"The problem is that when the brain gets those pain signals, it responds by producing the neurotransmitter serotonin to help control that pain," said senior investigator Zhou-Feng Chen, director of the centre for the study of itch at Washington University School of Medicine in St Louis.

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 did not scratch as much as normal mice.

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.

"Itch and pain signals are transmitted through different but related pathways," Chen maintained.

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

But it is not practical to try to treat itching by trying to block the release of serotonin as it is involved in growth, ageing, bone metabolism and in regulating mood.

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

The findings were reported in the journal Neuron.

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