Neurobehavioral effects of lead and manganese individually and in combination in developmentally exposed rats

Sex-specific associations of a ferroalloy metal mixture with motor function in Italian adolescents

Background

Motor function is critical for children's health, yet remains an understudied neurodevelopmental domain. Exposure to metals has been linked with motor function, but no study has examined the joint effects of metal mixtures.

Methods

We evaluated cross-sectional associations between a metal mixture and motor function among 569 adolescents (10-14 years old) living near the ferroalloy industry. Concentrations of blood lead, hair manganese, hair copper, and hair chromium were quantified using inductively coupled plasma mass spectrometry. Neuropsychologists administered multiple fine motor function assessments: pursuit aiming, finger tapping, visual reaction time (VRT), and subtests from the Luria Nebraska battery. We estimated associations between motor function and the metal mixture using quantile-based g-computation and multivariable linear regression, adjusting for child age, sex, and socioeconomic status. We explored sex-specific associations in stratified models.

Results

Associations between the metal mixture and motor function were mostly null but were modified by sex. We observed a beneficial association among females: a quartile increase in all metals in the mixture was associated with a 2.6% faster average response time on the VRT (95% confidence interval [CI] = -4.7%, -0.5%), driven by Cu and Cr. In contrast, this association was adverse among males (ß = 1.5% slower response time [95% CI = -0.7%, 3.9%]), driven by Cu and Mn.

Conclusions

Results suggest that males may be more susceptible to the adverse effects of metal exposure on motor function during adolescence than females. Future studies, particularly prospective study designs, are warranted to further understand the associations of metal mixtures with motor function.

Relationship Between a High-Fat Diet, Reduced Mobility, and Trace Element Overload in the Olfactory Bulbs of C57BL/6J and DBA/2J Mice

The dysregulation of trace elements in the brain, which can be caused by genetic or environmental factors, has been associated with disease and compromised mobility. Research regarding trace elements and motor function has focused mainly on the basal ganglia, but few studies have examined the olfactory bulb in this context. Diets high in fat have been shown to have consequences of dysregulated iron and manganese in the brain and disrupted motor activity. The aim of our study was to examine the relationship between mobility and trace element disruption in the olfactory bulb in male and female C57BL/6J and DBA/2J mice fed a high-fat diet. Mobility was significantly reduced in male C57BL/6Js, but the correlation between iron and manganese in the olfactory bulb with velocity, distance travelled, and habituation was not statistically significant. However, there appears to be an overall pattern of a high-fat diet having a statistically significant impact individually on elevated iron and manganese in the olfactory bulb, reduced velocity, reduced distance travelled, and reduced habituation mainly in the male C57BL/6J strain. We found similar trends within the scientific literature to suggest that dysregulated trace element status in the olfactory bulb may be related to motor function in both humans and animals and that males may be more susceptible to the negative outcomes. Our findings contribute new information regarding the impact of diet on the brain, behavior, and potential connection between trace element dysregulation in the olfactory bulb with mobility.

Synergistic suppression of BDNF via epigenetic mechanism deteriorating learning and memory impairment caused by Mn and Pb co-exposure

Microglia, the resident immune cells of the central nervous system (CNS), play a dual role in neurotoxicity by releasing the NLR Family Pyrin Domain Containing 3 (NLRP3) inflammasome and brain-derived neurotrophic factor (BDNF) in response to environmental stress. Suppression of BDNF is implicated in learning and memory impairment induced by exposure to manganese (Mn) or lead (Pb) individually. Methyl CpG Binding Protein 2 (MeCp2) and its phosphorylation status are related to BDNF suppression. Protein phosphatase2A (PP2A), a member of the serine/threonine phosphatases family, dephosphorylates substrates based on the methylation state of its catalytic C subunit (PP2Ac). However, the specific impairment patterns and molecular mechanisms resulting from co-exposure to Mn and Pb remain unclear. Therefore, the purpose of this study was to explore the effects of Mn and Pb exposure, alone and in combination, on inducing neurotoxicity in the hippocampus of mice and BV2 cells, and to determine whether simultaneous exposure to both metals exacerbate their toxicity. Our findings reveal that co-exposure to Mn and Pb leads to severe learning and memory impairment in mice, which correlates with the accumulation of metals in the hippocampus and synergistic suppression of BDNF. This suppression is accompanied by up-regulation of the epigenetic repressor MeCp2 and its phosphorylation status, as well as demethylation of PP2Ac. Furthermore, inhibition of PP2Ac demethylation using ABL127, an inhibitor for its protein phosphatase methylesterase1 (PME1), or knockdown of MeCp2 via siRNA transfection in vitro effectively increases BDNF expression and mitigates BV2 cell damage induced by Mn and Pb co-exposure. We also observe abnormal activation of microglia characterized by enhanced release of the NLRP3 inflammasome, Casepase-1 and pro-inflammatory cytokines IL-1β, in the hippocampus of mice and BV2 cells. In summary, our experiments demonstrate that simultaneous exposure to Mn and Pb results in more severe hippocampus-dependent learning and memory impairment, which is attributed to epigenetic suppression of BDNF mediated by PP2A regulation.

Neurobehavioral deficits, histoarchitectural alterations, parvalbumin neuronal damage and glial activation in the brain of male Wistar rat exposed to Landfill leachate

Concerns about inappropriate disposal of waste into unsanitary municipal solid waste landfills around the world have been on the increase, and this poses a public health challenge due to leachate production. The neurotoxic effect of Gwagwalada landfill leachate (GLL) was investigated in male adult Wistar rats. Rats were exposed to a 10% concentration of GLL for 21 days. The control group received tap water for the same period of the experiment. Our results showed that neurobehavior, absolute body and brain weights and brain histomorphology as well as parvalbumin interneurons were severely altered, with consequent astrogliosis and microgliosis after 21 days of administrating GLL. Specifically, there was severe loss and shrinkage of Purkinje cells, with their nucleus, and severe diffused vacuolations of the white matter tract of GLL-exposed rat brains. There was severe cell loss in the granular layer of the cerebellum resulting in a reduced thickness of the layer. Also, there was severe loss of dendritic arborization of the Purkinje cells in GLL-exposed rat brains, and damage as well as reduced populations of parvalbumin-containing fast-spiking GABAergic interneurons in various regions of the brain. In conclusion, data from the present study demonstrated the detrimental effects of Gwagwalada landfill leachate on the brain which may be implicated in neuropsychological conditions.

Sensorimotor dysfunction due to developmental manganese exposure is less severe in adult female than male rats and partially improved by acute methylphenidate treatment

Epidemiological studies have reported associations between elevated manganese (Mn) exposure and poorer psychomotor performance in children. Our studies in adult male rats have established that this relationship is causal and that prolonged methylphenidate (MPH) treatment is efficacious in treating this area of dysfunction. However, it is unclear if sensitivity to these Mn deficits differs between females and males, and whether existing pharmacological therapies are efficacious in improving sensorimotor dysfunction in females. To address these questions, we used our rat model of childhood environmental Mn exposure and the Montoya staircase test to determine whether 1) there are sex differences in the lasting sensorimotor dysfunction caused by developmental Mn exposure, and 2) MPH treatment is efficacious in ameliorating the sensorimotor deficits in females. Female and male neonates were treated orally with Mn (50 mg Mn/kg/d) from postnatal day 1 to 21 and evaluated for skilled forelimb sensorimotor performance as adults. Subsequently, the efficacy of acute oral MPH treatment (doses of 0, 0.5, and 3.0 mg MPH/kg/d) was assessed in females using a within-subject MPH treatment design. Developmental postnatal Mn exposure produced lasting sensorimotor reaching and grasping deficits that were milder in females than in males. Acute MPH treatment of Mn-exposed females with the 0.5 mg/kg/d dose attenuated the reaching dysfunction without alleviating grasping dysfunction. These findings show sex-based variations in sensitivity to the sensorimotor impairment caused by developmental Mn exposure, and they are consistent with prior studies showing less vulnerability of females to Mn-induced dysfunction in other functional domains, possibly due to the protective effects of estrogen. Given our previous work showing the efficacy of MPH treatment to alleviate Mn-induced inattention, impulsiveness, and sensorimotor dysfunctions in adult male rats, they also highlight the need for further research into sex-based differences in cognitive and behavioral areas of brain function, and the efficacy of therapeutics in treating behavioral dysfunction in females. Supported by NIEHS R01ES028369.

