Serum testosterone, agonistic behavior, and dominance in inbred strains of mice

The impact of cage dividers on mouse aggression, dominance and hormone levels

Home cage aggression in group-housed male mice is a major welfare concern and may compromise animal research. Conventional cages prevent flight or retreat from sight, increasing the risk that agonistic encounters will result in injury. Moreover, depending on social rank, mice vary in their phenotype, and these effects seem highly variable and dependent on the social context. Interventions that reduce aggression, therefore, may reduce not only injuries and stress, but also variability between cage mates. Here we housed male mice (Balb/c and SWISS, group sizes of three and five) with or without partial cage dividers for two months. Mice were inspected for wounding weekly and home cages were recorded during housing and after 6h isolation housing, to assess aggression and assign individual social ranks. Fecal boli and fur were collected to quantify steroid levels. We found no evidence that the provision of cage dividers improves the welfare of group housed male mice; The prevalence of injuries and steroid levels was similar between the two housing conditions and aggression was reduced only in Balb/c strain. However, mice housed with cage dividers developed less despotic hierarchies and had more stable social ranks. We also found a relationship between hormone levels and social rank depending on housing type. Therefore, addition of cage dividers may play a role in stabilizing social ranks and modulating the activation of hypothalamic-pituitary-adrenal (HPA) and hypothalamic-pituitary-gonadal (HPG) axes, thus reducing phenotypic variability between mice of different ranks.

Manifestations of domination: Assessments of social dominance in rodents

Social hierarchies are ubiquitous features of virtually all animal groups. The varying social ranks of members within these groups have profound effects on both physical and emotional health, with lower-ranked individuals typically being the most adversely affected by their respective ranks. Thus, reliable measures of social dominance in preclinical rodent models are necessary to better understand the effects of an individual's social rank on other behaviors and physiological processes. In this review, we outline the primary methodologies used to assess social dominance in various rodent species: those that are based on analyses of agonistic behaviors, and those that are based on resource competition. In synthesizing this review, we conclude that assays based on resource competition may be better suited to characterize social dominance in a wider variety of rodent species and strains, and in both males and females. Lastly, albeit expectedly, we demonstrate that similarly to many other areas of preclinical research, studies incorporating female subjects are lacking in comparison to those using males. These findings emphasize the need for an increased number of studies assessing social dominance in females to form a more comprehensive understanding of this behavioral phenomenon.

Who's the Boss? Assessing Convergent Validity of Aggression Based Dominance Measures in Male Laboratory Mice, Mus Musculus

Aggression among group housed male mice continues to challenge laboratory animal researchers because mitigation strategies are generally applied at the cage level without a good understanding of how it affects the dominance hierarchy. Aggression within a group is typically displayed by the dominant mouse targeting lower ranking subordinates; thus, the strategies for preventing aggression may be more successful if applied specifically to the dominant mouse. Unfortunately, dominance rank is often not assessed because of time intensive observations or tests. Several correlates of dominance status have been identified, but none have been directly compared to home cage behavior in standard housing. This study assessed the convergent validity of three dominance correlates (urinary darcin, tube test score, preputial gland to body length ratio) with wound severity and rankings based on home cage behavior, using factor analysis. Discriminant validity with open field measures was assessed to determine if tube test scores are independent of anxiety. Cages were equally split between SJL and albino C57BL/6 strains and group sizes of 3 or 5 (N = 24). Home cage behavior was observed during the first week, and dominance measures were recorded over the second. After controlling for strain and group size, darcin and preputial ratio had strong loadings on the same factor, which was a significant predictor of home cage ranking showing strong convergent validity. Tube test scores were not significantly impacted by open field data, showing discriminant validity. Social network analysis revealed that despotic power structures were prevalent, aggressors were typically more active and rested away from cage mates, and the amount of social investigation and aggression performed by an individual were highly correlated. Data from this study show that darcin and preputial ratio are representative of home cage aggression and provide further insight into individual behavior patterns in group housed male mice.

