Hypothalamic oxytocin mediates social buffering of the stress response

Oxytocin and Anxiety Disorders: Translational and Therapeutic Aspects

PURPOSE OF REVIEW

This review aims to evaluate the most recent literature examining the oxytocin (OXT) system's role in human anxiety by surveying various fields of preclinical and clinical research supporting this role, and queries whether the OXT system might be a target for novel anxiolytics.

RECENT FINDINGS

Evidence from the diverse body of literature presented here, from translational research, genetic and neuroimaging studies, to clinical trials of intranasal (IN) OXT reveals a positive association. In addition, some moderators (e.g., sex, specificities to cues) of OXT's anxiolytic effects can have an important influence on its outcomes, awaiting further research. Evidence for the role of OXT in regulating anxiety is undeniable. We expect that the diverse particularities of the OXT system will help broaden our understanding of anxiety and stress-related disorders. We conclude that OXT promises an enticing treatment option for human anxiety disorders especially those associated with socio-emotional dysfunctions.

Social information use in threat perception: Social buffering, contagion and facilitation of alarm responses

Group living animals can use the behavior of others as cues for the presence of threat in the environment and adjust their behavior accordingly. Therefore, different social phenomena that modulate the response to threat, such as social buffering, social transmission (contagion), and facilitation of alarm responses can be seen as different manifestations of social information use in threat detection. Thus, social phenomena that are functionally antagonistic, such as social buffering and social transmission of fear, may rely on shared neurobehavioral mechanisms related to the use of social information in decision-making about the presence of threat in the environment. Here, we propose a unifying conceptual framework for the study of social information use in threat perception based on signal detection theory.

Neuroendocrine control in social relationships in non-human primates: Field based evidence

Primates maintain a variety of social relationships and these can have fitness consequences. Research has established that different types of social relationships are unpinned by different or interacting hormonal systems, for example, the neuropeptide oxytocin influences social bonding, the steroid hormone testosterone influences dominance relationships, and paternal care is characterized by high oxytocin and low testosterone. Although the oxytocinergic system influences social bonding, it can support different types of social bonds in different species, whether pair bonds, parent-offspring bonds or friendships. It seems that selection processes shape social and mating systems and their interactions with neuroendocrine pathways. Within species, there are individual differences in the development of the neuroendocrine system: the social environment individuals are exposed to during ontogeny alters their neuroendocrine and socio-cognitive development, and later, their social interactions as adults. Within individuals, neuroendocrine systems can also have short-term effects, impacting on social interactions, such as those during hunting, intergroup encounters or food sharing, or the likelihood of cooperating, winning or losing. To understand these highly dynamic processes, extending research beyond animals in laboratory settings to wild animals living within their natural social and ecological setting may bring insights that are otherwise unreachable. Field endocrinology with neuropeptides is still emerging. We review the current status of this research, informed by laboratory studies, and identify questions particularly suited to future field studies. We focus on primate social relationships, specifically social bonds (mother-offspring, father-offspring, cooperative breeders, pair bonds and adult platonic friendships), dominance, cooperation and in-group/out-group relationships, and examine evidence with respect to the 'tend and defend' hypothesis.

Mechanisms of social buffering of fear in zebrafish

Some humans thrive whereas others resign when exposed to threatening situations throughout life. Social support has been identified as an important modulator of these discrepancies in human behaviour, and other social animals also exhibit phenomena in which individuals recover better from aversive events when conspecifics are present - aka social buffering. Here we studied social buffering in zebrafish, by exposing focal fish to an aversive stimulus (alarm substance - AS) either in the absence or presence of conspecific cues. When exposed to AS in the presence of both olfactory (shoal water) and visual (sight of shoal) conspecific cues, focal fish exhibited a lower fear response than when tested alone, demonstrating social buffering in zebrafish. When separately testing each cue's effectiveness, we verified that the visual cue was more effective than the olfactory in reducing freezing in a persistent threat scenario. Finally, we verified that social buffering was independent of shoal size and coincided with a distinct pattern of co-activation of brain regions known to be involved in mammalian social buffering. Thus, this study suggests a shared evolutionary origin for social buffering in vertebrates, bringing new evidence on the behavioural, sensory and neural mechanisms underlying this phenomenon.

Forced swimming-induced oxytocin release into blood and brain: Effects of adrenalectomy and corticosterone treatment

The oxytocin (OXT) system is functionally linked to the HPA axis in a reciprocal and complex manner. Certain stressors are known to cause the simultaneous release of OXT and adrenocorticotrophic hormone (ACTH) followed by corticosterone (CORT). Furthermore, brain OXT attenuates ACTH and CORT responses. Although there are some indications of CORT influencing OXT neurotransmission, specific effects of CORT on neurohypophyseal or intra-hypothalamic release of OXT have not been studied in detail. In the present set of experiments, adult male rats were adrenalectomized (ADX) or sham-operated and fitted with a jugular vein catheter and/or microdialysis probe targeting the hypothalamic paraventricular nucleus (PVN). Blood samples and dialysates were collected before and after forced swimming (FS) and analyzed for CORT, ACTH and AVP concentrations (in plasma) and OXT concentrations (in plasma and dialysates). Experimental treatments included acute infusion of CORT (70 or 175μg/kg i.v.) 5min prior to FS, or subcutaneous placement of 40% CORT pellets resulting in stable CORT levels in the normal basal range. Although ADX did not alter basal OXT concentrations either in plasma or in microdialysates from the PVN, it did cause an exaggerated peripheral secretion of OXT and a blunted intra-PVN release of OXT in response to FS. CORT pellets did not influence either of these ADX-induced effects, while acute infusion of 175μg/kg CORT rescued the stress-induced rise in OXT release within the PVN and modestly increased peripheral OXT secretion. In conclusion, these results indicate that CORT regulates both peripheral and intracerebral OXT release, but in an independent manner. Whereas the peripheral secretion of OXT occurs simultaneously to HPA axis activation in response to FS and is modestly influenced by CORT, HPA axis activation and circulating CORT strongly contribute to the stress-induced stimulation of OXT release within the PVN.

