Scratching an itch does provide some relief because the scratching creates pain signals that travel to the brain. But it's only a temporary reprieve. Scratching an itch only makes it worse, according to new research reported online last week in the journal Neuron.
In tests on mice, scientists at Washington University School of Medicine in the US state of Missouri have found that those pain signals created by scratching cause the brain to secrete a neurotransmitter that intensifies the itch sensation. The same vicious cycle of itching and scratching is also thought to occur in people.
The neurotransmitter, serotonin, is produced to help control the pain signals. "But as serotonin spreads from the brain into the spinal cord, we found the chemical can 'jump the tracks', moving from pain-sensing neurons to nerve cells that influence itch intensity," says senior investigator Professor Chen Zhou-feng, director of Washington University's Centre for the study of itch.
Serotonin's role in pain control was established decades ago, but this is the first time the chemical messenger has been linked to itching, says Chen.
For the study, the researchers bred a strain of mice that lacked the genes to make serotonin. These mice were then injected with a substance that normally makes the skin itch.