serotonin
Serotonin is a messenger chemical best known for its influence on mood, but it also helps regulate sleep, appetite, anxiety, and even gut movement. In fact most of the body's serotonin is in the gut, not the brain — a reminder that one molecule can do very different jobs depending on where it acts.
Serotonin, formally 5-hydroxytryptamine (5-HT), is a monoamine neurotransmitter. It acts on a large family of serotonin receptors (5-HT1 through 5-HT7), nearly all G-protein-coupled except the 5-HT3 ion channel. After release it is cleared from the synapse by the serotonin transporter (SERT), which is the key handle that several CNS drugs grab.
Drugs targeting serotonin are central to psychiatry and beyond: SSRIs and SNRIs block its reuptake to treat depression and anxiety, certain antipsychotics block 5-HT2A receptors, triptans activate 5-HT receptors to abort migraines, and 5-HT3 antagonists like ondansetron control nausea.
Pushing serotonin too high carries a specific danger: serotonin syndrome, a potentially life-threatening reaction with agitation, fever, tremor, rapid heart rate, and muscle rigidity, usually triggered by combining serotonergic drugs such as an SSRI with an MAO inhibitor.
Combining an SSRI with an MAO inhibitor is avoided because both raise serotonin, and together they can push levels into dangerous serotonin syndrome.
Stacking serotonergic drugs is the classic trigger for serotonin syndrome.
Serotonin's reach beyond the brain explains common SSRI side effects: because the gut is rich in serotonin and 5-HT3 receptors, raising serotonin there often causes nausea, especially early in treatment.