Neural background of glucocorticoid dysfunction-induced abnormal aggression in rats: involvement of fear- and stress-related structures

Toward understanding the neural mechanisms involved in early life stress-induced aggression: A Highlight for "Maternal separation early in life induces excessive activity of the central amygdala related to abnormal aggression"

Early life stress, such as childhood abuse and neglect, is one of the major risk factors for the development of antisocial behavior. In rat models, repeated maternal separation (MS) stress, in which the pups are separated from the dams for a few hours each day during the first 2-3 weeks of life, increases aggressive behavior in adult males. This Editorial highlights an article in the current issue of the Journal of Neurochemistry that demonstrates the involvement of the central nucleus of the amygdala (CeA) in the escalation of aggressive behavior in the MS model. The authors show that MS rats exhibit higher c-Fos expression in the CeA during an aggressive encounter compared to non-isolated control rats. Unexpectedly, other amygdala subnuclei did not show differential activation between MS and control groups. Using optogenetics, they provide direct evidence that activation of CeA neurons increases intermale aggressive behavior and that bilateral CeA activation shifts behavioral patterns toward more qualitatively intense aggressive behavior than unilateral CeA activation. These findings highlight the important role of the CeA in the development of abnormal aggression and indicate that this region may be an important therapeutic target for human aggression induced by early life stress.

Understanding the Role of Oxidative Stress, Neuroinflammation and Abnormal Myelination in Excessive Aggression Associated with Depression: Recent Input from Mechanistic Studies

Aggression and deficient cognitive control problems are widespread in psychiatric disorders, including major depressive disorder (MDD). These abnormalities are known to contribute significantly to the accompanying functional impairment and the global burden of disease. Progress in the development of targeted treatments of excessive aggression and accompanying symptoms has been limited, and there exists a major unmet need to develop more efficacious treatments for depressed patients. Due to the complex nature and the clinical heterogeneity of MDD and the lack of precise knowledge regarding its pathophysiology, effective management is challenging. Nonetheless, the aetiology and pathophysiology of MDD has been the subject of extensive research and there is a vast body of the latest literature that points to new mechanisms for this disorder. Here, we overview the key mechanisms, which include neuroinflammation, oxidative stress, insulin receptor signalling and abnormal myelination. We discuss the hypotheses that have been proposed to unify these processes, as many of these pathways are integrated for the neurobiology of MDD. We also describe the current translational approaches in modelling depression, including the recent advances in stress models of MDD, and emerging novel therapies, including novel approaches to management of excessive aggression, such as anti-diabetic drugs, antioxidant treatment and herbal compositions.

Topiramate inhibits offensive aggression through targeting ventrolateral periaqueductal gray

Topiramate is an approved antiepileptic drug clinically used to treat epilepsy and prevent migraines. Currently, topiramate has been found to be effective in treating aggressive symptoms in neuropsychiatric patients. In preclinical studies, however, the effects and mechanisms of topiramate on offensive aggression are still largely uninvestigated. Our previous studies indicated that glutamatergic transmission in the ventrolateral periaqueductal gray (vlPAG) plays a crucial role in regulating elements of offensive aggressive behaviors. In the present work, we investigated the actions of topiramate on vlPAG glutamatergic transmission and aggressive behaviors in group-housed (GH) and socially isolated (SI) rats. The results suggested that a single injection of topiramate systemically and dose-dependently inhibited elements of offensive aggressive behaviors of both GH and SI rats in the resident-intruder test (RIT), with long-lasting effective time profiles in SI rats. Moreover, systemic single administration of topiramate reduced the frequency and amplitude of miniature excitatory postsynaptic currents (mEPSCs) in the vlPAG. Bath perfusion of topiramate directly decreased the frequency and amplitude of mEPSCs and shortened the amplitude of evoked excitatory postsynaptic currents (EPSCs) in the vlPAG. Furthermore, intra-vlPAG single microinjection of topiramate dose-dependently inhibited offensive aggressive behaviors in GH and SI rats in a time-dependent manner. Additionally, both systemic and local topiramate inhibited offensive aggressive behaviors in a (2R,6R)-hydroxynorketamine (HNK)-dependent rat model. In conclusion, the present results suggest that topiramate exerts anti-aggressive roles through its inhibitory actions on glutamatergic activities in the vlPAG. These preclinical results support topiramate as a candidate drug to treat patients with heightened offensive aggression.

Agonistic behaviors and neuronal activation in sexually naïve female Mongolian gerbils

Agonistic interaction is important for establishing social hierarchy and determining access to limited resources. Although there are substantial studies investigating the neural mechanisms of aggressive or defensive behavior in male rodents, little attention has been paid to the mechanisms underlying agonistic behaviors in females. In the present study, we depicted patterns of agonistic behaviors in sexually naïve female Mongolian gerbils (Meriones unguiculatus) and examined the neuronal activation in the brain by Fos-immunoreactive (Fos-ir) staining. We found that the winner-loser relationship was established rapidly. Winners displayed higher levels of aggression, environmental exploration, scent marking, and self-grooming, but less defensive behavior, in comparison to losers. Several patterns of Fos-ir expression emerged following agonistic interactions. Winners had the number of Fos-ir cells in the ventrolateral subnucleus of the ventromedial hypothalamus (VMHvl) and dorsal periaqueductal grey (PAGd) more than the controls but less than the losers. Losers also had more Fos-ir cells in the paraventricular nucleus of the hypothalamus (PVN), anterior medial (BSTam) and anteriolateral (BSTal) subnuclei of the bed nucleus of the stria terminalis (BST), and the ventral subnucleus of the lateral septum (LSv), as well as less Fos-ir cells in the dentate gyrus of the hippocampus (DG), compared to the controls. In addition, the number of Fos-ir cells showed similar increases in the principal nucleus (BSTpr) and interfascicular nucleus (BSTif) of the BST and amygdala (AMYG) in both the winners and losers, compared to the controls. Together, these data illustrate the patterns of altered neuronal activation in a behavior-, social status-, and brain region-specific manner, implicating potential roles of the brain neural circuit in mediating agonistic interactions in female Mongolian gerbils.

