Our nerves are wonderful cells, transmitting all sorts of information from the outside world, through our spinal columns, into our brains where we turn stimuli into sensations such as taste, smell, sight, and more. We don’t always fully appreciate these sensations, though. Pain is a great example: it can warn us of dangerous situations, but chronic pain can be debilitating, which is why we have so many drugs dedicated to pain reduction.
Chronic itching can be just as crippling as chronic pain, but with only a handful of effective treatments, chronic itching doesn’t get nearly the attention of its cousin, chronic pain. In the search for better treatments, a team of scientists led by Steeve Bourane of the Salk Institute in La Jolla investigated the differences in the two main types of itching, mechanical and chemical, in a paper published last month in Science.
Two types of itching? Indeed, there is more than one itch that can come from external stimuli. Chemical itching is the kind induced by allergens, triggering histamines and best treated with antihistamines such as Benadryl. Mechanical itching is caused by something touching the tiny “vellus” hairs humans have all over their body. Imagine a mosquito or fly landing on your arm: the instinct you have to swat it away is due to the itch triggered by the hairs it touches.
Bourane and his research partners identified “a dedicated spinal cord inhibitory pathway,” or set of neurons headed towards the spinal column, “that gates the transmission of mechanical itch,” meaning that an inhibitor keeps our mechanical itch receptors turned off when there’s nothing to stimulate them. After identifying the neurotransmitter NPY as the inhibitor of the mechanical itch trigger, the group bred mice without NPY to test what would happen. The mice started to scratch themselves constantly, and the mice were no more susceptible to chemical itch signals, suggesting that they had found the neural path responsible for this type of itching.
This means a lot for people who suffer from chronic pruritus, a constant itching on part or all of their body. Discovering the neural pathways responsible for the mechanical itching is the first step in treating their condition, just as an understanding of how pain receptors work in the brain have led to a glut of pain medications in recent decades. And the next time you have an itch, imagine that NPY transmitter doing its thing deep in your spinal column—it may be enough of a distraction to keep you from scratching!
Image: Steve Babb/Flickr