Modeling Vestibular Compensation: Neural Plasticity Upon Thalamic Lesion

The present study in rats was conducted to identify brain regions affected by the interruption of vestibular transmission and to explore selected aspects of their functional connections. We analyzed, by positron emission tomography (PET), the regional cerebral glucose metabolism (rCGM) of cortical, and subcortical cerebral regions processing vestibular signals after an experimental lesion of the left laterodorsal thalamic nucleus, a relay station for vestibular input en route to the cortical circuitry. PET scans upon galvanic vestibular stimulation (GVS) were conducted in each animal prior to lesion and at post-lesion days (PLD) 1, 3, 7, and 20, and voxel-wise statistical analysis of rCGM at each PLD compared to pre-lesion status were performed. After lesion, augmented metabolic activation by GVS was detected in cerebellum, mainly contralateral, and in contralateral subcortical structures such as superior colliculus, while diminished activation was observed in ipsilateral visual, entorhinal, and somatosensory cortices, indicating compensatory processes in the non-affected sensory systems of the unlesioned side. The changes in rCGM observed after lesion resembled alterations observed in patients suffering from unilateral thalamic infarction and may be interpreted as brain plasticity mechanisms associated with vestibular compensation and substitution. The second set of experiments aimed at the connections between cortical and subcortical vestibular regions and their neurotransmitter systems. Neuronal tracers were injected in regions processing vestibular and somatosensory information. Injections into the anterior cingulate cortex (ACC) or the primary somatosensory cortex (S1) retrogradely labeled neuronal somata in ventral posteromedial (VPM), posterolateral (VPL), ventrolateral (VL), posterior (Po), and laterodorsal nucleus, dorsomedial part (LDDM), locus coeruleus, and contralateral S1 area. Injections into the parafascicular nucleus (PaF), VPM/VPL, or LDDM anterogradely labeled terminal fields in S1, ACC, insular cortex, hippocampal CA1 region, and amygdala. Immunohistochemistry showed tracer-labeled terminal fields contacting cortical neurons expressing the mu-opioid receptor. Antibodies to tyrosine hydroxylase, serotonin, substance P, or neuronal nitric oxide-synthase did not label any of the traced structures. These findings provide evidence for opioidergic transmission in thalamo-cortical transduction

In vitro and in vivo structure-property relationship of ga-68-labeled schiff base derivatives for functional myocardial pet imaging

SPECT (e.g., with Tc-99m-sestamibi) is routinely used for imaging myocardial damage, even though PET could offer a higher spatial resolution. Using the generator-gained isotope Ga-68 would allow a rapid supply of the tracer in the diagnostic unit. For this reason, the aim of the study was to develop Ga-68-labeled PET tracers based on different Schiff base amines and to evaluate the cardiomyocyte uptake in vitro as well as the biodistribution of the tracers in vivo. Fifteen different Schiff bases (basing on 3 different backbones) were synthesized and labeled with Ga-68. Lipophilicity varied between 0.87 +/- 0.24 and 2.72 +/- 0.14 (logD value). All tracers were positively charged and stable in plasma and apo-transferrin solution. In vitro uptake into cardiomyocytes was assessed in HL-1 cells in the absence and presence of the ionophor valinomycin. In vivo accumulation in the heart and in various organs was assessed by small animal PET imaging as well as by ex vivo biodistribution. The results were compared with Tc-99m-sestamibi and F-18-flurpiridaz. All cationic Schiff bases were taken up into cardiomyocytes but the amount varied by a factor of 10. When destroying the membrane potential, the cellular uptake was markedly reduced in most of the tracers, indicating the applicability of these tracers for identifying ischemic myocardium. PET imaging revealed that the in vivo myocardial uptake reached a constant value approximately 10 min after injection but the intracardial amount of the tracer varied profoundly (SUV 0.46 to 3.35). The most suitable tracers showed a myocardial uptake which was comparable to that of Tc-99m-sestamibi. Ga-68-based Schiff bases appear suitable for myocardial PET images with uptake comparable to Tc-99m-sestamibi but offering higher spatial resolution. By systematical variation of the backbone and the side chains, tracers with optimal properties can be identified for further clinical evaluation

Evaluation of p-glycoprotein (abcb1a/b) modulation of [f-18]fallypride in micropet imaging studies

