Oxytocin-Oxytocin Receptor Systems Facilitate Social Defeat Posture in Male Mice

Systematic review: pain, cognition, and cardioprotection-unpacking oxytocin's contributions in a sport context

Introduction

This systematic review investigates the interplay between oxytocin and exercise; in terms of analgesic, anti-inflammatory, pro-regenerative, and cardioprotective effects. Furthermore, by analyzing measurement methods, we aim to improve measurement validity and reliability.

Methods

Utilizing PRISMA, GRADE, and MECIR protocols, we examined five databases with a modified SPIDER search. Including studies on healthy participants, published within the last 20 years, based on keywords "oxytocin," "exercise" and "measurement," 690 studies were retrieved initially (455 unique records). After excluding studies of clinically identifiable diseases, and unpublished and reproduction-focused studies, 175 studies qualified for the narrative cross-thematic and structural analysis.

Results

The analysis resulted in five categories showing the reciprocal impact of oxytocin and exercise: Exercise (50), Physiology (63), Environment (27), Social Context (65), and Stress (49). Exercise-induced oxytocin could promote tissue regeneration, with 32 studies showing its analgesic and anti-inflammatory effects, while 14 studies discussed memory and cognition. Furthermore, empathy-associated OXTR rs53576 polymorphism might influence team sports performance. Since dietary habits and substance abuse can impact oxytocin secretion too, combining self-report tests and repeated salivary measurements may help achieve precision.

Discussion

Oxytocin's effect on fear extinction and social cognition might generate strategies for mental training, and technical, and tactical development in sports. Exercise-induced oxytocin can affect the amount of stress experienced by athletes, and their response to it. However, oxytocin levels could depend on the type of sport in means of contact level, exercise intensity, and duration. The influence of oxytocin on athletes' performance and recovery could have been exploited due to its short half-life. Examining oxytocin's complex interactions with exercise paves the way for future research and application in sports science, psychology, and medical disciplines.

Systematic Review Registration

https://www.crd.york.ac.uk/prospero/display_record.php?RecordID=512184, identifier CRD42024512184.

Giftedness and atypical sexual differentiation: enhanced perceptual functioning through estrogen deficiency instead of androgen excess

Syndromic autism spectrum conditions (ASC), such as Klinefelter syndrome, also manifest hypogonadism. Compared to the popular Extreme Male Brain theory, the Enhanced Perceptual Functioning model explains the connection between ASC, savant traits, and giftedness more seamlessly, and their co-emergence with atypical sexual differentiation. Overexcitability of primary sensory inputs generates a relative enhancement of local to global processing of stimuli, hindering the abstraction of communication signals, in contrast to the extraordinary local information processing skills in some individuals. Weaker inhibitory function through gamma-aminobutyric acid type A (GABAA) receptors and the atypicality of synapse formation lead to this difference, and the formation of unique neural circuits that process external information. Additionally, deficiency in monitoring inner sensory information leads to alexithymia (inability to distinguish one's own emotions), which can be caused by hypoactivity of estrogen and oxytocin in the interoceptive neural circuits, comprising the anterior insular and cingulate gyri. These areas are also part of the Salience Network, which switches between the Central Executive Network for external tasks and the Default Mode Network for self-referential mind wandering. Exploring the possibility that estrogen deficiency since early development interrupts GABA shift, causing sensory processing atypicality, it helps to evaluate the co-occurrence of ASC with attention deficit hyperactivity disorder, dyslexia, and schizophrenia based on phenotypic and physiological bases. It also provides clues for understanding the common underpinnings of these neurodevelopmental disorders and gifted populations.

Social neuroscience: How we learn to avoid the bully

During social interactions, individuals evaluate relationships with their peers and switch from approach to avoidance, particularly in response to aggressive encounters. A new study in mice investigated the underlying brain mechanisms and identified oxytocin as a key regulator of social avoidance learning.

Leveraging the unique social organization of California mice to study circuit-specific effects of oxytocin on behavior

Oxytocin is a versatile neuropeptide that modulates many different forms of social behavior. Recent hypotheses pose that oxytocin enhances the salience of rewarding and aversive social experiences, and the field has been working to identify mechanisms that allow oxytocin to have diverse effects on behavior. Here we review studies conducted on the California mouse (Peromyscus californicus) that shed light on how oxytocin modulates social behavior following stressful experiences. In this species, both males and females exhibit high levels of aggression, which has facilitated the study of how social stress impacts both sexes. We review findings of short- and long-term effects of social stress on the reactivity of oxytocin neurons. We also consider the results of pharmacological studies which show that oxytocin receptors in the bed nucleus of the stria terminalis and nucleus accumbens have distinct but overlapping effects on social approach behaviors. These findings help explain how social stress can have different behavioral effects in males and females, and how oxytocin can have such divergent effects on behavior. Finally, we consider how new technological developments and innovative research programs take advantage of the unique social organization of California mice to address questions that can be difficult to study in conventional rodent model species. These new methods and questions have opened new avenues for studying the neurobiology of social behavior.

A dedicated hypothalamic oxytocin circuit controls aversive social learning

To survive in a complex social group, one needs to know who to approach and, more importantly, who to avoid. In mice, a single defeat causes the losing mouse to stay away from the winner for weeks1. Here through a series of functional manipulation and recording experiments, we identify oxytocin neurons in the retrochiasmatic supraoptic nucleus (SOROXT) and oxytocin-receptor-expressing cells in the anterior subdivision of the ventromedial hypothalamus, ventrolateral part (aVMHvlOXTR) as a key circuit motif for defeat-induced social avoidance. Before defeat, aVMHvlOXTR cells minimally respond to aggressor cues. During defeat, aVMHvlOXTR cells are highly activated and, with the help of an exclusive oxytocin supply from the SOR, potentiate their responses to aggressor cues. After defeat, strong aggressor-induced aVMHvlOXTR cell activation drives the animal to avoid the aggressor and minimizes future defeat. Our study uncovers a neural process that supports rapid social learning caused by defeat and highlights the importance of the brain oxytocin system in social plasticity.

