ALLOSTERIC INTERACTIONS BETWEEN CYCLOTHIAZIDE AND AMPA KAINATE RECEPTOR ANTAGONISTS/

1 Cyclothiazide blocks lpha-amino-3-hydroxy-5-methyl-4-isoxazolepropio nic acid (AMPA) receptor desensitization and potentiates AMPA receptor gated currents. Interactions between cyclothiazide, and the non-compe titive antagonist GYKI52466 (GYKI) and competitive antagonist 2,3-dihy droxy-6-nitro-7-sulphamoyl-benzo (F) quinoxaline (NBQX) were studied a t native and recombinant AMPA/kainate receptors using whole-cell recor ding in order to characterize the modulation by cyclothiazide of these two antagonist sites. 2 GYKI 100 mu M, which is sufficient to elimina te virtually hippocampal kainate (100 mu M) currents, failed to preven t access of cyclothiazide to its site of potentiation, and was unable to enhance removal of cyclothiazide potentiation. However, cyclothiazi de reduced GYKI (30 mu M) block from 84+/-8.3% to 38+/-12%, and slowed the onset of the block with a time course much faster than the time c ourse for onset and offset of potentiation induced by cyclothiazide. C yclothiazide had qualitatively similar effects upon antagonism by NBQX 1 mu M. 3 Kainate activated desensitizing currents in dorsal root gan glion (DRG) neurones, which were unaffected by cyclothiazide. GYKI blo cked these kainate currents with lower affinity (IC50 > 120 mu M) than for hippocampal neurones (IC50 < 30 mu M), and cyclothiazide did not affect GYKI antagonism. 4 Steady-state AMPA currents from homomeric Gl uRA-D-flip receptors in HEK 293 cells were dramatically potentiated (u p to 216 fold) by cyclothiazide via reduction of desensitization. In c ontrast, kainate-gated currents in HEK 293 cells expressing GluR6R rec eptors exhibited pronounced desensitization that was unaffected by cyc lothiazide. GYKI retains its inhibition at both recombinant AMPA and k ainate receptors. 5 These results indicate that cyclothiazide alloster ically influences two important antagonist sites on AMPA receptors. In addition, AMPA/kainate receptor subunit composition influences the af finity of GYKI for the receptor.