Investigating the molecular mechanisms of L-DOPA-induced dyskinesia in the mouse

Arrestin-3-assisted activation of JNK3 mediates dopaminergic behavioral sensitization

In rodents with unilateral ablation of neurons supplying dopamine to the striatum, chronic treatment with the dopamine precursor L-DOPA induces a progressive increase of behavioral responses, a process known as behavioral sensitization. This sensitization is blunted in arrestin-3 knockout mice. Using virus-mediated gene delivery to the dopamine-depleted striatum of these mice, we find that the restoration of arrestin-3 fully rescues behavioral sensitization, whereas its mutant defective in c-Jun N-terminal kinase (JNK) activation does not. A 25-residue arrestin-3-derived peptide that facilitates JNK3 activation in cells, expressed ubiquitously or selectively in direct pathway striatal neurons, also fully rescues sensitization, whereas an inactive homologous arrestin-2-derived peptide does not. Behavioral rescue is accompanied by the restoration of JNK3 activity, as reflected by JNK-dependent phosphorylation of the transcription factor c-Jun in the dopamine-depleted striatum. Thus, arrestin-3-assisted JNK3 activation in direct pathway neurons is a critical element of the molecular mechanism underlying sensitization upon dopamine depletion and chronic L-DOPA treatment.

Effects of curcumin to counteract levodopa-induced toxicity in zebrafish

Parkinson's disease (PD) is a neurodegenerative disorder characterized by motor dysfunction due to the death of dopaminergic neurons in the substantia nigra pars compacta. Currently, treatment of PD has focused on increasing dopamine levels, using a dopamine precursor, levodopa (L-DOPA) or stimulation of dopaminergic receptors. Prolonged use of L-DOPA is associated with the occurrence of motor complications and dyskinesia, attributed to neurotoxic effects of this drug. The aim of this study was to investigate the effects of curcumin (CUR), a lipophilic polyphenol, to counteract L-DOPA induced toxicity. Zebrafish larvae were pre-treated with CUR (0.05 µM) or vehicle dimethyl sulfoxide (DMSO) for 24 hr and subsequently exposed to L-DOPA (1 mM) or vehicle. Immediately and 24 hr after L-DOPA exposure, spontaneous swimming and dark/light behavioral tests were performed. In addition, levels of reactive oxygen species (ROS) and lipid peroxidation products were determined at the end of treatment. CUR significantly improved the motor impairment induced by 24 hr L-DOPA treatment, and reduced levels of ROS and lipoperoxidation products in zebrafish larvae. In conclusion, our results suggest that CUR acts as a neuroprotector against toxicity initiated by L-DOPA. Evidence suggests the observed effects of CUR are associated with its antioxidant properties.

Oral Administration of Silybin Protects Against MPTP-Induced Neurotoxicity by Reducing Pro-inflammatory Cytokines and Preserving BDNF Levels in Mice

Parkinson's disease (PD) is the second most frequent neurodegenerative disease associated with motor dysfunction secondary to the loss of dopaminergic neurons in the nigrostriatal axis. Actual therapy consists mainly of levodopa; however, its long-term use promotes secondary effects. Consequently, finding new therapeutic alternatives, such as neuroprotective molecules, is necessary. Among these alternatives is silybin (Sb), the major bioactive flavonolignan in silymarin. Both exert neuroprotective effects, preserving dopamine levels and dopaminergic neurons when administered in the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) mouse PD model, being probably Sb the potential therapeutic molecule behind this effect. To elucidate the role of Sb in the PD model, we determined the dose-dependent conservation of striatal dopamine content following Sb oral administration. Then, we evaluated motor deficit tests using the best dopamine conservative dose of Sb and determined a cytokine-dependent inflammatory profile status, malondialdehyde as an oxidative stress product, and neurotrophic factors content in the MPTP-induced mouse PD model. Our results show that oral Sb at 100 mg/kg dose conserved about 60% dopamine levels. Also, Sb improved motor deficits, preserved neurotrophic factors content and mitochondrial function, reduced lipid peroxidation, diminished proinflammatory cytokines to basal levels, enhanced fractalkine production in the striatum and substantia nigra, and increased IL-10 and IL-4 levels in the substantia nigra in the MPTP mice. Thus, oral Sb may be a potential pharmacological PD treatment alternative.