Social dominance differentially alters gene expression in the medial prefrontal cortex without affecting adult hippocampal neurogenesis or stress and anxiety-like behavior

Social hierarchies are crucial for a group's survival and can influence the way an individual behaves and relates to a given social context. The study of social rank has been classically based on ethological and observational paradigms, but it recently has taken advantage of the use of other approaches, such as the tube test that measures territorial dominance without the display of in situ aggression and is executable in group-living animals. However, little is known about how previous basal individual differences affect the development of dominance hierarchy measured in the tube test. We have analyzed in male mice body weight, locomotion, anxiety, and serum corticosterone both before and after the tube test, as well as adult hippocampal neurogenesis and transcriptome in the prefrontal cortex after the hierarchy had been established. We found differential gene expression between dominants and subordinates but no association between the other parameters and social status, neither pre- nor posttest. Our findings reveal that social rank in mice is stable along time and is not related to basal differences in stress, mood, or physical features. Lastly, real-time quantitative PCR analysis confirmed differential expression of vomeronasal and olfactory receptors in the cerebral cortex between dominant and subordinate individuals, suggesting that differential brain gene expression in the medial prefrontal cortex could potentially be used as a biomarker of social dominance.-Pallé, A., Zorzo, C., Luskey, V. E., McGreevy, K. R., Fernández, S., Trejo, J. L. Social dominance differentially alters gene expression in the medial prefrontal cortex without affecting adult hippocampal neurogenesis or stress and anxiety-like behavior.

Hierarchical Status Predicts Behavioral Vulnerability and Nucleus Accumbens Metabolic Profile Following Chronic Social Defeat Stress

Extensive data highlight the existence of major differences in individuals' susceptibility to stress [1-4]. While genetic factors [5, 6] and exposure to early life stress [7, 8] are key components for such neurobehavioral diversity, intriguing observations revealed individual differences in response to stress in inbred mice [9-12]. This raised the possibility that other factors might be critical in stress vulnerability. A key challenge in the field is to identify non-invasively risk factors for vulnerability to stress. Here, we investigated whether behavioral factors, emerging from preexisting dominance hierarchies, could predict vulnerability to chronic stress [9, 13-16]. We applied a chronic social defeat stress (CSDS) model of depression in C57BL/6J mice to investigate the predictive power of hierarchical status to pinpoint which individuals will exhibit susceptibility to CSDS. Given that the high social status of dominant mice would be the one particularly challenged by CSDS, we predicted and found that dominant individuals were the ones showing a strong susceptibility profile as indicated by strong social avoidance following CSDS, while subordinate mice were not affected. Data from ¹H-NMR spectroscopy revealed that the metabolic profile in the nucleus accumbens (NAc) relates to social status and vulnerability to stress. Under basal conditions, subordinates show lower levels of energy-related metabolites compared to dominants. In subordinates, but not dominants, levels of these metabolites were increased after exposure to CSDS. To the best of our knowledge, this is the first study that identifies non-invasively the origin of behavioral risk factors predictive of stress-induced depression-like behaviors associated with metabolic changes.

Social context-dependent relationships between mouse dominance rank and plasma hormone levels

The associations between social status and endogenous testosterone and corticosterone have been well-studied across taxa, including rodents. Dominant social status is typically associated with higher levels of circulating testosterone and lower levels of circulating corticosterone but findings are mixed and depend upon numerous contextual factors. Here, we determine that the social environment is a key modulator of these relationships in Mus musculus. In groups of outbred CD-1 mice living in stable dominance hierarchies, we found no evidence of simple linear associations between social rank and corticosterone or testosterone plasma levels. However, in social hierarchies with highly despotic alpha males that socially suppress other group members, testosterone levels in subordinate males were significantly lower than in alpha males. In less despotic hierarchies, where all animals engage in high rates of competitive interactions, subordinate males had significantly elevated testosterone compared to agonistically inhibited subordinates from despotic hierarchies. Subordinate males from highly despotic hierarchies also had elevated levels of corticosterone compared to alpha males. In pair-housed animals, the relationship was the opposite, with alpha males exhibiting elevated levels of corticosterone compared to subordinate males. Notably, subordinate males living in social hierarchies had significantly higher levels of plasma corticosterone than pair-housed subordinate males, suggesting that living in a large group is a more socially stressful experience for less dominant individuals. Our findings demonstrate the importance of considering social context when analyzing physiological data related to social behavior and using ethologically relevant behavioral paradigms to study the complex relationship between hormones and social behavior.

