Serotonin-dependent depression in Parkinson's disease: a role for the subthalamic nucleus?

Mood disturbances in Parkinson's disease: From prodromal origins to application of animal models

Parkinson's disease (PD) is a complex illness with a constellation of environmental insults and genetic vulnerabilities being implicated. Strikingly, many studies only focus on the cardinal motor symptoms of the disease and fail to appreciate the major non-motor features which typically occur early in the disease process and are debilitating. Common comorbid psychiatric features, notably clinical depression, as well as anxiety and sleep disorders are thought to emerge before the onset of prominent motor deficits. In this review, we will delve into the prodromal stage of PD and how early neuropsychiatric pathology might unfold, followed by later motor disturbances. It is also of interest to discuss how animal models of PD capture the complexity of the illness, including depressive-like characteristics along with motor impairment. It remains to be determined how the underlying PD disease processes contributes to such comorbidity. But some of the environmental toxicants and microbial pathogens implicated in PD might instigate pro-inflammatory effects favoring α-synuclein accumulation and damage to brainstem neurons fueling the evolution of mood disturbances. We posit that comprehensive animal-based research approaches are needed to capture the complexity and time-dependent nature of the primary and co-morbid symptoms. This will allow for the possibility of early intervention with more novel and targeted treatments that fit with not only individual patient variability, but also with changes that occur over time with the evolution of the disease.

Neurobiology of depression in Parkinson's disease: Insights into epidemiology, molecular mechanisms and treatment strategies

Parkinson's disease (PD) is characterized mainly by motor dysfunctions due to the progressive loss of dopaminergic neurons. However, PD patients experience a multitude of debilitating non-motor symptoms, including depression, which may have deleteriously detrimental effects on life. Depression is multifactorial and exhibits a bimodal progression in PD, but its underlying molecular mechanisms are poorly understood. Studies demonstrating the pathophysiology of depression in PD and the specific treatment strategies for depression-like symptoms in PD patients are largely lacking, often underrated, under-recognized and, consequently, inadequately/under-treated. Nevertheless, reports suggest that the incidence of depression is approximately 20-30% of PD patients and may precede the onset of motor symptoms. Diagnosing depression in PD becomes difficult due to the clinical overlap in symptomatology between the two diseases, and the nigrostriatal dysfunction alone is insufficient to explain depressive symptoms in PD. Therefore, the current study provides an overview of the molecular mechanisms underlying the development of depression in PD and new insights into developing current antidepressant strategies to treat depression in PD. This review will identify and understand the molecular pathological mechanisms of depression in PD that will fundamentally help tailoring therapeutic interventions for depressive symptoms in PD.

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.

Metabolic depletion of sphingolipids inhibits agonist-induced endocytosis of the serotonin1A receptor

G protein-coupled receptors (GPCRs) are vital cellular signaling machinery and currently represent ~40% drug targets. Endocytosis of GPCRs is an important process that allows stringent spatiotemporal control over receptor population on the cell surface. Although the role of proteins in GPCR endocytosis is well addressed, the contribution of membrane lipids in this process is rather unexplored. Sphingolipids are essential functional lipids in higher eukaryotes and are implicated in several neurological functions. To understand the role of sphingolipids in GPCR endocytosis, we subjected cells expressing human serotonin_1A receptors (an important neurotransmitter GPCR involved in cognitive and behavioral functions) to metabolic sphingolipid depletion using fumonisin B₁ , an inhibitor of sphingolipid biosynthetic pathway. Our results, using flow cytometric analysis and confocal microscopic imaging, show that sphingolipid depletion inhibits agonist-induced endocytosis of the serotonin_1A receptor in a concentration-dependent manner, which was restored when sphingolipid levels were replenished. We further show that there was no change in the internalization of transferrin, a marker for clathrin-mediated endocytosis, under sphingolipid-depleted condition, highlighting the specific requirement of sphingolipids for endocytosis of serotonin_1A receptors. Our results reveal the regulatory role of sphingolipids in GPCR endocytosis and highlight the importance of neurotransmitter receptor trafficking in health and disease.

The role of neurotransmitter systems in mediating deep brain stimulation effects in Parkinson’s disease

Deep brain stimulation (DBS) is among the most successful paradigms in both translational and reverse translational neuroscience. DBS has developed into a standard treatment for movement disorders such as Parkinson’s disease (PD) in recent decades, however, specific mechanisms behind DBS’s efficacy and side effects remain unrevealed. Several hypotheses have been proposed, including neuronal firing rate and pattern theories that emphasize the impact of DBS on local circuitry but detail distant electrophysiological readouts to a lesser extent. Furthermore, ample preclinical and clinical evidence indicates that DBS influences neurotransmitter dynamics in PD, particularly the effects of subthalamic nucleus (STN) DBS on striatal dopaminergic and glutamatergic systems; pallidum DBS on striatal dopaminergic and GABAergic systems; pedunculopontine nucleus DBS on cholinergic systems; and STN-DBS on locus coeruleus (LC) noradrenergic system. DBS has additionally been associated with mood-related side effects within brainstem serotoninergic systems in response to STN-DBS. Still, addressing the mechanisms of DBS on neurotransmitters’ dynamics is commonly overlooked due to its practical difficulties in monitoring real-time changes in remote areas. Given that electrical stimulation alters neurotransmitter release in local and remote regions, it eventually exhibits changes in specific neuronal functions. Consequently, such changes lead to further modulation, synthesis, and release of neurotransmitters. This narrative review discusses the main neurotransmitter dynamics in PD and their role in mediating DBS effects from preclinical and clinical data.

Modeling Parkinson's disease-related symptoms in alpha-synuclein overexpressing mice

BACKGROUND

Intracellular deposition of alpha-synuclein (α-syn) as Lewy bodies and Lewy neurites is a central event in the pathogenesis of Parkinson's disease (PD) and other α-synucleinopathies. Transgenic mouse models overexpressing human α-syn, are useful research tools in preclinical studies of pathogenetic mechanisms. Such mice develop α-syn inclusions as well as neurodegeneration with a topographical distribution that varies depending on the choice of promoter and which form of α-syn that is overexpressed. Moreover, they display motor symptoms and cognitive disturbances that to some extent resemble the human conditions.

