The C-Terminal Domain of LRRK2 with the G2019S Substitution Increases Mutant A53T α-Synuclein Toxicity in Dopaminergic Neurons In Vivo.

Alpha-synuclein (α-syn) and leucine-rich repeat kinase 2 (LRRK2) play crucial roles in Parkinson's disease (PD). They may functionally interact to induce the degeneration of dopaminergic (DA) neurons via mechanisms that are not yet fully understood. We previously showed that the C-terminal portion of LRRK2 (ΔLRRK2) with the G2019S mutation (ΔLRRK2G2019S) was sufficient to induce neurodegeneration of DA neurons in vivo, suggesting that mutated LRRK2 induces neurotoxicity through mechanisms that are (i) independent of the N-terminal domains and (ii) "cell-autonomous". Here, we explored whether ΔLRRK2G2019S could modify α-syn toxicity through these two mechanisms. We used a co-transduction approach in rats with AAV vectors encoding ΔLRRK2G2019S or its "dead" kinase form, ΔLRRK2DK, and human α-syn with the A53T mutation (AAV-α-synA53T). Behavioral and histological evaluations were performed at 6- and 15-weeks post-injection. Results showed that neither form of ΔLRRK2 alone induced the degeneration of neurons at these post-injection time points. By contrast, injection of AAV-α-synA53T alone resulted in motor signs and degeneration of DA neurons. Co-injection of AAV-α-synA53T with AAV-ΔLRRK2G2019S induced DA neuron degeneration that was significantly higher than that induced by AAV-α-synA53T alone or with AAV-ΔLRRK2DK. Thus, mutated α-syn neurotoxicity can be enhanced by the C-terminal domain of LRRK2G2019 alone, through cell-autonomous mechanisms.

TSPO imaging in animal models of brain diseases.

Over the last 30 years, the 18-kDa TSPO protein has been considered as the PET imaging biomarker of reference to measure increased neuroinflammation. Generally assumed to image activated microglia, TSPO has also been detected in endothelial cells and activated astrocytes. Here, we provide an exhaustive overview of the recent literature on the TSPO-PET imaging (i) in the search and development of new TSPO tracers and (ii) in the understanding of acute and chronic neuroinflammation in animal models of neurological disorders. Generally, studies testing new TSPO radiotracers against the prototypic [11C]-R-PK11195 or more recent competitors use models of acute focal neuroinflammation (e.g. stroke or lipopolysaccharide injection). These studies have led to the development of over 60 new tracers during the last 15 years. These studies highlighted that interpretation of TSPO-PET is easier in acute models of focal lesions, whereas in chronic models with lower or diffuse microglial activation, such as models of Alzheimer's disease or Parkinson's disease, TSPO quantification for detection of neuroinflammation is more challenging, mirroring what is observed in clinic. Moreover, technical limitations of preclinical scanners provide a drawback when studying modest neuroinflammation in small brains (e.g. in mice). Overall, this review underlines the value of TSPO imaging to study the time course or response to treatment of neuroinflammation in acute or chronic models of diseases. As such, TSPO remains the gold standard biomarker reference for neuroinflammation, waiting for new radioligands for other, more specific targets for neuroinflammatory processes and/or immune cells to emerge.

Complete spatial characterisation of N-glycosylation upon striatal neuroinflammation in the rodent brain.

BACKGROUND: Neuroinflammation is an underlying pathology of all neurological conditions, the understanding of which is still being comprehended. A specific molecular pathway that has been overlooked in neuroinflammation is glycosylation (i.e., post-translational addition of glycans to the protein structure). N-glycosylation is a specific type of glycosylation with a cardinal role in the central nervous system (CNS), which is highlighted by congenital glycosylation diseases that result in neuropathological symptoms such as epilepsy and mental retardation. Changes in N-glycosylation can ultimately affect glycoproteins' functions, which will have an impact on cell machinery. Therefore, characterisation of N-glycosylation alterations in a neuroinflammatory scenario can provide a potential target for future therapies. METHODS: With that aim, the unilateral intrastriatal injection of lipopolysaccharide (LPS) in the adult rat brain was used as a model of neuroinflammation. In vivo and post-mortem, quantitative and spatial characterisation of both neuroinflammation and N-glycome was performed at 1-week post-injection of LPS. These aspects were investigated through a multifaceted approach based on positron emission tomography (PET), quantitative histology, reverse transcription-quantitative polymerase chain reaction (RT-qPCR), liquid chromatography and matrix-assisted laser desorption ionisation mass spectrometry imaging (MALDI-MSI). RESULTS: In the brain region showing LPS-induced neuroinflammation, a significant decrease in the abundance of sialylated and core fucosylated structures was seen (approximately 7.5% and 8.5%, respectively), whereas oligomannose N-glycans were significantly increased (13.5%). This was confirmed by MALDI-MSI, which provided a high-resolution spatial distribution of N-glycans, allowing precise comparison between normal and diseased brain hemispheres. CONCLUSIONS: Together, our data show for the first time the complete profiling of N-glycomic changes in a well-characterised animal model of neuroinflammation. These data represent a pioneering step to identify critical targets that may modulate neuroinflammation in neurodegenerative diseases.

