Päivi Marjamäki

Age-dependent decline in human brain dopamine D1 and D2 receptors.

Dopamine D1 and D2 receptor binding was investigated in the caudate nucleus and putamen in 65 individuals ranging from 6 to 93 years of age. [3H]SCH 23390 and [3H]spiroperidol were used as ligands for D1 and D2 receptors, respectively. A full Scatchard analysis with several ligand concentrations was performed yielding Bmax and Kd values. The number of D1 and D2 receptors declined with age both in the caudate nucleus and putamen without any change in the mean dissociation constant. The ratio of D1:D2 receptors remained unchanged. The results show that the dopaminergic system degenerates in the aging striatum and may contribute to the frequent occurrence of extrapyramidal symptoms in the elderly.

Detection of Microglial Activation in an Acute Model of Neuroinflammation Using PET and Radiotracers 11C-(R)-PK11195 and 18F-GE-180

It remains unclear how different translocator protein (TSPO) ligands reflect the spatial extent of astrocyte or microglial activation in various neuroinflammatory conditions. Here, we use a reproducible lipopolysaccharide (LPS)-induced model of acute central nervous system inflammation to compare the binding performance of a new TSPO ligand 18F-GE-180 with 11C-(R)-PK11195. Using immunohistochemistry, we also explore the ability of the TSPO ligands to detect activated microglial cells and astrocytes. Methods: Lewis rats (n = 30) were microinjected with LPS (1 or 10 μg) or saline (1 μL) into the left striatum. The animals were imaged in vivo at 16 h after the injection using PET radiotracers 18F-GE-180 or 11C-(R)-PK11195 (n = 3 in each group) and were killed afterward for autoradiography of the brain. Immunohistochemical assessment of OX-42 and glial fibrillary acidic protein (GFAP) was performed to identify activated microglial cells and reactive astrocytes. Results: In vivo PET imaging revealed an increase in the ipsilateral TSPO binding, compared with binding in the contralateral hemisphere, after the microinjection of 10 μg of LPS. No increase was observed with vehicle. By autoradiography, the TSPO radiotracer binding potential in the injected hemisphere was increased after striatal injection of 1 or 10 μg of LPS. However, the significant increase was observed only when using 18F-GE-180. The area of CD11b-expressing microglial cells extended beyond that of enhanced GFAP staining and mapped more closely to the extent of 18F-GE-180 binding than to 11C-(R)-PK11195 binding. The signal from either PET ligand was significantly increased in regions of increased GFAP immunoreactivity and OX-42 colocalization, meaning that the presence of both activated microglia and astrocytes in a given area leads to increased binding of the TSPO radiotracers. Conclusion: 18F-GE-180 is able to reveal sites of activated microglia in both gray and white matter. However, the signal is increased by the presence of activated astrocytes. Therefore, 18F-GE-180 is a promising new fluorinated longer-half-life tracer that reveals the presence of activated microglia in a manner that is superior to 11C-(R)-PK11195 due to the higher binding potential observed for this ligand.

Human adipose tissue glucose uptake determined using [18F]-fluoro-deoxy-glucose ([18F]FDG) and PET in combination with microdialysis

Abstract.Aims/hypothesis: To determine the lumped constant (LC), which accounts for the differences in the transport and phosphorylation between [18F]-2-fluoro-2-deoxy-d-glucose ([18F]FDG) and glucose, for [18F]FDG in human adipose tissue. Methods: [18F]FDG-PET was combined with microdialysis. Seven non-obese (29 ± 2 years of age, BMI 24 ± 1 kg/m2) and seven obese (age 32 ± 2 years of age, BMI 31 ± 1 kg/m2) men were studied during euglycaemic hyperinsulinaemia (1 mU/kg · min–1 for 130 min). Abdominal adipose tissue [18F]FDG uptake (rGUFDG) and femoral muscle glucose uptake were measured using [18F]FDG-PET. Adipose tissue perfusion was measured using [15O]-labelled water and PET, and interstitial glucose concentration using microdialysis. Glucose uptake (by microdialysis, rGUMD) was calculated by multiplying glucose extraction by regional blood flow. The LC was determined as the ratio of rGUFDG to rGUMD. Results: Rates of adipose tissue glucose uptake (rGUMD) were 36 % higher in the non-obese than in the obese patients (11.8 ± 1.7 vs 7.6 ± 0.8 μmol/kg · min–1, p < 0.05, respectively) and a correlation between rGUMD and rGUFDG was found (r = 0.82, p < 0.01). The LC averaged 1.14 ± 0.11, being similar in the obese and the non-obese subjects (1.01 ± 0.15 vs 1.26 ± 0.15, respectively, NS). Muscle glucose uptake was fourfold to fivefold higher than adipose tissue glucose uptake in both groups. Conclusion/interpretation: [18F]FDG-PET seems a feasible tool to investigate adipose tissue glucose metabolism in human beings. Direct measurements with [18F]FDG-PET and microdialysis suggest a LC value of 1.14 for [18F]FDG in human adipose tissue during insulin stimulation and the LC does not appear to be altered in insulin resistance. Furthermore, the obese patients show insulin resistance in both adipose tissue and skeletal muscle. [Diabetologia (2001) 44: 2171–2179]

