Humans, Rodents Pay Close Attention to Fear, Anxiety Expressed by Parents
Abstract
The neurobiology of intergenerational transmission of emotional trauma suggests a social, and not genetic, mechanism.
“Fear is a social phenomenon, especially among humans, and social context affects fear acquisition,” Jacek Debiec, M.D., Ph.D., a professor of psychiatry at the University of Michigan, told attendees of the 10th Annual Amygdala, Stress, and PTSD Conference at the Uniformed Services University in Bethesda, Md., in April.
Rats and humans both use social referencing to learn fear and safety, basing their response on that of a trusted caretaker, says Jacek Debiec, M.D., Ph.D., a professor of psychiatry at the University of Michigan.
Debiec, who treats children and mothers with anxiety and other conditions and studies the neurobiological mechanisms of emotional learning in rodents, described how his research into fear transmission in rodents suggests that infants pick up on their mother’s fear response early in life.
Similar findings have been found in humans, Debiec noted, pointing to the recent study of human twins in Sweden that suggested that children and adolescents learn anxious behavior from their parents .
The interest in intergenerational transmission of fear and anxiety began many decades ago with Holocaust studies, said Debiec.
“Psychiatrists recognized that the children of Holocaust survivors exhibited symptoms of what is now called PTSD,” he said. “The children did not meet all the criteria, but there was something dysfunctional going on.”
While there are many possible mechanisms for such transmission of fear, Debiec’s recent experiments using smell to look at fear conditioning in rats highlight the role of the social transmission of fear from a mother to her pups.
Like many species (including humans), rats regulate their behavior by observing their caregivers’ emotional expression during novel situations. In a study published August 19, 2014, in the Proceedings of the National Academy of Sciences (PNAS), Debiec and colleagues described how they taught female rats to fear the smell of peppermint—an odor that is typically neutral for rats—by linking it with an electric foot shock before the animals became pregnant. After the animals gave birth, the researchers once again exposed the mothers to peppermint, but without the shocks, in order to provoke the fear response.
If the mother was not re-exposed to the peppermint or had not been conditioned to fear the odor, infant rats did not exhibit avoidance to the peppermint odor. However, when the pups were exposed to the freezing reactions of their fear-conditioned mothers to the peppermint scent, the pups also learned to avoid the smell. The fear-conditioned mothers were later less nurturing to the pups than control animals as well.
The pups did not even need to be around their mother when she smelled the peppermint odor—simply catching a whiff of the scent of their mother reacting to the odor was enough to make them fear the same thing. According to the researchers, the pups’ fear response continued through adolescence.
Further investigation showed that both the olfactory bulb and the amygdala play a role in this process. Inactivating the amygdala with the GABA-A receptor agonist muscimol prevented the transmission of fear from the mother to the pup; noradrinergic blockade with propanolol did the same.
Similar to Debiec’s findings of the social transmission of fear from mothers to pups, a twin study published April 23 in AJP in Advance suggests that children model the anxious behavior of their parents.
“The association between parental and offspring anxiety largely arises because of a direct association between parents and their children independent of genetic confounds,” concluded Thalia Eley, Ph.D., a professor of developmental behavioral genetics in the Institute of Psychiatry at King’s College London, and colleagues.
During his talk, Debiec described how powerful social fear learning is, how early on in life it occurs, and how long it lasts.
“Babies learn from parents about threats before they can even fully express fear to these threats,” he said. “The young organism has to rely on emotion conveyed by caretakers when young to learn about what is safe and not safe about the world, but in situations where the caretaker is traumatized, the perspective is distorted, the response is not adaptive.” ■
