Exposure of rabbits to 50 ppm carbonyl sulfide. A biochemical and histomorphological study

Inhalational Exposure to Carbonyl Sulfide Produces Altered Brainstem Auditory and Somatosensory-Evoked Potentials in Fischer 344N Rats

Carbonyl sulfide (COS), a chemical listed by the original Clean Air Act, was tested for neurotoxicity by a National Institute of Environmental Health Sciences/National Toxicology Program and U.S. Environmental Protection Agency collaborative investigation. Previous studies demonstrated that COS produced cortical and brainstem lesions and altered auditory neurophysiological responses to click stimuli. This paper reports the results of expanded neurophysiological examinations that were an integral part of the previously published experiments (Morgan et al., 2004, Toxicol. Appl. Pharmacol. 200, 131-145; Sills et al., 2004, Toxicol. Pathol. 32, 1-10). Fisher 334N rats were exposed to 0, 200, 300, or 400 ppm COS for 6 h/day, 5 days/week for 12 weeks, or to 0, 300, or 400 ppm COS for 2 weeks using whole-body inhalation chambers. After treatment, the animals were studied using neurophysiological tests to examine: peripheral nerve function, somatosensory-evoked potentials (SEPs) (tail/hindlimb and facial cortical regions), brainstem auditory-evoked responses (BAERs), and visual flash-evoked potentials (2-week study). Additionally, the animals exposed for 2 weeks were examined using a functional observational battery (FOB) and response modification audiometry (RMA). Peripheral nerve function was not altered for any exposure scenario. Likewise, amplitudes of SEPs recorded from the cerebellum were not altered by treatment with COS. In contrast, amplitudes and latencies of SEPs recorded from cortical areas were altered after 12-week exposure to 400 ppm COS. The SEP waveforms were changed to a greater extent after forelimb stimulation than tail stimulation in the 2-week study. The most consistent findings were decreased amplitudes of BAER peaks associated with brainstem regions after exposure to 400 ppm COS. Additional BAER peaks were affected after 12 weeks, compared to 2 weeks of treatment, indicating that additional regions of the brainstem were damaged with longer exposures. The changes in BAERs were observed in the absence of altered auditory responsiveness in FOB or RMA. This series of experiments demonstrates that COS produces changes in brainstem auditory and cortical somatosensory neurophysiological responses that correlate with previously described histopathological damage.

Review of the toxicology of carbonyl sulfide, a new grain fumigant

Carbonyl sulfide (COS) is a new grain fumigant which has been developed to replace methyl bromide, being phased out due to its ozone depletion properties, and to supplement phosphine gas which is experiencing increased insect resistance. Treatment of commodities with COS, a highly effective fumigant, results in residues that are near or indistinguishable to natural background levels of this compound. COS is a naturally occurring gas, being the predominant sulfur moiety in the atmosphere, occurs naturally in food and is a normal by-product of mammalian aerobic metabolism. COS has low acute inhalational toxicity but with a steep dose response curve; COS is neither genotoxic nor a developmental toxicant but does reversibly impair male fertility. Prolonged, repeated exposure to COS is likely to present similar neurotoxicity hazards to that of the structurally and toxicologically related compound carbon disulfide. Although the occupational risks presented by COS as a fumigant of bulk grain are significant, these are, as they have been for a considerable time for phosphine and methyl bromide, manageable by good occupational safety practices. Consideration may need to be given to scrubbing of ventilated COS and its breakdown product hydrogen sulfide, at the completion of fumigation to minimise worker and bystander exposure. In terms of classical regulatory toxicology studies, the available database for COS is deficient in many aspects and registration in most jurisdictions will depend on sound scientific argument built upon the totality of the existing scientific data as there are strong arguments supporting the registration of this compound.

Contribution of magnetic resonance microscopy in the 12-week neurotoxicity evaluation of carbonyl sulfide in Fischer 344 rats

In this carbonyl sulfide (COS) study, magnetic resonance microscopy (MRM) and detailed light microscopic evaluation effectively functioned in parallel to assure that the distribution and degree of pathology in the brain was accurately represented. MRM is a powerful imaging modality that allows for excellent identification of neuroanatomical structures coupled with the ability to acquire 200 or more cross-sectional images of the brain, and the ability to display them in multiple planes. F344 rats were exposed to 200-600 ppm COS for up to 12 weeks. Prior to MRM, rats were anesthetized and cardiac perfused with McDowell Trump's fixative containing a gadolinium MR contrast medium. Fixed specimens were scanned at the Duke Center for In Vivo Microscopy on a 9.4 Tesla magnetic resonance system adapted explicitly for microscopic imaging. An advantage of MRM in this study was the ability to identify lesions in rats that appeared clinically normal prior to sacrifice and the opportunity to identify lesions in areas of the brain which would not be included in conventional studies. Other advantages include the ability to examine the brain in multiple planes (transverse, dorsal, sagittal) and obtain and save the MRM images in a digital format that allows for postexperimental data processing and manipulation. MRM images were correlated with neuroanatomical and neuropathological findings. All suspected MRM images were compared to corresponding H&E slides. An important aspect of this study was that MRM was critical in defining our strategy for sectioning the brain, and for designing mechanistic studies (cytochrome oxidase evaluations) and functional assessments (electrophysiology studies) on specifically targeted anatomical sites following COS exposure.

Myocardial morphology in rabbits exposed to various gas-phase constituents of tobacco smoke--an ultrastructural study

Rabbits were continuously exposed to 200 ppm carbon monoxide. Using the same criteria as applied by earlier investigators for morphological myocardial damage, no histotoxic effect on myocardial morphology could be demonstrated when electron-microscopic investigations were performed blindly. Similarly, exposure to 0.5 ppm hydrogen cyanide, 0.5 ppm hydrogen cyanide + 200 ppm carbon monoxide, 0.5 ppm hydrogen cyanide + 200 ppm carbon monoxide + 5 ppm nitric oxide and to 50 ppm carbonyl sulphide for 1-7 weeks had no significant effect on myocardial ultrastructure.