The serotonergic system in motor and non-motor manifestations of Parkinson’s disease

Serotonin 5-HT1A receptors and their interactions with adenosine A2A receptors in Parkinson's disease and dyskinesia

The dopamine neuronal loss that characterizes Parkinson's Disease (PD) is associated to changes in neurotransmitters, such as serotonin and adenosine, which contribute to the symptomatology of PD and to the onset of dyskinetic movements associated to levodopa treatment. The present review describes the role played by serotonin 5-HT_1A receptors and the adenosine A_2A receptors on dyskinetic movements induced by chronic levodopa in PD. The focus is on preclinical and clinical results showing the interaction between serotonin 5-HT_1A receptors and other receptors such as 5-HT_1B receptors and adenosine A_2A receptors. 5-HT_1A/1B receptor agonists and A_2A receptor antagonists, administered in combination, contrast dyskinetic movements induced by chronic levodopa without impairing motor behaviour, suggesting that this drug combination might be a useful therapeutic approach for counteracting the PD motor deficits and dyskinesia associated with chronic levodopa treatment. This article is part of the Special Issue on "The receptor-receptor interaction as a new target for therapy".

Characterizing the relationship between L-DOPA-induced-dyskinesia and psychosis-like behaviors in a bilateral rat model of Parkinson's disease

Parkinson's disease associated psychosis (PDAP) is a prevalent non-motor symptom (NMS) that significantly erodes patients' and caregivers' quality of life yet remains vastly understudied. One potential source of PDAP in late-stage Parkinson's disease (PD) is the common dopamine (DA) replacement therapy for motor symptoms, Levodopa (L-DOPA). Given the high incidence of L-DOPA-induced dyskinesia (LID) in later phases of PD, this study sought to characterize the relationship between PDAP and LID in a bilateral medial forebrain bundle 6-hydroxydopamine hydrobromide (6-OHDA) lesion rat model. To assess PDAP in this model, prepulse inhibition (PPI), a well-validated assay of sensorimotor gating, was employed. First, we tested whether a bilateral lesion alone or after chronic L-DOPA treatment was sufficient to induce PPI dysfunction. Rats were also monitored for LID development, using the abnormal involuntary movements (AIMs) test, to examine PPI and LID associations. In experiment 2, Vilazodone (VZD), a serotonin transporter (SERT) blocker and 1A receptor (5-HT_1A) partial agonist was administered to test its potential efficacy in reducing LID and PPI dysfunction. Once testing was complete, tissue was collected for high performance liquid chromatography (HPLC) to examine the monoamine levels in motor and non-motor circuits. Results indicate that bilateral DA lesions produced motor deficits and that chronic L-DOPA induced moderate AIMs; importantly, rats that developed more severe AIMs were more likely to display sensorimotor gating dysfunction. In addition, VZD treatment dose-dependently reduced L-DOPA-induced AIMs without impairing L-DOPA efficacy, although VZD's effects on PPI were limited. Altogether, this project established the bilateral 6-OHDA lesion model accurately portrayed LID and PDAP-like behaviors, uncovered their potential relationship, and finally, demonstrated the utility of VZD for reducing LID.

The Neurodegenerative Elderly Syndrome (NES) hypothesis: Alzheimer and Parkinson are two faces of the same disease

Increasing evidence suggests that Alzheimer's disease (AD) and Parkinson's disease (PD) share monoamine and alpha-synuclein (αSyn) dysfunctions, often beginning years before clinical manifestations onset. The triggers for these impairments and the causes leading these early neurodegenerative processes to become AD or PD remain unclear. We address these issues by proposing a radically new perspective to frame AD and PD: they are different manifestations of one only disease we call "Neurodegenerative Elderly Syndrome (NES)". NES goes through three phases. The seeding stage, which starts years before clinical signs, and where the part of the brain-body affected by the initial αSyn and monoamine dysfunctions, influences the future possible progression of NES towards PD or AD. The compensatory stage, where the clinical symptoms are still silent thanks to compensatory mechanisms keeping monoamine concentrations homeostasis. The bifurcation stage, where NES becomes AD or PD. We present recent literature supporting NES and discuss how this hypothesis could radically change the comprehension of AD and PD comorbidities and the design of novel system-level diagnostic and therapeutic actions.

Neurotoxicological Profiling of Paraquat in Zebrafish Model

Paraquat is a polar herbicide protecting plant products against invasive species, it requires careful manipulation and restricted usage because of its harmful potentials. Exposure to paraquat triggers oxidative damage in dopaminergic neurons and subsequently causes a behavioral defect in vivo. Thereby, persistent exposure to paraquat is known to increase Parkinson's disease risk by dysregulating dopaminergic systems in humans. Therefore, most studies have focused on the dopaminergic systems to elucidate the neurotoxicological mechanism of paraquat poisoning, and more comprehensive neurochemistry including histaminergic, serotonergic, cholinergic, and GABAergic systems has remained unclear. Therefore, in this study, we investigated the toxicological potential of paraquat poisoning using a variety of approaches such as toxicokinetic profiles, behavioral effects, neural activity, and broad-spectrum neurochemistry in zebrafish larvae after short-term exposure to paraquat and we performed the molecular modeling approach. Our results showed that paraquat was slowly absorbed in the brain of zebrafish after oral administration of paraquat. In addition, paraquat toxicity resulted in behavioral impairments, namely, reduced motor activity and led to abnormal neural activities in zebrafish larvae. This locomotor deficit came with a dysregulation of dopamine synthesis induced by the inhibition of tyrosine hydroxylase activity, which was also indirectly confirmed by molecular modeling studies. Furthermore, short-term exposure to paraquat also caused simultaneous dysregulation of other neurochemistry including cholinergic and serotonergic systems in zebrafish larvae. The present study suggests that this neurotoxicological profiling could be a useful tool for understanding the brain neurochemistry of neurotoxic agents that might be a potential risk to human and environmental health.

Parkinson and depression: review and outlook

Depression in Parkinson's Disorder (DPD) has been estimated to appear in up to 40% of people with PD and negatively impacts quality of life, motor and cognitive deficits and functional disability. Knowledge of the pathophysiology of DPD is unclear, DPD may be related to dysfunction in subcortical nuclei and the prefrontal cortex, striatal-thalamic-prefrontal and basotemporal limbic circuits, brainstem monoamine, and indolamine (i.e. dopamine, serotonin, and norepinephrine) systems. DPD is characterized by sadness, loss of interest, increased exhaustibility, feelings of helplessness, reduced drive, dysphoria, irritability, and pessimism about future. The diagnosis is complicated by overlap with PD symptoms, Detection of depression in PD should be made by psychometric depression scales. DPD is underrecognized and undertreated in clinical practice. Treatment mainly includes antidepressive medications and behavioral interventions as psychotherapy. Dopamine agonists showed some antidepressant effects, there are no sufficient numbers of RCTs. Important randomized clinical trials (RCTs) are summarized. SSRIs and SNRIs have a satisfying efficacy in DPD. TCAs are also good for improving depression. Side effects of different antidepressants (e.g. TCAs, SSRIs, SNRIs, bupropion, MAOIs) and potential interactions should be considered. In existing guidelines so far no statements, algorithms and recommendations are given for diagnosis and treatment of DPD. Methodologically adequate designed RCTs and comparative studies (NIS) which offer evidence-based results are urgently needed having the impact of DPD in mind.

