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

Manganese in autism spectrum disorder and attention deficit hyperactivity disorder: The state of the art

The objective of the present narrative review was to synthesize existing clinical and epidemiological findings linking manganese (Mn) exposure biomarkers to autism spectrum disorder (ASD) and attention deficit hyperactivity disorder (ADHD), and to discuss key pathophysiological mechanisms of neurodevelopmental disorders that may be affected by this metal. Existing epidemiological data demonstrated both direct and inverse association between Mn body burden and ASD, or lack of any relationship. In contrast, the majority of studies revealed significantly higher Mn levels in subjects with ADHD, as well as direct relationship between Mn body burden with hyperactivity and inattention scores in children, although several studies reported contradictory results. Existing laboratory studies demonstrated that impaired attention and hyperactivity in animals following Mn exposure was associated with dopaminergic dysfunction and neuroinflammation. Despite lack of direct evidence on Mn-induced neurobiological alterations in patients with ASD and ADHD, a plethora of studies demonstrated that neurotoxic effects of Mn overexposure may interfere with key mechanisms of pathogenesis inherent to these neurodevelopmental disorders. Specifically, Mn overload was shown to impair not only dopaminergic neurotransmission, but also affect metabolism of glutamine/glutamate, GABA, serotonin, noradrenaline, thus affecting neuronal signaling. In turn, neurotoxic effects of Mn may be associated with its ability to induce oxidative stress, apoptosis, and neuroinflammation, and/or impair neurogenesis. Nonetheless, additional detailed studies are required to evaluate the association between environmental Mn exposure and/or Mn body burden and neurodevelopmental disorders at a wide range of concentrations to estimate the potential dose-dependent effects, as well as environmental and genetic factors affecting this association.

Sex-dependent metal accumulation and immunoexpression of Hsp70 and Nrf2 in rats' brain following manganese exposure

Manganese (Mn), although important for multiple cellular processes, has posed environmental health concerns due to its neurotoxic effects. In recent years, there have been extensive studies on the mechanism of Mn-induced neuropathology, as well as the sex-dependent vulnerability to its neurotoxic effects. Nonetheless, cellular mechanisms influenced by sex differences in susceptibility to Mn have yet to be adequately characterized. Since oxidative stress is a key mechanism of Mn neurotoxicity, here, we have probed Hsp70 and Nrf2 proteins to investigate the sex-dependent changes following exposure to Mn. Male and female rats were administered intraperitoneal injections of MnCl2 (10 mg/kg and 25 mg/kg) 48 hourly for a total of eight injections (15 days). We evaluated changes in body weight, as well as Mn accumulation, Nrf2 and Hsp70 expression across four brain regions; striatum, cortex, hippocampus and cerebellum in both sexes. Our results showed sex-specific changes in body-weight, specifically in males but not in females. Additionally, we noted sex-dependent accumulation of Mn in the brain, as well as in expression levels of Nrf2 and Hsp70 proteins. These findings revealed sex-dependent susceptibility to Mn-induced neurotoxicity corresponding to differential Mn accumulation, and expression of Hsp70 and Nrf2 across several brain regions.

Impacts of a perinatal exposure to manganese coupled with maternal stress in rats: Learning, memory and attentional function in exposed offspring

