Interaction of Quantal Inhibitory and Excitatory Neurotransmission in Regulation of Synaptic Homeostasis via Molecular Signaling

The processing and flow of information through brain circuits is undeniably shaped by individual neurons and synapses. The type of synapse between any two given neurons determines the reliability, speed, and strength of signal transfer. Therefore, studying the organization and signaling capacity of the variety of synapses present in our nervous system is critical to understanding how information is shaped and communicated between neurons. Neurotransmission can be classified into two broad types: spontaneous and evoked. Much of the work examining these types of neurotransmission and their properties has been done in excitatory synapses. Yet the variety of inhibitory neurons is so vast that Ramon y Cajal referred to them as the "Butterflies of the Soul." In order to help paint a more complete picture of the brain, this PhD thesis is focused on the structure, function, and regulation of synapses and neurotransmission, with a focus on inhibitory synapses. We first expanded the tools available to study synapses by demonstrating that lentiviral CRISPR could be used in postmitotic cells to knockout synaptic proteins and study non-cell autonomous phenotypes. Next, we took advantage of the use-dependent properties of the GABAA receptor antagonist picrotoxin, demonstrating that this drug could be used to interrogate inhibitory synapse parameters, such as presynaptic release probability. By utilizing picrotoxin, we were able to examine the postsynaptic organization of spontaneous and evoked neurotransmission at inhibitory synapses, and discovered a partial segregation of the postsynaptic receptors activated by evoked and spontaneous neurotransmission. This result implied that spontaneous and evoked neurotransmission at inhibitory synapses may have partially non-overlapping functions. Because the function of inhibitory spontaneous neurotransmission is largely unknown, we next examined the signaling capacity of inhibitory spontaneous neurotransmission. We discovered that modulation of inhibitory, but not excitatory, spontaneous neurotransmission alters transcription of certain activity-induced genes, including Bdnf. Furthermore, blockade of inhibitory spontaneous neurotransmission leads to downscaling of excitatory synapses through a BDNF-dependent mechanism. Finally, we examined the regulation of synapse function by miRNAs regulated by MeCP2, a gene in which loss of function mutations are the primary cause of Rett Syndrome. We characterized the effects of two candidate miRNAs on synapse function. One of these miRNAs, miR-101a, has opposing impacts on excitatory and inhibitory synapses, suggesting a role in regulating excitatory/inhibitory balance, a feature that is often altered in ASD and ASD-like disorders such as Rett Syndrome. In summary, we have contributed to the study of synapses by expanding the tools available, improving understanding of inhibitory synapse structure and function, and examining the broad regulatory capacity of certain miRNAs over synaptic function.