Neurotoxin exposure of zebrafish larvae has been used to mimic a Parkinson’s disease (PD) phenotype and to facilitate high-throughput drug screening. However, the vulnerability of zebrafish to various neurotoxins was shown to be variable. Here, we provide a direct comparison of ablative effectiveness in order to identify the optimal neurotoxin-mediated dopaminergic (DAnergic) neuronal death in larval zebrafish. Transgenic zebrafish, Tg(dat:eGFP), were exposed to different concentrations of the neurotoxins MPTP, MPP⁺, paraquat, 6-OHDA, and rotenone for four days, starting at three days post-fertilization. The LC₅₀ of each respective neurotoxin concentration was determined. Confocal live imaging on Tg(dat:eGFP) showed that MPTP, MPP⁺, and rotenone caused comparable DAnergic cell loss in the ventral diencephalon (vDC) region while, paraquat and 6-OHDA caused fewer losses of DAnergic cells. These results were further supported by respective gene expression analyses of dat, th, and p53. Importantly, the loss of DAnergic cells from exposure to MPTP, MPP⁺, and rotenone impacted larval locomotor function. MPTP induced the largest motor deficit, but this was accompanied by the most severe morphological impairment. We conclude that, of the tested neurotoxins, MPP⁺ recapitulates a substantial degree of DAnergic ablation and slight locomotor perturbations without systemic defects indicative of a Parkinsonian phenotype.

Glial cell line-derived neurotrophic factor (GDNF) was initially described as important for dopaminergic neuronal survival and is involved in many other essential functions in the central nervous system. Characterization of GDNF phenotype in mammals is well described; however, studies in non-mammalian vertebrate models are scarce. Here, we characterized the anatomical distribution of gdnf-expressing cells in adult zebrafish brain by means of combined in situ hybridization (ISH) and immunohistochemistry. Our results revealed that gdnf was widely dispersed in the brain. gdnf transcripts were co-localized with radial glial cells along the ventricular area of the telencephalon and in the hypothalamus. Interestingly, Sox2 positive cells expressed gdnf in the neuronal layer but not in the ventricular zone of the telencephalon. A subset of GABAergic precursor cells labeled with dlx6a-1.4kbdlx5a/6a: green fluorescence protein (GFP) in the pallium, parvocellular preoptic nucleus, and the anterior and dorsal zones of the periventricular hypothalamus also showed expression with gdnf mRNA. In addition, gdnf signals were detected in subsets of dopaminergic neurons, including those in the ventral diencephalon, similar to what is seen in mammalian brain. Our work extends our knowledge of gdnf action sites and suggests a potential role for gdnf in adult brain neurogenesis and regeneration.

Neurotrophic factors are known to play a fundamental role in the development and maintenance of the nervous system, including growth, maturation, survival= and differentiation. There is potential in harnessing neurotrophic factors to prevent neurodegenerative damage. We aim to decipher how neurotrophic factors brain-derived neurotrophic factor (BDNF) and glial cell-derived neurotrophic (GDNF) originating from glial cells are affecting the dopaminergic neuron degeneration in the zebrafish larval brain. Spatiotemporal expression of bdnf, gdnf, tyrosine hydroxylase (th), and dopamine transporter (dat) genes during zebrafish embryonic development was carried out from 1 cell to 10 days post fertilization (dpf) in wildtype embryos and larvae. Larvae at 3 dpf were exposed to platelet derived growth factor receptor (PDGFR) inhibitor, for 24 hours to modulate neurotrophic factor levels prior to neurotoxic insult (MPTP) at 4 dpf. Larvae were assessed for locomotor responses and relevant gene expressions, and whole mount immunofluorescence staining at 6 dpf. Results showed bdnf and dat were expressed as early as the 1 cell stage, suggesting presence of maternal mRNA. Interestingly, gdnf was expressed following zygotic genome activation at 4 hpf, which is 4 hours earlier than a strong th expression at 8 hpf. By 24 hpf, all genes (bdnf, gdnf, th, and dat) were consistently expressed. Although there were no significant changes in the behavior and expression levels of bdnf, gdnf, th or dat in larvae pre-treated with the PDGFR inhibitor, immunofluorescent staining showed marked decrease in th-positive cell numbers in larvae compared to controls, suggestive of neurodegeneration being heightened by exposure to PDGFR inhibitor. In conclusion, it can be inferred that PDGFR may be critically involved in modulating the extent of neurodegeneration induced by MPTP. However, further refinements are needed to determine the dose of PDGFR inhibitor modulation that would impact on locomotor response and neurotrophic levels.