L-DOPA-induced dyskinesia in a rat model of Parkinson's disease is associated with the fluctuational release of norepinephrine in the sensorimotor striatum

Noradrenergic alterations in Parkinson's disease: a combined 11C-yohimbine PET/neuromelanin MRI study

Degeneration of the noradrenergic system is now considered a pathological hallmark of Parkinson's disease, but little is known about its consequences in terms of parkinsonian manifestations. Here, we evaluated two aspects of the noradrenergic system using multimodal in vivo imaging in patients with Parkinson's disease and healthy controls: the pigmented cell bodies of the locus coeruleus with neuromelanin sensitive MRI; and the density of α2-adrenergic receptors (ARs) with PET using 11C-yohimbine. Thirty patients with Parkinson's disease and 30 age- and sex-matched healthy control subjects were included. The characteristics of the patients' symptoms were assessed using the Movement Disorder Society Unified Parkinson's Disease Rating Scale (MDS-UPDRS). Patients showed reduced neuromelanin signal intensity in the locus coeruleus compared with controls and diminished 11C-yohimbine binding in widespread cortical regions, including the motor cortex, as well as in the insula, thalamus and putamen. Clinically, locus coeruleus neuronal loss was correlated with motor (bradykinesia, motor fluctuations, tremor) and non-motor (fatigue, apathy, constipation) symptoms. A reduction of α2-AR availability in the thalamus was associated with tremor, while a reduction in the putamen, the insula and the superior temporal gyrus was associated with anxiety. These results highlight a multifaceted alteration of the noradrenergic system in Parkinson's disease since locus coeruleus and α2-AR degeneration were found to be partly uncoupled. These findings raise important issues about noradrenergic dysfunction that may encourage the search for new drugs targeting this system, including α2-ARs, for the treatment of Parkinson's disease.

Behavioral effects of 6-hydroxydopamine-induced damage to nigro-striatal pathway and Locus coeruleus as a rodent model of Parkinson's disease

Parkinson's disease (PD) is a chronic and progressive neurodegenerative disorder characterized by the loss of dopaminergic neurons in the Substantia nigra pars compacta (SNpc), which leads to motor and non-motor symptoms (NMS). NMS can appear many years before the classical motor symptoms and are associated with the neurodegeneration of several nuclei; in this work, we highlight the neurodegeneration of Locus coeruleus (LC) in PD. The aim was to investigate the effects of depleting SNpc and LC catecholaminergic neurons on behavioral and neurobiological endpoints. Here we used 6-hydroxydopamine (6-OHDA) in order to induced neurotoxic damage in three independent experimental groups: SNpc lesion group, which 6-OHDA was injected into CPu (CPu-6-OHDA), LC lesion group, which 6-OHDA was injected directly on LC to selectively caused a damage on this nucleus (LC-6-OHDA), and the combined SNpc and LC lesion group (CL-6-OHDA). Next, the behavioral studies were performed using the Morris water maze (MWM), open field (OF), and elevated plus maze (EPM). After stereotaxic surgeries, the animals showed a loss of 67% and 77% of Tyrosine hydroxylase (TH) reactive neurons in the SNpc and LC, respectively. The behavioral analysis showed the anxiety-like behavior in CL-6-OHDA group in the EPM test; in the MWM test, the combined lesions (CL-6-OHDA) showed an impairment in memory acquisition and spatial memory; and no changes were observed in locomotor activity in all the tests. Furthermore, our investigation demonstrating the effects of depleting SN and LC catecholaminergic neurons on behavioral and neurobiological parameters. All these data together lead us to believe that a bilateral PD model including a LC bilateral degeneration is potentially a more accurate model to evaluate the NMS in the pathological development of the disease in rodents.

Amphiregulin blockade decreases the levodopa-induced dyskinesia in a 6-hydroxydopamine Parkinson's disease mouse model

BACKGROUND

Levodopa (L-DOPA) is considered the most reliable drug for treating Parkinson's disease (PD) clinical symptoms. Regrettably, long-term L-DOPA therapy results in the emergence of drug-induced abnormal involuntary movements (AIMs) in most PD patients. The mechanisms underlying motor fluctuations and dyskinesia induced by L-DOPA (LID) are still perplexing.

