Neurotransmitter release at rapid synapses

Abstract

The classical concept of the vesicular hypothesis for acetylcholine (ACh) release, one quantum resulting from exocytosis of one vesicle, is becoming more complicated than initially thought. 1) synaptic vesicles do contain ACh, but the cytoplasmic pool of ACh is the first to be used and renewed on stimulation. 2) The vesicles store not only ACh, but also ATP and Ca²⁺ and they are critically involved in determining the local Ca²⁺ microdomains which trigger and control release. 3) The number of exocytosis pits does increase in the membrane upon nerve stimulation, but in most cases exocytosis happens after the precise time of release, while it is a change affecting intramembrane particles which reflects more faithfully the release kinetics. 4) The SNARE proteins, which dock vesicles close to Ca²⁺ channels, are essential for the excitation-release coupling, but quantal release persists when the SNAREs are inactivated or absent. 5) The quantum size is identical at the neuromuscular and nerve-electroplaque junctions, but the volume of a synaptic vesicle is eight times larger in electric organ; at this synapse there is enough ACh in a single vesicle to generate 15–25 large quanta, or 150–200 subquanta. These contradictions may be only apparent and can be resolved if one takes into account that an integral plasmalemmal protein can support the formation of ACh quanta. Such a protein has been isolated, characterised and called mediatophore. Mediatophore has been localised at the active zones of presynaptic nerve terminals. It is able to release ACh with the expected Ca²⁺-dependency and quantal character, as demonstrated using mediatophore-transfected cells and other reconstituted systems. Mediatophore is believed to work like a pore protein, the regulation of which is in turn likely to depend on the SNARE-vesicle docking apparatus.

Introduction

The release of peptides, monoamines or rapid neurotransmitters displays a variety of different kinetics and Ca²⁺-dependencies even when tested on the same cell, suggesting differences in the underlying molecular mechanisms [1], [2]. In the ‘kiss-and-run’ process the vesicles recycle without undergoing full fusion with the plasma membrane, release occurring through a fusion pore [3], [4]. In another case, the core of the granules is transiently exposed at the external surface of the secreting cell but is re-internalised by endocytosis a few minutes after the secretion stimulation [5]. Also channels located in vesicle membrane and/or at the plasmalemma were proposed to modulate secretion and to bring appropriate charges for exchange with secretion products bound in the granule matrix [6]. Channels have also been proposed at the plasma membrane to establish fusion pores, connecting the vesicle lumen with the extracellular space [7]. In the present review, it will be shown that transmitter-specific channels may ensure the formation of neurotransmitter quanta [8]. Such a diversity will probably help to solve many discrepancies encountered in the past among the data obtained using different approaches and different preparations. Our aim here is to gather ancient and recent observations concerning the release of acetylcholine (ACh) in rapid synapses, which are defined as synapses operating in the millisecond range and where, in addition to a vesicular compartment, the cytosolic concentration of neurotransmitter is relatively high (mM range).