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

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

Sexual differentiation of neural mechanisms of stress sensitivity during puberty

Anxiety disorders are a major public health concern and current treatments are inadequate for many individuals. Anxiety is more common in women than men and this difference arises during puberty. Sex differences in physiological stress responses may contribute to this variability. During puberty, gonadal hormones shape brain structure and function, but the extent to which these changes affect stress sensitivity is unknown. We examined how pubertal androgens shape behavioral and neural responses to social stress in California mice (Peromyscus californicus), a model species for studying sex differences in stress responses. In adults, social defeat reduces social approach and increases social vigilance in females but not males. We show this sex difference is absent in juveniles, and that prepubertal castration sensitizes adult males to social defeat. Adult gonadectomy does not alter behavioral responses to defeat, indicating that gonadal hormones act during puberty to program behavioral responses to stress in adulthood. Calcium imaging in the medioventral bed nucleus of the stria terminalis (BNST) showed that social threats increased neural activity and that prepubertal castration generalized these responses to less threatening social contexts. These results support recent hypotheses that the BNST responds to immediate threats. Prepubertal treatment with the nonaromatizable androgen dihydrotestosterone acts in males and females to reduce the effects of defeat on social approach and vigilance in adults. These data indicate that activation of androgen receptors during puberty is critical for programming behavioral responses to stress in adulthood.

Effect of Neurosteroids on Basal and Stress-Induced Oxytocin Secretion in Luteal-Phase and Pregnant Sheep

Oxytocin (OT) is a neuropeptide synthesized in the hypothalamic nuclei that modulates both behavioral and reproductive functions, associated with the increased neurosteroid synthesis in the brain. Therefore, the present study tested the hypothesis that manipulation of central neurosteroid levels could affect oxytocin synthesis and release in non-pregnant and pregnant sheep under both basal and stressful conditions. In Experiment 1, luteal-phase sheep were subjected to a series of intracerebroventricular (icv.) infusions of allopregnanolone (AL, 4 × 15 μg/60 μL/30 min) for 3 days. In Experiment 2, pregnant animals (4th month) received a series of infusions of the neurosteroid synthesis blocker, finasteride (4 × 25 μg/60 μL/30 min), conducted for 3 days. In non-pregnant sheep AL alone was shown to differentially modulate OT synthesis in basal conditions, and strongly inhibit OT response to stress (p < 0.001). In contrast, in pregnant animals, basal and stress-induced OT secretion was significantly (p < 0.001) increased during finasteride infusion compared to controls. In conclusion, we showed that neurosteroids were involved in the control of OT secretion in sheep, particularly under stress and pregnancy conditions and are part of an adaptive mechanism which is responsible for protecting and maintaining pregnancy in harmful situations.

An analgesic pathway from parvocellular oxytocin neurons to the periaqueductal gray in rats

The hypothalamic neuropeptide oxytocin (OT) exerts prominent analgesic effects via central and peripheral action. However, the precise analgesic pathways recruited by OT are largely elusive. Here we discovered a subset of OT neurons whose projections preferentially terminate on OT receptor (OTR)-expressing neurons in the ventrolateral periaqueductal gray (vlPAG). Using a newly generated line of transgenic rats (OTR-IRES-Cre), we determined that most of the vlPAG OTR expressing cells targeted by OT projections are GABAergic. Ex vivo stimulation of parvocellular OT axons in the vlPAG induced local OT release, as measured with OT sensor GRAB. In vivo, optogenetically-evoked axonal OT release in the vlPAG of as well as chemogenetic activation of OTR vlPAG neurons resulted in a long-lasting increase of vlPAG neuronal activity. This lead to an indirect suppression of sensory neuron activity in the spinal cord and strong analgesia in both female and male rats. Altogether, we describe an OT-vlPAG-spinal cord circuit that is critical for analgesia in both inflammatory and neuropathic pain models.

L-4-Fluorophenylglycine produces antidepressant-like effects and enhances resilience to stress in mice

D-serine has attracted increasing attention for its possible role in depression. L-4-Fluorophenylglycine (L-4FPG), an inhibitor of the neutral amino acid transporter ASCT1/2, has been shown to regulate extracellular D-serine levels. The present study aimed to explore the potential antidepressant effects of L-4FPG. First, the acute effects of L-4FPG on the forced swimming test, elevated plus maze test, and novelty-suppressed feeding test were examined. L-4FPG showed antidepressant-like effects, which could be reversed by rapamycin, a mammalian target of rapamycin (mTOR) inhibitor, and 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), an alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptor antagonist. The phosphorylation levels of mTOR and GluR1 in the hippocampus were also increased after L-4FPG treatment. Next, the therapeutic effects of L-4FPG were examined in a chronic social defeat stress (CSDS) model of depression. L-4FPG ameliorated depression-like behaviors in mice subjected to CSDS. Furthermore, treatment with L-4FPG prior to each social defeat stress session not only decreased defensive behaviors but also prevented CSDS-induced social avoidance and anxiety-like and depression-like behaviors. These findings suggest that L-4FPG may be useful not only in alleviating depression but also in protecting against chronic stress-related psychiatric disorders.

An oxytocinergic neural pathway that stimulates thermogenic and cardiac sympathetic outflow

Oxytocin alters autonomic functions besides social behaviors. However, the central neuronal links between hypothalamic oxytocinergic neurons and the autonomic nervous system remain unclear. Here we show that oxytocinergic neurons in the rat paraventricular hypothalamic nucleus (PVH), a pivotal site for energy homeostasis, innervate sympathetic premotor neurons in the rostral medullary raphe region (rMR) to stimulate brown adipose tissue (BAT) thermogenesis and cardiovascular functions. Oxytocin receptor stimulation in the rMR evokes BAT thermogenesis and tachycardia. In vivo optogenetic stimulation of the PVH→rMR long-range oxytocinergic pathway, using a virus-mediated system for amplified gene expression in oxytocinergic neurons, not only elicits BAT thermogenic and cardiac responses but also potentiates sympathetic responses evoked by glutamatergic transmission in the rMR. The PVH→rMR oxytocinergic pathway connects the hypothalamic circuit for energy homeostasis to thermogenic and cardiac sympathetic outflow, and, therefore, its defects may cause obesity and impaired thermoregulation, as seen in Prader-Willi syndrome.

