Evaluation of 18F-nifene binding to α4β2 nicotinic receptors in the rat brain using microPET imaging
© Kant et al; licensee Springer. 2011
Received: 17 March 2011
Accepted: 20 June 2011
Published: 20 June 2011
MicroPET imaging studies using 18F-nifene, a new positron emission tomography (PET) radiotracer for nicotinic acetylcholinergic receptors (nAChR) α4β2 receptors in rats, have been carried out. Rats were imaged for 90 min after intravenous injection of 18F-nifene (0.8 to 1 mCi), and binding potential (BPND) was measured. 18F-Nifene binding to thalamic and extrathalamic brain regions was consistent with the α4β2 nAChR distribution in the rat brain. Using the cerebellum as a reference, the values for the thalamus varied less than 5% (BPND = 1.30, n = 3), confirming reproducibility of 18F-nifene binding. 18F-Nifene microPET imaging was also used to evaluate effects of nicotine in a group of Sprague-Dawley rats under isoflurane anesthesia. Nicotine challenge postadministration of 18F-nifene demonstrated reversibility of 18F-nifene binding in vivo. For α4β2 nAChR receptor occupancy (nAChROCC), various doses of nicotine (0, 0.02, 0.1, 0.25, and 0.50 mg/kg nicotine free base) 15 min prior to 18F-nifene were administered. Low-dose nicotine (0.02 mg) reached > 80% nAChROCC while at higher doses (0.25 mg) > 90% nAChROCC was measured. The small amount of 18F-nifene binding with reference to the cerebellum affects an accurate evaluation of nAChROCC. Efforts are underway to identify alternate reference regions for 18F-nifene microPET studies in rodents.
Nicotine has a high affinity for α4β2 nicotinic acetylcholinergic receptors (nAChR) receptors (K i = 1.68 nM, ). Cigarette smoking and nicotine (a major component of tobacco) have been shown to have a direct and significant occupancy of α4β2 nAChR receptors [5–7]. Studies have also shown an increase in α4β2 receptor density binding sites in rat and mice brains upon exposure to nicotine [8–10]. Chronic tobacco smoking increases the number of high affinity nAChRs in various brain areas . Human postmortem data have shown the presence of α4β2 nAChR receptors in the subiculum, which are upregulated in smokers . Human imaging studies, using SPECT imaging agent 5-123I-iodo-A-85380 and PET imaging agent 2-18F-fluoro-A-85380, have also identified an increase in receptor density among smokers versus nonsmokers, suggesting 2-18F-fluoro-A-85380 to be a reliable PET method for further tobacco studies [12, 13]. As reported recently, nicotine from typical cigarette smoking by daily smokers is likely to occupy a majority of α4β2 receptors and lend them to a desensitized state . Thus, noninvasive imaging is playing a major role in understanding nicotine dependency [14, 15].
The focus in this work is on in vivo evaluation of 18F-nifene binding to α4β2 nicotinic receptors in rodent brain regions using microPET. In an effort to establish 18F-nifene microPET studies in the rat model, our objectives were the following: (1) evaluate in vivo18F-nifene in the normal rat model using microPET and confirm by ex vivo microPET and autoradiography, (2) carry out test-retest microPET studies in the rat model in order to evaluate reproducibility of 18F-nifene microPET binding, and (3) measure changes in 18F-nifene binding in the rat model using microPET at different doses of nicotine. These findings will assist in our eventual goal to evaluate the role of α4β2 nAChR in nicotine dependency using the rodent model.
