Grantee Research Project Results
Final Report: Combined Effects of Metals and Stress on Central Nervous System Function
EPA Grant Number: R834578Title: Combined Effects of Metals and Stress on Central Nervous System Function
Investigators: Cory-Slechta, Deborah , Korfmacher Smith, Katrina
Institution: University of Rochester School of Medicine and Dentistry
EPA Project Officer: Hahn, Intaek
Project Period: October 1, 2010 through September 30, 2014 (Extended to September 30, 2015)
Project Amount: $1,250,000
RFA: Understanding the Role of Nonchemical Stressors and Developing Analytic Methods for Cumulative Risk Assessments (2009) RFA Text | Recipients Lists
Research Category: Human Health
Objective:
The overarching goal of this project was to begin to determine the potential for enhanced (i.e., cumulative) neurotoxicity of chemical and non-chemical stressors. It was based on the hypothesis that such enhancements could occur where either co-occurring or sequentially occurring stressors (risk factors) operated on the same biological systems, resulting in a convergence of effects downstream. The hypothesis stemmed from our previous studies in rat models demonstrating enhancement of some behavioral and neurotoxic effects of combined exposures to lead and to prenatal stress, as both of these risk factors act on brain mesocorticolimbic dopamine/glutamate (MESO) systems and on the hypothalamic-pituitary-adrenal (HPA) axis. The studies carried out with this funding, in conjunction with NIH funds, collectively sought to determine the generality of that hypothesis in a series of questions:
- Determine whether the enhanced behavioral toxicity of combined lead and prenatal stress as observed using fixed interval schedule controlled behavior, would generalize to other cognitive behaviors controlled by brain MESO systems, including assessment of impulsivity, a component of attention deficit.
- Determine whether enhanced effects of lead when combined with prenatal stress also would be seen (i.e., generalize) with other neurotoxic metals that likewise act on brain MESO systems and the HPA axis, achieved by examining methylmercury (MeHg) and prenatal stress under multiple conditions.
- Determine whether enhanced effects of lead and MeHg with prenatal stress also would occur with the use of other stress paradigms, and whether toxicity would be even further (cumulatively) enhanced by the combination of developmental exposure to a neurotoxic metal in conjunction with prenatal stress as well as with direct stress challenges in offspring. For this purpose, a series of experiments with the use of multiple metals (lead, MeHg), multiple behavioral paradigms related to domains of cognitive (impulsivity, Fixed Interval schedule-controlled behavior, novel object recognition), and different stress paradigms. These studies also specifically compared the impact of early behavioral adversity stress vs. early behavioral positive (non-stressful) experience in offspring.
- Determine whether enhanced toxicity of neurotoxic metals and stress are seen in mouse models and thus generalize beyond rat models. Our community-based participatory research component also focused on cumulative risk, specifically attempting to determine means of conveying this concept to communities.
Thus, the proposed studies sought to begin to determine the sets of biological conditions under which synergies would be expected, i.e., to begin to ascertain a biological algorithm to assist in defining appropriate biological rationales for exposures warranting consideration of cumulative risk conditions, thereby providing a strategy for advancing associated methods for human health protection.
Summary/Accomplishments (Outputs/Outcomes):
1. Does the enhanced behavioral toxicity of lead and prenatal stress seen with fixed interval schedule-controlled behavior extend to other executive function-type behaviors mediated by MESO circuitry? To address this, other behavioral profiles, including repeated learning and delay discounting, were examined.
Interestingly, enhanced behavioral toxicity was seen for lifetime lead exposure (beginning 2 months prior to breeding and continued for the duration of the experiment) and prenatal stress in a measure of repeated learning, i.e., a requirement to learn new three member response sequences in each successive experimental session, but only in females. Enhanced learning impairments were observed particularly for the sequences that were inherently more difficult to learn, and were primarily measured in an increase in the mean number of responses to earn each reward delivery. Furthermore, analyses of changes in brain biochemical and neurochemical markers revealed that this enhanced behavioral toxicity was, in fact, linked to the biological substrates on which lead and prenatal stress are known to act, specifically, plasma corticosterone levels, frontal cortex dopamine levels, and nucleus accumbens dopamine turnover, all of which were interrelated (Cory-Slechta, et al., 2010, Tox Sci 117:427-438).
