When an insect lands on your skin, it causes a tickling sensation that gives you a strong urge to itch. This is known as a mechanical itch, as opposed to the other type of itch experienced by animals that is induced by chemicals. But while the pathway that leads to chemical itches has been studied, the one controlling the mechanical itch is a little less understood. Now researchers have found that there are in fact two separate pathways that transmit itching: one for chemical stimuli and one for mechanical.
Some people suffer from chronic itching, an infuriating itch that scratching just cannot satisfy. Normally, uncontrolled itching is treated with antihistamine drugs as histamine compounds are known to be the cause of some chemical itches. But when given to people suffering from certain forms of chronic itching, they have no effect. This led some scientists to speculate that there might actually be distinct pathways that transmit the sensation of itching to the brain.
Mechanical itching is beneficial, as it can alert the animal to the presence of harmful insects and parasites. Wing-Chi Poon/Steel Bay/Wikimedia Commons
In a new study published in Science, researchers found this dedicated pathway for mechanical itching in neurons in the spinal cord. What it all comes down to is a small protein, known as a neurotransmitter, called neuropeptide Y (NPY). The spinal cord contains a number of different types of neurons, but one subset known as interneurons are responsible for transmitting the touch information from the skin to the brain. It was within these interneurons that the researchers found NPY. Although scientists already knew it was present in the brain and spinal cord, they weren't sure what role it played in the latter.
They suggest that this neurotransmitter acts in effect as a "gate control." The researchers found that by selectively removing the NPY neurons from the spinal cords of adult mice with a toxin, after two weeks the mice started uncontrollably scratching even from light touches. Yet when chemical itches were induced, they showed no such response. This indicates that there are indeed separate pathways for chemical and mechanical itching. The mice also displayed a similar response when they were bred to genetically lack the neurons that express NPY.
But it didn’t stop there. The researchers then found that the NPY neurons distinguish between a light touch on hair-covered skin, and a light touch on glabrous, or non-hairy, skin such as on the palms of the hands or soles of the feet. The neurons, it seems, can select to inhibit the signals coming from hairy skin, which are more likely to be stimulated by accident, suggesting that the mechanical itch pathway is further divided into two separate circuits. The researchers hope that by manipulating this specific pathway, it might offer a way to help people with itches they just can’t scratch.