Itch is one of the most common side effects of the anaesthetics used in procedures such as epidurals. One explanation is that itch and pain receptors are intrinsically connected. "Itch and pain are two sensations that antagonise each other," says Zhou-Feng Chen from Washington University in St Louis, Missouri. "By scratching you create a kind of mechanical pain and suppress the itch. Conversely, if you suppress pain you see more itching."
To understand this mechanism better, Chen used mice to study the action of morphine, a painkiller that can cause itching. Morphine works through a receptor called MOR, and Chen suspected that different variants of the receptor might be responsible for the itch and pain responses. His team bred mice lacking one form of this receptor, called MOR1D. These mice did not scratch themselves when given morphine, though they still felt its painkilling effect (Cell, DOI: 10.1016/j.cell.2011.08.043).
"It's quite exciting that we are able to segregate the two," says Cheng, who believes that separate pathways for pain and itch exist in humans too. "Our study suggests there are different ways that you can inhibit itch without interfering with analgesia."
Another puzzle is that some itches do not respond to anti-itch drugs called antihistamines, and a study published this week suggests why. Antihistamines have proved effective against the itch of mosquito bites, for example, but they do little to soothe the itching caused by kidney failure, liver disease or burns, says Matthias Ringkamp at Johns Hopkins University in Baltimore, Maryland.
Antihistamine-sensitive itches have been shown to activate nerve fibres called unmyelinated C-fibres. Ringkamp suspected that itches that do not respond to the drugs might be mediated by a different type of fibre called myelinated A-fibres.
The spines of a tropical plant called cowage can irritate the skin and produce an itch that does not respond to antihistamines. To find out if the cowage itch signal passes through the myelinated A-fibres, Ringkamp and his team placed a weighted band over volunteers' wrists to cut off conduction in the small A-fibres. When the team inserted cowage spines into the volunteers' fingertips, they found that itch was dramatically reduced in many - but not all - of them (Journal of Neuroscience, DOI: 10.1523/jneurosci.3005-11/2011).
"It is fascinating that this happens in some people and not others," says Glenn Geisler at the University of Minnesota in Minneapolis. "In future, drugs to treat itch would have to treat A and C fibres."