Chronic effects of single intrastriatal injections of 6-hydroxydopamine or 1-methyl-4-phenylpyridinium studied by microdialysis in freely moving rats

Genetically engineered mesenchymal stem cells with dopamine synthesis for Parkinson's disease in animal models

Although striatal delivery of three critical genes for dopamine synthesis by viruses is a potential clinical approach for treating Parkinson's disease (PD), the approach makes it difficult to finely control dopamine secretion amounts and brings safety concerns. Here, we generate genetically engineered mesenchymal stem cells encoding three critical genes for dopamine synthesis (DOPA-MSCs). DOPA-MSCs retain their MSC identity and stable ability to secrete dopamine during passaging. Following transplantation, DOPA-MSCs reinstate striatal dopamine levels and correct motor function in PD rats. Importantly, after grafting into the caudate and putamen, DOPA-MSCs provide homotopic reconstruction of midbrain dopamine pathways by restoring striatal dopamine levels, and safely and long-term (up to 51 months) correct motor disorders and nonmotor deficits in acute and chronic PD rhesus monkey models of PD even with advanced PD symptoms. The long-term benefits and safety results support the idea that the development of dopamine-synthesized engineered cell transplantation is an important strategy for treating PD.

Early increase in dopamine release in the ipsilateral striatum after unilateral intranigral administration of lactacystin produces spontaneous contralateral rotations in rats

Since the discovery of the role of the ubiquitin-proteasome system (UPS) in the pathogenesis of Parkinson's disease, UPS inhibitors, such as lactacystin have been used to investigate the relationship between UPS impairment and degeneration of dopamine (DA) neurons. However, mostly long-term neurotoxic effects of lactacystin have been studied in animal models. Therefore, the aim of our study was to investigate behavioral and biochemical changes related to the DA system during the first week following unilateral intranigral injection of lactacystin to rats. We found that lactacystin produced early spontaneous contralateral rotations which were inhibited by combined administration of DA D1 and D2 receptor antagonists. Simultaneously, an increase in the extracellular level of DA and its metabolites 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanilic acid (HVA) was found in the ipsilateral striatum. In contrast, one week after lesion, when turning behavior was no longer visible, a decrease in the extracellular level of DA, DOPAC and HVA was demonstrated. It was accompanied by a substantial reduction in the tissue levels of DA and its metabolites in the lesioned substantia nigra and striatum. We concluded that unilateral intranigral administration of lactacystin produces an early increase in DA neurotransmission which precedes a decrease in the striatal and nigral tissue DA content. It is manifested by the appearance of spontaneous contralateral rotations and an elevation of the extracellular DA level in the ipsilateral striatum. Since similar behavior was previously observed after intranigral administration of rotenone and MPP(+) but not 6-hydroxydopamine (6-OHDA), it may indicate a common mechanism of action shared by these neurotoxins.

Prosurvival effect of human wild-type alpha-synuclein on MPTP-induced toxicity to central but not peripheral catecholaminergic neurons isolated from transgenic mice

In the present work we report the generation of a new line of alpha-synuclein (alpha-SYN) transgenic mice in which the human wild-type alpha-SYN cDNA is expressed under the control of a tyrosine hydroxylase (TH) promoter. We provide evidence that the ectopic protein is found in TH expressing neurons of both central and peripheral nervous systems. The transgene is expressed very early in development coinciding with the activity of the TH promoter and in the adult brain the human protein distributes normally to the nerve endings and cell bodies of dopaminergic nigral neurons without any evidence of abnormal aggregation. Our results indicate that expression of human wild-type alpha-SYN does not affect normal development or maintenance of TH immunoreactive nigral neurons, striatal dopamine content, or locomotor activity. Systemic administration of the parkinsonian neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) induces a loss of TH immunoreactive nigral neurons and terminals and of dopamine levels to the same degree in both transgenic and non-transgenic adult mice. Intoxication also results in a similar loss of cardiac noradrenaline in both genotypes. Surprisingly, cultured transgenic ventral mesencephalic fetal dopaminergic neurons exhibit complete resistance to cell death induced by 1-methyl-4-phenylpyridinium ion (MPP(+)) intoxication, without changes in dopamine transporter (DAT) surface levels. Interestingly, this protection is not observed in other populations of catecholaminergic neurons such as peripheral sympathetic neurons, despite their high sensitivity to MPP(+)in vitro.

