Kinetics of Releasable Synaptic Vesicles and Their Plastic Changes at Hippocampal Mossy Fiber Synapses

Hippocampal mossy fiber boutons (hMFBs) are presynaptic terminals displaying various forms of synaptic plasticity. The presynaptic mechanisms underlying synaptic plasticity still remain poorly understood. Here, we have combined high temporal resolution measurements of presynaptic capacitance and excitatory postsynaptic currents (EPSCs) to measure the kinetics of exocytosis. In addition, total internal reflection fluorescence (TIRF) microscopy was employed to directly visualize dynamics of single synaptic vesicles adjacent to the plasma membrane at high spatial resolution. Readily releasable vesicles mostly consisted of already-tethered vesicles in the TIRF field. Vesicle replenishment had fast and slow phases, and TIRF imaging suggests that the fast phase depends on vesicle priming from already-tethered vesicles. Application of cyclic AMP (cAMP), a molecule crucial for LTP, mainly increases the vesicular release probability rather than the number of readily releasable vesicles or their replenishment rate, likely by changing the coupling between Ca²⁺ channels and synaptic vesicles. Thus, we revealed dynamic properties of synaptic vesicles at hMFBs.