PURPOSE

One of the main motives for assessing behavior in these mouse models is to evaluate the potential of new treatment strategies, including their impact on motor and cognitive symptoms. However, due to a high within-group variability with respect to such features, the behavioral studies need to be applied with caution. In this review, we discuss how to make appropriate choices in the experimental design and which tests that are most suitable for the evaluation of PD-related symptoms in such studies.

METHODS

We have evaluated published results on two selected transgenic mouse models overexpressing wild type (L61) and mutated (A30P) α-syn in the context of their validity and utility for different types of behavioral studies.

CONCLUSIONS

By applying appropriate behavioral tests, α-syn transgenic mouse models provide an appropriate experimental platform for studies of symptoms related to PD and other α-synucleinopathies.

Serotonin-Related Functional Genetic Variants Affect the Occurrence of Psychiatric and Motor Adverse Events of Dopaminergic Treatment in Parkinson’s Disease: A Retrospective Cohort Study

The serotonergic system is important in Parkinson’s disease (PD) pathogenesis as it can take over dopamine production after a large portion of dopaminergic neurons is lost through neurodegeneration. The aim of this study was to evaluate the effect of genetic variability of serotonergic genes on the occurrence of motor complications and psychiatric adverse events (AE) due to dopaminergic treatment. We enrolled 231 patients and their clinical data were collected. Genotyping was performed for eight genetic variants. Logistic regression was used for analysis. Carriers of the HTR1A rs6295 GC genotype (OR = 2.58; 95% CI = 1.15–5.78; p = 0.021), TPH2 rs4290270 AA genotype (OR = 2.78; 95% CI = 1.08–7.03; p = 0.034), and at least one TPH2 rs4570625 T allele (OR = 1.86; 95% CI = 1.00–3.44; p = 0.047) had increased risk for visual hallucinations (VH). Additionally, carriers of at least one SLC6A4 5-HTTPLR rs25531 S (OR = 0.52; 95% CI = 0.28–0.96; p = 0.037) or at least one L_G allele (OR = 0.37; 95% CI = 0.14–0.97; p = 0.044) had a decreased chance for VH. Constructed haplotypes of the TPH2 showed increased risk for VH (OR = 1.94; 95% CI = 1.06–3.55; p = 0.032) and impulse control disorders (OR = 5.20; 95% CI = 1.86–14.50; p = 0.002). Finally, individual gene–gene interactions showed decreased odds for the development of motor AE. Our findings suggest that the serotonergic pathway may play an important role in the development of AE resulting from dopaminergic treatment.

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.

Metabolic Depletion of Sphingolipids Reduces Cell Surface Population of the Human Serotonin1A Receptor due to Impaired Trafficking

Sphingolipids and their metabolites are increasingly implicated in the pathogenesis of many metabolic and neurological diseases. It has been postulated that sphingolipids coalesce with cholesterol to form laterally segregated lipid domains that are involved in protein sorting and trafficking. In this work, we have explored the effect of metabolic depletion of sphingolipids on cell surface expression of the human serotonin_1A receptor, a neurotransmitter G protein-coupled receptor. We used fumonisin B₁ (FB₁), a fungal mycotoxin, to inhibit sphingolipid biosynthesis in HEK-293 cells stably expressing the human serotonin_1A receptor. Our results obtained using flow cytometric analysis and confocal microscopic imaging show that the cell surface population of the serotonin_1A receptor is reduced under sphingolipid-depleted condition. Western blot analysis confirmed that there was no significant difference in total cellular level of the serotonin_1A receptor upon depletion of sphingolipids. Interestingly, the effect of FB₁ on serotonin_1A receptor population was reversed upon replenishment with sphingolipids. These results indicate that sphingolipid depletion does not alter total cellular receptor levels, but impairs serotonin_1A receptor trafficking to the cellular plasma membrane. These results could provide mechanistic insights into the role of sphingolipids in modulation of neurotransmitter receptor signaling and trafficking in health and disease.

Non-motor outcomes depend on location of neurostimulation in Parkinson's disease

Deep brain stimulation of the subthalamic nucleus is an effective and established therapy for patients with advanced Parkinson's disease improving quality of life, motor symptoms and non-motor symptoms. However, there is a considerable degree of interindividual variability for these outcomes, likely due to variability in electrode placement and stimulation settings. Here, we present probabilistic mapping data from a prospective, open-label, multicentre, international study to investigate the influence of the location of subthalamic nucleus deep brain stimulation on non-motor symptoms in patients with Parkinson's disease. A total of 91 Parkinson's disease patients undergoing bilateral deep brain stimulation of the subthalamic nucleus were included, and we investigated NMSScale, NMSQuestionnaire, Scales for Outcomes in Parkinson's disease-motor examination, -activities of daily living, and -motor complications, and Parkinson's disease Questionnaire-8 preoperatively and at 6-month follow-up after surgery. Leads were localized in standard space using the Lead-DBS toolbox and individual volumes of tissue activated were calculated based on clinical stimulation settings. Probabilistic stimulation maps and non-parametric permutation statistics were applied to identify voxels with significant above or below average improvement for each scale and analysed using the DISTAL atlas. All outcomes improved significantly at follow-up. Significant spatial distribution patterns of neurostimulation were observed for NMSScale total score and its mood/apathy and attention/memory domains. For both domains, voxels associated with below average improvement were mainly located dorsal to the subthalamic nucleus. In contrast, above average improvement for mood/apathy was observed in the ventral border region of the subthalamic nucleus and in its sensorimotor subregion and for attention/memory in the associative subregion. A trend was observed for NMSScale sleep domain showing voxels with above average improvement located ventral to the subthalamic nucleus. Our study provides evidence that the interindividual variability of mood/apathy, attention/memory, and sleep outcomes after subthalamic nucleus deep brain stimulation depends on the location of neurostimulation. This study highlights the importance of holistic assessments of motor and non-motor aspects of Parkinson's disease to tailor surgical targeting and stimulation parameter settings to patients' personal profiles.

