Can Fears Be Inherited?

Dana Smith gets excited about a study suggesting that fear memories may be heritable through little-understood epigenetic processes:

In the study, mice were trained to be afraid of acetophenone, a fruity smell that’s used in cherry, jasmine, honey-suckle and almond flavorings. The researchers paired this fragrance with a foot shock so that it soon became a warning signal to the mice, instilling fear and alerting them to an impending attack. The mice’s noses and brains also adapted accordingly, generating additional M71 neurons—cells receptive to this particular scent—so that they would be extra sensitive to the smell. So far, this is all basic Pavlovian conditioning and neural adaptation, nothing special yet.

However, the crazy part is that the offspring of these mice, who had never before been exposed to that smell, also showed increased fear and startle responses to the scent. This suggests that the learned association, connecting the smell with danger, was passed down from one generation to the next. And this second group’s offspring also showed heightened sensitivity to the odor. Thus, three generations of mice were affected by the conditioning, even though only one of them had actually experienced it. Behind these behavioral effects were similar changes in the noses of each of these offspring groups, with larger M71 receptors present and an increase in the number of M71 neurons available.

Joe Hanson rains on her parade:

It’s possible that there was an epigenetic change passed down. But it’s not for sure. Beyond that, the way that statistics are applied to mouse behavior studies make it possible that the differences they see are just due to sample sizes, or not including certain controls, or some other random factor like that the humidity on a particular day happened to make the mice very jumpy. There’s also the fact that there is no known way for nerve cell changes or chemical responses within the olfactory bulb to be communicated to the testes, where sperm are made (there’s literally a blood-testis barrier to prevent that kind of thing). … “More work needed” as they say!

Virginia Hughes provided a more in-depth look at the study earlier this month:

There are still some unanswered questions, [MIT research fellow Tomás] Ryan notes. For example, the researchers didn’t do a control experiment where the F0 animals are exposed to the fruity odor without the shock. So it’s unclear whether the “memory” they’re transmitting to their offspring is a fear memory, per se, or rather an increased sensitivity to an odor. This is an important distinction, because the brain uses many brain circuits outside of the olfactory bulb to encode fear memories. It’s difficult to imagine how that kind of complicated brain imprint might get passed down to the next generation. Ressler and Dias agree, and for that reason were careful not to refer to the transmitted information as a fear memory. “I don’t know if it’s a memory,” Dias says. “It’s a sensitivity, for now.”