Behavioral, neural and ultrastructural alterations in a graded-dose 6-OHDA mouse model of early-stage Parkinson's disease

Studying animal models furthers our understanding of Parkinson's disease (PD) pathophysiology by providing tools to investigate detailed molecular, cellular and circuit functions. Different versions of the neurotoxin-based 6-hydroxydopamine (6-OHDA) model of PD have been widely used in rats. However, these models typically assess the result of extensive and definitive dopaminergic lesions that reflect a late stage of PD, leading to a paucity of studies and a consequential gap of knowledge regarding initial stages, in which early interventions would be possible. Additionally, the better availability of genetic tools increasingly shifts the focus of research from rats to mice, but few mouse PD models are available yet. To address these, we characterize here the behavioral, neuronal and ultrastructural features of a graded-dose unilateral, single-injection, striatal 6-OHDA model in mice, focusing on early-stage changes within the first two weeks of lesion induction. We observed early onset, dose-dependent impairments of overall locomotion without substantial deterioration of motor coordination. In accordance, histological evaluation demonstrated a partial, dose-dependent loss of dopaminergic neurons of substantia nigra pars compacta (SNc). Furthermore, electron microscopic analysis revealed degenerative ultrastructural changes in SNc dopaminergic neurons. Our results show that mild ultrastructural and cellular degradation of dopaminergic neurons of the SNc can lead to certain motor deficits shortly after unilateral striatal lesions, suggesting that a unilateral dose-dependent intrastriatal 6-OHDA lesion protocol can serve as a successful model of the early stages of Parkinson's disease in mice.

Dopaminergic Progenitors Derived From Epiblast Stem Cells Function Similarly to Primary VM-Derived Progenitors When Transplanted Into a Parkinson's Disease Model

Neural transplantation in neurodegenerative diseases such as Parkinson’s disease (PD) offers to replace cells lost during the progression of the disease process. Primary fetal ventral mesencephalon (VM), the origin of bona fide midbrain dopaminergic (DAergic) precursors, is currently the gold standard source of cells for transplantation in PD. However, the use of tissue from this source raises ethical and logistical constraints necessitating the need for alternative supplies of donor cells. The requirement of any alternative donor cell source is to have the capability to generate authentic mature DAergic neurons, which could be utilized in cell-replacement strategies. Mouse pluripotent stem cells can efficiently generate electrochemically mature midbrain DAergic precursors in vitro using a stepwise control of FGF signaling. Here, we have compared DAergic transplants derived from two progenitor cell sources in an allograft system: mouse epiblast stem cells (EpiSC) and primary fetal mouse VM tissue. Cells were transplanted into the striatum of 6-OHDA lesioned mice pre-treated with L-DOPA. Drug-induced rotations, a number of motor tests and drug-induced abnormal involuntary movements (AIMs) were assessed. Functional improvements were demonstrated post-transplantation in some behavioral tests, with no difference in graft volume or the number of TH immuno-positive cells in the grafts of the two transplant groups. L-DOPA-induced AIMs and amphetamine-induced AIMs were observed in both transplant groups, with no differences in rate or severity between the two groups. Collectively, in this mouse-to-mouse allograft system, we report no significant differences in the functional ability between the gold standard primary VM derived and pluripotent stem cell-derived DAergic transplants.

mGluR5 Allosteric Modulation Promotes Neurorecovery in a 6-OHDA-Toxicant Model of Parkinson's Disease

