Variation in aggressiveness in house mouse populations

Behavioral coping with chronic defeat stress in mice: A systematic review of current protocols

Social stress is the most significant source of chronic stress in humans and is commonly associated with health impairment. Individual differences in the behavioral coping responses to stress have been proposed to mediate the negative effects of stress on physical, behavioral and mental health. Animal models, particularly mice, offer valuable insights into the physiological and neurobiological correlates of behavioral coping strategies in response to chronic social stress. Here we aim to identify differences and similarities among stress protocols in mice, with particular attention to how neuroendocrine and/or behavioral responses vary according to different coping strategies, while highlighting the need for standardized approaches in future research. A systematic review was undertaken following the Preferred Reporting Items for Systematic Reviews and Meta-Analysis (PRISMA statement). A total of 213 references were identified by electronic search, and after the screening, 18 articles were found to meet all the established criteria. We analyzed differences in the stress protocol, the characterization and classification of coping strategies and the physiological and behavioral differences according to coping. The results show that differences in behavioural expression under chronic social stress (coping) may also be associated with physiological differences and differential susceptibility to disease. However, this review also underlines the importance of a cautious interpretation of the results obtained. The lack of consistency in the nomenclature and procedures associated with the study of coping strategies for social stress, as well as the absence of a uniform classification, highlight the importance of using a common language when approaching the study of coping strategies. Thereby, this review encourages the development of a more defined method and criteria for assessing coping strategies, based on both behavioral and biological indicators.

Behavioural and psychological symptoms of dementia in mouse models of Alzheimer's disease-related pathology

Transgenic mouse models have been used extensively to model the cognitive impairments arising from Alzheimer's disease (AD)-related pathology. However, less is known about the relationship between AD-related pathology and the behavioural and psychological symptoms of dementia (BPSD) commonly presented by patients. This review discusses the BPSD-like behaviours recapitulated by several mouse models of AD-related pathology, including the APP/PS1, Tg2576, 3xTg-AD, 5xFAD, and APP23 models. Current evidence suggests that social withdrawal and depressive-like behaviours increase with progressive neuropathology, and increased aggression and sleep-wake disturbances are present even at early stages; however, there is no clear evidence to support increased anxiety-like behaviours, agitation (hyperactivity), or general apathy. Overall, transgenic mouse models of AD-related pathology recapitulate some of the BPSD-like behaviours associated with AD, but these behaviours vary by model. This reflects the patient population, where AD patients typically exhibit one or more BPSD, but rarely all symptoms at once. As a result, we suggest that transgenic mouse models are an important tool to investigate the pathology underlying BPSD in human AD patients.

Reduced social investigation and increased injurious behavior in transgenic 5xFAD mice

Social withdrawal and agitation/aggression are common behavioral and psychological symptoms of dementia presented by Alzheimer's disease (AD) patients, with males exhibiting more aggressive behaviors than females. Some transgenic mouse models of AD also exhibit social withdrawal and aggression, but many of these models only recapitulate the early stages of the disease. By comparison, the 5xFAD mouse model of AD exhibits rapid, progressive neurodegeneration, and is suitable for modeling cognitive and behavioral deficits at early, mid-, and late-stage disease progression. Anecdotal reports suggest that transgenic 5xFAD males exhibit high levels of aggression compared to wild-type controls, but to date, indirect genetic effects in this strain have not been studied. We measured home-cage behaviors in 5xFAD males housed in three different group-housing conditions (transgenic-only, wild-type only, and mixed-genotype) and social approach behaviors when exposed to a novel free-roaming or restrained, wild-type or transgenic conspecific. Transgenic-only home cages required earlier separation due to injuries arising from aggression compared to wild-type-only or mixed-genotype cages, despite no obvious increase in the frequency of aggressive behaviors. Transgenic 5xFAD males and females also spent less time investigating free-roaming conspecifics compared to wild-type controls, but they showed normal investigation of restrained conspecifics; the genotype of the conspecific did not affect approach behavior, and there was no aggression observed in transgenic males. These findings provide evidence in an animal model that amyloid pathology ultimately leads to avoidance of novel social stimuli, and that frequent interactions between individuals exhibiting an AD phenotype further exacerbates aggressive behaviors.

