Itching To Know More About Itch

It’s hard to follow the old rule “leaves of three, let it be” for avoiding poison ivy when brushing against greenery at top speed. David J. Kroll found that out the hard way.

As a result of a seemingly innocent run alongside a river near his home some years ago, Kroll, chair of the department of pharmaceutical sciences at North Carolina Central University, in Durham, acquired a horrific rash, accompanied by what he describes as an “unnerving” itch and some blisters on his chest and arms. “It was late September, so everything had grown really high,” Kroll says, remembering the narrow, overgrown trail.

The chest blisters were the result of Kroll running without a shirt: Plants whipped against him as he passed by, he says, likely giving the sinister urushiol oils produced by poison ivy leaves easy access to large patches of his skin. “Very smart,” he says, laughing now at the memory. “I didn’t think too much about it at the time because I was having a really good run.”

But four days later, when he was giving a talk on herbal medicines at the University of North Carolina’s continuing education center, in Chapel Hill, Kroll couldn’t forget the shirt he could have worn that would have saved him some torment. He felt like he was going to go crazy from the itch.

Fortunately for the pharmaceutical scientist, Joe and Terry Graedon, hosts of the public radio show “The People’s Pharmacy,” were in the audience that day. Longtime friends of Kroll’s, the Graedons approached him afterward, took one look at his sorry state, and recommended a hot water treatment.

According to the Graedons, who learned of the home remedy from an old dermatology textbook, hot water—as hot as can be tolerated without scalding the skin—provides a few hours’ reprieve from itch. Kroll, who went home immediately and took an extremely hot shower, swears by the treatment. “It was such a relief,” he says.

The book the Graedons referenced, “Dermatology: Diagnosis and Treatment” (Sulzberger, Marion B., et al. Chicago: Year Book Medical Publishers, 1961), turns out to have been ahead of its time. Not much was known about the molecular mechanisms behind itch when the book came out. But the theory behind the treatment, which researchers understand better now, is one of counterirritation, says Earl Carstens, a neuroscientist at the University of California, Davis. “The pain masks the itch,” he says. Other itch ointments that contain ingredients such as capsaicin, the pain-causing compound in hot peppers, work on a similar basis.

These days, neuroscientists know that the networks of nerves that cause itch and pain are hopelessly overlapped. Many of the itch-signaling protein receptors scientists have identified reside in the membranes of nerve cells, or neurons, that also transmit pain. In addition, some itch and pain receptors trigger the same membrane ion channels that cause neurons to fire. “The puzzle is how the brain separates out the pain from the itch” when it receives a signal generated by these neurons, says Robert H. LaMotte, a neurobiologist at Yale University.

LaMotte, Carstens, and other neuroscientists are trying to solve this puzzle. They’ve recently stepped up their efforts, thanks to molecular genetics techniques that can help pinpoint itch-regulating receptors. By removing from mice the genes that code for certain suspect receptors, researchers can evaluate whether those membrane proteins are part of the rodents’ itch circuits: in other words, inject the receptor-deficient mice with itch-causing compounds and see whether they still scratch. The itch receptors isolated from such tests could become drug targets.

Millions of people suffer from occasional itching or, as it’s called in the doctor’s office, pruritus, from the Latin prurire, meaning “to itch.” But it’s one of the most poorly understood sensations, LaMotte says. Scientists haven’t put that much effort into studying the common condition in the past because “it isn’t life threatening,” he says. “But it is life destroying. People are beginning to realize that.”

Patients with run-of-the-mill itch from poison ivy or bug bites, although in misery, don’t typically have their lives forever altered by itch because these cases are temporary. But people afflicted with chronic skin diseases and malignancies such as cutaneous T-cell lymphoma, a type of cancer in which malignant white blood cells concentrate in the skin, can itch uncontrollably if not treated. And even then, a small subset of those patients don’t respond to standard steroids and ultraviolet light therapies because their disease spreads beyond their skin. Those patients are so itchy, they can become suicidal, says Lynn A. Cornelius, a dermatologist at Washington University Medical Center, in St. Louis. “It is one of the most frustrating treatment dilemmas for both patient and physician,” she says.