Dynamic changes in social dominance and mPOA GnRH expression in male mice following social opportunity

Social competence - the ability of animals to dynamically adjust their social behavior dependent on the current social context - is fundamental to the successful establishment and maintenance of social relationships in group-living species. The social opportunity paradigm, where animals rapidly ascend a social hierarchy following the removal of more dominant individuals, is a well-established approach for studying the neural and neuroendocrine mechanisms underlying socially competent behavior. In the current study, we demonstrate that this paradigm can be successfully adapted for studying socially competent behavior in laboratory mice. Replicating our previous reports, we show that male laboratory mice housed in a semi-natural environment form stable linear social hierarchies. Novel to the current study, we find that subdominant male mice immediately respond to the removal of the alpha male from a hierarchy by initiating a dramatic increase in aggressive behavior towards more subordinate individuals. Consequently, subdominants assume the role of the alpha male. Analysis of brain gene expression in individuals 1h following social ascent indicates elevated gonadotropin-releasing hormone (GnRH) mRNA levels in the medial preoptic area (mPOA) of the hypothalamus compared to individuals that do not experience a social opportunity. Moreover, hormonal analyses indicate that subdominant individuals have increased circulating plasma testosterone levels compared to subordinate individuals. Our findings demonstrate that male mice are able to dynamically and rapidly adjust both behavior and neuroendocrine function in response to changes in social context. Further, we establish the social opportunity paradigm as an ethologically relevant approach for studying social competence and behavioral plasticity in mammals.

Sex chromosomes and brain gender

In birds and mammals, differences in development between the sexes arise from the differential actions of genes that are encoded on the sex chromosomes. These genes are differentially represented in the cells of males and females, and have been selected for sex-specific roles. The brain is a sexually dimorphic organ and is also shaped by sex-specific selection pressures. Genes on the sex chromosomes probably determine the gender (sexually dimorphic phenotype) of the brain in two ways: by acting on the gonads to induce sex differences in levels of gonadal secretions that have sex-specific effects on the brain, and by acting in the brain itself to differentiate XX and XY brain cells.

Age- and sex-dependent development of adrenocortical hyperactivity in a transgenic mouse model of Alzheimer's disease

In this study, we investigated mice of the TgCRND8 line, an APP transgenic mouse model of Alzheimer's disease (AD), with respect to behavioral, endocrinological, and neuropathological parameters. Our results show that transgenic and wild-type mice did not differ in their general health status, exploratory and anxiety related behavior as well as in the activity of their sympathetic-adrenomedullary system. Significant differences, however, were found regarding body weight, amyloid plaque formation, and the activity of the hypothalamic-pituitary-adrenocortical (HPA) axis. Continuous monitoring of glucocorticoid (GC) concentrations over a period of 120 days, utilizing a noninvasive technique to measure corticosterone metabolites in fecal samples, revealed that transgenic animals showed adrenocortical hyperactivity, starting very early in males (from day 30) and later in females (around day 90). It is hypothesized that these changes in the activity of the HPA axis are linked to amyloid-beta associated pathological alterations in the hippocampus, causing degenerations in the negative feedback regulation of the HPA axis leading to hypersecretion of GC. Thus, the development of adrenocortical hyperactivity might be a key-element in the understanding of AD.

Gonadal effects on plasma ACE activity in mice

Angiotensin-converting enzyme (ACE) regulates blood pressure and is an important target in the management of hypertension. Hypertension is a gender biased disease. Plasma ACE activity is significantly higher in male mice (309 U/l) than female mice (237 U/l) and is reduced significantly upon gonadectomy to 224 and 209 U/l, respectively. Although, the gonads influence plasma ACE activity in both male and female mice, the effect is more pronounced in male mice. Plasma ACE is derived from the cleavage of tissue ACE and lung has the highest concentration of tissue ACE. However, lung ACE activity is not gender dimorphic but increases significantly upon gonadectomy in both male and female. ACE mRNA level in the lung is not influenced by gender or gondaectomy. Therefore, the gonads affect plasma ACE activity by influencing cleavage of tissue ACE to plasma ACE and/or decrease stability of plasma ACE in gonadectomized mice is mediated.

Short-day increases in aggression are inversely related to circulating testosterone concentrations in male Siberian hamsters (Phodopus sungorus)

Many nontropical rodent species display seasonal changes in both physiology and behavior that occur primarily in response to changes in photoperiod. Short-day reductions in reproduction are due, in part, to reductions in gonadal steroid hormones. In addition, gonadal steroids, primarily testosterone (T), have been implicated in aggression in many mammalian species. Some species, however, display increased aggression in short days despite basal circulating concentrations of T. The goal of the present studies was to test the effects of photoperiod on aggression in male Siberian hamsters (Phodopus sungorus) and to determine the role of T in mediating photoperiodic changes in aggression. In Experiment 1, hamsters were housed in long and short days for either 10 or 20 weeks and aggression was determined using a resident-intruder model. Hamsters housed in short days for 10 weeks underwent gonadal regression and displayed increased aggression compared to long-day-housed animals. Prolonged maintenance in short days (i.e., 20 weeks), however, led to gonadal recrudescence and reduced aggression. In Experiment 2, hamsters were housed in long and short days for 10 weeks. Half of the short-day-housed animals were implanted with capsules containing T whereas the remaining animals received empty capsules. In addition, half of the long-day-housed animals were castrated whereas the remaining animals received sham surgeries. Short-day control hamsters displayed increased aggression compared to either castrated or intact long-day-housed animals. Short-day-housed T treated hamsters, however, did not differ in aggression from long-day-housed animals. Collectively, these results confirm previous findings of increased aggression in short-day-housed hamsters and suggest that short-day-induced increases in aggression are inversely related to gonadal steroid hormones.

