Physiological substrates of mammalian monogamy: the prairie vole model

Oxytocin receptor function regulates neural signatures of pair bonding and fidelity in the nucleus accumbens

The formation of enduring relationships dramatically influences future behavior, promoting affiliation between familiar individuals. How such attachments are encoded to elicit and reinforce specific social behaviors in distinct ethological contexts remains unknown. Signaling via the oxytocin receptor (Oxtr) in the nucleus accumbens (NAc) facilitates social reward as well as pair bond formation between mates in socially monogamous prairie voles 1-9 . How Oxtr function influences activity in the NAc during pair bonding to promote affiliative behavior with partners and rejection of other potential mates has not been determined. Using longitudinal in vivo fiber photometry in wild-type prairie voles and those lacking Oxtr, we demonstrate that Oxtr function sex-specifically regulates pair bonding behaviors and associated activity in the NAc. Oxtr function influences prosocial behavior in females in a state-dependent manner. Females lacking Oxtr demonstrate reduced prosocial behaviors and lower activity in the NAc during initial chemosensory investigation of novel males. Upon pair bonding, affiliative behavior with partners and neural activity in the NAc during these interactions increase, but these changes do not require Oxtr function. Conversely, males lacking Oxtr display increased prosocial investigation of novel females. Using the altered patterns of behavior and activity in the NAc of males lacking Oxtr during their first interactions with a female, we can predict their future preference for a partner or stranger days later. These results demonstrate that Oxtr function sex-specifically influences the early development of pair bonds by modulating prosociality and the neural processing of sensory cues and social interactions with novel individuals, unmasking underlying sex differences in the neural pathways regulating the formation of long-term relationships.

Effects of Oxytocin Receptor Agonism on Acquisition and Expression of Pair Bonding in Male Prairie Voles

There is much interest in targeting the activity in the oxytocin system to regulate social bonding. However, studies with exogenous administration of oxytocin face the caveats of its low stability, poor brain permeability and insufficient receptor specificity. The use of a small-molecule oxytocin receptor-specific agonist could overcome these caveats. Prior to testing the potential effects of a brain-penetrant oxytocin receptor agonist in clinical settings, it is important to assess how such an agonist would affect social bonds in animal models. The facultatively monogamous prairie voles (Microtus ochrogaster), capable of forming long-term social attachments between adult individuals, are an ideal rodent model for such testing. Therefore, in a series of experiments we investigated the effects of the recently developed oxytocin receptor-specific agonist LIT-001 on the acquisition and expression of partner preference, a well-established model of pair bonding, in prairie voles. LIT-001 (10 mg/kg, intraperitoneal), as expected, facilitated the acquisition of partner preference when administered prior to a 4-hour cohabitation. In contrast, while animals injected with vehicle after the 4-hour cohabitation exhibited significant partner preference, animals that were injected with LIT-001 did not show such partner preference. This result suggests that OXTR activation during expression of pair bonding can inhibit partner preference. The difference in effects of LIT-001 on acquisition versus expression was not due to basal differences in partner preference between the experiments, as LIT-001 had no significant effects on expression of partner preference if administered following a shorter (2 hour-long) cohabitation. Instead, this difference agrees with the hypothesis that the activation of oxytocin receptors acts as a signal of presence of a social partner. Our results indicate that the effects of pharmacological activation of oxytocin receptors crucially depend on the phase of social attachments.

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.

Chronic social defeat stress in prairie voles (Microtus ochrogaster): A preclinical model for the study of depression-related phenotypes

BACKGROUND

Stress-induced illnesses, like major depression, are among the leading causes of disability across the world. Consequently, there is a dire need for the validation of translationally-suited animal models incorporating social stress to uncover the etiology of depression. Prairie voles (Microtus ochrogaster) are more translationally relevant than many other rodent models as they display monogamous social and bi-parental behaviors. Therefore, we evaluated whether a novel social defeat stress (SDS) model in male prairie voles induces depression-relevant behavioral outcomes.

METHODS

Adult sexually-naïve male prairie voles experienced SDS bouts from a conspecific pair-bonded male aggressor, 10 min per day for 10 consecutive days. Non-stressed controls (same-sex siblings) were housed in similar conditions but never experienced physical stress. Twenty-four h later, voles were evaluated in social interaction, sucrose preference, and Morris water maze tests - behavioral endpoints validated to assess social withdrawal, anhedonia-related behavior, and spatial memory performance, respectively.

RESULTS

SDS-exposed voles displayed lower sociability and body weight, decreased preference for a sucrose solution, and impairment of spatial memory retrieval. Importantly, no differences in general locomotor activity were observed as a function of SDS exposure.

LIMITATIONS

This study does not include female voles in the experimental design.

CONCLUSIONS

We found that repeated SDS exposure, in male prairie voles, results in a depression-relevant phenotype resembling an anhedonia-like outcome (per reductions in sucrose preference) along with social withdrawal and spatial memory impairment - highlighting that the prairie vole is a valuable model with potential to study the neurobiology of social stress-induced depression-related outcomes.

Sexual coordination in a whole-brain map of prairie vole pair bonding

Sexual bonds are central to the social lives of many species, including humans, and monogamous prairie voles have become the predominant model for investigating such attachments. We developed an automated whole-brain mapping pipeline to identify brain circuits underlying pair-bonding behavior. We identified bonding-related c-Fos induction in 68 brain regions clustered in seven major brain-wide neuronal circuits. These circuits include known regulators of bonding, such as the bed nucleus of the stria terminalis, paraventricular hypothalamus, ventral pallidum, and prefrontal cortex. They also include brain regions previously unknown to shape bonding, such as ventromedial hypothalamus, medial preoptic area, and the medial amygdala, but that play essential roles in bonding-relevant processes, such as sexual behavior, social reward, and territorial aggression. Contrary to some hypotheses, we found that circuits active during mating and bonding were largely sexually monomorphic. Moreover, c-Fos induction across regions was strikingly consistent between members of a pair, with activity best predicted by rates of ejaculation. A novel cluster of regions centered in the amygdala remained coordinated after bonds had formed, suggesting novel substrates for bond maintenance. Our tools and results provide an unprecedented resource for elucidating the networks that translate sexual experience into an enduring bond.

