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.

The ratio of M1 to M2 microglia in the striatum determines the severity of L-Dopa-induced dyskinesias.

L-Dopa, while treating motor symptoms of Parkinson's disease, can lead to debilitating L-Dopa-induced dyskinesias, limiting its use. To investigate the causative relationship between neuro-inflammation and dyskinesias, we assessed if striatal M1 and M2 microglia numbers correlated with dyskinesia severity and whether the anti-inflammatories, minocycline and indomethacin, reverse these numbers and mitigate against dyskinesia. In 6-OHDA lesioned mice, we used stereology to assess numbers of striatal M1 and M2 microglia populations in non-lesioned (naïve) and lesioned mice that either received no L-Dopa (PD), remained non-dyskinetic even after L-Dopa (non-LID) or became dyskinetic after L-Dopa treatment (LID). We also assessed the effect of minocycline/indomethacin treatment on striatal M1 and M2 microglia and its anti-dyskinetic potential via AIMs scoring. We report that L-Dopa treatment leading to LIDs exacerbates activated microglia numbers beyond that associated with the PD state; the severity of LIDs is strongly correlated to the ratio of the striatal M1 to M2 microglial numbers; in non-dyskinetic mice, there is no M1/M2 microglia ratio increase above that seen in PD mice; and reducing M1/M2 microglia ratio using anti-inflammatories is anti-dyskinetic. Parkinson's disease is associated with increased inflammation, but this is insufficient to underpin dyskinesia. Given that L-Dopa-treated non-LID mice show the same ratio of M1/M2 microglia as PD mice that received no L-Dopa, and, given minocycline/indomethacin reduces both the ratio of M1/M2 microglia and dyskinesia severity, our data suggest the increased microglial M1/M2 ratio that occurs following L-Dopa treatment is a contributing cause of dyskinesias.

D2 dopamine receptors and the striatopallidal pathway modulate L-DOPA-induced dyskinesia in the mouse.

L-DOPA-induced dyskinesia (LID) remains a major complication of Parkinson's disease management for which better therapies are necessary. The contribution of the striatonigral direct pathway to LID is widely acknowledged but whether the striatopallidal pathway is involved remains debated. Selective optogenetic stimulation of striatonigral axon terminals induces dyskinesia in mice rendered hemiparkinsonian with the toxin 6-hydroxydopamine (6-OHDA). Here we show that optogenetically-induced dyskinesia is increased by the D2-type dopamine receptor agonist quinpirole. Although the quinpirole effect may be mediated by D2 receptor stimulation in striatopallidal neurons, alternative mechanisms may be responsible as well. To selectively modulate the striatopallidal pathway, we selectively expressed channelrhodopsin-2 (ChR2) in D2 receptor expressing neurons by crossing D2-Cre and ChR2-flox mice. The animals were rendered hemiparkinsonian and implanted with an optic fiber at the ipsilateral external globus pallidus (GPe). Stimulation of ChR2 at striatopallidal axon terminals reduced LID and also general motility during the off L-DOPA state, without modifying the pro-motor effect of low doses of L-DOPA producing mild or no dyskinesia. Overall, the present study shows that D2-type dopamine receptors and the striatopallidal pathway modulate dyskinesia and suggest that targeting striatopallidal axon terminals at the GPe may have therapeutic potential in the management of LID.

Dopamine Agonist Cotreatment Alters Neuroplasticity and Pharmacology of Levodopa-Induced Dyskinesia.

