Why does scratching an itch make it worse?

The act of scratching replaces the itching with a sensation of mild pain, which is why the brain releases the neurotransmitter serotonin, according to the researchers.

"But as serotonin spreads from the brain into the spinal cord, we found the chemical can 'jump the tracks,' moving from pain-sensing neurons to nerve cells that influence itch intensity," says senior investigator Zhou-Feng Chen, PhD, director of Washington University's Center for the Study of Itch.

Dr. Chen says the study is the first to link serotonin to the itching sensation.

In the study, Dr. Chen's team bred a line of mice that lacked the genes necessary to make serotonin.

When the researchers injected the mice with a substance that provokes itching, the mice didn't scratch as much as their non-genetically engineered counterparts.

Injecting these genetically engineered mice with serotonin, however, caused them to scratch every bit as much as the normal mice upon receiving itch-inducing compounds.

"Scratching can relieve itch by creating minor pain," says Dr. Chen. "But when the body responds to pain signals, that response actually can make itching worse."

Blocking serotonin, he says, is not the solution because it is involved in growth, aging, bone metabolism and in regulating mood as well as helping control pain.

The best solution, according to Dr. Chen, is to take Mom's advice and not to scratch.