The pathobiological basis of depression in Parkinson disease: challenges and outlooks

Depression, with an estimated prevalence of about 40% is a most common neuropsychiatric disorder in Parkinson disease (PD), with a negative impact on quality of life, cognitive impairment and functional disability, yet the underlying neurobiology is poorly understood. Depression in PD (DPD), one of its most common non-motor symptoms, can precede the onset of motor symptoms but can occur at any stage of the disease. Although its diagnosis is based on standard criteria, due to overlap with other symptoms related to PD or to side effects of treatment, depression is frequently underdiagnosed and undertreated. DPD has been related to a variety of pathogenic mechanisms associated with the underlying neurodegenerative process, in particular dysfunction of neurotransmitter systems (dopaminergic, serotonergic and noradrenergic), as well as to disturbances of cortico-limbic, striato-thalamic-prefrontal, mediotemporal-limbic networks, with disruption in the topological organization of functional mood-related, motor and other essential brain network connections due to alterations in the blood-oxygen-level-dependent (BOLD) fluctuations in multiple brain areas. Other hypothetic mechanisms involve neuroinflammation, neuroimmune dysregulation, stress hormones, neurotrophic, toxic or metabolic factors. The pathophysiology and pathogenesis of DPD are multifactorial and complex, and its interactions with genetic factors, age-related changes, cognitive disposition and other co-morbidities awaits further elucidation.

Othello syndrome in Parkinson's disease: a systematic review and report of a case series

INTRODUCTION

Psychosis in Parkinson's disease (PD) is common and consists of hallucinations, illusions, and delusions. Among the latter, delusional jealousy, also named Othello syndrome (OS), might impair the quality of life of both patients and their partners. We aimed to perform a systematic review and report a series of PD patients presenting with OS.

METHODS

A systematic review research was performed in PubMed database, excluding non-English articles, single case reports, reviews and neuropathology articles, comments, and articles concerning OS associated with deep brain stimulation (DBS) and levodopa-carbidopa intestinal gel infusion. We also described eleven PD patients (9 M and 2 F) with OS, identified in a cohort of consecutive 153 patients, comparing them with eleven matched no OS (nOS) PD subjects taken from the same cohort.

RESULTS

We included eight articles (four case series and four cross-sectional studies). OS resulted more common among males than females. We did not find higher levodopa dose and levodopa equivalent dose for dopamine agonists and for all anti-parkinsonian drugs in our OS group. In our case series, OS patients showed visual hallucinations (p=0.001) and a trend to have depression (p=0.080) more frequently than nOS ones.

CONCLUSIONS

OS is not a rare disorder in PD, probably due not only to abnormal dopaminergic stimulation but also to serotonergic dysfunction in biologically predisposed subjects. Visual hallucinations and other concomitant psychiatric diseases, in particular depression, might represent a risk factor for the OS development.

Altered regional homogeneity and connectivity in cerebellum and visual-motor relevant cortex in Parkinson's disease with rapid eye movement sleep behavior disorder

OBJECTIVE

Rapid eye movement sleep behavior disorder (RBD) frequently occurs in Parkinson's disease (PD), however, the exact pathophysiological mechanism underlying its occurrence is not clear. In this study, we explored whether there is abnormal spontaneous neuronal activities and connectivity maps in some brain areas under resting-state in PD patients with RBD.

METHODS

We recruited 38 PD patients (19 PD with RBD and 19 PD without RBD), and 20 age- and gender-matched normal controls. We used resting-state functional magnetic resonance imaging (RS-fMRI) to analyze regional homogeneity (ReHo) and functional connectivity (FC), and further to reveal the neuronal activity in all subjects.

RESULTS

Compared with the PD without RBD patients, the PD with RBD patients showed a significant increase in regional homogeneity in the left cerebellum, the right middle occipital region and the left middle temporal region, and decreased regional homogeneity in the left middle frontal region. The REM sleep behavioral disorders questionnaire scores were significantly positively correlated with the ReHo values of the left cerebellum. The functional connectivity analysis in which the four regions described above were used as regions of interest revealed increased functional activity between the left cerebellum and bilateral occipital regions, bilateral temporal regions and bilateral supplementary motor area.

CONCLUSION

The pathophysiological mechanism of PD with RBD may be related to abnormal spontaneous neuronal activity patterns with strong synchronization of cerebellar and visual-motor relevant cortex, and the increased connectivity of the cerebellum with the occipital and motor regions.

Serotonergic imaging in Parkinson's disease

Parkinson's disease (PD) is a neurodegenerative disorder characterized by the progressive degeneration of monoaminergic central pathways such as the serotonergic. The degeneration of serotonergic signaling in striatal and extrastriatal brain regions is an early feature of PD and is associated with several motor and non-motor complications of the disease. Molecular imaging techniques with Positron Emission Tomography (PET) have greatly contributed to the investigation of biological changes in vivo and to the understanding of the extent of serotonergic pathology in patients or individuals at risk for PD. Such discoveries provide with opportunities for the identification of new targets that can be used for the development of novel disease-modifying drugs or symptomatic treatments. Future studies of imaging serotonergic molecular targets will better clarify the importance of serotonergic pathology in PD, including progression of pathology, target-identification for pharmacotherapy, and relevance to endogenous synaptic serotonin levels. In this article, we review the current status and understanding of serotonergic imaging in PD.

Granisetron, a selective 5-HT3 antagonist, reduces L-3,4-dihydroxyphenylalanine-induced abnormal involuntary movements in the 6-hydroxydopamine-lesioned rat

Administration of L-3,4-dihydroxyphenylalanine (L-DOPA) provides Parkinson's disease patients with effective symptomatic relief. However, long-term L-DOPA therapy is often marred by complications such as dyskinesia. We have previously demonstrated that serotonin type 3 (5-HT3) receptor blockade with the clinically available and highly selective antagonist ondansetron alleviates dyskinesia in the 6-hydroxydopamine (6-OHDA)-lesioned rat. Here, we sought to explore the antidyskinetic efficacy of granisetron, another clinically available 5-HT3 receptor antagonist. Rats were rendered hemi-parkinsonian by 6-OHDA injection in the medial forebrain bundle. Following induction of stable abnormal involuntary movements (AIMs), granisetron (0.0001, 0.001, 0.01, 0.1 and 1 mg/kg) or vehicle was acutely administered in combination with L-DOPA and the severity of AIMs, both duration and amplitude, was determined. We also assessed the effect of granisetron on L-DOPA antiparkinsonian action by performing the cylinder test. Adding granisetron (0.0001, 0.001, 0.01, 0.1 and 1 mg/kg) to L-DOPA resulted in a significant reduction of AIMs duration and amplitude, with certain parameters being reduced by as much as 38 and 45% (P < 0.05 and P < 0.001, respectively). The antidyskinetic effect of granisetron was not accompanied by a reduction of L-DOPA antiparkinsonian action. These results suggest that 5-HT3 blockade may reduce L-DOPA-induced dyskinesia without impairing the therapeutic efficacy of L-DOPA. However, a U-shaped dose-response curve obtained with certain parameters may limit the therapeutic potential of this strategy and require further investigation.

