Terbium modulation of single gamma-aminobutyric acid-activated chloride channels in rat dorsal root ganglion neurons.

We have previously reported that lanthanides markedly potentiate the GABA-induced chloride current by acting at a distinct site on the GABAA receptor-channel complex (Ma and Narahashi, 1993a,b). These studies have now been extended to the single-channel level and changes in gating kinetics of GABAA receptor currents caused by 100 microM terbium (Tb3+) are reported. The GABA-induced currents were recorded from outside-out membrane patches isolated from rat dorsal root ganglion neurons in primary culture at a holding potential of -60 mV. At least two conductance levels were recorded, a main conductance of about 26 pS (70-80% of events) and a subconductance of about 19 pS (20-30% of events). These two conductances and the ratio of main- and subconductance state currents with respect to the number of events were not changed by Tb3+. The frequency of channel openings was also unchanged in the presence of Tb3+. The frequency histograms of open, close, and burst durations of the main-conductance state were best fitted by a sum of three exponential functions. All of the time constants remained unchanged by application of Tb3+ while the relative proportions of the longest open and burst duration time constants were increased and the relative proportion of longest closed time constant was decreased. We suggest that Tb3+ binds to an allosteric site on the GABAA receptor-channel complex to increase the apparent mean open time of the channel by increasing the affinity of GABA for the GABA binding site, and/or by shifting the distribution toward the open states so that the frequency of occurrence of longer open states is stabilized.(ABSTRACT TRUNCATED AT 250 WORDS)