The effects of Topiramate on isolation-induced aggression: a behavioral and immunohistochemical study in mice

Topiramate, an antiepileptic drug, has been found to be useful for the treatment of aggression in clinical populations. Most preclinical studies related to Topiramate have been focused exclusively on the quantitative aspects of the aggressive behavior between mice. However, there is still limited knowledge regarding the effects of Topiramate on neuronal mechanisms occurring in aggressive mice. The present work aims to understand further the effects of the antiepileptic drug Topiramate on aggressive behaviors, and on the neural correlates underlying such behaviors. To achieve this, we combined the resident-intruder model of isolation-induced aggression in mice with two drug regimens of Topiramate administration (30.0 mg/kg; acute and sub-chronic treatments). Our data showed that both acute and subchronic treatments decreased the intensity of agonistic encounters and reinforced social behavior. By using C-fos immunoreactivity, we investigated the neuronal activation of several brain regions involved in aggressive behavior following subchronic treatment. We found that Topiramate produced activation in several cortical areas and in the lateral septum of resident brain mice compared with their controls. However, Topiramate induced inhibition in the medial nucleus of the amygdala, the dorsomedial nucleus of the periaqueductal gray, and especially in the anterior hypothalamic nucleus. Finally, we performed microinfusion of Topiramate (0.1 and 0.3 mM) into the lateral septum and anterior hypothalamus on offensive behaviors in isolation-induced-aggression paradigm. Interestingly, the microinfusion of Topiramate into the lateral septum has the capacity to alleviate aggressive behavior, without affecting social behavior. However, the microinfusion of Topiramate into the anterior hypothalamus decreased aggressive behavior and slightly reinforced social behavior. Our observations supported that the dose of 0.1 mM of Topiramate appeared more efficacy to treat aggression in adult mice. These pharmacological characteristics may account for Topiramate efficacy on aggressive symptoms in psychiatric patients.

Hypothalamic Control of Conspecific Self-Defense

Active defense against a conspecific aggressor is essential for survival. Previous studies revealed strong c-Fos expression in the ventrolateral part of the ventromedial hypothalamus (VMHvl) in defeated animals. Here, we examined the functional relevance and in vivo responses of the VMHvl during conspecific defense. We found that VMHvl cells expressing estrogen receptor α (Esr1) are acutely excited during active conspecific defense. Optogenetic inhibition of the cells compromised an animal’s ability to actively defend against an aggressor, whereas activating the cells elicited defense-like behaviors. Furthermore, the VMHvl is known for its role in aggression. In vivo recording and c-Fos mapping revealed differential organization of the defense and aggression-responsive cells in the VMHvl. Specifically, defense-activated cells are concentrated in the anterior part of the VMHvl, which preferentially targets the periaqueductal gray (PAG). Thus, our study identified an essential neural substrate for active conspecific defense and expanded the function of the VMHvl.

Active defense against conspecific aggressors is essential for survival, but its underlying neural substrates remain largely unknown. Through a series of in vivo recordings and functional manipulations, Wang et al. demonstrate that cells expressing estrogen receptor α in a small medial hypothalamic nucleus are essential for defense against a bully.

The effects of pubertal exposure to bisphenol-A on social behavior in male mice

Puberty is a crucial developmental period for structural modifications of brain and activation of the neural circuits underlying sex differences in social behavior. It is possible that pubertal exposure to bisphenol-A (BPA), a common EED with a weak estrogenic activity, influences social behavior. After being exposed to BPA at 0.04, 0.4, 4 mg kg^-1 for 18 days, the 7-week-old male mice were tested with social play and three-chamber. The results showed that pubertal BPA exposure decreased social play between adolescent males and sociability of adolescent males. Further, pubertal BPA exposure reduced sociability and inhibited social novel preferences of adult males. BPA inhibited social interactions with opposite sex but improved socio-sexual exploration and the low-intensity mating behavior (mounting) with same sex in adult males. In residential-intruder test, BPA-exposed adult males showed a decrease in aggressiveness and an enhancement in prosocial behavior with intruder. Western blot analysis showed that BPA (especially at 4 mg/kg/d) down-regulated the levels of AR in the amygdala and the striatum but up-regulated the levels of DR1 and DAT proteins in the striatum of adult males. BPA at 4 mg kg^-1 decreased the levels of T in the serum and the brain. These results suggest that pubertal BPA exposure affects social play and sociability of adolescent males and even results in long-term effects on social behavior of adult males. BPA-induced down-regulations of the levels of AR in the amygdala and the striatum and up-regulation of the levels of DR1 and DAT in the striatum may be involved.

The influence of early postnatal chronic mild stress stimulation on the activation of amygdala in adult rat

Amygdala is a limbic structure involved in the stress response. The immunohistochemical and morphometric methods were used to examine whether the chronic mild psychological stress during the early postnatal period would change activation of amygdaloid nuclei in response to the same stressor in adult. In the study we focused on the role of neurons containing calbindin (CB), calretinin (CR), parvalbumin (PV) and nitric oxide synthase (NOS). The rats were divided into three groups: control non-stressed animals and two experimental: EI consisted of animals that were exposed to acute stress in the high-light, open-field test (HL-OF) at P90 (P - postnatal day) and EII consisted of rats that were exposed to chronic stress in HL-OF, daily during the first 21 postnatal days and then once at P90. The scheme of activation of amygdaloid nuclei under stress in EI and EII group was similar. The highest density of c-Fos-ir cells (c-Fos - a marker of neuronal activation) was demonstrated by the medial nucleus (Me) and bed nucleus of the accessory olfactory tract (BAOT). The amygdaloid nuclei diversity after HL-OF was determined by the high activation of the NOS-ir cells in the Me and NOS- and CR-ir cells in the BAOT. These are probably projection neurons involved in modulation of defensive, reproductive and autonomic behavior in stress response and creation/storage of aversive memory. However, in comparison with EI group, significant decrease in density of c-Fos-ir cells, in almost all amygdaloid nuclei of EII group was revealed. Particularly in BAOT and Me the strong decrease of activity of NOS- and CR-ir neurons was observed. It probably results in attenuation of stress responses what, depending on the circumstances, can be adaptive or maladaptive.

Aggressive behavior and brain neuronal activation in sexually naïve male Mongolian gerbils

Aggressive behavior plays an important role in animal's survival and reproductive success. Although there has been growing interests in studying neural mechanisms underlying aggressive behavior using traditional laboratory animal models, little is known about mechanisms controlling naturally occurring aggression in sexually naïve animals. In the present study, we characterized aggressive behavior displayed by sexually naïve male Mongolian gerbils (Meriones unguiculatus) and examined the subsequent neuronal activation in the brain measured by Fos-immunoreactive (Fos-ir) staining. We found that resident males initiated attacks and showed intense levels of aggression (including chase, bite, offensive sideway, lunge and on-top) towards a conspecific male intruder. Furthermore, attacks from the resident males towards the intruder produced a nonrandom distribution of bites, with the most on the rump, flank, back and tail and few on the limbs, ventrum and head. In contrast, control males that were exposed to a woodblock (control for novelty) never attacked the woodblock and showed higher levels of object/environmental investigation. Male gerbils exposed to an intruder had significantly higher levels of Fos-ir density in the medial (MeA) and anterior cortical (ACo) subnuclei of the amygdala, principal nucleus (BSTpr) and interfascicular nucleus (BSTif) of the bed nucleus of the stria terminalis, ventrolateral subdivision of the ventromedial hypothalamus (VMHvl), and paraventricular nucleus of the hypothalamus (PVN), compared to control males. Together, our results indicate that sexually naïve, group housed male gerbils naturally display aggression towards conspecific strangers, and such aggressive behavior is associated with special patterns of neuronal activation in the brain.

