Post-weaning social isolation alters anxiety-related behavior and neurochemical gene expression in the brain of male prairie voles

A gut (microbiome) feeling about addiction: Interactions with stress and social systems

In recent years, an increasing attention has given to the intricate and diverse connection of microorganisms residing in our gut and their impact on brain health and central nervous system disease. There has been a shift in mindset to understand that drug addiction is not merely a condition that affects the brain, it is now being recognized as a disorder that also involves external factors such as the intestinal microbiota, which could influence vulnerability and the development of addictive behaviors. Furthermore, stress and social interactions, which are closely linked to the intestinal microbiota, are powerful modulators of addiction. This review delves into the mechanisms through which the microbiota-stress-immune axis may shape drug addiction and social behaviors. This work integrates preclinical and clinical evidence that demonstrate the bidirectional communication between stress, social behaviors, substance use disorders and the gut microbiota, suggesting that gut microbes might modulate social stress having a significance in drug addiction.

Voluntary wheel running ameliorates abnormalities in social behavior induced by social isolation: involvement of neural and neurochemical responses

Social isolation (SI) can lead to devastating behavioral effects. Increasing evidence has demonstrated that physical activity can improve sociability and brain functions, but whether voluntary exercise can ameliorate SI-induced abnormalities in social behavior and its underlying neuronal mechanisms remains unknown. The present study found that SI during adulthood increased aggression in the resident-intruder test and motivation for social exploration in the three-chamber test. Voluntary wheel running (VWR) could reverse the alterations in social behavior induced by SI in male mice. In addition, SI reduced the number of c-Fos-immunoreactive neurons and increased c-Fos/AVP-labeled neurons in the PVN and c-Fos/TPH2-labeled neurons in the DRN. These alterations could be reversed by VWR. Together, our results reveal that voluntary exercise could ameliorate SI-induced negative effects on social behavior, possibly via alterations of neuronal activation in the brain. This finding provides a potential therapy and targets to prevent or treat the psychological diseases associated with abnormalities in social behaviors.

Modulation of social behavior by distinct vasopressin sources

The neuropeptide arginine-vasopressin (AVP) is well known for its peripheral effects on blood pressure and antidiuresis. However, AVP also modulates various social and anxiety-related behaviors by its actions in the brain, often sex-specifically, with effects typically being stronger in males than in females. AVP in the nervous system originates from several distinct sources which are, in turn, regulated by different inputs and regulatory factors. Based on both direct and indirect evidence, we can begin to define the specific role of AVP cell populations in social behavior, such as, social recognition, affiliation, pair bonding, parental behavior, mate competition, aggression, and social stress. Sex differences in function may be apparent in both sexually-dimorphic structures as well as ones without prominent structural differences within the hypothalamus. The understanding of how AVP systems are organized and function may ultimately lead to better therapeutic interventions for psychiatric disorders characterized by social deficits.

Limosilactobacillus reuteri administration alters the gut-brain-behavior axis in a sex-dependent manner in socially monogamous prairie voles

Research on the role of gut microbiota in behavior has grown dramatically. The probiotic L. reuteri can alter social and stress-related behaviors - yet, the underlying mechanisms remain largely unknown. Although traditional laboratory rodents provide a foundation for examining the role of L. reuteri on the gut-brain axis, they do not naturally display a wide variety of social behaviors. Using the highly-social, monogamous prairie vole (Microtus ochrogaster), we examined the effects of L. reuteri administration on behaviors, neurochemical marker expression, and gut-microbiome composition. Females, but not males, treated with live L. reuteri displayed lower levels of social affiliation compared to those treated with heat-killed L. reuteri. Overall, females displayed a lower level of anxiety-like behaviors than males. Live L. reuteri-treated females had lower expression of corticotrophin releasing factor (CRF) and CRF type-2-receptor in the nucleus accumbens, and lower vasopressin 1a-receptor in the paraventricular nucleus of the hypothalamus (PVN), but increased CRF in the PVN. There were both baseline sex differences and sex-by-treatment differences in gut microbiome composition. Live L. reuteri increased the abundance of several taxa, including Enterobacteriaceae, Lachnospiraceae NK4A136, and Treponema. Interestingly, heat-killed L. reuteri increased abundance of the beneficial taxa Bifidobacteriaceae and Blautia. There were significant correlations between changes in microbiota, brain neurochemical markers, and behaviors. Our data indicate that L. reuteri impacts gut microbiota, gut-brain axis and behaviors in a sex-specific manner in socially-monogamous prairie voles. This demonstrates the utility of the prairie vole model for further examining causal impacts of microbiome on brain and behavior.

The Influence of Environmental Enrichment on Affective and Neural Consequences of Social Isolation Across Development

Social stress is associated with depression and anxiety, physiological disruptions, and altered brain morphology in central stress circuitry across development. Environmental enrichment strategies may improve responses to social stress. Socially monogamous prairie voles exhibit analogous social and emotion-related behaviors to humans, with potential translational insight into interactions of social stress, age, and environmental enrichment. This study explored the effects of social isolation and environmental enrichment on behaviors related to depression and anxiety, physiological indicators of stress, and dendritic structural changes in amygdala and hippocampal subregions in young adult and aging prairie voles. Forty-nine male prairie voles were assigned to one of six groups divided by age (young adult vs. aging), social structure (paired vs. isolated), and housing environment (enriched vs. non-enriched). Following 4 weeks of these conditions, behaviors related to depression and anxiety were investigated in the forced swim test and elevated plus maze, body and adrenal weights were evaluated, and dendritic morphology analyses were conducted in hippocampus and amygdala subregions. Environmental enrichment decreased immobility duration in the forced swim test, increased open arm exploration in the elevated plus maze, and reduced adrenal/body weight ratio in aging and young adult prairie voles. Age and social isolation influenced dendritic morphology in the basolateral amygdala. Age, but not social isolation, influenced dendritic morphology in the hippocampal dentate gyrus. Environmental enrichment did not influence dendritic morphology in either brain region. These data may inform interventions to reduce the effects of social stressors and age-related central changes associated with affective behavioral consequences in humans.

The neural circuits of monogamous behavior

The interest in studying the neural circuits related to mating behavior and mate choice in monogamous species lies in the parallels found between human social structure and sexual behavior and that of other mammals that exhibit social monogamy, potentially expanding our understanding of human neurobiology and its underlying mechanisms. Extensive research has suggested that social monogamy, as opposed to non-monogamy in mammals, is a consequence of the neural encoding of sociosensory information from the sexual partner with an increased reward value. Thus, the reinforced value of the mate outweighs the reward value of mating with any other potential sexual partners. This mechanism reinforces the social relationship of a breeding pair, commonly defined as a pair bond. In addition to accentuated prosocial behaviors toward the partner, other characteristic behaviors may appear, such as territorial and partner guarding, selective aggression toward unfamiliar conspecifics, and biparental care. Concomitantly, social buffering and distress upon partner separation are also observed. The following work intends to overview and compare known neural and functional circuits that are related to mating and sexual behavior in monogamous mammals. We will particularly discuss reports on Cricetid rodents of the Microtus and Peromyscus genus, and New World primates (NWP), such as the Callicebinae subfamily of the titi monkey and the marmoset (Callithrix spp.). In addition, we will mention the main factors that modulate the neural circuits related to social monogamy and how that modulation may reflect phenotypic differences, ultimately creating the widely observed diversity in social behavior.