Identification of hippocampal area CA2 in hamster and vole brain

Prairie voles (Microtus ochrogaster) and Syrian, or golden, hamsters (Mesocricetus auratus) are closely related to mice (Mus musculus) and rats (Rattus norvegicus, for example) and are commonly used in studies of social behavior including social interaction, social memory, and aggression. The CA2 region of the hippocampus is known to play a key role in social memory and aggression in mice and responds to social stimuli in rats, likely owing to its high expression of oxytocin and vasopressin 1b receptors. However, CA2 has yet to be identified and characterized in hamsters or voles. In this study, we sought to determine whether CA2 could be identified molecularly in vole and hamster. To do this, we used immunofluorescence with primary antibodies raised against known molecular markers of CA2 in mice and rats to stain hippocampal sections from voles and hamsters in parallel with those from mice. Here, we report that, like in mouse and rat, staining for many CA2 proteins in vole and hamster hippocampus reveals a population of neurons that express regulator of G protein signaling 14 (RGS14), Purkinje cell protein 4 (PCP4) and striatal-enriched protein tyrosine phosphatase (STEP), which together delineate the borders with CA3 and CA1. These cells were located at the distal end of the mossy fiber projections, marked by the presence of Zinc Transporter 3 (ZnT-3) and calbindin in all three species. In addition to staining the mossy fibers, calbindin also labeled a layer of CA1 pyramidal cells in mouse and hamster but not in vole. However, Wolframin ER transmembrane glycoprotein (WFS1) immunofluorescence, which marks all CA1 neurons, was present in all three species and abutted the distal end of CA2, marked by RGS14 immunofluorescence. Staining for two stress hormone receptors-the glucocorticoid (GR) and mineralocorticoid (MR) receptors-was also similar in all three species, with GR staining found primarily in CA1 and MR staining enriched in CA2. Interestingly, although perineuronal nets (PNNs) are known to surround CA2 cells in mouse and rat, we found that staining for PNNs differed across species in that both CA2 and CA3 showed staining in voles and primarily CA3 in hamsters with only some neurons in proximal CA2 showing staining. These results demonstrate that, like in mouse, CA2 in voles and hamsters can be molecularly distinguished from neighboring CA1 and CA3 areas, but PNN staining is less useful for identifying CA2 in the latter two species. These findings reveal commonalities across species in molecular profile of CA2, which will facilitate future studies of CA2 in these species. Yet to be determined is how differences in PNNs might relate to differences in social behavior across species.

Comparative analysis of gonadal hormone receptor expression in the postnatal house mouse, meadow vole, and prairie vole brain

The socially monogamous prairie vole (Microtus ochrogaster) and promiscuous meadow vole (Microtus pennsylvanicus) are closely related, but only prairie voles display long-lasting pair bonds, biparental care, and selective aggression towards unfamiliar individuals after pair bonding. These social behaviors in mammals are largely mediated by steroid hormone signaling in the social behavior network (SBN) of the brain. Hormone receptors are reproducible markers of sex differences that can provide more information than anatomy alone and can even be at odds with anatomical dimorphisms. We reasoned that behaviors associated with social monogamy in prairie voles may emerge in part from unique expression patterns of steroid hormone receptors in this species, and that these expression patterns would be more similar across males and females in prairie than in meadow voles or the laboratory mouse. To obtain insight into steroid hormone signaling in the developing prairie vole brain, we assessed expression of estrogen receptor alpha (Esr1), estrogen receptor beta (Esr2), and androgen receptor (Ar) within the SBN, using in situ hybridization at postnatal day 14 in mice, meadow, and prairie voles. We found species-specific patterns of hormone receptor expression in the hippocampus and ventromedial hypothalamus, as well as species differences in the sex bias of these markers in the principal nucleus of the bed nucleus of the stria terminalis. These findings suggest the observed differences in gonadal hormone receptor expression may underlie species differences in the display of social behaviors.

Maladaptive cardiac and behavioral reactivity to repeated vicarious stress exposure in socially bonded male prairie vole siblings

Behaviors, emotions, and cardiovascular functions are influenced by stress. But these detrimental effects are not exclusive to an individual that directly experiences stress. Stress is also experienced vicariously through observation of another individual undergoing stress. The current study used the strong social bonds in socially monogamous prairie voles to determine effects of repeated vicarious stress on cardiac and behavioral outcomes. Male prairie voles were exposed to either a 5-minute open field chamber alone [separate (control)] or while concurrently witnessing their sibling undergo a tail-suspension stressor [concurrent (experimental)], repeated across 4 sessions. Cardiac responses in animals in the open field were evaluated for heart rate and heart rate variability prior to, during, and after each test session, and behaviors were evaluated for motion, exploration, stress reactivity, and anxiety-relevant behaviors during each test session. The concurrent condition (versus separate) displayed increased heart rate and reduced heart rate variability during repeated test sessions, and impaired recovery of these parameters following the test sessions. The pattern of disturbances suggests that both increased sympathetic and reduced parasympathetic influence contributed to the cardiac responses. Animals in the concurrent condition (versus separate) displayed disrupted rearing, grooming, and motion; reduced duration of center section exploration; and increased freezing responses across repeated test sessions. Collectively, cardiac and behavioral stress reactivity are increased as a function of vicarious stress in prairie voles, which are evident across repeated experiences of stress. These results inform our understanding of the experience of vicarious stress in social species, including humans.

SEX-SPECIFIC IMPACTS OF EARLY LIFE SLEEP DISRUPTION: ETHANOL SEEKING, SOCIAL INTERACTION, AND ANXIETY ARE DIFFERENTIALLY ALTERED IN ADOLESCENT PRAIRIE VOLES

Early life sleep is important for neuronal development and maturation. Using the highly social prairie vole rodent model, we have previously reported that early-life sleep disruption (ELSD) during the pre-weaning period postnatal day (P)14 to 21 results in adult interference with social bonding and increases ethanol consumption following a stressor. Furthermore, we have reported increased parvalbumin expression and reduced glutamatergic neurotransmission in cortical regions in adult prairie voles that experienced this paradigm. To understand the impact of ELSD on the lifespan, examination of an earlier time in life is necessary. Thus, the aim of the present study was to examine the behavioral outcomes of ELSD on adolescent prairie voles. Here we hypothesized that anxiety and reward related behaviors, as measured by light/dark box, 2-bottle choice and social interactions, would be negatively impacted by ELSD in adolescent male and female prairie voles. Male ELSD voles were no different from control voles in measures of anxiety and ethanol preference or consumption, but affiliative social interactions were significantly reduced. ELSD differentially impacted female prairie voles, with increased anxiety-like behavior and reductions in ethanol consumption compared to Controls, but no impact on ethanol preference or social interactions. Together, these results suggest both male and female prairie voles experience differential changes to reward seeking behaviors, but only female prairie voles showed increases in anxiety-like behavior. These results further suggest that early-life sleep is critically important for neurotypical behaviors in adolescence, a time where reward-seeking and risky behaviors are adaptive for learning and promoting survival.

