Chronic lead exposure accelerates decay of long-term potentiation in rat dentate gyrus in vivo

The Future of Neurotoxicology: A Neuroelectrophysiological Viewpoint

Neuroelectrophysiology is an old science, dating to the 18th century when electrical activity in nerves was discovered. Such discoveries have led to a variety of neurophysiological techniques, ranging from basic neuroscience to clinical applications. These clinical applications allow assessment of complex neurological functions such as (but not limited to) sensory perception (vision, hearing, somatosensory function), and muscle function. The ability to use similar techniques in both humans and animal models increases the ability to perform mechanistic research to investigate neurological problems. Good animal to human homology of many neurophysiological systems facilitates interpretation of data to provide cause-effect linkages to epidemiological findings. Mechanistic cellular research to screen for toxicity often includes gaps between cellular and whole animal/person neurophysiological changes, preventing understanding of the complete function of the nervous system. Building Adverse Outcome Pathways (AOPs) will allow us to begin to identify brain regions, timelines, neurotransmitters, etc. that may be Key Events (KE) in the Adverse Outcomes (AO). This requires an integrated strategy, from in vitro to in vivo (and hypothesis generation, testing, revision). Scientists need to determine intermediate levels of nervous system organization that are related to an AO and work both upstream and downstream using mechanistic approaches. Possibly more than any other organ, the brain will require networks of pathways/AOPs to allow sufficient predictive accuracy. Advancements in neurobiological techniques should be incorporated into these AOP-base neurotoxicological assessments, including interactions between many regions of the brain simultaneously. Coupled with advancements in optogenetic manipulation, complex functions of the nervous system (such as acquisition, attention, sensory perception, etc.) can be examined in real time. The integration of neurophysiological changes with changes in gene/protein expression can begin to provide the mechanistic underpinnings for biological changes. Establishment of linkages between changes in cellular physiology and those at the level of the AO will allow construction of biological pathways (AOPs) and allow development of higher throughput assays to test for changes to critical physiological circuits. To allow mechanistic/predictive toxicology of the nervous system to be protective of human populations, neuroelectrophysiology has a critical role in our future.

Inducing a long-term potentiation in the dentate gyrus is sufficient to produce rapid antidepressant-like effects

Recent hypotheses propose that one prerequisite to obtain a rapid antidepressant (AD) effect would reside in processes of synaptic reinforcement occurring within the dentate gyrus (DG) of the hippocampus independently from neurogenesis. However, to date no relationship has been established between an increased DG synaptic plasticity, and rapid AD-like action. To the best of our knowledge, this study shows for the first time that inducing a long-term potentiation (LTP) within the DG by stimulating the perforant pathway (PP) is sufficient to induce such effects. Thus, Sprague-Dawley rats having undergone a successful LTP displayed a significant reduction of immobility when passed acutely 3 days thereafter in the forced swimming test (FST). Further, in a longitudinal paradigm using the pseudo-depressed Wistar-Kyoto rat strain, LTP elicited a decrease of FST immobility after only 2 days, whereas the AD desipramine was not effective before 16 days. In both models, the influence of LTP was transient, as it was no more observed after 8-9 days. No effects were observed on the locomotor activity or on anxiety-related behavior. Theta-burst stimulation of a brain region anatomically adjacent to the PP remained ineffective in the FST. Immunoreactivity of DG cells for phosphorylated histone H3 and doublecortin were not modified three days after LTP, indicating a lack of effect on both cell proliferation and neurogenesis. Finally, depleting brain serotonin contents reduced the success rate of LTP but did not affect its subsequent AD-like effects. These results confirm the 'plastic DG' theory of rapid AD efficacy. Beyond, they point out stimulations of the entorhinal cortex, from which the PP originates, as putative new approaches in AD research.

δ-Aminolevulinic acid dehydratase single nucleotide polymorphism 2 (ALAD2) and peptide transporter 2*2 haplotype (hPEPT2*2) differently influence neurobehavior in low-level lead exposed children

Delta-aminolevulinic acid dehydratase single nucleotide polymorphism 2 (ALAD2) and peptide transporter haplotype 2*2 (hPEPT2*2) through different pathways can increase brain levels of delta-aminolevulinic acid and are associated with higher blood lead burden in young children. Past child and adult findings regarding ALAD2 and neurobehavior have been inconsistent, and the possible association of hPEPT2*2 and neurobehavior has not yet been examined. Mean blood lead level (BLL), genotype, and neurobehavioral function (fine motor dexterity, working memory, visual attention and short-term memory) were assessed in 206 males and 215 females ages 5.1-11.8years. Ninety-six percent of children had BLLs<5.0μg/dl. After adjusting for covariates (sex, age and mother's level of education) and sibling exclusion (N=252), generalized linear mixed model analyses showed opposite effects for the ALAD2 and hPEPT2*2 genetic variants. Significant effects for ALAD2 were observed only as interactions with BLL and the results suggested that ALAD2 was neuroprotective. As BLL increased, ALAD2 was associated with enhanced visual attention and enhanced working memory (fewer commission errors). Independent of BLL, hPEPT2*2 predicted poorer motor dexterity and poorer working memory (more commission errors). BLL alone predicted poorer working memory from increased omission errors. The findings provided further substantiation that (independent of the genetic variants examined) lowest-level lead exposure disrupted early neurobehavioral function, and suggested that common genetic variants alter the neurotoxic potential of low-level lead. ALAD2 and hPEPT2*2 may be valuable markers of risk, and indicate novel mechanisms of lead-induced neurotoxicity. Longitudinal studies are needed to examine long-term influences of these genetic variants on neurobehavior.

