On the Evolution of Neurochemical Transmission

A discussion of the evolution of neurochemical transmission is divided into three main topics: evolution of biochemical signalling devices, evolution of neurotransmitter substances, and evolution of signal meaning. Models of signalling devices are developed from a primitive chemoceptive process through open and closed loop communications to a neuronal commications network and to its development into a symbolic logic exchange. The evolution of neurotransmitter substances is extrapolated from experimental evidence which has been obtained under primitive earth atmosphere conditions. Examples from comparative biology suggest that the evolution of transmitter use was not unidirective and that purine derivatives may well have been the primordial transmitter substances. The classical neurotransmitters, such as acetylcholine and norepinephrine have a limited information content in their molecular structure, whereas inherent message content of peptidergic transmitters is potentially significant. If there are mnemotypic genes, they may be expressed as informational macromolecules which specify behavioral patterns. Such information transfer would represent a second order of neurochemical transmission and its evolution would be closely coupled to that of molecules which contain a universal meaning.