Actions of Harmonia axyridis (Coleoptera: Coccinellidae) alkaloids on vertebrate and invertebrate nicotinic acetylcholine receptors

Abstract

The productivity of livestock and crops worldwide is dramatically reduced by pest organisms and pesticides are an important front-line control. Current pesticides raise environmental concerns and are threatened by the development of resistance so there is a great need for alternatives. Previous work has shown that defensive alkaloids secreted by the Harlequin ladybird beetle, Harmonia axyridis (H. axyridis), act on insect and human nicotinic acetylcholine receptors (nAChRs). Selectivity for insect nAChRs, which are proven pesticide targets, was demonstrated so the alkaloids show promise as leads for the development of novel pesticides. Our aim was to ascertain whether these alkaloids had any anthelmintic effects by examining the effects on the pharynx and body wall neuromuscular systems of the nematode worm, Caenorhabditis elegans. Subsequent analysis of mutant strains was used to identify a possible target. Following this we aimed to demonstrate the effects on different vertebrate and invertebrate nAChRs directly using the Xenopus laevis oocyte expression system. The alkaloid extract of Harmonia axyridis inhibited the rate of pharyngeal pumping after 1 h (IC50 = 1.087 mg/ml) and 24 h (IC50 = 0.406 mg/ml). Body wall movement was also affected after 2 h (IC50 = 0.64 mg/ml), 4 h (IC50 = 0.63 mg/ml) and 6 h (IC50 = 0.58 mg/ml). Examination of mutant strains revealed the alkaloids may be acting through EAT-18 and ACR-16. This was confirmed by electrophysiological recordings of ACR-16 nAChRs expressed in Xenopus oocytes (IC50 = 9.83x10-6M) but no selectivity is apparent when compared with human α7 nAChRs (IC50 = 9.09x10-6M). Selectivity does appear to exist with a hybrid Drosophila/chicken nAChR (IC50 = 3.16x10-7M). This confirms the nAChR target of the alkaloids and that the selectivity is due to action at the nAChR. The action of alkaloids on nematodes in vivo and on nematode and insect nAChRs provides the potential for the alkaloids to serve as lead compounds for novel pesticides