Neuropeptide diversity and the regulation of social behavior in New World primates

From molecular variations to behavioral adaptations: Unveiling adaptive epistasis in primate oxytocin system

OBJECTIVE

Our primary objective was to investigate the variability of oxytocin (OT) and the GAMEN binding motif within the LNPEP oxytocinase in primates.

MATERIALS AND METHODS

We sequenced the LNPEP segment encompassing the GAMEN motif in 34 Platyrrhini species, with 21 of them also sequenced for the OT gene. Our dataset was supplemented with primate sequences of LNPEP, OT, and the oxytocin receptor (OTR) sourced from public databases. Evolutionary analysis and coevolution predictions were made followed by the macroevolution analysis of relevant amino acids associated with phenotypic traits, such as mating systems, parental care, and litter size. To account for phylogenetic structure, we utilized two distinct statistical tests. Additionally, we calculated binding energies focusing on the interaction between Callithtrix jacchus VAMEN and Pro8OT.

RESULTS

We identified two novel motifs (AAMEN and VAMEN), challenging the current knowledge of motif conservation in placental mammals. Coevolution analysis demonstrated a correlation between GAMEN, AAMEN, and VAMEN and their corresponding OTs and OTRs. Callithrix jacchus exhibited a higher binding energy between VAMEN and Pro8OT than orthologous molecules found in humans (GAMEN and Leu8OT).

DISCUSSION

The coevolution of AAMEN and VAMEN with their corresponding OTs and OTRs suggests a functional relationship that could have contributed to specific reproductive and adaptive behaviors, including paternal care, social monogamy, and twin births, prominent traits in Cebidae species, such as marmosets and tamarins. Our findings underscore the coevolution of taxon-specific amino acids among the three studied molecules, shedding light on the oxytocinergic system as an adaptive epistatic repertoire in primates.

Oxytocin and vasopressin signaling in health and disease

Neurohypophysial peptides are ancient and evolutionarily highly conserved neuropeptides that regulate many crucial physiological functions in vertebrates and invertebrates. The human neurohypophysial oxytocin/vasopressin (OT/VP) signaling system with its four receptors has become an attractive drug target for a variety of diseases, including cancer, pain, cardiovascular indications, and neurological disorders. Despite its promise, drug development faces hurdles, including signaling complexity, selectivity and off-target concerns, translational interspecies differences, and inefficient drug delivery. In this review we dive into the complexity of the OT/VP signaling system in health and disease, provide an overview of relevant pharmacological probes, and discuss the latest trends in therapeutic lead discovery and drug development.

Genetic determinants of individual variation in the superior temporal sulcus of chimpanzees (Pan troglodytes)

The superior temporal sulcus (STS) is a conserved fold that divides the middle and superior temporal gyri. In humans, there is considerable variation in the shape, folding pattern, lateralization, and depth of the STS that have been reported to be associated with social cognition and linguistic functions. We examined the role that genetic factors play on individual variation in STS morphology in chimpanzees. The surface area and depth of the STS were quantified in sample of 292 captive chimpanzees comprised of two genetically isolated population of individuals. The chimpanzees had been previously genotyped for AVPR1A and KIAA0319, two genes that play a role in social cognition and communication in humans. Single nucleotide polymorphisms in the KIAA0319 and AVPR1A genes were associated with average depth as well as asymmetries in the STS. By contrast, we found no significant effects of these KIA0319 and AVPR1A polymorphism on surface area and depth measures for the central sulcus. The overall findings indicate that genetic factors account for a small to moderate amount of variation in STS morphology in chimpanzees. These findings are discussed in the context of the role of the STS in social cognition and language in humans and their potential evolutionary origins.

