Cellular Origin of [18F]FDG-PET Imaging Signals During Ceftriaxone-Stimulated Glutamate Uptake: Astrocytes and Neurons.

Ceftriaxone stimulates astrocytic uptake of the excitatory neurotransmitter glutamate, and it is used to treat glutamatergic excitotoxicity that becomes manifest during many brain diseases. Ceftriaxone-stimulated glutamate transport was reported to drive signals underlying [18F]fluorodeoxyglucose-positron emission tomographic ([18F]FDG-PET) metabolic images of brain glucose utilization and interpreted as supportive of the notion of lactate shuttling from astrocytes to neurons. This study draws attention to critical roles of astrocytes in the energetics and imaging of brain activity, but the results are provocative because (1) the method does not have cellular resolution or provide information about downstream pathways of glucose metabolism, (2) neuronal and astrocytic [18F]FDG uptake were not separately measured, and (3) strong evidence against lactate shuttling was not discussed. Evaluation of potential metabolic responses to ceftriaxone suggests lack of astrocytic specificity and significant contributions by pre- and postsynaptic neuronal compartments. Indeed, astrocytic glycolysis may not make a strong contribution to the [18F]FDG-PET signal because partial or complete oxidation of one glutamate molecule on its uptake generates enough ATP to fuel uptake of 3 to 10 more glutamate molecules, diminishing reliance on glycolysis. The influence of ceftriaxone on energetics of glutamate-glutamine cycling must be determined in astrocytes and neurons to elucidate its roles in excitotoxicity treatment.