It is pretty clear to most people when they are in pain and when they have a bothersome itch, and if you want to live a long life you don’t want either as a chronic condition. However, the neurological events that lead to the experience of having a pain or having an itch have been only partially understood. General information on pruritus (itchiness) can be found here and here. “Studies have been done to show that itch receptors are only found on the top two skin layers, the epidermis and the epidermal/dermal transition layers. — Itch is never felt in muscle, joints, or inner organs, which show that deep tissue does not contain itch signaling apparatuses(ref).” Itch is a serious topic of continuing study as you could find out from the Report from the 4th International Workshop for the Study of Itch attended by more than 130 scientists and physicians from throughout the world.
For some time, researchers have thought that pain signals and itch signals traveled to the brain through two different pathways. “Unmyelinated nerve fibers for itch and pain both originate in the skin; however, information for them is conveyed centrally in two distinct systems that both use the same peripheral nerve bundle and spinothalamic tract(ref) .” The spinothalamic tract consists of a set of cells that travel up the spine to the brain’s thalamus.
Two years ago, a team at the Washington University School of Medicine in St. Louis, Missouri discovered a gene that, expressed in neurons, appeared to be necessary for itch, but not pain, called the gastrin-releasing peptide receptor (GRPR) gene. Neurons expressing GRPR were thought possibly to be itch neurons. Interestingly, GRPR – is involved in regulation of “–numerous functions of the gastrointestinal and central nervous systems, including release of gastrointestinal hormones, smooth muscle cell contraction, and epithelial cell proliferation and is a potent mitogen for neoplastic tissues. — The receptor is aberrantly expressed in numerous cancers such as those of the lung, colon, and prostate(ref).” Also curiously, “ – the presence of 2 expressed copies of the GRPR gene in females may be a factor in the increased susceptibility of women to tobacco-induced lung cancer(ref).”
In a new study reported in the Aug 6 issue of Science Express, the same Washington University team focused on whether neurons expressing GRPR were the long-sought itch-only neurons. The team selectively killed GRPR-expressing lamina I neurons and then exposed the mice to all kinds of itchy stimuli that would normally cause them to scratch themselves like crazy. The mice did not scratch but responded as usual to various kinds of induced pain. So, the inference is that GRPR-expressing neurons are the itch response neurons(ref). “This finding has very important therapeutic implications,” says Zhou-Feng Chen, Ph.D., the study’s principal investigator. “We’ve shown that particular neurons are critical for the itching sensation but not for pain, which 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(ref).”
There is still a lot more to be learned about itch. ““We’ve shown that these GRPR neurons are important for itching sensation and not for pain, but we really don’t know much more about them,” Chen says. “We still have a lot of questions, and we are very interested to find more answers(ref).”