Delayed effects of methylmercury on the mitochondria of dopaminergic neurons and developmental toxicity in zebrafish larvae (Danio rerio)

• A novel zebrafish line was used to evaluate the neurotoxic effects of MeHg.
• MeHg latently reduced dopaminergic mitochondria number in specific brain regions.
• The affected brain regions are similar to target regions of MPTP, a potent dopaminergic toxicant.
• Dopaminergic neuron function rather than neuron numbers was likely compromised.

Methylmercury (MeHg) is a known neurotoxicant affecting the central nervous system but effects on dopaminergic (DA) neurons are not well understood. Wild-type zebrafish (Danio rerio) and two transgenic lines: Tg(dat:eGFP) expressing enhanced green fluorescent protein (eGFP) in DA neuron clusters and Tg(dat:tom20 MLS-mCherry) expressing red fluorescence (mCherry) targeted to mitochondria of DA neurons were used to evaluate the effects of micromolar MeHg exposure on DA neuron and whole animal motor function during early development. Three-day-old larvae were exposed to micromolar concentrations of MeHg (0.03, 0.06, and 0.3 μM) in system water. Exposure to 0.3 μM MeHg caused mortality and significant morphological abnormalities including edema, curvature of the spine, and hemorrhages in zebrafish larvae after a 48 h exposure period. At 0.06 μM MeHg, the appearance of morphological abnormalities was delayed for 72 h and far less severe, whereas 0.03 μM MeHg did not cause any morphological defects or mortalities. A delayed but significant reduction in locomotor ability and mCherry fluorescence in specific brain regions in the 0.06 μM MeHg exposed larvae suggests that DA neuron function rather than neuron numbers was compromised. Double immunolabeling with tyrosine hydroxylase and pan neural staining showed no effect of MeHg exposure. We have established Tg(dat:tom20 MLS-mCherry) zebrafish larvae as a model which can be used to assess MeHg neurotoxicity and that exposure to low dose MeHg (0.06 μM) during development may predispose DA neurons to impairment caused by changes in mitochondrial dynamics.