Presynaptic mechanisms of motor fluctuations in Parkinson’s disease: a probabilistic model

Patient-specific models link neurotransmitter receptor mechanisms with motor and visuospatial axes of Parkinson's disease

Parkinson's disease involves multiple neurotransmitter systems beyond the classical dopaminergic circuit, but their influence on structural and functional alterations is not well understood. Here, we use patient-specific causal brain modeling to identify latent neurotransmitter receptor-mediated mechanisms contributing to Parkinson's disease progression. Combining the spatial distribution of 15 receptors from post-mortem autoradiography with 6 neuroimaging-derived pathological factors, we detect a diverse set of receptors influencing gray matter atrophy, functional activity dysregulation, microstructural degeneration, and dendrite and dopaminergic transporter loss. Inter-individual variability in receptor mechanisms correlates with symptom severity along two distinct axes, representing motor and psychomotor symptoms with large GABAergic and glutamatergic contributions, and cholinergically-dominant visuospatial, psychiatric and memory dysfunction. Our work demonstrates that receptor architecture helps explain multi-factorial brain re-organization, and suggests that distinct, co-existing receptor-mediated processes underlie Parkinson's disease.

Multimodal Imaging of Substantia Nigra in Parkinson's Disease with Levodopa-Induced Dyskinesia

BACKGROUND

Degeneration of the substantia nigra (SN) may contribute to levodopa-induced dyskinesia (LID) in Parkinson's disease (PD), but the exact characteristics of SN in LID remain unclear.

OBJECTIVE

To further understand the pathogenesis of patients with PD with LID (PD-LID), we explored the structural and functional characteristics of SN in PD-LID using multimodal magnetic resonance imaging (MRI).

METHODS

Twenty-nine patients with PD-LID, 37 patients with PD without LID (PD-nLID), and 28 healthy control subjects underwent T1-weighted MRI, quantitative susceptibility mapping, neuromelanin-sensitive MRI, multishell diffusion MRI, and resting-state functional MRI. Different measures characterizing the SN were obtained using a region of interest-based approach.

RESULTS

Compared with patients with PD-nLID and healthy control subjects, the quantitative susceptibility mapping values of SN pars compacta (SNpc) were significantly higher (P = 0.049 and P = 0.00002), and the neuromelanin contrast-to-noise ratio values in SNpc were significantly lower (P = 0.012 and P = 0.000002) in PD-LID. The intracellular volume fraction of the posterior SN in PD-LID was significantly higher compared with PD-nLID (P = 0.037). Resting-state fMRI indicated that PD-LID in the medication off state showed higher functional connectivity between the SNpc and putamen compared with PD-nLID (P = 0.031), and the functional connectivity changes in PD-LID were positively correlated with Unified Dyskinesia Rating Scale total scores (R = 0.427, P = 0.042).

CONCLUSIONS

Our multimodal imaging findings highlight greater neurodegeneration in SN and the altered nigrostriatal connectivity in PD-LID. These characteristics provide a new perspective into the role of SN in the pathophysiological mechanisms underlying PD-LID. © 2023 The Authors. Movement Disorders published by Wiley Periodicals LLC on behalf of International Parkinson and Movement Disorder Society.

Hallmarks of neurodegenerative disease: A systems pharmacology perspective

Age-related central neurodegenerative diseases, such as Alzheimer's and Parkinson's disease, are a rising public health concern and have been plagued by repeated drug development failures. The complex nature and poor mechanistic understanding of the etiology of neurodegenerative diseases has hindered the discovery and development of effective disease-modifying therapeutics. Quantitative systems pharmacology models of neurodegeneration diseases may be useful tools to enhance the understanding of pharmacological intervention strategies and to reduce drug attrition rates. Due to the similarities in pathophysiological mechanisms across neurodegenerative diseases, especially at the cellular and molecular levels, we envision the possibility of structural components that are conserved across models of neurodegenerative diseases. Conserved structural submodels can be viewed as building blocks that are pieced together alongside unique disease components to construct quantitative systems pharmacology (QSP) models of neurodegenerative diseases. Model parameterization would likely be different between the different types of neurodegenerative diseases as well as individual patients. Formulating our mechanistic understanding of neurodegenerative pathophysiology as a mathematical model could aid in the identification and prioritization of drug targets and combinatorial treatment strategies, evaluate the role of patient characteristics on disease progression and therapeutic response, and serve as a central repository of knowledge. Here, we provide a background on neurodegenerative diseases, highlight hallmarks of neurodegeneration, and summarize previous QSP models of neurodegenerative diseases.

Why do 'OFF' periods still occur during continuous drug delivery in Parkinson's disease?

Continuous drug delivery (CDD) is used in moderately advanced and late-stage Parkinson’s disease (PD) to control motor and non-motor fluctuations (‘OFF’ periods). Transdermal rotigotine is indicated for early fluctuations, while subcutaneous apomorphine infusion and levodopa-carbidopa intestinal gel are utilised in advanced PD. All three strategies are considered examples of continuous dopaminergic stimulation achieved through CDD. A central premise of the CDD is to achieve stable control of the parkinsonian motor and non-motor states and avoid emergence of ‘OFF’ periods. However, data suggest that despite their efficacy in reducing the number and duration of ‘OFF’ periods, these strategies still do not prevent ‘OFF’ periods in the middle to late stages of PD, thus contradicting the widely held concepts of continuous drug delivery and continuous dopaminergic stimulation. Why these emergent ‘OFF’ periods still occur is unknown. In this review, we analyse the potential reasons for their persistence. The contribution of drug- and device-related involvement, and the problems related to site-specific drug delivery are analysed. We propose that changes in dopaminergic and non-dopaminergic mechanisms in the basal ganglia might render these persistent ‘OFF’ periods unresponsive to dopaminergic therapy delivered via CDD.

Striatal synaptic adaptations in Parkinson's disease

The striatum is densely innervated by mesencephalic dopaminergic neurons that modulate acquisition and vigor of goal-directed actions and habits. This innervation is progressively lost in Parkinson's disease (PD), contributing to the defining movement deficits of the disease. Although boosting dopaminergic signaling with levodopa early in the course of the disease alleviates these deficits, later this strategy leads to the emergence of debilitating dyskinesia. Here, recent advances in our understanding of how striatal cells and circuits adapt to this progressive de-innervation and to levodopa therapy are discussed. First, we discuss how dopamine (DA) depletion triggers cell type-specific, homeostatic changes in spiny projection neurons (SPNs) that tend to normalize striatal activity but also lead to disruption of the synaptic architecture sculpted by experience. Second, we discuss the roles played by cholinergic and nitric oxide-releasing interneurons in these adaptations. Third, we examine recent work in freely moving mice suggesting that alterations in the spatiotemporal dynamics of striatal ensembles contributes to PD movement deficits. Lastly, we discuss recently published evidence from a progressive model of PD suggesting that contrary to the classical model, striatal pathway imbalance is necessary but not sufficient to produce frank parkinsonism.

