Hormone found to regulate aggression in mothers

Researchers at the University of Otago have discovered a new function for the hormone prolactin — it limits a new mum’s investment in aggressive behaviour and instead focuses them on interacting with their offspring.

Otago researcher Dr Rosemary Brown says one of the behavioural changes that take place in all mammalian mothers is an increase in protective behaviour so mothers can guard their infant from danger or a perceived threat. She noted, “Despite occurring in so many different species during motherhood, we didn’t know how this protective behaviour is regulated.”

Researchers from the laboratories of Dr Brown and Professor Dave Grattan set about trying to understand how hormones can change this behaviour in mothers. Their starting point, said Dr Brown, was the knowledge that prolactin could act on cells in an area of the brain called the ventromedial hypothalamic nucleus — a region known to regulate aggressive behaviour in males and females.

“Prolactin is very high during pregnancy and lactation and so we initially thought that prolactin might induce this maternal protective behaviour,” she said. “In fact, what we have actually revealed is a role for this hormone in restraining aggressive behaviour so mothers don’t get too distracted by the things happening around them and can concentrate more on caring for their offspring.”

Prolactin is produced by the pituitary gland, which sits at the bottom of the brain, and one of its primary responsibilities is milk production. But while the hormone can alter many other biological processes, Dr Brown said its role in protective behaviour was unknown.

“Although protective behaviour is important, it comes at a cost, as the mother may risk her own health and expend too much time and energy in dealing with perceived threats,” she said.

“Prolactin limits investment in unnecessary protective behaviour and promotes investment into interacting with infants. She will still protect her babies, but will do this in a much more controlled and focused manner.”

How the brain adapts to motherhood is very similar in mammals, so researchers used an animal model and new neuroscience tools to block prolactin from acting on its target cells in just this region of the adult brain. Dr Brown said, “We found that the cells active during protective behaviour are able to respond to prolactin and prolactin powerfully acts on these cells to restrict the amount of aggressive and protective behaviour shown by mothers.”

Fellow researcher Dr Teodora Georgescu said the finding, now published in the journal Proceedings of the National Academy of Sciences, is significant.

“These studies are really important in enabling us to understand how behaviour and mood is regulated in mothers,” Dr Georgescu said. “Up to one in five new mothers struggle with some form of mood disorder. Our hope is that by understanding when and how hormones are changing behaviour in mothers, in the future we can develop new effective ways of helping these women.”

Image credit: ©stock.adobe.com/au/dubova

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