Neuromelanin of the substantia nigra: a neuronal black hole with protective and toxic characteristics.

Neuromelanin accumulates in dopaminergic neurons during normal aging, and in Parkinson's disease, neurons with this pigment are those that selectively degenerate. Intraneuronal neuromelanin could play a protective role during its synthesis by preventing the toxic accumulation of cytosolic catechol derivatives and, in addition, by its ability to scavenge reactive metals, pesticides and other toxins to form stable adducts. However, dying neurons in Parkinson's disease that release neuromelanin might induce a vicious cycle of chronic neuroinflammation and neuronal loss.