Friday, May 24, 2013

Natriuretic Polypeptide b Makes You Itch

Scientists have known for a long time that sensory neurons called TRPV1 cells can detect itchy substances on the skin, says Mark Hoon, a neuroscientist at National Institute of Dental and Craniofacial Research in Bethesda, Maryland. Because TRPV1 neurons also respond to hot and painful stimuli, however, it wasn't clear whether the neurons that respond to itch are unique, or if itch might simply be low-grade pain. That's made it difficult to develop treatments that target itch without affecting other sensory systems, Hoon says.

While analyzing molecules excreted by TRPV1 cells in search of anything that might be itch-specific, Hoon and his colleagues came across a small group of the neurons that produce natriuretic polypeptide b (Nppb), a hormone that regulates heart function and can also act as a neurotransmitter. "We wondered what those cells were doing," Hoon says. To find out, the team genetically modified mice to block production of Nppb in TRPV1 neurons, then injected the skin on their shoulders with a range of itch-inducing compounds, including histamine, an inflammatory molecule involved in immune responses, and the malaria drug chloroquine. Normally, these substances make mice scratch nonstop, but the knockout mice hardly scratched themselves at all after the injection, showing that Nppb was required to produce the sensation of itch, the authors report online today in Science. No other sensory systems appeared to be affected in the knockout mice, Hoon says.
A new way to block itching would be a real boon, especially to those people who itch so badly that they scratch themselves to pieces. But as usual this is just mice we're talking about, so it remains to be seen if this pathway is as important in humans.

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