Low-level lead exposure selectively enhances dopamine overflow in nucleus accumbens: an in vivo electrochemistry time course assessment

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.

The intertwining between lead and ethanol in the model organism Caenorhabditis elegans

Caenorhabditis elegans (C. elegans) is a model organism widely used to evaluate the mechanistic aspects of toxicants with the potential to predict responses comparable to those of mammals. We report here the consequences of developmental lead (Pb) exposure on behavioral responses to ethanol (EtOH) in C. elegans. In addition, we present data on morphological alterations in the dopamine (DA) synapse and DA-dependent behaviors aimed to dissect the neurobiological mechanisms that underlie the relationship between these neurotoxicants. Finally, the escalation to superior animals that parallels the observed effects in both experimental models with references to EtOH metabolism and oxidative stress is also discussed. Overall, the literature revised here underpins the usefulness of C. elegans to evidence behavioral responses to a combination of neurotoxicants in mechanistic-orientated studies.

Neurotoxicants, the Developing Brain, and Mental Health

While life in urban environments may confer a number of benefits, it may also result in a variety of exposures, with toxic consequences for neurodevelopment and neuropsychological health. Neurotoxicants are any of a large number of chemicals or substances that interfere with normal function and/or compromise adaptation in the central and/or peripheral nervous system. Evidence suggests that neurotoxicant effects have a greater effect when occurring in utero and during early childhood. Recent findings exploring neural-level mechanisms provide a crucial opportunity to explore the ways in which environmental conditions may get "under the skin" to impact a number of psychological behaviors and cognitive processes, ultimately allowing for greater synergy between macro- and microlevel efforts to improve mental health in the presence of neurotoxicant exposures. In this review, we provide an overview of 3 types of neurotoxicants related to the built environment and relevant to brain development during childhood and adolescence: lead exposure, outdoor particulate matter pollution, and endocrine-disrupting chemicals. We also discuss mechanisms through which these neurotoxicants affect central nervous system function, including recent evidence from neuroimaging literature. Furthermore, we discuss neurotoxicants and mental health during development in the context of social determinants and how differences in the spatial distribution of neurotoxicant exposures result in health disparities that disproportionately affect low-income and minority populations. Multifaceted approaches incorporating social systems and their effect on neurotoxicant exposures and downstream mental health will be key to reduce societal costs and improve quality of life for children, adolescents, and adults.

Review of rodent models of attention deficit hyperactivity disorder

Attention deficit hyperactivity disorder (ADHD) is a polygenic neurodevelopmental disorder that affects 8-12 % of children and >4 % of adults. Environmental factors are believed to interact with genetic predispositions to increase susceptibility to ADHD. No existing rodent model captures all aspects of ADHD, but several show promise. The main genetic models are the spontaneous hypertensive rat, dopamine transporter knock-out (KO) mice, dopamine receptor subtype KO mice, Snap-25 KO mice, guanylyl cyclase-c KO mice, and latrophilin-3 KO mice and rats. Environmental factors thought to contribute to ADHD include ethanol, nicotine, PCBs, lead (Pb), ionizing irradiation, 6-hydroxydopamine, neonatal hypoxia, some pesticides, and organic pollutants. Model validation criteria are outlined, and current genetic models evaluated against these criteria. Future research should explore induced multiple gene KOs given that ADHD is polygenic and epigenetic contributions. Furthermore, genetic models should be combined with environmental agents to test for interactions.

Morphine-element interactions - The influence of selected chemical elements on neural pathways associated with addiction

Addiction is a pressing social problem worldwide and opioid dependence can be considered the strongest and most difficult addiction to treat. Mesolimbic and mesocortical dopaminergic pathways play an important role in modulation of cognitive processes and decision making and, therefore, changes in dopamine metabolism are considered the central basis for the development of dependence. Disturbances caused by excesses or deficiency of certain elements have a significant impact on the functioning of the central nervous system (CNS) both in physiological conditions and in pathology and can affect the cerebral reward system and therefore, may modulate processes associated with the development of addiction. In this paper we review the mechanisms of interactions between morphine and zinc, manganese, chromium, cadmium, lead, fluoride, their impact on neural pathways associated with addiction, and on antinociception and morphine tolerance and dependence.

Neurobehavioral effects of acute and chronic lead exposure in a desert rodent Meriones shawi: Involvement of serotonin and dopamine

Lead (Pb) is a non physiological metal that has been implicated in toxic processes affecting several organs and biological systems, including the central nervous system. Several studies have focused on changes in lead-associated neurobehavioral and neurochemical alterations that occur due to Pb exposure. The present study evaluates the effects of acute and chronic Pb acetate exposure on serotoninergic and dopaminergic systems within the dorsal raphe nucleus, regarding motor activity and anxiety behaviours. Experiments were carried out on adult male Meriones shawi exposed to acute lead acetate intoxication (25 mg/kg b.w., 3 i.p. injections) or to a chronic lead exposure (0,5%) in drinking water from intrauterine age to adult age. Immunohistochemical staining demonstrated that both acute and chronic lead exposure increased anti-serotonin (anti-5HT) and tyrosine hydroxylase (anti-TH) immuno-reactivities in the dorsal raphe nucleus. In parallel, our results demonstrated that a long term Pb-exposure, but not an acute lead intoxication, induced behavioural alterations including, hyperactivity (open field test), and anxiogenic like-effects. Such neurobehavioral impairments induced by Pb-exposure in Meriones shawi may be related to dopaminergic and serotoninergic injuries identified in the dorsal raphe nucleus.

An Important Personality Trait Varies with Blood and Plumage Metal Concentrations in a Free-Living Songbird

Metal pollution is a global problem, which threatens to seriously disrupt behavioral patterns and health in humans and wildlife. Nonetheless, little is known regarding how exposure to metal pollution affects animal personalities, as defined by repeatable among individual differences in behavior. We used a large dataset to investigate the relationship between individual blood and feather metal concentrations and three personality traits (exploration behavior, territorial aggressiveness, and aggressiveness during nest defense) in great tits (Parus major), a model species for animal personality research. We previously demonstrated slower exploration behavior at highly polluted study sites, where exposure to lead, cadmium, and arsenic is high. Here, we demonstrate the across-year repeatability of exploration behavior and aggressiveness during nest defense, providing strong evidence for the existence of personalities in our populations. Furthermore, we demonstrate that individuals with high blood lead concentrations and high concentrations of multiple metals in the feathers exhibit slower exploration behavior but no differences in territorial aggressiveness or nest defense relative to less exposed birds. The mechanism underlying the relationship between metal exposure and exploration behavior remains to be determined but could involve neurotoxic effects. Our study highlights that metal pollution could have underappreciated effects on animal personalities, with implications for individual fitness and societal function.

