Effects of thiocyanate and AMPA receptor ligands on (S)-5-fluorowillardiine, (S)-AMPA and (R,S)-AMPA binding.

AMPA receptors can be labeled using the agonist radioligands 3H-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (3H-AMPA), 3H-AMPA or 3H-5-fluorowillardiine. In the presence of KSCN, 3H-AMPA and 3H-AMPA bind to a single population of sites in rat brain membranes, whereas 3H-5-fluorowillardiine binds with two affinity components. KSCN increased the affinity of the low affinity 3H-5-fluorowillardiine component > 4-fold and increased the density of both components 1.5-1.7-fold, arguing against KSCN-induced interconversion of low to high affinity states. KSCN, which promotes receptor desensitization, increased the potency of AMPA isomers, (S)-5-fluorowillardiine, quisqualate and cyclothiazide for inhibition of 3H-5-fluorowillardiine binding suggesting that these ligands discriminate desensitized and nondesensitized receptors. In contrast, KSCN did not greatly affect the potency of glutamate, kainate, or competitive antagonists suggesting that these ligands do not discriminate desensitized and nondesensitized receptors. In the presence of KSCN, the rank order potency for agonists and antagonists was similar or identical in all assays indicating that the three radioligands bind identical glutamate recognition sites, a conclusion supported by their identical total receptor density. However, AMPA isomers displayed 6-10-fold higher potency for displacement of 3H- or (R,S)-AMPA relative to 3H-5-fluorowillardiine binding. This finding, coupled with the marked two component binding by 3H-5-fluorowillardiine but not 3H- or (R,S)-AMPA, suggests qualitative differences between the interaction of these ligands with the agonist recognition site.