Social stress shortens lifespan in mice

Stress and low socioeconomic status in humans confer increased vulnerability to morbidity and mortality. However, this association is not mechanistically understood nor has its causation been explored in animal models thus far. Recently, cellular senescence has been suggested as a potential mechanism linking lifelong stress to age‐related diseases and shorter life expectancy in humans. Here, we established a causal role for lifelong social stress on shortening lifespan and increasing the risk of cardiovascular disease in mice. Specifically, we developed a lifelong chronic psychosocial stress model in which male mouse aggressive behavior is used to study the impact of negative social confrontations on healthspan and lifespan. C57BL/6J mice identified through unbiased cluster analysis for receiving high while exhibiting low aggression, or identified as subordinate based on an ethologic criterion, had lower median and maximal lifespan, and developed earlier onset of several organ pathologies in the presence of a cellular senescence signature. Critically, subordinate mice developed spontaneous early‐stage atherosclerotic lesions of the aortic sinuses characterized by significant immune cells infiltration and sporadic rupture and calcification, none of which was found in dominant subjects. In conclusion, we present here the first rodent model to study and mechanistically dissect the impact of chronic stress on lifespan and disease of aging. These data highlight a conserved role for social stress and low social status on shortening lifespan and increasing the risk of cardiovascular disease in mammals and identify a potential mechanistic link for this complex phenomenon.

Development of the social behavior of two mice species with contrasting social systems

The house mouse (Mus musculus) and the mound-building mouse (M. spicilegus) differ in their mating and social systems. The M. musculus is polygynous and females tend to breed cooperatively while M. spicilegus is known to be monogamous and famous for its unique cooperative behavior; the communal overwintering. Mus spicilegus is considered as a highly aggressive species in the genus Mus. In the present study, we attempted to analyze the development of aggressive and sociable behavior in these species and to discover how familiarity with the target moderates the development of sociable and aggressive behaviors and moderates the differences between the species. Dyadic social interaction tests in neutral cages were performed on 21-, 60-, and 120-day-old mice of both genders and both species. Each individual was tested against a sibling and an unfamiliar individual at all three ages. Our results showed that the development of aggressive and sociable behavior with age differed between the species and sexes; however, familiarity with the other mouse moderated the effect of species on aggression. At 21 days, both genders of M. spicilegus were more aggressive against strangers than siblings. This became true of both species at 60 days. When facing a stranger, both sexes of M. spicilegus were more aggressive than M. musculus at 120 days. However, when facing a sibling, neither gender of M. spicilegus was more agonistic than M. musculus, indicating that either kinship or early social experiences elicit tolerance.

No postnatal maternal effect on male aggressiveness in wild-derived strains of house mice

Male aggressiveness is a complex behavior influenced by a number of genetic and non-genetic factors. Traditionally, the contribution of each of these factors has been established from experiments using artificially selected strains for high/low aggressive phenotypes. However, little is known about the factors underlying aggressive behavior in natural populations. In this study, we assess the influence of genetic background vs. postnatal maternal environment using a set of cross-fostering experiments between two wild-derived inbred strains, displaying high (STRA, derived from Mus musculus domesticus) and low (BUSNA, derived from Mus musculus musculus) levels of aggressiveness. The role of maternal environment was tested in males with the same genetic background (i.e. strain origin) reared under three different conditions: unfostered (weaned by mother), infostered (weaned by an unfamiliar dam from the same strain), and cross-fostered (weaned by a dam from a different strain). All males were tested against non-aggressive opponents from the A/J inbred strain. Resource-holding potential was assessed through body weight gains and territory ownership. The STRA males were shown to be aggressive in both neutral cage and resident-intruder tests. On the contrary, the BUSNA males were less aggressive in all tests. We did not find a significant effect of postnatal maternal environment; however, we detected significant maternal effect on body weight with differences between the strains, fostering type and interactions between these factors. We conclude that the aggressiveness preserved in the two strains has significant genetic component whose genetic basis can be dissected by quantitative trait loci analysis.