[F-18]Fallypride ([F-18]FP) is an important and routinely used D-2/D-3 antagonist for quantitative imaging of dopaminergic neurotransrnission in vivo. Recently it was shown that the brain uptake of the structurally related [C-11]raclopride is modulated by P-glycoprotein (P-gp), an important efflux transporter at the blood brain barrier. The purpose of this study was to determine whether the brain uptake of [F-18]FP is influenced by P-gp. For examination of this possible modulation microPET studies were performed in a rat and a mouse model. Hence, [F-18]FP was applied to Sprague Dawley rats, half of them being treated with the P-gp inhibitor cyclosporine A (CsA). In a second experimental series the tracer was applied to three different groups of FVB/N mice: wild type, P-gp double knockout (abcbl a/1 b (-/-)) and CsA-treated mice. In CsA-treated Sprague Dawley rats [F-18]FP showed an elevated standard uptake value in the striatum compared to the control animals. In FVB/N mice a similar effect was observed, showing an increasing uptake from wild type to CsA-treated and double knockout mice. Since genetically or pharmacologically induced reduction of P-gp activity increased the uptake of [F-18]FP markedly, we conclude that [F-18]FP is indeed a substrate of P-gp and that the efflux pump modulates its brain uptake. This effect if true for humans may have particular impact on clinical studies using [F-18]FP for assessment of D-2/3 receptor occupancy by antipsychotic drugs. This article is part of the Special Issue Section entitled 'Neuroimaging in Neuropharmacology'

Quantification of the Cannabinoid Type 1 Receptor Availability in the Mouse Brain

INTRODUCTION: The endocannabinoid system is involved in several diseases such as addictive disorders, schizophrenia, post-traumatic stress disorder, and eating disorders. As often mice are used as the preferred animal model in translational research, in particular when using genetically modified mice, this study aimed to provide a systematic analysis of in vivo cannabinoid type 1 (CB1) receptor ligand-binding capacity using positron emission tomography (PET) using the ligand [18F]MK-9470. We then compared the PET results with literature data from immunohistochemistry (IHC) to review the consistency between ex vivo protein expression and in vivo ligand binding. METHODS: Six male C57BL/6J (6–9 weeks) mice were examined with the CB1 receptor ligand [18F]MK-9470 and small animal PET. Different brain regions were evaluated using the parameter %ID/ml. The PET results of the [18F]MK-9470 accumulation in the mouse brain were compared with immunohistochemical literature data. RESULTS: The ligand [18F]MK-9470 was taken up into the mouse brain within 5 min after injection and exhibited slow kinetics. It accumulated highly in most parts of the brain. PET and IHC classifications were consistent for most parts of the telencephalon, while brain regions of the diencephalon, mesencephalon, and rhombencephalon were rated higher with PET than IHC. CONCLUSIONS: This preclinical [18F]MK-9470 study demonstrated the radioligand’s applicability for imaging the region-specific CB1 receptor availability in the healthy adult mouse brain and thus offers the potential to study CB1 receptor availability in pathological conditions

Iodine-124 PET quantification of organ-specific delivery and expression of NIS-encoding RNA

Background!#!RNA-based vaccination strategies tailoring immune response to specific reactions have become an important pillar for a broad range of applications. Recently, the use of lipid-based nanoparticles opened the possibility to deliver RNA to specific sites within the body, overcoming the limitation of rapid degradation in the bloodstream. Here, we have investigated whether small animal PET/MRI can be employed to image the biodistribution of RNA-encoded protein. For this purpose, a reporter RNA coding for the sodium-iodide-symporter (NIS) was in vitro transcribed in cell lines and evaluated for expression. RNA-lipoplex nanoparticles were then assembled by complexing RNA with liposomes at different charge ratios, and functional NIS protein translation was imaged and quantified in vivo and ex vivo by Iodine-124 PET upon intravenous administration in mice.!##!Results!#!NIS expression was detected on the membrane of two cell lines as early as 6 h after transfection and gradually decreased over 48 h. In vivo and ex vivo PET/MRI of anionic spleen-targeting or cationic lung-targeting NIS-RNA lipoplexes revealed a visually detectable rapid increase of Iodine-124 uptake in the spleen or lung compared to control-RNA-lipoplexes, respectively, with minimal background in other organs except from thyroid, stomach and salivary gland.!##!Conclusions!#!The strong organ selectivity and high target-to-background acquisition of NIS-RNA lipoplexes indicate the feasibility of small animal PET/MRI to quantify organ-specific delivery of RNA

