Quantitative mapping reveals age and sex differences in vasopressin, but not oxytocin, immunoreactivity in the rat social behavior neural network

The medial amygdala's neural circuitry: Insights into social processing and sex differences

The amygdala, a critical part of the limbic system, is essential for processing social stimuli and regulating stress responses. Among its various neuronal nuclei, the medial amygdala (MeA) remains one of the least studied in humans. The MeA plays a key role in receiving inputs from the olfactory system through pheromones, as well as from crucial areas such as the hypothalamus, hippocampus, and reward system. This allows the MeA to integrate external stimuli with the organism's internal state, finetuning social interactions, endocrine responses, and innate behaviors. Recent advances in neuroscience have highlighted the sex differences of the MeA and how they influence behavior and environmental perception. Understanding these sexspecific variations in brain structures, like the MeA in rodents, is vital for applying this knowledge to humans and could help bridge gaps in our understanding and treatment of mental health disorders, which often differ between sexes in both prevalence and presentation.

Vasopressin regulates social play behavior in sex-specific ways through glutamate modulation in the lateral septum

Understanding the neural basis of social play in juvenile rats may ultimately help restore social play deficits in autistic children. We previously found that administration of a vasopressin (AVP) V1a receptor (V1aR) antagonist into the lateral septum (LS) increased social play behavior in male juvenile rats and decreased it in females. Here, we demonstrate that glutamate, but not GABA, is involved in this sex-specific regulation. First, we found a sex difference in extracellular LS glutamate/GABA ratio (lower in females) that was eliminated by V1aR antagonist infusion in the LS that caused an increase in glutamate release in females only. Second, infusion of the glutamate receptor agonist L-glutamic acid into the LS mimicked the V1aR antagonist-induced decrease in female social play while preventing the increase in male social play. Third, infusion of the glutamate receptor antagonists AP-5 and CNQX into the LS prevented the V1aR antagonist-induced decrease in female social play. Fourth, there were no sex differences in extracellular GABA release in the LS upon either V1aR antagonist infusion or in social play expression upon infusion of the GABA-A receptor agonist muscimol into the LS, suggesting that GABA is not involved in the sex-specific regulation of social play by the LS-AVP system. Last, we found no sex differences in the type (GAD1/2, somatostatin, calbindin 1, Sox9) of V1aR-expressing LS cells, suggesting other cellular mechanisms mediating the sex-specific effects on glutamate release in the LS by the LS-AVP system. In conclusion, we demonstrate that the LS-AVP system regulates social play sex-specifically via glutamatergic neurotransmission. These findings have relevance for potential sex-specific effects of AVP-based treatment of social deficits in children.

Comparative review of the brain development in Acomys cahirinus

Acomys cahirinus (referred to as "acomys" in this article) is a precocial rodent, born well-developed and mobile, capable of feeding independently and escaping predators shortly after birth. Notable for its advanced regenerative abilities and menstrual cycle, acomys serves as a unique model for studying diverse aspects of physiology and neuroscience, including developmental and regenerative neuroscience. Despite its significance, only sporadic and unsystematic data on the structure and development of the acomys brain are available. Therefore, the aim of this study was to systematically organize the existing information on the structure and development of the acomys brain and to compare it with that of commonly studied altricial rodent species (rats, mice, hamsters, and gerbils). This review is organized into several sections, focusing on general aspects of brain development, such as myelination and brain growth. It also discusses the development of brain structures involved in sensory processing (olfactory, visual, and auditory), motor control, learning and memory, and social behavior.

Arginine vasopressin activates serotonergic neurons in the dorsal raphe nucleus during neonatal development in vitro and in vivo

Birth stress is a risk factor for psychiatric disorders and associated with exaggerated release of the stress hormone arginine vasopressin (AVP) into circulation and in the brain. In perinatal hippocampus, AVP activates GABAergic interneurons which leads to suppression of spontaneous network events and suggests a protective function of AVP on cortical networks during birth. However, the role of AVP in developing subcortical networks is not known. Here we tested the effect of AVP on the dorsal raphe nucleus (DRN) 5-hydroxytryptamine (5-HT, serotonin) system in male and female neonatal rats, since early 5-HT homeostasis is critical for the development of cortical brain regions and emotional behaviors. We show that AVP is strongly excitatory in neonatal DRN: it increases excitatory synaptic inputs of 5-HT neurons via V1A receptors in vitro and promotes their action potential firing through a combination of its effect on glutamatergic synaptic transmission and a direct effect on the excitability of these neurons. Furthermore, we identified two major firing patterns of neonatal 5-HT neurons in vivo, tonic regular firing and low frequency oscillations of regular spike trains and confirmed that these neurons are also activated by AVP in vivo. Finally, we show that the sparse vasopressinergic innervation in neonatal DRN originates exclusively from cell groups in medial amygdala and bed nucleus of stria terminalis. Hyperactivation of the neonatal 5-HT system by AVP during birth stress may impact its own functional development and affect the maturation of cortical target regions, which may increase the risk for psychiatric conditions later on.

Like sisters but not twins - vasopressin and oxytocin excite BNST neurons via cell type-specific expression of oxytocin receptor to reduce anxious arousal

Interoceptive signals dynamically interact with the environment to shape appropriate defensive behaviors. Hypothalamic hormones arginine-vasopressin (AVP) and oxytocin (OT) regulate physiological states, including water and electrolyte balance, circadian rhythmicity, and defensive behaviors. Both AVP and OT neurons project to dorsolateral bed nucleus of stria terminalis (BNSTDL), which expresses oxytocin receptors (OTR) and vasopressin receptors and mediates fear responses. However, understanding the integrated role of neurohypophysial hormones is complicated by the cross-reactivity of AVP and OT and their mutual receptor promiscuity. Here, we provide evidence that the effects of neurohypophysial hormones on BNST excitability are driven by input specificity and cell type-specific receptor selectivity. We show that OTR-expressing BNSTDL neurons, excited by hypothalamic OT and AVP inputs via OTR, play a major role in regulating BNSTDL excitability, overcoming threat avoidance, and reducing threat-elicited anxious arousal. Therefore, OTR-BNSTDL neurons are perfectly suited to drive the dynamic interactions balancing external threat risk and physiological needs.

Sex-dependent effects of acute stress and alcohol exposure during adolescence on mRNA expression of brain signaling systems involved in reward and stress responses in young adult rats

BACKGROUND

Adolescent stress and alcohol exposure increase the risk of maladaptive behaviors and mental disorders in adulthood, with distinct sex-specific differences. Understanding the mechanisms underlying these early events is crucial for developing targeted prevention and treatment strategies.

METHODS

Male and female Wistar rats were exposed to acute restraint stress and intermittent alcohol during adolescence. We assessed lasting effects on plasma corticosterone (CORT) and adrenocorticotropic hormone (ACTH) levels, and mRNA expression of genes related to corticotropin releasing hormone (CRH), neuropeptide Y (NPY), corticoid, opioid, and arginine vasopressin systems in the amygdala and hypothalamus.

RESULTS

The main findings are as follows: (1) blood alcohol concentrations (BAC) increased after the final alcohol administration, but stressed males had lower BAC than non-stressed males; (2) Males gained significantly more weight than females; (3) Stressed females showed higher ACTH levels than non-stressed females, with no changes in males; (4) Stress increased CORT levels in males, while stressed, alcohol-treated females had lower CORT levels than non-stressed females; (5) CRH: Females had lower Crhr1 levels in the amygdala, while alcohol reduced Crhr2 levels in males but not females. Significant interactions among sex, stress, and alcohol were found in the hypothalamus, with distinct patterns between sexes; (6) NPY: In the amygdala, stress reduced Npy and Npy1r levels in males but increased them in females. Alcohol decreased Npy2r levels in males, with varied effects in females. Similar sex-specific patterns were observed in the hypothalamus; (7) Corticoid system: Stress and alcohol had complex, sex-dependent effects on Pomc, Nr3c1, and Nr3c2 in both brain regions; (8) Opioid receptors: Stress and alcohol blunted the elevated expression of Oprm1, Oprd1, and Oprk1 in the amygdala of males and the hypothalamus of females; (8) Vasopressin: Stress and alcohol interacted significantly to affect Avp and Avpr1a expression in the amygdala, with stronger effects in females. In the hypothalamus, alcohol increased Avp levels in females.

