Role of the oxytocin system in amygdala subregions in the regulation of social interest in male and female rats

Mating-induced release of oxytocin in the mouse lateral septum: Implications for social fear extinction

In mammals, some physiological conditions are associated with the high brain oxytocin (OXT) system activity. These include lactation in females and mating in males and females, both of which have been linked to reduced stress responsiveness and anxiolysis. Also, in a murine model of social fear conditioning (SFC), enhanced brain OXT signaling in lactating mice, specifically in the lateral septum (LS), was reported to underlie reduced social fear expression. Here, we studied the effects of mating in male mice on anxiety-related behaviour, social (and cued) fear expression and its extinction, and the activity of OXT neurons reflected by cFos expression and OXT release in the LS and amygdala. We further focused on the involvement of brain OXT in the mating-induced facilitation of social fear extinction. We could confirm the anxiolytic effect of mating in male mice irrespective of the occurrence of ejaculation. Further, we found that only successful mating resulting in ejaculation (Ej+) facilitated social fear extinction, whereas mating without ejaculation (Ej-) did not. In contrast, mating did not affect cues fear expression. Using the cellular activity markers cFos and pErk, we further identified the ventral LS (vLS) as a potential region participating in the effect of ejaculation on social fear extinction. In support, microdialysis experiments revealed a rise in OXT release within the LS, but not the amygdala, during mating. Finally, infusion of an OXT receptor antagonist into the LS before mating or into the lateral ventricle (icv) after mating demonstrated a significant role of brain OXT receptor-mediated signaling in the mating-induced facilitation of social fear extinction.

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.

Oxytocin Attenuates Yohimbine-Induced Reinstatement of Alcohol-Seeking in Female Rats via the Central Amygdala

Alcohol use disorder is a significant public health concern, further exacerbated by an increased risk of relapse due to stress. In addition, factors such as biological sex may contribute to the progression of addiction, as females are especially susceptible to stress-induced relapse. While there have been many studies surrounding potential pharmacological interventions for male stress-induced ethanol reinstatement, research regarding females is scarce. Recently, the neuropeptide oxytocin has gained interest as a possible pharmacological intervention for relapse. The present study examines how oxytocin affects yohimbine-induced reinstatement of ethanol-seeking in female rats using a self-administration paradigm. Adult female rats were trained to press a lever to access ethanol in daily self-administration sessions. Rats then underwent extinction training before a yohimbine-induced reinstatement test. Rats administered with yohimbine demonstrated significantly higher lever response indicating a reinstatement of ethanol-seeking behavior. Oxytocin administration, both systemically and directly into the central amygdala, attenuated the effect of yohimbine-induced reinstatement of ethanol-seeking behavior. The findings from this study establish that oxytocin is effective at attenuating alcohol-relapse behavior mediated by the pharmacological stressor yohimbine and that this effect is modulated by the central amygdala in females. This provides valuable insight regarding oxytocin's potential therapeutic effect in female stress-induced alcohol relapse.

Intraamygdaloid Oxytocin Increases Time Spent on Social Interaction in Valproate-Induced Autism Animal Model

Autism spectrum disorder (ASD) is a pervasive neurodevelopmental disorder that affects about 1.5% of children worldwide. One of the core symptoms is impaired social interaction. Since proper treatment has not been found yet, an investigation of the exact pathophysiology of autism is essential. The valproate (VPA)-induced rat model can be an appropriate way to study autism. Oxytocin (OT) may amend some symptoms of ASD since it plays a key role in developing social relationships. In the present study, we investigated the effect of the intraamygdaloid OT on sham and intrauterine VPA-treated rats' social interaction using Crawley's social interaction test. Bilateral guide cannulae were implanted above the central nucleus of the amygdala (CeA), and intraamygdaloid microinjections were carried out before the test. Our results show that male Wistar rats prenatally exposed to VPA spent significantly less time on social interaction. Bilateral OT microinjection increased the time spent in the social zone; it also reached the level of sham-control animals. OT receptor antagonist blocked this effect of the OT but in itself did not significantly influence the behavior of the rats. Based on our results, we can establish that intraamygdaloid OT has significantly increased time spent on social interaction in the VPA-induced autism model, and its effect is receptor-specific.

