Three small vesicular pools in sequence govern synaptic response dynamics during action potential trains

Abstract

During prolonged trains of presynaptic action potentials (APs), synaptic release reaches a stable level that reflects the speed of replenishment of the readily releasable pool (RRP). Determining the size and filling dynamics of vesicular pools upstream of the RRP has been hampered by a lack of precision of synaptic output measurements during trains. Using the recent technique of tracking vesicular release in single active zone synapses, we now developed a method that allows the sizes of the RRP and upstream pools to be followed in time. We find that the RRP is fed by a small-sized pool containing approximately one to four vesicles per docking site at rest. This upstream pool is significantly depleted by short AP trains, and reaches a steady, depleted state for trains of >10 APs. We conclude that a small, highly dynamic vesicular pool upstream of the RRP potently controls synaptic strength during sustained stimulation.

A sequential two-step priming scheme reproduces diversity in synaptic strength and short-term plasticity

“…Release mediated by SV TSs has many features in common with the release contributed by so-called ‘pre-primed’ SVs, postulated for glutamatergic synapses in the cortex (80). Likewise, the sequential scheme proposed for cerebellar parallel fiber-molecular layer interneuron synapses (25, 26) is related to the sequential scheme favored here: Both assume reversible priming steps in sequence, postulate certain fractions of upstream SV pools being transferred to a downstream pool upon AP arrival and separating contributions to release dependent on the state of SVs prior to stimulation.…”

“…To more faithfully reproduce the multifaceted features of STP, different multiple-state and/or multiple-site schemes of transmitter release have been proposed (19)(20)(21)(22)(23)(24)(25)(26), which fall into two principal categories: (i) parallel schemes in which more than one kind of SVs can bind to one or more kinds of release sites, or (ii) sequential schemes in which SVs migrate between different kinds of release sites, or else undergo changes of maturation, reflected by distinct SV states, while being docked to a given site.…”

Prioritized docking of synaptic vesicles provided by a rapid recycling pathway

“…1A , right; Tran et al, 2022). Furthermore, we have shown that IP recovery occurs on a timescale of seconds, and is calcium-dependent (Tran et al, 2022). To explore further the nature and the size of SV pools upstream of the IP, we applied 8-AP trains repetitively, while changing the inter-train time interval from a baseline value of 10 s to various test values.…”

“…By changing the inter-train time interval in twin train experiments, we have recently shown that a single 8-AP train significantly reduces the size of an SV pool upstream of the RRP (called the intermediate pool or IP; Fig. 1A , right; Tran et al, 2022). Furthermore, we have shown that IP recovery occurs on a timescale of seconds, and is calcium-dependent (Tran et al, 2022).…”

“…Similar to the majority of synapses in the brain, individual GC-MLI synapses typically contain one active zone (Masugi-Tokita et al, 2007; Xu-Friedman et al, 2001) and a relatively small number of SVs (Eguchi et al, 2020). However, compared to others, vesicular release statistics and SV pool dynamics have been better characterized at these synapses due to recent methodological advances (Malagon et al, 2016, 2020; Miki et al, 2016, 2018; Tran et al, 2022). Here, we found evidence for a rapid route of SV recycling during repeated train stimulation with short inter-train intervals.…”