Increased vulnerability to psychosocial stress in heterozygous serotonin transporter knockout mice

Epidemiological evidence links exposure to stressful life events with increased risk for mental illness. However, there is significant individual variability in vulnerability to environmental risk factors, and genetic variation is thought to play a major role in determining who will become ill. Several studies have shown, for example, that individuals carrying the S (short) allele of the serotonin transporter (5-HTT) gene-linked polymorphic region (5-HTTLPR) have an increased risk for major depression following exposure to stress in adulthood. Identifying the molecular mechanisms underlying this gene-by-environment risk factor could help our understanding of the individual differences in resilience to stress. Here, we present a mouse model of the 5-HTT-by-stress risk factor. Wild-type and heterozygous 5-HTT knockout male mice were subjected to three weeks of chronic psychosocial stress. The 5-HTT genotype did not affect the physiological consequences of stress as measured by changes in body temperature, body weight gain and plasma corticosterone. However, when compared with wild-type littermates, heterozygous 5-HTT knockout mice experiencing high levels of stressful life events showed significantly depressed locomotor activity and increased social avoidance toward an unfamiliar male in a novel environment. Heterozygous 5-HTT knockout mice exposed to high stress also showed significantly lower levels of serotonin turnover than wild-type littermates, selectively in the frontal cortex, which is a structure that is known to control fear and avoidance responses, and that is implicated in susceptibility to depression. These data may serve as a useful animal model for better understanding the increased vulnerability to stress reported in individuals carrying the 5-HTTLPR S allele, and suggest that social avoidance represents a behavioral endophenotype of the interaction between 5-HTT and stress.

Individual housing of mice--impact on behaviour and stress responses

The replicability of results derived from studies in rodents might be influenced by stress caused by inappropriate housing conditions. Here we compared the experimental behaviour and stress response (circulating corticosterone level and adrenal tyrosine hydroxylase activity) of individually-housed male and female inbred mice with that of animals housed in social groups. All mice were behaviourally tested in the modified hole board test (mHB). Male C57BL/6, BALB/c and A mice housed in groups of 3 were compared with individually-housed mice. In a subsequent experiment female C57BL/6 and BALB/c mice were housed under similar conditions. To exclude the possible effects of within-cage order of testing, only one individual per group was behaviourally tested. Neither male nor female mice housed individually showed stronger signs of stress than their socially-housed counterparts. However, we observed a within-cage order effect on the hormonal stress response (corticosterone) in socially-housed female C57BL/6 mice. No effects of individual housing on behaviour in the mHB were found.

There's no place like home: cage odours and place preference in subordinate CD-1 male mice

Prior studies using mice have shown that scent marks are an important source of information and can cause behavioural changes in other individuals. Studies have also shown that scent marks in the environment can affect the outcome of social interactions between mice. We used conditioned place preference tests to investigate whether CD-1 male mice (Mus musculus) are reinforced by olfactory cues from the home cage. Soiled bedding from the home cage was presented in the initially less preferred chamber of the apparatus to determine whether this association would reduce the unconditioned preference for one chamber over the other. We tested the effects of social rank and housing condition by comparing the performance of dyads that were polarised into dominant and subordinate relationships, both when paired and when separated, with mice that were isolated throughout. The development of conditioned place preference (CPP) supported by home cage odours was influenced by social rank but not by housing condition. Only subordinate mice showed CPP to home cage odours, and this effect was seen irrespective of whether they were housed with a dominant cage mate or alone. Neither dominant (paired or separated) nor isolated mice showed any change in their preference for the chamber associated with home cage odours. This suggests that the smell of home is a more powerful reinforcer for subordinate mice in that it can produce contextual conditioning to the environment in which it is experienced.

Chronic psychosocial stress-induced down-regulation of immunity depends upon individual factors

The effect of chronic stress on immune functions is strongly biased by individual factors. Mice were subjected to a new model of chronic psychosocial stress in which four different subcategories of stressed animals may be identified: Resident Dominants (RD), Resident Subordinates (RS), Intruder Dominants (InD), and Intruder Subordinates (InS). After 7 days of stress, mice were immunized with keyhole limpet hemocyanine (KLH). Their immune functions were investigated 14 days later with stress continuing trough. Importantly, RS mice, which are mice losing territory ownership, were the more affected, having lower IgG, proliferation, and IL-2. RD and InD showed lower IgG while InS showed no immune alteration. In conclusion, loss of resources could be a key factor in determining individual vulnerability to stressful events.

