Improvement of levodopa-induced dyskinesia by propranolol in Parkinson's disease

Locus coeruleus injury modulates ventral midbrain neuroinflammation during DSS-induced colitis

Parkinson's disease (PD) is characterized by a decades-long prodrome, consisting of a collection of non-motor symptoms that emerges prior to the motor manifestation of the disease. Of these non-motor symptoms, gastrointestinal dysfunction and deficits attributed to central norepinephrine (NE) loss, including mood changes and sleep disturbances, are frequent in the PD population and emerge early in the disease. Evidence is mounting that injury and inflammation in the gut and locus coeruleus (LC), respectively, underlie these symptoms, and the injury of these systems is central to the progression of PD. In this study, we generate a novel two-hit mouse model that captures both features, using dextran sulfate sodium (DSS) to induce gut inflammation and N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine (DSP-4) to lesion the LC. We first confirmed the specificity of DSP-4 for central NE using neurochemical methods and fluorescence light-sheet microscopy of cleared tissue, and established that DSS-induced outcomes in the periphery, including weight loss, gross indices of gut injury and systemic inflammation, the loss of tight junction proteins in the colonic epithelium, and markers of colonic inflammation, were unaffected with DSP-4 pre-administration. We then measured alterations in neuroimmune gene expression in the ventral midbrain in response to DSS treatment alone as well as the extent to which prior LC injury modified this response. In this two-hit model we observed that DSS-induced colitis activates the expression of key cytokines and chemokines in the ventral midbrain only in the presence of LC injury and the typical DSS-associated neuroimmune is blunted by pre-LC lesioning with DSP-4. In all, this study supports the growing appreciation for the LC as neuroprotective against inflammation-induced brain injury and draws attention to the potential for NEergic interventions to exert disease-modifying effects under conditions where peripheral inflammation may compromise ventral midbrain dopaminergic neurons and increase the risk for development of PD.

Locus coeruleus injury modulates ventral midbrain neuroinflammation during DSS-induced colitis

Parkinson's disease (PD) is characterized by a decades-long prodrome, consisting of a collection of non-motor symptoms that emerges prior to the motor manifestation of the disease. Of these non-motor symptoms, gastrointestinal dysfunction and deficits attributed to central norepinephrine (NE) loss, including mood changes and sleep disturbances, are frequent in the PD population and emerge early in the disease. Evidence is mounting that injury and inflammation in the gut and locus coeruleus (LC), respectively, underlie these symptoms, and the injury of these systems is central to the progression of PD. In this study, we generate a novel two-hit mouse model that captures both features, using dextran sulfate sodium (DSS) to induce gut inflammation and N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine (DSP-4) to lesion the LC. We first confirmed the specificity of DSP-4 for central NE using neurochemical methods and fluorescence light-sheet microscopy of cleared tissue, and established that DSS-induced outcomes in the periphery, including weight loss, gross indices of gut injury and systemic inflammation, the loss of tight junction proteins in the colonic epithelium, and markers of colonic inflammation, were unaffected with DSP-4 pre-administration. We then measured alterations in neuroimmune gene expression in the ventral midbrain in response to DSS treatment alone as well as the extent to which prior LC injury modified this response. In this two-hit model we observed that DSS-induced colitis activates the expression of key cytokines and chemokines in the ventral midbrain only in the presence of LC injury and the typical DSS-associated neuroimmune is blunted by pre-LC lesioning with DSP-4. In all, this study supports the growing appreciation for the LC as neuroprotective against inflammation-induced brain injury and draws attention to the potential for NEergic interventions to exert disease-modifying effects under conditions where peripheral inflammation may compromise ventral midbrain dopaminergic neurons and increase the risk for development of PD.

Levodopa-Induced Dyskinesias in Parkinson's Disease: An Overview on Pathophysiology, Clinical Manifestations, Therapy Management Strategies and Future Directions

Since its first introduction, levodopa has become the cornerstone for the treatment of Parkinson's disease and remains the leading therapeutic choice for motor control therapy so far. Unfortunately, the subsequent appearance of abnormal involuntary movements, known as dyskinesias, is a frequent drawback. Despite the deep knowledge of this complication, in terms of clinical phenomenology and the temporal relationship during a levodopa regimen, less is clear about the pathophysiological mechanisms underpinning it. As the disease progresses, specific oscillatory activities of both motor cortical and basal ganglia neurons and variation in levodopa metabolism, in terms of the dopamine receptor stimulation pattern and turnover rate, underlie dyskinesia onset. This review aims to provide a global overview on levodopa-induced dyskinesias, focusing on pathophysiology, clinical manifestations, therapy management strategies and future directions.

Levodopa responsive freezing of gait is associated with reduced norepinephrine transporter binding in Parkinson's disease

BACKGROUND

Freezing of gait (FOG) is a major cause of falling in Parkinson's disease (PD) and can be responsive or unresponsive to levodopa. Pathophysiology is poorly understood.

OBJECTIVE

To examine the link between noradrenergic systems, the development of FOG in PD and its responsiveness to levodopa.

METHODS

We examined norepinephrine transporter (NET) binding via brain positron emission tomography (PET) to evaluate changes in NET density associated with FOG using the high affinity selective NET antagonist radioligand [¹¹C]MeNER (2S,3S)(2-[α-(2-methoxyphenoxy)benzyl]morpholine) in 52 parkinsonian patients. We used a rigorous levodopa challenge paradigm to characterize PD patients as non-freezing (NO-FOG, N = 16), levodopa responsive freezing (OFF-FOG, N = 10), and levodopa-unresponsive freezing (ONOFF-FOG, N = 21), and also included a non-PD FOG group, primary progressive freezing of gait (PP-FOG, N = 5).

RESULTS

Linear mixed models identified significant reductions in whole brain NET binding in the OFF-FOG group compared to the NO-FOG group (-16.8%, P = 0.021) and regionally in the frontal lobe, left and right thalamus, temporal lobe, and locus coeruleus, with the strongest effect in right thalamus (P = 0.038). Additional regions examined in a post hoc secondary analysis including the left and right amygdalae confirmed the contrast between OFF-FOG and NO-FOG (P = 0.003). A linear regression analysis identified an association between reduced NET binding in the right thalamus and more severe New FOG Questionnaire (N-FOG-Q) score only in the OFF-FOG group (P = 0.022).

