Dopamine Melanin Research Articles

Purified human neuromelanin, synthetic dopamine melanin as a potential model pigment, and the normal human substantia nigra: characterization by electron paramagnetic resonance spectroscopy.

Neuromelanin is a poorly understood pigment that accumulates in catecholaminergic neurons during normal aging. Electron paramagnetic resonance spectroscopy, an especially effective technique for investigating melanins, is used in the present study to show unambiguously that neuromelanin is a melanin; however, it is not well modeled by synthetic dopamine melanin and thus is an atypical melanin. Some of the unusual features of neuromelanin can be explained by postulating two distinct sources for its free radicals, the dominant one possibly derived from a precursor containing sulfur. Examination of human substantia nigra by electron paramagnetic resonance spectroscopy during the purification of neuromelanin also demonstrates, contrary to some other studies, that a portion of the paramagnetic metal ions in this tissue are bound to the pigment in situ. Combined with previous histochemical data, these observations have implications for the mechanism through which neuromelanin accumulates in vivo and are consistent with its having a cytoprotective function under normal conditions, but a cytotoxic role at advanced ages and in patients with Parkinson's disease. Other results of this study show that homogenizing tissues during the purification of any natural pigment may cause contamination of the pigment by extraneous metal ions and that subsequent incubation in hot acid, though most effective in removing metal ions and hydrolyzing proteins, leads to degradation of melanin. A purification procedure using incubation in acid at room temperature, however, is well suited for identifying and characterizing unknown natural pigments by electron paramagnetic resonance spectroscopy.

Iron, melanin and dopamine interaction: relevance to parkinson's disease

1. Interaction between iron and melanin may provide a reasonable explanation for the vulnerability of the melanin containing dopaminergic neurons in the substantia nigra (SN) to neurodegeneration in Parkinson's disease (PD). 2. Scatchard analysis of the binding of iron to synthetic dopamine melanin revealed a high-affinity (K D = 13 nM) and a lower affinity (K D = 200 nM) binding sites. 3. The binding of iron to melanin is dependent on the concentration of melanin and on pH. 4. Iron chelators, U74500A, desferrioxamine and to a lesser extent 1,10-phenanthroline and chlorpromazine could displace iron from melanin. In contrast, 1-methyl-4-phenyl-1,2,3,6-tetrahdropyridine (MPTP) and its metabolite 1-methyl-4-phenyl-pyridinium (MPP+), which cause Parkinsonism, were unable to displace iron. 5. Melanin alone reduced lipid peroxidation in rat cortical membrane preparations. However, iron induced lipid peroxidation, which could he inhibited by desferrioxamine, was potentiated by melanin. 6. Iron bound to neuromelanin in melanized dopamine neurons was detected only in parkinsonian brains and not in controls. The interaction of iron with neuromelanin as identified by x-ray defraction technique was identical to iron interaction with synthetic dopamine melanin. 7. In the absence of an identified exogenous or endogenous neurotoxin in idiopathic Parkinson's disease, iron-melanin interaction in the SN may serve as a candidate for the oxygen-radical induced neurodegeneration of the melanin containing dopaminergic neurons.

Iron‐Melanin Interaction and Lipid Peroxidation: Implications for Parkinson's Disease

The vulnerability of substantia nigral (SN) melaninized dopamine neurons to neurodegeneration in Parkinson's disease and the selective increases of iron and basal lipid peroxidation in SN indicate that iron-melanin interaction could be crucial to the pathogenesis of this disease. The present study describes, for the first time, the identification and characterization of a high-affinity (KD = 13 nM) and a lower affinity (KD = 200 nM) binding site for iron on dopamine melanin. The binding of iron to melanin is dependent on pH and the concentration of melanin. Iron chelators, U74500A, desferrioxamine, and to less extent 1,10-phenanthroline and chlorpromazine, but not the Parkinson-inducing neurotoxin, 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine, can inhibit the binding of iron to melanin and iron-induced lipid peroxidation. Although melanin alone diminishes basal lipid peroxidation in rat cortical homogenates, it can also potentiate that initiated by iron, a reaction inhibited by desferrioxamine. In the absence of an identifiable exogenous or endogenous neurotoxin in idiopathic Parkinson's disease, iron-melanin interaction in pars compacta of SN may be a strong candidate for the cytotoxic component of oxygen radical-induced neurodegeneration of melaninized dopamine neurons.

Autoradiography of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP): uptake in the monoaminergic pathways and in melanin containing tissues.

