Interaction Between Chronic Bronchitis and Blood Cadmium Levels on the Prevalence of Myocardial Infarction in US Adults: The National Health and Nutritional Examination Survey, 2005-2016

OBJECTIVE

To explore the interaction between chronic bronchitis and blood cadmium on the prevalence of myocardial infarction.

METHODS

We used weighted US-NHANES data. Multivariate survey logistic regression was used to examine the associations between myocardial infarction, cadmium concentration and chronic bronchitis. Adjusted odds ratios, 95% confidence intervals were computed.

RESULTS

There was a significant interaction (OR=1.33, CI = [1.01, 1.74]) between chronic bronchitis and blood cadmium level on the presence of myocardial infarction. For 1 μg/L increase in cadmium level, people with chronic bronchitis had 1.65 (1.24 × 1.33) times the odds of having myocardial infarction, while those without chronic bronchitis would be only 1.24 times as likely having the outcome (OR = 1.24, CI = [1.05, 1.46]).

CONCLUSION

Findings highlights the role of chronic bronchitis on the relationship between blood cadmium concentration and myocardial infarction. Prospective cohort designs are needed to confirm these findings.

Early-life lead exposure and neurodevelopmental disorders

Lead (Pb²⁺) exposure is a global public health problem of major proportion with an alarming number of children with blood Pb²⁺ levels > 10 >g/dL, twice the current CDC reference level for Pb²⁺ exposure. Mounting evidence from population-based studies suggests an association between chronic early life Pb²⁺ exposure (CELLE) and psychiatric disorders, specifically schizophrenia (SZ). Preclinical studies suggest a common mechanism in the pathophysiology of CELLE and SZ, NMDA receptor hypofunction. Here we describe human and experimental animal studies providing the evidence for such an association. Further, recent preclinical studies indicate that Pb²⁺-induced changes in neurotransmitter receptors that mediate the action(s) of drugs of abuse are increased in brain regions associated with addiction circuits in adolescence, a period of increased susceptibility to drug use and abuse and expression of psychiatric disease in humans. In summary, the relationship between the global burden of childhood Pb²⁺ exposure and the latent onset of psychiatric disorders and predisposition to drug use requires further investigations in human populations.

PET imaging of dopamine release in the frontal cortex of manganese-exposed non-human primates

Humans and non-human primates exposed to excess levels of manganese (Mn) exhibit deficits in working memory and attention. Frontal cortex and fronto-striatal networks are implicated in working memory and these circuits rely on dopamine for optimal performance. Here, we aimed to determine if chronic Mn exposure alters in vivo dopamine release (DAR) in the frontal cortex of non-human primates. We used [¹¹ C]-FLB457 positron emission tomography with amphetamine challenge to measure DAR in Cynomolgus macaques. Animals received [¹¹ C]-FLB457 positron emission tomography scans with and without amphetamine challenge prior to Mn exposure (baseline), at different time points during the Mn exposure period, and after 10 months of Mn exposure cessation. Four of six Mn-exposed animals expressed significant impairment of frontal cortex in vivo DAR relative to baseline. One Mn animal had no change in DAR and another Mn animal expressed increased DAR relative to baseline. In the reversal studies, one Mn-exposed animal exhibited complete recovery of DAR while the second animal had partial recovery. In both animals, frontal cortex Mn concentrations normalized after 10 months of exposure cessation based on T1-weighted magnetic resonance imaging. D1-dopamine receptor (D1R) autoradiography in frontal cortex tissue indicates that Mn animals that experienced cessation of Mn exposure expressed D1R levels that were approximately 50% lower than Mn animals that did not experience cessation of Mn exposure or control animals. The present study provides evidence of Mn-induced alterations in frontal cortex DAR and D1R that may be associated with working memory and attention deficits observed in Mn-exposed subjects.

Imaging of Glial Cell Activation and White Matter Integrity in Brains of Active and Recently Retired National Football League Players

Importance

Microglia, the resident immune cells of the central nervous system, play an important role in the brain's response to injury and neurodegenerative processes. It has been proposed that prolonged microglial activation occurs after single and repeated traumatic brain injury, possibly through sports-related concussive and subconcussive injuries. Limited in vivo brain imaging studies months to years after individuals experience a single moderate to severe traumatic brain injury suggest widespread persistent microglial activation, but there has been little study of persistent glial cell activity in brains of athletes with sports-related traumatic brain injury.

Objective

To measure translocator protein 18 kDa (TSPO), a marker of activated glial cell response, in a cohort of National Football League (NFL) players and control participants, and to report measures of white matter integrity.

Design, Setting, and Participants

This cross-sectional, case-control study included young active (n = 4) or former (n = 10) NFL players recruited from across the United States, and 16 age-, sex-, highest educational level-, and body mass index-matched control participants. This study was conducted at an academic research institution in Baltimore, Maryland, from January 29, 2015, to February 18, 2016.

Main Outcomes and Measures

Positron emission tomography-based regional measures of TSPO using [11C]DPA-713, diffusion tensor imaging measures of regional white matter integrity, regional volumes on structural magnetic resonance imaging, and neuropsychological performance.

Results

The mean (SD) ages of the 14 NFL participants and 16 control participants were 31.3 (6.1) years and 27.6 (4.9) years, respectively. Players reported a mean (SD) of 7.0 (6.4) years (range, 1-21 years) since the last self-reported concussion. Using [11C]DPA-713 positron emission tomographic data from 12 active or former NFL players and 11 matched control participants, the NFL players showed higher total distribution volume in 8 of the 12 brain regions examined (P < .004). We also observed limited change in white matter fractional anisotropy and mean diffusivity in 13 players compared with 15 control participants. In contrast, these young players did not differ from control participants in regional brain volumes or in neuropsychological performance.

Conclusions and Relevance

The results suggest that localized brain injury and repair, indicated by higher TSPO signal and white matter changes, may be associated with NFL play. Further study is needed to confirm these findings and to determine whether TSPO signal and white matter changes in young NFL athletes are related to later onset of neuropsychiatric symptoms.

In Vitro Effects of Lead on Gene Expression in Neural Stem Cells and Associations between Up-regulated Genes and Cognitive Scores in Children

BACKGROUND

Lead (Pb) adversely affects neurodevelopment in children. Neural stem cells (NSCs) play an essential role in shaping the developing brain, yet little is known about how Pb perturbs NSC functions and whether such perturbation contributes to impaired neurodevelopment.

OBJECTIVES

We aimed to identify Pb-induced transcriptomic changes in NSCs and to link these changes to neurodevelopmental outcomes in children who were exposed to Pb.

METHODS

We performed RNA-seq-based transcriptomic profiling in human NSCs treated with 1 μM Pb. We used qRT-PCR, Western blotting, ELISA, and ChIP (chromatin immunoprecipitation) to characterize Pb-induced gene up-regulation. Through interrogation of a genome-wide association study, we examined the association of gene variants with neurodevelopment outcomes in the ELEMENT birth cohort.

