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We Finally Know Why Scratching an Itch Makes It Worse

Serotonin's role in the sensation of itch has been described for the first time.

"Scratching is one of the sweetest gratifications of nature," the 16th-century French philosopher Michel de Montaigne once wrote. "But repentance follows too annoyingly close at its heels." Never has there been a more agreeable piece of philosophy: Scratching just makes the itch worse, and now, scientists finally think they know why.

Like lighting and thunder, itching and the resultant scratching will be forever linked. But scientists still don't really know all that much about itching, why we do it, what evolutionary purpose it serves, etc. In a wonderful New Yorker piece written by surgeon Atul Gawande back in 2008 (which, full disclosure, is where I originally saw that Montaigne quote), he notes that itching is a "peculiar and diabolical sensation."

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"Scientists believe that itch, and the accompanying scratch reflex, evolved in order to protect us from insects and clinging plant toxins—from such dangers as malaria, yellow fever, and dengue, transmitted by mosquitoes; from tularemia, river blindness, and sleeping sickness, transmitted by flies; from typhus-bearing lice, plague-bearing fleas, and poisonous spiders," he wrote. "But how, exactly, itch works has been a puzzle."

Itch is a cousin of pain, but it is not pain in and of itself. That was proven back in the late 1980s, when German researchers artificially made people super itchy by stimulating histamine production (an itch-producing substance) in the bodies of test subjects. Even when they got extremely itchy, they did not report being in any more pain, however, leading researcher H. O. Handwerker to note that itch and pain are different sensations with different neurological pathways.

That background brings us to today's new development. While itching isn't pain, scratching certainly causes pain, and that's key.

In a new paper published in Neuron, Zhou-Feng Chen of Washington University of St. Louis' Center for the Study of Itch (it's a thing) notes the pathway that causes scratching to cause more itching.

"Scratching behavior, as a noxious mechanical stimulus, can inhibit itch sensation and spinal projection neurons. However, it also intensifies skin inflammation, which in turn provokes more intense itch sensation as well as an uncontrollable urge to scratch," he wrote.

What happens, specifically, is that the brain gets the pain signals that come from a scratch, ramps up the production of serotonin (which, among other things, can modulate pain). But serotonin also can influence itch sensitivity, his study found.

To prove this, he genetically altered mice so that they did not make serotonin. They itched much less than normal mice, even when they were injected with a substance that normally makes them very itchy. Those mice were then given serotonin injections, and began to itch as normal.

So, that's a cool finding. But, it's not good news for anyone hoping we can somehow figure out how to stop making mosquito bites itch or something like that: Serotonin is an incredibly important neurotransmitter that's associated with all sorts of things such as mood, appetite, sleep, pain, etc. Screwing with serotonin levels is probably worth it if, say, you're severely depressed; less so if you think mosquitoes are annoying.

For now, mind over matter and some chamomile lotion will remain the best treatment for itches that aren't all that bad.