Experimental models of Parkinson's disease: from the static to the dynamic

NMDA receptor remodeling and nNOS activation in mice after unilateral striatal injury with 6-OHDA

Parkinson's disease (PD) is a progressive neurodegenerative disorder characterized by selective dopaminergic loss. Non dopaminergic neurotransmitters such as glutamate are also involved in PD progression. NMDA receptor/postsynaptic density protein 95 (PSD-95)/neuronal nitric oxide synthase (nNOS) activation is involved in neuronal excitability in PD. Here, we are focusing on the evaluating these post-synaptic protein levels in the 6-OHDA model of PD. Adult male C57BL/6 mice subjected to unilateral striatal injury with 6-OHDA were assessed at 1-, 2-, or 4-weeks post-lesion. Animals were subjected to an apomorphine-induced rotation test followed by the analysis of protein content, synaptic structure, and NOx production. All biochemical analysis was performed comparing the control versus lesioned sides of the same animal. 6-OHDA mice exhibited contralateral rotation activity, difficulties in coordinating movements, and changes in Iba-1 and glial fibrillary acidic protein (GFAP) expression during the whole period. At one week of survival, the mice showed a shift in NMDA composition, favoring the GluN2A subunit and increased PSD95 and nNOS expression and NOx formation. After two-weeks, a decrease in the total number of synapses was observed in the lesioned side. However, the number of excitatory synapses was increased with a higher content of GluN1 subunit and PSD95. After four weeks, NMDA receptor subunits restored to control levels. Interestingly, NOx formation in the serum increased. This study reveals, for the first time, the temporal course of behavioral deficits and glutamatergic synaptic plasticity through NMDAr subunit shift. Together, these data demonstrate that dopamine depletion leads to a fine adaptive response over time, which can be used for further studies of therapeutic management adjustments with the progression of PD.

The role of voltage-gated calcium channels in the pathogenesis of Parkinson's disease

Aim: Voltage-gated calcium (CaV) channels play an essential role in maintaining calcium homeostasis and regulating numerous physiological processes in neurons. Therefore, dysregulation of calcium signaling is relevant in many neurological disorders, including Parkinson's disease (PD). This review aims to introduce the role of CaV channels in PD and discuss some novel aspects of channel regulation and its impact on the molecular pathophysiology of the disease.

Methods: an exhaustive search of the literature in the field was carried out using the PubMed database of The National Center for Biotechnology Information. Systematic searches were performed from the initial date of publication to May 2022.

Results: Although α-synuclein aggregates are the main feature of PD, L-type calcium (CaV1) channels seem to play an essential role in the pathogenesis of PD. Changes in the functional expression of CaV1.3 channels alter Calcium homeostasis and contribute to the degeneration of dopaminergic neurons. Furthermore, recent studies suggest that CaV channel trafficking towards the cell membrane depends on the activity of the ubiquitin-proteasome system (UPS). In PD, there is an increase in the expression of L-type channels associated with a decrease in the expression of Parkin, an E3 enzyme of the UPS. Therefore, a link between Parkin and CaV channels could play a fundamental role in the pathogenesis of PD and, as such, could be a potentially attractive target for therapeutic intervention.

Conclusion: The study of alterations in the functional expression of CaV channels will provide a framework to understand better the neurodegenerative processes that occur in PD and a possible path toward identifying new therapeutic targets to treat this condition.

6PPD-quinone exposure induces neuronal mitochondrial dysfunction to exacerbate Lewy neurites formation induced by α-synuclein preformed fibrils seeding

The emerging toxicant N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine quinone (6PPD-Q) is of wide concern due to its ubiquitous occurrence and high toxicity. Despite regular human exposure, limited evidence exists about its presence in the body and potential health risks. Herein, we analyzed cerebrospinal fluid (CSF) samples from Parkinson's disease (PD) patients and controls. The CSF levels of 6PPD-Q were twice as high in PD patients compared to controls. Immunostaining assays performed with primary dopaminergic neurons confirm that 6PPD-Q at environmentally relevant concentrations can exacerbate the formation of Lewy neurites induced by α-synuclein preformed fibrils (α-syn PFF). Assessment of cellular respiration reveals a considerable decrease in neuronal spare respiratory and ATP-linked respiration, potentially due to changes in mitochondrial membrane potential. Moreover, 6PPD-Q-induced mitochondrial impairment correlates with an upsurge in mitochondrial reactive oxygen species (mROS), and Mito-TEMPO-driven scavenging of mROS can lessen the amount of pathologic phospho-serine 129 α-synuclein. Untargeted metabolomics provides supporting evidence for the connection between 6PPD-Q exposure and changes in neuronal metabolite profiles. In-depth targeted metabolomics further unveils an overall reduction in glycolysis metabolite pool and fluctuations in the quantity of TCA cycle intermediates. Given its potentially harmful attributes, the presence of 6PPD-Q in human brain could potentially be a risk factor for PD.

Animal models of Parkinson's disease: a guide to selecting the optimal model for your research

Parkinson's disease (PD) is a complex, multisystem disorder characterised by α-synuclein (SNCA) pathology, degeneration of nigrostriatal dopaminergic neurons, multifactorial pathogenetic mechanisms and expression of a plethora of motor and non-motor symptoms. Animal models of PD have already been instructive in helping us unravel some of these aspects. However, much remains to be discovered, requiring continued interrogation by the research community. In contrast with the situation for many neurological disorders, PD benefits from of a wide range of available animal models (pharmacological, toxin, genetic and α-synuclein) but this makes selection of the optimal one for a given study difficult. This is especially so when a study demands a model that displays a specific combination of features. While many excellent reviews of animal models already exist, this review takes a different approach with the intention of more readily informing this decision-making process. We have considered each feature of PD in turn - aetiology, pathology, pathogenesis, motor dysfunctions and non-motor symptoms (NMS) - highlighting those animal models that replicate each. By compiling easily accessible tables and a summary figure, we aim to provide the reader with a simple, go-to resource for selecting the optimal animal model of PD to suit their research needs.

