Parcellation of the human substantia nigra based on anatomical connectivity to the striatum

Substantia nigra/ventral tegmental area (SN/VTA) subregions, defined by dopaminergic projections to the striatum, are differentially affected by health (e.g. normal aging) and disease (e.g. Parkinson's disease). This may have an impact on reward processing which relies on dopaminergic regions and circuits. We acquired diffusion tensor imaging (DTI) with probabilistic tractography in 30 healthy older adults to determine whether subregions of the SN/VTA could be delineated based on anatomical connectivity to the striatum. We found that a dorsomedial region of the SN/VTA preferentially connected to the ventral striatum whereas a more ventrolateral region connected to the dorsal striatum. These SN/VTA subregions could be characterised by differences in quantitative structural imaging parameters, suggesting different underlying tissue properties. We also observed that these connectivity patterns differentially mapped onto reward dependence personality trait. We show that tractography can be used to parcellate the SN/VTA into anatomically plausible and behaviourally meaningful compartments, an approach that may help future studies to provide a more fine-grained synopsis of pathological changes in the dopaminergic midbrain and their functional impact

Utilidade dos exames de ressonância magnética no diagnóstico da doença de Parkinson : revisão sistemática e meta análise

Tese de mestrado, Neurociências, Faculdade de Medicina, Universidade de Lisboa, 2014Com a atual tendência para o envelhecimento da população e sendo a doença de Parkinson (DP) uma patologia que atinge cerca de 2% da população acima dos 65 anos, podemos prever um aumento da sua prevalência. Por outro lado devido às questões éticas e à morbilidade que implicariam as biopsias cerebrais como método de diagnóstico definitivo, a necessidade de encontrar métodos de diagnóstico precoce e não invasivos são de extrema importância. Mesmo nos melhores centros de diagnóstico há uma percentagem importante de desacordo entre o diagnóstico efetuado em vida, de base clínica e o diagnóstico pós-morte, que é histopatológico. A Ressonância Magnética, nas suas diferentes modalidades, proporciona-nos um meio de investigar “in vivo” as regiões corticais e subcorticais que se sabem estarem afetadas na DP. Diversos estudos recentes procuram demonstrar a utilidade destes testes como bio marcadores de diagnóstico e progressão da doença de Parkinson. Procurei efetuar uma revisão sistemática e meta análise em relação aos exames de Imagens de Ressonância Magnética (IRM) de modo a verificar a sua utilidade no diagnóstico da doença de Parkinson e se poderão ter potencial para serem considerados bio marcadores de diagnóstico e seguimento dos doentes. Objetivo Revisão sistemática dos estudos que compararem, a precisão das diferentes modalidades de ressonância magnética no diagnóstico da doença de Parkinson, com diagnóstico clínico e controlos saudáveis, explorando suas potencialidades como bio marcadores. Material e métodos Procedeu-se a uma pesquisa na literatura publicada sobre a temática, recorrendo à base de dados PubMed/MEDLINE, Embase, B-on, Google Scholar e ainda na bibliografia dos estudos considerados relevantes. Foram utilizadas as palavras-chave "Parkinson", "Magnetic resonance imaging", "MRI", "DTI", "Diffusion tensor imaging", "Spectroscopy", "MRI of Iron", "fMRI", "bold", “Neuromelanin”, combinados com operadores booleanos apropriados para cada pesquisa. Foram selecionados estudos redigidos em Inglês, Francês, Espanhol e Português. Critérios de inclusão: Estudos neurorradiológicos, com mais de cinco pacientes, até a presente data, que envolvam Imagens de ressonância magnética (MRI) de diferentes modalidades tais como: Exames Estruturais (T1, T2, Neuromelanina, Ferro e outras técnicas); Diffusion Tensor Imaging (DTI); Espectroscopia em Ressonância Magnética (MRSI); Ressonância Magnética Funcional (fMRI) no diagnóstico da Doença de Parkinson e que comparem a precisão do teste com diagnóstico clínico e controlos saudáveis, explorando suas potencialidades como bio marcadores. Foram excluídos, estudos que não usaram critérios formais de diagnóstico clínico, estudos incluindo pacientes submetidos a estimulação cerebral profunda, parkinsonismo idiopático ou vascular, casos relatados, editoriais, comentários, cartas, estudos em animais, estudos de diagnóstico diferencial com outras síndromes parkinsonianos e