Bicuculline methiodide (BIC-Mel) (10-100 microM) altered the kinetics of N-methyl-D-aspartate (NMDA) responses in single-channel and whole-cell recordings. The principal effect of BIC-Mel (10-100 microM) on NMDA channels was a dose-dependent decrease in mean channel open time (tau o), accompanied by the introduction of a new closed time (tau B) of 14.0 +/- 3.5 msec (mean +/- standard deviation; n = 14) in closed time distributions, which was independent of BIC-Mel concentration. BIC-Mel (10-100 microM) increased the frequency of NMDA channel opening in a dose-dependent manner, offsetting the decrease in tau o, such that the total time spent in the open state per minute was unchanged, and thus the total charge/min through NMDA channels was unchanged. Similarly, the amplitudes of NMDA whole-cell current responses were not noticeably affected by 10-80 microM BIC-Mel, even though power spectra density analysis of the whole-cell NMDA-stimulated noise revealed changes in the underlying channel kinetics in the presence of BIC-Mel. Taken together, the effects of 10-80 microM BIC-Mel on NMDA responses were consistent with the predictions of the sequential block model; however, the effects of BIC-Mel exhibited no obvious voltage dependence. In addition to the low-dose effects of BIC-Mel, 100 and 200 microM BIC-Mel inhibited whole-cell NMDA responses. The inhibition by 100 microM BIC-Mel was not large, but it was augmented from 15% to 30% by increasing the NMDA concentration from 10 microM NMDA to 20 microM NMDA, indicating that channel activation was necessary for BIC-Mel-mediated inhibition. Preliminary single-channel experiments performed under conditions conducive to trapping of an open channel blocker at its binding site indicated that the effect of BIC-Mel on tau o persisted after the removal of the blocker, consistent with use dependence of the dissociation of BIC-Mel from the NMDA receptor-channel complex.
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