Why Does Scratching An Itch Make It Worse?

Van Corey, 'Gotta ... Get That ... ITCH!' via Flickr. CC BY-NC-ND 2.0

It feels *so* gosh darn good when you scratch that awful itch. Until it starts itching more and drives you crazy. Have you ever wondered why that might be? Of course, a bunch of scientists have, and they think they’ve cracked it.

After investigating itching in mice, researchers from Washington University in St. Louis found that scratching causes the brain to release your “happy hormone” serotonin, which actually makes the itch worse. Although they haven’t looked at humans yet, they think that the same scratch cycles might occur in us. If so, then researchers might be able to eventually disrupt this cycle and develop treatments for individuals with chronic itching. The work has been published in Neuron.

Itching, or pruritus, can be caused by a variety of different things, from irritation to skin conditions such as eczema. Even some cancers and chemotherapies can cause chronic itching, a condition which is far from an annoyance; it seriously affects quality of life.

Scientists have known for some time that scratching soothes itches because it produces a small amount of pain. This causes nerve cells, or neurons, in the spinal cord to transmit pain signals to the brain rather than itch signals, providing temporary relief.

It’s also well established that one of the many roles of the neurotransmitter, or chemical messenger, serotonin is to help control pain. So is serotonin also involved in the itching process? To answer this burning question, researchers started out by genetically engineering mice so that they lacked the genes for serotonin production.

When they injected mice with a known itch-inducing chemical, the engineered mice exhibited far less scratching than the normal mice in the control group. However, when they injected the former with serotonin, they started scratching as normal. Furthermore, normal mice given a substance which stops serotonin escaping the brain also barely scratched when injected with the same skin-irritating chemical as before.

“So this fits very well with the idea that itch and pain signals are transmitted through different but related pathways,” senior investigator Zhou-Feng Chen said in a news release. “Scratching can relieve itch by creating minor pain. But when the body responds to pain signals, that response actually can make itching worse.”

Chen goes on to explain that while serotonin helps alleviate pain in the brain, it also eventually spreads to the spinal cord. From there, it moves from pain-sensing neurons to neurons that influence itch intensity, making itching worse.

Because serotonin plays so many crucial roles in the body, such as mood regulation and bone metabolism, blocking its release is not a viable treatment option for itching. A more practical approach might be to disrupt the communication between serotonin and the spinal cord neurons that carry the itch signals. This could be achieved by disrupting one of the receptors used by serotonin to activate these cells, which are known as GRPR neurons. But there are many components to the itching cycle, so it will probably be a while before this work in mice can help us find a valid therapeutic target.

[Via BBC News, New Scientist, WUSTL, Neuron]

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