Antagonizing L-type Ca2+ channel reduces development of abnormal involuntary movement in the rat model of L-3,4-dihydroxyphenylalanine-induced dyskinesia

l-DOPA receptor GPR143 inhibits neurite outgrowth via L-type calcium channels in PC12 cells

The gene product of ocular albinism 1 (OA1)/G-protein-coupled receptor (GPR)143 is a receptor for L-3,4-dihydroxyphenylanine (l-DOPA), the most effective agent for Parkinson's disease. When overexpressed, human wild-type GPR143, but not its mutants, inhibits neurite outgrowth in PC12 cells. We investigated the downstream signaling pathway for GPR143-induced inhibition of neurite outgrowth. Nifedipine restored GPR143-induced neurite outgrowth inhibition to the level of control transfectant but did not affect outgrowth in GPR143-knockdown cells. Cilnidipine and flunarizine also suppressed the GPR143-induced inhibition, but their effects at higher concentrations still occurred even in GPR143-knockdown cells. These results suggest that GPR143 regulates neurite outgrowth via L-type calcium channel(s).

Control of Dopamine Signal in High-Order Receptor Complex on Striatal Astrocytes

The receptor-receptor interaction (RRI) of G protein-coupled receptors (GPCRs) leads to new functional entities that are conceptually distinct from the simple addition of signals mediated by the activation of the receptors that form the heteromers. Focusing on astrocytes, there is evidence for the existence of inhibitory and facilitatory RRIs, including the heteromers formed by the adenosine A2A and the dopamine D2 receptors, by A2A and the oxytocin receptor (OTR), and the D2-OTR heteromers. The possible involvement of these receptors in mosaicism has never been investigated in striatal astrocytes. By biophysical and functional approaches, we focused our attention on the existence of an A2A-D2-OTR high-order receptor complex and its role in modulating cytosolic calcium levels and endogenous glutamate release, when striatal astrocyte processes were stimulated with 4-aminopyridine. Functional data indicate a permissive role of OTR on dopamine signaling in the regulation of the glutamatergic transmission, and an inhibitory control mediated by A2A on both the D2-mediated signaling and on the OTR-facilitating effect on D2. Imaging biochemical and bioinformatic evidence confirmed the existence of the A2A-D2-OTR complex and its ternary structure in the membrane. In conclusion, the D2 receptor appears to be a hotspot in the control of the glutamate release from the astrocytic processes and may contribute to the regulation and integration of different neurotransmitter-mediated signaling in the striatum by the A2A-D2-OTR heterotrimers. Considering the possible selectivity of allosteric interventions on GPCRs organized as receptor mosaics, A2A-D2-OTR heterotrimers may offer selective pharmacological targets in neuropsychiatric disorders and neurodegenerative diseases.

Distributed dopaminergic signaling in the basal ganglia and its relationship to motor disability in Parkinson's disease

The degeneration of mesencephalic dopaminergic neurons that innervate the basal ganglia is responsible for the cardinal motor symptoms of Parkinson's disease (PD). It has been thought that loss of dopaminergic signaling in one basal ganglia region - the striatum - was solely responsible for the network pathophysiology causing PD motor symptoms. While our understanding of dopamine (DA)'s role in modulating striatal circuitry has deepened in recent years, it also has become clear that it acts in other regions of the basal ganglia to influence movement. Underscoring this point, examination of a new progressive mouse model of PD shows that striatal dopamine DA depletion alone is not sufficient to induce parkinsonism and that restoration of extra-striatal DA signaling attenuates parkinsonian motor deficits once they appear. This review summarizes recent advances in the effort to understand basal ganglia circuitry, its modulation by DA, and how its dysfunction drives PD motor symptoms.

Quinpirole inhibits levodopa-induced dyskinesias at structural and behavioral levels: Efficacy negated by co-administration of isradipine

Dopamine depletion associated with parkinsonism induces plastic changes in striatal medium spiny neurons (MSN) that are maladaptive and associated with the emergence of the negative side-effect of standard treatment: the abnormal involuntary movements termed levodopa-induced dyskinesia (LID). Prevention of MSN dendritic spine loss is hypothesized to diminish liability for LID in Parkinson's disease. Blockade of striatal CaV1.3 calcium channels can prevent spine loss and significantly diminish LID in parkinsonian rats. While pharmacological antagonism with FDA approved CaV1 L-type channel antagonist dihydropyridine (DHP) drugs (e.g, isradipine) are potentially antidyskinetic, pharmacologic limitations of current drugs may result in suboptimal efficacy. To provide optimal CaV1.3 antagonism, we investigated the ability of a dual pharmacological approach to more potently antagonize these channels. Specifically, quinpirole, a D2/D3-type dopamine receptor (D2/3R) agonist, has been demonstrated to significantly reduce calcium current activity at CaV1.3 channels in MSNs of rats by a mechanism distinct from DHPs. We hypothesized that dual inhibition of striatal CaV1.3 channels using the DHP drug isradipine combined with the D2/D3 dopamine receptor agonist quinpirole prior to, and in conjunction with, levodopa would be more effective at preventing structural modifications of dendritic spines and providing more stable LID prevention. For these proof-of-principle studies, rats with unilateral nigrostriatal lesions received daily administration of vehicle, isradipine, quinpirole, or isradipine + quinpirole prior to, and concurrent with, levodopa. Development of LID and morphological analysis of dendritic spines were assessed. Contrary to our hypothesis, quinpirole monotherapy was the most effective at reducing dyskinesia severity and preventing abnormal mushroom spine formation on MSNs, a structural phenomenon previously associated with LID. Notably, the antidyskinetic efficacy of quinpirole monotherapy was lost in the presence of isradipine co-treatment. These findings suggest that D2/D3 dopamine receptor agonists when given in combination with levodopa and initiated in early-stage Parkinson's disease may provide long-term protection against LID. The negative interaction of isradipine with quinpirole suggests a potential cautionary note for co-administration of these drugs in a clinical setting.

Rethinking the network determinants of motor disability in Parkinson's disease

For roughly the last 30 years, the notion that striatal dopamine (DA) depletion was the critical determinant of network pathophysiology underlying the motor symptoms of Parkinson's disease (PD) has dominated the field. While the basal ganglia circuit model underpinning this hypothesis has been of great heuristic value, the hypothesis itself has never been directly tested. Moreover, studies in the last couple of decades have made it clear that the network model underlying this hypothesis fails to incorporate key features of the basal ganglia, including the fact that DA acts throughout the basal ganglia, not just in the striatum. Underscoring this point, recent work using a progressive mouse model of PD has shown that striatal DA depletion alone is not sufficient to induce parkinsonism and that restoration of extra-striatal DA signaling attenuates parkinsonian motor deficits once they appear. Given the broad array of discoveries in the field, it is time for a new model of the network determinants of motor disability in PD.

Therapeutic Potential of Heterocyclic Compounds Targeting Mitochondrial Calcium Homeostasis and Signaling in Alzheimer's Disease and Parkinson's Disease

Alzheimer's disease (AD) and Parkinson's disease (PD) are the two most common neurodegenerative diseases in the elderly. The key histopathological features of these diseases are the presence of abnormal protein aggregates and the progressive and irreversible loss of neurons in specific brain regions. The exact mechanisms underlying the etiopathogenesis of AD or PD remain unknown, but there is extensive evidence indicating that excessive generation of reactive oxygen species (ROS) and reactive nitrogen species (RNS), along with a depleted antioxidant system, mitochondrial dysfunction, and intracellular Ca²⁺ dyshomeostasis, plays a vital role in the pathophysiology of these neurological disorders. Due to an improvement in life expectancy, the incidence of age-related neurodegenerative diseases has significantly increased. However, there is no effective protective treatment or therapy available but rather only very limited palliative treatment. Therefore, there is an urgent need for the development of preventive strategies and disease-modifying therapies to treat AD/PD. Because dysregulated Ca²⁺ metabolism drives oxidative damage and neuropathology in these diseases, the identification or development of compounds capable of restoring Ca²⁺ homeostasis and signaling may provide a neuroprotective avenue for the treatment of neurodegenerative diseases. In addition, a set of strategies to control mitochondrial Ca²⁺ homeostasis and signaling has been reported, including decreased Ca²⁺ uptake through voltage-operated Ca²⁺ channels (VOCCs). In this article, we review the modulatory effects of several heterocyclic compounds on Ca²⁺ homeostasis and trafficking, as well as their ability to regulate compromised mitochondrial function and associated free-radical production during the onset and progression of AD or PD. This comprehensive review also describes the chemical synthesis of the heterocycles and summarizes the clinical trial outcomes.

