Manipulations of vasopressin alter aggression differently across testing conditions in monogamous and non-monogamousPeromyscus mice

Chronic intranasal oxytocin increases acoustic eavesdropping and adult neurogenesis

Social information gathering is a complex process influenced by neuroendocrine-modulated neural plasticity. Oxytocin (OXT) is a key regulator of social decision-making processes such as information gathering, as it contextually modulates social salience and can induce long-term structural plasticity, including neurogenesis. Understanding the link between OXT-induced plasticity and communicative awareness is crucial, particularly because OXT is being considered for treatment of social pathologies. We investigated the role of chronic OXT-dependent plasticity in attention to novel social information by manipulating the duration of time following cessation of intranasal treatment to allow for the functional integration of adult-born neurons resulting from OXT treatment. Following a 3-week delay, chronic intranasal OXT (IN-OXT) increased approach behavior of both female and male mice towards aggressive vocal playbacks of two unseen novel conspecifics, while no effect was observed after a 3-day delay. Immature neurons increased in the ventral hippocampus of females and males treated with chronic IN-OXT after the 3-week delay, indicating a potential association between ventral hippocampal neurogenesis and approach/acoustic eavesdropping. The less the mouse approached, the higher the level of neurogenesis. Contrary to expectations, the correlation between ventral hippocampal neurogenesis and approach behavior was not affected by IN-OXT, suggesting that other plasticity mechanisms underlie the long-term effects of chronic OXT on social approach. Furthermore, we found a negative correlation between ventral hippocampal neurogenesis and freezing behavior. Overall, our results demonstrate that chronic IN-OXT-induced long-term plasticity can influence approach to vocal information and we further reinforced the link between neurogenesis and anxiety.

Intranasal oxytocin reduces pre-courtship aggression and increases paternal response in California mice (Peromyscus californicus)

Oxytocin (OXT) is a neuropeptide that can facilitate prosocial behavior and decrease social stress and anxiety but can also increase aggression in some contexts. We investigated whether acute pulses of intranasal (IN) OXT influenced social behavior during social challenges that are likely to occur throughout the lifespan of a wild mouse. To test this, we examined the acute effects of IN OXT in the male California mouse (Peromyscus californicus), a monogamous, biparental, and territorial rodent, using a within-subjects longitudinal design. Social challenges included a pre-courtship male-female encounter conducted during the (1) initial aggressive and not the following affiliative phase of courtship, (2) same-sex resident intruder test, and (3) parental care test. Consecutive tests and doses were separated by at least two weeks. Males were treated with intranasal infusions of 0.8 IU/kg OXT or saline controls 5-min before each behavioral test, receiving a total of three treatments of either IN OXT or saline control. We predicted that IN OXT would 1) decrease aggression and increase affiliation during the pre-courtship aggression phase, 2) increase aggression during resident intruder paradigms, and 3) increase paternal care and vocalizations during a paternal care test. As predicted, during pre-courtship aggression with a novel female, IN OXT males displayed less contact aggression than control males, although with no change in affiliative behavior. However, post-pairing, during the resident intruder test, IN OXT males did not differ from control males in contact aggression. During the paternal care test, IN OXT males were quicker to approach their pups than control males but did not differ in vocalizations produced, unlike our previous research demonstrating an effect on vocalizations in females. In summary, during pre-courtship aggression and the paternal care test, IN OXT reduced antisocial behavior; however, during the resident intruder test, IN OXT did not alter antisocial behavior. These data suggest that IN OXT promotes prosocial behavior specifically in social contexts that can lead to affiliation.

