Proteins that bind to the cytoplasmic tails of AMPA receptors control receptor trafficking and thus the strength of postsynaptic responses. Here we show that AP2, a clathrin adaptor complex important for endocytosis, associates with a region of GluR2 that overlaps the NSF binding site. Peptides used previously to interfere with NSF binding also antagonize GluR2-AP2 interaction. Using GluR2 mutants and peptide variants that dissociate NSF and AP2 interaction, we find that AP2 is involved specifically in NMDA receptor-induced (but not ligand-dependent) internalization of AMPA receptors, and is essential for hippocampal long-term depression (LTD). NSF function, on the other hand, is needed to maintain synaptic AMPA receptor responses, but is not directly required for NMDA receptor-mediated internalization and LTD.