Oxidative stress as the trigger for menthol-induced developmental alterations in zebrafish (Danio rerio) model

Menthol is a natural origin organic compound obtained from the peppermint plant whose effects at the embryonic development level are not completely known. In this work, the effects of an acute exposure (96 h) to menthol were evaluated in zebrafish embryos based on the calculated lethal concentration (LC50, 27.5 mg L-1). The results showed a decreased hatching rate and heart rate as well as a notable increase in malformations and mortality. After menthol exposure, an increased production of reactive oxygen species (ROS) accompanied by an increase in the activity of antioxidant enzymes (superoxide dismutase, catalase, glutathione peroxidase and reductase) and a decrease in glutathione-S-transferase and protein carbonyls was detected. At the metabolic level, a decrease in ATPase activity and an increase in lactate dehydrogenase activity were perceived. Further-more, mitochondrial membrane potential (delta psi m) was increased after menthol exposure as well as the expression of Nrf2. At the behavioural level, shorter distance, speed, and percentage of active time were observed in menthol-treated larvae. In addition, an increase in the distance to the centre of the well and in the turn angle was observed supporting an anxiety-like behaviour induced by menthol exposure. Furthermore, there was a decrease in dopaminergic neurons and in the activity of the enzyme acetylcholinesterase (AChE) suggesting neuro-behavioral impairment. Overall, while further studies are required, menthol compromised the development of zebrafish by mediating early development through the regulation of Nrf2 by mitochondrial ROS production with the consequent teratogenic outcomes.