Dopamine receptors and l-dopa-induced dyskinesia

Neurobiological and Pharmacological Perspectives of D3 Receptors in Parkinson’s Disease

The discovery of the D3 receptor (D3R) subtypes of dopamine (DA) has generated an understandable increase in interest in the field of neurological diseases, especially Parkinson’s disease (PD). Indeed, although DA replacement therapy with l-DOPA has provided an effective treatment for patients with PD, it is responsible for invalidating abnormal involuntary movements, known as L-DOPA-induced dyskinesia, which constitutes a serious limitation of the use of this therapy. Of particular interest is the finding that chronic l-DOPA treatment can trigger the expression of D1R–D3R heteromeric interactions in the dorsal striatum. The D3R is expressed in various tissues of the central nervous system, including the striatum. Compelling research has focused on striatal D3Rs in the context of PD and motor side effects, including dyskinesia, occurring with DA replacement therapy. Therefore, this review will briefly describe the basal ganglia (BG) and the DA transmission within these brain regions, before going into more detail with regard to the role of D3Rs in PD and their participation in the current treatments. Numerous studies have also highlighted specific interactions between D1Rs and D3Rs that could promote dyskinesia. Finally, this review will also address the possibility that D3Rs located outside of the BG may mediate some of the effects of DA replacement therapy.

Dopamine D2 and Adenosine A2A Receptors Interaction on Ca2+ Current Modulation in a Rodent Model of Parkinsonism

Adenosine A1 and A2A receptors are expressed in striatal projection neurons (SPNs). A1 receptors are located in direct (dSPN) and indirect SPNs (iSNP). A2A receptors are only present in iSPNs. Dopamine D2 receptors are also expressed in iSPNs and interactions between D2 and A2A receptors have received attention. iSPNs activity increases during parkinsonism (PD) and A2A receptors may be responsible by enhancing Ca2+ currents (iCa2+). Therefore, A2A receptors blockade is a therapeutic approach. We asked whether A2A receptors need the interaction with D2 receptors (D2R) to exert their actions. By using isolated and identified iSPNs to avoid indirect influences, we show that D2R action habilitates A2A receptors (A2AR) modulation. iCa2+ through voltage gated Ca2+ channels (CaV) was used as a signal to observe this interaction. Voltage-clamp recordings in acutely dissociated iSPNs, current-clamp recordings in slices and calcium imaging in transgenic A2A-Cre mice, showed that D2R reduction in iCa2+ endows A2AR to restore iCa2+ on iSPNs showing an antagonistic interaction between D2 and A2A receptors. A2A receptors were blocked by the antagonist istradefylline, however, this blockade differed in control and dopamine-depleted iSPNs: istradefylline reduced D2R modulation in parkinsonian animals as compared to controls. Calcium imaging recordings show that istradefylline occludes D2R actions in the parkinsonian circuitry and this effect depends on the order of drugs application. Thus, while D2 activation enables A2A receptors action, blockade of A2AR induces a reduction in the action of D2 agonists, confirming a complex interaction.

Summary Statement

A2A receptor required previous D2 receptor activation to modulate Ca2+ currents. Istradefylline decreases pramipexole modulation on Ca2+ currents. Istradefylline reduces A2A + neurons activity in striatial microcircuit, but pramipexole failed to further reduce neuronal activity.

Marijuana improved motor impairments and changes in synaptic plasticity-related molecules in the striatum in 6-OHDA-treated rats

Using marijuana has become popular and is allowed for medical purposes in some countries. The effect of marijuana on Parkinson's disease is controversial and Medical marijuana may benefit for motor and non-motor symptoms of patients with Parkinson's disease. No research has been conducted to fully prove the benefits, risks, and uses of marijuana as a treatment for patients with Parkinson's disease. In the present study, several different approaches, including behavioral measures and the western blot method for protein level assay, were used to investigate whether exposure to marijuana affects the motor and synaptic plasticity impairment induced by 6-OHDA. Marijuana consumption significantly decreased apomorphine-induced contralateral rotation, beam travel time, beam freeze time, and catalepsy time, but significantly increased latency to fall in the rotarod test, balance time, and protein level of PSD-95 and dopamine receptor D1 in the 6-OHDA + marijuana group. These results suggest that marijuana may be helpful for motor disorders and synaptic changes in patients with Parkinson's disease.