Neurobiological findings underlying depressive behavior in Parkinson's disease: A review

Parkinson's disease (PD) is one of the most prevalent neurodegenerative diseases in the world with a harmful impact on the quality of life. Although its clinical diagnosis is based on motor symptoms such as resting tremor, postural instability, slow gait, and muscle stiffness, this disorder is also characterized by the presence of early emotional impairment, including features such as depression, anxiety, fatigue, and apathy. Depression is the main emotional manifestation associated with PD and the mechanisms involved in its pathophysiology have been extensively investigated however, it is not yet completely elucidated. In addition to monoaminergic imbalance, immunological and gut microbiota changes have been associated with depression in PD. Besides, a patient group appears be refractory to the treatment available currently. This review emphasizes the mainly neuromolecular findings of the PD-associated depression as well as discuss novel and potential pharmacological and non-pharmacological therapeutic strategies.

Elucidating Critical Proteinopathic Mechanisms and Potential Drug Targets in Neurodegeneration

Neurodegeneration entails progressive loss of neuronal structure as well as function leading to cognitive failure, apathy, anxiety, irregular body movements, mood swing and ageing. Proteomic dysregulation is considered the key factor for neurodegeneration. Mechanisms involving deregulated processing of proteins such as amyloid beta (Aβ) oligomerization; tau hyperphosphorylation, prion misfolding; α-synuclein accumulation/lewy body formation, chaperone deregulation, acetylcholine depletion, adenosine 2A (A2A) receptor hyperactivation, secretase deregulation, leucine-rich repeat kinase 2 (LRRK2) mutation and mitochondrial proteinopathies have deeper implications in neurodegenerative disorders. Better understanding of such pathological mechanisms is pivotal for exploring crucial drug targets. Herein, we provide a comprehensive outlook about the diverse proteomic irregularities in Alzheimer's, Parkinson's and Creutzfeldt Jakob disease (CJD). We explicate the role of key neuroproteomic drug targets notably Aβ, tau, alpha synuclein, prions, secretases, acetylcholinesterase (AchE), LRRK2, molecular chaperones, A2A receptors, muscarinic acetylcholine receptors (mAchR), N-methyl-D-aspartate receptor (NMDAR), glial cell line-derived neurotrophic factor (GDNF) family ligands (GFLs) and mitochondrial/oxidative stress-related proteins for combating neurodegeneration and associated cognitive and motor impairment. Cross talk between amyloidopathy, synucleinopathy, tauopathy and several other proteinopathies pinpoints the need to develop safe therapeutics with ability to strike multiple targets in the aetiology of the neurodegenerative disorders. Therapeutics like microtubule stabilisers, chaperones, kinase inhibitors, anti-aggregation agents and antibodies could serve promising regimens for treating neurodegeneration. However, drugs should be target specific, safe and able to penetrate blood-brain barrier.

Progress in neuromodulation of the brain: A role for magnetic nanoparticles?

The field of neuromodulation is developing rapidly. Current techniques, however, are still limited as they i) either depend on permanent implants, ii) require invasive procedures, iii) are not cell-type specific, iv) involve slow pharmacokinetics or v) have a restricted penetration depth making it difficult to stimulate regions deep within the brain. Refinements into the different fields of neuromodulation are thus needed. In this review, we will provide background information on the different techniques of neuromodulation discussing their latest refinements and future potentials including the implementation of nanoparticles (NPs). In particular we will highlight the usage of magnetic nanoparticles (MNPs) as transducers in advanced neuromodulation. When exposed to an alternating magnetic field (AMF), certain MNPs can generate heat through hysteresis. This MNP heating has been promising in the field of cancer therapy and has recently been introduced as a method for remote and wireless neuromodulation. This indicates that MNPs may aid in the exploration of brain functions via neuromodulation and may eventually be applied for treatment of neuropsychiatric disorders. We will address the materials chemistry of MNPs, their biomedical applications, their delivery into the brain, their mechanisms of stimulation with emphasis on MNP heating and their remote control in living tissue. The final section compares and discusses the parameters used for MNP heating in brain cancer treatment and neuromodulation. Concluding, using MNPs for nanomaterial-mediated neuromodulation seem promising in a variety of techniques and could be applied for different neuropsychiatric disorders when more extensively investigated.

Combining Gene Transfer and Nonhuman Primates to Better Understand and Treat Parkinson's Disease

Parkinson’s disease (PD) is a progressive CNS disorder that is primarily associated with impaired movement. PD develops over decades and is linked to the gradual loss of dopamine delivery to the striatum, via the loss of dopaminergic (DA) neurons in the substantia nigra pars compacta (SNpc). While the administration of L-dopa and deep brain stimulation are potent therapies, their costs, side effects and gradual loss of efficacy underlines the need to develop other approaches. Unfortunately, the lack of pertinent animal models that reproduce DA neuron loss and behavior deficits—in a timeline that mimics PD progression—has hindered the identification of alternative therapies. A complementary approach to transgenic animals is the use of nonhuman primates (NHPs) combined with the overexpression of disease-related genes using viral vectors. This approach may induce phenotypes that are not influenced by developmental compensation mechanisms, and that take into account the personality of animals. In this review article, we discuss the combination of gene transfer and NHPs to develop “genetic” models of PD that are suitable for testing therapeutic approaches.

