smFRET histograms showing the population distribution of the AMPA receptor labeled at site 23 of the amino-terminal domain under A) desensitized condition (presence of 1mM glutamate) with corresponding representative smFRET traces shown in B, C) open condition (presence of 1mM glutamate + 100 μM CTZ) and D) in tandem with γ2, in the desensitized condition (presence of 1mM glutamate). State 1, yellow peak; State 2, magenta peak; State 3, cyan peak; State 4, blue peak. Each state in A is accompanied by a cartoon representation of the corresponding distance between sites 23 of the B and D subunits of the AMPA receptor.