The developmental effects of chemicals that co-occur in vulnerable populations with elevated psychological stress are of increasing concern to the public. To investigate these concerns, we developed a rodent model of co-occurring perinatal manipulations and conducted a series of cognitive assessments in male and female offspring. Manganese (Mn), a neurodevelopmental toxicant when exceeding physiological requirements, was delivered in the drinking water (0, 2, or 4 mg Mn/mL) of rats from gestational day (GD) 7 to postnatal day (PND) 22. A variable perinatal stress paradigm was applied to half of the animals from GD13 to PND9. Novel object recognition (NOR), Morris water maze (MWM), differential reinforcement of low-rates procedure (DRL) and cued and uncued choice reaction time (CRT) tests were used to assess cognitive functions in offspring. Mn (4 mg/mL) and stress impaired NOR in adolescent males but facilitated NOR performance in females. However, when stress and Mn were combined these effects were attenuated in both sexes. During training for the DRL, Mn (2 mg/mL) facilitated, while stress impaired, lever press learning in both sexes. Few effects related to the treatments were found on DRL or MWM. During cued CRT, Mn (2 and 4 mg/mL) and stress reduced accuracy in males, while stress and Mn (2 mg/mL) increased anticipatory responding and slowed decision time in both sexes. Stress combined with Mn (2 mg/mL) improved cued accuracy and decision time, and Mn attenuated the effect of stress on anticipatory responding in both sexes. Stress slowed female movement time but when combined with Mn (4 mg/mL) the effect of stress was attenuated. During uncued CRT, except for decision time (which replicated effects observed with the cued task), no other effects of Mn or its combination with stress occurred. Females remained negatively affected by stress in most uncued CRT performance measures, while stressed improved male uncued accuracy. Taken together these data do not support increased cognitive impairment produced by Mn when combined with stress. However, the effects of perinatal stress alone, on these cognitive functions may hinder the detection of effects due to chemical exposures and underscores the need to consider the psychological health and wellbeing of the mother and her environment in risk assessment for developmental neurotoxicity of chemicals.

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.

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.

Sex-specific neurotoxic effects of heavy metal pollutants: Epidemiological, experimental evidence and candidate mechanisms

The heavy metals lead (Pb), mercury (Hg), and cadmium (Cd) are ubiquitous environmental pollutants and are known to exert severe adverse impacts on the nervous system even at low concentrations. In contrast, the heavy metal manganese (Mn) is first and foremost an essential nutrient, but it becomes neurotoxic at high levels. Neurotoxic metals also include the less prevalent metalloid arsenic (As) which is found in excessive concentrations in drinking water and food sources in many regions of the world. Males and females often differ in how they respond to environmental exposures and adverse effects on their nervous systems are no exception. Here, we review the different types of sex-specific neurotoxic effects, such as cognitive and motor impairments, that have been attributed to Pb, Hg, Mn, Cd, and As exposure throughout the life course in epidemiological as well as in experimental toxicological studies. We also discuss differential vulnerability to these metals such as distinctions in behaviors and occupations across the sexes. Finally, we explore the different mechanisms hypothesized to account for sex-based differential susceptibility including hormonal, genetic, metabolic, anatomical, neurochemical, and epigenetic perturbations. An understanding of the sex-specific effects of environmental heavy metal neurotoxicity can aid in the development of more efficient systematic approaches in risk assessment and better exposure mitigation strategies with regard to sex-linked susceptibilities and vulnerabilities.

Early Postnatal Manganese Exposure Reduces Rat Cortical and Striatal Biogenic Amine Activity in Adulthood

Growing evidence from studies with children and animal models suggests that elevated levels of manganese during early development lead to lasting cognitive and fine motor deficits. This study was performed to assess presynaptic biogenic amine function in forebrain of adult Long-Evans rats exposed orally to 0, 25, or 50 mg Mn/kg/day over postnatal day 1-21 or continuously from birth to the end of the study (approximately postnatal day 500). Intracerebral microdialysis in awake rats quantified evoked outflow of biogenic amines in the right medial prefrontal cortex and left striatum. Results indicated that brain manganese levels in the early life exposed groups (postnatal day 24) largely returned to control levels by postnatal day 66, whereas levels in the lifelong exposed groups remained elevated 10%-20% compared with controls at the same ages. Manganese exposure restricted to the early postnatal period caused lasting reductions in cortical potassium-stimulated extracellular norepinephrine, dopamine, and serotonin, and reductions in striatal extracellular dopamine. Lifelong manganese exposure produced similar effects with the addition of significant decreases in cortical dopamine that were not evident in the early postnatal exposed groups. These results indicate that early postnatal manganese exposure produces persistent deficits in cortical and striatal biogenic amine function. Given that these same animals exhibited lasting impairments in attention and fine motor function, these findings suggest that reductions in catecholaminergic activity are a primary factor underlying the behavioral effects caused by manganese, and indicate that children exposed to elevated levels of manganese during early development are at the greatest risk for neuronal deficiencies that persist into adulthood.

