Early presymptomatic and long-term changes of rest activity cycles and cognitive behavior in a MPTP-monkey model of Parkinson's disease

The importance of social behavior in nonhuman primate studies of aging: A mini-review

Social behavior plays an important role in supporting both psychological and physical health across the lifespan. People's social lives change as they age, and the nature of these changes differ based on whether people are on healthy aging trajectories or are experiencing neurodegenerative diseases that cause dementia, such as Alzheimer's disease and Parkinson's disease. Nonhuman primate models of aging have provided a base of knowledge comparing aging trajectories in health and disease, but these studies rarely emphasize social behavior changes as a consequence of the aging process. What data exist hold particular value, as negative effects of disease and aging on social behavior are likely to have disproportionate impacts on quality of life. In this mini review, we examine the literature on nonhuman primate models of aging with a focus on social behavior, in the context of both health and disease. We propose that adopting a greater focus on social behavior outcomes in nonhuman primates will improve our understanding of the intersection of health, aging and sociality in humans.

Quantification of Non-Motor Symptoms in Parkinsonian Cynomolgus Monkeys

BACKGROUND

Parkinson's disease (PD) is a neurodegenerative disorder that features motor and non-motor deficits. The use of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced dopamine neuron degeneration has been widely practiced to produce reliable animal models of PD. However, most previous preclinical studies focused on motor dysfunction, and few non-motor symptoms were evaluated. Thus far, there is a lack of comprehensive investigations of the non-motor symptoms in animal models.

OBJECTIVES

In this study, we aim to use a battery of behavioral methods to evaluate non-motor symptoms in MPTP-induced non-human primate PD models.

METHODS

Cognitive function, sleep, and psychiatric behaviors were evaluated in MPTP-treated cynomolgus monkeys. The tests consisted of a delayed matching-to-sample (DMTS) task, the use of a physical activity monitor (PAM), an apathy feeding task (AFT), the human intruder test (HIT), novel fruit test (NFT), and predator confrontation test (PCT). In addition, we tested whether the dopamine receptor agonist pramipexole (PPX) can improve these non-motor symptoms.

RESULTS

The present results show that the MPTP-treated monkeys exhibited cognitive deficits, abnormal sleep, and anxiety-like behaviors when compared to the control monkeys. These symptoms were relieved partially by PPX.

CONCLUSIONS

These results suggest that MPTP-induced PD monkeys displayed non-motor symptoms that were similar to those found in PD patients. PPX treatment showed moderate therapeutic effects on these non-motor symptoms. This battery of behavioral tests may provide a valuable model for future preclinical research.

The impact of biological clock and sex hormones on the risk of disease

Molecular clocks are responsible for defining 24-h cycles of behaviour and physiology that are called circadian rhythms. Several structures and tissues are responsible for generating these circadian rhythms and are named circadian clocks. The suprachiasmatic nucleus of the hypothalamus is believed to be the master circadian clock receiving light input via the optic nerve and aligning internal rhythms with environmental cues. Studies using both in vivo and in vitro methodologies have reported the relationship between the molecular clock and sex hormones. The circadian system is directly responsible for controlling the synthesis of sex hormones and this synthesis varies according to the time of day and phase of the estrous cycle. Sex hormones also directly interact with the circadian system to regulate circadian gene expression, adjust biological processes, and even adjust their own synthesis. Several diseases have been linked with alterations in either the sex hormone background or the molecular clock. So, in this chapter we aim to summarize the current understanding of the relationship between the circadian system and sex hormones and their combined role in the onset of several related diseases.

Mitochondrial autophagy in the sleeping brain

A significant percentage of the mitochondrial mass is replaced on a daily basis via mechanisms of mitochondrial quality control. Through mitophagy (a selective type of autophagy that promotes mitochondrial proteostasis) cells keep a healthy pool of mitochondria, and prevent oxidative stress and inflammation. Furthermore, mitophagy helps adapting to the metabolic demand of the cells, which changes on a daily basis.

Core components of the mitophagy process are PINK1 and Parkin, which mutations are linked to Parkinson’s Disease. The crucial role of PINK1/Parkin pathway during stress-induced mitophagy has been extensively studied in vitro in different cell types. However, recent advances in the field allowed discovering that mitophagy seems to be only slightly affected in PINK1 KO mice and flies, putting into question the physiological relevance of this pathway in vivo in the whole organism. Indeed, several cell-specific PINK1/Parkin-independent mitophagy pathways have been recently discovered, which appear to be activated under physiological conditions such as those that promote mitochondrial proteome remodeling during differentiation or in response to specific physiological stimuli.

In this Mini Review we want to summarize the recent advances in the field, and add another level of complexity by focusing attention on a potentially important aspect of mitophagy regulation: the implication of the circadian clock. Recent works showed that the circadian clock controls many aspects of mitochondrial physiology, including mitochondrial morphology and dynamic, respiratory activity, and ATP synthesis. Furthermore, one of the essential functions of sleep, which is controlled by the clock, is the clearance of toxic metabolic compounds from the brain, including ROS, via mechanisms of proteostasis. Very little is known about a potential role of the clock in the quality control mechanisms that maintain the mitochondrial repertoire healthy during sleep/wake cycles. More importantly, it remains completely unexplored whether (dys)function of mitochondrial proteostasis feedbacks to the circadian clockwork.