BACKGROUND: Current models of levodopa (L-dopa)-induced dyskinesia (LID) are obtained by treating dopamine-depleted animals with L-dopa. However, patients with LID receive combination therapies that often include dopamine agonists. OBJECTIVE: Using 6-hydroxydopamine-lesioned rats as a model, we aimed to establish whether an adjunct treatment with the D2/3 agonist ropinirole impacts on patterns of LID-related neuroplasticity and drug responses. METHODS: Different regimens of L-dopa monotreatment and L-dopa-ropinirole cotreatment were compared using measures of hypokinesia and dyskinesia. Striatal expression of ∆FosB and angiogenesis markers were studied immunohistochemically. Antidyskinetic effects of different drug categories were investigated in parallel groups of rats receiving either L-dopa monotreatment or L-dopa combined with ropinirole. RESULTS: We defined chronic regimens of L-dopa monotreatment and L-dopa-ropinirole cotreatment inducing overall similar abnormal involuntary movement scores. Compared with the monotreatment group, animals receiving the L-dopa-ropinirole combination exhibited an overall lower striatal expression of ∆FosB with a distinctive compartmental distribution. The expression of angiogenesis markers and blood-brain barrier hyperpermeability was markedly reduced after L-dopa-ropinirole cotreatment compared with L-dopa monotreatment. Moreover, significant group differences were detected upon examining the response to candidate antidyskinetic drugs. In particular, compounds modulating D1 receptor signaling had a stronger effect in the L-dopa-only group, whereas both amantadine and the selective NMDA antagonist MK801 produced a markedly larger antidyskinetic effect in L-dopa-ropinirole cotreated animals. CONCLUSIONS: Cotreatment with ropinirole altered LID-related neuroplasticity and pharmacological response profiles. The impact of adjuvant dopamine agonist treatment should be taken into consideration when investigating LID mechanisms and candidate interventions in both clinical and experimental settings. © 2023 The Authors. Movement Disorders published by Wiley Periodicals LLC on behalf of International Parkinson and Movement Disorder Society.

PT320, a Sustained-Release GLP-1 Receptor Agonist, Ameliorates L-DOPA-Induced Dyskinesia in a Mouse Model of Parkinson's Disease.

To determine the efficacy of PT320 on L-DOPA-induced dyskinetic behaviors, and neurochemistry in a progressive Parkinson's disease (PD) MitoPark mouse model. To investigate the effects of PT320 on the manifestation of dyskinesia in L-DOPA-primed mice, a clinically translatable biweekly PT320 dose was administered starting at either 5 or 17-weeks-old mice. The early treatment group was given L-DOPA starting at 20 weeks of age and longitudinally evaluated up to 22 weeks. The late treatment group was given L-DOPA starting at 28 weeks of age and longitudinally observed up to 29 weeks. To explore dopaminergic transmission, fast scan cyclic voltammetry (FSCV) was utilized to measure presynaptic dopamine (DA) dynamics in striatal slices following drug treatments. Early administration of PT320 significantly mitigated the severity L-DOPA-induced abnormal involuntary movements; PT320 particularly improved excessive numbers of standing as well as abnormal paw movements, while it did not affect L-DOPA-induced locomotor hyperactivity. In contrast, late administration of PT320 did not attenuate any L-DOPA-induced dyskinesia measurements. Moreover, early treatment with PT320 was shown to not only increase tonic and phasic release of DA in striatal slices in L-DOPA-naïve MitoPark mice, but also in L-DOPA-primed animals. Early treatment with PT320 ameliorated L-DOPA-induced dyskinesia in MitoPark mice, which may be related to the progressive level of DA denervation in PD.

Subregion-Specific Regulation of Dopamine D1 Receptor Signaling in the Striatum: Implication for L-DOPA-Induced Dyskinesia.