Dopamine-independent development and maintenance of mouse striatal medium spiny neuron dendritic spines

Striatal medium spiny neurons (MSNs) and striatal dopamine (DA) innervation are profoundly important for brain function such as motor control and cognition. A widely accepted theory posits that striatal DA loss causes (or leads to) MSN dendritic atrophy. However, examination of the literature indicates that the data from Parkinson's disease (PD) patients and animal PD models were contradictory among studies and hard to interpret. Here we have re-examined the potential effects of DA activity on MSN morphology or lack thereof. We found that in 15-day, 4- and 12-month old Pitx3 null mutant mice that have severe DA denervation in the dorsal striatum while having substantial residual DA innervation in the ventral striatum, MSN dendrites and spine numbers were similar in dorsal and ventral striatum, and also similar to those in normal mice. In 15-day, 4- and 12-month old tyrosine hydroxylase knockout mice that cannot synthesize L-dopa and thus have no endogenous DA in the entire brain, MSN dendrites and spine numbers were also indistinguishable from age-matched wild-type (WT) mice. Furthermore, in adult WT mice, unilateral 6-OHDA lesion at 12 months of age caused an almost complete striatal DA denervation in the lesioned side, but MSN dendrites and spine numbers were similar in the lesioned and control sides. Taken together, our data indicate that in mice, the development and maintenance of MSN dendrites and spines are DA-independent such that DA depletion does not trigger MSN dendritic atrophy; our data also suggest that the reported MSN dendritic atrophy in PD may be a component of neurodegeneration in PD rather than a consequence of DA denervation.

Downregulation of striatal CaV1.3 inhibits the escalation of levodopa-induced dyskinesia in male and female parkinsonian rats of advanced age

In the past 25 years, the prevalence of Parkinson's disease (PD) has nearly doubled. Age remains the primary risk factor for PD and as the global aging population increases this trend is predicted to continue. Even when treated with levodopa, the gold standard dopamine (DA) replacement therapy, individuals with PD frequently develop therapeutic side effects. Levodopa-induced dyskinesia (LID), a common side effect of long-term levodopa use, represents a significant unmet clinical need in the treatment of PD. Previously, in young adult (3-month-old) male parkinsonian rats, we demonstrated that the silencing of Ca_V1.3 (Cacan1d) L-type voltage-gated calcium channels via striatal delivery of rAAV-Ca_V1.3-shRNA provides uniform protection against the induction of LID, and significant reduction of established severe LID. With the goal of more closely replicating a clinical demographic, the current study examined the effects of Ca_V1.3-targeted gene therapy on LID escalation in male and female parkinsonian rats of advanced age (18-month-old at study completion). We tested the hypothesis that silencing aberrant Ca_V1.3 channel activity in the parkinsonian striatum would prevent moderate to severe dyskinesia with levodopa dose escalation. To test this hypothesis, 15-month-old male and female F344 rats were rendered unilaterally parkinsonian and primed with low-dose (3-4 mg/kg) levodopa. Following the establishment of stable, mild dyskinesias, rats received an intrastriatal injection of either the Cacna1d-specific rAAV-Ca_V1.3-shRNA vector (CAV-shRNA), or the scramble control rAAV-SCR-shRNA vector (SCR-shRNA). Daily (M-Fr) low-dose levodopa was maintained for 4 weeks during the vector transduction and gene silencing window followed by escalation to 6 mg/kg, then to 12 mg/kg levodopa. SCR-shRNA-shRNA rats showed stable LID expression with low-dose levodopa and the predicted escalation of LID severity with increased levodopa doses. Conversely, complex behavioral responses were observed in aged rats receiving CAV-shRNA, with approximately half of the male and female subjects-therapeutic 'Responders'-demonstrating protection against LID escalation, while the remaining half-therapeutic 'Non-Responders'-showed LID escalation similar to SCR-shRNA rats. Post-mortem histological analyses revealed individual variability in the detection of Cacna1d regulation in the DA-depleted striatum of aged rats. However, taken together, male and female therapeutic 'Responder' rats receiving CAV-shRNA had significantly less striatal Cacna1d in their vector-injected striatum relative to contralateral striatum than those with SCR-shRNA. The current data suggest that mRNA-level silencing of striatal Ca_V1.3 channels maintains potency in a clinically relevant in vivo scenario by preventing dose-dependent dyskinesia escalation in rats of advanced age. As compared to the uniform response previously reported in young male rats, there was notable variability between individual aged rats, particularly females, in the current study. Future investigations are needed to derive the sex-specific and age-related mechanisms which underlie variable responses to gene therapy and to elucidate factors which determine the therapeutic efficacy of treatment for PD.

The Role of Voltage-Gated Calcium Channels in Basal Ganglia Neurodegenerative Disorders

Calcium (Ca2+) plays a central role in regulating many cellular processes and influences cell survival. Several mechanisms can disrupt Ca2+ homeostasis to trigger cell death, including oxidative stress, mitochondrial damage, excitotoxicity, neuroinflammation, autophagy, and apoptosis. Voltage-gated Ca2+ channels (VGCCs) act as the main source of Ca2+ entry into electrically excitable cells, such as neurons, and they are also expressed in glial cells such as astrocytes and oligodendrocytes. The dysregulation of VGCC activity has been reported in both Parkinson's disease (PD) and Huntington's (HD). PD and HD are progressive neurodegenerative disorders (NDs) of the basal ganglia characterized by motor impairment as well as cognitive and psychiatric dysfunctions. This review will examine the putative role of neuronal VGCCs in the pathogenesis and treatment of central movement disorders, focusing on PD and HD. The link between basal ganglia disorders and VGCC physiology will provide a framework for understanding the neurodegenerative processes that occur in PD and HD, as well as a possible path towards identifying new therapeutic targets for the treatment of these debilitating disorders.

The Role of Striatal Cav1.3 Calcium Channels in Therapeutics for Parkinson's Disease

Parkinson's disease (PD) is a relentlessly progressive neurodegenerative disorder with typical motor symptoms that include rigidity, tremor, and akinesia/bradykinesia, in addition to a host of non-motor symptoms. Motor symptoms are caused by progressive and selective degeneration of dopamine (DA) neurons in the SN pars compacta (SNpc) and the accompanying loss of striatal DA innervation from these neurons. With the exception of monogenic forms of PD, the etiology of idiopathic PD remains unknown. While there are a number of symptomatic treatment options available to individuals with PD, these therapies do not work uniformly well in all patients, and eventually most are plagued with waning efficacy and significant side-effect liability with disease progression. The incidence of PD increases with aging, and as such the expected burden of this disease will continue to escalate as our aging population increases (Dorsey et al. Neurology 68:384-386, 2007). The daunting personal and socioeconomic burden has pressed scientists and clinicians to find improved symptomatic treatment options devoid side-effect liability and meaningful disease-modifying therapies. Federal and private sources have supported clinical investigations over the past two-plus decades; however, no trial has yet been successful in finding an effective therapy to slow progression of PD, and there is currently just one FDA approved drug to treat the antiparkinsonian side-effect known as levodopa-induced dyskinesia (LID) that impacts approximately 90% of all individuals with PD. In this review, we present biological rationale and experimental evidence on the potential therapeutic role of the L-type voltage-gated Cav1.3 calcium (Ca2+) channels in two distinct brain regions, with two distinct mechanisms of action, in impacting the lives of individuals with PD. Our primary emphasis will be on the role of Cav1.3 channels in the striatum and the compelling evidence of their involvement in LID side-effect liability. We also briefly discuss the role of these same Ca2+ channels in the SNpc and the longstanding interest in Cav1.3 in this brain region in halting or delaying progression of PD.

Pathway-Specific Remodeling of Thalamostriatal Synapses in a Mouse Model of Parkinson's Disease

BACKGROUND

The network pathophysiology underlying the motor symptoms of Parkinson's disease (PD) is poorly understood. In models of late-stage PD, there is significant cell-specific remodeling of corticostriatal, axospinous glutamatergic synapses on principal spiny projection neurons (SPNs). Neurons in the centrolateral nucleus (CLN) of the thalamus that relay cerebellar activity to the striatum also make axospinous synapses on SPNs, but the extent to which they are affected in PD has not been definitively characterized.

