A developmental-genetic analysis of aggressive behavior in mice. II. Cross-sex inheritance

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.

Transcriptome analysis of genes and gene networks involved in aggressive behavior in mouse and zebrafish

Despite moderate heritability estimates, the molecular architecture of aggressive behavior remains poorly characterized. This study compared gene expression profiles from a genetic mouse model of aggression with zebrafish, an animal model traditionally used to study aggression. A meta-analytic, cross-species approach was used to identify genomic variants associated with aggressive behavior. The Rankprod algorithm was used to evaluated mRNA differences from prefrontal cortex tissues of three sets of mouse lines (N = 18) selectively bred for low and high aggressive behavior (SAL/LAL, TA/TNA, and NC900/NC100). The same approach was used to evaluate mRNA differences in zebrafish (N = 12) exposed to aggressive or non-aggressive social encounters. Results were compared to uncover genes consistently implicated in aggression across both studies. Seventy-six genes were differentially expressed (PFP < 0.05) in aggressive compared to non-aggressive mice. Seventy genes were differentially expressed in zebrafish exposed to a fight encounter compared to isolated zebrafish. Seven genes (Fos, Dusp1, Hdac4, Ier2, Bdnf, Btg2, and Nr4a1) were differentially expressed across both species 5 of which belonging to a gene-network centred on the c-Fos gene hub. Network analysis revealed an association with the MAPK signaling cascade. In human studies HDAC4 haploinsufficiency is a key genetic mechanism associated with brachydactyly mental retardation syndrome (BDMR), which is associated with aggressive behaviors. Moreover, the HDAC4 receptor is a drug target for valproic acid, which is being employed as an effective pharmacological treatment for aggressive behavior in geriatric, psychiatric, and brain-injury patients. © 2016 Wiley Periodicals, Inc.

Sex-dependent alterations in social behaviour and cortical synaptic activity coincide at different ages in a model of Alzheimer's disease

Besides memory deficits, Alzheimer's disease (AD) patients suffer from neuropsychiatric symptoms, including alterations in social interactions, which are subject of a growing number of investigations in transgenic models of AD. Yet the biological mechanisms underlying these behavioural alterations are poorly understood. Here, a social interaction paradigm was used to assess social dysfunction in the triple-transgenic mouse model of AD (3xTg-AD). We observed that transgenic mice displayed dimorphic behavioural abnormalities at different ages. Social disinhibition was observed in 18 months old 3xTg-AD males compared to age and sex-matched control mice. In 3xTg-AD females, social disinhibition was present at 12 months followed by reduced social interactions at 18 months. These dimorphic behavioural alterations were not associated with alterations in AD neuropathological markers such as Aβ or tau levels in the frontal cortex. However, patch-clamp recordings revealed that enhanced social interactions coincided temporally with an increase in both excitatory and inhibitory basal synaptic inputs to layer 2-3 pyramidal neurons in the prefrontal cortex. These findings uncover a novel pattern of occurrence of psychiatric-like symptoms between sexes in an AD model. Our results also reveal that functional alterations in synapse activity appear as a potentially significant substrate underlying behavioural correlates of AD.

Developmental explanation and the ontogeny of birdsong: Nature/nurture redux

Despite several decades of criticism, dichotomous thinking about behavioral development (the view that the behavioral phenotype can be partitioned into inherited and acquired components) remains widespread and influential. This is particularly true in study of birdsong development, where it has become increasingly common to diagnose songs, elements of songs, or precursors of songs (song templates) as either innate or learned on the basis of isolation-rearing experiments. The theory of sensory templates has encouraged both the dichotomous approach (by providing a role for genetic blueprints to guide song learning) and an emphasis on structural rather than functional aspects of song development. As a result, potentially important lines of investigation have been overlooked and the interpretation of existing data is often flawed. Evidence for a genetic origin of behavioraldifferencesis frequently interpreted as evidence for the genetic determination of behavioralcharacters. The technique of isolation rearing remains the methodology of choice for many investigators, despite the fact that it offers only a rather crude analysis of the contribution of experience to song development and provides no information at all about genetic contributions to development. The latter could in principle be elucidated by the application of developmental-genetic techniques, but it is unlikely that these can easily be applied to the study of birdsong. Because developmental questions are so often posed in terms of the learned–innate dichotomy, “experience” is taken to be synonymous with “learning” and the possible role of nonobvious contributions to song development has largely been ignored. An alternative approach, based on Daniel Lehrman's interactionist theory of development, permits a more thorough appreciation of the problems that have yet to be addressed, and provides a more secure conceptual foundation for theories of song development.

