Vasopressin cell groups exhibit strongly divergent responses to copulation and male-male interactions in mice

A vasopressin circuit that modulates mouse social investigation and anxiety-like behavior in a sex-specific manner

One of the largest sex differences in brain neurochemistry is the expression of the neuropeptide arginine vasopressin (AVP) within the vertebrate brain, with males having more AVP cells in the bed nucleus of the stria terminalis (BNST) than females. Despite the long-standing implication of AVP in social and anxiety-like behaviors, the circuitry underlying AVP's control of these behaviors is still not well defined. Using optogenetic approaches, we show that inhibiting AVP BNST cells reduces social investigation in males, but not in females, whereas stimulating these cells increases social investigation in both sexes, but more so in males. These cells may facilitate male social investigation through their projections to the lateral septum (LS), an area with the highest density of sexually differentiated AVP innervation in the brain, as optogenetic stimulation of BNST AVP → LS increased social investigation and anxiety-like behavior in males but not in females; the same stimulation also caused a biphasic response of LS cells ex vivo. Blocking the vasopressin 1a receptor (V1aR) in the LS eliminated all these responses. Together, these findings establish a sexually differentiated role for BNST AVP cells in the control of social investigation and anxiety-like behavior, likely mediated by their projections to the LS.

Adolescent social isolation disrupts developmental tuning of neuropeptide circuits in the hypothalamus to amygdala regulating social and defensive behavior

Engaging in positive social (i.e., prosocial) interactions during adolescence acts to modulate neural circuits that determine adult adaptive behavior. While accumulating evidence indicates that a strong craving for prosocial behavior contributes to sustaining neural development, the consequences of social deprivation during adolescence on social neural circuits, including those involving oxytocin (OXT) and vasopressin (AVP), are poorly characterized. We evaluated adaptive behaviors in socially isolated mice, including anxiety-like, social, and defensive behaviors, along with OXT and AVP neural profiles in relevant brain regions. Social isolation from postnatal day (P-)22 to P-48 induced enhanced defensive and exploratory behaviors, in nonsocial and social contexts. Unlike OXT neurons, AVP+ cell density in the paraventricular nucleus of the hypothalamus increases with age in males. Social isolation also modulated gene expression in the medial amygdala (MeA), including the upregulation of OXT receptors in males and the downregulation of AVP1a receptors in both sexes. Socially isolated mice showed an enhanced defensive, anogenital approach toward a novel adult female during direct social interactions. Subsequent c-Fos mapping revealed diminished neural activity in restricted brain areas, including the MeA, lateral septum, and posterior intralaminar nucleus of the thalamus, in socially isolated mice. These data indicate that neural signals arising from daily social interactions invoke region-specific modification of neuropeptide expression that coordinates with altered defensiveness and neural responsivities, including OXT- and AVP-projecting regions. The present findings indicate an involvement of OXT and AVP circuits in adolescent neural and behavioral plasticity that is tuned by daily social interaction.

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.

Vasotocin but not isotocin is involved in the emergence of the dominant-subordinate status in males of the weakly electric fish, Gymnotus omarorum

The establishment of the dominant-subordinate status implies a clear behavioral asymmetry between contenders that arises immediately after the resolution of the agonistic encounter and persists during the maintenance of stable dominance hierarchies. Changes in the activity of the brain social behavior network (SBN) are postulated to be responsible for the establishment and maintenance of the dominant-subordinate status. The hypothalamic nonapeptides of the vasopressin (AVP) and oxytocin (OT) families are known to modulate the activity of the SBN in a context-dependent manner across vertebrates, including status-dependent modulations. We searched for status-dependent asymmetries in AVP-like (vasotocin, AVT) and OT-like (isotocin, IT) cell number and activation immediately after the establishment of dominance in males of the weakly electric fish, Gymnotus omarorum, which displays the best understood example of non-breeding territorial aggression among teleosts. We used immunolabeling (FOS, AVT, and IT) of preoptic area (POA) neurons after dyadic agonistic encounters. This study is among the first to show in teleosts that AVT, but not IT, is involved in the establishment of the dominant-subordinate status. We also found status-dependent subregion-specific changes of AVT cell number and activation. These results confirm the involvement of AVT in the establishment of dominance and support the speculation that AVT is released from dominants' AVT neurons.

A vasopressin circuit that modulates sex-specific social interest and anxiety-like behavior in mice

One of the largest sex differences in brain neurochemistry is the male-biased expression of the neuropeptide arginine vasopressin (AVP) within the vertebrate social brain. Despite the long-standing implication of AVP in social and anxiety-like behavior, the precise circuitry and anatomical substrate underlying its control are still poorly understood. By employing optogenetic manipulation of AVP cells within the bed nucleus of the stria terminalis (BNST), we have unveiled a central role for these cells in promoting social investigation, with a more pronounced role in males relative to females. These cells facilitate male social investigation and anxiety-like behavior through their projections to the lateral septum (LS), an area with the highest density of sexually-dimorphic AVP fibers. Blocking the vasopressin 1a receptor (V1aR) in the LS eliminated stimulation-mediated increases in these behaviors. Together, these findings establish a distinct BNST AVP → LS V1aR circuit that modulates sex-specific social interest and anxiety-like behavior.

Modulation of social behavior by distinct vasopressin sources

The neuropeptide arginine-vasopressin (AVP) is well known for its peripheral effects on blood pressure and antidiuresis. However, AVP also modulates various social and anxiety-related behaviors by its actions in the brain, often sex-specifically, with effects typically being stronger in males than in females. AVP in the nervous system originates from several distinct sources which are, in turn, regulated by different inputs and regulatory factors. Based on both direct and indirect evidence, we can begin to define the specific role of AVP cell populations in social behavior, such as, social recognition, affiliation, pair bonding, parental behavior, mate competition, aggression, and social stress. Sex differences in function may be apparent in both sexually-dimorphic structures as well as ones without prominent structural differences within the hypothalamus. The understanding of how AVP systems are organized and function may ultimately lead to better therapeutic interventions for psychiatric disorders characterized by social deficits.

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.

