Controlling the nonequilibrium interlayer exchange coupling in asymmetric magnetic tunnel junctions.

We predict an oscillatory bias behavior of the fieldlike spin torque, T(perpendicular), in magnetic tunnel junctions, which can be selectively controlled via the asymmetry in band filling between the ferromagnetic leads. This can lead to a linear or quadratic low-bias behavior, including tuning the bias-induced reversal of T(perpendicular). These findings reconcile the apparently contradictory experimental results recently reported in the literature. The underlying mechanism for the nonequilibrium interlayer exchange coupling (IEC) of noncollinear configurations is the interplay of four independent IEC for the majority- and minority-spin bands of the leads solely in the ferromagnetic configuration.

Anomalous bias dependence of spin torque in magnetic tunnel junctions.

We predict an anomalous bias dependence of the spin transfer torque parallel to the interface, Tparallel, in magnetic tunnel junctions, which can be selectively tuned by the exchange splitting. It may exhibit a sign reversal without a corresponding sign reversal of the bias or even a quadratic bias dependence. We demonstrate that the underlying mechanism is the interplay of spin currents for the ferromagnetic (antiferromagnetic) configurations, which vary linearly (quadratically) with bias, respectively, due to the symmetric (asymmetric) nature of the barrier. The spin transfer torque perpendicular to interface exhibits a quadratic bias dependence.

Increased pathology incidence in the forestomach of rats maintained on a diet containing ivermectin and given a single dose of N-methyl-N1-nitro-N-nitrosoguanidine.

Ivermectin is widely used against parasitic infections in veterinary and human medicine and was found to promote the growth of lesions leading to neoplasia when given continuously in the diet to Wistar rats receiving a single low dose of N-methyl-N1-nitro-N-nitrosoguanidine (MNNG). No tumors or pathological lesions were observed in the forestomach of the control animals or those given ivermectin alone. However, compared to animals receiving MNNG alone, rats maintained on a diet containing ivermectin (2 ppm) and given MNNG (12.5 mg/kg) by gavage showed an increased number of neoplasms (9/26 vs 3/18; p = 0.30) and a statistically significant fourfold increase in the number of pathological lesions (18/26 vs 3/18; p = 0.002), which include preneoplasia in the forestomach. In all cases, the pathological lesions were more severe in the animals receiving ivermectin and MNNG, compared to those receiving MNNG alone.

Oncogenic studies with felbamate (2-phenyl-1,3-propanediol dicarbamate).

Felbamate, 2-phenyl-1,3-propanediol dicarbamate, is a novel anticonvulsant that is effective against both chemically and electrically induced seizures in laboratory animals. Oncogenic studies were conducted in mice and rats to establish a preclinical safety profile for this drug. There was an increased incidence of hepatic cell adenoma in male and female mice and in female rats. There was an increased incidence of interstitial cell tumors of the testes in the male rat.

Oncogenicity studies of piperonyl butoxide in rats and mice.

1. The oncogenicity of Piperonyl butoxide (PBO) has been studied in the mouse and rat. CD-1 mice were administered PBO in the diet at target doses of 0, 30, 100 and 300 mg/kg/day for 79 weeks and Sprague-Dawley rats 0, 30, 100 and 500 mg/kg/day for 104/105 weeks. 2. At termination of the study in the mouse there was evidence of increased liver weights and an increased incidence of eosinophilic adenomas at 100 and 300 mg/kg/day in males and 300 mg/kg/day in females. 3. In rats there was increased liver weights at 100 and 500 mg/kg/day associated with hepatocyte hypertrophy in both male and female rats. There was no increased incidence of neoplasia at any site. Hypertrophy and hyperplasia of thyroid follicles was observed at 500 mg/kg/day in both sexes. 4. The observations reflect the expected changes related to the induction of the mixed function oxygenase group of enzymes. In the mouse the increased incidence of eosinophilic adenomas is not considered relevant for human risk evaluation.

Effect of piperonyl butoxide on cell replication and xenobiotic metabolism in the livers of CD-1 mice and F344 rats.

