New research from Japan suggests that mice are empathetic and can become depressed by watching other rodents being bullied.
A team at the Tokyo University of Science believes that exposure to vicarious stress causes deterioration of neurons in the hippocampus, which they see as a key factor in depression.
This so-called “second-hand” depression could provide insight into how people develop mental health problems and how to treat them, the scientists said.
“I want people to think about how stress can change not only the brains of people with depression, but ours – those who watch -,” lead author Akiyoshi Saitoh, professor of pharmacology, said Vice.
Mice created to watch other rodents being bullied developed symptoms of depression, including social withdrawal and an inability to experience pleasure
The researchers decided to look at how stress affected neurogenesis in the hippocampus, a small but complex brain structure that sits at the bottom of each temporal lobe and is linked to learning, emotion and memory.
“We decided to focus on the possible mechanism of psychological stress in hippocampal neurogenesis in adults to understand its role in major depressive disorder,” Saitoh says. said in a release.
Neurogenesis is the process by which new neurons are formed in the brain.
Saitoh and his colleague’s research exposed mice to “chronic vicarious social defeat stress” by repeatedly having them watch another mouse being overpowered.
They put two mice in a cage, one larger than the other, and pushed the smaller one into the larger rodent’s territory.
In the brains of the spectator mice, new neurons had a reduced survival rate in the dentate gyrus, a region of the hippocampus linked to memory and sensory perception. The authors believe that decline contributed greatly to their depression
The larger mouse would aggressively defend its space, but it wasn’t the smaller mouse that the team was interested in: it was a third mouse, safe in its own cage, watching the entire ordeal.
Although they were not at risk, the spectator mice developed depressive symptoms.
After 10 days, they showed signs of anhedonia, or the inability to experience pleasure — a key symptom of depression in humans.
“If you give mice the choice of drinking sweet or plain water, they will almost always go for the sweet drink because it tastes better,” Saitoh says. told Vice. “But the mouse that witnessed social defeat did not, indicating a decreased desire for pleasure,” he said.
The study was recently published in the journal Behavioral Brain Research.
Saitoh described the witnesses as “almost hikikomori,” young adults in Japan who become so withdrawn into their parents’ home that they are unable to work or attend school.
“They weren’t interested in other mice, which is unusual given that mice are social creatures,” he said.
There was also a physiological change: New neurons in the spectator mice had a reduced survival rate in the dentate gyrus, a region of the hippocampus linked to memory and sensory perception.
The authors believe that decline was a major contributor to their depression.
The neurons’ survival rate recovered after spectators were given the antidepressant fluoxetine, the selective serotonin reuptake inhibitor commonly known as Prozac.
But they “didn’t make a sudden, full recovery,” Saitoh said. In some cases, the symptoms even returned weeks later.
That’s important to understand how treating depression in humans isn’t a quick fix, he added.
Co-author Toshinori Yoshioka said in the release that the team believes the experiment will “play an important role in elucidating the pathophysiology of depression and in developing a corresponding new drug.”
The dentate gyrus is one of the few brain organs where neuron growth continues into adulthood for many mammals, including mice and primates.