Nanobody-based RFP-dependent Cre recombinase for selective anterograde tracing in RFP-expressing transgenic animals

Transgenic animals expressing fluorescent proteins are widely used to label specific cells and proteins. By using a split Cre recombinase fused with mCherry-binding nanobodies or designed ankyrin repeat proteins, we created Cre recombinase dependent on red fluorescent protein (RFP) (Cre-DOR). Functional binding units for monomeric RFPs are different from those for polymeric RFPs. We confirmed selective target RFP-dependent gene expression in the mouse cerebral cortex using stereotaxic injection of adeno-associated virus vectors. In estrogen receptor-beta (Esr2)-mRFP1 mice and gastrin-releasing peptide receptor (Grpr)-mRFP1 rats, we confirmed that Cre-DOR can be used for selective tracing of the neural projection from RFP-expressing specific neurons. Cellular localization of RFPs affects recombination efficiency of Cre-DOR, and light and chemical-induced nuclear translocation of an RFP-fused protein can modulate Cre-DOR efficiency. Our results provide a method for manipulating gene expression in specific cells expressing RFPs and expand the repertory of nanobody-based genetic tools.

Synergistic consequences of early-life social isolation and chronic stress impact coping and neural mechanisms underlying male prairie vole susceptibility and resilience

Chronic stress can be challenging, lead to maladaptive coping strategies, and cause negative mental and physical health outcomes. Early-life adversity exposes developing young to physical or psychological experiences that risks surpassing their capacity to effectively cope, thereby impacting their lifetime physical and mental wellbeing. Sensitivity to stressful events, like social isolation, has the potential to magnify stress-coping. Chronic stress through social defeat is an established paradigm that models adverse early-life experiences and can trigger enduring alterations in behavioral and neural phenotypes. To assess the degree to which stress resilience and sensitivity stemming from early-life chronic stress impact sociability, we exposed male prairie voles to chronic social defeat stress (CSDS) during adolescence. We simultaneously exposed subjects to either social isolation (CSDS+Isol) or group housing (CSDS+Soc) during this crucial time of development. On PND41, all subjects underwent a social approach test to examine the immediate impact of isolation, CSDS, or their combined effects on sociability. Unlike the CSDS+Isol group which primarily displayed social avoidance, the CSDS+Soc group was split by individuals exhibiting susceptible or resilient stress phenotypes. Notably, the Control+Soc and CSDS+Soc animals and their cage-mates significantly gained body weight between PND31 and PND40, whereas the Control+Isol and CSDS+Isol animals did not. These results suggest that the effects of early-life stress may be mitigated by having access to social support. Vasopressin, oxytocin, and opioids and their receptors (avpr1a, oxtr, oprk1, oprm1, and oprd1) are known to modulate social and stress-coping behaviors in the lateral septum (LS). Therefore, we did an mRNA expression analysis with RT-qPCR of the avpr1a, oxtr, oprk1, oprm1, and oprd1 genes to show that isolation and CSDS, or their collective influence, can potentially differentially bias sensitivity of the LS to early-life stressors. Collectively, our study supports the impact and dimensionality of early-life adversity because the type (isolation vs. CSDS), duration (acute vs. chronic), and combination (isolation + CSDS) of stressors can dynamically alter behavioral and neural outcomes.

Species-typical group size differentially influences social reward neural circuitry during nonreproductive social interactions

We investigated whether nonreproductive social interactions may be rewarding for colonial but not non-colonial species. We found that the colonial spiny mouse (Acomys cahirinus) is significantly more gregarious, more prosocial, and less aggressive than its non-colonial relative, the Mongolian gerbil (Meriones unguiculatus). In an immediate-early gene study, we examined oxytocin (OT) and tyrosine hydroxylase (TH) neural responses to interactions with a novel, same-sex conspecific or a novel object. The paraventricular nucleus of the hypothalamus (PVN) OT cell group was more responsive to interactions with a conspecific compared to a novel object in both species. However, the ventral tegmental area (VTA) TH cell group showed differential responses only in spiny mice. Further, PVN OT and VTA TH neural responses positively correlated in spiny mice, suggesting functional connectivity. These results suggest that colonial species may have evolved neural mechanisms associated with reward in novel, nonreproductive social contexts to promote large group-living.

Long-Term Effects of Repeated Social Defeat Stress on Brain Activity during Social Interaction in BALB/c Mice

Understanding the long-term effects of stress on brain function is crucial for understanding the mechanisms of depression. The BALB/c mouse strain has high susceptibility to stress and is thus an effective model for depression. The long-term effects of repeated social defeat stress (SDS) on BALB/c mice, however, are not clear. Here, we investigated the effects of repeated SDS in male BALB/c mice over the subsequent two weeks. Some defeated mice immediately exhibited social avoidance, whereas anxiety-like behavior was only evident at later periods. Furthermore, defeated mice segregated into two groups based on the level of social avoidance, namely, avoidant and nonavoidant mice. The characteristic of avoidance or nonavoidance in each individual was not fixed over the two weeks. In addition, we developed a semi-automated method for analyzing c-Fos expression in the mouse brain to investigate the effect of repeated SDS on brain activity more than two weeks after the end of the stress exposure. Following social interaction, c-Fos expression was reduced in several brain regions in the defeated mice compared with control mice. The correlation of c-Fos expression among these brain areas, with exception of the medial prefrontal cortex (mPFC) and central amygdala (CeA), was increased in defeated mice, suggesting increased synchrony. Notably, c-Fos expression in the lateral habenula (LHb) was different between mice that exhibited social avoidance from immediately after the repeated SDS and those that exhibited social avoidance only at later periods. These observations provide insight into the long-term effects of social stress on behavior and brain activity.