The protective effects of social bonding on behavioral and pituitary-adrenal axis reactivity to chronic mild stress in prairie voles

Positive social interactions may protect against stress. This study investigated the beneficial effects of pairing with a social partner on behaviors and neuroendocrine function in response to chronic mild stress (CMS) in 13 prairie vole pairs. Following 5 days of social bonding, male and female prairie voles were exposed to 10 days of CMS (mild, unpredictable stressors of varying durations, for instance, strobe light, white noise, and damp bedding), housed with either the social partner (paired group) or individually (isolated group). Active and passive behavioral responses to the forced swim test (FST) and tail-suspension test (TST), and plasma concentrations of adrenocorticotropic hormone (ACTH) and corticosterone, were measured in all prairie voles following the CMS period. Both female and male prairie voles housed with a social partner displayed lower durations of passive behavioral responses (immobility, a maladaptive behavioral response) in the FST (mean ± SEM; females: 17.3 ± 5.4 s; males: 9.3 ± 4.6 s) and TST (females: 56.8 ± 16.4 s; males: 40.2 ± 11.3 s), versus both sexes housed individually (females, FST: 98.6 ± 12.9 s; females, TST: 155.1 ± 19.3 s; males, FST: 92.4 ± 14.1 s; males, TST: 158.9 ± 22.0 s). Female (but not male) prairie voles displayed attenuated plasma stress hormones when housed with a male partner (ACTH: 945 ± 24.7 pg/ml; corticosterone: 624 ± 139.5 ng/ml), versus females housed individually (ACTH: 1100 ± 23.2 pg/ml; corticosterone: 1064 ± 121.7 ng/ml). These results may inform understanding of the benefits of social interactions on stress resilience. Lay Summary: Social stress can lead to depression. The study of social bonding and stress using an animal model will inform understanding of the protective effects of social bonds. This study showed that social bonding in a rodent model can protect against behavioral responses to stress, and may also be protective against the elevation of stress hormones. This study provides evidence that bonding and social support are valuable for protecting against stress in humans.

Paternal deprivation affects social behaviors and neurochemical systems in the offspring of socially monogamous prairie voles

Early life experiences, particularly the experience with parents, are crucial to phenotypic outcomes in both humans and animals. Although the effects of maternal deprivation on offspring well-being have been studied, paternal deprivation (PD) has received little attention despite documented associations between father absence and children health problems in humans. In the present study, we utilized the socially monogamous prairie vole (Microtus ochrogaster), which displays male-female pair bonding and bi-parental care, to examine the effects of PD on adult behaviors and neurochemical expression in the hippocampus. Male and female subjects were randomly assigned into one of two experimental groups that grew up with both the mother and father (MF) or with the mother-only (MO, to generate PD experience). Our data show that MO subjects received less parental licking/grooming and carrying and were left alone in the nest more frequently than MF subjects. At adulthood (∼75days of age), MO subjects displayed increased social affiliation (SOA) toward a conspecific compared to MF subjects, but the two groups did not differ in social recognition (SOR) and anxiety-like behavior. Interestingly, MO subjects showed consistent increases in both gene and protein expression of the brain-derived neurotrophic factor (BDNF) and tropomyosin receptor kinase B (TrkB) as well as the levels of total histone 3 and histone 3 acetylation in the hippocampus compared to MF subjects. Further, PD experience increased glucocorticoid receptor beta (GRβ) protein expression in the hippocampus of females as well as increased corticotrophin receptor 2 (CRHR2) protein expression in the hippocampus of males, but decreased CRHR2 mRNA in both sexes. Together, our data suggest that PD has a long-lasting, behavior-specific effect on SOA and alters hippocampal neurochemical systems in the vole brain. The functional role of such altered neurochemical systems in social behaviors and the potential involvement of epigenetic events should be further studied.

Social Decision-Making and the Brain: A Comparative Perspective

The capacity and motivation to be social is a key component of the human adaptive behavioral repertoire. Recent research has identified social behaviors remarkably similar to our own in other animals, including empathy, consolation, cooperation, and strategic deception. Moreover, neurobiological studies in humans, nonhuman primates, and rodents have identified shared brain structures (the so-called 'social brain') apparently specialized to mediate such functions. Neuromodulators may regulate social interactions by 'tuning' the social brain, with important implications for treating social impairments. Here, we survey recent findings in social neuroscience from a comparative perspective, and conclude that the very social behaviors that make us human emerge from mechanisms shared widely with other animals, as well as some that appear to be unique to humans and other primates.

Homeodomain protein Otp affects developmental neuropeptide switching in oxytocin neurons associated with a long-term effect on social behavior

Proper response to stress and social stimuli depends on orchestrated development of hypothalamic neuronal circuits. Here we address the effects of the developmental transcription factor orthopedia (Otp) on hypothalamic development and function. We show that developmental mutations in the zebrafish paralogous gene otpa but not otpb affect both stress response and social preference. These behavioral phenotypes were associated with developmental alterations in oxytocinergic (OXT) neurons. Thus, otpa and otpb differentially regulate neuropeptide switching in a newly identified subset of OXT neurons that co-express the corticotropin-releasing hormone (CRH). Single-cell analysis revealed that these neurons project mostly to the hindbrain and spinal cord. Ablation of this neuronal subset specifically reduced adult social preference without affecting stress behavior, thereby uncoupling the contribution of a specific OXT cluster to social behavior from the general otpa^-/- deficits. Our findings reveal a new role for Otp in controlling developmental neuropeptide balance in a discrete OXT circuit whose disrupted development affects social behavior.

Oxytocin and Social Relationships: From Attachment to Bond Disruption

Social relationships throughout life are vital for well-being and physical and mental health. A significant amount of research in animal models as well as in humans suggests that oxytocin (OT) plays an important role in the development of the capacity to form social bonds, the mediation of the positive aspects of early-life nurturing on adult bonding capacity, and the maintenance of social bonding. Here, we focus on the extensive research on a socially monogamous rodent model organism, the prairie vole (Microtus ochrogaster). OT facilitates mating-induced pair bonds in adults through interaction with the mesolimbic dopamine system. Variation in striatal OT receptor density predicts resilience and susceptibility to neonatal social neglect in female prairie voles. Finally, in adults, loss of a partner results in multiple disruptions in OT signaling, including decreased OT release in the striatum, which is caused by an activation of the brain corticotropin releasing factor (CRF) system. The dramatic behavioral consequence of partner loss is increased depressive-like behavior reminiscent of bereavement. Importantly, infusions of OT into the striatum of adults prevents the onset of depressive-like behavior following partner loss, and evoking endogenous OT release using melanocortin agonists during neonatal social isolation rescues impairments in social bonding in adulthood. This work has important translational implications relevant to the disruptions of social bonds in childhood and in adults.