Activation and blockade of 5-HT6 receptor in the medial septum-diagonal band recover working memory in the hemiparkinsonian rats

Working memory impairment is a common symptom occurred in Parkinson's disease (PD). The medial septum-diagonal band (MS-DB) complex and 5-HT₆ receptor are involved in modulation of cognition. However, their roles in working memory in PD are still unknown. Here, we used behavioral, neurochemical and immunohistochemical approaches to assess the role of MS-DB 5-HT₆ receptor in working memory in unilateral 6-hydroxydopamie (6-OHDA)-induced PD rats. Intra-MS-DB injection of 5-HT₆ receptor agonist WAY208466 (3, 6 and 12 μg/rat) enhanced working memory and increased dopamine (DA) and noradrenaline (NA) levels in the medial prefrontal cortex (mPFC) and hippocampus in sham and 6-OHDA-lesioned rats. The dose that produced significant effect on working memory in 6-OHDA-lesioned rats was lower than that in sham rats, indicating hypersensitivity of 5-HT₆ receptor after lesioning. Intra-MS-DB injection of 5-HT₆ receptor antagonist SB258585 (2, 4 and 8 μg/rat) alleviated working memory deficits and increased DA level in the mPFC and hippocampus and NA level in the mPFC in 6-OHDA-lesioned rats while having no effect in sham rats, suggesting that SB258585 did not change normal cognitive status. These results suggest that activation and blockade of MS-DB 5-HT₆ receptor recovered working memory in 6-OHDA-lesioned rats, which is probably related to changes in monoamine levels in the mPFC and hippocampus.

Non-Dopaminergic Treatments for Motor Control in Parkinson's Disease: An Update

Glutamatergic, noradrenergic, serotonergic, and cholinergic systems play a critical role in the basal ganglia circuitry. Targeting these non-dopaminergic receptors remains a focus of ongoing research to improve Parkinson's disease (PD) motor symptoms, without the potential side effects of dopamine replacement therapy. This review updates advancements in non-dopaminergic treatments for motor control in PD since 2013. To date, no non-dopaminergic selective drug has shown significant long-term efficacy as monotherapy in PD. The largest area of development in non-dopaminergic targets has been for motor complications of dopamine replacement therapy (motor fluctuations and dyskinesia). For treatment of motor fluctuations, safinamide, zonisamide, and istradefylline are currently approved, and novel glutamatergic and serotonergic drugs are in development. Long-acting formulations of amantadine are approved for treating dyskinesia. Several non-dopaminergic drugs have failed to show anti-dyskinetic efficacy, while some are still in development. Non-dopaminergic targets are also being pursued to treat specific motor symptoms of PD. For example, CX-8998 (a calcium channel modulator) is being evaluated for PD tremor and rivastigmine may improve gait dysfunction in PD. Drug repurposing continues to be a key strategy for non-dopaminergic targets in PD, but the field needs to increase discovery and availability of such drugs.

High-resolution PET imaging reveals subtle impairment of the serotonin transporter in an early non-depressed Parkinson's disease cohort

PURPOSE

The serotonin transporter (SERT) is a biochemical marker for monoaminergic signaling in brain and has been suggested to be involved inthe pathophysiology of Parkinson's disease (PD). The aim of this PET study was to examine SERT availability in relevant brain regions in early stages ofnon-depressed PD patients.

METHODS

In a cross-sectional study, 18 PD patients (13 M/5F, 64 ± 7 years, range 46-74 years, disease duration 2.9 ± 2.6 years; UPDRS motor 21.9 ± 5.2) and 20 age- and gender-matched healthy control (HC) subjects (15 M/5F, 61 ± 7 years, range 50-72 years) were included. In a subsequent longitudinal phase, ten of the PD patients (7 M/3F, UPDRS motor 20.6 ± 6.9) underwent a second PET measurement after 18-24 months. After a 3-T MRI acquisition, baseline PET measurements were performed with [11C]MADAM using a high-resolution research tomograph. The non-displaceablebinding potential (BPND) was chosen as the outcome measure and was estimated at voxel level on wavelet-aided parametric images, by using the Logan graphical analysis and the cerebellum as reference region. A molecular template was generated to visualize and define different subdivisions of the raphe nuclei in the brainstem. Subortical and cortical regions of interest were segmented using FreeSurfer. Univariate analyses and multivariate network analyses were performed on the PET data.

RESULTS

The univariate region-based analysis showed no differences in SERT levels when the PD patients were compared with the HC neither at baseline or after 2 years of follow-up. The multivariate network analysis also showed no differences at baseline. However, prominent changes in integration and segregation measures were observed at follow-up, indicating a disconnection of the cortical and subcortical regions from the three nuclei of the raphe.

CONCLUSION

We conclude that the serotoninergic system in PD patients seems to become involved with a network dysregulation as the disease progresses, suggesting a disturbed serotonergic signaling from raphe nuclei to target subcortical and cortical regions.

Interactions Between the Serotonergic and Other Neurotransmitter Systems in the Basal Ganglia: Role in Parkinson's Disease and Adverse Effects of L-DOPA

Parkinson's disease (PD) is characterized by the progressive loss of dopaminergic neurons in the substantia nigra. However, other non-dopaminergic neuronal systems such as the serotonergic system are also involved. Serotonergic dysfunction is associated with non-motor symptoms and complications, including anxiety, depression, dementia, and sleep disturbances. This pathology reduces patient quality of life. Interaction between the serotonergic and other neurotransmitters systems such as dopamine, noradrenaline, glutamate, and GABA controls the activity of striatal neurons and are particularly interesting for understanding the pathophysiology of PD. Moreover, serotonergic dysfunction also causes motor symptoms. Interestingly, serotonergic neurons play an important role in the effects of L-DOPA in advanced PD stages. Serotonergic terminals can convert L-DOPA to dopamine, which mediates dopamine release as a "false" transmitter. The lack of any autoregulatory feedback control in serotonergic neurons to regulate L-DOPA-derived dopamine release contributes to the appearance of L-DOPA-induced dyskinesia (LID). This mechanism may also be involved in the development of graft-induced dyskinesias (GID), possibly due to the inclusion of serotonin neurons in the grafted tissue. Consistent with this, the administration of serotonergic agonists suppressed LID. In this review article, we summarize the interactions between the serotonergic and other systems. We also discuss the role of the serotonergic system in LID and if therapeutic approaches specifically targeting this system may constitute an effective strategy in PD.