Impact of preweaning stress on long-term neurobehavioral outcomes in Sprague-Dawley rats: Differential effects of barren cage rearing, pup isolation, and the combination

Two developmental stressors were compared in preweaning rats exposed to either one stressor or both. Stressors were barren cage rearing or maternal separation (pup isolation). 40 gravid Sprague-Dawley CD/IGS rats were randomly assigned to two cage conditions: standard (Std) cage or barren cage (Bar), 20 litters/condition throughout gestation and lactation. After delivery, litters were randomly culled to 4 males and 4 females. The second stressor was maternal separation: Two male/female pairs per litter were isolated from their dam 4 h/day (Iso) and two pairs were not (Norm). Hence, there were 4 conditions: Std-Norm, Std-Iso, Bar-Norm, and Bar-Iso. One pair/litter/stress condition received the following: elevated zero-maze (EZM), open-field, swim channel, Cincinnati water maze, conditioned fear, and open-field with methamphetamine challenge. The second pair/litter/condition received the light-dark test, swim channel, Morris water maze, forced swim, and EZM with diazepam challenge. Barren rearing reduced EZM time-in-open, whereas isolation rearing reduced open-field activity in males and increased it in females. Effects on straight channel swimming were minor. In the Cincinnati water maze test of egocentric learning, isolation rearing increased errors whereas barren cage housing reduced errors in combination with normal rearing. Barren cage with maternal separation (pup isolation) increased Cincinnati water maze escape latency but not errors. Barren cage housing reduced hyperactivity in response to methamphetamine. Isolation rearing increased time in open in the EZM after diazepam challenge. Trends were seen in the Morris water maze. These suggested that barren cage and isolation rearing in combination reduced latency on acquisition on days 1 and 2 in males, whereas females had increased latency on days 2 and 3. Combined exposure to two developmental stressors did not induce additive or synergistic effects, however the data show that these stressors had long-term effects with some evidence that the combination of both caused effects when either stressor alone did not, but synergism was not observed.

Association between Serum Essential Metal Elements and the Risk of Schizophrenia in China

Numerous essential metal elements (EMEs) are necessary to maintain the proper function of human body. In this case-control study, we investigated the associations of 11 EMEs [Calcium (Ca), potassium (K), magnesium (Mg), sodium (Na), manganese (Mn), selenium (Se), cobalt (Co), Molybdenum (Mo), copper (Cu), zinc (Zn), and iron (Fe)] in serum with the risk of schizophrenia. We recruited first-episode and drug-naïve schizophrenic patients (cases = 99) and age-sex-matched normal subjects (controls = 99) from Tangshan, Hebei Province, China. The 11 EMEs in serum from cases and controls were quantified by inductively coupled plasma atomic emission spectrometry and inductively coupled plasma mass spectrometry. We observed that a higher level of Mn (OR = 2.390; 95%CI: 1.504–3.796) and lower levels of Ca (OR = 0.939; 95%CI: 0.890–0.990), Mg (OR = 0.806; 95%CI: 0.669–0.972), Na (OR = 0.995; 95%CI: 0.993–0.998), and Se (OR = 0.954; 95%CI: 0.937–0.972) were associated with an elevated risk of schizophrenia. Dose–response relationships between serum EME concentrations and the risk of schizophrenia were observed in most of the schizophrenia-associated EMEs. Moreover, the serum concentrations of these schizophrenia-associated EMEs in patients were correlated with the severity of their clinical symptoms. Significant correlations were found between EMEs and biomarkers associated with schizophrenia related to metabolic and oxidative stress. This study suggested that the concentration and profile of EMEs were different between schizophrenic patients and normal controls and revealed potential metabolisms associated with EMEs and schizophrenia, suggesting EMEs might act as biomarkers of schizophrenia to improve the current situation of diagnosis and treatment.