Intergenerational transmission of sociality: the role of parents in shaping social behavior in monogamous and non-monogamous species

Social bonds are necessary for many mammals to survive and reproduce successfully. These bonds (i.e. pair-bonds, friendships, filial bonds) are characterized by different periods of development, longevity and strength. Socially monogamous species display certain behaviors not seen in many other mammals, such as adult pair-bonding and male parenting. In our studies of prairie voles (Microtus ochrogaster) and titi monkeys (Callicebus cupreus), we have examined the neurohormonal basis of these bonds. Here, we discuss the evidence from voles that aspects of adolescent and adult social behavior are shaped by early experience, including changes to sensory systems and connections, neuropeptide systems such as oxytocin and vasopressin, and alterations in stress responses. We will compare this with what is known about these processes during development and adulthood in other mammalian species, both monogamous and non-monogamous, and how our current knowledge in voles can be used to understand the development of and variation in social bonds. Humans are endlessly fascinated by the variety of social relationships and family types displayed by animal species, including our own. Social relationships can be characterized by directionality (either uni- or bi-directional), longevity, developmental epoch (infant, juvenile or adult) and strength. Research on the neurobiology of social bonds in animals has focused primarily on 'socially monogamous' species, because of their long-term, strong adult affiliative bonds. In this Review, we attempt to understand how the ability and propensity to form these bonds (or lack thereof), as well as the display of social behaviors more generally, are transmitted both genomically and non-genomically via variation in parenting in monogamous and non-monogamous species.

Oxytocin pathways in the intergenerational transmission of maternal early life stress

Severe stress in early life, such as childhood abuse and neglect, constitutes a major risk factor in the etiology of psychiatric disorders and somatic diseases. Importantly, these long-term effects may impact the next generation. The intergenerational transmission of maternal early life stress (ELS) may occur via pre-and postnatal pathways, such as alterations in maternal-fetal-placental stress physiology, maternal depression during pregnancy and postpartum, as well as impaired mother-offspring interactions. The neuropeptide oxytocin (OT) has gained considerable attention for its role in modulating all of these assumed transmission pathways. Moreover, central and peripheral OT signaling pathways are highly sensitive to environmental exposures and may be compromised by ELS with implications for these putative transmission mechanisms. Together, these data suggest that OT pathways play an important role in the intergenerational transmission of maternal ELS in humans. By integrating recent studies on gene-environment interactions and epigenetic modifications in OT pathway genes, the present review aims to develop a conceptual framework of intergenerational transmission of maternal ELS that emphasizes the role of OT.

Neuropeptide Regulation of Social Attachment: The Prairie Vole Model

Social attachments are ubiquitous among humans and integral to human health. Although great efforts have been made to elucidate the neural underpinnings regulating social attachments, we still know relatively little about the neuronal and neurochemical regulation of social attachments. As a laboratory animal research model, the socially monogamous prairie vole (Microtus ochrogaster) displays behaviors paralleling human social attachments and thus has provided unique insights into the neural regulation of social behaviors. Research in prairie voles has particularly highlighted the significance of neuropeptidergic regulation of social behaviors, especially of the roles of oxytocin (OT) and vasopressin (AVP). This article aims to review these findings. We begin by discussing the role of the OT and AVP systems in regulating social behaviors relevant to social attachments, and thereafter restrict our discussion to studies in prairie voles. Specifically, we discuss the role of OT and AVP in adult mate attachments, biparental care, social isolation, and social buffering as informed by studies utilizing the prairie vole model. Not only do these studies offer insight into social attachments in humans, but they also point to dysregulated mechanisms in several mental disorders. We conclude by discussing these implications for human health. © 2017 American Physiological Society. Compr Physiol 7:81-104, 2017.

The neurobiology of pair bond formation, bond disruption, and social buffering

Enduring social bonds play an essential role in human society. These bonds positively affect psychological, physiological, and behavioral functions. Here, we review the recent literature on the neurobiology, particularly the role of oxytocin and dopamine, of pair bond formation, bond disruption, and social buffering effects on stress responses, from studies utilizing the socially monogamous prairie vole (Microtus ochrogaster).

The neurocircuitry involved in oxytocin modulation of methamphetamine addiction

The role of oxytocin in attenuating the abuse of licit and illicit drugs, including the psychostimulant methamphetamine, has been examined with increased ferocity in recent years. This is largely driven by the potential application of oxytocin as a pharmacotherapy. However, the neural mechanisms by which oxytocin modulates methamphetamine abuse are not well understood. Recent research identified an important role for the accumbens core and subthalamic nucleus in this process, which likely involves an interaction with dopamine, glutamate, GABA, and vasopressin. In addition to providing an overview of methamphetamine, the endogenous oxytocin system, and the effects of exogenous oxytocin on drug abuse, we propose a neural circuit through which exogenous oxytocin modulates methamphetamine abuse, focusing on its interaction with neurochemicals within the accumbens core and subthalamic nucleus. A growing understanding of exogenous oxytocin effects at a neurochemical and neurobiological level will assist in its evaluation as a pharmacotherapy for drug addiction.

Neurobiological underpinnings of dogs' human-like social competence: How interactions between stress response systems and oxytocin mediate dogs' social skills

Domestic dogs (Canis familiaris) have been suggested as a natural model for human social cognition, possessing social skills that are in many ways functionally analogous to those of young humans. Researchers have debated the origins of dogs' human-like social competence and the underlying cognitive mechanisms, but only recently have researchers begun to explore their neurobiological underpinnings. In this review, findings from behavioral studies are integrated with what is known about the biological basis of dogs' human-directed social competence, with an emphasis on how stress-mediating systems, particularly the hypothalamic-pituitary-adrenal (HPA) axis, interact with oxytocin and underlying neural systems to facilitate dogs' interspecific social-cognitive abilities. The working model presented in this paper offers a biological explanation for many of the inconsistent findings from past work on social cognition in dogs and generates questions for future research in the field of canine social competence.

Sex-Specific Effects of Stress on Oxytocin Neurons Correspond With Responses to Intranasal Oxytocin

BACKGROUND

Oxytocin (OT) is considered to be a stress-buffering hormone, dampening the physiologic effects of stress. However, OT can also be anxiogenic. We examined acute and long-lasting effects of social defeat on OT neurons in male and female California mice.

METHODS

We used immunohistochemistry for OT and c-fos cells to examine OT neuron activity immediately after defeat (n = 6-9) and 2 weeks (n = 6-9) and 10 weeks (n = 4-5) later. We quantified Oxt messenger RNA with quantitative polymerase chain reaction (n = 5-9). Intranasal OT was administered to naïve and stressed mice tested in social interaction and resident-intruder tests (n = 8-14).

RESULTS

Acute exposure to a third episode of defeat increased OT/c-fos colocalizations in the paraventricular nucleus of both sexes. In the medioventral bed nucleus of the stria terminalis, defeat increased Oxt messenger RNA, total OT neurons, and OT/c-fos colocalizations in female mice but not male mice. Intranasal OT failed to reverse stress-induced social withdrawal in female mice and reduced social interaction behavior in female mice naïve to defeat. In contrast, intranasal OT increased social interaction in stressed male mice and reduced freezing in the resident-intruder test.

