Ionic flow enhances low-affinity binding: a revised mechanistic view into Mg2+block of NMDA receptors

The N-methyl-d-aspartate receptor (NMDAR) channel is one of the major excitatory amino acid receptors in the mammalian brain. Since external Mg2+ blocks the channel in an apparently voltage-dependent fashion, this ligand-gated channel displays intriguing voltage-dependent control of Na+ and Ca2+ permeability and thus plays an important role in synaptic physiology. We found that the essential features of Mg2+ block could not be solely envisaged by binding of a charged blocker in the membrane electric field. Instead, the blocking effect of Mg2+ is critically regulated by, and quantitatively correlated with, the relative tendency of outward and inward ionic fluxes. The 'intrinsic' affinity of Mg2+ to the binding sites, however, is low (in the millimolar range) in the absence of net ionic flow at 0 mV. Besides, extracellular and intracellular Mg2+ blocks the channel at distinct sites of electrical distances similar to 0.7 and similar to 0.95 from the outside, respectively. The two sites are separated by a high energy barrier for the movement of Mg2+ (but not Na+ or the other ions), and functionally speaking, each could accommodate similar to 1.1 and similar to 0.8 coexisting permeating ions, respectively. Mg2+ block of the ionic flow thus is greatly facilitated by the flux-coupling effect or the ionic flow (the preponderant direction of permeant ion movement) per se, as if the poorly permeable Mg2+ is 'pushed' against a high energy barrier by the otherwise permeating ions. Extracellular and intracellular Mg2+ block then is in essence 'use dependent', more strongly inhibiting both Na+ and Ca2+ fluxes with stronger tendencies of influx and efflux, respectively. In conclusion, although permeant ions themselves could compete with Mg2+, the flow or the tendency of movement of the permeant ions may actually enhance rather than interfere with Mg2+ block, making the unique current-voltage relationship of NMDAR and the molecular basis of many important neurobiological phenomena.