In Vivo PET Measurements with [11C]PE2I to Evaluate Fetal Mesencephalic Transplantations to Unilateral 6-OHDA-Lesioned Rats

Modulating Microglia/Macrophage Activation by CDNF Promotes Transplantation of Fetal Ventral Mesencephalic Graft Survival and Function in a Hemiparkinsonian Rat Model

Parkinson’s disease (PD) is characterized by the loss of dopaminergic neurons in substantia nigra pars compacta, which leads to the motor control deficits. Recently, cell transplantation is a cutting-edge technique for the therapy of PD. Nevertheless, one key bottleneck to realizing such potential is allogenic immune reaction of tissue grafts by recipients. Cerebral dopamine neurotrophic factor (CDNF) was shown to possess immune-modulatory properties that benefit neurodegenerative diseases. We hypothesized that co-administration of CDNF with fetal ventral mesencephalic (VM) tissue can improve the success of VM replacement therapies by attenuating immune responses. Hemiparkinsonian rats were generated by injecting 6-hydroxydopamine (6-OHDA) into the right medial forebrain bundle of Sprague Dawley (SD) rats. The rats were then intrastriatally transplanted with VM tissue from rats, with/without CDNF administration. Recovery of dopaminergic function and survival of the grafts were evaluated using the apomorphine-induced rotation test and small-animal positron emission tomography (PET) coupled with [¹⁸F] DOPA or [¹⁸F] FE-PE2I, respectively. In addition, transplantation-related inflammatory response was determined by uptake of [¹⁸F] FEPPA in the grafted side of striatum. Immunohistochemistry (IHC) examination was used to determine the survival of the grated dopaminergic neurons in the striatum and to investigate immune-modulatory effects of CDNF. The modulation of inflammatory responses caused by CDNF might involve enhancing M2 subset polarization and increasing fractal dimensions of 6-OHDA-treated BV2 microglial cell line. Analysis of CDNF-induced changes to gene expressions of 6-OHDA-stimulated BV2 cells implies that these alternations of the biomarkers and microglial morphology are implicated in the upregulation of protein kinase B signaling as well as regulation of catalytic, transferase, and protein serine/threonine kinase activity. The effects of CDNF on 6-OHDA-induced alternation of the canonical pathway in BV2 microglial cells is highly associated with PI3K-mediated phagosome formation. Our results are the first to show that CDNF administration enhances the survival of the grafted dopaminergic neurons and improves functional recovery in PD animal model. Modulation of the polarization, morphological characteristics, and transcriptional profiles of 6-OHDA-stimualted microglia by CDNF may possess these properties in transplantation-based regenerative therapies.

Role of Nuclear Imaging to Understand the Neural Substrates of Brain Disorders in Laboratory Animals: Current Status and Future Prospects

Molecular imaging, which allows the real-time visualization, characterization and measurement of biological processes, is becoming increasingly used in neuroscience research. Scintigraphy techniques such as single photon emission computed tomography (SPECT) and positron emission tomography (PET) provide qualitative and quantitative measurement of brain activity in both physiological and pathological states. Laboratory animals, and rodents in particular, are essential in neuroscience research, providing plenty of models of brain disorders. The development of innovative high-resolution small animal imaging systems together with their radiotracers pave the way to the study of brain functioning and neurotransmitter release during behavioral tasks in rodents. The assessment of local changes in the release of neurotransmitters associated with the performance of a given behavioral task is a turning point for the development of new potential drugs for psychiatric and neurological disorders. This review addresses the role of SPECT and PET small animal imaging systems for a better understanding of brain functioning in health and disease states. Brain imaging in rodent models faces a series of challenges since it acts within the boundaries of current imaging in terms of sensitivity and spatial resolution. Several topics are discussed, including technical considerations regarding the strengths and weaknesses of both technologies. Moreover, the application of some of the radioligands developed for small animal nuclear imaging studies is discussed. Then, we examine the changes in metabolic and neurotransmitter activity in various brain areas during task-induced neural activation with special regard to the imaging of opioid, dopaminergic and cannabinoid receptors. Finally, we discuss the current status providing future perspectives on the most innovative imaging techniques in small laboratory animals. The challenges and solutions discussed here might be useful to better understand brain functioning allowing the translation of preclinical results into clinical applications.

