Differences in the capacity of male odours to affect investigatory behaviour and different urinary marking patterns in two strains of mice, selectively bred for high and low aggressiveness

The neurobiological basis of human aggression: A review on genetic and epigenetic mechanisms

Aggression is an evolutionary conserved behavior present in most species including humans. Inadequate aggression can lead to long-term detrimental personal and societal effects. Here, we differentiate between proactive and reactive forms of aggression and review the genetic determinants of it. Heritability estimates of aggression in general vary between studies due to differing assessment instruments for aggressive behavior (AB) as well as age and gender of study participants. In addition, especially non-shared environmental factors shape AB. Current hypotheses suggest that environmental effects such as early life stress or chronic psychosocial risk factors (e.g., maltreatment) and variation in genes related to neuroendocrine, dopaminergic as well as serotonergic systems increase the risk to develop AB. In this review, we summarize the current knowledge of the genetics of human aggression based on twin studies, genetic association studies, animal models, and epigenetic analyses with the aim to differentiate between mechanisms associated with proactive or reactive aggression. We hypothesize that from a genetic perspective, the aminergic systems are likely to regulate both reactive and proactive aggression, whereas the endocrine pathways seem to be more involved in regulation of reactive aggression through modulation of impulsivity. Epigenetic studies on aggression have associated non-genetic risk factors with modifications of the stress response and the immune system. Finally, we point to the urgent need for further genome-wide analyses and the integration of genetic and epigenetic information to understand individual differences in reactive and proactive AB. © 2015 Wiley Periodicals, Inc.

Changes in male odours and urinary marking patterns due to inhibition of aggression in male mice

The effects of active inhibition of aggression on male odours and urinary marking patterns were studied in mice belonging to a highly aggressive strain the TA (Turku Aggressive), which has been developed by selective breeding through 37 generations. These males were defeated by trained fighters until they showed no aggression. Individually housed TA males served as controls. Mice from the parental or Normal Strain, which is intermediate in aggression, were exposed to the odours. The males from the Normal Strain were tested for aggression against male castrates to which urine from the two types of TA males or water had been applied. The urine from the highly aggressive control TA males evoked most aggression. The Normal males were later tested against castrates on soiled sawdust. Fewer attacks occured on sawdust soiled by the urine from the control TA males. The preferences for areas covered with soiled sawdust were also assessed. The males from the Normal Strain preferred areas soiled by the TA males trained to nonaggressiveness while the females preferred areas soiled by the highly aggressive control TA males. Subsequently the size and number of urinary marks deposited were examined. The TA males trained to nonaggressiveness voided urine in fewer but larger pools. The differences showed the same direction as those previously found between the TA and TNA strains, selectively bred for aggression and non-aggression, respectively. In mice the odour signals and urinary marking patterns seem to be correlated with the level of aggressiveness, either hereditarily determined or acquired through learning.

Social housing influences the composition of volatile compounds in the preputial glands of male rats

In rodents the preputial glands are one of the major sources of pheromones. These volatile chemosignaling compounds are known to elicit specific behavioral and physiological effects in their conspecifics. While social stress can alter both the behavior and hormonal status of rodents, little is known about its influence on the volatile constituents of the preputial glands. We have examined the composition of volatile compounds in the preputial glands of gonadally intact male rats housed for 70 days in either unisex triads (three/cage) or singly. The rank status of triad-housed rats was based on quantitative behavioral assessments taken during the initial 30 min of triad housing. Dominant rats had heavier preputial glands compared to subdominant and subordinate rats. Capillary gas chromatography-mass spectrometry identified 56 volatile preputial compounds, of these 17 did not differ between groups while 26 compounds were significantly higher in the single-housed compared to the triad-housed rats. Six additional volatile compounds were higher in the dominant compared to the other 3 groups, while another six compounds were higher in both the dominant and single-housed rats compared to the subdominant and subordinate rats. It can be concluded that both housing condition and social rank status have significant but different effects on the composition of volatile compounds found in preputial glands of male rats. The physiological and behavioral significance of these changes in preputial gland volatile compound composition in rats remain to be investigated.

Aggression from a developmental perspective: genes, environments and interactions

Genetic influences on the social behaviours of non-human mammals are ubiquitous, powerful and readily detected. But demonstrations that social behaviours are influenced by genes constitute only part of the story. Developmental findings have helped complete the picture. Specifically, these studies show that: (1) genetic effects for aggressive behaviours are highly malleable over the course of development; (2) genetic influences on aggressive behaviour are more dynamic, easily achieved and open to rapid manipulation than has been recognized in current models of social evolution and behavioural genetics; (3) developmental timing has a significant impact upon the nature of the genetic effects observed in aggressive behaviours. These empirical results are consistent with the view that social behaviours are among the first features to be influenced by genetic selection and by environmental experience. Social actions have distinctive properties in adaptation because they organize the space between the organism and the environment and promote rapid, selective and novel accommodations. The modern integrative view of the development of individual-in-context brings attention to the correlation between constraints within and external to individuals. This paper reviews findings on how these processes become integrated over time in individuals and species, and their implications for the nurture of nature.

