Serotonin was a neurotransmitter.
In 2152, Phlox's cortical scan of Travis Mayweather revealed elevated levels of serotonin and several other neurotransmitters in his brain. (ENT: "Singularity")
One of the effects of the Ktarian game was increased serotonin production, generating a feeling of euphoria that added to its addictive properties. (TNG: "The Game")
Tampering by solanogen-based lifeforms caused elevated levels of serotonin to concentrate in the hippocampus of several crewmembers of the USS Enterprise-D. Doctor Beverly Crusher realized that this was caused by a neuro-sedative used by the aliens to sedate the victims of their experiments. (TNG: "Schisms")
Tom Paris exhibited elevated serotonin levels in his hypothalamus after his transwarp flight in 2372. (VOY: "Threshold")
Ensign Vorik had elevated serotonin levels during Pon farr in 2373. (VOY: "Blood Fever")
In 2375, The Doctor found that Seven of Nine's serotonin levels were low during a regeneration cycle. He described this as normal and thus it did not account for interruptions in the cycle. (VOY: "Infinite Regress")
In 2379, Dr. Crusher found that Deanna Troi's serotonin levels were slightly elevated after a telepathic attack by the Reman Viceroy. (Star Trek Nemesis)