Development of social recognition ability in female rats: Effect of pubertal ovarian hormones

The ability to recognize previously encountered conspecifics is crucial for social interaction. This social recognition ability is well characterized in adult rodents of both sexes but remains largely unexplored in juveniles. Using the social discrimination test of social recognition with short intervals (30 min and 1 h), we first found that juvenile female rats do not display a difference in investigation directed toward a novel vs. familiar stimulus rat. Using the social discrimination test with a 30-minute interval, we then showed that social recognition is established by the time of adolescence in female rats. Based on these findings, we hypothesized that social recognition is dependent on the initiation of ovarian hormone release during puberty. To test this, we ovariectomized females prior to puberty and found that prepubertal ovariectomy prevented the development of social recognition ability in adulthood. Administration of estradiol benzoate, 48 h prior to testing, to juvenile females or prepubertally ovariectomized adult females did not restore social recognition, suggesting that ovarian hormones organize the neural circuitry regulating this behavior during adolescence. These findings provide the first evidence of an effect of pubertal development on social recognition ability in female rats and highlight the importance of considering sex and age when interpreting results from behavioral paradigms initially designed for use in adult males.

Limosilactobacillus reuteri administration alters the gut-brain-behavior axis in a sex-dependent manner in socially monogamous prairie voles

Research on the role of gut microbiota in behavior has grown dramatically. The probiotic L. reuteri can alter social and stress-related behaviors - yet, the underlying mechanisms remain largely unknown. Although traditional laboratory rodents provide a foundation for examining the role of L. reuteri on the gut-brain axis, they do not naturally display a wide variety of social behaviors. Using the highly-social, monogamous prairie vole (Microtus ochrogaster), we examined the effects of L. reuteri administration on behaviors, neurochemical marker expression, and gut-microbiome composition. Females, but not males, treated with live L. reuteri displayed lower levels of social affiliation compared to those treated with heat-killed L. reuteri. Overall, females displayed a lower level of anxiety-like behaviors than males. Live L. reuteri-treated females had lower expression of corticotrophin releasing factor (CRF) and CRF type-2-receptor in the nucleus accumbens, and lower vasopressin 1a-receptor in the paraventricular nucleus of the hypothalamus (PVN), but increased CRF in the PVN. There were both baseline sex differences and sex-by-treatment differences in gut microbiome composition. Live L. reuteri increased the abundance of several taxa, including Enterobacteriaceae, Lachnospiraceae NK4A136, and Treponema. Interestingly, heat-killed L. reuteri increased abundance of the beneficial taxa Bifidobacteriaceae and Blautia. There were significant correlations between changes in microbiota, brain neurochemical markers, and behaviors. Our data indicate that L. reuteri impacts gut microbiota, gut-brain axis and behaviors in a sex-specific manner in socially-monogamous prairie voles. This demonstrates the utility of the prairie vole model for further examining causal impacts of microbiome on brain and behavior.

Oxytocin, Erectile Function and Sexual Behavior: Last Discoveries and Possible Advances

A continuously increasing amount of research shows that oxytocin is involved in numerous central functions. Among the functions in which oxytocin is thought to be involved are those that play a role in social and sexual behaviors, and the involvement of central oxytocin in erectile function and sexual behavior was indeed one of the first to be discovered in laboratory animals in the 1980s. The first part of this review summarizes the results of studies done in laboratory animals that support a facilitatory role of oxytocin in male and female sexual behavior and reveal mechanisms through which this ancient neuropeptide participates in concert with other neurotransmitters and neuropeptides in this complex function, which is fundamental for the species reproduction. The second part summarizes the results of studies done mainly with intranasal oxytocin in men and women with the aim to translate the results found in laboratory animals to humans. Unexpectedly, the results of these studies do not appear to confirm the facilitatory role of oxytocin found in male and female sexual behavior in animals, both in men and women. Possible explanations for the failure of oxytocin to improve sexual behavior in men and women and strategies to attempt to overcome this impasse are considered.

Maturation of amygdala inputs regulate shifts in social and fear behaviors: A substrate for developmental effects of stress

Stress can negatively impact brain function and behaviors across the lifespan. However, stressors during adolescence have particularly harmful effects on brain maturation, and on fear and social behaviors that extend beyond adolescence. Throughout development, social behaviors are refined and the ability to suppress fear increases, both of which are dependent on amygdala activity. We review rodent literature focusing on developmental changes in social and fear behaviors, cortico-amygdala circuits underlying these changes, and how this circuitry is altered by stress. We first describe changes in fear and social behaviors from adolescence to adulthood and parallel developmental changes in cortico-amygdala circuitry. We propose a framework in which maturation of cortical inputs to the amygdala promote changes in social drive and fear regulation, and the particularly damaging effects of stress during adolescence may occur through lasting changes in this circuit. This framework may explain why anxiety and social pathologies commonly co-occur, adolescents are especially vulnerable to stressors impacting social and fear behaviors, and predisposed towards psychiatric disorders related to abnormal cortico-amygdala circuits.