The pharmacokinetics of [18F]UCB-H revisited in the healthy non-human primate brain.

BACKGROUND: Positron Emission Tomography (PET) imaging of the Synaptic Vesicle glycoprotein (SV) 2A is a new tool to quantify synaptic density. [18F]UCB-H was one of the first promising SV2A-ligands to be labelled and used in vivo in rodent and human, while limited information on its pharmacokinetic properties is available in the non-human primate. Here, we evaluate the reliability of the three most commonly used modelling approaches for [18F]UCB-H in the non-human cynomolgus primate, adding the coupled fit of the non-displaceable distribution volume (VND) as an alternative approach to improve unstable fit. The results are discussed in the light of the current state of SV2A PET ligands. RESULTS: [18F]UCB-H pharmacokinetic data was optimally fitted with a two-compartment model (2TCM), although the model did not always converge (large total volume of distribution (VT) or large uncertainty of the estimate). 2TCM with coupled fit K1/k2 across brain regions stabilized the quantification, and confirmed a lower specific signal of [18F]UCB-H compared to the newest SV2A-ligands. However, the measures of VND and the influx parameter (K1) are similar to what has been reported for other SV2A ligands. These data were reinforced by displacement studies using [19F]UCB-H, demonstrating only 50% displacement of the total [18F]UCB-H signal at maximal occupancy of SV2A. As previously demonstrated in clinical studies, the graphical method of Logan provided a more robust estimate of VT with only a small bias compared to 2TCM. CONCLUSIONS: Modeling issues with a 2TCM due to a slow component have previously been reported for other SV2A ligands with low specific binding, or after blocking of specific binding. As all SV2A ligands share chemical structural similarities, we hypothesize that this slow binding component is common for all SV2A ligands, but only hampers quantification when specific binding is low.

Comparative test-retest variability of outcome parameters derived from brain [18F]FDG PET studies in non-human primates.

INTRODUCTION: Knowledge of the repeatability of quantitative parameters derived from [18F]FDG PET images is essential to define the group size and allow correct interpretation. Here we tested repeatability and accuracy of different [18F]FDG absolute and relative quantification parameters in a standardized preclinical setup in nonhuman primates (NHP). MATERIAL AND METHODS: Repeated brain [18F]FDG scans were performed in 6 healthy NHP under controlled experimental factors likely to account for variability. Regional cerebral metabolic rate of glucose (CMRglu) was calculated using a Patlak plot with blood input function Semi-quantitative approaches measuring standard uptake values (SUV, SUV×glycemia and SUVR (SUV Ratio) using the pons or cerebellum as a reference region) were considered. Test-retest variability of all quantification parameters were compared in different brain regions in terms of absolute variability and intra-and-inter-subject variabilities. In an independent [18F]FDG PET experiment, robustness of these parameters was evaluated in 4 naive NHP. RESULTS: Experimental conditions (injected dose, body weight, animal temperature) were the same at both imaging sessions (p >0.4). No significant difference in the [18F]FDG quantification parameters was found between test and retest sessions. Absolute variability of CMRglu, SUV, SUV×glycemia and normalized SUV ranged from 25 to 43%, 16 to 21%, 23 to 28%, and 7 to 14%, respectively. Intra-subject variability largely explained the absolute variability of all quantitative parameters. They were all significantly correlated to each other and they were all robust. Arterial and venous glycemia were highly correlated (r = 0.9691; p<0.0001). CONCLUSION: [18F]FDG test-retest studies in NHP protocols need to be conducted under well-standardized experimental conditions to assess and select the most reliable and reproducible quantification approach. Furthermore, the choice of the quantification parameter has to account for the transversal or follow-up study design. If pons and cerebellum regions are not affected, non-invasive SUVR is the most favorable approach for both designs.

