Developmental neurotoxic effects of dibutyl phthalate and its metabolite mono-n-butyl phthalate on Danio rerio

Mono-n-butyl phthalate (MBP) is a major active metabolite of the plasticizer dibutyl phthalate (DBP), a prevalent environmental contaminant. Although toxicity studies on DBP have been conducted, the toxicity of MBP remains largely understudied. Therefore, we aimed to elucidate the neurotoxic effects of MBP and DBP on zebrafish (Danio rerio) larvae. To this end, wild-type and transgenic lines (tg(huC:eGFP) and tg(mbp:mGFP)) were exposed to DBP and MBP, respectively, during the early developmental stages (4 h past fertilization(hpf) - 120 hpf). Behavioral changes such as tail coiling, touch-evoked responses, and locomotor activity (individual and group) were observed. Neurodevelopment was evaluated using fluorescence images from two transgenic lines and eight related gene transcripts. Additionally, two neurotransmitters were analyzed. Oxidative markers were evaluated to determine the mechanisms underlying neurotoxicity. Our findings revealed decreases in neurogenesis (tg(huC:eGFP)) and myelination (tg(mbp:mGFP)), as well as alterations in zebrafish behavior, including reduction of touch-evoked responses and distance moved. Dopamine levels decreased and oxidative stress increased after exposure to MBP. Both DBP and MBP exhibited neurotoxic effects in this study. Notably, the metabolite MBP exhibited a neurotoxic profile similar to, or in some endpoints greater than, its parent compound. These findings highlight the importance of considering metabolite-mediated toxicity. Our results suggest that regulatory frameworks should be expanded beyond monitoring parent chemicals to integrate metabolite toxicity into safety assessments, thereby mitigating ecological and public health risks.