Role of serotonergic 1A receptor dysfunction in depression associated with Parkinson's disease

Exploring Parkinson's Disease-Associated Depression: Role of Inflammation on the Noradrenergic and Serotonergic Pathways

Parkinson's disease (PD) is a multifactorial disease, with genetic and environmental factors contributing to the disease onset. Classically, PD is a movement disorder characterized by the loss of dopaminergic neurons in the nigrostriatal pathway and intraneuronal aggregates mainly constituted of the protein α-synuclein. However, PD patients also display non-motor symptoms, including depression, which have been linked to functional abnormalities of non-dopaminergic neurons, including serotonergic and noradrenergic ones. Thus, through this comprehensive literature review, we shed light on the noradrenergic and serotonergic impairment linked to depression in PD, focusing on the putative involvement of inflammatory mechanisms.

Investigating affective neuropsychiatric symptoms in rodent models of Parkinson's disease

Affective neuropsychiatric disorders such as depression, anxiety and apathy are among the most frequent non-motor symptoms observed in people with Parkinson's disease (PD). These conditions often emerge during the prodromal phase of the disease and are generally considered to result from neurodegenerative processes in meso-corticolimbic structures, occurring in parallel to the loss of nigrostriatal dopaminergic neurons. Depression, anxiety, and apathy are often treated with conventional medications, including selective serotonin reuptake inhibitors, tricyclic antidepressants, and dopaminergic agonists. The ability of these pharmacological interventions to consistently counteract such neuropsychiatric symptoms in PD is still relatively limited and the development of reliable experimental models represents an important tool to identify more effective treatments. This chapter provides information on rodent models of PD utilized to study these affective neuropsychiatric symptoms. Neurotoxin-based and genetic models are discussed, together with the main behavioral tests utilized to identify depression- and anxiety-like behaviors, anhedonia, and apathy. The ability of various therapeutic approaches to counteract the symptoms observed in the various models is also reviewed.

Anxiety in synucleinopathies: neuronal circuitry, underlying pathomechanisms and current therapeutic strategies

Synucleinopathies are neurodegenerative disorders characterized by alpha-synuclein (αSyn) accumulation in neurons or glial cells, including Parkinson's disease (PD), dementia with Lewy bodies (DLB), and multiple system atrophy (MSA). αSyn-related pathology plays a critical role in the pathogenesis of synucleinopathies leading to the progressive loss of neuronal populations in specific brain regions and the development of motor and non-motor symptoms. Anxiety is among the most frequent non-motor symptoms in patients with PD, but it remains underrecognized and undertreated, which significantly reduces the quality of life for patients. Anxiety is defined as a neuropsychiatric complication with characteristics such as nervousness, loss of concentration, and sweating due to the anticipation of impending danger. In patients with PD, neuropathology in the amygdala, a central region in the anxiety and fear circuitry, may contribute to the high prevalence of anxiety. Studies in animal models reported αSyn pathology in the amygdala together with alteration of anxiety or fear learning response. Therefore, understanding the progression, extent, and specifics of pathology in the anxiety and fear circuitry in synucleinopathies will suggest novel approaches to the diagnosis and treatment of neuropsychiatric symptoms. Here, we provide an overview of studies that address neuropsychiatric symptoms in synucleinopathies. We offer insights into anxiety and fear circuitry in animal models and the current implications for therapeutic intervention. In summary, it is apparent that anxiety is not a bystander symptom in these disorders but reflects early pathogenic mechanisms in the cortico-limbic system which may even contribute as a driver to disease progression.

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".

The Role of α-Synuclein in the Regulation of Serotonin System: Physiological and Pathological Features

In patients affected by Parkinson's disease (PD), up to 50% of them experience cognitive changes, and psychiatric disturbances, such as anxiety and depression, often precede the onset of motor symptoms and have a negative impact on their quality of life. Pathologically, PD is characterized by the loss of dopamine (DA) neurons in the substantia nigra pars compacta (SNc) and the presence of intracellular inclusions, called Lewy bodies and Lewy neurites, composed mostly of α-synuclein (α-Syn). Much of PD research has focused on the role of α-Syn aggregates in the degeneration of SNc DA neurons due to the impact of striatal DA deficits on classical motor phenotypes. However, abundant Lewy pathology is also found in other brain regions including the midbrain raphe nuclei, which may contribute to non-motor symptoms. Indeed, dysfunction of the serotonergic (5-HT) system, which regulates mood and emotional pathways, occurs during the premotor phase of PD. However, little is known about the functional consequences of α-Syn inclusions in this neuronal population other than DA neurons. Here, we provide an overview of the current knowledge of α-Syn and its role in regulating the 5-HT function in health and disease. Understanding the relative contributions to α-Syn-linked alterations in the 5-HT system may provide a basis for identifying PD patients at risk for developing depression and could lead to a more targeted therapeutic approach.

Botulinum neurotoxin A ameliorates depressive-like behavior in a reserpine-induced Parkinson's disease mouse model via suppressing hippocampal microglial engulfment and neuroinflammation

Depression is one of the common non-motor symptoms of Parkinson's disease (PD). In the clinic, botulinum neurotoxin A (BoNT/A) has been used to treat depression. In this study, we investigated the mechanisms underlying the anti-depressive effect of BoNT/A in a PD mouse model. Mice were administered reserpine (3 μg/mL in the drinking water) for 10 weeks. From the 10^th week, BoNT/A (10 U·kg^-1·d^-1) was injected into the cheek for 3 consecutive days. We showed that chronic administration of reserpine produced the behavioral phenotypes of depression and neurochemical changes in the substantia nigra pars compacta (SNpc) and striatum. BoNT/A treatment significantly ameliorated the depressive-like behaviors, but did not improve TH activity in SNpc of reserpine-treated mice. We demonstrated that BoNT/A treatment reversed reserpine-induced complement and microglia activation in the hippocampal CA1 region. Furthermore, BoNT/A treatment significantly attenuated the microglial engulfment of presynaptic synapses, thus ameliorating the apparent synapse and spine loss in the hippocampus in the reserpine-treated mice. Moreover, BoNT/A treatment suppressed microglia-mediated expression of pro-inflammatory cytokines TNF-α and IL-1β in reserpine-treated mice. In addition, we showed that BoNT/A (0.1 U/mL) ameliorated reserpine-induced complement and microglia activation in mouse BV2 microglial cells in vitro. We conclude that BoNT/A ameliorates depressive-like behavior in a reserpine-induced PD mouse model through reversing the synapse loss mediated by classical complement induced-microglial engulfment as well as alleviating microglia-mediated proinflammatory responses. BoNT/A ameliorates depressive-like behavior, and reverses synapse loss mediated by classical complement pathway-initiated microglia engulfment as well as alleviates microglia-mediated proinflammatory response in the reserpine-induced Parkinson's disease mouse model.

Brain Serotonin Release Is Reduced in Patients With Depression: A [11C]Cimbi-36 Positron Emission Tomography Study With a d-Amphetamine Challenge

BACKGROUND

The serotonin hypothesis of depression proposes that diminished serotonergic (5-HT) neurotransmission is causal in the pathophysiology of the disorder. Although the hypothesis is over 50 years old, there is no firm in vivo evidence for diminished 5-HT neurotransmission. We recently demonstrated that the 5-HT_2A receptor agonist positron emission tomography (PET) radioligand [¹¹C]Cimbi-36 is sensitive to increases in extracellular 5-HT induced by an acute d-amphetamine challenge. Here we applied [¹¹C]Cimbi-36 PET to compare brain 5-HT release capacity in patients experiencing a major depressive episode (MDE) to that of healthy control subjects (HCs) without depression.

