Disruption of social bonds induces behavioral and physiological dysregulation in male and female prairie voles

Hypothalamic vasopressin neural densities are higher in male Mongolian gerbils after separation from a pair bond partner and may facilitate behavior to form a new bond

Although pair bonding has been studied for several decades, only somewhat recently have researchers began studying the neural consequences of separation from a pair bond partner. Here we examined the impact of partner separation on the socially monogamous Mongolian gerbil. Using a within-subjects design, we assessed nonsocial, nonreproductive, and reproductive behavior in male gerbils pre- and post- either 4 weeks of cohabitation with or separation from a pair bond partner. We then conducted an immediate early gene study to examine the influence of partner separation on hypothalamic oxytocin and vasopressin neural responses to interactions with a novel, opposite-sex conspecific.

A translational neuroscience perspective on loneliness: Narrative review focusing on social interaction, illness and oxytocin

This review addresses key findings on loneliness from the social, neurobiological and clinical fields. From a translational perspective, results from studies in humans and animals are included, with a focus on social interaction, mental and physical illness and the role of oxytocin in loneliness. In terms of social interactions, lonely individuals tend to exhibit a range of abnormal behaviors based on dysfunctional social cognitions that make it difficult for them to form meaningful relationships. Neurobiologically, a link has been established between loneliness and the hypothalamic peptide hormone oxytocin. Since social interactions and especially social touch regulate oxytocin signaling, lonely individuals may have an oxytocin imbalance, which in turn affects their health and well-being. Clinically, loneliness is a predictor of physical and mental illness and leads to increased morbidity and mortality. There is evidence that psychopathology is both a cause and a consequence of loneliness. The final section of this review summarizes the findings from social, neurobiological and clinical perspectives to present a new model of the complex construct of loneliness.

Aging leads to sex-dependent effects on pair bonding and increased number of oxytocin-producing neurons in monogamous prairie voles

Pair bonds powerfully modulate health, which becomes particularly important when facing the detrimental effects of aging. To examine the impact of aging on relationship formation and response to loss, we examined behavior in 6-, 12-, and 18-month male and female prairie voles, a monogamous species that forms mating-based pair bonds. We found that older males (18-months) bonded quicker than younger voles, while similarly aged female voles increased partner directed affiliative behaviors. Supporting sex differences in bonding behaviors, we found that males were more likely to sample both partner and novel voles while females were more likely to display partner preference during the initial 20 minutes of the test. Using partner separation to study loss, we observed an erosion of partner preference only in 12-month females, but an overall decrease in partner-directed affiliation in females across all groups, but not in males. Finally, we found that the number of oxytocin, but not vasopressin, cells in the paraventricular hypothalamus increased during aging. These results establish prairie voles as a novel model to study the effects of normal and abnormal aging on pair bonding.

Highlights

18-month male voles demonstrate accelerated bond formation18-month female voles increase partner-directed huddling after 2 wksBonds erode faster in 12-month female voles after partner separationFemale behavior from partner preference tests is reflected in free interactionThe number of paraventricular hypothalamus oxytocin cells increase during aging.

Prairie voles as a model for adaptive reward remodeling following loss of a bonded partner

Loss of a loved one is a painful event that substantially elevates the risk for physical and mental illness and impaired daily function. Socially monogamous prairie voles are laboratory-amenable rodents that form life-long pair bonds and exhibit distress upon partner separation, mirroring phenotypes seen in humans. These attributes make voles an excellent model for studying the biology of loss. In this review, we highlight parallels between humans and prairie voles, focusing on reward system engagement during pair bonding and loss. As yearning is a unique feature that differentiates loss from other negative mental states, we posit a model in which the homeostatic reward mechanisms that help to maintain bonds are disrupted upon loss, resulting in yearning and other negative impacts. Finally, we synthesize studies in humans and voles that delineate the remodeling of reward systems during loss adaptation. The stalling of these processes likely contributes to prolonged grief disorder, a diagnosis recently added to the Diagnostic and Statistical Manual for Psychiatry.

Synaptic pruning during adolescence shapes adult social behavior in both males and females

Evolutionarily conserved, peer-directed social behaviors are essential to participate in many aspects of human society. These behaviors directly impact psychological, physiological, and behavioral maturation. Adolescence is an evolutionarily conserved period during which reward-related behaviors, including social behaviors, develop via developmental plasticity in the mesolimbic dopaminergic "reward" circuitry of the brain. The nucleus accumbens (NAc) is an intermediate reward relay center that develops during adolescence and mediates both social behaviors and dopaminergic signaling. In several developing brain regions, synaptic pruning mediated by microglia, the resident immune cells of the brain, is important for normal behavioral development. We previously demonstrated that during adolescence, in rats, microglial synaptic pruning shapes the development of NAc and social play behavior in males and females. In this report, we hypothesize that interrupting microglial pruning in NAc during adolescence will have persistent effects on male and female social behavior in adulthood. We found that inhibiting microglial pruning in the NAc during adolescence had different effects on social behavior in males and females. In males, inhibiting pruning increased familiar exploration and increased nonsocial contact. In females, inhibiting pruning did not change familiar exploration behavior but increased active social interaction. This leads us to infer that naturally occurring NAc pruning serves to reduce social behaviors toward a familiar conspecific in both males and females.

Behavioural variability, physical activity, rumination time, and milk characteristics of dairy cattle in response to regrouping

In the commercial dairy industry worldwide, it is common practice to periodically regroup cows as part of their management strategy within housed systems. While this animal husbandry practice is intended to improve management efficiency, cows may experience social stress as a result of the social environment changes, which may have an impact on their behavioural patterns, performance, and welfare. We investigated whether regrouping altered dairy cows' behaviour and impacted their cortisol concentration (a physiological marker of stress), oxytocin, milk yield, and quality in a robotic milking system. Fifty-two lactating cows (17 primiparous; 35 multiparous) were moved in groups of 3-5 individuals into established pens of approximately 100 cows. Behaviour of the regrouped cows was directly observed continuously for 4 h/day across 4-time blocks (day-prior (d-1), day-of regrouping (d0), day-after (d + 1), and 6-days after (d + 6) regrouping). Cows were categorised as being with others, alone, or feeding every 2.5 min prior to the assessment of behavioural dynamics. Milk yield (MY) and composition, total daily activity, and rumination time (RUM) data were extracted from the Lely T4C management program (Lely Industries, Maassluis, the Netherlands), and milk samples were collected for cortisol and oxytocin concentration analyses; data were analysed using linear mixed-effect modelling. Primiparous cows were less likely to be interacting with others on d + 1 than d-1 compared with multiparous. However, average bout duration (minutes) between being alone and feeding activity states were similar on d-1, d + 1, and d + 6, for both primiparous and multiparous cows. A reduction in the average alone and feeding bout duration was observed on d0. Multiparous cows spent significantly more total time being alone on d0 compared to d-1. Neither regrouping nor parity statistically influenced milk DM content, energy, or cortisol concentration. Primiparous cows produced 3.80 ± 2.42 kg (12.2%) less MY on d + 1 compared to their d-1, whereas multiparous cows did not change MY. A significant decrease of 0.2% fat was found in both parity groups following regrouping and remained low up to d + 6. Daily activity in both parity groups increased significantly and RUM reduced after regrouping. A significant decrease in oxytocin concentration was observed in all cows on d + 1. The results, specifically for primiparous cows, indicated a negative impact of regrouping on social interactions, due to changes in the social environment which may lead to short-term social instability. Multiparous cows may benefit from previous regrouping experiences.

