Membrane cholesterol regulates different modes of synaptic vesicle release and retrieval at the frog neuromuscular junction

We investigated the effects of cholesterol removal on spontaneous and KCl-evoked synaptic vesicle recycling at the frog neuromuscular junction. Cholesterol removal by methyl--cyclodextrin (MCD) induced an increase in the frequency of miniature end-plate potentials (MEPPs) and spontaneous destaining of synaptic vesicles labeled with the styryl dye FM1-43. Treatment with MCD also increased the size of MEPPs without causing significant changes in nicotinic receptor clustering. At the ultrastructural level, synaptic vesicles from nerve terminals treated with MCD were larger than those from control. In addition, treatment with MCD reduced the fusion of synaptic vesicles that are mobilized during KCl-evoked stimulation, but induced recycling of those vesicles that fuse spontaneously. We therefore suggest that MCD might favor the release of vesicles that belong to a pool that is different from that involved in the KCl-evoked release. These results reveal fundamental differences in the synaptic vesicle cycle for spontaneous and evoked release, and suggest that deregulation of cholesterol affects synaptic vesicle biogenesis and increases transmitter packing.