Scientists find why light touch evokes intense itch

U.S. researchers have found why a light touch, rather than a hard brushing movement on our hairy skin can invoke a strong urge to scratch, a finding that could potentially lead to treatments for many chronic itch patients.

In a study published this week in the U.S. journal Science, researchers from the Harvard University and Salk Institute reported identifying nerve cells within the spinal cord that curb the irksome light tickling sensation.

There are two types of itch: one that is induced by chemicals, or another, a mechanical itch, that occurs when tiny hairs on the skin's surface are lightly touched, for example by an insect.

"For the itch field, current studies have been focusing on itch evoked by chemicals such as histamine," co-corresponding author Professor Qiufu Ma of the Dana-Farber Cancer Institute and Harvard Medical School, told Xinhua.

"However, few people in the itch field pay attention to itch evoked by" light touch, which might be evolved to detect the movement of disease-bearing insects on the skin, he said.

In the study, Ma's team, working with researchers at the Salk Institute, used a modern genetic tool to selectively eliminate a type of nerve cells that produce a small protein called neuropeptide Y, or NPY for short, in the spinal cord of adult mice.

Within a week, these mice showed excessive scratching in response to light touch and "eventually resulted in severe skin lesion," Ma said.

The researchers saw similar behaviors when they used a chemical genetic strategy to silence the NPY-producing nerve cells in the spinal cord.

More importantly, the NPY-deficient mice responded to pain or chemicals that evoke itchiness normally, suggesting that light touch uses its own pathway in the nervous system to evoke scratching, Ma said.

He said the newly discovered pathway may help explain why some people affected by itch are unresponsive to commonly used antihistamine drugs.

"Our study has not yet led to immediate new treatment, but it is conceivable that drugs capable of enhancing NPY inhibitory neuron activity can help to suppress spontaneous mechanical itch," said the Harvard professor.

A Cell paper published by Ma's team last December found another type of spinal inhibitory nerve cells that can curb mechanical pain.

"Together with this study, we now found two separate populations of spinal inhibitory neurons that are required to inhibit mechanical pain and mechanical itch, respectively," Ma said.

"In the future, by mapping spinal circuits that transmit or inhibit mechanical pain or itch, we hope we could eventually identify new molecular targets to treat chronic mechanical pain or itch, a problem still affecting the life quality of millions of patients worldwide." Enditem