The number and subunit compositions of AMPA receptors (AMPARs), hetero- or homotetramers composed of four subunits GluA1–4, in the synapse is carefully tuned to sustain basic synaptic activity. This enables stimulation-induced synaptic plasticity, which is central to learning and memory. The AMPAR tetramers have been widely believed to be stable from their formation in the endoplasmic reticulum until their proteolytic decomposition. However, by observing GluA1 and GluA2 at the level of single molecules, we find that the homo- and heterotetramers are metastable, instantaneously falling apart into monomers, dimers, or trimers (in 100 and 200 ms, respectively), which readily form tetramers again. In the dendritic plasma membrane, GluA1 and GluA2 monomers and dimers are far more mobile than tetramers and enter and exit from the synaptic regions. We conclude that AMPAR turnover by lateral diffusion, essential for sustaining synaptic function, is largely done by monomers of AMPAR subunits, rather than preformed tetramers.

Memory and learning require structural synaptic plasticity. Among the key players for synaptic plasticity are the alpha-amino-3-hydroxy-5-methyl-4isoxazolepropionic acid (AMPA) subtype of ionotropic glutamate receptors (AMPARs), which are concentrated in the synapse to mediate excitatory transmission. AMPARs are composed of four subunits (GluA1-4), and work as ion channels in the forms of homo-and hetero-tetramers. These tetramers are formed at their exit from the endoplasmic reticulum and it was widely believed that these tetramers are very stable entities. However, we considered that the direct evidence for the existence of stable tetramers in the plasma membrane (PM) is lacking, and that if the tetramers were very stable entities, modulating the tetramer compositions in the synapse, which is known to occur rapidly after stimulation, would be difficult. We examined this issue using single-molecule imaging and tracking. We found that, at variance with the general belief, many GluA1 and GluA2 molecules expressed in the PM of HEK293 cells exist as monomers, and form metastable homo-and heterotetramers (100 and 200 ms, respectively), while they instantaneously fall apart into monomers, dimers, or trimers, which again form tetramers readily. In the dendritic-shaft PM of mouse hippocampal neurons, GluA1 and GluA2 also dynamically merged into greater oligomers on similar time scales to those in the HEK293-PM, suggesting that they form only metastable tetramers in neuronal PM. GluA1 and GluA2 monomers and dimers were much more mobile than tetramers in the dendritic-shaft PM and entered the spines quite readily. These results suggest a novel mechanism for synaptic plasticity: During synaptic stimulation, the AMPAR subunit numbers and compositions in the postsynaptic membranes could be regulated readily by recruiting the preferred subunits pooled in dendritic-shaft PM, mostly in the forms of monomers and dimers. N-methyl-d-aspartate (NMDA) ionotropic glutamate receptors are universal in mammalian central neurons that are engaged with neuronal advancement, synaptic versatility and memory. The integrity of mixture of pre-and post-synaptic action by NMDA receptors requires a coupling between receptor's binding and ion channel opening. Experimental studies have shown efficient interactions of various bioactive compounds for therapeutic applications. We have performed all-atom molecular dynamics (MD) simulations of two different NMDA receptors. Unbiased MD simulations of apo as well as agonist-and antagonist-bound proteins were used to characterize the local conformational dynamics of NMDA glutamate receptor mostly within the ligand binding site, while nonequilibrium pulling simulations were used to study the coupling between the local and the global conformational changes associated with the channel activation process. These pulling simulations were employed to trigger the opening of the channel by imposing rotational changes on the orientation of transmembrane (TM) alpha helices. The agonist-bound simulation has shown a clo...