The non-neuronal cholinergic system in humans: Expression, function and pathophysiology

Acetylcholine, a prime example of a neurotransmitter, has been detected in bacteria, algae, protozoa, and primitive plants, indicating an extremely early appearance in the evolutionary process (about 3 billion years). In humans, acetylcholine and/or the synthesizing enzyme, choline acetyltransferase (ChAT), have been found in epithelial cells (airways, alimentary tract, urogenital tract, epidermis), mesothelial (pleura, pericardium), endothelial, muscle and immune cells (mononuclear cells, granulocytes, alveolar macrophages, mast cells). The widespread expression of non-neuronal acetylcholine is accompanied by the ubiquitous presence of cholinesterase and receptors (nicotinic, muscarinic). Thus, the non-neuronal cholinergic system and non-neuronal acetylcholine, acting as a local cellular signaling molecule, has to be discriminated from the neuronal cholinergic system and neuronal acetylcholine, acting as neurotransmitter. In the human placenta anti-ChAT immunoreactivity is found in multiple subcellular compartments like the cell membrane (microvilli, coated pits), endosomes, cytoskeleton, mitochondria and in the cell nucleus. These locations correspond with the results of experiments where possible functions of non-neuronal acetylcholine have been identified (proliferation, differentiation, organization of the cytoskeleton and the cell-cell contact, locomotion, migration, ciliary activity, immune functions). In the human placenta acetylcholine release is mediated by organic cation transporters. Thus, structural and functional differences are evident between the non-neuronal and neuronal cholinergic system. Enhanced levels of acetylcholine are detected in inflammatory diseases. In conclusion, it is time to revise the role of acetylcholine in humans. Its biological and pathobiological roles have to be elucidated in more detail and possibly, new therapeutical targets may become available.