Contemporary Options for the Management of Motor Complications in Parkinson's Disease: Updated Clinical Review

Parkinson's disease (PD) is a chronic, progressive condition affecting around 1% of the population older than 60 years. Upon long-term treatment with levodopa, the mainstay of treatment in PD, most patients, especially younger ones exposed to higher doses, will experience symptoms related to end-of-dose deterioration, peak-dose dyskinesias, and other motor fluctuations. Therapeutic strategies are grounded on modification of oral levodopa pharmacokinetics to extend levodopa benefit and development of new routes of drug delivery (e.g., levodopa/carbidopa intestinal gel infusion) or long-acting formulations of existing dopaminergic drugs to prolong the duration of striatal dopamine receptors stimulation. As our understanding of the pathophysiology of motor complications evolves, our therapeutic armamentarium is actively expanding and the focus of research is now actively pointing to the new non-dopaminergic agents acting both within the basal ganglia and in other brain regions (e.g., drugs acting on glutamate, GABA, serotonin, and calcium channels). Despite the fact that trials comparing the different therapeutic strategies are lacking, we aimed at devising practical evidence- and experience-guided suggestions for the clinical management of motor complications, emphasizing that this should always be an individualized endeavor. This review summarizes the pharmacological management of motor complications in PD, including new formulations and routes of delivery, and the newer released drugs such as istradefylline, opicapone, safinamide, and zonisamide. Advanced therapeutic strategies for selected cases such as treatment with apomorphine and surgical techniques (deep brain stimulation) are also discussed. A comprehensive knowledge of the available options and evidence is fundamental for the successful management of these challenging complications.

Dystonia and dopamine: From phenomenology to pathophysiology

A line of evidence suggests that the pathophysiology of dystonia involves the striatum, whose activity is modulated among other neurotransmitters, by the dopaminergic system. However, the link between dystonia and dopamine appears complex and remains unclear. Here, we propose a physiological approach to investigate the clinical and experimental data supporting a role of the dopaminergic system in the pathophysiology of dystonic syndromes. Because dystonia is a disorder of motor routines, we first focus on the role of dopamine and striatum in procedural learning. Second, we consider the phenomenology of dystonia from every angle in order to search for features giving food for thought regarding the pathophysiology of the disorder. Then, for each dystonic phenotype, we review, when available, the experimental and imaging data supporting a connection with the dopaminergic system. Finally, we propose a putative model in which the different phenotypes could be explained by changes in the balance between the direct and indirect striato-pallidal pathways, a process critically controlled by the level of dopamine within the striatum. Search strategy and selection criteria References for this article were identified through searches in PubMed with the search terms « dystonia », « dopamine", « striatum », « basal ganglia », « imaging data », « animal model », « procedural learning », « pathophysiology », and « plasticity » from 1998 until 2018. Articles were also identified through searches of the authors' own files. Only selected papers published in English were reviewed. The final reference list was generated on the basis of originality and relevance to the broad scope of this review.

Neural substrates and potential treatments for levodopa-induced dyskinesias in Parkinson's disease

Parkinson's disease (PD) is primarily a motor disorder that involves the gradual loss of motor function. Symptoms are observed initially in the extremities, such as hands and arms, while advanced stages of the disease can effect blinking, swallowing, speaking, and breathing. PD is a neurodegenerative disease, with dopaminergic neuronal loss occurring in the substantia nigra pars compacta, thus disrupting basal ganglia functions. This leads to downstream effects on other neurotransmitter systems such as glutamate, γ-aminobutyric acid, and serotonin. To date, one of the main treatments for PD is levodopa. While it is generally very effective, prolonged treatments lead to levodopa-induced dyskinesia (LID). LID encompasses a family of symptoms ranging from uncontrolled repetitive movements to sustained muscle contractions. In many cases, the symptoms of LID can cause more grief than PD itself. The purpose of this review is to discuss the possible clinical features, cognitive correlates, neural substrates, as well as potential psychopharmacological and surgical (including nondopaminergic and deep brain stimulation) treatments of LID.

Expanding the repertoire of L-DOPA's actions: A comprehensive review of its functional neurochemistry

