Neurotransmitter
A neurotransmitter is a neurogenic chemical in the brain of a lifeform (humanoids for example) found inside neurons. These chemicals facilitate the transmission of neural energy between synapses. 

When Captain Jean-Luc Picard was under the influence of the Kataan probe, his neurotransmitter production was off the scale, presumably because his brain was processing Kamin's lifetime of memories in a very short time. ()

Following the death of Sev Maylor, Doctor Beverly Crusher discovered abnormally high neurotransmitter levels in her cerebral cortex. Later, when Ves Alkar was using Counselor Deanna Troi as a vessel for his negative emotions in the same manner he had used Maylor, Nurse Alyssa Ogawa found her neurotransmitter levels to be at 300% above normal and rising. They eventually reached over 420%, prior to her temporary death induced in order to force Alkar to terminate his link with her. Upon revival, Crusher and Ogawa performed a decontamination sequence to purge the elevated levels. ()

In 2371, The Doctor found Tom Paris neurotransmitter levels were normal when looking for evidence that Paris' memories had been tampered with. ()

In 2375, Seven of Nine began experiencing a Borg equivalent of multiple personality disorder due to proximity to a Borg vinculum. The vinculum was taken aboard  in order to try to disable its effects. When Seven of Nine came near it, she began hearing voices, but The Doctor adjusted her neurotransmitter levels and the effects faded. ()

Types of neurotransmitters 
Acetylcholine
Dopamine
Endorphin
Epinephrine
Isoboramine
Norepinephrine
Psilosynine
Serotonin

External link 
 

de:Neurotransmitter
Biochemical compounds