Title: Bridging Synaptic Plasticity and Neural Network Activity underlying Short-term Memory.

Synapses are the foundational unit of neural connectivity through which transient functional activity is translated into lasting structural changes in the postsynaptic neurons. While several seminal studies delineated the relationship between synaptic plasticity and neural activity in vivo, the technical difficulties in achieving the spatial resolution required to resolve synaptic proteins significantly biased these studies to focus on a small, superficial part of the brain (Roth et al., 2020, Hayashi-Takagi et al., 2015). To circumvent the limitation, we developed a combined ex-vivo and in-vivo pipeline that enables a whole-brain, unbiased approach to studying synaptic proteins. First, we cleared whole brains of knock-in mice in which glutamate AMPA receptor subunit 1 or 2, are tagged with green fluorescence (SEP-GluA1 or SEP-GluA2 mice). These cleared brains were then imaged in synaptic resolution using light-sheet microscopy to pinpoint a brain region of interest for in-vivo imaging in an unbiased way. Our preliminary data highlight a cluster of neurons in the piriform cortex in which SEP-GluA2 expression, but not SEP-GluA1, is specifically enriched in the cell body. These SEP-GluA2 enriched neurons were only present in the piriform cortex and nowhere else in the brain, providing a scientific justification to zoom into the area in imaging GluA2 in vivo. Concurrently, we also developed a microprism-guided 2-photon imaging method that enables deep brain imaging in synaptic resolution. Gradient index micro lens (GRIN lens) are often used for imaging deep brain structures, but its low NA limits the spatial resolution. Instead, we implanted a microprism with high NA to acquire synaptic resolution images from deep brain structures. Ongoing work is to complete light-sheet imaging data collection/analysis and apply synaptic resolution in-vivo imaging pipeline to the piriform cortex to close the loop.