- Star Trek: The Motion Picture
- Star Trek II: The Wrath of Khan
- Star Trek III: The Search for Spock
- Star Trek IV: The Voyage Home
- Star Trek V: The Final Frontier
- Star Trek VI: The Undiscovered Country
- Star Trek Generations
Biochemical substance that, among other unknown effects, acts as a central neurotransmitter in humanoid nervous systems, making them susceptible to influence. On stardate 45208.2, the addictive Ktaran game initiated a serotonin cascade in the frontal lobe of the brain, purposefully designed to control those who played the "game."