Ming‐Yie Liu, J. Casida
May 1, 1993
Citations
2
Influential Citations
244
Citations
Quality indicators
Journal
Pesticide Biochemistry and Physiology
Abstract
Abstract l-(6-Chloronicotinyl)-2-nitroimino-imidazolidine (imidacloprid or IMI) is a potent insecticide of a new chemical class considered from earlier studies to act at the insect nicotinic acetylcholine receptor. In a direct approach to the mode of action of IMI, it was radiolabeled and [ 3 H]IMI was examined in binding studies to elucidate its pharmacological profile. [ 3 H]IMI undergoes high affinity specific binding in house fly head P 2 membranes with 95% specific binding, a dissociation constant of 1.2 n M , and a maximal binding site capacity of 853 fmol/mg protein. The standard binding assay consisted of 1 n M [ 3 H]IMI and 200 μg membrane protein in 50 m M NaCl, 10 m M sodium phosphate (pH 7.4) containing 0.1% Triton X-100 with incubation for 60 min at 22°C prior to filtration. The radioligand undergoes rapid biphasic association and dissociation consistent with a two-stage sequential reaction in each case. [ 3 H]IMI binding is very sensitive to carbachol (IC 50 1.9 μ M ) and other choline esters (IC 50 s 0.2 to 0.5 μ M for acetylcholine, propionylcholine, and butyrylcholine in the presence of paraoxon as a cholinesterase inhibitor). The pharmacological profile for [ 3 H]IMI binding indicates inhibition by both nicotinic and muscarinic agents with IC 50 s of 0.6 μ M for (-)-nicotine, 2.2 μ M for α-bungarotoxin, 30 μ M for D-tubocurarine, 90 μ M for atropine, 275 μ M for quinuclidinyl benzilate, and 288 μ M for dexetimide. Lineweaver-Burk plots establish competitive inhibition kinetics for [ 3 H]IMI with acetylcholine, α-bungarotoxin, and quinuclidinyl benzilate. Detection of [ 3 H]IMI binding in membrane preparations from several insects but not from the vertebrates examined is consistent with the selective toxicity of the nitromethylene and nitroimine insecticides.