demências da DP. Resultados Dos 834 estudos identificados e atendendo aos critérios de seleção foram separados 109 estudos que após a leitura dos “Abstracts” verificou-se que apenas 52 preenchiam os requisitos dos critérios de inclusão. Destes estudos, foram obtidas as versões integrais publicadas, que foram integralmente lidas e sempre que existentes registados por mim, os seguintes dados: a referência, o ano, o título, o número de pacientes e controlos, a idade média, o estádio Hoehn & Yahr, a medicação, a modalidade de IRM, a região estudada, a intensidade do campo magnético do sistema, as conclusões e ainda se possível, a especificidade, a sensibilidade, a área sob a curva ROC e valores-p (Sigma) do teste t-Student (t-test) da comparação entre os valores obtidos dos exames dos pacientes com DP e a dos controlos saudáveis. Os resultados obtidos foram divididos em quatro grupos, em função da modalidade de estudo de IRM (imagens de ressonância magnética), para avaliação: 1º exames estruturais utilizando os métodos clássicos e IRM do ferro e da Neuromelanina; 2º exames utilizando DTI (Diffusion Tensor Imaging); 3º exames de espectroscopia de RMN; 4º fMRI (ressonância magnética funcional) incluindo a de em estado de repouso (RS-fMRI). 1º Exames estruturais utilizando os métodos clássicos e IRM, do ferro e Neuromelanina. Neste grupo de estudos podemos verificar que, utilizando T1 imagens Inversão de recuperação (a área) 24; T2W (o volume) 33; T1p (sensível á perda neuronal) 46; MRI sensível á Neuromelanina (medição do volume) 23, encontramos uma diminuição significativa na SN (substância nigra) dos pacientes com DP, quando comparados com controlos saudáveis pareados por idade, e um aumento significativo dos valores do R2 * (= 1/T2 *) e T2p, (sensível à deposição de ferro) em pacientes com DP 41, 27, quando comparados com controlos saudáveis. É de salientar que estas alterações se mantem ainda que tenhamos valores de sistemas com diferentes intensidades dos campos magnéticos 3T; 4T; 7T. 2º Exames utilizando DTI (Diffusion Tensor Imaging). Com este tipo de exames podemos detetar em pacientes com DP, alterações na AF (anisotropia fracionada) e DM (difusibilidade média) em todo o cérebro 74, mas que são mais pronunciadas na substância branca frontal e parietal refletindo deste modo um dano microestrutural generalizado. Estas alterações ocorrem nos estádios iniciais da PD75, em fibras de projeção do tálamo 11. Os valores da MK (mean kurtosis) e da AF foram significativamente menores no cíngulo anterior 22, na área motora, na pré-motora e motora suplementar do córtex 64,nas áreas de substância branca próximas das áreas motoras suplementares, cápsulas externa e interna, tálamo direito, putamen esquerdo 65 e como se demonstra na meta-análise há uma redução significativa da AF na SN. 3º Exames de espectroscopia de RMN Na espectroscopia dos metabolitos Substância Nigra na doença de Parkinson foram observadas diferenças significativas entre doentes PD e controlos saudáveis nas razões, NAA / Cr, NAA / Cho, NAA / (Co + Cr) 66, 76. Com estes exames podemos obter um in perfil neuro-químico “in vivo”, incluindo neurotransmissores (Glu e GABA) e os níveis de antioxidantes (GSH), que estão em excelente concordância com a literatura neuro química 70. Na pré-SMA, a razão NAA / Cr diminuiu seletivamente, em paralelo com disfunção neuronal nos DP (P = 0,045) 73. No putamen e mesencéfalo foi encontrada uma redução bilateral de fosfatos de alta energia, como adenosina trifosfato e fosfocreatina como recetores finais da energia da fosforilação oxidativa mitocondrial 71. 4º Exames de fMRI ressonância magnética funcional incluindo os de em estado de repouso (RS-fMRI). Usando diferentes paradigmas e comparando pacientes com DP, com controlos saudáveis, encontramos nos diferentes estudos uma redução da percentagem de mudança de sinal em todos os núcleos dos gânglios da base contra lateral e ipsilateral, tálamo lateral e medial, M1 (córtex motor primário) e área motora suplementar. Foram detetadas correlações negativas significativas entre a UPDRS e a ativação BOLD bilateralmente nos núcleos, caudado e putamen, segmento externo contra lateral do globo pálido, bilateralmente nos núcleos sub-talâmicos, substantia nigra e tálamo contra lateral. A bradicinesia é o sintoma que mais consistentemente previu a ativação BOLD nos gânglios da base e tálamo. Além disso, a ativação BOLD no globo pálido interno contra lateral, estava relacionada com tremor. A atividade cortical