Though a multi-facetted disorder, Parkinson's disease is prototypically characterized by neurodegeneration of nigrostriatal dopaminergic neurons of the substantia nigra pars compacta, leading to a severe disruption of motor function. Accordingly, L-DOPA, the metabolic precursor of dopamine (DA), is well-established as a treatment for the motor deficits of Parkinson's disease despite long-term complications such as dyskinesia and psychiatric side-effects. Paradoxically, however, despite the traditional assumption that L-DOPA is transformed in residual striatal dopaminergic neurons into DA, the mechanism of action of L-DOPA is neither simple nor entirely clear. Herein, focussing on its influence upon extracellular DA and other neuromodulators in intact animals and experimental models of Parkinson's disease, we highlight effects other than striatal generation of DA in the functional profile of L-DOPA. While not excluding a minor role for glial cells, L-DOPA is principally transformed into DA in neurons yet, interestingly, with a more important role for serotonergic than dopaminergic projections. Moreover, in addition to the striatum, L-DOPA evokes marked increases in extracellular DA in frontal cortex, nucleus accumbens, the subthalamic nucleus and additional extra-striatal regions. In considering its functional profile, it is also important to bear in mind the marked (probably indirect) influence of L-DOPA upon cholinergic, GABAergic and glutamatergic neurons in the basal ganglia and/or cortex, while anomalous serotonergic transmission is incriminated in the emergence of L-DOPA elicited dyskinesia and psychosis. Finally, L-DOPA may exert intrinsic receptor-mediated actions independently of DA neurotransmission and can be processed into bioactive metabolites. In conclusion, L-DOPA exerts a surprisingly complex pattern of neurochemical effects of much greater scope that mere striatal transformation into DA in spared dopaminergic neurons. Their further experimental and clinical clarification should help improve both L-DOPA-based and novel strategies for controlling the motor and other symptoms of Parkinson's disease.

Effects and molecular mechanism of chitosan-coated levodopa nanoliposomes on behavior of dyskinesia rats

BACKGROUND

Chitosan, the N-deacetylated derivative of chitin, is a cationic polyelectrolyte due to the presence of amino groups, one of the few occurring in nature. The use of chitosan in protein and drug delivery systems is being actively researched and reported in the literature.

RESULTS

In this study, we used chitosan-coated levodopa liposomes to investigate the behavioral character and the expression of phosphorylated extracellular signal-regulated kinase (ERK1/2), dopamine- and cAMP-regulated phosphoprotein of 32 kDa (DARPP-32) and FosB/ΔFosB in striatum of rat model of levodopa-induced dyskinesia (LID). We found that scores of abnormal involuntary movement (AIM) decreased significantly in liposome group (P < 0.05), compared with levodopa group. Levels of phospho-ERK1/2, phospho-Thr34 DARPP-32 and FosB/ΔFosB in striatum decreased significantly in liposome group lesion side compared with levodopa group (P < 0.05). However, both of two groups above have significantly differences compared with the control group (P < 0.05).

CONCLUSION

Chitosan-coated levodopa liposomes may be useful in reducing dyskinesias inducing for Parkinson disease. The mechanism might be involved the pathway of signaling molecular phospho-ERK1/2, phospho-Thr34 DARPP-32 and ΔFosB in striatum.

A GIPC1-Palmitate Switch Modulates Dopamine Drd3 Receptor Trafficking and Signaling

Palmitoylation is involved in several neuropsychiatric and movement disorders for which a dysfunctional signaling of the dopamine D3 receptor (Drd3) is hypothesized. Computational modeling of Drd3's homologue, Drd2, has shed some light on the putative role of palmitoylation as a reversible switch for dopaminergic receptor signaling. Drd3 is presumed to be palmitoylated, based on sequence homology with Drd2, but the functional attributes afforded by Drd3 palmitoylation have not been studied. Since these receptors are major targets of antipsychotic and anti-Parkinsonian drugs, a better characterization of Drd3 signaling and posttranslational modifications, like palmitoylation, may improve the prospects for drug development. Using molecular dynamics simulations, we evaluated in silico how Drd3 palmitoylation could elicit significant remodeling of the C-terminal cytoplasmic domain to expose docking sites for signaling proteins. We tested this model in cellulo by using the interaction of Drd3 with the G-alpha interacting protein (GAIP) C terminus 1 (GIPC1) as a template. From a series of biochemical studies, live imaging, and analyses of mutant proteins, we propose that Drd3 palmitoylation acts as a molecular switch for Drd3-biased signaling via a GIPC1-dependent route, which is likely to affect the mode of action of antipsychotic drugs.

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.