Assessment of simplified methods for quantification of [18F]-DPA-714 using 3D whole-brain TSPO immunohistochemistry in a non-human primate.

The 18 kDa translocator protein (TSPO) is the main molecular target to image neuroinflammation by positron emission tomography (PET). However, TSPO-PET quantification is complex and none of the kinetic modelling approaches has been validated using a voxel-by-voxel comparison of TSPO-PET data with the actual TSPO levels of expression. Here, we present a single case study of binary classification of in vivo PET data to evaluate the statistical performance of different TSPO-PET quantification methods. To that end, we induced a localized and adjustable increase of TSPO levels in a non-human primate brain through a viral-vector strategy. We then performed a voxel-wise comparison of the different TSPO-PET quantification approaches providing parametric [18F]-DPA-714 PET images, with co-registered in vitro three-dimensional TSPO immunohistochemistry (3D-IHC) data. A data matrix was extracted from each brain hemisphere, containing the TSPO-IHC and TSPO-PET data for each voxel position. Each voxel was then classified as false or true, positive or negative after comparison of the TSPO-PET measure to the reference 3D-IHC method. Finally, receiver operating characteristic curves (ROC) were calculated for each TSPO-PET quantification method. Our results show that standard uptake value ratios using cerebellum as a reference region (SUVCBL) has the most optimal ROC score amongst all non-invasive approaches.

Gene Therapy for Parkinson's Disease: Preclinical Evaluation of Optimally Configured TH:CH1 Fusion for Maximal Dopamine Synthesis.

A recent phase I-II, open-label trial of ProSavin, a lentiviral vector delivering the key enzymes in the dopamine biosynthetic pathway to non-dopaminergic striatal neurons, demonstrated safety and improved motor function in parkinsonian patients. However, the magnitude of the effect suggested that optimal levels of dopamine replacement may not have been achieved. OXB-102, a lentiviral vector with an optimized expression cassette for dopamine biosynthesis, has been shown to achieve a significantly higher dopamine yield than ProSavin. We assessed the efficacy of OXB-102 in the MPTP macaque model of Parkinson's disease (PD). At 6 months post-vector administration, all treated animals showed significant improvements in clinical scores and spontaneous locomotor activity compared to controls, with the highest recovery observed in the OXB-102 high-dose (HD) group. Positron emission tomography quantification of 6-[18F]-fluoro-L-m-tyrosine uptake showed a significant increase in amino acid decarboxylase activity for all treated animals, compared with controls, where the OXB-102 HD group showed the highest level of dopaminergic activity. A toxicology study in macaques demonstrated that the vector was safe and well tolerated, with no associated clinical or behavioral abnormalities and no immune response mounted against any transgene products. Overall, these data support the further clinical development of OXB-102 for the treatment of PD.

Longitudinal characterization of cognitive and motor deficits in an excitotoxic lesion model of striatal dysfunction in non-human primates.

As research progresses in the understanding of the molecular and cellular mechanisms underlying neurodegenerative diseases like Huntington's disease (HD) and expands towards preclinical work for the development of new therapies, highly relevant animal models are increasingly needed to test new hypotheses and to validate new therapeutic approaches. In this light, we characterized an excitotoxic lesion model of striatal dysfunction in non-human primates (NHPs) using cognitive and motor behaviour assessment as well as functional imaging and post-mortem anatomical analyses. NHPs received intra-striatal stereotaxic injections of quinolinic acid bilaterally in the caudate nucleus and unilaterally in the left sensorimotor putamen. Post-operative MRI scans showed atrophy of the caudate nucleus and a large ventricular enlargement in all 6 NHPs that correlated with post-mortem measurements. Behavioral analysis showed deficits in 2 analogues of the Wisconsin card sorting test (perseverative behavior) and in an executive task, while no deficits were observed in a visual recognition or an episodic memory task at 6 months following surgery. Spontaneous locomotor activity was decreased after lesion and the incidence of apomorphine-induced dyskinesias was significantly increased at 3 and 6 months following lesion. Positron emission tomography scans obtained at end-point showed a major deficit in glucose metabolism and D2 receptor density limited to the lesioned striatum of all NHPs compared to controls. Post-mortem analyses revealed a significant loss of medium-sized spiny neurons in the striatum, a loss of neurons and fibers in the globus pallidus, a unilateral decrease in dopaminergic neurons of the substantia nigra and a loss of neurons in the motor and dorsolateral prefrontal cortex. Overall, we show that this robust NHP model presents specific behavioral (learning, execution and retention of cognitive tests) and metabolic functional deficits that, to the best of our knowledge, are currently not mimicked in any available large animal model of striatal dysfunction. Moreover, we used non-invasive, translational techniques like behavior and imaging to quantify such deficits and found that they correlate to a significant cell loss in the striatum and its main input and output structures. This model can thus significantly contribute to the pre-clinical longitudinal evaluation of the ability of new therapeutic cell, gene or pharmacotherapy approaches in restoring the functionality of the striatal circuitry.