Bipolar spectrum disorders in neurologic disorders

Psychiatric symptoms frequently predate or complicate neurological disorders, such as neurodegenerative diseases. Symptoms of bipolar spectrum disorders (BSD), like mood, behavioral, and psychotic alterations, are known to occur - individually or as a syndromic cluster - in Parkinson's disease and in the behavioral variant of frontotemporal dementia (FTD). Nonetheless, due to shared pathophysiological mechanisms, or genetic predisposition, several other neurological disorders show significant, yet neglected, clinical and biological overlaps with BSD like neuroinflammation, ion channel dysfunctions, neurotransmission imbalance, or neurodegeneration. BSD pathophysiology is still largely unclear, but large-scale network dysfunctions are known to participate in the onset of mood disorders and psychotic symptoms. Thus, functional alterations can unleash BSD symptoms years before the evidence of an organic disease of the central nervous system. The aim of our narrative review was to illustrate the numerous intersections between BSD and neurological disorders from a clinical-biological point of view and the underlying predisposing factors, to guide future diagnostic and therapeutical research in the field.

Baseline cognitive profile is closely associated with long-term motor prognosis in newly diagnosed Parkinson's disease

OBJECTIVES

To investigate the association between cognitive function at baseline and the progression of motor disability in Parkinson's disease (PD).

METHODS

We consecutively enrolled 257 drug-naïve patients with early-stage PD (follow-up > 2 years) who underwent a detailed neuropsychological test at initial assessment. Factor analysis was conducted to yield four cognitive function factors and composite scores thereof: Factor 1 (visual memory/visuospatial), Factor 2 (verbal memory), Factor 3 (frontal/executive), and Factor 4 (attention/working memory/language). The global cognitive composite score of each patient was calculated based on these factors. Subsequently, we assessed the effect of baseline cognitive function on long-term motor outcomes, namely levodopa-induced dyskinesia (LID), wearing-off, freezing of gait (FOG), and rate of longitudinal increases in levodopa-equivalent dose (LED).

RESULTS

Cox regression analysis demonstrated that higher Factor 3 (frontal/executive) composite scores (i.e., better cognitive performance) were associated with early development of LID [hazard ratio (HR), 1.507; p = 0.003], whereas higher Factor 1 (visual memory/visuospatial) composite scores (i.e., better cognitive performance) were associated with a lower risk for FOG (HR 0.683; p = 0.017). We noted that higher global cognitive composite scores were associated with a lower risk for developing FOG (HR 0.455; p = 0.045). The linear mixed model demonstrated that higher global cognitive composite scores and better cognitive performance in visual memory/visuospatial function were associated with slower longitudinal increases in LED.

CONCLUSIONS

These findings suggest that baseline cognitive profiles have prognostic implications on several motor aspects in patients with PD.

Granisetron, a selective 5-HT3 antagonist, reduces L-3,4-dihydroxyphenylalanine-induced abnormal involuntary movements in the 6-hydroxydopamine-lesioned rat

Administration of L-3,4-dihydroxyphenylalanine (L-DOPA) provides Parkinson's disease patients with effective symptomatic relief. However, long-term L-DOPA therapy is often marred by complications such as dyskinesia. We have previously demonstrated that serotonin type 3 (5-HT3) receptor blockade with the clinically available and highly selective antagonist ondansetron alleviates dyskinesia in the 6-hydroxydopamine (6-OHDA)-lesioned rat. Here, we sought to explore the antidyskinetic efficacy of granisetron, another clinically available 5-HT3 receptor antagonist. Rats were rendered hemi-parkinsonian by 6-OHDA injection in the medial forebrain bundle. Following induction of stable abnormal involuntary movements (AIMs), granisetron (0.0001, 0.001, 0.01, 0.1 and 1 mg/kg) or vehicle was acutely administered in combination with L-DOPA and the severity of AIMs, both duration and amplitude, was determined. We also assessed the effect of granisetron on L-DOPA antiparkinsonian action by performing the cylinder test. Adding granisetron (0.0001, 0.001, 0.01, 0.1 and 1 mg/kg) to L-DOPA resulted in a significant reduction of AIMs duration and amplitude, with certain parameters being reduced by as much as 38 and 45% (P < 0.05 and P < 0.001, respectively). The antidyskinetic effect of granisetron was not accompanied by a reduction of L-DOPA antiparkinsonian action. These results suggest that 5-HT3 blockade may reduce L-DOPA-induced dyskinesia without impairing the therapeutic efficacy of L-DOPA. However, a U-shaped dose-response curve obtained with certain parameters may limit the therapeutic potential of this strategy and require further investigation.

Selective blockade of the 5-HT3 receptor acutely alleviates dyskinesia and psychosis in the parkinsonian marmoset

In Parkinson's disease (PD), management of L-3,4-dihydroxyphenylalanine (l-DOPA)-related complications, such as l-DOPA induced dyskinesia and psychosis, remains inadequate, which poses a significant burden on the quality of life of patients. We have shown, in the hemi-parkinsonian rat model of PD, that the selective serotonin type 3 (5-HT₃) receptor antagonists ondansetron and granisetron decreased the severity of established dyskinesia, and ondansetron even attenuated the development of dyskinesia. Here, we seek to confirm these favourable data on dyskinesia and to explore the effect of ondansetron on the severity of psychosis-like behaviours (PLBs) in the gold standard model of PD, the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-lesioned non-human primate. We first determined the pharmacokinetic profile of ondansetron in the marmoset. Subsequently, six MPTP-lesioned marmosets were administered l-DOPA chronically until they exhibited stable and reproducible dyskinesia and PLBs upon each administration of l-DOPA. On behavioural assessment days, ondansetron (0.01, 0.1 and 1 mg/kg) or vehicle was administered in conjunction with l-DOPA, and the severity of dyskinesia, PLBs and parkinsonism was evaluated. Ondansetron 0.1 mg/kg alleviated global dyskinesia severity by 73% (P < 0.0001) and decreased duration of on-time with disabling dyskinesia by 88% (P = 0.0491). Ondansetron 0.1 mg/kg reduced the severity of global PLBs by 80% (P < 0.0001) and suppressed on-time with disabling PLBs (P = 0.0213). Ondansetron enhanced the anti-parkinsonian action of l-DOPA, reducing global parkinsonism by 53% compared to l-DOPA (P = 0.0004). These results suggest that selective blockade of the 5-HT₃ receptor with ondansetron may be an effective approach to alleviate l-DOPA-related complications.

Optogenetic inactivation of the entopeduncular nucleus improves forelimb akinesia in a Parkinson's disease model

We performed optogenetic inactivation of rats' entopeduncular nucleus (EP, homologous to primates' globus pallidus interna (GPi)) and investigated the therapeutic effect in a rat model of PD. 6-Hydroxydopamine (6-OHDA)-induced hemiparkinsonian rats were injected with either a virus for halorhodopsin expression that is used to inactivate GABAergic neurons or a control virus injection and received optic fiber insertion. All the rats were illuminated by 590 nm of light. Each rat was then subjected to sequential sessions of stepping tests under controlled illumination patterns. The stepping test is a reliable evaluation method for forelimb akinesia. The number of adjusting steps was significantly higher in experimental (optogene with reporter gene expression) (5Hz - 10ms: 15.7 ± 1.9, 5Hz - 100ms: 16.0 ± 1.8, continuous: 21.6 ± 1.9) than control rats (reporter gene expression) (5Hz-10ms: 1.9 ± 1.1, 5Hz-100ms: 2.6 ± 1.0, continuous: 2.5 ± 1.2) (p < 0.001). Continuous EP illumination showed a significantly higher improvement of forelimb akinesia than other illumination patterns (p < 0.01). Optogene expression in the GABAergic neurons of the EP was confirmed by immunohistochemistry. Optogenetic inhibition of EP was effective to improve contralateral forelimb akinesia. However, further studies using prolonged illumination are needed to investigate the best illumination pattern for optogenetic stimulation.

