Nondopaminergic mechanisms in levodopa-induced dyskinesia

Activation of mGlu 2/3 receptors with the orthosteric agonist LY-404,039 alleviates dyskinesia in experimental parkinsonism

LY-404,039 is an orthosteric agonist at metabotropic glutamate 2 and 3 (mGlu 2/3 ) receptors, with a possible additional agonist effect at dopamine D 2 receptors. LY-404,039 and its pro-drug, LY-2140023, have previously been tested in clinical trials for psychiatric indications and could therefore be repurposed if they were shown to be efficacious in other conditions. We have recently demonstrated that the mGlu 2/3 orthosteric agonist LY-354,740 alleviated L-3,4-dihydroxyphenylalanine (L-DOPA)-induced abnormal involuntary movements (AIMs) in the 6-hydroxydopamine (6-OHDA)-lesioned rat without hampering the anti-parkinsonian action of L-DOPA. Here, we seek to take advantage of a possible additional D 2 -agonist effect of LY-404,039 and see if an anti-parkinsonian benefit might be achieved in addition to the antidyskinetic effect of mGlu 2/3 activation. To this end, we have administered LY-404,039 (vehicle, 0.1, 1 and 10 mg/kg) to 6-OHDA-lesioned rats, after which the severity of axial, limbs and oro-lingual (ALO) AIMs was assessed. The addition of LY-404,039 10 mg/kg to L-DOPA resulted in a significant reduction of ALO AIMs over 60-100 min (54%, P  < 0.05). In addition, LY-404,039 significantly enhanced the antiparkinsonian effect of L-DOPA, assessed through the cylinder test (76%, P  < 0.01). These results provide further evidence that mGlu 2/3 orthosteric stimulation may alleviate dyskinesia in PD and, in the specific case of LY-404,039, a possible D 2 -agonist effect might also make it attractive to address motor fluctuations. Because LY-404,039 and its pro-drug have been administered to humans, they could possibly be advanced to Phase IIa trials rapidly for the treatment of motor complications in PD.

Effects of Intracerebral Aminophylline Dosing on Catalepsy and Gait in an Animal Model of Parkinson's Disease

Parkinson's disease (PD) is a progressive disorder characterized by the apoptosis of dopaminergic neurons in the basal ganglia. This study explored the potential effects of aminophylline, a non-selective adenosine A1 and A2A receptor antagonist, on catalepsy and gait in a haloperidol-induced PD model. Sixty adult male Swiss mice were surgically implanted with guide cannulas that targeted the basal ganglia. After seven days, the mice received intraperitoneal injections of either haloperidol (experimental group, PD-induced model) or saline solution (control group, non-PD-induced model), followed by intracerebral infusions of aminophylline. The assessments included catalepsy testing on the bar and gait analysis using the Open Field Maze. A two-way repeated-measures analysis of variance (ANOVA), followed by Tukey's post hoc tests, was employed to evaluate the impact of groups (experimental × control), aminophylline (60 nM × 120 nM × saline/placebo), and interactions. Significance was set at 5%. The results revealed that the systemic administration of haloperidol in the experimental group increased catalepsy and dysfunction of gait that paralleled the observations in PD. Co-treatment with aminophylline at 60 nM and 120 nM reversed catalepsy in the experimental group but did not restore the normal gait pattern of the animals. In the non-PD induced group, which did not present any signs of catalepsy or motor dysfunctions, the intracerebral dose of aminophylline did not exert any interference on reaction time for catalepsy but increased walking distance in the Open Field Maze. Considering the results, this study highlights important adenosine interactions in the basal ganglia of animals with and without signs comparable to those of PD. These findings offer valuable insights into the neurobiology of PD and emphasize the importance of exploring novel therapeutic strategies to improve patient's catalepsy and gait.

Suppression of presynaptic corticostriatal glutamate activity attenuates L-dopa-induced dyskinesia in 6-OHDA-lesioned Parkinson's disease mice

A common adverse effect of Parkinson's disease (PD) treatment is L-dopa-induced dyskinesia (LID). This condition results from both dopamine (DA)-dependent and DA-independent mechanisms, as glutamate inputs from corticostriatal projection neurons impact DA-responsive medium spiny neurons in the striatum to cause the dyskinetic behaviors. In this study, we explored whether suppression of presynaptic corticostriatal glutamate inputs might affect the behavioral and biochemical outcomes associated with LID. We first established an animal model in which 6-hydroxydopamine (6-OHDA)-lesioned mice were treated daily with L-dopa (10 mg/kg, i.p.) for 2 weeks; these mice developed stereotypical abnormal involuntary movements (AIMs). When the mice were pretreated with the NMDA antagonist, amantadine, we observed suppression of AIMs and reductions of phosphorylated ERK1/2 and NR2B in the striatum. We then took an optogenetic approach to manipulate glutamatergic activity. Slc17a6 (vGluT2)-Cre mice were injected with pAAV5-Ef1a-DIO-eNpHR3.0-mCherry and received optic fiber implants in either the M1 motor cortex or dorsolateral striatum. Optogenetic inactivation at either optic fiber implant location could successfully reduce the intensity of AIMs after 6-OHDA lesioning and L-dopa treatment. Both optical manipulation strategies also suppressed phospho-ERK1/2 and phospho-NR2B signals in the striatum. Finally, we performed intrastriatal injections of LDN 212320 in the dyskenesic mice to enhance expression of glutamate uptake transporter GLT-1. Sixteen hours after the LDN 212320 treatment, L-dopa-induced AIMs were reduced along with the levels of striatal phospho-ERK1/2 and phospho-NR2B. Together, our results affirm a critical role of corticostriatal glutamate neurons in LID and strongly suggest that diminishing synaptic glutamate, either by suppression of neuronal activity or by upregulation of GLT-1, could be an effective approach for managing LID.

Future Directions for Developing Non-dopaminergic Strategies for the Treatment of Parkinson's Disease

The symptomatic treatment of Parkinson's disease (PD) has been dominated by the use of dopaminergic medication, but significant unmet need remains, much of which is related to non-motor symptoms and the involvement of non-dopaminergic transmitter systems. As such, little has changed in the past decades that has led to milestone advances in therapy and significantly improved treatment paradigms and patient outcomes, particularly in relation to symptoms unresponsive to levodopa. This review has looked at how pharmacological approaches to treatment are likely to develop in the near and distant future and will focus on two areas: 1) novel non-dopaminergic pharmacological strategies to control motor symptoms; and 2) novel non-dopaminergic approaches for the treatment of non-motor symptoms. The overall objective of this review is to use a 'crystal ball' approach to the future of drug discovery in PD and move away from the more traditional dopamine-based treatments. Here, we discuss promising non-dopaminergic and 'dirty drugs' that have the potential to become new key players in the field of Parkinson's disease treatment.

