The study, published in the Journal of Neuroscience, said a laboratory test could confirm a depression diagnosis and tell if antidepressant therapy is working.

"We discovered that in depressed individuals a signaling protein is located in specific areas of the cell membrane called lipid rafts," study leader Mark Rasenick of the University of Illinois at Chicago College of Medicine said in a statement. "This protein -- Gs alpha -- activates adenylyl cyclase, a link in signal transduction, and is responsible for the action of neurotransmitters such as serotonin."

Rasenick defines "rafts" as viscous "almost gluey" areas, that either help or slow down communication between membrane molecules. When Gs alpha is caught in them, adenylyl cyclase activation is reduced.

"This new study shows that in depressed humans, Gs alpha protein is confined in lipid rafts, where it's less likely to mediate the action of neurotransmitters, and that antidepressants have the opposite effect," Rasenick said. "In simple language -- we may be able to tell you if you are depressed and more importantly, whether you are responding to the chosen antidepressant therapy."