In vivo characterization of a novel dopamine D3 receptor agonist to treat motor symptoms of Parkinson's disease

Synthetic dopaminergic agents have found utility in treating neurological and neuropsychiatric disorders since the beginning of 19th century. The discovery of Levodopa (l-dopa) to effectively treat motor symptoms of Parkinson's disease (PD) revolutionized the therapy and remains a gold standard for treating PD. However, l-dopa therapy has been implicated in worsening of the non-motor symptoms including cognition and long-term therapy leads to plasticity and development of abnormal involuntary movements (AIMs) that are collectively called l-dopa induced dyskinesias (LID). Studies in rodents and non-human primates with PD have supported a role for dopamine D3 receptors in the etiology of both the motor symptoms and LID. We have recently developed SK609, a selective dopamine D3 receptor agonist with atypical signaling properties. In this study, we further characterized this novel small molecule using the unilateral lesioned rodent model of PD. In the forepaw stepping test paradigm, SK609 significantly improved the performance of the impaired paw and also normalized the bilateral asymmetry associated with the hemiparkinson rat. In addition, a chronic treatment of SK609 did not induce any AIMs and when used adjuvantly with l-dopa significantly reduced AIMs induced by l-dopa. Further, an optimal dose combination of SK609 with l-dopa was determined by dose dependent titrations of both SK609 and l-dopa that produced minimal AIMs and maximized the effect on improving motor symptoms. Results from this study suggest that SK609 is a novel dopaminergic agent that has the therapeutic potential to treat PD and LID. This article is part of the Special Issue entitled 'Synaptopathy--from Biology to Therapy'.

Levetiracetam Ameliorates L-DOPA-Induced Dyskinesia in Hemiparkinsonian Rats Inducing Critical Molecular Changes in the Striatum

L-DOPA-induced dyskinesias (LID) remain a major problem of long-term therapy of Parkinson's disease. Levetiracetam, a new antiepileptic drug, has been shown to reduce LID, but the mechanisms underlying its effects are unknown. In this study, we assessed the effect of levetiracetam on key mediators of LID in rats with 6-hydroxydopamine (6-OHDA) lesions. Following chronic administration of L-DOPA (12 mg/kg, twice daily for 14 days), rats developed abnormal involuntary movements (AIMs), but co-administration of levetiracetam (15, 30, and 60 mg/kg) with equivalent L-DOPA dosing significantly reduced AIMs scores in a dose dependent manner. The effects of levetiracetam were associated with changes in striatal expression of ΔFosB, phosphorylated extracellular signal-regulated kinases 1 and 2 (p-ERK1/2), and phosphorylated cAMP-regulated phosphoprotein of 32 kDa (p-DARPP-32). These data support that levetiracetam acts at multiple sites in the pathogenetic cascade of LID, and that further understanding of these actions of antiepileptics may contribute to developing new LID therapies.

Levodopa/benserazide microsphere (LBM) prevents L-dopa induced dyskinesia by inactivation of the DR1/PKA/P-tau pathway in 6-OHDA-lesioned Parkinson's rats

L-3, 4-dihydroxyphenylalanine (L-dopa) is the gold standard for symptomatic treatment of Parkinson's disease (PD), but long-term therapy is associated with the emergence of L-dopa-induced dyskinesia (LID). In the present study, L-dopa and benserazide were loaded by poly (lactic-co-glycolic acid) microspheres (LBM), which can release levodopa and benserazide in a sustained manner in order to continuous stimulate dopaminergic receptors. We investigated the role of striatal DR1/PKA/P-tau signal transduction in the molecular event underlying LID in the 6-OHDA-lesioned rat model of PD. We found that animals rendered dyskinetic by L-dopa treatment, administration of LBM prevented the severity of AIM score, as well as improvement in motor function. Moreover, we also showed L-dopa elicits profound alterations in the activity of three LID molecular markers, namely DR1/PKA/P-tau (ser396). These modifications are totally prevented by LBM treatment, a similar way to achieve continuous dopaminergic delivery (CDD). In conclusion, our experiments provided evidence that intermittent administration of L-dopa, but not continuous delivery, and DR1/PKA/p-tau (ser396) activation played a critical role in the molecular and behavioural induction of LID in 6-OHDA-lesioned rats. In addition, LBM treatment prevented the development of LID by inhibiting the expression of DR1/PKA/p-tau, as well as PPEB mRNA in dyskintic rats.

Interactions of CaMKII with dopamine D2 receptors: roles in levodopa-induced dyskinesia in 6-hydroxydopamine lesioned Parkinson's rats