Effects of acute cannabidiol administration on anxiety and tremors induced by a Simulated Public Speaking Test in patients with Parkinson's disease

BACKGROUND

Cannabidiol (CBD) is one of the main components of Cannabis sativa and has anxiolytic properties, but no study has been conducted to evaluate the effects of CBD on anxiety signs and symptoms in patients with Parkinson's disease (PD). This study aimed to evaluate the impacts of acute CBD administration at a dose of 300 mg on anxiety measures and tremors induced by a Simulated Public Speaking Test (SPST) in individuals with PD.

METHODS

A randomised, double-blinded, placebo-controlled, crossover clinical trial was conducted. A total of 24 individuals with PD were included and underwent two experimental sessions within a 15-day interval. After taking CBD or a placebo, participants underwent the SPST. During the test, the following data were collected: heart rate, systemic blood pressure and tremor frequency and amplitude. In addition, the Visual Analog Mood Scales (VAMS) and Self-Statements during Public Speaking Scale were applied. Statistical analysis was performed by repeated-measures analysis of variance (ANOVA) while considering the drug, SPST phase and interactions between these variables.

RESULTS

There were statistically significant differences in the VAMS anxiety factor for the drug; CBD attenuated the anxiety experimentally induced by the SPST. Repeated-measures ANOVA showed significant differences in the drug for the variable related to tremor amplitude as recorded by the accelerometer.

CONCLUSION

Acute CBD administration at a dose of 300 mg decreased anxiety in patients with PD, and there was also decreased tremor amplitude in an anxiogenic situation.

Markers of impaired motor and cognitive volition in Parkinson's disease: Correlates of dopamine dysregulation syndrome, impulse control disorder, and dyskinesias

INTRODUCTION

Dopaminergic therapy in Parkinson's disease (PD) can be associated with both motoric (e.g., dyskinesias) and neuropsychiatric adverse effects. Examples of the latter include Dopamine Dysregulation Syndrome (DDS) and impulse control disorder (ICD), which are separate but related behavioral/psychiatric complications of treatment in PD. Dysregulation of volition characterizes both dyskinesias and DDS/ICD; thus, we analyzed potential disease-related correlates in a large PD cohort.

METHODS

We analyzed cross-sectional data from 654 participants collected through the NINDS Parkinson's Disease Biomarkers Program. DDS/ICD symptoms and dyskinesias were assessed using the Movement Disorders Society (revised) Unified Parkinson's Disease Rating Scale. Potential associated variables were selected from PD-validated or PD-specific scales of neuropsychiatric or motoric status. Multivariable models with DDS/ICD or dyskinesia presence outcomes were produced with backward stepwise regression to identify factors independently associated with DDS/ICD and/or dyskinesias.

RESULTS

Fifty-three (8.1%) participants endorsed DDS and/or ICD symptoms and 150 (22.9%) were dyskinetic. In multivariable analysis, psychosis was independently associated with both dyskinesias (p = 0.006) and DDS/ICD (p < 0.001). Unpredictable motor fluctuations (p = 0.026) and depression (p = 0.023) were also associated with DDS/ICD; female sex (p = 0.025), low tremor score (p = 0.001) and high akinesia-rigidity score (p < 0.001) were associated with dyskinesias.

CONCLUSIONS

Our findings suggest that psychosis may be an important marker of impaired volition across motor and cognitive domains. Unpredictable motor fluctuations, psychosis, and depression may together comprise a phenotypic profile of patients at increased risk for DDS/ICD. Similarly, dyskinetic PD patients should be closely monitored for psychotic symptoms and treated appropriately.

Parkinson's disease psychosis: presentation, diagnosis and management

Parkinson's disease is a neurodegenerative disorder characterized by motor and nonmotor symptoms. Psychosis is a common feature of Parkinson's disease. Parkinson's disease psychosis (PDP) encompasses minor phenomena (illusions, passage hallucinations and presence hallucinations), visual and nonvisual hallucinations and delusions. PDP is associated with reduced function and quality of life. The initial management approach should focus on identification and treatment of any contributory medical factors, reduction or discontinuation of medications with potential to induce or worsen psychosis, nonpharmacological strategies and consideration of acetylcholinesterase inhibitor treatment in the setting of dementia. Pimavanserin, quetiapine and clozapine may all be considered for use in PDP. In this review, we discuss the presentation, diagnosis and management of PDP.

Pimavanserin: novel pharmacotherapy for Parkinson's disease psychosis

INTRODUCTION

Pimavanserin is the first FDA-approved atypical antipsychotic drug indicated for the treatment of hallucinations and delusions associated with Parkinson's disease psychosis (PDP). Areas covered: This review focuses on the preclinical discovery of pimavanserin. It analyzes the pharmacological, behavioral and molecular mechanisms of pimavanserin and their contribution to the therapeutic advantages of the drug as reported in published preclinical and clinical studies, press releases and product labels. Expert opinion: Pimavanserin exhibits a unique pharmacological profile with nanomolar affinity at serotonin 5-HT_2A and 5-HT_2C receptors. Functionally, it acts as a potent inverse agonist at 5-HT_2A receptors, with selectivity over 5-HT_2C receptors and no appreciable activity at other neurotransmitter receptors. Behavioral studies found that pimavanserin reversed impaired behaviors in animal models predictive of antipsychotic activity, and with no impairment of motor functions. The drug exhibits long plasma half-life (57 hours), which support its once/day administration. A pivotal phase III clinical trial demonstrated significant improvement in PDP symptoms in patients receiving pimavanserin compared to placebo-treated patients. The drug also displayed relatively benign safety and tolerability profiles. Pimavanserin's mechanism of action might contribute to its unique psychopharmacological properties in the improved treatment of PDP, and perhaps psychosis in other diseases including schizophrenia and dementia-related psychosis.

Neuropathology of Nonmotor Symptoms of Parkinson's Disease

Parkinson's disease (PD), a multiorgan neurodegenerative disorder associated with α-synuclein deposits throughout the nervous system and many organs, is clinically characterized by motor and nonmotor features, many of the latter antedating motor dysfunctions by 20 or more years. The causes of the nonmotor manifestations such as olfactory, autonomic, sensory, neuropsychiatric, visuospatial, sleep, and other disorders are unlikely to be related to single lesions. They are mediated by the involvement of both dopaminergic and nondopaminergic systems, and diverse structures outside the nigrostriatal system that is mainly responsible for the motor features of PD. The nonmotor alterations appear in early/prodromal stages of the disease and its further progression, suggesting a topographical and chronological spread of the lesions. This lends further support for the notion that PD is a multiorgan proteinopathy, although the exact relationship between presymptomatic and later developing nonmotor features of PD and neuropathology awaits further elucidation.

