Neurotransmitters are naturally occurring brain chemicals that likely play a role in clinical depression. When these chemicals are out of balance, it may be associated with depressive symptoms. Of the 30 or so neurotransmitters that have been identified, researchers have discovered links between clinical depression and the function of three main ones: serotonin, norepinephrine, and dopamine ("Biological Causes of Clinical depression," 2013). These three neurotransmitters function within parts of the brain that regulate emotions, reactions to stress, and the physical desires of sleep, appetite, and sexuality. Structures that have received a great deal of attention from clinical depression researchers include the limbic system and hypothalamus ("Biological Causes of Clinical depression," 2013).
Many researchers of clinical depression have been interested in the limbic system, which is the area of the brain that regulates emotions, physical and sexual drives, and the response to stress ("Biological Causes of Clinical depression," 2013). The hypothalamus is a small structure located at the base of the brain and is responsible for many basic functions such as body temperature, sleep, appetite, sexual drive, stress reaction, and the regulation of other activities. The hypothalamus also controls the function of the pituitary gland which in turn controls key hormones. Because the activities of the limbic are so important and complex that irritation to any part of it, including how neurotransmitters function, could affect a person's mood and behavior ("Biological Causes of Clinical depression," 2013.
Research shows that the hippocampus is smaller in some depressed people. For example, in one fMRI study (author, date) published in The Journal of Neuroscience, investigators studied 24 women who had a history of clinical depression ("What causes clinical depression?" 2009). On average, the hippocampus was 9% to 13% smaller in depressed women compared with those who were not depressed ("What causes clinical depression?" 2009).