Melanin, the pigment in hair, skin, eyes, and feathers, protects external tissue from damage by UV light. In contrast, neuromelanin (NM) is found in deep brain regions, specifically in loci that degenerate in Parkinson's disease. Although this distribution suggests a role for NM in Parkinson's disease neurodegeneration, the biosynthesis and function of NM have eluded characterization because of lack of an experimental system. We induced NM in rat substantia nigra and PC12 cell cultures by exposure to L-dihydroxyphenylalanine, which is rapidly converted to dopamine (DA) in the cytosol. This pigment was identical to human NM as assessed by paramagnetic resonance and was localized in double membrane autophagic vacuoles identical to NM granules of human substantia nigra. NM synthesis was abolished by adenoviral-mediated overexpression of the synaptic vesicle catecholamine transporter VMAT2, which decreases cytosolic DA by increasing vesicular accumulation of neurotransmitter. The NM is in a stable complex with ferric iron, and NM synthesis was inhibited by the iron chelator desferrioxamine, indicating that cytosolic DA and dihydroxyphenylalanine are oxidized by iron-mediated catalysis to membrane-impermeant quinones and semiquinones. NM synthesis thus results from excess cytosolic catecholamines not accumulated into synaptic vesicles. The permanent accumulation of excess catechols, quinones, and catechol adducts into a membrane-impermeant substance trapped in organelles may provide an antioxidant mechanism for catecholamine neurons. However, NM in organelles associated with secretory pathways may interfere with signaling, as it delays stimulated neurite outgrowth in PC12 cells. P arkinson's disease (PD) results from the death of neuromelanin (NM)-containing neurons in the substantia nigra pars compacta (SNC) (1) and the locus coeruleus (2). The presence of NM provides the characteristic pigmented appearance indicated by the names of these brain regions. NM appears in SNC DAergic neurons within 3 years of birth and increases with age (3). Because the neuronal populations that contain NM are those that die in PD, there has long been speculation that NM underlies PD pathogenesis. In one hypothesis, NM binds neurotoxins such as 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) (4) or paraquat (5), providing a pool of toxin within pigmented cells. Similarly, NM binds iron and toxic metals that could promote neurodegeneration (6, 7). Finally, NM could itself produce toxic free radicals (8). However, NM cannot be the sole causal factor in PD pathogenesis because all humans accumulate NM with age.Beyond the suggestions that NM underlies PD, there has been no suggestion of a biological function for this substance. Very little is known about NM biosynthesis, and it is not known where NM is synthesized in the cell, which intracellular or extracellular catecholamine pools are involved, or what triggers its formation (9-12). Although previous studies on NM synthesis have been performed on synthetic polymers arising from sponta...

To determine the importance of the NMDA receptor (NMDAR) in pain hypersensitivity after injury, the NMDAR1 (NR1) subunit was selectively deleted in the lumbar spinal cord of adult mice by the localized injection of an adenoassociated virus expressing Cre recombinase into floxed NR1 mice. NR1 subunit mRNA and dendritic protein are reduced by 80% in the area of the virus injection, and NMDA currents, but not AMPA currents, are reduced 86-88% in lamina II neurons. The spatial NR1 knock-out does not alter heat or cold paw-withdrawal latencies, mechanical threshold, or motor function. However, injury-induced pain produced by intraplantar formalin is reduced by 70%. Our results demonstrate conclusively that the postsynaptic NR1 receptor subunit in the lumbar dorsal horn of the spinal cord is required for central sensitization, the central facilitation of pain transmission produced by peripheral injury.