[The effects of prolonged selection for behavior on the stress response and Activity of the reproductive system of male Brown rats (Rattus norvegicus)].

Two Norway rat (Rattus norvegicus) populations contrasting in behavior have been raised at the Institute of Cytology and Genetics, Novosibirsk, Russia with long-term selection for the absence or enhancement of an aggressive response towards humans. They are designated as tame and aggressive, respectively. In this work we investigated the effects of the selection on behavior, stress responsiveness, and fertility in males of the 78th generation. It is shown that the difference between the strains in their response to humans remains the same as in previous generations. However, the differences in stress response and maturation age contradict earlier data. For the first time, we note a higher glucocorticoid-mediated response to restriction stress and retarded sexual maturation in tame rats compared to aggressive ones, according to morphometric indices of gonads and testosterone levels. It is conceivable that the change in selection effects is determined by the disjunction of the directions of selection for behavior and the modification of the stress response. This study is the first to characterize males recently (six or seven generations of propagation in captivity) caught in the wild with regard to the indices under consideration and used as a control group. Wild rats have the highest stress response and rate of sexual maturation as compared to those selected.

[Aggression and acoustic startle response in adult and infant rats with genetically defined aggression towards man and its absence].

The aim of the current study was to investigate the association between expression of acoustic startle response and fear-induced aggression. The intensity of aggressive response towards man and acoustic startle response in adult and 15-day old Norway rats selectively bred for 70 generation for high level or the lack of fear-induced aggression was studied. Adult rats of aggressive strain demonstrated high aggression and increased amplitude of acoustic startle response compared to rats selectively bred for the lack of fear-induced aggression. It was found that, in contrast to infant rats of tame strain, 15-day old rat pups of aggressive strain demonstrated distinct aggressive response towards man, although this response was not as intensive as in adult rats of this strain. There was no considerable difference between aggressive and tame infants in amplitude of acoustic startle response. Significant habituation of startle response was shown in adult rats of aggressive strain, whereas in 15-day old rat pups of aggressive rat strain amplitude of startle response remained unaltered. Both adult and infant rats of tame strain demonstrated considerable habituation of startle response. Thus, defensive aggression in rats with genetic predisposition to fear-induced aggression appears quite early--at eyes opening they demonstrate a pronounced aggressive response toward man. The acoustic startle reflex is one of the elements of complex behavior--fear-induced aggression, and this element is involved in the enhancement of aggressive response towards man in adult rats but does not play any essential role in infant aggressiveness.

[Functional correlates of the brain 5-HT2A and 5-HT2C serotonin receptors and expression of 5-HT2A AND 5-HT2C genes in aggressive and nonaggressive Norway rats].

Functional activity of the brain 5-HT2A and 5-HT2C serotonin receptors and expression of 5-HT2A and 5-HT2C genes in Norway rats selectively bred for 60 generations for high level and for a lack of fear-induced aggression towards humans were studied. There were no essential differences between aggressive and tame rats in 5-HT2A receptor mRNA level in the midbrain, hippocampus and frontal cortex as well as in the intensity of head-twitch induced by selective 5-HT2A agonist DOI. At the same time, administration of 5-HT2C receptor selective agonist MK-212 reduced the amplitude of acoustic startle response in tame but not in aggressive animals. The increase in 5-HT2C receptor mRNA level in the frontal cortex and hippocampus of nonaggressive rats compared to aggressive animals was shown. The data indicate involvement of the brain 5-HT2C receptor rather than 5-HT2C receptors in the mechanisms of genetically predetermined suppression of fear-induced aggressive behavior.