Scratching an itch might feel good - but it will only make it worse, scientists claim

Scientists have learnt that scratching releases serotonin which only intensifies the 'itch cycle'

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Scientists have finally worked out exactly why scratching an itch will only make it worse.

Scratching an itch triggers the release of a nerve chemical called serotonin that intensifies the itch sensation and causes you to scratch again, and again, in an uncomfortable cycle.

Relieving an itch by scratching causes temporary pain that makes nerve cells in the spinal cord to carry pain signals to the brain instead of itch signals, known as GRPR neurons. The brain responds with serotonin – resulting in the ‘itch cycle’.

US lead scientist Professor Zhou-Feng Chen, director of Washington University's Centre for the Study of Itch, said to The Telegraph: ''The problem is that when the brain gets those pain signals, it responds by producing the neurotransmitter serotonin to help control that pain.

''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.

''This fits very well with the idea that itch and pain signals are transmitted through different but related pathways. Scratching can relieve itch by creating minor pain. But when the body responds to pain signals, that response actually can make itching worse.''

The research, published in journal Neuron, monitored genetically engineered mice to see how they reacted when injected with a substance that makes the skin itch. Scientists found the modified mice – who were breed without serotonin – did not scratch as much as normal control mice.

When the modified mice were injected with serotonin they then scratched as much as the control group.

Despite these results, Professor Chen admits blocking the release of serotonin is not practical in humans.

Serotonin is important for growth, aging, bone metabolism – and in regulating mood. Anti-depressants, such as Prozac, Zoloft or Paxil, use the chemical. Additionally, blocking the nerve chemical would mean individuals would be unable to feel pain. 

Instead, the researchers believe that it may be possible to interrupt the communication between serotonin and the nerve cells in the spinal cord, by blocking the receptor 5HT1A which activated the GRPR neurons causing itching.

Dr Chen added to The Daily Mail: “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 findings shows that it also makes the itch worse by activating GRPR neurons through 5HT1A receptors.”