| Release of neurotransmitter from presynaptic terminals is triggered by the influx of Ca2+ in response to the neuronal action potential. Neurotransmitter is packaged in synaptic vesicles, which undergo a series of preparatory steps before exocytosis. This process is termed the vesicle cycle. Synaptic plasticity is the ability of a presynaptic terminal to change its efficiency of release during or following activity. Neurotransmission and synaptic plasticity are thought to underlie learning and memory, and the cellular and molecular mechanisms that occur during these processes are not completely understood.;The vesicle cycle is very highly regulated. This regulation is performed by numerous presynaptic proteins, and the exact functions of these proteins are not fully understood. Two proteins, Synapsin II and Rab3a, have been identified as modulators of the vesicle cycle. Synapsins are a neuron specific and highly abundant family of phosphoproteins that through interactions with the cytoskeleton maintain a reserve pool of vesicles. Synapsins are thought to regulate the availability of vesicles for release. Rabs are small GTP-binding proteins that are thought to play a role in vesicle targeting through protein-protein interactions. Rab3 isoforms are neuron specific, and Rab3a is the most abundant isoform. Although the classical view of the activity of these proteins is thought to be pre-docking, both Synapsin and Rab3a proteins have recently been implicated in acting in post-docking steps of the vesicle cycle. Additionally, Rab3a and the Synapsins have been shown to interact, and the physiological implications of this interaction have not been characterized.;This dissertation work studied the role of Synapsins and Rab3a in regulation of synaptic transmission and short-term synaptic plasticity using the diaphragm muscle of WT, Synapsin knockout, and Rab3a knockout mice. Additionally, a new genetic knockout mouse (the Synapsin II and Rab3a double knockout mouse) was created. The main results of this work demonstrate that Synapsin II and Rab3a generally play opposing regulatory roles in maintaining vesicle distribution within nerve terminals, as well as in maintaining synaptic transmission. Using the new double-knockout mouse, this work has provided evidence that the presence of both Rab3a and Synapsin II is critical for maintaining normal release characteristics. |
