Anxious mother rats give off an odor that teaches their newborn babies to be afraid.
Researchers studied mother rats who had learned to fear the smell of peppermint and saw them teach this fear to their babies in their first days of life by using an alarm odor that is released during distress.
The scientists pinpointed the specific area of the brain where this fear transmission takes root in the earliest days of life. Their findings in animals may help explain a phenomenon that has puzzled mental health experts for generations: how a mother’s traumatic experience can affect her children in profound ways, even when an event happened long before the children were born.
The researchers say they hope their work will lead to a better understanding of why all children of traumatized mothers, or of mothers with major phobias, other anxiety disorders, or major depression, don’t experience the same effects.
Mothers’ memories
“During the early days of an infant rat’s life, they are immune to learning information about environmental dangers. But if their mother is the source of threat information, we have shown they can learn from her and produce lasting memories,” says Jacek Debiec, assistant professor of child and adolescent psychiatry.
“Our research demonstrates that infants can learn from maternal expression of fear, very early in life. Before they can even make their own experiences, they basically acquire their mothers’ experiences. Most importantly, these maternally-transmitted memories are long-lived, whereas other types of infant learning, if not repeated, rapidly perish.”
Research with rats allows scientists to see what’s going on inside the brain during fear transmission, in ways they could never do in humans, says Debiec, who began his research during a fellowship at New York University with Regina Marie Sullivan, senior author of the new paper that is published in the Proceedings of the National Academy of Sciences.
Peppermint fears
Researchers taught female rats to fear the smell of peppermint by exposing them to mild, unpleasant electric shocks while they smelled the scent, before they were pregnant. Then after they gave birth, the team exposed the mothers to just the minty smell, without the shocks, to provoke the fear response. They also used a comparison group of female rats that didn’t fear peppermint.
They exposed the pups of both groups of mothers to the peppermint smell, under many different conditions with and without their mothers present.
Using special brain imaging, and studies of genetic activity in individual brain cells and cortisol in the blood, they zeroed in on a brain structure called the lateral amygdala as the key location for learning fears. During later life, this area is key to detecting and planning response to threats—so it makes sense that it would also be the hub for learning new fears.
But the fact that these fears could be learned in a way that lasted, during a time when the baby rat’s ability to learn any fears directly was naturally suppressed, is what makes the new findings so interesting, says Debiec.
Mother’s scent
The newborns could learn their mothers’ fears even when the mothers weren’t present. Just the piped-in scent of their mother reacting to the peppermint odor she feared was enough to make them fear the same thing.
And when the researchers gave the baby rats a substance that blocked activity in the amygdala, they failed to learn the fear of peppermint smell from their mothers. This suggests that there may be ways to intervene to prevent children from learning irrational or harmful fear responses from their mothers, or reduce their impact.
The new research builds on what scientists have learned over time about the fear circuitry in the brain, and what can go wrong with it. That work has helped psychiatrists develop new treatments for human patients with phobias and other anxiety disorders—for instance, exposure therapy that helps them overcome fears by gradually confronting the thing or experience that causes their fear.
In much the same way, Debiec hopes that exploring the roots of fear in infancy, and how maternal trauma can affect subsequent generations, could help human patients. While it’s too soon to know if the same odor-based effect happens between human mothers and babies, the role of a mother’s scent in calming human babies has been shown.
Debiec, who hails from Poland, recalls working with the grown children of Holocaust survivors, who experienced nightmares, avoidance instincts, and even flashbacks related to traumatic experiences they never had themselves. While they would have learned about the Holocaust from their parents, this deeply ingrained fear suggests something more at work, he says.
Going forward, he hopes to observe human infants and their mothers, and also work with military families.
The National Institutes of Health and the Brain and Behavior Research Foundation supported the research.
Source: University of Michigan