Melatonin modulates the differentiation of neural stem cells exposed to manganese via SIRT1/β-catenin signaling

Excessive exposure of children to manganese (Mn) in the environment has a bearing on developmental neurotoxicity. Although melatonin (Mel) can play a neuroprotective role by modulating the differentiation of neural stem cells (NSCs) in the developing brain, its specific mechanism under Mn overexposure remains to be explored. Here, we cultured primary NSCs as an available model to investigate the relevant molecular mechanism of Mel mitigation on Mn-induced disorder of NSCs differentiation through sirtuin 1 (SIRT1)/β-catenin pathway. It was found that Mel could facilitate the differentiation of Mn-treated NSCs into neurons. Further, our results uncovered that the pro-differentiation mechanism of Mel depended upon ascending the activity of SIRT1, thereby weakening β-catenin acetylation and increasing phosphorylation of β-catenin ser675 in the cytoplasm, which facilitates the nuclear translocation of β-catenin. Furthermore, the role of SIRT1 in Mel-mediated signal transduction was investigated through the pretreatment of NSCs using a highly specific SIRT1 inhibitor, EX527. After EX527 pretreatment, Mel could not maintain its protective effect. Overall, our results revealed that Mel could alleviate Mn-induced disorder of NSCs differentiation through the activation of the SIRT1/β-catenin pathway.

Scientific opinion on the tolerable upper intake level for manganese

Following a request from the European Commission (EC), the EFSA Panel on Nutrition, Novel Foods and Food Allergens (NDA) was asked to deliver a scientific opinion on the tolerable upper intake level (UL) for manganese. Systematic reviews of the literature of human and animal data were conducted to assess evidence regarding excess manganese intake (including authorised manganese salts) and the priority adverse health effect, i.e. manganese-induced neurotoxicity. Available human and animal studies support neurotoxicity as a critical effect, however, data are not sufficient and suitable to characterise a dose-response relationship and identify a reference point for manganese-induced neurotoxicity. In the absence of adequate data to establish an UL, estimated background dietary intakes (i.e. manganese intakes from natural dietary sources only) observed among high consumers (95th percentile) were used to provide an indication of the highest level of intake where there is reasonable confidence on the absence of adverse effects. A safe level of intake of 8 mg/day was established for adults ≥ 18 years (including pregnant and lactating women) and ranged between 2 and 7 mg/day for other population groups. The application of the safe level of intake is more limited than an UL because the intake level at which the risk of adverse effects starts to increase is not defined.

Early life manganese exposure and reported attention-related behaviors in Italian adolescents

Background

Manganese (Mn) is an essential nutrient and neurotoxicant, and the neurodevelopmental effects of Mn may depend on exposure timing. Less research has quantitatively compared the impact of Mn exposure on neurodevelopment across exposure periods.

Methods

We used data from 125 Italian adolescents (10-14 years) from the Public Health Impact of Metals Exposure Study to estimate prospective associations of Mn in three early life exposure periods with adolescent attention-related behaviors. Mn was quantified in deciduous teeth using laser ablation-inductively coupled plasma-mass spectrometry to represent prenatal (2nd trimester-birth), postnatal (birth ~1.5 years), and childhood (~1.5-6 years) exposure. Attention-related behavior was evaluated using the Conners Behavior Rating Scales in adolescence. We used multivariable linear regression models to quantify associations between Mn in each exposure period, and multiple informant models to compare associations across exposure periods.

Results

Median tooth Mn levels (normalized to calcium) were 0.4 area under the curve (AUC) 55Mn:43Ca × 104, 0.1 AUC 55Mn:43Ca × 104, and 0.0006 55Mn:43Ca for the prenatal, postnatal, and childhood periods. A doubling in prenatal tooth Mn levels was associated with 5.3% (95% confidence intervals [CI] = -10.3%, 0.0%) lower (i.e., better) teacher-reported inattention scores, whereas a doubling in postnatal tooth Mn levels was associated with 4.5% (95% CI = -9.3%, 0.6%) and 4.6% (95% CI = -9.5%, 0.6%) lower parent-reported inattention and attention deficit/hyperactivity disorder index scores, respectively. Childhood Mn was not beneficially associated with reported attention-related behaviors.

Conclusion

Protective associations in the prenatal and postnatal periods suggest Mn is beneficial for attention-related behavior, but not in the childhood period.

Associations between urinary heavy metals and anxiety among adults in the National Health and Nutrition Examination Survey (NHANES), 2007-2012

BACKGROUND

Few studies have investigated the associations between heavy metals and anxiety. The purpose of this study was to examine the associations between single and combined exposure to heavy metals and anxiety.

METHODS

This study employed data from the National Health and Nutrition Examination Survey (NHANES) from 2007 to 2012. Anxiety was assessed by patients self-reporting the number of anxious days per month. First, we evaluated the associations between 10 heavy metals single exposure and anxiety by multivariable logistic regression. We then selected 5 heavy metals (cadmium, antimony, cobalt, tungsten, and uranium) for further analysis by elastic net regression. Subsequently, principal component analysis (PCA), weighted quantile regression (WQS), and Bayesian kernel machine regression (BKMR) were utilized to evaluate the associations between 5 heavy metals co-exposure and anxiety.

RESULTS

This study included 4512 participants, among whom 1206 participants were in an anxiety state. Urinary cadmium and antimony were separately related to an increased risk of anxiety (p for trend <0.01 and < 0.01, respectively). In PCA analysis, PC1 was associated with an increased risk of anxiety (p for trend <0.001). In WQS analysis, the positive WQS index was substantially linked with the risk of anxiety (OR (95%CI): 1.23 (1.04,1.39)). In BKMR analysis, the overall effects of co-exposure to heavy metals were positively connected with anxiety.

CONCLUSION

Our study identified a positive correlation between individual exposure to cadmium and antimony and the risk of anxiety. Additionally, the co-exposure to cadmium, antimony, cobalt, tungsten, and uranium was associated with an increased risk of anxiety.

Anxiety, aggression, reward sensitivity, and forebrain dopamine receptor expression in a laboratory rat model of early-life disadvantage

Despite early-life disadvantage (ELD) in humans being a highly heterogenous construct, it consistently predicts negative neurobehavioral outcomes. The numerous environmental contributors and neural mechanisms underlying ELD remain unclear, though. We used a laboratory rat model to evaluate the effects of limited resources and/or heavy metal exposure on mothers and their adult male and female offspring. Dams and litters were chronically exposed to restricted (1-cm deep) or ample (4-cm deep) home cage bedding postpartum, with or without lead acetate (0.1%) in their drinking water from insemination through 1-week postweaning. Restricted-bedding mothers showed more pup-directed behaviors and behavioral fragmentation, while lead-exposed mothers showed more nestbuilding. Restricted bedding-raised male offspring showed higher anxiety and aggression. Either restricted bedding or lead exposure impaired goal-directed performance in a reinforcer devaluation task in females, whereas restricted bedding alone disrupted it in males. Lead exposure, but not limited bedding, also reduced sucrose reward sensitivity in a progressive ratio task in females. D1 and D2 receptor mRNA in the medial prefrontal cortex and nucleus accumbens (NAc) were each affected by the early-life treatments and differently between the sexes. Most notably, adult males (but not females) exposed to both early-life treatments had greatly increased D1 receptor mRNA in the NAc core. These results illuminate neural mechanisms through which ELD threatens neurobehavioral development and highlight forebrain dopamine as a factor.

Associations of Prenatal Chemical and Nonchemical Stressors with Early-Adulthood Anxiety and Depressive Symptoms

BACKGROUND

Prenatal exposure to environmental chemicals may increase risk of childhood internalizing problems, but few studies have explored the potential for longer-term consequences of such exposures.

OBJECTIVE

We evaluated associations between prenatal organochlorine and metal levels and early adulthood internalizing symptoms, considering whether sociodemographic/nonchemical stressors modified these associations.

METHODS

Participants were 209 young adults, born (1993-1998) to mothers residing in or near New Bedford, Massachusetts. As part of the early-adult assessment, self-reported anxiety (7-item Generalized Anxiety Disorder scale) and depressive (8-item Patient Health Questionnaire) symptoms (≥10: elevated symptoms) were ascertained. We previously analyzed levels of cord serum organochlorines [hexachlorobenzene, dichlorodiphenyldichloroethylene (p,p'-DDE), polychlorinated biphenyls (ΣPCB4: sum of congeners 118, 138, 153, 180)] and whole blood lead shortly after participants' birth, and levels of cord whole blood manganese from archived samples at the time of the adolescent study visit. We used modified Poisson regression models and quantile g-computation, adjusting for sociodemographics, and explored whether biological sex, race/ethnicity (proxy for unmeasured consequences of racism), prenatal social disadvantage (assessed when participants were neonates), and quality of the home environment (assessed during adolescence) modified these associations.

