Scientists research itching from scratch
The hunt is on for the cruel feedback loop between itching and scratching and itching. Samantha Page reports.
Researchers are a step closer to understanding how – if not why – scratching an itch makes it itch even more.
Working with mice, scientists at the Chinese Academy of Sciences in Shanghai, China, show that neurons in an area called the periaqueductal gray, part of the midbrain linked to pain modulation, are activated by itch-evoked scratching behaviour.
“Our study provides the circuit basis for the positive feedback regulation of itch, whereby initial itch signalling and scratching may induce a further elevation of the itch sensation, leading to the itch-scratching cycle,” the scientists say in their new study, which appears now in the journal Neuron.
“The top-down pathway might be recruited during stress-induced enhancement of itchiness, thus representing a potential central therapeutic target for breaking the vicious itch-scratching cycle associated with chronic itch.”
The researchers note that it is difficult to treat chronic itching, which reportedly affects as much as 10% of the population. Severe cases of chronic itching can lead to skin irritation, damage and even lesions.
“Effective treatment for chronic itch is still lacking, which is largely due to our limited knowledge about the neural mechanism of itch,” says senior study author Yan-Gang Sun.
“Our study provides the starting point to further decipher how itch is processed and modulated in the brain. Eventually this might lead to the identification of new therapeutic targets.”
In the study, researchers stimulated itching, using histamine or a medication. They found that the resultant scratching behavior “tracked the activity of a specific set of neurons”.
When these neurons were ablated, or damaged, by the researchers, the scratching behaviour decreased significantly. To the contrary, stimulating the neurons made mice spontaneously exhibit scratching behaviour, even without an itch trigger.
The neurons involved produce a neurotransmitter, glutamate, and a neuropeptide called tachykinin 1 (Tac1), which are all involved in muscle contractions.
“We have identified an important pathway for the descending regulation of itch signal processing,” the researchers conclude.