Changes in COX histochemistry in the brain of mice and rats exposed to chronic subcutaneous rotenone

Exposure of experimental animals to the mitochondrial toxin rotenone is considered to be a model of environmental progression of Parkinson's disease (PD). We investigated the differential vulnerability of various brain regions to generalized inhibition of complex I, induced by subcutaneous rotenone injections for the duration of 1, 3 and 7 days in both rats (2 mg/kg dosage) and mice (4 mg/kg dosage). To examine patterns of metabolic activity changes in the brain, histochemical evaluation of cytochrome C oxidase (COX) activity was performed in post mortem brain sections. Animals displayed a similar time course of neuronal loss in substantia nigra pars compacta (SNpc), reaching 44 % in mice and 42 % in rats by the 7th day. The pattern of COX activity changes, however, was different for the two species. In both experiments, metabolic changes were evident not only in the substantia nigra, but also in non-specific structures (cortex and hippocampus). In mice, a decrease in COX activity was shown mostly for the non-specific areas (V1 cortex and ventral hippocampus) after the single exposure to rotenone. Data from the experiment conducted on rats demonstrated both an acute metabolic decrease in mesencephalic structures (SNpc and nucleus ruber) after a single injection of rotenone and secondary changes in cortical structures (S1 cortex and dorsal hippocampus) after chronic 7 day exposure. These changes reflect the general effect of rotenone on neuronal metabolic rate.

An up-to-date overview of computational polypharmacology in modern drug discovery

INTRODUCTION

In recent years, computational polypharmacology has gained significant attention to study the promiscuous nature of drugs. Despite tremendous challenges, community-wide efforts have led to a variety of novel approaches for predicting drug polypharmacology. In particular, some rapid advances using machine learning and artificial intelligence have been reported with great success.

AREAS COVERED

In this article, the authors provide a comprehensive update on the current state-of-the-art polypharmacology approaches and their applications, focusing on those reports published after our 2017 review article. The authors particularly discuss some novel, groundbreaking concepts, and methods that have been developed recently and applied to drug polypharmacology studies.

EXPERT OPINION

Polypharmacology is evolving and novel concepts are being introduced to counter the current challenges in the field. However, major hurdles remain including incompleteness of high-quality experimental data, lack of in vitro and in vivo assays to characterize multi-targeting agents, shortage of robust computational methods, and challenges to identify the best target combinations and design effective multi-targeting agents. Fortunately, numerous national/international efforts including multi-omics and artificial intelligence initiatives as well as most recent collaborations on addressing the COVID-19 pandemic have shown significant promise to propel the field of polypharmacology forward.

Pharmacological Effects of Melatonin as Neuroprotectant in Rodent Model: A Review on the Current Biological Evidence

The progressive loss of structure and functions of neurons, including neuronal death, is one of the main factors leading to poor quality of life. Promotion of functional recovery of neuron after injury is a great challenge in neuroregenerative studies. Melatonin, a hormone is secreted by pineal gland and has antioxidative, anti-inflammatory, and anti-apoptotic properties. Besides that, melatonin has high cell permeability and is able to cross the blood-brain barrier. Apart from that, there are no reported side effects associated with long-term usage of melatonin at both physiological and pharmacological doses. Thus, in this review article, we summarize the pharmacological effects of melatonin as neuroprotectant in central nervous system injury, ischemic-reperfusion injury, optic nerve injury, peripheral nerve injury, neurotmesis, axonotmesis, scar formation, cell degeneration, and apoptosis in rodent models.

Local transgene expression and whole-body transgenesis to model brain diseases in nonhuman primate

Animal model is an essential tool in the life sciences research, notably in understanding the pathogenesis of the diseases and for further therapeutic intervention success. Rodents have been the most frequently used animals to model human disease since the establishment of gene manipulation technique. However, they remain inadequate to fully mimic the pathophysiology of human brain disease, partially due to huge differences between rodents and humans in terms of anatomy, brain function, and social behaviors. Nonhuman primates are more suitable in translational perspective. Thus, genetically modified animals have been generated to investigate neurologic and psychiatric disorders. The classical transgenesis technique is not efficient in that model; so, viral vector-mediated transgene delivery and the new genome-editing technologies have been promoted. In this review, we summarize some of the technical progress in the generation of an ad hoc animal model of brain diseases by gene delivery and real transgenic nonhuman primate.

Parkinson's disease research: adopting a more human perspective to accelerate advances

Parkinson's disease (PD) affects 1% of the population over 60 years old and, with global increases in the aging population, presents huge economic and societal burdens. The etiology of PD remains unknown; most cases are idiopathic, presumed to result from genetic and environmental risk factors. Despite 200 years since the first description of PD, the mechanisms behind initiation and progression of the characteristic neurodegenerative processes are not known. Here, we review progress and limitations of the multiple PD animal models available and identify advances that could be implemented to better understand pathological processes, improve disease outcome, and reduce dependence on animal models. Lessons learned from reducing animal use in PD research could serve as guideposts for wider biomedical research.

Animal models of early-stage Parkinson's disease and acute dopamine deficiency to study compensatory neurodegenerative mechanisms

Parkinson's disease is a common neurodegenerative disease characterized by a widely variety of motor and non-motor symptoms. While the motor deficits are only visible following a severe dopamine depletion, neurodegenerative process and some non-motor symptoms are manifested years before the motor deficits. Importantly, chronic degeneration of dopaminergic neurons leads to the development of compensatory mechanisms that play roles in the progression of the disease and the response to anti-parkinsonian therapies. The identification of these mechanisms will be of great importance for improving our understanding of factors with important contributions to the disease course and the underlying adaptive process. To date, most of the data obtained from animal models reflect the late, chronic, dopamine-depleted states, when compensatory mechanisms have already been established. Thus, adequate animal models with which researchers are able to dissect early- and late-phase mechanisms are necessary. Here, we reviewed the literature related to animal models of early-stage PD and pharmacological treatments capable of inducing acute dopamine impairments and/or depletion, such as reserpine, haloperidol and tetrodotoxin. We highlighted the advantages, limitations and the future prospective uses of these models, as well as their applications in the identification of novel agents for treating this neurological disorder.