Advanced imaging of transplant survival, fate, differentiation, and integration.

Stem cell-based therapy trials for the treatment of neurodegenerative diseases such as Parkinson's and Huntington's disease are being actively prepared both at the preclinical and clinical level. Preclinical validation of these stem cell transplantations necessitates to implement a translational continuum to take one pluripotent stem cell through the point of "first-in-man" clinical trial. Main steps along this translational continuum include stem cell GMP production, in vitro optimization of differentiation protocols necessary to direct stem cells to the desired neuronal phenotype, and evaluation of functional efficacy in animal models including large animal models. Whereas the first two steps only require in vitro techniques and can be expedited smoothly, the last step is generally time consuming as functional assessment requires transplantation of the animal models with sufficient time for cells to develop, reconnect, and exert their therapeutic effect (around 40 weeks for human cells in the rodent brain). In such a context, brain imaging techniques that allow noninvasive longitudinal evaluation/characterization of the grafted cells in the living animals have the potential to speed-up the evaluation of candidate cell lines. This chapter describes an assemblage of imaging methods that over the years proved useful in preclinical and clinical applications for cell therapy, providing in a noninvasive way unique insights on graft cellular composition, phenotypic differentiation, as well as signs of hyperproliferation and/or immunological rejection and inflammation.

Long-chain n-3 PUFAs from fish oil enhance resting state brain glucose utilization and reduce anxiety in an adult nonhuman primate, the grey mouse lemur.

Decreased brain content of DHA, the most abundant long-chain n-3 polyunsaturated fatty acid (n-3 LCPUFA) in the brain, is accompanied by severe neurosensorial impairments linked to impaired neurotransmission and impaired brain glucose utilization. In the present study, we hypothesized that increasing n-3 LCPUFA intake at an early age may help to prevent or correct the glucose hypometabolism observed during aging and age-related cognitive decline. The effects of 12 months' supplementation with n-3 LCPUFA on brain glucose utilization assessed by positron emission tomography was tested in young adult mouse lemurs (Microcebus murinus). Cognitive function was tested in parallel in the same animals. Lemurs supplemented with n-3 LCPUFA had higher brain glucose uptake and cerebral metabolic rate of glucose compared with controls in all brain regions. The n-3 LCPUFA-supplemented animals also had higher exploratory activity in an open-field task and lower evidence of anxiety in the Barnes maze. Our results demonstrate for the first time in a nonhuman primate that n-3 LCPUFA supplementation increases brain glucose uptake and metabolism and concomitantly reduces anxiety.

Human ESC-derived dopamine neurons show similar preclinical efficacy and potency to fetal neurons when grafted in a rat model of Parkinson's disease.

Considerable progress has been made in generating fully functional and transplantable dopamine neurons from human embryonic stem cells (hESCs). Before these cells can be used for cell replacement therapy in Parkinson's disease (PD), it is important to verify their functional properties and efficacy in animal models. Here we provide a comprehensive preclinical assessment of hESC-derived midbrain dopamine neurons in a rat model of PD. We show long-term survival and functionality using clinically relevant MRI and PET imaging techniques and demonstrate efficacy in restoration of motor function with a potency comparable to that seen with human fetal dopamine neurons. Furthermore, we show that hESC-derived dopamine neurons can project sufficiently long distances for use in humans, fully regenerate midbrain-to-forebrain projections, and innervate correct target structures. This provides strong preclinical support for clinical translation of hESC-derived dopamine neurons using approaches similar to those established with fetal cells for the treatment of Parkinson's disease.