Dopamine Transporter, Age, and Motor Complications in Parkinson's Disease: A Clinical and Single-Photon Emission Computed Tomography Study

BACKGROUND

Previous molecular imaging studies comparing dopamine function in vivo between early-onset PD and late-onset PD patients have shown contradictory results, presumably attributable to the aging-related decline in nigrostriatal function.

OBJECTIVES

(1) To investigate baseline dopamine transporter availability in early-onset PD (<55 years) and late-onset PD (>70 years) patients, z-scores values of putamen and caudate [¹²³ I]-ioflupane uptake were calculated using the respective age-matched controls in order to correct for early presynaptic compensatory mechanisms and age-related dopamine neuron loss; (2) to examine the associations of such baseline single-photon emission computed tomography measures with the emergence of late-disease motor complications.

METHODS

In this retrospective study, 105 de novo PD patients who underwent [¹²³ I]-ioflupane single-photon emission computed tomography at time of diagnosis were divided into three tertile groups according to age at disease onset (35 early-onset PD and 40 late-onset PD patients). Z-scores were compared between the two groups, and their predictive power for motor complications (during a mean follow-up of 7 years) was evaluated using Cox proportional hazard models.

RESULTS

Despite a less-severe motor phenotype, early-onset PD patients exhibited more reduced [¹²³ I]-ioflupane binding in the putamen and had a higher and earlier risk for developing motor complications than those with late-onset PD. Lower [¹²³ I]-Ioflupane uptake in the putamen and caudate increased the risk of motor complications.

CONCLUSIONS

Our findings indicate that a lower dopamine transporter binding in early-onset PD predicts the later development of motor complications, but it is not related to severity of motor symptoms, suggesting age-related differences in striatal compensatory mechanisms in PD. © 2020 International Parkinson and Movement Disorder Society.

Inhaled levodopa for intermittent treatment of OFF episodes in patients with Parkinson's disease

Introduction: Many patients with advanced Parkinson's disease (PD) have inadequate control of motor symptoms despite optimized treatment. Predictable and unpredictable OFF periods severely interfere with the quality of life. A drug that easily and rapidly reverts the OFF state is still needed. Subcutaneous apomorphine, the only approved drug for this indication, although efficacious, is not widely used probably due to its potential side effects and complicated administration.Levodopa is the most efficacious drug for the treatment of PD motor symptoms. However, issues related to the oral route and intestinal absorption in later disease stages render this route lengthy and inefficacious.Areas covered: Literature on the development of an inhaled formulation of levodopa has been reviewed. Significant advances in the field of pulmonary delivery systems and in dry powders have enabled the development of a new formulation of levodopa that can be inhaled and adequate blood levels rapidly achieved, bypassing intestinal absorption. Several clinical trials have reported efficacy, safety, and tolerability data. Some pulmonary-related adverse events have been reported but are mostly mild.Expert opinion: This new way of administering levodopa is likely to be very welcome and may fill a gap for OFF rescue treatments, at least for some patients.

Dyskinesia matters

Levodopa-induced dyskinesia (LID) represents a significant source of discomfort for people with Parkinson's disease (PD). It negatively affects quality of life, it is associated with both motor and nonmotor fluctuations, and it brings an increased risk of disability, balance problems, and falls. Although the prevalence of severe LID appears to be lower than in previous eras (likely owing to a more conservative use of oral levodopa), we have not yet found a way to prevent the development of this complication. Advanced surgical therapies, such as deep brain stimulation, ameliorate LID, but only a minority of PD patients qualify for these interventions. Although some have argued that PD patients would rather be ON with dyskinesia than OFF, the deeper truth is that patients would very much prefer to be ON without dyskinesia. As researchers and clinicians, we should aspire to make that goal a reality. To this end, translational research on LID is to be encouraged and persistently pursued. © 2019 International Parkinson and Movement Disorder Society.

Blood Flow and Glucose Metabolism Dissociation in the Putamen Is Predictive of Levodopa Induced Dyskinesia in Parkinson's Disease Patients

Background: The forefront treatment of Parkinson's disease (PD) is Levodopa. When patients are treated with Levodopa cerebral blood flow is increased while cerebral metabolic rate is decreased in key subcortical regions including the putamen. This phenomenon is especially pronounced in patients with Levodopa-induced dyskinesia (LID). Method: To study the effect of clinically-determined anti-parkinsonian medications, 10 PD patients (5 with LID and 5 without LID) have been scanned with FDG-PET (a probe for glucose metabolism) and perfusion MRI (a probe for cerebral blood flow) both when they are ON and OFF medications. Patients additionally underwent resting state fMRI to detect changes in dopamine-mediated cortico-striatal connectivity. The degree of blood flow-glucose metabolism dissociation was quantified by comparing the FDG-PET and perfusion MRI data. Results: A significant interaction effect (imaging modality × medication; blood flow-glucose metabolism dissociation) has been found in the putamen (p = 0.023). Post-hoc analysis revealed that anti-parkinsonian medication consistently normalized the pathologically hyper-metabolic state of the putamen while mixed effects were observed in cerebral blood flow changes. This dissociation was especially predominant in patients with LID compared to those without. Unlike the prior study, this differentiation was not observed when cortico-striatal functional connectivity was assessed. Conclusion: We confirmed striatal neurovascular dissociation between FDG-PET and perfusion MRI in response to clinically determined anti-parkinsonian medication. We further proposed a novel analytical method to quantify the degree of dissociation in the putamen using only the ON condition scans, Putamen-to-thalamus Hyper-perfusion/hypo-metabolism Index (PHI), which may have the potential to be used as a biomarker for LID (correctly classifying 8 out 10 patients). For wider use of PHI, a larger validation study is warranted.

Are There Benefits in Adding Catechol-O Methyltransferase Inhibitors in the Pharmacotherapy of Parkinson's Disease Patients? A Systematic Review

BACKGROUND

A qualified consensus suggests that a combination of levodopa with a peripherally acting dopa decarboxylase inhibitor continues to present the gold standard treatment of Parkinson's disease (PD). However, as the disease progresses the therapeutic window of levodopa becomes narrowed. Pharmacological strategies for motor fluctuations are focused on providing less pulsatile and more continuous dopaminergic stimulation. Peripheral catechol-O-methyltransferase (COMT) inhibition improves the bioavailability of levodopa and results in a prolonged response.

OBJECTIVE

The primary aim of this study was to investigate the efficacy and safety of the two available COMT inhibitors; entacapone and tolcapone and the recently introduced opicapone.