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

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

Blocking olfactory input alters aggression in male and female California mice (Peromyscus californicus)

Olfactory input into the brain can be disrupted by a variety of environmental factors, including exposure to pathogens or environmental contaminants. Olfactory cues are often eliminated in laboratory rats and mice through highly invasive procedures like olfactory bulbectomy, which may also disrupt accessory olfactory pathways and detection of non-volatile odors. In the present study, we tested whether inducing anosmia through intranasal infusion of zinc gluconate alters aggression in a monogamous, biparental rodent species, the California mouse (Peromyscus californicus). This less invasive method of manipulating olfaction selectively targets the olfactory epithelium and reduces the detection of volatile odors. Treatment with zinc gluconate extended the time required for male and female California mice to find hidden pieces of apple and reduced the amount of time spent investigating bedding that was soiled by unfamiliar males. Moreover, inhibition of olfaction with zinc gluconate reduced aggressiveness in both sexes as demonstrated by an increased attack latency in the resident-intruder test among same-sex dyads from the same treatment group. These results suggest that volatile olfactory cues are necessary for agonistic responses in both male and female California mice. Therefore, even in species with complex social systems that include territorial aggression and monogamy, volatile olfactory cues modulate agonistic behavior.

The influence of social context on pinyon mouse (Peromyscus truei) vocalizations

Mice in the diverse genus Peromyscus are emerging as important models in the study of acoustic communication. However, reports on vocal repertoires exist for only 8 of the 56 currently recognized species. Descriptions of acoustic content and context are necessary to facilitate comparative studies. In this study, we present the first recordings of wild-captured pinyon mice (Peromyscus truei) in the laboratory in different social contexts. Similar to other Peromyscus species, pinyon mice produced four general types of vocalizations, including sustained vocalizations (SVs), barks, simple sweeps, and complex sweeps. SVs were produced primarily by females in social isolation, highlighting the potential significance of SVs in female advertisement behavior. While agonistic barks were rare, simple and complex sweeps were reliably produced in response to olfactory cues from same- and opposite-sex mice, and in paired contexts. Opposite-sex pairs produced significantly more complex sweeps than same-sex pairs, consistent with the well-supported function of sweeps in coordinating close-distance interactions. In addition, females produced sweeps with lower peak frequencies following separation from their social partner, indicative of a sex-specific mechanism to facilitate advertisement to potential mates and competitors. Together, our findings highlight the importance of social context in rodent vocal production, the significance of female vocal behavior, and the continued need to sample understudied species to better understand similarities and differences in rodent communication systems.

Transmission of paternal retrieval behavior from fathers to sons in a biparental rodent

Transmission of maternal behavior across generations occurs, but less is known about paternal behavior. In biparental species like the California mouse (Peromyscus californicus), paternal care contributes to the well-being of offspring with lasting consequences on the brain and behavior. Paternal huddling/grooming behavior can be passed on to future generations, but whether paternal retrieval, which removes young from potential harm, is transmitted independently is unclear. We manipulated paternal retrieval experience through pup displacement manipulations, then examined whether males exposed to higher levels of paternal retrieval in development altered their adult retrieval behavior with their offspring. Males exposed to heightened paternal retrievals, as compared to reduced retrievals, retrieved their offspring more often but huddled/groomed offspring less during undisturbed natural observations. No differences were observed following a pup displacement challenge. The high paternal retrieval group also exhibited more physical activity and stereotypy. Our results are consistent with the hypothesis that paternal retrieval levels are transmitted across generations and may function via mechanisms separate from huddling/grooming. One modifying factor may be anxiety because increased activity and stereotypy occurred in the high retrieval group. We speculate how the transmission of paternal retrievals may inform a protective parenting style.

Sex-dependent effects of social status on the regulation of arginine-vasopressin (AVP) V1a, oxytocin (OT), and serotonin (5-HT) 1A receptor binding and aggression in Syrian hamsters (Mesocricetus auratus)

