A new study by neuroscientist Zhou-Feng Chen and his colleagues at Washington University in St Louis, Missouri, gives strong evidence in favour of this theory. Two years ago the team discovered that a cell-surface protein called the gastrin-releasing peptide receptor (GRPR) is important for sensing itchiness, but not pain in mice. The new paper published by Science magazine reports on their continued study of neurons expressing GRPR.

Chen’s team used a toxin attached to a protein, which easily binds to GRPR, to destroy neurons bearing GRPR in the spinal cords of adult mice. Then they made a series of tests to check the mice’s response to pain and itching. Mice subjected to this injection responded to pain just like normal mice. However, the reaction to chemicals which cause scratching behaviour, such as histamine, was greatly inhibited. The more damage to GRPR-producing neurons they suffered, the less they reacted to an itching sensation.

“This is the first behavioral evidence that there are itch-specific neurons,” says Chen. “People have been looking for these for many years.”

Other scientists are not convinced that the evidence is conclusive. They point out that pain and itch might not be relayed by neural signals, but rather by neuron firing patterns. Chen acknowledges it, but also notes that “so far there is no evidence to support that theory.”

The study may give new insight on treatment for irritating ailments. Currently, patients with chronic itching are usually prescribed pain drugs.