Bonding against the odds: Male prairie vole response to the "widow effect" among females

Although pair bonding is the preferred mating tactic among socially monogamous prairie voles, naturalistic observations have demonstrated many males remain non-bonded. Moreover, although males readily re-bond after the loss of a partner, females do not (i.e., the "widow effect'). Few studies have attempted to address why so many males remain non-bonded or if a reluctance of re-bonding in females contributes to this outcome. We investigate how female bonding history impacts male pair bond formation. Specifically, we test two alternative hypotheses for how sexually naïve males will behave when paired with widow females. The fecundity hypothesis predicts males will avoid bonding with widow females and be more receptive to novel bond-naïve females. The preference to bond hypothesis predicts males will choose to bond and express a partner preference, irrespective of if a pair-mate is a widow or sexually naïve. Our results demonstrated that males expressed a partner preference for females regardless of their social history. These data support the preference to bond hypothesis and suggest natural variation in bonding may not be strongly due to males forgoing bonding opportunities.

Understanding social attachment as a window into the neural basis of prosocial behavior

The representation and demonstration of human values are intimately tied to our status as a social species. Humans are relatively unique in our ability to form enduring social attachments, characterized by the development of a selective bond that persists over time. Such relationships include the bonds between parents and offspring, pair bonds between partners and other affiliative contacts, in addition to group relationships to which we may form direct and symbolic affiliations. Many of the cognitive and behavioral processes thought to be linked to our capacity for social attachment-including consolation, empathy, and social motivation, and the implicated neural circuits mediating these constructs, are shared with those thought to be important for the representation of prosocial values. This perspective piece will examine the hypothesis that our ability to form such long-term bonds may play an essential role in the construction of human values and ethical systems, and that components of prosocial behaviors are shared across species. Humans are one of a few species that form such long-term and exclusive attachments and our understanding of the neurobiology underlying attachment behavior has been advanced by studying behavior in non-human animals. The overlap in behavioral and affective constructs underlying attachment behavior and value representation is discussed, followed by evidence from other species that demonstrate attachment behavior that supports the overlapping neurobiological basis for social bonds and prosocial behavior. The understanding of attachment biology has broad implications for human health as well as for understanding the basis for and variations in prosocial behavior.

Attachment across the lifespan: Examining the intersection of pair bonding neurobiology and healthy aging

Increasing evidence suggests that intact social bonds are protective against age-related morbidity, while bond disruption and social isolation increase the risk for multiple age-related diseases. Social attachments, the enduring, selective bonds formed between individuals, are thus essential to human health. Socially monogamous species like the prairie vole (M. ochrogaster) form long-term pair bonds, allowing us to investigate the mechanisms underlying attachment and the poorly understood connection between social bonds and health. In this review, we explore several potential areas of focus emerging from data in humans and other species associating attachment and healthy aging, and evidence from prairie voles that may clarify this link. We examine gaps in our understanding of social cognition and pair bond behavior. Finally, we discuss physiologic pathways related to pair bonding that promote resilience to the processes of aging and age-related disease. Advances in the development of molecular genetic tools in monogamous species will allow us to bridge the mechanistic gaps presented and identify conserved research and therapeutic targets relevant to human health and aging.

A systematic review and meta-analysis of 90 cohort studies of social isolation, loneliness and mortality

The associations between social isolation, loneliness and the risk of mortality from all causes, cardiovascular disease (CVD) and cancer are controversial. We systematically reviewed prospective studies on the association between social isolation, loneliness and mortality outcomes in adults aged 18 years or older, as well as studies on these relationships in individuals with CVD or cancer, and conducted a meta-analysis. The study protocol was registered with PROSPERO (reg. no. CRD42022299959). A total of 90 prospective cohort studies including 2,205,199 individuals were included. Here we show that, in the general population, both social isolation and loneliness were significantly associated with an increased risk of all-cause mortality (pooled effect size for social isolation, 1.32; 95% confidence interval (CI), 1.26 to 1.39; P < 0.001; pooled effect size for loneliness, 1.14; 95% CI, 1.08 to 1.20; P < 0.001) and cancer mortality (pooled effect size for social isolation, 1.24; 95% CI, 1.19 to 1.28; P < 0.001; pooled effect size for loneliness, 1.09; 95% CI, 1.01 to 1.17; P = 0.030). Social isolation also increased the risk of CVD mortality (1.34; 95% CI, 1.25 to 1.44; P < 0.001). There was an increased risk of all-cause mortality in socially isolated individuals with CVD (1.28; 95% CI, 1.10 to 1.48; P = 0.001) or breast cancer (1.51; 95% CI, 1.34 to 1.70; P < 0.001), and individuals with breast cancer had a higher cancer-specific mortality owing to social isolation (1.33; 95% CI, 1.02 to 1.75; P = 0.038). Greater focus on social isolation and loneliness may help improve people's well-being and mortality risk.

The Neurobiology of Love and Pair Bonding from Human and Animal Perspectives

Love is a powerful emotional experience that is rooted in ancient neurobiological processes shared with other species that pair bond. Considerable insights have been gained into the neural mechanisms driving the evolutionary antecedents of love by studies in animal models of pair bonding, particularly in monogamous species such as prairie voles (Microtus ochrogaster). Here, we provide an overview of the roles of oxytocin, dopamine, and vasopressin in regulating neural circuits responsible for generating bonds in animals and humans alike. We begin with the evolutionary origins of bonding in mother-infant relationships and then examine the neurobiological underpinnings of each stage of bonding. Oxytocin and dopamine interact to link the neural representation of partner stimuli with the social reward of courtship and mating to create a nurturing bond between individuals. Vasopressin facilitates mate-guarding behaviors, potentially related to the human experience of jealousy. We further discuss the psychological and physiological stress following partner separation and their adaptive function, as well as evidence of the positive health outcomes associated with being pair-bonded based on both animal and human studies.

