Nonstationary noise analysis of M currents simulated and recorded in PC12 cells

M current relaxations recorded in PC12 cells were subjected to nonstationary noise analysis (NSNA) to obtain estimates of single-channel current (i), channel number (N), and open probability (Po) for the channels responsible for M current. The analysis was constrained such that N and single-channel conductance were the same at two potentials. The relation between variance and current indicated that the fraction of channels open was 0.58 +/- 0.06 (mean +/- SD) and 0.05 +/- 0.04 (mean +/- SD: n = 9) at -33 and -63 mV, respectively. The single M channel conductance was 4.0 pS, and a density of 1 functional M channel per 4 microm2 was estimated. Monte Carlo simulations of a two-state model of M channels were used to obtain sets of simulated macroscopic M currents that were subjected to the same NSNA procedure so as to evaluate the accuracy of M channel parameters obtained with this method. The influence of current rundown and filter frequency on estimates of i, N, and Po were evaluated. The single-channel parameters estimated from the simulations differed by < 10% from actual values at any level of current rundown, N, or Po. The dispersion in the estimation of N and Po increased as Po decreased. Decreasing filter frequency caused an underestimation of i, paralleled by an overestimation of N. The estimation of Po was relatively immune to the filter frequency, especially for data simulated with Po = 0.77.