Monoamine Transmitter Release from Small Synaptic and Large Dense-Core Vesicles

Abstract

Publisher Summary Most studies of neurotransmitter release utilize electrophysiological measurements of postsynaptic responses to infer presynaptic events. Furthermore, electrical measurements of the membrane capacitance have been used to follow directly the changes in cell-surface area associated with exocytosis of vesicles in a wide variety of cells, such as mast cells, chromaffin cells, and vertebrate neurons. Because of the large size of these cultured neurons (diameter of about SO pm), the releasing cell surface can be directly approached with a carbon fiber. Furthermore, the fiber can be positioned over discrete regions of the cell, such as the soma or the axon stump, the latter being the pr eferential site of synapse formation when these neurons are co-cultured with a chemoreceptive “follower” cell. The findings suggest that the signals represent Ca 2+ -triggered exocytosis of quantal packets of serotonin. In close correlation, electron microscopy demonstrated that two types of vesicles, small synaptic vesicles (SSVs) and large dense-core vesicles (LDCVs), are present at these axonal recording sites. The morphological data showed that SSVs are clustered, directly apposing the plasma membrane, whereas LDCVs often surround these clusters and are scattered throughout the cytoplasm. Several investigators have noted that the initial stage of exocytosis in nonneuronal secretory cells is the formation of a fusion pore, which establishes an electrical and diffusional connection between the vesicle interior and the extracellular spaces.