GABA synthesis in developing hippocampus: SNAT1 surfaces as a dynamic regulator of inhibitory synaptic transmission
Brown, Molly Nicole
GABA functions as the primary inhibitory neurotransmitter in the mammalian brain. In hippocampus, GABA serves multiple roles during development and throughout adulthood, which include: 1) orchestrate synapse maturation and synaptogenesis, 2) maintain inhibitory synaptic transmission, and 3) regulate the excitability and synchronize the neuronal output of principal neurons. Because normal brain function requires intact inhibitory synaptic transmission, interneurons must possess mechanisms to adapt synaptic GABA during a wide range of activity states. I hypothesized that one effective mechanism may involve regulating GABA synthesis and vesicular GABA content through modulating the supply of glutamine, an indirect GABAergic metabolic precursor, by System A glutamine transporters. In addition, because GABA serves critical roles during early hippocampal development we hypothesized that the role of System A transporters at inhibitory synapses also may be developmentlally regulated. Therefore, this dissertation explored if modulation of System A surface activity dynamically regulated GABA synthesis and inhibitory synaptic transmission in response to changing developmental and neuronal activity states. Using electrophysiology in hippocampal slices in conjunction with protein expression studies and uptake assays in synaptosomes, I demonstrate that System A transporters serve both a constitutive and activity-dependent role in modulating vesicular GABA content and inhibitory synaptic strength. Synaptic depolarization up-regulates the surface activity of System A transporters and thus induces an increase in vesicular GABA content in both immature and mature hippocampus. However, because System A’s basal uptake activity and therefore its constitutive contribution to vesicular GABA content diminishes over the first two postnatal weeks, its role in mature hippocampus is only manifest in an activity-dependent manner. Furthermore, my results support that these constitutive and activitydependent roles are likely mediated by the SNAT1 subtype of System A transporters. Therefore, my findings strongly support the hypothesis that the surface activity of SNAT1, regulated both by depolarization and by developmental cues, is the key component in a novel mechanism to dynamically link metabolic demand for GABA with vesicular GABA content and inhibitory synaptic strength.