Homeostatic signaling systems operate at synapses to enable flexible yet stable information transfer in the nervous system. Defects in homeostatic signaling contribute to seizures, excitotoxicity, cognitive decline, and neurodegeneration. Although much has been learned in recent years about the expression mechanisms synapses employ to counteract perturbations to neurotransmission, the pathways that rapidly initiate and chronically maintain homeostatic signaling remains poorly understood. Here, we propose to determine the induction mechanisms mediating homeostatic synaptic plasticity using the Drosophila neuromuscular junction as a unique and powerful model system. At this glutamatergic synapse, pharmacologic or genetic disruption to postsynaptic neurotransmitter receptors triggers a retrograde signaling system that leads to a compensatory increase in presynaptic glutamate release to maintain stable synaptic strength, referred to as presynaptic homeostatic potentiation (PHP). This process parallels similar phenomena observed in a variety of other organisms, including mammalian central synapses. We have recently discovered an E3 ubiquitin ligase adaptor that targets substrates in the postsynaptic compartment and enables retrograde homeostatic signaling. We propose to first identify and characterize postsynaptic targets of the homeostatic signaling system. Preliminary data suggests a key component of the postsynaptic density is necessary for retrograde homeostatic signaling. Next, we will define the induction mechanisms mediating the chronic expression of PHP and determine the role of CaMKII in this process. Finally, we will interrogate the pharmacological induction of PHP and test a hypothesis that trans-synaptic complexes mediate rapid retrograde homeostatic signaling. These studies will leverage a synergistic combination of molecular genetic, electrophysiological, and innovative functional imaging approaches at confocal, super resolution, and ultrastructural levels to determine the induction mechanisms that initiate and maintain retrograde homeostatic signaling. Together, these experiments will advance our understanding of the fundamental mechanisms that endow synapses with the capacity to sense perturbations to neurotransmission and adaptively modulate synaptic function to stabilize information transfer in the nervous system.