Messenger Substances of Neurons and Paraneurons: Their Chemical Nature and the Routes and Ranges of Their Transport to Targets

Neurons and paraneurons (peptide/amine producing endocrine and sensory cells) share cell-biological features and chemistry of secretions. The bioactive substances or chemieal messengers secreted by neurons and paraneurons (peptides, amines, acetylcholine, ATP, etc.) are conveyed to their targets which may be opposed only several nanometers apart or may be located at more or less remote sites in the body. The modes and ranges of the transport of different messengers from different neurons and paraneurons are reviewed. In order to visualize the gradational ranges of messenger transport, the following terms were defined: synaptocrinia, paracrinia, microhemocrinia (pericrinia), mesohemocrinia (mesocrinia) and macrohemocrinia (telecrinia). Transmitters and hormones are thus continuous to each other both in their chemical nature and in the routes and ranges of their transport to the targets. This review further analyses the implications of the concept of internal secretion (endocrinia). The classical concept of endocrinia seems unable to account for the endocrine-exocrine partition of secretions ('leakage' of hormones into exocrine lumen and that of exocrine secretions into blood circulation) of which the mechanism remains to be elucidated in future studies.