Immunohistochemical Expression of Cyclin D1, Cytokeratin 20, and Uroplakin III in Proliferative Urinary Bladder Lesions Induced by o-Nitroanisole in Fischer 344/N Rats.

o-Nitroanisole is an intermediate in the manufacture of azo dyes. In a National Toxicology Program stop-exposure study,o-nitroanisole induced hyperplasia, papillomas, and papillary carcinomas in the urinary bladder of Fischer 344/N rats.o-Nitroanisole was investigated since occupational or environmental exposure to aniline and azo dyes is a risk factor for urinary bladder cancer in humans. The current study describes the morphology of urinary bladder neoplasms seen in rats with respect to those observed in humans. This study also evaluated immunohistochemical expression of the cell cycle-related proteins cyclin D1 and p53 and the differentiation markers cytokeratin 20 and uroplakin III in hyperplastic (n= 11) and neoplastic (n= 6 papillomas,n= 11 carcinomas) lesions of the urinary bladder epithelium from rats treated with o-nitroanisole and in normal (n= 6) urinary bladders from untreated rats. The tumors observed were more similar to the papillary type rather than the muscle-invasive type of urinary bladder cancer in humans. The preneoplastic and neoplastic lesions observed suggest progression from hyperplasia to papilloma to papillary carcinoma. With neoplastic progression (hyperplasia to papilloma to carcinoma), cyclin D1 immunoreactivity progressively increased in intensity, percentage of cells staining, and distribution. Overexpression of p53 was not found. Cytokeratin 20 staining decreased in superficial cells, while uroplakin III staining increased in intermediate and basal cells with progression from hyperplasia to carcinoma. The results are consistent with increased cell cycle dysregulation or proliferation (cyclin D1), decreased differentiation (cytokeratin 20), and abnormal differentiation (uroplakin III) as lesions progress toward malignancy.

Mass spectrometric analysis of rat cerebrospinal fluid proteins following exposure to the neurotoxicant carbonyl sulfide.

RATIONALE: Using a proteomic-based approach we have investigated possible altered expression of a range of cerebral spinal fluid (CSF) proteins following exposure to the neurotoxicant carbonyl sulfide (COS). CSF is ideal for the investigation of markers of brain injury or disease since it is secreted from several central nervous system structures and changes in the CSF composition may reflect brain insult and many pathological processes. METHODS: Animals were placed in exposure chambers and were exposed to 0 ppm or 500 ppm COS for 1, 2 or 3 days, 6 h per day. After the last inhalation exposure, 50-70 µL CSF sample was obtained by lumbar puncture. CSF samples were analyzed by electrospray ionization mass spectrometry (ESI-MS) on either a Premier quadrupole time-of-flight (QTOF) or an Agilent 6340 ion trap and by matrix-assisted laser desorption/ionization (MALDI)-MS on a 4800 MALDI-TOF/TOF analyzer. RESULTS: The dynamic range of abundance of the identified proteins spanned over more than three orders of magnitude. The four most abundant proteins identified (albumin, cystatin C, serotransferrin, transthyretin) are major proteins that are present in both CSF and blood at high levels but the fifth most abundant protein identified (prostaglandin H2D isomerase) is the second most abundant protein in human CSF and is secreted and synthesized in the rat central nervous system. No significant differences were observed between COS-treated CSF samples and the control CSF samples because of blood contamination. CONCLUSIONS: Quantitative MS protein analyses of rat CSF is limited by the low sample volumes that can practicably be obtained from rats and the low protein concentrations in rat CSF. Results of this work suggest a clear need for CSF collection that would minimize blood contamination. Published in 2014. This article is a U.S. Government work and is in the public domain in the USA.

Wildtype Kras2 can inhibit lung carcinogenesis in mice.