A post-mortem study on striatal dopamine receptors in Parkinson's disease

Striatal dopamine D1 and D2 receptors were investigated in 49 patients with Parkinsons disease (PD) and 33 age-matched controls with [3H]SCH 23390 and [3H]spiroperidol as ligands respectively. A full Scatchard analysis giving Bmax and Kd values was performed. In the caudate nucleus, a small but significant decrease in the number of D1 and D2 receptors was seen, whereas in the putamen the number of dopamine receptors was unchanged. Treatment with neuroleptics was found to increase the number of D2 receptors both in the caudate nucleus and putamen. The number of neither D1 nor D2 receptors correlated neither with the duration of disease or levodopa treatment of the patients nor with the post-mortem delay or storage time of the samples. Furthermore, no association was found between either D1 or D2 receptor number and clinical variables of the patients. The activity of choline acetyltransferase (ChAT) was found to be unchanged in the striatum, whereas a marked decline was seen in the hippocampus and cortical areas, indicating that intrinsic striatal cholinergic neurons are not affected in PD. The present results suggest that there is a modest decline in the number of striatal dopamine D2 receptors in advanced patients with PD at the end stage of the disease.

Non-specific binding of [18F]FDG to calcifications in atherosclerotic plaques: experimental study of mouse and human arteries.

Purpose[18F]FDG has been used as an inflammation marker and shown to accumulate in inflammatory atherosclerotic plaques. The aim of this study was to investigate the uptake and location of [18F]FDG in atherosclerotic plaque compartments.MethodsThe biodistribution of intravenously administered [18F]FDG was analysed in atherosclerotic LDLR/ApoB48 mice (n=11) and control mice (n=9). Digital autoradiography was used to detect the ex vivo distribution in frozen aortic sections. In vitro binding of [18F]FDG in human atherosclerotic arteries was also examined.ResultsThe uptake of [18F]FDG was significantly higher in the aorta of atherosclerotic mice as compared with the control mice. Autoradiography of excised arteries showed higher [18F]FDG uptake in the plaques than in the healthy vessel wall (mean ratio ±SD 2.7±1.1). The uptake of [18F]FDG in the necrotic, calcified sites of the advanced atherosclerotic lesions was 6.2±3.2 times higher than that in the healthy vessel wall. The in vitro studies of human arterial sections showed marked binding of [18F]FDG to the calcifications but not to other structures of the artery wall.ConclusionIn agreement with previous studies, we observed [18F]FDG uptake in atherosclerotic plaques. However, prominent non-specific binding to calcified structures was found. This finding warrants further studies to clarify the significance of this non-specific binding in human plaques in vivo.

In Vivo PET Imaging Demonstrates Diminished Microglial Activation After Fingolimod Treatment in an Animal Model of Multiple Sclerosis

There is a great need for the monitoring of microglial activation surrounding multiple sclerosis lesions because the activation of microglia is thought to drive widespread neuronal damage. Recently, second-generation PET radioligands that can reveal the extent of microglial activation by quantifying the increased expression of the 18-kDa translocator protein have been developed. Here, we investigate whether PET imaging can be used to demonstrate the reduction in microglial activation surrounding a chronic focal multiple sclerosis (MS)–like lesion after treatment with fingolimod, an established MS therapy. Methods: Chronic focal experimental autoimmune encephalitis (EAE)–like lesions were induced in Lewis rats (n = 24) via stereotactic intrastriatal injection of heat-killed bacillus Calmette–Guérin (BCG) and subsequent activation using an intradermal injection of BCG in complete Freund adjuvant. This process resulted in a delayed-type hypersensitivity (DTH)–like EAE lesion. The extent of neuroinflammation surrounding the lesion was measured using 18F-GE180 as a PET radioligand. The imaging was performed before and after treatment with fingolimod (0.3 mg/kg/d by mouth, 28 d) or vehicle as a control. In addition to imaging, autoradiography and immunohistochemistry experiments were performed to verify the in vivo results. Results: The chronic DTH EAE lesion led to increased ligand binding in the ipsilateral, compared with contralateral, hemisphere when PET imaging was performed with the translocator protein–binding radioligand 18F-GE180. Treatment with fingolimod led to a highly significant reduction in the binding potential, which could be demonstrated using both in vivo and ex vivo imaging (fingolimod vs. vehicle treatment, P < 0.0001). The area of increased 18F-GE180 signal mapped closely to the area of activated microglial cells detected by immunohistochemistry. Conclusion: PET imaging, unlike MR imaging, can be used to visualize the microglial activation surrounding a chronic DTH EAE lesion. Importantly, the treatment effect of fingolimod can be monitored in vivo by measuring the degree of microglial activation surrounding the chronic DTH EAE lesion. This work gives promise for the introduction of new outcome measures applicable in treatment studies of progressive MS.