Partner-seeking and limbic dopamine system are enhanced following social loss in male prairie voles (Microtus ochrogaster)

Death of a loved one is recognized as one of life's greatest stresses, and 10%-20% of bereaved individuals will experience a complicated or prolonged grieving period that is characterized by intense yearning for the deceased. The monogamous prairie vole (Microtus ochrogaster) is a rodent species that forms pair bonds between breeding partners and has been used to study the neurobiology of social behaviors and isolation. Male prairie voles do not display distress after isolation from a familiar, same-sex conspecific; however, separation from a bonded female partner increases emotional, stress-related, and proximity-seeking behaviors. Here, we tested the investigatory response of male voles to partner odor during a period of social loss. We found that males who lost their partner spent significantly more time investigating partner odor but not non-partner social odor or food odor. Bachelor males and males in intact pairings did not respond uniquely to any odor. Furthermore, we examined dopamine (DA) receptor mRNA expression in the anterior insula cortex (aIC), nucleus accumbens (NAc), and anterior cingulate (ACC), regions with higher activation in grieving humans. While we found some effects of relationship type on DRD1 and DRD2 expression in some of these regions, loss of a high-quality opposite-sex relationship had a significant effect on DA receptor expression, with pair-bonded/loss males having higher expression in the aIC and ACC compared with pair-bonded/intact and nonbonded/loss males. Together, these data suggest that both relationship type and relationship quality affect reunion-seeking behavior and motivational neurocircuits following social loss of a bonded partner.

Lack of evidence for coevolution between oxytocin receptor N-terminal variants and monogamy in placental mammals

Oxytocin (OXT) is a neurohypophyseal hormone that influences a wide range of affiliative behaviors, such as pair-bonding and infant care, across mammals. The effects of OXT depend significantly on an adequate interaction with its receptor, OXTR. OXTR belongs to the G-protein coupled receptor family. The extracellular N-terminal domain of OXTR interacts with the linear C-terminal tail of OXT and is required for OXT binding. Across mammalian species there is a genetic diversity in OXTR terminal sequence. Previous work on primates has shown an association between OXTR phylogeny and monogamy. However, it is not clear whether this variation coevolved with either mating system (monogamy) or infant care behaviors (such as allomaternal care). Here, we take a phylogenetic comparative and evolutionary modeling approach across a wide range of placental mammals (n = 60) to test whether OXTR N-terminal variants co-evolved with either monogamy or allomaternal care behaviors. Our results indicate that the diversity in OXTR N-terminal region is unlikely to provide the underlying genetic bases for variation in mating system and/or allomaternal behavior as we find no evidence for co-evolution between protein sequence and affiliative behaviors. Hence, the role played by OXT in influencing affiliative behaviors is unlikely to be mediated by the genetic diversity of its receptor.

Tales from the life and lab of a female social neuroscientist

This narrative review charts my unconventional path to becoming a social neuroscientist and describes my research findings - some baffling, some serendipitous, some pivotal - in the field of neuropeptide biology. I trace my childhood as a Bell Labs "brat" to my adolescence as a soccer-playing party girl, to my early days as a graduate student, when I first encountered oxytocin and vasopressin. These two molecules instantly captivated - and held - my attention and imagination. For more than 25 years, a core goal of my research program has been to better understand how these neuropeptides regulate social functioning across a range of species (e.g., meadow voles, mice, squirrel monkeys, rhesus monkeys, and humans), and to translate fundamental insights from this work to guide development of novel pharmacotherapies to treat social impairments in clinical populations. I also discuss my experience of being a woman and a mother in STEM, and identify the important people and events which helped shape my career and the scientist I am today.

Sexual coordination in a whole-brain map of prairie vole pair bonding

Sexual bonds are central to the social lives of many species, including humans, and monogamous prairie voles have become the predominant model for investigating such attachments. We developed an automated whole-brain mapping pipeline to identify brain circuits underlying pair-bonding behavior. We identified bonding-related c-Fos induction in 68 brain regions clustered in seven major brain-wide neuronal circuits. These circuits include known regulators of bonding, such as the bed nucleus of the stria terminalis, paraventricular hypothalamus, ventral pallidum, and prefrontal cortex. They also include brain regions previously unknown to shape bonding, such as ventromedial hypothalamus, medial preoptic area and the medial amygdala, but that play essential roles in bonding-relevant processes, such as sexual behavior, social reward and territorial aggression. Contrary to some hypotheses, we found that circuits active during mating and bonding were largely sexually monomorphic. Moreover, c-Fos induction across regions was strikingly consistent between members of a pair, with activity best predicted by rates of ejaculation. A novel cluster of regions centered in the amygdala remained coordinated after bonds had formed, suggesting novel substrates for bond maintenance. Our tools and results provide an unprecedented resource for elucidating the networks that translate sexual experience into an enduring bond.

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.

Close encounters with oxytocin

The purpose of this narrative review is to use a personal perspective to describe unanticipated and pivotal findings that drew the author into the study oxytocin. Oxytocin was originally described as a "female reproductive hormone." However, supporting reproduction is only one of a myriad of functions now attributed to oxytocin. Oxytocin promotes survival and resilience in both sexes and across the lifespan, especially in the context of stress or trauma and helps to explain the health benefits of relationships. Oxytocin works in the context of individual histories and in conjunction with other molecules, as well as the autonomic nervous system and immune factors. The chemical properties of oxytocin make it biologically active, but difficult to measure. As a deeper understanding of the biology of oxytocin is emerging, we may use knowledge of the properties of oxytocin to uncover adaptive strategies that protect and heal in the face of stress and adversity in both males and females.

A test of the social behavior network reveals differential patterns of neural responses to social novelty in bonded, but not non-bonded, male prairie voles

The social behavior network (SBN) has provided a framework for understanding the neural control of social behavior. The original SBN hypothesis proposed this network modulates social behavior and should exhibit distinct patterns of neural activity across nodes, which correspond to distinct social contexts. Despite its tremendous impact on the field of social neuroscience, no study has directly tested this hypothesis. Thus, we assessed Fos responses across the SBN of male prairie voles (Microtus ochrogaster). Virgin/non-bonded and pair bonded subjects were exposed to a sibling cagemate or pair bonded partner, novel female, novel male, novel meadow vole, novel object, or no stimulus. Inconsistent with the original SBN hypothesis, we did not find profoundly different patterns of neural responses across the SBN for different contexts, but instead found that the SBN generated significantly different patterns of activity in response to social novelty in pair bonded, but not non-bonded males. These findings suggest that non-bonded male prairie voles may perceive social novelty differently from pair bonded males or that SBN functionality undergoes substantial changes after pair bonding. This study reveals novel information about bond-dependent, context-specific neural responsivity in male prairie voles and suggests that the SBN may be particularly important for processing social salience. Further, our study suggests there is a need to reconceptualize the framework of how the SBN modulates social behavior.