Knockdown of sexually differentiated vasopressin expression in the bed nucleus of the stria terminalis reduces social and sexual behaviour in male, but not female, mice

The neuropeptide arginine-vasopressin (AVP) has long been implicated in the regulation of social behaviour and communication, but the sources of AVP release relevant for behaviour have not been precisely determined. Ablations of the sexually dimorphic AVP cells within the bed nucleus of the stria terminalis (BNST), which are more numerous in males, affect social behaviour differently in males and females. However, it is unknown whether these behavioural effects are caused by a reduction of AVP or of other factors associated with these cells. To test the role of AVP specifically, we used an shRNA viral construct to knock down AVP gene expression within the BNST of wild-type male and female mice, using scrambled sequence virus as a control, and evaluated subsequent changes in social behaviours (social investigation, ultrasonic vocalization (USV), scent marking, copulation, and aggression), or anxiety-like behaviours (elevated plus maze). We observed that, in males, knockdown of AVP expression in the BNST strongly reduced investigation of novel males, aggressive signalling towards other males (tail rattling, USV), and copulatory behaviour, but did not alter attack initiation, other measures of social communication, or anxiety-like behaviours. In females, however, BNST AVP knockdown did not alter any of these behaviours. These results point to differential involvement of AVP derived from the BNST in social behaviour.

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.

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.

Aggression: Perspectives from social and systems neuroscience

Exhibiting behavioral plasticity in order to mount appropriate responses to dynamic and novel social environments is crucial to the survival of all animals. Thus, how animals regulate flexibility in the timing, duration, and intensity of specific behaviors is of great interest to biologists. In this review, we discuss how animals rapidly respond to social challenges, with a particular focus on aggression. We utilize a conceptual framework to understand the neural mechanisms of aggression that is grounded in Wingfield and colleagues' Challenge Hypothesis, which has profoundly influenced how scientists think about aggression and the mechanisms that allow animals to exhibit flexible responses to social instability. Because aggressive behavior is rooted in social interactions, we propose that mechanisms modulating prosocial behavior may be intricately tied to mechanisms of aggression. Therefore, in order to better understand how aggressive behavior is mediated, we draw on perspectives from social neuroscience and discuss how social context, species-typical behavioral phenotype, and neural systems commonly studied in relation to prosocial behavior (i.e., neuropeptides) contribute to organizing rapid responses to social challenges. Because complex behaviors are not the result of one mechanism or a single neural system, we consider how multiple neural systems important for prosocial and aggressive behavior (i.e., neuropeptides and neurosteroids) interact in the brain to produce behavior in a rapid, context-appropriate manner. Applying a systems neuroscience perspective and seeking to understand how multiple systems functionally integrate to rapidly modulate behavior holds great promise for expanding our knowledge of the mechanisms underlying social behavioral plasticity.

An updated field guide for snark hunting: Comparative contributions to behavioral neuroendocrinology in the era of model organisms

Studying neuroendocrine behavioral regulatory mechanisms in a variety of species across vertebrate groups is critical for determining how they work in natural contexts, how they evolved, and ultimately what can be generalized from them, potentially even to humans. All of the above are difficult, at best, if work within our field is exclusively done in traditional laboratory organisms. The importance of comparative approaches for understanding the relationships between hormones and behavior has been recognized and advocated for since our field's inception through a series of papers centered upon a poetic metaphor of Snarks and Boojums, all of which have articulated the benefits that come from studying a diverse range of species and the risks associated with a narrow focus on "model organisms." This mini-review follows in the footsteps of those powerful arguments, highlighting some of the comparative work since the latest interactions of the metaphor that has shaped how we think about three major conceptual frameworks within our field, two of them formalized - the Organization/Activation Model of sexual differentiation and the Social Brain Network - and one, context-dependency, that is generally associated with virtually all modern understandings of how hormones affect behavior. Comparative approaches are broadly defined as those in which the study of mechanism is placed within natural and/or evolutionary contexts, whether they directly compare different species or not. Studies are discussed in relation to how they have either extended or challenged generalities associated with the frameworks, how they have shaped subsequent work in model organisms to further elucidate neuroendocrine behavioral regulatory mechanisms, and how they have stimulated work to determine if and when similar mechanisms influence behavior in our own species.

Testosterone Acts Within the Medial Amygdala of Rats to Reduce Innate Fear to Predator Odor Akin to the Effects of Toxoplasma gondii Infection

Rats infected with the protozoan Toxoplasma gondii exhibit a reduced aversion to cat odor. This behavioral change is thought to increase trophic transmission of the parasite. Infected male rats also show a greater testicular synthesis of testosterone and epigenetic change in arginine vasopressin within the medial amygdala. Here, we show that exogenous supply of testosterone within MeA of uninfected castrates recapitulates reduction in innate fear akin to behavioral change attributed to the parasite. We also show that castration post establishment of chronic infection precludes changes in fear and medial amygdala arginine vasopressin in the infected male rats. These observations support the role of gonadal hormones and pursuant neuroendocrine changes in mediating the loss of fear in the infected rats. This work also demonstrates that testosterone acting specifically within the medial amygdala sufficiently explains reduced defensive behaviors often observed during the appetitive component of reproductive behaviors.

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.

Testosterone Reduces Fear and Causes Drastic Hypomethylation of Arginine Vasopressin Promoter in Medial Extended Amygdala of Male Mice

Testosterone reduces anxiety-like behaviors in rodents and increases exploration of anxiogenic parts of the environment. Effects of testosterone on innate defensive behaviors remain understudied. Here, we demonstrate that exogenous testosterone reduces aversion to cat odor in male mice. This is reflected as increased exploration of area containing cat urine when castrated male mice are supplied with exogenous testosterone. We also report that exogenous testosterone leads to DNA hypomethylation of arginine vasopressin (AVP) promoter in posterodorsal medial amygdala (MePD) and medial bed nucleus of stria terminalis (BNST). Our observations suggest that testosterone acting on AVP system within extended medial amygdala might regulate defensive behaviors in mice.

