Oxytocin and vasopressin modulate the social response to threat: a preclinical study

Arginine vasopressin in mood disorders: A potential biomarker of disease pathology and a target for pharmacologic intervention

Vasopressin or arginine-vasopressin (AVP) is a neuropeptide molecule known for its antidiuretic effects and serves to regulate plasma osmolality and blood pressure. The existing literature suggests that AVP plays a multifaceted-though less well-known-role in the central nervous system (CNS), particularly in relation to the pathophysiology and treatment of mood disorders. Animal models have demonstrated that AVP is implicated in regulating social cognition, affiliative and prosocial behaviors, and aggression, often in conjunction with oxytocin. In humans, AVP is implicated in mood disorders through its effects on the hypothalamic-pituitary-adrenal (HPA) axis as well as on the serotoninergic and glutamatergic systems. Measuring plasma AVP has yielded interesting but mixed results in mood and stress-related disorders. Recent advances have led to the development of copeptin as a stable and reliable surrogate biomarker for AVP. Another interesting but relatively unexplored issue is the interaction between the osmoregulatory system and mood disorder pathophysiology, given that psychotropic medications often cause dysregulation of AVP receptor expression or signaling that can subsequently lead to clinical syndromes like syndrome of inappropriate diuresis and diabetes insipidus. Finally, pharmaceutical trials of agents that act on V1a and V1b receptor antagonists are still underway. This narrative review summarizes: (1) the neurobiology of the vasopressinergic system in the CNS; (2) the interaction between AVP and the monoaminergic and glutamatergic pathways in the pathophysiology and treatment of mood disorders; (3) the iatrogenic AVP dysregulation caused by psychotropic medications; and (4) the pharmaceutical development of AVP receptor antagonists for the treatment of mood disorders.

Chronic sensory contact with subordinated conspecifics promotes splenic glucocorticoid resistance in experimentally wounded C57BL/6N male mice

Chronic psychosocial stress induced by the chronic subordinate colony housing (CSC, 19 Days) paradigm promotes functional splenic in vitro glucocorticoid (GC) resistance, but only if associated with significant bite wounding or prior abdominal transmitter implantation. Moreover, sensory contact to social defeat of conspecifics represents a social stressor for the observer individual. As the occurence and severity of bite wounding is not adequately controllable, the present study aimed to develop an animal model, allowing a bite wound-independent, more reliable generation of chronically-stressed mice characterized by functional splenic in vitro GC resistance. Therefore, male C57BL/6N mice received a standardized sterile intraperitoneal (i.p.) incision surgery or SHAM treatment one week prior to 19-days of (i) CSC, (ii) witnessing social defeat during CSC exposure in sensory contact (SENS) or (iii) single-housing for control (SHC), before assessing basal and LPS-induced splenic in vitro cell viability and GC resistance. Our results indicate that individually-housed SENS but not CSC mice develop mild signs of splenic in vitro GC resistance, when undergoing prior i.p.-wounding. Taken together and considering that future studies are warranted, our findings support the hypothesis that the combination of repeated standardized i.p.-wounding with chronic sensory stress exposure represents an adequate tool to induce functional splenic in vitro GC resistance independent of the occurrence of uncontrollable bite wounds required in social stress paradigms to induce a comparable phenotype.

The Relationship Between Oxytocin and Alcohol Dependence

The hypothalamic neuropeptide oxytocin (OT) is well known for its prosocial, anxiolytic, and ameliorating effects on various psychiatric conditions, including alcohol use disorder (AUD). In this chapter, we will first introduce the basic neurophysiology of the OT system and its interaction with other neuromodulatory and neurotransmitter systems in the brain. Next, we provide an overview over the current state of research examining the effects of acute and chronic alcohol exposure on the OT system as well as the effects of OT system manipulation on alcohol-related behaviors in rodents and humans. In rodent models of AUD, OT has been repeatedly shown to reduce ethanol consumption, particularly in models of acute alcohol exposure. In humans however, the results of OT administration on alcohol-related behaviors are promising but not yet conclusive. Therefore, we further discuss several physiological and methodological limitations to the effective application of OT in the clinic and how they may be mitigated by the application of synthetic OT receptor (OTR) agonists. Finally, we discuss the potential efficacy of cutting-edge pharmacology and gene therapies designed to specifically enhance endogenous OT release and thereby rescue deficient expression of OT in the brains of patients with severe forms of AUD and other incurable mental disorders.

Social Synchronization of Conditioned Fear in Mice Requires Ventral Hippocampus Input to the Amygdala

BACKGROUND

Social organisms synchronize behaviors as an evolutionary-conserved means of thriving. Synchronization under threat, in particular, benefits survival and occurs across species, including humans, but the underlying mechanisms remain unknown because of the scarcity of relevant animal models. Here, we developed a rodent paradigm in which mice synchronized a classically conditioned fear response and identified an underlying neuronal circuit.

METHODS

Male and female mice were trained individually using auditory fear conditioning and then tested 24 hours later as dyads while allowing unrestricted social interaction during exposure to the conditioned stimulus under visible or infrared illumination to eliminate visual cues. The synchronization of the immobility or freezing bouts was quantified by calculating the effect size Cohen's d for the difference between the actual freezing time overlap and the overlap by chance. The inactivation of the dorsomedial prefrontal cortex, dorsal hippocampus, or ventral hippocampus was achieved by local infusions of muscimol. The chemogenetic disconnection of the hippocampus-amygdala pathway was performed by expressing hM4D(Gi) in the ventral hippocampal neurons and infusing clozapine N-oxide in the amygdala.

RESULTS

Mice synchronized cued but not contextual fear. It was higher in males than in females and attenuated in the absence of visible light. Inactivation of the ventral but not dorsal hippocampus or dorsomedial prefrontal cortex abolished fear synchronization. Finally, the disconnection of the hippocampus-amygdala pathway diminished fear synchronization.

