Peripheral pulses of oxytocin increase partner preferences in female, but not male, prairie voles

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.

Conditioned preferences: Gated by experience, context, and endocrine systems

Central to the navigation of an ever-changing environment is the ability to form positive associations with places and conspecifics. The functions of location and social conditioned preferences are often studied independently, limiting our understanding of their interplay. Furthermore, a de-emphasis on natural functions of conditioned preferences has led to neurobiological interpretations separated from ecological context. By adopting a naturalistic and ethological perspective, we uncover complexities underlying the expression of conditioned preferences. Development of conditioned preferences is a combination of motivation, reward, associative learning, and context, including for social and spatial environments. Both social- and location-dependent reward-responsive behaviors and their conditioning rely on internal state-gating mechanisms that include neuroendocrine and hormone systems such as opioids, dopamine, testosterone, estradiol, and oxytocin. Such reinforced behavior emerges from mechanisms integrating past experience and current social and environmental conditions. Moreover, social context, environmental stimuli, and internal state gate and modulate motivation and learning via associative reward, shaping the conditioning process. We highlight research incorporating these concepts, focusing on the integration of social neuroendocrine mechanisms and behavioral conditioning. We explore three paradigms: 1) conditioned place preference, 2) conditioned social preference, and 3) social conditioned place preference. We highlight nonclassical species to emphasize the naturalistic applications of these conditioned preferences. To fully appreciate the complex integration of spatial and social information, future research must identify neural networks where endocrine systems exert influence on such behaviors. Such research promises to provide valuable insights into conditioned preferences within a broader naturalistic context.

Naturally occurring low sociality in female rhesus monkeys: A tractable model for autism or not?

BACKGROUND

Autism spectrum disorder (ASD) is characterized by persistent social interaction impairments and is male-biased in prevalence. We have established naturally occurring low sociality in male rhesus monkeys as a model for the social features of ASD. Low-social male monkeys exhibit reduced social interactions and increased autistic-like trait burden, with both measures highly correlated and strongly linked to low cerebrospinal fluid (CSF) arginine vasopressin (AVP) concentration. Little is known, however, about the behavioral and neurochemical profiles of female rhesus monkeys, and whether low sociality in females is a tractable model for ASD.

METHODS

Social behavior assessments (ethological observations; a reverse-translated autistic trait measurement scale, the macaque Social Responsiveness Scale-Revised [mSRS-R]) were completed on N = 88 outdoor-housed female rhesus monkeys during the non-breeding season. CSF and blood samples were collected from a subset of N = 16 monkeys across the frequency distribution of non-social behavior, and AVP and oxytocin (OXT) concentrations were quantified. Data were analyzed using general linear models.

RESULTS

Non-social behavior frequency and mSRS-R scores were continuously distributed across the general female monkey population, as previously found for male monkeys. However, dominance rank significantly predicted mSRS-R scores in females, with higher-ranking individuals showing fewer autistic-like traits, a relationship not previously observed in males from this colony. Females differed from males in several other respects: Social behavior frequencies were unrelated to mSRS-R scores, and AVP concentration was unrelated to any social behavior measure. Blood and CSF concentrations of AVP were positively correlated in females; no significant relationship involving any OXT measure was found.

LIMITATIONS

This study sample was small, and did not consider genetic, environmental, or other neurochemical measures that may be related to female mSRS-R scores.

CONCLUSIONS

Dominance rank is the most significant predictor of autistic-like traits in female rhesus monkeys, and CSF neuropeptide concentrations are unrelated to measures of female social functioning (in contrast to prior CSF AVP findings in male rhesus monkeys and male and female autistic children). Although preliminary, this evidence suggests that the strong matrilineal organization of this species may limit the usefulness of low sociality in female rhesus monkeys as a tractable model for ASD.

Oxytocin prevents dysregulation of the acute stress response and glucocorticoid-induced oxidative stress in chronically isolated prairie voles

Chronic social isolation can lead to dysregulation of many physiological and psychological processes, including the ability to respond to acute stressors. Previous work in our lab reported that six weeks of social isolation in prairie voles (Microtus ochrogaster) caused increased glucocorticoid levels, oxidative damage, telomere degradation and anhedonia, and that oxytocin treatment prevented all of these changes. Following these results, we investigated how chronic social isolation with and without oxytocin treatment affected glucocorticoid (CORT) and oxidative stress responses to an acute stressor, a 5-min resident-intruder (R-I) test at the end of the social isolation period. To investigate the effect of a brief acute stressor on CORT and oxidative stress, baseline blood samples were collected following six weeks of social isolation, 24-hrs before the R-I test. Two more blood samples were collected 15-min after the end of the R-I test, and again 25-min later to measure peak and recovery responses, respectively. Isolated animals had higher baseline, peak, recovery, and integrated levels of CORT and reactive oxygen metabolites (ROMs, a measure of oxidative stress), compared to animals that did not experience isolation. Importantly, oxytocin treatment throughout the isolation period prevented these elevations in CORT and ROMs. No significant changes were observed in total antioxidant capacity (TAC). Levels of CORT and ROMs at the peak and recovery time points were positively correlated. These data show that acute stress in chronically isolated prairie voles, then, is associated with increased glucocorticoid-induced oxidative stress (GiOS), and that oxytocin mitigates isolation-induced dysregulation of glucocorticoid and oxidative stress acute stress responses.

Microbiome and Metabolome Variation as Indicator of Social Stress in Female Prairie Voles

Social isolation is detrimental to the health of social mammals inducing neurochemical and hormonal changes related to depression and anxiety, as well as impairments of cardiovascular and immune functioning. Likewise, perceptions of loneliness are increasingly recognized as detrimental to human psychological well-being, cognitive functioning, and physical health. Few studies, however, have examined the impact of social isolation on the intestinal microbiome and metabolome. To better understand the impact of social isolation on these systems, intestinal microbiota, and the systemic impact via the gut-brain axis, we employed prairie voles. Physiological stress on female prairie voles (n = 22) either with a same-sex sibling (n = 11) or in isolation (n = 11) for four weeks demonstrated behavioral indicators of increased anxiety and depression in isolated voles (p ≤ 0.01). Bacterial DNA from fecal and colon samples, collected at five time points (T0-4), were sequenced for all nine hypervariable regions of the 16S rRNA gene. Microbiome analyses revealed several differences in gut communities of paired and isolated voles with greater differences at T4. Notably, several taxa associated with host health including Anaerostipes and Lactobacillaceae were more prevalent in paired voles, whereas several taxa associated with known pathogens (e.g., Staphylococcaceae and Enterococcus) or disease were elevated in isolated animals. Similarly, metabolome analyses suggested isolated voles, when compared to paired animals, exhibited differences in metabolites associated with diabetes and colitis. These findings further contribute to our understanding of the harmful effects of social isolation, which cause perturbations in the gut microbiome and serum metabolites.

