ABSTRACT Alzheimer's disease (AD) is a debilitating progressive neurodegenerative disorder affecting over 6 million Americans. It is characterized by deficits in memory, which have been attributed to dysfunctional Hebbian plasticity, a compelling synaptic mechanism supporting memory formation. Neurons also continuously adapt their excitability and synaptic weights as response to network activity, harnessing homeostatic mechanisms to preserve network stability and information flow. Our work on the parent NIMH R01 recently uncovered multiple overlapping molecular players between Hebbian plasticity and synaptic homeostasis, providing a valuable clue that homeostatic adaptation may also be disrupted in AD. It is therefore a natural extension of our main project to explore how synaptic homeostasis is dysregulated in AD. Our preliminary findings demonstrate that synaptic upscaling, a classical in vitro model of adaptation to chronic inactivity, is non-functional in neurons derived from 3xTg-AD mice. Under AIM 1, we will address the molecular players preventing synaptic upscaling in 3xTg-AD by focusing on key signaling pathways uncovered by the parent R01 project. Mindful that gene transcription is required for full development of synaptic homeostasis, we further discovered that transcriptomic changes observed in wild-type (WT) neurons were largely absent in 3xTg-AD neurons. AIM 2 will therefore investigate how this molecular program is disrupted, notably by attempting pharmacological interventions to rescue the transcriptional program. We will use FK506 to block calcineurin and force biochemical events normally recruited in the induction of synaptic upscaling. This will provide an avenue to probe where the defect in 3xTg- AD neurons occurs. Finally, we have adopted a clinically-relevant, in vivo model of chronic activity, using isoflurane as an exemplar of the volatile anesthetics used for surgery. This is experimentally useful because isoflurane inhibits sodium channels, much like chronic TTX application, and clinically interesting because of the oft-reported clinical association between general anesthesia and cognitive decline, particularly in the context of AD. Under AIM 3, we discovered that while some transcriptomic changes are conserved across the main cell types in the hippocampal formation, multiple dysregulated genes are observed in a cell type-specific manner. Therefore, neuronal response to chronic inactivity needs to be understood in the context of cell type-specific adaptations. Future experiments will strengthen the link between specific transcriptional changes and homeostasis of neuronal activity or the lack thereof in 3xTg-AD animals. Furthermore, we will provide much needed insight into the molecular underpinnings contributing to post-operative cognitive decline following general anesthesia. Together, the proposed research will shed light on the mechanisms involved in the dysfunctional homeostatic regulation of neuronal activity in a w...