CONCLUSIONS

Social defeat induces long-lasting increases in OT production and OT/c-fos cells in the medioventral bed nucleus of the stria terminalis of female mice but not male mice. Intranasal OT largely reversed the effects of stress on behavior in male mice, but effects were mixed in female mice. These results suggest that changes in OT-sensitive networks contribute to sex differences in behavioral responses to stress.

Dopamine and opioid systems interact within the nucleus accumbens to maintain monogamous pair bonds

Prairie vole breeder pairs form monogamous pair bonds, which are maintained through the expression of selective aggression toward novel conspecifics. Here, we utilize behavioral and anatomical techniques to extend the current understanding of neural mechanisms that mediate pair bond maintenance. For both sexes, we show that pair bonding up-regulates mRNA expression for genes encoding D1-like dopamine (DA) receptors and dynorphin as well as enhances stimulated DA release within the nucleus accumbens (NAc). We next show that D1-like receptor regulation of selective aggression is mediated through downstream activation of kappa-opioid receptors (KORs) and that activation of these receptors mediates social avoidance. Finally, we also identified sex-specific alterations in KOR binding density within the NAc shell of paired males and demonstrate that this alteration contributes to the neuroprotective effect of pair bonding against drug reward. Together, these findings suggest motivational and valence processing systems interact to mediate the maintenance of social bonds.

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

Oxytocin (OT) and vasopressin (AVP) are important hypothalamic neuropeptides that regulate peripheral physiology, and have emerged as important modulators of brain function, particularly in the social realm. OT structure and the genes that ultimately determine structure are highly conserved among diverse eutherian mammals, but recent discoveries have identified surprising variability in OT and peptide structure in New World monkeys (NWM), with five new OT variants identified to date. This review explores these new findings in light of comparative OT/AVP ligand evolution, documents coevolutionary changes in the oxytocin and vasopressin receptors (OTR and V1aR), and highlights the distribution of neuropeptidergic neurons and receptors in the primate brain. Finally, the behavioral consequences of OT and AVP in regulating NWM sociality are summarized, demonstrating important neuromodulatory effects of these compounds and OT ligand-specific influences in certain social domains.

Moderate Stress-Induced Social Bonding and Oxytocin Signaling are Disrupted by Predator Odor in Male Rats

In times of stress, social support can serve as a potent buffering mechanism that enhances resilience. In humans, stress can promote protective affiliative interactions and prosocial behavior. Yet, stress also precipitates psychopathologies characterized by social withdrawal such as post-traumatic stress disorder (PTSD) and depression. The factors that drive adaptive vs maladaptive social responses to stress are not yet clear. Rodent studies have focused on pair-bonded, opposite-sex mates and suggest that a variety of stressors can induce social support-like behaviors. However, between same-sex conspecifics-particularly males-stress effects on social bonding are less understood and often associated with aggression and social unrest. We thus sought to investigate if a moderate stressor-3 h of acute immobilization-impacts social-support behaviors differently when experienced in a neutral vs more innately threatening context (ie, paired with predator odor). We found that moderate stress increased social support-seeking behavior in rat cagemates and facilitated long-term sharing of a limited water resource, decreased aggression, and strongly defined dominance ranks (an indicator of home cage stability). In contrast, experiencing the same stressor in the presence of predator odor eliminated the positive behavioral effects of moderate stress. Importantly, hypothalamic oxytocin (OT) signaling increased coincident with stress in a neutral-but not a predator odor-context. Our results define a novel rodent model of divergent stress effects on social affiliation and OT signaling dependent on odor context with particularly strong relevance to stress-related disorders such as PTSD, which are characterized by a disrupted ability to seek and maintain social bonds.

Sex differences in the effects of social defeat on brain and behavior in the California mouse: Insights from a monogamous rodent

Women are nearly twice as likely as men to be diagnosed with major depressive disorder, yet the use of female animal models in studying the biological basis of depression lags behind that of males. The social defeat model uses social stress to generate depression-like symptoms in order to study the neurobiological mechanisms. In general, social defeat is difficult to apply in female rodents. However, male and female California mice (Peromyscus californicus) are territorial. This allows defeat to be studied in both sexes. Males exposed to defeat tend to exhibit proactive coping mechanisms and demonstrate aggression and reduced cognitive flexibility. Females exposed to defeat engage more in reactive coping mechanisms which is highlighted by social avoidance and low aggression. Importantly, effects of defeat on social interaction behavior in females is independent of adult gonadal steroids. These behavioral phenotypes are associated with sex-specific changes in arginine vasopressin (AVP) and oxytocin (OT), closely related peptides that regulate social behavior and stress reactivity. In brain regions associated with stress responses and social behavior, defeat induced long term decreases in AVP activity and increases in OT activity in males and females respectively. Intranasal OT administration was shown to mimic the effects of defeat-induced increases in endogenous OT activity, causing social withdrawal in undefeated females. This suggests that inhibition of OT activity could reduce the impact of stress on behavior in females. These results highlight the value of maintaining diverse rodent models in the search for sex-specific pharmacological approaches to treating mood disorders.

Breast cancer and social environment: getting by with a little help from our friends

Social environment is a well-recognized determinant in health and wellbeing. Among breast cancer patients, inadequate social support is associated with a substantial increase in cancer-related mortality. A common explanation is that socially isolated individuals fare worse due to reduced instrumental support (i.e., assistance meeting the demands of treatment). However, the ability to replicate the detrimental effects of social isolation on mammary tumor growth in rodents strongly suggests an alternative explanation; i.e., socially isolated individuals have a physiological milieu that promotes tumor growth. This review summarizes the clinical and basic science literature supporting social influences on breast cancer, and provides a conceptual physiological framework for these effects. We propose that social environment contributes to the vast individual differences in prognosis among breast cancer survivors because social environment is capable of altering basic physiological processes, which in turn can modulate tumor growth. Appreciation of the role of social environment in breast cancer progression could promote the identification of patients at increased risk for poor outcomes. In addition, characterization of the underlying physiological mechanisms could lead to targeted disruption of detrimental pathways that promote tumor progression in socially isolated individuals, or exploitation of protective pathways activated through social engagement as novel therapeutic complements to contemporary treatments.