Assessments of tight junction proteins occludin, claudin 5 and scaffold proteins ZO1 and ZO2 in endothelial cells of the rat blood-brain barrier: cellular responses to neurotoxicants malathion and lead acetate

The blood-brain barrier (BBB) is essential for central nervous system (CNS) normal function. It is formed by endothelial cells with special characteristics, which confer the BBB with low permeability and high transendothelial electrical resistance (TEER). We previously demonstrated that malathion and lead, two neurotoxicants widely present in the environment, decrease TEER and increase permeability in in vitro models of the BBB. In this study we assessed tight junction disruption at the protein and gene expression levels using a rat brain microvascular endothelial cell line (RBE4) exposed to lead acetate at 10(-5)M and 10(-6)M, malathion at 10(-5)M, malaoxon at 10(-6)M, and their combinations. Cells were incubated with treatments for 2h, 4h, 8h, 16h, and 24h periods. Immunoblotting assessments demonstrated that protein levels of tight junction proteins occludin and claudin 5, and scaffold proteins ZO1 and ZO2 were decreased after treatments. Gene expression determinations did not correlate with the decreases in protein, indicating that the effects on these proteins were post-translational.

Chronic lead exposure alters presynaptic calcium regulation and synaptic facilitation in Drosophila larvae

Prolonged exposure to inorganic lead (Pb(2+)) during development has been shown to influence activity-dependent synaptic plasticity in the mammalian brain, possibly by altering the regulation of intracellular Ca(2+) concentration ([Ca(2+)](i)). To explore this possibility, we studied the effect of Pb(2+) exposure on [Ca(2+)](i) regulation and synaptic facilitation at the neuromuscular junction of larval Drosophila. Wild-type Drosophila (CS) were raised from egg stages through the third larval instar in media containing either 0 microM, 100 microM or 250 microM Pb(2+) and identified motor terminals were examined in late third-instar larvae. To compare resting [Ca(2+)](i) and the changes in [Ca(2+)](i) produced by impulse activity, the motor terminals were loaded with a Ca(2+) indicator, either Oregon Green 488 BAPTA-1 (OGB-1) or fura-2 conjugated to a dextran. We found that rearing in Pb(2+) did not significantly change the resting [Ca(2+)](i) nor the Ca(2+) transient produced in synaptic boutons by single action potentials (APs); however, the Ca(2+) transients produced by 10 Hz and 20 Hz AP trains were larger in Pb(2+)-exposed boutons and decayed more slowly. For larvae raised in 250 microM Pb(2+), the increase in [Ca(2+)](i) during an AP train (20 Hz) was 29% greater than in control larvae and the [Ca(2+)](i) decay tau was 69% greater. These differences appear to result from reduced activity of the plasma membrane Ca(2+) ATPase (PMCA), which extrudes Ca(2+) from these synaptic terminals. These findings are consistent with studies in mammals showing a Pb(2+)-dependent reduction in PMCA activity. We also observed a Pb(2+)-dependent enhancement of synaptic facilitation at these larval neuromuscular synapses. Facilitation of EPSP amplitude during AP trains (20 Hz) was 55% greater in Pb(2+)-reared larvae than in controls. These results showed that Pb(2+) exposure produced changes in the regulation of [Ca(2+)](i) during impulse activity, which could affect various aspects of nervous system development. At the mature synapse, this altered [Ca(2+)](i) regulation produced changes in synaptic facilitation that are likely to influence the function of neural networks.

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.

Lead hazards for pregnant women and children: part 1: immigrants and the poor shoulder most of the burden of lead exposure in this country. Part 1 of a two-part article details how exposure happens, whom it affects, and the harm it can do

Poor, urban, and immigrant populations are at far greater risk for lead exposure than are other groups in the United States. Children with even slightly elevated blood lead levels are at increased risk for significant neurobehavioral problems that can extend through adolescence. Research has shown that elevated blood lead levels in pregnant women, even those well below 10 micrograms per deciliter-the Centers for Disease Control and Prevention's "level of concern"-can cause miscarriage, premature birth, low birth weight, and subsequent developmental delays in their children. Despite these well-established dangers, routine prenatal lead screening and lead education is not a standard of care. Part 1 of this two-part article presents a short case example of a pregnant mother with lead poisoning and describes the epidemiology of lead exposure in the United States, the main sources of lead exposure, and the effects of lead on the pregnant mother and the developing fetus and child. Prevention is crucial. Treatment options such as chelation must be used selectively and will not reverse damage once it's occurred. Part 2 will describe recommendations for screening, education, nutrition, reducing environmental exposures, and treatment.

Influences of different developmental periods of taurine supplements on synaptic plasticity in hippocampal CA1 area of rats following prenatal and perinatal lead exposure

Background

Previous study has demonstrated that dietary taurine supplement protected rats from impairments of synaptic plasticity induced by postnatal lead exposure. However, little is known about the role of taurine in the presence of prenatal and perinatal lead exposure. We investigated the possible effect of taurine supplement on prenatal and perinatal lead-induced synaptic plasticity deficit and determined developmental periods critical for the effect of taurine.

Results

In the present study, taurine was administrated to prenatal and perinatal lead-exposed rats in different developmental periods: from prenatal to weaning (Lead+PW-Tau), from weaning to life (Lead+WL-Tau), and from prenatal to life (Lead+PL-Tau). We examined the input-output (I/O) function, paired-pulse facilitation (PPF) and the long-term potentiation (LTP) of field excitatory postsynaptic potential (fEPSP) in the hippocampal CA1 area of rats on postnatal days 18–25 (P18–25) or days 60–75 (P60–75). We found that (1) on P18–25, taurine had no evident effect on I/O functions and PPF ratios of lead-exposed rats but caused a 12.0% increase in the LTP amplitudes of these animals; (2) on P60–75, taurine significantly elevated lead depressed I/O functions and PPF ratios in Lead+PW-Tau and Lead+PL-Tau rats, but failed in Lead+WL-Tau rats. The amplitudes of LTP of lead-exposed rats were all significantly increased by additional taurine supplement in any developmental period compared with untreated rats. Thus, taurine appeared to have the most effect during the prenatal and lactation periods and its effects on younger rats would not be manifest until the adult life; and (3) the level of lead deposition in hippocampus was evidently reduced by additional treatment of taurine in lead-exposed rats, compared with untreated rats.

