Quasielastic laser light scattering and laser Doppler electrophoresis as probes of synaptic and secretory terminal function

Abstract

The intensity fluctuations of scattered laser light provide a rapid, nondestructive probe of hydrodynamic size and polydispersity of nerve-ending subcellular particles. This method yields information on diffusive and directed motion of secretory granules and synaptic vesicles. It also enables accurate measurements of surface charge properties of isolated granules and plasma membranes. Recently, a fast, total intensity light-scattering change, sensitive to many of the factors that influence secretion, has been detected in neurosecretory terminals. Here a slower, but equally reversible, light-scattering change is detected using an intact invertebrate neurosecretory organ. Scattering at rest is dominated by particles of diameter similar to that of secretory vesicles detected in electron micrographs of the same tissue. Simultaneous release and light-scattering observations demonstrate an increase in the proportion of total scatter derived from mobile as opposed to fixed components, with no change in mean particle diameter. Since the major changes in intensity fluctuations are detected during the declining phase of the release of secretory product, they may represent longer term reorganization of terminals following intense secretory activity. As a prelude to dissecting the origin of the fast light-scattering changes, the hydrodynamic properties of secretosomes and purified secretory granules from mouse neurohypophysis have been characterized. In this way, size, dispersity and aggregation can be followed, and subpopulations can be characterized. Application of an electric field to various subcellular fractions enables measurement of electrophoretic mobility, and hence calculation of surface-charge properties of granules and plasma membranes. The effects of divalent cations on granule surface charge in vitro indicate that surface-charge neutralization is not a calcium-specific event in exocytosis. The possibility that some of the light-scattering signal reflects dynamic changes in cytoskeletal components of the secretory terminal is examined.