Cannabinoids in Medicine: A Multifaceted Exploration of Types, Therapeutic Applications, and Emerging Opportunities in Neurodegenerative Diseases and Cancer Therapy

In this review article, we embark on a thorough exploration of cannabinoids, compounds that have garnered considerable attention for their potential therapeutic applications. Initially, this article delves into the fundamental background of cannabinoids, emphasizing the role of endogenous cannabinoids in the human body and outlining their significance in studying neurodegenerative diseases and cancer. Building on this foundation, this article categorizes cannabinoids into three main types: phytocannabinoids (plant-derived cannabinoids), endocannabinoids (naturally occurring in the body), and synthetic cannabinoids (laboratory-produced cannabinoids). The intricate mechanisms through which these compounds interact with cannabinoid receptors and signaling pathways are elucidated. A comprehensive overview of cannabinoid pharmacology follows, highlighting their absorption, distribution, metabolism, and excretion, as well as their pharmacokinetic and pharmacodynamic properties. Special emphasis is placed on the role of cannabinoids in neurodegenerative diseases, showcasing their potential benefits in conditions such as Alzheimer's disease, Parkinson's disease, Huntington's disease, and multiple sclerosis. The potential antitumor properties of cannabinoids are also investigated, exploring their potential therapeutic applications in cancer treatment and the mechanisms underlying their anticancer effects. Clinical aspects are thoroughly discussed, from the viability of cannabinoids as therapeutic agents to current clinical trials, safety considerations, and the adverse effects observed. This review culminates in a discussion of promising future research avenues and the broader implications for cannabinoid-based therapies, concluding with a reflection on the immense potential of cannabinoids in modern medicine.

Identification of novel proteins associated with movement-related adverse antipsychotic effects by integrating GWAS data and human brain proteomes

Genome-wide association studies (GWAS) have identified some variants for movement-related adverse antipsychotic effects (MAAE), while how these variants confer MAAE remains unclear. We used the probabilistic Mendelian randomization (PMR) method to identify candidate proteins for MAAE by integrating MAAE GWASs and protein quantitative trait loci (pQTL) data. An independent pQTL data from the Banner project and brain-derived eQTL data were used to perform confirmatory PMR. A total of 56 proteins were identified as candidate targets for MAAE after false discovery rates (FDR) correction, such as GRIN2B, ADRA1A, and PED4B. 12 genes were replicated in the confirmatory PMR, and 18 genes had consistent evidence at the transcript level. Furthermore, we investigated the associations between candidate proteins and the motor symptoms of Parkinson's disease (PD). There were 24, 38, and 10 candidate proteins that were significantly associated with PD, PD motor subtypes, and PD motor progression, respectively. Enrichment analysis identified 34 GO terms and 17 pathways that may be involved in MAAE, such as glutamatergic synapse, glutamate receptor complex, and GABAergic synapse. Our study identified multiple candidate genes and pathways that were associated with MAAE, providing new insights into the biological mechanism of MAAE and targets for further mechanistic and therapeutic studies.

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.

Dyskinesia and Parkinson's disease: animal model, drug targets, and agents in preclinical testing

INTRODUCTION

Parkinson's disease (PD) is the second most common neurodegenerative disease after Alzheimer's disease. PD patients exhibit a classic spectrum of motor symptoms, arising when dopamine neurons in the substantia nigra pars compacta are reduced by 60%. The dopamine precursor L-DOPA represents the most effective therapy for improving PD motor dysfunctions, thus far available. Unfortunately, long-term treatment with L-DOPA is associated with the development of severe side effects, resulting in abnormal involuntary movements termed levodopa-induced dyskinesia (LID). Amantadine is the only drug currently approved for the treatment of LID indicating that LID management is still an unmet need in PD and encouraging the search for novel anti-dyskinetic drugs or the assessment of combined therapies with different molecular targets.

AREAS COVERED

This review provides an overview of the main preclinical models used to study LID and of the latest preclinical evidence on experimental and clinically available pharmacological approaches targeting non-dopaminergic systems.

EXPERT OPINION

LIDs are supported by complex molecular and neurobiological mechanisms that are still being studied today. This complexity suggests the need of developing personalized pharmacological approach to obtain an effective amelioration of LID condition and improve the quality of life of PD patients.

Molecular Mechanisms and Therapeutic Strategies for Levodopa-Induced Dyskinesia in Parkinson’s Disease: A Perspective Through Preclinical and Clinical Evidence

Parkinson’s disease (PD) is the second leading neurodegenerative disease that is characterized by severe locomotor abnormalities. Levodopa (L-DOPA) treatment has been considered a mainstay for the management of PD; however, its prolonged treatment is often associated with abnormal involuntary movements and results in L-DOPA-induced dyskinesia (LID). Although LID is encountered after chronic administration of L-DOPA, the appearance of dyskinesia after weeks or months of the L-DOPA treatment has complicated our understanding of its pathogenesis. Pathophysiology of LID is mainly associated with alteration of direct and indirect pathways of the cortico-basal ganglia-thalamic loop, which regulates normal fine motor movements. Hypersensitivity of dopamine receptors has been involved in the development of LID; moreover, these symptoms are worsened by concurrent non-dopaminergic innervations including glutamatergic, serotonergic, and peptidergic neurotransmission. The present study is focused on discussing the recent updates in molecular mechanisms and therapeutic approaches for the effective management of LID in PD patients.

Tyrosinyl-amantadine: A New Amantadine Derivative With an Ameliorative Effect in a 6-OHDA Experimental Model of Parkinson's Disease in Rats

The neuroprotective capacity of newly synthesized amantadine derivative tyrosinyl-amantadine (Tyr-Am) with expected antiparkinsonian properties was evaluated in a 6-hydroxydopamine (6-OHDA) model of Parkinson's disease. Male Wistar rats were divided into the following groups: sham-operated (SO), striatal 6-OHDA-lesioned control group, 6-OHDA-lesioned rats pretreated for 6 days with Tyr-Am (16 mg/kg administered intraperitoneally, i.p.), and 6-OHDA-lesioned rats pretreated for 6 days with amantadine (40 mg/kg i.p.), used as a referent. On the first, second and third week post-lesion, the animals were subjected to some behavioral tests (apomorphine-induced rotation, rotarod, and passive avoidance test). The acetylcholinesterase (AChE) activity and key oxidative stress parameters including lipid peroxidation levels (LPO) and superoxide dismutase (SOD) were measured in brain homogenates. The results showed that the neuroprotective effect of Tyr-Am was comparable to that of amantadine, improving neuromuscular coordination and learning and memory performance even at a 2.5-fold lower dose. Tyr-Am demonstrated significant antioxidant properties via decreased LPO levels but had no effect on AChE activity. We can conclude that the newly synthesized amantadine derivative Tyr-Am demonstrated significant antiparkinsonian activity in a 6-OHDA experimental model.

GOCOVRI® (amantadine) extended-release capsules in Parkinson's disease

Amantadine is an old, antiviral compound, which moderately improves motor behavior in Parkinson's disease. Its current resurgence results from an innovative, delayed uptake and extended release amantadine hydrochloride capsule, given at bedtime once daily. It is the only approved compound for reduction of involuntary movements, so called dyskinesia, in fluctuating orally levodopa treated patients. It additionally ameliorates 'off'-intervals characterized by impaired motor behavior. These beneficial effects result from higher and more continuous brain delivery of amantadine. Future clinical research is warranted on preventive effects of this amantadine capsule combined with enzyme blockers of central monoamine oxidase B and peripheral catechol-O-methyltransferase on motor complications in orally levodopa treated patients, as all these pharmacological principles support the concept of continuous dopamine substitution.

