Transplacental effect of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) on brain dopaminergic neurons in the mouse. An immunohistochemical study

Developmental origins of Parkinson disease: Improving the rodent models

Numerous pesticides are inhibitors of the oxidative phosphorylation system. Oxidative phosphorylation dysfunction adversely affects neurogenesis and often accompanies Parkinson disease. Since brain development occurs mainly in the prenatal period, early exposure to pesticides could alter the development of the nervous system and increase the risk of Parkinson disease. Different rodent models have been used to confirm this hypothesis. However, more precise considerations of the selected strain, the xenobiotic, its mode of administration, and the timing of animal analysis, are necessary to resemble the model to the human clinical condition and obtain more reliable results.

Acute MPTP treatment decreases dendritic spine density of striatal Medium Spiny Neurons via SNK-SPAR pathway in C57BL/6 mice

Parkinson's disease (PD) is a well-known neurodegenerative disorder associated with a high risk in middle-aged and elderly individuals, severely impacting the patient's quality of life. 1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) is frequently used to establish PD in animals. Dendritic spines are dendritic processes that form the foundation of learning and memory. Reportedly, dendritic spine density of striatal medium spiny neurons (MSNs) declines in PD, and this decline has been associated with PD progression; however, the underlying mechanism remains elusive. Herein, we used the MPTP animal model to examine whether serum-induced kinase (SNK) and spine-associated Rap guanosine triphosphatase (SPAR) contribute to decreased dendritic spine density in striatal MSNs. MPTP was used to establish the animal model, which exhibits motor function impairment and dopaminergic cell loss. To assess spine density, Golgi staining was performed to count striatal dendritic spines, which were reduced in the MPTP group when compared with those in the normal control group. Immunohistochemistry was performed to analyze changes in SNK and SPAR expression. MPTP treatment significantly increased the expression of SNK in striatal MSNs, whereas that of SPAR was significantly decreased when compared with the normal control group. These findings offer clues to further explore the mechanism of declining dendritic spine density in patients with PD and provide evidence for potential target identification in PD. This article is protected by copyright. All rights reserved.

Prenatal exposure to oxidative phosphorylation xenobiotics and late-onset Parkinson disease

Late-onset Parkinson disease is a multifactorial and multietiological disorder, age being one of the factors implicated. Genetic and/or environmental factors, such as pesticides, can also be involved. Up to 80% of dopaminergic neurons of the substantia nigra are lost before motor features of the disorder begin to appear. In humans, these neurons are only formed a few weeks after fertilization. Therefore, prenatal exposure to pesticides or industrial chemicals during crucial steps of brain development might also alter their proliferation and differentiation. Oxidative phosphorylation is one of the metabolic pathways sensitive to environmental toxicants and it is crucial for neuronal differentiation. Many inhibitors of this biochemical pathway, frequently found as persistent organic pollutants, affect dopaminergic neurogenesis, promote the degeneration of these neurons and increase the risk of suffering late-onset Parkinson disease. Here, we discuss how an early, prenatal, exposure to these oxidative phosphorylation xenobiotics might trigger a late-onset, old age, Parkinson disease.

Early prenatal exposure to MPTP does not affect nigrostrial neurons in macaque monkey

The discovery of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), a toxin that induces parkinsonism in both human and primate, has prompted the search for environmental toxins potentially responsible for idiopathic Parkinson's disease (PD). The present study reports the ultimate effects of MPTP intoxication of a female macaque monkey, which unraveled to be pregnant after parkinsonism had developed, upon its fetus. Detailed examination of the offpsring nigrostriatal pathway showed that tyrosine hydroxylase immunoreactivity in caudate-putamen nuclei and substantia nigra compacta (SNc) was not different from an age-matched control. Biochemical analysis of the tissue content of dopaminergic markers further suggested modification of metabolism in the MPTP-exposed monkey. These data suggest that early prenatal intoxication does not destroy nigrostriatal neurons, most likely because dopamine neurons had not developed yet when exposed to MPTP.