Crocus sativus restores dopaminergic and noradrenergic damages induced by lead in Meriones shawi: A possible link with Parkinson's disease

Lead (Pb) is a metal element released into the atmosphere and a major source of environmental contamination. The accumulation and concentration of this metal in a food web may lead to the intoxication of the body, more precisely, the nervous system (NS). In addition, Pb-exposure can cause structural and functional disruption of the NS. Studies have shown that Pb-exposure could be a risk factor in the development of Parkinson's disease (PD). The latter is related to dopaminergic deficiency that may be triggered by genetic and environmental factors such as Pb intoxication. In this study, we have evaluated, in one hand, the neurotoxic effect of Pb (25 mg / kg B.W i.p) for three consecutive days on dopaminergic system and locomotor performance in Merione shawi. In the other hand, the possible restorative potential of C. sativus (CS) (50 mg / kg BW) by oral gavage. The immunohistochemical approach has revealed that Pb-intoxicated Meriones show a significant increase of Tyrosine Hydroxylase (TH) levels within the Substantia Nigra compacta (SNc), Ventral Tegmental Area (VTA), Locus Coeruleus (LC), Dorsal Striatum (DS) and Medial Forebrain Bundle (MFB), unlike the control meriones, a group intoxicated and treated with Crocus sativus hydroethanolic extract (CSHEE) and treated group by CSHEE. Treatment with CSHEE, has shown a real potential to prevent all Pb-induced damages. In fact, restores the TH levels by 92%, 90%, 88%, 90% and 93% in SNc, VTA, LC, DS and MFB respectively, similarly, locomotor activity dysfunction in Pb-intoxicaed meriones was reinstated by 90%. In this study, we have revealed a new pharmacological potential of Crocus sativus that can be used as a neuroprotective product for neurodegenerative disorders, especially, which implying dopaminergic and noradrenergic injuries, like PD, trigged by heavy metals.

"Turn-on" Pb2+ sensing and rapid detection of biothiols in aqueous medium and real samples

Detection of lead has continued to be of immense interest in the present industrial as well as environmental diaspora. To this end, we report a prudent Schiff base which enables the sensitive detection of Pb2+ ions in mixed aqueous medium. The probe is afforded by simple synthetic and purification processes. Further, the probe employs simple mechanistic detection of Pb2+ and also successively detects a host of biothiols. The sensor is also used to detect Pb2+ ions in real water samples and consecutive detection of a variety of biothiols, including functionalized thiouracils, in onion and garlic extract. The fact that the Pb2+-L ensemble is 'non-selective' towards any particular thiol substituted analyte in real samples could as well prove to be an interesting contribution for chemical and biological detoxification processes.

Endocrine active metals, prenatal stress and enhanced neurobehavioral disruption

Metals, including lead (Pb), methylmercury (MeHg) and arsenic (As), are long-known developmental neurotoxicants. More recently, environmental context has been recognized to modulate metals toxicity, including nutritional state and stress exposure. Modulation of metal toxicity by stress exposure can occur through shared targeting of endocrine systems, such as the hypothalamic-pituitary-adrenal axis (HPA). Our previous rodent research has identified that prenatal stress (PS) modulates neurotoxicity of two endocrine active metals (EAMs), Pb and MeHg, by altering HPA and CNS systems disrupting behavior. Here, we review this research and further test the hypothesis that prenatal stress modulates metals neurotoxicity by expanding to test the effect of developmental As ± PS exposure. Serum corticosterone and behavior was assessed in offspring of dams exposed to As ± PS. PS increased female offspring serum corticosterone at birth, while developmental As exposure decreased adult serum corticosterone in both sexes. As + PS induced reductions in locomotor activity in females and reduced response rates on a Fixed Interval schedule of reinforcement in males, with the latter suggesting unique learning deficits only in the combined exposure. As-exposed males showed increased time in the open arms of an elevated plus maze and decreased novel object recognition whereas females did not. These data further confirm the hypothesis that combined exposure to chemical (EAMs) and non-chemical (PS) stressors results in enhanced neurobehavioral toxicity. Given that humans are exposed to multiple environmental risk factors that alter endocrine function in development, such models are critical for risk assessment and public health protection, particularly for children.

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.

Interaction between DRD2 and lead exposure on the cortical thickness of the frontal lobe in youth with attention-deficit/hyperactivity disorder

BACKGROUND

The dopamine receptor D2 receptor (DRD2) gene and lead exposure are both thought to contribute to the pathophysiology of attention-deficit/hyperactivity disorder (ADHD). ADHD is characterized by delay in brain maturation, most prominent in the prefrontal cortex (PFC). The D2 receptor is also mainly located in the PFC, and animal studies show that lead exposure affects the dopaminergic system of the frontal lobe, indicating an overlap in neural correlates of ADHD, DRD2, and lead exposure. We examined the interaction effects of DRD2 rs1800497 and lead exposure on the cortical thickness of the frontal lobe in patients with ADHD.

METHODS

A 1:1 age- and gender-matched sample of 75 participants with ADHD and 75 healthy participants was included in the analysis. The interaction effects of DRD2 and lead exposure on the cortical thickness of 12 regions of interest in the frontal lobe were examined by multivariable linear regression analyses.

RESULTS

When we investigated the DRD2×lead effects in the ADHD and HC groups separately, significant DRD2×lead effects were found in the ADHD group, but not in the healthy control group in multiple ROIs of the frontal lobe. There was a significant negative correlation between the cortical thickness of the right superior frontal gyrus and inattention scores.

CONCLUSIONS

The present findings demonstrated significant interaction effects of DRD2 and lead exposure on the cortical thickness of the frontal lobe in ADHD. Replication studies with larger sample sizes, using a prospective design, are warranted to confirm these findings.

New Research Strategy for Measuring Pre- and Postnatal Metal Dysregulation in Psychotic Disorders

While previous studies have found evidence for detrimental effects of metals on neurodevelopment, the long-term effects on mental health remain unclear. The objective was to explore the effect of early metal exposure on risk of psychotic disorder and on symptom severity following illness onset. Through the use of validated tooth-biomarkers, we estimated pre- and postnatal exposure levels of essential elements (copper, magnesium, manganese, and zinc) and elements associated with neurotoxicity (lead, arsenic, lithium, and tin). We found consistently higher levels of lithium in patients compared to controls. Higher levels of magnesium and lower levels of zinc were associated with more severe psychopathology over 20 years after metal exposure. The results show promise for the use of teeth biomarkers in examining early environmental risk for psychosis and underscore the relevance of studying metal exposure during critical neurodevelopmental periods.

