Selective Toxicity of Nicotinic Insecticides: Responsibility of Varied Amino Acid(s) in the Ligand-Docking Pocket between Insect versus Mammalian Nicotinic Acetylcholine Receptors

Nicotinic insecticides are utilized throughout the world for crop protection and animal care. The selective toxicity of nicotinic insecticides is principally attributable to the specificity of insect versus mammalian nicotinic acetylcholine receptors (nAChRs). The goal of this investigation is to evaluate the responsibilities of varied amino acids in the insecticide-binding spheres (located at an interface connecting α and non-α subunits) between insect and mammalian nAChR subtypes. Insect nAChR loop C region on the α subunit has Ser or Val, which is replaced by Glu on the mammalian α4 subunit, and insect loop D on the β subunit has Arg, whereas Thr takes this position on the mammalian β2 subunit. The present study, therefore, provides six recombinant nAChRs consisting of peach-potato aphid <i>Myzus persicae</i> α2 and <i>Rattus norvegicus</i> β2 subunits (WT, V225S, V225E, T77R, V225S+T77R, and V225E+T77R) to compare the binding affinity of nicotinic insecticides in an equilibrium state. [³H]imidacloprid (IMI) specifically binds to the six recombinant nAChRs with a single dissociation constant in each mutant, i.e., 3.2, 2.7, 6.0, 3.5, 2.2, or 8.2 nM for the WT, V225S, V225E, T77R, V225S+T77R, or V225E+T77R receptor, respectively. Relative to the binding affinity of the nicotinic ligand as a displacer of [³H]IMI binding, an insecticide with a <i>N</i>-nitroimine, <i>N</i>-cyanoamidine, <i>N</i>-cyanosulfoximine, or butenolide moiety (IMI, acetamiprid, sulfoxaflor, or flupyradifurone, respectively) fundamentally prefers nAChRs providing Ser (or Val) on the loop C region over those with Glu. Intriguingly, flupyrimin with a <i>N</i>-trifluoroacetamide pharmacophore shows higher affinities for the nAChRs having Arg on loop D than those with Thr. These results prompt us to establish the selective molecular recognition models of nicotinic insecticides at the insect nAChR.