Omega images at the active zone may be endocytotic rather than exocytotic: implications for the vesicle hypothesis of transmitter release

JH Koenig, K Yamaoka, K Ikeda - Proceedings of the …, 1998 - National Acad Sciences
JH Koenig, K Yamaoka, K Ikeda
Proceedings of the National Academy of Sciences, 1998National Acad Sciences
A Ca2+-dependent synaptic vesicle-recycling pathway emanating from the plasma
membrane adjacent to the dense body at the active zone has been demonstrated by
blocking pinch-off of recycling membrane by using the Drosophila mutant, shibire. Exposure
of wild-type Drosophila synapses to low Ca2+/high Mg2+ saline is shown here to block this
active zone recycling pathway at the stage in which invaginations of the plasma membrane
develop adjacent to the dense body. These observations, in combination with our previous …
A Ca2+-dependent synaptic vesicle-recycling pathway emanating from the plasma membrane adjacent to the dense body at the active zone has been demonstrated by blocking pinch-off of recycling membrane by using the Drosophila mutant, shibire. Exposure of wild-type Drosophila synapses to low Ca2+/high Mg2+ saline is shown here to block this active zone recycling pathway at the stage in which invaginations of the plasma membrane develop adjacent to the dense body. These observations, in combination with our previous demonstration that exposure to high Ca2+ causes “docked” vesicles to accumulate in the identical location where active zone endocytosis occurs, suggest the possibility that a vesicle-recycling pathway emanating from the active zone may exist that is stimulated by exposure to elevated Ca2+, thereby causing an increase in vesicle recycling, and is suppressed by exposure to low Ca2+ saline, thereby blocking newly forming vesicles at the invagination stage. The presence of a Ca2+-dependent endocytotic pathway at the active zone opens up the following possibilities: (i) electron microscopic omega-shaped images (and their equivalent, freeze fracture dimples) observed at the active zone adjacent to the dense body could represent endocytotic images (newly forming vesicles) rather than exocytotic images; (ii) vesicles observed attached to the plasma membrane adjacent to the dense body could represent newly formed vesicles rather than vesicles “docked” for release of transmitter.
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