Genetic influences on brain gene expression in rats selected for tameness and aggression.

Interindividual differences in many behaviors are partly due to genetic differences, but the identification of the genes and variants that influence behavior remains challenging. Here, we studied an F2 intercross of two outbred lines of rats selected for tame and aggressive behavior toward humans for >64 generations. By using a mapping approach that is able to identify genetic loci segregating within the lines, we identified four times more loci influencing tameness and aggression than by an approach that assumes fixation of causative alleles, suggesting that many causative loci were not driven to fixation by the selection. We used RNA sequencing in 150 F2 animals to identify hundreds of loci that influence brain gene expression. Several of these loci colocalize with tameness loci and may reflect the same genetic variants. Through analyses of correlations between allele effects on behavior and gene expression, differential expression between the tame and aggressive rat selection lines, and correlations between gene expression and tameness in F2 animals, we identify the genes Gltscr2, Lgi4, Zfp40, and Slc17a7 as candidate contributors to the strikingly different behavior of the tame and aggressive animals.

c-Fos activation and intermale aggression in rats selected for behavior toward humans.

Tame and aggressive rat lines were created during the long-term selection of Norway rats for elimination and enhancement of aggressiveness toward humans, respectively. Our previous experiments have demonstrated that selection for the elimination of defensive aggression toward humans is associated with attenuated intraspecific intermale aggression. However, the neuronal mechanisms underlying low and high intermale aggression in the tame and aggressive rats remain unclear. Here, we used c-Fos immunoreactivity to evaluate neuronal activation patterns in the main aggression-related areas in selected lines under basal conditions and after the resident-intruder (R-I) test. Although agonistic behavior of the tame and the aggressive rats differed significantly, social encounter caused similar brain activation patterns in both groups; we observed increased neuronal activation in the bed nucleus of stria terminalis, the hypothalamic attack area, and the medial amygdala 1h after the R-I test. However, neuronal activation in the hypothalamic attack area was significantly higher in the aggressive males compared to their tame counterparts. We propose that lower activation of the hypothalamic area is associated with the attenuation of intraspecific intermale aggression during selection for the elimination of aggressiveness toward humans.

A comparison of brain gene expression levels in domesticated and wild animals.

Domestication has led to similar changes in morphology and behavior in several animal species, raising the question whether similarities between different domestication events also exist at the molecular level. We used mRNA sequencing to analyze genome-wide gene expression patterns in brain frontal cortex in three pairs of domesticated and wild species (dogs and wolves, pigs and wild boars, and domesticated and wild rabbits). We compared the expression differences with those between domesticated guinea pigs and a distant wild relative (Cavia aperea) as well as between two lines of rats selected for tameness or aggression towards humans. There were few gene expression differences between domesticated and wild dogs, pigs, and rabbits (30-75 genes (less than 1%) of expressed genes were differentially expressed), while guinea pigs and C. aperea differed more strongly. Almost no overlap was found between the genes with differential expression in the different domestication events. In addition, joint analyses of all domesticated and wild samples provided only suggestive evidence for the existence of a small group of genes that changed their expression in a similar fashion in different domesticated species. The most extreme of these shared expression changes include up-regulation in domesticates of SOX6 and PROM1, two modulators of brain development. There was almost no overlap between gene expression in domesticated animals and the tame and aggressive rats. However, two of the genes with the strongest expression differences between the rats (DLL3 and DHDH) were located in a genomic region associated with tameness and aggression, suggesting a role in influencing tameness. In summary, the majority of brain gene expression changes in domesticated animals are specific to the given domestication event, suggesting that the causative variants of behavioral domestication traits may likewise be different.

Behavioral effects of bidirectional selection for behavior towards human in virgin and lactate Norway rats.

Although numerous studies have demonstrated strong differences in behavioral, hormonal and neurobiological characteristics between male rats selected for elimination (tame) and enhancement (aggressive) of aggressiveness towards humans, few studies have examined changes in female behavior under this selection. The objective of the current work was to evaluate the effects of bidirectional selection for aggressiveness towards humans on behavioral profiles of virgin and lactating rats compared with the behavior in tame, aggressive and unselected (wild-type) females. The behavior of virgin females was studied using the light-dark box, the startle response test and the modified glove test. Tame females were less anxious and more tolerant towards humans than unselected and aggressive rats. Principal component analysis of all behavioral parameters produced three independent factors, explaining 66.37% of the total variability. The measures of behavior towards humans and the measures of anxiety mainly loaded on PC1 (first principal component) which separated the tame females from the unselected and aggressive ones. These data suggest the genetic correlation between the selected behavior towards humans and anxiety-related behavior in virgin rats. No significant effect of line was found for PC2 scores, associated with risk assessment behavior. Measurements of freezing behavior mainly loaded on PC3, and this component separated rats of different genetic groups from each other. The behavior of lactating rats was studied in maternal defense and pup retrieval tests. Females of selected lines did not significantly differ in behavioral measurements of these tests and were characterized by higher maternal motivation than unselected rats. It is suggested that long-term breeding of tame and aggressive rats in captivity has reduced the threshold for maternal behavior.

