Abstract
The mechanisms responsible for the local increase in brain glucose utilization during functional activation remain unknown. Recent in vitro studies have identified a new signaling pathway involving an activation of glial glutamate transporters and enhancement of neuron-astrocyte metabolic interactions that suggest a putative coupling mechanism. The aim of the present study was to determine whether one of the glutamate transporters exclusively expressed in astrocytes, GLAST, is involved in the neurometabolic coupling in vivo. For this purpose, rats were microinjected into the posteromedial barrel subfield (PMBSF) of the somatosensory cortex with GLAST antisense or random phosphorothioate oligonucleotides. The physiologic activation was performed by stimulating the whisker-to-barrel pathway in anesthetized rats while measuring local cerebral glucose utilization by quantitative autoradiography in the PMBSF. Twenty-four hours after injection of two different antisense GLAST oligonucleotide sequences, and despite the presence of normal whisker-related neuronal activity in the PMBSF, the metabolic response to whisker stimulation was decreased by more than 50%. Injection of the corresponding random sequences still allowed a significant increase in glucose utilization in the activated area. The present study highlights the contribution of astrocytes to neurometabolic coupling in vivo. It provides evidence that glial glutamate transporters are key molecular components of this coupling and that neuronal glutamatergic activity is an important determinant of energy utilization. Results indicate that astrocytes should also be considered as possible sources of altered brain metabolism that could explain the distinct imaging signals observed in some pathologic situations.