New research suggests that the "feel-good" hormone that makes scratching so satisfying also traps us in a never-ending cycle of intensifying itching.
Scratching that pesky itch is one of the most euphoric feelings, only to be disrupted by the itch coming back - again, and again. Itching can be caused by a number of things, including dry skin, irritation by dust and insect bites. When we feel itchy, we tend to scratch, which temporarily relieves the itch.
It is a well known fact amongst scientists that scratching an itch gets the nerves to carry pain signals instead of itch signals to the brain, but they weren't sure until now what then caused the itch to come back again so strongly. Scientists have now found that the brain responds to these minor pain signals by releasing serotonin, the neurotransmitter that works as a pain blocker - but it also appears to make itching worse.
To test the action of the chemical messenger, researchers from Washington University School of Medicine in the US, bred a strain of genetically engineered mice that lacked serotonin. When the mice were injected with a substance that causes their skin to normally itch, they showed no signs of feeling itchy. The mice were then injected with serotonin, and they immediately began to scratch in response to the itch-inducing substance, the authors report in the journal Neuron.
"The problem is that when the brain gets those pain signals, it responds by producing the neurotransmitter serotonin to help control that pain," said Zhou-Feng Chen, biologist and lead author of the study, in a press release. "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."
Unfortunately blocking serotonin to help relieve people who suffer from chronic itching isn't an option. Serotonin is a "feel good" hormone, that's linked to depression and wellbeing, and without it, people would not be able to control pain naturally.
A better target, the scientists decided, would be to to interfere with the spinal cord pathway between serotonin and nerve cells - these are called GRPR neurons and they're responsible for communicating the itch signals from the skin to the brain.
To explore this, the team injected mice with an itch-inducing substance as well as compounds to activate different serotonin receptors on GRPR. After testing several different receptors, they found that a receptor called 5HT1A was the one that activated the GRPR neurons. The mice were then given a compound that blocks the 5HT1A receptor, and sure enough, they scratched less.
"Serotonin does more than only inhibit pain. Our new finding shows that it also makes itch worse by activating GRPR neurons through 5HT1A receptors," said Chen.
The team is now trying to get a better understanding of the molecular and cellular mechanisms behind the pathway. In the mean time, try not to scratch, even if this article triggers an inconvenient itch on the centre of your back.