Although the ras genes have long been established as proto-oncogenes, the dominant role of activated ras in cell transformation has been questioned. Previous studies have shown frequent loss of the wildtype Kras2 allele in both mouse and human lung adenocarcinomas. To address the possible tumor suppressor role of wildtype Kras2 in lung tumorigenesis, we have carried out a lung tumor bioassay in heterozygous Kras2-deficient mice. Mice with a heterozygous Kras2 deficiency were highly susceptible to the chemical induction of lung tumors when compared to wildtype mice. Activating Kras2 mutations were detected in all chemically induced lung tumors obtained from both wildtype and heterozygous Kras2-deficient mice. Furthermore, wildtype Kras2 inhibited colony formation and tumor development by transformed NIH/3T3 cells and a mouse lung tumor cell line containing an activated Kras2 allele. Allelic loss of wildtype Kras2 was found in 67% to 100% of chemically induced mouse lung adenocarcinomas that harbor a mutant Kras2 allele. Finally, an inverse correlation between the level of wildtype Kras2 expression and extracellular signal-regulated kinase (ERK) activity was observed in these cells. These data strongly suggest that wildtype Kras2 has tumor suppressor activity and is frequently lost during lung tumor progression.

H-ras consensus sequence and mutations in primary hepatocellular carcinomas of lemurs and lorises.

The authors have determined a consensus sequence for exons 1 and 2 of H-ras from captive lemurs and lorises and evaluated samples of nonneoplastic liver and hepatocellular carcinomas (HCC) from affected animals for mutations in these exons. Frozen liver samples were collected from 20 animals representing 9 different species with a sex distribution of 10 males and 10 females. A total of 26 liver samples, including 11 normal livers, 9 HCC, and 6 samples from nonneoplastic regions of liver from animals with HCC, were evaluated. This is the first report of the consensus sequence for exons 1 and 2 of H-ras in prosimians, and the authors have determined that it is identical to that of human H-ras and differs only slightly from the chimpanzee sequence. Point mutations were identified in 6 of the 9 HCC samples examined with codons 7, 22, 32, 56, 61, 84, and 96 affected. Two carcinomas had double mutations, and one tumor had triple mutations. One HCC had a mutation in codon 61, which is identical to a recognized affected codon for an H-ras "hot spot" in rodent neoplasia that has also been reported in human tumors. Although not statistically different, metastasis occurred in 5 of 6 HCC with H-ras mutation and only 1 of 3 HCC without mutations. There were 4 silent mutations that did not contain changes in the encoded amino acids, 2 of which were found in nonneoplastic regions of tumor-bearing liver.

Gene expression and mutation assessment provide clues of genetic and epigenetic mechanisms in liver tumors of oxazepam-exposed mice.

Liver tumors from a previous National Toxicology Program study were examined using global gene expression and mutation analysis to define the mechanisms of carcinogenesis in mice exposed to oxazepam. Five hepatocellular adenomas and 5 hepatocellular carcinomas from male B6C3F1 mice exposed to 5000 ppm oxazepam and 6 histologically normal liver samples from control animals were examined. One of the major findings in the study was upregulation of the Wnt/ß-catenin signaling pathway. Genes that activate ß-catenin, such as Sox4, were upregulated, whereas genes that inhibit Wnt signaling, such as APC and Crebbp, were downregulated. In addition, liver tumors from oxazepam-exposed mice displayed ß-catenin mutations and increased protein expression of glutamine synthetase, a downstream target in the Wnt signaling pathway. Another important finding in this study was the altered expression of oxidative stress-related genes, specifically increased expression of cytochrome p450 genes, including Cyp1a2 and Cyp2b10, and decreased expression of genes that protect against oxidative stress, such as Sod2 and Cat. Increased oxidative stress was confirmed by measuring isoprostane expression using mass spectrometry. Furthermore, global gene expression identified altered expression of genes that are associated with epigenetic mechanisms of cancer. There was decreased expression of genes that are hypermethylated in human liver cancer, including tumor suppressors APC and Pten. Oxazepam-induced tumors also exhibited decreased expression of genes involved in DNA methylation (Crebbp, Dnmt3b) and histone modification (Sirt1). These data suggest that formation of hepatocellular adenomas and carcinomas in oxazepam-exposed mice involves alteration of the Wnt signaling pathway, oxidative stress, and potential epigenetic alterations.

Re-evaluation of kidney histopathology from 13-week toxicity and two-year carcinogenicity studies of melamine in the F344 rat: morphologic evidence of retrograde nephropathy.

