Functional MRI characterization of animal models of parkinsonism

Parkinson's disease (PD) is the second most common neurological disorder. It is characterized by the progressive development of motor symptoms - bradykinesia, resting tremor, muscular rigidity and difficulty with postural control - which serve as criterias for its clinical diagnosis. However, there is a need for biomarkers to detect PD early before the appearance of the symptoms, but also to evaluate efficacy of treatments. Such biomarkers would also to evaluate the translational value of models of the disease. In recent years, magnetic resonance imaging (MRI) has been used by researchers to identify biomarkers of PD in the patients' brain. One MRI method that is gradually becoming more popular is resting-state functional MRI (rs-fMRI). It consists in tracking the activity of brain by acquiring the MRI signal of the brain over time for several minutes while the patient is at rest, i.e. when he/she tries not to think about anything in particular. Compared to task-based fMRI, it is advantageous for studying PD as patients have problems to perform tasks, both because of motor symptoms but also cognitive symptoms which are common in PD. In this thesis, after successfully demonstrating the translational value of rs-fMRI by comparing a set of functional networks in naive Sprague-Dawley and healthy human subjects (paper I), several rat models of parkinsonism were characterized. These models consisted in a well-established model, the unilateral 6-hydroxydopamine (6-OHDA) model (paper II), and two progressive models of parkinsonism, the alpha-synuclein adeno-associated virus overexpression model, a genetic model (paper III), and the β-sitosterol-β-D-glucoside model, a new toxin-based model (paper IV). By acquiring rs-fMRI datasets and analysing them using seed-based correlation analysis, functional connectivity maps were generated. We could reproducibly demonstrate that sensorimotor corticostriatal functional connectivity is increased in the 6-OHDA lesioned animals compared to their control counterparts, while in models with milder parkinsonian pathology, the sensorimotor corticostriatal functional connectivity is decreased. We therefore emit the hypothesis that there is a U-shaped function describing corticostriatal functional connectivity relative to the level of striatal dopaminergic innervation. We also observed in both models of mild parkinsonism a reinforcement of negative functional connectivity between the prefrontal cortex, in particular the orbital cortex, and the primary somatosensory cortex compared to their healthy counterparts. These results demonstrate that rs-fMRI is a valid method to observe alterations in the brain related to parkinsonism in animals and that both motor and non-motor areas of the brain are affected by the loss of dopaminergic neurons. Further investigations must be conducted to understand the mechanisms involved in these changes and evaluate their translational value

Multimodal view on resting-state brain activity in Parkinson’s disease: examining the relation between functional resting-state networks and metabolic network activity

