Effect of probe size on the concentration of brain extracellular uric acid monitored with carbon paste electrodes

We have investigated further the anomalously high concentration of brain extracellular uric acid detected with in vivo sampling probes reported recently. The contribution by uric acid and 5-hydroxyindoleacetic acid (5-HIAA) to peak 2 recorded in rat striatum with chronically implanted carbon paste electrodes (CPEs) of different sizes was estimated by comparing peak current densities and the effect of the monoamine oxidase inhibitor pargyline. The concentration of uric acid in the extracellular fluid was some 50 times greater for 320-microns-diameter CPEs than for 160-microns-diameter electrodes, where the urate level was estimated at approximately 1 microM. The concentration of 5-HIAA was similar for 320-, 260-, and 160-microns-diameter CPEs. These data provide an explanation for the previously observed differences in 5-HIAA/urate ratios recorded with 320-microns-diameter CPEs and smaller carbon fibre electrodes. The results also indicate that chronically implanted sampling probes of diameter > 160 microns perturb the surrounding tissue, which produces uric acid by a mechanism yet unknown, although preliminary histological data suggest that glial cells may be involved.