Intrastriatal infusion of the Parkinsonian neurotoxin, MPP+, induces damage of striatal cell nuclei in Sprague–Dawley rats

The potent Parkinsonian neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine is known to destroy dopaminergic neurons of the basal ganglia. Its neurotoxically active metabolite, 1-methyl-4-phenyl pyridinium (MPP(+)), has been examined in the present study to verify whether administration of the neurotoxin that depletes about 70% of the striatal dopamine (DA) can cause damage to nuclear components of the cells at the terminal region, the striatum. Unilateral intrastriatal infusion of MPP(+) (100 and 200 nmol in 4 microl saline) caused a dose-dependent depletion of striatal DA (69 and 92%, respectively), as measured employing HPLC electrochemistry. It also resulted in the loss of tyrosine hydroxylase (TH) immunoreactivity in the striatum and in the perikarya at substantia nigra pars compacta (SNpc) and acetylcholinesterase histoenzymological staining in the striatum. Specific nuclear staining employing Hoechst 33342 and acridine orange revealed distorted and spindle shaped nuclei, and perinuclear positioning of nucleolus, respectively, for the former and latter dyes in several of the cell populations in the ipsilateral striatum compared to the contralateral side. Existence of a widened lateral ventricle at the side that received the neurotoxin, as well as denser cellular population, as compared to the contralateral side under transmission electron microscope evidenced general shrinkage of the striatum. Extensive damage of the nuclei was visible in the cell bodies in the treated side. These results demonstrate non-specific damage extending to the cellular groups including cholinergic neurons in addition to dopaminergic neurons in the striatum to intrastriatal administration of the Parkinsonian neurotoxin, MPP(+).

Gene expression profiling in the midbrain of striatal 6-hydroxydopamine-injected mice

In order to clarify mechanisms underlying dopaminergic neuronal death in Parkinson's disease (PD), a gene expression profiling study was performed in a rodent model of PD. In this model, mice are intrastriatally injected with 6-hydroxydopamine (6-OHDA) and dopaminergic neurons in the substantia nigra (SN) gradually die by retrograde degeneration. The SN were removed 2 h, 24 h, or 14 days after 6-OHDA administration. Levels of mRNAs related to cell death or survival were quantified using adaptor-tagged competitive PCR (ATAC-PCR). The cyclin D1 gene showed an immediate increase in mRNA expression. After 24 h, when dopaminergic neurons were under intense degeneration, levels of caspase 8 mRNA and p53 apoptosis effecter related to pmp 22 (PERP) mRNA increased and, conversely, FAS mRNA decreased. After 14 days, when the degeneration was attenuated, levels of PERP mRNA and serum- and glucocorticoid-regulated kinase (SGK) mRNA still increased. SGK has a similarity to AKT, which is an important molecule involved in nerve growth factor signal transduction. AKT mRNA levels are low in dopaminergic neurons. These results suggest that an increase in cyclin D1 mRNA triggers dopaminergic neurons to enter an abnormal cell cycle, which leads to neuronal degeneration and cell death, possibly induced by PERP and caspase 8. In addition to cell death-related genes, several survival-related genes are activated. SGK might function as a key enzyme for the survival of dopaminergic neurons.

Behavioral improvement and dopamine release in a Parkinsonian rat model

Glial cell-line derived neurotrophic factor (GDNF) gene therapy might offer new strategies for the treatment of Parkinson's disease (PD). GDNF is a potent dopaminergic (DA) neurotrophic factor. The effect of GDNF gene therapy was assessed using anatomical, behavioral, and neurochemical approaches. We examined the protective effect of increased striatal GDNF levels achieved by delivery of an adenoviral vector (Ad-) encoding human GDNF (Ad-GDNF). Animals were injected with Ad-GDNF prior to striatal lesion. Striatal DA concentration was measured by microdialysis. Animals receiving 6-hydroxydopamine (6-OHDA) only showed a significant decrease in rotation when compared to those receiving Ad-GDNF prior to the 6-OHDA neurotoxin. Under basal conditions, the Ad-GDNF group showed a significant (P < or = 0.05) increase (1880%) in DA concentration when compared to the 6-OHDA group. Amphetamine challenge induced a significantly (P < or = 0.05) higher release of DA in the Ad-GDNF group than in the 6-OHDA group. These findings show that long-term delivery of GDNF protein in the striatum provides significant cell, behavioral, and neurochemical protection.