Sexually Dimorphic Vasopressin Cells Modulate Social Investigation and Communication in Sex-Specific Ways

The neuropeptide arginine vasopressin (AVP) has long been implicated in the regulation of social behavior and communication, but precisely which AVP cell groups are involved is largely unknown. To address whether the sexually dimorphic AVP cell group in the bed nucleus of the stria terminalis (BNST) is important for social communication, we deleted BNST AVP cells by viral delivery of a Cre-dependent caspase-3 cell-death construct in AVP-iCre-positive mice using AVP-iCre negative littermate as controls, and assessed social, sexual, aggressive and anxiety-related behaviors. In males, lesioning BNST AVP cells reduced social investigation of other males and increased urine marking (UM) in the presence of a live female, without altering ultrasonic vocalizations (USVs), resident-intruder aggression, copulatory behavior, anxiety, or investigation of females or their odor cues. In females, which have significantly fewer AVP cells in the BNST, these injections influenced copulatory behavior but otherwise had minimal effects on social behavior and communication, indicating that these cells contribute to sex differences in social behavioral function.

Neural responses to familiar conspecifics are modulated by a nonapeptide receptor in a winter flocking sparrow

The social behavior network, a collection of reciprocally connected areas within the basal forebrain and midbrain, plays a conserved role in the regulation of vertebrate social behavior. Specific behaviors are associated with patterns of activity across the network, and these activity profiles vary with species and context. We investigated how the social behavior network responds to familiar social stimuli in a seasonally flocking songbird. Further, we explored how socially-induced neural responses are modulated by endogenous nonapeptide receptor blockade. Winter flocking dark-eyed juncos were exposed to either familiar conspecifics or a familiar empty aviary following a peripheral injection of either saline or [desGly-NH₂,d(CH₂)₅, Tyr(Me)²,Thr⁴]-ornithine vasotocin, an VT3 receptor antagonist. Socially-exposed animals exhibited greater Fos induction across the social behavior network. Sex and drug effects were site-specific, with females tending to exhibit greater Fos responses to social stimuli and a greater sensitivity to VT3 antagonism. We suggest that in flocking animals, VT3 activation during social interaction may shift the pattern of neural activity towards the dorsocaudal lateral septum and rostral arcopallium and away from the extended amygdala, anterior and ventromedial hypothalamus, and the caudal ventral/ventrolateral lateral septum.

Injections to pregnant mice produce prenatal stress that affects aggressive behavior in their adult male offspring

Maternal stress could reprogram the developing fetal nervous system. A common target of maternal glucocorticoids is fetal neuro-endocrine axis. In the present study, pregnant mice were exposed to stress by injection and their male offspring were tested for sexual and aggressive behaviors in adult life. Three groups of pregnant mice were exposed to stress by sham syringe injection. The first group was injected on days 13, 14, and 15 p.c., the second group was injected on days 17 and 18 p.c., and the third group was injected daily from days 13 to 18 p.c. while control mice were not injected. Male offspring that were exposed to stress on days 13-18 p.c. and 17-18 p.c. were less aggressive and had lower blood testosterone levels in comparison to the control group. In male sexual behavior, there were no statistically significant differences between the groups. Body weight differed significantly with groups injected on days 13-18 p.c. and 13-15 p.c. having significantly higher body weight in adult life than the other two groups. After behavioral testing, brains were processed for immunohistochemical staining with antibodies against vasopressin (AVP) and calbindin (CALB). The expression of AVP and CALB in the lateral septum and in the preoptic area, respectively, did not differ between groups, suggesting that these two masculinization markers were not affected by prenatal stress. Present study therefore shows that even presumably mild and short prenatal stress weakens aggressive behavior of adult male mice, possibly due to reduced testosterone levels in blood.

The role of vasopressin in olfactory and visual processing

Neural vasopressin is a potent modulator of behaviour in vertebrates. It acts at both sensory processing regions and within larger regulatory networks to mediate changes in social recognition, affiliation, aggression, communication and other social behaviours. There are multiple populations of vasopressin neurons within the brain, including groups in olfactory and visual processing regions. Some of these vasopressin neurons, such as those in the main and accessory olfactory bulbs, anterior olfactory nucleus, piriform cortex and retina, were recently identified using an enhanced green fluorescent protein-vasopressin (eGFP-VP) transgenic rat. Based on the interconnectivity of vasopressin-producing and sensitive brain areas and in consideration of autocrine, paracrine and neurohormone-like actions associated with somato-dendritic release, we discuss how these different neuronal populations may interact to impact behaviour.

Putting it in Context: Linking Auditory Processing with Social Behavior Circuits in the Vertebrate Brain

Context is critical to the adaptive value of communication. Sensory systems such as the auditory system represent an important juncture at which information on physiological state or social valence can be added to communicative information. However, the neural pathways that convey context to the auditory system are not well understood. The serotonergic system offers an excellent model to address these types of questions. Serotonin fluctuates in the mouse inferior colliculus (IC), an auditory midbrain region important for species-specific vocalizations, during specific social and non-social contexts. Furthermore, serotonin is an indicator of the valence of event-based changes within individual social interactions. We propose a model in which the brain's social behavior network serves as an afferent effector of the serotonergic dorsal raphe nucleus in order to gate contextual release of serotonin in the IC. Specifically, discrete vasopressinergic nuclei within the hypothalamus and extended amygdala that project to the dorsal raphe are functionally engaged during contexts in which serotonin fluctuates in the IC. Since serotonin strongly influences the responses of IC neurons to social vocalizations, this pathway could serve as a feedback loop whereby integrative social centers modulate their own sources of input. The end result of this feedback would be to produce a process that is geared, from sensory input to motor output, toward responding appropriately to a dynamic external world.

Mechanistic substrates of a life history transition in male prairie voles: Developmental plasticity in affiliation and aggression corresponds to nonapeptide neuronal function

Although prairie vole (Microtus ochrogaster) social behavior is well-characterized in adults, surprisingly little is known about the development of social behavior in voles. Further, the overwhelming majority of studies in prairie voles examine social behavior in a reproductive context. Here, we examine developmental plasticity in affiliation and aggression and their underlying neural correlates. Using sexually naïve males, we characterized interactions with an age-matched, novel, same-sex conspecific in four different age groups that span pre-weaning to adulthood. We found that prosocial behavior decreased and aggression increased as males matured. Additionally, pre-weaning males were more prosocial than nonsocial, whereas post-weaning males were more nonsocial than prosocial. We also examined nonapeptide neural activity in response to a novel conspecific in brain regions important for promoting sociality and aggression using the immediate early gene cFos. Assessment of developmental changes in neural activity showed that vasopressin neurons in the medial bed nucleus of the stria terminalis exhibit functional plasticity, providing a potential functional mechanism that contributes to this change in sociality as prairie voles mature. This behavioral shift corresponds to the transition from a period of allopatric cohabitation with siblings to a period of time when voles disperse and presumably attempt to establish and defend territories. Taken together our data provide a putative mechanism by which brain and behavior prepare for the opportunity to pairbond (characterized by selective affiliation with a partner and aggression toward unfamiliar conspecifics) by undergoing changes away from general affiliation and toward selective aggression, accounting for this important life history event.