Developmental Lead and/or Prenatal Stress Exposures Followed by Different Types of Behavioral Experience Result in the Divergence of Brain Epigenetic Profiles in a Sex, Brain Region, and Time-Dependent Manner: Implications for Neurotoxicology

Over a lifetime, early developmental exposures to neurocognitive risk factors, such as lead (Pb) exposures and prenatal stress (PS), will be followed by multiple varied behavioral experiences. Pb, PS and behavioral experience can each influence brain epigenetic profiles. Our recent studies show a greater level of complexity, however, as all three factors interact within each sex to generate differential adult variation in global post-translational histone modifications (PTHMs), which may result in fundamentally different consequences for life-long learning and behavioral function. We have reported that PTHM profiles differ by sex, brain region and time point of measurement following developmental exposures to Pb±PS, resulting in different profiles for each unique combination of these parameters. Imposing differing behavioral experience following developmental Pb±PS results in additional divergence of PTHM profiles, again in a sex, brain region and time-dependent manner, further increasing complexity. Such findings underscore the need to link highly localized and variable epigenetic changes along single genes to the highly-integrated brain functional connectome that is ultimately responsible for governing behavioral function. Here we advance the idea that increased understanding may be achieved through iterative reductionist and holistic approaches. Implications for experimental design of animal studies of developmental exposures to neurotoxicants include the necessity of a 'no behavioral experience' group, given that epigenetic changes in response to behavioral testing can confound effects of the neurotoxicant itself. They also suggest the potential utility of the inclusion of salient behavioral experiences as a potential effect modifier in epidemiological studies.

Lead exposure is related to hypercortisolemic profiles and allostatic load in Brazilian older adults

Lead levels (Pb) have been linked to both hyper- and hypo-reactivity of hypothalamic-pituitary-adrenal axis (HPA) axis to acute stress in animals and humans. Similarly, allostatic load (AL), the 'wear and tear' of chronic stress, is associated with inadequate HPA axis activity. We examined whether Pb levels would be associated with altered diurnal cortisol profile, as a primary mediator of AL, during aging. Pb levels were measured from blood samples (BPb) of 126 Brazilian individuals (105 women), between 50 and 82 years old. Six neuroendocrine, metabolic, and anthropometric biomarkers were analyzed and values were transformed into an AL index using clinical reference cut-offs. Salivary samples were collected at home over 2 days at awakening, 30-min after waking, afternoon, and evening periods to determine cortisol levels. A multiple linear regression model showed a positive association between BPb as the independent continuous variable and cortisol awakening response (R²=0.128; B=0.791; p=0.005) and overall cortisol concentration (R²=0.266; B=0.889; p<0.001) as the outcomes. Repeated measures ANOVA showed that individuals with high BPb levels showed higher cortisol at 30min after awakening (p=0.003), and in the afternoon (p=0.002) than those with low BPb values. Regarding AL, regression model showed that BPb was positively associated with AL index (R²=0.100; B=0.204; p=0.032). Correlation analyzes with individual biomarkers showed that BPb was positively correlated with HDL cholesterol (p=0.02) and negatively correlated with DHEA-S (p=0.049). These findings suggest that Pb exposure, even at levels below the reference blood lead level for adults recommended by the National Institute for Occupational Safety and Health and by the Center for Disease Control and Prevention, may contribute to AL and dysregulated cortisol functioning in older adults. Considering these findings were based on cross-sectional data future research is needed to confirm our exploratory results.

Association between heavy metal exposure and poor working memory and possible mediation effect of antioxidant defenses during aging

BACKGROUND

Inverse associations have been observed between memory performance and blood concentrations of cadmium (Cd) and lead (Pb). Low antioxidant cell activity has also been linked to decline in memory due to aging. However, it has not yet been established whether the heavy metal-memory relationship is mediated by differences in antioxidant activity.

METHODS

We examined Cd and Pb levels, as well as oxidative stress parameters, in blood samples from 125 older adults (age range 50-82years). The Counting Span Test (CST) was used to evaluate working memory capacity (WMC). The Monte Carlo Method for Assessing Mediation (MCMAM) was used to analyze the mediation role of antioxidant activity in the heavy metals-memory association.

RESULTS

High blood Cd (BCd) concentration alone, and in combination with elevated blood Pb (BPb) concentration, was associated with poor WMC (p≤0.001) and low enzymatic antioxidant defenses (p≥0.006). The variance in WMC accounted for by BCd or by BCd combine with BPb was 20.6% and 18.6%, respectively. The MCMAM revealed that the influence of BCd and BPb concentrations on WMC was mediated by low antioxidant capacity (confidence interval - CI: 0.072 to -0.064 for BCd; CI: -0.062 to -0.045 for BPb).

CONCLUSION

These findings showed Pb and Cd blood concentration in older adults, even at levels below the current recommended threshold, was negatively associated with WMC and that this relationship may be partly mediated by low antioxidant defenses. Knowledge on the environmental factors that negatively influence brain and cognition during aging can help inform public policy strategies to prevent and control the adverse effects of environmental contaminant exposure during aging.

Early-life metal exposure and schizophrenia: A proof-of-concept study using novel tooth-matrix biomarkers

BACKGROUND

Despite evidence for the effects of metals on neurodevelopment, the long-term effects on mental health remain unclear due to methodological limitations. Our objective was to determine the feasibility of studying metal exposure during critical neurodevelopmental periods and to explore the association between early-life metal exposure and adult schizophrenia.

METHODS

We analyzed childhood-shed teeth from nine individuals with schizophrenia and five healthy controls. We investigated the association between exposure to lead (Pb(2+)), manganese (Mn(2+)), cadmium (Cd(2+)), copper (Cu(2+)), magnesium (Mg(2+)), and zinc (Zn(2+)), and schizophrenia, psychotic experiences, and intelligence quotient (IQ). We reconstructed the dose and timing of early-life metal exposures using laser ablation inductively coupled plasma mass spectrometry.

