NONSTATIONARY NOISE-ANALYSIS OF M-CURRENTS SIMULATED AND RECORDED IN PC12 CELLS

M current relaxations recorded in PC12 cells were subjected to nonstat ionary noise analysis (NSNA) to obtain estimates of single-channel cur rent (i), channel number (N), and open probability (P-o) for the chann els 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 c hannels 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 co nductance was 4.0 pS, and a density of 1 functional M channel per 4 mu m(2) 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 influe nce of current rundown and filter frequency on estimates of i, N, and P-o were evaluated. The single-channel parameters estimated from the s imulations differed by <10% from actual values at any level of current rundown, N, or P-o. The dispersion in the estimation of N and P-o inc reased as P-o decreased. Decreasing filter frequency caused an underes timation of i, paralleled by an overestimation of N. The estimation of P-o was relatively immune to the filter frequency, especially for dat a simulated with P-o = 0.77.