JUANA SUMMERS, HOST:
There is new evidence that pregnancy really does change the brain. NPR's Jon Hamilton reports on a study in mice showing that hormones produced during pregnancy alter the brain circuits involved in parenting.
JON HAMILTON, BYLINE: Female mice are not born mothers. Jonny Kohl of the Francis Crick Institute in London says until they mate, they really don't care much about babies.
JONNY KOHL: Sexually inexperienced female mice typically either ignore pups or show very low levels of spontaneous parental behavior.
HAMILTON: They don't groom the pups or retrieve them when they leave the nest. In the wild, the virgin female may even kill a baby. Kohl says pregnancy changes all that.
KOHL: Mothers are intensely parental. They spent most of their waking hours taking care of young.
HAMILTON: Kohl says this behavior is controlled by brain networks known as parenting circuits.
KOHL: Parenting circuits are dedicated networks of neurons in the brain, the purpose of which is to ensure taking optimal care of the young.
HAMILTON: Kohl thought these circuits might be switched on by hormones produced during pregnancy, so he and a team focused on special brain cells called galanin neurons, which are known to affect mating and parenting. The team found that pregnancy hormones changed galanin neurons in two ways. Kohl says one made them more responsive to the odors and sounds of baby mice.
KOHL: The salience, the relevance of the pup is more obvious to the animals. So at the level of these neurons, for instance, it's easier to decode the identity, the presence of a pup.
HAMILTON: The hormones also caused galanin neurons to form new connections, apparently streamlining the parenting circuits. Rachida Ammari, a member of Kohl's lab, says the changes depended on the presence of two types of hormone receptors on galanin neurons.
RACHIDA AMMARI: The two different hormonal receptors have different role in increasing parenting behaviors.
HAMILTON: One receptor responds to estradiol, a form of estrogen that prepares the uterus for a fetus. The other receptor responds to progesterone, a sex hormone that helps maintain a pregnancy. Ammari says experiments showed that both receptors were needed to change a mouse's behavior.
AMMARI: So when we delete those receptor, well, the programming to become a mother is completely abolished.
HAMILTON: Mice who gave birth didn't even try to nurse their pups. Meanwhile, other experiments have shown that artificially activating these hormone receptors causes virgin mice to act like mothers. The study appears in the journal Science. And Margaret McCarthy of the University of Maryland says it may help explain how pregnancy affects the brains of other species, like humans.
MARGARET MCCARTHY: Every woman who's been pregnant will tell you her brain was never the same.
HAMILTON: But McCarthy says the human brain is vastly more complicated than a mouse brain. Also, unlike mice, she says, humans spend years observing parenting behavior before becoming a parent is even possible.
MCCARTHY: Humans are so impacted by experience, and so we can never sort of isolate out what that impact has on human mothering versus just purely hormonal.
HAMILTON: Even so, McCarthy says, it's important to recognize that hormones play a part. For example, she says, they could be one reason that some mothers develop postpartum depression or fail to bond with their child.
MCCARTHY: When mothering fails in humans, it is so harshly judged, and it's like you're a failure as a woman. But if there is this hormonal biological contribution to mothering, it can definitely go wrong.
HAMILTON: McCarthy says it's possible that hormones also affect the brains of human fathers. One clue, she says, is research showing that becoming a father leads to a sharp decrease in testosterone levels. Jon Hamilton, NPR News.
(SOUNDBITE OF DUSTIN TEBBUTT'S "IN FADING LIGHT") Transcript provided by NPR, Copyright NPR.
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