Two endocytic recycling routes selectively fill two vesicle pools in frog motor nerve terminals

We have identified and characterized two vesicle recycling pathways in frog motor nerve terminals. We exploited the differential staining properties o f FM dyes of varying hydrophobicity to label selectively two different vesi cle pools, using optical imaging and electron microscopy of photoconverted dyes. During a 1 min tetanus, a rapidly recycling route places vesicles sel ectively into a small readily releasable pool comprising about 20% of vesic les. After the tetanus, a much slower pathway (from which FM2-10 but not FM 1-43 can be rinsed) delivers vesicles via infoldings and cisternae selectiv ely to a reserve pool with a halftime of about 8 min. Mixing between the tw o pools is slow. During stimulation at 30 Hz, 10-15 s is required to mobili ze and release dye from the reserve pool.