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Synapsins are highly abundant presynaptic proteins that play a crucial role in neurotransmission and plasticity via the clustering of synaptic vesicles. The synapsin III isoform is usually downregulated after development, but in hippocampal mossy fiberboutonsit persists in adulthood. Mossy fiberboutonsexpress presynaptic forms of short- and long-term plasticity, which are thought to underlie different forms of learning. Previous research on synapsins at this synapse focused on synapsin isoforms I and II. Thus, a complete picture regarding the role of synapsins in mossy fiber plasticity is still missing. Here, we investigated presynaptic plasticity at hippocampal mossy fiberboutonsby combining electrophysiological field recordings and transmission electron microscopy in a mouse model lacking all synapsin isoforms. We found decreased short-term plasticity - i.e. decreased facilitation and post-tetanic potentiation - but increased long-term potentiation in male synapsin triple knockout mice. At the ultrastructural level, we observed more dispersed vesicles and a higher density of active zones in mossy fiberboutonsfrom knockout animals. Our results indicate that all synapsin isoforms, including synapsin III, are required for fine regulation of short- and long-term presynaptic plasticity at the mossy fiber synapse.

Significance statement

Synapsins cluster vesicles at presynaptic terminals and shape presynaptic plasticity at giant hippocampal mossy fiberboutons. Deletion of all synapsin isoforms results in decreased short- but increased long-term plasticity.