Acetylcholine

Acetylcholine was a neurotransmitter common to the neural structures of most humanoid brains. It played a role in promoting aggressive tendencies. (VOY: "The Chute")

When the crew of Enterprise (NX-01) was possessed by Wisps, the possessed state was characterized by elevated acetylcholine levels in the autonomic nervous system. (ENT: "The Crossing")

Spock performed an acetylcholine test on a huge spaceborne amoeba creature that destroyed the Gamma 7A System in 2268, although McCoy felt the test was improperly done. (TOS: "The Immunity Syndrome")

As it is a choline compound, acetylcholine can be used to stabilize synapses in cases of nervous system collapse. However, it was not effective against the effects of an alien weapon that killed USS Voyager Ensign Ahni Jetal. (VOY: "Latent Image")

When acetylcholine absorption is stimulated, it increases tensile strength of the muscle tissue. This was done during a neuromuscular adaptation treatment. (DS9: "Melora")

An increase of acetylcholine within the hippocampus could indicate that the person had accumulated memories. A thirteen percent increase would indicate an accumulation of two days' worth of memories. (TNG: "All Good Things...")

The Akritirian clamp stimulates the production of acetylcholine in the hypothalamus, causing irritation and aggression. (VOY: "The Chute")