RESULTS

We identified 19 genes with significantly altered expression, including many known targets of NRF2-the master transcriptional factor for the oxidative stress response. Pb induced the expression of SPP1 (secreted phosphoprotein 1), which has known neuroprotective effects. We demonstrated that SPP1 is a novel direct NRF2 target gene. Single nucleotide polymorphisms (SNPs) (rs12641001) in the regulatory region of SPP1 exhibited a statistically significant association (p = 0.005) with the Cognitive Development Index (CDI).

CONCLUSION

Our findings revealed that Pb induces an NRF2-dependent transcriptional response in neural stem cells and identified SPP1 up-regulation as a potential novel mechanism linking Pb exposure with neural stem cell function and neurodevelopment in children.

Sex and genetic differences in the effects of acute diesel exhaust exposure on inflammation and oxidative stress in mouse brain

In addition to increased morbidity and mortality caused by respiratory and cardiovascular diseases, air pollution may also contribute to central nervous system (CNS) diseases. Traffic-related air pollution is a major contributor to global air pollution, and diesel exhaust (DE) is its most important component. DE contains more than 40 toxic air pollutants and is a major constituent of ambient particulate matter (PM), particularly of ultrafine-PM. Limited information suggests that exposure to DE may cause oxidative stress and neuroinflammation in the CNS. We hypothesized that males may be more susceptible than females to DE neurotoxicity, because of a lower level of expression of paraoxonase 2 (PON2), an intracellular anti-oxidant and anti-inflammatory enzyme. Acute exposure of C57BL/6 mice to DE (250-300μg/m³ for 6h) caused significant increases in lipid peroxidation and of pro-inflammatory cytokines (IL-1α, IL-1β, IL-3, IL-6, TNF-α) in various brain regions (particularly olfactory bulb and hippocampus). In a number of cases the observed effects were more pronounced in male than in female mice. DE exposure also caused microglia activation, as measured by increased Iba1 (ionized calcium-binding adapter molecule 1) expression, and of TSPO (translocator protein) binding. Mice heterozygotes for the modifier subunit of glutamate cysteine ligase (the limiting enzyme in glutathione biosynthesis; Gclm^+/- mice) appeared to be significantly more susceptible to DE-induced neuroinflammation than wild type mice. These findings indicate that acute exposure to DE causes neuroinflammation and oxidative stress in brain, and suggest that sex and genetic background may play important roles in modulating susceptibility to DE neurotoxicity.

Chronic early life lead (Pb2+) exposure alters presynaptic vesicle pools in hippocampal synapses

Background

Lead (Pb²⁺) exposure has been shown to impair presynaptic neurotransmitter release in both in vivo and in vitro model systems. The mechanism by which Pb²⁺ impairs neurotransmitter release has not been fully elucidated. In previous work, we have shown that Pb²⁺ exposure inhibits vesicular release and reduces the number of fast-releasing sites in cultured hippocampal neurons. We have also shown that Pb²⁺ exposure inhibits vesicular release and alters the distribution of presynaptic vesicles in Shaffer Collateral – CA1 synapses of rodents chronically exposed to Pb²⁺ during development.

Methods

In the present study, we used transmission electron microscopy to examine presynaptic vesicle pools in Mossy Fiber-CA3 synapses and in Perforant Path-Dentate Gyrus synapses of rats to determine if in vivo Pb²⁺ exposure altered presynaptic vesicle distribution in these hippocampal regions. Data were analyzed using T-test for each experimental endpoint.

Results

We found that Pb²⁺ exposure significantly reduced the number of vesicles in the readily releasable pool and recycling pool in Mossy Fiber-CA3 terminals. In both Mossy Fiber-CA3 terminals and in Perforant Path-Dentate Gyrus terminals, Pb²⁺ exposure significantly increased vesicle nearest neighbor distance in all vesicular pools (Rapidly Releasable, Recycling and Resting). We also found a reduction in the size of the postsynaptic densities of CA3 dendrites in the Pb²⁺ exposed group.

Conclusions

In our previous work, we have demonstrated that Pb²⁺ exposure impairs vesicular release in Shaffer Collateral - CA1 terminals of the hippocampus and that the number of docked vesicles in the presynaptic active zone was reduced. Our current data shows that Pb²⁺ exposure reduces the number of vesicles that are in proximity to release sites in Mossy Fiber- CA3 terminals. Furthermore, Pb²⁺ exposure causes presynaptic vesicles to be further from one another, in both Mossy Fiber- CA3 terminals and in Perforant Pathway – Dentate Gyrus terminals, which may interfere with vesicle movement and release. Our findings provide a novel in vivo mechanism by which Pb²⁺ exposure impairs vesicle dynamics and release in the hippocampus.

Translational Biomarkers of Neurotoxicity: A Health and Environmental Sciences Institute Perspective on the Way Forward

Neurotoxicity has been linked to a number of common drugs and chemicals, yet efficient and accurate methods to detect it are lacking. There is a need for more sensitive and specific biomarkers of neurotoxicity that can help diagnose and predict neurotoxicity that are relevant across animal models and translational from nonclinical to clinical data. Fluid-based biomarkers such as those found in serum, plasma, urine, and cerebrospinal fluid (CSF) have great potential due to the relative ease of sampling compared with tissues. Increasing evidence supports the potential utility of fluid-based biomarkers of neurotoxicity such as microRNAs, F₂-isoprostanes, translocator protein, glial fibrillary acidic protein, ubiquitin C-terminal hydrolase L1, myelin basic protein, microtubule-associated protein-2, and total tau. However, some of these biomarkers such as those in CSF require invasive sampling or are specific to one disease such as Alzheimer’s, while others require further validation. Additionally, neuroimaging methodologies, including magnetic resonance imaging, magnetic resonance spectroscopy, and positron emission tomography, may also serve as potential biomarkers and have several advantages including being minimally invasive. The development of biomarkers of neurotoxicity is a goal shared by scientists across academia, government, and industry and is an ideal topic to be addressed via the Health and Environmental Sciences Institute (HESI) framework which provides a forum to collaborate on key challenging scientific topics. Here we utilize the HESI framework to propose a consensus on the relative potential of currently described biomarkers of neurotoxicity to assess utility of the selected biomarkers using a nonclinical model.

Effects of chronic manganese exposure on attention and working memory in non-human primates

Manganese (Mn) is essential for a variety of physiological processes, but at elevated levels, can be neurotoxic. While cognitive dysfunction has been recently appreciated to occur as a result of chronic Mn exposures, it is still unclear as to which cognitive domains are most susceptible to disruption by Mn exposure. We previously described early appearing Mn-induced changes in performance on a paired associate learning task in monkeys chronically exposed to Mn and suggested that performance of this task might be a sensitive tool for detecting cognitive dysfunction resulting from Mn exposure. As chronic Mn exposure has been suggested to be associated with attention, working memory and executive function deficits, the present study was conducted to assess the extent to which detrimental effects of chronic Mn exposure could be detected using tasks specifically designed to preferentially assess attention, working memory, and executive function. Six cynomolgus monkeys received Mn exposure over an approximate 12 month period and three served as control animals. All animals were trained to perform a self-ordered spatial search (SOSS) task and a five choice serial reaction time (5-CSRT) task. Deficits in performance of the SOSS task began to appear by the fourth month of Mn exposure but only became consistently significantly impaired beginning at the ninth month of Mn exposure. Performance on the 5-CSRT became significantly affected by the third month of Mn exposure. These data suggest that in addition to the paired associate learning task, cognitive processing speed (as measured by the 5-CSRT) may be a sensitive measure of Mn toxicity and that brain circuits involved in performance of the SOSS task may be somewhat less sensitive to disruption by chronic Mn exposure.

