itch
October 31, 2014

Scientists Solve The Mystery Of Why Scratching Makes You Feel Even Itchier

Chuck Bednar for redOrbit.com - Your Universe Online

Scratching an itch should make you feel better, but often times it only intensifies the feeling – and now scientists from Washington University School of Medicine in St. Louis have discovered what is responsible for this paradox.

Writing in the latest edition of the journal Neuron, senior investigator Zhou-Feng Chen, director of the university’s Center for the Study of Itch, and his colleagues found that the act of scratching causes the brain to release serotonin, which intensifies the itch sensation.

According to BBC News, Chen’s team conducted research in mice that discovered that these so-called scratch cycles are harder to break as more serotonin is released into the system. While their work has yet to be tested in humans, their findings indicate that blocking specific serotonin receptors in the spine could reduce chronic itching, and dermatologists believe that it could lead to effective itch control.

Scientists have long known that scratching creates a mild amount of pain in the skin, the researchers said, and that pain can halt itching temporarily by causing nerve cells in the spinal cord to carry pain signals to the brain instead of itch signals. While serotonin’s role in pain control has long been known, this study marks the first time that its release from the brain has been associated with the sensation of itch.

“The problem is that when the brain gets those pain signals, it responds by producing the neurotransmitter serotonin to help control that pain,” Chen said in a statement Thursday. “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.”

The research team, which also included scientists from the University of Toledo, Wuhan University, the Academy of Chinese Science, Guangzhou Medical University, the University of California, and the Xi’an Jiaotong University School of Medicine, genetically engineered mice that lacked the genes required to make serotonin.

When those rodents were injected with a substance that normally caused the skin to become itchy, the mice did not scratch as much as their unmodified littermates. However, when the genetically altered mice were injected with serotonin, they scratched as much as regular mice do 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,” explained Chen, a professor of anesthesiology, psychiatry and developmental biology at the university. “Scratching can relieve itch by creating minor pain. But when the body responds to pain signals, that response actually can make itching worse.”

However, Chen told BBC News that it is not feasible to completely block serotonin release in humans, since the chemical plays a key role in growth, aging, bone metabolism and mood. However, the research does indicate that disrupting the communication between serotonin and the cells responsible for transmitting itch signals to the brain could well be one of the most promising ways of controlling chronic itching, the British news organization added.

“We always have wondered why this vicious itch-pain cycle occurs,” the professor 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.”

Related Reading:

> The Molecule That Makes You Itch
> Knockout Mouse - Genetically Modified Organisms Reference Library

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