Tests shows how scratching brings relief

By Julie Steenhuysen

CHICAGO (Reuters) - Scratching an itchy spot turns off an itch "switch" in the spinal cord, U.S. researchers said on Monday in a finding they think could lead to better treatments for itching disorders.

Tests on monkeys showed that scratching short-circuits itch signals to the brain.

Understanding how this works may lead to new treatments for people with diseases such as AIDS or Hodgkin's disease that cause itching not easily relieved by antihistamines or steroid creams.

"There are more than 50 diseases that produce itch that can't be easily treated," Glenn Giesler Jr. of the University of Minnesota, whose study appears in the journal Nature Neuroscience, said in a telephone interview.

In earlier studies, he and colleagues showed that neurons in a special part of the spinal cord known as the spinothalamic tract become very active when itchy substances are put on the skin.

Giesler said itch sensations -- which are a form of pain signal -- are sent to a region of the brain known as the thalamus.

But it has not been clear how scratching interferes with the itch signal. "We wanted to known how scratch works. How does it relieve itch?" Giesler said.

His team studied monkeys to find out what happens to these nerves when an itch is scratched. They painstakingly attached tiny electrodes to individual nerve fibres responsible for transmitting the itch signal to the brain.