CONCLUSIONS

This study demonstrates that adolescent acute stress and alcohol exposure induce lasting, sex-specific alterations in systems involved in reward and stress responses. These findings emphasize the importance of considering sex differences in the prevention and management of HPA dysfunction and psychiatric disorders.

Neuronal biomarkers as potential therapeutic targets for drug addiction related to sex differences in the brain: Opportunities for personalized treatment approaches

Biological sex disparities manifest at various stages of drug addiction, including craving, substance abuse, abstinence, and relapse. These discrepancies are underpinned by notable distinctions in neurobiological substrates, encompassing brain structures, functions, and neurotransmitter systems implicated in drug addiction. Neuronal biomarkers, such as neurotransmitters, signaling proteins, and genes may be associated with the diagnosis, prognosis, and treatment outcomes in both biological sexes afflicted by drug abuse. Sex differences in the neural reward system, mainly through dopaminergic transmission during drug abuse, can be attributed to modifications in neurotransmitter systems and signaling pathways. This results in distinct patterns of neural activation and responsiveness to addictive substances in males and females. Sex hormones, the estrus/menstrual cycle, and cerebral neurochemistry contribute to the progression of psychological and physiological dependence in both male and female individuals grappling with addiction. Moreover, the alteration of sex hormone balance and neurotransmitter release plays a pivotal role in substance use disorders, subsequently modulating cognitive functions pertinent to reward, including memory formation, decision-making, and locomotor activity. Comparative investigations reveal distinctions in brain region volume, gene expression, neuronal firing, and circuitry in substance use disorders affecting individuals of both biological sexes. This review examines prevalent substance use disorders to elucidate the impact of sex hormones as therapeutic biomarkers on the mesocorticolimbic neurotransmitter systems via diverse mechanisms within the addicted brain. We underscore the imperative necessity of considering these variations to gain a deeper comprehension of addiction mechanisms and potentially discern sex-specific neuronal biomarkers for tailored therapeutic interventions.

Sex differences in the structure and function of the vasopressin system in the ventral pallidum are associated with the sex-specific regulation of social play behavior in juvenile rats

Vasopressin (AVP) regulates various social behaviors, often in sex-specific ways, including social play behavior, a rewarding behavior displayed primarily by juveniles. Here, we examined whether and how AVP acting in the brain's reward system regulates social play behavior in juvenile rats. Specifically, we focused on AVP signaling in the ventral pallidum (VP), a brain region that is a part of the reward system. First, we examined the organization of the VP-AVP system in juvenile rats and found sex differences, with higher density of both AVP-immunoreactive fibers and AVP V1a receptor (V1aR) binding in males compared to females while females show a greater number of V1aR-expressing cells compared to males. We further found that, in both sexes, V1aR-expressing cells co-express a GABA marker to a much greater extent (approx. 10 times) than a marker for glutamate. Next, we examined the functional involvement of V1aR-expressing VP cells in social play behavior. We found that exposure to social play enhanced the proportion of activated V1aR-expressing VP cells in males only. Finally, we showed that infusion of a specific V1aR antagonist into the VP increased social play behaviors in juvenile male rats while decreasing these behaviors in juvenile female rats. Overall, these findings reveal structural and functional sex differences in the AVP-V1aR system in the VP that are associated with the sex-specific regulation of social play behavior.

Long-range projections of oxytocin neurons in the marmoset brain

The neurohormone oxytocin (OT) has become a major target for the development of novel therapeutic strategies to treat psychiatric disorders such as autism spectrum disorder because of its integral role in governing many facets of mammalian social behavior. Whereas extensive work in rodents has produced much of our knowledge of OT, we lack basic information about its neurobiology in primates making it difficult to interpret the limited effects that OT manipulations have had in human patients. In fact, previous studies have revealed only limited OT fibers in primate brains. Here, we investigated the OT connectome in marmoset using immunohistochemistry, and mapped OT fibers throughout the brains of adult male and female marmoset monkeys. We found extensive OT projections reaching limbic and cortical areas that are involved in the regulation of social behaviors, such as the amygdala, the medial prefrontal cortex, and the basal ganglia. The pattern of OT fibers observed in marmosets is notably similar to the OT connectomes described in rodents. Our findings here contrast with previous results by demonstrating a broad distribution of OT throughout the marmoset brain. Given the prevalence of this neurohormone in the primate brain, methods developed in rodents to manipulate endogenous OT are likely to be applicable in marmosets.

Characterization of oxytocin and vasopressin receptors in the Southern giant pouched rat and comparison to other rodents

Vasopressin and oxytocin are well known and evolutionarily ancient modulators of social behavior. The distribution and relative densities of vasopressin and oxytocin receptors are known to modulate the sensitivity to these signaling molecules. Comparative work is needed to determine which neural networks have been conserved and modified over evolutionary time, and which social behaviors are commonly modulated by nonapeptide signaling. To this end, we used receptor autoradiography to determine the distribution of vasopressin 1a and oxytocin receptors in the Southern giant pouched rat (Cricetomys ansorgei) brain, and to assess the relative densities of these receptors in specific brain regions. We then compared the relative receptor pattern to 23 other species of rodents using a multivariate ANOVA. Pouched rat receptor patterns were strikingly similar to hamsters and voles overall, despite the variation in social organization among species. Uniquely, the pouched rat had dense vasopressin 1a receptor binding in the caudate-putamen (i.e., striatum), an area that might impact affiliative behavior in this species. In contrast, the pouched rat had relatively little oxytocin receptor binding in much of the anterior forebrain. Notably, however, oxytocin receptor binding demonstrated extremely dense binding in the bed nucleus of the stria terminalis, which is associated with the modulation of several social behaviors and a central hub of the social decision-making network. Examination of the nonapeptide system has the potential to reveal insights into species-specific behaviors and general themes in the modulation of social behavior.

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.

Beyond the binary: Characterizing the relationships between sex and neuropeptide receptor binding density measures in the rat brain

Sex differences exist in numerous parameters of the brain. Yet, sex-related factors are part of a large set of variables that interact to affect many aspects of brain structure and function. This raises questions regarding how to interpret findings of sex differences at the level of single brain measures and the brain as a whole. In the present study, we reanalyzed two datasets consisting of measures of oxytocin, vasopressin V1a, and mu opioid receptor binding densities in multiple brain regions in rats. At the level of single brain measures, we found that sex differences were rarely dimorphic and were largely persistent across estrous stage and parental status but not across age or context. At the level of aggregates of brain measures showing sex differences, we tested whether individual brains are 'mosaics' of female-typical and male-typical measures or are internally consistent, having either only female-typical or only male-typical measures. We found mosaicism for measures showing overlap between females and males. Mosaicism was higher a) with a larger number of measures, b) with smaller effect sizes of the sex difference in these measures, and c) in rats with more diverse life experiences. Together, these results highlight the limitations of the binary framework for interpreting sex effects on the brain and suggest two complementary pathways to studying the contribution of sex to brain function: (1) focusing on measures showing dimorphic and persistent sex differences and (2) exploring the relations between specific brain mosaics and specific endpoints.