Effect of domestication on aggression in gray Norway rats.

A comparative analysis of intermale aggression in the resident-intruder test was conducted with gray rats from a wild unselected population bred at the laboratory for three generations and gray rats selected for elimination (tame) and enhancement (aggressive) of aggressiveness towards human for 71-72 generations. Males from the laboratory line Wistar were used as neutral opponents. Rats from the tame line were characterized by reduced aggression manifest as longer attack latency, decreased number of attacks, upright postures, chases, kicks, and shorter total time of aggressive behavior compared to unselected males. There was no significant difference in the attack latency and the total time of aggression between rats of the aggressive line and unselected rats. A trend to decrease in the number of attacks, chases and upright postures and to increase in contribution of lateral threat postures to the total time of aggression was observed for males of the aggressive line. Plasma corticosterone in unselected males not presented with intruders and after their presentation was higher than in males of both selected lines. Comparative behavioral analysis of agonistic behaviors in rats from the aggressive and tame lines to opponents of different lines (Wistar, tame, aggressive) showed that the presence of an intruder from the aggressive line can enhance aggressive responses in residents from the tame line. Thus, selection for domestication of gray rats caused a significant attenuation of aggressive behavior without affecting the basic agonistic repertoire.

Conserved methylation of the glucocorticoid receptor gene exon 1(7) promoter in rats subjected to a maternal methyl-supplemented diet.

It is well known that the early life experiences affect stress responses and other physiological and behavioral traits in adulthood. Both rat and human studies have shown that early postnatal effects are associated with methylation of the hippocampal glucocorticoid receptor gene exon 1(7) (rat) and 1-F (human) promoters. Methylation of these sites is also seen following methionine administration in adult rats. However, it remains unclear whether similar alterations in DNA methylation profiles can result from prenatal influences. To address this question, we fed pregnant rats a methyl-supplemented diet that resulted in alteration of the stress response. However, methylation analysis revealed no effect of methyl supplements on methylation patterns of the glucocorticoid receptor gene exon 1(7) promoter in offspring. These results suggest that the pre- and postnatal effects of methyl supplementation have different mechanisms.

Cross-fostering effects on weight, exploratory activity, acoustic startle reflex and corticosterone stress response in Norway gray rats selected for elimination and for enhancement of aggressiveness towards human.

Two rat lines, one tame, the other aggressive, differing by many behavioral features and stress reactivity were developed by long-term selection of wild gray rats for elimination and enhancement of aggressiveness towards humans. The aim of this work was to study the role of the maternal environment in the expression of these differences between the two rat lines using the cross-fostering paradigm. Fostering of tame rats of both sexes by aggressive mothers and aggressive females by tame mothers was without effect on behavior score towards humans, but the cross-fostered aggressive males had a small, yet significant, increase in aggressiveness score. Cross-fostering revealed that exploratory behavior in the hole-board test and the acoustic startle amplitude were weakly affected by maternal interactions, although there was an effect on body weight and on the stress corticosterone response. Body weight was decreased in tame males fostered by aggressive mothers only and it was increased in cross-fostered aggressive rats of both sexes. Fostering of tame males and females by an aggressive mother enhanced almost twofold the corticosterone response immediately after stress, while fostering of aggressive ratlings of both sexes by a tame mother was without effect. The current results demonstrated that the maternal postnatal environment had no substantial effect on the behavioral responses of both tame and aggressive rats, but it possibly contributed to the development of the corticosterone response to restraint stress in the tame, and not the aggressive rats, i.e. these effects of cross-fostering were dependent on ratling genotype.

Phenotypic differences in behavior, physiology and neurochemistry between rats selected for tameness and for defensive aggression towards humans.

To better understand the biology of tameness, i.e. tolerance of human presence and handling, we analyzed two lines of wild-derived rats (Rattus norvegicus) artificially selected for tameness and defensive aggression towards humans. In response to a gloved human hand, tame rats tolerated handling, whereas aggressive rats attacked. Cross-fostering showed that these behavioral differences are not caused by postnatal maternal effects. Tame rats were more active and explorative and exhibited fewer anxiety-related behaviors. They also had smaller adrenal glands, larger spleens and lower levels of serum corticosterone. Blood glucose levels were lower in tame rats, whereas the concentrations of nine amino acids were higher. In the brain, tame rats had lower serotonin and higher taurine levels than aggressive rats. Our findings reinforce the notion that tameness is correlated with differences in stress response and will facilitate future efforts to uncover the genetic basis for animal tameness.