Male CD- 1 mice were fed diets containing 0 (control), 10, 30, 100, and 300 mg/kg/day piperonyl butoxide (PBO) and 0.05% sodium phenobarbital (NaPB) and male F344 rats were fed diets containing 0 (control), 100, 550, 1050, and 1850 mg/kg/day PBO and 0.5% NaPB for periods of 7 and 42 days. In both species PBO and NaPB increased relative liver weight and whereas PBO produced a midzonal (mouse) or periportal/midzonal (rat) hypertrophy, NaPB produced a centrilobular hypertrophy. In the rat, individual cell necrosis was also observed at 42 days after high doses of PBO. Replicative DNA synthesis, assessed as the hepatocyte labeling index following implantation of 7-day osmotic pumps containing 5-bromo-2'-deoxyuridine during Study Days 0-7 and 35-42, was increased in mice given 300 mg/kg/day PBO and NaPB for 7 days and in rats given 550 and 1050 mg/kg/day PBO and NaPB for 7 days and 1050 mg/kg/day PBO for 42 days. While PBO had no effect on body weights in mice, the body weights of rats given 550, 1050, and 1850 mg/kg/day PBO for 42 days were reduced to 92, 89, and 70% of control, respectively. PBO induced microsomal cytochrome P450 content and mixed function oxidase activities in the mouse and rat, although the effects were less marked than those produced by NaPB. In summary, this data demonstrates that PBO can produce liver enlargement in the mouse and the rat which is associated with induction of xenobiotic metabolism, hypertrophy, and hyperplasia. The hepatic effects of PBO in the mouse were similar to but less marked than those produced by NaPB. In the rat high doses of PBO were hepatotoxic and resulted in a marked reduction in body weight. Thus while the reported formation of eosinophilic nodules in mouse liver by PBO may occur by a mechanism(s) similar to that of NaPB and other nongenotoxic enzyme inducers, the reported tumor formation in rats at greater than the maximum tolerated dose is most likely associated with marked enzyme induction in conjunction with a regenerative hyperplasia resulting from PBO-induced hepatotoxicity.

Mesenchymal tumors of the mouse urinary bladder with vascular and smooth muscle differentiation.

Bifenthrin, a synthetic pyrethroid insecticide/miticide, has been fed to male and female Swiss Webster mice at levels of 0, 50, 200, 500, and 600 ppm in the diet for between 604 and 644 days. Tumors of the urinary bladder were observed and initially reported as leiomyosarcomas. Subsequently, the bladders were reviewed and the tumors showed a pattern of both epithelioid cells and spindle cells forming irregular vascular channels. The tumors appeared to arise from the trigone of the bladder and, in some cases, invaded the bladder wall. No metastases were recorded. The tumor is usually considered rare; however, in this study, it was commonly observed in all groups but predominantly in males. The histogenesis of the tumor is uncertain, but from its pleomorphic histological features, including smooth muscle and vascularity, it is probably derived from vascular mesenchyme.

Lack of genotoxicity of piperonyl butoxide.

The genotoxicity of piperonyl butoxide has been investigated in bacterial mutation assays using tester strains TA98, TA100, TA1535, TA1537 and TA1538. The assays were conducted both with and without metabolic activation. Piperonyl butoxide was tested for mutation with and without metabolic activation in the CHO/HGPRT assay. Chromosomal aberrations were investigated also using Chinese hamster ovary (CHO) cells and effects on DNA were evaluated by in vitro unscheduled DNA synthesis (UDS) test using rat liver primary cell cultures. Piperonyl butoxide was not shown to be genotoxic in any assay system. The data presented supports the view that the liver tumors observed in rodents at dose levels above the maximally tolerated dose (MTD) result from a secondary non-genotoxic mechanism.

Lack of effect of piperonyl butoxide on unscheduled DNA synthesis in precision-cut human liver slices.