Social Isolation: A Narrative Review on the Dangerous Liaison between the Autonomic Nervous System and Inflammation

Social isolation and feelings of loneliness are related to higher mortality and morbidity. Evidence from studies conducted during space missions, in space analogs, and during the COVID-19 pandemic underline the possible role of the autonomic nervous system in mediating this relation. Indeed, the activation of the sympathetic branch of the autonomic nervous system enhances the cardiovascular response and activates the transcription of pro-inflammatory genes, which leads to a stimulation of inflammatory activation. This response is adaptive in the short term, in that it allows one to cope with a situation perceived as a threat, but in the long term it has detrimental effects on mental and physical health, leading to mood deflection and an increased risk of cardiovascular disease, as well as imbalances in immune system activation. The aim of this narrative review is to present the contributions from space studies and insights from the lockdown period on the relationship between social isolation and autonomic nervous system activation, focusing on cardiovascular impairment and immune imbalance. Knowing the pathophysiological mechanisms underlying this relationship is important as it enables us to structure effective countermeasures for the new challenges that lie ahead: the lengthening of space missions and Mars exploration, the specter of future pandemics, and the aging of the population.

Impaired microglia-mediated synaptic pruning in the nucleus accumbens during adolescence results in persistent dysregulation of familiar, but not novel social interactions in sex-specific ways

Evolutionarily conserved, peer-directed social behaviors are essential to participate in many aspects of human society. These behaviors directly impact psychological, physiological, and behavioral maturation. Adolescence is an evolutionarily conserved period during which reward-related behaviors, including social behaviors, develop via developmental plasticity in the mesolimbic dopaminergic 'reward' circuitry of the brain. The nucleus accumbens (NAc) is an intermediate reward relay center that develops during adolescence and mediates both social behaviors and dopaminergic signaling. In several developing brain regions, synaptic pruning mediated by microglia, the resident immune cells of the brain, is important for normal behavioral development. In rats, we previously demonstrated that microglial synaptic pruning also mediates NAc and social development during sex-specific adolescent periods and via sex-specific synaptic pruning targets. In this report, we demonstrate that interrupting microglial pruning in NAc during adolescence persistently dysregulates social behavior towards a familiar, but not novel social partner in both sexes, via sex-specific behavioral expression. This leads us to infer that naturally occurring NAc pruning serves to reduce social behaviors primarily directed toward a familiar conspecific in both sexes, but in sex-specific ways.

Neural and behavioral reactions to partners and strangers in monogamous female titi monkeys (Plecturocebus cupreus)

Pair bonding in humans and other socially monogamous species can have positive effects on health and well-being. These attachments also come with the potential for challenges such as separation, jealousy, or grief. Much of the work on the neurobiology of pair bonding in non-human primates has been carried out in coppery titi monkeys (Plecturocebus cupreus), a monogamous South American monkey, although these studies have been primarily in males. In the current study, we utilized female titi monkeys to experimentally examine responses to their monogamous male partner vs. a male stranger or being alone. Positron emission tomography (PET) scans were performed on eight adult female titi monkeys from well-established pairs. We used a within-subjects design in which each female underwent three different conditions after the fluorodeoxyglucose F18 (FDG) injection: a) the subject was reunited with her partner, b) encountered a stranger, or c) was alone in the experimental cage. Behavioural observations were recorded, and plasma assayed for cortisol. Females housed alone showed higher cortisol compared with either the partner or stranger conditions. FDG uptake was higher in the amygdala and hippocampus when interacting with the stranger than the partner. Proximity modulated the relationship between social condition and FDG uptake in several areas. Females entered into mutual proximity more frequently with the partner than with the stranger. Female titi monkeys have different physiological, neural, and behavioural reactions to being with their partner, a stranger male, or being alone.

Prolonged partner separation erodes nucleus accumbens transcriptional signatures of pair bonding in male prairie voles

The loss of a spouse is often cited as the most traumatic event in a person's life. However, for most people, the severity of grief and its maladaptive effects subside over time via an understudied adaptive process. Like humans, socially monogamous prairie voles (Microtus ochrogaster) form opposite-sex pair bonds, and upon partner separation, show stress phenotypes that diminish over time. We test the hypothesis that extended partner separation diminishes pair bond-associated behaviors and causes pair bond transcriptional signatures to erode. Opposite-sex or same-sex paired males were cohoused for 2 weeks and then either remained paired or were separated for 48 hours or 4 weeks before collecting fresh nucleus accumbens tissue for RNAseq. In a separate cohort, we assessed partner-directed affiliation at these time points. We found that these behaviors persist despite prolonged separation in both same-sex and opposite-sex paired voles. Opposite-sex pair bonding led to changes in accumbal transcription that were stably maintained while animals remained paired but eroded following prolonged partner separation. Eroded genes are associated with gliogenesis and myelination, suggesting a previously undescribed role for glia in pair bonding and loss. Further, we pioneered neuron-specific translating ribosomal affinity purification in voles. Neuronally enriched transcriptional changes revealed dopaminergic-, mitochondrial-, and steroid hormone signaling-associated gene clusters sensitive to acute pair bond disruption and loss adaptation. Our results suggest that partner separation erodes transcriptomic signatures of pair bonding despite core behavioral features of the bond remaining intact, revealing potential molecular processes priming a vole to be able to form a new bond.

Behavioral trajectories of aging prairie voles (Microtus ochrogaster): Adapting behavior to social context wanes with advanced age

Several studies using mice have examined the effects of aging on cognitive tasks, as well as sensory and motor functions. However, few studies have examined the influence of aging on social behavior. Prairie voles (Microtus ochrogaster) are a socially monogamous and biparental rodent that live in small family groups and are now among the most popular rodent models for studies examining social behavior. Although the social behavioral trajectories of early-life development in prairie voles have been well-studied, how social behavior may change throughout adulthood remains unknown. Here we examined behavior in virgin male and female prairie voles in four different age groups: postnatal day (PND) 60–80, 140–160, 220–240, and 300–320. All animals underwent testing in a novel object task, a dominance test, a resident-intruder test, and several iterations of social approach and social interaction tests with varying types of social stimuli (i.e., novel same-sex conspecific, novel opposite-sex conspecific, familiar same-sex sibling/cagemate, small group of novel same-sex conspecifics). We found that age influenced neophobia and dominance, but not social approach behavior. Further, we found that young adult, but not older adult, prairie voles adapt prosocial and aggressive behavior relative to social context, and that selective aggression occurs in relation to age even in the absence of a pair bond. Our results suggest that prairie voles calibrate social phenotype in a context-dependent manner in young adulthood and stop adjusting behavior to social context in advanced age, demonstrating that social behavior is plastic not only throughout early development, but also well into adulthood. Together, this study provides insight into age-related changes in social behavior in prairie voles and shows that prairie voles may be a viable model for studying the cognitive and physiological benefits of social relationships and social engagement in advanced age.

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.

