Exposure of pregnant rats to uranium and restraint stress: effects on postnatal development and behavior of the offspring

Cumulative effects of prenatal-exposure to exogenous chemicals and psychosocial stress on fetal growth: Systematic-review of the human and animal evidence

BACKGROUND

Adverse effects of prenatal stress or environmental chemical exposures on fetal growth are well described, yet their combined effect remains unclear.

OBJECTIVES

To conduct a systematic review on the combined impact and interaction of prenatal exposure to stress and chemicals on developmental outcomes.

METHODS

We used the first three steps of the Navigation Guide systematic review. We wrote a protocol, performed a robust literature search to identify relevant animal and human studies and extracted data on developmental outcomes. For the most common outcome (fetal growth), we evaluated risk of bias, calculated effect sizes for main effects of individual and combined exposures, and performed a random effects meta-analysis of those studies reporting on odds of low birthweight (LBW) by smoking and socioeconomic status (SES).

RESULTS

We identified 17 human- and 22 animal-studies of combined chemical and stress exposures and fetal growth. Human studies tended to have a lower risk of bias across nine domains. Generally, we found stronger effects for chemicals than stress, and these exposures were associated with reduced fetal growth in the low-stress group and the association was often greater in high stress groups, with limited evidence of effect modification. We found smoking associated with significantly increased odds of LBW, with a greater effect for high stress (low SES; OR 4.75 (2.46-9.16)) compared to low stress (high SES; OR 1.95 (95% CI 1.53-2.48)). Animal studies generally had a high risk of bias with no significant combined effect or effect modification.

CONCLUSIONS

We found that despite concern for the combined effects of environmental chemicals and stress, this is still an under-studied topic, though limited available human studies indicate chemical exposures exert stronger effects than stress, and this effect is generally larger in the presence of stress.

The neurotoxicology of uranium

The brain is a target of environmental toxic pollutants that impair cerebral functions. Uranium is present in the environment as a result of natural deposits and release by human applications. The first part of this review describes the passage of uranium into the brain, and its effects on neurological functions and cognitive abilities. Very few human studies have looked at its cognitive effects. Experimental studies show that after exposure, uranium can reach the brain and lead to neurobehavioral impairments, including increased locomotor activity, perturbation of the sleep-wake cycle, decreased memory, and increased anxiety. The mechanisms underlying these neurobehavioral disturbances are not clearly understood. It is evident that there must be more than one toxic mechanism and that it might include different targets in the brain. In the second part, we therefore review the principal mechanisms that have been investigated in experimental models: imbalance of the anti/pro-oxidant system and neurochemical and neurophysiological pathways. Uranium effects are clearly specific according to brain area, dose, and time. Nonetheless, this review demonstrates the paucity of data about its effects on developmental processes and the need for more attention to the consequences of exposure during development.

Stress exposure during the preimplantation period affects blastocyst lineages and offspring development

We found retardation of preimplantation embryo growth after exposure to maternal restraint stress during the preimplantation period in our previous study. In the present study, we evaluated the impact of preimplantation maternal restraint stress on the distribution of inner cell mass (ICM) and trophectoderm (TE) cells in mouse blastocysts, and its possible effect on physiological development of offspring. We exposed spontaneously ovulating female mice to restraint stress for 30 min three times a day during the preimplantation period, and this treatment caused a significant increase in blood serum corticosterone concentration. Microscopic evaluation of embryos showed that restraint stress significantly decreased cell counts per blastocyst. Comparing the effect of restraint stress on the two blastocyst cell lineages, we found that the reduction in TE cells was more substantial than the reduction in ICM cells, which resulted in an increased ICM/TE ratio in blastocysts isolated from stressed dams compared with controls. Restraint stress reduced the number of implantation sites in uteri, significantly delayed eye opening in delivered mice, and altered their behavior in terms of two parameters (scratching on the base of an open field test apparatus, time spent in central zone) as well. Moreover, prenatally stressed offspring had significantly lower body weights and in 5-week old females delivered from stressed dams, fat deposits were significantly lower. Our results indicate that exposure to stress during very early pregnancy can have a negative impact on embryonic development with consequences reaching into postnatal life.