Maintaining Translational Relevance in Animal Models of Manganese Neurotoxicity

Manganese is an essential metal, but elevated brain Mn concentrations produce a parkinsonian-like movement disorder in adults and fine motor, attentional, cognitive, and intellectual deficits in children. Human Mn neurotoxicity occurs owing to elevated exposure from occupational or environmental sources, defective excretion (e.g., due to cirrhosis), or loss-of-function mutations in the Mn transporters solute carrier family 30 member 10 or solute carrier family 39 member 14. Animal models are essential to study Mn neurotoxicity, but in order to be translationally relevant, such models should utilize environmentally relevant Mn exposure regimens that reproduce changes in brain Mn concentrations and neurological function evident in human patients. Here, we provide guidelines for Mn exposure in mice, rats, nematodes, and zebrafish so that brain Mn concentrations and neurobehavioral sequelae remain directly relatable to the human phenotype.

Metal bashing: iron deficiency and manganese overexposure impact on peripheral nerves

Iron (Fe) deficiency (FeD) and manganese (Mn) overexposure (MnOE) may result in several neurological alterations in the nervous system. Iron deficiency produces unique neurological deficits due to its elemental role in central nervous system (CNS) development and myelination, which might persist after normalization of Fe in the diet. Conversely, MnOE is associated with diverse neurocognitive deficits. Despite these well-known neurotoxic effects on the CNS, the influence of FeD and MnOE on the peripheral nervous system (PNS) remains poorly understood. The aim of the present investigation was to examine the effects of developmental FeD and MnOE or their combination on the sciatic nerve of young and adult rats. The parameters measured included divalent metal transporter 1 (DMT1), transferrin receptor (TfR), myelin basic protein (MBP) and peripheral myelin protein 22 (PMP22) expression, as well as Fe levels in the nerve. Our results showed that FeD produced a significant reduction in MBP and PMP22 content at P29, which persisted at P60 after Fe-sufficient diet replenishment regardless of Mn exposure levels. At P60 MnOE significantly increased sciatic nerve Fe content and DMT1 expression. However, the combination of FeD and MnOE produced no marked motor skill impairment. Evidence indicates that FeD appears to hinder developmental peripheral myelination, while MnOE may directly alter Fe homeostasis. Further studies are required to elucidate the interplay between these pathological conditions.

Effects of Preweaning Manganese in Combination with Adult Striatal Dopamine Lesions on Monoamines, BDNF, TrkB, and Cognitive Function in Sprague-Dawley Rats

Manganese (Mn) is an essential nutrient especially during development, but Mn overexposure (MnOE) produces long-term cognitive deficits. Evidence of long-term changes in dopamine in the neostriatum was found in rats from developmental MnOE previously. To examine the relationship between MnOE and dopamine, we tested whether the effects of developmental MnOE would be exaggerated by dopamine reductions induced by 6-hydroxydopamine (6-OHDA) neostriatal infusion when the rats were adults. The experiment consisted of four groups of females and males: Vehicle/Sham, MnOE/Sham, Vehicle/6-OHDA, and MnOE/6-OHDA. Both MnOE/Sham and Vehicle/6-OHDA groups displayed egocentric and allocentric memory deficits, whereas MnOE+6-OHDA had additive effects on spatial memory in the Morris water maze and egocentric learning in the Cincinnati water maze. 6-OHDA reduced dopamine in the neostriatum and nucleus accumbens, reduced norepinephrine in the hippocampus, reduced TH+ cells and TrkB and TH expression in the substantia nigra pars compacta (SNpc), but increased TrkB in the neostriatum. MnOE alone had no effect on monoamines or TrkB in the neostriatum or hippocampus but reduced BDNF in the hippocampus. A number of sex differences were noted; however, only a few significant interactions were found for MnOE and/or 6-OHDA exposure. These data further implicate dopamine and BDNF in the cognitive deficits arising from developmental MnOE.

