Imaging transmitter release. I. Peeking at the steps preceding membrane fusion

Over the recent year we have witnessed considerable advances in the study of neurotransmitter release. This progress has been severalfold as different techniques have allowed us to characterise many steps along the process of exocytosis, membrane fusion, formation of the fusion pore, and have given insight in the kinetics of release and membrane re-uptake. Patch clamping provided quantitative measurements of the capacitance changes as the membrane of the secretory vesicle is added to the surface of the cell during secretion, and the change in the opposite direction when membrane is retrieved back into the cell during exocytosis. Carbon-fibre microelectrodes have measured electrochemically the release of oxidisable transmitters into the extracellular space. Differential interference contrast microscopy has given us spatially resolved images of the cell surface during exocytosis; real-time images that are suggestive of bubbles breaking the surface of a boiling pot of water. The interest in novel techniques stems from the fact that existing approaches can provide only indirect evidence on the steps preceding membrane fusion. The vesicular dynamics just beneath the plasma membrane are out of the reach of capacitance measurements or amperometric detection. What we have needed is a tool that would allow us to look just below the cell surface. This much-needed tool appears to be evanescent-wave microscopy. This review describes how laser microscopy can be used to study exocytosis at single-vesicle resolution. A companion paper deals with the practical aspects of evanescent-wave imaging.