Channel Open Probability Controls Allosteric Modulation Of Potency And Efficacy

Kinetic models of NMDA receptors (NMDARs) that separate binding and gating into two distinct steps predict that the enhancement of open probability will shift the equilibrium to activated states and therefore enhance agonist EC50. The tetrahydroisoquinoline CIQ is a positive allosteric modulator (PAM) of GluN2C- and GluN2D-containing NMDARs, increasing the current response to maximal effective concentrations of agonist by 3-fold. Surprisingly, CIQ did not detectably alter glutamate potency at GluN2D (Mullasseril et al., 2010). We recently identified a CIQ analogue, (-)1180-55, that also potentiates GluN2B-containing NMDARs. This compound enhances the response of GluN2D-containing NMDARs by 3-fold but again has no detectable effect on glutamate EC50 for GluN2D-containing NMDARs, which was 0.5±0.07 μM (n=19) in control and 0.4±0.01 μM (n=13) in 20 μM (-)1180-55. By contrast, (-)1180-55 increased the response of GluN2B-containing NMDARs to maximally concentrations of agonist by 2-fold and increased glutamate potency by 2-fold from 2.0±0.16 μM (n=30) in control to 1.2±0.07 μM (n=22) in 20 μM (-)1180-55. To explore why this PAM showed variable actions on agonist potency in GluN2B and GluN2D NMDARs, we modelled the potentiation of efficacy for a de Castillo and Katz model of channel function that separated binding and gating steps. Evaluation of the shift in EC50 as a function of open probability revealed that the PAM-induced changes in agonist potency following an increase in efficacy are only detectable when the open probability is greater than 0.1. For exceptionally low open probability receptors such as GluN2D, doubling open probability from 0.01 to 0.02 does not detectably alter the EC50. The relationship between open probability and allosteric modulation is thus critical to understanding whether PAMs will alter agonist potency by shifting the gating/binding equilibria, or whether their binding pose directly impacts the ligand binding domain.