Riluzole Administration to Rats with Levodopa-Induced Dyskinesia Leads to Loss of DNA Methylation in Neuronal Genes

Dyskinesias are characterized by abnormal repetitive involuntary movements due to dysfunctional neuronal activity. Although levodopa-induced dyskinesia, characterized by tic-like abnormal involuntary movements, has no clinical treatment for Parkinson’s disease patients, animal studies indicate that Riluzole, which interferes with glutamatergic neurotransmission, can improve the phenotype. The rat model of Levodopa-Induced Dyskinesia is a unilateral lesion with 6-hydroxydopamine in the medial forebrain bundle, followed by the repeated administration of levodopa. The molecular pathomechanism of Levodopa-Induced Dyskinesia is still not deciphered; however, the implication of epigenetic mechanisms was suggested. In this study, we investigated the striatum for DNA methylation alterations under chronic levodopa treatment with or without co-treatment with Riluzole. Our data show that the lesioned and contralateral striata have nearly identical DNA methylation profiles. Chronic levodopa and levodopa + Riluzole treatments led to DNA methylation loss, particularly outside of promoters, in gene bodies and CpG poor regions. We observed that several genes involved in the Levodopa-Induced Dyskinesia underwent methylation changes. Furthermore, the Riluzole co-treatment, which improved the phenotype, pinpointed specific methylation targets, with a more than 20% methylation difference relative to levodopa treatment alone. These findings indicate potential new druggable targets for Levodopa-Induced Dyskinesia.

Hydrogen sulfide attenuates induction and prevents progress of the 6-hydroxydopamine-induced Parkinsonism in rat through activation of ATP-sensitive potassium channels and suppression of ER stress

PURPOSE

Studies argue in favor of hydrogen sulfide (H₂S) as the next potent therapeutic agent for neurodegenerative diseases. In present study, we investigated the effect of long term treatment with NaHS (as donor of H₂S) on induction and progress of the 6-hydroxydopamine (6-OHDA) -induced Parkinsonism in rat.

METHODS

The 6-OHDA was injected into medial forebrain bundle of right hemisphere by stereotaxic surgery. Behavioral tests and treatments were carried out to eight weeks after the toxin. Immunohistochemistry and western blotting were carried out to evaluate the survival of tyrosine hydroxylase (TH) -positive neurons in substantia nigra (SN) and also expression of glucose-regulated protein 78 (GRP78) and C/EBP homologous protein (CHOP), the markers of endoplasmic reticulum (ER) stress, in striatum and SN.

RESULTS

Eight weeks assessment of the behavioral symptoms showed that NaHS especially at dose of 100 μmol/kg attenuates remarkably induction of the Parkinsonism and prevents its progress. NaHS also increased the survival of TH- positive neurons and suppressed 6-OHDA- induced overexpression of GRP78 and CHOP. Blockade of ATP-sensitive potassium (K-ATP) channels with glibenclamide (Glib) prevented markedly the effect of NaHS on both the induction phase and survival of TH- positive neurons. But Glib did not affect the preventing effect of NaHS on the progress phase and its suppressing effect on the overexpression of ER stress markers.

CONCLUSION

H₂S attenuates induction of the 6-OHDA- induced Parkinsonism and also increases the survival of dopaminergic neurons through activation of K-ATP channels. H₂S also prevents progress of the Parkinsonism probably through suppression of ER stress.

Autophagy-dependent removal of α-synuclein: a novel mechanism of GM1 ganglioside neuroprotection against Parkinson's disease

GM1 ganglioside is particularly abundant in the mammalian central nervous system and has shown beneficial effects on neurodegenerative diseases. In this study, we investigated the therapeutic effect of GM1 ganglioside in experimental models of Parkinson's disease (PD) in vivo and in vitro. Mice were injected with MPTP (30 mg·kg-1·d-1, i.p.) for 5 days, resulting in a subacute model of PD. PD mice were treated with GM1 ganglioside (25, 50 mg·kg-1·d-1, i.p.) for 2 weeks. We showed that GM1 ganglioside administration substantially improved the MPTP-induced behavioral disturbance and increased the levels of dopamine and its metabolites in the striatal tissues. In the MPP+-treated SH-SY5Y cells and α-synuclein (α-Syn) A53T-overexpressing PC12 (PC12α-Syn A53T) cells, treatment with GM1 ganglioside (40 μM) significantly decreased α-Syn accumulation and alleviated mitochondrial dysfunction and oxidative stress. We further revealed that treatment with GM1 ganglioside promoted autophagy, evidenced by the autophagosomes that appeared in the substantia nigra of PD mice as well as the changes of autophagy-related proteins (LC3-II and p62) in the MPP+-treated SH-SY5Y cells. Cotreatment with the autophagy inhibitor 3-MA or bafilomycin A1 abrogated the in vivo and in vitro neuroprotective effects of GM1 ganglioside. Using GM1 ganglioside labeled with FITC fluorescent, we observed apparent colocalization of GM1-FITC and α-Syn as well as GM1-FITC and LC3 in PC12α-Syn A53T cells. GM1 ganglioside significantly increased the phosphorylation of autophagy regulatory proteins ATG13 and ULK1 in doxycycline-treated PC12α-Syn A53T cells and the MPP+-treated SH-SY5Y cells, which was inhibited by 3-MA. Taken together, this study demonstrates that the anti-PD role of GM1 ganglioside resulted from activation of autophagy-dependent α-Syn clearance.

The effect of intra-striatal administration of GPR55 agonist (LPI) and antagonist (ML193) on sensorimotor and motor functions in a Parkinson's disease rat model

OBJECTIVE

G protein-coupled receptor 55 (GPR55) is an orphan G protein-coupled receptor with various physiological functions. Recent evidence suggests that this receptor may be involved in the control of motor functions. Therefore, in the present study, we evaluated the effects of intra-striatal administration of GPR55 selective ligands in a rat model of Parkinson's disease.

METHODS

Experimental Parkinson was induced by unilateral intra-striatal administration of 6-hydroxydopamine (6-OHDA, 10 µg/rat). L-α-lysophosphatidylinositol (LPI, 1 and 5 µg/rat), an endogenous GPR55 agonist, and ML193 (1 and 5 µg/rat), a selective GPR55 antagonist, were injected into the striatum of 6-OHDA-lesioned rats. Motor performance and balance skills were evaluated using the accelerating rotating rod and the ledged beam tests. The sensorimotor function of the forelimbs and locomotor activity were assessed by the adhesive removal and open field tests, respectively.

RESULTS

6-OHDA-lesioned rats had impaired behaviours in all tests. Intra-striatal administration of LPI in 6-OHDA-lesioned rats increased time on the rotarod, decreased latency to remove the label, with no significant effect on slip steps, and locomotor activity. Intra-striatal administration of ML193 also increased time on the rotarod, decreased latency to remove the label and slip steps in 6-OHDA-lesioned rats mostly at the dose of 1 µg/rat.

CONCLUSIONS

This study suggests that the striatal GPR55 is involved in the control of motor functions. However, considering the similar effects of GPR55 agonist and antagonist, it may be concluded that this receptor has a modulatory role in the control of motor deficits in an experimental model of Parkinson.

Dysfunction in superior frontal gyrus associated with diphasic dyskinesia in Parkinson's disease

Alterations in brain function in Parkinson's disease (PD) patients with diphasic dyskinesia have not been investigated. We aimed to explore the alterations in regional brain function. Each of 53 levodopa (LD)-treated PD patients had two resting-state functional magnetic resonance imaging (rs-fMRI) scans in the same morning, before and after taking LD. The regional homogeneity (ReHo) approach was used to reveal local synchronization changes. Two-way factorial repeated measures analysis of covariance, with group as a between-subject factor and LD effect as a within-subject factor, was performed to explore the two main effects and interaction. Interactive analysis was used to show outcomes that combined disease status and LD effect. Spearman's correlations were used to detect associations between interactive brain regions and severity of dyskinetic symptoms, assessed by the Unified Dyskinesia Rating Scale (UDyRS) scores, along with moderation analyses. There was no significant difference in the main group effect analysis. Significantly different clusters obtained from main LD effect analysis were in left caudate nucleus and putamen. ReHo values decreased in the caudate nucleus and increased in the putamen during the ON state after taking LD. Interaction between group and LD effect was found in left medial superior frontal gyrus (mSFG), where there were the lowest ReHo values, and was negatively correlated with UDyRS scores in the diphasic dyskinetic group during the ON state. The relationship was independent of LD dose. Abnormal local synchronization in the mSFG is closely associated with the development of diphasic dyskinesia in PD patients.