METHODS

Here, we first performed the analysis on the microarray data set (GSE55096) from the gene expression omnibus (GEO) repository and identified the differentially expressed genes (DEGs) using linear models for microarray analysis (Limma) R packages from the Bioconductor project. 12 genes (Nr4a2, Areg, Tinf2, Ptgs2, Pdlim1, Tes, Irf6, Tgfb1, Serpinb2, Lipg, Creb3l1, Lypd1) were found to be upregulated. Six genes were validated on quantitative polymerase chain reaction and subsequently, Amphiregulin (Areg) was selected (based on log2 fold change) for further experiments to unravel its involvement in LID. Areg LV_shRNA was used to knock down Areg to explore its therapeutic role in the LID model.

RESULTS

Western blotting and immunofluorescence results show that AREG is significantly expressed in the LID group relative to the control. Dyskinetic movements in LID mice were alleviated by Areg knockdown, and the protein expression of delta FOSB, the commonly attributable protein in LID, was decreased. Moreover, Areg knockdown reduced the protein expression of P-ERK. In order to ascertain whether the inhibition of the ERK pathway (a common pathway known to mediate levodopa-induced dyskinesia) could also impede Areg, the animals were injected with an ERK inhibitor (PD98059). Afterward, the AIMs, AREG, and ERK protein expression were measured relative to the control group. A group treated with ERK inhibitor had a significant decrease of AREG and phosphorylated ERK protein expression relative to the control group.

CONCLUSION

Taken together, our results indicate unequivocal involvement of Areg in levodopa-induced dyskinesia, thus a target for therapy development.

The Dynamics of Dopamine D2 Receptor-Expressing Striatal Neurons and the Downstream Circuit Underlying L-Dopa-Induced Dyskinesia in Rats

L-dopa (l-3,4-dihydroxyphenylalanine)-induced dyskinesia (LID) is a debilitating complication of dopamine replacement therapy for Parkinson's disease. The potential contribution of striatal D2 receptor (D2R)-positive neurons and downstream circuits in the pathophysiology of LID remains unclear. In this study, we investigated the role of striatal D2R+ neurons and downstream globus pallidus externa (GPe) neurons in a rat model of LID. Intrastriatal administration of raclopride, a D2R antagonist, significantly inhibited dyskinetic behavior, while intrastriatal administration of pramipexole, a D2-like receptor agonist, yielded aggravation of dyskinesia in LID rats. Fiber photometry revealed the overinhibition of striatal D2R+ neurons and hyperactivity of downstream GPe neurons during the dyskinetic phase of LID rats. In contrast, the striatal D2R+ neurons showed intermittent synchronized overactivity in the decay phase of dyskinesia. Consistent with the above findings, optogenetic activation of striatal D2R+ neurons or their projections in the GPe was adequate to suppress most of the dyskinetic behaviors of LID rats. Our data demonstrate that the aberrant activity of striatal D2R+ neurons and downstream GPe neurons is a decisive mechanism mediating dyskinetic symptoms in LID rats.

Striatal D1 Dopamine Neuronal Population Dynamics in a Rat Model of Levodopa-Induced Dyskinesia

Background

The pathophysiology of levodopa-induced dyskinesia (LID) in Parkinson’s disease (PD) is not well understood. Experimental data from numerous investigations support the idea that aberrant activity of D₁ dopamine receptor-positive medium spiny neurons in the striatal direct pathway is associated with LID. However, a direct link between the real-time activity of these striatal neurons and dyskinetic symptoms remains to be established.

Methods

We examined the effect of acute levodopa treatment on striatal c-Fos expression in LID using D₁-Cre PD rats with dyskinetic symptoms induced by chronic levodopa administration. We studied the real-time dynamics of striatal D₁⁺ neurons during dyskinetic behavior using GCaMP₆-based in vivo fiber photometry. We also examined the effects of striatal D₁⁺ neuronal deactivation on dyskinesia in LID rats using optogenetics and chemogenetic methods.

