Mechanisms of NMDA receptor inhibition by diarylamidine compounds

Pentamidine, diminazene and 4',6-diamidino-2-phenylindole (DAPI) are antiprotozoal diarylamidine compounds. In the present work, we have studied their action on native N-methyl-D-aspartate (NMDA) receptors in rat hippocampal pyramidal neurons. All three compounds inhibited NMDA receptors at -80 mV holding voltage with IC₅₀ of 0.41 ± 0.08, 13 ± 3 and 3.1 ± 0.6 μM, respectively. The inhibition by pentamidine was strongly voltage-dependent, while that of DAPI was practically voltage-independent. Inhibition by diminazene had both voltage-dependent and voltage-independent components. Diminazene and DAPI demonstrated tail currents and overshoots suggesting "foot-in-the-door" mechanism of action. In contrast, pentamidine was partially trapped in the closed NMDA receptor channels. Such difference in the mechanism of action can be explained by the difference in the 3D structure of compounds. In the pentamidine molecule, two benzamidine groups are connected with a flexible linker, which allows the molecule to fold up and fit in the cavity of a closed NMDA receptor channel. Diminazene and DAPI, in contrast, have an extended form and could not be trapped.