reduzida no córtex motor primário e na área motora suplementar nos pacientes com DP recém-diagnosticada, não se relacionam com sintomas motores 58. Durante a execução de movimentos automáticos, os pacientes com doença de Parkinson em comparação com os controlos saudáveis, necessitam de mais atividade cerebral no cerebelo, na área pré-motora, no córtex parietal, no precuneus e córtex pré-frontal para compensar a disfunção dos gânglios basais 51. Usando RS-fMRI para estudar a conectividade funcional (CF), verificou-se que os pacientes PD apresentam uma disrupção da rede motora. O aumento CF em estado de repouso entre os núcleos sub-talâmicos (NST) e áreas motoras corticais e os sintomas de rigidez e tremor na PD podem estar relacionados a um acoplamento anormal dessas áreas. Com estudos selecionados foram efetuadas meta análises ponderando o efeito de tamanho da amostra nos 1º e 2º grupo, tendo-se verificado que neles há diferenças significativas (em t-test utilizando p-value) no que respeita á redução de volume e da anisotropia fracionada (AF) da Substância Nigra (SN) entre os doentes de Parkinson e os controlos saudáveis. Foi detetada uma redução média de volume da SN, estimada pela tamanho do efeito das IRM Estruturais de (-0,877, 95% intervalo de confiança de -1,049 a -0.705, p <0.0001) apresentando os estudos um baixo nível de heterogeneidade (Q [12] =14,598 p =0,264 I2 =17,795). Na AF da SN em DTI a redução média dos valores da AF estimada atendendo ao efeito tamanho dos estudos foi de (-0,811, 95% intervalo de confiança de -1,036 a -0,586, p <0,0001) com um baixo nível de heterogeneidade entre os estudos (Q [6] =7,327, p =0,396 I2 = 4,465). Conclusões Estes resultados são encorajadores pois pode-se concluir que os exames de imagem de ressonância magnética possuem uma boa capacidade discriminativa dos doentes de Parkinson em relação aos controlos saudáveis e poderão desempenhar um papel importante na deteção, na monitorização da progressão e no impacto terapêutico na DP. Entretanto, serão necessários estudos longitudinais e prospetivos com um número mais elevado de doentes utilizando as várias modalidades, isoladamente ou em associação, para melhorar a acuidade diagnóstica e confirmar a sua utilização como bio marcadores.Objectives: We performed a systematic review of the studies comparing the accuracy of the different modalities of magnetic resonance imaging in the diagnosis of Parkinson's disease with clinical diagnosis and healthy controls, exploring its potentials as biomarkers. Methods: We searched for studies and research reviews in, the MEDLINE, EMBASE, B-on (the online knowledge Library) databases, and in bibliography cited in relevant studies, comparing the MRI differences between Parkinson’s disease patients and healthy controls to access the accuracy of the different methods, the results were extracted and estimates were pooled by random-effects meta-analysis. Results: 834 studies were identified using MRI in PD but only 48 studies were eligible for inclusion, with a total of 1362 Parkinson’s disease patients and 1023 healthy controls, whose results were divided into four groups: 1st- Structural, Iron and Neuromelanin MRI; 2nd- DTI (diffusion tensor imaging) with FA and MD; 3rd- Spectroscopy (MRS); 4th- fMRI that includes RS-fMRI (resting state fMRI). It was found changes in basal ganglia, thalamus, white and gray matter in the different MRI modalities. In the 1st and 2nd group we performed a meta-analysis for the Volume and Fractional Anisotropy (FA) of the Substantia Nigra (SN) respectively. A good effect size of the reduction was found for both in the PD patients versus controls in structural MRI (-0,877, 95% confidence interval -1,049 to -0.705, p < 0.0001) and in DTI (-0,811, 95% confidence interval -1,036 to -0,586, p < 0,0001). With a low level of heterogeneity. Conclusions: Magnetic Resonance Imaging has a good accuracy in separate PD patients from Healthy Controls, and could have a role in detecting pre manifest disease, monitoring progression and drug therapeutic impact. Larger prospective and longitudinal studies using DTI, Spectroscopy, fMRI, RS-fMRI and other modalities of MRI on larger cohorts of patients with Parkinson´s disease are needed to investigate some of the actual encouraging preliminary findings. Standardization of protocols is a need and will be a reality in the future and that will help us to get better and comparable results. Combination of modalities could improve the diagnostic accuracy