Long range projections of oxytocin neurons in the marmoset brain

The neurohormone oxytocin (OT) has become a major target for the development of novel therapeutic strategies to treat psychiatric disorders such as autism spectrum disorder because of its integral role in governing many facets of mammalian social behavior. Whereas extensive work in rodents has produced much of our knowledge of OT, we lack basic information about its neurobiology in primates making it difficult to interpret the limited effects that OT manipulations have had in human patients. In fact, previous studies have revealed only limited OT fibers in primate brains. Here, we investigated the OT connectome in marmoset using immunohistochemistry, and mapped OT fibers throughout the brains of adult male and female marmoset monkeys. We found extensive OT projections reaching limbic and cortical areas that are involved in the regulation of social behaviors, such as the amygdala, the medial prefrontal cortex and the basal ganglia. The pattern of OT fibers observed in marmosets is notably similar to the OT connectomes described in rodents. Our findings here contrast with previous results by demonstrating a broad distribution of OT throughout the marmoset brain. Given the prevalence of this neurohormone in the primate brain, methods developed in rodents to manipulate endogenous OT are likely to be applicable in marmosets.

Prenatal allergic inflammation in rats confers sex-specific alterations to oxytocin and vasopressin innervation in social brain regions

Prenatal exposure to inflammation via maternal infection, allergy, or autoimmunity increases one's risk for developing neurodevelopmental and psychiatric disorders. Many of these disorders are associated with altered social behavior, yet the mechanisms underlying inflammation-induced social impairment remain unknown. We previously found that a rat model of acute allergic maternal immune activation (MIA) produced deficits like those found in MIA-linked disorders, including impairments in juvenile social play behavior. The neuropeptides oxytocin (OT) and arginine vasopressin (AVP) regulate social behavior, including juvenile social play, across mammalian species. OT and AVP are also implicated in neuropsychiatric disorders characterized by social impairment, making them good candidate regulators of social deficits after MIA. We profiled how acute prenatal exposure to allergic MIA changed OT and AVP innervation in several brain regions important for social behavior in juvenile male and female rat offspring. We also assessed whether MIA altered additional behavioral phenotypes related to sociality and anxiety. We found that allergic MIA increased OT and AVP fiber immunoreactivity in the medial amygdala and had sex-specific effects in the nucleus accumbens, bed nucleus of the stria terminalis, and lateral hypothalamic area. We also found that MIA reduced ultrasonic vocalizations in neonates and increased the stereotypical nature of self-grooming behavior. Overall, these findings suggest that there may be sex-specific mechanisms underlying MIA-induced behavioral impairment and underscore OT and AVP as ideal candidates for future mechanistic studies.

Vasopressin as Possible Treatment Option in Autism Spectrum Disorder

Autism spectrum disorder (ASD) is rather common, presenting with prevalent early problems in social communication and accompanied by repetitive behavior. As vasopressin was implicated not only in salt-water homeostasis and stress-axis regulation, but also in social behavior, its role in the development of ASD might be suggested. In this review, we summarized a wide range of problems associated with ASD to which vasopressin might contribute, from social skills to communication, motor function problems, autonomous nervous system alterations as well as sleep disturbances, and altered sensory information processing. Beside functional connections between vasopressin and ASD, we draw attention to the anatomical background, highlighting several brain areas, including the paraventricular nucleus of the hypothalamus, medial preoptic area, lateral septum, bed nucleus of stria terminalis, amygdala, hippocampus, olfactory bulb and even the cerebellum, either producing vasopressin or containing vasopressinergic receptors (presumably V1a). Sex differences in the vasopressinergic system might underline the male prevalence of ASD. Moreover, vasopressin might contribute to the effectiveness of available off-label therapies as well as serve as a possible target for intervention. In this sense, vasopressin, but paradoxically also V1a receptor antagonist, were found to be effective in some clinical trials. We concluded that although vasopressin might be an effective candidate for ASD treatment, we might assume that only a subgroup (e.g., with stress-axis disturbances), a certain sex (most probably males) and a certain brain area (targeting by means of virus vectors) would benefit from this therapy.

Neuropeptidergic control circuits in the spinal cord for male sexual behaviour: Oxytocin-gastrin-releasing peptide systems

The neuropeptidergic mechanisms controlling socio-sexual behaviours consist of complex neuronal circuitry systems in widely distributed areas of the brain and spinal cord. At the organismal level, it is now becoming clear that "hormonal regulations" play an important role, in addition to the activation of neuronal circuits. The gastrin-releasing peptide (GRP) system in the lumbosacral spinal cord is an important component of the neural circuits that control penile reflexes in rats, circuits that are commonly referred to as the "spinal ejaculation generator (SEG)." Oxytocin, long known as a neurohypophyseal hormone, is now known to be involved in the regulation of socio-sexual behaviors in mammals, ranging from social bonding to empathy. However, the functional interaction between the SEG neurons and the hypothalamo-spinal oxytocin system remains unclear. Oxytocin is known to be synthesised mainly in hypothalamic neurons and released from the posterior pituitary into the circulation. Oxytocin is also released from the dendrites of the neurons into the hypothalamus where they have important roles in social behaviours via non-synaptic volume transmission. Because the most familiar functions of oxytocin are to regulate female reproductive functions including parturition, milk ejection, and maternal behaviour, oxytocin is often thought of as a "feminine" hormone. However, there is evidence that a group of parvocellular oxytocin neurons project to the lower spinal cord and control male sexual function in rats. In this report, we review the functional interaction between the SEG neurons and the hypothalamo-spinal oxytocin system and effects of these neuropeptides on male sexual behaviour. Furthermore, we discuss the finding of a recently identified, localised "volume transmission" role of oxytocin in the spinal cord. Findings from our studies suggest that the newly discovered "oxytocin-mediated spinal control of male sexual function" may be useful in the treatment of erectile and ejaculatory dysfunction.

Oxytocin, Vasopressin, and Social Behavior: From Neural Circuits to Clinical Opportunities

Oxytocin and vasopressin are peptide hormones secreted from the pituitary that are well known for their peripheral endocrine effects on childbirth/nursing and blood pressure/urine concentration, respectively. However, both peptides are also released in the brain, where they modulate several aspects of social behaviors. Oxytocin promotes maternal nurturing and bonding, enhances social reward, and increases the salience of social stimuli. Vasopressin modulates social communication, social investigation, territorial behavior, and aggression, predominantly in males. Both peptides facilitate social memory and pair bonding behaviors in monogamous species. Here we review the latest research delineating the neural circuitry of the brain oxytocin and vasopressin systems and summarize recent investigations into the circuit-based mechanisms modulating social behaviors. We highlight research using modern molecular genetic technologies to map, monitor activity of, or manipulate neuropeptide circuits. Species diversity in oxytocin and vasopressin effects on social behaviors are also discussed. We conclude with a discussion of the translational implications of oxytocin and vasopressin for improving social functioning in disorders with social impairments, such as autism spectrum disorder.

