When you have a pesky itch, scratching brings blissful relief—at first. But then the more you scratch, the more you itch, in a vicious cycle that ends up being more frustrating than satisfying.
Now scientists at Washington University School of Medicine in St. Louis think they know why this happens. It all comes down to serotonin, a chemical messenger in the nervous system.
Itching for More Answers
At one time, scientists believed that itching and pain were transmitted through the same pathway in the nervous system. More recently, however, scientists have shown that itching signals are relayed over a separate pathway with its own itch-specific nerve cells and receptors. What’s more, the pathway for pain can override the one for itching. This helps explain why scratching, which produces mild pain, can suppress an itch, at least temporarily.
Now a new study in the journal Neuron takes those findings a step further. Senior author Zhou-Feng Chen, PhD, and his colleagues at Washington University’s Center for the Study of Itch looked at what happens after pain signals from scratching reach the brain. Although the research was conducted in mice, Chen believes a similar process occurs in humans.
Of Itchy Mice and Men
According to the researchers, when the brain receives mild pain signals due to scratching, it responds by producing serotonin. But as serotonin moves from the brain into the spinal cord, the researchers found, it can “jump the tracks,” moving from the pain pathway to the itching pathway. When serotonin reacts with receptors on itch-sensing cells in the spinal cord, it can ratchet up the intensity of the itching.
For this study, researchers used a strain of mice specifically bred to lack the genes for making serotonin. When injected with a substance that usually causes itchy skin, these particular mice didn’t scratch as much as other littermates. But when they were also injected with serotonin, the genetically-engineered mice scratched as much as would normally be expected.
In a statement about the study, Chen said that his research may one day lead to better treatments for people plagued by chronic itching. Blocking serotonin isn’t an option: In addition to fighting pain, it serves other crucial functions, such as helping to regulate mood, appetite, and sleep. But Chen said that it might be possible to block communication between serotonin and nerve cells in the spinal cord that relay itching signals.
A New Science...From Scratch
Chen’s group isn’t the only team working to construct a new science of itching. At the Wake Forest University School of Medicine, scientists used functional MRI brain imaging in human volunteers to study why scratching an itch feels great. They found that the brain’s reward circuits are involved in both the pleasure of scratching and the relief of itching.
These same circuits play a role in craving, pleasure, and even addiction, which helps explain how the urge to scratch—and the initial relief it brings—can feel so strong. It might also provide a clue to why scratching an itch leaves you wanting more.
Scientists admit that they are just starting to...scratch the surface of the complex neurobiology that underlies itching. But what they have learned thus far offers some fascinating insights into an experience everyone has had, yet no one completely understands.