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For at least half a century, alteration of synaptic strength through growth at specific nerve terminals has been favored as the mechanism underlying long-term changes in behavior and synaptic plasticity. Although new proteins for synapses can either be synthesized locally or transported from the cell body, recent work on the postsynaptic element (dendritic spines) of cortical excitatory synapses indicates that transmission can also be modified by controlling the density of α-amino-3-hydroxy-5-methylisoxazole-4-proprionic acid receptors (AMPARs) in the postsynaptic density (PSD). This regulation is mediated by mono-ubiquitination, which governs turnover of AMPAR subunits by determining whether the endocytosed subunits are sent to lysosomes to be degraded or recycled back to the membrane by exocytosis. Also important is activity-dependent multi-ubiquitination and degradation of proteins that make up the scaffolding complexes that confine receptors to the PSD.