Tracking down the source of an itch
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London - We may no longer need to scratch each others' backs. Scientists have discovered a key substance in the central nervous-system responsible for transmitting the sensation of an itch from the skin to the brain - a finding that raises the prospect of new treatments for serious itching conditions.
Medical researchers have found that the neuro-transmitter, a small molecule that transmits signals between nerve cells, plays a crucial role in the perception of an itch, which in some people can lead to chronic, long-term scratching.
Removing the neurotransmitter - called natriuretic polypeptide b, or NPPB for short - causes an itch to disappear.
And in turn, replacing NPPB within the spinal cord results in the itch reappearing, said Mark Hoon, a lead investigator at the US National Institute of Dental and Craniofacial Research in Washington DC.
“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,” Dr Hoon said.
“Overall, a better understanding of the biology of itch and the molecules involved can only mean we are closer to finding a treatment for chronic itching. Most people think of an itch as an inconvenience, but there are patients who have a poor quality of life because of chronic scratching,” he said.
Patients suffering from chronic skin-disorders such as psoriasis and eczema often experience long-term itching, but the irritating sensation can also be brought about by a variety of temporary conditions such as fungal infections, scabies, sun burn or just dry skin.
Very severe itching over long periods of time can cause serious harm. Some patients have scratched their skin until it bleeds while others have scratched through to the bone, and even through the scalp to the brain.
The study, published in the journal Science, involved testing various neurotransmitters of the central nervous-system to study their roles in the perception of temperature, pain and itching. They found that one, NPPB, appeared to be involved only in itch, said Santosh Mishra, a researcher in Dr Hoon's laboratory.
Laboratory mice that had been genetically modified so that they did not have the gene for making the NPPB protein were incapable of responding to itchy sensations, although they were still capable of feeling other sensations such as pain and heat.
“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,” Dr Mishra said.
The scientists also found that there were dedicated nerve-fibres in the spinal cord that responded to the NPPB neurotransmitter. When their nerve fibres were removed, their ability to feel an itch was also eliminated, Dr Hoon said. These findings are strong evidence to support the widely-held view that itch is not a special type of pain, but a unique sensation in itself.
“We can see the differences between the sensation of pain and of itch in a laboratory mouse, which will scratch itself if it feels an itch but lick itself, gnaw at its skin or flinch at a painful stimulus,” Dr Hoon said.
“Now the challenge is to find similar biocircuitry in people, evaluate what's there and to identify molecules that can be targeted to turn off chronic itch without causing unwanted side-effects,” he said.
“So this is a start, not a finish.”
EXPLAINING AN ITCH
An itch is not a tickle, although some people may think it is funny. Neither, according to the latest research, is an itch a form of pain, although try telling that to people tormented by chronic, long-term scratching.
The scientific evidence for one of life's odder sensations is that the feeling of an itch depends on a dedicated set of nerve cells leading from the spinal cord to the brain, complete with their own chemical messenger, a neurotransmitter known as Nppb. Take away Nppb and the itch disappears. Put it back again in the spinal cord, and the irritating sensation returns. It is easy to see a potential for a drug that blocks Nppb in chronic scratchers.
The worst type of itch most of us will endure is the temporary sort that comes after a mosquito bite. This is a result of the immune system releasing a substance called histamine into the skin to counteract all the nasty ingredients found in a mosquito's saliva.
Most itches - even the seven-year variety - eventually wane and disappear, especially with a bit of an effort to avoid scratching them too much. But there are some itches that just won't go away - such as the one that led a distressed Massachusetts woman to scratch right through her skull, exposing her brain.
An itch is loosely and simply defined as an unpleasant sensation that provokes a desire to scratch. It is thought to have evolved as a defence against parasites trying to burrow through the skin, but the sensation seems to have a psychological as well as a physical component - an itch can be brought on merely at the thought of something itchy.
Certainly the fear of chronic itching inspired one of the tortures described in Dante's Inferno: “the burning rage of fierce itching that nothing could relieve”.
One of the worst things about an itch, of course, is that once you develop the habit, it's tough to stop. Scientists call this the “itch-scratch cycle”. So what may have begun as a pleasantly relieving hobby becomes an painfully addictive occupation. - The Independent