Is replenishment of the readily releasable pool associated with vesicular movement?

Abstract

At the excitatory synapse of rat hippocampus the short-term synaptic depression observed during long high-frequency stimulation is associated with slower replenishment of the readily-releasable pool. Given that the replenishment rate is also not [Ca(++)]o sensitive this puts into question a widely held notion that the vesicles-constrained by the cytoskeleton and rendered free from such constraints by Ca(++) entry that renders them more mobile-are important in the replenishment of the readily-releasable pool. This raises a question-Is vesicular replenishment of the readily releasable pool associated with significant movement? To answer this question we evaluated how okadaic acid and staurosporine (compounds known to affect vesicular mobility) influence the replenishment rate. We used patterned stimulation on the Schaffer collateral fiber pathway and recorded the excitatory post-synaptic currents (EPSCs) from rat CA1 neurons, in the absence and presence of these drugs. The parameters of a circuit model with two vesicular pools were estimated by minimizing the squared difference between the ESPC amplitudes and simulated model output. [Ca(2+)]o did not influence the progressive decrease of the replenishment rate during long, high frequency stimulation. Okadaic acid did not significantly affect any parameters of the vesicular storage and release system, including the replenishment rate. Staurosporine reduced the replenishment coupling, but not the replenishment rate, and this is owing to the fact that it also reduces the ability of the readily releasable pool to contain quanta. Moreover, these compounds were ineffective in influencing how the replenishment rate decreases during long, high frequency stimulation. In conclusion at the excitatory synapses of rat hippocampus the replenishment of the readily releasable pool does not appear to be associated with a significant vesicular movement, and during long high frequency stimulation [Ca(++)]o does not influence the progressive decrease of vesicular replenishment.

Temperature dependence of vesicular dynamics at excitatory synapses of rat hippocampus

“…The replenishment coupling increases with temperature (*twofold when the temperature changes from 22 to 32°C), but this is more than counterbalanced by the greater RRP size (RRP size rises *fivefold as temperature increases by 10°C). Note that this provides additional evidence supporting the notion that the replenishment of the readily releasable pool is not associated with vesicular movement (Bui and Glavinovic 2013b). Unlike vesicular mobility, which increases greatly with temperature (Shtrahman et al 2005), the replenishment rate diminishes with temperature.…”

Secreted C-type lectin regulation of neuromuscular junction synaptic vesicle dynamics modulates coordinated movement

“…In future studies, we plan to dissect LGC1 mechanisms by focusing on SV pools and trafficking, as well as testing the possible involvement in signaling via Eiger, EAAT1 and downstream SV Synaptobrevin changes. Our work indicates that LGC1-dependent signaling fine-tunes the multiple presynaptic pools that exchange SVs differentially based on activity levels and release demand to control SV release probability (Bui and Glavinovic, 2014;Guo et al, 2015;Millar et al, 2002). Beyond this LGC1 mechanism, we hope this new model will allow further exploration of SV cyclingdependent NMJ function in neuromuscular disease states.…”

“…At the Drosophila NMJ, high-frequency neurotransmission challenges the synaptic release machinery, especially at high-probability release sites, which exhibit activity-dependent depression (Kauwe and Isacoff, 2013). Following high activity levels, new SV generation and SV recruitment enables recovery of neurotransmission amplitudes within tens of seconds (Bui and Glavinovic, 2014). We therefore hypothesized that the LGC1 mutant SV cycling defects determined with FM1-43 imaging (Fig.…”

Increased presynaptic excitability in a migraine with aura mutation

“…Replenishment of the RRP determines release in response to consecutive stimuli, regulating short-term synaptic adaptation to multiple stimuli 52 . The RRP is never completely depleted following repeated stimulations, as it is replenished from a distinct pool of vesicles known as the reserve pool 23 , through a delicate balance between vesicular endo- and exocytosis 53 . Although disparate at first, the rates of depletion and replenishment of the RRP reach equilibrium eventually, resulting in steady state post-synaptic responses.…”