Death of dopaminergic neurones in the rat substantia nigra can be induced by damage to globus pallidus

An Introspective Approach: A Lifetime of Parkinson's Disease Research and Not Much to Show for it Yet?

I feel part of a massive effort to understand what is wrong with motor systems in the brain relating to Parkinson’s disease. Today, the symptoms of the disease can be modified slightly, but dopamine neurons still die; the disease progression continues inexorably. Maybe the next research phase will bring the power of modern genetics to bear on halting, or better, preventing cell death. The arrival of accessible human neuron assemblies in organoids perhaps will provide a better access to the processes underlying neuronal demise.

Occupational exposures and parkinsonism

In recent years, the contribution of exposure to environmental toxicants has been recognized as a significant contributor to the etiopathogenesis of parkinsonism. Of these toxicants, exposure to pesticides, metals, solvents used in manufacturing processes, as well as flame-retardant chemicals used in consumer and commercial products, has received the greatest attention as possible risk factors. Related to this, individuals who are exposed to these compounds at high concentrations or for prolonged periods of time in an occupational setting appear to be one of the more vulnerable populations to these effects. Our understanding of which compounds are involved and the potential molecular pathways that are susceptible to these chemicals and may underlie the pathogenesis has greatly improved. However, there are still hundreds of chemicals that we are exposed to in the environment for which we do not have any information on their potential neurotoxicity on the nigrostriatal dopamine system. Thus, using our past accomplishments as a blueprint, future endeavors should focus on elaborating upon these initial findings in order to identify specific and relevant chemical toxicants in our environment that can impact the risk of parkinsonism and work towards a means to attenuate or abolish their effects on the human population.

Industrial toxicants and Parkinson's disease

The exposure of the human population to environmental contaminants is recognized as a significant contributing factor for the development of Parkinson's disease (PD) and other forms of parkinsonism. While pesticides have repeatedly been identified as risk factors for PD, these compounds represent only a subset of environmental toxicants that we are exposed to on a regular basis. Thus, non-pesticide contaminants, such as metals, solvents, and other organohalogen compounds have also been implicated in the clinical and pathological manifestations of these movement disorders and it is these non-pesticide compounds that are the subject of this review. As toxic exposures to these classes of compounds can result in a spectrum of PD or PD-related disorders, it is imperative to appreciate shared clinico-pathological characteristics or mechanisms of action of these compounds in order to further delineate the resultant disorders as well as identify improved preventive strategies or therapeutic interventions.

Delayed-onset diurnal bruxism, psychic akinesia and depression after carbon monoxide poisoning: a case report

Diagnosing the delayed neuropsychological sequelae of carbon monoxide (CO) poisoning is a clinical challenge because of its varied presentations. Pallidal lesions, the most common site of involvement in CO poisoning [Clin Radiol. 2000;55(4):273-80] can cause psychic akinesia [Mov Disord. 2001;16(5):810-4; J Neurol Neurosurg Psychiatry 1984;47(4):377-85]. We present a patient with diurnal bruxism, psychic akinesia and depression that were delayed manifestations of CO poisoning.

Slowly progressive dopamine cell loss--a model on which to test neuroprotective strategies for Parkinson's disease?

Making animal models of human disease is a very flawed process. Aspects of the disease can be imitated but models do not necessarily give reliable leads for treatment strategies. When Ungerstedt in Sweden first described the 6-hydroxydopamine (6-OHDA) treated rat model of Parkinson's disease /89/ we knew that the symptoms would not map readily to those of the human disease--rats have four legs after all. On the other hand, the neuropathology looked exactly like end-stage Parkinsonian pathology. That remained true even as we explored other types of neuropathology in the rats /24,43-46,80/. Many of today's treatments for Parkinsonism are developed from pharmacological studies on that model of rats with a chemically induced lesion. However, the 6-OHDA model does not address the important issue of a cure for the disease. The triggers and the time-course of dopamine (DA) cell death in rats are known for nearly every disease model - but for the human disease there is no equivalent knowledge. In the human, the neurons have been dying for a considerable time before the symptoms become obvious and they go on dying even with adequate symptomatic relief /94/, but after intracerebral administration of 6-OHDA to an animal the cells die quickly; all cells are destroyed in less than 5 days /42,88,89/. Thus, we were interested in developing an animal model of DA cell death with a slower time-course. After ibotenic acid injections into rat globus pallidus (GP), DA cells are lost from the ipsilateral substantia nigra over the slower time scale of about six weeks. This time scale has allowed us to test some interventions to prevent the cells from dying. Although some attempts have succeeded, cell death is prevented only for three weeks -beyond that treatments fail and DA cells die. At the moment, this model has at least opened a window into causes of neuronal death in a slower time scale /94/ than previous rodent models.

