Methodological considerations of microdialysis sampling for determination of cerebral basal glucose concentration in extracellular space of freely moving rat

Abstract

In the recent past there has been a controversy about the basal glucose levels in the brain of freely moving rats. Kinetic rate constants and transport parameters across the blood brain barrier and in the brain tissue obtained by using different techniques support a glucose value of 2-4mM. One paper, using microdialysis in awake freely moving animals has reported a value of 0.47 mM which is several fold lower than the currently accepted value.

Microdialysis was used to determine the basal concentration of glucose in the striatum of freely moving rats. While there are several approaches to calibrating brain microdialysis probes that subsequently allow determination of the tissue concentration, the zero net flux method at a fixed perfusion flow rate, was used to make such determinations. The basal glucose concentrations were found to vary with variation in perfusion flow rate. The in vivo recoveries did not equal in vivo deliveries. To probe glucose dynamics in the brain, pharmacological agents such as glucose inhibitors and sodium channel blockers were used. In addition, to determine concentration profiles with the microdialysis probes inserted in tissue, a mathematical model was developed which took into account transport and metabolism of glucose.

In conclusion, the measured basal values of glucose depend on the perfusion flow rate when a zero net flux calibration is performed. The dialysate glucose levels are closely related to neuronal activity. When non linear kinetics dominate in the tissue, in vivo recoveries and deliveries are not equal, at least for endogenous glucose. Increases in in vivo delivery at higher flow rates could be due to factors such as saturable transport of glucose across the blood brain barrier and its subsequent uptake by neuronal cells.