The effect of stress on toxicological investigations

Evaluation of Variability across Rat Acute Oral Systemic Toxicity Studies

Regulatory agencies rely upon rodent in vivo acute oral toxicity data to determine hazard categorization, require appropriate precautionary labeling, and perform quantitative risk assessments. As the field of toxicology moves toward animal-free new approach methodologies (NAMs), there is a pressing need to develop a reliable, robust reference data set to characterize the reproducibility and inherent variability in the in vivo acute oral toxicity test method, which would serve to contextualize results and set expectations regarding NAM performance. Such a data set is also needed for training and evaluating computational models. To meet these needs, rat acute oral LD₅₀ data from multiple databases were compiled, curated, and analyzed to characterize variability and reproducibility of results across a set of up to 2441 chemicals with multiple independent study records. Conditional probability analyses reveal that replicate studies only result in the same hazard categorization on average at 60% likelihood. Although we did not have sufficient study metadata to evaluate the impact of specific protocol components (eg, strain, age, or sex of rat, feed used, treatment vehicle, etc.), studies were assumed to follow standard test guidelines. We investigated, but could not attribute, various chemical properties as the sources of variability (ie, chemical structure, physiochemical properties, functional use). Thus, we conclude that inherent biological or protocol variability likely underlies the variance in the results. Based on the observed variability, we were able to quantify a margin of uncertainty of ±0.24 log₁₀ (mg/kg) associated with discrete in vivo rat acute oral LD₅₀ values.

Subcellular energy balance of Odontesthes bonariensis exposed to a glyphosate-based herbicide

Water pollution by agrochemicals is currently one of the most critical problems for the conservation of aquatic ecosystems. Glyphosate [N-(phosphonomethyl) glycine); PMG] is the main broad-spectrum post emergence herbicide used for the control of a wide range of pests in soybean crops. Adenylate energy charge (AEC) reflects the energy balance of the cells, a measure of the energy available from the adenylate pool: adenosine triphosphate (ATP), adenosine diphosphate (ADP) and adenosine monophosphate (AMP). Background adenylates, phosphagens and the AEC index of two year old Odontesthes bonariensis were determined in some tissues by HPLC, and the impact on subcellular energy balance of sublethal glyphosate-based herbicide exposure was analyzed. The doses used were 0 (control tank), 1 or 10mg PMGL(-1), trials were carried out during 15 days. AEC values in brain, liver and muscle from control fish were 0.37 ± 0.02, 0.49 ± 0.05 and 0.56 ± 0.03, respectively (means ± SEM). While brain ATP concentrations were undetectable (hence low values of AEC), the muscle tissue showed the highest concentrations of the more energetic molecules: 0.18 μmole ATP g(-1) and 8 μmole phosphocreatine g(-1) (PCrg(-1)). In the brain, no significant changes were detected in exposed fish compared to controls. Instead, in both the liver and muscle of animals exposed to the highest concentration of the herbicide, significant changes in the AEC (reduction of 26% and 15%, p<0.05) with respect to the control group were determined. Chronic exposure (15 days) of Odontesthes bonariensis to 1 and 10mgL(-1) of formulated glyphosate did not affect brain AEC. However, the highest concentration of the herbicide produced a significant decrease in liver and muscle AEC manifesting adverse sublethal effects on the energy metabolism. These results suggest the usefulness of AEC as a biomarker of fish glyphosate exposure.