Modulation of social behavior by distinct vasopressin sources

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

A tale of two hierarchies: Hormonal and behavioral factors underlying sex differences in social dominance in cooperative breeding callitrichids

Callitrichid primates are recognized for high levels of sociality in small groups, their great behavioral flexibility, and single-female dominant hierarchies. Previous work has highlighted that dominant, breeding callitrichids engage in behavioral and hormonal reproductive suppression of related and unrelated subordinates by both producing more offspring, having higher levels of ovulatory hormones, and accessing more sociosexual opportunities. This suppression constitutes a nexus of changes in pituitary responsiveness, ovarian cyclicity, sexual behavior, affiliation, and aggression. In this review, I will highlight important features that characterize callitrichid social hierarchies across broad social contexts. Dominant females sometimes exert reproductive suppression on subordinate nonbreeding females, but this suppression varies across callitrichids based on social stability and changes in group composition, particularly related to the number, experience, and age of nonbreeding subordinates. Meanwhile, dominant males may induce suppression of reproduction in subordinate males, but these effects occur by different behavioral and endocrine mechanisms and to a much lesser extent than their female counterparts; While dominant female callitrichids usually show higher levels of aggression relative to their male counterparts, callitrichids show a general absence of intersexual dominance, likely as an effort of maintaining a cohesive breeding pair within a stable social group and social cooperation. Future efforts are needed to identify precise neuroendocrine mechanisms underlying the presence of sex differences in callitrichid behavior separate from peripheral reproductive function. This is especially important with regard to parental experience, social relationships, development and aging, with larger implications toward understanding sex differences in overall health and wellbeing.

The neural circuits of monogamous behavior

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

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.

The neuropeptide Pth2 modulates social behavior and anxiety in zebrafish

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

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.

Modified Sawhorse Waveform for the Voltammetric Detection of Oxytocin

Carbon fiber microelectrodes (CFMEs) have been used to detect neurotransmitters and other biomolecules using fast-scan cyclic voltammetry (FSCV) for the past few decades. This technique measures neurotransmitters such as dopamine and, more recently, physiologically relevant neuropeptides. Oxytocin, a pleiotropic peptide hormone, is physiologically important for adaptation, development, reproduction, and social behavior. This neuropeptide functions as a stress-coping molecule, an anti-inflammatory agent, and serves as an antioxidant with protective effects especially during adversity or trauma. Here, we measure tyrosine using the Modified Sawhorse Waveform (MSW), enabling enhanced electrode sensitivity for the amino acid and oxytocin peptide. Applying the MSW, decreased surface fouling and enabled codetection with other monoamines. As oxytocin contains tyrosine, the MSW was also used to detect oxytocin. The sensitivity of oxytocin detection was found to be 3.99 ± 0.49 nA/μM, (n=5). Additionally, we demonstrate that applying the MSW on CFMEs allows for real time measurements of exogenously applied oxytocin on rat brain slices. These studies may serve as novel assays for oxytocin detection in a fast, sub-second timescale with possible implications for in vivo measurements and further understanding of the physiological role of oxytocin.

The neuropeptide Pth2 dynamically senses others via mechanosensation

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

Oxytocin amino acid variation within Neotropical primates: new genetic variants in hormone and receptor sequences and evidence for evolutionary forces driving this unexpected diversity

Oxytocin is a mammalian neuropeptide hormone that mediates behaviours important to reproduction. Despite almost universal amino acid sequence conservation across most groups of mammals, several unique forms have been reported across Neotropical primates. To explore sequence diversity, we investigated the genes encoding oxytocin and its receptor across the Atelidae, which was known to contain at least three unique oxytocin sequences. Additionally, we included the genus Cebus, within the Cebidae, to further explore the ubiquity of the Pro8 variant in this family. We found a novel amino acid variant (Val3) within the Atelidae radiation, bringing the total number of oxytocin sequences within Neotropical primates to seven. Analyses of physicochemical properties revealed conservative substitutions that are likely tolerated within the selective constraints imposed by receptor binding. Furthermore, we report radical substitutions at the eighth codon and evidence for co-evolution between Pro8 and a ligand-binding region of the oxytocin receptor in the Atelidae, supporting the notion that this variant may affect binding specificity. Overall, we suggest that selective constraint on binding specificity may maintain proper oxytocin function and that the diversification of amino acid sequence is likely due to a variety of processes such as relaxed constraint, neutral mutation, positive selection and coevolution.

