Optimization of 4'-ethynyl-4-n-[3H]propylbicycloorthobenzoate ([3H]EBOB) and reexamination of [35S]t-butylbicyclophosphorothionate ([35S]TBPS) as radioligands for the GABA receptor complex of house fly head membranes established their respective binding parameters as follows: specific binding, 75 and 50-60%; apparent KDs, 2 and 145 nM; Bmaxs, 0.35 and 2.4 pmol/mg protein. Five groups of insecticides, all inhibitory to [3H]EBOB binding, were evaluated for potency at the [3H]EBOB and [35S]TBPS binding sites and competitive or noncompetitive inhibition with [3H]-EBOB. They are (i) polychlorocycloalkanes with emphasis on lindane analogs; (n) 1-aryl-trioxabicyclooctanes, 2-aryl-dithiane, aryl-silatrane, and picrotoxinin; (iii) trioxabicyclooctanes including bicyclophosphorus esters and trithiabicyclooctanes and dithianes with smaller terminal substituents; (iv) phenylpyrazoles; and (v) avermectins. Groups i-iv are putative channel blockers and v is a channel activator. Additional observations were made on poisoning signs, temperature coefficient of poisoning, and cross-resistance in a dieldrin-resistant strain. The findings are interpreted in light of our earlier differentiation, from studies with mice, of "Type A" action for compounds in groups i and ii and "Type B" action for the trioxabicyclooctanes in group iii. In house flies, Type A action involves the EBOB site as lexicologically relevant for i and ii, and there are characteristic hyperexcitation signs, a positive temperature coefficient, and cross-resistance with dieldrin. Type B action (for some of the compounds in group iii) presumably involves the TBPS site, although its toxicological relevance is not established, and there are different poisoning signs, a negative temperature coefficient, and no cross-resistance. Mixed Types A and B action is suggested for other compounds in group in. Phenylpyrazoles (iv) are related to Type A action except for noncompetitive inhibition of [3H]EBOB binding in fly membranes and low potency at the [3H]EBOB site in mammals and are therefore designated as Type C. The action of the avermectins (v), designated Type D, is coupled to the EBOB but not the TBPS site with poisoning signs of sedation and diuresis and no cross-resistance. © 1993 by Academic Press, Inc.
