Reproducibility of striatal and thalamic dopamine D2 receptor binding using [11C]raclopride with high-resolution positron emission tomography

Positron emission tomography (PET) imaging of small striatal brain structures such as the ventral striatum (VST) has been hampered by low spatial resolution causing partial-volume effects. The high-resolution research tomograph (HRRT) is a brain-dedicated PET scanner that has considerably better spatial resolution than its predecessors. However, its superior spatial resolution is associated with a lower signal-to-noise ratio. We evaluated the test–retest reliability of the striatal and thalamic dopamine D₂ receptor binding using the HRRT scanner. Seven healthy male volunteers underwent two [¹¹C]raclopride PET scans with a 2.5-hour interval. Dopamine D₂ receptor availability was quantified as binding potential (BP_ND) using the simplified reference tissue model. To evaluate the reproducibility of repeated BP_ND estimations, absolute variability (VAR) and intraclass correlation coefficients (ICCs) were calculated. VAR values indicated fairly good reproducibility and were 3.6% to 4.5% for the caudate nucleus and putamen and 4.5% to 6.4% for the lateral and medial part of the thalamus. In the VST, the VAR value was 5.8% when the definition was made in the coronal plane. However, the ICC values were only moderate, in the range of 0.34 to 0.66, for all regions except the putamen (0.87). Experimental signal processing methods improved neither ICC nor VAR values significantly.     

Measuring α4β2* nicotinic acetylcholine receptor density in vivo with [18F]nifene PET in the nonhuman primate

[¹⁸F]Nifene is an agonist PET radioligand developed to image α4β2* nicotinic acetylcholine receptors (nAChRs). This work aims to quantify the receptor density (B_max) of α4β2* nAChRs and the in vivo (apparent) dissociation constant (K_Dapp) of [¹⁸F]nifene. Multiple-injection [¹⁸F]nifene experiments with varying cold nifene masses were conducted on four rhesus monkeys with a microPET P4 scanner. Compartment modeling techniques were used to estimate regional B_max values and a global value of K_Dapp. The fast kinetic properties of [¹⁸F]nifene also permitted alternative estimates of B_max and K_Dapp at transient equilibrium with the same experimental data using Scatchard-like methodologies. Averaged across subjects, the compartment modeling analysis yielded B_max values of 4.8±1.4, 4.3±1.0, 1.2±0.4, and 1.2±0.3 pmol/mL in the regions of antereoventral thalamus, lateral geniculate, frontal cortex, and subiculum, respectively. The K_Dapp of nifene was 2.4±0.3 pmol/mL. The Scatchard analysis based on graphical evaluation of the data after transient equilibrium yielded B_max estimations comparable to the modeling results with a positive bias of 28%. These findings show the utility of [¹⁸F]nifene for measuring α4β2* nAChR B_max in vivo in the rhesus monkey with a single PET experiment.     

Quantitation of translocator protein binding in human brain with the novel radioligand [18F]-FEPPA and positron emission tomography

This article describes the kinetic modeling of [¹⁸F]-FEPPA binding to translocator protein 18 kDa in the human brain using high-resolution research tomograph (HRRT) positron emission tomography. Positron emission tomography scans were performed in 12 healthy volunteers for 180 minutes. A two-tissue compartment model (2-CM) provided, with no exception, better fits to the data than a one-tissue model. Estimates of total distribution volume (V_T), specific distribution volume (V_S), and binding potential (BP_ND) demonstrated very good identifiability (based on coefficient of variation (COV)) for all the regions of interest (ROIs) in the gray matter (COV V_T<7%, COV V_S<8%, COV BP_ND<11%). Reduction of the length of the scan to 2 hours is feasible as V_S and V_T showed only a small bias (6% and 7.5%, respectively). Monte Carlo simulations showed that, even under conditions of a 500% increase in specific binding, the identifiability of V_T and V_S was still very good with COV<10%, across high-uptake ROIs. The excellent identifiability of V_T values obtained from an unconstrained 2-CM with data from a 2-hour scan support the use of V_T as an appropriate and feasible outcome measure for [¹⁸F]-FEPPA.     

