Simultaneous monitoring of 3,4-dihydroxyphenylacetic acid (DOPAC) and 5-hydroxyindoleacetic acid (5-HIAA) levels in the brains of freely moving rats by differential pulse voltammetry technique.

Differential pulse voltammetry with a newly devised carbon fiber electrode was used to study the nature of striatal electrochemical signals. Voltammograms recorded from the striatum of unanesthetized rats usually yielded the combined oxidation peak (1 + 2) and peak 3. Peaks 1 and 2 could be separated by eliminating peak 1 for ascorbate by electrochemical oxidation in the brain to allow clear monitoring of peak 2 at + 120 mV for catechols and peak 3 at + 270 mV for indoles. The changes in the oxidation potentials and the amplitudes of peaks 2 and 3 corresponded to those of 3,4-dihydroxyphenylacetic acid (DOPAC) and 5-hydroxyindoleacetic acid (5-HIAA) in vivo because: the oxidation potentials of peak 2 (+ 120 mV) and peak 3 (+ 270 mV) coincided with those of DOPAC and 5-HIAA in vitro; increases in the heights of peaks 2 and 3 were observed after micro-infusion of DOPAC and 5-HIAA, respectively, into the striatum; and peak 2 height increased after injection of haloperidol and gamma-butyrolactone and decreased after amphetamine and pargyline, while peak 3 amplitude increased following injection of gamma-butyrolactone, probenecid and 5-hydroxytryptophan and decreased after pargyline. Thus, the in vivo voltammetry method enabled simultaneous and stable monitoring of the dynamic changes in DOPAC and 5-HIAA levels in the brains of freely moving rats.