Science figures out why we know when to stop scratching an itch

By Stephen Beech

Relief for millions of eczema patients is a major step nearer after scientists discovered why we know when to stop scratching an itch.

Researchers have identified a sensory channel that acts as the body's built-in "enough scratching" signal.

They say the breakthrough could help develop more effective treatments for chronic itching conditions such as eczema, psoriasis, and kidney disease.

The research team explained that when we scratch an itch, something tells our brain when to stop.

But that moment of relief - when scratching feels “enough” - is not accidental.

Scientists have now identified a key molecular and neural mechanism behind the built-in braking system, shedding new light on how the body regulates itch and why that control fails in chronic conditions.

The research team, led by Professor Roberta Gualdani, of the University of Louvain in Brussels, Belgium, found an an unexpected role for the ion channel TRPV4 in mechanically evoked itch.

Gualdani said: “We were initially studying TRPV4 in the context of pain.

“But instead of a pain phenotype, what emerged very clearly was a disruption of itch, specifically, how scratching behaviour is regulated.”

She explained that TRPV4 belongs to a family of ion channels that act as "molecular gates" in the membranes of sensory neurons, allowing ions to flow in response to physical or chemical stimuli.

Gualdani says the channels help the nervous system detect temperature, pressure, and tissue stress.

She said: "While TRPV4 has long been suspected to participate in mechanosensation, its role in itch, and especially in chronic itch, has remained controversial."

To address this question with precision, Gualdani’s team engineered a genetic mouse model, selectively deleting TRPV4 only in sensory neurons.

The neuron-specific approach avoided a major limitation of earlier studies, in which TRPV4 was removed from all tissues, making it difficult to pinpoint where the channel was actually acting.

The research team demonstrated that TRPV4 is expressed in neurons classically associated with touch, called Aβ-LTMRs, as well as in subsets of sensory neurons linked to itch and pain pathways, including those expressing TRPV1.

When the team induced a chronic itch condition resembling atopic dermatitis, Gualdani said the results were "striking".

Mice lacking neuronal TRPV4 scratched less frequently, but each scratching bout lasted much longer than normal.

Gualdani said: “At first glance, that seems paradoxical.

“But it actually reveals something very important about how itch is regulated.”

She said the data suggests that TRPV4 does not simply generate itch.

Instead, in mechanosensory neurons, it helps trigger a negative feedback signal, a neural message that tells the spinal cord and brain that scratching has been sufficient.

Without this signal, the sensation of relief is blunted, and scratching continues excessively.

In other words, TRPV4 acts as part of the nervous system’s internal "stop-scratching" circuit.

Gualdani said: “When we scratch an itch, at some point we stop because there's a negative feedback signal that tells us we're satisfied.

“Without TRPV4, the mice don't feel this feedback, so they continue scratching much longer than normal.”

She said the findings suggest that TRPV4's role in itch is more complex than previously thought.

While the channel in skin cells appears to trigger itch sensations, the same channel in neurons seems to help regulate and restrain them. This dual role has important implications for drug development.

Gualdani said: “This means that broadly blocking TRPV4 may not be the solution."

She added: “Future therapies may need to be much more targeted - perhaps acting only in the skin, without interfering with the neuronal mechanisms that tell us when to stop scratching.”

The findings were presented at the 70th Biophysical Society annual meeting at San Francisco in the United States.