Sex matters: The impact of oxytocin on healthy conditions and psychiatric disorders

Oxytocin (OT) is involved in the regulation of physiological processes and emotional states, with increasing evidence for its beneficial actions being mediated by the autonomic and immune systems. Growing evidence suggests that OT plays a role in the pathophysiology of different psychiatric disorders. Given the limited information in humans the aim of this study was to retrospectively explore plasma OT levels in psychiatric patients, particularly focusing on sex-related differences, as compared with healthy controls. The patients studied here were divided into three groups diagnosed with obsessive-compulsive disorder (OCD), post-traumatic stress disorder (PTSD) or major depressive disorder (MDD). Plasma OT levels were significantly different between healthy men and women, with the latter showing higher values, while none of the three psychiatric groups showed sex-related differences in the parameters measured here. The intergroup analyses showed that the OT levels were significantly higher in OCD, lower in PTSD and even more reduced in MDD patients than in healthy subjects. These differences were also confirmed when gender was considered, with the exception of PTSD men, in whom OT levels were similar to those of healthy men. The present results indicated that OT levels were higher amongst healthy women than men, while a sex difference was less apparent or reversed in psychiatric patients. Reductions in sex differences in psychopathologies may be related to differential vulnerabilities in processes associated with basic adaptive and social functions.

Fear, love, and the origins of canid domestication: An oxytocin hypothesis

The process of dog domestication likely involved at least two functional stages. The initial stage occurred when subpopulations of wolves became synanthropes, benefiting from life nearby or in human environments. The second phase was characterized by the evolution of novel forms of interspecific cooperation and social relationships between humans and dogs. Here, we discuss possible roles of the oxytocin system across these functional stages of domestication. We hypothesize that in early domestication, oxytocin played important roles in attenuating fear and stress associated with human contact. In later domestication, we hypothesize that oxytocin's most critical functions were those associated with affiliative social behavior, social engagement, and cooperation with humans. We outline possible neurobiological changes associated with these processes and present a Siberian fox model of canid domestication in which these predictions can be tested. Lastly, we identify limitations of current studies on the neuroendocrinology of domestication and discuss challenges and opportunities for future research.

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We propose various roles for oxytocin across canid domestication.

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In early domestication, oxytocin primarily regulated fear and anxiety toward humans.

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In late domestication, oxytocin facilitated interspecific social bonds and cooperation.

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Comparative neurobiology is critical for understanding oxytocin's roles in domestication.

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Experimentally domesticated Siberian foxes provide a powerful model for these studies.

Blending oxytocin and dopamine with everyday creativity

Converging evidence suggests that oxytocin (OT) is associated with creative thinking (CT) and that release of OT depends on ADP ribosyl-cyclases (CD38 and CD157). Neural mechanisms of CT and OT show a strong association with dopaminergic (DA) pathways, yet the link between CT and CD38, CD157, dopamine receptor D2 (DRD2) and catechol-O-methyltransferase (COMT) peripheral gene expression remain inconclusive, thus limiting our understanding of the neurobiology of CT. To address this issue, two principal domains of CT, divergent thinking (AUT), were assessed. In men, both AUT is associated with gene expression of CD38, CD157, and their interaction CD38 × CD157. There were no significant associations for DA expression (DRD2, COMT, DRD2 × COMT) on both CT measures. However, analysis of the interactions of OT and DA systems reveal significant interactions for AUT in men. The full model explained a sizable 39% of the variance in females for the total CT score. The current findings suggest that OT and DA gene expression contributed significantly to cognition and CT phenotype. This provides the first empirical foundation of a more refined understanding of the molecular landscape of CT.

Assessing effects of oxytocin on alcohol consumption in socially housed prairie voles using radio frequency tracking

Alcohol use disorder affects millions of people each year. Currently approved pharmacotherapies have limited success in treating this disorder. Evidence suggests that this lack of success is partly due to how these pharmacotherapies are tested in preclinical settings. The vast majority of preclinical studies assessing the effects of pharmacotherapies on alcohol or drug self-administration are done in individually housed animals. However, it is known that alcohol and drug intake are heavily influenced by social settings. Here, we adapted radio frequency tracking technology to determine the effects of oxytocin, a potential therapy for alcohol use disorder, on alcohol consumption in socially housed male and female prairie voles. Voluntary alcohol consumption in these animals resulted in high daily alcohol intakes, blood ethanol concentrations that are considered intoxicating, and central changes in FosB immunoreactivity, indicative of changes in neural activity. Prairie voles that received oxytocin temporarily reduced alcohol consumption but not alcohol preference, compared with control prairie voles regardless whether their cagemates received a similar treatment or not. Our results demonstrate that oxytocin can decrease consummatory behaviors in the presence of peers that are not receiving this treatment, and therefore, its potential use in clinical trials is warranted. Moreover, effectiveness of other pharmacotherapies in preclinical studies can be tested in mixed-treatment socially housed animals similarly to clinical studies in humans.

Oxytocin in Schizophrenia: Pathophysiology and Implications for Future Treatment

Schizophrenia is a form of mental disorder that is behaviorally characterized by abnormal behavior, such as social function deficits or other behaviors that are disconnected from reality. Dysregulation of oxytocin may play a role in regulating the expression of schizophrenia. Given oxytocin’s role in social cognition and behavior, a variety of studies have examined the potential clinical benefits of oxytocin in improving the psychopathology of patients with schizophrenia. In this review, we highlight the evidence for the role of endogenous oxytocin in schizophrenia, from animal models to human studies. We further discuss the potential of oxytocin as a therapeutic agent for schizophrenia and its implication in future treatment.

Sex differences of oxytocin and vasopressin in social behaviors

The neuropeptides oxytocin (OT) and vasopressin (VP) are known to mediate social cognition and behaviors in a sex-dependent manner. This chapter reviews the sex-dependent influence of OT and VP on social behaviors, focusing on (1) partner preference and sexual orientation, (2) memory modulation, (3) emotion regulation, and (4) trust-related behaviors. Most studies suggest that OT promotes familiar (opposite-sex) partner preference, strengthens memory, relieves anxiety, and increases trust. However, VP-regulated social cognition has been studied less than OT. VP facilitates familiar (opposite-sex) partner preference, enhances memory, induces anxiety, and influences happiness/anger perception. Detailed sex differences of these effects are reviewed. There is a male preponderance in the use of animal models and many study results are too complex to draw firm conclusions. Clarifying the complex interplay between the OT/VP system and sex hormones in the regulation of social behaviors is needed.

Nature and nurture of sexual partner preference: Teachings from prenatal administration of acetaminophen in male rats

The organizational-activational hypothesis indicates that activation of adult sexual behavior in males depends on organization of the masculine brain during the perinatal sensitive period. In the medial preoptic area such masculinization depends on a neuroendocrine cascade that includes exposure to testosterone, aromatization to estradiol, activation of estrogen receptors, synthesis of cyclooxygenase (COX), increase of prostaglandins, release of glutamate, and activation of AMPA receptors that result in the formation of more dendritic spines. Thus, in the present study we assessed the sexual partner preference (SPP) of adult male rats prenatally treated with acetaminophen (APAP), an analgesic/antipyretic drug that inhibits COX-2 and is commonly used and prescribed during pregnancy. Female rats received either saline (2 ml/kg s.c.) or APAP (50 mg/kg s.c.) every 12 h, during days 16-20 of pregnancy. At postnatal day PD60 half of the male offspring were exposed to sexual experience with receptive females during 5 trials, and the other half remained sexually naïve. At PD90 all them were tested for SPP with one sexually receptive female and one stud male. The results indicated that only APAP-naïve males failed to display SPP. However, APAP-experienced males displayed SPP for females. We discuss the effects of prenatal APAP in the disruption of unconditioned responses towards females (nature mechanisms), and the effects of sexual experience (nurture mechanisms) in the development of conditioned heterosexual preference.