RESULTS

Participants were (mean±standard deviation) 22.1±1.5 y old, 76% Non-Hispanic White, and 67% female. Prenatal hexachlorobenzene, p,p'-DDE, and lead exposures were moderately associated with increased risk of elevated anxiety symptoms. There were strata-specific associations for prenatal social disadvantage and quality of home environment such that adverse associations of p,p'-DDE and lead and the overall mixture with anxiety and depressive symptoms were largely only evident in those with lower nonchemical stress [e.g., risk ratio and 95% confidence interval (CI) per doubling p,p'-DDE for anxiety: 1.54 (95% CI: 1.20, 1.99) in high-quality home environments and 0.77 (95% CI: 0.51, 1.16) in low-quality home environments]. Associations between prenatal hexachlorobenzene and p,p'-DDE and anxiety symptoms were stronger for underrepresented racial/ethnic group participants vs. Non-Hispanic Whites. We found minimal evidence for sex-specific effects, and no consistent associations with manganese or ΣPCB4.

DISCUSSION

Prenatal organochlorine pesticides and lead exposure possibly increases risk of internalizing problems, particular anxiety symptoms, in young adults. Varying risk was observed by sociodemographic/nonchemical stressor strata, demonstrating the importance of considering interactions between chemical and other stressors. https://doi.org/10.1289/EHP11171.

Effects of co-exposure to lead and manganese on learning and memory deficits

Lead (Pb) and manganese (Mn) are common neurotoxins. However, individuals are subject to co-exposures in real life, and it is therefore important to study these metals in combination. Weaning Sprague-Dawley rats were given ad libitum access to drinking water solutions containing Pb (100 mg/L), Mn (2.5 mg/mL) or a mixture, and each treatment had its own minocycline (50 mg/(kg•day)) supplement group. The results showed a significant difference in spatial memory and induction levels of hippocampal long-term potentiation (LTP) in all exposure groups when compared with controls. The combined-exposure group exhibited the most pronounced effect when compared with each of the single-metal exposure groups. Microglia displayed activation at day 3 after exposure alone or in combination, while astrocytes showed activation at day 5, accompanied by decreased expression levels of GLAST, GLT-1, and GS. Furthermore, the levels of glutamate in the synaptic cleft increased significantly. When microglial activation was inhibited by minocycline, the activation of astrocytes and the expression of GLAST, GLT-1, and GS were both reversed. In addition, upon minocycline treatment, hippocampal LTP impairment and cognitive injury were significantly alleviated in each of the exposure groups. These results suggest that combined exposure to Pb and Mn can cause greater effects on cognition and synaptic plasticity when compared to single-metal exposure groups. The reason may involve abnormal activation of microglia leading to excessive regulation of astrocytes, resulting in glutamate reuptake dysfunction in astrocytes and leading to perturbed cognition and synaptic plasticity.

Prenatal exposure to a mixture of organochlorines and metals and internalizing symptoms in childhood and adolescence

BACKGROUND

Although prenatal chemical exposures influence neurobehavior, joint exposures are not well explored as risk factors for internalizing disorders through adolescence.

OBJECTIVE

To evaluate associations of prenatal organochlorine and metal exposures, considered individually and as a mixture, with mid-childhood and adolescent internalizing symptoms.

METHODS

Participants were 468 children from a prospective cohort recruited at birth (1993-1998) in New Bedford, Massachusetts. Organochlorines (hexachlorobenzene, p,p'-dichlorodiphenyl dichloroethylene, polychlorinated biphenyls) and metals (lead, manganese) were analyzed in cord blood. Internalizing symptoms (anxiety, depressive, somatic) were assessed via multiple informants on the Conners' Rating Scale (CRS) at 8-years and Behavior Assessment System for Children, Second Edition (BASC-2) at 15-years; higher T-scores indicate greater symptoms. Overall and sex-specific covariate-adjusted associations were evaluated using Bayesian Kernel Machine Regression (BKMR) and five-chemical linear regression models.

RESULTS

The cohort was socioeconomically diverse (35% household income <$20,000; 55% maternal ≤ high school education at birth). Most chemical concentrations were consistent with background levels [e.g., median (range) cord blood lead: 1.1 (0-9.4) μg/dL]. BKMR suggested linear associations and no interactions between chemicals. The overall mixture was positively associated with Conners' Parent Rating Scale (CPRS) and BASC-2 Self Report of Personality (SRP) anxiety and depressive symptoms, and negatively with somatic symptoms. Prenatal lead was positively associated with adolescent anxiety symptoms [1.56 (95% CI: 0.50, 2.61) BASC-2 SRP Anxiety score increase per doubling lead]. For CRPS and BASC-2 SRP, a doubling of cord blood manganese was positively associated with internalizing symptoms for girls [e.g., 3.26 (95% CI: 0.27, 6.25) BASC-2 SRP Depression score increase], but not boys. Organochlorine exposures were not adversely associated with internalizing symptoms.

DISCUSSION

Low-level prenatal lead exposure was positively associated with adolescent anxiety symptoms, and prenatal manganese exposure was positively associated with internalizing symptoms for girls from mid-childhood through adolescence. In utero neurotoxicant metal exposures may contribute to the emergence of anxiety and depression.

Role of Environmental Toxicants on Neurodegenerative Disorders

Neurodegeneration leads to the loss of structural and functioning components of neurons over time. Various studies have related neurodegeneration to a number of degenerative disorders. Neurological repercussions of neurodegeneration can have severe impacts on the physical and mental health of patients. In the recent past, various neurodegenerative ailments such as Alzheimer’s and Parkinson’s illnesses have received global consideration owing to their global occurrence. Environmental attributes have been regarded as the main contributors to neural dysfunction-related disorders. The majority of neurological diseases are mainly related to prenatal and postnatal exposure to industrially produced environmental toxins. Some neurotoxic metals, like lead (Pb), aluminium (Al), Mercury (Hg), manganese (Mn), cadmium (Cd), and arsenic (As), and also pesticides and metal-based nanoparticles, have been implicated in Parkinson’s and Alzheimer’s disease. The contaminants are known for their ability to produce senile or amyloid plaques and neurofibrillary tangles (NFTs), which are the key features of these neurological dysfunctions. Besides, solvent exposure is also a significant contributor to neurological diseases. This study recapitulates the role of environmental neurotoxins on neurodegeneration with special emphasis on major neurodegenerative disorders such as Alzheimer’s and Parkinson’s disease.

Impacts of a perinatal exposure to manganese coupled with maternal stress in rats: Tests of untrained behaviors

Manganese (Mn), an element that naturally occurs in the environment, has been shown to produce neurotoxic effects on the developing young when levels exceed physiological requirements. To evaluate the effects of this chemical in combination with non-chemical factors pregnant Long-Evans rats were treated with 0, 2, or 4 mg/mL Mn in their drinking water from gestational day (GD) 7 to postnatal day (PND) 22. Half of the dams received a variable stress protocol from GD13 to PND9, that included restraint, small cage with reduced bedding, exposure to predator odor, intermittent intervals of white noise, lights on for 24 h, intermittent intervals of lights on during dark cycle and cages with grid floors and reduced bedding. One male and one female offspring from each litter were tested to assess untrained behavior. Ultrasonic vocalizations (USV) were recorded from PND13 pups while they were isolated from the litter. Locomotor activity (MA) was measured in figure-eight mazes at PND 17, 29, and 79 (different set of rats at each time point). Social approach (SA) was tested at PND48. Acoustic startle response (ASR) and pre-pulse inhibition (PPI) were measured starting at PND58. At PND53 a sweetness preference for a chocolate flavored milk solution was assessed. There were sex related differences on several parameters for the USVs. There was also a Mn by stress by sex interaction with the females from the 4 mg/mL stressed dams having more frequency modulated (FM) call elements than the 4 mg/mL non-stressed group. There was an effect of Mn on motor activity but only at PND29 with the 2 mg/mL group having higher counts than the 0 mg/mL group. The social approach test showed sex differences for both the habituation and test phase. There was an effect of Mn, with the 4 mg/mL males having a greater preference for the stimulus rat than did the 0 mg/mL males. There was also a stress by sex interaction. The ASR and PPI had only a sex effect. Thus, with only the FM call elements having a Mn by stress effect, and the PND29 MA and SA preference index having a Mn effect but at different doses requires further investigation.

Sexually dimorphic associations between prenatal blood lead exposure and performance on a behavioral testing battery in children

BACKGROUND

Associations between lead (Pb) and neurodevelopment have been studied widely in the context of global measures of cognitive function, such as IQ. Operant test batteries consist of behavioral tasks that can be used to target discrete cognitive and behavioral mechanisms, which contribute to global cognitive faculties.