Social touch promotes interfemale communication via activation of parvocellular oxytocin neurons

Oxytocin (OT) is a great facilitator of social life but, although its effects on socially relevant brain regions have been extensively studied, OT neuron activity during actual social interactions remains unexplored. Most OT neurons are magnocellular neurons, which simultaneously project to the pituitary and forebrain regions involved in social behaviors. In the present study, we show that a much smaller population of OT neurons, parvocellular neurons that do not project to the pituitary but synapse onto magnocellular neurons, is preferentially activated by somatosensory stimuli. This activation is transmitted to the larger population of magnocellular neurons, which consequently show coordinated increases in their activity during social interactions between virgin female rats. Selectively activating these parvocellular neurons promotes social motivation, whereas inhibiting them reduces social interactions. Thus, parvocellular OT neurons receive particular inputs to control social behavior by coordinating the responses of the much larger population of magnocellular OT neurons.

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.

Comparison of the distribution of oxytocin and vasopressin 1a receptors in rodents reveals conserved and derived patterns of nonapeptide evolution

Oxytocin (OT) and vasopressin (VP) are known modulators of social behaviour across rodents. Research has revealed the location of action of these nonapeptides through localization of their associated receptors, which include the oxytocin receptor (OTR) and the vasopressin 1a receptor (V1aR). As research into these complex systems has progressed, studies investigating how these systems modulate behaviour have remained relatively narrow in scope (ie, focused on how a single brain region shapes behaviour in only a handful of species). However, the brain regions that regulate social behaviour are part of interconnected neural networks for which coordinated activity enables behavioural variation. Thus, to better understand how nonapeptide systems have evolved under different selective pressures among rodent species, we conducted a meta-analysis using a multivariate comparative method to examine the patterns of OTR and V1aR density expression in this taxon. Several brain regions were highly correlated based on their OTR and V1aR binding patterns across species, supporting the notion that the distribution of these receptors is highly conserved in rodents. However, our results also revealed that specific patterns of V1aR density differed from OTR density, and within-genus variance for V1aR was low compared to between-genus variance, suggesting that these systems have responded and evolved quite differently to selective pressures over evolutionary time. We propose that, in addition to examining single brain regions of interest, taking a broad comparative approach when studying the OT and VP systems is important for understanding how the systemic action of nonapeptides modulate social behaviour across species.

Oxytocin attenuates stress-induced reinstatement of alcohol seeking behavior in male and female mice

RATIONALE

The neuropeptide oxytocin (OXT) has emerged as a potential therapeutic intervention in the treatment of both alcohol use disorder (AUD) and stress-related psychiatric illnesses.

OBJECTIVES

The present study evaluates the effects of systemically administered (intraperitoneal (i.p.)) OXT treatment on alcohol relapse-like behavior in male and female mice.

METHODS

Adult male and female C57BL/6J mice were trained to lever respond in operant conditioning chambers for alcohol in daily self-administration sessions. Once lever responding and alcohol intake stabilized, mice were tested under extinction conditions for 14 days before reinstatement testing. All mice underwent stress-induced reinstatement testing using either predator odor (2,3,5-trimethyl-3-thiazoline (TMT)) or the α-2 adrenergic receptor agonist yohimbine. In study 1, mice were exposed to TMT for 15 min and then immediately placed into operant conditioning chambers to examine alcohol-seeking behavior under extinction conditions. At 30 min prior to test session, separate groups of mice were injected with vehicle or OXT (0.1, 0.5, 1 mg/kg). In study 2, mice were injected with yohimbine (0.3, 0.625 mg/kg) 1 h prior to reinstatement testing. At 30 min post-yohimbine injection, mice are injected (i.p.) with vehicle or OXT (1 mg/kg).

RESULTS

OXT attenuated alcohol-seeking behavior in a dose-related manner in male and female mice in response to acute challenge with a predator odor. Additionally, OXT administration produced a similar decrease in alcohol relapse-like behavior triggered by the pharmacological stressor yohimbine in both sexes.

CONCLUSIONS

Systemic oxytocin administration attenuates stress-induced reinstatement of alcohol seeking in male and female mice.

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.