RESULTS

We found higher early-life Pb(2+) exposure among patients with schizophrenia than controls. The differences in log Mn(2+) and log Cu(2+) changed relatively linearly over time to postnatal negative values. There was a positive correlation between early-life Pb(2+) levels and psychotic experiences in adulthood. Moreover, we found a negative correlation between Pb(2+) levels and adult IQ.

CONCLUSIONS

In our proof-of-concept study, using tooth-matrix biomarker that provides direct measurement of exposure in the fetus and newborn, we provide support for the role of metal exposure during critical neurodevelopmental periods in psychosis.

Environmental exposure to metals, neurodevelopment, and psychosis

PURPOSE OF REVIEW

This article presents a new hypothesis about the possible relation between early life exposure to metals and psychosis. We review limitations of available research, and discuss novel approaches to overcome previous methodological barriers.

RECENT FINDINGS

Mechanistic studies suggest a possible association between excess lead, manganese, cadmium, arsenic, or copper, and zinc deficiency, and several biochemical disturbances related to psychosis, such as altered neurotransmitters levels, excitotoxicity, and inflammation. Furthermore, studies suggest that some metals (lead, manganese, cadmium excess, and zinc deficiency) are associated with schizophrenia or psychosis-related phenotype. However, previous studies had multiple methodological limitations. Importantly, metal exposure was often measured after disease development and seldom determined during critical developmental periods. Most studies fell short of depicting the exact timing of exposure and the change in exposure over time. Here, we propose several methods to overcome these methodological limitations.

SUMMARY

There is a plausible role of early life exposure to metals in the cause of psychosis. Owing to methodological limitations in exposure measurement, this has not been well characterized. Considering the wide exposure to metals and the high cost of psychosis to society, this hypothesis should be rigorously examined.

Lead neurotoxicity: exploring the potential impact of lead substitution in zinc-finger proteins on mental health

This critical review focuses on one possible link between the cellular biology of lead and its neurotoxic effects: the link between Pb²⁺substitution for Zn²⁺in zinc-finger proteins and mental illness in adulthood.

Cytotoxic, genotoxic, and neurotoxic effects of Mg, Pb, and Fe on pheochromocytoma (PC-12) cells

Metals such as lead (Pb), magnesium (Mg), and iron (Fe) are ubiquitous in the environment as a result of natural occurrence and anthropogenic activities. Although Mg, Fe, and others are considered essential elements, high level of exposure has been associated with severe adverse health effects including cardiovascular, hematological, nephrotoxic, hepatotoxic, and neurologic abnormalities in humans. In the present study we hypothesized that Mg, Pb, and Fe are cytotoxic, genotoxic and neurotoxic, and their toxicity is mediated through oxidative stress and alteration in protein expression. To test the hypothesis, we used the pheochromocytoma (PC-12) cell line as a neuro cell model and performed the LDH assay for cell viability, Comet assay for DNA damage, Western blot for oxidative stress, and HPLC-MS to assess the concentration levels of neurological biomarkers such as glutamate, dopamine (DA), and 3-methoxytyramine (3-MT). The results of this study clearly show that Mg, Pb, and Fe, respectively in the form of MgSO4 , Pb(NO3 )2 , FeCl2 , and FeCl3 induce cytotoxicity, oxidative stress, and genotoxicity in PC-12 cells. In addition, exposure to these metallic compounds caused significant changes in the concentration levels of glutamate, dopamine, and 3-MT in PC-12 cells. Taken together the findings suggest that MgSO4 , Pb(NO3 )2 , FeCl2 , and FeCl3 have the potential to induce substantial toxicity to PC-12 cells.

Early-life lead exposure recapitulates the selective loss of parvalbumin-positive GABAergic interneurons and subcortical dopamine system hyperactivity present in schizophrenia

Environmental factors have been associated with psychiatric disorders and recent epidemiological studies suggest an association between prenatal lead (Pb(2+)) exposure and schizophrenia (SZ). Pb(2+) is a potent antagonist of the N-methyl-D-aspartate receptor (NMDAR) and converging evidence indicates that NMDAR hypofunction has a key role in the pathophysiology of SZ. The glutamatergic hypothesis of SZ posits that NMDAR hypofunction results in the loss of parvalbumin (PV)-positive GABAergic interneurons (PVGI) in the brain. Loss of PVGI inhibitory control to pyramidal cells alters the excitatory drive to midbrain dopamine neurons increasing subcortical dopaminergic activity. We hypothesized that if Pb(2+) exposure in early life is an environmental risk factor for SZ, it should recapitulate the loss of PVGI and reproduce subcortical dopaminergic hyperactivity. We report that on postnatal day 50 (PN50), adolescence rats chronically exposed to Pb(2+) from gestation through adolescence exhibit loss of PVGI in SZ-relevant brain regions. PV and glutamic acid decarboxylase 67 kDa (GAD67) protein were significantly decreased in Pb(2+) exposed rats with no apparent change in calretinin or calbindin protein levels suggesting a selective effect on the PV phenotype of GABAergic interneurons. We also show that Pb(2+) animals exhibit a heightened locomotor response to cocaine and express significantly higher levels of dopamine metabolites and D2-dopamine receptors relative to controls indicative of subcortical dopaminergic hyperactivity. Our results show that developmental Pb(2+) exposure reproduces specific neuropathology and functional dopamine system changes present in SZ. We propose that exposure to environmental toxins that produce NMDAR hypofunction during critical periods of brain development may contribute significantly to the etiology of mental disorders.

Assessment of association between the dopamine D2 receptor (DRD2) polymorphism and neurodevelopment of children exposed to lead

The mechanism of lead (Pb) neurotoxicity has not been illustrated over the years. People pay more attention to dopaminergic neurotransmission, specifically dopamine receptor-2 (DRD2) Taq IA polymorphism, but no consensus has been reached. A total of 258 three-year-old children in Guiyu (exposed group) and Nanao (reference group), China were examined and their concentrations of blood lead (BPb) were determined. Cognitive and language scores of children were assessed using the Bayley Scales of Infant Development, third edition (BSID-III). Genotyping for the DRD2 polymorphism was carried out using a polymerase chain reaction (PCR) re-sequencing platform. The logistic stepwise regression analysis and stepwise regression analysis was used to explore associations among lead, neurodevelopment of children, and DRD2 Taq IA categories. Median values of Pb in Guiyu was higher than that of the reference group (11.30 ± 5.38 μg/dL vs. 5.77 ± 2.51 μg/dL, P < 0.001). Compared with the reference group, children from e-waste exposed area have lower cognitive scale scores (100 ± 25 vs 120 ± 20, P < 0.001) and lower language scale scores (99.87 ± 7.52 vs 111.39 ± 7.02, P < 0.001). The three kinds of genotype, A1/A1, A1/A2, and A2/A2, had no significant influences on BPb, cognitive scores and language scores (P > 0.05). Exposure of inhabitants, especially children to Pb from informal e-waste recycling activities might have contributed to higher levels of BPb and reduced cognitive and language scores observed in local children, however, the result obtained showed no significant association between DRD2 polymorphism and neurodevelopment of children exposed to lead.

