During pregnancy, it’s not women’s bodies alone that undergo change. Turns out the brain also gets ‘rewired’ to prepare for motherhood, finds a study in mice.
The findings, published in the journal Science, show that both oestrogen and progesterone act on a small population of neurons in the brain to switch on parental behaviour even before offspring arrive. These adaptations resulted in stronger and more selective responses to pups.
It is well known that while virgin female rodents do not show much interaction with pups, mothers spend most of their time looking after young. It was thought that hormones released when giving birth are most crucial for this onset of maternal behaviour.
But earlier research also showed that rats who have given birth by Caesarean section, and virgin mice exposed to pregnancy hormones, still display this maternal behaviour, suggesting that hormone changes already during pregnancy may be more important.
In the current study, the researchers from the Francis Crick Institute found that female mice indeed showed increased parental behaviour during late pregnancy, and that exposure to pups wasn’t necessary for this change in behaviour.
They found that a population of nerve cells (galanin-expressing neurons) in an area of the brain called the medial preoptic area (MPOA) in the hypothalamus, associated with parenting, was impacted by oestrogen and progesterone.
"We know that the female body changes during pregnancy to prepare for bringing up young. One example is the production of milk, which starts long before giving birth. Our research shows that such preparations are taking place in the brain, too,” said Jonny Kohl, from the Crick.
“We think that these changes, often referred to as ‘baby brain’, cause a change in priority -- virgin mice focus on mating, so don’t need to respond to other females’ pups, whereas mothers need to perform robust parental behaviour to ensure pup survival. What’s fascinating is that this switch doesn’t happen at birth -- the brain is preparing much earlier for this big life change,” Kohl added.
In the study, brain recordings showed that oestrogen simultaneously reduced the baseline activity of these neurons and made them more excitable, whereas progesterone rewired their inputs, by recruiting more synapses (sites of communication between neurons).
Making these neurons insensitive to hormones completely removed the onset of parental behaviour during pregnancy. Mice failed to show parental behaviour even after giving birth, suggesting there is a critical period during pregnancy when these hormones take effect.
While some of these changes lasted for at least a month after giving birth, others seem to be permanent, suggesting pregnancy can lead to long-term rewiring of the female brain.
The researchers believe the brain may also be rewired in a similar way during pregnancy in humans, as the same hormonal changes are expected to impact the same areas of the brain. This could influence parental behaviour alongside environmental and social cues.