BINDING OF THE NEW RADIOLIGAND (S)-[H-3]AMPA TO RAT-BRAIN SYNAPTIC-MEMBRANES - EFFECTS OF A SERIES OF STRUCTURAL ANALOGS OF THE NON-NMDA RECEPTOR AGONIST WILLARDIINE

This study examined the binding of (S)[H-3]AMPA, the radiolabelled active isomer of AMPA, to rat brain synaptic membranes. Under non-chaotropic conditions specific binding of 10 nM (S)-[H-3]AMPA represented 33 +/- 2% of the total; this increased to 74 +/- 1% in the presence of 100 mM KSCN. (S)-[H-3]AMPA binding was inhibited by non-NMDA receptor agonists and the antagonists NBQX and CNQX, with the following rank order of potency: NBQX > (S)-AMPA greater than or equal to quisqualate > CNQX > L-glutamate > domoate greater than or equal to kainate > (R)-AMPA. NMDA, and the metabotropic glutamate receptor agonist (1S,3R)-ACPD, up to 100 mu M, did not inhibit (S)-[H-3]AMPA. binding. A number of willardiine analogues all effectively inhibited (S)-[H-3]AMPA binding with the rank order of potency: (S)-5-fluorowillardiine > (S)-5-nitrowillardiine > (S)-5-trifluoromethylwillardiine > (S)-5-bromowillardiine approximate to (S)-5-chlorowillardiine > (S)-5-cyanowillardiine > (S)-willardiine > (S)-5-iodowillardiine > (S)6-methylwillardiine > (S)-5-methylwillardiine. This rank order closely reflects data from equilibrium measurements made, under voltage clamp, on cultured hippocampal neurons. In contrast the respective (R)-enantiomers and the racemate mixtures of (R,S)-3, 5 and 6-isowillardiine were relatively inactive. Similar IC50 values and thus rank orders of potency for the willardiines were observed in the presence of 100 mM KSCN.