In contrast to the females, however, males actually showed enhanced learning on the repeated learning paradigm under conditions of lead exposure (either maternal or lifetime) in combination with stress (Cory-Slechta, et al., 2012). Analyses of behavioral mechanisms showed this derived from the already higher response rates evoked by lead, stress, and the combination, meaning that they were obtaining a higher reinforcement density. While this might seem counterintuitive, those higher response rates were observed in males in other behavioral paradigms where they cause detrimental effects, meaning they are very behavioral context dependent.
We examined another behavior, self-control (impulsivity), using a delayed discounting task also influenced by brain MESO systems (Weston, et al., 2014). In this case, impaired function was seen exclusively in males where lead ± stress slowed acquisition of delayed discounting performance and increased numbers of failures to initiate trials or increased latency to initiate trials, findings consistent with impaired learning/behavioral flexibility during the transition across behavioral testing conditions. These effects were associated with reductions in serotonin, alterations in monoamine profiles and reductions in BDNF and NMDAR 2A levels in brain MESO systems. In this case, however, stress did not enhance the lead effects, and these deficits were produced by both lead and prenatal stress, but not further enhanced by the combination. Thus, certain behaviors may be more vulnerable to cumulative behavioral toxicity and this appears to be sex-dependent.
2. Does the enhanced toxicity seen following combined exposures to lead and prenatal stress extend to other neurotoxic metals that act on brain MESO systems? To address this question, we extended studies to examine the effects of developmental MeHg exposures combined with stress in both rats and mice.
Female-specific and non-monotonic enhancement and unmasking of MeHg neurotoxicity by prenatal stress were observed in rats (Weston, et al., 2014), with behavioral toxicity that included impaired learning and short-term memory. Similarly, many MeHg-induced changes in brain monoamine levels were only seen under conditions of combined MeHg and prenatal stress, as in striatum in males, and in hippocampus and nucleus accumbens in females. Notably, nucleus accumbens and hippocampus are critical to mediation of both learning and short-term memory, suggesting their potential involvement in the behavioral toxicity. Mechanisms by which prenatal stress enhances MeHg neurotoxicity remain to be determined, but do not appear to arise from increases in brain Hg levels, nor differences in pre-behavioral testing levels of corticosterone.
MeHg-induced behavioral toxicity also was found to be enhanced and/or unmasked by prenatal stress in mice at even lower MeHg levels), in this case in both sexes (manuscript in preparation), with impaired learning effects of MeHg + prenatal stress most evident in males (data presented at the 2014 SOT meeting). Correspondingly, in analyses completed to date, MeHg-induced changes in monoamines in frontal cortex, a region key to learning, were enhanced by prenatal stress in males. Non-monotonic effects of MeHg (greater effects at lower doses) were seen in mice as had also been seen in rats.
3. Does the enhanced toxicity of lead with stress extend to stressors other than maternal prenatal stress, such as stressors to offspring? If so, how do different stressors influence the outcome? To address this question, we undertook studies examining early behavioral experience (negative vs. positive; positive vs. no experience) in offspring as a stressor and studied its impact on the consequences of metal ± prenatal stress effects across the life span.
