Serotonin was a chemical neurotransmitter present in the brains of many animals, including Humans and other Humanoid species.
Doctor Jo Stern noted elevated levels of serotonin and several other neurotransmitters in Ven Kaldarren when he was "possessed" by Uramtali, one of the dithparu discovered trapped within the Well of Souls. (TLE novel: Well of Souls)