Dominance status in hamsters is driven by interactions between arginine-vasopressin V1a, oxytocin (OT), and serotonin 1A (5-HT1A) receptors. Activation of V1a and OT receptors in the anterior hypothalamus (AH) increases aggression in males, while decreasing aggression in females. In contrast, activation of 5-HT1A receptors in the AH decreases aggression in males and increases aggression in females. The mechanism underlying these differences is not known. The purpose of this study was to determine if dominance status and sex interact to regulate V1a, OT, and 5-HT1A receptor binding. Same-sex hamsters (N = 47) were paired 12 times across six days in five min sessions. Brains from paired and unpaired (non-social control) hamsters were collected immediately after the last interaction and processed for receptor binding using autoradiography. Differences in V1a, OT, and 5-HT1A receptor binding densities were observed in several brain regions as a function of social status and sex. For example, in the AH, there was an interaction between sex and social status, such that V1a binding in subordinate males was lower than in subordinate females and V1a receptor density in dominant males was higher than in dominant females. There was also an interaction in 5-HT1A receptor binding, such that social pairing increased 5-HT1A binding in the AH of males but decreased 5-HT1A binding in females compared with unpaired controls. These results indicate that dominance status and sex play important roles in shaping the binding profiles of key receptor subtypes across the neural circuitry that regulates social behavior.

The Monogamy Paradox: What Do Love and Sex Have to Do With It?

Genetic monogamy is rare-at least at the level of a species-and monogamy can exist in the absence of sexual fidelity. Rather than focusing on mating exclusivity, it has become common to use the term "social monogamy" to describe a cluster of social features, including the capacity for selective and lasting social bonds, central to what humans call "love." Socially monogamous mammals often exhibit selective aggression toward strangers and form extended families. These features of social monogamy in mammals are supported by patterns of hormonal function originating in the neurobiology of maternity, including oxytocin, as well as a more primitive vasopressin pathway. Another key feature of social monogamy is reduced sexual dimorphism. Processes associated with sexual differentiation offer clues to the mysteries surrounding the evolution of monogamy. Although there is consistency in the necessary ingredients, it is likely that there is no single recipe for social monogamy. As reviewed here, genes for steroids and peptides and their receptors are variable and are subject to epigenetic regulation across the lifespan permitting individual, gender and species variations and providing substrates for evolution. Reduced sensitivity to gonadal androgens, and a concurrent increased reliance on vasopressin (for selective defense) and oxytocin (for selective affiliation) may have offered pathways to the emergence of social monogamy.

Endogenous Oxytocin, Vasopressin, and Aggression in Domestic Dogs

Aggressive behavior in dogs poses public health and animal welfare concerns, however the biological mechanisms regulating dog aggression are not well understood. We investigated the relationships between endogenous plasma oxytocin (OT) and vasopressin (AVP)-neuropeptides that have been linked to affiliative and aggressive behavior in other mammalian species-and aggression in domestic dogs. We first validated enzyme-linked immunosorbent assays (ELISAs) for the measurement of free (unbound) and total (free + bound) OT and AVP in dog plasma. In Experiment 1 we evaluated behavioral and neuroendocrine differences between a population of pet dogs with a history of chronic aggression toward conspecifics and a matched control group. Dogs with a history of aggression exhibited more aggressive behavior during simulated encounters with conspecifics, and had lower free, but higher total plasma AVP than matched controls, but there were no group differences for OT. In Experiment 2 we compared OT and AVP concentrations between pet dogs and a population of assistance dogs that have been bred for affiliative and non-aggressive temperaments, and investigated neuroendocrine predictors of individual differences in social behavior within the assistance dog population. Compared to pet dogs, assistance dogs had higher free and total OT, but there were no differences in either measure for AVP. Within the assistance dog population, dogs who behaved more aggressively toward a threatening stranger had higher total AVP than dogs who did not. Collectively these data suggest that endogenous OT and AVP may play critical roles in shaping dog social behavior, including aspects of both affiliation and aggression.

