Single molecule tracking of AMPA receptors shows the role of synaptic insertion during maintenance of chemical LTP

Long term potentiation (LTP) likely contributes to memory formation. Early expression of LTP involves insertion of AMPA receptors (AMPARs) to the extrasynaptic membrane followed by their lateral diffusion into the synaptic membrane. However, whether a similar mechanism mediates the maintenance of LTP is unclear. Using single-molecule microscopy, we quantified that 6 GluA1- and 11 GluA2-containing endogenous AMPARs were added per synapse in cultured hippocampal neurons at 20 min following chemical LTP (cLTP) induction for 10 min, resulting in a 54% increase for both subunits. Single molecular tracking of transfected subunits revealed that the number of exocytosed subunits at the synapse increased by 15-18% from 5 to 20 min following cLTP induction, but their lateral exchange between synaptic and extrasynaptic membranes was minimal. These findings suggest that cLTP maintenance is contributed largely by synaptic insertion of AMPARs rather than the surface diffusion of exocytosed AMPARs from extrasynaptic to synaptic regions.