OBJECTIVE

To fill this gap, transgenic mice in which CLN neurons express Cre recombinase were used in conjunction with optogenetic and circuit mapping approaches to determine changes in the CLN projection to SPNs in a unilateral 6-hydroxydopamine (6-OHDA) model of late-stage PD.

METHODS

Adeno-associated virus vectors carrying Cre-dependent opsin expression constructs were stereotaxically injected into the CLN of Grp-KH288 mice in which CLN, but not parafascicular nucleus neurons, expressed Cre recombinase. The properties of this projection to identify direct pathway spiny projection neurons (dSPNs) and indirect pathway spiny projection neurons (iSPNs) were then studied in ex vivo brain slices of the dorsolateral striatum from control and 6-OHDA lesioned mice using anatomic, optogenetic, and electrophysiological approaches.

RESULTS

Optogenetically evoked excitatory synaptic currents in both iSPNs and dSPNs were reduced in lesioned mice; however, the reduction was significantly greater in dSPNs. In iSPNs, the reduction in evoked responses was attributable to synaptic pruning, because synaptic channelrhodopsin assisted circuit mapping (sCRACm) revealed fewer synapses per cell after lesioning. In contrast, sCRACm mapping of CLN inputs to dSPNs failed to detect any change in synapse abundance in lesioned mice. However, the ratio of currents through α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors to those through N-methyl-D-aspartate receptors was significantly reduced in dSPNs. Moreover, the distribution of currents evoked by optical stimulation of individual synapses shifted toward smaller amplitudes by lesioning, suggesting that they had undergone long-term depression.

CONCLUSIONS

Taken together, our results demonstrate that the CLN projection to the striatum undergoes a pathway-specific remodeling that could contribute to the circuit imbalance thought to drive the hypokinetic features of PD. © 2022 The Authors. Movement Disorders published by Wiley Periodicals LLC on behalf of International Parkinson and Movement Disorder Society.

Striatal synaptic adaptations in Parkinson's disease

The striatum is densely innervated by mesencephalic dopaminergic neurons that modulate acquisition and vigor of goal-directed actions and habits. This innervation is progressively lost in Parkinson's disease (PD), contributing to the defining movement deficits of the disease. Although boosting dopaminergic signaling with levodopa early in the course of the disease alleviates these deficits, later this strategy leads to the emergence of debilitating dyskinesia. Here, recent advances in our understanding of how striatal cells and circuits adapt to this progressive de-innervation and to levodopa therapy are discussed. First, we discuss how dopamine (DA) depletion triggers cell type-specific, homeostatic changes in spiny projection neurons (SPNs) that tend to normalize striatal activity but also lead to disruption of the synaptic architecture sculpted by experience. Second, we discuss the roles played by cholinergic and nitric oxide-releasing interneurons in these adaptations. Third, we examine recent work in freely moving mice suggesting that alterations in the spatiotemporal dynamics of striatal ensembles contributes to PD movement deficits. Lastly, we discuss recently published evidence from a progressive model of PD suggesting that contrary to the classical model, striatal pathway imbalance is necessary but not sufficient to produce frank parkinsonism.

Identification of antiparkinsonian drugs in the 6-hydroxydopamine zebrafish model

Parkinson's disease (PD) is known as a movement disorder due to characteristic motor features. Existing therapies for PD are only symptomatic, and their efficacy decreases as disease progresses. Zebrafish, a vertebrate in which parkinsonism has been modelled, offers unique features for the identification of molecules with antiparkinsonian properties. Here, we developed a screening assay for the selection of neuroactive agents with antiparkinsonian potential. First, we performed a pharmacological validation of the phenotypes exhibited by the 6-hydroxydopamine zebrafish model, by testing the effects of known antiparkinsonian agents. These drugs were also tested for disease-modifying properties by whole mount immunohistochemistry to TH⁺ neurons and confocal microscopy in the dopaminergic diencephalic cluster of zebrafish. Next, we optimized a phenotypic screening using the 6-hydroxydopamine zebrafish model and tested 1600 FDA-approved bioactive drugs. We found that 6-hydroxydopamine-lesioned zebrafish larvae exhibit bradykinetic and dyskinetic-like behaviours that are rescued by the administration of levodopa, rasagiline, isradipine or amantadine. The rescue of dopaminergic cell loss by isradipine was also verified, through the observation of a higher number of TH⁺ neurons in 6-OHDA-lesioned zebrafish larvae treated with this compound as compared to untreated lesioned larvae. The phenotypic screening enabled us to identify several compounds previously positioned for PD, as well as, new molecules with potential antiparkinsonian properties. Among these, we selected stavudine, tapentadol and nabumetone as the most promising candidates. Our results demonstrate the functional similarities of the motor impairments exhibited by 6-hydroxydopamine-lesioned zebrafish with mammalian models of PD and with PD patients, and highlights novel molecules with antiparkinsonian potential.

Genetic silencing of striatal CaV1.3 prevents and ameliorates levodopa dyskinesia

BACKGROUND

Levodopa-induced dyskinesias are an often debilitating side effect of levodopa therapy in Parkinson's disease. Although up to 90% of individuals with PD develop this side effect, uniformly effective and well-tolerated antidyskinetic treatment remains a significant unmet need. The pathognomonic loss of striatal dopamine in PD results in dysregulation and disinhibition of striatal CaV1.3 calcium channels, leading to synaptopathology that appears to be involved in levodopa-induced dyskinesias. Although there are clinically available drugs that can inhibit CaV1.3 channels, they are not adequately potent and have only partial and transient impact on levodopa-induced dyskinesias.

METHODS

To provide unequivocal target validation, free of pharmacological limitations, we developed a CaV1.3 shRNA to provide high-potency, target-selective, mRNA-level silencing of striatal CaV1.3 channels and examined its ability to impact levodopa-induced dyskinesias in severely parkinsonian rats.

RESULTS

We demonstrate that vector-mediated silencing of striatal CaV1.3 expression in severely parkinsonian rats prior to the introduction of levodopa can uniformly and completely prevent induction of levodopa-induced dyskinesias, and this antidyskinetic benefit persists long term and with high-dose levodopa. In addition, this approach is capable of ameliorating preexisting severe levodopa-induced dyskinesias. Importantly, motoric responses to low-dose levodopa remained intact in the presence of striatal CaV1.3 silencing, indicating preservation of levodopa benefit without dyskinesia liability.

DISCUSSION

The current data provide some of the most profound antidyskinetic benefit reported to date and suggest that genetic silencing of striatal CaV1.3 channels has the potential to transform treatment of individuals with PD by allowing maintenance of motor benefit of levodopa in the absence of the debilitating levodopa-induced dyskinesia side effect. © 2019 The Authors. Movement Disorders published by Wiley Periodicals, Inc. on behalf of International Parkinson and Movement Disorder Society.

New Therapeutics to Modulate Mitochondrial Function in Neurodegenerative Disorders

BACKGROUND

Mitochondrial function and energy metabolism are impaired in neurodegenerative diseases. There is evidence for these functional declines both within the brain and systemically in Alzheimer's disease, Parkinson's disease, and Amyotrophic Lateral Sclerosis. Due to these observations, therapeutics targeted to alter mitochondrial function and energy pathways are increasingly studied in pre-clinical and clinical settings.

METHODS

The goal of this article was to review therapies with specific implications on mitochondrial energy metabolism published through May 2016 that have been tested for treatment of neurodegenerative diseases.

RESULTS

We discuss implications for mitochondrial dysfunction in neurodegenerative diseases and how this drives new therapeutic initiatives.

CONCLUSION

Thus far, treatments have achieved varying degrees of success. Further investigation into the mechanisms driving mitochondrial dysfunction and bioenergetic failure in neurodegenerative diseases is warranted.

