How Traumatic Experiences Get Inherited

Bianca Jones Marlin never wants to let a captivating idea pass her by — which is why she keeps waterproof notepads in her shower at home. It’s also why, after earning dual bachelor’s degrees in biology and adolescent education in 2009, she gave up a secure job as a high-school science teacher to pursue a Ph.D. in neuroscience at New York University. “I loved teaching — my students gave me such energy,” she says. “But I wanted to know more.”

Now a postdoctoral researcher in the department of neuroscience at Columbia University, Marlin investigates how the traumatic experiences of parents can get passed down to multiple generations of offspring through epigenetic pathways. An edited version of my interview with her follows.

What inspires your interest in the science of mothering and parenthood?

There were two components. One was my experience as a teacher: I was always intrigued as to how the different backgrounds of students — their home life, their siblings — dictated their performance in my classes. The other was my own home life: My biological parents were also foster parents, so we had siblings who had lived through really traumatizing experiences, experiences that I heard about first hand when we would pretend to be sleeping, but talk for hours. Looking back, I’m so thankful that my foster siblings shared their lives with me, it changed me to my core.

I think those two experiences together made me really interested in how prior experiences in the environment dictate behavior, how we learn and how we operate throughout life. I took a real keen interest in wanting to figure out how to ameliorate some of the stressors that my siblings had to go through. And the best way for me to do that was through science.

Oxytocin is often simplistically described as “the love hormone.” What made you want to investigate its role in maternal behavior?

The reason we looked at oxytocin to begin with is because we know that oxytocin is released during uterine contractions and when the mice feed their young. So that seemed like a good neuromodulator to target. We also know that “virgin” and mother mice behave very differently toward baby mice. When a mouse pup leaves its nest, it’ll start to cry, which alerts the mom, who comes to pick the pup up because it can’t thermoregulate itself, and also because these vocalizations can alert predators. Whereas a virgin, upon hearing a pup’s ultrasonic vocalization, will either ignore it, run away to the opposite side of the cage or sometimes even cannibalize the pup. Behaviors from the mother and virgin can stop the sound, but which is better for your genetic lineage?

What did you discover?

We looked for oxytocin receptors in the brain, and we saw that on the left side of the auditory cortex, there were more oxytocin receptors than on the right. When I added oxytocin to the left hearing center of a virgin mouse’s brain, we saw that the neurons that usually fire aberrantly in response to pup calls became time-locked: Every time a pup cried, there’d be a spike. It also changed the output behavior in virgin mice: Instead of cannibalizing the pup, they’d act like a mother and pick them up. That was just mind-blowingly exciting.

Your current research focuses on the epigenetics of trauma. Does that mean bad experiences can change our DNA?

“Epi-” means “around.” So when we talk about epigenetics, we’re not talking about changing DNA. But in order for the genes in DNA to be expressed, those genes need to be able to be read. Sometimes there are things that happen in response to experience that physically block a part of the genome so it can’t be read, which means the proteins encoded by that gene can’t be made. Moreover, traits associated with them are not going forward into the future — even though the DNA hasn’t been changed.

This phenomenon has been documented in humans. In 1944–45, during World War II, the Netherlands were cut off from outside food supplies, and a lot of people suffered from extreme starvation for nine months. Because the Dutch took such good notes on their population, they saw that children who were in utero during the famine and those children’s children also suffered from extreme metabolic problems and were more likely to suffer from hypertension and diabetes. They also found that children of males who were undernourished in utero during the famine grew up to be more obese than offspring of males and females who were not undernourished in the womb. What the Dutch concluded was that these effects appeared in the descendants because their bodies were still primed to be in a place where there is no food.

So one generation’s physical reaction to starvation was passed down to their descendants, even though those descendants never experienced those stresses?

Yes. The descendants didn’t get these problems because they were overeating. They were on a normal diet. But for some reason their bodies still “remembered” that trauma, that starvation, that their parents or grandparents experienced.

How can we study this effect in the lab?

Behavioral scientists know that you can cause a mouse to fear a smell by pairing that odor with a mild shock. Scientists have shown that an odor paired with an unpleasant experience changes the number of cells that sense that odor in the nose of the mouse. Moreover, it seems that the offspring of those mice are born with more cells that sense the “scary” odor without ever experiencing the odor or shock for themselves. This is intriguing.  We are exploring how these changes could occur, absent the stimulus and the shock. Where is that biological memory stored? How is it passed down?

What are you doing now to explore how these biological memories get passed down?

I pair a mouse with a specific odor of a flower. The mouse is transgenic, which means we’ve changed its genes to provide markers that help us keep track of them. In this case, the receptor cells in its nose that respond to the flower odor have been turned green, so we can easily examine them. When the animal smells the flower, it’s given an electric shock. What we want to know is: If the number of green cells in the mouse’s children and grandchildren change in response to this trauma, then how?

If there were more of those green cells, it could mean one of two things. The next time you come across a flower, having more of these cells may allow you to learn more quickly. Or you could sense it at lower quantities, becoming more sensitive to it and then avoid it. I think that difference is very important to parse out. Either you’re becoming hyperintelligent about a smell because of something your parent or grandparent experienced, or you’re developing PTSD related to something you never experienced yourself.

What would these findings in mice imply for humans?

If we know what biological factors could make mice be unable to care for pups, then maybe we can find out what similar things may be going awry in mothers that can’t find it in them to take care of their children. We also know that trauma that happens in young children is more likely to be replicated in their children. So I’m interested in learning if we can stop these cycles on a biological level — in no way blaming mothers and parents.

Will the findings lead to a medical treatment for emotional trauma?

We all want our basic research to matter to the public, but I’m not going to discover something and then immediately have it in the hospital. That’s not the way wet-bench biological science works. And I’m OK with that, because everything therapeutic that we do have that’s been implemented to help people had to start somewhere. And I’m OK with being that starting point.