (e.g., maltreatment) and variation in genes related to neuroendocrine, dopaminergic as well as serotonergic systems increase the risk to develop AB. In this review, we summarize the current knowledge of the genetics of human aggression based on twin studies, genetic association studies, animal models, and epigenetic analyses with the aim to differentiate between mechanisms associated with proactive or reactive aggression. We hypothesize that from a genetic perspective, the aminergic systems are likely to regulate both reactive and proactive aggression, whereas the endocrine pathways seem to be more involved in regulation of reactive aggression through modulation of impulsivity. Epigenetic studies on aggression have associated non-genetic risk factors with modifications of the stress response and the immune system. Finally, we point to the urgent need for further genome-wide analyses and the integration of genetic and epigenetic information to understand individual differences in reactive and proactive AB. © 2015 Wiley Periodicals, Inc.

NMDA receptor antagonists attenuate the proconvulsant effect of juvenile social isolation in male mice

Experiencing psychosocial stress in early life, such as social isolation stress (SIS), is known to have negative enduring effects on the development of the brain and behavior. In addition to anxiety and depressive-like behaviors, we previously showed that juvenile SIS increases susceptibility to pentylenetetrazole (PTZ)-induced seizures in mice through enhancing the nitrergic system activity in the hippocampus. In this study, we investigated the possible involvement of N-methyl-D-aspartate (NMDA) receptors in proconvulsant effects of juvenile SIS. Applying 4 weeks of SIS to juvenile male mice at postnatal day 21-23, we observed an increased susceptibility to PTZ as well as anxiety and depressive-like behaviors in adult mice. Intraperitoneal (i.p.) administration of NMDA receptor antagonists, MK-801 (0.05 mg/kg) and ketamine (0.5mg/kg), reversed the proconvulsant effects of SIS in Isolated (and not social) housed animals. Co-administration of non-effective doses of nitric oxide synthase (NOS) inhibitors, 7NI (25mg/kg) and L-NAME (10mg/kg), with NMDA receptor antagonists, MK-801 (0.01 mg/kg) and ketamine (0.1mg/kg) attenuated the proconvulsant effects of juvenile SIS only in isolated housed mice. Also, using real time RT-PCR, we showed that hippocampal upregulation of NR2B subunit of NMDA receptor may play a critical role in proconvulsant effects of juvenile SIS by dysregulation of NMDA/NO pathway. In conclusion, results of present study revealed that experiencing SIS during adolescence predisposes the co-occurrence of seizure disorders with psychiatric comorbidities and also, alteration of NMDA receptor structure and function in hippocampus plays a role in proconvulsant effects of juvenile SIS through enhancing the NMDA/NO pathway.

Brain regional differences in social encounter-induced Fos expression in male and female rats after post-weaning social isolation

Early life adversity has been related to a number of psychological disorders including mood and other disorders that can manifest as inappropriate or aggressive responses to social challenges. The present study used post-weaning social isolation (PSI) in rats, a model of early life adversity, to examine its effects on Fos protein expression produced by exposure to a novel social encounter. We have previously reported that the social encounter-induced increase in Fos expression in the medial prefrontal cortex observed in group-housed controls (GRP) was attenuated in rats that had experienced PSI. Here we assessed Fos expression in other brain regions thought to be involved in emotion regulation and social behavior. Male and female rats were housed in same-sex groups or in isolation (ISO) for 4 weeks beginning on postnatal day (P) 21 and were exposed to a single 15 min social encounter with a novel same-sex conspecific on P49. Fos positive cells were assessed using immunohistochemistry in 16 regions within the forebrain. Exposure to a novel conspecific increased Fos expression in the forebrain of GRP rats in a region- and sex-specific fashion. This increase was blunted or absent in ISO rats within many regions including cortical regions, thalamus, habenula, dentate gyrus, lateral septum, and basolateral amygdala. In several regions, the increase in Fos was greater in male than in female group housed rats. Negative relationships were observed between social interactions and Fos in some regions. Forebrain hypofunction produced by early-life adversity may be involved in socially inappropriate behavior.

Chronic social isolation decreases glutamate and glutamine levels and induces oxidative stress in the rat hippocampus

Social isolation (SI) rearing of rodents is a developmental manipulation, which is commonly compared with the psychological stressors in humans as it produces several behavioral outcomes similar to those observed in humans with early life stress. To explain the SI-induced behavioral outcomes, animal studies have been performed to examine the dopaminergic and glutamatergic systems in the brain. In this study, we measured possible changes in levels of glutamate and glutamine of SI-rats using proton magnetic resonance spectroscopy. We also assessed the oxidative stress parameters in certain brain regions to see if glutamate and/or glutamine changes, if any, are associated with oxidative stress. SI rearing for 8 weeks decreased the activities of antioxidant enzymes catalase, glutathione peroxidase, superoxide dismutase, and the total antioxidant capacity, but increased levels of hydrogen peroxide, in certain brain regions, of which prefrontal cortex and hippocampus were most vulnerable. It also decreased levels of glutamate, glutamine, N-acetyl-l-aspartate (NAA), and phosphocreatine in the dorsal hippocampus, but not in the cerebral cortex. Decreased phosphocreatine and NAA indicate energy metabolism deficit in brain cells; the latter also suggests the neuronal viability was inhibited. Decreased glutamate and glutamine may suggest the neuron-glial integrity was implicated by chronic SI. These neurochemical and biochemical changes may contribute to the SI-induced behavioral abnormalities including a high level of anxiety, social interaction deficit, and impaired spatial working memory shown in this study.