Research focusing on the pathophysiology of neurodegenerative disorders has undergone a fundamental shift towards a network perspective in the last decades. Besides regional aggregation of misfolded proteins and changes in cellular metabolism, accompanying changes of synaptic activity evolve and evoke dysregulation within neural circuits including remote brain regions. Modern theories of neurodegeneration propose a stereotypic pattern of these cerebral pathologies, which partly are in vivo accessible by multimodal neuroimaging techniques. The most often used indirect measurement of functional network integrity is resting-state functional magnetic resonance imaging, which depends on a complex interplay of hemodynamics, blood volume, and blood flow. Less is known about a potential metabolic component underlying resting-state networks in healthy brains and changes thereof in neurodegeneration and the influence of different transmitter systems. The current work therefore sought to investigate the association between functional resting-state networks and metabolic network activity and focused on metabolic consequences of nigrostriatal and striatocortical dysfunction in Parkinson’s disease. In the current work, a multimodal data set of the TP10 KFO219 cohort was analyzed regarding 1) the impact of nigrostriatal dopamine depletion on resting-state networks and 2) the relation between changes in functional connectivity and metabolic network activity. The first study addressed the subset of the KFO219 TP10 cohort who completed the trimodal imaging protocol (42 patients vs. 14 controls). Dopamine deficiency in Parkinson’s patients was examined by voxel-wise comparison of 6-[18F]fluoro-L-Dopa positron emission tomography scans. Resulting clusters served as seeds for restingstate functional connectivity maps that were compared between both groups by voxelwise t-tests. Metabolic activity was extracted from 2-[18F]fluoro-2-deoxy-D-glucose positron emission tomography scans for respective cortical clusters with striatocortical dysconnectivity and the relation to functional connectivity values was analyzed. In a separate study, functional and metabolic resting-state networks were obtained by performing spatial independent component analyses in a subset of the same cohort who underwent 2-[18F]fluoro-2-deoxy-D-glucose positron emission tomography and functional magnetic resonance imaging (56 vs 16) and completed neuropsychological testing. Multimodally obtained regions of interest in the default mode network were defined and metabolic activity as well as metabolic connectivity compared to functional connectivity differences between patients without or with mild cognitive impairment and healthy controls. Moreover, a third study was initiated in the context of the present work with the aim of establishing a dynamic 2-[18F]fluoro-2-deoxy-D-glucose positronemission tomography acquisition with a constant infusion protocol for examining interregional metabolic connectivity on single subject level and enable comparable analysis of hemodynamic and metabolic fluctuations in Parkinson’s disease. In the first study, a significant association between striatocortical functional connectivity changes of the data-driven defined dopamine depleted posterior putamen and metabolic activity of the cortical target area in the inferior parietal cortex was found in Parkinson’s disease. Interestingly, striatocortical connectivity of the inferior parietal cortex was associated with motor and cognitive impairment. In a second study, the multivariate approach revealed a moderate spatial convergence for the posterior default mode network in functional and metabolic data. For all multimodally obtained default mode network regions, a significant trend towards an increment of metabolic deficits from healthy controls via unimpaired patients to patients with mild cognitive impairment was identified. In addition, posterior default mode network regions with the strongest metabolic deficits and gradual decline in comparison to controls, also showed the strongest increases in both metabolic and functional connectivity compared to controls. The verification of the applicability of a constant infusion dynamic 2-[18F]fluoro-2-deoxy- D-glucose positron emission tomography protocol in Parkinson’s disease patients was started in a self-initiated study, which finished the acquisition phase with 10 participants per group by the time the current work was submitted. Together the first two studies highlight the added value of multimodal imaging in investigating human brain function and the pathophysiology of neurodegenerative disorders, in particular their great potential for identifying links between individual pathologies. The second study partly continued, and answered questions raised in response to the first study, which hinted at an involvement of default mode network regions in cognitive symptoms of Parkinson’s disease and a relation between functional network degeneration and metabolic activity. The current work shows exemplary the complementarity of both measures of brain network activity and their individual significance for cognitive symptoms in Parkinson’s disease. The presented work highlights how multimodal resting-state studies can provide new insights into the (patho-)physiological network organization of brain activity by confirming insights obtained by one modality and deepen our understanding of disease processes. The selfinitiated study further laid the ground for multimodal characterization of metabolic and hemodynamic network changes on single-subject level and the evaluation of dynamic positron emission tomography-based connectivity as metabolic network marker for Parkinson’s disease

Cerebellar atrophy in Parkinson's disease and its implication for network connectivity.

Pathophysiological and atrophic changes in the cerebellum are documented in Parkinson's disease. Without compensatory activity, such abnormalities could potentially have more widespread effects on both motor and non-motor symptoms. We examined how atrophic change in the cerebellum impacts functional connectivity patterns within the cerebellum and between cerebellar-cortical networks in 42 patients with Parkinson's disease and 29 control subjects. Voxel-based morphometry confirmed grey matter loss across the motor and cognitive cerebellar territories in the patient cohort. The extent of cerebellar atrophy correlated with decreased resting-state connectivity between the cerebellum and large-scale cortical networks, including the sensorimotor, dorsal attention and default networks, but with increased connectivity between the cerebellum and frontoparietal networks. The severity of patients' motor impairment was predicted by a combination of cerebellar atrophy and decreased cerebellar-sensorimotor connectivity. These findings demonstrate that cerebellar atrophy is related to both increases and decreases in cerebellar-cortical connectivity in Parkinson's disease, identifying potential cerebellar driven functional changes associated with sensorimotor deficits. A post hoc analysis exploring the effect of atrophy in the subthalamic nucleus, a cerebellar input source, confirmed that a significant negative relationship between grey matter volume and intrinsic cerebellar connectivity seen in controls was absent in the patients. This suggests that the modulatory relationship of the subthalamic nucleus on intracerebellar connectivity is lost in Parkinson's disease, which may contribute to pathological activation within the cerebellum. The results confirm significant changes in cerebellar network activity in Parkinson's disease and reveal that such changes occur in association with atrophy of the cerebellum