METHODS

Seventeen antidepressant-free patients with MDE (3 female/14 male, mean age 44 ± 13 years, Hamilton Depression Rating Scale score 21 ± 4 [range 16-30]) and 20 HCs (3 female/17 male, mean age 32 ± 9 years) underwent 90-minute dynamic [¹¹C]Cimbi-36 PET before and 3 hours after a 0.5-mg/kg oral dose of d-amphetamine. Frontal cortex (main region of interest) 5-HT_2A receptor nondisplaceable binding was calculated from kinetic analysis using the multilinear analysis-1 approach with the cerebellum as the reference region.

RESULTS

Following d-amphetamine administration, frontal nondisplaceable binding potential (BP_ND) was significantly reduced in the HC group (1.04 ± 0.31 vs. 0.87 ± 0.24, p < .001) but not in the MDE group (0.97 ± 0.25 vs. 0.92 ± 0.22, not significant). ΔBP_ND of the MDE group was significantly lower than that of the HC group (HC: 15% ± 14% vs. MDE: 6.5% ± 20%, p = .041).

CONCLUSIONS

This first direct assessment of 5-HT release capacity in people with depression provides clear evidence for dysfunctional serotonergic neurotransmission in depression by demonstrating reduced 5-HT release capacity in patients experiencing an MDE.

Synthesis and Preclinical Evaluation of [11C]AZ11895530 for PET Imaging of the Serotonin 1A Receptor

The serotonin 1A (5-HT_1A) receptor is a G-protein-coupled receptor implicated in the pathophysiology of several neuropsychiatric and neurodegenerative disorders. We here report the preparation of two candidate 5-HT_1A radioligands, [¹¹C]AZ11132132 ([¹¹C]3) and [¹¹C]AZ11895530 ([¹¹C]4), and their subsequent evaluation in vitro using autoradiography and in vivo using positron emission tomography (PET). Compounds 3 and 4 were radiolabeled at high radiochemical purity (>99%) and high molar activity (>38 GBq/μmol) by heteroatom methylation with [¹¹C]methyl iodide. Autoradiography on whole hemispheres from post-mortem human brain revealed substantial nonspecific binding of [¹¹C]3, while the binding of [¹¹C]4 to brain tissue was consistent with the distribution of 5-HT_1A receptors and sensitive to co-incubation with the reference 5-HT_1A antagonist WAY-100635 (10 μM). Following intravenous injection of [¹¹C]4 into a cynomolgus monkey, brain radioactivity concentration (C_max ∼ 2.2 SUV) was high whereafter it decreased rapidly. The regional binding potential (BP_ND) values were calculated using the simplified reference tissue model with cerebellum as reference region. The values varied between 0.2 and 1.0 for temporal cortex, raphe nuclei, frontal cortex, and hippocampus which is consistent with the known 5-HT_1A expression pattern. After pretreatment with WAY100635 (0.5 mg/kg), a homogeneous distribution of radioactivity was observed in non-human primate (NHP) brain. Although [¹¹C]4 fulfilled important criteria for successful in vivo neuroimaging, including good blood-brain-barrier permeability and high specific binding in vitro to human brain tissue, the regional BP_ND values for [¹¹C]4 in NHP brain were low when compared to those obtained with existing radioligands and thus do not merit further investigation of [¹¹C]4. Evaluation of structurally related analogues is underway in our laboratory to identify improved candidates for clinical imaging.

Depression in Patients with Parkinson's Disease: Current Understanding of its Neurobiology and Implications for Treatment

Depression is one of the most frequent and burdensome non-motor symptoms in Parkinson’s disease (PD), across all stages. Even when its severity is mild, PD depression has a great impact on quality of life for these patients and their caregivers. Accordingly, accurate diagnosis, supported by validated scales, identification of risk factors, and recognition of motor and non-motor symptoms comorbid to depression are critical to understanding the neurobiology of depression, which in turn determines the effectiveness of dopaminergic drugs, antidepressants and non-pharmacological interventions. Recent advances using in vivo functional and structural imaging demonstrate that PD depression is underpinned by dysfunction of limbic networks and monoaminergic systems, depending on the stage of PD and its associated symptoms, including apathy, anxiety, rapid eye movement sleep behavior disorder (RBD), cognitive impairment and dementia. In particular, the evolution of serotonergic, noradrenergic, and dopaminergic dysfunction and abnormalities of limbic circuits across time, involving the anterior cingulate and orbitofrontal cortices, amygdala, thalamus and ventral striatum, help to delineate the variable expression of depression in patients with prodromal, early and advanced PD. Evidence is accumulating to support the use of dual serotonin and noradrenaline reuptake inhibitors (desipramine, nortriptyline, venlafaxine) in patients with PD and moderate to severe depression, while selective serotonin reuptake inhibitors, repetitive transcranial magnetic stimulation and cognitive behavioral therapy may also be considered. In all patients, recent findings advocate that optimization of dopamine replacement therapy and evaluation of deep brain stimulation of the subthalamic nucleus to improve motor symptoms represents an important first step, in addition to physical activity. Overall, this review indicates that increasing understanding of neurobiological changes help to implement a roadmap of tailored interventions for patients with PD and depression, depending on the stage and comorbid symptoms underlying PD subtypes and their prognosis.

Structural Plasticity of the Hippocampus in Neurodegenerative Diseases

Neuroplasticity is the capacity of neural networks in the brain to alter through development and rearrangement. It can be classified as structural and functional plasticity. The hippocampus is more susceptible to neuroplasticity as compared to other brain regions. Structural modifications in the hippocampus underpin several neurodegenerative diseases that exhibit cognitive and emotional dysregulation. This article reviews the findings of several preclinical and clinical studies about the role of structural plasticity in the hippocampus in neurodegenerative diseases, including Alzheimer’s disease, Parkinson’s disease, Huntington’s disease, and multiple sclerosis. In this study, literature was surveyed using Google Scholar, PubMed, Web of Science, and Scopus, to review the mechanisms that underlie the alterations in the structural plasticity of the hippocampus in neurodegenerative diseases. This review summarizes the role of structural plasticity in the hippocampus for the etiopathogenesis of neurodegenerative diseases and identifies the current focus and gaps in knowledge about hippocampal dysfunctions. Ultimately, this information will be useful to propel future mechanistic and therapeutic research in neurodegenerative diseases.

Altered Neural Network Connectivity Predicts Depression in de novo Parkinson’s Disease

Background

Depression, one of the most frequent non-motor symptoms in Parkinson’s disease (PD), was proposed to be related to neural network dysfunction in advanced PD patients. However, the underlying mechanisms in the early stage remain unclear. The study was aimed to explore the alterations of large-scale neural networks in de novo PD patients with depression.

Methods

We performed independent component analysis (ICA) on the data of resting-state functional magnetic resonance imaging from 21 de novo PD patients with depression (dPD), 34 de novo PD patients without depression (ndPD), and 43 healthy controls (HCs) to extract functional networks. Intranetwork and internetwork connectivity was calculated for comparison between groups, correlation analysis, and predicting the occurrence of depression in PD.