Presynaptic involvement in the nicotine prevention of the dopamine loss provoked by 6-OHDA administration in the substantia nigra

While nicotine, through stimulation of a specific sub-population of nicotinic acetylcholine receptors (nAChR) appears to protect cells in culture against a variety of insults, studies in vivo show controversial results. In a previous paper we have shown that in the 6-hydroxydopamine (6-OHDA) model of experimental parkinsonism, an intermittent administration schedule of nicotine (4 h before and 20, 44 and 68 h after 6-OHDA) was able to prevent the decrease of dopamine (DA) concentration in the corpus striatum (CS) provoked by the partial lesion of the substantia nigra (50% neuronal death after 6 micro g of 6-OHDA). To further analyze the mechanisms of nicotine effects, we performed a microdialysis study of striatal extracellular DA concentrations utilizing the nicotine administration schedule that was able to prevent DA decrease. Basal extracellular DA concentrations in the CS were maintained after 6-OHDA and were not modified by nicotine. Basal DOPAC levels were decreased after the neurotoxic administration. The response of extracellular DA to potassium chloride (KCl) challenge was significantly lower after 6-OHDA than in control animals. Nicotine significantly reversed this decrease. As previous studies have shown, the striatal DA terminals surviving the 6-OHDA toxic effect are able to keep extracellular DA concentrations close to normal, likely increasing DA synthesis. Nevertheless, the application of a releasing factor such as KCl shows the fragility of this equilibrium, exposing a decrease in the terminal number. Nicotine, through a further activation of tyrosine hydroxylase and DA synthesis or by prolonging the life of DA terminals, could reverse the effect of 6-OHDA.

The reverse transport of DA, what physiological significance?

It is well established that midbrain dopamine neurons innervating the striatum, release their neurotransmitter through an exocytotic process triggered by the neural firing and involving a transient calcium entry in the terminals. Long ago, it had been proposed, however, that another mechanism of release could co-exist with classical exocytosis, involving the reverse-transport of the cytosolic amine by the carrier, ordinarily responsible for uptake function. This atypical mode of release could be evoked directly at the preterminal level by multiple environmental endogenous factors involving transient alterations of the sodium gradient. It cannot be excluded that this mode of release participates in the firing-induced release. In contrast with the classical exocytosis of a preformed DA pool, the reverse-transport of DA requires simultaneous alterations of intraterminal amine metabolism including synthesis and displacement from storage compartment. The concept of a reverse-transport of dopamine is coming from the observations that releasing substances, such as amphetamine-related molecules, actually induce this type of transport. A large set of arguments advocates that reverse-transport plays a role in the maintenance of basal extracellular DA concentration in striatum. It was also often evoked in physiopathological situations including ischemia, neurodegenerative processes, etc. The most recent studies suggest that this release could occur mainly outside the synapses, and thus could constitute a major feature in the paracrine transmission, sometimes evoked for DA.

Regulation of GAP-43 protein and mRNA in nigrostriatal dopaminergic neurons after the partial destruction of dopaminergic terminals with intrastriatal 6-hydroxydopamine

Changes in the level of GAP-43 and its mRNA in nigrostriatal dopaminergic neurons in an animal model of the presymptomatic period of Parkinson's disease were measured to find the characteristic features of GAP-43 in nigrostriatal dopaminergic neurons. Since the dopaminergic neurons possess a relatively large amount of GAP-43 protein and mRNA, the dopaminergic neurons must be endowed with specific functions related to those of GAP-43. In this study, dopaminergic axon terminals were partially destroyed by intrastriatal 6-hydroxydopamine (6-OHDA). Rats were decapitated 3, 14, and 56 days following treatment. Levels of GAP-43 and tyrosine hydroxylase (TH) in the striatum were detected by immunoblotting and quantified. The number of GAP-43 mRNA-positive neurons and that of TH mRNA-positive neurons in the substantia nigra pars compacta (SNc) were detected by in situ hybridization using alkaline phosphatase (ALP)-labeled probes. Levels of GAP-43 in the striatum showed no significant alteration during the period of the experiment, although levels of TH were gradually restored. The number of GAP-43 mRNA-positive neurons as well as that of TH mRNA-positive neurons in the SNc decreased. These results suggests that dopaminergic neurons restore their axon terminals with little change in GAP-43, and that transcription and/or stability of GAP-43 mRNA in the dopaminergic neurons are susceptible to the toxin, although the dopaminergic neurons can maintain the translational product in the terminals. This feature may be related with a degeneration of dopaminergic neurons in Parkinson's disease.