Roles of Oxytocin in Stress Responses, Allostasis and Resilience

Oxytocin has been revealed to work for anxiety suppression and anti-stress as well as for psychosocial behavior and reproductive functions. Oxytocin neurons are activated by various stressful stimuli. The oxytocin receptor is widely distributed within the brain, and oxytocin that is released or diffused affects behavioral and neuroendocrine stress responses. On the other hand, there has been an increasing number of reports on the role of oxytocin in allostasis and resilience. It has been shown that oxytocin maintains homeostasis, shifts the set point for adaptation to a changing environment (allostasis) and contributes to recovery from the shifted set point by inducing active coping responses to stressful stimuli (resilience). Recent studies have suggested that oxytocin is also involved in stress-related disorders, and it has been shown in clinical trials that oxytocin provides therapeutic benefits for patients diagnosed with stress-related disorders. This review includes the latest information on the role of oxytocin in stress responses and adaptation.

Oxytocin as an Anti-obesity Treatment

Obesity is a growing health concern, as it increases risk for heart disease, hypertension, type 2 diabetes, cancer, COVID-19 related hospitalizations and mortality. However, current weight loss therapies are often associated with psychiatric or cardiovascular side effects or poor tolerability that limit their long-term use. The hypothalamic neuropeptide, oxytocin (OT), mediates a wide range of physiologic actions, which include reproductive behavior, formation of prosocial behaviors and control of body weight. We and others have shown that OT circumvents leptin resistance and elicits weight loss in diet-induced obese rodents and non-human primates by reducing both food intake and increasing energy expenditure (EE). Chronic intranasal OT also elicits promising effects on weight loss in obese humans. This review evaluates the potential use of OT as a therapeutic strategy to treat obesity in rodents, non-human primates, and humans, and identifies potential mechanisms that mediate this effect.

The oxytocin system and early-life experience-dependent plastic changes

Early-life experience influences social and emotional behaviour in adulthood. Affiliative tactile stimuli in early life facilitate the development of social and emotional behaviour, whereas early-life adverse stimuli have been shown to increase the risk of various diseases in later life. On the other hand, oxytocin has been shown to have organizational actions during early-life stages. However, the detailed mechanisms of the effects of early-life experience and oxytocin remain unclear. Here, we review the effects of affiliative tactile stimuli during the neonatal period and neonatal oxytocin treatment on the activity of the oxytocin-oxytocin receptor system and social or emotional behaviour in adulthood. Both affiliative tactile stimuli and early-life adverse stimuli in the neonatal period acutely activate the oxytocin-oxytocin receptor system in the brain but modulate social behaviour and anxiety-related behaviour apparently in an opposite direction in adulthood. Accumulating evidence suggests that affiliative tactile stimuli and exogenous application of oxytocin in early-life stages induce higher activity of the oxytocin-oxytocin receptor system in adulthood, although the effects are dependent on experimental procedures, sex, dosages and brain regions examined. On the other hand, early-life stressful stimuli appear to induce reduced activity of the oxytocin-oxytocin receptor system, possibly leading to adverse actions in adulthood. It is possible that activation of a specific oxytocin system can induce beneficial actions against early-life maltreatments and thus could be used for the treatment of developmental psychiatric disorders.

Chronic intranasal oxytocin reverses stress-induced social avoidance in female prairie voles

Social anxiety disorder (SAD) is a prevalent mental illness in both men and women, but current treatment approaches with selective serotonin reuptake inhibitors (SSRI) have limited success. The neuropeptide oxytocin (OXT) has become a therapeutic target due to its prosocial and anxiolytic effects. Nevertheless, no research has focused on the impact of chronic OXT treatment in animal models of SAD. Social defeat stress is an animal model of social conflict that reliably induces a social avoidance phenotype, reflecting symptoms observed in individuals suffering from SAD. Here, we used the socially monogamous prairie vole, which exhibits aggressive behavior in both sexes, to examine the effects of OXT and SSRI treatment following social defeat stress in males and females. Defeated voles became avoidant in unfamiliar social situations as early as one day after defeat experience, and this phenotype persisted for at least eight weeks. OXT receptor (OXTR) binding in mesocorticolimbic and paralimbic regions was reduced in defeated females during the eight-week recovery period. In males, serotonin 1A receptor binding was decreased in the basolateral amygdala and dorsal raphe nucleus starting at one week and four weeks post-defeat, respectively. Chronic intranasal treatment with OXT had a negative effect on sociability and mesolimbic OXTR binding in non-defeated females. However, chronic intranasal OXT promoted social engagement and increased mesolimbic OXTR binding in defeated females but not males. SSRI treatment led to only modest effects. This study identifies a sex-specific and stress-dependent function of intranasal OXT on mesolimbic OXTR and social behaviors.

The Comprehensive Neural Mechanism of Oxytocin in Analgesia

Oxytocin (OXT) is a nine amino acid neuropeptide hormone that has become one of the most intensively studied molecules in the past few decades. The vast majority of OXT is synthesized in the periventricular nucleus and supraoptic nucleus of the hypothalamus, and a few are synthesized in some peripheral organs (such as the uterus, ovaries, adrenal glands, thymus, pancreas, etc.) OXT modulates a series of physiological processes, including lactation, parturition, as well as some social behaviors. In addition, more and more attention has recently been focused on the analgesic effects of oxytocin. It has been reported that OXT can relieve tension and pain without other adverse effects. However, the critical role and detailed mechanism of OXT in analgesia remain unclear. This review aims to summarize the mechanism of OXT in analgesia and some ideas about the mechanism.

Roles of fibroblast growth factor 21 in the control of depression-like behaviours after social defeat stress in male rodents

Fibroblast growth factor 21 (FGF21) modulates energy metabolism and neuroendocrine stress responses. FGF21 synthesis is increased after environmental or metabolic challenges. Detailed roles of FGF21 in the control of behavioural disturbances under stressful conditions remain to be clarified. Here, we examined the roles of FGF21 in the control of behavioural changes after social defeat stress in male rodents. Central administration of FGF21 increased the number of tyrosine hydroxylase-positive catecholaminergic cells expressing c-Fos protein, an activity marker of neurones, in the nucleus tractus solitarius and area postrema. Double in situ hybridisation showed that some catecholaminergic neurones in the dorsal medulla oblongata expressed β-Klotho, an essential co-receptor for FGF21, in male mice. Social defeat stress increased FGF21 concentrations in the plasma of male mice. FGF21-deficient male mice showed social avoidance in a social avoidance test with C57BL/6J mice (background strain of FGF21-deficient mice) and augmented immobility behaviour in a forced swimming test after social defeat stress. On the other hand, overexpression of FGF21 by adeno-associated virus vectors did not significantly change behaviours either in wild-type male mice or FGF21-deficient male mice. The present data are consistent with the view that endogenous FGF21, possibly during the developmental period, has an inhibitory action on stress-induced depression-like behaviour in male rodents.