Restraint stress does not enhance the uranium-induced developmental and behavioral effects in the offspring of uranium-exposed male rats

The influence of stress on postnatal development and behavior was assessed in the offspring of male rats exposed to uranium (U). Eight groups of adult animals received uranyl acetate dihydrate (UAD) in the drinking water at doses of 0, 10, 20 and 40 mg/kg/day during 3 months. One half of rats in each group were concurrently subjected to restraint stress during 2 h per day throughout the study. At the end of the experimental period, male rats were mated with untreated females (1:2). On gestation day 14, one half of pregnant rats were euthanized in order to evaluate maternal toxicity and gestational parameters. The remaining dams were allowed to deliver and wean their offspring. Pups were evaluated for physical development, neuromotor maturation, as well as for behavioral effects. Restraint significantly increased the gravid uterine weight at 40 mg/kg/day. However, no significant interactions between restraint and U could be established in the remaining parameters of maternal toxicity. In the offspring, no remarkable effects of U, restraint or their combination were noted on developmental landmarks, or in the passive avoidance and water maze test. It is concluded that at the current U doses, restraint stress did not enhance the few uranium-induced physical, neuromotor and behavioral changes in the offspring of UAD-exposed male rats.

Combined action of uranium and stress in the rat

The influence of restraint stress on uranium (U)-induced behavioral effects was assessed in adult male rats. Eight groups of animals received uranyl acetate dihydrate (UAD) in the drinking water at doses of 0, 10, 20 and 40 mg/kg/day during 3 months. Rats in four groups were concurrently subjected to restraint during 2 h per day throughout the study. At the end of the period of uranium exposure, the following behavioral tests were carried out: open-field activity, passive avoidance and Morris water maze. Uranium concentrations in brain were also determined. At 10 and 20 mg/kg/day of UAD restraint significantly affected the total distance traveled in the open-field during the first and third periods tested, respectively, while no significant differences between groups were observed on the passive avoidance test. In the Morris water maze test, the influence of restraint was only significant on the latency time measured on Day 3 in rats exposed at 10 mg/kg/day. Restraint stress did not affect significantly the uranium levels in brain of rats. Although the results of the present study scarcely show uranium-induced behavioral effects at the oral doses of UAD here administered, these effects, as well as the slight influence of restraint stress noted in some tests should not be underrated.

Combined action of uranium and stress in the rat

The effects of stress on the potential reproductive toxicity of long-term exposure to uranyl acetate dihydrate (UAD) were assessed in adult male rats. Six groups of animals were given UAD at 10, 20, and 40 mg/kg/day in the drinking water during 3 months. Animals in three of these groups were also subjected to restraint for 2 h/day during the same period. Control groups included restrained and unrestrained male rats not exposed to UAD. To evaluate the fertility, male rats were mated with untreated females for 2 weeks. Although body weight was not affected by uranium at any dose, there was a significant (not dose-related) decrease in the pregnancy rate. Moreover, spermatid number/testis was significantly decreased by uranium administration. Histopathological examination of the testes in rats killed after 3 months of treatment revealed few differences in the tubule and interstitial alterations (focal atrophy, binucleated cells) between control and uranium-exposed animals. The results of this investigation show that at the current UAD doses, restraint stress did not enhance the uranium-induced adverse effects on reproduction in male rats.

Concurrent exposure to aluminum and stress during pregnancy in rats: Effects on postnatal development and behavior of the offspring

The present study was conducted to assess the potential combined influence of maternal restraint stress and aluminum (Al) exposure on postnatal development and behavior in the offspring of exposed rats. Female rats were concurrently exposed to 0 (control group), 50 or 100 mg/kg/day of Al administered as Al nitrate nonahydrate in drinking water with citric acid (355 or 710 mg/kg/day) for a period of 15 days prior to mating with untreated males. Aluminum exposure was maintained throughout the gestational, lactational and post-weaning periods. On days 6-20 of gestation, one-half of the pregnant animals in each group were restrained for 2 h/day. Food consumption and maternal body weight were decreased in the groups exposed to restraint only or combined with the highest Al dose. All of the animals were allowed to deliver and wean their offspring. The pups were evaluated for physical development and neuromotor maturation. Moreover, open-field activity, passive avoidance, and spatial learning in a water maze were also determined on postnatal days 30, 35 and 60, respectively. Body weight of pups treated with 100 mg/kg/day of Al was decreased relative to controls from postnatal day 12 through 21, sexual maturation was delayed in Al treated females and in males exposed to 100 mg/kg/day. Forelimb grip strength was reduced in males exposed to 100 mg/Al/kg/day and in females exposed to this Al dose plus prenatal restraint. Learning in a passive avoidance task indicated facilitated performance for Al treated rats at 100 mg/kg/day combined with prenatal restraint as evidenced by longer avoidance latencies, while learning in a water maze task showed a shorter latency to find the platform on acquisition day 2 for Al treated rats. However, no effects of Al on water maze performance were detected during the retention probe trial in which the only effect noted was an increase in the platform quadrant swim time for the prenatal restraint group. In general terms, the results of the present study did not show a notable influence of maternal restraint on the Al-induced postnatal developmental and behavioral effects in the offspring of prenatally Al-exposed rats.

