Study on the combined neurotoxicity of propiconazole and four other common residual pesticides in citrus fruits

ObjectiveExploring the neurotoxic effects of combined exposure to commonly detected pesticide residues in citrus， including profenofos （PFF） ，avermectin （ABM）， difenoconazole （DIF）， bifenthrin （BIF）， and imidacloprid （IMI）， respectively， on Caenorhabditis elegans.MethodsWild type N2 and transgenic EG1285 （unc-47：：GFP） C. elegans were exposed to mycotoxins in the liquid environment for 24-72 h， and the body length inhibition rates of five pesticides were measured. Based on the single toxic effect， low， medium and high （1/10 EC50， 1/2 EC50， EC50） dose groups were designed to investigate the effects of PFF mixed with four other pesticides， respectively. On the motility behavior of C. elegans， followed by using Chou-Talalay model to analyze the type of combined toxicity. Finally， a combined neurotoxicity study was conducted on the PFF-ABM mixture which showing the significantly synergistic effects to further explore its effects on the chemotaxis， predation rate， ATP and GABA levels and GABAergic neurons.ResultsThe toxic effects of the five pesticides on the body length of nematodes （24 h） were in the order of ABM （EC50=0.015 μg/mL）>PFF （EC50=0.458 μg/mL）>DIF （EC50=10.585 μg/mL）>IMI （EC50=39.981 μg/mL）>BIF （EC50=46.623 μg/mL）. The results of the combined toxicity effects of pesticide mixtures showed that all pesticide combination treatment groups had inhibitory effects on the motility behavior of C. elegans. The combined toxic effects of PFF-DIF， PFF-BIF and PFF-IMI were synergistic at low concentrations whereas antagonistic at high concentrations. However， the combined toxic effect of PFF-ABM was synergistic at all the tested concentration. The ATP and GABA levels of C. elegans in the medium and high-dose groups of PFF-ABM were significantly lower than those in the PFF and ABM groups alone （P<0.05）. In the experiment of neuronal developmental damage， the fluorescence intensity of GABAergic neurons in transgenic C. elegans in the high-dose group of PFF-ABM decreased by 43.38% and 33.20%， respectively， compared to the PFF and ABM alone groups.ConclusionThe combined neurotoxic effects of PFF and ABM mixture was synergistic and the combined neurotoxic effects of the mixtures with the other three pesticides showed a trend of shifting from synergistic to antagonistic effects with the increasing exposure concentration. PFF-ABM combined exposure induced synergistic effects by reducing the ATP and GABA levels and damaging GABAergic neurons in nematodes.