Confirmation of functional zones within the human subthalamic nucleus: Patterns of connectivity and sub-parcellation using diffusion weighted imaging

The subthalamic nucleus (STN) is a small, glutamatergic nucleus situated in the diencephalon. A critical component of normal motor function, it has become a key target for deep brain stimulation in the treatment of Parkinson's disease. Animal studies have demonstrated the existence of three functional sub-zones but these have never been shown conclusively in humans. In this work, a data driven method with diffusion weighted imaging demonstrated that three distinct clusters exist within the human STN based on brain connectivity profiles. The STN was successfully sub-parcellated into these regions, demonstrating good correspondence with that described in the animal literature. The local connectivity of each sub-region supported the hypothesis of bilateral limbic, associative and motor regions occupying the anterior, mid and posterior portions of the nucleus respectively. This study is the first to achieve in-vivo, non-invasive anatomical parcellation of the human STN into three anatomical zones within normal diagnostic scan times, which has important future implications for deep brain stimulation surgery

Feasibility of diffusion and probabilistic white matter analysis in patients implanted with a deep brain stimulator.

Deep brain stimulation (DBS) for Parkinson\u27s disease (PD) is an established advanced therapy that produces therapeutic effects through high frequency stimulation. Although this therapeutic option leads to improved clinical outcomes, the mechanisms of the underlying efficacy of this treatment are not well understood. Therefore, investigation of DBS and its postoperative effects on brain architecture is of great interest. Diffusion weighted imaging (DWI) is an advanced imaging technique, which has the ability to estimate the structure of white matter fibers; however, clinical application of DWI after DBS implantation is challenging due to the strong susceptibility artifacts caused by implanted devices. This study aims to evaluate the feasibility of generating meaningful white matter reconstructions after DBS implantation; and to subsequently quantify the degree to which these tracts are affected by post-operative device-related artifacts. DWI was safely performed before and after implanting electrodes for DBS in 9 PD patients. Differences within each subject between pre- and post-implantation FA, MD, and RD values for 123 regions of interest (ROIs) were calculated. While differences were noted globally, they were larger in regions directly affected by the artifact. White matter tracts were generated from each ROI with probabilistic tractography, revealing significant differences in the reconstruction of several white matter structures after DBS. Tracts pertinent to PD, such as regions of the substantia nigra and nigrostriatal tracts, were largely unaffected. The aim of this study was to demonstrate the feasibility and clinical applicability of acquiring and processing DWI post-operatively in PD patients after DBS implantation. The presence of global differences provides an impetus for acquiring DWI shortly after implantation to establish a new baseline against which longitudinal changes in brain connectivity in DBS patients can be compared. Understanding that post-operative fiber tracking in patients is feasible on a clinically-relevant scale has significant implications for increasing our current understanding of the pathophysiology of movement disorders, and may provide insights into better defining the pathophysiology and therapeutic effects of DBS

Neuroimaging biomarkers associated with clinical dysfunction in Parkinson disease