Amphetamine exposure alters behaviors, and neuronal and neurochemical activation in the brain of female prairie voles

Previous studies have shown that 3-day d-amphetamine (AMPH) treatment effectively induced conditioned place preferences (CPP) and impaired pair bonding behaviors in prairie voles (Microtus ochrogaster). Using this established animal model and treatment regimen, we examined the effects of the demonstrated threshold rewarding dose of AMPH on various behaviors and their potential underlying neurochemical systems in the brain of female prairie voles. Our data show that 3-day AMPH injections (0.2 mg/kg/day) impaired social recognition and decreased depressive-like behavior in females without affecting their locomotion and anxiety-like behaviors. AMPH treatment also decreased neuronal activation indicated by the labeling of the early growth response protein 1 (Egr-1) as well as the number of neurons double-labeled for Egr-1 and corticotrophin-releasing hormone (CRH) in the dentate gyrus (DG) of the hippocampus and paraventricular nucleus of the hypothalamus (PVN) in the brain. Further, AMPH treatment decreased the number of neurons double-labeled for Egr-1 and tyrosine hydroxylase (TH) but did not affect oxytocinergic neurons in the PVN or cell proliferation and neurogenesis markers in the DG. These data not only demonstrate potential roles of the brain CRH and dopamine systems in mediating disrupted social recognition and depressive-like behaviors by AMPH in female prairie voles, but also further confirm the utility of the prairie vole model for studying interactions between psychostimulants and social behaviors.

The influence of early-life and adulthood stressors on brain neuropeptide-S system

Central administered neuropeptide-S (NPS) was shown to reduce stress response in rodents. This study aimed to investigate the alterations in NPS system upon chronic exposure to early-life and adulthood stressors. Newborn pups underwent maternal separation (MS) from postnatal day 1 to 14 comprised of daily 3-h separations. In the adulthood, 90-min of restraint stress was loaded to males as an acute stress (AS) model. For chronic homotypic stress (CHS), same stressor was applied for 5 consecutive days. The changes in the expression and the release of NPS were monitored by immunohistochemistry and microdialysis, respectively. Throughout the CHS, heart rate variability (HRV) was analyzed on a daily basis. The immunoreactivity for NPS receptor (NPSR) was detected in basolateral amygdala (BLA) and hypothalamic paraventricular nucleus (PVN) by immunofluorescence staining. The NPS expression in the brainstem was increased upon AS which was more prominent following CHS, whereas these responses were found to be blunted in MS counterparts. Similar to histological data, the stress-induced release of NPS in BLA was attenuated in MS rats. CHS-induced elevations in sympatho-vagal balance were alleviated in control rats; which was not observed in MS rats. The expression of NPSR in BLA and PVN was down-regulated in MS rats. The brain NPS/NPSR system appears to be susceptible to the early-life stressors and the subsequent chronic stress exposure in adulthood which results in altered autonomic outflow.

The neuropeptide Pth2 modulates social behavior and anxiety in zebrafish

Behavior is context-dependent and often modulated by an animal’s internal state. In particular, different social contexts can alter anxiety levels and modulate social behavior. The vertebrate-specific neuropeptide parathyroid hormone 2 (pth2) is regulated by the presence of conspecifics in zebrafish. As its cognate receptor, the parathyroid hormone 2 receptor (pth2r), is widely expressed across the brain, we tested fish lacking the functional Pth2 peptide in several anxiety-related and social behavior paradigms. Here, we show that the propensity to react to sudden stimuli with an escape response was increased in pth2^−/− zebrafish, consistent with an elevated anxiety level. While overall social preference for conspecifics was maintained in pth2^−/− fish until the early juvenile stage, we found that both social preference and shoaling were altered later in development. The data presented suggest that the neuropeptide Pth2 modulates several conserved behaviors and may thus enable the animal to react appropriately in different social contexts.

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Lack of the neuropeptide pth2 increases startle responsiveness in zebrafish

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Social preference is decreased at the late juvenile stage in pth2 mutants

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Pth2 mutant fish form less cohesive shoals

Barriers and Breakthroughs in Targeting the Oxytocin System to Treat Alcohol Use Disorder

Development of better treatments for alcohol use disorder (AUD) is urgently needed. One promising opportunity for this development is the potential of targeting the oxytocin peptide system. Preclinical studies showed that administration of exogenous oxytocin or, more recently, stimulation of neurons expressing endogenous oxytocin lead to a decreased alcohol consumption across several rodent models. Initial clinical studies also showed that administration of oxytocin decreased craving for alcohol and heavy alcohol drinking. However, several more recent clinical studies were not able to replicate these effects. Thus, although targeting the oxytocin system holds promise for the treatment of AUD, more nuanced approaches toward development and application of these treatments are needed. In this mini-review we discuss potential caveats resulting in differential success of attempts to use oxytocin for modulating alcohol use disorder-related behaviors in clinical studies and evaluate three directions in which targeting the oxytocin system could be improved: (1) increasing potency of exogenously administered oxytocin, (2) developing oxytocin receptor agonists, and (3) stimulating components of the endogenous oxytocin system. Both advances and potential pitfalls of these directions are discussed.

Post-weaning Social Isolation in Male and Female Prairie Voles: Impacts on Central and Peripheral Immune System

The socially monogamous prairie vole (Microtus ochrogaster) offers a unique opportunity to examine the impacts of adolescent social isolation on the brain, immune system, and behavior. In the current study, male and female prairie voles were randomly assigned to be housed alone or with a same-sex cagemate after weaning (i.e., on postnatal day 21-22) for a 6-week period. Thereafter, subjects were tested for anxiety-like and depressive-like behaviors using the elevated plus maze (EPM) and Forced Swim Test (FST), respectively. Blood was collected to measure peripheral cytokine levels, and brain tissue was processed for microglial density in various brain regions, including the Nucleus Accumbens (NAcc), Medial Amygdala (MeA), Central Amygdala (CeA), Bed Nucleus of the Stria Terminalis (BNST), and Paraventricular Nucleus of the Hypothalamus (PVN). Sex differences were found in EPM and FST behaviors, where male voles had significantly lower total arm entries in the EPM as well as lower latency to immobility in the FST compared to females. A sex by treatment effect was found in peripheral IL-1β levels, where isolated males had a lower level of IL-1β compared to cohoused females. Post-weaning social isolation also altered microglial density in a brain region-specific manner. Isolated voles had higher microglial density in the NAcc, MeA, and CeA, but lower microglial density in the dorsal BNST. Cohoused male voles also had higher microglial density in the PVN compared to cohoused females. Taken together, these data suggest that post-weaning social housing environments can alter peripheral and central immune systems in prairie voles, highlighting a potential role for the immune system in shaping isolation-induced alterations to the brain and behavior.

Behavioral, neurochemical, and neuroimmune changes associated with social buffering and stress contagion

Social buffering can provide protective effects on stress responses and their subsequent negative health outcomes. Although social buffering is beneficial for the recipient, it can also have anxiogenic effects on the provider of the social buffering - a phenomena referred to as stress contagion. Social buffering and stress contagion usually occur together, but they have traditionally been studied independently, thus limiting our understanding of this dyadic social interaction. In the present study, we examined the effects of preventative social buffering and stress contagion in socially monogamous prairie voles (Microtus ochrogaster). We tested the hypothesis that this dynamic social interaction is associated with coordinated alterations in behaviors, neurochemical activation, and neuroimmune responses. To do so, adult male prairie voles were stressed via an acute immobilization restraint tube (IMO) either alone (Alone) or with their previously pair-bonded female partner (Partner) in the cage for 1 h. In contrast, females were placed in a cage containing either an empty IMO tube (Empty) or one that contained their pair-bonded male (Partner). Anxiety-like behavior was tested on the elevated plus maze (EPM) following the 60-mins test and brain sections were processed for neurochemical/neuroimmune marker labeling for all subjects. Our data indicate that females in the Partner group were in contact with and sniffed the IMO tube more, showed fewer anxiety-like behaviors, and had a higher level of oxytocin expression in the paraventricular nucleus of the hypothalamus (PVN) compared to the Empty group females. Males in the Partner group had lower levels of anxiety-like behavior during the EPM test, greater activation of corticotropin-releasing hormone expressing neurons in the PVN, lower activation of serotonin neurons in the dorsal raphe, and lower levels of microgliosis in the nucleus accumbens. Taken together, these data suggest brain region- and neurochemical-specific alterations as well as neuroinflammatory changes that may be involved in the regulation of social buffering and stress contagion behaviors.