Cross-generational influences on childhood anxiety disorders: pathways and mechanisms

Anxiety disorders are common across the lifespan, cause severe distress and impairment, and usually have their onset in childhood. Substantial clinical and epidemiological research has demonstrated the existence of links between anxiety and its disorders in children and parents. Research on the pathways and mechanisms underlying these links has pointed to both behavioral and biological systems. This review synthesizes and summarizes several major aspects of this research. Behavioral systems include vicarious learning, social referencing, and modeling of parental anxiety; overly protective or critical parenting styles; and aspects of parental responses to child anxiety including family accommodation of the child's symptoms. Biological systems include aspects of the prenatal environment affected by maternal anxiety, development and functioning of the oxytocinergic system, and genetic and epigenetic transmission. Implications for the prevention and treatment of child anxiety disorders are discussed, including the potential to enhance child anxiety treatment outcomes through biologically informed parent-based interventions.

Social buffering ameliorates conditioned fear responses in female rats

The stress experienced by an animal is ameliorated when the animal is exposed to distressing stimuli along with a conspecific animal(s). This is known as social buffering. Previously, we found that the presence of an unfamiliar male rat induced social buffering and ameliorated conditioned fear responses of a male rat subjected to an auditory conditioned stimulus (CS). However, because our knowledge of social buffering is highly biased towards findings in male subjects, analyses using female subjects are crucial for comprehensively understanding the social buffering phenomenon. In the present studies, we assessed social buffering of conditioned fear responses in female rats. We found that the estrus cycle did not affect the intensity of the rats' fear responses to the CS or their degree of vigilance due to the presence of a conspecific animal. Based on these findings, we then assessed whether social buffering ameliorated conditioned fear responses in female rats without taking into account their estrus cycles. When fear conditioned female rats were exposed to the CS without the presence of a conspecific, they exhibited behavioral responses, including freezing, and elevated corticosterone levels. By contrast, the presence of an unfamiliar female rat suppressed these responses. Based on these findings, we conclude that social buffering can ameliorate conditioned fear responses in female rats.

The ties that bond: neurochemistry of attachment in voles

In socially monogamous prairie voles (Microtus ochrogaster), mating induces three primary types of behavior; namely, partner preference, selective aggression toward conspecific strangers, and bi-parental care, making this rodent an ideal model system to study sociality and underlying neurochemical mechanisms associated with monogamous mating strategies. Here, we highlight species differences in neurochemical receptor distributions associated with mating experience leading to the establishment of stable pair-bonds. Specifically, we illustrate the role of nucleus accumbens dopamine in programming the formation and maintenance of monogamous bonds and describe the role of anterior hypothalamic vasopressin in the regulation of selective aggression. We conclude by discussing recent molecular work in voles and emphasize the importance of this rodent for future research in the behavioral neurobiology field.

New opportunities in vasopressin and oxytocin research: a perspective from the amygdala

In the present review, we discuss how the evolution of oxytocin and vasopressin from a single ancestor peptide after gene duplication has stimulated the development of the vertebrate social brain. Separate production sites became possible with a hypothalamic development, which, interestingly, is triggered by the same transcription factors that underlie the development of various subcortical regions where vasopressin and oxytocin receptors are adjacently expressed and which are connected by inhibitory circuits. The opposite modulation of their output by vasopressin and oxytocin could thus create a dynamic equilibrium for rapid responsiveness to external stimuli. At the level of the individual, nurturing early in life can long-lastingly program oxytocin signaling, maintaining a capability of learning and sensitivity to external stimuli that contributes to development of social behavior in adulthood. Oxytocin and vasopressin are thus important for the development of a vertebrate brain that supports bonding between individuals and building of an interactive community.

Oxytocin promotes functional coupling between paraventricular nucleus and both sympathetic and parasympathetic cardioregulatory nuclei

The neuropeptide oxytocin (OXT) facilitates prosocial behavior and selective sociality. In the context of stress, OXT also can down-regulate hypothalamic-pituitary-adrenal (HPA) axis activity, leading to consideration of OXT as a potential treatment for many socioaffective disorders. However, the mechanisms through which administration of exogenous OXT modulates social behavior in stressful environmental contexts are not fully understood. Here, we investigate the hypothesis that autonomic pathways are components of the mechanisms through which OXT aids the recruitment of social resources in stressful contexts that may elicit mobilized behavioral responses. Female prairie voles (Microtus ochrogaster) underwent a stressor (walking in shallow water) following pretreatment with intraperitoneal OXT (0.25mg/kg) or OXT antagonist (OXT-A, 20mg/kg), and were allowed to recover with or without their sibling cagemate. Administration of OXT resulted in elevated OXT concentrations in plasma, but did not dampen the HPA axis response to a stressor. However, OXT, but not OXT-A, pretreatment prevented the functional coupling, usually seen in the absence of OXT, between paraventricular nucleus (PVN) activity as measured by c-Fos immunoreactivity and HPA output (i.e. corticosterone release). Furthermore, OXT pretreatment resulted in functional coupling between PVN activity and brain regions regulating both sympathetic (i.e. rostral ventrolateral medulla) and parasympathetic (i.e. dorsal vagal complex and nucleus ambiguous) branches of the autonomic nervous system. These findings suggest that OXT increases central neural control of autonomic activity, rather than strictly dampening HPA axis activity, and provides a potential mechanism through which OXT may facilitate adaptive and context-dependent behavioral and physiological responses to stressors.

Stress impairs new but not established relationships in seasonally social voles

Affiliative social relationships are impacted by stressors and can shape responses to stress. However, the effects of stress on social relationships in different contexts are not well understood. Meadow voles provide an opportunity to study these effects on peer relationships outside of a reproductive context. In winter months, female meadow voles cohabit with peers of both sexes, and social huddling is facilitated by exposure to short, winter-like day lengths in the lab. We investigated the role of stress and corticosterone (cort) levels in social behavior in short day-housed female meadow voles. A brief forced swim elevated cort levels, and we assessed the effects of this stressor on new and established relationships between females. In pairs formed following exposure to swim stress, the stressor significantly reduced the fraction of huddling time subjects spent with a familiar partner. Swim stress did not affect partner preferences in pairs established prior to the stressor. Finally, we examined fecal glucocorticoid metabolite levels via immunoassay in voles housed under short day (10h light) versus long day (14 h light) conditions and detected higher glucocorticoid levels in long day-housed voles. These findings support a role for stress regulation in the formation of social relationships in female meadow voles, and are consistent with a potential role for seasonal variation in cort in the behavioral transition from solitary to social. Together they highlight the importance of stress and possibly glucocorticoid signaling for social behavior.