The histopathologic changes induced in F344 rat kidney by oral administration of melamine for 13-week and 2-year periods in studies conducted by the National Toxicology Program, NIH,(25) from 1976 to 1983 have been re-evaluated and described in detail. A constellation of tubule changes extending from papilla to cortex consistently included tubule dilatation and tubule basophilia as salient features at the subchronic time point. By 2 years, these lesions had usually resolved into fibrotic scars, in which tubule loss and collagen deposition were prominent, running from superficial cortex into the medulla. These fibrotic lesions required discrimination from chronic scars resulting from infarcts and foci of chronic progressive nephropathy (CPN). A case is presented here for interpreting the constellation of histologic changes induced in rats by melamine as representing an ascending form of nephropathy. The term retrograde nephropathy is considered to be the appropriate nomenclature for both the acute and chronic lesions. The cause for the reflux, emanating from the lower urinary tract, appeared not to be infection as an inflammatory response was not prominent. It can be speculated that melamine precipitation in the lower urinary tract created pressure effects through transient obstruction leading to the renal changes. These changes were different from those involved in a major US outbreak of renal disease and death in cats and dogs associated with triazine-contaminated pet food, in which crystalluria from insoluble melamine/cyanuric acid complexes occurred in the kidney. However, the rat findings may be relevant to melamine-associated kidney disease recently reported in infants in China.

Beta-catenin mutations and protein accumulation in all hepatoblastomas examined from B6C3F1 mice treated with anthraquinone or oxazepam.

The molecular pathogenesis of hepatoblastomas in the B6C3F1 mouse is unclear but may involve alterations in the beta-catenin/Wnt signaling pathway as was recently described for chemically induced hepatocellular neoplasms and human liver cancers. The objective of this study was to characterize the mutation frequency and spectrum of beta-catenin mutations and the intracellular localization of beta-catenin protein accumulation in chemically induced hepatoblastomas. In this study, beta-catenin mutations were identified in all 19 anthraquinone-induced hepatoblastomas and all 8 oxazepam-induced hepatoblastomas examined. Although several hepatoblastomas had multiple deletion and/or point mutations, the pattern of mutations in the hepatoblastomas did not differ from that identified in hepatocellular neoplasms. In a majority of the hepatoblastomas (six of seven) examined by immunohistochemical methods, both nuclear and cytoplasmic localization of beta-catenin protein were detected, whereas in hepatocellular adenomas, carcinomas, and normal liver only membrane staining was observed. Our data suggest that beta-catenin mutations and the subsequent translocation of beta-catenin protein from the cell membrane to the cytoplasm and nucleus may be critical steps in providing hepatocellular proliferative lesions with the growth advantage to progress to hepatoblastoma.

Isoprene, an endogenous hydrocarbon and industrial chemical, induces multiple organ neoplasia in rodents after 26 weeks of inhalation exposure.

Isoprene, the 2-methyl analogue of 1,3-butadiene, is a high production chemical used largely in the manufacture of synthetic rubber and is the major endogenous hydrocarbon exhaled in human breath. Thirteen-week inhalation toxicology studies of isoprene were conducted in male and female F344 rats and B6C3F1 mice at exposure concentrations of 0, 70, 220, 700, 2200, and 7000 ppm (6 h/day; 5 days/week). In addition, 26-week inhalation studies at the same exposure levels, followed by a 26-week recovery period, were conducted in male rats and mice. The 13-week exposures produced no discernible exposure-related toxic effects in rats. Interstitial cell hyperplasia of the testis was observed in all male rats in the 7000 ppm group after 26 weeks of exposure; following the 26-week recovery period the only effect in rats was a marginal increase in benign testicular interstitial cell tumors. In mice, isoprene induced toxic and carcinogenic effects at multiple organ sites. Following the 26-week exposure and 26-week recovery periods, incidences of neoplastic lesions in the liver, lung, forestomach, and harderian gland were significantly increased. Neoplastic effects were observed at 700 ppm and higher exposures. Non-neoplastic lesions in mice exposed to isoprene included spinal cord degeneration, testicular atrophy, degeneration of the olfactory epithelium, and epithelial hyperplasia of the forestomach. A partial hindlimb paralysis and a nonresponsive macrocytic anemia were also seen in mice. Most of the toxic and carcinogenic effects caused by isoprene, as well as the species' difference in response, had been observed after inhalation exposures to 1,3-butadiene.