Longitudinal functional connectivity changes related to dopaminergic decline in Parkinson’s disease

Background: Resting-state functional magnetic resonance imaging (fMRI) studies have demonstrated that basal ganglia functional connectivity is altered in Parkinson’s disease (PD) as compared to healthy controls. However, such functional connectivity alterations have not been related to the dopaminergic deficits that occurs in PD over time. Objectives: To examine whether functional connectivity impairments are correlated with dopaminergic deficits across basal ganglia subdivisions in patients with PD both cross-sectionally and longitudinally. Methods: We assessed resting-state functional connectivity of basal ganglia subdivisions and dopamine transporter density using 11C-PE2I PET in thirty-four PD patients at baseline. Of these, twenty PD patients were rescanned after 19.9 ± 3.8 months. A seed-based approach was used to analyze resting-state fMRI data. 11CPE2I binding potential (BPND) was calculated for each participant. PD patients were assessed for disease severity. Results: At baseline, PD patients with greater dopaminergic deficits, as measured with 11C-PE2I PET, showed larger decreases in posterior putamen functional connectivity with the midbrain and pallidum. Reduced functional connectivity of the posterior putamen with the thalamus, midbrain, supplementary motor area and sensorimotor cortex over time were significantly associated with changes in DAT density over the same period. Furthermore, increased motor disability was associated with lower intraregional functional connectivity of the posterior putamen. Conclusions: Our findings suggest that basal ganglia functional connectivity is related to integrity of dopaminergic system in patients with PD. Application of resting-state fMRI in a large cohort and longitudinal scanning may be a powerful tool for assessing underlying PD pathology and its progression

Longitudinal functional connectivity changes related to dopaminergic decline in Parkinson's disease.

BACKGROUND: Resting-state functional magnetic resonance imaging (fMRI) studies have demonstrated that basal ganglia functional connectivity is altered in Parkinson's disease (PD) as compared to healthy controls. However, such functional connectivity alterations have not been related to the dopaminergic deficits that occurs in PD over time. OBJECTIVES: To examine whether functional connectivity impairments are correlated with dopaminergic deficits across basal ganglia subdivisions in patients with PD both cross-sectionally and longitudinally. METHODS: We assessed resting-state functional connectivity of basal ganglia subdivisions and dopamine transporter density using 11C-PE2I PET in thirty-four PD patients at baseline. Of these, twenty PD patients were rescanned after 19.9 ± 3.8 months. A seed-based approach was used to analyze resting-state fMRI data. 11C-PE2I binding potential (BPND) was calculated for each participant. PD patients were assessed for disease severity. RESULTS: At baseline, PD patients with greater dopaminergic deficits, as measured with 11C-PE2I PET, showed larger decreases in posterior putamen functional connectivity with the midbrain and pallidum. Reduced functional connectivity of the posterior putamen with the thalamus, midbrain, supplementary motor area and sensorimotor cortex over time were significantly associated with changes in DAT density over the same period. Furthermore, increased motor disability was associated with lower intraregional functional connectivity of the posterior putamen. CONCLUSIONS: Our findings suggest that basal ganglia functional connectivity is related to integrity of dopaminergic system in patients with PD. Application of resting-state fMRI in a large cohort and longitudinal scanning may be a powerful tool for assessing underlying PD pathology and its progression