METHODS

Electronic databases were systematically searched for original studies published within the last 37 years. In addition, lists of identified studies, reviews and their references were examined.

RESULTS

Twelve studies fulfilled the inclusion criteria. 3701 patients with PD were included in this systematic review.

CONCLUSIONS

Adjuvant treatment of PD patients experiencing motor fluctuations with entacapone resulted in improvement of motor function and was well tolerated. Therefore, entacapone presented an acceptable benefit to risk ratio. Tolcapone appeared to result in a greater therapeutic effect. However, this was not consistent across all motor variables and studies, and thus would not support its use, given the current onerous monitoring that is required. Opicapone was not associated with adverse reactions in a phase III trial but did not present a greater efficacy than entacapone, and thus further studies are required in order to illustrate its cost effectiveness.

Molecular Imaging of the Dopamine Transporter

Dopamine transporter (DAT) single-photon emission tomography (SPECT) with (123)Ioflupane is a widely used diagnostic tool for patients with suspected parkinsonian syndromes, as it assists with differentiating between Parkinson’s disease (PD) or atypical parkinsonisms and conditions without a presynaptic dopaminergic deficit such as essential tremor, vascular and drug-induced parkinsonisms. Recent evidence supports its utility as in vivo proof of degenerative parkinsonisms, and DAT imaging has been proposed as a potential surrogate marker for dopaminergic nigrostriatal neurons. However, the interpretation of DAT-SPECT imaging may be challenged by several factors including the loss of DAT receptor density with age and the effect of certain drugs on dopamine uptake. Furthermore, a clear, direct relationship between nigral loss and DAT decrease has been controversial so far. Striatal DAT uptake could reflect nigral neuronal loss once the loss exceeds 50%. Indeed, reduction of DAT binding seems to be already present in the prodromal stage of PD, suggesting both an early synaptic dysfunction and the activation of compensatory changes to delay the onset of symptoms. Despite a weak correlation with PD severity and progression, quantitative measurements of DAT binding at baseline could be used to predict the emergence of late-disease motor fluctuations and dyskinesias. This review addresses the possibilities and limitations of DAT-SPECT in PD and, focusing specifically on regulatory changes of DAT in surviving DA neurons, we investigate its role in diagnosis and its prognostic value for motor complications as disease progresses.

Neuromelanin or DaT-SPECT: which is the better marker for discriminating advanced Parkinson's disease?

Background and purpose

Whether the neuromelanin‐positive substantia nigra pars compacta area (NM‐SNc) on neuromelanin magnetic resonance imaging (NM‐MRI) and the specific binding ratio (SBR) on ¹²³I‐N‐v‐fluoropropyl‐2b‐carbomethoxy3b‐(4‐iodophenyl)nortropane single photon emission computed tomography (DaT‐SPECT) can be correlated with motor fluctuations (MFs) in advanced Parkinson's disease (PD) was investigated.

Methods

Thirty‐five PD patients (60 ± 13 years) and 23 healthy individuals as controls (59 ± 19 years) were enrolled. The relationships between NM‐MRI and DaT‐SPECT were prospectively examined in two subgroups divided according to the presence or absence of MFs. Multivariate analysis was performed using the Cox proportional hazard model to screen for association factors.

Results

The NM‐SNc size was correlated with the SBR (Spearman's ρ = 0.43, P < 0.05). The NM‐SNc size was significantly reduced in PD with MFs compared with the subgroup without (P < 0.001), whereas the SBR did not significantly differ between the groups. NM‐SNc size was a significant association factor for MFs (hazard ratio 0.94, P = 0.04). In receiver operating characteristic analysis of the factors for MF occurrence, the area under the receiver operating characteristic curve of the NM‐SNc size showed a significant difference of 0.89 (P < 0.05) but no significant difference was found in the SBR.

Conclusions

NM‐SNc size was significantly correlated with the SBR in PD, but several factors in advanced PD were more closely associated with NM‐SNc size than the SBR. NM‐MRI might reflect the status of advanced PD more accurately than DaT‐SPECT. Therefore, NM‐MRI appears to provide a better marker for discriminating advanced PD than DaT‐SPECT.

Objective Measurement and Characterization of Sleep Benefit in Parkinson's Disease

Background

Sleep benefit (SB) in Parkinson's disease refers to improved motor symptoms upon waking despite an entire night without medications. Although it was first proposed 30 years ago, this phenomenon proved difficult to investigate, and its true prevalence and underlying mechanisms remain unclear.

Objective

This study aimed to identify and quantify SB through measurement of motor function using a validated smartphone application and to identify disease characteristics that predicted SB.

Methods

Ninety-two patients recruited from 2 Movement Disorder Services were clinically assessed at home using a validated smartphone application. Each patient was tested in the on-state, at the end of dose, and on waking (before medications) 3 times. Differences between the 3 states were used to determine the impact of sleep and levodopa on motor function. SB was considered to be a "measurable improvement in parkinsonism from the end of dose."

Results

The morning waking motor function of 20 patients (22%) improved compared with the end-of-dose function, with 9 patients demonstrating superior function compared with their on-state. No clinical features predicted SB. Although all participants subjectively reported motor fluctuations, only 35 patients (38%) demonstrated an objective improvement with levodopa. Patients who had SB more often demonstrated objective motor fluctuations compared with those who did not (65% vs. 31%; P = 0.008).

Conclusions

SB is a genuine motor phenomenon: 1 in 5 patients have a measurable improvement in motor function on waking. It remains questionable whether this improvement is a direct effect of sleep. Until its underlying mechanism is better understood, it is more appropriate to refer to this phenomenon as simply morning improvement or diurnal fluctuation of motor symptoms.

Monoaminergic and Histaminergic Strategies and Treatments in Brain Diseases

The monoaminergic systems are the target of several drugs for the treatment of mood, motor and cognitive disorders as well as neurological conditions. In most cases, advances have occurred through serendipity, except for Parkinson's disease where the pathophysiology led almost immediately to the introduction of dopamine restoring agents. Extensive neuropharmacological studies first showed that the primary target of antipsychotics, antidepressants, and anxiolytic drugs were specific components of the monoaminergic systems. Later, some dramatic side effects associated with older medicines were shown to disappear with new chemical compounds targeting the origin of the therapeutic benefit more specifically. The increased knowledge regarding the function and interaction of the monoaminergic systems in the brain resulting from in vivo neurochemical and neurophysiological studies indicated new monoaminergic targets that could achieve the efficacy of the older medicines with fewer side-effects. Yet, this accumulated knowledge regarding monoamines did not produce valuable strategies for diseases where no monoaminergic drug has been shown to be effective. Here, we emphasize the new therapeutic and monoaminergic-based strategies for the treatment of psychiatric diseases. We will consider three main groups of diseases, based on the evidence of monoamines involvement (schizophrenia, depression, obesity), the identification of monoamines in the diseases processes (Parkinson's disease, addiction) and the prospect of the involvement of monoaminergic mechanisms (epilepsy, Alzheimer's disease, stroke). In most cases, the clinically available monoaminergic drugs induce widespread modifications of amine tone or excitability through neurobiological networks and exemplify the overlap between therapeutic approaches to psychiatric and neurological conditions. More recent developments that have resulted in improved drug specificity and responses will be discussed in this review.