The osmoresponsiveness of oxytocin and vasopressin neurones: Mechanisms, allostasis and evolution

In the rat supraoptic nucleus, every oxytocin cell projects to the posterior pituitary, and is involved both in reflex milk ejection during lactation and in regulating uterine contractions during parturition. All are also osmosensitive, regulating natriuresis. All are also regulated by signals that control appetite, including the neural and hormonal signals that arise from the gut after food intake and from the sites of energy storage. All are also involved in sexual behaviour, anxiety-related behaviours and social behaviours. The challenge is to understand how a single population of neurones can coherently regulate such a diverse set of functions and adapt to changing physiological states. Their multiple functions arise from complex intrinsic properties that confer sensitivity to a wide range of internal and environmental signals. Many of these properties have a distant evolutionary origin in multifunctional, multisensory neurones of Urbilateria, the hypothesised common ancestor of vertebrates, insects and worms. Their properties allow different patterns of oxytocin release into the circulation from their axon terminals in the posterior pituitary into other brain areas from axonal projections, as well as independent release from their dendrites.

Adolescent intermittent ethanol exposure: Effects on pubertal development, novelty seeking, and social interaction in adulthood

Alcohol use initiated early in adolescence is a major predictor for the development of alcohol use disorders. This risk may be increased when drinking is initiated around the time of puberty, given evidence of bidirectional relationships between alcohol and gonadal hormones. The current study examined the effects of adolescent intermittent ethanol exposure (AIE) on pubertal timing and expression of novelty-seeking and peer-directed behaviors as well as neural correlates of these behaviors. AIE did not affect pubertal timing or the later expression of novelty-seeking and peer-directed behaviors. AIE increased corticosterone (CORT) levels in females not tested behaviorally in adulthood or tested in the novel-object exploration paradigm, whereas social interaction blunted CORT levels in AIE females. Delays in pubertal timing and decreases in CORT levels were correlated, however, with increased novelty seeking in adult males - a phenotype associated with increased addiction vulnerability. In females, social testing elevated oxytocin receptor (OXTR) mRNA expression in the central amygdala (CeA), with this social testing-associated elevation evident in the lateral septum (LS), regardless of sex. Vasopressin receptor 1a (AVP-1aR) mRNA expression in the CeA was enhanced by social testing in females, but not males, with expression of this gene suppressed by social testing in the LS in males, but not females. Together, these data demonstrate that behavioral and neural alterations that may serve as risk factors in later drug vulnerabilities are likely not the result of a single insult, but may reflect interactions among several variables including sex, pubertal timing, stress reactivity, and test circumstances.

Oxytocin and vasopressin modulation of social anxiety following adolescent intermittent ethanol exposure

RATIONALE

Adolescent intermittent ethanol exposure (AIE) produces lasting, sex-specific social anxiety-like alterations in male, but not female rats. Oxytocin (OXT) and vasopressin (AVP) brain systems play opposite roles in regulating social preference/avoidance, with OXT increasing approach to, and AVP increasing avoidance of social stimuli.

OBJECTIVES

To test the hypothesis that social anxiety-like alterations seen in adult males after AIE are associated with a shift in the balance between OXT and AVP toward AVP, effectiveness of pharmacological activation of the OXT system and blockade of endogenous activity at AVP receptors for reversing AIE-induced social anxiety-like alterations was assessed, along with examination of the effects of AIE on OXT, vasopressin V1a, and V1b receptor (OXT-R, V1a-R, and V1b-R) surface expression in the hypothalamus.

METHODS

Sprague-Dawley male and female rats were given 4 g/kg ethanol (AIE) or water intragastrically every 48 h for a total of 11 exposures during postnatal days (P) 25-45. On P70-72, animals were given a social interaction test following administration of a selective OXT-R agonist WAY-267464, selective V1a-R antagonist SR-49059, or V1b-R antagonist SSR-149415, and hypothalamic tissue was collected.

RESULTS

Social anxiety-like behavior was induced by AIE in males but not females, and was selectively reversed by the selective OXT-R agonist and V1b-R antagonist, but not V1a-R antagonist. AIE was also found to decrease OXT-R, but increase V1b-R neuronal surface expression relative to water-exposed controls in the hypothalamus of males, but not females.

CONCLUSIONS

These findings demonstrate that AIE induces changes in OXT-R and AVP-R surface expression in the hypothalamus along with social anxiety-like alterations in male rats. These social anxiety-like alterations can be reversed either by activation of the OXT system or by suppression of the AVP system, data that support the hypothesis that social anxiety-like alterations induced by adolescent alcohol exposure in male rats are associated at least in part with an OXT/AVP imbalance.