Mutation of beta-catenin is an early event in chemically induced mouse hepatocellular carcinogenesis.

beta-catenin activation, and subsequent upregulation of Wnt-signaling, is an important event in the development of certain human and rodent cancers. Recently, mutations in the beta-catenin gene in the region of the serine-threonine glycogen kinase (GSK)-3beta phosphorylation target sites have been identified in hepatocellular neoplasms from humans and transgenic mice. In this study we examined 152 hepatocellular neoplasms from B6C3F1 mice included in five chemical treatment groups and controls for mutations in the beta-catenin gene. Twenty of 29 hepatocellular neoplasms from mice treated with methyleugenol had point mutations at codons 32, 33, 34 or 41, sites which are mutated in colon and other cancers. Likewise, nine of 24 methylene chloride-induced hepatocellular neoplasms and 18 of 42 oxazepam-induced neoplasms exhibited similar mutations. In contrast, only three of 18 vinyl carbamate-induced liver tumors, one of 18 TCDD-induced liver tumors, and two of 22 spontaneous liver neoplasms had mutations in beta-catenin. Thus, there appears to be a chemical specific involvement of beta-catenin activation in mouse hepatocellular carcinogenesis. Expression analyses using Western blot and immunohistochemistry indicate that beta-catenin protein accumulates along cell membranes following mutation. The finding of mutations in both adenomas and carcinomas from diverse chemical treatment groups and the immunostaining of beta-catenin protein in an altered hepatocellular focus suggest that these alterations are early events in mouse hepatocellular carcinogenesis.

Magnetic fields and mammary cancer in rodents: a critical review and evaluation of published literature.

Epidemiological data suggesting a possible increase in breast cancer risk in male electricians have raised concerns about the relationship between exposure to power-frequency magnetic fields and breast cancer. In this paper, we review the results of animal studies that are relevant to identifying possible increases in breast cancer risk resulting from exposure to 50 or 60 Hz magnetic fields. Three large-scale chronic bioassays of carcinogenesis in rats or mice exposed to magnetic fields for 2 years demonstrated no increases in the incidence of mammary cancer; it is generally accepted that power-frequency magnetic fields have little or no activity as a complete carcinogen in the rodent mammary gland. Findings from one laboratory, though inconsistent, suggest that magnetic fields may stimulate mammary neoplasia in rats treated with a chemical carcinogen. However, studies conducted in two other laboratories failed to confirm these findings; rats exposed to magnetic fields demonstrated patterns of tumor incidence, multiplicity, size and latency that were generally similar to those in sham-exposed controls. Where differences were seen, the groups exposed to magnetic fields generally had fewer mammary tumors than did sham-exposed controls. On this basis, evaluations of the activity of 50 or 60 Hz magnetic fields in models of multistage mammary cancer in rodents have generally been negative; positive findings have been reported from only one laboratory. The totality of rodent data does not support the hypothesis that power-frequency magnetic-field exposure enhances mammary cancer in rodents, nor does it provide experimental support for possible epidemiological associations between magnetic-field exposure and increased breast cancer risk.

Leukemia and lymphoma incidence in rodents exposed to low-frequency magnetic fields.

A weak association between residential or occupational exposure to electric and magnetic fields (50/60 Hz fields) and an increased incidence of leukemia has been reported. Numerous animal studies have evaluated the potential association between magnetic-field exposure and leukemia. These include long-term (up to 2(1/2) years) bioassays, initiation/promotion studies, investigations in transgenic models, and tumor growth studies. Exposure to 60 Hz circularly polarized magnetic fields at 1,400 microT for 28 months did not affect lymphoma incidence in mice. The study included over 2000 C57BL/6J mice. In another study, 1000 B6C3F(1) mice exposed to 60 Hz magnetic fields up to 1000 microT for 2 years showed no increase in lymphomas. Approximately 400 transgenic Emu-Pim1 mice exposed to 50 Hz fields up to 1000 microT for up to 18 months had no increased incidence of leukemia. Similarly, Trp53(+/-) mice and Pim1transgenic mice exposed to 60 Hz magnetic fields for 23 weeks showed no increased incidence of lymphoma. Three studies in F344 rats exposed to 50 or 60 Hz magnetic fields up to 5 mT showed no increased incidence of leukemia. The combined animal bioassay results are nearly uniformly negative for magnetic-field exposures enhancing leukemia and weaken the possible epidemiological association between magnetic-field exposures and leukemia in humans as suggested by epidemiological data.