Indispensable role of the oxytocin receptor for allogrooming toward socially distressed cage mates in female mice

Social contact reduces stress responses in social animals. Mice have been shown to show allogrooming behaviour toward distressed conspecifics. However, the precise neuronal mechanisms underlying allogrooming behaviour remain unclear. In the present study, we examined whether mice show allogrooming behaviour towards distressed conspecifics in a social defeat model and we also determined whether oxytocin receptor-expressing neurons were activated during allogrooming by examining the expression of c-Fos protein, a marker of neurone activation. Mice showed allogrooming behaviour toward socially defeated conspecifics. After allogrooming behaviour, the percentages of oxytocin receptor-expressing neurones expressing c-Fos protein were significantly increased in the anterior olfactory nucleus, cingulate cortex, insular cortex, lateral septum and medial amygdala of female mice, suggesting that oxytocin receptor-expressing neurones in these areas were activated during allogrooming behaviour toward distressed conspecifics. The duration of allogrooming was correlated with the percentages of oxytocin receptor-expressing neurones expressing c-Fos protein in the anterior olfactory nucleus, insular cortex, lateral septum and medial amygdala. In oxytocin receptor-deficient mice, allogrooming behaviour toward socially defeated cage mates was markedly reduced in female mice but not in male mice, indicating the importance of the oxytocin receptor for allogrooming behaviour in female mice toward distressed conspecifics. The results suggest that the oxytocin receptor, possibly in the anterior olfactory nucleus, insular cortex, lateral septum and/or medial amygdala, facilitates allogrooming behaviour toward socially distressed familiar conspecifics in female mice.

Oxytocin and vasopressin: Signalling, behavioural modulation and potential therapeutic effects

Oxytocin (OT) and vasopressin (AVP) are endogenous ligands for OT and AVP receptors in the brain and in the peripheral system. Several studies demonstrate that OT and AVP have opposite roles in modulating stress, anxiety and social behaviours. Interestingly, both peptides and their receptors exhibit high sequence homology which could account for the biased signalling interaction of the peptides with OT and AVP receptors. However, how and under which conditions this crosstalk occurs in vivo remains unclear. In this review we shed light on the complexity of the roles of OT and AVP, by focusing on their signalling and behavioural differences and exploring the crosstalk between the receptor systems. Moreover, we discuss the potential of OT and AVP receptors as therapeutic targets to treat human disorders, such as autism, schizophrenia and drug abuse.

Convergent neuroendocrine mechanisms of social buffering and stress contagion

Social interactions play a key role in modulating the impact of stressful experiences. In some cases, social interactions can result in social buffering, the process in which the presence of one individual reduces the physiological and behavioral impact of stress in another individual. On the other hand, there is growing evidence that a key initiating factor of social buffering behaviors is the initiation of an anxiogenic state in the individual that was not directly exposed to the stress. This is referred to as stress contagion (a form of emotion contagion). Both processes involve the transmission of social information, suggesting that contagion and buffering could share similar neural mechanisms. In general, mechanistic studies of contagion and buffering are considered separately, even though behavioral studies show that a degree of contagion is usually necessary for social buffering behaviors to occur. Here we consider the extent to which the neuropeptides corticotropin releasing hormone and oxytocin are involved in contagion and stress buffering. We also assess the importance that frontal cortical areas such as the anterior cingulate cortex and infralimbic cortex play in these behavioral processes. We suggest that further work that directly compares neural mechanisms during stress contagion and stress buffering will be important for identifying what appear to be distinct but overlapping circuits mediating these processes.

Oxytocin Signaling as a Target to Block Social Defeat-Induced Increases in Drug Abuse Reward

There is huge scientific interest in the neuropeptide oxytocin (OXT) due to its putative capacity to modulate a wide spectrum of physiological and cognitive processes including motivation, learning, emotion, and the stress response. The present review seeks to increase the understanding of the role of OXT in an individual’s vulnerability or resilience with regard to developing a substance use disorder. It places specific attention on the role of social stress as a risk factor of addiction, and explores the hypothesis that OXT constitutes a homeostatic response to stress that buffers against its negative impact. For this purpose, the review summarizes preclinical and clinical literature regarding the effects of OXT in different stages of the addiction cycle. The current literature affirms that a well-functioning oxytocinergic system has protective effects such as the modulation of the initial response to drugs of abuse, the attenuation of the development of dependence, the blunting of drug reinstatement and a general anti-stress effect. However, this system is dysregulated if there is continuous drug use or chronic exposure to stress. In this context, OXT is emerging as a promising pharmacotherapy to restore its natural beneficial effects in the organism and to help rebalance the functions of the addicted brain.

Detection of neuropeptides in vivo and open questions for current and upcoming fluorescent sensors for neuropeptides

During a stress response, various neuropeptides are secreted in a spatiotemporally coordinated way in the brain. For a precise understanding of peptide functions in a stress response, it is important to investigate when and where they are released, how they diffuse, and how they are broken down in the brain. In the past two decades, genetically encoded fluorescent calcium indicators have greatly advanced our knowledge of the functions of specific neuronal activity in regulation of behavioral changes and physiological responses during stress. In addition, various kinds of structural information on G-protein-coupled receptors (GPCRs) for neuropeptides have been revealed. Recently, genetically encoded fluorescent sensors have been developed for detection of neurotransmitters by making use of conformational changes induced by ligand binding. In this review, we summarize the recent and upcoming advances of techniques for detection of neuropeptides and then present several open questions that will be solved by application of recent or upcoming technical advances in detection of neuropeptides in vivo.

