July 26, 2007 — -- Scientists have scratched the surface of what appears to be a gene that makes us itch.
A newly released study, published in the current issue of Nature, found that mice without the gene were much less likely to scratch themselves when provoked.
"It's quite interesting because itching is such a common phenomenon. And it's also a very big problem, clinically," said study author Zhou-Feng Chen, associate professor of anesthesiology at the Washington University School of Medicine Pain Center.
"But," he added, "nobody knew which gene was involved in the sensation."
Now Chen and his team have caught the gene for a protein, called GRPR, red-handed.
GRPR is a specific molecule that resides in spinal cord nerve cells, thought to be responsible for communicating messages of itchiness to the brain.
The scientists looked at mice that had normal versions of GRPR and compared them to mice that had no GRPR at all.
Both groups of mice responded in the same way to heat, pressure and pain. But mice who were missing GRPR scratched themselves much less often after receiving injections of itch-causing chemicals.
"I think it's a really interesting, possibly breakthrough kind of report," said itch expert Earl Carstens, a professor of neurobiology at the University of California at Davis.
"It suggests that there must be a separate itch-producing pathway."
Although itchiness and pain inform the brain in similar ways, the two feelings are biologically different.
"For a long time, people thought that itch may be some sort of minor pain," said Chen. "The relationship between pain and itch is quite interesting because imaging studies have shown that lots of [itch] pathways in the central nervous system seem to overlap with pain pathways."
In the past decade or so, however, new research has shed light on the idea that these two sensations are physically distinct.
"The animal responses to pain and itch are quite different," said Chen.
"When we feel pain, we withdraw our hands and feet to protect ourselves from painful stimuli, to protect ourselves from being hurt. When you feel an itch, instead of withdrawing, you use your hand to scratch that area -- it's an opposite response."
Carstens agrees, pointing out that scratching can serve multiple purposes when it comes to resolving the itch.
"You put your hand on the hot stove -- your first reaction is to move your hand so you don't get burned. Pain is like a warning system," said Carstens.
"You can think of itch in the same way; it's a warning system. Only you pay attention to the itch and you direct your attention to it, see what's causing the itch, and try to brush it away."
Often, that means scratching the skin in an attempt to remove the offending bug or chemical causing the itchy feeling.
Scratching an itch is also important because it can produce a temporary pain, which serves to distract the brain in hopes that it will forget about the itch and move on to something more important.
"Not only is the scratching a protective mechanism to remove a potential source of irritation, it's also a mild form of pain, and pain suppresses the itch," said Carstens.
Chen and his group actually started out in search of a pain receptor, and instead stumbled upon this new finding.
"This itchy gene was kind of a surprise because we initially were looking for something different," he said.
"Now we can think about designing a compound that will treat itch but that will not interfere with other sensory pathways."
Many doctors see the promise of medicines developed from this gene that may help stop itching without affecting important pain receptors within the body.
"They tested to see if pain is in anyway affected, and they show there was no change," said Dr. Arjunan Ganesh, an assistant professor of anesthesiology at the Children's Hospital of Philadelphia.
This could mean that patients who are on pain medication and also experiencing symptoms of itchiness may one day be able to find relief without worrying about interactions with their pain relievers.
"I would be interested to see if this study could lead to a drug that would relieve itching," said Ganesh.
Although such a drug may be years away, this study represents an advancement in understanding the process underlying itch and provides one part of a much larger picture.
"It's like a big puzzle," said Carstens. "Stay tuned -- hopefully there'll be a lot of new developments in the field."