CONCLUSION

This is the first study to examine brain noradrenergic innervation using NET-PET in PD patients with and without FOG. Based on the normal regional distribution of noradrenergic innervation and pathological studies in the thalamus of PD patients, the implications of our findings suggest that noradrenergic limbic pathways may play a key role in OFF-FOG in PD. This finding could have implications for clinical subtyping of FOG as well as development of therapies.

Inhibition of VMAT2 by β2-adrenergic agonists, antagonists, and the atypical antipsychotic ziprasidone

Vesicular monoamine transporter 2 (VMAT2) is responsible for packing monoamine neurotransmitters into synaptic vesicles for storage and subsequent neurotransmission. VMAT2 inhibitors are approved for symptomatic treatment of tardive dyskinesia and Huntington's chorea, but despite being much-studied inhibitors their exact binding site and mechanism behind binding and inhibition of monoamine transport are not known. Here we report the identification of several approved drugs, notably β2-adrenergic agonists salmeterol, vilanterol and formoterol, β2-adrenergic antagonist carvedilol and the atypical antipsychotic ziprasidone as inhibitors of rat VMAT2. Further, plausible binding modes of the established VMAT2 inhibitors reserpine and tetrabenazine and hit compounds salmeterol and ziprasidone were identified using molecular dynamics simulations and functional assays using VMAT2 wild-type and mutants. Our findings show VMAT2 as a potential off-target of treatments with several approved drugs in use today and can also provide important first steps in both drug repurposing and therapy development targeting VMAT2 function.

Deciphering Intertwined Molecular Pathways Underlying Metabolic Syndrome Leading to Parkinson's Disease

Parkinson's disease (PD) is a neurodegenerative disorder that gradually develops over time in a progressive manner. The main culprit behind the disease pathology is dopaminergic deficiency in Substantia nigra Pars Compacta (SNpc) due to neuronal degeneration. However, there are other factors that are not only associated with it but also somehow responsible for inception of pathology. Metabolic syndrome is one such risk factor for PD. Metabolic syndrome is a cluster of diseases mainly including diabetes, hypertension, obesity, and hyperlipidemia which pose a risk for developing cardiovascular disorders. All of these disorders have their own pathological pathways that intertwine with PD pathology. This leads to alpha-synuclein aggregation, neuroinflammation, mitochondrial dysfunction, and oxidative stress which are facets in initiating PD pathology. Although few reports are available, this area is underexplored and has contradictory views. Hence, further studies are needed in order to establish a definite relationship between PD and metabolic syndrome. In this review, we aim to elucidate the molecular mechanisms to confirm the association between them and pave the way for potential repurposing of therapies.

Manipulation of vocal communication and anxiety through pharmacologic modulation of norepinephrine in the Pink1-/- rat model of Parkinson disease

Vocal deficits and anxiety are common, co-occurring, and interacting signs of Parkinson Disease (PD) that have a devastating impact on quality of life. Both manifest early in the disease process. Unlike hallmark motor signs of PD, neither respond adequately to dopamine replacement therapies, suggesting that their disease-specific mechanisms are at least partially extra-dopaminergic. Because noradrenergic dysfunction is also a defining feature of PD, especially early in the disease progression, drug therapies targeting norepinephrine are being trialed for treatment of motor and non-motor impairments in PD. Research assessing the effects of noradrenergic manipulation on anxiety and vocal impairment in PD, however, is sparse. In this pre-clinical study, we quantified the influence of pharmacologic manipulation of norepinephrine on vocal impairment and anxiety in Pink1-/- rats, a translational model of PD that demonstrates both vocal deficits and anxiety. Ultrasonic vocalization acoustics, anxiety behavior, and limb motor activity were tested twice for each rat: after injection of saline and after one of three drugs. We hypothesized that norepinephrine reuptake inhibitors (atomoxetine and reboxetine) and a β receptor antagonist (propranolol) would decrease vocal impairment and anxiety compared to saline, without affecting spontaneous motor activity. Our results demonstrated that atomoxetine and reboxetine decreased anxiety behavior. Atomoxetine also modulated ultrasonic vocalization acoustics, including an increase in vocal intensity, which is almost always reduced in animal models and patients with PD. Propranolol did not affect anxiety or vocalization. Drug condition did not influence spontaneous motor activity. These studies demonstrate relationships among vocal impairment, anxiety, and noradrenergic systems in the Pink1-/- rat model of PD.

Propranolol Relieves L-Dopa-Induced Dyskinesia in Parkinsonian Mice

BACKGROUND

Parkinsonism is caused by dopamine (DA) insufficiency and results in a hypokinetic movement disorder. Treatment with L-Dopa can restore DA availability and improve motor function, but patients can develop L-Dopa-induced dyskinesia (LID), a secondary hyperkinetic movement disorder. The mechanism underlying LID remains unknown, and new treatments are needed. Experiments in mice have shown that DA deficiency promotes an imbalance between striatal acetylcholine (ACh) and DA that contributes to motor dysfunction. While treatment with L-Dopa improves DA availability, it promotes a paradoxical rise in striatal ACh and a further increase in the ACh to DA ratio may promote LID.

METHODS

We used conditional Slc6a3^DTR/+ mice to model progressive DA deficiency and the β-adrenergic receptor (β-AR) antagonist propranolol to limit the activity of striatal cholinergic interneurons (ChIs). DA-deficient mice were treated with L-Dopa and the dopa decarboxylase inhibitor benserazide. LID and motor performance were assessed by rotarod, balance beam, and open field testing. Electrophysiological experiments characterized the effects of β-AR ligands on striatal ChIs.

RESULTS

LID was observed in a subset of DA-deficient mice. Treatment with propranolol relieved LID and motor hyperactivity. Electrophysiological experiments showed that β-ARs can effectively modulate ChI firing.

CONCLUSIONS

The work suggests that pharmacological modulation of ChIs by β-ARs might provide a therapeutic option for managing LID.