Oxytocin reverses ethanol consumption and neuroinflammation induced by social defeat in male mice

Oxytocin (OXT) modulates social interactions, attenuates stressful responses and can decrease drug-seeking and taking behaviors. In previous studies, we observed that social defeat (SD) induced a long-lasting increase in ethanol intake and neuroinflammation in male mice. We also know that OXT blocks the increase in cocaine reward induced by SD. Therefore, in the present study we aimed to evaluate the effect of 1 mg/kg of OXT administered 30 min before each episode of SD on ethanol consumption and the neuroinflammatory response in adult male mice. Three weeks after the last SD, mice underwent oral ethanol self-administration (SA) procedure, and striatal levels of the two chemokines CX3CL1 and CXCL12 were measured after the last SD and at the end of the ethanol SA. OXT administration blocked the increase in voluntary ethanol consumption observed in defeated mice, although it did not affect motivation for ethanol. An increase in the striatal levels of CX3CL1 and CXCL12 was observed in defeated animals immediately after the last defeat and after the ethanol SA. However, defeated mice treated with OXT did not show this increase in the neuroinflammatory response. In conclusion, OXT treatment can be a powerful therapeutic target to reduce the negative effects of social stress on ethanol consumption and the neuroinflammatory process.

Mapping Central Projection of Oxytocin Neurons in Unmated Mice Using Cre and Alkaline Phosphatase Reporter

Oxytocin, a neuropeptide and peptide hormone, is produced by neurons in the hypothalamus and released by the posterior pituitary to control breastfeeding and labor. Recent studies have revealed that oxytocin in the central nervous system is also involved in modulating social interaction. To understand the potential role and innervation pattern of oxytocin neurons before sexual interaction, here we used transgenic mice which have the Cre recombinase under the control of an endogenous oxytocin promoter and Cre-dependent human placental alkaline phosphatase (AP) reporter to label the oxytocin neurons in the naive mouse brain. Since AP is located on the membrane of oxytocin neurons, AP histochemistry staining enabled us to observe the fine axonal terminals and the innervation pattern of oxytocin neurons in the thick serial coronal brain slices. Here we show that the number of AP-labeled cells varies with staining reaction time and ranges from 30% of the oxytocin immune-positive cell count to slightly higher than the oxytocin immune-positive cell count. Using AP staining with extended reaction time, which may not label all oxytocin neurons, we confirmed many innervation targets of oxytocin neurons from the anterior olfactory nucleus, some cortex regions, the limbic system, the hypothalamus, and the hindbrain, while the cell bodies were exclusively located in the hypothalamus and the bed nucleus of the stria terminalis. Finally, we observe some individual variance at the olfactory area, isocortex, striatum, paraventricular nucleus of thalamus, locus coeruleus, and Barrington's nucleus.

Extrahypothalamic oxytocin neurons drive stress-induced social vigilance and avoidance

Oxytocin increases the salience of both positive and negative social contexts and it is thought that these diverse actions on behavior are mediated in part through circuit-specific action. This hypothesis is based primarily on manipulations of oxytocin receptor function, leaving open the question of whether different populations of oxytocin neurons mediate different effects on behavior. Here we inhibited oxytocin synthesis in a stress-sensitive population of oxytocin neurons specifically within the medioventral bed nucleus of the stria terminalis (BNSTmv). Oxytocin knockdown prevented social stress-induced increases in social vigilance and decreases in social approach. Viral tracing of BNSTmv oxytocin neurons revealed fibers in regions controlling defensive behaviors, including lateral hypothalamus, anterior hypothalamus, and anteromedial BNST (BNSTam). Oxytocin infusion into BNSTam in stress naïve mice increased social vigilance and reduced social approach. These results show that a population of extrahypothalamic oxytocin neurons plays a key role in controlling stress-induced social anxiety behaviors.

Social approach and social vigilance are differentially regulated by oxytocin receptors in the nucleus accumbens

Oxytocin is currently being considered as a novel therapeutic for anxiety disorders due to its ability to promote affiliative behaviors. In the nucleus accumbens (NAc) activation of oxytocin receptors (OTR) promotes social approach (time spent near an unfamiliar individual). Here, we show that stressful social experiences reduce the expression of NAc OTR mRNA, coinciding with decreases in social approach. Social stressors also increase social vigilance, characterized as orienting to an unfamiliar individual without approaching. Vigilance is a key component of behavioral inhibition, a personality trait that is a risk factor for anxiety disorders. To understand whether NAc OTR can modulate both social approach and vigilance, we use pharmacological approaches to assess the impact of activation or inhibition of NAc OTR downstream pathways on these behaviors. First, we show that in unstressed male and female California mice, inhibition of OTR by an unbiased antagonist (L-368,899) reduces social approach but does not induce social vigilance. Next, we show that infusion of Atosiban, an OTR-Gq antagonist/OTR-Gi agonist, has the same effect in unstressed females. Finally, we show that Carbetocin, a biased OTR-Gq agonist, increases social approach in stressed females while simultaneously inhibiting social vigilance. Taken together these data suggest that OTR in the NAc differentially modulate social approach and social vigilance, primarily through an OTR-Gq mechanism. Importantly, pharmacological inhibition of OTR alone is insufficient to induce vigilance in unstressed mice, suggesting that mechanisms modulating social approach may be distinct from mechanisms modulating social vigilance.

Anxious to see you: Neuroendocrine mechanisms of social vigilance and anxiety during adolescence

Social vigilance is a behavioral strategy commonly used in adverse or changing social environments. In animals, a combination of avoidance and vigilance allows an individual to evade potentially dangerous confrontations while monitoring the social environment to identify favorable changes. However, prolonged use of this behavioral strategy in humans is associated with increased risk of anxiety disorders, a major burden for human health. Elucidating the mechanisms of social vigilance in animals could provide important clues for new treatment strategies for social anxiety. Importantly, during adolescence the prevalence of social anxiety increases significantly. We hypothesize that many of the actions typically characterized as anxiety behaviors begin to emerge during this time as strategies for navigating more complex social structures. Here, we consider how the social environment and the pubertal transition shape neural circuits that modulate social vigilance, focusing on the bed nucleus of the stria terminalis and prefrontal cortex. The emergence of gonadal hormone secretion during adolescence has important effects on the function and structure of these circuits, and may play a role in the emergence of a notable sex difference in anxiety rates across adolescence. However, the significance of these changes in the context of anxiety is still uncertain, as not enough studies are sufficiently powered to evaluate sex as a biological variable. We conclude that greater integration between human and animal models will aid the development of more effective strategies for treating social anxiety.