All chemicals and solvents were purchased from Aldrich Chemical (Aldrich Chemical Company, Wilwaukee, WI, USA) and Fisher Scientific (Fisher Scientific UK Ltd., Leicestershire, UK). Deionized water was acquired from Millipore Milli-Q Water Purification System (Millipore, Billerica, MA, USA). Gilson high-performance liquid chromatography (HPLC) was used for the semipreparative reverse phase column chromatography. Fluorine-18 fluoride was produced via MC-17 cyclotron using oxygen-18-enriched water. Radioactivity was counted using a Capintec dose calibrator while low level counting was done using a well counter. Inveon preclinical Dedicated PET (Siemen's Inc., Munich, Germany) was used for the microPET studies which has a resolution of 1.45 mm . Both in vivo and ex vivo images of the rat brains were obtained using the Inveon microPET scanner and were analyzed using the Acquisition Sinogram Image Processing (ASIPRO, Siemens Medical Solutions USA, Inc., Knoxville, TN, USA) and Pixelwise Modeling Software (PMOD Technologies, Zurich, Switzerland). Slices of the rat brain were prepared at 10 to 40-μm thick using the Leica 1850 cryotome (Leica Instruments, Nussloch, Germany). In vitro- or ex vivo-labeled brain sections were exposed to phosphor films (Perkin Elmer Multisensitive, Medium MS) and were read using the Cyclone Phosphor Imaging System (Packard Instruments, Meriden, CT, USA). An analysis of in vitro or ex vivo autoradiographs was done using the Optiquant Acquisition and Analysis software (Packard Instruments, Meriden, CT, USA). All animal studies have been approved by the Institutional Animal Health Care and Use Committee of the University of California, Irvine.
A synthesis of 18F-nifene was carried out following reported procedures (Pichika et al. 2006). The automated radiosynthesis of 18F-nifene was carried out in the chemistry processing control unit box. An Alltech C18 column (10 μm, 250 × 10 mm2) was used for reverse phase HPLC purification and specific activity of 18F-nifene was approximately 2,000 Ci/mmol.
MicroPET 18F-nifene studies
Male Sprague-Dawley rats were fasted 24 h prior to the time of scan. On the day of the study, rats were anesthetized using 4.0% isoflurane. The rat was then positioned on the scanner bed by placing it on a warm water circulating heating pad, and anesthesia was applied using a nose cone. A transmission scan was subsequently acquired. The preparation of the dose injection was as follows: 0.7-1.0 mCi of 18F-nifene was drawn into a 1-mL syringe with a 25-gauge needle and was diluted with sterile saline to a final volume of 0.3 mL. The dose was injected intravenously into the tail vein of the rat. Isoflurane was reduced and maintained at 2.5% following the injection. The scans were carried out for 90 min and were acquired by the Inveon microPET in full list mode. The list mode data were collected dynamically which were rebinned using a Fourier rebinning algorithm. The images were reconstructed using a two-dimensional Filter Back Projection using a Hanning Filter with a Nyquist cutoff at 0.5, and were corrected for attenuation using the Co-57 attenuation scan data. A calibration was conducted to Becquerel per cubic centimeter units using a germanium-68 phantom which was scanned in the Inveon microPET and was reconstructed under the same parameters as the subjects. Analyses of all data were carried out using the Acquisition Sinogram Image Processing IDL's virtual machine (ASIPRO VM) and Pixelwise Modeling software (PMOD 3.0). The test and retest microPET studies on the same animal were carried out within an interval of approximately 2 weeks.
Blood was collected at four different time points (5, 15, 60, and 90 min) after the injection of 18F-nifene. The blood was centrifuged for 5 min at 3,000 g. The plasma was separated and counted. Acetonitrile was added to the blood samples, and the organic layer was spotted on the analytical thin layer chromatography (TLC) plates (silica-coated plates, Baker-Flex, Phillipsburg, NJ, USA) and was developed in 15% methanol in dichloromethane. A sample of the plasma was also collected prior to the injection of 18F-nifene and was spiked with the tracer and was used as a standard.
Male Sprague-Dawley rats were injected intravenously (IV) with 0.5 mCi of 18F-nifene in a total volume of 0.3 mL and were sacrificed 40 min after injection. The brain was extracted and dissected into two hemispheres. The sagittal sections of 40-μm thickness were obtained from the left hemisphere using the Leica 1850 cryotome and were exposed to phosphor films overnight. The films were read using the Cyclone Phosphor Imaging System and were analyzed using the Optiquant software. The right hemisphere was homogenized with 1.15% KCl (2 mL), and this homogenized mixture was vortexed with 2% acetic acid in methanol (2 mL). This mixture was centrifuged for 10 min at 10,000 g, and the supernatant was removed for analysis. RadioTLC (9:1, dichloromethane and methanol) was obtained for both 18F-nifene standard and the brain extract.
