Specificity in Sociality: Mice and Prairie Voles Exhibit Different Patterns of Peer Affiliation

The effects of social loss and isolation on partner odor investigation and dopamine and oxytocin receptor expression in female prairie voles

In humans, grief is characterized by intense sadness, intrusive thoughts of the deceased, and intense longing for reunion with the deceased. Human fMRI studies show hyperactivity in emotional pain and motivational centers of the brain when an individual is reminded of a deceased attachment figure, but the molecular underpinnings of these changes in activity are unknown. Prairie voles (Microtus ochrogaster), which establish lifelong social bonds between breeding pairs, also display distress and motivational shifts during periods of prolonged social loss, providing a model to investigate these behavioral and molecular changes at a mechanistic level. Here, a novel odor preference test was used to assess social vs non-social odor investigation, and a sucrose preference test was used to assess non-social, reward-driven motivation. Females that lost a male partner investigated partner- and food-associated cues significantly more than females that lost a female cagemate or remained intact with a male partner. However, females experiencing the loss of a male partner did not change investigation of stranger-associated cues. Western blotting revealed significant increases of dopamine receptor type 1 (DRD1) and oxytocin receptor protein content in specific brain regions in response to the loss of distinct social relationships. Such effects included an increase in DRD1 in the medial preoptic area of the hypothalamus (mPOA) in females experiencing loss of a male partner compared to all other conditions. Pharmacological antagonism of DRD1 in the mPOA blocked the loss-associated increase of investigation of the partner odor but did not affect investigation of food or stranger odors. This reveals a novel dopamine-mediated mechanism for partner-seeking behavior during periods of partner loss in female prairie voles.

Protective effects of exercise on responses to combined social and environmental stress in prairie voles

The combination of social and environmental stressors significantly influences psychological and physical health in males and females, and contributes to both depression and cardiovascular diseases. Animal models support these findings. Voluntary exercise may protect against some forms of stress; however, the protective effects of exercise against social stressors require further investigation. This study evaluated the influence of exercise on the impact of combined social and environmental stressors in socially monogamous prairie voles. Following a period of social isolation plus additional chronic environmental stress, prairie voles were either allowed access to a running wheel in a larger cage for 2 weeks or remained in sedentary conditions. A behavioral stress task was conducted prior to and following exercise or sedentary conditions. Heart rate (HR) and HR variability were evaluated after exercise or sedentary conditions. Group-based analyses indicated that exercise prevented elevated resting HR and promoted autonomic control of the heart. Exercise was also effective against social and environmental stress-induced forced swim test immobility. Some minor sex differences in behavior were observed in response to exercise intensity. This research informs our understanding of the protective influence of physical exercise against social and environmental stressors in male and female humans.

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 naive 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 stranger voles while females were more likely to display partner preference during the initial 20 min of the test. We also found that male voles of all ages show enduring bonding behavior despite four weeks of partner separation while females show an overall decrease in partner-directed affiliation, including an erosion of partner preference in 12-month females. Finally, we found that the number of oxytocin, but not vasopressin, cells in the paraventricular hypothalamus increased at 18 months of age. These results establish prairie voles as a novel model to study the effects of normal and abnormal aging on pair bonding.

Prairie voles seek social contact with peer companions during immune challenge

Selection for group living has occurred across taxa, despite inherent risk of disease transmission. Behavioral and immune responses to sickness affect social interactions and can be altered by social contexts. However, the majority of research on sickness behavior has focused on species that do not form selective social relationships. Prairie voles (Microtus ochrogaster) form selective social relationships with mates and peers and provide a useful study system to examine effects of sickness on social seeking in established relationships. We used peripheral injections of lipopolysaccharide (LPS) of E. coli to stimulate the innate immune system and verified effects on activity, core temperature, and corticosterone concentrations for 6 h following treatment. We demonstrated that male and female same-sex pairs of prairie voles increase social contact when sick and that this increase persists when contact is initiated by the sick vole. Finally, we assessed social motivation following immune challenge using operant choice chambers equipped with two levers and side chambers. Voles worked to gain access to chambers with social and non-social rewards. While overall effort decreased following LPS injection, only immune-challenged voles worked significantly harder for their companion than for a non-social chamber. LPS treatment also increased proportion of rewards earned for the partner versus a stranger and again led to increased huddling behavior. Prior studies in other rodent species have shown decreased social interaction when sick; the present results demonstrate an alternative outcome of sickness in the context of dyadic bonds and lay the foundation for future work in peer companions.

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. Using the highly social prairie vole rodent model, we have previously reported that early life sleep disruption (ELSD) during the preweaning period results in interference with social bonding and increases ethanol consumption following a stressor in adulthood. Furthermore, ELSD increases parvalbumin expression and reduces glutamatergic neurotransmission in cortical regions in adult prairie voles. To understand the impact of ELSD on the lifespan, an examination of an earlier time in life is necessary. The aim of the present study was to examine behavioral outcomes of ELSD on adolescent prairie voles. Given the known effects of ELSD on development of neuronal systems involved in mood and social motivation, we hypothesized that anxiety, risk, and reward-related behaviors would be impacted by ELSD in adolescent prairie voles. We report that both male and female adolescent prairie voles that experienced ELSD showed heightened anxiety-like behavior compared to age-matched controls (CONs) as measured by a light-dark box. Additionally, both male and female ELSD voles showed reductions in both ethanol preference and consumption, and affiliative behavior compared to CONs. These results suggest that adolescent prairie voles of both sexes experience heightened anxiety-like behavior and reduced reward-seeking behaviors after ELSD. These results further suggest that early life sleep is critically important for neurotypical behaviors in adolescence.

