Morpho-Anatomical Degeneration of Dopaminergic Neurons in Adult Zebrafish Brain after Exposure to Nickel

Chronic exposure to heavy metals has been widely demonstrated to induce pathological features in different tissues and, in particular, in the central nervous system. Specific neurons, including dopaminergic neurons, were observed to be more susceptible to toxic agents. Several previous studies performed on zebrafish (Danio rerio) models observed that exposure to nickel (one of the most abundant heavy metals) induces impairment of memory and anxious-like behaviors. Nevertheless, this phenotypical evidence has not been associated with dopaminergic system damage, and no reports showing the effects of nickel on dopaminergic neurons are available. In this study, we aim to analyze the precise distribution and variation in dopaminergic neurons in adult zebrafish after chronic (96 h) exposure to nickel ions dissolved in water at different sub-lethal doses (0.4 mg L−1; 2 mg L−1 and 4 mg L−1). The effects of treatment on dopaminergic neurons were evaluated by measuring transcript and protein levels of tyrosine hydroxylase (TH), described as a dopaminergic neuron marker. As shown, the expression of the th1 and th2 genes was reduced in the entire brain of zebrafish treated with nickel. Immunostaining analysis allowed us to localize TH-expressing neurons mainly in the posterior tuberculum, where they were observed to be reduced after nickel treatment in a dose-dependent fashion. Consistently, the TUNEL assay revealed a significant increase in apoptosis of TH-expressing cells after treatment with 2 mg L−1 and 4 mg L−1 of nickel. Our findings represent the first evidence of the effect of nickel on the dopaminergic system.