Consequences of pandemic-associated social restrictions: Role of social support and the oxytocin system

During pandemics, governments take drastic actions to prevent the spreading of the disease, as seen during the present COVID-19 crisis. Sanctions of lockdown, social distancing and quarantine urge people to exclusively work and teach at home and to restrict social contacts to a minimum; lonely people get into further isolation, while families` nerves are strained to the extreme. Overall, this results in a dramatic and chronic increase in the level of psychosocial stress over several months mainly caused by i) social isolation and ii) psychosocial stress associated with overcrowding, social tension in families, and domestic violence. Moreover, pandemic-associated social restrictions are accompanied by loss of an essential stress buffer and important parameter for general mental and physical health: social support. Chronic psychosocial stress and, in particular, social isolation and lack of social support affect not only mental health, but also the brain oxytocin system and the immune system. Hence, pandemic-associated social restrictions are expected to increase the risk of developing psychopathologies, such as depression, anxiety-related and posttraumatic stress disorders, on the one hand, but also to induce a general inflammatory state and to impair the course of infectious disorders on the other. Due to its pro-social and stress-buffering effects, resulting in an anti-inflammatory state in case of disease, the role of the neuropeptide oxytocin will be discussed and critically considered as an emerging treatment option in cases of pandemic-induced psychosocial stress, viral infection and during recovery. In this review, we aim to critically focus on possible short- and long-term consequences of social restrictions on mental health and the immune system, while discussion oxytocin as a possible treatment option.

Sex-dependent effects of pair bond interruption on anxiety- and depression-like behaviors in adult mandarin voles

Stable and positive social bonds are pretty vital to the development of animals. Instability and disruptions of social bonds, such as maternal separation and social isolation, always produce disastrous influence on physiology, neuroendocrine and behaviors. Pair bond is one of the most important social bonds in adulthood. But the different effects of pair bond interruption between males and females are rarely studied. In the present study, the monogamous mandarin voles (Microtus mandarinus) were used to confirm the time window of pair bond formation. After that, voles were separated from their partner for 1 or 2 weeks. Then anxiety- and depression-like behaviors were investigated by using open field test, light-dark box test, tail suspension test and forced swimming test, respectively. The results showed that: (1) cohabitation for 5 days is sufficient and necessary for mandarin voles to form pair bond; (2) loss of partner is always crucial for the effects of pair bond interruption, while social isolation works in certain behavioral tests.; (3) pair bond interruption for 2 weeks significantly increased the level of anxiety and depression in adult males, but not female mandarin voles. Overall, this research suggested that loss of partner plays a key role in pair bond interruption in male mandarin voles.

Social isolation and group size are associated with divergent gene expression in the brain of ant queens

Social life and isolation pose a complex suite of challenges to organisms prompting significant changes in neural state. However, plasticity in how brains respond to social challenges remains largely unexplored. The fire ants Solenopsis invicta provide an ideal scenario for examining this. Fire ant queens may found colonies individually or in groups of up to 30 queens, depending on key factors such as density of newly mated queens and availability of nesting sites. We artificially manipulated availability of nesting sites to test how the brain responds to social versus solitary colony founding at two key timepoints (early vs. late colony founding) and to group size (large vs. small groups). We adopted a powerful neurogenomic approach to identify even subtle differences of gene expression between treatment groups, and we built a global gene co-expression network of the fire ant brain to identify gene modules specifically associated with the different components of the social environment. The difference between group and single founding queens involves only one gene when founding behavior is still plastic and queens can switch from one modality to another, while hundreds of genes are involved later in the process, when behaviors have lost the initial plasticity and are more canalized. Furthermore, we find that large groups are associated with greater changes in gene expression than small groups, showing that even potentially subtle differences in the social environment can be linked to different neurogenomic states.

Early social deprivation does not affect cortisol response to acute and chronic stress in zebrafish

Social isolation is a well-established technique for inducing early adversity but, in rodent models, the need of parental care makes it difficult to distinguish the effects of social deprivation from the consequences of nutritional deficiencies. Zebrafish do not require parental care, allowing separation of social deprivation from nutritional deprivation, and have emerged as a promising model to study ontogeny of normal and pathological behaviors relevant for human neuropsychological disorders. Previous reports of life-long isolation in zebrafish showed some consistency with mammalian literature, depicting later social deficits and locomotor hyperactivity. However, unlike reports of higher anxiety and stress behavior in isolated rodents and primates, behavioral responses were tapered in isolated fish. To examine whether life-long developmental isolation has a dampening effect on zebrafish endocrine stress response, we applied stressors to zebrafish siblings that were either isolated or socialized, and compared their whole-body cortisol levels with non-stressed control siblings kept in low-housing densities. Utilizing previously validated paradigms (exposure to novel tank and unpredictable chronic stress), we exposed separate groups (n = 9-14, mixed-sex) of social and isolated zebrafish to acute and chronic stressors and measured their cortisol levels. A univariate ANOVA and post-hoc Tukey's HSD tests confirmed that compared to socially raised control fish, developmental isolation did not increase baseline cortisol levels in zebrafish. Additionally, compared to the non-stressed condition, application of both acute and chronic stressors significantly increased cortisol levels in isolated fish and, to a similar degree, to socially raised fish. Our findings suggest that zebrafish isolation studies may help separate effects of social deprivation from nonsocial aspects of early adversity. These studies further substantiate the use of developmental isolation in zebrafish, particularly with acute and chronic stress paradigms, for modeling neuropsychological disorders.LAY SUMMARYA difficult childhood can make humans react more frequently or severely to later stress and modeling this effect in animals can help explain how and why early stress affects subsequent mental and physical health. Early social isolation does not affect later response to stressful situations in adult zebrafish, providing us with a model of psychiatric disorders that allows separation of effects of poor physical environments (lacking food, shelter, etc.) from poor social environments (lack of appropriate socialization).

Behavioral and neuroendocrine consequences of disrupting a long-term monogamous social bond in aging prairie voles

Social support from a spouse, long-term partner, or someone who provides emotional or instrumental support may protect against consequences of aging, including mediating behavioral stress reactivity and altering neurobiological process that underlie short-term stress responses. Therefore, long-term social bonding may have behavioral and neurobiological benefits. The socially monogamous prairie vole provides a valuable experimental model for investigating the benefits of long-term social bonds on short-term stress reactivity in aging animals, given their unique social structure of forming enduring opposite-sex bonds, living in family groups, and bi-parental rearing strategies. Male-female pairs of long-term, cohabitating prairie voles were investigated for short-term behavioral and neuroendocrine stress reactivity following either long-term social pairing (control), or a period of social isolation. In Experiment 1, social isolation was associated with altered behavioral reactivity to an acute swim stressor, and greater neural activation in the hypothalamic paraventricular nucleus, as well as specifically the parvocellular region, following the swim stressor (vs. control). In Experiment 2, social isolation was associated with greater corticosterone reactivity following an acute restraint stressor (vs. control). No sex differences were observed. Exploratory correlation and subgroup analyses revealed systematic relationships among various demographic variables (such as age of the subjects, amount of time the pair cohabitated together, and number of litters the pair reared together) and the behavioral and neuroendocrine outcome measures. These findings may inform our understanding of the benefits of long-term social bonding on modulating short-term behavioral and neuroendocrine responses to stress.LAY SUMMARYReceiving social support from a long-term spouse or partner, or having a strong support network from friends, may have important health benefits as people age. In aging monogamous prairie voles, social isolation from a long-term social partner disrupted behaviors and short-term stress responses, whereas living with a long-term partner protected against these disruptions. This research is important for our understanding of the benefits of social support on stress responses as we age.

