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 fibers responsible for transmitting the itch signal to the brain.

Then they injected a substance under the skin to make the monkeys itchy. When they scratched the itch, Giesler said the itch signal "magically" disappeared.

"The stimulation at that point stopped the neurons from firing or slowed the activity way down for 20 to 30 to 40 seconds -- the period where you feel relief from scratching," Giesler said.

"That suggests these neurons in the spinal cord are shut off during scratching," he said.

Giesler said the next step is to figure out how that circuit works, and what are the chemical messengers or neurotransmitters that are involved in this process.

"How can we activate this system without scratching?" Giesler asked. "Then we can conceivably develop something that might be a treatment for all of these forms of itch that are not helped by antihistamines."

(Editing by Maggie Fox and Vicki Allen)

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• Itching