Science just bolstered the wisdom that it’s never a good idea to bring your stress home with you. A new study suggests that stress changes certain brain structures, and those changes are mirrored in the brains of others. If that's true, the effect would help explain why we seem susceptible to “catching” another’s stress.
This was a mouse study, so the customary cautions about drawing too many conclusions for humans are warranted. But what makes these results interesting for us humans is that the same neuronal structures that were affected in the mice brains are also present in our brains, and a similar emotional contagion effect could be at play.
We know from past research, for example, that humans “spread” a grab-bag of emotions, including anger, fear and happiness (to greater or lesser extents, depending on the research you’re referencing), and there’s at least a strong theoretical framework involving specific brain areas to explain why this happens.
In the latest study, the researchers paired sets of mice together and then removed one mouse from each pair and subjected them to a mild amount of stress. They then returned the stressed mouse to the pair and observed the brains of both mice. The results showed that the stressed mouse experienced changes in a group of neurons located in the hippocampus, a brain area that plays a central role in memory and emotional response. The brain of the other mouse that hadn’t been stressed, but was now in the presence of its stressed partner, rapidly showed the same neuronal changes in its hippocampus. In effect, the brains of the unstressed mice mirrored the brains of the stressed mice.
"The neurons that control the brain's response to stress showed changes in unstressed partners that were identical to those we measured in the stressed mice,” said Toni-Lee Sterley of the Hotchkiss Brain Institute and the study’s first author.
The researchers think the mechanism behind this effect in mice is the release of a "putative alarm pheromone" from the stressed mouse that signals a response in other mice. That's not so surprising, since we know that animals give off a variety of cues, chemical and otherwise, that signal a response in others to avoid danger (think of a bird signaling to the flock to abruptly change direction). What's newer here, and the part of this study that may be revealing for humans, is an observation of changes in brain structure in response to stress that are then mirrored in other brains. How the signals are transmitted between human brains is still an open question.
"Stress circuits in mice and humans are very similar. In particular, the cells we investigated in mice play the exact same role in humans - they control the hormonal response to stress," senior study author Jaideep Bains, PhD told me in an email.
Bains says it's possible that chemical communication is also happening between humans. "Although pheromones or chemical signals are not widely studied in humans, there are recent observations suggesting that they transmit emotional information in subtle, perhaps even subconscious ways."
The good news is that the effect seems reversible—at least for female mice.
The researchers noticed that when the female partners of the stressed mice were placed among other mice, the changes in the hippocampus were reversed. Social interaction erased the brain-altering effects of stress, but not for the male mice. They instead held onto the stress and the brain changes accompanying it regardless of how many other mice they visited.
Again, with due caution about applying these results to humans, the researchers think there are some clues here that could help us develop more effective ways of treating stress.
"This (reversal effect) suggests that there are sex-specific differences that could be useful when thinking about approaches for treating stress disorders," added Bains. "What we can begin to think about is whether other people's experiences or stresses may be changing us in a way that we don't fully understand."
The study was published in the March edition of the journal Nature Neuroscience.
The newly revised and updated 2018 edition of What Makes Your Brain Happy and Why You Should Do the Opposite is now available.