Are there disciplinary boundaries in the comparative study of primate cognition?

A view continues to gain momentum that regards investigation of the cognition of great apes in captive settings as affording us a model for human cognitive evolution. Researchers from disciplines such as comparative psychology, anthropology, and even archaeology, seem eager to put their theories to the test by using great apes as their chosen experimental model. Questions addressed currently by comparative psychologists have long been the object of attention by neurophysiologists, psychobiologists and neuroscientists, who, however, often use rodents and monkeys as the species of choice. Whereas comparative psychology has been influenced greatly by ethology, much neuroscience has developed against a background of physiology and medicine. This separation of the intellectual contexts wherein they have arisen and flourished has impeded the development of fluid interaction between comparative psychologists and researchers in the other disciplines. We feel that it would be beneficial for comparative psychologists and neuroscientists to combine research endeavours far more often, in order to address common questions of interest related to cognition. We regard interdisciplinary cross-pollination to be particularly desirable, even if many comparative psychologists lack deep expertise about the workings of the brain, and even if many neuroscientists lack expert knowledge about the behaviour of different species. Furthermore, we believe that anthropology, archaeology, human evolutionary studies, and related disciplines, may well provide us with significant contextual knowledge about the physical and temporal background to the evolution in humans of specific cognitive skills. To that end, we urge researchers to dismantle methodological, conceptual and historical disciplinary boundaries, in order to strengthen cross-disciplinary cooperation in order to broaden and deepen our insights into the cognition of nonhuman and human primates.

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.

A CRISPR perspective of the oxytocin receptor in prairie voles

The oxytocin receptor has long been considered critical for social bonding and parenting in prairie voles. In this issue of Neuron, Berendzen et al.¹ show that oxytocin receptor-null prairie voles display normal bonding and parental behaviors, thus challenging the prevailing understanding of the receptor's role in these behaviors.

Wild brains: The value of neuroethological approaches in preclinical behavioral neuroscience animal models

For three decades, IBNS has provided a forum for the dissemination of behavioral neuroscience research, broadly defined. Throughout this time, research presented at the annual meetings has reflected representative trends in the field with an emphasis on relevant preclinical animal models. From its inception, IBNS has contributed to my professional development and evolving research interests. Unsurprisingly, throughout the three decades of its existence, IBNS annual programs have reflected research trends that have been thoughtfully evaluated, challenged, and, in some cases, recalibrated. An emphasis in my lab, for example, has slowly navigated toward the inclusion of more diverse species (e.g., nonhuman primate models, wild rats, wild and captive raccoons) assessed in settings that reflect more ethological relevance than typically observed in traditional laboratory settings. Consequently, my research interests are pivoting from laboratory animal model exclusive (L.A.M.E.) endeavors to more natural, diverse, ethoexperimental approaches. As progress toward translational findings for psychiatric and neurological conditions is considered, it is recommended that researchers remain open to nontraditional methodological approaches that incorporate diverse animal models and assessments to inform laboratory-generated findings.

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.

[How oxytocin became overtime the attachment-mediating hormone]

Oxytocin is a pleiotropic molecule which, in addition to its facilitating action during parturition and milk ejection, is involved in social and prosocial behaviors such as attachment. This article presents, after a brief historical review, the action of oxytocin during the milk ejection reflex. Oxytocin is indeed essential for this vital function in mammals. It is both a neurohormone released into the bloodstream by the axon terminals of the posterior pituitary and a neuromodulator released in the hypothalamus by the soma and dendrites of oxytocinergic magnocellular neurons. In addition, oxytocin is also released by the axon terminals of parvocellular neurons and axon collaterals of magnocellular neurons in the brain. Both maternal attachment in rats and ewes and attachment between sexual partners in the prairie vole, one of the few monogamous rodent species, are mediated by central oxytocin. However, neither administering oxytocin into the brain nor increasing expression of the oxytocin receptor in the nucleus accumbens using a gene transfer technique converts polygamous voles to monogamous ones. Unfortunately, translation of animal data to human remains problematic due to still unsolved difficulties in modifying the level of oxytocin in the brain.

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.

Sex, love and oxytocin: Two metaphors and a molecule

Dozens of studies, most conducted in the last four decades, have implicated oxytocin, as well as vasopressin and their receptors, in processes that mediate selective sociality and the consequences of early experience. Oxytocin is critical for the capacity to experience emotional safety and healthy sexuality. Oxytocin also plays a central role in almost every aspect of physical and mental health, including the coordination of sociality and loving relationships with physiological reactions to challenges across the lifespan. Species, including prairie voles, that share with humans the capacity for selective social bonds have been a particularly rich source of insights into the behavioral importance of peptides. The purpose of this historical review is to describe the discovery of a central role for oxytocin in behavioral interactions associated with love, and in the capacity to use sociality to anticipate and cope with challenges across the lifespan - a process that here is called "sociostasis."

Rethinking the Architecture of Attachment: New Insights into the Role for Oxytocin Signaling

Social attachments, the enduring bonds between individuals and groups, are essential to health and well-being. The appropriate formation and maintenance of social relationships depend upon a number of affective processes, including stress regulation, motivation, reward, as well as reciprocal interactions necessary for evaluating the affective state of others. A genetic, molecular, and neural circuit level understanding of social attachments therefore provides a powerful substrate for probing the affective processes associated with social behaviors. Socially monogamous species form long-term pair bonds, allowing us to investigate the mechanisms underlying attachment. Now, molecular genetic tools permit manipulations in monogamous species. Studies using these tools reveal new insights into the genetic and neuroendocrine factors that design and control the neural architecture underlying attachment behavior. We focus this discussion on the prairie vole and oxytocinergic signaling in this and related species as a model of attachment behavior that has been studied in the context of genetic and pharmacological manipulations. We consider developmental processes that impact the demonstration of bonding behavior across genetic backgrounds, the modularity of mechanisms underlying bonding behaviors, and the distributed circuitry supporting these behaviors. Incorporating such theoretical considerations when interpreting reverse genetic studies in the context of the rich ethological and pharmacological data collected in monogamous species provides an important framework for studies of attachment behavior in both animal models and studies of human relationships.

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.