In this study the effect of piperonyl butoxide (PBO) on unscheduled DNA synthesis in precision-cut human liver slices has been examined. Liver slices prepared from tissue samples from five human donors were cultured in medium containing [3H]thymidine and 0-2.5 mM PBO using a dynamic organ culture system. After 24 h the liver slices were processed for autoradiographic examination of UDS. As positive controls, liver slices were also cultured with three known genotoxic agents, namely 2-acetylaminofluorene (2-AAF), aflatoxin B1 (AFB1) and 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP). UDS was quantified as the net grain count in centrilobular hepatocytes and as the percentage of centrilobular hepatocyte nuclei with > 5 and > 10 net grains. Compared to control liver slice cultures PBO had no effect on UDS. In contrast, treatment with 0.02 and 0.05 mM 2-AAF, 0.002 and 0.02 mM AFB1 and 0.005 and 0.05 mM PhIP produced significant increases in net grain counts of centrilobular hepatocytes. The greatest induction of UDS was observed in liver slices treated with 0.05 mM PhIP. Treatment with 2-AAF, AFB1 and PhIP also produced increases in the number of centrilobular hepatocyte nuclei with > 5 and > 10 net grains. At the concentrations examined neither PBO, 2-AAF nor PhIP had any significant effect on replicative DNA synthesis in 24 h cultured human liver slices. In cultured liver slices treated with 0.02, but not 0.002, mM AFB1 a significant reduction in the rate of replicative DNA synthesis was observed. These results demonstrate that PBO does not induce UDS in cultured human liver slices. However, all three positive control compounds produced marked significant increases in UDS, thus confirming the functional viability of the human liver slice preparations used in this study. In conclusion, these results provide further evidence that PBO is a non-genotoxic agent which does not damage DNA in human liver.

A review of the hepatic tumors related to mixed-function oxidase induction in the mouse.

Mixed-function oxidase (MFO) induction in the mouse liver results in a rapid and sustained centrilobular hypertrophy associated with a hyperplastic response. In many studies, the long-term sequela of prolonged exposure is an increased incidence of lesions considered to be adenomas. Studies have shown in aged control mice that the burden of adenomas usually consists of lesions with basophilic cytoplasic staining and a uniform population of hepatocyte nuclei. With long-term feeding of MFO inducers, there is an additional burden of lesions diagnosed as adenomas having a different histological appearance with increased eosinophilic cytoplasm and pleomorphic nuclei. The incidence of hepatocarcinomas usually is not modified by the increased incidence of eosinophilic adenomas. Studies into the behavior of the eosinophilic lesions show that the hepatocytes approximate in their behavior to normal and not neoplastic cells. It is suggested that these lesions should not be considered a carcinogenic response to the chemical.

A 90-day feeding study of lupin (Lupinus angustifolius) flour spiked with lupin alkaloids in the rat.

Three groups of 20 male and 20 female Sprague-Dawley rats were given diets based on lupin (Lupinus angustifolius) flour (55.4 g/100 g diet) that had been spiked to provide dietary concentrations of 250, 1050 or 5050 mg lupin alkaloids/kg diet. A control group of 20 males and 20 females received 50 mg/kg (derived from the background level of alkaloid in lupin flour). The rats were treated for a minimum of 90-98 days. A dose-related reduction in red blood cell count and haematocrit (HCT) occurred in both sexes after 45 days, and the mean cell volume (MCV) was decreased in all the male treatment groups. The reductions in HCT and MCV persisted in the males until termination of the study when decreased haemoglobin levels were also observed in the top-dose males. The relative liver weights of female rats showed a dose-related increase. Altered foci of liver parenchymal cells were seen in five females receiving dietary levels of 5050 mg/kg, in one female fed 250 mg/kg and in one male from each of the 250 mg/kg and 1050 mg/kg treatment groups. No foci were seen in the control group. Basophilic foci are uncommon in young rats suggesting that the low incidence in this study is compound related.

Growth characteristics and Ha-ras mutations of cell cultures isolated from chemically induced mouse liver tumours.