OBJECTIVES

The goals of this study were to identify Pb-associated deficits in cognitive development and determine the underlying mechanisms involved, utilizing an operant test battery. We evaluated effect modification by child sex.

METHODS

This study utilized data from a prospective cohort in Mexico City. We included 549 participants aged 6-to-7 years with complete data on prenatal blood Pb measurements, Operant Test Battery (OTB) tasks, and demographic covariates. General linear models were used to examine the association of Pb levels at each prenatal timepoint and OTB performance. Effect modification by child sex was evaluated using 2-way interaction terms.

RESULTS

In three of the operant tasks, we observed that higher late-pregnancy blood Pb concentrations were associated with greater response latencies. In the temporal processing task, we observed that higher late-pregnancy Pb exposure was associated with worse overall task performance. Further, in two operant tasks, the effects of Pb were dependent on the sex of the child, such that the effects of Pb were more pronounced in females in the condition position responding task, but stronger in males in the temporal processing task.

CONCLUSIONS

Our results suggest that prenatal Pb concentrations yield broad dysregulation of executive functions, which can be attributed to dysregulation of temporal processing. In addition, we observed sex differences in two operant tasks suggesting that some Pb effects on neurocognitive function may be sexually dimorphic.

Impacts of a perinatal exposure to manganese coupled with maternal stress in rats: Maternal somatic measures and the postnatal growth and development of rat offspring

Psychological stress experienced by the mother during pregnancy has been associated with emotional and cognitive disorders in children such as depression and anxiety. Socioeconomically disadvantaged populations are vulnerable to adverse life experiences and can also be disproportionally exposed to environmental contaminants. To better understand the neurodevelopmental impacts of an environmental toxicant coupled with elevated psychological stress, we exposed pregnant rats to a series of perinatal stressors. Manganese (Mn), a neurotoxicant at excessive concentrations was delivered through drinking water (0, 2, or 4 mg/mL) from gestational day (GD) 7 to postnatal day (PND) 22. A variable stress paradigm was applied to half of the animals from GD13 to PND9. Measurements of somatic development and behavior were examined in the offspring at different developmental stages. No evidence of overt maternal toxicity was observed although the 4 mg/mL Mn-exposed dams gained less body weight during gestation compared to the other dams. Stress also reduced gestational maternal weight gain. Daily fluid consumption normalized for body weight was decreased in the Mn-exposed dams in a dose-dependent manner but was not altered by the stress paradigm. Maternal stress and/or Mn exposure did not affect litter size or viability, but pup weight was significantly reduced in the 4 mg/mL Mn-exposed groups on PNDs 9 through 34 when compared to the other offspring groups. The efficacy of the manipulations to increase maternal stress levels was determined using serum corticosterone as a biomarker. The baseline concentration was established prior to treatment (GD7) and levels were low and similar in all treatment groups. Corticosterone levels were elevated in the perinatal-stress groups compared to the no-stress groups, regardless of Mn exposure, on subsequent time points (GD16, PND9), but were only significantly different on GD16. An analysis of tissue concentrations revealed Mn was elevated similarly in the brain and blood of offspring at PND2 and at PND22 in a significant dose-dependent pattern. Dams also showed a dose-dependent increase in Mn concentrations in the brain and blood; the addition of stress increased the Mn concentrations in the maternal blood but not the brain. Perinatal stress did not alter the effects of Mn on the maternal or offspring somatic endpoints described here.

Metal brain bioaccumulation and neurobehavioral effects on the wild rodent Liomys irroratus inhabiting mine tailing areas

Ecotoxicological studies are necessary in order to evaluate the effects of environmental exposure of chemicals on wild animals and their ecological consequences. Particularly, neurobehavioral effects of heavy metal elements on wild rodents have been scarcely investigated. In the present study, we analyzed the effect of metal bioaccumulation (Pb, As, Mg, Ni, and Zn) in the brain and in the liver on exploratory activity, learning, memory, and on some dopaminergic markers in the wild rodent Liomys irroratus living inside mine tailings, at Huautla, Morelos, Mexico. We found higher Pb concentration but lower Zn in striatum, nucleus accumbens, midbrain, and hippocampus in exposed animals in comparison to rodents from the reference site. Exposed rodents exhibited anxious behavior evaluated in the open field, while no alterations in learning were found. However, they displayed slight changes in the memory test in comparison to reference group. The neurochemical evaluation showed higher levels of dopamine and 5-hydroxyindolacetic acid in midbrain, while lower levels of metabolites dihydroxyphenyl acetic acid and homovanillic acid in striatum of exposed rodents. In addition, mRNA expression levels of dopaminergic D2 receptors in nucleus accumbens were lower in animals from the mining zone than in animals from the reference zone. This is the first study that shows that chronic environmental exposure to metals results in behavioral and neurochemical alterations in the wild rodent L. irroratus, a fact that may comprise the survival of the individuals resulting in long-term effects at the population level. Finally, we suggest the use of L. irroratus as a sentinel species for environmental biomonitoring of mining sites.

Sex-Dependent Effects of Developmental Lead Exposure in Wistar Rats: Evidence from Behavioral and Molecular Correlates

Lead (Pb) exposure in early life affects brain development resulting in cognitive and behavioral deficits. Epidemiologic and experimental evidence of sex as an effect modifier of developmental Pb exposure is emerging. In the present study, we investigated Pb effects on behavior and mechanisms of neuroplasticity in the hippocampus and potential sex differences. To this aim, dams were exposed, from one month pre-mating to offspring weaning, to Pb via drinking water at 5 mg/kg body weight per day. In the offspring of both sexes, the longitudinal assessment of motor, emotional, and cognitive end points was performed. We also evaluated the expression and synaptic distribution of N-methyl-D-Aspartate receptor (NMDA) and α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor subunits at post-natal day (pnd) 23 and 70 in the hippocampus. Neonatal motor patterns and explorative behavior in offspring were affected in both sexes. Pb effects in emotional response and memory retention were observed in adult females only, preceded by increased levels of GluN2A and GluA1 subunits at the post-synapse at pnd 23. These data suggest that Pb exposure during development affects glutamatergic receptors distribution at the post-synaptic spine in females. These effects may contribute to alterations in selected behavioral domains.

Environmental exposure to low-level lead (Pb) co-occurring with other neurotoxicants in early life and neurodevelopment of children

Lead (Pb) is a worldwide environmental contaminant that even at low levels influences brain development and affects neurobehavior later in life; nevertheless it is only a small fraction of the neurotoxicant (NT) exposome. Exposure to environmental Pb concurrent with other NT substances is often the norm, but their joint effects are challenging to study during early life. The aim of this review is to integrate studies of Pb-containing NT mixtures during the early life and neurodevelopment outcomes of children. The Pb-containing NT mixtures that have been most studied involve other metals (Mn, Al, Hg, Cd), metalloids (As), halogen (F), and organo-halogen pollutants. Co-occurring Pb-associated exposures during pregnancy and lactation depend on the environmental sources and the metabolism and half-life of the specific NT contaminant; but offspring neurobehavioral outcomes are also influenced by social stressors. Nevertheless, Pb-associated effects from prenatal exposure portend a continued burden on measurable neurodevelopment; they thus favor increased neurological health issues, decrements in neurobehavioral tests and reductions in the quality of life. Neurobehavioral test outcomes measured in the first 1000 days showed Pb-associated negative outcomes were frequently noticed in infants (<6 months). In older (preschool and school) children studies showed more variations in NT mixtures, children's age, and sensitivity and/or specificity of neurobehavioral tests; these variations and choice of statistical model (individual NT stressor or collective effect of mixture) may explain inconsistencies. Multiple exposures to NT mixtures in children diagnosed with 'autism spectrum disorders' (ASD) and 'attention deficit and hyperactivity disorders' (ADHD), strongly suggest a Pb-associated effect. Mixture potency (number or associated NT components and respective concentrations) and time (duration and developmental stage) of exposure often showed a measurable impact on neurodevelopment; however, net effects, reversibility and/or predictability of delays are insufficiently studied and need urgent attention. Nevertheless, neurodevelopment delays can be prevented and/or attenuated if public health policies are implemented to protect the unborn and the young child.