Oxytocin receptor controls distinct components of pair bonding and development in prairie voles

Oxytocin receptor (Oxtr) signaling influences complex social behaviors in diverse species, including social monogamy in prairie voles. How Oxtr regulates specific components of social attachment behaviors and the neural mechanisms mediating them remains unknown. Here, we examine prairie voles lacking Oxtr and demonstrate that pair bonding comprises distinct behavioral modules: the preference for a bonded partner, and the rejection of novel potential mates. Our longitudinal study of social attachment shows that Oxtr sex-specifically influences early interactions between novel partners facilitating the formation of partner preference. Additionally, Oxtr suppresses promiscuity towards novel potential mates following pair bonding, contributing to rejection. Oxtr function regulates coordinated patterns of gene expression in regions implicated in attachment behaviors and regulates the expression of oxytocin in the paraventricular nucleus of the hypothalamus, a principal source of oxytocin. Thus, Oxtr controls genetically separable components of pair bonding behaviors and coordinates development of the neural substrates of attachment.

Egalitarian cooperation linked to central oxytocin levels in communal breeding house mice

Relationships between adult females are fundamental to understanding diversity in animal social systems. While cooperative relationships between kin are known to promote fitness benefits, the proximate mechanisms underlying this are not well understood. Here we show that when related female house mice (Mus musculus domesticus) cooperate to rear young communally, those with higher endogenous oxytocin levels have more egalitarian and successful cooperative relationships. Sisters with higher oxytocin concentrations in the paraventricular nucleus (PVN) of the hypothalamus weaned significantly more offspring, had lower reproductive skew and spent more equal proportions of time in the nest. By contrast, PVN oxytocin was unrelated to the number of weaned offspring produced in the absence of cooperation, and did not vary in response to manipulation of nest site availability or social cues of outgroup competition. By linking fitness consequences of cooperation with oxytocin, our findings have broad implications for understanding the evolution of egalitarian social relationships.

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.

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.

Paternal deprivation induces vigilance-avoidant behavior and accompanies sex-specific alterations in stress reactivity and central proinflammatory cytokine response in California mice (Peromyscus californicus)

RATIONALE

Early-life stress (ELS) can increase anxiety, reduce prosocial behaviors, and impair brain regions that facilitate emotional and social development. This knowledge greatly stems from assessing disrupted mother-child relationships, while studies investigating the long-term effects of father-child relationships on behavioral development in children are scarce. However, available evidence suggests that fathers may uniquely influence a child's behavioral development in a sex-specific manner. Rodent models examining mother-offspring interaction demonstrate relationships among ELS, neuroinflammatory mediators, and behavioral development; yet, the role paternal care may play in neuroimmune functioning remains unreported.

OBJECTIVES

Using the biparental California mouse (Peromyscus californicus), we examined to what extent paternal deprivation impairs social and anxiety-like behaviors, augments peripheral corticosterone (CORT) response, and alters central proinflammatory cytokine production following an acute stressor in adulthood.

METHODS

Biparentally reared and paternally deprived (permanent removal of the sire 24 h post-birth) adult mice were assessed for sociability, preference for social novelty, social vigilance, and social avoidance behaviors, followed by novelty-suppressed feeding (NSF) testing for general anxiety-like behavior. Following an acute stressor, circulating CORT concentrations and region-specific proinflammatory cytokine concentrations were determined via radioimmunoassay and Luminex multianalyte analysis, respectively.

RESULTS

In response to a novel same-sex conspecific, social vigilance behavior was associated with reduced sociability and increased avoidance in paternally deprived mice-an effect not observed in biparentally reared counterparts. Yet, in response to a familiar same-sex conspecific, social vigilance persisted but only in paternally deprived females. The latency to consume during NSF testing was not significantly altered by paternal deprivation. In response to an acute physical stressor, lower circulating CORT concentrations were observed in paternally deprived females. Compared to control-reared males, paternal deprivation increased hypothalamic interleukin-1β, but decreased hippocampal IL-6 protein concentration.

CONCLUSION

Greater social vigilance behavior was demonstrated in paternally deprived mice while they avoided social interaction with a novel same-sex conspecific; however, in response to a familiar same-sex conspecific, paternal deprivation increased social vigilance behavior but only in females. It is possible that different neurobiological mechanisms underlie these observed behavioral outcomes as sex-specific central proinflammatory cytokine and stress responsivity were observed in paternally deprived offspring.

Corticotropin-releasing hormone signaling from prefrontal cortex to lateral septum suppresses interaction with familiar mice

Social preference, the decision to interact with one member of the same species over another, is critical to optimize social interactions. Thus, adult rodents favor interacting with novel conspecifics over familiar ones, but whether this social preference stems from neural circuits facilitating interactions with novel individuals or suppressing interactions with familiar ones remains unknown. Here, we identify neurons in the infra-limbic area (ILA) of the mouse prefrontal cortex that express the neuropeptide corticotropin-releasing hormone (CRH) and project to the dorsal region of the rostral lateral septum (rLS). We show how release of CRH during familiar encounters disinhibits rLS neurons, thereby suppressing social interactions with familiar mice and contributing to social novelty preference. We further demonstrate how the maturation of CRH expression in ILA during the first 2 post-natal weeks enables the developmental shift from a preference for littermates in juveniles to a preference for novel mice in adults.

