Incomplete vesicular docking limits synaptic strength under high release probability conditions

Abstract

Central mammalian synapses release synaptic vesicles in dedicated structures called docking/release sites. It has been assumed that when voltage-dependent calcium entry is sufficiently large, synaptic output attains a maximum value of one synaptic vesicle per action potential and per site. Here we use deconvolution to count synaptic vesicle output at single sites (mean site number per synapse: 3.6). When increasing calcium entry with tetraethylammonium in 1.5 mM external calcium concentration, we find that synaptic output saturates at 0.22 vesicle per site, not at 1 vesicle per site. Fitting the results with current models of calcium-dependent exocytosis indicates that the 0.22 vesicle limit reflects the probability of docking sites to be occupied by synaptic vesicles at rest, as only docked vesicles can be released. With 3 mM external calcium, the maximum output per site increases to 0.47, indicating an increase in docking site occupancy as a function of external calcium concentration.

A dual role for α-synuclein in facilitation and depression of dopamine release from substantia nigra neurons in vivo

“…An alternate mechanism for α-Syn–dependent presynaptic facilitation is suggested by a role for synucleins in facilitating secretory vesicle exocytosis by enhancing the rate of dilation and collapse of the vesicle membrane during fusion at active zones ( 9 , 13 ). Multiple studies indicate that activity-dependent increase of presynaptic calcium promotes refilling of competent synaptic vesicle/active zone complexes ( 50 , 51 ). A synuclein-dependent enhancement of this refilling rate initiated during burst firing-induced calcium entry could enable facilitation by a more rapid replacement of synaptic vesicles/active zone complexes ( Fig.…”

Calcium dependence of neurotransmitter release at a high fidelity synapse

“…The steps preceding the fusion of synaptic vesicles are in general still poorly understood (Südhof, 2013). There is evidence that some steps preceding the fusion are strongly Ca 2+dependent (Neher and Sakaba, 2008), as has been demonstrated at chromaffin cells (Voets, 2000;Walter et al, 2013) and at several types of synapses such as the calyx of Held (Awatramani et al, 2005;Hosoi et al, 2007), the crayfish neuromuscular junctions (Pan and Zucker, 2009), parallel fiber to molecular layer interneuron synapses (Malagon et al, 2020), and cultured hippocampal neurons (Chang et al, 2018;Stevens and Wesseling, 1998). In previous reports, the Ca 2+ -dependence of vesicle priming at cMFBs was analyzed more indirectly with the Ca 2+ chelator EGTA (Ritzau-Jost et al, 2014;Ritzau-Jost et al, 2018) and the obtained results could be explained by Ca 2+ -dependent models but surprisingly also by Ca 2+ -independent models (Hallermann et al, 2010;Ritzau-Jost et al, 2018).…”

“…Sakaba and Neher, 2001). However, recent studies indicate very fast vesicle recruitment steps (Blanchard et al, 2020;Chang et al, 2018;Doussau et al, 2017;Hallermann et al, 2010;Lee et al, 2012;Malagon et al, 2020;Miki et al, 2016;Miki et al, 2018;Saviane and Silver, 2006;Valera et al, 2012). These findings further complicate the dissection between fusion, priming, and recruitment steps.…”

α-Synuclein facilitates dopamine release during burst firing of substantia nigra neurons in vivo

“…An alternate mechanism for α-Syn-dependent presynaptic facilitation is suggested by a role for synucleins in facilitating secretory vesicle exocytosis by enhancing the rate of dilation and collapse of the vesicle membrane during fusion at active zones (Logan et al, 2017;Sulzer and Edwards, 2019). Multiple studies indicate that activity-dependent increase of presynaptic calcium promotes refilling of competent synaptic vesicle/active zone complexes (Byczkowicz et al, 2018;Malagon et al, 2020). A synucleindependent enhancement of this refilling rate initiated during burst firing-induced calcium entry could enable facilitation by a more rapid replacement of synaptic vesicles/active zone complexes (Fig 6).…”