Jenny Häggkvist

Performance Evaluation of the Small-Animal nanoScan PET/MRI System

nanoScan is a high-resolution integrated system for consecutive PET and MR imaging of small laboratory animals. We evaluated the performance of the system, using the NEMA NU 4-2008 protocol for the PET component and the NEMA MS 1-2007, MS 2-2008, and MS 3-2007 standards for the MR imaging component. Methods: The imaging system uses magnetically shielded position-sensitive photomultiplier tubes and a compact 1-T permanent-magnet MR imaging platform. Spatial resolution, sensitivity, counting rate capabilities, and image quality parameters were evaluated in accordance with the aforementioned NEMA standards. Further in vivo evaluation experiments complement the physical validation results. Results: The spatial resolution of the PET system enabled the 0.8-mm rods of a Derenzo phantom to be resolved. With point source and 2-dimensional filtered backprojection reconstruction, the resolution varied from 1.50 to 2.01 mm in full width at half maximum in the radial direction and from 1.32 to 1.65 mm in the tangential direction within the radius of 25 mm. Peak absolute sensitivity was 8.41%. Scatter fraction was 17.3% and 34.0%, and maximum noise-equivalent counting rate was 406 and 119 kcps in the mouselike and ratlike phantom, respectively. The image quality test found a nonuniformity of 3.52% and a spillover ratio of 6.2% and 5.8% in water and air, respectively. In testing of the MR imaging component, artifact-free images with high signal-to-noise ratio were recorded. Geometric distortion was below 5%, and image uniformity was at least 94.5% and 96.6% for the 60- and 35-mm radiofrequency coils, respectively. Conclusion: The nanoScan integrated small-animal PET/MR imaging system has excellent spatial resolution and sensitivity. The performance characteristics of the PET and the MR imaging components are not compromised as a result of their integration onto a single platform. Because of its combination of features and performance parameters, the system provides crucial advantages for preclinical imaging studies over existing PET/CT systems, especially in neurologic and oncologic research.

The opioid receptor antagonist naltrexone attenuates reinstatement of amphetamine drug-seeking in the rat

Amphetamine produces its rewarding effects by enhancing dopamine transmission in the mesocorticolimbic pathway. Several studies have also suggested the involvement of the endogenous opioid system in mediating the neurochemical and behavioural effects of amphetamine. The aim of this study was to investigate the effect of the unselective opioid receptor antagonist naltrexone (NTX) on reinstatement of amphetamine self-administration in the rat. Animals were trained to self-administer amphetamine under a fixed ratio 1 (FR1) schedule (0.1mg/kg/infusion). After receiving a stable drug intake the amphetamine was replaced with saline and the animals went through an extinction period. After reaching the extinction criteria, animals were pre-treated with NTX (0, 0.3, 1.0 and 3.0mg/kg, s.c.) 30min before giving a priming dose of amphetamine (0.5mg/kg s.c). To study the effects of NTX on operant behaviour, animals were trained to lever press for food pellets under a FR1 schedule of reinforcement. Results from the present study shows that a single injection of amphetamine reinstated self-administration behaviour. NTX (0.3 and 1.0mg/kg) significantly attenuated the amphetamine-induced reinstatement but NTX had no effect at any dose studied on food taking behaviour. These results show that NTX attenuates reinstatement of amphetamine self-administration in rats without suppressing general behaviour, implicating a functional role for opioid receptors in modulating amphetamine seeking behaviour.

The effect of naltrexone on amphetamine-induced conditioned place preference and locomotor behaviour in the rat.

Whereas amphetamine and other psychostimulants primarily act on the dopamine system, there is also evidence that other neurotransmitter systems, such as the endogenous opioid system, modulate psychostimulant‐induced effects. Several studies have investigated the role of opioid antagonists on cocaine‐induced conditioned place preference (CPP), but there is limited information about the interaction with amphetamines. The aim of the present study was to investigate the effect of the opioid receptor antagonist, naltrexone (NTX) on the conditioning, expression and reinstatement of amphetamine‐induced place preference. In addition, the effect of NTX on locomotor behaviour was measured during all sessions. During training, animals were conditioned with amphetamine (2 mg/kg) to induce place preference. In order to extinguish the conditioned behaviour, animals received saline for 12 days. Reinstatement of CPP was induced by a priming dose of amphetamine (0.5 mg/kg). The interaction of NTX and amphetamine was evaluated using three paradigms of CPP: with NTX (vehicle, 0.3, 1.0 and 3.0 mg/kg) administered either 30 minutes prior to amphetamine conditioning, or 30 minutes before the expression, or 30 minutes before the amphetamine priming to induce reinstatement. Naltrexone had no effect on the conditioning, the expression or the reinstatement induced by a priming dose of amphetamine. Further, NTX by itself did not induce place preference or place aversion. In contrast, NTX significantly attenuated the locomotor response to a priming dose of amphetamine without affecting general locomotor behaviour. The results suggest differences in opioid modulation of amphetamine‐induced behaviours in the rat.

