RNAi knockdown of oxytocin receptor in the nucleus accumbens inhibits social attachment and parental care in monogamous female prairie voles

Limbic oxytocin receptor expression alters molecular signaling and social avoidance behavior in female prairie voles (Microtus ochrogaster)

Introduction

The social defeat paradigm is the most representative animal model to study social anxiety disorder (SAD) and its underlying neuronal mechanisms. We have previously reported that defeat progressively reduces oxytocin receptors (OXTR) in limbic regions of the brain over an eight-week period in female prairie voles (Microtus ochrogaster). Oxytocin receptors activate the mitogen-activated protein kinase (MAPK) pathway, which has been previously associated with the anxiolytic effects of oxytocin. Here, we assessed the functional significance of OXTR in stress-induced social avoidance and the response of the MAPK signaling pathway in the nucleus accumbens (NAc), anterior cingulate cortex (ACC), and basolateral amygdala (BLA) of female prairie voles.

Methods

In experiment 1, Sexually naïve adult female prairie voles were defeated for three consecutive days and tested a week after for social preference/avoidance (SPA) test. Control subjects were similarly handled without defeat conditioning. In experiment 2, sexually and stress naïve adult female prairie voles were bilaterally injected into the NAc, ACC, or the BLA with a CRISPR/Cas9 virus targeting the Oxtr coding sequence to induce OXTR knockdown. Two weeks post-surgery, subjects were tested for SPA behavior. Viral control groups were similarly handled but injected with a control virus. A subgroup of animals from each condition in both experiments were similarly treated and euthanized without being tested for SPA behavior. Brains were harvested for OXTR autoradiography, western blot analysis of MAPK proteins and quantification of local oxytocin content in the NAc, BLA, ACC, and PVN through ELISA.

Results

Social defeat reduced OXTR binding in the NAc and affected MAPK pathway activity and oxytocin availability. These results were region-specific and sensitive to exposure to the SPA test. Additionally, OXTR knockdown in the NAc, ACC, and BLA induced social avoidance and decreased basal MAPK activity in the NAc. Finally, we found that OXTR knockdown in these regions was associated with less availability of oxytocin in the PVN.

Conclusion

Dysregulation of the oxytocin system and MAPK signaling pathway in the NAc, ACC, and BLA are important in social behavior disruptions in female voles. This dysregulation could, therefore, play an important role in the etiology of SAD in women.

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 4hr cohabitation. In contrast, while animals injected with vehicle after the 4hr 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 (2hr-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.

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.

Re-wiring of the bonded brain: Gene expression among pair bonded female prairie voles changes as they transition to motherhood

Motherhood is a costly life-history transition accompanied by behavioral and neural plasticity necessary for offspring care. Motherhood in the monogamous prairie vole is associated with decreased pair bond strength, suggesting a trade-off between parental investment and pair bond maintenance. Neural mechanisms governing pair bonds and maternal bonds overlap, creating possible competition between the two. We measured mRNA expression of genes encoding receptors for oxytocin (oxtr), dopamine (d1r and d2r), mu-opioids (oprm1a), and kappa-opioids (oprk1a) within three brain areas processing salience of sociosensory cues (anterior cingulate cortex; ACC), pair bonding (nucleus accumbens; NAc), and maternal care (medial preoptic area; MPOA). We compared gene expression differences between pair bonded prairie voles that were never pregnant, pregnant (~day 16 of pregnancy), and recent mothers (day 3 of lactation). We found greater gene expression in the NAc (oxtr, d2r, oprm1a, and oprk1a) and MPOA (oxtr, d1r, d2r, oprm1a, and oprk1a) following the transition to motherhood. Expression for all five genes in the ACC was greatest for females that had been bonded for longer. Gene expression within each region was highly correlated, indicating that oxytocin, dopamine, and opioids comprise a complimentary gene network for social signaling. ACC-NAc gene expression correlations indicated that being a mother (oxtr and d1r) or maintaining long-term pair bonds (oprm1a) relies on the coordination of different signaling systems within the same circuit. Our study suggests the maternal brain undergoes changes that prepare females to face the trade-off associated with increased emotional investment in offspring, while also maintaining a pair bond.

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.

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.

Social regulation of arginine vasopressin and oxytocin systems in a wild group-living fish

The neuropeptides arginine vasopressin (AVP) and oxytocin (OXT) are key regulators of social behaviour across vertebrates. However, much of our understanding of how these neuropeptide systems interact with social behaviour is centred around laboratory studies which fail to capture the social and physiological challenges of living in the wild. To evaluate relationships between these neuropeptide systems and social behaviour in the wild, we studied social groups of the cichlid fish Neolamprologus pulcher in Lake Tanganyika, Africa. We first used SCUBA to observe the behaviour of focal group members and then measured transcript abundance of key components of the AVP and OXT systems across different brain regions. While AVP is often associated with male-typical behaviours, we found that dominant females had higher expression of avp and its receptor (avpr1a2) in the preoptic area of the brain compared to either dominant males or subordinates of either sex. Dominant females also generally had the highest levels of leucyl-cystinyl aminopeptidase (lnpep)-which inactivates AVP and OXT-throughout the brain, potentially indicating greater overall activity (i.e., production, release, and turnover) of the AVP system in dominant females. Expression of OXT and its receptors did not differ across social ranks. However, dominant males that visited the brood chamber more often had lower preoptic expression of OXT receptor a (oxtra) suggesting a negative relationship between OXT signalling and parental care in males of this species. Overall, these results advance our understanding of the relationships between complex social behaviours and neuroendocrine systems under natural settings.

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.

