Prior fighting experience increases aggression in Syrian hamsters: implications for a role of dopamine in the winner effect

Aggression modulator: Understanding the multifaceted role of the dorsal raphe nucleus

Aggressive behavior is instinctively driven behavior that helps animals to survive and reproduce and is closely related to multiple behavioral and physiological processes. The dorsal raphe nucleus (DRN) is an evolutionarily conserved midbrain structure that regulates aggressive behavior by integrating diverse brain inputs. The DRN consists predominantly of serotonergic (5-HT:5-hydroxytryptamine) neurons and decreased 5-HT activity was classically thought to increase aggression. However, recent studies challenge this 5-HT deficiency model, revealing a more complex role for the DRN 5-HT system in aggression. Furthermore, emerging evidence has shown that non-5-HT populations in the DRN and specific neural circuits contribute to the escalation of aggressive behavior. This review argues that the DRN serves as a multifaceted modulator of aggression, acting not only via 5-HT but also via other neurotransmitters and neural pathways, as well as different subsets of 5-HT neurons. In addition, we discuss the contribution of DRN neurons in the behavioral and physiological aspects implicated in aggressive behavior, such as arousal, reward, and impulsivity, to further our understanding of DRN-mediated aggression modulation.

Learning from fights: Males' social dominance status impact reproductive success in Drosophila melanogaster

In animals, the access to vital resources often relies on individuals' behavioural personality, strength, motivation, past experiences and dominance status. Dominant individuals would be more territorial, providing them with a better access to food resources and mate. The so-called winner and loser effects induce individuals' behavioural changes after experiencing a victory or a defeat, and lead to an individual persistent state influencing the outcome of subsequent fights. However, whether and how development of winner and loser effects affect individuals' fitness is controversial. The aim of this study is to evaluate how individuals' fitness can be influenced by previous fighting experience in Drosophila melanogaster. In this study, we assess various behavioural performances as indicators for dominant and subordinate fitness. Our results show that subordinates are less territorial than dominants although their locomotor abilities are not affected. We also demonstrate that in a non-competitive context, experiencing a defeat reduces males' motivation to court females but not the reproductive success while in a competitive context, it negatively affects males' reproductive success. However, we found no impact upon either males' ability to distinguish potential mates nor on females' choice of a specific mating partner. Overall, these results indicate that previous defeats reduce reproductive success, a commonly used estimate of individual fitness.

Fish self-awareness: limits of current knowledge and theoretical expectations

Animal self-awareness is divided into three levels: bodily, social, and introspective self-awareness. Research has focused mainly on the introspection of so-called higher organisms such as mammals. Herein, we turn our attention to fish and provide opinions on their self-awareness based on a review of the scientific literature. Our specific aims are to discuss whether fish (A) could have a neural substrate supporting self-awareness and whether they display signs of (B) social and (C) introspective self-awareness. The present knowledge does not exclude the possibility that fish could have a simple neocortex or other structures that support certain higher cognitive processes, as the function of the primate cerebral cortex can be replaced by other neurological structures. Fish are known to display winner, loser, and audience effects, which could be interpreted as signs of social self-awareness. The audience effect may be explained not only by ethological cost and benefit theory but also by the concept of public self-awareness, which comes from human studies. The behavioural and neural manifestations of depression may be induced in fish under social subordination and may be viewed as certain awareness of a social status. The current findings on fish introspective self-awareness have been debated in the scientific community and, therefore, demand replication to provide more evidence. Further research is needed to verify the outlined ideas; however, the current knowledge indicates that fish are capable of certain higher cognitive processes, which raises questions and implications regarding ethics and welfare in fish-related research and husbandry.

