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

Changes in peripheral oxytocin and vasopressin during a silent month-long Insight meditation retreat

Background

Given its putative roles in mediating prosocial behavior, attachment bonds, and stress physiology, oxytocin modulation has been hypothesized to be a biological correlate of the salubrious effects of meditation practice. Here we investigated the effects of a month-long silent meditation retreat on changes in oxytocin, and the related hormone and vasopressin, in relation to psychosocial changes in attachment style, anxiety, personality measures, and feelings of social connectedness with fellow meditators.

Methods

Plasma oxytocin and vasopressin and self-report questionnaires were measured in retreat participants (n = 28) at the beginning of, and 3 weeks into, a residential meditation retreat. Control participants (n = 34), who were similar in age, gender, and meditation experience, were also assessed across a 3-week interval. Linear mixed effects models were used to assess outcomes.

Results

The retreat group showed a small but significant decrease in oxytocin compared to controls who showed no change. In the retreat group, higher openness to experience at Time 1 predicted greater reductions in oxytocin during the retreat, and lower oxytocin at Time 2 was related to stronger feelings of personal connection with fellow meditators. The changes in oxytocin were not related to attachment style or anxiety. Vasopressin decreased over time across both groups, suggesting no specific effect of retreat.

Conclusion

These preliminary findings suggest that meditation training in the context of a silent residential retreat may reduce circulating levels of oxytocin. We interpret this finding from multiple theoretical perspectives, discussing key measurement limitations and proposing future study designs that may help to differentiate the effects of different meditation practices and contexts on oxytocin signaling.

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.

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

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

From grouping and cooperation to menstruation: Spiny mice (Acomys cahirinus) are an emerging mammalian model for sociality and beyond

While spiny mice are primarily used as a model for Type II diabetes and for studying complex tissue regeneration, they are also an emerging model for a variety of studies examining hormones, behavior, and the brain. We began studying the spiny mouse to take advantage of their highly gregarious phenotype to examine how the brain facilitates large group-living. However, this unique rodent can be readily bred and maintained in the lab and can be used to ask a wide variety of scientific questions. In this brief communication we provide an overview of studies that have used spiny mice for exploring physiology and behavior. Additionally, we describe how the spiny mouse can serve as a useful model for researchers interested in studying precocial development, menstruation, cooperation, and various grouping behaviors. With increasingly available technological advancements for non-traditional organisms, spiny mice are well-positioned to become a valuable organism in the behavioral neuroscience community.

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.

A test of the social behavior network reveals differential patterns of neural responses to social novelty in bonded, but not non-bonded, male prairie voles

The social behavior network (SBN) has provided a framework for understanding the neural control of social behavior. The original SBN hypothesis proposed this network modulates social behavior and should exhibit distinct patterns of neural activity across nodes, which correspond to distinct social contexts. Despite its tremendous impact on the field of social neuroscience, no study has directly tested this hypothesis. Thus, we assessed Fos responses across the SBN of male prairie voles (Microtus ochrogaster). Virgin/non-bonded and pair bonded subjects were exposed to a sibling cagemate or pair bonded partner, novel female, novel male, novel meadow vole, novel object, or no stimulus. Inconsistent with the original SBN hypothesis, we did not find profoundly different patterns of neural responses across the SBN for different contexts, but instead found that the SBN generated significantly different patterns of activity in response to social novelty in pair bonded, but not non-bonded males. These findings suggest that non-bonded male prairie voles may perceive social novelty differently from pair bonded males or that SBN functionality undergoes substantial changes after pair bonding. This study reveals novel information about bond-dependent, context-specific neural responsivity in male prairie voles and suggests that the SBN may be particularly important for processing social salience. Further, our study suggests there is a need to reconceptualize the framework of how the SBN modulates social 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.

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.

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

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

A consideration of brain networks modulating social behavior

A primary goal of the field of behavioral neuroendocrinology is to understand how the brain modulates complex behavior. Over the last 20 years we have proposed various brain networks to explain behavioral regulation, however, the parameters by which these networks are identified are often ill-defined and reflect our personal scientific biases based on our area of expertise. In this perspective article, I question our characterization of brain networks underlying behavior and their utility. Using the Social Behavior Network as a primary example, I outline issues with brain networks commonly discussed in the field of behavioral neuroendocrinology, argue that we reconsider how we identify brain networks underlying behavior, and urge the future use of analytical tools developed by the field of Network Neuroscience. With modern statistical/mathematical tools and state of the art technology for brain imaging, we can strive to minimize our bias and generate brain networks that may more accurately reflect how the brain produces behavior.

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.

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.

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.

Dynamic modulation of sociality and aggression: an examination of plasticity within endocrine and neuroendocrine systems

Endocrine and neuroendocrine systems are key mediators of behavioural plasticity and allow for the ability to shift social behaviour across dynamic contexts. These systems operate across timescales, modulating both rapid responses to environmental changes and developmental plasticity in behavioural phenotypes. Thus, not only do endocrine systems mediate behavioural plasticity, but also the systems themselves exhibit plasticity in functional capabilities. This flexibility at both the mechanistic and behavioural levels can be crucial for reproduction and survival. Here, we discuss how plasticity in nonapeptide and steroid actions may influence the expression of, and allow rapid shifts between, sociality and aggression-behavioural shifts that can be particularly important for social interactions. Recent findings of overlap in the mechanisms that modulate social and aggressive behaviour suggest the potential for a mechanistic continuum between these behaviours. We briefly discuss the potential for a sociality-aggression continuum and novel techniques that will enable probing of the functional connectivity of social behaviours. From an evolutionary perspective, we suggest that plasticity in endocrine and neuroendocrine mechanisms of behaviour may be important targets of selection, and discuss the conditions under which we would predict selection to have resulted in differences in endocrine plasticity across species that differ in social organization.This article is part of the themed issue 'Physiological determinants of social behaviour in animals'.

