Short-Term Plasticity at Hippocampal Mossy Fiber Synapses Is Induced by Natural Activity Patterns and Associated with Vesicle Pool Engram Formation

Post-tetanic potentiation (PTP) is an attractive candidate mechanism for hippocampus-dependent short-term memory. Although PTP has a uniquely large magnitude at hippocampal mossy fiber-CA3 pyramidal neuron synapses, it is unclear whether it can be induced by natural activity and whether its lifetime is sufficient to support short-term memory. We combined in vivo recordings from granule cells (GCs), in vitro paired recordings from mossy fiber terminals and postsynaptic CA3 neurons, and “flash and freeze” electron microscopy. PTP was induced at single synapses and showed a low induction threshold adapted to sparse GC activity in vivo. PTP was mainly generated by enlargement of the readily releasable pool of synaptic vesicles, allowing multiplicative interaction with other plasticity forms. PTP was associated with an increase in the docked vesicle pool, suggesting formation of structural “pool engrams.” Absence of presynaptic activity extended the lifetime of the potentiation, enabling prolonged information storage in the hippocampal network.

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Natural activity patterns in hippocampal GCs in vivo induce PTP at mossy fiber synapses

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PTP is primarily caused by an increase in the readily releasable vesicle pool

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PTP is associated with an increase in the number of docked vesicles at active zones

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Sparse activity extends pool engram lifetime, increasing overlap with short-term memory

Functional Electron Microscopy, "Flash and Freeze," of Identified Cortical Synapses in Acute Brain Slices

How structural and functional properties of synapses relate to each other is a fundamental question in neuroscience. Electrophysiology has elucidated mechanisms of synaptic transmission, and electron microscopy (EM) has provided insight into morphological properties of synapses. Here we describe an enhanced method for functional EM (“flash and freeze”), combining optogenetic stimulation with high-pressure freezing. We demonstrate that the improved method can be applied to intact networks in acute brain slices and organotypic slice cultures from mice. As a proof of concept, we probed vesicle pool changes during synaptic transmission at the hippocampal mossy fiber-CA3 pyramidal neuron synapse. Our findings show overlap of the docked vesicle pool and the functionally defined readily releasable pool and provide evidence of fast endocytosis at this synapse. Functional EM with acute slices and slice cultures has the potential to reveal the structural and functional mechanisms of transmission in intact, genetically perturbed, and disease-affected synapses.

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Functional EM may be applied to acute brain slices and organotypic slice cultures

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Docked vesicle pool and RRP are overlapping

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Smaller-diameter vesicles have higher release probability than larger vesicles

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Endocytic pits after moderate stimulation suggest fast endocytosis