Enhancement by GABA of the association rate of picrotoxin and tert-butylbicyclophosphorothionate to the rat cloned alpha 1 beta 2 gamma 2 GABAA receptor subtype.

1. We examined how gamma-aminobutyric acid (GABA) influences interaction of picrotoxin and tert-butylbicyclophosphorothionate (TBPS) with recombinant rat alpha 1 beta 2 gamma 2 GABAA receptors stably expressed in human embryonic kidney cells (HEK293), as monitored with changes in Cl- currents measured by the whole-cell patch clamp technique. 2. During application of GABA (5 microM) for 15 s, picrotoxin and TBPS dose-dependently accelerated the decay of inward GABA-induced currents (a holding potential of -60 mV under a symmetrical Cl- gradient). The drugs, upon preincubation with the receptors, also reduced the initial current amplitude in a preincubation time and concentration-dependent manner. This indicates their interaction with both GABA-bound and resting receptors. 3. The half maximal inhibitory concentration for picrotoxin and TBPS at the beginning of a 15 s GABA (5 microM) pulse was several times greater than that obtained at the end of the pulse. GABA thus appears to enhance picrotoxin and TBPS potency, but only at concentrations leading to occupancy of both high and low affinity GABA sites, i.e., 5 microM. Preincubation of the receptors with the drugs in the presence of GABA at 200 nM, which leads to occupancy of only high affinity GABA sites in the alpha 1 beta 2 gamma 2 subtype, produced no appreciable change in potency of picrotoxin or TBPS. This indicates that they preferentially interact with multiliganded, but not monoliganded receptors, unlike U-93631, a novel ligand to the picrotoxin site, which has higher affinity to both mono- and multiliganded receptors than resting receptors. 4. The time-dependent decay and preincubation time-dependent reduction of initial amplitude of GABA-induced Cl- currents followed monoexponential time courses, and time constants thus obtained displayed a linear relationship with drug concentration. Analysis of the data using a kinetic model with a single drug site showed that GABA (5 microM) enhanced the association rate for picrotoxin and TBPS nearly 100 fold, but their dissociation rate only 10 fold. The dissociation rate obtained from current recovery from picrotoxin or TBPS block yielded nearly identical values to the above analysis.5. We conclude that picrotoxin and TBPS interact with both resting and GABA-bound receptors, but their affinity for the latter is about 10 times greater than that for the former, largely due to a markedly increased association rate to the multiliganded receptors (but not monoliganded ones). This and our earlier study with U-93631 improves our understanding of functional coupling between GABA and picrotoxin sites, which appears to be useful in characterizing the mode of interaction for various picrotoxin site ligands.