Effect of in vitro inorganic lead on dopamine release from superfused rat striatal synaptosomes

Trace Elements Levels in Major Depressive Disorder-Evaluation of Potential Threats and Possible Therapeutic Approaches

The multifactorial etiology of major depressive disorder (MDD) includes biological, environmental, genetic, and psychological aspects. Recently, there has been an increasing interest in metallomic studies in psychiatry, aiming to evaluate the role of chosen trace elements in the MDD etiology as well as the progression of symptoms. This narrative review aims to summarize the available literature on the relationship between the concentration of chosen elements in the serum of patients with MDD and the onset and progression of this psychiatric condition. The authors reviewed PubMed, Web of Science, and Scopus databases searching for elements that had been investigated so far and further evaluated them in this paper. Ultimately, 15 elements were evaluated, namely, zinc, magnesium, selenium, iron, copper, aluminium, cadmium, lead, mercury, arsenic, calcium, manganese, chromium, nickel, and phosphorus. The association between metallomic studies and psychiatry has been developing dynamically recently. According to the results of current research, metallomics might act as a potential screening tool for patients with MDD while at the same time providing an assessment of the severity of symptoms. Either deficiencies or excessive amounts of chosen elements might be associated with the progression of depressive symptoms or even the onset of the disease among people predisposed to MDD.

Effect of subchronic exposure to ambient fine and ultrafine particles on rat motor activity and ex vivo striatal dopaminergic transmission

Alterations in dopaminergic transmission are associated with neurological disorders, such as depression, autism, and Parkinson's disease. Exposure of rats to ambient fine (FP) or ultrafine (UFP) particles induces oxidative and inflammatory responses in the striatum, a neuronal nucleus with dense dopaminergic innervation and critically involved in the control of motor activity.Objectives: We used an ex vivo system to evaluate the effect of in vivo inhalation exposure to FP and UFP on motor activity and dopaminergic transmission.Materials and Methods: Male adult Wistar rats were exposed to FP, UFP, or filtered air for 8 weeks (subchronic exposure; 5 h/day, 5 days/week) in a particle concentrator. Motor activity was evaluated using the open-field test. Uptake and release of [³H]-dopamine were assessed in striatal synaptosomes, and dopamine D₂ receptor (D₂R) affinity for dopamine was evaluated by the displacement of [³H]-spiperone binding to striatal membranes.Results: Exposure to FP or UFP significantly reduced spontaneous motor activity (ambulatory distance: FP -25%, UFP -32%; ambulatory time: FP -24%, UFP -22%; ambulatory episodes: FP -22%, UFP -30%), decreased [³H]-dopamine uptake (FP -18%, UFP -24%), and increased, although not significantly, [³H]-dopamine release (113.3 ± 16.3 and 138.6 ± 17.3%). Neither FP nor UFP exposure affected D₂R density or affinity for dopamine.Conclusions: These results indicate that exposure to ambient particulate matter reduces locomotion in rats, which could be related to altered striatal dopaminergic transmission: UFP was more potent than FP. Our results contribute to the evidence linking environmental factors to changes in brain function that could turn into neurological and psychiatric disorders.HIGHLIGHTSYoung adult rats were exposed to fine (FP) or ultrafine (UFP) particles for 40 days.Exposure to FP or UFP reduced motor activity.Exposure to FP or UFP reduced dopamine uptake by striatal synaptosomes.Neither D₂R density or affinity for dopamine was affected by FP or UFP.UFP was more potent than FP to exert the effects reported.

Metal mixtures are associated with increased anxiety during pregnancy

BACKGROUND

Exposure to low-dose toxic metals in the environment is ubiquitous. Several murine studies have shown metals induce anxiety-like behaviors, and mechanistic research supports that metals disrupt neurotransmitter signaling systems implicated in the pathophysiology of anxiety. In this study, we extend prior research by examining joint exposure to six metals in relation to maternal anxiety symptoms during pregnancy.

METHODS

The sample includes 380 participants enrolled in the PRogramming of Intergenerational Stress Mechanisms (PRISM) pregnancy cohort. Spot urine was collected during pregnancy (mean ± standard deviation: 31.1 ± 6.1 weeks), and concentrations of six metals (barium [Ba], cadmium [Cd], chromium [Cr], cesium [Cs], lead [Pb], antimony [Sb]) were measured by Inductively Coupled Plasma - Mass Spectrometry. Trait anxiety symptoms were measured during pregnancy using a short version of the Spielberger State Trait Anxiety Inventory (STAI-T) and information on covariates was collected by questionnaire. We used weighted quantile sum (WQS) regression as the primary modeling approach to examine metals, treated as a mixture, in relation to higher (≥20) vs. lower anxiety symptoms while adjusting for urinary creatinine and key sociodemographic variables.

RESULTS

The sample is socioeconomically and racially/ethnically diverse. Urinary metal concentrations were log-normally distributed and 25% of the sample had an STAI-T score ≥20. Joint exposure to metals was associated with elevated anxiety symptoms (OR_WQS = 1.56, 95% CI: 1.24, 1.96); Cd (61.8%), Cr (14.7%), and Cs (12.7%) contributed the greatest weight to the mixture effect.

CONCLUSION

Exposure to metals in the environment may be associated with anxiety symptoms during pregnancy. This is a public health concern, as anxiety disorders are highly prevalent and associated with significant co-morbidities, especially during pregnancy when both the mother and developing fetus are susceptible to adverse health outcomes.

Lead (Pb) exposure induces physiological alterations in the serotoninergic and vasopressin systems causing anxiogenic-like behavior in Meriones shawi: Assessment of BDMC as a neuroprotective compound for Pb-neurotoxicity and kidney damages

BACKGROUND

Studies have shown that lead (Pb) is one of hazardous heavy metals with various adverse effects on human health including mental health; Pb can induce psychiatric disorders like anxiety. In the present work, we examined the potential of bisdemethoxycurcumin (BDMC) as a neuroprotective agent against lead induced anxiety inMeriones shawi (M. shawi).

METHODS

We asses, the potential of three consecutive day exposure to Pb (25 mg/kg body weight) in inducing anxiogenic effect, serotoninergic and vasopressinergic disruptions inM. shawi. This was done using neurobehavioral tests (open field, elevated plus maze), immunohistochemestry by anti-serotonin (5-HT), and anti-vasopressin (AVP) antibodies. We also measured the possible restorative potential of BDMC (30 mg/kg body weight), delivered by oral gavage. After that, a biochemical and histopathological studies were done.

RESULTS

Our results showed that lead exposure for three consecutive days increases significantly the 5-HT-immunoreactivity in dorsal raphe nucleus (DRN) accompanied with a significant enhancement of AVP-immunoreactivity in the cell bodies and fibers in the supraoptic (SON) and paraventricular (PVN) nuclei of the hypothalamus. In the collecting tube, AVP binds to the V2 receptor of the epithelial cells and increases the water permeability. Our results showed clearly the epithelial cells degeneration after lead exposure, then we suggest that the increased AVP could be a response to the hydric balance disrupted after degenerative effect of lead exposure on epithelial cells. BDMC produced an anxiolytic effect in meriones. Moreover, it restored 5-HT and AVP immunoreactivity within studying nuclei. The biochemical and histopathological studies showed that Pb induced renal damages. In addition, BDMC restored the renal alterations.

CONCLUSION

According to the obtained results, we suggest new pharmacological effects of BDMC; while it has an anxiolytic effect against Pb-induced anxiety by working on serotoninergic and vasopressinergic systems with an obvious restoration of the renal injuries induced by lead exposure.