Ca²⁺/calmodulin-dependent protein kinase II is a synapse-enriched kinase in mammalian brains. This kinase interacts with various synaptic proteins to regulate expression and function of interacting proteins and thereby modulates synaptic transmission. CaMKII and its interacting partners are also believed to play a pivotal role in the pathogenesis of various neurological and neurodegenerative disorders, such as Parkinson's disease (PD). In this study, we found that CaMKIIα binds to dopamine D2 receptors (D2R) in vitro. A distal region in the D2R third intracellular loop harbors CaMKIIα binding. Endogenous CaMKIIα was also found to interact with native D2Rs in rat striatal neurons in which D2Rs are expressed at a high level. In addition, in a rat 6-hydroxydopamine lesioned model of PD, chronic levodopa administration induced characteristic dyskinesia. In parallel, levodopa induced an increase in CaMKIIα-D2R interactions in striatal neurons. Intrastriatal injection of a Tat-fusion and CaMKIIα-D2R interaction-dead peptide (Tat-D2Ri) reversed this increase in the interaction between two proteins. Tat-D2Ri also alleviated dyskinetic behaviors induced by levodopa. These results reveal a new interaction between CaMKIIα and D2Rs in striatal neurons which is sensitive to long-term administration of levodopa in PD rats. Prevention of the response of CaMKIIα-D2R interactions to levodopa can alleviate levodopa-induced dyskinesia.

An amino acid residue in the second extracellular loop determines the agonist-dependent tolerance property of the human D3 dopamine receptor

The D3 dopamine receptor is a therapeutic target for treating various nervous system disorders such as schizophrenia, Parkinson's disease, depression, and addictive behaviors. The crystal structure of the D3 receptor bound to an antagonist was recently described; however, the structural features that contribute to agonist-induced conformational changes and signaling properties are not well understood. We have previously described the conformation-dependent tolerance and slow response termination (SRT) signaling properties of the D3 receptor and identified the C147 residue in the second intracellular loop (IL2) of the D3 receptor as important for the tolerance property. Interestingly, while IL2 and the C147 residue, in particular, were important for dopamine- and quinpirole-induced tolerance, this residue did not affect the severe tolerance induced by the high affinity, D3 receptor-selective agonist, PD128907. Here, we used D2/D3 receptor chimeras and site-specific D3 receptor mutants to identify another residue, D187, in the second extracellular loop (EC2) of the human D3 receptor that mediates the tolerance property induced by PD128907, quinpirole, pramipexole, and dopamine. Molecular dynamics simulations confirmed the distinct conformation adopted by D3 receptor during tolerance and suggested that in the tolerant D3 receptor the D187 residue in EC2 forms a salt bridge with the H354 residue in EC3. Indeed, site-directed mutation of the H354 residue resulted in loss of PD1287907-induced tolerance. The mapping of specific amino acid residues that contribute to agonist-dependent conformation changes and D3 receptor signaling properties refines the agonist-bound D3 receptor pharmacophore model which will help develop novel D3 receptor agonists.

Metabotropic glutamate receptors for Parkinson's disease therapy

Excessive glutamatergic signalling within the basal ganglia is implicated in the progression of Parkinson's disease (PD) and inthe emergence of dyskinesia associated with long-term treatment with L-DOPA. There is considerable research focus on the discovery and development of compounds that modulate glutamatergic signalling via glutamate receptors, as treatments for PD and L-DOPA-induced dyskinesia (LID). Although initial preclinical studies with ionotropic glutamate receptor antagonists showed antiparkinsonian and antidyskinetic activity, their clinical use was limited due to psychiatric adverse effects, with the exception of amantadine, a weak N-methyl-d-aspartate (NMDA) antagonist, currently used to reduce dyskinesia in PD patients. Metabotropic receptor (mGlu receptor) modulators were considered to have a more favourable side-effect profile, and several agents have been studied in preclinical models of PD. The most promising results have been seen clinically with selective antagonists of mGlu5 receptor and preclinically with selective positive allosteric modulators of mGlu4 receptor. The growing understanding of glutamate receptor crosstalk also raises the possibility of more precise modulation of glutamatergic transmission, which may lead to the development of more effective agents for PD.

Multisite intracerebral microdialysis to study the mechanism of L-DOPA induced dopamine and serotonin release in the parkinsonian brain

L-DOPA is currently one of the best medications for Parkinson's disease. It was assumed for several years that its benefits and side effects were related to the enhancement of dopamine release in the dopamine-depleted striatum. The use of intracerebral microdialysis combined with a pharmacological approach has led to the discovery that serotonergic neurons are responsible for dopamine release induced by L-DOPA. The subsequent use of multisite microdialysis has further revealed that L-DOPA-stimulated dopamine release is widespread and related to the serotonergic innervation. The present Review emphasizes the functional impact of extrastriatal release of dopamine induced by L-DOPA in both the therapeutic and side effects of L-DOPA.