Cells have been isolated from liver tumours that have arisen in control C3H/He mice, in mice given 10 micrograms diethylnitrosamine (DEN) during the neonatal period or in mice given a diet containing phenobarbitone (PB) to allow a daily intake of 85 mg/kg/day. The cells were grown to the 8 degrees subculture when their growth characteristics were investigated in monolayer culture and following suspension in soft agar and on transplantation into nude mice. In addition, DNA was isolated from the cultures and from tumours that grew in nude mice and analysed for mutations at codon 61 of the Ha-ras oncogene. All cells derived from DEN-induced hepatocellular carcinomas (HCC) demonstrated a lack of density inhibition of growth in monolayer culture, grew in soft agar and formed tumours in nude mice with an average mean latency of 29 days. Three of the seven lines showed mutations in Ha-ras: two were CAA-->AAA transversions and one showed a CAA-->CTA transversion. In contrast, cells isolated from eosinophilic nodules in mice given PB showed inhibition of growth at confluence, did not grow in soft agar and only four of eight formed tumours in nude mice with a mean average latent period of 181 days. Cells grown from HCC in mice given PB showed a lack of density inhibition of growth, however, they did not grow in soft agar nor did they form tumours in nude mice. A single spontaneous HCC from a control mouse showed a similar growth pattern to HCC cells isolated from mice given PB. Cells from a basophilic nodule, taken from a control untreated mouse grew vigorously in culture and in soft agar and formed tumours in nude mice with a latency of 6 days. None of the cells isolated from control mice or from mice given PB showed evidence of mutations at codon 61 of Ha-ras. These data confirm that there are fundamental differences in the biology of cells grown from tumours that develop in mice under different treatment regimes. These studies also demonstrate the utility of cell culture and molecular biology in addressing the fundamental mechanism of mouse hepatic neoplasia.

Vigabatrin-induced lesions in the rat brain demonstrated by quantitative magnetic resonance imaging.

Rats treated with 250 mg/kg/day vigabatrin showed lesions detected by magnetic resonance imaging (MRI) in the cerebellar white matter in vivo. No lesions were seen in any control animal. As well as these visually apparent lesions, quantitative T2 relaxation time measurements showed a 12 ms increase in cerebellar white matter from 66 +/- 4 ms (SD, n = 5) to 78 +/- 2 ms (SD, n = 7). This region, as expected from previous studies, showed microvacuolation on post-mortem pathology. Additionally, significant increases in T2 relaxation times of 4-9 ms were found in the cerebral cortex, thalamus and hippocampus. Microvacuolation was not detected by post-mortem histopathology in the cerebral cortex or hippocampus, however, immunohistochemical staining for glial fibrillary acidic protein and for macrophages (ED1) showed reactive astrocytes (gliosis) and in more severe cases, microglial proliferation in these regions; such changes were also seen in association with the microvacuoles. No T2 increase was found in the cerebellar grey matter or olfactory bulbs. MRI techniques, including T2 relaxometry, are therefore sensitive for detecting vigabatrin-induced changes, including reactive astrocytosis, microglial proliferation and vacuolation in the rat brain. These results suggest that quantitative MRI should be a useful method for evaluating whether vigabatrin has neuropathological effects when given to patients.

Critique of the toxicology and carcinogenesis studies of iodinated glycerol in F344/N rats and B6C3F1 mice.

Carter-Wallace conducted a detailed audit and evaluation of the data available from the carcinogenicity studies with iodinated glycerol conducted in the B6C3F1 mouse and the F344/N rat by the National Toxicology Program (NTP). We conclude that there is no evidence of carcinogenicity of the compound in either the B6C3F1 mouse or the F344/N rat.

N-methyl-N'-nitro-N-nitrosoguanidine-induced carcinogenesis: differential pattern of upper gastrointestinal tract tumours in Wistar rats after single or chronic oral doses.

