There are two major types of itching. One, caused by bug bites or allergic reactions, is linked to the presence of the chemical histamine. However, most chronic, severe itching is not linked to the chemical - and does not respond to standard anti-histamine treatment. [2]
Zhou-Feng Chen at Washington University and other researchers at Peking University Third Hospital have found specific nerve cells responsible for itchiness (itch sensation), thus, itch and pain are not necessary closely related. They created itch-free mice by killing itch-specific neurons. When the mice then were exposed to things that caused itching, they did not scratch. But they felt pain.
This means those cells may contain several itch-specific receptors or signaling molecules that can be explored or identified as targets for future treatment or management of chronic itching.
In 2007 the researchers identified the first itch gene called gastrin-releasing peptide receptor (GRPR). It was active in the spinal cord. So they destroyed nerve cells that had active GRPR, using a toxin that attaches to GRPR and injecting it into the spinal cords of mice.
However, Professor Gil Yosipovitch at Wake Forest University said the pathway uncovered by this study was not the only one that could transmit the itch sensation. There were other pathways which transmitted both the sensations of itch and pain.
Reference:
[1] Eric Beech Scientists find nerve cells responsible for itch Reuters Aug 6, 2009.
[2] Scientists find an itchiness cell BBC. News 6 August 2009
