Serotonin is a monoamine neurotransmitter that is synthesized in the central nervous system and can also be found in the digestive tract and platelets of the animals and humans.
It's known that in the central nervous system, it has an important role as a neurotransmitter, taking part in the inhibition of anger, body temperature, sleep, vomiting, sexuality, and appetite. The antidepressant drugs can alter levels of serotonin.
In the episode Distractions, the medical case is related with the antidepressants and the serotonin.