The antiparasitic bephenium is a potent agonist of Caenorhabditis elegans levamisole-sensitive nicotinic receptors

Abstract

Nicotinic acetylcholine receptors (nAChRs) are ligand-gated ion channels involved in neuromuscular transmission. In nematodes, muscle nAChRs are main targets of antiparasitic drugs. Nematode parasites contain three pharmacological classes of muscle nAChRs, which are activated by levamisole (L-type), nicotine (N-type) and bephenium (B-type). The free-living nematode Caenorhabditis elegans is a model of parasitic nematodes, useful for drug discovery. Because in C. elegans muscle only the N-AChR and L-AChR classes have been described, we explored the behavioral (by paralysis assays) and molecular actions (by patch clamp recordings) of the antiparasitic bephenium. As in parasites, bephenium produced spastic paralysis. A mutant strain lacking the L-AChR showed full resistance to bephenium, indicating that this receptor is the drug target. Bephenium activated L-AChRs from isolated larvae muscle cells, eliciting channel activity as that elicited by levamisole. The analysis revealed that it is a potent agonist of the L-AChR and an open-channel blocker at higher concentrations. In contrast, we demonstrated that it is a very low efficacious agonist of the mammalian muscle nAChR. Molecular docking studies proposed that bephenium can form key interactions required for activation in mammalian and nematode nAChRs, revealed differences with ACh binding, and provided explanations for the experimental results.