Ex vivo microPET
In order to ascertain the brain uptake of 18F-nifene, after completion of the in vivo microPET scans, the rats were sacrificed and the brain was extracted for ex vivo microPET imaging. The whole brain was placed in a hexagonal polystyrene weighing boat (top edge side length, 4.5 cm; bottom edge side length, 3 cm) and was covered with powdered dry ice. This boat was placed securely on the scanner bed, and a transmission scan was acquired. Subsequently, a 60-min emission scan was acquired by the Inveon microPET scanner in full list mode. The list mode was collected in a single frame, and a reconstruction of the images was similar to the procedure described previously in the section "MicroPET 18F-nifene studies." The images were analyzed using the ASIPRO VM and PMOD 3.0 software.
Ex vivo autoradiography
The brain after the ex vivo microPET acquisition in the section "Ex vivo microPET" was removed from the dry ice and was rapidly prepared for sectioning. Horizontal sections (40-μm thick) containing brain regions of the thalamus, subiculum, cortex, striatum, hippocampus, and cerebellum were cut using the Leica CM1850 cryotome. The sections were air-dried and exposed to phosphor films overnight. The films were read using the Cyclone Phosphor Imaging System. The regions of interest of the same size were drawn and analyzed on the brain regions rich in α4β2 nicotinic receptors using the OptiQuant software, and the binding of 18F-nifene was measured in digital light units per square millimeter.
MicroPET studies of nicotine challenge
Nicotine challenge experiments were of two types. In order to demonstrate reversibility of bound 18F-nifene and to measure the off-rate, the postinjection nicotine effects were first measured. Sprague-Dawley rats were injected with 18F-nifene (0.2 to 0.5 mCi, IV) and at approximately 30 min postinjection of the 18F-nifene, 0.3 mg/kg of nicotine free base (administered as a ditartarate salt from Sigma Chemical Company, St. Louis, MO, USA) was administered intravenously. The total time of scan was 90 min and was acquired in full list mode, similar to the protocol for the control scans described in "MicroPET 18F-nifene studies." Before and after images were analyzed using the PMOD 3.0 software, and a time-activity curve was generated.
The second set of nicotine challenge experiments were designed to measure α4β2 nAChR receptor occupancy (nAChROCC) by nicotine. Male Sprague-Dawley rats were preinjected intravenously with nicotine using saline for baseline, and four different doses of nicotine (0.02, 0.1, 0.25, and 0.5 mg/kg free base, administered as a ditartarate salt) were diluted in a total volume of 0.3 mL sterile saline. Nicotine was injected 15 min prior to intravenous injection of 18F-nifene (0.8-1.0 mCi). Once anesthetized, the rats were scanned for 90 min using the Inveon microPET scanner in full list mode. Dynamic data were reconstructed and analyzed as described in the section "MicroPET 18F-nifene studies." Time-activity curves were measured and analyzed using the ASIPRO VM and PMOD 3.0 software. Percent occupancy was calculated from: (Thalcont - Thalnic/Thalcont]) × 100, where Thalcont is the percent injected dose of 18F-nifene in the brain regions of the control study, and Thalnic is the percent injected dose of 18F-nifene in the brain regions of the nicotine study at 60 min postinjection of 18F-nifene.
MicroPET 18F-nifene binding studies
Radiochromatograms were attained from running brain extracts and were compared to the peak to the parent compound providing evidence that the primary species within the brain of the rat was 18F-nifene. After sacrificing the rat, the brain was excised and dissected into the left and right hemispheres. Figure 3C,D shows the sagittal brain slices of the left hemisphere representing the total binding of 18F-nifene revealing maximal binding in the thalamus followed by extrathalamic regions such as the cortex and subiculum. The cerebellum had the least amount of activity. A thin layer chromatographic analysis of the extract of the homogenized right hemisphere shown in Figure 3F closely correlates with the retention of 18F-nifene standard (Figure 3D). No other significant metabolite peak was observed in the brain extract.
