<unk> s have the distinct structure of a benzene ring with two hydroxyl groups, an intermediate ethyl chain, and a terminal amine group. Phenylethanolamines such as norepinephrine have a hydroxyl group on the ethyl chain.
Catecholamine
Octopamine
Synephrine
Dopamine 
Dopamine
Serotonin
Somatostatin
Neuropil