Results

We observed an ordered decrease of connectivity among groups within the ventral attention network (VAN) (dPD < ndPD < HCs), mainly located in the left middle temporal cortex. Besides, dPD patients exhibited hypoconnectivity between the auditory network (AUD) and default mode network (DMN) or VAN compared to ndPD patients or healthy controls. Correlation analysis revealed that depression severity was negatively correlated with connectivity value within VAN and positively correlated with the connectivity value of AUD-VAN in dPD patients, respectively. Further analysis showed that the area under the curve (AUC) for dPD prediction was 0.863 when combining the intranetwork connectivity in VAN and internetwork connectivity in AUD-DMN and AUD-VAN.

Conclusion

Our results demonstrated that early dPD may be associated with abnormality of attention bias and especially auditory attention processing. Altered neural network connectivity is expected to be a potential neuroimaging biomarker to predict depression in PD.

Human α-synuclein overexpression in mouse serotonin neurons triggers a depressive-like phenotype. Rescue by oligonucleotide therapy

Anxiety and depression affect 35-50% of patients with Parkinson's disease (PD), often precede the onset of motor symptoms, and have a negative impact on their quality of life. Dysfunction of the serotonergic (5-HT) system, which regulates mood and emotional pathways, occurs during the premotor phase of PD and contributes to a variety of non-motor symptoms. Furthermore, α-synuclein (α-Syn) aggregates were identified in raphe nuclei in the early stages of the disease. However, there are very few animal models of PD-related neuropsychiatric disorders. Here, we develop a new mouse model of α-synucleinopathy in the 5-HT system that mimics prominent histopathological and neuropsychiatric features of human PD. We showed that adeno-associated virus (AAV5)-induced overexpression of wild-type human α-Syn (h-α-Syn) in raphe 5-HT neurons triggers progressive accumulation, phosphorylation, and aggregation of h-α-Syn protein in the 5-HT system. Specifically, AAV5-injected mice displayed axonal impairment in the output brain regions of raphe neurons, and deficits in brain-derived neurotrophic factor (BDNF) expression and 5-HT neurotransmission, resulting in a depressive-like phenotype. Intracerebroventricular treatment with an indatraline-conjugated antisense oligonucleotide (IND-ASO) for four weeks induced an effective and safe reduction of h-α-Syn synthesis in 5-HT neurons and its accumulation in the forebrain, alleviating early deficits of 5-HT function and improving the behavioural phenotype. Altogether, our findings show that α-synucleinopathy in 5-HT neurons negatively affects brain circuits that control mood and emotions, resembling the expression of neuropsychiatric symptoms occurring at the onset of PD. Early preservation of 5-HT function by reducing α-Syn synthesis/accumulation may alleviate PD-related depressive symptoms.

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.

Cholinergic Receptor Modulation as a Target for Preventing Dementia in Parkinson's Disease

Parkinson’s disease (PD) is a neurodegenerative condition characterized by the loss of dopaminergic neurons in the substantia nigra pars compacta (SNpc) in the midbrain resulting in progressive impairment in cognitive and motor abilities. The physiological and molecular mechanisms triggering dopaminergic neuronal loss are not entirely defined. PD occurrence is associated with various genetic and environmental factors causing inflammation and mitochondrial dysfunction in the brain, leading to oxidative stress, proteinopathy, and reduced viability of dopaminergic neurons. Oxidative stress affects the conformation and function of ions, proteins, and lipids, provoking mitochondrial DNA (mtDNA) mutation and dysfunction. The disruption of protein homeostasis induces the aggregation of alpha-synuclein (α-SYN) and parkin and a deficit in proteasome degradation. Also, oxidative stress affects dopamine release by activating ATP-sensitive potassium channels. The cholinergic system is essential in modulating the striatal cells regulating cognitive and motor functions. Several muscarinic acetylcholine receptors (mAChR) and nicotinic acetylcholine receptors (nAChRs) are expressed in the striatum. The nAChRs signaling reduces neuroinflammation and facilitates neuronal survival, neurotransmitter release, and synaptic plasticity. Since there is a deficit in the nAChRs in PD, inhibiting nAChRs loss in the striatum may help prevent dopaminergic neurons loss in the striatum and its pathological consequences. The nAChRs can also stimulate other brain cells supporting cognitive and motor functions. This review discusses the cholinergic system as a therapeutic target of cotinine to prevent cognitive symptoms and transition to dementia in PD.

The role of neuroimaging in Parkinson's disease

Parkinson's disease (PD) is a neurodegenerative disorder that affects millions of people worldwide. Two hallmarks of PD are the accumulation of alpha-synuclein and the loss of dopaminergic neurons in the brain. There is no cure for PD, and all existing treatments focus on alleviating the symptoms. PD diagnosis is also based on the symptoms, such as abnormalities of movement, mood, and cognition observed in the patients. Molecular imaging methods such as magnetic resonance imaging (MRI), single-photon emission computed tomography (SPECT), and positron emission tomography (PET) can detect objective alterations in the neurochemical machinery of the brain and help diagnose and study neurodegenerative diseases. This review addresses the application of functional MRI, PET, and SPECT in PD patients. We provide an overview of the imaging targets, discuss the rationale behind target selection, the agents (tracers) with which the imaging can be performed, and the main findings regarding each target's state in PD. Molecular imaging has proven itself effective in supporting clinical diagnosis of PD and has helped reveal that PD is a heterogeneous disorder, which has important implications for the development of future therapies. However, the application of molecular imaging for early diagnosis of PD or for differentiation between PD and atypical parkinsonisms has remained challenging. The final section of the review is dedicated to new imaging targets with which one can detect the PD-related pathological changes upstream from dopaminergic degeneration. The foremost of those targets is alpha-synuclein. We discuss the progress of tracer development achieved so far and challenges on the path toward alpha-synuclein imaging in humans.

Serotonergic and Dopaminergic Lesions Underlying Parkinsonian Neuropsychiatric Signs

BACKGROUND

Parkinson's disease (PD) is characterized by heterogeneous motor and nonmotor manifestations related to alterations in monoaminergic neurotransmission systems. Nevertheless, the characterization of concomitant dopaminergic and serotonergic dysfunction after different durations of Parkinson's disease, as well as their respective involvement in the expression and severity of neuropsychiatric signs, has gained little attention so far.

METHODS

To fill this gap, we conducted a cross-sectional study combining clinical and dual-tracer positron emission tomography (PET) neuroimaging approaches, using radioligands of dopamine ([¹¹ C]-N-(3-iodoprop-2E-enyl)-2-beta-carbomethoxy-3-beta-(4-methylphenyl)-nortropane) ([¹¹ C]PE2I) and serotonin ([¹¹ C]-N,N-dimethyl-2-(-2-amino-4-cyanophenylthio)-benzylamine) ([¹¹ C]DASB) reuptake, after different durations of Parkinson's disease (ie, in short-disease duration drug-naive de novo (n = 27, 0-2 years-duration), suffering from apathy (n = 14) or not (n = 13); intermediate-disease duration (n = 15, 4-7 years-duration) and long-disease duration, non-demented (n = 15, 8-10 years-duration) patients). Fifteen age-matched healthy subjects were also enrolled.

RESULTS

The main findings are threefold: (1) both dopaminergic and serotonergic lesions worsen with the duration of Parkinson's disease, spreading from midbrain/subcortical to cortical regions; (2) the presence of apathy at PD onset is associated with more severe cortical and subcortical serotonergic and dopaminergic disruption, similar to the denervation pattern observed in intermediate-disease duration patients; and (3) the severity of parkinsonian apathy, depression, and trait-anxiety appears primarily related to serotonergic alteration within corticostriatal limbic areas.

