Researchers discovered why scratching causes people to experience more itching, and have found a potential solution to the problem.
A research team found scratching g causes the brain to release serotonin, which intensifies itching sensations, the Washington University in St. Louis reported.
Scratching works by getting the nerve cells to send 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 senior investigator Zhou-Feng Chen, director of Washington University's Center for the Study of Itch. "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."
To make their findings the researchers bred a strain of mice that lacked the gene to make serotonin; when these mice were injected with an itch-inducing substance they did not scratch as much their littermates that did have the gene. When these mice were injected with serotonin they scratched as much as the normal mice did.
"So this fits very well with the idea that itch and pain signals are transmitted through different but related pathways," Chen said. "Scratching can relieve itch by creating minor pain. But when the body responds to pain signals, that response actually can make itching worse."
The researchers said that while interfering with serotonin production made mice less sensitive to itch, it is not practical to treat chronic itching this way. Serotonin is involved in "growth, aging, bone metabolism and in regulating mood."
In order to overcome this, the researchers injected the mice with the itch-inducing substance and also administered compounds that activated different serotonin receptors to determine which ones were linked to itching. Interfering with the communication between nerve cells and serotonin could help reduce the itchy effects of scratching.
"We always have wondered why this vicious itch-pain cycle occurs," Chen said. "Our findings suggest that the events happen in this order. First, you scratch, and that causes a sensation of pain. Then you make more serotonin to control the pain. But serotonin does more than only inhibit pain. Our new finding shows that it also makes itch worse by activating GRPR neurons through 5HT1A receptors."