Are there common biochemical and molecular mechanisms controlling manganism and parkisonism

Over the past several decades there has been considerable progress in our basic knowledge as to the mechanisms and factors regulating Mn toxicity. The disorder known as manganism is associated with the preferential accumulation of Mn in the globus pallidus of the basal ganglia which is generally considered to be the major and initial site of injury. Because the area of the CNS comprising the basal ganglia is very complex and dependent on the precise function and balance of several neurotransmitters, it is not surprising that symptoms of manganism often overlap with that of Parkinson's disease. The fact that neurological symptoms and onset of Mn toxicity are quite broad and can vary unpredictably probably reflects specific genetic variance of the physiological and biochemical makeup within the basal ganglia in any individual. Differences in response to Mn overexposure are, thus, likely due to underlying genetic variability which ultimately presents in deviations in both susceptibility as well as the characteristics of the neurological lesions and symptoms expressed. Although chronic exposure to Mn is not the initial causative agent provoking Parkinsonism, there is evidence suggesting that persistent exposure can predispose an individual to acquire dystonic movements associated with Parkinson's disease. As noted in this review, there appears to be common threads between the two disorders, as mutations in the genes, parkin and ATP13A2, associated with early onset of Parkinsonism, may also predispose an individual to develop Mn toxicity. Mutations in both genes appear to effect transport of Mn into the cell. These genetic difference coupled with additional environmental or nutritional factors must also be considered as contributing to the severity and onset of manganism.

Inhalation of divalent and trivalent manganese mixture induces a Parkinson's disease model: immunocytochemical and behavioral evidences

The present study investigates the effects of divalent and trivalent manganese (Mn(2+)/Mn(3+)) mixture inhalation on mice to obtain a novel animal model of Parkinson disease (PD) inducing bilateral and progressive cell death in the substantia nigra compacta (SNc) and correlating these alterations with motor disturbances. CD-1 male mice inhaled a mixture of 0.04 M manganese chloride (MnCl(2)) and manganese acetate (Mn(OAc)(3)), 1 h twice a week for 5 months. Before Mn exposure, animals were trained to perform motor function tests and were evaluated each week after the exposure. By doing this, overall behavior was assessed by ratings and by videotaped analyses; by the end of Mn exposure period, animals were killed. The mesencephalon was processed for tyrosine hydroxylase (TH) immunocytochemistry. After 5 months of Mn mixture inhalation, mice developed evident deficits in their motor performance manifested as akinesia, postural instability and action tremor. SNc of the Mn-exposed animals showed an important decrease (67.58%) in the number of TH-immunopositive neurons. Our data provide evidence that MnCl(2) and Mn(OAc)(3) mixture inhalation produces similar morphological and behavioral alterations to those observed in PD providing a useful experimental model for the study of this neurodegenerative disease.

