Cellular restoration and locomotor recovery upon endogenous dopaminergic neuroregeneration in the 6-OHDA-lesioned adult zebrafish Parkinson’s disease model

Parkinson’s disease (PD) is the fastest-growing neurological disorder characterised by progressive degeneration of midbrain dopaminergic neurons (DpN) and consequent motor dysfunction. Unfortunately, there remains no cure for PD and current treatment modalities do not modify the disease’s course. These limitations raise the need to focus on regeneration of DpN as a potential approach of PD management. The process is, however, inefficient in mammals, making mammalian-based models unsuited for understanding of neuroregeneration. As such, adult zebrafish is greatly favoured for its close brain homology with human and neuronal self-renewal capability. As part of the effort in elucidating cellular events involved in neuroregeneration in vivo, we established and characterised a 6-hydroxydopamine (6-OHDA)-induced zebrafish model of PD which resulted in the targeted ablation of over 85% of tyrosine hydroxylase (TH)-positive DpN within the ventral diencephalon (vDn) day-3 post-lesion. Notably, full cellular restoration was observed at the lesioned site by day-30 post-lesion, indicating effective dopaminergic regeneration of adult zebrafish brain. Locomotor assessment using the open field test revealed >65% reduction in both distance travelled and mean speed (p < 0.0001) at day-3 post-lesion, followed by motor recovery by day30 post-lesion, indicating functional recovery concurrent upon cellular regeneration. In order to examine the proliferative and neurogenic dynamics underlying this recovery, a double-pulse chase using 5-Bromo-2’-deoxyuridine (BrdU) and 5-ethynyl-2’deoxyuridine (EdU) was employed across early (days 5–7) and late (days 11–13) proliferative phases. BrdU-positive cells were predominantly detected in anterior brain regions such as the olfactory bulb (OB) and pallium-subpallium (Pa-SPa), while EdUpositive cells accumulated in the subpallium (SPa) and preoptic area (POA), adjacent to the lesion site. These findings along with whole brain mapping of both sagittal and coronal planes, indicated a potential migration of newly generated cells from OB and telencephalon (Tel) towards vDn