Basal ganglia function in parkinsonism and dystonia

Parkinsonism and dystonia are movement disorders linked with abnormal function of the basal ganglia. The most common cause of parkinsonism, Parkinson’s disease (PD), is caused by loss of dopaminergic neurons in the nigrostriatal tract, leading to dopamine depletion in the striatum. The pathophysiology in dystonia is largely unknown, although a major role of the basal ganglia has been suspected. Both syndromes can be treated with deep brain stimulation (DBS) in specific targets in the basal ganglia. The aim of this thesis was to study the function of the basal ganglia in parkinsonism and dystonia using single-photon emission computed tomography (SPECT) and positron emission tomography (PET). The work in this thesis was broadly divided into two sets of experiments. The basal ganglia function of patients with PD, non-degenerative parkinsonism and healthy controls were evaluated using dopamine transporter (DAT) imaging. In the other experiment, basal ganglia function in dystonia was investigated in patients with cervical dystonia undergoing globus pallidus interna (GPi) DBS using 18F-fluoro-deoxyglucose-positron emission tomography (FDG-PET). The results of this thesis showed that DAT binding does not predict the number of preserved neurons in the striatum in PD. Moreover, patients with a non-degenerative condition seemed to have higher DAT binding compared to healthy controls. Bupropion, even with a recommended wash-out time, caused clearly abnormal DAT binding in a patient without a neurodegenerative disorder affecting the dopamine system. In cervical dystonia, GPi-DBS increased glucose metabolism at the stimulation site and in other basal ganglia structures as well as in the primary sensorimotor cortex. Metabolic changes in the cortical regions, including the primary sensorimotor cortex and the supplementary motor area (SMA), correlated with acute and long-term therapeutic benefits, respectively. The symptoms returned gradually to the preoperative level after cessation of treatment. The results of this thesis indicate that DAT imaging reflects dopamine function of the striatum rather than neuron count. Moreover, DAT binding is affected by several factors that should be controlled for in both clinical work and in research settings. The findings also suggest that dystonia involves brain regions outside the basal ganglia, which may play a critical role in motor symptoms of dystonia and contribute to slow neuroplastic changes associated with DBS.Tyvitumakkeiden toiminta parkinsonismissa ja dystoniassa Parkinsonismi ja dystonia ovat neurologisia liikehäiriösairauksia, jotka yhdistetään tyvitumakkeiden eli aivojen liikehäiriökeskuksen toimintahäiriöihin. Yleisimmässä parkinsonismissa, Parkinsonin taudissa, aivojen striatumin ja mustatumakkeen dopamiinisolut tuhoutuvat. Dystonian etiologia on sen sijaan edelleen epäselvä, mutta sen on epäilty johtuvan tyvitumakkeiden poikkeavasta toiminnasta. Molempien sairauksien vaikeita muotoja voidaan hoitaa tyvitumakealueelle kohdennettavalla syväaivostimulaattorilla. Tässä tutkimuksessa tyvitumakkeiden toimintaa tutkittiin isotooppikuvantamisella parkinsonismissa ja dystoniassa. Väitöskirjassa tutkittiin Parkinsonin tautia sairastavia henkilöitä, oireisia ja dopamiinitoiminnaltaan terveitä henkilöitä sekä terveitä ja oireettomia henkilöitä dopamiinitransportterikuvantamisella. Lisäksi syväaivostimulaatiohoitoa saavia dystoniapotilaita tutkittiin aivojen sokeriaineenvaihduntaa kuvaavalla PET-tutkimuksella. Tulokset osoittivat, että aivojen dopamiinitransportterisitoutuminen ei ennusta säilyneiden hermosolujen määrää. Sitoutumisarvot saattavat myös olla korkeampia dopamiinitoiminnallaan terveillä, mutta oireisilla potilailla kuin terveillä ja oireettomilla henkilöillä. Lisäksi bupropion saattaa aiheuttaa virheellisiä tuloksia dopamiinikuvantamiseen. Syväaivostimulaattori lisää dystoniapotilalla aivojen sokeriaineenvaihduntaa stimulaatiokohdassa ja lisäksi viereisissä rakenteissa tyvitumakealueella sekä aivokuorella tunto- ja liikeaivokuorella. Oirekuvan nopea korjaantuminen korreloi aineenvaihdunnan lisääntymiseen tunto- ja liikeaivokuorella ja pitkäaikainen hoitovaste lisääntymiseen suplementaarisella motorisella aivokuorella. Oirekuvan hitaampaa palautumista kahden vuorokauden hoitotauon aikana ennusti nuori ikä. Tulokset osoittavat, että tyvitumakkeiden dopamiinikuvantamisessa tulosten tulkinta kliinisessä työssä ja tutkimuksessa ei ole johtopäätösten kannalta yksiselitteistä ja tulee tehdä mahdolliset virhelähteet huomioiden. Dystoniassa myös tyvitumakkeen ulkopuoliset aivoalueet saattavat olla tärkeitä oirekuvan synnyssä sekä hoitovasteen kehittymisessä syväaivostimulaatiohoidossa

