Mapping and longitudinal imaging of potentiated synapses in the mouse hippocampus using SynActive-eGRASP

Identifying individual synapses that undergo long-term potentiation following learning at specific circuits remains a major challenge in neuroscience. To address this problem, we developed SA-eGRASP, a genetically encoded reporter that combines SynActive (SA) targeting with enhanced GFP Reconstitution Across Synaptic Partners (eGRASP). SA drives the expression of the postsynaptic half of eGRASP at potentiated spines via the 5' and 3' untranslated regions (UTRs) of the immediate-early gene Arc, thus enabling selective labeling of individual synapses that are both structurally connected and have undergone potentiation through transsynaptic GFP reconstitution. Here, we validated SA-eGRASP and combined it with high-resolution optical imaging to generate a detailed map of hippocampal CA3-CA1 synapses subjected to potentiation following contextual fear conditioning in mice. The high signal-to-noise ratio of SA-eGRASP, allowed us to track individual potentiated dendritic spines over the course of several days, to investigate the long-term dynamics of CA3-CA1 potentiated dendritic spines, and to study the selective potentiation of a subset of spines upon experience. SA-eGRASP paves the way to the investigation of synaptic engrams by enabling longitudinal, circuit-specific investigation of synapses potentiated during learning in mice performing naturalistic behaviors.