Rats with minimal hepatic encephalopathy due to portacaval shunt show differential increase of translocator protein (18 kDa) binding in different brain areas, which is not affected by chronic MAP-kinase p38 inhibition

Neuroinflammation plays a main role in neurological deficits in rats with minimal hepatic encephalopathy (MHE) due to portacaval shunt (PCS). Treating PCS rats with SB239063, an inhibitor of MAP-kinase-p38, reduces microglial activation and brain inflammatory markers and restores cognitive and motor function. The translocator protein-(18-kDa) (TSPO) is considered a biomarker of neuroinflammation. TSPO is increased in brain of PCS rats and of cirrhotic patients that died in hepatic coma. Rats with MHE show strong microglial activation in cerebellum and milder in other areas when assessed by MHC-II immunohistochemistry. This work aims were assessing: 1) whether binding of TSPO ligands is selectively increased in cerebellum in PCS rats; 2) whether treatment with SB239063 reduces binding of TSPO ligands in PCS rats; 3) which cell type (microglia, astrocytes) increases TSPO expression. Quantitative autoradiography was used to assess TSPO-selective (3)H-(R)-PK11195 binding to different brain areas. TSPO expression increased differentially in PCS rats, reaching mild expression in striatum or thalamus and very high levels in cerebellum. TSPO was expressed in astrocytes and microglia. Treatment with SB239063 did not reduces (3)[H]-PK11195 binding in PCS rats. SB239063 reduces microglial activation and levels of inflammatory markers, but not binding of TSPO ligands. This indicates that SB239063-induced neuroinflammation reduction in PCS rats is not mediated by effects on TSPO. Also, enhanced TSPO expression is not always associated with cognitive or motor deficits. If enhanced TSPO expression plays a role in mechanisms leading to neurological alterations in MHE, SB239063 would interfere these mechanisms at a later step.

Neuroinflammation and brain atrophy in former NFL players: An in vivo multimodal imaging pilot study

There are growing concerns about potential delayed, neuropsychiatric consequences (e.g, cognitive decline, mood or anxiety disorders) of sports-related traumatic brain injury (TBI). Autopsy studies of brains from a limited number of former athletes have described characteristic, pathologic changes of chronic traumatic encephalopathy (CTE) leading to questions about the relationship between these pathologic and the neuropsychiatric disturbances seen in former athletes. Research in this area will depend on in vivo methods that characterize molecular changes in the brain, linking CTE and other sports-related pathologies with delayed emergence of neuropsychiatric symptoms. In this pilot project we studied former National Football League (NFL) players using new neuroimaging techniques and clinical measures of cognitive functioning. We hypothesized that former NFL players would show molecular and structural changes in medial temporal and parietal lobe structures as well as specific cognitive deficits, namely those of verbal learning and memory. We observed a significant increase in binding of [(11)C]DPA-713 to the translocator protein (TSPO), a marker of brain injury and repair, in several brain regions, such as the supramarginal gyrus and right amygdala, in 9 former NFL players compared to 9 age-matched, healthy controls. We also observed significant atrophy of the right hippocampus. Finally, we report that these same former players had varied performance on a test of verbal learning and memory, suggesting that these molecular and pathologic changes may play a role in cognitive decline. These results suggest that localized brain injury and repair, indicated by increased [(11)C]DPA-713 binding to TSPO, may be linked to history of NFL play. [(11)C]DPA-713 PET is a promising new tool that can be used in future study design to examine further the relationship between TSPO expression in brain injury and repair, selective regional brain atrophy, and the potential link to deficits in verbal learning and memory after NFL play.

Chronic exposure of mutant DISC1 mice to lead produces sex-dependent abnormalities consistent with schizophrenia and related mental disorders: a gene-environment interaction study

The glutamatergic hypothesis of schizophrenia suggests that hypoactivity of the N-methyl-D-aspartate receptor (NMDAR) is an important factor in the pathophysiology of schizophrenia and related mental disorders. The environmental neurotoxicant, lead (Pb(2+)), is a potent and selective antagonist of the NMDAR. Recent human studies have suggested an association between prenatal Pb(2+) exposure and the increased likelihood of schizophrenia later in life, possibly via interacting with genetic risk factors. In order to test this hypothesis, we examined the neurobehavioral consequences of interaction between Pb(2+) exposure and mutant disrupted in schizophrenia 1 (mDISC1), a risk factor for major psychiatric disorders. Mutant DISC1 and control mice born by the same dams were raised and maintained on a regular diet or a diet containing moderate levels of Pb(2+). Chronic, lifelong exposure of mDISC1 mice to Pb(2+) was not associated with gross developmental abnormalities but produced sex-dependent hyperactivity, exaggerated responses to the NMDAR antagonist, MK-801, mildly impaired prepulse inhibition of the acoustic startle, and enlarged lateral ventricles. Together, these findings support the hypothesis that environmental toxins could contribute to the pathogenesis of mental disease in susceptible individuals.

Regional brain distribution of translocator protein using [(11)C]DPA-713 PET in individuals infected with HIV

Imaging the brain distribution of translocator protein (TSPO), a putative biomarker for glial cell activation and neuroinflammation, may inform management of individuals infected with HIV by uncovering regional abnormalities related to neurocognitive deficits and enable non-invasive therapeutic monitoring. Using the second-generation TSPO-targeted radiotracer, [(11)C]DPA-713, we conducted a positron emission tomography (PET) study to compare the brains of 12 healthy human subjects to those of 23 individuals with HIV who were effectively treated with combination antiretroviral therapy (cART). Compared to PET data from age-matched healthy control subjects, [(11)C]DPA-713 PET of individuals infected with HIV demonstrated significantly higher volume-of-distribution (VT) ratios in white matter, cingulate cortex, and supramarginal gyrus, relative to overall gray matter VT, suggesting localized glial cell activation in susceptible regions. Regional TSPO abnormalities were evident within a sub-cohort of neuro-asymptomatic HIV subjects, and an increase in the VT ratio within frontal cortex was specifically linked to individuals affected with HIV-associated dementia. These findings were enabled by employing a gray matter normalization approach for PET data quantification, which improved test-retest reproducibility, intra-class correlation within the healthy control cohort, and sensitivity of uncovering abnormal regional findings.