Dyskinesias and impulse control disorders in Parkinson's disease: From pathogenesis to potential therapeutic approaches

Dopaminergic treatment in Parkinson's disease (PD) reduces the severity of motor symptoms of the disease. However, its chronic use is associated with disabling motor and behavioral side effects, among which levodopa-induced dyskinesias (LID) and impulse control disorders (ICD) are the most common. The underlying mechanisms and pathological substrate of these dopaminergic complications are not fully understood. Recently, the refinement of imaging techniques and the study of the genetics and molecular bases of LID and ICD indicate that, although different, they could share some features. In addition, animal models of parkinsonism with LID have provided important knowledge about mechanisms underlying such complications. In contrast, animal models of parkinsonism and abnormal impulsivity, although useful regarding some aspects of human ICD, do not fully resemble the clinical phenotype of ICD in patients with PD, and until now have provided limited information. Studies on animal models of addiction could complement the previous models and provide some insights into the background of these behavioral complications given that ICD are regarded as behavioral addictions. Here we review the most relevant advances in relation to imaging, genetics, biochemistry and pharmacological interventions to treat LID and ICD in patients with PD and in animal models with a view to better understand the overlapping and unique maladaptations to dopaminergic therapy that are associated with LID and ICD.

Effects of the beta-adrenergic receptor antagonist Propranolol on dyskinesia and L-DOPA-induced striatal DA efflux in the hemi-parkinsonian rat

Dopamine (DA) replacement therapy with L-DOPA continues to be the primary treatment of Parkinson's disease; however, long-term therapy is accompanied by L-DOPA-induced dyskinesias (LID). Several experimental and clinical studies have established that Propranolol, a β-adrenergic receptor antagonist, reduces LID without affecting L-DOPA's efficacy. However, the exact mechanisms underlying these effects remain to be elucidated. The aim of this study was to evaluate the anti-dyskinetic profile of Propranolol against a panel of DA replacement strategies, as well as elucidate the underlying neurochemical mechanisms. Results indicated that Propranolol, in a dose-dependent manner, reduced LID, without affecting motor performance. Propranolol failed to alter dyskinesia produced by the D1 receptor agonist, SKF81297 (0.08 mg/kg, sc), or the D2 receptor agonist, Quinpirole (0.05 mg/kg, sc). These findings suggested a pre-synaptic mechanism for Propranolol's anti-dyskinetic effects, possibly through modulating L-DOPA-mediated DA efflux. To evaluate this possibility, microdialysis studies were carried out in the DA-lesioned striatum of dyskinetic rats and results indicated that co-administration of Propranolol (20 mg/kg, ip) was able to attenuate L-DOPA- (6 mg/kg, sc) induced DA efflux. Therefore, Propranolol's anti-dyskinetic properties appear to be mediated via attenuation of L-DOPA-induced extraphysiological efflux of DA.

Molecular imaging of levodopa-induced dyskinesias

Levodopa-induced dyskinesias (LIDs) occur in the majority of patients with Parkinson's disease (PD) following years of levodopa treatment. The pathophysiology underlying LIDs in PD is poorly understood, and current treatments generate only minor benefits for the patients. Studies with positron emission tomography (PET) molecular imaging have demonstrated that in advanced PD patients, levodopa administration induces sharp increases in striatal dopamine levels, which correlate with LIDs severity. Fluctuations in striatal dopamine levels could be the result of the attenuated buffering ability in the dopaminergically denervated striatum. Lines of evidence from PET studies indicate that serotonergic terminals could also be responsible for the development of LIDs in PD by aberrantly processing exogenous levodopa and by releasing dopamine in a dysregulated manner from the serotonergic terminals. Additionally, other downstream mechanisms involving glutamatergic, cannabinoid, opioid, cholinergic, adenosinergic, and noradrenergic systems may contribute in the development of LIDs. In this article, we review the findings from preclinical, clinical, and molecular imaging studies, which have contributed to our understanding the pathophysiology of LIDs in PD.

Effects of noradrenergic denervation by anti-DBH-saporin on behavioral responsivity to L-DOPA in the hemi-parkinsonian rat

Dopamine (DA) replacement with l-DOPA remains the most effective pharmacotherapy for motor symptoms of Parkinson's disease (PD) including tremor, postural instability, akinesia, and bradykinesia. Prolonged L-DOPA use frequently leads to deleterious side effects including involuntary choreic and dystonic movements known as L-DOPA induced dyskinesias (LID). DA loss in PD is frequently accompanied by concomitant noradrenergic (NE) denervation of the locus coeruleus (LC); however, the effects of NE loss on L-DOPA efficacy and LID remain controversial and are often overlooked in traditional animal models of PD. The current investigation examined the role of NE loss in L-DOPA therapy by employing the NE specific neurotoxin anti-DA-beta hydroxylase saporin (αDBH) in a rat model of PD. Rats received unilateral 6-hydroxydopamine lesions of the medial forebrain bundle to deplete nigral DA and intraventricular injection of vehicle (DA lesioned rats) or αDBH (DANE lesioned rats) to destroy NE neurons bilaterally. Results indicated that αDBH infusion drastically reduced NE neuron markers within the LC compared to rats that received vehicle treatment. Behaviorally, this loss did not alter the development or expression of L-DOPA- or DA agonist-induced dyskinesia. However, rats with additional NE lesions were less responsive to L-DOPA's pro-motor effects. Indeed, DANE lesioned animals rotated less and showed less attenuation of parkinsonian stepping deficits following high doses of L-DOPA than DA lesioned animals. These findings suggest that severe NE loss may reduce L-DOPA treatment efficacy and demonstrate that degradation of the NE system is an important consideration when evaluating L-DOPA effects in later stage PD.

Treatment of advanced Parkinson’s disease

Parkinson's disease is a progressive neurological disorder characterized by tremor, bradykinesia, rigidity, gait and postural instability and a variety of nonmotor symptoms. While these and other motor signs typically improve with levodopa, the so-called axial signs, such as dysarthria, dysphagia, postural instability and freezing, and most nonmotor signs, such as depression, cognitive decline and dysautonomia, usually do not respond satisfactorily to levodopa. Furthermore, the use of levodopa may be limited by the development of motor fluctuations, dyskinesias and other adverse effects. This manuscript reviews the medical management of advanced Parkinson's disease, focusing on the treatment of motor fluctuations and dyskinesias and of nonmotor and nonlevodopa responsive symptoms.

The role of β-adrenergic blockers in Parkinson's disease: possible genetic and cell-signaling mechanisms

Genetic studies have identified numerous factors linking β-adrenergic blockade to Parkinson's disease (PD), including human leukocyte antigen genes, the renin-angiotensin system, poly(adenosine diphosphate-ribose) polymerase 1, nerve growth factor, vascular endothelial growth factor, and the reduced form of nicotinamide adenine dinucleotide phosphate. β-Adrenergic blockade has also been implicated in PD via its effects on matrix metalloproteinases, mitogen-activated protein kinase pathways, prostaglandins, cyclooxygenase 2, and nitric oxide synthase. β-Adrenergic blockade may have a significant role in PD; therefore, the characterization of β-adrenergic blockade in patients with PD is needed.

