Neurotoxicity of low-level lead exposure: History, mechanisms of action, and behavioral effects in humans and preclinical models.

Lead is a neurotoxin that produces long-term, perhaps irreversible, effects on health and well-being. This article summarizes clinical and preclinical studies that have employed a variety of research techniques to examine the neurotoxic effects of low levels of lead exposure. A historical perspective is presented, followed by an overview of studies that examined behavioral and cognitive outcomes. In addition, a short summary of potential mechanisms of action is provided with a focus on calcium-dependent processes. The current level of concern, or reference level, set by the CDC is 5 µg/dL of lead in blood and a revision to 3.5 µg/dL has been suggested. However, levels of lead below 3 µg/dL have been shown to produce diminished cognitive function and maladaptive behavior in humans and animal models. Because much of the research has focused on higher concentrations of lead, work on low concentrations is needed to better understand the neurobehavioral effects and mechanisms of action of this neurotoxic metal.

Lead exposure-induced cognitive impairment through RyR-modulating intracellular calcium signaling in aged rats.

Lead is widely distributed in the environment and has become a global public health issue. It is well known that lead exposure induces not only neurodevelopmental toxicity but also neurodegenerative diseases, with learning and memory impairment in the later stage. However, the molecular mechanisms remain elusive. The present study investigated the effects of early life and lifetime lead exposure on cognition and identified the molecular mechanisms involved in aged rats. The results herein demonstrated that the lead concentration in peripheral blood and brain tissues in aged rats was significantly increased in a lead dose-dependent manner. High-dose lead exposure caused cognitive functional impairment in aged rats, concomitant with a longer escape latency and a lower frequency of crossing the platform via Morris water maze testing compared to those in the control and low-dose lead exposure groups. Importantly, neuron functional defects were still observed even in early life lead exposure during the prenatal and weaning periods in aged rats. The neurotoxicity induced by lead exposure was morphologically evidenced by a recessed nuclear membrane, a swollen endoplasmic reticulum, and mitochondria in the neurons. Mechanistically, the exposure of aged rats to lead resulted in increasing free calcium concentration, reactive oxygen species, and apoptosis in the hippocampal neurons. Lead exposure increased RyR3 expression and decreased the levels of p-CaMKIIα/CaMKIIα and p-CREB/CREB in the hippocampus of aged rats. These findings indicated that early life lead exposure-induced cognition disorder was irreversible in aged rats. Lead-induced neurotoxicity might be related to the upregulation of RyR3 expression and high levels of intracellular free calcium with increasing lead concentration in injured neurons.

Lead-induced effects on learning/memory and fear/anxiety are correlated with disturbances in specific cholinesterase isoform activity and redox imbalance in adult brain.

The aim of the present study was to investigate whether the underlying mechanism of lead (Pb)-induced effects on learning/memory and fear/anxiety behavior involves changes either on AChE G4 (most abundant in brain) or on G1 isoform activity, and/or to a putative local disruption of oxidant/antioxidant balance. Adult male mice were randomly divided into two groups (18 animals/group): a vehicle group [500ppm (mg/L) CH3COONa/day for 4weeks in their drinking water] and a Pb-treated group [500ppm Pb(CH3COO)2/day for 4weeks in their drinking water]. At the end of the treatment period, mice were subjected to the behavioral tasks. Learning/memory was tested by step-through passive avoidance test, whereas fear/anxiety was studied using the elevated plus-maze and thigmotaxis tests. Pb levels in mice brain were determined using atomic absorption spectrometry. AChE activity was determined colorimetrically, and GSH and MDA levels fluorometrically in whole brain minus cerebellum, cerebral cortex, midbrain, hippocampus, striatum and cerebellum. The possible correlations between learning/memory or fear/anxiety behavior with the AChE activity and/or the lipid peroxidation levels and GSH content were also examined. Pb consumption caused significant deficits on mice learning/memory ability and increased anxiety. The consumption of the Pb solution inhibited the activity of the two AChE isoforms in all brain regions tested. Moreover, Pb exposure increased lipid peroxidation and decreased GSH levels in all brain regions examined. Spearman correlation analysis revealed that the coefficients between the particular behaviors, AChE activity and redox balance were brain region- and AChE isoform-specific.

Cumulative lead exposure in community-dwelling adults and fine motor function: comparing standard and novel tasks in the VA normative aging study.

