Quantitative MRI measures of multiexponential T 2 relaxation and magnetization transfer were acquired from six samples of excised and fixed rat spinal cord and compared with quantitative histology. MRI and histology data were analyzed from six white matter tracts, each of which possessed unique microanatomic characteristics (axon diameter and myelin thickness, in particular) but a relatively constant volume fraction of myelin. The results indicated that multiexponential T 2 relaxation characteristics varied substantially with variation of microanatomy, while the magnetization transfer characteristics remained close to constant. The most-often-cited multiexponential T 2 relaxation metric, myelin water fraction, varied by almost a factor of 2 between two regions with myelin volume fractions that differed by only % 12%. Based on the quantitative histology, the proposed explanation for this variation was intercompartmental water exchange, which caused the underestimation of myelin water fraction and T 2 values and is, presumably, a greater factor in white matter regions where axons are small and myelin is thin. In contrast to the multiexponential T 2 relaxation observations, magnetization transfer metrics were relatively constant across white matter tracts and concluded to be relatively insensitive to intercompartmental water exchange. Magn Reson Med 63:902-909, 2010. V C 2010 Wiley-Liss, Inc.Key words: MRI; multiexponential T 2 ; magnetization transfer; histology; myelin Multiexponential T 2 (MET 2 ) and quantitative magnetization transfer (qMT) have been proposed as potential MRI-derived reporters of myelin content in white matter (WM) and nerve. With MET 2 , the relative size of shortlived T 2 component (typically, T 2 ¼ 8-50 ms, depending on the amplitude of static field) has been defined as the myelin water fraction (MWF) and interpreted as a direct measure of myelin content (1,2). Similarly, in WM, the macromolecular protons that exchange magnetization with water protons are thought to be substantially constituents of myelin (3,4). That is, the qMT measure of this macromolecular pool size relative to the total water signal, sometimes called the pool-size-ratio (PSR) (5), is believed to be a measure of relative myelin content. While both MWF and PSR have been found to correlate with myelin content when comparing normal myelinated tissue with demyelinated or dysmyelinated tissue (6-11 and many others), the exact relationship between these measures and myelin content is not well understood.The interpretation of MWF as a measure of myelin content is predicated on the assumption of slow exchange between myelin water and water in the intraand interaxonal spaces, referred to herein as other water. If the rate of exchange between myelin water and other water is dependent upon the dimensions of these compartments (i.e., axon diameter [AxD], myelin thickness [MyTh], interaxonal distance), then the slow exchange model may be more suitable in some myelinated tissues than others. For example, peripheral nerve tends to have...

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Previous studies have demonstrated the ability of the dithiocarbamate, disulfiram, to produce a peripheral neuropathy in humans and experimental animals and have also provided evidence that N,N-diethyldithiocarbamate (DEDC) is a proximate toxic species of disulfiram. The ability of DEDC to elevate copper levels in the brain suggests that it may also elevate levels of copper in peripheral nerve, possibly leading to oxidative stress and lipid peroxidation from redox cycling of copper. The study presented here investigates the potential of DEDC to promote copper accumulation and lipid peroxidation in peripheral nerve. Rats were administered either DEDC or deionized water by ip osmotic pumps and fed a normal diet or diet containing elevated copper, and the levels of metals, isoprostanes, and the severity of lesions in peripheral nerve and brain were assessed by ICP-AES/AAS, GC/MS, and light microscopy, respectively. Copper was the only metal that demonstrated any significant compound-related elevations relative to controls, and total copper was increased in both brain and peripheral nerve in animals administered DEDC on both diets. In contrast, lesions and elevated F2-isoprostanes were significantly increased only in peripheral nerve for the rats administered DEDC on both diets. Autometallography staining of peripheral nerve was consistent with increased metal content along the myelin sheath, but in brain, focal densities were observed, and a periportal distribution occurred in liver. These data are consistent with the peripheral nervous system being more sensitive to DEDC-mediated demyelination and demonstrate the ability of DEDC to elevate copper levels in peripheral nerve. Additionally lipid peroxidation appears to either be a contributing event in the development of demyelination, possibly through an increase of redox active copper, or a consequence of the myelin injury.