Increased vasopressin expression in the BNST accompanies paternally induced territoriality in male and female California mouse offspring

While developmental consequences of parental investment on species-typical social behaviors has been extensively characterized in same-sex parent-offspring interactions, the impact of opposite-sex relationships is less clear. In the bi-parental California mouse (Peromyscus californicus), paternal retrieval behavior induces territorial aggression and the expression of arginine vasopressin (AVP) in adult male offspring. Although similar patterns of territorially emerge among females, the sexually dimorphic AVP system has not been considered since it is generally thought to regulate male-typical behavior. However, we recently demonstrated that male and female P. californicus offspring experience increases in plasma testosterone following paternal retrieval. Since AVP expression is androgen-dependent during development, we postulate that increases in AVP expression may accompany territoriality in female, as well as male offspring. To explore this aim, adult P. californicus offspring that received either high or low levels of paternal care (retrievals) during early development were tested for territoriality and immunohistochemical analysis of AVP within the bed nucleus of the stria terminalis (BNST), paraventricular nucleus (PVN), and supraoptic nucleus (SON). Consistent with previous studies, high care offspring were more aggressive than low care offspring. Moreover, high care offspring had significantly more AVP immunoreactive (AVP-ir) cells within the BNST than low care offspring. This pattern was observed within female as well as male offspring, suggesting an equally salient role for paternal care on female offspring physiology. Regardless of early social experience, sex differences in AVP persisted in the BNST, with males having greater expression than females.

What can animal research tell us about the link between androgens and social competition in humans?

A contribution to a special issue on Hormones and Human Competition. The relationship between androgenic hormones, like testosterone (T), and aggression is extensively studied in human populations. Yet, while this work has illuminated a variety of principals regarding the behavioral and phenotypic effects of T, it is also hindered by inherent limitations of performing research on people. In these instances, animal research can be used to gain further insight into the complex mechanisms by which T influences aggression. Here, we explore recent studies on T and aggression in numerous vertebrate species, although we focus primarily on males and on a New World rodent called the California mouse (Peromyscus californicus). This species is highly territorial and monogamous, resembling the modern human social disposition. We review (i) how baseline and dynamic T levels predict and/or impact aggressive behavior and disposition; (ii) how factors related to social and physical context influence T and aggression; (iii) the reinforcing or "rewarding" aspects of aggressive behavior; and (iv) the function of T on aggression before and during a combative encounter. Included are areas that may need further research. We argue that animal studies investigating these topics fill in gaps to help paint a more complete picture of how androgenic steroids drive the output of aggressive behavior in all animals, including humans.

Vasotocin increases dominance in the weakly electric fish Brachyhypopomus gauderio

Animals establish social hierarchies through agonistic behavior. The recognition of the own and others social ranks is crucial for animals that live in groups to avoid costly constant conflicts. Weakly electric fish are valuable model systems for the study of agonistic behavior and its neuromodulation, given that they display conspicuous electrocommunication signals that are generated by a very well-known electromotor circuit. Brachyhypopomus gauderio is a gregarious electric fish, presents a polygynous breeding system, morphological and electrophysiological sexual dimorphism during the breeding season, and displays a typical intrasexual reproduction-related aggression. Dominants signal their social status by increasing their electric organ discharge (EOD) rate after an agonistic encounter (electric dominance). Subordinates only occasionally produce transient electric signals (chirps and offs). The hypothalamic neuropeptide arginine-vasotocin (AVT) and its mammalian homologue, arginine- vasopressin (AVP) are key modulators of social behavior across vertebrates. In this study, we focus on the role of AVT on dominance establishment in Brachyhypopomus gauderio by analyzing the effects of pharmacological manipulations of the AVT system in potential dominants. AVT exerts a very specific direct effect restricted only to EOD rate, and is responsible for the electric dominance. Unexpectedly, AVT did not affect the intensity of aggression in either contender. Nor was the time structure affected by AVT administration. We also present two interesting examples of the interplay between contenders by evaluating how AVT modulations, even when directed to one individual, affect the behavior of the dyad as a unit. First, we found that V1a AVT receptor antagonist Manning Compound (MC) induces a reversion in the positive correlation between dominants' and subordinates' attack rates, observed in both control and AVT treated dyads, suggesting that an endogenous AVT tone modulates aggressive interactions. Second, we confirmed that AVT administered to dominants induces an increase in the submissive transient electric signals in subordinates.

