Ultrastructural basis of monoamine and acetylcholine function in CNS

Recent immunoelectron microscopic studies of the acetylcholine innervation in adult rat cerebral cortex, neostriatum and hippocampus reveal a frequency of synaptic relationships as low for these axon terminals (varicosities) as previously demonstrated for the noradrenaline innervation in cerebral cortex or hippocampus, and the serotonin innervation in neostriatum. The dopamine system is more synaptic, but shows regional variability in this regard (50–100% synaptic in cerebral cortex and 30–40% in neostriatum). The microenvironment of non-synaptic varicosities consistently lacks the enrichment in dendritic spines commonly observed around control populations of unlabeled terminals randomly selected from the same electron micrographs. It is hypothesized that in brain regions densely innervated by varicosities that are mostly non-junctional (e.g. acetylcholine and dopamine in neostriatum), a basal level of transmitter is permanently maintained around all cellular elements, contributing to the modulatory properties of the corresponding systems.