Individual Differences in Social Behavior and Cortical Vasopressin Receptor: Genetics, Epigenetics, and Evolution

Social behavior is among the most complex and variable of traits. Despite its diversity, we know little about how genetic and developmental factors interact to shape natural variation in social behavior. This review surveys recent work on individual differences in the expression of the vasopressin 1a receptor (V1aR), a major regulator of social behavior, in the neocortex of the socially monogamous prairie vole. V1aR exhibits profound variation in the retrosplenial cortex (RSC), a region critical to spatial and contextual memory. RSC-V1aR abundance is associated with patterns of male space-use and sexual fidelity in the field: males with high RSC-V1aR show high spatial and sexual fidelity to partners, while low RSC-V1aR males are significantly more likely to mate outside the pair-bond. Individual differences in RSC-V1aR are predicted by a set of linked single nucleotide polymorphisms within the avpr1a locus. These alternative alleles have been actively maintained by selection, suggesting that the brain differences represent a balanced polymorphism. Lastly, the alleles occur within regulatory sequences, and result in differential sensitivity to environmental perturbation. Together the data provide insight into how genetic, epigenetic and evolutionary forces interact to shape the social brain.

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

The neuropeptides vasopressin (AVP) and oxytocin (OT) have been implicated in the regulation of numerous social behaviors in adult and juvenile animals. AVP and OT signaling predominantly occur within a circuit of interconnected brain regions known collectively as the "social behavior neural network" (SBNN). Importantly, AVP and OT signaling within the SBNN has been shown to differentially regulate diverse social behaviors, depending on the age and/or sex of the animal. We hypothesized that variation in the display of these behaviors is due in part to age and sex differences in AVP and OT synthesis within the SBNN. However, a thorough characterization of AVP and OT-immunoreactive (ir) fibers and cell bodies across age and sex within the SBNN has been lacking in rats. We therefore quantified AVP- and OT-ir fibers and cell bodies in 22 subregions of the forebrain SBNN in juvenile and adult, male and female rats. We found numerous age (16 subregions) and sex (10 subregions) differences in AVP-ir fiber fractional areas, and AVP-ir cell body numbers, which were mainly observed in the medial amygdala/bed nucleus of the stria terminalis to lateral septum circuit. In contrast to AVP, we observed no age or sex differences in OT-ir fiber fractional areas or cell bodies in any of the 22 subregions of the forebrain SBNN. Thus, unlike the static pattern observed for OT, AVP innervation of the forebrain SBNN appears to undergo developmental changes, and is highly sexually dimorphic, which likely has significant functional consequences for the regulation of social behavior.

An immunohistochemical study on the distribution of vasotocin neurons in the brain of two weakly electric fish, Gymnotus omarorum and Brachyhypopomus gauderio

Hypothalamic nonapeptides (arginin vasotocin-vasopressin, oxytocin-isotocin) are known to modulate social behaviors across vertebrates. The neuroanatomical conservation of nonapeptide systems enables the use of novel vertebrate model species to identify general strategies of their functional mechanisms. We present a detailed immunohistochemical description of vasotocin (AVT) cell populations and their projections in two species of weakly electric fish with different social structure, Gymnotus omarorum and Brachyhypopomus gauderio. Strong behavioral, pharmacological, and electrophysiological evidence support that AVT modulation of electric behavior differs between the gregarious B. gauderio and the solitary G. omarorum. This functional diversity does not necessarily depend on anatomical differences of AVT neurons. To test this, we focus on interspecific comparisons of the AVT system in basal non-breeding males along the brain. G. omarorum and B. gauderio showed similar AVT somata sizes and comparable distributions of AVT somata and fibers. Interestingly, AVT fibers project to areas related to the control of social behavior and electromotor displays in both species. We found that no gross anatomical differences in the organization of the AVT system account for functional differences between species, which rather shall depend on the pattern of activation of neurons embedded in the same basic anatomical organization of the AVT system.

Serotonin, estrus, and social context influence c-Fos immunoreactivity in the inferior colliculus

A fundamental task of sensory systems is to extract relevant social information from a range of environmental stimuli in the face of changing behavioral contexts and reproductive states. Neuromodulatory pathways that interact with such contextual variables are 1 mechanism for achieving this. In the mouse inferior colliculus (IC), a midbrain auditory region, the neuromodulator serotonin increases in females interacting with courting males, but events downstream of serotonin release have not been investigated. Here, we manipulated serotonin levels in female mice with the serotonin releaser fenfluramine or the serotonin depleter para-chlorophenylalaninemethyl ester (pCPA). Females were then exposed to an empty cage, a male partner, or a playback of courtship vocalizations, and the numbers of neurons in the IC with positive immunoreactivity for the immediate early gene product c-Fos were measured. The effects of drug treatments depended on social context and estrous state. Fenfluramine had greater effects in the nonsocial than in the partner social treatments. Females in proestrus or estrus and given fenfluramine had higher densities of c-Fos immunoreactive neurons, while females in diestrus had fewer immunoreactive neurons. The drug pCPA had the expected opposite effect of fenfluramine, causing a decreased response in pro/estrus females and an increased response in diestrus females. These findings show that the effects of serotonin on c-Fos activity in the IC of females is dependent on both external context and reproductive state, and suggest that these effects occur downstream of serotonin release. (PsycINFO Database Record

Differential activation of vasotocin neurons in contexts that elicit aggression and courtship

Despite continued study on the neurobiological bases of aggressive and sexual behaviors, it is still not well understood how the brain integrates social information with physiological and neural states to produce context-specific behavioral outcomes. In fishes, manipulation of endogenous levels of arginine vasotocin (AVT) through peripheral and intracerebroventricular pharmacological injections results in significant changes in social behaviors, including aggressive and reproduction-related behaviors. In addition, many features of AVT neurons have been shown to correlate with social status and associated behavioral phenotypes. In this study, we used the immediate early gene egr-1 as a marker for neuronal activity and quantified the number of AVT neurons that were positive for egr-1 mRNA by in situ hybridization in Astatotilapia burtoni males that were exposed to either a social context that would elicit aggression or to one that would elicit courtship. In these social settings, focal males readily displayed context- appropriate bouts of aggression (towards the opponent) or bouts of courting (towards females). We found that males that fought had higher levels of egr-1 expression in the preoptic area compared to courting males. A greater proportion of AVT cells was positive for egr-1 after a fight than after a bout of courting. We mapped mRNA distribution of AVT V1a receptor subtypes v1a1 and v1a2 in the brain and identified overlapping areas of expression in nuclei in the ventral telencephalon, hypothalamus and thalamus as key areas for AVT signaling in males.

