How is it that a child of a Holocaust survivor can suffer nightmares or flashbacks from experiences he never had, or a parent with pathological fears can pass them along to a child?
Odor is just one of the cues that mothers use to "teach" their offspring about what they need to fear in a dangerous world, researchers say in a paper published Monday in the journal PNAS.
Although such maternal cues are intended to help offspring adapt to their environment, they can also cause problems if the caregiver suffers from post-traumatic stress disorder, the authors say.
"Infants can learn from maternal expression of fear, very early in life," said Dr. Jacek Debiec, lead study author and University of Michigan child psychiatrist.
"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," Debiec said in a press release.
Debiec, and co-author Regina Marie Sullivan, a professor of psychiatry at New York University, say that while their odor hypothesis is difficult to demonstrate in humans, they did succeed in proving it with rats.
The authors said they conditioned non-pregnant female rats to fear the smell of peppermint by administering a mild electric shock to their feet while also exposing them to the peppermint odor.
The rats later gave birth to litters of pups that were used in a variety of experiments along with their mothers, as well as other female rats that did not undergo fear conditioning.
The researchers measured levels of the stress hormone coritsol in the young rats' blood, and also dissected their brains to examine the amygdala, a portion of the brain associated with fear and response to threats.
Under normal conditions, infant rats are calmed by the presence of their mother, and their fear response is suppressed. However, researchers found that when fear-conditioned mothers and their pups were exposed to the peppermint smell, cortisol levels rose in the pups. The young rats also developed an aversion to the odor when their mother wasn't around, essentially moving away from the smell when they encountered it in a maze.
When researchers examined the baby rats' amygdalae, they found evidence of increased activation as well.
To determine whether the young rats were responding to the peppermint smell or an "alarm" odor their mother produced, they were placed in special containers away from their mothers, while the smell of their stressed mother was piped into the container. Researchers said the young rats again responded with high levels of cortisol and amygdala activation.
"Our behavioral results show that the odor of the frightened mother triggers pup stresss response and supports infant social fear learning," the authors wrote.
Although the exact mechanisms through which odor instructs the pups to become fearful have yet to be determined, the authors argued that their findings are likely to apply to humans, and could help to treat psychiatric problems in the future.
"These findings provide a model characterizing how parental adaptive and pathological fear may be transmitted to their offspring, such as in PTSD and specific phobias," the authors wrote.
"Understanding of the neural and molecular mechanisms controlling intergenerational transmission of fear will help to develop better preventive and therapeutic methods," they concluded.