Induced Cognitive Impairments Reversed by Grafts of Neural Precursors: A Longitudinal Study in a Macaque Model of Parkinson's Disease

Parkinson's disease (PD) evolves over an extended and variable period in humans; years prior to the onset of classical motor symptoms, sleep and biological rhythm disorders develop, significantly impacting the quality-of-life of patients. Circadian-rhythm disorders are accompanied by mild cognitive deficits that progressively worsen with disease progression and can constitute a severe burden for patients at later stages. The gold-standard 6-methyl-1-methyl-4-phenyl-1,2,3,6-tetrahydropyridin (MPTP) macaque model of PD recapitulates the progression of motor and nonmotor symptoms over contracted periods of time. Here, this multidisciplinary/multiparametric study follows, in five animals, the steady progression of motor and nonmotor symptoms and describes their reversal following grafts of neural precursors in diverse functional domains of the basal ganglia. Results show unprecedented recovery from cognitive symptoms in addition to a strong clinical motor recuperation. Both motor and cognitive recovery and partial circadian rhythm recovery correlate with the degree of graft integration, and in a subset of animals, with in vivo levels of striatal dopaminergic innervation and function. The present study provides empirical evidence that integration of neural precursors following transplantation efficiently restores function at multiple levels in parkinsonian nonhuman primates and, given interindividuality of disease progression and recovery, underlines the importance of longitudinal multidisciplinary assessments in view of clinical translation.

Experimental Models of Cognitive Impairment for Use in Parkinson's Disease Research: The Distance Between Reality and Ideal

Parkinson’s disease (PD) is the second most common neurodegenerative disease. Cognitive impairment is one of the key non-motor symptoms of PD, affecting both mortality and quality of life. However, there are few experimental studies on the pathology and treatments of PD with mild cognitive impairment (PD-MCI) and PD dementia (PDD) due to the lack of representative models. To identify new strategies for developing representative models, we systematically summarized previous studies on PD-MCI and PDD and compared differences between existing models and diseases. Our initial search identified 5432 articles, of which 738 were duplicates. A total of 227 articles met our inclusion criteria and were included in the analysis. Models fell into three categories based on model design: neurotoxin-induced, transgenic, and combined. Although the neurotoxin-induced experimental model was the most common type that was used during every time period, transgenic and combined experimental models have gained significant recent attention. Unfortunately, there remains a big gap between ideal and actual experimental models. While each model has its own disadvantages, there have been tremendous advances in the development of PD models of cognitive impairment, and almost every model can verify a hypothesis about PD-MCI or PDD. Finally, our proposed strategies for developing novel models are as follows: a set of plans that integrate symptoms, biochemistry, neuroimaging, and other objective indicators to judge and identify that the novel model plays a key role in new strategies for developing representative models; novel models should simulate different clinical features of PD-MCI or PDD; inducible α-Syn overexpression and SH-SY5Y-A53T cellular models are good candidate models of PD-MCI or PDD.

Cortical Serotonergic and Catecholaminergic Denervation in MPTP-Treated Parkinsonian Monkeys

Decreased cortical serotonergic and catecholaminergic innervation of the frontal cortex has been reported at early stages of Parkinson's disease (PD). However, the limited availability of animal models that exhibit these pathological features has hampered our understanding of the functional significance of these changes during the course of the disease. In the present study, we assessed longitudinal changes in cortical serotonin and catecholamine innervation in motor-symptomatic and asymptomatic monkeys chronically treated with low doses of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). Densitometry and unbiased stereological techniques were used to quantify changes in serotonin and tyrosine hydroxylase (TH) immunoreactivity in frontal cortices of 3 control monkeys and 3 groups of MPTP-treated monkeys (motor-asymptomatic [N = 2], mild parkinsonian [N = 3], and moderate parkinsonian [N = 3]). Our findings revealed a significant decrease (P < 0.001) in serotonin innervation of motor (Areas 4 and 6), dorsolateral prefrontal (Areas 9 and 46), and limbic (Areas 24 and 25) cortical areas in motor-asymptomatic MPTP-treated monkeys. Both groups of symptomatic MPTP-treated animals displayed further serotonin denervation in these cortical regions (P < 0.0001). A significant loss of serotonin-positive dorsal raphe neurons was found in the moderate parkinsonian group. On the other hand, the intensity of cortical TH immunostaining was not significantly affected in motor asymptomatic MPTP-treated monkeys, but underwent a significant reduction in the moderate symptomatic group (P < 0.05). Our results indicate that chronic intoxication with MPTP induces early pathology in the corticopetal serotonergic system, which may contribute to early non-motor symptoms in PD.