Results

Striatal D₁⁺ neurons in LID rats showed increased expression of c-Fos, a widely used marker for neuronal activation, following levodopa injection. Fiber photometry revealed synchronized overactivity of striatal D₁⁺ neurons during dyskinetic behavior in LID rats following levodopa administration. Consistent with these observations, optogenetic deactivation of striatal D₁⁺ neurons was sufficient to inhibit most of the dyskinetic behaviors of LID animals. Moreover, chemogenetic inhibition of striatal D₁⁺ neurons delayed the onset of dyskinetic behavior after levodopa administration.

Conclusion

Our data demonstrated that aberrant activity of striatal D₁⁺ neuronal population was causally linked with real-time dyskinetic symptoms in LID rats.

Human Embryonic Stem Cell-Derived Dopaminergic Grafts Alleviate L-DOPA Induced Dyskinesia

BACKGROUND

First-in-human studies to test the efficacy and safety of human embryonic stem cells (hESC)-derived dopaminergic cells in the treatment of Parkinson's disease (PD) are imminent. Pre-clinical studies using hESC-derived dopamine neuron transplants in rat models have indicated that the benefits parallel those shown with fetal tissue but have thus far failed to consider how ongoing L-DOPA administration might impact on the graft.

OBJECTIVE

To determine whether L-DOPA impacts on survival and functional recovery following grafting of hESC-derived dopaminergic neurons.

METHODS

Unilateral 6-OHDA lesioned rats were administered with either saline or L-DOPA prior to, and for 18 weeks following surgical implantation of dopaminergic neural progenitors derived from RC17 hESCs according to two distinct protocols in independent laboratories.

RESULTS

Grafts from both protocols elicited reduction in amphetamine-induced rotations. Reduced L-DOPA-induced dyskinesia preceded the improvement in amphetamine-induced rotations. Furthermore, L-DOPA had no effect on overall survival (HuNu) or dopaminergic neuron content of the graft (TH positive cells) but did lead to an increase in the number of GIRK2 positive neurons.

CONCLUSION

Critically, we found that L-DOPA was not detrimental to graft function, potentially enhancing graft maturation and promoting an A9 phenotype. Early improvement of L-DOPA-induced dyskinesia suggests that grafts may support the handling of exogenously supplied dopamine earlier than improvements in amphetamine-induced behaviours indicate. Given that one of the protocols will be employed in the production of cells for the European STEM-PD clinical trial, this is vital information for the management of patients and achieving optimal outcomes following transplantation of hESC-derived grafts for PD.

Behavioral and neurochemical interactions of the tricyclic antidepressant drug desipramine with L-DOPA in 6-OHDA-lesioned rats. Implications for motor and psychiatric functions in Parkinson's disease

RATIONALE

The pharmacological effects of antidepressants in modulating noradrenergic transmission as compared to serotonergic transmission in a rat model of Parkinson's disease under chronic L-DOPA therapy are insufficiently explored.

OBJECTIVES

The aim of the present study was to investigate the effect of the tricyclic antidepressant desipramine administered chronically alone or jointly with L-DOPA, on motor behavior and monoamine metabolism in selected brain structures of rats with the unilateral 6-OHDA lesion.

METHODS

The antiparkinsonian activities of L-DOPA and desipramine were assessed behaviorally using a rotation test and biochemically based on changes in the tissue concentrations of noradrenaline, dopamine and serotonin and their metabolites, evaluated separately for the ipsi- and contralateral motor (striatum, substantia nigra) and limbic (prefrontal cortex, hippocampus) structures of rat brain by HPLC method.

RESULTS

Desipramine administered alone did not induce rotational behavior, but in combination with L-DOPA, it increased the number of contralateral rotations more strongly than L-DOPA alone. Both L-DOPA and desipramine + L-DOPA significantly increased DA levels in the ipsilateral striatum, substantia nigra, prefrontal cortex and the ipsi- and contralateral hippocampus. The combined treatment also significantly increased noradrenaline content in the ipsi- and contralateral striatum, while L-DOPA alone decreased serotonin level on both sides of the hippocampus.

