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Russian scientists discover aggression causes new nerve cells to be generated in brain

February 03, 16:51 UTC+3 MOSCOW
The new publication confirms a previous theory - mice that are accustomed to fighting not only behave differently, but their brain starts to function in a different way
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© ITAR-TASS/Alexander Ryumin

MOSCOW, 3 февраля. /TASS /. A group of neurobiologists from Russia and the United States have proven experimentally that aggression has an influence on the production of new nerve cells in the brain. The scientists conducted a series of experiments on male mice and published their findings in the journal Frontiers in Neuroscience, the Moscow Institute of Physics and Technology (MIPT) said in a press release.

Researchers from the Institute of Cytology and Genetics of the Siberian Branch of the Russian Academy of Sciences (ICG SB RAS), MIPT, Cold Spring Harbor Laboratory, and Stony Brook University and School of Medicine studied the changes that occurred in the brains of mice demonstrating aggressive behavior, which attacked other mice and won in fights. After a win, these mice became even more aggressive, and new neurons appeared in their hippocampus - one of the key structures of the brain; in addition to this, in mice that were allowed to continue fighting certain changes were observed in the activity of their nerve cells. The scientists hope that the new information on the neurobiological bases of aggression will not only help in understanding this important phenomenon, but will also encourage research in other areas - and even help in finding causes of autism and other similar disorders in humans.

The experimental scheme

This is how the experiment itself was conducted: pairs of male mice were placed in a cage bisected by a partition. The partition allowed the animals to see, hear, and smell each other, but did not allow physical contact. Every day, in the early afternoon, the partition was removed and the observations began: it did not normally take long for fights to break out. After two or three encounters the winner was established and was then (after three minutes, or sometimes less to avoid injuries to the defeated male) separated from its neighbor again. After repeating the process for three days in a row, the scientists changed the mice in the cages, randomly placing defeated males with a new neighbor (but, most importantly, each time a defeated male was placed in the same cage as another winning male). In one group, after three weeks of these rotations, winners were prevented from entering into confrontation, and in another group the mice continued to fight with one another.

The scientists also conducted a series of tests to demonstrate the effect of aggression not on the brain, but on behavior. For example, the mice were placed in a cross-shaped maze (plus-maze) where one corridor was closed and the other was an open space. The more time that the mice preferred to spend in the dark, closed space, the more their behavior could be described as "avoiding risk".

The mice were placed in a cage with a transparent partition and another male on the other side - the more time the mice spent close to the barrier, the higher the level of potential aggression. This interpretation is consistent with the fact that the active animals in the study tend to attack their partners if the opportunity arises (tests were also performed to prove this).

All the tests showed that males with winning experience in a number of fights display a more "brazen" attitude - they approach the transparent partition more often and initiate an attack on their opponents more quickly. If the mice were deprived of fighting for a period of time before the test, they became even more aggressive: the latency to the first attack was almost three times less, and the fights themselves lasted for longer. But what is particularly interesting is that at the same time their level of anxiety increased - a male who succeeded in tearing out patches of hair from the back of a weaker mouse would rather avoid open spaces, preferring to sit in the dark wherever possible.

The influence of aggression on the brain structure

Simply observing individual neurons, or even groups of neurons, does not give a complete picture. The location of the cells needs to be taken into account. The activity of neurons in different regions of the brain may vary significantly, as these regions perform different functions.

In this particular study, the scientists examined the hippocampus and the amygdala. It is often said that the amygdala is associated with emotions, and the hippocampus with memory, and this is generally true - but it should be clarified that despite this, memory is not localized in the hippocampus, and to experience emotions even mice need more than just the amygdala.

Comparing the activity of the amygdala and the hippocampus enabled scientists to trace the influence of the aggression experiment on two key structures at once. Past evidence suggested that in aggressive and socially active mice, more new neurons are produced in the hippocampus, and in specially bred lines of mice with increased aggressive behavior, the level of neurogenesis is also higher than those who were selected on the basis of reduced aggression.

In this experiment, scientists discovered that with repeated fights the level of the c-fos protein increases in the hippocampus, but decreases in the amygdala. And if the mouse is prevented from being involved in further fights, these changes do not occur in the function of immediate early genes, although new neurons still develop. The researchers also conducted a number of additional tests and experiments to interpret the observations made.

The new publication confirms a previous theory - mice that are accustomed to fighting not only behave differently, but their brain starts to function in a different way. The number of new cells of the hippocampus increases, and if the mice are allowed to continue fighting, the activity of existing cells also changes. New cells seem to be one of the key mechanisms of the increase in aggression and, perhaps, also anxiety - although scientists are not yet certain of this: the winning reputation of an aggressive and dominant mouse would almost certainly need to be backed up by new fights, but this is not something that will help to reduce anxiety.

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