Polypharmacology by Design: A Medicinal Chemist's Perspective on Multitargeting Compounds

Multitargeting compounds comprising activity on more than a single biological target have gained remarkable relevance in drug discovery owing to the complexity of multifactorial diseases such as cancer, inflammation, or the metabolic syndrome. Polypharmacological drug profiles can produce additive or synergistic effects while reducing side effects and significantly contribute to the high therapeutic success of indispensable drugs such as aspirin. While their identification has long been the result of serendipity, medicinal chemistry now tends to design polypharmacology. Modern in vitro pharmacological methods and chemical probes allow a systematic search for rational target combinations and recent innovations in computational technologies, crystallography, or fragment-based design equip multitarget compound development with valuable tools. In this Perspective, we analyze the relevance of multiple ligands in drug discovery and the versatile toolbox to design polypharmacology. We conclude that despite some characteristic challenges remaining unresolved, designed polypharmacology holds enormous potential to secure future therapeutic innovation.

Nanoparticles restore lysosomal acidification defects: Implications for Parkinson and other lysosomal-related diseases

Lysosomal impairment causes lysosomal storage disorders (LSD) and is involved in pathogenesis of neurodegenerative diseases, notably Parkinson disease (PD). Strategies enhancing or restoring lysosomal-mediated degradation thus appear as tantalizing disease-modifying therapeutics. Here we demonstrate that poly(DL-lactide-co-glycolide) (PLGA) acidic nanoparticles (aNP) restore impaired lysosomal function in a series of toxin and genetic cellular models of PD, i.e. ATP13A2-mutant or depleted cells or glucocerebrosidase (GBA)-mutant cells, as well as in a genetic model of lysosomal-related myopathy. We show that PLGA-aNP are transported to the lysosome within 24 h, lower lysosomal pH and rescue chloroquine (CQ)-induced toxicity. Re-acidification of defective lysosomes following PLGA-aNP treatment restores lysosomal function in different pathological contexts. Finally, our results show that PLGA-aNP may be detected after intracerebral injection in neurons and attenuate PD-related neurodegeneration in vivo by mechanisms involving a rescue of compromised lysosomes.

FosB and ΔFosB expression in brain regions containing differentially susceptible dopamine neurons following acute neurotoxicant exposure

Parkinson disease (PD) is characterized by progressive neuronal degeneration, in particular nigrostriatal dopamine (NSDA) neurons and consequent deficits in movement. In mice and non-human primates, NSDA neurons preferentially degenerate following exposure to the neurotoxicant 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). Tuberoinfundibular (TI) DA neurons, in contrast, appear to be unaffected in PD and recover following acute MPTP exposure-induced injury (Behrouz et al., 2007; Benskey et al., 2012). The recovery of the TIDA neurons is dependent on de novo protein synthesis and positively correlated with an increase in parkin mRNA and protein expression (Benskey et al., 2012, 2015). Inhibition of parkin upregulation renders TIDA neurons susceptible to degeneration following MPTP exposure. In addition to parkin, other potentially protective proteins are likely to be differentially regulated in TIDA and NSDA neurons following neurotoxicant exposure. The regulation of potential transcription factors for parkin and other neuroprotective pathway genes are of interest since they may provide novel targets for PD disease modifying therapies. As such, we sought to determine if there are time-dependent differences in the expression of AP-1 transcription factors c-Fos, c-Jun, FosB, ΔFosB and JunD in TIDA and NSDA neurons of mice following acute MPTP exposure. We observed that both FosB and ΔFosB expression increase in brain regions containing TIDA, but not NSDA neurons. Furthermore, the nuclear and long-term expression of ΔFosB is consistent with its role as a transcription factor that may influence parkin transcription, which may underlie the unique ability of TIDA neurons to recovery from an injury that leads NSDA neurons to degeneration.

Modeling Parkinson's disease in the common marmoset (Callithrix jacchus): overview of models, methods, and animal care

The common marmoset (Callithrix jacchus) is a small-bodied, popular New World monkey and is used widely in reproductive biology, neuroscience, and drug development, due to its comparative ease of handling, high reproductive efficiency, and its unique behavioral characters. In this review, we discuss the marmoset models in Parkinson's disease (PD), which is a neurological movement disorder primarily resulting from a degeneration of dopaminergic neurons with clinical features of tremor, rigidity, postural instability, and akinesia. The most common PD models involve the administration of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) or 6-hydroxydopamine to study the pathogenesis and to evaluate novel therapies. Following the systemic or local administration of these neurotoxins, the marmosets with very severe Parkinson's symptoms are recommended to be placed in an intensive care unit with artificial feeding to increase survival rate. All procedures with MPTP should be conducted in a special room with enclosed cages under negative-pressure by trained researchers with personal protection. Behavioral tests are conducted to provide an external measure of the brain pathology. Along with several biomarkers, including α-synuclein and DJ-1, non-invasive neuroimaging techniques such as positron emission tomography and magnetic resonance imaging are used to evaluate the functional changes associated with PD. With the recent growing interest in potential and novel therapies such as stem cell and gene therapy for PD in Korea, the marmoset can be considered as a suitable non-human primate model in PD research to bridge the gap between rodent studies and clinical applications.

Pathophysiology of L-dopa-induced motor and non-motor complications in Parkinson's disease

Involuntary movements, or dyskinesia, represent a debilitating complication of levodopa (L-dopa) therapy for Parkinson's disease (PD). L-dopa-induced dyskinesia (LID) are ultimately experienced by the vast majority of patients. In addition, psychiatric conditions often manifested as compulsive behaviours, are emerging as a serious problem in the management of L-dopa therapy. The present review attempts to provide an overview of our current understanding of dyskinesia and other L-dopa-induced dysfunctions, a field that dramatically evolved in the past twenty years. In view of the extensive literature on LID, there appeared a critical need to re-frame the concepts, to highlight the most suitable models, to review the central nervous system (CNS) circuitry that may be involved, and to propose a pathophysiological framework was timely and necessary. An updated review to clarify our understanding of LID and other L-dopa-related side effects was therefore timely and necessary. This review should help in the development of novel therapeutic strategies aimed at preventing the generation of dyskinetic symptoms.

MPTP-induced changes in hippocampal synaptic plasticity and memory are prevented by memantine through the BDNF-TrkB pathway

BACKGROUND AND PURPOSE

Mild cognitive deficit in early Parkinson's disease (PD) has been widely studied. Here we have examined the effects of memantine in preventing memory deficit in experimental PD models and elucidated some of the underlying mechanisms.

