The Grapevine

Why Some People Maybe More Aggressive Than Others: Neurons Could Be Responsible

From shouting matches to full-blown wars, aggressive behavior has been commonly observed throughout history. In a study with mice, researchers from Karolinska Institutet in Sweden identified a relatively unknown group of neurons in the ventral premammillary nucleus (PMv) that may be responsible for aggression.

The paper, titled "A neural network for intermale aggression to establish social hierarchy" was published in the journal Nature Neuroscience on May 25. 

"Aggressive behavior and violence cause injury and lasting mental trauma for many people, with costly structural and economic consequences for society," said study leader Christian Broberger, an associate professor from the Department of Neuroscience at the institute. "Our study adds fundamental biological knowledge about its origins."

While it is known that aggression stems from the brain, the exact mechanism and identity of neurons involved remained unclear. In the new study, the researchers examined male mice that displayed aggression when a new male mouse was placed in their cage, finding them to have more active PMv neurons.

Next, they used optogenetics (a technique which uses light to control cells) and found they were able to control aggressive behavior. That is, by activating the PMv to initiate aggression in situations where animals do not normally attack and by hindering the PMv to interrupt an ongoing attack. In addition, other brain regions (such as reward centers) were shown to be activated by the mapping of the PMv neurons.

"That could explain why mice naturally make their way to a place where they have experienced an aggressive situation," said lead author Stefanos Stagkourakis, a doctoral student from the Department of Neuroscience at the institute.

In the experiments, a brief activation of the PMv neurons was able to trigger a prolonged outburst. Referring to it as "something we all recognize," Stagkourakis believed the mechanism may explain "how after a quarrel has ended, the feeling of antagonism can persist for a long time."

Aggression is somewhat of a ritual among male mice. The animals are less concerned about causing harm and more interested in using their aggression to establish a group hierarchy. This is done so by determining which mouse is the strongest. Researchers study this phenomenon with the use of a tube test which puts two mice face-to-face in a narrow corridor, allowing them to observe the submission and dominance. 

"One of the most surprising findings in our study was that the role-switch we achieved by manipulating PMv activity during an encounter lasted up to two weeks," said Broberger. By restraining the PMv cells in a dominant mouse and stimulating the same cells in a submissive mouse, the researchers were able to invert their hierarchical status.

The researchers hoped to see the creation of new strategies for managing aggression as the new findings may offer a deeper understanding of how neurons initiate and organize such behavior.