Effect of early life social adversity on drug abuse vulnerability: Focus on corticotropin-releasing factor and oxytocin

Early life adversity can set the trajectory for later psychiatric disorders, including substance use disorders. There are a host of neurobiological factors that may play a role in the negative trajectory. The current review examines preclinical evidence suggesting that early life adversity specifically involving social factors (maternal separation, adolescent social isolation and adolescent social defeat) may influence drug abuse vulnerability by strengthening corticotropin-releasing factor (CRF) systems and weakening oxytocin (OT) systems. In adulthood, pharmacological and genetic evidence indicates that both CRF and OT systems are directly involved in drug reward processes. With early life adversity, numerous studies show an increase in drug abuse vulnerability measured in adulthood, along a concomitant strengthening of CRF systems and a weakening of OT systems. Mechanistic studies, while relatively few in number, are generally consistent with the theme that strengthened CRF systems and weakened OT systems mediate, at least in part, the link between early life adversity and drug abuse vulnerability. Establishing a direct role of CRF and OT in mediating the relation between early life social stressors and drug abuse vulnerability will inform clinical researchers and practitioners toward the development of intervention strategies to reduce risk among those suffering from early life adversities. This article is part of the special issue on 'Vulnerabilities to Substance Abuse'.

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.

The neuropeptide Pth2 dynamically senses others via mechanosensation

Species that depend on membership in social groups for survival exhibit changes in neuronal gene expression and behaviour when they face restricted social interactions or isolation^1-3. Here we show that, across the lifespan of zebrafish (Danio rerio), social isolation specifically decreased the level of transcription of pth2, the gene that encodes the vertebrate-specific neuropeptide Pth2. However, 30 minutes of exposure to conspecifics was sufficient to initiate a significant rescue of pth2 transcript levels in previously isolated zebrafish. Transcription of pth2 exhibited bidirectional dynamics; following the acute isolation of socially reared fish, a rapid reduction in the levels of pth2 was observed. The expression of pth2 tracked not only the presence of other fish but also the density of the group. The sensory modality that controls the expression of pth2 was neither visual nor chemosensory in origin but instead was mechanical, induced by the movements of neighbouring fish. Chemical ablation of the mechanosensitive neuromast cells within the lateral line of fish prevented the rescue of pth2 levels that was induced by the social environment. In addition, mechanical perturbation of the water at frequencies similar to the movements of the zebrafish tail was sufficient to rescue the levels of pth2 in previously isolated fish. These data indicate a previously underappreciated role for the relatively unexplored neuropeptide Pth2 in both tracking and responding to the population density of the social environment of an animal.

Social isolation alters behavior, the gut-immune-brain axis, and neurochemical circuits in male and female prairie voles

The absence of social support, or social isolation, can be stressful, leading to a suite of physical and psychological health issues. Growing evidence suggests that disruption of the gut-immune-brain axis plays a crucial role in the negative outcomes seen from social isolation stress. However, the mechanisms remain largely unknown. The socially monogamous prairie vole (Microtus ochrogaster) has been validated as a useful model for studying negative effects of social isolation on the brain and behaviors, yet how the gut microbiome and central immune system are altered in isolated prairie voles are still unknown. Here, we utilized this social rodent to examine how social isolation stress alters the gut-immune-brain axis and relevant behaviors. Adult male and female prairie voles (n = 48 per sex) experienced social isolation or were cohoused with a same-sex cagemate (control) for six weeks. Thereafter, their social and anxiety-like behaviors, neuronal circuit activation, neurochemical expression, and microgliosis in key brain regions, as well as gut microbiome alterations from the isolation treatment were examined. Social isolation increased anxiety-like behaviors and impaired social affiliation. Isolation also resulted in sex- and brain region-specific alterations in neuronal activation, neurochemical expression, and microgliosis. Further, social isolation resulted in alterations to the gut microbiome that were correlated with key brain and behavioral measures. Our data suggest that social isolation alters the gut-immune-brain axis in a sex-dependent manner and that gut microbes, central glial cells, and neurochemical systems may play a critical, integrative role in mediating negative outcomes from social isolation.

Effect of environmental exposure to a maternally-learned odorant on anxiety-like behaviors at weaning in mice

Early sensory experience, such as exposure to maternal or other environmental factors, is considered to influence neurocognitive development and behaviors. In many species, exposure to odorants during pregnancy or lactation impacts the morpho-functional development of the olfactory circuitry with changes in olfactory sensitivity, feeding behavior and food preferences at birth or later. However, few studies have investigated the impact of a perinatal exposure to odorants on the anxiety-like behavior of animals to stressfull stimuli. Here, we exposed mice to heptaldehyde (HEP) during pregnancy and lactation and measured the anxiety-like behavior of their offspring to stress-inducing novel stimuli at weaning in presence or absence of odorants. We applied a combined social and maternal separation as a stressor and measured the anxiety-like behavior in an open field (OF) in presence of two odorants, HEP or α-pinene (AP) as a control odorant. Although the presence of the odorant during the social separation did not influence anxiety-like behavior, we found that, if mice born to non-odorized mothers exhibited a decreased exploratory behavior in the presence of both odorants, the effect was restricted to AP for the mice perinatally exposed to HEP. These results show that anxiety-like behaviors during a stress-inducing event could be reduced by the presence of a familiar odorant. We propose that the recall of an early olfactory experience could contribute to the improvement of animal welfare in various situations associated with husbandry practices.

Sex-specific effects of perinatal FireMaster® 550 (FM 550) exposure on socioemotional behavior in prairie voles

The rapidly rising incidence of neurodevelopmental disorders with social deficits is raising concern that developmental exposure to environmental contaminants may be contributory. Firemaster 550 (FM 550) is one of the most prevalent flame-retardant (FR) mixtures used in foam-based furniture and baby products and contains both brominated and organophosphate components. We and others have published evidence of developmental neurotoxicity and sex specific effects of FM 550 on anxiety-like and exploratory behaviors. Using a prosocial animal model, we investigated the impact of perinatal FM 550 exposure on a range of socioemotional behaviors including anxiety, attachment, and memory. Virtually unknown to toxicologists, but widely used in the behavioral neurosciences, the prairie vole (Microtus ochrogaster) is a uniquely valuable model organism for examining environmental factors on sociality because this species is spontaneously prosocial, biparental, and displays attachment behaviors including pair bonding. Dams were exposed to 0, 500, 1000, or 2000 μg of FM 550 via subcutaneous (sc) injections throughout gestation, and pups were directly exposed beginning the day after birth until weaning. Adult offspring of both sexes were then subjected to multiple tasks including open field, novel object recognition, and partner preference. Effects were dose responsive and sex-specific, with females more greatly affected. Exposure-related outcomes in females included elevated anxiety, decreased social interaction, decreased exploratory motivation, and aversion to novelty. Exposed males also had social deficits, with males in all three dose groups failing to show a partner preference. Our studies demonstrate the utility of the prairie vole for investigating the impact of chemical exposures on social behavior and support the hypothesis that developmental FR exposure impacts the social brain. Future studies will probe the possible mechanisms by which these effects arise.

