Figure 8.

Application of a gating reaction mechanism of NMDA receptors. (A) Individual responses from a recombinant GluN1/2B channel in an excised outside-out patch activated by 1 ms application of maximally effective glutamate and glycine (indicated by the gray vertical bar and the open tip recording above the channel recordings). The patch contained a single active channel, which allowed analysis of the variable delay before channel opening. NMDA receptors bind agonist rapidly and subsequently open after a multimillisecond delay that reflects transition through kinetically distinct protein conformations before pore dilation (i.e., channel gating). Note that although application of maximal glutamate and glycine always produces a binding event, not all binding events lead to channel opening. Reproduced from Erreger et al. (2005a). (B) The cumulative plot of latency to opening after application of 1 mM glutamate for 1 ms. (C) The average of all individual recordings of single activations produced a macroscopic waveform with a characteristic rise time. (D) Evaluation of closed periods within the GluN1/2B activation suggested a model where two pregating steps can occur in any order and explosive opening of the pore, which occurs faster than the resolution of the recordings, is assumed to happen instantaneously once both pregating steps have been traversed. (E) Simulation of a single activation for a GluN1/2B channel (using the model in D) illustrates how brief gaps can contain information about forward rates for the fast kinetically distinct pregating step. The color above the simulation indicates occupancy in the corresponding closed state of the model in D. The slow step often reverses again through the fast state (green) before reopening.