Optogenetic Inhibition of the Subthalamic Nucleus Reduces Levodopa-Induced Dyskinesias in a Rat Model of Parkinson's Disease

BACKGROUND

The inhibition of neuronal activity by electrical deep brain stimulation is one of the mechanisms explaining the amelioration of levodopa-induced dyskinesia. However, electrical deep brain stimulation cannot specifically activate or inactivate selected types of neurons.

OBJECTIVES

We applied optogenetics as an alternative treatment to deep brain stimulation for levodopa-induced dyskinesia, and also to confirm that the mechanism of levodopa-induced dyskinesia amelioration by subthalamic nucleus deep brain stimulation is mediated through neuronal inhibition.

METHODS

6-hydroxydopamine-induced hemiparkinsonian rats received injections of hSynapsin1-NpHR-YFP adeno-associated virus (AAV) or hSynapsin1-YFP AAV. Two weeks after viral injections, all rats were treated with daily injections of levodopa. Then, the optic fiber was implanted into the ipsilateral subthalamic nucleus. We performed various behavioral tests to evaluate the changes in levodopa-induced dyskinesias after optogenetic expression and illumination in the subthalamic nucleus.

RESULTS

The behavioral tests revealed that optical inhibition of the subthalamic nucleus significantly ameliorated levodopa-induced dyskinesia by reducing the duration of the dyskinesias as well as the severity of axial dyskinesia.

CONCLUSIONS

These findings will provide a useful foundation for the future development of optogenetic modulation systems that could be considered as an approach to dyskinesia therapy.

Polymorphism of the dopamine transporter type 1 gene modifies the treatment response in Parkinson's disease

After more than 50 years of treating Parkinson's disease with l-DOPA, there are still no guidelines on setting the optimal dose for a given patient. The dopamine transporter type 1, now known as solute carrier family 6 (neurotransmitter transporter), member 3 (SLC6A3) is the most powerful determinant of dopamine neurotransmission and might therefore influence the treatment response. We recently demonstrated that methylphenidate (a dopamine transporter inhibitor) is effective in patients with Parkinson's disease with motor and gait disorders. The objective of the present study was to determine whether genetic variants of the dopamine transporter type 1-encoding gene (SLC6A3) are associated with differences in the response to treatment of motor symptoms and gait disorders with l-DOPA and methylphenidate (with respect to the demographic, the disease and the treatment parameters and the other genes involved in the dopaminergic neurotransmission). This analysis was part of a multicentre, parallel-group, double-blind, placebo-controlled, randomized clinical trial of methylphenidate in Parkinson's disease (Protocol ID:2008-005801-20; ClinicalTrials.gov:NCT00914095). We scored the motor Unified Parkinson's Disease Rating Scale and the Stand-Walk-Sit Test before and after a standardized acute l-DOPA challenge before randomization and then after 3 months of methylphenidate treatment. Patients were screened for variants of genes involved in dopamine metabolism: rs28363170 and rs3836790 polymorphisms in the SLC6A3 gene, rs921451 and rs3837091 in the DDC gene (encoding the aromatic L-amino acid decarboxylase involved in the synthesis of dopamine from l-DOPA), rs1799836 in the MAOB gene (coding for monoamine oxidase B) and rs4680 in the COMT gene (coding for catechol-O-methyltransferase). Investigators and patients were blinded to the genotyping data throughout the study. Eighty-one subjects were genotyped and 61 were analysed for their acute motor response to l-DOPA. The SLC6A3 variants were significantly associated with greater efficacy of l-DOPA for motor symptoms. The SLC6A3 variants were also associated with greater efficacy of methylphenidate for motor symptoms and gait disorders in the ON l-DOPA condition. The difference between motor Unified Parkinson's Disease Rating Scale scores for patients with different SLC6A3 genotypes was statistically significant in a multivariate analysis that took account of other disease-related, treatment-related and pharmacogenetic parameters. Our preliminary results suggest that variants of SLC6A3 are genetic modifiers of the treatment response to l-DOPA and methylphenidate in Parkinson's disease. Further studies are required to assess the possible value of these genotypes for (i) guiding l-DOPA dose adaptations over the long term; and (ii) establishing the risk/benefit balance associated with methylphenidate treatment for gait disorders.

Risk factors and safe dosage of levodopa for wearing-off phenomenon in Chinese patients with Parkinson's disease

The objective of this study was to investigate the risk factors of wearing-off phenomenon in Parkinson's disease (PD) and propose safe dosage of levodopa to reduce wearing-off development based on Chinese cohort. Patients with PD who had taken levodopa (L-dopa) for at least 1 month were recruited. Wearing-off was diagnosed based on validated Chinese version of a patient self-rated 9-question Wearing-Off Questionnaire (WOQ-9) and clinical definition. Eleven variables (gender, disease duration at L-dopa initiation, disease duration at assessment, age at onset, age at assessment, H-Y stage, UPDRS III, L-dopa daily total dosage and dosage adjusted to weight, duration of L-dopa treatment, initial drug recipe) were included in our analysis. Univariate analysis, multivariate logistic regression analysis and decision tree classification model(DTC) were used to detect risk factors of wearing-off. Receiver operating characteristic (ROC) curve and DTC were used to investigate cut-off value of L-dopa to best predict wearing-off. Two hundred and thirty-four patients were investigated in our study, among whom 111 developed wearing-off. Patients with wearing-off tended to receive higher L-dopa dosage and endure longer duration of L-dopa treatment. L-Dopa dosage as 281 mg/day and 4.2 mg/kg/day by ROC, as well as 269 mg/day and 3.2 mg/kg/day by DTC were cut-off values for wearing-off. L-Dopa dosage and duration of L-dopa treatment were related to increased wearing-off development. Cumulative L-dopa dosage and L-dopa daily dosage were better predictive of wearing-off. Inadequate evidence was present for delayed L-dopa initiation. L-Dopa daily dosage no more than 275 mg or 4.2 mg/kg was regarded as safe.

Three mechanisms by which striatal denervation causes breakdown of dopamine signaling

Progressive loss of nigrostriatal dopamine (DA) neurons is the neuropathological hallmark of Parkinson's disease (PD). Symptoms of the disease can often be treated by DA D2 agonists and thus seem related to disinhibition of the indirect striatal pathway. However, there is no evidence that symptoms arise by low extracellular DA concentration or are associated with reduced D2 receptor binding. Here I provide a theoretical analysis of the pathophysiology and postsynaptic adaptation resulting from striatal DA denervation. I found that progressive denervation may alter DA signaling by three independent mechanisms depending on degree of denervation and macroscopic morphology of the lesion. As long as the remaining innervation stays anatomically coherent, denervation reduces phasic variations in extracellular DA, but the DA tone is not changed. The reduction of phasic signaling can be partially compensated by upregulating postsynaptic signaling cascades. However, changes in DA dynamics evade compensation. With 80-99% denervation, a persistent aberrant signal develops in D2-regulated pathways caused by random fluctuations in tonic DA release. Permanent low DA levels occur in regions completely void of innervation. Simulation of l-dopa therapy reduced the aberrant D2 signal. With a high degree of denervation, l-dopa enhanced another aberrant signal, this time in the D1 pathway. This analysis provides a quantitative, physiologically consistent view of the early and late stages of PD, the effect of main therapeutic medications, and potential side effects. The mechanisms described here may also provide an explanation to currently inexplicable pathological phenomena such as psycho stimulant-induced contraversive rotations in animal models.