Effects of prenatal exposure to chronic mild stress and toluene in rats

The aim of the present study was to elucidate whether prenatal chronic stress, in combination with exposure to a developmental neurotoxicant, would increase effects in the offspring compared with the effects of either exposure alone. Development and neurobehavioral effects were investigated in female offspring of pregnant rats (Mol:WIST) exposed to chronic mild stress (CMS) during gestational days (GD) 9-20, or 1500 ppm toluene, 6 h/day during gestational days 7-20, or a combination of the two. Prenatal CMS was associated with decreased thymic weight and increased auditory startle response. The corticosterone response to restraint seemed modified by prenatal exposure to toluene. Lactational body weight was decreased in offsprings subjected to CMS, primarily due to effects in the combined exposure group. Cognitive function was investigated in the Morris water maze, and some indications of improved function due to CMS were observed. In the present experimental setting, there was no indication of the two exposures potentiating each other with respect to adverse effects on the nervous system. However, the effects of prenatal CMS indicate that stress during fetal life may interfere with the development of the thymus and increase the reactivity (startle reflex) of the offspring.

Effects of restraint stress in gestation: implications for rodent developmental toxicology studies

Restraint has been used as a procedure to study the effects of stress on gestation outcome in rodents. The effects of restraint could potentially be used as a model for the impact of general stress produced by high doses of toxicants and other interventions. In mice, restraint in the peri-implantation period leads to implantation failure, and restraint at appropriate times in organogenesis produces cleft palate, supernumerary ribs, and resorption. In rats, there is some evidence for an association with restraint for implantation failure, but not for the morphological anomalies. Restraint in late gestation alters adult sexual behavior of male rat offspring, but consequences for their fertility are not known. Intrauterine growth retardation is not commonly associated with gestational restraint. In the few studies where they have been directly compared, different restraint procedures produced graded, qualitatively different, or no effects. Adrenocortical hormones have been implicated as mediating the effect of restraint on cleft palate, but not on supernumerary ribs, implantation failure, or sexual differentiation. Given the variety of restraint procedures and the varying species-dependent consequences, it is not possible to infer a generalizable pattern of developmental effects due to gestational stress from the restraint literature. As an alternative approach, contemporary methods in gene expression and developmental biology could profitably be applied to understanding different patterns of stress-mediated effects of toxicant exposures on intrauterine development.

Effects of prenatal exposure to manganese on postnatal development and behavior in mice: influence of maternal restraint

Manganese (Mn) is an essential trace element whose deficiency and excess have been reported to cause central nervous system (CNS) disturbances. On the other hand, during pregnancy, maternal stress has been shown to enhance the developmental toxicity of a number of metals. In this study, the maternal toxicity and developmental effects of a concurrent exposure to Mn and restraint stress were evaluated in mice. Pregnant animals were divided into three groups and received subcutaneous injections of manganese chloride tetrahydrate (MnCl2.4H2O) at 0, 1 and 2 mg/kg/day on Gestation Days 6-18. Each group was divided into two subgroups. Mice in one subgroup were subjected to restraint for 2 h/day on Days 6-18 of gestation. Pregnant mice were allowed to deliver, and pups were evaluated for physical and neuromotor maturation. Subsequently, adult mice were also evaluated for activity and learning. A significant increase in perinatal mortality was observed at 2 mg/kg/day Mn. A delay in some developmental landmarks (eye opening, testes descent) due to Mn exposure (2 mg/kg/day) was also seen in both restrained and unrestrained animals. No differences in motor resistance and coordination, or in learning at the passive avoidance test, were noted in adult mice. At the current Mn doses, combined exposure to Mn and stress during the prenatal period did not produce long-lasting effects on adult mice.

