Alpha-latrotoxin channels in neuroblastoma cells

Inhibition of endocytosis by elevated internal calcium in a synaptic terminal

During synaptic transmission in the nervous system, synaptic vesicles fuse with the plasma membrane of presynaptic terminals, releasing neurotransmitter by exocytosis. The vesicle membrane is then retrieved by endocytosis and recycled into new transmitter-containing vesicles. Exocytosis in synaptic terminals is calcium-dependent, and we now report that endocytosis also is regulated by the intracellular calcium concentration ([Ca2+]i). Capacitance measurements in synaptic terminals of retinal bipolar neurons revealed that endocytosis was strongly inhibited by elevated [Ca2+]i in the range achieved by Ca(2+)-current activation. The rate of membrane retrieval was steeply dependent on [Ca2+]i, with a Hill coefficient of 4 and half-inhibition at approximately 500 nM. At [Ca2+]i > or = 900 nM, endocytosis was entirely absent. The action of internal calcium on endocytosis represents a novel negative-feedback mechanism controlling the rate of membrane recovery in synaptic terminals after neurotransmitter secretion. As membrane retrieval is the first step in vesicle recycling, this mechanism may contribute to activity-dependent synaptic depression.

Intermediates in synaptic vesicle recycling revealed by optical imaging of Drosophila neuromuscular junctions

We show that uptake and release of the styryl dye FM1-43 may be used to monitor synaptic vesicle exocytosis and recycling at Drosophila larval neuromuscular junctions. At Drosophila nerve terminals, FM1-43 specifically labels subsynaptic domains enriched in synaptotagmin, in a manner that requires Ca2+, membrane depolarization, and shibire (shi) function. Endocytosis rates, very low in unstimulated synapses, are induced severalfold by the exocytosis of synaptic vesicles. Using shi(ts)1 mutant synapses to separate synaptic vesicle fusion and recycling temporally, we show that recycling events subsequent to the shi block do not require extracellular Ca2+. We suggest that two distinct intermediate stages in vesicle recycling may be trapped and analyzed at Drosophila neuromuscular junctions.