AMPA RECEPTOR FLIP FLOP MUTANTS AFFECTING DEACTIVATION, DESENSITIZATION, AND MODULATION BY CYCLOTHIAZIDE, ANIRACETAM, AND THIOCYANATE/

AMPA receptor GluRA subunits with mutations at position 750, a residue shown previously to control allosteric regulation by cyclothiazide, w ere analyzed for modulation of deactivation and desensitization by cyc lothiazide, aniracetam, and thiocyanate, Point mutations from Ser to A sn, Ala, Asp, Gly, Gin, Met, Cys, Thr, Leu, Val, and Tyr were construc ted in GluRA(flip). The last four of these mutants were not functional ; S750D was active only in the presence of cyclothiazide, and the rema ining mutants exhibited altered rates of deactivation and desensitizat ion for control responses to glutamate, and showed differential modula tion by cyclothiazide and aniracetam. Results from kinetic analysis ar e consistent with aniracetam and cyclothiazide acting via distinct mec hanisms. Our experiments demonstrate for the first time the functional importance of residue 750 in regulating intrinsic channel-gating kine tics and emphasize the biological significance of alternative splicing in the M3-M4 extracellular loop.