Pathology of Parkinson's disease. Changes other than the nigrostriatal pathway

The relevance of dopaminergic level in nocturnal disability in Parkinson's disease: implications of continuous dopaminergic stimulation at night to treat the symptoms

Sleep problems are an under-emphasized cause of disability in Parkinson's disease (PD). Difficult sleep maintenance (light and fragmented sleep) and difficulties in initiating sleep are often the earliest and the most frequent symptoms observed in PD patients. In fluctuating patients, nocturnal akinesia, dystonia, painful cramps, and parasomnias may aggravate nocturnal problems. Treatment of sleep problems can be complex and challenging for the physicians. Dopaminergic treatment may improve some of the nocturnal symptoms in PD. In this paper, the effect of drugs and technique that ensure a more continuous delivery of dopaminergic drugs on sleep problems in PD is reviewed.

Mapping track density changes in nigrostriatal and extranigral pathways in Parkinson's disease

In Parkinson's disease (PD) the demonstration of neuropathological disturbances in nigrostriatal and extranigral brain pathways using magnetic resonance imaging remains a challenge. Here, we applied a novel diffusion-weighted imaging approach-track density imaging (TDI). Twenty-seven non-demented Parkinson's patients (mean disease duration: 5 years, mean score on the Hoehn & Yahr scale=1.5) were compared with 26 elderly controls matched for age, sex, and education level. Track density images were created by sampling each subject's spatially normalized fiber tracks in 1mm isotropic intervals and counting the fibers that passed through each voxel. Whole-brain voxel-based analysis was performed and significance was assessed with permutation testing. Statistically significant increases in track density were found in the Parkinson's patients, relative to controls. Clusters were distributed in disease-relevant areas including motor, cognitive, and limbic networks. From the lower medulla to the diencephalon and striatum, clusters encompassed the known location of the locus coeruleus and pedunculopontine nucleus in the pons, and from the substantia nigra up to medial aspects of the posterior putamen, bilaterally. The results identified in brainstem and nigrostriatal pathways show a large overlap with the known distribution of neuropathological changes in non-demented PD patients. Our results also support an early involvement of limbic and cognitive networks in Parkinson's disease.

Differential effects of intrastriatal 6-hydroxydopamine on cell number and morphology in midbrain dopaminergic subregions of the rat

The midbrain dopaminergic perikarya are differentially affected in Parkinson׳s disease (PD). This study compared the effects of a partial unilateral intrastriatal 6-hydroxydopamine (6-OHDA) lesion model of PD on the number, morphology, and nucleolar volume of dopaminergic cells in the substantia nigra pars compacta (SNpc), ventral tegmental area (VTA), and retrorubral field (RRF). Adult, male rats (n=10) underwent unilateral intrastriatal infusion of 6-OHDA (12.5μg). Lesions were verified by amphetamine-stimulated rotation 7 days post-infusion. Rats were euthanized 14 days after treatment with 6-OHDA and brains were stained with a tyrosine hydroxylase-silver nucleolar (TH-AgNOR) stain. Dopaminergic cell number and morphology in the lesioned and intact hemispheres were quantified using stereological methods. The magnitude of decrease in planimetric volume, neuronal number, cell density, and neuronal volume resulting from 6-OHDA lesion differed between regions, with the SNpc exhibiting the greatest loss of neurons (46%), but the smallest decrease in neuronal volume (13%). The lesion also resulted in a decrease in nucleolar volume that was similar in all three regions (22-26%). These findings indicate that intrastriatal 6-OHDA lesion differentially affects dopaminergic neurons in the SNpc, VTA, and RRF; however, the resulting changes in nucleolar morphology suggest a similar cellular response to the toxin in all three cell populations.

Imaging changes associated with cognitive abnormalities in Parkinson's disease

The current study investigates both gray and white matter changes in non-demented Parkinson's disease (PD) patients with varying degrees of mild cognitive deficits and elucidates the relationships between the structural changes and clinical sequelae of PD. Twenty-six PD patients and 15 healthy controls (HCs) were enrolled in the study. Participants underwent T1-weighted and diffusion tensor imaging (DTI) scans. Their cognition was assessed using a neuropsychological battery. Compared with HCs, PD patients showed significant cortical thinning in sensorimotor (left pre- and postcentral gyri) and cognitive (left dorsolateral superior frontal gyrus [DLSFG]) regions. The DLSFG cortical thinning correlated with executive and global cognitive impairment in PD patients. PD patients showed white matter abnormalities as well, primarily in bilateral frontal and temporal regions, which also correlated with executive and global cognitive impairment. These results seem to suggest that both gray and white matter changes in the frontal regions may constitute an early pathological substrate of cognitive impairment of PD providing a sensitive biomarker for brain changes in PD.

Dyskinesias in Parkinson's disease: views from positron emission tomography studies

Levodopa-induced dyskinesias (LIDs) and graft-induced dyskinesias (GIDs) are serious and common complications of Parkinson's disease (PD) management following chronic treatment with levodopa or intrastriatal transplantation with dopamine-rich foetal ventral mesencephalic tissue, respectively. Positron emission tomography (PET) molecular imaging provides a powerful in vivo tool that has been employed over the past 20 years for the elucidation of mechanisms underlying the development of LIDs and GIDs in PD patients. PET used together with radioligands tagging molecular targets has allowed the functional investigation of several systems in the brain including the dopaminergic, serotonergic, glutamatergic, opioid, endocannabinoid, noradrenergic and cholinergic systems. In this article the role of PET imaging in unveiling pathophysiological mechanisms underlying the development of LIDs and GIDs in PD patients is reviewed.

