Antidepressants such as Prozac are not instant mood-lifters. But researchers have found clues to the delayed response and common return of depressive symptoms when taking serotonin-related antidepressants. Rather than activating all of the brain's serotonin receptors, as current drugs do, their study suggests there's just one critical serotonin receptor important to relieving depression and anxiety. It opens the door to more effective treatment for the 20.9 million Americans with depressive disorders.