Sex Differences in Acute Neuroendocrine Responses to Stressors in Rodents and Humans

Sex differences in the neuroendocrine response to acute stress occur in both animals and humans. In rodents, stressors such as restraint and novelty induce a greater activation of the hypothalamic-pituitary-adrenal axis (HPA) in females compared to males. The nature of this difference arises from steroid actions during development (organizational effects) and adulthood (activational effects). Androgens decrease HPA stress responsivity to acute stress, while estradiol increases it. Androgenic down-regulation of HPA responsiveness is mediated by the binding of testosterone (T) and dihydrotestosterone (DHT) to the androgen receptor, as well as the binding of the DHT metabolite, 3β-diol, to the β form of the estrogen receptor (ERβ). Estradiol binding to the α form of the estrogen receptor (ERα) increases HPA responsivity. Studies of human sex differences are relatively few and generally employ a psychosocial paradigm to measure stress-related HPA activation. Men consistently show greater HPA reactivity than women when being evaluated for achievement. Some studies have found greater reactivity in women when being evaluated for social performance. The pattern is inconsistent with rodent studies but may involve the differential nature of the stressors employed. Psychosocial stress is nonphysical and invokes a significant degree of top-down processing that is not easily comparable to the types of stressors employed in rodents. Gender identity may also be a factor based on recent work showing that it influences the neural processing of positive and negative emotional stimuli independent of genetic sex. Comparing different types of stressors and how they interact with gender identity and genetic sex will provide a better understanding of sex steroid influences on stress-related HPA reactivity.

CRISPR-Cas9 editing of the arginine-vasopressin V1a receptor produces paradoxical changes in social behavior in Syrian hamsters

Studies from a variety of species indicate that arginine–vasopressin (AVP) and its V1a receptor (Avpr1a) play a critical role in the regulation of a range of social behaviors by their actions in the social behavior neural network. To further investigate the role of AVPRs in social behavior, we performed CRISPR-Cas9–mediated editing at the Avpr1a gene via pronuclear microinjections in Syrian hamsters (Mesocricetus auratus), a species used extensively in behavioral neuroendocrinology because they produce a rich suite of social behaviors. Using this germ-line gene-editing approach, we generated a stable line of hamsters with a frame-shift mutation in the Avpr1a gene resulting in the null expression of functional Avpr1as. Avpr1a knockout (KO) hamsters exhibited a complete lack of Avpr1a-specific autoradiographic binding throughout the brain, behavioral insensitivity to centrally administered AVP, and no pressor response to a peripherally injected Avpr1a-specific agonist, thus confirming the absence of functional Avpr1as in the brain and periphery. Contradictory to expectations, Avpr1a KO hamsters exhibited substantially higher levels of conspecific social communication (i.e., odor-stimulated flank marking) than their wild-type (WT) littermates. Furthermore, sex differences in aggression were absent, as both male and female KOs exhibited more aggression toward same-sex conspecifics than did their WT littermates. Taken together, these data emphasize the importance of comparative studies employing gene-editing approaches and suggest the startling possibility that Avpr1a-specific modulation of the social behavior neural network may be more inhibitory than permissive.

Rescue of Vasopressin Synthesis in Magnocellular Neurons of the Supraoptic Nucleus Normalises Acute Stress-Induced Adrenocorticotropin Secretion and Unmasks an Effect on Social Behaviour in Male Vasopressin-Deficient Brattleboro Rats

The relevance of vasopressin (AVP) of magnocellular origin to the regulation of the endocrine stress axis and related behaviour is still under discussion. We aimed to obtain deeper insight into this process. To rescue magnocellular AVP synthesis, a vasopressin-containing adeno-associated virus vector (AVP-AAV) was injected into the supraoptic nucleus (SON) of AVP-deficient Brattleboro rats (di/di). We compared +/+, di/di, and AVP-AAV treated di/di male rats. The AVP-AAV treatment rescued the AVP synthesis in the SON both morphologically and functionally. It also rescued the peak of adrenocorticotropin release triggered by immune and metabolic challenges without affecting corticosterone levels. The elevated corticotropin-releasing hormone receptor 1 mRNA levels in the anterior pituitary of di/di-rats were diminished by the AVP-AAV-treatment. The altered c-Fos synthesis in di/di-rats in response to a metabolic stressor was normalised by AVP-AAV in both the SON and medial amygdala (MeA), but not in the central and basolateral amygdala or lateral hypothalamus. In vitro electrophysiological recordings showed an AVP-induced inhibition of MeA neurons that was prevented by picrotoxin administration, supporting the possible regulatory role of AVP originating in the SON. A memory deficit in the novel object recognition test seen in di/di animals remained unaffected by AVP-AAV treatment. Interestingly, although di/di rats show intact social investigation and aggression, the SON AVP-AAV treatment resulted in an alteration of these social behaviours. AVP released from the magnocellular SON neurons may stimulate adrenocorticotropin secretion in response to defined stressors and might participate in the fine-tuning of social behaviour with a possible contribution from the MeA.

Systemic oxytocin increases glutamate efflux in the nucleus accumbens core of cocaine-experienced male and female rats but only increases dopamine efflux in males

Oxytocin attenuates cocaine-seeking when administered both systemically and directly into the nucleus accumbens core. This effect is blocked by intra-accumbens antagonism of mGlu2/3 and, together with our finding that intra-accumbens oxytocin increases glutamate concentrations in this brain region, indicates that pre-synaptic regulation of glutamate release by oxytocin influences cocaine relapse. However, mGlu2/3 receptors also regulate dopamine release in the nucleus accumbens. Here we aimed to determine whether systemic oxytocin increases glutamate and dopamine concentrations in the nucleus accumbens core of cocaine-experienced and cocaine-naïve male and female rats. A subset of rats self-administered cocaine (0.5 mg/kg/infusion) and then underwent extinction training for 2-3 weeks. Rats were implanted with microdialysis probes in the accumbens core and samples were collected for a baseline period, and following saline (1 mL/kg), and oxytocin (1 mg/kg, IP) injections. Locomotion was assessed during microdialysis. In cocaine-experienced rats, oxytocin increased glutamate concentrations in the accumbens core to the same extent in males and females but only increased dopamine concentrations in male rats. Oxytocin did not alter glutamate levels in cocaine-naïve rats. Oxytocin did not produce sedation. These results extend previous findings that systemic oxytocin increases nucleus accumbens dopamine in a sex-specific manner in cocaine-experienced rats. These data are the first to find that systemic oxytocin increases nucleus accumbens glutamate after cocaine experience, providing a mechanism of action by which oxytocin attenuates the reinstatement of cocaine seeking in both male and female rats.

Differential fate between oxytocin and vasopressin cells in the developing mouse brain

Oxytocin (OXT) and arginine vasopressin (AVP), two neuropeptides involved in socio-emotional behaviors have been anatomically defined in the adult brain. Yet their spatial organization during postnatal development is not clearly defined. We built a developmental atlas using 3D imaging of cleared immunolabeled tissue over four early postnatal (P) stages, from birth (P0, P3, P7, P14) to young adulthood (≥P56). Our atlas-based mapping revealed that the number of OXT neurons doubles according to unique temporal dynamics in selective hypothalamic regions, namely, the periventricular and paraventricular nuclei, and in a novel location we named the antero-lateral preoptic. In the paraventricular nucleus, single-cell densities and fluorescence analysis demonstrated selective expansion of OXT cells in the antero-ventral division, whereas the postero-dorsal division contained cells present at birth. No changes were observed for AVP neurons. Our findings show the coexisting of innate and plastic OXT/AVP brain circuits probably triggered by environmental adaptation of the social brain.

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The OXT system continues to mature during early development but not the AVP system

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OXT hypothalamic nuclei emerge at different rates after birth

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PVN cells gradually acquire an oxytocinergic phenotype

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OXT cells are organized into innate and adaptive neural networks

Top-down regulation of motivated behaviors via lateral septum sub-circuits

How does cognition regulate innate behaviors? While the cognitive functions of the cortex have been extensively studied, we know much less about how cognition can regulate innate motivated behaviors to fulfill physiological, safety and social needs. Selection of appropriate motivated behaviors depends on external stimuli and past experiences that helps to scale priorities. With its abundant inputs from neocortical and allocortical regions, the lateral septum (LS) is ideally positioned to integrate perception and experience signals in order to regulate the activity of hypothalamic and midbrain nuclei that control motivated behaviors. In addition, LS receives numerous subcortical modulatory inputs, which represent the animal internal states and also participate in this regulation. In this perspective, we argue that LS sub-circuits regulate distinct motivated behaviors by integrating neural activity from neocortical, allocortical and neuromodulatory inputs. In addition, we propose that lateral inhibition between LS sub-circuits may allow the emergence of functional units that orchestrates competing motivated behaviors.