Comparison of the pharmacological profiles of arginine vasopressin and oxytocin analogs at marmoset, macaque, and human vasopressin 1a receptor

Arginine vasopressin (AVP) and oxytocin (OT) are nonapeptides that bind to G-protein coupled receptors and influence social behaviors. Consensus mammalian AVP and OT (Leu⁸-OT) sequences are highly conserved. In marmosets, an amino acid change in the 8th position of the peptide (Pro⁸-OT) exhibits unique structural and functional properties. There is ∼85 % structural homology between the OT receptor (OTR) and vasopressin 1a receptor (V1aR) resulting in significant cross-reactivity between the ligands and receptors. Chinese hamster ovary (CHO) cells expressing marmoset (mV1aR), macaque (qV1aR), or human vasopressin receptor 1a (hV1aR) were used to assess AVP, Leu⁸-OT and Pro⁸-OT pharmacological profiles. To assess activation of Gq, functional assays were performed using Fluo-3 to measure ligand-induced Ca²⁺ mobilization. In all three V1aR-expressing cell lines, AVP was more potent than the OT ligands. To assess ligand-induced hyperpolarization, FLIPR Membrane Potential (FMP) assays were performed. In all three V1aR lines, AVP was more potent than the OT analogs. The distinctive U-shaped concentration-response curve displayed by AVP may reflect enhanced desensitization of the mV1aR and hV1aR, which is not observed with qV1aR. Evaluation of Ca²⁺-activated potassium (K⁺) channels using the inhibitors apamin, paxilline, and TRAM-34 demonstrated that both intermediate and large conductance Ca²⁺-activated K⁺ channels contributed to membrane hyperpolarization, with different pharmacological profiles identified for distinct ligand-receptor combinations. Taken together, these data suggest differences in ligand-receptor signaling that may underlie differences in social behavior. Integrative studies of behavior, genetics and ligand-receptor interaction will help elucidate the connection between receptor pharmacology and social behaviors.

Comparison of the pharmacologic profiles of arginine vasopressin and oxytocin analogs at marmoset, titi monkey, macaque, and human oxytocin receptors

The oxytocin-arginine vasopressin (OT-AVP) ligand-receptor family influences a variety of physiological, behavioral, and social behavioral processes in the brain and periphery. The OT-AVP family is highly conserved in mammals, but recent discoveries have revealed remarkable diversity in OT ligands and receptors in New World Monkeys (NWMs) providing a unique opportunity to assess the effects of genetic variation on pharmacological signatures of peptide ligands. The consensus mammalian OT sequence has leucine in the 8^th position (Leu⁸-OT), whereas a number of NWMs, including the marmoset, have proline in the 8^th position (Pro⁸-OT) resulting in a more rigid tail structure. OT and AVP bind to OT’s cognate G-protein coupled receptor (OTR), which couples to various G-proteins (G_i/o, G_q, G_s) to stimulate diverse signaling pathways. CHO cells expressing marmoset (mOTR), titi monkey (tOTR), macaque (qOTR), or human (hOTR) OT receptors were used to compare AVP and OT analog-induced signaling. Assessment of G_q-mediated increase in intracellular calcium (Ca²⁺) demonstrated that AVP was less potent than OT analogs at OTRs from species whose endogenous ligand is Leu⁸-OT (tOTR, qOTR, hOTR), relative to Pro⁸-OT. Likewise, AVP-induced membrane hyperpolarization was less potent at these same OTRs. Evaluation of (Ca²⁺)-activated potassium (K⁺) channels using the inhibitors apamin, paxilline, and TRAM-34 demonstrated that both intermediate and large conductance Ca²⁺-activated K⁺ channels contributed to membrane hyperpolarization, with different pharmacological profiles identified for distinct ligand-receptor combinations. Understanding more fully the contributions of structure activity relationships for these peptide ligands at vasopressin and OT receptors will help guide the development of OT-mediated therapeutics.

