Actions of quinolizidine alkaloids on Periplaneta americana nicotinic acetylcholine receptors

Abstract

Botanical insecticides do not play a major role as crop protectants, but they are beneficial in some applications. The authors investigated the actions of naturally occurring alkaloids on insect nicotinic acetylcholine (ACh) receptors (nAChRs) by evaluating their abilities to inhibit specific binding of [(3)H]imidacloprid (IMI) to nerve-cord membranes from Periplaneta americana L. Two alkaloids were also tested for their actions on nAChRs expressed by cockroach neurons using patch-clamp electrophysiology. Four natural quinolizidine alkaloids (matrine, sophocarpine, cytisine and aloperine) exhibited more than 50% inhibition of [(3)H]IMI binding at 10 microM, although other compounds were found to have no or low inhibitory activity. The rank order of potency based on concentration-inhibition curves was cytisine > sophocarpine >or= aloperine >or= matrine. Patch-clamp analysis indicated that sophocarpine and aloperine were not agonists of nAChRs expressed in P. americana neurons, yet, at 10 microM, aloperine, but not sophocarpine, suppressed ACh-induced inward currents significantly. Three of the four natural alkaloids tested possess structural moieties that are necessary for interaction with P. americana nAChRs. Aloperine, which possesses a unique structure and showed a distinctive dose-response curve, was found to act as an antagonist. Appropriate modifications of these alkaloids might result in novel insecticidal nAChR ligands.