What are oxytocin assays measuring? Epitope mapping, metabolites, and comparisons of wildtype & knockout mouse urine

Oxytocin has become a popular analyte in behavioral endocrinology in recent years, due in part to its roles in social behavior, stress physiology, and cognition. Urine samples have the advantage of being non-invasive and minimally disruptive to collect, allowing for oxytocin measurements even in some wild populations. However, methods for urinary oxytocin immunoassay have not been sufficiently optimized and rigorously assessed for their potential limitations. Using samples from oxytocin knockout (KO) and wildtype (WT) mice, we find evidence of considerable interference in unextracted urine samples, with similar distributions of measured oxytocin in both genotypes. Importantly, although this interference can be reduced by a reversed-phase solid-phase extraction (SPE), this common approach is not sufficient for eliminating false-positive signal on three immunoassay kits. To better understand the source of the observed interference, we conducted epitope mapping of the Arbor Assays antibody and assessed its cross-reactivity with known, biologically active fragments of oxytocin. We found considerable cross-reactivity (0.5-52% by-molarity) for three fragments of oxytocin that share the core epitope, with more cross-reactivity for longer fragments. Given the presence of some cross-reactivity for even the tripeptide MIF-1, it is likely that many small protein metabolites might be sufficiently similar to the epitope that at high concentrations they interfere with immunoassays. We present a new mixed-mode cation-exchange SPE method that minimizes interference-with knockout samples measuring below the assay's limit of detection-while effectively retaining oxytocin from the urine of wildtype mice. This method demonstrates good parallelism and spike recovery across multiple species (mice, dogs, sifakas, humans). Our results suggest that immunoassays of urine samples may be particularly susceptible to interference, even when using common extraction protocols, but that this interference can be successfully managed using a novel mixed-mode cation exchange extraction. These findings imply that previous conclusions based on urinary oxytocin measurements-especially those involving unextracted samples-may need to be reassessed.

Oxytocin, Vasopressin, and Social Behavior: From Neural Circuits to Clinical Opportunities

Oxytocin and vasopressin are peptide hormones secreted from the pituitary that are well known for their peripheral endocrine effects on childbirth/nursing and blood pressure/urine concentration, respectively. However, both peptides are also released in the brain, where they modulate several aspects of social behaviors. Oxytocin promotes maternal nurturing and bonding, enhances social reward, and increases the salience of social stimuli. Vasopressin modulates social communication, social investigation, territorial behavior, and aggression, predominantly in males. Both peptides facilitate social memory and pair bonding behaviors in monogamous species. Here we review the latest research delineating the neural circuitry of the brain oxytocin and vasopressin systems and summarize recent investigations into the circuit-based mechanisms modulating social behaviors. We highlight research using modern molecular genetic technologies to map, monitor activity of, or manipulate neuropeptide circuits. Species diversity in oxytocin and vasopressin effects on social behaviors are also discussed. We conclude with a discussion of the translational implications of oxytocin and vasopressin for improving social functioning in disorders with social impairments, such as autism spectrum disorder.

Clomiphene citrate treatment during perinatal development alters adult partner preference, mating behaviour and androgen receptor and vasopressin in the male mandarin vole Microtus mandarinus

During development, many aspects of behaviour, including partner preferences and sexual behaviour, are "organized" by neural aromatization of androgen and oestrogen. This study aimed to analyse these processes in the mandarin vole (Microtus mandarinus); this is a novel mammalian model exhibiting strong monogamous pair bonds. Male pups were treated daily with a sesame oil only (MC) or the oestrogen receptor blocker-clomiphene citrate sesame oil mixture (MT) from prenatal day 14 to postnatal day 10. Female pups were treated daily with sesame oil only (FC). Partner preferences, sexual behaviour, and the expression of androgen receptor (AR) and arginine vasopressin (AVP) were examined when animals were 3 months old. The MT and FC groups exhibited male-directed partner preferences and feminized behaviour. AR-immunoreactive neurons (AR-IRs) in the medial preoptic area (mPOA), bed nucleus of stria terminalis (BNST), and medial amygdaloid nucleus (MeA) were reduced in MT males compared to MC males, and there was no significant difference in the number of AR-IRs between MT males and FC females. AVP-immunoreactive neurons (AVP-IRs) in the paraventricular nucleus (PVN) and supraoptic nucleus (SON) were reduced in MT males compared to MC males, and there were no significant differences in the number of AVP-IRs between MT males and FC females. The results indicate a significant role of hormone signalling in the development of male mate preference in the novel monogamous mammal model.

Intranasal oxytocin reduces pre-courtship aggression and increases paternal response in California mice (Peromyscus californicus)

Oxytocin (OXT) is a neuropeptide that can facilitate prosocial behavior and decrease social stress and anxiety but can also increase aggression in some contexts. We investigated whether acute pulses of intranasal (IN) OXT influenced social behavior during social challenges that are likely to occur throughout the lifespan of a wild mouse. To test this, we examined the acute effects of IN OXT in the male California mouse (Peromyscus californicus), a monogamous, biparental, and territorial rodent, using a within-subjects longitudinal design. Social challenges included a pre-courtship male-female encounter conducted during the (1) initial aggressive and not the following affiliative phase of courtship, (2) same-sex resident intruder test, and (3) parental care test. Consecutive tests and doses were separated by at least two weeks. Males were treated with intranasal infusions of 0.8 IU/kg OXT or saline controls 5-min before each behavioral test, receiving a total of three treatments of either IN OXT or saline control. We predicted that IN OXT would 1) decrease aggression and increase affiliation during the pre-courtship aggression phase, 2) increase aggression during resident intruder paradigms, and 3) increase paternal care and vocalizations during a paternal care test. As predicted, during pre-courtship aggression with a novel female, IN OXT males displayed less contact aggression than control males, although with no change in affiliative behavior. However, post-pairing, during the resident intruder test, IN OXT males did not differ from control males in contact aggression. During the paternal care test, IN OXT males were quicker to approach their pups than control males but did not differ in vocalizations produced, unlike our previous research demonstrating an effect on vocalizations in females. In summary, during pre-courtship aggression and the paternal care test, IN OXT reduced antisocial behavior; however, during the resident intruder test, IN OXT did not alter antisocial behavior. These data suggest that IN OXT promotes prosocial behavior specifically in social contexts that can lead to affiliation.