Chronic manganese exposure impairs visuospatial associative learning in non-human primates

Manganese (Mn) is an essential trace metal nutrient, however, excess Mn can be neurotoxic. The degree to which chronic environmental or occupational exposures to Mn in adults cause neuropsychological dysfunction is of considerable interest. Descriptions of neuropsychological dysfunction following chronic Mn exposure have been somewhat inconsistent though, likely owing to different measures of exposure in different populations, complicated by factors of mixed exposures and differences in neuropsychological tests administered. We previously described up-regulation of the mRNA expression for amyloid-beta (A-beta) precursor-like protein 1 (APLP1) and the presence of A-beta diffuse plaques in frontal cortex of Mn-exposed monkeys. The present study examined Mn-induced changes in performance on a paired associate learning (PAL) task that has been suggested as a marker for preclinical Alzheimer's disease. Aspects of performance of this task were affected early following initiation of Mn exposure. Thus, PAL performance may be a sensitive and valuable tool for the early, preclinical detection of incipient dementia and it may also be a sensitive tool for detecting cognitive dysfunction from Mn exposure. The current cognitive data, combined with our previous findings, suggest that frontal cortex may be a particularly sensitive target for the effects of Mn on cognition and that chronic Mn exposure may initiate or accelerate a process that could lead to or predispose to Alzheimer's like pathology and cognitive dysfunction.

Mechanisms of lead and manganese neurotoxicity

Human exposure to neurotoxic metals is a global public health problem. Metals which cause neurological toxicity, such as lead (Pb) and manganese (Mn), are of particular concern due to the long-lasting and possibly irreversible nature of their effects. Pb exposure in childhood can result in cognitive and behavioural deficits in children. These effects are long-lasting and persist into adulthood even after Pb exposure has been reduced or eliminated. While Mn is an essential element of the human diet and serves many cellular functions in the human body, elevated Mn levels can result in a Parkinson's disease (PD)-like syndrome and developmental Mn exposure can adversely affect childhood neurological development. Due to the ubiquitous presence of both metals, reducing human exposure to toxic levels of Mn and Pb remains a world-wide public health challenge. In this review we summarize the toxicokinetics of Pb and Mn, describe their neurotoxic mechanisms, and discuss common themes in their neurotoxicity.

Astrocytes release D-serine by a large vesicle

Long-term potentiation (LTP) of synaptic transmission in the CA1 region of the hippocampus depends on the activation of N-methyl-D-aspartate receptors (NMDARs), which can be regulated by Ca²⁺-dependent release of D-serine from astrocytes. The detailed mechanism underlying astrocytic d-serine release is still unknown. In hippocampal slices prepared from Sprague-Dawley rats, we found that clamping astrocytic [Ca²⁺] at 100-150 nM or puffing artificial cerebrospinal fluid (ACSF) into the extracellular space (weak mechanical stimulation) enhanced the synaptic activation of NMDARs. The enhancement was blocked by the NMDAR glycine site antagonist 5,7-dichlorokynurenic acid, glycine saturation, and infusion of astrocytes with D-amino acid oxidase and the serine racemase inhibitor L-erythro-3-hydroxyaspartate, suggesting the involvement of astrocytic D-serine release. Intracellular 100-150 nM [Ca²⁺] or puffing ACSF stimulated astrocytes to generate D-serine-containing large vesicles (1-3 μm), exocytotic fusion of which released D-serine. The formation of astrocytic large vesicles involved the intracellular fusion of small vesicles and/or other organelles. Spontaneous fusion of large vesicles occurred occasionally in astrocytes at rest, contributing to baseline D-serine levels, which increased the rising slope of NMDAR post-burst potentiation (PBP) without altering the PBP peak amplitude. Thus, under physiological conditions, astrocytic D-serine release by large vesicles facilitated weak theta-burst (TBS consisting of five bursts), but not strong TBS (TBS consisting of 10 bursts) stimulation-induced LTP.

The innate immune response to coxsackievirus B3 predicts progression to cardiovascular disease and heart failure in male mice

Background

Men are at an increased risk of dying from heart failure caused by inflammatory heart diseases such as atherosclerosis, myocarditis and dilated cardiomyopathy (DCM). We previously showed that macrophages in the spleen are phenotypically distinct in male compared to female mice at 12 h after infection. This innate immune profile mirrors and predicts the cardiac immune response during acute myocarditis.

Methods

In order to study sex differences in the innate immune response, five male and female BALB/c mice were infected intraperitoneally with coxsackievirus B3 (CVB3) or phosphate buffered saline and their spleens were harvested 12 h later for microarray analysis. Gene expression was determined using an Affymetrix Mouse Gene 1.0 ST Array. Significant gene changes were verified by quantitative real-time polymerase chain reaction or ELISA.

Results

During the innate immune response to CVB3 infection, infected males had higher splenic expression of genes which are important in regulating the influx of cholesterol into macrophages, such as phospholipase A₂ (PLA₂) and the macrophage scavenger receptor compared to the infected females. We also observed a higher expression in infected males compared to infected females of squalene synthase, an enzyme used to generate cholesterol within cells, and Cyp2e1, an enzyme important in metabolizing cholesterol and steroids. Infected males also had decreased levels of the translocator protein 18 kDa (TSPO), which binds PLA₂ and is the rate-limiting step for steroidogenesis, as well as decreased expression of the androgen receptor (AR), which indicates receptor activation. Gene differences were not due to increased viral replication, which was unaltered between sexes.

Conclusions

We found that, compared to females, male mice had a greater splenic expression of genes which are important for cholesterol metabolism and activation of the AR at 12 h after infection. Activation of the AR has been linked to increased cardiac hypertrophy, atherosclerosis, myocarditis/DCM and heart failure in male mice and humans.

Synthesis of [(125)I]iodoDPA-713: a new probe for imaging inflammation

[(125)I]IodoDPA-713 [(125)I]1, which targets the translocator protein (TSPO, 18 kDa), was synthesized in seven steps from methyl-4-methoxybenzoate as a tool for quantification of inflammation in preclinical models. Preliminary in vitro autoradiography and in vivo small animal imaging were performed using [(125)I]1 in a neurotoxicant-treated rat and in a murine model of lung inflammation, respectively. The radiochemical yield of [(125)I]1 was 44+/-6% with a specific radioactivity of 51.8 GBq/micromol (1400 mCi/micromol) and >99% radiochemical purity. Preliminary studies showed that [(125)I]1 demonstrated increased specific binding to TSPO in a neurotoxicant-treated rat and increased radiopharmaceutical uptake in the lungs of an experimental inflammation model of lung inflammation. Compound [(125)I]1 is a new, convenient probe for preclinical studies of TSPO activity.

Effects of chronic manganese exposure on working memory in non-human primates

Human exposure to manganese (Mn) has been associated with a variety of cognitive deficits including learning and memory deficits. However, results from epidemiological studies have been inconsistent in describing the nature of such cognitive deficits. The present study was conducted to evaluate the effects of chronic Mn exposure on memory functioning in non-human primates and to correlate behavioral outcome with brain Mn levels in an attempt to explain outcome variability seen in prior studies. Cynomolgus macaque monkeys were trained to perform memory-related tasks (spatial working memory, non-spatial working memory, reference memory) and exposed to manganese sulfate (15-20 mg/kg/week) over an exposure period lasting 227.5+/-17.3 days. Blood manganese levels were in the upper range of levels reported for human environmental, medical or occupational exposures. By the end of the manganese exposure period, animals developed mild deficits in spatial working memory, more significant deficits in non-spatial working memory and no deficits in reference memory. Linear regression analyses showed that for most brain regions sampled, there was a significant inverse relationship between working memory task performance and brain Mn concentration. These results suggest that chronic exposure to levels of manganese achieved in this study may have detrimental effects on working memory and that Mn levels achieved in several brain regions are inversely related to working memory performance.

