Researchers identify peach-fuzz pathway tied to itch
University of Michigan scientists say fine hairs and linked nerve cells form a sensory route that may help explain chronic itch.
By Lucas Ferreira · Science & Environment Writer
3 min read
University of Michigan researchers have identified a sensory pathway that appears to connect fine, touch-sensitive hairs with itch signals. The finding matters because chronic itch, including itch tied to inflammatory skin disease, remains hard to treat with current medicines.
The work, published in Neuron and supported in part by the National Institutes of Health, was done mainly in mouse models. The team found a class of vellus-like hairs in mice and a group of specialized nerve cells that respond to movement of those hairs, according to the university.
Bo Duan, an associate professor in the university’s Department of Molecular, Cellular, and Developmental Biology, said the pathway may play a role in both short-term and long-lasting itch. The researchers described it as a mechanical itch system, meaning itch triggered by touch rather than by chemical irritants.
How the pathway was tested
To study the system, Duan’s team examined mice with chronic skin inflammation, a model the university compared with eczema in people. Mice that had the specialized neurons scratched in response to itch stimulation, while mice without those neurons, or with the neurons turned off, scratched far less, according to the researchers.
The team also created a way to produce mechanical itch in mice by gently stimulating the vellus-like hairs with a small loop of thread. Because mice cannot report itch, the researchers used scratching behavior as the observable response.
After identifying the nerve cells involved, the scientists genetically altered them so they could be switched on with blue light. Shining blue light on the mice caused scratching similar to the response seen when the hairs were mechanically stimulated, the university said.
Why fine hairs can feel itchy
Vellus hairs are the short, pale, fine hairs often called peach fuzz in humans. The mouse hairs studied by the team resemble those human hairs and are found in areas such as behind the ears, below the lips and near the base of the paws, according to the university.
Duan uses a classroom demonstration to show the sensation: a finely twisted tissue brushed lightly over the tiny hairs near the lips can produce itch when it touches fine hairs rather than thicker hair. The study offers a biological explanation for that effect by linking those hairs to a dedicated nerve pathway.
The university said unusual vellus-like hairs in mice were first described more than a century ago, but they had not been a major focus of sensory research. Duan’s group had to develop methods to study this kind of itch in animals.
Possible relevance for eczema and other itch disorders
Current treatments can work for chemical itch caused by triggers such as mosquito bites or poison ivy, according to the university. They are less effective for persistent itch associated with chronic skin inflammation.
Duan said the newly identified neurons could become a future treatment target, and the lab is continuing related work. The researchers have not directly shown the same pathway in people, but the university said several findings point in that direction.
Humans have genes needed to make the specialized touch-sensitive neurons, according to the team. The researchers also identified proteins in mice that transmit itch signals from the hairs toward the spinal cord, and human neurons grown in laboratory cultures responded to those proteins in similar ways.
Duan’s earlier work may help explain why people are not constantly itchy despite having vellus hair across much of the body. The university said spinal cord “gating” circuits can suppress mechanical itch signals and allow them through only under certain conditions.
This story draws on original reporting from ScienceDaily.