Effect of isotocin on shoaling behaviour of the Guppy (Poecilia reticulata)

Pro-social effects of oxytocin and its homologues are well-documented in birds and mammals. However, in fishes, the effect of isotocin, the homologue of oxytocin, on social behaviour is less clear. Studies in fishes have generally shown no effect of isotocin on social behaviours or even an anti-social effect. In our study, we measured association preference for conspecifics in 92 adult guppies (46 females and 46 males), half of which were injected with isotocin and the other half with an isotocin antagonist. We found that individuals injected with isotocin spent 29% more time associating with conspecifics than individuals injected with an isotocin antagonist. The effect of isotocin on association time did not differ between males and females. Our study provides some of the first evidence of a pro-social effects of isotocin in a fish and suggests that in fishes, isotocin may have a homologous role to oxytocin, at least in promoting shoaling behaviour.

Fos expression is increased in oxytocin neurons of female rats with a sexually conditioned mate preference for an individual male rat

Evidence suggests an important role of Pavlovian learning in sexual partner selection. Female rats that experience paced copulation with a male scented with a neutral odor selectively solicit and receive ejaculations from the scented male relative to an unscented male. Exposure to the conditioned odor alone induces Fos protein in regions of the brain associated with sexual excitation. Here we tested whether female rats can be conditioned to show a sexual preference for an unscented male rat of the same strain. Female Long-Evans rats were given 10 copulatory trials with either a one-hole pacing divider or a four-hole pacing divider in a unilevel chamber with the same conspecific male (n = 16). Females were then given an open-field partner preference test with the paired male versus a novel male. After two reconditioning trials females were exposed to the partner or a novel male to induce Fos expression. Females that paced with the one-hole divider received the first ejaculation and more ejaculations overall from the paired compared to novel male. Fos immunoreactivity within oxytocin neurons in the PVN, mPOA, and VMH was increased in females with a preference that were exposed to the paired male. These data indicate that female rats can form selective sexual preferences for an individual conspecific and that their formation depends on the type of pacing during conditioning. These findings further suggest the involvement of oxytocin in the display of conditioned preferences. Thus, early copulatory experience appears to determine the mating strategy used by female rats.

Long-Acting and Selective Oxytocin Peptide Analogs Show Antidiabetic and Antiobesity Effects in Male Mice

Oxytocin (OXT) has been shown to suppress appetite, induce weight loss, and improve glycemic control and lipid metabolism in several species, including humans, monkeys, and rodents. However, OXT's short half-life in circulation and lack of receptor selectivity limit its application and efficacy. In this study, we report an OXT peptide analog (OXTGly) that is potent and selective for the OXT receptor (OXTR). OXT, but not OXTGly, activated vasopressin receptors in vitro and acutely increased blood pressure in vivo when administered IP. OXT suppressed food intake in mice, whereas OXTGly had a moderate effect on food intake when administered IP or intracerebroventricularly. Both OXT (IP) and OXTGly (IP) improved glycemic control in glucose tolerance tests. Additionally, both OXT (IP) and OXTGly (IP) stimulated insulin, glucagon-like peptide 1, and glucagon secretion in mice. We generated lipid-conjugated OXT (acylated-OXT) and OXTGly (acylated-OXTGly) and demonstrated that these molecules have significantly extended half-lives in vivo. Compared with OXT, 2-week treatment of diet-induced obese mice with acylated-OXT [subcutaneous(ly) (SC)] resulted in enhanced body weight reduction, an improved lipid profile, and gene expression changes consistent with increased lipolysis and decreased gluconeogenesis. Treatment with acylated-OXTGly (SC) also resulted in a statistically significant weight loss, albeit to a lesser degree compared with acylated-OXT treatment. In conclusion, we demonstrate that selective activation of the OXTR pathway results in both acute and chronic metabolic benefits, whereas potential activation of vasopressin receptors by nonselective OXT analogs causes physiological stress that contributes to additional weight loss.

Sexual dimorphism of oxytocin and vasopressin in social cognition and behavior

The neuropeptides oxytocin (OT) and vasopressin (VP) are hormones that are known to mediate social behavior and cognition, but their influence may be sex-dependent. This paper aims to provide a comprehensive review of the sex-related influence of OT and VP on social cognition, focusing on partner preference and sexual orientation, trust and relevant behaviors, memory modulation, and emotion regulation. Most studies have suggested that OT facilitates familiar-partner preference in both sexes, with females being more significant, increased trust in others, especially for male, enhanced memory in either sex, and reduced anxious emotion in males. However, VP-regulated social cognition has been less studied. Other relevant studies have indicated that VP facilitated familiar-partner preference, improved memory, induced empathy formation, increased positive-emotion recognition, and induced anxiety without any sex difference. However, there was a male preponderance among studies, and results were often too complex to draw firm conclusions. Clarifying the interplay between OT/VP and sex hormones in the regulation of social cognition is necessary for further applications.

Oxytocin administration prevents cellular aging caused by social isolation

Chronic stressors, such as chronic isolation in social mammals, can elevate glucocorticoids, which can affect cellular mechanisms of aging, including increased levels of oxidative stress and shortened telomere lengths. Recent work in the selectively social prairie vole (Microtus ochrogaster) suggests that oxytocin and social support may mitigate some of the negative consequences of social isolation, possibly by reducing glucocorticoid levels. We investigated the influences of isolation, social support, and daily oxytocin injections in female prairie voles. Glucocorticoid levels, oxidative damage, telomere length, and anhedonia, a behavioral index of depression, were measured throughout the study. We found that six weeks of chronic isolation led to increased glucocorticoid levels, oxidative damage, telomere degradation and anhedonia. However, daily oxytocin injections in isolated voles prevented these negative consequences. These findings demonstrate that chronic social isolation in female prairie voles is a potent stressor that results in depression-like behavior and accelerated cellular aging. Importantly, oxytocin can completely prevent the negative consequences of social isolation.

Evidence for intranasal oxytocin delivery to the brain: recent advances and future perspectives

The neuropeptide oxytocin plays an evolutionarily conserved role in mammalian social behavior. Despite striking effects on animal social behavior after intracerebroventricular drug delivery, this delivery mode is impractical in humans. Intranasal oxytocin delivery provides a noninvasive alternative to increase central oxytocin activity, and has shown promise as a treatment for psychiatric illnesses. Intranasal oxytocin delivery is purported to increase central oxytocin concentrations via channels surrounding trigeminal and olfactory nerve fibers, which may facilitate increased activity at central oxytocin receptors. This report outlines the evidence for intranasal oxytocin delivery increasing central concentrations or activity, identifies current knowledge gaps and highlights future research opportunities. Recent efforts to enhance intranasal oxytocin delivery via improved intranasal delivery technology and dose-ranging studies are discussed.

Sex-Related Differences in Plasma Oxytocin Levels in Humans

Background:

Increasing evidence supports a key role of Oxytocin (OT) as a modulator of social relationships in mammals.

Objective:

The aim of the present study was to investigate possible sex-related differences in plasma OT levels in human beings.

