Neuromelanin (NM) is a dark insoluble polymer pigment produced in specific populations of catecholaminergic neurons in the brain. Humans have the largest amount of NM, which is present in lesser amounts in other primates, and totally absent in many other species. However, the biological function remains unknown, although human NM has been shown to efficiently bind transition metals such as iron, as well as other potentially toxic molecules. Therefore, it may play crucial roles in apoptosis and the related Parkinson's disease.
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Dark pigments in the substantia nigra were first described in 1838 by PurkynÄ›,and the term neuromelanin was proposed in 1957 by Lillie, though it has been thought to serve no function until recently. It is now believed to play a vital role in preventing cell death in certain parts of the brain. It has been linked to Parkinson's disease and because of this possible connection, neuromelanin has been heavily researched in the last decade."

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New research (2020) suggests that Neuromelanin (NM) may be playing a role as "a very efficient quencher for toxic molecules". At the same time, however, an excessive build-up of NM in brain tissue can be also be toxic itself :

Thus, the paradox of NM is that its greatest virtue is its worst defect. NM can play either a cytoprotective or a neurotoxic role depending on its cellular and extracellular context and the load that carries."

See: The Neuromelanin Paradox and Its Dual Role in Oxidative Stress and Neurodegeneration Antioxidants Journal, Volume 10, Issue 1.