The Effect of Sertoli Cells on Xenotransplantation and Allotransplantation of Ventral Mesencephalic Tissue in a Rat Model of Parkinson's Disease

Intra-striatal transplantation of fetal ventral mesencephalic (VM) tissue has a therapeutic effect on patients with Parkinson’s disease (PD). Sertoli cells (SCs) possess immune-modulatory properties that benefit transplantation. We hypothesized that co-graft of SCs with VM tissue can attenuate rejection. Hemi-parkinsonian rats were generated by injecting 6-hydroxydopamine into the right medial forebrain bundle of Sprague Dawley (SD) rats. The rats were then intrastriatally transplanted with VM tissue from rats or pigs (rVM or pVM), with/without a co-graft of SCs (rVM+SCs or pVM+SCs). Recovery of dopaminergic function and survival of the grafts were evaluated using the apomorphine-induced rotation test and small animal-positron emission tomography (PET) coupled with [¹⁸F] DOPA or [¹⁸F] FE-PE2I, respectively. Immunohistochemistry (IHC) examination was used to determine the survival of the grafted dopaminergic neurons in the striatum and to investigate immune-modulatory effects of SCs. The results showed that the rVM+SCs and pVM+SCs groups had significantly improved drug-induced rotational behavior compared with the VM alone groups. PET revealed a significant increase in specific uptake ratios (SURs) of [¹⁸F] DOPA and [¹⁸F] FE-PE2I in the grafted striatum of the rVM+SCs and pVM+SCs groups as compared to that of the rVM and pVM groups. SC and VM tissue co-graft led to better dopaminergic (DA) cell survival. The co-grafted groups exhibited lower populations of T-cells and activated microglia compared to the groups without SCs. Our results suggest that co-graft of SCs benefit both xeno- and allo-transplantation of VM tissue in a PD rat model. Use of SCs enhanced the survival of the grafted dopaminergic neurons and improved functional recovery. The enhancement may in part be attributable to the immune-modulatory properties of SCs. In addition, [¹⁸F]DOPA and [¹⁸F]FE-PE2I coupled with PET may provide a feasible method for in vivo evaluation of the functional integrity of the grafted DA cell in parkinsonian rats.

Grafted Miniature-Swine Neural Stem Cells of Early Embryonic Mesencephalic Neuroepithelial Origin can Repair the Damaged Neural Circuitry of Parkinson's Disease Model Rats

Although recent progress in the use of human iPS cell-derived midbrain dopaminergic progenitors is remarkable, alternatives are essential in the strategies of treatment of basal-ganglia-related diseases. Attention has been focused on neural stem cells (NSCs) as one of the possible candidates of donor material for neural transplantation, because of their multipotency and self-renewal characteristics. In the present study, miniature-swine (mini-swine) mesencephalic neuroepithelial stem cells (M-NESCs) of embryonic 17 and 18 days grafted in the parkinsonian rat striatum were assessed immunohistochemically, behaviorally and electrophysiologically to confirm their feasibility for the neural xenografting as a donor material. Grafted mini-swine M-NESCs survived in parkinsonian rat striatum at 8 weeks after transplantation and many of them differentiated into tyrosine hydroxylase (TH)-positive cells. The parkinsonian model rats grafted with mini-swine M-NESCs exhibited a functional recovery from their parkinsonian behavioral defects. The majority of donor-derived TH-positive cells exhibited a matured morphology at 8 weeks. Whole-cell recordings from donor-derived neurons in the host rat brain slices incorporating the graft revealed the presence of multiple types of neurons including dopaminergic. Glutamatergic and GABAergic post-synaptic currents were evoked in the donor-derived cells by stimulation of the host site, suggesting they receive both excitatory and inhibitory synaptic inputs from host area. The present study shows that non-rodent mammalian M-NESCs can differentiate into functionally active neurons in the diseased xenogeneic environment and could improve the parkinsonian behavioral defects over the species. Neuroepithelial stem cells could be an attractive candidate as a source of donor material for neural transplantation.