CONCLUSIONS

Mice synchronize expression of conditioned fear relying on the ventral hippocampus-amygdala pathway, suggesting that the hippocampus transmits social information to the amygdala to synchronize threat response.

Similar behavioral but different endocrine responses to conspecific interactions in hand-raised wolves and dogs

Domestication has altered dogs' conspecific social organization compared to their closest, non-domesticated relatives, gray wolves. Wolves live in packs whose survival depends on coordinated behavior, but dogs rely less on conspecifics, which predicts greater cohesiveness in wolf than dog packs. Endocrine correlates such as oxytocin and glucocorticoids modulate group cohesion resulting in species-specific differences in social interactions. We found that although wolves' and dogs' observable behavioral reactions to a territorial threat and separation from the pack were similar, hormonal responses differed. Wolves' but not dogs' oxytocin and glucocorticoid concentrations correlated positively with territorial behaviors and only wolves showed increased glucocorticoid concentrations after separation from their pack. Together, results suggest stronger emotional activation to threats to group integrity in wolves than dogs, in line with their socio-ecology.

Sucrose intake by rats affected by both intraperitoneal oxytocin administration and time of day

RATIONALE

Daily limited access to palatable food or drink at a fixed time is commonly used in rodent models of bingeing. Under these conditions, entrainment may modulate intake patterns. Oxytocin is involved in circadian patterns of intake and, when administered peripherally, reduces sucrose intake. However, oxytocin's effects on intake under limited-access conditions and its potential interaction with entrainment have not been explored.

OBJECTIVES

This study examined the role of entrainment on intake patterns, oxytocin's effects on sucrose intakes and locomotor activity and whether oxytocin's effects were mediated by its actions at the oxytocin receptor.

METHODS

Sated rats received daily 1-h access to 10% sucrose solution either at a fixed or varied time of day. Rats received intraperitoneal oxytocin (0 mg/kg, 0.3 mg/kg, 1 mg/kg, 3 mg/kg) prior to sucrose access, and spontaneous locomotor activity was assessed in an open-field test. Rats were then pre-treated with an oxytocin receptor antagonist, L368,899, prior to oxytocin before sucrose access.

RESULTS

Intake patterns did not differ between fixed- or varied-time presentations; rats consumed more sucrose solution in the middle as opposed to the early-dark phase. Oxytocin dose-dependently reduced sucrose intakes, but also reduced locomotor activity. There was some evidence of partial blockade of oxytocin-induced sucrose intake reductions by L368,899, but the results were unclear.

CONCLUSIONS

Time of day and oxytocin impact sucrose solution intake under daily limited access in rats and the sedative-like effects of oxytocin should be considered in future studies on oxytocin and food intake.

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.

Family and Community Support, Brain-Derived Neurotrophic Factor, and Cognitive Performance in Older Adults: Findings From the Health, Wellbeing and Aging Study Population-Based Cohort

Background: Social networks can modulate physiological responses, protects against the detrimental consequences of prolonged stress, and enhance health outcomes. Family ties represent an essential source of social networks among older adults. However, the impact of family support on cognitive performance and the biological factors influencing that relationship is still unclear. We aimed to determine the relationship between family support, cognitive performance and BDNF levels. Methods: Cross-sectional data from three-hundred, eight-six individuals aged on average 60 years enrolled in the Health, Wellbeing and Aging Study (SABE), a population-cohort study, were assessed for family support, community support and cognitive performance. Structural and functional family support was evaluated based on family size and interactions allied to scores in the Family APGAR questionnaire. Community assistance (received or provided) assessed the community support. Cognitive performance was determined using the Mini-Mental State Examination (MMSE), verbal fluency (animals per minute) and backward digital span. Blood samples were obtained to determine BDNF levels. Results: Multivariate analysis showed that functional family support, but not structural, was associated with higher MMSE, verbal fluency and digit span scores, even controlling for potential cofounders (p < 0.001). Providing support to the community, rather than receiving support from others, was associated with better cognitive performance (p < 0.001). BDNF concentration was not associated with community support, family function, or cognitive performance. Conclusion: These findings suggest that emotional components of functional family and community support (e.g., loving and empathic relationship) may be more significant to cognitive health than size and frequency of social interactions.

Effect of early life social adversity on drug abuse vulnerability: Focus on corticotropin-releasing factor and oxytocin

Early life adversity can set the trajectory for later psychiatric disorders, including substance use disorders. There are a host of neurobiological factors that may play a role in the negative trajectory. The current review examines preclinical evidence suggesting that early life adversity specifically involving social factors (maternal separation, adolescent social isolation and adolescent social defeat) may influence drug abuse vulnerability by strengthening corticotropin-releasing factor (CRF) systems and weakening oxytocin (OT) systems. In adulthood, pharmacological and genetic evidence indicates that both CRF and OT systems are directly involved in drug reward processes. With early life adversity, numerous studies show an increase in drug abuse vulnerability measured in adulthood, along a concomitant strengthening of CRF systems and a weakening of OT systems. Mechanistic studies, while relatively few in number, are generally consistent with the theme that strengthened CRF systems and weakened OT systems mediate, at least in part, the link between early life adversity and drug abuse vulnerability. Establishing a direct role of CRF and OT in mediating the relation between early life social stressors and drug abuse vulnerability will inform clinical researchers and practitioners toward the development of intervention strategies to reduce risk among those suffering from early life adversities. This article is part of the special issue on 'Vulnerabilities to Substance Abuse'.

