Hippocampal mossy fiber boutons are giant, very plastic synapses that provide a strong and sparse input onto CA3 that is thought to be important for learning and memory. These synapses have a low basal release probability and display various forms of plasticity: frequency facilitation, post-tetanic potentiation (PTP), and a cAMP-dependent presynaptic form of long-term potentiation (LTP).Synapsins are the most abundant neuronal phosphoproteins. Due to their phosphorylation-dependent association with synaptic vesicles they play an important role in synaptic transmission and plasticity.Differently from most synapses, mossy fiber boutons retain synapsin III expression in the mature brain. We sought to determine if the knockout of all synapsin isoforms would influence mossy fiber boutons’ structure and function. To this aim we combined transmission electron microscopy and local field potential recordings of acute hippocampal slices from synapsin triple knockout (SynTKO) and wild type (WT) mice.We observed that both frequency facilitation and the PTP after high-frequency stimulation were lower in SynTKO animals, in line with previous experiments from synapsin double knock-out animals. This phenomenon is likely a consequence of the smaller reserve pool of vesicles observed at the electron microscope.We also measured LTP and saw that SynTKO mossy fibers displayed an increased potentiation 30 minutes after induction accompanied by an increase in AZ density at the ultrastructural level. Further experiments investigating mossy fiber plasticity at synapsin III knockout mice will help us define whether this specific isoform plays a role in presynaptic potentiation at hippocampal mossy fiber boutons.