An aggregation-induced emission based “turn-on” fluorescent chemodosimeter for the selective detection of Pb2+ ions

An aggregation-induced emission (AIE) based “turn-on” fluorescent chemodosimeter for selective detection of Pb²⁺ ions has been developed. The probe is a phosphate functionalized tetraphenylethylene derivative and the resulting lead–TPE complex has very low solubility in working solvent and triggers AIE and shows a low detection limit of 10 ppb.

Epigenetic histone modification regulates developmental lead exposure induced hyperactivity in rats

Lead (Pb) exposure was commonly considered as a high environmental risk factor for the development of attention-deficit/hyperactivity disorder (ADHD). However, the molecular basis of this pathological process still remains elusive. In light of the role of epigenetics in modulating the neurological disease and the causative environment, the alterations of histone modifications in the hippocampus of rats exposed by various doses of lead, along with concomitant behavioral deficits, were investigated in this study. According to the free and forced open field test, there showed that in a dosage-dependent manner, lead exposure could result in the increased locomotor activity of rats, that is, hyperactivity: a subtype of ADHD. Western blotting assays revealed that the levels of histone acetylation increased significantly in the hippocampus by chronic lead exposure, while no dramatic changes were detected in terms of expression yields of ADHD-related dopaminergic proteins, indicating that histone acetylation plays essential roles in this toxicant-involved pathogenesis. In addition, the increased level of histone acetylation might be attributed to the enzymatic activity of p300, a typical histone acetyltransferase, as the transcriptional level of p300 was significantly increased upon higher-dose Pb exposure. In summary, this study first discovered the epigenetic mechanism bridging the environmental influence (Pb) and the disease itself (ADHD) in the histone modification level, paving the way for the comprehensive understanding of ADHD's etiology and in further steps, establishing the therapy strategy of this widespread neurological disorder.

Environmental toxins and Parkinson's disease

Parkinson's disease (PD) is a chronic, progressive, disabling neurodegenerative disorder that begins in mid to late life and is characterized by motor impairment, autonomic dysfunction, and, in many, psychological and cognitive changes. Recent advances have helped delineate pathogenetic mechanisms, yet the cause of PD in most individuals is unknown. Although at least 15 genes and genetic loci have been associated with PD, identified genetic causes are responsible for only a few percent of cases. Epidemiologic studies have found increased risk of PD associated with exposure to environmental toxicants such as pesticides, solvents, metals, and other pollutants, and many of these compounds recapitulate PD pathology in animal models. This review summarizes the environmental toxicology of PD, highlighting the consistency of observations across cellular, animal, and human studies of PD pathogenesis.

Effect modification by transferrin C2 polymorphism on lead exposure, hemoglobin levels, and IQ

BACKGROUND

Iron deficiency and lead exposure remain significant public health issues in many parts of the world and are both independently associated with neurocognitive deficits. Polymorphisms in iron transport pathways have been shown to modify the absorption and toxicity of lead.

OBJECTIVE

We hypothesized that the transferrin (TF) C2 polymorphism modifies the effects of lead and hemoglobin on intelligence.

METHODS

Children aged 3-7 years (N=708) were enrolled from 12 primary schools in Chennai, India. The Binet-Kamath Scale of Intelligence were administered to ascertain intelligence quotient (IQ). Venous blood was analyzed for lead and hemoglobin levels. Genotyping for the TF C2 polymorphism (rs1049296) was carried out using a MassARRAY iPLEXTM platform. Stratified analyses and interaction models, using generalized estimating equations, were examined to explore interactions between lead, hemoglobin, and TF C2 categories.

RESULTS

A one-unit increase in log blood lead and 1g/dl higher hemoglobin was associated with -77 (95% CI: -136, -18) and 17 (95% CI 14, 21) IQ points, respectively, among children carrying the C2 variant. In comparison, among children who had the homozygous wildtype allele, the same increment of lead and hemoglobin were associated with -21(95% CI: -65, 24) and 28 (95% CI: 15, 40) IQ points, respectively. There was a significant interaction between lead (p=0.04) and hemoglobin (p=0.07) with the C2 variant.

CONCLUSION

Children who carry the TF C2 variant may be more susceptible to the neurotoxic effects of lead exposure and less protected by higher levels of hemoglobin.

Is lead exposure in early life an environmental risk factor for Schizophrenia? Neurobiological connections and testable hypotheses

Schizophrenia is a devastating neuropsychiatric disorder of unknown etiology. There is general agreement in the scientific community that schizophrenia is a disorder of neurodevelopmental origin in which both genes and environmental factors come together to produce a schizophrenia phenotype later in life. The challenging questions have been which genes and what environmental factors? Although there is evidence that different chromosome loci and several genes impart susceptibility for schizophrenia; and epidemiological studies point to broad aspects of the environment, only recently there has been an interest in studying gene × environment interactions. Recent evidence of a potential association between prenatal lead (Pb(2+)) exposure and schizophrenia precipitated the search for plausible neurobiological connections. The most promising connection is that in schizophrenia and in developmental Pb(2+) exposure there is strong evidence for hypoactivity of the N-methyl-d-aspartate (NMDA) subtype of excitatory amino acid receptors as an underlying neurobiological mechanism in both conditions. A hypofunction of the NMDA receptor (NMDAR) complex during critical periods of development may alter neurobiological processes that are essential for brain growth and wiring, synaptic plasticity and cognitive and behavioral outcomes associated with schizophrenia. We also describe on-going proof of concept gene-environment interaction studies of early life Pb(2+) exposure in mice expressing the human mutant form of the disrupted in schizophrenia 1 (DISC-1) gene, a gene that is strongly associated with schizophrenia and allied mental disorders.

A dopamine receptor (DRD2) but not dopamine transporter (DAT1) gene polymorphism is associated with neurocognitive development of Mexican preschool children with lead exposure

OBJECTIVE

To investigate the effects of prenatal and postnatal lead exposure and polymorphisms in dopamine metabolism genes on neurocognitive development of Mexican children at 24 months (n = 220) and 48 months (n = 186) of age.

