Summary/Abstract The hippocampal mossy fiber (MF) synapse exhibits robust short term presynaptic plasticity that is responsible for gating the flow of information into the CA3 region. MF axons make synapses with both CA3 pyramidal cells (PCs) and stratum lucidum interneurons (SLINs), producing both direct excitation and feedforward inhibition onto the local network. The properties of these synaptic connections have divergent phenotypes with MF-PC synapses showing facilitation and the MF-SLIN synapse depressing during a burst of granule cell activity. This results in a net effect of reduced inhibition and increased excitation during bursts of high frequency presynaptic firing that causes suprathreshold activation of CA3 PCs. Despite a large body of research on these synaptic mechanisms, it is not clear what underlies this divergent synaptic behavior of synapses formed by the same axon and in some cases by different compartments of the same presynaptic terminal. The goal of this fellowship is to determine if the high affinity Ca2+ sensor synaptotagmin 7 (Syt7), is responsible for creating this high pass filter in the area CA3. To address this question I will utilize a newly created conditional knockout mouse line where Syt7 is lost in MF presynaptic terminals. I will determine if Syt7 demonstrates compartmentalized activity at MF to PC and MF to SLIN connections, and thus constitutes the molecular mechanism for this circuit's behavior. Using genetic replacement approaches I will determine whether the C2 Ca2+ binding domains are necessary to explain the function of Syt7 in MF synapses. Furthermore, using high resolution biochemical and microscopy approaches I will determine the subcellular location of Syt7 in MF synapses to understand how the protein’s localization informs its function. Identifying the presynaptic mechanisms responsible for information transfer will allow future work to directly test the molecular processes underlying complex hippocampal-dependent behaviors.