Expanding the test set: Chemicals with potential to disrupt mammalian brain development

High-throughput test methods including molecular, cellular, and alternative species-based assays that examine critical events of normal brain development are being developed for detection of developmental neurotoxicants. As new assays are developed, a "training set" of chemicals is used to evaluate the relevance of individual assays for specific endpoints. Different training sets are necessary for each assay that would comprise a developmental neurotoxicity test battery. In contrast, evaluation of the predictive ability of a comprehensive test battery requires a set of chemicals that have been shown to alter brain development after in vivo exposure ("test set"). Because only a small number of substances have been well documented to alter human neurodevelopment, we have proposed an expanded test set that includes chemicals demonstrated to adversely affect neurodevelopment in animals. To compile a list of potential developmental neurotoxicants, a literature review of compounds that have been examined for effects on the developing nervous system was conducted. The search was limited to mammalian studies published in the peer-reviewed literature and regulatory studies submitted to the U.S. EPA. The definition of developmental neurotoxicity encompassed changes in behavior, brain morphology, and neurochemistry after gestational or lactational exposure. Reports that indicated developmental neurotoxicity was observed only at doses that resulted in significant maternal toxicity or were lethal to the fetus or offspring were not considered. As a basic indication of reproducibility, we only included a chemical if data on its developmental neurotoxicity were available from more than one laboratory (defined as studies originating from laboratories with a different senior investigator). Evidence from human studies was included when available. Approximately 100 developmental neurotoxicity test set chemicals were identified, with 22% having evidence in humans.

Susceptibility to a parkinsonian toxin varies during primate development

Symptoms of Parkinson's disease typically emerge later in life when loss of nigrostriatal dopamine neuron function exceeds the threshold of compensatory mechanisms in the basal ganglia. Although nigrostriatal dopamine neurons are lost during aging, in Parkinson's disease other detrimental factors must play a role to produce greater than normal loss of these neurons. Early development has been hypothesized to be a potentially vulnerable period when environmental or genetic abnormalities may compromise central dopamine neurons. This study uses a specific parkinsonian neurotoxin, 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), to probe the relative vulnerability of nigrostriatal dopamine neurons at different stages of primate development. Measures of dopamine, homovanillic acid, 1-methyl-pyridinium concentrations and tyrosine hydroxylase immunoreactive neurons indicated that at mid-gestation dopamine neurons are relatively vulnerable to MPTP, whereas later in development or in the young primate these neurons are resistant to the neurotoxin. These studies highlight a potentially greater risk to the fetus of exposure during mid-gestation to environmental agents that cause oxidative stress. In addition, the data suggest that uncoupling protein-2 may be a target for retarding the progressive loss of nigrostriatal dopamine neurons that occurs in Parkinson's disease and aging.

Age-associated changes in the dopamine synthesis as determined by GTP cyclohydrolase I inhibitor in the brain of senescence-accelerated mouse-prone inbred strains (SAMP8)

Our objective in this study was to elucidate the mechanism underlying the decrease in dopamine (DA) levels in the brain with ageing We administered 2,4-diamino-6-hydroxypyrimidine (DAHP), an inhibitor of GTP cyclohydrolase I to senescence-accelerated mouse-prones (SAMP8), to inhibit DA and serotonin syntheses, and following immunohistochemical staining, analyzed the immunoreactive intensities (IR-Is) for DA in the nigrostriatal dopaminergic neurons by microphotometry. The DA-IR-Is in the substantia nigra pars compacta and neostriatum of young mice (2 months old) reached a minimal value 3 h after DAHP administration and returned to the control value 12 h after the administration. However, in aged mice (10 months old), the minimal value was reached 6 h after the administration and the value remained at approximately 70 and 80% of the control value at 24 and 72 h, respectively, after DAHP administration. The results suggest that DA turnover is lower in aged mice than in young mice.

