Space & Innovation

Pain Could Be Socially Contagious

After smelling the bedding of mice in pain, healthy mice became sensitive to pain as well.

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"I feel for you" could be a literal statement. Behavioral neuroscientists from Oregon Health and Science University unintentionally discovered that mice in the lab can transfer pain to each other socially.

Professor Andrey Ryabinin and his colleagues didn't set out to put this idea to the test. Instead, they were actually looking at alcohol withdrawal in mice to find ways to help humans struggling with addiction. In humans, withdrawal usually produces generalized body pain so the team was trying to recreate that phenomenon in lab mice, Ryabinin explained to Science.

Their experiment setup does sound uncomfortable. The researchers started with two groups of mice located in the same room. One group of mice received as much ethanol and water solution as they wanted, which was then taken away to prompt withdrawal. A control group located in a separate cage a few feet away only got water. To test how both groups of mice responded to pain, the researchers did things like dip their tails in hot water, tickle their feet with fine hairs and inject an irritant into a paw.

Oddly enough, the control group seemed to behave as if they were also going through withdrawal. The two groups didn't show much difference in their hypersensitivity scores at all.

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Before throwing in the towel, the researchers decided to put the control group of mice in a different room. When, scientists repeated the tests that involved dipping tails in hot water, tickling feet and injecting irritants in the paws, the control group did not appear as hypersensitive. Their scores were much lower than the mice in pain.

The results were closer to expectations, but again, it raised more questions. Why did moving the control group to entirely different room reduce their sensitivity to pain? More experimentation began to provide a clue.

Ryabinin and his colleagues transferred bedding from the cage of the mice in pain to the cage of the control mice in the other room. Again, the researchers repeated the hypersensitivity tests and the scores from the control group nearly matched those of the mice in pain in the other room. Just smelling the bedding that had been used in the cage with mice in withdrawal seemed to heighten the pain sensitivity in the control group that only drank water, Science reported.

"We've shown for the first time that you don't need an injury or inflammation to develop a pain state," Ryabinin told New Scientist. "Pain can develop simply because of social cues."

The implications for humans aren't entirely clear at this point because smell might not be as much of a factor for us. However, the findings could change the way researchers design future lab experiments involving mice. And they may eventually help prevent heightened stress in those living with people who have chronic pain. Whether we like it or not, we really are all in this cage together.

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