The flexible synapse – How mossy fiber architecture adapts to changing needs

Abstract

Hippocampal mossy fiber boutons are unique, highly plastic synapses within the hippocampal circuitry. Despite mossy fiber bouton’s potential role in learning and memory processes, the precise underlying mechanisms leading to their strengthened synaptic connections are still not fully understood. Here, we provide an overview of the structural changes occurring during long-term potentiation of large presynaptic terminals formed by mossy fiber onto CA3 pyramidal cells. Such changes encompass (1) adaptations in the number, shape and size of the bouton; (2) changes in availability of synaptic vesicles as well as the number and occupancy of release sites within single boutons; and (3) nano-architectural changes in the molecular composition and spatial arrangements within active zones. We describe these changes and possible implications for mossy fiber function. Furthermore, we discuss open questions, current methodology, and possible future directions.