The influence of social environment in early life on the behavior, stress response, and reproductive system of adult male Norway rats selected for different attitudes to humans.

The influence of social disturbance in early life on behavior, response of blood corticosterone level to restraint stress, and endocrine and morphometric indices of the testes was studied in 2-month Norway rat males from three populations: not selected for behavior (unselected), selected for against aggression to humans (tame), and selected for increased aggression to humans (aggressive). The experimental social disturbance included early weaning, daily replacement of cagemates from days 19 to 25, and subsequent housing in twos till the age of 2months. The social disturbance increased the latent period of aggressive behavior in the social interaction test in unselected males and reduced relative testis weights in comparison to the corresponding control groups. In addition, experimental unselected rats had smaller diameters of seminiferous tubules and lower blood testosterone levels. In the experimental group, tame rats had lower basal corticosterone levels, and aggressive animals had lower hormone levels after restraint stress in comparison to the control. The results suggest that the selection in two directions for attitude to humans modifies the response of male rats to social disturbance in early life. In this regard, the selected rat populations may be viewed as a model for investigation of (1) neuroendocrinal mechanisms responsible for the manifestation of aggression and (2) interaction of the hypothalamic-pituitary-adrenal and hypothalamic-pituitary-gonadal systems in stress.

Targeted resequencing of a genomic region influencing tameness and aggression reveals multiple signals of positive selection.

The identification of the causative genetic variants in quantitative trait loci (QTL) influencing phenotypic traits is challenging, especially in crosses between outbred strains. We have previously identified several QTL influencing tameness and aggression in a cross between two lines of wild-derived, outbred rats (Rattus norvegicus) selected for their behavior towards humans. Here, we use targeted sequence capture and massively parallel sequencing of all genes in the strongest QTL in the founder animals of the cross. We identify many novel sequence variants, several of which are potentially functionally relevant. The QTL contains several regions where either the tame or the aggressive founders contain no sequence variation, and two regions where alternative haplotypes are fixed between the founders. A re-analysis of the QTL signal showed that the causative site is likely to be fixed among the tame founder animals, but that several causative alleles may segregate among the aggressive founder animals. Using a formal test for the detection of positive selection, we find 10 putative positively selected regions, some of which are close to genes known to influence behavior. Together, these results show that the QTL is probably not caused by a single selected site, but may instead represent the joint effects of several sites that were targets of polygenic selection.

Functional characteristics of serotonin 5-HT2A and 5-HT2C receptors in the brain and the expression of the 5-HT2A and 5-HT2C receptor genes in aggressive and non-aggressive rats.

The functional activity of serotonin 5-HT(2A) and 5-HT(2C) receptors and the expression of the genes encoding them were studied in Norway rats bred for 60 generations for the presence and absence of high levels of stress-evoked aggression to humans. There were no significant differences in the levels of 5-HT(2A) receptor mRNA in the midbrain, frontal cortex, and hippocampus and the extents of head twitching evoked by the 5-HT(2A) agonist DOI in rats with and without genetically determined high levels of aggression. Administration of the selective 5-HT(2C) agonist MK-212 weakened reflex startle in response to an acoustic signal (the acoustic startle response) in non-aggressive animals but had no significant effects on the response in aggressive animals. Increases in the level of 5-HT(2C) receptor mRNA were seen in the frontal cortex and hippocampus in non-aggressive rats as compared with aggressive animals. Increases in the expression of the 5-HT(2C) receptor gene and the functional state of 5-HT(2C) receptors were seen in the brains of non-aggressive rats, without any changes in the 5-HT(2A) receptor mRNA level or receptor sensitivity; this is evidence for the involvement of 5-HT(2C) receptors in the mechanisms inhibiting fear-evoked aggressive behavior.

Expression of serotonin transporter gene and startle response in rats with genetically determined fear-induced aggression.

The study was carried out on Norway rats selected through more than 60 generations for high and low fear-induced aggressiveness towards humans (Institute of Cytology and Genetics). The intensity of aggressive behavior towards man, reflex startle response, and expression of serotonin transporter (5-HTT) gene were studied. Selection for high aggression was associated with more intense startle response. The expression of 5-HTT gene in the frontal cortex was reduced significantly in rats selected for high aggression to humans in comparison with nonaggressive rats. The authors conclude that 5-HTT is involved in the regulation of genetically determined fear-induced aggression.

Some features of learning in the Morris water test in rats selected for responses to humans.

The characteristics of learning in the Morris water test were studied in gray rats subjected to prolonged selection for elimination (the tame strain) and enhanced (the aggressive strain) aggressivity towards humans. Blood corticosterone levels at different stages of learning were also estimated. Tame rats learned to locate the invisible platform better than aggressive rats. The time spent seeking the platform by aggressive rats increased because they spent more time at the periphery of the basin. The duration of vertical investigative activity while on the platform was greater in tame rats than in aggressive rats. Fixation of the memory trace was demonstrated by the observation that rats of both strains spent more time in the sector in which the platform had been located during the training period. Rats of the two strains showed essentially no difference in terms of the time spent seeking the platform when it was placed in the opposite sector. After one day of training, blood corticosterone was significantly lower in tame than in aggressive rats. On subsequent training days, hormone levels in tame animals increased and were no different from those in aggressive rats. It is suggested that decreased emotionality and stress reactivity facilitated the learning process in tame rats in the Morris water test.