Varied impacts of social relationships on neuroendocrine state

Social relationships, affiliative social attachments, are important for many species. The best studied types of relationships are monogamous pair bonds. However, it remains unclear how generalizable models of pair bonding are across types of social attachments. Zebra finches are a fascinating system to explore the neurobiology of social relationships because they form various adult bonds with both same- and opposite-sex partners. To test whether different bonds are supported by a single brain network, we quantified individuals' neuroendocrine state after either 24 h or 2 weeks of co-housing with a novel same- or opposite-sex partner. We defined neuroendocrine state by the expression of 22 genes related to 4 major signaling pathways (dopamine, steroid, nonapeptide, and opioid) in six brain regions associated with affiliation or communication [nucleus accumbens (NAc), nucleus taeniae of the amygdala (TnA), medial preoptic area (POM), and periaqueductal gray (PAG), ventral tegmental area, and auditory cortex]. Overall, we found dissociable effects of social contexts (same- or opposite-sex partnerships) and duration of co-housing. Social bonding impacted the neuroendocrine state of four regions in males (NAc, TnA, POM, and PAG) and three regions in females (NAc, TnA, and POM). Monogamous pair bonding specifically appeared to impact male NAc. However, the patterns of gene expression in zebra finches were different than has previously been reported in mammals. Together, our results support the view that there are numerous mechanisms regulating social relationships and highlight the need to further our understanding of how social interactions shape social bonds.

Development of the hippocampal CA2 region and the emergence of social recognition

Social memories formed in early life, like those for family and unrelated peers, are known to contribute to healthy social interactions throughout life, although how the developing brain supports social memory remains relatively unexplored. The CA2 subregion of the hippocampus is involved in social memory function, but most literature on this subject is restricted to studies of adult rodents. Here, we review the current literature on the embryonic and postnatal development of hippocampal subregion CA2 in mammals, with a focus on the emergence of its unusual molecular and cellular characteristics, including its notably high expression of plasticity-suppressing molecules. We also consider the connectivity of the CA2 with other brain areas, including intrahippocampal regions, such as the dentate gyrus, CA3, and CA1 regions, and extrahippocampal regions, such as the hypothalamus, ventral tegmental area, basal forebrain, raphe nuclei, and the entorhinal cortex. We review developmental milestones of CA2 molecular, cellular, and circuit-level features that may contribute to emerging social recognition abilities for kin and unrelated conspecifics in early life. Lastly, we consider genetic mouse models related to neurodevelopmental disorders in humans in order to survey evidence about whether atypical formation of the CA2 may contribute to social memory dysfunction.

Early-life sleep disruption impairs subtle social behaviours in prairie voles: a pose-estimation study

Early-life sleep disruption (ELSD) has been shown to have long-lasting effects on social behaviour in adult prairie voles (Microtus ochrogaster), including impaired expression of pair bonding during partner preference testing. However, due to the limitations of manual behaviour tracking, the effects of ELSD across the time course of pair bonding have not yet been described, hindering our ability to trace mechanisms. Here, we used pose estimation to track prairie voles during opposite-sex cohabitation, the process leading to pair bonding. Male-female pairs were allowed to interact through a mesh divider in the home cage for 72 h, providing variables of body direction, distance-to-divider and locomotion speed. We found that control males displayed periodic patterns of body orientation towards females during cohabitation. In contrast, ELSD males showed reduced duration and ultradian periodicity of these body orientation behaviours towards females. Furthermore, in both sexes, ELSD altered spatial and temporal patterns of locomotion across the light/dark cycles of the 72 h recordings. This study allows a comprehensive behavioural assessment of the effects of ELSD on later life sociality and highlights subtle prairie vole behaviours. Our findings may shed light on neurodevelopmental disorders featuring sleep disruption and social deficits, such as autism spectrum disorders.

Acute intranasal oxytocin dose enhances social preference for parents over peers in male but not female peri-adolescent California mice (Peromyscus californicus)

Peri-adolescence is a critical developmental stage marked by profound changes in the valence of social interactions with parents and peers. We hypothesized that the oxytocin (OXT) and vasopressin (AVP) systems, known for influencing social behavior, would be involved in the maintenance and breaking of bonding behavior expressed by very early peri-adolescent males and females. In rodents, OXT is associated with mother-pup bonding and may promote social attachment to members of the natal territory. AVP, on the other hand, can act in contrasting ways to OXT and has been associated with aggression and territoriality. Specifically, we predicted that in peri-adolescent male and female juveniles of the biparental and territorial California mouse (Peromyscus californicus), a) OXT would increase the social preferences for the parents over unfamiliar age-matched peers (one male and one female), and b) AVP would break the parent-offspring bond and either increase time in the neutral chamber and/or approach to their unfamiliar and novel peers. We examined anxiety and exploratory behavior using an elevated plus maze and a novel object task as a control. Peri-adolescent mice were administered an acute intranasal (IN) treatment of 0.5 IU/kg IN AVP, 0.5 IU/kg IN OXT, or saline control; five minutes later, the behavioral tests were conducted. As predicted, we found that IN OXT enhanced social preference for parents; however, this was only in male and not female peri-adolescent mice. IN AVP did not influence social preference in either sex. These effects appear specific to social behavior and not anxiety, as neither IN OXT nor AVP influenced behavior during the elevated plus maze or novel object tasks. To our knowledge, this is the first evidence indicating that OXT may play a role in promoting peri-adolescent social preferences for parents and delaying weaning in males.