A two-compartment mathematical model of neuroglial metabolism using [1-(11)C] acetate

The purpose of this study was to develop a two-compartment metabolic model of brain metabolism to assess oxidative metabolism from [1-¹¹C] acetate radiotracer experiments, using an approach previously applied in ¹³C magnetic resonance spectroscopy (MRS), and compared with an one-tissue compartment model previously used in brain [1-¹¹C] acetate studies. Compared with ¹³C MRS studies, ¹¹C radiotracer measurements provide a single uptake curve representing the sum of all labeled metabolites, without chemical differentiation, but with higher temporal resolution. The reliability of the adjusted metabolic fluxes was analyzed with Monte-Carlo simulations using synthetic ¹¹C uptake curves, based on a typical arterial input function and previously published values of the neuroglial fluxes V_tca^g, V_x, V_nt, and V_tcaⁿ measured in dynamic ¹³C MRS experiments. Assuming V_x^g=10 × V_tca^g and V_xⁿ=V_tcaⁿ, it was possible to assess the composite glial tricarboxylic acid (TCA) cycle flux V_gt^g (V_gt^g=V_x^g × V_tca^g/(V_x^g+V_tca^g)) and the neurotransmission flux V_nt from ¹¹C tissue-activity curves obtained within 30 minutes in the rat cortex with a beta-probe after a bolus infusion of [1-¹¹C] acetate (n=9), resulting in V_gt^g=0.136±0.042 and V_nt=0.170±0.103 μmol/g per minute (mean±s.d. of the group), in good agreement with ¹³C MRS measurements.     

Measuring drug occupancy in the absence of a reference region: the Lassen plot re-visited

Quantitative estimation of neuroreceptor occupancy by exogenous drugs using positron emission tomography is based on the reduction in the total volume of distribution (V_T) of site-specific radioligands after drug administration. An estimate of the distribution volume of free and nonspecifically bound radioligand (V_ND) is also required to distinguish specific from total binding. However, a true reference region, devoid of specific binding, is often not available. We present a transformation of a graphical method, originally introduced by Lassen, using regional estimates of V_T alone to determine occupancy, together with an extension that does not require baseline data.     

Partial volume correction using structural–functional synergistic resolution recovery: comparison with geometric transfer matrix method

We validated the use of a novel image-based method for partial volume correction (PVC), structural–functional synergistic resolution recovery (SFS-RR) for the accurate quantification of dopamine synthesis capacity measured using [¹⁸F]DOPA positron emission tomography. The bias and reliability of SFS-RR were compared with the geometric transfer matrix (GTM) method. Both methodologies were applied to the parametric maps of [¹⁸F]DOPA utilization rates (k_i^cer). Validation was first performed by measuring repeatability on test–retest scans. The precision of the methodologies instead was quantified using simulated [¹⁸F]DOPA images. The sensitivity to the misspecification of the full-width-half-maximum (FWHM) of the scanner point-spread-function on both approaches was also assessed. In the in-vivo data, the k_i^cer was significantly increased by application of both PVC procedures while the reliability remained high (intraclass correlation coefficients >0.85). The variability was not significantly affected by either PVC approach (<10% variability in both cases). The corrected k_i^cer was significantly influenced by the FWHM applied in both the acquired and simulated data. This study shows that SFS-RR can effectively correct for partial volume effects to a comparable degree to GTM but with the added advantage that it enables voxelwise analyses, and that the FWHM used can affect the PVC result indicating the importance of accurately calibrating the FWHM used in the recovery model.     

Adenosine A2A receptors in secondary progressive multiple sclerosis: a [11C]TMSX brain PET study

In this study, positron emission tomography (PET) imaging with a radioligand to adenosine A2A receptors (A2AR)—a potent regulator of inflammation—was used to gain insight into the molecular alterations in normal-appearing white matter (NAWM) and gray matter (GM) in secondary progressive multiple sclerosis (SPMS). Normal-appearing white matter and GM, despite seeming normal in conventional mangnetic resonance imaging (MRI), are important loci of widespread inflammation, neuronal damage, and source of progressive disability in multiple sclerosis (MS). Dynamic PET imaging using A2AR-specific [¹¹C]TMSX and brain MRI with diffusion tensor imaging were performed to eight SPMS patients and seven healthy controls. Distribution volumes (V_T) of [¹¹C]TMSX were analyzed from 13 regions of interest using Logan plot with arterial plasma input. The SPMS patients had significantly increased [¹¹C]TMSX-V_T in NAWM compared with controls (mean (s.d.): 0.55 (±0.08) vs. 0.45 (±0.05); P=0.036). Both the increased V_T and the decreased fractional anisotropy (FA) in NAWM were associated with higher expanded disability status scale (EDSS) scores (P=0.030 and P=0.012, respectively), whereas the T2-lesion load of SPMS patients did not correlate with EDSS. This study shows, that A2ARs are increased in the brain of SPMS patients, and that [¹¹C]TMSX-PET provides a novel approach to learn about central nervous system pathology in SPMS in vivo.     