Association between experience of aggression and anxiety in male mice

The sensory contact technique increases aggressiveness in male mice and allows an aggressive type of behavior to be formed as a result of repeated experience of social victories in daily agonistic confrontations. In the low aggressive and high emotional mice of CBA/Lac strain, repeated positive fighting experience leads to increased plus maze anxiety in the winners after 10 days of experience of victories and much more after 20 days. Behavioral reactivity to other conspecifics was significantly increased as revealed by the parameters of partition test, which measures aggressive motivation in the winners. Thus, anxiety as a consequence of repeated experience of aggression is associated with the increase of aggressive motivation in CBA/Lac mice. It is concluded, that: (1) Repeated experience of aggression provokes the development of anxiety in male mice. (2) The level of anxiety as well as its behavioral realization depends on the duration of aggressive experience and genetic strain. Genetically defined features of innate anxiety (trait or state) in individuals may determine the kind of association between aggressive experience, aggressive motivation and anxiety.

Ethological methods to study the effects of maternal exposure to estrogenic endocrine disrupters: a study with methoxychlor

There has been increasing interest, both at the scientific and regulatory level, in the use of ethological methods for evaluating neural effects of endocrine disrupters. We present a series of ethological studies on the effects of maternal exposure to low, environmentally relevant doses (0.02, 0.2, and 2 microg/g mother bw/day) of the estrogenic pesticide methoxychlor (MXC) on behavior. From gestation day 11 to 17, female mice spontaneously drank oil with or without MXC; their maternal behavior was examined from postpartum days 2 to 15. MXC treatment during pregnancy produced slight changes in the expression of maternal behavior: females fed the lower MXC dose spent less time nursing the pups as compared to control dams. Their maternally exposed offspring were subjected to a series of behavioral tests at different ages. Maternal exposure to MXC affected behavioral responses to novelty in both sexes at periadolescence. The onset of male intrasex aggression was delayed in males prenatally exposed to low doses of MXC, since exposed males showed low levels of aggressive interactions during early adolescence but not after they reached adulthood. When adults, MXC-exposed females, but not males showed increased exploration in an unfamiliar open-field. While a sex difference was observed in the control group, with males being significantly more active in the open field than females, prenatal treatment with some MXC doses tended to decrease the sexual dimorphism in activity levels in the novel environment. Ethology, as the evolutionary study of behavior, may provide a framework for integrating a functional perspective (i.e., evolutionary significance) to studies on proximate mechanisms that can account for behavioral alterations induced by developmental exposure to endocrine disrupters.

Modulation of Aggression in Male Mice: Influence of Cage Cleaning Regime and Scent Marks

Group housing of male laboratory mice often leads to welfare problems due to aggressive behaviour. From a welfare perspective, individual housing is not a preferred solution to these problems - and so we sought other ways of reducing aggression between male mice. Aggression peaks after disturbances such as cage cleaning. Transfer of olfactory cues during cage cleaning procedures has been repeatedly proposed as a means of reducing these peaks in aggression. In this study, the aggression-modulating properties of olfactory cues were studied by investigating the effects of their source and distribution on aggression after cage cleaning in groups of male BALB/c mice. The physiological effects of aggression on individuals within a group were also monitored.

Our results indicated that neither kinship nor distribution of urine marks affected aggression. Olfactory cues from nesting and bedding material, however, affected aggression to a marked degree: transfer of nesting material reduced aggression significantly, while transfer of sawdust containing urine and faeces seemed to intensify aggression. None of the physiological data revealed any differences between dominant and subordinate animals, nor any correlations with aggressiveness, except that dominant animals gained weight more rapidly than subordinate ones. We conclude that the transfer of nesting material will reduce aggression, or at least slow down its development, and thus aid the reduction of social tension due to cage cleaning.

Agonistic behavior: a model, experimental studies, and perspectives

Agonistic (competitive) behavior includes the manifestations of aggression and submissiveness by individuals in conflict situations and is a universal form of behavior found in animals of different species. The sensory contact model allows aggressive and submissive (inhibited, suppressed) types of behavior to be formed in male mice as a result of acquisition of repeated experience of social conquests or defeats in daily aggressive interactions. Chronic experience of aggression is accompanied by total activation of the dopaminergic systems in the victors. At the same time, experience of social defeat leads to changes in the state of the serotoninergic and noradrenergic systems of various parts of the brains of the defeated animals. As a consequence, significant differences in emotional expression, movement activity, investigative activity, communicative ability, alcohol consumption, and many physiological aspects were found in animals of opposite social groups. The extent and nature of these changes depended on the type of social behavior formed by the animals (victors, vanquished), the duration of confrontational experience, and the genetically determined characteristics of the animals (i.e., which line of mice). The possibilities and perspectives of this model of sensory contact in medical-biological and basic investigations is discussed.