Peer Social Environment Impacts Behavior and Dopamine D1 Receptor Density in Prairie Voles (Microtus ochrogaster)

Prairie voles (Microtus ochrogaster) are socially monogamous rodents that form selective, long-lasting relationships with mates and with same-sex peers. It is unknown to what extent mechanisms supporting 'peer relationships' are similar to those involved in mate relationships. The formation of pair bonds is dependent on dopamine neurotransmission, whereas the formation of peer relationships is not, providing evidence of relationship type-specificity. The current study assessed endogenous structural changes in dopamine D1 receptor density in male and female voles across different social environments, including long-term same-sex partnerships, new same-sex partnerships, social isolation, and group housing. We also related dopamine D1 receptor density and social environment to behavior in social interaction and partner preference tests. Unlike prior findings in mate pairs, voles paired with new same-sex partners did not exhibit upregulated D1 binding in the nucleus accumbens (NAcc) relative to controls paired from weaning. This is consistent with differences in relationship type: D1 upregulation in pair bonds aids in maintaining exclusive relationships through selective aggression, and we found that formation of new peer relationships did not enhance aggression. Isolation led to increases in NAcc D1 binding, and even across socially housed voles, individuals with higher D1 binding exhibited increased social avoidance. These findings suggest that elevated D1 binding may be both a cause and a consequence of reduced prosociality. These results highlight the neural and behavioral consequences of different non-reproductive social environments and contribute to growing evidence that the mechanisms underlying reproductive and non-reproductive relationship formation are distinct. Elucidation of the latter is necessary to understand mechanisms underlying social behavior beyond a mating context.

Partner separation rescues pair bond-induced decreases in hypothalamic oxytocin neural densities

Studies in prairie voles (Microtus ochrogaster) have shown that although formation of the pair bond is accompanied by a suite of behavioral changes, a bond between two voles can dissolve and individuals can form new pair bonds with other conspecifics. However, the neural mechanisms underlying this behavioral flexibility have not been well-studied. Here we examine plasticity of nonapeptide, vasopressin (VP) and oxytocin (OT), neuronal populations in relation to bonding and the dissolution of bonds. Using adult male and female prairie voles, animals were either pair bonded, co-housed with a same-sex sibling, separated from their pair bond partner, or separated from their sibling. We examined neural densities of VP and OT cell groups and observed plasticity in the nonapeptide populations of the paraventricular nucleus of the hypothalamus (PVN). Voles that were pair bonded had fewer PVN OT neurons, suggesting that PVN OT neural densities decrease with pair bonding, but increase and return to a pre-pair bonded baseline after the dissolution of a pair bond. Our findings suggest that the PVN nonapeptide cell groups are particularly plastic in adulthood, providing a mechanism by which voles can exhibit context-appropriate behavior related to bond status.

Oxytocin receptor is not required for social attachment in prairie voles

Prairie voles are among a small group of mammals that display long-term social attachment between mating partners. Many pharmacological studies show that signaling via the oxytocin receptor (Oxtr) is critical for the display of social monogamy in these animals. We used CRISPR mutagenesis to generate three different Oxtr-null mutant prairie vole lines. Oxtr mutants displayed social attachment such that males and females showed a behavioral preference for their mating partners over a stranger of the opposite sex, even when assayed using different experimental setups. Mothers lacking Oxtr delivered viable pups, and parents displayed care for their young and raised them to the weanling stage. Together, our studies unexpectedly reveal that social attachment, parturition, and parental behavior can occur in the absence of Oxtr signaling in prairie voles.

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

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

Comparing the development of cortex-wide gene expression patterns between two species in a common reference frame

Advances in sequencing techniques have made comparative studies of gene expression a current focus for understanding evolutionary and developmental processes. However, insights into the spatial expression of genes have been limited by a lack of robust methodology. To overcome this obstacle, we developed methods and software tools for quantifying and comparing tissue-wide spatial patterns of gene expression within and between species. Here, we compare cortex-wide expression of RZRβ and Id2 mRNA across early postnatal development in mice and voles. We show that patterns of RZRβ expression in neocortical layer 4 are highly conserved between species but develop rapidly in voles and much more gradually in mice, who show a marked expansion in the relative size of the putative primary visual area across the first postnatal week. Patterns of Id2 expression, by contrast, emerge in a dynamic and layer-specific sequence that is consistent between the two species. We suggest that these differences in the development of neocortical patterning reflect the independent evolution of brains, bodies, and sensory systems in the 35 million years since their last common ancestor.

Social recognition and short-term memory in two taxa of striped mouse with differing social systems

The ability to distinguish between familiar and strange conspecifics is important in group‐living animals and influences the types of interactions between conspecifics. Social systems differ in sister taxa of the striped mouse genus Rhabdomys originating from different environments. Xeric‐adapted R. pumilio displays facultative group‐living whereas the mesic‐adapted R. d. chakae is solitary. We assessed social recognition and attraction to strangers in females of two populations each of R. pumilio and R. d. chakae by means of a social discrimination task. We used a three‐chamber apparatus developed in an established protocol and measured the latency of test females to approach and the duration of their investigation of stimulus females. Differences in social recognition of and preference for unfamiliar conspecifics in group‐living and solitary‐living taxa occurred at the taxon‐level, even though constituent populations occurring kilometers apart showed similar responses. Females differed in the latency (testing phase) and duration of investigation (familiarization and testing phases) inter‐specifically but not intra‐specifically. Female R. pumilio approached stimulus females faster than female R. d. chakae. Female R. pumilio also investigated stimulus females for longer, regardless of stimulus type compared to R. d. chakae, but both taxa spent more time investigating familiar females than novel females and approached the familiar females faster than novel females. Social recognition, short‐term memory, and social preference do not appear to differ between closely related taxa and differences in behavior between the two taxa might be related to inherent personality and social proclivity.

Tested social memory and attraction in two striped mouse taxa (solitary vs. group‐living).

Taxa did not differ in social recognition memory.

Group‐living taxa more motivated to investigate due to personality differences between taxa.