The nicotine-mediated decline in l-dopa-induced dyskinesias is associated with a decrease in striatal dopamine release

l-dopa-induced dyskinesias (LIDs) are a side effect of Parkinson's disease therapy that is thought to arise, at least in part, because of excessive dopaminergic activity. Thus, drugs that regulate dopaminergic tone may provide an approach to manage LIDs. Our previous studies showed that nicotine treatment reduced LIDs in Parkinsonian animal models. This study investigates whether nicotine may exert its beneficial effects by modulating pre-synaptic dopaminergic function. Rats were unilaterally lesioned by injection of 6-hydroxydopamine (6-OHDA) (2 × 3 ug per site) into the medial forebrain bundle to yield moderate Parkinsonism. They were then implanted with minipumps containing vehicle or nicotine (2.0 mg/kg/d) and rendered dyskinetic with l-dopa (8 mg/kg plus 15 mg/kg benserazide). Lesioning alone decreased the striatal dopamine transporter, nicotinic receptor (nAChR) levels, and nAChR-mediated (3)H-dopamine release, consistent with previous results. Nicotine administration reduced l-dopa-induced abnormal involuntary movements throughout the course of the study (4 months). Nicotine treatment led to declines in the striatal dopamine transporter, α6β2* nAChRs and various components of α6β2* and α4β2* nAChR-mediated release. l-dopa treatment had no effect. These data suggest that nicotine may improve LIDs in Parkinsonian animal models by dampening striatal dopaminergic activity.

PSD-95 expression controls L-DOPA dyskinesia through dopamine D1 receptor trafficking

L-DOPA-induced dyskinesia (LID), a detrimental consequence of dopamine replacement therapy for Parkinson's disease, is associated with an alteration in dopamine D1 receptor (D1R) and glutamate receptor interactions. We hypothesized that the synaptic scaffolding protein PSD-95 plays a pivotal role in this process, as it interacts with D1R, regulates its trafficking and function, and is overexpressed in LID. Here, we demonstrate in rat and macaque models that disrupting the interaction between D1R and PSD-95 in the striatum reduces LID development and severity. Single quantum dot imaging revealed that this benefit was achieved primarily by destabilizing D1R localization, via increased lateral diffusion followed by increased internalization and diminished surface expression. These findings indicate that altering D1R trafficking via synapse-associated scaffolding proteins may be useful in the treatment of dyskinesia in Parkinson's patients.

Understanding and prevention of "therapy-" induced dyskinesias

L-dopa is the most effective, currently available treatment for Parkinson's disease (PD), but it leads to the development of involuntary movements known as L-dopa-induced dyskinesia (LID) in the majority of patients after long-term use. Both gene and cell therapy approaches are the subject of multiple ongoing studies as potential ways of relieving symptoms of PD without the complication of dyskinesia. However, the spectre of dyskinesia in the absence of L-dopa, the so-called "off-phase" or graft-induced dyskinesia (GID), remains a major obstacle particularly in the further development of cell therapy in PD, but it is also a concern for proponents of gene therapy approaches. LID results from nonphysiological dopamine release, supersensitivity of dopamine receptors, and consequent abnormal signalling through mechanisms of synaptic plasticity. Restoration of physiological circuitry within the basal ganglia loops is ultimately the aim of all cell and gene therapy approaches but each using distinctive strategies and accompanied by risks of exacerbation of LID or development of "off-phase"/GID. In this paper we discuss the details of what is understood regarding the development of dyskinesias with relevance to cell and gene therapy and potential strategies to minimize their occurrence.

The 5-HT1A-receptor agonist flibanserin reduces drug-induced dyskinesia in RGS9-deficient mice

Drug-induced dyskinesia is a major complication of dopamine replacement therapy in advanced Parkinson's disease consisting of dystonia, chorea and athetosis. Agonists at 5-HT1A-receptors attenuate levodopa-induced motor complications in non-human primates. Mice with increased dopamine D2 receptor (DRD2) signalling due to the lack of expression of the regulator of G-protein signalling 9 (RGS9) also develop dyskinesia following levodopa treatment. We investigated whether the 5-HT1A-receptor agonist flibanserin compared with buspirone reduces motor abnormalities induced by levodopa or quinelorane, a selective dopamine D2-receptor agonist. Following dopamine depletion via reserpine, 40 mice (20 wild-type and 20 RGS9 knock-out) were treated with flibanserin or buspirone in combination with levodopa or quinelorane. Motor behaviour was analysed using open field analysis. RGS9 knock-out mice displayed significantly more drug-induced dystonia (p < 0.04; t test) than wild type. In quinelorane-treated wild-type mice flibanserin as well as buspirone significantly reduced dystonia (p < 0.05). In RGS9 knock-out animals again both reduced quinelorane-induced dystonia. However, flibanserin was significantly more effective (p = 0.003). Following reserpine pretreatment and administration of levodopa wild-type and RGS 9 knock-out mice showed mild to moderate dystonia. Surprisingly, 10 mg/kg buspirone increased dystonia in both animal groups, whereas it was decreased by 10 mg/kg flibanserin. However, compared with levodopa alone only the increase of dystonia by buspirone was significant (p < 0.04). Flibanserin showed promising antidyskinetic effects in a model of drug-induced dyskinesia. Our data underline the possible benefit of 5-HT1A agonists in drug-induced dyskinesia.

