IONIC PERMEABILITY CHARACTERISTICS OF THE N-METHYL-D-ASPARTATE RECEPTOR-CHANNEL

N-methyl-D-aspartate (NMDA) receptor channels in cultured CAI hippocam pal neurons were studied using patch-clamp techniques. The purpose of the research was to determine the occupancy of the channel by permeant cations and to determine the influence of charged residues in or near the pore. The concentration dependence of permeability ratios, the mo le-fraction dependence of permeability ratios, the concentration depen dence of the single-channel conductance, and a single-channel analysis of Mg2+ block all independently indicated that the NMDA receptor beha ves as a singly-occupied channel. More precisely, there is one permean t cation at a time occupying the site or sites that are in the narrow region of the pore directly in the permeation pathway. Permeability-ra tio measurements in mixtures of monovalent and divalent cations indica ted that local charges in or near the pore do not produce a large loca l surface potential in physiologic solutions. In low ionic strength so lutions, a local negative surface potential does influence the ionic e nvironment near the pore, but in normal physiologic solutions the surf ace potential appears too small to significantly influence ion permeat ion. The results indicate that the mechanism for the high Ca2+ conduct ance of the NMDA receptor channel is not the same as for the voltage-d ependent Ca2+ channel (VDCC). The VDCC has two high affinity, interact ing binding sites that provide high Ca2+ selectivity and conductance. The binding site of the NMDA receptor is of lower affinity. Therefore, the selectivity for Ca2+ is net as high, but the lower affinity of bi nding provides a faster off rate so that interacting sites are not req uired for high conductance.