What really causes itching, and how it works, is surprisingly little understood. But researchers at the National Institutes of Health will publish a paper tomorrow in Science that details the discovery of a small molecule released in the spinal cord that triggers the sensation of itching in mice. The culprit molecule is called natriuretic polypeptide b, or Nppb.

Itching was long regarded as simply a less intense form of pain. In 1987, H.O. Handwerker, a German scientist, used histamine to induce itchiness in participants. Those participants itched until they couldn't take it, but did not feel an increase in pain, suggesting itching and pain were transmitted along different pathways.