CONCLUSIONS

Altogether, these findings highlight the prominent role of serotonergic degeneration in the expression of several neuropsychiatric symptoms occurring after different durations of Parkinson's disease. © 2021 International Parkinson and Movement Disorder Society.

Asymmetry of the insula-sensorimotor circuit in Parkinson's disease

Patients with Parkinson's disease (PD) experience motor and non-motor symptoms, suggesting alterations of the motor and/or limbic system or more probably of their communications. We hypothesized that the communication between the insula (part of the limbic system) and sensorimotor cortex in PD is altered and hemispheric asymmetric. Furthermore, that this asymmetry relates to non-motor symptoms, and specifically, that apathy-related asymmetry is unique to PD. To test these hypotheses, we used a novel multivariate time-frequency analysis method applied to resting-state functional magnetic resonance imaging (MRI) data of 28 controls and 25 participants with PD measured in their OFF medication state. The analysis infers directionality of coupling, that is, afferent or efferent, among four anatomical regions, thus defining directed pathways of information flow, which enables the extension of symmetry measures to include directionality. A major right asymmetry reduction of the dorsal-posterior insula efferent and a slight bilateral increase of insula afferent pathways were observed in participants with PD versus controls. Between-group pathways that correlated with mild cognitive impairments combined the central-executive and default-mode networks through the right insula. Apathy-correlated pathways of the posterior insula in participants with PD versus controls exhibited reduced right efferent and increased left afferent. Because apathy scores were comparable between the groups and effects of the other motor and non-motor symptoms were statistically removed by the analysis, the differences in apathy-correlated pathways were suggested as unique to PD. These pathways could be predictors in the pre-symptomatic phase in patients with apathy.

Different Alterations of Agonist and Antagonist Binding to 5-HT1A Receptor in a Rat Model of Parkinson’s Disease and Levodopa-Induced Dyskinesia: A MicroPET Study

Background: The gold-standard treatment for Parkinson’s disease is L-DOPA, which in the long term often leads to levodopa-induced dyskinesia. Serotonergic neurons are partially responsible for this, by converting L-DOPA into dopamine leading to its uncontrolled release as a “false neurotransmitter”. The stimulation of 5-HT1A receptors can reduce involuntary movements but this mechanism is poorly understood. Objective: This study aimed to investigate the functionality of 5-HT1A receptors using positron emission tomography in hemiparkinsonian rats with or without dyskinesia induced by 3-weeks daily treatment with L-DOPA. Imaging sessions were performed “off” L-DOPA. Methods: Each rat underwent a positron emission tomography scan with [18F]F13640, a 5-HT1AR agonist which labels receptors in a high affinity state for agonists, or with [18F]MPPF, a 5-HT1AR antagonist which labels all the receptors. Results: There were decreases of [18F]MPPF binding in hemiparkinsonian rats in cortical areas. In dyskinetic animals, changes were slighter but also found in other regions. In hemiparkinsonian rats, [18F]F13640 uptake was decreased bilaterally in the globus pallidus and thalamus. On the non-lesioned side, binding was increased in the insula, the hippocampus and the amygdala. In dyskinetic animals, [18F]F13640 binding was strongly increased in cortical and limbic areas, especially in the non-lesioned side. Conclusion: These data suggest that agonist and antagonist 5-HT1A receptor-binding sites are differently modified in Parkinson’s disease and levodopa-induced dyskinesia. In particular, these observations suggest a substantial involvement of the functional state of 5-HT1AR in levodopa-induced dyskinesia and emphasize the need to characterize this state using agonist radiotracers in physiological and pathological conditions.

The effect of 5-HT1A receptor agonists on the entopeduncular nucleus is modified in 6-hydroxydopamine-lesioned rats

BACKGROUND AND PURPOSE

l-DOPA prolonged treatment leads to disabling motor complications as dyskinesia that could be decreased by drugs acting on 5-HT1A receptors. Since the internal segment of the globus pallidus, homologous to the entopeduncular nucleus in rodents, seems to be involved in the etiopathology of l-DOPA-induced dyskinesia, we investigated whether the entopeduncular nucleus is modulated by the 5-HT1A receptor partial and full agonists, buspirone, and 8-hydroxy-2-(di-n-propylamino)-tetralin (8-OH-DPAT) in control and 6-hydroxydopamine (6-OHDA)-lesioned rats with or without long-term l-DOPA treatment.

EXPERIMENTAL APPROACH

Extracellular single-unit electrocorticogram and local field potential recordings under anaesthesia, immunostaining assays and optogenetic manipulation coupled to electrophysiological recordings were performed.

KEY RESULTS

Systemic buspirone reduced the entopeduncular nucleus firing rate in the sham animals and burst activity in the 6-OHDA-lesioned rats (with or without l-DOPA treatment), while local administration reduced entopeduncular nucleus activity in all the groups, regardless of DA integrity. Systemic 8-OH-DPAT also induced inhibitory effects only in the sham animals. Effects triggered by buspirone and 8-OH-DPAT were reversed by the 5-HT1A receptor antagonist, WAY-100635. Neither buspirone nor 8-OH-DPAT modified the low-frequency oscillatory activity in the entopeduncular nucleus or its synchronization with the motor cortex. Buspirone did not alter the response induced by subthalamic nucleus opto-stimulation in the entopeduncular nucleus.

CONCLUSION AND IMPLICATIONS

Systemic 5-HT1A receptor activation elicits different effects on the electrophysiological properties of the entopeduncular nucleus depending on the integrity of the nigrostriatal pathway and it does not alter the relationship between subthalamic nucleus and entopeduncular nucleus neuron activity.

Changes in Degree Centrality of Network Nodes in Different Frequency Bands in Parkinson's Disease With Depression and Without Depression

Background

Depression induces an early onset of Parkinson’s disease (PD), aggravates dyskinesia and cognitive impairment, and accelerates disease progression. However, it is very difficult to identify and diagnose PD with depression (PDD) in the early clinical stage. Few studies have suggested that the changes in neural networks are associated with PDD, while degree centrality (DC) has been documented to be effective in detecting brain network changes.

Objectives

The objectives of this study are to explore DC changes between patients with PDD and without depression (PDND) and to find the key brain hubs involved with depression in PD patients.

Methods

One hundred and four PD patients and 54 healthy controls (HCs) underwent brain resting-state functional magnetic resonance imaging. The Data Processing and Analysis of Brain Imaging and Resting-State Functional Magnetic Resonance Data Analysis Toolkit were used for processing and statistical analysis. The DC value of each frequency band was calculated. One-way analysis of variance and a two-sample t-test for post hoc comparison were used to compare the differences of the DC values in different frequency bands among PDD, PDND, and healthy control group. Gaussian random field was used for multiple comparison correction. Pearson correlation analysis was performed between each individual’s DC map and clinical indicators.

Results

The DC value of different brain regions changed in PDD and PDND in different frequency bands. The prefrontal lobe, limbic system, and basal ganglia were the main brain regions involved. PDD patients showed a wider range and more abnormal brain areas in the slow-4 frequency band (0.027–0.073 Hz) compared to the HCs. PDD showed a decreased DC value in the medial frontal gyrus, bilateral cuneus gyrus, right lingual gyrus, bilateral supplementary motor area (SMA), bilateral superior frontal gyrus, and left paracentral lobule, but an increased DC value in the bilateral brainstem, midbrain, bilateral parahippocampal gyrus, cerebellum, left superior temporal gyrus, bilateral insula, left fusiform gyrus, and left caudate nucleus in the traditional frequency band (0.01–0.08 Hz) compared to PDND patients. PDND patients displayed more abnormal functions in the basal ganglia in the slow-4 frequency band.