Social Stress and Aggression in Murine Models

Throughout life, animals engage in a variety of social interactions ranging from the affiliative mother-offspring interaction and juvenile play to aggressive conflict. Deprivation of the appropriate social interaction during early development is stressful and disrupts the development of appropriate social behaviors and emotional responses later in life. Additionally, agonistic encounters can induce stress responses in both dominant and subordinate individuals. This review focuses on the social stress that escalates aggressive behavior of animals and discusses the known neurobiological and physiological mechanisms underlying the link between social stress and aggression. Social instigation, a brief exposure to a rival without physical contact, induces aggressive arousal in dominant animals and escalates aggressive behaviors in the following agonistic encounter. Furthermore, the experience of winning an aggressive encounter is known to be as rewarding as addictive drugs, and the experience of repeatedly winning induces addiction-like behavioral and neurobiological changes and leads to abnormal aggressive behaviors. Social isolation stress in early development from neonatal to juvenile and adolescent periods also affects aggressive behavior, but these effects largely depend on the strain, sex, and species as well as the stage of development in which isolation stress is experienced. In conclusion, understanding neurobiological mechanisms underlying the link between social stress and aggression will provide an important insight for the development of more effective and tolerable treatments for maladaptive aggression in humans.

Evolution of stress responses refine mechanisms of social rank

Social rank functions to facilitate coping responses to socially stressful situations and conditions. The evolution of social status appears to be inseparably connected to the evolution of stress. Stress, aggression, reward, and decision-making neurocircuitries overlap and interact to produce status-linked relationships, which are common among both male and female populations. Behavioral consequences stemming from social status and rank relationships are molded by aggressive interactions, which are inherently stressful. It seems likely that the balance of regulatory elements in pro- and anti-stress neurocircuitries results in rapid but brief stress responses that are advantageous to social dominance. These systems further produce, in coordination with reward and aggression circuitries, rapid adaptive responding during opportunities that arise to acquire food, mates, perch sites, territorial space, shelter and other resources. Rapid acquisition of resources and aggressive postures produces dominant individuals, who temporarily have distinct fitness advantages. For these reasons also, change in social status can occur rapidly. Social subordination results in slower and more chronic neural and endocrine reactions, a suite of unique defensive behaviors, and an increased propensity for anxious and depressive behavior and affect. These two behavioral phenotypes are but distinct ends of a spectrum, however, they may give us insights into the troubling mechanisms underlying the myriad of stress-related disorders to which they appear to be evolutionarily linked.

Experimental competition induces immediate and lasting effects on the neurogenome in free-living female birds

Periods of social instability can elicit adaptive phenotypic plasticity to promote success in future competition. However, the underlying molecular mechanisms have primarily been studied in captive and laboratory-reared animals, leaving uncertainty as to how natural competition among free-living animals affects gene activity. Here, we experimentally generated social competition among wild, cavity-nesting female birds (tree swallows, Tachycineta bicolor). After territorial settlement, we reduced the availability of key breeding resources (i.e., nest boxes), generating heightened competition; within 24 h we reversed the manipulation, causing aggressive interactions to subside. We sampled females during the peak of competition and 48 h after it ended, along with date-matched controls. We measured transcriptomic and epigenomic responses to competition in two socially relevant brain regions (hypothalamus and ventromedial telencephalon). Gene network analyses suggest that processes related to energy mobilization and aggression (e.g., dopamine synthesis) were up-regulated during competition, the latter of which persisted 2 d after competition had ended. Cellular maintenance processes were also down-regulated after competition. Competition additionally altered methylation patterns, particularly in pathways related to hormonal signaling, suggesting those genes were transcriptionally poised to respond to future competition. Thus, experimental competition among free-living animals shifts gene expression in ways that may facilitate the demands of competition at the expense of self-maintenance. Further, some of these effects persisted after competition ended, demonstrating the potential for epigenetic biological embedding of the social environment in ways that may prime individuals for success in future social instability.

Rage Against the Machine: Advancing the study of aggression ethology via machine learning

RATIONALE

Aggression, comorbid with neuropsychiatric disorders, exhibits with diverse clinical presentations and places a significant burden on patients, caregivers, and society. This diversity is observed because aggression is a complex behavior that can be ethologically demarcated as either appetitive (rewarding) or reactive (defensive), each with its own behavioral characteristics, functionality, and neural basis that may transition from adaptive to maladaptive depending on genetic and environmental factors. There has been a recent surge in the development of preclinical animal models for studying appetitive aggression-related behaviors and identifying the neural mechanisms guiding their progression and expression. However, adoption of these procedures is often impeded by the arduous task of manually scoring complex social interactions. Manual observations are generally susceptible to observer drift, long analysis times, and poor inter-rater reliability, and are further incompatible with the sampling frequencies required of modern neuroscience methods.

