Serotonin is a neurotransmitter with functions in various systems of the body. In the central nervous system serotonin works to regulate mood, appetite, sleep, memory and learning. When present, the neurotransmitter acts as something of a connector between cells, and is associated with a feeling of happiness or decreased anxiety. Serotonin is a major part of many popular drug treatments for depression and anxiety. SSRI's or Selective Serotonin Re-Uptake Inhibitors (like Prozac and Zoloft) help to maintain the serotonin connection between cells.