The primary objective of this study was to verify the suitability of reference tissue-based quantification methods of the metabotropic glutamate receptor type 5 (mGluR(5)) with [(11)C]ABP688. This study presents in vivo (Positron Emission Tomography (PET)) and in vitro (autoradiography) measurements of mGluR(5) densities in the same rats and evaluates both noninvasive and blood-dependent pharmacokinetic models for the quantification of [(11)C]ABP688 binding. Eleven rats underwent [(11)C]ABP688 PET scans. In five animals, baseline scans were compared with blockade experiments with the antagonist 1,2-methyl-6-(phenylethynyl)-pyridine (MPEP), and arterial blood samples were drawn and corrected for metabolites. Afterward, saturation-binding autoradiography was performed. Blocking with MPEP resulted in an average decrease of the total distribution volume (V(T)) between 43% and 58% (thalamus and caudate-putamen, respectively) but had no significant effect on cerebellar V(T) (mean reduction: -0.01%). Comparing binding potential (BP(ND)) based on the V(T) with noninvasively determined BP(ND) revealed an average negative bias of 0.7% in the caudate-putamen and an average positive bias of 3.1% in the low-binding regions. Scan duration of 50 minutes is required. The cerebellum is a suitable reference region for the quantification of mGluR(5) availability as measured with [(11)C]ABP688 PET in rats. Blood-based and reference region-based PET quantification shows a significant linear relationship to autoradiographic determinations.

The present results highlight complexities underlying brain adaptations during the aging process, and support the notion that certain aspects of neurotransmission remain stable during the adult life span.

BackgroundThe hypothetical model of dynamic biomarkers for Alzheimer’s disease (AD) describes high amyloid deposition and hypometabolism at the mild cognitive impairment (MCI) stage. However, it remains unknown whether brain amyloidosis and hypometabolism follow the same trajectories in MCI individuals. We used the concept of early MCI (EMCI) and late MCI (LMCI) as defined by the Alzheimer’s disease Neuroimaging Initiative (ADNI)-Go in order to compare the biomarker profile between EMCI and LMCI.ObjectivesTo examine the global and voxel-based neocortical amyloid burden and metabolism among individuals who are cognitively normal (CN), as well as those with EMCI, LMCI and mild AD.MethodsIn the present study, 354 participants, including CN (n = 109), EMCI (n = 157), LMCI (n = 39) and AD (n = 49), were enrolled between September 2009 and November 2011 through ADNI-GO and ADNI-2. Brain amyloid load and metabolism were estimated using [18F]AV45 and [18F]fluorodeoxyglucose ([18F]FDG) PET, respectively. Uptake ratio images of [18F]AV45 and [18F]FDG were calculated by dividing the summed PET image by the median counts of the grey matter of the cerebellum and pons, respectively. Group differences of global [18F]AV45 and [18F]FDG were analyzed using ANOVA, while the voxel-based group differences were estimated using statistic parametric mapping (SPM).ResultsEMCI patients showed higher global [18F]AV45 retention compared to CN and lower uptake compared to LMCI. SPM detected higher [18F]AV45 uptake in EMCI compared to CN in the precuneus, posterior cingulate, medial and dorsal lateral prefrontal cortices, bilaterally. EMCI showed lower [18F]AV45 retention than LMCI in the superior temporal, inferior parietal, as well as dorsal lateral prefrontal cortices, bilaterally. Regarding to the global [18F]FDG, EMCI patients showed no significant difference from CN and a higher uptake ratio compared to LMCI. At the voxel level, EMCI showed higher metabolism in precuneus, hippocampus, entorhinal and inferior parietal cortices, as compared to LMCI.ConclusionsThe present results indicate that brain metabolism remains normal despite the presence of significant amyloid accumulation in EMCI. These results suggest a role for anti-amyloid interventions in EMCI aiming to delay or halt the deposition of amyloid and related metabolism impairment.