BACKGROUND AND AIMS: Lead exposure in children and occupationally exposed adults has been associated with reduced visuomotor and fine motor function. However, associations in environmentally exposed adults remain relatively unexplored. To address this, we examined the association between cumulative lead exposure-as measured by lead in bone-and performance on the grooved pegboard (GP) manual dexterity task, as well as on handwriting tasks using a novel assessment approach, among men in the VA Normative Aging Study (NAS). METHODS: GP testing was done with 362 NAS participants, and handwriting assessment with 328, who also had tibia and patella lead measurements made with K-X-Ray Fluorescence (KXRF). GP scores were time (s) to complete the task with the dominant hand. The handwriting assessment approach assessed the production of signature and cursive lowercase l and m letter samples. Signature and lm task scores reflect consistency in repeated trials. We used linear regression to estimate associations and 95% confidence intervals (CI) with adjustment for age, smoking, education, income and computer experience. A backward elimination algorithm was used in the subset with both GP and handwriting assessment to identify variables predictive of each outcome. RESULTS: The mean (SD) participant age was 69.1 (7.2) years; mean patella and tibia concentrations were 25.0 (20.7)µg/g and 19.2 (14.6)µg/g, respectively. In multivariable-adjusted analyses, GP performance was associated with tibia (ß per 15µg/g bone=4.66, 95% CI: 1.73, 7.58, p=0.002) and patella (ß per 20µg/g=3.93, 95% CI: 1.11, 6.76, p=0.006). In multivariable adjusted models of handwriting production, only the lm-pattern task showed a significant association with tibia (ß per 15µg/g bone=1.27, 95% CI: 0.24, 2.29, p=0.015), such that lm pattern production was more stable with increasing lead exposure. GP and handwriting scores were differentially sensitive to education, smoking, computer experience, financial stability, income and alcohol consumption. CONCLUSIONS: Long-term cumulative environmental lead exposure was associated with deficits in GP performance, but not handwriting production. Higher lead appeared to be associated with greater consistency on the lm task. Lead sensitivity differences could suggest that lead affects neural processing speed rather than motor function per se, or could result from distinct brain areas involved in the execution of different motor tasks.

δ-Aminolevulinic acid dehydratase genotype predicts toxic effects of lead on workers' peripheral nervous system.

There is a wide variation in sensitivity to lead (Pb) exposure, which may be due to genetic susceptibility towards Pb. We investigated whether a polymorphism (rs1800435) in the δ-aminolevulinic acid dehydratase (ALAD) gene affected the toxicokinetics and toxicodynamics of Pb. Among 461 Chinese Pb-exposed storage battery and 175 unexposed workers, allele frequencies for the ALAD1 and ALAD2 alleles were 0.968 and 0.032, respectively. The Pb-exposed workers had a higher fraction of the ALAD1-2/2-2 genotype than unexposed workers (7.8% vs. 2.3%, p=0.01). The Pb levels in blood (B-Pb) and urine (U-Pb) were higher in Pb-exposed workers carrying the ALAD2 allele compared to homozygotes for ALAD1 (median B-Pb: 606 vs. 499 µg/L; U-Pb: 233 vs. 164 µg/g creatinine), while there was no statistically significant difference in the unexposed controls (median: 24 vs. 37 µg/L, and 3.9 vs. 6.4µg/g creatinine, respectively). High B-Pb and U-Pb were associated with statistically significantly lower sensory and motor conduction velocities in the median, ulnar and peroneal nerves. At the same B-Pb and U-Pb, ALAD1 homozygotes had lower conduction velocities than the ALAD2 carriers. There were similar trends for toxic effects on haem synthesis (zinc protoporphyrin and haemoglobin in blood) and renal function (albumin and N-acetyl-d-ß-acetylglucosaminidase in urine), but without statistical significance. There was no difference in Pb toxicokinetics and toxicodynamics associated with VDR BsmI polymorphism. Our results show that the ALAD genotype modifies the relationship between Pb and its toxic effects on the peripheral nervous system. This must be considered in the assessment of risks at Pb exposure.

Severe neurotoxicity following ingestion of tetraethyl lead.

Organic lead compounds are potent neurotoxins which can result in death even from small exposures. Traditionally, these compounds are found in fuel stabilizers, anti-knock agents, and leaded gasoline. Cases of acute organic lead intoxication have not been reported for several decades. We report a case of a 13-year-old Iraqi male who unintentionally ingested a fuel stabilizer containing 80-90% tetraethyl lead, managed at our combat support hospital. The patient developed severe neurologic symptoms including agitation, hallucinations, weakness, and tremor. These symptoms were refractory to escalating doses of benzodiazepines and ultimately required endotracheal intubation and a propofol infusion. Adjunctive therapies included chelation, baclofen, and nutrition provided through a gastrostomy tube. The patient slowly recovered and was discharged in a wheelchair 20 days after ingestion, still requiring tube feeding. Follow-up at 62 days post-ingestion revealed near-resolution of symptoms with residual slurred speech and slight limp. This case highlights the profound neurotoxic manifestations of acute organic lead compounds.

p16 promoter methylation in Pb2+ -exposed individuals.