The Impact of Early Postnatal and Juvenile Social Environments on the Effects of Chronic Intranasal Oxytocin in the Prairie Vole

Interactions between social experiences at different stages of development (e.g., with parents as juveniles and peers as subadults) can profoundly shape the expression of social behavior. Rarely are the influences of more than one stage of developmental sensitivity to social environment investigated simultaneously. Furthermore, oxytocin (OT) has an extraordinary effect on a breadth of social behaviors, activationally or organizationally. The use of intranasal OT (IN-OT) has become increasingly common therapeutically in humans and scientifically in non-human experiments, however very little attention has been paid to the potential developmental consequences on social behavior that might result. We investigated the effects of early-life social environments and the impact of chronic IN-OT on social behavior at different stages of development in male prairie voles (Microtus ochrogaster). We raised animals under two conditions: “socially enriched” (in which they were biparentally reared and then weaned into group housing as subadults), or “socially limited” (in which they were reared by a single-mother, and that were then weaned into social isolation). Males raised under each condition were either administered daily doses of IN-OT or a saline control for 21 days from postnatal day (PND) 21–42. During this time, we assessed the prosocial behavior subjects demonstrated by evaluating juvenile affiliation (as subadults), alloparental care (as adults no longer being exposed to IN-OT), and partner preference tests to assess tendencies to form adult monogamous pairbonds. We found that “socially limited” males, exhibited increased social contact in juvenile affiliation tests at PND 35 and 42. These males were also more likely to form a partner preference than “socially enriched” males and formed stronger partner preferences overall. IN-OT did not alter these behavioral effects. We also found that “socially limited” males exhibited a distinct response to chronic IN-OT treatment. When compared to all other treatment groups, “socially limited” males that received IN-OT exhibited a greater amount of huddling behavior in the alloparental care test. This effect was, in part, explained by an absence of attack behavior, found only in these males. This study contributes to understanding the complex interactions between the developmental social environment, oxytocin, and social behavior.

The Impact of Ethologically Relevant Stressors on Adult Mammalian Neurogenesis

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

Sexual cues influence cocaine-induced locomotion, anxiety and the immunoreactivity of oestrogen receptor alpha and tyrosine hydroxylase in both sexes

Dyadic physical social interaction influences cocaine-seeking behaviour, although whether limited sexual cues (LSC) from an opposite-sex partner influence the behavioural responses to cocaine is unclear. We investigated this issue using a cylindrical wire cage containing a stimulus mouse; the subject mouse (of the opposite sex) had access to this stimulus mouse during a "binge" injection pattern (injected with cocaine or saline vehicle twice a day at 6-hour intervals). Following the second injection, locomotion and anxiety-like behaviours were examined using the open-field and elevated plus maze test, at the same time as oestrogen receptor (ER)α and tyrosine hydroxylase (TH) immunoreactivities were also examined. The data indicate that LSC enhanced cocaine-stimulated locomotion in both sexes and inhibited the levels of anxiety caused by cocaine in males only. Accompanying these changes, the interaction between LSC and cocaine altered ERα immunoreactivity in the ventral medial nuclei of the hypothalamus (VMH) and medial amygdaloid nucleus (MeA) of males, whereas such interaction effects occurred in the VMH, MeA, arcuate nucleus (AR), bed nucleus of the stria terminalis (BNST) and lateral septum (LS) of females. LSC increased cocaine-induced ERα immunoreactivity in the VMH in males and reduced cocaine-induced ERα immunoreactivity in the AR and LS in females. LSC up-regulated cocaine-induced increases in ventral tegmental area (VTA) TH immunoreactivity in females only. Our present data suggest that interactions between LSC and cocaine led to changes in ERα and TH immunoreactivity in a brain region-specific manner, which showed subtle differences in both sexes. The effects of LSC-mediated cocaine-induced locomotion and anxiety may be associated with alterations in ERα and dopamine activation.

Postweaning Isolation Rearing Alters the Adult Social, Sexual Preference and Mating Behaviors of Male CD-1 Mice

Objective: No study has comprehensively evaluated the effect of postweaning isolation on the social and sexual behaviors of a certain strain of rodents in ethology. The present study aims to explore how and to what extent isolation rearing after postweaning affects the social and sexual behaviors of male CD-1 mice in adulthood systematically.

Methods: Male CD-1 mice were randomly assigned to two groups: isolation reared (IS, one mouse per cage, n = 30) and group housed (GH, five mice per cage, n = 15). The mice underwent isolation rearing from postnatal day 23 to day 93. Then, social affiliation, recognition and memory were measured through selection task experiments. Social interaction under a home cage and novel environment were measured via resident–intruder and social interaction test, respectively. Furthermore, sexual preference, homosexual and heterosexual behaviors were measured.

Results: Our study found that postweaning isolation increased the social affiliation for conspecifics (p = 0.001), reduced social recognition (p = 0.042) and impaired social memory. As for social interaction, isolated mice showed a remarkably increased aggression toward the intruder male in a home cage or novelty environment. For instance, isolated mice presented a short attack latency (p < 0.001), high attack frequency (p < 0.001) and long attack duration (p < 0.001). In addition, isolated mice exhibited further social avoidance. Contrastingly, isolated mice displayed a reduced sexual preference for female (IS: 61.47 ± 13.80%, GH: 70.33 ± 10.06%, p = 0.038). As for heterosexual behavior, isolated mice have a short mating duration (p = 0.002), long mounting latency (p = 0.002), and long intromission latency (p = 0.015). However, no association was observed between postweaning isolation and homosexual behavior in male CD-1 mouse.

Conclusion: Postweaning isolation increased the social affiliation, impaired the social cognition and considerably increased the aggression in social interaction of adult male CD-1 mice. Postweaning isolation induced a decreased sexual preference for female in adulthood. Postweaning isolation extended the latency to mate, thereby reducing mating behavior. No association was observed between isolation and homosexual behavior.

Consequences of prenatal exposure to valproic acid in the socially monogamous prairie voles

Environmental risk factors contribute to autism spectrum disorders (ASD) etiology. In particular, prenatal exposure to the highly teratogenic anticonvulsant valproic acid (VPA) significantly increases ASD prevalence. Although significant discoveries on the embryopathology of VPA have been reported, its effects on the ability to form enduring social attachment—characteristic of ASD but uncommonly displayed by rats and mice—remains unknown. We aimed to examine the effects of prenatal VPA exposure in the social, monogamous prairie voles (Microtus ochrogaster). Compared to prenatal vehicle-exposed controls, prenatal VPA-exposed prairie voles had lower body weight throughout postnatal development, engaged in fewer social affiliative behaviors in a familial context, exhibited less social interactions with novel conspecifics, and showed enhanced anxiety-like behavior. Along these behavioral deficits, prenatal VPA exposure downregulated prefrontal cortex vasopressin receptor (V1aR) and methyl CpG-binding protein 2 (MeCP2) mRNA expression, but did not alter spine density in adults. Remarkably, adult social bonding behaviors, such as partner preference formation and selective aggression, were not disrupted by prenatal VPA exposure. Collectively, these studies suggest that, in this animal model, VPA alters only certain behavioral domains such as sex-naive anxiety and affiliative behaviors, but does not alter other domains such as social bonding with opposite sex individuals.

Effects of group size on learning and memory in the honey bee, Apis mellifera

In animals that experience interactions with conspecifics while young, social interactions appear to be a necessary prerequisite for typical behaviour. Eusocial insects have large colonies where individuals experience a great deal of social interactions with nest mates during all life stages, making them excellent candidates for understanding the effects of social isolation on brain development and behaviour. Here we used the honey bee Apis mellifera to study the effect of social isolation and group size on reward perception and discrimination learning and memory. We confined day old adult workers into three different size groups (1 bee, 8 or 32 bees) for six days during a critical period associated with adult behavioural maturation. We quantified their sucrose responsiveness, their ability to use and remember olfactory cues to discriminate between sucrose and salt (i.e. discrimination learning), and four biogenic amines in the brain. We found that the smaller the group size, the more responsive a worker was to the sucrose reward. Honey bees raised in groups of 32 performed the best in the learning trials and had the highest levels of dopamine. We found no effect of group size on memory. The observed group size effect on learning but not memory supports the hypothesis that social interactions modulate learning through the dopaminergic system.

Vasopressin and alcohol: a multifaceted relationship

BACKGROUND

Arginine vasopressin (VP) has been implicated in a number of neuropsychiatric disorders with an emphasis on situations where stress increased the severity of the disorder. Based on this hypothesized role for VP in neuropsychiatric disorders, much research is currently being undertaken in humans and animals to test VP as a target for treatment of a number of these disorders including alcohol abuse.