Methods:

Forty-five healthy men and 45 women (mean age: 34.9 ± 6.2 years), were included in the study. Plasma preparation, peptide extraction and OT radioimmunoassay were carried out according to standardized methods.

Results:

The results showed that OT plasma levels (pg / ml, mean ± SD) were significantly higher in women than in men (4.53 ± 1.18 vs 1.53 ± 1.19, p ˂ 0.001).

Conclusions:

The present finding demonstrates sex-related differences in plasma OT levels in humans. It is tempting to hypothesize that such differences might be related to behaviours, attitudes, as well as susceptibility to stress response, resilience and social emotions specific of women and men.

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

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

Oxytocin and vasopressin increase male-directed threats and vocalizations in female macaques

In a previous study, we reported that intranasal delivery of both oxytocin (OT) and arginine vasopressin (AVP) to male macaques relaxes spontaneous social interactions, flattens the existing dominance hierarchy, and increases behavioral synchrony with other monkeys. Here we report that intranasal OT and AVP administration modulates the behaviors of female macaque monkeys, but in robustly different ways from males. Most notably, both neuropeptides increase threatening and vocalization behaviors of females when they encounter males, and these behaviors effectively increase the social status of females over males. While OT and AVP heighten the confrontational nature of intersexual encounters, both peptides relax interactions between females. Finally, as previously reported for males, treating an individual female monkey with OT or AVP significantly modulates the behavior of her non-treated partner. Together, these findings show that OT and AVP can either inhibit or promote aggression, depending on sex and behavioral context, and call for a more careful, systematic examination of the functions of these neuropeptides in both sexes, especially in the context of therapeutics for human social disorders.

Novel unconditioned prosocial behavior in prairie voles (Microtus ochrogaster) as a model for empathy

OBJECTIVE

In this study, empathy is quantified using a novel social test. Empathy and prosocial behavior are linked to the expression of oxytocin in humans and rodent models. Specifically, prosocial behavior in prairie voles (Microtus ochrogaster) has been linked to the expression of oxytocin in the paraventricular nucleus of the hypothalamus. The animal's behavior was considered empathic if it spends significantly more time attempting to remove a loos fitting restraint (tether) from the stimulus animal than time in contact with a, simultaneously presented, non-social object similar to the tether. The behavioral data was cross-referenced with the number of neurons expressing oxytocin and arginine vasopressin, as well as the density of dopaminergic neurons (identified by the expression of tyrosine hydroxylase), in the paraventricular nucleus of the hypothalamus. These proteins influence empathic behavior in humans, non-human primates, rats, mice, and prairie voles.

RESULTS

The consistency between neuroanatomical mechanisms linked to empathy, and the durations of time spent engaging in empathic contact, support the prediction that the empathic contact in this test is a distinct prosocial behavior, lacking prior behavioral training or the naturally occurring ethological relevance of other prosocial behaviors, and is a measure of empathy.

Making waves: Comparing Morris water task performance in rats and prairie voles

Spatial processing is a critical component for survival. This domain of information processing has been extensively studied in rats and mice. Limited work has examined the capacity of other rodent species, like the prairie vole (Microtus ochrogaster), to process spatial information. The Morris water task (MWT) is a classic spatial task that has been used to examine spatial cognition in rodents. This task involves an animal developing configural relationships between extra-maze cues and the location of a hidden platform to successfully escape from a pool of water. The current study compared performance in the MWT between rats and prairie voles. Rats were observed to outperform prairie voles in key aspects of the task including latency to find the platform, directness of swim paths to the platform, and degrees of heading error. These results may be attributed to potential interspecies differences in spatial cognition, stress reactivity, physiology, or motivation. This study provides the foundation for future work investigating the spatial cognition of prairie voles and the factors that contribute to water task performance in rodents.

Inhibiting ERα expression in the medial amygdala increases prosocial behavior in male meadow voles (Microtus pennsylvanicus)

This study tested the hypothesis that site-specific estrogen receptor alpha (ERα) expression is a critical factor in the expression of male prosocial behavior and aggression. Previous studies have shown that in the socially monogamous prairie vole (Microtus ochrogaster) low levels of ERα expression, in the medial amygdala (MeA), play an essential role in the expression of high levels of male prosocial behavior and that increasing ERα expression reduced male prosocial behavior. We used an shRNA adeno-associated viral vector to knock down/inhibit ERα in the MeA of the polygynous male meadow vole (M. pennsylvanicus), which displays significantly higher levels of ERα in the MeA than its monogamous relative. Control males were transfected with a luciferase expressing AAV vector. After treatment males participated in three social behavior tests, a same-sex dyadic encounter, an opposite-sex social preference test and an alloparental test. We predicted that decreasing MeA ERα would increase male meadow vole's prosocial behavior and reduce aggression. The results generally supported the hypothesis. Specifically, MeA knockdown males displayed lower levels of defensive aggression during dyadic encounters and increased levels of overall side-x-side physical contact with females during the social preference test, eliminating the partner preference observed in controls. There was no effect on pup interactions, with both treatments expressing low levels of alloparental behavior. Behaviors affected were similar to those in male prairie voles with increased ERα in the BST rather than the MeA, suggesting that relative changes of expression within these nuclei may play a critical role in regulating prosocial behavior.

Oxytocin and vasopressin flatten dominance hierarchy and enhance behavioral synchrony in part via anterior cingulate cortex

The neuropeptides oxytocin (OT) and arginine vasopressin (AVP) influence social functions in many mammals. In humans and rhesus macaques, OT delivered intranasally can promote prosocial behavior in certain contexts. Yet the precise neural mechanisms mediating these behavioral effects remain unclear. Here we show that treating a group of male macaque monkeys intranasally with aerosolized OT relaxes their spontaneous social interactions with other monkeys. OT reduces differences in social behavior between dominant and subordinate monkeys, thereby flattening the status hierarchy. OT also increases behavioral synchrony within a pair. Intranasal delivery of aerosolized AVP reproduces the effects of OT with greater efficacy. Remarkably, all behavioral effects are replicated when OT or AVP is injected focally into the anterior cingulate gyrus (ACCg), a brain area linked to empathy and other-regarding behavior. ACCg lacks OT receptors but is rich in AVP receptors, suggesting exogenous OT may shape social behavior, in part, via nonspecific binding. Notably, OT and AVP alter behaviors of both the treated monkey and his untreated partner, consistent with enhanced feedback through reciprocal social interactions. These findings bear important implications for use of OT in both basic research and as a therapy for social impairments in neurodevelopmental disorders.

The Influence of Environmental Enrichment on Cardiovascular and Behavioral Responses to Social Stress

OBJECTIVE

Stress is linked to negative cardiovascular consequences and increases in depressive behaviors. Environmental enrichment (EE) involves exposure to novel items that provide physical and cognitive stimulation. EE has behavioral, cognitive, and neurobiological effects that may improve stress responses in humans and animal models. This study investigated the potential protective effects of EE on behavior and cardiovascular function in female prairie voles after a social stressor.

METHODS

Radiotelemetry transmitters were implanted into female prairie voles to measure heart rate (HR) and heart rate variability (HRV) throughout the study. All females were paired with a male partner for 5 days, followed by separation from their partner for 5 additional days, and a 10-day treatment period. Treatment consisted of continued isolation, isolation with EE, or re-pairing with the partner (n = 9 per group). After treatment, animals were observed in the forced swim test (FST) for measures of stress coping behaviors.

