In frightening situations, people tend to freeze, but not recent moms, who charge ahead. Now a new study shows how the brain speedily delivers the hormone oxytocin — which new mothers have in elevated levels, starting with childbirth — to where it's needed, freeing them to protect their young.
The study, done in rats, revealed that oxytocin rushes to the brain region governing fear, called the amygdala, courtesy of special cells that act like a neurological expressway.
Further, when the researchers provoked these cells into sending oxytocin to the amygdala, it diminished the rats' fearful responses to being startled.
The findings "could have implications for autism, anxiety and fear disorders," said study researcher Ron Stoop, a psychiatric neuroscientist at the University of Lausanne in Switzerland. The work may also spur scientists to look more closely at the brain's activity at moments when oxytocin levels are high, such as during childbirth and lactation, Stoop said.
The study is published in the February issue of the journal Neuron.
A hole in the wall
Oxytocin is produced in the hypothalamus, a marble-size region at the bottom of the brain, and released into blood. But the hormone also somehow makes its way into the rest of the brain, including the amygdala — a fact that has long-puzzled scientists, because the blood-brain barrier blocks oxytocin in the blood from moving into the brain.
From a previous experiment, Stoop's team knew that oxytocin in the amygdala causes rats to remain in motion when they are scared, instead of freezing as they normally would.
"The main question was, 'How does it get from hypothalamus to the amygdala?'" Stoop said. One idea was that oxytocin slowly diffused through the intervening brain tissue. But oxytocin affects the amygdala in "like, two seconds," Stoop said — far quicker than the time it would take for diffusion.
Oxytocin had to be reaching its destination another way. To investigate, Stoop's team infected rat hypothalamus cells with a virus that caused the cells to produce a glowing green protein whenever they produced oxytocin.
Afterward, when they dissected the rat brains, they saw "this beautiful network of green fluorescent protein," Stoop said, which included fibers that reached all the way from the hypothalamus to the amygdala. They had found the oxytocin's hole in the wall.
The next step was to see this speedy delivery system in action. The researchers induced the newly discovered fibers to deliver oxytocin to the amygdala, and the moment they did, the frozen-in-fear rats began to move freely, Stoop said. "When we stop… they stop moving." It was a living demonstration of how oxytocin gets to where it needs to go to control fear.
Oxytocin's dampening effect on fear is especially relevant for lactating mothers, who have high oxytocin levels, and can best defend their offspring from a threat when not frozen in terror. Similarly, during childbirth, elevated oxytocin delivery to the amygdala "may be important in reducing anxiety and fear levels," Stoop said.
Fear and the brain
The experiment was an "incredibly elegant approach to neurobiology," said C. Sue Carter, a behavioral neurobiologist at the University of Illinois at Chicago, who was not involved with the study.
The oxytocin-delivery system suggests that the hormone's role in our response to fear "is faster than we recognized," Carter said.
The findings also raise more questions, such as whether this system varies among individuals, Stoop said. It's possible that people have different numbers of oxytocin receptors in the amygdala, which could explain why some people are more anxious than others, he said, though more studies are needed to show that.
Certainly, some mental illnesses have their roots in fear, Carter said. "The literature suggests that individuals that have disorders — like autism, and certain forms of schizophrenia and a number of anxiety disorders — are all experiencing a sense of fear or threat, even when there's nothing there."
The oxytocin-delivery system, or a failure of this system to perform as it should, may be involved in these illnesses, Carter said.
Pass it on: Oxytocin, the molecule that promotes bonding between mother and child, is also essential to the brain's ability to control fear.