The third chemically distinct form of neurotransmitters are the peptides-- chains
of amino acids linked in peptide chains of 2-44 amino acids.  Some peptides share
short  sequences with  identical amino acids; such structurally related peptides
form "families", such as the opioid peptides (endorphins), the substance P-K
peptides, or the pancreatic polypeptide family.  Peptides frequently (always?) 
co-exist within synapses with non-petide transmitters.  Peptides serve in  still
undefined ways to embellish those small molecule transmitters'  effects. The
peptides are found in amounts of pM/mg protein.