Prenatal alcohol exposure disrupts male adolescent social behavior and oxytocin receptor binding in rodents

Social behavior deficits resulting from prenatal alcohol exposure (PAE) emerge early in life and become more pronounced across development. Maturational changes associated with adolescence, including pubertal onset, can have significant consequences for social behavior development, making adolescence a unique period of increased vulnerability to social behavior dysfunction. Unfortunately, little is known about the underlying neurobiology supporting PAE-related social behavior impairments, particularly in the context of adolescence, when the transition to a more complex social environment may exacerbate existing deficits in social behavior function. Here we perform a comprehensive evaluation of social behavior development in PAE animals during two different periods in adolescence using three separate but related tests of social behavior in increasingly complex social contexts: the social interaction test, the social recognition memory test (i.e. habituation-dishabituation test), and the social discrimination test. Additionally, we investigated the underlying neurobiology of the oxytocin (OT) and vasopressin (AVP) systems following PAE, given their well-documented role in mediating social behavior. Our results demonstrate that compared to controls, early adolescent PAE animals showed impairments on the social recognition memory test and increased OT receptor binding in limbic networks, while late adolescent PAE animals exhibited impairments on the social discrimination test and increased OTR binding in forebrain reward systems. Taken together, these data indicate that PAE impairs adolescent social behavior - especially with increasing complexity of the social context - and that impairments are associated with altered development of the OT but not the AVP system.

Role of central oxytocin and dopamine systems in nociception and their possible interactions: suggested hypotheses

Central oxytocin and dopamine have an important role in the process of nociception at the spinal level as well as supraspinal structures, e.g. anterior cingulate cortex, insular cortex, amygdala, nucleus accumbens, and hypothalamus. Many studies have pointed out the importance of both systems in the pain descending modulatory system and in pain-related symptoms in some chronic disorders, e.g. Parkinson disease and fibromyalgia. The interaction between oxytocin and dopamine systems has been addressed in some motivational behaviors, e.g. maternal and sexual behaviors, pair bonding, and salience. In this aspect, we propose that an oxytocin-dopamine interaction could be present in nociception, and we also explain the possible hypotheses of such an interaction between these systems.

Stereological Analysis of Microglia in Aged Male and Female Fischer 344 Rats in Socially Relevant Brain Regions

Aging is associated with a substantial decline in the expression of social behavior as well as increased neuroinflammation. Since immune activation and subsequent increased expression of cytokines can suppress social behavior in young rodents, we examined age and sex differences in microglia within brain regions critical to social behavior regulation (PVN, BNST, and MEA) as well as in the hippocampus. Adult (3-month) and aged (18-month) male and female F344 (N = 26, n = 5-8/group) rats were perfused and Iba-1 immunopositive microglia were assessed using unbiased stereology and optical density. For stereology, microglia were classified based on the following criteria: (1) thin ramified processes, (2) thick long processes, (3) stout processes, or (4) round/ameboid shape. Among the structures examined, the highest density of microglia was evident in the BNST and MEA. Aged rats of both sexes displayed increased total number of microglia number exclusively in the MEA. Sex differences also emerged, whereby aged females (but not males) displayed greater total number of microglia in the BNST relative to their young adult counterparts. When morphological features of microglia were assessed, aged rats exhibited increased soma size in the BNST, MEA, and CA3. Together, these findings provide a comprehensive characterization of microglia number and morphology under ambient conditions in CNS sites critical for the normal expression of social processes. To the extent that microglia morphology is predictive of reactivity and subsequent cytokine release, these data suggest that the expression of social behavior in late aging may be adversely influenced by heightened inflammation.

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.