Melanocortin agonism in a social context selectively activates nucleus accumbens in an oxytocin-dependent manner

Social deficits are debilitating features of many psychiatric disorders, including autism. While time-intensive behavioral therapy is moderately effective, there are no pharmacological interventions for social deficits in autism. Many studies have attempted to treat social deficits using the neuropeptide oxytocin for its powerful neuromodulatory abilities and influence on social behaviors and cognition. However, clinical trials utilizing supplementation paradigms in which exogenous oxytocin is chronically administered independent of context have failed. An alternative treatment paradigm suggests pharmacologically activating the endogenous oxytocin system during behavioral therapy to enhance the efficacy of therapy by facilitating social learning. To this end, melanocortin receptor agonists like Melanotan II (MTII), which induces central oxytocin release and accelerates formation of partner preference, a form of social learning, in prairie voles, are promising pharmacological tools. To model pharmacological activation of the endogenous oxytocin system during behavioral therapy, we administered MTII prior to social interactions between male and female voles. We assessed its effect on oxytocin-dependent activity in brain regions subserving social learning using Fos expression as a proxy for neuronal activation. In non-social contexts, MTII only activated hypothalamic paraventricular nucleus, a primary site of oxytocin synthesis. However, during social interactions, MTII selectively increased oxytocin-dependent activation of nucleus accumbens, a site critical for social learning. These results suggest a mechanism for the MTII-induced acceleration of partner preference formation observed in previous studies. Moreover, they are consistent with the hypothesis that pharmacologically activating the endogenous oxytocin system with a melanocortin agonist during behavioral therapy has potential to facilitate social learning.

Stimulating oxytocin receptors in the basolateral amygdala enhances stimulus processing: Differential and consistent effects for stimuli paired with fear versus sucrose in extinction and reversal learning

Oxytocin (OT) influences a range of social behaviors by enhancing the salience of social cues and regulating the expression of specific social behaviors (e.g., maternal care versus defensive aggression). We previously showed that stimulating OT receptors in the basolateral amygdala of rats also enhanced the salience of fear conditioned stimuli: relative to rats given vehicle infusions, rats infused with [Thr4,Gly7]-oxytocin (TGOT), a selective OT receptor agonist, showed greater discrimination between a cue predictive of danger, and one that signaled safety. In the present series of experiments using male rats, the effects of OT receptor activation in the basolateral amygdala on stimulus processing were examined further using conditioning protocols that consist of changes in stimulus-outcome contingencies (i.e., extinction and reversal), and with stimuli paired with aversive (i.e., foot shock) and appetitive (i.e., sucrose) outcomes. It was revealed that the effects of OTR stimulation diverge for aversive and appetitive learning - enhancing the former but not the latter. However, across both types of learning, OTR stimulation enhanced the detection of conditioned stimuli. Overall, these results are consistent with an emerging view of OT's effects on stimulus salience; facilitating the detection of meaningful stimuli while reducing responding to those that are irrelevant.

Balance Between Projecting Neuronal Populations of the Nucleus Accumbens Controls Social Behavior in Mice

BACKGROUND

Deficient social interactions are a hallmark of major neuropsychiatric disorders, and accumulating evidence points to altered social reward and motivation as key underlying mechanisms of these pathologies. In the present study, we further explored the role of the balance of activity between D1 and D2 receptor-expressing striatal projection neurons (D1R- and D2R-SPNs) in the control of social behavior, challenging the hypothesis that excessive D2R-SPN activity, rather than deficient D1R-SPN activity, compromises social behavior.

METHODS

We selectively ablated D1R- and D2R-SPNs using an inducible diphtheria toxin receptor-mediated cell targeting strategy and assessed social behavior as well as repetitive/perseverative behavior, motor function, and anxiety levels. We tested the effects of optogenetic stimulation of D2R-SPNs in the nucleus accumbens (NAc) and pharmacological compounds repressing D2R-SPN.

RESULTS

Targeted deletion of D1R-SPNs in the NAc blunted social behavior in mice, facilitated motor skill learning, and increased anxiety levels. These behaviors were normalized by pharmacological inhibition of D2R-SPN, which also repressed transcription in the efferent nucleus, the ventral pallidum. Ablation of D1R-SPNs in the dorsal striatum had no impact on social behavior but impaired motor skill learning and decreased anxiety levels. Deletion of D2R-SPNs in the NAc produced motor stereotypies but facilitated social behavior and impaired motor skill learning. We mimicked excessive D2R-SPN activity by optically stimulating D2R-SPNs in the NAc and observed a severe deficit in social interaction that was prevented by D2R-SPN pharmacological inhibition.

CONCLUSIONS

Repressing D2R-SPN activity may represent a promising therapeutic strategy to relieve social deficits in neuropsychiatric disorders.

Detection, processing and reinforcement of social cues: regulation by the oxytocin system

Many social behaviours are evolutionarily conserved and are essential for the healthy development of an individual. The neuropeptide oxytocin (OXT) is crucial for the fine-tuned regulation of social interactions in mammals. The advent and application of state-of-the-art methodological approaches that allow the activity of neuronal circuits involving OXT to be monitored and functionally manipulated in laboratory mammals have deepened our understanding of the roles of OXT in these behaviours. In this Review, we discuss how OXT promotes the sensory detection and evaluation of social cues, the subsequent approach and display of social behaviour, and the rewarding consequences of social interactions in selected reproductive and non-reproductive social behaviours. Social stressors - such as social isolation, exposure to social defeat or social trauma, and partner loss - are often paralleled by maladaptations of the OXT system, and restoring OXT system functioning can reinstate socio-emotional allostasis. Thus, the OXT system acts as a dynamic mediator of appropriate behavioural adaptations to environmental challenges by enhancing and reinforcing social salience and buffering social stress.