Distinct anti-dyskinetic effects of amantadine and group II metabotropic glutamate receptor agonist LY354740 in a rodent model: An electrophysiological perspective

L-DOPA-induced dyskinesia (LID) is a major complication of long-term dopamine replacement therapy in Parkinson's disease. Characteristic neural oscillation and abnormal activity of striatal projection neurons (SPNs) are typical pathological events of LID, which would be reliable biomarkers for assessment of novel anti-dyskinetic approach if fully profiled. Glutamate dysregulation plays a critical role in the development of LID, and the group II metabotropic glutamate receptors (mGluR2/3) is believed to regulate the release of glutamate on the presynaptic terminals and inhibits postsynaptic excitation. However, the anti-dyskinetic effect of modulating mGluR2/3 is still unclear. In this study, rats with unilateral dopaminergic lesion were injected with L-DOPA (12 mg/kg, i.p.) for seven days, while motor behavior was correlated with in vivo electrophysiology analyzing LFP and single-cell activity in both primary motor cortex and dorsolateral striatum. Our study showed that as LID established, high γ oscillation (hγ) predominated during LID, the number of unstable responses of SPN to dopamine increased, and the coherence between these patterns of oscillation and spiking activity also increased. We found that pretreatment of NMDA receptor antagonist, amantadine 60 mg/kg, i.p. (AMAN) significantly reduced abnormal involuntary movements (AIMs), in parallel with the reduction of hγ oscillation, and more markedly with a decrease in unstable responses of SPNs. In contrast, a mGluR2/3 agonist, LY354740 12 mg/kg, i.p. (LY) significantly shortened the duration of LID but merely exhibited a weak effect in diminishing the intensity of LID or reversing SPN responses. Together results indicate that AIMs in the rat model of PD are associated with abnormal corticostriatal signaling, which could be reversed by NMDAR antagonism more efficiently than mGluR2/3 agonism.

Roles of Glutamate Receptors in Parkinson's Disease

Parkinson's disease is a progressive neurodegenerative disorder resulting from the degeneration of pigmented dopaminergic neurons in the substantia nigra pars compacta. It induces a series of functional modifications in the circuitry of the basal ganglia nuclei and leads to severe motor disturbances. The amino acid glutamate, as an excitatory neurotransmitter, plays a key role in the disruption of normal basal ganglia function regulated through the interaction with its receptor proteins. It has been proven that glutamate receptors participate in the modulation of neuronal excitability, transmitter release, and long-term synaptic plasticity, in addition to being related to the altered neurotransmission in Parkinson's disease. Therefore, they are considered new targets for improving the therapeutic strategies used to treat Parkinson's disease. In this review, we discuss the biological characteristics of these receptors and demonstrate the receptor-mediated neuroprotection in Parkinson's disease. Pharmacological manipulation of these receptors during anti-Parkinsonian processes in both experimental studies and clinical trials are also summarized.

Continuous subcutaneous apomorphine infusion in Parkinson's disease: causes of discontinuation and subsequent treatment strategies

Continuous subcutaneous apomorphine infusion (CSAI) is a well-recognized therapeutic option for the management of motor fluctuations in Parkinson's disease (PD), although clinical experience suggests that most patients discontinue CSAI after a variable amount of time due to several causes and circumstances. The objective of the present study was to evaluate the reasons of CSAI discontinuation and to investigate which treatment was adopted afterwards. Two independent raters retrospectively reviewed the electronic medical record of 114 patients treated with CSAI for at least 6 months. The records were reviewed regarding efficacy, safety, and evolution of CSAI treatment. Most of PD patients on CSAI had a significant improvement in their clinical condition. Lack of improvement of dyskinesia was the most frequent causes of treatment discontinuation. The second reason for CSAI discontinuation was cognitive deterioration. At CSAI discontinuation, younger patients were more likely to undergo deep brain stimulation (DBS), while older patients and patients with cognitive impairment were more likely switched to oral therapy alone (OTA). CSAI is an effective treatment that unfortunately must be discontinued in a great number of patients with advanced PD. As older age is the main limiting factor for accessing second-level therapies at CSAI discontinuation, CSAI treatment should not be postponed to older age. CSAI might be considered a good first-line and fast strategy in patients undergoing rapid deterioration of their quality of life while waiting for DBS or levodopa/carbidopa intestinal gel therapy.

Evaluating ADS5102 (amantadine) for the treatment of Parkinson's disease patients with dyskinesia

Introduction: Amantadine is an old, antiviral compound that moderately ameliorates impaired motor behaviour in Parkinson's disease. Its current resurgence results from the novel retarded release amantadine hydrochloride formulation, ADS5102, which has also received approval for the treatment of levodopa-related involuntary movements known as dyskinesia. Areas covered: This non-systematic, narrative drug evaluation discusses the value of ADS5102 for patients with Parkinson's disease. ADS5102 is orally applied once daily in the evening. This capsule provides higher and more continuous amantadine plasma concentrations than conventional amantadine immediate release formulations with their two to three times daily intake plan. Expert opinion: ADS5102 was superior to placebo in clinical trials. They aimed for the amelioration of motor complications, particularly at 'OFF' periods and with dyskinesia in fluctuating levodopa treated patients with Parkinson's disease. Side effects and tolerability were similar to the well-known effects of conventional amantadine formulations. ADS5102 simplifies treatment and improves compliance problems in the long run. The marketing of ADS5102 outside the US will be complex for return of research costs and investments required for its manufacturing. Indeed, worldwide institutional price regulation scenarios often only consider new therapeutic mode of actions as being innovative as opposed to old drugs with improved pharmacokinetic behaviour.

Chemical management of levodopa-induced dyskinesia in Parkinson's disease patients

INTRODUCTION

Levodopa-induced dyskinesias (LID) appears in more than 50% of Parkinson's disease patients after 5 years of treatment and clinicians always have to ensure that there is a balance between the beneficial effect of the treatment and the potential complications.

AREAS COVERED

In this review, the authors discuss the treatment of LID. Treatment can be divided into strategies for preventing their occurrence, modification of dopaminergic therapy, and providing more continuous dopaminergic stimulation as well as the use of nondopaminergic drugs.

EXPERT OPINION

Amantadine is currently considered the most effective drug for the treatment of LID. Several compounds developed to target adenosine, adrenergic, glutamatergic, and serotonergic receptors have shown to significantly decrease dyskinesias in animal models. However, despite promising preclinical results, translation to clinical practice remains challenging and majority of these compounds failed to decrease LID in randomized controlled trials with moderate-to-advanced parkinsonian patients. Despite promising results with nondopaminergic drugs, treatment of dyskinesias is still challenging and largely due to their side effects. Future research should focus on developing treatments that can provide continuous dopaminergic delivery throughout the day in a noninvasive manner. Studies on the impact of the early administration of long-acting formulations of levo-3,4-dihydroxy-phenylalanine on dyskinesias are also necessary.