Social isolation and oxytocin antagonism increase emotion-related behaviors and heart rate in female prairie voles

Social isolation influences depression- and anxiety-related disorders and cardiac function. Oxytocin may mediate these conditions through interactions with social behavior, emotion, and cardiovascular function, via central and/or peripheral mechanisms. The present study investigated the influence of oxytocin antagonism using L-368,899, a selective oxytocin receptor antagonist that crosses the blood-brain barrier, on depression- and anxiety-related behaviors and heart rate in prairie voles. This rodent species has translational value for investigating interactions of social stress, behavior, cardiac responses, and oxytocin function. Adult female prairie voles were socially isolated or co-housed with a sibling for 4 weeks. A subset of animals in each housing condition was subjected to 4 sessions of acute L-368,899 (20 mg/kg, ip) or saline administration followed by a depression- or anxiety-related behavioral assessment. A subset of co-housed animals was evaluated for cardiac function following acute administration of L-368,899 (20 mg/kg, ip) and during behavioral assessments. Social isolation (vs. co-housing) increased depression- and anxiety-related behaviors. In isolated animals, L-368,899 (vs. vehicle) did not influence anxiety-related behaviors but exacerbated depression-related behaviors. In co-housed animals, L-368,899 exacerbated depression-related behaviors and increased heart rate at baseline and during behavioral tests. Social isolation produces emotion-related behaviors in prairie voles; central and/or peripheral oxytocin antagonism exacerbates these behavioral signs. Oxytocin antagonism induces depression-relevant behaviors and increases basal and stressor-reactive heart rate in co-housed prairie voles, similar to the consequences of social isolation demonstrated in this model. These results provide translational value for humans who experience behavioral and cardiac consequences of loneliness or social stress.

Effects of a D2 receptor antagonist on repeated pair bond formation in the male prairie vole

Repeated formation and subsequent dissolution of romantic relationships is common in humans across a lifetime. The socially monogamous prairie vole (Microtus ochrogaster) is used to study mechanisms of these bonds. At least in the laboratory, male prairie voles form bonds with a new female partner after loss of a previous partner. Initial bond formation depends on activation of dopamine D2-like receptors in the nucleus accumbens. Blocking activity of this receptor subtype disrupts formation of an animal's first pair bond. It is not known if these same D2-like receptors facilitate pair bonding with a subsequent partner after previous partner loss. This study examined the effects of D2-like receptor blockade on repeated pair bonding in male prairie voles. Males were paired with an initial female and allowed to mate before being separated. After a 5-day separation, males were then treated with either saline or eticlopride, a selective D2-receptor antagonist, prior to being paired with a second female and being allowed to mate. After a second separation, males were tested to determine if they developed a preference for spending time with their first or second mate. Eticlopride-treated males spent more time in a cage containing one of their previous partners compared to time in an empty cage but did not form a selective preference for either partner. Saline-treated males preferred their second, more recent partner. D2 receptor antagonism, then, disrupts bond formation in a second pairing but does not help to maintain a bond with the initial partner.

Reciprocal processes of sensory perception and social bonding: an integrated social-sensory framework of social behavior

Organisms filter the complexity of natural stimuli through their individual sensory and perceptual systems. Such perceptual filtering is particularly important for social stimuli. A shared "social umwelt" allows individuals to respond appropriately to the expected diversity of cues and signals during social interactions. In this way, the behavioral and neurobiological mechanisms of sociality and social bonding cannot be disentangled from perceptual mechanisms and sensory processing. While a degree of embeddedness between social and sensory processes is clear, our dominant theoretical frameworks favor treating the social and sensory processes as distinct. An integrated social-sensory framework has the potential to greatly expand our understanding of the mechanisms underlying individual variation in social bonding and sociality more broadly. Here we leverage what is known about sensory processing and pair bonding in two common study systems with significant species differences in their umwelt (rodent chemosensation and avian acoustic communication). We primarily highlight that (1) communication is essential for pair bond formation and maintenance, (2) the neural circuits underlying perception, communication and social bonding are integrated, and (3) candidate neuromodulatory mechanisms that regulate pair bonding also impact communication and perception. Finally, we propose approaches and frameworks that more fully integrate sensory processing, communication, and social bonding across levels of analysis: behavioral, neurobiological, and genomic. This perspective raises two key questions: (1) how is social bonding shaped by differences in sensory processing?, and (2) to what extent is sensory processing and the saliency of signals shaped by social interactions and emerging relationships?

Emergent intra-pair sex differences and organized behavior in pair bonded prairie voles (Microtus ochrogaster)

In pair bonding animals, coordinated behavior between partners is required for the pair to accomplish shared goals such as raising young. Despite this, experimental designs rarely assess the behavior of both partners within a bonded pair. Thus, we lack an understanding of the interdependent behavioral dynamics between partners that likely facilitate relationship success. To identify intra-pair behavioral correlates of pair bonding, we used socially monogamous prairie voles (Microtus ochrogaster) and tested both partners using social choice and non-choice tests at short- and long-term pairing timepoints. Females developed a preference for their partner more rapidly than males, with preference driven by different behaviors in each sex. Further, as bonds matured, intra-pair behavioral sex differences and organized behavior emerged-females consistently huddled more with their partner than males did regardless of overall intra-pair affiliation levels. When animals were allowed to freely interact with a partner or a novel vole in sequential free interaction tests, pairs spent more time interacting together than either animal did with a novel vole, consistent with partner preference in the more commonly employed choice test. Total pair interaction in freely moving voles was correlated with female, but not male, behavior. Via a social operant paradigm, we found that pair-bonded females, but not males, are more motivated to access and huddle with their partner than a novel vole. Together, our data indicate that as pair bonds mature, sex differences and organized behavior emerge within pairs, and that these intra-pair behavioral changes are likely organized and driven by the female animal.

Individual differences in social attachment: A multi-disciplinary perspective

Social behavior varies across both individuals and species. Research to explain this variation falls under the purview of multiple disciplines, each with its own theoretical and empirical traditions. Integration of these disciplinary traditions is key to developing a holistic perspective. Here, we review research on the biology of social attachment, a phenomena in which individuals develop strong affective connections to one another. We provide a historical overview of research on social attachment from psychological, ethological and neurobiological perspectives. As a case study, we describe work on pair-bonding in prairie voles, a socially monogamous rodent. This specific topic takes advantage of many biological perspectives and techniques to explain social bonds. Lastly, we conclude with an overview of multi-dimensional conceptual frameworks that can be used to explain social phenomena, and we propose a new framework for research on individual variation in attachment behavior. These conceptual frameworks originate from philosophy, physics, ethology, cognitive science and neuroscience. The application and synthesis of such frameworks offers a rich opportunity to advance understanding of social behavior and its mechanisms.

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.

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.