Effects of combined arsenic and lead exposure on the brain monoaminergic system and behavioral functions in rats: Reversal effect of MiADMSA

In this study, we evaluated the therapeutic efficacy of monoisoamyldimercaptosuccinic acid (MiADMSA) against individual and combined effects of arsenic (As) and lead (Pb) on the monoaminergic system and behavioral functions in rats. Pregnant rats were exposed to sodium metaarsenite (50 ppm) and lead acetate (0.2%) individually and in combination (As = 25 ppm + Pb = 0.1%) via drinking water from gestation day (GD) 6 to postnatal day (PND) 21. MiADMSA (50 mg/kg body weight) was given orally through gavage for 3 consecutive days to pups from PND 18 to PND 20. The results showed increases in synaptosomal epinephrine, dopamine, and norepinephrine levels with individual metal exposures and decreases with combined exposure to As and Pb in the cortex, cerebellum, and hippocampus in PND 21, PND 28, and 3 months age-group rats. We found decreased activity of mitochondrial monoamine oxidase in the selected brain regions following individual and combined exposures to Pb and As. In addition, rats treated with Pb and As alone or in combination showed significant deficits in open-field behavior, grip strength, locomotor activity, and exploratory behavior at PND 28 and 3 months of age. However, MiADMSA administration showed reversal effects against the As- and/or Pb-induced impairments in the monoaminergic system as well as in behavioral functions of rats. Our data demonstrated that the mixture of Pb and As induced synergistic toxicity to developing brain leading to impairments in neurobehavioral functions and also suggest therapeutic efficacy of MiADMSA against Pb- and/or As-induced developmental neurotoxicity.

Dentine biomarkers of prenatal and early childhood exposure to manganese, zinc and lead and childhood behavior

BACKGROUND

Metal exposure alters neurodevelopmental outcomes; little is known about critical windows of susceptibility when exposure exerts the strongest effect.

OBJECTIVE

To examine associations between dentine biomarkers of manganese (Mn), zinc (Zn) and lead (Pb) and later childhood behaviors.

METHODS

Subjects enrolled in a longitudinal birth cohort study in Mexico City provided naturally shed deciduous teeth. We estimated weekly prenatal and postnatal dentine Mn, Zn and Pb concentrations in teeth using laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) and measured behavior at ages 8-11 years of age using the Behavior Assessment System for Children, 2nd edition (BASC-2). We used distributed lag models and lagged weighted quantile sum regression to identify the role of individual and combined dentine biomarkers of Mn, Zn and Pb on behavioral outcomes controlling for maternal education and gestational age.

RESULTS

Among the 133 subjects included in this study, prenatal and early postnatal dentine Mn appeared protective against childhood behavioral problems, specifically hyperactivity and attention. Postnatal dentine Mn was associated with increased reporting of internalizing problems, specifically anxiety. At 6 months, a 1-unit increase (unit = 1 SD of log concentration) in Mn was associated with a 0.18-unit (unit = 1 SD of BASC-2 score) increase in internalizing symptoms score and a 0.25-unit increase in anxiety. Postnatal Pb was associated with increasing anxiety symptoms; at 12 months, a 1-unit increase in Pb was associated with a 0.4 unit increase in anxiety symptoms. When examined as a metal mixture, we observed two potential windows of susceptibility to increased anxiety symptoms: the first window (0-8 months) appeared driven by Mn, the second window (8-12 months) was driven by the metal mixture and dominated by Pb. A 1-unit increase in the mixture index was associated with a 0.7-unit increase in SD of anxiety symptoms.

CONCLUSIONS

Childhood behaviors may demonstrate postnatal windows of susceptibility to individual and mixed metal concentrations measured in deciduous teeth. Prenatal dentine Mn may be protective, while excessive early postnatal Mn may increase risk for adverse behaviors. In combination, higher concentrations of Mn, Zn and Pb may have an adverse impact on behavior.

Arsenic aggravated reproductive toxicity in male rats exposed to lead during the perinatal period

The aim of this study was to assess the reproductive toxic effects of arsenic on adult Wistar rats exposed to lead during the perinatal period. The pregnant rats were allowed ad libitum access to tap water containing 819 mg of lead (Pb) per L or without Pb from conception until weaning. Litter size, survival rate and developmental milestones of the pups delivered by Pb exposed dams were comparable to those of the control rats. Conversely, the pups exposed to Pb during the perinatal period exhibited significant delay in cliff avoidance, negative geotaxis, surface righting reflex, ascending wire mesh and testis descent. The control and perinatal Pb-exposed male rats were maintained on tap water containing 2.3 mg of arsenite (As) per L or without arsenite from the pubertal period (post-natal day 55) to adulthood (post-natal day 115) and assessed for reproductive end points. The results revealed that the (1) relative weights of the testis, epididymis, seminiferous tubules and ventral prostate; (2) daily sperm production; (3) epididymal sperm density and (4) numbers of motile, viable, and HOS tail swelled sperm declined significantly in the rats exposed to either Pb or As. The activity levels of testicular 3β- and 17β-hydroxysteroid dehydrogenases were also significantly decreased in the experimental rats. Significant elevation in the levels of reactive oxygen species and lipid peroxidation in association with reduced activities of antioxidant enzymes in the testis and different epididymal regions was recorded in the experimental rats. In the fertility study, although each male in the control and experimental groups produced a copulatory plug and impregnated a female, the mean conception time significantly increased in the experimental groups. The mean number of implantations decreased significantly in the females mated with the experimental males. Moreover, the results of the present study also indicate that reproductive alterations were more deteriorated in the Pb-exposed rats treated with arsenic when compared to individual exposures. In conclusion, the data clearly suggest that reproductive toxicity in male rats exposed to Pb during the perinatal period is exacerbated by As treatment during the pubertal period.

Maternal and cord blood manganese (Mn) levels and birth weight: The MIREC birth cohort study

Epidemiological studies have hypothesized that both insufficient and excess blood manganese (Mn) levels during pregnancy are associated with reduced fetal growth. This literature is characterized by inconsistent results and a limited focus on women with exposures representative of the general North American population. We examined the relationship between maternal and cord blood Mn levels and fetal growth among women enrolled in the Maternal-Infant Research on Environmental Chemicals Study (MIREC). Mothers with singleton, term infants and complete maternal first and third trimester blood Mn data were eligible for inclusion in the present study (n = 1519). Mean birth weight and odds ratios of small for gestational age (SGA) births according to maternal and cord blood Mn levels (low (<10), referent (10-<90), high (≥90) percentiles) were estimated. We also evaluated the association between the ratio of cord and maternal blood Mn and birth weight. Women with low (<0.82 μg/dL) maternal blood third trimester Mn levels had infants that weighed an average of 64.7 g (95% CI: -142.3,12.8) less than infants born to women in the referent exposure group. This association was strengthened and became statistically significant when adjusted for toxic metals (lead, mercury, arsenic, and cadmium) [-83.3 g (95% CI: -162.4, -4.1)]. No statistically significant associations were observed in models of maternal first trimester or cord blood Mn. A one unit increase in the cord/maternal blood Mn ratio was associated with a 29.4 g (95% CI: -50.2, -8.7), when adjusted for maternal and neonatal characteristics. Our findings motivate additional research regarding the relation between Mn exposure and fetal growth. Further inquiry is necessary to determine whether an exposure threshold exists, how growth related effects of maternal and fetal Mn may differ, and how concurrent exposure to other toxic metals may impact the association between Mn and growth.

Developmental manganese, lead, and barren cage exposure have adverse long-term neurocognitive, behavioral and monoamine effects in Sprague-Dawley rats

Developmental stress, including low socioeconomic status (SES), can induce dysregulation of the hypothalamic-pituitary-adrenal axis and result in long-term changes in stress reactivity. Children in lower SES households experience more stress and are more likely to be exposed to environmental neurotoxins such as lead (Pb) and manganese (Mn) than children in higher SES households. Co-exposure to stress, Pb, and Mn during early development may increase the risk of central nervous system dysfunction compared with unexposed children. To investigate the potential interaction of these factors, Sprague-Dawley rats were bred, and litters born in-house were culled on postnatal day (P)1 to 6 males and 6 females. One male and female within each litter were assigned to one of the following groups: 0 (vehicle), 10 mg/kg Pb, 100 mg/kg Mn, or 10 mg/kg Pb + 100 mg/kg Mn (PbMn), water gavage, and handled only from P4-28 with half the litters reared in cages with standard bedding (29 litters) and half with no bedding (Barren; 27 litters). Mn and PbMn groups had decreased anxiety, reduced acoustic startle, initial open-field hypoactivity, increased activity following (+)-methamphetamine, deficits in egocentric learning in the Cincinnati water maze (CWM), and deficits in latent inhibition conditioning. Pb increased anxiety and reduced open-field activity. Barren-reared rats had decreased anxiety, CWM deficits, increased startle, and initial open-field hyperactivity. Mn, PbMn, Pb Barren-reared groups had impaired Morris water maze performance. Pb altered neostriatal serotonin and norepinephrine, Mn increased hippocampal serotonin in males, Mn + Barren-rearing increased neostriatal serotonin, and Barren-rearing decreased neostriatal dopamine in males. At the doses used here, most effects were in the Mn and PbMn groups. Few interactions between Mn, Pb, and rearing stress were found, indicating that the interaction of these three variables is not as impactful as hypothesized.