Prenatal exposure to low doses of the estrogenic chemicals diethylstilbestrol and o,p'-DDT alters aggressive behavior of male and female house mice

Exposure to estrogenic chemicals during critical periods in fetal life can alter the development of reproductive organs, the neuroendocrine system, and subsequent behavior. We examined the effects of prenatal exposure to the estrogenic chemicals, o,p'-DDT (the estrogenic contaminant in commercial DDT) and the drug diethystilbestrol (DES), as a positive control, on different forms of aggressive behavior in both male and female house mice. We also examined effects of these chemicals on male reproductive organs. From gestation days 11-17 female mice were fed an average concentration (dissolved in oil) 0.018 and 0.18 ng/g body weight of DES. Doses of o,p'-DDT were 18 and 180 ng/g body weight, based on the prediction that the in vivo potency of o,p'-DDT would be approximately 1000-times lower than DES. We found that prenatal exposure to DES increased the frequency of both males and females that responded aggressively to a same-sex conspecific. Preputial glands in males exposed to the 0.018 ng/g dose of DES were significantly enlarged relative to controls. Males exposed to the 18 ng/g dose of DDT had smaller testes than controls. The possible implications of perturbing the development of social behaviors, such as aggression, on individuals reproductive success and social structure of the population are discussed.

Prenatal exposure to endocrine disrupting chemicals: effects on behavioral development

Numerous chemicals released into the environment by man are able to disrupt the functioning of the endocrine system by binding to hormonal receptors. Exposure to estrogenic endocrine disruptors during critical periods in fetal life can alter the development of reproductive organs, the neuroendocrine system and subsequent behavior. We present a series of studies on the effects of exposure during fetal life to low, environmentally relevant doses of two pesticides, o,p'DDT and methoxychlor, and of low doses of the synthetic estrogen, diethylstilbestrol on subsequent neuro-behavioral development in house mice. The main findings can be summarized as follows: (1) Mice prenatally exposed to methoxychlor showed changes in reflex development. Exposure to a very low dose of methoxychlor appeared to produce an increased reactivity during early postnatal life. (2) Methoxychlor exposed periadolescent mice showed a decreased reaction time exploring both a novel environment and a novel object. (3) The onset of male intrasex aggression appeared to be delayed in males prenatally exposed to low doses of methoxychlor, since exposed males showed low levels of aggressive interactions during early adolescence but not after they reached adulthood. (4) The rate of depositing urine marks in a novel environment was increased in males prenatally exposed to DES, and also to o,p'DDT and methoxychlor. (5) The proportion of both males and females attacking a same-sex conspecific was increased in mice prenatally exposed to low doses of DES and, marginally, to o,p'DDT. This effect appeared to be related to a decreased latency to attack. However, males prenatally exposed to o,p'DDT displayed a decreased intensity of aggression. The possible implications of perturbing the hormonal milieu during fetal development on the modulation of developmental turnpoints and future behavioral responses are discussed.

