House fly head GABA‐gated chloride channel: [3 H] α‐Endosulfan binding in relation to polychlorocycloalkane insecticide action

Abstract

AbstractThis study attempts to use [3H] α‐endosulfan to examine directly the binding site(s) for cyclodienes, lindane and toxaphene (collectively referred to as the polychlorocycloalkane or PCCA insecticides) in the 4‐aminobutyric acid (GABA)‐gated chloride channel. [3H] α‐Endosulfan was prepared by reduction of hexachloronorbornenedicarboxylic anhydride with sodium borotritide, then coupling the labelled alcohol with thionyl chloride followed by HPLC purification (35 Ci mmol−1, > 99% radiochemical purity). This new candidate radioligand readily partitions into lipid membranes and undergoes indiscriminate adsorption to surfaces resulting in high levels of non‐specific binding. This makes it very difficult to differentiate the small portion of specific binding at the site relevant to toxic action. This problem was partially circumvented by incubating [3H] α‐endosulfan (0.1 nM) with house‐fly head membranes (0.2 mg protein) for 70 min at 22°C giving 23 (±4)% specific binding (40 fmol mg−1 protein) determined as the difference between the radioligand alone and on preincubation for 15 min with unlabelled α‐endosulfan (final concentration 100 nM). This procedure is not appropriate for determination of saturation isotherms and standard binding kinetics. However, the effectiveness of 16 PCCAs (also at 100 nM final concentration) in blocking the specific binding of [3H] α‐endosulfan is generally consistent with their relative potencies as inhibitors of 4‐[3H] propyl‐1‐(4‐ethynylphenyl)‐2,6,7‐trioxabicyclo[2.2.2] octane ([3H]EBOB) binding suggesting that the binding site for both [3H]α‐endosulfan and the PCCAs is part of the GABA‐gated chloride channel. Insecticidal channel blockers of other types (e.g. picrotoxinin, trioxabicyclooctanes, a dithiane, and phenylpyrazoles) and GABA are poor inhibitors of [3H] α‐endosulfan binding relative to their potencies as inhibitors of [3H] EBOB binding. It therefore appears that the PCCAs compete directly for the [3H] α‐endosulfan site, whereas the other channel blockers bind with different inhibition kinetics or at a site more closely coupled to the EBOB than the α‐endosulfan binding domain.