HEXACHLOROPHENE-INDUCED LESIONS IN THE DEVELOPING PERIPHERAL NERVOUS SYSTEM OF MICE

Chemically Induced Myelinopathies

The diverse, structurally unrelated chemicals that cause toxic myelinopathies have been investigated and can be categorized into two types of primary demyelinators. Some demyelinating chemicals seem to leave intact the myeli-nating cells (oligodendrocytes in the central nervous system and Schwann cells in the peripheral nervous system), while others damage the myelinating cells as well as the myelin. The significance between the two is that with the myelinating cells still in tact, repair of the myelin sheath can occur. However, if the myelinating cells are destroyed, repair and reversal of the neuropathy may not occur. Histologically, these chemicals produce an edema of the white matter of the brain, and in some cases the peripheral nervous system, that appears spongy by light microscopy. By electron microscopy, vacuoles can be seen in the myelin surrounding axons. These vacuoles are characterized as fluid-filled separations (splitting) of myelin lamellae at the intraperiod line. In some cases these vacuoles can degenerate further to full demyelination, affecting conduction through those axons. Regeneration of the myelin layers can occur, and in some cases occurs at the same time other axons are undergoing toxic demyelination. Several of these chemicals, however, have been shown to increase cerebrospinal fluid pressure in the brain, optic nerve, and spinal cord, and/or intraneuronal pressure in the perineurium surrounding the axons in the peripheral nervous system. This increased pressure has been correlated with decreased conduction capacity through the axon, ischemia to the neuronal tissue from decreased blood flow because of pressure against the blood vessels, and, if unrelieved, permanent axonal damage. Several of these chemicals havebeen shown to inhibit oxidative phosphorylation, while others uncouple oxidative phosphorylation. One chemical appears to inhibit an enzyme critical to cholesterol synthesis, thus destabilizing myelin. Another hypothesis for a mechanism of action may be in the ability of these compounds to alter membrane permeability.

Ultrastructure of 6-aminonicotinamide (6-AN)-induced lesions in the central nervous system of rats. II. Alterations of the nervous susceptibility with aging

Lesions in the CNS induced by 6-aminonicotinamide (6-AN) presented a spongy state of the gray matter and neuronal chromatolysis. With aging of the experimental animals the lesions extended from the phylogenetically early developed structures to those developed later, i.e., from spinal gray matter, dentate nuclei, and brain stem nuclei through limbic structures and striatum to the cerebral cortex. Changes of the neurons were more prominent with aging. Lesions in the CNS of rats at the age, corresponding to the involutional period in the human, were similar to those of Creutzfeldt-Jakob disease (C-J) disease) in the presenile age. In recent years, the resemblance between C-J disease and pellagra encephalopathy had been noted by several authors, and they resemble the lesions caused by 6-AN, an antimetabolite of nicotinamide used in our experiment. This evidence, therefore, has led to the hypothesis that dysfunction of NAD(H)- or NADP(H)-dependent enzymes in the CNS of the aged, even if not the primary cause, may be one possible pathogenetic factor of C-J disease.

Studies on hexachlorophene-induced myelin lesions in the trigeminal root transitional region in developing and adult mice

Hexachlorophene (HCP)-induced intramyelinic vacuolation was studied in the transitional region of the trigeminal root of suckling and adult mice. HCP produced an extensive vacuolation in the central compartment of the region in both suckling and adult mice, while in the peripheral compartment myelin lesions were only seen in mice less than 16 days of age. Gas chromatographic measurements showed that in suckling mice the blood concentration of HCP decreased with age, apparently reflecting a faster elimination of HCP from the blood. By substantially increasing the HCP dose, higher blood concentrations were obtained in adult than less the 16-day-old mice; in spite of this, PNS myelin changes occurred only in the latter. Thus, by observing the HCP effect on the transitional region, where CNS and PNS directly meet, it is concluded that CNS of both suckling and adult mice is more severely effected by HCP than PNS, and that the reaction of the PNS myelin is age-dependent during the period of myelinogenesis; it is particularly vulnerable to a cytotoxic edema inducing substance.