Parkinson's disease is a neurodegenerative disease characterized by a loss of dopaminergic substantia nigra neurons and depletion of dopamine. To date, current therapeutic approaches focus on managing motor symptoms and trying to slow neurodegeneration, with minimal capacity to promote neurorecovery. mGluR5 plays a key role in neuroplasticity, and altered mGluR5 signaling contributes to synucleinopathy and dyskinesia in patients with Parkinson's disease. Here, we tested whether the mGluR5-negative allosteric modulator, (2-chloro-4-[2[2,5-dimethyl-1-[4-(trifluoromethoxy) phenyl] imidazol-4-yl] ethynyl] pyridine (CTEP), would be effective in improving motor deficits and promoting neural recovery in a 6-hydroxydopamine (6-OHDA) mouse model. Lesions were induced by 6-ODHA striatal infusion, and 30 days later treatment with CTEP (2 mg/kg) or vehicle commenced for either 1 or 12 weeks. Animals were subjected to behavioral, pathological, and molecular analyses. We also assessed how long the effects of CTEP persisted, and finally, using rapamycin, determined the role of the mTOR pathway. CTEP treatment induced a duration-dependent improvement in apomorphine-induced rotation and performance on rotarod in lesioned mice. Moreover, CTEP promoted a recovery of striatal tyrosine hydroxylase-positive fibers and normalized FosB levels in lesioned mice. The beneficial effects of CTEP were paralleled by an activation of mammalian target of rapamycin (mTOR) pathway and elevated brain-derived neurotrophic factor levels in the striatum of lesioned mice. The mTOR inhibitor, rapamycin (sirolimus), abolished CTEP-induced neurorecovery and rescue of motor deficits. Our findings indicate that mTOR pathway is a useful target to promote recovery and that mGluR5 allosteric regulators may potentially be repurposed to selectively target this pathway to enhance neuroplasticity in patients with Parkinson's disease.

The Rodent Models of Dyskinesia and Their Behavioral Assessment

Dyskinesia, a major motor complication resulting from dopamine replacement treatment, manifests as involuntary hyperkinetic or dystonic movements. This condition poses a challenge to the treatment of Parkinson's disease. So far, several behavioral models based on rodent with dyskinesia have been established. These models have provided an important platform for evaluating the curative effect of drugs at the preclinical research level over the past two decades. However, there are differences in the modeling and behavioral testing procedures among various laboratories that adversely affect the rat and mouse models as credible experimental tools in this field. This article systematically reviews the history, the pros and cons, and the controversies surrounding rodent models of dyskinesia as well as their behavioral assessment protocols. A summary of factors that influence the behavioral assessment in the rodent dyskinesia models is also presented, including the degree of dopamine denervation, stereotaxic lesion sites, drug regimen, monitoring styles, priming effect, and individual and strain differences. Besides, recent breakthroughs like the genetic mouse models and the bilateral intoxication models for dyskinesia are also discussed.

The abnormal activation of D1R/Shp-2 complex involved in levodopa-induced dyskinesia in 6-hydroxydopamine-lesioned Parkinson's rats

BACKGROUND

Levodopa-induced dyskinesia (LID) is a troublesome problem in the treatment of Parkinson's disease (PD). The mechanisms of LID are still mysterious. Recently, the interaction between Shp-2 and D1 dopamine receptor (D1R) has been identified to be indispensable in the D1R-mediated extracellular signal-regulated kinases 1 and 2 (ERK1/2) activation and the occurrence of LID. However, the role of Shp-2 in the D1R-mediated signaling pathway of dyskinetic rat models is not fully clear. We designed this study with the purpose of exploring the role of D1R/Shp-2 complex in the D1R-mediated signaling pathway in the occurrence of LID.

MATERIALS AND METHODS

The 6-hydroxydopamine (6-OHDA) was injected unilaterally to produce the rat models of PD. Successful PD rat models were randomly divided into three groups to receive the treatment with L-3,4-dihydroxyphenylalanine (l-DOPA) + benserazide, l-DOPA + benserazide + D1R antagonist (SCH23390) or D1R agonist (SKF38393). Abnormal involuntary movements were assessed in different groups during the treatment. The interaction between D1R and Shp-2 was confirmed in the sham and LID rats through the methods of coimmunoprecipitation. In addition, the levels of p-Shp-2, p-ERK1/2 and p-mTOR were determined by Western blot in different groups.

