Scientists, including one of Indian-origin, have found that a small molecule released in the spinal cord in mice triggers a process that is later experienced in the brain as the sensation of itch.
Scientists at the National Institutes of Health in US conducted mouse studies and discovered that 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.
When Nppb or its nerve cell was removed, mice stopped scratching at a broad array of itch-inducing substances. The signal wasn't going through.
Since the nervous systems of mice and humans are similar, the scientists said a comparable biocircuit for itch likely is present in people.
If correct, 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 the millions of people with chronic itch conditions, such eczema and psoriasis.
The paper, published online in the journal Science, also helps to solve a lingering scientific issue.
"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 Mark Hoon, the senior author on the paper and a scientist at the National Institute of Dental and Craniofacial Research, part of the National Institutes of Health.
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 fibres extending into the skin, muscle, and other tissues, help to monitor a range of external conditions, from extreme temperature changes to detecting pain.
Hoon's 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.
"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," said Santosh Mishra, lead author on the study and a researcher in the Hoon laboratory.