In vitro exposure to ambient fine and ultrafine particles alters dopamine uptake and release, and D2 receptor affinity and signaling

The exposure to environmental pollutants, such as fine and ultrafine particles (FP and UFP), has been associated with increased risk for Parkinson's disease, depression and schizophrenia, disorders related to altered dopaminergic transmission. The striatum, a neuronal nucleus with extensive dopaminergic afferents, is a target site for particle toxicity, which results in oxidative stress, inflammation, astrocyte activation and modifications in dopamine content and D₂ receptor (D₂R) density. In this study we assessed the in vitro effect of the exposure to FP and UFP on dopaminergic transmission, by evaluating [³H]-dopamine uptake and release by rat striatal isolated nerve terminals (synaptosomes), as well as modifications in the affinity and signaling of native and cloned D₂Rs. FP and UFP collected from the air of Mexico City inhibited [³H]-dopamine uptake and increased depolarization-evoked [³H]-dopamine release in striatal synaptosomes. FP and UFP also enhanced D₂R affinity for dopamine in membranes from either rat striatum or CHO-K1 cells transfected with the long isoform of the human D₂R (hD_2LR)2LR). In CHO-K1-hD2L In CHO-K1-hD_2LR cells or striatal slices, FP and UFP increased the potency of dopamine or the D₂R agonist quinpirole, respectively, to inhibit forskolin-induced cAMP formation. The effects were concentration-dependent, with UFP being more potent than FP. These results indicate that FP and UFP directly affect dopaminergic transmission.

Interference of pH buffer with Pb2+-peripheral domain interactions: obstacle or opportunity?

Pb²⁺ is a xenobiotic metal ion that competes for Ca²⁺-binding sites in proteins. Using the peripheral Ca²⁺-sensing domains of Syt1, we show that the chelating pH buffer Bis-Tris enables identification and functional characterization of high-affinity Pb²⁺ sites that are likely to be targeted by bioavailable Pb²⁺.

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.

High affinity interactions of Pb2+ with synaptotagmin I

Lead (Pb) is a potent neurotoxin that disrupts synaptic neurotransmission. We report that Synaptotagmin I (SytI), a key regulator of Ca2+-evoked neurotransmitter release, has two high-affinity Pb2+ binding sites that belong to its cytosolic C2A and C2B domains. The crystal structures of Pb2+-complexed C2 domains revealed that protein-bound Pb2+ ions have holodirected coordination geometries and all-oxygen coordination spheres. The on-rate constants of Pb2+ binding to the C2 domains of SytI are comparable to those of Ca2+ and are diffusion-limited. In contrast, the off-rate constants are at least two orders of magnitude smaller, indicating that Pb2+ can serve as both a thermodynamic and kinetic trap for the C2 domains. We demonstrate, using NMR spectroscopy, that population of these sites by Pb2+ ions inhibits further Ca2+ binding despite the existing coordination vacancies. Our work offers a unique insight into the bioinorganic chemistry of Pb(ii) and suggests a mechanism by which low concentrations of Pb2+ ions can interfere with the Ca2+-dependent function of SytI in the cell.

Gestational lead exposure selectively decreases retinal dopamine amacrine cells and dopamine content in adult mice

Gestational lead exposure (GLE) produces supernormal scotopic electroretinograms (ERG) in children, monkeys and rats, and a novel retinal phenotype characterized by an increased number of rod photoreceptors and bipolar cells in adult mice and rats. Since the loss of dopaminergic amacrine cells (DA ACs) in GLE monkeys and rats contributes to supernormal ERGs, the retinal DA system was analyzed in mice following GLE. C57BL/6 female mice were exposed to low (27 ppm), moderate (55 ppm) or high (109 ppm) lead throughout gestation and until postnatal day 10 (PN10). Blood [Pb] in control, low-, moderate- and high-dose GLE was ≤ 1, ≤ 10, ~25 and ~40 μg/dL, respectively, on PN10 and by PN30 all were ≤ 1 μg/dL. At PN60, confocal-stereology studies used vertical sections and wholemounts to characterize tyrosine hydroxylase (TH) expression and the number of DA and other ACs. GLE dose-dependently and selectively decreased the number of TH-immunoreactive (IR) DA ACs and their synaptic plexus without affecting GABAergic, glycinergic or cholinergic ACs. Immunoblots and confocal revealed dose-dependent decreases in retinal TH protein expression and content, although monoamine oxidase-A protein and gene expression were unchanged. High-pressure liquid chromatography showed that GLE dose-dependently decreased retinal DA content, its metabolites and DA utilization/release. The mechanism of DA selective vulnerability is unknown. However, a GLE-induced loss/dysfunction of DA ACs during development could increase the number of rods and bipolar cells since DA helps regulate neuronal proliferation, whereas during adulthood it could produce ERG supernormality as well as altered circadian rhythms, dark/light adaptation and spatial contrast sensitivity.

Association of cumulative lead exposure with Parkinson's disease

BACKGROUND

Research using reconstructed exposure histories has suggested an association between heavy metal exposures, including lead, and Parkinson's disease (PD), but the only study that used bone lead, a biomarker of cumulative lead exposure, found a nonsignificant increase in risk of PD with increasing bone lead.

OBJECTIVES

We sought to assess the association between bone lead and PD.

METHODS

Bone lead concentrations were measured using 109Cd excited K-shell X-ray fluorescence from 330 PD patients (216 men, 114 women) and 308 controls (172 men, 136 women) recruited from four clinics for movement disorders and general-community cohorts. Adjusted odds ratios (ORs) for PD were calculated using logistic regression.

RESULTS

The average age of cases and controls at bone lead measurement was 67 (SD = 10) and 69 (SD = 9) years of age, respectively. In primary analyses of cases and controls recruited from the same groups, compared with the lowest quartile of tibia lead, the OR for PD in the highest quartile was 3.21 [95% confidence interval (CI), 1.17-8.83]. Results were similar but slightly weaker in analyses restricted to cases and controls recruited from the movement disorders clinics only (fourth-quartile OR = 2.57; 95% CI, 1.11-5.93) or when we included controls recruited from sites that did not also contribute cases (fourth-quartile OR = 1.91; 95% CI, 1.01-3.60). We found no association with patella bone lead.

CONCLUSIONS

These findings, using an objective biological marker of cumulative lead exposure among typical PD patients seen in our movement disorders clinics, strengthen the evidence that cumulative exposure to lead increases the risk of PD.

Blood lead levels and major depressive disorder, panic disorder, and generalized anxiety disorder in US young adults

CONTEXT

Lead is a ubiquitous neurotoxicant, and adverse cognitive and behavioral effects are well-documented in children and occupationally exposed adults but not in adults with low environmental exposure.

OBJECTIVE

To investigate the association of current blood lead levels with 3 common psychiatric disorders-major depression, panic, and generalized anxiety-in young adults.

DESIGN

Cross-sectional epidemiologic survey.

SETTING

Nationally representative sample of US adults.

PARTICIPANTS

A total of 1987 adults aged 20 to 39 years who responded to the National Health and Nutrition Examination Survey (1999-2004).

MAIN OUTCOME MEASURES

Twelve-month DSM-IV criteria-based diagnoses of major depressive disorder, panic disorder, and generalized anxiety disorder assessed using the Composite International Diagnostic Interview.

RESULTS

The mean (SD) blood lead level was 1.61 (1.72) microg/dL (range, 0.3-37.3 microg/dL) (to convert to micromoles per liter, multiply by 0.0483). Increasing blood lead levels were associated with higher odds of major depression (P = .05 for trend) and panic disorder (P = .02 for trend) but not generalized anxiety disorder (P = .78 for trend) after adjustment for sex, age, race/ethnicity, education status, and poverty to income ratio. Persons with blood lead levels in the highest quintile had 2.3 times the odds of major depressive disorder (95% confidence interval [CI], 1.13-4.75) and 4.9 times the odds of panic disorder (1.32-18.48) as those in the lowest quintile. Cigarette smoking was associated with higher blood lead levels and outcome, but models that excluded current smokers also resulted in significantly increased odds of major depression (P = .03 for trend) and panic disorder (P = .01 for trend) with higher blood lead quintiles.

CONCLUSIONS

In these young adults with low levels of lead exposure, higher blood lead levels were associated with increased odds of major depression and panic disorders. Exposure to lead at levels generally considered safe could result in adverse mental health outcomes.

