Developmental exposure to lead causes inherent changes on voltage-gated sodium channels in rat hippocampal CA1 neurons

Lead exerts a depression of neurotransmitter release through a blockade of voltage dependent calcium channels in chromaffin cells

The present work, using chromaffin cells of bovine adrenal medullae (BCCs), aims to describe what type of ionic current alterations induced by lead (Pb2+) underlies its effects reported on synaptic transmission. We observed that the acute application of Pb2+ lead to a drastic depression of neurotransmitters release in a concentration-dependent manner when the cells were stimulated with both K+ or acetylcholine, with an IC50 of 119,57 μM and of 5,19 μM, respectively. This effect was fully recovered after washout. Pb2+ also blocked calcium channels of BCCs in a time- and concentration-dependent manner with an IC50 of 6,87 μM. This blockade was partially reversed upon washout. This compound inhibited the calcium current at all test potentials and shows a shift of the I-V curve to more negative values of about 8 mV. The sodium current was not blocked by acute application of high Pb2+ concentrations. Voltage-dependent potassium current was also shortly affected by high Pb2+. Nevertheless, the calcium- and voltage-dependent potassium current was drastically depressed in a dose-dependent manner, with an IC50 of 24,49 μM. This blockade was related to the prevention of Ca2+ influx through voltage-dependent calcium channels coupled to Ca2+-activated K+-channels (BK) instead a direct linking to these channels. Under current-clamp conditions, BCCs exhibit a resting potential of -52.7 mV, firing spontaneous APs (1-2 spikes/s) generated by the opening of Na+ and Ca2+-channels, and terminated by the activation of K+ channels. In spite of the effect on ionic channels exerted by Pb2+, we found that Pb2+ didn't alter cellular excitability, no modification of the membrane potential, and no effect on action potential firing. Taken together, these results point to a neurotoxic action evoked by Pb2+ that is associated with changes in neurotransmitter release by blocking the ionic currents responsible for the calcium influx.

Low dose lead exposure induces alterations on heterochromatin hallmarks persisting through SH-SY5Y cell differentiation

Lead (Pb) is a commonly found heavy metal due to its historical applications. Recent studies have associated early-life Pb exposure with the onset of various neurodegenerative disease. The molecular mechanisms of Pb conferring long-term neurotoxicity, however, is yet to be elucidated. In this study, we explored the persistency of alteration in epigenetic marks that arise from exposure to low dose of Pb using a combination of image-based and gene expression analysis. Using SH-SY5Y as a model cell line, we observed significant alterations in global 5-methycytosine (5 mC) and histone 3 lysine 27 tri-methylation (H3K27me3) and histone 3 lysine 9 tri-methylation (H3K9me3) levels in a dose-dependent manner immediately after Pb exposure. The changes are partially associated with alterations in epigenetic enzyme expression levels. Long term culturing (14 days) after cease of exposure revealed persistent changes in 5 mC, partial recovery in H3K9me3 and overcompensation in H3K27me3 levels. The observed alterations in H3K9me3 and H3K27me3 are reversed after neuronal differentiation, while reduction in 5 mC levels are amplified with significant changes in patterns as identified via texture clustering analysis. Moreover, correlation analysis demonstrates a strong positive correlation between trends of 5 mC alteration after differentiation and neuronal morphology. Collectively, our results suggest that exposure to low dose of Pb prior to differentiation can result in persistent epigenome alterations that can potentially be responsible for the observed phenotypic changes. Our work reveals that Pb induced changes in epigenetic repressive marks can persist through neuron differentiation, which provides a plausible mechanism underlying long-term neurotoxicity associated with developmental Pb-exposure.

High Content of Lead Is Associated with the Softness of Drinking Water and Raised Cardiovascular Morbidity: A Review

Daily ingestion of lead (Pb), even through piped drinking water, has long time been an important issue of concern, attracting for decades research in environmental science and toxicology, and again comes to prominence because of recent high-profile cases of exposure of populations in several countries to Pb-contaminated water. Numerous studies have reported an association between Pb in water and the risk of cardiovascular pathologies. Low levels of magnesium and calcium, i.e., low degree of hardness of the drinking water, may accentuate Pb leaching from water pipes and furthermore increase Pb absorption. This review evaluates the evidence for an association between Pb exposure from drinking water and cardiovascular end points in human populations.

Lead poisoning: acute exposure of the heart to lead ions promotes changes in cardiac function and Cav1.2 ion channels

Lead ions (Pb²⁺) possess characteristics similar to Ca²⁺. Because of this and its redox capabilities, lead causes different toxic effects. The neurotoxic effects have been well documented; however, the toxic effects on cardiac tissues remain allusive. We utilized isolated guinea pig hearts and measured the effects of Pb²⁺ on their contractility and excitability. Acute exposure to extracellular Pb²⁺ had a negative inotropic effect and increased diastolic tension. The speed of contraction and relaxation were affected, though the effects were more dramatic on the speed of contraction. Excitability was also altered. Heart beat frequency increased and later diminished after lead ion exposure. Pro-arrhytmic events, such as early after-depolarization and a reduction of the action potential plateau, were also observed. In isolated cardiomyocytes and tsA 201 cells, extracellular lead blocked currents through Cav1.2 channels, diminished their activation, and enhanced their fast inactivation, negatively affecting their gating currents. Thus, Pb²⁺ was cardiotoxic and reduced cardiac contractility, making the heart prone to arrhythmias. This was due, in part, to Pb²⁺ effects on the Cav1.2 channels; however, other channels, transporters or pathways may also be involved. Acute cardiotoxic effects were observed at Pb²⁺ concentrations achievable during acute lead poisoning. The results suggest how Cav1.2 gating can be affected by divalent cations, such as Pb², and also suggest a more thorough evaluation of heart function in individuals affected by lead poisoning.