RESULTS

After the treatment with l-DOPA + benserazide for 22 days, PD rats presented with dyskinesia. D1R agonist, SKF38393, induced similar involuntary movements in PD rats. In contrast, the dyskinetic movements were not induced by coadministration of l-DOPA + D1R antagonist (SCH23390). The interaction between D1R and Shp-2 in the normal rats was kept stable after the long-term use of l-DOPA. Moreover, we found that the pulsatile levodopa administration induced hyperphosphorylation of Shp-2, ERK1/2 and mTOR, while the coadministration of l-DOPA and D1R antagonist, SCH23390, did not induce the hyperphosphorylation of these proteins.

CONCLUSION

These data verified the existence of D1R/Shp-2 complex and its crucial role in the D1R-mediated signaling pathway in dyskinetic rats. Focus on the D1R/Shp-2 complex might be a potential treatment of LID in the future.

A Meta-Analysis of Adenosine A2A Receptor Antagonists on Levodopa-Induced Dyskinesia In Vivo

Background

Long-term use of levodopa (l-dopa) is inevitably complicated with highly disabling fluctuations and drug-induced dyskinesias, which pose major challenges to the existing drug therapy of Parkinson's disease.

Methods

In this study, we conducted a systematic review and meta-analysis to assess the efficacy of A2A receptor antagonists on reducing l-dopa-induced dyskinesias (LID).

Results

Nine studies with a total of 152 animals were included in this meta-analysis. Total abnormal involuntary movements (AIM) score, locomotor activity, and motor disability were reported as outcome measures in 5, 5, and 3 studies, respectively. Combined standardized mean difference (SMD) estimates were calculated using a random-effects model. We pooled the whole data and found that, when compared to l-dopa alone, A2A receptor antagonists plus l-dopa treatment showed no effect on locomotor activity (SMD -0.00, 95% confidence interval (CI): -2.52 to 2.52, p = 1.0), superiority in improvement of motor disability (SMD -5.06, 95% CI: -9.25 to -0.87, p = 0.02) and more effective in control of AIM (SMD -1.82, 95% CI: -3.38 to -0.25, p = 0.02).

Conclusion

To sum up, these results demonstrated that A2A receptor antagonists appear to have efficacy in animal models of LID. However, large randomized clinical trials testing the effects of A2A receptor antagonists in LID patients are always warranted.

Animal models of L-DOPA-induced dyskinesia: the 6-OHDA-lesioned rat and mouse

Appearance of L-DOPA-induced dyskinesia (LID) represents a major limitation in the pharmacological therapy with the dopamine precursor L-DOPA. Indeed, the vast majority of parkinsonian patients develop dyskinesia within 9-10 years of L-DOPA oral administration. This makes the discovery of new therapeutic strategies an important need. In the last decades, several animal models of Parkinson's disease (PD) have been developed, to both study mechanisms underlying PD pathology and treatment-induced side effects (i.e., LID) and to screen for new potential anti-parkinsonian and anti-dyskinetic treatments. Among all the models developed, the 6-OHDA-lesioned rodents represent the models of choice to mimic PD motor symptoms and LID, thanks to their reproducibility and translational value. Under L-DOPA treatment, rodents sustaining 6-OHDA lesions develop abnormal involuntary movements with dystonic and hyperkinetic features, resembling what seen in dyskinetic PD patients. These models have been extensively validated by the evidence that dyskinetic behaviors are alleviated by compounds reducing dyskinesia in patients and non-human primate models of PD. This article will focus on the translational value of the 6-OHDA rodent models of LID, highlighting their main features, advantages and disadvantages in preclinical research.

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.

