Chronic itching: how a handful of neurons control the discomfort – a potential approach to treating inflammatory diseases
For the time being, the evidence is purely experimental. But in future, these nerve cells could become a new therapeutic target
Vellus hair. Or, if you prefer, fine down. Bear this definition in mind, as it refers to a specific sensory system responsible for the sensation of itching. In the future, according to research by experts at the University of Michigan led by Bo Duan, it is precisely by starting with these touch-sensitive hairs and targeting their neural interfaces – which decode the sensation that prompts us to scratch – that we may find new avenues for treating chronic and more intractable forms of itching. This is the hope raised by the study published in Neuron. It must be said, however, that we are currently only at the stage of animal experimentation. Nevertheless, the research appears fascinating, so much so that Duan himself, in a university press release, points out that this is a pathway which could play a truly important role in the aetiology of acute and chronic itching.
A handful of neurons
In mice, researchers have identified not only the tips of these thin, fine, short and light-coloured hairs, but also a specialised group of touch-sensitive nerve cells connected to them. These hairs resemble typical human vellus hair. The function of the neurons involved in this new biological pathway was studied in animals with chronic skin inflammation, and it was found that in mice possessing these specialised neurons, the classic scratching response was observed. However, when the animals lacked these specialised nerve cells or when these cells were ‘switched off’, there was a marked reduction in scratching. “We need a new pathway to target if we are to treat chronic itch,” commented Duan. “And our research suggests that this population of neurons could be a target in the future.”
Does this happen to men as well?
The hope is that these findings can also be replicated in humans. And whilst it may not be possible to test the exact same biological pathway in humans, there is a good chance that a similar reaction occurs in our bodies too, given that humans possess the genes necessary to produce these specialised touch-sensitive neurons. Not only that: researchers have also identified proteins in animals that transmit itch signals from the hair to the spinal cord via these neurons. When human neurons grown in the laboratory were exposed to the same proteins, they reacted in a similar way. One thing is certain: in everyday life, facial hair can be a source of itching. If you wish to try a test recommended by Duan himself, you can roll up a corner of a handkerchief to form a long, thin point, and then gently run it over the fine hairs around your lips. If you lightly touch the fine hairs, it may trigger an itching sensation.
What might change
The transmission of the mechanical signals associated with itching helps us to understand why we do not scratch ourselves constantly and also (it is hoped) how, in future, we might counteract this sensation when it becomes pathological. In the experimental study, the animals’ vellus hairs were gently stimulated, and the nerve cells responsible for the response were modified so that they could be activated by blue light. Simply exposing mice to blue light triggered the same scratching behaviour observed during mechanical stimulation, providing strong evidence that these neurons directly produce the sensation of itch. Whilst it is true that in humans the spinal cord ‘filters’ what is happening via specific circuits to suppress the mechanical signals of itching—which are therefore only transmitted under specific conditions—in the future, perhaps by acting on these nerve cells, we will be able to better combat inflammatory skin diseases, offering new treatment options.