Hazard characterisation of chemicals in food and diet. dose response, mechanisms and extrapolation issues

Hazard characterisation of low molecular weight chemicals in food and diet generally use a no-observed-adverse-effect level (NOAEL) or a benchmark dose as the starting point. For hazards that are considered not to have thresholds for their mode of action, low-dose extrapolation and other modelling approaches may be applied. The default position is that rodents are good models for humans. However, some chemicals cause species-specific toxicity syndromes. Information on quantitative species differences is used to modify the default uncertainty factors applied to extrapolate from experimental animals to humans. A central theme for extrapolation is unravelling the mode of action for the critical effects observed. Food can be considered as an extremely complex and variable chemical mixture. Interactions among low molecular weight chemicals are expected to be rare given that the exposure levels generally are far below their NOAELs. Hazard characterisation of micronutrients must consider that adverse effects may arise from intakes that are too low (deficiency) as well as too high (toxicity). Interactions between different nutrients may complicate such hazard characterisations. The principle of substantial equivalence can be applied to guide the hazard identification and hazard characterisation of macronutrients and whole foods. Macronutrients and whole foods must be evaluated on a case-by-case basis and cannot follow a routine assessment protocol.

Effects of social housing condition on chemotherapeutic efficacy in a Shionogi carcinoma (SC115) mouse tumor model: influences of temporal factors, tumor size, and tumor growth rate

OBJECTIVE

The objective of this study was to investigate 1) whether social housing condition, tumor size, and tumor growth rate alter responses to chemotherapy and 2) whether the timing of tumor cell injection or chemotherapy initiation (relative to housing condition formation) influences tumor growth rate or the efficacy of chemotherapy.

METHODS

Mice were reared individually (I) or in groups (G). In experiment 1, mice were rehoused (IG or GI) or left in group housing (GG) immediately after tumor cell injection. In experiment 2, housing conditions (II, IG, GG, or GI) were formed when tumors weighed 1 g. Chemotherapy (adriamycin 4 mg/kg and cyclophosphamide 61.5 mg/kg IP) and exposure to acute novelty stress (15 min/d, 5 d/wk) were initiated 1 day after housing condition formation.

RESULTS

If chemotherapy was initiated when the tumor burden was undetectable (experiment 1), housing condition did not alter tumor response to chemotherapy, although IG mice lost the most weight and overall had the lowest probability of survival. If chemotherapy was initiated when tumors weighed 1 g (experiment 2), both tumor and host responses to chemotherapy were poorest for IG mice. Timing of tumor cell injection relative to housing condition formation also differentially influenced the rate of tumor growth in mice treated with the drug vehicle; in experiment 1, tumor growth rate was faster in GI and GG mice than in IG mice, whereas in experiment 2, the rate of tumor growth was faster in II mice than in GG and IG mice.

CONCLUSIONS

Altering the temporal relationships among social housing condition formation, tumor cell injection, and chemotherapy initiation differentially influences the rate of tumor growth and the efficacy of chemotherapy. Effects of housing condition are independent of tumor growth rate at chemotherapy initiation and, in terms of host responses, independent of tumor burden.