The striatum is the main structure of the basal ganglia. The striatum receives inputs from various cortical areas, and its subregions play distinct roles in motor and emotional functions. Recently, striatal maps based on corticostriatal connectivity and striosome-matrix compartmentalization were developed, and we were able to subdivide the striatum into seven subregions. Dopaminergic modulation of the excitability of medium spiny neurons (MSNs) is critical for striatal function. In this study, we investigated the functional properties of dopamine signaling in seven subregions of the striatum from male mice. By monitoring the phosphorylation of PKA substrates including DARPP-32 in mouse striatal slices, we identified two subregions with low D1 receptor signaling: the dorsolateral portion of the intermediate/rostral part (DL-IR) and the intermediate/caudal part (IC). Low D1 receptor signaling in the two subregions was maintained by phosphodiesterase (PDE)10A and muscarinic M4 receptors. In an animal model of 6-hydroxydopamine (6-OHDA)-induced hemi-parkinsonism, D1 receptor signaling was upregulated in almost all subregions including the DL-IR, but not in the IC. When L-DOPA-induced dyskinesia (LID) was developed, D1 receptor signaling in the IC was upregulated and correlated with the severity of LID. Our results suggest that the function of the striatum is maintained through the subregion-specific regulation of dopamine D1 receptor signaling and that the aberrant activation of D1 receptor signaling in the IC is involved in LID. Future studies focusing on D1 receptor signaling in the IC of the striatum will facilitate the development of novel therapeutics for LID.SIGNIFICANCE STATEMENT Recent progress in striatal mapping based on corticostriatal connectivity and striosome-matrix compartmentalization allowed us to subdivide the striatum into seven subregions. Analyses of D1 receptor signaling in the seven subregions identified two unique subregions with low D1 receptor signaling: the dorsolateral portion of the intermediate/rostral part (DL-IR) and the intermediate/caudal part (IC). Aberrant activation of D1 receptor signaling in the IC is involved in L-DOPA-induced dyskinesia (LID). Previous studies of LID have mainly focused on the DL-IR, but not on the IC of the striatum. Future studies to clarify aberrant D1 receptor signaling in the IC are required to develop novel therapeutics for LID.

Abnormal Cortico-Basal Ganglia Neurotransmission in a Mouse Model of l-DOPA-Induced Dyskinesia.

l-3,4-dihydroxyphenylalanine (l-DOPA) is an effective treatment for Parkinson's disease (PD); however, long-term treatment induces l-DOPA-induced dyskinesia (LID). To elucidate its pathophysiology, we developed a mouse model of LID by daily administration of l-DOPA to PD male ICR mice treated with 6-hydroxydopamine (6-OHDA), and recorded the spontaneous and cortically evoked neuronal activity in the external segment of the globus pallidus (GPe) and substantia nigra pars reticulata (SNr), the connecting and output nuclei of the basal ganglia, respectively, in awake conditions. Spontaneous firing rates of GPe neurons were decreased in the dyskinesia-off state (≥24 h after l-DOPA injection) and increased in the dyskinesia-on state (20-100 min after l-DOPA injection while showing dyskinesia), while those of SNr neurons showed no significant changes. GPe and SNr neurons showed bursting activity and low-frequency oscillation in the PD, dyskinesia-off, and dyskinesia-on states. In the GPe, cortically evoked late excitation was increased in the PD and dyskinesia-off states but decreased in the dyskinesia-on state. In the SNr, cortically evoked inhibition was largely suppressed, and monophasic excitation became dominant in the PD state. Chronic l-DOPA treatment partially recovered inhibition and suppressed late excitation in the dyskinesia-off state. In the dyskinesia-on state, inhibition was further enhanced, and late excitation was largely suppressed. Cortically evoked inhibition and late excitation in the SNr are mediated by the cortico-striato-SNr direct and cortico-striato-GPe-subthalamo-SNr indirect pathways, respectively. Thus, in the dyskinesia-on state, signals through the direct pathway that release movements are enhanced, while signals through the indirect pathway that stop movements are suppressed, underlying LID.SIGNIFICANCE STATEMENT Parkinson's disease (PD) is caused by progressive loss of midbrain dopaminergic neurons, characterized by tremor, rigidity, and akinesia, and estimated to affect around six million people world-wide. Dopamine replacement therapy is the gold standard for PD treatment; however, control of symptoms using l-3,4-dihydroxyphenylalanine (l-DOPA) becomes difficult over time because of abnormal involuntary movements (AIMs) known as l-DOPA-induced dyskinesia (LID), one of the major issues for advanced PD. Our electrophysiological data suggest that dynamic changes in the basal ganglia circuitry underlie LID; signals through the direct pathway that release movements are enhanced, while signals through the indirect pathway that stop movements are suppressed. These results will provide the rationale for the development of more effective treatments for LID.