Enhanced striatal β1-adrenergic receptor expression following hormone loss in adulthood is programmed by both early sexual differentiation and puberty: a study of humans and rats

After reproductive senescence or gonadectomy, changes occur in neural gene expression, ultimately altering brain function. The endocrine mechanisms underlying these changes in gene expression beyond immediate hormone loss are poorly understood. To investigate this, we measured changes in gene expression the dorsal striatum, where 17β-estradiol modulates catecholamine signaling. In human caudate, quantitative PCR determined a significant elevation in β1-adrenergic receptor (β1AR) expression in menopausal females when compared with similarly aged males. No differences were detected in β2-adrenergic and D1- and D2-dopamine receptor expression. Consistent with humans, adult ovariectomized female rats exhibited a similar increase in β1AR expression when compared with gonadectomized males. No sex difference in β1AR expression was detected between intact adults, prepubertal juveniles, or adults gonadectomized before puberty, indicating the necessity of pubertal development and adult ovariectomy. Additionally, increased β1AR expression in adult ovariectomized females was not observed if animals were masculinized/defeminized with testosterone injections as neonates. To generate a model system for assessing functional impact, increased β1AR expression was induced in female-derived cultured striatal neurons via exposure to and then removal of hormone-containing serum. Increased β1AR action on cAMP formation, cAMP response element-binding protein phosphorylation and gene expression was observed. This up-regulation of β1AR action was eliminated with 17β-estradiol addition to the media, directly implicating this hormone as a regulator of β1AR expression. Beyond having implications for the known sex differences in striatal function and pathologies, these data collectively demonstrate that critical periods early in life and at puberty program adult gene responsiveness to hormone loss after gonadectomy and potentially reproductive senescence.

Clinical features, pathophysiology, and treatment of levodopa-induced dyskinesias in Parkinson's disease

Dyskinetic disorders are characterized by excess of motor activity that may interfere with normal movement control. In patients with Parkinson's disease, the chronic levodopa treatment induces dyskinetic movements known as levodopa-induced dyskinesias (LID). This paper analyzed the pathophysiology, clinical manifestations, pharmacological treatments, and surgical procedures to treat hyperkinetic disorders. Surgery is currently the only treatment available for Parkinson's disease that may improve both parkinsonian motor syndrome and LID. However, this paper shows the different mechanisms involved are not well understood.

The emerging role of norepinephrine in cognitive dysfunctions of Parkinson's disease

Parkinson's disease (PD) is the second most common neurodegenerative disorder, affecting 1% of the population over age 60. In those patients cognitive dysfunction is a persistent issue that impairs quality of life and productivity. Neuropathological studies demonstrate significant damage in brain regions outside the nigral dopamine (DA) system, including early degeneration of locus coeruleus norepinephrine (LC-NE) neurons, yet discussion of PD and treatment focus has remained dopaminergic-based. Motor symptoms benefit from DA replacement for many years, but other symptoms including several cognitive deficits continue unabated. Recent interest in non-DA substrates of PD highlights early involvement of LC-NE neurons and provides evidence for a prodromal phase, with cognitive disturbance, even in sporadic PD. We outline insights from basic research in LC-NE function to clinical and pathological evidence highlighting a role for NE in PD cognitive dysfunction. We propose that loss of LC-NE regulation, particularly in higher cortical regions, critically underlies certain cognitive dysfunctions in early PD. As a major unmet need for patients, research and use of NE drugs in PD may provide significant benefits for cognitive processing.

Norepinephrine: the next therapeutics frontier for Parkinson's disease

Tissue concentrations of norepinephrine (NE) are markedly decreased in various regions of the Parkinson's disease (PD) brain. As in the substantia nigra pars compacta, neuronal dropout and Lewy bodies are prominent changes affecting the locus coeruleus, which is the source of ascending NErgic projections. Despite the major roles of NE throughout the brain, there has been only minimal exploration of pharmacological intervention with NErgic neurotransmission. Cognitive operations, "freezing" of gait, tremor, dyskinesia, REM sleep regulation, and other aspects of brain function are tied into signaling by NE, and there is also evidence that it may have a role in the neurodegenerative process itself. This article reviews the reported pharmacological experience in PD therapeutics.

Treatment of motor symptoms in advanced Parkinson's disease: a practical approach

Patients with advanced Parkinson's disease (PD) are known to develop motor complications after a few years of levodopa (L-dopa) therapy. Motor fluctuations develop with increasing severity of the disease, owing to loss of dopaminergic neurons and loss of the buffering capacity of the neurons to fluctuating dopamine levels. Dyskinesias develop as a result of pulsatile stimulation of the receptors and alterations in neuronal firing patterns. L-dopa remains the gold standard medication for the treatment of patients with advanced PD. However, once motor complications on L-dopa therapy emerge, clinicians may add on other classes of antiparkinsonian drugs such as dopamine agonists, catechol-O-methyl transferase inhibitors (COMTIs) or monoamine oxidase type B inhibitors (MAOBIs). The individualisation of the treatment seems to be the key for the best approach of advanced PD patients. The present review provides the most important current clinical data in the pharmacological treatment of motor symptoms in advanced PD and provides the clinician a simple algorithm in order to determine the best suitable treatment to advanced parkinsonian patients.

Nondopaminergic treatments for Parkinson’s disease

SUMMARY The main treatment strategy for Parkinson’s disease (PD) is focused on dopamine replacement. However, PD is no longer seen purely as a disease of the dopaminergic system, as the pathological processes involve neurodegeneration and altered neurotransmission of several nondopaminergic systems that are involved in both motor and nonmotor features of the disease. This article reviews current and experimental nondopaminergic pharmacological approaches to treatments for PD with a focus on motor symptoms, treatments of L-dopa-induced motor complications and treatments of nonmotor symptoms including mood disorders, cognition, psychosis and autonomic problems.