Selection, evolution of behavior and animal models in behavioral neuroscience

We investigated whether genetic differences in various forms of intraspecific aggression and anxiety in four different genetic lines of mice (i.e. wild, outbred Swiss-CD1, inbred DBA/2 and inbred C57/BL6N) may reflect modifications in behavioral strategy. Experiments 1 and 2 used ethologically based paradigms to analyze aggressive and anxiety responses both in social (i.e. aggression) and non-social (i.e. novel environment exploration) contexts. In Experiment 3, an anxiolytic drug (chlordiazepoxide (CDP)) was used to examine possible differences in proximal mechanisms underlying anxiety-related behaviors. The data show that intrasexual aggression, infanticide and maternal aggressions are related and covarying. Genetic lines with the highest levels of intermale attack (i.e. Wild and Swiss-CD1) also have highest levels of infanticide, interfemale attack and maternal aggression but, interestingly, the lowest levels of anxiety. In fact, exploratory behavior is lower and risk assessment behavior markedly higher in DBA/2 and C57/BL6N mice (i.e. the less aggressive strains) compared to Swiss and Wild genetic lines. Although reproductive status influences anxiety levels in female mice, our findings show that (contrary to previous studies) lactating mice are more anxious than virgin females in terms of risk assessment activities. These data demonstrate the importance of studying behavior in a more ecologically-relevant context which emphasizes the function of behavior in a specific situation. Moreover, differential strain sensitivity to the behavioral effects of CDP suggests that genetic lines of mice may differ in the underlying mechanisms mediating behavior. It is therefore possible that artificial selection of different genotypes has resulted in differences in proximate mechanisms modulating the levels of aggression and anxiety, thereby leading to modification of social behavior. Overall, the results presented here suggest that subtle genetic alterations in specific underlying neural mechanisms are likely to cause profound effects on behavioral responses and their adaptive significance. Implications for behavioral neuroscience research that seeks to understand both the proximal and ultimate mechanisms of behavior are discussed.

Effects of haloperidol on communicative and aggressive behavior in male mice with different experiences of aggression

Effects of two doses of haloperidol (0.1 and 0.4 mg/kg, 30 min and 24 h, IP) on communicative and aggressive behavior in C57BL/6J male mice have been studied. Some of the mice were without prior experience of aggression ("recruits"); the others had been victorious in 20 daily aggressive confrontations ("experienced winners"). Communicative behavior was estimated as the behavioral reaction to a standard tester (loser) in the partition test. Haloperidol in either dose significantly reduced communicative behavior in the "recruits." but not in the "experienced winners." Significantly fewer attacks, less total attacking time, and total time of aggressive behavior (aggressive grooming + attacks) were demonstrated by the "experiences winners," than by the "recruits," while the latency of the first attack, the number, the total and average duration of aggressive grooming events were significantly higher. In the "recruits," haloperidol dose dependently increased the latency and decreased the number of attacks, the total attacking time, and the total time of aggressive behavior 30 min and 24 h after injection. However, haloperidol did not affect the average or total time of aggressive grooming. Neither dose significantly affected any measure of aggressive behavior in the "experienced winners." It has been concluded that repeated aggression experience reduces the pharmacological sensitivity of the dopamine receptors.

An evolutionary approach to behavioral pharmacology: using drugs to understand proximate and ultimate mechanisms of different forms of aggression in mice

Psychoactive drugs (Fluprazine and Chlordiazepoxide--CDP) were used as probes to test both differences or similarities in neurochemical substrates (proximal causations) and adaptive significance (ultimate causations) of different forms of intraspecific aggression in wild mice and laboratory Swiss CD-1 counterparts. Fluprazine (1-5 mg/kg) inhibited maternal attack on female, but not on male intruders. Thus, phenotypically different attack behaviors (offence and defence respectively) which have different functions may be modulated by different neurochemical substrates. Intrasexual attack and infanticide which are phenotypically different, but share similar functions (i.e. competition for mates and resources) were equally inhibited by Fluprazine (2 mg/kg) both in males and females of wild and laboratory mice. This indicates that the neural substrates of these behaviors are related and similarly regulated in the two sexes. Fluprazine was used to test the prediction of the evolutionary model on fighting strategies in male-male asymmetric contests as far as fighting ability and resource value (mating and cohabitation with a female) are concerned. Fluprazine inhibited the intensity of fighting (i.e. more 'defensive' behavioral phenotype of attack) only in animals without previous positive fighting experience, suggesting that different behavioral strategies are based on different neurochemical modulation. Experience of attack also influenced the effects of CDP (2.5-5 mg/kg) in both lactating females and male resident mice. The reported proaggressive effects of benzodiazepines were observed only in animals with prior fighting experience in both cases. Thus the understanding of the effects of drugs on behavior demands consideration of the biological variability (e.g. genetic, previous experience and/or interindividual differences) and the adaptive significance of behavior in the experimental context. On this background ethopharmacology can be defined as an evolutionary approach to the study of a drugs effect on neurochemical mechanisms and functions of behavior.

