Differentially poised vesicles underlie fast and slow components of release at single synapses

Abstract

In several types of central mammalian synapses, sustained presynaptic stimulation leads to a sequence of two components of synaptic vesicle release, reflecting the consecutive contributions of a fast-releasing pool (FRP) and of a slow-releasing pool (SRP). Previous work has shown that following common depletion by a strong stimulation, FRP and SRP recover with different kinetics. However, it has remained unclear whether any manipulation could lead to a selective enhancement of either FRP or SRP. To address this question, we have performed local presynaptic calcium uncaging in single presynaptic varicosities of cerebellar interneurons. These varicosities typically form “simple synapses” onto postsynaptic interneurons, involving several (one to six) docking/release sites within a single active zone. We find that strong uncaging laser pulses elicit two phases of release with time constants of ∼1 ms (FRP release) and ∼20 ms (SRP release). When uncaging was preceded by action potential–evoked vesicular release, the extent of SRP release was specifically enhanced. We interpret this effect as reflecting an increased likelihood of two-step release (docking then release) following the elimination of docked synaptic vesicles by action potential–evoked release. In contrast, a subthreshold laser-evoked calcium elevation in the presynaptic varicosity resulted in an enhancement of the FRP release. We interpret this latter effect as reflecting an increased probability of occupancy of docking sites following subthreshold calcium increase. In conclusion, both fast and slow components of release can be specifically enhanced by certain presynaptic manipulations. Our results have implications for the mechanism of docking site replenishment and the regulation of synaptic responses, in particular following activation of ionotropic presynaptic receptors.

Calcium dependence of neurotransmitter release at a high fidelity synapse

“…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.…”

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

“…In particular, it remains unclear whether the SRP, the primed FRP, and the superprimed FRP release their SVs according to a parallel scheme, to a sequential scheme, or to a mixed parallel-sequential scheme ( 5 , 8 , 12 , 20 – 25 ), even though recent studies often favor sequential schemes ( 9 , 26 , 27 ). In certain additional synapses [cerebellar parallel fiber to molecular layer interneuron ( 28 , 29 ), molecular layer interneuron to molecular layer interneuron ( 30 ), and hippocampal mossy fiber to interneuron ( 31 )], studies carried out at the level of individual AZs (“simple synapse recording”: ref. 32 ) suggest a sequential binding of incoming SVs to a “replacement site” and an associated “docking site” before release.…”

Analogue signaling of somato-dendritic synaptic activity to axon enhances GABA release in young cerebellar molecular layer interneurons

“…This is different from what was found by Bouhours et al, who showed that the analog-digital coupling was dependent on the activation of PKC. In a recent work, Blanchard et al 25 showed that the modulation of basal calcium with local calcium photolysis in a single MLI axonal varicosity can increase release through a modulation of the docking site occupancy. In the present work, both the amplitude and the half-width of the axAP seem to be very resistant to various manipulations, like an increase in the stimulation frequency ( supplementary figure 4 ) and somatic depolarization ( Figure 4 ), which is in agreement with what has been described by Alle and Geiger (see above) and by Ritzau-Jost et al 26 , who showed that in neocortical (L5) fast-spiking GABAergic cells the width of the axAP does not change when the stimulation frequency increases.…”