Muscarinic cholinergic modulation of synaptic transmission and plasticity in rat hippocampus following chronic lead exposure

The cholinergic system is believed to be associated with learning and memory functions. Lead (Pb2+) is a well-known neurotoxic metal that causes irreversible damage to the central nervous system (CNS). To investigate whether Pb2+ interferes with cholinergic modulation, we examined the effects of carbachol (CCh), a muscarinic cholinergic agonist, on synaptic transmission and plasticity in the CA1 area of the hippocampus of developmentally Pb2+-exposed rats. The results showed that: (1) In both control and Pb2+-exposed rats, 0.1 microM CCh significantly enhanced tetanus-induced long-term potentiation (LTP), while 5 microM CCh induced a reversible depression of field excitatory postsynaptic potentials (fEPSPs). However, both the enhancement of LTP and depression of fEPSPs were significantly smaller in Pb2+-exposed rats than in controls, suggesting that the extent of the effect of CCh on the cholinergic system was depressed by Pb2+. (2) In Pb2+-exposed rats, the enhancement of LTP induced by 0.1 microM CCh was attenuated by pirenzepine, a M1AChR antagonist, but was not affected by methoctramine tetrahydrochloride (M-105), a M2/4AChR antagonist. The depression of fEPSPs induced by 5 microM CCh was reduced by either pirenzepine or M-105. (3) Furthermore, paired-pulse facilitation (PPF) was not affected by 0.1 microM CCh in control and Pb2+-exposed rats but was increased by 5 microM CCh in either group; the increase in PPF was less pronounced in Pb2+-treated when compared to control rats. These results suggested that cholinergic modulation could be impaired by Pb2+, and this kind of impairment might occur via different mAChR subtypes. Our study delineated the effects of Pb2+ on muscarinic modulation, and this might be one of the underlying mechanisms by which Pb2+ impairs learning and memory.

Low-level human equivalent gestational lead exposure produces sex-specific motor and coordination abnormalities and late-onset obesity in year-old mice

BACKGROUND

Low-level developmental lead exposure is linked to cognitive and neurological disorders in children. However, the long-term effects of gestational lead exposure (GLE) have received little attention.

OBJECTIVES

Our goals were to establish a murine model of human equivalent GLE and to determine dose-response effects on body weight, motor functions, and dopamine neurochemistry in year-old offspring.

METHODS

We exposed female C57BL/6 mice to water containing 0, 27 (low), 55 (moderate), or 109 ppm (high) of lead from 2 weeks prior to mating, throughout gestation, and until postnatal day 10 (PN10). Maternal and litter measures, blood lead concentrations ([BPb]), and body weights were obtained throughout the experiment. Locomotor behavior in the absence and presence of amphetamine, running wheel activity, rotarod test, and dopamine utilization were examined in year-old mice.

RESULTS

Peak [BPb] were < 1, < or = 10, 24-27, and 33-42 microg/dL in control, low-, moderate- and high-dose GLE groups at PN0-10, respectively. Year-old male but not female GLE mice exhibited late-onset obesity. Similarly, we observed male-specific decreased spontaneous motor activity, increased amphetamine-induced motor activity, and decreased rotarod performance in year-old GLE mice. Levels of dopamine and its major metabolite were altered in year-old male mice, although only forebrain utilization increased. GLE-induced alterations were consistently larger in low-dose GLE mice.

CONCLUSIONS

Our novel results show that GLE produced permanent male-specific deficits. The nonmonotonic dose-dependent responses showed that low-level GLE produced the most adverse effects. These data reinforce the idea that lifetime measures of dose-response toxicant exposure should be a component of the neurotoxic risk assessment process.

The challenge posed to children's health by mixtures of toxic waste: the Tar Creek superfund site as a case-study

In the United States, many of the millions of tons of hazardous wastes that have been produced since World War II have accumulated in sites throughout the nation. Citizen concern about the extent of this problem led Congress to establish the Superfund Program in 1980 to locate, investigate, and clean up the worst sites nationwide. Most such waste exists as a complex mixture of many substances. This article discusses the issue of toxic mixtures and children's health by focusing on the specific example of mining waste at the Tar Creek Superfund Site in Northeast Oklahoma.

Cumulative lead exposure and cognitive performance among elderly men

BACKGROUND

Recent evidence suggests that cumulative lead exposure among adults in nonoccupational settings can adversely affect cognitive function. Which cognitive domains are affected has not been explored in detail.

METHODS

We used nonlinear spline regressions and linear repeated-measures analysis to assess the association between scores on a battery of cognitive tests over time and both blood and bone lead concentrations in the Normative Aging Study, a cohort of community-dwelling elderly men. Bone lead was measured from 1991 through 1999 with K-shell x-ray fluorescence. A total of 1089 men with a mean (+/-standard deviation) age of 68.7 (+/-7.4) years with blood lead measurements, 761 of whom also had valid bone lead measurements, completed at least one of a battery of cognitive tests. Approximately 3.5 years later, 69% of the men had at least one repeat test. Cognitive testing was performed from 1993 through 2001.

RESULTS

On a cross-sectional basis, there was little association between blood or bone lead and cognitive test scores. Change in performance over time on virtually all tests worsened as bone lead increased, with the most robust effects on performance and reaction time scores on visuospatial/visuomotor tests.

CONCLUSIONS

Low-level cumulative exposure to lead in nonoccupational settings may adversely affect cognitive function, particularly in the visuospatial/visuomotor domain.

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.

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.

Waterborne Lead Exposure Affects Brain Endocannabinoid Content in Male but not Female Fathead Minnows (Pimephales promelas)

There are several similarities between the behavioral and neurochemical effects of lead (Pb2+) and the cannabinoids. Both Pb2+ exposure and cannabinoid treatment decrease exploratory behavior. Pb2+-induced hyperactivity has been observed in rats and fish. By comparison, cannabinoids increase locomotor activity at higher doses in rats. Moreover, Pb2+ exposure produces learning and memory impairments as do the cannabinoids. Many of the behavioral effects of Pb2+ are thought to be due, in part, to the ability of Pb2+ to either inhibit or mimic the actions of calcium (Ca2+). At low concentrations, Pb2+ enhances basal release of neurotransmitter from presynaptic terminals by increasing intracellular free Ca2+ concentrations. Pb2+ also decreases evoked neurotransmitter release due to blockade of voltage-gated Ca2+ channels. Interestingly, the endocannabinoids (eCBs) including N-arachidonylethanolamine (AEA) and 2-arachidonylglycerol (2-AG) are synthesized in response to increases in intracellular Ca2+ and activate the CB1 receptor that inhibits voltage-gated Ca2+ channels. We tested the hypothesis that waterborne Pb2+ exposure significantly affects whole-brain eCB content in adult male and female fathead minnows (Pimephales promelas). Waterborne Pb2+ exposure (1.0 ppm) resulted in a time-dependent accumulation of Pb2+ in bone in both males and females. Brain AEA and 2-AG content were significantly greater in females compared to males. Pb2+ exposure increased brain AEA content in males at 7 and 14 days of exposure and increased brain 2-AG content at 14 days. Pb2+ exposure had no effect on either brain AEA or 2-AG content in females at any of the time points examined. As eCBs serve as activity-dependent retrograde inhibitors of neurotransmitter release, the increase in brain eCB content would accentuate Pb2+-induced decreases in evoked neurotransmitter release in male but not female fathead minnows.

Cumulative lead exposure and prospective change in cognition among elderly men: the VA Normative Aging Study

Lead exposure has been found to affect cognitive function in several different populations. Whether chronic low-level environmental exposure to lead results in cognitive decline among adults has not been examined. The authors assessed the relation between biomarkers of lead exposure and change in Mini-Mental State Examination (MMSE) scores in the Normative Aging Study, a cohort of elderly US men. Bone lead was measured with K-shell x-ray fluorescence. A total of 466 men aged 67.4 (standard deviation, 6.6) years took the MMSE on two occasions that were an average of 3.5 (standard deviation, 1.1) years apart during the period 1993-2001 and had bone lead concentrations measured during the period 1991-2002. A one-interquartile range (20 microg/g of bone mineral) higher patella bone lead concentration was associated with a change in MMSE score of -0.24 (95% confidence interval: -0.44, -0.05) after adjustment for age, education, smoking, alcohol intake, and time between MMSE tests. This effect is approximately equivalent to that of aging 5 years in relation to the baseline MMSE score in study data. The association with tibia lead was weaker and that with blood lead was absent. The data suggest that higher patella bone lead levels, a marker of mobilizable accumulated lead burden, are associated with a steeper decline over time in performance on the MMSE test among nonoccupationally exposed elderly men.