Parkinson's disease psychosis as a serotonin-dopamine imbalance syndrome

Parkinson's disease psychosis (PDP) is theoretically a serotonin-dopamine imbalance syndrome due to disruption of the normal balance between the serotonergic and dopaminergic neurotransmitter systems in key brain circuits.

Mechanism of action of pimavanserin in Parkinson's disease psychosis: targeting serotonin 5HT2A and 5HT2C receptors

Pimavanserin, a novel agent approved for the treatment of Parkinson's disease psychosis, has potent actions as an antagonist/inverse agonist at serotonin 5HT2A receptors and less potent antagonist/inverse agonist actions at 5HT2C receptors.

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

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

Loss of dopaminergic neurons occurs in the ventral tegmental area and hypothalamus of rats following chronic stress: Possible pathogenetic loci for depression involved in Parkinson's disease

Parkinson's disease (PD) is a neurodegenerative disease characterized by loss of dopaminergic (DA) neurons in the nigrostriatal and mesolimbic pathways including ventral tegmental area (VTA). Although several factors for the neuronal loss have been suggested, most of the PD cases are sporadic and idiopathic. In our previous study, we demonstrated the first evidence that solely chronic restraint stress (RS) induced the DA neuronal loss in the substantia nigra (SN). In this study, we further investigated whether chronic stress could affect other major DA systems, VTA and tuberoinfundibular system (TIDA), by using immunohistochemical and in situ hybridization techniques. The present study showed that, in the VTA, tyrosine hydroxylase (TH) immunoreactive neurons decreased by 9.8% at 2nd week, 19.2% at 4th week, 39.5% at 8th week, and 40.6% at 16th week during chronic RS as compared to control. Similarly, in the TIDA, the TH neurons decreased by 10.9% at 2nd week, 38.2% at 4th week, 56.3% at 8th week, and 57.1% at 16th week. The in situ hybridization results consistently demonstrated decreases in Th mRNA expressing cells in the VTA and TIDA in a comparable time dependent manner. Thus, exposure to chronic stress may simultaneously induce multiple neuronal loss of DA systems.

Chemical anatomy of pallidal afferents in primates

Neurons of the globus pallidus receive massive inputs from the striatum and the subthalamic nucleus, but their activity, as well as those of their striatal and subthalamic inputs, are modulated by brainstem afferents. These include serotonin (5-HT) projections from the dorsal raphe nucleus, cholinergic (ACh) inputs from the pedunculopontine tegmental nucleus, and dopamine (DA) afferents from the substantia nigra pars compacta. This review summarizes our recent findings on the distribution, quantitative and ultrastructural aspects of pallidal 5-HT, ACh and DA innervations. These results have led to the elaboration of a new model of the pallidal neuron based on a precise knowledge of the hierarchy and chemical features of the various synaptic inputs. The dense 5-HT, ACh and DA innervations disclosed in the associative and limbic pallidal territories suggest that these brainstem inputs contribute principally to the planification of motor behaviors and the regulation of attention and mood. Although 5-HT, ACh and DA inputs were found to modulate pallidal neurons and their afferents mainly through asynaptic (volume) transmission, genuine synaptic contacts occur between these chemospecific axon varicosities and pallidal dendrites, revealing that these brainstem projections have a direct access to pallidal neurons, in addition to their indirect input through the striatum and subthalamic nucleus. Altogether, these findings reveal that the brainstem 5-HT, ACh and DA pallidal afferents act in concert with the more robust GABAergic inhibitory striatopallidal and glutamatergic excitatory subthalamopallidal inputs. We hypothesize that a fragile equilibrium between forebrain and brainstem pallidal afferents plays a key role in the functional organization of the primate basal ganglia, in both health and disease.

Severely impaired hippocampal neurogenesis associates with an early serotonergic deficit in a BAC α-synuclein transgenic rat model of Parkinson's disease

Parkinson's disease (PD) is a multisystem disorder, involving several monoaminergic neurotransmitter systems resulting in a broad range of motor and non-motor symptoms. Pathological hallmarks of PD are the loss of dopaminergic neurons and the accumulation of alpha-synuclein, however also being present in the serotonergic raphe nuclei early in the disease course. The dysfunction of the serotonergic system projecting to the hippocampus may contribute to early non-motor symptoms such as anxiety and depression. The adult hippocampal dentate gyrus (DG), a unique niche of the forebrain continuously generating new neurons, may particularly present enhanced susceptibility towards accumulating alpha-synuclein levels. The underlying molecular mechanisms in the context of neuronal maturation and survival of new-born neurons are yet not well understood. To characterize the effects of overexpression of human full-length alpha-synuclein on hippocampal cellular and synaptic plasticity, we used a recently generated BAC alpha-synuclein transgenic rat model showing important features of PD such as widespread and progressive alpha-synuclein aggregation pathology, dopamine loss and age-dependent motor decline. At the age of four months, thus prior to the occurrence of the motor phenotype, we observed a profoundly impaired dendritogenesis of neuroblasts in the hippocampal DG resulting in severely reduced survival of adult new-born neurons. Diminished neurogenesis concurred with a serotonergic deficit in the hippocampus as defined by reduced levels of serotonin (5-HT) 1B receptor, decreased 5-HT neurotransmitter levels, and a loss of serotonergic nerve terminals innervating the DG/CA3 subfield, while the number of serotonergic neurons in the raphe nuclei remained unchanged. Moreover, alpha-synuclein overexpression reduced proteins involved in vesicle release, in particular synapsin-1 and Rab3 interacting molecule (RIM3), in conjunction with an altered ultrastructural architecture of hippocampal synapses. Importantly, BAC alpha-synuclein rats showed an early anxiety-like phenotype consisting of reduced exploratory behavior and feeding. Taken together, these findings imply that accumulating alpha-synuclein severely affects hippocampal neurogenesis paralleled by impaired 5-HT neurotransmission prior to the onset of aggregation pathology and overt motor deficits in this transgenic rat model of PD.

