The study, if confirmed in humans, will help in developing treatments for chronic itch, including itch caused by life-saving medications.
At the heart of the discovery is a type of sensory nerve cell whose endings receive information from the skin and relay it to other nerves in the spinal cord, which then coordinates a response to the stimulus.
Published in Nature Neuroscience, the study suggests that even when the itch-specific nerve cells receive stimuli that are normally pain-inducing, the message they send isn't "That hurts!" but rather "That itches!"
Pain and itch are both important sensations that help organisms survive. And pain is arguably more important because it tells us to withdraw the pained body part in order to prevent tissue damage.
However, itch also warns us of the presence of irritants, as in an allergic reaction.
However, "when either of these sensations continues for weeks or months, they are no longer helpful. We even see patients stop taking life-saving medications because they cause such horrible itchiness all over," said Xinzhong Dong, from the Johns Hopkins University School of Medicine.
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"And sometimes when we try to suppress chronic pain, with morphine for example, we end up causing chronic itchiness. So the two sensations are somehow related, and this study has begun to untangle them," he said in a statement.
Because nerve cells send their messages as electrical currents that flow through them just as they would through wires, scientists can plug tiny monitors into individual nerve cells to detect the moment of stimulation.
The scientific controversy over pain and itch centers around a group of nerve cells known to respond electrically to painful stimuli such as molecules of capsaicin, the fiery ingredient in chili peppers.
A small subset of these nerve cells also responds electrically to itchy stimuli because they have on their surfaces receptors for molecules like histamine. One of these itchy receptors, called MrgA3, binds the anti-malaria drug chloroquine, causing serious itchiness in many patients.