Striatal Gαolf/cAMP Signal-Dependent Mechanism to Generate Levodopa-Induced Dyskinesia in Parkinson's Disease

The motor symptoms of Parkinson’s disease (PD) result from striatal dopamine (DA) deficiency due to a progressive degeneration of nigral dopaminergic cells. Although DA replacement therapy is the mainstay to treat parkinsonian symptoms, a long-term daily administration of levodopa often develops levodopa-induced dyskinesia (LID). LID is closely linked to the dysregulation of cyclic adenosine monophosphate (cAMP) signaling cascades in the medium spiny neurons (MSNs), the principal neurons of the striatum, which are roughly halved with striatonigral MSNs by striatopallidal MSNs. The olfactory type G-protein α subunit (Gα_olf) represents an important regulator of the cAMP signal activities in the striatum, where it positively couples with D₁-type dopamine receptor (D₁R) and adenosine A_2A receptor (A_2AR) to increase cAMP production in the MSNs. Notably, D₁Rs are primarily expressed in striatonigral MSNs, whereas D₂Rs and A_2ARs are expressed in striatopallidal MSNs. Based on the evidence obtained from parkinsonian mice, we hypothesized that in the DA-denervated striatum with D₁R hypersensitivity, a repeated and pulsatile exposure to levodopa might cause a usage-induced degradation of Gα_olf proteins in striatal MSNs, resulting in increased and decreased levels of Gα_olf protein in the striatonigral and striatopallidal MSNs, respectively. As a principal cause for generating LID, this might lead to an increased responsiveness to levodopa exposure in both striatonigral and striatopallidal MSNs. Our hypothesis reinforces the long-standing concept that LID might result from the reduced activity of the striatopallidal pathway and has important clinical implications.

Altered adenosine 2A and dopamine D2 receptor availability in the 6-hydroxydopamine-treated rats with and without levodopa-induced dyskinesia

Several lines of evidence imply alterations in adenosine signaling in Parkinson's disease (PD). Here, we investigated cerebral changes in adenosine 2A receptor (A_2AR) availability in 6-hydroxydopamine (6-OHDA)-lesioned rats with and without levodopa-induced dyskinesia (LID) using positron-emission tomography (PET) with [¹¹C]preladenant. In parallel dopamine type 2 receptor (D₂R) imaging with [¹¹C]raclopride PET and behavioral tests for motor and cognitive function were performed.

METHODS

Parametric A_2AR and D₂R binding potential (BP_ND) images were reconstructed using reference tissue models with midbrain and cerebellum as reference tissue, respectively. All images were anatomically standardized to Paxinos space and analyzed using volume-of-interest (VOI) and voxel-based approaches. The behavioral alternations were assessed with the open field test, Y-maze, novel object recognition test, cylinder test, and abnormal involuntary movement (AIM) score. In total, 28 female Wistar rats were included.

RESULTS

On the behavioral level, 6-OHDA-lesioned rats showed asymmetry in forepaw use and deficits in spatial memory and explorative behavior as compared to the sham-operated animals. 15-Days of levodopa (L-DOPA) treatment induced dyskinesia but did not alleviate motor deficits in PD rats. Intranigral 6-OHDA injection significantly increased D₂R binding in the lesioned striatum (BP_ND: 2.69 ± 0.40 6-OHDA vs. 2.31 ± 0.18 sham, + 16.6%; p = 0.03), whereas L-DOPA treatment did not affect the D₂R binding in the ipsilateral striatum of the PD rats. In addition, intranigral 6-OHDA injection tended to decrease the A_2AR availability in the lesioned striatum. The decrease became significant when data were normalized to the non-affected side (BP_ND: 4.32 ± 0.41 6-OHDA vs. 4.58 ± 0.89 sham; NS, ratio: 0.94 ± 0.03 6-OHDA vs. 1.00 ± 0.02 sham; - 6.1%; p = 0.01). L-DOPA treatment significantly increased A_2AR binding in the affected striatum (BP_ND: 6.02 ± 0.91 L-DOPA vs. 4.90 ± 0.76 saline; + 23.4%; p = 0.02). In PD rats with LID, positive correlations were found between D₂R and A_2AR BP_ND values in the ipsilateral striatum (r = 0.88, p_peak = 8.56.10^-4 uncorr), and between AIM score and the D₂R BP_ND in the contralateral striatum (r = 0.98; p_peak = 9.55.10^-5 uncorr).

CONCLUSION

A_2AR availability changed in drug-naïve and in L-DOPA-treated PD rats. The observed correlations of striatal D₂R availability with A_2AR availability and with AIM score may provide new knowledge on striatal physiology and new possibilities to further unravel the functions of these targets in the pathophysiology of PD.

Investigational drugs in Phase I and Phase II for Levodopa-induced dyskinesias

INTRODUCTION

Prolonged treatment of Parkinson's disease (PD) with levodopa (L-DOPA) results in motor complications, including motor fluctuations and involuntary movements known as L-DOPA induced dyskinesias (LIDs). LIDs represent an additional cause of disability for PD patients and a major challenge for the clinical neurologist. Preclinical research has provided invaluable insights into the molecular and neural substrates of LIDs, identifying a number of potential targets for new anti-dyskinetic strategies. Areas covered: This review article is centered on drugs currently in Phase I and II clinical trials for LIDs and their relative pharmacological targets, which include glutamate, acetylcholine, serotonin, adrenergic receptors and additional targets of potential therapeutic interest. Expert opinion: LIDs are sustained by complex molecular and neurobiological mechanisms that are difficult to disentangle or target, unless one or more prevalent mechanisms are identified. In this context, the role of the serotonergic system and mGluR5 glutamate receptors seem to stand out. Interesting results have been obtained, for example, with partial 5-HT1A/5-HT1B receptor agonist eltoprazine and mGluR5 negative allosteric modulator dipraglurant. Confirmation of these results through large-scale, Phase III clinical trials will be needed, to obtain new pharmacological tools that may be used to optimize the treatment of PD patients with motor complications.

Non-human primate models of PD to test novel therapies

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

Dysregulation of BET proteins in levodopa-induced dyskinesia

Levodopa (L-DOPA) remains the most effective pharmacological treatment for Parkinson Disease (PD) but its use is limited by the development of debilitating drug-related side effects, particularly L-DOPA induced dyskinesia (LID). LID is a consequence of long-term L-DOPA use, and in model systems is characterized by a "priming effect", whereby initial administrations of L-DOPA trigger a sensitized biochemical and transcriptional response upon subsequent dopaminergic stimulation. Preliminary studies into the mechanisms underlying this cellular memory have indicated an important role for epigenetic change but many of the downstream mechanisms remain unknown. The family of bromodomain and extraterminal (BET) proteins, which bind acetylated histones, play a critical effector role in the regulation of transcription. BET proteins have been implicated in several forms of neural plasticity, but their potential relevance to LID remains unexplored. Using the 6-OHDA rodent model of LID, we show that dyskinesia development induces alterations in BET protein expression along with enhanced occupation of sites at the promoter and enhancer regions of genes dysregulated during dyskinesia development. When BET function was blocked using the pharmacologic inhibitor JQ1, LID was prevented. In addition, we found that JQ1 treatment blocked the transcriptional upregulation of several immediate-early genes known to participate in the pathogenesis of dyskinesia. Together, these results demonstrate an essential role for BET protein activity as an epigenetic "reader" of the altered histone acetylation required for LID development and suggest that modulation of BET protein function is a potential therapeutic avenue for the prevention or reversal of LID in PD.