STUDY DESIGN

We genotyped the dopamine transporter gene (DAT1; SLC6A3) variable nucleotide tandem repeat and the dopamine receptor D2 (DRD2) Taq1A single nucleotide polymorphism. Children were assessed at 24 months with Bayley Scales of Infant Development (Mental Development Index and Psychomotor Development Index) and at 48 months with McCarthy Scales of Children's Abilities.

RESULTS

Blood lead concentration (BLL) in umbilical cord was 6.6 ± 3.3 μg/dL (measured in 1995-96), 8.1 ± 4.4 μg/dL at 24 months, and 8.1 ± 3.6 μg/dL at 48 months. Cord BLL was negatively associated with Mental Development Index (P < .01) and Psychomotor Development Index (P < .1), but not McCarthy scores. The 48-month BLL, but not the 24-month BLL, was negatively associated with children's scores. Children with DRD2 TT genotype (variant) scored higher than children with CC genotype (wild type) on the Mental Development Index and McCarthy memory scale. Neither polymorphism modified the relationship between BLL (either prenatal or postnatal) and neurocognitive development.

CONCLUSION

Lead exposure was adversely associated with neurocognitive measures, whereas the DRD2 Taq1A TT variant was positively associated with neurocognitive measures. We found no evidence of gene-environment interactions on developmental outcomes in early childhood.

A transdisciplinary perspective of chronic stress in relation to psychopathology throughout life span development

The allostatic load (AL) model represents an interdisciplinary approach to comprehensively conceptualize and quantify chronic stress in relation to pathologies throughout the life cycle. This article first reviews the AL model, followed by interactions among early adversity, genetics, environmental toxins, as well as distinctions among sex, gender, and sex hormones as integral antecedents of AL. We next explore perspectives on severe mental illness, dementia, and caregiving as unique human models of AL that merit future investigations in the field of developmental psychopathology. A complimenting transdisciplinary perspective is applied throughout, whereby we argue that the AL model goes beyond traditional stress–disease theories toward the advancement of person-centered research and practice that promote not only physical health but also mental health.

Hemoglobin, lead exposure, and intelligence quotient: effect modification by the DRD2 Taq IA polymorphism

BACKGROUND

Anemia and lead exposure remain significant public health issues in many parts of the world, often occurring together. Animal studies suggest that the dopamine D2 receptor (DRD2) mediates the effects of both lead and iron on cognition and behavior.

OBJECTIVE

We tested the hypothesis that the DRD2 Taq IA polymorphism modifies the effects of lead and hemoglobin on intelligence quotient (IQ) among children.

METHODS

Blood lead and hemoglobin were assessed in 717 children 3-7 years of age attending 12 schools in Chennai, India. IQ was determined using the Binet-Kamat scales of intelligence. Genotyping for the DRD2 polymorphism was carried out using a MassARRAY iPLEX platform. Stratified analyses and interaction models, using generalized estimating equations (GEEs), were used to explore interactions between lead and hemoglobin, and DRD2 Taq IA categories [homozygous variant (A1) vs. presence of wild-type allele (A2)].

RESULTS

After we controlled for potential confounders, a one-unit increase in log blood lead was associated with a decrease of 9 IQ points [95% confidence interval (CI), -18.08 to -0.16] in the homozygous-variant children (n = 73) compared with a decrease of 4 IQ points (95% CI, -7.21 to -0.69) among those with the wild-type allele (n = 644). Higher hemoglobin levels were associated with higher IQ in the children who carried the wild-type allele DRD2, but in children homozygous for the variant allele, an increase of 1 g/dL hemoglobin was associated with a decrease in 1.82 points of IQ (95% CI, -5.28 to 1.64; interaction term p-value = 0.02).

CONCLUSION

The results of this study suggest that the DRD2 Taq IA polymorphism disrupts the protective effect of hemoglobin on cognition and may increase the susceptibility to the deficits in IQ due to lead exposure.

Lead and PCBs as risk factors for attention deficit/hyperactivity disorder

OBJECTIVES

Attention deficit/hyperactivity disorder (ADHD) is the most frequently diagnosed neurobehavioral disorder of childhood, yet its etiology is not well understood. In this review we present evidence that environmental chemicals, particularly polychlorinated biphenyls (PCBs) and lead, are associated with deficits in many neurobehavioral functions that are also impaired in ADHD.

DATA SOURCES

Human and animal studies of developmental PCB or lead exposures that assessed specific functional domains shown to be impaired in ADHD children were identified via searches of PubMed using "lead" or "PCB exposure" in combination with key words, including "attention," "working memory," "response inhibition," "executive function," "cognitive function," "behavior," and "ADHD."

DATA SYNTHESIS

Children and laboratory animals exposed to lead or PCBs show deficits in many aspects of attention and executive function that have been shown to be impaired in children diagnosed with ADHD, including tests of working memory, response inhibition, vigilance, and alertness. Studies conducted to date suggest that lead may reduce both attention and response inhibition, whereas PCBs may impair response inhibition to a greater degree than attention. Low-level lead exposure has been associated with a clinical diagnosis of ADHD in several recent studies. Similar studies of PCBs have not been conducted.

CONCLUSIONS

We speculate that exposures to environmental contaminants, including lead and PCBs, may increase the prevalence of ADHD.

Prenatal lead exposure enhances methamphetamine sensitization in rats

Adult female rats were exposed to lead-free sodium acetate via gavage [0 mg (vehicle control)] or to 16 mg lead as lead acetate for 30 days prior to breeding. Following confirmation of breeding, the female animals continued to be exposed to their respective doses throughout gestation and lactation. When weaned, 16 control and 16 lead-exposed offspring were placed on regular water and food (lead-exposure was discontinued) until postnatal day (PND) 70. At this time, one-half of the control animals and one-half of the lead-treatment animals received intraperitoneal (i.p.) injections of the vehicle (saline) for 10 successive days and the remaining animals in each exposure conditions received daily injections of 1.0 mg/kg (+)-methamphetamine (METH) for 10 days (N=8/group). Locomotion in automated chambers was monitored daily for 45 min post-injection. Subsequently, during dose-effect testing, all animals received consecutive daily i.p. injections of 0, 1.0, 2.0, and then 4.0 mg/kg METH. The results of the experiment showed that both control and lead-exposed animals exhibited heightened locomotor activity (i.e. behavioral sensitization) to the repeated administration of 1.0 mg/kg METH. More importantly, animals developmentally (perinatally) exposed to lead showed more rapid sensitization than did their control counterparts. These data indicate that early lead exposure increases sensitivity to the locomotor-stimulating effects of METH. In contrast, identically exposed lead animals exhibit diminished METH dose-effect responding when tested in an intravenous (i.v.) self-administration paradigm [Rocha A., Valles R., Bratton G.R., Nation J.R. Developmental lead exposure alters methamphetamine self-administration in the male rat: acquisition and reinstatement. Drug Alcohol Depend 2008a;95:23-29, Rocha A., Valles R., Hart N., Bratton G.R., Nation J.R. Developmental lead exposure attenuates methamphetamine dose-effect self-administration performance and progressive ratio responding in the male rat. Pharmacol Biochem Behav 2008b;89:508-514].

