Regulation of Synaptic Vesicle Trafficking at Central Synapses
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Synapses are where electrical information is converted to chemical signaling, allowing for careful regulation of inter-neuronal communication in the brain. At presynaptic terminals, synaptic vesicles fuse with plasma membrane in response to electrical stimulation, followed by rapid retrieval to the terminal and re-organization for reuse. Thus, synaptic vesicle trafficking is of interest as to where presynaptic regulations of synaptic transmission begins to occur. The first two chapters explored a novel secretagogue, lanthanum (La³⁺), and its potential usage as a probe to study vesicle recycling at central synapses. Chapter two describes the characteristics of La³⁺-evoked transmission at hippocampal synapses. La³⁺ has two separate actions on transmission, with a different time course and underlying mechanism of action. This newly characterized rapid action of La³⁺ is intracellular Ca²⁺-independent, in contrast to its delayed action, yet requires functional SNARE complex formation. Therefore, chapter three took advantage of La³⁺-evoked transmission as a tool to investigate the coupling between exo- and endocytosis in SNARE-dependent fusion. Using multifaceted approaches, I propose that La³⁺ induces transmitter release via narrow fusion pore opening and closure, or a ‘kiss-and-run’ mode of exo- and endocytosis. Chapter four investigates the molecular requirement for the synaptic vesicle recycling pathway. I analyzed the impact of one of main players in endocytosis, dynamin in different forms of release. Acute inhibition of dynamin in central synapses impairs activity-dependent synaptic vesicle recycling while leaves spontaneous recycling intact, suggesting the operation of two parallel recycling pathways in central synapses as well as proposing the molecular signature between spontaneously and activity-dependently recycling pathways. In chapter five, I further investigated the origins of spontaneously recycling synaptic vesicles by simultaneous monitoring of spectrally separable FM dyes, as chapter suggested four that they are originated from an isolated pool. This chapter includes comprehensive analysis of the endocytic pathway operating at rest and its molecular participants –specifically dynamin, which was implicated to play a role in the endocytic pathway from observations I made in chapter four. Chapter six expands the investigation as to how presynaptic signaling regulates synaptic vesicle trafficking in glutamatergic synapses. I focused on the impact of ambient glutamate concentration on vesicle recycling as a feedback signal to rapid synaptic reuse to impact short-term synaptic plasticity. Taken together, these results suggest that synaptic vesicle trafficking is an actively regulated process, impacting various aspects of information cascades between neurons.