Properties of fast endocytosis at hippocampal synapses

Regulation of synaptic transmission is a widespread means for dynamic alter ations in nervous system function. In several cases, this regulation target s vesicular recycling in presynaptic terminals and may result in substantia l changes in efficiency of synaptic transmission. Traditionally, experiment al accessibility of the synaptic vesicle cycle in central neuronal synapses has been largely limited to the exocytotic side, which can be monitored wi th electrophysiological responses to neurotransmitter release. Recently, ph ysiological measurements on the endocytotic portion of the cycle have been made possible by the introduction of styryl dyes such as FM1-43 as fluoresc ent markers for recycling synaptic vesicles. Here we demonstrate the existe nce of fast endocytosis in hippocampal nerve terminals and derive its kinet ics from fluorescence measurements using dyes with varying rates of membran e departitioning. The rapid mode of vesicular retrieval was greatly speeded by exposure to staurosporine or elevated extracellular calcium. The effect ive time-constant for retrieval can be <2 seconds under appropriate conditi ons. Thus, hippocampal synapses capitalize on efficient mechanisms for endo cytosis and their vesicular retrieval is subject to modulatory control.