Conclusion

The DC changes in PDD and PDND are frequency dependent and frequency specific. The medial frontal gyrus, SMA, and limbic system may be the key hubs for depression in PD.

Networks Are Associated With Depression in Patients With Parkinson's Disease: A Resting-State Imaging Study

Background

Disturbance of networks was recently proposed to be associated with the occurrence of depression in Parkinson's disease (PD). However, the neurobiological mechanism of depression underlying PD remains unclear.

Objective

This study was conducted to investigate whether intra-network and inter-network brain connectivity is differently changed in PD patients with and without depression (PDD and PDND patients, respectively).

Methods

Forty-one PDD patients, 64 PDND patients, and 55 healthy controls (HCs) underwent resting-state functional magnetic resonance imaging (fMRI). The default mode network (DMN), executive control network (ECN), salience network (SN), precuneus network (PCUN), and sensorimotor network (SMN) were extracted using independent component analysis (ICA), and then the functional connectivity (FC) values within and between these networks were measured.

Results

PDD patients exhibited abnormal FC values within the DMN, ECN, SN, PCUN, and SMN. In addition, PDD patients demonstrated decreased connectivity between anterior SN (aSN) and bilateral ECN, between posterior SN (pSN) and dorsal DMN (dDMN), and between PCUN and dDMN/SMN/bilateral ECN. Connectivity within the left hippocampus of dDMN and the right medial superior frontal gyrus of aSN was a significant predictor of depression level in PD patients.

Conclusions

Aberrant intra- and inter-network FC is involved in several important hubs in the large-scale networks, which can be a biomarker for distinguishing PDD from PDND.

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.

Breathing new life into neurotoxic-based monkey models of Parkinson's disease to study the complex biological interplay between serotonin and dopamine

Numerous clinical studies have shown that the serotonergic system also degenerates in patients with Parkinson's disease. The causal role of this impairment in Parkinson's symptomatology and the response to treatment remains to be refined, in particular thanks to approaches allowing the two components DA and 5-HT to be isolated if possible. We have developed a macaque monkey model of Parkinson's disease exhibiting a double lesion (dopaminergic and serotonergic) thanks to the sequential use of MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine) and MDMA (3,4-methylenedioxy-N-methamphetamine) (or MDMA prior MPTP). We characterized this monkey model by multimodal imaging (PET, positron emission tomography with several radiotracers; DTI, diffusion tensor imaging), behavioral assessments (parkinsonism, dyskinesia, neuropsychiatric-like behavior) and post-mortem analysis (with DA and 5-HT markers). When administrated after MPTP, MDMA damaged the 5-HT presynaptic system without affecting the remaining DA neurons. The lesion of 5-HT fibers induced by MDMA altered rigidity and prevented dyskinesia and neuropsychiatric-like symptoms induced by levodopa therapy in MPTP-treated animals. Interestingly also, prior MDMA administration aggravates the parkinsonian deficits and associated DA injury. Dystonic postures, action tremor and global spontaneous activities were significantly affected. All together, these data clearly indicate that late or early lesions of the 5-HT system have a differential impact on parkinsonian symptoms in the macaque model of Parkinson's disease. Whether MDMA has an impact on neuropsychiatric-like symptoms such as apathy, anxiety, depression remains to be addressed. Despite its limitations, this toxin-based double-lesioned monkey model takes on its full meaning and provides material for the experimental study of the heterogeneity of patients.

Metformin as a Potential Neuroprotective Agent in Prodromal Parkinson's Disease-Viewpoint

To date, there are no clinically effective neuroprotective or disease-modifying treatments that can halt Parkinson's disease (PD) progression. The current clinical approach focuses on symptomatic management. This failure may relate to the complex neurobiology underpinning the development of PD and the absence of true translational animal models. In addition, clinical diagnosis of PD relies on presentation of motor symptoms which occur when the neuropathology is already established. These multiple factors could contribute to the unsuccessful development of neuroprotective treatments for PD. Prodromal symptoms develop years prior to formal diagnosis and may provide an excellent tool for early diagnosis and better trial design. Patients with idiopathic rapid eye movement behavior disorder (iRBD) have the highest risk of developing PD and could represent an excellent group to include in neuroprotective trials for PD. In addition, repurposing drugs with excellent safety profiles is an appealing strategy to accelerate drug discovery. The anti-diabetic drug metformin has been shown to target diverse cellular pathways implicated in PD progression. Multiple studies have, additionally, observed the benefits of metformin to counteract other age-related diseases. The purpose of this viewpoint is to discuss metformin's neuroprotective potential by outlining relevant mechanisms of action and the selection of iRBD patients for future clinical trials in PD.

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.

Synthesis, in vitro and in vivo evaluation of 11C-O-methylated arylpiperazines as potential serotonin 1A (5-HT1A) receptor antagonist radiotracers

Background

Serotonin 1A (5-HT_1A) receptors are implicated in the pathogenesis of several psychiatric and neurodegenerative disorders motivating the development of suitable radiotracers for in vivo positron emission tomography (PET) neuroimaging. The gold standard PET imaging agent for this target is [carbonyl-¹¹C]WAY-100635, labeled via a technically challenging multi-step reaction that has limited its widespread use. While several antagonist and agonist-based PET radiotracers for 5-HT _1A receptors have been developed, their clinical translation has been hindered by methodological challenges and/or and non-specific binding. As a result, there is continued interest in the development of new and more selective 5-HT_1A PET tracers having a relatively easier and reliable radiosynthesis process for routine production and with favorable metabolism to facilitate tracer-kinetic modeling. The purpose of the current study was to develop and characterize a radioligand with suitable characteristics for imaging 5-HT_1A receptors in the brain. The current study reports the in vitro characterization and radiosyntheses of three candidate 5-HT_1A receptor antagonists, DF-100 (1), DF-300 (2) and DF-400 (3), to explore their suitability as potential PET radiotracers.

Results

Syntheses of 1–3 and corresponding precursors for radiolabeling were achieved from isonicotinic, picolinic acid or picolino nitrile. In vitro binding studies demonstrated nanomolar affinity of the compounds for 5-HT_1A receptors. Binding of 1–3 for other biogenic amines, neurotransmitter receptors, and transporters was negligible with the exception of moderate affinities for α₁-adrenergic receptors (4–6-fold less potent than that for 5-HT_1A receptor). Radioligands [¹¹C]1–3 were efficiently prepared by ¹¹C-O-methylation of the corresponding phenolic precursor in non-decay corrected radiochemical yields of 7–11% with > 99% chemical and radiochemical purities. Dynamic PET studies in rats demonstrated negligible brain uptake of [¹¹C]1 and [¹¹C]2. In contrast, significant brain uptake of [¹¹C]3 was observed with an early peak SUV of 4–5. However, [¹¹C]3 displayed significant off-target binding attributed to α₁-adrenergic receptors based on regional distribution (thalamus>hippocampus) and blocking studies.

Conclusion

Despite efficient radiolabeling, results from PET imaging experiments limit the application of [¹¹C]3 for in vivo quantification of 5-HT_1A receptors. Nevertheless, derivatives of compound 3 may provide a scaffold for alternative PET radiotracers with improved selectivity for 5-HT _1A receptors or α₁-adrenergic receptors.

Neuropsychiatric Disorders in Parkinson's Disease: What Do We Know About the Role of Dopaminergic and Non-dopaminergic Systems?

