Health

How the Brain Turns an Itch Into a Scratch

An indirect neural pathway communicates the sensation of itchiness to the brain, and the discovery means researchers believe itch-suppressing therapies might be possible.

Maria Fuchs/Getty Images

Mosquitoes, poison ivy, dry skin, wool sweaters.

You know the feeling.

Yet despite being one of the most common sensations of the human body, the physiological underpinnings of itchiness have remained largely mysterious to scientists even after decades of research.

Itching isn’t just an annoyance. Chronic, unchecked scratching — the kind prompted by certain skin or liver diseases — can cause significant bodily harm. Truly effective itch-relief treatment would be both a boon for medical practitioners and a godsend for anyone spending two months in a cast. It could also, quite likely, represent a commercial retail bonanza.

Like the feeling of pain, itching is an unpleasant but important way the body signals that something is amiss and requires attention. As with pain, however, the feeling quickly becomes unbearable when it’s too intense.

Recent studies have made progress identifying some of the neurochemistry involved. But a central question remained: How does the itch signal actually travel from the skin to the brain?

Now, scientists in China have provided some clarity and discovered that by interrupting the signal, they could stop the itching.

Itch-mediating spinal neurons, which express the gastrin-releasing peptide receptor, are disynaptically connected to the parabrachial nucleus via glutamatergic spinal projection neurons. | Sun Yangang's lab

They found that although a certain set of spinal neurons with gastrin-releasing peptide receptors (GRPR) were crucial for itch-signal processing, those neurons did not send the itch signal directly to the brain.

Instead, GRPR neurons sent the signal through an intermediary patch of neurons in the spine, which act as a pathway into a brainstem region known as the parabrachial nucleus, or PBN. The PBN then serves as the first central relay point for itch signal information entering the brain, according to the study.

"Our study revealed a long-range neural circuit that is critical for transmitting itch signal from the spinal cord to brain," Dr. Sun Yangang of the Chinese Academy of Sciences' Institute of Neuroscience, who led the study, told China’s Xinhua news agency.

The intermediary pathway between the GRPR neurons and the brain could itself be manipulated to block the itch signal, researchers found.

In their study, published last week in the journal Science, the scientists were able to suppress itching and scratching in mice by inhibiting those intermediary pathway neurons through optogenetics — a technique that uses light to stimulate or suppress the activity of neurons in living tissue.

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Mice were bred with genetically modified neurons that react to light, giving researchers the power to  activate or deactivate the spinal pathway involved in relaying information about itchiness.

In other words, the researchers found they could turn off the sensation of itchiness by disabling the mid-stage neurons that were transmitting itch signals to the brain, effectively blocking the link between the GRPR neurons and the PBN in the brainstem.

That finding points to the potential for new forms of long-term therapy, providing effective ways can be found to interrupt the analogous itch-signal pathway in humans.

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