Hepatocarcinogenesis in female Sprague-Dawley rats following discontinuous treatment with 2,3,7,8-tetrachlorodibenzo-p-dioxin.

In this study, we investigated the time course of promotion of tumors and putatively preneoplastic altered hepatic foci in the livers of diethylnitrosamine (DEN)-initiated female Sprague-Dawley rats. These rats had been treated with 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) under different dosing regimens, but we used the same administered biweekly dose of 1.75 microg/kg of body weight. Animals were treated continuously for up to 60 weeks, or continuously for 30 weeks, followed by cessation of treatment for up to 30 weeks. In addition, TCDD treatment in these groups was begun either 2 or 18 weeks after initiation with DEN. Liver tumors were only observed in animals after 60 weeks on the study and were increased by continuous TCDD treatment, relative to controls. The incidence of hepatocellular adenoma and carcinoma combined, in animals treated with TCDD for 30 weeks followed by no TCDD treatment for 30 weeks (17%), was lower than in animals receiving either TCDD (79%) or vehicle control (corn oil) alone (55%) for 60 weeks. The lower liver-tumor incidence after cessation of TCDD treatment paralleled time-dependent decreases in the volume fraction occupied by placental glutathione S-transferase-positive altered hepatic foci and the number of foci per unit volume, but not the mean focus volume that exhibited a time-dependent increase after cessation of TCDD treatment. Cessation of TCDD treatment led to reductions in liver TCDD levels, and these changes were reflected in a cessation of reduced body weight because of TCDD treatment. These data indicate that liver-tumor promotion by TCDD in female rats is dependent upon continuous exposure to TCDD, and that alterations in patterns of TCDD exposure can have significant effects on tumor incidence not reflected by standard measures of dioxin exposure.

Neurotoxicity produced by dibromoacetic acid in drinking water of rats.

An evaluation of potential adverse human health effects of disinfection byproducts requires study of both cancer and noncancer endpoints; however, no studies have evaluated the neurotoxic potential of a common haloacetic acid, dibromoacetic acid (DBA). This study characterized the neurotoxicity of DBA during 6-month exposure in the drinking water of rats. Adolescent male and female Fischer 344 rats were administered DBA at 0, 0.2, 0.6, and 1.5 g/l. On a mg/kg/day basis, the consumed dosages decreased greatly over the exposure period, with average intakes of 0, 20, 72, and 161 mg/kg/day. Weight gain was depressed in the high-concentration group, and concentration-related diarrhea and hair loss were observed early in exposure. Testing with a functional observational battery and motor activity took place before dosing and at 1, 2, 4, and 6 months. DBA produced concentration-related neuromuscular toxicity (mid and high concentrations) characterized by limb weakness, mild gait abnormalities, and hypotonia, as well as sensorimotor depression (all concentrations), with decreased responses to a tail-pinch and click. Other signs of toxicity at the highest concentration included decreased activity and chest clasping. Neurotoxicity was evident as early as one month, but did not progress with continued exposure. The major neuropathological finding was degeneration of spinal cord nerve fibers (mid and high concentrations). Cellular vacuolization in spinal cord gray matter (mostly) and in white matter (occasionally) tracts was also observed. No treatment-related changes were seen in brain, eyes, peripheral nerves, or peripheral ganglia. The lowest-observable effect level for neurobehavioral changes was 20 mg/kg/day (produced by 0.2 g/l, lowest concentration tested), whereas this dosage was a no-effect level for neuropathological changes. These studies suggest that neurotoxicity should be considered in the overall hazard evaluation of haloacetic acids.

Inhalation toxicity and carcinogenicity studies of cobalt sulfate.