RESULTS

Isolation elevated HR and reduced HRV relative to baseline for all groups (p < .001). HR and HRV returned to baseline in the EE and re-paired groups, but not in the continued isolation group (p < .001). Animals in the EE and re-paired groups displayed significantly lower immobility time (p < .001) and HR (p < .03) during the FST, with a shorter latency for HR to return to baseline levels after the FST, relative to the continued isolation group (p < .001).

CONCLUSIONS

EE and re-pairing reversed the negative behavioral and cardiovascular consequences associated with social isolation.

A comparison of methods to measure central and peripheral oxytocin concentrations in human and non-human primates

Oxytocin (OT) concentration in the blood is considered to be a marker of its action in the brain. However, two problems have emerged when measuring OT level in the blood. First, it is unclear whether different methods of assessment lead to similar OT values. Second, it is unclear if plasma OT concentrations is informative on what OT does in the brain. To clarify these issues, we collected cerebrospinal fluid (CSF) from the brain ventricle of 25 patients during surgery to compare with plasma OT after simultaneous blood withdrawal. Additionally, we collected 12 CSF and blood samples from non-human primates while awake or under anaesthesia. We used four methods to assay OT concentrations: Commercial EIA with/without extraction, laboratory developed EIA with filtration and RIA with extraction. Three of these methods showed a positive correlation between plasma and CSF OT, suggesting a link between plasma and central OT, at least under specific testing conditions. However, none of the methods correlated to each other. Our results show major disagreements among methods used here to measure peripheral and brain OT and therefore they call for more caution when plasma OT is taken as a marker of central OT.

Intranasal Oxytocin May Improve High-Level Social Cognition in Schizophrenia, But Not Social Cognition or Neurocognition in General: A Multilevel Bayesian Meta-analysis

While there is growing interest in the potential for intranasal oxytocin (IN-OT) to improve social cognition and neurocognition (ie, nonsocial cognition) in schizophrenia, the extant literature has been mixed. Here, we perform a Bayesian meta-analysis of the efficacy of IN-OT to improve areas of social and neurocognition in schizophrenia. A systematic search of original research publications identified randomized controlled trials (RCTs) of IN-OT as a treatment for social and neurocognitive deficits in schizophrenia for inclusion. Standardized mean differences (SMD) and corresponding variances were used in multilevel Bayesian models to obtain meta-analytic effect-size estimates. Across a total of 12 studies (N = 273), IN-OT did not improve social cognition (SMD = 0.07, 95% credible interval [CI] = [-0.06, 0.17]) or neurocognition (SMD = 0.12, 95% CI = [-0.12, 0.34]). There was moderate between study heterogeneity for social cognition outcomes (τs= 0.12). Moderator analyses revealed that IN-OT had a significantly larger effect on high-level social cognition (ie, mentalizing and theory of mind) compared to low-level social cognition (ie, social cue perception) (b = 0.19, 95% CI = [0.05, 0.33]). When restricting our analysis to outcomes for high-level social cognition, there was a significant effect of IN-OT (SMD = 0.20, 95 % CI = [0.05, 0.33]) but the effect was not robust to sensitivity analyses. The present analysis indicates that IN-OT may have selective effects on high-level social cognition, which provides a more focused target for future studies of IN-OT.

C-tactile afferents: Cutaneous mediators of oxytocin release during affiliative tactile interactions?

Low intensity, non-noxious, stimulation of cutaneous somatosensory nerves has been shown to trigger oxytocin release and is associated with increased social motivation, plus reduced physiological and behavioural reactivity to stressors. However, to date, little attention has been paid to the specific nature of the mechanosensory nerves which mediate these effects. In recent years, the neuroscientific study of human skin nerves (microneurography studies on single peripheral nerve fibres) has led to the identification and characterisation of a class of touch sensitive nerve fibres named C-tactile afferents. Neither itch nor pain receptive, these unmyelinated, low threshold mechanoreceptors, found only in hairy skin, respond optimally to low force/velocity stroking touch. Notably, the speed of stroking which C-tactile afferents fire most strongly to is also that which people perceive to be most pleasant. The social touch hypothesis posits that this system of nerves has evolved in mammals to signal the rewarding value of physical contact in nurturing and social interactions. In support of this hypothesis, we review the evidence that cutaneous stimulation directly targeted to optimally activate C-tactile afferents reduces physiological arousal, carries a positive affective value and, under healthy conditions, inhibits responses to painful stimuli. These effects mirror those, we also review, which have been reported following endogenous release and exogenous administration of oxytocin. Taken together this suggests C-tactile afferent stimulation may mediate oxytocin release during affiliative tactile interactions.

Effects of intranasal oxytocin on symptoms of schizophrenia: A multivariate Bayesian meta-analysis

Schizophrenia is a heterogeneous disorder in which psychiatric symptoms are classified into two general subgroups-positive and negative symptoms. Current antipsychotic drugs are effective for treating positive symptoms, whereas negative symptoms are less responsive. Since the neuropeptide oxytocin (OT) has been shown to mediate social behavior in animals and humans, it has been used as an experimental therapeutic for treating schizophrenia and in particular negative symptoms which includes social deficits. Through eight randomized controlled trials (RCTs) and three meta-analyses, evidence for an effect of intranasal OT (IN-OT) has been inconsistent. We therefore conducted an updated meta-analysis that offers several advantages when compared to those done previously: (1) We used a multivariate analysis which allows for comparisons between symptoms and accounts for correlations between symptoms; (2) We controlled for baseline scores; (3) We used a fully Bayesian framework that allows for assessment of evidence in favor of the null hypothesis using Bayes factors; and (4) We addressed inconsistencies in the primary studies and previous meta-analyses. Eight RCTs (n=238) were included in the present study and we found that oxytocin did not improve any aspect of symptomology in schizophrenic patients and there was moderate evidence in favor of the null (no effect of oxytocin) for negative symptoms. Multivariate comparisons between symptom types revealed that oxytocin was not especially beneficial for treating negative symptoms. The effect size estimates were not moderated, publication bias was absent, and our estimates were robust to sensitivity analyses. These results suggest that IN-OT is not an effective therapeutic for schizophrenia.

Oxytocin Effect on Collective Decision Making: A Randomized Placebo Controlled Study

Collective decision making often benefits both the individuals and the group in a variety of contexts. However, for the group to be successful, individuals should be able to strike a balance between their level of competence and their influence on the collective decisions. The hormone oxytocin has been shown to promote trust, conformism and attention to social cues. We wondered if this hormone may increase participants’ (unwarranted) reliance on their partners’ opinion, resulting in a reduction in collective benefit by disturbing the balance between influence and competence. To test this hypothesis we employed a randomized double-blind placebo-controlled design in which male dyads self-administered intranasal oxytocin or placebo and then performed a visual search task together. Compared to placebo, collective benefit did not decrease under oxytocin. Using an exploratory time dependent analysis, we observed increase in collective benefit over time under oxytocin. Moreover, trial-by-trial analysis showed that under oxytocin the more competent member of each dyad was less likely to change his mind during disagreements, while the less competent member showed a greater willingness to change his mind and conform to the opinion of his more reliable partner. This role-dependent effect may be mediated by enhanced monitoring of own and other’s performance level under oxytocin. Such enhanced social learning could improve the balance between influence and competence and lead to efficient and beneficial collaboration.

