Why Scratching Makes You Itch More

Stop that scratching! You'll only make it worse. You've probably heard this line at least once in your life and now, scientists have found out that it just might be true. They've discovered that scratching an intch only makes it itchier due to a release of serotonin, which intensifies the "itch" sensation.

Scientists have known for decades that scratching creates a mild amount of pain in the skin. This pain can interfere with itching-at least temporary. It causes 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," said Zhou-Feng Chen, one of the researchers, in a news release. "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."

In order to better understand the sensation of itch, the scientists bred a strain of mice that lacked genes to make serotonin. When the mice were injected with a substance that normally makes the skin itch, the mice didn't scratch as much as their littermates. But when the genetically altered mice were injected with serotonin, they scratched as much as their counterparts.

"So this fits very well with the idea that itch and pain signals are transmitted through different but related pathways," said Chen. "Scratching can relieve itch by creating minor pain. But when the body responds to pain signals, that response actually can make itching worse."

Serotonin is involved in growth, aging, bone metabolism and in regulating mood, so blocking it entirely isn't a good idea. Yet it may be possible to interfere with the communication between serotonin and nerve cells in the spinal cord that specifically transmit itch in order to reduce the "itch" sensation.

The findings are published in the journal Neuron.