J. Neurochem. (2011) 10.1111/j.1471‐4159.2011.07518.x

Planarians have robust regenerative ability dependent on X‐ray‐sensitive pluripotent stem cells, called neoblasts. Here, we report that planarians can regenerate dopaminergic neurons after selective degeneration of these neurons caused by treatment with a dopaminergic neurotoxin (6‐hydroxydopamine; 6‐OHDA). This suggests that planarians have a system to sense the degeneration of dopaminergic neurons and to recruit stem cells to produce dopaminergic neurons to recover brain morphology and function. We confirmed that X‐ray‐irradiated planarians do not regenerate brain dopaminergic neurons after 6‐OHDA‐induced lesioning, suggesting that newly generated dopaminergic neurons are indeed derived from pluripotent stem cells. However, we found that the majority of regenerated dopaminergic neurons were 5‐bromo‐2′‐deoxyuridine‐negative cells. Therefore, we carefully analyzed when proliferating stem cells became committed to become dopaminergic neurons during regeneration by a combination of 5‐bromo‐2′‐deoxyuridine pulse‐chase experiments, immunostaining/in situ hybridization, and 5‐fluorouracil treatment. The results strongly suggested that G₂‐phase stem cells become committed to dopaminergic neurons in the mesenchymal space around the brain, after migration from the trunk region following S‐phase. These new findings obtained from planarian regeneration provide hints about how to conduct cell‐transplantation therapy for future regenerative medicine.