Functional neuroimaging of the 6-OHDA lesion rat model of Parkinson's disease

We characterized the unilaterally 6-hydroxydopamine (6-OHDA)-lesioned rat, a well-known acute model of Parkinson's disease (PD), with [(18)F]-fluoro-2-deoxy-d-glucose (FDG) small-animal positron emission tomography (PET), which we compared with a drug-induced rotation behavioral test. In the 6-OHDA model, significant glucose hypometabolism was present in the primary motor cortex, substantia nigra, and pedunculopontine tegmental nucleus on the ipsilateral side. In contrast, neuronal activations were observed in the primary somatosensory cortex and ventral caudate-putamen area after lesioning. Correlation analysis revealed a significant relationship between the behavioral results and the degree of glucose metabolism impairment in the primary motor cortex, substantia nigra, and pedunculopontine tegmental nucleus. In addition, the pedunculopontine tegmental nucleus correlated significantly with the primary somatosensory cortex, the ventral caudate-putamen, the substantia nigra, and the primary motor cortex. Furthermore, the primary motor cortex also showed significant correlations with the substantia nigra. In conclusion, In vivo cerebral mapping of the 6-OHDA-lesioned rats using [(18)F]-FDG PET showed correspondence at the functional levels to the cortico-subcortical network impairment observed in PD patients.

Use of [18F]FDOPA-PET for in vivo evaluation of dopaminergic dysfunction in unilaterally 6-OHDA-lesioned rats

Background

We evaluated the utility of L-3,4-dihydroxy-6-[¹⁸F]fluoro-phenylalanine ([¹⁸F]FDOPA) positron emission tomography (PET) as a method for assessing the severity of dopaminergic dysfunction in unilaterally 6-hydroxydopamine (6-OHDA)-lesioned rats by comparing it with quantitative biochemical, immunohistochemical, and behavioral measurements.

Methods

Different doses of 6-OHDA (0, 7, 14, and 28 μg) were unilaterally injected into the right striatum of male Sprague-Dawley rats. Dopaminergic functional activity in the striatum was assessed by [¹⁸F]FDOPA-PET, measurement of striatal dopamine (DA) and DA metabolite levels, tyrosine hydroxylase (TH) immunostaining, and methamphetamine-induced rotational testing.

Results

Accumulation of [¹⁸F]FDOPA in the bilateral striatum was observed in rats pretreated with both aromatic L-amino acid decarboxylase and catechol-O-methyltransferase (COMT) inhibitors. Unilateral intrastriatal injection of 6-OHDA produced a significant site-specific reduction in [¹⁸F]FDOPA accumulation. The topological distribution pattern of [¹⁸F]FDOPA accumulation in the ipsilateral striatum agreed well with the pattern in TH-stained corresponding sections. A significant positive relationship was found between Patlak plot K_i values and striatal levels of DA and its metabolites (r = 0.958). A significant negative correlation was found between both K_i values (r = -0.639) and levels of DA and its metabolites (r = -0.719) and the number of methamphetamine-induced rotations.

Conclusions

K_i values determined using [¹⁸F]FDOPA-PET correlated significantly with the severity of dopaminergic dysfunction. [¹⁸F]FDOPA-PET makes it possible to perform longitudinal evaluation of dopaminergic function in 6-OHDA-lesioned rats, which is useful in the development of new drugs and therapies for Parkinson's disease (PD).