Protective effect of quinacrine on striatal dopamine levels in 6-OHDA and MPTP models of Parkinsonism in rodents

Recent studies provide evidence that phospholipase A2 (PLA2) may play a role in the development of experimental parkinsonism. In this investigation an attempt was made to determine a possible protective effect of quinacrine (QNC), a PLA2 inhibitor on MPTP as well as 6-hydroxydopamine (6-OHDA)-induced neurotoxicity in rodents. For MPTP studies, adult male mice (C57 BL) were treated with MPTP (30 mg/kg, i.p.) daily for 5 days. QNC was injected i.p. in the doses of 0, 10, 30 and 60 mg/kg daily 30 min before MPTP in four different groups. Two other groups of mice received either vehicle (control) or a high dose of QNC (60 mg/kg). Two hours after the last injection of MPTP, striata were collected for the analysis of dopamine (DA), 3,4-dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA) and glutathione (GSH). For the 6-OHDA study, male Wistar rats were infused with 6-OHDA (60 microg) in the right striatum under chloral hydrate anesthesia. The rats in different groups were treated with 0, 5, 15 and 30 mg/kg QNC (i.p.) for 4 days, while first injection was given 30 min before 6-OHDA. On day 5, rats were sacrificed and striata were stored at -80 degrees C. Administration of MPTP or 6-OHDA significantly reduced striatal DA, which was significantly attenuated by QNC. Concomitant treatment with QNC also protected animals against MPTP or 6-OHDA-induced depletion of striatal GSH. Our findings clearly suggest the role of PLA2 in MPTP and 6-OHDA induced neurotoxicity and oxidative stress. However, further studies are warranted to explore the therapeutic potential of PLA2 inhibitors for the treatment of Parkinson's disease.

MPP(+)-induced mitochondrial dysfunction is potentiated by dopamine

MPP(+), the major metabolite of the Parkinsonism-inducing compound MPTP, responsible for the destruction of the nigrostriatal pathway in primates and rodents, has been assayed in isolated rat liver mitochondria in the presence of physiological concentrations of dopamine or analogous concentrations of melanin-dopamine. 5 microM MPP(+) in the presence of 70 microM dopamine or melanin-dopamine, but not alone, decreased the heat production and oxygen consumption of a mitochondrial suspension activated with succinate and ADP. Both dopamine and oxidized dopamine plus MPP(+) also decreased the mitochondrial reductive power measured with MTT. Mitochondrial swelling was observed, associated with an increase in membrane mitochondrial potential, as a synergistic effect between low concentrations of MPP(+) and dopamine. It is suggested that cytosolic dopamine, by itself or via its autooxidation products, may play a relevant role in the mitochondrial toxicity of MPP(+). A failure in the regulation of the storage/release of dopamine could aggravate a mitochondrial damage and trigger the neurodegenerative process underlying MPTP toxicity and Parkinson's disease.

Astrocyte line SVG-TH grafted in a rat model of Parkinson's disease

The present review describes gene transfer into the brain using extraneuronal cells with an ex vivo approach. The mild immunological reactions in the central nervous system to grafts provided the rationale and empirical basis for brain-transplantation, to replace dying cells, of potential clinical relevance. Fetal human astrocytes were genetically engineered to express tyrosine hydroxylase, the rate-limiting enzyme for the synthesis of catecholamines. These cells were also found to produce constitutively and secrete GDNF and interleukins. Therefore, these cells may prove as a drug-delivery system for the treatment of neurological degenerative conditions such as Parkinson's disease (PD). The field of neuronal reconstruction has reached a critical threshold and there is a need to evaluate the variables that will become critical as the field matures. One of the needs is to characterize the neurochemical alterations in the microenvironment in the context of grafted-host connectivity. This review discusses the functional effects of the pharmacologically-active construct, which consists of astrocytes producing L-DOPA and GDNF. The striatum in PD that lacks the dopaminergic projection from the substantia nigra metabolizes and releases dopamine differently from normal tissue and may react to different factors released by the grafted cells. Moreover, neurochemicals of the host tissue may effect grafted cells as well. An understanding of the way in which these neurochemicals are abnormal in PD and their role in the grafted brain is critical to the improvement of reconstructive strategies using cellular therapeutic strategies.

