NMDA receptor pharmacology: perspectives from molecular biology.

The NMDA receptor is an important target for drug development, with agents from many different classes acting on this receptor. While the severe side effects associated with complete NMDA receptor blockade have limited clinical usefulness of most antagonists, the understanding of the multiple forms of NMDA receptors provides an opportunity for development of subtype specific agents with potentially fewer side effects. Different NMDA receptor subtypes are assembled from combinations of NR1 and NR2 subunits with each subunit conveying distinct properties. The NRI subunit is the glycine binding subunit and exists as 8 splice variants of a single gene. The glutamate binding subunit is the NR2 subunit, which is generated as the product of four distinct genes, and provides most of the structural basis for heterogeneity in NMDA receptors. Pharmacological heterogeneity results from differences in the structure of ligand binding regions, as well as structural differences between subtypes in a modulatory region called the LIVBP-like domain. This region in NR1 and NR2B controls the action of NR2B-selective drugs like ifenprodil, while this domain in receptors containing the NR2A subunit controls the action of NR2A-selective drugs such as zinc. This suggests that NMDA receptor subtype selective drugs can be created, and further understanding of subtype specific mechanisms ultimately may allow successful use of NMDA receptor antagonists as therapeutic agents.