Background:

DL-AP5 is the racemic version of the selective N-methyl-D-aspartate (NMDA) receptor antagonist, D-AP5.[1] Whereas D-AP5 is the active (?)-stereoisomer that competitively inhibits the glutamate binding site of NMDA receptors (Kd = 1.4 μM), the (+)-isomer (L-AP5) demonstrates considerably less potent NMDA receptor antagonist activity.[1] AP5 has been widely used to study the activity of NMDA receptors particularly in regard to researching synaptic plasticity, learning, and memory.[2]

Reference:
[1]. Evans, R.H., Francis, A.A., Jones, A.W., et al. The effects of a series of ω-phosphonic α-carboxylic amino acids on electrically evoked and excitant amino acid-induced responses in isolated spinal cord preparations. Br. J. Pharmacol. 75(1), 65-75 (1982).
[2]. Morris, R.J. Synaptic plasticity and learning: Selective impairment of learning rats and blockade of long-term potentiation in vivo by the N-methyl-D-aspartate receptor antagonist AP5. J. Neurosci. 9(9), 3040-3057 (1989).