Presynaptic Mechanisms of l-DOPA-Induced Dyskinesia: The Findings, the Debate, and the Therapeutic Implications

The dopamine (DA) precursor l-DOPA has been the most effective treatment for Parkinson’s disease (PD) for over 40 years. However, the response to this treatment changes with disease progression, and most patients develop dyskinesias (abnormal involuntary movements) and motor fluctuations within a few years of l-DOPA therapy. There is wide consensus that these motor complications depend on both pre- and post-synaptic disturbances of nigrostriatal DA transmission. Several presynaptic mechanisms converge to generate large DA swings in the brain concomitant with the peaks-and-troughs of plasma l-DOPA levels, while post-synaptic changes engender abnormal functional responses in dopaminoceptive neurons. While this general picture is well-accepted, the relative contribution of different factors remains a matter of debate. A particularly animated debate has been growing around putative players on the presynaptic side of the cascade. To what extent do presynaptic disturbances in DA transmission depend on deficiency/dysfunction of the DA transporter, aberrant release of DA from serotonin neurons, or gliovascular mechanisms? And does noradrenaline (which is synthetized from DA) play a role? This review article will summarize key findings, controversies, and pending questions regarding the presynaptic mechanisms of l-DOPA-induced dyskinesia. Intriguingly, the debate around these mechanisms has spurred research into previously unexplored facets of brain plasticity that have far-reaching implications to the treatment of neuropsychiatric disease.

Are human dental papilla-derived stem cell and human brain-derived neural stem cell transplantations suitable for treatment of Parkinson's disease?

Transplantation of neural stem cells has been reported as a possible approach for replacing impaired dopaminergic neurons. In this study, we tested the efficacy of early-stage human dental papilla-derived stem cells and human brain-derived neural stem cells in rat models of 6-hydroxydopamine-induced Parkinson's disease. Rats received a unilateral injection of 6-hydroxydopamine into right medial forebrain bundle, followed 3 weeks later by injections of PBS, early-stage human dental papilla-derived stem cells, or human brain-derived neural stem cells into the ipsilateral striatum. All of the rats in the human dental papilla-derived stem cell group died from tumor formation at around 2 weeks following cell transplantation. Postmortem examinations revealed homogeneous malignant tumors in the striatum of the human dental papilla-derived stem cell group. Stepping tests revealed that human brain-derived neural stem cell transplantation did not improve motor dysfunction. In apomorphine-induced rotation tests, neither the human brain-derived neural stem cell group nor the control groups (PBS injection) demonstrated significant changes. Glucose metabolism in the lesioned side of striatum was reduced by human brain-derived neural stem cell transplantation. [¹⁸F]-FP-CIT PET scans in the striatum did not demonstrate a significant increase in the human brain-derived neural stem cell group. Tyrosine hydroxylase (dopaminergic neuronal marker) staining and G protein-activated inward rectifier potassium channel 2 (A9 dopaminergic neuronal marker) were positive in the lesioned side of striatum in the human brain-derived neural stem cell group. The use of early-stage human dental papilla-derived stem cells confirmed its tendency to form tumors. Human brain-derived neural stem cells could be partially differentiated into dopaminergic neurons, but they did not secrete dopamine.

Maladaptive plasticity in levodopa-induced dyskinesias and tardive dyskinesias: old and new insights on the effects of dopamine receptor pharmacology

Maladaptive plasticity can be defined as behavioral loss or even development of disease symptoms resulting from aberrant plasticity changes in the human brain. Hyperkinetic movement disorders, in the neurological or psychiatric realms, have been associated with maladaptive neural plasticity that can be expressed by functional changes such as an increase in transmitter release, receptor regulation, and synaptic plasticity or anatomical modifications such as axonal regeneration, sprouting, synaptogenesis, and neurogenesis. Recent evidence from human and animal models provided support to the hypothesis that these phenomena likely depend on altered dopamine turnover induced by long-term drug treatment. However, it is still unclear how and where these altered mechanisms of cortical plasticity may be localized. This study provides an up-to-date overview of these issues together with some reflections on future studies in the field, particularly focusing on two specific disorders (levodopa-induced dyskinesias in Parkinson’s disease patients and tardive dyskinesias in schizophrenic patients) where the modern neuroimaging approaches have recently provided new fundamental insights.

Parkinson's disease: dopaminergic nerve cell model is consistent with experimental finding of increased extracellular transport of α-synuclein

Background

Parkinson’s disease is an age-related disease whose pathogenesis is not completely known. Animal models exist for investigating the disease but not all results can be easily transferred to humans. Therefore, mathematical or probabilistic models for the human disease are to be constructed in silico in order to predict specific processes within a cell, such as the dopamine metabolism and transport processes in a neuron.

Results

We present a Systems Biology Markup Language (SBML) model of a whole dopaminergic nerve cell consisting of 139 reactions and 111 metabolites which includes, among others, the dopamine metabolism and transport, oxidative stress, aggregation of α-synuclein (αSYN), lysosomal and proteasomal degradation, and mitophagy. The predictive power of the model was investigated using flux balance analysis for the identification of steady model states. To this end, we performed six experiments: (i) investigation of the normal cell behavior, (ii) increase of O₂, (iii) increase of ATP, (iv) influence of neurotoxins, (v) increase of αSYN in the cell, and (vi) increase of dopamine synthesis. The SBML model is available in the BioModels database with identifier MODEL1302200000.

Conclusion

It is possible to simulate the normal behavior of an in vivo nerve cell with the developed model. We show that the model is sensitive for neurotoxins and oxidative stress. Further, an increased level of αSYN induces apoptosis and an increased flux of αSYN to the extracellular space was observed.

Imaging of Dopamine in PD and Implications for Motor and Neuropsychiatric Manifestations of PD

Parkinson’s disease (PD) is characterized by dopamine depletion in the putamen, which leads to motor dysfunction. As the disease progresses, a substantial degree of dopamine depletion also occurs in caudate and nucleus accumbens. This may explain a number of neuropsychiatric manifestations, including depression, apathy, and cognitive decline. Dopamine replacement therapy partially restores motor function but long-term treatment is often associated with motor complications (motor fluctuations and dyskinesias). Positron emission tomography (PET) studies suggest that the dopamine release rate is substantially higher in PD subjects with motor complications compared to stable responders. Notably, this differential pattern of dopamine release is already present in the early stages of the disease, before motor complications become clinically apparent. Converging evidence suggests that striatal dopamine depletion in PD leads to reduced plasticity in the primary motor cortex and, presumably, in non-motor cortical areas as well. Although dopamine replacement therapy tends to restore physiological plasticity, treatment-induced motor, and neuropsychiatric complications could be related to abnormalities in corticostriatal synaptic plasticity.