Selenoprotein P is an abundant extracellular protein that is expressed in liver, brain, and other tissues. Studies in mice with the selenoprotein P gene deleted (Sepp-/- mice) have implicated the protein in maintaining brain selenium. Sepp-/- mice fed a normal or low selenium diet develop severe motor impairment and die, but Sepp-/- mice fed a high selenium diet remain clinically unimpaired. As an initial step to evaluate the effect of selenoprotein P deletion on central nervous system architecture, the brains and cervical spinal cords of Sepp-/- and Sepp+/+ mice fed low or high selenium diets were examined by light and electron microscopy. Brains of Sepp-/- mice demonstrated no gross abnormalities. At the light microscopic level, however, Sepp-/- mice fed either the selenium deficient diet or the high selenium diet had enlarged dystrophic axons and degenerated axons in their brainstems and cervical spinal cords. No axonal lesions were observed in the Sepp+/+ mice fed either diet. Electron microscopy demonstrated that the enlarged axons in the Sepp-/- mice were packed with organelles, suggesting a deficit in fast axonal transport. The similar severity of axonal lesions observed in Sepp-/- mice fed the 2 diets suggests that axonal dystrophy is a common phenotype for deletion of selenoprotein P regardless of selenium intake and that additional studies will be required to determine the pathogenesis of the neurological signs and mortality observed in Sepp-/- mice fed a low selenium diet.

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Thiocarbamates are a major class of herbicides used extensively in the agricultural industry. It has been shown that thiocarbamates can form reactive sulfoxide and sulfone intermediates, which may be involved in the toxicity of thiocarbamates through covalent modification of cysteine and serine active sites of enzymes. Molinate has been shown to generate an S-hexahydro-1H-azepine-1-carbonyl adduct on the Cys-125 residue of the beta2- and beta3-chains of rat globin analogous to that reported for disulfiram and to inhibit aldehyde dehydrogenase and nonspecific esterase activity. The present study examined whether other thiocarbamate herbicides produce similar covalent protein modifications and enzyme inhibition to that reported for molinate and whether S-(N,N-dialkylaminocarbonyl)cysteine adduct levels are correlated to enzyme inhibition or the structure of thiocarbamate herbicides. Additionally, the potential of molinate to act as a peripheral demyelinating agent similar to disulfiram was evaluated. To address these aims, rats were exposed ip to molinate, vernolate, ethiolate, EPTC, or butylate for 5 days after which hemogloblin was isolated and analyzed for protein adducts using HPLC and matrix-assisted laser desorption ionization time-of-flight mass spectrometry. In addition, brain, liver, and testes mitochondrial and microsomal fractions were assayed for nonspecific esterase, low Km ALDH, or total ALDH activities, and S-(N,N-dialkylaminocarbonyl)cysteine adducts were measured by LC/MS/MS. For the neurotoxicity assessments, rats were administered molinate parenterally for subchronic periods and morphological evaluations performed on peripheral nerves. All of the thiocarbamates except butylate produced S-(N,N-dialkylaminocarbonyl)cysteine adducts on globin and the quantity of adducts detected decreased with increasing size of the nitrogen substituents. In contrast, a clear relationship between cysteine modification in mitochondrial and microsomal samples to nitrogen substituents was not evident, and although molinate produced relatively high levels of adducts and esterase inhibition and butylate low levels of adducts and esterase inhibition for most samples, in general, the level of S-(N,N-dialkylaminocarbonyl)cysteine adducts did not appear to be related to enzyme inhibition. Molinate did not produce segmental demyelination in peripheral nerve, suggesting that molinate and possibly other thiocarbamates do not share the neurotoxic potential of dithiocarbamates.

Dithiocarbamates have a wide spectrum of applications in industry, agriculture and medicine with new applications being actively investigated. One adverse effect of dithiocarbamates is the neurotoxicity observed in humans and experimental animals. Results from previous studies have suggested that dithiocarbamates elevate copper and promote lipid oxidation within myelin membranes. In the current study, copper levels, lipid oxidation, protein oxidative damage and markers of inflammation were monitored as a function of N,N-diethyldithiocarbmate (DEDC) exposure duration in an established model for DEDC-mediated myelinopathy in the rat. Intraabdominal administration of DEDC was performed using osmotic pumps for periods of 2, 4, and 8 weeks. Metals in brain, liver and tibial nerve were measured using ICP-MS and lipid oxidation assessed through HPLC measurement of malondialdehyde in tibial nerve, and GC/MS measurement of F 2 isoprostanes in sciatic nerve. Protein oxidative injury of sciatic nerve proteins was evaluated through quantification of 4-hydroxynonenal protein adducts using immunoassay, and inflammation monitored by quantifying levels of IgGs and activated macrophages using immunoassay and immunhistochemistry methods, respectively. Changes in these parameters were then correlated to the onset of structural lesions, determined by light and electron microscopy, to delineate the temporal relationship of copper accumulation and oxidative stress in peripheral nerve to the onset of myelin lesions. The data provide evidence that DEDC mediates lipid oxidation and elevation of total copper in peripheral nerve well before myelin lesions or activated macrophages are evident. This relationship is consistent with copper-mediated oxidative stress contributing to the myelinopathy.

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