Involvement of brain serotonin 5-HT1A receptors in genetic predisposition to aggressive behavior.

Experiments were performed on Norwegian rats selected over more than 59 generations for high and low levels of high-affective defensive aggressivity and on highly aggressive (offensive) Tg8 mice with irreversible monoamine oxidase A knockout. There were significant differences in the functional state and expression of 5-HT(1A) receptors between highly aggressive and non-aggressive animals. Functional activity assessed in terms of hypothermia evoked by a 5-HT(1A) agonist was significantly greater in non-aggressive rats and mice than in aggressive animals. The high level of functional activity in non-aggressive rats coincided with a greater level of expression of 5-HT(1A) receptors in the midbrain. The level of 5-HT(1A) receptor mRNA in aggressive mice was unchanged in the midbrain and hypothalamus and was increased in the frontal cortex and amygdaloid complex. These results led to the conclusion that 5-HT(1A) receptors play a significant role in the mechanisms of genetic predisposition to aggressive behavior.

A maternal methyl-containing diet alters learning ability in the Morris swimming test in adult rats.

Maternal choline diet is known to affect the processes of spatial learning. We report here our studies of learning ability in the Morris swimming test in the adult offspring of maternal rats given a methyl-containing supplement enriched with choline and betaine during pregnancy and lactation. Increases in the time taken to find the invisible platform and the duration of swimming close to the vessel walls were seen, these demonstrating worsening of learning ability in response to the maternal diet. Changes in the platform search strategy were not associated with increases in anxiety in male rats. The possible role of a maternal methyl-containing diet in altering the expression of genes controlling the development of the nervous system is discussed.

Responses of the hypophyseal-adrenal system to stress and immune stimuli in gray rats selected for behavior.

The characteristics of the responses of the hypothalamo-hypophyseal-adrenal system to restriction stress and administration of lipopolysaccharide and interleukin-2 were studied in gray rats selected for the maintenance and absence of aggressive behavior in relation to humans. These experiments demonstrated decreased levels of corticosterone and ACTH in the plasma of tame rats in restriction stress and after administration of lipopolysaccharide as compared with non-tame rats. After administration of interleukin-2, the corticosterone level was identical in both groups of animals, though it reached the basal level in tame rats more quickly than in rats selected for maintaining aggressive behavior. Thus, selection of gray rats for tame behavior induces not only decreases in the responses of the hypophyseal-adrenal system to restriction stress, but also changes its response to immune stimuli and also, perhaps, its interaction with the immune system.

Directional left-sided asymmetry of adrenals in experimentally domesticated animals.

Directional left-sided asymmetry of the adrenals was typical of black and silver foxes, American minks, and gray rats selected by their behavior. In domesticated, but to a greater extent, in aggressive animals, the weight of the left adrenal and the width of its medulla and cortex markedly surpassed the corresponding parameters of the right adrenal. In aggressive animals enlargement of the left adrenal cortex was associated with widening of the zona reticularis, while in domesticated animals with enlargement of the zona fasciculata.

Specific [3H]8-OH-DPAT binding in brain regions of rats genetically predisposed to various defense behavior strategies.

Distribution of 5-HT1A receptors was studied in rats genetically predisposed to two basic defense strategies--passive (freezing) or active (aggression) defensive behavior. Specific [3H]8-OH-DPAT binding was assayed in the brain structures of rat strains bred for 40 generations from Wistar stock for predisposition to freezing (catalepsy), and in wild rats bred for low and high aggression to humans. Considerable changes in [3H]8-OH-DPAT binding were found in the brain of rats with hereditary predisposition to catalepsy. A significant decrease in Bmax of specific receptor binding of [3H]8-OH-DPAT in the frontal cortex, and in the striatum as well as an increase in Kd in the hippocampus of cataleptic rats was shown. A clear-cut tendency to decrease of 5-HT1A receptor density was observed in the midbrain and hypothalamus of these rats. A comparison of wild Norway rats bred for aggressiveness against humans with those bred for the absence of affective aggressiveness showed a Bmax decrease without Kd change in the frontal cortex, hypothalamus, and amygdala of aggressive animals. It is hypothesized that 5-HT1A and probably 5-HT1A-like 5-HT7 serotonin receptors are involved in the mechanisms of both active and passive defense reactions, and the high expression of fear-induced defense is associated with their decrease in the frontal cortex. At the same time, the genetically determined preference for a certain defense behavior strategy depends either on the peculiarities of distribution of these receptor types in the brain regions or on some other types of serotonin receptors.