Uncovering neurodevelopmental windows of susceptibility to manganese exposure using dentine microspatial analyses

BACKGROUND

Associations between manganese (Mn) and neurodevelopment may depend on dose and exposure timing, but most studies cannot measure exposure variability over time well.

OBJECTIVE

We apply temporally informative tooth-matrix biomarkers to uncover windows of susceptibility in early life when Mn is associated with visual motor ability in childhood. We also explore effect modification by lead (Pb) and child sex.

METHODS

Participants were drawn from the ELEMENT (Early Life Exposures in MExico and NeuroToxicology) longitudinal birth cohort studies. We reconstructed dose and timing of prenatal and early postnatal Mn and Pb exposures for 138 children by analyzing deciduous teeth using laser ablation-inductively coupled plasma-mass spectrometry. Neurodevelopment was assessed between 6 and 16 years of age using the Wide Range Assessment of Visual Motor Abilities (WRAVMA). Mn associations with total WRAVMA scores and subscales were estimated with multivariable generalized additive mixed models. We examined Mn interactions with Pb and child sex in stratified models.

RESULTS

Levels of dentine Mn were highest in the second trimester and declined steeply over the prenatal period, with a slower rate of decline after birth. Mn was positively associated with visual spatial and total WRAVMA scores in the second trimester, among children with lower (< median) tooth Pb levels: one standard deviation (SD) increase in ln-transformed dentine Mn at 150 days before birth was associated with a 0.15 [95% CI: 0.04, 0.26] SD increase in total score. This positive association was not observed at high Pb levels. In contrast to the prenatal period, significant negative associations were found in the postnatal period from ~ 6 to 12 months of age, among boys only: one SD increase in ln-transformed dentine Mn was associated with a 0.11 [95% CI: - 0.001, - 0.22] to 0.16 [95% CI: - 0.04, - 0.28] SD decrease in visual spatial score.

CONCLUSIONS

Using tooth-matrix biomarkers with fine scale temporal profiles of exposure, we found discrete developmental windows in which Mn was associated with visual-spatial abilities. Our results suggest that Mn associations are driven in large part by exposure timing, with beneficial effects found for prenatal levels and toxic effects found for postnatal levels.

Sex-dependent differences in voluntary physical activity

Numbers of overweight and obese individuals are increasing in the United States and globally, and, correspondingly, the associated health care costs are rising dramatically. More than one-third of children are currently considered obese with a predisposition to type 2 diabetes, and it is likely that their metabolic conditions will worsen with age. Physical inactivity has also risen to be the leading cause of many chronic, noncommunicable diseases (NCD). Children are more physically inactive now than they were in past decades, which may be due to intrinsic and extrinsic factors. In rodents, the amount of time engaged in spontaneous activity within the home cage is a strong predictor of later adiposity and weight gain. Thus, it is important to understand primary motivators stimulating physical activity (PA). There are normal sex differences in PA levels in rodents and humans. The perinatal environment can induce sex-dependent differences in PA disturbances. This Review considers the current evidence for sex differences in PA in rodents and humans. The rodent studies showing that early exposure to environmental chemicals can shape later adult PA responses are discussed. Next, whether there are different motivators stimulating exercise in male vs. female humans are examined. Finally, the brain regions, genes, and pathways that modulate PA in rodents, and possibly by translation in humans, are described. A better understanding of why each sex remains physically active through the life span could open new avenues for preventing and treating obesity in children and adults. © 2016 Wiley Periodicals, Inc.

Alleviation of ginsenoside Rg1 on hematopoietic homeostasis defects caused by lead-acetate

The hematopoietic system is one of the potential targets of lead intoxication. Recognition of the lead-related deleterious outcomes promotes us to explore the potential therapeutic intervention. Here, we show that ginsenoside Rg1 (Rg1), extracted from the Chinese herb Panax ginseng, remarkably ameliorates the lead acetate-caused hematological index distortion as well as advanced hematopoietic stem cells (HSCs) aging and aging associated inflammation response. Specifically, Rg1 administration alleviated the increment of aging associated p53-p21-Rb signaling in aged HSCs induced by lead acetate. It also led a reduction of the lead acetate-induced increased inflammation level in blood plasma, which also partly contribute the aged HSCs rejuvenation. In conclusion, our results indicate that ginsenoside Rg1 therapeutically alleviated the essential blood deficits and advanced HSCs aging by lead acetate exposure, by which it could be viewed as a potential candidate for lead acetate-caused blood toxicity treatment.

Polymorphisms in manganese transporters show developmental stage and sex specific associations with manganese concentrations in primary teeth

BACKGROUND

Manganese (Mn) is an essential metal that can become neurotoxic at elevated levels with negative consequences on neurodevelopment. We have evaluated the influence of single nucleotide polymorphisms (SNPs) in Mn transporter genes SLC30A10 and SLC39A8 on Mn concentrations in dentine, a validated biomarker that reflects Mn tissue concentrations early in life.

METHODS

The study included 195 children with variable environmental Mn exposure. Mn concentrations in dentine representing fetal, early postnatal and early childhood developmental periods were measured using laser ablation-inductively coupled plasma mass spectrometry. SLC30A10 rs12064812 (T/C) and SLC39A8 rs13107325 (C/T) were genotyped by TaqMan real time PCR and SLC30A10 rs1776029 (G/A) by pyrosequencing; and SNPs were analyzed in association with Mn in dentine.

RESULTS

SLC39A8 rs13107325 rare allele (T) carriers had significantly higher Mn concentrations in postnatal dentine (110%, p=0.008). For all SNPs we also observed non-significant associations with Mn concentrations in dentine in opposite directions for fetal and early postnatal periods. Furthermore, there were significant differences in the influence of SLC30A10 rs1776929 genotypes on Mn concentrations in dentine between sexes.

DISCUSSION

The findings from this study indicate that common SNPs in Mn transporters influence Mn homeostasis in early development and may therefore be important to consider in future studies of early life Mn exposure and health effects. Our results also suggest that the influence of these transporters on Mn regulation may differ by developmental stage, as well as between girls and boys.

"Manganese-induced neurotoxicity: a review of its behavioral consequences and neuroprotective strategies"

Manganese (Mn) is an essential heavy metal. However, Mn’s nutritional aspects are paralleled by its role as a neurotoxicant upon excessive exposure. In this review, we covered recent advances in identifying mechanisms of Mn uptake and its molecular actions in the brain as well as promising neuroprotective strategies. The authors focused on reporting findings regarding Mn transport mechanisms, Mn effects on cholinergic system, behavioral alterations induced by Mn exposure and studies of neuroprotective strategies against Mn intoxication. We report that exposure to Mn may arise from environmental sources, occupational settings, food, total parenteral nutrition (TPN), methcathinone drug abuse or even genetic factors, such as mutation in the transporter SLC30A10. Accumulation of Mn occurs mainly in the basal ganglia and leads to a syndrome called manganism, whose symptoms of cognitive dysfunction and motor impairment resemble Parkinson’s disease (PD). Various neurotransmitter systems may be impaired due to Mn, especially dopaminergic, but also cholinergic and GABAergic. Several proteins have been identified to transport Mn, including divalent metal tranporter-1 (DMT-1), SLC30A10, transferrin and ferroportin and allow its accumulation in the central nervous system. Parallel to identification of Mn neurotoxic properties, neuroprotective strategies have been reported, and these include endogenous antioxidants (for instance, vitamin E), plant extracts (complex mixtures containing polyphenols and non-characterized components), iron chelating agents, precursors of glutathione (GSH), and synthetic compounds that can experimentally afford protection against Mn-induced neurotoxicity.

Measuring the impact of manganese exposure on children's neurodevelopment: advances and research gaps in biomarker-based approaches

BACKGROUND

Children's exposure to manganese (Mn) is a public health concern and consistent policy guidelines for safe levels of Mn exposure is lacking. The complexity of establishing exposure thresholds for Mn partially relates to its dual role as an essential micronutrient with low levels required for good health, but also as a neurotoxin at high levels. Questions exist about the age-related susceptibility to excess Mn, particularly for children, and how best to measure chronic exposures. To address this concern we conducted a systematic review of studies examining children's exposure to Mn and neurodevelopmental outcomes focused on selection of biomarker-based and environmental measurements of Mn exposure to identify the scientific advances and research gaps.

