1. Howard Hughes Medical Institute and Departments of Pharmacology and Cell Biology, Yale University Medical School, New Haven, Connecticut 06510, USA

Abstract

NEUROTRANSMITTER release is mediated by Ca²⁺ dependent exocytosis of synaptic vesicles¹. Neither the amount of transmitter released from individual synaptic vesicles nor the kinetics of this process have yet been directly determined. Using carbon fibres as electrochemical detectors^2,3, we have measured release of the neurotransmitter serotonin from cultured neurons of the leech⁴. This technique allowed us to monitor transmitter discharge from single synaptic vesicles as spike-like oxidation currents at high time resolution, providing new insight into the mechanism of neuronal exocytosis. Two types of signals were characterized, corresponding to exocytosis of small clear and large dense core vesicles present in these cells. A small vesicle discharges about 4,700 transmitter molecules with a time constant in the region of 260 s, whereas large vesicles release their content of approximately 80,000 molecules with a time constant of about 1.3 ms. Release from both vesicle types is initiated rapidly, with a rise time of less than 60 s, suggesting an abrupt opening of a preassembled fusion pore.