MORPHOLOGICALLY DOCKED SYNAPTIC VESICLES ARE REDUCED IN SYNAPTOTAGMINMUTANTS DROSOPHILA

Nerve terminal specializations include mechanisms for maintaining a su bpopulation of vesicles in a docked, fusion-ready state. We have inves tigated the relationship between synaptotagmin and the number of morph ologically docked vesicles by an electron microscopic analysis of Dros ophila synaptotagmin (syt) mutants. The overall number of synaptic ves icles in a terminal was reduced, although each active zone continued t o have a cluster of vesicles in its vicinity, in addition, there was a n increase in the number of large vesicles near synapses. Examining th e clusters, we found that the pool of synaptic vesicles immediately ad jacent to the presynaptic membrane, the pool that includes the docked population, was reduced to 24 +/- 5% (means +/- SEM) of control in the syt(null) mutation. To separate contributions of overall vesicle depl etion and increased spontaneous release from direct effects of synapto tagmin on morphological docking, we examined syt mutants in an altered genetic background. Recombining syt alleles onto a second chromosome bearing an as yet uncharacterized mutation resulted in the expected de crease in evoked release but suppressed the increase in spontaneous re lease frequency. Motor nerve terminals in this genotype contained more synaptic vesicles than control, yet the number of vesicles immediatel y adjacent to the presynaptic membrane near active zones was still red uced (33 +/- 4% of control). Our findings demonstrate that there is a decrease in the number of morphologically docked vesicles seen in syt mutants. The decreases in docking and evoked release are independent o f the increase in spontaneous release. These results support the hypot hesis that synaptotagmin stabilizes the docked slate.