OBJECTIVES

In this review, we discuss recent advances in the preclinical study of appetitive aggression in mice, paired with our perspective on the potential for machine learning techniques in producing automated, robust scoring of aggressive social behavior. We discuss critical considerations for implementing valid computer classifications within behavioral pharmacological studies.

KEY RESULTS

Open-source automated classification platforms can match or exceed the performance of human observers while removing the confounds of observer drift, bias, and inter-rater reliability. Furthermore, unsupervised approaches can identify previously uncharacterized aggression-related behavioral repertoires in model species.

DISCUSSION AND CONCLUSIONS

Advances in open-source computational approaches hold promise for overcoming current manual annotation caveats while also introducing and generalizing computational neuroethology to the greater behavioral neuroscience community. We propose that currently available open-source approaches are sufficient for overcoming the main limitations preventing wide adoption of machine learning within the context of preclinical aggression behavioral research.

Effect of Winning Experience on Aggression Involving Dangerous Fighting Behavior in Anastatus disparis (Hymenoptera: Eupelmidae)

Aggressive behavior is widely observed in animal species for acquiring important resources and usually includes both dangerous and nondangerous fighting patterns. Only a few species show dangerous fighting patterns that are defined by fights ending with contestants being severely injured or killed. Prior experience, an important factor in many species, has been demonstrated to affect a contestant's subsequent fighting behavior. Few studies have focused on the effect of experience on aggression involving dangerous fighting patterns. Here, an egg parasitoid wasp, Anastatus disparis, which shows extreme and dangerous fighting behavior to acquire mating opportunities, was used as an experimental model. Our results showed that the fighting intensity of the winning males significantly decreased subsequent fighting behavior, which was inconsistent with general predictions. Transcriptomic analyses showed that many genes related to energy metabolism were downregulated in winners, and winners increased their fighting intensity after dietary supplementation. Our study suggested that fighting in A. disparis is a tremendous drain on energy. Thus, although males won at combat, significant reductions in available energy constrained the intensity of subsequent fights and influenced strategic decisions. In addition, winners might improve their fighting skills and abilities from previous contests, and their fighting intensity after dietary supplementation was significantly higher than that of males without any fighting experience. Generally, in A. disparis, although winners increased their fighting ability with previous experience, the available energy in winners was likely to be a crucial factor affecting the intensity and strategic decisions in subsequent fights.

Orexin signaling in GABAergic lateral habenula neurons modulates aggressive behavior in male mice

Heightened aggression is characteristic of multiple neuropsychiatric disorders and can have various negative effects on patients, their families and the public. Recent studies in humans and animals have implicated brain reward circuits in aggression and suggest that, in subsets of aggressive individuals, domination of subordinate social targets is reinforcing. In this study, we showed that, in male mice, orexin neurons in the lateral hypothalamus activated a small population of glutamic acid decarboxylase 2 (GAD2)-expressing neurons in the lateral habenula (LHb) via orexin receptor 2 (OxR2) and that activation of these GAD2 neurons promoted male-male aggression and conditioned place preference for aggression-paired contexts. Moreover, LHb GAD2 neurons were inhibitory within the LHb and dampened the activity of the LHb as a whole. These results suggest that the orexin system is important for the regulation of inter-male aggressive behavior and provide the first functional evidence of a local inhibitory circuit within the LHb.

Sociobiology: Changing the Dominance Hierarchy

One fundamental question is to understand what neural circuits are involved when social hierarchies are established, maintained and modified. Now, a new study shows that a previously subordinate animal can become dominant after optogenetic stimulation of the dorsomedial prefrontal cortex, demonstrating that this brain region is necessary and sufficient to quickly induce winning during social competitions.