Dopamine-Induced Changes in Gαolf Protein Levels in Striatonigral and Striatopallidal Medium Spiny Neurons Underlie the Genesis of l-DOPA-Induced Dyskinesia in Parkinsonian Mice

The dopamine precursor, l-3,4-dihydroxyphenylalanine (l-DOPA), exerts powerful therapeutic effects but eventually generates l-DOPA-induced dyskinesia (LID) in patients with Parkinson’s disease (PD). LID has a close link with deregulation of striatal dopamine/cAMP signaling, which is integrated by medium spiny neurons (MSNs). Olfactory type G-protein α subunit (Gα_olf), a stimulatory GTP-binding protein encoded by the GNAL gene, is highly concentrated in the striatum, where it positively couples with dopamine D₁ (D₁R) receptor and adenosine A_2A receptor (A_2AR) to increase intracellular cAMP levels in MSNs. In the striatum, D₁Rs are mainly expressed in the MSNs that form the striatonigral pathway, while D₂Rs and A_2ARs are expressed in the MSNs that form the striatopallidal pathway. Here, we examined the association between striatal Gα_olf protein levels and the development of LID. We used a hemi-parkinsonian mouse model with nigrostriatal lesions induced by 6-hydroxydopamine (6-OHDA). Using quantitative immunohistochemistry (IHC) and a dual-antigen recognition in situ proximity ligation assay (PLA), we here found that in the dopamine-depleted striatum, there appeared increased and decreased levels of Gα_olf protein in striatonigral and striatopallidal MSNs, respectively, after a daily pulsatile administration of l-DOPA. This leads to increased responsiveness to dopamine stimulation in both striatonigral and striatopallidal MSNs. Because Gα_olf protein levels serve as a determinant of cAMP signal-dependent activity in striatal MSNs, we suggest that l-DOPA-induced changes in striatal Gα_olf levels in the dopamine-depleted striatum could be a key event in generating LID.

Polymorphisms of Dopamine Receptor Genes and Risk of L-Dopa-Induced Dyskinesia in Parkinson's Disease

L-dopa–induced dyskinesia (LID) is a frequent motor complication of Parkinson’s disease (PD), associated with a negative prognosis. Previous studies showed an association between dopamine receptor (DR) gene (DR) variants and LID, the results of which have not been confirmed. The present study is aimed to determine whether genetic differences of DR are associated with LID in a small but well-characterized cohort of PD patients. To this end we enrolled 100 PD subjects, 50 with and 50 without LID, matched for age, gender, disease duration and dopaminergic medication in a case-control study. We conducted polymerase chain reaction for single nucleotide polymorphisms (SNP) in both D1-like (DRD1A48G; DRD1C62T and DRD5T798C) and D2-like DR (DRD2G2137A, DRD2C957T, DRD3G25A, DRD3G712C, DRD4C616G and DRD4nR VNTR 48bp) analyzed genomic DNA. Our results showed that PD patients carrying allele A at DRD3G3127A had an increased risk of LID (OR 4.9; 95% CI 1.7–13.9; p = 0.004). The present findings may provide valuable information for personalizing pharmacological therapy in PD patients.

Long-Term Effects of Safinamide on Dyskinesia in Mid- to Late-Stage Parkinson's Disease: A Post-Hoc Analysis

Background: Safinamide is a novel α-aminoamide with dopaminergic and non-dopaminergic properties developed as adjunctive therapy for patients with PD. Results from a 24-month double-blind controlled study suggested that as add-on to levodopa (and other PD medications) the benefits of safinamide on dyskinesia may be related to severity of dyskinesia at baseline.

Objective: This post-hoc analysis further characterized the effects of safinamide on dyskinesia in mid- to late-stage PD patients.

Methods: Patients were stratified by the presence or absence of dyskinesia at baseline, and by whether or not the dose of levodopa had been changed during the 24-month treatment period. Differences between safinamide and placebo were evaluated using the Wilcoxon rank-sum test.

Results: For the overall treated population (with or without baseline dyskinesia), safinamide 100 mg/day significantly improved the dyskinesia rating scale score, compared with placebo, in the subgroup of patients with no change in levodopa dose (p = 0.0488). For patients with baseline dyskinesia, improvements over placebo were also significant (p = 0.0153) in patients with or without changes in levodopa dose, and nearly significant (p = 0.0546) in patients with no change in levodopa dose, suggesting that these improvements were not due to levodopa dose reductions.

Conclusions: While no statistically significant difference in mean DRS scores was seen between safinamide and placebo in the original study population, the present post-hoc analysis helps to provide a meaningful interpretation of the long-term effects of safinamide on dyskinesia. These results may be related to safinamide state- and use-dependent inhibition of sodium channels and stimulated glutamate release, and are unlikely due to reduced dopaminergic stimulation.

Dynamic DNA Methylation Regulates Levodopa-Induced Dyskinesia

Levodopa-induced dyskinesia (LID) is a persistent behavioral sensitization that develops after repeated levodopa (l-DOPA) exposure in Parkinson disease patients. LID is a consequence of sustained changes in the transcriptional behavior of striatal neurons following dopaminergic stimulation. In neurons, transcriptional regulation through dynamic DNA methylation has been shown pivotal to many long-term behavioral modifications; however, its role in LID has not yet been explored. Using a rodent model, we show LID development leads to the aberrant expression of DNA demethylating enzymes and locus-specific changes to DNA methylation at the promoter regions of genes aberrantly transcribed following l-DOPA treatment. Looking for dynamic DNA methylation in LID genome-wide, we used reduced representation bisulfite sequencing and found an extensive reorganization of the dorsal striatal methylome. LID development led to significant demethylation at many important regulatory areas of aberrantly transcribed genes. We used pharmacologic treatments that alter DNA methylation bidirectionally and found them able to modulate dyskinetic behaviors. Together, these findings demonstrate that l-DOPA induces widespread changes to striatal DNA methylation and that these modifications are required for the development and maintenance of LID.

SIGNIFICANCE STATEMENT

Levodopa-induced dyskinesia (LID) develops after repeated levodopa (l-DOPA) exposure in Parkinson disease patients and remains one of the primary obstacles to effective treatment. LID behaviors are a consequence of striatal neuron sensitization due to sustained changes in transcriptional behavior; however, the mechanisms responsible for the long-term maintenance of this cellular priming remain uncertain. Regulation of dynamic DNA methylation has been shown pivotal to the maintenance of several long-term behavioral modifications, yet its role in LID has not yet been explored. In this work, we report a pivotal role for the reorganization of DNA methylation in the development of LID and show that modification of DNA methylation may be a novel therapeutic target for use in preventing or reversing dyskinetic behaviors.

How close are we to individualized medicine for Parkinson's disease?

INTRODUCTION

There is a considerable inter-individual heterogeneity in clinical features, disease course, and treatment response in Parkinson's disease (PD), which can be explained not only by disease process and clinical variables, but also by an impact from genetic factors. Evidence-based medicine relies on large randomized control trials and meta-analysis-average medicine, which ignores individual differences. However, we are now in the early phases of a paradigm shift in medicine relating to individuality and variability. The purpose of individualized medicine is to predict patients' responses to targeted therapy using diagnostic tests based on genetics or other molecular mechanisms, thus providing the right drug at the right dose at the right time.

AREAS COVERED

In this article, we outline current state of individualized medicine for PD. Expert Commentary: Pharmacogenomics, an important element of individualized medicine, is just beginning to be considered in PD. To advance the clinical use of pharmacogenomics, big data cohort for genomic research and multidisciplinary team approaches are necessary.

