The epigenetics of fear

I learned something new today about fearfulness, which it turns out has an even more complicated set of causes than I had previously known. And I had previously thought that fearfulness (made up of a whole lot of little genetic causes as well as almost impossible to fully comprehend environmental causes) was pretty damn complicated. The findings I’m going to describe are in mice, but this stuff is totally relevant to fearful dogs, at least in the opinion of this dog zombie.

My story begins earlier today when I received email from an ex asking if a recently published study is too crazy to be for real. (I do actually enjoy being the translator of Nature Neuroscience articles for the ex-boyfriends of the world.) My ex had encountered a National Geographic Phenomena article which covers the Nature Neuroscience article “Parental olfactory experience influences behavior and neural structure in subsequent generations." That is quite a title — let’s try it again. “When mice are trained to fear a particular smell, the brains and behavior of their offspring are affected.” (The Phenomena article, by the way, is detailed and provides some nice snippets of interviews with the researchers who did this study, but misses some of the nuances of the experimental setup. So while I do recommend you read it if you’re interested in this study, you should probably take it with a grain of salt.)

Dias B.G. & Ressler K.J. (2013). Parental olfactory experience influences behavior and neural structure in subsequent generations, Nature Neuroscience, DOI: 10.1038/nn.3594

These researchers took a group of mice and fear conditioned them to the smell of a chemical called acetophenone. Then they bred them and tested their offspring. The offspring could detect acetophenone at lower concentrations than other mice; they had more receptors in their nose for detecting acetophenone than other mice did; and they were more reactive to loud noises after having been exposed to acetophenone. The smell was inherently scary to these offspring mice, even though they had not previously encountered it.

For the record, I am totally down with the first few changes. Offspring are adapted to the parent's environment by being better at smelling a relevant smell? Freaky as hell, but that is what epigenetics is and why we all find it so fascinating. But a change in behavior? That is beyond the usual freakiness of epigenetics. That's not just passing along more scent receptors for a particular smell. That's passing along the emotional content of the parent's experience with the smell. How is it possible!

Well, first, some details about the experiment:

How severe was the fear conditioning? Not all that severe, it turns out (which makes the results even more surprising to me). Mice were only trained over three days, with only five trials each day. A trial consisted of exposure to the odor, followed by a “mild” foot shock. I don't have a feel for how traumatic this experience was for the mice, and I'd be curious to know more. Was the shock really “mild”? You know, according to the mice, not according to the researchers, because we have all seen instances in which the animal's perception differs from the human's. Was being in the training chamber itself somewhat traumatic? Maybe the animals hadn’t been out of their home cages before. And so forth. But it was certainly a short period of training.

To test the startle response, the researchers put offspring mice into a startle chamber. The mice were habituated to the chamber for a few days before testing began. Then a few startle trials were run, in which the mice were exposed to sudden loud noises, and their responses were recorded. Next the mice were exposed to acetophenone, and then some more startling noises. The difference in their response was what was important: how much more did they startle after having been exposed to the smell, as compared to before exposure to it? Note that the mice were not actually startling just after exposure to the smell; there was a loud noise which triggered the startle. But they seem to have been primed by their reaction to acetophenone to startle more at subsequent noises.

Now, there are a zillion different possible explanations for why these mice could have appeared to be afraid of a smell that they had never encountered before. "Because my dad was afraid of it” is not the first thing that comes to mind, and the researchers tested a whole lot of other possibilities.

Were these mice particularly reactive to all smells? The researchers actually tested two groups of mice on two different smells. The group which was descended from mice trained on smell A reacted to smell A and not smell B. And vice versa for the other group. It really was just that particular smell.

Were these mice more anxious over all, possibly due to their father’s experience, having nothing to do with acetophenone? We might already have rejected this idea as the mice only reacted to the relevant smell, not the control smell. But the researchers also performed a test to see if the mice were particularly anxious over all, by examining the mice's fear of open spaces. The mice were no more afraid of open spaces than average, suggesting that they were not particularly anxious individuals in general.

Was there some social influence passed down from the fathers? The researchers had begun by fear conditioning male mice, who never had direct contact with their offspring, but did have direct contact with the mothers. It was possible that the fathers had somehow taught the mothers to fear the smell of acetophenone, and the mothers had passed this down to the offspring. To control for this, the researchers artificially inseminated some mice so that the females never interacted with the males, and had the offspring raised at an entirely different lab. They also fear conditioned mothers, and then fostered the offspring to mothers who had not been fear conditioned (and fostered offspring from normal mothers onto fear conditioned mothers). None of this changed the findings: the phenomenon appeared to be genetic, not social.

The researchers had chosen this particular smell because they knew what gene controlled the receptor for it. So they looked at the mice’s brains, and indeed the offspring of fear-conditioned mice did have more of the receptors for the relevant smell, which is why the mice were able to detect it at lower concentrations, even though they had never been exposed to the smell previously. Looking at the DNA for the gene controlling this receptor, they found epigenetic changes, specifically less methylation — basically, less stuff on the DNA, making it easier to express genes from. This is a plausible explanation for how the receptor changes happened.

Which means the story goes like this: mouse is trained to fear a smell; there are changes to the mouse’s DNA, marking a particular gene as one that should be expressed more often; these epigenetic changes are passed on to the mouse's offspring; that offspring generates more of a particular kind of smell receptor, because that gene is marked as “important, make lots!” And I am okay with that, as far as it goes. But how do we get from “make lots of receptors for this smell” to “when you smell this smell, be prepared for Bad Things to Happen”?

There is some precedent for this sort of thing, although it's limited. Primates are known to be primed to recognize snakes, although it's less clear if we are primed to fear them. Mice fed acetophenone while pregnant produced offspring who preferred the smell. Neither of these phenomena are epigenetic, which makes them inherently less freaky. It's particularly interesting to me that mice will “prefer” acetophenone if their mothers have eaten it: another case of inherited emotional content or salience, although in this case due to the in utero environement, not to epigenetics.

But in the end we don't really know how the salience of the smell was transmitted. More receptors for the smell don’t cause salience: just because you can smell it better doesn’t mean you’ll like or fear it. The researchers don't try to make a guess at how this happens, but they do comment on its importance for future research: “Such a phenomenon may contribute to the etiology and potential intergenerational transmission of risk for neuropsychiatric disorders, such as phobias, anxiety and post-traumatic stress disorder.” And dogs! Mice are actually probably a better model for dogs than for humans in this case, because dogs are so much more scent-oriented than we are.

So what does this mean for fearful dogs? We all want to know what makes a fearful dog fearful. How much of it is environment (poor socialization) versus genetics (starting life having been dealt a bad hand)? Well, first of all, remember that this was a very simple stimulus — a very specific smell and very straightforward classical conditioning. That’s why the researchers chose it. Could fear of the mailman be passed on as well? It would be harder, since there is not a single receptor to recognize the mailman, controlled by a single gene which can be expressed more frequently. (I love the idea of a mailman receptor, though.) So I wouldn't extrapolate these findings to non-scent stimuli quite yet. But that doesn't mean that this weird epigenetic force is not out there, interacting with the other poorly-understood forces of environment and genetics, in a crazy storm of things we can't separate out.