Inhaling xenon ameliorates l-dopa-induced dyskinesia in experimental parkinsonism

Parkinson's disease motor symptoms are treated with levodopa, but long-term treatment leads to disabling dyskinesia. Altered synaptic transmission and maladaptive plasticity of corticostriatal glutamatergic projections play a critical role in the pathophysiology of dyskinesia. Because the noble gas xenon inhibits excitatory glutamatergic signaling, primarily through allosteric antagonism of the N-methyl-d-aspartate receptors, we aimed to test its putative antidyskinetic capabilities. We first studied the direct effect of xenon gas exposure on corticostriatal plasticity in a murine model of levodopa-induced dyskinesia We then studied the impact of xenon inhalation on behavioral dyskinetic manifestations in the gold-standard rat and primate models of PD and levodopa-induced dyskinesia. Last, we studied the effect of xenon inhalation on axial gait and posture deficits in a primate model of PD with levodopa-induced dyskinesia. This study shows that xenon gas exposure (1) normalized synaptic transmission and reversed maladaptive plasticity of corticostriatal glutamatergic projections associated with levodopa-induced dyskinesia, (2) ameliorated dyskinesia in rat and nonhuman primate models of PD and dyskinesia, and (3) improved gait performance in a nonhuman primate model of PD. These results pave the way for clinical testing of this unconventional but safe approach. © 2018 The Authors. Movement Disorders published by Wiley Periodicals, Inc. on behalf of International Parkinson and Movement Disorder Society.

Synaptic plasticity and levodopa-induced dyskinesia: electrophysiological and structural abnormalities

Parkinson's disease (PD) is a neurodegenerative disorder characterized by progressive degeneration of dopaminergic neurons located in the midbrain. The gold-standard therapy for PD is the restoration of dopamine (DA) levels through the chronic administration of the DA precursor levodopa (L-DOPA). Although levodopa therapy is the main therapeutic approach for PD, its use is limited by the development of very disabling dyskinetic movements, mainly due to the fluctuation of DA cerebral content. Experimental animal models of PD identified in DA D1/ERK-signaling pathway aberrant activation, occurring in striatal projection neurons, coupled with structural spines abnormalities, the molecular and neuronal basis of L-DOPA-induced dyskinesia (LIDs) occurrence. Different electrophysiological approaches allowed the identification of  the alteration of homeostatic structural and synaptic changes, the neuronal bases of LIDs either in vivo in parkinsonian patients or in vitro in experimental animals. Here, we report the most recent studies showing electrophysiological and morphological evidence of aberrant synaptic plasticity in parkinsonian patients during LIDs in different basal ganglia nuclei and also in cortical transmission, accounting for the complexity of the synaptic changes during dyskinesias. All together, these studies suggest that LIDs are associated with a loss of homeostatic synaptic mechanisms.

Neuroprotection and neurorestoration as experimental therapeutics for Parkinson's disease

Disease-modifying treatments remain an unmet medical need in Parkinson's disease (PD). Such treatments can be operationally defined as interventions that slow down the clinical evolution to advanced disease milestones. A treatment may achieve this outcome by either inhibiting primary neurodegenerative events ("neuroprotection") or boosting compensatory and regenerative mechanisms in the brain ("neurorestoration"). Here we review experimental paradigms that are currently used to assess the neuroprotective and neurorestorative potential of candidate treatments in animal models of PD. We review some key molecular mediators of neuroprotection and neurorestoration in the nigrostriatal dopamine pathway that are likely to exert beneficial effects on multiple neural systems affected in PD. We further review past and current strategies to therapeutically stimulate these mediators, and discuss the preclinical evidence that exercise training can have neuroprotective and neurorestorative effects. A future translational task will be to combine behavioral and pharmacological interventions to exploit endogenous mechanisms of neuroprotection and neurorestoration for therapeutic purposes. This type of approach is likely to provide benefit to many PD patients, despite the clinical, etiological, and genetic heterogeneity of the disease.

Levodopa treatment and dendritic spine pathology

Parkinson's disease (PD) is a neurodegenerative disorder associated with the progressive loss of nigrostriatal dopaminergic neurons. Levodopa is the most effective treatment for the motor symptoms of PD. However, chronic oral levodopa treatment can lead to various motor and nonmotor complications because of nonphysiological pulsatile dopaminergic stimulation in the brain. Examinations of autopsy cases with PD have revealed a decreased number of dendritic spines of striatal neurons. Animal models of PD have revealed altered density and morphology of dendritic spines of neurons in various brain regions after dopaminergic denervation or dopaminergic denervation plus levodopa treatment, indicating altered synaptic transmission. Recent studies using rodent models have reported dendritic spine head enlargement in the caudate‐putamen, nucleus accumbens, primary motor cortex, and prefrontal cortex in cases where chronic levodopa treatment following dopaminergic denervation induced dyskinesia‐like abnormal involuntary movement. Hypertrophy of spines results from insertion of alpha‐amino‐2,3‐dihydro‐5‐methyl‐3‐oxo‐4‐isoxazolepropanoic acid receptors into the postsynaptic membrane. Such spine enlargement indicates hypersensitivity of the synapse to excitatory inputs and is compatible with a lack of depotentiation, which is an electrophysiological hallmark of levodopa‐induced dyskinesia found in the corticostriatal synapses of dyskinetic animals and the motor cortex of dyskinetic PD patients. This synaptic plasticity may be one of the mechanisms underlying the priming of levodopa‐induced complications such as levodopa‐induced dyskinesia and dopamine dysregulation syndrome. Drugs that could potentially prevent spine enlargement, such as calcium channel blockers, N‐methyl‐D‐aspartate receptor antagonists, alpha‐amino‐2,3‐dihydro‐5‐methyl‐3‐oxo‐4‐isoxazolepropanoic acid receptor antagonists, and metabotropic glutamate receptor antagonists, are candidates for treatment of levodopa‐induced complications in PD. © 2017 The Authors. Movement Disorders published by Wiley Periodicals, Inc. on behalf of International Parkinson and Movement Disorder Society.

Changes in kynurenine pathway metabolism in Parkinson patients with L-DOPA-induced dyskinesia

L-3,4-Dihydroxyphenylalanine (L-DOPA) is the most effective drug in the symptomatic treatment of Parkinson's disease, but chronic use is associated with L-DOPA-induced dyskinesia in more than half the patients after 10 years of treatment. L-DOPA treatment may affect tryptophan metabolism via the kynurenine pathway. Altered levels of kynurenine metabolites can affect glutamatergic transmission and may play a role in the development of L-DOPA-induced dyskinesia. In this study, we assessed kynurenine metabolites in plasma and cerebrospinal fluid of Parkinson's disease patients and controls. Parkinson patients (n = 26) were clinically assessed for severity of motor symptoms (UPDRS) and L-DOPA-induced dyskinesia (UDysRS). Plasma and cerebrospinal fluid samples were collected after overnight fasting and 1-2 h after intake of L-DOPA or other anti-Parkinson medication. Metabolites were analyzed in plasma and cerebrospinal fluid of controls (n = 14), Parkinson patients receiving no L-DOPA (n = 8), patients treated with L-DOPA without dyskinesia (n = 8), and patients with L-DOPA-induced dyskinesia (n = 10) using liquid chromatography-mass spectrometry. We observed approximately fourfold increase in the 3-hydroxykynurenine/kynurenic acid ratio in plasma of Parkinson's patients with L-DOPA-induced dyskinesia. Anthranilic acid levels were decreased in plasma and cerebrospinal fluid of this patient group. 5-Hydroxytryptophan levels were twofold increased in all L-DOPA-treated Parkinson's patients. We conclude that a higher 3-hydroxykynurenine/kynurenic acid ratio in plasma may serve as a biomarker for L-DOPA-induced dyskinesia. Longitudinal studies including larger patients cohorts are needed to verify whether the changes observed here may serve as a prognostic marker for L-DOPA-induced dyskinesia.

Loss and remodeling of striatal dendritic spines in Parkinson's disease: from homeostasis to maladaptive plasticity?