Compared to what: What can we say about nonapeptide function and social behavior without a frame of reference?

Our understanding of behavior and mechanism is undermined by the absence of a frame of reference because relationships between individuals and species are without context. We highlight a need to be more comparative, using nonapeptide (vasopressin and oxytocin) modulation of social behavior as an example. We reconsider the use of model organisms and the term 'social' in this context, contrasting two popular models for nonapeptide regulation of social behavior. We then propose that a frame of reference should be established by studying mechanisms of behavior across taxa along the same continua. If we are to ever establish a unifying theory of behavior, we must transcend individual examples and determine the relative relationships of behavior and mechanism among and between species.

Nesting behavior is associated with VIP expression and VIP-Fos colocalization in a network-wide manner

Many species, including humans, engage in a series of behaviors that are preparatory to the arrival of offspring. Such "nesting behaviors" are of obvious importance, but relevant neuroendocrine mechanisms remain little studied. We here focus on the potential roles of vasoactive intestinal polypeptide (VIP) in the performance of appetitive and consummatory nesting behaviors in male and female zebra finches (Taeniopygia guttata). Using combined immunocytochemistry for Fos and in situ hybridization for VIP, we now show that many VIP cell groups show increased transcriptional activity in response to nest building in male and female zebra finches. Particularly strong data come from the preoptic area (medial preoptic area and medial preoptic nucleus), where VIP-Fos co-expression correlates positively with three different measures of nesting behavior, as does the number of VIP-expressing cells. Remarkably, we find that VIP mRNA and/or VIP-Fos co-expression is correlated with nesting behavior in virtually every brain area that we examined, including the medial amygdala (anterior and posterior), medial bed nucleus of the stria terminalis, medial preoptic area, medial preoptic nucleus, anterior hypothalamus, ventromedial hypothalamus, periaqueductal gray complex (central gray and nucleus intercollicularis), and ventral tegmental area. Near-significant effects are also obtained in the tuberoinfundibular hypothalamus. Although most correlations are positive, negative correlations are observed for the VIP cell group of the anterior hypothalamus, a population that selectively promotes aggression, and also the periaqueductal gray complex. These data demonstrate a network-wide relationship between peptide production and social behavior that is, to our knowledge, unparalleled by other peptidergic modulators.

Vasopressin Proves Es-sense-tial: Vasopressin and the Modulation of Sensory Processing in Mammals

As mammals develop, they encounter increasing social complexity in the surrounding world. In order to survive, mammals must show appropriate behaviors toward their mates, offspring, and same-sex conspecifics. Although the behavioral effects of the neuropeptide arginine vasopressin (AVP) have been studied in a variety of social contexts, the effects of this neuropeptide on multimodal sensory processing have received less attention. AVP is widely distributed through sensory regions of the brain and has been demonstrated to modulate olfactory, auditory, gustatory, and visual processing. Here, we review the evidence linking AVP to the processing of social stimuli in sensory regions of the brain and explore how sensory processing can shape behavioral responses to these stimuli. In addition, we address the interplay between hormonal and neural AVP in regulating sensory processing of social cues. Because AVP pathways show plasticity during development, early life experiences may shape life-long processing of sensory information. Furthermore, disorders of social behavior such as autism and schizophrenia that have been linked with AVP also have been linked with dysfunctions in sensory processing. Together, these studies suggest that AVP's diversity of effects on social behavior across a variety of mammalian species may result from the effects of this neuropeptide on sensory processing.

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.

The effects of vasopressin deficiency on aggression and impulsiveness in male and female rats

The role of vasopressin in aggression received much attention in recent years. However, vasopressin has complex roles on social behavior, which are affected by social experience, motivation and hormonal background, suggesting that its effects depend on the condition of subjects. This hypothesis was tested here by studying the impact of vasopressin deficiency on aggressiveness in reproductively naive and reproductively experienced males, as well as in lactating females, with special reference to the patterns and contexts of attack behavior. We also studied effects on impulsiveness, a behavioral feature strongly related to aggression. Vasopressin deficiency did not affect aggressiveness in reproductively experienced males, decreased the share of violent attacks in reproductively inexperienced males without affecting total attack counts, and suppressed maternal aggression in both early and late phases of lactation; violent forms of attack were decreased in the latter but not the former phase. Changes in aggression appeared unrelated to general changes in maternal behaviors. Impulsivity in the delay discounting task was markedly decreased by vasopressin deficiency in lactating females but not males. Taken together, our findings confirm that vasopressin has an impact on aggressiveness, but show that this impact depends on the condition of subjects, and suggest that the effects of vasopressin on maternal aggression develop in conjunction with impulsivity. Interestingly, overall effects on aggression and specific effects on violent attacks dissociated in both males and females, which hints to the possibility that vasopressin has distinct roles in the development of escalated forms of aggression.

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.