Orthotopic transplantation of immortalized mesencephalic progenitors (CSM14.1 cells) into the substantia nigra of hemiparkinsonian rats induces neuronal differentiation and motoric improvement

Neural progenitor cell grafting is a promising therapeutic option in the treatment of Parkinson's disease. In previous experiments we grafted temperature-sensitive immortalized CSM14.1 cells, derived from the ventral mesencephalon of E14-rats, bilaterally in the caudate putamen of adult hemiparkinsonian rats. In these studies we were not able to demonstrate either a therapeutic improvement or neuronal differentiation of transplanted cells. Here we examined whether CSM14.1 cells grafted bilaterally orthotopically in the substantia nigra of hemiparkinsonian rats have the potential to differentiate into dopaminergic neurons. Adult male rats received 6-hydroxydopamine into the right medial forebrain bundle, and successful lesions were evaluated with apomorphine-induced rotations 12 days after surgery. Two weeks after a successful lesion the animals received bilateral intranigral grafts consisting of either about 50 000 PKH26-labelled undifferentiated CSM14.1 cells (n = 16) or a sham-graft (n = 9). Rotations were evaluated 3, 6, 9 and 12 weeks post-grafting. Animals were finally perfused with 4% paraformaldehyde. Cryoprotected brain slices were prepared for immunohistochemistry using the freeze-thaw technique to preserve PKH26-labelling. Slices were immunostained against neuronal epitopes (NeuN, tyrosine hydroxylase) or glial fibrillary acidic protein. The CSM14.1-cell grafts significantly reduced the apomorphine-induced rotations 12 weeks post-grafting compared to the sham-grafts (P < 0.05). There was an extensive mediolateral migration (400-700 microm) of the PKH26-labelled cells within the host substantia nigra. Colocalization with NeuN or glial fibrillary acidic protein in transplanted cells was confirmed with confocal microscopy. No tyrosine hydroxylase-immunoreactive grafted cells were detectable. The therapeutic effect of the CSM14.1 cells could be explained either by their glial cell-derived neurotrophic factor-expression or their neural differentiation with positive effects on the basal ganglia neuronal networks.

The influence of the subthalamic nucleus upon the damage to the dopamine system following lesions of globus pallidus in rats

Lesioning or stimulating the subthalamic nucleus (STN) in patients with Parkinson's disease, or in animal models of parkinsonism, alleviates many of the symptoms and so it is tempting to think of the STN as a part of the cause of Parkinson's disease. The globus pallidus (GP) is thought to have a tonic inhibitory action on the STN. An ibotenic acid injection into the GP in rats removes the cells of the GP and, over the following 6 weeks, a progressive loss of dopamine cells (counted stereologically in sections stained for tyrosine hydroxylase) develops in substantia nigra (SN). In this investigation we show that, when animals have the STN cells destroyed by very small ibotenic acid injections, their dopamine neurons are not damaged. Furthermore, if a lesion to the GP follows a lesion of STN then the dopamine cells also survive this double insult, at least for the first 3 weeks following the lesion. The experiments provide good reason to suspect that, at least in the short term, increased activity in the STN is a contributory cause of the loss of dopamine cells which follows the lesion of the GP in rats. Whether or not this is part of the mechanism of cell loss in Parkinson's disease, the rats with GP lesions at least provide an opportunity to test strategies that might protect dopamine cells from slowly developing damage. Removing the STN seems to be neuroprotective in this new model of dopamine degeneration.

Transforming growth factor beta2 haploinsufficient mice develop age-related nigrostriatal dopamine deficits

The transforming growth factor-betas (TGF-betas) regulate the induction of dopaminergic neurons and are elevated in the CSF of Parkinson's patients. We report here that mice with TGF-beta2 haploinsufficiency (TGF-beta2+/-) have subclinical defects in the dopaminergic neurons of their substantia nigra pars compacta. At 6 weeks of age, the TGF-beta2+/- mice had 12% fewer dopaminergic neurons than wild-type littermates. No additional loss of neurons occurred during the next 5 months, although striatal dopamine declined to 70% of normal. The level of 3,4-dihydroxphenylacetic acid was normal in the TGF-beta2+/- mice, indicating that a compensatory mechanism maintains dopamine stimulation of their striatum. The TGF-beta2+/- mice had normal sensitivity to the neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine, despite having reduced levels of monoamine oxidase-B. These results raise the possibility that people with naturally low levels of TGF-beta2 may have less functional reserve in their nigrostriatal pathway, causing them to be at increased risk of developing Parkinson disease.