Peripubertal stress-induced heightened aggression: modulation of the glucocorticoid receptor in the central amygdala and normalization by mifepristone treatment

Despite the enormous negative impact of excessive aggression for individuals and societies, there is a paucity of treatments. Here, using a peripubertal stress model of heightened aggression in rats, we investigated the involvement of the glucocorticoid system and tested the effectiveness of antiglucocorticoid treatment to normalize behavior. We assessed peripubertal stress-induced changes in glucocorticoid (GR) and mineralocorticoid (MR) gene expression in different amygdala nuclei and hippocampus, and report a specific increase in GR mRNA expression in the central amygdala (CeA). Administration of mifepristone (10 mg/kg), a GR antagonist, before stressor exposure at peripuberty prevented the habituation of plasma corticosterone responses observed throughout the stress protocol. This treatment also prevented the increase in aggression and GR expression in the CeA observed in peripubertally stressed rats at adulthood. Viral downregulation of CeA GR expression at adulthood led to reduced aggression. Subsequently, we showed that a brief, 3-day, treatment with mifepristone at adulthood was effective to normalize the abnormal aggression phenotype in peripubertally stressed rats. Our results support a key role for GR actions during peripubertal stress for the long-term programming of heightened aggression. Strikingly, they also support the translational interest of testing the effectiveness of mifepristone treatment to diminish reactive aggression in early adversity-related human psychopathologies.

Neurobiology of Aggressive Behavior-Role of Autoantibodies Reactive With Stress-Related Peptide Hormones

Adrenocorticotropic hormone together with arginine vasopressin and oxytocin, the neuropeptides regulating the stress response and the hypothalamic-pituitary-adrenal axis activity, are known to modulate aggressive behavior. The functional role of the adrenocorticotropic hormone immunoglobulin G autoantibodies in peptidergic signaling and motivated behavior, including aggression, has been shown in experimental and in vitro models. This review summarizes some experimental data implicating autoantibodies reactive with stress-related peptides in aggressive behavior.

Non-invasive genotyping with a massively parallel sequencing panel for the detection of SNPs in HPA-axis genes

We designed a genotyping panel for the investigation of the genetic underpinnings of inter-individual differences in aggression and the physiological stress response. The panel builds on single nucleotide polymorphisms (SNPs) in genes involved in the three subsystems of the hypothalamic-pituitary-adrenal (HPA)-axis: the catecholamine, serotonin and corticoid metabolism. To promote the pipeline for use with wild animal populations, we used non-invasively collected faecal samples from a wild population of Assamese macaques (Macaca assamensis). We targeted loci of 46 previously reported SNPs in 21 candidate genes coding for elements of the HPA-axis and amplified and sequenced them using next-generation Illumina sequencing technology. We compared multiple bioinformatics pipelines for variant calling and variant effect prediction. Based on this strategy and the application of different quality thresholds, we identified up to 159 SNPs with different types of predicted functional effects among our natural study population. This study provides a massively parallel sequencing panel that will facilitate integrating large-scale SNP data into behavioural and physiological studies. Such a multi-faceted approach will promote understanding of flexibility and constraints of animal behaviour and hormone physiology.

The Neural Mechanisms of Sexually Dimorphic Aggressive Behaviors

Aggression is a fundamental social behavior that is essential for competing for resources and protecting oneself and families in both males and females. As a result of natural selection, aggression is often displayed differentially between the sexes, typically at a higher level in males than females. Here, we highlight the behavioral differences between male and female aggression in rodents. We further outline the aggression circuits in males and females, and compare their differences at each circuit node. Lastly, we summarize our current understanding regarding the generation of sexually dimorphic aggression circuits during development and their maintenance during adulthood. In both cases, gonadal steroid hormones appear to play crucial roles in differentiating the circuits by impacting on the survival, morphology, and intrinsic properties of relevant cells. Many other factors, such as environment and experience, may also contribute to sex differences in aggression and remain to be investigated in future studies.

Constitutive differences in glucocorticoid responsiveness to stress are related to variation in aggression and anxiety-related behaviors

Glucocorticoids coordinate responses that enable an individual to cope with stressful challenges and, additionally, mediate adaptation following cessation of a stressor. There are important individual differences in the magnitude of glucocorticoid responsiveness to stressors. However, whether individual differences in glucocorticoid responsiveness to stress are linked to different behavioral strategies in coping with social and non-social challenges is not easily studied, owing to the lack of appropriate animal models. To address this, we generated three lines of Wistar rats selectively bred for the magnitude of their glucocorticoid responses following exposure to a variety of stressors over three consecutive days at juvenility. Here, we present findings following observations of a high level of variation in glucocorticoid responsiveness to stress in outbred Wistar rats, and the strong response to selection for this trait over a few generations. When challenged with different stressful challenges, rats from the three lines differed in their coping behaviors. Strikingly, the line with high glucocorticoid responsiveness to stress displayed enhanced aggression and anxiety-like behaviors. In addition, these rats also showed alterations in the expression of genes within both central and peripheral nodes of the hypothalamic-pituitary-adrenal (HPA) axis and enhanced reactivity to acute stress exposure. Together, these findings strongly link differences in glucocorticoid responsiveness to stress with marked differences in coping styles. The developed rat lines are thus a promising model with which to examine the relationship between variation in reactivity of the HPA axis and stress-related pathophysiology and could be employed to assess the therapeutic potential of treatments modulating stress habituation to ameliorate psychopathology.

The role of central and medial amygdala in normal and abnormal aggression: A review of classical approaches

The involvement of the amygdala in aggression is supported by overwhelming evidence. Frequently, however, the amygdala is studied as a whole, despite its complex internal organization. To reveal the role of various subdivisions, here we review the involvement of the central and medial amygdala in male rivalry aggression, maternal aggression, predatory aggression, and models of abnormal aggression where violent behavior is associated with increased or decreased arousal. We conclude that: (1) rivalry aggression is controlled by the medial amygdala; (2) predatory aggression is controlled by the central amygdala; (3) hypoarousal-associated violent aggression recruits both nuclei, (4) a specific upregulation of the medial amygdala was observed in hyperarousal-driven aggression. These patterns of amygdala activation were used to build four alternative models of the aggression circuitry, each being specific to particular forms of aggression. The separate study of the roles of amygdala subdivisions may not only improve our understanding of aggressive behavior, but also the differential control of aggression and violent behaviors of various types, including those associated with various psychopathologies.

Recent advances in understanding the role of the hypothalamic circuit during aggression

The hypothalamus was first implicated in the classic “fight or flight” response nearly a century ago, and since then, many important strides have been made in understanding both the circuitry and the neural dynamics underlying the generation of these behaviors. In this review, we will focus on the role of the hypothalamus in aggression, paying particular attention to recent advances in the field that have allowed for functional identification of relevant hypothalamic subnuclei. Recent progress in this field has been aided by the development of new techniques for functional manipulation including optogenetics and pharmacogenetics, as well as advances in technology used for chronic in vivo recordings during complex social behaviors. We will examine the role of the hypothalamus through the complimentary lenses of (1) loss of function studies, including pharmacology and pharmacogenetics; (2) gain of function studies, including specific comparisons between results from classic electrical stimulation studies and more recent work using optogenetics; and (3) neural activity, including both immediate early gene and awake-behaving recordings. Lastly, we will outline current approaches to identifying the precise role of the hypothalamus in promoting aggressive motivation and aggressive action.