Association between parental behaviors and structural plasticity in the brain of male rodents

In recent decades, human fathers across the globe have shown a substantial increase in their engagement in paternal caregiving behaviors. Despite the growing interest, the precise neurobiological mechanisms underlying caregiving behaviors in males remain unclear. Neurobiological studies conducted on rodents have advanced our understanding of the molecular, cellular, and circuit-level mechanisms. Typically, sexually naïve males exhibit aggression toward offspring, while fathers display parental behaviors. This drastic behavioral plasticity may be associated with changes in connections among specific regions or cell types. Recent studies have begun to describe this structural plasticity by comparing neural connections before and after fatherhood. In this Perspective, we summarize the findings from four well-studied rodent species, namely prairie voles, California mice, laboratory rats, and laboratory mice, with a view toward integrating past and current progress. We then review recent advances in the understanding of structural plasticity for parental behaviors. Finally, we discuss remaining questions that require further exploration to gain a deeper understanding of the neural mechanisms underlying paternal behaviors in males, including their possible implications for the human brain.

Natural variation in oxytocin receptor signaling causes widespread changes in brain transcription: a link to the natural killer gene complex

Oxytocin (OXT) is a highly conserved neuropeptide that modulates social cognition, and variation in its receptor gene (Oxtr) is associated with divergent social phenotypes. The cellular mechanisms connecting Oxtr genotype to social phenotype remain obscure. We exploit an association between Oxtr polymorphisms and striatal-specific OXTR density in prairie voles to investigate how OXTR signaling influences the brain transcriptome. We discover widespread, OXTR signaling-dependent transcriptomic changes. Interestingly, OXTR signaling robustly modulates gene expression of C-type lectin-like receptors (CTLRs) in the natural killer gene complex, a genomic region associated with immune function. CTLRs are positioned to control microglial synaptic pruning; a process important for shaping neural circuits. Similar relationships between OXTR RNA and CTLR gene expression were found in human striatum. These data suggest a potential molecular mechanism by which variation in OXTR signaling due to genetic background and/or life-long social experiences, including nurturing/neglect, may affect circuit connectivity and social behavior.

Perinatal opioid exposure leads to decreased social play in adolescent male and female rats: Potential role of oxytocin signaling in brain regions associated with social reward

Over the last two decades, the number of infants exposed to opioids in utero has quadrupled in the United States, with some states reporting rates as high as 55 infants per 1000 births. Clinical studies report that children previously exposed to opioids during gestation show significant deficits in social behavior, including an inability to form friendships or other social relationships. To date, the neural mechanisms whereby developmental opioid exposure disrupts social behavior remain unknown. Using a novel paradigm of perinatal opioid administration, we tested the hypothesis that chronic opioid exposure during critical developmental periods would disrupt juvenile play. As oxytocin is a major regulator of sociability, the impact of perinatal morphine exposure on oxytocin peptide expression was also examined. Juvenile play was assessed in vehicle- or morphine-exposed male and female rats at P25, P35, and P45. Classical features of juvenile play were measured, including time spent engaged in social play, time not in contact, number of pins, and number of nape attacks. We report that morphine-exposed males and females spend less time engaged in play behavior than control males and females, with a corresponding increase in time spent alone. Morphine-exposed males and females also initiated fewer pins and nape attacks. Together, these data suggest that male and female rats exposed to morphine during critical developmental periods are less motivated to participate in social play, potentially due to alterations in oxytocin-mediated reward signaling.

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

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

An AAV-CRISPR/Cas9 strategy for gene editing across divergent rodent species: Targeting neural oxytocin receptors as a proof of concept

A major issue in neuroscience is the poor translatability of research results from preclinical studies in animals to clinical outcomes. Comparative neuroscience can overcome this barrier by studying multiple species to differentiate between species-specific and general mechanisms of neural circuit functioning. Targeted manipulation of neural circuits often depends on genetic dissection, and use of this technique has been restricted to only a few model species, limiting its application in comparative research. However, ongoing advances in genomics make genetic dissection attainable in a growing number of species. To demonstrate the potential of comparative gene editing approaches, we developed a viral-mediated CRISPR/Cas9 strategy that is predicted to target the oxytocin receptor (Oxtr) gene in >80 rodent species. This strategy specifically reduced OXTR levels in all evaluated species (n = 6) without causing gross neuronal toxicity. Thus, we show that CRISPR/Cas9-based tools can function in multiple species simultaneously. Thereby, we hope to encourage comparative gene editing and improve the translatability of neuroscientific research.

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.

Providing or receiving alloparental care promote partner preference and alter central oxytocin and dopamine systems in adult mandarin voles

Juveniles of cooperative breeding species usually remain in the natal area and provide care to younger siblings, a behavior considered one form of alloparenting in the natural condition. Previous studies have demonstrated the effects of providing or receiving alloparental care on adult behaviors, including anxiety-like behavior, social interaction, and parental behavior, but little is known about the influences on species-typical bonding behaviors, such as pair-bond formation. In this study, we explored this concept using socially monogamous mandarin voles (Lasiopodomys mandarinus). As the oxytocin (OT) and dopamine systems are involved in alloparental and pair-bonding behaviors, we also examined the levels of central OT and tyrosine hydroxylase (TH), as well as OT receptor (OTR) and dopamine D1-type and D2-type receptors (D1R and D2R) mRNA expression in the nucleus accumbens (NAcc) and amygdala to investigate the underlying mechanisms. Our results show that mandarin voles providing alloparental care to younger siblings displayed facilitation of partner preference formation, lower levels of OT expression in the paraventricular nucleus of the hypothalamus (PVN) and lateral hypothalamus (LH), and increased OTR and D2R mRNA expression in the NAcc compared to controls. Individuals receiving alloparental care also demonstrated facilitation of partner preference formation in adult voles. Additionally, alloparental care enhanced OT expression in the PVN, anterior medial preoptic nucleus (MPOAa), medial amygdala (MeA), and TH expression in the ventral tegmental area (VTA) and zona incerta (ZI). Furthermore, males displayed decreased D1R mRNA expression in the NAcc, whereas females showed slightly increased D2R expression in the amygdala. These results demonstrate that providing or received alloparental care can promote partner preference formation in monogamous species and that these changes are associated with altered OT and dopamine levels and their receptors in specific brain regions.

