Role of Human GABAA Receptor β3 Subunit in Insecticide Toxicity

Abstract

The γ-aminobutyric acid type A (GABAA) receptor is the target for the major insecticides α-endosulfan, lindane, and fipronil and for many analogs. Their action as chloride channel blockers is directly measured by binding studies with [3H]ethynylbicycloorthobenzoate ([3H]EBOB). This study tests the hypothesis that GABAA receptor subunit composition determines the sensitivity and selectivity of insecticide toxicity. Human receptor subtypes were expressed individually (α1, α6, β1, β3, and γ2) and in combination in insect Sf9 cells. Binding parameters were similar for [3H]EBOB in the β3 homooligomer, α1β3γ2 heterooligomer, and native brain membranes, but toxicological profiles were very different. Surprisingly, α-endosulfan, lindane, and fipronil were all remarkably potent on the recombinant β3 homooligomeric receptor (IC50 values of 0.5–2.4 nM), whereas they were similar in potency on the α1β3γ2 subtype (IC50 values of 16–33 nM) and highly selective on the native receptor (IC50 values of 7.3, 306, and 2470 nM, respectively). The selectivity order for 29 insecticides and convulsants as IC50 ratios for native/β3 or α1β3γ2/β3 was as follows: fipronil > lindane > 19 other insecticides including α-endosulfan and picrotoxinin > 4 trioxabicyclooctanes and dithianes (almost nonselective) > tetramethylenedisulfotetramine, 4-chlorophenylsilatrane, or α-thujone. Specificity between mammals and insects at the target site (fipronil > lindane > α-endosulfan) paralleled that for toxicity. Potency at the native receptor is more predictive for inhibition of GABA-stimulated chloride uptake than that at the β3 or α1β3γ2 receptors. Therefore, the β3 subunit contains the insecticide target and other subunits differentially modulate the binding to confer compound-dependent specificity and selective toxicity.