Competitive ability during mate competition relates to unique patterns of dopamine-related gene expression in the social decision-making network of male zebra finches

Aggressive interactions usually reveal individual differences in the competitive ability of contest participants. Individuals with higher competitive ability often gain priority access to resources such as food, territory, and/or mates. Individuals with lower competitive ability usually have reduced access to these resources and limited mating opportunities. Despite the importance of contest performance to the reproductive success of individuals, the neuroendocrine factors associated with individual differences in competitive ability have not been fully elucidated. Here, we investigate the relationship between dopamine (DA)-related gene expression and competitive ability during mate competition in male zebra finches. Males demonstrating high competitive ability (HCA) had higher tyrosine hydroxylase mRNA levels in the ventral tegmental area and higher D1 receptor (D1-R) mRNA levels in the preoptic area than low competitive ability (LCA) males. Additionally, HCA males had lower levels of D1-R mRNA in the anterior hypothalamus relative to LCA males. These data suggest that there are dynamic and region-specific changes in DA function that relate to variation in competitive ability during mate competition.

Social defeat stress and escalation of cocaine and alcohol consumption: Focus on CRF

Both the ostensibly aversive effects of unpredictable episodes of social stress and the intensely rewarding effects of drugs of abuse activate the mesocorticolimbic dopamine systems. Significant neuroadaptations in interacting stress and reward neurocircuitry may underlie the striking connection between stress and substance use disorders. In rodent models, recurring intermittent exposure to social defeat stress appears to produce a distinct profile of neuroadaptations that translates most readily to the repercussions of social stress in humans. In the present review, preclinical rodent models of social defeat stress and subsequent alcohol, cocaine or opioid consumption are discussed with regard to: (1) the temporal pattern of social defeat stress, (2) male and female protocols of social stress-escalated drug consumption, and (3) the neuroplastic effects of social stress, which may contribute to escalated drug-taking. Neuroadaptations in corticotropin-releasing factor (CRF) and CRF modulation of monoamines in the ventral tegmental area and the bed nucleus of the stria terminalis are highlighted as potential mechanisms underlying stress-escalated drug consumption. However, the specific mechanisms that drive CRF-mediated increases in dopamine require additional investigation as do the stress-induced neuroadaptations that may contribute to the development of compulsive patterns of drug-taking.

Honey bee neurogenomic responses to affiliative and agonistic social interactions

Social interactions can be divided into two categories, affiliative and agonistic. How neurogenomic responses reflect these opposing valences is a central question in the biological embedding of experience. To address this question, we exposed honey bees to a queen larva, which evokes nursing, an affiliative alloparenting interaction, and measured the transcriptomic response of the mushroom body brain region at different times after exposure. Hundreds of genes were differentially expressed at distinct time points, revealing a dynamic temporal patterning of the response. Comparing these results to our previously published research on agonistic aggressive interactions, we found both shared and unique transcriptomic responses to each interaction. The commonly responding gene set was enriched for nuclear receptor signaling, the set specific to nursing was enriched for olfaction and neuron differentiation, and the set enriched for aggression was enriched for cytoskeleton, metabolism, and chromosome organization. Whole brain histone profiling after the affiliative interaction revealed few changes in chromatin accessibility, suggesting that the transcriptomic changes derive from already accessible areas of the genome. Although only one stimulus of each type was studied, we suggest that elements of the observed transcriptomic responses reflect molecular encoding of stimulus valence, thus priming individuals for future encounters. This hypothesis is supported by behavioral analyses showing that bees responding to either the affiliative or agonistic stimulus exhibited a higher probability of repeating the same behavior but a lower probability of performing the opposite behavior. These findings add to our understanding of the biological embedding at the molecular level.

Advances in understanding neural mechanisms of social dominance

Dominance hierarchy profoundly impacts social animals' survival, physical and mental health and reproductive success. As the measurements of dominance hierarchy in rodents become established, it is now possible to understand the neural mechanism mediating the intrinsic and extrinsic factors determining social hierarchy. This review summarizes the latest advances in assay development for measuring dominance hierarchy in laboratory mice. It also reviews our current understandings on how activity and plasticity of specific neural circuits shape the dominance trait and mediate the 'winner effect'.