BACKGROUND: One of the principle symptoms of lead poisoning is the development of neurological disorders. Neuronal response is closely related to DNA methylation changes. Aim. In this study, we estimated p16 methylation in nine individuals exposed to lead using methylation-specific polymerase chain reaction followed by analysis of the methylated cytosine content of the product by thermal denaturation. RESULTS: We found that, based on lead blood concentration, lead-exposed individuals were divided into two groups. Among highly exposed individuals (blood Pb(2+) concentration = 51-100 microg/dL), we observed complete CpG methylation, whereas for low Pb(2+) concentrations (blood Pb(2+) concentration = 6-11 microg/dL), we observed partial methylation. CONCLUSION: Our results show that among lead-overexposed individuals, p16 methylation is frequent and extensive, and suggest that DNA methylation could be involved in the mechanism by which lead induces neurotoxicity.

Goya's deafness.

Francisco Goya (1746-1828), a major Spanish artist, became profoundly deaf aged 46 years, following an acute illness. Despite this, his success continued and he eventually died aged 82 years. His illness is sketchily documented in letters written during his convalescence, describing headache, deafness, tinnitus, unsteadiness and visual disturbance with recovery (apart from deafness) over three months. There was a milder similar illness two years before, suggesting a relapsing condition. Vogt-Koyanagi-Harada syndrome, although previously accepted as Goya's diagnosis, is not supported by the limited evidence. Susac's syndrome or Cogan's syndrome, although both rare, are more likely explanations.

Impact of cognitive reserve on the relationship of lead exposure and neurobehavioral performance.

BACKGROUND: Cognitive reserve (CR)--a construct studied in many neurologic disorders--refers to the maintenance of cognitive performance in spite of ongoing underlying brain pathology. OBJECTIVE: We hypothesized that a dose-effect relationship would exist between chronic occupational lead exposure and cognitive effects in workers with low CR but not in workers with high CR and identical lead exposure, and that level of CR would not influence the relationship between lead exposure and motor performance. METHODS: We stratified currently employed lead smelter workers by Wide Range Achievement Test-R for reading (WRAT), a recognized measure of CR, into loCR and hiCR groups. From these two groups we matched 56 pairs on working lifetime weighted blood lead (TWA). We performed a factor analysis on 14 neuropsychological outcome variables. Within each CR group regression analyses after adjusting for age, alcohol use, and depression scale score tested for dose-effect relationships between TWA and outcome variables. RESULTS: Both CR groups had comparable age, years employed, alcohol use, and current blood lead levels. Factor analysis provided three factors and five tests used in the regression analyses. Significant dose-effect relationships between TWA and cognitive tests present only in the loCR group included Attention Factor and Digit Symbol. Both CR groups demonstrated significant dose-effect relationships on the Motor Factor. CONCLUSION: This study found that cognitive reserve protects against the effect of chronic lead exposure on select measures of cognitive performance but not on motor performance.

Effects of lead on the adult brain: a 15-year exploration.

BACKGROUND: Historically, there has been minimal concern about the effect of adult lead exposure on the brain. Evidence from recent longitudinal studies raise concerns about the long-term effects of past exposure. METHODS: We initiated three independent longitudinal studies to determine whether cumulative lead exposure was associated with persistent or progressive brain effects. The studies include 1,109 former U.S. organolead manufacturing workers, 803 current and former inorganic lead workers in Korea, and 1,140 50- to 70-year-old Baltimore residents with environmental lead exposure. The organolead workers had past exposure to inorganic and tetraethyl lead (TEL); in the other two studies, exposure was to inorganic lead. In each of these studies, we measured blood lead and tibia and patella lead by 109Cd K-shell-induced X-ray fluorescence. RESULTS: Higher tibia lead was consistently associated with poorer measures of cognitive function. Longitudinal analysis of the Korean and organolead cohort indicate that the effect of lead is persistent. Moreover, MRI data on organolead workers indicates a possible progressive effect from past exposure; higher tibia lead was associated with lower brain volume. The latter study indicates that a difference in tibia lead equivalent to about one-sixth of the overall range was associated with a mean difference in these cognitive tests that was equivalent, on average, to what was observed for a five-year age difference. CONCLUSIONS: Our data suggest that a significant proportion of what is considered to be "normal" age-related cognitive decline may, in fact, be due to past exposure to neurotoxicants such as lead.

Autonomic dysfunction due to lead poisoning.