Interactions between the κ opioid system, corticotropin-releasing hormone and oxytocin in partner loss

Selective adult social attachments, or ‘pair bonds’, represent central relationships for individuals in a number of social species, including humans. Loss of a pair mate has emotional consequences that may or may not diminish over time, and that often translate into impaired psychological and physical health. In this paper, we review the literature on the neuroendocrine mechanisms for the emotional consequences of partner loss, with a special focus on hypothesized interactions between oxytocin, corticotropin-releasing hormone and the κ opioid system.

This article is part of the theme issue ‘Interplays between oxytocin and other neuromodulators in shaping complex social behaviours’.

Synergistic consequences of early-life social isolation and chronic stress impact coping and neural mechanisms underlying male prairie vole susceptibility and resilience

Chronic stress can be challenging, lead to maladaptive coping strategies, and cause negative mental and physical health outcomes. Early-life adversity exposes developing young to physical or psychological experiences that risks surpassing their capacity to effectively cope, thereby impacting their lifetime physical and mental wellbeing. Sensitivity to stressful events, like social isolation, has the potential to magnify stress-coping. Chronic stress through social defeat is an established paradigm that models adverse early-life experiences and can trigger enduring alterations in behavioral and neural phenotypes. To assess the degree to which stress resilience and sensitivity stemming from early-life chronic stress impact sociability, we exposed male prairie voles to chronic social defeat stress (CSDS) during adolescence. We simultaneously exposed subjects to either social isolation (CSDS+Isol) or group housing (CSDS+Soc) during this crucial time of development. On PND41, all subjects underwent a social approach test to examine the immediate impact of isolation, CSDS, or their combined effects on sociability. Unlike the CSDS+Isol group which primarily displayed social avoidance, the CSDS+Soc group was split by individuals exhibiting susceptible or resilient stress phenotypes. Notably, the Control+Soc and CSDS+Soc animals and their cage-mates significantly gained body weight between PND31 and PND40, whereas the Control+Isol and CSDS+Isol animals did not. These results suggest that the effects of early-life stress may be mitigated by having access to social support. Vasopressin, oxytocin, and opioids and their receptors (avpr1a, oxtr, oprk1, oprm1, and oprd1) are known to modulate social and stress-coping behaviors in the lateral septum (LS). Therefore, we did an mRNA expression analysis with RT-qPCR of the avpr1a, oxtr, oprk1, oprm1, and oprd1 genes to show that isolation and CSDS, or their collective influence, can potentially differentially bias sensitivity of the LS to early-life stressors. Collectively, our study supports the impact and dimensionality of early-life adversity because the type (isolation vs. CSDS), duration (acute vs. chronic), and combination (isolation + CSDS) of stressors can dynamically alter behavioral and neural outcomes.

Alone, in the dark: The extraordinary neuroethology of the solitary blind mole rat

On the social scale, the blind mole rat (BMR; Spalax ehrenbergi) is an extreme. It is exceedingly solitary, territorial, and aggressive. BMRs reside underground, in self-excavated tunnels that they rarely leave. They possess specialized sensory systems for social communication and navigation, which allow them to cope with the harsh environmental conditions underground. This review aims to present the blind mole rat as an ideal, novel neuroethological model for studying aggressive and solitary behaviors. We discuss the BMR's unique behavioral phenotype, particularly in the context of 'anti-social' behaviors, and review the available literature regarding its specialized sensory adaptations to the social and physical habitat. To date, the neurobiology of the blind mole rat remains mostly unknown and holds a promising avenue for scientific discovery. Unraveling the neural basis of the BMR's behavior, in comparison to that of social rodents, can shed important light on the underlying mechanisms of psychiatric disorders in humans, in which similar behaviors are displayed.

Neurobiology of Loneliness, Isolation, and Loss: Integrating Human and Animal Perspectives

In social species such as humans, non-human primates, and even many rodent species, social interaction and the maintenance of social bonds are necessary for mental and physical health and wellbeing. In humans, perceived isolation, or loneliness, is not only characterized by physical isolation from peers or loved ones, but also involves negative perceptions about social interactions and connectedness that reinforce the feelings of isolation and anxiety. As a complex behavioral state, it is no surprise that loneliness and isolation are associated with dysfunction within the ventral striatum and the limbic system - brain regions that regulate motivation and stress responsiveness, respectively. Accompanying these neural changes are physiological symptoms such as increased plasma and urinary cortisol levels and an increase in stress responsivity. Although studies using animal models are not perfectly analogous to the uniquely human state of loneliness, studies on the effects of social isolation in animals have observed similar physiological symptoms such as increased corticosterone, the rodent analog to human cortisol, and also display altered motivation, increased stress responsiveness, and dysregulation of the mesocortical dopamine and limbic systems. This review will discuss behavioral and neuropsychological components of loneliness in humans, social isolation in rodent models, and the neurochemical regulators of these behavioral phenotypes with a neuroanatomical focus on the corticostriatal and limbic systems. We will also discuss social loss as a unique form of social isolation, and the consequences of bond disruption on stress-related behavior and neurophysiology.

Cardiovascular Reactivity to a Novel Stressor: Differences on Susceptible and Resilient Rats to Social Defeat Stress

Prolonged and heightened responses to stress are known factors that influence the development of mood disorders and cardiovascular diseases. Moreover, the coping strategies related to the experience of adverse events, i.e., resilience or the susceptibility to stress, are determinants for the individual risk of developing such diseases. Susceptible rats to the social defeat stress (SDS), identified by the social interaction test (SIT), show behavioral and cardiovascular alterations after SDS exposure that are not found in resilient rats. However, it is not elucidated yet how the cardiovascular system of susceptible and resilient phenotypes responds to a new stressor after SDS exposure. Thus, using the SDS exposure followed by the SIT, we evaluated heart rate, blood pressure (BP), tail skin temperature, and circulating corticosterone responses to an acute session of restraint stress in susceptible and resilient rats to SDS. Susceptible rats showed resting tachycardia and exaggerated BP response to restraint stress, while resilient rats did not present such alterations. In contrast, both phenotypes showed increased plasma corticosterone and a drop in tail skin temperature to restraint stress, which was similar to that observed in control animals. Our results revealed an increased cardiovascular reactivity in response to a new stressful stimulus in susceptible rats, which might be related to a greater risk for the development of cardiovascular diseases.

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.