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.

Dopamine receptor agonists for Parkinson's disease

INTRODUCTION

Prolonged administration of l-3,4-dihydroxyphenylalanine (l-DOPA) for Parkinson's disease (PD) is hampered by motor complications related to the progressive incapacity of residual nigrostriatal neurons to properly utilize the drug. Direct stimulation of dopaminergic (DAergic) receptors with specific compounds (DA agonists) has, therefore, become an additional therapeutic tool for PD.

AREAS COVERED

DA agonists have considerable anti-parkinsonian symptomatic efficacy, although they are less potent than l-DOPA. This review summarizes pre-clinical and clinical data on DA agonists and their role in treating PD. Specific focus was put on second-generation, first-line non-ergolinic DA agonists and their motor, non-motor and putative neuroprotective effects. The anti-parkinsonian potential of recently developed DA agonists that reached Phase II and III clinical trials was also addressed.

EXPERT OPINION

DA agonists can be useful along the whole natural course of PD, as monotherapy in the initial phase or combined with l-DOPA in advanced PD. Extended-release formulations have been developed for second-generation DA agonists, which are better appreciated by patients. Neuroprotective properties have been proposed for DA agonists, based on pre-clinical studies, but never convincingly demonstrated in patients. New DA agonists, with better symptomatic efficacy and devoid of the side effects that characterize current compounds, are needed.

Parkinson disease and exercise

Parkinson disease (PD) is a progressive, neurodegenerative movement disorder. PD was originally attributed to neuronal loss within the substantia nigra pars compacta, and a concomitant loss of dopamine. PD is now thought to be a multisystem disorder that involves not only the dopaminergic system, but other neurotransmitter systems whose role may become more prominent as the disease progresses (189). PD is characterized by four cardinal symptoms, resting tremor, rigidity, bradykinesia, and postural instability, all of which are motor. However, PD also may include any combination of a myriad of nonmotor symptoms (195). Both motor and nonmotor symptoms may impact the ability of those with PD to participate in exercise and/or impact the effects of that exercise on those with PD. This article provides a comprehensive overview of PD, its symptoms and progression, and current treatments for PD. Among these treatments, exercise is currently at the forefront. People with PD retain the ability to participate in many forms of exercise and generally respond to exercise interventions similarly to age-matched subjects without PD. As such, exercise is currently an area receiving substantial research attention as investigators seek interventions that may modify the progression of the disease, perhaps through neuroprotective mechanisms.

Depression impairs learning, whereas the selective serotonin reuptake inhibitor, paroxetine, impairs generalization in patients with major depressive disorder

To better understand how medication status and task demands affect cognition in major depressive disorder (MDD), we evaluated medication-naïve patients with MDD, medicated patients with MDD receiving the selective serotonin reuptake inhibitors (SSRI) paroxetine, and healthy controls. All three groups were administered a computer-based cognitive task with two phases, an initial phase in which a sequence is learned through reward-based feedback (which our prior studies suggest is striatal-dependent), followed by a generalization phase that involves a change in the context where learned rules are to be applied (which our prior studies suggest is hippocampal-region dependent). Medication-naïve MDD patients were slow to learn the initial sequence but were normal on subsequent generalization of that learning. In contrast, medicated patients learned the initial sequence normally, but were impaired at the generalization phase. We argue that these data suggest (i) an MDD-related impairment in striatal-dependent sequence learning which can be remediated by SSRIs and (ii) an SSRI-induced impairment in hippocampal-dependent generalization of past learning to novel contexts, not otherwise seen in the medication-naïve MDD group. Thus, SSRIs might have a beneficial effect on striatal function required for sequence learning, but a detrimental effect on the hippocampus and other medial temporal lobe structures is critical for generalization.

Trigger medications and patient-related risk factors for Parkinson disease psychosis requiring anti-psychotic drugs: a retrospective cohort study

Background

Psychoses such as hallucinations are a frequent non-motor problem in patients with Parkinson disease (PD) and serious psychosis requires anti-psychotic medications that worsen Parkinsonism. Although psychosis could be associated with patient-related or biological factors such as cognition, age, and severity of PD, it can also be associated with medications.

Therefore we aimed to investigate patient-related and medication-related risks of psychosis requiring anti-psychotic medications (serious psychosis).

Methods

A retrospective cohort of 331 PD patients was followed for 2 years. Patient-related factors associated with risk of psychosis were identified by a survival time analysis. In patients who developed psychosis, medications during the hazard period (1-14 days before psychosis) were contrasted with those during the control periods (1 and 3 months before psychosis) using a case–crossover analysis to identify medication-related risks of psychosis.

Results

Serious psychosis was detected in 52 patients and the incidence was estimated to be 116 (95% confidence interval [CI], 85-148) per 1,000 person-years. Analyses of baseline characteristics revealed the risk to be higher in patients with a modified Hoehn–Yahr stage of ≥4 (hazard ratio [HR], 2.22; 95% CI, 1.11-4.40), those with a longer duration of PD (HR, 1.25; 95% CI, 1.00-1.55, per 5 years) and those with Mini-Mental State Examination scores of ≤24 (HR, 2.66; 95% CI, 1.37-5.16). The case-crossover analysis revealed that anti-cholinergics use (HR, 19.7; 95% CI, 2.39-162) elevated the risk, while donepezil use reduced it (HR, 0.48; 95% CI, 0.27-0.85).

