Preliminary morphological and histochemical changes in rat spinal cord neurons following arsenic ingestion

Aqueous sodium arsenite was administered in doses of 1.0, 1.5 and 2.0 mg/kg body weight (bw)/day by intragastric route to adult Wistar rats, (groups II, III and IV, respectively, n=6 animals/group) for a period of 12 weeks. Controls (group I) received distilled water without added arsenite. At the end of experimental period, four animals from each group were perfusion fixed with 4% paraformaldehyde and two animals from each group were used for fresh tissue purposes (for SDH localization). Fixed tissue blocks from the cervical and lumbar spinal cords were processed for paraffin and cryostat sectioning. Measurements of nuclear area were carried out in cresyl violet (CV) stained paraffin sections by Image Analysis. Cryostat sections (20 microm) of the cervical and lumbar spinal cords of fixed and fresh frozen tissue blocks (n=2/group) were stained for localization of cytochrome oxidase (CO) and succinic dehydrogenase (SDH) activities, respectively. Quantitative estimations of enzyme activities was determined from mean grey values obtained by Image Analysis. The results revealed a dose related reduction in the mean log nuclear area and in the two enzyme activities. The differences noted between the cervical and lumbar motor neurons, viz. (i) the decrease in mean log nuclear area were observed only in cervical motor neurons of animals receiving 2.0 mg/kg bw whereas for the lumbar motor neurons this was true of all doses of arsenite; (ii) in general, CO expression was higher in the cervical than in lumbar spinal cord and (iii) reduction in SDH activity was more pronounced in the cervical spinal cord with doses of 1.5 and 2.0 mg/kg bw as compared to lumbar spinal cord. These observations may throw some light on the earlier observations [Jenkins RB. Inorganic arsenic and the nervous system. Brain 1996;89:479-98] reporting differences in the affliction to arsenic ingestion between the fore limb and hind limb musculature (in human subjects). Our observations ascertain that the neurons, which innervate these musculature, also differ in some characteristics.