Peripubertal stress-induced behavioral changes are associated with altered expression of genes involved in excitation and inhibition in the amygdala

Early-life stress is a critical risk factor for developing psychopathological alterations later in life. This early adverse environment has been modeled in rats by exposure to stress during the peripubertal period-that is, corresponding to childhood and puberty-and has been shown to lead to increased emotionality, decreased sociability and pathological aggression. The amygdala, particularly its central nucleus (CeA), is hyperactivated in this model, consistent with evidence implicating this nucleus in the regulation of social and aggressive behaviors. Here, we investigated potential changes in the gene expression of molecular markers of excitatory and inhibitory neurotransmission in the CeA. We found that peripubertal stress led to an increase in the expression of mRNA encoding NR1 (the obligatory subunit of the N-methyl D-aspartate (NMDA) receptor) but to a reduction in the level of mRNA encoding glutamic acid decarboxylase 67 (GAD67), an enzyme that is critically involved in the activity-dependent synthesis of GABA, and to an increase in the vesicular glutamate transporter 1 (VGLUT1)/vesicular GABA transporter (VGAT) ratio in the CeA. These molecular alterations were present in addition to increased novelty reactivity, sociability deficits and increased aggression. Our results also showed that the full extent of the peripubertal protocol was required for the observed behavioral and neurobiological effects because exposure during only the childhood/prepubertal period (Juvenile Stress) or the male pubertal period (Puberty Stress) was insufficient to elicit the same effects. These findings highlight peripuberty as a period in which stress can lead to long-term programming of the genes involved in excitatory and inhibitory neurotransmission in the CeA.

The glucocorticoid/aggression relationship in animals and humans: an analysis sensitive to behavioral characteristics, glucocorticoid secretion patterns, and neural mechanisms

Glucocorticoids control a wide array of biological processes from glucose homeostasis to neuronal function. The mechanisms mediating their effects are similarly varied and include rapid and transient nongenomic effects on calcium trafficking, various neurotransmitter receptors, and other membrane/cytoplasmic proteins, as well as slowly developing but durable genomic effects that are mediated by a large number of glucocorticoid-sensitive genes that are affected after variable lag-times. Given this complexity, we suggest that the aggression/glucocorticoid relationship cannot be reduced to the simple "stimulation/inhibition" question. Here, we review the effects of glucocorticoids on aggression by taking into account the complexities of glucocorticoid actions. Acute and chronic effects were differentiated because these are mediated by different mechanisms. The effects of chronic increases and decreases in glucocorticoid production were discussed separately, because the activation of mechanisms that are not normally activated and the loss of normal functions should not be confounded. Findings in healthy/normal subjects and those obtained in subjects that show abnormal forms of behavior or psychopathologies were also differentiated, because the effects of glucocorticoids are indirect, and largely depend on the properties of neurons they act upon, which are altered in subjects with psychopathologies. In addition, the conditions of glucocorticoid measurements were also thoroughly evaluated. Although the role of glucocorticoids in aggression is perceived as controversial by many investigators, a detailed analysis that is sensitive to glucocorticoid and behavioral measure as well as to the mediating mechanism suggests that this role is rather clear-cut; moreover, there is a marked similarity between animal and human findings.

Hypothalamic attack: a wonderful artifact or a useful perspective on escalation and pathology in aggression? A viewpoint

W.R. Hess' early demonstration of aggressive responses evoked by electrical stimulation in the cat's hypothalamus had a significant impact on the development of psychological and behavioral concepts. Many ideas on behavioral routines, allegedly organized in the brainstem, derive from his observation. Similar responses have since been evoked from the hypothalamus of many different species, suggesting that the mechanism mediating these responses is evolutionarily well preserved. However, these effects have also been portrayed as artificial responses to an artificial stimulus in an artificial environment. True enough; after many years of research, crucial questions on the underlying mechanism remain unanswered. Questions such as: How do they emerge in the first place? What neuronal elements mediate these responses? What is their role in "spontaneous" aggression? In the first part of this chapter we show methodology to study such questions in a consistent way using behavioral, physiological, anatomical, and pharmacological findings on hypothalamic attack in rats. In the second part we suggest that one important function of the underlying mechanism is to match the dynamics of the endocrine stress response with the dynamics of the behavioral and physiological requirements of coping with conflicts. This neuroendocrine-behavioral matching seems crucial right from the first emergence of the aggressive response in inexperienced animals, up to the full-blown violent responding in fully experienced animals. Impeding these essential functions results in inadequate coping with conflicts. The stress response during a first conflict in an inexperienced individual in an unfamiliar environment seems to rapidly initialize a crucial change in a mechanism involved in the appraisal of social signals during conflict. That change has enduring consequences for future conflict strategies. This concept opens another perspective on "escalated" or "pathological" aggression, especially so in individuals with a dysfunctional stress response.

Excessive aggression as model of violence: a critical evaluation of current preclinical methods

RATIONALE

Preclinical experimental models of pathological aggressive behavior are a sorely understudied and difficult research area.

OBJECTIVES

How valid, reliable, productive, and informative are the most frequently used animal models of excessive aggressive behavior?

METHODS

The rationale, key methodological features, supporting data, and arguments as well as their disadvantages and limitations of the most frequently used animal models for excessive aggressive behavior are summarized and their validity and reliability are evaluated.

RESULTS

Excessive aggressive behavior is validly and reliably seen in (1) a proportion of feral-derived rats and selectively bred mice; (2) rats with compromised adrenal function resulting in a hypoglucocorticoid state; (3) a significant minority of mice, rats, and monkeys after consumption of a moderate dose of alcohol; and (4) resident animals of various species after social instigation. Limitations of these procedures include restrictive animal research regulations, the requirement of expertise in surgical, pharmacological, and behavioral techniques, and the behaviorally impoverished mouse strains that are used in molecular genetics research. Promising recent initiatives for novel experimental models include aggressive behaviors that are evoked by optogenetic stimulation and induced by the manipulation of early social experiences such as isolation rearing or social stress.

CONCLUSIONS

One of the most significant challenges for animal models of excessive, potentially abnormal aggressive behavior is the characterization of distinctive neurobiological mechanisms that differ from those governing species-typical aggressive behavior. Identifying novel targets for effective intervention requires increased understanding of the distinctive molecular, cellular, and circuit mechanisms for each type of abnormal aggressive behavior.