In utero and Lactational Exposure to Acetamiprid Induces Abnormalities in Socio-Sexual and Anxiety-Related Behaviors of Male Mice

Neonicotinoids, a widely used group of pesticides designed to selectively bind to insect nicotinic acetylcholine receptors, were considered relatively safe for mammalian species. However, they have been found to activate vertebrate nicotinic acetylcholine receptors and could be toxic to the mammalian brain. In the present study, we evaluated the developmental neurotoxicity of acetamiprid (ACE), one of the most widely used neonicotinoids, in C57BL/6J mice whose mothers were administered ACE via gavage at doses of either 0 mg/kg (control group), 1.0 mg/kg (low-dose group), or 10.0 mg/kg (high-dose group) from gestational day 6 to lactation day 21. The results of a battery of behavior tests for socio-sexual and anxiety-related behaviors, the numbers of vasopressin-immunoreactive cells in the paraventricular nucleus of the hypothalamus, and testosterone levels were used as endpoints. In addition, behavioral flexibility in mice was assessed in a group-housed environment using the IntelliCage, a fully automated mouse behavioral analysis system. In adult male mice exposed to ACE at both low and high doses, a significant reduction of anxiety level was found in the light-dark transition test. Males in the low-dose group also showed a significant increase in sexual and aggressive behaviors. In contrast, neither the anxiety levels nor the sexual behaviors of females were altered. No reductions in the testosterone level, the number of vasopressin-immunoreactive cells, or behavioral flexibility were detected in either sex. These results suggest the possibility that in utero and lactational ACE exposure interferes with the development of the neural circuits required for executing socio-sexual and anxiety-related behaviors in male mice specifically.

Integrating resource defence theory with a neural nonapeptide pathway to explain territory-based mating systems

The ultimate-level factors that drive the evolution of mating systems have been well studied, but an evolutionarily conserved neural mechanism involved in shaping behaviour and social organization across species has remained elusive. Here, we review studies that have investigated the role of neural arginine vasopressin (AVP), vasotocin (AVT), and their receptor V1a in mediating variation in territorial behaviour. First, we discuss how aggression and territoriality are a function of population density in an inverted-U relationship according to resource defence theory, and how territoriality influences some mating systems. Next, we find that neural AVP, AVT, and V1a expression, especially in one particular neural circuit involving the lateral septum of the forebrain, are associated with territorial behaviour in males of diverse species, most likely due to their role in enhancing social cognition. Then we review studies that examined multiple species and find that neural AVP, AVT, and V1a expression is associated with territory size in mammals and fishes. Because territoriality plays an important role in shaping mating systems in many species, we present the idea that neural AVP, AVT, and V1a expression that is selected to mediate territory size may also influence the evolution of different mating systems. Future research that interprets proximate-level neuro-molecular mechanisms in the context of ultimate-level ecological theory may provide deep insight into the brain-behaviour relationships that underlie the diversity of social organization and mating systems seen across the animal kingdom.

Hypothalamic vasopressin systems are more sensitive to the long term effects of social defeat in males versus females

Vasopressin signaling has important effects on the regulation of social behaviors and stress responses, and is considered a promising pathway to target for new therapeutics of stress-induced psychiatric disorders. Although there is evidence for sex differences in the behavioral effects of arginine vasopressin (AVP), few data have directly compared the effects of stress on endogenous AVP signaling in males and females. We used California mice (Peromyscus californicus) to study the short and long term effects of social defeat stress on AVP immunoreactive cells in the paraventricular nucleus (PVN) and the posteromedial bed nucleus of the stria terminalis (BNSTmp). Acute exposure to defeat increased AVP/c-fos cells in the PVN and SON of both males and females. In contrast, there were sex differences in the long term effects of defeat. Males but not females exposed to defeat had less avp mRNA in the PVN, and in two experiments defeat reduced the number of AVP positive cells in the caudal PVN of males but not females. Interestingly, during relatively benign social encounters with a target mouse, there was a rapid decrease in AVP percent staining (including cell bodies and fibers) in the PVN of males but not females. Defeat reduced AVP percent staining in males, but did not block the socially induced decrease in percent staining. When mice were tested in resident-intruder tests, males exposed to defeat were no less aggressive than control males whereas aggression was abolished in females. However, bouts of aggression were positively correlated with the number of AVP neurons in the BNSTmp of control males but not stressed males, suggesting that different mechanisms mediate aggression in control and stressed males. These data show that while acute AVP responses to defeat are similar in males and females, the long term effects of defeat on AVP are stronger in males.

Social isolation-induced increase in NMDA receptors in the hippocampus exacerbates emotional dysregulation in mice

Epidemiological studies have shown that early life adverse events have long-term effects on the susceptibility to subsequent stress exposure in adolescence, but the precise mechanism is unclear. In the present study, mice on postnatal day 21-28 were randomly assigned to either a group or isolated cages for 8 weeks. The socially isolated (SI) mice exhibited a higher level of spontaneous locomotor activity, a longer duration of immobility in the forced swimming test (FST), significantly less prepulse inhibition (PPI) and an increase in aggressive (but not attack) behavior. However, acute stress markedly exacerbated the attack counts of the SI mice but did not affect the group housing (GH) mice. SI mice exhibited higher synaptosomal NR2A and NR2B levels in the hippocampus as compared to the GH mice. Whole-cell patch clamp recordings of CA1 neurons in hippocampal slices showed that the SI mice exhibited a higher input-output relationship of NMDAR-EPSCs as compared to the GH mice. Application of the NR2B -specific antagonist ifenprodil produced a greater attenuating effect on NMDAR-EPSCs in slices from the SI mice. NMDAR EPSCs recorded from the SI mice had a slower deactivation kinetic. MK-801, CPP and ifenprodil, the NMDA antagonists, reversed acute stress-induced exaggeration of aggressive and depressive behaviors. Furthermore, acute stress-induced exacerbation of attack behavior in the SI mice was abolished after the knockdown of NR2B expression. These results suggest that social isolation-induced increased expression of NMDA receptors in the hippocampus involves stress exacerbation of aggressive behaviors. Amelioration of aggressive behaviors by NMDA antagonists may open a new avenue for the treatment of psychopathologies that involve outbursts of emotional aggression in neglected children.

