Evanescent-wave microscopy: a new tool to gain insight into the control of transmitter release

Evanescent-wave excitation was used to visualize individual fluorescently labelled vesicles in an optical slice near the plasma membrane of bovine adrenal chromaffin cells. A standard upright microscope was modified to accommodate the optics used for directing a laser beam under a supracritical angle on to the glass-water interface on top of which the cells are grown. Whereas epi-illumination images appeared blurred and structureless, evanescent-wave excitation highlighted acridine orange-labelled vesicles as individual pinpoints. Three-dimensional (3D) trajectories of individual vesicles were obtained from time-resolved image stacks and used to characterize vesicles in terms of their average fluorescence F and mobility, expressed here as the 3D diffusion coefficient D(3). Based on the single-vesicle analysis, two groups of vesicles were identified. Transitions between these states were studied before and after stimulation of exocytosis by repetitive or maintained membrane depolarizations by elevated extracellular [K+]. Findings were interpreted as sequential transitions between the previously characterized pools of vesicles preceding the fusion step. The observed approach of vesicles to their docking sites was not explained in terms of free diffusion: most vesicles moved unidirectionally as if directed to their binding sites at the plasma membrane. Vesicle mobility at the membrane was low, such that the sites of docking and fusion were in close vicinity. Both the rim region and confined areas in the centre of the footprint region were the site of intense vesicle trafficking.