Levodopa-induced dyskinesia is strongly associated with resonant cortical oscillations

The standard pharmacological treatment for Parkinson's disease using the dopamine precursor levodopa is unfortunately limited by gradual development of disabling involuntary movements for which the underlying causes are poorly understood. Here we show that levodopa-induced dyskinesia in hemiparkinsonian rats is strongly associated with pronounced 80 Hz local field potential oscillations in the primary motor cortex following levodopa treatment. When this oscillation is interrupted by application of a dopamine antagonist onto the cortical surface the dyskinetic symptoms disappear. The finding that abnormal cortical oscillations are a key pathophysiological mechanism calls for a revision of the prevailing hypothesis that links levodopa-induced dyskinesia to an altered sensitivity to dopamine only in the striatum. Apart from having important implications for the treatment of Parkinson's disease, the discovered pathophysiological mechanism may also play a role in several other psychiatric and neurological conditions involving cortical dysfunction.

Neurogenetics of aggressive behavior: studies in rodents

Aggressive behavior is observed in many animal species, such as insects, fish, lizards, frogs, and most mammals including humans. This wide range of conservation underscores the importance of aggressive behavior in the animals' survival and fitness, and the likely heritability of this behavior. Although typical patterns of aggressive behavior differ between species, there are several concordances in the neurobiology of aggression among rodents, primates, and humans. Studies with rodent models may eventually help us to understand the neurogenetic architecture of aggression in humans. However, it is important to recognize the difference between the ecological and ethological significance of aggressive behavior (species-typical aggression) and maladaptive violence (escalated aggression) when applying the findings of aggression research using animal models to human or veterinary medicine. Well-studied rodent models for aggressive behavior in the laboratory setting include the mouse (Mus musculus), rat (Rattus norvegicus), hamster (Mesocricetus auratus), and prairie vole (Microtus ochrogaster). The neural circuits of rodent aggression have been gradually elucidated by several techniques, e.g., immunohistochemistry of immediate-early gene (c-Fos) expression, intracranial drug microinjection, in vivo microdialysis, and optogenetics techniques. Also, evidence accumulated from the analysis of gene-knockout mice shows the involvement of several genes in aggression. Here, we review the brain circuits that have been implicated in aggression, such as the hypothalamus, prefrontal cortex (PFC), dorsal raphe nucleus (DRN), nucleus accumbens (NAc), and olfactory system. We then discuss the roles of glutamate and γ-aminobutyric acid (GABA), excitatory and inhibitory amino acids in the brain, as well as their receptors, in controlling aggressive behavior, focusing mainly on recent findings. At the end of this chapter, we discuss how genes can be identified that underlie individual differences in aggression, using the so-called forward genetics approach.

High and abnormal forms of aggression in rats with extremes in trait anxiety--involvement of the dopamine system in the nucleus accumbens

A better neurobiological understanding of high and abnormal aggression based on adequate animal models is essential for novel therapy and prevention. Selective breeding of rats for extremes in anxiety-related behavior resulted in two behavioral phenotypes with high and abnormal forms of aggression. Rats bred for low anxiety-related behavior (LAB) consistently show highest levels of aggression and little social investigation in the resident-intruder (RI) test, compared with non-selected low-aggressive (NAB) rats. High anxiety-related (HAB) rats also show higher levels of aggression than NAB rats, but to a lesser extent than LAB rats. Accordingly, extremes in inborn anxiety in both directions are linked to an increased aggression level. Further, both LAB and HAB, but not NAB males, display abnormal aggression (attacks towards vulnerable body parts, females or narcotized males), which is particularly prominent in LABs. Also, only in LAB rats, the nucleus accumbens (NAc) was found to be strongly activated in response to the RI test as reflected by increased c-fos and zif268 mRNA expression, and higher local dopamine release compared with NAB males, without differences in local dopamine receptor binding. Consequently, local pharmacological manipulation by infusion of an anesthetic (lidocaine, 20 μg/μl) or a dopamine D2 (haloperidol, 10 ng/μl), but not D1 (SCH-23390 10 ng/μl), receptor antagonist significantly reduced high aggression in LAB rats. Thus, LAB rats are an adequate model to study high and abnormal aggression. In LAB males, this is likely to be linked to hyper-activation of the reward system, as found in psychopathic patients. Specifically, activation of the accumbal dopamine system is likely to underlie the high aggression observed in LAB rats.

The neurobiology of abnormal manifestations of aggression--a review of hypothalamic mechanisms in cats, rodents, and humans

Aggression research was for long dominated by the assumption that aggression-related psychopathologies result from the excessive activation of aggression-promoting brain mechanisms. This assumption was recently challenged by findings with models of aggression that mimic etiological factors of aggression-related psychopathologies. Subjects submitted to such procedures show abnormal attack features (mismatch between provocation and response, disregard of species-specific rules, and insensitivity toward the social signals of opponents). We review here 12 such laboratory models and the available human findings on the neural background of abnormal aggression. We focus on the hypothalamus, a region tightly involved in the execution of attacks. Data show that the hypothalamic mechanisms controlling attacks (general activation levels, local serotonin, vasopressin, substance P, glutamate, GABA, and dopamine neurotransmission) undergo etiological factor-dependent changes. Findings suggest that the emotional component of attacks differentiates two basic types of hypothalamic mechanisms. Aggression associated with increased arousal (emotional/reactive aggression) is paralleled by increased mediobasal hypothalamic activation, increased hypothalamic vasopressinergic, but diminished hypothalamic serotonergic neurotransmission. In aggression models associated with low arousal (unemotional/proactive aggression), the lateral but not the mediobasal hypothalamus is over-activated. In addition, the anti-aggressive effect of serotonergic neurotransmission is lost and paradoxical changes were noticed in vasopressinergic neurotransmission. We conclude that there is no single 'neurobiological road' to abnormal aggression: the neural background shows qualitative, etiological factor-dependent differences. Findings obtained with different models should be viewed as alternative mechanisms rather than conflicting data. The relevance of these findings for understanding and treating of aggression-related psychopathologies is discussed. This article is part of a Special Issue entitled 'Extrasynaptic ionotropic receptors'.

Maternal neglect with reduced depressive-like behavior and blunted c-fos activation in Brattleboro mothers, the role of central vasopressin

Early mother-infant relationships exert important long-term effects in offspring and are disturbed by factors such as postpartum depression. We aimed to clarify if lack of vasopressin influences maternal behavior paralleled by the development of a depressive-like phenotype. We compared vasopressin-deficient Brattleboro mothers with heterozygous and homozygous normal ones. The following parameters were measured: maternal behavior (undisturbed and separation-induced); anxiety by the elevated plus maze; sucrose and saccharin preference and forced swim behavior. Underlying brain areas were examined by c-fos immunocytochemistry among rest and after swim-stress. In another group of rats, vasopressin 2 receptor agonist was used peripherally to exclude secondary changes due to diabetes insipidus. Results showed that vasopressin-deficient rats spend less time licking-grooming their pups through a centrally driven mechanism. There was no difference between genotypes during the pup retrieval test. Vasopressin-deficient mothers tended to explore more the open arms of the plus maze, showed more preference for sucrose and saccharin and struggled more in the forced swim test, suggesting that they act as less depressive. Under basal conditions, vasopressin-deficient mothers had more c-fos expression in the medial preoptic area, shell of nucleus accumbens, paraventricular nucleus of the hypothalamus and amygdala, but not in other structures. In these areas the swim-stress-induced activation was smaller. In conclusion, vasopressin-deficiency resulted in maternal neglect due to a central effect and was protective against depressive-like behavior probably as a consequence of reduced activation of some stress-related brain structures. The conflicting behavioral data underscores the need for more sex specific studies.