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.

Manganese in teeth and neurobehavior: Sex-specific windows of susceptibility

BACKGROUND

Manganese (Mn) is an essential element required for growth and development, but higher body burdens have been associated with neurobehavioral decrements in children.

OBJECTIVES

We examined whether prenatal or postnatal Mn measured in deciduous teeth was associated with scores on a test of visuospatial learning and memory.

METHODS

Deciduous teeth were collected from 142 participants (ages 10-14years) residing near varied ferro‑manganese industry in Italy. Mn concentrations were measured in prenatal and postnatal tooth regions by laser ablation inductively coupled plasma mass spectrometry (ICP-MS). The Virtual Radial Arm Maze (VRAM), an animal-human analogue task, was used to assess visuospatial learning and memory. We used generalized additive, linear and zero-inflated Poisson mixed regression models to estimate associations between prenatal or postnatal Mn concentrations and repeated measures of all four VRAM outcomes: time, distance, working and reference memory errors. Effect measure modification by sex was examined in stratified models.

RESULTS

U-shaped associations between prenatal Mn and VRAM outcomes were observed among girls only (pGAMM=0.001 to 0.02 in stratified models). Compared to the mid-tertile of prenatal Mn, girls in the highest tertile took 7.7s [95% CI: -6.1, 21.5] longer to complete the task, traveled 2.3 maze units [0.1, 4.4] farther, and committed more working and reference memory errors (β for count ratio=1.33 [1.01, 1.83]; 1.10 [0.98, 1.24], respectively). This association was not observed among boys. In contrast, for postnatal Mn, no significant associations were found, and patterns were similar for boys and girls.

CONCLUSIONS

The prenatal period may be a critical window for the impact of environmental Mn on visuospatial ability and executive function, especially for females.

Effects of Housing on Methamphetamine-Induced Neurotoxicity and Spatial Learning and Memory

Severe stress potentiates methamphetamine (MA) neurotoxicity. However, whether moderate stress increases or decreases the neurotoxic effects of MA is unknown. We assessed the effects of MA (4 × 10 mg/kg at 2 h intervals) in combination with prior barren-cage housing in adult male Sprague-Dawley rats on monoamines and glial fibrillary acid protein (GFAP) in one cohort and spatial learning and memory in the Morris water maze in another cohort. MA reduced dopamine (DA) and serotonin (5-HT) in the neostriatum and nucleus accumbens, 5-HT in the hippocampus, and increased GFAP in neostriatum and nucleus accumbens compared with saline controls. In neostriatum, barren-cage housing protected against MA-induced increases in GFAP, but it did not prevent DA and 5-HT reductions, although it did increase hippocampal norepinephrine. MA impaired spatial learning during acquisition, reversal, and shift phases and impaired reference memory on reversal and shift probe trials. Barren-cage housing enhanced performance during acquisition but not during reversal or shift or on probe trials. The data indicate that prior barren-cage housing moderates MA-induced neostriatal astrogliosis and initial spatial learning, but has no protective effect when the platform is smaller and relocated and therefore requires cognitive flexibility in relearning.

Manganese in Drinking Water and Cognitive Abilities and Behavior at 10 Years of Age: A Prospective Cohort Study

BACKGROUND

Cross-sectional studies have indicated impaired neurodevelopment with elevated drinking water manganese concentrations (W-Mn), but potential susceptible exposure windows are unknown.

OBJECTIVES

We prospectively evaluated the effects of W-Mn, from fetal life to school age, on children's cognitive abilities and behavior.

METHODS

We assessed cognitive abilities and behavior in 1,265 ten-year-old children in rural Bangladesh using the Wechsler Intelligence Scale for Children (WISC-IV) and the Strengths and Difficulties Questionnaire (SDQ), respectively. Manganese in drinking water used during pregnancy and by the children at 5 y and 10 y was measured using inductively coupled plasma mass spectrometry.

