Drosophila GABA-gated chloride channel: Modified [³H]EBOB binding site associated with Ala → Ser or Gly mutants of Rdl subunit

The non-competitive blocker site of the GABA-gated chloride ion channel in normal susceptible strains of Drosophila melanogaster and simulans binds 4-n [³H]propyl-4′-ethynylbicycloorthobenzoate ([³H]EBOB) at specific sites with K_ds of 1.6-1.9 nM and B_maxs of 171-181 fmol/mg protein. This specific binding of [³H]EBOB is strongly inhibited by: a large number and variety of insecticidal channel blockers at 20 nM (lindane, α-endosulfan, dieldrin, 12-ketoendrin, fipronil, and a representative bicycloorthobenzoate and dithiane) or 200 nM (picrotoxinin); the insecticidal channel activators avermectin and moxidectin at 20 nM; muscimol at 30 μM and GABA at 300 μM. Cyclodiene resistance in D. melanogaster has been attributed to a mutation resulting in an Ala³⁰² → Ser replacement in the Rdl GABA receptor subunit and in D. simulans to an homologous Ala → Ser or Gly replacement. These mutations are shown here to greatly reduce [³H]EBOB binding, i.e. lower affinity and apparent number of binding sites. The Ala → Ser replacement with both melanogaster and simulans almost always reduces the potency in inhibiting [³H]EBOB binding of each of eight channel blockers and of muscimol and GABA. The Ala → Gly replacement in D. simulans is generally less effective than the Ala → Ser modification in reducing sensitivity to the channel blockers and to muscimol and GABA. The channel activators avermectin and moxidectin usually retain their inhibitory potency in the Rdl subunit mutants. Thus, it appears that replacement of Ala by Ser generally modifies the non-competitive blocker site and its coupling to the GABA-recognition site with less effect on the channel activator site. In contrast, the Ala → Gly replacement has less impact in protecting the chloride channel from the action of insecticidal blockers. Each of the resistant strains has the same level of resistance to the lethal action of the five channel blockers examined but none to avermectins and muscimol.