Developmental shift to a mechanism of synaptic vesicle endocytosis requiring nanodomain Ca2+

Ca2+ is thought to be essential for the exocytosis and endocytosis of synaptic vesicles. However, the manner in which Ca2+ coordinates these processes remains unclear, particularly at mature synapses. Using membrane capacitance measurements from calyx of Held nerve terminals in rats, we found that vesicle endocytosis is initiated primarily in Ca2+ nanodomains around Ca2+ channels, where exocytosis is triggered. Bulk Ca2+ outside of the domain could also be involved in endocytosis at immature synapses, although only after extensive exocytosis at more mature synapses. This bulk Ca2+-dependent endocytosis required calmodulin and calcineurin activation at immature synapses, but not at more mature synapses. Similarly, GTP-independent endocytosis, which occurred after extensive exocytosis at immature synapses, became negligible after maturation. We propose that nanodomain Ca2+ simultaneously triggers exocytosis and endocytosis of synaptic vesicles and that the molecular mechanisms underlying Ca2+-dependent endocytosis undergo major developmental changes at this fast central synapse.