Role of human GABA(A) receptor beta 3 subunit in insecticide toxicity

The gamma -aminobutyric acid type A (GABA(A)) receptor is the target for th e major insecticides alpha -endosulfan, lindane, and fipronil and for many analogs. Their action as chloride channel blockers is directly measured by binding studies with [H-3]ethynylbicycloorthobenzoate ([H-3]EBOB). This stu dy tests the hypothesis that GABA(A) receptor subunit composition determine s the sensitivity and selectivity of insecticide toxicity. Human receptor s ubtypes were expressed individually (alpha1, alpha6, beta1, beta3, and gamm a2) and in combination in insect Sf9 cells. Binding parameters were similar for [H-3]EBOB in the beta3 homooligomer, alpha1 beta3 gamma2 heterooligome r, and native brain membranes, but toxicological profiles were very differe nt. Surprisingly, alpha -endosulfan, lindane, and fipronil were all remarka bly potent on the recombinant beta3 homooligomeric receptor (IC50 values of 0.5-2.4 nM), whereas they were similar in potency on the alpha1 beta3 gamm a2 subtype (IC50 values of 16-33 nM) and highly selective on the native rec eptor (IC50 values of 7.3, 306, and 2470 nM, respectively). The selectivity order for 29 insecticides and convulsants as IC50 ratios for native/beta3 or alpha1 beta3 gamma2/beta3 was as follows: fipronil > lindane > 19 other insecticides including alpha -endosulfan and picrotoxinin > 4 trioxabicyclo octanes and dithianes (almost nonselective) > tetramethylenedisulfotetramin e, 4-chlorophenylsilatrane, or alpha -thujone, Specificity between mammals and insects at the target site (fipronil > lindane > alpha -endosulfan) par alleled that for toxicity. Potency at the native receptor is more predictiv e for inhibition of GABA-stimulated chloride uptake than that at the beta3 or alpha1 beta3 gamma2 receptors, Therefore, the beta3 subunit contains the insecticide target and other subunits differentially modulate the binding to confer compound-dependent specificity and selective toxicity. (C) 2001 A cademic Press.