Why scratching makes us itchy!

We all know that a good scratch will get rid of an itch - only for it to come back with a vengeance.

Now scientists claim to have finally worked out exactly why scratching always makes us itch even more.

Research indicates that scratching causes the brain to release serotonin, which intensifies the itch sensation.

Scientists claim to have answered one of life's nagging questions: why scratching makes us itch even more Scratching causes the brain to release serotonin, which intensifies the itch sensation

The findings, in mice, indicate the same vicious cycle of itching and scratching is thought to occur in humans.

And the research also provides new clues that may finally help break that cycle - particularly in people who experience chronic itching.

Scientists have known for decades that scratching creates a mild amount of pain in the skin.

Senior investigator Doctor Zhou-Feng Chen, director of Washington University's Centre for the Study of Itch, said that pain can interfere with itching - at least temporarily - by getting nerve cells in the spinal cord to carry pain signals to the brain instead of itch signals.

Dr Chen said: 'The problem is that when the brain gets those pain signals, it responds by producing the neurotransmitter serotonin to help control that pain.'

As serotonin spreads from the brain into the spinal cord, the chemical can 'jump the tracks' – moving from neurons that sense pain to nerve cells that influence the intensity of an itch, she added.

Dr Chen said scientists uncovered serotonin's role in controlling pain decades ago, but this is the first time the release of the chemical messenger from the brain has been linked to itching.

As part of the study, the researchers bred a strain of mice that lacked the genes to make serotonin.

 

Scientists worked out that scratching causes a sensation of pain. This causes the brain to release serotonin is to control the pain. But serotonin activates itch-transmitting cells, making the itch worse

When those genetically engineered mice were injected with a substance that normally makes the skin itch, the mice didn't scratch as much as their normal littermates.

But when the genetically altered mice were injected with serotonin, they scratched as much as would be expected in response to the itch-inducing compound.

Dr Chen said: '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.'

Although interfering with serotonin made mice less sensitive to itch, Dr Chen said it's not practical to try to treat itching by trying to block the release of serotonin.

Serotonin is involved in growth, ageing, bone metabolism and in regulating mood.

Anti-depressants - such as Prozac, Zoloft and Paxil - increase serotonin levels to control depression.

Blocking serotonin would have far-reaching consequences throughout the body, and people wouldn't have a natural way to control pain.

In humans, it is not possible to block serotonin to control itching as it is important for pain-control

Instead, Dr Chen explained, it might be possible to interfere with the communication between serotonin and nerve cells in the spinal cord that specifically transmit itch. 

Those itch-transmitting cells, known as GRPR neurons, relay itch signals from the skin to the brain.

To work toward that goal, Dr Chen's team isolated the receptor used by serotonin to activate GRPR neurons.

To do this, the researchers injected mice with a substance that causes itching.

They also gave the mice compounds that activated various serotonin receptors on nerve cells.

They learned that the receptor known as 5HT1A was the key to activating the itch-specific GRPR neurons in the spinal cord.

To prove they had the correct receptor, Dr Chen's team also treated mice with a compound that blocked the 5HT1A receptor, and those mice scratched much less.

Dr Chen added: 'We always have wondered why this vicious itch-pain cycle occurs.

'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 findings shows that it also makes the itch worse by activating GRPR neurons through 5HT1A receptors.'

The research was published in the journal Neuron. 

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