Ca 2+ Influx through Distinct Routes Controls Exocytosis and Endocytosis at Drosophila Presynaptic Terminals

Abstract

Endocytosis of synaptic vesicles follows exocytosis, and both processes require external Ca 2+. However, it is not known whether Ca 2+ influx through one route initiates both processes. At larval Drosophila neuromuscular junctions, we separately measured exocytosis and endocytosis using FM1-43. In a temperature-sensitive Ca 2+ channel mutant, cacophony TS2 , exocytosis induced by high K + decreased at nonpermissive temperatures, while endocytosis remained unchanged. In wild-type larvae, a spider toxin, PLTXII, preferentially inhibited exocytosis, whereas the Ca 2+ channel blockers flunarizine and La 3+ selectively depressed endocytosis. None of these blockers affected exocytosis or endocytosis induced by a Ca 2+ ionophore. Evoked synaptic potentials were depressed regardless of stimulus frequency in cacophony TS2 at nonpermissive temperatures and in wild-type by PLTXII, whereas flunarizine or La 3+ gradually depressed synaptic potentials only during high-frequency stimulation, suggesting depletion of synaptic vesicles due to blockade of endocytosis. In shibire ts1 , a dynamin mutant, flunarizine or La 3+ inhibited assembly of clathrin at the plasma membrane during stimulation without affecting dynamin function.