Cocaine: An in vivo microdialysis evaluation of its acute action on dopamine transmission in rat striatum

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Abstract
The effects of cocaine on dopamine (DA) neurotransmission were evaluated by in vivo microdialysis in the striatum of halothane-anesthetized rats. Intravenous cocaine produced a dose-dependent, transient increase of the extracellular concentration of DA, with a peak response within 10 min and a return to control level by 30 min. The sharp DA response pattern was abolished in a calcium-free environment, indicating that DA release enhanced by cocaine originates from a vesicular storage pool. Continuous administration of cocaine (via the perfusion medium) directly into the nigrostriatal terminal region also produced a dose-dependent increase in DA release. Low concentrations (10−5 M and 10−6 M) of cocaine maintained DA at a constant stable level, consistent with the effects observed after potent DA uptake inhibitory agents (e.g., nomifensine and Lu19005). However, continuous exposure to high concentrations (< 10−4 M) induced a transient elevation of DA within 20 min, following which DA decreased to a stable but high level; this decrease might reflect tolerance to the effect of cocaine. Administration of cocaine (10−3 M) into the substantia nigra did not change striatal DA release. The local striatal action of cocaine was less potent than amphetamine in elevating DA overflow and in its effect on DA metabolism. These findings suggest that the fast transient enhancement of DA by intravenous cocaine is most likely a consequence of the transient presence of cocaine in the terminal region, correlating with the well-known rapid pharmacokinetic and behavioral aspects of the drug.