Acute effects of the imidacloprid metabolite desnitro-imidacloprid on human nACh receptors relevant for neuronal signaling

Dominik Loser,Paul Walker,Iain Gardner,Anna Forsby,Karin Grillberger,Jonathan Blum,Timm Danker,Gerhard F Ecker,Susanne H Bennekou,Maria G Hinojosa,Annette Nicke,Marcel Leist,Udo Kraushaar,Clemens Möller,Yves Haufe,Caroline Bauch,Ylva Johansson

doi:10.1007/s00204-021-03168-z

Abstract

Several neonicotinoids have recently been shown to activate the nicotinic acetylcholine receptor (nAChR) on human neurons. Moreover, imidacloprid (IMI) and other members of this pesticide family form a set of diverse metabolites within crops. Among these, desnitro-imidacloprid (DN-IMI) is of special toxicological interest, as there is evidence (i) for human dietary exposure to this metabolite, (ii) and that DN-IMI is a strong trigger of mammalian nicotinic responses. We set out here to quantify responses of human nAChRs to DN-IMI and an alternative metabolite, IMI-olefin. To evaluate toxicological hazards, these data were then compared to those of IMI and nicotine. Ca2+-imaging experiments on human neurons showed that DN-IMI exhibits an agonistic effect on nAChRs at sub-micromolar concentrations (equipotent with nicotine) while IMI-olefin activated the receptors less potently (in a similar range as IMI). Direct experimental data on the interaction with defined receptor subtypes were obtained by heterologous expression of various human nAChR subtypes in Xenopus laevis oocytes and measurement of the transmembrane currents evoked by exposure to putative ligands. DN-IMI acted on the physiologically important human nAChR subtypes α7, α3β4, and α4β2 (high-sensitivity variant) with similar potency as nicotine. IMI and IMI-olefin were confirmed as nAChR agonists, although with 2–3 orders of magnitude lower potency. Molecular docking studies, using receptor models for the α7 and α4β2 nAChR subtypes supported an activity of DN-IMI similar to that of nicotine. In summary, these data suggest that DN-IMI functionally affects human neurons similar to the well-established neurotoxicant nicotine by triggering α7 and several non-α7 nAChRs.

Highlights

The toxicological assessment of many pesticides is complicated by the fact that there is exposure to the original substances, and to their many metabolites formed in the environment
We explored whether DN-IMI possesses a potential neurotoxicity or developmental neurotoxicity hazard, by acting on nicotinic acetylcholine receptor (nAChR) of human neurons
DN-IMI appeared at least as potent as ACh and nicotine (pEC50 values of ~ 6.0 in LUHMES neurons) (Loser et al 2021a). These signaling data are in line with published binding data that suggest a similar affinity of DN-IMI and nicotine for mammalian nAChRs (Tomizawa and Casida 1999; D’Amour and Casida 1999; Tomizawa et al 2000)

Summary

Introduction

The toxicological assessment of many pesticides is complicated by the fact that there is exposure to the original substances, and to their many metabolites formed in the environment This applies to the neonicotinoids, a class of insecticides with long persistence within crops (Simon-Delso et al 2015; Craddock et al 2019; Thompson et al 2020). A broad toxicological debate has been triggered by the observation that acetamiprid and IMI activated the nAChRs on neonatal rat neurons in the low μM range (Kimura-Kuroda et al 2012). The relevance of this finding for human toxicology is further supported by a recent study using cultured human neurons. Clear nAChR signaling and pronounced receptor desensitization were demonstrated for several neonicotinoids at concentrations that may be reached by dietary or accidental exposure (Loser et al 2021a)

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