Brain Network Metabolic Changes in Patients with Parkinsonian Tremors

Functional neuroimaging and modern multivariate analysis techniques have greatly contributed to research into the pathophysiology, diagnosis, and new treatments of neurodegenerative diseases, such as Parkinson’s disease (PD). The pathogenesis of PD symptoms, especially akinesia and rigidity, is associated with abnormalities of cortico-striato-pallido-thalamocortical circuits. Although a resting tremor is one of the cardinal features of PD, the pathophysiology underlying this symptom is unclear and is thought to differ from those of akinesia and rigidity. The application of network analyses to metabolic positron emission tomography scans of patients with PD has provided valuable information concerning functional neural connectivity and identified the patterns of covariance that are specific to the motor manifestations and many nonmotor features of the disease, such as cognitive dysfunction. Functional imaging methods have revealed PD-specific brain activation patterns, including a parkinsonian tremor-related network. Network-based algorithms might aid in the clinical diagnosis of patients with PD from early symptoms and provide objective evidence of treatment responses

Rat Model of Pre-Motor Parkinson\u27s Disease: Behavioral and MRI Characterization.

Background: Parkinson\u27s disease (PD) is a chronic, progressive, neurodegenerative disorder with currently no known cure. PD has a significant impact on quality of life of the patients, as well as, the caregivers and family members. It is the second most common cause of chronic neurological disability in US and Europe. According to National Parkinson\u27s Foundation, there are almost 1 million patients in the Unites States and 50,000 to 60,000 new cases of PD are diagnosed each year. The total number of cases of PD is predicted to double by 2030. The annual cost associated with this disease is estimated to be $10.8 billion in the United States, including the cost of treatment and the cost of the disability. Although it is primarily thought of as a movement-disorder and is clinically diagnosed based on motor symptoms, non-motor symptoms such as cognitive and emotional deficits are thought to precede the clinical diagnosis by almost 20 years. By the time of clinical diagnosis, there is 80% loss in the dopamine content in the striatum and 50% degeneration of the substantia nigra dopamine cells. The research presented in this thesis was an attempt to develop an animal model of PD in its pre-motor stages. Such a model would allow us to develop pre-clinical markers for PD, and facilitate the development and testing of potential treatment strategies for the non-motor symptoms of the disorder. Specific Aims: There were five specific aims for this research: * The first specific aim dealt with development of a rat model of PD with slow, progressive onset of motor deficits, determination of timeline for future studies, and quantification the dopamine depletion in this model at a pre-motor stage. * The second and the third specific aims focused on testing for emotional (aversion) deficits and cognitive (executive functioning) deficits in this rat model at the 3 week timepoint determined during specific aim 1. * The fourth specific aim was to determine the brain network changes associated with the behavioral changes observed our rat model using resting state connectivity as a measure. * The fifth and the final specific aim was to test sodium butyrate, a drug from the histone deacetylase inhibitor family, as a potential treatment option for cognitive deficits in PD. Results: The 6-hydroxy dopamine based stepwise striatal lesion model of pre-motor PD, developed during this research, exhibits delayed onset of Parkinsonian gait like symptoms by week 4 after the lesions. At 3 weeks post lesion (3WKPD), the rats exhibit 27% reduction in striatal dopamine and 23%reduction in substantia nigra dopamine cells, with lack of any apparent motor deficits. The 3WKPD rats also exhibited changes in aversion. The fMRI study with the aversive scent pointed towards possible amygdala dysfunction sub-serving the aversion deficits. The executive function deficits tested using a rat analog of the Wisconsin card sorting test, divulged an extra-dimensional set shifting deficit in the 3WKPD rats similar to those reported in PD patients. The resting state connectivity study indicated significant changes in the 3WKPD rats compared to age matched controls. We observed increased overall connectivity of the motor cortex and increased CPu connectivity with prefrontal cortex, cingulate cortex, and hypothalamus in the 3WKPD rats compared to the controls. These observations parallel the observations in unmedicated early-stage PD patients. We also observed negative correlation between amygdala and prefrontal cortex as reported in humans. This negative correlation was lost in 3WKPD rats. Sodium butyrate treatment, tested in the cognitive deficit study, was able to ameliorate the extra-dimensional set shifting deficit observed in this model. This treatment also improved the attentional set formation. Conclusion: Taken together, our observations indicate that, the model of pre-motor stage PD developed during this research is a very high face validity rat model of late Braak stage 2 or early Braak stage 3 PD. Sodium butyrate was able to alleviate the cognitive deficits observed in our rat model. Hence, along with the prior reports of anti-depressant and neuroprotective effects of this drug, our results point towards a possible treatment strategy for the non-motor deficits of PD