A Review of Oxytocin's Effects on the Positive, Negative, and Cognitive Domains of Schizophrenia

Schizophrenia is a disabling, heterogeneous disorder with clinical features that can be parsed into three domains: positive symptoms, negative symptoms, and cognitive deficits. Current antipsychotic drugs produce fairly robust clinical benefit against positive symptoms but typically have minimal therapeutic effects on negative symptoms and cognitive deficits. Oxytocin (OT) is a nonapeptide that, in addition to its role as a hormone regulating peripheral reproductive-relevant functions, acts as a neurotransmitter in the brain. Several lines of preclinical and clinical research suggest that the OT system may play a role in regulating the expression of schizophrenia spectrum disorders and that targeting the central OT system may yield novel treatments to address these symptoms. In this review, we summarize the extant preclinical and clinical evidence relevant to the role of OT in schizophrenia with particular emphasis on its putative therapeutic effects on each of the three above-mentioned clinical domains.

Sex and diagnosis specific associations between DNA methylation of the oxytocin receptor gene with emotion processing and temporal-limbic and prefrontal brain volumes in psychotic disorders

BACKGROUND

The oxytocin (OT) system, including receptor epigenetic mechanisms, has been shown to influence emotion processing, especially in females. Whether OT receptor (OXTR) epigenetic alterations occur across psychotic disorders in relation to illness-related disturbances in social cognition and brain anatomy is unknown.

METHODS

Participants with affective and nonaffective psychotic disorders (92 women, 75 men) and healthy controls (38 women, 37 men) from the Chicago site of the BSNIP study completed the Penn Emotion Recognition Test (ER-40), a facial emotion recognition task. We measured cytosine methylation at site -934 upstream of the OXTR start codon in DNA from whole blood, and for the first time their relationship with plasma OT levels assessed by enzyme-immunoassay. Volumes of brain regions supporting social cognition were measured from MRI scans using FreeSurfer.

RESULTS

Patients with prototypic schizophrenia features showed higher levels of DNA methylation than those with prototypic bipolar features. Methylation was higher in women than men, and was associated with poorer emotion recognition only in female patients and controls. Greater methylation was associated with smaller volumes in temporal-limbic and prefrontal regions associated previously with social cognition, but only in healthy women and females with schizophrenia.

CONCLUSION

DNA methylation of the OXTR site -934 was higher in schizophrenia spectrum than bipolar patients. Among patients, it was linked to behavioral deficits in social cognition and neuroanatomic structures known to support emotion processing only in schizophrenia spectrum individuals.

Peripheral oxytocin administration reduces ethanol consumption in rats

The neuropeptide oxytocin interacts with mesolimbic dopamine neurons to mediate reward associated with filial behaviors, but also other rewarding behaviors such as eating or taking drugs of abuse. Based on its efficacy to decrease intake of other abused substances, oxytocin administration is implicated as a possible treatment for excessive alcohol consumption. We tested this hypothesis by measuring ethanol intake in male Sprague-Dawley rats injected with oxytocin or saline using two different ethanol self-administration paradigms. First, a dose-response curve was constructed for oxytocin inhibition of fluid intake using a modified drinking-in-the-dark model with three bottles containing .05% saccharine, 10% ethanol in saccharine, and 15% ethanol in saccharine. Doses of oxytocin tested were 0.05, 0.1, 0.3, and 0.5mg/kg (I.P.). Next, rats received 0.3mg/kg oxytocin preceding operant sessions in which they were trained to lever-press for either plain gelatin or ethanol gelatin in order to compare oxytocin inhibition of ethanol intake versus caloric intake. For the three-bottle choice study, rats consumed significantly less ethanol when treated with the three higher doses of oxytocin on the injection day. In the operant study, 0.3mg/kg oxytocin significantly decreased ethanol gel consumption to a greater extent than plain gel consumption, both in terms of the amount of gel eaten and calories consumed. These data affirm oxytocin's efficacy for decreasing ethanol intake in rats, and confirm clinical studies suggesting oxytocin as a potential treatment for alcoholism.

Marmosets treated with oxytocin are more socially attractive to their long-term mate

Adult male-female bonds are partly characterized by initiating and maintaining close proximity with a social partner, as well as engaging in high levels of affiliative and sociosexual behavior. Oxytocin (OXT), a neuromodulatory nonapeptide, plays a critical role in the facilitation of social bonding and prosocial behavior toward a social partner (Feldman, 2012). However, less attention has been given to whether augmentation of OXT levels in an individual alters others' perceptions and behavior toward an OXT-treated social partner. We examined social dynamics in well-established male-female pairs of marmoset monkeys (Callithrix jacchus) in which one member of the pair was administered an intranasal OXT agonist, an OXT antagonist (OXTA), or saline. OXT treatment did not alter the expression of affiliative toward an untreated partner. However, OXT did significantly influence the expression of proximity and grooming behavior with a treated partner, as a function of OXT treatment and sex. Female interest in initiating and maintaining proximity with a pair-mate was altered by OXT treatment. Untreated female marmosets departed from their saline-treated partner more frequently than they approached them, as indicated by a low proximity index score. However, when males received an intranasal OXT agonist they had a significantly increased proximity index score relative to saline, indicating that their untreated partner approached them more often than they departed from them). Saline-treated females initiated and received equivalent levels of grooming behavior. However, when female marmosets were treated with an OXT agonist their untreated partner groomed them proportionately more often, for a greater total duration, and for more time per bout, than they initiated grooming behavior. These results suggest that intranasal OXT altered male and female marmosets' stimulus properties in such a way as to increase the amount of grooming behavior that females received from their long-term mate, as well as increase female interest in initiating and maintaining proximity with their long-term mate. Furthermore, these results support the notion that central OXT activity plays an important neuromodulatory role in the maintenance of long-lasting male-female relationships.

Sensitive periods in human social development: New insights from research on oxytocin, synchrony, and high-risk parenting

Sensitive periods (SP) in behavioral development appeared in the biological sciences during the first decade of the 20th century, and research in animal models beginning in the 1950s provide terminology and evidence for SP effects. This paper proposes a rigorous program for human SP research and argues that the complexity of the human brain and variability of the human ecology necessitate that SP effects must be studied in humans, employ longitudinal designs starting at birth, test mechanism-based hypotheses based on animal studies that manipulate early environments, and utilize high-risk conditions as “natural experiments.” In light of research on the molecular basis of critical periods and their sequential cascades, it is proposed that the oxytocin (OT) system, an ancient and integrative system that cross-talks with the stress, reward, immune, and brain stem mediated homeostatic systems and supports mammalian sociality, plays a unique role in experience-dependent plasticity that buttresses SP effects due to its (a) dendritic mode of release leading to autoregulated functioning primed by early experience, (b) pulsatile pattern of activity, and (c) special role in neural plasticity at the molecular and network assembly levels. Synchrony, the coordination of biology and behavior during social contact, is suggested as a mechanism by which SP exert their effect on OT functionality, the social brain, and adult sociality. Findings from four high-risk birth cohorts, each followed repeatedly from birth to 10 years, provide unique “natural experiments” for human SP research based on specific programs in animal models. These include prematurity (maternal proximity), multiple birth (peer rearing), postpartum depression (low licking and grooming), and chronic unpredictable trauma (maternal rotation, variable foraging demands). In each cohort, hypotheses are based on the missing environmental component during SP, and findings on social synchrony, OT functionality, stress response, emotion regulation, and mental health accord with the multilevel and dynamic principles of developmental psychopathology. The results on the potential for reparation versus chronicity following early deprivation highlight a flexible conceptualization of resilience based on human SP research. Consideration of SP effects at the molecular, endocrine, brain, and behavioral levels and in relation to the neural plasticity and multifinality of human social functions may assist in fine-tuning early detection and the construction of targeted individualized interventions.