Targeting the D1-N-methyl-D-aspartate receptor complex reduces L-dopa-induced dyskinesia in 6-hydroxydopamine-lesioned Parkinson's rats

L-3,4-dihydroxyphenylalanine (l-dopa) remains the most effective therapy for Parkinson’s disease (PD), but its long-term administration is associated with the development of debilitating motor complications known as l-dopa-induced dyskinesia (LID). Enhanced function of dopamine D1 receptor (D1R) and N-methyl-d-aspartate receptor (NMDAR) is believed to participate in the pathogenesis of LID. Given the existence of physical and functional interactions between D1R and NMDAR, we explored the effects of uncoupling D1R and NMDA GluN1 (GluN1) interaction on LID by using the Tat-conjugated interfering peptide (Tat-D1-t2). In this study, we demonstrated in 6-hydroxydopamine (6-OHDA)-lesioned PD rat model that intrastriatal injection of Tat-D1-t2 alleviated dyskinetic behaviors and downregulated the phosphorylation of DARPP-32 at Thr34 induced by levodopa. Moreover, we also showed intrastriatal administration of Tat-D1-t2 elicited alterations in membranous GluN1 and D1R expression. These findings indicate that D1R/GluN1 complexes may be a molecular target with therapeutic potential for the treatment of dyskinesia in Parkinson’s patients.

[Pain fluctuations in Parkinson's disease]

OBJECTIVE

To specify the character of pain syndromes and determine their relationship with main symptoms of ON and OFF periods in patients with complications of long-term treatment with levodopa medications.

MATERIAL AND METHODS

Authors examined 40 patients with Parkinson's disease (27 women and 13 men), mean age 69.2±8 years (from 62 to 85 years), illness duration 9.2±1.2 years (from 7 to 11 years), duration of treatment with levodopa 6.9±1.2 years (from 5 to 9 years). The severity of disease course, character and intensity of pain syndrome were assessed.

RESULTS AND CONCLUSION

We described pain syndromes that had differences in the pathogenesis and localization. Adjustment of the antiparkinsonian treatment resulted in the decrease in their severity. Motor fluctuations and drug-induced dyskinesia that was accompanied by pain sensations were the most frequent signs of Parkinson's disease during its progression. The pain syndrome was related to main symptoms of the ON and OFF period in patients with complications of long-term treatment with levodopa. The pain fluctuations had both nociceptive and central neuropathic pain phenotypes.

Amelioration of L-Dopa-Associated Dyskinesias with Triterpenoic Acid in a Parkinsonian Rat Model

Levo-Dopa (L-Dopa) is widely used for the oral treatment of Parkinson's disease. However, chronic treatment with L-Dopa produces abnormal involuntary movements (AIMs) known as dyskinesias. In this study, commercially available oleanolic acid (OA) that has been previously shown to ameliorate the toxic effects of 6-hydroxydopamine (6-OHDA) in preconditioning studies was used to treat AIMs in a rat model for Parkinson's disease. The forelimb-use asymmetry test was used to measure Parkinson's disease-associated motor impairment. AIMs were measured after 21 days of L-Dopa administration. Glutathione levels were measured in blood, and catalase levels were measured in the substantia nigra and striatum of both the left and right hemispheres. We found that L-Dopa alone as well as L-Dopa and OA combination treatment attenuated the limb-use asymmetry caused by the unilateral injection of 6-OHDA. Chronic L-Dopa administration produced AIMs which were attenuated by treatment with OA. Catalase concentration decreased significantly in the striatum but not in the substantia nigra of the lesioned hemisphere. L-Dopa alone as well as the combined L-Dopa and OA treatment ameliorated the effects of 6-OHDA on catalase concentration. However, intervention with L-Dopa alone as well as with L-Dopa and OA did not affect plasma glutathione concentration. These results suggest that OA administration enhances the effect of catalase on reactive oxygen species following 6-OHDA injection. OA may provide possibilities as an adjunct treatment to prevent or attenuate the development of AIMs following chronic L-Dopa treatment in Parkinson's disease.

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

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

New treatments for the motor symptoms of Parkinson's disease

Levodopa remains the most potent drug to treat motor symptoms in Parkinson's disease (PD); however, motor fluctuations and levodopa-induced dyskinesia that occur with long-term use restrict some of its therapeutic value. Despite these limitations, the medical treatment of PD strives for continuous relief of symptoms using different strategies throughout the course of the illness: increasing the half-life of levodopa, using 'levodopa-sparing agents' and adding non-dopaminergic drugs. New options to 'improve' delivery of levodopa are under investigation, including long-acting levodopa, nasal inhalation and continuous subcutaneous or intrajejunal administration of levodopa. Long-acting dopamine agonists were recently developed and are undergoing further comparative studies to investigate potential superiority over the immediate-release formulations. Non-dopaminergic drugs acting on adenosine receptors, cholinergic, adrenergic, serotoninergic and glutamatergic pathways are newly developed and many are being evaluated in Phase II and Phase III trials. This article focuses on promising novel therapeutic approaches for the management of PD motor symptoms and motor complications. We will provide an update since 2011 on new formulations of current drugs, new drugs with promising results in Phase II and Phase III clinical trials, old drugs with new possibilities and some new potential strategies that are currently in Phase I and II of development (study start date may precede 2011 but are included as study is still ongoing or full data have not yet been published). Negative Phase II and Phase III clinical trials published since 2011 will also be briefly mentioned.

Dual κ-agonist/μ-antagonist opioid receptor modulation reduces levodopa-induced dyskinesia and corrects dysregulated striatal changes in the nonhuman primate model of Parkinson disease

OBJECTIVE

Effective medical management of levodopa-induced dyskinesia (LID) remains an unmet need for patients with Parkinson disease (PD). Changes in opioid transmission in the basal ganglia associated with LID suggest a therapeutic opportunity. Here we determined the impact of modulating both mu and kappa opioid receptor signaling using the mixed agonist/antagonist analgesic nalbuphine in reducing LID and its molecular markers in the nonhuman primate model.

METHODS

1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine-treated macaques with advanced parkinsonism and reproducible LID received a range of nalbuphine doses or saline subcutaneously as: (1) monotherapy, (2) acute coadministration with levodopa, and (3) chronic coadministration for 1 month. Animals were assessed by blinded examiners for motor disability and LID severity using standardized rating scales. Plasma levodopa levels were determined with and without nalbuphine, and postmortem brain samples were subjected to Western blot analyses.

RESULTS

Nalbuphine reduced LID in a dose-dependent manner by 48% (p < 0.001) without compromising the anti-PD effect of levodopa or changing plasma levodopa levels. There was no tolerance to the anti-LID effect of nalbuphine given chronically. Nalbuphine coadministered with levodopa was well tolerated and did not cause sedation. Nalbuphine monotherapy had no effect on motor disability. Striatal tissue analyses showed that nalbuphine cotherapy blocks several molecular correlates of LID, including overexpression of ΔFosB, prodynorphin, dynorphin A, cyclin-dependent kinase 5, and increased phosphorylation of DARPP-32 at threonine-34.

INTERPRETATION

Nalbuphine reverses the molecular milieu in the striatum associated with LID and is a safe and effective anti-LID agent in the primate model of PD. These findings support repurposing this analgesic for the treatment of LID.

Dopamine heteroreceptor complexes as therapeutic targets in Parkinson's disease

INTRODUCTION

Several types of D2R and D1R heteroreceptor complexes were discovered in the indirect and direct pathways of the striatum, respectively. The hypothesis is given that changes in the function of the dopamine heteroreceptor complexes may help us understand the molecular mechanisms underlying the motor complications of long-term therapy in Parkinson's disease (PD) with l-DOPA and dopamine receptor agonists.

AREAS COVERED

In the indirect pathway, the potential role of the A2AR-D2R, A2AR-D2R-mGluR5 and D2R-NMDAR heteroreceptor complexes in PD are covered and in the direct pathway, the D1R-D3R, A1R-D1R, D1R-NMDAR and putative A1R-D1R-D3R heteroreceptor complexes.

