Determination of Oxidative Glucose Metabolism In Vivo in the Young Rat Brain Using Localized Direct-Detected 13C NMR Spectroscopy

Determination of oxidative metabolism in the brain using in vivo C-13 NMR spectroscopy (C-13 MRS) typically requires repeated blood sampling throughout the study to measure blood glucose concentration and fractional enrichment (input function). However, drawing blood from small animals, such as young rats, placed deep inside the magnet is technically difficult due to their small total blood volume. In the present study, a custom-built animal holder enabled temporary removal of the animal from the magnet for blood collection, followed by accurate repositioning in the exact presampling position without degradation of B-0 shimming. C-13 label incorporation into glutamate C4 and C3 positions during a 120 min [1,6-C-13(2)] glucose infusion was determined in 28-day-old rats (n = 4) under alpha-chloralose sedation using localized, direct-detected in vivo C-13 MRS at 9.4T. The tricarboxylic acid cycle activity rate (V-TCA) determined using a one-compartment metabolic modeling was 0.67 +/- 0.13 mu mol/g/min, a value comparable to previous ex vivo studies. This methodology opens the avenue for in vivo measurements of brain metabolic rates using C-13 MRS in small animals.