How does serotonin affect mood?

The answer

Serotonin (5-hydroxytryptamine or 5-HT) is a neurotransmitter that modulates mood, anxiety, sleep, appetite, and social behavior. Despite its association with the brain, approximately 90% of the body's serotonin is produced by enterochromaffin cells in the gastrointestinal tract, where it regulates gut motility and secretion. In the brain, serotonin is synthesized primarily in the raphe nuclei of the brainstem and projected widely to the cortex, limbic system, and other regions. Low serotonin activity has been associated with depression, anxiety, and impulsivity, which is why selective serotonin reuptake inhibitors (SSRIs) are a major class of antidepressants. However, the 'chemical imbalance' theory of depression is an oversimplification; serotonin interacts with at least 14 different receptor subtypes and works in concert with many other neurotransmitter systems.