Calibration factors for cationic and anionic neurochemicals at carbon-fiber microelectrodes are oppositely affected by the presence of Ca2+ and Mg2+

Carbon-fiber microelectrodes and voltammetric methods have been used extens ively for detection of neurochemical substances in brain tissue. We previou sly reported that calibration factors for dopamine obtained with these elec trodes and fast-scan cyclic voltammetry are 2-3-fold higher in nonphysiolog ical phosphate-buffered saline (PBS) than in artificial cerebrospinal fluid (ACSF) containing Ca2+ and Mg2+. In the present report, we describe the me dia-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 neu rotransmitters were roughly 2-fold higher in ACSF than PBS. The data are co nsistent with adsorption or repulsion of these molecules by a negatively ch arged carbon-fiber electrode surface. One consequence of this pattern is th at electrodes calibrated in PBS, rather than Ca2+- and Mg2+-containing medi a, will have a 2-6-fold lower amine-to-metabolite selectivity ratio in brai n tissue than expected from in vitro calibration. More generally, the data suggest that the sensitivity of carbon-fiber-based microsensors to many org anic molecules will be media-dependent. Thus, calibration in appropriate ph ysiological media is essential for accurate tissue measurements.