Social stress effects on territorial marking and ultrasonic vocalizations in mice

Acute social defeat (SD) leads to transient and persistent physiological and behavioral changes. We examined the effects of acute SD on territorial urine marking and ultrasonic courtship vocalizations in DBA/2 male mice. Both behaviors are considered androgen dependent and are influenced by social status, with dominant mice displaying more of both behaviors. In Experiment 1, male mice that received SD displayed prolonged inhibition of territorial urine marking, relative to nondefeated control mice (NOSD). In addition, territorial marking increased with repeated tests. In Experiment 2, male mice that received 3 successive days of SD displayed fewer ultrasonic courtship vocalizations at 30 min. post-SD1 and 30 min. post-SD2, relative to NOSD mice. In Experiment 2, we also observed decreased territorial marking 4 weeks post-SD. In sum, SD induced prolonged inhibition of territorial marking, but had only transient effects on ultrasonic courtship vocalizations, suggesting that different mechanisms may mediate the maintenance of these behaviors.

Effects of chronic treatment with testosterone propionate on aggression and hormonal levels in intact male mice

Effects of testosterone propionate, an anabolic-androgenic steroid (AAS), on aggression in gonadally intact male mice were examined. Animals were given weekly injections of 3.75, 7.5, 15, and 30 mg/kg of drug or sesame oil for 10 weeks. During the last 3 weeks, behavioral tests were conducted and at the end of the experiment, body, liver and testes weight and hormonal data were collected. The treatment had minimal behavioral and endocrine effects. It resulted in shorter latencies of 'threat' only in the last agonistic encounter, increases in testosterone levels and decreases in testes weight in a non-linear dose-dependant way. The action of treatment was different on threat and attack, the latter being unaffected. The behavioral effects in the total sample were only found in aggressive animals selected on the basis of their latency of attack in the first encounter.

Aggression in humans: what is its biological foundation?

Although human aggression is frequently inferred to parallel aggression based on testosterone in nonprimate mammals, there is little concrete support for this position. High- and low-aggression individuals do not consistently differ in serum testosterone. Aggression does not change at puberty when testosterone levels increase. Aggression does not increase in hypogonadal males (or females) when exogenous testosterone is administered to support sexual activity. Similarly, there are no reports that aggression increases in hirsute females even though testosterone levels may rise to 200% above normal. Conversely, castration or antiandrogen administration to human males is not associated with a consistent decrease in aggression. Finally, changes in human aggression associated with neuropathology are not consistent with current knowledge of the neural basis of testosterone-dependent aggression. In contrast, human aggression does have a substantial number of features in common with defensive aggression seen in nonprimate mammals. It is present at all age levels, is displayed by both males and females, is directed at both males and females, and is not dependent on seasonal changes in hormone levels or experiential events such as sexual activity. As would be expected from current knowledge of the neural system controlling defensive aggression, aggression in humans increases with tumors in the medial hypothalamus and septal region, and with seizure activity in the amygdala. It decreases with lesions in the amygdala. The inference that human aggression has its roots in the defensive aggression of nonprimate mammals is in general agreement with evidence on the consistency of human aggressiveness over age, with similarities in male and female aggressiveness in laboratory studies, and with observations that some neurological disturbances contribute to criminal violence. This evidence suggests that human aggression has its biological roots in the defensive aggression of nonprimate mammals and not in hormone-dependent aggression based on testosterone.