Neuroendocrine regulation of female aggression

Classically the neurobiology of aggression has been studied exclusively in males. Thus, females have been considered mildly aggressive except during lactation. Interestingly, recent studies in rodents and humans have revealed that non-lactating females can show exacerbated and pathological aggression similarly to males. This review provides an overview of recent findings on the neuroendocrine mechanisms regulating aggressive behavior in females. In particular, the focus will be on novel rodent models of exaggerated aggression established in non-lactating females. Among the neuromodulatory systems influencing female aggression, special attention has been given to sex-steroids and sex-steroid-sensitive neuronal populations (i.e., the core nuclei of the neural pathway of aggression) as well as to the neuropeptides oxytocin and vasopressin which are major players in the regulation of social behaviors.

Distribution of Vasopressin and Oxytocin Neurons in the Basal Forebrain and Midbrain of Spiny Mice (Acomys cahirinus)

The nonapeptides vasopressin (VP) and oxytocin (OT) are present in some form in most vertebrates. VP and OT play critical roles in modulating physiology and are well-studied for their influences on a variety of social behaviors, ranging from affiliation to aggression. Their anatomical distributions have been mapped for numerous species across taxa, demonstrating relatively strong evolutionary conservation in distributions throughout the basal forebrain and midbrain. Here we examined the distribution of VP-immunoreactive (-ir) and OT-ir neurons in a gregarious, cooperatively breeding rodent species, the spiny mouse (Acomys cahirinus), for which nonapeptide mapping does not yet exist. Immunohistochemical techniques revealed VP-ir and OT-ir neuronal populations throughout the hypothalamus and amygdala of males and females that are consistent with those of other rodents. However, a novel population of OT-ir neurons was observed in the median preoptic nucleus of both sexes, located dorsally to the anterior commissure. Furthermore, we found widespread sex differences in OT neuronal populations, with males having significantly more OT-ir neurons than females. However, we observed a sex difference in only one VP cell group - that of the bed nucleus of the stria terminalis (BST), a VP neuronal population that exhibits a phylogenetically widespread sexual dimorphism. These findings provide mapping distributions of VP and OT neurons in Acomys cahirinus. Spiny mice lend themselves to the study of mammalian cooperation and sociality, and the nonapeptide neuronal mapping presented here can serve as a basic foundation for the study of nonapeptide-mediated behavior in a group of highly social rodents.

Oxytocin and vasopressin within the ventral and dorsal lateral septum modulate aggression in female rats

In contrast to male rats, aggression in virgin female rats has been rarely studied. Here, we established a rat model of enhanced aggression in females using a combination of social isolation and aggression-training to specifically investigate the involvement of the oxytocin (OXT) and arginine vasopressin (AVP) systems within the lateral septum (LS). Using neuropharmacological, optogenetic, chemogenetic as well as microdialysis approaches, we revealed that enhanced OXT release within the ventral LS (vLS), combined with reduced AVP release within the dorsal LS (dLS), is required for aggression in female rats. Accordingly, increased activity of putative OXT receptor-positive neurons in the vLS, and decreased activity of putative AVP receptor-positive neurons in the dLS, are likely to underly aggression in female rats. Finally, in vitro activation of OXT receptors in the vLS increased tonic GABAergic inhibition of dLS neurons. Overall, our data suggest a model showing that septal release of OXT and AVP differentially affects aggression in females by modulating the inhibitory tone within LS sub-networks.

Long-term depression of excitatory transmission in the lateral septum

Neurons in the lateral septum (LS) integrate glutamatergic synaptic inputs, primarily from hippocampus, and send inhibitory projections to brain regions involved in reward and the generation of motivated behavior. Motivated learning and drugs of abuse have been shown to induce long-term changes in the strength of glutamatergic synapses in the LS, but the cellular mechanisms underlying long-term synaptic modification in the LS are poorly understood. Here, we examined synaptic transmission and long-term depression (LTD) in brain slices prepared from male and female C57BL/6 mice. No sex differences were observed in whole cell patch-clamp recordings of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPA-R)- and N-methyl-d-aspartate receptor (NMDA-R)-mediated currents. Low-frequency stimulation of the fimbria fiber bundle (1 Hz 15 min) induced LTD of the LS field excitatory postsynaptic potential (fEPSP). Induction of LTD was blocked by the NMDA-R antagonist (d)-2-amino-5-phosphonovaleric acid (APV), but not the selective antagonist of GluN2B-containing NMDA-Rs ifenprodil. These results demonstrate the NMDA-R dependence of LTD in the LS. The LS is a sexually dimorphic structure, and sex differences in glutamatergic transmission have been reported in vivo; our results suggest sex differences observed in vivo result from network activity rather than intrinsic differences in glutamatergic transmission.NEW & NOTEWORTHY The lateral septum (LS) integrates information from hippocampus and other regions to provide context-dependent (top down or higher order) regulation of mood and motivated behavior. Learning and drugs of abuse induce long-term changes in the strength of glutamatergic projections to the LS; however, the cellular mechanisms underlying such changes are poorly understood. Here, we demonstrate there are no apparent sex differences in fast excitatory transmission and that long-term synaptic depression in the LS is NMDA-R dependent.

Transport of oxytocin to the brain after peripheral administration by membrane-bound or soluble forms of receptors for advanced glycation end-products

Oxytocin (OT) is a neuropeptide hormone. Single and repetitive administration of OT increases social interaction and maternal behaviour in humans and mammals. Recently, it was found that the receptor for advanced glycation end-products (RAGE) is an OT-binding protein and plays a critical role in the uptake of OT to the brain after peripheral OT administration. Here, we address some unanswered questions on RAGE-dependent OT transport. First, we found that, after intranasal OT administration, the OT concentration increased in the extracellular space of the medial prefrontal cortex (mPFC) of wild-type male mice, as measured by push-pull microperfusion. No increase of OT in the mPFC was observed in RAGE knockout male mice. Second, in a reconstituted in vitro blood-brain barrier system, inclusion of the soluble form of RAGE (endogenous secretory RAGE [esRAGE]), an alternative splicing variant, in the luminal (blood) side had no effect on the transport of OT to the abluminal (brain) chamber. Third, OT concentrations in the cerebrospinal fluid after i.p. OT injection were slightly higher in male mice overexpressing esRAGE (esRAGE transgenic) compared to those in wild-type male mice, although this did not reach statistical significance. Although more extensive confirmation is necessary because of the small number of experiments in the present study, the reported data support the hypothesis that RAGE may be involved in the transport of OT to the mPFC from the circulation. These results suggest that the soluble form of RAGE in the plasma does not function as a decoy in vitro.

Central and peripheral release of oxytocin: Relevance of neuroendocrine and neurotransmitter actions for physiology and behavior

The hypothalamic neuropeptide oxytocin (OT) is critically involved in the modulation of socio-emotional behavior, sexual competence, and pain perception and anticipation. While intracellular signaling of OT and its receptor (OTR), as well as the functional connectivity of hypothalamic and extra-hypothalamic OT projections, have been recently explored, it remains elusive how one single molecule has pleotropic effects from cell proliferation all the way to modulation of complex cognitive processes. Moreover, there are astonishing species-dependent differences in the way OT regulates various sensory modalities such as touch, olfaction, and vision, which can be explained by differences in OTR expression in brain regions processing sensory information. Recent research highlights a small subpopulation of OT-synthesizing cells, namely, parvocellular cells, which merely constitute 1% of the total number of OT cells but act as "master cells' that regulate the activity of the entire OT system. In this chapter, we summarize the latest advances in the field of OT research with a particular focus on differences between rodents, monkeys and humans and highlight the main differences between OT and its "sister" peptide arginine-vasopressin, which often exerts opposite effects on physiology and behavior.

