Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. AP5 or APV ((2R)-amino-5-phosphonovaleric acid; (2R)-amino-5-phosphonopentanoate) is a selective NMDA receptor antagonist that competitively inhibits the ligand (glutamate) binding site of NMDA receptors. AP5 blocks the cellular analog of classical conditioning in the sea slug Aplysia californica, and has similar effects on Aplysia long-term potentiation, since NMDA receptors are required for both. It is sometimes used in conjunction with the calcium chelator BAPTA to determine whether NMDARs are required for a particular cellular process.