Conclusions

Risk of psychosis was elevated by increasing severity of PD, cognitive dysfunction and duration of the disease. It was elevated by use of anti-cholinergic drugs and reduced by use of donepezil. The medication-related risk was higher in patients aged ≥ 70 years. In contrast, there was no significant medication-related risk in younger patients, suggesting different pathomechanisms between young and old patients.

Protocol for a randomised controlled trial: efficacy of donepezil against psychosis in Parkinson's disease (EDAP)

INTRODUCTION

Psychosis, including hallucinations and delusions, is one of the important non-motor problems in patients with Parkinson's disease (PD) and is possibly associated with cholinergic neuronal degeneration. The EDAP (Efficacy of Donepezil against Psychosis in PD) study will evaluate the efficacy of donepezil, a brain acetylcholine esterase inhibitor, for prevention of psychosis in PD.

METHODS AND ANALYSIS

Psychosis is assessed every 4 weeks using the Parkinson Psychosis Questionnaire (PPQ) and patients with PD whose PPQ-B score (hallucinations) and PPQ-C score (delusions) have been zero for 8 weeks before enrolment are randomised to two arms: patients receiving donepezil hydrochloride or patients receiving placebo. The patients are then followed for 96 weeks. The primary outcome measure is the time to the event, defined as getting 2 points or more on the PPQ-B score or PPQ-C score, which is assessed using a survival time analysis. The hypothesis being tested is that donepezil prevents psychosis in patients with PD. Efficacy will be tested statistically using the intention-to-treat analysis including a log-rank test or Cox proportional hazard models. Secondary outcomes, such as changes of PPQ scores and Unified Parkinson's Disease Rating Scale scores from baseline will be assessed.

ETHICS AND DISSEMINATION

Ethics approval was received from the Central Review Board of the National Hospital Organization, Tokyo, Japan. The trial was declared and registered to the Pharmaceuticals and Medical Devices Agency(PMDA), Japan (No. 22-4018). All participants will receive a written informed consent that was approved by the Central Review. A completed written informed consent is required to enrol in the study. Severe adverse events will be monitored by investigators and in cases where a severe adverse event was previously unreported, it will be reported to the PMDA.

CLINICAL TRIAL REGISTRATION NUMBER

UMIN000005403.

Characterization of cognitive deficits in rats overexpressing human alpha-synuclein in the ventral tegmental area and medial septum using recombinant adeno-associated viral vectors

Intraneuronal inclusions containing alpha-synuclein (a-syn) constitute one of the pathological hallmarks of Parkinson's disease (PD) and are accompanied by severe neurodegeneration of A9 dopaminergic neurons located in the substantia nigra. Although to a lesser extent, A10 dopaminergic neurons are also affected. Neurodegeneration of other neuronal populations, such as the cholinergic, serotonergic and noradrenergic cell groups, has also been documented in PD patients. Studies in human post-mortem PD brains and in rodent models suggest that deficits in cholinergic and dopaminergic systems may be associated with the cognitive impairment seen in this disease. Here, we investigated the consequences of targeted overexpression of a-syn in the mesocorticolimbic dopaminergic and septohippocampal cholinergic pathways. Rats were injected with recombinant adeno-associated viral vectors encoding for either human wild-type a-syn or green fluorescent protein (GFP) in the ventral tegmental area and the medial septum/vertical limb of the diagonal band of Broca, two regions rich in dopaminergic and cholinergic neurons, respectively. Histopathological analysis showed widespread insoluble a-syn positive inclusions in all major projections areas of the targeted nuclei, including the hippocampus, neocortex, nucleus accumbens and anteromedial striatum. In addition, the rats overexpressing human a-syn displayed an abnormal locomotor response to apomorphine injection and exhibited spatial learning and memory deficits in the Morris water maze task, in the absence of obvious spontaneous locomotor impairment. As losses in dopaminergic and cholinergic immunoreactivity in both the GFP and a-syn expressing animals were mild-to-moderate and did not differ from each other, the behavioral impairments seen in the a-syn overexpressing animals appear to be determined by the long term persisting neuropathology in the surviving neurons rather than by neurodegeneration.

The role of the cerebellum in the pathophysiology of Parkinson's disease

Parkinson's disease (PD), the most common neurodegenerative movement disorder, has traditionally been considered a "classic" basal ganglia disease, as the most obvious pathology is seen in the dopaminergic cells in the substantia nigra pars compacta. Nevertheless recent discoveries in anatomical connections linking the basal ganglia and the cerebellum have led to a re-examination of the role of the cerebellum in the pathophysiology of PD. This review summarizes the role of the cerebellum in explaining many curious features of PD: the significant variation in disease progression between individuals; why severity of dopaminergic deficit correlates with many features of PD such as bradykinesia, but not tremor; and why PD subjects with a tremor-predominant presentation tend to have a more benign prognosis. It is clear that the cerebellum participates in compensatory mechanisms associated with the disease and must be considered an essential contributor to the overall pathophysiology of PD.

Behavioral characterization of the 6-hydroxidopamine model of Parkinson's disease and pharmacological rescuing of non-motor deficits

BACKGROUND

Parkinson's disease (PD) is a chronic neurodegenerative condition that is characterized by motor symptoms as a result of dopaminergic degeneration, particularly in the mesostriatal pathway. However, in recent years, a greater number of clinical studies have focused on the emergence of non-motor symptoms in PD patients, as a consequence of damage on the mesolimbic and mesocortical dopaminergic networks, and on their significant impact on the quality of life of PD patients. Herein, we performed a thorough behavioral analysis including motor, emotional and cognitive dimensions, of the unilateral medial forebrain bundle (MFB) 6-hydroxidopamine (6-OHDA)-lesioned model of PD, and further addressed the impact of pharmacological interventions with levodopa and antidepressants on mood dimensions.

