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Dogs do it. Birds do it. Some people do it a lot. They itch. But scientists don't know very much about this sensation, so they're trying to learn more by studying mice that don't seem to itch. NPR's Jon Hamilton has the story.

JON HAMILTON: Here's what scientists do know about an itch. It begins when something, like a mosquito bite, activates nerve cells in the skin.

Dr. ROBERT LAMONT(ph) (Itch specialist, Yale): And they send electrical signals into the spinal cord.

HAMILTON: Where, says Yale itch specialist Robert Lamont, the signals reach other nerve cells.

Dr. LAMONT: And those presumably send signals to the brain.

HAMILTON: Lamont doesn't have a lot more to say about the process.

Dr. LAMONT: Because, you know, there hasn't been much itch research.

HAMILTON: Which is why itch researchers are pretty excited about the results of a mouse experiment at Washington University in St. Louis.

First, normal mice were given substances known to cause itching in people. You can't ask a mouse whether something itches, so researcher Zhou-Feng Chen said the team watched closely.

Professor ZHOU-FENG CHEN (Anesthesiology, Washington University): All animal scratch when they itch. So all you needed to do is to count how many times the animals scratched.

HAMILTON: And all the normal mice scratched like crazy. Then the team tried the experiment on mice that had been treated to eliminate a certain type of nerve cell in their spinal cords.

Prof. CHEN: Very surprisingly, we found that those mice, they didn't scratch much no matter what kind of itchy stimuli they were given.

HAMILTON: They still felt pain. They moved around fine and they appeared completely normal. They just didn't seem to itch.

The result suggests that Chen's team has identified specific cells in the spinal cord that are part of an itch pathway between the skin and brain. That's a big step forward for itch research. And it also could turn out to be a big deal for people with skin diseases or conditions that cause chronic itching.

Gil Yosipovitch is a dermatologist at Wake Forest University who treats some of these people.

Dr. GIL YOSIPOVITCH (Dermatologist, Wake Forest University): There are patients who cannot sleep at night and wake up with itching. These patients, the only way that can relieve them of their itch is by scratching and causing it to bleed.

HAMILTON: The new research is preliminary. Also, it's hard to know whether what works in mice will work in people. But Yosipovitch says knowing which cells communicate itchiness should give researchers a starting point to develop a drug for patients with chronic itch.

Dr. YOSIPOVITCH: If there is a receptor that we can target and inhibit itch without causing any pain or any adverse effects, that would be very helpful for those patients.

HAMILTON: And the research adds to evidence that itching has some real survival value. If it didn't, evolution would have been unlikely to designate specific cells to convey the itch sensation.

Yosipovitch says it's remarkable how many different species seem to itch and scratch.

Dr. YOSIPOVITCH: Even observations that fish scratch themselves in the aquarium.

HAMILTON: And Yosipovitch says it's easy to imagine how itching might help animals protect themselves.

Dr. YOSIPOVITCH: Most probably, this was related to scratching and insect bites and fleas in animals to remove them. So itching and scratching served some kind of protective measures.

HAMILTON: The itch research appears in the journal Science Express.

Jon Hamilton, NPR News.

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