Sex differences in the effects of social defeat on brain and behavior in the California mouse: Insights from a monogamous rodent

Women are nearly twice as likely as men to be diagnosed with major depressive disorder, yet the use of female animal models in studying the biological basis of depression lags behind that of males. The social defeat model uses social stress to generate depression-like symptoms in order to study the neurobiological mechanisms. In general, social defeat is difficult to apply in female rodents. However, male and female California mice (Peromyscus californicus) are territorial. This allows defeat to be studied in both sexes. Males exposed to defeat tend to exhibit proactive coping mechanisms and demonstrate aggression and reduced cognitive flexibility. Females exposed to defeat engage more in reactive coping mechanisms which is highlighted by social avoidance and low aggression. Importantly, effects of defeat on social interaction behavior in females is independent of adult gonadal steroids. These behavioral phenotypes are associated with sex-specific changes in arginine vasopressin (AVP) and oxytocin (OT), closely related peptides that regulate social behavior and stress reactivity. In brain regions associated with stress responses and social behavior, defeat induced long term decreases in AVP activity and increases in OT activity in males and females respectively. Intranasal OT administration was shown to mimic the effects of defeat-induced increases in endogenous OT activity, causing social withdrawal in undefeated females. This suggests that inhibition of OT activity could reduce the impact of stress on behavior in females. These results highlight the value of maintaining diverse rodent models in the search for sex-specific pharmacological approaches to treating mood disorders.

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.

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.

Naturally occurring variation in vasopressin immunoreactivity is associated with maternal behavior in female Peromyscus mice

In many mammals, species-appropriate social behavior is necessary for an individual's ability to survive and reproduce. In the present study, we examined whether arginine-vasopressin (AVP) pathways that have been associated with social behavior differed between two closely related species of Peromyscus mice with different patterns of maternal behavior. We also tested whether individual levels of AVP-immunoreactive staining (AVP-ir) were associated with individual levels of maternal behavior as measured using a composite score consisting of huddling, nursing, grooming and time spent inside the nest (HNGI score). In addition, we examined whether these associations between vasopressin and behavior differed between species. Females from the highly biparental species, California mice, displayed higher AVP-ir in the bed nucleus of the stria terminalis (BNST), which corresponded with a higher level of nest building and a higher HNGI score than was found in the less parental white-footed mice. The HNGI score was positively associated with AVP-ir in the medial amygdala in female California mice but not white-footed mice. Finally, we examined whether AVP-ir in these pathways varied based on the species-specific rearing environments by reciprocally cross-fostering California mice and white-footed mice. In contrast to previous research with male California mice, cross-fostering itself had no effect on maternal behavior or any consistent effect on AVP-ir staining in brain areas such as the BNST and associated brain areas. This suggests that there is little plasticity in maternal behavior and that the underlying AVP system in females does not respond to the postnatal environment provided by the parents. The positive associations between maternal behavior and AVP-ir indicate that AVP may regulate maternal behavior despite the lack of plasticity in AVP and maternal behavior.

Paternal responsiveness is associated with, but not mediated by reduced neophobia in male California mice (Peromyscus californicus)

Hormones associated with pregnancy and parturition have been implicated in facilitating the onset of maternal behavior via reductions in neophobia, anxiety, and stress responsiveness. To determine whether the onset of paternal behavior has similar associations in biparental male California mice (Peromyscus californicus), we compared paternal responsiveness, neophobia (novel-object test), and anxiety-like behavior (elevated plus maze, EPM) in isolated virgins (housed alone), paired virgins (housed with another male), expectant fathers (housed with pregnant pairmate), and new fathers (housed with pairmate and pups). Corticotropin-releasing hormone (CRH) and Fos immunoreactivity (IR) were quantified in brain tissues following exposure to a predator-odor stressor or under baseline conditions. New fathers showed lower anxiety-like behavior than expectant fathers and isolated virgins in EPM tests. In all housing conditions, stress elevated Fos-IR in the hypothalamic paraventricular nucleus (PVN). Social isolation reduced overall (baseline and stress-induced) Fos- and colocalized Fos/CRH-IR, and increased overall CRH-IR, in the PVN. In the central nucleus of the amygdala, social isolation increased stress-induced CRH-IR and decreased stress-induced activation of CRH neurons. Across all housing conditions, paternally behaving males displayed more anxiety-related behavior than nonpaternal males in the EPM, but showed no differences in CRH- or Fos-IR. Finally, the latency to engage in paternal behavior was positively correlated with the latency to approach a novel object. These results suggest that being a new father does not reduce anxiety, neophobia, or neural stress responsiveness. Low levels of neophobia, however, were associated with, but not necessary for paternal responsiveness.