CONCLUSIONS

The performed analysis of the level of monoamines and their metabolites in the selected brain structures suggests that co-modulation of noradrenergic and dopaminergic transmission in Parkinson's disease by the combined therapy with desipramine + L-DOPA may have some positive implications for motor and psychiatric functions but further research is needed to exclude potential negative effects.

Inhibition of striatal dopamine D5 receptor attenuates levodopa-induced dyskinesia in a rat model of Parkinson's disease

Levodopa-induced dyskinesia (LID) is experienced by most patients of Parkinson's disease (PD) upon the long-term use of the dopamine precursor levodopa. Striatal dopaminergic signaling plays a critical role in the pathogenesis of LID through its interactions with dopamine receptors. The specific roles of striatal dopaminergic D5 receptors in the pathophysiological process of LID are still poorly established. In the study, we investigated the role of striatal dopamine D5 receptor in LID by using PD rats with or without dyskinetic symptoms after chronic levodopa administration. The experimental results showed that the expression level of D5 receptors in the sensorimotor striatum of dyskinetic rats is significantly higher than that of the non-dyskinetic controls. The administration of levodopa increased c-Fos expression in a subpopulation of sensorimotor striatum neurons of dyskinetic rats, but not in non-dyskinetic rats. The majority of the c-Fos+ neurons activated by levodopa in the striatum are positive for D5 receptor staining. Intrastriatal injection of D1-like (D1 and D5) dopamine receptor antagonist, SCH-23390, significantly inhibited dyskinetic behavior in dyskinetic rats after the injection of levodopa, meanwhile, intrastriatal administration of SKF-83959, a partial D5 receptor agonist, yielded significant dyskinetic movements in dyskinetic rats without levodopa. In contrast, intrastriatal perfusion of small interfering RNA directed against DRD5 downregulated D5 receptors expression and moderately inhibited dyskinetic behavior of dyskinetic animals. Our data suggested that the striatal dopamine D5 receptor might play a novel role in the pathophysiology of LID.

The Noradrenergic System in Parkinson's Disease

Nowadays it is well accepted that in Parkinson’s disease (PD), the neurodegenerative process occurs in stages and that damage to other areas precedes the neuronal loss in the substantia nigra pars compacta, which is considered a pathophysiological hallmark of PD. This heterogeneous and progressive neurodegeneration may explain the diverse symptomatology of the disease, including motor and non-motor alterations. In PD, one of the first areas undergoing degeneration is the locus coeruleus (LC). This noradrenergic nucleus provides extensive innervation throughout the brain and plays a fundamental neuromodulator role, participating in stress responses, emotional memory, and control of motor, sensory, and autonomic functions. Early in the disease, LC neurons suffer modifications that can condition the effectiveness of pharmacological treatments, and importantly, can lead to the appearance of common non-motor symptomatology. The noradrenergic system also exerts anti-inflammatory and neuroprotective effect on the dopaminergic degeneration and noradrenergic damage can consequently condition the progress of the disease. From the pharmacological point of view, it is also important to understand how the noradrenergic system performs in PD, since noradrenergic medication is often used in these patients, and drug interactions can take place when combining them with the gold standard drug therapy in PD, L-3,4-dihydroxyphenylalanine (L-DOPA). This review provides an overview about the functional status of the noradrenergic system in PD and its contribution to the efficacy of pharmacological-based treatments. Based on preclinical and clinical publications, a special attention will be dedicated to the most prevalent non-motor symptoms of the disease.

