Mice Develop Depression Watching Others Suffer, Study Finds

Purely witnessing a stressful situation can cause depression, a new study found.

Using mice, researchers at the Tokyo University of Science came to this conclusion by simulating what’s known as the social defeat model. The scientists put two mice, one larger than the other, in a cage. Then they forced the smaller mouse to intrude on the bigger mouse’s territory, prompting the larger rodent to aggressively defend its space.

But the humans didn’t really care about the action inside the cage; they wanted to know how a third mouse, watching the exchange from an adjacent compartment, felt about it. They found that the witness developed depressive symptoms, likely because it was empathetic to the plight of the weaker mouse. The findings were published online last month in Behavioural Brain Research.

“I want people to think about how stress can not only change the brains of those with depression, but can change ours—those watching—as well,” Akiyoshi Saitoh, a lead author of the study and a pharmaceutical sciences professor, told VICE World News. 

After 10 days of experiencing second-hand stress, the witnessing mouse became “almost like hikikomori,” Saitoh said, referring to people with acute social withdrawal.

“They weren’t interested in other mice, which is unusual, as mice are sociable creatures,” he said.

The witnessing mouse also exhibited anhedonia, the inability to enjoy pleasure. “If you give mice the choice of drinking sweet or regular water, they will almost always go for the sweet drink because it’s more enjoyable. But the mouse that witnessed social defeat didn’t, which indicated a decreased desire for pleasure,” he said. 

Notably, the mice also developed fewer neurons in their hippocampus, the area of the brain associated with learning and emotion, which the paper’s authors believe is a contributing factor in developing depression. 

Though mouse models are imperfect analogies to humans, highly regarded techniques such as the social defeat model offer insight into people’s mental well-being. 

Brandon Warren, an assistant professor at the University of Florida's department of pharmacodynamics who is not affiliated with the study, said, “This is an important model, because it allows the experimenter to rule out the effects of inflammation, physical pain, etc. when looking at the effects of social defeat, since vicarious social defeated mice watch from the safety of a plexiglass compartment that prevents them from being hurt,” he told VICE World News. 

But, Warren said, the study wasn’t “revolutionary.”  

“We have known for many years (decades) that neurogenesis is decreased after chronic stress, and that antidepressants reverse this effect. We don’t really understand how (or if) this causes depression, but we know it is involved in the disorder,” he said in an email.

Saitoh anticipates that with further research, workplaces and societies can implement better options to assist those struggling with mental health. 

“Mice experiencing second-hand social defeat didn’t make sudden, full recoveries once they were removed from that stress setting, placed back into their normal environment and treated with antidepressants. Alarmingly, some developed their symptoms again, but four weeks later,” he said. 

When an employee is experiencing poor mental health, it’s common for workplaces to offer one to two weeks off, Saitoh said. But this study indicates how ineffective this solution can be, as symptoms can return, or even worsen, he said. 

Saitoh further added that the damage that stress causes to our brains is long-lasting and, if untreated, could continue weeks after being freed from an anxiety-ridden environment. 

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