METHODS

PubMed and EMBASE databases were searched through March 2016 for studies that were published in English, used a biomarker-based or environmental measurement of Mn exposure, and measured at least one neurological outcome for children aged 0-18 years. Ultimately, thirty-six papers from 13 countries were selected. Study designs were cross-sectional (24), prospective cohorts (9), and case control (3). Neurodevelopmental outcomes were first assessed for Mn exposure in infants (6 papers), toddlers or preschoolers (3 papers) and school-age children (27 papers).

RESULTS

Studies of school-aged children most frequently measured Intelligence Quotient (IQ) scores using Mn biomarkers of hair or blood. Higher hair concentrations of Mn were consistently associated with lower IQ scores while studies of blood biomarkers and IQ scores had inconsistent findings. Studies of infants and toddlers most frequently measured mental and psychomotor development; findings were inconsistent across biomarkers of Mn (hair, cord blood, tooth enamel, maternal or child blood and dentin). Although few studies measured environmental sources of Mn, hair biomarkers were associated with Mn in drinking water and infant formula. Only one paper quantified the associations between environmental sources of Mn and blood concentrations.

CONCLUSION

Hair-Mn was the more consistent and valid biomarker of Mn exposure in school-aged children. Accurate measurement of children's exposure to Mn is crucial for addressing these knowledge gaps in future studies. However, research on biomarkers feasible for fetuses and infants is urgently needed given their unique vulnerability to excessive Mn.

Developmental manganese exposure in combination with developmental stress and iron deficiency: Effects on behavior and monoamines

Manganese (Mn) is an essential element but neurotoxic at higher exposures, however, Mn exposure seldom occurs in isolation. It often co-occurs in populations with inadequate dietary iron (Fe) and limited resources that result in stress. Subclinical FeD affects up to 15% of U.S. children and exacerbates Mn toxicity by increasing Mn bioavailability. Therefore, we investigated Mn overexposure (MnOE) in rats in combination with Fe deficiency (FeD) and developmental stress, for which we used barren cage rearing. For barren cage rearing (BAR), rats were housed in cages with a wire grid floor or standard bedding material (STD) from embryonic day (E)7 through postnatal day (P)28. For FeD, dams were fed a 90% Fe-deficient NIH-07 diet from E15 through P28. Within each litter, different offspring were treated with 100mg/kg Mn (MnOE) or vehicle (VEH) by gavage every other day from P4-28. Behavior was assessed at two ages and consisted of: open-field, anxiety tests, acoustic startle response (ASR) with prepulse inhibition (PPI), sociability, sucrose preference, tapered beam crossing, and the Porsolt's forced swim test. MnOE had main effects of decreasing activity, ASR, social preference, and social novelty. BAR and FeD transiently modified MnOE effects. BAR groups weighed less and showed decreased anxiety in the elevated zero maze, had increased ASR and decreased PPI, and exhibited reduced sucrose preference compared with the STD groups. FeD animals also weighed less and had increased slips on the tapered beam. Most of the monoamine effects were dopaminergic and occurred in the MnOE groups. The results showed that Mn is a pervasive developmental neurotoxin, the effects of which are modulated by FeD and/or BAR cage rearing.

Mechanisms of divalent metal toxicity in affective disorders

Metals are required for proper brain development and play an important role in a number of neurobiological functions. The divalent metal transporter 1 (DMT1) is a major metal transporter involved in the absorption and metabolism of several essential metals like iron and manganese. However, non-essential divalent metals are also transported through this transporter. Therefore, altered expression of DMT1 can modify the absorption of toxic metals and metal-induced toxicity. An accumulating body of evidence has suggested that increased metal stores in the brain are associated with elevated oxidative stress promoted by the ability of metals to catalyze redox reactions, resulting in abnormal neurobehavioral function and the progression of neurodegenerative diseases. Metal overload has also been implicated in impaired emotional behavior, although the underlying mechanisms are not well understood with limited information. The current review focuses on psychiatric dysfunction associated with imbalanced metabolism of metals that are transported by DMT1. The investigations with respect to the toxic effects of metal overload on behavior and their underlying mechanisms of toxicity could provide several new therapeutic targets to treat metal-associated affective disorders.

Expanding the test set: Chemicals with potential to disrupt mammalian brain development

High-throughput test methods including molecular, cellular, and alternative species-based assays that examine critical events of normal brain development are being developed for detection of developmental neurotoxicants. As new assays are developed, a "training set" of chemicals is used to evaluate the relevance of individual assays for specific endpoints. Different training sets are necessary for each assay that would comprise a developmental neurotoxicity test battery. In contrast, evaluation of the predictive ability of a comprehensive test battery requires a set of chemicals that have been shown to alter brain development after in vivo exposure ("test set"). Because only a small number of substances have been well documented to alter human neurodevelopment, we have proposed an expanded test set that includes chemicals demonstrated to adversely affect neurodevelopment in animals. To compile a list of potential developmental neurotoxicants, a literature review of compounds that have been examined for effects on the developing nervous system was conducted. The search was limited to mammalian studies published in the peer-reviewed literature and regulatory studies submitted to the U.S. EPA. The definition of developmental neurotoxicity encompassed changes in behavior, brain morphology, and neurochemistry after gestational or lactational exposure. Reports that indicated developmental neurotoxicity was observed only at doses that resulted in significant maternal toxicity or were lethal to the fetus or offspring were not considered. As a basic indication of reproducibility, we only included a chemical if data on its developmental neurotoxicity were available from more than one laboratory (defined as studies originating from laboratories with a different senior investigator). Evidence from human studies was included when available. Approximately 100 developmental neurotoxicity test set chemicals were identified, with 22% having evidence in humans.

Developmental effects of parental exposure to soil contaminated with urban metals

Soil is a highly complex material, and because of rapid population growth, intense industrial activity and petrochemical development, it has suffered from contamination with substances of various origins. These environmental contaminants may have detrimental effects on human health, particularly during development. Due to the ability to transmit contaminants to the fetus, evaluating the effects of exposure of pregnant women on the psychomotor development of their offspring is of particular interest. Therefore, this study aimed to investigate the effects of exposure of female rats to an urban soil influenced by the dispersion of air contaminants during periods of pre-pregnancy, pregnancy and lactation on offspring development. Using physiological, behavioral and hematological parameters, deleterious effects on offspring were assessed. In behavioral parameters, parental exposure during pregnancy and lactation resulted in no significant differences in the evaluated parameters when compared to the control group. In contrast, pups from the pre-pregnancy group displayed decreased locomotor and exploratory activity in addition to increased levels of anxiety. Furthermore, offspring of rats exposed to contaminated urban soil during pre-pregnancy demonstrated significant changes in weight gain and development length and a reduction in the number of platelets compared to controls. Significantly, pups born to mothers exposed to contaminated urban soil during the pregnancy displayed changes in birth weight, weight gain during the growth, development length, incisor eruption and opening of the ears in addition to a reduction in their physical performance and a change in the number of lymphocytes. These results clearly show the negative influence of parental exposure to contaminated urban soil on the general development of the rats during the periods studied. These data indicate that developing organisms are highly sensitive to external factors. Further, they demonstrate the utility of these various biomarkers for identifying and displaying toxic effects of exposure to contaminated soils.

Exposure to Mixtures of Metals and Neurodevelopmental Outcomes: A Multidisciplinary Review Using an Adverse Outcome Pathway Framework

Current risk assessment guidance calls for an individual chemical-by-chemical approach that fails to capture potential interactive effects of exposure to environmental mixtures and genetic variability. We conducted a review of the literature on relationships between prenatal and early life exposure to mixtures of lead (Pb), arsenic (As), cadmium (Cd), and manganese (Mn) with neurodevelopmental outcomes. We then used an adverse outcome pathway (AOP) framework to integrate lines of evidence from multiple disciplines based on evolving guidance developed by the Organization for Economic Cooperation and Development (OECD). Toxicological evidence suggests a greater than additive effect of combined exposures to As-Pb-Cd and to Mn with any other metal, and several epidemiologic studies also suggest synergistic effects from binary combinations of Pb-As, Pb-Cd, and Pb-Mn. The exposure levels reported in these epidemiologic studies largely fall at the high-end (e.g., 95th percentile) of biomonitoring data from the National Health and Nutrition Examination Survey (NHANES), suggesting a small but significant potential for high-end exposures. This review integrates multiple data sources using an AOP framework and provides an initial application of the OECD guidance in the context of potential neurodevelopmental toxicity of several metals, recognizing the evolving nature of regulatory interpretation and acceptance.