Parkinson disease (PD) is the second most common neurodegenerative disorder in the world, directly affecting 2-3% of the population over the age of 65. People diagnosed with the disorder can experience motor, autonomic, cognitive, sensory and neuropsychiatric symptoms that can significantly impact quality of life. Uncertainty still exists about the pathophysiological mechanisms that underlie a range of clinical features of the disorder, linked to structural as well as functional brain changes. This thesis thus aimed to uncover neuroimaging biomarkers associated with clinical dysfunction in PD. A 'hubs-and-spokes' neural circuit-based approach can contribute to this aim, by analysing the component elements and also the interconnections of important brain networks. This thesis focusses on structures within basal ganglia-thalamocortical neuronal circuits that are linked to a range functions impacted in the disorder, and that are vulnerable to the consequences of PD pathology. This thesis investigated neuronal 'hubs' by studying the morphology of the caudate nucleus, putamen, thalamus and neocortex. The caudate nucleus, putamen and thalamus are all vital subcortical 'hubs' that play important roles in a number of functional domains that are compromised in PD. The neocortex, on the other hand, has a range of 'hubs' spread across it, regions of the brain that are crucial for neuronal signalling and communication. The interconnections, or 'spokes', between these hubs and other brain regions were investigated using seed-based resting-state functional connectivity analyses. Finally, a morphological analysis was used to investigate possible structural changes to the corpus callosum, the major inter-hemispheric white matter tract of the brain, crucial to effective higher-order brain processes. This thesis demonstrates that the caudate nucleus, putamen, thalamus, corpus callosum and neocortex are all atrophied in PD participants with dementia. PD participants also demonstrated a significant correlation between volumes of the caudate nuclei and general cognitive functioning and speed, while putamina volumes were correlated with general motor function. Cognitively unimpaired PD participants demonstrated minimal morphological alterations compared to control participants, however they demonstrated significant increases in functional connectivity of the caudate nucleus, putamen and thalamus with areas across the frontal lobe, and decreases in functional connectivity with parietal and cerebellar regions. PD participants with mild cognitive impairment and dementia show decreased functional connectivity of the thalamus with paracingulate and posterior cingulate cortices, respectively. This thesis contributes a deeper understanding of the relationship between structures of basal ganglia-thalamocortical neuronal circuits, corpus callosal and neocortical morphology, and the clinical dysfunction associated with PD. This thesis suggests that functional connectivity changes are more common in early stages of the disorder, while morphological alterations are more pronounced in advanced disease stages

The whole-brain pattern of magnetic susceptibility perturbations in Parkinson's disease

Although iron-mediated oxidative stress has been proposed as a potential pathomechanism in Parkinson's disease, the global distribution of iron accumulation in Parkinson's disease has not yet been elucidated. This study used a new magnetic resonance imaging contrast, quantitative susceptibility mapping, and state-of-the-art methods to map for the first time the whole-brain landscape of magnetostatic alterations as a surrogate for iron level changes in n = 25 patients with idiopathic Parkinson's disease versus n = 50 matched controls. In addition to whole-brain analysis, a regional study including sub-segmentation of the substantia nigra into dorsal and ventral regions and qualitative assessment of susceptibility maps in single subjects were also performed. The most remarkable basal ganglia effect was an apparent magnetic susceptibility increase-consistent with iron deposition-in the dorsal substantia nigra, though an effect was also observed in ventral regions. Increased bulk susceptibility, additionally, was detected in rostral pontine areas and in a cortical pattern tightly concordant with known Parkinson's disease distributions of α-synuclein pathology. In contrast, the normally iron-rich cerebellar dentate nucleus returned a susceptibility reduction suggesting decreased iron content. These results are in agreement with previous post-mortem studies in which iron content was evaluated in specific regions of interest; however, extensive neocortical and cerebellar changes constitute a far more complex pattern of iron dysregulation than was anticipated. Such findings also stand in stark contrast to the lack of statistically significant group change using conventional magnetic resonance imaging methods namely voxel-based morphometry, cortical thickness analysis, subcortical volumetry and tract-based diffusion tensor analysis; confirming the potential of whole-brain quantitative susceptibility mapping as an in vivo biomarker in Parkinson's disease

Relationship between neuromelanin and dopamine terminals within the Parkinson's nigrostriatal system.