Naltrexone attenuates amphetamine‐induced locomotor sensitization in the rat

Amphetamine, and other stimulants, readily induces behavioral sensitization, an effect hypothesized to reflect neurobiological changes that may underlie certain aspects of drug addiction. Apart from the effects on the dopamine system, previous studies have also shown that amphetamine interacts with other neurotransmitters, including the endogenous opioid system. The unselective opioid receptor antagonist naltrexone (NTX) modulates amphetamine‐induced effects in both laboratory animals and humans. To further examine this interaction, the aim of the present study was to investigate the effect of NTX on the expression of locomotor sensitization and conditioned locomotor response in animals previously conditioned with amphetamine. Sensitization was induced by repeated administration of amphetamine (2 mg/kg) for 10 consecutive days. After a 10‐day drug‐free period, the rats were administered NTX (3 mg/kg) 30 minutes prior to the administration of a challenge dose of either amphetamine (0.5 mg/kg) (test for drug‐induced sensitization) or saline (test for conditioned locomotor response). NTX had no effect on acute amphetamine‐induced locomotor activity or on general locomotor activity in animals without a history of amphetamine conditioning. However, animals previously conditioned with amphetamine showed a sensitized locomotor response to the amphetamine challenge following the 10‐day drug‐free period. This sensitized response was significantly inhibited by NTX pre‐treatment. In addition, NTX pre‐treatment blocked the conditioned locomotor response when the amphetamine‐conditioned animals were placed in the previously amphetamine‐paired context. This study showed that NTX attenuates drug‐ and cue‐induced locomotor behavior in amphetamine‐conditioned animals, supporting recent clinical findings that indicated a potential role of NTX as a treatment for amphetamine dependence.

Molecular Imaging of PDE10A Knockout Mice with a Novel PET Radiotracer: [ 11 C]T-773

Purpose[11C]T-773 is a new radioligand for positron emission tomography (PET) targeting the phosphodiesterase 10A enzyme (PDE10A). PDE10A is highly expressed in the striatum by medium spiny neurons, and it has been demonstrated to be involved in the regulation of striatal signaling through the reduction of medium spiny neuronal sensitivity towards glutamatergic excitation. PDE10A is associated with Parkinson’s disease and different neuropsychiatric disorders such as Huntington’s disease, obsessive-compulsive disorders (OCD) and schizophrenia. Studies have indicated that the inhibition of PDE10A may represent a novel therapeutic approach to the treatment of the aforementioned diseases characterized by the reduced activity of medium spiny neurons. An appropriate PET radioligand for PDE10A would help to facilitate drug development and drug evaluation.ProceduresWe have evaluated the [11C]T-773 ligand in PDE10A knockout mice (heterozygous [HET] and homozygous [HOM]) as well as in normal control animals (WILD) with PET.ResultsThe regional percent standardized uptake values (%SUV; mean ± SD) in the striatum were 48.2 ± 1.0 (HOM), 63.6 ± 5.3 (HET) and 85.1 ± 6.3 (WILD). Between each animal group the striatal %SUV values were significantly different (p < 0.0001). The striatal BPND values (mean ± SD) were 0.0 ± 0.0 (HOM), 0.14 ± 0.07 (HET) and 0.56 ± 0.15 (WILD). The BPND values were significantly lower in homozygous and heterozygous animals compared to wild type (p < 0.0001).ConclusionsThe novel PDE10A radioligand [11C]T-773 shows increased signals with higher levels of PDE10A and acceptable binding in the striatum in control animals compared to knockout mice.