Identification of neuronal loci involved with displays of affective aggression in NC900 mice

Aggression is a complex behavior that is essential for survival. Of the various forms of aggression, impulsive violent displays without prior planning or deliberation are referred to as affective aggression. Affective aggression is thought to be caused by aberrant perceptions of, and consequent responses to, threat. Understanding the neuronal networks that regulate affective aggression is pivotal to development of novel approaches to treat chronic affective aggression. Here, we provide a detailed anatomical map of neuronal activity in the forebrain of two inbred lines of mice that were selected for low (NC100) and high (NC900) affective aggression. Attack behavior was induced in male NC900 mice by exposure to an unfamiliar male in a novel environment. Forebrain maps of c-Fos+ nuclei, which are surrogates for neuronal activity during behavior, were then generated and analyzed. NC100 males rarely exhibited affective aggression in response to the same stimulus, thus their forebrain c-Fos maps were utilized to identify unique patterns of neuronal activity in NC900s. Quantitative results indicated robust differences in the distribution patterns and densities of c-Fos+ nuclei in distinct thalamic, subthalamic, and amygdaloid nuclei, together with unique patterns of neuronal activity in the nucleus accumbens and the frontal cortices. Our findings implicate these areas as foci regulating differential behavioral responses to an unfamiliar male in NC900 mice when expressing affective aggression. Based on the highly conserved patterns of connections and organization of neuronal limbic structures from mice to humans, we speculate that neuronal activities in analogous networks may be disrupted in humans prone to maladaptive affective aggression.

Temporal changes in c-Fos activation patterns induced by conditioned fear

Mechanisms underlying shock-induced conditioned fear - a paradigm frequently used to model posttraumatic stress disorder, PTSD - are usually studied shortly after shocks. Some of the brain regions relevant to conditioned fear were activated in all the c-Fos studies published so far, but the overlap between the activated regions was small across studies. We hypothesized that discrepant findings were due to dynamic neural changes that followed shocks, and a more consistent picture would emerge if consequences were studied after a longer interval. Therefore, we exposed rats to a single session of footshocks and studied their behavioral and neural responses one and 28 days later. The neuronal activation marker c-Fos was studied in 24 brain regions relevant for conditioned fear, e.g. in subdivisions of the prefrontal cortex, hippocampus, amygdala, hypothalamic defensive system, brainstem monoaminergic nuclei and periaqueductal gray. The intensity of conditioned fear (as shown by the duration of contextual freezing) was similar at the two time-points, but the associated neuronal changes were qualitatively different. Surprisingly, however, Multiple Regression Analyses suggested that conditioned fear-induced changes in neuronal activation patterns predicted the duration of freezing with high accuracy at both time points. We suggest that exposure to electric shocks is followed by a period of plasticity where the mechanisms that sustain conditioned fear undergo qualitative changes. Neuronal changes observed 28 days but not 1 day after shocks were consistent with those observed in human studies performed in PTSD patients.

Gene expression in aminergic and peptidergic cells during aggression and defeat: relevance to violence, depression and drug abuse

In this review, we examine how experiences in social confrontations alter gene expression in mesocorticolimbic cells. The focus is on the target of attack and threat due to the prominent role of social defeat stress in the study of coping mechanisms and victimization. The initial operational definition of the socially defeated mouse by Ginsburg and Allee (1942) enabled the characterization of key endocrine, cardiovascular, and metabolic events during the initial response to an aggressive opponent and during the ensuing adaptations. Brief episodes of social defeat stress induce an augmented response to stimulant challenge as reflected by increased locomotion and increased extracellular dopamine (DA) in the nucleus accumbens (NAC). Cells in the ventral tegmental area (VTA) that project to the NAC were more active as indicated by increased expression of c-fos and Fos-immunoreactivity and BDNF. Intermittent episodes of social defeat stress result in increased mRNA for MOR in brainstem and limbic structures. These behavioral and neurobiological indices of sensitization persist for several months after the stress experience. The episodically defeated rats also self-administered intravenous cocaine during continuous access for 24 h ("binge"). By contrast, continuous social stress, particularly in the form of social subordination stress, leads to reduced appetite, compromised endocrine activities, and cardiovascular and metabolic abnormalities, and prefer sweets less as index of anhedonia. Cocaine challenges in subordinate rats result in a blunted psychomotor stimulant response and a reduced DA release in NAC. Subordinate rats self-administer cocaine less during continuous access conditions. These contrasting patterns of social stress result from continuous vs. intermittent exposure to social stress, suggesting divergent neuroadaptations for increased vulnerability to cocaine self-administration vs. deteriorated reward mechanisms characteristic of depressive-like profiles.

Exogenous cortisol facilitates responses to social threat under high provocation

Stress is one of the most important promoters of aggression. Human and animal studies have found associations between basal and acute levels of the stress hormone cortisol and (abnormal) aggression. Irrespective of the direction of these changes--i.e., increased or decreased aggressive behavior--the results of these studies suggest dramatic alterations in the processing of threat-related social information. Therefore, the effects of cortisol and provocation on social information processing were addressed by the present study. After a placebo-controlled pharmacological manipulation of acute cortisol levels, we exposed healthy individuals to high or low levels of provocation in a competitive aggression paradigm. Influences of cortisol and provocation on emotional face processing were then investigated with reaction times and event-related potentials (ERPs) in an emotional Stroop task. In line with previous results, enhanced early and later positive, posterior ERP components indicated a provocation-induced enhanced relevance for all kinds of social information. Cortisol, however, reduced an early frontocentral bias for angry faces and--despite the provocation-enhancing relevance--led to faster reactions for all facial expressions in highly provoked participants. The results thus support the moderating role of social information processing in the 'vicious circle of stress and aggression'.

