The urge to scratch a mosquito bite or skin rash can be maddening. Now, scientists have pinpointed a group of neurons that signal it's time to relieve the itch.
Disabling the neurons eliminated itching in mice, which are thought to be a good analogue to humans for neurobiology studies.
The work could pave the way for treatment of serious human itches, such as psoriasis and eczema, said study scientist Zhou-Feng Chen of Washington University School of Medicine in St. Louis.
Until now, scientists have puzzled over whether the tickly torture is just a less intense version of pain. The new finding suggests it's not.
Mice are "one of the best models for itching, because so far we have found that all substances which can cause itching sensations in humans can also cause scratching behaviors in mice," Chen told LiveScience.
The research builds on work by Chen and his colleagues reported in 2007 in which they identified the first gene in the spinal cords of mice linked with itching, called gastrin-releasing peptide receptor (GRPR). Humans and other animals, such as frogs and chickens, also have this gene.
Here's how it works: Something causes the skin to itch, releasing a chemical that binds with the receptor, in turn, activating a nerve cell in the spinal cord that transmits this itch information to nerve cells in the brain.
But like all cells, the neuron linked with GRPR contains many genes. So the researchers wanted to find out if this neuron was exclusively sending itch information or if it contained other genes perhaps related to pain.
"A key question was whether those GRPR neurons also were transmitting pain signals," Chen said.
Chen's team essentially killed the GRPR neurons in some mice and then exposed the rodents to both itchy and painful stimuli. The modified mice didn't scratch but still responded to pain; in fact, their scratching behaviors were reduced by more than 80 percent or completely eliminated in some instances compared with their normal littermates, the researchers note in the Aug. 6 issue of Science Express, the advance online publication of the journal Science.
The findings held for all types of itch-causing substances, including those that release histamine (think bug bites and allergic reactions that can be treated with Benadryl) and other agents responsible for chronic, severe itching not eased by antihistamines.
Chen said he has many more questions, such as how the GRPR neuron passes information to the brain and what other itch-specific genes the neuron contains.
The research was supported by the National Institutes of Health and Washington University.