EXPERT OPINION

One explanation for the more powerful ability of l-DOPA treatment versus treatment with the partial dopamine receptor agonist/antagonist activity to induce dyskinesias, may be that dopamine formed from l-DOPA acts as a full agonist. The field of D1R and D2R heteroreceptor complexes in the CNS opens up a new understanding of the wearing off of the antiparkinson actions of l-DOPA and dopamine receptor agonists and the production of l-DOPA-induced dyskinesias. It can involve a reorganization of the D1R and D2R heteroreceptor complexes and a disbalance of the D1R and D2R homomers versus non-dopamine receptor homomers in the direct and indirect pathways.

Pharmacological strategies for the management of levodopa-induced dyskinesia in patients with Parkinson's disease

L-Dopa-induced dyskinesias (LID) are the most common adverse effects of long-term dopaminergic therapy in Parkinson's disease (PD). However, the exact mechanisms underlying dyskinesia are still unclear. For a long time, nigrostriatal degeneration and pulsatile stimulation of striatal postsynaptic receptors have been highlighted as the key factors for the development of LID. In recent years, PD models have revealed a wide range of non-dopaminergic neurotransmitter systems involved in pre- and postsynaptic changes and thereby contributing to the pathophysiology of LID. In the current review, we focus on therapeutic LID targets, mainly based on agents acting on dopaminergic, glutamatergic, serotoninergic, adrenergic, and cholinergic systems. Despite a large number of clinical trials, currently only amantadine and, to a lesser extent, clozapine are being used as effective strategies in the treatment of LID in clinical settings. Thus, in the second part of the article, we review the placebo-controlled trials on LID treatment in order to disentangle the changing scenario of drug development. Promising results include the extension of L-dopa action without inducing LID of the novel monoamine oxidase B- and glutamate-release inhibitor safinamide; however, this had no obvious effect on existing LID. Others, like the metabotropic glutamate-receptor antagonist AFQ056, showed promising results in some of the studies; however, confirmation is still lacking. Thus, to date, strategies of continuous dopaminergic stimulation seem the most promising to prevent or ameliorate LID. The success of future therapeutic strategies once moderate to severe LID occur will depend on the translation from preclinical experimental models into clinical practice in a bidirectional process.

Selective loss of bi-directional synaptic plasticity in the direct and indirect striatal output pathways accompanies generation of parkinsonism and l-DOPA induced dyskinesia in mouse models

Parkinsonian symptoms arise due to over-activity of the indirect striatal output pathway, and under-activity of the direct striatal output pathway. l-DOPA-induced dyskinesia (LID) is caused when the opposite circuitry problems are established, with the indirect pathway becoming underactive, and the direct pathway becoming over-active. Here, we define synaptic plasticity abnormalities in these pathways associated with parkinsonism, symptomatic benefits of l-DOPA, and LID. We applied spike-timing dependent plasticity protocols to cortico-striatal synapses in slices from 6-OHDA-lesioned mouse models of parkinsonism and LID, generated in BAC transgenic mice with eGFP targeting the direct or indirect output pathways, with and without l-DOPA present. In naïve mice, bidirectional synaptic plasticity, i.e. LTP and LTD, was induced, resulting in an EPSP amplitude change of approximately 50% in each direction in both striatal output pathways, as shown previously. In parkinsonism and dyskinesia, both pathways exhibited unidirectional plasticity, irrespective of stimulation paradigm. In parkinsonian animals, the indirect pathway only exhibited LTP (LTP protocol: 143.5±14.6%; LTD protocol 177.7±22.3% of baseline), whereas the direct pathway only showed LTD (LTP protocol: 74.3±4.0% and LTD protocol: 63.3±8.7%). A symptomatic dose of l-DOPA restored bidirectional plasticity on both pathways to levels comparable to naïve animals (Indirect pathway: LTP protocol: 124.4±22.0% and LTD protocol: 52.1±18.5% of baseline. Direct pathway: LTP protocol: 140.7±7.3% and LTD protocol: 58.4±6.0% of baseline). In dyskinesia, in the presence of l-DOPA, the indirect pathway exhibited only LTD (LTP protocol: 68.9±21.3% and LTD protocol 52.0±14.2% of baseline), whereas in the direct pathway, only LTP could be induced (LTP protocol: 156.6±13.2% and LTD protocol 166.7±15.8% of baseline). We conclude that normal motor control requires bidirectional plasticity of both striatal outputs, which underlies the symptomatic benefits of l-DOPA. Switching from bidirectional to unidirectional plasticity drives global changes in striatal pathway excitability, and underpins parkinsonism and dyskinesia.

(6aR)-11-amino-N-propyl-noraporphine, a new dopamine D2 and serotonin 5-HT1A dual agonist, elicits potent antiparkinsonian action and attenuates levodopa-induced dyskinesia in a 6-OHDA-lesioned rat model of Parkinson's disease

Parkinson's disease (PD) drug therapy remains a challenge. Dual modulation of dopamine and 5-HT receptors has emerged as a promising approach in anti-PD drug development. Taking advantage of the newly discovered aporphine analogue(s), (6aR)-11-amino-N-propyl-noraporphine (SOMCL-171), which exhibited dual D2/5-HT1A receptor agonistic activity, we studied the effects of the compound on levodopa-induced dyskinesia (LID) in a PD animal model. The results demonstrated that SOMCL-171 elicited a potent anti-PD effect in a 6-OHDA-lesioned rat model. Chronic use of SOMCL-171 reduced LID without compromising the antiparkinsonian efficacy. Furthermore, we found that the antidyskinesia effect of SOMCL-171 is associated with its 5-HT1A agonistic activity and the up-regulation of the striatal 5-HT1A receptor. The present data indicated that chronic SOMCL-171 alone produced potent antiparkinsonian effects with weak dyskinesia, compared with that of levodopa. In addition, chronic SOMCL-171 application attenuated the development of levodopa-induced LID at no expense to the antiparkinsonian efficacy. Taken together, our data suggested that dual modulation of D2/5-HT1A receptors may provide a novel approach for drug development in PD and LID.

Brain morphometry and the neurobiology of levodopa-induced dyskinesias: current knowledge and future potential for translational pre-clinical neuroimaging studies

Dopamine replacement therapy in the form of levodopa results in a significant proportion of patients with Parkinson’s disease developing debilitating dyskinesia. This significantly complicates further treatment and negatively impacts patient quality of life. A greater understanding of the neurobiological mechanisms underlying levodopa-induced dyskinesia (LID) is therefore crucial to develop new treatments to prevent or mitigate LID. Such investigations in humans are largely confined to assessment of neurochemical and cerebrovascular blood flow changes using positron emission tomography and functional magnetic resonance imaging. However, recent evidence suggests that LID is associated with specific morphological changes in the frontal cortex and midbrain, detectable by structural MRI and voxel-based morphometry. Current human neuroimaging methods however lack sufficient resolution to reveal the biological mechanism driving these morphological changes at the cellular level. In contrast, there is a wealth of literature from well-established rodent models of LID documenting detailed post-mortem cellular and molecular measurements. The combination therefore of advanced neuroimaging methods and rodent LID models offers an exciting opportunity to bridge these currently disparate areas of research. To highlight this opportunity, in this mini-review, we provide an overview of the current clinical evidence for morphological changes in the brain associated with LID and identify potential cellular mechanisms as suggested from human and animal studies. We then suggest a framework for combining small animal MRI imaging with rodent models of LID, which may provide important mechanistic insights into the neurobiology of LID.