RESULTS

Based on apomorphine-induced turning behaviour and degree of dopaminergic degeneration, animals submitted to MFB lesions were subdivided in complete and incomplete lesion groups. Importantly, this division also translated into a different severity of motor and exploratory impairments and depressive-like symptoms; in contrast, no deficits in anxiety-like and cognitive behaviors were found in MFB-lesioned animals. Subsequently, we found that the exploratory and the anhedonic behavioural alterations of MFB-lesioned rats can be partially improved with the administration of both levodopa or the antidepressant bupropion, but not paroxetine.

CONCLUSIONS

Our results suggest that this model is a relevant tool to study the pathophysiology of motor and non-motor symptoms of PD. In addition, the present data shows that pharmacological interventions modulating dopaminergic transmission are also relevant to revert the non-motor behavioral deficits found in the disease.

In vivo neurochemical imaging of olfactory dysfunction in Parkinson's disease

Olfactory dysfunction is common in Parkinson's disease (PD) and has been attributed to early deposition of α-synuclein pathology in olfactory areas. The pathophysiology of olfactory dysfunction in PD, however, remains poorly understood. Changes in odor identification suggest in part impairment in odor memory, possibly due to hippocampal dysfunction. Olfactory dysfunction occurs also in Alzheimer's disease (AD) and increases with severity of dementia. Cholinergic degeneration is not only a feature of AD but can also occur in PD, at least in a subset of patients with cognitive changes. We reported previously that impaired odor identification in early PD is more closely correlated with hippocampal dopaminergic than nigrostriatal dopaminergic denervation. Results of our multi-tracer PET studies show that odor identification deficits in PD are best predicted by cholinergic denervation and to a lesser extent by dopaminergic denervation. These results suggest that olfactory dysfunction in PD may have multiple components including hippocampal dysfunction secondary to cholinergic and dopaminergic denervations. Olfactory dysfunction in PD may be the most marked in subjects at risk of incipient dementia, and may reflect the transition of PD toward a stage with more heterogeneous multi-system neurodegenerations. Our preliminary imaging data do not support a significant contribution of amyloidopathy or serotoninergic denervation to abnormal olfactory functions in PD, at least in the absence of dementia. We outline how progressive changes in olfaction may be used as a biomarker of cholinergic denervation and cognitive decline in PD patients. We will discuss also the utility of olfactory testing as an early screening test for neurodegeneration.

Neural and immune mechanisms in the pathogenesis of Parkinson's disease

Although almost 50 years have passed since impaired dopaminergic transmission was identified as the main neurochemical defect in Parkinson's disease (PD), the cause of the disease remains unknown. A restricted number of biological mechanisms are likely to contribute to the process of cell death in the nigrostriatal pathway. These mechanisms include mitochondrial defects and enhanced formation of reactive oxygen species--leading to oxidative damage--and abnormal protein aggregation. In addition to or, possibly, intermingled with these mechanisms of neuronal damage there is another crucial factor: neuroinflammation. The inflammatory response associated with cell loss in the dopaminergic nigrostriatal tract and, more in general, the role of immune mechanisms are increasingly recognized in PD pathogenesis. Neuroinflammatory changes have been repeatedly demonstrated, in both neurotoxic and transgenic animal models of PD, as well as in PD patients. Transgenic models based on α-synuclein overexpression, in particular, have provided crucial insights into the correlation between this protein and the dichotomous response that microglia can activate, with the polarization toward a cytotoxic (M1) or cytoprotective (M2) phenotype. Full understanding of such mechanisms may set the ground for a fine tuning of the neuroinflammatory process that accompanies and sustains neurodegeneration, thereby opening new therapeutic perspectives for PD.

Alpha-synuclein pathology and axonal degeneration of the peripheral motor nerves innervating pharyngeal muscles in Parkinson disease

Parkinson disease (PD) is a neurodegenerative disease primarily characterized by cardinal motor manifestations and CNS pathology. Current drug therapies can often stabilize these cardinal motor symptoms, and attention has shifted to the other motor and nonmotor symptoms of PD that are resistant to drug therapy. Dysphagia in PD is perhaps the most important drug-resistant symptom because it leads to aspiration and pneumonia, the leading cause of death. Here, we present direct evidence for degeneration of the pharyngeal motor nerves in PD. We examined the cervical vagal nerve (cranial nerve X), pharyngeal branch of nerve X, and pharyngeal plexus innervating the pharyngeal muscles in 14 postmortem specimens, that is, from 10 patients with PD and 4 age-matched control subjects. Synucleinopathy in the pharyngeal nerves was detected using an immunohistochemical method for phosphorylated α-synuclein. Alpha-synuclein aggregates were revealed in nerve X and the pharyngeal branch of nerve X, and immunoreactive intramuscular nerve twigs and axon terminals within the neuromuscular junctions were identified in all of the PD patients but in none of the controls. These findings indicate that the motor nervous system of the pharynx is involved in the pathologic process of PD. Notably, PD patients who have had dysphagia had a higher density of α-synuclein aggregates in the pharyngeal nerves than those without dysphagia. These findings indicate that motor involvement of the pharynx in PD is one of the factors leading to oropharyngeal dysphagia commonly seen in PD patients.