Sex-Specific Differences in Oxytocin Receptor Expression and Function for Parental Behavior

Parental care is among the most profound behavior expressed by humans and other animals. Despite intense interest in understanding the biological basis of parental behaviors, it remains unknown how much of parenting is encoded by the genome and which abilities instead are learned or can be refined by experience. One critical factor at the intersection between innate behaviors and experience-dependent learning is oxytocin, a neurohormone important for maternal physiology and neuroplasticity. Oxytocin acts throughout the body and brain to promote prosocial and maternal behaviors and modulates synaptic transmission to affect neural circuit dynamics. Recently we developed specific antibodies to mouse oxytocin receptors, found that oxytocin receptors are left lateralized in female auditory cortex, and examined how oxytocin enables maternal behavior by sensitizing the cortex to infant distress sounds. In this study we compare oxytocin receptor expression and function in male and female mice. Receptor expression is higher in adult female left auditory cortex than in right auditory cortex or males. Developmental profiles and mRNA expression were comparable between males and females. Behaviorally, male and female mice began expressing parental behavior similarly after cohousing with experienced females; however, oxytocin enhanced parental behavior onset in females but not males. This suggests that left lateralization of oxytocin receptor expression in females provides a mechanism for accelerating maternal behavior onset, although male mice can also effectively co-parent after experience with infants. The sex-specific pattern of oxytocin receptor expression might genetically predispose female cortex to respond to infant cues, which both males and females can also rapidly learn.

Sex differences in neural activation following different routes of oxytocin administration in awake adult rats

The neuropeptide oxytocin (OT) regulates social behavior in sex-specific ways across species. OT has promising effects on alleviating social deficits in sex-biased neuropsychiatric disorders. However little is known about potential sexually dimorphic effects of OT on brain function. Using the rat as a model organism, we determined whether OT administered centrally or peripherally induces sex differences in brain activation. Functional magnetic resonance imaging was used to examine blood oxygen level-dependent (BOLD) signal intensity changes in the brains of awake rats during the 20min following intracerebroventricular (ICV; 1μg/5μl) or intraperitoneal (IP; 0.1mg/kg) OT administration as compared to baseline. ICV OT induced sex differences in BOLD activation in 26 out of 172 brain regions analyzed, with 20 regions showing a greater volume of activation in males (most notably the nucleus accumbens and insular cortex), and 6 regions showing a greater volume of activation in females (including the lateral and central amygdala). IP OT also elicited sex differences in BOLD activation with a greater volume of activation in males, but this activation was found in different and fewer (10) brain regions compared to ICV OT. In conclusion, exogenous OT modulates neural activation differently in male versus female rats with the pattern and magnitude, but not the direction, of sex differences depending on the route of administration. These findings highlight the need to include both sexes in basic and clinical studies to fully understand the role of OT on brain function.

Oxytocin differentially alters resting state functional connectivity between amygdala subregions and emotional control networks: Inverse correlation with depressive traits

The hypothalamic neuropeptide oxytocin (OT) has received increasing attention for its role in modulating social-emotional processes across species. Previous studies on using intranasal-OT in humans point to a crucial engagement of the amygdala in the observed neuromodulatory effects of OT under task and rest conditions. However, the amygdala is not a single homogenous structure, but rather a set of structurally and functionally heterogeneous nuclei that show distinct patterns of connectivity with limbic and frontal emotion-processing regions. To determine potential differential effects of OT on functional connectivity of the amygdala subregions, 79 male participants underwent resting-state fMRI following randomized intranasal-OT or placebo administration. In line with previous studies OT increased the connectivity of the total amygdala with dorso-medial prefrontal regions engaged in emotion regulation. In addition, OT enhanced coupling of the total amygdala with cerebellar regions. Importantly, OT differentially altered the connectivity of amygdala subregions with distinct up-stream cortical nodes, particularly prefrontal/parietal, and cerebellar down-stream regions. OT-induced increased connectivity with cerebellar regions were largely driven by effects on the centromedial and basolateral subregions, whereas increased connectivity with prefrontal regions were largely mediated by right superficial and basolateral subregions. OT decreased connectivity of the centromedial subregions with core hubs of the emotional face processing network in temporal, occipital and parietal regions. Preliminary findings suggest that effects on the superficial amygdala-prefrontal pathway were inversely associated with levels of subclinical depression, possibly indicating that OT modulation may be blunted in the context of increased pathological load. Together, the present findings suggest a subregional-specific modulatory role of OT on amygdala-centered emotion processing networks in humans.

Challenges for the in vivo quantification of brain neuropeptides using microdialysis sampling and LC-MS

In recent years, neuropeptides and their receptors have received an increased interest in neuropharmacological research. Although these molecules are considered relatively small compared with proteins, their in vivo quantification using microdialysis is more challenging than for small molecules. Low microdialysis recoveries, aspecific adsorption and the presence of various multiply charged precursor ions during ESI-MS/MS detection hampers the in vivo quantification of these low abundant biomolecules. Every step in the workflow, from sampling until analysis, has to be optimized to enable the sensitive analysis of these compounds in microdialysates.