Repeatability of parametric methods for [18F]florbetapir imaging in Alzheimer’s disease and healthy controls: A test–retest study

Accumulation of amyloid beta (Aβ) is one of the pathological hallmarks of Alzheimer’s disease (AD), which can be visualized using [¹⁸F]florbetapir positron emission tomography (PET). The aim of this study was to evaluate various parametric methods and to assess their test-retest (TRT) reliability. Two 90 min dynamic [¹⁸F]florbetapir PET scans, including arterial sampling, were acquired (n = 8 AD patient, n = 8 controls). The following parametric methods were used; (reference:cerebellum); Logan and spectral analysis (SA), receptor parametric mapping (RPM), simplified reference tissue model2 (SRTM2), reference Logan (rLogan) and standardized uptake value ratios (SUV_r(50–70)). BP_ND+1, DVR, V_T and SUVr were compared with corresponding estimates (V_T or DVR) from the plasma input reversible two tissue compartmental (2T4k_V_B) model with corresponding TRT values for 90-scan duration. RPM (r² = 0.92; slope = 0.91), Logan (r² = 0.95; slope = 0.84) and rLogan (r² = 0.94; slope = 0.88), and SRTM2 (r² = 0.91; slope = 0.83), SA (r² = 0.91; slope = 0.88), SUVr (r² = 0.84; slope = 1.16) correlated well with their 2T4k_V_B counterparts. RPM (controls: 1%, AD: 3%), rLogan (controls: 1%, AD: 3%) and SUV_r(50–70) (controls: 3%, AD: 8%) showed an excellent TRT reliability. In conclusion, most parametric methods showed excellent performance for [¹⁸F]florbetapir, but RPM and rLogan seem the methods of choice, combining the highest accuracy and best TRT reliability.     

Kinetic modeling of the monoamine oxidase B radioligand [11C]SL25.1188 in human brain with high-resolution positron emission tomography

This article describes the kinetic modeling of [¹¹C]SL25.1188 ([(S)-5-methoxymethyl-3-[6-(4,4,4-trifluorobutoxy)-benzo[d]isoxazol-3-yl]-oxazolidin-2-[¹¹C]one]) binding to monoamine oxidase B (MAO-B) in the human brain using high-resolution positron emission tomography (PET). Seven healthy subjects underwent two separate 90- minute PET scans after an intravenous injection of [¹¹C]SL25.1188. Complementary arterial blood sampling was acquired. Radioactivity was quickly eliminated from plasma with 80% of parent compound remaining at 90 minutes. Metabolites were more polar than the parent compound. Time-activity curves showed high brain uptake, early peak and washout rate consistent with known regional MAO-B concentration. A two-tissue compartment model (2-TCM) provided better fits to the data than a 1-TCM. Measurement of total distribution volume (V_T) showed very good identifiability (based on coefficient of variation (COV)) for all regions of interest (ROIs) (COV(V_T)<8%), low between-subject variability (∼20%), and quick temporal convergence (within 5% of final value at 45 minutes). Logan graphical method produces very good estimation of V_T. Regional V_T highly correlated with previous postmortem report of MAO-B level (r²=⩾0.9). Specific binding would account from 70% to 90% of V_T. Hence, V_T measurement of [¹¹C]SL25.1¹88 PET is an excellent estimation of MAO-B concentration.     