Optopharmacological tools for precise spatiotemporal control of oxytocin signaling in the central nervous system and periphery

Oxytocin is a neuropeptide critical for maternal physiology and social behavior, and is thought to be dysregulated in several neuropsychiatric disorders. Despite the biological and neurocognitive importance of oxytocin signaling, methods are lacking to activate oxytocin receptors with high spatiotemporal precision in the brain and peripheral mammalian tissues. Here we developed and validated caged analogs of oxytocin which are functionally inert until cage release is triggered by ultraviolet light. We examined how focal versus global oxytocin application affected oxytocin-driven Ca2+ wave propagation in mouse mammary tissue. We also validated the application of caged oxytocin in the hippocampus and auditory cortex with electrophysiological recordings in vitro, and demonstrated that oxytocin uncaging can accelerate the onset of mouse maternal behavior in vivo. Together, these results demonstrate that optopharmacological control of caged peptides is a robust tool with spatiotemporal precision for modulating neuropeptide signaling throughout the brain and body.

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.

Perinatal Opioid Exposure Leads to Decreased Social Play in Adolescent Male and Female Rats: Potential Role of Oxytocin Signaling in Brain Regions Associated with Social Reward

Over the last two decades, the number of infants exposed to opioids in utero has quadrupled in the United States, with some states reporting rates as high as 55 infants per 1000 births. Clinical studies report that children previously exposed to opioids during gestation show significant deficits in social behavior, including an inability to form friendships or other social relationships. To date, the neural mechanisms whereby developmental opioid exposure disrupts social behavior remain unknown. Using a novel paradigm of perinatal opioid administration, we tested the hypothesis that chronic opioid exposure during critical developmental periods would disrupt juvenile play. As oxytocin is a major regulator of sociability, the impact of perinatal morphine exposure on oxytocin peptide and receptor expression was also examined. Juvenile play was assessed in vehicle- or morphine-exposed male and female rats at P25, P35, and P45. Classical features of juvenile play were measured, including time spent engaged in social play, time not in contact, number of pins, and number of nape attacks. We report that morphine-exposed females spend less time engaged in play behavior than control males and females, with a corresponding increase in time spent alone. Morphine-exposed females also initiated fewer pins and nape attacks. Oxytocin receptor binding was reduced in morphine-exposed females in the nucleus accumbens, a brain region critical for social reward. Together, these data suggest that females exposed to morphine during critical developmental periods are less motivated to participate in social play, potentially due to alterations in oxytocin-mediated reward signaling.

Oxytocin treatments or activation of the paraventricular nucleus-the shell of nucleus accumbens pathway reduce adverse effects of chronic social defeat stress on emotional and social behaviors in Mandarin voles

Chronic social stress can cause psychological disease. Although oxytocin (OT) has been showed to modulate effects of chronic social defeat stress (CSDS) on emotional and social behaviors, however, how OT circuits mediate effects of CSDS on emotional and social abnormalities remains unclear. Here, we found that repeated intraperitoneal OT administration in the process of CSDS buffered adverse effects of CSDS on emotional and social behaviors in mandarin voles (Microtus mandarinus) of both sexes except no effect on depression-like behavior of males. Repeated OT treatments during CSDS prevented decrease of oxytocin receptors in nucleus accumbens (NAc) in females, but produced no effects on males. Furthermore, using designer receptors exclusively activated by designer drugs (DREADDs)-based chemogenetic tools, we determined that the activation of the paraventricular nucleus (PVN)-the shell of NAc (NAcs) projections before social defeat during CSDS process significantly prevented the increase of the anxiety-like behaviors and social avoidance induced by CSDS in both sexes, and reversed the depressive-like behaviors induced by CSDS only in females. Besides, optogenetic activation of PVN-NAcs projections after CSDS reduced anxiety-like behaviors and increased levels of sociality. Collectively, we suggest that PVN-NAcs projections modulate emotional and social behaviors during or after the process of CSDS sex-specifically, although AAV viruses did not specifically infect OT neurons. These findings offer potential targets for preventing or treating emotional and social disorders induced by chronic stress.

Distribution of vasopressin 1a and oxytocin receptor protein and mRNA in the basal forebrain and midbrain of the spiny mouse (Acomys cahirinus)

The nonapeptide system modulates numerous social behaviors through oxytocin and vasopressin activation of the oxytocin receptor (OXTR) and vasopressin receptor (AVPR1A) in the brain. OXTRs and AVPR1As are widely distributed throughout the brain and binding densities exhibit substantial variation within and across species. Although OXTR and AVPR1A binding distributions have been mapped for several rodents, this system has yet to be characterized in the spiny mouse (Acomys cahirinus). Here we conducted receptor autoradiography and in situ hybridization to map distributions of OXTR and AVPR1A binding and Oxtr and Avpr1a mRNA expression throughout the basal forebrain and midbrain of male and female spiny mice. We found that nonapeptide receptor mRNA is diffuse throughout the forebrain and midbrain and does not always align with OXTR and AVPR1A binding. Analyses of sex differences in brain regions involved in social behavior and reward revealed that males exhibit higher OXTR binding densities in the lateral septum, bed nucleus of the stria terminalis, and anterior hypothalamus. However, no association with gonadal sex was observed for AVPR1A binding. Hierarchical clustering analysis further revealed that co-expression patterns of OXTR and AVPR1A binding across brain regions involved in social behavior and reward differ between males and females. These findings provide mapping distributions and sex differences in nonapeptide receptors in spiny mice. Spiny mice are an excellent organism for studying grouping behaviors such as cooperation and prosociality, and the nonapeptide receptor mapping here can inform the study of nonapeptide-mediated behavior in a highly social, large group-living rodent.

