Monitoring synaptic vesicle recycling in frog motor nerve terminals with FM dyes

Ultrastructural observations made in the study of the frog neuromuscular junction (NMJ) almost three decades ago showed that synaptic vesicle cycling functions through a slow pathway, requiring the use of clathrin-coated vesicles and an endosomal compartment. Simultaneously, a conceptually simpler model emerged, postulating rapid retrieval of vesicle membrane through a mechanism similar to a reversal of vesicle fusion. With the advent of fluorescence imaging which allows the investigator to monitor recycling in living nerve-muscle preparations, new data appeared which reconcile at least in part the two models, indicating that both may be important at this synapse. Two different synaptic vesicle pools can be defined, a readily releasable pool (RRP), consisting of quanta that are immediately available for release, and a reserve pool (RP) that is exocytosed only after prolonged stimulation. Vesicles in the RRP recycle through a fast endocytic pathway, which does not rely on an endosomal compartment, while vesicles in the RP cycle more slowly through formation of infoldings and endosomes and their subsequent severance into vesicles. The two pools mix slowly, and their recycling may be regulated by different mechanisms.