We present a case history of a 24 years old male who developed autonomic dysfunction, intestinal pseudo-obstruction and anemia due to lead poisoning. Concomitant recording of blood levels of lead and autonomic function showed a gradual decline in blood lead level (98.8 microg/dL at week 0, 56 microg/dL at week 6, and 40 microg/dL at week 52) and gradual improvement in autonomic functions. Decrease in blood lead levels with DMSA (Meso-2, 3-dimercaptosuccinic acid) therapy showed improvement in autonomic functions. At week 0, the patient had severe loss of autonomic tone and autonomic reactivity which improved at week 6. At the 52nd week, most of the autonomic parameters had normalized except for the persistence of mild loss of parasympathetic reactivity.

Comparison of patella lead with blood lead and tibia lead and their associations with neurobehavioral test scores.

OBJECTIVE: Lead exposure in adults is associated with worse cognitive function in cross-sectional and longitudinal studies. Previous studies have mainly examined relations with blood lead or cortical bone lead; few have examined trabecular bone lead. METHODS: We performed a cross-sectional analysis of the relations of patella lead and other lead biomarkers with measures of neurobehavioral and peripheral nervous system function in 652 lead workers. RESULTS: Patella lead was found to be associated with worse performance on seven of 19 tests of manual dexterity, sensory vibration threshold, and depressive symptoms. The associations of patella lead with cognitive function were essentially similar to those with blood lead or tibia lead but of somewhat lower magnitude. CONCLUSIONS: In this study, measurement of patella lead did not aid causal inference regarding cognitive effects when compared with blood lead and tibia lead.

The relationship between blood lead levels and neurobehavioral test performance in NHANES III and related occupational studies.

OBJECTIVES: The goals of this study were two-fold: (1) to assess the relationship between blood lead levels and neurobehavioral test performance in a nationally sample of adults from the third National Health and Nutrition Evaluation Survey and (2) to analyze the results from previously published studies of occupational lead exposure that used the same neurobehavioral tests as those included in the survey. METHODS: Regression models were used to test and estimate the relationships between measurements of blood lead and performance on a simple reaction time, a symbol-digit substitution, and a serial digit learning test in adults aged 20-59 years who participated the survey. Mixed models were used to analyze the data from the occupational studies. RESULTS: The blood lead levels of those participating in the survey ranged from 0.7 to 41.8 microg/dl. The estimated geometric mean was 2.51 microg/dl, and the estimated arithmetic mean was 3.30 microg/dl. In the survey, no statistically significant relationships were found between blood lead concentration and performance on the three neurobehavioral tests when adjusted for covariates. In the occupational studies, the groups exposed to lead consistently performed worse than control groups on the simple reaction time and digit-symbol substitution tests. CONCLUSIONS: The results from the survey and the occupational studies do not provide evidence for impairment of neurobehavioral test performance at levels below 25 microg/dl, the concentration that the Centers for Disease Control and Prevention define as elevated in adults. The average blood lead level of the exposed groups in the occupational studies was 41.07 microg/dl, less than 50 microg/dl, the minimum concentration that the Occupational Safety and Health Administration requires for medical removal from the workplace. Given the evidence of impaired neurobehavioral performance in these groups, the 50 microg/dl limit should be reevaluated.

Pattern of blood lead levels over working lifetime and neuropsychological performance.

The authors examined the impact on neuropsychological performance of past high lead exposure, followed by lower proximate lead exposure, in 2 groups of smelter workers selected on the basis of their patterns of blood lead levels (BLLs) over time. Prior to 1980 (past exposure), both groups had more than 90% of their BLLs > or = 40 microg/dl. During and subsequent to 1980 (proximate exposure), those subjects with more than 90% of their BLLs remaining at > or = 40 microg/dl were assigned to the high-high (H-H) pattern group (n = 40); whereas those with 90% of levels below 40 microg/dl were assigned to the high-low (H-L) pattern group (n = 40). Means (and standard deviations) for pre-1980 time-integrated blood lead (IBL) levels were similar for the H-H pattern [633.2 (202.2) microg/yr-dl] and the H-L pattern [556.5 (144.8) microg/yr x dl]; however, IBLs from 1980 on were significantly different [H-H pattern = 646.9 (58.70) microg/yr x dl and H-L pattern = 408.8 (46.37) microg/yr x dl; p < 0.0001]. Age, education, and years of employment were similar for both groups. Examination of 5 neuropsychological measures revealed that verbal memory was significantly better in the H-L pattern group than in the H-H group. Multivariate examination of the data showed that pattern of exposure contributed significantly to verbal memory performance, after adjustment for the covariates, current BLL, and IBL. A partial correlation analysis between verbal memory and IBL for past high exposure showed an association with H-H pattern, but none with H-L pattern. Pattern of BLLs over a working lifetime contributed unique variance to verbal memory. Absence of an association between past high lead exposure and verbal memory in the H-L pattern group suggests that reversibility of function may occur when proximate BLL is maintained below 40 microg/dl.