Medial Amygdala Arginine Vasopressin Neurons Regulate Innate Aversion to Cat Odors in Male Mice

Aversion to environmental cues of predators is an integral part of defensive behaviors in many prey animals. It enhances their survival and probability of future reproduction. At the same time, animals cannot be maximally defended because imperatives of defense usually trade-off with behaviors required for sexual reproduction like display of dominance and production of sexual pheromones. Here, we approach this trade-off through the lens of arginine vasopressin (AVP) neurons within the posterodorsal medial amygdala (MePD) of mice. This neuronal population is known to be involved in sexual behaviors like approach to sexually salient cues. We show that chemogenetic partial ablation of this neuronal population increases aversion to predator odors. Moreover, overexpression of AVP within this population is sufficient to reduce aversion to predator odors. The loss of fear of the predator odor occurs in parallel with increased recruitment of AVP neurons within the MePD. These observations suggest that AVP neurons in the medial aspect of the extended amygdala are a proximate locus for the reduction in innate fear during life stages dominated by reproductive efforts.

Chemoarchitecture of the bed nucleus of the stria terminalis: Neurophenotypic diversity and function

The bed nucleus of the stria terminalis (BNST) is a compact but neurophenotypically complex structure in the ventral forebrain that is structurally and functionally linked to other limbic structures, including the amygdala nuclear complex, hypothalamic nuclei, hippocampus, and related midbrain structures, to participate in a wide range of functions, especially emotion, emotional learning, stress-related responses, and sexual behaviors. From a variety of sensory inputs, the BNST acts as a node for signal integration and coordination for information relay to downstream central neuroendocrine and autonomic centers for appropriate homeostatic physiological and behavioral responses. In contrast to the role of the amygdala in fear, the BNST has gained wide interest from work suggesting that it has main roles in mediating sustained responses to diffuse, unpredictable and/or long-duration threats that are typically associated with anxiety-related responses. Further, some BNST subregions are highly sexually dimorphic which appear contributory to the differential stress and social interactive behaviors, including reproductive responses, between males and females. Notably, maladaptive BNST neuroplasticity and function have been implicated in chronic pain, depression, anxiety-related abnormalities, and other psychopathologies including posttraumatic stress disorders. The BNST circuits are predominantly GABAergic-the glutaminergic neurons represent a minor population-but the complexity of the system results from an overlay of diverse neuropeptide coexpression in these neurons. More than a dozen neuropeptides may be differentially coexpressed in BNST neurons, and from variable G protein-coupled receptor signaling, may inhibit or activate downstream circuit activities. The mechanisms and roles of these peptides in modulating intrinsic BNST neurocircuit signaling and BNST long-distance target cell projections are still not well understood. Nevertheless, an understanding of some of the principal players may allow assembly of the circuit interactions.

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.

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.

Is Oxytocin "Nature's Medicine"?

Oxytocin is a pleiotropic, peptide hormone with broad implications for general health, adaptation, development, reproduction, and social behavior. Endogenous oxytocin and stimulation of the oxytocin receptor support patterns of growth, resilience, and healing. Oxytocin can function as a stress-coping molecule, an anti-inflammatory, and an antioxidant, with protective effects especially in the face of adversity or trauma. Oxytocin influences the autonomic nervous system and the immune system. These properties of oxytocin may help explain the benefits of positive social experiences and have drawn attention to this molecule as a possible therapeutic in a host of disorders. However, as detailed here, the unique chemical properties of oxytocin, including active disulfide bonds, and its capacity to shift chemical forms and bind to other molecules make this molecule difficult to work with and to measure. The effects of oxytocin also are context-dependent, sexually dimorphic, and altered by experience. In part, this is because many of the actions of oxytocin rely on its capacity to interact with the more ancient peptide molecule, vasopressin, and the vasopressin receptors. In addition, oxytocin receptor(s) are epigenetically tuned by experience, especially in early life. Stimulation of G-protein-coupled receptors triggers subcellular cascades allowing these neuropeptides to have multiple functions. The adaptive properties of oxytocin make this ancient molecule of special importance to human evolution as well as modern medicine and health; these same characteristics also present challenges to the use of oxytocin-like molecules as drugs that are only now being recognized. SIGNIFICANCE STATEMENT: Oxytocin is an ancient molecule with a major role in mammalian behavior and health. Although oxytocin has the capacity to act as a "natural medicine" protecting against stress and illness, the unique characteristics of the oxytocin molecule and its receptors and its relationship to a related hormone, vasopressin, have created challenges for its use as a therapeutic drug.

Rescue of oxytocin response and social behaviour in a mouse model of autism

One of the most fundamental challenges in developing treatments for autism-spectrum disorders is the heterogeneity of the condition. More than one hundred genetic mutations confer high risk for autism, with each individual mutation accounting for only a small fraction of autism cases^1–3. Subsets of risk genes can be grouped into functionally-related pathways, most prominently synaptic proteins, translational regulation, and chromatin modifications. To possibly circumvent this genetic complexity, recent therapeutic strategies have focused on the neuropeptides oxytocin and vasopressin^4–6 which regulate aspects of social behavior in mammals⁷. However, whether genetic risk factors might predispose to autism due to modification of oxytocinergic signaling remains largely unknown. Here, we report that an autism-associated mutation in the synaptic adhesion molecule neuroligin-3 (Nlgn3) results in impaired oxytocin signaling in dopaminergic neurons and in altered social novelty responses in mice. Surprisingly, loss of Nlgn3 is accompanied by a disruption of translation homeostasis in the ventral tegmental area. Treatment of Nlgn3^KO mice with a novel, highly specific, brain-penetrant inhibitor of MAP-kinase interacting kinases resets mRNA translation and restores oxytocin and social novelty responses. Thus, this work identifies an unexpected convergence between the genetic autism risk factor Nlgn3, translational regulation, and oxytocinergic signaling. Focus on such common core plasticity elements might provide a pragmatic approach to reduce the heterogeneity of autism. Ultimately, this would allow for mechanism-based stratification of patient populations to increase the success of therapeutic interventions.

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.

Oxytocin has sex-specific effects on social behaviour and hypothalamic oxytocin immunoreactive cells but not hippocampal neurogenesis in adult rats

Oxytocin regulates social behaviours, pair bonding and hippocampal neurogenesis but most studies have used adult males. Our study investigated the effects of oxytocin on social investigation and adult hippocampal neurogenesis in male and female rats. Oxytocin has poor penetration of the blood-brain barrier, therefore we tested a nanoparticle drug, TRIOZAN™ (Ovensa Inc.), which permits greater blood-brain-barrier penetration. Adult male and female rats were injected daily (i.p.) for 10 days with either: oxytocin in PBS (0.5 or 1.0 mg/kg), oxytocin in TRIOZAN™ (0.5 or 1.0 mg/kg), or vehicle (PBS) and tested for social investigation. Oxytocin decreased body mass and increased social investigation and number of oxytocin-immunoreactive cells in the supraoptic nucleus (SON) of the hypothalamus in male rats only. In both sexes, oxytocin decreased the number of immature neurons (doublecortin+ cells) in the ventral hippocampus and reduced plasma 17β-estradiol levels in a dose- and delivery-dependent way. Oxytocin in TRIOZAN™ reduced "sedation" observed post-injection and increased certain central effects (oxytocin levels in the hypothalamus and neurogenesis in the ventral hippocampus) relative to oxytocin in PBS, indicating that the nanoparticle may be used as an alternative brain delivery system. We showed that oxytocin has sex-specific effects on social investigation, body mass, "sedation", and the oxytocin system. In contrast, similar effects were observed in both sexes in neurogenesis and plasma 17β-estradiol. Our work suggests that sex differences in oxytocin regulation of brain endpoints is region-specific (hypothalamus versus hippocampus) and that oxytocin does not promote social investigation in females.

