Gating Scheme for Single GABA-activated Cl- Channels Determined from Stability Plots, Dwell-time Distributions, and Adjacent-interval Durations

To study the gating of a GABA-activated Cl- channel, currents from single channels activated by 1.0 microM GABA were examined in patches of membrane excised from cultured chick cerebral neurons. The distributions of open and shut interval durations were each described by the sum of 3 exponential components, suggesting that the channel normally enters at least 3 open and 3 shut states. Five different 6-state gating schemes were found that could describe, all equally well, the observed distributions of open and shut interval durations. Plots of the mean duration of open intervals adjacent to shut intervals of specified durations revealed that, on the average, openings of brief duration were adjacent to closings of long duration. This observation indicated 2 or more independent transition pathways between the open and shut states. Examination of the distributions of open intervals adjacent to shut intervals of specified durations revealed that the time constants of the exponential components describing these conditional open-interval distributions were independent of the durations of the adjacent shut intervals. In contrast, the areas changed in a manner consistent with open states of briefer mean lifetimes typically making transitions to shut states of longer mean lifetimes. Four of the 5 gating schemes considered were rejected because they did not predict the relationship between adjacent intervals or because they predicted that the channel should switch between 2 gating modes with markedly different mean open and shut times, which was not a characteristic of the experimental data. The single remaining kinetic scheme could account for the observed kinetic properties of the GABA channel.