The Development of Severe and Chronic Violence Among Youth: The Role of Psychopathic Traits and Reward Processing

Psychopathic traits are a manifestation of a personality pathology that comprises a core affective-interpersonal dysfunction (callous-unemotional traits) and an impulsive-antisocial behavioral component. Of particular importance, psychopathic traits are associated with the perpetration of some of the most severe acts of violence, and they appear to indicate a subset of youth at risk for earlier onset, greater frequency, and persistence of violent offending. Although these youth represent a minority of the population, they commit a significant proportion of the violence in the general community. In our review, we highlight evidence of a unique neurobiological predisposition that underlies the core affective deficits and describe contemporary accounts for the developmental processes leading to the antisocial behavior associated with psychopathy. Current evidence suggests that, for this subset of youth, the structure and function of neural circuitry supporting emotion processing, reward learning, decision making, and the development of emotion related to empathy may be crucial to understanding why they are at risk for violence. In particular, a reward dominant pattern of neurobehavioral conditioning may explain how these youth progress to some of the most severe and persistent forms of violence. However, this pattern of conditioning may also be essential to the primary prevention of such deleterious behavior. We suspect that effective strategies to prevent such violence may ultimately be informed by understanding these affective and motivational mechanisms.

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.

An emerging role for the lateral habenula in aggressive behavior

Inter-male aggression is an essential component of social behavior in organisms from insects to humans. However, when expressed inappropriately, aggression poses significant threats to the mental and physical health of both the aggressor and the target. Inappropriate aggression is a common feature of numerous neuropsychiatric disorders in humans and has been hypothesized to result from the atypical activation of reward circuitry in response to social targets. The lateral habenula (LHb) has recently been identified as a major node of the classical reward circuitry and inhibits the release of dopamine from the midbrain to signal negative valence. Here, we discuss the evidence linking LHb function to aggression and its valence, arguing that strong LHb outputs to the ventral tegmental area (VTA) and the dorsal raphe nucleus (DRN) are likely to play roles in aggression and its rewarding components. Future studies should aim to elucidate how various inputs and outputs of the LHb shape motivation and reward in the context of aggression.

Competition, testosterone, and adult neurobehavioral plasticity

Motivation in performance is often measured via competitions. Winning a competition has been found to increase the motivation to perform in subsequent competitions. One potential neurobiological mechanism that regulates the motivation to compete involves sex hormones, such as the steroids testosterone and estradiol. A wealth of studies in both nonhuman animals and humans have shown that a rise in testosterone levels before and after winning a competition enhances the motivation to compete. There is strong evidence for acute behavioral effects in response to steroid hormones. Intriguingly, a substantial testosterone surge following a win also appears to improve an individual's performance in later contests resulting in a higher probability of winning again. These effects may occur via androgen and estrogen pathways modulating dopaminergic regions, thereby behavior on longer timescales. Hormones thus not only regulate and control social behavior but are also key to adult neurobehavioral plasticity. Here, we present literature showing hormone-driven behavioral effects that persist for extended periods of time beyond acute effects of the hormone, highlighting a fundamental role of sex steroid hormones in adult neuroplasticity. We provide an overview of the relationship between testosterone, motivation measured from objective effort, and their influence in enhancing subsequent effort in competitions. Implications for an important role of testosterone in enabling neuroplasticity to improve performance will be discussed.

Nuance and behavioral cogency: How the Visible Burrow System inspired the Stress-Alternatives Model and conceptualization of the continuum of anxiety

By creating the Visible Burrow System (VBS) Bob Blanchard found a way to study the interaction of genetics, physiology, environment, and adaptive significance in a model with broad validity. The VBS changed the way we think about anxiety and affective disorders by allowing the mechanisms which control them to be observed in a dynamic setting. Critically, Blanchard used the VBS and other models to show how behavioral systems like defense are dependent upon context and behavioral elements unique to the individual. Inspired by the VBS, we developed a Stress Alternatives Model (SAM) to further explore the multifaceted dynamics of the stress response with a dichotomous choice condition. Like the VBS, the SAM is a naturalistic model built upon risk assessment and defensive behavior, but with a choice of response: escape or submission to a large conspecific aggressor. The anxiety of novelty during the first escape must be weighed against fear of the aggressor, and a decision must be made. Both outcomes are adaptively significant, evidenced by a 50/50 split in outcome across several study systems. By manipulating the variables of the SAM, we show that a gradient of anxiety exists that spans the contextual settings of escaping an open field, escaping from aggression, and submitting to aggression. These findings correspond with increasing levels of corticosterone and increasing levels of NPS and BDNF in the central amygdala as the context changes.Whereas some anxiolytics were able to reduce the latency to escape for some animals, only with the potent anxiolytic drug antalarmin (CRF1R-blocker) and the anxiogenic drug yohimbine (α2 antagonist) were we able to reverse the outcome for a substantial proportion of individuals. Our findings promote a novel method for modeling anxiety, offering a distinction between low-and-high levels, and accounting for individual variability. The translational value of the VBS is immeasurable, and it guided us and many other researchers to seek potential clinical solutions through a deeper understanding of regional neurochemistry and gene expression in concert with an ecological behavioral model.

