Calibration Factors for Cationic and Anionic Neurochemicals at Carbon-Fiber Microelectrodes are Oppositely Affected by the Presence of Ca2+and Mg2+

Abstract

Carbon-fiber microelectrodes and voltammetric methods have been used extensively for detection of neurochemical substances in brain tissue. We previously reported that calibration factors for dopamine obtained with these electrodes and fast-scan cyclic voltammetry are 2–3-fold higher in nonphysiological phosphate-buffered saline (PBS) than in artificial cerebrospinal fluid (ACSF) containing Ca2+and Mg2+. In the present report, we describe the media-dependence of calibration factors for other neurochemicals. Strikingly, whereas electrode sensitivities to dopamine and the indoleamine serotonin were lower in ACSF than in PBS, those for the acid metabolites of these neuro-transmitters were roughly 2-fold higher in ACSF than PBS. The data are consistent with adsorption or repulsion of these molecules by a negatively charged carbon-fiber electrode surface. One consequence of this pattern is that electrodes calibrated in PBS, rather than Ca2+- and Mg2+-containing media, will have a 2–6-fold lower amine-to-metabolite selectivity ratio in brain tissue than expected from in vitro calibration. More generally, the data suggest that the sensitivity of carbon-fiber-based microsensors to many organic molecules will be media-dependent. Thus, calibration in appropriate physiological media is essential for accurate tissue measurements.