MRS Measurement of Glutamate and GABA Metabolism in Brain

13C is a stable (i.e., non-radioactive) isotope of carbon with a natural abundance of approximately 1%. Following infusion of [13C]glucose and/or [13C]acetate, in vivo MRS (magnetic resonance spectroscopy) can monitor the rate of flux of the 13C atom from glucose and/or acetate to glutamate to glutamine. Thus, this procedure can provide measure of glutamate (GLU) and glutamine (GLN) turnover in brain. We have established parameters to obtain these measurements in nonhuman primate brain. The current protocol seeks approval to optimize MRS parameters and to develop new MRS techniques for human brain using the GE 3T, the Siemens 3T, and the Siemens 7T device.

Study population: All subjects will be aged 18 - 65 years, without serious medical illnesses and meet criteria listed under 'Subjects'.

Design: Subjects will receive either oral administration of [13C]glucose or an intravenous infusion of [13C]glucose and/or [13C]acetate to approximately double their plasma glucose levels. The plasma acetate level will remain within the physiological range observed in humans. While lying in the 3T or 7T device, serial data acquisitions will be obtained over approximately 2 h to optimize the experimental conditions so as to measure the 13C signals from GLU, GLN and other metabolisms in brain.

Outcome measures: The primary goal of this study is to measure GLU/GLN turnover in brain. With no additional data acquisition, we can also obtain information on the synthesis of GABA, the major inhibitory neurotransmitter in brain. GLU is converted to GABA via the enzyme glutamic acid decarboxylase (GAD). While monitoring the transfer of 13C signal from GLU to GLN, we can simultaneously measure the transfer of 13C signal from GLU to GABA and thereby measure the activity of GAD. In addition to directly measure 13C signals, 13C labeling to brain metabolites can also be measured indirectly by detecting proton MRS during infusion of [13C]glucose and/or [13C]acetate.