Brain-Region Specific Metabolic Abnormalities in Parkinson's Disease and Levodopa-Induced Dyskinesia

Several lines of evidence point to alteration in brain metabolic homeostasis in Parkinson’s disease (PD) and levodopa-induced dyskinesia (LID), yet the metabolic mechanism in different brain regions underlying PD and LID remains largely unknown. The present study aimed to uncover the metabolic pathways across anatomical regions in the brain of PD and LID. Using an NMR-based metabolomic approach, we generated the metabolomics profiling data from six different brain regions of PD rats and following the onset of LIDs. The diversity of metabolite patterns across the brain and its relation to PD and LID were further investigated through principal component analysis (PCA) and multivariate general linear model. Compared with control rats, dopamine loss in PD rats produced a marked and persistent metabolic disturbance in neurotransmitter metabolism and energy pathway, resulting in a metabolic imbalance among different brain regions. In LID rats, levodopa replacement did not restore the midbrain-striatum metabolic crosstalk and metabolic disturbance throughout the brain was involved in levodopa related involuntary movements. Most notably, the midbrain and right cortex were identified as the primary regions of metabolic abnormalities in PD and LID rats. Neurochemical differences in metabolic phenotypes were mainly defined by various neurotransmitters including glutamate, glutamine and aspartate. Accordingly, we found that the PD and LID rats exhibited lower levels of synaptophysin (SYP), a marker for synaptic plasticity, compared with control rats. These findings provide key insights into the metabolic mechanism underlying PD and LID by defining brain-region specific metabolic phenotype, with implications for developing targeted therapies.

Tapentadol Prevents Motor Impairments in a Mouse Model of Dyskinesia

The motor features in Parkinson's disease (PD) are associated with the degeneration of dopaminergic cells in the substantia nigra in the brain. Thus, the gold-standard in PD therapeutics still consists of dopamine replacement with levodopa. However, as the disease progresses, this therapeutic option becomes less effective and can be accompanied by levodopa-induced complications. On the other hand, several other neuronal pathways have been implicated in the pathological mechanisms of PD. In this context, the development of alternative therapeutic options that modulate non-dopaminergic targets is emerging as a major goal in the field. In a phenotypic-based screen in a zebrafish model of PD, we identified tapentadol as a candidate molecule for PD. The therapeutic potential of an agent that modulates the opioid and noradrenergic systems has not been explored, despite the implication of both neuronal pathways in parkinsonism. Therefore, we assessed the therapeutic properties of this µ-opioid receptor agonist and norepinephrine reuptake inhibitor in the 6-hydroxydopamine mouse model of parkinsonism. We further submitted 6-hydroxydopamine-lesioned mice to chronic treatment with levodopa and evaluated the effects of tapentadol during levodopa OFF states and on levodopa-induced dyskinesia. Importantly, we found that tapentadol halted the aggravation of dyskinesia and improved the motor impairments during levodopa OFF states. Altogether, our findings raise the hypothesis that concomitant modulation of µ-opioid receptor and norepinephrine transporter might constitute relevant intervention strategies in PD and that tapentadol holds therapeutic potential that may be translated into the clinical practice.

L-DOPA in Parkinson's Disease: Looking at the "False" Neurotransmitters and Their Meaning

L-3,4-dihydroxyphenylalanine (L-DOPA) has been successfully used in the treatment of Parkinson’s disease (PD) for more than 50 years. It fulfilled the criteria to cross the blood–brain barrier and counteract the biochemical defect of dopamine (DA). It remarkably worked after some adjustments in line with the initial hypothesis, leaving a poor place to the plethora of mechanisms involving other neurotransmitters or mechanisms of action beyond newly synthesized DA itself. Yet, its mechanism of action is far from clear. It involves numerous distinct cell populations and does not mimic the mechanism of action of dopaminergic agonists. L-DOPA-derived DA is mainly released by serotonergic neurons as a false neurotransmitter, and serotonergic neurons are involved in L-DOPA-induced dyskinesia. The brain pattern and magnitude of DA extracellular levels together with this status of false neurotransmitters suggest that the striatal effects of DA via this mechanism would be minimal. Other metabolic products coming from newly formed DA or through the metabolism of L-DOPA itself could be involved. These compounds can be trace amines and derivatives. They could accumulate within the terminals of the remaining monoaminergic neurons. These “false neurotransmitters,” also known for some of them as inducing an “amphetamine-like” mechanism, could reduce the content of biogenic amines in terminals of monoaminergic neurons, thereby impairing the exocytotic process of monoamines including L-DOPA-induced DA extracellular outflow. The aim of this review is to present the mechanism of action of L-DOPA with a specific attention to “false neurotransmission.”