A New Tool to Study Parkinsonism in the Context of Aging: MPTP Intoxication in a Natural Model of Multimorbidity

The diurnal rodent Octodon degus (O. degus) is considered an attractive natural model for Alzheimer's disease and other human age-related features. However, it has not been explored so far if the O. degus could be used as a model to study Parkinson's disease. To test this idea, 10 adult male O. degus were divided into control group and MPTP-intoxicated animals. Motor condition and cognition were examined. Dopaminergic degeneration was studied in the ventral mesencephalon and in the striatum. Neuroinflammation was also evaluated in the ventral mesencephalon, in the striatum and in the dorsal hippocampus. MPTP animals showed significant alterations in motor activity and in visuospatial memory. Postmortem analysis revealed a significant decrease in the number of dopaminergic neurons in the ventral mesencephalon of MPTP animals, although no differences were found in their striatal terminals. We observed a significant increase in neuroinflammatory responses in the mesencephalon, in the striatum and in the hippocampus of MPTP-intoxicated animals. Additionally, changes in the subcellular expression of the calcium-binding protein S100β were found in the astrocytes in the nigrostriatal pathway. These findings prove for the first time that O. degus are sensitive to MPTP intoxication and, therefore, is a suitable model for experimental Parkinsonism in the context of aging.

Measurement of baseline locomotion and other behavioral traits in a common marmoset model of Parkinson's disease established by a single administration regimen of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine: providing reference data for efficacious preclinical evaluations

Supplemental Digital Content is available in the text.

Baseline locomotion and behavioral traits in the common marmoset Parkinson's disease model were examined to provide basic information for preclinical evaluations of medical treatments. A single regimen of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine at a cumulative dose of 5 mg/kg as the free base over three consecutive days was administered subcutaneously to 10 marmosets. Data obtained from these marmosets were compared to pre-1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine levels or 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine free marmosets. After the single regimen, reduced daily locomotion, a measure of immobility (a primary sign of Parkinsonism), was observed for more than a year. A moving tremor was also observed by visual inspection during this period. When apomorphine (0.13 mg/kg, s.c.) was administered, either right or left circling behavior was observed in a cylindrical chamber in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine marmosets, suggestive of unequal neural damage between the two brain hemispheres to different extents. MRI revealed that T1 relaxation time in the right substantia nigra correlated with right circling in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine marmosets. Histology was supportive of dopaminergic neural loss in the striatum. These results increase our understanding of the utility and limitations of the Parkinson's disease model in marmosets with a single 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine regimen, and provide reference data for efficacious preclinical evaluations.

Shared cerebral metabolic pathology in non-transgenic animal models of Alzheimer's and Parkinson's disease

Parkinson's disease (PD) and Alzheimer's disease (AD) are the most common chronic neurodegenerative disorders, characterized by motoric dysfunction or cognitive decline in the early stage, respectively, but often by both symptoms in the advanced stage. Among underlying molecular pathologies that PD and AD patients have in common, more attention is recently paid to the central metabolic dysfunction presented as insulin resistant brain state (IRBS) and altered cerebral glucose metabolism, both also explored in animal models of these diseases. This review aims to compare IRBS and alterations in cerebral glucose metabolism in representative non-transgenic animal PD and AD models. The comparison is based on the selectivity of the neurotoxins which cause experimental PD and AD, towards the cellular membrane and intracellular molecular targets as well as towards the selective neurons/non-neuronal cells, and the particular brain regions. Mitochondrial damage and co-expression of insulin receptors, glucose transporter-2 and dopamine transporter on the membrane of particular neurons as well as astrocytes seem to be the key points which are further discussed in a context of alterations in insulin signalling in the brain and its interaction with dopaminergic transmission, particularly regarding the time frame of the experimental AD/PD pathology appearance and the correlation with cognitive and motor symptoms. Such a perspective provides evidence on IRBS being a common underlying metabolic pathology and a contributor to neurodegenerative processes in representative non-transgenic animal PD and AD models, instead of being a direct cause of a particular neurodegenerative disorder.

Degeneration of human photosensitive retinal ganglion cells may explain sleep and circadian rhythms disorders in Parkinson's disease

Parkinson's disease (PD) patients often suffer from non-motor symptoms like sleep dysregulation, mood disturbances or circadian rhythms dysfunction. The melanopsin-containing retinal ganglion cells are involved in the control and regulation of these processes and may be affected in PD, as other retinal and visual implications have been described in the disease. Number and morphology of human melanopsin-containing retinal ganglion cells were evaluated by immunohistochemistry in eyes from donors with PD or control. The Sholl number of intersections, the number of branches, and the number of terminals from the Sholl analysis were significantly reduced in PD melanopsin ganglion cells. Also, the density of these cells significantly decreased in PD compared to controls. Degeneration and impairment of the retinal melanopsin system may affect to sleep and circadian dysfunction reported in PD pathology, and its protection or stimulation may lead to better disease prospect and global quality of life of patients.

Transplantation in the nonhuman primate MPTP model of Parkinson's disease: update and perspectives

In order to calibrate stem cell exploitation for cellular therapy in neurodegenerative diseases, fundamental and preclinical research in NHP (nonhuman primate) models is crucial. Indeed, it is consensually recognized that it is not possible to directly extrapolate results obtained in rodent models to human patients. A large diversity of neurological pathologies should benefit from cellular therapy based on neural differentiation of stem cells. In the context of this special issue of Primate Biology on NHP stem cells, we describe past and recent advances on cell replacement in the NHP model of Parkinson's disease (PD). From the different grafting procedures to the various cell types transplanted, we review here diverse approaches for cell-replacement therapy and their related therapeutic potential on behavior and function in the NHP model of PD.