AVPR1A variation is linked to gray matter covariation in the social brain network of chimpanzees

The vasopressin system has been implicated in the regulation of social behavior and cognition in humans, nonhuman primates and other social mammals. In chimpanzees, polymorphisms in the vasopressin V1a receptor gene (AVPR1A) have been associated with social dimensions of personality, as well as to responses to sociocommunicative cues and mirror self-recognition. Despite evidence of this association with social cognition and behavior, there is little research on the neuroanatomical correlates of AVPR1A variation. In the current study, we tested the association between AVPR1A polymorphisms in the RS3 promotor region and gray matter covariation in chimpanzees using magnetic resonance imaging and source-based morphometry. The analysis identified 13 independent brain components, three of which differed significantly in covariation between the two AVPR1A genotypes (DupB-/- and DupB+/-; P < .05). DupB+/- chimpanzees showed greater covariation in gray matter in the premotor and prefrontal cortex, basal forebrain, lunate and cingulate cortex, and lesser gray matter covariation in the superior temporal sulcus and postcentral sulcus. Some of these regions were previously found to differ in vasopressin and oxytocin neural fibers between nonhuman primates, and in AVPR1A gene expression in humans with different RS3 alleles. This is the first report of an association between AVPR1A and gray matter covariation in nonhuman primates, and specifically links an AVPR1A polymorphism to structural variation in the social brain network. These results further affirm the value of chimpanzees as a model species for investigating the relationship between genetic variation, brain structure and social cognition with relevance to psychiatric disorders, including autism.

Leu8 and Pro8 oxytocin agonism differs across human, macaque, and marmoset vasopressin 1a receptors

Oxytocin (OXT) is an important neuromodulator of social behaviors via activation of both oxytocin receptors (OXTR) and vasopressin (AVP) 1a receptors (AVPR1a). Marmosets are neotropical primates with a modified OXT ligand (Pro⁸-OXT), and this ligand shows significant coevolution with traits including social monogamy and litter size. Pro⁸-OXT produces more potent and efficacious responses at primate OXTR and stronger behavioral effects than the consensus mammalian OXT ligand (Leu⁸-OXT). Here, we tested whether OXT/AVP ligands show differential levels of crosstalk at primate AVPR1a. We measured binding affinities and Ca²⁺ signaling responses of AVP, Pro⁸-OXT and Leu⁸-OXT at human, macaque, and marmoset AVPR1a. We found that AVP binds with higher affinity than OXT across AVPR1a, and marmoset AVPR1a show a 10-fold lower OXT binding affinity compared to human and macaque AVPR1a. Both Leu⁸-OXT and Pro⁸-OXT produce a less efficacious response than AVP at human AVPR1a and higher efficacious response than AVP at marmoset AVPR1a. These data suggest that OXT might partially antagonize endogenous human AVPR1a signaling and enhance marmoset AVPR1a signaling. These findings aid in further understanding inconsistencies observed following systemic intranasal administration of OXT and provide important insights into taxon-specific differences in nonapeptide ligand/receptor coevolution and behavior.

AVPR1b variation and the emergence of adaptive phenotypes in Platyrrhini primates

Platyrrhini (New World monkeys, NWm) are a group of primates characterized by behavioral and reproductive traits that are otherwise uncommon among primates, including social monogamy, direct paternal care, and twin births. As a consequence, the study of Platyrrhine primates is an invaluable tool for the discovery of the genetic repertoire underlying these taxon-specific traits. Recently, high conservation of vasopressin (AVP) sequence, in contrast with high variability of oxytocin (OXT), has been described in NWm. AVP and OXT functions are possible due to interaction with their receptors: AVPR1a, AVPR1b, AVPR2, and OXTR; and the variability in this system is associated with the traits mentioned above. Understanding the variability in the receptors is thus fundamental to understand the function and evolution of the system as a whole. Here we describe the variability of AVPR1b coding region in 20 NWm species, which is well-known to influence behavioral traits such as aggression, anxiety, and stress control in placental mammals. Our results indicate that 4% of AVPR1b sites may be under positive selection and a significant number of sites under relaxed selective constraint. Considering the known role of AVPR1b, we suggest that some of the changes described here for the Platyrrhini may be a part of the genetic repertoire connected with the complex network of neuroendocrine mechanisms of AVP-OXT system in the modulation of the HPA axis. Thus, these changes may have promoted the emergence of social behaviors such as direct paternal care in socially monogamous species that are also characterized by small body size and twin births.