Issues in the search for candidate genes in mice as potential animal models of human aggression

Conceptual and methodological issues in the search for candidate genes for mouse aggression and for the development of animal models of human aggression are considered. First, the focus is on genetic and then behavioural aspects of the search for candidate genes in mice. For the genetic aspect, two approaches are presented. In mice, these are chromosome mapping of polymorphic genes and evaluation of gene (polymorphic or monomorphic) function using knockout mutants. For the behavioural aspect, several parameters, including the type of aggression, measure of aggression, test situation and opponent type can have effects on the obtained genetics. This is illustrated for the offence type of attack behaviour in mice. The current combination of sophisticated genetic and behavioural analyses will result in time in the identification of many of the genes with effects on variation and development of one or more types of murine aggression. Since mouse and humans have many homologous genes mapped to homologous chromosome regions, it is conceivable that individual genes identified for one or more types of mouse aggression may be developed as animal models for human aggression. Genetic, physiological and behavioural limitations and uses of such models are discussed.

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.

Artificial selection for increased maternal defense behavior in mice

Maternal aggression is directed towards intruders by lactating females and is critical for defense of offspring. Within-family selection for increased maternal defense in outbred house mice (Mus domesticus; Hsd:ICR strain) was applied to one selected (S) line, using total duration of attacks in a 3-min test as the selection criterion. One control (C) line was maintained and both lines were propagated by 13 families in each generation. Prior to selection, heritability of maternal aggression was estimated to be 0.61 based on mother-offspring regression. Duration of attacks responded to selection with a mean realized heritability of 0.40 (corrected for within-family selection) after eight generations. At generation 5, the S and C line also differed significantly for litter size at birth and at mid-lactation (both lower in S), average individual pup mass at midlactation (higher in S), and pup retrieval latency (longer in S), but not for other maternal measures that we studied (e.g., dam mass). Additionally, number of entries to middle and closed plus maze compartments was significantly higher in S mice in Generation 5. This is the first study to select for high maternal defense and these mice will be made available as a tool for understanding the genetic and neural basis of maternal aggression.

Predatory aggression, but not maternal or intermale aggression, is associated with high voluntary wheel-running behavior in mice

Predatory (towards crickets), intermale, and maternal aggression were examined in four replicate lines of mice that had been selectively bred for high wheel-running (S) and in four random-bred control lines (C). In generation 18, individual differences in both predatory and intermale aggression were significantly consistent across four trial days, but predatory and intermale aggression were uncorrelated both at the individual level and among the eight line means. Latencies to attack crickets were significantly lower in S lines as a group. Intermale aggression, however, did not differ between S and C lines. S lines were significantly smaller in body mass, but did not differ in either testes mass or plasma testosterone. In generations 28 and 30, respectively, S and C lines did not differ in either maternal or intermale aggression. However, significant differences among the individual lines were found for maternal aggression, and one S line exhibited an extremely high mean time of aggression (>120 sec for a 5-min test). Maternal and intermale aggression were not correlated among the eight line means or at the level of individual variation. Overall, our results suggest: (1) predatory aggression and voluntary wheel-running are positively related at the genetic level; (2) predatory and intermale aggression are unrelated at a genetic level; and (3) maternal and intermale aggression are not tightly related at the genetic level. Possible relationships between predatory aggression, dopamine, and wheel-running behavior are discussed.

