A new study has found a system in the brain that explains why female mice, who are not normally aggressive, suddenly exhibit this behaviour after becoming pregnant and giving birth.

Researchers at Stockholm University and Karolinska Institutet have found a system in the brain that can explain why female mice, who are not normally aggressive, suddenly and dramatically gain access to this behaviour after becoming pregnant and giving birth (so-called maternal aggression). The study shows that a group of neurons that control aggression in the normally aggressive males are turned off in the non-pregnant female, but switch to active mode when she becomes a mother. When the researchers silenced the neurons, the mother stopped attacking cage intruders. The study also shows that oxytocin and prolactin, the hormones that control maternal bodily functions such as lactation, can powerfully activate these neurons.

The study was conducted on animals, and should be interpreted with caution in a human context. But the findings touch on a larger conceptual question: How can an individual gain access to a behaviour that is outside their normal repertoire, during a limited phase of their life? The example studied here, with a brain circuit that can be turned on and off depending on whether an animal needs access to certain behaviour (regardless of sex) for its (or its offspring’s) survival, may have general significance for how the plastic brain functions also in humans.

“Surprisingly, it turned out that the same network of cells that drive aggression in male mice lies dormant in females – until motherhood flips the switch of this hormone-sensitive system,” says the article’s first author Stefanos Stagkourakis, who now leads his own research group at SciLifeLab and Karolinska Institutet.

“This is a study on laboratory mice, and we do not currently know whether the results can be transferred to humans. But the mechanism that we identify here how a behaviour that is normally outside an individual's repertoire can become available for a limited period of its life – may reflect a principle of brain flexibility with relevance beyond maternal aggression,” says Christian Broberger, professor of neurochemistry at Stockholm University.

The study was conducted under the leadership of Christian Broberger, Professor of Neurochemistry at Stockholm University. The experiments were carried out at Karolinska Institutet, where his laboratory was previously located.

Read article in Nature Communications: Maternal aggression driven by the transient mobilisation of a dormant hormone-sensitive circuit

Read more about Christian Broberger's research