In Parkinson's disease (PD) patients and animal models of PD, the progressive degeneration of the nigrostriatal dopamine (DA) projection leads to two major changes in the morphology of striatal projection neurons (SPNs), i.e., a profound loss of dendritic spines and the remodeling of axospinous glutamatergic synapses. Striatal spine loss is an early event tightly associated with the extent of striatal DA denervation, but not the severity of parkinsonian motor symptoms, suggesting that striatal spine pruning might be a form of homeostatic plasticity that compensates for the loss of striatal DA innervation and the resulting dysregulation of corticostriatal glutamatergic transmission. On the other hand, the remodeling of axospinous corticostriatal and thalamostriatal glutamatergic synapses might represent a form of late maladaptive plasticity that underlies changes in the strength and plastic properties of these afferents and the resulting increased firing and bursting activity of striatal SPNs in the parkinsonian state. There is also evidence that these abnormal synaptic connections might contribute to the pathophysiology of L-DOPA-induced dyskinesia. Despite the significant advances made in this field over the last thirty years, many controversial issues remain about the striatal SPN subtypes affected, the role of spine changes in the altered activity of SPNs in the parkinsonisn state, and the importance of striatal spine plasticity in the pathophysiology of L-DOPA-induced dyskinesia. In this review, we will examine the current state of knowledge of these issues, discuss the limitations of the animal models used to address some of these questions, and assess the relevance of data from animal models to the human-diseased condition.

Intrastriatal injection of ionomycin profoundly changes motor response to l-DOPA and its underlying molecular mechanisms

Long-term l-DOPA treatment of Parkinson's disease is accompanied with fluctuations of motor responses and l-DOPA-induced dyskinesia (LID). Phosphorylation of the dopamine and c-AMP regulated phosphoprotein of 32kDa (DARPP-32) plays a role in the pathogenesis of LID, and thus dephosphorylation of this protein by activated calcineurin may help reduce LID. One important activator of calcineurin is the Ca²⁺ ionophore ionomycin. Here, we investigated whether intrastriatal injection of ionomycin to hemiparkinsonian rats produced changes in l-DOPA responses including LID. We also analyzed the effects of ionomycin on key molecular mediators of LID. Results confirmed our hypothesis that ionomycin could downregulate the phosphorylation of DARPP32 at Thr-34 and reduce LID. Besides, ionomycin decreased two established molecular markers of LID, FosB/ΔFosB and phosphorylated ERK1/2. Ionomycin also decreased the phosphorylation of three main subunits of the NMDA receptor, NR1 phosphorylated at ser896, NR2A phosphorylated at Tyr-1325, and NR2B phosphorylated at Tyr-1472. Furthermore, the anti-LID effect of striatally injected ionomycin was not accompanied by reduction of the antiparkinsonian action of l-DOPA. These data indicate that ionomycin largely interacts with striatal mechanisms that are critical to the l-DOPA motor response highlighting the role of protein dephosphorylation by calcineurin.

Regulation of Postsynaptic Stability by the L-type Calcium Channel CaV1.3 and its Interaction with PDZ Proteins

Alterations in dendritic spine morphology and postsynaptic structure are a hallmark of neurological disorders. Particularly spine pruning of striatal medium spiny neurons and aberrant rewiring of corticostriatal synapses have been associated with the pathology of Parkinson’s disease and L-DOPA induced dyskinesia, respectively. Owing to its low activation threshold the neuronal L-type calcium channel Ca_V1.3 is particularly critical in the control of neuronal excitability and thus in the calcium-dependent regulation of neuronal functions. Ca_V1.3 channels are located in dendritic spines and contain a C-terminal class 1 PDZ domain-binding sequence. Until today the postsynaptic PDZ domain proteins shank, densin-180, and erbin have been shown to interact with Ca_V1.3 channels and to modulate their current properties. Interestingly experimental evidence suggests an involvement of all three PDZ proteins as well as Ca_V1.3 itself in regulating dendritic and postsynaptic morphology. Here we briefly review the importance of Ca_V1.3 and its proposed interactions with PDZ proteins for the stability of dendritic spines. With a special focus on the pathology associated with Parkinson’s disease, we discuss the hypothesis that Ca_V1.3 L-type calcium channels may be critical modulators of dendritic spine stability.

Inhibiting Lateral Habenula Improves L-DOPA-Induced Dyskinesia

BACKGROUND

A systematic search of brain nuclei putatively involved in L-3,4-dihydroxyphenylalanine (L-DOPA)-induced dyskinesia (LID) in Parkinson's disease shed light, notably, upon the lateral habenula (LHb), which displayed an overexpression of the ∆FosB, ARC, and Zif268 immediate-early genes only in rats experiencing abnormal involuntary movements (AIMs). We thus hypothesized that LHb might play a role in LID.

METHODS

∆FosB immunoreactivity, 2-deoxyglucose uptake, and firing activity of LHb were studied in experimental models of Parkinson's disease and LID. ΔFosB-expressing LHb neurons were then targeted using the Daun02-inactivation method. A total of 18 monkeys and 55 rats were used.

RESULTS

LHb was found to be metabolically modified in dyskinetic monkeys and its neuronal firing frequency significantly increased in ON L-DOPA dyskinetic 6-hydroxydopamine-lesioned rats, suggesting that increased LHb neuronal activity in response to L-DOPA is related to AIM manifestation. Therefore, to mechanistically test if LHb neuronal activity might affect AIM severity, following induction of AIMs, 6-hydroxydopamine rats were injected with Daun02 in the LHb previously transfected with ß-galactosidase under control of the FosB promoter. Three days after Daun02 administration, animals were tested daily with L-DOPA to assess LID and L-DOPA-induced rotations. Inactivation of ∆FosB-expressing neurons significantly reduced AIM severity and also increased rotations. Interestingly, the dopaminergic D1 receptor was overexpressed only on the lesioned side of dyskinetic rats in LHb and co-localized with ΔFosB, suggesting a D1 receptor-mediated mechanism supporting the LHb involvement in AIMs.

CONCLUSIONS

This study highlights the role of LHb in LID, offering a new target to innovative treatments of LID.

Striatal Plasticity in L-DOPA- and Graft-Induced Dyskinesia; The Common Link?

One of the major symptoms of the neurodegenerative condition Parkinson's disease (PD) is a slowness or loss of voluntary movement, yet frustratingly therapeutic strategies designed to restore movement can result in the development of excessive abnormal movements known as dyskinesia. These dyskinesias commonly develop as a result of pharmacotherapy in the form of L-DOPA administration, but have also been identified following deep brain stimulation (DBS) and intrastriatal cell transplantation. In the case of L-DOPA these movements can be treatment limiting, and whilst they are not long lasting or troubling following DBS, recognition of their development had a near devastating effect on the field of cell transplantation for PD.Understanding the relationship between these therapeutic approaches and the development of dyskinesia may improve our ability to restore function without disabling side effects. Interestingly, despite the fact that dopaminergic cell transplantation repairs many of the changes induced by the disease process and through L-DOPA treatment, there appears to be a relationship between the two. In rodent models of the disease, the severity of dyskinesia induced by L-DOPA prior to the transplantation procedure correlated with post-transplantation, graft-induced dyskinesia. A review of clinical data also suggested that the worse preoperational dyskinesia causes worsened graft-induced dyskinesia (GID). Understanding how these aberrant behaviors come about has been of keen interest to open up these therapeutic options more widely and one major underlying theory is the effects of these approaches on the plasticity of synapses within the basal ganglia. This review uniquely brings together developments in understanding the role of striatal synaptic plasticity in both L-DOPA and GID to guide and stimulate further investigations on the important striatal plasticity.

Targeting β-arrestin2 in the treatment of L-DOPA-induced dyskinesia in Parkinson's disease

Parkinson's disease (PD) is characterized by severe locomotor deficits and is commonly treated with the dopamine (DA) precursor l-3,4-dihydroxyphenylalanine (L-DOPA), but its prolonged use causes dyskinesias referred to as L-DOPA-induced dyskinesias (LIDs). Recent studies in animal models of PD have suggested that dyskinesias are associated with the overactivation of G protein-mediated signaling through DA receptors. β-Arrestins desensitize G protein signaling at DA receptors (D1R and D2R) in addition to activating their own G protein-independent signaling events, which have been shown to mediate locomotion. Therefore, targeting β-arrestins in PD L-DOPA therapy might prove to be a desirable approach. Here we show in a bilateral DA-depletion mouse model of Parkinson's symptoms that genetic deletion of β-arrestin2 significantly limits the beneficial locomotor effects while markedly enhancing the dyskinesia-like effects of acute or chronic L-DOPA treatment. Viral rescue or overexpression of β-arrestin2 in knockout or control mice either reverses or protects against LIDs and its key biochemical markers. In other more conventional animal models of DA neuron loss and PD, such as 6-hydroxydopamine-treated mice or rats and 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-treated nonhuman primates, β-arrestin2 overexpression significantly reduced dyskinesias while maintaining the therapeutic effect of L-DOPA. Considerable efforts are being spent in the pharmaceutical industry to identify therapeutic approaches to block LIDs in patients with PD. Our results point to a potential therapeutic approach, whereby development of either a genetic or pharmacological intervention to enhance β-arrestin2- or limit G protein-dependent D1/D2R signaling could represent a more mechanistically informed strategy.