The regulation of social recognition, social communication and aggression: vasopressin in the social behavior neural network

Neuropeptides in the arginine vasotocin/arginine vasopressin (AVT/AVP) family play a major role in the regulation of social behavior by their actions in the brain. In mammals, AVP is found within a circuit of recriprocally connected limbic structures that form the social behavior neural network. This review examines the role played by AVP within this network in controlling social processes that are critical for the formation and maintenance of social relationships: social recognition, social communication and aggression. Studies in a number of mammalian species indicate that AVP and AVP V1a receptors are ideally suited to regulate the expression of social processes because of their plasticity in response to factors that influence social behavior. The pattern of AVP innervation and V1a receptors across the social behavior neural network may determine the potential range and intensity of social responses that individuals display in different social situations. Although fundamental information on how social behavior is wired in the brain is still lacking, it is clear that different social behaviors can be influenced by the actions of AVP in the same region of the network and that AVP can act within multiple regions of this network to regulate the expression of individual social behaviors. The existing data suggest that AVP can influence social behavior by modulating the interpretation of sensory information, by influencing decision making and by triggering complex motor outputs. This article is part of a Special Issue entitled Oxytocin, Vasopressin, and Social Behavior.

Winning territorial disputes selectively enhances androgen sensitivity in neural pathways related to motivation and social aggression

Winning aggressive disputes can enhance future fighting ability and the desire to seek out additional contests. In some instances, these effects are long lasting and vary in response to the physical location of a fight. Thus, in principle, winning aggressive encounters may cause long-term and context-dependent changes to brain areas that control the output of antagonistic behavior or the motivation to fight (or both). We examined this issue in the territorial California mouse (Peromyscus californicus) because males of this species are more likely to win fights after accruing victories in their home territory but not after accruing victories in unfamiliar locations. Using immunocytochemistry and real-time quantitative PCR, we found that winning fights either at home or away increases the expression of androgen receptors (AR) in the medial anterior bed nucleus of the stria terminalis, a key brain area that controls social aggression. We also found that AR expression in brain regions that mediate motivation and reward, nucleus accumbens (NAcc) and ventral tegmental area (VTA), increases only in response to fights in the home territory. These effects of winning were likely exclusive to the neural androgenic system because they have no detectible impact on the expression of progestin receptors. Finally, we demonstrated that the observed changes in androgen sensitivity in the NAcc and VTA are positively associated with the ability to win aggressive contests. Thus, winning fights can change brain phenotype in a manner that likely promotes future victory and possibly primes neural circuits that motivate individuals to fight.

Estrous phase alters social behavior in a polygynous but not a monogamous Peromyscus species

The social organization of rodent species determines behavioral patterns for both affiliative and agonistic encounters. The neuropeptide oxytocin has been implicated in the mediation of social behavior; however, variability in both neuropeptide expression and social behavior within a single species indicates an additional mediating factor. The purpose of the present comparative study was to investigate social behaviors in naïve mixed-sex pairs of monogamous Peromyscus californicus and polygynous Peromyscus leucopus. We identified substantial inter- and intra-specific variability in the expression of affiliative and agonistic behaviors. Although all P. californicus tested engaged in frequent and prolonged intervals of social contact and rarely engaged in aggressive behaviors, P. leucopus exhibited significant variability in both measures of social behaviors. The naturally occurring differences in social behavior displayed by P. leucopus vary across the estrous cycle, and correspond to hypothalamic oxytocin, as well as circulating oxytocin and glucocorticoid concentrations. These results provide evidence for a rhythm in social behavior across the estrous cycle in polygynous, but not monogamous, Peromyscus species.