Conclusion

Taurine supplement can protect the adult rats from synaptic plasticity deficits following prenatal and perinatal lead exposure, and the protective effects are critical for the prenatal and lactation periods of lead-exposed rats.

Environmental lead exposure during early life alters granule cell neurogenesis and morphology in the hippocampus of young adult rats

Exposure to environmentally relevant levels of lead (Pb(2+)) during early life produces deficits in hippocampal synaptic plasticity in the form of long-term potentiation (LTP) and spatial learning in young adult rats [Nihei MK, Desmond NL, McGlothan JL, Kuhlmann AC, Guilarte TR (2000) N-methyl-D-aspartate receptor subunit changes are associated with lead-induced deficits of long-term potentiation and spatial learning. Neuroscience 99:233-242; Guilarte TR, Toscano CD, McGlothan JL, Weaver SA (2003) Environmental enrichment reverses cognitive and molecular deficits induced by developmental lead exposure. Ann Neurol 53:50-56]. Other evidence suggests that the performance of rats in the Morris water maze spatial learning tasks is associated with the level of granule cell neurogenesis in the dentate gyrus (DG) [Drapeau E, Mayo W, Aurousseau C, Le Moal M, Piazza P-V, Abrous DN (2003) Spatial memory performance of aged rats in the water maze predicts level of hippocampal neurogenesis. Proc Natl Acad Sci U S A 100:14385-14390]. In this study, we examined whether continuous exposure to environmentally relevant levels of Pb(2+) during early life altered granule cell neurogenesis and morphology in the rat hippocampus. Control and Pb(2+)-exposed rats received bromodeoxyuridine (BrdU) injections (100 mg/kg; i.p.) for five consecutive days starting at postnatal day 45 and were killed either 1 day or 4 weeks after the last injection. The total number of newborn cells in the DG of Pb(2+)-exposed rats was significantly decreased (13%; P<0.001) 1 day after BrdU injections relative to controls. Further, the survival of newborn cells in Pb(2+)-exposed rats was significantly decreased by 22.7% (P<0.001) relative to control animals. Co-localization of BrdU with neuronal or astrocytic markers did not reveal a significant effect of Pb(2+) exposure on cellular fate. In Pb(2+)-exposed rats, immature granule cells immunolabeled with doublecortin (DCX) displayed aberrant dendritic morphology. That is, the overall length-density of the DCX-positive apical dendrites in the outer portion of the DG molecular layer was significantly reduced up to 36% in the suprapyramidal blade only. We also found that the area of Timm's-positive staining representative of the mossy fibers terminal fields in the CA3 stratum oriens (SO) was reduced by 26% in Pb(2+)-exposed rats. These findings demonstrate that exposure to environmentally relevant levels of Pb(2+) during early life alters granule cell neurogenesis and morphology in the rat hippocampus. They provide a cellular and morphological basis for the deficits in synaptic plasticity and spatial learning documented in Pb(2+)-exposed animals.

Developmental lead (Pb) exposure reduces the ability of the NMDA antagonist MK-801 to suppress long-term potentiation (LTP) in the rat dentate gyrus, in vivo

Chronic developmental lead (Pb) exposure increases the threshold and enhances decay of long-term potentiation (LTP) in the dentate gyrus of the hippocampal formation. MK-801 and other antagonists of the N-methyl-D-aspartate (NMDA) glutamate receptor subtype impair induction of LTP. In addition, Pb exposure reduces presynaptic glutamate release and is associated with alterations in NMDA receptor expression. This study examined LTP in Pb-exposed animals challenged with a low dose of MK-801 to assess the sensitivity of this receptor to inhibition. Pregnant rats received 0.2% Pb acetate in the drinking water beginning on gestational day 16, and this regimen was continued through lactation. Adult male offspring maintained on this solution from weaning were prepared with indwelling electrodes in the perforant path and dentate gyrus. Several weeks later, input/output (I/O) functions were collected in awake animals before and after saline or MK-801 administration (0.05 mg/kg, s.c.). LTP was induced using suprathreshold train stimuli 60 min post-drug. Post-train I/O functions were reassessed 1 and 24 h after train delivery. Upon full decay of any induced LTP, drug conditions were reversed such that each animal was tested under saline and MK-801. I/O functions measured 1 and 24 h after train induction as well as immediate post-train responses revealed significant LTP of comparable magnitude that was induced in both control and Pb-exposed animals tested under saline conditions. In contrast, MK-801 reduced LTP in control but not in Pb-exposed animals. The broadening of the excitatory postsynaptic potential evident in responses evoked by train stimuli is NMDA-dependent. Pb exposure attenuated the MK-801-induced reduction in area of this NMDA component by approximately 50%. These findings are consistent with other neurochemical and behavioural observations and suggest that up-regulation of postsynaptic NMDA receptors produces subsensitivity to the inhibitory effects of MK-801 on hippocampal LTP following chronic developmental Pb exposure.