Predator odor exposure increases social contact in adolescents and parental behavior in adulthood in Brandt's voles

Research suggests that predation risk during adolescence can program adult stress response and emotional behavior; however, little is known about the short-term and lasting residual effects of this experience on social behavior. We explored this concept in social Brandt's voles (Lasiopodomys brandtii). Adolescent male and female voles were exposed to distilled water, rabbit urine (as a non-predator stimulus), and cat urine for 60 min daily from postnatal day (PND) 28-49. Social play tests were conducted immediately following exposure on PND 28, 35, 42, and 49. In the social play test, repeated cat odor (CO) exposure enhanced the contact behavior of voles with their cagemate. Adolescent exposure to CO did not affect behavioral responses toward unrelated pups in the alloparental behavior test or same-sex individuals in the social interaction test. However, exposure to CO significantly enhanced the licking/grooming behavior of voles towards their own pups in the home cage parental behavior test. Repeated CO exposure significantly inhibited weight gain in male voles during adolescence. This effect was transmitted to the next generation, with lower weight gain in offspring before weaning. Following repeated CO exposure, males tended to have more female offspring whereas females produced more offspring, suggesting an adaptive strategy to increase inclusive fitness under predatory risk. These findings demonstrate that adolescent exposure to predatory risk augments adolescent social contact and adult parental behavior and suggest a role for improved inclusive fitness in mediating long-term outcomes.

Divergent pathways mediate 5-HT1A receptor agonist effects on close social interaction, grooming and aggressive behaviour in mice: Exploring the involvement of the oxytocin and vasopressin systems

BACKGROUND

5-HT_1A receptor (5-HT_1AR) abnormalities are implicated in aggression, and there has been considerable interest in developing 5-HT_1AR agonists for treating aggression. Endogenous oxytocin (OXT) released upon stimulation of 5-HT_1ARs in the hypothalamus mediates at least some of the effects of 5-HT_1AR agonists on social behaviour.

AIMS

Given 5-HT_1AR, OXT receptor (OXTR) and vasopressin V1a receptor (V1aR) agonists can all reduce aggression, the current study aimed to determine whether the anti-aggressive effects of 5-HT_1AR stimulation can also be explained by downstream actions at OXTRs and/or V1aRs in a mouse model of non-territorial, hyper-aggressive behaviour.

METHODS

Male Swiss mice (N=80) were socially isolated or group housed for six weeks prior to the start of testing. Testing involved placing two unfamiliar weight- and condition-matched mice together in a neutral context for 10 minutes.

RESULTS

Social isolation led to a pronounced increase in aggressive behaviour, which was dose-dependently inhibited by the 5-HT_1AR agonist 8-OH-DPAT (0.1, 0.3 and 1 mg/kg intraperitoneally (i.p.)), with accompanying increases in close social contact (huddling) and grooming. The effects of 8-OH-DPAT on aggression, huddling and grooming were blocked by pretreatment with a selective 5-HT_1AR antagonist (WAY-100635; 0.1 mg/kg i.p.). The anti-aggressive effects of 8-OH-DPAT were unaffected by an OXTR antagonist (L-368,899; 10 mg/kg i.p.), whereas the effects on huddling and grooming were inhibited. Pretreatment with a V1aR antagonist (SR49059; 20 mg/kg i.p.) had no effect.

CONCLUSIONS

Our study suggests that stimulation of endogenous oxytocin is involved in the effects of 5-HT_1AR activation on close social contact and grooming but not aggression.

Oxytocin delivered nasally or intraperitoneally reaches the brain and plasma of normal and oxytocin knockout mice

Intranasal delivery of oxytocin (Oxt) has been identified as a potential therapeutic to target human conditions characterized by social deficits, yet the ability of this administrative route to deliver to the brain is unconfirmed. Oxt knockout (Oxt KO) and wildtype C57BL/6 J male mice received Oxt (12 μg total amount) either by nasal or intraperitoneal administration. Oxt concentrations were monitored for 2 h after administration in circulation via a jugular vein catheter and in the brain by two intracerebral microdialysis probes. Group sizes varied from 4 to 7 mice (n = 22 total). We document for the first time that Oxt applied to the nasal mucosa after nasal administration is delivered to the extracellular fluid in the brain. After nasal application, Oxt concentrations in circulation and in the extracellular fluid of the amygdala and, to an extent, the dorsal hippocampus, rose within the first 30 min and remained elevated for the subsequent hour. These findings were confirmed in an Oxt KO mouse line, establishing that the circulating and brain Oxt elevations derive from the administered dose. Interestingly, the pharmacokinetics of Oxt were slightly biased to the brain after nasal administration and to the periphery following intraperitoneal injection. No change in vasopressin levels was detected. These findings have stimulating implications for the interpretation of various behavioral and physiological effects described in animal and human studies after nasal administration of Oxt and provide the pharmacokinetics necessary to develop this drug delivery route for therapeutic purposes.

Involvement of oxytocin and GABA in consolation behavior elicited by socially defeated individuals in mandarin voles

Consolation, which entails comforting contact directed toward a distressed party, is a common empathetic response in humans and other species with advanced cognition. Here, using the social defeat paradigm, we provide empirical evidence that highly social and monogamous mandarin voles (Microtus mandarinus) increased grooming toward a socially defeated partner but not toward a partner who underwent only separation. This selective behavioral response existed in both males and females. Accompanied with these behavioral changes, c-Fos expression was elevated in many of the brain regions relevant for emotional processing, such as the anterior cingulate cortex (ACC), bed nucleus of the stria terminalis, paraventricular nucleus (PVN), basal/basolateral and central nucleus of the amygdala, and lateral habenular nucleus in both sexes; in the medial preoptic area, the increase in c-Fos expression was found only in females, whereas in the medial nucleus of the amygdala, this increase was found only in males. In particular, the GAD67/c-Fos and oxytocin (OT)/c-Fos colocalization rates were elevated in the ACC and PVN, indicating selective activation of GABA and OT neurons in these regions. The "stressed" pairs matched their anxiety-like behaviors in the open-field test, and their plasma corticosterone levels correlated well with each other, suggesting an empathy-based mechanism. This partner-directed grooming was blocked by pretreatment with an OT receptor antagonist or a GABA_A receptor antagonist in the ACC but not by a V1a subtype vasopressin receptor antagonist. We conclude that consolation behavior can be elicited by the social defeat paradigm in mandarin voles, and this behavior may be involved in a coordinated network of emotion-related brain structures, which differs slightly between the sexes. We also found that the endogenous OT and the GABA systems within the ACC are essential for consolation behavior in mandarin voles.