Striatal Synaptic Dysfunction in Dystonia and Levodopa-Induced Dyskinesia.

This review provides an overview of the synaptic dysfunctions of neuronal circuits and underlying neurochemical alterations observed in the hyperkinetic movement disorders, dystonia and dyskinesia. These disorders exhibit similar changes in expression of synaptic plasticity and neuromodulation. This includes alterations in physical attributes of synapses, synaptic protein expression, and neurotransmitter systems, such as glutamate and gamma-aminobutyric acid (GABA), and neuromodulators, such as dopamine, acetylcholine, serotonin, adenosine, and endocannabinoids. A full understanding of the mechanisms and consequences of disruptions in synaptic function and plasticity will lend insight into the development of these disorders and new ways to combat maladaptive changes.

D1/D5 Inverse Agonists Restore Striatal Cholinergic Interneuron Physiology in Dyskinetic Mice.

BACKGROUND: In advanced stages of Parkinson's disease (PD), dyskinesia and motor fluctuations become seriously debilitating and therapeutic options become scarce. Aberrant activity of striatal cholinergic interneurons (SCIN) has been shown to be critical to PD and dyskinesia, but the systemic administration of cholinergic medications can exacerbate extrastriatal-related symptoms. Thus, targeting the mechanisms causing pathological SCIN activity in severe PD with motor fluctuations and dyskinesia is a promising therapeutic alternative. METHODS: We used ex vivo electrophysiological recordings combined with pharmacology to study the alterations in intracellular signaling that contribute to the altered SCIN physiology observed in the 6-hydroxydopamine mouse model of PD treated with levodopa. RESULTS: The altered phenotypes of SCIN of parkinsonian mice during the "off levodopa" state resulting from aberrant Kir/leak and Kv1.3 currents can be rapidly reverted by acute inhibition of cAMP-ERK1/2 signaling. Inverse agonists that inhibit the ligand-independent activity of D5 receptors, like clozapine, restore Kv1.3 and Kir/leak currents and SCIN normal physiology in dyskinetic mice. CONCLUSION: Our work unravels a signaling pathway involved in the dysregulation of membrane currents causing SCIN hyperexcitability and burst-pause activity in parkinsonian mice treated with levodopa (l-dopa). These changes persist during off-medication periods due to tonic mechanisms that can be acutely reversed by pharmacological interventions. Thus, targeting the D5-cAMP-ERK1/2 signaling pathway selectively in SCIN may have therapeutic effects in PD and dyskinesia by restoring the normal SCIN function. © 2022 International Parkinson and Movement Disorder Society.

Pharmacological targeting of G protein-coupled receptor heteromers.

A main rationale for the role of G protein-coupled receptor (GPCR) heteromers as targets for drug development is the putative ability of selective ligands for specific GPCRs to change their pharmacological properties upon GPCR heteromerization. The present study provides a proof of concept for this rationale by demonstrating that heteromerization of dopamine D1 and D3 receptors (D1R and D3R) influences the pharmacological properties of three structurally similar selective dopamine D3R ligands, the phenylpiperazine derivatives PG01042, PG01037 and VK4-116. By using D1R-D3R heteromer-disrupting peptides, it could be demonstrated that the three D3R ligands display different D1R-D3R heteromer-dependent pharmacological properties: PG01042, acting as G protein-biased agonist, counteracted D1R-mediated signaling in the D1R-D3R heteromer; PG01037, acting as a D3R antagonist cross-antagonized D1R-mediated signaling in the D1R-D3R heteromer; and VK4-116 specifically acted as a ß-arrestin-biased agonist in the D1R-D3R heteromer. Molecular dynamics simulations predicted potential molecular mechanisms mediating these qualitatively different pharmacological properties of the selective D3R ligands that are dependent on D1R-D3R heteromerization. The results of in vitro experiments were paralleled by qualitatively different pharmacological properties of the D3R ligands in vivo. The results supported the involvement of D1R-D3R heteromers in the locomotor activation by D1R agonists in reserpinized mice and L-DOPA-induced dyskinesia in rats, highlighting the D1R-D3R heteromer as a main pharmacological target for L-DOPA-induced dyskinesia in Parkinson's disease. More generally, the present study implies that when suspecting its pathogenetic role, a GPCR heteromer, and not its individual GPCR units, should be considered as main target for drug development.

