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 ‘dock’ to refill vacated release sites. 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. Fusions during synchronous release are distributed throughout the active zone, whereas fusions during asynchronous release are biased toward the center of the active zone. After stimulation, the total number of docked vesicles across all synapses decreases by ~40%. Within 14 ms, new vesicles are recruited and fully replenish the docked pool, but this docking 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.

SynapsEM: Computer-Assisted Synapse Morphometry

“…For example, synaptic vesicles are 30-50 nm in diameter (Zhang et al, 1998), and a few hundred vesicles are clustered within a synaptic bouton (Schikorski and Stevens, 1997;Shepherd and Harris, 1998), which is only ∼0.5-1 µm in diameter. Moreover, a vesicle may move only a few nanometers to fully engage the active zone membrane during docking (Hammarlund et al, 2007;Imig et al, 2014), and this state is quite dynamic (Chang et al, 2018;Kusick et al, 2020). Given these dimensions, synaptic morphometry requires the resolution of electron microscopy.…”

Excitatory postsynaptic calcium transients at Aplysia sensory-motor neuron synapses allow for quantal examination of synaptic strength over multiple days in culture

“…Our results show that even with the improved resolution of the membrane delimited Src-GCAMP6s, many EPSCaTs still appear to be multiquantal. While we cannot rule out the potential contribution of intrasite multivesicular release (Kusick et al 2020), it is likely this largely results from multiple independent synapses existing in close proximity and such arrangements would not be resolvable with the methods used here. The propensity for sites to occur in clusters that are resolvable as in figure 4 further supports the idea that the ROIs with multiquantal EPSCaTs are a tight cluster of individual synapses.…”

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.…”

“…These currents reflect alterations in presynaptic glutamate release and subsequent activation of post-synaptic α-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) and α-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (NMDA) receptors (Turrigiano and Nelson, 2004). Notably, fluctuation in the release of presynaptic vesicles has been reported to profoundly affect both the timing of sEPSCs (Chanaday et al , 2019; Chuhma and Ohmori, 1998) and synaptic vesicle distribution along the active zone (Kusick et al , 2020). The changes we observed in sEPSCs in Tau35 hippocampal neurons could, therefore, caused by asynchronous release of presynaptic vesicles.…”