Oxytocin receptors are widely distributed in the prairie vole (Microtus ochrogaster) brain: Relation to social behavior, genetic polymorphisms, and the dopamine system

Oxytocin regulates social behavior via direct modulation of neurons, regulation of neural network activity, and interaction with other neurotransmitter systems. The behavioral effects of oxytocin signaling are determined by the species-specific distribution of brain oxytocin receptors. The socially monogamous prairie vole has been a useful model organism for elucidating the role of oxytocin in social behaviors, including pair bonding, response to social loss, and consoling. However, there has been no comprehensive mapping of oxytocin receptor-expressing cells throughout the prairie vole brain. Here, we employed a highly sensitive in situ hybridization, RNAscope, to construct an exhaustive, brain-wide map of oxytocin receptor mRNA-expressing cells. We found that oxytocin receptor mRNA expression was widespread and diffused throughout the brain, with specific areas displaying a particularly robust expression. Comparing receptor binding with mRNA revealed that regions of the hippocampus and substantia nigra contained oxytocin receptor protein but lacked mRNA, indicating that oxytocin receptors can be transported to distal neuronal processes, consistent with presynaptic oxytocin receptor functions. In the nucleus accumbens, a region involved in oxytocin-dependent social bonding, oxytocin receptor mRNA expression was detected in both the D1 and D2 dopamine receptor-expressing subtypes of cells. Furthermore, natural genetic polymorphisms robustly influenced oxytocin receptor expression in both D1 and D2 receptor cell types in the nucleus accumbens. Collectively, our findings further elucidate the extent to which oxytocin signaling is capable of influencing brain-wide neural activity, responses to social stimuli, and social behavior. KEY POINTS: Oxytocin receptor mRNA is diffusely expressed throughout the brain, with strong expression concentrated in certain areas involved in social behavior. Oxytocin receptor mRNA expression and protein localization are misaligned in some areas, indicating that the receptor protein may be transported to distal processes. In the nucleus accumbens, oxytocin receptors are expressed on cells expressing both D1 and D2 dopamine receptor subtypes, and the majority of variation in oxytocin receptor expression between animals is attributable to polymorphisms in the oxytocin receptor gene.

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.

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.

Sex-specific and social experience-dependent oxytocin-endocannabinoid interactions in the nucleus accumbens: implications for social behaviour

Oxytocin modulates social behaviour across diverse vertebrate taxa, but the precise nature of its effects varies across species, individuals and lifetimes. Contributing to this variation is the fact that oxytocin's physiological effects are mediated through interaction with diverse neuromodulatory systems and can depend on the specifics of the local circuits it acts on. Furthermore, those effects can be influenced by both genetics and experience. Here we discuss this complexity through the lens of a specific neuromodulatory system, endocannabinoids, interacting with oxytocin in the nucleus accumbens to modulate prosocial behaviours in prairie voles. We provide a survey of current knowledge of oxytocin-endocannabinoid interactions in relation to social behaviour. We review in detail recent research in monogamous female prairie voles demonstrating that social experience, such as mating and pair bonding, can change how oxytocin modulates nucleus accumbens glutamatergic signalling through the recruitment of endocannabinoids to modulate prosocial behaviour toward the partner. We then discuss potential sex differences in experience-dependent modulation of the nucleus accumbens by oxytocin in voles based on new data in males. Finally, we propose that future oxytocin-based precision medicine therapies should consider how prior social experience interacts with sex and genetics to influence oxytocin actions. This article is part of the theme issue 'Interplays between oxytocin and other neuromodulators in shaping complex social behaviours'.

Oxytocin receptor expression patterns in the human brain across development

Oxytocin plays a vital role in social behavior and homeostatic processes, with animal models indicating that oxytocin receptor (OXTR) expression patterns in the brain influence behavior and physiology. However, the developmental trajectory of OXTR gene expression is unclear. By analyzing gene expression data in human post-mortem brain samples, from the prenatal period to late adulthood, we demonstrate distinct patterns of OXTR gene expression in the developing brain, with increasing OXTR expression along the course of the prenatal period culminating in a peak during early childhood. This early life OXTR expression peak pattern appears slightly earlier in a comparative macaque sample, which is consistent with the relative immaturity of the human brain during early life compared to macaques. We also show that a network of genes with strong spatiotemporal couplings with OXTR is enriched in several psychiatric illness and body composition phenotypes. Taken together, these results demonstrate that oxytocin signaling plays an important role in a diverse set of psychological and somatic processes across the lifespan.

Oxytocin has 'tend-and-defend' functionality in group conflict across social vertebrates

Across vertebrate species, intergroup conflict confronts individuals with a tension between group interests best served by participation in conflict and personal interest best served by not participating. Here, we identify the neurohormone oxytocin as pivotal to the neurobiological regulation of this tension in distinctly different group-living vertebrates, including fishes, birds, rodents, non-human primates and humans. In the context of intergroup conflict, a review of emerging work on pro-sociality suggests that oxytocin and its fish and birds homologues, isotocin and mesotocin, respectively, can elicit participation in group conflict and aggression. This is because it amplifies (i) concern for the interests of genetically related or culturally similar ‘in-group’ others and (ii) willingness to defend against outside intruders and enemy conspecifics. Across a range of social vertebrates, oxytocin can induce aggressive behaviour to ‘tend-and-defend’ the in-group during intergroup contests.

This article is part of the theme issue ‘Intergroup conflict across taxa’.

