Serotonin was a neurotransmitter.
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")
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")