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 [H-3](R,S )-alpha-amino-3-hydroxy-5-methylisoxazole acid-4-propionic acid ([H-3] (R,S)-AMPA), [H-3](S)-AMPA or [H-3](S)-5-fluorowillardiine. In the pre sence of KSCN, [H-3](R,S)-AMPA and [H-3](S)-AMPA bind to a single popu lation of sites in rat brain membranes, whereas [H-3](S)-5-fluorowilla rdiine binds with two affinity components. KSCN increased the affinity of the low affinity [H-3](S)-5-fluorowillardiine component > 4-fold a nd increased the density of both components 1.5-1.7-fold, arguing agai nst KSCN-induced interconversion of low to high affinity states. KSCN, which promotes receptor desensitization, increased the potency of AMP A isomers, (S)-5-fluorowillardiine, quisqualate and cyclothiazide for inhibition of [H-3](S)-5-fluorowillardiine binding suggesting that the se ligands discriminate desensitized and nondesensitized receptors. In contrast, KSCN did not greatly affect the potency of glutamate, kaina te, or competitive antagonists suggesting that these ligands do not di scriminate desensitized and nondesensitized receptors. In the presence of KSCN, the rank order potency for agonists and antagonists was simi lar 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 displaye d 6-10-fold higher potency for displacement of [H-3](S)- or (R,S)-AMPA relative to [H-3](S)-5-fluorowillardiine binding. This finding, coupl ed with the marked two component binding by [H-3](S)-5-fluorowillardii ne but not [H-3](S)- or (R,S)-AMPA, suggests qualitative differences b etween the interaction of these ligands with the agonist recognition s ite. (C) 1997 Elsevier Science B.V.