Chronic MPTP administration regimen in monkeys: a model of dopaminergic and non-dopaminergic cell loss in Parkinson's disease

Parkinson's disease (PD) is a progressive neurodegenerative disorder clinically characterized by cardinal motor deficits including bradykinesia, tremor, rigidity and postural instability. Over the past decades, it has become clear that PD symptoms extend far beyond motor signs to include cognitive, autonomic and psychiatric impairments, most likely resulting from cortical and subcortical lesions of non-dopaminergic systems. In addition to nigrostriatal dopaminergic degeneration, pathological examination of PD brains, indeed, reveals widespread distribution of intracytoplasmic inclusions (Lewy bodies) and death of non-dopaminergic neurons in the brainstem and thalamus. For that past three decades, the MPTP-treated monkey has been recognized as the gold standard PD model because it displays some of the key behavioral and pathophysiological changes seen in PD patients. However, a common criticism raised by some authors about this model, and other neurotoxin-based models of PD, is the lack of neuronal loss beyond the nigrostriatal dopaminergic system. In this review, we argue that this assumption is largely incorrect and solely based on data from monkeys intoxicated with acute administration of MPTP. Work achieved in our laboratory and others strongly suggest that long-term chronic administration of MPTP leads to brain pathology beyond the dopaminergic system that displays close similarities to that seen in PD patients. This review critically examines these data and suggests that the chronically MPTP-treated nonhuman primate model may be suitable to study the pathophysiology and therapeutics of some non-motor features of PD.

Neurodegeneration and the Circadian Clock

Despite varied etiologies and symptoms, several neurodegenerative diseases—specifically, Alzheimer’s (AD), Parkinson’s (PD), and Huntington’s diseases (HDs)—share the common feature of abnormal circadian rhythms, such as those in behavior (e.g., disrupted sleep/wake cycles), physiological processes (e.g., diminished hormone release) and biochemical activities (e.g., antioxidant production). Circadian disturbances are among the earliest symptoms of these diseases, and the molecular mechanisms of the circadian system are suspected to play a pivotal, and possibly causal, role in their natural histories. Here, we review the common circadian abnormalities observed in ADs, PDs and HDs, and summarize the evidence that the molecular circadian clockwork directly influences the course of these disease states. On the basis of this research, we explore several circadian-oriented interventions proposed as treatments for these neurological disorders.

Non-human primate models of PD to test novel therapies

Non-human primate (NHP) models of Parkinson disease show many similarities with the human disease. They are very useful to test novel pharmacotherapies as reviewed here. The various NHP models of this disease are described with their characteristics including the macaque, the marmoset, and the squirrel monkey models. Lesion-induced and genetic models are described. There is no drug to slow, delay, stop, or cure Parkinson disease; available treatments are symptomatic. The dopamine precursor, L-3,4-dihydroxyphenylalanine (L-Dopa) still remains the gold standard symptomatic treatment of Parkinson. However, involuntary movements termed L-Dopa-induced dyskinesias appear in most patients after chronic treatment and may become disabling. Dyskinesias are very difficult to manage and there is only amantadine approved providing only a modest benefit. In this respect, NHP models have been useful to seek new drug targets, since they reproduce motor complications observed in parkinsonian patients. Therapies to treat motor symptoms in NHP models are reviewed with a discussion of their translational value to humans. Disease-modifying treatments tested in NHP are reviewed as well as surgical treatments. Many biochemical changes in the brain of post-mortem Parkinson disease patients with dyskinesias are reviewed and compare well with those observed in NHP models. Non-motor symptoms can be categorized into psychiatric, autonomic, and sensory symptoms. These symptoms are present in most parkinsonian patients and are already installed many years before the pre-motor phase of the disease. The translational usefulness of NHP models of Parkinson is discussed for non-motor symptoms.

Circadian Dysregulation in Parkinson's Disease

Parkinson's disease (PD) is the second most common neurodegenerative disorder that affects over one million individuals in the US alone. PD is characterized by a plethora of motor and non-motor manifestations, resulting from a progressive degeneration of dopaminergic neurons and disbalance of several other neurotransmitters. A growing body of evidence points to significant alterations of the circadian system in PD. This is not surprising given the pivotal role that dopamine plays in circadian regulation as well as the role of circadian influences in dopamine metabolism. In this review we present basic and clinical investigations that examined the function of the circadian system in PD.

Prefrontal Markers and Cognitive Performance Are Dissociated during Progressive Dopamine Lesion

Dopamine is thought to directly influence the neurophysiological mechanisms of both performance monitoring and cognitive control-two processes that are critically linked in the production of adapted behaviour. Changing dopamine levels are also thought to induce cognitive changes in several neurological and psychiatric conditions. But the working model of this system as a whole remains untested. Specifically, although many researchers assume that changing dopamine levels modify neurophysiological mechanisms and their markers in frontal cortex, and that this in turn leads to cognitive changes, this causal chain needs to be verified. Using longitudinal recordings of frontal neurophysiological markers over many months during progressive dopaminergic lesion in non-human primates, we provide data that fail to support a simple interaction between dopamine, frontal function, and cognition. Feedback potentials, which are performance-monitoring signals sometimes thought to drive successful control, ceased to differentiate feedback valence at the end of the lesion, just before clinical motor threshold. In contrast, cognitive control performance and beta oscillatory markers of cognitive control were unimpaired by the lesion. The differing dynamics of these measures throughout a dopamine lesion suggests they are not all driven by dopamine in the same way. These dynamics also demonstrate that a complex non-linear set of mechanisms is engaged in the brain in response to a progressive dopamine lesion. These results question the direct causal chain from dopamine to frontal physiology and on to cognition. They imply that biomarkers of cognitive functions are not directly predictive of dopamine loss.