Zooming in on the small: the plasticity of striatal dendritic spines in L-DOPA-induced dyskinesia

The spiny dendrites of striatal projection neurons integrate synaptic inputs of different origins to regulate movement. It has long been known that these dendrites lose spines and display atrophic features in Parkinson's disease (PD), but the significance of these morphological changes has remained unknown. Some recent studies reveal a remarkable structural plasticity of striatal spines in parkinsonian rodents treated with L-3,4-dihydroxyphenylalanine (L-DOPA), and they demonstrate an association between this plasticity and the development of dyskinesia. These studies used different approaches and animal models, which possibly explains why they emphasize different plastic changes as being most closely linked to dyskinesia (such as a growth of new spines in neurons of the indirect pathway, or a loss of spines in neurons of the direct pathway, or the appearance of spines with aberrant synaptic features). Clearly, further investigations are required to reconcile these intriguing findings and integrate them in a coherent pathophysiological model. Nevertheless, these studies may mark the beginning of a new era for dyskinesia research. In addition to addressing neurochemical and molecular events that trigger involuntary movements, there is a need to better understand the long-lasting structural reorganization of cells and circuits that maintain the brain in a "dyskinesia-prone" state. This may lead to the identification of new efficacious approaches to prevent the complications of dopaminergic therapies in PD.

Drug therapy for Parkinson’s disease: An update

Parkinson’s disease (PD) is the most common neurodegenerative movement disorder, affecting about 1% of the population above the age of 65. PD is characterized by a selective degeneration of the dopaminergic neurons of the substantia nigra pars compacta. This results in a marked loss of striatal dopamine and the development of the characteristic features of the disease, i.e., bradykinesia, rest tremor, rigidity, gait abnormalities and postural instability. Other types of neurons/neurotransmitters are also involved in PD, including cholinergic, serotonergic, glutamatergic, adenosine, and GABAergic neurotransmission which might have relevance to the motor, non-motor, neuropsychiatric and cognitive disturbances that occur in the course of the disease. The treatment of PD relies on replacement therapy with levodopa (L-dopa), the precursor of dopamine, in combination with a peripheral decarboxylase inhibitor (carbidopa or benserazide). The effect of L-dopa, however, declines over time together with the development of motor complications especially dyskinesia in a significant proportion of patients within 5 years of therapy. Other drugs include dopamine-receptor-agonists, catechol-O-methyltransferase inhibitors, monoamine oxidase type B (MAO-B) inhibitors, anticholinergics and adjuvant therapy with the antiviral drug and the N-methyl-D-aspartate glutamate receptor antagonist amantadine. Although, these medications can result in substantial improvements in parkinsonian symptoms, especially during the early stages of the disease, they are often not successful in advanced disease. Moreover, dopaminergic cell death continues over time, emphasizing the need for neuroprotective or neuroregenerative therapies. In recent years, research has focused on non-dopaminergic approach such as the use of A_2A receptor antagonists: istradefylline and preladenant or the calcium channel antagonist isradipine. Safinamide is a selective and reversible inhibitor of MAO-B, a glutamate receptor inhibitor as well as sodium and calcium channel blocker. Minocycline and pioglitazone are other agents which have been shown to prevent dopaminergic nigral cell loss in animal models of PD. There is also an evidence to suggest a benefit from iron chelation therapy with deferiprone and from the use of antioxidants or mitochondrial function enhancers such as creatine, alpha-lipoic acid, l-carnitine, and coenzyme Q10.

Activation of DREAM (downstream regulatory element antagonistic modulator), a calcium-binding protein, reduces L-DOPA-induced dyskinesias in mice

BACKGROUND

Previous studies have implicated the cyclic adenosine monophosphate/protein kinase A pathway as well as FosB and dynorphin-B expression mediated by dopamine D1 receptor stimulation in the development of 3,4-dihydroxyphenyl-L-alanine (L-DOPA)-induced dyskinesia. The magnitude of these molecular changes correlates with the intensity of dyskinesias. The calcium-binding protein downstream regulatory element antagonistic modulator (DREAM) binds to regulatory element sites called DRE in the DNA and represses transcription of target genes such as c-fos, fos-related antigen-2 (fra-2), and prodynorphin. This repression is released by calcium and protein kinase A activation. Dominant-active DREAM transgenic mice (daDREAM) and DREAM knockout mice (DREAM(-/-)) were used to define the involvement of DREAM in dyskinesias.

METHODS

Dyskinesias were evaluated twice a week in mice with 6-hydroxydopamine lesions during long-term L-DOPA (25 mg/kg) treatment. The impact of DREAM on L-DOPA efficacy was evaluated using the rotarod and the cylinder test after the establishment of dyskinesia and the molecular changes by immunohistochemistry and Western blot.

RESULTS

In daDREAM mice, L-DOPA-induced dyskinesia was decreased throughout the entire treatment. In correlation with these behavioral results, daDREAM mice showed a decrease in FosB, phosphoacetylated histone H3, dynorphin-B, and phosphorylated glutamate receptor subunit, type 1 expression. Conversely, genetic inactivation of DREAM potentiated the intensity of dyskinesia, and DREAM(-/-) mice exhibited an increase in expression of molecular markers associated with dyskinesias. The DREAM modifications did not affect the kinetic profile or antiparkinsonian efficacy of L-DOPA therapy.

CONCLUSIONS

The protein DREAM decreases development of L-DOPA-induced dyskinesia in mice and reduces L-DOPA-induced expression of FosB, phosphoacetylated histone H3, and dynorphin-B in the striatum. These data suggest that therapeutic approaches that activate DREAM may be useful to alleviate L-DOPA-induced dyskinesia without interfering with the therapeutic motor effects of L-DOPA.

Astrocytosis in parkinsonism: considering tripartite striatal synapses in physiopathology?

The current concept of basal ganglia organization and function in physiological and pathophysiological conditions excludes the most numerous cells in the brain, i.e., the astrocytes, present with a ratio of 10:1 neuron. Their role in neurodegenerative condition such as Parkinson's disease (PD) remains to be elucidated. Before embarking into physiological investigations of the yet-to-be-identified "tripartite" synapses in the basal ganglia in general and the striatum in particular, we therefore characterized anatomically the PD-related modifications in astrocytic morphology, the changes in astrocytic network connections and the consequences on the spatial relationship between astrocytic processes and asymmetric synapses in normal and PD-like conditions in experimental and human PD. Our results unravel a dramatic regulation of striatal astrocytosis supporting the hypothesis of a key role in (dys) regulating corticostriatal transmission. Astrocytes and their various properties might thus represent a therapeutic target in PD.

RGS4 is involved in the generation of abnormal involuntary movements in the unilateral 6-OHDA-lesioned rat model of Parkinson's disease

Regulators of G-protein signalling (RGS) proteins are implicated in striatal G-protein coupled receptor (GPCR) sensitisation in the pathophysiology of l-DOPA-induced abnormal involuntary movements (AIMs), also known as dyskinesia (LID), in Parkinson's disease (PD). In this study, we investigated RGS protein subtype 4 in the expression of AIMs in the unilateral 6-hydroxydopamine (6-OHDA)-lesioned rat model of LID. The effects of RGS4 antisense brain infusion on the behavioural and molecular correlates of l-DOPA priming in 6-OHDA-lesioned rats were assessed. In situ hybridisation revealed that repeated l-DOPA/benserazide treatment caused an elevation of RGS4 mRNA levels in the striatum, predominantly in the lateral regions. The increased expression of RGS4 mRNA in the rostral striatum was found to positively correlate with the behavioural (AIM scores) and molecular (pre-proenkephalin B, PPE-B expression) markers of LID. We found that suppressing the elevation of RGS4 mRNA in the striatum by continuous infusion of RGS4 antisense oligonucleotides, via implanted osmotic mini-pumps, during l-DOPA priming, reduced the induction of AIMs. Moreover, ex vivo analyses of the rostral dorsolateral striatum showed that RGS4 antisense infusion attenuated l-DOPA-induced elevations of PPE-B mRNA and dopamine-stimulated [(35)S]GTPγS binding, a marker used for measuring dopamine receptor super-sensitivity. Taken together, these data suggest that (i) RGS4 proteins play an important pathophysiological role in the development and expression of LID and (ii) suppressing the elevation of RGS4 mRNA levels in l-DOPA priming attenuates the associated pathological changes in LID, dampening its physiological expression. Thus, modulating RGS4 proteins could prove beneficial in the treatment of dyskinesia in PD.