Oxytocin and vasopressin inhibit hyper-aggressive behaviour in socially isolated mice

Despite the high prevalence of aggression across a wide range of disorders, there is a severe lack of pharmacological treatments. Recent rodent studies have shown both centrally and peripherally administered oxytocin is effective in reducing territorial aggression, an adaptive form of aggression not reflective of pathological hyper-aggression. The current study tested i.p. administered oxytocin and vasopressin in a model of non-territorial hyper-aggression and examined the involvement of oxytocin receptors (OXTR) and vasopressin V1a receptors (V1aR). Male Swiss mice (N = 160) were either socially isolated or group housed for 6 weeks prior to the commencement of testing; wherein two unfamiliar weight and condition matched mice were placed into a neutral context for 10 min. Socially isolated mice exhibited heightened aggression that was powerfully and dose-dependently inhibited by oxytocin and vasopressin and that was accompanied by dose-dependent increases in close social contact (huddling) and grooming. These anti-aggressive effects of oxytocin were blocked by pre-treatment with a higher dose of selective V1aR antagonist SR49059 (20 mg/kg i.p.), but not a lower dose of SR49059 (5 mg/kg i.p.) or selective OXTR antagonist L-368,899 (10 mg/kg i.p.). This is consistent with a growing number of studies linking a range of effects of exogenous oxytocin to actions at the V1a receptor. Interestingly, the highest dose of the OXTR agonist TGOT (10 mg/kg) also reduced isolation-induced aggression. These results suggest that while activation of the V1a receptor appears critical for the anti-aggressive effects of oxytocin, activation of the oxytocin receptor cannot be excluded. This article is part of the Special Issue entitled 'Current status of the neurobiology of aggression and impulsivity.'

Higher plasma levels of CT-proAVP are linked to less anxiety in men but not women with cardiovascular risk factors: Results from the observational Diast-CHF study

AIM

Using data from the multicenter, observational Diast-CHF (Diagnostic Trial on Prevalence and Clinical Course of Diastolic Dysfunction and Heart Failure) study, this post-hoc analysis aimed at assessing the association between serum concentrations of C-terminal pro-arginine vasopressin (CT-proAVP) and anxiety in patients with cardiovascular risk factors.

BACKGROUND

Animal studies have demonstrated that centrally released AVP is involved in the development of anxiety-like behaviors, however, it is unknown whether, also in humans, CT-proAVP used as a proxy for the co-secreted AVP is associated with self-reported anxiety.

METHODS

In 1463 study participants with cardiovascular risk factors (mean age 66.7 ± 8.1 years, 51.3% males, mean left ventricular ejection fraction 59.8 ± 8.3%), serum concentrations of CT-proAVP were measured by means of an ELISA assay, and anxiety was assessed using the Hospital Anxiety and Depression Scale (HADS).

RESULTS

Data showed that there was a significant and inverse correlation between HADS anxiety and CT-proAVP (rho = -0.074; p = 0.005). Serum CT-proAVP and the HADS anxiety differed between the two sexes: men displayed lower anxiety (4.7 ± 3.5 versus 5.5 ± 3.7) and had higher CT-proAVP levels (5.8 pmol/L, interquartile range 3.5-9.9 pmol/L versus 3.0 pmol/L, interquartile range 2.0-4.7) than women (both, p < 0.001). Using univariate ANOVA adjusted for age, body-mass index, estimated glomerular filtration rate, left ventricular ejection fraction, 6-minute walking distance, SF-36 physical functioning, and the natriuretic peptides NT-proBNP and MR-proANP, the interaction term sex*CT-proAVP was significantly associated with anxiety (p = 0.006). Further analysis showed that CT-proAVP was inversely related to anxiety only in men (B = -0.991; 95%CI = -1.650 to -0.331; p = 0.003), but not in women (p = 0.335).

CONCLUSION

In male study participants with cardiovascular risk factors, serum concentrations of CT-proAVP showed an inverse association with anxiety, which was independent from the severity of physical impairment.

Prosocial effects of an oxytocin metabolite, but not synthetic oxytocin receptor agonists, in a mouse model of autism

Currently, there are no established pharmaceutical strategies that effectively treat social deficits in autism spectrum disorder (ASD). Oxytocin, a neurohormone that plays a role in multiple types of social behaviors, has been proposed as a possible therapeutic against social impairment and other symptoms in ASD. However, from the standpoint of pharmacotherapy, oxytocin has several liabilities as a standard clinical treatment, including rapid metabolism, low brain penetrance, and activity at the vasopressin (antidiuretic hormone) receptors. The present studies describe findings from a preclinical screening program to evaluate oxytocin receptor (OXTR) agonists and oxytocin metabolites for potential clinical use as more optimal treatments. We first investigated two synthetic oxytocin analogs, TC-OT-39 and carbetocin, using in vitro cell-based assays for pharmacological characterization and behavioral tests in the BALB/cByJ mouse model of ASD-like social deficits. Although both TC-OT-39 and carbetocin selectively activate the OXTR, neither synthetic agonist had prosocial efficacy in the BALB/cByJ model. We next evaluated two oxytocin metabolites: OT(4-9) and OT(5-9). While OT(5-9) failed to affect social deficits, the metabolite OT(4-9) led to significant social preference in the BALB/cByJ model, in a dose-dependent manner. The increased sociability was observed at both 24 h and 12 days following the end of a subchronic regimen with OT(4-9) (2.0 mg/kg). Overall, these results suggest that the prosocial effects of oxytocin could be mediated by downstream activity of oxytocin metabolites, raising the possibility of new pathways to target for drug discovery relevant to ASD.

