Toxicity and nicotinic acetylcholine receptor interaction of imidacloprid and its metabolites in Apis mellifera (Hymenoptera : Apidae)

Acute oral and contact toxicity tests of imidacloprid, an insecticide actin g agonistically on nicotinic acetylcholine receptors (nAChR), to adult hone ybees, Apis mellifera L var carnica, were carried out by seven different Eu ropean research facilities. Results indicated that the 48-h oral LD50 of im idacloprid is between 41 and >81 ng per bee, and the contact LD50 between 4 9 and 102 ng per bee. The ingested amount of imidacloprid-containing sucros e solution decreased with increasing imidacloprid concentrations and may be attributed to dose-related sub-lethal intoxication symptoms or to antifeed ant responses. Some previously reported imidacloprid metabolites occurring at low levels in planta after seed dressing, ie olefine-, 5-OH- and 4,5-OH- imidlacloprid, showed lower oral LD50 values (>36, >49 and 159ng per bee, r espectively) compared with the concurrently tested parent molecule (41ng pe r bee). The urea metabolite and 6-chloronicotinic acid (6-CNA) exhibited LD 50 values of >99 500 and >121 500ng per bee, respectively. The pharmacological profile of the [H-3]imidacloprid binding site in honeyb ee head membrane preparations is consistent with that anticipated for a nAC hR. IC50 values for the displacement of [3H]imidacloprid by several metabol ites such as olefine, 5-OH-, 4,4-OH-imidacloprid, urea and 6-CNA were 0.45, 24, 6600, > 100 000, and > 100 000 nM, respectively. Displacement of [3H]i midacloprid by imidacloprid revealed an IC50 value of 2.9 nM, thus correlat ing well with the observed acute oral toxicity of the compounds in honeybee s. Neurons isolated from the antennal lobe of A mellifera and subjected to who le-cell voltage clamp electrophysiology responded to the application of 100 muM acetylcholine with a fast inward current of between 30 and 1600 pA at -70mV clamp potential. Imidacloprid and two of the metabolites (olefine- an d 5-OH-imidacloprid) acted agonistically on these neurons, whereas the othe rs did not induce currents at test conencentrations up to. 3 mM. The electr ophysiological data revealed Hill coefficients of approximately 1, indicati ng a single binding site responsible for an activation of the receptor and no direct cooperativity or allosteric interaction with a second binding sit e. (C) 2001 Society of Chemical Industry.