Summary: Prolactin, a hormone responsible for milk production in mothers, reduces a mother’s investment in aggressive behaviors and helps to increase protective behaviors.
Source: University of Otago
University of Otago researchers have discovered a new function for the hormone prolactin—it limits a new mom’s investment in aggressive behavior and instead focuses them on interacting with their babies.
University of Otago researcher Dr. Rosemary Brown, of the Department of Physiology, says one of the behavioral changes that takes place in all mammalian mothers is an increase in protective behavior so mothers can guard their infant from danger or a perceived threat.
“Mothers are prepared to take significant risks to aggressively protect their babies. Despite occurring in so many different species during motherhood, we didn’t know how this protective behavior is regulated.”
Researchers from the laboratories of Dr. Brown and Professor Dave Grattan have set about trying to understand how hormones can change this behavior in mothers.
Researchers had previously observed that prolactin could act on cells in an area of the brain called the ventromedial hypothalamic nucleus, a region known to regulate aggressive behavior in males and females, Dr. Brown says.
“Prolactin is very high during pregnancy and lactation and so we initially thought that prolactin might induce this maternal protective behavior. In fact, what we have actually revealed is a role for this hormone in restraining aggressive behavior so mothers don’t get too distracted by the things happening around them and can concentrate more on caring for their offspring,” she says.
The hormone is produced by the pituitary gland which sits at the bottom of the brain and one of its primary responsibilities is milk production.
“Prolactin can alter many other biological processes but a role in protective behavior was unknown. Although protective behavior 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.
“Prolactin limits investment in unnecessary protective behavior 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,” Dr. Brown says.
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.
“We found that the cells active during protective behavior are able to respond to prolactin and prolactin powerfully acts on these cells to restrict the amount of aggressive and protective behavior shown by mothers.”
Fellow researcher Dr. Teodora Georgescu, of the Department of Anatomy, says the finding is significant.
“These studies are really important in enabling us to understand how behavior and mood is regulated in mothers. 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 behavior in mothers, in the future we can develop new effective ways of helping these women,” Dr. Georgescu says.
Prolactin-mediated restraint of maternal aggression in lactation
Aggressive behavior is rarely observed in virgin female mice but is specifically triggered in lactation where it facilitates protection of offspring. Recent studies demonstrated that the hypothalamic ventromedial nucleus (VMN) plays an important role in facilitating aggressive behavior in both sexes.
Here, we demonstrate a role for the pituitary hormone, prolactin, acting through the prolactin receptor in the VMN to control the intensity of aggressive behavior exclusively during lactation. Prolactin receptor deletion from glutamatergic neurons or specifically from the VMN resulted in hyperaggressive lactating females, with a marked shift from intruder-directed investigative behavior to very high levels of aggressive behavior.
Prolactin-sensitive neurons in the VMN project to a wide range of other hypothalamic and extrahypothalamic regions, including the medial preoptic area, paraventricular nucleus, and bed nucleus of the stria terminalis, all regions known to be part of a complex neuronal network controlling maternal behavior. Within this network, prolactin acts in the VMN to specifically restrain male-directed aggressive behavior in lactating females.
This action in the VMN may complement the role of prolactin in other brain regions, by shifting the balance of maternal behaviors from defense-related activities to more pup-directed behaviors necessary for nurturing offspring.