Cross-talk among oxytocin and arginine-vasopressin receptors: Relevance for basic and clinical studies of the brain and periphery

Oxytocin (OT) and arginine-vasopressin (AVP) act in the brain to regulate social cognition/social behavior and in the periphery to influence a variety of physiological processes. Although the chemical structures of OT and AVP as well as their receptors are quite similar, OT and AVP can have distinct or even opposing actions. Here, we review the increasing body of evidence that exogenously administered and endogenously released OT and AVP can activate each other's canonical receptors (i.e., cross-talk) and examine the possibility that receptor cross-talk following the synaptic and non-synaptic release of OT and AVP contributes to their distinct roles in the brain and periphery. Understanding the consequences of cross-talk between OT and AVP receptors will be important in identifying how these peptides control social cognition and behavior and for the development of drugs to treat a variety of psychiatric disorders.

The impact of early life stress on the central oxytocin system and susceptibility for drug addiction: Applicability of oxytocin as a pharmacotherapy

Early life trauma is strongly associated with an increased vulnerability to abuse illicit drugs and the impairment of neural development. This includes alterations to the development of the oxytocin system, which plays a pivotal role in the regulation of social behaviours and emotion. Dysregulation of this important system also contributes to increased susceptibility to develop drug addiction. In this review, we provide an overview of the animal models of early life stress that are widely used, and discuss the impact that early life stress has on drug-taking behaviour in adolescence and adulthood in both sexes. We link this to the changes that early life stress has on the endogenous oxytocin system, and how exogenously administered oxytocin may help to re-establish functioning of the system, and in turn, reduce drug-taking behaviour.

Oxytocin and Rodent Models of Addiction

Interest for the use of oxytocin as a treatment for addiction began over 40years ago. Better known for its roles in parturition, lactation and pair bonding, oxytocin also has anxiolytic properties, reduces immune and inflammatory responses, and has a role in learning and memory. In this chapter, oxytocin effects on addiction processes are described by highlighting research findings that have used oxytocin within current preclinical animal models of addiction, relapse, or craving. First, we provide a brief background of the endogenous oxytocin system followed by descriptions of the behavioral models used to study addiction, including models of drug taking and seeking. Then we review recent preclinical studies that have used oxytocin as a therapeutic intervention throughout multiple stages of the addiction cycle from a behavioral and neurobiological perspective. These models encompass the entire range of the addiction cycle including acquisition and maintenance of drug taking, withdrawal and craving during periods of drug abstinence, and ultimately relapse. We then posit several theories about how oxytocin interacts with both drug and social reward, as well as presenting a mechanistic account of how specific oxytocin receptor localization may contribute to oxytocin's efficacy as an addiction therapeutic.

Oxytocin and the warm outer glow: Thermoregulatory deficits cause huddling abnormalities in oxytocin-deficient mouse pups

Oxytocin is a social and reproductive hormone that also plays critical roles in a range of homeostatic processes, including thermoregulation. Here, we examine the role of oxytocin (OT) as a mediator of brown adipose tissue (BAT) thermogenesis, cold-induced huddling, and thermotaxis in eight-day-old (PD8) OT 'knock out' (OTKO) mouse pups. We tested OTKO and wildtype (WT) pups in single- and mixed-genotype groups of six, exposing these to a period of ambient warmth (~35°C) followed by a period of cold (~21.5°C). Whether huddling exclusively with other OTKO or alongside WT pups, OTKO pups showed reduced BAT thermogenesis and were significantly cooler when cold-challenged. Huddles of OTKO pups were also significantly less cohesive than WT huddles during cooling, suggesting that thermoregulatory deficits contribute to contact abnormalities in OTKO pups. To further explore this issue, we examined thermotaxis in individuals and groups of four OTKO or WT pups placed on the cool end of a thermocline and permitted to freely locomote for 2h. When tested individually, male OTKO pups displayed abnormal thermotaxis, taking significantly longer to move up the thermocline and settling upon significantly lower temperatures than WT pups during the 2h test. OTKO mouse pups thus appear to have deficits in both thermogenesis and thermotaxis-the latter deficit being specific to males. Our results add to a growing body of work indicating that OT plays critical roles in thermoregulation and also highlight the entanglement of social and thermoregulatory processes in small mammals such as mice.

Anxiety-like behavior and neuropeptide receptor expression in male and female prairie voles: The effects of stress and social buffering

Strong social support can negate negative health outcomes - an effect defined as 'social buffering'. In the present study, using the socially monogamous prairie vole (Microtus ochrogaster), we examined whether the presence of a bonded partner during a stressful event can reduce stress responses. Adult, pair-bonded female and male voles were assigned into experimental groups that were either handled (Control), experienced a 1-h immobilization (IMO) stress alone (IMO-Alone), or experienced IMO with their partner (IMO-Partner). Thereafter, subjects were tested for anxiety-like behavior, and brain sections were subsequently processed for oxytocin receptor (OTR) and vasopressin V1a-type receptor (V1aR) binding. Our data indicate that while IMO stress significantly decreased the time that subjects spent in the open arms of an elevated plus maze, partner's presence prevented this behavioral change - this social buffering on anxiety-like behavior was the same for both male and female subjects. Further, IMO stress decreased OTR binding in the nucleus accumbens (NAcc), but a partner's presence dampened this effect. No effects were found in V1aR binding. These data suggest that the neuropeptide- and brain region-specific OTR alterations in the NAcc may be involved in both the mediation and social buffering of stress responses. Some sex differences in the OTR and V1aR binding were also found in selected brain regions, offering new insights into the sexually dimorphic roles of the two neuropeptides. Overall, our results suggest a potential preventative approach in which the presence of social interactions during a stressor may buffer typical negative outcomes.