Lead neurotoxicity: From exposure to molecular effects

The effects of lead (Pb(2+)) on human health have been recognized since antiquity. However, it was not until the 1970s that seminal epidemiological studies provided evidence on the effects of Pb(2+) intoxication on cognitive function in children. During the last two decades, advances in behavioral, cellular and molecular neuroscience have provided the necessary experimental tools to begin deciphering the many and complex effects of Pb(2+) on neuronal processes and cell types that are essential for synaptic plasticity and learning and memory in the mammalian brain. In this review, we concentrate our efforts on the effects of Pb(2+) on glutamatergic synapses and specifically on the accumulating evidence that the N-methyl-D-aspartate type of excitatory amino acid receptor (NMDAR) is a direct target for Pb(2+) effects in the brain. Our working hypothesis is that disruption of the ontogenetically defined pattern of NMDAR subunit expression and NMDAR-mediated calcium signaling in glutamatergic synapses is a principal mechanism for Pb(2+)-induced deficits in synaptic plasticity and in learning and memory documented in animal models of Pb(2+) neurotoxicity. We provide an introductory overview of the magnitude of the problem of Pb(2+) exposure to bring forth the reality that childhood Pb(2+) intoxication remains a major public health problem not only in the United States but worldwide. Finally, the latest research offers some hope that the devastating effects of childhood Pb(2+) intoxication in a child's ability to learn may be reversible if the appropriate stimulatory environment is provided.

Lead Exposure and (n-3) Fatty Acid Deficiency during Rat Neonatal Development Affect Subsequent Spatial Task Performance and Olfactory Discrimination

Docosahexaenoic acid [22:6(n-3), DHA] is important for optimal infant central nervous system development, and lead (Pb) exposure during development can produce neurological deficits. Long-Evans strain rats were fed either an (n-3) deficient [(n-3) Def] diet to produce brain DHA deficiency, or an adequate [(n-3) Adq] diet through 2 generations. At the birth of the 2nd generation, the dams were subdivided into 4 groups and supplied drinking water containing either 5.27 mmol/L (Pb) or sodium (Na) acetate until weaning. Rats were killed at 3 wk (weaning) and 11 wk (maturity) for brain Pb and fatty acid analysis. Spatial task and olfactory-cued behavioral assessments were initiated at 9 wk. Rats in the (n-3) Def group had a 79% lower concentration of brain DHA compared with the (n-3) Adq group with no effect of Pb exposure. At weaning, Pb concentrations were 7.17 +/- 0.47 nmol Pb/g of brain (wet weight) in the (n-3) Adq-Pb group and 6.49 +/- 0.63 nmol Pb/g of brain (wet weight) in the (n-3) Def-Pb group. At maturity, the brains contained 1.30 +/- 0.22 and 1.07 +/- 0.12 nmol Pb/g (wet weight), respectively. In behavioral testing, significant effects of both Pb and DHA deficiency were observed in the Morris water maze probe trial and in 2-odor olfactory discrimination acquisition and olfactory-based reversal learning tasks. Both lactational Pb exposure and (n-3) fatty acid deficiency led to behavioral deficits with additive effects observed only in the acquisition of 2-odor discriminations.

Calcium/calmodulin-dependent protein kinase II activity and expression are altered in the hippocampus of Pb2+-exposed rats

In the present study, we examined whether calcium/calmodulin-dependent protein kinase II (CaMKII) is affected by chronic developmental Pb2+ exposure. The effects of Pb2+ exposure on rat hippocampal CaMKII were assessed by measuring CaMKII activity, phosphorylation of CaMKII at threonine-286, and CaMKII alpha and beta protein levels. In the hippocampus of Pb2+-exposed 50-day-old rats known to exhibit deficits in hippocampal long-term potentiation (LTP) and spatial learning, there was a marked reduction (41%) in the apparent maximal velocity (Vmax) of CaMKII and a significant increase (22%) in apparent affinity of the enzyme. These Pb2+-induced changes in CaMKII activity could not be explained by changes in enzyme phosphorylation at threonine-286 or sensitivity to calmodulin. In vitro incubation of hippocampal homogenates from control rats, but not from Pb2+-exposed rats, with Pb2+ prior to assay recapitulated the increase in the affinity of the enzyme observed with in vivo exposure to Pb2+. Western blots of cytosolic and membrane fractions from hippocampus showed a significant decrease in the levels of CaMKII-beta but not alpha protein in the cytosolic fraction of Pb2+-exposed rats. These findings indicate effects of developmental Pb2+ exposure on CaMKII, a component of calcium signaling associated with synaptic plasticity, learning, and memory.

Chronic developmental lead exposure reduces neurogenesis in adult rat hippocampus but does not impair spatial learning

The dentate granule cell (DG) layer of the hippocampal formation has the distinctive property of ongoing neurogenesis that continues throughout adult life. Although the function of these newly generated neurons and the mechanisms that control their birth are unknown, age, activity, diet and psychosocial stress have all been demonstrated to regulate this type of neurogenesis. Little information on the impact of environmental insults on this process has appeared to date. Developmental lead (Pb) exposure has been well documented to impair cognitive function in children and animals and reduce activity-dependent synaptic plasticity in the hippocampus of rodents. Therefore, we examined the effects of this classic environmental neurotoxicant on hippocampal-dependent learning and adult neurogenesis in the hippocampus. Pregnant rats were exposed to a low level of Pb-acetate (0.2%) via the drinking water from late gestation (GD 16) until weaning on postnatal day 21 (PN 21). At weaning, half of the Pb-exposed animals were weaned to control drinking water and the remainder were maintained on Pb water until termination of the study. Animals were paired- housed and on PN 75 were administered a series of injections of a thymidine analog bromodeoxyuridine (BrdU), a marker of DNA synthesis that labels proliferating cells and their progeny. At 12-h intervals for 12 days, rats received an ip injection of BrdU (50 mg/kg). Subjects were sacrificed and perfused 24 h and 28 days after the last injection. Spatial learning was assessed in an independent group of animals beginning on PN 110 using a Morris water maze. No Pb-induced impairments were evident in water maze learning. Immunohistochemistry for the detection of BrdU-labeled cells was performed on 40-microm coronal sections throughout the hippocampus. Continuous exposure to Pb (Life) reduced the total number of BrdU-positive cells at 28 days without affecting the total number of labeled cells evident 24 h after the last injection. No differences in the number of progenitor cells labeled or surviving were seen between control and treated animals whose Pb exposure was terminated at weaning. Double labeling with BrdU and the glial specific marker, glial acidic fibrillary protein (GFAP) indicated that the bulk of the surviving cells were of a neuronal rather than a glial phenotype. These data reveal that chronic low-level Pb exposure reduces the capacity for neurogenesis in the adult hippocampus. Despite deficits in synaptic plasticity previously reported from our laboratory, and now structural plasticity, no significant impact on spatial learning was detected.