Presynaptic disruption of transmitter release by lead

Low concentrations of inorganic lead ions (Pb2+) disrupt transmitter release by causing aberrant augmentation of spontaneous and suppression of evoked release. These effects result from high affinity interactions of Pb2+ with the voltage-gated calcium channels (VGCC) as well as Ca2+ binding proteins which regulate the synaptic vesicle mobilization, docking, and exocytosis processes. Augmentation of spontaneous release may involve stimulation of vesicle mobilization consequent to Pb2+ activation of CaMKII-dependent phosphorylation of synapsin I and/or stimulation of asynchronous exocytosis via direct Pb2+ activation of the putative exocytotic Ca2+-sensor protein synaptotagmin I. In addition, synergistic stimulation of PLC and DAG/Pb2+-dependent activation of PKC may enhance the secretagogue effects of Pb2+ by increasing metal sensitivity of exocytosis and/or modulating calcium channel activity. In contrast to intracellularly-mediated actions of Pb2+ resulting in augmentation of spontaneous release, the inhibition of evoked transmitter release by Pb2+ is largely attributable to extracellular block of the voltage-gated calcium channels.

Inorganic lead exposure in the rat activates striatal cFOS expression at lower blood levels and inhibits amphetamine-induced cFOS expression at higher blood levels

The impact of inorganic lead exposure on dopamine (DA) neurotransmission in the basal ganglia was examined. Amphetamine (AMPH)-induced cFOS immunoreactivity (cFOS-IR) in the striatum was determined after a 3-week exposure to lead acetate (0, 50, or 250 ppm). On the 21st day of lead exposure, rats were challenged with AMPH (4 mg/kg i.p.) or saline vehicle (Veh) and were assayed for presence of cFOS-IR. In the untreated control (Con) group, AMPH challenge (Con/AMPH) increased cFOS-IR expression by approximately 35-fold over Veh challenge (Con/Veh) (P < 0.01). In the Pb50/Veh group, cFOS-IR expression was approximately 7-fold greater than in the Con/Veh group (P < 0.05). Given that there was negligible cFOS-IR expression in the Con/Veh group, this indicates that the Pb50 exposure induced cFOS expression. The increase in cFOS-IR in the Pb50/AMPH was also significant (P < 0.01), but it was not different from the Con/AMPH (P > 0.20). Neither the Pb250/Veh group nor the Pb250/AMPH group had a significant increase in cFOS-IR relative to Con/Veh (P > 0.20). These results indicate that chronic 50 ppm lead exposure induced a low but statistically significantly level of cFOS gene activation and that it did not affect the AMPH-induced cFOS activation. However, chronic 250 ppm lead exposure inhibited AMPH-induced activation of cFOS in the striatum by about 89%. Therefore, lead is capable of both activating cFOS expression at low levels of exposure (mean blood lead level 21.6 +/- 1.9 microg/dl) and inhibiting AMPH-induced cFOS expression at higher levels of exposure (mean blood lead level 47.4 +/- 2.6 microg/dl).

Relationship of bone and blood lead levels to psychiatric symptoms: the normative aging study

Blood and bone lead levels were used to investigate lead's potential effect on psychiatric symptoms among middle-aged to elderly men from the Normative Aging Study. Symptoms were assessed using the Brief Symptom Inventory (BSI) and analyzed as individual outcomes as well as a measure that combined anxiety, depression, and phobic anxiety. Blood and bone lead averaged 6.3 microg/dL (standard deviation [SD] = 4.16), 21.9 microg/g (SD = 13.5), and 32.1 microg/g (SD = 19.8) for blood, tibia, and patella lead, respectively. In logistic regression models that adjusted for age, alcohol intake, employment status, and education status, we found that patella bone lead was significantly associated with an increased risk of phobic anxiety and the combined outcome measure at the P </= 0.05 level. Tibia and blood lead had similar associations. We conclude that cumulative lead exposure, which bone lead levels reflect, could be a risk factor for psychiatric symptoms even at modest levels of exposure.

Lead exposure biomarkers and mini-mental status exam scores in older men

BACKGROUND

Lead is neurotoxic; yet, whether cognitive decline in older persons is associated with lead exposure is unknown. We studied whether lead exposure biomarkers are associated with cognitive test scores, as well as the modifying effects of age on the lead-cognition relationship.

METHODS

Lead exposure biomarkers and Mini-Mental Status Exam (MMSE) scores were measured among subjects in the Normative Aging Study. Multiple linear and logistic regression analyses were performed to examine the cross-sectional association of these 2 variables.

RESULTS

We found an odds ratio (OR) of 2.1 for MMSE <24 with an increase from the lowest to the highest quartile of patella lead levels (95% confidence interval [CI] = 1.1 to 4.1). From the lowest to the highest quartile of blood lead the OR for low MMSE was 3.4 (CI = 1.6 to 6.2). There was an interaction between lead biomarkers and age. Among subjects in the lowest quartile of patella lead levels, MMSE score decreased by 0.03 points per year (CI = -0.07 to 0.005), whereas in the highest quartile, MMSE score decreased by 0.13 points per year (CI = -0.19 to -0.07). Similar interactions were found between blood lead levels and age.

CONCLUSIONS

Increased levels of lead in bone and blood are inversely associated with cognitive performance among older men. Lead exposure might accelerate age-associated cognitive decline.

Vascular endothelial growth factor mediates vasogenic edema in acute lead encephalopathy

Brain injury from inorganic Pb(2+) is considered the most important environmental childhood health hazard worldwide. The microvasculature of the developing brain is uniquely susceptible to high level Pb(2+) toxicity (ie, Pb(2+) encephalopathy) characterized by cerebellar hemorrhage, increased blood-brain barrier permeability, and vasogenic edema. However, the specific molecular mediators of Pb(2+) encephalopathy have been elusive. We found that Pb(2+) induces vascular endothelial growth factor/vascular permeability factor (VEGF) in cultured astrocytes (J Biol Chem, 2000;275:27874-27882). The study presented here asks if VEGF dysregulation contributes mechanistically to Pb(2+) encephalopathy. Neonatal rats exposed to 4% Pb-carbonate develop the histopathological features of Pb(2+) encephalopathy seen in children. Cerebellar VEGF expression increased approximately twofold (p < 0.01) concurrent with the development of cerebellar microvascular hemorrhage, enhanced vascular permeability to serum albumin, and vasogenic cerebellar edema (p < 0.01). No change in VEGF expression occurred in cerebral cortex that does not develop these histopathological complications of acute Pb(2+) intoxication. Pb(2+) exposure increased phosphorylation of cerebellar Flk-1 VEGF receptors and the Flk-1 inhibitor CEP-3967 completely blocked cerebellar edema formation without affecting microhemorrhage formation or blood-brain barrier permeability. This establishes that Pb(2+)-induced vasogenic edema formation develops via a Flk-1-dependent mechanism and suggests that the vascular permeability caused by Pb(2+) is Flk-1 independent.

Effect of acute and chronic lead exposure on apomorpine-induced sniffing in rats

Sniffing is a behaviour which can be induced by dopamine D1/D2 receptor agonists. In order to test the effect of chronic lead exposure on dopamine receptor subtypes, we studied the effects of acute and chronic lead exposure on sniffing induced by apomorphine, a dopamine receptor agonist. Intraperitoneal injection of the dopaminergic receptor agonist, apomorphine (0.25-1 mg/kg), induced dose-dependent the sniffing behaviour in rats. Acute administration of lead acetate (50, 100 and 200 mg/kg) deceased the apomorphine-induced sniffing. Chronic lead (0.25%) exposure also decreased the apomophine response Dopamine D1 or D2 receptor antagonists reduced the apomorphine effect. Lead exposure could not potentiate the blockade induced by the dopamine receptor antagonists. It is concluded that the response of lead is not mediated by alteration of dopamine receptors.