Distinct correlations of vasopressin release within the lateral septum and the bed nucleus of the stria terminalis with the display of intermale aggression

Arginine vasopressin (AVP) has been implicated in a wide variety of social behaviors ranging from affiliation to aggression. However, the precise functional involvement of AVP in intermale aggression is still a matter of debate. In fact, very little is known about AVP release patterns within distinct brain regions during the display of intermale aggression and, in turn, the behavioral consequences of such release. We used intracerebral microdialysis to monitor local AVP release within the lateral septum (LS) and the bed nucleus of the stria terminalis (BST) of adult male Wistar rats during the resident-intruder (RI) test. Resident males were cohabitated with a female prior to the RI test to stimulate intermale aggression toward the intruder male. AVP release within the LS correlated positively with intermale aggression. The specific AVP V1a receptor antagonist d(CH(2))(5)Tyr(Me)AVP (10 microg/ml) administered via retrodialysis (3.3 microl/min, 30 min) into the LS of high-aggressive rats prior to the second RI test, prevented an increase in aggression in the second compared with the first RI test as seen in vehicle-treated high-aggressive rats. In contrast to the LS, AVP release within the BST correlated negatively with intermale aggression. Moreover, retrodialysis of synthetic AVP (1 microg/ml) administered into the BST of high-aggressive rats significantly reduced the display of aggression during the second RI test. These data reveal that AVP can both promote and inhibit intermale aggression, depending upon the brain region in which AVP is released. Although challenging the general view that central AVP release enhances intermale aggression in rodents, our data support a model in which AVP coordinates a range of social behaviors by eliciting region-specific effects.

Treatment with arginine vasotocin alters mating calls and decreases call attractiveness in male túngara frogs

The peptide hormone arginine vasotocin (AVT) and its mammalian homolog arginine vasopressin modulate a variety of social behaviors in vertebrates. In anurans, AVT influences the production of advertisement calls, the acoustic signals that males use to attract females and repel rival males. In this study, we investigate the effects of AVT on call characteristics in the túngara frog (Physalaemus pustulosus). Túngara frogs produce a "whine" that is important for species recognition; they may also produce a second, attractive call component, the "chuck". We used a field playback experiment to determine changes in male calling behavior following treatment with AVT. A previous study showed that AVT alters call rate and the production of chucks; in the current analysis, we focus on changes in the whine. Males produce shorter whines with higher initial frequencies following treatment with AVT. Call changes do not vary with a social stimulus. We also used female phonotaxis experiments to investigate the effects of call changes on female mate choice. Females disfavor the calls produced by males treated with exogenous AVT. We suggest that AVT influences motivation to call and the motor control of call production, but that over-stimulation of the vocal system limited the production of attractive calls in this experimental context.

The 'home advantage' is necessary for a full winner effect and changes in post-encounter testosterone

Winning aggressive contests can both enhance future winning ability and change post-encounter hormones; however, it remains unclear if the context of a fight also influences such winner effects and hormone changes. We investigated this issue by using California mice (Peromyscus californicus) to test if the effect of residency status is necessary to improve future winning ability and alter post-encounter hormones. Male mice were subjected to an aggressive contest and their blood was collected 45 min after the fight. Upon contest initiation, focal mice had a 'home advantage' and three prior winning experiences, only one of these factors, or neither factor. Only individuals with a 'home advantage' and prior winning experience showed a full winner effect. Post-encounter changes in testosterone and progesterone resulted from an interaction between residency status and winning experience. These data indicate that in male California mice a 'home advantage' is necessary to produce the full winner effect and that residency status helps regulate inter-individual variation in the expression of post-encounter testosterone pulses and progesterone changes. Furthermore, these respective behavioral and physiological phenomena might be modulated in a context-specific manner, in part by the surrounding physical environment.

