Real-time characterization of dopamine overflow and uptake in the rat striatum

The rate of overflow and disappearance of dopamine from the extracellular fluid of the rat striatum has been measured during neuronal stimulation. Overflow of dopamine was induced by electrical stimulation of the medial forebrain bundle with biphasic pulse trains. The instantaneous concentration of dopamine was measured with a Nafion-coated, carbon fiber microelectrode implanted in the brain. The measurement technique, fast-scan cyclic voltammetry, samples the concentration of dopamine in less than 10 ms at 100 ms intervals. Identification of dopamine is made with cyclic voltammetry. Stimulated overflow was measured as a function of electrode position, stimulation duration, stimulation frequency, and after administration of L-DOPA and nomifensine. The observed concentration during a 2-s, 60-Hz stimulation was found to alter with position of the carbon fiber electrode. For stimuli of 3 s or less the amount of overflow was found to be a linear function of stimulus duration at a fixed electrode position. The observed overflow was found to be steady-state at a frequency of 30 Hz, suggesting a balance between uptake and synaptic overflow under these conditions. The experimental data was found to be successfully modelled when the balance of uptake and stimulated overflow was considered. It was assumed that each stimulus pulse releases a constant amount of dopamine (125 nM), and that uptake follows a Michaelis-Menten model for a single uptake site with Km = 200 nM and Vmax = 5 microM/s. The increase in stimulated overflow observed after L-DOPA (250 mg/kg) could be modelled by a 1.6-fold increase in the amount of dopamine release with no alteration of the uptake parameters. The increase in modelled by an increase in Km. In addition, the fit of the modelled data to the experimental data was improved when diffusion from the release and uptake sites was considered.