Compound Fusion at Ribbon Synapses
 
How ribbon synapses in photoreceptors and retinal bipolar cells can release vesicles extremely rapidly has long been a mystery.  Diffusion of undocked vesicles to the plasma membrane is too slow to account for the speed of release.  Matthews and Sterling now provide electron-microscopic evidence that compound fusion - in which vesicles fuse to each other before fusing with the plasma membrane - occurs at ribbon synapses of goldfish bipolar cells.  Stimulated ribbon synapses had significantly fewer vesicles tethered to the ribbon than those in which exocytosis was blocked.  Although most vesicles had similar sizes in stimulated and unstimulated synapses, vesicles up to three times larger appeared after stimulation.  The largest of these occurred at the base of the ribbon, adjacent to the plasma membrane, whereas normal-sized vesicles predominated at the top of the ribbon.  Invaginations of the plasma membrane near ribbon synapses also appeared after stimulation, and like vesicles, these were tethered to the ribbon.

The Journal Of Neuroscience

Gary Matthews and Peter Sterling