Some non-skin-related disorders—particularly kidney and liver disease—also cause itch. Not much is known about how the sensation comes about in these patients, Cornelius adds. Doctors can use disease-targeted drugs on this type of itch, but “basically, what you try to do is treat the underlying disease and hope that if the disease gets better, then the itch gets better, which isn’t always the case,” she says.

Although the mechanisms of pain and itch appear to be intimately entwined, doctors and researchers have a better understanding of pain, a more obvious, compelling sensation, Carstens says. They’ve simply been studying it for longer. In 1974, researchers came together to form the International Association for the Study of Pain, which now has more than 7,000 members. It wasn’t until 2005 that itch got a society: the International Forum for the Study of Itch. That organization currently has only about 100 members.

Itch-related studies today are “where pain research was maybe 20 or 30 years ago,” says Cornelius, who is also codirector of the Center for the Study of Itch, which opened just this year in St. Louis. “People are only now starting to identify the various receptors involved in itch and unravel the pathways that transmit the sensation.”

So far, scientists know that common cases of itch begin when a chemical or mechanical stimulus contacts the skin. In the case of Kroll’s poison ivy, urushiol, an oily mixture of alkyl-substituted catechols, triggers an immune reaction at the skin surface when its components oxidize and become orthoquinones that bind to proteins on skin cells. For people like Kroll who are allergic to urushiol, inflammatory skin cells called mast cells respond by rushing in and releasing histamine, a notorious itch-causing chemical.

The histamine then binds to a histamine receptor—typically a G-protein-coupled receptor (GPCR) called H1R—in the membrane of a sensory nerve cell in the skin. A conformational change in H1R then triggers a cascade of events that eventually opens ion channels in the cell’s membrane, causing the nerve to send an electrical signal to the brain.

The ion channel activated by histamine, called TRPV1, has been investigated by hundreds of labs, according to Xinzhong Dong, a neuroscientist at Johns Hopkins University School of Medicine. But it wasn’t until a couple of years ago that researchers realized it was involved in itch. Up until that point, TRPV1, typically referred to as the “capsaicin receptor,” was thought of as a regulator of pain because it opens in response to the hot-pepper compound, Dong says.

But this channel also resides “downstream” of H1R in the same neuron membrane. According to Dong, histamine activates H1R, which works with a G protein to drive some phospholipases into action. These enzymes, which behave like middlemen in a business transaction, hydrolyze lipids that cause TRPV1 to open and generate an electrical signal.

But not all itch pathways are regulated by histamine, Dong says. “More than 70% of itch conditions in people are histamine independent,” he adds. That’s why antihistamines, which block the histamine-H1R interaction, work only in a limited number of cases.

Dong’s lab recently isolated in mice a GPCR, called MrgprA3, that does not respond to histamine but triggers itch when activated by the antimalarial compound chloroquine (Cell, DOI: 10.1016/j.cell.2009.11.034). This drug has eliminated malaria in many places around the globe. But chloroquine-induced itch can be so unbearable for some patients that they stop taking the drug, Dong says.

To figure out how the drug induces itch, Dong has collaborated with Diana M. Bautista, a pain researcher at UC Berkeley. They found that MrgprA3 activates an ion channel other than TRPV1 (Nat. Neurosci., DOI: 10.1038/nn.2789). This channel, TRPA1, is similar to the capsaicin receptor in that it responds to pain-causing compounds, but it reacts to molecules such as the isothiocyanate components of mustard oil and wasabi instead of the hot-pepper chemical. Bautista and Dong believe that phospholipases are not involved in communicating MrgprA3’s activation to the downstream TRPA1; a set of G proteins do all the go-between work.

A variety of other protein receptors have been implicated in itch, including the serotonin receptors called 5HTRs and a receptor called PAR2, which is activated by a protease enzyme from the tropical plant cowhage.

Scientists still know relatively little about the workings of all of these and other itch receptors in the sensory neurons of the skin. But they know even less about the receptors those nerves activate when they meet up with nerves in the spinal cord. Zhou-Feng Chen, an anesthesiologist at Washington University in St. Louis, has had some luck in that department, though. In 2007, he and his group reported that ­gastrin-releasing peptide receptor (GRPR)—another GPCR, but one that is expressed in spinal cord nerves—regulates itch (Nature, DOI: 10.1038/nature06029).