RasGRP1 promotes amphetamine-induced motor behavior through a Rhes interaction network ("Rhesactome") in the striatum

The striatum of the brain coordinates motor function. Dopamine-related drugs may be therapeutic to patients with striatal neurodegeneration, such as Huntington's disease (HD) and Parkinson's disease (PD), but these drugs have unwanted side effects. In addition to stimulating the release of norepinephrine, amphetamines, which are used for narcolepsy and attention-deficit/hyperactivity disorder (ADHD), trigger dopamine release in the striatum. The guanosine triphosphatase Ras homolog enriched in the striatum (Rhes) inhibits dopaminergic signaling in the striatum, is implicated in HD and L-dopa-induced dyskinesia, and has a role in striatal motor control. We found that the guanine nucleotide exchange factor RasGRP1 inhibited Rhes-mediated control of striatal motor activity in mice. RasGRP1 stabilized Rhes, increasing its synaptic accumulation in the striatum. Whereas partially Rhes-deficient (Rhes+/-) mice had an enhanced locomotor response to amphetamine, this phenotype was attenuated by coincident depletion of RasGRP1. By proteomic analysis of striatal lysates from Rhes-heterozygous mice with wild-type or partial or complete knockout of Rasgrp1, we identified a diverse set of Rhes-interacting proteins, the "Rhesactome," and determined that RasGRP1 affected the composition of the amphetamine-induced Rhesactome, which included PDE2A (phosphodiesterase 2A; a protein associated with major depressive disorder), LRRC7 (leucine-rich repeat-containing 7; a protein associated with bipolar disorder and ADHD), and DLG2 (discs large homolog 2; a protein associated with chronic pain). Thus, this Rhes network provides insight into striatal effects of amphetamine and may aid the development of strategies to treat various neurological and psychological disorders associated with the striatal dysfunction.

Differential induction of dyskinesia and neuroinflammation by pulsatile versus continuous l-DOPA delivery in the 6-OHDA model of Parkinson's disease

Neuroinflammation is associated with l-DOPA treatment in Parkinson's disease (PD), suggesting a role in l-DOPA-induced dyskinesia (LID), however it is unclear whether increased inflammation is specifically related to the dyskinetic outcome of l-DOPA treatment. Diversely from oral l-DOPA, continuous intrajejunal l-DOPA infusion is associated with very low dyskinetic outcome in PD patients. We reproduced these regimens of administration in 6-OHDA-lesioned hemiparkinsonian rats, where dyskinetic responses and striatal neuroinflammation induced by chronic pulsatile (DOPAp) or continuous (DOPAc) l-DOPA were compared. Moreover, we investigated the contribution of a peripheral inflammatory challenge with lipopolysaccharide (LPS), to DOPAp-induced dyskinetic and neuroinflammatory responses. Rats 6-OHDA-infused in the medial forebrain bundle received two weeks treatment with DOPAp, DOPAc via subcutaneous osmotic minipumps, or DOPAp followed by DOPAc. l-DOPA plasma levels were measured in all experimental groups. An independent group of rats received one peripheral dose of LPS 24h before DOPAp treatment. Abnormal involuntary movements (AIMs) were evaluated as a rat model of LID. Immunoreactivity (IR) for OX-42, microglial and neuronal TNF-α, iNOS and GFAP was quantified in denervated and contralateral striatum. In addition, serum TNF-α was measured. The 6-OHDA denervation induced a mild microgliosis in the striatum two weeks after neurotoxin infusion, and increased TNF-α IR in microglia. Rats receiving the DOPAp treatment developed AIMs and displayed increased striatal OX-42, microglial TNF-α, iNOS and GFAP. Moreover, TNF-α IR was also increased in a subpopulation of striatal neurons. Conversely, DOPAc did not induce AIMs or inflammatory responses in either drug-naïve animals or rats that were previously dyskinetic when exposed to DOPAp. Serum TNF-α was not altered by any l-DOPA treatment. LPS pre-treatment increased the degree of DOPAp-induced AIMs and striatal IR for OX-42, TNF-α, iNOS and GFAP. Altogether the present findings indicate that in the 6-OHDA model, chronic l-DOPA induces striatal inflammatory responses, which however depend upon the administration regimen and the dyskinetic outcome of drug treatment. The potentiation of dyskinetic responses by LPS suggests a reciprocal causal link between neuroinflammation and LID.