Repurposing drugs to treat l-DOPA-induced dyskinesia in Parkinson's disease

In this review, we discuss the opportunity for repurposing drugs for use in l-DOPA-induced dyskinesia (LID) in Parkinson's disease. LID is a particularly suitable indication for drug repurposing given its pharmacological diversity, translatability of animal-models, availability of Phase II proof-of-concept (PoC) methodologies and the indication-specific regulatory environment. A compound fit for repurposing is defined as one with appropriate human safety-data as well as animal safety, toxicology and pharmacokinetic data as found in an Investigational New Drug (IND) package for another indication. We first focus on how such repurposing candidates can be identified and then discuss development strategies that might progress such a candidate towards a Phase II clinical PoC. We discuss traditional means for identifying repurposing candidates and contrast these with newer approaches, especially focussing on the use of computational and artificial intelligence (AI) platforms. We discuss strategies that can be categorised broadly as: in vivo phenotypic screening in a hypothesis-free manner; in vivo phenotypic screening based on analogy to a related disorder; hypothesis-driven evaluation of candidates in vivo and in silico screening with a hypothesis-agnostic component to the selection. To highlight the power of AI approaches, we describe a case study using IBM Watson where a training set of compounds, with demonstrated ability to reduce LID, were employed to identify novel repurposing candidates. Using the approaches discussed, many diverse candidates for repurposing in LID, originally envisaged for other indications, will be described that have already been evaluated for efficacy in non-human primate models of LID and/or clinically. This article is part of the Special Issue entitled 'Drug Repurposing: old molecules, new ways to fast track drug discovery and development for CNS disorders'.

Current Nondopaminergic Therapeutic Options for Motor Symptoms of Parkinson's Disease

Objective:

The aim of this study was to summarize recent studies on nondopaminergic options for the treatment of motor symptoms in Parkinson's disease (PD).

Data Sources:

Papers in English published in PubMed, Cochrane, and Ovid Nursing databases between January 1988 and November 2016 were searched using the following keywords: PD, nondopaminergic therapy, adenosine, glutamatergic, adrenergic, serotoninergic, histaminic, and iron chelator. We also reviewed the ongoing clinical trials in the website of clinicaltrials.gov.

Study Selection:

Articles related to the nondopaminergic treatment of motor symptoms in PD were selected for this review.

Results:

PD is conventionally treated with dopamine replacement strategies, which are effective in the early stages of PD. Long-term use of levodopa could result in motor complications. Recent studies revealed that nondopaminergic systems such as adenosine, glutamatergic, adrenergic, serotoninergic, histaminic, and iron chelator pathways could include potential therapeutic targets for motor symptoms, including motor fluctuations, levodopa-induced dyskinesia, and gait disorders. Some nondopaminergic drugs, such as istradefylline and amantadine, are currently used clinically, while most such drugs are in preclinical testing stages. Transitioning of these agents into clinically beneficial strategies requires reliable evaluation since several agents have failed to show consistent results despite positive findings at the preclinical level.

Conclusions:

Targeting nondopaminergic transmission could improve some motor symptoms in PD, especially the discomfort of dyskinesia. Although nondopaminergic treatments show great potential in PD treatment as an adjunct therapy to levodopa, further investigation is required to ensure their success.

Identification of metabolite biomarkers for L-DOPA-induced dyskinesia in a rat model of Parkinson's disease by metabolomic technology

