Extracellular Glucose Concentrations in the Rat Hippocampus Measured byZero‐Net‐Flux

: The concentration of glucose in the brain's extracellularfluid remains controversial, with recent estimates and measurements rangingfrom 0.35 to 3.3 m M . In the present experiments, we used the methodof zero‐net‐flux microdialysis to determine glucose concentration in thehippocampal extracellular fluid of awake, freely moving rats. In addition, thepoint of zero‐net‐flux was measured across variations in flow rate to confirmthat the results for glucose measurement were robust to such variations. In3‐month‐old male Sprague‐Dawley rats, the concentration of glucose in thehippocampal extracellular fluid was found to be 1.00 ± 0.05m M , which did not vary with changes in flow rate. Three‐month‐old and24‐month‐old Fischer‐344 rats both showed a significantly higher hippocampalextracellular fluid glucose concentration, at 1.24 ± 0.07 and 1.21± 0.04 m M , respectively ; there was no significant differencebetween the two age groups. The present data demonstrate variation inextracellular brain glucose concentration between rat strains. When takentogether with previous data showing a striatal extracellular glucoseconcentration on the order of 0.5 m M , the data also demonstratevariation in extracellular glucose between brain regions. Traditional modelsof brain glucose transport and distribution, in which extracellularconcentration is assumed to be constant, may require revision.