Potentiation of the α7 nAChR by divalent ions at the single-channel level. (a) Trace showing single-channel currents recorded in the presence of 10 µM ACh in the absence of divalent cations plus 1 mM EGTA. Recording was obtained in the cell-attached patch configuration with an applied potential of −100 mV and Gaussian filter of 25 kHz (see Materials and methods). Channel openings are upward deflections from baseline. Left: Plot of the durations of individual channel openings as a function of their time of occurrence during the recording. Right: A histogram of open dwell times fitted by the sum of two exponentials with the following time constants and relative weights: τ0 = 1.2 × 10−5 s, a0 = 0.855; τ1 = 9.9 × 10−5 s, a1 = 0.145. For the population of channel openings from this patch, the mean current amplitude was 18.6 ± 1.3 pA. (b) Trace showing single-channel currents in the presence of 10 µM ACh and 1.8 mM Ca2+; the cell-attached patch for this recording was obtained from the same cell as in a. Left: Plot of the durations of individual channel openings as a function of time of occurrence during the recording. Right: A histogram of open dwell times fitted by the sum of two exponentials with the following time constants and relative weights: τ0 = 1.4 × 10−5 s, a0 = 0.658; τ1 = 8.3 × 10−5 s, a1 = 0.342. For the population of channel openings from this patch, the mean current amplitude was 14.5 ± 1.8 pA. (c) Trace showing single-channel currents in the presence of 10 µM ACh and 1.8 mM Ba2+; the cell-attached patch for this recording was obtained from the same cell as in a and b. Left: Plot of the durations of individual channel openings as a function of time of occurrence during the recording. Right: A histogram of open dwell times fitted by the sum of two exponentials with the following time constants and relative weights: τ0 = 1.2 × 10−5 s, a0 = 0.611; τ1 = 9.7 × 10−5 s, a1 = 0.389. For the population of channel openings from this patch, the mean current amplitude was 14.3 ± 2.0 pA.