Social stress buffering by friends in childhood and adolescence: Effects on HPA and oxytocin activity

Previous research has demonstrated that before puberty, parents are able to buffer, and often completely block, cortisol responses to social evaluative stressors (e.g., Trier Social Stress Test; TSST). However, after puberty, parents no longer provide a powerful buffer of the HPA axis from a social-evaluative stressor. The current study investigates whether friends can buffer the HPA axis in both children and adolescents compared to parents and whether similar stress-ameliorating patterns can also be observed in oxytocin activity. A total of 109 participants (54 children aged 9-10 and 55 adolescents aged 15-16; half of each sex) completed the TSST and were randomly assigned to prepare for their speech with their parent or friend for 5 minutes beforehand. Salivary cortisol and urinary oxytocin were measured before and after the TSST. For children, cortisol responses were comparable regardless of who helped the child prepare the speech. For adolescents, however, friends actually amplified the cortisol response compared to parents. In addition, adolescents produced less oxytocin than children, as did males compared to females. Notably, for boys, oxytocin levels decreased across the session if participants prepared with a friend rather than their parent. The mean change was in the same direction but not significant for girls. These results indicate that friends do not take over the social buffering role by age 15-16, which may inform interventions in at-risk children and adolescents.

Oxytocin-dependent consolation behavior in rodents

Consolation behavior toward distressed others is common in humans and great apes, yet our ability to explore the biological mechanisms underlying this behavior is limited by its apparent absence in laboratory animals. Here, we provide empirical evidence that a rodent species, the highly social and monogamous prairie vole (Microtus ochrogaster), greatly increases partner-directed grooming toward familiar conspecifics (but not strangers) that have experienced an unobserved stressor, providing social buffering. Prairie voles also match the fear response, anxiety-related behaviors, and corticosterone increase of the stressed cagemate, suggesting an empathy mechanism. Exposure to the stressed cagemate increases activity in the anterior cingulate cortex, and oxytocin receptor antagonist infused into this region abolishes the partner-directed response, showing conserved neural mechanisms between prairie vole and human.

Oxytocin in the nucleus accumbens shell reverses CRFR2-evoked passive stress-coping after partner loss in monogamous male prairie voles

Loss of a partner can have severe effects on mental health. Here we explore the neural mechanisms underlying increased passive stress-coping, indicative of depressive-like behavior, following the loss of the female partner in the monogamous male prairie vole. We demonstrate that corticotropin-releasing factor receptor 2 (CRFR2) in the nucleus accumbens shell mediates social loss-induced passive coping. Further, we show that partner loss compromises the oxytocin system through multiple mechanisms. Finally, we provide evidence for an interaction of the CRFR2 and oxytocin systems in mediating the emotional consequences of partner loss. Our results suggest that chronic activation of CRFR2 and suppression of striatal oxytocin signaling following partner loss result in an aversive emotional state that may share underlying mechanisms with bereavement. We propose that the suppression of oxytocin signaling is likely adaptive during short separations to encourage reunion with the partner and may have evolved to maintain long-term partnerships. Additionally, therapeutic strategies targeting these systems should be considered for treatment of social loss-mediated depression.

Relations between plasma oxytocin and cortisol: The stress buffering role of social support

Stress responses in humans can be attenuated by exogenous oxytocin administration, and these stress-buffering properties may be moderated by social factors. Yet, the influence of acute stressors on circulating endogenous oxytocin levels have been inconsistent, and limited information is available concerning the influence of social support in moderating this relationship. In the current investigation, undergraduate women (N = 67) were assessed in the Trier Social Stress Test (TSST) with either social support available from a close female friend or no social support being available. An additional set of women served as controls. The TSST elicited marked elevations of state anxiety and negative emotions, which were largely attenuated among women who received social support. Furthermore, baseline oxytocin levels were inversely related to women's general feelings of distrust, as well as basal plasma cortisol levels. Despite these associations, oxytocin levels were unaffected by the TSST, and this was the case irrespective of oral contraceptive use or estrogen levels. In contrast, plasma cortisol elevations were elicited by the psychosocial stressor, but only in women using oral contraceptives, an effect that was prevented when social support was available. Taken together, these data provisionally suggest that changes in plasma oxytocin might not accompany the stress attenuating effects of social support on cortisol levels. Moreover, as plasma oxytocin might not reliably reflect brain oxytocin levels, the linkage between oxytocin and prosocial behaviors remains tenuous.

Local oxytocin tempers anxiety by activating GABAA receptors in the hypothalamic paraventricular nucleus

Oxytocin (Oxt) is released in various hypothalamic and extrahypothalamic brain areas in response to anxiogenic stimuli to regulate aspects of emotionality and stress coping. We examined the anxiolytic action of Oxt in the hypothalamic paraventricular nucleus (PVN) while appraising if Oxt recruits GABA neurons to inhibit the behavioral, hormonal, and neuronal response to stress in female prairie voles (Microtus ochrogaster). Voles received an injection of Oxt in the PVN either before or after an elevated platform stress to determine a time-course for the effects of Oxt on the hormonal stress response. Subsequently, we evaluated if ante-stress injections of Oxt affected anxiety-like behaviors as well as neuronal activity in the PVN, using real-time in-vivo retrodialysis and immunohistochemistry with c-Fos expression as a biomarker of neural activity. In addition, we exposed voles to Oxt and a GABAA receptor antagonist, concurrently, to evaluate the impact of pharmacological blockade of GABAA receptors on the anxiolytic effects of Oxt. Elevated platform stress amplified anxiety-like behaviors and hypothalamic-pituitary-adrenal (HPA) axis activity-catalyzing corticotrophin-releasing hormone (CRH) neuronal activity and augmenting corticosterone release in circulation. Ante-stress Oxt injections in the PVN blocked these stress effects while promoting PVN GABA activity and release. Post-stress Oxt treatments were ineffective. The anxiolytic effects of Oxt were hindered by concurrent pharmacological blockade of GABAA receptors. Together, our data demonstrate ante-stress treatments of Oxt in the PVN inhibit stress activation of the HPA axis through recruitment of GABAergic neurons, providing insights to the local circuitry and potential therapeutically-relevant mechanisms.