Activation and blockade of serotonin7 receptors in the prelimbic cortex regulate depressive-like behaviors in a 6-hydroxydopamine-induced Parkinson's disease rat model

The role of serotonin7 (5-HT7) receptors in the regulation of depression is poorly understood, particularly in Parkinson's disease-associated depression. Here we examined whether 5-HT7 receptors in the prelimbic (PrL) sub-region of the ventral medial prefrontal cortex (mPFC) involve in the regulation of depressive-like behaviors in sham-operated rats and rats with unilateral 6-hydroxydopamine lesions of the medial forebrain bundle. The lesion induced depressive-like responses as measured by the sucrose preference and forced swim tests when compared to sham-operated rats. Intra-PrL injection of 5-HT7 receptor agonist AS19 (0.5, 1 and 2 μg/rat) increased sucrose consumption, and decreased immobility time in sham-operated and the lesioned rats, indicating the induction of antidepressant-like effects. Further, intra-PrL injection of 5-HT7 receptor antagonist SB269970 (1.5, 3 and 6 μg/rat) decreased sucrose consumption, and increased immobility time, indicating the induction of depressive-like responses. However, the doses producing these effects in the lesioned rats were higher than those in sham-operated rats. Neurochemical results showed that intra-PrL injection of AS19 (2 μg/rat) increased dopamine, 5-hydroxytryptamine (5-HT) and noradrenaline (NA) levels in the mPFC, habenula and ventral hippocampus (vHip) in sham-operated and the lesioned rats; whereas SB269970 (6 μg/rat) decreased 5-HT levels in the habenula and vHip, and the levels of NA in the mPFC, habenula and vHip in the two groups of rats. The results suggest that 5-HT7 receptors in the PrL play an important role in the regulation of these behaviors, which attribute to changes in monoamine levels in the limbic and limbic-related brain regions after activation and blockade of 5-HT7 receptors.

Dopaminergic, serotonergic, and noradrenergic deficits in Parkinson disease

OBJECTIVE

People with Parkinson disease (PD) frequently develop dementia, which is associated with neocortical deposition of alpha-synuclein (α-syn) in Lewy bodies and Lewy neurites. In addition, neuronal loss and deposition of aggregated α-syn also occur in multiple subcortical nuclei that project to neocortical, limbic, and basal ganglia regions. Therefore, we quantified regional deficits in innervation from these PD-affected subcortical nuclei, by measuring the neurotransmitters and neurotransmitter transporter proteins originating from projections of dopaminergic neurons in substantia nigra pars compacta, serotonergic neurons in dorsal raphé nuclei, noradrenergic neurons in locus coeruleus, and cholinergic neurons in nucleus basalis of Meynert.

METHODS

High-performance liquid chromatography and novel enzyme-linked immunosorbent assays were performed to quantify dopaminergic, serotonergic, noradrenergic, and cholinergic innervation in postmortem brain tissue. Eight brain regions from 15 PD participants (with dementia and Braak stage 6 α-syn deposition) and six age-matched controls were tested.

RESULTS

PD participants compared to controls had widespread reductions of dopamine transporter in caudate, amygdala, hippocampus, inferior parietal lobule (IPL), precuneus, and visual association cortex (VAC) that exceeded loss of dopamine, which was only significantly reduced in caudate and amygdala. In contrast, PD participants had comparable deficits of both serotonin and serotonin transporter in caudate, middle frontal gyrus, IPL, and VAC. PD participants also had significantly reduced norepinephrine levels for all eight brain regions tested. Vesicular acetylcholine transporter levels were only quantifiable in caudate and hippocampus and did not differ between PD and control groups.

INTERPRETATION

These results demonstrate widespread deficits in dopaminergic, serotonergic, and noradrenergic innervation of neocortical, limbic, and basal ganglia regions in advanced PD with dementia.

Subthalamic Nucleus Deep Brain Stimulation Modulate Catecholamine Levels with Significant Relations to Clinical Outcome after Surgery in Patients with Parkinson's Disease

Aims

Although subthalamic nucleus deep brain stimulation (STN-DBS) is effective in patients with advanced Parkinson’s disease (PD), its physiological mechanisms remain unclear. Because STN-DBS is effective in patients with PD whose motor symptoms are dramatically alleviated by L-3,4-dihydroxyphenylalanine (L-DOPA) treatment, the higher preoperative catecholamine levels might be related to the better clinical outcome after surgery. We aimed to examine the correlation between the preoperative catecholamine levels and postoperative clinical outcome after subthalamic nucleus deep brain stimulation. The effectiveness of STN-DBS in the patient who responded well to dopaminergic medication suggest the causal link between the dopaminergic system and STN-DBS. We also examined how catecholamine levels were modulated after subthalamic stimulation.

Methods

In total 25 patients with PD were enrolled (Mean age 66.2 ± 6.7 years, mean disease duration 11.6 ± 3.7 years). Mean levodopa equivalent doses were 1032 ± 34.6 mg before surgery. Cerebrospinal fluid and plasma catecholamine levels were measured an hour after oral administration of antiparkinsonian drugs before surgery. The mean Unified Parkinson’s Disease Rating Scale scores (UPDRS) and the Parkinson’s disease Questionnaire-39 (PDQ-39) were obtained before and after surgery. Of the 25 patients, postoperative cerebrospinal fluid and plasma were collected an hour after oral administration of antiparkinsonian drugs during on stimulation at follow up in 11 patients.

Results

Mean levodopa equivalent doses significantly decreased after surgery with improvement in motor functions and quality of life. The preoperative catecholamine levels had basically negative correlations with postoperative motor scores and quality of life, suggesting that higher preoperative catecholamine levels were related to better outcome after STN-DBS. The preoperative plasma levels of L-DOPA had significantly negative correlations with postoperative UPDRS- III score in off phase three months after STN-DBS. The preoperative cerebrospinal fluid (CSF) 3,4-dihydroxyphenylacetic acid (DOPAC) and 5-hydroxytryptamine (5-HT) levels had significantly negative correlations with postoperative UPDRS- III score in off phase one year after STN-DBS and the preoperative CSF homovanilic acid (HVA) levels had significant negative correlations with postoperative UPDRS- III score in on phase three months after STN-DBS. In PDQ-39 SI (summary index), preoperative plasma dopamine (DA) level had significantly negative correlations with postoperative PDQ-39 SI one year after STN-DBS suggesting that higher preoperative plasma DA level resulted in better quality of life (QOL) one year after STN-DBS. The stepwise multiple linear regression study revealed that higher preoperative plasma HVA levels had negative influence on the postoperative motor symptoms (i.e., increase in the score of UPDRS), whereas higher preoperative CSF L-DOPA levels had positive influence on the postoperative motor symptoms and QOL (decrease in the score of UPDRS and PDQ-39 SI) The catecholamine levels were not significantly reduced postoperatively in 11 patients despite the significant reduction in levodopa equivalent doses. Unexpectedly, CSF HVA levels significantly increased from 0.00089±0.0003 ng/μl to 0.002±0.0008 ng/μl after STN-DBS.

Conclusion

The preoperative catecholamine levels might affect the postoperative motor symptoms and quality of life. The catecholamine levels were not significantly reduced postoperatively despite the significant reduction in levodopa equivalent doses.