Positron Emission Tomography studies with [11C]PBR28 in the Healthy Rodent Brain: Validating SUV as an Outcome Measure of Neuroinflammation

Molecular imaging of the 18 kD Translocator protein (TSPO) with positron emission tomography (PET) is of great value for studying neuroinflammation in rodents longitudinally. Quantification of the TSPO in rodents is, however, quite challenging. There is no suitable reference region and the use of plasma-derived input is not an option for longitudinal studies. The aim of this study was therefore to evaluate the use of the standardized uptake value (SUV) as an outcome measure for TSPO imaging in rodent brain PET studies, using [11C]PBR28. In the first part of the study, healthy male Wistar rats (n = 4) were used to determine the correlation between the distribution volume (VT, calculated with Logan graphical analysis) and the SUV. In the second part, healthy male Wistar rats (n = 4) and healthy male C57BL/6J mice (n = 4), were used to determine the test-retest variability of the SUV, with a 7-day interval between measurements. Dynamic PET scans of 63 minutes were acquired with a nanoScan PET/MRI and nanoScan PET/CT. An MRI scan was made for anatomical reference with each measurement. The whole brain VT of [11C]PBR28 in rats was 42.9 ± 1.7. A statistically significant correlation (r2 = 0.96; p < 0.01) was found between the VT and the SUV. The test-retest variability in 8 brain region ranged from 8 to 20% in rats and from 7 to 23% in mice. The interclass correlation coefficient (ICC) was acceptable to excellent for rats, but poor to acceptable for mice. In conclusion: The SUV of [11C]PBR28 showed a high correlation with VT as well as good test-retest variability. For future longitudinal small animal PET studies the SUV can thus be used to describe [11C]PBR28 uptake in healthy brain tissue. Based on the present observations, further studies are needed to explore the applicability of this approach in small animal disease models, with special regard to neuroinflammatory models.

Naltrexone modulates dopamine release following chronic, but not acute amphetamine administration: a translational study

The opioid antagonist naltrexone has been shown to attenuate the subjective effects of amphetamine. However, the mechanisms behind this modulatory effect are currently unknown. We hypothesized that naltrexone would diminish the striatal dopamine release induced by amphetamine, which is considered an important mechanism behind many of its stimulant properties. We used positron emission tomography and the dopamine D2-receptor radioligand [11C]raclopride in healthy subjects to study the dopaminergic effects of an amphetamine injection after pretreatment with naltrexone or placebo. In a rat model, we used microdialysis to study the modulatory effects of naltrexone on dopamine levels after acute and chronic amphetamine exposure. In healthy humans, naltrexone attenuated the subjective effects of amphetamine, confirming our previous results. Amphetamine produced a significant reduction in striatal radioligand binding, indicating increased levels of endogenous dopamine. However, there was no statistically significant effect of naltrexone on dopamine release. The same pattern was observed in rats, where an acute injection of amphetamine caused a significant rise in striatal dopamine levels, with no effect of naltrexone pretreatment. However, in a chronic model, naltrexone significantly attenuated the dopamine release caused by reinstatement of amphetamine. Collectively, these data suggest that the opioid system becomes engaged during the more chronic phase of drug use, evidenced by the modulatory effect of naltrexone on dopamine release following chronic amphetamine administration. The importance of opioid-dopamine interactions in the reinforcing and addictive effects of amphetamine is highlighted by the present findings and may help to facilitate medication development in the field of stimulant dependence.

Discovery and Preclinical Validation of [11C]AZ13153556, a Novel Probe for the Histamine Type 3 Receptor

UNLABELLED The histamine type 3 receptor (H3) is a G protein-coupled receptor implicated in several disorders of the central nervous system. Herein, we describe the radiolabeling and preclinical evaluation of a candidate radioligand for the H3 receptor, 4-(1S,2S)-2-(4-cyclobutylpiperazine-1-carbonyl)cyclopropyl]-N-methyl-benzamide (5), and its comparison with one of the frontrunner radioligands for H3 imaging, namely, GSK189254 (1). Compounds 1 and 5 were radiolabeled with tritium and carbon-11 for in vitro and in vivo imaging experiments. The in vitro binding of [(3)H]1 and [(3)H]5 was examined by (i) saturation binding to rat and nonhuman primate brain tissue homogenate and (ii) in vitro autoradiography on tissue sections from rat, guinea pig, and human brain. The in vivo binding of [(11)C]1 and [(11)C]5 was examined by PET imaging in mice and nonhuman primates. Bmax values obtained from Scatchard analysis of [(3)H]1 and [(3)H]5 binding were in good agreement. Autoradiography with [(3)H]5 on rat, guinea pig, and human brain slices showed specific binding in regions known to be enhanced in H3 receptors, a high degree of colocalization with [(3)H]1, and virtually negligible nonspecific binding in tissue. PET measurements in mice and nonhuman primates demonstrated that [(11)C]5 binds specifically and reversibly to H3 receptors in vivo with low nonspecific binding in brain tissue. Whereas [(11)C]1 showed similar binding characteristics in vivo, the binding kinetics appeared faster for [(11)C]5 than for [(11)C]1. CONCLUSIONS [(11)C]5 has suitable properties for quantification of H3 receptors in nonhuman primate brain and has the potential to offer improved binding kinetics in man compared to [(11)C]1.

