What happens when you see someone scratch a mosquito bite? You may start to feel an itch come out of nowhere. Then you might start to scratch, too. Mice, new data show, suffer the same strange phenomenon. It’s called contagious scratching.
Tests with mice provide the first clear evidence that contagious scratching can spread mouse-to-mouse, says Zhou-Feng Chen. As a neuroscientist, he studies the brain. He works at the Washington University School of Medicine in St. Louis, Mo. His mice started scratching after watching an itchy neighbor. They even did this after just watching videos of scratching mice.
In discovering this, “there were lots of surprises,” Chen notes. One was that mice would even pay attention to some scratching neighbor. After all, mice are nocturnal. They mostly sniff and use their whiskers to feel their way through the dark. Yet Chen had his own irresistible itch to test the “crazy idea: that mice might share an urge to scratch, he says. And it paid off.
The quirk isn’t just a weird little finding, though. It opens new possibilities for exploring the basis in the brain for picking up such contagious behaviors.
Chen’s group described its new discoveries in the March 10 issue of Science.
How they did it
The researchers housed mice super-itchy mice within sight of some that didn’t scratch much. Then they recorded the animals on video. Shortly after a normal mouse looked at a neighbor scratching, it scratched, too. For a comparison, researchers gave others of these mice a not-very-itchy neighbor. These comparison mice also looked at their neighbor now and then. Rarely, however, would they scratch right after that glance.
And the neighbor didn’t have to be real. Sometimes it was just a mouse video. Animals that viewed the video of scratchers responded the same way. More audience itching and scratching followed the film of a mouse with itchy skin than one of a mouse just poking around on other rodent business.
Next, researchers looked for where contagious itching plays out in the animal’s nervous system.
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Mice recently struck by contagious urges to scratch showed extra brain activity in several spots. These included, surprisingly, a pair of nerve cell clusters called the SCN, for suprachiasmatic nuclei (SU-per-ky-az-MAT-ik NU-klee-eye). These clusters are better known as the body’s internal clock. They keep they body’s daily — or circadian (Sur-KAY-dee-un) — rhythms on time. They respond to light. They use that daily signal to reset the clock so that it can keep the body’s internal activities in sync. (People have these SCN clusters, too. They’re found deep in the brain roughly behind the eyes.)
Other tests linked contagious itching with a small brain molecule nicknamed GRP. Scientists had known that GRP helps carry itch information elsewhere in a mouse’s nervous system. In the new tests, mice didn’t catch the urge to scratch if they had no genes to make this GRP or to make the molecule that detects it. Yet the mice still scratched when the researchers tickled their skin.
Also, in normal mice, injecting a dose of GRP into the SCN provoked scratching. And this was true even when there was no itchy neighbor in sight. Injecting salt water into the SCN did not set off much pawing. This seems to confirm that the brain’s clock can play a central role in triggering the scratch response — at least in mice.
How convincing are the new data?
Chen’s team has done fine work, says Gil Yosipovitch. He is a dermatologist, or skin specialist, in Florida where he directs the Miami Itch Center at the University of Miami. Still, he’s not sure how the mouse discovery might apply to people. So far, brain imaging in his own work has not turned up signs of a role for the SCN in contagious itching in people, he says.
Henning Holle is also puzzled by why the SCN alone might control behavior based on seeing a mouse scratch. Holle is a psychologist and neuroscientist at the University of Hull in England. Contagious scratching is “a very specific and rich visual stimulus,” he says. And research suggests other brain regions play a role in contagious itching in people, he says.
Tracking down how scratching spreads by sight is more than a science puzzle, Yosipovitch points out. Some people are troubled by strong, constant itching. These people can often catch the need to scratch very easily just by watching someone else scratch. And responding this way can make their own itching even worse. New ideas that come from studying these mice might provide clues to easing the misery that these people with a non-stop itch endure.
behavior The way a person or other organism acts towards others, or conducts itself.
cell The smallest structural and functional unit of an organism. Typically too small to see with the unaided eye, it consists of a watery fluid surrounded by a membrane or wall. Depending on their size, animals are made of anywhere from thousands to trillions of cells. Some organisms, such as yeasts, molds, bacteria and some algae, are composed of only one cell.
circadian rhythm Biological functions such as body temperature and sleeping/waking times that operate on a roughly 24-hour cycle.
colleague Someone who works with another; a co-worker or team member.
contagious An adjective for some disease that can be spread by direct contact with an infected individual or the germs that they shed into the air, their clothes or their environment. Such diseases are referred to as contagious.
factor Something that plays a role in a particular condition or event; a contributor.
frequency The number of times a specified periodic phenomenon occurs within a specified time interval.
gene (adj. genetic) A segment of DNA that codes, or holds instructions, for a cell’s production of a protein. Offspring inherit genes from their parents. Genes influence how an organism looks and behaves.
itch A sensation on the skin that causes a person or animal to want to scratch. Itch sensations can be short-lived, such as when they are caused by a mosquito bite, or they can be chronic, even lasting for years without relief.
mechanism The steps or process by which something happens or “works.” It may be the spring that pops something from one hole into another. It could be the squeezing of the heart muscle that pumps blood throughout the body. It could be the friction (with the road and air) that slows down the speed of a coasting car. Researchers often look for the mechanism behind actions and reactions to understand how something functions.
molecule An electrically neutral group of atoms that represents the smallest possible amount of a chemical compound. Molecules can be made of single types of atoms or of different types. For example, the oxygen in the air is made of two oxygen atoms (O2), but water is made of two hydrogen atoms and one oxygen atom (H2O).
nerve A long, delicate fiber that communicates signals across the body of an animal. An animal’s backbone contains many nerves, some of which control the movement of its legs or fins, and some of which convey sensations such as hot, cold or pain.
nervous system The network of nerve cells and fibers that transmits signals between parts of the body.
neuroscience The field of science that deals with the structure or function of the brain and other parts of the nervous system. Researchers in this field are known as neuroscientists.
nocturnal An adjective for something that is done, occurring or active at night.
persistent An adjective for something that is long-lasting.
phenomenon Something that is surprising or unusual.
psychologist A scientist or mental-health professional who studies the human mind, especially in relation to actions and behaviors.
rodent A mammal of the order Rodentia, a group that includes mice, rats, squirrels, guinea pigs, hamsters and porcupines.
stimulus (plural: stimuli) Something that prompts a response in a living organism or in a controlled environment (including a chemical or physical test system).