L-DOPA-induced dyskinesia (LID) is a frequent complication of chronic L-DOPA therapy in the clinical treatment of Parkinson's disease (PD). The pathogenesis of LID involves complex molecular mechanisms in the striatum. Metabolomics can shed light on striatal metabolic alterations in LID. In the present study, we compared metabolomics profiles of striatum tissue from Parkinsonian rats with or without dyskinetic symptoms after chronic L-DOPA administration. A liquid chromatography-mass spectrometry based global metabolomics method combined with multivariate statistical analyses were used to detect candidate metabolites associated with LID. 36 dysregulated metabolites in the striatum of LID rats, including anandamide, 2-arachidonoylglycerol, adenosine, glutamate and sphingosine1-phosphate were identified. Furthermore, IMPaLA metabolite set analysis software was used to identify differentially regulated metabolic pathways. The results showed that the metabolic pathways of "Retrograde endocannabinoid signaling", "Phospholipase D signaling pathway", "Glycerophospholipid metabolism" and "Sphingolipid signaling", etc. were dysregulated in LID rats compared to non-LID controls. Moreover, integrated pathway analysis based on results from the present metabolomics and our previous gene expression data in LID rats further demonstrates that aberrant "Retrograde endocannabinoid signaling" pathway might be involved in the development of LID. The present results provide a new profile for the understanding of the pathological mechanism of LID.

Involvement of lateral habenula α1 subunit-containing GABAA receptor-mediated inhibitory transmission in the regulation of depression-related behaviors in experimental Parkinson's disease

The lateral habenula (LHb) plays an important role in the regulation of depression. At present, it is not clear whether GABA_A receptor-mediated inhibitory transmission in the LHb is involved in Parkinson's disease (PD)-associated depression. In this study, unilateral 6-hydroxydopamine lesions of the substantia nigra in rats induced depressive-like behaviors and led to hyperactivity of LHb neurons compared to sham-operated rats, which attribute to depletion of dopamine, and decreased synthesis and release of GABA and increased release of glutamate in the LHb. Intra-LHb injection of GABA_A receptor agonist muscimol produced antidepressant-like effects, while the injection of GABA_A receptor antagonist picrotoxin induced or increased the expression of depressive-like behaviors in sham-operated and the lesioned rats. However, the doses producing these behavioral effects in the lesioned rats were lower than those in sham-operated rats. Intra-LHb injection of muscimol decreased the firing rate of LHb neurons and increased the medial prefrontal cortex serotonin (5-HT) release; conversely, picrotoxin increased the firing rate of the neurons and decreased 5-HT release in two groups of rats. Compared to sham-operated rats, the duration of muscimol and picrotoxin action on the firing rate of the neurons and 5-HT release was prolonged in the lesioned rats. These changes in the lesioned rats were associated with up-regulation of the expression of α1 subunit-containing GABA_A receptors and reduction of GABA release in the LHb. Collectively, our findings suggest that degeneration of the nigrostriatal pathway impairs GABA_A receptor-mediated inhibitory transmission in the LHb, and the transmission is important for regulating PD-associated depression.

Comparison of neuropathology in Parkinson's disease subjects with and without deep brain stimulation

BACKGROUND

The aim of this postmortem study was to compare, in Parkinson's disease subjects with and without bilateral subthalamic nucleus deep brain stimulation (STN-DBS), the loss of pigmented neurons within the substantia nigra and pathological alpha-synuclein density within the SN and other brain regions.

METHODS

PD subjects were identified from the Arizona Study of Aging and Neurodegenerative Disorders database (STN-DBS = 11, non-DBS = 156). Pigmented neuron loss scores within the substantia nigra as well as alpha-synuclein density scores within the substantia nigra and 9 other brain regions were compared, the latter individually and in summary as the Lewy body brain load score.

RESULTS

DBS subjects had higher alpha-synuclein density scores within the substantia nigra, olfactory bulb, and locus ceruleus, as well as higher total Lewy body brain load scores when compared with non-DBS subjects. No differences in substantia nigra pigmented neuron loss scores were found.

CONCLUSIONS

STN-DBS subjects tend to have higher alpha-synuclein density scores, but do not have a differential loss of substantia nigra pigmented neurons. © 2016 International Parkinson and Movement Disorder Society.

