Itch is probably the most frequently experienced sensation within a human lifetime. I, for one, know I experience it on a regular basis whether it is a bug-bite, contact dermatitis (exposure to metal, plant-based chemicals such as urishiol from poison ivy/oak/sumac), sunburn, or eczema. For many patients, itch may be a sign of a more worrying, underlying issue such as renal failure, liver disease, allergies (to food, to medication, to the environment), cancer, phantom sensations, diabetes, stress, emotional distress, or other neurologic disturbances. In fact, itch is so pervasive in our lives that the thought of itch may cause some of us to start scratching, just like yawning!
It is estimated that one in ten people will experience chronic itch at some point in their lives (if they have not, or are not experiencing it at this moment). Unfortunately, modern medicine has not made great leaps and advances in anti-itch medications. Many patients rely on antihistamines, low-percentage steroid creams, and homeopathic solutions such a calamine or oatmeal baths. As of yet, this is no cure.
For a long time, scientists believed that the sensation of “itch” (pruritus) was directly linked to the sensation of pain, with itch being a low level pain response. However, with novel research being conducted around the world, it is becoming increasingly clear that itch is in a field entirely its own. For instance, with itch, the human response is to scratch; but when experiencing pain the instinct is to withdraw, an opposite impulse. Also, the sensation of itch can sometimes be relieved by the induction of pain (like when we use hot or cold packs to relieve itch), meaning that painful sensations may actually override those of itch. Though there is a very complex network of cell signaling pathways that relate itch to pain (in fact the pathways may even use the same receptors and signaling molecules, though the different triggers cause different responses), much eludes present-day scientists on the exact neurological hardwiring of the itch sensation in the human nervous system.
The NIH and other research laboratories throughout the USA, have had successes in isolating molecules that may actually be responsible for itch such as the ion channel TRPV1, capsaicin and wasabi receptors, and the neuropeptide Nppb. In mouse models, mice that did not express Nppb did not react to itch-inducing agents; however, if Nppb was re-introduced into their system, they experienced itch. This research definitively proved Nppb was an itch-selective neuropeptide and opened new doors for the identification of the neuronal pathways dedicated to the itch sensation. It’s safe to say the topic of itch is very hot!
In fact, with the new BRAIN Initiative, it may finally be possible to understand how itch and pain sensations travel from the point of origin to the brain, as well as the complex interrelationship between itch and pain. Not only may scientists be able to delineate the routes to the brain, but they may also ask just how the brain interpret these signals into a response. With this information in hand, researchers may finally have the tools they need to create treatments combatting itch. So fear not itch-sufferers, scientists are hard at work trying to determine a way to alleviate that most aggravating, most excruciating, irritating, intolerable, insufferable (I might be getting carried away here) sensation of itch.
Written by:
Margit Lai Wun Juhasz
Mount Sinai Medical Student