Posted: 02 May 2017
Comment on “Why You Have To Scratch That Itch” on Morning Edition for 24 May 2013. Scientists have researched why we scratch an itch. In order to find out what is causing that itch, they could not just focus on what happens on the surface of the skin. They realized that nerves had to trigger the feeling of itch and there are none that come out onto the surface of the skin. So they had to induce itching from under the skin. They found a surprising molecule inside neurons responsible for telling a nervous system that there is an itch. That is just the hypothesized start of a cascade of nervous signals that eventually make one conscious of an itch. What happens next?
Mishra & Hoon (2013) studied what causes mice to scratch. NPR reporter Rhitu Chatterjee tells us that they discovered an “itch molecule,” natriuretic polypeptide b (Nppb), which normally resides in the heart to regulate blood pressure, also resides inside specific neurons which sense an itch on the surface of the body. This molecule is a neurotransmitter which triggers the beginning of a series of neuron signals to the brain to make one conscious of the itch. They injected irritants composed of chloroquine (movie), histamine, and substance P (supplemental movies) into the dermis of “wild type” and genetically modified (knockout or lacking Nppb neurotransmitter) mice. Wild type mice scratched in response to the injections, but knockout mice did not. When the researchers injected Nppb into both types of mice, the both started to scratch. They showed that Nppb was the substance that triggered a scratch response. However, they did not show what caused Nppb to be triggered by any of these irritants, in other words, what was happening chemically to trigger these neurons to send Nppb to the spinal cord. They do not know if humans have this molecule and if it plays the same role in humans as it does in mice.
My Comments at NPR (NPR discontinued comments in 2016)
How does dialysis cause itching? Maybe it is closely related to what these researchers are doing by injecting itch-causing substances intradermally. The important detail is that the itch substances are injected into the dermis. The dermis is where most nerve endings are. Touch neurons enter the bottom layers of the epidermis, but pain, heat, cold, pressure, and proprioception (position or stretch) receptors are in the dermis or deeper (e.g. hypodermis). Moreover, there is a strong physical and chemical connection between the hypodermal fluids and those traveling within the layers of the dermis. There is a whole new world in the hypodermis that needs a lot of investigation and Traditional Chinese Medicine (acupuncture) may open a window into that world, along with a different perspective that can combine both Western and Eastern medicine.
Mishra & Hoon (2013) clearly are on the right track. One can suspect that a combination of sensations like fine touch and pressure were involved. However, the researchers did not address why a person itches. In other words, why evolve an “itch molecule” (Nppb) which makes one want to scratch an itch? There is no adaptive reason for creating an itch if that feeling is conveyed to the conscious brain and there is no response to it, other than close observation of the location of the itch, e.g. the skin. How and why does that molecule get produced when a mosquito or gnat or “no-see-em” injects its poison under our skin? or does it respond to the inflammation and swelling caused by that poison and not the poison itself? If so, why not to other causes of inflammation, e.g. to joints? What exactly happens when an itch is scratched? Sometimes the itch goes away when scratched, if for only a few seconds. What exactly is happening? A theory involving local controls of non-vascular fluid flow that explains both how scratching an itch works and how dialysis causes itching is proposed at What is the Hypodermis?
Buchen, L. (2009). The itch without the pain. Nature News 6 Aug 2009 doi:10.1038/news.2009.802. [Freely Available].
Mishra, S. K. & Hoon, M. A.( 2013). The cells and circuitry for itch responses in mice. Science 340(6135), 968-971. Supplementary Information
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