Intrastriatal grafts of fetal mesencephalic cell suspensions in MPP+-lesioned rats: a microdialysis study in vivo

The striatum of rats was lesioned by unilateral administration of MPP+. Two weeks later, a suspension of fetal mesencephalic cells (FMC), obtained from 14-day rat embryos, was injected into the lesioned striatum. Two weeks after grafting, the success of implantation and recovery of dopamine function were assessed by tyrosine hydroxylase immunocytochemistry (TH) and the measurement of striatal dopamine content. In addition, the extracellular concentrations of dopamine and dopamine metabolites were studied by microdialysis in vivo before and after perfusion of MPP+ to induce dopamine release from vesicular stores. TH+ cell bodies were seen in the lesioned grafted striata, indicating that fetal cells survived in these striata. In addition, there was a marked increase in TH-immunoreactivity in the neuronal fibers and terminals in the area surrounding the cell implant, suggesting a compensatory response of the host tissue which may involve fiber sprouting. Grafting induced a recovery in indices of dopamine function, including recovery in dopamine content, and basal and MPP+-induced dopamine release. Thus, grafts of FMC may provide a significant recovery of dopamine function in MPP+-lesioned striata.

7-Nitroindazole prevents dopamine depletion caused by low concentrations of MPP+ in rat striatal slices

A significant loss of dopamine was found in rat striatal slices incubated with 1-methyl-4-phenylpyridinium ion (MPP+) at a concentration of 2 microM or higher. The addition of 7-nitroindazole, a specific inhibitor of neuronal nitric oxide synthase (nNOS), prevented this effect on dopamine when the concentration of MPP+ was between 2-5 microM, but not at higher concentrations. This protection was reproduced with other less specific NOS-inhibitors, such as nitro-arginine and nitro-arginine methylester. 7-nitroindazole did not protect against the dopamine depletion caused by the non-specific mitochondrial chain blocker rotenone. Neither MPP- nor rotenone significantly increased the nitrite concentration in striatal slices, measured as an index of nitric oxide production. The basal production of nitric oxide may be enough to trigger the dopamine depletion at very low concentrations of MPP+, probably acting synergistically with cytosolic calcium increase. Higher concentrations of MPP+ are toxic by themselves without the mediation of nitric oxide. The inhibition of nNOS may protect against dopamine loss at early stages of a neurodegenerative process, and it could then be considered in the treatment or prevention of neurodegenerative human processes such as Parkinson's disease.

Striatal infarction in the rat causes a transient reduction of tyrosine hydroxylase immunoreactivity in the ipsilateral substantia nigra

Dopaminergic neurons of the substantia nigra pars compacta were examined in the rat brain following striatal infarction subsequent to transient focal cerebral ischemia. Rats had the middle cerebral artery occluded for 2 h or were sham-operated, and tyrosine hydroxylase immunoreactivity was evaluated by Western blot and immunohistochemistry at different times ranging from 1 to 60 days after ischemia. The number of tyrosine hydroxylase-immunoreactive cells in the substantia nigra pars compacta was counted under the light microscope and compared to that in the contralateral side and controls. No changes of tyrosine hydroxylase immunoreactivity were detected in the ipsilateral versus the contralateral substantia nigra of sham-operated rats or 1 day after ischemia. However, a statistically significant reduction of tyrosine hydroxylase-immunoreactive cells became apparent in the ipsilateral compared with the contralateral substantia nigra at 7 and 14 days after ischemia. This reduction showed a clear recovery at 30 days after ischemia, and no signs of difference between the ipsilateral and the contralateral side were apparent by 60 days. Therefore, the reduction of tyrosine hydroxylase immunoreactivity in the ipsilateral substantia nigra was only transiently seen from 1 to 2 weeks following ischemia. The observed loss of tyrosine hydroxylase was not accompanied by signs of cell death or gliosis in the ipsilateral pars compacta. The present results show a transitory reduction of tyrosine hydroxylase immunoreactivity in the ipsilateral substantia nigra pars compacta after focal ischemia and suggest that striatal infarction causes a transient deficit of dopaminergic function.