Altered resting-state functional connectivity of the dentate nucleus in Parkinson's disease

We used functional magnetic resonance imaging (fMRI) to measure functional connectivity of the dentate nucleus (DN) between patients with Parkinson's disease (PD) and normal controls who were studied in a resting state. Images were acquired in 18 PD patients and in age- and sex-matched normal controls. Connectivity of the bilateral DN was calculated and compared between patients and controls, connectivity of the bilateral DN within the cerebellum was compared between rigidity and bradykinesia-dominant patients (PD(AR)) and tremor-dominant patients (PD(T)), and correlation analysis was performed between the connectivity strength and behavioral measures within the cerebellum. Some regions in the cerebellum showed enhanced connectivity with the bilateral DN in PD patients, and decreased connectivity of the DN with the bilateral cerebellar posterior lobe was observed in PD(T) as compared to PD(AR). A set of regions consistent with the default mode network showed disrupted connectivity with the DN. Decreased connectivity between the inferior parietal lobule and the DN was also observed in PD patients. Additional analyses did not show any significant correlations between functional connectivity within the cerebellum and Unified Parkinson's Disease Rating Scale-III scores. Our findings suggest that connectivity of the DN in the resting state is disrupted in PD, and there may be a compensatory cerebellar connectivity mechanism in the resting state in PD. Further study of the cerebellum may clarify the pathophysiology of PD.

Biomarkers, mechanisms, and potential prevention of catecholamine neuron loss in Parkinson disease

This chapter is on biomarkers, mechanisms, and potential treatment of catecholamine neuron loss in Parkinson disease (PD). PD is characterized by a movement disorder from loss of nigrostriatal dopamine neurons. An intense search is going on for biomarkers of the disease process. Theoretically, cerebrospinal fluid (CSF) levels of the deaminated DA metabolite, 3,4-dihydroxyphenylacetic acid (DOPAC), should be superior to other neurochemical indices of loss of central dopamine. CSF DOPAC is low in PD-even in patients with recent onset of Parkinsonism. Cardiac norepinephrine depletion is as severe as the loss of putamen dopamine. PD importantly involves nonmotor manifestations, including anosmia, dementia, REM behavior disorder, and orthostatic hypotension, and all of these nonmotor features are associated with neuroimaging evidence for cardiac sympathetic denervation, which seems to occur independently of the movement disorder and striatal dopaminergic lesion. Analogy to a bank robber's getaway car conveys the catecholaldehyde hypothesis, according to which buildup of the dopamine metabolite 3,4-dihydroxyphenylacetaldehyde (DOPAL), the immediate product of the action of monoamine oxidase on cytosolic dopamine, causes or contributes to the death of dopamine neurons. Decreased vesicular uptake of dopamine and decreased DOPAL detoxification by aldehyde dehydrogenase (ALDH) determine this buildup. Vesicular uptake is also markedly decreased in the heart in PD. Multiple factors influence vesicular uptake and ALDH activity. Evidence is accruing for aging-related induction of positive feedback loops and an autotoxic final common pathway in the death of catecholamine neurons, mediated by metabolites produced continuously in neuronal life. The catecholaldehyde hypothesis also leads to testable experimental therapeutic ideas.

Association study on pathological gambling and polymorphisms of dopamine D1, D2, D3, and D4 receptor genes in a Korean population

Several studies have reported that some dopaminergic receptor polymorphisms are associated with pathological gambling (PG). Considering that there are major race and ethnic group difference in dopaminergic polymorphisms, the result of genetic association studies should be confirmed in more homogeneous population to avoid problems of population stratification. The present study aimed to investigate whether selected polymorphisms in the dopamine receptors genes (DRD1, DRD2, DRD3, and DRD4) are associated with PG in Korean population which is consisted of only Korean ancestry. Subjects were 104 men with a diagnosis of PG and 114 unrelated age-matched normal control men. Genotyping was performed for the DRD1 gene -48 A/G, DRD2 gene TaqI A, DRD3 gene Ser9Gly, and DRD4 gene exon III variable number tandem repeat polymorphisms. The method of multifactor dimensionality reduction (MDR) was used to analyze gene-gene interactions. There were no differences in the frequencies of any studied polymorphisms between patients with PG and normal controls. MDR analysis did not show a significant effect of the 4 dopamine receptor gene polymorphisms on susceptibility to PG (P > 0.05). The present study suggests that the analyzed polymorphisms of the dopamine receptor genes might not be associated with PG in a Korean population.

Higher iron in the red nucleus marks Parkinson's dyskinesia

Dopamine cell loss and increased iron in the substantia nigra (SN) characterize Parkinson's disease (PD), with cerebellar involvement increasingly recognized, particularly in motor compensation and levodopa-induced dyskinesia (LID) development. Because the red nucleus (RN) mediates cerebellar circuitry, we hypothesized that RN iron changes might reflect cerebellum-related compensation, and/or the intrinsic capacity for LID development. We acquired high resolution magnetic resonance images from 23 control and 38 PD subjects (12 with PD and history of LID [PD+DYS]) and 26 with PD and no history of LID (PD-DYS). Iron content was estimated from bilateral RN and SN transverse relaxation rates (R2*). PD subjects overall displayed higher R2* values in both the SN and RN. RN R2* values correlated with off-drug Unified Parkinson's Disease Rating Scale-motor scores, but not disease duration or drug dosage. RN R2* values were significantly higher in PD+DYS compared with control and PD-DYS subjects; control and PD-DYS subjects did not differ. The association of higher RN iron content with PD-related dyskinesia suggests increased iron content is involved in, or reflects, greater cerebellar compensatory capacity and thus increased likelihood of LID development.

