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AMPA/Kainate Receptors (updated March 18, 2002) |
| AMPA and Kainate receptors are built from closely related subunits (AMPA - GluR1-4; Kainate - GluR5-7, KA-1,2). The diagnostic agonists for these two receptors (AMPA and kainate, respectively) also activate the other receptor (see below). The synthesis of compounds that can discriminate bwetween these closely related receptors has thus proved a great challenge to medicinal chemists. A further challenge has been to develop pharmacological agent that can discriminate between AMPA or kainate receptor that contain particular subunits. The information detailed below has been obtained by expressing the human isoforms of the receptor subunits to create homomeric channels. Many of the compounds described are available through Tocris Cookson. Click on the link for details of compounbds that act on NMDA and mGlu receptors Compounds named in Red are as yet commercially unavailable; click on the y symbol to follow a PubMed link. |
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AMPA Receptor Agonists As would be expected, AMPA itself shows a good selectivity for AMPA receptors over kainate receptors (10-20 fold higher affinity for GluR1-4 over a representative kainate subunit, GluR5). However, it shows no selectivity for different AMPA receptor subunits. In contrast, compounds based on willardine are not only more selective for AMPA receptors than AMPA itself (5-fluorowillardiine has a 70-150 fold higher affinity for GluR1,2 over GluR5), but also shows selectivity for AMPA receptor subunits. 5-fluorowillardiine shows a 10-20 fold selectivity for GluR1 and 2 over GluR4, while 5-chloro-6-azawillardiine shows some selectivity for GluR4 over GluR1 and 2, but at the cost of losing selectivity for AMPA over kainate receptors. 5-iodo-6-azawillardiine shows a similar pattern of selectivity (see below). |
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AMPA |
5-F-Will |
5-Cl-6-Azawilly | |
|
Subunit |
Ki Values (nM) for Displacement of [ 3 H]-AMPA Binding | ||
|
hGluR1 |
103 ± 13 |
14.7 ± 1.3 |
7.1 6 ± 2.2 |
|
hGluR2 |
107 ± 16 |
25.1 ± 5.2 |
16.7 ± 6.6 |
|
hGluR4 |
155 ± 10 |
305 ± 107 |
3.6 ± 1.4 |
|
Ki Values (nM) for Displacement of [ 3 H]-Kainate Binding | |||
|
hGluR5 |
2144 ± 416 |
1820 ± 149 |
30.9 ± 5.4 |
| (5-Cl-6-azawillardine is available from Dr David Jane.) | |||
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Kainate Receptor Agonists A number of agonists show selectivity for kainate receptors over AMPA receptors. Apart from kainate itself, ATPA and 5-iodowillardiine show high degrees of selectivity for GluR5 over AMPA receptor subunits (1400 and 700 fold, respectively). Neither ATPA or 5-iodowillardiine significantly activate GluR6; however, both have been shown to have weak agonist activity at GluR6/KA2 heteromers. 5-iodowillardiine also has weak agonist activity at GluR7/KA2 heteromeric receptors. Interestingly, 5-iodowillardiine also shows a degree of selectivity for GluR1 and 2 over GluR4, whereras this is reversed for 5-iodo-6-azawillardiine (compare this with 5-Cl-6-azawill). This compound also shows a similar affinity for GluR5. Thus far there are no selective agonists for any other kainate receptor subunits. | |||
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Kainate |
ATPA |
5-I-Will |
5-I-6-Azawilly | |
|
Subunit |
Ki Values (nM) for Displacement of [3H]-AMPA Binding
| |||
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hGluR1 |
7450 ± 2020 |
}6000-14000 |
163 ± 42 |
73.9 ± 4.8 |
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hGluR2 |
12200 ± 2740 |
176 ± 29 |
101 ± 37 | |
|
hGluR4 |
1710 ± 170 |
972 ± 155 |
19.9 ± 8.2 | |
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Ki Values (nM) for Displacement of [ 3 H]-Kainate Binding | ||||
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hGluR5 |
177 ± 22 |
4.3 ± 1.1 |
0.24 ± 0.06 |
19.8 ± 6.0 |
