Spatiotemporal Kinetics of AMPAR Trafficking in Single Spines

Abstract

Learning and memory is one of the critical components of the human experience. In one model of memory, hippocampal LTP, it is believed that the trafficking of AMPA receptors to the synapse is a fundamental process, yet the spatiotemporal kinetics of the process remain under dispute. In this work, we imaged the trafficking of AMPA receptors by combining two-photon glutamate uncaging on single spines with a fluorescent reporter for surface AMPA receptors. We found that AMPA receptors are trafficked to the spine at the same time as the spine size is increasing. Using a bleaching protocol, we found that the receptors that reach the spine come from a combination of the surface and endosomal pools. Imaging exocytosis in real time, we found that the exocytosis rate increases briefly (~1 min.), both in the spine and neighbouring dendrite. Finally, we performed pharmacological and genetic manipulations of signaling pathways, and found that the Ras-ERK signaling pathway is necessary for AMPAR exocytosis. In a set of related experiments, we also investigated the capacity of single spines to undergo potentiation multiple times. By stimulating spines twice using glutamate uncaging, we found that there is a refractory period for synaptic plasticity in spines during which they cannot further be potentiated. We furthermore found that inducing plasticity in a given spine inhibits plasticity at nearby spines.