D.W. Anderson

Nigrostriatal dopamine system dysfunction and subtle motor deficits in manganese-exposed non-human primates

We tested the hypothesis that movement abnormalities induced by chronic manganese (Mn) exposure are mediated by dysfunction of the nigrostriatal dopamine system in the non-human primate striatum. Motor function and general activity of animals was monitored in parallel with chronic exposure to Mn and Positron Emission Tomography (PET) studies of in vivo dopamine release, dopamine transporters and dopamine receptors in the striatum. Analysis of metal concentrations in whole blood and brain was obtained and post-mortem analysis of brain tissue was used to confirm the in vivo PET findings. Chronic Mn exposure resulted in subtle motor function deficits that were associated with a marked decrease of in vivo dopamine release in the absence of a change in markers of dopamine (DA) terminal integrity or dopamine receptors in the striatum. These alterations in nigrostriatal DA system function were observed at blood Mn concentrations within the upper range of environmental, medical and occupational exposures in humans. These findings show that Mn-exposed non-human primates that exhibit subtle motor function deficits have an apparently intact but dysfunctional nigrostriatal DA system and provide a novel mechanism of Mn effects on the dopaminergic system.

Enriched environment during development is protective against lead-induced neurotoxicity

It is known that children of lower socioeconomic status have a disproportionately higher risk of being exposed to lead and have a more negative outcome from that exposure than children who are raised under more fortunate circumstances. Yet, little is known about how environmental factors may influence the injurious effects on the brain of a neurotoxin such as lead. The present study used an animal model of lead poisoning to examine the extent to which different environmental milieus may modify the effects of lead on the developing brain. Young rats were raised in either enriched or impoverished environments and drank either distilled water or water with lead. Lead-exposed rats raised in the impoverished environment had spatial learning deficits and significantly decreased neurotrophic factor gene expression in the hippocampus. In contrast, the animals raised in the enriched environment were significantly protected against the behavioral and neurochemical toxicity of lead. These results demonstrate that impoverished environment may accentuate while enriched environment may ameliorate neurobehavioral and neurochemical toxicity from developmental lead exposure.

Influence of developmental lead exposure on expression of DNA methyltransferases and methyl cytosine-binding proteins in hippocampus

Developmental exposure to lead (Pb) has adverse effects on cognitive functioning and behavior that can persist into adulthood. Exposures that occur during fetal or early life periods may produce changes in brain related to physiological re-programming from an epigenetic influence such as altered DNA methylation status. Since DNA methylation is regulated by DNA methyltransferases and methyl cytosine-binding proteins, this study assessed the extent to which developmental Pb exposure might affect expression of these proteins in the hippocampus. Long Evans dams were fed chow with or without added Pb acetate (0, 150, 375, 750 ppm) prior to breeding and remained on the same diet through weaning (perinatal exposure group). Other animals were exposed to the same doses of Pb but exposure started on postnatal day 1 and continued through weaning (early postnatal exposure group). All animals were euthanized on day 55 and hippocampi were removed. Western blot analyses showed significant effects of Pb exposure on DNMT1, DNMT3a, and MeCP2 expression, with effects often seen at the lowest level of exposure and modified by sex and developmental window of Pb exposure. These data suggest potential epigenetic effects of developmental Pb exposure on DNA methylation mediated at least in part through dysregulation of methyltransferases.

Neuroprotection in Parkinson models varies with toxin administration protocol.

Numerous factors contribute to substantia nigra pars compacta (SNc) dopamine (DA) neuron death in Parkinsons disease (PD), thus complicating the search for effective neuroprotective agents for this disease. Although the 1‐methyl‐4‐phenyl‐1,2,3,6‐tetrahydropyridine (MPTP)‐treated mouse has been widely used for assessing neuroprotective agents for PD, the pathological processes resulting from MPTP exposure can vary greatly depending upon the MPTP administration protocol. This study assessed the degree to which the neuroprotective efficacy of particular agents may depend upon the MPTP administration protocol (i.e. acute vs. subacute toxin administration). Endpoints analysed were changes in tyrosine hydroxylase (TH) and NeuN cell numbers in the SNc, striatal DA and metabolite levels, and striatal TH+ fiber density. The efficacy of putative neuroprotective agents [i.e. LIGA 20, nicotinamide and pramipexole (PPX)] varied depending upon the MPTP administration protocol. LIGA 20 spared striatal DA levels in both MPTP models, while nicotinamide was only effective in the acute toxin administration model and PPX was only effective in the subacute model. In both MPTP models, LIGA 20 and nicotinamide significantly spared DAergic neurons; PPX only spared DAergic neurons in the subacute model. Only acute MPTP‐treated mice that received nicotinamide had a significant sparing of striatal DAergic fibers. These results underscore the need to assess putative neuroprotective agents for PD in multiple animal models using multiple endpoints. This strategy may better identify compounds with broad neuroprotective/neurorestorative profiles that may be more likely to be clinically effective.

