A diurnal rhythm in glucose uptake in brown adipose tissue revealed by in vivo PET-FDG imaging

Abstract

Using a micro-positron emission tomography (PET)/computerized tomography scanner, we have measured (18)F-fluorodeoxyglucose (FDG) uptake in interscapular brown adipose tissue (iBAT) in C57Bl/6 mice at intervals across a 24-h light-dark cycle. Our data reveal a strong 24-h profile of glucose uptake of iBAT, peaking at ~9 h into the light phase of the 12-h light, 12-h dark day. BAT is increasingly gaining attention as being involved in metabolic phenotypes and obesity, where BAT, as observed by PET analysis, negatively correlates with obesity and age. Conversely, animals that show perturbations in circadian clocks, behavior, and physiology show metabolic phenotypes. The observation of a 24-h rhythm in glucose uptake in iBAT makes this tissue a candidate site of interaction between metabolic and circadian systems.

Figure 1. PET imaging reveals time-of-day specific variation in interscapular brown adipose tissue (iBAT) FDG uptake

Panel A shows a representative PET acquired image at ZT 12 of FDG uptake in a sagittal plane. From rostral to caudal, tissues that exhibit high uptake are the injections site (right eye)/brain (dorsal), tongue (ventral), iBAT (dorsal), heart (ventral), paraspinal muscle and bladder. Panel B shows four representative coronal (middle) and transverse (bottom) sections at the level of the iBAT for animals imaged at four alternative times of 24-hour day: ZT 0, 6, 12 and 18. Kidneys can also be observed in these images. Individual values (panel C) and group means (panel D) of the average voxel value in the iBAT are plotted against time of measurement. Values for FDG uptake in those animals that are part of the group of animals that are measured around the dark to light transition (ZT 0/24), but fall in the light phase are plotted both early in the light phase and after the end of the dark phase. Cosinor fitting of the data predicts a peak phase of FDG uptake at ZT 9.3 and a trough at ZT 21.3. **P < 0.01 against ZT 0 and 18. Grey area indicates the dark phase of the 24-hours LD cycle. The data did not show any effects of age, gender, iBAT volume, and body mass on average voxel value FDG uptake (P’s > 0.05).