Serotonin
From Memory Alpha, the free Star Trek reference.
Serotonin is a neurotransmitter.
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 solanagen-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")