Naked Mole-Rat Social Phenotypes Vary in Investigative and Aggressive Behavior in a Laboratory Partner Preference Paradigm

Here we employed the partner preference test (PPT) to examine how naked mole-rat non-breeding individuals of different behavioral phenotypes make social decisions. Naked mole-rats from six colonies were classified into three behavioral phenotypes (soldiers, dispersers, and workers) using a battery of behavioral tests. They then participated in a 3 h long PPT, where they could freely interact with a tethered familiar or tethered unfamiliar conspecific. By comparing the three behavioral phenotypes, we tested the hypothesis that the PPT can be used to interrogate social decision-making in this species, revealing individual differences in behavior that are consistent with discrete social phenotypes. We also tested whether a shorter, 10 min version of the paradigm is sufficient to capture group differences in behavior. Overall, soldiers had higher aggression scores toward unfamiliar conspecifics than both workers and dispersers at the 10 min and 3 h comparison times. At the 10 min comparison time, workers showed a stronger preference for the familiar animal’s chamber, as well as for investigating the familiar conspecific, compared to both dispersers and soldiers. At the 3 h time point, no phenotype differences were seen with chamber or investigation preference scores. Overall, all phenotypes spent more time in chambers with another animal vs. being alone. Use of the PPT in a comparative context has demonstrated that the test identifies species and group differences in affiliative and aggressive behavior toward familiar and unfamiliar animals, revealing individual differences in social decision-making and, importantly, capturing aspects of species-specific social organization seen in nature.

Helping behavior in prairie voles: A model of empathy and the importance of oxytocin

Several studies suggest that rodents show empathic responses and helping behavior toward others. We examined whether prairie voles would help conspecifics who were soaked in water by opening a door to a safe area. Door-opening latency decreased as task sessions progressed. Female and male voles stayed close to the soaked voles' side at equal rates and opened the door with similar latencies. When the conspecific was not soaked in water, the door-opening latency did not decrease. This suggests that the distress of the conspecific is necessary for learning to open the door and that the door-opening performed by prairie voles corresponds to helping behavior. Additionally, we examined the helping behavior in prairie voles in which oxytocin receptors were genetically knocked out. Oxytocin receptor knockout voles demonstrated less learning of the door-opening behavior and less interest in soaked conspecifics. This suggests that oxytocin is important for the emergence of helping behavior.

•

Prairie voles demonstrated helping behavior toward a cagemate in distress

•

There was no difference in helping behavior depending on the helper’s sex

•

Learning of the helping behavior was prevented when cagemates were not in distress

•

Oxytocin receptor knockout prairie voles demonstrated less helping behavior

Selectivity and Sociality: Aggression and Affiliation Shape Vole Social Relationships

The formation of selective social relationships is not a requirement of group living; sociality can be supported by motivation for social interaction in the absence of preferences for specific individuals, and by tolerance in place of social motivation. For species that form selective social relationships, these can be maintained by preference for familiar partners, as well as by avoidance of or aggression toward individuals outside of the social bond. In this review, we explore the roles that aggression, motivation, and tolerance play in the maintenance of selective affiliation. We focus on prairie voles (Microtus ochrogaster) and meadow voles (Microtus pennsylvanicus) as rodent species that both exhibit the unusual tendency to form selective social relationships, but differ with regard to mating system. These species provide an opportunity to investigate the mechanisms that underlie social relationships, and to compare mechanisms supporting pair bonds with mates and same-sex peer relationships. We then relate this to the role of aggression in group composition in a comparative context.

Sex differences in the reward value of familiar mates in prairie voles

The rewarding properties of social interactions facilitate relationship formation and maintenance. Prairie voles are one of the few laboratory species that form selective relationships, manifested as "partner preferences" for familiar partners versus strangers. While both sexes exhibit strong partner preferences, this similarity in outward behavior likely results from sex-specific neurobiological mechanisms. We recently demonstrated that in operant trials, females worked hardest for access to familiar conspecifics of either sex, while males worked equally hard for access to any female, indicating a sex difference in social motivation. As tests were performed with one social target at a time, males might have experienced a ceiling effect, and familiar females might be more relatively rewarding in a choice scenario. Here we performed an operant social choice task in which voles lever-pressed to gain temporary access to either the chamber containing their mate or one containing a novel opposite-sex vole. Females worked hardest to access their mate, while males pressed at similar rates for either female. Individual male behavior was heterogeneous, congruent with multiple mating strategies in the wild. Voles exhibited preferences for favorable over unfavorable environments in a non-social operant task, indicating that lack of social preference does not reflect lack of discrimination. Natural variation in oxytocin receptor genotype at the intronic single nucleotide polymorphism NT213739 was associated with oxytocin receptor density, and predicted individual variation in stranger-directed aggressive behavior. These findings suggest that convergent preference behavior in male and female voles results from sex-divergent pathways, particularly in the realm of social motivation.

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?

Familiarity and Mate Preference Assessment with the Partner Preference Test

In contrast to traditional laboratory animals, prairie voles form socially monogamous partnerships in the wild and exhibit lasting social preferences for familiar individuals-both mates and same-sex peers-in the laboratory. Decades of research into the mechanisms supporting pair bonding behavior have made prairie voles an important model organism for the study of social relationships. The partner preference test is a laboratory test of familiarity preference that takes place over an extended interval (typically 3 hr), during which test subjects can directly interact with conspecifics and often engage in resting side-by-side contact (i.e., huddling). The use of this test has enabled study of the neural pathways and mechanisms involved in promoting or impairing relationship formation. The tendency to form partner preferences is also used as a behavioral indicator of the effects of early life experiences and environmental exposures. While this test was developed to assess the extent of social preference for mates in prairie voles, it has been adapted for use in other social contexts and in multiple other species. This article provides instructions for conducting the classic partner preference test, as well as variations including same-sex "peer" partner preference tests. The effects of several protocol variations are examined, including duration of cohousing, separation interval, use of tethers versus barriers, linear versus branched apparatus configuration, and duration of the test. The roles of social variables including sex of the focal individual, sex of conspecifics, reproductive state, and use of the test in other species are then considered. Finally, sample data are provided along with discussion of scoring and statistical analysis of partner preference tests. © 2021 Wiley Periodicals LLC. Basic Protocol: Partner preference test Support Protocol: Behavioral scoring.

