N-acetylaspartate is an axon-specific marker of mature white matter in vivo: a biochemical and immunohistochemical study on the rat optic nerve.

Axonal pathology is a major cause of neurological disability in multiple sclerosis. Axonal transection begins at disease onset but remains clinically silent because of compensatory brain mechanisms. Noninvasive surrogate markers for axonal injury are therefore essential to monitor cumulative disease burden in vivo. The neuronal compound N-acetylaspartate, as measured by magnetic resonance spectroscopy, is currently the best and most specific noninvasive marker of axonal pathology in multiple sclerosis. The possibility has been raised, however, that N-acetylaspartate is expressed also by oligodendroglial lineage cells. In order to investigate N-acetylaspartate specificity for white matter axons, transected rat optic nerves were analyzed by high-performance liquid chromatography and immunohistochemistry. In transected adult nerves, N-acetylaspartate and N-acetyl aspartylglutamate decreased in concordance with axonal degeneration and were undetectable 24 days posttransection. Nonproliferating oligodendrocyte progenitor cells, oligodendrocytes, and myelin were abundant in these axon-free nerves. At 24 days posttransection, N-acetylaspartate was increased (42%; p = 0.02) in nontransected contralateral nerves. After transection at postnatal day 4, total N-acetylaspartate decreased by 80% (P14; p = 0.002) and 94% (P20; p = 0.003). In these developing axon-free nerves, 25 to 33% of oligodendrocyte progenitor cells were proliferating. These data validate magnetic resonance spectroscopy measurements of N-acetylaspartate as an axon-specific monitor of central nervous system white matter in vivo. In addition, the results indicate that neuronal adaptation can increase N-acetylaspartate levels, and that 5 to 20% of the N-acetylaspartate in developing white matter is synthesized by proliferating oligodendrocyte progenitor cells.     

Evidence of melanogenesis in porcine retinal pigment epithelial cells in vitro

Several weeks after porcine retinal pigment epithelial (RPE) cell cultures attain confluence, macroscopically visible brown foci appear. The cuboidal cells that form the foci contain numerous phase dark granules that do not exhibit the autofluorescence characteristic of lipofuscin. The data described here indicate that the granules are melanosomes. Electron microscopy revealed three types of electron-dense granules in these cells: simple spheres 0.3-0.5 microns in diameter, large spheres 1-2 microns in diameter, and lysosomal aggregations of the smaller spheres. The matrix of both spheres is composed of 40-nm microvesicles that were also found free in the cytoplasm and aggregated within vacuolar structures. Reversed-phase high-performance liquid chromatography of RPE cells and their media detected melanogens, i.e. intermediates of melanin biosynthesis, including several indole derivatives. The porcine RPE cultures therefore may be a useful system for studying melanogenic regulation.     

Pharmacokinetics, hepatic biotransformation and biliary and urinary excretion of bromosulfophthalein (BSP) in an experimental liver disease mimicking biliary cirrhosis

We studied the hepatic handling of bromosulfophthalein in healthy rabbits with hepatic coccidiosis 28 days after an experimental infection with sporulated oocysts of Eimeria stiedai, an experimental model of liver disease histopathologically resembling primary biliary cirrhosis in man. A pharmacokinetic study of the results was performed following a multicompartmental model with 7 transfer constants to describe the physiological disposition of the dye. The study showed that the plasma disappearance, distribution volume (Vi), hepatic biotransformation and the biliary and urinary elimination of conjugated (BSPc) and unconjugated (BSPu) bromosulfophthalein were markedly altered. Whereas Vi and urinary excretion of the dye were significantly increased, the hepatic clearance, biotransformation and biliary excretion of BSPc and BSPu were drastically reduced in infected rabbits. Satisfactory agreement was obtained between the experimental and estimated data, particularly those relating to biotransformation clearance and biliary and urinary excretion of the dye. These results demonstrate that severe liver disease in rabbits with histopathological liver alterations resembling several hepatic dysfunctions in man markedly reduce hepatic uptake, metabolism and biliary excretion of a xenobiotic such as BSP.