IRC-082451, a novel multitargeting molecule, reduces L-DOPA-induced dyskinesias in MPTP Parkinsonian primates.

The development of dyskinesias following chronic L-DOPA replacement therapy remains a major problem in the long-term treatment of Parkinson's disease. This study aimed at evaluating the effect of IRC-082451 (base of BN82451), a novel multitargeting hybrid molecule, on L-DOPA-induced dyskinesias (LIDs) and hypolocomotor activity in a non-human primate model of PD. IRC-082451 displays multiple properties: it inhibits neuronal excitotoxicity (sodium channel blocker), oxidative stress (antioxidant) and neuroinflammation (cyclooxygenase inhibitor) and is endowed with mitochondrial protective properties. Animals received daily MPTP injections until stably parkinsonian. A daily treatment with increasing doses of L-DOPA was administered to parkinsonian primates until the appearance of dyskinesias. Then, different treatment regimens and doses of IRC-082451 were tested and compared to the benchmark molecule amantadine. Primates were regularly filmed and videos were analyzed with specialized software. A novel approach combining the analysis of dyskinesias and locomotor activity was used to determine efficacy. This analysis yielded the quantification of the total distance travelled and the incidence of dyskinesias in 7 different body parts. A dose-dependent efficacy of IRC-082451 against dyskinesias was observed. The 5 mg/kg dose was best at attenuating the severity of fully established LIDs. Its effect was significantly different from that of amantadine since it increased spontaneous locomotor activity while reducing LIDs. This dose was effective both acutely and in a 5-day sub-chronic treatment. Moreover, positron emission tomography scans using radiolabelled dopamine demonstrated that there was no direct interference between treatment with IRC-082451 and dopamine metabolism in the brain. Finally, post-mortem analysis indicated that this reduction in dyskinesias was associated with changes in cFOS, FosB and ARC mRNA expression levels in the putamen. The data demonstrates the antidyskinetic efficacy of IRC-082451 in a primate model of PD with motor complications and opens the way to the clinical application of this treatment for the management of LIDs.

Reactive astrocytes overexpress TSPO and are detected by TSPO positron emission tomography imaging.

Astrocytes and microglia become reactive under most brain pathological conditions, making this neuroinflammation process a surrogate marker of neuronal dysfunction. Neuroinflammation is associated with increased levels of translocator protein 18 kDa (TSPO) and binding sites for TSPO ligands. Positron emission tomography (PET) imaging of TSPO is thus commonly used to monitor neuroinflammation in preclinical and clinical studies. It is widely considered that TSPO PET signal reveals reactive microglia, although a few studies suggested a potential contribution of reactive astrocytes. Because astrocytes and microglia play very different roles, it is crucial to determine whether reactive astrocytes can also overexpress TSPO and yield to a detectable TSPO PET signal in vivo. We used a model of selective astrocyte activation through lentiviral gene transfer of the cytokine ciliary neurotrophic factor (CNTF) into the rat striatum, in the absence of neurodegeneration. CNTF induced an extensive activation of astrocytes, which overexpressed GFAP and become hypertrophic, whereas microglia displayed minimal increase in reactive markers. Two TSPO radioligands, [(18)F]DPA-714 [N,N-diethyl-2-(2-(4-(2-[(18)F]fluoroethoxy)phenyl)-5,7-dimethylpyrazolo[1,5-a]pyrimidin-3-yl)acetamide] and [(11)C]SSR180575 (7-chloro-N,N-dimethyl-5-[(11)C]methyl-4-oxo-3-phenyl-3,5-dihydro-4H-pyridazino[4,5-b]indole-1-acetamide), showed a significant binding in the lenti-CNTF-injected striatum that was saturated and displaced by PK11195 [N-methyl-N-(1-methylpropyl)-1-(2-chlorophenyl)-isoquinoline-3-carboxamide]. The volume of radioligand binding matched the GFAP immunopositive volume. TSPO mRNA levels were significantly increased, and TSPO protein was overexpressed by CNTF-activated astrocytes. We show that reactive astrocytes overexpress TSPO, yielding to a significant and selective binding of TSPO radioligands. Therefore, caution must be used when interpreting TSPO PET imaging in animals or patients because reactive astrocytes can contribute to the signal in addition to reactive microglia.

