BEHAVIOR
November 6, 2014

Itch. Scratch. Repeat.

The urge to scratch an itch is irresistible, but it really does make the itch worse. Here's why.

The intense urge to scratch an itch — whether it's the result of poison ivy or oak, chicken pox, hives or something else — is nearly irresistible, even though it's also likely that someone has told you that scratching your itch can actually make it more intense in the long run.

And that bit of advice is totally accurate: New research from Washington University School of Medicine sheds light on the origin of this vicious cycle of itching, scratching, and more itching.

Scratching can relieve an itch by creating minor pain, but when the body responds to pain signals by releasing serotonin, the itching worsens.

Scratching is recognized by the brain as mild pain. It has long been known that the brain responds to these pain signals by releasing serotonin. The research team found that this release of serotonin also has a role in the sensation of itchiness.

When scientists injected an itch-causing substance into mice that had been bred to be unable to produce serotonin, the animals did not scratch as much as their serotonin-producing counterparts. And when these same mice received supplemental serotonin injections, their impulse to scratch returned.

The findings support the idea that itch and pain signals are transmitted through different, but related, pathways. Scratching can relieve an itch by creating minor pain, but when the body responds to pain signals by releasing serotonin, the itching worsens.

Serotonin is involved in many biological processes, including regulating mood. [Serotonin reuptake inhibitors (SSRIs), like Prozac, Paxil, Zololf and Celexa are used for mood regulation.] The neurotransmitter is also key in aging and bone metabolism, so blocking the release of serotonin as a means to treat chronic itch is not a practical therapeutic option.

Instead, the researchers hoped to interfere with the communication between serotonin and the nerve cells that transmit the itch signal. They activated different serotonin receptors involved in relaying itch signals from the skin to the brain, identifying one that was primarily responsible. This was confirmed by blocking this receptor in mice and observing a reduction in scratching.

“We always have wondered why this vicious itch-pain cycle occurs,” Zhou-Feng Chen, of Washington University said in a statement. “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.”

Dr. Chen and his colleagues hope to leverage these findings to find therapeutic treatments for people suffering from chronic itch. Until then, they advise avoiding that temptation to scratch. At the very least, don't use your fingernails to scratch vigorously. Instead gently rub the itchy area with your fingertips.

The study is published in Neuron.
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