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Ever felt like yawning after seeing someone else yawn? If you have, then you’ve experienced the phenomenon known as contagious yawning — the urge to yawn didn’t exist until you saw someone else yawn! Similarly, itching is also a social contagion, and, more likely than not, you’ve fallen victim to it before as well. When we see someone reach to scratch themselves or even hear a mention of itchiness, we may simultaneously be filled with a compelling urge to itch ourselves.

Recently, scientists at Washington University in St. Louis School of Medicine have discovered a molecular and neural basis of itching — in other words, itching may be also hardwired into our brains and not simply a result of psychological triggers. As documented in their study, principal investigator Zhou-Feng Chen, Ph.D. and his research team examined the neural mechanisms behind socially contagious itching.

In their experiments, the team placed mice into enclosures with screens showing videos of other mice scratching themselves. Soon, the mouse inside the enclosure would begin scratching. According to Chen, this behavior was surprising because mice are known for their poor vision; however, the mice in the enclosure were able to see the screen and begin scratching. 

 The researchers used mice as their model system.

Image source: Adam Gault

To further investigate this fascinating observation, the researchers analyzed brain activity of the mice after engaging in the contagious scratching. They found that the suprachiasmatic nucleus (SCN), a region of the brain controlling the circadian cycle, released a molecule called gastrin-releasing peptide (GRP). Chen and his colleagues had previously determined that GRP was involved in transmitting itch signals between the skin and the spinal cord.

Chen’s team blocked GRP and the receptor that GRP binds to in the mice and found that the mice did not scratch themselves in response to seeing other mice scratch. However, these mice were still fully capable of itching, as they scratched themselves normally after being injected with itch-inducing substances. From these results, the team concluded that GRP and its receptor are necessary for the transmission of itch signals.

 Itching is hardwired into mice’s brains.

Image source: PM Images 

The research team’s findings have significant implications for future research on the neural mechanisms controlling socially contagious behaviors such as itching. As Chen put it, “the next time you scratch or yawn in response to someone else doing it, remember it’s not really a choice nor a psychological response; it’s hardwired into your brain.”

Although direct conclusions cannot yet be extended to the human brain, in which these processes are more complex and nuanced, Chen’s results shed new light on the molecular and neurobiological basis of socially contagious behavior among research that has historically been founded on psychological approaches.

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