What makes us itch?

Scientists  now know the secret. In what will be a boon for millions of people with chronic itch conditions, such as eczema and psoriasis, a small molecule released in the spinal cord has now been found to trigger a process that is later experienced in the brain as the sensation of itch.


The finding made by Santosh Mishra and Mark Hoon of the National Institute of Dental andCraniofacial Research in Bethesda, Maryland — which is part of the US National Institutes of Health (NIH) — has raised hopes this start switch would provide a natural place to look for unique molecules that can be targeted with drugs to turn off the sensation more efficiently in patients of chronic itching.

The small molecule, called natriuretic polypeptide b (Nppb), streams ahead and selectively plugs into a specific nerve cell in the spinal cord, which sends the signal onward through the central nervous system.

Researchers made this discovery in mouse models. When Nppb or its nerve cell was removed, mice stopped scratching at a broad array of itch-inducing substances. The signal didn’t going through.

Scientists at the NIH say because the nervous systems of mice and humans are similar, a comparable bio circuit for itch likely is present in people.

The paper has been published online in the journal Science.

“Our work shows that itch, once thought to be a low-level form of pain, is a distinct sensation that is uniquely hardwired into the nervous system with the biochemical equivalent of its own dedicated land line to the brain,” said Hoon.

Santosh, who is a researcher in Hoon’s lab, said: “We tested Nppb for its possible role in various sensations without success. When we exposed the Nppb-deficient mice to several itch-inducing substances, it was amazing to watch. Nothing happened. The mice wouldn’t scratch.”

Hoon said his group’s findings began with searching for the signalling components on a class of nerve cells, or neurons, that contain a molecule called TRPV1.

These neurons — with their long nerve fibers extending into the skin, muscle and other tissues — help to monitor a range of external conditions, from extreme temperature changes to detecting pain. Yet little is known about how these neurons recognize the various sensory inputs and know how to route them correctly to the appropriate pathway to the brain.

To fill in more of the details, Hoon said his laboratory identified in mice some of the main neurotransmitters that TRPV1 neurons produce. A neurotransmitter is a small molecule that neurons selectively release when stimulated, like a quick pulse of water from a faucet, to communicate sensory signals to other nerve cells. The scientists screened the various neurotransmitters, including Nppb, to see which ones corresponded with transmitting sensation.

Further experiments established that Nppb was essential to initiate the sensation of itch, known clinically as pruritus.

Photo credit: Time

Via Times of India