A Comparison of the Ability of Leu8- and Pro8-Oxytocin to Regulate Intracellular Ca2+ and Ca2+-Activated K+ Channels at Human and Marmoset Oxytocin Receptors

The neurohypophyseal hormone oxytocin (OT) regulates biologic functions in both peripheral tissues and the central nervous system. In the central nervous system, OT influences social processes, including peer relationships, maternal-infant bonding, and affiliative social relationships. In mammals, the nonapeptide OT structure is highly conserved with leucine in the eighth position (Leu⁸-OT). In marmosets (Callithrix), a nonsynonymous nucleotide substitution in the OXT gene codes for proline in the eighth residue position (Pro⁸-OT). OT binds to its cognate G protein-coupled receptor (OTR) and exerts diverse effects, including stimulation (G_s) or inhibition (G_i/o) of adenylyl cyclase, stimulation of potassium channel currents (G_i), and activation of phospholipase C (G_q). Chinese hamster ovary cells expressing marmoset or human oxytocin receptors (mOTRs or hOTRs, respectively) were used to characterize OT signaling. At the mOTR, Pro⁸-OT was more efficacious than Leu⁸-OT in measures of G_q activation, with both peptides displaying subnanomolar potencies. At the hOTR, neither the potency nor efficacy of Pro⁸-OT and Leu⁸-OT differed with respect to G_q signaling. In both mOTR- and hOTR-expressing cells, Leu⁸-OT was more potent and modestly more efficacious than Pro⁸-OT in inducing hyperpolarization. In mOTR cells, Leu⁸-OT-induced hyperpolarization was modestly inhibited by pretreatment with pertussis toxin (PTX), consistent with a minor role for G_i/o activation; however, the Pro⁸-OT response in mOTR and hOTR cells was PTX insensitive. These findings are consistent with membrane hyperpolarization being largely mediated by a G_q signaling mechanism leading to Ca²⁺-dependent activation of K⁺ channels. Evaluation of the influence of apamin, charybdotoxin, paxilline, and TRAM-34 demonstrated involvement of both intermediate and large conductance Ca²⁺-activated K⁺ channels.

Oxytocin modulates mate-guarding behavior in marmoset monkeys

In socially-monogamous species, intolerance of interactions between a pairmate and a sexual rival (i.e., mate-guarding) promotes the preservation of long-lasting partnerships. One promising neurobiological candidate for the regulation of mate-guarding behavior in monogamous primates is the oxytocin (OT) system, given its established role in both the development of monogamous bonds and the behavioral processes that facilitate the preservation of those bonds. In this study, male and female marmosets were exposed to a same-sex intruder in their home environment during conditions when their pairmate was present and absent, and across three treatment conditions (OT receptor agonist; saline control; OT receptor antagonist). Saline-treated marmosets spent significantly more time in proximity to the intruder, relative to the empty pairmate enclosure, when their pairmate was absent. However, when marmosets received OT they spent less time in proximity to the intruder, indicating that OT may reduce interest in a same-sex stranger in a territorial context. When their pairmate was present, saline-treated marmosets spent equal time in proximity to both intruder and pairmate; yet when they received OT they spent significantly more time in proximity to the intruder, indicating that OT may increase interest in a same-sex stranger in a mate-guarding context. While OT treatment did not directly influence the expression of aggression, OT system manipulations impacted the expression of selective social interest during an intruder challenge, suggesting that OT may enhance adaptive responses to social challenges. Moreover, these findings add to the converging evidence that the OT system regulates behavioral processes that underlie the preservation of established relationships.