Effect of vitamin E on lead exposure-induced learning and memory impairment in rats

Chronic lead (Pb(2+)) exposure has been associated with learning and memory impairments, whereas vitamin E improves cognitive deficits. In this study, using a passive avoidance learning model in rats, we investigated the effects of vitamin E on Pb(2+) exposure-induced learning and memory impairments in rats. In the present study, 56 Wistar male rats (weighting 230-250g) were divided into eight groups (n=7). The Pb(2+) exposure involved gavages of lead acetate solution using three different doses (0.05%, 0.1%, and 0.2%) and the vitamin E consisted of three different doses (10, 25, 50μg/rat) for 30days. After the 30-day period, the rats were tested using a passive avoidance task (acquisition test). In a retrieval test conducted 48h after the training, step through latency (STL) and time in the dark compartment (TDC) were recorded. The statistical analysis of data was performed using ANOVA followed by Tukey's post hoc analysis. In all cases, differences were considered significant if p<0.05. The results of the present study showed that chronic exposure to high doses of Pb(2+) significantly increased both the number of trails required for learning and the TDC, whereas it decreased the STL in the passive avoidance test. Administration of vitamin E ameliorated the effects of Pb(2+) on animal behavior in the passive avoidance learning and memory task. Our results indicate that impairments of learning and memory in Pb(2+)-exposed rats are dose dependent and can be inhibited by antioxidants such as vitamin E.

Pb2+: an endocrine disruptor in Drosophila?

Environmental exposure to Pb(2+) affects hormone-mediated responses in vertebrates. To help establish the fruit fly, Drosophila melanogaster, as a model system for studying such disruption, we describe effects of Pb(2+) on hormonally regulated traits. These include duration of development, longevity, females' willingness to mate, fecundity and adult locomotor activity. Developmental Pb(2+) exposure has been shown to affect gene expression in a specific region of the Drosophila genome (approximately 122 genes) involved in lead-induced changes in adult locomotion and to affect regulation of intracellular calcium levels associated with neuronal activity at identified synapses in the larval neuromuscular junction. We suggest ways in which Drosophila could become a new model system for the study of endocrine disruptors at genetic, neural and behavioral levels of analysis, particularly by use of genomic methods. This will facilitate efforts to distinguish between behavioral effects of Pb(2+) caused by direct action on neural mechanisms versus effects of Pb(+2) on behavior mediated through endocrine disruption.

Variations at a quantitative trait locus (QTL) affect development of behavior in lead-exposed Drosophila melanogaster

We developed Drosophila melanogaster as a model to study correlated behavioral, neuronal and genetic effects of the neurotoxin lead, known to affect cognitive and behavioral development in children. We showed that, as in vertebrates, lead affects both synaptic development and complex behaviors (courtship, fecundity, locomotor activity) in Drosophila. By assessing differential behavioral responses to developmental lead exposure among recombinant inbred Drosophila lines (RI), derived from parental lines Oregon R and Russian 2b, we have now identified a genotype by environment interaction (GEI) for a behavioral trait affected by lead. Drosophila Activity Monitors (TriKinetics, Waltham, MA), which measure activity by counting the number of times a single fly in a small glass tube walks through an infrared beam aimed at the middle of the tube, were used to measure activity of flies, reared from eggs to 4 days of adult age on either control or lead-contaminated medium, from each of 75 RI lines. We observed a significant statistical association between the effect of lead on Average Daytime Activity (ADA) across lines and one marker locus, 30AB, on chromosome 2; we define this as a Quantitative Trait Locus (QTL) associated with behavioral effects of developmental lead exposure. When 30AB was from Russian 2b, lead significantly increased locomotor activity, whereas, when 30AB was from Oregon R, lead decreased it. 30AB contains about 125 genes among which are likely "candidate genes" for the observed lead-dependent behavioral changes. Drosophila are thus a useful, underutilized model for studying behavioral, synaptic and genetic changes following chronic exposure to lead or other neurotoxins during development.

Effects of SO2 derivatives on sodium currents in acutely isolated rat hippocampal lead-exposed neurons

In this study, the effects of acute SO2 derivatives and chronic lead exposure together on sodium currents (I(Na)) were investigated in acutely isolated rat hippocampal neurons by using the whole-cell patch clamp techniques. We found that chronic lead exposure hardly reduced the amplitudes of I(Na). In the normal condition, sodium current started to appear at around -70 mV, and reached the peak current at around -40 mV. After chronic lead exposure, the data changed to -70 and -30 mV. After adding SO2 derivatives, the data changed to -80 and -40 mV, respectively. SO2 derivatives caused a significant increase of I(Na) in hippocampal chronic-lead exposed neurons. Chronic lead exposure induced a right shift of the activation curve and a left shift of the inactivation curve of sodium channels. SO2 derivatives caused negative shifts of the activation and inactivation curves of I(Na) in hippocampal chronic-lead exposed neurons. Lead exposure put off the time reaching the peak of I(Na) activation. SO2 derivatives increased the time constants of inactivation after lead exposure. The interaction of lead and SO2 derivatives with voltage-dependent sodium channels may lead to changes in electrical activity and contribute to worsening the neurotoxicological damage.