Multicomponent T2 analysis of dithiocarbamate-mediated peripheral nerve demyelination.

Standard light microscope histological evaluation of peripheral nerve lesions has been used routinely to assess peripheral nerve demyelination; however, the development of magnetic resonance (MR) methodology for assessing peripheral nerve may provide complementary information, with less expense and in less time than nerve histology methods. In this study, the utility of multicomponent NMR T(2) relaxation analysis for assessing myelin injury in toxicology studies was examined using two dithiocarbamates, N,N-diethyldithiocarbamate (DEDC) and pyrrolidine dithiocarbamate (PDTC), known to produce myelin injury and elevate copper in the nervous system. T(2) analysis was used in conjunction with standard histological methods to assess myelin injury and determine if dithiocarbamate-mediated copper accumulation in peripheral nerve was associated with more severe myelin lesions. Male Sprague-Dawley rats were administered i.p. DEDC for 8 weeks and maintained on either a diet containing normal (13 ppm) or elevated (200 ppm) copper. Another group of male Sprague-Dawley rats was administered oral PDTC and a 200 ppm copper diet, with controls given only the 200 ppm copper diet, for 47 weeks. Following exposures, the morphology of sciatic nerve was evaluated using light microscopy and multicomponent T(2) analysis of excised fixed nerves; and copper levels in sciatic nerve were determined using ICP-AES. Light microscopy demonstrated the presence of a primary myelinopathy in dithiocarbamate-exposed rats characterized by intramyelinic edema, demyelination, and secondary axonal degeneration. Both the nerve copper level and number of degenerated axons, as ascertained by ICP-AES and microscopy, respectively, were augmented by dietary copper supplementation in conjunction with administration of DEDC or PDTC. T(2) analysis revealed a decreased contribution from the shortest T(2) component in multicomponent T(2) spectra obtained from animals administered DEDC or PDTC, consistent with decreased myelin content; and the decrease of the myelin water component was inversely correlated to the levels of nerve copper and myelin lesion counts. Also, the T(2) analysis showed reduced variability compared to histological assessment. These studies support multicomponent T(2) analysis as a complementary method to light microscopic evaluations that may also be applicable to in vivo assessments.

Dietary copper enhances the peripheral myelinopathy produced by oral pyrrolidine dithiocarbamate.

The neurotoxic hazard of a dithiocarbamate is influenced by route of exposure and acid stability of the dithiocarbamate. As an example, oral administration of the acid labile dithiocarbamate N,N-diethyldithiocarbamate (DEDC) causes a central-peripheral axonopathy thought to result from acid-promoted decomposition to CS2 in the stomach. In contrast, parenteral administration of DEDC, which bypasses the acidic environment of the stomach, causes a primary demyelination that is thought to be mediated through the intact parent dithiocarbamate. The relative acid stability of pyrrolidine dithiocarbamate (PDTC) suggests that a significant portion of a dose can be absorbed intact following oral exposure with the potential to produce a primary myelin injury. The present study was performed to characterize the neurotoxicity of PDTC and evaluate the possible role of copper in dithiocarbamate-mediated demyelination. Male Sprague Dawley rats were administered PDTC in drinking water and given either a normal- or high-copper diet for 18, 47, or 58 weeks. Examination of peripheral nerve by light microscopy and electron microscopy at the end of exposures revealed primary myelin lesions and axonal degeneration in the PDTC groups, with a significant increase in the severity of several lesions observed for the PDTC, high-copper group relative to the PDTC normal-copper diet. ICP-AES metal analysis determined that the PDTC groups had significantly increased brain copper, and at 58 weeks a significant increase in copper was seen in the sciatic nerve of PDTC high-copper animals relative to PDTC normal-copper diet animals. Although RP-HPLC analysis could not detect globin alkylaminocarbonyl cysteine modifications analogous to those seen with parenteral DEDC, LC/MS/MS identified (pyrrolidin-1-yl carbonyl)cysteine adducts on PDTC-exposed rat globin. These findings are consistent with previous studies supporting the ability of acid-stable dithiocarbamates to mediate myelin injury following oral exposure. The greater severity of lesions associated with dietary copper supplementation and elevated copper levels in nerve also suggests that perturbation of copper homeostasis may contribute to the development of myelin lesions.

Brainstem axonal degeneration in mice with deletion of selenoprotein p.

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.