Recruitment of release sites underlies chemical presynaptic potentiation at hippocampal mossy fiber boutons.

Marta Orlando,Dietmar Schmitz,Marta Maglione,Benjamin R Rost,Stephan J Sigrist,Felicitas Bruentgens,Anton Dvorzhak,Jörg Breustedt,Matthew B. Dalva

doi:10.1371/journal.pbio.3001149

Abstract

Synaptic plasticity is a cellular model for learning and memory. However, the expression mechanisms underlying presynaptic forms of plasticity are not well understood. Here, we investigate functional and structural correlates of presynaptic potentiation at large hippocampal mossy fiber boutons induced by the adenylyl cyclase activator forskolin. We performed 2-photon imaging of the genetically encoded glutamate sensor iGluu that revealed an increase in the surface area used for glutamate release at potentiated terminals. Time-gated stimulated emission depletion microscopy revealed no change in the coupling distance between P/Q-type calcium channels and release sites mapped by Munc13-1 cluster position. Finally, by high-pressure freezing and transmission electron microscopy analysis, we found a fast remodeling of synaptic ultrastructure at potentiated boutons: Synaptic vesicles dispersed in the terminal and accumulated at the active zones, while active zone density and synaptic complexity increased. We suggest that these rapid and early structural rearrangements might enable long-term increase in synaptic strength.

Highlights

The term synaptic plasticity describes the ability of synapses to change their strength and efficacy over time
By means of 2-photon fluorescent imaging of glutamate release, Stimulated emission depletion (STED) microscopy, and 3D transmission electron microscopy (EM) analysis, we addressed the following questions: Do the addition of release sites and the rearrangement of active zone (AZ) nano-architecture play a role in presynaptic potentiation? How does glutamate release dynamics change upon presynaptic potentiation?
Our data show that an increase in the number of available release sites—and in release probability—is instrumental for forskolininduced mossy fiber presynaptic potentiation

Summary

Introduction

The term synaptic plasticity describes the ability of synapses to change their strength and efficacy over time. Long-term forms of synaptic plasticity are postulated as cellular mechanisms responsible for learning and memory [1,2]. Changes in synaptic strength are paralleled by changes in the structure of neuronal contacts that underlie long-term circuit reorganization [3,4]. A long-term increase in synaptic strength (long-term potentiation [LTP]) can be expressed postsynaptically, importantly by changes in postsynaptic receptor number or properties [5], and presynaptically, by changes in neurotransmitter release [4].

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Conclusion