By INNA GAISLER-SALOMON
WE intuitively understand, and scientific studies confirm, that if a woman experiences stress during her pregnancy, it can affect the health of her baby. But what about stress that a woman experiences before getting pregnant — perhaps long before?
It may seem unlikely that the effects of such stress could be directly transmitted to the child. After all, stress experienced before pregnancy is not part of a mother’s DNA, nor does it overlap with the nine months of fetal development.
Nonetheless, it is undeniable that stress experienced during a person’s lifetime is often correlated with stress-related problems in that person’s offspring — and even in the offspring’s offspring. Perhaps the best-studied example is that of the children and grandchildren of Holocaust survivors. Research shows that survivors’ children have greater-than-average chances of having stress-related psychiatric illnesses like post-traumatic stress disorder, even without being exposed to higher levels of stress in their own lives.
Similar correlations are found in other populations. Studies suggest that genocides in Rwanda, Nigeria, Cambodia, Armenia and the former Yugoslavia have brought about distinct psychopathological symptoms in the offspring of survivors.
What explains this pattern? Does trauma lead to suboptimal parenting, which leads to abnormal behavior in children, which later affects their own parenting style? Or can you biologically inherit the effects of your parents’ stress, after all?
It may be the latter. In a study that I, together with my colleagues Hiba Zaidan and Micah Leshem, recently published in the journal Biological Psychiatry, we found that a relatively mild form of stress in female rats, before pregnancy, affected their offspring in a way that appeared to be unrelated to parental care.
When rats or mice are put under duress, particularly during early development, their second and third generation offspring exhibit behavioral irregularities. Studies have shown that stress to male mice — which are minimally involved in their offspring’s care — also affects their offspring’s behavior, suggesting that “parenting style” is not the issue.
For our study, we looked at adolescent female rats that went through a mild stress procedure for seven days. Two weeks later, these rats and their nonstressed controls were mated with nonstressed male rats. Our study confirmed that there were indeed behavioral differences between the adult progeny of stressed and nonstressed females, particularly in tests of emotional and exploratory behavior.
How was this stress effect transmitted? We focused on a gene that encodes a molecule involved in the body’s response to stress. Unsurprisingly, we detected larger amounts of the molecular product of this gene in the brains of the previously stressed female rats than in the nonstressed controls. But we also found that this molecular product was present in the ova, or eggs, of the stressed females in significantly increased concentrations.
Traditionally, scientists have understood that the ova transfer only genetic information (that is, DNA) from parent to child. But our hunch was that the ova were also transferring “soft-wired” information — specifically, information about the stress experience — by way of the molecule coded by the gene. And indeed, we found that at birth, the offspring of stressed female rats already had more of this same molecular product in their brains than did the offspring of controls. This suggested that they may have “inherited” the effect of their mothers’ stress.
An additional and unexpected finding of our research was that the expression of the gene in adult offspring did not depend on maternal stress alone, but was also influenced by the stress the offspring themselves experienced. It also differed between males and females.
Our findings suggest that some stress-related changes in newborn rats are unrelated to maternal care. The emerging field of epigenetics, which attempts to understand how information not coded by the DNA sequence is inherited, may be able to explain exactly how this happens.
Many psychiatric illnesses, including post-traumatic stress disorder and schizophrenia, are related to stress. Hopefully, a better understanding of the transgenerational effects of stress will improve the prediction, diagnosis and treatment of human illnesses.