Transplantation of Human Neural Progenitor Cells (NPC) into Putamina of Parkinsonian Patients: A Case Series Study, Safety and Efficacy Four Years after Surgery

Individuals with Parkinson’s disease (PD) suffer from motor and mental disturbances due to degeneration of dopaminergic and non-dopaminergic neuronal systems. Although they provide temporary symptom relief, current treatments fail to control motor and non-motor alterations or to arrest disease progression. Aiming to explore safety and possible motor and neuropsychological benefits of a novel strategy to improve the PD condition, a case series study was designed for brain grafting of human neural progenitor cells (NPCs) to a group of eight patients with moderate PD. A NPC line, expressing Oct-4 and Sox-2, was manufactured and characterized. Using stereotactic surgery, NPC suspensions were bilaterally injected into patients’ dorsal putamina. Cyclosporine A was given for 10 days prior to surgery and continued for 1 month thereafter. Neurological, neuropsychological, and brain imaging evaluations were performed pre-operatively, 1, 2, and 4 years post-surgery. Seven of eight patients have completed 4-year follow-up. The procedure proved to be safe, with no immune responses against the transplant, and no adverse effects. One year after cell grafting, all but one of the seven patients completing the study showed various degrees of motor improvement, and five of them showed better response to medication. PET imaging showed a trend toward enhanced midbrain dopaminergic activity. By their 4-year evaluation, improvements somewhat decreased but remained better than at baseline. Neuropsychological changes were minor, if at all. The intervention appears to be safe. At 4 years post-transplantation we report that undifferentiated NPCs can be delivered safely by stereotaxis to both putamina of patients with PD without causing adverse effects. In 6/7 patients in OFF condition improvement in UPDRS III was observed. PET functional scans suggest enhanced putaminal dopaminergic neurotransmission that could correlate with improved motor function, and better response to L-DOPA. Patients’ neuropsychological scores were unaffected by grafting. Trial Registration: Fetal derived stem cells for Parkinson’s disease https://doi.org/10.1186/ISRCTN39104513Reg#ISRCTN39104513

Effects of chronic aspartame consumption on MPTP-induced Parkinsonism in male and female mice

BACKGROUND

Parkinson's disease is a progressive neurodegenerative disorder. Aspartame (l-aspartyl-l-phenylalanine methyl ester), a low calorie sweetener used in foods and beverages.

OBJECTIVES

This study investigated the effect of chronic aspartame intake on Parkinsonism induced by 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP).

METHOD

Forty-eight mice (24 males and 24 females): control, aspartame, MPTP, and aspartame + MPTP groups tested by Y-maze, stepping, forced swimming and olfactory preference tests. Brain tissues examined for dopamine content, tyrosine hydroxylase, inducible nitric oxide synthase (iNOS), glutathione peroxidase, phosphorylated tau and α-synuclein protein. Histopathological evaluation of brain sections at the level of basal ganglia was done.

RESULTS

Decreased dopamine content, tyrosine hydroxylase expression, glutathione peroxidase expression and increased iNOS, tau and α-synuclein expression in groups received aspartame, MPTP or both agents simultaneously in both males and females group.

CONCLUSIONS

Increased dopaminergic degeneration and complications with chronic aspartame consumption and more injury in male groups.

Deep Brain Stimulation Modified Autism-Like Deficits via the Serotonin System in a Valproic Acid-Induced Rat Model

Deep brain stimulation (DBS) is known to be a promising treatment for resistant depression, which acts via the serotonin (5-hydroxytryptamine, 5-HT) system in the infralimbic prefrontal cortex (ILPFC). Previous study revealed that dysfunction of brain 5-HT homeostasis is related to a valproate (VPA)-induced rat autism spectrum disorder (ASD) model. Whether ILPFC DBS rescues deficits in VPA-induced offspring through the 5-HT system is not known. Using VPA-induced offspring, we therefore explored the effect of DBS in autistic phenotypes and further investigated the underlying mechanism. Using combined behavioral and molecular approaches, we observed that applying DBS and 5-HT_1A receptor agonist treatment with 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) reversed sociability deficits, anxiety and hyperactivity in the VPA-exposed offspring. We then administered the selective 5-HT_1A receptor antagonist N-[2-[4-(2-Methoxyphenyl)-1-piperazinyl]ethyl]-N-2-pyridinylcyclohexanecarboxamide maleate (WAY 100635), following which the effect of DBS in terms of improving autistic behaviors was blocked in the VPA-exposed offspring. Furthermore, we found that both 8-OH-DPAT and DBS treatment rescued autistic behaviors by decreasing the expressions of NR2B subunit of N-methyl-D-aspartate receptors (NMDARs) and the β₃ subunit of γ-aminobutyric acid type A receptors (GABA_AR) in the PFC region. These results provided the first evidence of characteristic behavioral changes in VPA-induced offspring caused by DBS via the 5-HT system in the ILPFC.

Early Life Stress, Depression And Parkinson's Disease: A New Approach

This review aims to shed light on the relationship that involves exposure to early life stress, depression and Parkinson's disease (PD). A systematic literature search was conducted in Pubmed, MEDLINE, EBSCOHost and Google Scholar and relevant data were submitted to a meta-analysis . Early life stress may contribute to the development of depression and patients with depression are at risk of developing PD later in life. Depression is a common non-motor symptom preceding motor symptoms in PD. Stimulation of regions contiguous to the substantia nigra as well as dopamine (DA) agonists have been shown to be able to attenuate depression. Therefore, since PD causes depletion of dopaminergic neurons in the substantia nigra, depression, rather than being just a simple mood disorder, may be part of the pathophysiological process that leads to PD. It is plausible that the mesocortical and mesolimbic dopaminergic pathways that mediate mood, emotion, and/or cognitive function may also play a key role in depression associated with PD. Here, we propose that a medication designed to address a deficiency in serotonin is more likely to influence motor symptoms of PD associated with depression. This review highlights the effects of an antidepressant, Fluvoxamine maleate, in an animal model that combines depressive-like symptoms and Parkinsonism.

