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 ∼0.7 and ∼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 ∼1.1 and ∼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.