The effects of serotonin depletion on the voltammetric response to amphetamine

On the significance of brain extracellular uric acid detected with in-vivo monitoring techniques: a review

The concentration of uric acid [UA] in the extracellular fluid (ECF) estimated with in-vivo voltammetry and microdialysis data is compared for probes of different diameters from the day of implantation (acute) to several days (chronic) or even months after surgery. For small probes (diameter < 160 microns) the acute [UA] of ca. 5 microM decreased significantly to ca. 1 microM under chronic conditions. For larger probes (e.g., 320-microns diameter) the acute [UA] was also ca. 5 microM, but this value significantly increased to ca. 50 microM under chronic conditions. Associated with this difference in [UA], there were parallel differences in the extent of gliosis around the probes. These findings are discussed in terms of possible sources of extracellular UA and their implications for in-vivo monitoring techniques in behaving animals.

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.

Anomalously high concentrations of brain extracellular uric acid detected with chronically implanted probes: implications for in vivo sampling techniques

The height of peak 2, h2, recorded using linear sweep voltammetry with 350-micron-diameter carbon paste electrodes in rat striatum was measured from the day of implantation (day 0) to 4 months after surgery. The value of h2 was at a minimum on day 0 (0.6 +/- 0.2 nA; n = 20), rose sharply to a maximum on day 2 (6.3 +/- 0.9 nA; n = 12), and decreased to a stable level by day 7 (3.3 +/- 0.7 nA; n = 16), which lasted for 4 months (3.2 +/- 0.6 nA; n = 9). These changes were shown by microinfusion of uricase to be due to variations in the concentrations of extracellular uric acid, although h2 appears to have a small baseline contribution of approximately 0.3 nA from 5-hydroxyindoleacetic acid. The stable value of h2 recorded under chronic conditions was estimated to correspond to a minimal uric acid concentration of 50 mumol/L, which represents a 10-fold increase in the extracellular level of this purine metabolite compared with the initial (acute) value. Very similar results were obtained using a microdialysis technique that detected uric acid directly. These estimates of striatal uric acid concentration are in marked contrast to those obtained using 40-micron diameter carbon fiber electrodes, which showed a decrease from the acute preparation to less than 1 mumol/L under chronic conditions. Large values of h2 were also recorded with chronically implanted paste electrodes in the hippocampus and frontal cortex.(ABSTRACT TRUNCATED AT 250 WORDS)

Uric acid levels and dopamine transmission in rat striatum: diurnal changes and effects of drugs

Carbon paste disc electrodes were used to detect voltammetrically changes in the extracellular concentration of the purine metabolite, uric acid, and the dopamine metabolite, homovanillic acid (HVA), in the striatum of unanaesthetized, unrestrained rats under a variety of conditions. The motor activity level for each rat was recorded between the electrochemical scans. In totally unperturbed animals, there was a significant correlation between the levels of the two metabolites during the bright, relatively inactive, period of the diurnal cycle. During much of the dark (active) phase of the cycle, however, the uric acid signal showed no significant change compared with the light-on period, in contrast to the HVA signal which showed a marked increase. Significant variations in the concentration of striatal uric acid were observed during the switch-over from light to dark and dark to light conditions. The unilateral infusion of gamma-aminobutyric acid, taurine and haloperidol into the substantia nigra caused increases in the height of both the uric acid and HVA peak in the ipsilateral striatum; the size of these changes showed a significant correlation. Variable changes occurred on the contralateral side where no correlation was observed. Intraperitoneal administration of the mixed dopamine-receptor agonist, apomorphine, and the mixed antagonist, haloperidol, did not affect striatal uric acid levels significantly. These results suggest that, although there are conditions where parallel changes in dopamine release/receptor-activation and uric acid levels do occur in the striatum, neither the release of dopamine nor activation of dopamine receptors need necessarily lead to changes in the extracellular concentration of uric acid.

The effects of haloperidol and amphetamine on ascorbic acid and uric acid in caudate and nucleus accumbens of rats as measured by voltammetry in vivo

The ability of haloperidol (0.1 mg/kg) to reduce the amphetamine-induced (2 and 5 mg/kg) increase in ascorbic and uric acid in anterior caudate and in nucleus accumbens was tested using voltammetry in vivo. In both areas, haloperidol reduced the amphetamine-induced increase in uric acid. In both areas, haloperidol only marginally affected the amphetamine-induced increase in ascorbic acid. Amphetamine-induced increases in uric acid were more nearly dose-related than changes in ascorbic acid. Of the two compounds, uric acid seems more likely to be associated with dopamine.

Corticostriatal and thalamic regulation of amphetamine-induced ascorbate release in the neostriatum

Lesions of cerebral cortex and ventromedial nucleus (VM) of the thalamus were made in rats to investigate the contribution of these structures to amphetamine (AMPH)-induced ascorbate (AA) release in the neostriatum as measured by in vivo voltammetry. Following a recovery period of at least one week, rats were anesthetized, and electrochemically modified, carbon-fiber electrodes were lowered into the neostriatum. Compared to data obtained from sham-operated and unoperated controls, bilateral aspiration lesions of cerebral cortex significantly lowered both the basal level of AA and the amount of AA released by AMPH in the neostriatum. Similar results were obtained after bilateral, but not unilateral electrolytic lesions of the VM thalamus. Collectively, these results suggest that the corticostriatal pathway and the VM thalamic nuclei participate in the regulation of basal and AMPH-induced AA release in the neostriatum.