Management of motor and non-motor symptoms in Parkinson's disease

Parkinson's disease (PD) is the second most common neurodegenerative disorder that affects approximately 1 % of people over the age of 60 years. Accurate diagnosis and individualized assessment of the risks and benefits of available antiparkinsonian medications as well as specific clinical features and the phase of disease should guide treatment for patients with PD. Levodopa still remains the gold standard for the treatment of motor symptoms of PD but dopamine agonists (DAs), catechol-O-methyltransferase (COMT) inhibitors and monoamine oxidase B (MAO-B) inhibitors have also been developed to provide more continuous oral delivery of dopaminergic stimulation in order to improve motor outcomes and decrease the risk of levodopa-induced motor complications. Deep-brain stimulation as well as other invasive therapies can be used for the treatment of drug-refractory levodopa-induced motor complications. Despite all of the therapeutic advances achieved within the last 20 years, PD continues to be a progressive disorder leading to severe disability caused by motor and non-motor symptoms. To date, neuroprotective interventions able to modify PD progression are not available. This review focuses on medical and invasive treatment strategies for early and advanced stages of PD as well as on the treatment of PD non-motor symptoms such as mood and behavioural disorders, cognitive and autonomic dysfunction, and sleep disorders, which can antedate PD motor symptoms for years.

Role of Serotonin Neurons in L-DOPA- and Graft-Induced Dyskinesia in a Rat Model of Parkinson's Disease

L-DOPA, the most effective drug to treat motor symptoms of Parkinson's disease, causes abnormal involuntary movements, limiting its use in advanced stages of the disease. An increasing body of evidence points to the serotonin system as a key player in the appearance of L-DOPA-induced dyskinesia (LID). In fact, exogenously administered L-DOPA can be taken up by serotonin neurons, converted to dopamine and released as a false transmitter, contributing to pulsatile stimulation of striatal dopamine receptors. Accordingly, destruction of serotonin fibers or silencing serotonin neurons by serotonin agonists could counteract LID in animal models. Recent clinical work has also shown that serotonin neurons are present in the caudate/putamen of patients grafted with embryonic ventral mesencephalic cells, producing intense serotonin hyperinnervation. These patients experience graft-induced dyskinesia (GID), a type of dyskinesia phenotypically similar to the one induced by L-DOPA but independent from its administration. Interestingly, the 5-HT_1A receptor agonist buspirone has been shown to suppress GID in these patients, suggesting that serotonin neurons might be involved in the etiology of GID as for LID. In this paper we will discuss the experimental and clinical evidence supporting the involvement of the serotonin system in both LID and GID.

Signal transduction pathway activity compensates dopamine D₂/D₃ receptor density changes in Parkinson's disease: a preliminary comparative human brain receptor autoradiography study with [³H]raclopride and [³⁵S]GTPγS

The degeneration of dopaminergic nigrostriatal pathway in Parkinson's disease (PD) results in alterations of the dopamine receptor system. In the present study we have investigated the relationship between the disease related changes of expressed dopamine D₂/D₃ receptor density and the corresponding intracellular signal transduction route in cortical and sub-cortical brain structures in the human brain. Dopamine D₂/D₃ receptor autoradiography (ARG), using [³H]raclopride, and agonist stimulated [³⁵S]GTPγS (guanosine 5'-O-[γ-thio]triphosphate) binding autoradiography have been performed in human striatum, cingulate gyrus and medial frontal gyrus samples obtained from six deceased PD patients and six age matched control subjects. Receptor densities were expressed as fmol/gram tissue protein for [³H]raclopride; agonist stimulated [³⁵S]GTPγS binding was expressed in fmol/gram tissue and its change was expressed in percentage values above basal binding. Our results indicate that whereas there is a decrease of the dopamine D₂/D₃ receptors in the striatum demonstrated by classical receptor autoradiography (controls and PD: 24.08±2.06 fmol/gram (mean±SEM) and 18.43±2.82 fmol/gram, respectively; p<0.05), the corresponding agonist stimulated [³⁵S]GTPγS binding autoradiography shows unchanged basal [³⁵S]GTPγS binding (controls and PD: 199±17 fmol/g and 198±21 fmol/g, respectively; n.s.) and, at the same time, no change in stimulation (controls and PD: 0.40±4.57% and 1.51±2.27%, respectively; n.s.). In cingular gyrus and medial frontal gyrus neither the dopamine D₂/D₃ receptor densities nor the [³⁵S]GTPγS binding displayed significant differences between PD and age matched control brain samples, whereas the [³⁵S]GTPγS binding values were markedly higher in PD. These preliminary findings may indicate a possible compensatory mechanism in striatal regions of PD brains: the loss of the dopamine receptors in the striatum appears to be compensated by an increased post-synaptic intracellular signal transduction route activity. However, the accurate interpretation of the present findings requires detailed further studies.

Anatomy of Graft-induced Dyskinesias: Circuit Remodeling in the Parkinsonian Striatum

The goal of researchers and clinicians interested in re-instituting cell based therapies for PD is to develop an effective and safe surgical approach to replace dopamine (DA) in individuals suffering from Parkinson's disease (PD). Worldwide clinical trials involving transplantation of embryonic DA neurons into individuals with PD have been discontinued because of the often devastating post-surgical side-effect known as graft-induced dyskinesia (GID). There have been many review articles published in recent years on this subject. There has been a tendency to promote single factors in the cause of GID. In this review, we contrast the pros and cons of multiple factors that have been suggested from clinical and/or preclinical observations, as well as novel factors not yet studied that may be involved with GID. It is our intention to provide a platform that might be instrumental in examining how individual factors that correlate with GID and/or striatal pathology might interact to give rise to dysfunctional circuit remodeling and aberrant motor output.

Effects of amphetamine on subcellular distribution of dopamine and DOPAC

Amphetamine effects on distribution of dopamine, 3,4-dihydroxyphenylacetic acid (DOPAC), and amphetamine in vesicular, cytosolic, and extracellular compartments associated with a striatal varicosity were estimated through use of a computer simulation model. In addition, contribution to overall effects of amphetamine by each of five actions--transport by dopamine transporter (DAT), transport by vesicular monoamine transporter, stimulation of reverse transport, inhibition of monoamine oxidase (MAO), and slowing of dopamine cell firing rate--were evaluated. Amphetamine enters a varicosity almost entirely by DAT and accumulates to very high levels within the varicosity. Both reverse transport by DAT and passive diffusion contribute to continual amphetamine egress across the plasma membrane. Amphetamine enters storage vesicles by both transport and diffusion. The transport portion competes with dopamine storage, resulting in redistribution of approximately half of dopamine from vesicles to cytosol. The high concentration of amphetamine in the cytosol inhibits MAO, protecting cytosolic dopamine. A very small fraction of cytosolic dopamine is moved to extracellular compartment via reverse transport by DAT. The amount of dopamine moved by reverse transport is limited because of competition by very high cytosolic levels of amphetamine. In the presence of amphetamine, rate of dopamine transfer to extracellular compartment is less than control; however, high levels of extracellular dopamine are maintained because amphetamine occupies the DAT, thus limiting dopamine reuptake. Simulation output from a model using exchange-diffusion mechanism of reverse transport does not match all published data that were simulated, suggesting that inward transport of a substrate is not required to initiate reverse transport.

