25 IN VIVO MAGNETIC RESONANCE SPECTROSCOPY STUDIES OF THE GLUTAMATE AND GABA NEUROTRANSMITTER CYCLES AND FUNCTIONAL NEUROENERGETICS

In the last 5 years there has been a renewed interest in the role of metabolism in supporting brain function. Much of this interest is based on the development of functional positron emission tomography (PET) and magnetic resonance imaging (MRI). Although often incorrectly described as directly mapping neuronal activity, both functional PET and MRI actually measure changes in either glucose metabolism or physiologic parameters coupled to glucose metabolism such as blood flow and volume (1). A major limitation in interpreting functional imaging is that the relationship between neuronal activity and the neuroenergetic processes supported by glucose metabolism is poorly defined (2,3). The term neuronal activity applies to a spectrum of energyrequiring processes including action potential propagation, neurotransmitter release and uptake, vesicular recycling, and maintenance of membrane potentials (4). All of these processes are involved in short-term neuronal information transfer, and the relative distribution of energy among them remains an open question. There is also uncertainty as to how the different classes of neurons in a region contribute to the overall energy consumption. While an increase in the imaging signal is usually assigned to an increase in neuronal excitation, this interpretation is confounded by both inhibi-