Sex-related coping responses in mice selectively bred for aggression

Sex differences in strategies of coping with novel situations were studied in three strains of mice with regard to metabolism and open-field and maze activity as well as learning-induced adjustment. The 140 mice were selectively bred for high (Turku Aggressive [TA]) and low (Turku Nonaggressive [TNA]) levels of aggressiveness and originated from a Swiss albino stock normally distributed [N] for aggressiveness. The results indicated that TNA sex differences are more similar to those of the control N mice as compared to those of TA mice. In maze learning, however, the sex differences of TA mice are more in agreement with those of the N strain. Recordings of metabolism and open-field as well as maze activity were correlates of both gender and strain. Sex differences in learning-induced open-field coping behavior were unrelated to strain.

Selective breeding for isolation-induced intermale aggression in mice: associated responses and environmental influences

Aggressive (TA) and nonaggressive (TNA) lines of mice were established by selective breeding for isolation-induced intermale aggression. This paper summarizes and updates studies performed on the TA and TNA lines. The genetic analysis revealed that in these lines the genes for aggression are located on the autosomes and demonstrate a Mendelian segregation. The genes are expressed only in the presence of androgens which are normally present only in males. Behavioral and biological responses associated with high and low levels of aggression in TA and TNA mice are reviewed. Line differences have been found in olfactory communication and marking behavior, in maternal and predatory aggression in females, in locomotor activity, and in learning abilities. Also, correlated neurochemical and endocrinological responses to the selection have been detected. Maternal factors during the preweaning period do not significantly affect the development of aggression in TA and TNA males, while early postweaning exposure to aggression or sex enhanced later aggressive and sexual activity. Early experience and genetic disposition for aggression are correlated, with TA males showing the greatest increase in the behaviors studied.

Effects of acute administration of the 5-HT3 receptor antagonist, BRL 46470A, on the behaviour of mice in a two compartment light-dark box and during social interactions in their home cage and an unfamiliar neutral cage

Adult male CD1 mice received the 5-HT3 receptor antagonist, BRL 46470A, by intraperitoneal injection at three dose levels (2.5 mg/kg, 25 and 2.5 micrograms/kg). Controls were injected with physiological saline. At 30 min after injection, the behaviour of each mouse was examined by ethological procedures, when encountering an untreated partner for 5 min in its home cage and for 5 min in the more aversive situation of an unfamiliar neutral cage. The behaviour of each mouse also was monitored for 5 min in a two compartment light-dark box. At all doses tested, BRL 46470A increased the time spent in the light compartment of the light-dark box. At the smallest dose (2.5 micrograms/kg), the number of transitions between light and dark compartments was increased and there also was an increase (per unit time) in the numbers of squares crossed and number of scans in the light compartment. At all doses tested, BRL 46470A increased social investigation and reduced non-social exploratory activity in both the home cage and the unfamiliar neutral cage. In both test situations, increase of social investigation was maximum at 25 micrograms/kg, and at this dose, aggressive behaviour was also enhanced. In the neutral cage, digging in the sawdust by drug-treated mice showed a progressive dose-related increase. These results indicate potent anxiolytic-like activity by BRL 46470A and also demonstrate increased reactivity to unfamiliar environmental stimuli, such as novel sawdust. The significance of these findings is discussed.

Open-field parameters and maze learning in aggressive and nonaggressive male mice

Significant differences were observed in thigmotaxis, ambulation, and latency to move (time to start ambulating) between highly aggressive (TA) and low aggressive (TNA) male mice. The former displayed more thigmotaxis, ambulated more, and had a shorter latency to move than the TNA animals. Also they voided a greater number of urinary spots and defecated less than TNA. Further they were superior to the TNA mice in maze-learning capacity. The tendency to enter inner partitions of the field as well as total ambulation increased after learning by TA mice. The training toward nonaggressiveness of TA mice suppressed aggressive responses, thigmotaxis, and the number of urinary spots but enhanced defecation. All measures returned to their initial levels after one month of rest. The attacking behaviour of TA animals increased both thigmotaxis and ambulation.

Highly potent inhibitory effects of 5-HT3 receptor antagonist, GR38032F, on non-opioid defeat analgesia in male mice

Behavioural and pharmacological evidence indicates that non-opioid analgesia in defeated male mice is initiated by anxiety and that serotongergic (5-HT) substrates are implicated. In the present study, the effects of the novel putative 5-HT3 anxiolytic, GR38032F, on this form of adaptive inhibition of pain have been examined. The results showed that defeat analgesia was totally inhibited by 1 microgram/kg-1 mg/kg of GR38032F, with partial inhibition evident over the dose range of 0.0001-0.1 microgram/kg and loss of efficacy at smaller doses. These highly potent effects of GR38032F are consistent with its anxiolytic profile in animal models and cannot be accounted for by indirect actions on basal nociception. These findings point to a potentially important modulatory role for 5-HT3 receptor mechanisms in defeat analgesia and, more generally, provide further evidence for the involvement of 5-HT in the mediation of non-opioid forms of environmentally-induced antinociception.

Adaptive pain inhibition in murine resident-intruder interactions

In dyadic encounters with aggressive resident conspecifics, male intruder mice display an initial acute nonopioid analgesia followed by a more enduring opioid analgesia. The former reaction occurs in association with active defense (flight or fight) and can be seen in response to the scent of an aggressive conspecific or defeat experience per se. In contrast, the latter (opioid) reaction is associated with passive defense (immobility) and occurs in response to extended conspecific attack. The mechanisms underlying these two ecologically-relevant forms of pain inhibition are contrasted and the phenomena are discussed in relation to the question of adaptive significance.