Animal behavioral assessments in current research of Parkinson's disease

Parkinson's disease (PD), a neurodegenerative disorder, is traditionally classified as a movement disorder. Patients typically suffer from many motor dysfunctions. Presently, clinicians and scientists recognize that many non-motor symptoms are associated with PD. There is an increasing interest in both motor and non-motor symptoms in clinical studies on PD patients and laboratory research on animal models that imitate the pathophysiologic features and symptoms of PD patients. Therefore, appropriate behavioral assessments are extremely crucial for correctly understanding the mechanisms of PD and accurately evaluating the efficacy and safety of novel therapies. This article systematically reviews the behavioral assessments, for both motor and non-motor symptoms, in various animal models involved in current PD research. We addressed the strengths and weaknesses of these behavioral tests and their appropriate applications. Moreover, we discussed potential mechanisms behind these behavioral tests and cautioned readers against potential experimental bias. Since most of the behavioral assessments currently used for non-motor symptoms are not particularly designed for animals with PD, it is of the utmost importance to greatly improve experimental design and evaluation in PD research with animal models. Indeed, it is essential to develop specific assessments for non-motor symptoms in PD animals based on their characteristics. We concluded with a prospective view for behavioral assessments with real-time assessment with mobile internet and wearable device in future PD research.

Individual and Familial Susceptibility to MPTP in a Common Marmoset Model for Parkinson's Disease

INTRODUCTION

Insight into susceptibility mechanisms underlying Parkinson's disease (PD) would aid the understanding of disease etiology, enable target finding and benefit the development of more refined disease-modifying strategies.

METHODS

We used intermittent low-dose MPTP (0.5 mg/kg/week) injections in marmosets and measured multiple behavioral and neurochemical parameters. Genetically diverse monkeys from different breeding families were selected to investigate inter- and intrafamily differences in susceptibility to MPTP treatment.

RESULTS

We show that such differences exist in clinical signs, in particular nonmotor PD-related behaviors, and that they are accompanied by differences in neurotransmitter levels. In line with the contribution of a genetic component, different susceptibility phenotypes could be traced back through genealogy to individuals of the different families.

CONCLUSION

Our findings show that low-dose MPTP treatment in marmosets represents a clinically relevant PD model, with a window of opportunity to examine the onset of the disease, allowing the detection of individual variability in disease susceptibility, which may be of relevance for the diagnosis and treatment of PD in humans.

Progression and recovery of Parkinsonism in a chronic progressive MPTP-induction model in the marmoset without persistent molecular and cellular damage

Chronic exposure to low-dose 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) in marmoset monkeys was used to model the prodromal stage of Parkinson's disease (PD), and to investigate mechanisms underlying disease progression and recovery. Marmosets were subcutaneously injected with MPTP for a period of 12weeks, 0.5mg/kg once per week, and clinical signs of Parkinsonism, motor- and non-motor behaviors were recorded before, during and after exposure. In addition, postmortem immunohistochemistry and proteomics analysis were performed. MPTP-induced parkinsonian clinical symptoms increased in severity during exposure, and recovered after MPTP administration was ended. Postmortem analyses, after the recovery period, revealed no alteration of the number and sizes of tyrosine hydroxylase (TH)-positive dopamine (DA) neurons in the substantia nigra. Also levels of TH in putamen and caudate nucleus were unaltered, no differences were observed in DA, serotonin or nor-adrenalin levels in the caudate nucleus, and proteomics analysis revealed no global changes in protein expression in these brain areas between treatment groups. Our findings indicate that parkinsonian symptoms can occur without detectable damage at the cellular or molecular level. Moreover, we show that parkinsonian symptoms may be reversible when diagnosed and treated early.

Symptomatic Models of Parkinson's Disease and L-DOPA-Induced Dyskinesia in Non-human Primates

Models of Parkinson's disease (PD) can be produced in several non-human primate (NHP) species by applying neurotoxic lesions to the nigrostriatal dopamine pathway. The most commonly used neurotoxin is MPTP, a compound accidentally discovered as a contaminant of street drugs. Compared to other neurotoxins, MPTP has the advantage of crossing the blood-brain barrier and can thus be administered systemically. MPTP-lesioned NHPs exhibit the main core clinical features of PD. When treated with L-DOPA, these NHP models develop involuntary movements resembling the phenomenology of human dyskinesias. In old-world NHP species (macaques, baboons), choreic and dystonic dyskinesias can be readily distinguished and quantified with specific rating scales. More recently, certain non-motor symptoms relevant to human PD have been described in L-DOPA-treated MPTP-NHPs, including a range of neuropsychiatric abnormalities and sleep disturbances. The main shortcomings of MPTP-NHP models consist in a lack of progression of the underlying neurodegenerative lesion, along with an inability to model the intracellular protein-inclusion pathology typical of PD. The strength of MPTP-NHP models lies in their face and predictive validity for symptomatic treatments of parkinsonian motor features. Indeed, these models have been instrumental to the development of several medical and surgical approaches that are currently applied to treat PD.