Effects of noradrenergic denervation on L-DOPA-induced dyskinesia and its treatment by α- and β-adrenergic receptor antagonists in hemiparkinsonian rats

While L-3,4-dihydroxyphenylalanine (L-DOPA) remains the standard treatment for Parkinson's disease (PD), long-term efficacy is often compromised by L-DOPA-induced dyskinesia (LID). Recent research suggests that targeting the noradrenergic (NE) system may provide relief from both PD and LID, however, most PD patients exhibit NE loss which may modify response to such strategies. Therefore this investigation aimed to characterize the development and expression of LID and the anti-dyskinetic potential of the α2- and β-adrenergic receptor antagonists idazoxan and propranolol, respectively, in rats receiving 6-OHDA lesions with (DA lesion) or without desipramaine protection (DA+NE lesion). Male Sprague-Dawley rats (N=110) received unilateral 6-hydroxydopamine lesions. Fifty-three rats received desipramine to protect NE neurons (DA lesion) and 57 received no desipramine reducing striatal and hippocampal NE content 64% and 86% respectively. In experiment 1, the development and expression of L-DOPA-induced abnormal involuntary movements (AIMs) and rotations were examined. L-DOPA efficacy using the forepaw adjusting steps (FAS) test was also assessed in DA- and DA+NE-lesioned rats. In experiment 2, DA- and DA+NE-lesioned rats received pre-treatments of idazoxan or propranolol followed by L-DOPA after which the effects of these adrenergic compounds were observed. Results demonstrated that moderate NE loss reduced the development and expression of AIMs and rotations but not L-DOPA efficacy while anti-dyskinetic efficacy of α2- and β-adrenergic receptor blockade was maintained. These findings suggest that the NE system modulates LID and support the continued investigation of adrenergic compounds for the improved treatment of PD.

Behavioral and cellular modulation of L-DOPA-induced dyskinesia by beta-adrenoceptor blockade in the 6-hydroxydopamine-lesioned rat

Chronic dopamine replacement therapy in Parkinson's disease (PD) leads to deleterious motor sequelae known as L-DOPA-induced dyskinesia (LID). No known therapeutic can eliminate LID, but preliminary evidence suggests that dl-1-isopropylamino-3-(1-naphthyloxy)-2-propanol [(±)propranolol], a nonselective β-adrenergic receptor (βAR) antagonist, may reduce LID. The present study used the rat unilateral 6-hydroxydopamine model of PD to characterize and localize the efficacy of (±)propranolol as an adjunct to therapy with L-DOPA. We first determined whether (±)propranolol was capable of reducing the development and expression of LID without impairing motor performance ON and OFF L-DOPA. Coincident to this investigation, we used reverse-transcription polymerase chain reaction techniques to analyze the effects of chronic (±)propranolol on markers of striatal activity known to be involved in LID. To determine whether (±)propranolol reduces LID through βAR blockade, we subsequently examined each enantiomer separately because only the (-)enantiomer has significant βAR affinity. We next investigated the effects of a localized striatal βAR blockade on LID by cannulating the region and microinfusing (±)propranolol before systemic L-DOPA injections. Results showed that a dose range of (±)propranolol reduced LID without deleteriously affecting motor activity. Pharmacologically, only (-)propranolol had anti-LID properties indicating βAR-specific effects. Aberrant striatal signaling associated with LID was normalized with (±)propranolol cotreatment, and intrastriatal (±)propranolol was acutely able to reduce LID. This research confirms previous work suggesting that (±)propranolol reduces LID through βAR antagonism and presents novel evidence indicating a potential striatal locus of pharmacological action.

Modulation of L-DOPA-induced abnormal involuntary movements by clinically tested compounds: further validation of the rat dyskinesia model

L-DOPA-induced dyskinesia (LID) is a major complication of the pharmacotherapy of Parkinson's Disease. A model of LID has recently been described in rats with unilateral 6-hydroxydopamine (6-OHDA) lesions. In the present study, the model was used in order to compare the efficacies of some clinically available compounds that have shown antidyskinetic effects in nonhuman primate models of LID and/or in patients, namely, amantadine (20 and 40 mg/kg), buspirone (1, 2 and 4 mg/kg), clonidine (0.01, 0.1 and 1 mg/kg), clozapine (4 and 8 mg/kg), fluoxetine (2.5 and 5 mg/kg), propranolol (5, 10 and 20mg/kg), riluzole (2 and 4 mg/kg), and yohimbine (2 and 10 mg/kg). Rats were treated for 3 weeks with L-DOPA for an induction and monitoring of abnormal involuntary movements (AIMs) prior to the drug screening experiments. The antidyskinetic drugs or their vehicles were administered together with L-DOPA, and their effects were evaluated according to a randomized cross-over design both on the AIM rating scale and on the rotarod test. Most of the compounds under investigation attenuated the L-DOPA-induced axial, limb and orolingual AIM scores. However, the highest doses of many of these substances (but for amantadine and riluzole) had also detrimental motor effects, producing a reduction in rotarod performance and locomotor scores. Since the present results correspond well to existing clinical and experimental data, this study indicates that axial, limb and orolingual AIMs possess predictive validity for the preclinical screening of novel antidyskinetic treatments. Combining tests of general motor performance with AIMs ratings in the same experiment allows for selecting drugs that specifically reduce dyskinesia without diminishing the anti-akinetic effect of L-DOPA.

Drug Insight: new drugs in development for Parkinson's disease

For many years, levodopa has given most patients with Parkinson's disease excellent symptomatic benefit. This agent does not slow down the progression of the disease, however, and it can induce motor fluctuations and dyskinesias in the long term. The other available antiparkinsonian agents also have drawbacks, and as a consequence research into antiparkinsonian drugs is expected to take new and different directions in the coming years. The most promising approaches include the development of 'neuroprotective' drugs that are capable of blocking or at least slowing down the degenerative process that is responsible for cellular death; 'restorative' strategies intended to restore normal brain function; more-effective agents for replacing dopamine loss; and symptomatic and antidyskinetic drugs that act on neurotransmitters other than dopamine or target brain areas other than the striatum. In this Review, we discuss the numerous drugs in development that target the primary motor disorder in Parkinson's disease.

Medical management of advanced Parkinson's disease

The management of advancing Parkinson's disease (PD) is a daunting task, complicated by dynamic medication responses, side effects, and treatment-refractory symptoms in an aging patient population. The motor and nonmotor complications of advancing PD are reviewed, and practical treatment strategies are provided. Careful assessment in the context of the known natural history of advancing PD and rational treatment choices can create significant improvement in the lives of patients who have advancing PD.