EXPERIMENTAL APPROACHES

I.p. injection of 1-methyl-4- phenyl-1,2,3,6-tetrahydro pyridine (MPTP) in C57BL/6 mice was used to produce models of PD. We used behavioural tasks to test memory. In vitro, we used slices of hippocampus, with electrophysiological, Western blotting, real time PCR, elisa and immunochemical techniques.

KEY RESULTS

Following MPTP injection, long-term memory was impaired and these changes were prevented by pre-treatment with memantine. In hippocampal slices from MPTP treated mice, long-term potentiation (LTP) -induced by θ burst stimulation (10 bursts, 4 pulses) was decreased, while long-term depression (LTD) induced by low-frequency stimulation (1 Hz, 900 pulses) was enhanced, compared with control values. A single dose of memantine (i.p., 10 mg·kg(-1) ) reversed the decreased LTP and the increased LTD in this PD model. Activity-dependent changes in tyrosine kinase receptor B (TrkB), ERK and brain-derived neurotrophic factor (BDNF) expression were decreased in slices from mice after MPTP treatment. These effects were reversed by pretreatment with memantine. Incubation of slices in vitro with 1-methyl-4-phenylpyridinium (MPP(+) ) decreased depolarization-induced expression of BDNF. This effect was prevented by pretreatment of slices with memantine or with calpain inhibitor III, suggesting the involvement of an overactivated calcium signalling pathway.

CONCLUSIONS AND IMPLICATIONS

Memantine should be useful in preventing loss of memory and hippocampal synaptic plasticity in PD models.

Extrapyramidal system neurotoxicity: animal models

The central nervous system's extrapyramidal system provides involuntary motor control to the muscles of the head, neck, and limbs. Toxicants that affect the extrapyramidal system are generally clinically characterized by impaired motor control, which is usually the result of basal ganglionic dysfunction. A variety of extrapyramidal syndromes are recognized in humans and include Parkinson's disease, secondary parkinsonism, other degenerative diseases of the basal ganglia, and clinical syndromes that result in dystonia, dyskinesia, essential tremor, and other forms of tremor and chorea. This chapter briefly reviews the anatomy of the extrapyramidal system and discusses several naturally occurring and experimental models that target the mammalian (nonhuman) extrapyramidal system. Topics discussed include extrapyramidal syndromes associated with antipsychotic drugs, carbon monoxide, reserpine, cyanide, rotenone, paraquat, 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), and manganese. In most cases, animals are used as experimental models to improve our understanding of the toxicity and pathogenesis of these agents. Another agent discussed in this chapter, yellowstar thistle poisoning in horses, however, represents an important spontaneous cause of parkinsonism that naturally occurs in animals. The central focus of the chapter is on animal models, especially the concordance between clinical signs, neurochemical changes, and neuropathology between animals and people.

Tea polyphenols alleviate motor impairments, dopaminergic neuronal injury, and cerebral α-synuclein aggregation in MPTP-intoxicated parkinsonian monkeys

Tea polyphenols (TPs) are bioactive flavanol-related catechins that have been shown to protect dopaminergic (DAergic) neurons against neurotoxin-induced injury in mouse Parkinson's disease (PD) models. However, the neuroprotective efficacy of TP has not been investigated in nonhuman PD primates, which can more accurately model the neuropathology and motor impairments of human PD patients. Here, we show that oral administration of TP alleviates motor impairments and DAergic neuronal injury in the substantia nigra in N-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-intoxicated PD monkeys, indicating an association between protection against motor deficits and preservation of DAergic neurons. We also show a significant inhibition of MPTP-induced accumulation of neurotoxic α-synuclein (α-syn) oligomers in the striatum and other brain regions, which may contribute to the neuroprotection and improved motor function conferred by TP. The association between reduced α-syn oligomerization and neuroprotection was confirmed in cultured DAergic cells. The most abundant and bioactive TP in the mixture used in vivo, (-)-epigallocatechin-3-gallate, reduced intracellular levels of α-syn oligomers in neurons treated with α-syn oligomers, 1-methyl-4-phenylpyridiniumion, or both, accompanied by increased cell viability. The present study provides the first evidence that TP can alleviate motor impairments, DAergic neuronal injury, and α-syn aggregation in nonhuman primates.

Time perception impairs sensory-motor integration in Parkinson's disease

It is well known that perception and estimation of time are fundamental for the relationship between humans and their environment. However, this temporal information processing is inefficient in patients with Parkinson' disease (PD), resulting in temporal judgment deficits. In general, the pathophysiology of PD has been described as a dysfunction in the basal ganglia, which is a multisensory integration station. Thus, a deficit in the sensorimotor integration process could explain many of the Parkinson symptoms, such as changes in time perception. This physiological distortion may be better understood if we analyze the neurobiological model of interval timing, expressed within the conceptual framework of a traditional information-processing model called "Scalar Expectancy Theory". Therefore, in this review we discuss the pathophysiology and sensorimotor integration process in PD, the theories and neural basic mechanisms involved in temporal processing, and the main clinical findings about the impact of time perception in PD.

Modeling Parkinson's disease in primates: The MPTP model

The 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) primate models of Parkinson's disease (PD) reproduce most, although not all, of the clinical and pathological hallmarks of PD. The present contribution presents the possibilities offered by the MPTP monkey models of PD to readers with minimal knowledge of PD, emphasizing the diversity of species, route and regimen of administration, symptoms and pathological features. Readers would eventually find out that there is not a single MPTP monkey model of PD but instead MPTP monkey models of PD, each addressing a specific experimental need.