Cobalt sulfate is a water-soluble cobalt salt with a variety of industrial and agricultural uses. Several cobalt compounds have induced sarcomas at injection sites in animals, and reports have suggested that exposure to cobalt-containing materials may cause lung cancer in humans. The present studies were done because no adequate rodent carcinogenicity studies had been performed with a soluble cobalt salt using a route relevant to occupational exposures. Groups of 50 male and 50 female F344/N rats and B6C3F1 mice were exposed to aerosols containing 0, 0.3, 1.0, or 3.0 mg/m3 cobalt sulfate hexahydrate, 6 h/day, 5 days/week, for 104 weeks. Survival and body weights of exposed rats and mice were generally unaffected by the exposures. In rats, proteinosis, alveolar epithelial metaplasia, granulomatous alveolar inflammation, and interstitial fibrosis were observed in the lung in all exposed groups. Nonneoplastic lesions of the nose and larynx were also attributed to exposure to all concentrations of cobalt sulfate. In 3.0 mg/m3 male rats and in female rats exposed to 1.0 or 3.0 mg/m3, the incidences of alveolar/bronchiolar neoplasms were increased over those in the control groups. Lung tumors occurred with significant positive trends in both sexes. The incidences of adrenal pheochromocytoma in 1.0 mg/m3 male rats and in 3.0 mg/m3 female rats were increased. Nonneoplastic lesions of the respiratory tract were less severe in mice than in rats. In mice, alveolar/bronchiolar neoplasms in 3.0 mg/m3 males and females were greater than those in the controls, and lung tumors occurred with significantly positive trends. Male mice had liver lesions consistent with a Helicobacter hepaticus infection. Incidences of liver hemangiosarcomas were increased in exposed groups of male mice; however, because of the infection, no conclusion could be reached concerning an association between liver hemangiosarcomas and cobalt sulfate. In summary, exposure to cobalt sulfate by inhalation resulted in increased incidence of alveolar/bronchiolar neoplasms and a spectrum of inflammatory, fibrotic, and proliferative lesions in the respiratory tracts of male and female rats and mice. Adrenal pheochromocytomas were increased in female rats, and possibly in male rats.

Exposure of C57BL/6 mice to carbon disulfide induces early lesions of atherosclerosis and enhances arterial fatty deposits induced by a high fat diet.

Even though atherosclerotic cardiovascular disease (ACVD) is the number one cause of death in the United States, the effects of environmental toxicants on this process are less well studied than the effects of chemicals on the second leading cause of death, cancer. There is considerable epidemiological evidence that workers exposed to carbon disulfide (CS2) have increased rates of ACVD, and there is conflicting evidence of the atherogenic potential of CS2 from animal studies. Chemical modification, such as oxidation of low-density lipoproteins (LDL), is tightly associated with increased LDL uptake by macrophages and the development of arterial fatty streaks. CS2 has been previously demonstrated to modify several proteins in vitro including LDL, and others in vivo through derivatization and covalent cross-linking. To investigate both the capacity of CS2 to induce arterial fatty deposits by itself, and its ability to enhance the rate of fatty deposit formation induced by a western style, high fat diet, groups of 20 female C57BL/6 mice were exposed to 0, 50, 500, or 800 ppm CS2 by inhalation. Half the animals in each group were placed on an atherogenic high fat diet and half on a control diet (NIH-07). Animals were sacrificed after 1, 4, 8, 12, 16, or 20 weeks of exposure, and the rates of fatty deposit formation under the aortic valve leaflets were evaluated. Exposure of mice on the control diet to 500 and 800 ppm CS2 induced a small but significant increase in the rate of fatty deposit formation over non-exposed controls. A more striking result was observed in the animals on the high fat diet. There was marked enhancement of the rate of fatty deposit formation in mice exposed to 500 and 800 ppm over the animals on the high fat diet alone. In addition, there was a small but significant enhancement in mice exposed to 50 ppm over the rate of fatty deposit formation induced by the high fat diet alone. Analysis of erythrocyte spectrin for protein cross-linking revealed a dose-dependent formation of alpha- and beta-heterodimers in animals on both diets. These data demonstrate that CS2 is atherogenic at high concentrations, but more importantly, suggest that, in conjunction with other risk factors, CS2 at relatively low concentrations can enhance atherogenesis.

Inhalation toxicity and carcinogenicity of isoprene in rats and mice: comparisons with 1,3-butadiene.