Reduced cognitive abilities in lead-exposed men.

OBJECTIVES: Since it is still controversial whether-low-to moderate long-term lead exposure below current threshold values causes neurobehavioural deficits in adults, we investigated executive functions of the prefrontal cortex, attention, and visuospatial and visuomotor functioning in lead-exposed subjects. METHODS: Forty-seven lead-exposed subjects with a mean blood lead (PbB) level of 30.8 microg/100 ml and 53 non-lead-exposed aged-matched subjects (PbB: 4.32 microg/100 ml) with the same socio-economic background were investigated. Both groups were also matched on verbal intelligence. Neuropsychological tests were done by the modified Wisconsin card sorting test, block design test, visual recognition test, simple reaction time, choice reaction and digit symbol substitution. Lead exposure was assessed by current and cumulative measures. RESULTS: While there were significant differences in the results of the Wisconsin, block design and visual recognition tests, no differences were found in simple reaction time, choice reaction and digit symbol substitution. Significant correlations existed between current exposure and cognitive deficits. No correlation was found between cumulative exposure measures and cognitive parameters. CONCLUSIONS: Our results show that PbB below 70 microg/100 ml reduce neurobehavioural abilities, particularly visuospatial abilities and executive functions referring to the prefrontal cortex. As neurobiological substrate of the prefrontal dysfunction, glutamatergic system disturbances are discussed.

Assessment of neurobehavioral performance as a function of current and cumulative occupational lead exposure.

A cross sectional field study was planned to assess neurotoxic effects caused by low-level occupational lead exposure. Two groups of 66 workers and 86 controls were examined with a battery including a questionnaire on neurotoxic symptoms, the measure of performance at neurobehavioral testing, the detection of visual contrast sensitivity, and the dosage of serum prolactin. Both current and cumulative exposure to lead were defined. The average PbB was 27.50 +/- 28 microg/dl (median 28, range 6-61) in the exposed and 8.11 +/- 4.47 microg/dl (median 7, range 2-21). The test results were controlled for possible confounders including age, schooling, alcohol and coffee intake. Significant differences were observed between exposed and controls regarding neurotoxic symptoms reporting, the exposed reporting more frequently mood changes and abnormal fatigue. The exposed subjects showed a decreased visual contrast sensitivity, and a marked increase of prolactin secretion. No changes emerged regarding neurobehavioral testing. The alterations observed resulted associated to the current lead exposure and not to the cumulative indices. A safe exposure level was calculated on the basis of dose-response relationship with prolactin alteration, yielding a PbB value of 10 microg/dl.

Distortion product oto-acoustic emissions in Andean children and adults with chronic lead intoxication.

Neuroauditory disorders and sensory-neural hearing loss have been suggested as possible etiologic factors in the neurodevelopmental learning disabilities attributed to lead (Pb) intoxication. However, studies relating hearing loss to Pb poisoning have presented disparate results, suggesting that auditory sensitivity may not be a reliable marker of Pb intoxication. Oto-acoustic emissions, sounds that can be recorded non-invasively from the ear canal and are preneural responses of the outer hair cells of the inner ear, have been found to be diminished in ears exposed to some toxic agents. In the current study, distortion product oto-acoustic emissions (DPOAEs) were obtained from 28 ears of 14 children and 10 ears of 5 adults living in a highly Pb-contaminated environment in remote villages in the Andes Mountains of Ecuador. Blood lead (PbB) levels for the children (ages: 5-14 years) ranged from 33.4 to 118.2 microg/dl (mean: 51.5; SD: 22.9 microg/dl), or 3-12 times higher than the U.S. Centers for Disease Control and Prevention's toxic level of 10 microg/dl. The PbB levels for the adults ranged from 19.2 to 55.7 microg/dl. Despite the high PbB levels, the children had normal hearing thresholds, and DPOAEs were present for the children at the following f2 frequencies: 1187, 1500, 1906, 2406, 3031, 3812, 4812 and 6031 Hz. Although there was a tendency for the children to have diminished DPOAEs, no consistent correlation of DPOAEs with PbB level was found. The adults had diminished DPOAEs that were consistent with their observed, probably noise-related hearing loss. Contrary to some reports in the literature, the current results show no unequivocal clinical or subclinical evidence that high PbB levels have a toxic effect on the cochlea.