Spinal cord lesions in sporadic Parkinson's disease

In this autopsy-based study, α-synuclein immunohistochemistry and lipofuscin pigment-Nissl architectonics in serial sections of 100 μm thickness were used to investigate the spinal cords and brains of 46 individuals: 28 patients with clinically and neuropathologically confirmed Parkinson's disease, 6 cases with incidental Lewy body disease, and 12 age-matched controls. α-Synuclein inclusions (particulate aggregations, Lewy neurites/bodies) in the spinal cord were present between neuropathological stages 2-6 in all cases whose brains were staged for Parkinson's disease-related synucleinopathy. The only individuals who did not have Lewy pathology in the spinal cord were a single stage 1 case (incidental Lewy body disease) and all controls. Because the Parkinson's disease-related lesions were observable in the spinal cord only after Lewy pathology was seen in the brain, it could be concluded that, within the central nervous system, sporadic Parkinson's disease does not begin in the spinal cord. In addition: (1) α-Synuclein-immunoreactive axons clearly predominated over Lewy bodies throughout the spinal cord and were visible in medial and anterior portions of the anterolateral funiculus. Their terminal axons formed dense α-synuclein-immunoreactive networks in the gray matter and were most conspicuous in the lateral portions of layers 1, 7, and in the cellular islands of layer 9. (2) Notably, this axonopathy increased remarkably in density from cervicothoracic segments to lumbosacral segments of the cord. (3) Topographically, it is likely that the spinal cord α-synuclein immunoreactive axonal networks represent descending projections from the supraspinal level setting nuclei (locus coeruleus, lower raphe nuclei, magnocellular portions of the reticular formation). (4) Following the appearance of the spinal cord axonal networks, select types of projection neurons in the spinal cord gray matter displayed α-synuclein-immunoreactive inclusions: chiefly, nociceptive neurons of the dorsal horn in layer 1, sympathetic and parasympathetic preganglionic neurons in layer 7, the cellular pools of α-motoneurons in layer 9, and the smaller motoneurons in Onuf's nucleus in layer 9 (ventral horn). The spinal cord lesions may contribute to clinical symptoms (e.g., pain, constipation, poor balance, lower urinary tract complaints, and sexual dysfunction) that occur during the premotor and motor phases of sporadic Parkinson's disease.

Parkinson's disease: from genetics to treatments

Parkinson's disease (PD) is a common neurodegenerative disease and typically presents with tremor, rigidity, bradykinesia, and postural instability. The hallmark pathological features of PD are loss of dopaminergic neurons in the substantia nigra (SN) and the presence of neuronal intracellular Lewy body (LB) inclusions. In general, PD is sporadic; however, familial PD, while uncommon, can be inherited in an autosomal dominant (AD) or autosomal recessive (AR) manner. The molecular investigations of proteins encoded by PD-linked genes have clarified that ADPD is associated with α-synuclein and LRRK2, while ARPD is linked to Parkin, PINK1, DJ1, and ATP13A2. Understanding these genes can bring insights into this disease and create possible genetic tests for early diagnosis. Long-term pharmacological treatment is so far disappointing, probably due to unwanted complications and decreasing drug efficacy. Several strategies have been proposed and tested as alternatives for PD. Cellular transplantation of dopamine-secreting stem cells opens the door to new therapeutic avenues for restoration of the functions of degenerative and/or damaged neurons in PD.

Cognitive impairment in Parkinson's disease: the dual syndrome hypothesis

Research into the heterogeneous nature of cognitive impairment documented in patients with Parkinson's disease (PD) has focused on disentangling deficits that vary between individuals, evolve and respond differentially to pharmacological treatments, and relate differentially to PD dementia (PDD). We summarise studies conducted in our laboratory over the last 2 decades, outlining the incremental development of our hypotheses, the starting point for which is our early work on executive deficits mirroring fronto-striatal dysfunction. We present subsequent findings linking these deficits to a model of dopaminergic function that conforms to an inverted curvilinear function. We review studies that investigated the range of dopamine-independent attentional and visuospatial memory deficits seen in PD, demonstrating that abnormalities in these domains more accurately predict PDD. We conclude with an exposition of the dual syndrome hypothesis, which distinguishes between dopaminergically mediated fronto-striatal executive impairments and a dementia syndrome with distinctive prodromal visuospatial deficits in which cholinergic treatments offer some clinical benefits.

Biology of Parkinson's disease: pathogenesis and pathophysiology of a multisystem neurodegenerative disorder

Parkinson's disease (PD) is the second most common movement disorder. The characteristic motor impairments - bradykinesia, rigidity, and resting tremor - result from degenerative loss of midbrain dopamine (DA) neurons in the substantia nigra, and are responsive to symptomatic treatment with dopaminergic medications and functional neurosurgery. PD is also the second most common neurodegenerative disorder. Viewed from this perspective, PD is a disorder of multiple functional systems, not simply the motor system, and of multiple neurotransmitter systems, not merely that of DA. The characteristic pathology - intraneuronal Lewy body inclusions and reduced numbers of surviving neurons - is similar in each of the targeted neuron groups, suggesting a common neurodegenerative process. Pathological and experimental studies indicate that oxidative stress, proteolytic stress, and inflammation figure prominently in the pathogenesis of PD. Yet, whether any of these mechanisms plays a causal role in human PD is unknown, because to date we have no proven neuroprotective therapies that slow or reverse disease progression in patients with PD. We are beginning to understand the pathophysiology of motor dysfunction in PD, but its etiopathogenesis as a neurodegenerative disorder remains poorly understood.