Effects of the beta-adrenergic receptor antagonist Propranolol on dyskinesia and L-DOPA-induced striatal DA efflux in the hemi-parkinsonian rat

Dopamine (DA) replacement therapy with L-DOPA continues to be the primary treatment of Parkinson's disease; however, long-term therapy is accompanied by L-DOPA-induced dyskinesias (LID). Several experimental and clinical studies have established that Propranolol, a β-adrenergic receptor antagonist, reduces LID without affecting L-DOPA's efficacy. However, the exact mechanisms underlying these effects remain to be elucidated. The aim of this study was to evaluate the anti-dyskinetic profile of Propranolol against a panel of DA replacement strategies, as well as elucidate the underlying neurochemical mechanisms. Results indicated that Propranolol, in a dose-dependent manner, reduced LID, without affecting motor performance. Propranolol failed to alter dyskinesia produced by the D1 receptor agonist, SKF81297 (0.08 mg/kg, sc), or the D2 receptor agonist, Quinpirole (0.05 mg/kg, sc). These findings suggested a pre-synaptic mechanism for Propranolol's anti-dyskinetic effects, possibly through modulating L-DOPA-mediated DA efflux. To evaluate this possibility, microdialysis studies were carried out in the DA-lesioned striatum of dyskinetic rats and results indicated that co-administration of Propranolol (20 mg/kg, ip) was able to attenuate L-DOPA- (6 mg/kg, sc) induced DA efflux. Therefore, Propranolol's anti-dyskinetic properties appear to be mediated via attenuation of L-DOPA-induced extraphysiological efflux of DA.

Modulation of L-DOPA's antiparkinsonian and dyskinetic effects by α2-noradrenergic receptors within the locus coeruleus

Long-term l-DOPA use for Parkinson's disease (PD) is frequently complicated by the emergence of a debilitating motor side effect known as l-DOPA-induced dyskinesia (LID). Accumulating evidence has implicated the norepinephrine (NE) system in the pathogenesis of LID. Here we used the unilateral 6-hydroxydopamine rat model of PD to determine the role of the α2-adrenoceptors (α2R) in l-DOPA's therapeutic and detrimental motor-inducing effects. First, we characterized the effects of systemic α2R stimulation with clonidine, or blockade with atipamezole, on LID using the rodent abnormal involuntary movements scale, and l-DOPA's therapeutic effects using the forepaw adjusting steps test and locomotor activity chambers. The anatomical locus of action of α2R in LID was investigated by directly infusing clonidine or atipamezole into the locus coeruleus prior to systemic l-DOPA administration. Results showed systemic clonidine treatment reduced LID and locomotor activity but did not interfere with l-DOPA's antiparkinsonian benefits. Conversely, systemic atipamezole pretreatment prolonged LID and locomotor activity but did not modulate l-DOPA's antiparkinsonian benefits. Intra-LC infusions of clonidine and atipamezole mirrored systemic effects where clonidine reduced, and atipamezole increased, LID. Collectively, these results demonstrate that α2R play an important modulatory role in l-DOPA-mediated behaviors and should be further investigated as a potential therapeutic target.

Presynaptic Mechanisms of l-DOPA-Induced Dyskinesia: The Findings, the Debate, and the Therapeutic Implications

The dopamine (DA) precursor l-DOPA has been the most effective treatment for Parkinson’s disease (PD) for over 40 years. However, the response to this treatment changes with disease progression, and most patients develop dyskinesias (abnormal involuntary movements) and motor fluctuations within a few years of l-DOPA therapy. There is wide consensus that these motor complications depend on both pre- and post-synaptic disturbances of nigrostriatal DA transmission. Several presynaptic mechanisms converge to generate large DA swings in the brain concomitant with the peaks-and-troughs of plasma l-DOPA levels, while post-synaptic changes engender abnormal functional responses in dopaminoceptive neurons. While this general picture is well-accepted, the relative contribution of different factors remains a matter of debate. A particularly animated debate has been growing around putative players on the presynaptic side of the cascade. To what extent do presynaptic disturbances in DA transmission depend on deficiency/dysfunction of the DA transporter, aberrant release of DA from serotonin neurons, or gliovascular mechanisms? And does noradrenaline (which is synthetized from DA) play a role? This review article will summarize key findings, controversies, and pending questions regarding the presynaptic mechanisms of l-DOPA-induced dyskinesia. Intriguingly, the debate around these mechanisms has spurred research into previously unexplored facets of brain plasticity that have far-reaching implications to the treatment of neuropsychiatric disease.