Assessment of metabolic fluxes in the mouse brain in vivo using 1H-[13C] NMR spectroscopy at 14.1 Tesla

¹³C magnetic resonance spectroscopy (MRS) combined with the administration of ¹³C labeled substrates uniquely allows to measure metabolic fluxes in vivo in the brain of humans and rats. The extension to mouse models may provide exclusive prospect for the investigation of models of human diseases. In the present study, the short-echo-time (TE) full-sensitivity ¹H-[¹³C] MRS sequence combined with high magnetic field (14.1 T) and infusion of [U-¹³C₆] glucose was used to enhance the experimental sensitivity in vivo in the mouse brain and the ¹³C turnover curves of glutamate C4, glutamine C4, glutamate+glutamine C3, aspartate C2, lactate C3, alanine C3, γ-aminobutyric acid C2, C3 and C4 were obtained. A one-compartment model was used to fit ¹³C turnover curves and resulted in values of metabolic fluxes including the tricarboxylic acid (TCA) cycle flux V_TCA (1.05±0.04 μmol/g per minute), the exchange flux between 2-oxoglutarate and glutamate V_x (0.48±0.02 μmol/g per minute), the glutamate-glutamine exchange rate V_gln (0.20±0.02 μmol/g per minute), the pyruvate dilution factor K_dil (0.82±0.01), and the ratio for the lactate conversion rate and the alanine conversion rate V_Lac/V_Ala (10±2). This study opens the prospect of studying transgenic mouse models of brain pathologies.     

Beta1-adrenergic receptor-mediated dilation of rat cerebral artery requires Shaker-type KV1 channels on PSD95 scaffold

Postsynaptic density-95 (PSD95) is a scaffolding protein in cerebral vascular smooth muscle cells (cVSMCs), which binds to Shaker-type K⁺ (K_V1) channels and facilitates channel opening through phosphorylation by protein kinase A. β1-Adrenergic receptors (β1ARs) also have a binding motif for PSD95. Functional association of β1AR with K_V1 channels through PSD95 may represent a novel vasodilator complex in cerebral arteries (CA). We explored whether a β1AR-PSD95-K_V1 complex is a determinant of rat CA dilation. RT-PCR and western blots revealed expression of β1AR in CA. Isoproterenol induced a concentration-dependent dilation of isolated, pressurized rat CA that was blocked by the β1AR blocker {"type":"entrez-protein","attrs":{"text":"CGP20712","term_id":"874704353","term_text":"CGP20712"}}CGP20712. Cranial window imaging of middle cerebral arterioles in situ showed isoproterenol- and norepinephrine-induced dilation that was blunted by β1AR blockade. Isoproterenol-induced hyperpolarization of cVSMCs in pressurized CA was blocked by {"type":"entrez-protein","attrs":{"text":"CGP20712","term_id":"874704353","term_text":"CGP20712"}}CGP20712. Confocal images of cVSMCs immunostained with antibodies against β1AR and PSD95 indicated strong colocalization, and PSD95 co-immunoprecipitated with β1AR in CA lysate. Blockade of K_V1 channels, β1AR or disruption of PSD95-K_V1 interaction produced similar blunting of isoproterenol-induced dilation in pressurized CA. These findings suggest that PSD95 mediates a vasodilator complex with β1AR and K_V1 channels in cVSMCs. This complex may be critical for proper vasodilation in rat CA.     

mGluR5 and GABAA receptor‐specific parametric PET atlas construction—PET/MR data processing pipeline,validation, and application

The glutamate and γ‐aminobutyric acid neuroreceptor subtypes mGluR₅ and GABA_A are hypothesized to be involved in the development of a variety of psychiatric diseases. However, detailed information relating to their in vivo distribution is generally unavailable. Maps of such distributions could potentially aid clinical studies by providing a reference for the normal distribution of neuroreceptors and may also be useful as covariates in advanced functional magnetic resonance imaging (MR) studies. In this study, we propose a comprehensive processing pipeline for the construction of standard space, in vivo distributions of non‐displaceable binding potential (BP _ND), and total distribution volume (V _T) based on simultaneously acquired bolus‐infusion positron emission tomography (PET) and MR data. The pipeline was applied to [¹¹C]ABP688‐PET/MR (13 healthy male non‐smokers, 26.6 ± 7.0 years) and [¹¹C]Flumazenil‐PET/MR (10 healthy males, 25.8 ± 3.0 years) data. Activity concentration templates, as well as V _T and BP _ND atlases of mGluR₅ and GABA_A, were generated from these data. The maps were validated by assessing the percent error δ from warped space to native space in a selection of brain regions. We verified that the average δ_ABP = 3.0 ± 1.0% and δ_FMZ = 3.8 ± 1.4% were lower than the expected variabilities σ of the tracers (σ_ABP = 4.0%–16.0%, σ_FMZ = 3.9%–9.5%). An evaluation of PET‐to‐PET registrations based on the new maps showed higher registration accuracy compared to registrations based on the commonly used [¹⁵O]H₂O‐template distributed with SPM12. Thus, we conclude that the resulting maps can be used for further research and the proposed pipeline is a viable tool for the construction of standardized PET data distributions.     