“We were trying to identify a new receptor in the spinal cord for pain therapeutics,” Chen recalls. “Initially, it was a disappointment because we didn’t find that the receptor had anything to do with pain sensation.” In preliminary studies, mice that Chen’s group engineered to lack GRPR still responded to pain normally.

But then the researchers injected gastrin-releasing peptide, the neurotransmitter ligand that activates GRPR, into the spinal cord of normal mice, and they started to scratch. “That was kind of an Aha! moment for us,” Chen says. “We realized, ‘Oh, we’ve got an itch receptor here.’ ” This was confirmed in later tests when the researchers injected GRPR-deficient mice with gastrin-releasing peptide and they scratched much less than their normal counterparts.

Since then, rather than just removing GRPRs from mice, Chen’s team has also gone so far as to destroy GRPR-expressing nerves in the spinal cords of mice to learn more about the itch pathway. With those neurons effectively removed, the mice scratched about 80 to 100% less than normal mice did when injected with itch-causing compounds such as histamine and chloroquine (Science, DOI: 10.1126/science.1174868). Meanwhile, the mice responded normally to pain.

“Our hypothesis is that the spinal cord may be the area that distinguishes between itch and pain,” Chen says. In the sensory neurons of the skin, “maybe it’s not their job to tell the difference,” he adds. “You don’t have to distinguish itch at every level. All you need is one place.”

The itch-pain connection has been debated for at least a century, UC Davis’ Carstens says. Thoughts on the matter of whether itch and pain have separate or identical pathways have “gone back and forth—like a ping-pong game,” he adds. On one side, neuroscientists originally thought that itch and pain were controlled by two completely different sets of neurons. But this theory has been ruled out, he adds.

On the other side, some neuroscientists thought at one time that itch and pain shared the same receptors and neurons. The theory goes that when those neurons fire at a high rate, the sensation a person experiences is pain, and when the neurons fire at a low rate, the sensation is itch. “Nobody believes that anymore either,” Carstens says.

What has emerged is a more complex, population-dependent view of pain and itch. Scientists now believe that some population of sensory neurons contains only pain receptors and responds only to pain. Then there are itch-regulating neurons that have receptors for both pain and itch. If a stimulus activates both neuron populations, pain occurs. But if an itchy stimulus triggers only the neurons that respond to both itch and pain, Carstens says, “what the brain interprets from that mix is an itch.”

Johns Hopkins’ Dong thinks he might have located the itch-regulating neurons of this population theory through his work with MrgprA3. By tagging that receptor in mice with various fluorescent proteins, he sees certain nerve fibers near the spinal cord and in the outer layer of the skin “light up.” Measuring those marked nerves with electrophysiology, Dong’s team has observed, in unpublished work, that those nerve cells respond to itch-causing compounds such as histamine as well as pain-generating compounds such as capsaicin.

The jury is still out on whether Chen’s GRPR work disagrees with the population theory, Carstens says. Just because Chen found an itch-regulating set of neurons that contains itch receptors, he adds, doesn’t mean that those neurons might not also carry some pain receptors.

But while Carstens mulls over Chen’s results and how they fit into the current itch-pain model, Chen is moving full-speed ahead to convert his findings into a treatment. Chen helped establish the Center for the Study of Itch in St. Louis and is now working with collaborator Robert H. Mach, a radiological chemist also at Washington University, to design compounds that block GRPR.

With the research arm of the itch center up and running, Chen’s codirector, Cornelius, now has the challenge of bringing the clinical arm up to speed. Until drug candidates are available, clinical trials remain on hold, Cornelius says. In the meantime, however, she explains that the center will start seeing patients, evaluating them, and banking their tissue and blood. That way, “we can have all this information gathered so that by the time peptides or other therapeutics are developed, we’ll have a whole group of patients ready for clinical trials.”

And desperate patients are already calling. “Our challenge right now,” Cornelius says, “is how to do these patients a service when we don’t have anything new to offer them.” At this point, she adds, “we need their help in finding out more about their diseases. Then we’ll be ready.”