Parkinson's disease is characterized by the progressive loss of pigmented dopaminergic neurons in the substantia nigra and associated striatal deafferentation. Neuromelanin content is thought to reflect the loss of pigmented neurons, but available data characterizing its relationship with striatal dopaminergic integrity are not comprehensive or consistent, and predominantly involve heterogeneous samples. In this cross-sectional study, we used neuromelanin-sensitive MRI and the highly specific dopamine transporter PET radioligand, 11C-PE2I, to assess the association between neuromelanin-containing cell levels in the substantia nigra pars compacta and nigrostriatal terminal density in vivo, in 30 patients with bilateral Parkinson's disease. Fifteen healthy control subjects also underwent neuromelanin-sensitive imaging. We used a novel approach taking into account the anatomical and functional subdivision of substantia nigra into dorsal and ventral tiers and striatal nuclei into pre- and post-commissural subregions, in accordance with previous animal and post-mortem studies, and consider the clinically asymmetric disease presentation. In vivo, Parkinson's disease subjects displayed reduced neuromelanin levels in the ventral (-30 ± 28%) and dorsal tiers (-21 ± 24%) as compared to the control group [F(1,43) = 11.95, P = 0.001]. Within the Parkinson's disease group, nigral pigmentation was lower in the ventral tier as compared to the dorsal tier [F(1,29) = 36.19, P < 0.001] and lower in the clinically-defined most affected side [F(1,29) = 4.85, P = 0.036]. Similarly, lower dopamine transporter density was observed in the ventral tier [F(1,29) = 76.39, P < 0.001] and clinically-defined most affected side [F(1,29) = 4.21, P = 0.049]. Despite similar patterns, regression analysis showed no significant association between nigral pigmentation and nigral dopamine transporter density. However, for the clinically-defined most affected side, significant relationships were observed between pigmentation of the ventral nigral tier with striatal dopamine transporter binding in pre-commissural and post-commissural striatal subregions known to receive nigrostriatal projections from this tier, while the dorsal tier correlated with striatal projection sites in the pre-commissural striatum (P < 0.05, Benjamini-Hochberg corrected). In contrast, there were no statistically significant relationships between these two measures in the clinically-defined least affected side. These findings provide important insights into the topography of nigrostriatal neurodegeneration in Parkinson's disease, indicating that the characteristics of disease progression may fundamentally differ across hemispheres and support post-mortem data showing asynchrony in the loss of neuromelanin-containing versus tyrosine hydroxylase positive nigral cells.The research leading to these results has received funding from the European Research Council under the European Union's Seventh Framework Programme (FP7/2007-2013) [FP7-242003], from the Medical Research Council (MRC) [MR/P025870/1] and from Parkinson’s UK [J-1204]. Infrastructure support for this research was provided by the NIHR Imperial Biomedical Research Centre (BRC) and NIHR Imperial CRF at Imperial College healthcare NHS trust. The views expressed are those of the authors and not necessarily those of the funder, the NHS, the NIHR, or the Department of Health. This work was also supported financially by a PhD studentship awarded to N.P.L-K from Parkinson’s UK

Neuroimaging at 7 Tesla: a pictorial narrative review

Neuroimaging using the 7-Tesla (7T) human magnetic resonance (MR) system is rapidly gaining popularity after being approved for clinical use in the European Union and the USA. This trend is the same for functional MR imaging (MRI). The primary advantages of 7T over lower magnetic fields are its higher signal-to-noise and contrast-to-noise ratios, which provide high-resolution acquisitions and better contrast, making it easier to detect lesions and structural changes in brain disorders. Another advantage is the capability to measure a greater number of neurochemicals by virtue of the increased spectral resolution. Many structural and functional studies using 7T have been conducted to visualize details in the white matter and layers of the cortex and hippocampus, the subnucleus or regions of the putamen, the globus pallidus, thalamus and substantia nigra, and in small structures, such as the subthalamic nucleus, habenula, perforating arteries, and the perivascular space, that are difficult to observe at lower magnetic field strengths. The target disorders for 7T neuroimaging range from tumoral diseases to vascular, neurodegenerative, and psychiatric disorders, including Alzheimer’s disease, Parkinson’s disease, multiple sclerosis, epilepsy, major depressive disorder, and schizophrenia. MR spectroscopy has also been used for research because of its increased chemical shift that separates overlapping peaks and resolves neurochemicals more effectively at 7T than a lower magnetic field. This paper presents a narrative review of these topics and an illustrative presentation of images obtained at 7T. We expect 7T neuroimaging to provide a new imaging biomarker of various brain disorders

Theoretical and experimental considerations of selective vulnerability In Parkinson's disease

