Synaptic vesicles transiently dock to refill release sites

Abstract

Synaptic vesicles fuse with the plasma membrane to release neurotransmitter following an action potential, after which new vesicles must refill vacated release sites. How many vesicles can fuse at a single active zone, where they fuse within the active zone, and how quickly they are replaced with new vesicles is not well-established. To capture synaptic vesicle exocytosis at cultured mouse hippocampal synapses, we induced single action potentials by electrical field stimulation then subjected neurons to high-pressure freezing to examine their morphology by electron microscopy. During synchronous release, multiple vesicles can fuse at a single active zone; this multivesicular release is augmented by increasing the extracellular calcium concentration. Synchronous fusions are distributed throughout the active zone, whereas asynchronous fusions are biased toward the center of the active zone. Immediately after stimulation a large fraction of vesicles become undocked. Between 8 and 14 ms, new vesicles are recruited to the plasma membrane and fully replenish the docked pool, but docking of these vesicles is transient and they either undock or fuse within 100 ms. These results demonstrate that recruitment of synaptic vesicles to release sites is rapid and reversible.3 When an action potential invades a synaptic bouton, voltage-gated calcium channels open and calcium influx triggers vesicle fusion to release neurotransmitter. Synaptic vesicle fusion takes place at a specialized membrane domain: the active zone 1 . It is believed that the active zone is organized into one or more release sites, which are individual units at which a single synaptic vesicle can fuse 2 . Ultrastructural studies demonstrate that some synaptic vesicles are in contact with the plasma membrane and define the 'docked' pool 3,4 . Since both docking and physiological readiness require engaged SNARE proteins 4-6 , docked vesicles are thought to represent fusion-competent vesicles. There are several features of fusion that could be addressed by ultrastructural studies if methods existed to capture such rapid events as fusion and docking, specifically: how many vesicles fuse in response to an action potential, when and where they fuse, and how vesicle docking refills fusion sites.How many vesicles fuse in response to an action potential has been intensely debated. A large body of work argues that, even when release probability is high, small central synapses act as binary operators: no more than one vesicle can fuse per action potential 7-10 . One possible mechanism is that the fusion of a vesicle blocks further fusions by 'lateral inhibition' 11,12 . By contrast, evidence from electrophysiology [13][14][15][16] , optical imaging 17-21 and electron microscopy 22,23 suggests that multiple vesicles can fuse at a synapse 24 .Evoked neurotransmitter release takes place in two phases, with a synchronous component that begins within a millisecond of an action potential followed by an asynchronous component that can last for hundreds of milliseconds 25 . As ...

What We Can Learn From Cumulative Numbers of Vesicular Release Events

“…Thus investigation of these AZ proteins may tell us localization, movement and regulation of DSs. In addition, nano-meter scale movements of synaptic vesicles have been observed after stimulation near the presynaptic membrane, indicating the speed and time course of vesicle recruitment (Midorikawa and Sakaba, 2015, 2017; Kusick et al, 2018). Those studies combined with the model simulation based on the quantal analysis may allow us to see the relationship between parameter values for predicted models and physical observations of DSs and synaptic vesicles.…”

“…More detailed description of DS models by the developed method may more clearly define molecular/morphological correlates of synaptic parameters. In addition, nanometer-resolution observations in mammalian CNS using super-resolution imaging and electron microscopy have revealed synaptic vesicle movement in presynaptic terminals (Midorikawa and Sakaba, 2015, 2017; Rothman et al, 2016; Maschi and Klyachko, 2017; Chang et al, 2018; Kusick et al, 2018), and distribution of synaptic vesicles and AZ proteins (Siksou et al, 2007; Imig et al, 2014; Nakamura et al, 2015; Tang et al, 2016), paving the way for a comparison between data and DS model simulations for a comprehensive understanding of vesicle release/replenishment at DSs if the vesicle recruitment process and the pool size of releasable/suppliable vesicles can be predicted accurately by the new method.…”

Tau-mediated synaptic dysfunction is coupled with HCN channelopathy

“…In Drosophila, distinct states of the active zone have been proposed to regulate neuronal communication by modulating synaptic vesicle positioning and release (Kittel and Heckmann, 2016). Within a given synapse, not all release sites are equally re-used during sustained synaptic activity (Kusick et al, 2020). The reduction in synaptic vesicles located at the active zone in Tau35 mouse brain may indicate disruption of synchronous vesicle release.…”

Control of Presynaptic Parallel Fiber Efficacy by Activity-Dependent Regulation of the Number of Occupied Release Sites

“…As detailed above, Doussau et al (2017) found evidence that the replenishment process is reversible on a slower time scale of ~200 ms. Interestingly, in a recent manuscript reporting results from electron microscopic analysis of hippocampal synapses, following stimulation and rapid freezing of cultured neurons, new SVs were recruited to the plasma membrane and fully replenished the docked pool of SVs within ~10 ms after stimulation (Kusick et al, 2018). The docking of these SVs was transient and they either undocked or fused within 100 ms.…”