Scratching an itch causes the brain to release the neurotransmitter serotonin which intensifies the itch sensation, a new study has found.

Serotonin's role in controlling pain is known, but this is the first time the release of the chemical messenger from the brain has been linked to itch, according to researchers from Washington University School of Medicine in St Louis.

Scratching creates a mild amount of pain in the skin, said senior investigator Zhou-Feng Chen, director of Washington University's Center for the Study of Itch.

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.

"The problem is that when the brain gets those pain signals, it responds by producing the neurotransmitter serotonin to help control that pain," Chen explained.

"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," Chen added.

As part of the study, the researchers bred a strain of mice that lacked the genes to make serotonin. 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 mice would be expected to in response to compounds designed to induce itching.

"So this fits very well with the idea that itch and pain signals are transmitted through different but related pathways," said Chen.

"Scratching can relieve itch by creating minor pain. But when the body responds to pain signals, that response actually can make itching worse," Chen added.

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

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

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

Those cells, known as GRPR neurons, relay itch signals from the skin to the brain. Chen's team isolated the receptor used by serotonin to activate GRPR neurons.

They injected mice with a substance that causes itching. They also gave the mice compounds that activated various serotonin receptors on nerve cells.

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

The findings are published in the journal Neuron.