Developmental toxicity of two organophosphate pesticides in Zebrafish embryo: Comparative and combinatorial assessment of neuro- and cardio-toxicity of sub-lethal concentrations of chlorpyrifos and malathion.

Organophosphates are one of the major and most widely used pesticides. Globally, two of the most widely used organophosphates are chlorpyrifos (CPF) and malathion (MAL). Reports on the effect of environmentally relevant concentrations of organophosphates during embryological development are very limited. In this study, we have exposed 2 h viable zebrafish to 100 µg/L, 200 µg/L, 400 µg/L concentrations of CPF or MAL. In addition, the embryos were also exposed to a mixture of CPF and MAL containing 200 µg/L of each of the pesticides (CPF+MAL). Morphological observations of the whole embryo, behavioural study and histopathological assessment of various organs in 120 hpf zebrafish larvae were conducted. mRNA expressions of the genes relevant to neuro and cardiac development were also analysed. Finally, intracellular ROS generation and protein expression of the oxidative stress responsive transcription factor NRF2 was assessed. Both the pesticides have been shown to induce various types of malformations related to cardiac and neural toxicity. A significant observation of this study is the differential effect of CPF and MAL on the hatching rate of zebrafish embryos and on the locomotor activity of zebrafish larvae. Among all the experimental exposures, CPF was the most toxic, even more than the combination group which we found to be very startling. Overall, we showed that sub-lethal concentrations of organophosphate pesticides can cause severe developmental toxicity and abnormal gene expressions without causing lethality in the embryos.