When longevity becomes chemistry: Why rodent brains lack neuromelanin

Background: The biology of neuromelanin (NMel) is linked to Parkinson's disease, in which pigment accumulation and release contribute to dopaminergic cell loss. Rodent brains lack detectable levels of NMel, despite possessing the molecular machinery required for its synthesis. The basis for this absence remains unknown. Objectives: To determine whether long-living rodents accumulate NMel and to test whether lifespan constraints explain the lack of pigmentation in mice and rats. Methods: Histological stains were conducted in brain samples of mole rats of different ages to disclose the potential pigmentation of catecholaminergic centers and to determine the chemical phenotype of pigmented neurons. Results: Mole rats exhibited clear age-dependent NMel accumulation beginning after two years of age, with the highest pigmentation levels observed in neurons of the locus coeruleus. Conclusions: Accumulation of NMel is a slow kinetic process that requires extended longevity. Common laboratory rodents lack pigmented brains simply because they do not live long enough to reach detectable NMel levels.