Since the NMDA receptor was first identified by the selective activation by N-methyl-D-aspartate (NMDA, see below), it has become clear that many different subtypes of this receptor could be expressed due to the multiple combinations of individual subunits. The challenge for medicinal chemists in recent years has thus been to synthesis compounds that can discriminate between these multiple sub-types. In addition to the glutamate (NMDA) binding site, there are also multiple binding sites on the NMDA receptor for modulatory compounds. Efficient NMDA receptor activation requires not only NMDA but also a co-agonist, glycine. Activation can also be modulated by the binding of polyamines. Each of the binding sites (glutamate, glycine, ployamine) has been used as a potential target for the development of both receptor and sub-type selective compounds. Many of the agonists and competitive antagonists described here have been developed by Prof. Jeff Watkins and Dr David Jane, here at Bristol and are available through Tocris Cookson. Click on the links for details of compounds that act on AMPA and mGlu receptorsCompounds named in Red are as yet commercially unavailable; click on the y symbol for a PubMed link.

NMDA Receptor Agonists

Most of the selective NMDA receptor agonists available are, not surprisingly, based on NMDA, the diagnostic ligand for these receptors. NMDA itself is an analogue of aspartate (can also act as a weak agonist at most glutamate receptors). Although this compound acts selectively at NMDA receptors, it cannot dicsriminate between receptor subtypes. Homoquinolinic acid, on the other hand, is a conformationally constrained analogue of glutamate. This compound shows higher affinity for NMDA receptors that contain NR2B and is thus the first NMDA receptor agonist to display sub-type selectivity. A radiolabelled form of homoquinolinic acid, available through Tocris Cookson, has beeen shown to selectively label regions of the brain that express NR2B-containing NMDA receptors.

Diagnostic ligand for the NMDA receptor - none selective

NMDA Receptor Competitive Antagonists

These compounds act at the glutamate binding site. (R)-AP5 is perhaps the most commonly used NMDA receptor antagonist, developed 20 years ago by Jeff Watkins. This compound shows a tendency to selectivity for NR2A and NR2B containing NMDA receptor complexes, with a 5-fold lower Ki value for NR1/NR2A over NR1/NR2D complexes. (R)-CPP-ene follows a similar pattern. However, the degree of selectivity is greater with the Ki value for NR1/NR2D complexes nearly 20-fold higher than that for NR1/NR2A receptors, although it cannot discriminate between NR2A and NR2B-containing complexes. PBPD, on the other hand is the first compound to begin to display selectivity for NR2D over NR2A and NR2C

NMDA Receptor Channel Blockers

These compounds act by binding to the pore of the NMDA receptor channel and are thus non-competitive antagonists. The highest affinity compound available to date is MK-801 with a binding affinity of 18 nM. Both Memantine and Ketamine are lower affinity antagonists. These compounds have proved very effective NMDA receptor antagonists and MK-801 has been used in a readiolabelled form to label NMDA receptor populations in brain slices. However, none of these compounds shows subunit selectivity.

NMDA Receptor Glycine Site Antagonists

Glycine is a co-agonist of glutamate at the NMDA receptor, increasing the affinity of the receptor for the endogenous agonist glutamate. Hence antagonism of this binding site will also antagonise NMDA receptor function. A series of high affinity antagonists have been developed for this binding site on the NMDA receptor. L-701,324, L-689,560 and GV96771A all displace [ ³ H]-glycine binding with affinities below 10 nM. None of these compounds, however, display sub-type selectivity in their actions. This is likely due to the glycine binding site being located on the NR1 subunit while the glutamate binding site is on the NR2 subunits.

NMDA Receptor Polyamine Site Antagonists

Polyamines modulate NMDA receptor function. For instance, spermine stimulates NMDA receptors and increases the affinity for glycine. Conversly, it may aslo decrease the affinity of the receptor for glutamate and generate a voltage dependent inhibition of the channel. The effects of ployamine site antagonists are specific for channels containing the NR2B subunit. Hence, ployamine site antagonists would be very useful as NR2B selective ligands. The highest affinity antagonists for this site is currently Ro 25-6981 that has a binding affinity of 6 nM.

Summary

In summary, probably the most widely used and best characterised of all the NMDA receptor antagonists is still D-AP5, although new compounds are now being developed that show greater subtype selectivity such as (R)-CPPene and PBPD. The polyamine site antagonists should be very useful as NR2B selective ligands whilst the glycine site antagonists currently display the highest affinity for this receptor.