Current understanding of the molecular mechanisms in Parkinson's disease: Targets for potential treatments

Gradual degeneration and loss of dopaminergic neurons in the substantia nigra, pars compacta and subsequent reduction of dopamine levels in striatum are associated with motor deficits that characterize Parkinson’s disease (PD). In addition, half of the PD patients also exhibit frontostriatal-mediated executive dysfunction, including deficits in attention, short-term working memory, speed of mental processing, and impulsivity. The most commonly used treatments for PD are only partially or transiently effective and are available or applicable to a minority of patients. Because, these therapies neither restore the lost or degenerated dopaminergic neurons, nor prevent or delay the disease progression, the need for more effective therapeutics is critical. In this review, we provide a comprehensive overview of the current understanding of the molecular signaling pathways involved in PD, particularly within the context of how genetic and environmental factors contribute to the initiation and progression of this disease. The involvement of molecular chaperones, autophagy-lysosomal pathways, and proteasome systems in PD are also highlighted. In addition, emerging therapies, including pharmacological manipulations, surgical procedures, stem cell transplantation, gene therapy, as well as complementary, supportive and rehabilitation therapies to prevent or delay the progression of this complex disease are reviewed.

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.

The lateral habenula and the serotonergic system

The habenula (Hb) is an epithalamic structure differentiated into two nuclear complexes, medial (MHb) and lateral habenula (LHb). After decades of relative neglect, interest in the Hb resurged when it was demonstrated that LHb neurons play a key role in encoding disappointments and expectation of punishments. Consistent with such a role, the LHb has been implicated in a broad array of functions and pathologic conditions, notably in mechanisms of stress and pain, as well as in the pathophysiology of mood disorders. So far, the vast majority of research involving the LHb has focused on its role in regulating midbrain dopamine release. However, the LHb is also robustly interconnected in a reciprocal manner with a set of rostral serotonin (5-HT) nuclei. Thus, there is increasing evidence that the LHb is amply linked to the dorsal (DR) and median raphe nucleus (MnR) by a complex network of parallel topographically organized direct and indirect pathways. Here, we summarize research about the interconnections of the LHb with different subregions of the DR and MnR, as well as findings about 5-HT-dependent modulation of LHb neurons. Finally, we discuss the contribution of distinct LHb-raphe loops to stress and stress-related psychiatric disorders including anxiety and depression.

Unilateral ultra-brief pulse electroconvulsive therapy for depression in Parkinson's disease

OBJECTIVES

Electroconvulsive therapy (ECT) has demonstrated efficacy in treating core symptoms of Parkinson's disease (PD); however, widespread use of ECT in PD has been limited due to concern over cognitive burden. We investigated the use of a newer ECT technology known to have fewer cognitive side effects (right unilateral [RUL] ultra-brief pulse [UBP]) for the treatment of medically refractory psychiatric dysfunction in PD.

MATERIALS AND METHODS

This open-label pilot study included 6 patients who were assessed in the motoric, cognitive, and neuropsychiatric domains prior to and after RUL UBP ECT. Primary endpoints were changes in total score on the HAM-D-17 and GDS-30 rating scales.

RESULTS

Patients were found to improve in motoric and psychiatric domains following RUL UBP ECT without cognitive side effects, both immediately following ECT and at 1-month follow-up.

CONCLUSIONS

This study demonstrates that RUL UBP ECT is safe, feasible, and potentially efficacious in treating multiple domains of PD, including motor and mood, without clear cognitive side effects.

Sphingolipids modulate the function of human serotonin1A receptors: Insights from sphingolipid-deficient cells

Sphingolipids are essential components of eukaryotic cell membranes and are known to modulate a variety of cellular functions. It is becoming increasingly clear that membrane lipids play a crucial role in modulating the function of integral membrane proteins such as G protein-coupled receptors (GPCRs). In this work, we utilized LY-B cells, that are sphingolipid-auxotrophic mutants defective in sphingolipid biosynthesis, to monitor the role of cellular sphingolipids in the function of an important neurotransmitter receptor, the serotonin_1A receptor. Serotonin_1A receptors belong to the family of GPCRs and are implicated in behavior, development and cognition. Our results show that specific ligand binding and G-protein coupling of the serotonin_1A receptor exhibit significant enhancement under sphingolipid-depleted conditions, which reversed to control levels upon replenishment of cellular sphingolipids. In view of the reported role of sphingolipids in neuronal metabolism and pathogenesis of several neuropsychiatric disorders, exploring the role of serotonin_1A receptors under conditions of defective sphingolipid metabolism assumes relevance, and could contribute to our overall understanding of such neuropsychiatric disorders. This article is part of a Special Issue entitled: Lipid order/lipid defects and lipid-control of protein activity edited by Dirk Schneider.

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.

Serotonergic modulation of the activity of mesencephalic dopaminergic systems: Therapeutic implications

Since their discovery in the mammalian brain, it has been apparent that serotonin (5-HT) and dopamine (DA) interactions play a key role in normal and abnormal behavior. Therefore, disclosure of this interaction could reveal important insights into the pathogenesis of various neuropsychiatric diseases including schizophrenia, depression and drug addiction or neurological conditions such as Parkinson's disease and Tourette's syndrome. Unfortunately, this interaction remains difficult to study for many reasons, including the rich and widespread innervations of 5-HT and DA in the brain, the plethora of 5-HT receptors and the release of co-transmitters by 5-HT and DA neurons. The purpose of this review is to present electrophysiological and biochemical data showing that endogenous 5-HT and pharmacological 5-HT ligands modify the mesencephalic DA systems' activity. 5-HT receptors may control DA neuron activity in a state-dependent and region-dependent manner. 5-HT controls the activity of DA neurons in a phasic and excitatory manner, except for the control exerted by 5-HT_2C receptors which appears to also be tonically and/or constitutively inhibitory. The functional interaction between the two monoamines will also be discussed in view of the mechanism of action of antidepressants, antipsychotics, anti-Parkinsonians and drugs of abuse.

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.

Depression in Parkinson's disease: the effectiveness and risk of pharmacotherapy. Clinical review

Parkinson's disease (PD) is a neurological disease with a heterogeneous pattern of neurological symptoms and concomitant psychiatric syndromes. These syndromes are triggered by alterations to neurotransmission that are likely common for both neurological and psychiatric symptoms. Syndromes such as depression, anxiety, or cognitive impairment can precede motor symptoms of PD and delay its diagnosis. Recently, questions related to aetiological factors and treatment strategies of depression in PD have become a growing concern of PD researchers. This article describes the main features of depression in PD and presents current hypotheses on its aetiology and recommended treatment modes.