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.

Multiple dimensions of social motivation in adult female degus

Many animals become more motivated to interact after a period of isolation. This phenomenon may involve general drives, e.g. for social touch or companionship, as well as drives that are specific to particular peers, and which ultimately serve to reestablish relationships between the individuals. Female degus are known to be affiliative with multiple other individuals, including unrelated and unfamiliar conspecifics, offering an opportunity to study social motivation independent from exclusive pair-bonds or overt, same-sex competition. We attempted to disentangle factors driving peer interaction by examining reunion behavior across several social isolation and separation manipulations. High levels of interaction were observed between adult females who had been separated even without isolation, revealing a drive to re-establish relationships with specific peers. The content of separation-only reunions differed from isolation, with the latter involving more early-session interaction, higher levels of allogrooming before rear-sniffing, and a higher ratio of chitter vocalizations. To assess whether post-isolation behavior was related to stress, we examined reunions following a non-social (footshock) stressor. Like isolation, footshock increased early-session interactions, but did not increase allogrooming before rear-sniffing or chittering, as compared with controls. To test whether separation-only reunion behavior shared qualities with relationship formation, we also examined reunions of new (stranger) dyads. Strangers exhibited higher levels of interaction than cagemates, with particularly high levels of late-session rear-sniffing. Like separation-only reunions, strangers showed more non-chitter vocalizations and lower levels of allogrooming before rear-sniffing. Across experiments, an exploratory clustering method was used to identify vocalizations that differed between conditions. This yielded promising leads for future investigation, including a chaff-type syllable that may have been more common during relationship renewal. Overall, results are consistent with the hypothesis that female degu reunions are supported by both general and peer-stimulus specific drives, expressed through the structure of physical and vocal interactions over time.

Comparative Assessment of Familiarity/Novelty Preferences in Rodents

Sociality-i.e., life in social groups-has evolved many times in rodents, and there is considerable variation in the nature of these groups. While many species-typical behaviors have been described in field settings, the use of consistent behavioral assays in the laboratory provides key data for comparisons across species. The preference for interaction with familiar or novel individuals is an important dimension of social behavior. Familiarity preference, in particular, may be associated with more closed, less flexible social groups. The dimension from selectivity to gregariousness has been used as a factor in classification of social group types. Laboratory tests of social choice range from brief (10 minutes) to extended (e.g., 3 hours). As familiarity preferences typically need long testing intervals to manifest, we used 3-hour peer partner preference tests to test for the presence of familiarity preferences in same-sex cage-mates and strangers in rats. We then conducted an aggregated analysis of familiarity preferences across multiple rodent species (adult male and female rats, mice, prairie voles, meadow voles, and female degus) tested with the same protocol. We found a high degree of consistency within species across data sets, supporting the existence of strong, species-typical familiarity preferences in prairie voles and meadow voles, and a lack of familiarity preferences in other species tested. Sociability, or total time spent near conspecifics, was unrelated to selectivity in social preference. These findings provide important background for interpreting the neurobiological mechanisms involved in social behavior in these 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.

Social preference in rats

Rats were given repeated choices between social and nonsocial outcomes, and between familiar and unfamiliar social outcomes. Lever presses on either of 2 levers in the middle chamber of a 3-chamber apparatus opened a door adjacent to the lever, permitting 45-s access to social interaction with the rat in the chosen side chamber. In Experiment 1, rats preferred (a) social over nonsocial options, choosing their cagemate rat over an empty chamber, and (b) an unfamiliar over a familiar rat, choosing a non-cagemate over their cagemate. These findings were replicated in Experiment 2 with 2 different non-cagemate rats. Rats preferred both non-cagemate rats to a similar degree when pitted against their cagemate, but were indifferent when the 2 non-cagemates were pitted against each other. Similar preference for social over nonsocial and non-cagemate over cagemate was seen in Experiment 3, with new non-cagemate rats introduced after every third session. Response rates (for both cagemate and non-cagemate rats) were elevated under conditions of nonsocial (isolated) housing compared to conditions of social (paired) housing, demonstrating a social deprivation effect. Together, the experiments contribute to an experimental analysis of social preference within a social reinforcement framework, drawing on methods with proven efficacy in the analysis of reinforcement more generally.

Social Preference Tests in Zebrafish: A Systematic Review

The use of animal models in biology research continues to be necessary for the development of new technologies and medicines, and therefore crucial for enhancing human and animal health. In this context, the need to ensure the compliance of research with the principles Replacement, Reduction and Refinement (the 3 Rs), which underpin the ethical and human approach to husbandry and experimental design, has become a central issue. The zebrafish (Danio rerio) is becoming a widely used model in the field of behavioral neuroscience. In particular, studying zebrafish social preference, by observing how an individual fish interacts with conspecifics, may offer insights into several neuropsychiatric and neurodevelopmental disorders. The main aim of this review is to summarize principal factors affecting zebrafish behavior during social preference tests. We identified three categories of social research using zebrafish: studies carried out in untreated wild-type zebrafish, in pharmacologically treated wild-type zebrafish, and in genetically engineered fish. We suggest guidelines for standardizing social preference testing in the zebrafish model. The main advances gleaned from zebrafish social behavior testing are discussed, together with the relevance of this method to scientific research, including the study of behavioral disorders in humans. The authors stress the importance of adopting an ethical approach that considers the welfare of animals involved in experimental procedures. Ensuring a high standard of animal welfare is not only good for the animals, but also enhances the quality of our science.