RESULTS

The median W-Mn was 0.20 mg/L (range 0.001–6.6) during pregnancy and 0.34mg/L (<0.001–8.7) at 10 y. In multivariable-adjusted linear regression analyses, restricted to children with low arsenic (As) exposure, none of the W-Mn exposures was associated with the children’s cognitive abilities. Stratifying by gender (p for interaction in general <0.081) showed that prenatal W-Mn (3 mg/L) was positively associated with cognitive ability measures in girls but not in boys. W-Mn at all time points was associated with an increased risk of conduct problems, particularly in boys (range 24–43% per mg/L). At the same time, the prenatal W-Mn was associated with a decreased risk of emotional problems [odds ratio (OR)=0.39 (95% CI: 0.19, 0.82)] in boys. In girls, W-Mn was mainly associated with low prosocial scores [prenatal W-Mn: OR=1.48 (95% CI: 1.06, 1.88)].

CONCLUSIONS

Elevated prenatal W-Mn exposure was positively associated with cognitive function in girls, whereas boys appeared to be unaffected. Early life W-Mn exposure appeared to adversely affect children's behavior. https://doi.org/10.1289/EHP631.

Polyphenolic Extract of Euphorbia supina Attenuates Manganese-Induced Neurotoxicity by Enhancing Antioxidant Activity through Regulation of ER Stress and ER Stress-Mediated Apoptosis

Manganese (Mn) is an important trace element present in human body, which acts as an enzyme co-factor or activator in various metabolic reactions. While essential in trace amounts, excess levels of Mn in human brain can produce neurotoxicity, including idiopathic Parkinson’s disease (PD)-like extrapyramidal manganism symptoms. This study aimed to investigate the protective role of polyphenolic extract of Euphorbia supina (PPEES) on Mn-induced neurotoxicity and the underlying mechanism in human neuroblastoma SKNMC cells and Sprague-Dawley (SD) male rat brain. PPEES possessed significant amount of total phenolic and flavonoid contents. PPEES also showed significant antioxidant activity in 1,1-diphenyl-2-picrylhydrazyl (DPPH) radical scavenging and reducing power capacity (RPC) assays. Our results showed that Mn treatment significantly reduced cell viability and increased lactate dehydrogenase (LDH) level, which was attenuated by PPEES pretreatment at 100 and 200 µg/mL. Additionally, PPEES pretreatment markedly attenuated Mn-induced antioxidant status alteration by resolving the ROS, MDA and GSH levels and SOD and CAT activities. PPEES pretreatment also significantly attenuated Mn-induced mitochondrial membrane potential (ΔΨm) and apoptosis. Meanwhile, PPEES pretreatment significantly reversed the Mn-induced alteration in the GRP78, GADD34, XBP-1, CHOP, Bcl-2, Bax and caspase-3 activities. Furthermore, administration of PPEES (100 and 200 mg/kg) to Mn exposed rats showed improvement of histopathological alteration in comparison to Mn-treated rats. Moreover, administration of PPEES to Mn exposed rats showed significant reduction of 8-OHdG and Bax immunoreactivity. The results suggest that PPEES treatment reduces Mn-induced oxidative stress and neuronal cell loss in SKNMC cells and in the rat brain. Therefore, PPEES may be considered as potential treat-ment in Mn-intoxicated patients.

Manganese and Developmental Neurotoxicity

Manganese (Mn) is an essential metal that plays a fundamental role for brain development and functioning. Environmental exposure to Mn may lead to accumulation in the basal ganglia and development of Parkinson-like disorders. The most recent research is focusing on early-life overexposure to Mn and the potential vulnerability of younger individuals to Mn toxicity also in regard to cognitive and executive functions through the involvement of the frontal cortex.Neurodevelopmental disturbances are increasing in the society, and understanding the potential role of environmental determinants is a key for prevention. Therefore, assessing the environmental sources of Mn exposure and the mechanisms of developmental neurotoxicity and defining appropriate biomarkers of exposure and early functional alterations represent key issues to improve and address preventive strategies. These themes will be reviewed in this chapter.