Neuroprotective effects of pramipexole in young and aged MPTP-treated mice.

This study examined the effect of pramipexole (PPX), a selective dopamine (DA) D(3)/D(2) agonist, on 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced damage to the nigrostriatal dopamine system in young (8-week-old) and aged (12-month-old) mice. Co-administration of PPX and MPTP to young or aged mice, followed by 2 or 14 days of additional PPX treatment, significantly attenuated MPTP-induced striatal DA loss. Pramipexole treatment also significantly attenuated the loss of tyrosine hydroxylase immunoreactive neurons (TH-IR) within the substantia nigra pars compacta (SNc) in both young and aged animals. Effects of PPX administration on dopaminergic cell survival were confirmed in Nissl-stained sections and by quantitation of retrogradely labeled Fluorogold-positive SNc neurons. Protective effects of PPX on striatal DA levels and SNc DA neuron survival were similar in young and aged animals, although the magnitude of these effects was significantly less in aged animals. These findings support the early initiation of PPX therapy in Parkinsons disease patients.

Broad neuroprotective profile of nicotinamide in different mouse models of MPTP-induced parkinsonism

The factors contributing to substantia nigra pars compacta (SNc) dopamine (DA) neuron death and striatal DA depletion in Parkinson’s disease (PD) are still poorly understood. However, mitochondrial dysfunction, cellular energy depletion and oxidative stress appear to play important roles in the pathogenesis of PD. In view of this, the current study examined the potential of nicotinamide, a form of the B‐complex vitamin niacin, to protect against 1‐methyl‐4‐phenyl‐1,2,3,6‐tetrahydropyridine (MPTP)‐induced SNc cell loss and striatal DA depletion in two mouse MPTP models that respond differently to putative neuroprotective agents. Adult male C57Bl/6 mice received nicotinamide (125, 250 or 500 mg/kg i.p.) prior to either acute (four injections in 1 day at 2‐h intervals) or sub‐acute (two injections per day at 4‐h intervals for 5 days) MPTP administration. Striatal DA levels, changes in numbers of tyrosine hydroxylase (TH)‐ and cresyl violet‐stained cells in the SNc at 2 and 6 weeks following the last MPTP exposure were analyzed. Nicotinamide administration resulted in a dose‐dependent sparing of striatal DA levels and SNc neurons in acute MPTP‐treated animals. Only the highest dose of nicotinamide had similar effects in sub‐acute MPTP‐treated animals. At 6 weeks after MPTP exposure, there was some spontaneous recovery of striatal DA levels in both models: neuroprotective effects were still apparent in acute but not sub‐acute MPTP‐treated animals. These results show neuroprotective effects of nicotinamide in different mouse Parkinson models associated with different forms of cell death and suggest that nicotinamide may have broad neuroprotective potential in PD.

Functional significance of aldehyde dehydrogenase ALDH1A1 to the nigrostriatal dopamine system.

Aldehyde dehydrogenase 1A1 (ALDH1A1) is a member of a superfamily of detoxification enzymes found in various tissues that participate in the oxidation of both aliphatic and aromatic aldehydes. In the brain, ALDH1A1 participates in the metabolism of catecholamines including dopamine (DA) and norepinephrine, but is uniquely expressed in a subset of dopaminergic (DAergic) neurons in the ventral mesencephalon where it converts 3,4-dihydroxyphenylacetaldehyde, a potentially toxic aldehyde, to 3,4-dihydroxyphenylacetic acid, a non toxic metabolite. Therefore, loss of ALDH1A1 expression could be predicted to alter DA metabolism and potentially increase neurotoxicity in ventral mesencephalic DA neurons. Recent reports of reduced levels of expression of both Aldh1a1 mRNA and protein in the substantia nigra (SN) of Parkinsons disease patients suggest possible involvement of ALDH1A1 in this progressive neurodegenerative disease. The present study used an Aldh1a1 null mouse to assess the influence of ALDH1A1 on the function and maintenance of the DAergic system. Results indicate that the absence of Aldh1a1 did not negatively affect growth and development of SN DA neurons nor alter protein expression levels of tyrosine hydroxylase, the DA transporter or vesicular monoamine transporter 2. However, absence of Aldh1a1 significantly increased basal extracellular DA levels, decreased KCl and amphetamine stimulated DA release and decreased DA re-uptake and resulted in more tyrosine hydroxylase expressing neurons in the SN than in wildtype animals. These data suggest that in young adult animals with deletion of the Aldh1a1 gene there is altered DA metabolism and dysfunction of the DA transporter and DA release mechanisms.

