Turns out your mom was right: Scratching an itch only makes it worse. New research from scientists at Washington University School of Medicine in St. Louis indicates that scratching causes the brain to release serotonin, which intensifies the itch sensation.
The findings, in mice, are reported online in the journal Neuron. The same vicious cycle of itching and scratching is thought to occur in humans, and the research provides new clues that may help break that cycle, particularly in people who experience chronic itching.
Scientists have known for decades that scratching creates a mild amount of pain in the skin, said senior investigator Zhou-Feng Chen, PhD, director of Washington University's Center for the Study of Itch. That pain can interfere with itching -- at least temporarily -- by getting nerve cells in the spinal cord to carry pain signals to the brain instead of itch signals.
"The problem is that when the brain gets those pain signals, it responds by producing the neurotransmitter serotonin to help control that pain," Chen explained. "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."
Scientists uncovered serotonin's role in controlling pain decades ago, but this is the first time the release of the chemical messenger from the brain has been linked to itch, Chen said.
As part of the study, the researchers bred a strain of mice that lacked the genes to make serotonin. When those genetically engineered mice were injected with a substance that normally makes the skin itch, the mice didn't scratch as much as their normal littermates. But when the genetically altered mice were injected with serotonin, they scratched as mice would be expected to in response to compounds designed to induce itching.
"So this fits very well with the idea that itch and pain signals are transmitted through different but related pathways," said Chen, a professor of anesthesiology, of psychiatry and of developmental biology. "Scratching can relieve itch by creating minor pain. But when the body responds to pain signals, that response actually can make itching worse."
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