Stroke penumbra defined by an MRI-based oxygen challenge technique: 2. Validation based on the consequences of reperfusion

Magnetic resonance imaging (MRI) with oxygen challenge (T₂^* OC) uses oxygen as a metabolic biotracer to define penumbral tissue based on CMRO₂ and oxygen extraction fraction. Penumbra displays a greater T₂^* signal change during OC than surrounding tissue. Since timely restoration of cerebral blood flow (CBF) should salvage penumbra, T₂^* OC was tested by examining the consequences of reperfusion on T₂^* OC-defined penumbra. Transient ischemia (109±20 minutes) was induced in male Sprague-Dawley rats (n=8). Penumbra was identified on T₂^*-weighted MRI during OC. Ischemia and ischemic injury were identified on CBF and apparent diffusion coefficient maps, respectively. Reperfusion was induced and scans repeated. T₂ for final infarct and T₂^* OC were run on day 7. T₂^* signal increase to OC was 3.4% in contralateral cortex and caudate nucleus and was unaffected by reperfusion. In OC-defined penumbra, T₂^* signal increased by 8.4%±4.1% during ischemia and returned to 3.25%±0.8% following reperfusion. Ischemic core T₂^* signal increase was 0.39%±0.47% during ischemia and 0.84%±1.8% on reperfusion. Penumbral CBF increased from 41.94±13 to 116.5±25 mL per 100 g per minute on reperfusion. On day 7, OC-defined penumbra gave a normal OC response and was located outside the infarct. T₂^* OC-defined penumbra recovered when CBF was restored, providing further validation of the utility of T₂^* OC for acute stroke management.     

Within-subject comparison of [11C]-(+)-PHNO and [11C]raclopride sensitivity to acute amphetamine challenge in healthy humans

[¹¹C]PHNO is a D₂/D₃ agonist positron emission tomography radiotracer, with higher in vivo affinity for D₃ than for D₂ receptors. As [¹¹C]-(+)-PHNO is an agonist, its in vivo binding is expected to be more affected by acute fluctuations in synaptic dopamine than that of antagonist radiotracers such as [¹¹C]raclopride. In this study, the authors compared the effects of an oral dose of the dopamine releaser amphetamine (0.3 mg/kg) on in vivo binding of [¹¹C]-(+)-PHNO and [¹¹C]raclopride in healthy subjects, using a within-subjects, counterbalanced, open-label design. In the dorsal striatum, where the density of D₃ receptors is negligible and both tracers predominantly bind to D₂ receptors, the reduction of [¹¹C]-(+)-PHNO binding potential (BP_ND) was 1.5 times larger than that of [¹¹C]raclopride. The gain in sensitivity associated with the agonist [¹¹C]-(+)-PHNO implies that ∼65% of D₂ receptors are in the high-affinity state in vivo. In extrastriatal regions, where [¹¹C]-(+)-PHNO predominantly binds to D₃ receptors, the amphetamine effect on [¹¹C]-(+)-PHNO BP_ND was even larger, consistent with the higher affinity of dopamine for D₃. This study indicates that [¹¹C]-(+)-PHNO is superior to [¹¹C]raclopride for studying acute fluctuations in synaptic dopamine in the human striatum. [¹¹C]-(+)-PHNO also enables measurement of synaptic dopamine in D₃ regions.     