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

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

Toxoplasma gondii infection reduces predator aversion in rats through epigenetic modulation in the host medial amygdala

Male rats (Rattus novergicus) infected with protozoan Toxoplasma gondii relinquish their innate aversion to the cat odours. This behavioural change is postulated to increase transmission of the parasite to its definitive felid hosts. Here, we show that the Toxoplasma gondii infection institutes an epigenetic change in the DNA methylation of the arginine vasopressin promoter in the medial amygdala of male rats. Infected animals exhibit hypomethylation of arginine vasopressin promoter, leading to greater expression of this nonapeptide. The infection also results in the greater activation of the vasopressinergic neurons after exposure to the cat odour. Furthermore, we show that loss of fear in the infected animals can be rescued by the systemic hypermethylation and recapitulated by directed hypomethylation in the medial amygdala. These results demonstrate an epigenetic proximate mechanism underlying the extended phenotype in the Rattus novergicus-Toxoplasma gondii association.

Arginine vasotocin, steroid hormones and social behavior in the green anole lizard (Anolis carolinensis)

Arginine vasotocin (AVT) is a potent regulator of social behavior in many species, but little is known about its role in reptilian behavior. Here we examine the effect of exogenous AVT on aggressive responding and courtship behavior in the green anole lizard (Anolis carolinensis). Aggressive behavior was stimulated in two ways: (1) mirror presentation (no relative status formed) and (2) size-matched pairs (where a social status is achieved). To elicit courtship behavior, a novel female was introduced into the home cage of a male. Regardless of the behavior condition, male anoles were injected i.p. with either reptile Ringer solution (vehicle) or AVT prior to testing. Animals treated with AVT performed fewer aggressive display bouts during mirror presentation but AVT treatment did not affect the overall number of aggressive display bouts within size-matched pairs. Male courtship behavior was not affected by AVT; however, untreated females displayed more frequently when paired with an AVT-treated male than a vehicle-injected control, suggesting that AVT-treated males were more attractive to females. Regardless of behavior condition, AVT injections led to increases in circulating corticosterone. Overall, we found that AVT tended to reduce aggressive behavior as has been reported for other territorial species. AVT did not perceptibly alter male courtship but did increase the display behavior of untreated females paired with treated males. Our study supports a role for AVT in the regulation of reptile social behavior.

Copulation or sensory cues from the female augment Fos expression in arginine vasopressin neurons of the posterodorsal medial amygdala of male rats

Background

The posterodorsal part of the medial amygdala is essential for processing reproductively salient sensory information in rodents. This is the initial brain structure where information from olfactory system and male hormones intersect. The neurochemical identity of the neurons participating in the sensory processing in medial amygdala remains presently undetermined. Many neurons in this brain structure express arginine vasopressin in a testosterone-dependent manner, suggesting that this neuropeptide is maintained by the androgenic milieu.

Method

Here we use Fos, a protein expressed by recently active neurons, to quantify activation of arginine vasopressin neurons after exposure to odor from physically inaccessible female. We compare it to mating with accessible female and to reproductively innocuous odor.

Results

We show that inaccessible female activate arginine vasopressin neurons in the male posterodorsal medial amygdala. The magnitude of activation is not further enhanced when physical access with resultant mating is granted, even though it remains undetermined if same population of AVP neurons is activated by both inaccessible female and copulation. We also show that arginine vasopressin activation cannot be fully accounted for by mere increase in the number of Fos and AVP neurons.

Conclusion

These observations posit a role for the medial amygdala arginine vasopressin in reproductive behaviors, suggesting that these neurons serve as integrative node between the hormonal status of the animal and the availability of reproductive opportunities.

Identification of arginine vasotocin (AVT) neurons activated by acute and chronic restraint stress in the avian septum and anterior diencephalon

Effects of acute and chronic psychological stress in the brain of domestic avian species have not been extensively studied. Experiments were performed using restraint stress to determine groups of neurons activated in the septum and diencephalon of chickens. Using FOS immunoreactivity six brain structures were shown activated by acute stress including: the lateral hypothalamic area (LHy), ventrolateral thalamic nucleus (VLT), lateral septum (LS), lateral bed nucleus of the stria terminalis (BSTL), nucleus of the hippocampal commissure (NHpC) and the core region of the paraventricular nucleus (PVNc). Additionally, the LHy and PVNc showed increased FOS immunoreactive (-ir) cells in the birds chronically stressed when compared to controls. In contrast, the NHpC showed decreased FOS-ir cells following the 10day chronic stress imposed. Thereafter, restraint stress experiments were performed to identify activated arginine vasotocin (AVT) neurons (parvocellular or magnocellular) using immunocytochemistry. Of the six FOS activated structures, the PVN was known to contain distinct size groups of AVT-ir neurons, parvocellular (small), medium sized and magnocellular (large). Using dual immunostaining (AVT/FOS), AVT-ir parvocellular neurons in the PVNc were found activated in both acute and chronic stress. To determine whether these AVT-ir parvocellular neurons are co-localized with corticotropin releasing hormone (CRH), an attempt was made to visualize CRH-ir neurons using colchicine. Although AVT-ir and CRH-ir parvocellular neurons occur in the PVNc, only a few neurons were shown co-localized with AVT and CRH after acute restraint stress. Results of this study suggest that the NHpC, LS, VLT, BSTL, LHy and AVT-ir parvocellular neurons in the PVNc are associated with psychological stress in birds.