L-Dihydroxyphenylalanine modulates the steady-state expression of mouse striatal tyrosine hydroxylase, aromatic L-amino acid decarboxylase, dopamine and its metabolites in an MPTP mouse model of Parkinson's disease

AIMS

l-3,4-Dihydroxyphenylalanine (L-DOPA) is the most effective symptomatic treatment for Parkinson's disease (PD), but PD patients usually experience a successful response to L-DOPA therapy followed by a progressive loss of response. L-DOPA efficacy relies on its decarboxylation by aromatic l-amino acid decarboxylase (AAAD) to form dopamine (DA). So exogenous L-DOPA drives the reaction and AAAD becomes the rate limiting enzyme in the supply of DA. In turn, exogenous L-DOPA regulates the expression and activity of AAAD as well as the synthesis of DA and its metabolites, changes that may be linked to the efficacy and side-effects of L-DOPA.

MAIN METHODS

One-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) mouse PD model was utilized to study the effects of L-DOPA on the steady-state level and activity of AAAD, tyrosine hydroxylase (TH), DA and the metabolites of DA. The MPTP and control mice were treated twice daily with PBS or with 100mg/kg of L-DOPA for 14days and the expression and activity of AAAD, the expression of TH and the levels of DA and its metabolites were determined 24h after L-DOPA or PBS treatment, when exogenous L-DOPA is eliminated.

KEY FINDINGS

In the MPTP model, L-DOPA reduced the steady-state expression and the activity of striatal AAAD by 52% and 50%, respectively, DA and metabolites were also significantly decreased.

SIGNIFICANCE

The outcome shows that while L-DOPA replenishes striatal DA it also down-regulates AAAD and the steady-state synthesis and metabolic capability of the dopaminergic system. These findings are important in the precipitation of L-DOPA induced side effects and the management of L-DOPA therapy.

Functions of the nigrostriatal dopaminergic synapse and the use of neurotransplantation in Parkinson's disease

While pharmaceutical options remain the overwhelmingly accepted treatment of choice for neurological and psychiatric diseases, significant accomplishments in regenerative neuroscience research have demonstrated the potential of cellular and synaptic functional repair in future therapies. Parkinson's disease stands out as an example in which repair by dopaminergic neurons appears a viable potential therapy. This article describes the basic neurobiological underpinnings of the rationale for cell therapy for Parkinson's disease and the challenges ahead for the use of regenerative medicine in the treatment for this disease.

Dopamine turnover increases in asymptomatic LRRK2 mutations carriers

Increase in dopamine (DA) turnover was found to occur early in symptomatic Parkinson's disease (PD) and to be functionally related to the dopamine transporter (DAT). The objectives of this study were to examine changes in DA turnover in the asymptomatic PD phase; to compare them with changes in other dopaminergic markers, and to investigate a possible relationship between DAT and DA turnover. Eight subjects from families at increased risk of PD due to LRRK2 mutation were investigated. Positron emission tomography imaging was performed with: ¹⁸F-fluorodopa to determine the effective DA distribution volume (EDV), the inverse of DA turnover, and the DA uptake rate K(occ), a marker of DA synthesis and storage; ¹¹C-methylphenidate (MP, a DAT marker) and ¹¹C-dihydrotetrabenazine (DTBZ, a VMAT2 marker) to estimate the binding potentials BP(ND_MP) and BP(ND_DTBZ). On average, EDV showed the largest reduction from age-matched control values (42%) followed by BP(ND_MP) (23%) and BP(ND_DTBZ) (17%), whereas K(occ) remained in the normal range for all subjects. No correlation was found between EDV and any other marker. DA turnover was found to be elevated in asymptomatic mutation carriers at increased risk of PD. Such change was determined to be larger than and statistically independent from changes observed with the other markers. These results support a compensatory role of increased DA turnover in presymptomatic disease and indicate that at this stage, in contrast to the symptomatic PD phase, increased turnover is not related to DAT.

Functional neuroimaging in Parkinson's disease

INTRODUCTION

Functional neuroimaging techniques have greatly contributed to improving our understanding of Parkinson's disease (PD) neurodegeneration and related compensatory mechanisms.

AREAS COVERED

In this paper, the authors analyze the role of functional neuroimaging as a diagnostic tool in PD and review functional neuroimaging studies on PD progression and compensatory adaptations. Through this, the article provides the reader with sensible approaches for the use of functional neuroimaging in the diagnosis of PD. The reader is also provided with knowledge on the time course of nigrostriatal dopamine dysfunction in PD as well as an overview of the potential beneficial and deleterious effects of increased dopamine turnover. Finally, the reader is provided with a critical discussion of the differential effects of levodopa and dopamine agonists on presynaptic dopamine markers and the implications for the interpretation of clinical trials.

EXPERT OPINION

Functional neuroimaging probably plays a limited role in the diagnosis of PD. Parkinson's disease pathology leads to an exponential decline in nigrostriatal dopamine function and a compensatory increase in dopamine turnover, which may help delay symptom onset. On the negative side, increased dopamine turnover contributes to the development of treatment-related motor complications. Presynaptic markers of dopamine function are subject to regulatory changes, compromising the direct interpretation of neuroimaging results in trials of neuroprotective therapies for PD.

Levetiracetam for the management of levodopa-induced dyskinesias in Parkinson's disease

The efficacy and safety of levetiracetam (LEV), administered for management of levodopa-induced dyskinesias (LID) in Parkinson's disease (PD), was examined using a multicenter, double-blind, placebo-controlled, parallel groups, crossover trial. Because of having a period effect, data after crossover point was excluded from analysis. Levodopa-treated PD participants with LID (n = 38) received LEV 500 mg/day, were assessed, titrated to 1,000 mg/day and reassessed, before and after crossover. The placebo group followed the same routine. Primary efficacy was defined from percent change in "On with LID" time from patient diaries. Secondary efficacy assessment used "On without LID," "Off" time, unified PD rating scale (UPDRS), clinical global impression (CGI), and Goetz dyskinesia scale after levodopa challenge. Safety measures were also performed. On with LID time decreased 37 minutes (95% confidence interval [CI] 0.59, 7.15; P = 0.02) at 500 mg/day, 7.85% 75 minutes (95% CI 3.3, 12.4; P = 0.002) at 1,000 mg/day. On without LID time increased by 46 minutes (95% CI -1.55, -0.03; P = 0.04) at 500 mg/day and 55 minutes (95% CI -10.39, -1.14; P = 0.018) at 1,000 mg/day. UPDRS 32 showed decreased dyskinesia duration mean change 0.35 (95% CI 0.09, 0.5; P = 0.009) at 1,000 mg/day. CGI showed LID decreased by 0.7 (95% CI 0.21, 1.18; P = 0.006) at 1,000 mg/day. Patient diaries and UPDRS show no increase in Off time. This exploratory trial provides evidence that LEV in 1,000 mg/day, slowly titrated, could be useful in improving LID as was assessed with patient diaries, UPDRS, and CGI scales, safely, with minimal side effects.