Morphological changes in the basolateral amygdala and behavioral disruptions associated with social isolation

Social isolation and the disruption of established social bonds contribute to several physical and psychological health issues. Animal models are a useful tool for investigating consequences of social stress, including social isolation. The current study examined morphological changes in the basolateral amygdala (BLA) and affect-related behavioral and endocrine changes due to prolonged social isolation, using the translational prairie vole model (Microtus ochrogaster). Adult male prairie voles were either socially paired (control) or isolated from a same-sex sibling for 4 weeks. Following this 4-week period, a subset of animals (n = 6 per condition) underwent a series of behavioral tasks to assess affective, social, and stress-coping behaviors. Plasma was collected following the last behavioral task for stressor-induced endocrine assays. Brains were collected from a separate subset of animals (n = 10 per condition) following the 4-week social housing period for dendritic structure analyses in the BLA. Social isolation was associated with depressive- and anxiety-like behaviors, as well as elevated oxytocin reactivity following a social stressor. Social isolation was also associated with altered amount of dendritic material in the BLA, with an increase in spine density. These results provide further evidence that social isolation may lead to the development of affective disorders. Dysfunction in the oxytocin system and BLA remodeling may mediate these behavioral changes. Further research will promote an understanding of the connections between oxytocin function and structural changes in the BLA in the context of social stress. This research can facilitate novel treatments for alleviating or preventing behavioral and physiological consequences of social stressors in humans.

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.

Mean Girls: Social Stress Models for Female Rodents

Social stressors are known to have strong negative impacts on mental health. There is a long history of preclinical social defeat stress studies in rodents focusing on males that has produced important insights into the neural mechanisms that modulate depression- and anxiety-related behavior. Despite these impressive results, a historical weakness of rodent social stress models has been an under-representation of studies in females. This is problematic because rates of depression and anxiety are higher in women versus men. Recently there has been a surge of interest in adapting social stress methods for female rodents. Here we review new rodent models that have investigated numerous facets of social stress in females. The different models have different strengths and weaknesses, with some model systems having stronger ethological validity with other models having better access to molecular tools to manipulate neural circuits. Continued use and refinement of these complementary models will be critical for addressing gaps in understanding the function of neural circuits modulating depression- and anxiety-related behavior in females.

Physical isolation and mental health among older US adults during the COVID-19 pandemic: longitudinal findings from the COVID-19 Coping Study

PURPOSE

We investigated the relationships between physical isolation at home during the period when many US states had shelter-in-place orders and subsequent longitudinal trajectories of depression, anxiety, and loneliness in older adults over a 6 month follow-up.

METHODS

Data were from monthly online questionnaires with US adults aged ≥ 55 in the nation-wide COVID-19 Coping Study (April through October 2020, N = 3978). Physical isolation was defined as not leaving home except for essential purposes (0, 1-3, 4-6, and 7 days in the past week), measured at baseline (April-May). Outcomes were depressive symptoms (8-item Center for Epidemiological Studies Depression Scale), anxiety symptoms (5-item Beck Anxiety Inventory), and loneliness (3-item UCLA loneliness scale), measured monthly (April-October). Multivariable, population- and attrition-weighted linear mixed-effects models assessed the relationships between baseline physical isolation with mental health symptoms at baseline and over time.

RESULTS

Physical isolation (7 days versus 0 days in the past week) was associated with elevated depressive symptoms (adjusted β = 0.85; 95% CI 0.10-1.60), anxiety symptoms (adjusted β = 1.22; 95% CI 0.45-1.98), and loneliness (adjusted β = 1.06; 95% CI 0.51-1.61) at baseline, but not with meaningful rate of change in these mental health outcomes over time. The symptom burden of each mental health outcome increased with increasing past-week frequency of physical isolation.

CONCLUSION

During the early COVID-19 pandemic, physical isolation was associated with elevated depressive symptoms, anxiety symptoms, and loneliness, which persisted over time. These findings highlight the unique and persistent mental health risks of physical isolation at home under pandemic control measures.

Separation from a bonded partner alters neural response to inflammatory pain in monogamous rodents

Pain experience is known to be modified by social factors, but the brain mechanisms remain unspecified. We recently established an animal model of social stress-induced hyperalgesia (SSIH) using a socially monogamous rodent, the prairie vole, in which males separated from their female partners (loss males) became anxious and displayed exacerbated inflammatory pain behaviors compared to males with partners (paired males). In the present study, to explore the neural pathways involved in SSIH, a difference in neuronal activation in pain-related brain regions, or "pain matrix", during inflammatory pain between paired and loss males was detected using Fos immunoreactivity (Fos-ir). Males were paired with a female and pair bonding was confirmed in all subjects using a partner preference test. During formalin-induced inflammatory pain, both paired and loss males showed a significant induction of Fos-ir throughout the analyzed pain matrix components compared to basal condition (without injection), and no group differences in immunoreactivity were found among the injected males in many brain regions. However, the loss males had significantly lower Fos-ir following inflammatory pain in the medial prefrontal cortex and nucleus accumbens shell than the paired males, even though base Fos-ir levels were comparable between groups. Notably, both regions with different Fos-ir are major components of the dopamine and oxytocin systems, which play critical roles in both pair bonding and pain regulation. The present results suggest the possibility that pain exacerbation by social stress emerges through alteration of signaling in social brain circuitry.

What is a pair bond?

Pair bonding is a psychological construct that we attempt to operationalize via behavioral and physiological measurements. Yet, pair bonding has been both defined differently in various taxonomic groups as well as used loosely to describe not just a psychological and affective phenomenon, but also a social structure or mating system (either social monogamy or just pair living). In this review, we ask the questions: What has been the historical definition of a pair bond? Has this definition differed across taxonomic groups? What behavioral evidence do we see of pair bonding in these groups? Does this observed evidence alter the definition of pair bonding? Does the observed neurobiology underlying these behaviors affect this definition as well? And finally, what are the upcoming directions in which the study of pair bonding needs to head?

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.

Microglia react to partner loss in a sex- and brain site-specific manner in prairie voles

Positive social relationships are paramount for the survival of mammals and beneficial for mental and physical health, buffer against stressors, and even promote appropriate immune system functioning. By contrast, impaired social relationships, social isolation, or the loss of a bonded partner lead to aggravated physical and mental health. For example, in humans partner loss is detrimental for the functioning of the immune system and heightens the susceptibility for the development of post-traumatic stress disorders, anxiety disorders, and major depressive disorders. To understand potential underlying mechanisms, the monogamous prairie vole can provide important insights. In the present study, we separated pair bonded male and female prairie voles after five days of co-housing, subjected them to the forced swim test on the fourth day following separation, and studied their microglia morphology and activation in specific brain regions. Partner loss increased passive stress-coping in male, but not female, prairie voles. Moreover, partner loss was associated with microglial priming within the parvocellular region of the paraventricular nucleus of the hypothalamus (PVN) in male prairie voles, whereas in female prairie voles the morphological activation within the whole PVN and the prelimbic cortex (PrL) was decreased, marked by a shift towards ramified microglial morphology. Expression of the immediate early protein c-Fos following partner loss was changed within the PrL of male, but not female, prairie voles. However, the loss of a partner did not affect the investigated aspects of the peripheral immune response. These data suggest a potential sex-dependent mechanism for the regulation of microglial activity following the loss of a partner, which might contribute to the observed differences in passive stress-coping. This study furthers our understanding of the effects of partner loss and its short-term impact on the CNS as well as the CNS immune system and the peripheral innate immune system in both male and female prairie voles.

