Why Do We Itch?
Animals itch for a variety of reasons. This annoying sensation can be caused by dry skin, skin irritation, dirty skin, insect bites, disease, parasites such as lice, or even for psychosomatic reasons. Researchers believe this sensation came about as a way of teaching us to avoid irritants and preserve the skin’s health.
Scientists have made progress in identifying neurotransmitters responsible for causing the itching impulses in the brain, a team from the National Institute of Health has been able to identify which of those factors is primarily responsible for driving the sensation. The research was led by Michael Iadarola of the NIH’s Department of Perioperative Medicine and the paper has been made open access in the journal Molecular Pain.
Itching is believed to be caused by gastrin releasing peptide (GRP), natriuritic precursor peptide B (NPPB), and neuromedin B (NMB). A study published in Science in May 2013 identified a significant part of the pathway that NPPB—which is produced in the heart—uses to trigger the itching sensation in mice. The authors of that paper worked on Iadarola’s team to make quantitative measurements about the potential role each neuropeptide plays in causing itching.
The current study used a technique called RNA Sequencing (RNA-Seq) to identify where and in what quantity the itch-causing neuropeptides were being expressed in mice, rats, and humans. The results supported the previous study in Science that implicated NPPB as a primary cause of itching—at least in mice. As it turns out, rats and humans don’t express NPPB in their sensory ganglia. However, all of the tested species expressed NPPA, which acts on the same peptide receptor as NPPB.
NMB was expressed in high levels at sensory ganglia across all three species, hinting that it has been evolutionarily conserved. The results also showed that GRP was not expressed in any of the three species, refuting the long-held notion that it plays a role in producing itching. The misidentification likely came from interactions between anti-GRP antibodies and NMB in the sensory ganglia. This could mean that previous studies based on GRP’s supposed role in itching need to be reevaluated.
While scratching acute itches can be really, really satisfying, certain conditions can trigger the desire to scratch excessively, which can break the skin and have serious consequences. Understanding the pathway involved in causing the itch sensation would allow researchers to potentially create a way to block those molecules. If the pathway could be altered, it could be used as a treatment and bring significant relief to those suffering from psoriasis, eczema, and other chronic skin conditions.