Extracellular superoxide dismutase is a growth regulatory mediator of tissue injury recovery.

Extracellular superoxide dismutase (SOD3) gene therapy has been shown to attenuate tissue damages and to improve the recovery of the tissue injuries, but the cellular events delivering the therapeutic response of the enzyme are not well defined. In the current work, we overexpressed SOD3 in rat hindlimb ischemia model to study the signal transduction and injury healing following the sod3 gene transfer. The data suggest a novel sod3 gene transfer-derived signal transduction cascade through Ras-Mek-Erk mitogenic pathway leading to activation of AP1 and CRE transcription factors, increased vascular endothelial growth factor (VEGF)-A and cyclin D1 expression, increased cell proliferation, and consequently improved metabolic functionality of the injured tissue. Increased cell proliferation could explain the improved metabolic performance and the healing of the tissue damages after the sod3 gene transfer. The present data is a novel description of the molecular mechanism of SOD3-mediated recovery of tissue injury and suggests a new physiological role for SOD3 as a Ras regulatory molecule in signal transduction.

Uptake of inflammatory cell marker [11C]PK11195 into mouse atherosclerotic plaques.

PurposeThe ligand [11C]PK11195 binds with high affinity and selectivity to peripheral benzodiazepine receptor, expressed in high amounts in macrophages. In humans, [11C]PK11195 has been used successfully for the in vivo imaging of inflammatory processes of brain tissue. The purpose of this study was to explore the feasibility of [11C]PK11195 in imaging inflammation in the atherosclerotic plaques.MethodsThe presence of PK11195 binding sites in the atherosclerotic plaques was verified by examining the in vitro binding of [3H]PK11195 onto mouse aortic sections. Uptake of intravenously administered [11C]PK11195 was studied ex vivo in excised tissue samples and aortic sections of a LDLR/ApoB48 atherosclerotic mice. Accumulation of the tracer was compared between the atherosclerotic plaques and non-atherosclerotic arterial sites by autoradiography and histological analyses.ResultsThe [3H]PK11195 was found to bind to both the atherosclerotic plaques and the healthy wall. The autoradiography analysis revealed that the uptake of [11C]PK11195 to inflamed regions in plaques was more prominent (p = 0.011) than to non-inflamed plaque regions, but overall it was not higher than the uptake to the healthy vessel wall. Also, the accumulation of 11C radioactivity into the aorta of the atherosclerotic mice was not increased compared to the healthy control mice.ConclusionsOur results indicate that the uptake of [11C]PK11195 is higher in inflamed atherosclerotic plaques containing a large number of inflammatory cells than in the non-inflamed plaques. However, the tracer uptake to other structures of the artery wall was also prominent and may limit the use of [11C]PK11195 in clinical imaging of atherosclerotic plaques.

Uptake of 11C-Choline in Mouse Atherosclerotic Plaques

The purpose of this study was to explore the feasibility of 11C-choline in the assessment of the degree of inflammation in atherosclerotic plaques. Methods: Uptake of 11C-choline was studied ex vivo in tissue samples and aortic sections excised from 6 atherosclerotic mice deficient for both low-density lipoprotein receptor and apolipoprotein B48 (LDLR−/−ApoB100/100) and 5 control mice. The autoradiographs were compared with the immunohistology of the arterial sites. Results: The uptake of 11C-choline (percentage of the injected activity per gram of tissue) in the atherosclerotic aortas of the LDLR−/−ApoB100/100 mice was significantly higher (1.9-fold, P = 0.0016) than that in the aortas of the control mice. The autoradiography analysis showed significantly higher uptake of 11C-choline in the plaques than in healthy vessel wall (mean ratio, 2.3 ± 0.6; P = 0.014), prominently in inflamed plaques, compared with noninflamed plaque areas. Conclusion: We observed a high 11C-choline uptake in the aortic plaques of atherosclerotic mice. Our data suggest that macrophages may be responsible for the uptake of 11C-choline in the plaques.

A novel electrophilic synthesis and evaluation of medium specific radioactivity (1R,2S)-4-[18F]fluorometaraminol, a tracer for the assessment of cardiac sympathetic nerve integrity with PET

(1R,2S)-4-[18F]fluorometaraminol (4-[18F]FMR), a tracer for cardiac sympathetic innervation, was synthesized by electrophilic aromatic substitution. A trimethylstannyl precursor, protected with tert-butoxycarbonyl protecting groups, was radiofluorinated with high specific radioactivity [18F]F2. Specific radioactivity of 4-[18F]FMR, in average 11.8 +/-3.3 GBq/micromol, was improved 40-800-fold in comparison to the previous electrophilic fluorinations. The biodistribution of 4-[18F]FMR in rat was in accordance with the known distribution of sympathetic innervation. 4-[18F]FMR showed no metabolic degradation in left ventricle of rat heart, where the uptake was high, rapid and specific.