Imaging glial cell activation with [11C]-R-PK11195 in patients with AIDS

Glial cell activation occurs in response to brain injury and is present in a wide variety of inflammatory processes including dementia associated with human immunodeficiency virus (HIV). HIV-infected glial cells release cytokines and chemokines that, along with viral neurotoxins, contribute to neuronal damage and apoptosis. The purpose of this study was to determine if glial cell activation in HIV-positive (HIV+) patients could be detected noninvasively, in vivo, using [11C]-R-PK11195 with positron emission tomography (PET). [11C]-R-PK11195 is a selective radioligand for the peripheral benzodiazepine receptor (PBR), and is known to reflect the extent of glial cell activation. A subaim was to determine if nondemented HIV+ patients could be distinguished from those with HIV-associated dementia (HAD) on the basis of [11C]-R-PK11195 binding. Five healthy volunteers and 10 HIV+ patients underwent PET with [11C]-R-PK11195. Time-radioactivity curves (TACs) were generated from dynamic PET images in nine regions of interest (ROIs) drawn on coregistered magnetic resonance imaging (MRI) scans. The average radioactivity was calculated in each ROI and was normalized to the average radioactivity in white matter. Patients with HAD showed significantly higher [11C]-R-PK11195 binding than controls in five out of eight brain regions (P < .05, Mann-Whitney U test). Nondemented HIV+ patients did not show significantly increased binding compared to controls. HIV+ patients overall (demented and nondemented) showed significantly higher radioligand binding than controls in five brain regions (P < 0.05). Patients with HAD did not show significant differences in binding when compared to HIV+ nondemented patients. The findings of this pilot study support a role for glial cell activation in HAD, and that PET with [11C]-R-PK11195 can detect the concomitants of neuronal damage in individuals infected with HIV.

Elevated peripheral benzodiazepine receptor expression in simian immunodeficiency virus encephalitis

Measurement of central nervous system (CNS) expression of the peripheral benzodiazepine receptor (PBR), a microglia and macrophage activation marker, by positron emission tomography (PET) would aid clinical management of human immunodeficiency virus (HIV)-infected patients. To evaluate the utility of examining PBR expression in the CNS as a cellular activation marker in HIV CNS disease, PBR levels were measured in frontal cortex of simian immunodeficiency virus (SIV)-infected macaques with encephalitis and uninfected animals via PK11195 ligand autoradiography. [(3)H]-(R)-PK11195 binding to both grey matter (P =.017) and white matter (P =.038) was significantly higher in animals with SIV encephalitis (n = 10) versus control animals (n = 3). When PK11195 binding was compared with other microglial/macrophage activation markers (obtained via quantitative image analysis), a strong, significant association was found for both HAM56 (P =.004) and KP-1 (anti-CD68; P =.006) immunostaining in white matter. In contrast, grey matter PK11195 binding did not correlate with HAM56 (P =.46), KP-1 (P =.06), or glial fibrillary acidic protein (GFAP) immunostaining for astrocytic activation (P =.09). The regional nature of these increases in activation within the brain illustrates the crucial need to focus functional neuroimaging analyses of HIV-infected individuals on subcortical white matter to assess activation of microglia and macrophages.

Methamphetamine-induced deficits of brain monoaminergic neuronal markers: distal axotomy or neuronal plasticity

We examined the effects of methamphetamine (METH) on monoaminergic (i.e. dopamine and serotonin) axonal markers and glial cell activation in the rat brain. Our findings indicate that the loss of dopamine transporters (DAT), serotonin transporters (5-HTT), vesicular monoamine transporter type-2 (VMAT-2) and glial cell activation induced by METH in the striatum and in the central gray are consistent with a degenerative process. Our novel finding of METH effects on monoaminergic neurons in the central gray may have important implications on METH-induced hyperthermia. In other brain regions examined, DAT and 5-HTT deficits after METH administration were present in the absence of lasting changes in VMAT-2 levels or glial cell activation. Brain regions exhibiting protracted deficits in DAT and/or 5-HTT and VMAT-2 levels also expressed increased levels of [(3)H]-R-PK11195 binding to peripheral benzodiazepine receptors, a quantitative marker of glial cell activation. Immunohistochemical assessment of microglia and astrocytes confirmed the PBR results. Microglia activation was more pronounced than astrocytosis in affected regions in most METH-exposed brains with the exception of a small number of rats that were most severely affected by METH based on loss of body weight. In these rats, both microglia and astrocytes were highly activated and expressed a distinct regional pattern suggestive of widespread brain injury. The reason for the pattern of glial cell activation in this group of rats is not currently known but it may be associated with METH-induced hyperthermia. In summary, our findings suggest two neurotoxic endpoints in the brain of METH-exposed animals. Brain regions exhibiting DAT and 5-HTT deficits that co-localize with decreased VMAT-2 levels and glial cell activation may represent monoaminergic terminal degeneration. However, the DAT and 5-HTT deficits in brain regions lacking a deficit in VMAT-2 and glial cell activation may reflect drug-induced modulation of these plasma membrane proteins.

Is methamphetamine abuse a risk factor in parkinsonism?

Parkinsons disease (PD) is a neurodegenerative disorder with increased incidence in individuals beyond 50 years of age. The etiology of PD is currently not known, but it appears that environmental factors may play an important role. The molecular basis of PD is the nearly complete loss of the neurotransmitter dopamine (DA) in the basal ganglia (caudate/putamen). The decrease in dopamine levels is the result of degeneration of dopamine-containing neurons in the substantia nigra. This biochemical deficit in the nigrostriatal pathway leads to the emergence of motor impairments typical of PD. Methamphetamine (METH) is a psychostimulant drug with increasing use in certain segments of the population in the United States and worldwide. In experimental animal models and human studies, METH administration has been shown to decrease markers of dopaminergic neuron terminal integrity in the basal ganglia. A long-standing question has been whether the reductions in dopaminergic markers induced by METH constitute degenerative changes or reflect drug-induced modulation. Resolving this question is important because the irreversible loss of dopaminergic function may increase the likelihood of Parkinsonism with advancing age.

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.

Low level Pb(2+) exposure affects hippocampal protein kinase C gamma gene and protein expression in rats

We examined the effect of chronic exposure to lead (Pb(2+)) on protein kinase C (PKC) in 50-day-old rat hippocampus. Cytosolic and membrane fractions of hippocampus from Pb(2+)-exposed rats showed reduced expression of PKC gamma protein. In contrast, a significant elevation of PKC gamma mRNA was observed in pyramidal and dentate granule cell layers. Protein expression of alpha, beta I, beta II and epsilon isoenzymes were unchanged in Pb(2+)-exposed rats, as was [(3)H]phorbol 12,13 dibutyrate (PDBu) binding in tissue slices. Differences were not observed in Ca(2+)-dependent or -independent PKC activity, or in PKC-specific back-phosphorylation of hippocampal homogenates from Pb(2+)-exposed rats. Reduced subcellular levels of PKC gamma in Pb(2+)-exposed rats suggest that signal transduction in the hippocampus may be selectively altered and may be important in manifesting Pb(2+)-induced impairments of synaptic plasticity, learning and memory.