Modes of action associated with uranium induced adverse effects in bone function and development

Uranium, a naturally occurring element used in military and industrial applications, accumulates in the skeletal system of animals and humans. Evidence from animal and in-vitro studies demonstrates that uranium exposure is associated with alterations in normal bone functions. The available studies suggest that upon absorption uranium directly affects bone development and maintenance by inhibiting osteoblast differentiation and normal functions, and indirectly by disrupting renal production of Vitamin D. Animal studies also provide evidence for increased susceptibility to uranium-induced bone toxicity during early life stages. The objective of this review is to provide a summary of uranium-induced bone toxicity and the potential mechanisms by which uranium can interfere with bone development and promote fragility. Since normal Vitamin D production and osteoblast functions are essential for bone growth and maintenance, young individuals and the elderly may represent potentially susceptible populations to uranium-induced bone damage.

Role of the olfactory receptor neurons in the direct transport of inhaled uranium to the rat brain

Uranium presents numerous industrial and military uses and one of the most important risks of contamination is dust inhalation. In contrast to the other modes of contamination, the inhaled uranium has been proposed to enter the brain not only by the common route of all modes of exposure, the blood pathway, but also by a specific inhalation exposure route, the olfactory pathway. To test whether the inhaled uranium enter the brain directly from the nasal cavity, male Sprague-Dawley rats were exposed to both inhaled and intraperitoneally injected uranium using the (236)U and (233)U, respectively, as tracers. The results showed a specific frontal brain accumulation of the inhaled uranium which is not observed with the injected uranium. Furthermore, the inhaled uranium is higher than the injected uranium in the olfactory bulbs (OB) and tubercles, in the frontal cortex and in the hypothalamus. In contrast, the other cerebral areas (cortex, hippocampus, cerebellum and brain residue) did not show any preferential accumulation of inhaled or injected uranium. These results mean that inhaled uranium enters the brain via a direct transfer from the nasal turbinates to the OB in addition to the systemic pathway. The uranium transfer from the nasal turbinates to the OB is lower in animals showing a reduced level of olfactory receptor neurons (ORN) induced by an olfactory epithelium lesion prior to the uranium inhalation exposure. These results give prominence to a role of the ORN in the direct transfer of the uranium from the nasal cavity to the brain.

Uranium in drinking water: effects on mouse oocyte quality

The aim of this work was to evaluate the reproductive toxicological effects of uranium (U) at 2.5, 5, and 10 mgU/kg/d chronically administered in drinking water for 40 d. Swiss female control mice (n = 28) and mice chronically contaminated with uranyl nitrate in drinking water (n = 36) were tested. The number and quality of ovulated oocytes, chromatin organization, and nuclear integrity were evaluated. No significant differences were obtained in the numbers of ovulated oocytes between the different groups. Nevertheless, in 1,520 of the oocytes examined, dysmorphism increased from 11.99% in the control group to 27.99%, 27.19%, and 27.43% in each of the contaminated groups, respectively, in a dose-independent manner. On the other hand, morphological chromatin organization from 880 oocytes examined showed an increase in metaphase plate abnormalities from 37.20% (+/-7.21) in the control group to 55.13% (+/-21.36), 58.29% (+/-21.72), and 64.10% (+/-12.62) in each of the contaminated groups, respectively. Cumulus cell (CC) micronucleation, a parameter of nuclear integrity, increased from 0.21% (+/-0.31) in the control group to 1.92 (+/-0.95), 2.98 (+/-0.97), and 3.2 (+/-0.98), respectively. Both metaphase plate abnormalities and CC micronucleation showed an increase in a dose-dependent manner (r = 0.9; p < 0.001). The oocyte and its microenvironment showed high sensitivity to uranium contamination by drinking water. The lowest observed adverse effect level for this system is estimated at a level below 2.5 mgU/kg/d for female mice.