Have an urge to scratch that itch? The cause is now being studied at even the cellular level.
The experiment was not for the squirmish. Volunteers were made to itch like crazy on one arm, but not allowed to scratch. Then they were whisked into an MRI scanner to see what parts of their brains lit up when they itched, when researchers scratched them and when they were finally allowed to scratch themselves.
The scientific question was this: Why does it feel so good to scratch an itch?
“It’s quite intriguing to see how many brain centers are activated,” said Dr. Gil Yosipovitch, chairman of dermatology at the Temple University School of Medicine and director of the Temple Center for Itch. “There is no one itch center. Everyone wants that target, but it doesn’t work in real life like that.”
Instead, itching and scratching engage brain areas involved not only in sensation, but also in mental processes that help explain why we love to scratch: motivation and reward, pleasure, craving and even addiction. What an itch turns on, a scratch turns off — and scratching oneself does it better than being scratched by someone else.
Itching was long overshadowed by pain in both research and treatment, and was even considered just a mild form of pain. But millions of people suffer from itching, and times have changed. Research has found nerves, molecules and cellular receptors that are specific for itching and set it apart from pain, and the medical profession has begun to take it seriously as a debilitating problem that deserves to be studied and treated.
Within the past decade, there has been a flurry of research into what causes itching and how to stop it. Along with brain imaging, studies have begun to look at gene activity and to map the signals that flow between cells in the skin, the immune system, the spinal cord and the brain.
The concern is not so much the fleeting nastiness of mosquito bites and poison ivy, but the unending misery caused by chronic itching — the kind that won’t go away, that torments people night and day and very often resists remedies like antihistamines and cortisone cream.
For the first time in the United States, itching research and treatment centers have opened: Temple’s in September, in Philadelphia, and Washington University’s Center for the Study of Itch, in 2011, in St. Louis.
“Itch is now where pain was probably 20 years ago,” said Dr. Lynn Cornelius, chief of the dermatology division at Washington University School of Medicine. “It used to be lumped together with pain.”
But now, she said, there is more interest in itching and in sorting out its different types, and more research money being spent on it.
“The science has to lead to treatment, I believe,” Cornelius said. “If that happens, it will translate to better and better, more targeted therapies, so clinicians won’t just look upon someone itching as someone who needs antihistamines.”
Scratching, and therefore itching, appear widespread in the animal kingdom — though no one knows for sure why animals claw, bite or peck themselves, or scrape against trees or fences.
Even fruit flies engage in “robust grooming behaviors” that look a lot like scratching when they are infected with mites, said Diana Bautista, an assistant professor of cell and developmental biology at the University of California, Berkeley. Her research includes studying various strains of itchy mice that are models for human ailments.
“I have a collection of movies showing different animals scratching,” Bautista said. “I’m hoping they will help me determine if there is a difference between itch-evoked scratching versus wiping and other behaviors in diverse species.”
One of her favorite videos shows a seal lying on the beach, briskly rubbing its head with a flipper.
Scratching that human itch
In people, there are different types of itching. The most familiar type, from a mosquito bite or hives, occurs when cells in the skin release histamine, which causes nerves in the skin to fire off signals to the spinal cord and brain. Antihistamine pills or creams usually bring relief.