Arginine Vasotocin Promotes Calling Behavior and Call Changes in Male Túngara Frogs

In the túngara frog, Physalaemus pustulosus, males alter calling behavior with changes in their social environment, adding 'chucks' to their advertisement calls in response to the calls of conspecific males. Other studies demonstrate that adding chucks increases the attractiveness of calls to females but also increases the risk of bat predation. In the current study, subcutaneous injections of the neuropeptide hormone arginine vasotocin (AVT) significantly increased chuck production in male túngara frogs. The effects of AVT on chuck production did not depend on the presence of playback stimuli, suggesting that AVT increased either the males' general motivation to produce chucks or their responsiveness to the calls of distant males. Injections of AVT also increased the probability that males would call and decreased the latency to call after injection, supporting the hypothesis that AVT influences motivation to call. Finally, AVT inhibited a drop in call rate after the termination of a playback stimulus and increased call rate at a lower dose of AVT. The effects of AVT on chucks and call rate appear to be independent of each other, as there was no correlation between change in chuck production and change in call rate in individual males. We conclude that AVT may play an important role in socially-mediated call changes that result from competition for mates. The behavioral changes induced by AVT might increase a male's attractiveness to females, and also may be consistent with an aggressive response to another túngara frog male.

Paternal behavior influences development of aggression and vasopressin expression in male California mouse offspring

Parental care has been demonstrated to have important effects on offspring behavioral development. California mice (Peromyscus californicus) are biparental, and correlational evidence suggests that pup retrieving by fathers has important effects on the development of aggressive behavior and extra-hypothalamic vasopressin systems. We tested whether retrievals affected these systems by manipulating paternal retrieval behavior between day 15 and 21 postpartum. Licking and grooming behavior affect behavioral development in rats, so we also experimentally reduced huddling and grooming behavior by castrating a subset of fathers. Experimentally increasing the frequency of paternal pup retrieving behavior decreased attack latency in resident-intruder in both male and female adult offspring, whereas experimental reduction of huddling and grooming had no effect. In a separate group of male offspring, we examined vasopressin immunoreactivity (AVP-ir) in two regions of the posterior bed nucleus of the stria terminalis (BNST): the dorsal fiber tracts (dBNST) and the ventral cell body-containing region (vBNST). Experimentally increasing retrievals led to an apparent shift in AVP-ir distribution. Specifically, offspring from the high retrieval group had more AVP-ir than offspring from the sham retrieval group in the dBNST, whereas the opposite was observed in the vBNST. Experimental reduction of paternal grooming was associated with increased AVP-ir in the paraventricular nucleus and also increased corticosterone and progesterone, similar to observed effects of maternal grooming on HPA function. This study provides further evidence that paternal behavior influences the development of aggression and associated neural substrates.

Production of ultrasonic vocalizations by Peromyscus mice in the wild

BACKGROUND

There has been considerable research on rodent ultrasound in the laboratory and these sounds have been well quantified and characterized. Despite the value of research on ultrasound produced by mice in the lab, it is unclear if, and when, these sounds are produced in the wild, and how they function in natural habitats.

RESULTS

We have made the first recordings of ultrasonic vocalizations produced by two free-living species of mice in the genus Peromyscus (P. californicus and P. boylii) on long term study grids in California. Over 6 nights, we recorded 65 unique ultrasonic vocalization phrases from Peromyscus. The ultrasonic vocalizations we recorded represent 7 different motifs. Within each motif, there was considerable variation in the acoustic characteristics suggesting individual and contextual variation in the production of ultrasound by these species.

CONCLUSION

The discovery of the production of ultrasonic vocalizations by Peromyscus in the wild highlights an underappreciated component in the behavior of these model organisms. The ability to examine the production of ultrasonic vocalizations in the wild offers excellent opportunities to test hypotheses regarding the function of ultrasound produced by rodents in a natural context.