Historically, the development of selective agonists and antagonists for AMPA receptors has been made difficult by their similarity to kainate receptors.
As would be expected, AMPA itself shows a good selectivity for AMPA receptors over kainate receptors (10-20 fold higher affinity for GluA1-4 over a representative kainate receptor subunit, GluK1). However, it shows no selectivity for different AMPA receptor subunits. In contrast, compounds based on willardiine are not only more selective for AMPA receptors than AMPA itself (5-fluorowillardiine has a 70-150 fold higher affinity for GluA1 and 2 over GluK1) but some also show a degree of selectivity for one or more AMPA receptor subunits. For example, 5-fluorowillardiine shows a 10-20 fold selectivity for GluA1 and 2 over GluA4. The importance of the halogen substitution at the 5-position can be clearly seen - replacing the fluorine with a chlorine atom (5-chlorowillardiine) maintains AMPA receptor subunit selectivity, but loses the selectivity between AMPA and kainate receptors. Indeed, substitution with an iodine atom (5-iodowillardiine) generates a GluK1 selective agonist.
| Compound | AMPA | 5-Fluorowillardiine | 5-Chlorowillardiine |
|---|---|---|---|
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 |
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|
| GluA1 | 103±13 | 14.7±1.3 | 65±11 |
| GluA2 | 107±16 | 25.1±5.2 | 53.1±4.4 |
| GluA4 | 155±10 | 305±107 | 451±65 |
| GluK1 | 2144±416 | 1820±149 | 57.1±23.5 |
| References | Jane et al 1997 | Jane et al 1997 | Jane et al 1997 |
The most commonly used AMPA receptor antagonists are the quinoxiline derivatives CNQX and NBQX, which show a 20-150 fold selectivity for AMPA over kainate receptor subunits. A further useful antagonist is (R)-3,4-DCPG. This compound has no detectable antagonist activity at kainate receptors, although it is active at NMDA receptors. However, this cross-reactivity is easily overcome by the use of AP-5, a selective NMDA receptor antagonist.
Another useful antagonist is GYKI53655, a non-competitive antagonist that shows good selectivity for AMPA receptors over kainate receptors. This compound can thus be used to distinguish these two receptor subtypes. Cyclothiazide, another commonly used modulator of AMPA/kainate receptors, blocks receptor desensitisation and thus potentiates the current passed by these receptor channels.
| Compound | NBQX | (R)-3,4-DCPG | GYKI53655 | Cyclothiazide |
|---|---|---|---|---|
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| GluA1-4 | Ki=0.1-0.9μM | Kd=77μM (AMPA) | IC50=6μM | |
| GluK1 | Ki=19.76μM | Kd>3000μM (Kainate) | >30%@100μM | |
| GluK2 | Ki=15.79μM | >30%@100μM | ||
| References | Thomas et al 1997 | Bleakman et al 1996 | ||
| Thomas et al 1998 | Bleakman et al 1999 |