Besides the hallmark motor symptoms (rest tremor, hypokinesia, rigidity, and postural instability), patients with Parkinson’s disease (PD) have non-motor symptoms, namely neuropsychiatric disorders. They are frequent and may influence the other symptoms of the disease. They have also a negative impact on the quality of life of patients and their caregivers. In this article, we will describe the clinical manifestations of the main PD-related behavioral disorders (depression, anxiety disorders, apathy, psychosis, and impulse control disorders). We will also provide an overview of the clinical and preclinical literature regarding the underlying mechanisms with a focus on the role of the dopaminergic and non-dopaminergic systems.

Overview of Therapeutic Drugs and Methods for the Treatment of Parkinson's Disease

Parkinson's Disease (PD) is a neurodegenerative disease involving degeneration of dopaminergic neurons of the nigrostriatal pathways. Over the past decades, most of the medications for the treatment of PD patients have been used to modulate dopamine concentrations in the basal ganglia. This includes levodopa and its inhibitory metabolizing enzymes. In addition to modulating dopamine concentrations in the brain, there are D2-like dopamine receptor agonists that mimic the action of dopamine to compensate for the deficit in dopamine found in PD patients. Muscarinic antagonists' drugs are used rarely due to some side effects. Monoamine oxidase inhibitors are among the first in line, and are considered popular drugs that reduce the metabolism of dopamine in PD patients. Furthermore, we discussed in this review the existence of certain glutamate receptor antagonists for the treatment of PD. Alternatively, we further discussed the potential therapeutic role of adenosine (2A) receptor antagonists, such as tozadenant and istradefylline in the treatment of PD. We also discussed the important role of serotonin1A receptor agonist, adrenergic autoreceptors (α2) antagonists and calcium channel blockers in the treatment of PD. Finally, neurotrophic factors, such as glial cell line-derived neurotrophic growth factor and brain-derived neurotrophic factor are considered the primary factors for neuroprotection in PD.

Damaged Insula Network Contributes to Depression in Parkinson's Disease

BACKGROUND

Depression is common in patients with Parkinson's disease (PD). Our previous studies suggest that depressed PD patients have altered insula structures. It is, however, still unknown whether the altered structures cause disruption of insula functional networks, further contributing to depression in PD.

METHODS

In the present study, 17 depressed PD patients, 17 non-depressed PD patients, and 17 normal controls were enrolled. All subjects went through neurological and psychiatric clinical assessments. Resting-state functional magnetic resonance imaging and seed-based insula functional analyses were performed to examine the insula functional connectivity alterations in PD patients.

RESULTS

We found that compared with normal controls, PD patients exhibited significantly decreased insula functional connectivity widely across the whole brain. Compared with non-depressed PD patients, depressed patients showed further decreased functional connectivity in the middle frontal gyrus and inferior parietal lobe. Furthermore, connectivity between the left anterior insula and middle frontal gyrus was positively correlated with the cognitive scale score.

CONCLUSION

These results suggest that insula networks were severely damaged in PD patients, and that the disrupted connection between the salience network and executive control network might contribute to depression in PD.

Functional connectivity markers of depression in advanced Parkinson's disease

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Patient-specific 42-ICNs template was created from 156 PD patients’ rs-fMRI data.

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6 FC markers significantly contributed to depression discrimination in PD.

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Classifiers achieved the mean accuracy of 82.4% for depression diagnosis in PD.

Background

Depression is a common comorbid condition in Parkinson's disease and a major contributor to poor quality of life. Despite this, depression in PD is under-diagnosed due to overlapping symptoms and difficulties in the assessment of depression in cognitively impaired old patients.

Objectives

This study is to explore functional connectivity markers of depression in PD patients using resting-state fMRI and help diagnose whether patients have depression or not.

Methods

We reviewed 156 advanced PD patients (duration > 5 years; 59 depressed ones) and 45 healthy control subjects who underwent a resting-state fMRI scanning. Functional connectivity analysis was employed to characterize intrinsic connectivity networks using group independent component analysis and extract connectivity features. Features were put into an all-relevant feature selection procedure within cross-validation loops, to identify features with significant discriminative power for classification. Random forest classifiers were built for depression diagnosis, on the basis of identified features.

Results

42 intrinsic connectivity networks were identified and arranged into subcortical, auditory, somatomotor, visual, cognitive control, default-mode and cerebellar networks. Six features were significantly relevant to classification. They were connectivity within posterior cingulate cortex, within insula, between posterior cingulate cortex and insula/hippocampus+amygdala, between insula and precuneus, and between superior parietal lobule and medial prefrontal cortex. The mean accuracy achieved with classifiers to discriminate depressed patients from the non-depressed was 82.4%.

Conclusions

Our findings provide preliminary evidence that resting-state functional connectivity can characterize depressed PD patients and help distinguish them from non-depressed ones.

Drug Choices and Advancements for Managing Depression in Parkinson's Disease

Depression is a frequent non-motor symptom of Parkinson’s disease (PD), and may even precede the onset of motor symptoms of parkinsonism. Beyond its negative influence on mood, depression in PD is frequently associated with other neuropsychiatric symptoms and with late-stage complications such as dementia. Despite its profound impact on the quality of life and cognitive functioning in PD, depression in PD is often under-recognized and poorly treated.

Pathophysiological studies demonstrated that depression in PD is associated with global dysfunction of interactions between discrete brain areas rather than focal structural or functional abnormalities, and that it is sustained by pathological changes of several neurotransmitter/receptor complexes.

In general, all traditional antidepressants and some dopamine agonists have been found to be safe and well-tolerated to treat depressive symptoms in PD, despite initial warning on worsening of parkinsonism. Available data suggest that the time-course of response differs among antidepressants. Efficacy results from clinical trials with antidepressant in PD are, however, rather uncertain, although pooled analysis suggests a moderate benefit. Several issues may critically impact the results of clinical trials with antidepressants in PD, including the correct psychiatric diagnosis, the overlap of symptoms between depression and PD, and the selection of appropriate end-points and rating scales.

Altered Cortico-Limbic Network Connectivity in Parkinsonian Depression: The Effect of Antidepressants

BACKGROUND

Depression is a common comorbidity of Parkinson's disease (PD); however, the impact of antidepressant status on cortical function in parkinsonian depression is not fully understood. While studies of resting state functional MRI in major depression have shown that antidepressant treatment affects cortical connectivity, data on connectivity and antidepressant status in PD is sparse.

OBJECTIVE

We tested the hypothesis that cortico-limbic network (CLN) resting state connectivity is abnormal in antidepressant-treated parkinsonian depression.

METHODS

Thirteen antidepressant-treated depressed PD and 47 non-depressed PD participants from the Parkinson's Progression Markers Initiative (PPMI) database were included. Data was collected using 3T Siemens TIM Trio MR scanners and analyzed using SPM and CONN functional connectivity toolbox. Volumetric analysis was also performed using BrainSuite.

RESULTS

We found decreased connectivity in the antidepressant-treated depressed PD group when compared to non-depressed PD between the left frontal operculum and bilateral insula, and also reduced connectivity between right orbitofrontal cortex and left temporal fusiform structures. Increased depression scores were associated with decreased insular-frontal opercular connectivity. No ROI volumetric differences were found between groups.

CONCLUSION

Given the relationship between depression scores and cortico-limbic connectivity in PD, the abnormal insular-frontal opercular hypoconnectivity in this cohort may be associated with persistent depressive symptoms or antidepressant effects.

