Why heat makes you itch

An itch, besides being an irresistible sensation, is one of the main symptoms of an inflammatory skin disease. Yet scratching only makes the problem worse. Warm weather doesn't help either — it is well known that heat exacerbates itching, however, little is known about the underlying biology.

Now, a team of researchers led by dermatologist Hiroyuki Murota of Osaka University in Japan, and including dermatologist Mustafa Abd El-Latif at Cairo University, has determined the mechanism by which heat worsens an itch.

First, they treated fibroblasts, or connective tissue cells, in culture with various chemicals linked to itching and pain, and measured gene expression in the cells. The neuropeptide substance P caused the cells to express a growth factor called artemin.

Next, the team examined sections of skin taken from patients with atopic dermatitis, a chronic skin condition characterised by scaly and itchy rashes. This revealed that artemin-expressing fibroblasts accumulate in the lesions.

They then injected artemin or substance P into the skin of healthy mice and found that both induced branching of itch-sensing nerve fibres under the surface of skin, causing the mice to become hypersensitive to warm temperatures.

This suggested to the researchers that artemin is somehow involved in temperature-dependent hypersensitivity. To confirm these findings, they performed behavioural tests on mutant mice lacking the artemin receptor, and found that heat doesn't make the itch worse.

"We are now investigating how artemin sensitizes the nervous system and leads to thermal hyperalgesia," says Murota. "We believe that this might provide some idea to develop novel therapeutic strategies. Topical artemin-neutralization might be one of the treatment strategies to attenuate warmth-provoked itch."

Sign-up to receive our e-alert update every two weeks to keep up with everything new on the portal.