Effects of maternal lead administration on monoaminergic, GABAergic and glutamatergic systems

The effects of perinatal exposure to lead (300 mg/l) on the development of monoaminergic and aminoacidergic systems were evaluated in the striatum, cerebral cortex (Cx), dorsal hippocampus (d-Hipp) and basal-medial hypothalamus. Maternal exposure to lead produced regional alterations in monoamine content, with increases in dopamine and serotonin or their metabolites. Further, decreased glutamate levels were seen in all brain regions studied, while GABA content decreased only in the Cx. Together, these results show that lead causes alterations to neurotransmitter systems during development. These may be related to lead-induced neurobehavioral impairment.

Synaptotagmin I is a molecular target for lead

Lead poisoning can cause a wide range of symptoms with particularly severe clinical effects on the CNS. Lead can increase spontaneous neurotransmitter release but decrease evoked neurotransmitter release. These effects may be caused by an interaction of lead with specific molecular targets involved in neurotransmitter release. We demonstrate here that the normally calcium-dependent binding characteristics of the synaptic vesicle protein synaptotagmin I are altered by lead. Nanomolar concentrations of lead induce the interaction of synaptotagmin I with phospholipid liposomes. The C2A domain of synaptotagmin I is required for lead-mediated phospholipid binding. Lead protects both recombinant and endogenous rat brain synaptotagmin I from proteolytic cleavage in a manner similar to calcium. However, lead is unable to promote the interaction of either recombinant or endogenous synaptotagmin I and syntaxin. Finally, nanomolar concentrations of lead are able to directly compete with and inhibit the ability of micromolar concentrations of calcium to induce the interaction of synaptotagmin I and syntaxin. Based on these findings, we conclude that synaptotagmin I may be an important, physiologically relevant target of lead.

Lead acetate potentiates brain catalase activity and enhances ethanol-induced locomotion in mice

Several reports have demonstrated that acute lead acetate administration enhances brain catalase activity in animals. Other reports have shown a role of brain catalase in ethanol-induced behaviors. In the present study we investigated the effect of acute lead acetate on brain catalase activity and on ethanol-induced locomotion, as well as whether mice treated with different doses of lead acetate, and therefore, with enhanced brain catalase activity, exhibit an increased ethanol-induced locomotor activity. Lead acetate or saline was injected IP in Swiss mice at doses of 50, 100, 150, or 200 mg/kg. At 7 days following this treatment, ethanol (0.0, 1.5, 2.0, 2.5, or 3.0 g/kg) was injected IP, and the animals were placed in the open-field chambers. Results indicated that the locomotor activity induced by ethanol was significantly increased in the groups treated with lead acetate. Maximum ethanol-induced locomotor activity increase was found in animals treated with 100 mg/kg of lead acetate and 2.5 g/kg of ethanol. Total brain catalase activity in lead-pretreated animals also showed a significant induction, which was maximum at 100 mg/kg of lead acetate treatment. No differences in blood ethanol levels were observed among treatment groups. The fact that brain catalase and ethanol-induced locomotor activity followed a similar pattern could suggest a relationship between both lead acetate effects and also a role for brain catalase in ethanol-induced behaviors.

Effects of chronic low-level oral lead exposure on prepulse inhibition of acoustic startle in the rat

Previous work has suggested that the behavioral effects of chronic low-level lead exposure on fixed interval (FI) operant behavior result from enhanced dopaminergic neurotransmission in the nucleus accumbens (Cory-Slechta et al., J Pharmacol Exp Ther 286: 794-805, 1998). The present studies were designed to further characterize the effects of chronic low-level oral lead exposure on another behavior that is modulated by dopaminergic neurotransmission in the nucleus accumbens. In these studies acoustic startle and the prepulse inhibition (PPI) of startle were studied in rats following chronic low-level oral lead exposure. Weanling male rats were treated for 5-6 weeks with lead via drinking water (250 ppm lead acetate; controls drank 250 ppm sodium acetate). Acoustic startle reactivity (95, 105, and 115 dB noise bursts) and PPI (prepulses of 1-8 dB over the 70-dB background) of startle were tested following lead exposure. Lead exposure did not affect body weight. Lead exposure also did not significantly affect baseline [i.e., no prepulse inhibition (NO-PPI)] acoustic startle as measured by 1) startle amplitude on the first startle trial (105 dB), 2) the average startle amplitude for the first ten trials (105 dB), or 3) the average startle amplitude for the NO-PPI trials during PPI testing (95, 105, and 115 dB). Lead exposure also did not affect the latency to onset for the startle response. In contrast, for both the 105 dB and 115 dB acoustic startle stimuli, chronic low-level oral lead exposure significantly attenuated the capacity of an acoustic prepulse to reduce the startle response. This effect was present whether the data were presented and analyzed as raw change from baseline or as the percentage of baseline startle. Given the strong link between the modulation of PPI and dopaminergic neurotransmission in the nucleus accumbens, the present data support the hypothesis that chronic low-level oral lead exposure facilitates dopamine neurotransmission in the nucleus accumbens.

Lead increases tetrodotoxin-insensitive spontaneous release of glutamate and GABA from hippocampal neurons

This study was aimed at investigating the effects of the environmental pollutant lead (Pb2+) on the tetrodotoxin (TTX)-insensitive release of neurotransmitters from hippocampal neurons. Evidence is provided that Pb2+ (>/=100 nM) increases the frequency of gamma-aminobutyric acid (GABA)- and glutamate-mediated miniature postsynaptic currents (MPSCs) recorded by means of the patch-clamp technique from cultured hippocampal neurons. Because Pb2+ changed neither the amplitude nor the decay-time constant of the MPSCs, Pb2+-induced changes in MPSC frequency are exclusively due to a presynaptic action of this heavy metal. Increase by Pb2+ of the action potential-independent release of GABA and glutamate was concentration dependent and was only partially reversible upon washing of the neurons with nominally Pb2+-free external solution. This effect was also Ca2+ independent and began approximately after 1-2-min exposure of the neurons to Pb2+. The latency for the onset of the Pb2+'s effect on the MPSC frequency and the inability of the chelator ethylenediaminetetraacetic acid (100 microM) to reverse the effect that remained after washing of the neurons with external solution suggested that Pb2+ acted via an intracellular mechanism. Of interest also was the finding that Pb2+ simultaneously increased the release of GABA and glutamate, overriding the ability of these neurotransmitters to decrease the release of one another. Given that synaptic activity is a key mechanism for the establishment of stable synaptic connections early in the development, it is possible that, by interfering with spontaneous transmitter release, Pb2+ has lasting effects on neuronal maturation and plasticity.

Nanomolar concentrations of lead inhibit glutamatergic and GABAergic transmission in hippocampal neurons

To investigate whether lead (Pb2+) affects the tetrodotoxin (TTX)-sensitive release of neurotransmitters, the whole-cell mode of the patch-clamp technique was applied to cultured hippocampal neurons. Pb2+ (>/=10 nM) reversibly blocked the TTX-sensitive release of glutamate and gamma-aminobutyric acid (GABA), as evidenced by the reduction of the amplitude and frequency of glutamate- and GABA-mediated postsynaptic currents (PSCs) evoked by spontaneous neuronal firing. This effect of Pb2+, which occurred 2-3 s after exposure of the neurons to Pb2+-containing external solution, was not related to changes in Na+-channel activity, and was quantified by measurements of changes in the amplitude of PSCs evoked when a 50-micros, 5-V stimulus was applied via a bipolar electrode to a neuron synaptically connected to the neuron under study. With an IC50 of approximately 68 nM, Pb2+ blocked the evoked release of glutamate and GABA. This effect was most likely mediated by Pb2+'s actions on extracellular targets, because there was a very short delay (<3 s) for its onset, and it could be completely reversed by the chelator ethylene diaminetetraacetic acid (EDTA). Given that Pb2+-induced blockade of evoked transmitter release could be reversed by 4-aminopyridine, it is suggested that the effect on release was mediated via the binding of Pb2+ to voltage-gated Ca2+ channels. Thus, it is most likely that the neurotoxic effects of Pb2+ in the mammalian brain involve a decrease of the TTX-sensitive, Ca2+-dependent release of neurotransmitters.

