Hugh Pickens writes: "A ticklish problem in neuroscience has been that although historically many scientists have regarded itching as just a less intense version of pain and decades searching for itch-specific nerve cells have been unfruitful. Now Nature reports that neuroscientist Zhou-Feng Chen and his colleagues at Washington University in St Louis, Missouri have found the first behavioral evidence that there are separate circuits of nerve cells to convey itchiness and pain and their studies suggest that itch and pain signals are transmitted along different pathways in the spinal cord. "Most people accept that there are specific, highly specialized neurons for sensations like taste," says Chen. "But for pain and itch this is much more controversial." Two years ago, Chen's group discovered that a cell-surface protein called the gastrin-releasing peptide receptor (GRPR) is important for sensing itchiness but not pain in mice. When Chen and his colleagues destroyed GRPR-bearing neurons by means of a cell toxin, the mice reacted to painful stimuli just like normal mice, licking themselves and flinching or jumping in response to heat, highly irritant chemicals and mechanical pressure. But when the researchers injected the animals with chemicals that normally cause scratching, such as histamine, they barely responded and the greater the number of GRPR-expressing neurons destroyed, the more subdued was the scratching response. "This is the first behavioral evidence that there are itch-specific neurons," says Chen. "People have been looking for these for many years." Although the present research only scratches the surface, the discovery highlights a new target for therapy and opens the field for specific treatments for itch that don't affect pain."
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