Altered intrinsic functional coupling between core neurocognitive networks in Parkinson\u27s disease

Parkinson3s disease (PD) is largely attributed to disruptions in the nigrostriatal dopamine system. These neurodegenerative changes may also have a more global effect on intrinsic brain organization at the cortical level. Functional brain connectivity between neurocognitive systems related to cognitive processing is critical for effective neural communication, and is disrupted across neurological disorders. Three core neurocognitive networks have been established as playing a critical role in the pathophysiology of many neurological disorders: the default-mode network (DMN), the salience network (SN), and the central executive network (CEN). In healthy adults, DMN–CEN interactions are anti-correlated while SN–CEN interactions are strongly positively correlated even at rest, when individuals are not engaging in any task. These intrinsic between-network interactions at rest are necessary for efficient suppression of the DMN and activation of the CEN during a range of cognitive tasks. To identify whether these network interactions are disrupted in individuals with PD, we used resting state functional magnetic resonance imaging (rsfMRI) to compare between-network connectivity between 24 PD participants and 20 age-matched controls (MC). In comparison to the MC, individuals with PD showed significantly less SN–CEN coupling and greater DMN–CEN coupling during rest. Disease severity, an index of striatal dysfunction, was related to reduced functional coupling between the striatum and SN. These results demonstrate that individuals with PD have a dysfunctional pattern of interaction between core neurocognitive networks compared to what is found in healthy individuals, and that interaction between the SN and the striatum is even more profoundly disrupted in those with greater disease severity

Molecular imaging in Parkinson's disease

The present work explores brain functional changes in drug-naïve Parkinson's disease (PD) patients by means of molecular imaging techniques. Thirty-one consecutive drug-naïve PD patients from the Neurological Clinic of the University of Flor-ence underwent clinical assessment, neuropsychological assessment, MRI, [123I]FP-CIT SPECT, [18F]FDG PET. First, [18F]FDG-PET was employed to identify in drug-naïve PD patients brain metabolic alteration uniquely related to disease process and not modulated by anti-parkinsonian therapeutic intervention. Second, [18F]FDG-PET and [123I]FP-CIT SPECT were employed together to explore the early functional changes in brain function related to dopaminergic depletion in the putamen and in the caudate nucleus