Hypothalamic oxytocin and vasopressin neurons exert sex-specific effects on pair bonding, gregariousness, and aggression in finches

Antagonism of oxytocin (OT) receptors (OTRs) impairs the formation of pair bonds in prairie voles (Microtus ochrogaster) and zebra finches (Taenioypygia guttata), and also reduces the preference for the larger of two groups ("gregariousness") in finches. These effects tend to be stronger in females. The contributions of specific peptide cell groups to these processes remain unknown, however. This issue is complicated by the fact that OTRs in finches and voles bind not only forms of OT, but also vasopressin (VP), and >10 cell groups produce each peptide in any given species. Using RNA interference, we found that knockdown of VP and OT production in the paraventricular nucleus of the hypothalamus exerts diverse behavioral effects in zebra finches, most of which are sexually differentiated. Our data show that knockdown of VP production significantly reduces gregariousness in both sexes and exerts sex-specific effects on aggression directed toward opposite-sex birds (increases in males; decreases in females), whereas OT knockdown produces female-specific deficits in gregariousness, pair bonding, and nest cup ownership; reduces side-by-side perching in both sexes; modulates stress coping; and induces hyperphagia in males. These findings demonstrate that paraventricular neurons are major contributors to the effects of VP-OT peptides on pair bonding and gregariousness; reveal previously unknown effects of sex-specific peptide on opposite-sex aggression; and demonstrate a surprising lack of effects on same-sex aggression. Finally, the observed effects of OT knockdown on feeding and stress coping parallel findings in mammals, suggesting that OT modulation of these processes is evolutionarily conserved across the amniote vertebrate classes.

Personality is tightly coupled to vasopressin-oxytocin neuron activity in a gregarious finch

Nonapeptides of the vasopressin-oxytocin family modulate social processes differentially in relation to sex, species, behavioral phenotype, and human personality. However, the mechanistic bases for these differences are not well understood, in part because multidimensional personality structures remain to be described for common laboratory animals. Based upon principal components (PC) analysis of extensive behavioral measures in social and nonsocial contexts, we now describe three complex dimensions of phenotype (“personality”) for the zebra finch, a species that exhibits a human-like social organization that is based upon biparental nuclear families embedded within larger social groups. These dimensions can be characterized as Social competence/dominance, Gregariousness, and Anxiety. We further demonstrate that the phasic Fos responses of nonapeptide neurons in the paraventricular nucleus of the hypothalamus and medial bed nucleus of the stria terminalis are significantly predicted by personality, sex, social context, and their interactions. Furthermore, the behavioral PCs are each associated with a distinct suite of neural PCs that incorporate both peptide cell numbers and their phasic Fos responses, indicating that personality is reflected in complex patterns of neuromodulation arising from multiple peptide cell groups. These findings provide novel insights into the mechanisms underlying sex- and phenotype-specific modulation of behavior, and should be broadly relevant, given that vasopressin-oxytocin systems are strongly conserved across vertebrates.

Behavioral relevance of species-specific vasotocin anatomy in gregarious finches

Despite substantial species differences in the vasotocin/vasopressin (VT/VP) circuitry of the medial bed nucleus of the stria terminalis (BSTm) and lateral septum (LS; a primary projection target of BSTm VT/VP cells), functional consequences of this variation are poorly known. Previous experiments in the highly gregarious zebra finch (Estrildidae: Taeniopygia guttata) demonstrate that BSTm VT neurons promote gregariousness in a male-specific manner and reduce anxiety in both sexes. However, in contrast to the zebra finch, the less gregarious Angolan blue waxbill (Estrildidae: Uraeginthus angolensis) exhibits fewer VT-immunoreactive cells in the BSTm as well as differences in receptor distribution across the LS subnuclei, suggesting that knockdown of VT production in the BSTm would produce behavioral effects in Angolan blue waxbills that are distinct from zebra finches. Thus, we here quantified social contact, gregariousness (i.e., preference for the larger of two groups), and anxiety-like behavior following bilateral antisense knockdown of VT production in the BSTm of male and female Angolan blue waxbills. We find that BSTm VT neurons promote social contact, but not gregariousness (as in male zebra finches), and that antisense effects on social contact are significantly stronger in male waxbills than in females. Knockdown of BSTm VT production has no effect on anxiety-like behavior. These data provide novel evidence that species differences in the VT/VP circuitry arising in the BSTm are accompanied by species-specific effects on affiliation behaviors.

Functional significance of a phylogenetically widespread sexual dimorphism in vasotocin/vasopressin production

Male-biased production of arginine vasotocin/vasopressin (VT/VP) in the medial bed nucleus of the stria terminalis (BSTm) represents one of the largest and most phylogenetically widespread sexual dimorphisms in the vertebrate brain. Although this sex difference was identified 30 years ago, the function of the dimorphism has yet to be determined. Because 1) rapid transcriptional activation of BSTm VT/VP neurons is observed selectively in response to affiliation-related stimuli, 2) BSTm VT/VP content and release correlates negatively with aggression, and 3) BSTm VT/VP production is often limited to periods of reproduction, we hypothesized that the sexual dimorphism serves to promote male-specific reproductive behaviors and offset male aggression in the context of reproductive affiliation. We now show that antisense knockdown of BSTm VT production in colony-housed finches strongly increases aggression in a male-specific manner and concomitantly reduces courtship. Thus, the widespread dimorphism may serve to focus males on affiliation in appropriate reproductive contexts (e.g., when courting) while concomitantly offsetting males' tendency for greater aggression relative to females.

Nongenomic effects of estradiol on aggression under short day photoperiods

In several vertebrate species, the effects of estrogens on male aggressive behavior can be modulated by environmental cues. In song sparrows and rodents, estrogens modulate aggression in the nonbreeding season or winter-like short days, respectively. The behavioral effects of estrogens are rapid, which generally is considered indicative of nongenomic processes. The current study further examined the hypothesis that estradiol acts nongenomically under short days by utilizing a protein synthesis inhibitor, cycloheximide (CX). Mice were housed in either short or long day photoperiods, and treated with an aromatase inhibitor. One hour before resident-intruder testing mice were injected with either CX or saline vehicle, and 30 min later were treated orally with either cyclodextrin conjugated estradiol or vehicle. Under short days, mice treated with estradiol showed a rapid decrease in aggressive behavior, independent of CX administration. CX alone had no effect on aggression. These results show that protein synthesis is not required for the rapid effects of estradiol on aggression, strongly suggesting that these effects are mediated by nongenomic processes. We also showed that estradiol suppressed c-fos immunoreactivity in the caudal bed nucleus of the stria terminalis under short days. No effects of estradiol on behavior or c-fos expression were observed in mice housed under long days. Previously we had also demonstrated that cage bedding influenced the directional effects of estrogens on aggression. Here, we show that the phenomenon of rapid action of estradiol on aggression under short days is a robust result that generalizes to different bedding conditions.