Transcriptomic Regulations Underlying Pair-bond Formation and Maintenance in the Socially Monogamous Male and Female Prairie Vole

BACKGROUND

The ability to form enduring social bonds is characteristic of human nature, and impairments in social affiliation are central features of severe neuropsychiatric disorders including autism spectrum disorder and schizophrenia. Owing to its ability to form long-term pair-bonds, the socially monogamous prairie vole has emerged as an excellent model to study the neurobiology of social attachment. Despite the enduring nature of the bond, however, surprisingly few genes have been implicated in the pair-bonding process in either sex.

METHODS

Male and female prairie voles (Microtus ochrogaster) were cohabitated with an opposite-sex partner for 24 hours or 3 weeks, and transcriptomic regulations in the nucleus accumbens were measured by RNA sequencing.

RESULTS

We found sex-specific response patterns despite similar behavioral indicators of pair-bond establishment. Indeed, 24 hours of cohabitation with an opposite-sex partner induced widespread transcriptomic changes that remained sustained to some extent in females after 3 weeks but returned to baseline before a second set of regulations in males. This led to a highly sexually biased nucleus accumbens transcriptome at 3 weeks related to processes such as neurotransmission, protein turnover, and DNA transcription. In particular, we found sex-specific alterations of mitochondrial dynamics following cohabitation, with a shift toward fission in males.

CONCLUSIONS

In addition to identifying the genes, networks, and pathways involved in the pair-bonding process in the nucleus accumbens, our work illustrates the vast extent of sex differences in the molecular mechanisms underlying pair-bonding in prairie voles and paves the way to further our understanding of the complex social bonding process.

Social selectivity and social motivation in voles

Selective relationships are fundamental to humans and many other animals, but relationships between mates, family members, or peers may be mediated differently. We examined connections between social reward and social selectivity, aggression, and oxytocin receptor signaling pathways in rodents that naturally form enduring, selective relationships with mates and peers (monogamous prairie voles) or peers (group-living meadow voles). Female prairie and meadow voles worked harder to access familiar versus unfamiliar individuals, regardless of sex, and huddled extensively with familiar subjects. Male prairie voles displayed strongly selective huddling preferences for familiar animals, but only worked harder to repeatedly access females versus males, with no difference in effort by familiarity. This reveals a striking sex difference in pathways underlying social monogamy and demonstrates a fundamental disconnect between motivation and social selectivity in males-a distinction not detected by the partner preference test. Meadow voles exhibited social preferences but low social motivation, consistent with tolerance rather than reward supporting social groups in this species. Natural variation in oxytocin receptor binding predicted individual variation in prosocial and aggressive behaviors. These results provide a basis for understanding species, sex, and individual differences in the mechanisms underlying the role of social reward in social preference.

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?

Pair Bond-Induced Affiliation and Aggression in Male Prairie Voles Elicit Distinct Functional Connectivity in the Social Decision-Making Network

Complex social behaviors are governed by a neural network theorized to be the social decision-making network (SDMN). However, this theoretical network is not tested on functional grounds. Here, we assess the organization of regions in the SDMN using c-Fos, to generate functional connectivity models during specific social interactions in a socially monogamous rodent, the prairie voles (Microtus ochrogaster). Male voles displayed robust selective affiliation toward a female partner, while exhibiting increased threatening, vigilant, and physically aggressive behaviors toward novel males and females. These social interactions increased c-Fos levels in eight of the thirteen brain regions of the SDMN. Each social encounter generated a distinct correlation pattern between individual brain regions. Thus, hierarchical clustering was used to characterize interrelated regions with similar c-Fos activity resulting in discrete network modules. Functional connectivity maps were constructed to emulate the network dynamics resulting from each social encounter. Our partner functional connectivity network presents similarities to the theoretical SDMN model, along with connections in the network that have been implicated in partner-directed affiliation. However, both stranger female and male networks exhibited distinct architecture from one another and the SDMN. Further, the stranger-evoked networks demonstrated connections associated with threat, physical aggression, and other aversive behaviors. Together, this indicates that distinct patterns of functional connectivity in the SDMN can be detected during select social encounters.

FireMaster® 550 (FM 550) exposure during the perinatal period impacts partner preference behavior and nucleus accumbens core medium spiny neuron electrophysiology in adult male and female prairie voles, Microtus ochrogaster

One of the most widely used flame retardant (FR) mixtures in household products is Firemaster 550 (FM 550). FM 550 leaches from items such as foam-based furniture and infant products, resulting in contamination of the household environment and biota. Previous studies indicate sex-specific behavioral deficits in rodents and zebrafish in response to developmental FM 550 exposure. These deficits include impacts on social and attachment behaviors in a prosocial rodent: the prairie vole (Microtus ochrogaster). The prairie vole is a laboratory-acclimated rodent that exhibits spontaneous attachment behaviors including pair bonding. Here we extend previous work by addressing how developmental exposure to FM 550 impacts pair bonding strength via an extended-time partner preference test, as well as neuron electrophysiological properties in a region implicated in pair bond behavior, the nucleus accumbens (NAcc) core. Dams were exposed to vehicle or 1000 μg of FM 550 via subcutaneous injections throughout gestation, and female and male pups were directly exposed beginning the day after birth until weaning. Pair bond behavior of adult female and male offspring was assessed using a three hour-long partner preference test. Afterwards, acute brain slices of the NAcc core were produced and medium spiny neuron electrophysiological attributes recorded via whole cell patch-clamp. Behavioral impacts were sex-specific. Partner preference behavior was increased in exposed females but decreased in exposed males. Electrophysiological impacts were similar between sexes and specific to attributes related to input resistance. Input resistance was decreased in neurons recorded from both sexes exposed to FM 550 compared to vehicle. This study supports the hypothesis that developmental exposure to FM 550 impacts attachment behaviors and demonstrates a novel FM 550 effect on neural electrophysiology.

Intranasal oxytocin may help maintain romantic bonds by decreasing jealousy evoked by either imagined or real partner infidelity

BACKGROUND

While romantic jealousy may help to maintain relationships, following partner infidelity and an irretrievable loss of trust it can also promote break-ups. The neuropeptide oxytocin can enhance the maintenance of social bonds and reduce couple conflict, although its influence on jealousy evoked by imagined or real infidelity is unclear.

AIMS

This study aimed to investigate the effects of intranasal oxytocin (24 IU) on romantic jealousy in both males and females in imagined and real contexts.

METHODS

Seventy heterosexual couples participated in this double-blind, placebo-controlled, between-subject design study. Jealousy was firstly quantified in the context of subjects imagining partner infidelity and secondly in a Cyberball game where their partner interacted preferentially with an opposite-sexed rival stranger to simulate partner exclusion, or rejected a neutral stranger but not the partner.

