20 March 2015

Scratch That Itch?

Welcome back. I’ve got an itch. It’s not a yen. It’s a dry-skin, winter itch that I’d put lotion on if I could reach it or scratch if I didn’t know better. I don’t remember when I was told or learned that scratching itches makes them worse; it had to be a long time ago. I passed that pearl of wisdom to my offspring, of course, though they probably had to learn it themselves anyway.

Bear scratching its back from USGS
Northern Divide Bear Project video.
Knowing the punishing consequences of scratching an itch is not the same as knowing why that happens. For that, we needed a study with 21 contributors from Washington University in St. Louis. (Several of those contributors are now elsewhere--the University of Toledo in Ohio, University of California, San Francisco and different academic and medical institutions in China.)

To learn why scratching makes itches worse, the researchers worked with mice. Although their findings hadn’t been tested with humans at the time the study was reported online, they seemed to have nailed it. They blame it on serotonin.


You may have heard of serotonin. It’s a neurotransmitter found in the gastrointestinal tract, platelets and central nervous system. In general, neurotransmitters activate receptors, which in turn may excite or inhibit transmission between cells. Because serotonin receptors are involved in a variety of biological and neurological processes (e.g., anxiety, appetite, memory, pain, sleep), they’re the focus of different drugs, including antidepressants and antipsychotics.

Serotonin and Itching

Scratching relieves an itch temporarily by causing minor skin pain. That pain prompts nerve cells in the spinal cord to carry pain signals instead of itch signals to the brain. The brain responds by producing serotonin to aid in pain control. That’s where the reason for scratching gets befuddled.

The researchers found that, as serotonin spreads from the brain into the spinal cord, it can do more that activate pain-sensing nerve cells. It can also activate nerve cells that affect itch intensity and thus worsen the itch.

They bred mice that lacked the genes needed to produce serotonin. These mice scratched much less than unaltered control mice when injected with a compound that causes itching. When the genetically altered mice were subsequently injected with serotonin, they scratched the same as the control mice.

Interrupting the Itch Pain Cycle

The obvious solution to reduce itching is to reduce serotonin; but since serotonin gets involved in assorted biological and neurological processes, that’s a no-no. Instead, the investigators targeted the spinal cord nerve cells that transmit itch signals. Specifically, they isolated and targeted the serotonin receptor that activates the itch-transmitting GRPR nerve cells.

Singling out the correct receptor was done by monitoring the response after injecting mice with an itch-causing compound and compounds that activated different serotonin receptors. To confirm the winning receptor, which is known as 5HT1A, they also treated mice with a compound that blocked that receptor.

Wrap Up

So what have we got? Scratching an itch causes pain, the brain produces serotonin to control the pain and the serotonin worsens the itch by activating the itch-transmitting nerve cells through the serotonin receptor. Not to worry if you didn’t get that. All you really have to remember is don't scratch that itch. Thanks for stopping by.


Washington University study in Neuron journal:
Articles on the study on Science Daily and Medical News Today websites:
Examples of other research on serotonin and pain:
2009: www.ncbi.nlm.nih.gov/pmc/articles/PMC2811866/
2014: www.cell.com/neuron/abstract/S0896-6273%2813%2901141-0
Background on serotonin: en.wikipedia.org/wiki/Serotonin

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