Parkinson's disease and parkinsonism: neuropathology

Parkinsonism, the clinical term for a disorder with prominent bradykinesia and variable associated extrapyramidal signs and symptoms, is accompanied by degeneration of the nigrostriatal dopaminergic system, with neuronal loss and reactive gliosis in the substantia nigra found at autopsy. Parkinsonism is pathologically heterogeneous, with the most common pathologic substrates related to abnormalities in the presynaptic protein α-synuclein or the microtubule binding protein tau. In idiopathic Parkinson's disease (PD), α-synuclein accumulates in neuronal perikarya (Lewy bodies) and neuronal processes (Lewy neurites). The disease process is multifocal and involves select central nervous system neurons and peripheral autonomic nervous system neurons. The particular set of neurons affected determines nonmotor clinical presentations. Multiple system atrophy (MSA) is the other major α-synucleinopathy. It is also associated with autonomic dysfunction and in some cases with cerebellar signs. The hallmark histopathologic feature of MSA is accumulation of α-synuclein within glial cytoplasmic inclusions (GCI). The most common of the Parkinsonian tauopathies is progressive supranuclear palsy (PSP), which is clinically associated with severe postural instability leading to early falls. The tau pathology of PSP also affects both neurons and glia. Given the population frequency of PD, α-synuclein pathology similar to that in PD, but not accompanied by neuronal loss, is relatively common (10% of people over 65 years of age) in neurologically normal individuals, leading to proposed staging schemes for PD progression. Although MSA-like and PSP-like pathology can be detected in neurologically normal individuals, such cases are too infrequent to permit assessment of patterns of disease progression.

Altered pharyngeal muscles in Parkinson disease

Dysphagia (impaired swallowing) is common in patients with Parkinson disease (PD) and is related to aspiration pneumonia, the primary cause of death in PD. Therapies that ameliorate the limb motor symptoms of PD are ineffective for dysphagia. This suggests that the pathophysiology of PD dysphagia may differ from that affecting limb muscles, but little is known about potential neuromuscular abnormalities in the swallowing muscles in PD. This study examined the fiber histochemistry of pharyngeal constrictor and cricopharyngeal sphincter muscles in postmortem specimens from 8 subjects with PD and 4 age-matched control subjects. Pharyngeal muscles in subjects with PD exhibited many atrophic fibers, fiber type grouping, and fast-to-slow myosin heavy chain transformation. These alterations indicate that the pharyngeal muscles experienced neural degeneration and regeneration over the course of PD. Notably, subjects with PD with dysphagia had a higher percentage of atrophic myofibers versus with those without dysphagia and controls. The fast-to-slow fiber-type transition is consistent with abnormalities in swallowing, slow movement of food, and increased tone in the cricopharyngeal sphincter in subjects with PD. The alterations in the pharyngeal muscles may play a pathogenic role in the development of dysphagia in subjects with PD.

Tongue force and tongue motility are differently affected by unilateral vs bilateral nigrostriatal dopamine depletion in rats

In addition to its cardinal symptoms of bradykinesia, muscle rigidity, resting tremor and postural disturbances, Parkinson's disease (PD) also affects orolingual motor function. Orolingual motor deficits can contribute to dysphagia, which increases morbidity and mortality in this population. Previous preclinical studies describing orolingual motor deficits in animal models of PD have focused on unilateral nigrostriatal dopamine (DA) depletion. In this study we compared the effects of unilateral vs bilateral 6-hydroxydopamine (6-OHDA)-induced DA depletion in rats trained to lick water from an isometric force-sensing disc. Rats received either unilateral or bilateral 6-OHDA into the medial forebrain bundle and were tested for four weeks post-lesion. Dependent variables included task engagement (the number of licks per session), tongue force (mean and maximum), and tongue motility (the number of licks per second). While both lesion groups exhibited decreased tongue force output, tongue motility deficits were present in only the group that received unilateral nigrostriatal DA depletion. Task engagement was not significantly diminished by 6-OHDA. Analysis of striatal DA tissue content revealed that DA depletion was ∼97% in the unilateral group and ∼90% in the bilateral group. These results suggest that while nigrostriatal DA depletion affects tongue force output, deficits in tongue motility may instead result from a functional imbalance in neural pathways affecting this midline structure.

Mathematical insights into the effects of levodopa

Parkinson’s disease has been traditionally thought of as a dopaminergic disease in which cells of the substantia nigra pars compacta (SNc) die. However, accumulating evidence implies an important role for the serotonergic system in Parkinson’s disease in general and in physiological responses to levodopa therapy, the first line of treatment. We use a mathematical model to investigate the consequences of levodopa therapy on the serotonergic system and on the pulsatile release of dopamine (DA) from dopaminergic and serotonergic terminals in the striatum. Levodopa competes with tyrosine and tryptophan at the blood-brain barrier and is taken up by serotonin neurons in which it competes for aromatic amino acid decarboxylase. The DA produced competes with serotonin (5HT) for packaging into vesicles. We predict the time courses of LD, cytosolic DA, and vesicular DA in 5HT neurons during an LD dose. We predict the time courses of DA and 5HT release from 5HT cell bodies and 5HT terminals as well as the changes in 5HT firing rate due to lower 5HT release. We compute the time course of DA release in the striatum from both 5HT and DA neurons and show how the time course changes as more and more SNc cells die. This enables us to explain the shortening of the therapeutic time window for the efficacy of levodopa as Parkinson’s disease progresses. Finally, we study the effects 5HT1a and 5HT1b autoreceptor agonists and explain why they have a synergistic effect and why they lengthen the therapeutic time window for LD therapy. Our results are consistent with and help explain results in the experimental literature and provide new predictions that can be tested experimentally.