The role of dopamine signaling in prairie vole peer relationships

Dopamine signaling mediates the formation of some types of social relationships, including reproductive pair bonds in the socially monogamous prairie vole (Microtus ochrogaster). In addition to these pair bonds with mates, prairie voles demonstrate selective preferences for familiar same-sex peers. The dependence of peer relationships on dopamine signaling has not been tested, and the mechanisms supporting these relationships may differ from those underlying pair bonds. We examined the effects of pharmacological manipulations of dopamine signaling on peer partner preference and socially conditioned place preference in female prairie voles. Haloperidol blockade of dopamine receptors at multiple doses did not alter selective preferences for familiar same-sex partners, suggesting that dopamine neurotransmission is not necessary for the formation of prairie vole peer relationships, unlike mate relationships. Dopamine receptor agonist apomorphine facilitated peer partner preferences under conditions normally insufficient for partner preference formation; however, in the absence of effects from blockade, it is difficult to distinguish between a role for dopamine in partner preference formation and the generally rewarding properties of a dopamine agonist. Prairie voles exhibited socially conditioned place preferences for new but not long-term same-sex peers, and these preferences were not blocked by haloperidol. These results suggest that prairie vole peer relationships are less dependent on dopamine signaling than pair bonds, while still being rewarding. The data support distinct roles of dopamine and motivation in prairie vole peer relationships relative to mate relationships, suggesting that reproductive bonds are mediated differently from non-reproductive ones.

What's in a Moment: What Can Be Learned About Pair Bonding From Studying Moment-To-Moment Behavioral Synchrony Between Partners?

Our understanding of the behavioral and physiological mechanisms of monogamy largely comes from studies of behavioral interactions unique to pair-bonded individuals. By focusing on these highly marked behaviors, a remarkable conservation in the mechanisms underlying pair bonding has been revealed; however, we continue to know very little about the range of behavioral and neurobiological mechanisms that could explain the great diversity of pair-bonding phenotypes that exists both within and across species. In order to capture the dynamic nature of bonds over time and across contexts, we need specific, operationally-defined behavioral variables relevant across such a diversity of scenarios. Additionally, we need to be able to situate these behavioral variables within broader frameworks that allow us to interpret and compare patterns seen across species. Here I review what is known about behavioral synchrony with respect to pair bonding and discuss using synchrony as such a variable as well as a framework to expand on our understanding of pair bonding across timescales, contexts and species. First, I discuss the importance of behavioral synchrony and parental coordination for reproductive success in monogamous biparental bird species. Second, I highlight research documenting the critical importance of interpersonal coordination for human social relationships. Finally, I present recent work that experimentally bridges these lines of research by quantifying moment-to-moment behavioral synchrony during brief social interactions in zebra finch dyads. All together, these distinct perspectives support the notion that synchrony (1) is a shared premise for sociality across species, (2) is deeply shaped by social experiences, and (3) exists across timescales, behaviors, and levels of physiology. Conceptualizing pair bonding through the framework of behavioral synchrony is likely to facilitate a deeper understanding of the nuances of how social experiences and interactions impact the brain and behavior.

Inducing Partner Preference in Mice by Chemogenetic Stimulation of CA2 Hippocampal Subfield

Social recognition is fundamental for social decision making and the establishment of long-lasting affiliative behaviors in behaviorally complex social groups. It is a critical step in establishing a selective preference for a social partner or group member. C57BL/6J lab mice do not form monogamous relationships, and typically do not show prolonged social preferences for familiar mice. The CA2 hippocampal subfield plays a crucial role in social memory and optogenetic stimulation of inputs to the dorsal CA2 field during a short memory acquisition period can enhance and extend social memories in mice. Here, we show that partner preference in mice can be induced by chemogenetic selective stimulation of the monosynaptic projections from the hypothalamic paraventricular nucleus (PVN) to the CA2 during the cohabitation period. Specifically, male mice spend more time in social contact, grooming and huddling with the partner compared to a novel female. Preference was not induced by prolonging the cohabitation period and allowing more time for social interactions and males to sire pups with the familiar female. These results suggest that PVN-to-CA2 projections are part of an evolutionarily conserved neural circuitry underlying the formation of social preference and may promote behavioral changes with appropriate stimulation.

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.

Female degus show high sociality but no preference for familiar peers

Group-living animals vary in social behavior across multiple dimensions, including in the selectivity of social interactions with familiar versus unfamiliar peers. Standardized behavioral tests can be used to tease apart different dimensions of behavior. These serve a dual function-on one hand, helping to isolate behavioral factors that may support collective behavior in natural habitats, and, on another, providing a basis for comparative approaches to understanding physiological mechanisms of behavior. Degus (Octodon degus) are South American caviomorph rodents that nest and forage in groups with relatively low genetic relatedness. Flexibility in group membership is likely supported by gregariousness toward strangers, but the relative preference for strangers compared with familiar individuals has not been systematically tested. We assessed the specificity of social preferences in female degus using a same-sex partner preference test. Degus huddled extensively with both familiar and unfamiliar peers, with no average preference for one over the other. Detailed analysis of social interactions demonstrated an effect of familiarity on social investigation and aggressive behaviors, indicating that degus distinguished between familiar and unfamiliar conspecifics, even though it did not impact huddling. This behavioral profile is thus far unique to degus; in similar tests, meadow and prairie voles exhibit strong partner preferences for known peers, while mice exhibit low social huddling and spend relatively less time in social chambers. Understanding how group-living species differ in specific aspects of social behavior such as familiarity/novelty preference and propensity for social contact will offer a foundation to interpret differences in neural systems supporting sociality.