Other pharmacological treatments for motor complications and dyskinesias

Controlling motor complications becomes increasingly difficult with disease progression. The "wearing-off" phenomenon is the most-common motor fluctuation. Wearing-off can be treated by dietary manipulation, shortening the dosing interval, substituting sustained-release levodopa, adding amantadine, or monoamine oxidase type B inhibitors, and other options, including catechol-O-methyltransferase inhibitors and the approved dopamine agonists addressed in another chapter. The rotigotine constant-delivery system is being developed to treat wearing-off symptoms. Istradefylline (KW-6002), an adenosine A(2A) receptor antagonist, has been studied for wearing-off and the results will be discussed. The on-off fluctuations can be treated with liquid levodopa and the rescue therapy of injectable apomorphine. Patients may also suffer from dyskinesias. Dyskinesias can be treated with small doses of liquefied levodopa-carbidopa, amantadine, and clozapine, an atypical neuroleptic.

Neurokinin B, Neurotensin, and Cannabinoid Receptor Antagonists and Parkinson Disease

The neuropeptides neurokinin B, neurotensin, and anandamide, the endogenous ligands of NK3, NT1, and CB1 receptors respectively, are known to interact with brain dopaminergic transmission. This study evaluated the effects of these three antagonists of the NK3 (SR 142801), neurotensin (SR 48692), and cannabinoid (SR 141716) receptors on the severity of motor symptoms and levodopa-induced dyskinesias after administration of a single dose of levodopa in 24 patients with Parkinson disease. In this exploratory randomized, double-blind, placebo-controlled study, at the dose used, the drugs tested were well tolerated and could not improve parkinsonian motor disability.

Noradrenergic Drugs for Levodopa-Induced Dyskinesia

Dyskinesia frequently mars the long-term therapeutic response to levodopa (LD) in Parkinson's disease (PD). New treatment strategies for levodopa-induced dyskinesia (LID) currently being investigated include some that target the nondopaminergic pathways. Indeed, LID in parkinsonism can be modulated by drugs acting on different neurotransmitters including glutamate, gamma-aminobutyric acid, noradrenaline, acetylcholine, serotonin, adenosine, and cholecystokinin. In many cases, the possibility of using specific compounds to counteract LID was raised by the previously shown efficacy of such compounds in the treatment of other types of dyskinesia. More data are now available on drugs that act on the noradrenergic system. Two studies have recently shown how the alpha-2 adrenoreceptor antagonist idazoxan can significantly reduce LID in the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-lesioned primate model of parkinsonism and in patients with advanced PD. The experimental paper, which studied the antagonistic action of idazoxan on dyskinesia induced by both LD and apomorphine in marmosets with MPTP-induced parkinsonism, showed that the pharmacologic mechanisms underlying LID and apomorphine-induced dyskinesia in PD are probably distinct. LD, although not apomorphine-induced, dyskinesia was found to be influenced by adrenoreceptor antagonists. Indeed, the action of alpha-2 adrenoreceptor antagonists may involve the blockade of the action of noradrenaline synthesized from LD. The hypothesis is that because dopamine agonists are not metabolized to noradrenaline, alpha-2 adrenoreceptor antagonists do not reduce dyskinesia produced by such agents. This finding is particularly relevant in planning clinical studies in which LD or dopamine agonist challenges are used to assess the potential antidyskinetic properties of new drugs. The clinical study assessed the effects of idazoxan on LID in 18 patients with advanced PD: An improvement in LID, without the reappearance of parkinsonian symptoms, was observed. The practical outcome of this research is that, although the mechanisms underlying the manifestations and the priming process for dyskinesia have yet to be fully elucidated, a nondopaminergic approach to therapy may provide an effective way of preventing, or at least limiting, the expression of involuntary movements in PD.

Neuroleptic withdrawal versus serotonergic syndrome in an 8-year-old child

There appears to be considerable symptomatic overlap between neuroleptic withdrawal reactions and the serotonin syndrome. This case report is of an 8-year-old boy who developed symptoms compatible with both conditions while discontinuing pimozide and starting fluoxetine. It illustrates how the use of neuroleptic medication in young children is not without the risk of serious adverse drug events and can complicate diagnostic issues. This case report supports the suggestion that adverse drug reactions related to neuroleptics and serotonergic agents could be part of the same clinical and neurophysiological spectrum.

Parkinson's disease: medical treatment of moderate to advanced disease

Parkinson's disease, a common neurodegenerative disorder, results in significant morbidity 10 to 15 years after disease onset and increased mortality. Levodopa is the mainstay of therapy and provides benefit for the duration of the illness. However, within 5 years, up to 50% of individuals develop fluctuations, including dyskinesias, wearing off, and "on/off" effects. Optimal management of Parkinson's disease patients requires careful titration of medications, with use of polypharmacy, including levodopa, dopamine agonists, catechol-O-methyltransferase inhibitors, amantadine, and anticholinergics in order to maintain good motor function and quality of life. With advancing disease, problems such as dysphagia, dysarthria, and gait and balance abnormalities occur, which are not responsive to dopaminergic medication. Due to extradopaminergic neuronal system degeneration, autonomic dysfunction can also be prominent. Recognition and management of these problems is helpful in improving quality of life in late-stage disease. In very late stages, dementia may complicate treatment, requiring discontinuation of combination therapy and use of low-dose levodopa with atypical neuroleptics.

Parkinson's disease: medical and surgical treatment

It has been over three decades since the introduction of L-dihydroxyphenylalanine or levodopa therapy for Parkinson's disease (PD). The early levodopa trials were driven by recognition of a profound cerebral dopamine deficiency state in this disorder. Whereas dopamine fails to cross the blood brain barrier and hence is ineffective as therapy, the amino acid precursor, dopa, is transported across this barrier and provides a substrate for dopamine synthesis. Levodopa is converted to dopamine within the brain by dopa decarboxylase, replenishing central dopamine stores and potentially reversing the motor symptoms of PD.

