ABSTRACT
Although the neonicotinoid insecticides have good selectivity towards insects rather than vertebrates, they have severe effects on honeybee production and pollination activities. Therefore, the effects of imidacloprid (IMI), the most used neonicotinoid, on the two main bioreceptors, acetylcholinesterase (AChE) and nicotinic acetylcholine receptor alpha subunit (nAChRα1) of honeybees were examined to identify their roles in honeybee toxicity and possible binding sites which assist in selecting and designing neonicotinoids. In vivo, IMI showed a high inhibitory effect on AChE (IC50 5.63 mg/L); however, the effect was much lower in vitro experiment (IC50 719 mg/L). This result induced us to examine the IMI effect on AChE gene expression which revealed that the AChE-2 gene expression was severely affected by IMI explaining the observed high enzyme inhibition. In addition, although toxicity increased by increasing exposure to IMI (LC50 2.9 mg/L after 4h and 0.75 mg/L after 48h), AChE was not elevated (IC50 5.63 and 5.52 mg/L respectively). Besides, Despite resuming most enzyme activity (77% during 2 h and 84.14% after 4 h), a high mortality level was observed with LC50 2.9 mg/L. These results reinforced that the observed high toxicity is a multifactor process. Accordingly, Molecular modeling and docking of IMI into honeybee AChE and nAChRα1were also performed to examine their possible interactions and identify the important binding sites. Results models indicated that the first two binding sites in AChE were found in the esteratic subunit in the active site explaining the observed in vitro inhibition. In nAChRα1, four of the highest five free energy binding sites are located in the large TM3-TM4 loop and one in the extracellular loops. Consequently, the present work revealed that IMI toxicity is attributed to various factors including direct interaction with both AChE and nAChRα1 as well as downregulating AChE-2 gene expression.