The role of oxytocin in regulation of appetitive behaviour, body weight and glucose homeostasis

Obesity and its associated complications have reached epidemic proportions in the USA and also worldwide, highlighting the need for new and more effective treatments. Although the neuropeptide oxytocin (OXT) is well recognised for its peripheral effects on reproductive behaviour, the release of OXT from somatodendrites and axonal terminals within the central nervous system (CNS) is also implicated in the control of energy balance. In this review, we summarise historical data highlighting the effects of exogenous OXT as a short-term regulator of food intake in a context-specific manner and the receptor populations that may mediate these effects. We also describe what is known about the physiological role of endogenous OXT in the control of energy balance and whether serum and brain levels of OXT relate to obesity on a consistent basis across animal models and humans with obesity. We describe recent data on the effectiveness of chronic CNS administration of OXT to decrease food intake and weight gain or to elicit weight loss in diet-induced obese (DIO) and genetically obese mice and rats. Of clinical importance is the finding that chronic central and peripheral OXT treatments both evoke weight loss in obese animal models with impaired leptin signalling at doses that are not associated with visceral illness, tachyphylaxis or adverse cardiovascular effects. Moreover, these results have been largely recapitulated following chronic s.c. or intranasal treatment in DIO non-human primates (rhesus monkeys) and obese humans, respectively. We also identify plausible mechanisms that contribute to the effects of OXT on body weight and glucose homeostasis in rodents, non-human primates and humans. We conclude by describing the ongoing challenges that remain before OXT-based therapeutics can be used as a long-term strategy to treat obesity in humans.

Increased fibroblast growth factor-21 in chronic kidney disease is a trade-off between survival benefit and blood pressure dysregulation

Circulating levels of fibroblast growth factor-21 (FGF21) start increasing in patients with chronic kidney disease (CKD) since early stages during the cause of disease progression. FGF21 is a liver-derived hormone that induces responses to stress through acting on hypothalamus to activate the sympathetic nervous system and the hypothalamus-pituitary-adrenal endocrine axis. However, roles that FGF21 plays in pathophysiology of CKD remains elusive. Here we show in mice that FGF21 is required to survive CKD but responsible for blood pressure dysregulation. When introduced with CKD, Fgf21^−/− mice died earlier than wild-type mice. Paradoxically, these Fgf21^−/− CKD mice escaped several complications observed in wild-type mice, including augmentation of blood pressure elevating response and activation of the sympathetic nervous system during physical activity and increase in serum noradrenalin and corticosterone levels. Supplementation of FGF21 by administration of an FGF21-expressing adeno-associated virus vector recapitulated these complications in wild-type mice and restored the survival period in Fgf21^−/− CKD mice. In CKD patients, high serum FGF21 levels are independently associated with decreased baroreceptor sensitivity. Thus, increased FGF21 in CKD can be viewed as a survival response at the sacrifice of blood pressure homeostasis.

Oxytocin in the periaqueductal gray mainly comes form the hypothalamic supraoptic nucleus to participate in pain modulation

Oxytocin (OXT) that effects the nociception process is mainly synthesized and secreted in the hypothalamic supraoptic nucleus (SON). Although the periaqueductal gray (PAG) hardly synthesizes OXT, OXT in PAG also plays a role in pain regulation. The communication investigates whether OXT in the PAG comes from SON to influence pain modulation. RT-PCR was used to analyze OXT mRNA expression and radioimmunoassay to measure OXT concentration. The results showed that (1) pain stimulation enhanced OXT mRNA expression in the SON at 10 min (268.1 ± 39.2%, p < 0.001) and 20 min (135.4±37.9%, p < 0.05) treatment and did not change in the PAG; (2) OXT level increase in SON perfusion liquid during pain stimulation [236.7±22.1% at 10 min (p < 0.001), 223.1±12.4% at 20 min (p<0.001), 56.1 ± 15.7% at 30 min (p < 0.01) and 11.2±14.2% at 40 min] was earlier than that in PAG perfusion liquid [17.8±9.7% at 10 min, 375.6±35.1% at 20 min (p <  0.001), 123.2±17.7% at 30 min (p <  0.001) and 52.7±22.4% at 40 min (p < 0.05)]; (3) SON excitation (L-glutamate sodium microinjection) induced OXT level increase in PAG perfusion liquid in a dose-dependent manner; (4) the bilateral SON cauterization completely controlled and the right SON cauterization partly reversed the pain stimulation induced-OXT concentration increase in PAG perfusion liquid. The data suggested that OXT in PAG came from SON, which might influence the pain process.

Emergent decision-making behaviour and rhythm generation in a computational model of the ventromedial nucleus of the hypothalamus

The ventromedial nucleus of the hypothalamus (VMN) has an important role in diverse behaviours. The common involvement in these of sex steroids, nutritionally-related signals, and emotional inputs from other brain areas, suggests that, at any given time, its output is in one of a discrete number of possible states corresponding to discrete motivational drives. Here we explored how networks of VMN neurons might generate such a decision-making architecture. We began with minimalist assumptions about the intrinsic properties of VMN neurons inferred from electrophysiological recordings of these neurons in rats in vivo, using an integrate-and-fire based model modified to simulate activity-dependent post-spike changes in neuronal excitability. We used a genetic algorithm based method to fit model parameters to the statistical features of spike patterning in each cell. The spike patterns in both recorded cells and model cells were assessed by analysis of interspike interval distributions and of the index of dispersion of firing rate over different binwidths. Simpler patterned cells could be closely matched by single neuron models incorporating a hyperpolarising afterpotential and either a slow afterhyperpolarisation or a depolarising afterpotential, but many others could not. We then constructed network models with the challenge of explaining the more complex patterns. We assumed that neurons of a given type (with heterogeneity introduced by independently random patterns of external input) were mutually interconnected at random by excitatory synaptic connections (with a variable delay and a random chance of failure). Simple network models of one or two cell types were able to explain the more complex patterns. We then explored the information processing features of such networks that might be relevant for a decision-making network. We concluded that rhythm generation (in the slow theta range) and bistability arise as emergent properties of networks of heterogeneous VMN neurons.

