Due to their extensive use and high biological activity, insecticides largely contribute to loss of biodiversity as well as environmental pollution. The regulation of insecticides mainly relies on lethal concentrations, however, also sub-lethal effects such as alterations in behavior and neurodevelopment can significantly affect fitness of the individual and population dynamics and therefore need consideration. Moreover, it is important to understand the impact of exposure timing during development, which currently lacks relevant knowledge. Here, we investigated whether there are periods during neurodevelopment of fish, which are particularly vulnerable to insecticide exposure. Therefore, we exposed zebrafish embryos to six different insecticides with a cholinergic mode of action for 24 hours during different periods of neurodevelopment and measured locomotor output using an age-matched behavior assay. We used the organophosphates diazinon and dimethoate, the carbamates pirimicarb and methomyl as well as the neonicotinoids thiacloprid and imidacloprid because they are abundant in the environment and cholinergic signaling plays a major role during key processes of neurodevelopment. We found that early embryonic motor behaviors, as measured by spontaneous tail coiling, increased upon exposure to most insecticides, while later movements, measured through touch-evoked response and light-dark transition assay, rather decreased for the same insecticides and exposure duration. Moreover, the observed effects were more pronounced when the exposure windows were temporally closer to the performing of the respective behavioral assay. However, the measured effects recovered after a short period, indicating that none of the exposure windows chosen here are particularly critical, but rather that insecticides acutely interfere with neuronal function at all stages as long as they are present. Overall, our results contribute to a better understanding of the risks posed by cholinergic insecticides to fish and provide an important basis for the development of safe regulations to improve environmental health.