The impact of metabotropic glutamate receptors into active neurodegenerative processes: A "dark side" in the development of new symptomatic treatments for neurologic and psychiatric disorders

Metabotropic glutamate (mGlu) receptor ligands are under clinical development for the treatment of CNS disorders with high social and economic burden, such as schizophrenia, major depressive disorder (MDD), and Parkinson's disease (PD), and are promising drug candidates for the treatment of Alzheimer's disease (AD). So far, clinical studies have shown symptomatic effects of mGlu receptor ligands, but it is unknown whether these drugs act as disease modifiers or, at the opposite end, they accelerate disease progression by enhancing neurodegeneration. This is a fundamental issue in the treatment of PD and AD, and is also an emerging theme in the treatment of schizophrenia and MDD, in which neurodegeneration is also present and contribute to disease progression. Moving from in vitro data and preclinical studies, we discuss the potential impact of drugs targeting mGlu2, mGlu3, mGlu4 and mGlu5 receptor ligands on active neurodegeneration associated with AD, PD, schizophrenia, and MDD. We wish to highlight that our final comments on the best drug candidates are not influenced by commercial interests or by previous or ongoing collaborations with drug companies. This article is part of the Special Issue entitled 'Metabotropic Glutamate Receptors, 5 years on'.

Inhibition of Glycogen Synthase Kinase-3β (GSK-3β) as potent therapeutic strategy to ameliorates L-dopa-induced dyskinesia in 6-OHDA parkinsonian rats

Levodopa (L-dopa) is the dominating therapy drug for exogenous dopaminergic substitution and can alleviate most of the manifestations of Parkinson's disease (PD), but long-term therapy is associated with the emergence of L-dopa-induced dyskinesia (LID). Evidence points towards an involvement of Glycogen Synthase Kinase-3β (GSK-3β) in development of LID. In the present study, we found that animals rendered dyskinetic by L-dopa treatment, administration of TDZD8 (2mg/kg) obviously prevented the severity of AIM score, as well as improvement in motor function (P < 0.05). Moreover, the TDZD8-induced reduction in dyskinetic behavior correlated with a reduction in molecular correlates of LID. TDZD8 reduced the phosphorylation levels of tau, DARPP32, ERK and PKA protein, which represent molecular markers of LID, as well as reduced L-dopa-induced FosB mRNA and PPEB mRNA levels in the lesioned striatum. In addition, we found that TDZD8 antidyskinetic properties were overcome by D1 receptor, as pretreatment with SKF38393 (5 mg/kg, 10 mg/kg, respectively), a D1 receptor agonist, blocked TDZD8 antidyskinetic actions. This study supported the hypothesis that GSK-3β played an important role in the development and expression of LID. Inhibition of GSK-3β with TDZD8 reduced the development of ALO AIM score and associated molecular changes in 6-OHDA-lesioned rats.

Modulation by Trace Amine-Associated Receptor 1 of Experimental Parkinsonism, L-DOPA Responsivity, and Glutamatergic Neurotransmission

Parkinson's disease (PD) is a movement disorder characterized by a progressive loss of nigrostriatal dopaminergic neurons. Restoration of dopamine transmission by l-DOPA relieves symptoms of PD but causes dyskinesia. Trace Amine-Associated Receptor 1 (TAAR1) modulates dopaminergic transmission, but its role in experimental Parkinsonism and l-DOPA responses has been neglected. Here, we report that TAAR1 knock-out (KO) mice show a reduced loss of dopaminergic markers in response to intrastriatal 6-OHDA administration compared with wild-type (WT) littermates. In contrast, the TAAR1 agonist RO5166017 aggravated degeneration induced by intrastriatal 6-OHDA in WT mice. Subchronic l-DOPA treatment of TAAR1 KO mice unilaterally lesioned with 6-OHDA in the medial forebrain bundle resulted in more pronounced rotational behavior and dyskinesia than in their WT counterparts. The enhanced behavioral sensitization to l-DOPA in TAAR1 KO mice was paralleled by increased phosphorylation of striatal GluA1 subunits of AMPA receptors. Conversely, RO5166017 counteracted both l-DOPA-induced rotation and dyskinesia as well as AMPA receptor phosphorylation. Underpinning a role for TAAR1 receptors in modulating glutamate neurotransmission, intrastriatal application of RO5166017 prevented the increase of evoked corticostriatal glutamate release provoked by dopamine deficiency after 6-OHDA-lesions or conditional KO of Nurr1. Finally, inhibition of corticostriatal glutamate release by TAAR1 showed mechanistic similarities to that effected by activation of dopamine D2 receptors. These data unveil a role for TAAR1 in modulating the degeneration of dopaminergic neurons, the behavioral response to l-DOPA, and presynaptic and postsynaptic glutamate neurotransmission in the striatum, supporting their relevance to the pathophysiology and, potentially, management of PD.