Semiquantitative analysis of immunoreactivities of tyrosine hydroxylase and aromatic l-amino acid decarboxylase in the locus coeruleus of desipramine-treated mice

The influence of antidepressant drugs on catecholaminergic neuron groups of the mouse brain was studied immunohistochemically, using a microphotometric semiquantitative method. The immunoreactive level of tyrosine hydroxylase (TH) and aromatic L-amino acid decarboxylase (AADC) in the locus coeruleus (LC) was significantly decreased after 2 weeks of antidepressant administration. Our results suggest that long-term exposure to an antidepressant drug, desipramine, significantly affects intracellular catecholamine-synthesizing enzymes. These results may suggest some significant roles of the catecholaminergic neuronal groups to the action of the tricyclic antidepressant drugs.

Neurotoxic effect of prenatal exposure to MPTP on the dopaminergic systems of the marmoset brain

MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine) was incidentally administered to pregnant marmosets during the whole gestational period, except for the last 15 days before term. The infant monkeys were killed 5 months after birth, and dopamine and its metabolites were measured in the striatum and the nucleus accumbens. Prenatal exposure to MPTP produced a marked dopamine depletion in these brain regions of the offspring, showing that MPTP is able to cross the placental barrier in primates.

Detection of tyrosine hydroxylase and phenylethanolamine-N-methyltransferase messenger RNAs in the mouse adrenal gland and the brain by in situ hybridization

To study the expression of tyrosine hydroxylase (TH) and phenylethanolamine-N-methyltransferase (PNMT) genes in the mouse adrenal gland and the brain, we performed in situ hybridization studies by using several types of complementary DNA probes recognizing coding regions of human TH (THc), the 3'-end of the human region of TH (TH3'), and the coding region of the human PNMT (PNMTc). THc mRNA was detected in the chromaffin cells of the mouse adrenal medulla and brain catecholaminergic neurons including the substantia nigra and ventral tegmental area. TH-immunopositive neurons were located in a similar pattern in adjacent sections. However no positive signals were detected by the TH3' probe. Using the PNMTc probe, the majority of cells in the adrenal medulla demonstrated positive labelling. Although the mouse TH and PNMT genes have not been fully isolated and sequenced, the present study strongly suggests that the sequence of the coding regions of TH and PNMT are similar in human and mouse. The THc and PNMTc probes are particularly useful in investigating the loci of gene transcription in mouse tissues.

Placental toxicity of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) in mice

1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) is a well-known model substance for inducing in humans and monkeys a severe extrapyramidal syndrome similar to Parkinson's disease. The neurotoxic action of MPTP can be exerted not only in adult animals but also during fetal development by diaplacental passage. Here we show that, during the gestation period of mice, the placenta is another important target organ of MPTP cytotoxicity. Pregnant NMRI mice on gestation day 15 received a single intraperitoneal dose of 20, 40, or 60 mg/kg MPTP. Developmental parameters of the fetuses and the placentas were determined on gestation day 18. Placental weight was consistently reduced in all experimental groups. Histology showed conspicuous alterations of the labyrinth layer; at 20 mg/kg MPTP there was already a significant reduction of the trabecular diameters and from 40 mg/kg onwards, severe necrosis of the syncytial trophoblast cells. In addition, there were necrotic alterations of the cells of the visceral yolk sac. The toxic effects are confined to the placenta at the doses used in the present experiments, leading at just 60 mg/kg to a marked placental insufficiency syndrome.

Transplacentally-transported 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) affects the catecholamine and indoleamine levels in the fetal mouse brain

The effects of a dopaminergic neurotoxin, 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) on the amounts of dopamine (DA), 3,4-dihydroxyphenylacetic acid (DOPAC), 5-hydroxytryptamine (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) were examined in the whole brains of fetal mice and maternal mice after its administration to pregnant mice. DA and DOPAC concentrations were decreased significantly in both the fetal and maternal brains. At 3 hr after injection, reduction of the DOPAC concentration was more marked than that of DA in both the fetal and maternal brains. Increase of 5-HT concentration was observed until 12 hr after injection in the fetal brains and 6 hr in the maternal brains. These results indicate that 1-methyl-4-phenyl-pyridinium ion (MPP+) and MPTP affect the levels of catechol- and indoleamines in the brain of premature stage as well as in the mature brain.