Role of striatal ΔFosB in l-Dopa-induced dyskinesias of parkinsonian nonhuman primates.

Long-term dopamine (DA) replacement therapy in Parkinson's disease (PD) leads to the development of abnormal involuntary movements known as l-Dopa-induced dyskinesia (LID). The transcription factor ΔFosB that is highly up-regulated in the striatum following chronic l-Dopa exposure may participate in the mechanisms of altered neuronal responses to DA generating LID. To identify intrinsic effects of elevated ΔFosB on l-Dopa responses, we induced transgenic ΔFosB overexpression in the striatum of parkinsonian nonhuman primates kept naïve of l-Dopa treatment. Elevated ΔFosB levels led to consistent appearance of LID since the initial acute l-Dopa tests. In line with this motor response, striatal projection neurons (SPNs) responded to DA with changes in firing frequency that reversed at the peak of the motor response, and these unstable SPN activity changes in response to DA are typically associated with the emergence of LID. Transgenic ΔFosB overexpression also induced up-regulation of other molecular markers of LID. These results support an autonomous role of striatal ΔFosB in the adaptive mechanisms altering motor responses to chronic DA replacement in PD.

Exercise decreases aberrant corticostriatal plasticity in an animal model of l-DOPA-induced dyskinesia.

Physical exercise attenuates the development of l-3,4-dihydroxyphenylalanine (l-DOPA)-induced dyskinesia (LID) in 6-hydroxydopamine-induced hemiparkinsonian mice through unknown mechanisms. We now tested if exercise normalizes the aberrant corticostriatal neuroplasticity associated with experimental murine models of LID. C57BL/6 mice received two unilateral intrastriatal injections of 6-hydroxydopamine (12 µg) and were treated after 3 wk with l-DOPA/benserazide (25/12.5 mg/kg) for 4 wk, with individualized moderate-intensity running (60%-70% V̇o2peak) or not (untrained). l-DOPA converted the pattern of plasticity in corticostriatal synapses from a long-term depression (LTD) into a long-term potentiation (LTP). Exercise reduced LID severity and decreased aberrant LTP. These results suggest that exercise attenuates abnormal corticostriatal plasticity to decrease LID.

Levodopa Causes Striatal Cholinergic Interneuron Burst-Pause Activity in Parkinsonian Mice.

BACKGROUND: Enhanced striatal cholinergic interneuron activity contributes to the striatal hypercholinergic state in Parkinson's disease (PD) and to levodopa-induced dyskinesia. In severe PD, dyskinesia and motor fluctuations become seriously debilitating, and the therapeutic strategies become scarce. Given that the systemic administration of anticholinergics can exacerbate extrastriatal-related symptoms, targeting cholinergic interneurons is a promising therapeutic alternative. Therefore, unraveling the mechanisms causing pathological cholinergic interneuron activity in severe PD with motor fluctuations and dyskinesia may provide new molecular therapeutic targets. METHODS: We used ex vivo electrophysiological recordings combined with pharmacological and morphological studies to investigate the intrinsic alterations of cholinergic interneurons in the 6-hydroxydopamine mouse model of PD treated with levodopa. RESULTS: Cholinergic interneurons exhibit pathological burst-pause activity in the parkinsonian "off levodopa" state. This is mediated by a persistent ligand-independent activity of dopamine D1/D5 receptor signaling, involving a cyclic adenosine monophosphate (cAMP) pathway. Dysregulation of membrane ion channels that results in increased inward-rectifier potassium type 2 (Kir2) and decreased leak currents causes the burst pause activity, which can be dampened by pharmacological inhibition of intracellular cAMP. A single challenge with a dyskinetogenic dose of levodopa is sufficient to induce persistent cholinergic interneuron burst-pause firing. CONCLUSION: Our data unravel a mechanism causing aberrant cholinergic interneuron burst-pause activity in parkinsonian mice treated with levodopa. Targeting D5-cAMP signaling and the regulation of Kir2 and leak channels may alleviate parkinsonism and dyskinesia by restoring normal cholinergic interneuron function. © 2021 International Parkinson and Movement Disorder Society.

