By LAURAN NEERGAARD (AP Medical Writer)
WASHINGTON, D.C. – Got an itch to scratch? Scientists have pinpointed a key group of cells that sends itch-alerts to the brain. When researchers at Washington University in St. Louis knocked out those cells in mice, it alleviated their itchiness without affecting their ability to sense pain – work that opens a possible new target for creating better itch relievers.
Don’t underestimate that need. The kind of itch caused by bug bites or allergies typically goes away with a little scratching or some antihistamines. But some people can scratch themselves raw without relieving serious, daily itching triggered by a variety of conditions, such as certain cancers, chronic kidney failure, and even use of certain narcotic pain relievers.
Indeed, pain and itch have been difficult to separate. Previous research has found various nerve pathways that seem involved in both.
But Thursday’s report in the journal is the first to identify itch-specific cells in the spinal cord, that highway that delivers sensation to the brain.
"It’s exciting," said well-known itch specialist Dr. Gil Yosipovitch of North Carolina’s Wake Forest University Baptist Medical Center, who wasn’t involved in the new research. "This comprehensive study opens the field."
Lead researcher Zhou-Feng Chen, a Washington University associate professor of anesthesiology, in 2007 discovered the first gene related to itchiness, named GRPR. They found that mice with an inactive version of the gene scratched less when exposed to itchy things than mice with an active gene.
But that didn’t prove that the spinal cord neurons, or nerve cells, that harbored this gene were itch-specific. They could also be important to genes related to pain sensation.
So this time, Chen’s team injected the spinal cords of mice with a neurotoxin that seeks out the GRPR receptor, sort of a docking site. Over about two weeks, the toxin killed about 80 percent of the cells that harbored that gene.
Before those injections, the mice scratched vigorously. But after the itch cells were killed off, their scratching plummeted – in some cases stopped completely – when Chen introduced one after another itchy substance.
They weren’t simply numbed: Their motor function remained normal, and so did their response to pain from heat and pressure in a series of common experiments that show animals flick their tails or pull away their paws during various stresses.
This isn’t the only itch pathway to the brain, stressed Wake Forest’s Yosipovitch. Nor does anyone know if this gene would behave similarly in people. But researchers have been hunting itch-specific receptors in hopes of eventually learning how to block their "scratch-me" signals to the brain and help relieve at least some types of itch.