RESULTS

Oxytocin primarily decreased jealousy and arousal ratings towards imagined emotional and sexual infidelity by a partner in both sexes. During the Cyberball game, while male and female subjects in both groups subsequently threw the ball least often to the rival stranger, under oxytocin they showed reduced romantic jealousy and arousal ratings for stranger players, particularly the rival one, and reported reduced negative and increased positive feelings while playing the game.

CONCLUSIONS

Together, our results suggest that oxytocin can reduce the negative emotional impact of jealousy in established romantic partners evoked by imagined or real infidelity or exclusive social interactions with others. This provides further support for oxytocin promoting maintenance of relationships.

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.

Female-female reproductive suppression: impacts on signals and behavior

Female-female reproductive suppression is evident in an array of mammals, including rodents, primates, and carnivores. By suppressing others, breeding females can benefit by reducing competition from other females and their offspring. There are neuroendocrinological changes during suppression which result in altered behavior, reproductive cycling, and communication. This review, which focuses on species in Rodentia, explores the current theoretical frameworks of female-female reproductive suppression, how female presence and rank impacts reproductive suppression, and some of the proposed mechanisms of suppression. Finally, the understudied role of olfactory communication in female-female reproductive suppression is discussed to identify current gaps in our understanding of this topic.

Cannabinoid receptor Type 1 densities reflect social organization in Microtus

Across many species, endocannabinoids play an important role in regulating social play, reward, and anxiety. These processes are mediated through at least two distinct cannabinoid receptors (CB), CB1 and CB2. CB1 expression is found in appreciable densities across regions of the brain that integrate memory with socio-spatial information; many of these regions have been directly linked to the neurobiology of pair bonding in monogamous species. Using receptor autoradiography, we provide the first distributional map of CB1 within the brains of closely related monogamous prairie voles and promiscuous meadow voles, and compare receptor densities across sexes and species in limbic regions. We observe CB1-specific signal using [3H] CP-55,940 and [3H] SR141716A, though the latter exhibited a lower signal to noise ratio. We confirmed the presence of CB2 in prairie vole spleen tissue using [3H] CP-55,940. However, we found no evidence of CB2 in the brain using either [3H] CP-55,940 or [3H] A-836,339. The overall distribution of putative CB1 in the brain was similar across vole species and followed the pattern of CB1 expression observed in other species-high intensity binding within the telencephalon, moderate binding within the diencephalon, and mild binding within the mesencephalon and metencephalon (aside from the cerebellar cortex). However, we found profound differences in CB1 densities across species, with prairie voles having higher CB1 binding in regions implicated in social attachment and spatial memory (e.g., periaqueductal gray, hippocampus). These findings suggest that CB1 densities, but not distribution, correlate with the social systems of vole species.

Natural variation in the oxytocin receptor gene and rearing interact to influence reproductive and nonreproductive social behavior and receptor binding

Individual variation in social behavior offers an opportunity to explore gene-by-environment interactions that could contribute to adaptative or atypical behavioral profiles (e.g., autism spectrum disorders). Outbred, socially monogamous prairie voles provide an excellent model to experimentally explore how natural variations in rearing and genetic diversity interact to shape reproductive and nonreproductive social behavior. In this study, we manipulated rearing (biparental versus dam-only), genotyped the intronic NT213739 single nucleotide polymorphism (SNP) of the oxytocin receptor gene (Oxtr), and then assessed how each factor and their interaction related to reciprocal interactions and partner preference in male and female adult prairie voles. We found that C/T subjects reared biparentally formed more robust partner preferences than T/T subjects. In general, dam-only reared animals huddled less with a conspecific in reproductive and nonreproductive contexts, but the effect of rearing was more pronounced in T/T animals. In line with previous literature, C/T animals exhibited higher densities of oxytocin receptor (OXTR) in the striatum (caudoputamen, nucleus accumbens) compared to T/T subjects. There was also a gene-by-rearing interaction in the striatum and insula of females: In the insula, T/T females expressed varying OXTR densities depending on rearing. Overall, this study demonstrates that significant differences in adult reproductive and nonreproductive social behavior and OXTR density can arise due to natural differences in Oxtr, experimental manipulations of rearing, and their interaction.

The role of social isolation in opioid addiction

Humans are social animals: social isolation hurts people both psychologically and physically. Strong, positive social bonds help people to live longer and healthier lives compared with their more isolated peers. Opioid use disorder is associated with feelings of social isolation, an increased risk of suicide and, at the community level, lower social capital. I propose a psychobiological mechanistic explanation that contributes to the association between opioid use and social isolation. The endogenous opioid system plays a central role in the formation and maintenance of social bonds across the life span and has been investigated primarily through the framework of the brain opioid theory of social attachment. In primates, maternal-infant bonding and social play are both impaired by the administration of naltrexone (an opioid antagonist), and in humans, the chronic use of opioids appears to be particularly (relative to other drugs) corrosive to close relationships. Social isolation may play a role in the development and exacerbation of opioid use disorder. Taken together, work on the brain’s opioid system suggests a possible mechanistic basis for bidirectional causal links between social isolation and opioid use disorder. Evaluation of this hypothesis would benefit from longitudinal psychosocial and neuropsychopharmacological investigations.

Neural correlates of mating system diversity: oxytocin and vasopressin receptor distributions in monogamous and non-monogamous Eulemur

Contemporary theory that emphasizes the roles of oxytocin and vasopressin in mammalian sociality has been shaped by seminal vole research that revealed interspecific variation in neuroendocrine circuitry by mating system. However, substantial challenges exist in interpreting and translating these rodent findings to other mammalian groups, including humans, making research on nonhuman primates crucial. Both monogamous and non-monogamous species exist within Eulemur, a genus of strepsirrhine primate, offering a rare opportunity to broaden a comparative perspective on oxytocin and vasopressin neurocircuitry with increased evolutionary relevance to humans. We performed oxytocin and arginine vasopressin 1a receptor autoradiography on 12 Eulemur brains from seven closely related species to (1) characterize receptor distributions across the genus, and (2) examine differences between monogamous and non-monogamous species in regions part of putative "pair-bonding circuits". We find some binding patterns across Eulemur reminiscent of olfactory-guided rodents, but others congruent with more visually oriented anthropoids, consistent with lemurs occupying an 'intermediary' evolutionary niche between haplorhine primates and other mammalian groups. We find little evidence of a "pair-bonding circuit" in Eulemur akin to those proposed in previous rodent or primate research. Mapping neuropeptide receptors in these nontraditional species questions existing assumptions and informs proposed evolutionary explanations about the biological bases of monogamy.