Lead stimulates ERK1/2 and p38MAPK phosphorylation in the hippocampus of immature rats

Lead (Pb(2+)) is widely recognized as a neurotoxicant whose mechanisms of action are not completely established. We have previously demonstrated that Pb(2+) can activate the p38(MAPK) pathway and increase the phosphorylation of Hsp27 in bovine adrenal chromaffin cells and human SH SY5Y cells over a short incubation period (1 h). In the present work we analyzed the effects of Pb(2+) administered in vivo on the level and the phosphorylation state of ERK1/2 and p38(MAPK) in the hippocampus of immature rats. Rats were treated with lead acetate (2, 8 or 12 mg/kg, i.p.) or saline (control) over the 8th to 12th postnatal days, and hippocampal slices were prepared on the 14th day. The Pb(2+) level in the lead-treated animals increased 2.5-6-fold in the blood (3.0-6.0 microg/dl) and 2.0-3.0-fold in the forebrain (78-103 ng/g wet weight), compared to control (saline). The phosphorylation of both ERK1/2 and p38(MAPK) was significantly increased by prior exposure to Pb(2+) in vivo. In in vitro experiments, hippocampal slices from 14-day-old rats were exposed to Pb(2+) (1-10 microM) for 1 and 3 h. There were no changes in the phosphorylation state of ERK and p38(MAPK) for 1-h incubation, whereas a significant increase of ERK1/2 and p38(MAPK) phosphorylation by Pb(2+) (5 microM) was observed for the 3-h incubation. Cell viability measured using MTT was not modified in any of the conditions tested. These results indicate that the phosphorylation of hippocampal ERK1/2 and p38(MAPK) is stimulated by lead in a period of rapid brain development, an effect that may underlie, at least in part, the neurotoxicty elicited by this metal.

Developmental neuropathology of environmental agents

The developing central nervous system (CNS) is more vulnerable to injury than the adult one. Although a great deal of research has been devoted to subtle effects of developmental exposure, such as neurobehavioral changes, this review instead focuses on a number of chemicals that have been shown, in several experimental models as well as humans, to cause morphological changes in the developing nervous system. Chemicals that are discussed include methylmercury (MeHg), lead (Pb), antiepileptic drugs, and ethanol. Additionally, the issue of silent neurotoxicity, i.e., persistent morphological and/or biochemical injury that remains clinically unapparent until later in life, is discussed.

Lead exposure alters cyclic-AMP response element binding protein phosphorylation and binding activity in the developing rat brain

We examined the effect of lead (Pb(2+)) exposure during development on cyclic-AMP response element binding protein (CREB) expression and phosphorylation in cortical and hippocampal nuclear extracts at postnatal (PN) days 7, 14, 21 and 50. We also examined the binding of CREB family proteins to the cyclic-AMP response element (CRE) using a novel filter-binding assay that provides a quantitative measure of binding kinetics. In the hippocampus and cerebral cortex of control rats, CREB and phospho-CREB (pCREB; serine-133) expression is highest at PN7 and decreases steadily until PN50. Developmental Pb(2+) exposure does not affect total CREB levels but decreases pCREB levels at PN14 and PN50 in the cortex and at PN50 in the hippocampus. Using the filter-binding assay, we measured a 30% decrease in B(max) and 38% decrease in the Kd of CREB family proteins for the CRE in PN50 hippocampal nuclear fractions prepared from Pb(2+)-exposed rats. A similar, but nonsignificant, trend is observed in the cortex of PN50 lead-exposed rats. In addition, a 70% increase in the B(max) was observed in the cortex of PN14 lead-exposed rats without a significant change in the Kd. These disruptions in pCREB expression and binding activity of CREB family members during the ontogeny of the rat brain begin to decipher intracellular mechanisms of Pb(2+) neurotoxicity.

Propylthiouracil (PTU)-induced hypothyroidism in the developing rat impairs synaptic transmission and plasticity in the dentate gyrus of the adult hippocampus

Reductions in thyroid hormone during critical periods of brain development can have devastating effects on neurological function that are permanent. Neurochemical, molecular and structural alterations in a variety of brain regions have been well documented, but little information is available on the consequences of developmental hypothyroidism on synaptic function. Developing rats were exposed to the thyrotoxicant, propylthiouracil (PTU: 0 or 15 ppm), through the drinking water of pregnant dams beginning on GD18 and extending throughout the lactational period. Male offspring were allowed to mature after termination of PTU exposure at weaning on PND21 and electrophyiological assessments of field potentials in the dentate gyrus were conducted under urethane anesthesia between 2 and 5 months of age. PTU dramatically reduced thyroid hormones on PND21 and produced deficits in body weight that persisted to adulthood. Synaptic transmission was impaired as evidenced by reductions in excitatory postsynaptic potential (EPSP) slope and population spike (PS) amplitudes at a range of stimulus intensities. Long-term potentiation of the EPSP slope was impaired at both modest and strong intensity trains, whereas a paradoxical increase in PS amplitude was observed in PTU-treated animals in response to high intensity trains. These data are the first to describe functional impairments in synaptic transmission and plasticity in situ as a result of PTU treatment and suggest that perturbations in synaptic function may contribute to learning deficits associated with developmental hypothyroidism.