Chemogenetic manipulation of the bed nucleus of the stria terminalis counteracts social behavioral deficits induced by early life stress in C57BL/6J mice

Trauma during critical periods of development can induce long-lasting adverse effects. To study neural aberrations resulting from early life stress (ELS), many studies utilize rodent maternal separation, whereby pups are intermittently deprived of maternal care necessary for proper development. This can produce adulthood behavioral deficits related to anxiety, reward, and social behavior. The bed nucleus of the stria terminalis (BNST) encodes aspects of anxiety-like and social behaviors, and also undergoes developmental maturation during the early postnatal period, rendering it vulnerable to effects of ELS. Mice underwent maternal separation (separation 4 hr/day during postnatal day (PD)2-5 and 8 hr/day on PD6-16) with early weaning on PD17, which induced behavioral deficits in adulthood performance on two-part social interaction task designed to test social motivation (choice between a same-sex novel conspecific or an empty cup) and social novelty preference (choice between the original-novel conspecific vs. a new-novel conspecific). We used chemogenetics to non-selectively silence or activate neurons in the BNST to examine its role in social motivation and social novelty preference, in mice with or without the history of ELS. Manipulation of BNST produced differing social behavior effects in non-stressed versus ELS mice; social motivation was decreased in non-stressed mice following BNST activation, but unchanged following BNST silencing, while ELS mice showed no change in social behavior after BNST activation, but exhibited enhancement of social motivation-for which they were deficient prior-following BNST silencing. Findings emphasize the BNST as a potential therapeutic target for social anxiety disorders instigated by childhood trauma.

Effect of Yokukansan and Yokukansankachimpihange on Aggressive Behavior, 5-HT Receptors and Arginine Vasopressin Expression in Social Isolation-Reared Mice

The traditional herbal medicines yokukansan (YKS) and yokukansankachimpihange (YKSCH) are prescribed for neurosis, insomnia or night crying and irritability in children. YKSCH comprises YKS and two additional herbs, a chimpi and a hange, and is used to treat digestive function deficiencies. However, the differences between the effects of YKS and YKSCH on brain function are unclear. The present study examined the effects of YKS and YKSCH on aggressive behavior in mice reared under a social isolation (SI) condition. Mice were housed individually for 6 weeks. YKS and YKSCH were administered orally for 2 weeks before aggression tests. SI increased aggressive behavior against naïve mice, and YKS, but not YKSCH, significantly attenuated this aggressive behavior. Because serotonin (5-HT)_2A and 5-HT_3A receptor antagonists are reported to have anti-aggressive effects, the mRNA levels of these receptors were examined. YKS attenuated the SI-induced increase in 5-HT_2A and 5-HT_3A receptor mRNA in the amygdala. On the other hand, YKSCH attenuated the SI-induced increase in 5-HT_1A receptor mRNA. YKS and YKSCH did not affect 5-HT and its metabolite 5-hydroxyindoleacetic acid content in the amygdala. However, YKSCH increased the mRNA level of arginine vasopressin (AVP), which is a neuropeptide that has been implicated in aggression, in the amygdala. These results suggest that YKS ameliorates aggressive behavior by decreasing 5-HT_2A and 5-HT_3A receptor expression. The YKSCH-induced increase in AVP may disrupt the anti-aggressive effect of YKS. YKS may be more effective than YKSCH for treating irritability if digestive function deficiencies are not considered.

The involvement of oxytocin in the effects of chronic social defeat stress on emotional behaviours in adult female mandarin voles

Chronic social defeat stress (CSDS) can induce anxiety and depression in male rodents, but the prevalence of anxiety and depression is much higher in females, and effects of CSDS on adult females and its underlying mechanism remain unclear. Oxytocin is a stress-buffering hormone in the brain that modulates the physiological effects of stress. Strikingly, research regarding the effect of oxytocin on emotional changes caused by CSDS is still lacking in females. Thus, we focused on the involvement of the oxytocin system in changes in emotional regulation induced by CSDS in female voles. Seventy-day-old female mandarin voles (Microtus mandarinus) were exposed to aggressive adult females for 14 days, and the effects of CSDS on emotion and regulation of oxytocin system were characterized. In addition, we injected vehicle, oxytocin and oxytocin receptor antagonist into the nucleus accumbens (Nacc) of female voles to investigate the involvement of Nacc oxytocin in the effect of CSDS on emotion. Herein, we reported that CSDS increased anxiety and depression-like behaviour and the circulating level of corticosterone, but decreased the number of oxytocin projections and the protein and mRNA expression levels of oxytocin receptor in the Nacc. Injection of oxytocin into the Nacc reversed the effects of CSDS on anxiety-like and depressive-like behaviour, whereas combined injections of oxytocin and oxytocin receptor antagonist eliminated these effects. In conclusion, CSDS increases the levels of anxiety and depression possibly via a reduction in oxytocin projections and the oxytocin receptor level in the Nacc. Nacc oxytocin may be involved in the effects of CSDS on emotional behaviours.

Subtle sex differences in vasopressin mRNA expression in the embryonic mouse brain

Arginine vasopressin (AVP) is a neuropeptide which acts centrally to modulate numerous social behaviors. One receptor subtype through which these effects occur is the AVP 1a receptor (AVPR1A). The modulatory effects of Avp via the AVPR1A varies by species as well as sex, since both AVP and the AVPR1A tend to be expressed more prominently in males. Beyond these neuromodulatory effects there are also indications that the AVP system may play a role in early development to, in part, organize sex-specific neural circuitry that is important to sexually dimorphic social behaviors in adulthood. However, to date, AVP's role in early development is poorly understood, particularly with respect to its differential effect on males and females. In order to determine the timing and distribution of the AVP system in early brain development, we examined the brains of male and female C57BL/6J mice between embryonic day (E) 12.5 and postnatal day (P) 2 and quantified Avp and Avpr1a mRNA using qPCR and AVPR1A protein using receptor autoradiography. The mRNA for Avp was measurable in males and females starting at E14.5, with males producing more than females, while Avpr1a mRNA was found as early as E12.5, with no difference in expression between sexes. AVPR1A binding was observed in both sexes starting at E16.5, and while there were no observed sex differences, binding density and the number of neuroanatomical areas did increase over time. These data are significant as they provide the first whole-brain characterization of the vasopressin system in the embryonic mouse. Further, these findings are consistent with data from other species, that have documented a sex difference in the vasopressin system during early brain formation.

One cranium, two brains not yet introduced: Distinct but complementary views of the social brain

Social behavior is pervasive across the animal kingdom, and elucidating how the brain enables animals to respond to social contexts is of great interest and profound importance. Our understanding of 'the social brain' has been fractured as it has matured. Two drastically different conceptualizations of the social brain have emerged with relatively little awareness of each other. In this review, we briefly recount the history behind the two dominant definitions of a social brain. The divide that has emerged between these visions can, in part, be attributed to differential attention to cortical or sub-cortical regions in the brain, and differences in methodology, comparative perspectives, and emphasis on functional specificity or generality. We discuss how these factors contribute to a lack of communication between research efforts, and propose ways in which each version of the social brain can benefit from the perspectives, tools, and approaches of the other. Interface between the two characterizations of social brain networks is sure to provide essential insight into what the social brain encompasses.

