Connecting the dots between two molecules whose levels are decreased in depression and increased by current antidepressants could yield new therapies, researchers say. Serotonin is a neurotransmitter that enables brain cells to communicate and brain-derived neurotropic factor, or BDNF, is a brain-nourishing molecule that also aids connectivity. Popular antidepressants such as Prozac, developed to increase levels of serotonin, have recently been found to also increase BDNF levels.