Cyclosporin A attenuates the decrease in tyrosine hydroxylase immunoreactivity in nigrostriatal dopaminergic neurons and in striatal dopamine content in rats with intrastriatal injection of 6-hydroxydopamine

To explore new therapeutic strategies for Parkinson's disease, we studied the possible protective effect of an immunosuppressant, cyclosporin A (CsA), treatment on changes in dopaminergic function in rats with intrastriatal injections of 6-hydroxydopamine (6-OHDA). Four weeks after injection of 6-OHDA, dopamine (DA) and dihydroxyphenylacetic acid in the striatum were depleted by 70-80%, and repeated high-dose CsA (20 mg/kg) treatment for 1 week significantly protected against these depletions. Tyrosine hydroxylase immunoreactivity (TH-IR) of the cell bodies in the substantia nigra pars compacta (SNc) ipsilateral to the injection were lower than on the contralateral side at 4 weeks but not at 1 week after 6-OHDA injection. The number of TH-positive cell bodies in the SNc decreased to 64% but CsA treatment increased this to 87%. The staining of microglia in the SN with OX42 and Griffonia simplicifolia B4 isolectin was intense at 3 days and gradually decreased by 28 days after injection. At 3 and 7 days after injection, the microglial staining in the SN was prominent and equal both in the 6-OHDA group and in ascorbic acid (SA)-injected controls. By 28 days postinjection, the staining had decreased to control levels in the SA group but was still above the control in the 6-OHDA group. CsA treatment did not affect this staining in either group. These results suggest that CsA protects against 6-OHDA-induced injury of nigrostriatal DA neurons by a mechanism not involving microglia.

Unlike MPP+, apoptosis induced by 6-OHDA in PC12 cells is independent of mitochondrial inhibition

The mechanisms of 6-hydroxydopamine (6-OHDA) cytotoxicity were studied in vitro using the PC12 cell line. Following a 24 h exposure, this neurotoxin induced apoptosis and a dose-dependent decrease in cell survival. The presence of monoamine oxidase inhibitors, tranylcypromine and clorgyline, together with 6-OHDA had neither synergistic nor protective effects. Unlike 1-methyl-4-phenylpyridinium (MPP+), 6-OHDA toxicity to PC12 cells remained unchanged when glycolysis was prevented by either depleting glucose from the culture medium or growing the cells in low-glucose medium containing 2-deoxy-glucose. These results suggest that the inhibition of mitochondrial respiration is not responsible for the cell death induced by 6-OHDA.

Animal models of Parkinson's disease: an empirical comparison with the phenomenology of the disease in man

Animal models are an important aid in experimental medical science because they enable one to study the pathogenetic mechanisms and the therapeutic principles of treating the functional disturbances (symptoms) of human diseases. Once the causative mechanism is understood, animal models are also helpful in the development of therapeutic approaches exploiting this understanding. On the basis of experimental and clinical findings. Parkinson's disease (PD) became the first neurological disease to be treated palliatively by neurotransmitter replacement therapy. The pathological hallmark of PD is a specific degeneration of nigral and other pigmented brainstem nuclei, with a characteristic inclusion, the Lewy body, in remaining nerve cells. There is now a lot of evidence that degeneration of the dopaminergic nigral neurones and the resulting striatal dopamine-deficiency syndrome are responsible for its classic motor symptoms akinesia and bradykinesia. PD is one of many human diseases which do not appear to have spontaneously arisen in animals. The characteristic features of the disease can however be more or less faithfully imitated in animals through the administration of various neurotoxic agents and drugs disturbing the dopaminergic neurotransmission. The cause of chronic nigral cell death in PD and the underlying mechanisms remain elusive. The partial elucidation of the processes underlie the selective action of neurotoxic substances such as 6-hydroxydopamine (6-OHDA) or 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), has however revealed possible molecular mechanisms that give rise to neuronal death. Accordingly, hypotheses concerning the mechanisms of these neurotoxines have been related to the pathogenesis of nigral cell death in PD. The present contribution starts out by describing some of the clinical, pathological and neurochemical phenomena of PD. The currently most important animal models (e.g. the reserpine model, neuroleptic-induced catalepsy, tremor models, experimentally-induced degeneration of nigrostriatal dopaminergic neurons with 6-OHDA, methamphetamine, MPTP, MPP+, tetrahydroisoquinolines, beta-carbolines, and iron) critically reviewed next, and are compared with the characteristic features of the disease in man.