Serotonin
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. ()

One of the effects of the Ktarian game was increased serotonin production, generating a feeling of euphoria that added to its addictive properties. ()

Tampering by solanogen-based lifeforms caused elevated levels of serotonin to concentrate in the hippocampus of several crewmembers of the . Doctor Beverly Crusher realized that this was caused by a neuro-sedative used by the aliens to sedate the victims of their experiments. ()

Tom Paris exhibited elevated serotonin levels in his hypothalamus after his transwarp flight in 2372. ()

Ensign Vorik had elevated serotonin levels during pon farr in 2373. ()

In 2374 The Doctor found that Kes' serotonin levels were 62% above normal, which accounted for her sleeping less and experiencing a lot of energy. ()

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. ()

In 2379, Dr. Crusher found that Deanna Troi's serotonin levels were slightly elevated after a telepathic attack by the Reman Viceroy. ()

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de:Serotonin
Biochemical compounds