Intravenous amantadine improves levadopa-induced dyskinesias: an acute double-blind placebo-controlled study

Experimental evidence suggests that glutamatergic receptor blockade may improve the motor response complications associated with long-term levodopa treatment in Parkinson's disease (PD) patients. Our objective was to evaluate the acute effect of amantadine, a noncompetitive antagonist of the N-methyl-D-aspartate (NMDA) receptor, on levodopa-induced dyskinesias, and to gain further insights into the antidyskinetic mechanism of this drug. Nine PD patients with motor fluctuations and severely disabling peak of dose dyskinesias received their first morning levodopa dose, followed by a 2-hour intravenous amantadine (200 mg) or placebo infusion, on two different days. Parkinsonian symptoms and dyskinesias were assessed every 15 minutes during the infusion and for 3 hours thereafter, while patients were taking their usual oral antiparkinsonian therapy, by means of Unified Parkinson's Disease Rating Scale (UPDRS, motor examination), tapping test, and a modified Abnormal Involuntary Movement Scale (AIMS). Intravenous amantadine acutely improved levodopa-induced dyskinesias by 50%without any loss of the anti-parkinsonian benefit from levodopa. This study confirms the antidyskinetic effect of amantadine and strengthens the rationale for using antiglutamatergic drugs in the treatment of parkinsonian motor fluctuations.

Non-dopaminergic drug treatment of Parkinson's disease

Several lines of evidence suggest that substitution of the dopaminergic striatal deficit only represents one important aspect of the treatment of Parkinson's disease (PD) because neurotransmitter systems other than the dopaminergic one also degenerate and aggravate parkinsonian motor, vegetative and cognitive symptoms. Thus, regulation and balance of altered non-dopaminergic neurotransmission could provide an additional benefit for parkinsonian patients (PP). Moreover, onset of motor complications, psychosis and loss of drug efficacy increasingly reduce parkinsonian quality of life in the course of long-term dopamine substitution. Indirect stimulation of the dopaminergic neurotransmission via non-dopaminergic systems is an upcoming interesting strategy to solve these problems. Treatment of L-dopa-associated dyskinesias represents a further important future task of non-dopaminergic drug therapy. NMDA antagonists are a promising therapeutic option but further trials are necessary to elucidate their efficacy. A further peripheral effect of L-dopa/dopa decarboxylase inhibitor (DDI) application is increased homocysteine synthesis with its putative hypothetical additional central impact on neurodegeneration and progression of PD. Long-term monitoring with subsequent therapeutic decrease of homocysteine levels with folic acid could result in substantial clinical benefits at reasonable costs for PP. Also, it could hypothetically influence altered dopaminergic and non-dopaminergic neurotransmission beside its impact on occurrence of vascular disease and altered striatal microvascularisation in PD. The interesting field of non-dopaminergic drug therapy is emerging and will hopefully lead to a better understanding of PD and subsequently improve drug therapy of parkinsonian symptoms, which do not respond to dopaminergic substitution or are long-term complications of dopamine substitution.

Low-dose olanzapine for levodopa induced dyskinesias

OBJECTIVE

To assess the usefulness of low-dose olanzapine (2.5 to 7. 5 mg/day) for Levodopa-induced-dyskinesias (LID) in patients with PD.

METHODS

Ten patients with PD and LID took part in this randomized, placebo-controlled, double blind, crossover trial. Patients received a 2-week course of olanzapine or placebo in each treatment phase with 1-week washout in between. Dyskinesias were assessed at baseline and after each treatment period with an acute dopaminergic challenge and unified PD rating scale (UPDRS) questionnaires. Patients also kept on/off and dyskinesia diaries for the last 3 days prior to each assessment.

RESULTS

There was a 41% difference in dyskinesia reduction on olanzapine compared to placebo, as measured by objective dyskinesia rating scales (mean percentage reduction abnormal involuntary movement score: 30% versus -11.2%, p < 0.02). Similar differences were demonstrated by patient diaries (mean reduction: 46% versus -2%, p < 0.02) and UPDRS items 32 and 33. Compared with placebo, treatment with olanzapine resulted in significant increases in 'off' time as measured by patient diaries (30% versus 2%) and reported adverse events (1.7 versus 0.1) including increased parkinsonism (1.1 versus 0.1) and a nonsignificant reported increase in drowsiness.

CONCLUSIONS

Low-dose olanzapine is effective in reducing dyskinesias in PD, but even at very low doses can result in unacceptable increases in parkinsonism and 'off' time.

Emergency department presentations of patients with Parkinson's disease

Parkinson's disease (PD) is a chronic progressive neurological disorder characterized by tremor, muscle rigidity, slowness of movement (bradykinesia), and gait instability. In early disease, PD is well managed in an office setting, however, as the disease progresses, a variety of syndromes may result in emergency department visits. The scenarios most likely to require an emergent evaluation are severe motor "off" periods with immobility, involuntary movements (dyskinesia), psychosis, acute confusion, panic disorder, and pain. Other less frequent presentations are also discussed. This article uses illustrative cases to provide a framework to discuss emergency department diagnosis and management issues in caring for these patients.

Medical treatment of later-stage motor problems of Parkinson disease

Parkinson disease progression is associated with the development of levodopa short-duration responses and dyskinesias, as well as gait freezing. Levodopa dose adjustment and adjunctive treatment with dopamine agonists form the major therapeutic strategies. Catechol O-methyltransferase inhibitors are also appropriate considerations, whereas other drugs, including selegiline, amantadine, anticholinergic agents, and propranolol, have a more minor role.

Quantification of dyskinesia in Parkinson's disease: validation of a novel instrumental method

We used a rotation-sensitive movement monitor (RoMM) to quantify and characterize dyskinesia in Parkinson's disease (PD). Both upper limbs of 22 patients with dyskinetic PD were recorded and videotaped simultaneously. Three neurologists reviewed the video segments and rated severity of dyskinesia on a four-point scale; they also assessed any asymmetry of dyskinesia between the right and left side as well as the dyskinesia type (choreic, dystonic, or mixed). Mean and median clinical ratings for severity, asymmetry, and type of dyskinesia were compared with (1) the total power of the frequency power spectrum (FPS, degrees/second), (2) the percent difference of FPS values between the right and left side, and (3) the frequency (Hz) of the predominant peak, respectively. Intra- and interrater reliability was determined and a test-retest analysis was performed. FPS values showed a statistically significant correlation with the clinical ratings for dyskinesia severity. FPS difference between both sides was more sensitive than raters in detecting dyskinesia asymmetry. A predominant frequency peak of dyskinesia was obtained in all cases and ranged from 0.25-3.25 Hz. There was a significant trend for high-frequency dyskinesia to correlate with choreic type and for low-frequency dyskinesia to correlate with dystonic type. Test-retest analysis indicated a high reliability. We conclude that the RoMM is a valid, reliable, and sensitive method to quantify and characterize dyskinesia. Examples are provided suggesting that this instrument may prove useful for long-term assessment of dyskinetic patients and as a standardized tool for assessing dyskinesia in pharmaceutical or surgical trials for PD.

