INTERACTION OF IMIDACLOPRID METABOLITES AND ANALOGS WITH THE NICOTINIC ACETYLCHOLINE-RECEPTOR OF MOUSE-BRAIN IN RELATION TO TOXICITY

The favorable selective toxicity of imidacloprid (IMI) to insects vers us mammals is attributed to differences in their binding affinity or p otency in the nicotinic acetylcholine receptor (nAChR), a proposal tes ted here by studies on the mechanism of toxicity of IMI metabolites an d analogs to mammals. IMI, its desnitro metabolite (DN-IMI), its nitro methylene analog (CH-IMI), and 26 other analogs and metabolites were e xamined for intraperitoneal toxicity to mice and potency for in vitro inhibition of the binding of [H-3]nicotine (the classical nAChR probe) in mouse brain membranes. IMI and 7 analogs with LD50 values of 7-50 mg/ kg (or intoxication signs at 50 mg/kg) inhibited [H-3]nicotine bin ding by 50% (IC50) at 12-800 nM whereas 21 other analogs that were not toxic at 50 mg/kg gave an IC50 of >1000 nM, thereby correlating the t oxicity with interaction at the [3H]nicotine binding site. The most po tent compounds were DN-IMI and CH-IMI (and its tetrahydropyrimidine an alog) with LD(50)s of 7-24 mg/kg and IC(50)s of 12-33 nM compared with values for IMI of 39-49 mg/kg and 806 nM, respectively. DN-IMI is the refore a candidate bioactivation product for IMI in mammals. Scatchard analyses indicated that CH-IMI in vitro and possibly DN-IMI in vitro and ex vivo compete for the nicotine site (which is at or near the ACh site). When used directly as radioligands, single, saturable, high-af finity binding sites were observed for [H-3]DN-IMI (K-D 13 nM, B-max 5 1 fmol/mg protein) and [H-3]CH-IMI (K-D 16 nM, B-max 20 fmol/mg protei n) using the conditions of [H-3]nicotine binding (K-D 7.8 nM, B-max 87 fmol/mg protein). [H-3]DN-IMI also binds to kidney membranes at a sit e where it is displaced by atropine (k(i) 0.5 mu M). [H-3]CH-IMI is pa rticularly useful for comparative studies because of high-affinity sit es in both insect and mammalian brain. (C) 1997 Academic Press.