Les maladies neurodégénératives sont typiquement caractérisées en fonction de leurs symptômes et des observations pathologiques effectuées après le décès. Les symptômes spécifiques à la maladie sont également normalement associés aux dysfonctionnements et à la dégénérescence de certaines sous- populations spécifiques de neurones dans le système nerveux. La maladie de Parkinson (MP) est une maladie neurodégénérative principalement caractérisée par des symptômes moteurs dus à la dégénérescence spécifique des neurones dopaminergiques (DA) de la substantia nigra pars compacta (SNpc/SNc). Il semble cependant que les neurones DA de la SNc ne soient pas la seule population de neurones qui dégénère dans la MP. L'analyse post-mortem, l'imagerie in vivo et les symptômes cliniques démontrent que le dysfonctionnement et la dégénérescence se produisent dans plusieurs autres régions du système nerveux, incluant par exemple des neurones noradrénergiques (NA) du locus coeruleus (LC), des neurones sérotoninergiques des noyaux du raphé et des neurones cholinergiques du noyau moteur dorsal du nerf vague (DMV) et du noyau pédonculopontin. Comme d'autres maladies neurodégénératives, la MP est causée par plusieurs facteurs, tant génétiques qu'environnementaux. De nombreuses observations suggèrent que ces facteurs soient associés au dysfonctionnement de plusieurs systèmes ou fonctions cellulaires incluant la production d’énergie par la mitochondrie, l’élimination de protéines dysfonctionnelles par le protéasome et le lysosome, la régulation de l’équilibre entre la production d'espèces oxydatives réactives et les mécanismes antioxydants, la régulation des niveaux de calcium intracellulaire et l’inflammation. Il semble donc que le dysfonctionnement de ces facteurs converge pour provoquer la dégénérescence neuronale dans le contexte du vieillissement. Ce qui rend les neurones de certaines régions du système nerveux intrinsèquement plus vulnérables aux facteurs associés à la MP est une question fondamentale qui n’est pas résolue pour le moment. Les travaux de cette thèse sont basés sur l’hypothèse proposant que cette vulnérabilité sélective découle de propriétés communes retrouvées chez les neurones vulnérables. En particulier, les neurones vulnérables auraient en commun d’être des neurones de projections dotés d’un axone complexe qui projette sur de longues distances, formant un nombre très élevé de terminaisons axonales sur de vastes territoires du système nerveux. De plus, ces neurones présenteraient des propriétés physiologiques distinctives, incluant notamment une décharge autonome (pacemaker). Ensemble, ces caractéristiques pourraient contribuer à placer ces neurones dans des conditions de fonctionnement aux limites de leur capacités bioénergétiques et homéostatiques, rendant difficile toute adaptation aux dysfonctionnements cellulaires associés au vieillissement et causés par les facteurs de risques de la MP. Dans cette thèse, je présenterai une revue systématique de la littérature sur la perte de neurones dans le cerveau des personnes atteintes de la maladie de Parkinson, montrant que l'identité neurochimique précise des neurones qui dégénèrent dans la maladie de Parkinson, et l'ordre temporel dans lequel cela se produit, n’est pas clair. Cependant, en analysant les points de vue présentés dans les publications citant cette revue, nous avons remarqué que la majorité de ceux-ci ne reflètent pas le message central de notre publication. Puisque l’identification de l’identité des neurones vulnérables et non vulnérables à la MP est fondamentale pour le développement de théories et hypothèses sur les causes de la MP, nous suivons cette première publication avec une lettre réaffirmant l'importance de faire face aux problèmes identifiés dans notre revue. Nous présentons ensuite des données in vitro montrant que les neurones vulnérables à la MP, comparés à ceux qui sont moins vulnérables, ont une capacité intrinsèque à développer des champs axonaux plus importants et plus complexes, avec un nombre plus élevé de sites actifs de libération de neurotransmetteurs. De plus, nous constatons que ces observations sont corrélées à une vulnérabilité plus élevée face à un stress oxydatif pertinent pour la MP. Ces données sont en accord avec l'hypothèse selon laquelle le domaine axonal, et en particulier le nombre de sites de libération de neurotransmetteurs par neurone, est un facteur important qui contribue à rendre un neurone sélectivement vulnérable dans le contexte de la MP. Enfin, nous présentons une méthode d’analyse d’image open-source visant à aider les biologistes et les neuroscientifiques à automatiser la quantification du nombre de neurones dans des cultures primaires de neurones, telle qu’utilisée dans les travaux de cette thèse. Nous proposons que cet algorithme simple — mais robuste — permettra aux biologistes d'automatiser le comptage des neurones