Evidence for a critical role of catecholamines for cardiomyocyte lineage commitment in murine embryonic stem cells

Catecholamine release is known to modulate cardiac output by increasing heart rate. Although much is known about catecholamine function and regulation in adults, little is known about the presence and role of catecholamines during heart development. The present study aimed therefore to evaluate the effects of different catecholamines on early heart development in an in vitro setting using embryonic stem (ES) cell-derived cardiomyocytes. Effects of catecholamine depletion induced by reserpine were examined in murine ES cells (line D3, αPIG44) during differentiation. Cardiac differentiation was assessed by immunocytochemistry, qRT-PCR, quantification of beating clusters, flow cytometry and pharmacological approaches. Proliferation was analyzed by EB cross-section measurements, while functionality of cardiomyocytes was studied by extracellular field potential (FP) measurements using microelectrode arrays (MEAs). To further differentiate between substance-specific effects of reserpine and catecholamine action via α- and β-receptors we proved the involvement of adrenergic receptors by application of unspecific α- and β-receptor antagonists. Reserpine treatment led to remarkable down-regulation of cardiac-specific genes, proteins and mesodermal marker genes. In more detail, the average ratio of ∼40% spontaneously beating control clusters was significantly reduced by 100%, 91.1% and 20.0% on days 10, 12, and 14, respectively. Flow cytometry revealed a significant reduction (by 71.6%, n = 11) of eGFP positive CMs after reserpine treatment. By contrast, reserpine did not reduce EB growth while number of neuronal cells in reserpine-treated EBs was significantly increased. MEA measurements of reserpine-treated EBs showed lower FP frequencies and weak responsiveness to adrenergic and muscarinic stimulation. Interestingly we found that developmental inhibition after α- and β-adrenergic blocker application mimicked developmental changes with reserpine. Using several methodological approaches our data suggest that reserpine inhibits cardiac differentiation. Thus catecholamines play a critical role during development.

IRC-082451, a novel multitargeting molecule, reduces L-DOPA-induced dyskinesias in MPTP Parkinsonian primates

The development of dyskinesias following chronic L-DOPA replacement therapy remains a major problem in the long-term treatment of Parkinson's disease. This study aimed at evaluating the effect of IRC-082451 (base of BN82451), a novel multitargeting hybrid molecule, on L-DOPA-induced dyskinesias (LIDs) and hypolocomotor activity in a non-human primate model of PD. IRC-082451 displays multiple properties: it inhibits neuronal excitotoxicity (sodium channel blocker), oxidative stress (antioxidant) and neuroinflammation (cyclooxygenase inhibitor) and is endowed with mitochondrial protective properties. Animals received daily MPTP injections until stably parkinsonian. A daily treatment with increasing doses of L-DOPA was administered to parkinsonian primates until the appearance of dyskinesias. Then, different treatment regimens and doses of IRC-082451 were tested and compared to the benchmark molecule amantadine. Primates were regularly filmed and videos were analyzed with specialized software. A novel approach combining the analysis of dyskinesias and locomotor activity was used to determine efficacy. This analysis yielded the quantification of the total distance travelled and the incidence of dyskinesias in 7 different body parts. A dose-dependent efficacy of IRC-082451 against dyskinesias was observed. The 5 mg/kg dose was best at attenuating the severity of fully established LIDs. Its effect was significantly different from that of amantadine since it increased spontaneous locomotor activity while reducing LIDs. This dose was effective both acutely and in a 5-day sub-chronic treatment. Moreover, positron emission tomography scans using radiolabelled dopamine demonstrated that there was no direct interference between treatment with IRC-082451 and dopamine metabolism in the brain. Finally, post-mortem analysis indicated that this reduction in dyskinesias was associated with changes in cFOS, FosB and ARC mRNA expression levels in the putamen. The data demonstrates the antidyskinetic efficacy of IRC-082451 in a primate model of PD with motor complications and opens the way to the clinical application of this treatment for the management of LIDs.

Animal models of Parkinson's disease: limits and relevance to neuroprotection studies

Over the last two decades, significant strides has been made toward acquiring a better knowledge of both the etiology and pathogenesis of Parkinson's disease (PD). Experimental models are of paramount importance to obtain greater insights into the pathogenesis of the disease. Thus far, neurotoxin-based animal models have been the most popular tools employed to produce selective neuronal death in both in vitro and in vivo systems. These models have been commonly referred to as the pathogenic models. The current trend in modeling PD revolves around what can be called the disease gene-based models or etiologic models. The value of utilizing multiple models with a different mechanism of insult rests on the premise that dopamine-producing neurons die by stereotyped cascades that can be activated by a range of insults, from neurotoxins to downregulation and overexpression of disease-related genes. In this position article, we present the relevance of both pathogenic and etiologic models as well as the concept of clinically relevant designs that, we argue, should be utilized in the preclinical development phase of new neuroprotective therapies before embarking into clinical trials.

A tale on animal models of Parkinson's disease

Parkinson's disease is a neurodegenerative disorder whose cardinal manifestations are due primarily to a profound deficit in brain dopamine. Since the 1980s, several therapeutic strategies have been discovered to treat the symptoms of this neurological disorder, but as of yet, none halts or retards the neurodegenerative process. In an attempt to shed light on the neurobiology of Parkinson's disease, a number of experimental models have been developed, especially during the last 25 years. They come essentially in 3 flavors: pharmacological (eg, reserpine), toxic (eg, 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine), and genetic (eg, transgenic synuclein mice). These models can also be recast as etiologic, pathogenic, and symptomatic/pathophysiologic, as each may contribute to our understanding of the cause, the mechanisms, and the treatment of Parkinson's disease. In this review, we will discuss the question of Parkinson's disease models, starting from the period when this journal was born to today. During this journey of 25 years, we will discuss both the significant contributions of the Parkinson's disease models and hurdles that remain to be overcome to one day cure this neurological disease.

A guide to neurotoxic animal models of Parkinson's disease

Parkinson's disease (PD) is a neurological movement disorder primarily resulting from damage to the nigrostriatal dopaminergic pathway. To elucidate the pathogenesis, mechanisms of cell death, and to evaluate therapeutic strategies for PD, numerous animal models have been developed. Understanding the strengths and limitations of these models can significantly impact the choice of model, experimental design, and data interpretation. The primary objectives of this article are twofold: First, to assist new investigators who are contemplating embarking on PD research to navigate through the available animal models. Emphasis will be placed on common neurotoxic murine models in which toxic molecules are used to lesion the nigrostriatal dopaminergic system. And second, to provide an overview of basic technical requirements for assessing the pathology, structure, and function of the nigrostriatal pathway.

Neuroanatomical study of the A11 diencephalospinal pathway in the non-human primate

BACKGROUND

The A11 diencephalospinal pathway is crucial for sensorimotor integration and pain control at the spinal cord level. When disrupted, it is thought to be involved in numerous painful conditions such as restless legs syndrome and migraine. Its anatomical organization, however, remains largely unknown in the non-human primate (NHP). We therefore characterized the anatomy of this pathway in the NHP.