Early stressful behavioral experiences have protracted negative consequences for the brain. Consequently, we compared the trajectory of effects of lifetime lead exposure ± prenatal stress on brain neurotransmitter levels and HPA axis function in offspring subjected to early negative behavioral experience (inescapable forced swim test) or to very positive behavioral experience (food rewarded performance on a fixed interval schedule) (Cory-Slechta, et al., 2013). The nature of that early behavioral experience was found to be a critical determinant of how lead ± prenatal stress influenced MESO neurotransmitters. Further, these neurotransmitter changes could be related directly to behavioral outcomes, particularly in males. Thus, early behavioral experience, not just prenatal stress, can be a nonchemical stressor that can modify lead exposure.
Of note, we have looked to determine whether early behavioral experience as a stressor influenced the effects of lead on learning in adulthood (Cory-Slechta, et al., in preparation; presented at 2014 SOT meeting). Those studies showed that positive behavioral experience yielded higher overall accuracy levels on a repeated learning paradigm than did early negative behavioral experience. Moreover, the detrimental effects of early negative behavioral stress were greater in females, with the greatest decrements in learning and in short-term memory (novel object recognition) paradigm seen in females who also had experienced maternal lead exposure + prenatal stress + early negative behavioral experience, consistent with a cumulative effect.
To ascertain the generality of early behavioral experience as a modifier of neurotoxic metal effect, we also carried out similar studies to those described above in mice that had been developmentally exposed to MeHg ± prenatal stress and subsequently given early positive behavioral experience that might mitigate effects of MeHg vs. no behavioral experience (Cory-Slechta, et al., in preparation). Results show that behavioral testing can alter the neurochemical consequences of MeHg relative to what occurs in the absence of behavioral testing, indicating an interaction of the neurochemical effects of behavioral experience with those of MeHg.
4. Do the enhanced effects produced by combined neurotoxic metals and stress generalize beyond rat models, i.e., are they also seen in mouse models?
Our earlier studies demonstrated enhanced behavioral toxicity of maternal lead exposure and prenatal stress on behavior measured on a fixed interval schedule of food reward, which were seen as increased overall response rates, an outcome that has no impact on numbers or timing of reward delivery and is thus inefficient. These effects were seen primarily in females, with some increases in overall rate produced by lead alone, but a further increase was seen in offspring that had been subjected to prenatal stress as well as a variety of offspring stress challenges. Importantly, we repeated this study using a mouse model (Cory-Slechta, et al., in preparation), and in the mouse model, selective increases in overall response rates occurred in response to combined lead + prenatal stress, but not to either alone, with a more prominent effect in males.
As described above, we also have now examined the effect of early negative behavioral experience as a modifier of lead in both mouse (Cory-Slechta, et al., 2013) and rat (Cory-Slechta, et al., in preparation) models. These have provided clear evidence that in both species, behavioral experience can modify the trajectory of effects of lead, prenatal stress, and the combination. These studies, which provide an even closer simulation of the trajectory of human conditions, set the stage for further studies aimed at defining what behavioral experiences may further exacerbate vs. mitigate the consequences of lead ± prenatal stress.
In addition, we presented a comparison of the impacts of MeHg ± prenatal stress in rats vs. mice (2015 SOT meeting); while in rats, females were impacted to a greater extent, a particular vulnerability of male mice to behavioral toxicity was unmasked by combined exposure to MeHg and prenatal stress.
5. Community Based Participatory Research
The CBPR project focused on disseminating information to identified key audiences (particularly perinatal health care and service providers) about interactions between chemical and non-chemical exposures, and worked with CBPR teams from other STAR CRA projects to integrate lessons learned from their experiences, which has been published in a joint journal article. The focus groups conducted in Rochester helped to clarify a theme that emerged from several of the CBPR projects: communicating about cumulative risks is complex due to the uncertainty, sensitivity, and complexity of exposure to chemical and non-chemical stressors, particularly in environmental justice communities. This suggests that communication strategies may be different from many other environmental health issues, and need to be carefully considered based on the interests, background, and capacity of the audience. Follow-up efforts to the CBPR project have tested and implemented these observations with medical professionals, high-risk pregnant women, and others.