Selective vulnerability of the developing brain to lead

Environmental lead exposure in young children who ingest household paint dust or other sources impairs their potential intelligence in a linear, dose-dependent fashion in contrast to its far more subtle effects on other neurologic functions. Basic investigations have identified three interrelated steps in synaptic neurotransmission at which low levels of lead can disrupt signal processing. Lead enhances background transmitter release, but impairs stimulated release, inhibits function at the N-methyl-D-aspartate-type glutamate receptor and stimulates background levels of the intracellular messenger protein kinase C. Taken together these effects have the effect of diminishing the synaptic signal to noise ratio. The ability of lead to enhance 'synaptic noise' during a critical early period of postnatal development may permanently disrupt the architecture of cortical processing units by depriving them of high resolution environmental signals needed to refine synaptic connections.

Effects of lead-arsenic combined exposure on central monoaminergic systems

Lead acetate (116 mg/kg/day), arsenic (11 or 13.8 mg/kg/day as sodium arsenite), a lead-arsenic mixture or vehicle were administered to adult mice through gastric intubation during 14 days. Then, the regional content of norepinephrine (NE), dopamine (DA), serotonin (5-HT), 3,4 dihydroxyphenyl-acetic acid (DOPAC), 5-hydroxyindole-3-acetic acid (5-HIAA), arsenic, and lead were quantified. Compared with the accumulation after single element exposures, the mixture elicited a higher accumulation of lead and a lower arsenic accumulation in the brain. Compared to controls, lead induced only an augmentation of DOPAC (200%) in the hypothalamus. By contrast, the mixture provoked increases of DOPAC in the hypothalamus (250%), DA and 5-HIAA in the striatum (67 and 187%, respectively) and NE decreased in the hypothalamus (45%). Although these alterations were similar to those produced by arsenic alone, the mixture provoked a 38% decrease of NE in the hippocampus and increases of 5-HT in midbrain and frontal cortex (100 and 90%, respectively) over control values, alterations that were not elicited by either metal alone. These results demonstrate an interaction arsenic/lead on the central monoaminergic systems of the adult mouse.

Induction of c-fos mRNA by lead in PC 12 cells

Addition of lead acetate to PC 12 pheochromocytoma cells elicits induction of c-fos, an immediate early response gene. Induction of c-fos was concentration- and time-dependent: the lowest concentration of lead acetate tested that induced c-fos was 10 microM; induction was observed after a 30 min incubation and remained high after 90 min. Treatment with lead acetate and cycloheximide superinduced c-fos mRNA. Actinomycin D, an inhibitor of mRNA transcription, decreased the level of c-fos mRNA induced by lead acetate by almost 80%. Cadmium chloride and zinc chloride did not induce c-fos mRNA. Since the c-fos gene encodes a transcription factor, Pb2+ has the potential to deregulate the expression of other genes.

Distinct mechanisms of neurotransmitter release from PC 12 cells exposed to lead

Two enzymes, protein kinase C and microsomal Ca(2+)-ATPase help regulate levels of Ca2+ in many types of cells. Since proteins that regulate Ca2+ often influence sensitivity to Pb2+, we determined the possible roles played by protein kinase C and microsomal Ca(2+)-ATPase for the Pb(2+)-evoked release of norepinephrine (NOR) in PC cells. NOR release was observed at 10 microM Pb2+ when PC 12 cells were stimulated with inhibitors of microsomal Ca(2+)-ATPase such as thapsigargin, cyclopiazonic acid, or 2,5-di-(t-butyl)-hydroquinone. At 5 microM, Pb2+ evoked the release of NOR in PC 12 cells stimulated with activators of protein kinase C such as phorbol 12-myristate 13-acetate (PMA) or (-)-7-octylindolactam. NOR release was observed at 1 microM Pb2+ in the presence of both PMA and thapsigargin. Ni2+ and Cd2+ blocked NOR release stimulated by Pb2+ in the presence of thapsigargin but not by PMA. NOR released by thapsigargin stimulation was not altered in PC 12 cells depleted of protein kinase C. Two proteins found in vesicles, chromogranin B and secretogranin-II were released with NOR. Our results indicate that in PC 12 cells, PB(2+)-evokes the release of neurotransmitters. Furthermore, thapsigargin and PMA increase the cell's sensitivity to Pb2+ by different pathways.

Chronic developmental lead exposure increases the threshold for long-term potentiation in rat dentate gyrus in vivo

Chronic developmental lead (Pb) exposure has been long associated with cognitive dysfunction in children and animals. In an attempt to more directly relate the behavioral observations of impaired cognitive ability to Pb-induced effects on neuronal activity, we utilized the long-term potentiation (LTP) model of neural plasticity to assess synaptic function. Male rats were chronically exposed to 0.2% Pb(2+)-acetate through the drinking water of the pregnant dam, and directly through their own water supply at weaning. As adults, field potentials evoked by perforant path stimulation were recorded in the dentate gyrus under urethane anesthesia. LTP threshold was determined by applying a series of stimulus trains of increasing intensities. Baseline testing of dentate gyrus field potentials indicated that input/output functions, maximal response amplitudes, and threshold currents required to evoke a population spike (PS) did not differ for control and Pb-exposed animals. Despite similarities in baseline synaptic transmission, Pb-exposed animals required a higher train intensity to evoke LTP than controls. With maximal train stimulation, however, control and Pb animals exhibited comparable levels of potentiation. These findings suggest that the mechanisms of LTP induction are preferentially impaired by Pb exposure. Although baseline synaptic transmission was not altered in Pb-exposed animals, decreases in glutamate release following high K+ perfusion and reductions in paired pulse facilitation have been reported in the intact animal. Pb-induced reductions in calcium influx through voltage-sensitive or N-methyl-D-aspartate (NMDA) receptor-dependent channels may mediate increases in LTP threshold. It is possible that the threshold changes in the induction of LTP reported here contribute to cognitive impairments associated with Pb exposure.

Presynaptic glutamatergic function in dentate gyrus in vivo is diminished by chronic exposure to inorganic lead

Reductions in membrane Ca2+ channel currents and depolarization-evoked neurotransmitter release have been repeatedly observed as a result of acute exposure to Pb2+. This study was performed to determine whether hippocampal glutamate and GABA release are impaired in intact animals chronically exposed to lead (Pb). As paired-pulse facilitation in the hippocampus is primarily mediated by an enhancement of glutamate release, this neurophysiological measure was also assessed in the dentate gyrus of Pb-exposed animals. Pregnant dams received 0.2% Pb acetate in the drinking water at parturition, and male offspring were weaned to the same solution as that given their dams. Control animals were maintained on distilled water. As adults, animals had intracerebral dialysis probes inserted through guide cannulae implanted 2-4 days previously and the hippocampal CAI-dentate area was perfused with modified Ringer's solution. Transmitter release was induced by perfusion with 150 mM K+ with half the animals in each group tested with Ca2+ present in the perfusate (total release) and the other half with Ca2+ absent (Ca(2+)-independent release). K(+)-stimulated total glutamate release was reduced in Pb-exposed animals relative to controls. No group differences were observed under Ca(2+)-free conditions, indicating that Ca(2+)-dependent glutamate release was decreased in exposed rats. In contrast no group differences in K(+)-stimulated total GABA release were evident, whereas an augmentation in GABA release under Ca(2+)-free conditions was revealed in Pb-exposed animals. The effects of exposure on the Ca(2+)-dependent components of release are consistent with in vitro evidence indicating an inhibitory action of Pb2+ at voltage-sensitive Ca2+ channels. A separate group of animals was prepared under urethane anesthesia with stimulating and recording electrodes placed in the perforant path and dentate gyrus, respectively. Pairs of stimulus pulses were delivered at interpulse intervals (IPI) of 10-250 ms. Pb exposure induced an increase in paired-pulse depression at the 20 ms 1PI and reduced paired-pulse facilitation at the 30 ms IPI. Decreases in paired-pulse facilitation could not be attributed to the reported effects of Pb2+ on N-methyl-D-aspartate (NMDA) receptors as MK-801 (1.0 mg/kg, s.c.) administration produced an opposing pattern of effects on paired-pulse measures. The Pb-induced suppression of paired-pulse facilitation is consistent with exposure-related decreases in total glutamate release. The impact of these effects of Pb exposure on hippocampal glutamatergic transmission may contribute to the reported effects of Pb on other forms of synaptic plasticity including long-term potentiation, a model of learning and memory.

