Itch-busting nerve cells could block urge to scratch

Mice studies show how brain deals with irritation of light touch on hairy skin

UP TO SCRATCH  A group of nerve cells in the spinal cord of a mouse (red) curb mechanical itch, a new study suggests. 

Salk Institute 

A fly tickling your arm hair can spark a maddening itch. Now, scientists have spotted nerve cells in mice that curb this light twiddling sensation. If humans possess similar itch-busters, the results, published in the Oct. 30 Science, could lead to treatments for the millions of people who suffer from intractable, chronic itch.

For many of these people, there are currently no good options. “This is a major problem,” says clinician Gil Yosipovitch of Temple University School of Medicine in Philadelphia and director of the Temple Itch Center. The new study shows that mice handle an itch caused by a fluttery touch differently than other kinds of itch. This distinction “seems to have clinical applications that clearly open our field,” Yosipovitch says.

In recent years, scientists have made progress teasing apart the pathways that carry itchy signals from skin to spinal cord to brain (SN: 11/22/2008, p. 16). But those itch signals often originate from chemicals, such as those delivered by mosquitoes. All that’s needed to spark a different sort of itch, called mechanical itch, is a light touch on the skin. The existence of this kind of itch is no surprise, Yosipovitch says. Mechanical itch may help explain why clothes or even dry, scaly skin can be itchy.

The new finding came from itchy mice engineered to lack a type of nerve cell in their spinal cords. Without prompting, these mice scratched so often that they developed sore bald patches on their skin. “They have the urge to scratch all the time,” says study coauthor Qiufu Ma, a neuroscientist at Harvard Medical School. A light touch from a filament caused the itchy mice to scratch themselves more than regular mice. Yet the itchy mice responded to pain and itchy chemicals normally, results that suggest the body has a dedicated, specific way of detecting mechanical itch, Ma says.

If a light touch taps into the itch accelerator, then these spinal cord nerve cells are the brakes, says neuroscientist Martyn Goulding of the Salk Institute for Biological Studies in La Jolla, Calif., also a study coauthor. Removing these nerve cells lets the itch signal proceed unchecked, he says.

With the discovery of these itch-blocking nerve cells, scientists can now start to piece together the rest of the pathway that detects mechanical itch on the skin and carries that signal to the brain, Goulding says. These nerve cells produce a chemical signal called neuropeptide Y. Experiments that scrutinize neuropeptide Y, as well as the cells that neuropeptide Y nerve cells influence, will help clarify how a mechanical itch makes itself known, he says.

It makes sense that human skin, particularly hairy skin, would develop the ability to detect an itchy tickle, Goulding says. “If you have parasites or disease-bearing insects that are on your skin biting you, they might introduce pathogens,” he says. A quick scratch, prompted by an itch, could prevent that. 

Laura Sanders is the neuroscience writer. She holds a Ph.D. in molecular biology from the University of Southern California.

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