Conditioned same-sex partner preference in male rats is facilitated by oxytocin and dopamine: effect on sexually dimorphic brain nuclei

Conditioned same-sex partner preference can develop in male rats that undergo cohabitation under the effects of quinpirole (QNP, D2 agonist). Herein, we assessed the development of conditioned same-sex social/sexual preference in males that received either nothing, saline, QNP, oxytocin (OT), or QNP+OT during cohabitation with another male (+) or single-caged (-). This resulted in the following groups: (1) Intact-, (2) Saline+, (3) QNP-, (4) OT-, (5) QNP+, (6) OT+ and (7) QNP/OT+. Cohabitation occurred during 24h in a clean cage with a male partner that bore almond scent on the back as conditioned stimulus. This was repeated every 4 days for a total of three trials. Social and sexual preference were assessed four days after the last conditioning trial in a drug-free test in which experimental males chose between the scented familiar male and a novel sexually receptive female. Results showed that males from groups Intact-, Saline+, QNP- and OT- displayed a clear preference for the female (opposite-sex), whereas groups QNP+, OT+ and QNP/OT+ displayed socio/sexual preference for the male partner (same-sex). In Experiment 2, the brains were processed for Nissl dye and the area size of two sexually dimorphic nuclei (SDN-POA and SON) was compared between groups. Males from groups OT-, OT+ and QNP/OT+ expressed a smaller SDN-POA and groups QNP+ and QNP/OT+ expressed a larger SON. Accordingly, conditioned same-sex social/sexual partner preference can develop during cohabitation under enhanced D2 or OT activity but such preference does not depend on the area size of those sexually dimorphic nuclei.

Long-term exposure to intranasal oxytocin in a mouse autism model

Oxytocin (OT) is a neuropeptide involved in mammalian social behavior. It is currently in clinical trials for the treatment of autism spectrum disorder (ASD). Previous studies in healthy rodents (prairie voles and C57BL/6J mice) have shown that there may be detrimental effects of long-term intranasal administration, raising the questions about safety and efficacy. To investigate the effects of OT on the aspects of ASD phenotype, we conducted the first study of chronic intranasal OT in a well-validated mouse model of autism, the BTBR T+ Itpr3tf/J inbred strain (BTBR), which displays low sociability and high repetitive behaviors. BTBR and C57BL/6J (B6) mice (N=94) were administered 0.8  IU/kg of OT intranasally, daily for 30 days, starting on day 21. We ran a well-characterized set of behavioral tasks relevant to diagnostic and associated symptoms of autism, including juvenile reciprocal social interactions, three-chambered social approach, open-field exploratory activity, repetitive self-grooming and fear-conditioned learning and memory, some during and some post treatment. Intranasal OT did not improve autism-relevant behaviors in BTBR, except for female sniffing in the three-chambered social interaction test. Male saline-treated BTBR mice showed increased interest in a novel mouse, both in chamber time and sniffing time, whereas OT-treated male BTBR mice showed a preference for the novel mouse in sniffing time only. No deleterious effects of OT were detected in either B6 or BTBR mice, except possibly for the lack of a preference for the novel mouse's chamber in OT-treated male BTBR mice. These results highlight the complexity inherent in understanding the effects of OT on behavior. Future investigations of chronic intranasal OT should include a wider dose range and early developmental time points in both healthy rodents and ASD models to affirm the efficacy and safety of OT.

Oxytocin and object preferences in the male prairie vole

The neuropeptide oxytocin has been previously associated with social attachment behaviors in various species. Studies in socially monogamous prairie voles (Microtus ochrogaster) and other species have implicated oxytocin in partner preferences and other social behaviors. In the present study male prairie voles were injected intraperitoneally with either oxytocin or the selective oxytocin antagonist, L-368,899, and were assessed for object preference (for small inanimate toys) 30-min after injection. Object preferences were assessed in animals tested alone or in the presence of their sibling cage mate. Saline-treated controls displayed preferences for the novel object, both when tested alone and in pairs, while oxytocin-treated voles did not demonstrate an object preference, regardless of whether tested alone or in pairs. Finally, oxytocin antagonist treated voles showed preference for the novel object, but only when tested in pairs. These data support a possible involvement of oxytocin and oxytocin receptors in object preference.

Oxytocin facilitates fidelity in well-established marmoset pairs by reducing sociosexual behavior toward opposite-sex strangers

Behavioral strategies that facilitate the maintenance of social bonds are critical for the preservation of high-quality social relationships. Central oxytocin (OT) activity modulates the behavioral features of socially monogamous relationships in a number of mammalian species (including marmoset monkeys), and plays a vital role in the behavioral maintenance of long-term social relationships. Two distinct variants of OT have been identified in some New World primates (including marmosets; Lee et al., 2011). The marmoset variant of the oxytocin ligand (Pro(8)-OT) is structurally distinct from the consensus mammalian variant of the oxytocin ligand (Leu(8)-OT), due to a proline substitution at the 8th amino-acid position. The goal of the present study was to determine if treating marmosets with Pro(8)-OT, relative to treatments with Leu(8)-OT, control saline, or an OT antagonist, had modulatory effects on the behavioral maintenance of long-term social relationships in marmosets. Treatment with the Pro(8) variant, but not the Leu(8) variant, of OT facilitated fidelity with a long-term partner by reducing time spent in close proximity with an opposite-sex stranger. However, this facilitative effect of Pro(8)-OT on proximity behavior manifested itself differently in male and female marmosets, such that females preferred to interact socially with their partner rather than a stranger when treated with Pro(8)-OT, while males spent less time in close proximity with both their partner and a stranger when treated with Pro(8)-OT. Furthermore, treatment with Pro(8)-OT, but not Leu(8)-OT, significantly delayed the expression of sexual solicitation behavior toward an opposite-sex stranger in both male and female marmosets, but had no effect on sociosexual behavior directed toward a long-term partner. These results suggest that the OT system is highly involved in reducing fidelity-threatening behaviors in well-established marmoset pairs, and that the effects were only produced by species-specific OT ligands.