Lead alters structure and function of mouse flexor muscle

To evaluate the effect of long-term exposure to heavy metals on skeletal muscle, chronic subcutaneous injections for 7 days of two level treatments (low dose, 0.1 mg/kg and high dose, 1 mg/kg) of lead acetate were investigated. Comparative analyses of in situ dorsiflexor muscle isometric contractile characteristics were studied in urethane-anesthetized (2 mg/g, i.p.) control and lead-exposed male mice. Control muscle-twitch tension reached an average of 1.81 +/- 0.06 g. Chronic lead (Pb2+) treatments did not affect muscle contractile speed, but reduced significantly the twitch tension in both high and low doses when compared to control animals. This effect was in a dose-dependent manner; 1.21 +/- 0.07 g for low dose and 0.90 +/- 0.05 g for high dose. These chronic Pb2+ treatments accelerated muscle fatigue after 250 stimuli (25 Hz for 10 sec) in both the low and high doses equally. However, marked elevation in tetanic (25 Hz) specific tension were observed in the high-dose, chronically treated animals, indicating some changes in contractile apparatus function. The high dose of chronic Pb2+ treatment induced ultrastructural changes, including reduced number of synaptic vesicles, disruption of mitochondria and increased number of smooth endoplasmic reticulum and myelin-like figures in the intramuscular axons and neuromuscular junctions. Chronic Pb2+ treatment caused extensive disruption of the sarcoplasmic mitochondria and increased the number of myelin-like figures in the muscle. These results suggest that exposure to Pb2+ at a low concentration can compromise the in situ skeletal muscle isometric contraction.

Lead alters inositol polyphosphate receptor activities: protection by ATP

Receptor-mediated phosphoinositide signaling pathway which generates a variety of second messengers is regulated by intracellular free Ca2+ concentrations. Since toxic metal cations like Pb2+ are known to alter Ca(2+)-dependent processes, the present study was initiated to study the effects of Pb2+ on inositol 1,4,5-trisphosphate (InsP3) and inositol 1,3,4,5-tetrakisphosphate (InsP4) receptor binding and InsP3-mediated Ca(2+)-release. Rat cerebellar membrane and microsomal fractions were incubated with various concentrations of Pb2+ (0.01-100 microM). Pb2+ significantly stimulated [3H]-InsP3 and [3H]-InsP4 receptor binding (EC50 22.7 and 13.5 microM respectively) as a function of metal concentrations. However, InsP3-mediated Ca2+ release, determined by measuring the changes in fluorescence intensity of Fura-2, was significantly inhibited by varying concentrations of Pb2+. Re-uptake of Ca2+ into the microsomes was also inhibited by Pb2+. A significant inhibition of microsomal Ca(2+)-pump by micromolar concentration of Pb2+ was also observed. ATP at 5-1000 microM concentration range inhibited [3H]-InsP3 and [3H]-InsP4 binding to the specific receptors. [3H]-InsP4 receptor binding was more sensitive to ATP inhibition as compared to [3H]-InsP3 receptor binding. Furthermore, varying concentrations of ATP also inhibited Pb(2+)-mediated increase in [3H]-InsP3 and [3H]-InsP4 receptor binding. The kinetic analysis of ATP effect on Pb(2+)-stimulated [3H]-InsP4 receptor binding revealed non-competitive type of interaction. The results of the present study suggest that Pb2+ may be increasing the binding of [3H]-InsP3 and [3H]-InsP4 to the specific receptors by modulating the conformation of the receptor sites. ATP may be playing a protective role in Pb2+ induced alteration of the receptor sites.

Lead (Pb) alters the norepinephrine-induced secretion of luteinizing hormone releasing hormone from the median eminence of adult male rats in vitro

In the present study, we evaluated the in vitro effects of lead (Pb) on basal and stimulated luteinizing hormone releasing hormone (LHRH) and Prostaglandin E2 (PGE2) secretion. Median eminences (ME) were removed from brains of adult male rats and preincubated for 15 minutes in Krebs-Ringer bicarbonate glucose buffer in an atmosphere of 95% O2-5% CO2. These media were discarded and all MEs were subjected to one of the following experiments. In Experiment 1, all MEs were incubated for 30 minutes in medium only. These media were collected and replaced with medium only (controls) or with medium containing Pb doses ranging from 5 to 20 microM. After this 60-minute incubation, media were collected, then replaced with new medium containing 60 microM norepinephrine (NE), or NE plus each dose of Pb, then incubated for a final 30-minute period. Experiment 2 was conducted as above, except PGE2 (2.8 microM) replaced the NE. In both experiments, the amounts of LHRH released was measured by RIA. In experiment 3, NE was again used for the challenge; however, this time, the amount of PGE2 released was measured by RIA. Results indicate that Pb did not alter basal LHRH release, but compared with controls, significantly blocked NE-induced LHRH release in a dose-related manner. Conversely, Pb had no effect on the PGE2-induced release of LHRH. Additionally, Pb did not alter basal PGE2 release; however, it significantly blocked the NE-induced release of PGE2. Since NE-induced LHRH release is mediated by PGE2, these results support the hypothesis that Pb is capable of altering the hypothalamus and suggest that this effect is due, at least in part, to the diminished PGE2 synthesis/release within the ME, resulting in diminished LHRH secretion.

Chronic exposure to environmental levels of lead impairs in vivo induction of long-term potentiation in rat hippocampal dentate

This study examined the effects of chronic developmental lead (Pb) exposure in rats on hippocampal long-term potentiation (LTP). Male offspring were exposed to 0.2% Pb acetate continuously from birth until testing at 85-105 days. Excitatory postsynaptic potential (EPSP) and population spike amplitudes were measured in the dentate hilar region in response to stimulation applied to the lateral perforant path. LTP was induced in control animals with an average maximal EPSP potentiation of 41%, which was significantly greater than the increase in EPSP amplitudes (2%) in exposed animals after tetanizing stimulation. Current-voltage curves in controls demonstrated significant increases in EPSPs and population spikes after application of pulse trains to induce LTP, while exposed rats exhibited no discernible change in responses. These findings suggest that induction or development of LTP in the dentate hilar region in vivo is impaired by chronic developmental exposure to environmentally relevant levels of Pb.

Postnatal lead exposure induces supersensitivity to the stimulus properties of a D2-D3 agonist

To examine the impact of lead (Pb) exposure during the ontogeny of dopaminergic (DA) systems on resultant DA function, rats were exposed postnatally (0-21 days of age) via the lactating dam to 0, 100 or 350 ppm Pb acetate in drinking water. At 2 months of age, the postnatally Pb-exposed rats were trained to discriminate the stimulus properties of either the D1 receptor agonist SKF38393 (6.0 mg/kg) or the D2-D3 receptor family subtype agonist quinpirole (0.05 mg/kg) from saline using a standard two-lever operant food-reinforced drug discrimination paradigm. In each training group, dose-effect curves describing drug lever responding to lower doses of the training drug and to preadministration of selective DA antagonists were obtained to examine Pb-induced changes in DA sensitivity, and doses of non-DA compounds were substituted to determine the specificity of any changes in DA sensitivity. In the D1/saline training condition, Pb exposure was not associated with any specific or consistent changes in DA sensitivity. In contrast, exposure to Pb was associated with D2-D3 receptor subtype supersensitivity as was indicated by significantly elevated levels of drug lever responding in the presence of quinpirole and haloperidol and to at least one dose of apomorphine. No differences in the dose-effect curves for either (+)-amphetamine or NMDA were observed in the D2-D3-trained control and Pb-exposed groups, but an increase in drug lever responding in the presence of pentobarbital was noted in the Pb-exposed group relative to control. Taken together, these findings are consistent with a Pb-induced functional D2-D3 supersensitivity possibly mediated via autoreceptors. Moreover, this functional D2-D3 supersensitivity necessarily represents a permanent effect of postnatal Pb exposure since both blood and brain Pb levels were negligible at the time drug discrimination training began.

