Receptor Binding Drives Toxicokinetic Processes: Insights from Two Amphipod Species Exposed to Insecticides Targeting the Nicotinic Acetylcholine Receptor

Abstract Insecticides targeting nicotinic acetylcholine receptors (nAChRs) are used because of their high neuroactive efficacy. However, they can also exert substantial toxic pressure on nontarget aquatic invertebrates. A previous study suggested that irreversible receptor binding is the main driver of thiacloprid bioaccumulation. To determine whether the observed elimination resistance applies to other nAChR-binding insecticides and species, this study investigates five compounds from different substance classes that target nAChRs. Using the two amphipod species, Hyalella azteca and Gammarus pulex, toxicokinetic experiments were combined with assessing receptor binding in vivo and in vitro. The three N-cyanoamidines and N-nitroguanidine neonicotinoids (thiacloprid, acetamiprid, and imidacloprid) and the metabolite imidacloprid-olefin showed resistance toward elimination (i.e., over 15 days of depuration in both amphipod species). This observation could be well described by a two-compartment toxicokinetic (TK)-receptor model (>0.89 R2) with integrated irreversible binding. In contrast, the butenolide flupyradifurone was fully eliminated, and the organic biocide spinosad was eliminated significantly but slowly, both fitted well with a one-compartment TK model (>0.71 R2). These findings demonstrate that receptor-binding specificity and affinity are highly compound class dependent. Although no direct link between receptor binding and acute toxicity could be established, elimination resistance indicates a risk for delayed, sublethal effects that should be critically considered in regulatory risk assessment.