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.

Lessons from the analysis of nonhuman primates for understanding human aging and neurodegenerative diseases

Animal models are necessary tools for solving the most serious challenges facing medical research. In aging and neurodegenerative disease studies, rodents occupy a place of choice. However, the most challenging questions about longevity, the complexity and functioning of brain networks or social intelligence can almost only be investigated in nonhuman primates. Beside the fact that their brain structure is much closer to that of humans, they develop highly complex cognitive strategies and they are visually-oriented like humans. For these reasons, they deserve consideration, although their management and care are more complicated and the related costs much higher. Despite these caveats, considerable scientific advances have been possible using nonhuman primates. This review concisely summarizes their role in the study of aging and of the mechanisms involved in neurodegenerative disorders associated mainly with cognitive dysfunctions (Alzheimer's and prion diseases) or motor deficits (Parkinson's and related diseases).

Social behavioral changes in MPTP-treated monkey model of Parkinson's disease

Parkinsonian patients experience not only the physical discomfort of motor disorders but also the considerable psychological distress caused by cognitive deficits and behavioral disorders. These two factors can result in a disruption of social relationships during the symptomatic and even the presymptomatic motor states of the disease. However, it remains difficult, if not impossible, to evaluate social relationships in presymptomatic patients. The present study focused on the evaluation of social relationships within a group of female long-tailed macaques during presymptomatic and symptomatic motor states induced by Chronic Low-Dose (CLD) and then Chronic High-Dose (CHD) systemic administration of 1-methyl-4-phenyl-l,2,3,6-tetrahydropyridine (MPTP). Dopaminergic denervation within basal ganglia and cortical areas was evaluated using Positron Emission Tomography (PET) scans with (18)F-DOPA (6-[18F]-fluoro-L-3,4-dihydroxyphenylalanine) radiotracer. Interestingly, social behavioral changes could be identified in the presymptomatic motor state before any motor and/or cognitive impairment occurred. Stronger effects were observed in subordinate animals compared to dominant animals. From baseline state to CLD-presymptomatic motor state, the frequency of emitted affiliative and aggressive behaviors increased. From CLD-presymptomatic to CHD-presymptomatic motor states, the frequency of the three categories of social behaviors (aggressive, submissive and affiliative) decreased. At this time, quantitative data analysis in PET scans highlighted a dopaminergic denervation in the insula and the posterior caudate nucleus. Finally, the frequency of the three categories of social behaviors decreased during the stable-symptomatic motor state compared to baseline and presymptomatic motor states; this was also associated with motor and cognitive disorders and a dopaminergic denervation in all the evaluated cortical and subcortical structures.

'The clocks that time us'--circadian rhythms in neurodegenerative disorders

Circadian rhythms are physiological and behavioural cycles generated by an endogenous biological clock, the suprachiasmatic nucleus. The circadian system influences the majority of physiological processes, including sleep-wake homeostasis. Impaired sleep and alertness are common symptoms of neurodegenerative disorders, and circadian dysfunction might exacerbate the disease process. The pathophysiology of sleep-wake disturbances in these disorders remains largely unknown, and is presumably multifactorial. Circadian rhythm dysfunction is often observed in patients with Alzheimer disease, in whom it has a major impact on quality of life and represents one of the most important factors leading to institutionalization of patients. Similarly, sleep and circadian problems represent common nonmotor features of Parkinson disease and Huntington disease. Clinical studies and experiments in animal models of neurodegenerative disorders have revealed the progressive nature of circadian dysfunction throughout the course of neurodegeneration, and suggest strategies for the restoration of circadian rhythmicity involving behavioural and pharmacological interventions that target the sleep-wake cycle. In this Review, we discuss the role of the circadian system in the regulation of the sleep-wake cycle, and outline the implications of disrupted circadian timekeeping in neurodegenerative diseases.

Loss of dopamine disrupts circadian rhythms in a mouse model of Parkinson's disease

Although a wide range of physiological functions regulated by dopamine (DA) display circadian variations, the role of DA in the generation and/or modulation of these rhythms is unknown. In Parkinson's disease (PD) patients, in addition to the classical motor symptoms, disturbances of the pattern of daily rest/wake cycles are common non-motor symptoms. We investigated daily and circadian rhythms of rest/activity behaviors in a transgenic MitoPark mouse model with selective inactivation of mitochondrial transcription factor A (Tfam) resulting in a slow and progressive degeneration of DA neurons in midbrain structures. Correlated with this, MitoPark mice show a gradual reduction in locomotor activity beginning at about 20weeks of age. In a light-dark cycle, MitoPark mice exhibit a daily pattern of rest/activity rhythms that shows an age-dependent decline in both the amplitude and the stability of the rhythm, coupled with an increased fragmentation of day/night activities. When the circadian system is challenged by exposure to constant darkness or constant light conditions, control littermates retain a robust free-running circadian locomotor rhythm, whereas in MitoPark mice, locomotor rhythms are severely disturbed or completely abolished. Re-exposure to a light/dark cycle completely restores daily locomotor rhythms. MitoPark mice and control littermates express similar masking behaviors under a 1h light/1h dark regime, suggesting that the maintenance of a daily pattern of rest/activity in arrhythmic MitoPark mice can be attributed to the acute inhibitory and stimulatory effects of light and darkness. These results imply that, in addition to the classical motor abnormalities observed in PD, the loss of the midbrain DA neurons leads to impairments of the circadian control of rest/activity rhythms.