Advances in non-dopaminergic treatments for Parkinson's disease

Since the 1960's treatments for Parkinson's disease (PD) have traditionally been directed to restore or replace dopamine, with L-Dopa being the gold standard. However, chronic L-Dopa use is associated with debilitating dyskinesias, limiting its effectiveness. This has resulted in extensive efforts to develop new therapies that work in ways other than restoring or replacing dopamine. Here we describe newly emerging non-dopaminergic therapeutic strategies for PD, including drugs targeting adenosine, glutamate, adrenergic, and serotonin receptors, as well as GLP-1 agonists, calcium channel blockers, iron chelators, anti-inflammatories, neurotrophic factors, and gene therapies. We provide a detailed account of their success in animal models and their translation to human clinical trials. We then consider how advances in understanding the mechanisms of PD, genetics, the possibility that PD may consist of multiple disease states, understanding of the etiology of PD in non-dopaminergic regions as well as advances in clinical trial design will be essential for ongoing advances. We conclude that despite the challenges ahead, patients have much cause for optimism that novel therapeutics that offer better disease management and/or which slow disease progression are inevitable.

L-type Ca2+ channels in heart and brain

L-type calcium channels (Cav1) represent one of the three major classes (Cav1–3) of voltage-gated calcium channels. They were identified as the target of clinically used calcium channel blockers (CCBs; so-called calcium antagonists) and were the first class accessible to biochemical characterization. Four of the 10 known α1 subunits (Cav1.1–Cav1.4) form the pore of L-type calcium channels (LTCCs) and contain the high-affinity drug-binding sites for dihydropyridines and other chemical classes of organic CCBs. In essentially all electrically excitable cells one or more of these LTCC isoforms is expressed, and therefore it is not surprising that many body functions including muscle, brain, endocrine, and sensory function depend on proper LTCC activity. Gene knockouts and inherited human diseases have allowed detailed insight into the physiological and pathophysiological role of these channels. Genome-wide association studies and analysis of human genomes are currently providing even more hints that even small changes of channel expression or activity may be associated with disease, such as psychiatric disease or cardiac arrhythmias. Therefore, it is important to understand the structure–function relationship of LTCC isoforms, their differential contribution to physiological function, as well as their fine-tuning by modulatory cellular processes.

Striatal shape in Parkinson's disease

Parkinson's disease (PD) is marked pathologically by nigrostriatal dopaminergic terminal loss. Histopathological and in vivo labeling studies demonstrate that this loss occurs most extensively in the caudal putamen and caudate head. Previous structural studies have suggested reduced striatal volume and atrophy of the caudate head in PD subjects. The spatial distribution of atrophy in the putamen, however, has not been characterized. We aimed to delineate the specific locations of atrophy in both of these striatal structures. T1- and T2-weighted brain MR (3T) images were obtained from 40 PD and 40 control subjects having no dementia and similar age and gender distributions. Shape analysis was performed using doubly segmented regions of interest. Compared to controls, PD subjects had lower putamen (p = 0.0003) and caudate (p = 0.0003) volumes. Surface contraction magnitudes were greatest on the caudal putamen (p ≤ 0.005) and head and dorsal body of the caudate (p ≤ 0.005). This spatial distribution of striatal atrophy is consistent with the known pattern of dopamine depletion in PD and may reflect global consequences of known cellular remodeling phenomena.

The changing landscape of voltage-gated calcium channels in neurovascular disorders and in neurodegenerative diseases

It is a common belief that voltage-gated calcium channels (VGCC) cannot carry toxic amounts of Ca²⁺ in neurons. Also, some of them as L-type channels are essential for Ca²⁺-dependent regulation of prosurvival gene-programs. However, a wealth of data show a beneficial effect of drugs acting on VGCCs in several neurodegenerative and neurovascular diseases. In the present review, we explore several mechanisms by which the “harmless” VGCCs may become “toxic” for neurons. These mechanisms could explain how, though usually required for neuronal survival, VGCCs may take part in neurodegeneration. We will present evidence showing that VGCCs can carry toxic Ca²⁺ when: a) their density or activity increases because of aging, chronic hypoxia or exposure to β-amyloid peptides or b) Ca²⁺-dependent action potentials carry high Ca²⁺ loads in pacemaker neurons. Besides, we will examine conditions in which VGCCs promote neuronal cell death without carrying excess Ca²⁺. This can happen, for instance, when they carry metal ions into the neuronal cytoplasm or when a pathological decrease in their activity weakens Ca²⁺-dependent prosurvival gene programs. Finally, we will explore the role of VGCCs in the control of nonneuronal cells that take part to neurodegeneration like those of the neurovascular unit or of microglia.

Immediate-early gene expression in structures outside the basal ganglia is associated to l-DOPA-induced dyskinesia

Long-term l-3,4-dihydroxyphenylalanine (l-DOPA) treatment in Parkinson's disease (PD) leads to l-DOPA-induced dyskinesia (LID), a condition thought to primarily involve the dopamine D1 receptor-expressing striatal medium spiny neurons. Activation of the D1 receptor results in increased expression of several molecular markers, in particular the members of the immediate-early gene (IEG) family, a class of genes rapidly transcribed in response to an external stimulus. However, several dopaminoceptive structures in the brain that are likely to be affected by the exogenously produced DA have received little attention although they might play a key role in mediating those l-DOPA-induced abnormal behaviours. ΔFosB, ARC, FRA2 and Zif268 IEGs expression patterns were thus characterised, using unbiased stereological methods, in the whole brain of dyskinetic and non-dyskinetic rats to identify brain nuclei displaying a transcriptional response specifically related to LID. Within the basal ganglia, the striatum and the substantia nigra pars reticulata showed an increased expression of all four IEGs in dyskinetic compared to non-dyskinetic rats. Outside the basal ganglia, there was a striking increased expression of the four IEGs in the motor cortex, the bed nucleus of the stria terminalis, the dorsal hippocampus, the pontine nuclei, the cuneiform nucleus and the pedunculopontine nuclei. Moreover, the zona incerta and the lateral habenula displayed an overexpression of ΔFosB, ARC and Zif268. Among these structures, the IEG expression in the striatum, the bed nucleus of the stria terminalis, the lateral habenula, the pontine nuclei and the cuneiform nucleus correlate with LID severity. These results illustrate a global transcriptional response to a dyskinetic state in the whole brain suggesting the possible involvement of these structures in LID.

Differential striatal spine pathology in Parkinson's disease and cocaine addiction: a key role of dopamine?

In the striatum, the dendritic tree of the two main populations of projection neurons, called "medium spiny neurons (MSNs)", are covered with spines that receive glutamatergic inputs from the cerebral cortex and thalamus. In Parkinson's disease (PD), striatal MSNs undergo an important loss of dendritic spines, whereas aberrant overgrowth of striatal spines occurs following chronic cocaine exposure. This review examines the possibility that opposite dopamine dysregulation is one of the key factors that underlies these structural changes. In PD, nigrostriatal dopamine degeneration results in a significant loss of dendritic spines in the dorsal striatum, while rodents chronically exposed to cocaine and other psychostimulants, display an increase in the density of "thin and immature" spines in the nucleus accumbens (NAc). In rodent models of PD, there is evidence that D2 dopamine receptor-containing MSNs are preferentially affected, while D1-positive cells are the main targets of increased spine density in models of addiction. However, such specificity remains to be established in primates. Although the link between the extent of striatal spine changes and the behavioral deficits associated with these disorders remains controversial, there is unequivocal evidence that glutamatergic synaptic transmission is significantly altered in both diseased conditions. Recent studies have suggested that opposite calcium-mediated regulation of the transcription factor myocyte enhancer factor 2 (MEF2) function induces these structural defects. In conclusion, there is strong evidence that dopamine is a major, but not the sole, regulator of striatal spine pathology in PD and addiction to psychostimulants. Further studies of the role of glutamate and other genes associated with spine plasticity in mediating these effects are warranted.