Evaluation of a novel assay of potential toxicity/neurotoxicity of carpet emissions (VOCs) in mice

A private testing laboratory utilizing the whole-body plethysmograph/head-only exposure apparatus outlined in the respiratory irritation assay ASTM E981-84, along with a novel exposure regimen, has reported neurotoxic effects and mortality in mice exposed to relatively low levels of volatile organic compounds (VOCs) emitted from a number of consumer products. This methodology was evaluated by exposing groups of mice, including unrestrained and sham-treated animals, to VOCs generated from a sample of carpet reported to be neurotoxic using the modified assay. General toxicological (haematological measurements, organ weights, gross pathology, histopathology) and specific neurotoxicity (functional observations, body temperature, histopathology of nervous tissues) parameters were evaluated. No effects related to exposure to carpet VOCs were observed in the mice. However, despite careful handling, a number of effects were observed which were attributed to the repeated restraint of mice in the ASTM E981 apparatus. These included a number of minor physical injuries, decreased body weights, altered thymus weights, compression damage to the liver and haemorrhage of the pituitary gland. It was concluded that the modification of the original ASTM E981 methodology may result in physical injuries and stress which may significantly affect any evaluation of toxicity and neurotoxicity in treated animals and result in inaccurate conclusions.

Effects of maternal stress on methylmercury-induced developmental toxicity in mice

The developmental toxicity of combined exposure to maternal restraint stress and methylmercury chloride (MMC) was assessed in Swiss mice. On day 10 of gestation, four groups of plug-positive female mice were treated (p.o.) with a single dose of 12.5 or 25 mg MMC/kg. Immediately after MMC exposure, two of those groups were subjected to restraint for 14 hr. Control groups included restrained and unrestrained pregnant mice nonexposed to MMC. Combined exposure to 25 mg MMC/kg and restraint enhanced MMC-induced maternal toxicity, which included deaths and decreased body weight gain and food consumption. The number of nonviable implants was also increased significantly following concurrent exposure to MMC (25 mg/kg) and restraint, with the percentage of postimplantation loss increased from 64% (MMC alone) to 100% (MMC plus restraint). However, the types and incidence of internal and skeletal anomalies observed after administration of 12.5 mg MMC/kg were not increased by maternal restraint. These results suggest that maternal stress would enhance the MMC-induced maternal and embryo/fetal toxicity at doses of MMC that are highly toxic to the dams, whereas at doses that are less acutely toxic the role of maternal stress would not be significant.

Enhancement of the teratogenicity of all-trans-retinoic acid by maternal restraint stress in mice as a function of treatment timing

In CD-1 mice, maternal restraint stress was combined with all-trans-retinoic acid (tRA) given during the restraint period (9:00 a.m. to 9:00 p.m.) to determine in what manner and to what degree teratogenesis might be affected by treatment timing within the stress period and to determine the optimum timing for stress-enhanced production of fetal defects. Eleven groups were treated on gestation day 9 (copulation plug = day 1): group 1, vehicle (corn oil) control (C); group 2, food/water deprived (FWD); group 3, restraint only (R); group 4, tRA plus food/water deprivation (tRA+FWD); groups 5 and 6, tRA at 0 or 4 hr after 9:00 a.m., i.e., tRA(0) and tRA(4), respectively; and groups 7-11, restraint plus tRA at 0, 2, 4, 8, or 12 hr after 9:00 a.m., (i.e., R+tRA(0), R+tRA(2), R+tRA(4), R+tRA(8), and R+tRA(12), respectively). The tRA dose was 20 mg/kg, PO; mice were restrained in the supine position. FWD mice were deprived for the same 12 hr as the restrained mice. All stated differences were significant (P < or = 0.05), based on litter incidences. The incidences of short tails (65%), fused ribs (62%), and fused vertebrae (37%) were elevated in the R+tRA(4) group in comparison with all others, and there appeared to be more exencephalies in R+tRA(2) litters than in any others. The incidence of supernumerary ribs was elevated in the R group in comparison with C and FWD; it was further elevated by tRA at all treatment times.(ABSTRACT TRUNCATED AT 250 WORDS)