Multiple chemical exposures: synergism vs. individual exposure levels

Maternal and early life stress effects on immune function: relevance to immunotoxicology

Stress is triggered by a variety of unexpected environmental stimuli, such as aggressive behavior, fear, forced physical activity, sudden environmental changes, social isolation or pathological conditions. Stressful experiences during very early life (particularly, maternal stress during fetal ontogeny) can permanently alter the responsiveness of the nervous system, an effect called programming or imprinting. Programming affects the hypothalamic-pituitary-adrenocortical (HPA) axis, brain neurotransmitter systems, sympathetic nervous system (SNS), and the cognitive abilities of the offspring, which can alter neural regulation of immune function. Prenatal or early life stress may contribute to the maladaptive immune responses to stress that occur later in life. This review focuses on the effect of maternal and early life stress on immune function in the offspring across life span. It highlights potential mechanisms by which prenatal stress impacts immune functions over life span. The literature discussed in this review suggests that psychosocial stress during pre- and early postnatal life may increase the vulnerability of infants to the effects of immunotoxicants or immune-mediated diseases, with long-term consequences. Neural-immune interactions may provide an indirect route through which immunotoxicants affect the developing immune system. A developmental approach to understanding how immunotoxicants interact with maternal and early life stress-induced changes in immunity is needed, because as the body changes physiologically across life span so do the effects of stress and immunotoxicants. In early and late life, the immune system is more vulnerable to the effects of stress. Stress can mimic the effects of aging and exacerbate age-related changes in immune function. This is important because immune dysregulation in the elderly is more frequently and seriously associated with clinical impairment and death. Aging, exposure to teratogens, and psychological stress interact to increase vulnerability and put the elderly at the greatest risk for disease.

Enhancement of developmental toxicity effects of chemicals by gestational stress. A review

Risk assessment of developmental toxicants is almost exclusively based on single chemicals studied in animals under controlled experimental conditions, as to reduce stress. Although humans may be exposed simultaneously to numerous hazards, little is known about the interaction of prenatal chemical exposures with other factors, such as maternal stress, itself a modifier of fetal development. Gestational stress has been hypothesized to enhance the developmental toxicity of chemicals. This review identified 36 animal studies investigating if maternal stress may enhance the effects of prenatal chemical exposure, and evaluated the presented hypothesis. Studies of a broad range of chemicals and developmental endpoints support the notion, that maternal stress is able to enhance the effects of developmental toxicants, although stress mitigated chemically induced effects in a few cases. Maternal stress most often enhanced chemical developmental toxicity at dose levels associated with severe maternal toxicity or where test agents were already above threshold for effect. Thus, LOAEL(chemical) was generally similar to LOAEL(chemical+stress), although not necessarily for the same endpoint. It should be noted that the database contained a limited number of studies, and only a single high dose level was applied in most studies, rendering establishment of NOAELs for combined exposures impossible. Furthermore, for some compounds, the margin between human exposure levels and the LOAEL(chemical+stress) seems small. Future studies are recommended to investigate compounds, for which maternal stress was already proven as an enhancer, at lower dose levels. Interactive response seems to depend on stressor severity and timing of chemical exposure relative to maternal stress which should be further scrutinized.

Developmental toxicity of combined ethylbenzene and methylethylketone administered by inhalation to rats

Pregnant Sprague-Dawley rats were exposed to ethylbenzene (EB; 0, 250, or 1000 ppm) and methylethylketone (MEK; 0, 1000, or 3000 ppm), alone and in combination, by inhalation, for 6h/day, during days 6-20 of gestation. Maternal toxicity, evidenced by decreased in body weight gain and food consumption, tended to be greater after simultaneous exposures to the high concentrations of 1000 ppm EB and 3000 ppm MEK, when compared to the treatments with individual compounds. No significant increase in embryo/fetal lethality or incidence of malformations and variations was observed in any of the treatment groups. Fetal body weight was significantly reduced after individual treatment with 1000 ppm EB or 3000 ppm MEK, and in the combined groups. There was no evidence of interaction between EB and MEK in causing developmental toxicity.

Interactive effects of cadmium and all-trans-retinoic acid on the induction of forelimb ectrodactyly in C57BL/6 mice

BACKGROUND

Most toxicological studies have tested single chemical agents at relatively high doses, and fewer studies have addressed the toxic effects of chemical interactions. It is important to understand the toxicity of chemical mixtures in order to assess the more realistic risks of environmental and occupational exposures. A number of chemicals are known to induce a predominantly postaxial forelimb ectrodactyly in C57BL/6 mice, including acetazolamide, ethanol, cadmium, valproic acid, carbon dioxide, dimethadione, phenytoin, and 13-cis-retinoic acid and all-trans-retinoic acid (RA). In the present study, the interactive effects of coadministration of cadmium and RA on developing limbs were investigated.