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.

Cellular and subcellular localization of peripheral benzodiazepine receptors after trimethyltin neurotoxicity

The peripheral benzodiazepine receptor (PBR) is currently used as a marker of inflammation and gliosis following brain injury. Previous reports suggest that elevated PBR levels in injured brain tissue are specific to activated microglia and infiltrating macrophages. We have produced hippocampal lesions using the neurotoxicant trimethyltin (TMT) to examine the cellular and subcellular nature of the PBR response. Degenerating, argyrophilic pyramidal neurons were observed in the hippocampus at 2 and 14 days after TMT exposure. Reactive microglia were also evident at both times with a maximal response observed at 14 days, subsiding by 6 weeks. Astrocytosis was observed at 14 days and 6 weeks, but not 2 days, after TMT administration, suggesting that the onset of the astroglia response is delayed, but more persistent, compared with microgliosis. Morphological evidence from [3H]PK11195 microautoradiography and PBR immunohistochemistry indicates that both astrocytes and microglia are capable of expressing high levels of PBR after injury. This was confirmed by double labeling of either Griffonia simplicifolia isolectin B4, a microglial-specific marker, or glial fibrillary acidic protein, an astrocyte-specific protein with PBR fluorescence immunohistochemistry. These results demonstrate that PBR expression is increased after brain injury in both activated microglia and astrocytes. Our findings also provide the first evidence for in situ nuclear localization of PBR in glial cells.

Hippocampal expression of N-methyl-D-aspartate receptor (NMDAR1) subunit splice variant mRNA is altered by developmental exposure to Pb(2+)

N-methyl-D-aspartate receptors (NMDAR) play an important role in synaptic plasticity and brain development. We have previously shown that NR1-pan mRNA is significantly increased in the hippocampus of rats chronically exposed to low levels of lead (Pb(2+)) 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]. It is not known whether this Pb(2+)-induced increase in NR1-pan mRNA is associated with changes in specific splice isoforms. To study this effect, we used in situ hybridization of oligonucleotides to probe for the NR1-a, NR1-b, NR1-1, NR1-2, and NR1-4 isoforms which are most abundantly expressed in the rat hippocampus. Developmental exposure to increasing levels of Pb(2+) resulted in significant increases in NR1-a mRNA throughout the pyramidal and granule cell layers of the rat hippocampus at postnatal day 14 (PN14). NR1-b mRNA was increased in the pyramidal cell layer of Pb(2+)-exposed rats at PN21. Splicing of the C-terminus cassettes was also regulated by developmental exposure to Pb(2+). NR1-2 mRNA was increased in CA4 pyramidal cells and in dentate granule cells of PN21 Pb(2+)-exposed rats. Notably, expression of NR1-4 mRNA in CA3 pyramidal cells was increased in Pb(2+)-exposed rats at PN14 and decreased at PN21. No significant Pb(2+) effect was measured for NR1-1 mRNA expression. These data indicate that alternative splicing of the NR1 gene shows selective anatomical and temporal regulation by Pb(2+) in the developing rat hippocampus. This study provides further support to the hypothesis that NMDARs are important targets for Pb(2+)-induced neurotoxicity.

Divalent cations modulate N-methyl-D-aspartate receptor function at the glycine site

The modulation of the N-methyl-D-aspartate (NMDA) receptor (NMDAR) by divalent cations was examined using (+)-5-methyl-10, 11-dihydro-5H-dibenzo[a,d]cyclohepten 5,10-imine maleate ([(3)H]MK-801) binding as a functional indicator of NMDAR function. Ca(2+) and Mg(2+) produce a biphasic effect on the binding of [(3)H]MK-801 to the NMDAR channel in extensively washed adult rat brain membranes. Concentrations of Ca(2+) and Mg(2+) between 1 and 600 microM potentiate binding, but higher concentrations inhibit binding. The potentiating effect of Ca(2+) and Mg(2+) on [(3)H]MK-801 binding is due to an increase in the maximal number of binding sites (B(max)) with no effect on binding affinity (K(d)). Ca(2+)- and Mg(2+)induced potentiation is the result of an apparent increase in the affinity of the NMDAR for glycine. The ontogeny of NMDAR potentiation by Ca(2+) and Mg(2+) was also investigated. The number of [(3)H]MK-801 binding sites associated with divalent cation potentiation are present at low levels shortly after birth, and increase to peak level at 17 days of age before declining to adult levels. The potency of Ca(2+) and Mg(2+) to stimulate [(3)H]MK-801 binding did not change as a function of age. Lead (Pb(2+)) and zinc (Zn(2+)), potent inhibitors of the NMDAR, antagonize NMDAR potentiation by Ca(2+) and Mg(2+). These findings indicate that divalent cations differentially regulate NMDAR function by modulation of the glycine site. The NMDAR glycine site may be important in the regulation of glutamatergic neurotransmission by physiologically and toxicologically relevant cations.

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.

Regional and temporal expression of the peripheral benzodiazepine receptor in MPTP neurotoxicity

We used the dopaminergic neurotoxicant 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) to evaluate the sensitivity and specificity of the peripheral benzodiazepine receptor (PBR) as a biomarker of chemical-induced neurotoxicity. Receptor autoradiography of [3H]-PK11195, a PBR selective ligand, indicated dose-dependent increases throughout the nigrostriatal dopaminergic system as early as 24 h after MPTP administration (10-80 mg/kg), which persisted for at least 21 days. The binding of [3H]-PK11195 was increased as much as 98% in the corpus striatum and 114% in the substantia nigra, following MPTP exposure. The integrity of nigrostriatal dopaminergic terminals in the corpus striatum was assessed by measuring high affinity dopamine transporter (DAT) levels and dopamine content. DAT levels were measured by [3H]-WIN 35,428 autoradiography, and dopamine content decreased with increasing MPTP dose. Reductions of both indices of dopaminergic terminal integrity correlated with increased levels of [3H]-PK11195-binding in the striatum (r2 = 0.84 for DAT and 0.93 for dopamine content). Tyrosine hydroxylase (TH) immunohistochemistry demonstrated dose-dependent reductions of dopaminergic neurons in the substantia nigra pars compacta, with a 67% loss measured 7 days after treatment with 80 mg/kg MPTP. The loss of TH-positive neurons was correlated (r2 = 0.95) with increased levels of [3H]-PK11195 binding in the substantia nigra. These findings demonstrate that the PBR is both sensitive and specific for identifying brain regions involved in MPTP neurotoxicity.

Understanding the NIH review process: a brief guide to writing grant proposals in neurotoxicology

During the past two years, the National Institutes of Health have made significant changes in the review process for investigator-initiated research grant applications in neurotoxicology. First, study sections that formerly dealt with toxicology and alcohol, respectively, have been merged. Neurotoxicology grant applications are now reviewed by ALTX-3, a study section in which the majority of members have expertise in the neuronal, biochemical or behavioral effects of alcohol, but usually not other neurotoxicants. Second, the NIH has instituted new review criteria, in which significance, approach, innovation, investigator expertise, and research environment must all be explicitly addressed by the reviews. In this article, past and present members of the ALTX-3 study section describe the NIH review process, with emphasis on how neurotoxicology applications are handled, and provide guidelines for preparing competitive applications.