Developmental Pb2+ exposure alters NMDAR subtypes and reduces CREB phosphorylation in the rat brain

In the present study we show that chronic exposure to low levels of lead (Pb(2+)) during development alters the type of N-methyl-D-aspartate receptor (NMDAR) expressed in the developing and young adult rat brain. Ifenprodil inhibition of [3H]MK-801 binding to the NMDAR channel in cortical and hippocampal neuronal membranes expressed high and low affinity components. Previous studies have shown that the high affinity component is associated with NR1/NR2B receptor complexes while the low affinity component is associated with the appearance and insertion of the NR2A subunit to NMDAR complexes. Pb(2+)-exposed rats express a greater number of [3H]MK-801 binding sites associated with the high affinity and low affinity components of ifenprodil inhibition. Further, [3H]ifenprodil saturation isotherms and Scatchard analysis in cortical and hippocampal membranes showed a higher number of binding sites (B(max)) with no change in binding affinity (K(d)) in Pb(2+)-exposed animals relative to controls. Quantitative [3H]MK-801 autoradiography in response to glutamate and glycine provided evidence that NMDAR complexes in Pb(2+)-exposed rat brain were maximally activated in situ. Higher levels of ifenprodil-sensitive binding sites and increased sensitivity to agonists are properties characteristic of NR1/NR2B recombinant receptors. Thus, our results strongly suggest that a greater proportion of the total number of NMDAR are NR1/NR2B receptors in the Pb(2+)-exposed rat brain. This Pb(2+)-induced change in NMDAR subtypes in the rat brain was associated with reduced CREB phosphorylation in cortical and hippocampal nuclear extracts. These findings demonstrate that chronic exposure to environmentally relevant levels of Pb(2+) altered the subunit composition of NMDAR complexes with subsequent effects on calcium-sensitive signaling pathways involved in CREB phosphorylation.

In vitro vs in vivo Pb effects on brain protein kinase C activity

Alteration of normal protein kinase C (PKC) function by environmental Pb exposure during neurodevelopment is hypothesized to be an important mechanism of toxicity underlying neurologic impairment. Previous studies have reported widely varying effects of Pb on PKC, possibly in part because of differences in in vitro and in vivo models used in those studies. Therefore, we tested the hypothesis that, with comparable tissue Pb levels, the effects of in vitro Pb exposure on brain PKC are the same as the effects caused by in vivo Pb exposure of intact animals. For chronic in vivo Pb exposure, female Long-Evans rats were exposed to Pb or vehicle from postnatal days 1 to 34-36 (n=10/treatment). For in vitro Pb exposure, homogenate of the frontal cortex region was exposed directly to Pb in an amount comparable to that accumulated in brain during chronic in vivo Pb exposure. Brain Pb levels were measured using ultraclean techniques and inductively coupled plasma mass spectrometry. PKC activity was subsequently determined in cytosolic and membrane subcellular fractions in the frontal cortex, hippocampus, and remaining brain regions. Results indicate that brain Pb levels following in vivo Pb exposure were increased approximately 20-fold above those of nonexposed animals (vehicle group [Pb] approximately 130ng Pb/g dry wt.). However, in vivo Pb exposure did not measurably alter brain PKC activity in the regions tested. In contrast, in vitro Pb exposure significantly increased PKC activity by approximately 20% in the frontal cortex homogenate membrane subcellular fraction. These results indicate that Pb added in vitro caused more dramatic effects than those produced by a comparable amount of Pb in the tissue from in vivo exposure. While the mechanisms underlying these outcomes are not clear, they suggest that in vitro models might not accurately reflect effects of chronic low-level in vivo Pb exposure.

Long-term consequences of developmental exposure to lead or polychlorinated biphenyls: Synaptic transmission and plasticity in the rodent CNS

Exposure to lead (Pb) or polychlorinated biphenyls (PCBs) during early development has been associated with deficits in cognitive function in children (Pediatrics 87 (1991) 219; N. Engl. J. Med. 335 (1996) 783). These effects persist in the child long after exposure has ceased and body burdens have diminished. Despite intensive research, no consensus on the mechanisms of neurotoxicity of these chemicals has resulted. As the primary neurotoxic action of these agents is to impair cognitive ability, a number of laboratories have examined and reported on the detrimental the effects of Pb or PCBs on hippocampal synaptic transmission and long-term potentiation (LTP) in animals exposed during the perinatal period. Use-dependent synaptic plasticity, of which hippocampal LTP is the primary model system, is a fundamental property of neuronal function. In forebrain structures such as amygdala and hippocampus, LTP and related processes are purported to represent a physiological substrate for memory. During brain ontogeny, this type of plasticity guides the establishment and maintenance of synaptic connections in cortical structures based on sensory input. We postulate that the actions of PCBs and Pb in the developing nervous system perturb activity-dependent plasticity and promote organizational changes in brain. Aberrant connectivity derived from perturbations in activity-dependent plasticity during development may manifest as impaired LTP and cognitive ability in the adult organism.

