Observations on 2'-chloro-2,4-dinitro-5',6-di(trifluoromethyl)-diphenylamine-induced edema in the white matter of the central nervous system of the rat

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.

Evaluation of the neurotoxic potential of chemicals in animals

This review provides a scientific view on how to evaluate effectively the neurotoxic potential of chemicals in order to provide adequate safeguards for human health. Detection of compounds that may cause direct, persistent, adverse effects on the nervous system should be given the most critical attention. Evaluation of the neurotoxic potential of a chemical should include descriptions of functional and morphological effects as well as the determination of the dose response, no-observed-effect level, time course and reversibility of effects. Evaluation of the nervous system in the context of standard toxicity studies that use a variety of species and study durations are appropriate screening tests (Tier 1) for the detection of potential neurotoxicity. Studies specifically designed to assess neurotoxicity (Tier 2) should be performed with chemicals for which there is an indication of neurotoxic potential and where the available data are inadequate for risk assessment. Tier 2 studies should evaluate the function and structure of the nervous system by comprehensive clinical examinations and neuropathological assessment. These studies may be conducted in conjunction with standard toxicity studies so that any potential neurotoxicity can be interpreted in the context of other systemic toxicity. More specific neurotoxicity tests (Tier 3) may be necessary for advanced characterization of discovered neurotoxicants.

The relationship between uncoupling of oxidative phosphorylation and neuronal necrosis within the CNS in rats dosed with trihalogenated imidazoles

The trihalogenated imidazoles, trichloroimidazole (TCI), tribromoimidazole (TBI), and triiodoimidazole (TII), are in vitro uncouplers of oxidative phosphorylation with similar activities. Although TCI and TBI are also uncouplers in vivo, some doubt exists for TII, which is much less toxic and produces atypical signs of poisoning. Dibromo- and monobromoimidazole do not uncouple oxidative phosphorylation either in vitro or in vivo. Dosing of TCI and TBI to rats resulted within 24-48 hr in neuronal necrosis within the CNS involving the vestibular nucleus, red nucleus, and outer parietal neocortex and ataxia of the hindlimbs. However, no neuronal necrosis or ataxia was observed after dosing of TII to rats, even when given at doses four times greater than for either TCI or TBI, resulting in much higher brain concentrations. Although TBI was equitoxic to rats, mice, hamsters, and gerbils, CNS damage and ataxia were observed only in the rat, even though comparable brain concentrations of TBI were found in the gerbil. Measurement of the concentration of TBI in the dissected rat brain gave no indication of localized concentrations of compound in the areas associated with neuronal damage. Doses of TBI and the classical uncoupler 3,5-dinitro-o-cresol (DNOC), matched for whole body O2 consumption, caused comparable changes in rat brain blood flow although DNOC does not cause brain damage. Changes in blood flow were not restricted to those brain areas susceptible to damage. Thus, although we were unable to completely dissociate CNS damage from uncoupling of oxidative phosphorylation produced by TBI and TCI in the rat, it is unlikely that such damage is primarily related to the uncoupling ability of these compounds.

The toxicity and neuropathology of 2,4,5-tribromoimidazole and its derivatives in rats

2,4,5-tribromoimidazole and its 1-n-butylcarboxylate and 1-dimethylcarbamoyl derivatives, when administered to rats, induced poisoning typical of uncouplers of oxidative phosphorylation. At 48 h rats surviving a single toxic dose of 20-60 mg/kg developed permanent incoordination of the hindlimbs in the absence of brain oedema. Neuropathologic examination of brain and spinal cord from perfused fixed rats at 24 h revealed neuronal necrosis and chromatolysis in the vestibular nucleus, the outer parietal neocortex and red nucleus. Chromatolysis and necrosis in these areas had increased at 72-96 h and were also observed in the deeper layers of the neocortex, the medial entorhinal cortex, the reticular formation, the grey matter of the spinal cord extending into the ventral horns, the dorsal, and ventral cochlear nuclei and the deep cerebellar nuclei, in decreasing order of severity. Neuronal necrosis was accompanied by an increased glial response, including neuronophagia and at 16 days with astroglial hypertrophy and hyperplasia.