Imaging behavioural complications of Parkinson's disease

In addition to motor symptoms, behavioural complications are commonly found in patients with Parkinson's disease (PD). Behavioural complications, including depression, anxiety, apathy, impulse control disorder and psychosis, together have a large impact on PD patient's quality of life. Many neuroimaging studies using PET, SPECT and MRI techniques have been conducted to study the underlying neural mechanisms of PD pathogenesis and pathophysiology in relation to its behavioural complications. This review will survey these PET, SPECT and MRI studies to describe the current understanding of the neuro-chemical, functional and structural changes associated with behavioural complications in PD patients.

Neuropathology and pathogenesis of extrapyramidal movement disorders: a critical update-I. Hypokinetic-rigid movement disorders

Extrapyramidal movement disorders include hypokinetic rigid and hyperkinetic or mixed forms, most of them originating from dysfunction of the basal ganglia (BG) and their information circuits. The functional anatomy of the BG, the cortico-BG-thalamocortical, and BG-cerebellar circuit connections are briefly reviewed. Pathophysiologic classification of extrapyramidal movement disorder mechanisms distinguish (1) parkinsonian syndromes, (2) chorea and related syndromes, (3) dystonias, (4) myoclonic syndromes, (5) ballism, (6) tics, and (7) tremor syndromes. Recent genetic and molecular-biologic classifications distinguish (1) synucleinopathies (Parkinson's disease, dementia with Lewy bodies, Parkinson's disease-dementia, and multiple system atrophy); (2) tauopathies (progressive supranuclear palsy, corticobasal degeneration, FTLD-17; Guamian Parkinson-dementia; Pick's disease, and others); (3) polyglutamine disorders (Huntington's disease and related disorders); (4) pantothenate kinase-associated neurodegeneration; (5) Wilson's disease; and (6) other hereditary neurodegenerations without hitherto detected genetic or specific markers. The diversity of phenotypes is related to the deposition of pathologic proteins in distinct cell populations, causing neurodegeneration due to genetic and environmental factors, but there is frequent overlap between various disorders. Their etiopathogenesis is still poorly understood, but is suggested to result from an interaction between genetic and environmental factors. Multiple etiologies and noxious factors (protein mishandling, mitochondrial dysfunction, oxidative stress, excitotoxicity, energy failure, and chronic neuroinflammation) are more likely than a single factor. Current clinical consensus criteria have increased the diagnostic accuracy of most neurodegenerative movement disorders, but for their definite diagnosis, histopathological confirmation is required. We present a timely overview of the neuropathology and pathogenesis of the major extrapyramidal movement disorders in two parts, the first one dedicated to hypokinetic-rigid forms and the second to hyperkinetic disorders.

Biomarkers for Dementia, Fatigue, and Depression in Parkinson's Disease

Parkinson's disease is a common multisystem neurodegenerative disorder characterized by typical motor and non-motor symptoms. There is an urgent need for biomarkers for assessment of disease severity, complications and prognosis. In addition, biomarkers reporting the underlying pathophysiology assist in understanding the disease and developing neuroprotective therapies. Ultimately, biomarkers could be used to develop a more efficient personalized approach for clinical trials and treatment strategies. With the goal to improve quality of life in Parkinson's disease it is essential to understand and objectively monitor non-motor symptoms. This narrative review provides an overview of recent developments of biomarkers (biofluid samples and imaging) for three common neuropsychological syndromes in Parkinson's disease: dementia, fatigue, and depression.

Degeneration of dopaminergic circuitry influences depressive symptoms in Lewy body disorders

Aims

Depression is commonly observed even in prodromal stages of Lewy body disorders (LBD), and is associated with cognitive impairment and a faster rate of cognitive decline. Given the role of dopamine in the development of movement disorders, but also in motivation and reward, we investigated neurodegenerative pathology in dopaminergic circuitry in Parkinson's disease (PD), PD with dementia (PDD) and dementia with Lewy bodies (DLB) patients in relation to depressive symptoms.

Methods

α‐synuclein, hyperphosphorylated tau and amyloid‐beta pathology was assessed in 17 DLB, 14 PDD and 8 PD cases within striatal and midbrain subregions, with neuronal cell density assessed in substantia nigra and ventral tegmental area. Additionally, we used a structural equation modeling (SEM) approach to investigate the extent to which brain connectivity might influence the deposition of pathological proteins within dopaminergic pathways.

Results

A significantly higher α‐synuclein burden was observed in the substantia nigra (P = 0.006), ventral tegmental area (P = 0.011) and nucleus accumbens (P = 0.031) in LBD patients with depression. Significant negative correlations were observed between cell density in substantia nigra with Lewy body (LB) Braak stage (P = 0.013), whereas cell density in ventral tegmental area showed negative correlations with LB Braak stage (P = 0.026) and neurofibrillary tangle Braak stage (P = 0.007).

Conclusions

Dopaminergic α‐synuclein pathology appears to drive depression. Selective targeting of dopaminergic pathways may therefore provide symptomatic relief for depressive symptoms in LBD patients.

The pathophysiological mechanisms of motivational deficits in Parkinson's disease

Parkinson's disease (PD) is a progressive degenerative disorder that leads to disabling motor symptoms and a wide variety of neuropsychiatric symptoms. Apathy is the most common psychiatric disorder in the early stages of untreated PD and can be defined as a hypodopaminergic syndrome, which also includes anxiety and depression. Apathy is also considered the core feature of the parkinsonian triad (apathy, anxiety and depression) of behavioural non-motor signs, including a motivational deficit. Moreover, apathy is recognised as a distinct chronic neuropsychiatric behavioural disorder based on specific diagnostic criteria. Given the prevalence of apathy in approximately 40% of the general Parkinson's disease population, this appears to be a contributing factor to dementia in PD; also, apathy symptoms are factors that potentially contribute to morbidity, leading to a major impairment of health-related quality of life, thus stressing the importance of understanding the pathophysiology of this disease. Several studies have clearly established a prominent role for DA-mediated signals in PD apathy. However, synergistic interaction between dopaminergic impairment resulting from the neurodegenerative process and deep brain stimulation of the subthalamic nucleus may cause or exacerbate apathy. Furthermore, serotoninergic mechanism signalling is also likely to be of importance in this pathophysiology.

Salience Network and Depressive Severities in Parkinson's Disease with Mild Cognitive Impairment: A Structural Covariance Network Analysis

Purpose: In Parkinson’s disease with mild cognitive impairment (PD-MCI), we investigated the clinical significance of salience network (SN) in depression and cognitive performance.

Methods: Seventy seven PD-MCI patients that fulfilled multi-domain and non-amnestic subtype were included. Gray matter structural covariance networks were constructed by 3D T1-magnetic resonance imaging and seed based analysis. The patients were divided into two groups by psychiatric interviews and screening of Geriatric Depression Scale (GDS): PD-MCI with depression (PD-MCI-D) or without depression (PD-MCI-ND). The seed or peak cluster volume, or the significant differences in the regression slopes in each seed-peak cluster correlation, were used to evaluate the significance with the neurobehavioral scores.

Results: This study is the first to demonstrate that the PD-MCI-ND group presented a larger number of voxels of structural covariance in SN than the PD-MCI-D group. The right fronto-insular seed volumes and the peak cluster of left lingual gyrus showed significant inverse correlation with the Geriatric Depression Scale (GDS; r = -0.231, P = 0.046).

Conclusions: This study is the first to validate the clinical significance of the SN in PD-MCI-D. The right insular seed value and the SN correlated with the severity of depression in PD-MCI.