Maternal behavior changes after immune challenge of neonates with developmental effects on adult social behavior

To examine whether maternal responsiveness during interactions with endotoxin-treated pups contributes to long-term effects on social development, neonatal mice were fostered on postnatal day 1 to dams from three selectively bred lines that differ in social behaviors. On day 5, neonates were administered saline or 0.5 mg/kg endotoxin (lipopolysaccharide, i.p.). Observations of undisturbed dams and litters on days 2, 4, 6, and 8 showed modest line differences in maternal behaviors. At the peak intensity of the transient illness induced by endotoxin (3 hr postinjection on day 5), dams increased licking and decreased time off-nest for endotoxin, but not saline-treated pups. As adults, fostered-reared males were observed in brief social interactions. Males exposed to endotoxin early in life showed changes in adult social behaviors that depended on foster dam line as well as individual differences in maternal responsiveness. Maternal responsiveness to stressed neonates can ameliorate the social-developmental effects of early illness.

Genetic-environment analysis of sensitivity and acute tolerance to ethanol in mice

The purpose of this study was to characterize initial sensitivity (IS), acute functional tolerance (AFT), and rate of tolerance development to ethanol in lines of mice selected for aggression mice as well as to investigate the impact of isolate housing on these phenotypes. The results showed that for IS, there were no differences among treatment groups. For acute tolerance and rate of tolerance development, a Line x Sex x Housing interaction was present, with the response to housing being more pronounced in the low aggressive line than the high aggressive line, and the females being more affected than the males. Correlational analysis showed low to moderate associations between rate of tolerance development and IS, as well as between rate of tolerance and AFT. Housing condition significantly influenced female expression of ethanol phenotypes as compared to males. The line of the subject also influenced the magnitude of expression of these phenotypes. These findings suggest that environmental and genetic influences interact to influence acute tolerance and rate of tolerance development.

Selective breeding for infant vocal response: a role for postnatal maternal effects?

N:NIH rats were selectively bred on the basis of high or low rates of ultrasonic vocalization (USV) response to isolation at 10 days of age (Brunelli et al., 1997: Dev Psychobiol 31:255-265). To examine the possibility of postnatal maternal effects in the generational transmission of divergent traits, pups were cross-fostered shortly after birth between dams of the two lines (Low- and High-USV). Controls were fostered to dams of the same line (in-fostered). Additional (population) control data were obtained from the entire 13th generation of the selectively bred lines. USV rates of cross-fostered pups in each line were not significantly different from rates of in-fostered pups of the same line. High USV line pups cross-fostered to Low USV line dams weighed significantly less than in-fostered pups, on the day of testing. The results provide no evidence for a postnatal maternal contribution to the USV phenotype. Prenatal and/or perinatal maternal effects have not been ruled out.

Differences in NK cell function in mice bred for high and low aggression: genetic linkage between complex behavioral and immunological traits?

In previous studies, we found differences in cellular immune responsiveness in Institute for Cancer Research (ICR) mice selectively bred for high and low levels of aggression. Compared to the high aggressive line, the low aggressive line had low levels of natural killer (NK) and T cell activity and increased susceptibility to tumor development. To dissect further this novel association, experiments were designed to test two competing hypotheses. The first hypothesis was that the phenotypic expression of the line differences in NK cell activity are dependent on and regulated by the expression of high and low levels aggressive behavior in the lines. The alternative hypothesis was that the differences in immune status are independent of the expression of aggression by the lines, suggesting linkage between a subset of genes involved in determining these complex behavioral and immunological traits or pleiotropic gene effects on both traits. In Experiment 1, three conditions of postweaning social experience (mice singly housed, group housed within line, or group housed between lines) were tested in males to determine whether experiential conditions which modify the expression of aggression would in turn modify the line differences in NK cell activity. This experiment revealed that the difference in NK cell activity between high aggressive and low aggressive male mice was attributable to line only. The different postweaning social conditions examined had no effect on modifying the differences in NK activity, and social dominance hierarchy did not correlate with levels of NK cell activity. Whereas males of the high and low lines exhibit differences in aggressive behaviors across most contexts, females do not exhibit such differences except in response to an intruder during the postpartum period. Therefore, in Experiment 2 we compared the NK cell activity of nulliparous females of the high and low aggressive lines. Under these conditions, females of the low aggressive line had low levels of NK activity compared to high aggressive females (differences comparable to those seen between males of the high and low lines). Taken together, these experiments lend support to the hypothesis that this association may be due to a genetic linkage between subsets of genes involved in determining these complex behavioral and immunological traits, or may possibly represent a fortuitous association which occurred during the selective breeding.

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.