Rebalancing the Addicted Brain: Oxytocin Interference with the Neural Substrates of Addiction

Drugs that act on the brain oxytocin (OXT) system may provide a much-needed treatment breakthrough for substance-use disorders. Targeting the brain OXT system has the potential to treat addiction to all major classes of addictive substance and to intervene across all stages of the addiction cycle. Emerging evidence suggests that OXT is able to interfere with such a wide range of addictive behaviours for such a wide range of addictive substances by rebalancing core neural systems that become dysregulated over the course of addiction. By improving our understanding of these interactions between OXT and the neural substrates of addiction, we will not only improve our understanding of addiction, but also our ability to effectively treat these devastating disorders.

Oxytocin in the medial prefrontal cortex attenuates anxiety: Anatomical and receptor specificity and mechanism of action

Numerous studies in animals and humans have established that oxytocin (OT) reduces anxiety. In rats, the prelimbic (PL) subregion of the medial prefrontal cortex (mPFC) is among the brain areas implicated in the anxiolytic actions of OT. However, questions remain about the anatomical and receptor specificity of OT and its mechanism of action. Here we assessed whether the regulation of anxiety by mPFC OT is restricted to the PL subregion and evaluated whether oxytocin receptor (OTR) activation is required for OT to have an anxiolytic effect. We also examined whether OT interacts with GABA in the mPFC to reduce anxiety and investigated the extent to which OT in the mPFC affects activation of mPFC GABA neurons as well as neuronal activation in the amygdala, a primary target of the mPFC which is part of the neural network regulating anxiety. We found that OT reduced anxiety-like behavior when delivered to the PL, but not infralimbic or anterior cingulate subregions of the mPFC. The anxiolytic effect of OT in the PL mPFC was blocked by pretreatment with an OTR, but not a vasopressin receptor, antagonist as well as with a GABA_A receptor antagonist. Lastly, administration of OT to the PL mPFC was accompanied by increased activation of GABA neurons in the PL mPFC and altered neuronal activation of the amygdala following anxiety testing. These results demonstrate that OT in the PL mPFC attenuates anxiety-related behavior and may do so by engaging GABAergic neurons which ultimately modulate downstream brain regions implicated in anxiety.

The Oxytocin-Vasopressin Pathway in the Context of Love and Fear

Vasopressin (VP) and oxytocin (OT) are distinct molecules; these peptides and their receptors [OT receptor (OTR) and V1a receptor (V1aR)] also are evolved components of an integrated and adaptive system, here described as the OT-VP pathway. The more ancient peptide, VP, and the V1aRs support individual survival and play a role in defensive behaviors, including mobilization and aggression. OT and OTRs have been associated with positive social behaviors and may function as a biological metaphor for social attachment or "love." However, complex behavioral functions, including selective sexual behaviors, social bonds, and parenting require combined activities of OT and VP. The behavioral effects of OT and VP vary depending on perceived emotional context and the history of the individual. Paradoxical or contextual actions of OT also may reflect differential interactions with the OTR and V1aR. Adding to the complexity of this pathway is the fact that OT and VP receptors are variable, across species, individuals, and brain region, and these receptors are capable of being epigenetically tuned. This variation may help to explain experience-related individual and sex differences in behaviors that are regulated by these peptides, including the capacity to form social attachments and the emotional consequences of these attachments.

The Multidimensional Therapeutic Potential of Targeting the Brain Oxytocin System for the Treatment of Substance Use Disorders

The neuropeptide oxytocin is released both into the blood and within the brain in response to reproductive stimuli, such as birth, suckling and sex, but also in response to social interaction and stressors. Substance use disorders, or addictions, are chronic, relapsing brain disorders and are one of the major causes of global burden of disease. Unfortunately, current treatment options for substance use disorders are extremely limited and a treatment breakthrough is sorely needed. There is mounting preclinical evidence that targeting the brain oxytocin system may provide that breakthrough. Substance use disorders are characterised by a viscous cycle of bingeing and intoxication, followed by withdrawal and negative affect, and finally preoccupation and anticipation that triggers relapse and further consumption. Administration of oxytocin has been shown to have a potential therapeutic benefit at each stage of this addiction cycle for numerous drugs of abuse. This multidimensional therapeutic utility is likely due to oxytocin's interactions with key biological systems that underlie the development and maintenance of addiction. Only a few human trials of oxytocin in addicted populations have been completed with the results thus far being mixed. There are numerous other trials underway, and the results are eagerly awaited. However, the ability to fully harness the potential therapeutic benefit of targeting the brain oxytocin system may depend on the development of molecules that selectively stimulate the oxytocin system, but that have superior pharmacokinetic properties to oxytocin itself.

ASD and Genetic Associations with Receptors for Oxytocin and Vasopressin-AVPR1A, AVPR1B, and OXTR

Background: There are limited treatments available for autism spectrum disorder (ASD). Studies have reported significant associations between the receptor genes of oxytocin (OT) and vasopressin (AVP) and ASD diagnosis, as well as ASD-related phenotypes. Researchers have also found the manipulation of these systems affects social and repetitive behaviors, core characteristics of ASD. Consequently, research involving the oxytocin/vasopressin pathways as intervention targets has increased. Therefore, further examination into the relationship between these neuropeptides and ASD was undertaken. In this study, we examined associations between variants in the receptor genes of vasopressin (AVPR1A, AVPR1B), oxytocin (OXTR), and ASD diagnosis along with related subphenotypes.

Methods: Probands were assessed using Autism Diagnostic Interview-Revised, Autism Diagnostic Observation Schedule, and clinical DSM-IV-TR criteria. Single nucleotide polymorphisms (SNPs) in AVPR1B and OXTR, and microsatellites in AVPR1A were genotyped in ~200 families with a proband with ASD. Family-based association testing (FBAT) was utilized to determine associations between variants and ASD. Haplotypes composed of OXTR SNPs (i.e., rs53576-rs2254298-rs2268493) were also analyzed due to previously published associations.

