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Socially contagious itching hardwired into brain

A mouse observing a video screen of a scratching demonstrator will begin to scratch itself, too.Chen lab, Washington University School of Medicine/Science

Certain behaviors seem to be contagious among social animals. Just seeing someone scratch or yawn can give you the urge to do so, too. Even seeing pictures of insects, scratch marks, and allergic reactions can evoke these reactions.

The brain mechanisms underlying the mimicking of certain behaviors isn’t fully understood. To study the molecules and brain circuits involved in contagious behaviors, a research team led by Dr. Zhou-Feng Chen of Washington University investigated whether itching is socially contagious in mice, as it is in people and other primates. The work was funded by NIH’s National Institute of Arthritis and Musculoskeletal and Skin Diseases (NIAMS), National Institute of Neurological Disorders and Stroke (NINDS), and National Institute on Drug Abuse (NIDA). Results appeared on March 10, 2017, in Science.

Researchers placed mice that scratched excessively from a chronic itch in a cage next to normal mice. Within seconds of seeing a scratchy mouse, the normal mice started scratching themselves, too. To test whether the mere sight of the scratchy mouse prompted the imitation—as opposed to a smell or sound—the researchers tried the same experiment with mice watching a scratchy mouse on a computer screen. As before, mice that saw the scratching behavior started scratching themselves.

Researchers examined the brain activity of the mice imitating the scratching behaviors. These animals showed significant increases in activity in several brain areas, including the suprachiasmatic nucleus (SCN). The SCN receives direct visual input from the environment, suggesting that it could be one of the first brain circuits involved in socially contagious itching.

The team investigated whether altering molecules within the SCN affected the imitative scratching behaviors. Gastrin-releasing peptide, or GRP, is a protein-like molecule known to influence SCN activity. Using various techniques, the researchers altered GRP signaling in the SCN. Mice that lacked GRP signaling showed almost no imitative scratching behaviors when exposed to a scratchy mouse, but still scratched themselves when exposed to a skin irritant. In contrast, artificially activating GRP signaling induced scratching in mice without the visual input of seeing a scratchy mouse.

These results suggest that contagious itching stems not from empathy, but rather from an automatic activation of neural circuits. “The mouse doesn’t see another mouse scratching and then think it might need to scratch, too,” Chen says. “Instead, its brain begins sending out itch signals using GRP as a messenger.”

More research is needed to fully understand the mechanisms of contagious behaviors in mice and whether these are the same for other social animals. Understanding the underlying pathways of itch may also suggest novel therapeutic strategies. Chronic itch, which occurs in many medical conditions and in response to certain drugs, affects millions of Americans.

—by Harrison Wein, Ph.D.