Involvement of the Protein Ras Homolog Enriched in the Striatum, Rhes, in Dopaminergic Neurons' Degeneration: Link to Parkinson's Disease.

Rhes is one of the most interesting genes regulated by thyroid hormones that, through the inhibition of the striatal cAMP/PKA pathway, acts as a modulator of dopamine neurotransmission. Rhes mRNA is expressed at high levels in the dorsal striatum, with a medial-to-lateral expression gradient reflecting that of both dopamine D2 and adenosine A2A receptors. Rhes transcript is also present in the hippocampus, cerebral cortex, olfactory tubercle and bulb, substantia nigra pars compacta (SNc) and ventral tegmental area of the rodent brain. In line with Rhes-dependent regulation of dopaminergic transmission, data showed that lack of Rhes enhanced cocaine- and amphetamine-induced motor stimulation in mice. Previous studies showed that pharmacological depletion of dopamine significantly reduces Rhes mRNA levels in rodents, non-human primates and Parkinson's disease (PD) patients, suggesting a link between dopaminergic innervation and physiological Rhes mRNA expression. Rhes protein binds to and activates striatal mTORC1, and modulates L-DOPA-induced dyskinesia in PD rodent models. Finally, Rhes is involved in the survival of mouse midbrain dopaminergic neurons of SNc, thus pointing towards a Rhes-dependent modulation of autophagy and mitophagy processes, and encouraging further investigations about mechanisms underlying dysfunctions of the nigrostriatal system.

Beneficial effects of the phytocannabinoid Δ9-THCV in L-DOPA-induced dyskinesia in Parkinson's disease.

The antioxidant and CB2 receptor agonist properties of Δ9-tetrahydrocannabivarin (Δ9-THCV) afforded neuroprotection in experimental Parkinson's disease (PD), whereas its CB1 receptor antagonist profile at doses lower than 5 mg/kg caused anti-hypokinetic effects. In the present study, we investigated the anti-dyskinetic potential of Δ9-THCV (administered i.p. at 2 mg/kg for two weeks), which had not been investigated before. This objective was investigated after inducing dyskinesia by repeated administration of L-DOPA (i.p. at 10 mg/kg) in a genetic model of dopaminergic deficiency, Pitx3ak mutant mice, which serves as a useful model for testing anti-dyskinetic agents. The daily treatment of these mice with L-DOPA for two weeks progressively increased the time spent in abnormal involuntary movements (AIMs) and elevated their horizontal and vertical activities (as measured in a computer-aided actimeter), signs that reflected the dyskinetic state of these mice. Interestingly, when combined with L-DOPA from the first injection, Δ9-THCV delayed the appearance of all these signs and decreased their intensity, with a reduction in the levels of FosB protein and the histone pAcH3 (measured by immunohistochemistry), which had previously been found to be elevated in the basal ganglia in L-DOPA-induced dyskinesia. In addition to the anti-dyskinetic effects of Δ9-THCV when administered at the onset of L-DOPA treatment, Δ9-THCV was also effective in attenuating the intensity of dyskinesia when administered for three consecutive days once these signs were already present (two weeks after the onset of L-DOPA treatment). In summary, our data support the anti-dyskinetic potential of Δ9-THCV, both to delay the occurrence and to attenuate the magnitude of dyskinetic signs. Although further studies are clearly required to determine the clinical significance of these data in humans, the results nevertheless situate Δ9-THCV in a promising position for developing a cannabinoid-based therapy for patients with PD.