METHODS AND FINDINGS

In situ hybridization of spinal dopamine receptors showed that D1 receptor mRNA is absent while D2 and D5 receptor mRNAs are mainly expressed in the dorsal horn and D3 receptor mRNA in both the dorsal and ventral horns. Unilateral injections of the retrograde tracer Fluoro-Gold (FG) into the cervical spinal enlargement labeled A11 hypothalamic neurons quasi-exclusively among dopamine areas. Detailed immunohistochemical analysis suggested that these FG-labeled A11 neurons are tyrosine hydroxylase-positive but dopa-decarboxylase and dopamine transporter-negative, suggestive of a L-DOPAergic nucleus. Stereological cell count of A11 neurons revealed that this group is composed by 4002±501 neurons per side. A 1-methyl-4-phenyl-1, 2, 3, 6-tetrahydropyridine (MPTP) intoxication with subsequent development of a parkinsonian syndrome produced a 50% neuronal cell loss in the A11 group.

CONCLUSION

The diencephalic A11 area could be the major source of L-DOPA in the NHP spinal cord, where it may play a role in the modulation of sensorimotor integration through D2 and D3 receptors either directly or indirectly via dopamine formation in spinal dopa-decarboxylase-positives cells.

Antagonists of the human A(2A) receptor. Part 5: Highly bio-available pyrimidine-4-carboxamides

A novel series of antagonists of the human A(2A) receptor have been identified and have been shown to display good potency and high degrees of selectivity over other receptor sub-types. Displaying in vivo potency in commonly used disease models and high oral bio-availability, this class of compounds may serve as clinically useful treatments for the relief of the symptoms associated with Parkinson's disease.

Antagonists of the human A(2A) adenosine receptor. 4. Design, synthesis, and preclinical evaluation of 7-aryltriazolo[4,5-d]pyrimidines

Antagonism of the human A(2A) receptor has been implicated as a point of therapeutic intervention in the alleviation of the symptoms associated with Parkinson's disease. This is thought to occur, at least in part, by increasing the sensitivity of the dopaminergic neurons to the residual, depleted levels of striatal dopamine. We herein describe a novel series of functionalized triazolo[4,5-d]pyrimidine derivatives that display functional antagonism of the A(2A) receptor. Optimization of these compounds has resulted in improvements in potency, selectivity, and the pharmacokinetic properties of key derivatives. These efforts have led to the discovery of 60 (V2006/BIIB014), which demonstrates strong oral activity in commonly used models of Parkinson's disease. Furthermore, this derivative has shown excellent preclinical pharmacokinetics and has successfully completed phase I clinical studies. This compound is presently undergoing further clinical evaluation in collaboration with Biogen Idec.

Priming for l-dopa-induced dyskinesia in Parkinson's disease: a feature inherent to the treatment or the disease?

Involuntary movements, or dyskinesia, represent a debilitating complication of levodopa therapy for Parkinson's disease ultimately experienced by the vast majority of patients. This article does not review the increased understanding of dyskinesia pathophysiology we have seen during the past few years but, instead, specifically focuses upon the very first molecular events thought to be responsible for the establishment of dyskinesia and generally grouped under the term of "priming". Priming is classically defined as the process by which the brain becomes sensitized such that administration of a dopaminergic therapy modifies the response to subsequent dopaminergic treatments. In this way, over time, with repeated treatment, the chance of dopaminergic stimulation eliciting dyskinesia is increased and once dyskinesia has been established, the severity of dyskinesia increases. In this opinion review, however, we aim at strongly opposing the common view of priming. We propose, and hopefully will demonstrate, that priming does not exist per se but is the direct and intrinsic consequence of the loss of dopamine innervation of the striatum (and other target structures), meaning that the first injections of dopaminergic drugs only exacerbate those mechanisms (sensitization) but do not induce them. Chronicity and pulsatility of subsequent dopaminergic treatment only exacerbates the likelihood of developing dyskinesia.

Antagonists of the human adenosine A2A receptor. Part 3: Design and synthesis of pyrazolo[3,4-d]pyrimidines, pyrrolo[2,3-d]pyrimidines and 6-arylpurines

A series of pyrazolo[3,4-d]pyrimidine, pyrrolo[2,3-d]pyrimidine and 6-arylpurine adenosine A(2A) antagonists is described. Many examples were highly selective against the human A(1) receptor sub-type and were active in an in vivo model of Parkinson's disease.

A call for clinically driven experimental design in assessing neuroprotection in experimental Parkinsonism

Parkinson's disease is a progressive neurodegenerative disorder. At present, only symptomatic treatments are of proven efficacy, whereas strategies that slow or stop the neurodegenerative process are currently not available. The selection of interesting drug candidates or surgical strategies should be based on the soundest clinically driven preclinical validation. My goal here is not to discuss the relative merits of the available models, but to simply raise the issue of the experimental design that has led to the demonstration of efficacy of given compounds in these models. As some compounds previously shown to be active in classic experimental designs fail when tested in clinically relevant experimental designs, I emphasize the need for progressive screening methods and for the use of different animal species before entering into the clinic.

Asymmetrically lesioned mesencephalon in healthy rodents: call for caution

Stereological counting of tyrosine-hydroxylase immunoreactive (TH-IR) neurons in the mesencephalon is a pivotal parameter in assessing the extent of lesioning in animal models of Parkinson's disease. We here show that the number of TH-IR neurons often appears abnormally decreased in healthy--commercially available--mice and rats, although both the number of Nissl-stained cells and the striatal dopaminergic innervation are unaffected. This potential bias in assessing extent of neurotoxin-induced lesion and subsequent protection by pharmacological manipulation prompts us to call for caution in setting up experimental designs.

sgk1, a member of an RNA cluster associated with cell death in a model of Parkinson's disease

In an effort to gain deeper insight into the molecular processes underlying neurodegeneration in Parkinson's disease, we performed gene expression profiling at several early time points after MPTP-injection into old (1-year) mice. We used a PCR-based gene expression profiling method, digital expression pattern display (DEPD), a method of very high sensitivity and reproducibility, which displays almost all transcripts of a tissue. To identify cell death-associated genes, we defined clusters of differentially expressed transcripts with expression behaviour that correlated with the temporal profile of cell death progression and characterized one of these cell death clusters further. We selected one of the strongest regulated genes, the serum and glucocorticoid-regulated kinase 1 (sgk1), and validated its differential expression by Northern blot analysis, semiquantitative PCR and in situ hybridization. Up-regulation of sgk1 (i) coincides with the onset of dopaminergic cell death in both the 8-week acute and 1-year subacute MPTP models, (ii) spans the entire brain, (iii) is attenuated by the l-deprenyl-mediated inhibition of the MPTP conversion to its active metabolite MPP+ and (iv) is not induced by dehydration. This study demonstrated that the combination of the DEPD technology, clustering analysis and a detailed histopathology is a useful tool for elucidating molecular pathways in neurodegenerative diseases.