Sex-based differences in gene expression in hippocampus following postnatal lead exposure.

The influence of sex as an effect modifier of childhood lead poisoning has received little systematic attention. Considering the paucity of information available concerning the interactive effects of lead and sex on the brain, the current study examined the interactive effects of lead and sex on gene expression patterns in the hippocampus, a structure involved in learning and memory. Male or female rats were fed either 1500 ppm lead-containing chow or control chow for 30 days beginning at weaning.Blood lead levels were 26.7±2.1 μg/dl and 27.1±1.7 μg/dl for females and males, respectively. The expression of 175 unique genes was differentially regulated between control male and female rats. A total of 167 unique genes were differentially expressed in response to lead in either males or females. Lead exposure had a significant effect without a significant difference between male and female responses in 77 of these genes. In another set of 71 genes, there were significant differences in male vs. female response. A third set of 30 genes was differentially expressed in opposite directions in males vs. females, with the majority of genes expressed at a lower level in females than in males. Highly differentially expressed genes in males and females following lead exposure were associated with diverse biological pathways and functions. These results show that a brief exposure to lead produced significant changes in expression of a variety of genes in the hippocampus and that the response of the brain to a given lead exposure may vary depending on sex.

Effects of Developmental Lead Exposure on the Hippocampal Transcriptome: Influences of Sex, Developmental Period, and Lead Exposure Level

Developmental lead (Pb) exposure has profound effects on cognition and behavior. Much is known about effects of Pb on hippocampal-mediated behaviors, but little is known about the molecular consequences of Pb exposure and the influences of developmental timing of exposure, level of exposure, and sex as effect modifiers of Pb exposure on the brain. The aim of this study was to examine the effects of different levels of Pb exposure (250 and 750 ppm Pb acetate) during perinatal (gestation/lactation) and postnatal (through postnatal day 45) periods on the hippocampal transcriptome in male and female Long Evans rats. Total RNA was extracted from hippocampus from four animals per experimental condition. RNA was hybridized to Affymetrix Rat Gene RNA Arrays using standard methods. Pb exposure per se influenced the expression of 717 transcripts (328 unique annotated genes), with many influenced in a sex-independent manner. Significant differences in gene expression patterns were also influenced by timing and level of exposure, with generally larger effects at the lower level of exposure across all groups. Statistically enriched biological functions included ion binding, regulation of RNA metabolic processes, and positive regulation of macromolecule biosynthetic processes. Processes of regulation of transcription and regulation of gene expression were preferentially enriched in males, regardless of timing or amount of Pb exposure. The effect on transcription factors and the diverse pathways or networks affected by Pb suggest a substantial effect of developmental Pb exposure on plasticity and adaptability, with these effects significantly modified by sex, developmental window of exposure, and level of Pb exposure.

Sex and Rearing Condition Modify the Effects of Perinatal Lead Exposure on Learning and Memory

Developmental lead (Pb) exposure is associated with cognitive impairments in humans and rodents alike. In particular, impaired spatial learning and memory, as assessed using the Morris water maze (MWM), has been noted in developmentally Pb-exposed rats. Although sex and rearing environment can influence MWM performance in normal animals, the interactions of sex and rearing environment on the impact of developmental Pb exposure on hippocampal-dependent processes has not been well characterized. The present study examined the effects of perinatal exposure (i.e., gestation through weaning) to different levels of Pb (250, 750 and 1500 ppm Pb acetate in food) in males and females raised in a non-enriched environment (standard cage with 3 animals and no toys) or an enriched environment (large cage containing a variety of toys that were changed twice weekly). Testing in the MWM began at postnatal day 55. Behavioral outcomes were influenced by sex and rearing environment, with complex interactions with Pb exposure. In non-Pb exposed control animals, beneficial effects of environmental enrichment on spatial learning and memory were observed in males and females, with greater effects in females. Pb exposure in females mitigated at least some of the benefits of enrichment on learning, particularly at the lowest and highest exposure levels. In males, enrichment conferred a modest learning advantage and for the most part, Pb exposure did not affect this. However, in males with the highest Pb exposure, enrichment did help to overcome detrimental effects of Pb on learning. In females, any potential benefit to reference memory contributed by enrichment was muted by exposure to Pb and for the most part, this was not reproduced in males. Thus, there are complex interactions between sex, environment, and Pb exposure on spatial learning and memory. Environmental manipulation is a potential risk modifier of developmental Pb exposure and interacts with other factors including sex and amount of Pb exposure to affect the functional influences of Pb on the brain.