A recently discovered neurotoxic compound, 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine, has been found to cause a parkinsonian-like syndrome in man and monkey, but not in laboratory animals such as rat, mouse and guinea pig. MPTP seems to selectively destroy the melanin containing dopaminergic cells in pars compacta of substantia nigra. Lower mammalian species do not have melanin in these cells, which indicates that the presence of neuromelanin may be of importance for the development of MPTP-induced lesions. By means of whole-body autoradiography of 3H-MPTP in mice, accumulation and retention was observed in the dopaminergic pathways, in locus caeruleus and in structures in the medulla oblongata and spinal cord. A high uptake was also seen in melanin-containing tissues such as in the eyes of pigmented mice. MPTP has earlier been found to have high affinity in vitro for dopamine melanin, which is similar to the pigment in substantia nigra. The typical features of the MPTP-induced neurotoxicity with destruction of pigmented nerve cells and development of parkinsonism may be to to accumulation and retention of MPTP and its metabolites in these cells. In species with pigmented nerve cells, such as man and monkey, the accumulation may be much more pronounced because of the melanin affinity of MPTP and its metabolites.

Melanin affinity of manganese.

The melanin affinity of manganese was studied in vitro using melanin isolated from beef eyes or human hair, and synthetic dopamine melanin, which is known to be structurally similar to the melanin present in the pigmented nerve cells in the human substantia nigra. In addition, the uptake of manganese in melanin containing tissues in vitro and in vivo was studied by whole body autoradiography. Manganese was bound to beef eye, human hair and synthetic dopamine melanin, and was taken up in pigmented tissues in mice and a monkey. Long-time exposure to manganese may cause a chronic extrapyramidal disorder. It is suggested that manganese, due to its neurotoxicity, may cause lesions in pigment containing neurones in the substantia nigra secondary to its accumulation on the neuromelanin.

Pathogenesis of Parkinson's Disease

Nucleolar volume and melanin pigment were measured in nerve cells of the substantia nigra in cases of idiopathic Parkinson's disease and in other cases of parkinsonism associated with cerebrovascular disease. In cerebrovascular parkinsonism, nucleolar volume is reduced by 16%, whereas melanin content is unchanged, reflecting local deficiencies in circulation on function of nerve cells, otherwise normal for age. In idiopathic parkinsonism, remaining cells are atrophied, and nucleolar volume is reduced by 16%. Melanin content is decreased by 22% because of preferential loss of the highly pigmented cells. The pathogenesis of idiopathic parkinsonism may lie with a secondary aggravation of changes that occurs, as part of the "normal process of aging," in relation to the cytotoxic effects of dopamine metabolism and melanin accumulation.

Effects of dopamine agonists and antagonists on murine melanoma:Correlation with dopamine binding activity

The biosynthetic pathway melanin is present in many melanomas. Previous investigations have suggested that pharmacologic levels of intermediates in this pathway (L-dopa, dopamine, and their analogues) may inhibit macromolecular synthesis in some tissue culture melanoma cell lines and prolong survival in tumor-bearing mice. Recently, a potent antidopamine drug (pimozide) has been developed. This study was designed to investigate the effects of these drugs on murine melanomas and to correlate effects on macromolecular synthesis with competitive dopamine binding activity (receptors) and melanin synthesis. Three murine melanomas (F1, F10, B16) were studied. The amelanotic B16 cell line showed no inhibition by dopa, dopamine, or pimozide when assayed for 14C-leu or 3H-TdR incorporation. Using a competitive binding assay, only low levels of dopamine binding were present. The very melanotic F1 cell line showed no inhibition by dopa or dopamine, but pimozide inhibited 14C-leu and 3H-TdR incorporation in a dose-response fashion; 50% inhibition was noted at 10(-9) M concentration with no loss in cell viability as tested by trypan blue exclusion or cell counting. Competitive dopamine binding was present (19 pmoles per g of wet tissue) with a Kd of 0.2 nM, figures approximating those seen in normal dog caudate nucleus controls. The F10 line, with melanin production between the B16 and F1, was intermediate in terms of inhibition of 14C-leu and 3H-TdR incorporation and dopamine binding. Purified melanin did not bind and, thus, does not appear to explain these binding results. These data suggest that pimozide, a drug which is a potent dopamine antagonist, may inhibit 14C-leu and 3H-TdR incorporation in murine melanoma cells and that competitive dopamine binding (receptor) appears to correlate with this inhibition. This drug and the dopamine binding assay may be useful in the study of human melanoma.