Increased DAT binding in the early stage of the dopaminergic lesion: a longitudinal [11C]PE2I binding study in the MPTP-monkey

The delayed appearance of motor symptoms in PD poses a crucial challenge for early detection of the disease. We measured the binding potential of the selective dopamine active transporter (DAT) radiotracer [(11)C]PE2I in MPTP-treated macaque monkeys, thus establishing a detailed profile of the nigrostriatal DA status following MPTP intoxication and its relation to induced motor and non-motor symptoms. Clinical score and cognitive performance were followed throughout the study. We measured longitudinally in vivo the non-displaceable binding potential to DAT in premotor, motor-recovered (i.e. both non-symptomatic) and symptomatic MPTP-treated monkeys. Results show an unexpected and pronounced dissociation between clinical scores and [(11)C]PE2I-BP(ND) during the premotor phase i.e. DAT binding in the striatum of premotor animals was increased around 20%. Importantly, this broad increase of DAT binding in the caudate, ventral striatum and anterior putamen was accompanied by i) deteriorated cognitive performance, showing a likely causal role of the observed hyperdopaminergic state (Cools, 2011; Cools and D'Esposito, 2011) and ii) an asymmetric decrease of DAT binding at a focal point of the posterior putamen, suggesting that increased DAT is one of the earliest, intrinsic compensatory mechanisms. Following spontaneous recovery from motor deficits, DAT binding was greatly reduced as recently shown in-vivo with other radiotracers (Blesa et al., 2010, 2012). Finally, high clinical scores were correlated to considerably low levels of DAT only after the induction of a stable parkinsonian state. We additionally show that the only striatal region which was significantly correlated to the degree of motor impairments is the ventral striatum. Further research on this period should allow better understanding of DA compensation at premature stages of PD and potentially identify new diagnosis and therapeutic index.

Progress and prospects for genetic modification of nonhuman primate models in biomedical research

The growing interest of modeling human diseases using genetically modified (transgenic) nonhuman primates (NHPs) is a direct result of NHPs (rhesus macaque, etc.) close relation to humans. NHPs share similar developmental paths with humans in their anatomy, physiology, genetics, and neural functions; and in their cognition, emotion, and social behavior. The NHP model within biomedical research has played an important role in the development of vaccines, assisted reproductive technologies, and new therapies for many diseases. Biomedical research has not been the primary role of NHPs. They have mainly been used for safety evaluation and pharmacokinetics studies, rather than determining therapeutic efficacy. The development of the first transgenic rhesus macaque (2001) revolutionized the role of NHP models in biomedicine. Development of the transgenic NHP model of Huntington's disease (2008), with distinctive clinical features, further suggested the uniqueness of the model system; and the potential role of the NHP model for human genetic disorders. Modeling human genetic diseases using NHPs will continue to thrive because of the latest advances in molecular, genetic, and embryo technologies. NHPs rising role in biomedical research, specifically pre-clinical studies, is foreseeable. The path toward the development of transgenic NHPs and the prospect of transgenic NHPs in their new role in future biomedicine needs to be reviewed. This article will focus on the advancement of transgenic NHPs in the past decade, including transgenic technologies and disease modeling. It will outline new technologies that may have significant impact in future NHP modeling and will conclude with a discussion of the future prospects of the transgenic NHP model.

Alteration of daily and circadian rhythms following dopamine depletion in MPTP treated non-human primates

Disturbances of the daily sleep/wake cycle are common non-motor symptoms of Parkinson's disease (PD). However, the impact of dopamine (DA) depletion on circadian rhythms in PD patients or non-human primate (NHP) models of the disorder have not been investigated. We evaluated alterations of circadian rhythms in NHP following MPTP lesion of the dopaminergic nigro-striatal system. DA degeneration was assessed by in vivo PET ([(11)C]-PE2I) and post-mortem TH and DAT quantification. In a light∶dark cycle, control and MPTP-treated NHP both exhibit rest-wake locomotor rhythms, although DA-depleted NHP show reduced amplitude, decreased stability and increased fragmentation. In all animals, 6-sulphatoxymelatonin peaks at night and cortisol in early morning. When the circadian system is challenged by exposure to constant light, controls retain locomotor rest-wake and hormonal rhythms that free-run with stable phase relationships whereas in the DA-depleted NHP, locomotor rhythms are severely disturbed or completely abolished. The amplitude and phase relations of hormonal rhythms nevertheless remain unaltered. Use of a light-dark masking paradigm shows that expression of daily rest-wake activity in MPTP monkeys requires the stimulatory and inhibitory effects of light and darkness. These results suggest that following DA lesion, the central clock in the SCN remains intact but, in the absence of environmental timing cues, is unable to drive downstream rhythmic processes of striatal clock gene and dopaminergic functions that control locomotor output. These findings suggest that the circadian component of the sleep-wake disturbances in PD is more profoundly affected than previously assumed.

