Itch-sensitive nerves discovered
A key part of the neurological signalling system that conveys itching sensations has been unravelled by US scientists, paving the way for the development of more effective treatments for eczema and other itchy conditions. That there are nerve cells that uniquely signal itching in the skin was already known, but these couldn't be identified from amongst the myriad other neurones supplying the body surface. Now, writing in Science, Santosh Mishra and Mark Hoon from the National Institute of Dental and Craniofacial Research in Bethesda, Maryland, have found a way to pick out these cells, and they've identified the nerve transmitter chemical that they use to send itch signals brainwards. The scientists made the breakthrough by coming through the genes that are routinely turned on in nerve cells that supply the skin. They noticed that a small population of the nerves that are equipped to pick up on pain and temperature stimuli were exclusively also expressing a gene called Nppb (natriuretic polypeptide b). Knocking out this gene produced mice that had normal responses to pain and temperature but were incapable of becoming itchy. The Nppb signal normally activates a second group of nerve cells inside the spinal cord, and these spinal neurones then relay the signal to the brain using a second nerve chemical, identified previously, called GRP (gastrin releasing peptide). Now that this pathway has been understood, and Nppb identified as the critical first chemical in the chain that sounds the itch alarm, it opens the door to developing novel therapeutics to selectively block chronic pruritus.