5‐HT3Receptor Antagonist MDL 72222 Dose‐Dependently Attenuates Cocaine‐ and Amphetamine‐Induced Elevations of Extracellular Dopamine in the Nucleus Accumbens and the Dorsal Striatum

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Abstract
The effects of a 5‐HT3receptor antagonist MDL 72222 on cocaine‐ and amphetamine‐induced increases in extracellular dopamine in the nucleus accumbens and the dorsal striatum were studied with microdialysis technique using halothane anaesthesized rats. Dopamine and its metabolites were measured by HPLC with electrochemical detection. Cocaine elevated extracellular dopamine in the nucleus accumbens and to a lesser extent in the dorsal striatum, but it did not affect dopamine metabolites,3,4‐dihydroxyphenylacetic acid (DOPAC) and homovanillic acid. Pretreatment with MDL 72222 (25‐100 μg/kg) dose‐dependently attenuated cocaine‐induced elevation of dopamine in both of the nuclei studied. Amphetamine elevated extracellular dopamine and reduced DOPAC and homovanillic acid equally in the nucleus accumbens and in the dorsal striatum. MDL 72222 also attenuated the amphetamine‐induced elevation of extracellular dopamine concentration in both brain areas studied, but first at a dose of 100 μg/kg. The different potencies of the interactions of the 5‐HT3receptor antagonist with cocaine and amphetamine could be related to the different mechanisms by which these drugs primarily elevate extracellular dopamine.