Kinetic analysis of striatal clearance of exogenous dopamine recorded by chronoamperometry in freely-moving rats

Previously, we developed technology that coupled high-speed chronoamperometry with microejections of dopamine (DA) to measure DA clearance in the brains of freely-behaving rats. Here, by varying the ejection volumes of DA across a 200-fold difference, the kinetics of striatal clearance were analyzed as a function of time and DA volume from 289 chronoamperometric signals (n=20 rats). Each DA clearance trace was fitted to a first-order exponential decay function to determine the rate constant for DA clearance (k). Additionally, the apparent Michaelis-Menten V_max and K_m kinetic constants were determined in freely-moving rats, enabling quantitative comparison of our values with other models of reuptake. The first-order rate constant for DA clearance, which reflects the V_max/K_m ratio or clearance efficiency, did not vary significantly when small volumes of DA were ejected resulting in peak DA signal amplitudes (A_max) of <5 μM. However, following nomifensine-induced DAT inhibition, A_max was increased and k was attenuated simultaneously with behavioral activation; and A_max and behavior remained elevated beyond the initial period. Our results indicate that the analysis of kinetic parameters from chronoamperometric DA signals may be useful for investigating drug-induced regulation of DAT kinetics in relation to the behavior of freely-moving rats.