METHODS

Pregnant C57BL/6 mice were treated with different intraperitoneal (IP) doses of cadmium chloride (CdCl2) and/or RA on gestational day (GD) 9.5, and fetuses were collected on GD 18 and double stained for examination of skeletal defects.

RESULTS

When RA was given simultaneously with cadmium, a significant increase in the incidence and severity of forelimb ectrodactyly (predominantly postaxial) was observed compared to the results with corresponding doses of cadmium or RA alone. When mice were exposed to subthreshold doses of both cadmium (0.5 mg/kg) and RA (1 mg/kg), the combined treatment exceeded the threshold, resulting in forelimb ectrodactyly in 19% of the fetuses. Moreover, coadministration of cadmium and RA at doses exceeding the respective thresholds showed a synergistic effect, that is, 92% of fetuses were found with the forelimb defect as opposed to 10% if the response were additive.

CONCLUSIONS

The findings demonstrate that concurrent exposure to these teratogens can have a synergistic effect and that subteratogenic doses may combine to exceed a threshold.

ADVANCES IN RISK ASSESSMENT AND COMMUNICATION

Risk analysis continues to evolve. There is increasing depth and breadth to each component of the four-step risk-assessment paradigm of hazard identification, dose-response analysis, exposure assessment, and risk characterization. Basic conceptual approaches to understanding how people perceive risk are being tested against a growing body of empirical observations, many involving stakeholders. Emerging ideas such as the precautionary principle have provided challenges that have led to a rethinking of the role of risk assessment in environmental health. Newer problems, such as intergenerational issues posed by long-lasting radiation pollution, environmental justice, and the assessment and communication of risks related to terrorism, have spurred innovative approaches to risk analysis.

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.

Effect of co-administration of retinoids on rat embryo development in vitro

BACKGROUND

Excess retinyl palmitate (vitamin A) induces a variety of malformations in many mammalian species, yet retinyl palmitate is not thought to be the proximate teratogen. Many metabolites are generated after oral dosing several of which are, individually teratogenic. It is not known whether these metabolites when present simultaneously interact in an additive or synergistic manner.

METHODS

Whole rat embryos were cultured with retinol, all-trans retinoic acid (tRA), 4-oxo all-trans retinoic acid (4otRA) or 9-cis retinoic acid (9cRA). Retinoids were added singly or in paired combinations at the start of culture and embryos were examined for evidence of dysmorphogenesis at the end of culture. Retinoids were added at both subthreshold and supra-threshold concentrations.

RESULTS

Co-administration of subthreshold concentrations of tRA and 4otRA, retinol and tRA or 4otRA and 4otRA and 9cRA resulted in a greater frequency of abnormal embryos compared to single administration of each retinoid.

CONCLUSIONS

Knowledge of the way in which retinyl palmitate metabolites interact should refine our ability to predict risk from a potentially teratogenic event e.g., accidental overdose with RP during human pregnancy.

Toxic effects of chemical mixtures

Exposures to chemical mixtures have reportedly produced unexpected effects. Examination of new case studies, as well as those previously reported, shows that when the human body is exposed to mixtures of chemicals that include lipophilic and hydrophilic species, the lipophiles facilitate the absorption of the hydrophiles at enhanced levels and produce effects that are not expected from an individual chemical. These effects include enhanced acute and chronic responses, low-level concentration response, and unexpected target organ attack. Octanol:water partition coefficients are predictive of relative lipophilicity and hydrophilicity. The findings have implications for safe drinking water standards, air quality standards, safe industrial and environmental exposure levels, product formulation, product labeling, and protocols for toxicity testing of chemical products.

Interactions in developmental toxicology: combined action of restraint stress, caffeine, and aspirin in pregnant mice

BACKGROUND

Stress can result in an increased use of substances such as caffeine and aspirin. The effect of maternal stress on concurrent exposure to caffeine and aspirin on prenatal development was assessed in mice.

METHODS

On gestational day 9, mice were assigned to three treatment groups orally exposed to caffeine (30 mg/kg), aspirin (250 mg/kg), or a combination of caffeine (30 mg/kg) and aspirin (250 mg/kg). Three additional groups of pregnant animals received similar caffeine and aspirin doses and were immediately subjected to restraint for 14 hr. Control groups included unrestrained and restrained pregnant mice not exposed to caffeine or aspirin. All dams were euthanized on gestational day 18. Live fetuses were evaluated for sex, body weight, and external, internal, and skeletal malformations and variations.

RESULTS

A single oral dose of caffeine or aspirin did not cause significant maternal toxicity. However, coadministration of these drugs with restraint produced some adverse maternal effects (i.e., reduction in maternal weight gain and food consumption on gestational days 9-11). In relation to embryo/fetal toxicity, the incidence of some skeletal defects was significantly increased after exposure to caffeine, aspirin, or maternal restraint, and their binary and ternary combinations.

CONCLUSIONS

Although caffeine and aspirin were given in a single dose in this study, the results suggest that prenatal stress could slightly exacerbate the maternal and developmental toxicity of the combination of these drugs in mice.