Facing the unique challenges of dyskinesias in Parkinson’s disease

Dyskinesia is among the most challenging complications of levodopa and dopaminergic drug therapy in advanced Parkinson’s disease. This symptom has a negative impact on the quality of life of patients with Parkinson’s disease and is hard to manage. Current advances in our understanding of the diverse phenomenology and complicated pathophysiology of dyskinesia have led to a number of novel strategies aimed at better control of this complication. Further insight has been gained from focusing on the characteristics of the rating scale used for assessment of dyskinesia and from the inherent susceptibility of dyskinesia to placebo effect. Here, we will briefly review the phenomenology, pathophysiology and the treatment of dyskinesia in Parkinson’s disease.

Dopamine receptors, motor responses, and dopaminergic agonists

Dopamine receptors are widely distributed within the central nervous system with its highest expression in the striatum. Two different families of dopamine receptors have been identified. The D₁ family comprises D₁ and D5 receptors, whereas D₂, D₃, and D₄ receptors form the D₂ family. These 2 families mediate different behavior patterns that are linked to activation of specific transduction pathways. The functional relevance of dopamine receptors derives from the reduced dopamine content found in the striatum of Parkinson disease (PD) patients and the ability of dopamine and dopamine receptors to reverse the motor deficits exhibited by PD patients. During the last 2 decades dopamine receptor agonists have been used either in de novo PD patients to prevent the appearance of dyskinesias or in PD patients with motor fluctuations to reduce the number of daily "off" hours. It seems that all dopamine receptors agonists produce similar motor responses and adverse effects, but data comparing their effectiveness in the treatment of PD are not available. In this article we summarize the main characteristics of dopamine receptors, their structure, their signaling pathways, and the responses mediated by their independent activation. Here is also described the therapeutic value of the different dopamine receptor agonists in the treatment of PD.

Imbalanced Dopaminergic Transmission Mediated by Serotonergic Neurons in L-DOPA-Induced Dyskinesia

L-DOPA-induced dyskinesias (LIDs) are one of the main motor side effects of L-DOPA therapy in Parkinson's disease. The review will consider the biochemical evidence indicating that the serotonergic neurons are involved in the dopaminergic effects of L-DOPA in the brain. The consequences are an ectopic and aberrant release of dopamine that follows the serotonergic innervation of the brain. After mid- to long-term treatment with L-DOPA, the pattern of L-DOPA-induced dopamine release is modified. In several brain regions, its effect is dramatically reduced while, in the striatum, its effect is quite preserved. LIDs could appear when the dopaminergic effects of L-DOPA fall in brain areas such as the cortex, enhancing the subcortical impact of dopamine and promoting aberrant motor responses. The consideration of the serotonergic system in the core mechanism of action of L-DOPA opens an important reserve of possible strategies to limit LIDs.

The locus coeruleus is directly implicated in L-DOPA-induced dyskinesia in parkinsonian rats: an electrophysiological and behavioural study

Despite being the most effective treatment for Parkinson’s disease, L-DOPA causes a development of dyskinetic movements in the majority of treated patients. L-DOPA-induced dyskinesia is attributed to a dysregulated dopamine transmission within the basal ganglia, but serotonergic and noradrenergic systems are believed to play an important modulatory role. In this study, we have addressed the role of the locus coeruleus nucleus (LC) in a rat model of L-DOPA-induced dyskinesia. Single-unit extracellular recordings in vivo and behavioural and immunohistochemical approaches were applied in rats rendered dyskinetic by the destruction of the nigrostriatal dopamine neurons followed by chronic treatment with L-DOPA. The results showed that L-DOPA treatment reversed the change induced by 6-hydroxydopamine lesions on LC neuronal activity. The severity of the abnormal involuntary movements induced by L-DOPA correlated with the basal firing parameters of LC neuronal activity. Systemic administration of the LC-selective noradrenergic neurotoxin N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine did not modify axial, limb, and orolingual dyskinesia, whereas chemical destruction of the LC with ibotenic acid significantly increased the abnormal involuntary movement scores. These results are the first to demonstrate altered LC neuronal activity in 6-OHDA lesioned rats treated with L-DOPA, and indicate that an intact noradrenergic system may limit the severity of this movement disorder.