In vivo imaging neurotransmitter function. The rat 6-hydroxydopamine model and its relevance for human Parkinson's disease

This article gives an overview of those small animal imaging studies which have been conducted on neurotransmitter function in the rat 6-hydoxydopamine (6-OHDA) model of Parkinson's disease, and discusses findings with respect to the outcome of clinical studies on Parkinsonian patients.

PE2I: a radiopharmaceutical for in vivo exploration of the dopamine transporter

The membrane dopamine transporter (DAT) has a pivotal role in the regulation of dopamine (DA) neurotransmission involved in a number of physiological functions and brain disorders. Molecular imaging techniques, such as positron emission tomography (PET) and single photon emission computerized tomography (SPECT), are relevant tools to explore the DAT, and we developed the cocaine derivative N-(3-iodopro-2E-enyl)-2beta-carbomethoxy-3beta-(4'-methylphenyl) nortropane (PE2I) that has proved to be a very potent radiopharmaceutical to image the DAT by these techniques. Several methods are available to obtain PE2I labeled with iodine-123 or -125, carbon-11 and tritium. The pharmacological properties of PE2I have demonstrated that it has good affinity for the DAT (4 nM) and is one of the most selective DAT ligands. [(125)I]PE2I characterized postmortem in human brains has revealed very intense and selective binding in the basal ganglia. Ex vivo autoradiography in rats has shown that high level of [(125)I]PE2I accumulates in the striatum and also in the substantia nigra and ventral tegmental area. [(125)I]PE2I accumulation in the rat striatum is rapid, high, and selective, providing a maximum striatum/cerebellum ratio of 10 during the first 30 min post injection. Using SPECT or PET, rapid, high, and selective accumulation of PE2I was found in the caudate nucleus and putamen in monkeys, whereas rapid wash out from the cerebellum was observed. In vivo investigations in healthy humans have demonstrated that PE2I has high striatal uptake, low nonspecific binding, low radiation exposure, and a fairly short scanning time. A number of findings in various animal models of Parkinson's disease in rats and monkeys have demonstrated the high efficacy of PE2I for detection of reduction in the density of DAT, thus showing the potential value of PE2I for early diagnosis and evaluation of treatment of this disease. The excellent properties of PE2I are basis for the development of new DAT tracers for use in future PET explorations using fluor-18.

Movies of dopamine transporter occupancy with ultra-high resolution focusing pinhole SPECT

A pivotal question in neuropharmacology is how the function of neurotransmitter systems relates to psychiatric diseases. In experimental neuropharmacology, we have dreamt about a looking glass that would allow us to see neurotransmitter systems in action, and about animals that would faithfully serve us as models for human psychiatric disease. Analysis of animal models has been limited by the availability of methods to study in vivo neurotransmitter dynamics. Now, a single photon emission computed tomography system called U-SPECT can localize dopamine transporters in sub-compartments of the mouse brain during a range of points in time. Applied to the midbrain dopamine system of different models of disease, this will aid the understanding of dynamic processes of this neurotransmitter that underlie brain functions and human brain pathology.

Metabolic-dopaminergic mapping of the 6-hydroxydopamine rat model for Parkinson's disease

PURPOSE

The unilateral 6-hydroxydopamine (6-OHDA) lesion rat model is a well-known acute model for Parkinson's disease (PD). Its validity has been supported by invasive histology, behavioral studies and electrophysiology. Here, we have characterized this model in vivo by multitracer imaging [glucose metabolism and dopamine transporter (DAT)] in relation to behavioral and histological parameters.