Natural variation in maternal care and cross-tissue patterns of oxytocin receptor gene methylation in rats

This article is part of a Special Issue "Parental Care". Since the first report of maternal care effects on DNA methylation in rats, epigenetic modifications of the genome in response to life experience have become the subject of intense focus across many disciplines. Oxytocin receptor expression varies in response to early experience, and both oxytocin signaling and methylation status of the oxytocin receptor gene (Oxtr) in blood have been related to disordered social behavior. It is unknown whether Oxtr DNA methylation varies in response to early life experience, and whether currently employed peripheral measures of Oxtr methylation reflect variation in the brain. We examined the effects of early life rearing experience via natural variation in maternal licking and grooming during the first week of life on behavior, physiology, gene expression, and epigenetic regulation of Oxtr across blood and brain tissues (mononucleocytes, hippocampus, striatum, and hypothalamus). Rats reared by "high" licking-grooming (HL) and "low" licking-grooming (LL) rat dams exhibited differences across study outcomes: LL offspring were more active in behavioral arenas, exhibited lower body mass in adulthood, and showed reduced corticosterone responsivity to a stressor. Oxtr DNA methylation was significantly lower at multiple CpGs in the blood of LL versus HL males, but no differences were found in the brain. Across groups, Oxtr transcript levels in the hypothalamus were associated with reduced corticosterone secretion in response to stress, congruent with the role of oxytocin signaling in this region. Methylation of specific CpGs at a high or low level was consistent across tissues, especially within the brain. However, individual variation in DNA methylation relative to these global patterns was not consistent across tissues. These results suggest that blood Oxtr DNA methylation may reflect early experience of maternal care, and that Oxtr methylation across tissues is highly concordant for specific CpGs, but that inferences across tissues are not supported for individual variation in Oxtr methylation.

Effects of social defeat on dopamine neurons in the ventral tegmental area in male and female California mice

Dopamine neurons in the ventral tegmental area (VTA) have important functions related to rewards but are also activated in aversive contexts. Electrophysiology studies suggest that the degree to which VTA dopamine neurons respond to noxious stimuli is topographically organized across the dorsal-ventral extent. We used c-fos immunohistochemistry to examine the responses of VTA dopamine neurons in contexts of social defeat and social approach. Studying monogamous California mice (Peromyscus californicus) allowed us to observe the effects of social defeat on both males and females. Females exposed to three episodes of defeat, but not a single episode, had more tyrosine hydroxylase (TH)/c-fos-positive cells in the ventral (but not dorsal) VTA compared with controls. This observation suggests that repeated exposure to aversive contexts is necessary to trigger activation of VTA dopamine neurons. Defeat did not affect TH/c-fos colocalizations in males. We also examined the long-term effects of defeat on c-fos expression in a social interaction test. As previously reported, defeat reduced social interaction in females but not males. Surprisingly, there were no effects of defeat stress on TH/c-fos colocalizations in any subregion of the VTA. However, females had more TH/c-fos-positive cells than males across the entire VTA, and also had greater c-fos-positive cell counts in posterior subregions of the nucleus accumbens shell. Our results show that dopamine neurons in the VTA are more responsive to social contexts in females and that the ventral VTA in particular is sensitive to aversive contexts.

Neuropeptidergic regulation of pair-bonding and stress buffering: Lessons from voles

This article is part of a Special Issue "SBN 2014". Interpersonal attachment is a critical component of the human experience. Pair-bonding ameliorates the severity of several mental and physical diseases. Thus, a better understanding of how the central nervous system responds to and encodes social-buffering during stress is a valuable research enterprise. The prairie vole (Microtus ochrogaster), as a laboratory animal model, provides the gold standard for the investigation of the neurobiology underlying attachment. Furthermore, emerging research in voles, additional laboratory rodents, transgenic mice, primates, and humans has provided novel insight into the neurochemical mechanisms underlying the therapeutic effects of social bonds reducing anxiety, depression, and drug abuse liability. In the present review, we highlight the work from this burgeoning field and focus on the role(s) of the neuropeptides oxytocin (OT), vasopressin (AVP), and corticotrophin releasing hormone (CRH) mediating stress buffering. Together, the data suggest that OT underlies social bonding to reduce stress-induced psychological illness while AVP and CRH facilitate arousal to enhance autonomic reactivity, increasing susceptibility to adverse mental and physical health.

Recent Developments in the Study of Social Relationships, Stress Responses, and Physical Health

This selective review aims to highlight some of the most recent empirical or theoretical advancements in the study of social relationships as buffers against stress and as protective factors against risk for disease, focusing on articles published between 2013 and 2015. The review summarizes novel findings showing that social relationships can protect individuals against negative health outcomes associated with chronic adversity and can be associated with reduced cumulative physiological damage (allostatic load). There is also evidence that some relationships can be a source of stress. Additionally, recent findings concerning the psychological and neurobiological mechanisms of action for social support, the developmental patterning of social stress-buffering and recent experimental studies attempting to change relationships to affect health are also reviewed.

Oxytocin Regulates Stress-Induced Crf Gene Transcription through CREB-Regulated Transcription Coactivator 3

The major regulator of the neuroendocrine stress response in the brain is corticotropin releasing factor (CRF), whose transcription is controlled by CREB and its cofactors CRTC2/3 (TORC2/3). Phosphorylated CRTCs are sequestered in the cytoplasm, but rapidly dephosphorylated and translocated into the nucleus following a stressful stimulus. As the stress response is attenuated by oxytocin (OT), we tested whether OT interferes with CRTC translocation and, thereby, Crf expression. OT (1 nmol, i.c.v.) delayed the stress-induced increase of nuclear CRTC3 and Crf hnRNA levels in the paraventricular nucleus of male rats and mice, but did not affect either parameter in the absence of the stressor. The increase in Crf hnRNA levels at later time points was parallel to elevated nuclear CRTC2/3 levels. A direct effect of Thr(4) Gly(7)-OT (TGOT) on CRTC3 translocation and Crf expression was found in rat primary hypothalamic neurons, amygdaloid (Ar-5), hypothalamic (H32), and human neuroblastoma (Be(2)M17) cell lines. CRTC3, but not CRCT2, knockdown using siRNA in Be(2)M17 cells prevented the effect of TGOT on Crf hnRNA levels. Chromatin-immunoprecipitation demonstrated that TGOT reduced CRTC3, but not CRTC2, binding to the Crf promoter after 10 min of forskolin stimulation. Together, the results indicate that OT modulates CRTC3 translocation, the binding of CRTC3 to the Crf promoter and, ultimately, transcription of the Crf gene.

SIGNIFICANCE STATEMENT

The neuropeptide oxytocin has been proposed to reduce hypothalamic-pituitary-adrenal (HPA) axis activation during stress. The underlying mechanisms are, however, elusive. In this study we show that activation of the oxytocin receptor in the paraventricular nucleus delays transcription of the gene encoding corticotropin releasing factor (Crf), the main regulator of the stress response. It does so by sequestering the coactivator of the transcription factor CREB, CRTC3, in the cytosol, resulting in reduced binding of CRTC3 to the Crf gene promoter and subsequent Crf gene expression. This novel oxytocin receptor-mediated intracellular mechanism might provide a basis for the treatment of exaggerated stress responses in the future.