Hormone-dependent aggression in male rats is proportional to serum testosterone concentration but sexual behavior is not

Male hooded rats were castrated and implanted with Silastic capsules (1.57 mm i.d.; 3.18 mm o.d.) having a testosterone-filled space 0, 7, 22, 60, or 90 mm long. All animals were returned to their original group cages for a three-week period to allow hormone concentrations and behavioral tendencies to stabilize. Each male was then housed with an intact female in a large cage. Aggression by the male toward an unfamiliar male was tested at weekly intervals for three weeks. Sexual behavior with an estrogen/progesterone-primed ovariectomized female was tested on each of the subsequent two weeks. Serum testosterone was measured during the following week. The frequency of aggression was correlated with serum testosterone concentration up to the normal level and did not increase with higher serum testosterone concentrations. In contrast, sexual behavior was virtually absent in animals with no testosterone replacement and normal in all other groups. These results demonstrate a clear dissociation in the dependence of hormone-dependent aggression and sexual behavior on serum testosterone concentration. In a male cohabiting with a female, sexual experience activates hormone-dependent aggression toward an unfamiliar male but the level of aggression that develops depends on the serum testosterone concentration in the resident male.

Y chromosome and aggression in strains of laboratory mice

Intermale attack behavior differences in laboratory strains of inbred mice have Y chromosome correlates in a small number of strain comparisons. Moreover, the Y correlates interact with autosomal or pseudoautosomal genes. Recent data on the genetics of the Y do not contradict these conclusions. The discovery of several polymorphic loci of the Y could pave the way for a direct confirmation of Y correlates of attack behavior by linkage detection. The involvement of the Y in this behavior has been put forward. Plasmatic testosterone concentration reactivity of some target organs to exogeneous testosterone appears to be correlated with two independent loci of the Y acting in an additive or interactive manner with autosomal or pseudoautosomal loci. However, the association between testosterone action and attack behavior in males needs further evidence, and in any case this association does not underline linear mechanisms.

Aggressive behavior in birds: an experimental model for studies of brain-steroid interactions

1. Although testosterone (T) stimulates aggressiveness in males of many vertebrate species, it is now known that the full expression of T actions in the central nervous system requires aromatization to estradiol (E2) and subsequent binding of formed E2 to its receptor. 2. We have recently confirmed these as rate-limiting steps in the control of sex-related and individual differences in aggressiveness of the Japanese quail (Coturnix coturnix japonica). 3. In this review, we describe some of the neuroendocrine factors which control aggression with a focus on our recent studies in quail.

Gonadal steroid influence upon sexual and aggressive behavior of female rats

The present experiment investigates the activation of aggressive and sexual behaviors by gonadal hormones in female rats of the S3-strain. In the first experiment three doses of testosterone propionate (TP) were chronically injected. In the second experiment effects of TP were compared to those of estradiol benzoate (EB) and methyltrienelone (R1881), a synthetic, unaromatizable androgen. Females of the S3-strain were tested against TP-treated female Wistar rats as opponents, and masculine and feminine sexual responses were assessed in the test for aggression as well as in separate tests with sexually active stimulus animals. The results of the first experiment indicate that TP in all doses, increased aggressive as well as sexual behavior equally, although plasma testosterone levels differed significantly between the groups. In the second experiment, EB significantly decreased overall aggression as compared to control-treatment. TP- and R1881-stimulated fighting, particularly, as the most offensive parameter of aggression, but did not increase overall levels of aggression. Tests for sexual preference in which the choice between a sexually active male or female was given, indicated that TP-treated females stayed near males with longer durations. Scentmarking frequencies, measured in the semiopenfield test, were effectively activated by TP-treatment. EB- and R1881-treatment resulted in intermediate levels of marking behavior.

Testicular hormones reduce individual differences in the aggressive behavior of male mice: a theory of hormone action

A chronology of theoretical development in studies of the role of testicular hormones in murine aggression is presented. Evidence which brings into question the generality of current theory is reviewed, and the implications of this evidence for the direction of future research are discussed. A new method by which to characterize individual differences in aggressive behavior is described, and recent data which provide the basis for the development of a new theory are presented. It is theorized that testicular hormones reduce individual differences in the aggressive behavior of male mice, and that this behavioral "homogenization" is mediated by a testicular-hormone regulated "discrimination mechanism," possibly localized within the mouse olfactory system. The generality of this theory and the implications for other hormone/behavior systems are also discussed.

Genetic analysis of the Y-chromosome of the mouse: evidence for two loci affecting androgen metabolism

Male mice from congenic lines carryY-chromosomes derived from two pairs of inbred strains (CBA/FaCamSt and C57BL/FaSt; PHL/St and PHL-YH/St) on various genetic backgrounds were compared. Serum testosterone levels, and the response of target organs in castrated animals to graded doses of exogenous testerone propionate were measured. These comparisons produced evidence for twoY-chromosomal loci influencing androgen metabolism. One of these affects serum testosterone levels, with variant alleles on theY-chromosomes derived from the PHL and PHL-YH strains. The other locus influences the response to testosterone of target organs, most significantly seminal vesicle, and variant alleles are found in the CBA and C57 strains. The effects of both loci are modulated by the genetic background. The relationship of these loci to otherY-chromosomal loci in the mouse is briefly discussed.

