The nitroguanidine insecticide imidacloprid (IMI) is known to act as an agonist at the insect nicotinic acetylcholine receptor (nAChR). Saturable binding of [H-3]IMI to membranes of the peach potato aphid Myzus persicae (R1 strain) revealed the presence of both high- and low-affinity [H-3]IMI binding sites having dissociation constants and maximal binding capacities of 0.14 nM and 284 fmol/mg and 12.6 nM and 485 fmol/mg, respectively with a Hill value of 0.61. Displacement of 500 pM [H-3]IMI binding to aphid membranes by the nicotinic ligands acetylcholine, (-)-nicotine, alpha-bungarotoxin (alpha-BGTX), (+/-)-epibatidine, methyllycaconitine (MLA) and IMI gave IC50 values (nM) of 2390, 643, 62.4, 21.9, 15.1, and 1.1, respectively. The pharmacology of the aphid high-affinity [3H]IMI binding site thus resembles that of nicotinic acetylcholine receptors. Low Hill values for ACh, alpha-BGTX, and MLA displacement suggest either that high-affinity [H-3]IMI binding occurs at multiple sites which have differing affinities for ACh, alpha-BGTX, or MLA or that binding to one high-affinity site is allosterically influenced by binding to another site on the same putative nAChR. Saturable [H-3]IMI binding was analyzed in a panel of insects, namely, Nephorettix cincticeps (Hemiptera: Cicadellidae), Periplaneta americana (Dictyoptera: Blattidae), Lucilia sericata (Diptera: Calliphoridae), Drosophila melanogaster (Canton S) (Diptera: Drosophiloidea),,Manduca sexta (Lepidoptera: Sphingidae), Heliothis virescens (Lepidoptera: Noctuidae), and Ctenocephalides felis (Siphonaptera: Pulicidae). The results demonstrate that of these, only aphids and green leaf hoppers (both Hemiptera) have very high-affinity binding sites for [H-3]IMI in addition to the lower affinity sites that are also seen in insects from other orders, which in these insects have Kd values of about 1 nM and Hill values of approximately 1. The presence of very high-affinity [H-3]IMI binding sites only in the hemipteran insects studied may explain why IMI is particularly useful in controlling sucking pests. (C) 1998 Academic Press.