OBJECTIVES

To provide a summary of the literature regarding the role of VP in alcohol- and stress-related behaviors including the use of drugs that target VP in clinical trials.

RESULTS

Changes in various components of the VP system occur with alcohol and stress. Manipulating VP or its receptors can alter alcohol- and stress-related behaviors including tolerance to alcohol, alcohol drinking, and anxiety-like behavior. Finally, the hypothalamic-pituitary-adrenal axis response to alcohol is also altered by manipulating the VP system. However, clinical trials of VP antagonists have had mixed results.

CONCLUSIONS

A review of VP's involvement in alcohol's actions demonstrates that there is much to be learned about brain regions involved in VP-mediated effects on behavior. Thus, future work should focus on elucidating relevant brain regions. By using previous knowledge of the actions of VP and determining the brain regions and/or systems involved in its different behavioral effects, it may be possible to identify a specific receptor subtype target, drug treatment combination, or specific clinical contexts that may point toward a more successful treatment.

Rapid nonapeptide synthesis during a critical period of development in the prairie vole: plasticity of the paraventricular nucleus of the hypothalamus

Vasopressin (VP) and oxytocin (OT) are involved in modulating basic physiology and numerous social behaviors. Although the anatomical distributions of nonapeptide neurons throughout development have been described, the functional roles of VP and OT neurons during development are surprisingly understudied, and it is unknown whether they exhibit functional changes throughout early development. We utilized an acute social isolation paradigm to determine if VP and OT neural responses in eight nonapeptide cell groups differ at three different stages of early development in prairie voles. We tested pups at ages that are representative of the three rapid growth stages of the developing brain: postnatal day (PND)2 (closed eyes; poor locomotion), PND9 (eye opening; locomotion; peak brain growth spurt), and PND21 (weaning). Neural responses were examined in pups that (1) were under normal family conditions with their parents and siblings, (2) were isolated from their parents and siblings and then reunited, and (3) were isolated from their parents and siblings. We found that VP and OT neural activity (as assessed via Fos co-localization) did not differ in response to social condition across development. However, remarkably rapid VP and OT synthesis in response to social isolation was observed only in the paraventricular nucleus of the hypothalamus (PVN) and only in PND9 pups. These results suggest that PVN nonapeptide neurons exhibit distinct cellular properties during a critical period of development, allowing nonapeptide neurons to rapidly upregulate peptide production in response to stressors on a much shorter timescale than has been observed in adult animals.

Anxiety-like behavior and neuropeptide receptor expression in male and female prairie voles: The effects of stress and social buffering

Strong social support can negate negative health outcomes - an effect defined as 'social buffering'. In the present study, using the socially monogamous prairie vole (Microtus ochrogaster), we examined whether the presence of a bonded partner during a stressful event can reduce stress responses. Adult, pair-bonded female and male voles were assigned into experimental groups that were either handled (Control), experienced a 1-h immobilization (IMO) stress alone (IMO-Alone), or experienced IMO with their partner (IMO-Partner). Thereafter, subjects were tested for anxiety-like behavior, and brain sections were subsequently processed for oxytocin receptor (OTR) and vasopressin V1a-type receptor (V1aR) binding. Our data indicate that while IMO stress significantly decreased the time that subjects spent in the open arms of an elevated plus maze, partner's presence prevented this behavioral change - this social buffering on anxiety-like behavior was the same for both male and female subjects. Further, IMO stress decreased OTR binding in the nucleus accumbens (NAcc), but a partner's presence dampened this effect. No effects were found in V1aR binding. These data suggest that the neuropeptide- and brain region-specific OTR alterations in the NAcc may be involved in both the mediation and social buffering of stress responses. Some sex differences in the OTR and V1aR binding were also found in selected brain regions, offering new insights into the sexually dimorphic roles of the two neuropeptides. Overall, our results suggest a potential preventative approach in which the presence of social interactions during a stressor may buffer typical negative outcomes.

Lost connections: Oxytocin and the neural, physiological, and behavioral consequences of disrupted relationships

In humans and rodent animal models, the brain oxytocin system is paramount for facilitating social bonds, from the formation and consequences of early-life parent-infant bonds to adult pair bond relationships. In social species, oxytocin also mediates the positive effects of healthy social bonds on the partners' well-being. However, new evidence suggests that the negative consequences of early neglect or partner loss may be mediated by disruptions in the oxytocin system as well. With a focus on oxytocin and its receptor, we review studies from humans and animal models, i.e. mainly from the biparental, socially monogamous prairie vole (Microtus ochrogaster), on the beneficial effects of positive social relationships both between offspring and parents and in adult partners. The abundance of social bonds and benevolent social relationships, in general, are associated with protective effects against psycho- and physiopathology not only in the developing infant, but also during adulthood. Furthermore, we discuss the negative effects on well-being, emotionality and behavior, when these bonds are diminished in quality or are disrupted, for example through parental neglect of the young or the loss of the partner in adulthood. Strikingly, in prairie voles, oxytocinergic signaling plays an important developmental role in the ability to form bonds later in life in the face of early-life neglect, while disruption of oxytocin signaling following partner loss results in the emergence of depressive-like behavior and physiology. This review demonstrates the translational value of animal models for investigating the oxytocinergic mechanisms that underlie the detrimental effects of developmental parental neglect and pair bond disruption, encouraging future translationally relevant studies on this topic that is so central to our daily lives.

Voluntary physical exercise protects against behavioral and endocrine reactivity to social and environmental stressors in the prairie vole

Physical activity can combat detrimental effects of stress. The current study examined the potential protective effects of exercise against a combination of social isolation and chronic mild stress (CMS) in a prairie vole model. Female voles were isolated for 4 weeks, with the addition of CMS during the final 2 weeks. Half of the voles were allowed access to a running wheel during this final 2 weeks, while the other half remained sedentary. Animals underwent behavioral tests to assess depressive- and anxiety-behaviors. In a subset of animals, plasma was collected 10 minutes after behavioral testing for corticosterone analysis. In a separate subset, brains were collected 2 hours after behavioral testing for cFos analysis in the paraventricular nucleus (PVN). Voles in the exercise group displayed significantly lower depressive- and anxiety-behaviors, and displayed significantly lower corticosterone levels, compared to animals in the sedentary group. There was no difference in PVN cFos activity between groups. Interestingly, animals that moderately exercised displayed lower levels of depressive-behavior and attenuated corticosterone reactivity compared to animals in the low and high activity subgroups. These findings suggest that physical activity can protect against a combination of social and environmental stressors.

Paternal deprivation affects social behaviors and neurochemical systems in the offspring of socially monogamous prairie voles

Early life experiences, particularly the experience with parents, are crucial to phenotypic outcomes in both humans and animals. Although the effects of maternal deprivation on offspring well-being have been studied, paternal deprivation (PD) has received little attention despite documented associations between father absence and children health problems in humans. In the present study, we utilized the socially monogamous prairie vole (Microtus ochrogaster), which displays male-female pair bonding and bi-parental care, to examine the effects of PD on adult behaviors and neurochemical expression in the hippocampus. Male and female subjects were randomly assigned into one of two experimental groups that grew up with both the mother and father (MF) or with the mother-only (MO, to generate PD experience). Our data show that MO subjects received less parental licking/grooming and carrying and were left alone in the nest more frequently than MF subjects. At adulthood (∼75days of age), MO subjects displayed increased social affiliation (SOA) toward a conspecific compared to MF subjects, but the two groups did not differ in social recognition (SOR) and anxiety-like behavior. Interestingly, MO subjects showed consistent increases in both gene and protein expression of the brain-derived neurotrophic factor (BDNF) and tropomyosin receptor kinase B (TrkB) as well as the levels of total histone 3 and histone 3 acetylation in the hippocampus compared to MF subjects. Further, PD experience increased glucocorticoid receptor beta (GRβ) protein expression in the hippocampus of females as well as increased corticotrophin receptor 2 (CRHR2) protein expression in the hippocampus of males, but decreased CRHR2 mRNA in both sexes. Together, our data suggest that PD has a long-lasting, behavior-specific effect on SOA and alters hippocampal neurochemical systems in the vole brain. The functional role of such altered neurochemical systems in social behaviors and the potential involvement of epigenetic events should be further studied.