Treating and preventing levodopa-induced dyskinesias: current and future strategies

Prevention of levodopa-induced dyskinesias is a therapeutic challenge for physicians. At present, it seems only possible to delay dyskinesias and motor fluctuations. In younger patients (aged <50 years), the strategy is to use a dopamine D2 agonist as monotherapy and then to add levodopa treatment when the parkinsonian symptoms progress. In older patients, (aged >50 years to <70 years), the therapeutic approach is to use early combination therapy of levodopa and a D2 agonist. The treatment of levodopa-induced dyskinesias must be considered in regard to the subtype and the severity of dyskinesias, and the patient. The general approach to the treatment of peak dose dyskinesias is to maintain dopamine brain stimulation at as stable a level as possible by keeping plasma and brain levodopa concentrations in the therapeutic range (above the therapeutic threshold but below the dyskinesia threshold). An appropriate strategy is to reduce the individual dose of levodopa, to spread out the daily levodopa dose and/or to try treatment with the sustained-release form of the drug. Combination treatment with the standard and sustained-release levodopa formulations is also possible. Stopping selegiline (deprenyl) therapy may reduce dyskinesias; reducing the dose of, or stopping treatment with, a dopamine agonist may also be beneficial. Anti-dyskinetic drugs such as amantadine, buspirone, fluoxetine, propanolol and principally clozapine may be used. In severe dyskinesias, apomorphine infusion may be tried. In refractory dyskinesia, surgical procedures such as pallidotomy and chronic deep brain stimulation (globus pallidus/subthalamic nucleus) may be proposed. Theoretically, treatment of diphasic dyskinesias requires the maintenance of plasma levodopa concentrations above the dyskinesia threshold. However, this approach leads to constant and severe dyskinesia after only a few weeks of treatment. Thus, the strategy used to treat diphasic dyskinesia is close to the treatment of peak-dose dyskinesias. Apomorphine (or the liquid form of levodopa) may be helpful to prevent diphasic dyskinesias. In selected patients, a midday rest in the 'off' phase may decrease the duration of dyskinesia. Treatment of early morning dystonia is based on the addition to the regimen of the sustained release formulation of levodopa before bedtime. Liquid levodopa and apomorphine injection may be used just before the appearance of the dystonic posture. Botulinum toxin may be helpful in severe dystonia.

Clinical rating of dyskinesias in Parkinson's disease: use and reliability of a new rating scale

Drug-induced dyskinesias (DID) manifested as hyperkinetic and/or dystonic movements or postures are common problems in Parkinson's disease (PD). Novel therapeutic interventions may offer possibilities to counteract these common adverse effects of an otherwise necessary treatment. To be able to evaluate the effects of such interventions on DID, reliable and relevant clinical assessment tools are needed. We tested the inter- and intrarater reliability of a new clinical dyskinesia rating scale consisting of separate ratings of different body parts, including lateralization and separate ratings of dystonia and hyperkinesias. Interrater reliability was tested both with and without a defined scoring code and clarification of the dystonia section. The nondefined version was also tested for intrarater reliability. Thirteen raters independently reviewed 23 videotape sequences showing PD patients performing standardized motor tests. Inter- and intrarater agreement was significant in all evaluations, and no differences were detected when comparing ratings performed with the defined and nondefined version of the scale. The rationale for, and the role and use of, the present scale are addressed.

Worsening of levodopa-induced dyskinesias by motor and mental tasks

Ten patients who had Parkinson's disease with disabling dyskinesia were included in this study to evaluate the role of mental (mental calculation) and motor (flexion/extension of right fingers, flexion/extension of left fingers, flexion/extension of the neck, speaking aloud) tasks on the worsening of peak-dose dyskinesia following administration of an effective single dose of apomorphine. Compared with the score at rest (1.3+/-0.3), a significant aggravation of the dyskinesia score was observed during speaking aloud (5.2+/-1.1, p<0.05), movements of right (4.5+/-1.0, p<0.05) and left (3.7+/-0.8, p<0.05) fingers, movements of the neck (5.1+/-1.0, p<0.05), and mental calculation (3.1+/-1.0, p<0.05). These results suggest that activation tasks such as "speaking aloud" could be used for objective assessment of dyskinesia severity.

New developments in understanding the etiology of Parkinson's disease and in its treatment

Important recent advances have been made in understanding the etiology and pathogenesis of Parkinson's disease, as well as in developing novel treatments. Two newly identified genes, alpha-synuclein and parkin, have been linked to parkinsonism. In addition, disturbances to the normal basal ganglia circuits in Parkinson's patients are being described at both anatomical and physiological levels. These developments provide a strong scientific basis for novel medical and surgical strategies to treat the profound motor disturbances in patients with Parkinson's disease.

Adjuncts to dopamine replacement: a pragmatic approach to reducing the problem of dyskinesia in Parkinson's disease

Dyskinesias following long-term dopamine replacement therapy are a major limitation of current treatments for Parkinson's disease. Recently, attention has been focused on the concept of using non-dopaminergic adjuncts to currently available therapies in an attempt to reduce the problem of dyskinesia. Thus, an enhanced understanding of the neural mechanisms underlying dyskinetic symptoms has led to the realization that it might be possible to manipulate non-dopaminergic systems and reduce dyskinesia without compromising the anti-parkinsonian efficacy of drugs such as L-dopa. This article discusses how non-dopaminergic manipulations could reverse the abnormalities in basal ganglia circuitry responsible for generating dyskinesia. It is proposed that potential anti-dyskinetic drugs might include glutamate (NMDA) receptor antagonists, opioid receptor antagonists, cannabinoid receptor agonists or antagonists, alpha2 adrenergic receptor antagonists, and 5-HT-enhancing agents.

Parkinson's disease. Second of two parts

At no time in the past have the basic and clinical sciences applied to Parkinson's disease been so active. Experimental therapies under study at present promise to improve on the limitations of existing treatments. Future progress in understanding the causation and pathogenesis of the disorder will permit the development of new treatments that will slow, halt, or even reverse the currently inexorable progressive course of Parkinson's disease.