ABC transporter-dependent brain uptake of the 5-HT1B receptor radioligand [11C]AZ10419369: a comparative PET study in mouse, rat, and guinea pig

BackgroundWe have explored the possibility that the serotonin 1B receptor radioligand [11C]AZ10419369 is a substrate for adenosine triphosphate (ATP)-binding cassette (ABC) transporters, such as P-glycoprotein (P-gp), Mrp4, and Bcrp, in rodents and whether there is a species difference regarding its blood-brain barrier (BBB) penetration.MethodsIn a series of preclinical positron emission tomography measurements, we have administered [11C]AZ10419369 to mice, rats, and guinea pigs under baseline conditions and, on separate experimental days, after administration of the ABC transporter inhibitor, cyclosporin A (CsA).ResultsDuring baseline conditions, the brain uptake was low in mice and rats, but not in guinea pigs. After CsA pretreatment, the peak whole brain uptake values of [11C]AZ10419369 increased by 207% in mice, 94% in rats, and 157% in guinea pigs. Binding potentials (BPND) could not be estimated during baseline conditions in mice and rats. After CsA pretreatment, the highest BPND values were obtained in the striatum and thalamus (BPND ≈ 0.4) in mice, while in rats, the highest binding areas were the striatum, thalamus, hypothalamus, and periaqueductal gray (BPND ≈ 0.5). In guinea pigs, we did not find any significant changes in BPND between baseline and CsA pretreatment, except in the striatum.ConclusionsThe results indicate that BBB penetration of [11C]AZ10419369 was hindered by ABC transporter activity in mouse, rat, and guinea pig. This study highlights the importance of ABC transporters in the design of preclinical positron emission tomography (PET) studies.

Longitudinal microPET imaging of the zQ175 mouse model of Huntington's disease shows in vivo changes of molecular targets in the striatum and the cerebral cortex.

Since the discovery of the HTT gene in 1993, numerous animal models have been developed to study the progression of Huntington disease (HD) and to evaluate potential new therapeutics. In the present study, we used small-animal PET to characterize the expression of molecular targets in the recently reported HD animal model, the zQ175 mouse model. Methods: Male heterozygous zQ175 (Htttm1Mfc/190JChdi, CHDI-81003003) and wild-type (WT, C57BL/6J) animals were imaged with the dopamine D2 receptor radioligand 11C-raclopride, the PDE10A radioligand 18F-MNI-659, the dopamine D1 receptor radioligand 11C-NNC 112, and the 5-HT2A radioligand 11C-MDL 100907 at 6 and 9 mo of age. The outcome measure was the binding potential (BPND), using the cerebellum as the reference region. Selected regions of interest were the striatum for all radioligands and additionally the striatum, rostral cortex, caudal cortex, and hippocampus for 11C-NNC 112 and 11C-MDL 100907. Results: At 6 mo of age, the BPND in the striatum was lower in zQ175 than WT animals by 40% for 11C-raclopride, by 52% for 18F-MNI-659, by 28% for 11C-NNC, and by 11% for 11C-MDL 100907. In the rostral cortex, D1 receptor binding was 22% lower in zQ175 than WT animals. We found an overall reduction in D1 and 5-HT2A binding in the hippocampus of zQ175 compared with WT animals. The BPND of 11C-MDL 100907 in the caudal cortex was also lower in zQ175 WT animals. At 9 mo, there was a slight further reduction of D1, D2, and 5-HT2A BPND in the striatum, whereas PDE10A reached a plateau. Cortical markers were also slightly further decreased at 9 mo in zQ175 animals. Conclusion: Our study indicates a marked reduction of ligand binding to D1 and D2 and 5-HT2A receptors as well as loss of PDE10A enzyme in the striatum of zQ175 mice as compared with WT animals, in agreement with data obtained in clinical PET studies of patients with HD. The zQ175 mouse model recapitulates the expression pattern seen in humans with HD and may have value in further elucidating pathophysiologic events and therapeutic strategies.