Lead and conditioned fear to contextual and discrete cues

Male Fischer 344 rats received either tap water or water containing 250 ppm lead for 90 days prior to training in either Pavlovian fear conditioning or consummatory contrast, an aversive reward reduction paradigm. In Experiment 1, lead-exposed and -unexposed rats were trained in operant chambers over a 6-min session. After 3 min elapsed, three tone-shock pairings were presented over the remainder of the session. Rats then received 7 days of extinction training in an identical procedure with only tones presented, no shocks. Lead-exposed rats exhibited greater behavioral suppression to both the contextual and auditory cues that predicted shock. In Experiment 2, rats were placed in operant chambers daily and allowed to consume either a 15% or a 5% fructose solution for 7 days. On Day 8, the rats consuming the 15% fructose solution were shifted to the 5% solution for 3 days. Lead-exposed rats did not differ from their controls in either initial consumption of either solution or in the suppression of their consumption after shifting to the 5% solution. Taken together, these findings suggest that lead impairs the extinction of fear conditioning and this finding is not due to a nonspecific increase in aversive emotionality.

Lead and spatial vs. cued open field performance

Male Fischer 344 rats received chronic exposure to either water containing 250 ppm lead or tap water. On the first day of the study, rats were allowed to habituate to a 1-m(2) open field arena with a rectilinear grid pattern of food wells on the floor for a 2-min session. On the following 7 days, half the rats were trained (four trials per day, 2-4-min intertrial interval) to find a food location based on extra-maze spatial cues and the other half were trained to find a food location based on a discrete intra-maze cue placed over the baited food well. While lead did not appear to significantly affect motor activity during the habituation phase, lead-exposed spatially trained rats exhibited superior acquisition and performance of the food-rewarded task compared to their controls and their cue-trained lead-exposed and counterparts. Furthermore, by the last day of training, Day 7, lead significantly reduced the relative amount of time spent on the periphery of the maze in spatially and cued-trained rats.

Molecular changes in glutamatergic synapses induced by Pb2+: association with deficits of LTP and spatial learning

What are the molecular bases for the neurotoxicity that occurs after developmental exposure to low levels of Pb2+, and are these effects persistent and detrimental in adults? Our inability to understand specific mechanisms behind Pb2+ neurotoxicity has long been one of many problem areas of this preventable childhood disease. The sensitivity of the developing brain to Pb2+-induced neurotoxicity is an outcome of the many unique characteristics that comprise the developing central nervous system. The developing brain can be exposed to significant concentrations of Pb2+ during vulnerable periods of development such as synapse formation, gene and protein expression, and other diverse molecular changes associated with these processes. Recently, changes in NMDA receptor subunits were identified in animals that showed cognitive deficits induced by exposure to Pb2+. This molecular association is important because it provides new evidence in the characterization of developmental Pb2+ neurotoxicity that supports physiological findings of impairments in synaptic plasticity and behavior. This review updates information from molecular studies that can be directly associated with impairments of behavior and synaptic plasticity, and outlines the functional consequences of molecular differences in Pb2+-exposed animals that illuminate potential mechanisms of Pb2+-induced neurotoxicity.

Developmental lead exposure: behavioral alterations in the short and long term

Wistar dams were exposed to 500 ppm of Pb, as Pb acetate, or 660 ppm Na acetate in drinking water during pregnancy and lactation. Male pups at 23 (weaned) or 70 days (adult) of age were submitted to behavioral evaluation and Pb determination. The behaviors evaluated were: locomotor activity (open-field test), motor coordination (rotarod test), exploratory behavior (holeboard test), anxiety (elevated plus maze and social interaction tests), and learning and memory (shuttle box). Pb levels were measured in the blood and cerebral regions (hippocampus and striatum) of dams and pups. The results of the present report demonstrated that exposure to Pb during pregnancy and lactation induces in weaned pups hyperactivity, decreased exploratory behavior, and impairment of learning and memory. These alterations were observed at blood Pb levels in the range that may be attained in children chronically exposed to low levels of Pb (21+/-3 microg/dl). Regarding adults, the results demonstrated that the regimen of exposure adopted induces anxiety in these animals at nondetectable blood Pb levels.

In vitro systems as simulations of in vivo conditions: the study of cognition and synaptic plasticity in neurotoxicology

Neuroscientists have been engaged for decades in the search for brain regions and brain processes that underlie learning and memory. The effects of regional brain stimulation and ablation on behavior have been documented and inferences made on the impact of these manipulations on the psychological constructs of "learning" and "memory". Discovery of an electrophysiological property, long-term potentiation (LTP), greatly expanded the ability to probe cellular aspects of how memories are represented in the brain. The study of LTP serves as an excellent example of how in vivo phenomena can be taken to more simplified in vitro test systems to directly address cellular and biochemical mechanisms of information storage in brain.

N-methyl-D-aspartate receptor subunit changes are associated with lead-induced deficits of long-term potentiation and spatial learning

The present study demonstrates that impairments of spatial learning and hippocampal long-term potentiation in rats chronically exposed to lead are associated with changes in gene and protein expression of N-methyl-D-aspartate receptor subunits. Rats exposed to 750 and 1500 ppm lead acetate were found to exhibit deficits in acquisition of a water maze spatial learning task. Furthermore, lead-exposed rats show dose-dependent reductions in the maintenance of in vivo hippocampal long-term potentiation induced in entorhinal cortex-dentate gyrus synapses. We found an unexpected, but significant (P<0.05), correlation between spatial learning and long-term potentiation when control and lead-exposed rats were analysed as a single, combined population. Dentate gyrus NR1 subunit messenger RNA was reduced 18% and 28% by exposure to 750 and 1500 ppm lead acetate, respectively. NR2A subunit messenger RNA was reduced 18% but only in the dentate gyrus of rats exposed to 1500 ppm lead acetate. No significant changes in dentate NR2B messenger RNA expression were measured in either of the lead-exposed groups. NR1 subunit protein was reduced 24% and 58% in hippocampal homogenates from rats exposed to 750 and 1500 ppm lead acetate. In contrast, no changes in NR2A or NR2B subunit protein were observed in the same hippocampal homogenates. These data show that reductions of specific N-methyl-D-aspartate receptor subunits are associated with deficits of both hippocampal long-term potentiation and spatial learning, induced in rats by chronic exposure to environmentally relevant levels of lead. These findings strongly suggest that the effects of lead on N-methyl-D-aspartate receptors may be the mechanistic basis for lead-induced deficits in cognitive function.