Non-random associations in group housed rats (Rattus norvegicus)

Ecological factors, such as predation, have traditionally been used to explain sociability. However, it is increasingly recognised that individuals within a group do not associate randomly, and that these non-random associations can generate fitness advantages. The majority of the empirical evidence on differentiated associations in group-living mammals, however, comes from a limited number of taxa and we still know very little about their occurrence and characteristics in some highly social species, such as rats (Rattus spp.). Here, using network analysis, we quantified association patterns in four groups of male fancy rats. We found that the associations between rats were not randomly distributed and that most individuals had significantly more preferred/avoided associates than expected by random. We also found that these preferences can be stable over time, and that they were not influenced by individuals’ rank position in the dominance hierarchy. Our findings are consistent with work in other mammals, but contrast with the limited evidence available for other rat strains. While further studies in groups with different demographic composition are warranted to confirm our findings, the occurrence of differentiated associations in all male groups of rats have important implications for the management and welfare of captive rat populations.

The social network: Neural control of sex differences in reproductive behaviors, motivation, and response to social isolation

Social animal species present a vast repertoire of social interactions when encountering conspecifics. Reproduction-related behaviors, such as mating, parental care, and aggression, are some of the most rewarding types of social interactions and are also the most sexually dimorphic ones. This review focuses on rodent species and summarizes recent advances in neuroscience research that link sexually dimorphic reproductive behaviors to sexual dimorphism in their underlying neuronal circuits. Specifically, we present a few possible mechanisms governing sexually-dimorphic behaviors, by hypothalamic and reward-related brain regions. Sex differences in the neural response to social isolation in adulthood are also discussed, as well as future directions for comparative studies with naturally solitary species.

A Neuroscientist's Guide to the Vole

Prairie voles have emerged as an important rodent model for understanding the neuroscience of social behavior. Prairie voles are well known for their capacity for pair bonding and alloparental care. These behavioral phenomena overlap with human social behavior but are not commonly observed in traditional rodent models. In this article, we highlight the many benefits of using prairie voles in neuroscience research. We begin by describing the advantages of using diverse and non-traditional study models. We then focus on social behaviors, including pair bonding, alloparental care, and peer interactions, that have brought voles to the forefront of social neuroscience. We describe many additional features of prairie vole biology and behavior that provide researchers with opportunities to address an array of research questions. We also survey neuroethological methods that have been used with prairie voles, from classic to modern techniques. Finally, we conclude with a discussion of other vole species, particularly meadow voles, and their own unique advantages for neuroscience studies. This article provides a foundation for researchers who are new to working with voles, as well as for experienced neuroscientists who want to expand their research scope. © 2021 Wiley Periodicals LLC.

Differential paraventricular nucleus activation and behavioral responses to social isolation in prairie voles following environmental enrichment with and without physical exercise

Social stressors produce neurobiological and emotional consequences in social species. Environmental interventions, such as environmental enrichment and exercise, may modulate physiological and behavioral stress responses. The present study investigated the benefits of environmental enrichment and exercise against social stress in the socially monogamous prairie vole. Female prairie voles remained paired with a sibling (control) or were isolated from a sibling for 4 weeks. The isolated groups were assigned to isolated sedentary, isolated with environmental enrichment, or isolated with both enrichment and exercise conditions. Behaviors related to depression, anxiety, and sociality were investigated using the forced swim test (FST), elevated plus maze (EPM), and a social crowding stressor (SCS), respectively. cFos expression was evaluated in stress-related circuitry following the SCS. Both enrichment and enrichment with exercise protected against depression-relevant behaviors in the FST and social behavioral disruptions in the SCS, but only enrichment with exercise protected against anxiety-related behaviors in the EPM and altered cFos expression in the hypothalamic paraventricular nucleus in isolated prairie voles. Enrichment may improve emotion-related and social behaviors, however physical exercise may be an important component of environmental strategies for protecting against anxiety-related behaviors and reducing neural activation as a function of social stress.

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.

The neural circuitry of social homeostasis: Consequences of acute versus chronic social isolation

Social homeostasis is the ability of individuals to detect the quantity and quality of social contact, compare it to an established set-point in a command center, and adjust the effort expended to seek the optimal social contact expressed via an effector system. Social contact becomes a positive or negative valence stimulus when it is deficient or in excess, respectively. Chronic deficits lead to set-point adaptations such that reintroduction to the previous optimum is experienced as a surplus. Here, we build upon previous models for social homeostasis to include adaptations to lasting changes in environmental conditions, such as with chronic isolation.

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.

How prior pair-bonding experience affects future bonding behavior in monogamous prairie voles

Monogamous prairie voles (Microtus ochrogaster) form mating-based pair bonds. Although wild prairie voles rarely re-pair following loss of a partner, laboratory studies have shown that previous pairing and mating does not negate the ability to form a new partner preference. However, little is known about how prior bond experience may alter the trajectory and display of a new pair bond. In the present study, we disrupted an initial pair bond by separating partners and then varied the amount of time before a new partner was introduced. We assessed how separation time affected the stability of partner preference over time and influenced decision-making in male voles performing a head-to-head partner preference test in which they chose between the first and second partner. We found that the ability to consistently display a preference for the second partner, supplanting the initial pair bond, depended on how long the test animal was separated from their first partner. Prior bonding experience also shaped the subsequent effects of mating on partner preference. Partner preference strength was sensitive to latency to mate with the second partner but not the first partner, irrespective of separation time. These results suggest that the ability to form a consistent, strong preference for a new partner after an initial pair bond depends upon the amount of time that has passed since separation from the first partner. These results provide valuable insight into how social bonds are dynamically shaped by prior social experience and identify variables that contribute to recovery from partner loss and the ability to form a new pair bond. They also delineate a behavioral trajectory essential for future work examining the hormonal and genetic changes that enable recovery from partner loss.

Neuroendocrine mechanisms of grief and bereavement: A systematic review and implications for future interventions

Bereavement is associated with many negative behavioural, psychological and physiological consequences and leads to an increased risk of mortality and morbidity. However, studies specifically examining neuroendocrine mechanisms of grief and bereavement have yet to be reviewed. This systematic review is a synthesis of the latest evidence in this field and aims to draw conclusions about the implications of neurobiological findings on the development of new interventions. PRISMA guidelines for systematic reviews were used to search for articles assessing neuroendocrine correlates of grief. Findings were qualitatively summarised. The National Heart, Lung, and Blood Institute Study Assessment Tool was used to assess the quality of the included studies. Out of 460 papers, 20 met the inclusion criteria. However, most were of fair quality only. As a neuroendocrine marker, the majority of the studies reported cortisol as the outcome measure and found elevated mean cortisol levels, flattened diurnal cortisol slopes and higher morning cortisol in bereaved subjects. Cortisol alterations were moderated by individual differences such as emotional reaction to grief, depressive symptoms, grief severity, closeness to the deceased and age or gender. Research on neuroendocrine mechanisms of grief is still in its early stages regarding grief measures and the use and timing of neuroendocrine assessments. Most of the studies focus on cortisol as outcome, and only limited data exist on other biomarkers such as oxytocin. Future research might consider assessing a broader range of neuroendocrine markers and use longitudinal designs with a focus on the psychobiological reactions to loss. Based on this, individually tailored psychosocial interventions, possibly in the palliative care context, might be developed to prevent prolonged grief disorder.

