The antiparasitic isoxazoline A1443 is a potent blocker of insect ligand-gated chloride channels

A structurally unique isoxazoline class compound, A1443, exhibits antiparasitic activity against cat fleas and dog ticks comparable to that of the commercial ectoparasiticide fipronil This isoxazoline compound inhibits specific binding of the gamma-aminobutyric acid (GABA) receptor channel blocker [H-3]4'-n-propylbicycloorthobenzoate (EBOB) to housefly-head membranes, with an IC50 value of 455 pM. In contrast, the IC50 value in rat-brain membranes is >10 mu M To study the mode of action ofthis isoxazoline, we utilized MdGBC1 and MdGluC1 cDNAs, which encode the subunits of housefly GABA- and glutamate-gated chloride channels, respectively. Two-electrode voltage clamp electrophysiology was used to confirm that A1443 blocks GABA- and glutamate-induced chloride cut rents in Xenopus oocytes expressing MdGBC1 or MdGluC1 channels, with IC50 values of 5.32 and 79 9 nM, respectively Blockade by A1443 was observed in A2'S MdGBC1 and S2'A-MdGluCl mutant channels at levels similar to those of the respective wild-types, and houseflies expressing A2'S-MdGBCl channels were as susceptible to A1443 as standard houseflies. These findings indicate that A1443 is a novel and specific blocker of insect ligand-gated chloride channels. (C) 2009 Elsevier Inc. All rights reserved.