Developmental exposure to a commercial PCB mixture (Aroclor 1254) produces a persistent impairment in long-term potentiation in the rat dentate gyrus in vivo

Developmental exposure to polycholorinated biphenyls (PCBs) has been associated with cognitive deficits in humans and laboratory animals. The present study sought to examine synaptic plasticity in the hippocampus, a brain region critical for some types of memory function, in animals exposed to PCBs early in development. Pregnant Long-Evans rats were administered either corn oil (control) or 6 mg/kg/day of a commercial PCB mixture, Aroclor 1254 (A1254) by gavage from gestational day (GD) 6 until pups were weaned on postnatal day (PND) 21. In adult male offspring (3-6 months of age), field potentials evoked by perforant path stimulation were recorded in the dentate gyrus under urethane anesthesia. Input/output (I/O) functions were assessed by averaging the response evoked in the dentate gyrus to stimulus pulses delivered to the perforant path in an ascending intensity series. Long-term potentiation (LTP) was induced by delivering a series of brief high frequency (400 Hz) train bursts to the perforant path at a moderate stimulus intensity and I/O functions were reassessed 1 h later. No differences in baseline synaptic population spike (PS) and minor effects on excitatory postsynaptic potential (EPSP) slope amplitudes were discerned between the groups prior to train delivery. Post-train I/O functions, however, revealed a 50% decrement in the magnitude of LTP in PCB-exposed animals. These data are the first to demonstrate persistent decrements in hippocampal synaptic plasticity in the intact animal following developmental exposure to PCBs. Disruption of early brain ontogeny due to developmental PCB exposure may underlie perturbations in the neurological substrates that support synaptic plasticity and contribute to deficits in LTP and learning that persist into adulthood.

Low-level lead exposure in the early postnatal period results in persisting neuroplastic deficits associated with memory consolidation

Prospective studies in humans and experimental investigations in animals have correlated elevated perinatal blood lead levels with enduring behavioural and cognitive perturbations. Although deficits in neuroplastic events necessary for long-term memory consolidation have been observed during the postnatal period, there is little evidence that these persist into adulthood in the absence of continued lead exposure. To address this issue, we exposed Wistar rat pups to 400 mg of PbCI2/L via their dams' drinking water from postnatal day 1 to 30. At postnatal day 80, the animals were trained in a one-trial, step-through, light-dark passive avoidance paradigm. Prior postnatal lead exposure resulted in a significant decline in recall latency on posttraining day 5, an effect that was specific to the learned response as no obvious behavioural alterations were apparent in open-field studies. As recall was unaffected in the immediate 48-h posttraining period, this suggested an enduring impairment in events associated with long-term memory storage. To investigate this further, we determined the influence of prior lead exposure on the transient modulations of hippocampal neural cell adhesion molecule polysialylation state that occur in the 10-12-h posttraining period, a neuroplastic event associated with memory consolidation. Direct quantification of polysialylated dentate neurons revealed prior lead exposure to have no effect on basal number but to significantly delay and blunt the transient increase observed in control animals at the 12-h posttraining time. These findings confirm that lead exposure in the postnatal period results in enduring neuroplastic deficits most likely associated with reordering of connections in pathways subservient to memory consolidation.

NMDAR-2A subunit protein expression is reduced in the hippocampus of rats exposed to Pb2+ during development

Chronic exposure to lead (Pb2+) produces deficits of learning and memory in children and spatial learning deficits in developing rats. The N-methyl-D-aspartate receptor (NMDAR) has been identified as a principal target for Pb2+-induced neurotoxicity. Age-dependent changes in NMDAR subunit gene expression were observed in hippocampi of rats chronically exposed to Pb2+ during development [T.R. Guilarte, J.L. McGlothan, Hippocampal NMDA receptor mRNA undergoes subunit specific changes during developmental lead exposure, Brain Res. 790 (1998) 98-107]. These changes were present at blood Pb2+ levels ranging from 20-60 microg/dl. Littermates were used in the present study to determine whether the changes in gene expression were reflected in protein levels. NR1, NR2A, and NR2B subunit protein levels were measured in rat hippocampus and cortex at post-natal days (PND) 7, 14, 21, and 28 by Western blot and densitometric analysis. A treatment effect was apparent for NR2A subunit protein expression in the hippocampus (F1,28=10.224, p<0.01). NR2A subunit protein was reduced by 40%, 19%, and 27% from control levels in PND14, 21, and 28 Pb2+-exposed rats, respectively. Mean comparisons indicated that rats at PND14 exhibited the most significant reduction of NR2A (p<0.001). These data concur with our previous finding of reduced NR2A mRNA found in hippocampal pyramidal and granule cells of Pb2+-exposed rats. Pb2+ exposure during development had no effect on NR1 or NR2B subunit protein expression in the hippocampus at any age. No effect was observed on any subunit in the cortex at any age. The developmental profile of the NMDAR-2A subunit protein in the hippocampus is specifically changed by chronic exposure to Pb2+. These data suggest that composition of subunits comprising NMDAR may be altered in Pb2+-exposed rats.

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.