Eltoprazine prevents levodopa-induced dyskinesias by reducing striatal glutamate and direct pathway activity

BACKGROUND

Preclinical and clinical evidence that the serotonergic system plays a major role in levodopa-induced dyskinesias has been provided. Selective serotonin (5-hydroxytryptamine; 5-HT) 5-HT1A or 5-HT1B receptor agonists, and, very recently, the mixed 5-HT1A /5-HT1B receptor agonist, eltoprazine, proved effective in inhibiting L-dopa-induced dyskinesias in experimental animals and parkinsonian patients. Here, we investigate the mechanisms underlying this effect.

METHODS

Microdialysis was employed in 6-hydroxydopamine-hemilesioned rats chronically treated with L-dopa alone or in combination with eltoprazine. Gamma-aminobutyric acid (GABA) and glutamate levels were monitored on L-dopa in the dopamine-depleted striatum and ipsilateral SNr. Motor activity on the rotarod was assessed, both off and on L-dopa. Western blot was used to quantify ex vivo striatal levels of phosphorylated extracellular signal-regulated kinase 1 and 2. Striatal and nigral amino acid levels, as well as striatal dopamine levels, were also monitored in L-dopa-primed dyskinetic rats acutely challenged with L-dopa and eltoprazine.

RESULTS

Eltoprazine attenuated the development and expression of dyskinesias, preserving motor coordination on the rotarod. Eltoprazine prevented the rise of nigral amino acids and striatal glutamate levels, as well as the increase in striatal phosphorylated extracellular signal-regulated kinase 1 and 2, associated with dyskinesias. However, eltoprazine did not affect the L-dopa-induced increase in striatal dopamine.

CONCLUSIONS

Eltoprazine inhibits the sensitization of striatonigral medium-sized GABA spiny neurons (the direct pathway) to L-dopa and their overactivation associated with dyskinesias appearance. Activation of 5-HT1A and 5-HT1B receptors regulating striatal glutamate transmission, but not striatal ectopic dopamine release, might underlie the symptomatic effect of eltoprazine.

Metabolomics Approach Reveals Integrated Metabolic Network Associated with Serotonin Deficiency

Serotonin is an important neurotransmitter that broadly participates in various biological processes. While serotonin deficiency has been associated with multiple pathological conditions such as depression, schizophrenia, Alzheimer's disease and Parkinson's disease, the serotonin-dependent mechanisms remain poorly understood. This study therefore aimed to identify novel biomarkers and metabolic pathways perturbed by serotonin deficiency using metabolomics approach in order to gain new metabolic insights into the serotonin deficiency-related molecular mechanisms. Serotonin deficiency was achieved through pharmacological inhibition of tryptophan hydroxylase (Tph) using p-chlorophenylalanine (pCPA) or genetic knockout of the neuronal specific Tph2 isoform. This dual approach improved specificity for the serotonin deficiency-associated biomarkers while minimizing nonspecific effects of pCPA treatment or Tph2 knockout (Tph2-/-). Non-targeted metabolic profiling and a targeted pCPA dose-response study identified 21 biomarkers in the pCPA-treated mice while 17 metabolites in the Tph2-/- mice were found to be significantly altered compared with the control mice. These newly identified biomarkers were associated with amino acid, energy, purine, lipid and gut microflora metabolisms. Oxidative stress was also found to be significantly increased in the serotonin deficient mice. These new biomarkers and the overall metabolic pathways may provide new understanding for the serotonin deficiency-associated mechanisms under multiple pathological states.

The highly-selective 5-HT(1A) agonist F15599 reduces L-DOPA-induced dyskinesia without compromising anti-parkinsonian benefits in the MPTP-lesioned macaque

L-3,4-dihydroxyphenylalanine (L-DOPA) is the most effective anti-parkinsonian agent available, but upon chronic administration, patients with Parkinson's disease (PD) experience abnormal involuntary movements, dyskinesia. Modulation of serotonin 1A (5-HT1A) receptors is regarded as an effective way to alleviate dyskinesia, yet this approach has been marred by a reduction of the therapeutic effectiveness of L-DOPA. We hypothesised that highly-selective 5-HT1A stimulation might be a way to alleviate dyskinesia without compromising L-DOPA anti-parkinsonian action. F15599 (also known as NLX-101) is a highly-selective 5-HT1A agonist that displays over 1000 × selectivity over off-target receptors. Seven cynomolgus macaques were administered MPTP and developed severe parkinsonism. Following chronic administration of L-DOPA, they developed severe and reproducible dyskinesia. F15599 (0.003, 0.01, 0.03 and 0.1 mg/kg) or vehicle was administered in combination with L-DOPA and its effect on dyskinesia and L-DOPA anti-parkinsonian was assessed. In combination with L-DOPA, F15599 (0.1 mg/kg) reduced the severity of peak-dose dyskinesia, by ≈45% (P < 0.001), compared to L-DOPA alone. F15599 (any dose) had no effect on duration of on-time or motor activity counts compared to L-DOPA alone. F15599 at 0.03 and 0.1 mg/kg significantly reduced duration of on-time with disabling dyskinesia (by ≈49% and ≈71%, P < 0.05 and P < 0.001, respectively). These results suggest that F15599, a highly-selective 5-HT1A receptor agonist, alleviates dyskinesia without exerting a deleterious effect on L-DOPA anti-parkinsonian action.

Nonmotor Symptoms and Subthalamic Deep Brain Stimulation in Parkinson's Disease

Subthalamic deep brain stimulation (STN DBS) is an established treatment for the motor symptoms in patients with advanced Parkinson’s disease (PD). In addition to improvements in motor symptoms, many studies have reported changes in various nonmotor symptoms (NMSs) after STN DBS in patients with PD. Psychiatric symptoms, including depression, apathy, anxiety, and impulsivity, can worsen or improve depending on the electrical stimulation parameters, the locations of the stimulating contacts within the STN, and changes in medications after surgery. Global cognitive function is not affected by STN DBS, and there is no increase in the incidence of dementia after STN DBS compared to that after medical treatment, although clinically insignificant declines in verbal fluency have been consistently reported. Pain, especially PD-related pain, improves with STN DBS. Evidence regarding the effects of STN DBS on autonomic symptoms and sleep-related problems is limited and remains conflicting. Many symptoms of nonmotor fluctuations, which are occasionally more troublesome than motor fluctuations, improve with STN DBS. Although it is clear that NMSs are not target symptoms for STN DBS, NMSs have a strong influence on the quality of life of patients with PD, and clinicians should thus be aware of these NMSs when deciding whether to perform surgery and should pay attention to changes in these symptoms after STN DBS to ensure the optimal care for patients.

Reduced plasma serotonin and 5-hydroxyindoleacetic acid levels in Parkinson's disease are associated with nonmotor symptoms

BACKGROUND

Accumulating evidence suggests that serotonergic system may be implicated in the pathophysiology of Parkinson's disease (PD), and particularly in nonmotor symptoms such as depression, fatigue, sleep disorders, sensory and autonomic dysfunction. This study aimed to evaluate plasma levels of serotonin (5-HT) and its metabolite 5-hydroxyindoleacetic acid (5-HIAA) in PD patients, and investigate their associations with nonmotor symptoms.