SIGNIFICANCE STATEMENT

Parkinson's disease (PD) is characterized by a progressive loss of nigrostriatal dopaminergic neurons. Restoration of dopamine transmission by l-DOPA relieves symptoms of PD but causes severe side effects. Trace Amine-Associated Receptor 1 (TAAR1) modulates dopaminergic transmission, but its role in PD and l-DOPA responses has been neglected. Here, we report that TAAR1 potentiates the degeneration of dopaminergic neurons and attenuates the behavioral response to l-DOPA and presynaptic and postsynaptic glutamate neurotransmission in the striatum, supporting the relevance of TAAR1 to the pathophysiology and, potentially, management of PD.

Gene therapy blockade of dorsal striatal p11 improves motor function and dyskinesia in parkinsonian mice

Complications of dopamine replacement for Parkinson's disease (PD) can limit therapeutic options, leading to interest in identifying novel pathways that can be exploited to improve treatment. p11 (S100A10) is a cellular scaffold protein that binds to and potentiates the activity of various ion channels and neurotransmitter receptors. We have previously reported that p11 can influence ventral striatal function in models of depression and drug addiction, and thus we hypothesized that dorsal striatal p11 might mediate motor function and drug responses in parkinsonian mice. To focally inhibit p11 expression in the dorsal striatum, we injected an adeno-associated virus (AAV) vector producing a short hairpin RNA (AAV.sh.p11). This intervention reduced the impairment in motor function on forced tasks, such as rotarod and treadmill tests, caused by substantia nigra lesioning in mice. Measures of spontaneous movement and gait in an open-field test declined as expected in control lesioned mice, whereas AAV.sh.p11 mice remained at or near normal baseline. Mice with unilateral lesions were then challenged with l-dopa (levodopa) and various dopamine receptor agonists, and resulting rotational behaviors were significantly reduced after ipsilateral inhibition of dorsal striatal p11 expression. Finally, p11 knockdown in the dorsal striatum dramatically reduced l-dopa-induced abnormal involuntary movements compared with control mice. These data indicate that focal inhibition of p11 action in the dorsal striatum could be a promising PD therapeutic target to improve motor function while reducing l-dopa-induced dyskinesias.

Pathophysiology of L-dopa-induced motor and non-motor complications in Parkinson's disease

Involuntary movements, or dyskinesia, represent a debilitating complication of levodopa (L-dopa) therapy for Parkinson's disease (PD). L-dopa-induced dyskinesia (LID) are ultimately experienced by the vast majority of patients. In addition, psychiatric conditions often manifested as compulsive behaviours, are emerging as a serious problem in the management of L-dopa therapy. The present review attempts to provide an overview of our current understanding of dyskinesia and other L-dopa-induced dysfunctions, a field that dramatically evolved in the past twenty years. In view of the extensive literature on LID, there appeared a critical need to re-frame the concepts, to highlight the most suitable models, to review the central nervous system (CNS) circuitry that may be involved, and to propose a pathophysiological framework was timely and necessary. An updated review to clarify our understanding of LID and other L-dopa-related side effects was therefore timely and necessary. This review should help in the development of novel therapeutic strategies aimed at preventing the generation of dyskinetic symptoms.

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

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