The role of progesterone in pregnancy-induced aggression in mice

A series of six experiments was performed in order to explore the potential involvement of progesterone (P) in pregnancy-induced aggression (PIA) displayed by Rockland-Swiss mice toward adult male intruders. In Experiment 1, circulating levels of P and aggression were low on gestation Days 6 and 10 while both the behavior and the steroid reached peak levels by gestation Day 14. By gestation Day 18 (the day prior to parturition), serum P was at its lowest level yet aggressive behavior was still intense. Also, individual differences in the display of fighting behavior by pregnant females were not related to circulating P. Experiments 2 and 3 showed that supplemental P treatment to early pregnant female mice did not advance the onset of aggression. Experiment 4 showed that P treatment promoted the onset and elevated the incidence of aggression in virgin mice, but only in those females with intact ovaries. Experiment 5 showed that the aggressive behavior of P-stimulated virgin females was qualitatively and quantitatively different from that exhibited by pregnant mice in that the former exhibited fewer attacks and lunges than the latter. Finally, Experiment 6 showed that the removal of P from aggressive, P-stimulated virgins dramatically attenuated levels of the behavior. This contrasts sharply with the continued fighting behavior observed in late pregnant P-deficient mice. Thus, although P augments aggression in female mice it apparently is not a sufficient stimulus for producing pregnancy-like aggressive behavior.

Behavioural examinations of the anti-aggressive drug fluprazine

The behavioural influence of the anti-aggressive drug Fluprazine (DU 27716) was examined using an ethological technique. The drug inhibited aggressive behaviour but not in an entirely specific way. Fluprazine also stimulated non-social and defensive/flight behaviours; there was a greater tendency for drug-treated animals to avoid their opponents. Using an automatic recording technique the drug's anti-aggressive action was monitored for 23 h. There was a potent anti-aggressive influence that lasted for up to 4 h. However, when the drug-effect wore off there were bouts of fighting. Over 23 h Fluprazine did not significantly decrease the total aggression recorded.

Y-chromosome length in sublines of two mouse strains

Differences in intermale aggression have been repeatedly reported for DBA/1 and C57BL/10 mice. The results of rreciprocal crosses combined with cross-fostering procedures suggest an involvement of the Y chromosome. In the present study, the length of the Y chromosome relative to that of chromosome 19 was ascertained in five sublines of DBA/1Bg, three sublines of C57BL/10Bg, and C57BL/10.DBA/1-Y congenic stock of mice, which carries the DBA/1Bg Y chromosome. With respect to the length of the Y chromosome relative to that of chromosome of 19, two of the DBA/1 sublines are shorter than the other three DBA/1 sublines, and all DBA/1 sublines are shorter than the three C57BL/10 sublines. This is attributable primarily to the length of the Y chromosome. The C57BL/10 sublines and the BL10.D1-Y congenic stock tested exhibit the same relative lengths of the Y chromosome, suggesting that its length has changed on the C57BL/10 genetic background. There is a parallel dependence on autosomal background of the effect of the Y chromosome on intermale aggression.

The effect of cyproterone acetate on motor activity, aggression, "emotionality", body weight and testes in wild mice

The literature reports on antiandrogenic and antigonadotropic effects of cyproterone acetate (CA) and on its inhibitory influence on the structure and function of the testis and reduction of the sexual drive in man and animals. No one has as yet been able to confirm a reduction of aggression which could be expected under CA. Isolated male wild mice received daily doses of 0.5 mg (17 mg/kg) CA subcutaneously over 32 days and were studied daily for aggressive behavior, motor activity and emotionality during the last 11 days. Aggression was reduced significantly by CA in the form of an increased latent time and continued to decrease during the test period with a reduction of fighting time and the correlated coarse motor activity. A latent period of approximately 4 weeks was determined for the aggression-inhibiting effect of CA. The detection rate was not changed under CA. It is doubted whether it is an adequate parameter of "emotionality". The overall motor activity was not reduced, so that an aggression-specific effect is assumed rather than a general reduction of drive. The body and testicular weight was reduced, the testicular tissue atrophies. Central points of attack of CA probably play a major role in the reduction of aggression.