Hippocampal NMDA receptor mRNA undergoes subunit specific changes during developmental lead exposure

The N-methyl-D-aspartate (NMDA) receptor has shown to play an important role in the cognitive deficits associated with developmental lead (Pb) exposure. In this study, we examined the effects of low-level Pb exposure on NMDA receptor subunit gene expression in the developing rat brain. The pattern of NR1, NR2A, NR2B, and NR2C subunit mRNA in situ hybridization was consistent with previous studies. Brain levels of NR1 and NR2A mRNAs were lowest shortly after birth, increasing to reach peak levels by 14 or 21 days of age and subsequently decreasing at 28 days of age. NR2B mRNA levels were highest during early development and decreased as the animals aged. NR2C subunit mRNA was restricted to the cerebellum and a signal was not detectable until the second week of life. Lead exposure resulted in significant and opposite effects in NR1 and NR2A subunit mRNA expression with no changes in NR2B or NR2C subunit expression. The Pb-induced changes in NR1 and NR2A subunit mRNA were mainly present in the hippocampus. Hippocampal NR1 mRNA levels were significantly increased in the CA1 (15.3%) and CA4 (26.8%) pyramidal cells from 14-day-old Pb-exposed rats. At 21 days of age, only the NR1 mRNA at the CA4 subfield remained significantly elevated (10.3%). Lead exposure caused reductions of NR2A mRNA levels (11.9-19.3%) in the pyramidal and granule cell layers of the hippocampus at 14 and 21 days of age. NR1 mRNA levels were also significantly increased (14.0%) in the cerebellum of 28-day-old rats with no change in NR2A mRNA at any age. No significant changes in subunit mRNA levels were present in cortical or subcortical regions at any age. The Pb-induced changes in hippocampal NMDA receptor subunit mRNA expression measured in the present study may lead to modifications in receptor levels or subtypes and alter the development of defined neuronal connections which require NMDA receptor activation.

Glutamatergic system and developmental lead neurotoxicity

The excitatory amino acid receptor subtype N-Methyl-D-Aspartate (NMDA) has been the focus of intense investigation during the last decade because of its role in brain development and synaptic plasticity and in pathological conditions associated with excitotoxicity. This receptor complex has recently been identified to be a target for lead (Pb2+) effects on the developing central nervous system. This presentation is an overview of studies demonstrating the interaction of Pb2+ with the NMDA receptor and its potential effects on cellular function and cognition.

Developmental lead exposure causes spatial learning deficits in adult rats

Groups of male rats exposed to lead (Pb) during different developmental periods were tested as adults in a water maze. A highly significant (P < 0.01) impairment in water maze performance was measured in rats exposed to Pb only during gestation and lactation (maternal exposure). At the time of testing (100-106 days old), blood and brain Pb concentrations were at control levels. Significant impairments (P < 0.05) were also present in rats continuously exposed to Pb from conception through adulthood. Post-weaning Pb exposure alone did not result in impaired performance despite significantly elevated blood and brain Pb levels at the time of testing. This study supports the hypothesis that a window of vulnerability to Pb neurotoxicity exists in the developing brain and that Pb exposure can result in long-term cognitive deficits.

Age-dependent effects of developmental lead exposure on performance in the Morris water maze

The neurobehavioral toxicity of developmental exposure to lead (Pb) was investigated by conducting tests of spatial learning in the Morris water maze. Female Long-Evans rats were exposed to 0 or 250 ppm Pb acetate in the diet beginning 10 days prior to breeding and continued throughout gestation and lactation. Pups were weaned onto the same diets as the dams at postnatal day 20 (PN20). Increased levels of Pb were detected in the hippocampus of the 250 ppm Pb acetate group relative to controls. The highest concentration of Pb measured in the hippocampus was at PN21 with decreasing levels at older ages. In the Morris Water Maze, a statistically significant (p < 0.03; female rats) or near significant (p < 0.07; male rats) increase in the time required to find the hidden platform (escape latency) was observed when Pb-treated rats were tested in a reference memory paradigm. This effect was only observed when rats were tested at PN21 and not at older ages. No significant effects of developmental Pb exposure were measured when rats were tested in a working memory paradigm of the Morris water maze at any age. These initial studies indicate an impairment of performance in the swim task in PN21 rats exposed to Pb during development. The age-dependent effect of Pb in this learning paradigm is consistent with previous studies in experimental animals and with the observation that children are more susceptible to Pb-induced cognitive deficits than adults. The Morris water maze may be useful for studying the effects of Pb on learning and memory, and their neurochemical basis.

Intrahippocampal administration of lead (Pb) impairs performance of rats in the Morris water maze

We examined spatial learning in the Morris water maze after daily acute bilateral micro-injection of 13.9 ng sodium acetate (NaAc) or 37.9 ng lead acetate (PbAc) in 1 microliter volumes into the dorsal hippocampus of normal adult rats. After six days of injections and water maze training, rats injected with NaAc were able to find a hidden platform in 8.3 s, and those injected with PbAc were significantly slower (15.2 s; p < 0.02). In a second experiment, rats were trained to find a hidden platform before injections began and then tested in order to determine if intrahippocampal injections of Pb affected the recall of a previously learned task. The escape latency on the first day after injections began was increased slightly when compared to the last day of training before injections, however the NaAc and PbAc groups were not significantly different over three days of injections. Both treatment groups performed as well as they did before injections began by the second day of injections. These results suggest that the direct injection of Pb into the hippocampus impairs the acquisition but not the recall of the spatial learning task in the Morris water maze.

The peripheral benzodiazepine receptor is a sensitive indicator of domoic acid neurotoxicity

To evaluate the utility of the peripheral benzodiazepine receptor (PBR) as a biomarker of neurotoxicity, we measured receptor levels after sub-seizure doses of domoic acid (0-3.0 mg/kg) in rats using [3H]PK-11195 autoradiography. PBR expression in limbic structures was significantly increased 5 days, but not 24 or 48 h after injection of 3.0 mg/kg domoic acid. The largest increase in [3H]PK-11195 binding (> 500% above control) was found in the CA3 subfield of the hippocampus. Other limbic structures including the CA1 hippocampal subfield, subiculum, dentate gyrus and amygdala also showed significant increases in PBR expression, as did the striatum and substantia nigra pars reticulata. Smaller but significant increases were also observed 5 days after injection of 1.5 mg/kg, but not in animals treated with 0.75 mg/kg domoic acid. No pathology was observed after routine histological staining of brain tissue. Spatial learning and memory, a process thought to be associated with the hippocampus, was assessed in the Morris water maze. Groups treated with 1.5 and 3.0 mg/kg, but not 0.75 mg/kg domoic acid were significantly impaired in water maze performance. These findings suggest that the PBR could provide a sensitive and specific biomarker of neurotoxicity.