In vivo imaging of the immune response upon systemic RNA cancer vaccination by FDG-PET

Abstract Background [18F]Fluoro-2-deoxy-2-d-glucose positron emission tomography (FDG-PET) is commonly used in the clinic for diagnosis of cancer and for follow-up of therapy outcome. Additional to the well-established value in tumor imaging, it bears potential to depict immune processes in modern immunotherapies. T cells enhance their glucose consumption upon activation and are crucial effectors for the success of such novel therapies. In this study, we analyzed the T cell immunity in spleen after antigen-specific stimulation of T cells via highly innovative RNA-based vaccines using FDG-PET/MRI. For this purpose, we employed systemic administration of RNA-lipoplexes encoding the endogenous antigen of Moloney murine leukemia virus (gp70) which have been previously shown to induce potent innate as well as adaptive immune mechanisms for cancer immunotherapy. Feasibility of clinical imaging of increased splenic FDG uptake was demonstrated in a melanoma patient participating in a clinical phase 1 trial of a tetravalent RNA-lipoplex cancer vaccine. Results We observed exclusive increase of glucose uptake in spleen compared to other organs thanks to liposome-mediated RNA targeting to this immune-relevant organ. In vivo and ex vivo FDG uptake analysis in the spleen of vaccinated mice correlated well with antigen-specific T cell activation. Moreover, the use of an irrelevant (antigen non-specific) RNA also resulted in enhanced FDG uptake early after vaccination through the activation of several other splenic cell populations. The glucose uptake was also dependent on the dose of RNA administered in line with the activation and frequencies of proliferating antigen-specific T cells as well as the general activation pattern of splenic cell populations. Conclusions Our preclinical results show rapid and transient vaccination-induced increase of FDG uptake within the spleen reflecting immune activation preceding T cell proliferation. FDG-PET/CT in patients is also capable to image this immune activation resulting in a new potential application of FDG-PET/CT to image immune processes in new immunological therapies

Selective binding to monoamine oxidase a : in vitro and in vivo evaluation of f-18-labeled beta-carboline derivatives

In this study we synthesized four different F-18-labeling precursors for the visualization of the monoamino oxidase A using harmol derivatives. Whereas two are for prosthetic group labeling using [F-18]fluoro-d(2)-methyl tosylate and 2-[F-18]fluoroethyl-tosylate, the other three precursors are for direct nucleophilic F-18-labeling. Additionally the corresponding reference compounds were synthesized. The syntheses of [F-18]fluoro-d(2)-methyl-harmol and 2-[F-18] fluoroethyl-harmol were carried out using harmol as starting material. For direct nucleophilic F-18-labeling of the tracers carrying oligoethyled spacers (PEG), a toluenesulfonyl leaving group was employed. The radiolabeling, purification and formulation for each tracer was optimized and evaluated in vitro and in vivo. Stability tests in human serum showed that all tracers were stable over the observation period of 60 min. mu PET studies using of the synthesized tracers revealed that the tracers carrying PEG spacers showed no sufficient brain uptake. Consequently, the F-18-fuoro alkylated tracers [F-18] fluoro-d2-methyl-harmol and 2-[F-18]fluoroethyl-harmol were further evaluated showing SUVs in the brain of 1.0 +/- 0.2 g/mL and 3.4 +/- 0.5 g/mL after 45 min, respectively. In blockade studies the selectivity and specificity of both tracers were demonstrated. However, for [F-18]fluoro-d(2)-methyl-harmol a rapid washout from the brain was also observed. In vitro binding assays revealed that 2-[F-18] fluoroethyl-harmol (IC50 = 0.54 +/- 0.06 nM) has a higher affinity than the F-18-fluoro-d(2)-methylated ligand (IC50 = 12.2 +/- 0.6 nM), making 2-[F-18] fluoroethyl-harmol superior to the other evaluated compounds and a promising tracer for PET imaging of the MAO A