[Circadian clock and non-visual functions: the role of light in humans]

Hormonal secretion, cognitive performance, motor activity, metabolic processes, the sleep wake cycle and, most recently shown, cell division and ADN repair show a 24 h rhythmicity that is driven by the circadian timing system (the biological clock). Their appropriate activity over the 24 h requires appropriate entrainment of the circadian clock, which is achieved through the synchronizing effects of ocular light exposure. The activation of melanopsin-expressing ganglion cells in the retina depends on timing, quality, intensity, and history of light exposure. Inappropriate lighting leads to inappropriate synchronization of the clock, and activation of non-visual functions (mood, wakefulness, cognition, etc.). In turn, a deficit of circadian entrainment to the 24 h is responsible for alterations of a large number of functions, and leads to altered sleep, wake, mood, neurobehavioral processes and cell division, but also to pathologies. The crucial role of the circadian clock and the nature of the non-visual functions activated by light give rise to the concept that light is a biological need fundamental to health. Without an appropriate light hygiene, the clock receives an odd tempo, and it is cacophony!

Motor Function in MPTP-Treated Tree Shrews (Tupaia belangeri chinensis)

The tree shrew, a new experimental animal model, has been used to study a variety of diseases, especially diseases of the nervous system. 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) is the gold standard for toxin-based animal models of Parkinson's disease (PD) because MPTP treatment replicates almost all of the pathological hallmarks of PD. Therefore, in this study, the effects of MPTP on the motor function of the tree shrew were examined. After five daily injections of a 3 mg/kg dose of MPTP, the motor function of MPTP-injected tree shrews decreased significantly, and the classic Parkinsonian symptoms of action and resting tremor, bradykinesia, posture abnormalities, and gait instability were observed in most MPTP-injected tree shrews. HPLC results also showed significantly reduced striatal dopamine and 3,4-dihydroxyphenylacetic acid levels in tree shrews after MPTP injection. Increased oxidative stress levels are usually considered to be the cause of dopaminergic neuron depletion in the presence of MPTP and were observed in the substantia nigra of MPTP-treated tree shrews, as indicated by a significant reduction in superoxide dismutase and glutathione peroxidase activity and increased levels of malondialdehyde. In addition, elevated α-synuclein mRNA levels in the midbrain of MPTP-treated tree shrews were observed. Furthermore, MPTP-treated tree shrews showed the classic Parkinsonian symptoms at a lower MPTP dosage compared with other animal models. Thus, the MPTP-treated tree shrew may be a potential animal model for studying the pathogenesis of PD.

Circadian dysfunction may be a key component of the non-motor symptoms of Parkinson's disease: insights from a transgenic mouse model

Sleep disorders are nearly ubiquitous among patients with Parkinson's disease (PD), and they manifest early in the disease process. While there are a number of possible mechanisms underlying these sleep disturbances, a primary dysfunction of the circadian system should be considered as a contributing factor. Our laboratory's behavioral phenotyping of a well-validated transgenic mouse model of PD reveals that the electrical activity of neurons within the master pacemaker of the circadian system, the suprachiasmatic nuclei (SCN), is already disrupted at the onset of motor symptoms, although the core features of the intrinsic molecular oscillations in the SCN remain functional. Our observations suggest that the fundamental circadian deficit in these mice lies in the signaling output from the SCN, which may be caused by known mechanisms in PD etiology: oxidative stress and mitochondrial disruption. Disruption of the circadian system is expected to have pervasive effects throughout the body and may itself lead to neurological and cardiovascular disorders. In fact, there is much overlap in the non-motor symptoms experienced by PD patients and in the consequences of circadian disruption. This raises the possibility that the sleep and circadian dysfunction experienced by PD patients may not merely be a subsidiary of the motor symptoms, but an integral part of the disease. Furthermore, we speculate that circadian dysfunction can even accelerate the pathology underlying PD. If these hypotheses are correct, more aggressive treatment of the circadian misalignment and sleep disruptions in PD patients early in the pathogenesis of the disease may be powerful positive modulators of disease progression and patient quality of life.

Classic and new animal models of Parkinson's disease

Neurological disorders can be modeled in animals so as to recreate specific pathogenic events and behavioral outcomes. Parkinson's Disease (PD) is the second most common neurodegenerative disease of an aging population, and although there have been several significant findings about the PD disease process, much of this process still remains a mystery. Breakthroughs in the last two decades using animal models have offered insights into the understanding of the PD disease process, its etiology, pathology, and molecular mechanisms. Furthermore, while cellular models have helped to identify specific events, animal models, both toxic and genetic, have replicated almost all of the hallmarks of PD and are useful for testing new neuroprotective or neurorestorative strategies. Moreover, significant advances in the modeling of additional PD features have come to light in both classic and newer models. In this review, we try to provide an updated summary of the main characteristics of these models as well as the strengths and weaknesses of what we believe to be the most popular PD animal models. These models include those produced by 6-hydroxydopamine (6-OHDA), 1-methyl-1,2,3,6-tetrahydropiridine (MPTP), rotenone, and paraquat, as well as several genetic models like those related to alpha-synuclein, PINK1, Parkin and LRRK2 alterations.