Recent failures of new potential symptomatic treatments for Parkinson's disease: causes and solutions

One major goal of current research in Parkinson's disease (PD) is the discovery of novel agents to improve symptomatic management. The object of these new treatments should be to provide effective symptom control throughout the course of the disease without the development of side effects such as motor and psychiatric complications. Results of several clinical trials of new treatment options reported in the past 2 years have shown negative or unsatisfactory results. Most of the drugs and surgical procedures used in these studies had been tested previously in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) monkeys as well as in the classic 6-hydroxydopamine-lesioned rat model. They raise several questions about the true reliability of animal studies, the adequacy of the working hypotheses and design of clinical trials, the validity of tools in current use to evaluate a specific effect, and the selectivity of the drugs used. All these factors may explain failure. This review focuses on pharmacological and surgical treatments tested to improve the management of patients with motor fluctuations and dyskinesias. Some of the recent trials and possible reasons for their lack of success are critically analysed. Finally, some suggestions to avoid further failures and improve results are proposed.

Gradients of dopamine D1- and D2/3-binding sites in the basal ganglia of pig and monkey measured by PET

The distributions of dopamine D1 and D2/3 binding sites in living brain have not been compared directly using positron emission tomography (PET). To map these binding sites, we first optimized methods for the assay of dopamine receptors in brain of Göttingen miniature pigs. The binding potentials (pB) of [11C]NNC 112 for dopamine D1 receptors and [11C]raclopride for dopamine D2/3 receptors were calculated in pig striatum volumes of interest using metabolite corrected arterial inputs or using cerebellum as a non-binding reference region. Depending upon the method for quantitation, the pB for [11C]NNC 112 was 1.2-5.1 in pig striatum, whereas the pB for [11C]raclopride was 1.0-1.8. We used the reference tissue method of Logan to calculate pB maps for the two tracers. The maps were co-registered to the common stereotaxic space for the pig brain and normalized to a global mean for pB in striatum; t-maps showed that dopamine D1 binding was relatively more abundant in the ventral-anterior striatum of the pig, while dopamine D2/3 binding was greater in the dorsal striatum. Similar comparisons were made for the pBs of [11C]Sch 23390 for dopamine D1 receptors and for [11C]raclopride in the brain of six rhesus monkeys. The magnitudes of pB for both binding sites in monkey brain were close to those in the pig. Consistent with the pig results, there were distinct gradients in the distributions of the two binding sites in monkey brain: D1 binding predominated in the ventral striatum, whereas D2/3 binding was relatively greater in the dorsal-posterior striatum. Gradients of dopamine receptor concentration within the striatum may be a general phenomenon of mammalian brain.

Neurotrophic actions of the novel AMPA receptor potentiator, LY404187, in rodent models of Parkinson's disease

Recent developments in the molecular biology and pharmacology of alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptors has led to the discovery of selective, potent and systemically active AMPA receptor potentiators. These molecules enhance synaptic transmission and evidence suggests that they play important roles in plasticity and cognitive processes. Activation of AMPA receptors also increases neuronal activation and activity-dependent signalling, which may increase brain-derived neurotrophic factor (BDNF) expression and enhance cell proliferation in the brain. We therefore hypothesised that an AMPA receptor potentiator may provide neurotrophic effects in rodent models of Parkinson's disease. In the present studies we report that the potent and selective AMPA receptor potentiator, R,S-N-2-(4-(4-Cyanophenyl)phenyl)propyl 2-propanesulfonamide (LY404187), provides both functional, neurochemical and histological protection against unilateral infusion of 6-hydroxydopamine into the substantia nigra or striatum of rats. The compound also reduced 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced toxicity in mice. Interestingly, we were also able to observe large functional and histological effects when we delayed treatment until after cell death had occurred (3 or 6 days after 6-hydroxydopamine infusion), supporting a neurotrophic mechanism of action. In addition, LY404187 provided a dose-dependent increase in growth-associated protein-43 expression in the striatum. Therefore, we propose that AMPA receptor potentiators offer the potential of a new therapy to halt the progression and perhaps repair the degeneration in Parkinson's disease.

The contribution of the MPTP-treated primate model to the development of new treatment strategies for Parkinson's disease

Current research into Parkinson's disease (PD) is directed at developing novel agents and strategies for improved symptomatic management. The aim of this research is to provide effective and maintained symptom control throughout the course of the disease without loss of efficacy and without priming the basal ganglia for the onset of dyskinesia. To achieve these objectives, it is important to have relevant animal models of PD in which new pharmacological agents and treatment strategies can be assessed prior to clinical assessment. At present, the most effective experimental model of PD is the methyl phenyl tetrahydropyridine (MPTP)-treated primate. Primates treated with MPTP develop motor disturbances resembling those seen in idiopathic PD, including bradykinesia, rigidity and postural abnormalities. In addition, MPTP-treated primates are responsive to all commonly used antiparkinsonian agents and display treatment-associated motor complications such as dyskinesia, wearing-off and on-off, which occur during the long-term treatment of the illness. This review examines how studies conducted in MPTP-treated primates have contributed to the development of dopaminergic therapies. There is now accumulating evidence that the pulsatile manner in which short-acting agents stimulate striatal dopamine receptors is a key contributing factor to the priming of the basal ganglia for dyskinesia induction. It has been suggested that providing more continuous stimulation of dopamine receptors will avoid the development of motor complications, particularly dyskinesia. So far, the actions of all commonly used antiparkinsonian drugs assessed in MPTP-treated primates have proved to be highly predictive of drug action in PD. These primate studies have demonstrated that long-acting dopamine agonists and levodopa given in combination with a catechol-O-methyl transferase (COMT) inhibitor (to increase its relatively short half-life), induce significantly less dyskinesia than occurs with standard levodopa therapy.