Oxytocin, Vasopressin, and the Motivational Forces that Drive Social Behaviors

The motivation to engage in social behaviors is influenced by past experience and internal state, but also depends on the behavior of other animals. Across species, the oxytocin (Oxt) and vasopressin (Avp) systems have consistently been linked to the modulation of motivated social behaviors. However, how they interact with other systems, such as the mesolimbic dopamine system, remains understudied. Further, while the neurobiological mechanisms that regulate prosocial/cooperative behaviors have been extensively examined, far less is understood about competitive behaviors, particularly in females. In this chapter, we highlight the specific contributions of Oxt and Avp to several cooperative and competitive behaviors and discuss their relevance to the concept of social motivation across species, including humans. Further, we discuss the implications for neuropsychiatric diseases and suggest future areas of investigation.

David vs. Goliath: Serotonin modulates opponent perception between smaller and larger rivals

During agonistic encounters, the perception of a larger opponent through morphological signaling typically suppresses aggression in the smaller individual, preventing contest intensity escalation. However, non-morphological factors such as central serotonin (5-HT) activity can influence individual aggression, potentially altering contest intensity despite initial size discrepancies. When male stalk-eyed flies (Teleopsis dalmanni) fight, contest escalation is directly proportional to similarity in body size, with escalation being lower in size-mismatched contests. We have shown that both high-intensity aggression and the probability of winning are increased in males with pharmacologically elevated 5-HT relative to size-matched non-treated opponents. Here, we hypothesized that, in size-mismatched contests, increasing brain 5-HT in the smaller opponent could similarly increase aggression and counteract the low contest intensity normally driven by size discrepancy. Size-mismatched male pairs (greater than 5% difference in eyestalk length) engaged in a forced fight paradigm, with the smaller fly either untreated or with pharmacologically elevated 5-HT levels. The expression of high-intensity aggressive behaviors was significantly increased in smaller treated opponents, but the probability of winning was not altered. This suggests that while elevated serotonergic activity can increase aggression and intensity despite perception of a larger opponent, this is not sufficient to overcome size biases with respect to contest outcome. However, the fact that larger opponents continued to win against smaller treated flies was not simply a function of size. Instead, untreated larger males adjusted their fighting strategy to match the increased aggression of their smaller treated opponent, suggesting contextual flexibility in behavior based on individual opponent assessment.

Dopamine in the nucleus accumbens modulates the memory of social defeat in Syrian hamsters (Mesocricetus auratus)

Conditioned defeat (CD) is a behavioral response that occurs in Syrian hamsters after they experience social defeat. Subsequently, defeated hamsters no longer produce territorial aggression but instead exhibit heightened levels of avoidance and submission, even when confronted with a smaller, non-aggressive intruder. Dopamine in the nucleus accumbens is hypothesized to act as a signal of salience for both rewarding and aversive stimuli to promote memory formation and appropriate behavioral responses to significant events. The purpose of the present study was to test the hypothesis that dopamine in the nucleus accumbens modulates the acquisition and expression of behavioral responses to social defeat. In Experiment 1, bilateral infusion of the non-specific D1/D2 receptor antagonist cis(z)flupenthixol (3.75 μg/150 nl saline) into the nucleus accumbens 5 min prior to defeat training significantly reduced submissive and defensive behavior expressed 24h later in response to a non-aggressive intruder. In Experiment 2, infusion of 3.75 μg cis-(Z)-flupenthixol 5 min before conditioned defeat testing with a non-aggressive intruder significantly increased aggressive behavior in drug-infused subjects. In Experiment 3, we found that the effect of cis-(Z)-flupenthixol on aggression was specific to defeated animals as infusion of drug into the nucleus accumbens of non-defeated animals did not significantly alter their behavior in response to a non-aggressive intruder. These data demonstrate that dopamine in the nucleus accumbens modulates both acquisition and expression of social stress-induced behavioral changes and suggest that the nucleus accumbens plays an important role in the suppression of aggression that is observed after social defeat.