Interindividual variability in Swiss male mice: relationship between social factors, aggression, and anxiety

In the present study we carried out a series of experiments in Swiss albino male mice to investigate a) the effects of previous social experience on the levels of anxiety in the elevated plus-maze (EPM) and b) whether the response of males in the EPM differs in relation to the different social status. In Experiment 1 we tested in the EPM male mice that received different social experience. Results showed that individually housing generally increased measures of anxiety in the EPM compared with the group-housing condition. Moreover, aggressive males, screened during dyadic encounters in a neutral cage, displayed the highest levels of anxiety relative to the other experimental conditions. In Experiment 2 male mice remained group-housed and were observed to record their social status. Results showed that those animals rated as socially dominant displayed a higher level of EPM anxiety relative to subordinates. From an ethological perspective our findings may be interpreted in terms of coping strategies, with aggressive/dominant animals typified by higher levels of risk assessment and open-arm avoidance than defensive/subordinate animals.

Ethotoxicology: an evolutionary approach to the study of environmental endocrine-disrupting chemicals

The focus of most studies in behavioral toxicology has been on effects of chemicals on learning and memory, sensory function, activity, and neuromuscular function. Behavior has also been used as a biomarker of chemical exposure, often without concern for the context in which the behavior was being observed. We have applied the strategies employed in ethological analyses of behavior and developed an alternative, ethotoxicological, approach to the study of effects of endocrine disruptors on behavior. This involves examining whether a behavioral alteration may be pathological from the perspective of the adaptive significance of the behavior under investigation. Pathological changes in behavior lead to reduced social adaptation and impaired responsiveness to environmental demands, with consequences for social structure and population dynamics. It thus follows that the ecological context in which the behavior would normally occur and the function of the behavior are of paramount importance when studying the impact of endocrine disruptors.

Effects of chlordiazepoxide on maternal aggression in mice depend on experience of resident and sex of intruder

Lactating mice respond differentially to intruders of differing sex, displaying defensive attack against the male and offensive attack against the female. Such a phenotypic dichotomy appears to have adaptive value in that unfamiliar males pose a much greater threat to the offspring than do females. The present study examined the effects of the benzodiazepine anxiolytic chlordiazepoxide (CDP) (2.5-10.0 mg/kg) on this differential response pattern in aggression-naive (nonscreened) (NS) and aggression-experienced (screened) (S) lactating female mice (Mus musculus domesticus) confronting intruders of either sex in a 10-min test. This procedure was used to evaluate the influence of both the type of opponent and previous aggressive experience on basal behavioural profiles and drug action. Results showed that both intruder sex and prior screening for attack modulated the behaviour of lactating females toward intruders. In turn, both variables strongly influenced CDP effects on maternal aggression. In particular, in S dams, CDP dose-dependently increased maternal attack against males but decreased attack against female intruders. Conversely, in NS dams, CDP decreased attack (and fear) against males but did not affect it against females. In both S and NS conditions, CDP modified the attack strategy of lactating females against the male, switching it from a defensive to an offensive pattern. Exploration, social investigation, eating, and immobility were differentially affected by the drug treatment, depending on screening and/or intruder sex condition. These differential effects of CDP between S and NS conditions, toward either male or female intruders, cannot be fully explained by differences in the baseline levels of these behaviours. Alternative hypotheses are discussed. These findings demonstrate that the effects of CDP on maternal attack behaviour depend on not only the drug but also the object of attack, and hence the function of attack and the prior experience of the attacker.

Involvement of the brain catecholaminergic system in the regulation of dominant behavior

The role of the brain catecholaminergic system in establishing dominant-subordinate relationships in mice of different genotypes was studied using inhibitors of tyrosine hydroxylase (alpha-methyl-p-tyrosine) or of dopamine-beta-hydroxylase (FLA-57) or FLA-57 plus the dopamine precursor, DOPA. Demotion in all dominant and subdominant animals was associated with decreased noradrenaline levels, but the aggressive behavior of dominant male mice depended on the noradrenaline/dopamine ratio. Alterations in this relationship seem to have specific effects on social dominance in animals in the micropopulation, as drug-treated mice do not exhibit changes in their general activity. It can be concluded that brain catecholamines are of prime importance in maintenance of dominance.