Kate Middleton is raging right now -- with oxytocin, the powerful hormone that acts a neurotransmitter in the brain and plays a big role in the warm and fuzzy feelings that promote mother-baby bonding.
As the duchess departed St. Mary's Hospital in London on Tuesday, 24 hours after giving birth, she said the experience had been "very emotional," adding that any parent would "know what this feeling feels like."
"That's a classic response," said Erica Lyon, creator of the video series Birth360 and author of the "Big Book of Birth." "Most women experience crying at cat food commercials, looking at their husbands and bursting into tears: 'You are the best dad in the world.'"
"It's very emotional," said Lyon, a 43-year-old mother of three. "You fall crazy in love in a way that blows everything else out of the water."
Often called the "love" hormone, oxytocin is released during childbirth, breast-feeding and sex. Whenever humans kiss or hug a loved one, oxytocin levels soar. Prairie voles produce oxytocin at some of the highest levels in nature and they are one of the monogamous species.
But oxytocin has a darker side, according to a study at Northwestern University Feinberg School of Medicine. The hormone can cause emotional pain, strengthening bad memories and increasing fear and anxiety.
While oxytocin can enhance a feeling of well-being under positive circumstances, it can exacerbate those perceptions in a negative and stressful environment.
Dr. Jelena Radulovic, who studies molecular and behavioral science at Northwestern, set out to study the positive social effects of oxytocin in two experiments with mice. But she said she soon discovered the results "ran the complete opposite of our hypothesis. ... It was a surprise and it proved more effective than the original one."
In the first study to link oxytocin to social stress, researchers have discovered the part of the brain region -- the lateral septum -- responsible for these effects and the route it takes to amplify fear.
"After positive social interaction, [oxytocin] is beneficial. It improves special memory and positive interaction," Radulovic said. But in a social context of stress, "it can exacerbate conditions."
Her research underscores the need for a "supportive environment" for women during child birth and lactation, when high oxytocin levels are high.
"My advice is to try to minimize social stress," she said.
Oxytocin, made in the brain's posterior pituitary, is both a neurotransmitter that has psychological effects, and a hormone that travels to other organs in the body, the uterus and breast tissue.
It is thought not only to trigger the flow of milk, but to enhance maternal attachment. Orgasm is also thought to be associated with oxytocin, creating similar feelings of attachment.
"It's long been known than oxytocin has a very important role in motherhood," said Dr. Suena Massey, assistant professor of psychiatry and behavioral sciences at Northwestern, who was not involved in the study. "Late in gestation during pregnancy, oxytocin is secreted from the brain and causes uterine contractions that lead to labor. And after birth, in response to suckling, the nipple and the breast tissue release milk.
"It is well studied -- the more attached a mother is to the child the more healthy the child will develop," she said.
Social memory also increases with oxytocin.
"Women often feel many different emotions during pregnancy and postpartum," Massey said. "Often, they relive emotions from their own childhood and are flooded by memories of mothering by their own mothers."
When those memories are not positive, oxytocin might enhance the negative ones, suggests Massey, who just received a grant to study the association between the hormone and post-partum depression.
The Northwestern study was done on male mice because they respond physically to social stress.
"Female mice don't fight," Radulovic said.
The first experiment was on the social effects of oxytocin on memory. Scientists placed three groups of mice individually in cages with aggressive mice so they would experiences "social defeat," which is stressful.
One group had no oxytocin receptors, which means the hormone could not reach their brain cells. The second group had an increased number of receptors, so theirs were flooded with oxytocin. The third was a control group with a normal number of receptors.
Six hours later, the mice were returned to cages with the aggressive mice. Those missing oxytocin receptors didn't seem to remember the mice and showed no fear. Those with the increased receptors reacted with intense fear and avoided the aggressive mice.
In the second experiment on fear and anxiety in a new stressful situation, again three groups of mice were exposed to more aggressive mice. This time, six hours later, they were put in a box and received a brief electrical shock, which startles them, but is not painful. Then, 24 hours later, the mice were returned to the same box, but with no shock.
Mice missing receptors showed no enhanced fear when they returned to the box where they had received the shock. Those with extra receptors showed much greater fear; the control group had an average fear response.
Radulovic said that the dual effect might play a biological role so that mothers can react in "fight or flight" situations and avoid dangerous situations to protect the baby. But today, when most stress is chronic in nature, the negative effects of oxytocin can be more menacing, she said.
"The memories can be long lasting, particularly nowadays when social stressors are the main cause of mental morbidity," she said. "What we are trying to do is to prove how this system operates under chronic conditions."