Male Wistar rats were treated with N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) either as a single dose of 50, 125 or 250 mg/kg given by gavage or via drinking water for 28 weeks at a concentration of 40, 80 or 160 micrograms/ml, in the case of the higher concentration reverting to 80 micrograms/ml after the first 8 weeks. The single dose regimen had no effect on water intake or body weight, but the chronic exposure led to a dose-dependent reduction in water intake that was paralleled by a slower weight gain, with the final body weights at approximately 90, 84 and 79% of the control weight values. The combined yield of benign and malignant tumours (79-100% of the animals treated) occurred in the forestomach in the case of the single doses, whereas the chronic exposure resulted in a maximum yield of tumours located in the pyloric region of the glandular stomach (64-100% of animals treated). The principal histological types of tumours induced were squamous cell papilloma and carcinoma in the forestomach and adenocarcinoma in the pylorus. There was a persistent, but low yield (25-30% of animals treated) of tumours in the jejunum, mainly adenocarcinoma, after administration via drinking water, whereas after single doses, multiple solitary cysts and cholangioma (30% and 25-70% respectively of the animals treated) were found in the liver. This report differs from earlier reports in that marked effects were noted on water consumption and body weight gain and that tumour induction can be achieved after much shorter periods of exposure than previously reported in the literature. These data confirm the tissue specificity of MNNG when given either as a single or chronic dose regimen and provide a suitable model for the investigation of the target cell specificity of tumour induction.

The histology and development of hepatic nodules and carcinoma in C3H/He and C57BL/6 mice following chronic phenobarbitone administration.

Male C3H/He and C57BL/6 mice were given diets containing sodium phenobarbitone (PB) to allow a daily intake of 85 mg/kg. Control and treated animals were killed at 5, 30, 40, 60, and 80 wk. Other mice were killed in extremis or at the end of the respective experiments: 91 wk for C3H/He and 100 wk for the C57BL/6 animals. A basophilic nodule was found in 1/5 control C3H/He mice at 30 wk; these nodules increased in number with time so that nodules of this type were found in approximately 70% of animals by 91 wk. Nodules were not found in control C57BL/6 mice until 80 wk, when they were found in 4% of mice. PB treatment markedly increased the number of hepatic nodules in both strains of mice. The additional nodule burden was due to the development of a second nodule type formed of large cells with a predominantly eosinophilic cytoplasm. C3H/He animals given PB for 60 wk and then returned to a control diet bore fewer nodules at 91 wk than treated mice killed at 60 or 91 wk. The cumulative incidence of carcinoma in control C3H/He and C57BL/6 mice was 28 and 4%, respectively. The incidence of carcinoma was not increased by PB treatment in either strain. It is concluded that both strains of mice behave in a qualitively similar way to PB administration, although they show considerable quantitative differences in terms of the time and number of nodules that develop. Furthermore, the increased nodule numbers associated with PB treatment were not accompanied by an increase in the number of carcinomas.

An ultrastructural study of spontaneous and phenobarbitone-induced nodules in the mouse liver.

Male C3H/He mice were given 0 (control) or 85 mg/kg/day phenobarbitone (PB) in the diet. At 40, 60 and 93 weeks, groups of mice were killed and the ultrastructure of spontaneous and PB-induced liver nodules was examined. Treated mice showed typical centrilobular hypertrophy and eosinophilic nodules which may be considered as an end stage lesion. The nodule cells were similar in appearance to those in areas of centrilobular hypertrophy except for the presence of convoluted membranes which are considered to be indicative of proliferation. The incidence of carcinoma was not increased by PB treatment. The carcinomas from control and treated animals differed in their ultrastructure in that increased levels of smooth endoplasmic reticulum (SER) were seen in the carcinomas of the PB animals. The presence of SER proliferation in the carcinomas of PB animals suggests that carcinoma may respond to the enzyme-inducing effects of PB.

Promutagenic lesions persist in the DNA of target cells for nitrosamine-induced carcinogenesis.

Immunohistochemical procedures for the location of O6-methylguanine (O6-meGua) permit detection of cells proficient for the metabolism of N-nitrosodimethylamine (NDMA) and deficient for the repair of this DNA lesion. Such cells are potentially at high risk for cancer induction and are present in various tissues. In animals maintained on a protein-deficient diet, the distribution and intensity of alkylation of individual cells is altered, particularly in liver where fewer cells apparently retain the capacity to metabolize the nitrosamine, thereby permitting increased levels of alkylation in other tissues. In the renal cortex, specific, O6-meGua-positive target cells for renal cancer induced by a single dose of NDMA in weanling rats persist at least up to the appearance of early lesions. Persistence of alkylated cells in several tissues indicates prospects for the detection of environmental exposure.