Aberrant restoration of spines and their synapses in L-DOPA-induced dyskinesia: involvement of corticostriatal but not thalamostriatal synapses

We examined the structural plasticity of excitatory synapses from corticostriatal and thalamostriatal pathways and their postsynaptic targets in adult Sprague-Dawley rats to understand how these striatal circuits change in l-DOPA-induced dyskinesias (LIDs). We present here detailed electron and light microscopic analyses that provide new insight into the nature of the structural and synaptic remodeling of medium spiny neurons in response to LIDs. Numerous studies have implicated enhanced glutamate signaling and persistent long-term potentiation as central to the behavioral sensitization phenomenon of LIDs. Moreover, experience-dependent alterations in behavior are thought to involve structural modifications, specifically alterations in patterns of synaptic connectivity. Thus, we hypothesized that in the striatum of rats with LIDs, one of two major glutamatergic pathways would form new or altered contacts, especially onto the spines of medium spiny neuron (MSNs). Our data provide compelling evidence for a dramatic rewiring of the striatum of dyskinetic rats and that this rewiring involves corticostriatal but not thalamostriatal contacts onto MSNs. There is a dramatic increase in corticostriatal contacts onto spines and dendrites that appear to be directly linked to dyskinetic behaviors, since they were not seen in the striatum of animals that did not develop dyskinesia. There is also an aberrant increase in spines receiving more than one excitatory contact(i.e., multisynaptic spines) in the dyskinetic animals compared with the 6-hydroxydopamine-treated and control rats. Such alterations could substantially impair the ability of striatal neurons to gate cortically driven signals and contribute to the loss of bidirectional synaptic plasticity.

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.

Methods for treating neurological conditions (WO2011159945)

This patent application claims that inhibition of p21-activated kinases (PAK) reverses, partially reverses or delays clinical signs in neurological conditions (main claim for Huntington's disease (HD), substance abuse and addiction, Parkinson's disease, depression, bipolar disorder, anxiety disorder, posttraumatic stress disorder and neurofibromatosis). Several compounds with a pyrido-[2,3-d]pyrimidine-7(8H)-one core and high affinity to the catalytic domain of PAK1-4 are described in the patent. These PAK inhibitors are hypothesized to exert beneficial effects on clinical symptoms via modulation of dendritic spine morphology and/or synaptic function. Preliminary preclinical data suggest that PAK inhibition may be an interesting approach for the treatment of HD, neurofibromatosis and fragile X syndrome, while data for other neurological conditions are missing. Current limitations call for a comprehensive characterization of the role of PAK dysfunction in neurological disorders before further testing the effect of PAK inhibitors in relevant preclinical models. If ever, it will probably take many years before the most promising compounds will head to the clinic for further assessment in patients with neurological disorders.

Role of vitamin d in Parkinson's disease

Parkinson's disease (PD) is the second most common form of neurodegeneration in the elderly population. Clinically, it is characterized by tremor, rigidity, slowness of movement, and postural imbalance. A significant association between low serum vitamin D and PD has been demonstrated, suggesting that elevated vitamin D levels might provide protection against PD. Genetic studies have helped identify a number of proteins linking vitamin D to PD pathology, including the major histocompatibility complex (MHC) class II, the vitamin D receptor (VDR), cytochrome P450 2D6 (CYP2D6), chromosome 22, the renin-angiotensin system (RAS), heme oxygenase-1 (HO-1), poly(ADP-ribose) polymerase-1 gene (PARP-1), neurotrophic factor (NTF), and Sp1 transcription factor. Vitamin D has also been implicated in PD through its effects on L-type voltage-sensitive calcium channels (L-VSCC), nerve growth factor (NGF), matrix metalloproteinases (MMPs), prostaglandins (PGs) and cyclooxygenase-2 (COX-2), reactive oxygen species (ROS), and nitric oxide synthase (NOS). A growing body of evidence suggests that vitamin D supplementation may be beneficial for PD patients. Among the different forms of vitamin D, calcitriol (1,25-dihydroxyvitamin D₃) is best indicated for PD, because it is a highly active vitamin D₃ metabolite with an appropriate receptor in the central nervous system (CNS).

L-DOPA impairs proteasome activity in parkinsonism through D1 dopamine receptor

Aberrant membrane localization of dopamine D(1) receptor (D1R) is associated with L-DOPA-induced dyskinesia (LID), a major complication of L-DOPA treatment in Parkinson's disease (PD). Since the proteasome plays a central role in modulating neuronal response through regulation of neurotransmitter receptor intraneuronal fate, we hypothesized that the ubiquitine-proteasome proteolytic pathway could be impaired in LID. Those LIDs are actually associated with a striatum-specific decrease in proteasome catalytic activity and accumulation of polyubiquitinated proteins in experimental rodent and monkey parkinsonism. We then demonstrated that such decreased proteasome catalytic activity (1) results from D1R activation and (2) feed-back the D1R abnormal trafficking, i.e., its exaggerated cell surface abundance. We further showed that the genetic invalidation of the E3 ubiquitin-protein ligase parkin PD gene leads to exaggerated abnormal involuntary movements compared with wild-type mice. We thus established in an unprecedented series of experimental models that impairment of the ubiquitine-proteasome system at specific nodes (E3 ligase parkin, polyubiquitination, proteasome catalytic activity) leads to the same phenomenon, i.e., aberrant behavioral response to dopamine replacement therapy in PD, highlighting the intimate interplay between dopamine receptor and proteasome activity in a nondegenerative context.

Thiamine and Parkinson's disease

Parkinson's disease (PD) is the second most common form of neurodegeneration in the elderly population. PD is clinically characterized by tremors, rigidity, slowness of movement and postural imbalance. A significant association has been demonstrated between PD and low levels of thiamine in the serum, which suggests that elevated thiamine levels might provide protection against PD. Genetic studies have helped identify a number of factors that link thiamine to PD pathology, including the DJ-1 gene, excitatory amino acid transporters (EAATs), the α-ketoglutarate dehydrogenase complex (KGDHC), coenzyme Q10 (CoQ10 or ubiquinone), lipoamide dehydrogenase (LAD), chromosome 7, transcription factor p53, the renin-angiotensin system (RAS), heme oxygenase-1 (HO-1), and poly(ADP-ribose) polymerase-1 gene (PARP-1). Thiamine has also been implicated in PD through its effects on L-type voltage-sensitive calcium channels (L-VSCC), matrix metalloproteinases (MMPs), prostaglandins (PGs), cyclooxygenase-2 (COX-2), reactive oxygen species (ROS), and nitric oxide synthase (NOS). Recent studies highlight a possible relationship between thiamine and PD. Genetic studies provide opportunities to determine which proteins may link thiamine to PD pathology. Thiamine can also act through a number of non-genomic mechanisms that include protein expression, oxidative stress, inflammation, and cellular metabolism. Further studies are needed to determine the benefits of using thiamine as a treatment for PD.

Effect of chronic L-dopa or melatonin treatments after dopamine deafferentation in rats: dyskinesia, motor performance, and cytological analysis

The present study examines the ability of melatonin to protect striatal dopaminergic loss induced by 6-OHDA in a rat model of Parkinson's disease, comparing the results with L-DOPA-treated rats. The drugs were administered orally daily for a month, their therapeutic or dyskinetic effects were assessed by means of abnormal involuntary movements (AIMs) and stepping ability. At the cellular level, the response was evaluated using tyrosine hydroxylase immunoreactivity and striatal ultrastructural changes to compare between L-DOPA-induced AIMs and Melatonin-treated rats. Our findings demonstrated that chronic oral administration of Melatonin improved the alterations caused by the neurotoxin 6-OHDA. Melatonin-treated animals perform better in the motor tasks and had no dyskinetic alterations compared to L-DOPA-treated group. At the cellular level, we found that Melatonin-treated rats showed more TH-positive neurons and their striatal ultrastructure was well preserved. Thus, Melatonin is a useful treatment to delay the cellular and behavioral alterations observed in Parkinson's disease.