Mecanismes cellulaires du metabolisme energetique cerebral: Implications pour l'imagerie fonctionnelle

Signals detected with functional brain imaging techniques are based on the coupling of neuronal activity with energy metabolism. Techniques such as position emission tomography (PET) and functional magnetic resonance imaging (fMRI) allow the visualization of brain areas that are activated by a variety of sensory, motor or cognitive tasks. Despite the technological sophistication of these brain imaging techniques, the precise mechanisms and cell types involved in coupling and in generating metabolic signals are still debated. Recent experimental data on the cellular and molecular mechanisms that underlie the fluorodeoxyglucose (FDG)-based PET imaging point to a critical role of a particular brain cell type, the astrocytes, in coupling neuronal activity to glucose utilization. Indeed, astrocytes possess receptors and reuptake sites for a variety of neurotransmitters, including glutamate, the predominant excitatory neurotransmitter in the brain. In addition, astrocytic end-feet, which surround capillaries, are enriched in the specific glucose transporter GLUT-1. These features allow astrocytes to 'sense' synaptic activity and to couple it with energy metabolism. In vivo and in vitro data support the following functional model: in response to glutamate released by active neurons, glucose is predominantly taken up by astrocytic end-feet; glucose is then metabolized to lactate which provides a preferred energy substrate for neurons. These data support the notion that astrocytes markedly contribute to the FDG-PET signal.