Involvement of ventral pallidal vasopressin in the sex-specific regulation of sociosexual motivation in rats

The ventral pallidum (VP) is a critical node of the mesocorticolimbic reward circuit and is known to modulate social behaviors in rodents. Arginine vasopressin (AVP) signaling via the V1A receptor (V1AR) within the VP is necessary for the expression of socially motivated affiliative behaviors in monogamous voles. However, whether the VP-AVP system regulates socially motivated behaviors in non-monogamous species remains unknown. Here, we determined the extent of AVP fiber innervation in the VP as well as the involvement of the VP-AVP system in sociosexual motivation in adult male and female rats. We found that males have nearly twice the density of AVP-immunoreactive (AVP-ir) fibers in the VP compared to females, suggesting the possibility that males experience enhanced AVP signaling in the VP. We further found that this sex difference in VP-AVP-ir fiber density likely arises from an observed sex difference (males > females) in the percentage of VP-projecting AVP-ir cell bodies located in the bed nucleus of the stria terminalis and medial amygdala. To determine the behavioral implications of this sex difference, we next blocked AVP signaling in the VP by antagonizing VP-V1ARs in male and female rats and tested their preference to investigate an unfamiliar male rat or unfamiliar estrus female rat confined to corrals located on opposite ends of a three-chamber apparatus. Under vehicle conditions, males showed a significantly greater innate preference to investigate an opposite sex over same sex conspecific than estrus females. Interestingly, VP-V1AR antagonism significantly reduced males' opposite sex preference, while enhancing estrus females' opposite sex preference. Importantly, all subjects reliably discriminated between male and female stimulus rats regardless of drug treatment, demonstrating a change in motivational state rather than a perceptual impairment induced by VP-V1AR blockade. These results provide a novel functional link between a sex difference in ventral pallidal AVP fiber density and the sex-specific regulation of a sexually motivated behavior necessary for reproductive success.

From adolescence to late aging: A comprehensive review of social behavior, alcohol, and neuroinflammation across the lifespan

The passage of time dictates the pace at which humans and other organisms age but falls short of providing a complete portrait of how environmental, lifestyle and underlying biological processes contribute to senescence. Two fundamental features of the human experience that change dramatically across the lifespan include social interactions and, for many, patterns of alcohol consumption. Rodent models show great utility for understanding complex interactions among aging, social behavior and alcohol use and abuse, yet little is known about the neural changes in late aging that contribute to the natural decline in social behavior. Here, we posit that aging-related neuroinflammation contributes to the insipid loss of social motivation across the lifespan, an effect that is exacerbated by patterns of repeated alcohol consumption observed in many individuals. We provide a comprehensive review of (i) neural substrates crucial for the expression of social behavior under non-pathological conditions; (ii) unique developmental/lifespan vulnerabilities that may contribute to the divergent effects of low-and high-dose alcohol exposure; and (iii) aging-associated changes in neuroinflammation that may sit at the intersection between social processes and alcohol exposure. In doing so, we provide an overview of correspondence between lifespan/developmental periods between common rodent models and humans, give careful consideration to model systems used to aptly probe social behavior, identify points of coherence between human and animal models, and point toward a multitude of unresolved issues that should be addressed in future studies. Together, the combination of low-dose and high-dose alcohol effects serve to disrupt the normal development and maintenance of social relationships, which are critical for both healthy aging and quality of life across the lifespan. Thus, a more complete understanding of neural systems-including neuroinflammatory processes-which contribute to alcohol-induced changes in social behavior will provide novel opportunities and targets for promoting healthy aging.

An insular view of the social decision-making network

Social animals must detect, evaluate and respond to the emotional states of other individuals in their group. A constellation of gestures, vocalizations, and chemosignals enable animals to convey affect and arousal to others in nuanced, multisensory ways. Observers integrate social information with environmental and internal factors to select behavioral responses to others via a process call social decision-making. The Social Decision Making Network (SDMN) is a system of brain structures and neurochemicals that are conserved across species (mammals, reptiles, amphibians, birds) that are the proximal mediators of most social behaviors. However, how sensory information reaches the SDMN to shape behavioral responses during a social encounter is not well known. Here we review the empirical data that demonstrate the necessity of sensory systems in detecting social stimuli, as well as the anatomical connectivity of sensory systems with each node of the SDMN. We conclude that the insular cortex is positioned to link integrated social sensory cues to this network to produce flexible and appropriate behavioral responses to socioemotional cues.

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.

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.

Interactions of sex and early life social experiences at two developmental stages shape nonapeptide receptor profiles

Early life social experiences are critical to behavioral and cognitive development, and can have a tremendous influence on developing social phenotypes. Most work has focused on outcomes of experiences at a single stage of development (e.g. perinatal or post-weaning). Few studies have assessed the impact of social experience at multiple developmental stages and across sex. Oxytocin and vasopressin are profoundly important for modulating social behavior and these nonapeptide systems are highly sensitive to developmental social experience, particularly in brain areas important for social behavior. We investigated whether oxytocin receptor (OTR) and vasopressin receptor (V1aR) distributions of prairie voles (Microtus ochrogaster) change as a function of parental composition within the natal nest or social composition after weaning. We raised pups either in the presence or absence of their fathers. At weaning, offspring were housed either individually or with a same-sex sibling. We also examined whether changes in receptor distributions are sexually dimorphic because the impact of the developmental environment on the nonapeptide system could be sex-dependent. We found that differences in nonapeptide receptor expression were region-specific, sex-specific and rearing condition-specific, indicating a high level of complexity in the ways that early life experiences shape the social brain. We found many more differences in V1aR density compared to OTR density, indicating that nonapeptide receptors demonstrate differential levels of neural plasticity and sensitivity to environmental and biological variables. Our data highlight that critical factors including biological sex and multiple experiences across the developmental continuum interact in complex ways to shape the social brain.

Post-weaning social isolation exacerbates aggression in both sexes and affects the vasopressin and oxytocin system in a sex-specific manner

Post-weaning social isolation (PWSI) is known to induce exaggerated and abnormal aggression in male rats. Here we aimed to assess the effects of PWSI on aggressiveness and social behavior in both male and female rats. Furthermore, we evaluated how PWSI affects the central oxytocin (OXT) and vasopressin (AVP) systems in both sexes. Wistar rats were isolated (IS) or group housed (GH) in same-sex groups immediately after weaning. After seven weeks, rats underwent an intruder test to assess aggression. In one group, brains were immediately dissected afterwards for in situ hybridization and receptor autoradiography. The other group underwent additional anxiety-like and social behavior tests. PWSI induced increased (abnormal) aggression and impaired social memory in both sexes. Especially IS females exhibited abnormal aggression towards juveniles. Furthermore, PWSI increased OXT mRNA expression in the paraventricular nucleus of the hypothalamus (PVN) and decreased OXTR binding in the anterior portion of the nucleus accumbens (NAcc), independent of the sex. V1a receptor binding was decreased in the lateral hypothalamus (LH) and dentate gyrus (DG) in IS rats, regardless of sex. However, V1a receptor binding in the anterior portion of the bed nucleus of stria terminalis (BNSTa) was decreased in IS females but increased in IS males. Taken together, our data support PWSI as a reliable model to exacerbate aggression not only in male but also in female rats. In addition, OXT receptors in the NAcca and V1a receptors in the LH, DG, and BNSTa may play a role in the link between PWSI and aggression. This article is part of the Special Issue entitled 'Current status of the neurobiology of aggression and impulsivity'.