Scratch That: Berkeley Biologist Traces Cellular Circuitry to Explain Itching

There’s no way into the animal mind, you say? No plumbing of a beast’s true feelings? Not so. Simply observe any footage of a bear rubbing its back against a tree, or even your own mutt vigorously scratching behind the ear with a hind paw. Observe the slack-jawed, stuporous expressions of relief and satisfaction. Yeah. We know what’s going on.

We are not bound by cosmic harmony: It is the Itch that unites us all.

But…just what is an itch, anyway? Sure, it can be a source of intense satisfaction, assuming you can get at it—or social misery, or even living hell, if you can’t. But what’s the physiology behind this most universal of physical irritations?

UC Berkeley biology professor Diana Bautista takes a stab (or scratch) at it in the newest issue of the journal Nature Neuroscience. Itch, it seems, is rather onion-like—the more layers you peel back, the more there are. Generally, it seems that certain nerve cells are geared to signal conditions that are ultimately translated by the brain as pain, while others detect circumstances that are itchy.

A detector cell is probably one or the other: If it’s programmed to recognize pain, it isn’t stimulated by itching, or vice versa. Nevertheless, such cells may overlap, which is why you can both itch and hurt in the same place.

Still, pain can trump itch—in fact, it may be essential to suppressing itchiness. Scratching, Bautista determined, may relieve itching because it produces a response in pain-actuated nerve cells, which can mute the response from the itch cells.

But some cells may switch hit, signaling an itch when slightly stimulated, then producing pain when really stressed. Or it could be that the pain and itch cells are fully differentiated, but the signals get mixed together like pancake batter in the spinal cord, resulting in a deeply textured sensation by the time they reach the brain.

In other words, itchiness, like love, is complicated—only itching comes with no apparent upside. Both can cause a lot of misery. Chronic itching plagues about 10 percent of the population at one point or another. So itch research isn’t aimed at just determining causes. Remedies are also a goal.

Bautista’s research points to some promising new possibilities. Typically an itch is treated with antihistamines, compounds that neutralize (unsurprisingly) histamines, which are substances implicated in many types of inflammation. But not all itches are caused by histamines, nor are they responsive to antihistamines. Bautista’s research indicates specific receptors on immune cells may cause such stubborn cases. Switching off these receptors by some as-yet-undetermined means may lead to relief.

In other words, stay tuned. And keep scratching.

— Glen Martin
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