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AMPA Receptor Antagonists 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. A compound that has been synthesised recently, LY293558, shows good selectivity for AMPA receptors over GluR6 homomeric channels and GluR7/KA-2 heteromeric channels. However, this compound, based on the decxahydroisoquinoline structural motif is active at GluR5 homomeric receptor channels with a similar Ki to AMPA receptor channels. This may reveal a template for the synthesis of kainate selective compounds (see LY382884 below). |
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NBQX |
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Ki (µM)
|
Kd (µM) * | ||
|
GluR1-4 |
0.1 - 0.9 |
3 -50 |
77 (AMPA) |
|
GluR5 |
19.76 |
4.8 |
>3000 (Kainate) |
|
GluR6 |
15.79 |
>100 |
472 (NMDA; RS mix) |
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GluR7/KA-2 |
- |
>100 |
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| * - for antagonism of agonist mediated depolarisations in the rat spinal cord preparation | |||
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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. |
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Subunit |
IC50 (µM) |
|
GluR1 1 |
6 | |
|
GluR4 1 |
5 | |
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GluR5 2 |
>30% @ 100 µM | |
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GluR6 2 |
>30% @ 100 µM | |
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GluR6/KA-2 2 |
>30% @ 100 µM | |
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1 - antagonism of AMPA-induced currents in HEK293 cells expressing the appropriate receptor 2 - % inhibition of kainate activated currents in HEK293 cells expressing the appropriate receptor | |
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Cyclothiazide |
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Kainate Receptor Antagonists Until recently, very few selective kainate receptor antagonists had been synthesised. The most commonly used antagonist has been NS-102, that shows a 20 fold selectivity for kainate receptors over AMPA receptors. This compound, however, suffers from serious solubility problems, and thus it's usefulness is limited. A much more useful compound is LY382884. This compound is selective for GluR5 comtaining receptor complexes and is inactive at AMPA receptors, GluR6 homomeric receptors and GluR7/KA-2 heteromers. It is interesting that this compound is closely related to LY293558, (see above), which also shows selective antagonism of GluR5 over GluR6 and GluR7/KA-2. A new antagonist UBP301, a novel willardiine derivatve, shows a nearly 30-fold selectivity for kainate over AMPA receptors in the spinal cord. |
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 |
 | ||
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UBP 301y |
NS-102 (available from Research Biochemicals International) |
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Kd (µM) 1 |
Ki (µM) 1 |
Ki (µM) 2 | ||
|
AMPA |
164 ± 25 (IC50) |
114 |
GluR1-4 |
>100 |
|
Kainate |
5.94 ± 0.63 |
6 |
GluR5 |
6.8 |
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GluR6 |
>100 | |||
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GluR7/KA-2 |
>100 | |||
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1 - antagonism of either AMPA or kainate receptor mediated current; |
2 - for GluR1-4, displacement of [ 3 H]-AMPA, otherwise displacement of [ 3 H]-kainate | |||
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Summary In summary, there are more pharmacological tools that are selective for AMPA receptors than for kainate receptors. These two receptor sub-types are difficult to distinguish from each other. The most commonly used antagonists, CNQX and NBQX, are now being superseded by more selective compounds such as GYKI53665 and LY382884. New developments in the synthesis of agonists has identified the willardiine derivatives as the best hope for the synthesis of subunit selective pharmacological agents as well as sub-type selective compounds. Thus kainate receptor selective agonists such as (5)-I-willardiine and antagonists like UBP301 should prove very useful in the analysis of the role of GluR5 containing kainate receptors. In addition, a further willardiine analoge, UBP282, can be used to isolate non-GluR5 containing kainate receptors as it has antagonist activity against GluR5 and AMPA receptors y. |