Emerging drugs for the treatment of L-DOPA-induced dyskinesia: an update.

INTRODUCTION: Prolonged treatment with L-3,4-dihydroxyphenylalanine (L-DOPA) leads to the development of uncontrolled movements (L-DOPA-induced dyskinesias (LID)) in Parkinson's disease (PD). There is currently only a single approved drug for the treatment of LID, a long-acting preparation of the NMDA antagonist, amantadine, that has variable benefits and side-effects. Therefore, new treatments for LID remain an unmet in PD. AREAS COVERED: We review the current strategies for the management of LID; the pathogenic mechanisms underlying the development of LID, which provides the rationale for clinical trials of novel targets for LID and provide a review of phase II/III trials for emerging drugs for LID, with either positive results, or ongoing studies, reported between January 2014 and December 2019. EXPERT OPINION: There are several ongoing studies for agents that showed possible benefit at phase Ib/IIa for reducing LID. However, there are no new positive phase III double-blind randomized controlled clinical trials (DBRCT) for emerging treatments for LID. Generating better preclinical models, more precise recruitment tools and better outcome measures remain a priority. The pharmacology of drugs investigated for LID may be too selective; therefore, evaluating combinations of drugs is worthy of consideration as is the repurposing of existing drugs with multiple pharmacological targets.

Research advances on L-DOPA-induced dyskinesia: from animal models to human disease.

L-3,4-dihydroxyphenylalanine (L-DOPA) was introduced about half a century ago and is still the most effective medicine for the treatment of Parkinson's disease (PD). However, such chronic treatment eventually leads to L-DOPA-induced dyskinesia (LID) on the majority of PD patients. Besides L-DOPA, dopamine agonists are able to induce dyskinesia as well. So far no drug is yet claimed to effectively curb LID, and amantadine has only a modest benefit on LID patients. Thus, understanding the molecular mechanisms behind LID is urgently needed, and developing new antiparkinsonian medications with low dyskinesia potential is necessarily required. In the last decades, several animal models have been generated for these aims. 1-Methyl-4-phenyl-1, 2, 3, 6-tetrahydropyridine (MPTP)-lesioned monkey models always considered as gold standard of PD studies are also applied well for the research of LID. Additionally, several rodent models were developed for such clinical needs. Of them, 6-hydroxydopamine (OHDA)-lesioned rats or mice exhibiting countable abnormal involuntary movements (AIMs) after L-DOPA treatments have becoming widely applicable tools for LID pathogenesis studies. Under investigating these models for years, multiple potential LID-associated genes and pathways have been innovatively identified, which largely advance the therapeutic and preventative strategies for the disease. In this review, we attempt to update the recent findings represented in LID animal models and trial studies, which may facilitate the mechanistic understanding, drug development, and clinical evaluation of this movement disorder.

L-DOPA for Parkinson's disease-a bittersweet pill.

3,4-dihydroxy-L-phenylalanine (L-DOPA) is the gold standard treatment for Parkinson's disease. It has earned that title through its highly effective treatment of some of the motor symptoms in the early stages of the disease but it is a far from perfect drug. The inevitable long-term treatment that comes with this chronic neurodegenerative condition raises the risk significantly of the development of motor fluctuations including disabling L-DOPA-induced dyskinesia. Being unsurpassed as a therapy means that understanding the mechanisms of dyskinesia priming and induction is vital to the search for therapies to treat these side effects and allow optimal use of L-DOPA. However, L-DOPA use may also have consequences (positive or negative) for the development of other interventions, such as cell transplantation, which are designed to treat or repair the ailing brain. This review looks at the issues around the use of L-DOPA with a focus on its potential impact on advanced reparative interventions.

Repurposing an established drug: an emerging role for methylene blue in L-DOPA-induced dyskinesia.

