Increased Stimulated Release and Uptake of Dopamine in Nucleus Accumbens After Repeated Cocaine Administration as Measured by In Vivo Voltammetry

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Abstract
Electrically stimulated dopamine (DA) release (overflow) and uptake were measured with in vivo voltam-metry in the nucleus accumbens (N ACC) of anesthetized rats that had previously received repeated cocaine treatments. Electrically stimulated DA release was induced by a 10-s stimulation in the medial forebrain bundle (2-ms, 200-/iA, biphasic pulses at 100 Hz). DA overflow and uptake were measured with fast chronoamperometry using a Nafion-plated, carbon fiber electrode. Animals given repeated doses of cocaine (10 mg/kg s.c. from day 1 to 5, 20 mg/kg s.c. from day 6 to 10) showed marked increases in DA uptake (5.47±0.28 vs. 2.93±0.26 μM/s) and in stimulated DA overflow (27.3±1.1 vs. 18.9±1.3 nM) compared with DA uptake and stimulated overflow in saline control animals. The increased uptake was shown to be independent of the increased overflow. Uptake was monitored as a function of stimulation current, and the data were extrapolated to zero stimulation, resulting in calculated rates of uptake of 2.43 and 3.71 μM/s in the control and cocaine-treated groups, respectively. These effects were found to be temporary, as there were no significant differences in stimulated release or uptake between saline control animals and animals given 10 days of cocaine followed by a 10-day abstinence period. These alterations in the N ACC produced by repeated cocaine administration may be a compensatory response to prolonged uptake blockade of synaptic DA.

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