March 12 (HealthDay News) -- Researchers say they've discovered a biomarker for depression that could lead to a quick lab test to determine whether a particular antidepressant is making headway against the disease.

"This may be a very simple biochemical indicator for depression," said study co-author Mark Rasenick, director of the Interdisciplinary Neuroscience Program at the University of Illinois at Chicago. The test "wouldn't tell you which [medication] to start, but it would tell you if the one you're taking is working."

It may even be possible to use the test to determine whether rounds of psychotherapy are reaping any benefit, he said.

For now, however, such a test is a hypothetical, pending further exploration of the finding reported in the March edition of The Journal of Neuroscience.

At issue is whether the brain itself shows physical or chemical signs of depression.

The researchers looked at the interaction of neurotransmitters and a protein called Gs alpha. In brain cells, the protein acts like a kind of butler, passing messages from neurotransmitters on the outside and amplifying their messages, Rasenick explained.

When the protein is working properly, it's like a butler whose "hands are just flying, cooking and cleaning at the same time," he said. But when the brain is depressed, "it just sits there in the corner."

In this post-mortem study, the researchers looked at the protein in the brains of 18 depressed people who committed suicide and compared them to the brains of non-depressed people. They found the protein would have worked less effectively in the brain cells of the suicide victims.

The findings raise the prospect of a blood test that would measure within days whether antidepressants are effectively treating depression, Rasenick said.

Now, it can take several weeks for patients and psychiatrists to figure out if an antidepressant is working properly. According to Rasenick, only about 30 percent of depression patients will respond to a specific drug.

"Unfortunately, we have a very poor ability to predict which antidepressant might be more effective for any individual," said Dr. Gregory Simon, a psychiatrist and mental health researcher with Group Health Cooperative in Seattle. "There's a long history of research using patterns of symptoms or biological measures -- chemicals measured in blood or spinal fluid -- to predict response to a particular antidepressant. None of those hoped-for predictors have significant value."

Genetic tests may provide some clues, he said, and the new study suggests there might be another approach. "It would not eliminate trial-and-error, but it would reduce the waiting time with each trial. But it's a long way from a study like this one to a test that's useful to patients and doctors."

Rasenick is hopeful, however, and he said the proposed test could do more than gauge whether drugs are working. It could conceivably measure the effectiveness of talking to a therapist, study co-author Rasenick said, since psychiatrists think psychotherapy has a physical effect on the brain.

Rasenick said more research and money are needed. The study was funded by the U.S. Public Health Service and the American Foundation for Suicide Prevention.

"The next step is to partner with people who are doing large-scale studies on individuals with depression and begin to look at this," he said. "We can begin to get hundreds of people and see if we can confirm that we're right."

SOURCES: Mark Rasenick, Ph.D., distinguished university professor, physiology and biophysics and psychiatry, and director, Interdisciplinary Neuroscience Program, University of Illinois at Chicago; Gregory Simon, M.D., M.P.H., psychiatrist and mental health researcher, Group Health Cooperative, Seattle; March 2008, The Journal of Neuroscience