Intranasal Oxytocin Selectively Modulates Social Perception, Craving, and Approach Behavior in Subjects With Alcohol Use Disorder

OBJECTIVES

A pharmacotherapy that both improves social abilities and promotes abstinence may be particularly helpful for the treatment of alcohol use disorder. Recent clinical and preclinical evidence suggests that oxytocin has prosocial and antiaddiction effects. We performed a pilot, laboratory-based, preclinical trial of oxytocin in subjects with alcohol abuse (as per Diagnostic and Statistical Manual of Mental Disorders, 4 Edition criteria) to evaluate therapeutic potential and assess tolerability.

METHODS

Social perceptual ability, cue-induced craving, and approach bias for alcohol and appetitive imagery were quantified after intranasal oxytocin and placebo administration to 32 nontreatment-seeking individuals with alcohol abuse in a double-blind, crossover study. Because attachment style can moderate the effects of oxytocin, we also explored whether attachment style moderated oxytocin's effects on our behavioral measures.

RESULTS

Oxytocin significantly improved recognition of easier items on a social perception task, but had no significant group-level effect on cue-induced craving. However, oxytocin effects on craving were moderated by attachment anxiety, with oxytocin reducing craving in more anxiously attached individuals and increasing craving in less anxiously attached individuals. Subjects did not display an approach bias to alcohol images on the placebo day, preventing meaningful analysis of this measure. Subjects did display an approach bias to appetitive images on the placebo day, which was significantly reduced by oxytocin administration. No adverse reactions were observed.

CONCLUSIONS

Intranasal oxytocin has potential to improve social perception, reduce cue-induced alcohol cravings, and reduce appetitive approach bias in subjects with alcohol abuse, and can be safely tolerated in this population. The effects of oxytocin are complex, however, and require further investigation.

Intergenerational transmission of sociality: the role of parents in shaping social behavior in monogamous and non-monogamous species

Social bonds are necessary for many mammals to survive and reproduce successfully. These bonds (i.e. pair-bonds, friendships, filial bonds) are characterized by different periods of development, longevity and strength. Socially monogamous species display certain behaviors not seen in many other mammals, such as adult pair-bonding and male parenting. In our studies of prairie voles (Microtus ochrogaster) and titi monkeys (Callicebus cupreus), we have examined the neurohormonal basis of these bonds. Here, we discuss the evidence from voles that aspects of adolescent and adult social behavior are shaped by early experience, including changes to sensory systems and connections, neuropeptide systems such as oxytocin and vasopressin, and alterations in stress responses. We will compare this with what is known about these processes during development and adulthood in other mammalian species, both monogamous and non-monogamous, and how our current knowledge in voles can be used to understand the development of and variation in social bonds. Humans are endlessly fascinated by the variety of social relationships and family types displayed by animal species, including our own. Social relationships can be characterized by directionality (either uni- or bi-directional), longevity, developmental epoch (infant, juvenile or adult) and strength. Research on the neurobiology of social bonds in animals has focused primarily on 'socially monogamous' species, because of their long-term, strong adult affiliative bonds. In this Review, we attempt to understand how the ability and propensity to form these bonds (or lack thereof), as well as the display of social behaviors more generally, are transmitted both genomically and non-genomically via variation in parenting in monogamous and non-monogamous species.

Neuropeptide Regulation of Social Attachment: The Prairie Vole Model

Social attachments are ubiquitous among humans and integral to human health. Although great efforts have been made to elucidate the neural underpinnings regulating social attachments, we still know relatively little about the neuronal and neurochemical regulation of social attachments. As a laboratory animal research model, the socially monogamous prairie vole (Microtus ochrogaster) displays behaviors paralleling human social attachments and thus has provided unique insights into the neural regulation of social behaviors. Research in prairie voles has particularly highlighted the significance of neuropeptidergic regulation of social behaviors, especially of the roles of oxytocin (OT) and vasopressin (AVP). This article aims to review these findings. We begin by discussing the role of the OT and AVP systems in regulating social behaviors relevant to social attachments, and thereafter restrict our discussion to studies in prairie voles. Specifically, we discuss the role of OT and AVP in adult mate attachments, biparental care, social isolation, and social buffering as informed by studies utilizing the prairie vole model. Not only do these studies offer insight into social attachments in humans, but they also point to dysregulated mechanisms in several mental disorders. We conclude by discussing these implications for human health. © 2017 American Physiological Society. Compr Physiol 7:81-104, 2017.

Species differences in behavior and cell proliferation/survival in the adult brains of female meadow and prairie voles

Microtine rodents display diverse patterns of social organization and behaviors, and thus provide a useful model for studying the effects of the social environment on physiology and behavior. The current study compared the species differences and the effects of oxytocin (OT) on anxiety-like, social affiliation, and social recognition behaviors in female meadow voles (Microtus pennsylvanicus) and prairie voles (Microtus ochrogaster). Furthermore, cell proliferation and survival in the brains of adult female meadow and prairie voles were compared. We found that female meadow voles displayed a higher level of anxiety-like behavior but lower levels of social affiliation and social recognition compared to female prairie voles. In addition, meadow voles showed lower levels of cell proliferation (measured by Ki67 staining) and cell survival (measured by BrdU staining) in the ventromedial hypothalamus (VMH) and amygdala (AMY), but not the dentate gyrus of the hippocampus (DG), than prairie voles. Interestingly, the numbers of new cells in the VMH and AMY, but not DG, also correlated with anxiety-like, social affiliation, and social recognition behaviors in a brain region-specific manner. Finally, central OT treatment (200 ng/kg, icv) did not lead to changes in behavior or cell proliferation/survival in the brain. Together, these data indicate a potential role of cell proliferation/survival in selected brain areas on different behaviors between vole species with distinct life strategies.

Perinatal and juvenile social environments interact to shape cognitive behaviour and neural phenotype in prairie voles

Social environments experienced at different developmental stages profoundly shape adult behavioural and neural phenotypes, and may have important interactive effects. We asked if social experience before and after weaning influenced adult social cognition in male prairie voles. Animals were raised either with or without fathers and then either housed singly or in sibling pairs. Males that were socially deprived before (fatherless) and after (singly housed) weaning did not demonstrate social recognition or dissociate spatial from social information. We also examined oxytocin and vasopressin receptors (OTR and V1aR) in areas of the forebrain associated with social behaviour and memory. Pre- and post-wean experience differentially altered receptor expression in several structures. Of note, OTR in the lateral septum-an area in which oxytocin inhibits social recognition-was greatest in animals that did not clearly demonstrate social recognition. The combination of absentee fathers on V1aR in the retrosplenial cortex and single housing on OTR in the septohippocampal nucleus produced a unique phenotype previously found to be associated with poor reproductive success in nature. We demonstrate that interactive effects of early life experiences throughout development have tremendous influence over brain-behaviour phenotype and can buffer potentially negative outcomes due to social deprivation.

Neuropeptidergic regulation of pair-bonding and stress buffering: Lessons from voles

This article is part of a Special Issue "SBN 2014". Interpersonal attachment is a critical component of the human experience. Pair-bonding ameliorates the severity of several mental and physical diseases. Thus, a better understanding of how the central nervous system responds to and encodes social-buffering during stress is a valuable research enterprise. The prairie vole (Microtus ochrogaster), as a laboratory animal model, provides the gold standard for the investigation of the neurobiology underlying attachment. Furthermore, emerging research in voles, additional laboratory rodents, transgenic mice, primates, and humans has provided novel insight into the neurochemical mechanisms underlying the therapeutic effects of social bonds reducing anxiety, depression, and drug abuse liability. In the present review, we highlight the work from this burgeoning field and focus on the role(s) of the neuropeptides oxytocin (OT), vasopressin (AVP), and corticotrophin releasing hormone (CRH) mediating stress buffering. Together, the data suggest that OT underlies social bonding to reduce stress-induced psychological illness while AVP and CRH facilitate arousal to enhance autonomic reactivity, increasing susceptibility to adverse mental and physical health.