From α-synuclein to synaptic dysfunctions: new insights into the pathophysiology of Parkinson's disease

Alpha-synuclein is a natively unfolded protein playing a key role in the regulation of several neuronal synaptic functions in physiological and pathological conditions. Many studies, over the past years, have shown that it is actively involved in PD pathophysiology. Alpha-synuclein is integrated in a complex network of neuronal processes through the interaction with cytosolic and synaptic proteins. Hence, it is not the sole α-synuclein pathology but its effects on diverse protein partners and specific cellular pathways in the membrane and/or cytosolic districts such as endoplasmic reticulum/Golgi, axonal and synaptic compartments of dopaminergic neurons, that may cause the onset of neuronal cell dysfunction and degeneration which are among the key pathological features of the PD brain. Here we summarize a series of experimental data supporting that α-synuclein aggregation may induce dysfunction and degeneration of synapses via these multiple mechanisms. Taken together, these data add new insights into the complex mechanisms underlying synaptic derangement in PD and other α-synucleinopathies. This article is part of a Special Issue entitled: Brain Integration.

Animal models of Parkinson's disease: a source of novel treatments and clues to the cause of the disease

Animal models of Parkinson's disease (PD) have proved highly effective in the discovery of novel treatments for motor symptoms of PD and in the search for clues to the underlying cause of the illness. Models based on specific pathogenic mechanisms may subsequently lead to the development of neuroprotective agents for PD that stop or slow disease progression. The array of available rodent models is large and ranges from acute pharmacological models, such as the reserpine- or haloperidol-treated rats that display one or more parkinsonian signs, to models exhibiting destruction of the dopaminergic nigro-striatal pathway, such as the classical 6-hydroxydopamine (6-OHDA) rat and 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) mouse models. All of these have provided test beds in which new molecules for treating the motor symptoms of PD can be assessed. In addition, the emergence of abnormal involuntary movements (AIMs) with repeated treatment of 6-OHDA-lesioned rats with L-DOPA has allowed for examination of the mechanisms responsible for treatment-related dyskinesia in PD, and the detection of molecules able to prevent or reverse their appearance. Other toxin-based models of nigro-striatal tract degeneration include the systemic administration of the pesticides rotenone and paraquat, but whilst providing clues to disease pathogenesis, these are not so commonly used for drug development. The MPTP-treated primate model of PD, which closely mimics the clinical features of PD and in which all currently used anti-parkinsonian medications have been shown to be effective, is undoubtedly the most clinically-relevant of all available models. The MPTP-treated primate develops clear dyskinesia when repeatedly exposed to L-DOPA, and these parkinsonian animals have shown responses to novel dopaminergic agents that are highly predictive of their effect in man. Whether non-dopaminergic drugs show the same degree of predictability of response is a matter of debate. As our understanding of the pathogenesis of PD has improved, so new rodent models produced by agents mimicking these mechanisms, including proteasome inhibitors such as PSI, lactacystin and epoximycin or inflammogens like lipopolysaccharide (LPS) have been developed. A further generation of models aimed at mimicking the genetic causes of PD has also sprung up. Whilst these newer models have provided further clues to the disease pathology, they have so far been less commonly used for drug development. There is little doubt that the availability of experimental animal models of PD has dramatically altered dopaminergic drug treatment of the illness and the prevention and reversal of drug-related side effects that emerge with disease progression and chronic medication. However, so far, we have made little progress in moving into other pharmacological areas for the treatment of PD, and we have not developed models that reflect the progressive nature of the illness and its complexity in terms of the extent of pathology and biochemical change. Only when this occurs are we likely to make progress in developing agents to stop or slow the disease progression. The overarching question that draws all of these models together in the quest for better drug treatments for PD is how well do they recapitulate the human condition and how predictive are they of successful translation of drugs into the clinic? This article aims to clarify the current position and highlight the strengths and weaknesses of available models.

New pharmacological avenues for the treatment of L-DOPA-induced dyskinesias in Parkinson's disease: targeting glutamate and adenosine receptors

INTRODUCTION

Parkinson's disease (PD) therapy is still centered on the use of L-3,4-dihydroxyphenylalanine (L-DOPA), which is hampered by numerous side effects, including abnormal involuntary movements known as L-DOPA-induced dyskinesias (LIDs). LIDs are the result of pre- and postsynaptic changes at the corticostriatal level, induced by chronic and pulsatile stimulation of striatal dopaminergic receptors. These changes impact on synaptic plasticity and involve also selected, nondopaminergic receptors expressed by striatal projection neurons.

AREAS COVERED

Among nondopaminergic receptors, glutamate receptors - NMDA and mGluR5 subtypes in particular - and adenosine A(2A) receptors are those most likely involved in LIDs. The aim of the present review is to summarize results of studies undertaken with specific antagonists of these receptors, first conducted in animal models of LIDs, which in selected cases have been translated into clinical trials.

EXPERT OPINION

Selected antagonists of glutamate and adenosine receptors have been proposed as anti-dyskinetic agents. Promising results have been obtained in preclinical investigations and in initial clinical trials, but long-term safety, tolerability and efficacy studies in patients are still required. The current development of novel antagonists, including tools able to act on receptor mosaics, may provide innovative tools for LIDs management in the next future.