The early-life stress induced by oxytocin inhibition in p53 knockout mouse dams increases adulthood tumorigenesis in first and second generations

BACKGROUND

Early-life stress due to poor parental care has been suggested to increase cancer risk, though, so far, no experimental evidence established a link between defective parental behavior and spontaneous tumorigenesis in progeny. Essential maternal behavior is regulated, in particular, by the oxytocin (OT) hormonal circuit, which in turn responds to stimuli from the offspring and impinges on the central nervous systems.

METHODS

By providing L-368,899 OT receptor (OTR) inhibitor to lactating mothers, we set up a model of defective maternal care in p53 knockout mice.

RESULTS

The progeny of these dams showed, later in life, higher cortisol levels, shortened life span and increased tumorigenic potential of bone marrow cells (BMC). Notably, these phenotypes were transmitted to the following generation.

CONCLUSIONS

Therefore, the inhibition of OT function in mothers is a novel paradigm of early-life stress that is inherited across generations and increases cancer risk in tumor-prone mice.

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.

Dysfunctions of brain oxytocin signaling: Implications for poor mothering

Good mothering has profound impact on both the mother's and the young's well-being. Consequently, experiencing inadequate maternal care - or even neglect - in the first stages of life is a major risk factor for the development of psychiatric disorders, and even for poor parenting towards the future offspring. Thus, understanding the neurobiological basis of maternal neglect becomes crucial. Along with other neurotransmitters and neuropeptides, oxytocin (OXT) has long been known as one of the main modulators of maternal behavior. In rodents, disruptions of central OXT transmission have been associated with poor maternal responses, like impaired onset of nursing behaviors, and reduced care and defense of the pups. Importantly, such behavioral and molecular deficits can be transmitted through generations, creating a vicious circle of low-quality maternal behavior. Similarly, evidence from human studies shows that OXT signaling is defective in conditions of inadequate mothering and child neglect. On those premises, this review aims at providing a comprehensive overview of animal and human studies linking perturbed OXT transmission to poor maternal behavior. Considering the important fallouts of inadequate maternal responses, we believe that unraveling the alterations in OXT transmission might provide useful insights for a better understanding of maternal neglect and, ultimately, for future intervention approaches.

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.

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Prairie voles demonstrated helping behavior toward a cagemate in distress

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

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

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Oxytocin receptor knockout prairie voles demonstrated less helping behavior

Social experience alters oxytocinergic modulation in the nucleus accumbens of female prairie voles

Social relationships are dynamic and evolve with shared and personal experiences. Whether the functional role of social neuromodulators also evolves with experience to shape the trajectory of relationships is unknown. We utilized pair bonding in the socially monogamous prairie vole as an example of socio-sexual experience that dramatically alters behaviors displayed toward other individuals. We investigated oxytocin-dependent modulation of excitatory synaptic transmission in the nucleus accumbens as a function of pair-bonding status. We found that an oxytocin receptor agonist decreases the amplitude of spontaneous excitatory postsynaptic currents (sEPSCs) in sexually naive virgin, but not pair-bonded, female voles, while it increases the amplitude of electrically evoked EPSCs in paired voles, but not in virgins. This oxytocin-induced potentiation of synaptic transmission relies on the de novo coupling between oxytocin receptor signaling and endocannabinoid receptor type 1 (CB1) receptor signaling in pair-bonded voles. Blocking CB1 receptors after pair-bond formation increases the occurrence of a specific form of social rejection-defensive upright response-that is displayed toward the partner, but not toward a novel individual. Altogether, our results demonstrate that oxytocin's action in the nucleus accumbens is changed through social experience in a way that regulates the trajectory of social interactions as the relationship with the partner unfolds, potentially promoting the maintenance of a pair bond by inhibiting aggressive responses. These results provide a mechanism by which social experience and context shift oxytocinergic signaling to impact neural and behavioral responses to social cues.

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?

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.

Multi-Level Effects Driving Cognitive and Behavioral Variability among Prairie Voles: Insights into Reproductive Decision-Making from Biological Levels of Organization

Behavioral phenotypes play an active role in maximizing fitness and shaping the evolutionary trajectory of species by offsetting the ecological and social environmental factors individuals experience. How these phenotypes evolve and how they are expressed is still a major question in ethology today. In recent years, an increased focus on the mechanisms that regulate the interactions between an individual and its environment has offered novel insights into the expression of alternative phenotypes. In this review, we explore the proximate mechanisms driving the expression of alternative reproductive phenotypes in the male prairie vole (Microtus ochrogaster) as one example of how the interaction of an individual's social context and internal milieu has the potential to alter behavior, cognition, and reproductive decision-making. Ultimately, integrating the physiological and psychological mechanisms of behavior advances understanding into how variation in behavior arises. We take a "levels of biological organization" approach, with prime focus placed on the level of the organism to discuss how cognitive processes emerge as traits, and how they can be studied as important mechanisms driving the expression of behavior.

Linking Central Gene Expression Patterns and Mental States Using Transcriptomics and Large-Scale Meta-Analysis of fMRI Data: A Tutorial and Example Using the Oxytocin Signaling Pathway

The measurement of gene expression levels in the human brain can help accelerate our understanding of complex mental states and psychiatric illnesses. Mental states are typically associated with whole-brain networks; however, gene expression levels from postmortem brain samples have traditionally been measured in a limited number of brain regions due to resource limitations. The recent availability of whole-brain gene expression data from the Allen Human Brain Atlas (AHBA) provides the opportunity to generate gene expression patterns for over 20,000 genes. By linking these expression patterns with brain activity patterns that are associated with specific mental states, researchers can better understand which genes may support given mental states, via forward inference. Conversely, reverse inference can also be used to determine which mental state activation patterns are most strongly associated with a given gene expression map. This chapter provides a step-by-step guide on how to use the AHBA in conjunction with the NeuroSynth fMRI meta-analysis tool to identify the mental state correlates of specific gene expression patterns, using genes from oxytocin signaling pathway as an example. We also demonstrate how to perform an out-of-sample validation and assess the specificity of results for genes of interest.

