Adamantane derivative IEM-1676 (Ad-N+H2-(CH2)5-N+Me3) causes open-channel block of Ca2+ permeable AMPA receptors when applied externally, but internal application results in both closed- and open-channel block. The relationships between blocking action of externally and internally applied IEM-1676 were studied using patch clamp technique. Extracellular action of IEM-1676 is decreased by its intracellular application, thus indicating overlap of the binding site at external and internal application. We demostrated that internal closed-channel block is voltage-dependent and occurs at the site where externally applied drug is trapped. We conclude that the selectivity filter of the closed AMPA receptor channel is not occluded and remains permeable for organic cations.
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