Alterations in glucocorticoid negative feedback following maternal Pb, prenatal stress and the combination: a potential biological unifying mechanism for their corresponding disease profiles

Combined exposures to maternal lead (Pb) and prenatal stress (PS) can act synergistically to enhance behavioral and neurochemical toxicity in offspring. Maternal Pb itself causes permanent dysfunction of the body's major stress system, the hypothalamic pituitary adrenal (HPA) axis. The current study sought to determine the potential involvement of altered negative glucocorticoid feedback as a mechanistic basis of the effects in rats of maternal Pb (0, 50 or 150 ppm in drinking water beginning 2 mo prior to breeding), prenatal stress (PS; restraint on gestational days 16-17) and combined maternal Pb+PS in 8 mo old male and female offspring. Corticosterone changes were measured over 24 h following an i.p. injection stress containing vehicle or 100 or 300 microg/kg (females) or 100 or 150 microg/kg (males) dexamethasone (DEX). Both Pb and PS prolonged the time course of corticosterone reduction following vehicle injection stress. Pb effects were non-monotonic, with a greater impact at 50 vs. 150 ppm, particularly in males, where further enhancement occurred with PS. In accord with these findings, the efficacy of DEX in suppressing corticosterone was reduced by Pb and Pb+PS in both genders, with Pb efficacy enhanced by PS in females, over the first 6 h post-administration. A marked prolongation of DEX effects was found in males. Thus, Pb, PS and Pb+PS, sometimes additively, produced hypercortisolism in both genders, followed by hypocortisolism in males, consistent with HPA axis dysfunction. These findings may provide a plausible unifying biological mechanism for the reported links between Pb exposure and stress-associated diseases and disorders mediated via the HPA axis, including obesity, hypertension, diabetes, anxiety, schizophrenia and depression. They also suggest broadening of Pb screening programs to pregnant women in high stress environments.

Lead-induced catalase activity differentially modulates behaviors induced by short-chain alcohols

Acute lead administration produces a transient increase in brain catalase activity. This effect of lead has been used to assess the involvement of brain ethanol metabolism, and therefore centrally formed acetaldehyde, in the behavioral actions of ethanol. In mice, catalase is involved in ethanol and methanol metabolism, but not in the metabolism of other alcohols such as 1-propanol or tert-butanol. In the present study, we assessed the specificity of the effects of lead acetate on catalase-mediated metabolism of alcohols, and the ability of lead to modulate the locomotion and loss of the righting reflex (LRR) induced by 4 different short-chain alcohols. Animals were pretreated i.p. with lead acetate (100 mg/kg) or saline, and 7 days later were injected i.p. with ethanol (2.5 or 4.5 g/kg), methanol (2.5 or 6.0 g/kg), 1-propanol (0.5 or 2.5 g/kg) or tert-butanol (0.5 or 2.0 g/kg) for locomotion and LRR, respectively. Locomotion induced by ethanol was significantly potentiated in lead-treated mice, while methanol-induced locomotion was reduced by lead treatment. The loss of righting reflex induced by ethanol was shorter in lead-treated mice, and lead produced the opposite effect in methanol-treated mice. There was no effect of lead on 1-propanol or tert-butanol-induced behaviors. Lead treatment was effective in inducing catalase activity and protein both in liver and brain. These results support the hypothesis that the effects of lead treatment on ethanol-induced behaviors are related to changes in catalase activity, rather than some nonspecific effect that generalizes to all alcohols.

Studying Toxicants as Single Chemicals: Does this Strategy Adequately Identify Neurotoxic Risk?

Despite the fact that virtually all chemicals exposure of humans are to mixtures, and that these mixed exposures occur in the context of numerous other risk modifiers, our current understanding of human health risks is based almost entirely on the evaluation of chemicals studied in isolation. This paper describes findings from our collaborative studies that prompt questions about these approaches in the context of neurotoxicology. The first section describes studies investigating the interactions of maternal Pb exposure with maternal stress. Examined across a range of outcome measures, it shows that maternal Pb can modulate the effects of maternal stress, and, conversely, stress modifies the effects of Pb. Further, effects of Pb+stress could be detected in the absence of an effect of either risk factor alone, and, moreover, the profile of effects of Pb alone differs notably from that of Pb+stress. Collectively, interactions were not systematic, but differed by brain region, gender and outcome measure. A second section describes outcomes of studies examining combined exposures to the pesticides paraquat (PQ) and maneb (MB) during development which likewise reveal potentiated effects of combined exposures. They also demonstrate examples of both progressive and cumulative neurotoxicity, including a marked vulnerability following gestational exposure to MB, to the effects of PQ, a pesticide with no structural relationship to MB. The ability of current hazard identification and risk assessment approaches to adequately identify and encompass such effects remains an important unanswered question. One consideration proposed for further evaluating potential interactions that may be of significance for the nervous system is based on a multi-hit hypothesis. It hypothesizes that the brain may readily compensate for the effects of an individual chemical itself acting on a particular target system, but when multiple target or functional sites within that one system are attacked by different mechanisms (i.e., multiple chemical exposures or chemical exposures combined with other risk factors), homeostatic capabilities may be restricted, thereby leading to sustained or cumulative damage.

