Brain Chemicals Explain Why People Scratch Even If They Aren't Itchy 


When you smile or yawn, reactive mimicry theory predicts your neighbors will smile or yawn in turn. Another contagious behavior is itching: If you scratch your nose, it’s likely that the coworker watching you will scratch as well. However, recent research on itching reveals that the reason why one itch leads to another is very different from the mechanism driving other contagious behaviors. While yawning is considered a subconscious social behavior revealing empathy, an itch is a hardwired physiological response that we — and the people around us — simply can’t control.

In a paper to be released Friday in Science, a team of scientists led by Dr. Zhou-Feng Chen, director of Washington University’s Center for the Study of Itch, explain that contagious itching is an uncontrollable reaction of the body. Chen and his team found that when mice watched videos of other mice scratching themselves, the mice watching would begin to scratch as well. These findings, Chen tells Inverse, could very likely apply to humans as well.

This part of the experiment was very cute.

Washington Center for the Study of Itch

In the study, Chen’s team scanned a region of the mouse brain called the suprachiasmatic nucleus in mice that watched videos of scratching mice. As they watched, their brains shot out gastrin-releasing peptide (GRP), a neuropeptide, and they began to scratch. This indicated to the researchers that the GRP works as a messenger to the brain to ignite “itch signals.” (The fact that they responded at all was surprising — mice have such poor vision that the researchers didn’t even anticipate that they’d be able to see the other mice.)

Chen anticipates that, because we are animals with the same peptide in our bodies, our brains likely send out GRP when we see an itch as well. But why the brain does this, says Chen, is still up for debate. He speculates that contagious itching is an evolutionary mechanism that falls under the umbrella of collective behaviors: Animals look to each other to see what’s working for others, and mimic that behavior in hopes it will keep them alive as well. If an animal is bitten by a mosquito, it learns to deal with the itch of that bite by watching other animals scratch.

Chen says that contagious scratching has long been observed in mice, but this is the first time the mechanism behind that scratching has been analyzed. He hopes to apply these findings to the center’s overall mission of understanding the underlying mechanism of itching — and then in turn discovering treatments for people whose itches are completely intractable.

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