Effects of the α subunit on imidacloprid sensitivity of recombinant nicotinic acetylcholine receptors

Abstract

Imidacloprid is a new insecticide with selective toxicity for insects over vertebrates. Recombinant (α4β2) chicken neuronal nicotinic acetylcholine receptors (AChRs) and a hybrid nicotinic AChR formed by co‐expression of a Drosophila melanogaster neuronal α subunit (SAD) with the chicken β2 subunit were heterologously expressed in Xenopus oocytes by nuclear injection of cDNAs. The agonist actions of imidacloprid and other nicotinic AChR ligands ((+)‐epibatidine, (−)‐nicotine and acetylcholine) were compared on both recombinant nicotinic AChRs by use of two‐electrode, voltage‐clamp electrophysiology. Imidacloprid alone of the 4 agonists behaved as a partial agonist on the α4β2 receptor; (+)‐epibatidine, (−)‐nicotine and acetylcholine were all full, or near full, agonists. Imidacloprid was also a partial agonist of the hybrid Drosophila SAD chicken β2 receptor, as was (−)‐nicotine, whereas (+)‐epibatidine and acetylcholine were full agonists. The EC₅₀ of imidacloprid was decreased by replacing the chicken α4 subunit with the Drosophila SAD α subunit. This α subunit substitution also resulted in an increase in the EC₅₀ for (+)‐epibatidine, (−)‐nicotine and acetylcholine. Thus, the Drosophila (SAD) α subunit contributes to the greater apparent affinity of imidacloprid for recombinant insect/vertebrate nicotinic AChRs. Imidacloprid acted as a weak antagonist of ACh‐mediated responses mediated by SADβ2 hybrid receptors and as a weak potentiator of ACh responses mediated by α4β2 receptors. This suggests that imidacloprid has complex effects upon these recombinant receptors, determined at least in part by the α subunit. British Journal of Pharmacology (1998) 123, 518–524; doi:10.1038/sj.bjp.0701618