Test-retest 18F-nifene binding potential in thalamus
Ex vivo studies
Measured 18F-nifene ratios of rat brain regions with reference to the cerebellum
In vivo microPETa
Ex vivo microPETb
Ex vivo autoradiographsc
3.13 ± 0.29
3.92 ± 0.49
4.60 ± 0.52
2.28 ± 0.24
2.39 ± 0.15
1.98 ± 0.10
2.05 ± 0.17
1.83 ± 0.19
1.52 ± 0.39
1.77 ± 0.28
1.46 ± 0.07
MicroPET studies of nicotine challenges
Nicotine dose effects on 18F-nifene binding
% Injected dose/cc
Our primary goal was to evaluate 18F-nifene binding to the α4β2 receptors in thalamic and extrathalamic brain regions of rodents using microPET imaging. 18F-Nifene, an agonist, was developed with fast binding kinetics and a shorter scan time in order to image the α4β2 nicotinic receptors. This is useful in the assessment of nicotinic receptors in neurological diseases. MicroPET studies in rats validated the faster binding profile of 18F-nifene thus providing shorter scan times. Maximum binding was found in the thalamus, while moderate binding is seen in the cortex, and minimal binding in the cerebellum. Time-activity curves for the thalamus, cortex, and cerebellum show that 18F-nifene peaks early into the scan, and nonspecific binding in the cerebellum cleared rapidly. Thalamus to cerebellum ratios were > 3.0 and cortex to cerebellum were approximately 2. Thus, 18F-nifene allows shorter duration PET studies for quantitative measures of α4β2 receptors compared to 2-18F-FA-85380 which has been shown to require 5 h to reach steady state in rodents .
No lipophilic metabolites of 18F-nifene were detected in plasma extracts, and a significant amount of 18F-nifene parent remained in the blood after 90 min of the PET study. The absence of lipophilic metabolites was also confirmed using brain extracts of rats injected with 18F-nifene. Only 18F-nifene was detected in the brain extracts.
The binding of 18F-nifene to α4β2 receptors of the rodent brain in microPET studies gave results consistent with the receptor distribution and was comparable with the autoradiographic slices done in vitro . Test-retest results of binding potentials, summarized in Table 1, remained consistent between scans thus confirming reproducibility of 18F-nifene with <5% standard deviation, suggesting 18F-nifene to be suitable for PET studies. Ex vivo images, both microPET and autoradiographic, confirmed binding of 18F-nifene to thalamic and extrathalamic regions seen in the in vivo microPET study.
Nicotine, because of its high affinity to α4β2 receptors, exhibited competition with 18F-nifene. Previous in vitro studies using 10 nM of nicotine displaced 60-65% in the thalamus region and 300 μM of nicotine, 95% elimination is seen in the thalamus . As expected, displacement of 18F-nifene binding was seen in the post-nicotine challenge similar to that reported for 2-[18F]F-A-85380 . Figure 6 clearly shows a drop in binding at the time of nicotine injection (30 min into the scan), displacing at least > 80% of 18F-nifene binding. The ability for nicotine to compete with 18F-nifene can be used to detect changes in receptor occupancy suggesting PET to be a valuable tool in assessing tobacco-related dependence . Pre-nicotine challenges at different dose levels of nicotine, demonstrated a steady decrease in 18F-nifene occupancy with respect to nicotine. At low doses of nicotine, 0.02 mg/kg, > 40% of receptors were occupied while at high doses (0.5 mg/kg) > 80% receptors were occupied with nicotine (Table 3). While the cerebellum was used as a reference region, some issues have risen questioning the validity of the cerebellum as a reference region. With the presence of nicotinic receptors in the rat cerebellum [17–19], measurement of binding potential can be complex. Studies using 2-[18F]F-A-85380 in rodents have reported nicotine displaceable component in the cerebellum , suggesting a need for arterial input function for accurate quantification.
Aside from the cerebellum, efforts have been underway to identify other regions of the brain, such as the corpus callosum and pons as reference regions . Efforts are underway in our rodent 18F-nifene studies to identify other reference regions in the brain, other than the cerebellum. Future work in the rodent model will incorporate arterial blood sampling for more accurate quantification.
18F-nifene binds to the α4β2 receptors in thalamic and extrathalamic regions in rat microPET studies. With its faster binding kinetics, short scan time, and reversible binding, 18F-nifene is an agonist radiotracer with potential for studying this receptor system in various rodent models.
This research was supported by the National Institutes of Health (NIH), U.S. Department of Health and Human Services, grant no. R01AG029479. We would like to thank Robert Coleman for the technical assistance.
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