Adverse neurodevelopmental effects and hearing loss in children associated with manganese in well water, North Carolina, USA

Aim

Heavy metals such as manganese, arsenic and lead can act as neurotoxins. There have been few human studies of neurobehavioral/neurodevelopmental effects of arsenic and manganese on children in the United States. Since 1998, North Carolina has tested all new private wells for manganese, arsenic and lead. This study was conducted to evaluate adverse neurodevelopmental effects (delayed milestones, speech/language disorders and hearing loss) in children and metal concentrations in well water.

Methods

A quasi-regression model of the number of children (0–35 months of age) with adverse neurodevelopmental effects as outcome measures and aggregate mean metal concentration (arsenic, lead, and manganese) in private well water in each county as exposures.

Results

Over 70,000 private well water samples from 1998 to 2011 were analyzed for metal content. From 2008 to 2011, an average of 17,000 children was enrolled in the Infant Toddler Program. On average, 1.7% of children in this age range in each county had a speech/language disorder, 0.24% had a diagnosis of delayed milestones, and 0.026% had a diagnosis of hearing loss. The county mean manganese concentration was significantly and positively associated with the prevalence of delayed milestones and hearing loss in the children. No association was found for metal concentrations and speech/language disorders.

Conclusion

This ecological study indicates that further investigation of manganese in well water and associated neurodevelopmental health outcomes in children is needed.

Maternal blood and hair manganese concentrations, fetal growth, and length of gestation in the ISA cohort in Costa Rica

BACKGROUND

Animal studies have shown that both deficiency and excess manganese (Mn) may result in decreased fetal size and weight, but human studies have reported inconsistent results.

METHODS

We examined the association of blood and hair Mn concentrations measured at different times during pregnancy with fetal growth among term births and length of gestation in a cohort of 380 mother-infant pairs living near banana plantations aerially sprayed with Mn-containing fungicides in Costa Rica. We used linear regression and generalized additive models to test for linear and nonlinear associations

RESULTS

Mean (± SD) blood Mn concentration was 24.4 ± 6.6 μg/L and geometric mean (geometric SD) hair Mn concentration was 1.8 (3.2) μg/g. Hair Mn concentrations during the second and third trimesters of gestation were positively related to infant chest circumference (β for 10-fold increase = 0.62 cm; 95% CI: 0.16, 1.08; and β = 0.55 cm; 95% CI: -0.16, 1.26, respectively). Similarly, average maternal hair Mn concentrations during pregnancy were associated with increased chest circumference (β for 10-fold increase = 1.19 cm; 95% CI: 0.43, 1.95) in infants whose mothers did not have gestational anemia, but not in infants of mothers who had gestational anemia (β = 0.39 cm; 95% CI: -0.32, 1.10; pINT=0.14). All these associations were linear. Blood Mn concentrations did not show consistent linear nor nonlinear relationships with any of the birth outcomes

CONCLUSIONS

Mn plays an important role in fetal development, but the extent to which environmental exposures may cause adverse health effects to the developing fetus is not well understood. Among women living near banana plantations in Costa Rica, we did not observe linear or nonlinear associations of Mn concentrations with lowered birth weight or head circumference, as reported in previous studies. However, we did find positive linear associations between maternal hair Mn concentrations during pregnancy and infant chest circumference.

Manganese-induced atypical parkinsonism is associated with altered Basal Ganglia activity and changes in tissue levels of monoamines in the rat

Manganese neurotoxicity is associated with motor and cognitive disturbances known as Manganism. However, the mechanisms underlying these deficits remain unknown. Here we investigated the effects of manganese intoxication on motor and non-motor parkinsonian-like deficits such as locomotor activity, motor coordination, anxiety and “depressive-like” behaviors. Then, we studied the impact of this intoxication on the neuronal activity, the globus pallidus (GP) and subthalamic nucleus (STN). At the end of experiments, post-mortem tissue level of the three monoamines (dopamine, norepinephrine and serotonin) has been determined. The experiments were carried out in adult Sprague-Dawley rats, daily treated with MnCl₂ (10 mg/kg/, i.p.) for 5 weeks. We show that manganese progressively reduced locomotor activity as well as motor coordination in parallel with the manifestation of anxiety and “depressive-like” behaviors. Electrophysiological results show that, while majority of GP and STN neurons discharged regularly in controls, manganese increased the number of GP and STN neurons discharging irregularly and/or with bursts. Biochemical results show that manganese significantly decreased tissue levels of norepinephrine and serotonin with increased metabolism of dopamine in the striatum. Our data provide evidence that manganese intoxication is associated with impaired neurotransmission of monoaminergic systems, which is at the origin of changes in basal ganglia neuronal activity and the manifestation of motor and non-motor deficits similar to those observed in atypical Parkinsonism.

Interaction of curcumin with manganese may compromise metal and neurotransmitter homeostasis in the hippocampus of young mice

Manganese (Mn) exposure is related to industrial activities, where absorption by inhalation has high relevance. Manganism, a syndrome caused as a result of excessive accumulation of Mn in the central nervous system, has numerous symptoms similar to those seen in idiopathic Parkinson disease (IPD). Some of these symptoms, such as learning, memory, sensorial, and neurochemical changes, appear before the onset of motor deficits in both manganism and IPD. The aim of this study was to evaluate the possible neuroprotective effects of curcumin against behavioral deficits induced by Mn toxicity in young (2 months old) Swiss mice. We evaluated the effect of chronic inhalation of a Mn mixture [Mn(OAc)3 and MnCl2 (20:40 mM)], 1 h/session, three times a week, over a 14-week period on behavioral and neurochemical parameters. Curcumin was supplemented in the diet (500 or 1,500 ppm in food pellets). The Mn disrupted the motor performance evaluated in the single-pellet reach task, as well as the short- and long-term spatial memory evaluated in the step-down inhibitory avoidance task. Surprisingly, curcumin also produced similar deleterious effects in such behavioral tests. Moreover, the association of Mn plus curcumin significantly increased the levels of Mn and iron, and decreased the levels of dopamine and serotonin in the hippocampus. These alterations were not observed in the striatum. In conclusion, the current Mn treatment protocol resulted in mild deficits in motor and memory functions, resembling the early phases of IPD. Additionally, curcumin showed no beneficial effects against Mn-induced disruption of hippocampal metal and neurotransmitter homeostasis.

Lead-induced morphological changes and amyloid precursor protein accumulation in adult rat hippocampus

Lead is an important environmental pollutant that exerts potent toxic effects on many organs. The toxic effects of lead are less well known for adult brain than for children. We investigated the morphological changes and amyloid precursor protein (APP) accumulation in the adult rat hippocampus following exposure to lead. Forty rats were divided into two groups of 20. One group was exposed to 580 parts per million (ppm) lead acetate and other group to an identical concentration of sodium acetate as a control group. After exposure to lead for 3 months, the hippocampus was examined by electron microscopy and APP levels in the hippocampus were detected using immunohistochemistry. Lead levels in the blood of rats exposed to lead were significantly higher than in the controls. The morphological changes in the hippocampus included mitochondrial degeneration, apoptosis and abnormal synapses in the rats exposed to lead. APP in hippocampus was increased significantly in the group exposed to lead compared to controls. We determined that lead exposure causes accumulation of APP and morphological changes in the adult rat hippocampus.

Effects of developmental manganese, stress, and the combination of both on monoamines, growth, and corticosterone

Developmental exposure to manganese (Mn) or stress can each be detrimental to brain development. Here, Sprague-Dawley rats were exposed to two housing conditions and Mn from postnatal day (P)4–28. Within each litter two males and two females were assigned to the following groups: 0 (vehicle), 50, or 100 mg/kg Mn by gavage every other day. Half the litters were reared in cages with standard bedding and half with no bedding. One pair/group in each litter had an acute shallow water stressor before tissue collection (i.e., standing in shallow water). Separate litters were assessed at P11, 19, or 29. Mn-treated rats raised in standard cages showed no change in baseline corticosterone but following acute stress increased more than controls on P19; no Mn effects were seen on P11 or P29. Mn increased neostriatal dopamine in females at P19 and norepinephrine at P11 and P29. Mn increased hippocampal dopamine at P11 and P29 and 5-HT at P29 regardless of housing or sex. Mn had no effect on hypothalamic dopamine, but increased norepinephrine in males at P29 and 5-HT in males at all ages irrespective of rearing condition. Barren reared rats showed no or opposite effects of Mn, i.e., barren rearing + Mn attenuated corticosterone increases to acute stress. Barren rearing also altered the Mn-induced changes in dopamine and norepinephrine in the neostriatum, but not in the hippocampus. Barren rearing caused a Mn-associated increase in hypothalamic dopamine at P19 and P29 not seen in standard reared Mn-treated groups. Developmental Mn alters monoamines and corticosterone as a function of age, stress (acute and chronic), and sex.