Studies on the mode of action and the selectivity of insecticides acting on ligand-gated ion channels in the nervous system

Abstract

The mode of action and the mechanisms for the selectivity of neurotoxic insecticides were investigated using computational chemistry, molecular biology, homology modeling of the target site and X-ray crystallography. Whole-cell, patch-clamp electrophysiology has been used to show that non-competitive antagonists (NCAs), such as 4-ethynyl-4-n-propylbicycloorthobenzoate (EBOB) and γ-hexachlorocyclohexane (γ-HCH), are more effective blockers of native cockroach (Periplaneta americana) and recombinant house fly (Musca domestica) γ-aminobutyric acid receptors (GABARs) than glutamate-gated chloride channels (GluCls). Site-directed mutagenesis studies have been used to show that that Ser278 in the second transmembrane region is important for determining the less potent actions of these NCAs on GluCls. The structure-activity relationship and the mechanism of the selectivity of neonicotinoids targeting nicotinic acetylcholine receptors (nAChRs) have also been investigated. Electrophysiological studies show that loops C and D of insect nAChRs are critical in determining the selectivity of neonicotinoids, results consistent with the crystal structures of Lymnaea stagnalis acetylcholine binding protein (Ls-AChBP) in complex with imidacloprid. Neonicotinoids show diverse actions on nAChRs as measured using voltage-clamp electrophysiology. Single channel recording and X-ray crystallography are helping to elucidate the mechanism of super agonist actions of clothianidin and an analog. © Pesticide Science Society of Japan.