Neuropathobiology of non-motor symptoms in Parkinson disease

Parkinson disease (PD) is a multisystem disorder associated with α-synuclein aggregates throughout the central, autonomic, and peripheral nervous system, clinically characterized by motor and non-motor (NM) symptoms. The NMS in PD, many of which antedating motor dysfunction and representing a preclinical phase spanning 20 or more years, are linked to widespread distribution of α-synuclein pathology not restricted to the dopaminergic nigrostriatal system that is responsible for core motor features of PD. The pathologic substrate of NM manifestations such as olfactory, autonomic (gastrointestinal, urogenital, cardia, respiratory), sensory, skin, sleep, visual, neuropsychiatric dysfunctions (cognitive, mood, dementia), and others are critically reviewed. In addition to non-nigral brainstem nuclei, α-synuclein pathology involves sympathetic and parasympathetic, enteric, cardiac and pelvic plexuses, and many other organs indicating a topographical and chronological spread, particularly in the prodromal stages of the disease. Few animal models recapitulate NMS in PD. The relationship between regional α-synuclein/Lewy pathology, neurodegeneration and the corresponding clinical deficits awaits further elucidation. Controlled clinicopathologic studies will refine the correlations between presymptomatic and late-developing NM features of PD and neuropathology, and new premotor biomarkers will facilitate early diagnosis of PD as a basis for more effective preventive and therapeutic options of this devastating disease.

Current perspectives on deep brain stimulation for severe neurological and psychiatric disorders

Deep brain stimulation (DBS) has become a well-accepted therapy to treat movement disorders, including Parkinson's disease, essential tremor, and dystonia. Long-term follow-up studies have demonstrated sustained improvement in motor symptoms and quality of life. DBS offers the opportunity to selectively modulate the targeted brain regions and related networks. Moreover, stimulation can be adjusted according to individual patients' demands, and stimulation is reversible. This has led to the introduction of DBS as a treatment for further neurological and psychiatric disorders and many clinical studies investigating the efficacy of stimulating various brain regions in order to alleviate severe neurological or psychiatric disorders including epilepsy, major depression, and obsessive-compulsive disorder. In this review, we provide an overview of accepted and experimental indications for DBS therapy and the corresponding anatomical targets.

Lateral habenula and the rostromedial tegmental nucleus innervate neurochemically distinct subdivisions of the dorsal raphe nucleus in the rat

The lateral habenula (LHb) is an epithalamic structure differentiated in a medial (LHbM) and a lateral division (LHbL). Together with the rostromedial tegmental nucleus (RMTg), the LHb has been implicated in the processing of aversive stimuli and inhibitory control of monoamine nuclei. The inhibitory LHb influence on midbrain dopamine neurons has been shown to be mainly mediated by the RMTg, a mostly GABAergic nucleus that receives a dominant input from the LHbL. Interestingly, the RMTg also projects to the dorsal raphe nucleus (DR), which also receives direct LHb projections. To compare the organization and transmitter phenotype of LHb projections to the DR, direct and indirect via the RMTg, we first placed injections of the anterograde tracer Phaseolus vulgaris leucoagglutinin into the LHb or the RMTg. We then confirmed our findings by retrograde tracing and investigated a possible GABAergic phenotype of DR-projecting RMTg neurons by combining retrograde tracing with in situ hybridization for GAD67. We found only moderate direct LHb projections to the DR, which mainly emerged from the LHbM and were predominantly directed to the serotonin-rich caudal DR. In contrast, RMTg projections to the DR were more robust, emerged from RMTg neurons enriched in GAD67 mRNA, and were focally directed to a distinctive DR subdivision immunohistochemically characterized as poor in serotonin and enriched in presumptive glutamatergic neurons. Thus, besides its well-acknowledged role as a GABAergic control center for the ventral tegmental area (VTA)-nigra complex, our findings indicate that the RMTg is also a major GABAergic relay between the LHb and the DR.

Mechanisms of body weight fluctuations in Parkinson's disease

Typical body weight changes are known to occur in Parkinson’s disease (PD). Weight loss has been reported in early stages as well as in advanced disease and malnutrition may worsen the clinical state of the patient. On the other hand, an increasing number of patients show weight gain under dopamine replacement therapy or after surgery. These weight changes are multifactorial and involve changes in energy expenditure, perturbation of homeostatic control, and eating behavior modulated by dopaminergic treatment. Comprehension of the different mechanisms contributing to body weight is a prerequisite for the management of body weight and nutritional state of an individual PD patient. This review summarizes the present knowledge and highlights the necessity of evaluation of body weight and related factors, as eating behavior, energy intake, and expenditure in PD.

Depression and Parkinson's disease: current knowledge

Depressive disturbances are common in patients with Parkinson's disease (PD) and influence many other clinical aspects of the disease. In addition to causing inherent emotional distress, depressive disorders negatively impact quality of life, motor and cognitive deficits, functional disability, and other psychiatric comorbidities in patients with PD. Knowledge of the pathophysiology of PD depression remains limited. However, clinical studies demonstrate the efficacy of medications and psychotherapies for PD depression, underscoring the importance of their timely detection and concerted management.

