Effect of styrene and other alkyl benzene derivatives on oxidation of FAD- and NAD-linked substrates in rat liver mitochondria

Chemically induced pheochromocytomas in rats: mechanisms and relevance for human risk assessment

Pheochromocytomas are tumors originating from chromaffin cells of the adrenal medulla, which have been observed in numerous carcinogenicity studies. The authors have evaluated pheochromocytoma concurrence with other effects and the possible mechanisms, in order to assess the relevance of such data for the classification of carcinogenic effects and their relevance to humans. The evaluation revealed that pheochromocytomas occur with relatively higher frequency in male rats, especially when the following conditions are involved: hypoxia, uncoupling of oxidative phosphorylation, disturbance in calcium homeostasis, and disturbance of the hypothalamic endocrine axis. The underlying biochemical mechanisms suggest that other substances that interfere with these biochemical endpoints also produce pheochromocytomas. Such endpoints include enzymes involved in catecholamine synthesis, receptor tyrosine kinase (RET), hypoxia-inducible factor (HIF), succinate dehydrogenase, fumarate hydratase, and pyruvate dehydrogenase. To date, there is no indication that the substances inducing pheochromocytomas in animal experiments also induce corresponding tumors in humans. Because the mechanisms of action identified in rats are to be expected in humans, pheochromocytomas may be induced after exposure conditions similar to those used in the animal studies. Whether hereditary mutations represent a risk factor in humans is not clear. Pheochromocytomas that occur in animal experiments currently appear to have little relevance for conditions at the work place. When sufficiently documented and evaluated, such secondary pheochromocytomas are not relevant for classification and human risk assessment.

NMDA sensitization and stimulation by peroxynitrite, nitric oxide, and organic solvents as the mechanism of chemical sensitivity in multiple chemical sensitivity

Multiple chemical sensitivity (MCS) is a condition where previous exposure to hydrophobic organic solvents or pesticides appears to render people hypersensitive to a wide range of chemicals, including organic solvents. The hypersensitivity is often exquisite, with MCS individuals showing sensitivity that appears to be at least two orders of magnitude greater than that of normal individuals. This paper presents a plausible set of interacting mechanisms to explain such heightened sensitivity. It is based on two earlier theories of MCS: the elevated nitric oxide/peroxynitrite theory and the neural sensitization theory. It is also based on evidence implicating excessive NMDA activity in MCS. Four sensitization mechanisms are proposed to act synergistically, each based on known physiological mechanisms: Nitric oxide-mediated stimulation of neurotransmitter (glutamate) release; peroxynitrite-mediated ATP depletion and consequent hypersensitivity of NMDA receptors; peroxynitrite-mediated increased permeability of the blood-brain barrier, producing increased accessibility of organic chemicals to the central nervous system; and nitric oxide inhibition of cytochrome P450 metabolism. Evidence for each of these mechanisms, which may also be involved in Parkinson's disease, is reviewed. These interacting mechanisms provide explanations for diverse aspects of MCS and a framework for hypothesis-driven MCS research.

Acute interactive pharmacologic effects of inhaled toluene and dichloromethane on rat brain electrophysiology

Toluene (TOL) and dichloromethane (DCM) are widely used industrial solvents and are common components of solvent mixtures that are voluntarily inhaled to produce altered states of consciousness. In previous studies we characterized some of the acute electrophysiologic effects of these solvents. Opposite effects were noted for some measures, suggesting that they might be antagonistic when combined. In this study we examined the solvents again singly (10,700 and 16,000 ppm) and also in combination (16,000 ppm: 33/67 and 67/33% TOL/DCM ratios). The single gases caused effects similar to those observed previously. Combined effects varied, dependent upon the particular variable examined and the major gas in the mixture. In some respects the solvents were concordant, exerting similar effects on a variable, e.g., both solvents prolonged the latencies of components of the brainstem auditory-evoked response. In other respects they were discordant, e.g., whereas toluene caused mean EEG frequency to increase, dichloromethane had the opposite effect. Sometimes the solvents had similar effects alone, but acted independently in combination. Nonindependent interactions were also observed-both additive/subtractive or positively or negatively synergistic. The results further demonstrate and emphasize the unique patterns of acute central nervous system effects that can be effected by solvents that might have a common behavioral endpoint such as anesthesia, and the results characterize a variety of electrophysiologic interactions between these two solvents. Although there were several variables exhibiting synergistic relationships, independent or additive interactions were the most common.