A complementary diffusion tensor imaging (DTI)-histological study in a model of Huntington's disease.

In vivo diffusion tensor imaging (DTI) was performed on the quinolinic acid (QUIN) rat model of Huntington's disease, together with behavioral assessment of motor deficits and histopathological characterization. DTI and histology revealed the presence of a cortical lesion in 53% of the QUIN animals (QUIN(+ctx)). Histologically, QUIN(+ctx) were distinguished from QUIN(-ctx) animals by increased astroglial reaction within a subregion of the caudate putamen and loss of white matter in the external capsula. Although both techniques are complementary, the quantitative character of DTI makes it possible to pick up subtle differences in tissue microstructure that are not identified with histology. DTI demonstrated differential changes of fractional anisotropy (FA), axial diffusivity (AD), radial diffusivity (RD), and mean diffusivity (MD) in the internal and external capsula, and within a subregion of the caudate putamen. It was suggested that FA increased due to a selective loss of the subcortical connections targeted by degenerative processes at the early stage of the disease, which might turn the striatum into a seemingly more organized structure. When tissue degeneration becomes more severe, FA decreased while AD, RD and MD increased.

Genotype specific age related changes in a transgenic rat model of Huntington's disease.

We aimed to characterize the transgenic Huntington rat model with in vivo imaging and identify sensitive and reliable biomarkers associated with early and progressive disease status. In order to do so, we performed a multimodality (DTI and PET) longitudinal imaging study, during which the same TgHD and wildtype (Wt) rats were repetitively scanned. Surprisingly, the relative ventricle volume was smaller but increased faster in TgHD compared to Wt animals. DTI (mean, axial, radial diffusivity) revealed subtle genotype-specific aging effects in the striatum and its surrounding white matter, already in the presymptomatic stage. Using ¹8F-FDG and ¹8F-Fallypride PET imaging, we were not able to demonstrate genotype-specific aging effects within the striatum. The outcome of this longitudinal study was somewhat surprising as it demonstrated a significant differential aging pattern in TgHD versus Wt animals. Although it seems that the TgHD rat model does not have a sufficient expression of disease yet at the age of 12 months, further validation of this model is highly beneficial since there is still an incomplete understanding of the early disease mechanisms of Huntington's disease.

In vivo imaging of neuroinflammation in the rodent brain with [11C]SSR180575, a novel indoleacetamide radioligand of the translocator protein (18 kDa).

PURPOSE: Neuroinflammation is involved in neurological disorders through the activation of microglial cells. Imaging of neuroinflammation with radioligands for the translocator protein (18 kDa) (TSPO) could prove to be an attractive biomarker for disease diagnosis and therapeutic evaluation. The indoleacetamide-derived 7-chloro-N,N,5-trimethyl-4-oxo-3-phenyl-3,5-dihydro-4H-pyridazino[4,5-b]indole-1-acetamide, SSR180575, is a selective high-affinity TSPO ligand in human and rodents with neuroprotective effects. METHODS: Here we report the radiolabelling of SSR180575 with (11)C and in vitro and in vivo imaging in an acute model of neuroinflammation in rats. RESULTS: The image contrast and the binding of [(11)C]SSR180575 are higher than that obtained with the isoquinoline-based TSPO radioligand, [(11)C]PK11195. Competition studies demonstrate that [(11)C]SSR180575 has high specific binding for the TSPO. CONCLUSION: [(11)C]SSR180575 is the first PET radioligand for the TSPO based on an indoleacetamide scaffold designed for imaging neuroinflammation in animal models and in the clinic.

Morphologic and functional changes in the unilateral 6-hydroxydopamine lesion rat model for Parkinson's disease discerned with microSPECT and quantitative MRI.