As with 1,3-butadiene (BD), inhalation exposure of B6C3F1 mice to isoprene (2-methyl-1,3-butadiene) caused a macrocytic anemia; induced increases in sister chromatid exchanges in bone marrow cells and in levels of micronucleated erythrocytes in peripheral blood; and produced degeneration of the olfactory epithelium, forestomach epithelial hyperplasia, and testicular atrophy. Most notable was the finding that like BD, isoprene induced neoplasms in the liver, lung, Harderian gland, and forestomach of mice. The carcinogenic effects of isoprene were observed after a 26-week exposure (6 h/day, 5 days/week) of male mice to 700 ppm or higher concentrations of isoprene followed by a 26-week recovery period. Unlike BD, isoprene did not induce lymphomas or hemangiosarcomas of the heart in mice under these conditions nor did it induce chromosomal aberrations in mouse bone marrow cells. No toxicological effects were evident in rats exposed for 13 weeks to either isoprene or BD at concentrations up to 7000 ppm or 8000 ppm, respectively. Interstitial cell hyperplasia of the testis was observed in male F344 rats exposed to 7000 ppm isoprene for 26 weeks, and following a 26-week recovery period, there was a marginal increase in benign testicular interstitial cell tumors.

Carbon disulfide neurotoxicity in rats: II. Toxicokinetics.

Carbon disulfide (CS2) is an important industrial chemical widely used in the production of rayon, cellophane, fungicides and biocides. The uptake and elimination kinetics of CS2 was characterized for a single i.v. dose and for a single inhalation exposure. The uptake of CS2 into the blood was rapid with half times of 6 to 9 minutes. Elimination was relatively quick with terminal elimination half times of 41 to 77 minutes. The plateau CS2 blood concentration was lower in females than in males and lower in the male 50 ppm treatment group than would be predicted by linear dose proportionality compared to the 500 ppm and 800 ppm treatments. The CS2 blood concentration for the female 50 ppm group was below the limit of detection. The total and central compartment apparent volumes of distribution, 4.2 l/kg and .9 l/kg, were estimated from a single 50 mg/kg i.v. dose. The concentration of CS2 in blood resulting from repeated exposure, was investigated in a 13 week inhalation study. Blood samples were taken in rats previously exposed to 0, 50, 500, and 800 ppm CS2 for 2, 4, 8, or 13 weeks. The concentration of CS2 in the blood of male rats remained relatively constant throughout study. However the female 500 and 800 ppm groups showed a marked decrease over the course of the 13 week study. The concentration of CS2 in the blood from the 500 and 800 ppm groups of both sexes at all time points was higher compared to the 50 ppm group, than would be predicted by linear dose proportionality. The concentration of 2-thiothiazolidine-4-carboxylic acid in urine collected from the same animals lacked dose proportionality between the treatment groups at all time points. CS2 exposure caused dose-related decreases in body weight gain in both male and female rats.

Carbon disulfide neurotoxicity in rats: VI. Electrophysiological examination of caudal tail nerve compound action potentials and nerve conduction velocity.

The effects of subchronic exposure to carbon disulfide (CS2) on ventral caudal tail nerve compound nerve action potential (CNAP) amplitudes and latencies, and nerve conduction velocity (NCV) in rats were examined. Male and female Fischer 344 rats were exposed to 0, 50, 500, or 800 ppm CS2 for 6 hrs/day, 5 days/week. Using separate groups, exposure duration was 2, 4, 8, or 13 weeks. Exposure to 500 or 800 ppm CS2 for 13 weeks decreased NCV compared to the 50 ppm CS2 group. CNAP amplitudes were increased, and peak P1P2 interpeak latency decreased, after exposure to 500 or 800 ppm CS2 for 13 weeks. Most of the changes in NCV and CNAPs were not attributable to differences in tail or colonic temperature. However, the increases in peak P1 amplitude may relate to the proximity of the electrodes to the tail nerves. Assessment of tail nerve morphology after 13 weeks exposure to 800 ppm CS2 revealed only minor changes compared to the extent of axonal swelling and degeneration observed in the muscular branch of the tibial nerve and axonal swelling in the spinal cord. As anticipated, in older animals the NCV increased, the CNAP amplitudes increased, and the CNAP latencies decreased. The biological basis for the changes in CNAPs produced by CS2 is under investigation. Future studies will focus on electrophysiological evaluation of spinal nerve function, to allow better correlation with pathological and behavioral endpoints.