<unk>  is<unk> line molecule that has been<unk> etylated at the oxygen atom. Because of the charged ammonium group,<unk>  does not penetrate lipid membranes. Because of this, when the molecule is introduced externally, it remains in the extracellular space and does not pass through the blood–brain barrier.
Acetylcholine
Serotonin
N-Acetylserotonin
Neuropil
Beta-Endorphin
Opioid peptide
Gluten exorphin
Glutamate 