Why is scratching contagious?

Like yawning, seeing someone scratch can be contagious and cause us to do the same. This is a behavior that exists in monkeys, but also in mice, which have been studied to understand this phenomenon.

In the journal Science, a study published this Thursday, March 9, 2017 is the “ first evidence that socially contagious itch also exists in rodents “, according to the researchers who have demonstrated that the phenomenon originates in a part of the brain of rodents.

 » Mice scratched when they saw other mice scratching says Zhoufeng Chen, researcher and director of the Center for the Study of Itch, Washington University School of Medicine.  » Just like human beings “, he adds. Indeed, in humans, even the mention of lice can cause a person to scratch their head. Rhesus macaques, too, tend to scratch when they see other monkeys scratching in front of them or even on video, according to a 2013 study in the Journal Acta Dermato Venereologica.

To determine whether mice also engage in this behavior, Zhoufeng Chen and his colleagues placed mice in front of conspecifics with a chronically itchy condition and others in front of silent video of mice scratching themselves. In both cases, the process of mimicry took place and the mice also scratched.

When these mice observed others scratching, brain scans of the rodents showed increased activity in a structure called the suprachiasmatic nucleus (SCN) that controls when the animals fall asleep and wake up. As the rodents watched their peers scratch, cells in the SCN released a chemical called bombesin (or GRP). Chen and his colleagues identified GRP as a key transmitter of itch signals between the skin and the spinal cord in a 2007 study published in the journal Nature.

GRP only affects “social” itching. Indeed, when the researchers used a technique to block the release of GRP as well as the receptor to which it is bound, the mice no longer scratched when they saw others doing so. But they were still able to feel the itch, since they scratched when they were injected with histamine, an itch-inducing substance.

Another observation is that GRP can cause “social” itching on its own. Indeed, when the researchers injected this additional substance into the SCNs of the rodents, the latter scratched vigorously even without observing this behavior in others.

These findings may allow scientists to understand the brain circuits that control socially contagious behaviors. For Zhoufeng Chen, explaining why this contagious behavior exists is not very clear, but it could be a protective mechanism.

 » It is possible that when a number of mice are scratching, it will warn other mice that this is an area full of insects and it is better to start scratching before it does. be too late “, he explains.

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