The role of oxidative stress in cardiovascular disease caused by social isolation and loneliness

Loneliness and social isolation are common sources of chronic stress in modern society. Epidemiological studies have demonstrated that loneliness and social isolation increase mortality risk as much as smoking or alcohol consumption and more than physical inactivity or obesity. Loneliness in human is associated with higher blood pressure whereas enhanced atherosclerosis is observed in animal models of social isolation. Loneliness and social isolation lead to activation of the hypothalamic-pituitary-adrenocortical (HPA) axis, enhanced sympathetic nerve activity, impaired parasympathetic function and a proinflammatory immune response. These mechanisms have been implicated in the development of cardiovascular disease conferred by social isolation although a causal relationship has not been established so far. There is evidence that oxidative stress is likely to be a key molecular mechanism linking chronic psychosocial stress to cardiovascular disease. NADPH oxidase-mediated oxidative stress in the hypothalamus has been shown to be required for social isolation-induced HPA axis activation in socially isolated rats. Oxidative stress in the rostral ventrolateral medulla is also a key regulator of sympathetic nerve activity. In the vasculature, oxidative stress increases vascular tone and promote atherogenesis through multiple mechanisms. Thus, preventing oxidative stress may represent a therapeutic strategy to reduce the detrimental effects of social stress on health.

Physiological and behavioral responses to observing a sibling experience a direct stressor in prairie voles

Uncontrollable stress precipitates negative mental and physical health outcomes. Furthermore, the vicarious experience of stress (e.g. observing another individual experience a direct stressor) can mimic the effects of directly experiencing the stressor. The current experiment examined the behavioral and physiological effects of the vicarious experience of stress using the socially monogamous prairie vole. Male prairie voles were exposed to either an empty open field chamber, or a chamber in which the animal observed a sibling undergoing a concurrent direct physical stressor (tail suspension test) for five minutes. Exploratory and anxiety-like behaviors were recorded in all observers during the test session. Cardiac indices of heart rate and heart rate variability were recorded in a subset of observers prior to, during, and following the test session. Corticosterone levels were measured in all observers and siblings following the test session. When compared to animals exposed to an empty open field chamber, animals that observed a sibling undergo a direct physical stressor exhibited increased heart rate and circulating corticosterone, and decreased heart rate variability. These physiological stress indicators were supported by behavioral changes, including increased freezing followed immediately by orienting of the head toward the center of the apparatus, and decreased locomotion, grooming, and rearing. These preliminary results suggest that prairie voles experience stress vicariously, and provide a foundation for additional studies focused on the underlying mechanisms of vicarious stress. The use of this model may inform our understanding of the social transmission of stress among social species, including humans.LAY SUMMARYThe experience of stress, including observing stress in a loved one, has negative consequences on mental and physical health. This study used a social rodent (prairie voles) to demonstrate that stress transfers among social individuals, consequently producing an increased physiological and behavioral stress response in prairie voles observing their siblings experience stress. This research informs our understanding of the interactions of social experiences and stress in humans.

Stress in groups: Lessons from non-traditional rodent species and housing models

A major feature of life in groups is that individuals experience social stressors of varying intensity and type. Social stress can have profound effects on health, social behavior, and ongoing relationships. Relationships can also buffer the experience of exogenous stressors. Social stress has most commonly been investigated in dyadic contexts in mice and rats that produce intense stress. Here we review findings from studies of diverse rodents and non-traditional group housing paradigms, focusing on laboratory studies of mice and rats housed in visible burrow systems, prairie and meadow voles, and mole-rats. We argue that the use of methods informed by the natural ecology of rodent species provides novel insights into the relationship between social stress, behavior and physiology. In particular, we describe how this ethologically inspired approach reveals how individuals vary in their experience of and response to social stress, and how ecological and social contexts impact the effects of stress. Social stress induces adaptive changes, as well as long-term disruptive effects on behavior and physiology.

Protective neuroendocrine effects of environmental enrichment and voluntary exercise against social isolation: evidence for mediation by limbic structures

Previous research indicates that loneliness and social isolation may contribute to behavioral disorders and neurobiological dysfunction. Environmental enrichment (EE), including both cognitive and physical stimulation, may prevent some behavioral, endocrine, and cardiovascular consequences of social isolation; however, specific neural mechanisms for these benefits are still unclear. Therefore, this study examined potential neuroendocrine protective effects of both EE and exercise. Adult female prairie voles were randomly assigned to one of four experimental conditions: paired control, social isolation/sedentary, social isolation/EE, and social isolation/voluntary exercise. All isolated animals were housed individually for 8 weeks, while paired animals were housed with their respective sibling for 8 weeks. Animals in the EE and voluntary exercise conditions received EE items (including a running wheel) and a running wheel only, respectively, at week 4 of the isolation period. At the end of the experiment, plasma and brains were collected from all animals for corticosterone and FosB and delta FosB (FosB/ΔFosB) - immunoreactivity in stress-related brain regions. Overall, social isolation increased neuroendocrine stress responses, as reflected by the elevation of corticosterone levels and increased FosB/ΔFosB-immunoreactivity in the basolateral amygdala (BLA) compared to paired animals; EE and voluntary exercise attenuated these increases. EE and exercise also increased FosB/ΔFosB-immunoreactivity in the medial prefrontal cortex (mPFC) compared to other conditions. Limbic structures statistically mediated hypothalamic immunoreactivity in EE and exercise animals. This research has translational value for socially isolated individuals by informing our understanding of neural mechanisms underlying responses to social stressors. Highlights Prolonged social isolation increased basal corticosterone levels and basolateral amygdala immunoreactivity. Environmental enrichment and exercise buffered corticosterone elevations and basolateral amygdala hyperactivity. Protective effects of environmental enrichment and exercise may be mediated by medial prefrontal cortex and limbic structures.