avec une grande précision, quelque chose de difficile à effectuer avec les autres approches open-source disponibles présentement. Nous espérons que les travaux présentés dans cette thèse permettront de contribuer à raffiner les théories visant à expliquer l’origine de la MP et à terme, de développer de nouvelles approches thérapeutiques.Neurodegenerative diseases are typically characterized based on their symptoms, and pathological factors identified after death. The disease-specific symptoms are due to the dysfunction and degeneration of specific subpopulations of neurons, which cause dysfunction in particular brain functions. Parkinson's disease (PD) is a neurodegenerative disease primarily characterized by motor symptoms due to the specific degeneration of dopamine (DA) neurons of the substantia nigra pars compacta (SNpc/SNc): a population of neurons essential for motor control. SNc DA neurons are, however, not the only population of neurons that degenerate in PD. Post-mortem analysis, in vivo imaging, and clinical symptoms demonstrate that dysfunction and degeneration occur in several other neuronal nuclei. These include, but are not limited to, noradrenergic (NA) locus coeruleus (LC) neurons, serotonin neurons of the raphe nuclei, and cholinergic neurons of the dorsal motor nucleus of the vagus (DMV) and pedunculopontine nucleus. Like other neurodegenerative diseases, PD is linked to several risk factors, both genetic and environmental. The evidence suggests that these risk factors are associated with the dysfunction in systems of cellular bioenergetics (including mitochondrial function); proteostatic homeostasis; endolysosomal function; an imbalance between the production of reactive oxidative species (ROS), and antioxidant mechanisms; calcium homeostasis; alpha-synuclein misfolding; and neuroinflammation. Consequently, together with aging, these risk factors converge on causing the selective degeneration of "PD-vulnerable" nuclei. What makes these neurons intrinsically vulnerable to PD-associated risk factors is a fundamental question — and understanding these neurons will reveal biological mechanisms that we can target to protect these cells from degeneration. Our best hypotheses to explain why these neurons are based on the observations that most PD- vulnerable neurons are long-range projection neuromodulatory neurons sharing common characteristics: projecting to voluminous territories, having very long and highly branched unmyelinated axonal domains with vast numbers of neurotransmitter release sites, and exhibiting a unique physiology such as pacemaker firing. Taken together, this suggests that these neurons function at the tail-end of their bioenergetic and homeostatic capacity, unable to tolerate any further demands, such as those incurred in the presence of risk factors associated with PD. In this thesis, I will present a systematic review on the literature on purported cell loss in PD brains, showing that — given the actual primary evidence — the precise neurochemical identity of neurons that degenerate in PD, and the temporal order of this degeneration, is far less clear than described by most publications. This review — at the time of writing — has gone on to be highly cited. However, analyzing the claims made in publications citing this review, we discover that the majority of claims do not reflect the core message of our publication. Since the identity of PD-vulnerable and non-PD-vulnerable neurons is fundamental to theory and hypotheses when trying to understand PD, we follow this first publication with a letter restating the importance to address our observations. We then present in vitro data showing that classically PD-vulnerable neurons, when compared to non-PD vulnerable neurons, have an intrinsic capacity to develop larger and more complex axonal domains, with higher numbers of active neurotransmitter release sites. Moreover, we find that these observations correlate to elevated vulnerability to PD-relevant stress assays. These data provide additional support for the hypothesis that the axonal domain — and in particular — the number of active neurotransmitter sites per neuron, is a cell-autonomous factor rendering a neuron selectively vulnerable in the context of PD. Finally, we present an open-source tool to support biologists and neuroscientists in automating the quantification of neuron numbers in medium-throughput primary cell cultures. Where the application of other open-source solutions is either too simplistic for the use-case or technically challenging to implement, this simple — yet robust algorithm — allows biologists with limited computational nous to automate neuron counting with high precision. We hope that the work presented in this thesis will contribute to the refinement of theories aimed at explaining the origin of PD and, ultimately, to the development of new therapeutic approaches