Abstract
NMDA receptors (NMDARs) are glutamate-gated ion channels that mediate fast excitatory synaptic transmission in the nervous system. Applying glutamate to outside-out patches containing a single NMDAR, we find that agonist-bound receptors transition to the open state via two conformations, an "unconstrained pre-active" state that contributes to fast synaptic events and a "constrained pre-active" state that does not. To define how glutamate drives these conformations, we decoupled the ligand-binding domains from specific transmembrane segments for GluN1 and GluN2A. Displacements of the pore-forming M3 segments define the energy of fast opening. However, to enter the unconstrained conformation and contribute to fast signaling, the GluN2 pre-M1 helix must be displaced before the M3 segments move. This pre-M1 displacement is facilitated by the flexibility of the S2-M4 of GluN1 and GluN2A. Thus, outer structures-pre-M1 and S2-M4-work in concert to remove constraints and prime the channel for rapid opening, facilitating fast synaptic transmission.