Alterations in blood glucose and plasma glucagon concentrations during deep brain stimulation in the shell region of the nucleus accumbens in rats

Deep brain stimulation (DBS) of the nucleus accumbens (NAc) is an effective therapy for obsessive compulsive disorder (OCD) and is currently under investigation as a treatment for eating disorders. DBS of this area is associated with altered food intake and pharmacological treatment of OCD is associated with the risk of developing type 2 diabetes. Therefore we examined if DBS of the NAc-shell (sNAc) influences glucose metabolism. Male Wistar rats were subjected to DBS, or sham stimulation, for a period of 1 h. To assess the effects of stimulation on blood glucose and glucoregulatory hormones, blood samples were drawn before, during and after stimulation. Subsequently, all animals were used for quantitative assessment of Fos immunoreactivity in the lateral hypothalamic area (LHA) using computerized image analysis. DBS of the sNAc rapidly increased plasma concentrations of glucagon and glucose while sham stimulation and DBS outside the sNAc were ineffective. In addition, the increase in glucose was dependent on DBS intensity. In contrast, the DBS-induced increase in plasma corticosterone concentrations was independent of intensity and region, indicating that the observed DBS-induced metabolic changes were not due to corticosterone release. Stimulation of the sNAc with 200 μA increased Fos immunoreactivity in the LHA compared to sham or 100 μA stimulated animals. These data show that DBS of the sNAc alters glucose metabolism in a region- and intensity- dependent manner in association with neuronal activation in the LHA. Moreover, these data illustrate the need to monitor changes in glucose metabolism during DBS-treatment of OCD patients.

Depression and impulse control disorders in Parkinson's disease: two sides of the same coin?

Depression and impulse control disorders (ICD) are two common neuropsychiatric features in Parkinson's disease (PD). Studies have revealed that both phenomena are associated with aberrations in ventral striatal dopamine signaling and concomitant dysfunction of the reward-related (limbic) cortico-striatal-thalamocortical (CSTC) circuit. Depression in PD seems associated with decreased activity in the limbic CSTC circuit, whereas ICD seem associated with increased limbic CSTC circuit activity, usually after commencing dopamine replacement therapy (DRT). Not all DRT using PD patients, however, develop symptoms of ICD, suggesting an additional underlying neurobiological susceptibility. Furthermore, the symptoms of depression and ICD frequently coincide even though they are related to seemingly contrasting limbic CSTC circuit activation states. The aim of this review is to provide an overview of the currently available literature on the neurobiology of PD-related depression and ICD and discusses possible susceptibility factors. Finally, we propose a neurobiological model that identifies ventral striatal dopaminergic denervation as a common underlying neurobiological substrate of depression and ICD and subsequent dysfunction of reward and motivation-related brain areas.

SLC6 transporters: structure, function, regulation, disease association and therapeutics

The SLC6 family of secondary active transporters are integral membrane solute carrier proteins characterized by the Na(+)-dependent translocation of small amino acid or amino acid-like substrates. SLC6 transporters, which include the serotonin, dopamine, norepinephrine, GABA, taurine, creatine, as well as amino acid transporters, are associated with a number of human diseases and disorders making this family a critical target for therapeutic development. In addition, several members of this family are directly involved in the action of drugs of abuse such as cocaine, amphetamines, and ecstasy. Recent advances providing structural insight into this family have vastly accelerated our ability to study these proteins and their involvement in complex biological processes.

Depression in Parkinson's disease: identification and management

Depression is a common psychiatric comorbidity in Parkinson's disease (PD) and contributes to significant impairments in cognitive, functional, motor, and social performance. This results in reduced quality of life, higher levels of care dependency, and increased caregiver burden. When treating depression, it is important to ensure that the patient's response to treatment will be adequately monitored. This can be accomplished in neurology or primary care settings, or in clinical settings with interdisciplinary treatment teams. Mental health services should be engaged early as a component of ongoing comprehensive care. This article reviews a general approach to treating the pharmacotherapy of depression in PD. Ultimately, clinicians should rely on empiric assessments of known risks and putative benefits to guide treatment decisions and should include a targeted and individualized multimodal approach that utilizes psychotherapeutic interventions along with pharmacologic therapies.

Biomarkers in Parkinson's disease (recent update)

Parkinson's disease (PD) is the second most common neurodegenerative disorder mostly affecting the aging population over sixty. Cardinal symptoms including, tremors, muscle rigidity, drooping posture, drooling, walking difficulty, and autonomic symptoms appear when a significant number of nigrostriatal dopaminergic neurons are already destroyed. Hence we need early, sensitive, specific, and economical peripheral and/or central biomarker(s) for the differential diagnosis, prognosis, and treatment of PD. These can be classified as clinical, biochemical, genetic, proteomic, and neuroimaging biomarkers. Novel discoveries of genetic as well as nongenetic biomarkers may be utilized for the personalized treatment of PD during preclinical (premotor) and clinical (motor) stages. Premotor biomarkers including hyper-echogenicity of substantia nigra, olfactory and autonomic dysfunction, depression, hyposmia, deafness, REM sleep disorder, and impulsive behavior may be noticed during preclinical stage. Neuroimaging biomarkers (PET, SPECT, MRI), and neuropsychological deficits can facilitate differential diagnosis. Single-cell profiling of dopaminergic neurons has identified pyridoxal kinase and lysosomal ATPase as biomarker genes for PD prognosis. Promising biomarkers include: fluid biomarkers, neuromelanin antibodies, pathological forms of α-Syn, DJ-1, amyloid β and tau in the CSF, patterns of gene expression, metabolomics, urate, as well as protein profiling in the blood and CSF samples. Reduced brain regional N-acetyl-aspartate is a biomarker for the in vivo assessment of neuronal loss using magnetic resonance spectroscopy and T2 relaxation time with MRI. To confirm PD diagnosis, the PET biomarkers include [(18)F]-DOPA for estimating dopaminergic neurotransmission, [(18)F]dG for mitochondrial bioenergetics, [(18)F]BMS for mitochondrial complex-1, [(11)C](R)-PK11195 for microglial activation, SPECT imaging with (123)Iflupane and βCIT for dopamine transporter, and urinary salsolinol and 8-hydroxy, 2-deoxyguanosine for neuronal loss. This brief review describes the merits and limitations of recently discovered biomarkers and proposes coenzyme Q10, mitochondrial ubiquinone-NADH oxidoreductase, melatonin, α-synculein index, Charnoly body, and metallothioneins as novel biomarkers to confirm PD diagnosis for early and effective treatment of PD.

