US scientists have pinpointed² a type of nerve cell in mice which appears to generate the itch⁴ sensation.

美国科学家在老鼠身上精确的找到一种可以产生痒痒感觉的神经细胞。

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Chronic itch can be a debilitating condition

The finding suggests itching⁷（瘙痒，渴望） is not simply a low-level variation of pain - but a distinct sensation.

A team from Washington University found itch and pain signals seem to be transmitted along different pathways in the spinal⁸ cord（脊髓）.

The study, published online by the journal Science, raises hopes of new treatments for itching.

Many scientists have regarded itching as just a less intense version of pain.

They spent decades searching in vain for itch-specific nerve cells to explain how the brain perceives（觉察，感觉） itch differently from pain.

The latest study finally pinpoints⁹ these cells - but shows that the low-level pain theory was wrong.

The researchers were able to knock out the itch response in mice without affecting the animals' ability to sense pain and attempt to avoid it.

Lead researcher Dr Zhou-Feng Chen said: "This finding has very important therapeutic¹⁰（治疗的） implications（含义，启示）.

"We have shown that particular neurons（神经原） are critical for the itching sensation but not for pain, which means those cells may contain several itch-specific receptors（受容器，感觉器官） or signalling molecules¹¹ that can be explored or identified as targets for future treatment or management of chronic（长期的，慢性的） itching."

First gene³

The same team identified an "itch gene", called GRPR, in 2007.

This time, they injected the spinal cord of mice with a toxin¹² that killed off cells in which the gene was active.

By doing this they were able to eliminate the scratch response in some animals completely.

However, the same animals continued to respond normally to pain.

This showed that the key cells were active in transmitting the sensation of itching, but not the sensation of pain.

There are two major types of itching. One, caused by bug¹³ bites or allergic¹⁴ reactions, is linked to the presence of the chemical histamine（组胺）.

But most chronic, severe itching is not linked to the chemical - and does not respond to standard anti-histamine treatment.

However, mice whose itch cells had been destroyed did not scratch, regardless of the type of itching agent to which they were exposed.

Dr Glenn Giesler, an expert in itch at the University of Minnesota, said: "I believe this work is very important.

"It could pinpoint¹ targets for future treatments for itch, a common and sometimes debilitating（使衰弱的） condition produced by more than 50 diseases."

However, Professor Gil Yosipovitch, another expert in the field at Wake Forest University in North Carolina, said the pathway uncovered by the latest study was not the only one that could transmit the itch sensation.

He said other work suggested there were other pathways which transmitted both the sensations of itch and pain.

He also warned that there was a long way to go from work on mice to the development of new drugs for humans.

However, he added: "It could surely help develop new treatments for itch.

"As yet there are no general purpose anti-pruritic (anti-itch) drugs that target the neural¹⁵ system."