A new protein may play a key role in helping to treat depression, say scientists studying the illness.

Serotonin is a mood-regulating chemical that allows nerve cells in the brain to communicate with each other.

Most antidepressants work by boosting levels of serotonin in the brain.

Little is known about how antidepressants work to relieve symptoms.

But scientists don't know what causes depression or exactly how serotonin works. Swedish, American and French researchers designed laboratory experiments to try and unravel the mystery.

The team, led by Nobel laureate and neuroscientist Paul Greengard, discovered that the new p11 protein increases the number of places serotonin can bind to on the surface of brain cells.

Lab rodents bred to have a depression-like illness showed lower levels of the protein, the researchers reported in Friday's issue of the journal Science. The depressed mice were not as mobile and less responsive.

Rats and mice given two older classes of antidepressants and shock therapy showed higher levels of the protein compared to control animals.

The three treatments work in different ways but all caused the same biochemical change, adding to the evidence that p11 is linked to the therapeutic benefits of antidepressant drugs, Greengard said.

"Overall, this finding represents compelling evidence that p11 has a pivotal role in both the cause of depression and perhaps its successful treatment," wrote Trevor Sharp, a pharmacologist at Oxford University in a journal commentary.

The findings won't immediately lead to new drug treatments but could offer future targets, said Per Svenningsson, a professor of physiology and pharmacology at the Karolinska Institute in Stockholm.

The research was sponsored by the U.S. Public Health Service and the Swedish Research Council. One of the study's authors works for the pharmaceutical company, Eli Lilly.