Depression, anxiety, and apathy in Parkinson's disease: insights from neuroimaging studies

Depression, anxiety and apathy are common mood disturbances in Parkinson's disease (PD) but their pathophysiology is unclear. Advanced neuroimaging has been increasingly used to unravel neural substrates linked to these disturbances. A systematic review is provided of neuroimaging findings in depression, anxiety and apathy in PD. A PubMed, MEDLINE and EMBASE search of peer‐reviewed original research articles on these mood disturbances in PD identified 38 studies on depression, eight on anxiety and 14 on apathy in PD. Most of the imaging studies used either position emission tomography or single‐photon emission computed tomography techniques. These studies generally suggest increased neural activity in the prefrontal regions and decreased functional connectivity between the prefrontal−limbic networks in depressed patients. Functional imaging studies revealed an inverse correlation between dopaminergic density in the caudate and putamen with the severity of anxiety in PD. There was no consistent correlation between dopaminergic density of thalamus and anxiety. Studies demonstrated both positive and inverse correlations between apathy and metabolism or activity in the striatum, amygdalar, prefrontal, temporal and parietal regions. The clinical variability of study subjects and differences in image pre‐processing and analytical strategies may contribute to discrepant findings in these studies. Both nigrostriatal and extra‐nigrostriatal pathways (in particular the frontal region and its connecting areas) are affected in mood disorders in PD. Identifying the relative contributions of these neural pathways in PD patients with overlapping motor and mood symptoms could provide new pathophysiological clues for the development of better therapeutic targets for affected patients.

Restorative Strategies in Movement Disorders: the Contribution of Imaging

Purpose of Review

The purpose of this review was to review the imaging, particularly positron emission tomography (PET), findings in neurorestoration studies in movement disorders, with specific focus on neural transplantation in Parkinson’s disease (PD) and Huntington’s disease (HD).

Recent Findings

PET findings in PD transplantation studies have shown that graft survival as reflected by increases in dopaminergic PET markers does not necessarily correlate with clinical improvement. PD patients with more denervated ventral striatum and more imbalanced serotonin-to-dopamine ratio in the grafted neurons tended to have worse outcome. In HD transplantation studies, variable graft survival and clinical responses may be related to host inflammatory/immune responses to the grafts.

Summary

Information gleaned from imaging findings in previous neural transplantation studies has been used to refine study protocol and patient selection in future trials. This includes identifying suitable candidates for transplantation using imaging markers, employing multiple and/or novel PET tracers to better assess graft functions and inflammatory responses to grafts.

Imaging the Etiology of Apathy, Anxiety, and Depression in Parkinson's Disease: Implication for Treatment

Apathy, depression, and anxiety are among the most important non-motor signs of Parkinson's disease (PD). This may be encountered at early stages of illness and represent a major source of burden. Understanding their pathophysiology is a major prerequisite for efficient therapeutic strategies. Anatomical and metabolic imaging studies have enabled a breakthrough by demonstrating that widespread abnormalities within the limbic circuits notably the orbitofrontal and anterior cingulate cortices, amygdala, thalamus, and ventral striatum are involved in the pathophysiology of depression, anxiety, and apathy in PD. Functional imaging has further shown that mesolimbic dopaminergic but also serotonergic lesions play a major role in the mechanisms of these three neuropsychiatric manifestations, which has direct therapeutic implications.

The role of neurotransmitters in the development of Parkinson's disease-related psychosis

Psychotic symptoms are common, disabling non-motor features of Parkinson's disease (PD). Despite noted heterogeneity in clinical features, natural history and therapy response, current dogma posits that psychosis generally progresses in a stereotypic manner through a cascade of events that begins with minor hallucinations and evolves to severe hallucinations and delusions. Further, the occurrence of psychotic symptoms is believed to indicate a poor prognosis. Here we propose a classification scheme that outlines the pathogenesis of psychosis as it relates to dysfunction of several neurotransmitter systems. We hypothesize that several subtypes exist, and that PD psychosis is not consistently indicative of a progressive cascade and poor prognosis. The literature was reviewed from 1990 to 2017. An overview of the features of PD psychosis is followed by a review of data indicating the existence of neurotransmitter-related subtypes of psychosis. We found that ample evidence exists to demonstrate the presence of multiple subtypes of PD psychosis, which are traced to dysfunction of the following neurotransmitter systems: dopamine, serotonin and acetylcholine. Dysfunction of each of these systems is recognizable through their clinical features and correlates, and the varied long-term prognoses. Identifying which neurotransmitter system is dysfunctional may help to develop targeted therapies. PD psychosis has various subtypes that differ in clinical features, underlying pathology and pathophysiology, treatment response and prognosis. A novel classification scheme is presented that describes the clinical subtypes with different outcomes, which could lead to the development of targeted therapies. Future research should focus on testing the viability of this classification.

Neurotransmission systems in Parkinson’s disease

Parkinson’s disease (PD) is histologically characterized by the accumulation of α-synuclein particles, known as Lewy bodies. The second most common neurodegenerative disorder, PD is widely known because of the typical motor manifestations of active tremor, rigidity, and postural instability, while several prodromal non-motor symptoms including REM sleep behavior disorders, depression, autonomic disturbances, and cognitive decline are being more extensively recognized. Motor symptoms most commonly arise from synucleinopathy of nigrostriatal pathway. Glutamatergic, γ-aminobutyric acid (GABA)ergic, cholinergic, serotoninergic, and endocannabinoid neurotransmission systems are not spared from the global cerebral neurodegenerative assault. Wide intrabasal and extrabasal of the basal ganglia provide enough justification to evaluate network circuits disturbance of these neurotransmission systems in PD. In this comprehensive review, English literature in PubMed, Science direct, EMBASE, and Web of Science databases were perused. Characteristics of dopaminergic and non-dopaminergic systems, disturbance of these neurotransmitter systems in the pathophysiology of PD, and their treatment applications are discussed.

Nonmotor Subtyping in Parkinson's Disease

Nonmotor symptoms are integral to Parkinson's disease. Several subtypes dominated by specific nonmotor symptoms have emerged. In this chapter, the rationale behind nonmotor subtyping and currently proposed nonmotor subgroups within Parkinson's disease based on data-driven cluster analysis and clinical observations will be summarized. Furthermore, the concept of seven clinical nonmotor subtypes will be discussed in detail including the clinical presentation, potential biomarkers, and the clinical relevance. In future, nonmotor subtypes will possibly play a major role within the aim to achieve personalized medicine.

Nonmotor fluctuations: phenotypes, pathophysiology, management, and open issues

Parkinson's disease (PD) is a neurodegenerative multisystem disorder characterized by progressive motor symptoms such as bradykinesia, tremor and muscle rigidity. Over the course of the disease, numerous non-motor symptoms, sometimes preceding the onset of motor symptoms, significantly impair patients' quality of life. The significance of non-motor symptoms may outweigh the burden through progressive motor incapacity, especially in later stages of the disease. The advanced stage of the disease is characterized by motor complications such as fluctuations and dyskinesias induced by the long-term application of levodopa therapy. In recent years, it became evident that various non-motor symptoms such as psychiatric symptoms, fatigue and pain also show fluctuations after chronic levodopa therapy (named non-motor fluctuations or NMFs). Although NMFs have moved into the focus of interest, current national guidelines on the treatment of PD may refer to non-motor symptoms and their management, but do not mention NMF, and do not contain recommendations on their management. The present article summarizes major issues related to NMF including clinical phenomenology and pathophysiology, and outlines a number of open issues and topics for future research.