What Causes Aggression? Scientists Turn Off Angry Behavior in Mice

Scientists have turned off aggressive behavior in mice for weeks at a time by harnessing the power of a little-understood group of brain cells.

In a new study, which sheds light on the potential biological cause of aggression, scientists tinkered with neurons in the brains of lab mice.

To arrive at their findings, the researchers at the Karolinska Institutet in Sweden investigated the role of a set of neurons in the ventral premammillary nucleus (PMv) of the hypothalamus.

Mice generally resort to aggression as a way to assert their dominance rather than cause harm, so the scientists tested their hypothesis using the so-called tube test. This involves pitting two mice against each other in a narrow corridor, in order to document their responses in this socially fraught situation.

The study, published in the journal Nature Neuroscience, showed when male mice were confronted with a new male in their home cage, the animals responded aggressively, and showed activity in the PMv neurons.

Using a technique where neurons are turned on and off using light, called optogenetics, the researchers were also able to trigger aggressive behavior in mice in scenarios where they were usually calm.

In turn, they could stop a mouse mid-attack by turning off the light. What's more, when a generally submissive male was faced with a dominant male, blocking the PMv cells inverted their statuses. As well as aggression, PMv neurons also appeared to be associated with other parts of the brain—the part linked to rewards.

"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," study co-author Christian Broberger, associate professor at the Department of Neuroscience, Karolinska Institutet, said in a statement.

The study's co-author Stefanos Stagkourakis, doctoral student at the department of neuroscience, Karolinska Institutet, added: "We also found that the brief activation of the PMv cells could trigger a protracted outburst, which may explain something we all recognize—how after a quarrel has ended, the feeling of antagonism can persist for a long time."

Although the study was carried out in mice, which are markedly different to other mammals, the team hopes that further research could reveal whether PMv neurons play a similar role in human aggression.

"Aggressive behavior and violence cause injury and lasting mental trauma for many people, with costly structural and economic consequences for society," said Broberger, adding: "our study adds fundamental biological knowledge about its origins."