Conclusions:
Summary and Public Health Significance: Collectively, these findings indicate that combined exposures to metals and stressors that share biological substrates (here the HPA axis and brain MESO circuitry) and that produce common adverse effects (cognitive deficits) can produce enhanced toxicity, or unmask the effects of chemical exposures, consistent with cumulative risk, providing proof of principle of the stated hypothesis of this study. The enhanced toxicity is not specific to lead exposures, but occurs with MeHg exposure combined with prenatal stress as well, suggesting the generality of this result. In addition, the enhanced neurotoxicity of developmental exposures to lead and MeHg are not restricted to prenatal stress, i.e., maternal stress, but can be exhibited under conditions of stress to the offspring, in this case early behavioral adversity vs. early behavioral positive experience. Importantly, our studies repeatedly demonstrate that such enhanced neurotoxicity is highly sex-dependent, not with sex-selective effects, but with different profiles of effects by sex. In some studies, a further enhancement of effects can be seen under conditions of developmental exposures to metals and prenatal stress combined with offspring stress. The findings have significant implications for: 1) defining susceptible populations, and 2) providing a biological basis for assessing what mixtures of chemicals and/or non-chemical stressors should be included in a cumulative risk paradigm.
Journal Articles on this Report : 7 Displayed | Download in RIS Format
Other project views: | All 43 publications | 7 publications in selected types | All 7 journal articles |
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Cory-Slechta DA, Weston D, Liu S, Allen JL. Brain hemispheric differences in the neurochemical effects of lead, prenatal stress, and the combination and their amelioration by behavioral experience. Toxicological Sciences 2013;132(2):419-430. |
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Cory-Slechta DA, Merchant-Borna K, Allen JL, Liu S, Weston D, Conrad K. Variations in the nature of behavioral experience can differentially alter the consequences of developmental exposures to lead, prenatal stress, and the combination. Toxicological Sciences 2013;131(1):194-205. |
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Kraft AD, Aschner M, Cory-Slechta DA, Bilbo SD, Caudle WM, Makris SL. Unmasking silent neurotoxicity following developmental exposure to environmental toxicants. Neurotoxicology and Teratology 2016;55:38-44. |
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Payne-Sturges DC, Korfmacher KS, Cory-Slechta DA, Jimenez M, Symanski E, Carr Shmool JL, Dotson-Newman O, Cloughtery JE, French R, Levy JI, Laumbach R, Rodgers K, Bongiovanni R, Scammell MK. Engaging communities in research on cumulative risk and social stress-environment interactions: lessons learned from EPA's STAR Program. Environmental Justice 2015;8(6):203-212. |
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Sobolewski M, Conrad K, Marvin E, Allen J, Cory-Slechta D. Endocrine active metals, prenatal stress and enhanced neurobehavioral disruption. HORMONES AND BEHAVIOR 2018;101:36-49. |
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Weston HI, Sobolewski ME, Allen JL, Weston D, Conrad K, Pelkowski S, Watson GE, Zareba G, Cory-Slechta DA. Sex-dependent and non-monotonic enhancement and unmasking of methylmercury neurotoxicity by prenatal stress. Neurotoxicology 2014;41:123-140. |
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Weston HI, Weston DD, Allen JL, Cory-Slechta DA. Sex-dependent impacts of low-level lead exposure and prenatal stress on impulsive choice behavior and associated biochemical and neurochemical manifestations. Neurotoxicology 2014;44:169-183. |
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Supplemental Keywords:
sensitive populations, environmental justice, stressors, public policy, neuroscienceRelevant Websites:
Cory-Slechta Lab | University of Rochester Medical Center Exit
Progress and Final Reports:
Original AbstractThe perspectives, information and conclusions conveyed in research project abstracts, progress reports, final reports, journal abstracts and journal publications convey the viewpoints of the principal investigator and may not represent the views and policies of ORD and EPA. Conclusions drawn by the principal investigators have not been reviewed by the Agency.