The nitric oxide (NO) system has been proven to be a valuable modulator of L-DOPA-induced dyskinesia in Parkinsonian rodents. NO activates the enzyme soluble guanylyl cyclase and elicits the synthesis of the second-messenger cGMP. Although we have previously described the anti-dyskinetic potential of NO synthase inhibitors on L-DOPA-induced dyskinesia, the effect of soluble guanylyl cyclase inhibitors remains to be evaluated. The aim of this study was to analyze whether the clinically available non-selective inhibitor methylene blue, or the selective soluble guanylyl cyclase inhibitor ODQ (1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one), could mitigate L-DOPA-induced dyskinesia in 6-hydroxydopamine-lesioned rats. Here, we demonstrated that methylene blue was able to reduce L-DOPA-induced dyskinesia incidence when chronically co-administered with L-DOPA during 3 weeks. Methylene blue chronic (but not acute) administration (2 weeks) was effective in attenuating L-DOPA-induced dyskinesia in rats rendered dyskinetic by a previous course of L-DOPA chronic treatment. Furthermore, discontinuous methylene blue treatment (e.g., co-administration of methylene blue and L-DOPA for 2 consecutive days followed by vehicle and L-DOPA co-administration for 5 days) was effective in attenuating dyskinesia. Finally, we demonstrated that microinjection of methylene blue or ODQ into the lateral ventricle effectively attenuated L-DOPA-induced dyskinesia. Taken together, these results demonstrate an important role of NO-soluble guanylyl cyclase-cGMP signaling on L-DOPA-induced dyskinesia. The clinical implications of this discovery are expected to advance the treatment options for patients with Parkinson's disease.

A Selective Phosphodiesterase 10A Inhibitor Reduces L-Dopa-Induced Dyskinesias in Parkinsonian Monkeys.

BACKGROUND: Phosphodiesterase 10A is a member of the phosphodiesterase family whose brain expression is restricted to the striatum. Phosphodiesterase 10A regulates cyclic adenosine monophosphate and cyclic guanosine monophosphate, which mediate responses to dopamine receptor activation, and the levels of these cyclic nucleotides are decreased in experimental models of l-dopa-induced dyskinesia. The elevation of cyclic adenosine monophosphate/cyclic guanosine monophosphate levels by phosphodiesterase 10A inhibition may thus be targeted to reduce l-dopa-induced dyskinesia. OBJECTIVES: The present study was aimed at determining the potential antidyskinetic effects of phosphodiesterase 10A inhibitors in a primate model of Parkinson's disease (PD). The experiments performed in this model were also intended to provide translational data for the design of future clinical trials. METHODS: Five MPTP-treated macaques with advanced parkinsonism and reproducible l-dopa-induced dyskinesia were used. MR1916, a selective phosphodiesterase 10A inhibitor, at doses 0.0015 to 0.05 mg/kg, subcutaneously, or its vehicle (control test) was coadministered with l-dopa methyl ester acutely (predetermined optimal and suboptimal subcutaneous doses) and oral l-dopa chronically as daily treatment for 5 weeks. Standardized scales were used to assess motor disability and l-dopa-induced dyskinesia by blinded examiners. Pharmacokinetics was also examined. RESULTS: MR1916 consistently reduced l-dopa-induced dyskinesia in acute tests of l-dopa optimal and suboptimal doses. Significant effects were present with every MR1916 dose tested, but the most effective was 0.015 mg/kg. None of the MR1916 doses tested affected the antiparkinsonian action of l-dopa at the optimal dose. The anti-l-dopa-induced dyskinesia effect of MR1916 (0.015 mg/kg, subcutaneously) was sustained with chronic administration, indicating that tolerance did not develop over the 5-week treatment. No adverse effects were observed after MR1916 administration acutely or chronically. CONCLUSIONS: Results show that regulation of striatal cyclic nucleotides by phosphodiesterase 10A inhibition could be a useful therapeutic approach for l-dopa-induced dyskinesia, and therefore data support further studies of selective phosphodiesterase 10A inhibitors for PD therapy. © 2018 International Parkinson and Movement Disorder Society.