Interactions of chronic lead exposure and intermittent stress: consequences for brain catecholamine systems and associated behaviors and HPA axis function

Elevated lead (Pb) burden and high stress levels are co-occurring risk factors in low socioeconomic status (SES) children. Our previous work demonstrated that maternal Pb exposure can permanently alter hypothalamic-pituitary-adrenal (HPA) axis function and responsivity to stress challenges in offspring. The current study sought to determine the consequences of chronic Pb exposures initiated later in development combined with variable intermittent stress challenges. Male rats were exposed chronically from weaning to 0, 50, or 150 ppm Pb acetate drinking solutions (producing blood Pb levels of <5, 9-15, and 23-27 mug/dl, respectively). Pb itself decreased basal plasma corticosterone, with greater effects at 50 than 150 ppm; 150 ppm reduced both cytosolic and nuclear glucocorticoid receptor binding. Responsivity to stress challenges including novelty, cold, and restraint, was measured as changes in Fixed Interval (FI) schedule-controlled behavior in a subset of rats within each group. FI performance was modified by novelty stress only in Pb-treated rats, whereas cold and restraint stress effects were comparable across groups. Novelty elevated corticosterone equivalently across groups, but cold stress markedly increased corticosterone only in Pb-treated groups. The pattern of Pb-induced changes in serotonin (5-HT) or its metabolite 5-HIAA in frontal cortex, nucleus accumbens, striatum, and hypothalamus resembled that observed for basal corticosterone levels indicating a relationship between these variables. In addition to suggesting the potential for HPA axis-mediated effects of Pb on the central nervous system, these findings also raise questions about whether single chemicals studied in isolation from other relevant risk factors can adequately identify neurotoxic hazards.

The role of acetaldehyde in the neurobehavioral effects of ethanol: A comprehensive review of animal studies

Acetaldehyde has long been suggested to be involved in a number of ethanol's pharmacological and behavioral effects, such as its reinforcing, aversive, sedative, amnesic and stimulant properties. However, the role of acetaldehyde in ethanol's effects has been an extremely controversial topic during the past two decades. Opinions ranged from those virtually denying any role for acetaldehyde in ethanol's effects to those who claimed that alcoholism is in fact "acetaldehydism". Considering the possible key role of acetaldehyde in alcohol addiction, it is critical to clarify the respective functions of acetaldehyde and ethanol molecules in the pharmacological and behavioral effects of alcohol consumption. In the present paper, we review the animal studies reporting evidence that acetaldehyde is involved in the pharmacological and behavioral effects of ethanol. A number of studies demonstrated that acetaldehyde administration induces a range of behavioral effects. Other pharmacological studies indicated that acetaldehyde might be critically involved in several effects of ethanol consumption, including its reinforcing consequences. However, conflicting evidence has also been published. Furthermore, it remains to be shown whether pharmacologically relevant concentrations of acetaldehyde are achieved in the brain after alcohol consumption in order to induce significant effects. Finally, we review current evidence about the central mechanisms of action of acetaldehyde.

Responsiveness of extracellular dopamine in the nucleus accumbens to systemic quinpirole and eticlopride is modulated by low-level lead exposure

Environmental lead exposure has been linked to learning and memory impairments as well as psychosocial deficits in children. Although the precise mechanisms by which lead exerts these effects are not completely understood, experimental animal studies suggest the involvement of mesolimbic dopamine system. Here, we investigated the effects of post weaning, 90-day exposure to 50ppm lead on the responsiveness of extracellular dopamine in the nucleus accumbens core to quinpirole and eticlopride using in vivo microdialysis in anesthetized Long-Evans rats. Attenuation of dopamine release by 1mg/kg quinpirole was more pronounced in lead-exposed rats compared to control rats. In contrast, eticlopride did not significantly modulate extracellular dopamine in lead-exposed rats as it did in the control group. Taken together, these results provides further evidence that exposure to environmentally relevant levels of lead disrupts mesolimbic dopamine system function, at least in part, through perturbations of D(2) receptor systems.

Postnatal Inorganic Lead Exposure Reduces Midbrain Dopaminergic Impulse Flow and Decreases Dopamine D1 Receptor Sensitivity in Nucleus Accumbens Neurons

Lead treatment via drinking water for 3 to 6 weeks at 250 ppm was found to significantly decrease the number of spontaneously active dopamine (DA) neurons in both the substantia nigra and ventral tegmental area that were recorded using standard extracellular electrophysiological recording techniques. Lead exposure did not affect the discharge rate or discharge pattern of these DA neurons. No significant decrease in the number of tyrosine hydroxylase immunopositive cells was detected in lead-treated animals relative to controls even though the length of lead exposure was extended beyond that of the electrophysiological studies. The significant lead-induced decrease in spontaneously active cells observed in the electrophysiological studies was, therefore, not due to cell death. An acute drug challenge with the DA receptor agonist apomorphine at a dose known to hyperpolarize midbrain DA neurons (50 mug/kg i.v.) was used to determine whether hyperpolarization would normalize the number of spontaneously active DA neurons. The results suggest that depolarization inactivation was most likely not the cause for this lead effect. The D(1) receptor agonist SKF-38393 [1-phenyl-2,3,4,5-tetrahydro-(1H)-3-benzazepine-7,8-diol] was iontophoretically applied to type I nucleus accumbens (Nacb) neurons. The results demonstrated that type I Nacb neurons have a significantly lower basal discharge rate in lead-treated animals relative to controls and that the Nacb DA D(1) receptors were significantly less sensitive to SKF-38393 in the lead-treated animals. Therefore, lead exposure decreases DA neuron impulse flow presynaptically and decreases DA D(1) receptor sensitivity postsynaptically in the nucleus accumbens.

Impaired Neuropsychological Functioning in Lead-Exposed Children

Neuropsychological functions were assessed in 174 children participating in a longitudinal study of low-level lead exposure. At age 5 1/2 years, children were administered the Working Memory and Planning Battery of the Cambridge Neuropsychological Testing Automated Battery. Measures of sociodemographic characteristics of the family, prenatal and perinatal risk, quality of caregiving and crowding in the home, and maternal and child intelligence were used as covariates to test the hypothesis that children with higher lifetime average blood lead concentrations would perform more poorly on tests of working memory, attentional flexibility, and planning and problem solving. The lifetime average blood lead level in this sample was 7.2 micrograms per deciliter (mug/dL; range: 0-20 mug/dL). Children with greater exposure performed more poorly on tests of executive processes. In both bivariate and multivariate analyses, children with higher lifetime average blood lead concentrations showed impaired performance on the tests of spatial working memory, spatial memory span, intradimensional and extradimensional shifts, and an analog of the Tower of London task. Many of the significant associations remained after controlling for children's intelligence test scores, in addition to the other covariates. These findings indicate that the effects of pediatric lead exposure are not restricted to global indexes of general intellectual functioning, and executive processes may be at particular risk of lead-induced neurotoxicity.