METHODS

Eighteen female adult Wistar rats (eight 6-OHDA-lesioned, ten controls) were investigated using multitracer [(18)F]-fluoro-2-deoxy-D: -glucose (FDG) and [(18)F]-FECT {2'-[(18)F]-fluoroethyl-(1R-2-exo-3-exe)-8-methyl-3-(4-chlorophenyl)-8-azabicyclo(3.2.1)-octane-2-carboxylate} small animal positron emission tomography (PET). Relative glucose metabolism and parametric DAT binding images were anatomically standardized to Paxinos space and analyzed on a voxel-basis using SPM2: , supplemented by a template-based predefined volumes-of-interest approach. Behavior was characterized by the limb-use asymmetry test; dopaminergic innervation was validated by in vitro tyrosine hydroxylase staining.

RESULTS

In the 6-OHDA model, significant glucose hypometabolism is present in the ipsilateral sensory-motor cortex (-6.3%; p = 4 x 10(-6)). DAT binding was severely decreased in the ipsilateral caudate-putamen, nucleus accumbens and substantia nigra (all p < 5 x 10(-9)), as confirmed by the behavioral and histological outcomes. Correlation analysis revealed a positive relationship between the degree of DAT impairment and the change in glucose metabolism in the ipsilateral hippocampus (p = 3 x 10(-5)), while cerebellar glucose metabolism was inversely correlated to the level of DAT impairment (p < 3 x 10(-4)).

CONCLUSIONS

In vivo cerebral mapping of 6-OHDA-lesioned rats using [(18)F]-FDG and [(18)F]-FECT small animal PET shows molecular-functional correspondence to the cortico-subcortical network impairments observed in PD patients. This provides a further molecular validation supporting the validity of the 6-OHDA lesion model to mimic multiple aspects of human PD.

Progressive changes of pre- and post-synaptic dopaminergic biomarkers in conscious MPTP-treated cynomolgus monkeys measured by positronemission tomography

Positron emission tomography (PET) is a useful technique for the consecutive investigation of the relationship between changes in neurotransmission biomarkers and behavioral signs in animal models of Parkinson's disease (PD). In this study, we aimed to investigate the threshold of dopamine (DA) neuron damage for the appearance of tremor by observing the longitudinal changes of pre- and post-synaptic DA biomarkers in awake monkeys using PET with multiple tracers. Three cynomolgus monkeys were treated with MPTP every 3-6 weeks until tremor was observed. Brain uptake of [11C]PE2I, [beta-11C]DOPA, and [11C]raclopride for DA transporter (DAT), DOPA utilization, and DA D2 receptor were measured using PET as a single set in awake condition. Sets of PET scans were repeated in parallel with continuous behavioral estimation. The pre-synaptic biomarkers of DA neuron in the striatum decreased [11C]PE2I binding and [beta-11C]DOPA uptake in an MPTP dose-dependent manner. Tremor was not observed until striatal [11C]PE2I binding was reduced to about 15% of the pretreatment level and [beta-11C]DOPA uptake was reduced to about 34%. DA D2 receptor measured by [11C]raclopride was not significantly changed throughout the experiment. Our results revealed that it is possible to quantitatively define the threshold of the onset of behavioral PD signs by monitoring spontaneous motor activity, and in vivo PET with DAT marker can be a biomarker for early diagnosis at the presymptomatic stage of PD and for high-risk groups.

Persistent dopamine functions of neurons derived from embryonic stem cells in a rodent model of Parkinson disease

The derivation of dopamine neurons is one of the best examples of the clinical potential of embryonic stem (ES) cells, but the long-term function of the grafted neurons has not been established. Here, we show that, after transplantation into an animal model, neurons derived from mouse ES cells survived for over 32 weeks, maintained midbrain markers, and had sustained behavioral effects. Microdialysis in grafted animals showed that dopamine (DA) release was induced by depolarization and pharmacological stimulants. Positron emission tomography measured the expression of presynaptic dopamine transporters in the graft and also showed that the number of postsynaptic DA D(2) receptors was normalized in the host striatum. These data suggest that ES cell-derived neurons show DA release and reuptake and stimulate appropriate postsynaptic responses for long periods after implantation. This work supports continued interest in ES cells as a source of functional DA neurons.