Early rearing experience is related to altered aggression and vasopressin production following chronic social isolation in the prairie vole

Parent-offspring interactions early in life can permanently shape the developmental path of those offspring. Manipulation of maternal care has long been used to alter the early-life environment of infants and impacts their later social behavior, aggression, and physiology. More recently, naturally occurring variation in maternal licking and grooming behavior has been shown to result in differences in social behavior and stress physiology in adult offspring. We have developed a model of natural variation in biparental care in the prairie vole (Microtus ochrogaster) and have demonstrated an association between the amount of early care received and later social behavior. In this study, we investigate the relationship between early life care and later aggression and neuroendocrine responses following chronic social isolation. Male and female offspring were reared by their high-contact (HC) or low-contact (LC) parents, then housed for 4 weeks post-weaning in social isolation. After 4 weeks, half of these offspring underwent an intrasexual aggression test. Brains and plasma were collected to measure corticotropin-releasing hormone (CRH) and vasopressin (AVP) immunoreactivity and plasma corticosterone (CORT). Male offspring of LC parents engaged in more aggressive behavior in the intrasexual aggression test compared to HC males. Female offspring of HC parents had higher plasma CORT levels after chronic social isolation and increases in the number and density of AVP-immunopositive cells in the supraoptic nucleus following an intrasexual aggression test. These findings show that the impact of early life biparental care on behavior and HPA activity following a social stressor is both sex-dependent and early experience-specific.

Breaking bonds in male prairie vole: long-term effects on emotional and social behavior, physiology, and neurochemistry

Social relationships are essential for many fundamental aspects of life while bond disruption can be detrimental to mental and physical health. Male prairie voles form enduring social bonds with their female partners, allowing the evaluation of partner loss on behavior, physiology, and neurochemistry. Males were evaluated for partner preference formation induced by 24h of mating, and half were separated from their partner for 4 wk. In Experiment 1, partner loss significantly increased anxiety-like behaviors in the elevated plus maze and light-dark box tests and marginally increased depressive-like behaviors in the forced swim test. In addition, while intruder-directed aggression is common in pair bonded prairie voles, separated males were affiliative and lacked aggression toward an unfamiliar female and an intruding male conspecific. Partner loss increased the density of oxytocin-immunoreactivity (-ir), vasopressin-ir, and corticotrophin-releasing hormone-ir cells in the paraventricular nucleus of the hypothalamus and oxytocin-ir cells in the supraoptic nucleus. Tyrosine hydroxylase-ir was not affected. In Experiment 2, partner preference was observed after 2 wk of partner loss but eliminated after 4 wk partner loss. Body weight gain and plasma corticosterone concentrations were elevated throughout the 4 wk. No effects were observed for plasma oxytocin or vasopressin. Together, partner loss elicits anxiety-like and depression-like behaviors, disrupts bond-related behaviors, and alters neuropeptide systems that regulate such behaviors. Thus, partner loss in male prairie voles may provide a model to better understand the behavior, pathology, and neurobiology underlying partner loss and grief.

Chemosensory cues affect amygdaloid neurogenesis and alter behaviors in the socially monogamous prairie vole

The current study examined the effects of pheromonal exposure on adult neurogenesis and revealed the role of the olfactory pathways on adult neurogenesis and behavior in the socially monogamous prairie vole (Microtus ochrogaster). Subjects were injected with a cell proliferation marker [5-bromo-2'-deoxyuridine (BrdU)] and then exposed to their own soiled bedding or bedding soiled by a same- or opposite-sex conspecific. Exposure to opposite-sex bedding increased BrdU labeling in the amygdala (AMY), but not the dentate gyrus (DG), of female, but not male, voles, indicating a sex-, stimulus-, and brain region-specific effect. The removal of the main olfactory bulbs or lesioning of the vomeronasal organ (VNOX) in females reduced BrdU labeling in the AMY and DG, and inhibited the male bedding-induced BrdU labeling in the AMY, revealing the importance of an intact olfactory pathway for amygdaloid neurogenesis. VNOX increased anxiety-like behavior and altered social preference, but it did not affect social recognition memory in female voles. VNOX also reduced the percentage of BrdU-labeled cells that co-expressed the neuronal marker TuJ1 in the AMY, but not the DG. Together, our data indicate the importance of the olfactory pathway in mediating brain plasticity in the limbic system as well as its role in behavior.

Breaking bonds in male prairie vole: long-term effects on emotional and social behavior, physiology, and neurochemistry

Social relationships are essential for many fundamental aspects of life while bond disruption can be detrimental to mental and physical health. Male prairie voles form enduring social bonds with their female partners, allowing the evaluation of partner loss on behavior, physiology, and neurochemistry. Males were evaluated for partner preference formation induced by 24h of mating, and half were separated from their partner for 4 wk. In Experiment 1, partner loss significantly increased anxiety-like behaviors in the elevated plus maze and light-dark box tests and marginally increased depressive-like behaviors in the forced swim test. In addition, while intruder-directed aggression is common in pair bonded prairie voles, separated males were affiliative and lacked aggression toward an unfamiliar female and an intruding male conspecific. Partner loss increased the density of oxytocin-immunoreactivity (-ir), vasopressin-ir, and corticotrophin-releasing hormone-ir cells in the paraventricular nucleus of the hypothalamus and oxytocin-ir cells in the supraoptic nucleus. Tyrosine hydroxylase-ir was not affected. In Experiment 2, partner preference was observed after 2 wk of partner loss but eliminated after 4 wk partner loss. Body weight gain and plasma corticosterone concentrations were elevated throughout the 4 wk. No effects were observed for plasma oxytocin or vasopressin. Together, partner loss elicits anxiety-like and depression-like behaviors, disrupts bond-related behaviors, and alters neuropeptide systems that regulate such behaviors. Thus, partner loss in male prairie voles may provide a model to better understand the behavior, pathology, and neurobiology underlying partner loss and grief.

Chronic variable stress in fathers alters paternal and social behavior but not pup development in the biparental California mouse (Peromyscus californicus)

Stress and chronically elevated glucocorticoid levels have been shown to disrupt parental behavior in mothers; however, almost no studies have investigated corresponding effects in fathers. The present experiment tested the hypothesis that chronic variable stress inhibits paternal behavior and consequently alters pup development in the monogamous, biparental California mouse (Peromyscus californicus). First-time fathers were assigned to one of three experimental groups: chronic variable stress (CVS, n=8), separation control (SC, n=7), or unmanipulated control (UC, n=8). The CVS paradigm (3 stressors per day for 7 days) successfully stressed mice, as evidenced by increased baseline plasma corticosterone concentrations, increased adrenal mass, decreased thymus mass, and a decrease in body mass over time. CVS altered paternal and social behavior of fathers, but major differences were observed only on day 6 of the 7-day paradigm. At that time point, CVS fathers spent less time with their pairmate and pups, and more time autogrooming, as compared to UC fathers; SC fathers spent more time behaving paternally and grooming the female mate than CVS and UC fathers. Thus, CVS blocked the separation-induced increase in social behaviors observed in the SC fathers. Nonetheless, chronic stress in fathers did not appear to alter survival or development of their offspring: pups from the three experimental conditions did not differ in body mass gain over time, in the day of eye opening, or in basal or post-stress corticosterone levels. These results demonstrate that chronic stress can transiently disrupt paternal and social behavior in P. californicus fathers, but does not alter pup development or survival under controlled, non-challenging laboratory conditions.