Oxytocin structure and function in New World monkeys: from pharmacology to behavior

Oxytocin (OT) is a hypothalamic nonapeptide that mediates a host of physiological and behavioral processes including reproductive physiology and social attachments. While the OT sequence structure is highly conserved among mammals, New World monkeys (NWMs) represent an unusual "hot spot" in OT structure variability among mammals. At least 6 distinct OT ligand variants among NWMs exist, yet it is currently unclear whether these evolved structural changes result in meaningful functional consequences. NWMs offer a new area to explore how these modifications to OT and its canonical G-protein coupled OT receptor (OTR) may mediate specific cellular, physiological and behavioral outcomes. In this review, we highlight relationships between OT ligand and OTR structural variability, specifically examining coevolution between OT ligands, OTRs, and physiological and behavioral phenotypes across NWMs. We consider whether these evolved modifications to the OT structure alter pharmacological profiles at human and marmoset OTRs, including changes to receptor binding, intracellular signaling and receptor internalization. Finally, we evaluate whether exogenous manipulation using OT variants in marmoset monkeys differentially enhance or impair behavioral processes involved in social relationships between pairmates, opposite-sex strangers, and parents and their offspring. Overall, it appears that changes to OT ligands in NWMs result in important changes ranging from cellular signaling to broad measures of social behavior.

Oxytocin regulates reunion affiliation with a pairmate following social separation in marmosets

While separation from significant social partners produces a host of neurobiological and behavioral perturbations, including behavioral distress and increased glucocorticoid production, positive social interactions upon reunion are critical for the reestablishment of normative relationship dynamics and the attenuation of the biobehavioral stress response. The hormone oxytocin has critical and pervasive roles in reproductive and behavioral processes across the lifespan, and plays a particularly prominent role in social bonding. In this study, we examined the extent that oxytocin modulates interactions with a pairmate following separation challenges that varied in both social context (isolation; separation) and duration (long; short), in marmosets. We demonstrated that the impact of pharmacological manipulations of the oxytocin system on the expression of affiliation upon reunion depended on both the context and duration of the separation challenge. Specifically, marmosets treated with an oxytocin antagonist spent less time in proximity with their pairmate upon reunion following a long-separation challenge. During the short-separation challenge, marmosets engaged in more social gaze when separated with an opposite-sex stranger, but not when separated with their mate. Furthermore, marmosets that received the most social gaze from opposite-sex strangers spent the most time in proximity with their long-term mate upon reunion. We also showed that marmosets treated with an OT agonist received increased levels of gaze from opposite-sex strangers, but not from their mate. Overall, these results suggest that marmosets are sensitive to the nature of the social interactions during separation, and subsequently alter their expression of affiliation upon reunion with their long-term mate. These findings further implicate oxytocin as a bond-enhancing molecule that regulates the reestablishment of normative levels of affiliation with a mate following separation, and add to the emerging literature that suggests the OT system underlies critical behavioral processes that contribute to the preservation of long-lasting social bonds.

Binding Characteristics of Two Oxytocin Variants and Vasopressin at Oxytocin Receptors from Four Primate Species with Different Social Behavior Patterns

A clade of New World monkeys (NWMs) exhibits considerable diversity in both oxytocin (OT) ligand and oxytocin receptor (OTR) structure. Most notable is the variant Pro⁸-OT, with proline instead of leucine at the eighth position, resulting in a rigid bend in the peptide backbone. A higher proportion of species that express Pro⁸-OT also engage in biparental care and social monogamy. When marmosets (genus Callithrix), a biparental and monogamous Pro⁸-OT NWM species, are administered the ancestral Leu⁸-OT, there is no change in social behavior compared with saline treatment. However, when Pro⁸-OT is administered, marmosets' sociosexual and prosocial behaviors are altered. The studies here tested the hypothesis that OTR binding affinities and OT-induced intracellular Ca²⁺ potencies would favor the native OT ligand in OTRs from four primate species, each representing a unique combination of ancestral lineage, breeding system, and native OT ligand: humans (Leu⁸-OT, monogamous, apes), macaques (Leu⁸-OT, nonmonogamous, Old World monkey), marmosets (Pro⁸-OT, monogamous, NWM), and titi monkeys (Leu⁸-OT, monogamous, NWM). OTRs were expressed in immortalized Chinese hamster ovary cells and tested for intact-cell binding affinities for Pro⁸-OT, Leu⁸-OT, and arginine vasopressin (AVP), as well as intracellular Ca²⁺ signaling after stimulation with Pro⁸-OT, Leu⁸-OT, and AVP. Contrary to our hypothesis, Pro⁸-OT bound at modestly higher affinities and stimulated calcium signaling at modestly higher potencies compared with Leu⁸-OT in all four primate OTRs. Thus, differences downstream from a ligand-receptor binding event are more likely to explain the different behavioral responses to these two ligands.