In-vivo measurement of LDOPA uptake,dopamine reserve and turnover in the rat brain using [18F]FDOPA PET

Longitudinal measurements of dopamine (DA) uptake and turnover in transgenic rodents may be critical when developing disease-modifying therapies for Parkinson''s disease (PD). We demonstrate methodology for such measurements using [¹⁸F]fluoro-3,4-dihydroxyphenyl-L-alanine ([¹⁸F]FDOPA) positron emission tomography (PET). The method was applied to 6-hydroxydopamine lesioned rats, providing the first PET-derived estimates of DA turnover for this species. Control (n=4) and unilaterally lesioned (n=11) rats were imaged multiple times. Kinetic modeling was performed using extended Patlak, incorporating a k_loss term for metabolite washout, and modified Logan methods. Dopaminergic terminal loss was measured via [¹¹C]-(+)-dihydrotetrabenazine (DTBZ) PET. Clear striatal [¹⁸F]FDOPA uptake was observed. In the lesioned striatum the effective DA turnover increased, shown by a reduced effective distribution volume ratio (EDVR) for [¹⁸F]FDOPA. Effective distribution volume ratio correlated (r>0.9) with the [¹¹C]DTBZ binding potential (BP_ND). The uptake and trapping rate (k_ref) decreased after lesioning, but relatively less so than [¹¹C]DTBZ BP_ND. For normal controls, striatal estimates were k_ref=0.037±0.005 per minute, EDVR=1.07±0.22 and k_loss=0.024±0.003 per minute (30 minutes turnover half-time), with repeatability (coefficient of variation) ≤11%. [¹⁸F]fluoro-3,4-dihydroxyphenyl-L-alanine PET enables measurements of DA turnover in the rat, which is useful for developing novel therapies for PD.     

A graphical method to compare the in vivo binding potential of PET radioligands in the absence of a reference region: application to [11C]PBR28 and [18F]PBR111 for TSPO imaging

Positron emission tomography (PET) radioligands for a reversible central nervous system (CNS) demand a high specific to nonspecific signal characterized by the binding potential (BP_ND). The quantification of BP_ND requires the determination of the nondisplaceable binding usually derived from a reference region devoid of the target of interest. However, for many CNS targets, there is no valid reference region available. In such cases, the total volume of distribution (V_T) is often used as the outcome measure, which includes both the specific and nonspecific binding signals. Here we present a graphical method that allows for direct comparison of the binding potential of ligands using the regional V_T data alone via linear regression. The method was first validated using literature data for five serotonin transporter ligands, for which a reference region exists, and then applied to two second generation 18 kDa translocator protein radioligands, namely [¹¹C]PBR28 and [¹⁸F]PBR111. The analysis determined that [¹¹C]PBR28 had a higher BP_ND than [¹⁸F]PBR111.     

Arterial input function derived from pairwise correlations between PET-image voxels

A metabolite corrected arterial input function is a prerequisite for quantification of positron emission tomography (PET) data by compartmental analysis. This quantitative approach is also necessary for radioligands without suitable reference regions in brain. The measurement is laborious and requires cannulation of a peripheral artery, a procedure that can be associated with patient discomfort and potential adverse events. A non invasive procedure for obtaining the arterial input function is thus preferable. In this study, we present a novel method to obtain image-derived input functions (IDIFs). The method is based on calculation of the Pearson correlation coefficient between the time-activity curves of voxel pairs in the PET image to localize voxels displaying blood-like behavior. The method was evaluated using data obtained in human studies with the radioligands [¹¹C]flumazenil and [¹¹C]AZ10419369, and its performance was compared with three previously published methods. The distribution volumes (V_T) obtained using IDIFs were compared with those obtained using traditional arterial measurements. Overall, the agreement in V_T was good (∼3% difference) for input functions obtained using the pairwise correlation approach. This approach performed similarly or even better than the other methods, and could be considered in applied clinical studies. Applications to other radioligands are needed for further verification.     

Quantitative imaging of neuroinflammation in human white matter: A positron emission tomography study with translocator protein 18 kDa radioligand, [18F]‐FEPPA

