Factors affecting in vivo electrochemistry: electrode-tissue interaction and the ascorbate amplification effect.

While in vivo electrochemistry has been shown to be useful for discovering new neurophysiological phenomena, there is still considerable controversy about the identity of the compounds being measured and the concentration of those compounds in extracellular fluid in brain. We have found that carbon paste electrodes undergo changes in sensitivity and specificity for dopamine and other compounds after being implanted in brain. We have also examined the effect of ascorbate on the selective enhancement of catecholamine peaks to provide an explanation for the apparently very high concentrations of dopamine measured in the extracellular fluid space. After temporary brain implantation (20 min), carbon paste electrodes tested in vitro showed increased sensitivity and lower oxidation potentials for dopamine, norepinephrine and serotonin. These brain-treated electrodes also detected 3,4-dihydroxyphenylacetic acid (DOPAC) as a distinct peak at +0.16 V, although the electrode sensitivity for DOPAC was some 25 times lower than that for dopamine. Brain treatment did not alter electrode sensitivity or oxidation potential for 5-HIAA. The oxidation current for ascorbic acid when processed as the semiderivative showed no distinct peak in the potential range -0.2 to +0.4V for either untreated or brain-treated electrodes. However ascorbic acid amplified the electrochemical peaks of catechols in direct proportion to the ratio of the concentration of ascorbate to the concentration of the catechol. In the physiologic concentration range of 300 microM ascorbate, the electrochemical signal for 1 microM dopamine was amplified 4250%. While ascorbate amplification improves detectability of dopamine and norepinephrine, it also introduces ambiguity since changing catechol concentrations cannot be distinguished from changing ascorbate concentrations.