Low level lead exposure increases sensitivity to the stimulus properties of dopamine D1 and D2 agonists

To examine the impact of Pb exposure on dopaminergic (DA) function, weanling rats were chronically exposed to 0, 50 or 250 ppm Pb acetate in drinking water. At 3 months of age, the rats were trained to discriminate the stimulus properties of either the D1 agonist SKF38393 (3.0 mg/kg i.p.; D1/sal) or the D2 agonist quinpirole (0.05 mg/kg i.p., D2/sal) from saline using a standard two-lever operant food-reinforced drug discrimination paradigm. Lead-exposed rats learned the discriminations faster than respective controls. Moreover, they exhibited greater levels of drug lever responding to lower doses of the training drugs (D1/sal and D2/sal), and to selected doses of other direct and indirect DA agonists (D2/sal only), including apomorphine, cocaine and (+)-amphetamine, and less blockade of drug lever responding by haloperidol (D2/sal). Taken together, these findings are consistent with a generalized DA supersensitivity. There were no differential Pb effects when non-DA compounds including morphine, pentobarbital and MK-801 were substituted for the training drugs, indicating the selectivity of the DA effects in the context of these experiments, and the improbability of a non-specific behavioral causation. Pb-exposed rats in the D2/sal group also showed a pronounced enhancement of drug lever responding when NMDA was substituted for quinpirole, suggesting the possibility of a Pb-induced NMDA supersensitivity as well.

Differential effects of triethyllead on synaptosomal [3H]dopamine vs. [3H]acetylcholine and [3H]gamma-aminobutyric acid release

In vitro exposure to tetraethyllead (Et4Pb, 10 microM) did not alter the release of [3H] dopamine (DA), [3H]acetylcholine (ACh), or [3H]gamma-aminobutyric acid (GABA) from superfused synaptosomes isolated from rat brain striatum, hippocampus, and cortex, respectively. On the other hand, a concentration-dependent increase in the spontaneous release of these transmitters was observed following exposure to triethyllead (Et3Pb, 0.1-10 microM). The magnitude of 1 microM Et3Pb-induced [3H]DA release was 5-fold greater than that observed for [3H]ACh or [3H]GABA release. Removal of [Ca2+]e did not alter the Et3Pb-induced increase in the release of these three transmitter substances, nor did Et3Pb alter synaptosomal 45Ca efflux. EtePb-induced [3H]ACh and [3H]GABA release, but not [3H]DA release, was blocked by lowering [Na+]e from 140 to 50 mM. Similarly, the release of [3H]ACh and [3H]GABA, but not [3H]DA, induced by either Na,K-ATPase inhibition or veratridine (a Na(+)-ionophore), was attenuated by lowering [Na+]e from 140 to 50 mM. However, Et3Pb did not inhibit isolated synaptic membrane Na,K-ATPase, nor did the magnitude or temporal patterns of Et3Pb-induced transmitter release resemble transmitter release induced by Na,K-ATPase inhibition. Et3Pb and veratridine, but not Na,K-ATPase inhibition, produced an increase in synaptosomal [3H] deoxyglucose phosphate (dGluP) efflux, suggesting that both compounds increase membrane permeability. A Et3Pb-induced increase in membrane permeability is further supported by electrophysiological studies using the frog neuromuscular junction in which Et3Pb was found to reduce both the input resistance and membrane potential of muscle cells. As with [3H]ACh and [3H]GABA release, the Et3Pb-induced increase in synaptosomal [3H]dGluP efflux was attenuated by lowering [Na+]e.(ABSTRACT TRUNCATED AT 250 WORDS)

Mechanisms of lead neurotoxicity

During the past several years, there has been a renewed interest in the mechanisms by which lead poisoning disrupts brain function. In part, this is related to clinical observations that imply an absence of threshold for toxicity in the immature brain. Many of the neurotoxic effects of lead appear related to the ability of lead to mimic or in some cases inhibit the action of calcium as a regulator of cell function. At a neuronal level, exposure to lead alters the release of neurotransmitter from presynaptic nerve endings. Spontaneous release is enhanced and evoked release is inhibited. The former may be due to activation of protein kinases in the nerve endings and the latter to blockade of voltage-dependent calcium channels. This disruption of neuronal activity may, in turn, alter the developmental processes of synapse formation and result in a less efficient brain with cognitive deficits. Brain homeostatic mechanisms are disrupted by exposure to higher levels of lead. The final pathway appears to be a breakdown in the blood-brain barrier. Again, the ability of lead to mimic or mobilize calcium and activate protein kinases may alter the behavior of endothelial cells in immature brain and disrupt the barrier. In addition to a direct toxic effect upon the endothelial cells, lead may alter indirectly the microvasculature by damaging the astrocytes that provide signals for the maintenance of blood-brain barrier integrity.

Ca2(+)-surrogate action of Pb2+ on acetylcholine release from rat brain synaptosomes

The effect of lead ions on the release of acetylcholine (ACh) was investigated in intact and digitonin-permeabilized rat cerebrocortical synaptosomes that had been prelabeled with [3H]choline. Release of ACh was inferred from the release of total 3H label or by determination of [3H]ACh. Application of 1 microM Pb2+ to intact synaptosomes in Ca2(+)-deficient medium induced 3H release, which was enhanced by K+ depolarization. This suggests that entry of Pb2+ into synaptosomes and Pb2(+)-induced ACh release can be augmented by activation of the voltage-gated Ca2+ channels in nerve terminals. The lead-induced release of [3H]ACh was blocked by treatment of synaptosomes with vesamicol, which prevents uptake of ACh into synaptic vesicles without affecting its synthesis in the synaptoplasm. This indicates that Pb2+ selectively activates the release of a vesicular fraction of the transmitter with little or no effect on the leakage of cytoplasmic ACh. Application of 1-50 nM (EC50 congruent to 4 nM) free Pb2+ to digitonin-permeabilized synaptosomes elicited release of 3H label that was comparable with the release induced by 0.2-5 microM (EC50 congruent to 0.5 microM) free Ca2+. This suggests that Pb2+ triggers transmitter exocytosis directly and that it is a some 100 times more effective activator of exocytosis than is the natural agonist Ca2+.

In vitro effect of organic and inorganic mercury on the serotonergic system

Interactions of organic (methyl mercury) and mineral (acetate, chloride, nitrate: Hg2+) mercury salts with the serotonergic systems were studied in vitro using synaptosomal fractions prepared from rat brain cortex preloaded with [3H]5-HT. In the absence of calcium, mercury salts induced a spontaneous release of the amine corresponding to a total depletion of the synaptosomal content. EC50 for the organic and mineral forms of the ion were 66 +/- 2 microM and 107 +/- 16 microM respectively. In the presence of calcium (2.4 mM) a similar pattern of release was observed except that an additional release was induced by mineral mercury at low concentration (EC50 = 8.4 +/- 1.3 microM) which corresponded to a maximal release of 18.6 +/- 3.5% of the synaptosomal content. Therefore, the latter release appears calcium-dependent. Parallelly, the effects of mercury salts have been examined on the binding of [3H]5-HT to its high affinity 5-HT1 sites; organic and mineral mercury inhibited the binding with IC50 of 27.8 +/- 3.2 microM respectively. These results show that mercury ions interact with the serotonergic system by different mechanisms depending on their concentrations and their ionic forms.