The Current Status of Drug Discovery for the Oxytocin Receptor

The oxytocin receptor plays a significant role in peripheral regulation of parturition and lactation. Given this important role, multiple drug discovery programs have been conducted to develop agonists and antagonists for peripheral activity. The role of the oxytocin receptor in the central nervous system is also significant, promoting social interaction, trust, and empathy in humans. As such, molecules that can access the central nervous system and target the oxytocin receptor are of significant interest. Due to the role of the oxytocin receptor in regulating social function and psychological well-being, agonists of this receptor have considerable promise for the treatment of numerous neuropsychiatric conditions. The poor pharmacokinetic properties and blood-brain barrier penetration of peptide-based molecules means nonpeptide compounds have more commonly been the focus for central nervous system activity. This chapter aims to summarize the current standing of peptide and nonpeptide drug discovery for antagonists and agonists of the oxytocin receptor and focusses on centrally active nonpeptidic agonists.

Integrative Studies of the Effects of Mothers on Offspring: An Example from Wild North American Red Squirrels

Animal species vary in whether they provide parental care or the type of care provided, and this variation in parental care among species has been a common focus of comparative studies. However, the proximate causes and ultimate consequences of within-species variation in parental care have been less studied. Most studies about the impacts of within-species variation in parental care on parental fitness have been in primates, whereas studies in laboratory rodents have been invaluable for understanding what causes inter-individual variation in parental care and its influence on offspring characteristics. We integrated both of these perspectives in our long-term study of North American red squirrels (Tamiasciurus hudsonicus) in the Yukon, Canada, where we have focused on understanding the impacts of mothers on offspring. This includes documenting the impacts that mothers or the maternal environment itself has on their offspring, identifying how changes in maternal physiology impact offspring characteristics, the presence of individual variation in maternal attentiveness toward offspring before weaning and its fitness consequences, and postweaning maternal care and its fitness consequences. We provide an overview of these contributions to understanding the impacts mothers have on their offspring in red squirrels using an integrative framework and contrast them with studies in the laboratory.

Oxytocin and the Neurobiology of Prosocial Behavior

Humans are an unusually prosocial species, who engage in social behaviors that include altruism-whereby an individual engages in costly or risky acts to improve the welfare of another person-care, and cooperation. Current perspectives on the neurobiology of human prosociality suggest that it is deeply rooted in the neuroendocrine architecture of the social brain and emphasize the modulatory role of the neuropeptide hormone oxytocin. In this review, we provide a conceptual overview of the neurobiology of prosocial behavior with a focus on oxytocin's modulatory role in human prosociality. Specifically, we aim to encourage a better understanding of the peptide's susceptibility to diverse factors that produce heterogeneity in outcomes and the resulting methodological implications for measuring the behavioral effects of oxytocin in humans. After providing an overview of the state-of-the-art research on oxytocin's exogenous use, we elaborate on the peptide's modulatory role in the context of care-based altruism, cooperation, and conflict and discuss its potential for therapeutic interventions in psychiatric disorders characterized by social dysfunction.

Distribution of brain oxytocin and vasopressin V1a receptors in chimpanzees (Pan troglodytes): comparison with humans and other primate species

Despite our close genetic relationship with chimpanzees, there are notable differences between chimpanzee and human social behavior. Oxytocin and vasopressin are neuropeptides involved in regulating social behavior across vertebrate taxa, including pair bonding, social communication, and aggression, yet little is known about the neuroanatomy of these systems in primates, particularly in great apes. Here, we used receptor autoradiography to localize oxytocin and vasopressin V1a receptors, OXTR and AVPR1a respectively, in seven chimpanzee brains. OXTR binding was detected in the lateral septum, hypothalamus, medial amygdala, and substantia nigra. AVPR1a binding was observed in the cortex, lateral septum, hypothalamus, mammillary body, entire amygdala, hilus of the dentate gyrus, and substantia nigra. Chimpanzee OXTR/AVPR1a receptor distribution is compared to previous studies in several other primate species. One notable difference is the lack of OXTR in reward regions such as the ventral pallidum and nucleus accumbens in chimpanzees, whereas OXTR is found in these regions in humans. Our results suggest that in chimpanzees, like in most other anthropoid primates studied to date, OXTR has a more restricted distribution than AVPR1a, while in humans the reverse pattern has been reported. Altogether, our study provides a neuroanatomical basis for understanding the function of the oxytocin and vasopressin systems in chimpanzees.

Paraventricular Nucleus Oxytocin Subsystems Promote Active Paternal Behaviors in Mandarin Voles

Paternal care plays a critical role in the development of brain and behaviors in offspring in monogamous species. However, the neurobiological mechanisms, especially the neuronal circuity, underlying paternal care is largely unknown. Using socially monogamous male mandarin voles (Microtus mandarinus) with high levels of paternal care, we found that paraventricular nucleus of the hypothalamus (PVN) to ventral tegmental area (VTA) or nucleus accumbens (NAc) oxytocin (OT) neurons are activated during paternal care. Chemogenetic activation/inhibition of the PVN OT projection to VTA promoted/decreased paternal care, respectively. Chemogenetic inhibition of the PVN to VTA OT pathway reduced dopamine (DA) release in the NAc of male mandarin voles during licking and grooming of pups as revealed by in vivo fiber photometry. Optogenetic activation/inhibition of the VTA to NAc DA pathway possibly enhanced/suppressed paternal behaviors, respectively. Furthermore, chemogenetic activation/inhibition of PVN to NAc OT circuit enhanced/inhibited paternal care. This finding is a first step toward delineating the neuronal circuity underlying paternal care and may have implications for treating abnormalities in paternal care associated with paternal postpartum depression or paternal abuse.SIGNIFICANCE STATEMENT Paternal behavior is essential for offspring survival and development in some mammalian species. However, the circuit mechanisms underlying the paternal brain are poorly understood. We show that manipulation of paraventricular nucleus of the hypothalamus (PVN) to ventral tegmental area (VTA) oxytocin (OT) projections as well as VTA to nucleus accumbens (NAc) DA projections promote paternal behaviors. Inhibition the PVN to VTA OT pathway reduces DA release in the NAc during pup licking and grooming. PVN to NAc OT circuit is also essential for paternal behaviors. Our findings identify two new neural circuits that modulate paternal behaviors.