Effects of Neonatal Methamphetamine and Stress on Brain Monoamines and Corticosterone in Preweanling Rats

Neonatal exposure to methamphetamine (MA) and developmental chronic stress significantly alter neurodevelopmental profiles that show a variety of long-term physiological and behavioral effects. In the current experiment, Sprague-Dawley rats were exposed to one of two housing conditions along with MA. Rats were given 0 (saline), 5, or 7.5 mg/kg MA, four times per day from postnatal day (P)11 to 15 or P11 to 20. Half of the litters were reared in cages with standard bedding and half with no bedding. Separate litters were assessed at P15 or P20 for organ weights (adrenals, spleen, thymus); corticosterone; and monoamine assessments (dopamine, serotonin, norepinephrine) and their metabolites within the neostriatum, hippocampus, and prefrontal cortex. Findings show neonatal MA altered monoamines, corticosterone, and organ characteristics alone, and as a function of developmental age and stress compared with controls. These alterations may in part be responsible for MA and early life stress-induced long-term learning and memory deficits.

Developmental manganese neurotoxicity in rats: Cognitive deficits in allocentric and egocentric learning and memory

Manganese (Mn) is an essential element but neurotoxic at higher exposure levels. The effects of Mn overexposure (MnOE) on hippocampal and striatal-dependent learning and memory in rats were tested in combination with iron deficiency (FeD) and developmental stress that often co-occur with MnOE. Moderate FeD affects up to 15% of U.S. children and developmental stress is common in lower socio-economic areas where MnOE occurs. Pregnant Sprague-Dawley rats and their litters were housed in cages with or without (barren cage (BAR)) standard bedding from embryonic day (E)7 to postnatal day (P)28. Dams were fed a 90% FeD or iron sufficient (FeS) diet from E15-P28. Within each litter, separate offspring were treated with 100mg/kg Mn (MnOE) or vehicle (VEH) by gavage on alternate days from P4-28. Offspring were tested as adults in the Morris and Cincinnati water mazes. FeD and developmental stress interactively impaired spatial learning in the Morris water maze. Developmental stress and MnOE impaired learning and memory in both mazes. MnOE resulted in reduced CA1 hippocampal long-term potentiation (LTP) and increased levels of α-synuclein. Preweaning MnOE resulted in cognitive deficits on multiple domains of learning and memory accompanied by impaired LTP and α-synuclein changes, effects worsened by developmental stress.

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.

Manganese in teeth and neurodevelopment in young Mexican-American children

INTRODUCTION

Manganese (Mn) is an essential nutrient but higher exposure has been associated with poorer neurodevelopment in children.

METHODS

We measured Mn levels in prenatal (Mnpre) (n=197) and postnatal (Mnpost) dentin (n=193) from children's shed teeth using laser ablation inductively coupled plasma mass spectroscopy and examined the relationship with children's scores on the Mental Development Index (MDI) and Psychomotor Development Index (PDI) on the Bayley Scales of Infant Development at 6, 12, and 24-months. We explored non-linear associations and interactions by sex, blood lead concentrations and maternal iron status during pregnancy.

RESULTS

A two-fold increase of Mnpost levels in dentin was associated with small decreases in MDI at 6-months and 12-months of age. We also observed a non-linear relationship between Mnpost levels and PDI at 6-months. We found effect modification by sex for Mnpost levels and neurodevelopment at 6-months with stronger effects among girls for both MDI (-1.5 points; 95% Confidence Interval (CI): -2.4, -0.6) and PDI (-1.8 points; 95% CI: -3.3, -0.3). Girls whose mothers had lower hemoglobin levels experienced larger decreases in MDI and PDI associated with Mnpre levels than girls whose mothers had higher hemoglobin levels (pinteraction=0.007 and 0.09, respectively). We did not observe interactions with blood lead concentrations or any relationships with neurodevelopment at 24-months.