Results: Using the additive inheritance model in FBAT we found associations between AVPR1B SNPs (rs28632197, p = 0.005, rs35369693, p = 0.025) and diagnosis. As in other studies, OXTR rs2268493 (p = 0.050) was associated with diagnosis. rs2268493 was also associated with ASD subphenotypes of social withdrawal (p = 0.013) and Insistence on Sameness (p = 0.039). Further analyses demonstrated that the haplotype, rs2254298–rs2268493 was found to be significantly associated with diagnosis (A-T; p = 0.026). FBAT was also used to analyze AVPR1A microsatellites (RS1 and RS3). Both length variants were found to be associated with restrictive, repetitive behaviors, but not overall diagnosis. Correction for multiple comparisons was performed for SNPs tested in each gene region, only AVPR1B SNPs remained significantly associated with ASD diagnosis.

Conclusions: Autism is a heterogeneous disorder with many genes and pathways that contribute to its development. SNPs and microsatellites in the receptor genes of OT and AVP are associated with ASD diagnosis and measures of social behavior as well as restricted repetitive behaviors. We reported a novel association with ASD and AVPR1B SNPs. Understanding of genotype-phenotype relationships may be helpful in the development of pharmacological interventions for the OT/AVP system.

Regional c-Fos expression induced by peripheral oxytocin administration is prevented by the vasopressin 1A receptor antagonist SR49059

Peripherally administered oxytocin induces a wide range of behavioural and physiological effects that are thought to be mediated by the oxytocin receptor (OTR). However, oxytocin also has considerable affinity for the vasopressin 1A receptor (V_1AR), such that various oxytocinergic effects may in fact be mediated by the V_1AR rather than the OTR. Here we used c-Fos immunohistochemistry to determine the extent to which the regional pattern of neuronal activation produced by peripheral oxytocin involves the V_1AR. Male Wistar rats were administered oxytocin (1mg/kg, IP) alone, or following pre-treatment with the V_1AR antagonist SR49059 (1mg/kg, IP), and were assessed for locomotor activity changes and for c-Fos expression across a number of brain regions. Oxytocin reduced the distance travelled by rats during a 70min test session, and this inhibitory behavioural effect was prevented by SR49059. Consistent with previous reports, oxytocin increased c-Fos expression in a number of brain regions. In several of these regions-the supraoptic and paraventricular (PVN) nuclei of the hypothalamus, locus coeruleus and nucleus of the solitary tract-the c-Fos response was prevented by SR49059 pre-treatment. Notably, SR49059 inhibited the c-Fos activation in oxytocin-synthesising magnocellular neurons in the PVN. However, c-Fos expression in the central amygdala to oxytocin was unaffected by SR49059. The current findings add to an increasing body of research suggesting that many of the functional effects of oxytocin may be V_1AR mediated.

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.

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.

Oxytocin and MDMA ('Ecstasy') enhance social reward in rats

RATIONALE

Oxytocin (OT), vasopressin (AVP) and 3,4 methylenedioxymethamphetamine (MDMA, 'Ecstasy') all increase social interaction in rats, perhaps by enhancing the rewarding value of social encounters.

OBJECTIVES

Here, we used the conditioned place preference (CPP) paradigm to assess the intrinsic rewarding effects of OT, AVP and MDMA, and whether these effects are enhanced by the presence of a conspecific, or a dynamic, tactile object (a tennis ball).

METHODS

Adult male rats received conditioning sessions in a CPP apparatus twice a day (vehicle at 10 a.m., drug at 3 p.m.). Experiment 1 involved conditioning with OT (0.5 mg/kg, intraperitoneal (i.p.)), AVP (0.005 mg/kg, i.p.) or MDMA (5 mg/kg, i.p.). Experiments 2 and 3 involved conditioning with the same treatments but in the presence of a conspecific receiving the same treatment (social-CPP) or in the presence of a tennis ball (object-CPP), respectively. Conditioned place preference was assessed 24 h, 2 weeks and 4 weeks later.

RESULTS

OT, AVP and MDMA did not produce a conventional CPP. However, when the conditioning environment also contained a conspecific both OT and MDMA induced a significant CPP lasting for at least 4 weeks. Rats given OT and MDMA also developed a more modest yet significant CPP for the environment where they encountered a tennis ball.

CONCLUSIONS

These results indicate that OT and MDMA can augment the rewarding effects of social interaction, but also interaction with a dynamic and tactile non-social object. AVP does not condition social- or object-CPPs and may promote social proximity by inducing generalized anxiety and defensive aggregation.

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.

Oxytocin prevents ethanol actions at δ subunit-containing GABAA receptors and attenuates ethanol-induced motor impairment in rats

Even moderate doses of alcohol cause considerable impairment of motor coordination, an effect that substantially involves potentiation of GABAergic activity at δ subunit-containing GABA(A) receptors (δ-GABA(A)Rs). Here, we demonstrate that oxytocin selectively attenuates ethanol-induced motor impairment and ethanol-induced increases in GABAergic activity at δ-GABA(A)Rs and that this effect does not involve the oxytocin receptor. Specifically, oxytocin (1 µg i.c.v.) given before ethanol (1.5 g/kg i.p.) attenuated the sedation and ataxia induced by ethanol in the open-field locomotor test, wire-hanging test, and righting-reflex test in male rats. Using two-electrode voltage-clamp electrophysiology in Xenopus oocytes, oxytocin was found to completely block ethanol-enhanced activity at α4β1δ and α4β3δ recombinant GABA(A)Rs. Conversely, ethanol had no effect when applied to α4β1 or α4β3 cells, demonstrating the critical presence of the δ subunit in this effect. Oxytocin had no effect on the motor impairment or in vitro effects induced by the δ-selective GABA(A)R agonist 4,5,6,7-tetrahydroisoxazolo(5,4-c)pyridin-3-ol, which binds at a different site on δ-GABA(A)Rs than ethanol. Vasopressin, which is a nonapeptide with substantial structural similarity to oxytocin, did not alter ethanol effects at δ-GABA(A)Rs. This pattern of results confirms the specificity of the interaction between oxytocin and ethanol at δ-GABA(A)Rs. Finally, our in vitro constructs did not express any oxytocin receptors, meaning that the observed interactions occur directly at δ-GABA(A)Rs. The profound and direct interaction observed between oxytocin and ethanol at the behavioral and cellular level may have relevance for the development of novel therapeutics for alcohol intoxication and dependence.

