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Endocytosis in Neurons: Editorial Introduction and Overview

Neurons transmit information by the regulated release of neurotransmitters that bind to specific receptors at the target cell, where they alter themembrane potential and/or regulate second messenger cascades. Before release, neurotransmitters are stored within presynaptic nerve endings in specialized storage compartments, the synaptic vesicles. When an action potential arrives in the nerve terminal, the presynaptic membrane depolarizes, voltagegated calcium channels open and the influx of calcium ions in turn triggers exocytosis of synaptic vesicles. Vesicles are then retrieved by endocytosis, and after being reloaded with transmitter by means of specific carriers in the vesicle membrane, they re-enter the pool of fusion-competent vesicles. There is an enormous variability between different types of synapses, ranging from small central nervous system synapses that release on an average less than one vesicle per action potential to the ‘industrial style’ giants among the synapses such as the neuromuscular junction or the Calyx of Held in the auditory pathway, which contain up to a few hundreds of thousands of vesicles.