CONCLUSIONS

Using Mn measurements in tooth dentin, a novel biomarker that provides prenatal and early postnatal levels, we observed negative transient associations between postnatal Mn levels and early neurodevelopment with effect modification by sex and interactions with prenatal hemoglobin.

Manganese Toxicity Upon Overexposure: a Decade in Review

Exposure to manganese (Mn) causes clinical signs and symptoms resembling, but not identical to, Parkinson's disease. Since our last review on this subject in 2004, the past decade has been a thriving period in the history of Mn research. This report provides a comprehensive review on new knowledge gained in the Mn research field. Emerging data suggest that beyond traditionally recognized occupational manganism, Mn exposures and the ensuing toxicities occur in a variety of environmental settings, nutritional sources, contaminated foods, infant formulas, and water, soil, and air with natural or man-made contaminations. Upon fast absorption into the body via oral and inhalation exposures, Mn has a relatively short half-life in blood, yet fairly long half-lives in tissues. Recent data suggest Mn accumulates substantially in bone, with a half-life of about 8-9 years expected in human bones. Mn toxicity has been associated with dopaminergic dysfunction by recent neurochemical analyses and synchrotron X-ray fluorescent imaging studies. Evidence from humans indicates that individual factors such as age, gender, ethnicity, genetics, and pre-existing medical conditions can have profound impacts on Mn toxicities. In addition to body fluid-based biomarkers, new approaches in searching biomarkers of Mn exposure include Mn levels in toenails, non-invasive measurement of Mn in bone, and functional alteration assessments. Comments and recommendations are also provided with regard to the diagnosis of Mn intoxication and clinical intervention. Finally, several hot and promising research areas in the next decade are discussed.

Effects of developmental exposure to manganese and/or low iron diet: Changes to metal transporters, sucrose preference, elevated zero-maze, open-field, and locomotion in response to fenfluramine, amphetamine, and MK-801

Manganese overexposure (MnOE) can be neurotoxic. In humans this can occur through occupational exposure, air or water contamination, well water, soy milk, and some baby formulas. In children MnOE has been associated with cognitive and behavioral deficits. The effects of MnOE may be modified by factors such as iron status. We hypothesized that developmental MnOE would be exacerbated by iron deficiency. A diet with a 90% decrease in iron (FeD) was given to gravid female rats starting on embryonic day 15 and continued through postnatal day (P)28. Mn (100 mg/kg) or vehicle (VEH) was administered by gavage every other day from P4-28. Metal transporters and receptors (divalent metal transporter-1 (DMT1), transferrin (Tf), transferrin receptor (TfR), and zip8 (zrt8)) were quantified in brain at P28. These markers were increased but the changes were specific: MnOE increased TfR and decreased Tf in hippocampus, whereas FeD increased TfR in neostriatum and increased TfR and DMT1 in the hippocampus, and the combination increased TfR in neostriatum (zip8 was unaffected). Identically treated animals were tested behaviorally at P29 or P60. The combination of FeD+MnOE increased head dips in an elevated zero-maze, reversed deficits in sucrose preference induced by MnOE alone, and increased spontaneous locomotion in an open-field. Rats were also evaluated for changes in locomotor activity after challenge with (±)-fenfluramine (FEN, a 5-HT agonist: 5 mg/kg), MK-801 (MK801, an NMDA antagonist: 0.2 mg/kg), or (+)amphetamine (AMPH, a dopamine agonist: 1 mg/kg). Compared with VEH animals, MnOE animals were more hyperactive after fenfluramine, amphetamine, or MK-801, regardless of FeD exposure. The results indicate persistent effects of developmental MnOE on brain and behavior but few interactions with dietary iron deficiency.