EFFECT OF COCAINE AND 5-HT3 RECEPTOR ANTAGONISTS ON 5-HT-INDUCED [H-3] DOPAMINE RELEASE FROM RAT STRIATAL SYNAPTOSOMES

The effect of serotonin (5-HT) on the release of tritium from striatal synaptosomes previously loaded with [H-3]dopamine ([H-3]DA) was studied. 5-HT stimulated both the spontaneous and Ca2+-evoked efflux of tritium in a concentration-dependent manner. This effect was not mimicked by the non-selective 5-HT agonist, d-lysergic acid diethylamide. Further, the stimulatory effects of 5-mu-M 5-HT were unaffected by the selective 5-HT3 receptor antagonists, MDL-72222 and GR-38032F. On the other hand, cocaine and the selective DA uptake inhibitor, nomifensine completely antagonized the effect of 5-mu-M 5-HT on spontaneous tritium efflux with IC50 values of 0.2 and 0.09-mu-M, respectively. The effect of 5-HT on Ca2+-evoked tritium efflux was also blocked by these DA uptake inhibitors, albeit at somewhat higher concentrations. These data support the hypothesis that 5-HT induces the release of DA from striatal nerve terminals via a mechanism involving the transport of 5-HT into the dopaminergic terminal, rather than by activating 5-HT3 receptors as has been proposed to account for the effect of 5-HT observed in striatal slices.