Handling alters aggression and "loser" effect formation in Drosophila melanogaster

In Drosophila, prior fighting experience influences the outcome of later contests: losing a fight increases the probability of losing second contests, thereby revealing “loser” effects that involve learning and memory. In these experiments, to generate and quantify the behavioral changes observed as consequences of losing fights, we developed a new behavioral arena that eliminates handling. We compared two commonly used fly handling procedures with this new chamber and demonstrated that handling influences aggressive behavior and prevents “loser” effect formation. In addition, we induced and observed novel aspects of learning associated with aggression such as the formation of robust winner effects.

Chronic enhancement of brain oxytocin levels causes enduring anti-aggressive and pro-social explorative behavioral effects in male rats

Oxytocin (OXT) has been implicated in the regulation of social behaviors, including intermale offensive aggression. Recently, we showed that acute enhancement of brain OXT levels markedly suppressed offensive aggression and increased social exploration in resident rats confronted with an intruder in their home territory. Moreover, a different responsivity to the exogenous OXTergic manipulation was observed among individuals based on their baseline aggression. In this study we aimed at evaluating the behavioral response to chronically enhancing or attenuating central OXT levels, and at scrutinizing whether the trait-aggression moderates the treatment-induced behavioral changes. To this end, resident male wild-type Groningen rats were continuously (via osmotic minipumps) intracerebroventricularly infused with synthetic OXT or a selective OXT receptor (OXTR) antagonist for 7days. Changes in behavior were assessed performing a resident-intruder test before and at the end of the treatment period, as well as after 7days of withdrawal. Chronic infusion of OXT was found to selectively suppress aggression and enhance social exploration. Chronic blockage of OXTRs instead increased introductory aggressive behavior (i.e. lateral threat), yet without affecting the total duration of the aggression. The magnitude of the anti-aggressive changes correlated positively with the level of baseline aggression. Interestingly, OXT-induced behavioral changes persisted 7days after cessation of the treatment. In conclusion, these findings provide further evidence that enhanced functional activity of the central OXTergic system decreases social offensive aggression while it increases social explorative behavior. The data also indicate that chronically enhancing brain OXT levels may cause enduring anti-aggressive and pro-social explorative behavioral effects.

Measuring virgin female aggression in the female intruder test (FIT): effects of oxytocin, estrous cycle, and anxiety

The costs of violence and aggression in our society have stimulated the scientific search for the predictors and causes of aggression. The majority of studies have focused on males, which are considered to be more aggressive than females in most species. However, rates of offensive behavior in girls and young women are considerable and are currently rising in Western society. The extrapolation of scientific results from males to young, non-maternal females is a priori limited, based on the profound sex differences in brain areas and functioning of neurotransmitters involved in aggression. Therefore, we established a paradigm to assess aggressive behavior in young virgin female rats, i.e. the female intruder test (FIT). We found that approximately 40% of un-manipulated adult (10-11 weeks old) female Wistar rats attack an intruder female during the FIT, independent of their estrous phase or that of their intruder. In addition, adolescent (7-8 weeks old) female rats selected for high anxiety behavior (HABs) displayed significantly more aggression than non-selected (NAB) or low-anxiety (LAB) rats. Intracerebroventricular infusion of oxytocin (OXT, 0.1 µg/5 µl) inhibited aggressive behavior in adult NAB and LAB, but not HAB females. Adolescent NAB rats that had been aggressive towards their intruder showed increased pERK immunoreactivity (IR) in the hypothalamic attack area and reduced pERK-IR in OXT neurons in the paraventricular hypothalamic nucleus compared to non-aggressive NAB rats. Taken together, aggressive behavior in young virgin female rats is partly dependent on trait anxiety, and appears to be under considerable OXT control.