DIMENSIONS OF THE NARROW PORTION OF A RECOMBINANT NMDA RECEPTOR-CHANNEL

Glutamate-activated single-channel and ensemble currents were recorded from Xenopus laevis oocytes and HEK 293 cells expressing a recombinan t NMDA receptor, assembled from NR1 and NR2A subunits. Cesium was the main charge carrier, and organic cations were used to determine the pr esence of vestibules of this channel and to estimate its pore diameter . The large organic cations tris-(hydroxymethyl)-aminomethane (Tris), N-methyl-glucamine (NMG), arginine (Arg), choline, and tetramethylammo nium (TMA), when added in millimolar concentrations to the extracellul ar or cytoplasmic side, produced a voltage-dependent blockade of singl e-channel Cs+ currents. These molecules behaved as impermeant ions tha t only partially traverse the channel from either side. The smaller ca tions trimethylammonium (TriMA) and dimethylammonium (DMA) produced a small and nearly voltage-independent reduction in current amplitude, s uggesting that they are permeant. In biionic experiments with Cs+ as t he reference ion, the large blocking cations NMG, Arg, Tris, TMA, chol ine, hexamethonium (Hme), triethylammonium (TriEA), and tetraethylammo nium (TEA) showed no measurable permeability. TriMA and smaller ammoni um derivatives were permeant. Both the permeability and single-channel conductance of organic cations, relative to Cs+, decreased as the ion size increased. The results suggest that the NMDA receptor has extrac ellular and cytoplasmic mouths that can accommodate large cations up t o 7.3 Angstrom in mean diameter. The narrow portion of the pore is est imated to have a mean diameter of 5.5 Angstrom.