Is oxytocin a maternal-foetal signalling molecule at birth? Implications for development

The neuropeptide oxytocin was first noted for its capacity to promote uterine contractions and facilitate delivery in mammals. The study of oxytocin has grown to include awareness that this peptide is a neuromodulator with broad effects throughout the body. Accumulating evidence suggests that oxytocin is a powerful signal to the foetus, helping to prepare the offspring for the extrauterine environment. Concurrently, the use of exogenous oxytocin or other drugs to manipulate labour has become common practice. The use of oxytocin to expedite labour and minimise blood loss improves both infant and maternal survival under some conditions. However, further investigations are needed to assess the developmental consequences of changes in oxytocin, such as those associated with pre-eclampsia or obstetric manipulations associated with birth. This review focuses on the role of endogenous and exogenous oxytocin as a neurochemical signal to the foetal nervous system. We also examine the possible developmental consequences, including those associated with autism spectrum disorder, that arise from exogenous oxytocin supplementation during labour.

A novel model for neuroendocrine toxicology: neurobehavioral effects of BPA exposure in a prosocial species, the prairie vole (Microtus ochrogaster)

Impacts on brain and behavior have been reported in laboratory rodents after developmental exposure to bisphenol A (BPA), raising concerns about possible human effects. Epidemiological data suggest links between prenatal BPA exposure and altered affective behaviors in children, but potential mechanisms are unclear. Disruption of mesolimbic oxytocin (OT)/vasopressin (AVP) pathways have been proposed, but supporting evidence is minimal. To address these data gaps, we employed a novel animal model for neuroendocrine toxicology: the prairie vole (Microtus ochrogaster), which are more prosocial than lab rats or mice. Male and female prairie vole pups were orally exposed to 5-μg/kg body weight (bw)/d, 50-μg/kg bw/d, or 50-mg/kg bw/d BPA or vehicle over postnatal days 8-14. Subjects were tested as juveniles in open field and novel social tests and for partner preference as adults. Brains were then collected and assessed for immunoreactive (ir) tyrosine hydroxylase (TH) (a dopamine marker) neurons in the principal bed nucleus of the stria terminalis (pBNST) and TH-ir, OT-ir, and AVP-ir neurons in the paraventricular nucleus of the hypothalamus (PVN). Female open field activity indicated hyperactivity at the lowest dose and anxiety at the highest dose. Effects on social interactions were also observed, and partner preference formation was mildly inhibited at all dose levels. BPA masculinized principal bed nucleus of the stria terminalis TH-ir neuron numbers in females. Additionally, 50-mg/kg bw BPA-exposed females had more AVP-ir neurons in the anterior PVN and fewer OT-ir neurons in the posterior PVN. At the 2 lowest doses, BPA eliminated sex differences in PVN TH-ir neuron numbers and reversed this sex difference at the highest dose. Minimal behavioral effects were observed in BPA-exposed males. These data support the hypothesis that BPA alters affective behaviors, potentially via disruption of OT/AVP pathways.

Oxytocin and social cognition in rhesus macaques: implications for understanding and treating human psychopathology

Converging evidence from humans and non-human animals indicates that the neurohypophysial hormone oxytocin (OT) evolved to serve a specialized function in social behavior in mammals. Although OT-based therapies are currently being evaluated as remedies for social deficits in neuropsychiatric disorders, precisely how OT regulates complex social processes remains largely unknown. Here we describe how a non-human primate model can be used to understand the mechanisms by which OT regulates social cognition and thereby inform its clinical application in humans. We focus primarily on recent advances in our understanding of OT-mediated social cognition in rhesus macaques (Macaca mulatta), supplemented by discussion of recent work in humans, other primates, and rodents. Together, these studies endorse the hypothesis that OT promotes social exploration both by amplifying social motivation and by attenuating social vigilance. This article is part of a Special Issue entitled Oxytocin and Social Behav.

Centrally-administered oxytocin promotes preference for familiar objects at a short delay in ovariectomized female rats

Oxytocin has been previously associated with social attachment behaviors in various species, however, most studies focused on partner preference in the socially-monogamous prairie vole. In these, oxytocin treatment was shown to promote partner preference, such that females receiving either central or pulsatile peripheral administration would spend more time with a familiar male. This behavioral outcome was blocked by oxytocin receptor antagonist treatment. The aim of the current study was to further explore the preference-inducing properties of oxytocin by examining its effects on object preference on ovariectomized female rats. In other words, we assessed whether these effects would apply to objects and if they would be persistent across species. Eight rats were infused with oxytocin into the left ventricle and object preference was assessed at two delays: 30min and 4h. At the 30min delay, oxytocin-treated animals showed preference for the familiar object, whereas saline-treated controls exhibited preference for the novel object. At the 4h delay, both groups showed novel-object preference. Our findings show that oxytocin modulates object preference in the female rat at a shorter delay, similar to the findings from partner-preference studies in the prairie vole, suggesting that the mechanisms driving object preference might be in part similar to those responsible for partner preference.

Arginine vasopressin plasma levels change seasonally in African striped mice but do not differ between alternative reproductive tactics

Arginine vasopressin (AVP) is an important hormone for osmoregulation, while as a neuropeptide in the brain it plays an important role in the regulation of social behaviors. Dry habitats are often the home of obligately sociable species such as meerkats and Damaraland mole-rats, leading to the hypothesis that high plasma AVP levels needed for osmoregulation might be associated with the regulation of social behavior. We tested this in a facultative sociable species, the African striped mouse (Rhabdomys pumilio). During the moist breeding season, both solitary- and group-living reproductive tactics occur in this species, which is obligatory sociable in the dry season. We collected 196 plasma samples from striped mice following different reproductive tactics both during the moist and the dry season. Solitary mice did not have lower AVP levels than sociable mice, rejecting the hypothesis that peripheral AVP is involved in the regulation of alternative reproductive tactics. However, we found significantly higher AVP levels during the dry season, with AVP levels correlated with the abundance of food plants, the main source of water for striped mice. Plasma AVP levels were not correlated with testosterone or corticosterone levels. Our study underlines the important role that AVP plays in osmoregulation, particularly for a free ranging mammal living under harsh arid conditions.

Social preference and maternal defeat-induced social avoidance in virgin female rats: sex differences in involvement of brain oxytocin and vasopressin

BACKGROUND

Research concerning non-reproductive sociability in rodents is mainly restricted to assessing the effects of oxytocin (OXT) and arginine-vasopressin (AVP) in male rats and mice. Comparable studies on natural social preference and social avoidance in females are substantially lacking.

NEW METHOD

Here, we adapted a behavioral paradigm for monitoring social preference of female rats consisting of two consecutive exposures to either non-social or social stimuli. Further, to induce stimulus-specific social avoidance, female rats were exposed to a single 10-min maternal defeat by a lactating dam.

RESULTS

Social preference towards same-sex conspecifics in female rats was shown to be independent of the estrous cycle and even more pronounced than in male rats. Intracerebroventricular (icv) application of OXT, AVP, or their selective receptor antagonists or agonists, did not alter naturally-occurring social preference in female rats. Stimulus-specific social avoidance could be induced by prior exposure to a lactating rat: an effect that could not be reversed/overcome by icv OXT.

COMPARISON WITH EXISTING METHOD(S)

The female social preference paradigm for rats established in this study detected subtle sex differences in social preference behavior of rats. Further, stimulus-specific social deficits could be induced in female rats using an acute exposure to social defeat - as previously observed in male rodents.

CONCLUSIONS

Female rats show strong social preference behavior, which can be prevented by social defeat, but does not seem to be regulated by the OXT or AVP systems. Accordingly, icv application of synthetic OXT does not reverse maternal defeat-induced social avoidance in female rats.