Oxytocin Receptor Exon III Methylation in the Umbilical Cord Blood of Newborns With Prenatal Exposure to Crack Cocaine

BACKGROUND

Prenatal cocaine exposure (PCE) is associated with behavioral, cognitive, and social consequences in children that might persist into later development. However, there are still few data concerning epigenetic mechanisms associated with the effects of gestational cocaine exposure, particularly in human newborns.

AIMS

We investigated the effects of PCE on DNA methylation patterns of the Oxytocin Receptor (OXTR) gene in the umbilical cord blood (UCB). The relationship between UCB DNA methylation levels and the severity of the mother's cocaine use during pregnancy was also evaluated.

METHODS

In this cross-sectional study, 28 UCB samples of newborns with a history of crack cocaine exposure in utero and 30 UCB samples of non-exposed newborns (NEC) were compared for DNA methylation levels at two genomic loci located in exon III of the OXTR gene (OXTR1 and OXTR2) through pyrosequencing. Maternal psychopathology was investigated using the Mini International Neuropsychiatric Interview, and substance use characteristics and addiction severity were assessed using the Smoking and Substance Involvement Screening Test (ASSIST).

RESULTS

No differences between newborns with a history of PCE and NEC were observed in OXTR1 or OXTR2 DNA methylation levels. However, regression analyses showed that maternal addiction severity for crack cocaine use predicted OXTR1 DNA methylation in newborns.

CONCLUSION

These data suggest that OXTR methylation levels in the UCB of children are affected by the severity of maternal crack cocaine usage. Larger studies are likely to detect specific changes in DNA methylation relevant to the consequences of PCE.

Facilitating mGluR4 activity reverses the long-term deleterious consequences of chronic morphine exposure in male mice

Understanding the neurobiological underpinnings of abstinence from drugs of abuse is critical to allow better recovery and ensure relapse prevention in addicted subjects. By comparing the long-term transcriptional consequences of morphine and cocaine exposure, we identified the metabotropic glutamate receptor subtype 4 (mGluR4) as a promising pharmacological target in morphine abstinence. We evaluated the behavioral and molecular effects of facilitating mGluR4 activity in abstinent mice. Transcriptional regulation of marker genes of medium spiny neurons (MSNs) allowed best discriminating between 4-week morphine and cocaine abstinence in the nucleus accumbens (NAc). Among these markers, Grm4, encoding mGluR4, displayed down-regulated expression in the caudate putamen and NAc of morphine, but not cocaine, abstinent mice. Chronic administration of the mGluR4 positive allosteric modulator (PAM) VU0155041 (2.5 and 5 mg/kg) rescued social behavior, normalized stereotypies and anxiety and blunted locomotor sensitization in morphine abstinent mice. This treatment improved social preference but increased stereotypies in cocaine abstinent mice. Finally, the beneficial behavioral effects of VU0155041 treatment in morphine abstinent mice were correlated with restored expression of key MSN and neural activity marker genes in the NAc. This study reports that chronic administration of the mGluR4 PAM VU0155041 relieves long-term deleterious consequences of morphine exposure. It illustrates the neurobiological differences between opiate and psychostimulant abstinence and points to pharmacological repression of excessive activity of D2-MSNs in the NAc as a promising therapeutic lever in drug addiction.

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.

Investigation of Oxtr-expressing Neurons Projecting to Nucleus Accumbens using Oxtr-ires-Cre Knock-in prairie Voles (Microtus ochrogaster)

Social bonds such as parent-infant attachment or pair bonds can be critical for mental and physical well-being. The monogamous prairie vole (Microtus ochrogaster) has proven useful for examining the neural substrates regulating social behaviors, including social bonding. Oxytocin (OXT) and oxytocin receptor (OXTR) play critical roles in alloparental care, pair bonding and consoling behavior in prairie voles. While OXTR in a few regions, such as the nucleus accumbnes (NAcc), prefrontal cortex (PFC) and anterior cingulate cortex (ACC), have been implicated in regulating these behaviors, the extent to which other OXT sensitive areas modulate social behaviors has not been investigated. The NAcc is a central hub for modulating OXTR dependent social behaviors. To identify neurons expressing Oxtr in prairie vole brain, we generated gene knock-in voles expressing Cre recombinase in tandem with Oxtr (Oxtr-ires-Cre) using CRISPR/Cas9 genome editing. We confirmed Oxtr and Cre mRNA co-localization in NAcc, validating this model. Next, we identified putative Oxtr-expressing neurons projecting to NAcc by infusing retrograde CRE-dependent EGFP AAV into NAcc and visualizing fluorescence. We found enhanced green fluorescent protein (EGFP) positive neurons in anterior olfactory nucleus, PFC, ACC, insular cortex (IC), paraventricular thalamus (PVT), basolateral amygdala (BLA), and posteromedial and posterolateral cortical amygdaloid area (PMCo, PLCo). The ACC to NAcc OXTR projection may represent a species-specific circuit since Oxtr-expressing neurons in the ACC of mice were reported not to project to the NAcc. This is the first delineation of Oxtr-expressing neural circuits in the prairie vole, and demonstrates the utility of this novel genetically modified organism for characterizing OXTR circuits involved in social behaviors.