Researchers at Washington University School of Medicine in St Louis found that itching caused by touch is directly related to the number of touch receptors embedded in the skin. The fewer the receptors, the more likely it is that touching will induce itching.

Studying mice, the researchers discovered that the number of touch receptors called Merkel cells in the skin declined as the animals aged. They also found fewer of these touch receptors in animals with dry skin. Not having as many Merkel cells made itch problems more likely when the animals were poked with a hairlike nylon device that scientists use to study itch responses.

"As the number of Merkel cells went down, problems with touch-related itch went up," said senior investigator Hongzhen Hu, an associate professor of anesthesiology at the university's Center for the Study of Itch. "What exactly Merkel cells do has not been clear, but our findings suggest they help control the itch response. When you lose these cells, their ability to inhibit itch also is lost."

In additional work, the researchers turned to genetically engineered mice whose Merkel cells could be activated with a chemical compound. When the animals were given the compound, they were less likely to scratch when touched with the hairlike device.

The researchers have identified a second potential therapeutic target, a protein on the Merkel cells that appears to control itch. The protein, called Piezo2, is made on the membranes of the cells. In the mouse experiments, the researchers found that the Piezo2 protein played a role in controlling Merkel cells as they tamped down itch.

The research was published on May 4 in the journal Science.