As you see one member of each pair of cotransmitters appears to be peptide. This fact and several unique features of neuropeptides indicate the possibility that the large variety of peptides in nerve terminals may not be neurotransmitters in the classical sense, that is, transmitting messages to the postsynaptic cell based on the frequency of action potentials arriving from the cell body. Rather, they may play some more subtle, long-term role in such information transfer, possibly enhancing the message of another “primary” transmitter.

The monoamines were the first CNS transmitters to be identified and rapid advanced came in the mid-1960s, during a remarkable decade of progress – the "monoamine years"- when a combination of neurochemistry and neuropharmacology led to many important discoveries about the role of CNS transmitters and about the ability of drugs to influence these systems.