Inhibitory effects of four neonicotinoid active ingredients on acetylcholine esterase activity

There is a great concern about the decline of pollinators, and neonicotinoids emerging bee disorders are assumed to play a significant role. Since changes in learning ability has been observed in honey bees exposed to some acetylcholine esterase (AChE) inhibitors, we therefore, tested in vitro the effect of four neonicotinoids on purified eel AChE. AChE activity was inhibited in a concentration-dependent manner, and calculated IC₅₀ values for thiamethoxam (IC₅₀ = 414 μM) and clothianidin (IC₅₀ = 160 μM) were found to be much higher compared to acetamiprid (IC₅₀ = 75.2 μM) and thiacloprid (IC₅₀ = 87.8 μM). The Lineweaver-Burk reciprocal plots for acetamiprid shows unchanged V_max and increased K_m values with inhibitor concentrations, while analysis of Michaelis-Menten plots shows predominantly competitive mechanism. The inhibition constant value (K_i = 24.3 μM) indicates strong binding of the acetamiprid complex to AChE. Finally, the four tested neonicotinoids are not a uniform group regarding their blocking ability. Our results suggest a previously not established, direct AChE blocking mechanism of neonicotinoids tested, thus the neuronal AChE enzyme is likely among the direct targets of the neonicotinoid insecticides. We conclude, that these AChE inhibitory effects may also contribute to toxic effects on the whole exposed animal.