L-DOPA: a scapegoat for accelerated neurodegeneration in Parkinson's disease?

There is consensus that amelioration of the motor symptoms of Parkinson's disease is most effective with L-DOPA (levodopa). However, this necessary therapeutic step is biased by an enduring belief that L-DOPA is toxic to the remaining substantia nigra dopaminergic neurons by itself, or by specific metabolites such as dopamine. The concept of L-DOPA toxicity originated from pre-clinical studies conducted mainly in cell culture, demonstrating that L-DOPA or its derivatives damage dopaminergic neurons due to oxidative stress and other mechanisms. However, the in vitro data remain controversial as some studies showed neuroprotective, rather than toxic action of the drug. The relevance of this debate needs to be considered in the context of the studies conducted on animals and in clinical trials that do not provide convincing evidence for L-DOPA toxicity in vivo. This review presents the current views on the pathophysiology of Parkinson's disease, focusing on mitochondrial dysfunction and oxidative/proteolytic stress, the factors that can be affected by L-DOPA or its metabolites. We then critically discuss the evidence supporting the two opposing views on the effects of L-DOPA in vitro, as well as the animal and human data. We also address the problem of inadequate experimental models used in these studies. L-DOPA remains the symptomatic 'hero' of Parkinson's disease. Whether it contributes to degeneration of nigral dopaminergic neurons, or is a 'scapegoat' for explaining undesirable or unexpected effects of the treatment, remains a hotly debated topic.

Prospects of statins in Parkinson disease

Parkinson disease (PD) is second only to Alzheimer disease as the most common neurodegenerative disorder in humans. Despite intense investigations, no effective therapy is available to halt the progression of PD. Although statins are widely used cholesterol-lowering drugs throughout the world, recent studies suggest that these drugs modulate neurodegeneration-related signaling processes and may be beneficial for PD. Simvastatin is the most potent statin in crossing the blood-brain barrier, and this particular statin drug negatively correlates with the incidence of PD and shows efficacy in animal models of PD. However, PD mainly occurs in the aging population, who are more vulnerable to cholesterol or lipid-related disorders, raising questions whether this possible beneficial effect of statins in PD patients is cholesterol dependent or cholesterol independent. This article presents data on the therapeutic efficacy of simvastatin in a chronic MPTP model of PD, reviews recent literature, and discusses the pros and cons of statin therapy in PD.

From the cell to the clinic: a comparative review of the partial D₂/D₃receptor agonist and α2-adrenoceptor antagonist, piribedil, in the treatment of Parkinson's disease

Though L-3,4-dihydroxyphenylalanine (L-DOPA) is universally employed for alleviation of motor dysfunction in Parkinson's disease (PD), it is poorly-effective against co-morbid symptoms like cognitive impairment and depression. Further, it elicits dyskinesia, its pharmacokinetics are highly variable, and efficacy wanes upon long-term administration. Accordingly, "dopaminergic agonists" are increasingly employed both as adjuncts to L-DOPA and as monotherapy. While all recognize dopamine D(2) receptors, they display contrasting patterns of interaction with other classes of monoaminergic receptor. For example, pramipexole and ropinirole are high efficacy agonists at D(2) and D(3) receptors, while pergolide recognizes D(1), D(2) and D(3) receptors and a broad suite of serotonergic receptors. Interestingly, several antiparkinson drugs display modest efficacy at D(2) receptors. Of these, piribedil displays the unique cellular signature of: 1), signal-specific partial agonist actions at dopamine D(2)and D(3) receptors; 2), antagonist properties at α(2)-adrenoceptors and 3), minimal interaction with serotonergic receptors. Dopamine-deprived striatal D(2) receptors are supersensitive in PD, so partial agonism is sufficient for relief of motor dysfunction while limiting undesirable effects due to "over-dosage" of "normosensitive" D(2) receptors elsewhere. Further, α(2)-adrenoceptor antagonism reinforces adrenergic, dopaminergic and cholinergic transmission to favourably influence motor function, cognition, mood and the integrity of dopaminergic neurones. In reviewing the above issues, the present paper focuses on the distinctive cellular, preclinical and therapeutic profile of piribedil, comparisons to pramipexole, ropinirole and pergolide, and the core triad of symptoms that characterises PD-motor dysfunction, depressed mood and cognitive impairment. The article concludes by highlighting perspectives for clarifying the mechanisms of action of piribedil and other antiparkinson agents, and for optimizing their clinical exploitation.