The Research Domain Criteria (RDoC) Project and Studies of Risk and Resilience in Maltreated Children

OBJECTIVE

The Research Domain Criteria (RDoC) project was initiated to develop, for research purposes, new ways of classifying mental disorders based on dimensions of observable behavior and neurobiological measures. This article reviews the rationale behind the RDoC program, its goals, and central tenets; discusses application of an RDoC framework to research with maltreated children; and highlights some clinical implications of this work.

METHOD

Published RDoC papers were reviewed, together with relevant preclinical and clinical studies that guide our work on risk and resilience in maltreated children.

RESULTS

The ultimate long-term goal of the RDoC initiative is precision medicine in psychiatry. In the interim, the RDoC initiative provides a framework to organize research to help develop the database required to derive a new psychiatric nomenclature that can appropriately match treatments to patients. The primary focus of RDoC is on neural circuitry, with levels of analyses that span from molecules to behavior. There has been some concern that the RDoC framework is reductionist, with an overemphasis on neural circuits and genetics; however, the briefly reviewed, burgeoning literature on neuroplasticity and epigenetics highlights that this concern is unwarranted, as one cannot study neural circuits and genetics without considering experience.

CONCLUSION

The study of maltreated children has a number of advantages for the RDoC project, including the following: study of a subset of patients who are often not responsive to standard interventions; examination of a relatively homogenous sample with onset of psychopathology proposed to be associated with stress-related mechanisms; and well-established, relevant animal models to facilitate translational research.

Social Support Can Buffer against Stress and Shape Brain Activity

Social support from close relationship partners is an important resource for coping with stress, particularly during childhood. We discuss ethical challenges associated with studying stress and its social buffering in the laboratory, as well as emerging evidence regarding two potential neural substrates for the social buffering of stress: hypothalamic oxytocin activity and activation of areas in the prefrontal cortex associated with effective self-regulation. We also address the role of early-life social experiences in shaping brain development, as well as recommendations for practice and policy that would advance the ethical treatment of children and reduce social inequalities in early-life experiences and opportunities-e.g., investing in programs that prevent child maltreatment and facilitating access to high-quality child care for economically disadvantaged families. We also debate the ethical implications of using oxytocin nasal sprays to simulate the stress-reducing properties of social support and advise waiting for more evidence before recommending their use.

Stress and the social brain: behavioural effects and neurobiological mechanisms

Stress often affects our social lives. When undergoing high-level or persistent stress, individuals frequently retract from social interactions and become irritable and hostile. Predisposition to antisocial behaviours - including social detachment and violence - is also modulated by early life adversity; however, the effects of early life stress depend on the timing of exposure and genetic factors. Research in animals and humans has revealed some of the structural, functional and molecular changes in the brain that underlie the effects of stress on social behaviour. Findings in this emerging field will have implications both for the clinic and for society.

Oxytocin: parallel processing in the social brain?

Early studies attempting to disentangle the network complexity of the brain exploited the accessibility of sensory receptive fields to reveal circuits made up of synapses connected both in series and in parallel. More recently, extension of this organisational principle beyond the sensory systems has been made possible by the advent of modern molecular, viral and optogenetic approaches. Here, evidence supporting parallel processing of social behaviours mediated by oxytocin is reviewed. Understanding oxytocinergic signalling from this perspective has significant implications for the design of oxytocin-based therapeutic interventions aimed at disorders such as autism, where disrupted social function is a core clinical feature. Moreover, identification of opportunities for novel technology development will require a better appreciation of the complexity of the circuit-level organisation of the social brain.

Neurobiological mechanisms of social attachment and pair bonding

Species have evolved diverse social behavior and mating strategies in response to selective forces in their environments. While promiscuity is the predominant mating strategy across most vertebrate taxa, convergent evolution of monogamous mating systems has occurred multiple times across distant lineages. Monogamous behavior is thought to be facilitated by a neurobiological capacity to form and maintain selective social attachments, or pair bonds, with a mating partner. The neural mechanisms of pair bonding behavior have been investigated most rigorously in Microtine rodents, which exhibit diverse social organizations. These studies have highlighted mesolimbic dopamine pathways, social neuropeptides (oxytocin and vasopressin), and other neural systems as integral factors in the formation, maintenance, and expression of pair bonds.

Stress, social behavior, and resilience: insights from rodents

The neurobiology of stress and the neurobiology of social behavior are deeply intertwined. The social environment interacts with stress on almost every front: social interactions can be potent stressors; they can buffer the response to an external stressor; and social behavior often changes in response to stressful life experience. This review explores mechanistic and behavioral links between stress, anxiety, resilience, and social behavior in rodents, with particular attention to different social contexts. We consider variation between several different rodent species and make connections to research on humans and non-human primates.

Oxytocin and Estrogen Receptor β in the Brain: An Overview

Oxytocin (OT) is a neuropeptide synthesized primarily by neurons of the paraventricular and supraoptic nuclei of the hypothalamus. These neurons have axons that project into the posterior pituitary and release OT into the bloodstream to promote labor and lactation; however, OT neurons also project to other brain areas where it plays a role in numerous brain functions. OT binds to the widely expressed OT receptor (OTR), and, in doing so, it regulates homeostatic processes, social recognition, and fear conditioning. In addition to these functions, OT decreases neuroendocrine stress signaling and anxiety-related and depression-like behaviors. Steroid hormones differentially modulate stress responses and alter OTR expression. In particular, estrogen receptor β activation has been found to both reduce anxiety-related behaviors and increase OT peptide transcription, suggesting a role for OT in this estrogen receptor β-mediated anxiolytic effect. Further research is needed to identify modulators of OT signaling and the pathways utilized and to elucidate molecular mechanisms controlling OT expression to allow better therapeutic manipulations of this system in patient populations.

Social functions of individual vasopressin-oxytocin cell groups in vertebrates: what do we really know?

Vasopressin-oxytocin (VP-OT) nonapeptides modulate numerous social and stress-related behaviors, yet these peptides are made in multiple nuclei and brain regions (e.g., >20 in some mammals), and VP-OT cells in these areas often exhibit overlapping axonal projections. Furthermore, the magnocellular cell groups release peptide volumetrically from dendrites and soma, which gives rise to paracrine modulation in distal brain areas. Nonapeptide receptors also tend to be promiscuous. Hence, behavioral effects that are mediated by any given receptor type (e.g., the OT receptor) in a target brain region cannot be conclusively attributed to either VP or OT, nor to a specific cell group. We here review what is actually known about the social behavior functions of nonapeptide cell groups, with a focus on aggression, affiliation, bonding, social stress, and parental behavior, and discuss recent studies that demonstrate a diversity of sex-specific contributions of VP-OT cell groups to gregariousness and pair bonding.