How Itching An Itch Makes It Itch Worse

Thinking of scratching that itch? Think again. A team of researchers led by Zhou-Feng Chen of Washington University School of Medicine in St. Louis has determined that scratching causes the brain to release serotonin, which in turn prods nerve cells in the spinal cord to amplify the itch sensation (Neuron 2014, DOI: 10.1016/j.neuron.2014.10.003). The researchers hope that therapeutically interrupting the itch-scratch cycle in the spinal cord might one day bring relief to patients suffering from chronic itching. “People would rather suffer a little pain than itch,” Chen says, explaining that the mild pain signal a scratch sends to the brain overrides itch. The problem is that the brain tries to dampen this pain by releasing serotonin. After eight years of study, Chen’s team demonstrated that serotonin not only dampens pain, it also increases itch. In one experiment, the researchers used a group of mice lacking serotonin receptors in their spinal cords. When injected with the itch-inducing compound chloroquine, these rodents scratched far less than their normal counterparts. Other experiments unveiled that serotonin-amplified itch occurs following the coactivation of two nerve cell receptors in the spine—the serotonin receptor 5-HT1A and the itch-related receptor GRPR—although how they interact remains unknown.