Pb2+ inhibits NMDA receptor function at high and low affinity sites: developmental and regional brain expression

Lead (Pb2+) is a potent inhibitor of the NMDA receptor complex. In the present study we have measured high and low affinity components of NMDA receptor inhibition by Pb2+ using [3H]-MI-801 binding as a biochemical indicator of NMDA receptor function. The inhibitory effects of Pb2+ were dependent on the age of the rat and the brain region analyzed. The number of [3H]-MK-801 binding sites associated with the high and low affinity sites of Pb2+ inhibition in the hippocampus increased as a function of age, peaking at postnatal day 28 and 21, respectively. In the cerebellum, a steady decrease in the number of both types of Pb(2+)-sensitive [3H]-MK-801 binding sites was measured from postnatal day 1 to adulthood. High and low affinity Pb(2+)-sensitive [3H]-MK-801 binding sites were measured in the cerebral cortex in early development, but the resolution of the two sites was not possible after postnatal day 14. The Pb2+ sensitivity of the NMDA receptor complex also appeared to be regulated developmentally.

Mice lacking tissue non-specific alkaline phosphatase die from seizures due to defective metabolism of vitamin B-6

In humans, deficiency of the tissue non-specific alkaline phosphatase (TNAP) gene is associated with defective skeletal mineralization. In contrast, mice lacking TNAP generated by homologous recombination using embryonic stem (ES) cells have normal skeletal development. However, at approximately two weeks after birth, homozygous mutant mice develop seizures which are subsequently fatal. Defective metabolism of pyridoxal 5'-phosphate (PLP), characterized by elevated serum PLP levels, results in reduced levels of the inhibitory neurotransmitter gamma-aminobutyric acid (GABA) in the brain. The mutant seizure phenotype can be rescued by the administration of pyridoxal and a semi-solid diet. Rescued animals subsequently develop defective dentition. This study reveals essential physiological functions of TNAP in the mouse.

Intrastriatal dopamine infusion reverses compensatory increases in D2-dopamine receptors in the 6-OHDA lesioned rat

Direct infusion of dopamine into the corpus striatum has been proposed as a potential approach for the treatment of Parkinson's disease. The present study examined the effect of intrastriatal dopamine infusion on D2-dopamine receptors in the 6-hydroxydopamine (6-OHDA) lesioned rat brain. The completeness of the 6-OHDA-induced nigrostriatal injury was confirmed using [3H]-mazindol autoradiography and apomorphine-induced behaviour. Intrastriatal infusion of three different dopamine doses significantly reduced the apomorphine-induced behaviour. [3H]-spiperone autoradiography performed one day after the termination of dopamine infusion into the striatum revealed a dramatic reduction of D2-dopamine receptor binding. The mean +/- SEM percent reduction of D2 receptor binding in the affected areas of the striatum was 28.8 +/- 1.0% for 4.74 micrograms dopamine/h infusion rate, 35.0 +/- 1.6% for 9.48 micrograms dopamine/h infusion rate and 33.3 +/- 5.0% for 14.22 micrograms dopamine/h infusion rate when compared to the unlesioned side. Infusion of vehicle alone did not have any effect. The present results support the concept that intrastriatal dopamine infusion may be a useful therapeutic approach for the treatment of Parkinson's disease.

Intrastriatal infusion of lisuride--a potential treatment for Parkinson's disease? Behavioral and autoradiographic studies in 6-OHDA lesioned rats

The present study examined the effect of chronic intrastriatal infusion of the dopamine receptor agonist lisuride on apomorphine-induced rotational behaviour and on D2-dopamine receptors in rats with unilateral 6-hydroxydopamine (6-OHDA) lesions of the nigrostriatal dopaminergic pathway. The completeness of the lesion of the right ascending nigrostriatal dopaminergic pathway was confirmed by apomorphine-induced rotation and [3H]-mazindol autoradiography. The intrastriatal infusion of lisuride (0.5 microgram/h) into the lesioned striatum for 2 weeks induced an immediate but temporary spontaneous contralateral rotation and a reduction of apomorphine-induced rotation of 47.2% relative to pre-lisuride infusion. The density of D2-receptors in the lisuride-infused striatum was significantly decreased by 40% relative to vehicle-infused 6-OHDA lesioned rats. The level of D2-dopamine receptors returned to normal levels 3 weeks after the termination of lisuride infusion. These results show that the intrastriatal infusion of lisuride reverses the behavioural and D2-dopamine receptor changes present in the 6-OHDA lesion rat model of Parkinson's disease.

Culture of arterial chemoreceptor cells from adult cats in defined medium

Recently patch clamp techniques and optical fluorometric techniques have been applied to freshly dissociated or cultured carotid body. However, very few studies have shown the effects of the dissociation and/or culture conditions on the health and function of the cells. The purpose of this study was to develop a culture method which support healthy and functioning carotid body cells from adult cats. Carotid bodies were dissociated with 0.1-0.2% collagenase and gentle trituration. The cells were plated on glass wells coated with poly-D-lysin and Matrigel, and cultured in chemically defined medium. Culture was maintained for up to 37 days without overgrowth of fibroblasts. Glomus cells extended their processes within and from clusters. Single glomus cells acquired the shape of neurons. Glomus cells synthesized dopamine and its secretion increased during exposure of the cells to hypoxia. Tyrosine hydroxylase was expressed throughout the culture period. These results indicate that glomus cells cultured under conditions described here are healthy and function in a manner similar to that in vivo.

Postnatal development and GABA allosteric modulation of benzodiazepine receptor binding in the vitamin B-6 deficient rat brain

We have measured the postnatal development and GABA modulation of benzodiazepine receptors in neuronal membranes from vitamin B-6 deficient and normal rats. In rats fed vitamin B-6 adequate and deficient diets there were age-dependent changes in [3H]flunitrazepam binding site affinity and in the number of binding sites. Vitamin B-6 deficiency produced a significant reduction in the potency of GABA to enhance [3H]flunitrazepam binding to cortical membranes prepared from 14 day old rats. These results suggests an uncoupling of the GABAa/benzodiazepine receptor at a developmental period when the animals are most susceptible to spontaneous seizures.

Assessment of brain muscarinic acetylcholinergic receptors in living mice using a simple probe, [125I]-4-iododexetimide and [125I]-4-iodolevetimide

This study describes assessment of brain muscarinic acetylcholinergic receptors in living mice using a single-crystal radiation detection system, the high-affinity antagonist [125I]-4-iododexetimide, and the inactive enantiomer [125I]-4-iodolevetimide. Kinetics of radioligand binding, as well as perturbation by atropine displacement, can be determined using this simple probe technique.

Chronic prenatal and postnatal Pb2+ exposure increases [3H]MK801 binding sites in adult rat forebrain

We have measured the binding of [3H]MK801 to the N-methyl-D-aspartate (NMDA) receptor-ion channel in membrane preparations from adult rat forebrain exposed to lead (Pb2+) during gestation, lactation, and postweaning. Our results indicate a 30.9% increase in the number of [3H]MK801 binding sites in Pb2+ exposed animals relative to controls. No significant changes in the affinity constant were observed. The level of blood Pb2+ for which such changes were measured was 13.9 +/- 2.8 micrograms/dl. These results indicate that alterations in the NMDA receptor-ion channel complex are present at blood Pb2+ levels which are environmentally relevant and suggest that chronic Pb2+ exposure during development can influence the NMDA receptor complex in the adult rat brain.