Pheromone exposure influences preoptic arginine vasotocin gene expression and inhibits social approach behavior in response to rivals but not potential mates

The nonapeptides arginine vasotocin (AVT) and vasopressin mediate a variety of social behaviors in vertebrates. However, the effects of these peptides on behavior can vary considerably both between and within species. AVT, in particular, stimulates aggressive and courtship responses typical of dominant males in several species, although it can also inhibit social interactions in some cases. Such differential effects may depend upon AVT influences within brain circuits that differ among species or between males that adopt alternative reproductive phenotypes and/or upon the differential activation of those circuits in different social contexts. However, to date, very little is known about how social stimuli that promote alternative behavioral responses influence AVT circuits within the brain. To address this issue, we exposed adult male goldfish to androstenedione (AD), a pheromonal signal that is released by both males and females during the breeding season, and measured social approach responses of males towards same- and other-sex individuals before and after AD exposure. In a second experiment, we measured AD-induced AVT gene expression using in situ hybridization. We found that brief exposure to AD induces social avoidance in response to rival males, but does not affect the level of sociality exhibited in response to sexually receptive females. Exposure to AD also increases AVT gene expression in the preoptic area of male goldfish, particularly in the parvocellular population of the preoptic nucleus. Together, these data suggest that AD is part of a social signaling system that induces social withdrawal specifically during male-male interactions by activating AVT neurons.

Extending the socio-sexual brain: arginine-vasopressin immunoreactive circuits in the telencephalon of mice

Quantitative analysis of the immunoreactivity for arginine-vasopressin (AVP-ir) in the telencephalon of male (intact and castrated) and female CD1 mice allows us to precisely locate two sexually dimorphic (more abundant in intact than castrated males and females) AVP-ir cell groups in the posterior bed nucleus of the stria terminalis (BST) and the amygdala. Chemoarchitecture (NADPH diaphorase) reveals that the intraamygdaloid AVP-ir cells are located in the intra-amygdaloid BST (BSTIA) rather than the medial amygdala (Me), as previously thought. Then, we have used for the first time tract tracing (combined with AVP immunofluorescence) and fiber-sparing lesions of the BST to analyze the projections of the telencephalic AVP-ir cell groups. The results demonstrate that the posterior BST originates the sexually dimorphic innervation of the lateral septum, the posterodorsal Me and a substance P-negative area in the medioventral striato-pallidum (mvStP).The BSTIA may also contribute to some of these terminal fields. Our material also reveals non-dimorphic AVP-ir processes in two locations of the amygdala. First, the ventral Me shows dendrite-like AVP-ir processes apparently belonging supraoptic neurons, whose possible functions are discussed. Second, the Ce shows sparse, thick AVP-ir axons with high individual variability in density and distribution, whose possible influence on stress coping in relation to the affiliative or agonistic behaviors mediated by the Me are discussed. Finally, we propose that the region of the mvStP showing sexually dimorphic AVP-ir innervation is part of the brain network for socio-sexual behavior, in which it would mediate motivational aspects of chemosensory-guided social interactions.

Deconstructing sociality, social evolution and relevant nonapeptide functions

Although behavioral neuroendocrinologists often discuss "sociality" as a unitary variable, the term encompasses a wide diversity of behaviors that do not evolve in a linked fashion across species. Thus grouping, monogamy, paternal care, cooperative breeding/alloparental care, and various other forms of social contact are evolutionarily labile and evolve in an almost cafeteria-like fashion, indicating that relevant neural mechanisms are at least partially dissociable. This poses a challenge for the study of the nonapeptides (vasopressin, oxytocin, and homologous neuropeptides), because nonapeptides are known to modulate all of these aspects of sociality in one species or another. Hence, we may expect substantial diversity in the behavioral functions of nonapeptides across species, and indeed this is the case. Further compounding this complexity is the fact that the pleiotropic contributions of nonapeptides to social behavior are matched by pleiotropic contributions to physiology. Given these considerations, single "model systems" approaches to nonapeptide function will likely not have strong predictive validity for humans or other species. Rather, if we are to achieve predictive validity, we must sample a wide diversity of species in an attempt to derive general principles. In the present review, I discuss what is known about functional evolution of nonapeptide systems, and critically evaluate general assumptions about bonding and other functions that are based on the model systems approach. From this analysis I attempt to summarize what can and cannot be generalized across species, and highlight critical gaps in our knowledge about the functional evolution of nonapeptide systems as it relates to dimensions of sociality.

Aggression- and sex-induced neural activity across vasotocin populations in the brown anole

Activity within the social behavior neural network is modulated by the neuropeptide arginine vasotocin (AVT) and its mammalian homologue arginine vasopressin (AVP). However, central AVT/AVP release causes different behavioral effects across species and social environments. These differences may be due to the activation of different neuronal AVT/AVP populations or to similar activity patterns causing different behavioral outputs. We examined neural activity (assessed as Fos induction) within AVT neurons in male brown anole lizards (Anolis sagrei) participating in aggressive or sexual encounters. Lizards possess simple amniote nervous systems, and their examination provides a comparative framework to complement avian and mammalian studies. In accordance with findings in other species, AVT neurons in the anole paraventricular nucleus (PVN) were activated during aggressive encounters; but unlike in other species, a positive correlation was found between aggression levels and activation. Activation of AVT neurons within the supraoptic nucleus (SON) occurred nonspecifically with participation in either aggressive or sexual encounters. Activation of AVT neurons in the preoptic area (POA) and bed nucleus of the stria terminalis (BNST) was associated with engagement in sexual behaviors. The above findings are congruent with neural activation patterns observed in other species, even when the behavioral outputs (i.e., aggression level) differed. However, aggressive encounters also increased activation of AVT neurons in the BNST, which is incongruous with findings in other species. Thus, some species differences involve the encoding of social stimuli as different neural activation patterns within the AVT/AVP network, whereas other behavioral differences arise downstream of this system.