People who experience post-traumatic stress disorder have unthinkable pain and stress. But they're not the only ones. Their families and friends may also be affected even though they have no first-hand history of the trauma.

Researchers may have figured out why stress like this is contagious.

Scientists at the Hotchkiss Brain Institute at the University of Calgary Cumming School of Medicine studied the impact of stress on male and female pairs of mice. They removed one mouse from each pair and exposed it to mild stress, then returned it to its partner. They found that certain brain cells in both the stressed mouse and its partner were changed in the same way.

"There has been other literature that shows stress can be transferred — and our study is actually showing the brain is changed by that transferred stress," Toni-Lee Sterley, a postdoctoral fellow and the study’s lead author, said in a statement. "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."

The research team found that when those neurons in the brains are activated, they cause the release of a chemical that functions as an "alarm pheromone." The partner mouse detects the signal, is alerted to the stress, and can go on to alert other members of their group.

"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," said Jaideep Bains, Ph.D., principal investigator and professor in the department of physiology and pharmacology. "The study also demonstrates that traits we think of as uniquely human are evolutionary conserved biological traits."

The study was published in the journal Nature Neuroscience.

Interestingly, the researchers found that the effects of stress on the brain were reversed in some of the mice after calmer social interactions. In female mice, the effects of stress on neurons were cut nearly in half after they spent time with partners that were untrue. Males didn't have the same response.

"If some of the effects of stress are erased through social interactions, but this benefit is limited to females, this may provide insights into how we design personalized approaches for the treatment of stress disorders in people," says Bains.

Mary Jo DiLonardo covers a wide range of topics focused on nature, health, science and anything that helps make the world a better place.

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