The negative effects of social bond disruption are partially ameliorated by sertraline administration in prairie voles

Negative social experiences influence both depression and cardiovascular dysfunction. Many individuals who experience negative mood states or cardiovascular conditions have limited social support. Therefore, investigation of drug treatments that may protect against the consequences of social stress will aid in designing effective treatment strategies. The current study used an animal model to evaluate the protective effect of sertraline administration on behavioral and cardiovascular consequences of social stress. Specifically, male prairie voles (Microtus ochrogaster), which are socially monogamous rodents that share several behavioral and physiological characteristics with humans, were isolated from a socially-bonded female partner, and treated with sertraline (16 mg/kg/day, ip) or vehicle during isolation. Unexpectedly, sertraline did not protect against depression-relevant behaviors, and it was associated with increased short- and long-term heart rate responses. However, sertraline administration improved heart rate variability recovery following a behavioral stressor, including increased parasympathetic regulation, and altered long-term neuronal activity in brain regions that modulate autonomic control and stress reactivity. These results indicate that sertraline may partially protect against the consequences of social stressors, and suggest a mechanism through which sertraline may beneficially influence neurobiological control of cardiac function.

Neural mechanisms of social homeostasis

Social connections are vital to survival throughout the animal kingdom and are dynamic across the life span. There are debilitating consequences of social isolation and loneliness, and social support is increasingly a primary consideration in health care, disease prevention, and recovery. Considering social connection as an "innate need," it is hypothesized that evolutionarily conserved neural systems underlie the maintenance of social connections: alerting the individual to their absence and coordinating effector mechanisms to restore social contact. This is reminiscent of a homeostatic system designed to maintain social connection. Here, we explore the identity of neural systems regulating "social homeostasis." We review findings from rodent studies evaluating the rapid response to social deficit (in the form of acute social isolation) and propose that parallel, overlapping circuits are engaged to adapt to the vulnerabilities of isolation and restore social connection. By considering the neural systems regulating other homeostatic needs, such as energy and fluid balance, we discuss the potential attributes of social homeostatic circuitry. We reason that uncovering the identity of these circuits/mechanisms will facilitate our understanding of how loneliness perpetuates long-term disease states, which we speculate may result from sustained recruitment of social homeostatic circuits.

Wheel access has opposing effects on stress physiology depending on social environment in female prairie voles (Microtus ochrogaster)

Physical exercise and chronic social stress are both known to impact general health and hypothalamic-pituitary-adrenal (HPA) axis function, albeit typically in opposing directions. Therefore, the question we investigated in this study was how these two factors - physical exercise and chronic social isolation - would interact when presented simultaneously in a female rodent model. Adult female prairie voles were separated into four experimental groups: (1) isolated without wheel access, (2) isolated with wheel access, (3) paired without wheel access, and (4) paired with wheel access. Plasma, hair, and adrenal glands were sampled to investigate changes in stress physiology. Our results indicate that, when isolated, wheel access had a mitigating effect on HPA activity. However, in paired animals, wheel access had the opposite effect, as both adrenal mass and increase in hair corticosterone concentrations were greater in paired animals with wheel access. Strong correlations were detected between change in hair corticosterone and adrenal mass, while no correlations were found between plasma corticosterone and either of the other markers. These results imply that the HPA axis is highly sensitive to both the social environment and the physical demands placed on the individual, and that when investigating the effects of chronic isolation, both hair corticosterone and adrenal mass may be more reliable markers than a single plasma corticosterone sample.

Behavioral and cardiovascular consequences of disrupted oxytocin communication in cohabitating pairs of male and female prairie voles

Negative social experiences may influence psychological and physiological health via altered central oxytocin communication. The prairie vole is valuable for investigating the potential influence of oxytocin on responses to social experiences. Prairie voles are socially monogamous, live in pairs or family groups, and respond negatively to changes in the social environment. This study investigated the hypothesis that disruptions of oxytocin in one prairie vole of a cohabitating male-female pair would alter social behavior in that specific animal; and these behavioral changes in turn would influence the untreated partner's behavior and physiology. Pharmacological antagonism of oxytocin with the receptor antagonist L-368,899 in the male prairie vole disrupted social behaviors between the male and his untreated female partner. This manipulation also negatively influenced the behavior and cardiovascular function in the untreated female partner, including increased: (a) depression-relevant behaviors in two behavioral stressors, (b) basal mean arterial pressure and heart rate, and (c) cardiovascular reactivity to the behavioral stressors. These results suggest that disruptions of oxytocin and social behavior in one animal may produce indicators of social stress in an untreated social partner. This preliminary research provides a foundation for future studies to investigate mechanisms underlying responses to social experiences in humans.

Neural Circuits Underlying Rodent Sociality: A Comparative Approach

All mammals begin life in social groups, but for some species, social relationships persist and develop throughout the course of an individual's life. Research in multiple rodent species provides evidence of relatively conserved circuitry underlying social behaviors and processes such as social recognition and memory, social reward, and social approach/avoidance. Species exhibiting different complex social behaviors and social systems (such as social monogamy or familiarity preferences) can be characterized in part by when and how they display specific social behaviors. Prairie and meadow voles are closely related species that exhibit similarly selective peer preferences but different mating systems, aiding direct comparison of the mechanisms underlying affiliative behavior. This chapter draws on research in voles as well as other rodents to explore the mechanisms involved in individual social behavior processes, as well as specific complex social patterns. Contrasts between vole species exemplify how the laboratory study of diverse species improves our understanding of the mechanisms underlying social behavior. We identify several additional rodent species whose interesting social structures and available ecological and behavioral field data make them good candidates for study. New techniques and integration across laboratory and field settings will provide exciting opportunities for future mechanistic work in non-model species.

Social environment affects central distribution of estrogen receptor-α in Peromyscus californicus

Social environment has well-established effects on an animal's social behavior and associated neuroendocrine responses. The presence of estrogen receptor alpha (ERα) in limbic system brain regions is related to the expression of a variety of social, reproductive and aggressive behaviors. We hypothesized that alterations to the social environment, specifically social isolation, would cause changes in ERα throughout the limbic system. The number of ERα immunoreactive (ERα-ir) cells within specific limbic system brain regions was quantified in male and female California mice (Peromyscus californicus), isolated or same sex pair-housed for 4 or 24 days. Peromyscus californicus is a highly social rodent species (monogamous and bi-parental) and therefore, may be particularly sensitive to manipulations of its social environment. Isolated males had a significantly greater number of ERα-ir cells in the ventromedial nucleus of the hypothalamus (VMH) and similar patterns within the bed nucleus of the stria terminalis (BST) and medial preoptic area (MPOA). Males housed for 24 days had a significantly greater number of ERα-ir cells in the BST, VMH, MPOA when compared with males housed for 4 days. Females housed for 24 days had significantly greater ERα-ir in the dentate gyrus of the hippocampus (DG) when compared with females housed for 4 days. No differences were found in the medial amygdala (MeA). These data demonstrate that social environment has region and sex specific effects on ERα-ir cells in this species. These results add to the comparative evidence regarding ERα, demonstrating a consistent role for ERα in species specific responsiveness to changes in the social environment.