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You Won't Believe What's Contagious!
Itching is contagious. To be exact, it is highly contagious. When you see someone scratch, you're likely to scratch, too. Just reading these words may have caused you to itch--and then scratch.

We've long known that laughing and yawning can be socially contagious. If you see someone yawn, you yawn. If you see a group of people laughing, chances are you will laugh. The same thing is true for scratching.

Find out why scratching stops an itch.

Previously, researchers assumed this was all in our minds. Now researchers from the Washington University School of Medicine in St. Louis, Missouri have proven that contagious itching is hardwired in the brain and not a form of empathy.

Led by Zhou-Feng Chen, PhD, director of the Washington University Center for the Study of Itch, the team studied mice. One by one, they put a single mouse in an enclosure with a computer screen and then played a video for the rodent that showed another mouse scratching.

"Within a few seconds, the mouse in the enclosure would start scratching, too," Chen said. "This was very surprising because mice are known for their poor vision. They use smell and touch to explore areas, so we didn't know whether a mouse would notice a video. Not only did it see the video, [but also] it could tell that the mouse in the video was scratching."

Next, the researchers identified a structure called the suprachiasmatic nucleus (SCN), a brain region that controls when animals fall asleep or wake up. The SCN was highly active after the mouse watched the video of the scratching mouse.

When the mouse saw other mice scratching--in the video and when placed near scratching littermates--the brain's SCN would release a chemical substance called GRP (gastrin-releasing peptide). In 2007, Chen's team identified GRP as a key transmitter of itch signals between the skin and the spinal cord.

"The mouse doesn't see another mouse scratching and then think it might need to scratch, too," Chen said. "Instead, its brain begins sending out itch signals using GRP as a messenger."

Chen's team also used various methods to block GRP or the receptor it binds to on neurons. Mice whose GRP or GRP receptor were blocked in the brain's SCN region did not scratch when they saw others scratch. But they maintained the ability to scratch normally when exposed to itch-inducing substances.

Chen believes the contagious itch behavior the mice engaged in is something the animals can't control.

"It's an innate behavior and an instinct," he said. "We've been able to show that a single chemical and a single receptor are all that's necessary to mediate this particular behavior. The next time you scratch or yawn in response to someone else doing it, remember it's really not a choice nor a psychological response; it's hardwired into your brain."

So what? The discovery may help scientists understand the neural circuits that control socially contagious behaviors.

The study findings were published in the journal Science.

Find out why scratching a mosquito bite just makes you want to claw your skin even more.

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