The relationship between basal and acute HPA axis activity and aggressive behavior in adults

The hypothalamic-pituitary-adrenal (HPA) axis seems to play a major role in the development, elicitation, and enhancement of aggressive behavior in animals. Increasing evidence suggests that this is also true for humans. However, most human research on the role of the HPA axis in aggression has been focusing on highly aggressive children and adolescent clinical samples. Here, we report on a study of the role of basal and acute HPA axis activity in a sample of 20 healthy male and female adults. We used the Taylor Aggression Paradigm to induce and measure aggression. We assessed the cortisol awakening response as a trait measure of basal HPA axis activity. Salivary free cortisol measures for the cortisol awakening response were obtained on three consecutive weekdays immediately following awakening and 30, 45, and 60 min after. Half of the subjects were provoked with the Taylor Aggression Paradigm to behave aggressively; the other half was not provoked. Acute HPA axis activity was measured four times, once before and three times after the induction of aggression. Basal cortisol levels were significantly and negatively related to aggressive behavior in the provoked group and explained 67% of the behavioral variance. Cortisol levels following the induction of aggression were significantly higher in the provoked group when baseline levels were taken into account. The data implicate that the HPA axis is not only relevant to the expression of aggressive behavior in clinical groups, but also to a large extent in healthy ones.

Early life stress, the development of aggression and neuroendocrine and neurobiological correlates: what can we learn from animal models?

Early life stress (child and adolescent abuse, neglect and trauma) induces robust alterations in emotional and social functioning resulting in enhanced risk for the development of psychopathologies such as mood and aggressive disorders. Here, an overview is given on recent findings in primate and rodent models of early life stress, demonstrating that chronic deprivation of early maternal care as well as chronic deprivation of early physical interactions with peers are profound risk factors for the development of inappropriate aggressive behaviors. Alterations in the hypothalamic-pituitary-adrenocortical (HPA), vasopressin and serotonin systems and their relevance for the regulation of aggression are discussed. Data suggest that social deprivation-induced inappropriate forms of aggression are associated with high or low HPA axis (re)activity and a generally lower functioning of the serotonin system in adulthood. Moreover, genetic and epigenetic modifications in HPA and serotonin systems influence the outcome of early life stress and may even moderate adverse effects of early social deprivation on aggression. A more comprehensive study of aggression, neuroendocrine, neurobiological and (epi)genetic correlates of early life stress using animal models is necessary to provide a better understanding of the invasive aggressive deficits observed in humans exposed to child maltreatment.

Neuroendocrine regulation and physical and relational aggression: the moderating roles of child maltreatment and gender

The purpose of the present investigation was to examine the association between circadian rhythms of cortisol and physical and relational aggression. Morning arrival, prelunch, and afternoon predeparture salivary cortisol were assessed among 418 maltreated and nonmaltreated children (52% maltreated; 49% female) attending a summer day camp. Counselors and peers rated participants' involvement in physically and relationally aggressive behaviors. Results indicated that physical aggression was associated with heightened cortisol following morning arrival and relatively steep declines in cortisol over the day, whereas relational aggression was associated with low cortisol following morning arrival and blunted diurnal change in cortisol. Moreover, maltreatment was a significant moderator of this relationship such that aggression was related to greater cortisol dysregulation among nonmaltreated than among maltreated children. The findings suggest that physiological correlates of aggression may differ for physical and relational forms of aggression and among maltreated versus nonmaltreated populations.

How can the study of biological processes help design new interventions for children with severe antisocial behavior?

Children with severe antisocial behavior have an increased risk of showing violently aggressive and other forms of problem behavior in adolescence and adulthood. It is well established that both biological and social factors are involved in the development of antisocial behavior. The primary aim of this paper is to discuss the evidence that specific neurobiological systems are involved in the etiology of childhood-onset antisocial behavior. These factors are responsible for the severity of the behavioral problems observed in antisocial children, but they also play a role in their persistence, because they influence children's interactions with their environment. We will discuss the possible causes of disruptions in neurobiological systems in childhood antisocial behavior and point out the implications of these findings for theory and clinical practice. We will argue that familial factors (e.g., genetic influences, early childhood adversity) are linked to negative behavioral outcomes (e.g., antisocial behavior problems) through the mediating and transactional interplay with neurobiological deficits. An investigation of neurobiological functioning in antisocial children might not only indicate which children are most likely to persist in engaging in severe antisocial behavior, but also guide the development of new interventions.

Lasting changes in social behavior and amygdala function following traumatic experience induced by a single series of foot-shocks

Neuronal plasticity within the amygdala mediates many behavioral effects of traumatic experience, and this brain region also controls various aspects of social behavior. However, the specific involvement of the amygdala in trauma-induced social deficits has never been systematically investigated. We exposed rats to a single series of electric foot-shocks--a frequently used model of trauma--and studied their behavior in the social avoidance and psychosocial stimulation tests (non-contact versions of the social interaction test) at different time intervals. Social interaction-induced neuronal activation patterns were studied in the prefrontal cortex (orbitofrontal and medial), amygdala (central, medial, and basolateral), dorsal raphe and locus coeruleus. Shock exposure markedly inhibited social behavior in both tests. The effect lasted at least 4 weeks, and amplified over time. As shown by c-Fos immunocytochemistry, social interactions activated all the investigated brain areas. Traumatic experience exacerbated this activation in the central and basolateral amygdala, but not in other regions. The tight correlation between the social deficit and amygdala activation patterns suggest that the two phenomena were associated. A real-time PCR study showed that CRF mRNA expression in the amygdala was temporarily reduced 14, but not 1 and 28 days after shock exposure. In contrast, amygdalar NK1 receptor mRNA expression increased throughout. Thus, the trauma-induced social deficits appear to be associated with, and possibly caused by, plastic changes in fear-related amygdala subdivisions.

Central actions of arginine vasopressin and a V1a receptor antagonist on maternal aggression, maternal behavior, and grooming in lactating rats

Maternal aggression is a robust type of aggression displayed by lactating female rats. Although arginine vasopressin (AVP) has been implicated in the control of male aggression, its involvement in maternal aggression has not been thoroughly investigated. Previous neuroanatomical studies suggest that AVP may mediate the display of aggression during lactation. In the current study, AVP and an AVP V1a receptor antagonist were centrally administered to primiparous rats on days 5 and 15 of lactation, and aggression, maternal behavior, and grooming were recorded. Although AVP did not affect the number of attacks or duration of aggression, it increased the latency to initiate aggression on day 5, in addition to decreasing maternal behavior and increasing grooming. Conversely, V1a antagonist treatment increased maternal aggression on both days of lactation, decreased maternal behavior on day 15, and decreased grooming on day 5. Thus, it appears that central AVP activity modulates maternal aggression, as well as maternal behavior and grooming behavior during lactation.

The effect of neurokinin1 receptor blockade on territorial aggression and in a model of violent aggression

BACKGROUND

Neurokinin1 (NK1) receptor blockers were recently proposed for the treatment of anxiety and depression. Disparate data suggest that NK1 receptors are also involved in the control of aggressiveness, but their role is poorly known.

METHODS

We evaluated the aggression-induced activation of NK1 neurons by double-labeling brain sections for NK1 receptors and c-Fos in two laboratory models of aggression. We also studied the effects of the NK1 antagonist L-703,606 in these models.