OBJECT: In the present study, we aimed to evaluate the impact of neurodegeneration of the nigrostriatal tract in a rodent model of Parkinson's disease on the different MR contrasts (T(2), T(1), CBF and CBV) measured in the striatum. MATERIAL AND METHODS: Animals were injected with 6-hydroxydopamine (6OHDA) in the substantia nigra resulting in massive loss of nigrostriatal neurons and hence dopamine depletion in the ipsilateral striatum. Using 7T MRI imaging, we have quantified T(2), T(1), CBF and CBV in the striata of 6OHDA and control rats. To validate the lesion size, behavioral testing, dopamine transporter muSPECT and tyrosine hydroxylase staining were performed. RESULTS: No significant differences were demonstrated in the absolute MRI values between 6OHDA animals and controls; however, 6OHDA animals showed significant striatal asymmetry for all MRI parameters in contrast to controls. CONCLUSIONS: These PD-related asymmetry ratios might be the result of counteracting changes in both intact and affected striatum and allowed us to diagnose PD lesions. As lateralization is known to occur also in PD patients and might be expected in transgenic PD models as well, we propose that MR-derived asymmetry ratios in the striatum might be a useful tool for in vivo phenotyping of animal models of PD.

In vivo imaging of neuroinflammation: a comparative study between [(18)F]PBR111, [ (11)C]CLINME and [ (11)C]PK11195 in an acute rodent model.

PURPOSE: The key role of neuroinflammation in acute and chronic neurological disorders has stimulated the search for specific radiotracers targeting the peripheral benzodiazepine receptor (PBR)/18 kDa translocator protein (TSPO), a hallmark of neuroinflammation. Here we evaluate the new radiotracer for positron emission tomography (PET) [(18)F]PBR111 in a rodent model of acute inflammation and compare it with [(11)C]CLINME, an (11)C-labelled tracer of the same chemical family, and with the isoquinolinic carboxamide [(11)C]PK11195. METHODS: We studied radiometabolites by HPLC, in vitro binding by autoradiography and in vivo brain kinetics as well as in vivo specificity of binding using PET imaging. RESULTS: We show that this radiotracer has a high in vitro specificity for PBR/TSPO versus central benzodiazepine receptors, as reflected by the drastic reduction of its binding to target tissue by addition of PK11195 or PBR111, while addition of flumazenil does not affect binding. Only intact [(18)F]PBR111 is detected in brain up to 60 min after i.v. injection, and PET imaging shows an increased uptake in the lesion as compared to the contralateral side as early as 6 min after injection. Administration of an excess of PK11195 and PBR111, 20 min after [(18)F]PBR111 administration, induces a rapid and complete displacement of [(18)F]PBR111 binding from the lesion. Modelling of the PET data using the simplified reference tissue model showed increased binding potential (BP) in comparison to [(11)C]PK11195. CONCLUSION: [(18)F]PBR111 is a metabolically stable tracer with a high specific in vitro and in vivo binding to TSPO. In addition, considering the longer half-life of (18)F over (11)C, these results support [(18)F]PBR111 as a promising PET tracer of the PBR/TSPO for neuroinflammation imaging.

Evaluation of the PBR/TSPO radioligand [(18)F]DPA-714 in a rat model of focal cerebral ischemia.

Focal cerebral ischemia leads to an inflammatory reaction involving an overexpression of the peripheral benzodiazepine receptor (PBR)/18-kDa translocator protein (TSPO) in the cerebral monocytic lineage (microglia and monocyte) and in astrocytes. Imaging of PBR/TSPO by positron emission tomography (PET) using radiolabeled ligands can document inflammatory processes induced by cerebral ischemia. We performed in vivo PET imaging with [(18)F]DPA-714 to determine the time course of PBR/TSPO expression over several days after induction of cerebral ischemia in rats. In vivo PET imaging showed significant increase in DPA (N,N-diethyl-2-(2-(4-(2-fluoroethoxy)phenyl)-5,7-dimethylpyrazolo[1,5-a]pyrimidin-3-yl)acetamide) uptake on the injured side compared with that in the contralateral area on days 7, 11, 15, and 21 after ischemia; the maximal binding value was reached 11 days after ischemia. In vitro autoradiography confirmed these in vivo results. In vivo and in vitro [(18)F]DPA-714 binding was displaced from the lesion by PK11195 and DPA-714. Immunohistochemistry showed increased PBR/TSPO expression, peaking at day 11 in cells expressing microglia/macrophage antigens in the ischemic area. At later times, a centripetal migration of astrocytes toward the lesion was observed, promoting the formation of an astrocytic scar. These results show that [(18)F]DPA-714 provides accurate quantitative information of the time course of PBR/TSPO expression in experimental stroke.