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.

Number of X-chromosome genes influences social behavior and vasopressin gene expression in mice

Sex differences in behavior are widespread and often caused by hormonal differences between the sexes. In addition to hormones, the composition and numbers of the sex chromosomes also affect a variety of sex differences. In humans, X-chromosome genes are implicated in neurobehavioral disorders (i.e. fragile-X, autism). To investigate the role of X-chromosome genes in social behavior, we used a mouse model that has atypical sex chromosome configurations resembling Turner (45, XO) and Klinefelter syndromes (47, XXY). We examined a number of behaviors in juvenile mice. Mice with only one copy of most X-chromosome genes, regardless of gonadal sex, were less social in dyadic interaction and social preference tasks. In the elevated plus maze, mice with one X-chromosome spent less time in the distal ends of the open arms as compared to mice with two copies of X-chromosome genes. Using qRTPCR, we noted that amygdala from female mice with one X-chromosome had higher expression levels of vasopressin (Avp) as compared to mice in the other groups. Finally, in plasma from girls with Turner syndrome we detected reduced vasopressin (AVP) concentrations as compared to control patients. These novel findings link sex chromosome genes with social behavior via concentrations of AVP in brain, adding to our understanding of sex differences in neurobehavioral disorders.

Effects of oxytocin on attention to emotional faces in healthy volunteers and highly socially anxious males

BACKGROUND

Evidence suggests that individuals with social anxiety demonstrate vigilance to social threat, whilst the peptide hormone oxytocin is widely accepted as supporting affiliative behaviour in humans.

METHODS

This study investigated whether oxytocin can affect attentional bias in social anxiety. In a double-blind, randomized, placebo-controlled, within-group study design, 26 healthy and 16 highly socially anxious (HSA) male volunteers (within the HSA group, 10 were diagnosed with generalized social anxiety disorder) were administered 24 IU of oxytocin or placebo to investigate attentional processing in social anxiety. Attentional bias was assessed using the dot-probe paradigm with angry, fearful, happy and neutral face stimuli.

RESULTS

In the baseline placebo condition, the HSA group showed greater attentional bias for emotional faces than healthy individuals. Oxytocin reduced the difference between HSA and non-socially anxious individuals in attentional bias for emotional faces. Moreover, it appeared to normalize attentional bias in HSA individuals to levels seen in the healthy population in the baseline condition. The biological mechanisms by which oxytocin may be exerting these effects are discussed.

CONCLUSIONS

These results, coupled with previous research, could indicate a potential therapeutic use of this hormone in treatment for social anxiety.

Possible involvement of oxytocin in modulating the stress response in lactating dairy cows

Oxytocin can attenuate the physiological and behavioral response to stress in animals. In this study we investigated the relationship between plasma oxytocin concentrations and the behavioral and physiological response of dairy cows to a repeated psychological stressor (novel environment). Twenty lactating multi-parous dairy cows were milked in a familiar milking parlor and in a novel environment. Blood samples were collected before and after milking in the familiar parlor (baseline) and on the second and fifth day in the novel parlor to measure plasma cortisol and oxytocin concentrations. Heart rate was recorded on all cows during milking in the familiar and novel environment. On all test days, the behavioral response of cows to milk cluster attachment was scored. On day 2 in the novel parlor, the oxytocin response, cortisol concentrations and heart rate were greater, and heart rate variability was lower than baseline values recorded in the familiar parlor. The results from this study suggest that oxytocin release is increased in response to exposure to a psychological stressor (novel environment) and that cows adapt to this stressor over time. After initial suppression, oxytocin levels increased over days of milking in a novel environment, whereas indicators of stress simultaneously decreased. Furthermore, the oxytocin increase was associated with habituation of the cortisol response in anticipation of milking in a novel environment, suggesting that oxytocin may be involved in habituation to a novel environment in dairy cows. This mechanism of habituation to novel environments may reflect an association between oxytocin and a "familiarization-habituation response" to repeated exposure to an initially novel environment that has previously been reported in humans.

Active coping toward predatory stress is associated with lower corticosterone and progesterone plasma levels and decreased methylation in the medial amygdala vasopressin system

An active coping style displayed under stress - which involves proactive investigatory responses toward environmental threats - has been associated with reduced vulnerability to psychiatric illness. However, the neurobiological determinants of coping styles are not well understood. When rats are exposed to a naturalistic stressor (cat fur) in a group, some individuals in the group show robust active investigation of the stimulus while others show a passive response involving retreat, immobility and close aggregation with conspecifics. Here we explored endocrine and epigenetic correlates of these contrasting coping styles. Male Wistar rats (n=48) were exposed to cat fur in groups of 4 and the passive and active responders were identified and assessed for endocrine and epigenetic differences. Three days after the final cat fur exposure, active responders had substantially lower plasma levels of corticosterone and progesterone than passive responders. Plasma and testicular testosterone levels did not differ between active and passive responders. Active responders had markedly less methylation of the AVP CGCG promoter region located at base 4970 in the posterodorsal region of the medial amygdala but did not differ in the methylation status of the CCGG sequence located at base 2243. This is in agreement with prior research suggesting that AVP and progesterone act in opposition within the medial amygdala to modulate stress-related behaviors. The present study reports striking endocrine and epigenetic differences between active and passive responders, providing insight into potential systems involved in the manifestation of differing coping styles.