Genetic evidence for an equilibrium between docked and undocked vesicles

By using the shibire mutation to block endocytosis in a temperature-depende nt fashion, we have manipulated the number of synaptic vesicles in a nerve terminal and have observed a remarkable proportionality the number of quant a released to the size of the total vesicle pool. In the experiments descri bed below we determine that approximately 0.3% of the vesicle pool is relea sed per stimulus. The data suggest that the pool of readily releasable dock ed vesicles does not represent the saturation of a limiting number of relea se sites, but instead represents a subset of vesicles that is in equilibriu m with the larger pool of vesicles. Before presenting this data and the sig nificance of the finding for the regulation of neurotransmission, we will b riefly review the use of Drosophila genetics as a tool for dissecting synap tic transmission.