Complementary Neural Circuits for Divergent Effects of Oxytocin: Social Approach Versus Social Anxiety

Oxytocin (OT) is widely known for promoting social interactions, but there is growing appreciation that it can sometimes induce avoidance of social contexts. The social salience hypothesis posed an innovative solution to these apparently opposing actions by proposing that OT enhances the salience of both positive and negative social interactions. The mesolimbic dopamine system was put forth as a likely system to evaluate social salience owing to its well-described role in motivation. Evidence from several sources supports the premise that OT acting within the nucleus accumbens and ventral tegmental area facilitates social reward and approach behavior. However, in aversive social contexts, additional pathways play critical roles in mediating the effects of OT. Recent data indicate that OT acts in the bed nucleus of the stria terminalis to induce avoidance of potentially dangerous social contexts. Here, we review evidence for neural circuits mediating the effects of OT in appetitive and aversive social contexts. Specifically, we propose that distinct but potentially overlapping circuits mediate OT-dependent social approach or social avoidance. We conclude that a broader and more inclusive consideration of neural circuits of social approach and avoidance is needed as the field continues to evaluate the potential of OT-based therapeutics.

Comparing vasopressin and oxytocin fiber and receptor density patterns in the social behavior neural network: Implications for cross-system signaling

Vasopressin (AVP) and oxytocin (OXT) regulate social behavior by binding to their canonical receptors, the vasopressin V1a receptor (V1aR) and oxytocin receptor (OTR), respectively. Recent studies suggest that these neuropeptides may also signal via each other's receptors. The extent to which such cross-system signaling occurs likely depends on anatomical overlap between AVP/OXT fibers and V1aR/OTR expression. By comparing AVP/OXT fiber densities with V1aR/OTR binding densities throughout the rat social behavior neural network (SBNN), we propose the potential for cross-system signaling in four regions: the medial amygdala (MeA), bed nucleus of the stria terminalis (BNSTp), medial preoptic area, and periaqueductal grey. We also discuss possible implications of corresponding sex (higher in males versus females) and age (higher in adults versus juveniles) differences in AVP fiber and OTR binding densities in the MeA and BNSTp. Overall, this review reveals the need to unravel the consequences of potential cross-system signaling between AVP and OXT systems in the SBNN for the regulation of social behavior.

Role of oxytocin in the control of stress and food intake

Oxytocin neurones in the hypothalamus are activated by stressful stimuli and food intake. The oxytocin receptor is located in various brain regions, including the sensory information-processing cerebral cortex; the cognitive information-processing prefrontal cortex; reward-related regions such as the ventral tegmental areas, nucleus accumbens and raphe nucleus; stress-related areas such as the amygdala, hippocampus, ventrolateral part of the ventromedial hypothalamus and ventrolateral periaqueductal gray; homeostasis-controlling hypothalamus; and the dorsal motor complex controlling intestinal functions. Oxytocin affects behavioural and neuroendocrine stress responses and terminates food intake by acting on the metabolic or nutritional homeostasis system, modulating emotional processing, reducing reward values of food intake, and facilitating sensory and cognitive processing via multiple brain regions. Oxytocin also plays a role in interactive actions between stress and food intake and contributes to adaptive active coping behaviours.

Oxytocin prevents the increase of cocaine-related responses produced by social defeat

The neuropeptide oxytocin (OXT) plays a critical role in the regulation of social and emotional behaviors. OXT plays a role in stress response and in drug reward, but to date no studies have evaluated its implication in the long-lasting increase of the motivational effects of cocaine induced by repeated social defeat (RSD). During the social defeat procedure, 1 mg/kg of OXT was administered 30 min before each episode of RSD. Three weeks after the last defeat, the effects of cocaine on the conditioned place preference (CPP), locomotor sensitization and the self-administration (SA) paradigms were evaluated. The influence of OXT on the levels of BDNF in the prefrontal cortex (PFC), striatum and hippocampus was also measured. Our results confirm that raising the levels of OXT during social defeat stress can block the long-lasting effects of this type of stress. OXT counteracts the anxiety induced by social defeat and modifies BDNF levels in all the structures we have studied. Moreover, OXT prevents RSD-induced increases in the motivational effects of cocaine. Administration of OXT before each social defeat blocked the social defeat-induced increment in the conditioned rewarding effects of cocaine in the CPP, favored the extinction of cocaine-associated memories in both the CPP and SA, and decreased reinstatement of cocaine-seeking behavior in the SA. In conclusion, the long-lasting effects of RSD are counteracted by administering OXT prior to stress, and changes in BDNF expression may underlie these protective effects.

The impact of sex as a biological variable in the search for novel antidepressants

A roadblock to successful treatment for anxiety and depression is the high proportion of individuals that do not respond to existing treatments. Different underlying neurobiological mechanisms may drive similar symptoms, so a more personalized approach to treatment could be more successful. There is increasing evidence that sex is an important biological variable modulating efficacy of antidepressants and anxiolytics. We review evidence for sex-specific effects of traditional monoamine based antidepressants and newer pharmaceuticals targeting kappa opioid receptors (KOR), oxytocin receptors (OTR), and N-methyl-D-aspartate receptors (ketamine). In some cases, similar behavioral effects are observed in both sexes while in other cases strong sex-specific effects are observed. Most intriguing are cases such as ketamine which has similar behavioral effects in males and females, perhaps through sex-specific neurobiological mechanisms. These results show how essential it is to include both males and females in both clinical and preclinical evaluations of novel antidepressants and anxiolytics.

Behavioral Modulation by Social Experiences in Rodent Models

The lasting behavioral changes elicited by social signals provide important adaptations for survival of organisms that thrive as a group. Unlike the rapid innate responses to social cues, such adaptations have been understudied. Here, the rodent models of the lasting socially induced behavioral changes are presented as either modulations or reinforcements of the distinct forms of learning and memory or non-associative changes of affective state. The purpose of this categorization is to draw attention to the potential mechanistic links between the neuronal pathways that process social cues and the neuronal systems that mediate the well-studied forms of learning and memory. © 2018 by John Wiley & Sons, Inc.