The ability to quantify translocator protein 18 kDa (TSPO) in white matter (WM) is important to understand the role of neuroinflammation in neurological disorders with WM involvement. This article aims to extend the utility of TSPO imaging in WM using a second‐generation radioligand, [¹⁸F]‐FEPPA, and high‐resolution research tomograph (HRRT) positron emission tomography (PET) camera system. Four WM regions of interests (WM‐ROI), relevant to the study of aging and neuroinflammatory diseases, were examined. The corpus callosum, cingulum bundle, superior longitudinal fasciculus, and posterior limb of internal capsule were delineated automatically onto subject's T₁‐weighted magnetic resonance image using a diffusion tensor imaging‐based WM template. The TSPO polymorphism (rs6971) stratified individuals to three genetic groups: high‐affinity binders (HAB), mixed‐affinity binders (MAB), and low‐affinity binders. [¹⁸F]‐FEPPA PET scans were acquired on 32 healthy subjects and analyzed using a full kinetic compartment analysis. The two‐tissue compartment model showed moderate identifiability (coefficient of variation 15–19%) for [¹⁸F]‐FEPPA total volume distribution (V_T) in WM‐ROIs. Noise affects V_T variability, although its effect on bias was small (6%). In a worst‐case scenario, ≤6% of simulated data did not fit reliably. A simulation of increased TSPO density exposed minimal effect on variability and identifiability of [¹⁸F]‐FEPPA V_T in WM‐ROIs. We found no association between age and [¹⁸F]‐FEPPA V_T in WM‐ROIs. The V_T values were 15% higher in HAB than in MAB, although the difference was not statistically significant. This study provides evidence for the utility and limitations of [¹⁸F]‐FEPPA PET to measure TSPO expression in WM. Synapse 68:536–547, 2014 . © 2014 Wiley Periodicals, Inc.     

Connexin channels provide a target to manipulate brain endothelial calcium dynamics and blood�Cbrain barrier permeability

The cytoplasmic Ca²⁺ concentration ([Ca²⁺]_i) is an important factor determining the functional state of blood–brain barrier (BBB) endothelial cells but little is known on the effect of dynamic [Ca²⁺]_i changes on BBB function. We applied different agonists that trigger [Ca²⁺]_i oscillations and determined the involvement of connexin channels and subsequent effects on endothelial permeability in immortalized and primary brain endothelial cells. The inflammatory peptide bradykinin (BK) triggered [Ca²⁺]_i oscillations and increased endothelial permeability. The latter was prevented by buffering [Ca²⁺]_i with BAPTA, indicating that [Ca²⁺]_i oscillations are crucial in the permeability changes. Bradykinin-triggered [Ca²⁺]_i oscillations were inhibited by interfering with connexin channels, making use of carbenoxolone, Gap27, a peptide blocker of connexin channels, and Cx37/43 knockdown. Gap27 inhibition of the oscillations was rapid (within minutes) and work with connexin hemichannel-permeable dyes indicated hemichannel opening and purinergic signaling in response to stimulation with BK. Moreover, Gap27 inhibited the BK-triggered endothelial permeability increase in in vitro and in vivo experiments. By contrast, [Ca²⁺]_i oscillations provoked by exposure to adenosine 5′ triphosphate (ATP) were not affected by carbenoxolone or Gap27 and ATP did not disturb endothelial permeability. We conclude that interfering with endothelial connexin hemichannels is a novel approach to limiting BBB-permeability alterations.     

Determination of [11C]PBR28 binding potential in vivo: a first human TSPO blocking study

Positron emission tomography (PET) targeting the 18 kDa translocator protein (TSPO) is used to quantify neuroinflammation. Translocator protein is expressed throughout the brain, and therefore a classical reference region approach cannot be used to estimate binding potential (BP_ND). Here, we used blockade of the TSPO radioligand [¹¹C]PBR28 with the TSPO ligand XBD173, to determine the non-displaceable volume of distribution (V_ND), and hence estimate the BP_ND. A total of 26 healthy volunteers, 16 high-affinity binders (HABs) and 10 mixed affinity binders (MABs) underwent a [¹¹C]PBR28 PET scan with arterial sampling. Six of the HABs received oral XBD173 (10 to 90 mg), 2 hours before a repeat scan. In XBD173-dosed subjects, V_ND was estimated via the occupancy plot. Values of BP_ND for all subjects were calculated using this V_ND estimate. Total volume of distribution (V_T) of MABs (2.94±0.31) was lower than V_T of HABs (4.33±0.29) (P<0.005). There was dose-dependent occupancy of TSPO by XBD173 (ED50=0.34±0.13 mg/kg). The occupancy plot provided a V_ND estimate of 1.98 (1.69, 2.26). Based on these V_ND estimates, BP_ND for HABs is approximately twice that of MABs, consistent with predictions from in vitro data. Our estimates of [¹¹C]PBR28 V_ND and hence BP_ND in the healthy human brain are consistent with in vitro predictions. XBD173 blockade provides a practical means of estimating V_ND for TSPO targeting radioligands.