The Oxytocin Receptor: From Intracellular Signaling to Behavior

The many facets of the oxytocin (OXT) system of the brain and periphery elicited nearly 25,000 publications since 1930 (see FIGURE 1 , as listed in PubMed), which revealed central roles for OXT and its receptor (OXTR) in reproduction, and social and emotional behaviors in animal and human studies focusing on mental and physical health and disease. In this review, we discuss the mechanisms of OXT expression and release, expression and binding of the OXTR in brain and periphery, OXTR-coupled signaling cascades, and their involvement in behavioral outcomes to assemble a comprehensive picture of the central and peripheral OXT system. Traditionally known for its role in milk let-down and uterine contraction during labor, OXT also has implications in physiological, and also behavioral, aspects of reproduction, such as sexual and maternal behaviors and pair bonding, but also anxiety, trust, sociability, food intake, or even drug abuse. The many facets of OXT are, on a molecular basis, brought about by a single receptor. The OXTR, a 7-transmembrane G protein-coupled receptor capable of binding to either Gαior Gαqproteins, activates a set of signaling cascades, such as the MAPK, PKC, PLC, or CaMK pathways, which converge on transcription factors like CREB or MEF-2. The cellular response to OXT includes regulation of neurite outgrowth, cellular viability, and increased survival. OXTergic projections in the brain represent anxiety and stress-regulating circuits connecting the paraventricular nucleus of the hypothalamus, amygdala, bed nucleus of the stria terminalis, or the medial prefrontal cortex. Which OXT-induced patterns finally alter the behavior of an animal or a human being is still poorly understood, and studying those OXTR-coupled signaling cascades is one initial step toward a better understanding of the molecular background of those behavioral effects.

Social Monogamy in Nonhuman Primates: Phylogeny, Phenotype, and Physiology

Monogamy as a social system has been both a scientific puzzle and a sociocultural issue for decades. In this review, we examine social monogamy from a comparative perspective with a focus on primates, our closest genetic relatives. We break down monogamy into component elements, including pair-bonding and partner preference, mate guarding or jealousy, social attachment, and biparental care. Our survey of primates shows that not all features are present in species classified as socially monogamous, in the same way that human monogamous relationships may not include all elements-a perspective we refer to as "monogamy à la carte." Our review includes a survey of the neurobiological correlates of social monogamy in primates, exploring unique or common pathways for the elemental components of monogamy. This compilation reveals that the components of monogamy are modulated by a suite of androgenic steroids, glucocorticoid hormones, the nonapeptide hormones oxytocin and vasopressin, and other neurotransmitter systems (e.g., dopamine and opioids). We propose that efforts to understand the biological underpinnings of complex human and animal sociosexual relationships will be well served by exploring individual phenotypic traits, as opposed to pursuing these questions with the assumption that monogamy is a unitary trait or a species-specific characteristic.

Titi Monkeys as a Novel Non-Human Primate Model for the Neurobiology of Pair Bonding

It is now widely recognized that social bonds are critical to human health and well-being. One of the most important social bonds is the attachment relationship between two adults, known as the pair bond. The pair bond involves many characteristics that are inextricably linked to quality of health, including providing a secure psychological base and acting as a social buffer against stress. The majority of our knowledge about the neurobiology of pair bonding comes from studies of a socially monogamous rodent, the prairie vole (Microtus ochrogaster), and from human imaging studies, which inherently lack control. Here, we first review what is known of the neurobiology of pair bonding from humans and prairie voles. We then present a summary of the studies we have conducted in titi monkeys (Callicebus cupreus)-a species of socially monogamous New World primates. Finally, we construct a neural model based on the location of neuropeptide receptors in the titi monkey brain, as well as the location of neural changes in our imaging studies, with some basic assumptions based on the prairie vole model. In this model, we emphasize the role of visual mating stimuli as well as contributions of the dopaminergic reward system and a strong role for the lateral septum. This model represents an important step in understanding the neurobiology of social bonds in non-human primates, which will in turn facilitate a better understanding of these mechanisms in humans.