The mouse MPTP model: gene expression changes in dopaminergic neurons

Parkinson's disease (PD) is a common neurodegenerative disorder, characterized by the progressive loss of dopaminergic neurons in the substantia nigra. Although valuable animal models have been developed, our knowledge of the aetiology and pathogenic factors implicated in PD is still insufficient to develop causal therapeutic strategies aimed at halting its progression. The neurotoxicity induced by 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) is one of the most valuable models for analysing pathological aspects of PD. In this paper we studied the gene expression patterns underlying the pathogenesis of MPTP-induced neurodegeneration. We treated young and old C57BL/6 mice with different schedules of MPTP to induce degenerative processes that vary in intensity and time-course. During the first week after intoxication we used nonradioactive in situ-hybridization to investigate the expression patterns of genes associated with (i) dopamine metabolism and signalling; (ii) familial forms of PD; (iii) protein folding and (iv) energy metabolism. MPTP injections induced different severities of neuronal injury depending on the age of the animals and the schedule of administration as well as a significant degeneration in the striatum. In situ hybridization showed that MPTP intoxication initiated a number of gene expression changes that (i) were restricted to the neurons of the substantia nigra pars compacta; (ii) were correlated in intensity and number of changes with the age of the animals and the severity of histopathological disturbances; (iii) displayed in each a significant down-regulation by the end of one week after the last MPTP injection, but (iv) varied within one MPTP regimen in expression levels during the observation period. The subacute injection of MPTP into one-year-old mice induced the most severe changes in gene expression. All genes investigated were affected. However, alpha-synuclein was the only gene that was exclusively up-regulated in MPTP-treated animals displaying cell death.

A simple method to measure stride length as an index of nigrostriatal dysfunction in mice

Reduced stride length characterizes Parkinsonian gait. We aimed to demonstrate that it could be measured simply and reliably in mice by pawprints and used as an index of basal ganglia dysfunction. In C57BL/6 mice, stride length measurements proved to be consistent across measurements and experimenters. It was slightly lower in the hindlimbs and was correlated to femur size and animal velocity. Dopamine depletion by reserpine and striatal dopamine receptor blockade by haloperidol resulted in reduced mean stride length in four limbs. Significant forelimb/hindlimb difference was also observed both in mice with 3-nitropropionic acid (3-NP) induced striatal lesions and in those with MPTP-induced nigral cell loss. Reduction of hindlimb stride length was correlated significantly with the magnitude of cell loss, either in the substantia nigra or in the lateral mid-striatum. Stride length is, therefore, a simple method to obtain an index of motor disorders due to basal ganglia dysfunction in mice.

Dopamine D2 receptor-mediated G-protein activation in rat striatum: functional autoradiography and influence of unilateral 6-hydroxydopamine lesions of the substantia nigra

Unilateral 6-hydroxydopamine (6-OHDA) lesions of substantia nigra pars compacta (SNPC) neurons in rats induce behavioural hypersensitivity to dopaminergic agonists. However, the role of specific dopamine receptors is unclear, and potential alterations in their transduction mechanisms remain to be evaluated. The present study addressed these issues employing the dopaminergic agonist, quinelorane, which efficaciously stimulated G-protein activation (as assessed by [35S]GTPgammaS binding) at cloned hD2 (and hD3) receptors. At rat striatal membranes, dopamine stimulated [35S]GTPgammaS binding by 1.9-fold over basal, but its actions were only partially reversed by the selective D2/D3 receptor antagonist, raclopride, indicating the involvement of other receptor subtypes. In contrast, quinelorane-induced stimulation (48% of the effect of dopamine) was abolished by raclopride, and by the D2 receptor antagonist, L741,626. Further, novel antagonists selective for D3 and D4 receptors, S33084 and S18126, respectively, blocked the actions of quinelorane at concentrations corresponding to their affinities for D2 receptors. Quinelorane potently induced contralateral rotation in unilaterally 6-OHDA-lesioned rats, an effect abolished by raclopride and L741,626, but not by D3 and D4 receptor-selective doses of S33084 and S18126, respectively. In functional ([35S]GTPgammaS) autoradiography experiments, quinelorane stimulated G-protein activation in caudate putamen and, to a lesser extent, in nucleus accumbens and cingulate cortex of naive rats. In unilaterally SNPC-lesioned rats, quinelorane-induced G-protein activation in the caudate putamen on the non-lesioned side was similar to that seen in naive animals (approximately 50% stimulation), but significantly greater on the lesioned side (approximately 80%). This increase was both pharmacologically and regionally specific since it was reversed by raclopride, and was not observed in nucleus accumbens or cingulate cortex. In conclusion, the present data indicate that, in rat striatum, the actions of quinelorane are mediated primarily by D2 receptors, and suggest that behavioural hypersensitivity to this agonist, induced by unilateral SNPC lesions, is associated with an increase in D2, but not D3 or D4, receptor-mediated G-protein activation.

Pallidal border cells: an anatomical and electrophysiological study in the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-treated monkey

A dopamine transporter-radioligand binding study demonstrated a dopaminergic innervation around the pallidal complex in the normal monkey (n=5), i.e. where a subpopulation of pallidal neurons known as "border cells" is classically identified. Surprisingly, this peripallidal binding persists in monkeys rendered parkinsonian (n=5) with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine treatment. The border cell electrophysiological activity was then analysed in normal and parkinsonian monkeys (n=2), either in the untreated state or following administration of levodopa. Pallidal border cell firing frequency was significantly decreased after 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine treatment (8.9+/-0.7 vs 31.4+/-1.6Hz, P<0.05). This decrease was partly corrected by levodopa administration (19.2+/-1.0Hz, P<0.05 vs both normal and parkinsonian situations). The peripallidal dopaminergic innervation suggests that pallidal border cells are under a direct dopaminergic control, arising from the ventral tegmental area and/or the basal forebrain magnocellular complex, the role of which remains unknown. Moreover, the relative sparing of these dopaminergic fibers in parkinsonian monkeys suggests that they would exhibit specific adaptive properties totally different from those described in the nigrostriatal pathway.