IN-VIVO DETERMINATION OF EXTRACELLULAR BRAIN ASCORBATE

A quantitative method consisting of the modification of the zero net f lux technique was used to determine the extracellular concentration of ascorbate. A carbon paste electrode (CPE) held at constant potential was combined with a microdialysis probe through which various concentr ations of ascorbate were locally infused. In experiments with bilatera l CPEs, one of which was combined with a dialysis probe, no difference was observed in either basal or stimulated ascorbate currents. Additi on of different concentrations of ascorbate to the perfusion fluid pro duced rapid changes in the voltammetric current due to ascorbate oxida tion. Irrespective of the concentration of ascorbate infused, the curr ent returned to basal level when the perfusion was stopped. The return was found to be more rapid after infusion of concentrations of ascorb ate higher than the exracellular concentration than after perfusion wi th ascorbate-free solutions. This demonstrates that homeostatsis of as corbate occurs in vivo. The calculated point of zero net flux was 416/-66 mu M (n = 6). This represents the extracellular concentration of ascorbate.