Diurnal variation of hypothalamic function and chronic subthalamic nucleus stimulation in Parkinson's disease

BACKGROUND

Deep brain stimulation of the subthalamic nucleus (STN-DBS) improves quality of life in patients with advanced Parkinson's disease (PD), but is associated with neuropsychiatric side effects and weight gain in some individuals. The pathomechanisms of these phenomena are still unknown. Considering anatomical and functional connections of the STN with the hypothalamic-pituitary (HP) system, we prospectively investigated whether chronic STN-DBS alters HP functioning in 11 PD patients.

METHODS

Basal hormone levels of the HP-adrenal (HPA), HP-gonadal and HP-somatotropic axis were determined before surgery as well as 3 and 6 months after electrode implantation. In addition, 24-hour cortisol profiles and dexamethasone suppression tests were obtained. Postoperative hormone changes were correlated with individual neuropsychological test performance, psychiatric status and anthropometric measures.

RESULTS

While PD patients experienced weight gain (p = 0.025) at follow-up, most neuropsychological data and basal HP hormone levels did not change over time. HPA regulation and diurnal rhythmicity of cortisol remained intact in all patients. The 24-hour mean cortisol levels decreased 6 months after surgery (p = 0.002) correlating with improved postoperative depression (p = 0.02).

CONCLUSIONS

Chronic application of high-frequency electrical stimuli in the STN was not associated with HP dysfunction in patients with advanced PD. The diurnal variability of peripheral cortisol secretion as one important element of the endogenous biological clock remained intact. Evening cortisol levels decreased after surgery reflecting a favorable regulation of the cortisol setpoint. STN-DBS can be considered safe from a neuroendocrine perspective, but the origin of unwanted side effects warrants further elucidation.

Deep brain stimulation of the subthalamic nucleus in Parkinson's disease: Why so successful?

The subthalamic nucleus (STN), historically referred to as the corpus Luysii, is a relatively small nucleus located in the junction between the diencephalon and midbrain. An important discovery was made in the late 1980s by Miller and DeLong putting the focus on the STN demonstrating abnormal hyperactivity in this area in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) treated non-human primates. Shortly after, Benazzouz and colleagues showed STN deep brain stimulation (DBS) to significantly improve MPTP induced parkinsonian symptoms, including rigidity and bradykinesia in monkeys. In the same year, Pollak et al. were the first to publish a French case report describing the potential of STN DBS in a patient with advanced Parkinson's disease (PD) in whom they observed improvement of akinesia. Many other prospective studies showed similar improvements of motor symptoms and the lowering of required levodopa dosage. The great success of STN DBS for the treatment of advanced PD is underlined by the growing number of patients treated. STN DBS also provided additional insight into the role of the STN, which is important not only in motor control but also in cognitive and emotional functions.

Effects of repeated deep brain stimulation on depressive- and anxiety-like behavior in rats: comparing entopeduncular and subthalamic nuclei

BACKGROUND

Deep brain stimulation (DBS) of the subthalamic nucleus (STN) or internal globus pallidus (GPi) has been routinely used for the treatment of some movement disorders. However, DBS may be associated with adverse psychiatric effects, such as depression, anxiety and impulsivity.

OBJECTIVE

To compare DBS applied to the entopeduncular nucleus (EPN; the rodent homolog of the GPi) and STN in terms of their effects on depressive- and anxiety-like behavior in rats.

METHODS

DBS was applied for 21 days (4 h a day) to either the STN or EPN. Rats then underwent behavioral testing on learned helplessness and elevated plus maze tasks before being sacrificed for brain analyses of zif268, BDNF and trkB mRNA as well as BDNF protein levels.

RESULTS

Repeated DBS of the STN, but not of the EPN, led to impaired performance in the learned helplessness task, suggesting that STN-DBS induces or potentiates depressive-like behavior. There was no effect of DBS on elevated plus maze or on open field behavior. Repeated STN-DBS, but not EPN-DBS, led to decreased levels of BDNF and trkB mRNA in hippocampus. Acute stimulation of the STN or EPN resulted in similar changes in zif268 levels in several brain areas, except for the raphe where decreases were seen only after STB-DBS.

CONCLUSIONS

Together these results indicate that the effects of STN- and EPN-DBS differ in behavioral and neurochemical respects. Results further suggest that the EPN may be a preferable target for clinical DBS when psychiatric side effects are considered insofar as it may be associated with a lower incidence of depressive-like behavior than the STN.

Contribution of decreased serotonin release to the antidyskinetic effects of deep brain stimulation in a rodent model of tardive dyskinesia: comparison of the subthalamic and entopeduncular nuclei

Mechanisms whereby deep brain stimulation (DBS) of the subthalamic nucleus (STN) or internal globus pallidus (GPi) reduces dyskinesias remain largely unknown. Using vacuous chewing movements (VCMs) induced by chronic haloperidol as a model of tardive dyskinesia (TD) in rats, we confirmed the antidyskinetic effects of DBS applied to the STN or entopeduncular nucleus (EPN, the rodent homolog of the GPi). We conducted a series of experiments to investigate the role of serotonin (5-HT) in these effects. We found that neurotoxic lesions of the dorsal raphe nuclei (DRN) significantly decreased HAL-induced VCMs. Acute 8-OH-DPAT administration, under conditions known to suppress raphe neuronal firing, also reduced VCMs. Immediate early gene mapping using zif268 in situ hybridization revealed that STN-DBS inhibited activity of DRN and MRN neurons. Microdialysis experiments indicated that STN-DBS decreased 5-HT release in the dorsolateral caudate-putamen, an area implicated in the etiology of HAL-induced VCMs. DBS applied to the EPN also suppressed VCMs but did not alter 5-HT release or raphe neuron activation. While these findings suggested a role for decreased 5-HT release in the mechanisms of STN DBS, further microdialysis experiments showed that when the 5-HT lowering effects of STN DBS were prevented by pretreatment with fluoxetine or fenfluramine, the ability of DBS to suppress VCMs remained unaltered. These results suggest that EPN- and STN-DBS have different effects on the 5-HT system. While decreasing 5-HT function is sufficient to suppress HAL-induced VCMs, 5-HT decrease is not necessary for the beneficial motor effects of DBS in this model.