Influence of membrane potential on conductance sublevels of chloride channels activated by GABA

Single-channel chloride currents activated by 0.5 microM GABA were recorded in cell-attached and inside-out membrane patches from rat cultured hippocampal neurons. The currents displayed multiple conductance states and outward rectification. The number and amplitude of conductance levels were determined over a range of potentials by using digital signal-processing techniques. It was found that, except for a level close to zero, subconductance levels were regularly spaced. There were fewer sublevels at hyperpolarized than at depolarized potentials, and the spacing between levels varied linearly with potential giving an incremental conductance of 8-10 pS that was independent of membrane potential. Outward rectification is related to the change in the number of conductance levels with potential. One hypothesis that is consistent with these observations is that a channel is composed of a number of synchronized, non-rectifying, conducting pores, and that the number of pores activated changes with membrane potential.