Functional New World monkey oxytocin forms elicit an altered signaling profile and promotes parental care in rats

The neurohormone oxytocin is a key player in the modulation of reproductive and social behavioral traits, such as parental care. Recently, a correlation between different forms of oxytocin and behavioral phenotypes has been described in the New World Monkeys (NWMs). Here, we demonstrate that, compared with the Leu⁸OXT found in most placental mammals, the Cebidae Pro⁸OXT and Saguinus Val³Pro⁸OXT taxon-specific variants act as equi-efficacious agonists for the G_q-dependent pathway but are weaker agonists for the β-arrestin engagement and subsequent endocytosis toward the oxytocin receptor (OXTR). Upon interaction with the AVPR1a, Pro⁸OXT and the common Leu⁸OXT yielded similar signaling profiles, being equally efficacious on G_q and β-arrestin, while Val³Pro⁸OXT showed reduced relative efficacy toward β-arrestin. Intranasal treatment with either of the variants increased maternal behavior and also promoted unusual paternal care in rats, as measured by pup-retrieval tests. We therefore suggest that Val³Pro⁸OXT and Pro⁸OXT are functional variants, which might have been evolutionarily co-opted as an essential part of the adaptive genetic repertoire that allowed the emergence of taxon-specific complex social behaviors, such as intense parental care in the Cebidae and the genus Saguinus.

Vasopressin and Oxytocin Reduce Food Sharing Behavior in Male, but Not Female Marmosets in Family Groups

Oxytocin (OT) is critical for lactation and maternal care, but OT and the related nonapeptide vasopressin are important for caregiving behaviors in fathers and alloparents as well. This experiment tested the effects of vasopressin and OT on food sharing in marmoset families. We treated caregivers (parents, siblings) with intranasal vasopressin, OT, or saline, and then paired them with the youngest marmoset in the family. Caregivers were given preferred food, and then observed for food sharing and aggressive behavior with young marmosets. OT reduced food sharing from male alloparents to youngest siblings, and fathers that received vasopressin refused to share food with their youngest offspring more often than when treated with OT. Vasopressin increased aggressive vocalizations directed toward potential food recipients in all classes of caregivers. These results indicate that vasopressin and OT do not always enhance prosocial behavior: modulation of food sharing depends on both sex and parental status.

Out-Group Threat Responses, In-Group Bias, and Nonapeptide Involvement Are Conserved across Vertebrates: (A Comment on Bruintjes et al., "Out-Group Threat Promotes Within-Group Affiliation in a Cooperative Fish")

The challenges and threats posed by out-groups have major effects on human social behavior and how individuals interact with one another. We briefly review evidence here that out-group threat similarly affects nonhuman animal behavior. Actual and potential threats posed by out-group individuals (unfamiliar and genetically nonrelated individuals of the same species) affect social behavior promoting "out-group" avoidance and "in-group" bias and enhancing in-group (familiar and/or genetically related individuals) affiliation and interactions. Individuals from out-groups present risks of pathogen exposure as well as being threats to resources, territory, and offspring. All of these threats function to promote in-group bias in humans and nonhumans. There are also striking similarities in the underlying neurobiological mechanisms mediating the responses to out-group threat and the expression of in-group bias. In particular, the evolutionarily conserved, hormone-regulated nonapeptide systems (oxytocin, arginine-vasopressin, and homologous neuropeptides and their receptors) are involved in the mediation of the detection and avoidance of out-groups and response to in-groups and facilitation of in-group responses across multiple vertebrate species. Consequently, comparative investigations of both the behavioral expression of and the mechanism underlying out-group avoidance and in-group bias are necessary for a full understanding of the evolution of social behavior and responses to in- and out-groups.