Small synaptic vesicles

This chapter introduces the classes of synaptic vesicle and discusses the structure and components of small synaptic vesicles (SSVs) in detail. It highlights the relationship of SSV structure to their dynamics in the nerve terminal, especially the mechanisms that organize SSV within the synaptic cytoplasm, and at the active zone where exocytosis takes place. It also discusses the recycling of SSV from components retrieved from synaptic plasma membranes by endocytosis. SSVs store and release the classical neurotransmitters such as acetylcholine, γ-aminobutyric acid (GABA), glutamate, and glycine. These vesicles are remarkably homogenous in both size and density and they account for approximately 6% of total brain protein. Electron microscopic studies reveal that they have a clear core, except for catecholamine containing SSVs, which appear to have an electron-opaque core under certain fixation conditions; these particular vesicles are sometimes referred to as small dense core vesicles. SSV exocytosis is initiated by the Ca 2+ influx that is mediated by voltage-sensitive Ca 2+ channels upon depolarization of the presynaptic membrane. Within 200 μsec of calcium influx, SSV exocytosis has occurred. A second essential requirement for exocytosis is guanosine triphosphate GTP.