METHODS

Eighty-two PD patients and sixty-four controls underwent a series of clinical assessments, including Hamilton Depression Scale, Fatigue Severity Scale, Pittsburgh Sleep Quality Index, Visual Analog Scale for Pain, and Scale for Outcomes in PD for Autonomic Symptoms. Plasma 5-HT and 5-HIAA levels were measured by HPLC-ECD.

RESULTS

PD patients exhibited worse performance on nonmotor symptom scales (all P-values <0.001) and presented lower plasma levels of 5-HT (P < 0.001) and 5-HIAA (P < 0.001) than control individuals. Within the PD group, decreased concentrations of plasma 5-HT and 5-HIAA were correlated with more severe depression (r = -0.447, P < 0.001; r = -0.407, P < 0.001, respectively) and pain (r = -0.485, P < 0.001; r = -0.416, P < 0.001, respectively). After performing multiple linear regression, plasma 5-HT (P = 0.01) and 5-HIAA (P = 0.006) remained significantly associated with depression.

CONCLUSIONS

Our results suggest that serotonergic dysfunction might exist in PD, and specifically correlated with depression and pain in PD. Plasma levels of 5-HT and 5-HIAA may be considered as peripheral markers for depression in PD.

Dorsal-to-Ventral Shift in Midbrain Dopaminergic Projections and Increased Thalamic/Raphe Serotonergic Function in Early Parkinson Disease

Loss of nigrostriatal neurons leading to dopamine depletion in the dorsal striatum is the pathologic hallmark of Parkinson disease contributing to the primary motor symptoms of the disease. However, Parkinson pathology is more widespread in the brain, affecting also other dopaminergic pathways and neurotransmitter systems, but these changes are less well characterized. This study aimed to investigate the mesencephalic striatal and extrastriatal dopaminergic projections together with extrastriatal serotonin transporter binding in Parkinson disease.

METHODS

Two hundred sixteen patients with Parkinson disease and 204 control patients (patients without neurodegenerative parkinsonism syndromes and normal SPECT imaging) were investigated with SPECT using the dopamine/serotonin transporter ligand (123)I-N-ω-fluoropropyl-2β-carbomethoxy-3β-(4-iodophenyl)nortropane ((123)I-FP-CIT) in the clinical setting. The group differences and midbrain correlations were analyzed voxel by voxel over the entire brain.

RESULTS

We found that Parkinson patients had lower (123)I-FP-CIT uptake in the striatum and ventral midbrain but higher uptake in the thalamus and raphe nuclei than control patients. In patients with Parkinson disease, the correlation of the midbrain tracer uptake was shifted from the putamen to widespread corticolimbic areas. All findings were highly significant at the voxel level familywise error-corrected P value of less than 0.05.

CONCLUSION

Our findings show that Parkinson disease is associated not only with the degeneration of the nigrostriatal dopamine neurotransmission, but also with a parallel shift toward mesolimbic and mesocortical function. Furthermore, Parkinson disease patients seem to have upregulation of brain serotonin transporter function at the early phase of the disease.

Monoamine reuptake inhibitors in Parkinson's disease

The motor manifestations of Parkinson's disease (PD) are secondary to a dopamine deficiency in the striatum. However, the degenerative process in PD is not limited to the dopaminergic system and also affects serotonergic and noradrenergic neurons. Because they can increase monoamine levels throughout the brain, monoamine reuptake inhibitors (MAUIs) represent potential therapeutic agents in PD. However, they are seldom used in clinical practice other than as antidepressants and wake-promoting agents. This review article summarises all of the available literature on use of 50 MAUIs in PD. The compounds are divided according to their relative potency for each of the monoamine transporters. Despite wide discrepancy in the methodology of the studies reviewed, the following conclusions can be drawn: (1) selective serotonin transporter (SERT), selective noradrenaline transporter (NET), and dual SERT/NET inhibitors are effective against PD depression; (2) selective dopamine transporter (DAT) and dual DAT/NET inhibitors exert an anti-Parkinsonian effect when administered as monotherapy but do not enhance the anti-Parkinsonian actions of L-3,4-dihydroxyphenylalanine (L-DOPA); (3) dual DAT/SERT inhibitors might enhance the anti-Parkinsonian actions of L-DOPA without worsening dyskinesia; (4) triple DAT/NET/SERT inhibitors might exert an anti-Parkinsonian action as monotherapy and might enhance the anti-Parkinsonian effects of L-DOPA, though at the expense of worsening dyskinesia.

Serotonergic markers in Parkinson's disease and levodopa-induced dyskinesias

Preclinical animal models implicate serotonin neurons in the pathophysiology of levodopa (l-dopa)-induced dyskinesias in Parkinson's disease (PD), but effective treatment remains elusive. We examined the relationship between serotonin and l-dopa-induced dyskinesias in a pathologically confirmed cohort of PD patients. We obtained brain tissue from 44 PD cases and 17 age-matched controls and assessed monoamine levels and the serotonin and dopamine transporters in the striatum, and the extent of dopaminergic and serotonergic cell preservation in the substantia nigra (SN) and the dorsal raphe nuclei (DRN), respectively. As expected, PD patients demonstrated a severe loss of all dopaminergic markers, including dopamine (P < 0.0001) and the dopamine transporter (P < 0.0001) in the striatum, and dopaminergic neurons (P < 0.001) in the SN, compared with controls. Marked serotonin loss was observed in the caudate (but not putamen) in PD patients compared with controls (P < 0.001), but no difference was found in the levels of the serotonin transporter in the striatum or density of serotonergic neurons in the DRN between these groups, suggesting a functional but not structural change in the serotonergic system in PD. No difference was seen in levels of serotonergic and dopaminergic markers in the striatum between PD patients with and without dyskinesias, or between cases separated according to the clinical severity of their dyskinesias. The absence of a correlation between striatal serotonin markers and the incidence and severity of l-dopa-induced dyskinesias suggests that an intact and functioning serotonergic system is not a risk factor for developing dyskinesias in PD.

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.

Recent progress of imaging agents for Parkinson's disease

Parkinson's disease (PD) is a common progressive, neurodegenerative brain disease that is promoted by mitochondrial dysfunction, oxidative stress, protein aggregation and proteasome dysfunction in the brain. Compared with computer tomography (CT) or magnetic resonance imaging (MRI), non-invasive nuclear radiopharmaceuticals have great significance for the early diagnosis of PD due to their high sensitivity and specificity in atypical and preclinical cases. Based on the development of coordination chemistry and chelator design, radionuclides may be delivered to lesions by attaching to PD-related transporters and receptors, such as dopamine, serotonin, and others. In this review, we comprehensively detailed the current achievements in radionuclide imaging in Parkinson's disease.