Effectiveness and Relationship between Biased and Unbiased Measures of Dopamine Release and Clearance

Fast-scan cyclic voltammetry (FSCV) is an effective tool for measuring dopamine release and clearance throughout the brain, especially the striatum where dopamine terminals are abundant and signals are heavily regulated by release machinery and the dopamine transporter (DAT). Peak height measurement is perhaps the most common method for measuring dopamine release, but it is influenced by changes in clearance. Michaelis-Menten-based modeling has been a standard in measuring dopamine clearance, but it is problematic in that it requires experimenter fitted modeling subject to experimenter bias. This study presents the use of the first derivative (velocity) of evoked dopamine signals as an alternative approach for measuring and distinguishing dopamine release from clearance. Maximal upward velocity predicts reductions in dopamine peak height due to D₂ and GABA_B receptor stimulation and by alterations in calcium concentrations. The Michaelis-Menten maximal velocity (V_max) measure, an approximation for DAT levels, predicts maximal downward velocity in slices and in vivo. Dopamine peak height and upward velocity were similar between wild-type and DAT knock-out (DATKO) mice. In contrast, downward velocity was lower and exponential decay (tau) was higher in DATKO mice, supporting the use of both measures for extreme changes in DAT activity. In slices, the competitive DAT inhibitors cocaine, PTT, and WF23 increased peak height and upward velocity differentially across increasing concentrations, with PTT and cocaine reducing these measures at high concentrations. Downward velocity and tau values decreased and increased respectively across concentrations, with greater potency and efficacy observed with WF23 and PTT. In vivo recordings demonstrated similar effects of WF23, PTT, and cocaine on measures of release and clearance. Tau was a more sensitive measure at low concentrations, supporting its use as a surrogate for the Michaelis-Menten measure of apparent affinity (K_m). Together, these results inform on the use of these various measures for dopamine release and clearance.