RESULTS

Aggressive encounters activated a large number of NK1 receptor-expressing neurons in areas relevant for aggression control. The activation was aggression-specific, because the effects of psychosocial encounters (that allowed sensory but not physical contacts) were markedly weaker. In the medial amygdala, the activation of neurons expressing NK1 receptors showed a marked positive correlation with the occurrence of violent attacks. In resident/intruder conflicts, NK1 blockade lowered the number of hard bites, without affecting milder forms of attack. In the model of violent aggression, attacks on vulnerable body parts of opponents (the main indicators of violence in this model) were decreased to the levels seen in control subjects. Autonomic deficits seen in the model of violent aggression were also ameliorated. The effects of the compound were not secondary to changes in locomotion or in the behavior of intruders.

CONCLUSIONS

Our data show that neurons expressing NK1 receptors are involved in the control of aggressiveness, especially in the expression of violent attacks. This suggests that NK1 antagonists-beyond anxiety and depression-might also be useful in the treatment of aggressiveness and violence.

Glucocorticoid hyper- and hypofunction: stress effects on cognition and aggression

It is now well documented that both increased and decreased stress responses can profoundly affect cognition and behavior. This mini review presents possible neural mechanisms subserving stress effects on memory and aggression, particularly focusing on glucocorticoid (GC) hyper- and hypofunction. First, uncontrollable stress impedes hippocampal memory and long-term potentiation (LTP). Because the hippocampus is important for the stability of long-term memory and because LTP has qualities desirable of an information storage mechanism, it has been hypothesized that stress-induced alterations in LTP contribute to memory impairments. Recent evidence suggests a neural-endocrine network comprising amygdala, prefrontal cortex (PFC), and glucocorticoids may be involved in regulating stress effects on hippocampal mnemonic functioning. Second, antisocial aggressiveness correlates with chronically decreased glucocorticoid production, and this condition leads in rats to behavioral-autonomic deficits reminiscent of the human disorder. Glucocorticoid deficiency-induced antisocial aggressiveness results from functional changes in the PFC, medial and central amygdala, and altered serotonin and substance P neurotransmissions. Accordingly, a neurobiological understanding of how stress and glucocorticoid deficiency alter brain, cognition, and behavior is an important challenge facing modern neuroscience with broad implications for individual and social well-being.

Neurobiological Mechanisms of Aggression and Stress Coping: A Comparative Study in Mouse and Rat Selection Lines

Aggression causes major health and social problems and constitutes a central problem in several psychiatric disorders. There is a close relationship between the display of aggression and stress coping strategies. In order to gain more insight into biochemical pathways associated with aggression and stress coping, we assessed behavioral and neurobiological responses in two genetically selected rodent models, namely wild house mice selectively bred for a short (SAL) and long (LAL) attack latency and Wistar rats bred for high (HAB) or low (LAB) anxiety-related behavior. Compared to their line counterparts, the SAL mice and the LAB rats display a high level of intermale aggression associated with a proactive coping style. Both the SAL mice and the LAB rats show a reduced hypothalamic-pituitary-adrenal (HPA) axis response to non-social stressors. However, when exposed to social stressors (resident-intruder, sensory contact), SAL mice show an attenuated HPA response, whereas LAB rats show an elevated HPA response. In both rodent lines, the display of aggression is associated with high neuronal activation in the central amygdala, but reduced neuronal activation in the lateral septum. Furthermore, in the lateral septum, SAL mice have a reduced vasopressinergic fiber network, and LAB rats show a decreased vasopressin release during the display of aggression. Moreover, the two lines show several indications of an increased serotonergic neurotransmission. The relevance of these findings in relation to high aggression and stress coping is discussed. In conclusion, exploring neurobiological systems in animals sharing relevant behavioral characteristics might be a useful approach to identify general mechanisms of action, which in turn can improve our understanding of specific behavioral symptoms in human psychiatric disorders.

The effect of buspirone on normal and hypoarousal-driven abnormal aggression in rats

Aggressiveness is associated with decreased glucocorticoid production, autonomic hypoarousal, and social deficits in antisocial personality disorder and its childhood antecedent conduct disorder. We showed previously that experimentally induced chronic glucocorticoid deficiency leads to abnormal forms of attack, autonomic hypoarousal, and social deficits in rats. We also showed that serotonergic neurotransmission, which downregulates aggressiveness in normal rats appears to lose its aggression-controlling role in glucocorticoid-deficient rats. We suggested that abnormal aggression develops in such rats as a consequence of serotonergic disturbances that result from chronic glucocorticoid deficiency. Here we assessed the effects of the serotonergic anxiolytic buspirone on aggressive behavior in normal and glucocorticoid-deficient rats. Noteworthy, this compound is frequently used in the clinic to control moderate aggression problems. As expected, buspirone dose-dependently reduced the duration of agonistic behaviors in normal rats exposed to resident/intruder conflicts. Similar to earlier experiments, glucocorticoid deficiency dramatically increased the share of attacks directed towards vulnerable body parts of the opponents (head, throat and belly). Surprisingly, 1 and 5 mg/kg buspirone dramatically increased the frequency of biting attacks in glucocorticoid-deficient rats. The share of vulnerable attacks remained as high as in vehicle-treated glucocorticoid-deficient rats. These data show that chronic glucocorticoid deficiency disturbs serotonergic neurotransmission, which reverses the aggression-related effects of the serotonergic agent buspirone. This finding is in line with disparate human findings on the effects of serotonergic agents on aggression in antisocial personality disordered people.

Low inborn anxiety correlates with high intermale aggression: link to ACTH response and neuronal activation of the hypothalamic paraventricular nucleus

Aggression constitutes a central problem in several psychopathologies, including anxiety and depression disorders and antisocial behaviors. In particular, the activity of the hypothalamic-pituitary-adrenocortical (HPA) axis has been associated with aggression-related disorders. The present study assessed whether genetically determined levels of anxiety-related behavior influence the level of intermale aggression and whether this is associated with differences in neuroendocrine responsiveness and neuronal activation in the brain. Adult male Wistar rats bred for high (HAB) or low (LAB) anxiety-related behavior were used, as well as non-selected rats (NAB) with an intermediate anxiety level. LAB residents displayed more aggressive behavior than HAB and NAB residents during the resident-intruder (RI) test. Moreover, an inverse correlation was found between the level of anxiety and the level of aggression. The plasma corticotropin (ACTH) response to RI-test exposure was significantly higher in LABs than in HABs and NABs, indicating that a higher level of aggression was linked to an elevated hormonal stress response. Furthermore, LAB residents showed more neuronal activation in the parvocellular part of the hypothalamic paraventricular nucleus (PVN) than HAB residents 1 h after the RI-test. In addition, a tendency toward a higher number of c-Fos-positive cells in LABs compared with HABs was observed in the medial amygdala, hypothalamic attack area and central amygdala, areas relevant for the regulation of aggression. These data demonstrate that low trait anxiety is correlated with high intermale aggression. Furthermore, the increased neuronal activation of the PVN along with the higher ACTH responsiveness might underlie the display of high aggression.