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

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

Oxytocin receptor knockout prairie voles generated by CRISPR/Cas9 editing show reduced preference for social novelty and exaggerated repetitive behaviors

Behavioral neuroendocrinology has benefited tremendously from the use of a wide range of model organisms that are ideally suited for particular questions. However, in recent years the ability to manipulate the genomes of laboratory strains of mice has led to rapid advances in our understanding of the role of specific genes, circuits and neural populations in regulating behavior. While genome manipulation in mice has been a boon for behavioral neuroscience, the intensive focus on the mouse restricts the diversity in behavioral questions that can be investigated using state-of-the-art techniques. The CRISPR/Cas9 system has great potential for efficiently generating mutants in non-traditional animal models and consequently to reinvigorate comparative behavioral neuroendocrinology. Here we describe the efficient generation of oxytocin receptor (Oxtr) mutant prairie voles (Microtus ochrogaster) using the CRISPR/Cas9 system, and describe initial behavioral phenotyping focusing on behaviors relevant to autism. Oxtr mutant male voles show no disruption in pup ultrasonic vocalization, anxiety as measured by the open field test, alloparental behavior, or sociability in the three chamber test. Mutants did however show a modest elevation in repetitive behavior in the marble burying test, and an impairment in preference for social novelty. The ability to efficiently generate targeted mutations in the prairie vole genome will greatly expand the utility of this model organism for discovering the genetic and circuit mechanisms underlying complex social behaviors, and serves as a proof of principle for expanding this approach to other non-traditional model organisms.

Comparative role of reward in long-term peer and mate relationships in voles

This is a contribution to SI: SBN/ICN meeting. In social species, relationships may form between mates, parents and their offspring, and/or social peers. Prairie voles and meadow voles both form selective relationships for familiar same-sex peers, but differ in mating system, allowing comparison of the properties of peer and mate relationships. Prairie vole mate bonds are dopamine-dependent, unlike meadow vole peer relationships, indicating potential differences in the mechanisms and motivation supporting these relationships within and/or across species. We review the role of dopamine signaling in affiliative behavior, and assess the role of behavioral reward across relationship types. We compared the reinforcing properties of mate versus peer relationships within a species (prairie voles), and peer relationships across species (meadow and prairie voles). Social reinforcement was assessed using the socially conditioned place preference test. Animals were conditioned using randomly assigned, equally preferred beddings associated with social (CS+) and solitary (CS-) housing. Prairie vole mates, but not prairie or meadow vole peers, conditioned toward the social cue. A second study in peers used counter-conditioning to enhance the capacity to detect low-level conditioning. Time spent on CS+ bedding significantly decreased in meadow voles, and showed a non-significant increase in prairie voles. These data support the conclusion that mate relationships are rewarding for prairie voles. Despite selectivity of preferences for familiar individuals in partner preference tests, peer relationships in both species appear only weakly reinforcing or non-reinforcing. This suggests important differences in the pathways underlying these relationship types, even within species.

Affiliation, Aggression, and Selectivity of Peer Relationships in Meadow and Prairie Voles

Relationships between adult peers are central to the structure of social groups. In some species, selective preferences for specific peers provide a foundation for consistent group composition. These preferences may be shaped by affiliation toward familiar individuals, and/or by aversion to unfamiliar individuals. We compared peer interactions in two vole species that form selective preferences for familiar same-sex individuals but differ in mating system. Prairie voles (Microtus ochrogaster) form pair bonds with mates and may reside in family groups. Meadow voles (Microtus pennsylvanicus) are promiscuous breeders that form communal winter groups in the wild, and exhibit greater social behavior in short day (SD) lengths in the laboratory. We characterized affiliative, anxiety-like, and aggressive interactions with familiar and novel same-sex conspecifics in meadow and prairie voles housed in summer- or winter-like photoperiods. Species differences in affective behaviors were pronounced, with prairie voles exhibiting more aggressive behavior and less anxiety-like behavior relative to meadow voles. Meadow voles housed in short (vs. long) day lengths were more affiliative and more interactive with strangers; prosocial behavior was also facilitated by a history of social housing. Prairie voles exhibited partner preferences regardless of sex or day length, indicating that selective peer preferences are the norm in prairie voles. Prairie vole females formed preferences for new same-sex social partners following re-pairing; males were often aggressive upon re-pairing. These data suggest that preferences for familiar peers in prairie voles are maintained in part by aggression toward unfamiliar individuals, as in mate partnerships. In contrast, social tolerance is an important feature of meadow vole peer affiliation, demonstrated by low aggression toward unfamiliar conspecifics, and consistent with field data on winter tolerance.

Frank Beach award winner: Neuroendocrinology of group living

Why do members of some species live in groups while others are solitary? Group living (sociality) has often been studied from an evolutionary perspective, but less is known about the neurobiology of affiliation outside the realms of mating and parenting. Colonial species offer a valuable opportunity to study nonsexual affiliative behavior between adult peers. Meadow voles (Microtus pennsylvanicus) display environmentally induced variation in social behavior, maintaining exclusive territories in summer months, but living in social groups in winter. Research on peer relationships in female meadow voles demonstrates that these selective preferences are mediated differently than mate relationships in socially monogamous prairie voles, but are also impacted by oxytocin and HPA axis signaling. This review addresses day-length dependent variation in physiology and behavior, and presents the current understanding of the mechanisms supporting selective social relationships in meadow voles, with connections to lessons from other species.

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.

Inclusion of females does not increase variability in rodent research studies

The underrepresentation of female subjects in animal research has gained attention in recent years, and new NIH guidelines aim to address this problem early, at the grant proposal stage. Many researchers believe that use of females will hamper research because of the need for increased sample sizes, and increased costs. Here I review empirical research across multiple rodent species and traits that demonstrates that females are not more variable than males, and that for most traits, female estrous cyclicity need not be considered. I present statistical simulations illustrating how factorial designs can reduce the need for additional research subjects, and discuss cultural issues around the inclusion of male and female subjects in research.